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Performance of Quaternized Polybenzimidazole-Cross-Linked Poly(vinylbenzyl chloride) Membranes in HT-PEMFCs
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Applications of Polymer, Composite, and Coating Materials

Performance of Quaternized Polybenzimidazole-Cross-Linked Poly(vinylbenzyl chloride) Membranes in HT-PEMFCs
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  • Funda Arslan*
    Funda Arslan
    Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany
    Department of Chemical and Biological Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0001-5273-192X
  • Khajidkhand Chuluunbandi
    Khajidkhand Chuluunbandi
    Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany
    Department of Chemical and Biological Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany
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  • Anna T.S. Freiberg
    Anna T.S. Freiberg
    Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany
    Department of Chemical and Biological Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany
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    Orcidhttps://orcid.org/0000-0002-7885-7632
  • Attila Kormanyos
    Attila Kormanyos
    Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany
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    Orcidhttps://orcid.org/0000-0002-2145-7419
  • Ferit Sit
    Ferit Sit
    Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany
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  • Serhiy Cherevko
    Serhiy Cherevko
    Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany
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    Orcidhttps://orcid.org/0000-0002-7188-4857
  • Jochen Kerres
    Jochen Kerres
    Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany
    Faculty of Natural Science, North-West University, Potchefstroom 2520, South Africa
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  • Simon Thiele
    Simon Thiele
    Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany
    Department of Chemical and Biological Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany
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    Orcidhttps://orcid.org/0000-0002-4248-2752
  • Thomas Böhm*
    Thomas Böhm
    Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0003-2036-2159
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ACS Applied Materials & Interfaces

Cite this: ACS Appl. Mater. Interfaces 2021, 13, 47, 56584–56596
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https://doi.org/10.1021/acsami.1c17154
Published November 16, 2021

Copyright © 2021 The Authors. Published by American Chemical Society. This publication is licensed under

CC-BY 4.0 .

Abstract

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High-temperature proton-exchange membrane fuel cells (HT-PEMFCs) are mostly based on acid-doped membranes composed of polybenzimidazole (PBI). A severe drawback of acid-doped membranes is the deterioration of mechanical properties upon increasing acid-doping levels. Cross-linking of different polymers is a way to mitigate stability issues. In this study, a new ion-pair-coordinated membrane (IPM) system with quaternary ammonium groups for the application in HT-PEMFCs is introduced. PBI cross-linked with poly(vinylbenzyl chloride) and quaternized with three amines (DABCO, quinuclidine, and quinuclidinol) are manufactured and compared to the state-of-the-art commercial Dapazol PBI membrane ex situ as well as by evaluating their HT-PEMFC performance. The IPMs show reduced swelling and better mechanical properties upon doping, which enables a reduction in membrane thickness while maintaining a comparably low gas crossover and mechanical stability. The HT-PEMFC based on the best-performing IPM reaches up to 530 mW cm–2 at 180 °C under H2/air conditions at ambient pressure, while Dapazol is limited to less than 430 mW cm–2 at equal parameters. This new IPM system requires less acid doping than conventional PBI membranes while outperforming conventional PBI membranes, which renders these new membranes promising candidates for application in HT-PEMFCs.

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1. Introduction

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Fuel cells (FCs) are promising energy conversion devices, gaining more importance with the world’s increasing energy demand and the need to reduce CO2 emission. Among different types of FCs, proton-exchange membrane FCs (PEMFCs) are the most promising and best developed due to their applicability in different areas such as the automotive application and in the energy storage sector. (1,2) PEMFCs can be divided into low-temperature (LT) PEMFCs (below 100 °C) and high-temperature (HT) PEMFCs (above 100 °C). (1) HT-PEMFCs offer various advantages over LT-PEMFCs, such as faster reaction kinetics and improved tolerance to carbon monoxide (CO). (1,3−5) Above 100 °C, no liquid water management is required when operating at ambient pressure, and the higher temperature simplifies the cooling system by the larger difference between FC and ambient temperature. (6)
Membranes used in LT-PEMFC application are mainly based on perfluorosulfonic acid polymers such as Nafion. Their proton conductivity depends on humidification, and therefore, these types of proton-exchange membranes (PEMs) lose ionic conductivity above 100 °C. (7) In this regard, basic polymers doped with nonvolatile inorganic acids have proven themselves as efficient HT-PEMs. (1) Polybenzimidazole (PBI) is currently the polymer of choice for HT-PEMFCs when doped with phosphoric acid (PA, H3PO4). (8) Among different types of PBIs, the most investigated as HT-PEM is poly[2,2′-(m-phenylene)5,5′-(bibenzimidazole)] (m-PBI) (see Figure S1), which is also commercially available in different forms such as coatings or fibers. (1,9) PBI shows excellent thermal and chemical stability, with a glass transition temperature above 400 °C. (6,10) Two basic N-sites per repeating unit can interact with PA molecules. Additionally, free PA molecules will enter the PBI polymer matrix and generate the main proton conductive network. The proton conduction in PBI/PA systems depends on the protonation equilibrium of the acid by building a network of N–H+···H2PO4–, H3PO4··· H2PO4–, and/or N–H+···H–OH, which enables proton hopping. (11−13,48)
Since PA is responsible for proton conductivity in HT-PEMs, the acid-doping level (ADL), which is defined as the number of PA molecules per repeating unit of the basic polymer, is a crucial parameter for HT-PEMFC performance. (12) While higher ADLs increase proton conductivity, they also deteriorate the mechanical strength of the membrane. (4−6,12) Alternatively, an acid uptake (AU) instead of the ADL is provided, which corresponds to the weight gain due to PA uptake relative to the dry weight of the membrane. (14) The AU is the preferred measure of the acid content for cross-linked HT-PEMs. This type of composite membrane includes not only a basic polymer such as PBI but also an additional polymer that is covalently or ionically cross-linked with the basic polymer. Cross-linked membranes aim at maintaining mechanical stability upon acid doping, enhancing chemical stability and degradation resistance, and lowering the acid loss of PEMs at high PA-doping levels. (11,15−20)
Ionic cross-links are formed by mixing a basic PBI-type polymer with acidic polymers (e.g., sulfonated polysulfones). (11,21) Covalent cross-linking can be achieved by using, for example, low-molecular compounds such as divinyl sulfone, (22) bisphenol A-diepoxide, (23) terephthaldehyde, (24) or dichloromethyl phosphinic acid. (21,25) Also, macromolecular cross-linkers are well-established in the literature, such as halomethylated polymers, (26) chloromethylated PSU, (27) bromomethylated polyetherketone, (28) or poly(vinylbenzyl chloride) (PVBC). (21,29)
Recently, PEMs containing quaternary ammonium (QA) functional groups are gaining significant attention for the application in HT-PEMFCs. (1) When doped with PA for use as an HT-PEM, these membrane systems are called ion-pair-coordinated membranes (IPMs). The QA moieties can form strong interactions with biphosphate anions. (11) These additional interactions can prevent PA leaching since the QA···H2PO4– interaction is 8 times stronger than the acid–base interaction between benzimidazole and PA. (15,30) In 2011, Ma et al. evaluated poly(arylene ether ketone) with pendant QA groups and doped with PA as HT-PEMs. Their results showed that QA groups show a better PA-doping ability than the imidazole groups in PBI while maintaining thermal and mechanical stability. (31) In 2014, Cho et al. cross-linked m-PBI with methylated PBI-OO bearing two QA groups. A stronger PA absorption for these membranes could be observed. (32) Furthermore, Lee et al. presented in 2016 a QA-biphosphate ion-pair-coordinated polyphenylene membrane that yielded a peak power density of 800 mW cm–2 at 180 °C during H2/O2 operation. (33) Using the same membrane material, Atanasov et al. recently optimized the catalyst layer of an HT-PEMFC using a phosphonated ionomeric binder, yielding peak power densities between 309 and 923 mW cm–2 at 120–240 °C under H2/air conditions. At 160 °C and at a current density of approximately 1600 mA cm–2, a peak power density of 540 mW cm–2 was achieved. (34) Hu et al. investigated PBIs with macromolecular hyperbranched cross-linkers and QA as HT-PEMs. (15) They cross-linked bromomethylated poly(p-xylene) with poly(4,4′-diphenylether-5,5′-bibenzimidazole) (OPBI) and used trimethylamine as a quaternizing agent. Their results showed that the basic QA moieties helped to dissociate PA, establishing acid–base pairs, which enhanced proton conductivity and acid retention. They reported a peak power density of 260 mW cm–2 at 160 °C during H2/air operation.
A potentially good amine for the application in IPMs is 1,4-diazabicyclo(2.2.2)octane (DABCO) (see Figure 1), which is, for example, known for its use in the research field of anion-exchange membranes. (35,36) In this work, its performance as a quaternizing agent for OPBI-c-PVBC cross-linked membrane systems for the application in HT-PEMFCs was investigated. Furthermore, the amines 1-azabicyclo(2.2.2)octan-3-ol (quinuclidinol) and 1-azabicyclo(2.2.2)octane (quinuclidine) (see Figure 1) were chosen as structural relatives to DABCO. The main difference between these three amines is the number of amine moieties (see Figure 1). The main difference between quinuclidine and quinuclidinol is the −OH group present in the latter, which may contribute to the hydrogen-bonding network between PA and its anions, leading to stronger interactions and less PA leaching.

Figure 1

Figure 1. Chemical structures of the three amines (quinuclidine, quinuclidinol, and DABCO) and the polymer blend membrane OPBI-c-PVBC (exemplary for quinuclidine). Cross-linking points are highlighted and numbered (1: covalent cross-linking, 2: imidazolium cation (Im+), and 3: QA). X– represents the counter anion, for example, Cl–, OH–, and H2PO4–.

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In this work, we study the effect of these three amines on cross-linked membranes comprising OPBI and PVBC in three different ratios. The prepared membranes were evaluated ex situ as well as within an HT-PEMFC. As a reference system, the commercial HT membrane Dapazol, from Danish Power Systems ApS, an m-PBI membrane, was chosen. (37)

2. Experimental Section

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2.1. Membrane Materials

OPBI was purchased from Fumatech BWT GmbH. PVBC was obtained from Scientific Polymer Products. DABCO and 85 wt % ortho-PA were purchased from Merck. 1-Azabicyclo(2.2.2)octan-3-ol (quinuclidinol, QOH) and 1-azabicyclo(2.2.2)octane (quinuclidine, Q) were procured from Sigma-Aldrich and Alfa Aesar, respectively. All chemicals were used as received. The Dapazol M40 membrane with a nominal dry thickness of 40 μm was purchased from Danish Power Systems ApS.

2.2. Membrane Preparation

5 wt % OPBI and 15 wt % PVBC solutions were prepared by dissolving the polymers in dimethyl sulfoxide. The OPBI solution was stirred at 50 °C overnight and the PVBC solution for 1 h at rt. Afterward, the solutions were blended to yield weight ratios of OPBI/PVBC of 60:40, 75:25, and 90:10. The blend solutions were stirred for 30 min at room temperature (rt) until homogenization was achieved, followed by casting onto a glass plate. The casting was carried out using an automated film applicator (ZAA 2300, Zehntner) with a blade length of 8 cm and a speed of 20 mm s–1. The heights were set to 450, 500, and 630 μm for the ratios 60:40, 75:25, and 90:10, respectively. The resulting membranes were dried at 80 °C for 24 h, followed by 5 h at 80 °C under reduced pressure. During the 24 h at 80 °C, membranes were covered with a plastic lid to slow down the evaporation rate and achieve cross-linking. (18) The dry thicknesses of all membranes were between 20 and 25 μm.
The blend membranes were quaternized by immersion in 0.5 M solutions of the three amines (DABCO, quinuclidinol, and quinuclidine) in ethanol for 24 h at 60 °C. Afterward, the membranes were rinsed with DI water.

2.3. Doping with PA

Doping was performed by the immersion of membrane pieces of 1 (for weighing), 4 [for ion-exchange capacity (IEC) measurements], or 9 cm2 (for membrane electrode assembly preparation) in 85 wt % aqueous PA at rt. All membranes were doped with PA until saturation was reached (2 days for IPMs and 4 days for Dapazol). PA-doped in-house-prepared membranes had a thickness of 40–50 μm, and Dapazol had a thickness of 75–80 μm.

2.4. Measurement of ADL

The 1 cm2 membrane pieces were dried at 110 °C and vacuum for 5 h, following the literature, (38) to determine their dry weight (wdry). Following, the samples were immersed in 4 mL of aqueous 85 wt % PA until saturation was reached. The drying process was repeated for the doped membrane pieces. This process guaranteed that the weight gain is purely due to PA uptake (wPA) and not caused by water uptake. The ADL represents the number of PA molecules per repeating unit of the basic polymer, here OPBI. (39,48) Therefore, eq 1 was used to calculate ADLs
(1)
with the mass fraction of OPBI (XOPBI), the molar mass of the repeating unit of OPBI (MOPBI), and the molar weight of PA (MPA). It should be noted that the interaction of OPBI with PVBC, which forms imidazolium cations (see 2 in Figure 1), is not considered in eq 1.
The membranes’ swelling when taking up PA was determined by measuring their x, y, and z dimensions before and after the doping process using eq 2. (48)
(2)
Vdoped and Vundoped represent the doped and undoped volumes, respectively, calculated by the multiplication of x, y, and z dimensions for the doped and undoped membrane pieces.

2.5. Measurement of IEC

IECs were determined by the back titration method. (40) First, the membranes were conditioned in 0.1 M HCl for 48 h at 90 °C to remove excess amine. Afterward, the membranes were immersed in a 1 M KOH solution at 90 °C for 24 h to exchange Cl– with OH–. The OH– form of the membranes was then dried at 110 °C and vacuum for 5 h to determine the dry weight (wdry). Afterward, the membranes were immersed in saturated sodium chloride (NaCl) at rt for 24 h. 3 mL of 0.1 M hydrochloric acid (HCl) was added to the saturated NaCl solution, including the membrane, and stirred at rt overnight. The membrane was washed with 25 mL of distilled H2O, which was added to the same solution. This solution was titrated back with a 0.1 M sodium hydroxide (NaOH) solution to determine the remaining HCl using an OMNIS autotitrator (Metrohm Instruments). Equation 3 was used to calculate the IECs. (40)
(3)
The concentrations of the HCl and NaOH solutions are represented by cHCl and cNaOH. VHCl and VNaOH are the used volumes. Wdry is the dry weight of the membrane (OH–-form).
The IECs of the in-house-prepared membranes are total IECs, to which the cross-linking points 2 and 3 (see Figure 1) and the imidazole groups of OPBI are contributing.

2.6. Tensile Testing

Uniaxial tensile tests were performed on a Shimadzu EZ-SX with a 100 N load cell. The dimensions of the samples were kept constant with 5 cm length and 1.5 cm width. Samples were measured pristine and doped with PA. Young’s modulus and elongations at the break were evaluated with stress–strain measurements under ambient conditions with a constant strain rate of 5 mm min–1. Young’s modulus (E) was determined by the stress (σ) and strain (ε) ratio at a small strain in the linear region (see eq 4).
(4)

2.7. Thermogravimetric Analysis

Thermogravimetric analysis (TGA) was carried out with samples of approximately 5 mg of the pristine membrane material (before doping with PA) using a TG 209 F1 Libra (NETZSCH). The measurements were carried out under oxidizing (65% O2, 35% N2) conditions to imitate the PA doping and FC measurement atmosphere. The flow rate was set to 40 mL min–1 over a temperature range of 30–1000 °C. The samples were loaded into Al2O3 crucibles.

2.8. Fabrication of Membrane Electrode Assemblies

Symmetrical membrane electrode assemblies (MEAs) were fabricated using identical electrodes for the anode and cathode. The ink for the fabrication of gas diffusion electrodes (GDEs) consisted of 1 wt % solid content in a solvent mixture of 50 wt % 2-propanol in water. The solid fraction consisted of 80 wt % Pt/C [Alfa Aesar, 60 wt % platinum (Pt) on carbon] and 20 wt % polytetrafluoroethylene (PTFE) binder (Fuel Cell Store, Teflon Dispersion DISP 30, 60 wt % PTFE). The ink was homogenized using an ultrasonic horn (Hielscher) at 90 W for 30 min at 0 °C. The catalyst ink was deposited onto a Freudenberg H23C8 gas diffusion layer (GDL) via spray coating (ExactaCoat, Sono-Tek). The temperature of the heating stage was set to 90 °C. The ink flow rate was set to 0.33 mL min–1 with an ultrasonication power of 5 W, resulting in a deposition rate of approximately 0.03 mgPt cm–2 per deposition cycle until a total Pt loading of 1 mg cm–2 was reached. The loading was determined by weighing the GDL and the GDE after the ink deposition. The resulting GDEs were sintered at 360 °C at atmospheric pressure for 20 min (OTF-1200X, MTI Corporation). (41)
For MEA production, the PA-doped membrane was placed between the GDEs and hot-pressed (Carver Inc.) at 150 °C and 10 MPa for 10 min using Pacopads (Pacothane) (3 × 3 cm2) to ensure an equal pressure distribution. (42) Due to the Pacopad dimensions, a total force of 9 kN was applied during hot pressing. The GDEs and hence the resulting active area had dimensions of 2 × 2 cm2 and the membrane had an area of 3 × 3 cm2.

2.9. Fuel Cell Testing

FC testing was performed using an 850e FC test system from Scribner Associates Inc. All FCs had an active area of 4 cm2 and were assembled using 150 μm glass-fiber-reinforced PTFE gaskets on both anode and cathode sides. A GDL compression of approximately 20% was reached when considering the electrodes as noncompressible. A torque of 5 N m was applied to assemble the cell. All tests were run at 180 °C under dry H2 (0.25 L min–1) and air (0.75 L min–1) at ambient pressure. For conditioning, the FCs were heated up to 120 °C. Following, the H2/air supply was switched on while heating up to 180 °C, and a constant load of 200 mA cm–2 was applied until the temperature was reached. Afterward, a constant load of 400 mA cm–2 was held for 30 min. (43) The polarization data were measured galvanostatically with a step size of 0.05 A per point in the activation region, 0.2 A in the Ohmic region, and 0.05 A in the mass transport region. The step time was 1 min per point. To evaluate the Ohmic cell resistance (RΩ), the on-line current-interrupt measurement method was used and recorded at each polarization data point. (44)

2.10. Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy was carried out using a Scribner 885-HS FC potentiostat (Scriber Associates Inc.). A constant current hold of 15 min was performed at the desired current density (400 and 600 mA cm–2) before the measurement. The frequency was swept from 10 kHz to 0.1 Hz with 20 steps per decade. The AC RMS amplitude was set to 10% of the DC current.

2.11. Linear Sweep Voltammetry

Linear sweep voltammetry (LSV) was carried out using a Biologic SP-300 potentiostat. Gas flows and cell temperature were controlled using a Scribner 850e (Scribner Associates). The anode was purged with 0.3 L min–1 H2 and the cathode with 0.3 L min–1 N2 at a cell temperature of 180 °C and atmospheric pressure. The measurements were conducted after polarization data were recorded. Before starting the LSV measurement, the cell was purged with H2 and N2 until the rest potential reached a pseudo-steady-state value. Afterward, the voltage was swept between that steady value and 0.5 V. Higher working electrode potentials were avoided to prevent Pt oxidation. The scan rate was 2 mV s–1. The limiting current density was defined after applying a short-circuit correction. (45) Therefore, a linear fit was carried out for the potential window from 0.30–0.40 V. The limiting current density (jH2cross) was quantified at 0.3 V. The internal short-circuit resistance (ISR) was estimated from the inverse of the slope of this linear fit. (48)
The hydrogen crossover flux (ηH2cross) was calculated from jH2cross applying Faraday’s law according to eq 5
(5)
with the FC active area A, the number of electrons taking part in the reaction n (here, 2e– per H2 molecule), and the Faraday constant F (96 485 C mol–1). (46,47)

2.12. Inductively Coupled Plasma–Mass Spectrometry

The phosphorous content of the liquid samples taken from both cell compartments was analyzed by inductively coupled plasma–mass spectrometry (ICP–MS) (PerkinElmer NexION 350x). Samples were prepared as follows: the exhaust gases from the cell passed through 400 mL of distilled water at 10 °C separately at the anode and cathode to collect the exhaust water containing leached PA. Water sampling was carried out after operating the FCs for 48 h at 400 mA cm–2. To quantify the phosphorous content, ICP–MS was operated using its dynamic reaction cell (DRC). O2 was used as a DRC cell gas. Instrument parameters were set based on previous literature data (RPa = 0.02 and RPq = 0.40). (48)16S32O+ served as the internal standard. ICP–MS was calibrated daily using a four-point calibration curve. Standards (both the calibration and internal standards) were prepared freshly from stock solutions (Merck Certipur P and S) containing the elements of interest in a given concentration in pure MilliQ water.

3. Results and Discussion

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3.1. OPBI-C-PVBC Membranes

Figure 1 represents the chemical structure of the cross-linked OPBI with PVBC (OPBI-c-PVBC) quaternized with the amines quinuclidine (Q), quinuclidinol (QOH), or DABCO. An SN2-type reaction can occur between the −CH2Cl moieties of PVBC and the amine group (−NH−) of PBI. (29) Hence, cross-linking probably occurs via N-alkylation of the benzimidazole moieties by −CH2Cl groups (see 1 in Figure 1). (35) Furthermore, the imine moiety in PBI (═N−) can also react with the −CH2Cl group and form imidazolium cations (Im+) (see 2 in Figure 1) at elevated temperatures, (49−51) which was verified by Lu et al. (35) Postquaternization was carried out by immersing the OPBI-c-PVBC membranes into amine solutions in ethanol. An excess of amine was given so that selective quaternization in the case of DABCO was favored, yielding the quaternization of only one nitrogen atom. (35)3 in Figure 1 represents the quaternizing exemplary with quinuclidine, which is analogous for quinuclidinol and DABCO.
OPBI and PVBC were cross-linked with the ratios 90:10, 75:25, and 60:40. The resulting blends were characterized by attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy (see Figure S2). For all ratios, the disappearance of the CH2–Cl bending peak at 1266 cm–1 could be observed. (52) Furthermore, the C–N stretching vibration appearing at 1014 cm–1 confirmed successful cross-linking. (52) A noticeable stretching vibration of the −CH2 group of PVBC at 2927 cm–1 and the C–Cl peak at 692 cm–1 loses intensity with the decreasing PVBC content in the blend. (26)
The OPBI-c-PVBC membranes were post-treated with three quaternizing agents (DABCO, quinuclidinol, and quinuclidine) and yielded nine membranes. These nine membranes were doped with PA and used as PEMs in HT FC application. The initial peak power densities achieved were compared to each other to evaluate the influence of the blend ratio and/or the quaternizing agent on FC performance.
As can be seen in Table 1, there is one general trend observable for this membrane matrix: the initial peak power density increases with the increasing PVBC content. This can be linked to the increased AU at higher PVBC contents, as shown in Table S1 of the Supporting Information. The AU is, in this context, a better suited measure than the ADL to compare acid contents within the membranes: the ADL calculated from eq 1 will increase when the absolute AU is kept constant if the amount of OPBI in the membrane is reduced. A higher proton conductivity can be expected from 90–10 to 75–25 to 60–40. The only outlier from this trend is the OPBI-c-PVBC 90:10 membrane with quinuclidinol. When comparing the effect of different QAs on the maximum peak power density for the maximum amount of PVBC studied in the cross-linked membrane, DABCO shows 44% lower performance compared to quinuclidine and quinuclidinol. DABCO possesses two amine moieties, and both could lead to cross-linking with PVBC. However, when an excess of DABCO is given, presumably, only one nitrogen will undergo cross-linking. (53) Nevertheless, at this point, we cannot exclude a possible formation of bis-quaternized DABCO, which could lead to a conductivity drop due to the instability of the ammonium groups. (54) Furthermore, Lu et al. reported lower IECs for this type of cross-linked membrane quaternized with DABCO compared to the three best-performing here. (35)
Table 1. Initial Peak Power Densities of the Cross-Linked Membranes OPBI-C-PVBC 90–10, 75–25, and 60–40 Quaternized with Quinuclidine, Quinuclidinol, and DABCOa
OPBI:PVBCquinuclidinequinuclidinolDABCO
90:10246 mW cm–2461 ± 11 mW cm–2254 mW cm–2
75:25341 mW cm–2381 mW cm–2362 mW cm–2
60:40529 ± 6 mW cm–2529 ± 13 mW cm–2372 mW cm–2
a

The corresponding polarization and power density curves can be seen in Figure S3. All measurements were carried out at 180 °C under 0.25 L min–1 dry H2 at the anode, and 0.75 L min–1 air at the cathode, with a Pt loading of 1 mg cm−2 at the anode and cathode. The bold values represent the HT-PEMFCs incorporating the best-performing membranes. These measurements were repeated three times, and the initial peak power densities are represented with their standard deviations.

The ratio 60 wt % OPBI to 40 wt % PVBC yielded for quinuclidine and quinuclidinol almost the same initial peak power densities. Therefore, these two membranes were chosen for further analysis to understand the influence of the two different amines, quinuclidine and quinuclidinol, on properties such as swelling, acid doping, mechanical properties, H2 gas crossover, and PA loss during 48 h FC operation. As a third in-house-prepared membrane, the 90 wt % OPBI and 10 wt % PVBC quaternized with quinuclidinol were chosen for further evaluation since it exhibited the third-highest peak power density. This is the only membrane prepared that does not fall into the overall line that the peak power density increases with the increasing PVBC content. At a low PVBC content and, therefore, at low quaternization levels, the OH-moiety could serve as an additional anchor point for the PA network or participate in proton conduction. At a higher PVBC content, with increasing PA uptake, the difference between the membranes quaternized with quinuclidine and quinuclidinol becomes less dominant, probably because now the effect of the amine is the dominant influence compared to the weak interaction of the OH group with PA.

3.2. Property Comparison of the Three Best-Performing Membranes and a Commercial Membrane

Membrane properties of the three best-performing OPBI-c-PVBC blend membranes (see Table 1) were compared to those of the commercial Dapazol membrane. Therefore, the ADLs, AUs, swelling, IECs, Young’s moduli (E), elongations at break (εb), and tensile strengths at break (σE) were investigated, and the results are shown in Table 2.
Table 2. ADLs, AU, Swelling, IECs, Young’s Moduli (E), Elongations at Break (εb), and Tensile Strengths at Break (σE) for the Membranes 60-40-Q, 60-40-QOH, and 90-10-QOHa
[wt % OPBI-wt % PVBC]-[Amine]ADL (molPA molPBI–1)AU (wt %)swelling (vol %)IEC (meq g–1)E (MPa) undopedE (MPa) dopedεb (%) undopedεb (%) dopedσE (MPa) undopedσE (MPa) doped
60-40-Q16.08 ± 0.19236 ± 3134 ± 33.14 ± 0.111684.20 ± 75.37100.87 ± 3.3618.44 ± 1.3025.62 ± 2.1457.75 ± 6.748.58 ± 1.60
60-40-QOH15.43 ± 0.64227 ± 9130 ± 63.82 ± 0.231957.43 ± 67.64135.51 ± 12.1311.78 ± 0.1723.82 ± 3.1174.99 ± 5.0510.10 ± 1.71
90-10-QOH8.05 ± 0.16177 ± 4130 ± 13.94 ± 0.052656.44 ± 39.61262.76 ± 31.9414.15 ± 2.8244.35 ± 2.53107.81 ± 9.6222.27 ± 0.31
Dapazol12.70 ± 0.89404 ± 28248 ± 106.49b2738.09 ± 68.5387.20 ± 4.1540.45 ± 3.09139.78 ± 5.33110.88 ± 3.2714.77 ± 2.16
a

Young’s moduli (E), elongations at break (εb), and tensile strength values at break (σE) are shown for doped and undoped samples at rt. The values are the mean of three replicates and are represented with their standard deviations. It should be noted that the cross-linking points 2 and 3 and the imidazole moieties of OPBI are contributing to the IECs of the in-house-prepared membranes.

b

Theoretically calculated (see the Supporting Information).

The terminology of the quaternized OPBI-c-PVBC membranes is [wt % OPBI]-[wt % PVBC]-[abbreviation of amine]. Exemplarily, a blend with 60 wt % OPBI and 40 wt % PVBC quaternized with quinuclidinol will be abbreviated as 60-40-QOH.
We first start by comparing the PA-doping characteristics of the four types of membranes. The achieved ADL for m-PBI (Dapazol) of around 13 (see Table 2) is comparable with the ADLs found in the literature for low-molecular-weight m-PBI. Yang et al. reported the ADL of intermediate-molecular-weight m-PBI to be around 10–11 and of low-molecular-weight m-PBI to be about 12–14 when doped in 85 wt % PA at rt. (55,56) The volumetric swelling of around 248 vol % (see Table 2) is also in good accordance with the literature-reported volumetric swelling of m-PBI ranging from 150–250 vol %. (1,57) As shown in Table 2, the ADL of 60-40-Q and 60-40-QOH is about 16 and higher than the ADL of m-PBI, which is caused by the normalization of the amount of PA absorbed to the amount of PBI in the membrane blends. Therefore, the ADL of 90-10-QOH is also only half of the 60–40 blends. With an absolute AU of around 230% for the 60–40 blends and 180 wt % for the 90-10-QOH membrane, the amount of acid present in the membrane is roughly half of the AU of Dapazol with about 400 wt %. This finding correlates directly with the reduced swelling of the blend membranes with roughly half the value of the commercial Dapazol (see Table 2). The low AU of 90-10-QOH is very promising since the long-term goal in HT-PEMFC development is an HT-PEM without (or less) doping: the PA is the limiting factor due to its evaporation and leaching during FC operation. Nonetheless, as shown in Table 1, 90-10-QOH within an HT-PEMFC showed a slightly lower initial peak power density than 60-40-Q and 60-40-QOH, indicating that the lower PA content in this membrane may pose a limiting factor for its performance.
It is known that cross-linking enhances the mechanical properties of membranes, reduces softening and volumetric swelling with the increasing PA content, and allows for higher ADLs. Generally, higher AUs while maintaining mechanical stability are desirable since high AUs are expected to lead to higher proton conductivities. (58) The same behavior can be observed for F6PBI. Li et al. reported an ADL of 7 for pure F6PBI when doped in 75 wt % PA at rt for 100 h, (59) while blends with F6PBI showed higher ADLs. In an earlier study, we have reported an ADL of 10 for a blend membrane of F6PBI and PWN [phosphonated poly(pentafluorostyrene)] (39) and Yang et al. reported an ADL of around 14 for a blend of F6PBI and chloromethyl polysulfone when doped at rt in 85 wt % PA for 120 h. (27) There are two possible reasons for achieving higher ADLs when using OPBI and quaternization instead of F6PBI. First, the more flexible structure of OPBI can increase the number of incorporated free PA molecules within the polymer matrix. Second, two cationic groups (quaternized amine and imidazolium, 3 and 2 in Figure 1) can stabilize the anions of PA and hence increase proton concentration within the membrane compared to a pure PBI membrane.
All membranes prepared in this work showed high IECs ranging from 3 to 4 meq g–1 (see Table 2). This is in agreement with the literature. Lu et al. reported for this type of cross-linked membrane an IEC of 2.51 meq g–1 for PBI-c-PVBC with DABCO as a quaternizing agent. (35) Kulkarni et al. reported an IEC of 2.95 meq g–1 for sulfone-substituted OPBI and of 4.2 meq g–1 for postsulfonated OPBI (sOPBI). (60) Since Dapazol is a pure PBI membrane, the IEC was calculated to be 6.49 meq g–1 (see the Supporting Information). As shown in Table 2, the IECs of the membranes 90-10-QOH and 60-40-QOH showed almost equal values.
The amount of PA in the membrane is important for the ionic conductivity during HT-PEMFC operation, and it also largely influences the mechanical properties. Thus, stress–strain tests were executed with the pristine and doped membrane samples. With a Young’s modulus of around 2700 MPa, undoped Dapazol shows the highest rigidity, closely followed by the undoped 90-10-QOH membrane. This is in agreement with the literature. Zhang et al. reported an elastic modulus of 2950 MPa for OPBI. (61) As the mechanical stability in the polymer blends stems predominantly from the OPBI chain, a strong decrease in Young’s moduli for the 60–40 blends is expected. (35) After PA doping, however, Dapazol loses most of its mechanical stability, and the Young’s modulus is dropping to only 3% of its value in the undoped state. As the commercial membrane is not cross-linked, the PA uptake results in a strong softening effect as the chains can now very easily slide off from each other. The Young’s moduli of the cross-linked 60–40 membrane blends also drop significantly to 6–7% of their undoped values, resulting in a higher Young’s modulus in the doped state compared to Dapazol. Both the reduced AU and the cross-linking could explain the higher maintenance of stability upon doping. Of special interest is the 90-10-QOH membrane as it keeps almost 10% of its high Young’s modulus in the completely doped state, therewith significantly outperforming the other membranes and resulting in a doped Young’s modulus 3 times higher compared to Dapazol. The tensile strengths at break (σE) of undoped 90-10-QOH and Dapazol are almost the same with 108–111 MPa. Zhang et al. reported a tensile strength of 129.5 MPa for undoped OPBI. (61) The tensile strength of undoped 90-10-QOH is in accordance with the literature when considering the 90 wt % OPBI in the membrane. Yang et al. reported a tensile strength of 116.8 MPa for an undoped m-PBI membrane. (27) Hence, the tensile strength of Dapazol is 110.88 MPa, in good accordance with the literature (see Table 2). 60-40-QOH and 60-40-Q exhibit lower tensile strengths of 74.99 and 57.75 MPa, respectively, presumably due to the lower OPBI content within the membrane. The tensile strength at break after doping is lower for all membranes due to the plasticizing effect of PA. (55) Dapazol with a tensile strength of 14.77 MPa at rt and an ADL of 12.70 is in accordance with the literature. (27,55) 90-10-QOH shows the highest tensile strength of 22.27 MPa after doping with PA due to its lower AU of less than 200 wt % compared to the other membranes (see Table 2). Furthermore, it can be seen that the elongations at break for Dapazol are significantly higher than those for the blend membranes, in both the doped and undoped states (see Table 2). Higher elongations at break of pure PBI compared to those of the blend membranes can be explained by the lack of cross-links within this membrane, which allows this polymer to stretch without breaking covalent bonds. In general, the elongations at break were larger when doped compared to the undoped state of the membranes. This finding can be attributed to the plasticizing effect of absorbed PA. (55) For the OPBI-c-PVBC membranes, the increase in elongations at break upon doping was around 2–3 times, which is lower than that for Dapazol, which exhibited an increase of 3.5 times. This result confirms the better mechanical properties of the OPBI-c-PVBC membranes and correlates with both the higher AU and the higher volumetric swelling of Dapazol compared to those of the blend membranes.
The thermal-oxidative stability of the PEMs was investigated in oxygen-enriched synthetic air (65% O2 and 35% N2) by TGA (see Figure 2). The mass loss until 100 °C is due to water evaporation. Both PBI-types, m-PBI and OPBI, exhibited a high thermal-oxidative stability up to 400 °C. The cross-linked membranes showed thermal-oxidative stabilities up to 260 °C, where mass losses due to the degradation of PVBC can be observed. Within this range also mass losses due to the amines occur. The membrane 90-10-QOH containing 90 wt % OPBI showed a mass loss of around 10% between 260 and 480 °C due to the full loss of PVBC. The membranes containing 60 wt % OPBI exhibited lower thermal-oxidative stability due to their higher PVBC content. In general, all membranes exhibited high thermal-oxidative stabilities, verifying that PEMFC operation at 180 °C poses no thermal limit for the blend membranes.

Figure 2

Figure 2. (A) TGA and (B) derivative of the TGA data for the membranes 60-40-Q, 60-40-QOH, 90-10-QOH, its blend components OPBI and PVBC, and the commercial Dapazol membrane under oxidative conditions (65% O2 and 35% N2).

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Furthermore, the membranes 60-40-Q, 60-40-QOH, and 90-10-QOH revealed an amorphous nature as verified by X-ray diffraction (see Figure S6). Additionally, Fenton’s test was carried out to investigate the oxidative stability of the membranes 60-40-Q, 60-40-QOH, 90-10-QOH, and Dapazol. The corresponding data can be found in the Supporting Information (Figure S7).

3.3. Fuel Cell Characterization

The polarization and power density curves for the HT-PEMFCs incorporating the PA-doped membranes prepared in this work as well as the commercial Dapazol are shown in Figure 3.

Figure 3

Figure 3. Initial polarization and power density curves for the FCs based on the membranes 60-40-Q, 60-40-QOH, 90-10-QOH, and Dapazol. All measurements were carried out at 180 °C under 0.25 L min–1 dry H2 at the anode and 0.75 L min–1 air at the cathode, with a Pt loading of 1 mg cm–2 at the cathode and anode. Each curve in (A,B) represents the mean of three measurements of three different MEAs. The shaded areas represent standard deviations. In (C,D), the high-frequency resistance (HFR)-free data of the mean of the three FC measurements are represented.

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It can be seen that the OPBI-c-PVBC incorporating FCs outperformed the FC with Dapazol (see Figure 3). The FCs with 60-40-Q and 60-40-QOH perform almost identically with an initial peak power density of 529 mW cm–2 at 1550 mA cm–2 (see Table 3). 90-10-QOH shows an initial peak power density of 461 mW cm–2 at a current density of 1440 mA cm–2. Dapazol shows the lowest initial peak power density with 424 mW cm–2 at 1210 mA cm–2 (see Figure 3B). FCs incorporating PA-doped Dapazol showed lower reproducibility, especially in the mass transport region at >1000 mA cm–2 (see Figure 3A). We assume that the poor reproducibility in the mass transport region was due to the weaker mechanical properties of the commercial membrane, such as increased swelling, which possibly leads to a blocking of the catalyst layer pores, hindering the transport of fuel, oxidant, and product water.
Table 3. OCV Values and Initial Peak Power Densities for Figure 3A with Their Standard Deviations of Three Replicatesa
wt % [OPBI-wt % PVBC]-[Amine]OCV (V)peak power density (mW cm–2)HFR (mΩ cm2)σ (mS cm–1)
60-40-Q0.83 ± 0.11529 ± 687.76 ± 9.8092.16 ± 17.17
60-40-QOH0.83 ± 0.06529 ± 1390.29 ± 5.1285.42 ± 8.22
90-10-QOH0.87 ± 0.08461 ± 11107.58 ± 9.2965.04 ± 8.99
Dapazol0.90 ± 0.12424 ± 20104.36 ± 8.49120.94 ± 15.64
a

HFR values were defined as the mean of six measurements (from impedance data at 400 and 600 mA cm–2, each three measurements). Membrane conductivities (σ) are derived from HFR values and correction for the electronic resistances. Here, the conductivities at 180 °C are given. A more detailed explanation and conductivities at 140 and 160 °C can be found in the Supporting Information (Figure S5).

At low current densities (below 100 mA cm–2), all polarization curves show a similar behavior. This result is in agreement with the usage of equal electrodes for all MEAs, irrespective of the membrane type. It should be noted that polarization data were recorded after a holding time of 30 min at 400 mA cm–2. Following the literature, the break-in process is considered to be around 48 to 100 h. (62) Hot-pressing is known to reduce the break-in time since it enhances the transfer of free PA from the doped PEM to the catalyst layer. (1) Initial polarization data were recorded after 30 min at 180 °C and 400 mA cm–2, as the data showed high reproducibility (compare Figure 3), while for PA loss, a 48 h constant current hold at 400 mA cm–2 was carried out.
To understand the influence of the PEM on FC performance, the Ohmic region was analyzed in more detail. Therefore, we investigated the HFRs and fuel inefficiency caused by fuel crossover. Impedance measurements were carried out at 400 and 600 mA cm–2 for each of the three replicates. The mean HFR and its standard deviations are shown in Table 3. The HFRs ranged between 88 and 104 mΩ cm2. 90-10-QOH and Dapazol showed almost the same HFRs, which are slightly higher than those for 60-40-Q and 60-40-QOH (see Table 3). The HFR is dependent on the AU and the membrane thickness, which explains the order with respect to the HFR of 60-40-Q ≈ 60–40QOH < 90-10-QOH ≈ Dapazol. Generally, it can be noted that the different performances of the FCs correlate with their HFRs and, therefore, with Ohmic resistances of the membranes, as can also be seen in the HFR-corrected polarization and power density plots in Figure 3C,D. When excluding contributions from the HFR, the performance of the MEAS with IPMs still outperforms the one with Dapazol, especially at high current densities. This phenomenon is most likely due to the almost 2 times higher volumetric swelling behavior of Dapazol when doped with PA compared to that of the in-house-prepared PEMs (see Table 2). While Dapazol, despite its increase in thickness, still shows a comparable HFR to the IPMs, excessive swelling presumably leads to characteristic mass transport-restricted voltage losses for potentials below 0.4 V (see Figure 3A).
Fuel crossover is an important parameter for membrane materials in FC application. Therefore, we carried out LSV to detect H2 gas crossover on one replicate per membrane type. The LSV data are represented in Figure 4A. Ideally, the current response follows a saturation curve, of which the maximum corresponds to the hydrogen crossover. However, the expected curvature at around 0.2 V is followed by a linear increase of the current density. This saturation regime is the sum of the hydrogen crossover and a superimposed short-circuit component. (63) The latter occurs with increased potential of the working electrode due to an electrical short across the PEM. Figure 4B represents the short-circuit-corrected linear sweep voltammograms (see Figure S4 for an exemplary short-circuit correction).

Figure 4

Figure 4. (A) Linear sweep voltammograms and (B) linear sweep voltammograms with internal short-circuit correction. All measurements were carried out at 180 °C, with 0.3 L min–1 dry H2 at the anode and 0.3 L min–1 N2 at the cathode with a scan rate of 2 mV s–1.

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LSV was carried out on one replicate, and the corresponding values can be found in Table 4. The cells used for LSV characterization showed OCVs of 0.94 (60-40-Q), 0.86 (60-40-QOH), 0.95 (90-10-QOH), and 0.93 V (Dapazol). 60-40-QOH showed the highest hydrogen crossover with 2.88 × 10–11 mol s–1. The lowest gas crossover was observed for 90-10-QOH with 7.26 × 10–12 mol s–1, which is almost 3 times smaller than the gas crossover of Dapazol (see Table 4). The OCVs are in accordance with the hydrogen gas crossover fluxes shown in Table 4. 90-10-QOH with the lowest hydrogen gas crossover exhibited the highest OCV. Just as the HFR, the H2 crossover is also dependent on the thickness of the membrane. Furthermore, the polymer ratio and the degree of cross-linking may have an influence. Minding the variation in OCV observed for replicates of the same PEMs (see Table 3), we do not want to put an emphasis on the exact values obtained by the H2 crossover LSV analysis. It is important, however, to stress that IPMs can obtain lower crossover rates and similar ISRs at lower absolute thicknesses compared to the commercial Dapazol membrane. As the casting process is not close to any commercial scale yet, the variance is not addressed in more detail here.
Table 4. Limiting Current Density jH2cross, Hydrogen Crossover Flux (ηH2cross), and Internal Short-Circuit Resistances (ISRs) for Single-Cell Measurementsa
[wt % OPBI-wt % PVBC]-[Amine]jH2 cross (mA cm–2)ηH2 cross (mol s–1)ISR (Ω cm2)OCV (V)
60-40-Q0.491.01 × 10–116250.94
60-40-QOH1.392.88 × 10–118990.86
90-10-QOH0.357.26 × 10–129060.95
Dapazol0.951.98 × 10–117690.93
a

ISRs represent the inverse of the slopes between 0.30 and 0.40 V for the IPMs and between 0.45 and 0.50 V for Dapazol of the linear sweep voltammograms (Figure 4A). Furthermore, the OCVs of the FCs used for LSV characterization are shown. The OCVs were taken from polarization data recorded after a 30 min hold at 400 mA cm–2 at 180 °C under 0.25 L min–1 H2 and 0.75 L min–1 air before LSV measurements.

Furthermore, hydrogen gas crossover depends on operating conditions, such as temperature and gas pressure. (64) All the obtained limiting current densities here are in good agreement with the literature and show even lower hydrogen permeations at a higher operating temperature (180 °C). Galbiati et al. reported for the BASF Celtec-P2100 MEA with an active area of 20 cm2 limiting current densities between 0.5 and 13.8 mA cm–2 for FC operation at 160 °C. (63) Pinar et al. reported for the Celtec-P1100W MEA with an active area of 20.25 cm2 a permeation flux of 2.46 × 10–7 mol s–1 for a FC operation temperature of 160 °C. (62)
By the inverse of the slope of the linear region (see Figure 4A), the ISR can be evaluated. 60-40-Q exhibited with 625 Ω cm2, the lowest ISR (see Table 4) followed by Dapazol. 90-10-QOH showed the highest ISR of 906 Ω cm2. This finding is noteworthy since Dapazol exceeds the thickness of the OPBI-c-PVBC membranes, which would, for a constant membrane material, typically result in a higher ISR. The fact that Dapazol shows a lower ISR than 90-10-QOH can be correlated with the worse mechanical properties, such as high swelling and low Young’s modulus of the commercial membrane. Again, due to variances in membrane casting, emphasis should not be put on exact values but on the overall possibility of improving the ISR by use of IPMs in HT-PEMFCs.

3.4. PA Loss over Time

For PA loss monitoring, high flow rates (0.25 L min–1 H2, 0.75 L min–1 air, and λH2/λair = 9/11) and an operating temperature of 180 °C were chosen. A constant current density of 400 mA cm–2 was held for 48 h, and the voltage profiles over time are shown in Figure 5. The amount of PA in the collected samples was quantified by ICP–MS.

Figure 5

Figure 5. (A) Cell voltage over time for 48 h at 400 mA cm–2 with 0.25 L min–1 H2 and 0.75 L min–1 air at 180 °C, (B) corresponding cell resistance by current interruption, and (C) cathodic area-specific acid loss rates during 48 h at 400 mA cm–2.

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As can be seen, all FCs showed a similar degradation behavior approximately within the first 20 h (see Figure 5A). Since the electrodes were kept the same for all measurements, a similar behavior regarding degradation was to be expected. Degradation reasons within this time frame are presumably caused by poisoning of the Pt catalyst by PA. The initially sharper drop in the voltage of the cell with Dapazol compared to the other cells can be explained by its higher AU (see Table 2) and its presumably deeper extension into the catalyst layer. Dapazol might thus poison the catalyst more strongly and more quickly than the other membranes. After a specific saturation is reached, the following continuous voltage decay can be assumed to correlate with the PA loss leading to lower conductivities.
For the further understanding of FC degradation, the cell resistances were investigated by the current interrupt method (see Figure 5B). Within the first up to 30 h, all cells showed a decrease in cell resistance, which indicates that the break-in of the cells was not completed yet. Hence, only the last 18 h were considered for further evaluation. The voltage decays and cell resistance increases for the last 18 h are listed in Table 5. There is an inverse correlation between a higher increase in cell resistance (see Table 5) and the AU of the PEMs (see Table 2). The FC with Dapazol, which exhibited an almost 2 times higher AU compared to the IPMs, showed the lowest increase of cell resistance. The more acid present in the membrane, the less vulnerable it will most likely be for the resistance to increase strongly upon the removal of small amounts of acid. In agreement with this, the IPM with the lowest AU, 90-10-QOH, exhibited the largest increase in cell resistance. The resistance increase of the 60–40 blends is rather similar and lies between the increase of the Dapazol and the 90-10-QOH IPM. Along with the increased cell resistance, the voltage decays. Also, here, the Dapazol membrane behaves most stable.
Table 5. Voltage Decay and Increase of Cell Resistance during Fuel Cell Operation, Cathodic Acid Loss Rate, and Total Phosphorus Losses for the Cathode and Anodea
[wt % OPBI-wt % PVBC]-[Amine]voltage decay (μV·h –1)increase of cell resistance (mΩ·cm2·h–1)cathodic acid loss rate (μg·m–2·s–1)cathodic total P loss (μg)anodic total P loss (μg)
60-40-Q–4790.0727.716055.11
60-40-QOH–5560.1021.39467-b
90-10-QOH–4900.1616.17353-b
Dapazol–1540.0221.80476-b
a

Voltage decay and cell resistance increase rates were defined for the time range from 30 to 48 h. The acid loss rate was calculated for a total of 48 h.

b

Untraceable. The limit of detection for P was 5 μg L–1.

The change in cell resistance throughout this constant current hold is seen to arise from a change in the Ohmic resistance of the membrane caused by acid removal. The Ohmic resistance is predominantly influenced by the PA content, which itself is a simple function of the acid level after doping and the removal of PA over time. The amounts of PA found in the cathode exhaust water are shown in Figure 5C and Table 5. It should be noted that over 120 °C PA, dimerization occurs, leading to the formation of pyrophosphoric acid (H4P2O7) and water production. This will lead to an increase in membrane conductivity due to elevated temperatures and an increase in resistance due to PA dimerization (65)
Even though the cell with the Dapazol membrane shows the most stable performance over time, it is not the one with the lowest but with the second largest PA loss. Due to the high amount of PA present in the membrane at the begin-of-test, however, this acid loss does not impair the steady-state performance significantly. Even though the constant current hold of the two 60–40 IPM blends showed similar stability values, the amount of PA leached in the case of 60-40-Q is almost 30% higher than the amount found in the exhaust of the 60-40-QOH cell. This can be explained by the main difference in their chemical structure: the OH-functionality of the amine quinuclidinol (see Figure 1). It may help to stabilize PA within the membrane since it is an additional anchor point to retain PA molecules through hydrogen bridging. The membrane with the lowest AU, 90-10-QOH, showed the highest resistance increase throughout the constant current hold. The absolute amount of PA found in the exhaust is, however, the lowest. This is seen to be a combination of both the higher affinity of the quniuclidinole QA to retain PA and the overall lower amount of PA present in the membrane at the begin-of-test. Considering the mass of PA for 4 cm2 membrane pieces, 60-40-Q loses 8.92%, 60-40-QOH 4.81%, 90-10-QOH 4.88%, and Dapazol 1.92% of PA (see Table S2). This confirms that the OH functionality may help to stabilize PA within the membrane since both 60-40-QOH and 90-10-QOH show an almost identical PA loss.
Søndergaard et al. showed that the acid loss is dependent on temperature, flow rate, and current density. (66) The reported acid loss rates for a total flow rate of 1 L min–1 (air + H2) at 180 °C are 17 μg m–2 s–1 at 200 mA cm–2, 48 μg m–2 s–1 at 600 mA cm–2, and 59 μg m–2 s–1 at 800 mA cm–2. We detected acid loss rates between 16.17 and 27.71 μg m–2 s–1 (see Table 5) for the same total flow rate but at 400 mA cm–2, which are in agreement with the literature. One main difference between the study of Søndergaard et al. and this study was FC operation time periods. Søndergaard et al. operated FC measurements for 42 000 h, while only operation times of 48 h were investigated in this work. Nonetheless, acid loss rates have been suggested to be lower after long-term operation compared to shorter periods of time. (67)
Interestingly, the anodic PA loss was very low or untraceable compared to the cathodic PA loss (see Table 5). Anodic P losses were either untraceable or a maximum of 5.11 μg. Becker et al. described in 2018 that PA anions migrate from the cathode to the anode and recombine with protons to PA. (68) This migration causes a concentration gradient, which in turn causes back diffusion of acid from the anode to the cathode but still favors acid loss on the cathode side. This explanation is in agreement with our results. It should be noted that the PA back diffusion rate will be more limiting at larger current densities. Furthermore, Kannan et al. reported a 2 times higher acid loss at the cathode exhaust compared to that at the anode with 1 order of magnitude higher cathode exit flow. (69)

4. Conclusions

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This study evaluated the properties and FC performances of cross-linked OPBI and PVBC membranes after decoration with QA groups from three different amines and doping with PA. FC tests revealed that quinuclidine and quinuclidinol resulted in higher power densities than DABCO. This result is interesting since DABCO is a more frequently used quaternizing agent, especially in the field of anion-exchange membranes. The best PEMs for FC application were the membranes with OPBI to PVBC ratios of 90:10 quaternized with quinuclidinol, 60:40 quaternized with quinuclidinol, and 60:40 quaternized with quinuclidine. All three membranes outperformed the PA-doped commercial Dapazol, with the last two showing the highest and almost identical initial peak power densities and the highest current density output when operated within an HT-PEMFC (530 mW cm–2 at 1550 mA cm–2 under H2/air operation at 180 °C and ambient pressure). The reproducibility of the FC resulting at high current densities was superior compared to that of the commercial Dapazol, which can be explained by the poor mechanical properties of Dapazol, upon doping with PA. With an AU of 404 wt %, Dapazol showed the lowest FC performance with an initial peak power density of 424 mW cm–2 at 1210 mA cm–2, while the membranes OPBI to PVBC 60:40 quaternized with quinuclidine and quinuclidinol showed AUs of roughly 230 wt %. With the membrane 90 wt % OPBI and 10 wt % PVBC quaternized with quinuclidinol, an initial peak power density of 461 mW cm–2 at 1440 mA cm–2 was achieved. The lower power density can be explained by its lower AU of 177 wt %, which resulted in larger Ohmic losses.
While the Ohmic contribution of Dapazol-based MEAs was roughly comparable to that of the MEAs with IPMs, the Dapazol-based MEAs suffered from an earlier onset of mass transport-related losses, which explain their limited peak power density. Thus, it can be concluded that the higher AU of Dapazol enabled this membrane to maintain an almost equal proton conductivity as the IPMs, despite its larger thickness, however, at the cost of poor mechanical properties, which can explain its pronounced mass transport resistance. The investigated IPMs offered superior mechanical properties at lower thicknesses, which allowed these membranes to provide a very reproducible FC performance and a hydrogen crossover that is comparable to Dapazol.
Regarding PA leaching, all membranes showed amounts of P in the off-gas analysis in the same order of magnitude. Due to the higher AU during doping, the acid content loss of the Dapazol membrane over 48 h is 1.92% less than that of the IPMs, resulting in a more stable electrochemical performance. It could be observed that the membranes quaternized with quinuclidinol (4.81–4.88% PA loss) showed better PA retention compared to the membrane quaternized with quinuclidine (8.92% PA loss). The additional −OH group of quinuclidinol compared to that of quinuclidine is a possible explanation for this finding, as the polar moiety may improve acid retention.
Taken together, we have found that PA/OPBI-c-PVBC decorated with QA groups is a very promising membrane system for HT-PEMFCs. The results indicate that it offers higher peak FC performances compared to commercial HT-PEMs. It requires a relatively small total PA uptake and shows high mechanical stability at low thicknesses. However, its low initial PA content also makes it more susceptible to acid loss compared to a membrane with a higher total AU. In order to obtain mechanically stable, low-acid, and long-term stable membranes, the acid retention capacity of IPMs with QA groups has to be addressed in the future work. Furthermore, the OPBI-c-PVBC membrane systems could be quaternized with different amines such as trimethylamine to have a better understanding of the influence of the used quaternizing agent on FC performance and acid retention.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.1c17154.

  • Chemical structures and molar masses of F6PBI, OPBI, m-PBI, and PVBC; ATR FT-IR spectra of the polymers OPBI and PVBC and the cross-linked membranes; polarization and power density curves for the FCs in Table 1; ADLs and AUs for all membranes discussed in this work; PA uptake in milligram, cathodic P and PA losses, and acid loss; exemplary short-circuit correction for the linear sweep voltammogram of 90-10-QOH; in situ proton conductivities at 140, 160, and 180 °C; XRD spectra; and Fenton’s test (PDF)

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Author Information

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  • Corresponding Authors
    • Funda Arslan - Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany;  Department of Chemical and Biological Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany;  Orcidhttps://orcid.org/0000-0001-5273-192X;  Email: [email protected]
    • Thomas Böhm - Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany;  Orcidhttps://orcid.org/0000-0003-2036-2159;  Email: [email protected]
  • Authors
    • Khajidkhand Chuluunbandi - Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany;  Department of Chemical and Biological Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany
    • Anna T.S. Freiberg - Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany;  Department of Chemical and Biological Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany;  Orcidhttps://orcid.org/0000-0002-7885-7632
    • Attila Kormanyos - Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany;  Orcidhttps://orcid.org/0000-0002-2145-7419
    • Ferit Sit - Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany
    • Serhiy Cherevko - Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany;  Orcidhttps://orcid.org/0000-0002-7188-4857
    • Jochen Kerres - Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany;  Faculty of Natural Science, North-West University, Potchefstroom 2520, South Africa
    • Simon Thiele - Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstraße 3, 91058 Erlangen, Germany;  Department of Chemical and Biological Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany;  Orcidhttps://orcid.org/0000-0002-4248-2752
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We thank Dillam Diaz Romero and Julian Stonawski for helpful discussions. We also gratefully thank the Bavarian Ministry of Economic Affairs, Regional Development and Energy through the project “Emissionsfreier und stark emissionsreduzierter Bahnverkehr auf nicht-elektrifizierten Strecken” for the financial support, which made this work possible. Furthermore, we thank Marco Sarcletti and Andreas Eigen from the Institute of Polymer Materials at the Friedrich-Alexander University Erlangen-Nürnberg for introduction to and usage of the instrument for TGA measurements.

References

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    Kerres, Jochen; Atanasov, Vladimir
    International Journal of Hydrogen Energy (2015), 40 (42), 14723-14735CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
    In this study different types of polybenzimidazole(PBI)-based High-T fuel cell membranes were investigated comparatively. The different membranes comprised: (1) ionically cross-linked PBI-excess blend membranes by mixing PBI (the polybenzimidazoles PBIOO and F6PBI) with different cation-exchange ionomers such as poly(tetrafluorostyrene-4-phosphonic acid), and different nonfluorinated and partially fluorinated sulfonated arylene main-chain polymers, where the cation-exchange groups form ionical cross-links with the imidazole groups of the PBI by proton transfer, (2) covalently cross-linked PBI-excess membranes by mixing PBI with different halomethylated arylene polymers where the halomethyl groups form covalent cross-links towards the imidazole group of the PBI by alkylation of the N-H group: polymer-CH2Br + PBI-imidazole-N-H → polymer-CH2-N-imidazole-PBI, (3) PBI-anion-exchange polymer blends, (4) covalent-ionically cross-linked PBI blend membranes by mixing PBI with a sulfonated polymer and a halomethylated polymer. The membranes were investigated in terms of: (i) chem. stability by Fentons Test (FT), (ii) extent of crosslinking by extn. with DMAc, (iii) thermal stability by TGA, (iv) H+-cond. in the T range 80-150 °C as H3PO4-doped membranes, and (v) fuel cell performance in a high-T H2/air fuel cell. The general results of the study were summarized as follows: (1) Most of the membranes showed excellent chem. stability in FT, (2) the PBI blends with F6PBI showed better chem. stabilities than the PBIOO-contg. blends, (3) the proton conductivities of all investigated membranes were in a range of 4-90 mS/cm at T from 80 to 150 °C, (4) the fuel cell test results of the membranes were promising.
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  12. 12
    Li, Q.; Pan, C.; Jensen, J. O.; Noyé, P.; Bjerrum, N. J. Cross-Linked Polybenzimidazole Membranes for Fuel Cells. Chem. Mater. 2007, 19, 350– 352,  DOI: 10.1021/cm0627793
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    Cross-Linked Polybenzimidazole Membranes for Fuel Cells
    Li, Qingfeng; Pan, Chao; Jensen, Jens Oluf; Noye, Pernille; Bjerrum, Niels J.
    Chemistry of Materials (2007), 19 (3), 350-352CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    In this communication, p-xylene dibromide is used as a crosslinking agent with contents of 0 (pure polybenzimidazole, PBI) 1.0, 3.0, and 10.0 wt.% in the prepd. PBI membranes samples. Assuming a complete reaction for each mole of the crosslink agent with 2 equiv of polybenzimidazole amine hydrogen, these wt. ratios correspond to a crosslinking degree of 0, 1,1,. 3.6, and 13.0 wt.%, resp., of the total amine hydrogen atoms in PBI. Crosslinking slowed the rate and extent of membrane dissoln. in N,N-dimethylacetamide. Doping with phosphoric acid more than doubled the proton cond. of the membranes, while lowering the tensile strength and modulus, but increasing the elongation to break. Crosslinking restored some of the tensile strength and modulus, while greatly reducing the elongation. Crosslinking also increases resistance of the membrane to oxidn.
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  13. 13
    Zeis, R. Materials and Characterization Techniques for High-Temperature Polymer Electrolyte Membrane Fuel Cells. Beilstein J. Nanotechnol. 2015, 6, 68– 83,  DOI: 10.3762/bjnano.6.8
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    Materials and characterization techniques for high-temperature polymer electrolyte membrane fuel cells
    Zeis, Roswitha
    Beilstein Journal of Nanotechnology (2015), 6 (), 68-83, 16 pp.CODEN: BJNEAH; ISSN:2190-4286. (Beilstein-Institut zur Foerderung der Chemischen Wissenschaften)
    A review. The performance of high-temp. polymer electrolyte membrane fuel cells (HT-PEMFC) is critically dependent on the selection of materials and optimization of individual components. A conventional high-temp. membrane electrode assembly (HT-MEA) primarily consists of a polybenzimidazole (PBI)-type membrane contg. phosphoric acid and two gas diffusion electrodes (GDE), the anode and the cathode, attached to the two surfaces of the membrane. This review article provides a survey on the materials implemented in state-of-the-art HT-MEAs. These materials must meet extremely demanding requirements because of the severe operating conditions of HT-PEMFCs. They need to be electrochem. and thermally stable in highly acidic environment. The polymer membranes should exhibit high proton cond. in low-hydration and even anhyd. states. Of special concern for phosphoric-acid-doped PBI-type membranes is the acid loss and management during operation. The slow oxygen redn. reaction in HT-PEMFCs remains a challenge. Phosphoric acid tends to adsorb onto the surface of the platinum catalyst and therefore hampers the reaction kinetics. Addnl., the binder material plays a key role in regulating the hydrophobicity and hydrophilicity of the catalyst layer. Subsequently, the binder controls the electrode-membrane interface that establishes the triple phase boundary between proton conductive electrolyte, electron conductive catalyst, and reactant gases. Moreover, the elevated operating temps. promote carbon corrosion and therefore degrade the integrity of the catalyst support. These are only some examples how materials properties affect the stability and performance of HT-PEMFCs. For this reason, materials characterization techniques for HT-PEMFCs, either in situ or ex situ, are highly beneficial. Significant progress has recently been made in this field, which enables us to gain a better understanding of underlying processes occurring during fuel cell operation. Various novel tools for characterizing and diagnosing HT-PEMFCs and key components are presented in this review, including FTIR and Raman spectroscopy, confocal Raman microscopy, synchrotron X-ray imaging, X-ray microtomog., and at. force microscopy.
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  14. 14
    Henkensmeier, D.; Aili, D. Techniques for PBI Membrane Characterization. In High Temperature Polymer Electrolyte Membrane Fuel Cells; Li, Q., Aili, D., Hjuler, H. A., Jensen, J. O., Eds.; Springer International Publishing, 2016.
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  15. 15
    Hu, M.; Li, T.; Neelakandan, S.; Wang, L.; Chen, Y. Cross-Linked Polybenzimidazoles Containing Hyperbranched Cross-Linkers and Quaternary Ammoniums as High-Temperature Proton Exchange Membranes: Enhanced Stability and Conductivity. J. Membr. Sci. 2020, 593, 117435,  DOI: 10.1016/j.memsci.2019.117435
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    15
    Cross-linked polybenzimidazoles containing hyperbranched cross-linkers and quaternary ammoniums as high-temperature proton exchange membranes: Enhanced stability and conductivity
    Hu, Meishao; Li, Tianyun; Neelakandan, Sivasubramaniyan; Wang, Lei; Chen, Yongming
    Journal of Membrane Science (2020), 593 (), 117435CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
    High proton cond. with sufficient stability for phosphoric acid (PA) doped polybenzimidazole membranes is crit. for applications in fuel cells. Macromol. cross-linkers with hyperbranched structures have large vols. and many functional groups, which can not only react with polymer backbones to form multiple cross-linked sites, but also provide favorable conditions for subsequent functional group modifications to regulate the interaction between polymers and PA; however, such cross-linkers are rarely exploited in fuel cells. Herein, a series of cross-linked polybenzimidazole membranes were successfully prepd. based on a novel hyperbranched cross-linker and the incorporation of numerous quaternary ammonium groups. These cross-linked polybenzimidazole membranes contg. a hyperbranched cross-linker and quaternary ammonium groups showed superior performance, in terms of mech. properties, oxidative resistance and proton cond. The tensile strength of the PA doped cross-linked membranes was >20.0 MPa. These cross-linked membranes showed only a slight wt. loss and no cracks after immersion in Fenton's reagent for 200 h, while the linear membrane was broken into pieces after immersion in Fenton's reagent for 100 h. With low acid loading, the membranes contg. cross-linked polybenzimidazole with quaternary ammonium groups still exhibited good cond. As a result of the excellent comprehensive properties, the obtained fuel cell based on the membrane with 15% of the hyperbranched cross-linker showed a great power d. of 260 mW/cm2, which was 36.8% higher than that of the fuel cell based on the corresponding linear membrane.
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  16. 16
    Luo, H.; Pu, H.; Chang, Z.; Wan, D.; Pan, H. Crosslinked polybenzimidazole via a Diels-Alder reaction for proton conducting membranes. J. Mater. Chem. 2012, 22, 20696,  DOI: 10.1039/c2jm33725h
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    Crosslinked polybenzimidazole via a Diels-Alder reaction for proton conducting membranes
    Luo, Haochuan; Pu, Hongting; Chang, Zhihong; Wan, Decheng; Pan, Haiyan
    Journal of Materials Chemistry (2012), 22 (38), 20696-20705CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    The crosslinking of polybenzimidazole (PBI) is a potential strategy to improve the mech. properties and dimensional stability of acid-doped membranes, as well as to retain additives in the membranes. An effective method to prep. crosslinked PBI with a well-defined structure via a Diels-Alder reaction between vinylbenzyl functionalized PBI (PBI-VB) and α,α'-difurfuryloxy-p-xylene (DFX) is proposed. The chem. structure of PBI-VB is confirmed by FTIR and 1H NMR. The model reaction of styrene and DFX is employed to clarify the crosslinking reaction of PBI and DFX. During the crosslinking process, three kinds of chem. reaction may happen. The first is a Diels-Alder reaction of DFX with the vinyl groups of PBI-VB. The second is the self-polymn. of vinyl groups. The third is the grafting of difuran groups via a Diels-Alder reaction. The first two reactions contribute the most to the crosslinking of the PBI membrane. With the addn. of DFX, there is competition between these two kinds of crosslinking reactions. When the feed ratio of DFX is below 20%, the tensile strength of the crosslinked membranes increases with increasing content of DFX. The crosslinking of the membrane is mainly a results of Diels-Alder reactions. When the feed ratio of DFX exceeds 20%, the tensile strength decreases slightly. Besides the crosslinking via Diels-Alder reactions, the crosslinking of the membrane is also contributed by the self-polymn. of vinyl groups and the grafting of difuran groups. The crosslinked PBI membrane exhibits improved mech. strength, higher phys. and chem. stability, as well as higher phosphoric acid (PA) retention ability. After doping with PA, the crosslinked membrane exhibits good proton cond. over a temp. range of 60 to 180 °C.
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  17. 17
    Liu, F.; Wang, S.; Chen, H.; Li, J.; Tian, X.; Wang, X.; Mao, T.; Xu, J.; Wang, Z. Cross-Linkable Polymeric Ionic Liquid Improve Phosphoric Acid Retention and Long-Term Conductivity Stability in Polybenzimidazole Based PEMs. ACS Sustainable Chem. Eng. 2018, 6, 16352– 16362,  DOI: 10.1021/acssuschemeng.8b03419
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    17
    Cross-Linkable Polymeric Ionic Liquid Improve Phosphoric Acid Retention and Long-Term Conductivity Stability in Polybenzimidazole Based PEMs
    Liu, Fengxiang; Wang, Shuang; Chen, Hao; Li, Jinsheng; Tian, Xue; Wang, Xu; Mao, Tiejun; Xu, Jingmei; Wang, Zhe
    ACS Sustainable Chemistry & Engineering (2018), 6 (12), 16352-16362CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
    Composite cross-linked membrane based on fluorine-contg. polybenzimidazole (6FPBI) and a cross-linkable polymeric ionic liq. (cPIL) were prepd. for high temp. proton exchange membrane (HT-PEM) applications. Particularly, the obtained composite cross-linked membranes showed excellent phosphoric acid doping ability and proton cond. From the trade-off between mech. strength and proton cond. of composite membranes, the optimal content of cPIL is 20% (6FPBI-cPIL 20 membrane). For instance, the 6FPBI-cPIL 20 membrane with a PA doping level of 27.8 exhibited a proton cond. of 0.106 S/cm at 170°, which is much higher than that of pristine 6FPBI membrane. The most outstanding contribution of this work is that the 6FPBI-cPIL membranes showed improved phosphoric acid retention and long-term cond. stability under harsh conditions (80°/40% RH) for 96 h. In particular, the proton cond. and PA doping level of the 6FPBI-cPIL 20 membrane remained at a high level of 0.064 S/cm and 8.5 after 96 h of the test, resp.
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  18. 18
    Hao, J.; Jiang, Y.; Gao, X.; Lu, W.; Xiao, Y.; Shao, Z.; Yi, B. Functionalization of Polybenzimidazole-Crosslinked Poly(vinylbenzyl chloride) with Two Cyclic Quaternary Ammonium Cations for Anion Exchange Membranes. J. Membr. Sci. 2018, 548, 1– 10,  DOI: 10.1016/j.memsci.2017.10.062
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    18
    Functionalization of polybenzimidazole-crosslinked poly(vinylbenzyl chloride) with two cyclic quaternary ammonium cations for anion exchange membranes
    Hao, Jinkai; Jiang, Yongyi; Gao, Xueqiang; Lu, Wangting; Xiao, Yu; Shao, Zhigang; Yi, Baolian
    Journal of Membrane Science (2018), 548 (), 1-10CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
    The anion exchange membranes (AEMs) with both high ionic cond. and good stability is always the research focus role for the long-term use of AEM fuel cells. A series of the mech. and chem. stable PVBC/PBI crosslinked membranes, functionalized with N1-Bu substituted BDABCO groups, were designed, prepd. and characterized. With the crosslinking by polybenzimidazole (PBI), the membranes showed good flexibility, strength and low swelling ratio (less than 18%). N1-Bu substituted doubly-charged BDABCO was introduced in the AEMs during the crosslinking reaction instead of the traditional dipping method, benefiting from the improvement compatibility between polymers and BDABCO groups. Attributing to the well-developed phase sepn. between hydrophilic domains and hydrophobic domains, the family of synthesized AEMs exhibited the higher conductivities than that of DABCO based membranes, which was proved by TEM and SAXS. The M-BDABCO-OH-1:3 with high BDABCO content displayed the highest ionic cond. of 29.3 and 91.4 mS cm-1 at 20 and 80 °C, resp. The results of alk. stability showed that the membranes had the superior chem. stability after immersing in a 1 mol L-1 KOH at 60 °C soln. for more than 550 h. Furthermore, the peak power d. of an H2/O2 single fuel cell using the optimized M-BDABCO-OH-1:3 was up to 340 mW cm-2 at 0.492 V with the EIS consisting of membrane resistance less than 0.1 Ω cm2 which was much smaller than the other AEMs. Overall, the developed membranes demonstrated the superior performance and would be a promising candidate material for AEMFCs.
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  19. 19
    Özdemir, Y.; Özkan, N.; Devrim, Y. Fabrication and Characterization of Cross-linked Polybenzimidazole Based Membranes for High Temperature PEM Fuel Cells. Electrochim. Acta 2017, 245, 1– 13,  DOI: 10.1016/j.electacta.2017.05.111
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    Fabrication and Characterization of Cross-linked Polybenzimidazole Based Membranes for High Temperature PEM Fuel Cells
    Ozdemir, Yagmur; Ozkan, Necati; Devrim, Yilser
    Electrochimica Acta (2017), 245 (), 1-13CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    In this study different types of crosslinked polybenzimidazole (PBI) membranes were compared as high temp. proton exchange membrane fuel cells (HT-PEMFC). Crosslinking of PBI was performed with different cross-linkers including bisphenol A diglycidyl ether (BADGE), ethylene glycol diglycidyl ether (EGDE), α-α'-dibromo-p-xylene (DBpX), and terephthalaldehyde (TPA). The crosslinked membranes were characterized by TGA, SEM, acid uptake and impedance analyses. The crosslinking of the PBI polymer matrix helps to improve the acid retention properties. PBI/BADGE presented the highest acid retention properties. Proton conductivities of the membranes were comparable to that of com. membranes. Cond. values up to 0.151 S cm-1 were obtained at 180° with PBI/DBpX membranes. Gas diffusion electrodes (GDE) were fabricated by an ultrasonic coating technique with 0.6 mg Pt.cm-2 catalyst loading for both anode and cathode. The crosslinked membranes were tested in a single HT-PEMFC with a 5. cm2 active area at 165° without humidification. PBI/BADGE crosslinked membranes demonstrated stability and high performance on single cell HT-PEMFC tests. The max. power d. for PBI/BADGE was detd. as 0.123 W cm-2. As a result, the exptl. results suggested that the PBI/BADGE and PBI/DBpX cross-linked membranes are promising electrolyte options for HT-PEMFC.
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  20. 20
    Tian, X.; Wang, S.; Li, J.; Liu, F.; Wang, X.; Chen, H.; Wang, D.; Ni, H.; Wang, Z. Benzimidazole Grafted Polybenzimidazole Cross-Linked Membranes with Excellent PA Stability for High-Temperature Proton Exchange Membrane Applications. Appl. Surf. Sci. 2019, 465, 332– 339,  DOI: 10.1016/j.apsusc.2018.09.170
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    20
    Benzimidazole grafted polybenzimidazole cross-linked membranes with excellent PA stability for high-temperature proton exchange membrane applications
    Tian, Xue; Wang, Shuang; Li, Jinsheng; Liu, Fengxiang; Wang, Xu; Chen, Hao; Wang, Di; Ni, Hongzhe; Wang, Zhe
    Applied Surface Science (2019), 465 (), 332-339CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Benzimidazole grafted polybenzimidazole crosslinked membranes (CPBIm-X) with outstanding phosphoric acid (PA) stability are successfully prepd. 2-chloromethyl benzimidazole (CMBelm) is grafted onto PBI mainchains and using 3-glycidoxypropyltrimethoxysilane (KH560) as a crosslinker. The benzimidazole sidechains can not only increase the basic sites but also allow the membrane to achieve higher phosphoric acid uptakes without sacrificing mech. strength. Moreover, compared with the imidazole rings in the PBI backbone, the side chain with imidazole rings are flexible, which benefit the proton transportation and lower the activation energy. In addn., the mech. strength of crosslinked membranes is excellent. For example, the tensile strength value of CPBIm-5 is 101.4 MPa, while that of the PBIm is 79.0 MPa. The proton cond. is enhanced because the hydrolysis of KH560 resulting Si-O-Si networks structure, which can absorb more phosphoric acid. The Si-O-Si networks in the matrix can efficiently improve the stability of phosphoric acid (PA), the remaining wt. of CPBIm-5 is 57% PA. Taking into account performance comprehensively, the wt. of 5% KH560 is the optimum content. For example, the proton cond. of CPBIm-5 is 0.092 S cm-1 at 180 °C. Compared to the pristine PBI, it is almost three-fold in proton cond.
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  21. 21
    Kerres, J. Applications of Acid-Base Blend Concepts to Intermediate Temperature Membranes. In High Temperature Polymer Electrolyte Membrane Fuel Cells; Li, Q., Aili, D., Hjuler, H. A., Jensen, J. O., Eds.; Springer International Publishing, 2016, pp 59– 89. DOI: 10.1007/978-3-319-17082-4_4 .
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    Aili, D.; Li, Q.; Christensen, E.; Jensen, J. O.; Bjerrum, N. J. Crosslinking of Polybenzimidazole Membranes by Divinylsulfone Post-Treatment for High-Temperature Proton Exchange Membrane Fuel Cell Applications. Polym. Int. 2011, 60, 1201– 1207,  DOI: 10.1002/pi.3063
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    Crosslinking of polybenzimidazole membranes by divinylsulfone post-treatment for high-temperature proton exchange membrane fuel cell applications
    Aili, David; Li, Qingfeng; Christensen, Erik; Jensen, Jens Oluf; Bjerrum, Niels J.
    Polymer International (2011), 60 (8), 1201-1207CODEN: PLYIEI; ISSN:0959-8103. (John Wiley & Sons Ltd.)
    Phosphoric acid-doped polybenzimidazole (PBI) has been suggested as a promising electrolyte for proton exchange membrane fuel cells operating at temps. up to 200°. This paper describes the development of a crosslinking procedure for PBI membranes by post-treatment with divinylsulfone. The crosslinking chem. was studied and optimized on a low-mol.-wt. model system and the results were used to optimize the crosslinking conditions of PBI membranes. The crosslinked membranes were characterized with respect to chem. and physiochem. properties, showing improved mech. strength and oxidative stability compared with their linear analogs. Fuel cell tests were further conducted in order to demonstrate the feasibility of the crosslinked membranes. Copyright © 2011 Society of Chem. Industry.
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    Wang, S.; Zhang, G.; Han, M.; Li, H.; Zhang, Y.; Ni, J.; Ma, W.; Li, M.; Wang, J.; Liu, Z.; Zhang, L.; Na, H. Novel Epoxy-Based Cross-Linked Polybenzimidazole for High Temperature Proton Exchange Membrane Fuel Cells. Int. J. Hydrogen Energy 2011, 36, 8412– 8421,  DOI: 10.1016/j.ijhydene.2011.03.147
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    23
    Novel epoxy-based cross-linked polybenzimidazole for high temperature proton exchange membrane fuel cells
    Wang, Shuang; Zhang, Gang; Han, Miaomiao; Li, Hongtao; Zhang, Yang; Ni, Jing; Ma, Wenjia; Li, Mingyu; Wang, Jing; Liu, Zhongguo; Zhang, Liyuan; Na, Hui
    International Journal of Hydrogen Energy (2011), 36 (14), 8412-8421CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
    An approach has been proposed to prep. the reinforced phosphoric acid-doped cross-linked polybenzimidazole membranes for high-temp. proton exchange membrane fuel cells, using 1,3-bis(2,3-epoxypropoxy)-2,2-dimethylpropane (NGDE) as the cross-linker. FTIR measurement and soly. test showed the successful completion of the crosslinking reaction. The resulting cross-linked membranes exhibited improved mech. strength, making it possible to obtain higher phosphoric acid doping levels and therefore relatively high proton cond. Moreover, the oxidative stability of the cross-linked membranes was significantly enhanced. For instance, in Fenton's reagent (3% H2O2 soln., 4 ppm Fe2+, 70°), the cross-linked PBI-NGDE-20% membrane did not break into pieces and kept its shape for more than 480 h and its remaining wt.% was ∼65%. In addn., the thermal stability was sufficient enough within the operation temp. of PBI-based fuel cells. The cross-linked PBI-NGDE-X% (X is the wt.% of epoxy resin in the cross-linked membranes) membranes displayed relatively high proton cond. under anhyd. conditions. For instance, PBI-NGDE-5% membrane with acid uptake of 193% exhibited a proton cond. of 0.017 S/cm at 200°. All the results indicated that it may be a suitable candidate for applications in high-temp. proton exchange membrane fuel cells.
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    Xu, H.; Chen, K.; Guo, X.; Fang, J.; Yin, J. Synthesis of Hyperbranched Polybenzimidazoles and their Membrane Formation. J. Membr. Sci. 2007, 288, 255– 260,  DOI: 10.1016/j.memsci.2006.11.022
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    Synthesis of hyperbranched polybenzimidazoles and their membrane formation
    Xu, Hongjie; Chen, Kangcheng; Guo, Xiaoxia; Fang, Jianhua; Yin, Jie
    Journal of Membrane Science (2007), 288 (1+2), 255-260CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
    A series of amine-terminated hyperbranched polybenzimidazoles (HBPBIs) were successfully synthesized by condensation polymn. of arom. dicarboxylic acids and an in situ synthesized arom. hexamine intermediate product from 1,3,5-benzenetricarboxylic acid (BTA) and 3,3'-diaminobenzidine (DAB) in polyphosphoric acid (PPA) at 190° for 20 h. HBPBI membranes were fabricated by soln. cast method in the presence of crosslinkers (ethylene glycol diglycidyl ether (EGDE) and terephthaldehyde (TPA)). The resulting HBPBI membranes displayed good mech. properties and good thermal stability. High proton cond. was obtained with the phosphoric acid-doped and TPA-cross-linked HBPBI membranes at 0% relative humidity.
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    Noyé, P.; Li, Q.; Pan, C.; Bjerrum, N. J. Cross-Linked Polybenzimidazole Membranes for High Temperature Proton Exchange Membrane Fuel Cells with Dichloromethyl Phosphinic Acid as a Cross-Linker. Polym. Adv. Technol. 2008, 19, 1270– 1275,  DOI: 10.1002/pat.1123
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    Cross-linked polybenzimidazole membranes for high temperature proton exchange membrane fuel cells with dichloromethyl phosphinic acid as a cross-linker
    Noye, Pernille; Li, Qingfeng; Pan, Chao; Bjerrum, Niels J.
    Polymers for Advanced Technologies (2008), 19 (9), 1270-1275CODEN: PADTE5; ISSN:1042-7147. (John Wiley & Sons Ltd.)
    Phosphoric acid-doped polybenzimidazole (PBI) membranes were covalently cross-linked with dichloromethyl phosphinic acid (DCMP). FT-IR measurements showed new bands originating from bonds between the hydrogen bearing nitrogen in the imidazole group of PBI and the CH2 group in DCMP. The produced cross-linked membranes show increased mech. strength, making it possible to achieve higher phosphoric acid doping levels and therefore higher proton cond. Oxidative stability is significantly improved and thermal stability is sufficient in a temp. range of up to 250°C, i.e. within the temp. range of operation of PBI-based fuel cells.
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    Harilal; Nayak, R.; Ghosh, P. C.; Jana, T. Cross-Linked Polybenzimidazole Membrane for PEM Fuel Cells. ACS Appl. Polym. Mater. 2020, 2, 3161– 3170,  DOI: 10.1021/acsapm.0c00350
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    Cross-Linked Polybenzimidazole Membrane for PEM Fuel Cells
    Harilal; Nayak, Ratikanta; Ghosh, Prakash Chandra; Jana, Tushar
    ACS Applied Polymer Materials (2020), 2 (8), 3161-3170CODEN: AAPMCD; ISSN:2637-6105. (American Chemical Society)
    Despite several unique advantages, high temp. proton exchange membrane fuel cell (HT-PEMFC) based on polybenzimidazole (PBI) membrane suffers from various drawbacks like weak chem. resistance, poor mech. strength, acid leaching etc. which eventually reduce the performance of the cell. In order to improve these drawbacks and to improve the cell performance, in this work proton exchange membrane (PEM) is developed in which pyridine-bridged-oxypolybenzimidazole (PyOPBI) and brominated polyphenylene oxide (BrPPO) were chem. cross-linked by an ex-situ methodol. Three cross-linked membranes P1, P2, and P3 consisting of 12.5, 25.0, and 37.5 wt. % BrPPO, resp. with respect to PyOBI were successfully fabricated and PEM properties were studied. These membranes showed much improved acid stability, oxidative stability, mech. strength and strong resistance to swelling in concd. phosphoric acid (PA) soln. They were found to be completely stable in the 85% PA whereas uncross-linked PyOPBI membrane readily dissolved in 60% PA. The reason for such stability has been ascribed to the cross-linked network structure of the membrane. The P1 membrane exhibited remarkably high proton cond. (0.123 S cm-1) whereas pristine PyOPBI membrane showed cond. 0.008 S cm-1 at 180°. The single cell measurement in anhyd. conditions at 160° of membrane electrode assembly (MEA) obtained from P1 membrane displayed good fuel cell efficiencies with power d. 290 mW cm-2 and c.d. 848.7 mA cm-3 at 0.3V whereas under the identical measurement condition MEA of pristine PyOPBI membrane showed 96.4 mW cm-2 power d. and 321.5 mA cm-2 c.d. at 0.3V. All these results endorsed that cross-linked membranes have a great potential to be used in the HT-PEMFC.
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  27. 27
    Yang, J.; Li, Q.; Cleemann, L. N.; Jensen, J. O.; Pan, C.; Bjerrum, N. J.; He, R. Crosslinked Hexafluoropropylidene Polybenzimidazole Membranes with Chloromethyl Polysulfone for Fuel Cell Applications. Adv. Energy Mater. 2013, 3, 622– 630,  DOI: 10.1002/aenm.201200710
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    27
    Crosslinked hexafluoropropylidene polybenzimidazole membranes with chloromethyl polysulfone for fuel cell applications
    Yang, Jingshuai; Li, Qingfeng; Cleemann, Lars N.; Jensen, Jens Oluf; Pan, Chao; Bjerrum, Niels J.; He, Ronghuan
    Advanced Energy Materials (2013), 3 (5), 622-630CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
    Hexafluoropropylidene polybenzimidazole (F6PBI) was synthesized with excellent chem. stability and improved soly. When doped with phosphoric acid, however, the F6PBI membranes showed plastic deformation at elevated temps. Further efforts were made to covalently crosslink F6PBI membranes with chloromethyl polysulfone as a polymeric crosslinker. Comparing with linear F6PBI and mPBI membranes, the polymer crosslinked F6PBI membranes exhibited little organo soly., excellent stability towards the radical oxidn., high resistance to swelling in concd. phosphoric acid solns., and improved mech. strength, esp. at elevated temps. The superior characteristics of crosslinked membranes allowed for higher acid doping levels and therefore increased proton cond. as well as significantly improved fuel cell performance and durability, as compared with the linear F6PBI and mPBI membranes.
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  28. 28
    Wang, S.; Zhao, C.; Ma, W.; Zhang, N.; Liu, Z.; Zhang, G.; Na, H. Macromolecular cross-linked polybenzimidazole based on bromomethylated poly (aryl ether ketone) with enhanced stability for high temperature fuel cell applications. J. Power Sources 2013, 243, 102– 109,  DOI: 10.1016/j.jpowsour.2013.05.181
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    28
    Macromolecular cross-linked polybenzimidazole based on bromomethylated poly (aryl ether ketone) with enhanced stability for high temperature fuel cell applications
    Wang, Shuang; Zhao, Chengji; Ma, Wenjia; Zhang, Na; Liu, Zhongguo; Zhang, Gang; Na, Hui
    Journal of Power Sources (2013), 243 (), 102-109CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    Macromol. cross-linked polybenzimidazole (PBI) membranes were successfully prepd. for the high temp. proton exchange membrane fuel cell (HT-PEMFC) applications. Bromomethylated poly(aryl ether ketone) (BrPAEK) is synthesized and used as a macromol. cross-linker, the crosslinking reaction can be accomplished at 160° using an easy facial heating treatment. The resulting cross-linked membranes CBrPBI-X (X is the wt. fraction of the cross-linker) display excellent mech. strength. After phosphoric acid (PA) doping, the mech. strength and proton cond. of the PA/CBrPBI-X membranes are both enhanced comparing with the pristine PA/PBI. Considering the tradeoff of the mech. strength and proton cond., 10% BrPAEK is an optimum content in the matrix. For instance, the proton cond. of PA/CBrPBI-10 is 0.038 S cm-1 at 200°, which is higher than that of pristine PA/PBI with the proton cond. of 0.029 S cm-1 at the same temp. Other properties of the cross-linked membranes are also studied in detail, including the oxidative stability, soly. and thermal stability. All the PA/CBrPBI-10 membrane has the potential application in HT-PEMFCs.
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  29. 29
    Yang, J.; Aili, D.; Li, Q.; Cleemann, L. N.; Jensen, J. O.; Bjerrum, N. J.; He, R. Covalently Cross-Linked Sulfone Polybenzimidazole Membranes with Poly(vinylbenzyl chloride) for Fuel Cell Applications. ChemSusChem 2013, 6, 275– 282,  DOI: 10.1002/cssc.201200716
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    29
    Covalently Cross-Linked Sulfone Polybenzimidazole Membranes with Poly(Vinylbenzyl Chloride) for Fuel Cell Applications
    Yang, Jingshuai; Aili, David; Li, Qingfeng; Cleemann, Lars N.; Jensen, Jens Oluf; Bjerrum, Niels J.; He, Ronghuan
    ChemSusChem (2013), 6 (2), 275-282CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Covalently cross-linked polymer membranes were fabricated from poly(aryl sulfone benzimidazole) (SO2PBI) and poly(vinylbenzyl chloride) (PVBCl) as electrolytes for high-temp. proton-exchange-membrane fuel cells. The crosslinking imparted organo insoly. and chem. stability against radical attack to the otherwise flexible SO2PBI membranes. Steady phosphoric acid doping of the cross-linked membranes was achieved at elevated temps. with little swelling. The acid-doped membranes exhibited increased mech. strength compared to both pristine SO2PBI and poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole] (mPBI). The superior characteristics of the cross-linked SO2PBI membranes allowed higher acid doping levels and, therefore, higher proton cond. Fuel-cell tests with the cross-linked membranes demonstrated a high open circuit voltage and improved power performance and durability.
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  30. 30
    Venugopalan, G.; Chang, K.; Nijoka, J.; Livingston, S.; Geise, G. M.; Arges, C. G. Stable and Highly Conductive Polycation-Polybenzimidazole Membrane Blends for Intermediate Temperature Polymer Electrolyte Membrane Fuel Cells. ACS Appl. Energy Mater. 2020, 3, 573– 585,  DOI: 10.1021/acsaem.9b01802
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    30
    Stable and Highly Conductive Polycation-Polybenzimidazole Membrane Blends for Intermediate Temperature Polymer Electrolyte Membrane Fuel Cells
    Venugopalan, Gokul; Chang, Kevin; Nijoka, Justin; Livingston, Sarah; Geise, Geoffrey M.; Arges, Christopher G.
    ACS Applied Energy Materials (2020), 3 (1), 573-585CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
    Intermediate-temp. polymer electrolyte membrane fuel cells (IT-PEMFCs), operating with phosphoric acid (H3PO4) doped polybenzimidazole (PBI), are severely limited by H3PO4 evapn. at high temps. and poor resiliency in the presence of water. Polycations (PCs), on the other hand, provide good acid retention due to strong ion-pair interactions but have low cond. due to lower ion-exchange capacity when compared to PBI. In this work, a class of H3PO4 doped PC-PBI membrane blends was prepd., and the optimal blend (50:50 ratio) exhibited remarkably high in-plane proton cond., near 0.3 S cm-1 at 240°C, while also displaying excellent thermal stability and resiliency to water vapor. Microwave dielec. spectroscopy demonstrated that incorporating PBI into the PCs raised the dielec. const. by 50-70% when compared to the PC by itself. This observation explains, in part, the high proton cond. of the optimal membrane blend. Finally, an all-polymeric membrane electrode assembly with the new materials gave a competitive IT-PEMFC performance of 680 mW cm-2 at 220°C under dry H2/O2. Importantly, the cell was stable for up to 30 h at 220°C and over 84 h at 180°C. The IT-PEMFC had reasonable performance (450 mW cm-2) with 25% carbon monoxide in the hydrogen fuel.
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  31. 31
    Ma, W.; Zhao, C.; Lin, H.; Zhang, G.; Ni, J.; Wang, J.; Wang, S.; Na, H. High-Temperature Water-Free Proton Conducting Membranes based on Poly(arylene ether ketone) containing Pendant Quaternary Ammonium Groups with Enhanced Proton Transport. J. Power Sources 2011, 196, 9331– 9338,  DOI: 10.1016/j.jpowsour.2011.08.003
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    High-temperature water-free proton conducting membranes based on poly(arylene ether ketone) containing pendant quaternary ammonium groups with enhanced proton transport
    Ma, Wenjia; Zhao, Chengji; Lin, Haidan; Zhang, Gang; Ni, Jing; Wang, Jing; Wang, Shuang; Na, Hui
    Journal of Power Sources (2011), 196 (22), 9331-9338CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    Poly(arylene ether ketone) contg. pendant quaternary ammonium groups (QPAEKs) are anion-conducting polymers synthesized from benzylmethyl-contg. poly(arylene ether ketone)s (PAEK-TM). QPAEK membranes doped with different concns. of H3PO4 were prepd. and evaluated as high temp. p exchange membranes. The H3PO4 doping ability of quaternary ammonium groups in QPAEK system is stronger than that of imidazole groups in polybenzimidazole system. The doping level of resulting QPAEK/H3PO4 composite membranes increases with both the concn. level of soaking H3PO4 soln. and the ion exchange capacity. For example, the highest doping level of composite membranes is 28.6, which is derived from QPAEK-5 with an ion exchange capacity of 2.02 mmol/g satd. with concd. H3PO4. A strong correlation between the doping level and the p cond. is obsd. for all the membranes. Besides their low cost, novel high temp. p exchange membranes, QPAEK/H3PO4, show really high p cond. and possess excellent thermal and mech. stability, suggesting a bright future for applications in high temp. fuel cells.
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  32. 32
    Cho, H.; Hur, E.; Henkensmeier, D.; Jeong, G.; Cho, E.; Kim, H. J.; Jang, J. H.; Lee, K. Y.; Hjuler, H. A.; Li, Q.; Jensen, J. O.; Cleemann, L. N. Meta-PBI/Methylated PBI-OO Blend Membranes for Acid Doped HT PEMFC. Eur. Polym. J. 2014, 58, 135– 143,  DOI: 10.1016/j.eurpolymj.2014.06.019
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    meta-PBI/methylated PBI-OO blend membranes for acid-doped high-temperature polymer electrolyte fuel cells (HT-PEMFC)
    Cho, Hyeongrae; Hur, Eun; Henkensmeier, Dirk; Jeong, Gisu; Cho, Eunae; Kim, Hyoung Juhn; Jang, Jong Hyun; Lee, Kwan Young; Hjuler, Hans Aage; Li, Qingfeng; Jensen, Jens Oluf; Cleemann, Lars Nielausen
    European Polymer Journal (2014), 58 (), 135-143CODEN: EUPJAG; ISSN:0014-3057. (Elsevier Ltd.)
    Methylation of polybenzimidazole leads to pos. charged polymer backbones, and moveable anions. Ion exchange of methylated PBI-OO in phosphoric acid (PA) shows that the resulting polymers dissolve. meta-PBI, however, absorbs ∼400 wt% PA while remaining a self supported membrane. We investigate the properties of blend membranes, employing meta-PBI for mech. integrity and methylated PBI-OO for high PA uptake and resulting proton cond. While small addns. of PBI-OO decrease the tensile strength of blend membranes (58 MPa for 10% PBI-OO), further addn. leads to an increase, and 50% blend membranes show again a tensile strength of 74 MPa, just 3 MPa lower than pure meta-PBI membranes. Thermal stability of iodide exchanged blend membranes appears to be remarkably high, probably because cleaved iodomethane does not evap. but methylates meta-PBI. PA concn. in doped membranes of 60-63% is reached by doping in 60% PA (blend; 6.3 PA/repeat unit) and 70% PA (meta-PBI; 4.6 PA/r.u.). This suggests that blends absorb PA more strongly. Both membranes show similar cond. between rt and 140 °C, indicating that PA concn. describes these membranes better than PA/r.u. In the fuel cell, blend membranes show similar or better performance than meta-PBI. In the TGA, blends doped in 20% PA showed a stable plateau between 115 and 180 °C, while meta-PBI lost wt. continuously.
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    Lee, K.-S.; Spendelow, J. S.; Choe, Y.-K.; Fujimoto, C.; Kim, Y. S. An Operationally Flexible Fuel Cell Based on Quaternary Ammonium-Biphosphate Ion Pairs. Nat. Energy 2016, 1, 447,  DOI: 10.1038/nenergy.2016.120
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    Atanasov, V.; Lee, A. S.; Park, E. J.; Maurya, S.; Baca, E. D.; Fujimoto, C.; Hibbs, M.; Matanovic, I.; Kerres, J.; Kim, Y. S. Synergistically Integrated Phosphonated Poly(pentafluorostyrene) for Fuel Cells. Nat. Mater. 2021, 20, 370– 377,  DOI: 10.1038/s41563-020-00841-z
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    Synergistically integrated phosphonated poly(pentafluorostyrene) for fuel cells
    Atanasov, Vladimir; Lee, Albert S.; Park, Eun Joo; Maurya, Sandip; Baca, Ehren D.; Fujimoto, Cy; Hibbs, Michael; Matanovic, Ivana; Kerres, Jochen; Kim, Yu Seung
    Nature Materials (2021), 20 (3), 370-377CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
    Modern electrochem. energy conversion devices require more advanced proton conductors for their broad applications. Phosphonated polymers have been proposed as anhyd. proton conductors for fuel cells. However, the anhydride formation of phosphonic acid functional groups lowers proton cond. and this prevents the use of phosphonated polymers in fuel cell applications. Here, we report a poly(2,3,5,6-tetrafluorostyrene-4-phosphonic acid) that does not undergo anhydride formation and thus maintains protonic cond. above 200°C. We use the phosphonated polymer in fuel cell electrodes with an ion-pair coordinated membrane in a membrane electrode assembly. This synergistically integrated fuel cell reached peak power densities of 1,130 mW cm-2 at 160°C and 1,740 mW cm-2 at 240°C under H2/O2 conditions, substantially outperforming polybenzimidazole- and metal phosphate-based fuel cells. Our result indicates a pathway towards using phosphonated polymers in high-performance fuel cells under hot and dry operating conditions.
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    Published Online: Dec. 7, 2020

  35. 35
    Lu, W.; Zhang, G.; Li, J.; Hao, J.; Wei, F.; Li, W.; Zhang, J.; Shao, Z.-G.; Yi, B. Polybenzimidazole-Crosslinked Poly(vinylbenzyl chloride) with Quaternary 1,4-Diazabicyclo (2.2.2) Octane Groups as High-Performance Anion Exchange Membrane for Fuel Cells. J. Power Sources 2015, 296, 204– 214,  DOI: 10.1016/j.jpowsour.2015.07.048
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    Polybenzimidazole-crosslinked poly(vinylbenzyl chloride) with quaternary 1,4-diazabicyclo (2.2.2) octane groups as high-performance anion exchange membrane for fuel cells
    Lu, Wangting; Zhang, Geng; Li, Jin; Hao, Jinkai; Wei, Feng; Li, Wenhui; Zhang, Jiying; Shao, Zhi-Gang; Yi, Baolian
    Journal of Power Sources (2015), 296 (), 204-214CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    Development of anion exchange membrane (AEM) with high cond., good dimensional stability, desirable toughness and long life-time simultaneously is still a challenge for the practical application of AEM fuel cells. Herein, a novel AEM (denoted as PBI-c-PVBC/OH) is fabricated by applying polybenzimidazole (PBI) and 1,4-diazabicyclo (2.2.2) octane (DABCO) as the macromol. crosslinker and quaternizing reagent for poly(vinylbenzyl chloride) (PVBC), resp. With the aid of crosslinking by PBI, PBI-c-PVBC/OH exhibits good flexibility and strength both in dry and water-satd. state. Moreover, high hydroxide cond. (>25 mS cm-1 at room temp.) and low swelling ratio (∼13%) is obtained, esp. the swelling ratio nearly does not increase with temp. The membrane is also advanced for the superior chem. stability in alk. environment due to the stable polymer backbone and ionic conductive group (only one nitrogen atom in a DABCO mol. is quaternized). Furthermore, a peak power d. of 230 mW cm-2 at 50 °C is obtained on the H2/O2 fuel cell using PBI-c-PVBC/OH, and the membrane presents high durability both in the const. current and continuous open circuit voltage testing. Therefore, it is considered that the PBI crosslinking together with DABCO quaternization can be regarded as a promising strategy in the development of AEM for fuel cells.
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    Qaisrani, N. A.; Ma, L.; Hussain, M.; Liu, J.; Li, L.; Zhou, R.; Jia, Y.; Zhang, F.; He, G. Hydrophilic Flexible Ether Containing, Cross-Linked Anion-Exchange Membrane Quaternized with DABCO. ACS Appl. Mater. Interfaces 2020, 12, 3510– 3521,  DOI: 10.1021/acsami.9b15435
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    Hydrophilic Flexible Ether Containing, Cross-Linked Anion-Exchange Membrane Quaternized with DABCO
    Qaisrani, Naeem Akhtar; Ma, Lingling; Hussain, Manzoor; Liu, Jiafei; Li, Lv; Zhou, Ruiting; Jia, Yabin; Zhang, Fengxiang; He, Gaohong
    ACS Applied Materials & Interfaces (2020), 12 (3), 3510-3521CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Anion-exchange membranes (AEM) with high ion content usually suffer from excessive water absorption and diln. effects that impair cond. and mech. properties. We herein report a novel ether contg. a crosslinking strategy without adopting high ion-exchange capacity (IEC). The ether-contg. crosslinks and the quaternized structure are created simultaneously by introducing an ether-contg. flexible hydrophilic spacer between two 1,4-diazabicyclo[2,2,2,2]octane or DABCO mols.; the resultant bi-DABCO structure was further employed to react with chloromethylated polysulfone. The long spacer with the ether moiety may benefit the hydroxide ion transport, and the crosslinks will control the swelling and water absorption of the AEM. The two ether groups in the long spacer of the crosslinks will also shield the DABCO cation from OH- attack due to an electron-donating effect. The prepd. membranes exhibited an improved cond. of 31 mS/cm (at 25°C) at a comparatively low IEC (1.08 mmol/g) with a rational water absorption and low swelling ratio (95.0 and 27.1%, resp.); they also displayed an enhanced alk. stability in 1 M NaOH aq. soln. at 80°C for 150 h. The d. functional theory study and phys. characterization after the alk. treatment further confirm the better chem. stability of the crosslinked membrane over its counterpart. Our work presents an effective strategy to balance AEM cond. and robustness.
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    Published Online: Jan. 10, 2020

  37. 37
    Park, J.-H.; Park, J.-S. KOH-Doped Porous Polybenzimidazole Membranes for Solid Alkaline Fuel Cells. Energies 2020, 13, 525,  DOI: 10.3390/en13030525
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    KOH-doped porous polybenzimidazole membranes for solid alkaline fuel cells
    Park, Jong-Hyeok; Park, Jin-Soo
    Energies (Basel, Switzerland) (2020), 13 (3), 525CODEN: ENERGA; ISSN:1996-1073. (MDPI AG)
    In this study the prepn. and properties of potassium hydroxide-doped meta-polybenzimidazole membranes with 20-30μm thickness are reported as anion conducting polymer electrolyte for application in fuel cells. Di-Bu phthalate as porogen forms an asym. porous structure of membranes along thickness direction. One side of the membranes has a dense skin layer surface with 1.5-15μm and the other side of the membranes has a porous one. It demonstrated that ion cond. of the potassium hydroxide-doped porous membrane with the porogen content of 47 wt.% (0.090 S cm-1), is 1.4 times higher than the potassium hydroxide-doped dense membrane (0.065 S cm-1). This is because the porous membrane allows 1.4 times higher potassium hydroxide uptake than dense membranes. Tensile strength and elongation studies confirm that doping by simply immersing membranes in potassium hydroxide solns. was sufficient to fill in the inner pores. The membrane-electrode assembly using the asym. porous membrane with 1.4 times higher ionic cond. than the dense non-doped polybenzimidazole (mPBI) membrane showed 1.25 times higher peak power d.
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    Li, Q.; He, R.; Berg, R. W.; Hjuler, H. A.; Bjerrum, N. J. Water Uptake and Acid Doping of Polybenzimidazoles as Electrolyte Membranes for Fuel Cells. Solid State Ionics 2004, 168, 177– 185,  DOI: 10.1016/j.ssi.2004.02.013
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    Water uptake and acid doping of polybenzimidazoles as electrolyte membranes for fuel cells
    Li, Qingfeng; He, Ronghuan; Berg, Rolf W.; Hjuler, Hans A.; Bjerrum, Niels J.
    Solid State Ionics (2004), 168 (1-2), 177-185CODEN: SSIOD3; ISSN:0167-2738. (Elsevier Science B.V.)
    Acid-doped polybenzimidazole (PBI) membranes have been demonstrated for fuel cell applications with advanced features such as high operating temps., little humidification, excellent CO tolerance, and promising durability. The water uptake and acid doping of PBI membranes have been studied. The water uptake of PBI from the vapor phase is only slightly increased as the atm. humidity increases up to unity (100%). Little difference is obsd. for the water uptake from vapor and liq. phases, behaving very differently from Nafion membranes. When doped with phosphoric acid at low levels (<2), the active sites of the imidazole ring are preferably occupied by the doping acid and the water uptake is consequently lower. At higher acid doping levels, the water uptake is influenced by the excess of hygroscopic acid and higher water uptake than for Nafion membranes is obsd. Upon doping, the acid is found to be concd. inside the polymer. Only two mols. of phosphoric acid are bonded to each repeat unit of PBI, corresponding to the two nitrogen sites available. IR and Raman spectra show the presence of strong hydrogen bonds between phosphoric acid and nitrogen atoms of the imidazole rings. The excessive doping acid is "free acid" that contributes to high cond. but suffers from a fast washing out when adequate liq. is present.
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    Arslan, F.; Böhm, T.; Kerres, J.; Thiele, S. Spatially and Temporally Resolved Monitoring of Doping Polybenzimidazole Membranes with Phosphoric Acid. J. Membr. Sci. 2021, 625, 119145,  DOI: 10.1016/j.memsci.2021.119145
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    Spatially and temporally resolved monitoring of doping polybenzimidazole membranes with phosphoric acid
    Arslan, Funda; Boehm, Thomas; Kerres, Jochen; Thiele, Simon
    Journal of Membrane Science (2021), 625 (), 119145CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
    Polybenzimidazole-based membranes in high temp. proton exchange membrane fuel cells require doping with phosphoric acid to enable proton cond. The level of phosphoric acid doping is traditionally detd. by titrn. or weighing, but these methods only provide limited information, since they do not offer spatial resoln. We show that confocal Raman microscopy can not only provide information on the level of doping, but in addn. about the spatial distribution of the dopant. We prove that doping is a diffusion-limited process, leading to a spatially inhomogeneous distribution of phosphoric acid unless doping is performed to satn. Further, evidence of a slow redistribution of the dopant within a freestanding membrane under ambient conditions is provided. Confocal Raman microscopy can be used as a non-invasive measurement tool to investigate status and progress of doping a membrane with phosphoric acid.
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    Cho, H.; Krieg, H.; Kerres, J. Performances of Anion-Exchange Blend Membranes on Vanadium Redox Flow Batteries. Membranes 2019, 9, 31,  DOI: 10.3390/membranes9020031
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    Performances of anion-exchange blend membranes on vanadium redox flow batteries
    Cho, Hyeongrae; Krieg, Henning M.; Kerres, Jochen A.
    Membranes (Basel, Switzerland) (2019), 9 (2), 31/1-31/14CODEN: MBSEB6; ISSN:2077-0375. (MDPI AG)
    Anion exchange blend membranes (AEBMs) were prepd. for use in Vanadium Redox Flow Batteries (VRFBs). These AEBMs consisted of 3 polymer components. Firstly, PBI-OO (nonfluorinated PBI) or F6-PBI (partially fluorinated PBI) were used as a matrix polymer. The second polymer, a bromomethylated PPO, was quaternized with 1,2,4,5-tetramethylimidazole (TMIm) which provided the anion exchange sites. Thirdly, a partially fluorinated polyether or a non-fluorinated poly (ether sulfone) was used as an ionical cross-linker. While the AEBMs were prepd. with different combinations of the blend polymers, the same wt. ratios of the three components were used. The AEBMs showed similar membrane properties such as ion exchange capacity, dimensional stability and thermal stability. For the VRFB application, comparable or better energy efficiencies were obtained when using the AEBMs compared to the com. membranes included in this study, i.e., Nafion (cation exchange membrane) and FAP 450 (anion exchange membrane). One of the blend membranes showed no capacity decay during a charge-discharge cycles test for 550 cycles run at 40 mA/cm2 indicating superior performance compared to the com. membranes tested.
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  41. 41
    Pan, C.; Li, Q.; Jensen, J. O.; He, R.; Cleemann, L. N.; Nilsson, M. S.; Bjerrum, N. J.; Zeng, Q. Preparation and Operation of Gas Diffusion Electrodes for High-Temperature Proton Exchange Membrane Fuel Cells. J. Power Sources 2007, 172, 278– 286,  DOI: 10.1016/j.jpowsour.2007.07.019
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    Preparation and operation of gas diffusion electrodes for high-temperature proton exchange membrane fuel cells
    Pan, Chao; Li, Qingfeng; Jensen, Jens Oluf; He, Ronghuan; Cleemann, Lars N.; Nilsson, Morten S.; Bjerrum, Niels J.; Zeng, Qingxue
    Journal of Power Sources (2007), 172 (1), 278-286CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    Gas diffusion electrodes for high-temp. PEMFCs based on acid-doped polybenzimidazole membranes were prepd. by tape-casting. The overall porosity of the electrodes was tailored at 38-59% by introducing porogens into the supporting and/or catalyst layers. The studied porogens include volatile NH4+ oxalate, carbonate and acetate and acid-sol. Zn oxide - NH4+ oxalate and ZnO are more effective in improving the overall electrode porosity. Effects of electrode porosity on fuel cell performance were studied in terms of the cathodic limiting c.d. and min. air stoichiometry, anodic limiting current and H use, as well as operations under different pressures and temps.
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    Lobato, J.; Rodrigo, M. A.; Linares, J. J.; Scott, K. Effect of the Catalytic Ink Preparation Method on the Performance of High Temperature Polymer Electrolyte Membrane Fuel Cells. J. Power Sources 2006, 157, 284– 292,  DOI: 10.1016/j.jpowsour.2005.07.040
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    Effect of the catalytic ink preparation method on the performance of high temperature polymer electrolyte membrane fuel cells
    Lobato, J.; Rodrigo, M. A.; Linares, J. J.; Scott, K.
    Journal of Power Sources (2006), 157 (1), 284-292CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    Two methods of prepn. of the membrane-electrode-assemblies based on polybenzimidazole membranes were studied for high temps. PEMFCs. One is called the colloidal method (using acetone as solvent), and the other is the soln. method (using dimethylacetamide as solvent). Phys. property studies (SEM micrographs and pore size distribution) and electrochem. analyses in half-cell (Electrochem. Impedance Spectroscopy, Polarization Curves for Oxygen Redn. and Cyclic Voltammetry) were carried out to characterize the structural and electrochem. behavior of both methods. A cell performance study, using electrodes prepd. by both methods was carried out at 3 different temps. (125, 150, and 175°), in a single PEMFC setup. A better behavior was obtained for the soln. method at the two highest temps. at intermediate current densities, whereas at 125° the best results were obtained with the colloidal method in all the current densities ranges. A discussion of the behaviors obsd. with the different characterization techniques is made.
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    Rau, M.; Niedergesäß, A.; Cremers, C.; Alfaro, S.; Steenberg, T.; Hjuler, H. A. Characterization of Membrane Electrode Assemblies for High-Temperature PEM Fuel Cells. Fuel Cells 2016, 16, 577– 583,  DOI: 10.1002/fuce.201500105
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    Characterization of Membrane Electrode Assemblies for High-Temperature PEM Fuel Cells
    Rau, M.; Niedergesaess, A.; Cremers, C.; Alfaro, S.; Steenberg, T.; Hjuler, H. A.
    Fuel Cells (Weinheim, Germany) (2016), 16 (5), 577-583CODEN: FUCEFK; ISSN:1615-6846. (Wiley-Blackwell)
    This paper will present the characterization of two types of membrane-electrode-assemblies (MEAs) for high-temp. polymer electrolyte membrane fuel cells (HT-PEMFC) working under reformate stream. The important aspects to be considered in the characterization of these MEAs are: (i) presence of contaminants, and (ii) compn. of the anode. Start/stop cycling test were performed for two different Dapozol MEAs using different GDL materials, using first hydrogen and then synthetic reformate as a fuel gas, both with a dew point of 80 °C. With these results the influence of contaminants present in the reformate was compared for the two types of MEAs, showing the superior performance of the Dapozol 101 MEA under these conditions. The possibility to further enhance the MEAs' resilience against the operation of reformates by changing the anode catalyst compn. was evaluated in a half MEA configuration, considering that the impact of the H2S present in the fuel presents a major issue. For this reason the hydrogen oxidn. reaction (HOR) was evaluated for two types of Pt-based electrocatalysts in an anodic half MEA configuration using different hydrogen-rich fuel mixts. These results provide valuable information for the optimization of the MEA and the anode catalyst for HT-PEMFC.
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    Cooper, K. R.; Smith, M. Electrical Test Methods for On-Line Fuel Cell Ohmic Resistance Measurement. J. Power Sources 2006, 160, 1088– 1095,  DOI: 10.1016/j.jpowsour.2006.02.086
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    Electrical test methods for on-line fuel cell ohmic resistance measurement
    Cooper, K. R.; Smith, M.
    Journal of Power Sources (2006), 160 (2), 1088-1095CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    The principles and trade-offs of 4 elec. test methods suitable for online measurement of the ohmic resistance, RΩ, of fuel cells is presented: current interrupt, a.c. resistance, high frequency resistance (HFR), and electrochem. impedance spectroscopy (EIS). The internal resistance of a p exchange membrane (PEM) fuel cell detd. with the current interrupt, HFR and EIS techniques is compared. The influence of the a.c. amplitude and frequency of the HFR measurement on the obsd. ohmic resistance is examd., as is the ohmic resistance extd. from the EIS data by modeling the spectra with a transmission line model for porous electrodes. The ohmic resistance of a H2/O2 PEM fuel cell detd. via the 3 methods was within 10-30% of each other. The current interrupt technique consistently produced measured cell resistances that exceeded those of the other 2 techniques. For the HFR technique, the frequency at which the measurement was conducted influenced the measured resistance (i.e., higher frequency providing smaller R Ω), whereas the a.c. amplitude did not effect the obsd. value. The difference in measured ohmic resistance between these techniques exceeds that reasonably accounted for by measurement error. The source of the discrepancy between current interrupt and impedance-based methods is attributed to the difference in the response of a nonuniformly polarized electrode, such as a porous electrode with non-negligible ohmic resistance, to a large perturbation (current interrupt event) as compared to a small perturbation (impedance measurement).
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    Kocha, S. S.; Deliang Yang, J.; Yi, J. S. Characterization of Gas Crossover and its Implications in PEM Fuel Cells. AIChE J. 2006, 52, 1916– 1925,  DOI: 10.1002/aic.10780
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    Characterization of gas crossover and its implications in PEM fuel cells
    Kocha, Shyam S.; Yang, J. Deliang; Yi, Jung S.
    AIChE Journal (2006), 52 (5), 1916-1925CODEN: AICEAC; ISSN:0001-1541. (John Wiley & Sons, Inc.)
    With pure hydrogen as the fuel, PEM fuel cell operation at or near 100% fuel utilization is desirable to achieve a high stack efficiency and zero emissions. However, typical membranes used in PEM fuel cells allow a finite amt. of permeation rates or crossover of hydrogen, oxygen, and nitrogen across the membrane. The hydrogen and oxygen that permeate through the membrane are consumed with the generation of heat and water but without the generating of useful work, leading to a fuel inefficiency. Nitrogen crossover, on the other hand, from the cathode side to the anode side accumulates at the exit of the anode flow fields, lowering the hydrogen concn. and resulting in local fuel starvation. In this study, an in-situ electrochem. technique was applied to det. the magnitude of the hydrogen crossover over a range of relevant fuel cell operating temps. and pressures. Permeability coeffs. thus obtained are compared to values reported in the literature. A math. model was developed to predict the extent of nitrogen accumulation along the anode flow fields, and fuel recycle as a mitigation method is simulated by improving hydrogen distribution. The model results were validated by comparison with exptl. results.
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    Cooper, K. R. Laboratory #4 – Fuel Crossover by Linear Sweep Voltammetry & Electrochemical Surface Area by Cyclic Voltammetry. Fuel Cell Mag. 2008.
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    High Temperature Polymer Electrolyte Membrane Fuel Cells; Li, Q., Aili, D., Hjuler, H. A., Jensen, J. O., Eds.; Springer International Publishing, 2016.
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    Bandura, D. R.; Baranov, V. I.; Tanner, S. D. Detection of Ultratrace Phosphorus and Sulfur by Quadrupole ICPMS with Dynamic Reaction Cell. Anal. Chem. 2002, 74, 1497– 1502,  DOI: 10.1021/ac011031v
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    Detection of ultratrace phosphorus and sulfur by quadrupole ICPMS with dynamic reaction cell
    Bandura, Dmitry R.; Baranov, Vladimir I.; Tanner, Scott D.
    Analytical Chemistry (2002), 74 (7), 1497-1502CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    A method of detection of ultratrace phosphorus and sulfur that uses reaction with O2 in a dynamic reaction cell (DRC) to oxidize S+ and P+ to allow their detection as SO+ and PO+ is described. The method reduces the effect of polyat. isobaric interferences at m/z = 31 and 32 by detecting P+ and S+ as the product oxide ions that are less interfered. Use of an axial field in the DRC improves transmission of the product oxide ions 4-6 times. With no axial field, detection limits (3σ, 5-s integration) of 0.20 and 0.52 ng/mL, with background equiv. concns. of 0.53 and 4.8 ng/mL, resp., are achieved. At an optimum axial field potential (200 V), the detection limits are 0.06 ng/mL for P and 0.2 ng/mL for S, resp. The method is used for detg. the degree of phosphorylation of β-casein, and regular and dephosphorylated α-caseins at 10-1000 fmol/μL concn., with 5-10% vol./vol. org. sample matrix (acetonitrile, formic acid, ammonium bicarbonate). The measured degree of phosphorylation for β-casein (4.9 phosphorus atoms/mol.) and regular α-casein (8.8 phosphorus atoms/mol.) are in good agreement with the structural data for the proteins. The P/S ratio for regular α-casein (1.58) is in good agreement with the ratio of the no. of phosphorylation sites to the no. of sulfur-contg. amino acid residues cysteine and methionine. The P/S ratio for com. available dephosphorylated α-casein is measured at 0.41 (∼26% residual phosphate).
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    Zhang, H.; Su, S.; Chen, X.; Lin, G.; Chen, J. Performance Evaluation and Optimum Design Strategies of an Acid Water Electrolyzer System for Hydrogen Production. Int. J. Hydrogen Energy 2012, 37, 18615– 18621,  DOI: 10.1016/j.ijhydene.2012.09.127
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    Performance evaluation and optimum design strategies of an acid water electrolyzer system for hydrogen production
    Zhang, Houcheng; Su, Shanhe; Chen, Xiaohang; Lin, Guoxing; Chen, Jincan
    International Journal of Hydrogen Energy (2012), 37 (24), 18615-18621CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
    The performance of a new acid water electrolyzer system for hydrogen prodn. is investigated, based on semi-empirical equations of a phosphoric acid water electrolyzer. The circulating electrolyte concns. under differently operating temps. are optimized so that the min. input voltages of the electrolyzer are detd. for other given conditions. The optimum electrochem. characteristics of the electrolyzer are revealed. Moreover, it is expounded that the Joule heat resulting from the irreversibilities inside the electrolyzer is larger than the thermal energy needed in the water splitting process. The general performance characteristics of the phosphoric acid water electrolyzer system are discussed, from which the lower bound of the operating c.d. is detd. The upper bound of the operating c.d. is further detd. by introducing a multi-objective function including the system efficiency and hydrogen prodn. rate. Consequently, some optimum design strategies of a phosphoric acid water electrolyzer system are obtained and may be chosen according to different practical requirements.
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    Lin, X.; Liang, X.; Poynton, S. D.; Varcoe, J. R.; Ong, A. L.; Ran, J.; Li, Y.; Li, Q.; Xu, T. Novel Alkaline Anion Exchange Membranes Containing Pendant Benzimidazolium Groups for Alkaline Fuel Cells. J. Membr. Sci. 2013, 443, 193– 200,  DOI: 10.1016/j.memsci.2013.04.059
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    Novel alkaline anion exchange membranes containing pendant benzimidazolium groups for alkaline fuel cells
    Lin, Xiaocheng; Liang, Xuhao; Poynton, Simon D.; Varcoe, John R.; Ong, Ai Lien; Ran, Jin; Li, Yan; Li, Qiuhua; Xu, Tongwen
    Journal of Membrane Science (2013), 443 (), 193-200CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
    Novel benzimidazolium (BIm) functionalized anion exchange membranes (AEMs) are synthesized and characterized for alk. fuel cells (AFCs). Poly(phenylene oxide) (PPO) is firstly brominated followed by nucleophilic substitution reaction with methylbenzimidazole to obtain the objective BIm-PPO AEMs. Such soln.-casting AEMs show good mech. and thermal stabilities as well as the favorable fuel cell-related indicators, including high ion exchange capacity, proper water uptake and high ionic cond. In addn., a single H2/O2 fuel cell test by employing the optimal BIm-PPO-0.54 AEM yields a peak power d. of 13 mW cm-2 at 35 °C, indicating the potential application of BIm-PPO AEMs in AFCs. Compared with the analogous AEMs based on PPO contg. the classical pendant quaternary ammonium and imidazolium cations, BIm-PPO AEMs show the advantages in dimensional, mech. and thermal stabilities, while simultaneously exhibiting the higher ionic cond. Compared with polybenzimidazolium based AEMs, where BIm cations distribute within the polymer backbone, AEMs herein present the higher ionic cond. and power d. (produced from a single cell test) due to the better mobility and aggregation abilities of pendant BIm cations attached to the backbone via a side chain relative to those distribute within the polymer backbone.
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    Hou, H.; Wang, S.; Liu, H.; Sun, L.; Jin, W.; Jing, M.; Jiang, L.; Sun, G. Synthesis and Characterization of a New Anion Exchange Membrane by a Green and Facile Route. Int. J. Hydrogen Energy 2011, 36, 11955– 11960,  DOI: 10.1016/j.ijhydene.2011.06.054
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    Synthesis and characterization of a new anion exchange membrane by a green and facile route
    Hou, Hongying; Wang, Suli; Liu, He; Sun, Lili; Jin, Wei; Jing, Mingyi; Jiang, Luhua; Sun, Gongquan
    International Journal of Hydrogen Energy (2011), 36 (18), 11955-11960CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
    A new anion exchange membrane was synthesized by a green and facile route, chem. grafting polybenzimidazole (PBI) membrane with BrCH2CH3. The obtained membrane was characterized by means of ex-situ and in-situ tests (EDX, FTIR, a.c. impedance, single cell test). The results suggested that the group of -CH2CH3 was successfully grafted onto N atom within PBI backbone, with Br- as counter ion. The corresponding ionic cond. and ethanol permeability were about 0.022 S/cm and 5.24 × 10-8 cm2/s, resp. Finally, single cell test suggested that air-breathing alk. direct ethanol fuel cell with quaternized PBI membrane can deliver a peak power d. of ∼11 mW/cm2 even at ambient temp. of 13°, which was better than those of air-breathing alk. direct methanol fuel cells in literature. In addn., a possible reaction mechanism was also proposed and discussed.
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    Ogunlaja, A. S.; Hosten, E. C.; Tshentu, Z. R. The Oxidation of Dibenzothiophene using Oxidovanadium(IV)-Containing Nanofibres as Catalyst. S. Afr. J. Chem. 2015, 68, 172– 180,  DOI: 10.17159/0379-4350/2015/v68a24
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    The oxidation of dibenzothiophene using oxidovanadium(IV)-containing nanofibres as catalyst
    Ogunlaja, Adeniyi S.; Hosten, Eric C.; Tshentu, Zenixole R.
    South African Journal of Chemistry (2015), 68 (), 172-180CODEN: SAJCDG; ISSN:0379-4350. (South African Chemical Institute)
    Polyvinylbenzylchloride nanofibres were fabricated by the electrospinning technique and subsequently functionalized with a tetradentate ligand, 2,2'-(1E,1'E)-(1,2-phenylenebis(azan-1-yl-1-ylidene))bis(methan-1-yl-1-ylidene)bis(4-aminophenol). VO2+ was then incorporated into the nanofibres to produce the catalyst VO-fibers. Microanal., TG and FT-IR were used for the characterization of VO-fiber, and EPR also confirmed the presence of oxidovanadium(IV) within the nanofibres. Oxidn. of dibenzothiophene (DBT) was investigated by varying the catalyst amt., substrate amt., oxidant and temp., and the progress of oxidn. was followed with a gas chromatograph fitted with a flame ionization detector.Anincrease in the amt. of oxidant caused an increase in the amt. of dibenzothiophene sulfone (DBTO2), while a decrease in the quantity of dibenzothiophene resulted in an increase in the overall yield of dibenzothiophene sulfone under a const. temp. and oxidant (H2O2) concn. Dibenzothiophene sulfone was confirmed as the oxidn. product through 1H-NMRspectroscopy and single crystal X-ray diffraction.
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    Couture, G.; Alaaeddine, A.; Boschet, F.; Ameduri, B. Polymeric Materials as Anion-Exchange Membranes for Alkaline Fuel Cells. Prog. Polym. Sci. 2011, 36, 1521– 1557,  DOI: 10.1016/j.progpolymsci.2011.04.004
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    Polymeric materials as anion-exchange membranes for alkaline fuel cells
    Couture, Guillaume; Alaaeddine, Ali; Boschet, Frederic; Ameduri, Bruno
    Progress in Polymer Science (2011), 36 (11), 1521-1557CODEN: PRPSB8; ISSN:0079-6700. (Elsevier Ltd.)
    A review. After summarizing the different fuel cells systems, including advantages and drawbacks, this review focuses on the prepn. of copolymers and polymeric materials as starting materials for solid alk. fuel cells membranes. The requirements for such membranes are also summarized. Then, different strategies are given to synthesize anion-exchange polymeric materials contg. cationic (esp. ammonium) groups. The first pathway focuses on heterogeneous membranes that consist in: (i) polymer blends and composites based on poly(alkene oxide)s and hydroxide salts or polybenzimidazole doped with potassium hydroxide, (ii) org.-inorg. hybrid membranes esp. those synthesized via a sol-gel process, and (iii) (semi)interpenetrated networks based on poly(epichlorhydrine), poly(acrylonitrile) and polyvinyl alc. for example, that have led to new polymeric materials for anion-exchange membranes. The second and main part concerns the homogeneous membranes divided into three categories. The first one consists in materials synthesized from (co)polymers obtained via direct (co)polymn., for example membranes based on poly(diallyldimethylammonium chloride). The second pathway concerns the modification of polymeric materials via radiografting or chem. reactions. These polymeric materials can be hydrogenated or halogenated. The radiografting of membranes means the irradn. via various sources - electron beam, X and γ rays, 60Co and 137Cs that lead to trapped radicals or macromol. peroxides or hydroperoxides, followed by the radical graft polymn. of specific monomers such as chloromethyl styrene. The third route deals with the chem. modifications of com. available hydrogenated aliph. and arom. (co)polymers, and the syntheses of fluorinated (co)polymers such as carboxylic and sulfonic perfluoropolymers. In addn., several approaches for the crosslinking of above-mentioned polymeric materials are also reported as this process enhances the properties of the resulting membranes. Moreover, electrochem. and thermal properties of various above ionomers are given and discussed.
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    Teresa Pérez-Prior, M.; Ureña, N.; Tannenberg, M.; del Río, C.; Levenfeld, B. DABCO-Functionalized Polysulfones as Anion-Exchange Membranes for Fuel Cell Applications: Effect of Crosslinking. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1326– 1336,  DOI: 10.1002/polb.24390
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    DABCO-functionalized polysulfones as anion-exchange membranes for fuel cell applications: Effect of crosslinking
    Teresa Perez-Prior, Maria; Urena, Nieves; Tannenberg, Monika; del Rio, Carmen; Levenfeld, Belen
    Journal of Polymer Science, Part B: Polymer Physics (2017), 55 (17), 1326-1336CODEN: JPBPEM; ISSN:0887-6266. (John Wiley & Sons, Inc.)
    A series of DABCO-functionalized polysulfones were synthesized and characterized. The effect that crosslinking has on the membrane properties contg. different degrees of functionalization was evaluated. These polymers showed good thermal stability below the fuel cell operation temp., T < 100°, reflected by the TOD, TFD, and thermal durability. The water uptake increased as the percentage of DABCO groups increased and the crosslinked membranes showed lower capacity to absorb water than the non-crosslinked ones favoring thus the dimensional stability of the first ones. Membranes in the chloride form contg. low degree of functionalization exhibited the highest tensile strength values. The ionic cond. of non-crosslinked membranes varied as a function of the functionalization degree until a value of around 100% achieving a max. value at 86%. However, the crosslinked ones showed satisfactory ionic conductivities for values higher than 100%. The behavior of these polymeric materials in alk. solns. revealed a great alk. stability necessary to be used as solid electrolytes in fuel cells. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017.
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    Yang, J. S.; Cleemann, L. N.; Steenberg, T.; Terkelsen, C.; Li, Q. F.; Jensen, J. O.; Hjuler, H. A.; Bjerrum, N. J.; He, R. H. High Molecular Weight Polybenzimidazole Membranes for High Temperature PEMFC. Fuel Cells 2014, 14, 7– 15,  DOI: 10.1002/fuce.201300070
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    High Molecular Weight Polybenzimidazole Membranes for High Temperature PEMFC
    Yang, J. S.; Cleemann, L. N.; Steenberg, T.; Terkelsen, C.; Li, Q. F.; Jensen, J. O.; Hjuler, H. A.; Bjerrum, N. J.; He, R. H.
    Fuel Cells (Weinheim, Germany) (2014), 14 (1), 7-15CODEN: FUCEFK; ISSN:1615-6846. (Wiley-Blackwell)
    High temp. operation of proton exchange membrane fuel cells under ambient pressure has been achieved by using phosphoric acid doped polybenzimidazole (PBI) membranes. To optimize the membrane and fuel cells, high performance polymers were synthesized of mol. wts. from 30 to 94 kDa with good soly. in org. solvents. Membranes fabricated from the polymers were systematically characterized in terms of oxidative stability, acid doping and swelling, cond., mech. strength and fuel cell performance and durability. With increased mol. wts. the polymer membranes showed enhanced chem. stability towards radical attacks under the Fenton test, reduced vol. swelling upon the acid doping and improved mech. strength at acid doping levels of as high as about 11 mol H3PO4 per M repeat polymer unit. The PBI-78kDa/10.8PA membrane, for example, exhibited tensile strength of 30.3 MPa at room temp. or 7.3 MPa at 130 °C and a proton cond. of 0.14 S cm-1 at 160 °C. Fuel cell tests with H2 and air at 160 °C showed high open circuit voltage, power d. and a low degrdn. rate of 1.5 μV h-1 at a const. load of 300 mA cm-2.
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    Aili, D.; Yang, J.; Jankova, K.; Henkensmeier, D.; Li, Q. From Polybenzimidazoles to Polybenzimidazoliums and Polybenzimidazolides. J. Mater. Chem. A 2020, 8, 12854– 12886,  DOI: 10.1039/D0TA01788D
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    From polybenzimidazoles to polybenzimidazoliums and polybenzimidazolides
    Aili, David; Yang, Jingshuai; Jankova, Katja; Henkensmeier, Dirk; Li, Qingfeng
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2020), 8 (26), 12854-12886CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    A review. Polybenzimidazoles represent a large family of high-performance polymers contg. benzimidazole groups as part of the structural repeat unit. New application areas in electrochem. cells and sepn. processes have emerged during the last two decades, which has been a major driver for the tremendous development of new polybenzimidazole chemistries and materials in recent years. This comprehensive treatise is devoted to an investigation of the structural scope of polybenzimidazole derivs., polybenzimidazole modifications and the acid-base behavior of the resulting materials. Advantages and limitations of different synthetic procedures and pathways are analyzed, with focus on homogeneous soln. polymn. The discussion extends to soln. properties and the challenges that are faced in connection to mol. wt. detn. and processing. Methods for polybenzimidazole grafting or crosslinking, in particular by N-coupling, are reviewed and successful polymer blend strategies are identified. The amphoteric nature of benzimidazole groups further enriches the chem. of polybenzimidazoles, as cationic or anionic ionenes are obtained depending on the pH. In the presence of protic acids, such as phosphoric acid, cationic ionenes in the form of protic polybenzimidazoliums are obtained, which dramatically changes the physicochem. properties of the material. Cationic ionenes are also derived by complete N-alkylation of a polybenzimidazole to the corresponding poly(dialkyl benzimidazolium), which has been intensively explored recently as a new direction in the field of anion exchange membranes. In the higher end of the pH scale in aq. hydroxide solns., anionic ionenes in the form of polybenzimidazolides are obtained as a result of deprotonation of the benzimidazole groups. The ionization of the polymer results in dramatically changed physicochem. properties as compared to the pristine material, which is described and discussed. From a technol. point of view, performance and stability targets continue to motivate further research and development of new polybenzimidazole chemistries and energy materials. The overall aim of this review is therefore to identify challenges and opportunities in this area from synthetic chem. and materials science perspectives to serve as a solid basis for further development prospects.
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    Composite membranes based on poly(2,2'(m-phenylene)-5,´5bibenzimidazole) (PBI) and sulfonated polyhedral oligosilsesquioxane (S-POSS) with S-POSS contents of 5 and 10% were prepd. by soln. casting as base materials for high temp. polymer electrolyte membrane fuel cells. With membranes based on pure PBI as a ref. point, the composite membranes were characterized with respect to spectroscopic and physicochem. properties. After doping with H3PO4, the composite membranes showed considerably improved ex situ proton cond. under anhyd. as well as under fully humidified conditions in the 120-180° temp. range. The cond. improvements were also confirmed by in situ fuel cell tests at 160° and further supported by the electrochem. impedance spectroscopy data based on the operating membrane electrode assemblies, demonstrating the tech. feasibility of the novel electrolyte materials.
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    Published Online: Nov. 27

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    Journal of the Electrochemical Society (2018), 165 (6), F3053-F3062CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    This work studies the long-term durability of high-temp. polymer electrolyte membrane fuel cells based on acid-doped polybenzimidazole membranes. The primary focus is on acid loss via the evapn. mechanism, which is a major cause of degrdn. in applications that involve long-term operation. Durability is assessed for 16 identically fabricated membrane electrode assemblies (MEAs), and evaluations are carried out using operating parameters as stressors with gas stoichiometries ranging from 2 to 25, current densities from 200 to 800 mA cm-2, and temps. of 160 or 180°C. Cell diagnostics are composed of time resolved polarization curves, post mortem anal., and in situ temp. measurements. A major part of the cell degrdn. during these steady-state tests can be ascribed to increasing area-specific series resistance. By means of post mortem acid-loss measurements, the degrdn. is correlated to the temp. and to the accumulated gas-flow vol. Such relations are indicative of acid loss via evapn. C.d. also plays a crit. role for the acid loss and, thus, for the overall cell degrdn. The effect of current is likely tied to mechanisms that involve water generation, migration of electrolyte ions, and locally elevated temp. inside the MEAs.
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    The passage of an elec. current through phosphoric acid doped polymer membranes involves parasitic migration of the acid, which imposes a crit. issue for long-term operation of the high temp. polymer electrolyte membranes fuel cell (HT-PEMFC). To elucidate the phenomenon, a three-layered membrane is constructed with embedded micro ref. electrodes to measure phosphoric acid redistribution in a polybenzimidazole based membrane. Under a const. load, a concn. gradient develops due to the acid migration, which drives the back diffusion of the acid and eventually reaches a steady state between migration and diffusion. The acid gradient is measured as a difference in local ohmic resistances of the anode- and cathode-layer membranes by electrochem. impedance spectroscopy. The phosphoric acid diffusion coeff. through the acid doped membrane is about 10-11 m2 s-1, at least one order of magnitude lower than that of aq. phosphoric acid solns. The anion (H2PO4-) transference no. is found to range up to 4% depending on c.d., temp. and atm. humidity of the cell, implying that careful control of the operating parameters is needed in order to suppress the vehicular proton conduction as a degrdn. mitigation strategy.
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    The durability of high-temp. polymer electrolyte membrane fuel cells (HT-PEMFCs) was studied with phosphoric acid doped membranes of polybenzimidazole (PBI). One of the challenges for this technol. is the loss and instability of phosphoric acid resulting in performance degrdn. after long-term operation. The effect of the gas diffusion layers (GDL) on acid loss was studied. Four different com. available GDLs were subjected to passive ex situ acid uptake by capillary forces and the acid distribution mapped over the cross-section. Materials with an apparent fine structure made from carbon black took up much more acid than materials with a more coarse apparent structure made from graphitized carbon. The same trend was evident from thermally accelerated fuel cell tests at 180 °C under const. load where degrdn. rates depended strongly on the choice of GDL material, esp. on the cathode side. Acid was collected from the fuel cell exhaust at rates clearly correlated to the fuel cell degrdn. rates, but amounted to less than 6% of the total acid content in the cell even after significant degrdn. Long-term durability of more than 5,500 h with a degrdn. rate of 12 μV h-1 at 180 °C and 200 mA cm-2 was demonstrated with the GDL that retained acid most efficiently.
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  • Abstract

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    Figure 1

    Figure 1. Chemical structures of the three amines (quinuclidine, quinuclidinol, and DABCO) and the polymer blend membrane OPBI-c-PVBC (exemplary for quinuclidine). Cross-linking points are highlighted and numbered (1: covalent cross-linking, 2: imidazolium cation (Im+), and 3: QA). X– represents the counter anion, for example, Cl–, OH–, and H2PO4–.

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    Figure 2

    Figure 2. (A) TGA and (B) derivative of the TGA data for the membranes 60-40-Q, 60-40-QOH, 90-10-QOH, its blend components OPBI and PVBC, and the commercial Dapazol membrane under oxidative conditions (65% O2 and 35% N2).

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    Figure 3

    Figure 3. Initial polarization and power density curves for the FCs based on the membranes 60-40-Q, 60-40-QOH, 90-10-QOH, and Dapazol. All measurements were carried out at 180 °C under 0.25 L min–1 dry H2 at the anode and 0.75 L min–1 air at the cathode, with a Pt loading of 1 mg cm–2 at the cathode and anode. Each curve in (A,B) represents the mean of three measurements of three different MEAs. The shaded areas represent standard deviations. In (C,D), the high-frequency resistance (HFR)-free data of the mean of the three FC measurements are represented.

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    Figure 4

    Figure 4. (A) Linear sweep voltammograms and (B) linear sweep voltammograms with internal short-circuit correction. All measurements were carried out at 180 °C, with 0.3 L min–1 dry H2 at the anode and 0.3 L min–1 N2 at the cathode with a scan rate of 2 mV s–1.

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    Figure 5

    Figure 5. (A) Cell voltage over time for 48 h at 400 mA cm–2 with 0.25 L min–1 H2 and 0.75 L min–1 air at 180 °C, (B) corresponding cell resistance by current interruption, and (C) cathodic area-specific acid loss rates during 48 h at 400 mA cm–2.

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  • References

    This article references 69 other publications.

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      Aili, D.; Henkensmeier, D.; Martin, S.; Singh, B.; Hu, Y.; Jensen, J. O.; Cleemann, L. N.; Li, Q. Polybenzimidazole-Based High-Temperature Polymer Electrolyte Membrane Fuel Cells: New Insights and Recent Progress. Electrochem. Energy Rev. 2020, 3, 793,  DOI: 10.1007/s41918-020-00080-5
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      Polybenzimidazole-Based High-Temperature Polymer Electrolyte Membrane Fuel Cells: New Insights and Recent Progress
      Aili, David; Henkensmeier, Dirk; Martin, Santiago; Singh, Bhupendra; Hu, Yang; Jensen, Jens Oluf; Cleemann, Lars N.; Li, Qingfeng
      Electrochemical Energy Reviews (2020), 3 (4), 793-845CODEN: EERLAM; ISSN:2520-8136. (Springer International Publishing AG)
      Abstr.: High-temp. proton exchange membrane fuel cells based on phosphoric acid-doped polybenzimidazole membranes are a technol. characterized by simplified construction and operation along with possible integration with, e.g., methanol reformers. Significant progress has been achieved in terms of key materials, components and systems. This review is devoted to updating new insights into the fundamental understanding and technol. deployment of this technol. Polymers are synthetically modified with basic functionalities, and membranes are improved through crosslinking and inorg.-org. hybridization. New insights into phosphoric acid along with its interactions with basic polymers, metal catalysts and carbon-based supports are recapped. Recognition of parasitic acid migration raises acid retention issues at high current densities. Acid loss via evapn. is estd. with respect to the acid inventory of membrane electrode assembly. Acid adsorption on platinum surfaces can be alleviated for platinum alloys and non-precious metal catalysts. Binders have been considered a key to the establishment of the triple-phase boundary, while recent development of binderless electrodes opens new avenues toward low Pt loadings. Often ignored microporous layers and water impacts are also discussed. Of special concern are durability issues including acid loss, platinum sintering and carbon corrosion, the latter being crit. during start/stop cycling with mitigation measures proposed. Long-term durability has been demonstrated with a voltage degrdn. rate of less than 1 μV h-1 under steady-state tests at 160 °C, while challenges remain at higher temps., current densities or reactant stoichiometries, particularly during dynamic operation with thermal, load or start/stop cycling. Graphic abstr.: [graphic not available: see fulltext].
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      Fuel cell vehicles: Status 2007
      von Helmolt, Rittmar; Eberle, Ulrich
      Journal of Power Sources (2007), 165 (2), 833-843CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Within the framework of this paper, a short motivation for hydrogen as a fuel is provided and recent developments in the field of fuel cell vehicles are described. In particular, the propulsion system and its efficiency, as well as the integration of the hydrogen storage system are discussed. A fuel cell drivetrain poses certain requirements (concerning thermodn. and engineering issues) on the operating conditions of the tank system. These limitations and their consequences are described. For this purpose, conventional and novel storage concepts will be shortly introduced and evaluated for their automotive viability and their potential impact. Eventually, GM's third generation vehicles (i.e. the HydroGen3) are presented, as well as the recent 4th generation Chevrolet Equinox Fuel Cell SUV. An outlook is given that addresses cost targets and infrastructure needs.
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      Zhu, Y.; Zhu, W. H.; Tatarchuk, B. J. Performance Comparison between High Temperature and Traditional Proton Exchange Membrane Fuel Cell Stacks using Electrochemical Impedance Spectroscopy. J. Power Sources 2014, 256, 250– 257,  DOI: 10.1016/j.jpowsour.2014.01.049
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      Performance comparison between high temperature and traditional proton exchange membrane fuel cell stacks using electrochemical impedance spectroscopy
      Zhu, Ying; Zhu, Wenhua H.; Tatarchuk, Bruce J.
      Journal of Power Sources (2014), 256 (), 250-257CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      A temp. above 100° is always desired for proton exchange membrane fuel cell operation. It not only improves kinetic and mass transport processes, but also facilitates thermal and water management in fuel cell systems. Increased carbon monoxide tolerance at higher operating temp. also simplifies the pretreatment of fuel supplement. The novel phosphoric acid-doped polybenzimidazole membranes achieve proton exchange membrane fuel cell operations above 100°. The performance of a com. high-temp. proton exchange membrane fuel cell stack module is studied by measuring its impedance under various current loads when the operating temp. is set at 160°. The contributions of kinetic and mass transport processes to stack impedance are analyzed qual. and quant. by equiv. circuit simulation. The performance of a traditional proton exchange membrane fuel cell stack module operated is also studied by impedance measurement and equiv. circuit simulation. The operating temp. is self-stabilized between 40° and 65°. An enhancement of the high-temp. proton exchange membrane fuel cell stack in polarization impedance is evaluated by comparing to the traditional proton exchange membrane fuel cell stack. The impedance study on two com. fuel cell stacks reveals the real situation of current fuel cell development.
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    4. 4
      Bose, S.; Kuila, T.; Nguyen, T. X. H.; Kim, N. H.; Lau, K.-t.; Lee, J. H. Polymer Membranes for High Temperature Proton Exchange Membrane Fuel Cell: Recent Advances and Challenges. Prog. Polym. Sci. 2011, 36, 813– 843,  DOI: 10.1016/j.progpolymsci.2011.01.003
      4
      Polymer membranes for high temperature proton exchange membrane fuel cell: Recent advances and challenges
      Bose, Saswata; Kuila, Tapas; Nguyen, Thi Xuan Hien; Kim, Nam Hoon; Lau, Kin-tak; Lee, Joong Hee
      Progress in Polymer Science (2011), 36 (6), 813-843CODEN: PRPSB8; ISSN:0079-6700. (Elsevier Ltd.)
      A review. Proton-exchange membrane fuel cells (PEMFCs) are considered to be a promising technol. for efficient power generation in the 21st century. Currently, high temp. proton exchange membrane fuel cells (HT-PEMFC) offer several advantages, such as high proton cond., low permeability to fuel, low electro-osmotic drag coeff., good chem./thermal stability, good mech. properties and low cost. Owing to the aforementioned features, high temp. proton exchange membrane fuel cells have been utilized more widely compared to low temp. proton exchange membrane fuel cells, which contain certain limitations, such as carbon monoxide poisoning, heat management, water leaching, etc. This review examines the inspiration for HT-PEMFC development, the technol. constraints, and recent advances. Various classes of polymers, such as sulfonated hydrocarbon polymers, acid-base polymers and blend polymers, have been analyzed to fulfill the key requirements of high temp. operation of proton exchange membrane fuel cells (PEMFC). The effect of inorg. additives on the performance of HT-PEMFC has been scrutinized. A detailed discussion of the synthesis of polymer, membrane fabrication and physicochem. characterizations is provided. The proton cond. and cell performance of the polymeric membranes can be improved by high temp. treatment. The mech. and water retention properties have shown significant improvement., However, there is scope for further research from the perspective of achieving improvements in certain areas, such as optimizing the thermal and chem. stability of the polymer, acid management, and the integral interface between the electrode and membrane.
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    5. 5
      Park, J.; Wang, L.; Advani, S. G.; Prasad, A. K. Mechanical Stability of H3PO4-Doped PBI/Hydrophilic-Pretreated PTFE Membranes for High Temperature PEMFCs. Electrochim. Acta 2014, 120, 30– 38,  DOI: 10.1016/j.electacta.2013.12.030
      5
      Mechanical Stability of H3PO4-Doped PBI/Hydrophilic-Pretreated PTFE Membranes for High Temperature PEMFCs
      Park, Jaehyung; Wang, Liang; Advani, Suresh G.; Prasad, Ajay K.
      Electrochimica Acta (2014), 120 (), 30-38CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      A novel polybenzimidazole (PBI)/poly(tetrafluoroethylene) (PTFE) composite membrane doped with H3PO4 was fabricated for high temp. operation in a polymer electrolyte membrane (PEM) fuel cell. A hydrophilic surface pretreatment was applied to the porous PTFE matrix film to improve its interfacial adhesion to the PBI polymer, thereby avoiding the introduction of Nafion ionomer which is traditionally used as a coupling agent. The pretreated PTFE film was embedded within the composite membrane during soln.-casting using 5% PBI/DMAc soln. The mech. stability and durability of three types of MEAs assembled with PBI only, PBI with pretreated PTFE, and PBI-Nafion with untreated PTFE membranes were evaluated under an accelerated degrdn. testing protocol employing extreme temp. cycling. Degrdn. was characterized by recording polarization curves, H crossover, and proton resistance. Cross sections of the membranes were examd. before and after thermal cycling by scanning electron microscope. Energy-dispersive x-ray spectroscopy verified that the PBI is dispersed homogeneously in the porous PTFE matrix. The PBI composite membrane with pretreated PTFE has a lower degrdn. rate than the Nafion/PBI membrane with untreated PTFE. Thus, the hydrophilic pretreatment employed here greatly improved the mech. stability of the composite membrane, which resulted in improved durability under an extreme thermal cycling regime.
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    6. 6
      Araya, S. S.; Zhou, F.; Liso, V.; Sahlin, S. L.; Vang, J. R.; Thomas, S.; Gao, X.; Jeppesen, C.; Kær, S. K. A Comprehensive Review of PBI-Based High Temperature PEM Fuel Cells. Int. J. Hydrogen Energy 2016, 41, 21310– 21344,  DOI: 10.1016/j.ijhydene.2016.09.024
      6
      A comprehensive review of PBI-based high temperature PEM fuel cells
      Araya, Samuel Simon; Zhou, Fan; Liso, Vincenzo; Sahlin, Simon Lennart; Vang, Jakob Rabjerg; Thomas, Sobi; Gao, Xin; Jeppesen, Christian; Kaer, Soeren Knudsen
      International Journal of Hydrogen Energy (2016), 41 (46), 21310-21344CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      The current status on the understanding of the various operational aspects of high temp. proton exchange membrane fuel cells (HT-PEMFC) has been summarized. The paper focuses on phosphoric acid-doped polybenzimidazole (PBI)-based HT-PEMFCs and an overview of the common practices of their design and characterization techniques at single cell, stack and system levels is given. The state-of-the-art concepts of different degrdn. mechanisms and methods of their mitigation are also discussed. Moreover, accelerated stress testing (AST) procedures for HT-PEMFCs available in literature are outlined. Catalyst degrdn. and electrolyte loss take place at higher rates in the beginning of life of the fuel cell. This is due to the smaller size of Pt particles and the presence of excess phosphoric acid in the beginning of life that favor the resp. degrdn. Therefore, the redistribution of phosphoric acid in the membrane and the electrodes is crucial for the proper activation of the fuel cell, and a startup procedure should take this into account in order to avoid beginning of life degrdn. Online monitoring of the fuel cell system's state of health using diagnostic tools can help detect fuel cell faults for targeted interventions based on the obsd. conditions to prevent sudden failures and to prolong the fuel cell's lifetime. However, the technol. is still under development and robust online diagnostics tools are hardly available. Currently, mitigation is mainly done based on favorable operating conditions and techniques to recover degrdn. and the development of more resistant components that can withstand the known degrdn. mechanisms.
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    7. 7
      Atanasov, V.; Gudat, D.; Ruffmann, B.; Kerres, J. Highly Phosphonated Polypentafluorostyrene: Characterization and Blends with Polybenzimidazole. Eur. Polym. J. 2013, 49, 3977– 3985,  DOI: 10.1016/j.eurpolymj.2013.09.002
      7
      Highly phosphonated polypentafluorostyrene: Characterization and blends with polybenzimidazole
      Atanasov, Vladimir; Gudat, Dietrich; Ruffmann, Bastian; Kerres, Jochen
      European Polymer Journal (2013), 49 (12), 3977-3985CODEN: EUPJAG; ISSN:0014-3057. (Elsevier Ltd.)
      The authors present results of the cond. and resistance to thermooxidative and condensation reactions of a highly phosphonated poly(pentafluorostyrene) (PWN2010) and of its blends with poly(benzimidazole)s (PBI). This polymer, which combines both: (i) a high degree of phosphonation (above 90%) and (ii) a relatively high acidity (pKa (-PO3H2 ↔ -PO3H-) ∼ 0.5) due to the F neighbors, is designed for low humidity operating fuel cell. This was confirmed by the cond. measurements for PWN2010 reaching σ = 5 × 10-4 S cm-1 at 150° in dry N2 and σ = 1 × 10-3 S cm-1 at 150° (λ = 0.75). Also, this polymer showed only 48% of anhydride formation when annealing it at T = 250° for 5 h and only 2% wt. loss during a 96 h Fenton test. These properties combined with the ability of the PWN2010 to form homogeneous blends with polybenzimidazoles resulting in stable and flexible polymer films, makes PWN2010 a very promising candidate as a polymer electrolyte for intermediate- and high-temp. fuel cell applications.
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    8. 8
      Li, Q.; He, R.; Jensen, J. O.; Bjerrum, N. J. PBI-Based Polymer Membranes for High Temperature Fuel Cells- Preparation, Characterization and Fuel Cell Demonstration. Fuel Cells 2004, 4, 147– 159,  DOI: 10.1002/fuce.200400020
      8
      PBI-based polymer membranes for high temperature fuel cells - preparation, characterization and fuel cell demonstration
      Li, Q.; He, R.; Jensen, J. O.; Bjerrum, N. J.
      Fuel Cells (Weinheim, Germany) (2004), 4 (3), 147-159CODEN: FUCEFK; ISSN:1615-6846. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Proton exchange membrane fuel cell (PEMFC) technol. based on perfluorosulfonic acid (PFSA) polymer membranes is briefly reviewed. The newest development in alternative polymer electrolytes for operation >100° is summarized and discussed. As one of the successful approaches to high operational temps., the development and evaluation of acid doped polybenzimidazole (PBI) membranes are reviewed, covering polymer synthesis, membrane casting, acid doping, physicochem. characterization and fuel cell testing. A high temp. PEMFC system, operational at up to 200° based on phosphoric acid-doped PBI membranes, is demonstrated. It requires little or no gas humidification and has a CO tolerance of up to several percent. The direct use of reformed hydrogen from a simple methanol reformer, without the need for any further CO removal, was demonstrated. A lifetime of continuous operation, for over 5000 h at 150°, and shutdown-restart thermal cycle testing for 47 cycles was achieved. Other issues such as cooling, heat recovery, possible integration with fuel processing units, assocd. problems and further development are discussed.
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    9. 9
      Quartarone, E.; Angioni, S.; Mustarelli, P. Polymer and Composite Membranes for Proton-Conducting, High-Temperature Fuel Cells: A Critical Review. Materials 2017, 10, 687,  DOI: 10.3390/ma10070687
      9
      Polymer and composite membranes for proton-conducting, high-temperature fuel cells: a critical review
      Quartarone, Eliana; Angioni, Simone; Mustarelli, Piercarlo
      Materials (2017), 10 (7), 687/1-687/17CODEN: MATEG9; ISSN:1996-1944. (MDPI AG)
      Polymer fuel cells operating above 100 °C (High Temp. Polymer Electrolyte Membrane Fuel Cells, HT-PEMFCs) have gained large interest for their application to automobiles. The HT-PEMFC devices are typically made of membranes with poly(benzimidazoles), although other polymers, such as sulfonated poly(ether ether ketones) and pyridine-based materials have been reported. In this crit. review, we address the state-of-the-art of membrane fabrication and their properties. A large no. of papers of uneven quality has appeared in the literature during the last few years, so this review is limited to works that are judged as significant. Emphasis is put on proton transport and the physico-chem. mechanisms of proton cond.
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    10. 10
      Aili, D.; Cleemann, L. N.; Li, Q.; Jensen, J. O.; Christensen, E.; Bjerrum, N. J. Thermal Curing of PBI Membranes for High Temperature PEM Fuel Cells. J. Mater. Chem. 2012, 22, 5444,  DOI: 10.1039/c2jm14774b
      10
      Thermal curing of PBI membranes for high temperature PEM fuel cells
      Aili, David; Cleemann, Lars N.; Li, Qingfeng; Jensen, Jens Oluf; Christensen, Erik; Bjerrum, Niels J.
      Journal of Materials Chemistry (2012), 22 (12), 5444-5453CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
      H3PO4 doped polybenzimidazole (PBI) is a promising electrolyte for p exchange membrane (PEM) fuel cells operating under anhyd. conditions at temps. of up to 200°. The limited long-term durability of the membrane electrode assemblies (MEAs) is currently hampering the com. viability of the technol. Thermoset PBI membranes were prepd. by curing the membranes under inert atm. at temps. of up to 350° prior to the acid doping. The membrane characterizations with respect to soly., H3PO4 doping, radical-oxidative resistance and mech. strength indicated that the PBI membranes were irreversibly cured by the thermal treatment. After curing, the PBI membranes demonstrated features that are characteristic of a thermoset resin including complete insoly., good resistance to swelling and improved mech. toughness. Addnl., the thermal treatment increases the degree of crystallinity of the membranes. The improved physicochem. characteristics of the membranes after curing were further illustrated by improved long-term durability of the corresponding fuel cell MEAs. During continuous operation for 1800 h at 160° and 600 mA/cm2, the av. cell voltage decay rate of the MEA based on the cured membrane was 43 μV/h. This should be compared with an av. cell voltage decay rate of 308 μV/h which was recorded for the MEA based on its non-cured counterpart.
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    11. 11
      Kerres, J.; Atanasov, V. Cross-linked PBI-Based High-Temperature Membranes: Stability, Conductivity and Fuel Cell Performance. Int. J. Hydrogen Energy 2015, 40, 14723– 14735,  DOI: 10.1016/j.ijhydene.2015.08.054
      11
      Cross-linked PBI-based high-temperature membranes: Stability, conductivity and fuel cell performance
      Kerres, Jochen; Atanasov, Vladimir
      International Journal of Hydrogen Energy (2015), 40 (42), 14723-14735CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      In this study different types of polybenzimidazole(PBI)-based High-T fuel cell membranes were investigated comparatively. The different membranes comprised: (1) ionically cross-linked PBI-excess blend membranes by mixing PBI (the polybenzimidazoles PBIOO and F6PBI) with different cation-exchange ionomers such as poly(tetrafluorostyrene-4-phosphonic acid), and different nonfluorinated and partially fluorinated sulfonated arylene main-chain polymers, where the cation-exchange groups form ionical cross-links with the imidazole groups of the PBI by proton transfer, (2) covalently cross-linked PBI-excess membranes by mixing PBI with different halomethylated arylene polymers where the halomethyl groups form covalent cross-links towards the imidazole group of the PBI by alkylation of the N-H group: polymer-CH2Br + PBI-imidazole-N-H → polymer-CH2-N-imidazole-PBI, (3) PBI-anion-exchange polymer blends, (4) covalent-ionically cross-linked PBI blend membranes by mixing PBI with a sulfonated polymer and a halomethylated polymer. The membranes were investigated in terms of: (i) chem. stability by Fentons Test (FT), (ii) extent of crosslinking by extn. with DMAc, (iii) thermal stability by TGA, (iv) H+-cond. in the T range 80-150 °C as H3PO4-doped membranes, and (v) fuel cell performance in a high-T H2/air fuel cell. The general results of the study were summarized as follows: (1) Most of the membranes showed excellent chem. stability in FT, (2) the PBI blends with F6PBI showed better chem. stabilities than the PBIOO-contg. blends, (3) the proton conductivities of all investigated membranes were in a range of 4-90 mS/cm at T from 80 to 150 °C, (4) the fuel cell test results of the membranes were promising.
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    12. 12
      Li, Q.; Pan, C.; Jensen, J. O.; Noyé, P.; Bjerrum, N. J. Cross-Linked Polybenzimidazole Membranes for Fuel Cells. Chem. Mater. 2007, 19, 350– 352,  DOI: 10.1021/cm0627793
      12
      Cross-Linked Polybenzimidazole Membranes for Fuel Cells
      Li, Qingfeng; Pan, Chao; Jensen, Jens Oluf; Noye, Pernille; Bjerrum, Niels J.
      Chemistry of Materials (2007), 19 (3), 350-352CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      In this communication, p-xylene dibromide is used as a crosslinking agent with contents of 0 (pure polybenzimidazole, PBI) 1.0, 3.0, and 10.0 wt.% in the prepd. PBI membranes samples. Assuming a complete reaction for each mole of the crosslink agent with 2 equiv of polybenzimidazole amine hydrogen, these wt. ratios correspond to a crosslinking degree of 0, 1,1,. 3.6, and 13.0 wt.%, resp., of the total amine hydrogen atoms in PBI. Crosslinking slowed the rate and extent of membrane dissoln. in N,N-dimethylacetamide. Doping with phosphoric acid more than doubled the proton cond. of the membranes, while lowering the tensile strength and modulus, but increasing the elongation to break. Crosslinking restored some of the tensile strength and modulus, while greatly reducing the elongation. Crosslinking also increases resistance of the membrane to oxidn.
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    13. 13
      Zeis, R. Materials and Characterization Techniques for High-Temperature Polymer Electrolyte Membrane Fuel Cells. Beilstein J. Nanotechnol. 2015, 6, 68– 83,  DOI: 10.3762/bjnano.6.8
      13
      Materials and characterization techniques for high-temperature polymer electrolyte membrane fuel cells
      Zeis, Roswitha
      Beilstein Journal of Nanotechnology (2015), 6 (), 68-83, 16 pp.CODEN: BJNEAH; ISSN:2190-4286. (Beilstein-Institut zur Foerderung der Chemischen Wissenschaften)
      A review. The performance of high-temp. polymer electrolyte membrane fuel cells (HT-PEMFC) is critically dependent on the selection of materials and optimization of individual components. A conventional high-temp. membrane electrode assembly (HT-MEA) primarily consists of a polybenzimidazole (PBI)-type membrane contg. phosphoric acid and two gas diffusion electrodes (GDE), the anode and the cathode, attached to the two surfaces of the membrane. This review article provides a survey on the materials implemented in state-of-the-art HT-MEAs. These materials must meet extremely demanding requirements because of the severe operating conditions of HT-PEMFCs. They need to be electrochem. and thermally stable in highly acidic environment. The polymer membranes should exhibit high proton cond. in low-hydration and even anhyd. states. Of special concern for phosphoric-acid-doped PBI-type membranes is the acid loss and management during operation. The slow oxygen redn. reaction in HT-PEMFCs remains a challenge. Phosphoric acid tends to adsorb onto the surface of the platinum catalyst and therefore hampers the reaction kinetics. Addnl., the binder material plays a key role in regulating the hydrophobicity and hydrophilicity of the catalyst layer. Subsequently, the binder controls the electrode-membrane interface that establishes the triple phase boundary between proton conductive electrolyte, electron conductive catalyst, and reactant gases. Moreover, the elevated operating temps. promote carbon corrosion and therefore degrade the integrity of the catalyst support. These are only some examples how materials properties affect the stability and performance of HT-PEMFCs. For this reason, materials characterization techniques for HT-PEMFCs, either in situ or ex situ, are highly beneficial. Significant progress has recently been made in this field, which enables us to gain a better understanding of underlying processes occurring during fuel cell operation. Various novel tools for characterizing and diagnosing HT-PEMFCs and key components are presented in this review, including FTIR and Raman spectroscopy, confocal Raman microscopy, synchrotron X-ray imaging, X-ray microtomog., and at. force microscopy.
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    14. 14
      Henkensmeier, D.; Aili, D. Techniques for PBI Membrane Characterization. In High Temperature Polymer Electrolyte Membrane Fuel Cells; Li, Q., Aili, D., Hjuler, H. A., Jensen, J. O., Eds.; Springer International Publishing, 2016.
      There is no corresponding record for this reference.
    15. 15
      Hu, M.; Li, T.; Neelakandan, S.; Wang, L.; Chen, Y. Cross-Linked Polybenzimidazoles Containing Hyperbranched Cross-Linkers and Quaternary Ammoniums as High-Temperature Proton Exchange Membranes: Enhanced Stability and Conductivity. J. Membr. Sci. 2020, 593, 117435,  DOI: 10.1016/j.memsci.2019.117435
      15
      Cross-linked polybenzimidazoles containing hyperbranched cross-linkers and quaternary ammoniums as high-temperature proton exchange membranes: Enhanced stability and conductivity
      Hu, Meishao; Li, Tianyun; Neelakandan, Sivasubramaniyan; Wang, Lei; Chen, Yongming
      Journal of Membrane Science (2020), 593 (), 117435CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      High proton cond. with sufficient stability for phosphoric acid (PA) doped polybenzimidazole membranes is crit. for applications in fuel cells. Macromol. cross-linkers with hyperbranched structures have large vols. and many functional groups, which can not only react with polymer backbones to form multiple cross-linked sites, but also provide favorable conditions for subsequent functional group modifications to regulate the interaction between polymers and PA; however, such cross-linkers are rarely exploited in fuel cells. Herein, a series of cross-linked polybenzimidazole membranes were successfully prepd. based on a novel hyperbranched cross-linker and the incorporation of numerous quaternary ammonium groups. These cross-linked polybenzimidazole membranes contg. a hyperbranched cross-linker and quaternary ammonium groups showed superior performance, in terms of mech. properties, oxidative resistance and proton cond. The tensile strength of the PA doped cross-linked membranes was >20.0 MPa. These cross-linked membranes showed only a slight wt. loss and no cracks after immersion in Fenton's reagent for 200 h, while the linear membrane was broken into pieces after immersion in Fenton's reagent for 100 h. With low acid loading, the membranes contg. cross-linked polybenzimidazole with quaternary ammonium groups still exhibited good cond. As a result of the excellent comprehensive properties, the obtained fuel cell based on the membrane with 15% of the hyperbranched cross-linker showed a great power d. of 260 mW/cm2, which was 36.8% higher than that of the fuel cell based on the corresponding linear membrane.
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    16. 16
      Luo, H.; Pu, H.; Chang, Z.; Wan, D.; Pan, H. Crosslinked polybenzimidazole via a Diels-Alder reaction for proton conducting membranes. J. Mater. Chem. 2012, 22, 20696,  DOI: 10.1039/c2jm33725h
      16
      Crosslinked polybenzimidazole via a Diels-Alder reaction for proton conducting membranes
      Luo, Haochuan; Pu, Hongting; Chang, Zhihong; Wan, Decheng; Pan, Haiyan
      Journal of Materials Chemistry (2012), 22 (38), 20696-20705CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
      The crosslinking of polybenzimidazole (PBI) is a potential strategy to improve the mech. properties and dimensional stability of acid-doped membranes, as well as to retain additives in the membranes. An effective method to prep. crosslinked PBI with a well-defined structure via a Diels-Alder reaction between vinylbenzyl functionalized PBI (PBI-VB) and α,α'-difurfuryloxy-p-xylene (DFX) is proposed. The chem. structure of PBI-VB is confirmed by FTIR and 1H NMR. The model reaction of styrene and DFX is employed to clarify the crosslinking reaction of PBI and DFX. During the crosslinking process, three kinds of chem. reaction may happen. The first is a Diels-Alder reaction of DFX with the vinyl groups of PBI-VB. The second is the self-polymn. of vinyl groups. The third is the grafting of difuran groups via a Diels-Alder reaction. The first two reactions contribute the most to the crosslinking of the PBI membrane. With the addn. of DFX, there is competition between these two kinds of crosslinking reactions. When the feed ratio of DFX is below 20%, the tensile strength of the crosslinked membranes increases with increasing content of DFX. The crosslinking of the membrane is mainly a results of Diels-Alder reactions. When the feed ratio of DFX exceeds 20%, the tensile strength decreases slightly. Besides the crosslinking via Diels-Alder reactions, the crosslinking of the membrane is also contributed by the self-polymn. of vinyl groups and the grafting of difuran groups. The crosslinked PBI membrane exhibits improved mech. strength, higher phys. and chem. stability, as well as higher phosphoric acid (PA) retention ability. After doping with PA, the crosslinked membrane exhibits good proton cond. over a temp. range of 60 to 180 °C.
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    17. 17
      Liu, F.; Wang, S.; Chen, H.; Li, J.; Tian, X.; Wang, X.; Mao, T.; Xu, J.; Wang, Z. Cross-Linkable Polymeric Ionic Liquid Improve Phosphoric Acid Retention and Long-Term Conductivity Stability in Polybenzimidazole Based PEMs. ACS Sustainable Chem. Eng. 2018, 6, 16352– 16362,  DOI: 10.1021/acssuschemeng.8b03419
      17
      Cross-Linkable Polymeric Ionic Liquid Improve Phosphoric Acid Retention and Long-Term Conductivity Stability in Polybenzimidazole Based PEMs
      Liu, Fengxiang; Wang, Shuang; Chen, Hao; Li, Jinsheng; Tian, Xue; Wang, Xu; Mao, Tiejun; Xu, Jingmei; Wang, Zhe
      ACS Sustainable Chemistry & Engineering (2018), 6 (12), 16352-16362CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
      Composite cross-linked membrane based on fluorine-contg. polybenzimidazole (6FPBI) and a cross-linkable polymeric ionic liq. (cPIL) were prepd. for high temp. proton exchange membrane (HT-PEM) applications. Particularly, the obtained composite cross-linked membranes showed excellent phosphoric acid doping ability and proton cond. From the trade-off between mech. strength and proton cond. of composite membranes, the optimal content of cPIL is 20% (6FPBI-cPIL 20 membrane). For instance, the 6FPBI-cPIL 20 membrane with a PA doping level of 27.8 exhibited a proton cond. of 0.106 S/cm at 170°, which is much higher than that of pristine 6FPBI membrane. The most outstanding contribution of this work is that the 6FPBI-cPIL membranes showed improved phosphoric acid retention and long-term cond. stability under harsh conditions (80°/40% RH) for 96 h. In particular, the proton cond. and PA doping level of the 6FPBI-cPIL 20 membrane remained at a high level of 0.064 S/cm and 8.5 after 96 h of the test, resp.
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    18. 18
      Hao, J.; Jiang, Y.; Gao, X.; Lu, W.; Xiao, Y.; Shao, Z.; Yi, B. Functionalization of Polybenzimidazole-Crosslinked Poly(vinylbenzyl chloride) with Two Cyclic Quaternary Ammonium Cations for Anion Exchange Membranes. J. Membr. Sci. 2018, 548, 1– 10,  DOI: 10.1016/j.memsci.2017.10.062
      18
      Functionalization of polybenzimidazole-crosslinked poly(vinylbenzyl chloride) with two cyclic quaternary ammonium cations for anion exchange membranes
      Hao, Jinkai; Jiang, Yongyi; Gao, Xueqiang; Lu, Wangting; Xiao, Yu; Shao, Zhigang; Yi, Baolian
      Journal of Membrane Science (2018), 548 (), 1-10CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      The anion exchange membranes (AEMs) with both high ionic cond. and good stability is always the research focus role for the long-term use of AEM fuel cells. A series of the mech. and chem. stable PVBC/PBI crosslinked membranes, functionalized with N1-Bu substituted BDABCO groups, were designed, prepd. and characterized. With the crosslinking by polybenzimidazole (PBI), the membranes showed good flexibility, strength and low swelling ratio (less than 18%). N1-Bu substituted doubly-charged BDABCO was introduced in the AEMs during the crosslinking reaction instead of the traditional dipping method, benefiting from the improvement compatibility between polymers and BDABCO groups. Attributing to the well-developed phase sepn. between hydrophilic domains and hydrophobic domains, the family of synthesized AEMs exhibited the higher conductivities than that of DABCO based membranes, which was proved by TEM and SAXS. The M-BDABCO-OH-1:3 with high BDABCO content displayed the highest ionic cond. of 29.3 and 91.4 mS cm-1 at 20 and 80 °C, resp. The results of alk. stability showed that the membranes had the superior chem. stability after immersing in a 1 mol L-1 KOH at 60 °C soln. for more than 550 h. Furthermore, the peak power d. of an H2/O2 single fuel cell using the optimized M-BDABCO-OH-1:3 was up to 340 mW cm-2 at 0.492 V with the EIS consisting of membrane resistance less than 0.1 Ω cm2 which was much smaller than the other AEMs. Overall, the developed membranes demonstrated the superior performance and would be a promising candidate material for AEMFCs.
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    19. 19
      Özdemir, Y.; Özkan, N.; Devrim, Y. Fabrication and Characterization of Cross-linked Polybenzimidazole Based Membranes for High Temperature PEM Fuel Cells. Electrochim. Acta 2017, 245, 1– 13,  DOI: 10.1016/j.electacta.2017.05.111
      19
      Fabrication and Characterization of Cross-linked Polybenzimidazole Based Membranes for High Temperature PEM Fuel Cells
      Ozdemir, Yagmur; Ozkan, Necati; Devrim, Yilser
      Electrochimica Acta (2017), 245 (), 1-13CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      In this study different types of crosslinked polybenzimidazole (PBI) membranes were compared as high temp. proton exchange membrane fuel cells (HT-PEMFC). Crosslinking of PBI was performed with different cross-linkers including bisphenol A diglycidyl ether (BADGE), ethylene glycol diglycidyl ether (EGDE), α-α'-dibromo-p-xylene (DBpX), and terephthalaldehyde (TPA). The crosslinked membranes were characterized by TGA, SEM, acid uptake and impedance analyses. The crosslinking of the PBI polymer matrix helps to improve the acid retention properties. PBI/BADGE presented the highest acid retention properties. Proton conductivities of the membranes were comparable to that of com. membranes. Cond. values up to 0.151 S cm-1 were obtained at 180° with PBI/DBpX membranes. Gas diffusion electrodes (GDE) were fabricated by an ultrasonic coating technique with 0.6 mg Pt.cm-2 catalyst loading for both anode and cathode. The crosslinked membranes were tested in a single HT-PEMFC with a 5. cm2 active area at 165° without humidification. PBI/BADGE crosslinked membranes demonstrated stability and high performance on single cell HT-PEMFC tests. The max. power d. for PBI/BADGE was detd. as 0.123 W cm-2. As a result, the exptl. results suggested that the PBI/BADGE and PBI/DBpX cross-linked membranes are promising electrolyte options for HT-PEMFC.
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    20. 20
      Tian, X.; Wang, S.; Li, J.; Liu, F.; Wang, X.; Chen, H.; Wang, D.; Ni, H.; Wang, Z. Benzimidazole Grafted Polybenzimidazole Cross-Linked Membranes with Excellent PA Stability for High-Temperature Proton Exchange Membrane Applications. Appl. Surf. Sci. 2019, 465, 332– 339,  DOI: 10.1016/j.apsusc.2018.09.170
      20
      Benzimidazole grafted polybenzimidazole cross-linked membranes with excellent PA stability for high-temperature proton exchange membrane applications
      Tian, Xue; Wang, Shuang; Li, Jinsheng; Liu, Fengxiang; Wang, Xu; Chen, Hao; Wang, Di; Ni, Hongzhe; Wang, Zhe
      Applied Surface Science (2019), 465 (), 332-339CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Benzimidazole grafted polybenzimidazole crosslinked membranes (CPBIm-X) with outstanding phosphoric acid (PA) stability are successfully prepd. 2-chloromethyl benzimidazole (CMBelm) is grafted onto PBI mainchains and using 3-glycidoxypropyltrimethoxysilane (KH560) as a crosslinker. The benzimidazole sidechains can not only increase the basic sites but also allow the membrane to achieve higher phosphoric acid uptakes without sacrificing mech. strength. Moreover, compared with the imidazole rings in the PBI backbone, the side chain with imidazole rings are flexible, which benefit the proton transportation and lower the activation energy. In addn., the mech. strength of crosslinked membranes is excellent. For example, the tensile strength value of CPBIm-5 is 101.4 MPa, while that of the PBIm is 79.0 MPa. The proton cond. is enhanced because the hydrolysis of KH560 resulting Si-O-Si networks structure, which can absorb more phosphoric acid. The Si-O-Si networks in the matrix can efficiently improve the stability of phosphoric acid (PA), the remaining wt. of CPBIm-5 is 57% PA. Taking into account performance comprehensively, the wt. of 5% KH560 is the optimum content. For example, the proton cond. of CPBIm-5 is 0.092 S cm-1 at 180 °C. Compared to the pristine PBI, it is almost three-fold in proton cond.
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    21. 21
      Kerres, J. Applications of Acid-Base Blend Concepts to Intermediate Temperature Membranes. In High Temperature Polymer Electrolyte Membrane Fuel Cells; Li, Q., Aili, D., Hjuler, H. A., Jensen, J. O., Eds.; Springer International Publishing, 2016, pp 59– 89. DOI: 10.1007/978-3-319-17082-4_4 .
      There is no corresponding record for this reference.
    22. 22
      Aili, D.; Li, Q.; Christensen, E.; Jensen, J. O.; Bjerrum, N. J. Crosslinking of Polybenzimidazole Membranes by Divinylsulfone Post-Treatment for High-Temperature Proton Exchange Membrane Fuel Cell Applications. Polym. Int. 2011, 60, 1201– 1207,  DOI: 10.1002/pi.3063
      22
      Crosslinking of polybenzimidazole membranes by divinylsulfone post-treatment for high-temperature proton exchange membrane fuel cell applications
      Aili, David; Li, Qingfeng; Christensen, Erik; Jensen, Jens Oluf; Bjerrum, Niels J.
      Polymer International (2011), 60 (8), 1201-1207CODEN: PLYIEI; ISSN:0959-8103. (John Wiley & Sons Ltd.)
      Phosphoric acid-doped polybenzimidazole (PBI) has been suggested as a promising electrolyte for proton exchange membrane fuel cells operating at temps. up to 200°. This paper describes the development of a crosslinking procedure for PBI membranes by post-treatment with divinylsulfone. The crosslinking chem. was studied and optimized on a low-mol.-wt. model system and the results were used to optimize the crosslinking conditions of PBI membranes. The crosslinked membranes were characterized with respect to chem. and physiochem. properties, showing improved mech. strength and oxidative stability compared with their linear analogs. Fuel cell tests were further conducted in order to demonstrate the feasibility of the crosslinked membranes. Copyright © 2011 Society of Chem. Industry.
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    23. 23
      Wang, S.; Zhang, G.; Han, M.; Li, H.; Zhang, Y.; Ni, J.; Ma, W.; Li, M.; Wang, J.; Liu, Z.; Zhang, L.; Na, H. Novel Epoxy-Based Cross-Linked Polybenzimidazole for High Temperature Proton Exchange Membrane Fuel Cells. Int. J. Hydrogen Energy 2011, 36, 8412– 8421,  DOI: 10.1016/j.ijhydene.2011.03.147
      23
      Novel epoxy-based cross-linked polybenzimidazole for high temperature proton exchange membrane fuel cells
      Wang, Shuang; Zhang, Gang; Han, Miaomiao; Li, Hongtao; Zhang, Yang; Ni, Jing; Ma, Wenjia; Li, Mingyu; Wang, Jing; Liu, Zhongguo; Zhang, Liyuan; Na, Hui
      International Journal of Hydrogen Energy (2011), 36 (14), 8412-8421CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      An approach has been proposed to prep. the reinforced phosphoric acid-doped cross-linked polybenzimidazole membranes for high-temp. proton exchange membrane fuel cells, using 1,3-bis(2,3-epoxypropoxy)-2,2-dimethylpropane (NGDE) as the cross-linker. FTIR measurement and soly. test showed the successful completion of the crosslinking reaction. The resulting cross-linked membranes exhibited improved mech. strength, making it possible to obtain higher phosphoric acid doping levels and therefore relatively high proton cond. Moreover, the oxidative stability of the cross-linked membranes was significantly enhanced. For instance, in Fenton's reagent (3% H2O2 soln., 4 ppm Fe2+, 70°), the cross-linked PBI-NGDE-20% membrane did not break into pieces and kept its shape for more than 480 h and its remaining wt.% was ∼65%. In addn., the thermal stability was sufficient enough within the operation temp. of PBI-based fuel cells. The cross-linked PBI-NGDE-X% (X is the wt.% of epoxy resin in the cross-linked membranes) membranes displayed relatively high proton cond. under anhyd. conditions. For instance, PBI-NGDE-5% membrane with acid uptake of 193% exhibited a proton cond. of 0.017 S/cm at 200°. All the results indicated that it may be a suitable candidate for applications in high-temp. proton exchange membrane fuel cells.
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    24. 24
      Xu, H.; Chen, K.; Guo, X.; Fang, J.; Yin, J. Synthesis of Hyperbranched Polybenzimidazoles and their Membrane Formation. J. Membr. Sci. 2007, 288, 255– 260,  DOI: 10.1016/j.memsci.2006.11.022
      24
      Synthesis of hyperbranched polybenzimidazoles and their membrane formation
      Xu, Hongjie; Chen, Kangcheng; Guo, Xiaoxia; Fang, Jianhua; Yin, Jie
      Journal of Membrane Science (2007), 288 (1+2), 255-260CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      A series of amine-terminated hyperbranched polybenzimidazoles (HBPBIs) were successfully synthesized by condensation polymn. of arom. dicarboxylic acids and an in situ synthesized arom. hexamine intermediate product from 1,3,5-benzenetricarboxylic acid (BTA) and 3,3'-diaminobenzidine (DAB) in polyphosphoric acid (PPA) at 190° for 20 h. HBPBI membranes were fabricated by soln. cast method in the presence of crosslinkers (ethylene glycol diglycidyl ether (EGDE) and terephthaldehyde (TPA)). The resulting HBPBI membranes displayed good mech. properties and good thermal stability. High proton cond. was obtained with the phosphoric acid-doped and TPA-cross-linked HBPBI membranes at 0% relative humidity.
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    25. 25
      Noyé, P.; Li, Q.; Pan, C.; Bjerrum, N. J. Cross-Linked Polybenzimidazole Membranes for High Temperature Proton Exchange Membrane Fuel Cells with Dichloromethyl Phosphinic Acid as a Cross-Linker. Polym. Adv. Technol. 2008, 19, 1270– 1275,  DOI: 10.1002/pat.1123
      25
      Cross-linked polybenzimidazole membranes for high temperature proton exchange membrane fuel cells with dichloromethyl phosphinic acid as a cross-linker
      Noye, Pernille; Li, Qingfeng; Pan, Chao; Bjerrum, Niels J.
      Polymers for Advanced Technologies (2008), 19 (9), 1270-1275CODEN: PADTE5; ISSN:1042-7147. (John Wiley & Sons Ltd.)
      Phosphoric acid-doped polybenzimidazole (PBI) membranes were covalently cross-linked with dichloromethyl phosphinic acid (DCMP). FT-IR measurements showed new bands originating from bonds between the hydrogen bearing nitrogen in the imidazole group of PBI and the CH2 group in DCMP. The produced cross-linked membranes show increased mech. strength, making it possible to achieve higher phosphoric acid doping levels and therefore higher proton cond. Oxidative stability is significantly improved and thermal stability is sufficient in a temp. range of up to 250°C, i.e. within the temp. range of operation of PBI-based fuel cells.
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    26. 26
      Harilal; Nayak, R.; Ghosh, P. C.; Jana, T. Cross-Linked Polybenzimidazole Membrane for PEM Fuel Cells. ACS Appl. Polym. Mater. 2020, 2, 3161– 3170,  DOI: 10.1021/acsapm.0c00350
      26
      Cross-Linked Polybenzimidazole Membrane for PEM Fuel Cells
      Harilal; Nayak, Ratikanta; Ghosh, Prakash Chandra; Jana, Tushar
      ACS Applied Polymer Materials (2020), 2 (8), 3161-3170CODEN: AAPMCD; ISSN:2637-6105. (American Chemical Society)
      Despite several unique advantages, high temp. proton exchange membrane fuel cell (HT-PEMFC) based on polybenzimidazole (PBI) membrane suffers from various drawbacks like weak chem. resistance, poor mech. strength, acid leaching etc. which eventually reduce the performance of the cell. In order to improve these drawbacks and to improve the cell performance, in this work proton exchange membrane (PEM) is developed in which pyridine-bridged-oxypolybenzimidazole (PyOPBI) and brominated polyphenylene oxide (BrPPO) were chem. cross-linked by an ex-situ methodol. Three cross-linked membranes P1, P2, and P3 consisting of 12.5, 25.0, and 37.5 wt. % BrPPO, resp. with respect to PyOBI were successfully fabricated and PEM properties were studied. These membranes showed much improved acid stability, oxidative stability, mech. strength and strong resistance to swelling in concd. phosphoric acid (PA) soln. They were found to be completely stable in the 85% PA whereas uncross-linked PyOPBI membrane readily dissolved in 60% PA. The reason for such stability has been ascribed to the cross-linked network structure of the membrane. The P1 membrane exhibited remarkably high proton cond. (0.123 S cm-1) whereas pristine PyOPBI membrane showed cond. 0.008 S cm-1 at 180°. The single cell measurement in anhyd. conditions at 160° of membrane electrode assembly (MEA) obtained from P1 membrane displayed good fuel cell efficiencies with power d. 290 mW cm-2 and c.d. 848.7 mA cm-3 at 0.3V whereas under the identical measurement condition MEA of pristine PyOPBI membrane showed 96.4 mW cm-2 power d. and 321.5 mA cm-2 c.d. at 0.3V. All these results endorsed that cross-linked membranes have a great potential to be used in the HT-PEMFC.
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    27. 27
      Yang, J.; Li, Q.; Cleemann, L. N.; Jensen, J. O.; Pan, C.; Bjerrum, N. J.; He, R. Crosslinked Hexafluoropropylidene Polybenzimidazole Membranes with Chloromethyl Polysulfone for Fuel Cell Applications. Adv. Energy Mater. 2013, 3, 622– 630,  DOI: 10.1002/aenm.201200710
      27
      Crosslinked hexafluoropropylidene polybenzimidazole membranes with chloromethyl polysulfone for fuel cell applications
      Yang, Jingshuai; Li, Qingfeng; Cleemann, Lars N.; Jensen, Jens Oluf; Pan, Chao; Bjerrum, Niels J.; He, Ronghuan
      Advanced Energy Materials (2013), 3 (5), 622-630CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
      Hexafluoropropylidene polybenzimidazole (F6PBI) was synthesized with excellent chem. stability and improved soly. When doped with phosphoric acid, however, the F6PBI membranes showed plastic deformation at elevated temps. Further efforts were made to covalently crosslink F6PBI membranes with chloromethyl polysulfone as a polymeric crosslinker. Comparing with linear F6PBI and mPBI membranes, the polymer crosslinked F6PBI membranes exhibited little organo soly., excellent stability towards the radical oxidn., high resistance to swelling in concd. phosphoric acid solns., and improved mech. strength, esp. at elevated temps. The superior characteristics of crosslinked membranes allowed for higher acid doping levels and therefore increased proton cond. as well as significantly improved fuel cell performance and durability, as compared with the linear F6PBI and mPBI membranes.
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    28. 28
      Wang, S.; Zhao, C.; Ma, W.; Zhang, N.; Liu, Z.; Zhang, G.; Na, H. Macromolecular cross-linked polybenzimidazole based on bromomethylated poly (aryl ether ketone) with enhanced stability for high temperature fuel cell applications. J. Power Sources 2013, 243, 102– 109,  DOI: 10.1016/j.jpowsour.2013.05.181
      28
      Macromolecular cross-linked polybenzimidazole based on bromomethylated poly (aryl ether ketone) with enhanced stability for high temperature fuel cell applications
      Wang, Shuang; Zhao, Chengji; Ma, Wenjia; Zhang, Na; Liu, Zhongguo; Zhang, Gang; Na, Hui
      Journal of Power Sources (2013), 243 (), 102-109CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Macromol. cross-linked polybenzimidazole (PBI) membranes were successfully prepd. for the high temp. proton exchange membrane fuel cell (HT-PEMFC) applications. Bromomethylated poly(aryl ether ketone) (BrPAEK) is synthesized and used as a macromol. cross-linker, the crosslinking reaction can be accomplished at 160° using an easy facial heating treatment. The resulting cross-linked membranes CBrPBI-X (X is the wt. fraction of the cross-linker) display excellent mech. strength. After phosphoric acid (PA) doping, the mech. strength and proton cond. of the PA/CBrPBI-X membranes are both enhanced comparing with the pristine PA/PBI. Considering the tradeoff of the mech. strength and proton cond., 10% BrPAEK is an optimum content in the matrix. For instance, the proton cond. of PA/CBrPBI-10 is 0.038 S cm-1 at 200°, which is higher than that of pristine PA/PBI with the proton cond. of 0.029 S cm-1 at the same temp. Other properties of the cross-linked membranes are also studied in detail, including the oxidative stability, soly. and thermal stability. All the PA/CBrPBI-10 membrane has the potential application in HT-PEMFCs.
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    29. 29
      Yang, J.; Aili, D.; Li, Q.; Cleemann, L. N.; Jensen, J. O.; Bjerrum, N. J.; He, R. Covalently Cross-Linked Sulfone Polybenzimidazole Membranes with Poly(vinylbenzyl chloride) for Fuel Cell Applications. ChemSusChem 2013, 6, 275– 282,  DOI: 10.1002/cssc.201200716
      29
      Covalently Cross-Linked Sulfone Polybenzimidazole Membranes with Poly(Vinylbenzyl Chloride) for Fuel Cell Applications
      Yang, Jingshuai; Aili, David; Li, Qingfeng; Cleemann, Lars N.; Jensen, Jens Oluf; Bjerrum, Niels J.; He, Ronghuan
      ChemSusChem (2013), 6 (2), 275-282CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Covalently cross-linked polymer membranes were fabricated from poly(aryl sulfone benzimidazole) (SO2PBI) and poly(vinylbenzyl chloride) (PVBCl) as electrolytes for high-temp. proton-exchange-membrane fuel cells. The crosslinking imparted organo insoly. and chem. stability against radical attack to the otherwise flexible SO2PBI membranes. Steady phosphoric acid doping of the cross-linked membranes was achieved at elevated temps. with little swelling. The acid-doped membranes exhibited increased mech. strength compared to both pristine SO2PBI and poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole] (mPBI). The superior characteristics of the cross-linked SO2PBI membranes allowed higher acid doping levels and, therefore, higher proton cond. Fuel-cell tests with the cross-linked membranes demonstrated a high open circuit voltage and improved power performance and durability.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXntFaqsg%253D%253D&md5=f5b83d8cee9ae7ba13846d84d872aa33
    30. 30
      Venugopalan, G.; Chang, K.; Nijoka, J.; Livingston, S.; Geise, G. M.; Arges, C. G. Stable and Highly Conductive Polycation-Polybenzimidazole Membrane Blends for Intermediate Temperature Polymer Electrolyte Membrane Fuel Cells. ACS Appl. Energy Mater. 2020, 3, 573– 585,  DOI: 10.1021/acsaem.9b01802
      30
      Stable and Highly Conductive Polycation-Polybenzimidazole Membrane Blends for Intermediate Temperature Polymer Electrolyte Membrane Fuel Cells
      Venugopalan, Gokul; Chang, Kevin; Nijoka, Justin; Livingston, Sarah; Geise, Geoffrey M.; Arges, Christopher G.
      ACS Applied Energy Materials (2020), 3 (1), 573-585CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
      Intermediate-temp. polymer electrolyte membrane fuel cells (IT-PEMFCs), operating with phosphoric acid (H3PO4) doped polybenzimidazole (PBI), are severely limited by H3PO4 evapn. at high temps. and poor resiliency in the presence of water. Polycations (PCs), on the other hand, provide good acid retention due to strong ion-pair interactions but have low cond. due to lower ion-exchange capacity when compared to PBI. In this work, a class of H3PO4 doped PC-PBI membrane blends was prepd., and the optimal blend (50:50 ratio) exhibited remarkably high in-plane proton cond., near 0.3 S cm-1 at 240°C, while also displaying excellent thermal stability and resiliency to water vapor. Microwave dielec. spectroscopy demonstrated that incorporating PBI into the PCs raised the dielec. const. by 50-70% when compared to the PC by itself. This observation explains, in part, the high proton cond. of the optimal membrane blend. Finally, an all-polymeric membrane electrode assembly with the new materials gave a competitive IT-PEMFC performance of 680 mW cm-2 at 220°C under dry H2/O2. Importantly, the cell was stable for up to 30 h at 220°C and over 84 h at 180°C. The IT-PEMFC had reasonable performance (450 mW cm-2) with 25% carbon monoxide in the hydrogen fuel.
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    31. 31
      Ma, W.; Zhao, C.; Lin, H.; Zhang, G.; Ni, J.; Wang, J.; Wang, S.; Na, H. High-Temperature Water-Free Proton Conducting Membranes based on Poly(arylene ether ketone) containing Pendant Quaternary Ammonium Groups with Enhanced Proton Transport. J. Power Sources 2011, 196, 9331– 9338,  DOI: 10.1016/j.jpowsour.2011.08.003
      31
      High-temperature water-free proton conducting membranes based on poly(arylene ether ketone) containing pendant quaternary ammonium groups with enhanced proton transport
      Ma, Wenjia; Zhao, Chengji; Lin, Haidan; Zhang, Gang; Ni, Jing; Wang, Jing; Wang, Shuang; Na, Hui
      Journal of Power Sources (2011), 196 (22), 9331-9338CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Poly(arylene ether ketone) contg. pendant quaternary ammonium groups (QPAEKs) are anion-conducting polymers synthesized from benzylmethyl-contg. poly(arylene ether ketone)s (PAEK-TM). QPAEK membranes doped with different concns. of H3PO4 were prepd. and evaluated as high temp. p exchange membranes. The H3PO4 doping ability of quaternary ammonium groups in QPAEK system is stronger than that of imidazole groups in polybenzimidazole system. The doping level of resulting QPAEK/H3PO4 composite membranes increases with both the concn. level of soaking H3PO4 soln. and the ion exchange capacity. For example, the highest doping level of composite membranes is 28.6, which is derived from QPAEK-5 with an ion exchange capacity of 2.02 mmol/g satd. with concd. H3PO4. A strong correlation between the doping level and the p cond. is obsd. for all the membranes. Besides their low cost, novel high temp. p exchange membranes, QPAEK/H3PO4, show really high p cond. and possess excellent thermal and mech. stability, suggesting a bright future for applications in high temp. fuel cells.
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    32. 32
      Cho, H.; Hur, E.; Henkensmeier, D.; Jeong, G.; Cho, E.; Kim, H. J.; Jang, J. H.; Lee, K. Y.; Hjuler, H. A.; Li, Q.; Jensen, J. O.; Cleemann, L. N. Meta-PBI/Methylated PBI-OO Blend Membranes for Acid Doped HT PEMFC. Eur. Polym. J. 2014, 58, 135– 143,  DOI: 10.1016/j.eurpolymj.2014.06.019
      32
      meta-PBI/methylated PBI-OO blend membranes for acid-doped high-temperature polymer electrolyte fuel cells (HT-PEMFC)
      Cho, Hyeongrae; Hur, Eun; Henkensmeier, Dirk; Jeong, Gisu; Cho, Eunae; Kim, Hyoung Juhn; Jang, Jong Hyun; Lee, Kwan Young; Hjuler, Hans Aage; Li, Qingfeng; Jensen, Jens Oluf; Cleemann, Lars Nielausen
      European Polymer Journal (2014), 58 (), 135-143CODEN: EUPJAG; ISSN:0014-3057. (Elsevier Ltd.)
      Methylation of polybenzimidazole leads to pos. charged polymer backbones, and moveable anions. Ion exchange of methylated PBI-OO in phosphoric acid (PA) shows that the resulting polymers dissolve. meta-PBI, however, absorbs ∼400 wt% PA while remaining a self supported membrane. We investigate the properties of blend membranes, employing meta-PBI for mech. integrity and methylated PBI-OO for high PA uptake and resulting proton cond. While small addns. of PBI-OO decrease the tensile strength of blend membranes (58 MPa for 10% PBI-OO), further addn. leads to an increase, and 50% blend membranes show again a tensile strength of 74 MPa, just 3 MPa lower than pure meta-PBI membranes. Thermal stability of iodide exchanged blend membranes appears to be remarkably high, probably because cleaved iodomethane does not evap. but methylates meta-PBI. PA concn. in doped membranes of 60-63% is reached by doping in 60% PA (blend; 6.3 PA/repeat unit) and 70% PA (meta-PBI; 4.6 PA/r.u.). This suggests that blends absorb PA more strongly. Both membranes show similar cond. between rt and 140 °C, indicating that PA concn. describes these membranes better than PA/r.u. In the fuel cell, blend membranes show similar or better performance than meta-PBI. In the TGA, blends doped in 20% PA showed a stable plateau between 115 and 180 °C, while meta-PBI lost wt. continuously.
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    33. 33
      Lee, K.-S.; Spendelow, J. S.; Choe, Y.-K.; Fujimoto, C.; Kim, Y. S. An Operationally Flexible Fuel Cell Based on Quaternary Ammonium-Biphosphate Ion Pairs. Nat. Energy 2016, 1, 447,  DOI: 10.1038/nenergy.2016.120
      There is no corresponding record for this reference.
    34. 34
      Atanasov, V.; Lee, A. S.; Park, E. J.; Maurya, S.; Baca, E. D.; Fujimoto, C.; Hibbs, M.; Matanovic, I.; Kerres, J.; Kim, Y. S. Synergistically Integrated Phosphonated Poly(pentafluorostyrene) for Fuel Cells. Nat. Mater. 2021, 20, 370– 377,  DOI: 10.1038/s41563-020-00841-z
      34
      Synergistically integrated phosphonated poly(pentafluorostyrene) for fuel cells
      Atanasov, Vladimir; Lee, Albert S.; Park, Eun Joo; Maurya, Sandip; Baca, Ehren D.; Fujimoto, Cy; Hibbs, Michael; Matanovic, Ivana; Kerres, Jochen; Kim, Yu Seung
      Nature Materials (2021), 20 (3), 370-377CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
      Modern electrochem. energy conversion devices require more advanced proton conductors for their broad applications. Phosphonated polymers have been proposed as anhyd. proton conductors for fuel cells. However, the anhydride formation of phosphonic acid functional groups lowers proton cond. and this prevents the use of phosphonated polymers in fuel cell applications. Here, we report a poly(2,3,5,6-tetrafluorostyrene-4-phosphonic acid) that does not undergo anhydride formation and thus maintains protonic cond. above 200°C. We use the phosphonated polymer in fuel cell electrodes with an ion-pair coordinated membrane in a membrane electrode assembly. This synergistically integrated fuel cell reached peak power densities of 1,130 mW cm-2 at 160°C and 1,740 mW cm-2 at 240°C under H2/O2 conditions, substantially outperforming polybenzimidazole- and metal phosphate-based fuel cells. Our result indicates a pathway towards using phosphonated polymers in high-performance fuel cells under hot and dry operating conditions.
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      Published Online: Dec. 7, 2020

    35. 35
      Lu, W.; Zhang, G.; Li, J.; Hao, J.; Wei, F.; Li, W.; Zhang, J.; Shao, Z.-G.; Yi, B. Polybenzimidazole-Crosslinked Poly(vinylbenzyl chloride) with Quaternary 1,4-Diazabicyclo (2.2.2) Octane Groups as High-Performance Anion Exchange Membrane for Fuel Cells. J. Power Sources 2015, 296, 204– 214,  DOI: 10.1016/j.jpowsour.2015.07.048
      35
      Polybenzimidazole-crosslinked poly(vinylbenzyl chloride) with quaternary 1,4-diazabicyclo (2.2.2) octane groups as high-performance anion exchange membrane for fuel cells
      Lu, Wangting; Zhang, Geng; Li, Jin; Hao, Jinkai; Wei, Feng; Li, Wenhui; Zhang, Jiying; Shao, Zhi-Gang; Yi, Baolian
      Journal of Power Sources (2015), 296 (), 204-214CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Development of anion exchange membrane (AEM) with high cond., good dimensional stability, desirable toughness and long life-time simultaneously is still a challenge for the practical application of AEM fuel cells. Herein, a novel AEM (denoted as PBI-c-PVBC/OH) is fabricated by applying polybenzimidazole (PBI) and 1,4-diazabicyclo (2.2.2) octane (DABCO) as the macromol. crosslinker and quaternizing reagent for poly(vinylbenzyl chloride) (PVBC), resp. With the aid of crosslinking by PBI, PBI-c-PVBC/OH exhibits good flexibility and strength both in dry and water-satd. state. Moreover, high hydroxide cond. (>25 mS cm-1 at room temp.) and low swelling ratio (∼13%) is obtained, esp. the swelling ratio nearly does not increase with temp. The membrane is also advanced for the superior chem. stability in alk. environment due to the stable polymer backbone and ionic conductive group (only one nitrogen atom in a DABCO mol. is quaternized). Furthermore, a peak power d. of 230 mW cm-2 at 50 °C is obtained on the H2/O2 fuel cell using PBI-c-PVBC/OH, and the membrane presents high durability both in the const. current and continuous open circuit voltage testing. Therefore, it is considered that the PBI crosslinking together with DABCO quaternization can be regarded as a promising strategy in the development of AEM for fuel cells.
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    36. 36
      Qaisrani, N. A.; Ma, L.; Hussain, M.; Liu, J.; Li, L.; Zhou, R.; Jia, Y.; Zhang, F.; He, G. Hydrophilic Flexible Ether Containing, Cross-Linked Anion-Exchange Membrane Quaternized with DABCO. ACS Appl. Mater. Interfaces 2020, 12, 3510– 3521,  DOI: 10.1021/acsami.9b15435
      36
      Hydrophilic Flexible Ether Containing, Cross-Linked Anion-Exchange Membrane Quaternized with DABCO
      Qaisrani, Naeem Akhtar; Ma, Lingling; Hussain, Manzoor; Liu, Jiafei; Li, Lv; Zhou, Ruiting; Jia, Yabin; Zhang, Fengxiang; He, Gaohong
      ACS Applied Materials & Interfaces (2020), 12 (3), 3510-3521CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Anion-exchange membranes (AEM) with high ion content usually suffer from excessive water absorption and diln. effects that impair cond. and mech. properties. We herein report a novel ether contg. a crosslinking strategy without adopting high ion-exchange capacity (IEC). The ether-contg. crosslinks and the quaternized structure are created simultaneously by introducing an ether-contg. flexible hydrophilic spacer between two 1,4-diazabicyclo[2,2,2,2]octane or DABCO mols.; the resultant bi-DABCO structure was further employed to react with chloromethylated polysulfone. The long spacer with the ether moiety may benefit the hydroxide ion transport, and the crosslinks will control the swelling and water absorption of the AEM. The two ether groups in the long spacer of the crosslinks will also shield the DABCO cation from OH- attack due to an electron-donating effect. The prepd. membranes exhibited an improved cond. of 31 mS/cm (at 25°C) at a comparatively low IEC (1.08 mmol/g) with a rational water absorption and low swelling ratio (95.0 and 27.1%, resp.); they also displayed an enhanced alk. stability in 1 M NaOH aq. soln. at 80°C for 150 h. The d. functional theory study and phys. characterization after the alk. treatment further confirm the better chem. stability of the crosslinked membrane over its counterpart. Our work presents an effective strategy to balance AEM cond. and robustness.
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      Published Online: Jan. 10, 2020

    37. 37
      Park, J.-H.; Park, J.-S. KOH-Doped Porous Polybenzimidazole Membranes for Solid Alkaline Fuel Cells. Energies 2020, 13, 525,  DOI: 10.3390/en13030525
      37
      KOH-doped porous polybenzimidazole membranes for solid alkaline fuel cells
      Park, Jong-Hyeok; Park, Jin-Soo
      Energies (Basel, Switzerland) (2020), 13 (3), 525CODEN: ENERGA; ISSN:1996-1073. (MDPI AG)
      In this study the prepn. and properties of potassium hydroxide-doped meta-polybenzimidazole membranes with 20-30μm thickness are reported as anion conducting polymer electrolyte for application in fuel cells. Di-Bu phthalate as porogen forms an asym. porous structure of membranes along thickness direction. One side of the membranes has a dense skin layer surface with 1.5-15μm and the other side of the membranes has a porous one. It demonstrated that ion cond. of the potassium hydroxide-doped porous membrane with the porogen content of 47 wt.% (0.090 S cm-1), is 1.4 times higher than the potassium hydroxide-doped dense membrane (0.065 S cm-1). This is because the porous membrane allows 1.4 times higher potassium hydroxide uptake than dense membranes. Tensile strength and elongation studies confirm that doping by simply immersing membranes in potassium hydroxide solns. was sufficient to fill in the inner pores. The membrane-electrode assembly using the asym. porous membrane with 1.4 times higher ionic cond. than the dense non-doped polybenzimidazole (mPBI) membrane showed 1.25 times higher peak power d.
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    38. 38
      Li, Q.; He, R.; Berg, R. W.; Hjuler, H. A.; Bjerrum, N. J. Water Uptake and Acid Doping of Polybenzimidazoles as Electrolyte Membranes for Fuel Cells. Solid State Ionics 2004, 168, 177– 185,  DOI: 10.1016/j.ssi.2004.02.013
      38
      Water uptake and acid doping of polybenzimidazoles as electrolyte membranes for fuel cells
      Li, Qingfeng; He, Ronghuan; Berg, Rolf W.; Hjuler, Hans A.; Bjerrum, Niels J.
      Solid State Ionics (2004), 168 (1-2), 177-185CODEN: SSIOD3; ISSN:0167-2738. (Elsevier Science B.V.)
      Acid-doped polybenzimidazole (PBI) membranes have been demonstrated for fuel cell applications with advanced features such as high operating temps., little humidification, excellent CO tolerance, and promising durability. The water uptake and acid doping of PBI membranes have been studied. The water uptake of PBI from the vapor phase is only slightly increased as the atm. humidity increases up to unity (100%). Little difference is obsd. for the water uptake from vapor and liq. phases, behaving very differently from Nafion membranes. When doped with phosphoric acid at low levels (<2), the active sites of the imidazole ring are preferably occupied by the doping acid and the water uptake is consequently lower. At higher acid doping levels, the water uptake is influenced by the excess of hygroscopic acid and higher water uptake than for Nafion membranes is obsd. Upon doping, the acid is found to be concd. inside the polymer. Only two mols. of phosphoric acid are bonded to each repeat unit of PBI, corresponding to the two nitrogen sites available. IR and Raman spectra show the presence of strong hydrogen bonds between phosphoric acid and nitrogen atoms of the imidazole rings. The excessive doping acid is "free acid" that contributes to high cond. but suffers from a fast washing out when adequate liq. is present.
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    39. 39
      Arslan, F.; Böhm, T.; Kerres, J.; Thiele, S. Spatially and Temporally Resolved Monitoring of Doping Polybenzimidazole Membranes with Phosphoric Acid. J. Membr. Sci. 2021, 625, 119145,  DOI: 10.1016/j.memsci.2021.119145
      39
      Spatially and temporally resolved monitoring of doping polybenzimidazole membranes with phosphoric acid
      Arslan, Funda; Boehm, Thomas; Kerres, Jochen; Thiele, Simon
      Journal of Membrane Science (2021), 625 (), 119145CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      Polybenzimidazole-based membranes in high temp. proton exchange membrane fuel cells require doping with phosphoric acid to enable proton cond. The level of phosphoric acid doping is traditionally detd. by titrn. or weighing, but these methods only provide limited information, since they do not offer spatial resoln. We show that confocal Raman microscopy can not only provide information on the level of doping, but in addn. about the spatial distribution of the dopant. We prove that doping is a diffusion-limited process, leading to a spatially inhomogeneous distribution of phosphoric acid unless doping is performed to satn. Further, evidence of a slow redistribution of the dopant within a freestanding membrane under ambient conditions is provided. Confocal Raman microscopy can be used as a non-invasive measurement tool to investigate status and progress of doping a membrane with phosphoric acid.
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    40. 40
      Cho, H.; Krieg, H.; Kerres, J. Performances of Anion-Exchange Blend Membranes on Vanadium Redox Flow Batteries. Membranes 2019, 9, 31,  DOI: 10.3390/membranes9020031
      40
      Performances of anion-exchange blend membranes on vanadium redox flow batteries
      Cho, Hyeongrae; Krieg, Henning M.; Kerres, Jochen A.
      Membranes (Basel, Switzerland) (2019), 9 (2), 31/1-31/14CODEN: MBSEB6; ISSN:2077-0375. (MDPI AG)
      Anion exchange blend membranes (AEBMs) were prepd. for use in Vanadium Redox Flow Batteries (VRFBs). These AEBMs consisted of 3 polymer components. Firstly, PBI-OO (nonfluorinated PBI) or F6-PBI (partially fluorinated PBI) were used as a matrix polymer. The second polymer, a bromomethylated PPO, was quaternized with 1,2,4,5-tetramethylimidazole (TMIm) which provided the anion exchange sites. Thirdly, a partially fluorinated polyether or a non-fluorinated poly (ether sulfone) was used as an ionical cross-linker. While the AEBMs were prepd. with different combinations of the blend polymers, the same wt. ratios of the three components were used. The AEBMs showed similar membrane properties such as ion exchange capacity, dimensional stability and thermal stability. For the VRFB application, comparable or better energy efficiencies were obtained when using the AEBMs compared to the com. membranes included in this study, i.e., Nafion (cation exchange membrane) and FAP 450 (anion exchange membrane). One of the blend membranes showed no capacity decay during a charge-discharge cycles test for 550 cycles run at 40 mA/cm2 indicating superior performance compared to the com. membranes tested.
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    41. 41
      Pan, C.; Li, Q.; Jensen, J. O.; He, R.; Cleemann, L. N.; Nilsson, M. S.; Bjerrum, N. J.; Zeng, Q. Preparation and Operation of Gas Diffusion Electrodes for High-Temperature Proton Exchange Membrane Fuel Cells. J. Power Sources 2007, 172, 278– 286,  DOI: 10.1016/j.jpowsour.2007.07.019
      41
      Preparation and operation of gas diffusion electrodes for high-temperature proton exchange membrane fuel cells
      Pan, Chao; Li, Qingfeng; Jensen, Jens Oluf; He, Ronghuan; Cleemann, Lars N.; Nilsson, Morten S.; Bjerrum, Niels J.; Zeng, Qingxue
      Journal of Power Sources (2007), 172 (1), 278-286CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Gas diffusion electrodes for high-temp. PEMFCs based on acid-doped polybenzimidazole membranes were prepd. by tape-casting. The overall porosity of the electrodes was tailored at 38-59% by introducing porogens into the supporting and/or catalyst layers. The studied porogens include volatile NH4+ oxalate, carbonate and acetate and acid-sol. Zn oxide - NH4+ oxalate and ZnO are more effective in improving the overall electrode porosity. Effects of electrode porosity on fuel cell performance were studied in terms of the cathodic limiting c.d. and min. air stoichiometry, anodic limiting current and H use, as well as operations under different pressures and temps.
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    42. 42
      Lobato, J.; Rodrigo, M. A.; Linares, J. J.; Scott, K. Effect of the Catalytic Ink Preparation Method on the Performance of High Temperature Polymer Electrolyte Membrane Fuel Cells. J. Power Sources 2006, 157, 284– 292,  DOI: 10.1016/j.jpowsour.2005.07.040
      42
      Effect of the catalytic ink preparation method on the performance of high temperature polymer electrolyte membrane fuel cells
      Lobato, J.; Rodrigo, M. A.; Linares, J. J.; Scott, K.
      Journal of Power Sources (2006), 157 (1), 284-292CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Two methods of prepn. of the membrane-electrode-assemblies based on polybenzimidazole membranes were studied for high temps. PEMFCs. One is called the colloidal method (using acetone as solvent), and the other is the soln. method (using dimethylacetamide as solvent). Phys. property studies (SEM micrographs and pore size distribution) and electrochem. analyses in half-cell (Electrochem. Impedance Spectroscopy, Polarization Curves for Oxygen Redn. and Cyclic Voltammetry) were carried out to characterize the structural and electrochem. behavior of both methods. A cell performance study, using electrodes prepd. by both methods was carried out at 3 different temps. (125, 150, and 175°), in a single PEMFC setup. A better behavior was obtained for the soln. method at the two highest temps. at intermediate current densities, whereas at 125° the best results were obtained with the colloidal method in all the current densities ranges. A discussion of the behaviors obsd. with the different characterization techniques is made.
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    43. 43
      Rau, M.; Niedergesäß, A.; Cremers, C.; Alfaro, S.; Steenberg, T.; Hjuler, H. A. Characterization of Membrane Electrode Assemblies for High-Temperature PEM Fuel Cells. Fuel Cells 2016, 16, 577– 583,  DOI: 10.1002/fuce.201500105
      43
      Characterization of Membrane Electrode Assemblies for High-Temperature PEM Fuel Cells
      Rau, M.; Niedergesaess, A.; Cremers, C.; Alfaro, S.; Steenberg, T.; Hjuler, H. A.
      Fuel Cells (Weinheim, Germany) (2016), 16 (5), 577-583CODEN: FUCEFK; ISSN:1615-6846. (Wiley-Blackwell)
      This paper will present the characterization of two types of membrane-electrode-assemblies (MEAs) for high-temp. polymer electrolyte membrane fuel cells (HT-PEMFC) working under reformate stream. The important aspects to be considered in the characterization of these MEAs are: (i) presence of contaminants, and (ii) compn. of the anode. Start/stop cycling test were performed for two different Dapozol MEAs using different GDL materials, using first hydrogen and then synthetic reformate as a fuel gas, both with a dew point of 80 °C. With these results the influence of contaminants present in the reformate was compared for the two types of MEAs, showing the superior performance of the Dapozol 101 MEA under these conditions. The possibility to further enhance the MEAs' resilience against the operation of reformates by changing the anode catalyst compn. was evaluated in a half MEA configuration, considering that the impact of the H2S present in the fuel presents a major issue. For this reason the hydrogen oxidn. reaction (HOR) was evaluated for two types of Pt-based electrocatalysts in an anodic half MEA configuration using different hydrogen-rich fuel mixts. These results provide valuable information for the optimization of the MEA and the anode catalyst for HT-PEMFC.
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    44. 44
      Cooper, K. R.; Smith, M. Electrical Test Methods for On-Line Fuel Cell Ohmic Resistance Measurement. J. Power Sources 2006, 160, 1088– 1095,  DOI: 10.1016/j.jpowsour.2006.02.086
      44
      Electrical test methods for on-line fuel cell ohmic resistance measurement
      Cooper, K. R.; Smith, M.
      Journal of Power Sources (2006), 160 (2), 1088-1095CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      The principles and trade-offs of 4 elec. test methods suitable for online measurement of the ohmic resistance, RΩ, of fuel cells is presented: current interrupt, a.c. resistance, high frequency resistance (HFR), and electrochem. impedance spectroscopy (EIS). The internal resistance of a p exchange membrane (PEM) fuel cell detd. with the current interrupt, HFR and EIS techniques is compared. The influence of the a.c. amplitude and frequency of the HFR measurement on the obsd. ohmic resistance is examd., as is the ohmic resistance extd. from the EIS data by modeling the spectra with a transmission line model for porous electrodes. The ohmic resistance of a H2/O2 PEM fuel cell detd. via the 3 methods was within 10-30% of each other. The current interrupt technique consistently produced measured cell resistances that exceeded those of the other 2 techniques. For the HFR technique, the frequency at which the measurement was conducted influenced the measured resistance (i.e., higher frequency providing smaller R Ω), whereas the a.c. amplitude did not effect the obsd. value. The difference in measured ohmic resistance between these techniques exceeds that reasonably accounted for by measurement error. The source of the discrepancy between current interrupt and impedance-based methods is attributed to the difference in the response of a nonuniformly polarized electrode, such as a porous electrode with non-negligible ohmic resistance, to a large perturbation (current interrupt event) as compared to a small perturbation (impedance measurement).
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    45. 45
      Kocha, S. S.; Deliang Yang, J.; Yi, J. S. Characterization of Gas Crossover and its Implications in PEM Fuel Cells. AIChE J. 2006, 52, 1916– 1925,  DOI: 10.1002/aic.10780
      45
      Characterization of gas crossover and its implications in PEM fuel cells
      Kocha, Shyam S.; Yang, J. Deliang; Yi, Jung S.
      AIChE Journal (2006), 52 (5), 1916-1925CODEN: AICEAC; ISSN:0001-1541. (John Wiley & Sons, Inc.)
      With pure hydrogen as the fuel, PEM fuel cell operation at or near 100% fuel utilization is desirable to achieve a high stack efficiency and zero emissions. However, typical membranes used in PEM fuel cells allow a finite amt. of permeation rates or crossover of hydrogen, oxygen, and nitrogen across the membrane. The hydrogen and oxygen that permeate through the membrane are consumed with the generation of heat and water but without the generating of useful work, leading to a fuel inefficiency. Nitrogen crossover, on the other hand, from the cathode side to the anode side accumulates at the exit of the anode flow fields, lowering the hydrogen concn. and resulting in local fuel starvation. In this study, an in-situ electrochem. technique was applied to det. the magnitude of the hydrogen crossover over a range of relevant fuel cell operating temps. and pressures. Permeability coeffs. thus obtained are compared to values reported in the literature. A math. model was developed to predict the extent of nitrogen accumulation along the anode flow fields, and fuel recycle as a mitigation method is simulated by improving hydrogen distribution. The model results were validated by comparison with exptl. results.
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    46. 46
      Cooper, K. R. Laboratory #4 – Fuel Crossover by Linear Sweep Voltammetry & Electrochemical Surface Area by Cyclic Voltammetry. Fuel Cell Mag. 2008.
      There is no corresponding record for this reference.
    47. 47
      High Temperature Polymer Electrolyte Membrane Fuel Cells; Li, Q., Aili, D., Hjuler, H. A., Jensen, J. O., Eds.; Springer International Publishing, 2016.
      There is no corresponding record for this reference.
    48. 48
      Bandura, D. R.; Baranov, V. I.; Tanner, S. D. Detection of Ultratrace Phosphorus and Sulfur by Quadrupole ICPMS with Dynamic Reaction Cell. Anal. Chem. 2002, 74, 1497– 1502,  DOI: 10.1021/ac011031v
      48
      Detection of ultratrace phosphorus and sulfur by quadrupole ICPMS with dynamic reaction cell
      Bandura, Dmitry R.; Baranov, Vladimir I.; Tanner, Scott D.
      Analytical Chemistry (2002), 74 (7), 1497-1502CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      A method of detection of ultratrace phosphorus and sulfur that uses reaction with O2 in a dynamic reaction cell (DRC) to oxidize S+ and P+ to allow their detection as SO+ and PO+ is described. The method reduces the effect of polyat. isobaric interferences at m/z = 31 and 32 by detecting P+ and S+ as the product oxide ions that are less interfered. Use of an axial field in the DRC improves transmission of the product oxide ions 4-6 times. With no axial field, detection limits (3σ, 5-s integration) of 0.20 and 0.52 ng/mL, with background equiv. concns. of 0.53 and 4.8 ng/mL, resp., are achieved. At an optimum axial field potential (200 V), the detection limits are 0.06 ng/mL for P and 0.2 ng/mL for S, resp. The method is used for detg. the degree of phosphorylation of β-casein, and regular and dephosphorylated α-caseins at 10-1000 fmol/μL concn., with 5-10% vol./vol. org. sample matrix (acetonitrile, formic acid, ammonium bicarbonate). The measured degree of phosphorylation for β-casein (4.9 phosphorus atoms/mol.) and regular α-casein (8.8 phosphorus atoms/mol.) are in good agreement with the structural data for the proteins. The P/S ratio for regular α-casein (1.58) is in good agreement with the ratio of the no. of phosphorylation sites to the no. of sulfur-contg. amino acid residues cysteine and methionine. The P/S ratio for com. available dephosphorylated α-casein is measured at 0.41 (∼26% residual phosphate).
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    49. 49
      Zhang, H.; Su, S.; Chen, X.; Lin, G.; Chen, J. Performance Evaluation and Optimum Design Strategies of an Acid Water Electrolyzer System for Hydrogen Production. Int. J. Hydrogen Energy 2012, 37, 18615– 18621,  DOI: 10.1016/j.ijhydene.2012.09.127
      49
      Performance evaluation and optimum design strategies of an acid water electrolyzer system for hydrogen production
      Zhang, Houcheng; Su, Shanhe; Chen, Xiaohang; Lin, Guoxing; Chen, Jincan
      International Journal of Hydrogen Energy (2012), 37 (24), 18615-18621CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      The performance of a new acid water electrolyzer system for hydrogen prodn. is investigated, based on semi-empirical equations of a phosphoric acid water electrolyzer. The circulating electrolyte concns. under differently operating temps. are optimized so that the min. input voltages of the electrolyzer are detd. for other given conditions. The optimum electrochem. characteristics of the electrolyzer are revealed. Moreover, it is expounded that the Joule heat resulting from the irreversibilities inside the electrolyzer is larger than the thermal energy needed in the water splitting process. The general performance characteristics of the phosphoric acid water electrolyzer system are discussed, from which the lower bound of the operating c.d. is detd. The upper bound of the operating c.d. is further detd. by introducing a multi-objective function including the system efficiency and hydrogen prodn. rate. Consequently, some optimum design strategies of a phosphoric acid water electrolyzer system are obtained and may be chosen according to different practical requirements.
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    50. 50
      Lin, X.; Liang, X.; Poynton, S. D.; Varcoe, J. R.; Ong, A. L.; Ran, J.; Li, Y.; Li, Q.; Xu, T. Novel Alkaline Anion Exchange Membranes Containing Pendant Benzimidazolium Groups for Alkaline Fuel Cells. J. Membr. Sci. 2013, 443, 193– 200,  DOI: 10.1016/j.memsci.2013.04.059
      50
      Novel alkaline anion exchange membranes containing pendant benzimidazolium groups for alkaline fuel cells
      Lin, Xiaocheng; Liang, Xuhao; Poynton, Simon D.; Varcoe, John R.; Ong, Ai Lien; Ran, Jin; Li, Yan; Li, Qiuhua; Xu, Tongwen
      Journal of Membrane Science (2013), 443 (), 193-200CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      Novel benzimidazolium (BIm) functionalized anion exchange membranes (AEMs) are synthesized and characterized for alk. fuel cells (AFCs). Poly(phenylene oxide) (PPO) is firstly brominated followed by nucleophilic substitution reaction with methylbenzimidazole to obtain the objective BIm-PPO AEMs. Such soln.-casting AEMs show good mech. and thermal stabilities as well as the favorable fuel cell-related indicators, including high ion exchange capacity, proper water uptake and high ionic cond. In addn., a single H2/O2 fuel cell test by employing the optimal BIm-PPO-0.54 AEM yields a peak power d. of 13 mW cm-2 at 35 °C, indicating the potential application of BIm-PPO AEMs in AFCs. Compared with the analogous AEMs based on PPO contg. the classical pendant quaternary ammonium and imidazolium cations, BIm-PPO AEMs show the advantages in dimensional, mech. and thermal stabilities, while simultaneously exhibiting the higher ionic cond. Compared with polybenzimidazolium based AEMs, where BIm cations distribute within the polymer backbone, AEMs herein present the higher ionic cond. and power d. (produced from a single cell test) due to the better mobility and aggregation abilities of pendant BIm cations attached to the backbone via a side chain relative to those distribute within the polymer backbone.
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    51. 51
      Hou, H.; Wang, S.; Liu, H.; Sun, L.; Jin, W.; Jing, M.; Jiang, L.; Sun, G. Synthesis and Characterization of a New Anion Exchange Membrane by a Green and Facile Route. Int. J. Hydrogen Energy 2011, 36, 11955– 11960,  DOI: 10.1016/j.ijhydene.2011.06.054
      51
      Synthesis and characterization of a new anion exchange membrane by a green and facile route
      Hou, Hongying; Wang, Suli; Liu, He; Sun, Lili; Jin, Wei; Jing, Mingyi; Jiang, Luhua; Sun, Gongquan
      International Journal of Hydrogen Energy (2011), 36 (18), 11955-11960CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      A new anion exchange membrane was synthesized by a green and facile route, chem. grafting polybenzimidazole (PBI) membrane with BrCH2CH3. The obtained membrane was characterized by means of ex-situ and in-situ tests (EDX, FTIR, a.c. impedance, single cell test). The results suggested that the group of -CH2CH3 was successfully grafted onto N atom within PBI backbone, with Br- as counter ion. The corresponding ionic cond. and ethanol permeability were about 0.022 S/cm and 5.24 × 10-8 cm2/s, resp. Finally, single cell test suggested that air-breathing alk. direct ethanol fuel cell with quaternized PBI membrane can deliver a peak power d. of ∼11 mW/cm2 even at ambient temp. of 13°, which was better than those of air-breathing alk. direct methanol fuel cells in literature. In addn., a possible reaction mechanism was also proposed and discussed.
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    52. 52
      Ogunlaja, A. S.; Hosten, E. C.; Tshentu, Z. R. The Oxidation of Dibenzothiophene using Oxidovanadium(IV)-Containing Nanofibres as Catalyst. S. Afr. J. Chem. 2015, 68, 172– 180,  DOI: 10.17159/0379-4350/2015/v68a24
      52
      The oxidation of dibenzothiophene using oxidovanadium(IV)-containing nanofibres as catalyst
      Ogunlaja, Adeniyi S.; Hosten, Eric C.; Tshentu, Zenixole R.
      South African Journal of Chemistry (2015), 68 (), 172-180CODEN: SAJCDG; ISSN:0379-4350. (South African Chemical Institute)
      Polyvinylbenzylchloride nanofibres were fabricated by the electrospinning technique and subsequently functionalized with a tetradentate ligand, 2,2'-(1E,1'E)-(1,2-phenylenebis(azan-1-yl-1-ylidene))bis(methan-1-yl-1-ylidene)bis(4-aminophenol). VO2+ was then incorporated into the nanofibres to produce the catalyst VO-fibers. Microanal., TG and FT-IR were used for the characterization of VO-fiber, and EPR also confirmed the presence of oxidovanadium(IV) within the nanofibres. Oxidn. of dibenzothiophene (DBT) was investigated by varying the catalyst amt., substrate amt., oxidant and temp., and the progress of oxidn. was followed with a gas chromatograph fitted with a flame ionization detector.Anincrease in the amt. of oxidant caused an increase in the amt. of dibenzothiophene sulfone (DBTO2), while a decrease in the quantity of dibenzothiophene resulted in an increase in the overall yield of dibenzothiophene sulfone under a const. temp. and oxidant (H2O2) concn. Dibenzothiophene sulfone was confirmed as the oxidn. product through 1H-NMRspectroscopy and single crystal X-ray diffraction.
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    53. 53
      Couture, G.; Alaaeddine, A.; Boschet, F.; Ameduri, B. Polymeric Materials as Anion-Exchange Membranes for Alkaline Fuel Cells. Prog. Polym. Sci. 2011, 36, 1521– 1557,  DOI: 10.1016/j.progpolymsci.2011.04.004
      53
      Polymeric materials as anion-exchange membranes for alkaline fuel cells
      Couture, Guillaume; Alaaeddine, Ali; Boschet, Frederic; Ameduri, Bruno
      Progress in Polymer Science (2011), 36 (11), 1521-1557CODEN: PRPSB8; ISSN:0079-6700. (Elsevier Ltd.)
      A review. After summarizing the different fuel cells systems, including advantages and drawbacks, this review focuses on the prepn. of copolymers and polymeric materials as starting materials for solid alk. fuel cells membranes. The requirements for such membranes are also summarized. Then, different strategies are given to synthesize anion-exchange polymeric materials contg. cationic (esp. ammonium) groups. The first pathway focuses on heterogeneous membranes that consist in: (i) polymer blends and composites based on poly(alkene oxide)s and hydroxide salts or polybenzimidazole doped with potassium hydroxide, (ii) org.-inorg. hybrid membranes esp. those synthesized via a sol-gel process, and (iii) (semi)interpenetrated networks based on poly(epichlorhydrine), poly(acrylonitrile) and polyvinyl alc. for example, that have led to new polymeric materials for anion-exchange membranes. The second and main part concerns the homogeneous membranes divided into three categories. The first one consists in materials synthesized from (co)polymers obtained via direct (co)polymn., for example membranes based on poly(diallyldimethylammonium chloride). The second pathway concerns the modification of polymeric materials via radiografting or chem. reactions. These polymeric materials can be hydrogenated or halogenated. The radiografting of membranes means the irradn. via various sources - electron beam, X and γ rays, 60Co and 137Cs that lead to trapped radicals or macromol. peroxides or hydroperoxides, followed by the radical graft polymn. of specific monomers such as chloromethyl styrene. The third route deals with the chem. modifications of com. available hydrogenated aliph. and arom. (co)polymers, and the syntheses of fluorinated (co)polymers such as carboxylic and sulfonic perfluoropolymers. In addn., several approaches for the crosslinking of above-mentioned polymeric materials are also reported as this process enhances the properties of the resulting membranes. Moreover, electrochem. and thermal properties of various above ionomers are given and discussed.
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    54. 54
      Teresa Pérez-Prior, M.; Ureña, N.; Tannenberg, M.; del Río, C.; Levenfeld, B. DABCO-Functionalized Polysulfones as Anion-Exchange Membranes for Fuel Cell Applications: Effect of Crosslinking. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1326– 1336,  DOI: 10.1002/polb.24390
      54
      DABCO-functionalized polysulfones as anion-exchange membranes for fuel cell applications: Effect of crosslinking
      Teresa Perez-Prior, Maria; Urena, Nieves; Tannenberg, Monika; del Rio, Carmen; Levenfeld, Belen
      Journal of Polymer Science, Part B: Polymer Physics (2017), 55 (17), 1326-1336CODEN: JPBPEM; ISSN:0887-6266. (John Wiley & Sons, Inc.)
      A series of DABCO-functionalized polysulfones were synthesized and characterized. The effect that crosslinking has on the membrane properties contg. different degrees of functionalization was evaluated. These polymers showed good thermal stability below the fuel cell operation temp., T < 100°, reflected by the TOD, TFD, and thermal durability. The water uptake increased as the percentage of DABCO groups increased and the crosslinked membranes showed lower capacity to absorb water than the non-crosslinked ones favoring thus the dimensional stability of the first ones. Membranes in the chloride form contg. low degree of functionalization exhibited the highest tensile strength values. The ionic cond. of non-crosslinked membranes varied as a function of the functionalization degree until a value of around 100% achieving a max. value at 86%. However, the crosslinked ones showed satisfactory ionic conductivities for values higher than 100%. The behavior of these polymeric materials in alk. solns. revealed a great alk. stability necessary to be used as solid electrolytes in fuel cells. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017.
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    55. 55
      Yang, J. S.; Cleemann, L. N.; Steenberg, T.; Terkelsen, C.; Li, Q. F.; Jensen, J. O.; Hjuler, H. A.; Bjerrum, N. J.; He, R. H. High Molecular Weight Polybenzimidazole Membranes for High Temperature PEMFC. Fuel Cells 2014, 14, 7– 15,  DOI: 10.1002/fuce.201300070
      55
      High Molecular Weight Polybenzimidazole Membranes for High Temperature PEMFC
      Yang, J. S.; Cleemann, L. N.; Steenberg, T.; Terkelsen, C.; Li, Q. F.; Jensen, J. O.; Hjuler, H. A.; Bjerrum, N. J.; He, R. H.
      Fuel Cells (Weinheim, Germany) (2014), 14 (1), 7-15CODEN: FUCEFK; ISSN:1615-6846. (Wiley-Blackwell)
      High temp. operation of proton exchange membrane fuel cells under ambient pressure has been achieved by using phosphoric acid doped polybenzimidazole (PBI) membranes. To optimize the membrane and fuel cells, high performance polymers were synthesized of mol. wts. from 30 to 94 kDa with good soly. in org. solvents. Membranes fabricated from the polymers were systematically characterized in terms of oxidative stability, acid doping and swelling, cond., mech. strength and fuel cell performance and durability. With increased mol. wts. the polymer membranes showed enhanced chem. stability towards radical attacks under the Fenton test, reduced vol. swelling upon the acid doping and improved mech. strength at acid doping levels of as high as about 11 mol H3PO4 per M repeat polymer unit. The PBI-78kDa/10.8PA membrane, for example, exhibited tensile strength of 30.3 MPa at room temp. or 7.3 MPa at 130 °C and a proton cond. of 0.14 S cm-1 at 160 °C. Fuel cell tests with H2 and air at 160 °C showed high open circuit voltage, power d. and a low degrdn. rate of 1.5 μV h-1 at a const. load of 300 mA cm-2.
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    56. 56
      Aili, D.; Yang, J.; Jankova, K.; Henkensmeier, D.; Li, Q. From Polybenzimidazoles to Polybenzimidazoliums and Polybenzimidazolides. J. Mater. Chem. A 2020, 8, 12854– 12886,  DOI: 10.1039/D0TA01788D
      56
      From polybenzimidazoles to polybenzimidazoliums and polybenzimidazolides
      Aili, David; Yang, Jingshuai; Jankova, Katja; Henkensmeier, Dirk; Li, Qingfeng
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2020), 8 (26), 12854-12886CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      A review. Polybenzimidazoles represent a large family of high-performance polymers contg. benzimidazole groups as part of the structural repeat unit. New application areas in electrochem. cells and sepn. processes have emerged during the last two decades, which has been a major driver for the tremendous development of new polybenzimidazole chemistries and materials in recent years. This comprehensive treatise is devoted to an investigation of the structural scope of polybenzimidazole derivs., polybenzimidazole modifications and the acid-base behavior of the resulting materials. Advantages and limitations of different synthetic procedures and pathways are analyzed, with focus on homogeneous soln. polymn. The discussion extends to soln. properties and the challenges that are faced in connection to mol. wt. detn. and processing. Methods for polybenzimidazole grafting or crosslinking, in particular by N-coupling, are reviewed and successful polymer blend strategies are identified. The amphoteric nature of benzimidazole groups further enriches the chem. of polybenzimidazoles, as cationic or anionic ionenes are obtained depending on the pH. In the presence of protic acids, such as phosphoric acid, cationic ionenes in the form of protic polybenzimidazoliums are obtained, which dramatically changes the physicochem. properties of the material. Cationic ionenes are also derived by complete N-alkylation of a polybenzimidazole to the corresponding poly(dialkyl benzimidazolium), which has been intensively explored recently as a new direction in the field of anion exchange membranes. In the higher end of the pH scale in aq. hydroxide solns., anionic ionenes in the form of polybenzimidazolides are obtained as a result of deprotonation of the benzimidazole groups. The ionization of the polymer results in dramatically changed physicochem. properties as compared to the pristine material, which is described and discussed. From a technol. point of view, performance and stability targets continue to motivate further research and development of new polybenzimidazole chemistries and energy materials. The overall aim of this review is therefore to identify challenges and opportunities in this area from synthetic chem. and materials science perspectives to serve as a solid basis for further development prospects.
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    57. 57
      Aili, D.; Allward, T.; Alfaro, S. M.; Hartmann-Thompson, C.; Steenberg, T.; Hjuler, H. A.; Li, Q.; Jensen, J. O.; Stark, E. J. Polybenzimidazole and Sulfonated Polyhedral Oligosilsesquioxane Composite Membranes for High Temperature Polymer Electrolyte Membrane Fuel Cells. Electrochim. Acta 2014, 140, 182– 190,  DOI: 10.1016/j.electacta.2014.03.047
      57
      Polybenzimidazole and sulfonated polyhedral oligosilsesquioxane composite membranes for high temperature polymer electrolyte membrane fuel cells
      Aili, David; Allward, Todd; Alfaro, Silvia Martinez; Hartmann-Thompson, Claire; Steenberg, Thomas; Hjuler, Hans Aage; Li, Qingfeng; Jensen, Jens Oluf; Stark, Edmund J.
      Electrochimica Acta (2014), 140 (), 182-190CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      Composite membranes based on poly(2,2'(m-phenylene)-5,´5bibenzimidazole) (PBI) and sulfonated polyhedral oligosilsesquioxane (S-POSS) with S-POSS contents of 5 and 10% were prepd. by soln. casting as base materials for high temp. polymer electrolyte membrane fuel cells. With membranes based on pure PBI as a ref. point, the composite membranes were characterized with respect to spectroscopic and physicochem. properties. After doping with H3PO4, the composite membranes showed considerably improved ex situ proton cond. under anhyd. as well as under fully humidified conditions in the 120-180° temp. range. The cond. improvements were also confirmed by in situ fuel cell tests at 160° and further supported by the electrochem. impedance spectroscopy data based on the operating membrane electrode assemblies, demonstrating the tech. feasibility of the novel electrolyte materials.
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    58. 58
      Aili, D.; Jensen, J. O.; Li, Q. Polybenzimidazole Membranes by Post Acid Doping. In High Temperature Polymer Electrolyte Membrane Fuel Cells; Li, Q., Aili, D., Hjuler, H. A., Jensen, J. O., Eds.; Springer International Publishing, 2016.
      There is no corresponding record for this reference.
    59. 59
      Li, Q. F.; Rudbeck, H. C.; Chromik, A.; Jensen, J. O.; Pan, C.; Steenberg, T.; Calverley, M.; Bjerrum, N. J.; Kerres, J. Properties, Degradation and High Temperature Fuel Cell Test of Different Types of PBI and PBI Blend Membranes. J. Membr. Sci. 2010, 347, 260– 270,  DOI: 10.1016/j.memsci.2009.10.032
      59
      Properties, degradation and high temperature fuel cell test of different types of PBI and PBI blend membranes
      Li, Q. F.; Rudbeck, H. C.; Chromik, A.; Jensen, J. O.; Pan, C.; Steenberg, T.; Calverley, M.; Bjerrum, N. J.; Kerres, J.
      Journal of Membrane Science (2010), 347 (1-2), 260-270CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      Polybenzimidazoles (PBIs) with modified structures and their blends with a partially fluorinated sulfonated arom. polyether were prepd. and characterized for high-temp. p exchange membrane fuel cells. Significant improvement in the polymer chem. stability in terms of the oxidative wt. loss, mol. wt. decrease and onset temps. for the thermal SO2 and CO splitting-off was achieved with the electron-deficient polybenzimidazoles contg. -S(O)2- and -C(CF3)2- bridging groups. Ionic crosslinking in acid-base blends further improved the polymer stability and assist maintaining membrane integrity. Upon acid doping membrane swelling was reduced for the modified PBI and their blend membranes, which, in turn, results in enhancement of the mech. strength, p cond. and high temp. fuel cell performance.
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    60. 60
      Kulkarni, M. P.; Thomas, O. D.; Peckham, T. J.; Holdcroft, S. High Ion Exchange Capacity, Sulfonated Polybenzimidazoles. In Polymers for energy storage and delivery: Polyelectrolytes for batteries and fuel cells ; [Symposium on “Polymers for Energy Storage and Delivery” held in March of 2011 as part of the 241st ACS national meeting & exposition; Page, K. A., Ed.; American Chemical Soc: Anaheim, CA, 2012; Vol. 1096, pp 221– 231. DOI: 10.1021/bk-2012-1096.ch013 .ACS Symposium Series
      There is no corresponding record for this reference.
    61. 61
      Zhang, R.; Shi, Z.; Liu, Y.; Yin, J. Synthesis and Characterization of Polybenzimidazole-Nanodiamond Hybrids via In Situ Polymerization Method. J. Appl. Polym. Sci. 2012, 125, 3191– 3199,  DOI: 10.1002/app.36497
      61
      Synthesis and characterization of polybenzimidazole-nanodiamond hybrids via in situ polymerization method
      Zhang, Ru; Shi, ZhiXing; Liu, Yang; Yin, Jie
      Journal of Applied Polymer Science (2012), 125 (4), 3191-3199CODEN: JAPNAB; ISSN:0021-8995. (John Wiley & Sons, Inc.)
      Poly[2,2'-(p-oxydiphenylene)-5,5'-bibenzimidazole] (OPBI) was polymd. in poly(phosphoric acid) (PPA) with the presence of the pristine nanodiamonds (NDs) (0.2-5 wt%) to fabricate NDs-g-OPBI/OPBI nanocomposites via Friedel-Crafts (F-C) reaction. The OPBI chains were successfully attached to the NDs through F-C reaction between carboxylic acid from OPBI and NDs, which was proved by NMR, X-ray photoelectron, and X-ray diffraction. NDs-g-OPBI/OPBI nanocomposites show more homogeneous dispersion than the phys. blending contg. pristine NDs and OPBI matrix, as showed through scanning electronic microscopy images. The mech. properties, including Young's modulus, yield strength, and tensile strength are all improved by the introduction of NDs (<1 wt%) without loss of ductility, which overcomes the brittleness brought by the addn. of inorg. reinforced agent in traditional composites. Dynamic mech. anal. results showed that the modulus of the ND-g-OPBI/OPBI nanocomposites was significantly higher than OPBI matrix, and the NDs-g-OPBI/OPBI nanocomposites displayed more pronounced improvement than the phys. blending, which could be ascribed to the homogeneous dispersion of NDs particles and the covalent bonding between NDs and OPBI via F-C reaction. Thermogravimetric anal. indicated that all the OPBI nanocomposites contg. NDs displayed the improved thermal stability. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012.
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    62. 62
      Pinar, F. J.; Rastedt, M.; Pilinski, N.; Wagner, P. Characterization of HT-PEM Membrane-Electrode-Assemblies. In High Temperature Polymer Electrolyte Membrane Fuel Cells; Li, Q., Aili, D., Hjuler, H. A., Jensen, J. O., Eds.; Springer International Publishing, 2016, pp 353– 386. DOI: 10.1007/978-3-319-17082-4_17
      There is no corresponding record for this reference.
    63. 63
      Galbiati, S.; Baricci, A.; Casalegno, A.; Marchesi, R. Degradation in Phosphoric Acid Doped Polymer Fuel Cells: A 6000 h Parametric Investigation. Int. J. Hydrogen Energy 2013, 38, 6469– 6480,  DOI: 10.1016/j.ijhydene.2013.03.012
      63
      Degradation in phosphoric acid doped polymer fuel cells: A 6000 h parametric investigation
      Galbiati, Samuele; Baricci, Andrea; Casalegno, Andrea; Marchesi, Renzo
      International Journal of Hydrogen Energy (2013), 38 (15), 6469-6480CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      This paper reports an exptl. study of the degrdn. of single PBI-based high temp. MEAs doped with phosphoric acid. The study is carried out by operating the single MEAs for long periods in steady state, the degrdn. is quantified considering the voltage decay rate. Besides the most common operating condition suggested by the MEAs producer (T = 160 °C, i = 0.2 A cm-2, λH2 = 1.2, λair = 2), the study also investigates higher operating temp. (T = 180 °C), higher c.d. (i = 0.4 A cm-2) and double air flow rate (λair = 4). A temp. of 180 °C accelerates the degrdn. of the MEA which increases from around 8 μV h-1 up to around 19 μV h-1. On the opposite side, operating the MEA at i = 0.4 A cm-2 reduces the voltage degrdn. rate down to 4 μV h-1 and increases the power output making this condition particularly interesting. EIS, CV and LSV are used to clarify the causes of degrdn. A consistent increase in the charge transfer resistance is obsd. and is related to the loss of catalyst active area due to catalyst agglomeration, carbon corrosion and possible acid leaching. Concerning the electrolyte membrane, a slight decrease in the proton cond. is measured, a major effect on degrdn. is played by the increasing gas crossover rate and by the short circuit current.
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    64. 64
      Mann, R. F.; Amphlett, J. C.; Peppley, B. A.; Thurgood, C. P. Henry’s Law and the solubilities of reactant gases in the modelling of PEM fuel cells. J. Power Sources 2006, 161, 768– 774,  DOI: 10.1016/j.jpowsour.2006.05.054
      64
      Henry's Law and the solubilities of reactant gases in the modelling of PEM fuel cells
      Mann, R. F.; Amphlett, J. C.; Peppley, B. A.; Thurgood, C. P.
      Journal of Power Sources (2006), 161 (2), 768-774CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Proton exchange membrane (PEM) fuel cells have been under development for many years and appear to be the potential soln. for many electricity supply applications. Modeling and computer simulation of PEM fuel cells have been equally-active areas of work as a means of developing better understanding of cell and stack operation, facilitating design improvements and supporting system simulation studies. In general, fuel cell models must be able to predict both activation and concn. polarizations at both anode and cathode. Normally these predictions require values of the concn. of the reactant gases (i.e., H2 and O2) at the interface between the catalyst and the electrolyte. Electrolytes of interest could include various dil. acids or polymeric membranes such as Nafion so that gas solubilities, in the form of Henry's Law consts., could be required for a diverse group of solvents. Published soly. data have been evaluated and a no. of Henry's Law correlations are proposed.
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    65. 65
      Haider, R.; Wen, Y.; Ma, Z.-F.; Wilkinson, D. P.; Zhang, L.; Yuan, X.; Song, S.; Zhang, J. High Temperature Proton Exchange Membrane Fuel Cells: Progress in Advanced Materials and Key Technologies. Chem. Soc. Rev. 2020, 50, 1138,  DOI: 10.1039/d0cs00296h
      There is no corresponding record for this reference.

      Published Online: Nov. 27

    66. 66
      Søndergaard, T.; Cleemann, L. N.; Becker, H.; Steenberg, T.; Hjuler, H. A.; Seerup, L.; Li, Q.; Jensen, J. O. Long-Term Durability of PBI-Based HT-PEM Fuel Cells: Effect of Operating Parameters. J. Electrochem. Soc. 2018, 165, F3053– F3062,  DOI: 10.1149/2.0081806jes
      66
      Long-Term Durability of PBI-Based HT-PEM Fuel Cells: Effect of Operating Parameters
      Sondergaard, Tonny; Cleemann, Lars Nilausen; Becker, Hans; Steenberg, Thomas; Hjuler, Hans Aage; Seerup, Larisa; Li, Qingfeng; Jensen, Jens Oluf
      Journal of the Electrochemical Society (2018), 165 (6), F3053-F3062CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      This work studies the long-term durability of high-temp. polymer electrolyte membrane fuel cells based on acid-doped polybenzimidazole membranes. The primary focus is on acid loss via the evapn. mechanism, which is a major cause of degrdn. in applications that involve long-term operation. Durability is assessed for 16 identically fabricated membrane electrode assemblies (MEAs), and evaluations are carried out using operating parameters as stressors with gas stoichiometries ranging from 2 to 25, current densities from 200 to 800 mA cm-2, and temps. of 160 or 180°C. Cell diagnostics are composed of time resolved polarization curves, post mortem anal., and in situ temp. measurements. A major part of the cell degrdn. during these steady-state tests can be ascribed to increasing area-specific series resistance. By means of post mortem acid-loss measurements, the degrdn. is correlated to the temp. and to the accumulated gas-flow vol. Such relations are indicative of acid loss via evapn. C.d. also plays a crit. role for the acid loss and, thus, for the overall cell degrdn. The effect of current is likely tied to mechanisms that involve water generation, migration of electrolyte ions, and locally elevated temp. inside the MEAs.
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    67. 67
      Schmidt, T. J. High-Temperature Polymer Electrolyte Fuel Cells: Durability Insights. In Polymer Electrolyte Fuel Cell Durability; Büchi, F. N., Inaba, M., Schmidt, T. J., Eds.; Springer: New York, 2009.
      There is no corresponding record for this reference.
    68. 68
      Becker, H.; Reimer, U.; Aili, D.; Cleemann, L. N.; Jensen, J. O.; Lehnert, W.; Li, Q. Determination of Anion Transference Number and Phosphoric Acid Diffusion Coefficient in High Temperature Polymer Electrolyte Membranes. J. Electrochem. Soc. 2018, 165, F863– F869,  DOI: 10.1149/2.1201810jes
      68
      Determination of anion transference number and phosphoric acid diffusion coefficient in high temperature polymer electrolyte membranes
      Becker, Hans; Reimer, Uwe; Aili, David; Cleemann, Lars N.; Jensen, Jens Oluf; Lehnert, Werner; Li, Qingfeng
      Journal of the Electrochemical Society (2018), 165 (10), F863-F869CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      The passage of an elec. current through phosphoric acid doped polymer membranes involves parasitic migration of the acid, which imposes a crit. issue for long-term operation of the high temp. polymer electrolyte membranes fuel cell (HT-PEMFC). To elucidate the phenomenon, a three-layered membrane is constructed with embedded micro ref. electrodes to measure phosphoric acid redistribution in a polybenzimidazole based membrane. Under a const. load, a concn. gradient develops due to the acid migration, which drives the back diffusion of the acid and eventually reaches a steady state between migration and diffusion. The acid gradient is measured as a difference in local ohmic resistances of the anode- and cathode-layer membranes by electrochem. impedance spectroscopy. The phosphoric acid diffusion coeff. through the acid doped membrane is about 10-11 m2 s-1, at least one order of magnitude lower than that of aq. phosphoric acid solns. The anion (H2PO4-) transference no. is found to range up to 4% depending on c.d., temp. and atm. humidity of the cell, implying that careful control of the operating parameters is needed in order to suppress the vehicular proton conduction as a degrdn. mitigation strategy.
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    69. 69
      Kannan, A.; Li, Q.; Cleemann, L. N.; Jensen, J. O. Acid Distribution and Durability of HT-PEM Fuel Cells with Different Electrode Supports. Fuel Cells 2018, 18, 103– 112,  DOI: 10.1002/fuce.201700181
      69
      Acid Distribution and Durability of HT-PEM Fuel Cells with Different Electrode Supports
      Kannan, A.; Li, Q.; Cleemann, L. N.; Jensen, J. O.
      Fuel Cells (Weinheim, Germany) (2018), 18 (2), 103-112CODEN: FUCEFK; ISSN:1615-6846. (Wiley-Blackwell)
      The durability of high-temp. polymer electrolyte membrane fuel cells (HT-PEMFCs) was studied with phosphoric acid doped membranes of polybenzimidazole (PBI). One of the challenges for this technol. is the loss and instability of phosphoric acid resulting in performance degrdn. after long-term operation. The effect of the gas diffusion layers (GDL) on acid loss was studied. Four different com. available GDLs were subjected to passive ex situ acid uptake by capillary forces and the acid distribution mapped over the cross-section. Materials with an apparent fine structure made from carbon black took up much more acid than materials with a more coarse apparent structure made from graphitized carbon. The same trend was evident from thermally accelerated fuel cell tests at 180 °C under const. load where degrdn. rates depended strongly on the choice of GDL material, esp. on the cathode side. Acid was collected from the fuel cell exhaust at rates clearly correlated to the fuel cell degrdn. rates, but amounted to less than 6% of the total acid content in the cell even after significant degrdn. Long-term durability of more than 5,500 h with a degrdn. rate of 12 μV h-1 at 180 °C and 200 mA cm-2 was demonstrated with the GDL that retained acid most efficiently.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.1c17154.

    • Chemical structures and molar masses of F6PBI, OPBI, m-PBI, and PVBC; ATR FT-IR spectra of the polymers OPBI and PVBC and the cross-linked membranes; polarization and power density curves for the FCs in Table 1; ADLs and AUs for all membranes discussed in this work; PA uptake in milligram, cathodic P and PA losses, and acid loss; exemplary short-circuit correction for the linear sweep voltammogram of 90-10-QOH; in situ proton conductivities at 140, 160, and 180 °C; XRD spectra; and Fenton’s test (PDF)


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