Comparing Ammonium and Tetraaminophosphonium Anion-Exchange Membranes Derived from Vinyl-Addition Polynorbornene Copolymers

Herein, we systematically examined how composition influenced the properties of vinyl addition polynorbornene anion exchange membranes (AEMs) prepared from 5-n-hexyl-2-norbornene and 5-(4-bromobutyl)-2-norbornene. Copolymerization kinetics revealed that 5-n-hexyl-2-norbornene is consumed faster than 5-(4-bromobutyl)-2-norbornene, leading to a portion of the chain being richer in bromoalkyl groups. The alkyl halide pendants can then be converted to either trimethylammonium or tetrakis(dialkylamino)phosphonium cations through straightforward substitution with trimethylamine or a tris(dialkylamino)phosphazene. A series of cationic ammonium polymers were synthesized first, where conductivity and water uptake increased as a function of increasing ionic content in the polymer. The optimized copolymer had a hydroxide conductivity of 95 ± 6 mS/cm at 80 °C. The living polymerization of the two monomers catalyzed by a cationic tert-butylphosphine palladium catalyst also enabled precise changes in the molecular weight while keeping the functional group concentration constant. Molecular weight did not have a significant impact on hydroxide conductivity over the range of ∼60–190 kg/mol (Mn). The optimized tetraaminophosphonium AEM had the highest conductivity for any tetraaminophosphonium polymer to date (70 ± 3 mS/cm at 80 °C). Clear phase separation and larger domains were observed for the phosphonium-based AEM compared to the ammonium at an identical composition, which is attributed to the larger occupied volume of the phosphorus cation. Fuel cell studies with the two membranes resulted in peak power densities of 1.59 and 0.79 W/cm2 for the ammonium and tetraaminophosphonium membrane electrode assemblies, respectively. The ammonium-based membrane was more water permeable as evidenced by water limiting current studies, which likely contributed to the improved performance.


NMR Analysis.
All NMR spectra were recorded on a 500 MHz Bruker Avance 3 Spectrometer or a 500 MHz Bruker Avance Neo with a Prodigy Cryoprobe.The 1 H NMR spectra of compounds and polymers were collected in either deuterated chloroform (CDCl3) or deuterated tetrachloroethane (TCE-d2) and referenced to residual protio solvents (7.26 ppm for CHCl3 and 6.00 ppm for TCE-d).The 31 P NMR spectra were referenced to the lock signal.

Gas chromatography-Mass Spectrometry (GC-MS) Analysis. GC-MS analysis was performed
on a Hewlett-Packard Agilent 6890-5973 GC-MS workstation.The GC column was a Restek fused silica capillary column (RTX-5).Helium was used as the carrier gas.The following conditions were used for all GC-MS analyses: injector temperature, 250 °C; initial temperature, 50 °C; temperature ramp, 10 °C/min; final temperature, 170 °C.For estimation of conversion, 0.05 mL aliquots were removed from polymerization reactions and quenched with 2 drops of acetonitrile in a scintillation vial.This was then diluted with 5 mL of ethyl acetate, which resulted in precipitation of the polymer from the solution.Once the polymer settled to the bottom of the vial, the solution was then pipetted into a 2 mL vial for analysis.Conversion was calculated by comparison of the monomer integrations with the internal standard (1,3,5-trimethoxybenzene). Thermogravimetric Analysis (TGA).TGA was carried out using a TA Instruments TGA Q50.

Gel-Permeation Chromatography (GPC
Samples were heated from 50 -800 °C at a rate of 10 °C/min under a N2 atmosphere.

Conductivity, Ion Exchange Capacity (IEC), and Water Uptake (WU). All measurements
5][6] IEC was measured using standard back titration methods. 4- 6 ater uptake was determined gravimetrically using the following equation: Small Angle X-Ray Scattering (SAXS).Dried thin film membranes in the bromide or chloride form were mounted on a membrane sample holder.SAXS was performed using an Anton Paar SAXSess mc 2 with narrow-slit beam collimation. 5,6 ogenic Transmission Electron Microscopy (cryo-TEM).Sectioning of the PNB-Hex-NMe3[Cl] and PNB-Hex-iPrMe[Cl] copolymers for cryo-TEM was performed using cryo-microtomy on a Leica EM UC7/FC7 Cryo-Ultra-microtome.The samples were cooled to temperatures between −60 to −95°C and cut into ~30 nm thick sections using a diamond knife.
These sections were then collected using copper TEM grids with lacey carbon support and stored at room temperature.Cryo-TEM images of the grids were taken on a Thermo Fisher Talos Arctica Renewable Energy Laboratory 10 was used as the ionomer at a target loading of 3 wt% in the catalyst ink dispersion.To fabricate the gas diffusion electrodes (GDEs), the catalyst ink was sprayed onto the gas diffusion layer with a SonoTek ultrasonic spray coating station.The gas diffusion layers (GDLs) were Toray paper (Toray Carbon paper 060 wet proofed) for the anode and Freudenberg H23C8 for the cathode.The target loading of anode and cathode electrodes were 0.8 mgPtRu/cm 2 and 0.4 mgPt/cm 2 , respectively.Previous fuel cell testing and optimization influenced the choice of gas diffusion layers.Before assembling the cell, GDEs and membranes were soaked in 1 M KOH solution for 12 h at room temperature and then assembled while still slightly wet with KOH solution.The membrane was sandwiched between two GDEs with 5 cm 2 active area, PTFE gaskets were used and a GDL compression of 25% was targeted.

Hydrogen Crossover Measurements
Hydrogen crossover measurements were conducted by flowing H2 at the anode and N2 at the cathode with a flow rate of 0.2 slpm before and after the durability test with a cell operating temperature = 70 °C, back pressure = 131 kPa, and 100% relative humidity.The H2 crossover current was measured by applying a voltage of 0.5 V across the cell to obtain H2 crossover limiting S8 current density (iH2).The Department of Energy has targeted that the maximum H2 crossover current density for a novel electrolyte membrane to be less than 2 mA/cm 2 at the beginning of life. 11

Water Limiting Current
Before measuring limiting current, 100% relative humidity H2 and N2 were supplied to anode and cathode, respectively at 500 ml/min until desired cell temperature (70 °C) was achieved.Then, the cathode flow was switched from N2 to O2.After OCV stabilized, a constant voltage of 0.5 V was applied until a plateau in the current density was observed.The limiting currents were measured with an applied anode flow rate of 500 mL/min and cathode flow rate of 200 ml/min.With the cathode relative humidity at 0% (dry), the cell potential was held at 0.4V for 5 min.Then the cell potential was scanned from 0.4 V to 0.15 V in 0.025 V steps and the current density was recorded after allowing the cell to stabilize for 100 s.
excess of methanol, which yielded an off-white fibrous polymer that was filtered and dried for 17 h in vacuo (2.51 g, 96% yield). 1 H NMR (500 MHz, CDCl3)  ppm: 3.4 (br s, 2H), 2.6 -0.3 (br, all aliphatic protons), 0.88 (br s, 3H).The1 H NMR shifts for each statistical copolymer in the series were all nearly identical, but integrations vary according to target monomer ratios and are indicated below for diagnostic -CH2Br and -CH3 signals.For the kinetic studies, copolymerizations were carried out with 1,3,5-trimethoxybenzene as an internal standard (10 mol % relative to the two starting monomers) and aliquots were removed periodically for analysis using GC-MS.
Percent Incorporation of NB-5-BuBr in PNB-Hex-Br copolymers.Incorporation of NB-5-BuBr in the PNB-Hex-Br copolymers was determined as in our prior report. 6The integrations of the -CH2Br signal from the bromobutyl chain and the -CH3 signal from the hexyl chain were compared after careful baseline correction of the spectrum to determine the relative ratio of the two monomers.Typically, ~50 mg of the polymer sample was dissolved in ~1 mL CDCl3 for this analysis (delay time = 2s).The integration for the methylene signal was set to 2 (corresponding to 1 repeat unit of NB-5-BuBr) and the integration value for the -CH3 signal from the hexyl chain was then divided by 3 to determine the relative ratio of the two monomers.A sample calculation is shown in Figure S6.In the 1 H NMR spectrum for the 67:33, 60:40 and 50:50 copolymers (NB-5-Hex:NB-5BrBu), ratios of 6:2, 4.5:2 and 3:2 should be observed for the terminal -CH3 as compared to -CH2Br signal.
Table S1.Mole fraction of monomer in the feed prior to initiating copolymerization of NB-5-Hex and NB-5-BuBr as determined by GC-MS relative to the internal standard.

Figure S5 .
Figure S5.GPC traces for DP 500 PNB-Hex-Br copolymers after the final timepoint in conversion analysis.

Figure S8. 1 H
Figure S8. 1 H NMR spectra (500 MHz) of DP 500 60:40 PNB-Hex-Br copolymer collected in CDCl3 (top) and the corresponding PNB-Hex-NMe3[Br] polymer after functionalization with NMe3 collected in 1:1 CDCl3:CD3OD (bottom).A 9:4.5 ratio should be observed for the Nmethyl groups of the trimethylammonium to the terminal methyl group from NB-5-Hex for a 40 mol% ionic copolymer.Accurate integrals were difficult to obtain due to overlapping solvent and signal broadness.

Table S2 .
Mole fraction monomer in polymer in copolymerizations NB-5-Hex and NB-5-BuBr after 5 minutes as determined by GC-MS (monomer consumption) relative to the internal standard.