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Accessing In Situ Photocorrosion under Realistic Light Conditions: Photoelectrochemical Scanning Flow Cell Coupled to Online ICP-MS
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Accessing In Situ Photocorrosion under Realistic Light Conditions: Photoelectrochemical Scanning Flow Cell Coupled to Online ICP-MS
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  • Ken J. Jenewein*
    Ken J. Jenewein
    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/0000-0001-8979-7252
  • Attila Kormányos
    Attila Kormányos
    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
  • Julius Knöppel
    Julius Knöppel
    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/0000-0001-8355-3062
  • Karl J. J. Mayrhofer
    Karl J. J. Mayrhofer
    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/0000-0002-4248-0431
  • Serhiy Cherevko*
    Serhiy Cherevko
    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/0000-0002-7188-4857
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ACS Measurement Science Au

Cite this: ACS Meas. Sci. Au 2021, 1, 2, 74–81
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https://doi.org/10.1021/acsmeasuresciau.1c00016
Published August 19, 2021

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Abstract

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High-impact photoelectrode materials for photoelectrochemical (PEC) water splitting are distinguished by synergistically attaining high photoactivity and stability at the same time. With numerous efforts toward optimizing the activity, the bigger challenge of tailoring the durability of photoelectrodes to meet industrially relevant levels remains. In situ photostability measurements hold great promise in understanding stability-related properties. Although different flow systems coupled to light-emitting diodes were introduced recently to measure time-resolved photocorrosion, none of the measurements were performed under realistic light conditions. In this paper, a photoelectrochemical scanning flow cell connected to an inductively coupled plasma mass spectrometer (PEC-ICP-MS) and equipped with a solar simulator, Air Mass 1.5 G filter, and monochromator was developed. The established system is capable of independently assessing basic PEC metrics, such as photopotential, photocurrent, incident photon to current efficiency (IPCE), and band gap in a high-throughput manner as well as the in situ photocorrosion behavior of photoelectrodes under standardized and realistic light conditions by coupling it to an ICP-MS. Polycrystalline platinum and tungsten trioxide (WO3) were used as model systems to demonstrate the operation under dark and light conditions, respectively. Photocorrosion measurements conducted with the present PEC-ICP-MS setup revealed that WO3 starts dissolving at 0.8 VRHE with the dissolution rate rapidly increasing past 1.2 VRHE, coinciding with the onset of the saturation photocurrent. The most detrimental damage to the photoelectrode is caused when subjecting it to a prolonged high potential hold, e.g., at 1.5 VRHE. By using standardized illumination conditions such as Air Mass 1.5 Global under 1 Sun, the obtained dissolution characteristics are translatable to actual devices under realistic light conditions. The gained insights can then be utilized to advance synthesis and design approaches of novel PEC materials with improved photostability.

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1. Introduction

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Photoelectrochemical (PEC) water splitting is a promising way of harnessing solar energy and to directly create energy-dense, storable fuels, e.g., hydrogen. (1) The generation of such fuels circumvents the intermittency of renewables and enables coupling of different energy sectors and thus contributing to their decarbonization. Unfortunately, PEC devices have yet to see widespread commercial application in part due to low efficiencies and poor durability of the employed photoactive components. (2) Most research efforts in the past decades have been devoted to improving the intrinsic activity of photoelectrocatalysts, but comparatively few investigations focused on the photostability. Hence, the instability of semiconductors in PEC environments is less understood than the catalytic processes. (3) From a material science perspective, only photoelectrodes accommodating high catalytic activity with long-term stability can be considered as a high-performance material.
Photostability is traditionally assessed by measuring the photocurrent retention over an extended chronoamperometic hold or potential cycling under illumination. Alternatively, morphological and structural changes in the sample can be investigated by post-mortem analysis using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). None of these analyses can provide insights into the degradation processes happening in situ. (3,4) Past reports on the revelation of in situ potential-dependent degradation processes in dark electrocatalysis involved a highly sensitive inductively coupled mass spectrometer (ICP-MS) coupled downstream to a scanning flow cell (SFC). (5,6) On the one hand, ICP-MS allows for the detection of multiple elements simultaneously with a very low detection limit. On the other hand, this system lacks the ability to provide information about the chemical state of dissolved or undissolved species. In more recent studies, flow cells have been equipped with light-emitting diodes (LEDs), channeled perpendicular to the photoelectrode surface, to investigate photocorrosion processes of WO3, (7) ZnO, (8) or BiVO4 (9) in a time-resolved fashion. While these studies provide great detail into degradation processes during PEC operation, the first two works irradiate the sample using one LED at a fixed wavelength. Alternatively, Zhang et al. (9) used a continuous light source filtered to 400–700 nm that recreates white light. In their work, the light is not channeled in into the cell but the sample is irradiated through the cell body, which could make exact adjustments of the irradiation condition at the sample area challenging. Photoelectrodes are commonly characterized under standardized 1 Sun Air Mass 1.5 Global (AM 1.5 G) conditions using a solar simulator. It is, therefore, inconclusive whether observed degradation behaviors using LEDs are also encountered under realistic light conditions. Notably, Sliozberg et al. have developed a scanning droplet cell system coupled to a solar simulator, yet, with clear focus on activity determination. (10) There remains a need for an experimental setup that combines simulated solar irradiation with a flow system, making in situ photocorrosion under standardized light conditions yet to be demonstrated for any phototelectode. Attempts to conduct in situ analysis on the photostability include a quartz crystal microbalance to detect mass losses during PEC operation. (11) Given the complexity of photodecomposition processes, a gravimetric approach is less conclusive of the underlying degradation mechanism, especially in multielement systems with diverse dissolution characteristics.
In this work, we developed a photoelectrochemical scanning flow cell connected to an inductively coupled plasma mass spectrometer (PEC-ICP-MS) equipped with a solar simulator, AM 1.5 G filter, and a monochromator. This system enables the assessment of basic PEC properties of a photoelectrode, such as photopotential, photocurrent, incident photon to current efficiency (IPCE), and band gap, while at the same time allows measuring real-time dissolution behavior under realistic solar irradiation. First, details on the hardware development are outlined together with the design concepts of the photoelectrochemical scanning flow cell (PEC-SFC) that allows for the coupling of a solar simulator to a flow cell. Then, initial system evaluations are conducted under dark electrocatalytic conditions to benchmark the setup against past literature reports for SFC systems. Finally, tungsten trioxide (WO3) is used as a model system to study its PEC performance and in situ photostability. WO3 is acknowledged to be a promising photoanode for solar water splitting due to its band gap being in the visible spectrum, electron transport properties, hole diffusion length, and chemical stability in acidic media. (12) Since maximum theoretical solar-to-hydrogen efficiencies are around 6%, PEC devices made solely of WO3 are not commercially viable. Nonetheless, modified WO3-based PEC devices have great potential to attain efficiencies beyond 10%, which would make such systems considerable for widespread use. (13)

2. Experimental Section

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2.1. PEC-ICP-MS Setup

The developed PEC-ICP-MS setup is schematically illustrated in Scheme 1. A photograph of the light components can be found in Figure S1. A 300 W ozone-free Xe lamp (I, Newport, 66984–300XF-R1) with a lens transmission range of 200–2500 nm was used as an illumination source. The emitted light was then passed through a focusing lens (II, Newport, 6199) and an AM 1.5 G filter (III, Newport, 81094) before channeling in the simulated sunlight into a monochromator with an integrated shutter (IV, Newport, Cornerstone, CS130B-1-MC). To ensure maximum light throughput, the distance between the focusing lens and the monochromator was adjusted in a way to channel as much light into the monochromator as possible. The monochromator can be operated in either a “bypass mode” in which the light passes through the device without any alteration or a “monochromatic mode” in which a selected wavelength can be extracted from the full light spectrum. The AM 1.5 G filter was removed when operating the monochromator in the latter mode. Having two modes available makes the setup flexible and modular, rendering the need to manually exchange the monochromator depending on the desired PEC experiment obsolete. The light exiting the monochromator was then fed into a filter wheel (V, Newport, USFW-100) equipped with long-pass filters of different cutoff wavelengths to exclude any higher-order harmonics produced on the diffraction grating in the monochromator. Finally, the collimated light source was converted into a fiber optic source by using a focusing assembly (VI, Newport, 77799) before being fed into a liquid light guide (VII, Newport, 77566) with a core diameter of 3 mm and a transmission range of 340–800 nm. Employing a liquid light guide is crucial especially when operating the setup in “monochromatic mode.” Here, light intensities drop significantly when filtering for just a single wavelength with few nanometers of bandwidth. Low light power can potentially manifest in very small photocurrents in the range of few microamperes, complicating data analysis. Hence, choosing a liquid light guide with a sufficient core diameter ensures high light throughput and superior longevity compared to glass fiber optics due to no risk of fiber breakage. White light was calibrated to 1 Sun (100 mW cm–2) at the PEC-SFC opening at the beginning of each measurement day using a reference solar cell (Newport, 91150V). Additionally, the light intensity of the solar simulator was measured at the end of a measurement day to verify no significant intensity losses over the course of a day. Power densities of the monochromatic light were measured using a photodiode sensor (Thorlabs, S120VC) coupled to a USB power and energy meter interface (Thorlabs, PM100USB).

Scheme 1

Scheme 1. Schematic Illustration of PEC-ICP-MS Setup Employed in This Studya
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aThe optical components consist of: (I) 300 W ozone-free Xe lamp, (II) focusing lens, (III) AM 1.5 G filter, (IV) monochromator, (V) filter wheel with different cutoff filters, (VI) focusing assembly, and (VII) liquid light guide.

Basic (photo)electrochemical experiments were controlled with a potentiostat (BioLogic, SP 150) using a Ag/AgCl electrode in 3 M KCl (Metrohm) as the reference and a glassy carbon rod (HTW, SIGRADUR G) as the counter electrode. Investigated samples were contacted through a metal wire to establish good electrical contact. All experiments were conducted in 0.1 M HClO4 prepared by diluting appropriate amounts of concentrated HClO4 (VWR, Suprapur, 70%) with ultrapure water (Merck Millipore, Milli-Q). The electrolyte was constantly purged with Ar in a separate reservoir and was transported through the system within Tygon tubings via a peristaltic pump (Ismatec, Reglo ICC). Measured potentials, EAg/AgCl, were all corrected to the reversible hydrogen electrode (RHE) scale by determining the pH of the electrolyte using a pH meter (Mettler Toledo, FiveEasy Plus) and applying the following equation:
Samples were mounted onto an XYZ stage (Physik Instrumente, M-403), allowing rapid and accurate positioning under the PEC-SFC. The cell was fixed on a force sensor (ME-Messsysteme GmbH, KD45) to control the contact pressure to the sample using a proportional–integral–derivative controller. The contacting force was set to 2 N. All components within the setup were controlled and synchronized through an in-house made LabVIEW software.
In situ photostability measurements using an online ICP-MS were performed with an Nexion 350X instrument (PerkinElmer). Calibration of the ICP-MS was carried out daily for all investigated metals by a four-point calibration curve (0, 0.5, 1, 5 μg L–1) from freshly prepared standard solutions (Merck Centripur). 187Re at a concentration of 10 μg L–1 served as an internal standard to ensure good system performance. The flow rate of the electrolyte into the ICP-MS was 3.55 μL s–1.

2.2. Cell Design

Several cell designs used for SFC applications have been previously reported. (6,14,15) However, due to the necessity to accommodate a liquid light guide with an 11 mm ferrule into an SFC, a new PEC-SFC design was conceptualized for this setup. The V-shaped channels in an SFC allow the electrolyte to continuously flow above a sample. Therefore, it is crucial to optimize the cell design for individual applications to ensure that dissolved species are transported downstream toward the ICP-MS for trace element detection. (16) Flow profiles simulated with COMSOL Multiphysics show that incorporating a third light channel perpendicular to the sample plane shifts the electrolyte velocity maximum further away from the SFC opening compared to a conventional arrangement used for dark electrocatalysis (Figure S2a, b). An insufficient flow velocity at the sample plane may cause issues due to dissolved ions not being flushed away to the outlet. To remedy this problem, the channel angles can be widened up to 90° to decrease the headspace above the cell opening, thus bringing the maximum flow velocity closer to the measurement area (Figure S2c).
Polytetrafluoroethylene (PTFE) was chosen as the cell material for its nontransparency, chemical stability, resistance to UV light, and plasticity. The latter is especially advantageous for creating a hermetic seal between the cell opening and the sample due to the ability of PTFE to deform when pressing the cell onto the sample with a set force. As long as the contacting force is within reasonable ranges (e.g., 1–2 N), the sealing by deformation can be repeated multiple times. Therefore, there is no need to equip the cell opening with an additional silicone O-ring to ensure complete closure, as was practiced with previous designs, (14) which reduces manual labor. Additional grinding of the cell opening was performed to ensure alignment with the sample to account for manufacturing inaccuracies. To further prevent any oxygen from diffusing into the measurement area, a gas pocket around the cell opening was designed, which can be purged with an inert gas. The space between the pocket walls and the cell opening plane was 0.5 mm to prevent any contact of the pocket walls with the sample. This feature could become useful when studying, for instance, hydrogen evolution or CO2 reduction reactions, where oxygen reduction could interfere with the measurement. (17)
As evident from the established PEC-SFC design (Figure 1), a small volume of air will be entrapped between the ferrule tip and the electrolyte flow. This raises the question of whether the flowing electrolyte moves up toward the ferrule rather than being transported out of the PEC-SFC. To quickly evaluate this potential issue, a simplified PEC-SFC was 3D-printed using a transparent polymer and its flow profile was checked visually. Figure S3a shows that the electrolyte only flows from the inlet to the outlet side without migrating toward the liquid light guide. This proves that the newly designed PEC-SFC is capable of operating as a flow cell.

Figure 1

Figure 1. Cross-sectional 3D design of a PEC-SFC. (I) Electrolyte outlet channel, (II) reference electrode channel, (III) inert gas pocket, (IV) liquid light guide channel, (V) electrolyte inlet channel, and (VI) inert gas inlet.

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The reference electrode was positioned on the outlet channel side to prevent potential contamination of chlorine due to diffusion from the reference electrode compartments, which would affect the measured stability. (18) The counter electrode was connected to the sample through the inlet side. A three-electrode configuration was established by contacting the sample through the cell opening. To determine the actual illuminated area of the PEC-SFC, a UV-sensitive paper (Astromedia, Solar-Fotopapier) was employed (Figure S3b). The illuminated spot shows homogeneous coverage of the total cell opening, verifying that the entire contacted sample area is also irradiated.

3. Results and Discussion

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3.1. Setup Validation under Dark Conditions

With a new cell design concept being established herein, reproducing results from older SFC designs is crucial to verify the viability of the current setup as a flow system. Hence, before benchmarking the setup for PEC applications, it was necessary to check that the system can reliably operate under dark electrocatalysis conditions. Initial system validation was performed by recording cyclic voltammograms (CVs) of a polycrystalline platinum foil (MaTeck, 99.99%) in 0.1 M HClO4. The surface of the foil was ground and polished with a 1 μm diamond suspension. Prior to the measurement, the surface was electrochemically cleaned by sweeping the working electrode potential between 0.05 and 1.4 VRHE with a scan rate of 200 mV s–1 for 30 cycles. Figure S4a shows a CV recorded at 10 mV s–1, indicating all expected oxidation and reduction peaks for Pt in an acidic medium. (19) Model samples such as polycrystalline platinum are susceptible to oxygen traces in the electrolyte, which manifest as a significant cathodic drop in the current below 1 VRHE due to concomitantly occurring oxygen reduction reactions. (20) Notably, the double-layer region between 0.4 and 0.6 VRHE lies close to 0 mA cm–2, indicating minimal diffusion of atmospheric oxygen into the measurement area. The employed air-shielding walls, together with the introduction of Ar around the cell opening, prove to be suitable strategies against gas contaminations entering the measurement area. Successive online dissolution measurements on the same sample (Figure S4b, c) revealed similar Pt dissolution traces to previous reports. (5) Dissolution peaks are slightly wider, originating from the larger PEC-SFC opening area employed here. Reproducing the reported Pt dissolution behavior encourages the suitability of the established PEC-SFC design for the application in online dissolution measurements.

3.2. Output Spectra

Figure 2 shows the corresponding light spectra for both operating modes (bypass and monochromatic) recorded with a concave grating spectrometer (StellarNet, BLACK-Comet) after the light has traversed through a 0.1 M HClO4 electrolyte. The relatively large bandwidth of the monochromatic spectra is a result of setting the slit size to its maximum width and height to achieve a compromise between light throughput and resolution. Using an AM 1.5 G filter reduces light intensities in the UV region below 350 nm and dampens the characteristic Xe peaks between 800 and 1000 nm in order to shape the emission spectrum close to the solar energy distribution.

Figure 2

Figure 2. Output spectrum of PEC-SFC setup operated in (a) monochromatic mode and (b) bypass mode. All spectra were recorded at the PEC-SFC opening after the light has traveled through a 0.1 M HClO4 solution.

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3.3. Setup Validation under Illuminated Conditions

Performing general PEC measurements on a photoactive material can aid in estimating the performance and suitability for further focused assessments. (21−23) By using a PEC-SFC, such general PEC measurements can be performed rapidly in a serial manner, presenting a high-throughput platform capable of discovering new materials for solar water splitting applications. The validation was conducted on spray-coated WO3 samples synthesized using a peroxotungstic acid-based route. (24−26) Detailed descriptions of the sample preparation can be found in the Supporting Information.
Measuring the photopotential is the very first step to check whether a semiconductor is light-active. The absorption of photons leads to electron–hole pairs that induce an unbending of the valence and the conduction band of the semiconductor, which manifests in different open circuit potentials (OCP) under dark and light conditions. (23) The semiconducting type can be estimated depending on whether the potential changes in the anodic or cathodic direction. (27) The magnitude of the photopotential also reveals the driving force for the photoelectrochemical reaction (28) and can help in assessing the flat band potential used to estimate the band edge positions in photoelectrodes. (29) The spray-coated WO3 exhibits a drop in OCP when illuminating the sample after a short period in dark conditions (Figure 3a) and equilibrates around 0.48 VRHE. A shift toward cathodic potentials confirms WO3 being a n-type semiconductor.

Figure 3

Figure 3. (a) Photopotential under dark and illuminated state. (b) Photovoltammogram recorded with a scan rate of 10 mV s–1 and a chopping frequency of 0.2 Hz. Inset showing magnified potential region around onset potential. (c) IPCE measurement at 1.23 VRHE. (d) (IPCE hν)0.5 vs hν (gray) with linear fit (red). All measurements were conducted on spray-coated WO3 in Ar-saturated 0.1 M HClO4. (a, b) were measured with the monochromator in bypass mode to access the polychromatic spectrum of the solar simulator calibrated to 1 Sun and (c) was measured using the monochromatic mode.

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The actual activity of the sample is most commonly assessed by recording photovoltammograms under chopped irradiation (Figure 3b). Here, it is crucial to utilize standardized conditions, such as 1 Sun AM 1.5 G, to ensure comparability between different studies. WO3 exhibits a broad oxidation peak around 0.4–0.6 VRHE representing the electrochromic redox response of HxWO3/WO3. (30,31) The onset potential for the photocurrent is around 0.5 VRHE, standing in agreement with previous reports (30,32) and the observed photopotential in Figure 3a. The photocurrent slowly increases with an inclining bias as more charge carriers get extracted from the semiconductor into the solution. The photocurrent starts to plateau past 1.2 VRHE indicating the maximum extractable charge carriers is reached. The measured photocurrent, which is the current density difference between a light-on and light-off state, is about 0.7 mA cm–2 and comparable with literature values. (33)
Another crucial diagnostic metric regarding material efficiency is the so-called incident photon to current efficiency (IPCE). This metric represents the semiconductor’s capability of converting the impinging photons into photoelectrochemical reaction products. Catalytic conversions are accounted for by the overall measured photocurrent (Jph(λ)) at a given wavelength, λ. The photon energy is given by the measured power density (Pmono(λ)).
Here, the faradaic efficiency is assumed to be 100% toward the oxygen evolution reaction (OER). The IPCE curve recorded at 1.23 VRHE (Figure 3c) shows a maximum efficiency of 36% at 340 nm and falls to 0% around 480 nm. Similar performances have been observed in other reports investigating WO3 in 1 M HClO4. (34) The decrease in IPCE is a sign of decreased photon absorption by the semiconductor as higher wavelengths cannot excite WO3 to form electron–hole pairs. The corresponding Tauc plot based on an indirect semiconductor ((IPCE hν)0.5) (35) is shown in Figure 3d. Extrapolating the linear region around the inflection point reveals the bandgap value of the semiconductor at the intercept with the x-axis (red fit in Figure 3d). Here, the bandgap energy is around 2.56 eV and is consistent with the indirect bandgap for crystalline WO3 known from literature. (35)
The validation of the developed PEC-SFC setup under PEC conditions demonstrated closely matching performance values for WO3 to those reported in literature where the measurements are conducted using bulk PEC cells. Thus, the current setup can give valid insights into PEC properties of a photoelectrode in an accurate and high-throughput manner.

3.4. In Situ Photocorrosion under Simulated Solar Irradiation

Besides measuring the intrinsic activity, it is equally important to assess the stability of photoelectrodes to better understand material properties and ultimately improve on the durability constrain. Achieving high activity while retaining good stability is one of the greatest challenges when designing novel materials for PEC applications. For this, the developed PEC-ICP-MS setup using a solar simulator serves as an excellent tool to assess these two important metrics simultaneously. By measuring photocorrosion under standardized illumination rather than monochromatic light, as it was previously performed, the determined photostability will be closer to what would be expected in a real-life application scenario.
To study the photocorrosion behavior under realistic operating conditions, a potential protocol consisting of two linear sweep voltammograms (LSV) and chronoamperometric (CA) holds was established. The first of the two LSVs and CA holds was performed in dark and the second in 1 Sun AM 1.5 G conditions, as depicted in Figure 4a. Measured currents (Figure 4b) in the dark correspond to the redox behavior of WO3. Greater performance is recorded under illumination, evident by visibly higher photocurrents. WO3 is well-known to be a stable photoanode in acidic conditions demonstrated by negligible dissolution rates during a dark LSV up to 1.5 VRHE shown in Figure 4c. A small dissolution peak is observed probably due to undercoordinated W sites dissolving at high potentials. Subjecting WO3 to harsher conditions such as a two minute CA hold at 1.5 VRHE in dark results in a noticeable W dissolution peak in dark. Assessing the real-time photocorrosion under illuminated LSVs reveals a W dissolution onset around 0.8 VRHE, which coincides with the steep increase in the photocurrent in this potential region. Previous PEC-ICP-MS setups utilizing a 385 nm LED detected an onset potential of dissolution around 1.0 VRHE for WO3. (7,36) With the emitted polychromatic spectrum from the solar simulator including wavelengths below 385 nm, it is suggested that the additional high energy irradiation leads to an altered dissolution characteristic compared to results using monochromatic light. Once the potential passes 1.2 VRHE (gray dashed line in Figure 4) the dissolution rate experiences a substantial incline indicated by the two red dashed lines in Figure 4c representing linear fits of the dissolution rate before and after 1.2 VRHE. Interestingly, 1.2 VRHE also marks the potential at which the saturation photocurrent starts to onset. This behavior highlights that when nearing the maximum extractable charge carriers for the photooxidation, the additional bias applied will contribute more to W dissolution than to the actual PEC reaction. Such photocorrosion behavior in conjunction with the current response was not observed in previous PEC-ICP-MS studies with WO3. (7,36) Encountered discrepancies in the photocorrosion behavior between 385 nm and AM 1.5 G implies the importance of utilizing realistic irradiation conditions to understand photostability properties for photoelectrodes in real devices. Applying light to a CA hold at 1.5 VRHE has a more detrimental effect on the photoanode, causing a steep increase in dissolution until peaking around 0.2 ng cm–2 s–1. Notably, WO3 exposed to an illuminated, high potential hold displays an elevated W baseline dissolution after returning to a cathodic hold of 0.4 VRHE in the dark. This observation might hint to permanent damage (e.g., pinhole) to the sample after harsh operating conditions. Newly generated imperfections in the photolectrode structure or lattice could promote irreversible material instability. (37,38) The degraded sample could also lose mechanical integrity, causing material detachment by the flowing electrolyte. The measurements were successfully repeated with Figure S5 summarizing obtained W dissolutions for the individual, potentiostatic techniques under dark and light with their respective reproducibility.

Figure 4

Figure 4. Current density and dissolution profile of spray-coated WO3 under dark and 1 Sun AM 1.5 G illuminated condition in 0.1 M HClO4. (a) Potential protocol with a scan rate of 10 mV s–1 for the LSVs. CA holds were performed for two minutes. Upper potential limit for all LSVs and CA holds was 1.5 VRHE. (b) Measured current density. (c) Dissolution rate of W (gray, measured signal; dashed red, linear fit of dissolution rate before and after dissolution rate increase; dashed gray, transition in dissolution rate). Yellow bars indicate the period of illumination.

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To translate the amount of dissolved ions into a comparable metric across different studies, the S-number concept has been introduced first for dark OER catalysis. (39) The S-number originally relates the produced oxygen, assuming 100% faradaic efficiency, to the total amount of dissolved species (n(M)). However, it remains disputed whether OER is the sole reaction occurring on the WO3 surface with HClO4 as the electrolyte, as ClO4– can undergo decomposition reactions to form radical species. (40) Therefore, the S-number concept was adapted to account for any oxidized species using the total number of transferred electrons (n(e–)). (36)
The S-number(e–) based on the second CA hold under illumination was calculated to be 8850 and is in the ballpark of previously reported values for WO3 in HClO4 conducted in our group. (36) The calculated S-number(e–) for the dark CA hold equals to about 21 700. The smaller value under light indicates that even after consideration of the total measured charge, WO3 is rendered more unstable by photogenerated charge carriers.
The developed PEC-ICP-MS system proves its viability as a tool to study in situ photocorrosion behaviors under AM 1.5 G conditions. This experimental setup is expected to play a vital role in unfolding photodecomposition phenomena guiding the synthesis and design of more stable photoactive materials. Moreover, flow cell systems are well suited for operation in a high-throughput manner. In fact, scanning droplet cells have been successfully employed for rapid (photo)electrochemical screening of (photo)electrocatalyst libraries. (10,41) We envision a full automation of the presented system to establish a platform that is capable of not only performing full sets of PEC measurements but at the same time assess the in situ photostability of a photoelectrode material library. Such a platform would enable material discovery, which is tailored to search not only for the most active but also for the most stable PEC material.

4. Conclusion

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In situ photodecomposition measurements of photoelectrodes are still a rarity in the field of photoelectrochemistry, yet they are crucial to unravel stability properties of promising photoelectrodes. Here, a PEC-ICP-MS setup was developed that is operated under standardized illumination conditions using a solar simulator and an AM 1.5 G filter. The established system was benchmarked for its suitability to function as a scanning flow design with expected observations under dark electrocatalytic conditions being met. Further, it was demonstrated that the PEC-SFC setup is capable of conducting general PEC measurements to assess photopotential, photocurrent, and IPCE of semiconducting materials, e.g., WO3. The obtained performance for spray-coated WO3 meets literature values. Finally, time-resolved PEC-ICP-MS measurements were successfully performed under simulated solar irradiation on WO3, confirming the pronounced dissolution of W under light compared to dark conditions. The dissolution behavior is closely tied to the photocurrent response as the dissolution onset coincides with the steep incline in the photocurrent. Applying additional bias after the saturation photocurrent is reached triggers an increase in W dissolution. Such behavior was not observed in the past when studying the in situ photocorrosion of WO3 at 385 nm, which demonstrates the importance of studying the time-resolved photostability under realistic light conditions.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsmeasuresciau.1c00016.

  • Electrolyte flow profile simulation, WO3 synthesis procedure, photograph of light components, photograph of transparent PEC-SFC with flowing electrolyte, photograph of UV sensitive paper to determine illuminated area, CV and dissolution profile of polycrystalline Pt, reproducibility of WO3 dissolution experiments (PDF)

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Author Information

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  • Corresponding Authors
    • Ken J. Jenewein - 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/0000-0001-8979-7252;  Email: [email protected]
    • 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;  Email: [email protected]
  • Authors
    • Attila Kormányos - Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstrasse 3, 91058 Erlangen, Germany;  Orcidhttps://orcid.org/0000-0002-2145-7419
    • Julius Knöppel - 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/0000-0001-8355-3062
    • Karl J. J. Mayrhofer - 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/0000-0002-4248-0431
  • Author Contributions

    K.J.J. developed the setup, conducted all measurements, simulated flow profiles, and wrote the manuscript, J.K. provided spray-coated WO3. A.K. and S.C. supervised the work. K.J.J.M., A.K., J.K., and S.C. participated in discussions and provided valuable comments on the manuscript. All authors have given approval to the final version of the manuscript.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors would like to thank Jonas Möller for developing the custom-made LabVIEW software employed for the setup. Furthermore, the authors would like to acknowledge Achim Mannke, Stefan Borlein, and Stefan Fiegl for technical support. We appreciate the help from Renee Timmins with editing the manuscript.

Abbreviations

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AM 1.5 G

air mass 1.5 global

CA

chronoamperometric

CV

cyclic voltammogram

ICP-MS

inductively coupled plasma mass spectrometer

IPCE

incident photon to current efficiency

LED

light-emitting diode

LSV

linear sweep voltammogram

OCP

open circuit potential

OER

oxygen evolution reaction

PEC

photoelectrochemical

PEC-ICP-MS

photoelectrochemical scanning flow cell connected to an inductively coupled plasma mass spectrometer

PEC-SFC

photoelectrochemical scanning flow cell

PTFE

polytetrafluoroethylene

RHE

reversible hydrogen electrode

SEM

scanning electron microscopy

SFC

scanning flow cell

TEM

transmission electron microscopy

XPS

X-ray photoelectron spectroscopy

XRD

X-ray diffraction

References

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    Shkirskiy, Viacheslav; Speck, Florian Dominik; Kulyk, Nadiia; Cherevko, Serhiy
    Journal of the Electrochemical Society (2019), 166 (16), H866-H870CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    Many of the recent advancements in the electrocatalysis research have been obtained by application of coupled electrochem./massspectrometry techniques. Representative example is the electrochem. flow cells coupled to inductively coupled plasma mass spectrometry (online ICP-MS) in electrocatalysis stability research. In this technique, unambiguous correlation of the concn. of dissolved species vs. potential/current represents a significant challenge due to different time scales of electrochem. and concn. transients. In this work, we address this issue by investigating the time resoln. of the scanning flow cell (SFC). For this, residence time distribution (RTD) is estd. using Cu dissoln. expts. Both expts. and numerical simulations show that RTD can be closely approximated by a bi-Gaussian distribution with asymmetry arising from the mass transport of species in the outlet channel. Studying the influence of cell geometry and exptl. conditions on RTD, it is found that the length of the outlet tubes of SFC should be as short as possible. Moreover, an optimum flow rate and angle between inlet and outlet channels are defined. To demonstrate practical applicability of our findings, obtained RTD was used to deconvolute previously reported platinum dissoln. transients during cycling voltammetry. Such data are of high importance in mechanistic studies of platinum dissoln.
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  17. 17
    Benn, E. E.; Gaskey, B.; Erlebacher, J. D. Suppression of Hydrogen Evolution by Oxygen Reduction in Nanoporous Electrocatalysts. J. Am. Chem. Soc. 2017, 139 (10), 3663– 3668,  DOI: 10.1021/jacs.6b10855
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    17
    Suppression of Hydrogen Evolution by Oxygen Reduction in Nanoporous Electrocatalysts
    Benn Ellen E; Gaskey Bernard; Erlebacher Jonah D
    Journal of the American Chemical Society (2017), 139 (10), 3663-3668 ISSN:.
    Electroreduction of small molecules in aqueous solution often competes with the hydrogen evolution reaction (HER), especially if the reaction is driven even moderately hard using a large overpotential. Here, the oxygen reduction reaction (ORR) was studied under proton diffusion-limited conditions in slightly acidic electrolytes: a model system to study the relative transport kinetics of protons and reactants to an electrocatalyst and the relationship between transport and catalytic performance. Using dealloyed nanoporous nickel-platinum (np-NiPt) electrodes, we find the hydrogen evolution reaction can be completely suppressed even at high overpotentials (-400 mV vs RHE). In addition, the mechanism of oxygen reduction can be changed by using buffered versus unbuffered solutions, suggesting the reaction selectivity is associated with a transient rise (or lack thereof) in the interface pH at the np-NiPt surface. Independently controlling reactant transport to electrocatalyst surfaces at high overpotentials exhibited a surprisingly rich phenomenology that may offer a generalizable strategy to increase activity and selectivity during electroreduction reactions.
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  18. 18
    Geiger, S.; Cherevko, S.; Mayrhofer, K. J. J. Dissolution of Platinum in Presence of Chloride Traces. Electrochim. Acta 2015, 179, 24– 31,  DOI: 10.1016/j.electacta.2015.03.059
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    18
    Dissolution of Platinum in Presence of Chloride Traces
    Geiger, Simon; Cherevko, Serhiy; Mayrhofer, Karl J. J.
    Electrochimica Acta (2015), 179 (), 24-31CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    One of the main issues on the way to a com. use of fuel cells is the durability of the cell compartments, in particular electrocatalysts. Chloride impurities, originating from airborne salts or synthesis residues, can have severe effects on degrdn. in general. The quant. impact on dissoln. of Pt, which is the most common electrocatalyst material in polymer electrolyte membrane fuel cells, and the mechanism are still not fully understood. In the current work, potentiodynamic and potentiostatic measurements of platinum in acidic electrolytes contg. chlorides in the range of 1 to 1000 μM were carried out utilizing a scanning flow cell (SFC) coupled to an inductively coupled plasma mass spectrometer (ICP-MS). We show that during potentiodynamic measurements dissoln. accelerates with increase in amt. of chlorides, as expected. Similarly as in Cl--free electrolytes, dissoln. is a predominantly transient process taking place during oxidn. or redn. While thus in general the mechanism remains the same as reported before in the absence of chlorides, evidence for addnl. dissoln. processes during oxide formation and redn. are obsd. This leads to a variation in the ratio between anodic and cathodic dissoln. when chlorides are present. Based on the exptl. results a tentative dissoln. mechanism is proposed.
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  19. 19
    Shinozaki, K.; Zack, J. W.; Richards, R. M.; Pivovar, B. S.; Kocha, S. S. Oxygen Reduction Reaction Measurements on Platinum Electrocatalysts Utilizing Rotating Disk Electrode Technique. J. Electrochem. Soc. 2015, 162 (10), F1144– F1158,  DOI: 10.1149/2.1071509jes
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    19
    Oxygen Reduction Reaction Measurements on Platinum Electrocatalysts Utilizing Rotating Disk Electrode Technique
    Shinozaki, Kazuma; Zack, Jason W.; Richards, Ryan M.; Pivovar, Bryan S.; Kocha, Shyam S.
    Journal of the Electrochemical Society (2015), 162 (10), F1144-F1158CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    The rotating disk electrode (RDE) technique is being extensively used as a screening tool to est. the activity of novel PEMFC electrocatalysts synthesized in lab.-scale (mg) quantities. Discrepancies in measured activity attributable to glassware and electrolyte impurity levels, as well as conditioning, protocols and corrections are prevalent in the literature. The electrochem. response to a broad spectrum of com. sourced HClO4 and the effect of acid molarity on impurity levels and soln. resistance were also assessed. The authors' findings reveal that an area specific activity (SA) exceeding 2.0 mA/cm2 (20 mV/s, 25°, 100 kPa, 0.1 M HClO4) for polished poly-Pt is an indicator of impurity levels that do not impede the accurate measurement of the ORR activity of Pt based catalysts. After exploring various conditioning protocols to approach max. use of the electrochem. area (ECA) and peak ORR activity without introducing catalyst degrdn., a study of measurement protocols for ECA and ORR activity was conducted. Down-selected protocols were based on the criteria of reproducibility, duration of expts., impurity effects and magnitude of pseudo-capacitive background correction. Statistical reproducibility of ORR activity for poly-Pt and Pt supported on high surface area C was demonstrated.
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  20. 20
    Klemm, S. O.; Karschin, A.; Schuppert, A. K.; Topalov, A. A.; Mingers, A. M.; Katsounaros, I.; Mayrhofer, K. J. J. Time and potential resolved dissolution analysis of rhodium using a microelectrochemical flow cell coupled to an ICP-MS. J. Electroanal. Chem. 2012, 677–680, 50– 55,  DOI: 10.1016/j.jelechem.2012.05.006
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    20
    Time and potential resolved dissolution analysis of rhodium using a microelectrochemical flow cell coupled to an ICP-MS
    Klemm, Sebastian O.; Karschin, Arndt; Schuppert, Anna K.; Topalov, Angel A.; Mingers, Andrea M.; Katsounaros, Ioannis; Mayrhofer, Karl J. J.
    Journal of Electroanalytical Chemistry (2012), 677-680 (), 50-55CODEN: JECHES; ISSN:1873-2569. (Elsevier B.V.)
    The dissoln. of polycryst. Rh in 0.1 M H2SO4 is quant. studied during potential cycling and potential step expts. using a novel setup which consists of a microelectrochem. scanning flow cell (SFC) linked to an inductively coupled plasma-mass spectrometer (ICP-MS). The time-resolved dissoln. profile during electrochem. treatment is presented for the 1st time and used for quant. detn. of area-normalized dissoln. rates. A high time resoln. and very low detection limits allow distinguishing between anodic and cathodic dissoln. at a sufficiently low scan rate. The redn. of surface oxides triggers a significantly higher mass loss than the oxide formation (∼4-10 fold), and this ratio and the overall extent of dissoln. are dependent on the upper potential limit. Also, a strong scan rate dependence was obsd. for both the anodic and cathodic dissoln. peaks, with a decrease of the amt. of Rh dissolved per cycle with increasing scan rate and a min. at potentiostatic step expts. between the resp. potential limits. Clear evidence for steady state dissoln. of the oxidized surface is presented as well, even though this value ranges around a few femtograms per s and square centimeter at potentials up to 1.4 VSHE. The differences in dissoln. cover more than two orders of magnitude, potentially providing valuable information for electrode operating conditions and degrdn. mechanisms of noble metal materials.
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  21. 21
    Chen, Z.; Jaramillo, T. F.; Deutsch, T. G.; Kleiman-Shwarsctein, A.; Forman, A. J.; Gaillard, N.; Garland, R.; Takanabe, K.; Heske, C.; Sunkara, M.; McFarland, E. W.; Domen, K.; Miller, E. L.; Turner, J. A.; Dinh, H. N. Accelerating materials development for photoelectrochemical hydrogen production: Standards for methods, definitions, and reporting protocols. J. Mater. Res. 2010, 25 (1), 3– 16,  DOI: 10.1557/JMR.2010.0020
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    Accelerating materials development for photoelectrochemical hydrogen production: standards for methods, definitions, and reporting protocols
    Chen, Zhebo; Jaramillo, Thomas F.; Deutsch, Todd G.; Kleiman-Shwarsctein, Alan; Forman, Arnold J.; Gaillard, Nicolas; Garland, Roxanne; Takanabe, Kazuhiro; Heske, Clemens; Sunkara, Mahendra; McFarland, Eric W.; Domen, Kazunari; Miller, Eric L.; Turner, John A.; Dinh, Huyen N.
    Journal of Materials Research (2010), 25 (1), 3-16CODEN: JMREEE; ISSN:0884-2914. (Materials Research Society)
    A review. Photoelectrochem. (PEC) H2O splitting for H prodn. is a promising technol. that uses sunlight and H2O to produce renewable H with O as a byproduct. In the expanding field of PEC H prodn., the use of standardized screening methods and reporting has emerged as a necessity. This article is intended to provide guidance on key practices in characterization of PEC materials and proper reporting of efficiencies. Presented here are the definitions of various efficiency values that pertain to PEC, with an emphasis on the importance of solar-to-H efficiency, as well as a flow chart with std. procedures for PEC characterization techniques for planar photoelectrode materials (i.e., not suspensions of particles) with a focus on single band gap absorbers. These guidelines serve as a foundation and prelude to a much more complete and in-depth discussion of PEC techniques and procedures presented elsewhere.
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  22. 22
    Shi, X.; Cai, L.; Ma, M.; Zheng, X.; Park, J. H. General Characterization Methods for Photoelectrochemical Cells for Solar Water Splitting. ChemSusChem 2015, 8 (19), 3192– 203,  DOI: 10.1002/cssc.201500075
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    General Characterization Methods for Photoelectrochemical Cells for Solar Water Splitting
    Shi, Xinjian; Cai, Lili; Ma, Ming; Zheng, Xiaolin; Park, Jong Hyeok
    ChemSusChem (2015), 8 (19), 3192-3203CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Photoelectrochem. (PEC) water splitting is a very promising technol. that converts water into clean hydrogen fuel and oxygen by using solar light. However, the characterization methods for PEC cells are diverse and a systematic introduction to characterization methods for PEC cells has rarely been attempted. Unlike most other review articles that focus mainly on the material used for the working electrodes of PEC cells, this review introduces general characterization methods for PEC cells, including their basic configurations and methods for characterizing their performance under various conditions, regardless of the materials used. Detailed exptl. operation procedures with theor. information are provided for each characterization method. The PEC research area is rapidly expanding and more researchers are beginning to devote themselves to related work. Therefore, the content of this review can provide entry-level knowledge to beginners in the area of PEC, which might accelerate progress in this area.
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  23. 23
    Bedoya-Lora, F. E.; Holmes-Gentle, I.; Hankin, A. Electrochemical techniques for photoelectrode characterisation. Current Opinion in Green and Sustainable Chemistry 2021, 29, 100463,  DOI: 10.1016/j.cogsc.2021.100463
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    Nanba, T.; Takano, S.; Yasui, I.; Kudo, T. Structural study of peroxopolytungstic acid prepared from metallic tungsten and hydrogen peroxide. J. Solid State Chem. 1991, 90 (1), 47– 53,  DOI: 10.1016/0022-4596(91)90170-M
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    Structural study of peroxopolytungstic acid prepared from metallic tungsten and hydrogen peroxide
    Nanba, Tokuro; Takano, Sanae; Yasui, Itaru; Kudo, Tetsuichi
    Journal of Solid State Chemistry (1991), 90 (1), 47-53CODEN: JSSCBI; ISSN:0022-4596.
    The structure of peroxotungstic acid (W-PTA) prepd. from metallic W and aq. H2O2 was investigated based on Raman, IR, and x-ray diffraction analyses. W-PTA was an amorphous compd. constructed of peroxo polytungstate anions, in which the anions were bound to each other through H bonding. Radial distribution function analyses suggested that the polyanion was W12O38(O2)616-, in which a 6-membered ring of corner-shared polyhedra, such as WO5(O2) or WO6, was sandwiched by 2 W3O10 units consisting of edge-shared WO6.
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  25. 25
    Kwong, W. L.; Savvides, N.; Sorrell, C. C. Electrodeposited nanostructured WO3 thin films for photoelectrochemical applications. Electrochim. Acta 2012, 75, 371– 380,  DOI: 10.1016/j.electacta.2012.05.019
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    25
    Electrodeposited nanostructured WO3 thin films for photoelectrochemical applications
    Kwong, W. L.; Savvides, N.; Sorrell, C. C.
    Electrochimica Acta (2012), 75 (), 371-380CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    Thin films of WO3 were deposited on FTO-coated glass substrates by electrodeposition using aq. solns. of peroxotungstic acid. The effects of varying the W concn. of peroxotungstic acid and deposition time on the mineralogical, microstructural, morphol., optical, and photoelectrochem. properties were detd. using x-ray diffraction, SEM, focused ion beam milling, UV-visible spectrophotometry, and linear potentiodynamic voltammetry, resp. The films consisted of monoclinic WO3 of grain sizes in the range 77-122 nm and thicknesses in the range 258-1394 nm; the true porosities were <5%. These microstructural and morphol. parameters depended largely upon the W concn. and deposition time. Some preferred orientation was obsd. and this was considered to result from crystallog. and microstructural factors. The optical transmission data revealed significant decreases in the optical indirect band gap, from 3.05 eV to 2.60 eV, as a function of increasing film thickness. This was considered to result from differential contributions from the surface and bulk band gap components as well as compressive stress. The voltammetry data and assocd. Butler plot revealed the establishment of a Schottky depletion layer and a flat-band potential of +0.2 V to +0.3 V vs. Ag/AgCl. Although the calcd. photoconversion efficiencies were in the range 0.02-0.14%, which is commensurate using a W-halogen light rather than Xe, there was a trend of increasing efficiency as a function of increasing film thickness. This was attributed to decreasing band gap and increasing light absorption. The shape of the curve of the preceding data supports the conclusion of differential contributions from the surface and bulk band gap components. Finally, evidence of photolysis in the absence of an external applied potential suggests the importance of the effect of grain size on the pH and its alteration of the flat-band potential.
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    Vidmar, T.; Topič, M.; Dzik, P.; Opara Krašovec, U. Inkjet printing of sol-gel derived tungsten oxide inks. Sol. Energy Mater. Sol. Cells 2014, 125, 87– 95,  DOI: 10.1016/j.solmat.2014.02.023
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    Inkjet printing of sol-gel derived tungsten oxide inks
    Vidmar, Tjasa; Topic, Marko; Dzik, Petr; Opara Krasovec, Ursa
    Solar Energy Materials & Solar Cells (2014), 125 (), 87-95CODEN: SEMCEQ; ISSN:0927-0248. (Elsevier B.V.)
    Tungsten (VI) oxide-WO3 is a widely studied functional inorg. semiconductor material with exceptional chromogenic properties. It is used in energy efficient systems such as smart windows, sensors, displays, storage units, photocatalysts and solar cells. Layers of WO3 are generally produced using expensive vacuum deposition techniques. In this paper, the inkjet printing of sol-gel derived tungsten inks on glass and transparent conductive oxide is reported. Peroxo sol-gel synthesis was used to prep. peroxopolytungstic acid sols which were then modified using different solvents to obtain a suitable jetting ink. Described are the rheol. and structural properties of WO3 inks, the dynamics of WO3 droplets and the morphol. and quality of WO3 printouts. The functionality of these transparent WO3 layers is successfully demonstrated in an electrochromic device.
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    Krishnan, R. Fundamentals of Semiconductor Electrochemistry and Photoelectrochemistry. In Encyclopedia of Electrochemistry; Bard, A.J., Ed.; Wiley, 2007.
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    White, J. R.; Fan, F. R. F.; Bard, A. J. Semiconductor Electrodes: LVI. Principles of Multijunction Electrodes and Photoelectrosynthesis at Texas Instruments’ p/n-Si Solar Arrays. J. Electrochem. Soc. 1985, 132 (3), 544– 550,  DOI: 10.1149/1.2113884
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    Semiconductor electrodes. LVI. Principles of multijunction electrodes and photoelectrosynthesis at Texas Instruments' p/n-silicon solar arrays
    White, James R.; Fan, Fu Ren F.; Bard, Allen J.
    Journal of the Electrochemical Society (1985), 132 (3), 544-50CODEN: JESOAN; ISSN:0013-4651.
    Multilayer semiconductor electrode structures involving several photoactive junctions, such as the title (TI) arrays based on Si p/n junctions, can show stable operation and produce sufficient photovoltages to promote energetic reactions, such as the decompn. of HCl to H and Cl. The current-voltage (I-V) behavior at the interface with the soln. is an important component of the cell performance, and metal surfaces can be selected to stabilize the electrode from photodecompn. and to catalyze the desired reactions. The behavior of multijunction devices can be obtained graphically from the I-V characteristics of each junction. Multijunction structures, such as coupled TI arrays, can produce photovoltages >2 V, and can drive energetic reactions. The application of these arrays to nonaq. solns. was also illustrated. Reactions considered include generation of Cl with the redn. of O or the generation of H, the photobromination of PhOH [108-95-2], and the photochlorination of cyclohexene [110-83-8] in MeCN.
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    Hankin, A.; Bedoya-Lora, F. E.; Alexander, J. C.; Regoutz, A.; Kelsall, G. H. Flat band potential determination: avoiding the pitfalls. J. Mater. Chem. A 2019, 7 (45), 26162– 26176,  DOI: 10.1039/C9TA09569A
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    Flat band potential determination: avoiding the pitfalls
    Hankin, Anna; Bedoya-Lora, Franky E.; Alexander, John C.; Regoutz, Anna; Kelsall, Geoff H.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2019), 7 (45), 26162-26176CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    The flat band potential is one of the key characteristics of photoelectrode performance. However, its detn. on nanostructured materials is assocd. with considerable uncertainty. The complexity, applicability and pitfalls assocd. with the four most common exptl. techniques used for evaluating flat band potentials, are illustrated using nanostructured synthetic hematite (α-Fe2O3) in strongly alk. solns. as a case study. The motivation for this study was the large variance in flat band potential values reported for synthetic hematite electrodes that could not be justified by differences in exptl. conditions, or by differences in their charge carrier densities. We demonstrate through theory and expts. that different flat band potential detn. methods can yield widely different results, so could mislead the anal. of the photoelectrode performance. We have examd.: (a) application of the Mott-Schottky (MS) equation to the interfacial capacitance, detd. by electrochem. impedance spectroscopy as a function of electrode potential and potential perturbation frequency; (b) Gartner-Butler (GB) anal. of the square of the photocurrent as a function of electrode potential; (c) detn. of the potential of transition between cathodic and anodic photocurrents during slow potentiodynamic scans under chopped illumination (CI); (d) open circuit electrode potential (OCP) under high irradiance. Methods GB, CI and OCP were explored in absence and presence of H2O2 as hole scavenger. The CI method was found to give reproducible and the most accurate results on hematite but our overall conclusion and recommendation is that multiple methods should be employed for verifying a reported flat band potential.
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  30. 30
    Amano, F.; Tian, M.; Wu, G.; Ohtani, B.; Chen, A. Facile preparation of platelike tungsten oxide thin film electrodes with high photoelectrode activity. ACS Appl. Mater. Interfaces 2011, 3 (10), 4047– 52,  DOI: 10.1021/am200897n
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    Facile Preparation of Platelike Tungsten Oxide Thin Film Electrodes with High Photoelectrode Activity
    Amano, Fumiaki; Tian, Min; Wu, Guo-Sheng; Ohtani, Bunsho; Chen, Ai-Cheng
    ACS Applied Materials & Interfaces (2011), 3 (10), 4047-4052CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    W trioxide (WO3) thin film electrodes with platelike structures were prepd. by a facile hydrothermal reaction of W sheets in a dil. HNO3 soln. at 100-180° and subsequent calcination at 450°. The calcination step facilitated the transformation of the crystal structure from W oxide hydrates (WO3·H2O or WO3·2H2O) to monoclinic WO3 without significant modification to the platelike structures. The photoelectrochem. performance of the thin film electrodes for H2O splitting that took place in a dil. H2SO4 was strongly dependent on both temp. and the time used for the hydrothermal reaction. Probably the thickness of the film influences the process of photoexcited electron transport. The time required for the hydrothermal reaction under higher temps. was reduced in the generation of thin film electrodes with high photoelectrode activity, because the crystal growth is accelerated at high temps. and the electron transport is restricted by a relatively thick compact layer that is comprised of WO3 nanoparticulates. The electrode exhibited sensitivity to the violet portion of the visible light spectrum due to the bandgap of 2.8 eV and high photoelectrode efficiency, as well as an incident photon-to-current conversion efficiency (IPCE) of 66.2%, for the photoelectrochem. oxidn. of H2O.
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    Cai, M.; Fan, P.; Long, J.; Han, J.; Lin, Y.; Zhang, H.; Zhong, M. Large-Scale Tunable 3D Self-Supporting WO3Micro-Nano Architectures as Direct Photoanodes for Efficient Photoelectrochemical Water Splitting. ACS Appl. Mater. Interfaces 2017, 9 (21), 17856– 17864,  DOI: 10.1021/acsami.7b02386
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    31
    Large-Scale Tunable 3D Self-Supporting WO3 Micro-Nano Architectures as Direct Photoanodes for Efficient Photoelectrochemical Water Splitting
    Cai, Mingyong; Fan, Peixun; Long, Jiangyou; Han, Jinpeng; Lin, Yi; Zhang, Hongjun; Zhong, Minlin
    ACS Applied Materials & Interfaces (2017), 9 (21), 17856-17864CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Hydrogen prodn. from water based on photoelectrochem. (PEC) reactions is feasible to solve the urgent energy crisis. Herein, hierarchical 3D self-supporting WO3 micro-nano architectures in situ grown on W plates are successfully fabricated via ultrafast laser processing hybrid with thermal oxidn. Owing to the large surface area and efficient interface charge transfer, the W plate with hierarchical porous WO3 nanoparticle aggregates has been directly employed as the photoanode for excellent PEC performance, which exhibits a high photocurrent d. of 1.2 mA cm-2 at 1.0 V vs Ag/AgCl (1.23 V vs RHE) under AM 1.5 G illumination and reveals excellent structural stability during long-term PEC water splitting reactions. The nanoscale and microscale features can be facilely tuned by controlling the laser processing parameters and the thermal oxidn. conditions to achieve improved PEC activity. The presented hybrid method is simple, cost-effective, and controllable for large-scale fabrication, which should provide a new and general route that how the properties of conventional metal oxides can be improved via hierarchical 3D micro-nano configurations.
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    Santato, C.; Ulmann, M.; Augustynski, J. Photoelectrochemical Properties of Nanostructured Tungsten Trioxide Films. J. Phys. Chem. B 2001, 105 (5), 936– 940,  DOI: 10.1021/jp002232q
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    Photoelectrochemical Properties of Nanostructured Tungsten Trioxide Films
    Santato, Clara; Ulmann, Martine; Augustynski, Jan
    Journal of Physical Chemistry B (2001), 105 (5), 936-940CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    The photoelectrochem. characteristics of highly transparent nanoporous WO3 films are described. The photocurrent vs. excitation wavelength spectra of these photoelectrodes exhibit a max. close to 400 nm and a significant photoresponse to the blue part of the visible spectrum. The obsd. conversion efficiencies attain 75% for the photogeneration of oxygen from 1M aq. HClO4 and reach 190% in the presence of methanol in the soln., denoting in the latter case the occurrence of a perfect photocurrent doubling. Expts. conducted under simulated solar AM 1.5 illumination resulted in steady-state anodic photocurrents of the order of several mA/cm2.
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  • Abstract

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    Scheme 1

    Scheme 1. Schematic Illustration of PEC-ICP-MS Setup Employed in This Studya
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    aThe optical components consist of: (I) 300 W ozone-free Xe lamp, (II) focusing lens, (III) AM 1.5 G filter, (IV) monochromator, (V) filter wheel with different cutoff filters, (VI) focusing assembly, and (VII) liquid light guide.

    Figure 1

    Figure 1. Cross-sectional 3D design of a PEC-SFC. (I) Electrolyte outlet channel, (II) reference electrode channel, (III) inert gas pocket, (IV) liquid light guide channel, (V) electrolyte inlet channel, and (VI) inert gas inlet.

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    Figure 2

    Figure 2. Output spectrum of PEC-SFC setup operated in (a) monochromatic mode and (b) bypass mode. All spectra were recorded at the PEC-SFC opening after the light has traveled through a 0.1 M HClO4 solution.

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    Figure 3

    Figure 3. (a) Photopotential under dark and illuminated state. (b) Photovoltammogram recorded with a scan rate of 10 mV s–1 and a chopping frequency of 0.2 Hz. Inset showing magnified potential region around onset potential. (c) IPCE measurement at 1.23 VRHE. (d) (IPCE hν)0.5 vs hν (gray) with linear fit (red). All measurements were conducted on spray-coated WO3 in Ar-saturated 0.1 M HClO4. (a, b) were measured with the monochromator in bypass mode to access the polychromatic spectrum of the solar simulator calibrated to 1 Sun and (c) was measured using the monochromatic mode.

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    Figure 4

    Figure 4. Current density and dissolution profile of spray-coated WO3 under dark and 1 Sun AM 1.5 G illuminated condition in 0.1 M HClO4. (a) Potential protocol with a scan rate of 10 mV s–1 for the LSVs. CA holds were performed for two minutes. Upper potential limit for all LSVs and CA holds was 1.5 VRHE. (b) Measured current density. (c) Dissolution rate of W (gray, measured signal; dashed red, linear fit of dissolution rate before and after dissolution rate increase; dashed gray, transition in dissolution rate). Yellow bars indicate the period of illumination.

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      Topalov, Angel A.; Cherevko, Serhiy; Zeradjanin, Aleksandar R.; Meier, Josef C.; Katsounaros, Ioannis; Mayrhofer, Karl J. J.
      Chemical Science (2014), 5 (2), 631-638CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Platinum is one of the most important electrode materials for continuous electrochem. energy conversion due to its high activity and stability. The resistance of this scarce material towards dissoln. is however limited under the harsh operational conditions that can occur in fuel cells or other energy conversion devices. In order to improve the understanding of dissoln. of platinum, we therefore investigate this issue with an electrochem. flow cell system connected to an inductively coupled plasma mass spectrometer (ICP-MS) capable of online quantification of even small traces of dissolved elements in soln. The electrochem. data combined with the downstream analytics are used to evaluate the influence of various operational parameters on the dissoln. processes in acidic electrolytes at room temp. Platinum dissoln. is a transient process, occurring during both pos.- and neg.-going sweeps over potentials of ∼1.1 VRHE and depending strongly on the structure and chem. of the formed oxide. The amt. of anodically dissolved platinum is thereby strongly related to the no. of low-coordinated surface sites, whereas cathodic dissoln. depends on the amt. of oxide formed and the timescale. Thus, a tentative mechanism for Pt dissoln. is suggested based on a place exchange of oxygen atoms from surface to sub-surface positions.
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    6. 6
      Klemm, S. O.; Topalov, A. A.; Laska, C. A.; Mayrhofer, K. J. J. Coupling of a high throughput microelectrochemical cell with online multielemental trace analysis by ICP-MS. Electrochem. Commun. 2011, 13 (12), 1533– 1535,  DOI: 10.1016/j.elecom.2011.10.017
      6
      Coupling of a high throughput microelectrochemical cell with online multielemental trace analysis by ICP-MS
      Klemm, Sebastian O.; Topalov, Angel A.; Laska, Claudius A.; Mayrhofer, Karl J. J.
      Electrochemistry Communications (2011), 13 (12), 1533-1535CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
      The successful coupling of a specially designed microelectrochem. cell and direct online multi-elemental trace anal. by ICP-MS is presented. The feasibility of this method is demonstrated by the example of copper dissoln. in HCl (1 and 10 mM), showing very high sensitivity and an excellent congruency between electrochem. expts. and copper concns. detected downstream. The complementary data allows for a precise detn. of the valence of Cu ions released during anodic dissoln., which undergoes changes depending on the electrolyte and the applied c.d. Moreover it provides a means of quantification of processes without net external currents that are not readily accessible by plain electrochem. techniques, in particular the exchange current densities at the open circuit potential, i.e. corrosion rate, and the dissoln. of native oxides. The system presented combines full spectrum electrochem. capabilities, convection control, and highly sensitive electrolyte anal. in an integrated, miniaturized arrangement under full computer control and automation.
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    7. 7
      Knöppel, J.; Zhang, S.; Speck, F. D.; Mayrhofer, K. J. J.; Scheu, C.; Cherevko, S. Time-resolved analysis of dissolution phenomena in photoelectrochemistry - A case study of WO3 photocorrosion. Electrochem. Commun. 2018, 96, 53– 56,  DOI: 10.1016/j.elecom.2018.09.008
      There is no corresponding record for this reference.
    8. 8
      Dworschak, D.; Brunnhofer, C.; Valtiner, M. Photocorrosion of ZnO Single Crystals during Electrochemical Water Splitting. ACS Appl. Mater. Interfaces 2020, 12 (46), 51530– 51536,  DOI: 10.1021/acsami.0c15508
      8
      Photocorrosion of ZnO Single Crystals during Electrochemical Water Splitting
      Dworschak, Dominik; Brunnhofer, Carina; Valtiner, Markus
      ACS Applied Materials & Interfaces (2020), 12 (46), 51530-51536CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Degrdn. and dissoln. of transparent semiconducting oxides is central to various areas, including design of catalysts and catalysis conditions, as well as passivation of metal surfaces. In particular, photocorrosion can be significant and plays a central role during photoelectrochem. activity of transparent semiconducting oxides. Here, we utilize an electrochem. flow cell combined with an inductively coupled plasma mass spectrometer (ICP-MS) to enable the in situ study of the time-resolved release of zinc into soln. under simultaneous radiation of UV-light. With this system we study the dissoln. of zinc oxide single crystals with (0001) and (1010) orientations. At acidic and alk. pH, we characterized potential dependent dissoln. rates into both the oxygen and the hydrogen evolving conditions. A significant influence of the UV radiation and the pH of the electrolyte was obsd. The obsd. dissoln. behavior agrees well with the surface chem. and stabilization mechanism of ZnO surfaces. In particular, polar ZnO(0001) shows ideal stability at low potentials and under hydrogen evolution conditions. Whereas ZnO(1010) sustains higher dissoln. rates, while it is inactive for water splitting. Our data demonstrates that surface design and fundamental understanding of surface chem. provides an effective path to rendering electroactive surfaces stable under operating conditions.
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    9. 9
      Zhang, S.; Rohloff, M.; Kasian, O.; Mingers, A. M.; Mayrhofer, K. J. J.; Fischer, A.; Scheu, C.; Cherevko, S. Dissolution of BiVO4 Photoanodes Revealed by Time-Resolved Measurements under Photoelectrochemical Conditions. J. Phys. Chem. C 2019, 123 (38), 23410– 23418,  DOI: 10.1021/acs.jpcc.9b07220
      9
      Dissolution of BiVO4 Photoanodes Revealed by Time-Resolved Measurements under Photoelectrochemical Conditions
      Zhang, Siyuan; Rohloff, Martin; Kasian, Olga; Mingers, Andrea M.; Mayrhofer, Karl J. J.; Fischer, Anna; Scheu, Christina; Cherevko, Serhiy
      Journal of Physical Chemistry C (2019), 123 (38), 23410-23418CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Photocorrosion imposes a fundamental limit to the longevity of devices that harvest energy from photons. As one of the best performing electrode materials for photoelectrochem. water oxidn. reaction, BiVO4 undergoes photocorrosion with various postulated mechanisms under debate. The time-resolved dissoln. measurements are presented to advance the mechanistic understanding, enabled by the recent development in illuminated scanning flow cell coupled to inductively coupled plasma mass spectrometry. The contact dissoln. of predominantly V was distinguished from the stoichiometric photoelectrochem. dissoln. of Bi and V. The citrate electrolyte was utilized to form sol. complexes with dissolved Bi and to act as hole scavengers that provide photocurrents at a wide range of potentials. The photoelectrochem. dissoln. rates remain similar between 0.4-1.6 V vs. reversible hydrogen electrode and become lower at the open circuit potential, 0.2 V. The time-resolved measurements support oxidn. of Bi(III) by photogenerated holes as the main mechanism for photocorrosion.
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    10. 10
      Sliozberg, K.; Schafer, D.; Erichsen, T.; Meyer, R.; Khare, C.; Ludwig, A.; Schuhmann, W. High-throughput screening of thin-film semiconductor material libraries I: system development and case study for Ti-W-O. ChemSusChem 2015, 8 (7), 1270– 8,  DOI: 10.1002/cssc.201402917
      10
      High-Throughput Screening of Thin-Film Semiconductor Material Libraries I: System Development and Case Study for Ti-W-O
      Sliozberg, Kirill; Schaefer, Dominik; Erichsen, Thomas; Meyer, Robert; Khare, Chinmay; Ludwig, Alfred; Schuhmann, Wolfgang
      ChemSusChem (2015), 8 (7), 1270-1278CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      An automated optical scanning droplet cell (OSDC) enables high-throughput quant. characterization of thin-film semiconductor material libraries. Photoelectrochem. data on small selected measurement areas are recorded including intensity-dependent photopotentials and currents, potentiodynamic and potentiostatic photocurrents, as well as photocurrent (action) spectra. The OSDC contains integrated counter and double-junction ref. electrodes and is fixed on a precise positioning system. A Xe lamp with a monochromator is coupled to the cell through a thin poly(Me methacrylate) (PMMA) optical fiber. A specifically designed polytetrafluoroethylene (PTFE) capillary tip is pressed on the sample surface and defines through its diam. the homogeneously illuminated measurement area. The overall and wavelength-resolved irradn. intensities and the cell surface area are precisely detd. and calibrated. System development and its performance are demonstrated by means of screening of a Ti-W-O thin film.
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    11. 11
      Choi, Y.; Yim, C.; Baek, S.; Choi, M.; Jeon, S.; Yong, K. In situ measurement of photostability of CdSe/CdS/ZnO nanowires photoelectrode for photoelectrochemical water splitting. Sens. Actuators, B 2015, 221, 113– 119,  DOI: 10.1016/j.snb.2015.06.092
      11
      In situ measurement of photostability of CdSe/CdS/ZnO nanowires photoelectrode for photoelectrochemical water splitting
      Choi, Youngwoo; Yim, Changyong; Baek, Seunghyeon; Choi, Mingi; Jeon, Sangmin; Yong, Kijung
      Sensors and Actuators, B: Chemical (2015), 221 (), 113-119CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
      Quantum dot sensitized photoanodes have drawn much attention due to their high potential as efficient anodes for photoelectrochem. (PEC) water splitting or solar cells. However the photocorrosion of QDs is the crucial barrier for applications in these devices. The in situ anal. of photocorrosion is important in understanding its mechanism and also developing the possible soln. for photocorrosion. In this study we have developed a novel, integrated anal. system for in situ measurements of photocorrosion and PEC performances. We have fabricated the CdSe/CdS/ZnO nanowire (NW) arrays on quartz crystal microbalance (QCM) as a platform for usage as PEC photoanodes and also mass anal. at the same time. The in situ measuring of photocurrents and mass changes were performed with continuous operation of PEC cells for CdSe/CdS/ZnO NWs photoanode. The study exhibited highly correlated tendency in photocurrent decrease and mass redn., due to photocorrosion of CdSe/CdS/ZnO NWs. Also to improve the photostability of CdSe/CdS/ZnO NWs, applications of passivation and catalysts were studied and their effects were discussed. Our integrated in situ anal. system is highly applicable to various semiconductor sensitized systems.
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    12. 12
      Wang, Y.; Tian, W.; Chen, C.; Xu, W.; Li, L. Tungsten Trioxide Nanostructures for Photoelectrochemical Water Splitting: Material Engineering and Charge Carrier Dynamic Manipulation. Adv. Funct. Mater. 2019, 29 (23), 1809036,  DOI: 10.1002/adfm.201809036
      There is no corresponding record for this reference.
    13. 13
      Zheng, G.; Wang, J.; Liu, H.; Murugadoss, V.; Zu, G.; Che, H.; Lai, C.; Li, H.; Ding, T.; Gao, Q.; Guo, Z. Tungsten oxide nanostructures and nanocomposites for photoelectrochemical water splitting. Nanoscale 2019, 11 (41), 18968– 18994,  DOI: 10.1039/C9NR03474A
      13
      Tungsten oxide nanostructures and nanocomposites for photoelectrochemical water splitting
      Zheng, Guangwei; Wang, Jinshu; Liu, Hu; Murugadoss, Vignesh; Zu, Guannan; Che, Haibing; Lai, Chen; Li, Hongyi; Ding, Tao; Gao, Qiang; Guo, Zhanhu
      Nanoscale (2019), 11 (41), 18968-18994CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Hydrogen prodn. from photoelectrochem. (PEC) water splitting using semiconductor photocatalysts has attracted great attention to realize clean and renewable energy from solar energy. The visible light response of WO3 with a long hole diffusion length (∼150 nm) and good electron mobility (∼12 cm2 V-1 s-1) makes it suitable as the photoanode. However, WO3 suffers from issues including rapid recombination of photoexcited electron-hole pairs, photo-corrosion during the photocatalytic process due to the formation of peroxo-species, sluggish kinetics of photogenerated holes, and slow charge transfer at the semiconductor/electrolyte interface. This work highlights the approaches to overcome these drawbacks of WO3 photoanodes, including: (i) the manipulation of nanostructured WO3 photoanodes to decrease the nanoparticle size to promote hole migration to the WO3/electrolyte interface which benefits the charge sepn.; (ii) doping or introducing oxygen vacancies to improve elec. cond.; exposing high energy crystal surfaces to promote the consumption of photogenerated holes on the high-active crystal face, thereby suppressing the recombination of photogenerated electrons and holes; (iii) decorating with co-catalysts to reduce the overpotential which inhibits the formation of peroxo-species; (iv) other methods such as coupling with narrow band semiconductors to accelerate the charge sepn. and controlling the crystal phase via annealing to reduce defects. These approaches are reviewed with detailed examples.
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    14. 14
      Schuppert, A. K.; Topalov, A. A.; Katsounaros, I.; Klemm, S. O.; Mayrhofer, K. J. J. A Scanning Flow Cell System for Fully Automated Screening of Electrocatalyst Materials. J. Electrochem. Soc. 2012, 159 (11), F670– F675,  DOI: 10.1149/2.009211jes
      14
      A scanning flow cell system for fully automated screening of electrocatalyst materials
      Schuppert, Anna K.; Topalov, Angel A.; Katsounaros, Ioannis; Klemm, Sebastian O.; Mayrhofer, Karl J. J.
      Journal of the Electrochemical Society (2012), 159 (11), F670-F675CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      Electrocatalysts play an important role in sustainable energy-related fields. As these catalysts still require improvement on activity and stability, a lot of effort is invested in developing new materials. Due to the enormous parameter space spanned by material compn. and exptl. conditions, there is a great demand for high-throughput screening of the material performance. To address this issue, a scanning flow cell (SFC) is developed and utilized for catalyst research for the first time. The adaptation of a homemade LabVIEW program to the SFC setup enables fully automated, computer-controlled high-throughput measurements. A gas purging system is introduced to sat. the electrolyte with gases in a time frame comparable to typical rotating disk electrode (RDE) systems. To demonstrate the capabilities of the setup, the oxygen redn. reaction on polycryst. platinum is investigated as a sample system. The cyclic voltammograms are consistent with those expected from investigations in conventional electrochem. cells. Repetitive measurements show high reproducibility. Considering the potential of the system toward improvements and extensions, the SFC will be a valuable screening tool for electrocatalyst research.
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    15. 15
      Grote, J. P.; Zeradjanin, A. R.; Cherevko, S.; Mayrhofer, K. J. Coupling of a scanning flow cell with online electrochemical mass spectrometry for screening of reaction selectivity. Rev. Sci. Instrum. 2014, 85 (10), 104101,  DOI: 10.1063/1.4896755
      15
      Coupling of a scanning flow cell with online electrochemical mass spectrometry for screening of reaction selectivity
      Grote, Jan-Philipp; Zeradjanin, Aleksandar R.; Cherevko, Serhiy; Mayrhofer, Karl J. J.
      Review of Scientific Instruments (2014), 85 (10), 104101/1-104101/5CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
      In this work the online coupling of a miniaturized electrochem. scanning flow cell (SFC) to a mass spectrometer is introduced. The system is designed for the detn. of reaction products in dependence of the applied potential and/or current regime as well as fast and automated change of the sample. The reaction products evap. through a hydrophobic PTFE membrane into a small vacuum probe, which is positioned only 50-100 μm away from the electrode surface. The probe is implemented into the SFC and directly connected to the mass spectrometer. This unique configuration enables fast parameter screening for complex electrochem. reactions, including investigation of operation conditions, compn. of electrolyte, and material compn. The tech. developments of the system are validated by initial measurements of hydrogen evolution during water electrolysis and electrochem. redn. of CO2 to various products, showcasing the high potential for systematic combinatorial screening by this approach. (c) 2014 American Institute of Physics.
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    16. 16
      Shkirskiy, V.; Speck, F. D.; Kulyk, N.; Cherevko, S. On the Time Resolution of Electrochemical Scanning Flow Cell Coupled to Downstream Analysis. J. Electrochem. Soc. 2019, 166 (16), H866– H870,  DOI: 10.1149/2.1401915jes
      16
      On the time resolution of electrochemical scanning flow cell coupled to downstream analysis
      Shkirskiy, Viacheslav; Speck, Florian Dominik; Kulyk, Nadiia; Cherevko, Serhiy
      Journal of the Electrochemical Society (2019), 166 (16), H866-H870CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      Many of the recent advancements in the electrocatalysis research have been obtained by application of coupled electrochem./massspectrometry techniques. Representative example is the electrochem. flow cells coupled to inductively coupled plasma mass spectrometry (online ICP-MS) in electrocatalysis stability research. In this technique, unambiguous correlation of the concn. of dissolved species vs. potential/current represents a significant challenge due to different time scales of electrochem. and concn. transients. In this work, we address this issue by investigating the time resoln. of the scanning flow cell (SFC). For this, residence time distribution (RTD) is estd. using Cu dissoln. expts. Both expts. and numerical simulations show that RTD can be closely approximated by a bi-Gaussian distribution with asymmetry arising from the mass transport of species in the outlet channel. Studying the influence of cell geometry and exptl. conditions on RTD, it is found that the length of the outlet tubes of SFC should be as short as possible. Moreover, an optimum flow rate and angle between inlet and outlet channels are defined. To demonstrate practical applicability of our findings, obtained RTD was used to deconvolute previously reported platinum dissoln. transients during cycling voltammetry. Such data are of high importance in mechanistic studies of platinum dissoln.
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    17. 17
      Benn, E. E.; Gaskey, B.; Erlebacher, J. D. Suppression of Hydrogen Evolution by Oxygen Reduction in Nanoporous Electrocatalysts. J. Am. Chem. Soc. 2017, 139 (10), 3663– 3668,  DOI: 10.1021/jacs.6b10855
      17
      Suppression of Hydrogen Evolution by Oxygen Reduction in Nanoporous Electrocatalysts
      Benn Ellen E; Gaskey Bernard; Erlebacher Jonah D
      Journal of the American Chemical Society (2017), 139 (10), 3663-3668 ISSN:.
      Electroreduction of small molecules in aqueous solution often competes with the hydrogen evolution reaction (HER), especially if the reaction is driven even moderately hard using a large overpotential. Here, the oxygen reduction reaction (ORR) was studied under proton diffusion-limited conditions in slightly acidic electrolytes: a model system to study the relative transport kinetics of protons and reactants to an electrocatalyst and the relationship between transport and catalytic performance. Using dealloyed nanoporous nickel-platinum (np-NiPt) electrodes, we find the hydrogen evolution reaction can be completely suppressed even at high overpotentials (-400 mV vs RHE). In addition, the mechanism of oxygen reduction can be changed by using buffered versus unbuffered solutions, suggesting the reaction selectivity is associated with a transient rise (or lack thereof) in the interface pH at the np-NiPt surface. Independently controlling reactant transport to electrocatalyst surfaces at high overpotentials exhibited a surprisingly rich phenomenology that may offer a generalizable strategy to increase activity and selectivity during electroreduction reactions.
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    18. 18
      Geiger, S.; Cherevko, S.; Mayrhofer, K. J. J. Dissolution of Platinum in Presence of Chloride Traces. Electrochim. Acta 2015, 179, 24– 31,  DOI: 10.1016/j.electacta.2015.03.059
      18
      Dissolution of Platinum in Presence of Chloride Traces
      Geiger, Simon; Cherevko, Serhiy; Mayrhofer, Karl J. J.
      Electrochimica Acta (2015), 179 (), 24-31CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      One of the main issues on the way to a com. use of fuel cells is the durability of the cell compartments, in particular electrocatalysts. Chloride impurities, originating from airborne salts or synthesis residues, can have severe effects on degrdn. in general. The quant. impact on dissoln. of Pt, which is the most common electrocatalyst material in polymer electrolyte membrane fuel cells, and the mechanism are still not fully understood. In the current work, potentiodynamic and potentiostatic measurements of platinum in acidic electrolytes contg. chlorides in the range of 1 to 1000 μM were carried out utilizing a scanning flow cell (SFC) coupled to an inductively coupled plasma mass spectrometer (ICP-MS). We show that during potentiodynamic measurements dissoln. accelerates with increase in amt. of chlorides, as expected. Similarly as in Cl--free electrolytes, dissoln. is a predominantly transient process taking place during oxidn. or redn. While thus in general the mechanism remains the same as reported before in the absence of chlorides, evidence for addnl. dissoln. processes during oxide formation and redn. are obsd. This leads to a variation in the ratio between anodic and cathodic dissoln. when chlorides are present. Based on the exptl. results a tentative dissoln. mechanism is proposed.
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    19. 19
      Shinozaki, K.; Zack, J. W.; Richards, R. M.; Pivovar, B. S.; Kocha, S. S. Oxygen Reduction Reaction Measurements on Platinum Electrocatalysts Utilizing Rotating Disk Electrode Technique. J. Electrochem. Soc. 2015, 162 (10), F1144– F1158,  DOI: 10.1149/2.1071509jes
      19
      Oxygen Reduction Reaction Measurements on Platinum Electrocatalysts Utilizing Rotating Disk Electrode Technique
      Shinozaki, Kazuma; Zack, Jason W.; Richards, Ryan M.; Pivovar, Bryan S.; Kocha, Shyam S.
      Journal of the Electrochemical Society (2015), 162 (10), F1144-F1158CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      The rotating disk electrode (RDE) technique is being extensively used as a screening tool to est. the activity of novel PEMFC electrocatalysts synthesized in lab.-scale (mg) quantities. Discrepancies in measured activity attributable to glassware and electrolyte impurity levels, as well as conditioning, protocols and corrections are prevalent in the literature. The electrochem. response to a broad spectrum of com. sourced HClO4 and the effect of acid molarity on impurity levels and soln. resistance were also assessed. The authors' findings reveal that an area specific activity (SA) exceeding 2.0 mA/cm2 (20 mV/s, 25°, 100 kPa, 0.1 M HClO4) for polished poly-Pt is an indicator of impurity levels that do not impede the accurate measurement of the ORR activity of Pt based catalysts. After exploring various conditioning protocols to approach max. use of the electrochem. area (ECA) and peak ORR activity without introducing catalyst degrdn., a study of measurement protocols for ECA and ORR activity was conducted. Down-selected protocols were based on the criteria of reproducibility, duration of expts., impurity effects and magnitude of pseudo-capacitive background correction. Statistical reproducibility of ORR activity for poly-Pt and Pt supported on high surface area C was demonstrated.
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    20. 20
      Klemm, S. O.; Karschin, A.; Schuppert, A. K.; Topalov, A. A.; Mingers, A. M.; Katsounaros, I.; Mayrhofer, K. J. J. Time and potential resolved dissolution analysis of rhodium using a microelectrochemical flow cell coupled to an ICP-MS. J. Electroanal. Chem. 2012, 677–680, 50– 55,  DOI: 10.1016/j.jelechem.2012.05.006
      20
      Time and potential resolved dissolution analysis of rhodium using a microelectrochemical flow cell coupled to an ICP-MS
      Klemm, Sebastian O.; Karschin, Arndt; Schuppert, Anna K.; Topalov, Angel A.; Mingers, Andrea M.; Katsounaros, Ioannis; Mayrhofer, Karl J. J.
      Journal of Electroanalytical Chemistry (2012), 677-680 (), 50-55CODEN: JECHES; ISSN:1873-2569. (Elsevier B.V.)
      The dissoln. of polycryst. Rh in 0.1 M H2SO4 is quant. studied during potential cycling and potential step expts. using a novel setup which consists of a microelectrochem. scanning flow cell (SFC) linked to an inductively coupled plasma-mass spectrometer (ICP-MS). The time-resolved dissoln. profile during electrochem. treatment is presented for the 1st time and used for quant. detn. of area-normalized dissoln. rates. A high time resoln. and very low detection limits allow distinguishing between anodic and cathodic dissoln. at a sufficiently low scan rate. The redn. of surface oxides triggers a significantly higher mass loss than the oxide formation (∼4-10 fold), and this ratio and the overall extent of dissoln. are dependent on the upper potential limit. Also, a strong scan rate dependence was obsd. for both the anodic and cathodic dissoln. peaks, with a decrease of the amt. of Rh dissolved per cycle with increasing scan rate and a min. at potentiostatic step expts. between the resp. potential limits. Clear evidence for steady state dissoln. of the oxidized surface is presented as well, even though this value ranges around a few femtograms per s and square centimeter at potentials up to 1.4 VSHE. The differences in dissoln. cover more than two orders of magnitude, potentially providing valuable information for electrode operating conditions and degrdn. mechanisms of noble metal materials.
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    21. 21
      Chen, Z.; Jaramillo, T. F.; Deutsch, T. G.; Kleiman-Shwarsctein, A.; Forman, A. J.; Gaillard, N.; Garland, R.; Takanabe, K.; Heske, C.; Sunkara, M.; McFarland, E. W.; Domen, K.; Miller, E. L.; Turner, J. A.; Dinh, H. N. Accelerating materials development for photoelectrochemical hydrogen production: Standards for methods, definitions, and reporting protocols. J. Mater. Res. 2010, 25 (1), 3– 16,  DOI: 10.1557/JMR.2010.0020
      21
      Accelerating materials development for photoelectrochemical hydrogen production: standards for methods, definitions, and reporting protocols
      Chen, Zhebo; Jaramillo, Thomas F.; Deutsch, Todd G.; Kleiman-Shwarsctein, Alan; Forman, Arnold J.; Gaillard, Nicolas; Garland, Roxanne; Takanabe, Kazuhiro; Heske, Clemens; Sunkara, Mahendra; McFarland, Eric W.; Domen, Kazunari; Miller, Eric L.; Turner, John A.; Dinh, Huyen N.
      Journal of Materials Research (2010), 25 (1), 3-16CODEN: JMREEE; ISSN:0884-2914. (Materials Research Society)
      A review. Photoelectrochem. (PEC) H2O splitting for H prodn. is a promising technol. that uses sunlight and H2O to produce renewable H with O as a byproduct. In the expanding field of PEC H prodn., the use of standardized screening methods and reporting has emerged as a necessity. This article is intended to provide guidance on key practices in characterization of PEC materials and proper reporting of efficiencies. Presented here are the definitions of various efficiency values that pertain to PEC, with an emphasis on the importance of solar-to-H efficiency, as well as a flow chart with std. procedures for PEC characterization techniques for planar photoelectrode materials (i.e., not suspensions of particles) with a focus on single band gap absorbers. These guidelines serve as a foundation and prelude to a much more complete and in-depth discussion of PEC techniques and procedures presented elsewhere.
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    22. 22
      Shi, X.; Cai, L.; Ma, M.; Zheng, X.; Park, J. H. General Characterization Methods for Photoelectrochemical Cells for Solar Water Splitting. ChemSusChem 2015, 8 (19), 3192– 203,  DOI: 10.1002/cssc.201500075
      22
      General Characterization Methods for Photoelectrochemical Cells for Solar Water Splitting
      Shi, Xinjian; Cai, Lili; Ma, Ming; Zheng, Xiaolin; Park, Jong Hyeok
      ChemSusChem (2015), 8 (19), 3192-3203CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Photoelectrochem. (PEC) water splitting is a very promising technol. that converts water into clean hydrogen fuel and oxygen by using solar light. However, the characterization methods for PEC cells are diverse and a systematic introduction to characterization methods for PEC cells has rarely been attempted. Unlike most other review articles that focus mainly on the material used for the working electrodes of PEC cells, this review introduces general characterization methods for PEC cells, including their basic configurations and methods for characterizing their performance under various conditions, regardless of the materials used. Detailed exptl. operation procedures with theor. information are provided for each characterization method. The PEC research area is rapidly expanding and more researchers are beginning to devote themselves to related work. Therefore, the content of this review can provide entry-level knowledge to beginners in the area of PEC, which might accelerate progress in this area.
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    23. 23
      Bedoya-Lora, F. E.; Holmes-Gentle, I.; Hankin, A. Electrochemical techniques for photoelectrode characterisation. Current Opinion in Green and Sustainable Chemistry 2021, 29, 100463,  DOI: 10.1016/j.cogsc.2021.100463
      There is no corresponding record for this reference.
    24. 24
      Nanba, T.; Takano, S.; Yasui, I.; Kudo, T. Structural study of peroxopolytungstic acid prepared from metallic tungsten and hydrogen peroxide. J. Solid State Chem. 1991, 90 (1), 47– 53,  DOI: 10.1016/0022-4596(91)90170-M
      24
      Structural study of peroxopolytungstic acid prepared from metallic tungsten and hydrogen peroxide
      Nanba, Tokuro; Takano, Sanae; Yasui, Itaru; Kudo, Tetsuichi
      Journal of Solid State Chemistry (1991), 90 (1), 47-53CODEN: JSSCBI; ISSN:0022-4596.
      The structure of peroxotungstic acid (W-PTA) prepd. from metallic W and aq. H2O2 was investigated based on Raman, IR, and x-ray diffraction analyses. W-PTA was an amorphous compd. constructed of peroxo polytungstate anions, in which the anions were bound to each other through H bonding. Radial distribution function analyses suggested that the polyanion was W12O38(O2)616-, in which a 6-membered ring of corner-shared polyhedra, such as WO5(O2) or WO6, was sandwiched by 2 W3O10 units consisting of edge-shared WO6.
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    25. 25
      Kwong, W. L.; Savvides, N.; Sorrell, C. C. Electrodeposited nanostructured WO3 thin films for photoelectrochemical applications. Electrochim. Acta 2012, 75, 371– 380,  DOI: 10.1016/j.electacta.2012.05.019
      25
      Electrodeposited nanostructured WO3 thin films for photoelectrochemical applications
      Kwong, W. L.; Savvides, N.; Sorrell, C. C.
      Electrochimica Acta (2012), 75 (), 371-380CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      Thin films of WO3 were deposited on FTO-coated glass substrates by electrodeposition using aq. solns. of peroxotungstic acid. The effects of varying the W concn. of peroxotungstic acid and deposition time on the mineralogical, microstructural, morphol., optical, and photoelectrochem. properties were detd. using x-ray diffraction, SEM, focused ion beam milling, UV-visible spectrophotometry, and linear potentiodynamic voltammetry, resp. The films consisted of monoclinic WO3 of grain sizes in the range 77-122 nm and thicknesses in the range 258-1394 nm; the true porosities were <5%. These microstructural and morphol. parameters depended largely upon the W concn. and deposition time. Some preferred orientation was obsd. and this was considered to result from crystallog. and microstructural factors. The optical transmission data revealed significant decreases in the optical indirect band gap, from 3.05 eV to 2.60 eV, as a function of increasing film thickness. This was considered to result from differential contributions from the surface and bulk band gap components as well as compressive stress. The voltammetry data and assocd. Butler plot revealed the establishment of a Schottky depletion layer and a flat-band potential of +0.2 V to +0.3 V vs. Ag/AgCl. Although the calcd. photoconversion efficiencies were in the range 0.02-0.14%, which is commensurate using a W-halogen light rather than Xe, there was a trend of increasing efficiency as a function of increasing film thickness. This was attributed to decreasing band gap and increasing light absorption. The shape of the curve of the preceding data supports the conclusion of differential contributions from the surface and bulk band gap components. Finally, evidence of photolysis in the absence of an external applied potential suggests the importance of the effect of grain size on the pH and its alteration of the flat-band potential.
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    26. 26
      Vidmar, T.; Topič, M.; Dzik, P.; Opara Krašovec, U. Inkjet printing of sol-gel derived tungsten oxide inks. Sol. Energy Mater. Sol. Cells 2014, 125, 87– 95,  DOI: 10.1016/j.solmat.2014.02.023
      26
      Inkjet printing of sol-gel derived tungsten oxide inks
      Vidmar, Tjasa; Topic, Marko; Dzik, Petr; Opara Krasovec, Ursa
      Solar Energy Materials & Solar Cells (2014), 125 (), 87-95CODEN: SEMCEQ; ISSN:0927-0248. (Elsevier B.V.)
      Tungsten (VI) oxide-WO3 is a widely studied functional inorg. semiconductor material with exceptional chromogenic properties. It is used in energy efficient systems such as smart windows, sensors, displays, storage units, photocatalysts and solar cells. Layers of WO3 are generally produced using expensive vacuum deposition techniques. In this paper, the inkjet printing of sol-gel derived tungsten inks on glass and transparent conductive oxide is reported. Peroxo sol-gel synthesis was used to prep. peroxopolytungstic acid sols which were then modified using different solvents to obtain a suitable jetting ink. Described are the rheol. and structural properties of WO3 inks, the dynamics of WO3 droplets and the morphol. and quality of WO3 printouts. The functionality of these transparent WO3 layers is successfully demonstrated in an electrochromic device.
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    27. 27
      Krishnan, R. Fundamentals of Semiconductor Electrochemistry and Photoelectrochemistry. In Encyclopedia of Electrochemistry; Bard, A.J., Ed.; Wiley, 2007.
      There is no corresponding record for this reference.
    28. 28
      White, J. R.; Fan, F. R. F.; Bard, A. J. Semiconductor Electrodes: LVI. Principles of Multijunction Electrodes and Photoelectrosynthesis at Texas Instruments’ p/n-Si Solar Arrays. J. Electrochem. Soc. 1985, 132 (3), 544– 550,  DOI: 10.1149/1.2113884
      28
      Semiconductor electrodes. LVI. Principles of multijunction electrodes and photoelectrosynthesis at Texas Instruments' p/n-silicon solar arrays
      White, James R.; Fan, Fu Ren F.; Bard, Allen J.
      Journal of the Electrochemical Society (1985), 132 (3), 544-50CODEN: JESOAN; ISSN:0013-4651.
      Multilayer semiconductor electrode structures involving several photoactive junctions, such as the title (TI) arrays based on Si p/n junctions, can show stable operation and produce sufficient photovoltages to promote energetic reactions, such as the decompn. of HCl to H and Cl. The current-voltage (I-V) behavior at the interface with the soln. is an important component of the cell performance, and metal surfaces can be selected to stabilize the electrode from photodecompn. and to catalyze the desired reactions. The behavior of multijunction devices can be obtained graphically from the I-V characteristics of each junction. Multijunction structures, such as coupled TI arrays, can produce photovoltages >2 V, and can drive energetic reactions. The application of these arrays to nonaq. solns. was also illustrated. Reactions considered include generation of Cl with the redn. of O or the generation of H, the photobromination of PhOH [108-95-2], and the photochlorination of cyclohexene [110-83-8] in MeCN.
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    29. 29
      Hankin, A.; Bedoya-Lora, F. E.; Alexander, J. C.; Regoutz, A.; Kelsall, G. H. Flat band potential determination: avoiding the pitfalls. J. Mater. Chem. A 2019, 7 (45), 26162– 26176,  DOI: 10.1039/C9TA09569A
      29
      Flat band potential determination: avoiding the pitfalls
      Hankin, Anna; Bedoya-Lora, Franky E.; Alexander, John C.; Regoutz, Anna; Kelsall, Geoff H.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2019), 7 (45), 26162-26176CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      The flat band potential is one of the key characteristics of photoelectrode performance. However, its detn. on nanostructured materials is assocd. with considerable uncertainty. The complexity, applicability and pitfalls assocd. with the four most common exptl. techniques used for evaluating flat band potentials, are illustrated using nanostructured synthetic hematite (α-Fe2O3) in strongly alk. solns. as a case study. The motivation for this study was the large variance in flat band potential values reported for synthetic hematite electrodes that could not be justified by differences in exptl. conditions, or by differences in their charge carrier densities. We demonstrate through theory and expts. that different flat band potential detn. methods can yield widely different results, so could mislead the anal. of the photoelectrode performance. We have examd.: (a) application of the Mott-Schottky (MS) equation to the interfacial capacitance, detd. by electrochem. impedance spectroscopy as a function of electrode potential and potential perturbation frequency; (b) Gartner-Butler (GB) anal. of the square of the photocurrent as a function of electrode potential; (c) detn. of the potential of transition between cathodic and anodic photocurrents during slow potentiodynamic scans under chopped illumination (CI); (d) open circuit electrode potential (OCP) under high irradiance. Methods GB, CI and OCP were explored in absence and presence of H2O2 as hole scavenger. The CI method was found to give reproducible and the most accurate results on hematite but our overall conclusion and recommendation is that multiple methods should be employed for verifying a reported flat band potential.
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    30. 30
      Amano, F.; Tian, M.; Wu, G.; Ohtani, B.; Chen, A. Facile preparation of platelike tungsten oxide thin film electrodes with high photoelectrode activity. ACS Appl. Mater. Interfaces 2011, 3 (10), 4047– 52,  DOI: 10.1021/am200897n
      30
      Facile Preparation of Platelike Tungsten Oxide Thin Film Electrodes with High Photoelectrode Activity
      Amano, Fumiaki; Tian, Min; Wu, Guo-Sheng; Ohtani, Bunsho; Chen, Ai-Cheng
      ACS Applied Materials & Interfaces (2011), 3 (10), 4047-4052CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      W trioxide (WO3) thin film electrodes with platelike structures were prepd. by a facile hydrothermal reaction of W sheets in a dil. HNO3 soln. at 100-180° and subsequent calcination at 450°. The calcination step facilitated the transformation of the crystal structure from W oxide hydrates (WO3·H2O or WO3·2H2O) to monoclinic WO3 without significant modification to the platelike structures. The photoelectrochem. performance of the thin film electrodes for H2O splitting that took place in a dil. H2SO4 was strongly dependent on both temp. and the time used for the hydrothermal reaction. Probably the thickness of the film influences the process of photoexcited electron transport. The time required for the hydrothermal reaction under higher temps. was reduced in the generation of thin film electrodes with high photoelectrode activity, because the crystal growth is accelerated at high temps. and the electron transport is restricted by a relatively thick compact layer that is comprised of WO3 nanoparticulates. The electrode exhibited sensitivity to the violet portion of the visible light spectrum due to the bandgap of 2.8 eV and high photoelectrode efficiency, as well as an incident photon-to-current conversion efficiency (IPCE) of 66.2%, for the photoelectrochem. oxidn. of H2O.
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    31. 31
      Cai, M.; Fan, P.; Long, J.; Han, J.; Lin, Y.; Zhang, H.; Zhong, M. Large-Scale Tunable 3D Self-Supporting WO3Micro-Nano Architectures as Direct Photoanodes for Efficient Photoelectrochemical Water Splitting. ACS Appl. Mater. Interfaces 2017, 9 (21), 17856– 17864,  DOI: 10.1021/acsami.7b02386
      31
      Large-Scale Tunable 3D Self-Supporting WO3 Micro-Nano Architectures as Direct Photoanodes for Efficient Photoelectrochemical Water Splitting
      Cai, Mingyong; Fan, Peixun; Long, Jiangyou; Han, Jinpeng; Lin, Yi; Zhang, Hongjun; Zhong, Minlin
      ACS Applied Materials & Interfaces (2017), 9 (21), 17856-17864CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Hydrogen prodn. from water based on photoelectrochem. (PEC) reactions is feasible to solve the urgent energy crisis. Herein, hierarchical 3D self-supporting WO3 micro-nano architectures in situ grown on W plates are successfully fabricated via ultrafast laser processing hybrid with thermal oxidn. Owing to the large surface area and efficient interface charge transfer, the W plate with hierarchical porous WO3 nanoparticle aggregates has been directly employed as the photoanode for excellent PEC performance, which exhibits a high photocurrent d. of 1.2 mA cm-2 at 1.0 V vs Ag/AgCl (1.23 V vs RHE) under AM 1.5 G illumination and reveals excellent structural stability during long-term PEC water splitting reactions. The nanoscale and microscale features can be facilely tuned by controlling the laser processing parameters and the thermal oxidn. conditions to achieve improved PEC activity. The presented hybrid method is simple, cost-effective, and controllable for large-scale fabrication, which should provide a new and general route that how the properties of conventional metal oxides can be improved via hierarchical 3D micro-nano configurations.
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    32. 32
      Santato, C.; Ulmann, M.; Augustynski, J. Photoelectrochemical Properties of Nanostructured Tungsten Trioxide Films. J. Phys. Chem. B 2001, 105 (5), 936– 940,  DOI: 10.1021/jp002232q
      32
      Photoelectrochemical Properties of Nanostructured Tungsten Trioxide Films
      Santato, Clara; Ulmann, Martine; Augustynski, Jan
      Journal of Physical Chemistry B (2001), 105 (5), 936-940CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      The photoelectrochem. characteristics of highly transparent nanoporous WO3 films are described. The photocurrent vs. excitation wavelength spectra of these photoelectrodes exhibit a max. close to 400 nm and a significant photoresponse to the blue part of the visible spectrum. The obsd. conversion efficiencies attain 75% for the photogeneration of oxygen from 1M aq. HClO4 and reach 190% in the presence of methanol in the soln., denoting in the latter case the occurrence of a perfect photocurrent doubling. Expts. conducted under simulated solar AM 1.5 illumination resulted in steady-state anodic photocurrents of the order of several mA/cm2.
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    33. 33
      Monllor-Satoca, D.; Borja, L.; Rodes, A.; Gomez, R.; Salvador, P. Photoelectrochemical behavior of nanostructured WO3 thin-film electrodes: The oxidation of formic acid. ChemPhysChem 2006, 7 (12), 2540– 51,  DOI: 10.1002/cphc.200600379
      33
      Photoelectrochemical behavior of nanostructured WO3 thin-film electrodes: the oxidation of formic acid
      Monllor-Satoca, Damian; Borja, Luis; Rodes, Antonio; Gomez, Roberto; Salvador, Pedro
      ChemPhysChem (2006), 7 (12), 2540-2551CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Nanostructured tungsten trioxide thin-film electrodes are prepd. on conducting glass substrates by either potentiostatic electrodeposition from aq. solns. of peroxotungstic acid or direct deposition of WO3 slurries. Once treated thermally in air at 450°, the electrodes are found to be composed of monoclinic WO3 grains with a particle size around 30-40 nm. The photoelectrochem. behavior of these electrodes in 1 M HCIO4 apparently reveals a low degree of electron-hole recombination. Upon addn. of formic acid, the electrode showed the current multiplication phenomenon together with a shift of the photocurrent onset potential towards less pos. values. Photoelectrochem. expts. devised on the basis of a kinetic model reported recently showed that an interfacial mechanism of inelastic, direct hole transfer takes place in the photooxidn. of formic acid. This behavior is attributed to the tendency of formic acid mols. to be specifically adsorbed on the WO3 nanoparticles, as evidenced by attenuated total reflection IR spectroscopy.
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    34. 34
      Yagi, M.; Maruyama, S.; Sone, K.; Nagai, K.; Norimatsu, T. Preparation and photoelectrocatalytic activity of a nano-structured WO3 platelet film. J. Solid State Chem. 2008, 181 (1), 175– 182,  DOI: 10.1016/j.jssc.2007.11.018
      34
      Preparation and photoelectrocatalytic activity of a nano-structured WO3 platelet film
      Yagi, Masayuki; Maruyama, Syou; Sone, Koji; Nagai, Keiji; Norimatsu, Takayoshi
      Journal of Solid State Chemistry (2008), 181 (1), 175-182CODEN: JSSCBI; ISSN:0022-4596. (Elsevier)
      A WO3 film was prepd. by calcination from a precursor paste including suspended (NH4)2WO4 and polyethylene glycol (PEG). The (NH4)2WO4 suspension was yielded by an acid-base reaction of tungstic acid and an ammonium soln. followed by deposition with EtOH addn. Thermogravimetric (TG) anal. showed that the TG profile of PEG is significantly influenced by deposited ammonium tungstate, suggesting that PEG interacts strongly with deposited ammonium tungstate in the suspension paste. XRD data indicated that the WO3 film is crystd. by sintering over 400°. The scanning electron microscopic (SEM) measurement showed that the film is composed of the nano-structured WO3 platelets. The semiconductor properties of the film were examd. by Mott-Schottky anal. to give flat band potential EFB = 0.30 V vs. satd. calomel ref. electrode (SCE) and donor carrier d. ND = 2.5 × 1022 cm-3, latter of which is higher than previous WO3 films by 2 orders of magnitude. The higher ND was explained by the large interfacial heterojunction area caused by the nano-platelet structure, which apparently increases capacitance per a unit electrode area. The WO3 film sintered at 550° produced 3.7 mA cm-2 of a photoanodic current at 1.2 V vs. SCE under illumination with a 500 W Xe lamp due to catalytic H2O oxidn. This photocurrent was 4.5-12.8 times higher than those for the other control WO3 films prepd. by similar but different procedures. The high catalytic activity could be explained by the nano-platelet structure. The photocurrent was generated on illumination of UV and visible light <470 nm, and the max. incident photon-to-current conversion efficiency (IPCE) was 47% at 320 nm at 1.2 V. Tech. important procedures for prepn. of nano-structured platelets are discussed.
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    35. 35
      Butler, M. A. Photoelectrolysis and physical properties of the semiconducting electrode WO2. J. Appl. Phys. 1977, 48 (5), 1914– 1920,  DOI: 10.1063/1.323948
      35
      Photoelectrolysis and physical properties of the semiconducting electrode tungsten trioxide
      Butler, M. A.
      Journal of Applied Physics (1977), 48 (5), 1914-20CODEN: JAPIAU; ISSN:0021-8979.
      The behavior of semiconducting electrodes for photoelectrolysis of H2O is examd., in terms of the phys. properties of the semiconductor. The semiconductor-electrolyte junction is treated as a simple Schottky barrier, and the photocurrent is described using this model. The approach is appropriate because large-band-gap semiconductors have an intrinsic O overpotential which removes the electrode reaction kinetics as the rate-limiting step. The model is successful in describing the wavelength and potential dependence of the photocurrent in WO3 and allows a detn. of the band gap, optical absorption depth, minority-carrier diffusion length, flat-band potential, and the nature of the fundamental optical transition (direct or indirect). For WO3, the minority-carrier diffusion plays a limited role in detg. the photoresponse of the semiconductor-electrolyte junction. There are indications that the diffusion length in this low carrier mobility material is detd. by diffusion-controlled bulk recombination processes rather than the more common trap-limited recombination. The fundamental optical transition is indirect and the band-gap energy depends relatively strongly on applied potential and electrolyte. This effect seems to be the result of field-induced crystallog. distortions in antiferroelec. WO3.
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    36. 36
      Knöppel, J.; Kormányos, A.; Mayerhöfer, B.; Hofer, A.; Bierling, M.; Bachmann, J.; Thiele, S.; Cherevko, S. Photocorrosion of WO3 Photoanodes in Different Electrolytes. ACS Phys. Chem. Au 2021,  DOI: 10.1021/acsphyschemau.1c00004
      There is no corresponding record for this reference.
    37. 37
      Samu, G. F.; Janaky, C. Photocorrosion at Irradiated Perovskite/Electrolyte Interfaces. J. Am. Chem. Soc. 2020, 142 (52), 21595– 21614,  DOI: 10.1021/jacs.0c10348
      37
      Photocorrosion at Irradiated Perovskite/Electrolyte Interfaces
      Samu, Gergely F.; Janaky, Csaba
      Journal of the American Chemical Society (2020), 142 (52), 21595-21614CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A review. Metal-halide perovskites transformed optoelectronics research and development during the past decade. They have also gained a foothold in photocatalytic and photoelectrochem. processes recently, but their sensitivity to the most commonly applied solvents and electrolytes together with their susceptibility to photocorrosion hinders such applications. Understanding the elementary steps of photocorrosion of these materials can aid the endeavor of realizing stable devices. In this Perspective, both thermodn. and kinetic aspects of photocorrosion processes occurring at the interface of perovskite photocatalysts and photoelectrodes with different electrolytes are discussed. Combined in situ and operando electrochem. techniques can reveal the underlying mechanisms. Finally, the authors also discuss emerging strategies to mitigate photocorrosion (such as surface protection, materials and electrolyte engineering, etc.).
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    38. 38
      Nandjou, F.; Haussener, S. Degradation in photoelectrochemical devices: review with an illustrative case study. J. Phys. D: Appl. Phys. 2017, 50 (12), 124002,  DOI: 10.1088/1361-6463/aa5b11
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      Degradation in photoelectrochemical devices: review with an illustrative case study
      Nandjou, Fredy; Haussener, Sophia
      Journal of Physics D: Applied Physics (2017), 50 (12), 124002/1-124002/23CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
      The durability, reliability, and robustness of photoelectrochem. (PEC) devices are key factors for advancing the practical large-scale implementation of cost-competitive solar fuel prodn. We review the known degrdn. mechanisms occurring in water-splitting photoelectrochem. devices. The degrdn. of single components is discussed in detail, and the parameters and conditions which influence it are presented. Device short-term durability depends on the semiconductor material and its interface with the electrolyte. Catalyst and electrolyte degrdns. are considerable challenges for long-term durability. We highlight how PEC device design choices can affect the salience of alternative degrdn. mechanisms. The PEC device architecture and the initial operating design point are crucial for obsd. device performance loss. Device degrdn. behavior is further impacted by irradn. intensity and concn., and by c.d. and concn. Enhancing a phys. understanding of degrdn. phenomena and investigating their effect on component properties is of utmost importance for predicting performance loss and tackling the durability challenge of PEC devices.
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    39. 39
      Geiger, S.; Kasian, O.; Ledendecker, M.; Pizzutilo, E.; Mingers, A. M.; Fu, W. T.; Diaz-Morales, O.; Li, Z.; Oellers, T.; Fruchter, L.; Ludwig, A.; Mayrhofer, K. J. J.; Koper, M. T. M.; Cherevko, S. The stability number as a metric for electrocatalyst stability benchmarking. Nature Catalysis 2018, 1 (7), 508– 515,  DOI: 10.1038/s41929-018-0085-6
      39
      The stability number as a metric for electrocatalyst stability benchmarking
      Geiger, Simon; Kasian, Olga; Ledendecker, Marc; Pizzutilo, Enrico; Mingers, Andrea M.; Fu, Wen Tian; Diaz-Morales, Oscar; Li, Zhizhong; Oellers, Tobias; Fruchter, Luc; Ludwig, Alfred; Mayrhofer, Karl J. J.; Koper, Marc T. M.; Cherevko, Serhiy
      Nature Catalysis (2018), 1 (7), 508-515CODEN: NCAACP; ISSN:2520-1158. (Nature Research)
      Reducing the noble metal loading and increasing the specific activity of the oxygen evolution catalysts are omnipresent challenges in proton-exchange-membrane water electrolysis, which have recently been tackled by utilizing mixed oxides of noble and non-noble elements. However, proper verification of the stability of these materials is still pending. Here we introduce a metric to explore the dissoln. processes of various iridium-based oxides, defined as the ratio between the amts. of evolved oxygen and dissolved iridium. The so-called stability no. is independent of loading, surface area or involved active sites and provides a reasonable comparison of diverse materials with respect to stability. The case study on iridium-based perovskites shows that leaching of the non-noble elements in mixed oxides leads to the formation of highly active amorphous iridium oxide, the instability of which is explained by the generation of short-lived vacancies that favor dissoln. These insights are meant to guide further research, which should be devoted to increasing the utilization of highly durable pure cryst. iridium oxide and finding solns. to stabilize amorphous iridium oxides.
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    40. 40
      Mi, Q.; Zhanaidarova, A.; Brunschwig, B. S.; Gray, H. B.; Lewis, N. S. A quantitative assessment of the competition between water and anion oxidation at WO3 photoanodes in acidic aqueous electrolytes. Energy Environ. Sci. 2012, 5 (2), 5694,  DOI: 10.1039/c2ee02929d
      40
      A quantitative assessment of the competition between water and anion oxidation at WO3 photoanodes in acidic aqueous electrolytes
      Mi, Qixi; Zhanaidarova, Almagul; Brunschwig, Bruce S.; Gray, Harry B.; Lewis, Nathan S.
      Energy & Environmental Science (2012), 5 (2), 5694-5700CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      The faradaic efficiency for O2(g) evolution at thin-film WO3 photoanodes has been evaluated in a series of acidic aq. electrolytes. In 1.0 M H2SO4, persulfate was the predominant photoelectrochem. oxidn. product, and no O2 was detected unless catalytic quantities of Ag+(aq) were added to the electrolyte. In contact with 1.0 M HClO4, dissolved O2 was obsd. with nearly unity faradaic efficiency, but addn. of a hole scavenger, 4-cyanopyridine N-oxide, completely suppressed O2 formation. In 1.0 M HCl, Cl2(g) was the primary oxidn. product. These results indicate that at WO3 photoanodes, water oxidn. is dominated by oxidn. of the acid anions in 1.0 M HCl, H2SO4, and HClO4, resp.
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    41. 41
      Gregoire, J. M.; Xiang, C.; Liu, X.; Marcin, M.; Jin, J. Scanning droplet cell for high throughput electrochemical and photoelectrochemical measurements. Rev. Sci. Instrum. 2013, 84 (2), 024102,  DOI: 10.1063/1.4790419
      41
      Scanning droplet cell for high throughput electrochemical and photoelectrochemical measurements
      Gregoire, John M.; Xiang, Chengxiang; Liu, Xiaonao; Marcin, Martin; Jin, Jian
      Review of Scientific Instruments (2013), 84 (2), 024102/1-024102/6CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
      High throughput electrochem. techniques are widely applied in material discovery and optimization. For many applications, the most desirable electrochem. characterization requires a three-electrode cell under potentiostat control. In high throughput screening, a material library is explored by either employing an array of such cells, or rastering a single cell over the library. To attain this latter capability with unprecedented throughput, the authors have developed a highly integrated, compact scanning droplet cell that is optimized for rapid electrochem. and photoelectrochem. measurements. Using this cell, the authors screened a quaternary oxide library as (photo)electrocatalysts for the O evolution (water splitting) reaction. High quality electrochem. measurements were carried out and key electrocatalytic properties were identified for each of 5456 samples with a throughput of 4 s per sample. (c) 2013 American Institute of Physics.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsmeasuresciau.1c00016.

    • Electrolyte flow profile simulation, WO3 synthesis procedure, photograph of light components, photograph of transparent PEC-SFC with flowing electrolyte, photograph of UV sensitive paper to determine illuminated area, CV and dissolution profile of polycrystalline Pt, reproducibility of WO3 dissolution experiments (PDF)


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