Gas-fed photoelectrochemical reactions sustained by phosphotungstic acid as an inorganic surface electrolyte

Abstract

Gas-fed photoelectrochemical (PEC) system with a porous photoelectrode and proton-exchange membrane (PEM) has the potential to produce hydrogen from water vapour and activate methane at room temperature. To effectively drive gas-phase PEM-PEC reactions, porous photoelectrodes should be coated with a solid electrolyte of perfluorinated sulfonic acid (PFSA) ionomers. However, fluorocarbon-based ionomers were not chemically stable in vapour-fed PEC systems. Herein, we report that polyoxometalate, an inorganic proton-conducting material, may be employed as the surface electrolyte of a WO₃ porous photoelectrode for vapour-fed water splitting and methane activation under visible light irradiation. We demonstrate that the porous WO₃ photoanode modified with phosphotungstic acid (H₃PW₁₂O₄₀) induces PEC reactions, including the oxygen evolution reaction and methane conversion, under gas feeding. Additionally, we demonstrated improved durability by using the inorganic surface electrolyte.

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Introduction

Photoelectrochemical (PEC) water splitting is a method of producing hydrogen using renewable energy with no carbon dioxide emissions. Water vapour harvesting from ambient humidity may solve the freshwater shortage problem in the world, making green H₂ sustainable.^1–3 The use of water vapour has the additional benefit of minimizing active water purification, liquid pumping, and bubble formation.^4–6 The gas-fed PEM-PEC reactions for producing H₂ necessitate the use of all-solid PEC devices with polymer electrolyte membranes that operate at low temperatures around 25 °C.

Prototype gas-fed PEC systems have been developed using a proton exchange membrane (PEM) with a porous photoanode and cathode electrocatalyst.^7–23 We found that the surface coating of the porous photoelectrodes with perfluorinated sulfonic acid (PFSA) ionomers significantly enhanced the PEC performance of the vapour-fed system operating without liquid electrolyte.^13–17 The surface ionomer decorated on the porous photoanodes forms a triple-phase boundary, allowing gas, semiconductor particles, and electrolyte to come into contact simultaneously, facilitating proton-coupled electron transfer (PCET) and charge transport in the gas-fed PEC cell. Surface proton conduction is promoted by the humidity-adsorbed water layer in the ionomer.^16,19,20,22

Surface coatings of the PFSA ionomers, such as Nafion® from Chemours and Aquivion® from Solvay, aid in PCET and proton transport in the functionalized porous photoelectrode.^{11,13–17,22,23} As a result, PFSA-functionalized photoanodes such as TiO₂ and SrTiO₃ can induce vapour-fed water splitting with as high efficiency as the conventional liquid electrolyte system.^14–16 The all-solid PEM-PEC system using porous photoanodes such as TiO₂ and WO₃ has also been employed in the CH₄ conversion reactions (steam reforming of methane and dehydrogenative coupling to ethane).^24,25 The selectivity for C₂H₆ from CH₄ was more than 50% on a carbon basis for the ionomer-coated WO₃ photoanode under visible light irradiation.

However, the photocurrent responses in the PEM-PEC cells gradually decreased with the accompanying CO₂ formation during vapour-fed water splitting reactions under UV irradiation.^14–16,22 This degradation suggests oxidative decomposition of the PFSA ionomers decorated on the photoanode surface by the photogenerated holes and hydroxyl radicals. Although the fluorocarbon backbones are chemically stable compared with other organic materials, PFSA ionomers are degraded by hydroxyl radicals during PEM fuel cell operation.²⁶ In the same way, the PFSA ionomer decorated on the photoanodes would be oxidized by the valence band holes and the photogenerated hydroxyl radical in the PEM-PEC system.

To avoid the degradation of the triple-phase boundary in the functionalized photoanodes, we explored polyoxometalate, an inorganic proton-conducting material that is oxidatively stable in comparison with other organic materials, as a surface electrolyte to replace the PFSA ionomer. In this study, we studied the surface modification of a porous WO₃ photoanode with phosphotungstic acid (PWA) to improve the photocurrent response in water vapour splitting and CH₄ conversion reactions in a gas-fed PEM-PEC system. PWA (H₃PW₁₂O₄₀) is a polyoxometalate with Brønsted acidity and exhibits proton conductivity under high humidity.^27–30 We demonstrated that PWA enhances the PEC response in gas-fed PEC systems under UV and visible light.

Experimental

Preparation of the porous WO₃ photoanodes

The porous WO₃ photoanode was prepared by using Ti fibre felt (Nikko Techno), which served as a gas-diffusion conductive substrate (Fig. 1a).^24,31 The Ti felt was dipped in an aqueous solution of (NH₄)₆H₂W₁₂O₄₀ (Nippon inorganic color & chemical) and polyethylene glycol 20 000 (Fujifilm Wako pure chemical) for 30 min. The dip-coated felt was then dried at 80 °C. This treatment was repeated three times. The obtained samples were then calcined at 650 °C in air for 2 h.

Fig. 1 (a) Preparation of porous WO₃ photoanode and surface electrolyte coating. (b) XRD patterns of PWA/WO₃ photoanodes and the standard lines of WO₃ (JCPDS No. 01-072-0677) and Ti (JCPDS No. 00-044-1294).

The porous WO₃ electrodes were functionalized with solutions of 5 wt% Nafion PESA ionomer (D520, fuel cell store) or H₃(PW₁₂O₄₀)⋅nH₂O (Fujifilm Wako pure chemicals) by drop casting. After applying 3.5–10 μL cm⁻² onto the surface, the drop-cast electrode was dried at 80 °C for 10 min. Subsequently, another cast was applied on the opposite side of the electrode and dried in the same manner. The loading weights of the surface electrolyte relative to WO₃ (8 mg cm⁻²) are 12 wt% and 2–20 wt% for PFSA/WO₃ and PWA/WO₃, respectively.

Characterization

X-ray diffraction patterns were recorded on a SmartLab diffractometer (Rigaku) using Cu Kα radiation. Micro-Raman spectra were recorded on an inVia Reflex (Renishaw) with a 532-nm green laser. Surface composition was analyzed using a JSM-7800F field-emission scanning electron microscope (JEOL) with an energy-dispersive X-ray spectrometer (EDS).

PEM-PEC reaction in the gas phase

The PEC measurements were performed in a two-electrode system using a HZ-Pro S4 workstation (Hokuto Denko). The cathode was composed of a ∼50 wt% Pt-loaded carbon black (Pt–CB, Tanaka Kikinzoku Kogyo) catalyst with an ionomer and Toray carbon paper (fuel cell store). A Nafion N117 film (Chemours) was sandwiched between the porous photoanode and cathode electrocatalyst by pressing at 140 °C for 4 min.

The PEC reaction was performed at 26 °C and a pressure of 1 bar. The gas flow rate was controlled using mass-flow controllers. The photoanode side was supplied with 20 mL min⁻¹ of high-purity CH₄ (Osaka gas liquid, >99.999%), which was passed through the liquid water at 24 °C. The water vapour pressure was monitored using a dew point meter, EE33 (Tekhne). The relative humidity (RH) in the reactor was 80–90%, without liquid condensation. The cathode side was supplied with 20 mL min⁻¹ of humidified Ar gas. The geometric surface area of the two electrodes was 4 cm², and that of the photoanode was 2 cm². Photoirradiation was performed using a 365-nm UV LED (Nitride semiconductor), and a 453-nm blue LED (OptoSupply). The incident photon-to-current conversion efficiency (IPCE) was calculated using the following equation:

where j_photo is the photocurrent density, λ is the wavelength (nm), and I₀ is the intensity of the incident light.

The gas products were analyzed using gas chromatography. H₂ and O₂ were quantified using a GC-8A (Shimadzu) instrument equipped with a molecular sieve 5A and a thermal conductivity detector in an Ar carrier. To quantify CO and CO₂, a He carrier was used in a Shincarbon ST column. Hydrocarbons were analyzed using a GC-2014 (Shimadzu) equipped with a GS-CarbonPLOT column, an N₂ carrier, and a flame ionization detector. The C₂H₆ selectivity on a carbon basis was calculated from the production rate of each carbon-containing product (r_i) as follows:

The C₂H₆ Faradaic efficiency (FE) value was calculated from the Faradaic constant (F) as follows:

Results and discussion

The crystal structure of H₃PW₁₂O₄₀·nH₂O varies depending on the hydration number (n = 6, 14, and 21).³² An edge-shared WO₆ shell enclosing the tetrahedrally coordinated phosphorus core, which is Keggin's anion unit (PW₁₂O₄₀³⁻), is surrounded by protonated water layers. XRD analysis revealed no peaks attributed to H₃PW₁₂O₄₀·nH₂O on the PWA/WO₃ photoanodes (Fig. 1b), indicating the high dispersion of H₃PW₁₂O₄₀ on the porous WO₃ photoanode. All the peaks were attributed to monoclinic WO₃ and hexagonal Ti from the felt substrate. The Raman spectra of the PWA/WO₃ electrode exhibited small bands of H₃PW₁₂O₄₀ at 1011 and 993 cm⁻¹ (Fig. 2), which are assigned to the symmetric and asymmetric stretching of the terminal W=O bond of the Keggin's unit, respectively.³³ The other Raman bands are attributed to monoclinic WO₃.³⁴ The XRD and Raman results indicate that H₃PW₁₂O₄₀ is deposited on the WO₃ surface in a highly dispersed state. The SEM observation supports the dispersed state of PWA on the WO₃ particles (Fig. S1 in ESI†).

Fig. 2 Raman spectra of the WO₃ photoanodes with and without the PWA (10 and 20 wt%) coating and H₃PW₁₂O₄₀·nH₂O crystalline powder.

The functionalized WO₃ photoanode and Pt–CB were attached to both sides of an N117 membrane and the prepared membrane electrode assembly was installed in a planar-type stainless-steel reactor with a glass window for irradiation (Fig. 3). The PEM-PEC measurements were performed in a two-electrode system, but the cathode potential is likely close to that of the reversible hydrogen electrode (RHE) because the overpotential for the hydrogen evolution reaction was very small over the Pt–CB electrocatalyst (Fig. S2, ESI†).

Fig. 3 (a) Membrane electrode assembly composed of the WO₃ photoanode, N117, and the Pt–CB electrocatalyst. (b) Planar-type stainless steel cell for gas-fed PEC reaction with an irradiation window of 2 cm².

Fig. 4a shows the chronoamperometric data of the PEM-PEC system at a constant bias of 1.2 V under 365-nm UV irradiation (area 2 cm²). The WO₃ photoanodes were employed in a continuous Ar flow with ∼3 kPa of water vapour at 26 °C (RH 90%). Humidified Ar flowed into the cathode side. Under this vapour-fed condition (RH 90%), the photocurrent response was negligibly small for the bare WO₃ photoanode without a surface electrolyte because proton transfer and transport were sluggish in the absence of a liquid electrolyte. In contrast, surface coating by the Nafion ionomer significantly increased the photocurrent response under vapour-fed conditions. PWA coating also enhanced the vapour-fed PEC performance of WO₃ photoanodes. The photocurrent response of PWA/WO₃ increased with increasing PWA loading from 2 to 5 wt%. The photocurrents were stable after a steep decay during the initial 10 min. The IPCE was calculated from the steady-state photocurrent density. The IPCE values of PWA/WO₃ were maximized at 5–10 wt% loading and decreased slightly at 20 wt% loading (Fig. 4b). At 365 nm, the IPCE of PWA/WO₃ was 29%, which was nearly identical to the values obtained for WO₃ in a liquid electrolyte (0.1 M H₂SO₄) at a potential of 1.2 V vs. RHE. Similar IPCE values have been reported for WO₃ photoanodes in conventional liquid electrolytes.^31,35–37 There was no significant difference in IPCE values between PWA/WO₃ and PFSA/WO₃ under vapour feeding. This indicates that the PWA acts as a surface electrolyte like PFSA ionomer for the vapour-fed condition.

Fig. 4 Photocurrent response of the WO₃ photoanodes in a continuous flow of humidified argon (RH < 90%) under UV irradiation (365 nm, 7.0 mW cm⁻², area 2 cm²). (a) Photocurrent response of WO₃ photoanodes coated with PFSA and PWA with different loading amount (wt%) at 1.2 V vs. Pt–CB cathode. (b) Incident photon-to-current conversion efficiency (IPCE) at 1.2 V.

Fig. 5a–c show the effect of applied voltage on the photocurrent response in the PEM-PEC system under visible light (453 nm). Initial decays of the photocurrent were significant when the applied potential was lower than 1.2 V (vs. cathode) for both PWA/WO₃ and PFSA/WO₃ photoanodes. The photocurrent response at the steady state was diminished below 0.6 V (vs. cathode). This result is consistent with the reported flat band potentials (0.3–0.5 V vs. RHE) for WO₃ electrodes.^35,38 There was no significant difference between PWA/WO₃ and PFSA/WO₃ on the applied voltage dependence. These results suggest that the charge separation efficiency is decreased when the potential drop in band bending is decreased at the steady state.

Fig. 5 Effect of applied voltage (V vs. Pt–CB cathode) on the photocurrent response of (a) PFSA/WO₃ and (b) PWA/WO₃ in the vapour-fed PEM-PEC system under visible light (453 nm, 22 mW cm⁻²). (c) Steady-state IPCE at 453 nm after 20 min of photoirradiation. (d) Schematic illustration of PEC triple-phase boundary over the PWA/WO₃ photoanode. The loading of PWA and PFSA were 5 and 12 wt% relative to WO₃.

We found that the polyoxometalate loaded on the porous WO₃ photoanode acted as a surface electrolyte like PFSA ionomer with proton conductivity. Fig. 5d illustrates the role of PWA in the PEC reaction in the presence of saturated water vapour. In PEC water oxidation, the photogenerated holes promote the oxygen evolution reaction (2H₂O → O₂ + 4H⁺ + 4e⁻) probably through the PCET process. Photoexcited electrons are transported toward the conductive Ti-fibre substrate through the semiconductor particles. In conventional liquid systems, aqueous electrolytes promote proton transfer/transport in the microenvironment of photoanode materials. In contrast, the absence of aqueous electrolytes causes proton transfer and transport to be the rate-determining steps in OER. Therefore, the PFSA ionomer is required as a surface electrolyte to promote proton transfer and transport under vapour-fed conditions.

Fig. 6 shows the nitrogen and water vapour adsorption/desorption isotherms of the WO₃ electrodes. The BET specific surface area of bare WO₃, PWA/WO₃, and PFSA/WO₃ layers on Ti felt was estimated from the nitrogen adsorption to be 3.6, 2.9, and 2.6 m² g⁻¹, respectively. The coating of the surface electrolyte slightly reduces the specific surface area of the WO₃ particles. In the PEM-PEC cell, one of the roles of PFSA ionomers is considered to be capturing humidity from the moisture air to supply enough water on the photoelectrodes.^16,22 As shown in the water vapour adsorption/desorption isotherms, the PFSA/WO₃ exhibits larger water uptake than those of bare WO₃ and PWA/WO₃. The water uptake of the WO₃ electrode was slightly increased by the loading of PWA, but the effect was not significant at higher RH regions (p/p₀ > 0.8). This result demonstrates that the activity enhanced by PWA is attributable to the proton transport property rather than the capture of the water vapour from humidity.

Fig. 6 (a) Nitrogen adsorption isotherms at −196 °C and (b) water vapour adsorption–desorption isotherms at 25 °C for bare WO₃, PWA/WO₃, and PFSA/WO₃ layers on Ti felt. The loading of PWA and PFSA were 5 and 12 wt% relative to WO₃.

The long-term stability of the PEM-PEC cells with PFSA/WO₃ and PWA/WO₃ was tested at 1.2 V under 453 nm visible light irradiation (area 16 cm²). Fig. 7 shows the time course of the current density and production rates in the cathode and photoanode compartments in the vapour-fed PEC water splitting. The photocurrent and production rate of PFSA/WO₃ gradually decreased, but was relatively stable for PWA/WO₃. After 20 h of photoirradiation, dark current was significantly increased for PFSA/WO₃. The IPCE value was calculated from the photocurrent density after subtraction of the dark current density when turning off the light. The IPCE of the PWA/WO₃ photoanode (2.8%) was higher than that of the PFSA/WO₃ photoanode (2.4%) at the 20-h PEC reaction. To further investigate the durability of the photoanodes, we compared the voltammograms before and after long-term PEC reactions (Fig. 7c and f). The photocurrent response of the PFSA/WO₃ photoanode was decreased after the stability test, but a significant difference was not observed for PWA/WO₃ between the fresh and the used photoanodes. These results indicated that stable water splitting was achieved by the PEM-PEC system using H₃PW₁₂O₄₀ as an inorganic solid electrolyte.

Fig. 7 Long-term stability test of (a)–(c) PFSA/WO₃ and (d)–(f) PWA/WO₃ photoanodes in the vapour-fed PEM-PEC system under visible light (453 nm, 12 mW cm⁻², area 16 cm²) at 1.2 V (vs. Pt–CB): (a) and (d) Production rate of H₂ in the cathode side, (b) and (e) production rate of O₂ and CO₂ in the photoanode side, and (c) and (f) cyclic voltammograms before and after the stability test.

The H₂ FE was close to 100% on the cathode side. In contrast, the O₂ FE was less than 100% probably owing to CO₂ formation and dark current (Table S1 in ESI†). However, PWA/WO₃ showed an O₂ FE of 95%, which was significantly higher than that of PFSA/WO₃ (O₂ FE of 85%). Continuous CO₂ evolution during PEC reactions was observed only for the PFSA/WO₃ photoanode. This CO₂ production leading to lower O₂ FE is assigned to the oxidative decomposition of PFSA ionomer and membrane.^15,16

The SEM-EDS analysis revealed that the fluorine content of the PFSA/WO₃ photoanode decreased from 36.5 to 18.5 mass% after the PEC reaction (Table S2, ESI†). Therefore, the CO₂ production primarily originated from the degradation of the PFSA ionomer coated on the photoanodes. In contrast, the P content (0.3 mass%) did not change during the PEC reaction for the PWA/WO₃ photoanode. The phosphorus emission mapping image revealed that the high dispersion of H₃PW₁₂O₄₀ on the porous WO₃ electrode was maintained even after the reaction (Fig. S3, ESI†). Raman spectra also confirmed the presence of two bands of H₃PW₁₂O₄₀ even after the vapour-fed PEC reaction (Fig. S4, ESI†). The long-term stability test was also performed under 385 nm UV light (irradiance 60 mW cm⁻², area 2 cm², Fig. S5, ESI†). After 20 h of reaction, the IPCE of the WO₃/PWA photoanode (2.4%) was significantly higher than that of the WO₃/Nafion photoanode (0.9%).

Finally, we employed the PEM-PEC cell for methane conversion reaction under visible light irradiation. Fig. 8 depicts the time course of PEC methane conversion at 1.2 V (vs. cathode). CH₄ gas containing water vapour (90% RH) was continuously fed over the WO₃ photoanodes. The H₂ FE on the cathode side was approximately 100% (Table S3, ESI†). O₂, CO₂, CO, and C₂H₆ were formed on the photoanode side. The rate of O₂ evolution was markedly reduced by the PWA coating, which enhanced the synthesis of CO_x and C₂H₆, as opposed to the PFSA ionomer coating (12 wt%). This suggests that CH₄ oxidation is more plausible for the PWA/WO₃ than for OER by water splitting. For PWA/WO₃, the FEs of CO₂, CO, and C₂H₆ were 71.3%, 9.2%, and 10.5%, respectively (Table S3, ESI†). The increased CO_x formation rate implies that steam reforming of methane (CH₄ + xH₂O → CO_x + (2 + x)H₂) is promoted even at 25 °C.^24,25,39 C₂H₆ is formed by CH₄ activation, implying the formation of methyl radicals and their coupling reaction.^40–43 The activation of the C–H bond of CH₄ is believed to be promoted by photogenerated holes and hydroxyl radicals.^24,44–47

Fig. 8 PEM-PEC methane conversion reactions using the PWA/WO₃ and PFSA/WO₃ photoanodes at 1.2 V (vs. Pt–CB) under visible light irradiation (453 nm, 22 mW cm⁻², and area 2 cm²): (a) photocurrent response, (b) H₂ production rate at the cathode, (c) O₂ production rate at the photoanode and (d) production rate of CO₂ and C₂H₆ at the photoanode.

Conclusions

We found that phosphotungstic acid can function as a novel surface solid electrolyte for WO₃ photoanodes in gas-phase PEM-PEC reactions. The H₃PW₁₂O₄₀/WO₃ photoanode exhibits a photocurrent efficiency as high as that of the porous WO₃ photoanode functionalized by PFSA ionomer. The enhanced activity by phosphotungstic acid functionalization is attributed to the proton transport property rather than the water uptake from humidity. In the vapour-fed water-splitting reaction, the H₃PW₁₂O₄₀/WO₃ photoanode, composed of inorganic materials, was more stable than the photoanode using the organic PFSA ionomer. The H₃PW₁₂O₄₀ surface electrolyte was also applied for the gas-phase PEC conversion of methane to ethane and carbon oxides. This study sheds light on the use of surface inorganic electrolytes to develop robust PEM-PEC systems for converting gaseous molecules in the absence of liquid electrolytes.

Author contributions

F. Amano: conceptualization, methodology, writing; K. Tsushiro and C. Akamoto: investigation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Japan Science and Technology Agency (JST), Precursory Research for Embryonic Science and Technology (PRESTO) (grant numbers JPMJPR15S1 and JPMJPR18T1) and the Japan Society for the Promotion of Science (JSPS), KAKENHI (grant number JP20H02525).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ya00568b

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