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Article

A WO3–CuCrO2 Tandem Photoelectrochemical Cell for Green Hydrogen Production under Simulated Sunlight

by
Ana K. Díaz-García
1,2 and
Roberto Gómez
1,*
1
Institut Universitari d’Electroquímica i Departament de Química Física, Universitat d’Alacant, Apartat 99, E-03080 Alicante, Spain
2
Facultad de Bioanálisis, Universidad Veracruzana, Xalapa C.P. 91010, Mexico
*
Author to whom correspondence should be addressed.
Molecules 2024, 29(18), 4462; https://doi.org/10.3390/molecules29184462
Submission received: 8 August 2024 / Revised: 9 September 2024 / Accepted: 10 September 2024 / Published: 20 September 2024
(This article belongs to the Section Electrochemistry)
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Abstract

:
The development of photoelectrochemical tandem cells for water splitting with electrodes entirely based on metal oxides is hindered by the scarcity of stable p-type oxides and the poor stability of oxides in strongly alkaline and, particularly, strongly acidic electrolytes. As a novelty in the context of transition metal oxide photoelectrochemistry, a bias-free tandem cell driven by simulated sunlight and based on a CuCrO2 photocathode and a WO3 photoanode, both unprotected and free of co-catalysts, is demonstrated to split water while working with strongly acidic electrolytes. Importantly, the Faradaic efficiency for H2 evolution for the CuCrO2 electrode is found to be about 90%, among the highest for oxide photoelectrodes in the absence of co-catalysts. The tandem cell shows no apparent degradation in short-to-medium-term experiments. The prospects of using a practical cell based on this configuration are discussed, with an emphasis on the importance of modifying the materials for enhancing light absorption.

1. Introduction

Developing carbon-free energy technologies that are not only efficient but also in line with the conservation of the natural environment is one of the most pressing current challenges for humanity [1]. In this context, photoelectrochemical water splitting is a promising way of, not only harvesting, but also storing solar energy through the production of H2 from H2O [2,3,4].
The key components of a photoelectrochemical water splitting device are the photoactive materials [5,6,7,8]. Among them, typical photoelectrodes belong to several families of binary and ternary metal oxides. In fact, n-type oxides, such as Fe2O3 [9], TiO2 [10], WO3 [11] and BiVO4 [12], and p-type oxides, such as Cu2O [13], CuFeO2 [14], CaFe2O4 [15], CuFe2O4 [16], LaFeO3 [17] and CuCrO2 [18], as well as other ternary oxides [19,20], have been investigated for their use as photoanodes and photocathodes, respectively, in water splitting. In this context, the difficulty of developing photocathodes for the water reduction reaction based on nontoxic earth-abundant elements and possessing high stability should be stressed [2,19]. In any case, some works about photoelectrochemical water splitting using tandem cells under simulated sunlight have been published for a number of years. Specifically, tandem cells for water splitting have been investigated since 1977 [21]. However, even when rather high efficiencies were obtained, some of the materials employed were expensive and/or unstable, which made the studied systems unviable for real applications. In this sense, we believe that tandem photoelectrochemical cells based on earth-abundant metal oxides should be further developed since they may lead to, not only an affordable price, but also a stability in contact with aqueous solutions higher than that of electrodes based on chalcogenides, arsenides or nitrides.
A preliminary work on the use of both n-type and p-type hematite electrodes for zero-bias water photoelectrolysis was published as early as 2006 [22]. Interestingly, the electrolyte used was a dilute sulfuric acid solution, which led to short-term stability of the electrodes, although it was sufficient for carrying out some measurements. Ishihara et al. [23] demonstrated the unassisted operation of a tandem photoelectrochemical cell for water splitting based on metal oxides: TiO2 was employed as a photoanode and CaFe2O4 as a photocathode. However, the fact that Pt was used as a conducting substrate (too expensive for real applications) together with the employment of alkaline media (which may suffer from carbonation) should be taken into account. A few years later, Sivula et al. [24] demonstrated the unassisted operation of a tandem cell employing BiVO4 as a photoanode and Cu2O as a photocathode. However, the electrolyte was buffered to pH 6, which, according to Lewis et al. [25,26], is not desirable in terms of efficiency or safety. Moreover, some elements used in the structure of the photocathode (i.e., Au and RuOx) were expensive for a scalable process. Photocurrents corresponding to a solar-to-hydrogen (STH) conversion efficiency of ca. 0.5% were found to decay over a time of some minutes. Subsequent studies have expanded the number of electrode materials and strategies for interface engineering. It is worth noting that some of the proposed configurations are only partially based on metal oxides, as they also include either chalcogenides [27,28] or silicon [29] in their electrode composition.
In this work, we demonstrate the successful operation of a WO3–CuCrO2 tandem photoelectrochemical cell for water splitting under simulated sunlight without the application of external bias. Importantly, the tandem cell showed stability over 3 h without significant degradation, not only in neutral but also in acidic electrolytes, which is a remarkable achievement when compared with similar cell configurations. As far as we know, this is the first tandem cell based on unprotected metal oxides that works in acidic media in a stable way. Such conditions preclude the use of other photocathodes such as those based on CaFe2O4 or CuFeO2. In addition, the favorable combination of electrodes and electrolyte proposed in this work could also be used for other artificial photosynthesis processes (i.e., the reduction of CO2). Admittedly, the conversion efficiency is low, mainly due to both the lack of adequate solar light absorption and fast recombination. The strategies to be followed to overcome these limitations are also briefly discussed.

2. Results and Discussion

2.1. Morphological Characterization of Thin-Film Electrodes

The electrochemical and sol–gel synthetic methods employed for the preparation of the WO3 and CuCrO2 electrodes, respectively, yield rather compact thin films, as shown in Figure 1, particularly for the former. As observed, the grain size is significantly larger in the case of CuCrO2. This type of electrodes, as opposed to nanoporous ones, has the advantage that they can support a space charge layer, thus facilitating charge carrier separation [30].
It is worth noting that both electrode materials were thoroughly characterized in two previous papers from our laboratory devoted separately to WO3 [31] and CuCrO2 [18]. Spectroscopic and microscopic characterization (including TEM and AFM) of the WO3 electrodes revealed that the thin films were rather flat and consisted of monoclinic WO3 nanocrystals with a rather narrow size (diameter) distribution centered around 30 nm. Optical characterization revealed both transparency and a band gap of 2.65–2.7 eV. In the case of the CuCrO2 electrodes, XRD, microscopic and spectroscopic characterization showed that the thin films were a compact layer of rather large grains (size of around 150 nm) with a high degree of crystallinity corresponding to the pure delafossite phase. Importantly, optical measurements revealed high transparency and a band gap of 3.15 eV.

2.2. Photoelectrochemical Properties of WO3 Photoanodes and CuCrO2 Photocathodes

The photoelectrochemical behavior of the WO3 and CuCrO2 electrodes was first analyzed in a separate way in contact with perchloric acid solutions. The dark cyclic voltammogram (CV) for WO3 depicted in Figure 2A shows relatively large pseudo-capacitive currents in the low potential region (below 0.2 V), as expected for an n-type electrode material capable of adsorbing/intercalating protons. In addition, Figure 2B shows a corresponding linear scan voltammogram performed under chopped illumination. Anodic photocurrents start to appear close to the onset of the accumulation region. The voltammetric behavior, both in the dark and under illumination, shows that the material behaves as an n-type semiconductor. On the other hand, a CV for the CuCrO2 electrode is shown in Figure 2C, exhibiting small capacitive currents above 0.75 VAg/AgCl, which correspond to an accumulation region. Figure 2D shows a linear scan voltammogram under chopped illumination for CuCrO2 electrodes, which is characterized by the existence of very small dark currents (in agreement with Figure 2C) together with significant cathodic photocurrents. The lack of significant dark currents is a first indication of the electrode stability in acidic electrolytes. The onset potential is located in this case at 0.90 VAg/AgCl. Based on the CV results, CuCrO2 behaves as a p-type semiconductor. It should be noted that both photoelectrodes show stable responses, not only in close-to-neutral, but also in acidic electrolytes. The onset potential for the WO3 photoanode is located at 0.37 V (0.63 V vs. RHE), while significant photocurrents appear from 0.55 V (0.81 V vs. RHE) in the case of the CuCrO2 photocathode. Further details on the photoelectrochemical behavior of these electrodes can be found in our previous works [18,31].
IPCE spectra were also obtained at applied potentials of 0.10 and 0.96 VAg/AgCl for the CuCrO2 and WO3 electrodes, respectively, as shown in Figure 3. It should be mentioned that while the IPCE values are rather high for the WO3 photoanodes (almost 45% at 350 nm), they are relatively low for the CuCrO2 photocathodes (10% at 320 nm). This fact together with its wider band gap converts the CuCrO2 electrode into the limiting one in the tandem cell configuration.
In addition, a gas product analysis revealed that the Faradaic efficiency for hydrogen evolution was around 90% for the CuCrO2 photocathode, with an estimated relative error of 10% (Figure S1 in the SD shows the evolution of the electrode potential during photoelectrolysis). To our knowledge, this is the highest reported value for an oxide photocathode in the absence of co-catalysts or plasmonic effects at the surface of the photoelectrode [16,20,32].
The expected tandem cell operating point at a short circuit was obtained as the crossing point of the absolute value of the photocurrent density vs. potential curves for both photoelectrodes, as shown in Figure 4, where the inset reveals that a very modest value of 5–10 μA cm−2 photocurrent can be expected from this tandem device without the application of bias and/or other optimization strategies.
The possibility of using these photoelectrodes in photoelectrochemical devices either as photoanodes or as photocathodes together with counter-electrodes working in the dark was first considered. In our measurements, Pt was used as a counter-electrode in both cases. In agreement with the results shown above, to obtain sizable photocurrents, a substantial bias (above 0.8 V) is needed in the case of WO3, while a smaller value (above 0.2 V) would be required for CuCrO2 (see SD, Figure S2). In any case, a bias-free device based on one photoelectrode is not possible.

2.3. Operation and Stability of the Tandem Cell

To check the functioning of the tandem photoelectrochemical system, a classical two-compartment H-type electrochemical cell equipped with fused silica windows was employed. Both electrodes were illuminated with the full output of a solar simulator at 1 sun (parallel illumination). The compartments were air-tight closed and purged with nitrogen prior to the measurements. They were separated by a Nafion membrane NM-117 (see SD, Figure S3). Figure 5A shows the photocurrent delivered by the device as a function of the applied bias (defined as the difference in potential between the photoanode and the photocathode) scanned at a rate of 2 mV s−1. As observed, the unbiased system delivers a photocurrent of 20 μA, which grows almost linearly with the applied bias. On the other hand, the stability of the cell was checked over a period of several hours, as shown in Figure 5B. After the decay of the photocurrent during the first hour, a remarkable stability was attained. In addition, with the aim of examining more thoroughly the efficiency and stability of the tandem photoelectrochemical cell, additional experiments were performed in close-to-neutral media. As observed, photocurrents and stability very similar to those observed in acidic media were found.
Despite the high value of Faradaic efficiency for hydrogen generation on the CuCrO2 electrode, photocurrents leading to only 0.006% solar-to-hydrogen (STH) conversion efficiency at a short circuit (zero bias) were determined [33]. Despite the very low STH conversion energy value, some key points should be highlighted. First, photoelectrochemical water splitting employing only oxide photoelectrodes is demonstrated for the first time in an acidic electrolyte. This overcomes the challenge of using either buffered or unbuffered near-neutral pH electrolytes [25,26], which could be unviable for real applications. It is worth noting that while using alkaline electrolytes may lead to carbonated media, working in acidic electrolytes avoids this disadvantage and, in addition, it minimizes ohmic drops in the electrolyte. Furthermore, the use of acidic electrolytes could be relevant in the potential application of the device for CO2 reduction without having to deal with the complexities derived from the formation of (bi)carbonate. Second, the photoelectrodes show remarkable short-to-medium-term stability (above 3 h), which is particularly relevant on account of the absence of any type of protective layer and the use of an acidic electrolyte. Third, even when no co-catalyst was used for the photocathode, a Faradaic efficiency for hydrogen evolution as high as 90% was determined, which is, as far as we know, the highest reported for a catalyst-free oxide photocathode. Fourth, both WO3 and CuCrO2 were synthesized by following a one-step wet procedure, without employing sophisticated physical methods, as for instance those needed in the case of Cu2O for increasing stability [34]. Finally, the electrodes were made of abundant elements, which adds potential applicability to the system.
Admittedly, in comparison with other tandem photoelectrochemical cells of similar configuration [7,24,27], a very modest STH conversion efficiency (0.006%) was obtained. It should be noted, though, that neither the electrodes nor the device design was engineered to optimize the efficiency. Among the all-oxide photoelectrochemical tandem cells, as far as we know, this is the only one showing stable operation in an acidic electrolyte. In the case of alkaline electrolytes, a relatively stable behavior with an efficiency as high as 0.15% was reported for an iron oxide–copper bismuth oxide photoelectrochemical cell [35]. In contrast, STH efficiencies over 4% have been reported for close-to-neutral electrolytes by using sophisticated electrode architectures [36]. In terms of stability, the device presented here is in line with the state of the art, even when working with a highly acidic electrolyte.
There are several ways that one could pursue for increasing the device performance in addition to the application of an external bias. Some can be easily understood based on the sketch presented in Figure 6. Any surface modification of the WO3 electrode that could shift the flat band potential of WO3 toward less positive values would be beneficial. In clear connection with this, a TiO2 electrode would probably give comparable or even higher conversion efficiencies on account of its significantly lower flat band potential, even when only UV light would be useful. In addition, some strategies to tailor the interfaces of the CuCrO2 with passivating and/or extracting layers and with co-catalysts could be useful for increasing the relatively low IPCE values characterizing this material [14,37]. In any case, one of the main limitations in the STH conversion efficiency for the tandem device proposed here lies in the low solar light absorption capability of both electrode materials. Extensive doping could be one of the ways to address this problem [38,39,40,41,42].

3. Materials and Methods

3.1. Electrode Preparation

Sol–gel CuCrO2 photocathodes were prepared as previously described [18]. Cu(CH3COO)2∙H2O (purity 99.99%+, Sigma-Aldrich, Saint Louis, MO, USA) and Cr(NO3)3∙9H2O (purity 99%, Sigma-Aldrich, Saint Louis, MO, USA) were used as metal precursors. The precursor solution was spin-coated over FTO substrates (U-type, Asahi Glass Co., Japan) and the samples were annealed at 400 °C in air for 1 h. A total of four layers were deposited for synthesizing one electrode (i.e., the process was repeated 4 times to prepare one electrode). Finally, the samples were post-annealed at 650 °C in an N2 atmosphere.
The electrodeposited WO3 photoanodes were prepared according to the procedure used by Luo and Hepel [43]. Briefly, tungsten powder (purity 99.99%, 12 μm, Sigma-Aldrich, Saint Louis, MO, USA) was dissolved in a concentrated hydrogen peroxide solution. Once the exothermic reaction was completed, ultrapure water and isopropanol (Merck p. a., Darmstadt, Germany) were added. This was the working solution used in a conventional three-electrode cell where the deposition was carried out. An Ag/AgCl/KCl(sat) electrode and a Pt wire were employed as a reference and as a counter-electrode, respectively, and a clean piece of FTO glass was used as a working electrode. A potential of −0.4 V vs. Ag/AgCl was applied to the FTO substrate for 30 min. Finally, the samples were post-annealed in air at 450 °C for 1 h.

3.2. Photoelectrochemical Measurements

Photoelectrochemical measurements were performed at room temperature using a cell based on a parallel illumination mode (a closed and sealed glass cell with two compartments separated by a NafionTM membrane (NM-117, DuPont, Wilmington, DE, USA) and a computer-controlled Autolab PGSTAT30 potentiostat (Metrohm Autolab B.V., Utrecht, The Netherlands). Depending on the experiment, an Ag/AgCl/KCl(sat) electrode and a Pt wire were employed as a reference and as a counter-electrode, respectively. Separate potentiostatic current–potential curves for both electrodes in this configuration were carried out in 0.1 M HClO4, which was prepared with ultrapure water and purged with N2 before the measurements. The illumination was carried out from the electrode substrate side by employing a solar simulator SUN 2000 (Abet Technologies, Milford, CN, USA). The light intensity was adjusted with a neutral density filter down to 100 mW cm−2 as measured with an optical power meter (Thorlabs model PM100D, Newton, NJ, USA). The illuminated area of each electrode was 2 cm2.

3.3. Tandem Cell Measurements

Currents through the cell were measured using two different working electrolytes: 0.5 M Na2SO4 and 0.1 M HClO4 under 1 sun illumination, as previously described. Perchloric acid was chosen as to avoid significant adsorption of the anion on the photoelectrodes. All the electrolytes were prepared with ultrapure water and purged with N2 before the experiments unless otherwise mentioned. The illuminated area of each electrode was 2 cm2.

3.4. Hydrogen Evolution Detection by Gas Chromatography

The analysis of hydrogen gas production was conducted using a Hewlett Packard 5890 gas chromatograph (Palo Alto, CA, USA) equipped with a thermal conductivity detector (TCD). The experiments were carried out in an airtight cell. The working electrode was a CuCrO2 photocathode (2 cm2 of illuminated area). In the counter-electrode compartment, a platinum sheet was placed, and in the reference compartment, an Ag/AgCl/KCl(sat) electrode was employed. The cell was filled with N2-purged 0.1 M HClO4, and prior to the experiments, its headspace was purged using a vacuum pump and filled with nitrogen gas several times. Samples of 300 μL were extracted from the headspace of the working electrode compartment.

4. Conclusions

In this work, we demonstrate that a WO3–CuCrO2 photoelectrochemical tandem cell is able to perform overall water splitting under simulated sunlight without the application of external bias. It is remarkable that the device works with strongly acidic electrolytes, which avoids complications linked to ohmic drops or electrolyte carbonation, showing remarkable short-to-medium-term stability (more than 3 h under operation without significant degradation). As far as we know, this is the first tandem cell with unprotected, co-catalyst-free metal oxide electrodes able to work in acidic media. In addition, it is worth noting that the Faradaic efficiency for hydrogen evolution is around 90% for the CuCrO2 photocathode without the use of co-catalysts, as far as we know the highest value reported to date for a co-catalyst-free oxide photocathode. Even though only a modest STH conversion efficiency was obtained for the tandem cell, some enhancement strategies apart from applying a bias could be pursued aiming to increase the solar light absorption, such as extensive doping of both electrode materials. In any case, this study illustrates the feasibility of overall water splitting working in acidic media with a tandem photoelectrochemical cell in which the electrodes are made of earth-abundant metal oxides by following simple and scalable procedures.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/molecules29184462/s1, Figure S1: Electrode potential vs. time curves during photoelectrolysis; Figure S2: Current density–voltage (j-V) curves; Figure S3: Photograph of the two-electrode PEC cell under operation.

Author Contributions

Conceptualization, R.G.; methodology, A.K.D.-G.; validation, A.K.D.-G. and R.G.; formal analysis, A.K.D.-G. and R.G.; investigation, A.K.D.-G.; resources, R.G.; data curation, A.K.D.-G. and R.G.; writing—original draft preparation, R.G.; writing—review and editing, A.K.D.-G. and R.G.; visualization, A.K.D.-G.; supervision, R.G.; project administration, R.G.; funding acquisition, R.G. All authors have read and agreed to the published version of the manuscript.

Funding

These results are part of the project TED2021–132697B–I00, funded by MCIN/AEI/10.13039/501100011033 and by the European Union “NextGenerationEU”/PRTR. We are also grateful to the Ministerio de Ciencia e Innovación/Agencia Estatal de Investigación/Fondos FEDER for financial support through project PID2021-128876OB-I00.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

A. K. D.-G. thanks the Mexican government (CONACYT) for the award of a doctoral grant.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. SEM images corresponding to (A) WO3 and (B) CuCrO2 thin-film electrodes.
Figure 1. SEM images corresponding to (A) WO3 and (B) CuCrO2 thin-film electrodes.
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Figure 2. Cyclic voltammograms for (A) WO3 and (C) CuCrO2 electrodes in the dark in N2-purged 0.1 M HClO4. Scan rate 20 mV s−1. Linear scan voltammograms for (B) WO3 and (D) CuCrO2 electrodes in 0.1 M HClO4 purged with N2 under chopped simulated solar illumination (100 mW cm−2). Scan rate 5 mV s−1.
Figure 2. Cyclic voltammograms for (A) WO3 and (C) CuCrO2 electrodes in the dark in N2-purged 0.1 M HClO4. Scan rate 20 mV s−1. Linear scan voltammograms for (B) WO3 and (D) CuCrO2 electrodes in 0.1 M HClO4 purged with N2 under chopped simulated solar illumination (100 mW cm−2). Scan rate 5 mV s−1.
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Figure 3. IPCE spectra for (A) CuCrO2 and (B) WO3 electrodes in 0.1 M HClO4 purged with N2 at 0.10 VAg/AgCl and 0.96 VAg/AgCl, respectively.
Figure 3. IPCE spectra for (A) CuCrO2 and (B) WO3 electrodes in 0.1 M HClO4 purged with N2 at 0.10 VAg/AgCl and 0.96 VAg/AgCl, respectively.
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Figure 4. Linear scan voltammograms under chopped illumination for CuCrO2 (blue line) and WO3 (green line) electrodes in 0.1 M HClO4 purged with N2. Note that both cathodic and anodic currents are plotted as positive. The inset shows a detail of the region of curve crossing.
Figure 4. Linear scan voltammograms under chopped illumination for CuCrO2 (blue line) and WO3 (green line) electrodes in 0.1 M HClO4 purged with N2. Note that both cathodic and anodic currents are plotted as positive. The inset shows a detail of the region of curve crossing.
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Figure 5. (A) Photocurrent vs. voltage curve (recorded at a scan rate of 2 mV s−1) for the tandem photoelectrochemical cell in 0.1 M HClO4 purged with N2 under illumination with a solar simulator (100 mW cm−2). (B) Comparative chronoamperometric curves for the tandem cell in either 0.1 M HClO4 or 0.5 M Na2SO4 (both purged with N2) under simulated solar illumination (100 mW cm−2) at zero bias. V = Ephotoanode − Ephotocathode.
Figure 5. (A) Photocurrent vs. voltage curve (recorded at a scan rate of 2 mV s−1) for the tandem photoelectrochemical cell in 0.1 M HClO4 purged with N2 under illumination with a solar simulator (100 mW cm−2). (B) Comparative chronoamperometric curves for the tandem cell in either 0.1 M HClO4 or 0.5 M Na2SO4 (both purged with N2) under simulated solar illumination (100 mW cm−2) at zero bias. V = Ephotoanode − Ephotocathode.
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Figure 6. Sketch of the different potential levels relevant for the photoelectrodes and the electrolyte for the WO3/CuCrO2 photoelectrochemical tandem device.
Figure 6. Sketch of the different potential levels relevant for the photoelectrodes and the electrolyte for the WO3/CuCrO2 photoelectrochemical tandem device.
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MDPI and ACS Style

Díaz-García, A.K.; Gómez, R. A WO3–CuCrO2 Tandem Photoelectrochemical Cell for Green Hydrogen Production under Simulated Sunlight. Molecules 2024, 29, 4462. https://doi.org/10.3390/molecules29184462

AMA Style

Díaz-García AK, Gómez R. A WO3–CuCrO2 Tandem Photoelectrochemical Cell for Green Hydrogen Production under Simulated Sunlight. Molecules. 2024; 29(18):4462. https://doi.org/10.3390/molecules29184462

Chicago/Turabian Style

Díaz-García, Ana K., and Roberto Gómez. 2024. "A WO3–CuCrO2 Tandem Photoelectrochemical Cell for Green Hydrogen Production under Simulated Sunlight" Molecules 29, no. 18: 4462. https://doi.org/10.3390/molecules29184462

APA Style

Díaz-García, A. K., & Gómez, R. (2024). A WO3–CuCrO2 Tandem Photoelectrochemical Cell for Green Hydrogen Production under Simulated Sunlight. Molecules, 29(18), 4462. https://doi.org/10.3390/molecules29184462

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