Search

Understanding the role of defect density in thick oxide passivation layer in electrolyte/oxide/semiconductor (EOS) junction photoanode system is critical for efficient photo-electrochemical water splitting with long-term stability.

The activity and selectivity for C2+ products from electrochemical CO2 reduction in a zero-gap membrane-electrode assembly (MEA) are improved using a synchronous KOH-activation and tailoring of Cu catalyst thickness.

We demonstrate Ni–N/C is an effective electrocatalyst for the direct conversion of captured CO2 in monoethanol amine-based aqueous absorbents showing high CO faradaic efficiency (78%) and its high selectivity is maintained in various amine solvents.

Bimetallic CuPd oxide alloy electrocatalysts can promote selective ammonia production from the nitrate reduction reaction by accelerating the rate-determining hydrogenation of nitrite, which is a critical intermediate.

The high capital cost of electrolyzers is one of the major obstacles encountered in the industrial adoption of electrochemical CO2 reduction (CO2R) systems. The anion exchange membrane (AEM) is a major contributor to the cost of electrolyzers. Herein, we propose a strategy for improving the economic feasibility by applying a low-cost porous membrane (PM). For a CO2R reaction (CO2RR), the activity and selectivity of an electrolyzer using a PM were found to be similar to those achieved using an AEM. Furthermore, the physical properties of PM, such as pore size, hydrophobicity, and thickness, can be tuned to suit the CO2RR. A techno-economic analysis of the entire process revealed that adopting a PM is economically advantageous compared to adopting an AEM. Our results not only provide new practical and economical insights into the CO2RR, but also demonstrate its potential extension to a variety of electrolytic systems.

Artificial nitrogen fixation causes excess nitrate (NO3−) production due to an unbalanced nitrogen cycle. Recently, the electrocatalytic nitrate reduction reaction (NO3RR) used to produce value-added chemicals such as ammonia (NH3) has attracted attention as a promising technology for energy and environmental reasons; however, the design of the catalytic material used in this reaction is yet to be fully understood for the production of NH3. Herein, the fundamentals of the NO3RR are introduced to understand the thermodynamics and kinetics of the NO3RR using heterogeneous electrocatalysts, and the analytical methods are explained to provide a precise evaluation of the NO3RR performance. The recent strategies used to design efficient and selective electrocatalysts have been reviewed, including the effects of facets, heterogeneous interfaces, alloying, strain, oxygen vacancies in metal oxides, single atom catalysts, and bio-inspired structures. The critical factors determining the NO3RR activity and selectivity are highlighted in terms of the nitrate adsorption, intermediate nitrite conversion, chemical environment, and intermediate species adsorption upon modifying the electronic and chemical states of the catalyst surface. The NO3RR is potentially applied for the electrochemical synthesis of nitrogen-containing chemicals.

Solar-chemical production is one of the most promising options for producing valuable chemicals from greenhouse gases. An economically attractive and industrially applicable solar-chemical production device not only requires catalyst and/or reactor design, but also auxiliary unit process design, process integration, and optimization. Herein, we report a state-of-the-art monolithic solar-chemical production device having 8.03% solar to CO conversion efficiency and 0.77 to 31.9% CO2 one path conversion. Since the monolithic device directly couples a photovoltaic cell and a CO2 electrolyzer, the power loss due to a current converter can be avoided. According to the solar-chemical production device, a comprehensive process design accounting for CO2 to CO conversion, unreacted CO2 separation, and recycling structure is provided. The process model shows good agreement with experimental data for CO2 conversion in the electrolyzer. A process level techno-economic evaluation and a comprehensive review are also presented to highlight the current state and the economic feasibility of the developed device. Thereafter, we provide a sensitivity analysis in terms of CO2 conversion, membrane cost, solar to chemical efficiency, and current density necessary for economically profitable CO production. The equivalent CO sales cost from a 4 MW production plant is estimated to be $10.9 per kg and the corresponding carbon tax compensating for the price gap of the current market price is $6.6 per kg CO2. The sensitivity analysis demonstrates that >80 mA cm−2 current density or 22% CO2 conversion is desirable to effectively compete with the conventional CO production process.

Electroreduction systems to convert CO2 into CO via Ag electrodes have been intensely studied as a means of producing carbon-neutral fuels or chemical products. However, despite many efforts to maximize the performance of CO-producing systems, the performance of electrochemical cells that produce CO has not yet reached the level of economic viability. Moreover, compared with electrode development attempts, studies on the optimization of large-scale CO-producing systems are lacking, thus impeding the commercialization of electrochemical CO2 reduction systems. In this study, we present optimization results of a pilot-scale CO production system. Operating conditions such as pressure, temperature, and cell voltage were considered as the optimization variables to improve the CO partial current density. To facilitate experiment-based optimization of the pilot-scale operation, we adopted an efficient design of the experiment, for which data points were decided by input–output relations. As a result, the maximum CO partial current reached 2.56 A using a 50 cm2 electrode within 25 experiments. In addition, regression analysis results were provided for future studies on the systematic optimization of electrochemical systems. The operating temperature and CO2 solubility were more highly correlated with the current density and selectivity than was the applied cell voltage, and the CO current density could be predicted with high accuracy.

Cu catalysts prepared by modifying bulk Cu foils have achieved high performance for value-added C2+ compounds from electrochemical CO2 reduction (CO2RR) but the transformation of active sites can be affected by the bulk substrate, which make it complex to design the catalyst. Herein, we newly introduce a simple electrospray pyrolysis method to take advantage of a facile wet-chemical synthesis applicable on non-copper substrates, such as a porous carbon paper, and demonstrate highly enhanced selectivity for C2H4 production from CO2RR. The electrosprayed copper oxide on the carbon paper showed uniquely improved C2 selectivity compared with that on the copper substrate. The improved performance is proposed to be related to the presence of Cu mixed state and retention of morphology of the electrosprayed catalyst on the carbon paper, showing the importance of the substrate. In addition, the C2 product selectivity can be tuned by the electrospray synthesis time as it affects the size of the surface nanostructure as well as the porosity of the catalyst, which can provide an effective way to regulate the C2/C1 ratio.

Electrochemical oxidation processes can affect the electronic structure and activate the catalytic performance of precious-metal and transition-metal based catalysts for the oxygen evolution reaction (OER). Also there are emerging requirements to develop OER electrocatalysts under various pH conditions in order to couple with different reduction reactions. Herein, we studied the effect of pH on the electroactivation of IrNi alloy nanoparticles supported on carbon (IrNi/C) and evaluated the electrocatalytic activities of the activated IrNiOx/C for water oxidation under neutral conditions. In addition, their electronic structures and atomic arrangement were analyzed by in situ/operando X-ray absorption spectroscopy (XAS) and identical location transmission electron microscopy techniques, showing the reconstruction of the metal elements during electroactivation due to their different stabilities depending on the electrolyte pH. IrNiOx/C activated under neutral pH conditions showed a mildly oxidized thin IrOx shell. Meanwhile, IrNiOx/C activated in acidic and alkaline electrolytes showed Ni-leached IrOx and Ni-rich IrNiOx surfaces, respectively. Particularly, the surface of IrNiOx/C activated under alkaline conditions shows IrOx with a high d-band hole and NiOx with a high oxidation state leading to excellent OER catalytic activity in neutral media (η = 384 mV at 10 mA cm−2) whereas much lower OER activity was reported under alkaline or acid conditions. Our results, which showed that electrochemically activated catalysts under different pH conditions exhibit a unique electronic structure by modifying the initial alloy catalyst, can be applied for the design of catalysts suitable for various electrochemical reactions.

Single-atom catalysts (SACs) possess the potential to achieve unique catalytic properties and remarkable catalytic mass activity by utilizing low-coordination and unsaturated active sites. However, smaller particles tend to aggregate into clusters or particles owing to their high surface energy. In addition, support materials that have strong interactions with isolated metal atoms, extremely large surface areas, and electrochemical stability are required. Therefore, sufficient information about these factors is needed to synthesize and utilize SACs. Herein, we review the recent investigations and advances in SACs for the oxygen evolution reaction (OER). We present not only the structural characterization of SACs, but also in situ/operando spectroscopic techniques and computational research for SACs to understand the mechanism and reveal the origin of their excellent OER activity. Furthermore, the OER catalytic activity and stability of SACs are summarized to evaluate the current level of SACs. Currently, research on single-atoms as OER catalysts is in the infant stage for synthesis, characterization and mechanism studies. We discuss some challenges for understanding the fundamentals of SACs and enhancing the catalytic performance of SACs for industrial applications.

Hydrogen peroxide production by enhanced electrocatalysts is an attractive alternative to the present commercial process. While the subnano/atomic dispersion in noble metal nanocatalysts is known to strongly enhance their catalytic efficiency and chemoselectivity, their excessive surface energy and consequent coarsening seriously compromise their physical/chemical stability. Here, we report a subnano/atomically dispersed Pt–Ag alloy (by a simply modified polyol process) that is resistant to agglomeration/Ostwald ripening. This catalyst does not follow a conventional four-electron oxygen reduction reaction (ORR) but selectively produces H2O2 without excessive degradation of its activity. We clarified the role of the alloying element, Ag, as follows: (1) selective activation of two-electron ORR by inhibiting O2 dissociation and (2) suppression of H2O2 decomposition by preventing the H2O2 adsorption. The present approach provides a convenient route for the direct generation of H2O2 as a simple byproduct of electricity generation by fuel-cell systems.

The gradual increase in the atmospheric CO2 concentration is an urgent issue that poses a threat to human beings. Recently, the electrochemical CO2 reduction reaction (eCO2RR) has arisen as a promising and eco-friendly strategy for the storage of electricity from renewable sources as permanent chemical energy, as well as for the conversion of atmospheric CO2 into value-added chemicals. Among various catalysts, transition metals have been employed as heterogeneous electrocatalysts to yield valuable carbon chemicals such as carbon monoxide, formic acid, ethylene, and ethanol. Recent developments in catalysts and electrochemical devices have boosted catalytic activities and product selectivities, bringing the eCO2RR to practically promising levels. However, a lack of study to secure stable catalysts for eCO2RR remains a major obstacle for further progress of this technology. This review focuses on efforts to improve the electrochemical stability of catalysts. First, the catalyst deactivation process, including contaminations by metal impurities or carbon derivatives and changes in catalyst morphology during the eCO2RR, is discussed to understand the origin of insufficient stability. Next, recent progress in strategies for the preparation of highly stable catalyst systems will be presented. The discussion of valuable approaches that effectively prevent deactivation processes, such as the exclusion of metal impurities, periodic electrochemical activation, and the design of stable catalysts through the manipulation of various factors, allows identification of several clues for long-term stability. We hope that this review will inspire future catalyst design and stimulate the development of new ideas for the improvement of electrochemical stability.

In the electrochemical CO2 reduction reaction (CO2RR), Cu has been spotlighted as the only electro-catalyst that can produce multi-carbon molecules, but the mechanism of the selective C2+ production reaction remains elusive. Here, we directly monitored CO2RR intermediates by employing time-resolved attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS), with particular attention to the C1 and C2+ pathways beyond the formation of *CO. Electrodeposited Cu and Cu(OH)2-derived Cu were synthesized, and subsequently employed as a C1 and C2+ activating catalyst and C2+ activating catalyst, respectively. For the first time, a kinetically linked dimer intermediate (*OCCO) was observed and identified as the C2+ path triggering intermediate. The ATR-SEIRAS results suggest that C–C coupling occurs exclusively by CO dimerization toward *OCCO, without the participation of *CHO, which is an intermediate for CH4 production. In the real-time measurements, CO dimerization occurred concurrently with CO adsorption (∼5 s), while proton-coupled reduction toward *CHO has slower kinetics (∼30 s). We demonstrated that the sites showing a high vibrational frequency of *CO on the fragmented Cu surface are the potential active sites for the fast dimerization of CO. This work provides mechanistic insights into the CO2RR pathways and enables the design of efficient C2+-producing catalysts.

The electrochemical reduction of CO2 stores intermittent renewable energy in valuable raw materials, such as chemicals and transportation fuels, while minimizing carbon emissions and promoting carbon-neutral cycles. Recent technoeconomic reports suggested economically feasible target products of CO2 electroreduction and the relative influence of key performance parameters such as faradaic efficiency (FE), current density, and overpotential in the practical industrial-scale applications. Furthermore, fundamental factors, such as available reaction pathways, shared intermediates, competing hydrogen evolution reaction, scaling relations of the intermediate binding energies, and CO2 mass transport limitations, should be considered in relation to the electrochemical CO2 reduction performance. Intensive research efforts have been devoted to designing and developing advanced electrocatalysts and improving mechanistic understanding. More recently, the research focus was extended to the catalyst environment, because the interfacial region can delicately modulate the catalytic activity and provide effective solutions to challenges that were not fully addressed in the material development studies. Herein, we discuss the importance of catalyst-electrolyte interfaces in improving key operational parameters based on kinetic equations. Furthermore, we extensively review previous studies on controlling organic modulators, electrolyte ions, electrode structures, as well as the three-phase boundary at the catalyst-electrolyte interface. The interfacial region modulates the electrocatalytic properties via electronic modification, intermediate stabilization, proton delivery regulation, catalyst structure modification, reactant concentration control, and mass transport regulation. We discuss the current understanding of the catalyst-electrolyte interface and its effect on the CO2 electroreduction activity.

The fabrication of large-area single crystalline monolayer transition metal dichalcogenides (TMDs) is essential for a range of electric and optoelectronic applications. Chemical vapor deposition (CVD) is a promising method to achieve this goal by employing orientation control or alignment along the crystalline lattice of the substrate such as sapphire. On the other hand, a fundamental understanding of the aligned-growth mechanism of TMDs is limited. In this report, we show that the controlled introduction of H2 during the CVD growth of MoSe2 plays a vital role in the step-edge aligned nucleation on a c-sapphire (0001) substrate. In particular, the MoSe2 domains nucleate along the [112[combining macron]0] step-edge orientation by flowing H2 subsequent to pure Ar. Systematic studies, including the H2 introduction time, flow rate, and substrate temperature, suggest that the step-edge aligned nucleation of MoSe2 can be controlled by the hydrogen concentration on the sapphire substrate. These results offer important insights into controlling the epitaxial growth of 2D materials on a crystalline substrate.

The concept of plant vision refers to the fact that plants are receptive to their visual environment, although the mechanism involved is quite distinct from the human visual system. The mechanism in plants is not well understood and has yet to be fully investigated. In this work, we have exploited the properties of TiO2 nanowires as a UV sensor to simulate the phenomenon of photosynthesis in order to come one step closer to understanding how plants see the world. To the best of our knowledge, this study is the first approach to emulate and depict plant vision. We have emulated the visual map perceived by plants with a single-pixel imaging system combined with a mechanical scanner. The image acquisition has been demonstrated for several electrolyte environments, in both transmissive and reflective configurations, in order to explore the different conditions in which plants perceive light.

Here, we successfully synthesized a Ta3N5 thin film using a simple metal–organic-precursor decomposition process followed by its conversion to nitride and studied its photoelectrochemical (PEC) properties to understand charge separation on the surface. Newly synthesized Ta3N5 photoanodes showed a significant difference in the PEC activity in relation to the annealing temperature under ammonia flow, although similar light absorption properties or electronic states were obtained. Charge separation related PEC properties were analyzed using intensity modulated photocurrent density spectroscopy (IMPS) and photocurrent measurements in the absence/presence of scavengers. The charge transfer and recombination rate constants which are related to the photogenerated charge-separation dynamics on the Ta3N5 surface were found to be more sensitively influenced by the ammonia annealing temperatures, and low temperature (700 °C) treated Ta3N5 showed a fast recombination rate constant (kr). In addition, high-efficiency charge injection into the electrolyte on the surface was critically associated with the greatly enhanced photocurrent density of Ta3N5 synthesized at a higher temperature (900 °C) of ammonia annealing.

An electrokinetic analysis of electrochemical CO2 reduction reactions provides information about the decoupled involvement of electron–proton transfer from the Tafel slope and reaction order analyses. Conventionally, a one-electron transfer to CO2 and a chemical proton transfer from HCO3− have been considered to be typical rate-limiting steps. These suggested reaction mechanisms are justified under several assumptions: (1) the bicarbonate ion is a major proton donor, (2) the gaseous CO2 is a carbon source, (3) the reaction mechanism is unaffected by the applied potentials outside the Tafel region, etc. However, recent electrokinetic studies combined with in situ and isotopic experiments raise a question that the above conventional assumptions may not always be valid. Furthermore, there are unresolved issues between the mechanisms suggested by electrokinetic studies. In this review, reported reaction mechanisms of the CO2 reduction reaction are summarized with CO and HCOO− formation as model reaction systems. The reaction pathways are also discussed with a theoretical consideration. A deep investigation into the mechanisms reveals the complex feature of reaction pathways and the difficulty in suggesting the mechanism solely from an electrokinetic analysis.

An electron-mediated CO2-to-chemical conversion system is regarded as one of the effective solutions for the depletion of fossil fuels and the accumulation of atmospheric CO2. In this process, the protons and electrons generated from the water-oxidation reaction at an anode are used during the reduction of CO2 at a cathode, in order to produce high-value hydrocarbon chemicals. Therefore, water oxidation is also a key reaction for the overall electron-mediated CO2-to-chemical conversion. In this work, a facile preparation method is developed for a highly efficient water oxidation electrocatalyst which stably operates in a neutral bicarbonate electrolyte optimized for CO2-reduction conditions. Ni-rich cratered structures were spontaneously formed on the stainless steel surface by harsh electro-oxidation, and the chemical composition changes of Fe and Ni on the catalyst surface dramatically enhance water-oxidation activity showing an overpotential value of 504 mV at 10 mA cm−2 in a CO2-saturated bicarbonate electrolyte. In contrast to a severe degradation in the phosphate electrolyte, the cratered stainless-steel (CSS) catalyst is very stable for an 18 h reaction in the bicarbonate electrolyte. Surface spectroscopic analyses of CSS consistently revealed that the active-surface structure of the NiOOH and adsorbed water molecules is remarkably stable throughout water-oxidation in the neutral bicarbonate electrolyte, while the destruction of Ni structures by the phosphate electrolyte is proposed to cause concomitant activity loss for water oxidation.

In order to realize the modulation of band-gap profile in low-cost and printable CuInxGa1−xS2 thin-film solar cells, a simple chemical route, namely a multi-stage paste coating method, was developed. In particular, with this method, multiple coatings with two precursor solution pastes with different compositions were applied. Elemental analysis techniques confirmed the formation of three different graded Ga distributions (front, back, and front-and-back gradient) throughout the absorber films depending on the coating sequence with two different pastes. The back gradient cell showed the largest power conversion efficiency of 7.29%, which was almost two times larger than those of the non-graded cells.

We prepared TiO2 nanorod (NR) arrays on a fluorine-doped tin oxide substrate and decorated with graphene oxide (GO) to study their photoelectrochemical (PEC) water oxidation activities in two different electrolytes. The PEC performances of GO-decorated TiO2 NR photoanodes were characterized by optical and electrochemical impedance spectroscopy measurements. In 1 M KOH, the photocurrent density of the TiO2 NR film decreased after deposition of GO, while in the neutral pH electrolyte (phosphate buffered 0.5 M Na2SO4), the TiO2 NR photoanode showed enhanced performance after deposition with the 2 wt% GO solution. This was a consequence of the decrease in charge transfer resistance between the electrode surface and the electrolyte. The improvement of photocurrents by GO decoration was obvious near the onset potential of the photocurrents in the neutral pH electrolyte. These opposite contributions of GO on the TiO2 NR photoanodes suggest that GO can promote water oxidation effectively in a neutral electrolyte because depending on the pH of the electrolyte, different chemical species interact with the surface of the photoanode in the water oxidation reaction.

The need for developing sustainable energy sources has generated academic and industrial attention in artificial photosynthesis, inspired by the natural process. In this study, we demonstrate a highly efficient solar energy to fuel conversion device using CO2 and water as feedstock. We developed a thin film photovoltaic technology for the light absorbing component using a low cost, solution based Cu(InxGa1−x)(SySe1−y)2 (CIGS) module fabrication method to provide sufficient potential for the conversion reactions. Our solar-fuel device uses cobalt oxide (Co3O4) nanoparticle thin film deposited with a low temperature coating method as the water oxidation catalyst and nanostructured gold film as the CO2 reduction to CO generation catalyst. We demonstrated that the integrated monolithic device operated by energy only from sunlight, in an absence of any external energy input. The individual components showed the following abilities: solar-to-power conversion efficiency of 8.58% for the CIGS photovoltaic module photoelectrode, overpotential reduction of water oxidation with the Co3O4 catalyst film by ∼360 mV at 5 mA cm−2, and Faradaic efficiency of over 90% by the nanostructured Au catalyst for CO2 reduction to CO. Remarkably, this is the first demonstration of a monolithic and standalone solar-fuel device whose solar-to-fuel conversion efficiency from CO2 and H2O is 4.23%, which is comparable with that of photosynthesis in nature.

A strategy to improve the power conversion efficiency (η) in a quantum dot solar cell (QDSC) is demonstrated with a model system of TiO2/CdS QDSCs. When the electrolyte is changed from polysulfide to a [Co(bpy)3]2+/3+ complex, a higher Voc and η are observed because of its low redox potential. To resolve the slow diffusion nature of [Co(bpy)3]2+/3+ complexes within dense TiO2 nanoparticle/CdS film, a TiO2 nanorod (NR) array anode is applied, which increases η by more than a factor of 3. In addition, introduction of hexanoic acid (HA) in TiO2 NR/CdS film is found to improve η (Jsc as well as Voc) by alleviating the back recombination loss between CoIII and the TiO2 surface. Electrochemical impedance spectroscopy indicates that the charge transfer resistance on the photoanode decreases by suppressing the interfacial charge recombination after HA treatment, although adding HA in the electrolyte impedes diffusion resistance.

To enable water splitting in photoelectrochemical cells, the conduction-band (CB) and valence-band edges must straddle the hydrogen-reduction and water-oxidation potentials; however, the CB edge potential of many photoanodic semiconductors is insufficient. Here, we demonstrate the nanostructured Bi2WO6 for photoanodic application, which has a high, flat-band potential of 0.15 V vs. RHE. Single-phase, orthorhombic Bi2WO6 nano-structures were successfully grown on an FTO substrate through a two-step hydrothermal synthesis with subsequent thermal treatment at 600 °C. The synthesized Bi2WO6 photoanode shows a lower onset potential and improved photocurrents which were enhanced further by a factor of three by coating the surface with Co-Pi. The low onset potential of the Bi2WO6/Co-Pi photoanode results in a higher operating photocurrent in a p/n photodiode cell combined with a p-Si/n-Si/Pt photocathode.

In a dual bandgap system such as WO3/BiVO4, the morphology of each component should be controlled by understanding its properties, in particular with respect to the charge flow in the system. For WO3/BiVO4 photoanodes, a porous BiVO4 film allows contact of an electrolyte to the bottom layer with enhanced surface area, thereby promoting the oxidation reaction, while one-dimensional (1-D) WO3 nanorods, directly grown on F-doped tin oxide, are advantageous for transporting electrons to the back contact. The morphology of the BiVO4 film covered by 1-D WO3 nanorods varies with the addition of organic additives such as ethylcellulose in the metal precursor solution. The cross-sectional images from scanning electron microscopy show that 1-D WO3 nanorods is coated with the BiVO4 layer, which forms a porous top layer that can effectively absorb visible light and enhance charge transfer resulting in enhanced photocurrents. We report on the highest photocurrent at a potential of 1.23 V versus a reversible hydrogen electrode (RHE) by means of a 1-D WO3/BiVO4/Co-Pi photoanode. The strategies for constructing such kind of heterojunctions are well applicable to other dual bandgap photoanodes.

We demonstrate a facile hydrothermal method to grow oriented WO3 nanorods on a transparent conductive substrate without the assistance of any seed layer or structure directing agent for photocatalytic applications. The effects of hydrothermal growth conditions such as reaction time, precursor solution, and post annealing temperature on the crystalline phase and morphology of the WO3 on the FTO substrate are discussed. XRD studies reveal that the as-prepared orthorhombic WO3·0.33H2O nanorods are transformed to the hexagonal phase by post annealing at 400 °C. Moreover, post annealing above 500 °C converts them to monoclinic WO3 nanorods on the FTO substrate, which is the photoactive crystal phase of WO3 for water oxidation. The synthesized WO3 nanorods were revealed to have a single crystalline structure by HRTEM analysis. The photoelectrochemical water splitting properties of the annealed WO3 nanorod arrays were investigated in 0.5 M Na2SO4 under AM 1.5G illumination. The optimized WO3 nanorod arrays exhibit a photocurrent of 2.26 mA cm−2 at 1.23 V (versus RHE), and an incident photon-to-current conversion efficiency (IPCE) of as high as 35% at 400 nm for the photoelectrochemical oxidation of water. This simple hydrothermal method can allow the use of WO3 for photoanodic applications with high efficiency.