Photoanode For Photoelectrochemical Water Oxidation Research Articles

Unraveling oxygen vacancy changes of WO3 photoanodes for promoting oxygen evolution reaction

Oxygen vacancy (VO) on semiconductor photoanode plays an important role in enhancing photoelectrochemical water oxidation performances. Nonetheless, there is still a lack of definitive elucidation regarding the structural changes and their impact on charge transport during the oxygen evolution reaction (OER). Herein, oxygen vacancies were rationally introduced on WO3 nanoflake photoanodes via Ar-plasma engraving, resulting in a threefold increase in the photocurrent density of 2.76 mA cm−2 at 1.23 VRHE under AM 1.5 G solar irradiation compared to the pristine WO3 photoanode. Comprehensive experiments and theoretical calculations reveal that the self-healing process of surface oxygen vacancies on WO3 photoanodes should be more easily achieved by capturing oxygen atoms from adsorbed H2O molecules. However, some survived oxygen vacancies in the subsurface could effectively increase the charge carrier density and provide the additional driving force to accelerate the interfacial charge transport, leading to enhanced photoelectrochemical (PEC) activities. More importantly, the oxygen vacancy self-healing on metal-oxide semiconductors is a universal phenomenon, which might bring new insights for design and construction of highly efficient photoanodes for PEC water oxidation.

Improving the charge separation efficiency by embedding the electron transfer layer of hematite photoanode for photoelectrochemical water oxidation

The photogenerated hole transfer from the bulk to the surface is highly valued in the rational design of advanced photoanodes. However, the transfer of photogenerated electrons to the FTO (fluorine-doped tin oxide glass) substrate has a vital influence on the separation behaviors of photogenerated charges as well, which would regulate the photoelectrochemical (PEC) performance of photoanodes in turn. Herein, the W-TiO2/Ti-Fe2O3 heterojunction was fabricated for the first time by inserting W-TiO2 as the electron transfer layer between the Ti-Fe2O3 photoanode and the FTO substrate. Benefitting from the unique electron transport channel of W-TiO2, the photogenerated electrons in Ti-Fe2O3 could smoothly migrate to FTO, while photogenerated holes water oxidation kinetics is promoted and then easily participate in water oxidation reaction. As expected, W-TiO2/Ti-Fe2O3 exhibits a remarkable negative onset potential and a slight improved photocurrent density.

BiVO4 Heterojunctions as Efficient Photoanodes for Photoelectrochemical Water Oxidation

AbstractPhotoelectrochemical (PEC) water splitting has attracted strong interest as sustainable technology by converting solar energy into hydrogen. The semiconductor photoelectrodes play an important role to increase the solar‐to‐hydrogen conversion efficiency. Bismuth vanadate (BiVO4) is an excellent candidate as a photoanode material for PEC water oxidation because of its visible light absorption, suitable band edge location, high stability and low cost. However, BiVO4 alone may undergo short carrier diffusion length, rapid recombination of photo‐induced charge carriers and photocorrosion. The heterojunction strategy established by combining two or more materials has provided an outstanding technique to address these issues. This Review focuses on recent important works with respect to BiVO4 heterojunction photoanodes for PEC water oxidation reactions, including interface junction and surface functional junction formation. Additionally, challenges faced and prospects for future research on BiVO4 heterojunction photoanodes in the field of solar‐to‐hydrogen conversion are proposed.

Monolithic FAPbBr3 photoanode for photoelectrochemical water oxidation with low onset-potential and enhanced stability

Despite considerable research efforts on photoelectrochemical water splitting over the past decades, practical application faces challenges by the absence of efficient, stable, and scalable photoelectrodes. Herein, we report a metal-halide perovskite-based photoanode for photoelectrochemical water oxidation. With a planar structure using mesoporous carbon as a hole-conducting layer, the precious metal-free FAPbBr3 photovoltaic device achieves 9.2% solar-to-electrical power conversion efficiency and 1.4 V open-circuit voltage. The photovoltaic architecture successfully applies to build a monolithic photoanode with the FAPbBr3 absorber, carbon/graphite conductive protection layers, and NiFe catalyst layers for water oxidation. The photoanode delivers ultralow onset potential below 0 V versus the reversible hydrogen electrode and high applied bias photon-to-current efficiency of 8.5%. Stable operation exceeding 100 h under solar illumination by applying ultraviolet-filter protection. The photothermal investigation verifies the performance boost in perovskite photoanode by photothermal effect. This study is significant in guiding the development of photovoltaic material-based photoelectrodes for solar fuel applications.

WO3 layer sensitized with BiVO4 and MIL-101(Fe) as photoanode for photoelectrochemical water oxidation

Thick tungsten oxide layers were prepared electrophoretically in order to be used as photoanodes in photoelectrochemical water oxidation applications. The highest photocurrent density was obtained for a WO3 layer with thickness of ∼900 nm. Additionally, WO3/BiVO4 and WO3/BiVO4/MIL-101(Fe) heterojunctions have been fabricated using WO3 layer as substrate. WO3/BiVO4 shows an increased value of the electrochemical active surface area indicating that more sites of this photoanode are activated through the formation of the heterojunction. The small V5+ signals observed in the V 2p XPS spectra of these heterojunctions were attributed to the substitution of V5+ atoms with W6+ atoms on the surface of BiVO4. Excessive W doping of the BiVO4 film determined the decrease of the photoelectrochemical performance of WO3/BiVO4 photoanode. The significant improvement of the photoconversion efficiency of the sample decorated with MIL-101(Fe) indicated that this co-catalyst provides sites more efficiently in the photoelectrochemical process. This performance was correlated with the reduced value of the charge transfer resistance at electrode/electrolyte interface obtained from electrochemical impedance spectroscopy investigation.

Photoelectrochemical water oxidation and methylene blue degradation enhanced by Nb doping and CoPi modification for hematite photoanodes

Nanostructured α-Fe2O3 films doped with niobium and supported with CoPi cocatalyst (Nb-α-Fe2O3 @CoPi) are prepared by hydrothermal growth together with electrochemical deposition. The effect of Nb doping and CoPi cocatalyst modification on the performance of photoelectrochemical water oxidation and methylene blue degradation of α-Fe2O3 is studied in detail. It shows that Nb doping and CoPi modification toward α-Fe2O3 not only enhances its light absorption, but also improves its electrical properties, including conductivity, bulk charge separation efficiency and charge transfer efficiency at the electrode/electrolyte interface. When used as photoanode for photoelectrochemical water oxidation, the Nb-α-Fe2O3 @CoPi photoanodes show a negative shift of 540 mV in the onset potential in comparison with the pristine α-Fe2O3 films. At the same time, the photocurrent density (at 1.6 V, vs. RHE) of Nb-α-Fe2O3@CoPi is more than three times that of the pristine α-Fe2O3, reaching 1.44 mA·cm−2. In addition, the obtained Nb-α-Fe2O3 @CoPi photoanodes also show excellent performance in visible light-driven photoelectrochemical degradation of methylene blue, a common pollutant in wastewater. For example, a degradation rate of 98% is achieved in 2 h by the Nb-α-Fe2O3 @CoPi, which is 3.6 times that of the pristine α-Fe2O3 (26%).

Temperature of photoanode for photoelectrochemical water oxidation

Solar-light-driven photoelectrochemical that can utilise water for high-performance artificial photosynthesis for low-cost green hydrogen fuel production. However, the interaction of photoanode with water is crucial to its heterogeneous water oxidation for sustainable fuel production for the replacement of fossil fuels. Here, we report experiment investigations of photoelectrode with temperature for solar energy conversion. These measurements revealed that a rise in temperature affects the catalytic generation of hydrogen in three routes, viz., minimising the required energy for water splitting, reducing bandgap energy and mitigating the resistance at the interface.

NiFePB-modified ZnO/BiVO4 photoanode for PEC water oxidation.

Photoelectrochemical (PEC) water splitting has been recognized as the most promising approach for directly converting solar energy into chemical energy, and substantial efforts have been made to develop a highly efficient and low-cost photoanode for enhancement of PEC water splitting efficiency due to sluggish water oxidation reaction kinetics. A ternary NiFePB-modified ZnO/BiVO4 heterojunction photoanode was simply assembled by low-temperature hydrothermal, metal-organic decomposition and electrodeposition methods to improve the water splitting efficiency; its photocurrent density for water oxidation reached 1.66 mA cm-2 at 1.23 V (vs. RHE); in comparison, that of ZnO is only 0.4 mA cm-2. The onset potential manifests a cathodic shift of ∼283 mV compared to ZnO. The IPCE and the ABPE respectively are 3.1 and 6.4 times those of ZnO, respectively. This improvement is ascribed to the efficient separation of photogenerated electrons and holes by the formation of a heterojunction between ZnO and BiVO4 and the enhancement in the oxygen evolution reaction kinetics by the decoration of the co-catalyst NiFePB as a hole acceptor.

Engineering the Interfacial Structure of Heavy Metal‐Free Colloidal Heterostructured Quantum Dots for High‐Efficiency Photoelectrochemical Water Oxidation without Co‐Catalyst

Solar‐driven photoelectrochemical (PEC) water splitting cell fabricated using colloidal quantum dots (QDs) is deemed as low‐cost and high‐efficiency solar‐to‐fuel conversion systems for future carbon neutrality. However, current QDs‐based PEC technologies are still hindered by several critical restrictions including the sluggish water oxidation kinetics and the frequent use of highly toxic QDs (e.g., Pb, Cd‐chalcogenides) as well as co‐catalysts, thus limiting their prospective commercial developments. Herein, the optoelectronic properties of heavy metal‐free InP/ZnSe core/shell QDs are tailored by introducing the interfacial GaP layers with variable thicknesses. As‐prepared InP/GaP/ZnSe core/shell QDs are used to sensitize TiO2 film as photoanodes for PEC water oxidation, showing an unprecedented photocurrent density of 4.1 mA cm−2 at 1.23 V versus reversible hydrogen electrode with excellent durability under one sun AM 1.5 G illumination. It is demonstrated that engineering the thickness of the interfacial GaP layer enables optimized optical properties and can introduce appropriate intermediate energy levels to promote the charge extraction from the InP core to the ZnSe shell for enhanced PEC water oxidation efficiency. This study paves the way for interfacial engineering of “green” core/shell QDs to realize cost‐effective, environment‐friendly, high‐performance, and co‐catalyst‐free QDs‐based PEC water splitting system.

Integration of an Electron Transport Layer and a p‐n Heterojunction in a ZnO photoanode for Photoelectrochemical Water Oxidation

AbstractZinc oxide (ZnO) has received extensive attention in the field of photoelectrochemical water splitting (PEC‐WS). However, ZnO has a narrow absorption range for visible light, easy recombination of photogenerated electrons and holes, and slow kinetics of surface water oxidation, limiting its further practical application. In this work, a FTO/TiO2/ZnO/NiO photoanode with a micro‐nano structure is built with ZnO as nanosheet clusters, TiO2 as the electron transport layer, and NiO for forming p‐n heterojunction between NiO and ZnO. This photoanode exhibits a photocurrent density of 1.91 mA/cm2 at 1.23 VRHE, which is three times that of the pure ZnO photoanode (0.65 mA/cm2). The detailed mechanism investigations demonstrate that the introduced TiO2 can reduce the interface defects between ZnO and FTO, and Ti doping to ZnO improves the conductivity, which simultaneously reduces the bottom surface and bulk charge recombination. The constructed p‐n heterojunction further significantly increases the carrier transfer efficiency.

Accelerated oxygen evolution kinetics on hematite by Zn2+ for boosting the photoelectrochemical water oxidation

The sluggish oxygen evolution reaction (OER) kinetics on hematite (α-Fe2O3) has constrained the photoelectrochemical (PEC) performance for water oxidation. Considering the regulatory effect of Zn for electrocatalytic OER, the Zn doped α-Fe2O3 nanowire is successfully prepared and served as the photoanode for PEC water oxidation. The results reveal the theoretical overpotential for the limiting step (from OH* to O*) is decreased by ca. 320 mV after Zn doing, which indicates an incredibly improved catalytic performance of OER. The Zn doped α-Fe2O3 displays PEC performance improvement over the bare α-Fe2O3, photocurrent density increases up to 0.88 mA/cm2(1.23 VRHE, 1 M NaOH), which shows a 44-fold increase than the α-Fe2O3. Meanwhile, the onset potential also shifts negatively by 300 mV, which also agrees well with the decreased OER overpotential. This work provides a deep understanding of the role of Zn in the α-Fe2O3 photoanodes and the strategy could be extended to other photoactive systems.

Reconstructing Oxygen Vacancies in the Bulk and Nickel Oxyhydroxide Overlayer to Promote the Hematite Photoanode for Photoelectrochemical Water Oxidation

Reconstructing Oxygen Vacancies in the Bulk and Nickel Oxyhydroxide Overlayer to Promote the Hematite Photoanode for Photoelectrochemical Water Oxidation

Elucidating the Role of Hypophosphite Treatment in Enhancing the Performance of BiVO4 Photoanode for Photoelectrochemical Water Oxidation.

Slow water oxidation kinetics and poor charge transport restrict the development of efficient BiVO4 photoanodes for photoelectrochemical (PEC) water splitting. Oxygen vacancy as an effective strategy can significantly enhance charge transport and improve conductivity in semiconductor photoanodes. Herein, we obtained BiVO4 photoanodes with appropriate oxygen vacancy by treating them with hypophosphite, which significantly improved the PEC performance. The synthesized photoanode exhibits a remarkable photocurrent density of 3.37 mA/cm2 at 1.23 V vs reversible hydrogen electrode with excellent stability. Interestingly, the performance improvement mainly originates from the oxygen vacancy rather than P doping. Our study provides insights in understanding the role of oxygen vacancy in PEC water splitting and strategies for designing more efficient photoelectrodes.

Effect of SnO2 incorporation on the photoelectrochemical properties of α-Fe2O3–SnO2 nanocomposites prepared by hydrothermal method

In this work, we report the effect of tin oxide (SnO2) incorporation on the photoelectrochemical (PEC) water oxidation activity of hematite-tin oxide (α-Fe2O3–SnO2) nanocomposites prepared by hydrothermal method. During synthesis of α-Fe2O3–SnO2, the proportions of SnO2 in the composites were varied from 5 wt% to 50 wt%. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), and UV–visible spectroscopy. XRD results showed diffraction signals from both α-Fe2O3 and SnO2 in the composite structure. The surface morphology and size features of α-Fe2O3 altered drastically with addition of SnO2. TEM analysis revealed that small-sized SnO2 were coated on the surface of α-Fe2O3, forming heterogeneous grain boundary interfaces. The α-Fe2O3–SnO2 nanocomposites were tested as photoanode for PEC water oxidation under standard 1 Sun illumination condition in 1 M NaOH. Among the various compositions, α-Fe2O3–SnO2 (50 wt%) displayed promising water splitting photocurrent density of 0.32 mAcm−2 at 1.4 V versus RHE, which is a 5-fold higher as compared to the photocurrent density of pure α-Fe2O3 (0.06 mAcm−2). This enhancement in photocurrent density can be attributed to the reduced electron-hole recombination as a result of better charge separation at local heterojunction between α-Fe2O3 and SnO2 in the composite.

Enhanced solar water oxidation by CoWO4-WO3 heterojunction photoanode

Tungsten oxide (WO3) is a low band-gap semiconductor with valence band maximum favorably located for photo-electrochemical water oxidation under sunlight but its performance suffers from fast recombination of photo-generated electrons and holes. In order to suppress recombination and reduce photocorrosion, herein, we report the fabrication of a facile heterojunction photoanode design by combining n-type WO3 nanoplates with p-type cobalt tungstate (CoWO4) nanocuboids produced by simple chemical precipitation methods followed by their successive deposition on a FTO glass substrate. The hybrid photoanode not only exhibited a reduction of the onset potential but also demonstrated up to 2.4 times increase in the photocurrent density compared to those with the bare WO3 electrode when tested for photo-electrochemical (PEC) water oxidation using techniques such as linear sweep voltammetry, transient photo amperometry, electrochemical impedance spectroscopy under 1.5 AM G simulated solar light. The band aligned hybrid design of the photoanode helped to suppress the recombination of electrons and holes before utilization in water oxidation. In addition, the use of back illumination ensured the avoidance of light shading of primary photo absorber (WO3) by the co-catalyst (CoWO4) as well as reduced photocorrosion of WO 3by electrolyte leading to enhanced and stabilized photocurrent. Thus, the CoWO4 -WO3 based novel photoanode developed in this work shows good promise as a facile, low-cost and durable photoanode for PEC water oxidation, especially at low bias as confirmed by the two-terminal measurements.

Synthesis of diketopyrrolopyrrole and anthraquinone-based polymers of D–A1–D–A2 architecture by direct arylation polycondensation and designing inorganic/organic nano-heterostructured photoanodes for visible light water splitting

Synthesis of D–A1–D–A2 type polymers by direct heteroarylation poly-condensation reaction to make N-doped TiO2 nanorod based inorganic/organic nano-heterostructured photoanode for photoelectrochemical water oxidation.

Synergy between Mn and Co in Mn/CoOx cocatalyst for enhanced photoelectrochemical water oxidation of hematite photoanode

Hematite (α-Fe2O3) photoanode is regarded widely as one of most promising photoanode candidates for photoelectrochemical (PEC) water splitting, while the poor surface reaction kinetics and severe charge recombination hinder its further application. Herein, a bimetallic Mn/CoOx film, prepared by high-temperature annealing of metal–organic frameworks (MOFs), is designed as an efficient cocatalyst to modify α-Fe2O3 photoanode (FTO/Sn@α-Fe2O3-Mn/CoOx) for PEC water oxidation. With an optimized content of Mn/CoOx cocatalyst, the FTO/Sn@α-Fe2O3-Mn/CoOx photoanode exhibits an excellent photocurrent density of 2.66 mA/cm2 at 1.23 VRHE , which is higher than that of CoOx (1.43 mA/cm2) or MnOx (1.86 mA/cm2) single-metal catalyst. The on-set potential achieves a remarkable cathodic shift of 0.11 V. The detailed mechanism studies demonstrate that the electron donation from Mn/CoOx cocatalyst to α-Fe2O3 photoanode can obtain high oxidation state of Mn and Co to enhance photogenerated carriers separation efficiency, reduce recombination and facilitate the OH– transformation, which synergically increases the catalytic activities of FTO/Sn@α-Fe2O3-Mn/CoOx photoanode for PEC water oxidation. Furthermore, a heterojunction, formed by Mn/CoOx doping in the surface α-Fe2O3, can further enhance the photogenerated carriers separation efficiency. This work provides valuable guidance for the design and construction of bimetallic cocatalyst on various photoelectrodes for outstanding PEC water splitting performance.

The kinetics of metal oxide photoanodes from charge generation to catalysis

Generating charge carriers with lifetimes long enough to drive catalysis is a critical aspect for photoelectrochemical and photocatalytic systems, and a key determinant of their efficiency. Metal oxides are widely explored as photoanodes for photoelectrochemical water oxidation. However, their application is limited by the disparity between the picosecond–nanosecond lifetimes of electrons and holes photoexcited in bulk metal oxides versus the millisecond–second timescale of water oxidation catalysis. This Review addresses the charge-carrier dynamics underlying the performance of metal oxide photoanodes and their ability to drive photoelectrochemical water oxidation, alongside comparison with metal oxide function in photocatalytic and electrocatalytic systems. We assess the dominant kinetic processes determining photoanode performance, namely, charge generation, polaron formation and charge trapping, bulk and surface recombination, charge separation and extraction, and, finally, the kinetics of water oxidation catalysis. We examine approaches to enhance performance, including material selection, doping, nanostructuring, junction formation and/or co-catalyst deposition. Crucially, we examine how such performance enhancements can be understood from analyses of carrier dynamics and propose design guidelines for further material or device optimization. Metal oxides are widely used in photoelectrochemical and photocatalytic systems for fuel synthesis and environmental remediation. In this Review, we examine the kinetic challenges, from charge generation to water oxidation catalysis, that determine the performance of metal oxide photo(electro)catalysts.

One-pot synthesis of TiO2/Sb2S3/RGO complex multicomponent heterostructures for highly enhanced photoelectrochemical water splitting

The development of new sources of renewable energy fuels like hydrogen remains challenging considering the nowadays society energy needs on a day-to-day basis. In this context, hybrid nanostructures conformed by TiO2 nanoparticles sensitized with bundles of rod-like Sb2S3 (stibnite) on the surface of reduced graphene oxide (TiO2/Sb2S3/RGO), can pave the way in this direction, as offering heterostructures that can be employed as the active phase in photo-anodes for photoelectrochemical water oxidation. For that, these TiO2/Sb2S3/RGO heterostructures are able to extend the light absorption to the visible range, enhance the charge separation and transportation, and improve the conductivity of the catalyst. Furthermore, the method of synthesis, though simple, implies a one-pot strategy by which the TiO2 nanoparticles and the Sb2S3 rod-like particles are independently produced at the surface of RGO sheets, warranties the proper improved function of the hybrids and offers the engineering of future chalcogenide-based catalysts with promising water splitting photoelectrochemical properties.

Amorphous type FeOOH modified defective BiVO4 photoanodes for photoelectrochemical water oxidation

Herein, a new approach of inducing oxygen vacancy in BiVO4/FeOOH nanostructures is designed, where metal trichalcogenide is introduced. The strategy involves integration of Bi2S3 quantum dots (QDs) in BiVO4/FeOOH to construct defect-rich BiVO4/FeOOH (denoted Vo-BiVO4/FeOOH) as high-performance photoanode for photoelectrochemical water oxidation. Both experimental and theoretical analysis confirm that the excellent PEC performance is due to the introduction of oxygen vacancies and FeOOH cocatalyst, which significantly improves the charge separation. As a result, the obtained Vo-BiVO4/FeOOH photoanode not only demonstrates an optimized Photoelectrochemical (PEC) performance for water oxidation, with photocurrent density of 4.71 mA cm−2 at 1.23 V versus RHE under AM 1.5 G illumination but the interaction of BiVO4 with FeOOH cocatlayst also significantly enhance the stability for Vo-BiVO4/FeOOH without any decrease after 11 h. The oxygen vacancy and FeOOH can effectively enhance the light conversion efficiency, charge transfer efficiencies and charge separation efficiency. This work highlights an effective strategy towards achieving efficient and stable BiVO4 photoanode for sustainable solar energy conversion.