Photoelectrochemical Water Oxidation Reaction Research Articles

Unbiased Photoelectrochemical H2O2 Coupled to H2 Production via Dual Sb2S3‐Based Photoelectrodes with Ultralow Onset Potential

AbstractThe productions of hydrogen peroxide (H2O2) and hydrogen (H2) in a photoelectrochemical (PEC) water splitting cell suffer from an onset potential that limits solar conversion efficiencies. Moreover, the formation of H2O2 through two‐electron PEC water oxidation reaction competes with four‐electron oxidation evolution reaction. Herein, we developed the surface selenium doped antimony trisulfide photoelectrode with the integrated ruthenium cocatalyst (Ru/Sb2(S,Se)3) to achieve the low onset potential and high Faraday efficiency (FE) for selective H2O2 production. The photoanode exhibits an outstanding average FE of 85 % in the potential range of 0.4–1.6 VRHE and the H2O2 yield of 1.01 μmol cm−2 min−1 at 1.6 VRHE, especially at low potentials of 0.1–0.55 VRHE with 80.4 % FE. Impressively, an unassisted PEC system that employs light and electrolyte was constructed to simultaneously produce H2O2 and H2 production on both the Ru/Sb2(S,Se)3 photoanode and the Pt/TiO2/Sb2S3 photocathode. The integrated system enables the average PEC H2O2 production rate of 0.637 μmol cm−2 min−1 without applying any addition bias. To our knowledge, this is the first demonstration that Sb2S3‐based photoelectrodes exhibit H2O2/H2 two‐side production with a strict key factor of the system, which represents its powerful platform to achieve high efficiency and productivity and the feasibility to facilitate value‐added products in neutral conditions.

Unbiased Photoelectrochemical H2O2 Coupled to H2 Production via Dual Sb2S3-Based Photoelectrodes with Ultralow Onset Potential.

The productions of hydrogen peroxide (H2O2) and hydrogen (H2) in a photoelectrochemical (PEC) water splitting cell suffer from an onset potential that limits solar conversion efficiencies. Moreover, the formation of H2O2 through two-electron PEC water oxidation reaction competes with four-electron oxidation evolution reaction. Herein, we developed the surface selenium doped antimony trisulfide photoelectrode with the integrated ruthenium cocatalyst (Ru/Sb2(S,Se)3) to achieve the low onset potential and high Faraday efficiency (FE) for selective H2O2 production. The photoanode exhibits an outstanding average FE of 85 % in the potential range of 0.4-1.6 VRHE and the H2O2 yield of 1.01 μmol cm-2 min-1 at 1.6 VRHE, especially at low potentials of 0.1-0.55 VRHE with 80.4 % FE. Impressively, an unassisted PEC system that employs light and electrolyte was constructed to simultaneously produce H2O2 and H2 production on both the Ru/Sb2(S,Se)3 photoanode and the Pt/TiO2/Sb2S3 photocathode. The integrated system enables the average PEC H2O2 production rate of 0.637 μmol cm-2 min-1 without applying any addition bias. To our knowledge, this is the first demonstration that Sb2S3-based photoelectrodes exhibit H2O2/H2 two-side production with a strict key factor of the system, which represents its powerful platform to achieve high efficiency and productivity and the feasibility to facilitate value-added products in neutral conditions.

Photoelectrochemical water oxidation reaction for coated and meta-chemical surface electrodes with Na3[Ru2(μ-CO3)4]

The most economical and efficient method to produce green hydrogen relies on electrochemistry, the electricity for which is of a green origin, and on photocatalytic water splitting. The latter technology involves the use of a photocatalyst or a photo-electrocatalyst to transform solar energy into chemical energy. Vast efforts have thus been dedicated to the pursuit of such a catalyst.The current study focused on the development of a heterogeneous photoelectrochemical system in which the Na3[Ru2(μ-CO3)4] complex was used as a WOC. To that end, two types of electrodes were prepared: coated indium tin oxide (ITO) and meta-chemical surface (MCS) electrodes. Under 420 nm illumination, both electrode types exhibited higher catalytic currents than were observed without light.Our novel application of the Na3[Ru2(μ-CO3)4] complex as a catalyst in the photoelectrochemical water oxidation reaction requires minute amounts of the complex. The study findings add to the knowledge about the water-splitting process, and ultimately, they may facilitate the broader adoption of hydrogen as an environmentally friendly and increasingly accessible energy source.

Boosting photoelectrocatalytic oxygen evolution activity of BiVO4 photoanodes via caffeic acid bridged to NiFeOOH

Bismuth vanadate (BiVO4) is a promising n-type photoanode material for photoelectrochemical (PEC) water splitting. However, its activity is impeded by poor charge carrier transport and sluggish oxygen evolution kinetics. In this study, we reports a Fe doping and caffeic acid (CA)/NiFeOOH (NiFe) modification with BiVO4 photoanode (NiFe/CA/Fe-BiVO4) to enhance the PEC water oxidation performance and stability. The modified BiVO4 demonstrates significant advantages in promoting the PEC performance and stability. Specifically, Fe doping via in-situ electro-deposition results in smaller crystal sizes, larger specific surface areas, and more exposed active sites. Furthermore, co-catalyst NiFe connects with Fe-BiVO4 through CA bridge facilitates a more uniform self-assembly distribution of NiFe on BiVO4 surface. The NiFe/CA/Fe-BiVO4 exhibits an excellent photocurrent density of 6.2 mA/cm2 at 1.23 VRHEand demonstrates good stability at 0.8 VRHE. In addition, the composite photoanode shows a high applied bias photon-to-current efficiency (ABPE) value of 2.15% at 0.65 VRHE. EPR and in-situ FTIR confirm the generation of superoxide active species (O2- and ·O2-) during the PEC water oxidation reaction.

Pentanuclear iron complex for water oxidation: Spectroscopic analysis of reactive intermediates in solution and catalyst immobilization into the MOF-based photoanode

Photoelectrochemical water splitting can produce green hydrogen for industrial use and CO2-neutral transportation, ensuring the transition from fossil fuels to green, renewable energy sources. The iron-based electrocatalyst [FeII4FeIII(μ-3-O)(μ-L)6]3+ (LH = 3,5-bis(2-pyridyl)pyrazole) (1), discovered in 2016, is one of the fastest molecular water oxidation catalysts (WOC) based on earth-abundant elements. However, its water oxidation reaction (WOR) mechanism has not been yet fully elucidated. Here, we present in situ X-ray spectroscopy and electron paramagnetic resonance (EPR) analysis of electrochemical WOR promoted by (1) in water-acetonitrile solution. We observed transient reactive intermediates during the in situ electrochemical WOR, consistent with a coordination sphere expansion prior to the onset of catalytic current. At a pre-catalytic (∼+1.1 V vs. Ag/AgCl) potential, the distinct g ∼ 2.0 EPR signal assigned to FeIII/FeIV interaction was observed. Prolonged bulk electrolysis at catalytic (∼+1.6 V vs. Ag/AgCl) potential leads to the further oxidation of Fe centers in (1). At the steady state achieved with such electrolysis, the formation of hypervalent FeVO and FeIVO catalytic intermediates was inferred with XANES and EXAFS fitting, detecting a short FeO bond at ∼ 1.6 Å. (1) was embedded into MIL-126 MOF with the formation of a (1)-MIL-126 composite. The latter was tested in photoelectrochemical WOR and demonstrated an increase in electrocatalytic current upon visible light irradiation in acidic (pH = 2) water solution. The presented spectroscopic analysis gives further insight into the catalytic pathways of multinuclear systems and should help the subsequent development of more energy- and cost-effective water-splitting catalysts based on earth-abundant metals. Photoelectrocatalytic activity of (1)-MIL-126 confirms the possibility of creating an assembly of (1) inside a solid support and harnessing solar irradiation towards industrial applications of the catalyst.

A Facile Design for Water-Oxidation Molecular Catalysts Precise Assembling on Photoanodes.

Regulating the interfacialcharge transfer behavior between cocatalysts and semiconductors remains a critical challenge for attaining efficient photoelectrochemical water oxidation reactions. Herein, using bismuth vanadate (BiVO4 ) photoanode as a model, it introduces an Au binding bridge as holes transfer channels onto the surfaces of BiVO4 , and the cyano-functionalized cobalt cubane (Co4 O4 ) molecules are preferentially immobilized on the Au bridge due to the strong adsorption of cyano groups with Au nanoparticles. This orchestrated arrangement facilitates the seamless transfer of photogenerated holes from BiVO4 to Co4 O4 molecules, forming an orderly charge transfer pathway connecting the light-absorbing layer to reactive sites. An exciting photocurrent density of 5.06mAcm-2 at 1.23V versus the reversible hydrogen electrode (3.4 times that of BiVO4 ) is obtained by the Co4 O4 @Au(A)/BiVO4 photoanode, where the surface charge recombination is almost completely suppressed accompanied by a surface charge transfer efficiency over 95%. This work represents a promising strategy for accelerating interfacial charge transfer and achieving efficient photoelectrochemical water oxidation reaction.

Photoelectrochemical Water Oxidation Using Cobalt Phosphate‐Modified Nitrogen‐Doped Titania Nanotube Arrays

The synthesis of cobalt phosphate (CoPi)‐modified, nitrogen‐doped TiO2 nanotube arrays (N‐TNAs) for photoelectrochemical (PEC) oxygen evolution under visible light is reported. Because of the nitrogen doping, the N‐TNAs exhibit enhanced visible‐light activity toward the PEC water oxidation reaction. However, the performance is diminished as a result of self‐oxidation by photogenerated holes in the valence band of the N‐TNAs. Compared with the N‐TNAs, the CoPi/N‐TNAs prepared under optimal conditions show a twofold improvement in photocurrent generation and are also sensitive to light with wavelengths as long as 580 nm. Stable oxygen evolution with a Faradaic efficiency approaching unity is demonstrated using the CoPi/N‐TNA photoanodes under simulated sunlight for at least 2 h of operation.

Nail-like α-SnWO4 Array Film with Increased Reactive Facets for Photoelectrochemical Water Splitting

α-SnWO4 is a good candidate for photoelectrochemical (PEC) water splitting due to its suitable band position that straddles the H+/H2 and O2/H2O redox potentials. However, its poor charge transfer ability results in unsatisfactory PEC performance. From the view of material design, a nail-like α-SnWO4 array film was constructed by a hydrothermal method assisted by halogen ions, and the corresponding physical and PEC properties are discussed. Density functional theory (DFT) calculations were also performed to help understand the effect of halogen ions (Cl–, F–) on the growth and exposure facet of α-SnWO4 and the underlying mechanism of PEC performance. The existence of F– decreases the surface energy of the {100} and {001} facets, directions along which the film has better charge transfer, and the {001} facet has a lower barrier in the PEC water oxidation reaction. The α-SnWO4 film photoanode after facet regulation presents better charge transfer ability, and the oxygen evolution rate is 0.16 μmol cm–2 h–1. In addition, a 1.9-fold photoresponse current is obtained during unbiased PEC water splitting.

Photoelectrochemical Water Oxidation over Novel Semiconducting Zinc-Based Metal–Thiolate Framework

Designing an efficient catalyst for a sustainable photoelectrochemical water oxidation reaction is very challenging in the context of renewable energy research. Here, we have introduced a new semiconducting porous zinc-thiolate framework via successful stitching of an "N" donor linker with a triazine-based tristhiolate secondary building unit in the overall architecture. The introduction of both linker and tristhiolate ligand synergistically modifies the architecture by making it a rigid, crystalline, three-dimensional, thermally stable, and porous framework. Our novel zinc-thiolate framework is used as an n-type semiconductor as revealed from the solid-state UV-vis DRS spectroscopic analysis, ac and dc conductivity analysis, and Mott-Schottky plot. This n-type semiconductor-based zinc-thiolate framework is utilized in the photoelectrochemical water oxidation reaction. It displayed a very high efficiency for a visible-light-driven oxygen evolution reaction (OER) in a KOH medium using standard Ag/AgCl as the reference electrode. The superiority of this material was further revealed from the low onset potential (0.822 mV vs RHE), high photocurrent density (0.204 mA cm-2), good stability, and high O2 evolution rate (77 μmol g-1 of oxygen evolution within 2 h), and a good efficiency (ABPE 0.42%, IPCE 29.6% and APCE 34.5%). Furthermore, the porosity in the overall framework seems to be a blessing to the photoelectrochemical performance due to better mass diffusion of the electrolyte. A detailed mechanism for the OER reaction was analyzed through density functional theory analysis suggesting the potential future of this Zn-thiolate framework for achieving a high efficiency in the sustainable water oxidation reaction.

Surface plasmon resonance metal-coupled biomass carbon modified TiO2 nanorods for photoelectrochemical water splitting

Exploring efficient and stable photoanode materials is a necessary link to realize the practical application of solar-driven photoelectrochemical (PEC) water splitting. Hence, we prepared rutile TiO 2 nanorods, with a width of 50 nm, which was growth in situ on carbon cloth (TiO 2 @CC) by hydrothermal reaction. And then, Ag nanoparticles (NPs) and biomass N,S-C NPs were chosen for the additional modification of the fabricated TiO 2 nanorods to produce broccoli-like Ag-N,S-C/TiO 2 @CC nanocomposites. According to the result of ultraviolet–visible diffuse reflectance spectroscopy (UV–vis) and PEC water splitting performance tests, Ag-N,S-C/TiO 2 @CC broadens the absorption region of TiO 2 @CC from the ultraviolet region to the visible region. Under AM 1.5G solar light irradiation, the photocurrent density of Ag-N,S-C/TiO 2 @CC is 89.8 μA·cm −2 , which is 11.8 times higher than TiO 2 @CC. Under visible light irradiation, the photocurrent density of Ag-N,S-C/TiO 2 @CC reaches to 12.6 μA·cm −2 , which is 21.0 times higher than TiO 2 @CC. Moreover, Ag-N,S-C/TiO 2 @CC shows a photocurrent responses in full pH range. It can be found that Ag NPs and N, S-C NPs play key roles in broaden the absorption range of TiO 2 nanorods to the visible light region and, promote the occurrence of PEC water oxidation reaction due to the surface plasmon resonance effect of Ag NPs and the synergistic effect of N,S-C NPs. The mechanism demonstrated that Ag-N,S-C/TiO 2 @CC can separate the photogenerated electron-hole pairs effectively and transfer the photogenerated electrons to the photocathode (Pt plate) in time. This research provides a new strategy for exploration surface plasma metal coupled biomass carbon materials in the field of PEC water splitting.

Transparent Ultramicroelectrodes for Studying Interfacial Charge-Transfer Kinetics of Photoelectrochemical Water Oxidation at TiO2 Nanorods with Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) has been extensively applied to the electrochemical analysis of the surfaces and interfaces of a photoelectrochemical (PEC) system. A semiconductor photoelectrode with a well-defined geometry and active surface area comparable to SECM's tip is highly desired for accurately quantifying interfacial charge-transfer activities and photoelectrochemically generated redox species, where the broadening effects due to the mass transfer gradient and nonlocal electron transfer at a planar semiconductor surface can be minimized. Here, we present a newly developed platform as a SECM substrate for investigating semiconductor PEC activities, which is based on a transparent ultramicroelectrode (UME) fabricated by using two-step photolithographic patterning and ion milling methods. This transparent UME with a 25 μm recessed disk shape is fully characterized with SECM for quantifying the interfacial charge-transfer rates of IrCl62-/IrCl63- by comparing with theoretical results from finite element simulations in COMSOL Multiphysics. When coated with TiO2 nanorods as a model semiconductor material, the transparent UME can be used to quantify the catalytic PEC water oxidation in a feedback mode of SECM by sampling tip and substrate current signals simultaneously. This transparent UME-SECM study provides insights into the potential-dependent PEC water oxidation reaction mechanism and the quantitative analysis of photocurrent contributions from water oxidation and the SECM tip-generated redox mediator. The transparent UME-SECM method can be potentially expanded to other SECM operation modes such as surface interrogation for understanding the dynamics of the electrode surfaces and interfaces of a PEC system.

Semiconductor‐based Photoanodes Modified with Metal‐Organic Frameworks and Molecular Catalysts as Cocatalysts for Enhanced Photoelectrochemical Water Oxidation Reaction

AbstractPhotoelectrochemical (PEC) water splitting is a promising approach to convert inexhaustible solar energy to hydrogen energy. In this system, the semiconductor is a crucial part as photoelectrode to harvest the solar light and achieve water splitting on the surface. For the surface modification of semiconductor, cocatalyst loading is an effective method to improve the sluggish reaction kinetics. Due to the specific structure of their organic ligands, the metal‐organic frameworks (MOFs) or molecular catalysts as cocatalysts can provide more reaction sites and overcome the interface recombination between semiconductors and cocatalyst layers. The inorganic metal species derived from the MOFs or molecular catalysts also possess porous structures. This review summarizes the recent developments of MOFs and molecular catalysts as well as their derivatives modifying several representative semiconductors for PEC water oxidation reaction. The synthesis methods and performance enhancements are discussed. Finally, this review addresses remarks on current existing changes and perspectives for the further development.

Engineering Nanostructure-Interface of Photoanode Materials Toward Photoelectrochemical Water Oxidation.

Photoelectrochemical (PEC) water oxidation based on semiconductor materials plays an important role in the production of clean fuel and value-added chemicals. Nanostructure-interface engineering has proven to be an effective way to construct highly efficient PEC water oxidation photoanodes with good light capture, carrier transport, and water oxidation kinetics. However, from theoretical and application perspectives, the relationship between the nanostructure and interface of photoanode materials and their PEC performance remains unclear. In this review, the PEC water oxidation reaction mechanism and evaluation criteria are briefly presented. The theoretical basis and research status of the nanostructure-interface engineering on constructing high-performance PEC water oxidation photoanodes are summarized and discussed. Finally, the current challenges and the future opportunities of nanostructure-interface engineering for the PEC reactions are pointed out.

Enhancing the Photoelectrochemical Water Oxidation Reaction of BiVO4 Photoanode by Employing Carbon Spheres as Electron Reservoirs

The rate-determining step of the photoelectrochemical (PEC) water splitting is the water oxidation reaction at the photoanode, which is 4 orders of magnitude slower than the water reduction reactio...

Better Together: Ilmenite/Hematite Junctions for Photoelectrochemical Water Oxidation.

Hematite (α-Fe2O3) is an earth-abundant indirect n-type semiconductor displaying a band gap of about 2.2 eV, useful for collecting a large fraction of visible photons, with frontier energy levels suitably aligned for carrying out the photoelectrochemical water oxidation reaction under basic conditions. The modification of hematite mesoporous thin-film photoanodes with Ti(IV), as well as their functionalization with an oxygen-evolving catalyst, leads to a 6-fold increase in photocurrent density with respect to the unmodified electrode. In order to provide a detailed understanding of this behavior, we report a study of Ti-containing phases within the mesoporous film structure. Using X-ray absorption fine structure and high-resolution transmission electron microscopy coupled with electron energy loss spectroscopy, we find that Ti(IV) ions are incorporated within ilmenite (FeTiO3) near-surface layers, thus modifying the semiconductor–electrolyte interface. To the best of our knowledge, this is the first time that an FeTiO3/α-Fe2O3 composite is used in a photoelectrochemical setup for water oxidation. In fact, previous studies of Ti(IV)-modified hematite photoanodes reported the formation of pseudobrookite (Fe2TiO5) at the surface. By means of transient absorption spectroscopy, transient photocurrent experiments, and electrochemical impedance spectroscopy, we show that the formation of the Fe2O3/FeTiO3 interface passivates deep traps at the surface and induces a large density of donor levels, resulting in a strong depletion field that separates electron and holes, favoring hole injection in the electrolyte. Our results provide the identification of a phase coexistence with enhanced photoelectrochemical performance, allowing for the rational design of new photoanodes with improved kinetics.

BiVO4 photoanode decorated with cobalt-manganese layered double hydroxides for enhanced photoelectrochemical water oxidation

Bismuth vanadate (BiVO4) is being widely identified as a leading n-type semiconductor material for photoelectrochemical (PEC) water splitting. Nevertheless, achieving efficient PEC water oxidation process through BiVO4 photoanode still faces serious challenge such as severe electron-hole recombination. In this case, PEC activity of BiVO4 photoanode was enhanced by decoration of three-dimensional CoMn-layered double hydroxide (CoMn-LDH) nanoflakes on the BiVO4 surface via a facile electrodeposition process. It was suggested that CoMn-LDH played a synergistic effect on broadening internal light absorption, which accelerated injection of holes carrier to electrolyte and alleviated the electron-hole recombination, resulting in expediting faster PEC water oxidation reaction kinetics. Consequently, the photocurrent density of BiVO4/CoMn-LDH photoanode achieved 2.69 mA cm−2 at 1.23 VRHE, 2.45 times higher than the pristine BiVO4. What's more, 220 mV negative-shift took place on onset potential that was further decreased to 0.31 VRHE. The vastly enhanced PEC performance was also prioritized to those of Co and Mn single relatives. This work demonstrated that the synergistic BiVO4/CoMn-LDH as a capable candidate material, can be utilized for effective PEC water splitting.

GaN nanowires grown by halide chemical vapour deposition as photoanodes for photo-electrochemical water oxidation reactions

Manifold morphologies of GaN nanowires (NWs) were fabricated using halide chemical vapour deposition (HCVD) on an n-Si (111) substrate and demonstrated to be a promising photoelectrode for photo-electrochemical (PEC) water splitting applications. We report a substantial enhancement in the photocurrent for vertically-grown GaN NWs on a buffer layer as compared to other counterparts such as GaN whiskers, tapered nanostructures and thin films. GaN NWs grown on Si have advantages due to the absorption of photons in a wide spectral range from ultraviolet to infrared and thus are directly involved in PEC reactions. A GaN NW photoanode was demonstrated with a saturation photocurrent density of 0.55 mA cm−2 under 1 sun of illumination, which is much greater than its counterparts. The role of the buffer layer and the carrier density on the PEC performance of vertically-grown GaN NW photoanodes is further elucidated. Photo-electrochemical impedance spectroscopy and Mott-Schottky characterizations were employed to further explain the PEC performance of GaN NW embedded photoanodes. Here, photoanodes based on diverse GaN nanostructures were examined for a better PEC evaluation in order to support the conclusion. The results may pave the way for the fabrication of efficient photoelectrodes and GaN as a protective layer against corrosion for improved photo-stability in an NaOH electrolyte for enhancing the efficiency of water splitting.

A New Porous Ni‐W Mixed Metal Phosphonate Open Framework Material for Efficient Photoelectrochemical OER

AbstractPhotoelectrochemical water oxidation or oxygen evolution reaction (PEC‐OER) at the semiconductor surfaces has attracted huge attention over the years as it plays key role for energetically demanding processes like fuel cells, water splitting, and rechargeable metal‐air batteries. Thus, development of sustainable, abundant and economical semiconductor nanomaterials as the catalyst for photoelectrochemical water oxidation reaction is very challenging in achieving high efficiency in the overall water splitting reaction. Herein, we report a new porous Ni−W mixed metal phosphonate material NiWPPA‐4 with open framework structure through simple hydrothermal route and its crystalline framework has been indexed into a new orthorhombic crystal with unit cell dimensions: a=14.5506 Å, b=17.6445 Å, c=20.1766 Å and α=β=γ=90°. NiWPPA‐4 has been successfully utilized for photoelectrochemical OER from moderately basic medium [1(M) KOH] under constant irradiation of 100 mWcm−1 light source. Semiconducting property together with nanoscale porosity in the Ni−W mixed metal phosphonate is responsible for photoelectrochemical OER in alkaline pH. To the best of our knowledge it is the first report on a mixed metal phosphonate material, which showed excellent catalytic activity in the photoelectrochemical OER. The onset‐potential of NiWPPA‐4 prepared over ITO coated glass was only −410 mV vs Ag/AgCl and the oxygen evolution rate is 275 μmol g−1. This study thus established efficient photoelectrochemical OER on porous mixed‐metal phosphonates and offer opportunities for the utilization of metal phosphonates as potential photocatalyst in the energy conversion processes.

Photoelectrochemical water oxidation in α-Fe2O3 thin films enhanced by a controllable wet-chemical Ti-doping strategy and Co–Pi co-catalyst modification

The poor electrical conductivity, short hole diffusion length, and slow water oxidation reaction kinetics have severely limited the photoelectrochemical performance of the hematite (α-Fe2O3) photoanodes. In this paper, we report a facile wet-chemical approach to prepare the Ti-doping controllable hematite photoanode with subsequent Co–Pi electrocatalysts modification for solar water splitting. By optimizing the Ti-doping in Fe2O3, the Ti–Fe2O3 photoanodes can retain the primary morphology with the pristine Fe2O3 nanostructures, while simultaneously reducing the photogenerated charge carrier recombination rate in the films. Besides, by further decorating with Co–Pi co-catalysts on Ti–Fe2O3 surface, the formed Ti–Fe2O3/Co–Pi photoanode demonstrated an accelerated electrode/electrolyte kinetics during photoelectrochemical water oxidation reaction. As expected, the Ti–Fe2O3/Co–Pi photoanode produced an improved photocurrent density of 0.76 mA/cm2 at 1.23 V vs RHE, which is much higher than the photocurrent density of individual Fe2O3 (0.25 mA/cm2) and Ti–Fe2O3 photoanode (0.51 mA/cm2). This work provides a good insight for designing the composite photoanode to simultaneously enhance the charge transport and surface water oxidation kinetics for efficient solar fuel production.

Cooperative Catalytic Behavior of SnO2 and NiWO4 over BiVO4 Photoanodes for Enhanced Photoelectrochemical Water Splitting Performance

n-BiVO4 is a favorable photoelectrode candidate for a photoelectrochemical (PEC) water splitting reaction owing to its suitable energy level edge locations for an oxygen evolution reaction. On the other hand, the sluggish water oxidation kinetics of BiVO4 photoanodes when used individually make it necessary to use a hole blocking layer as well as water oxidation catalysts to overcome the high kinetic barrier for the PEC water oxidation reaction. Here, we describe a very simple synthetic strategy to fabricate nanocomposite photoanodes that synergistically address both of these critical limitations. In particular, we examine the effect of a SnO2 buffer layer over BiVO4 films and further modify the photoanode surface with a crystalline nickel tungstate (NiWO4) nanoparticle film to boost PEC water oxidation. When NiWO4 is incorporated over BiVO4/SnO2 films, the PEC performance of the resultant triple-layer NiWO4/BiVO4/SnO2 films for the oxygen evolution reaction (OER) is further improved. The enhanced performance for the PEC OER is credited to the synergetic effect of the individual layers and the introduction of a SnO2 buffer layer over the BiVO4 film. The optimized NiWO4/BiVO4/SnO2 electrode demonstrated both enriched visible light absorption and achieves charge separation and transfer efficiencies of 23% and 30%, respectively. The photoanodic current density for the OER on optimized NiWO4/BiVO4/SnO2 photoanode shows a maximum photocurrent of 0.93 mA/cm2 at 1.23 V vs. RHE in a phosphate buffer solution (pH~7.5) under an AM1.5G solar simulator, which is an incredible five-fold and two-fold enhancement compared to its parent BiVO4 photoanode and BiVO4/SnO2 photoanodes, respectively. Further, the incorporation of the NiWO4 co-catalyst over the BiVO4/SnO2 film increases the interfacial electron transfer rate across the composite/solution interface.