Electronic States Modulation of BiVO4 with Transition Metal-Substituted Polyoxometalates to Activate Lattice Oxygen Mechanism for Efficient Water Oxidation.

Ti-based metal-organic frameworks (MOFs) are demonstrated as promising photosensitizers for photoelectrochemical (PEC) water splitting. Photocurrents of TiO2 nano wire photoelectrodes can be improved under visible light through sensitization with aminated Ti-based MOFs. As a host, other sensitizers or catalysts such as Au nanoparticles can be incorporated into the MOF layer thus further improving the PEC water splitting efficiency.

Photoelectrochemical(PEC) water splitting is a promising approachto directly convert solar energy to renewable and storable hydrogen.However, the very low photovoltage and serious corrosion of semiconductorphotoelectrodes in strongly acidic or alkaline electrolytes neededfor water splitting severely impede the practical application of thistechnology. In this work, we demonstrate a facile approach to fabricatea high-photovoltage, stable photoanode by depositing Ni(OH)2 cocatalyst on cubic silicon carbide (3C-SiC), followed by agingin 1.0 M NaOH at room temperature for 40 h without applying electrochemicalbias. The aged 3C-SiC/Ni(OH)2 photoanode achieves a record-highphotovoltage of 1.10 V, an ultralow onset potential of 0.10 V vs thereversible hydrogen electrode, and enhanced stability for PEC watersplitting in the strongly alkaline solution (pH = 13.6). This agedphotoanode also exhibits excellent in-air stability, demonstratingidentical PEC water-splitting performance for more than 400 days.We find that the aged Ni(OH)2 dramatically promotes thehole transport at the photoanode/electrolyte interface, thus significantlyenhancing the photovoltage and overall PEC performance. Furthermore,the oxygen evolution reaction (OER) activity and the phase transitionsof the Ni(OH)2 electrocatalyst before and after aging aresystematically investigated. We find that the aging process is criticalfor the formation of the relatively stable and highly active Fe-dopedβ-NiOOH, which accounts for the enhanced OER activity and stabilityof the PEC water splitting. This work provides a simple and effectiveapproach to fabricate high-photovoltage and stable photoanodes, bringingnew premise toward solar fuel development.

Although the oxygen evolution reaction (OER) activity of BiVO4 photoanodes has been significantly enhanced, achieving long-term photostability is still challenging due to the gradual dissolution of V5+ during photoelectrochemical (PEC) water splitting. Herein, we deliberately generate ligand defects in a (Co0.91V0.09)3(BTC)2 metal-organic framework (CoV-MOF) that creates more undercoordinated sites, forming strong chemical bonds with BiVO4. Consequently, the dissolution of V5+ from BiVO4 during PEC water splitting can be effectively suppressed, leading to significantly enhanced stability. The optimized Co3O4/CoV-MOF/BiVO4 photoanode exhibits a high photocurrent density of 6.0 mA cm-2 at 1.23 V vs the reversible hydrogen electrode (RHE). Impressively, the photoanode can stably operate for 500 h at 0.6 V vs RHE under AM 1.5 G illumination. This work demonstrates the proof-of-concept of anchoring V5+ in BiVO4 photoanodes achieving ultrastable PEC water splitting.

The rapid depletion of fossil fuels and increasing strain on the environment due to excess consumption of the fossil fuels has necessitated the exploration of new renewable energy sources to solve the global energy crisis.1 Hydrogen has been considered as the future promising energy source, capable of providing clean and reliable energy supply and thus, meeting the global energy demand. However, economic production of hydrogen still remains a challenge, limiting its commercialization as a fuel. Hence, the efficient and economic production of hydrogen using renewable energy sources, such as solar, wind, etc. has received special interest, in the aim of finding clean (low carbon footprint) fuel, that will have minimum impact on the environment. Among all promising renewable energy sources such as solar, geothermal, wind, solar derived energy remains a highly attractive approach since it is a de-concentrated and inexhaustible energy source. Hence, solar energy driven water splitting, also known as photo-electrochemical (PEC) water splitting is considered as a promising approach for economic and efficient hydrogen production, since water splitting does not involve any greenhouse gas emission or any toxic byproducts. In PEC water splitting, a semiconductor is used as photoanode, which absorbs solar energy and thus, provides, photogenerated carriers (electrons and holes) for hydrogen evolution reaction (HER) and water oxidation or oxygen evolution reaction (OER), respectively. The development of semiconductor material with low band gap (1.23 eV < Eg < 3 eV) to absorb visible light and thus, drive the HER and OER reactions with minimum over-potential, and minimum density of lattice defects is of considerable interest. This will also facilitate recombination of photo-generated carriers and thus, lower the PEC response, high electrical conductivity and high stability in aqueous electrolyte solution leading to efficient utilization of solar energy, resulting in efficient and economic hydrogen production. Thus far, well studied semiconductor materials such as TiO2, ZnO, Fe2O3 have issues such as wide band gap and poor stability for long term PEC water splitting operation. However, among all these materials, ZnO is a promising material due to its higher electron mobility than TiO2 (~155 cm2 V-1 s-1 for ZnO vs. ~10-5 cm2 V-1 s-1 for TiO2). The systematic band gap engineering of ZnO will help in lowering the band gap of ZnO (3.2 eV), which will improve light absorption in the visible region of interest and number of carriers available for the reaction. In this study, vertically aligned ZnO nanowires (NWs) co-doped with Co and N are explored as photoanode (semiconductor) for PEC water splitting for the first time. These NWs are synthesized using well-known hydrothermal route, followed by heat treatment in NH3 at 400oC, 500oC 600oC and 700oC. These compositions are denoted as (Zn0.95Co0.05)O:N-y (y=400, 500, 600) NWs, where y is the temperature of heat treatment in NH3 atmosphere. The cross-sectional SEM image of (Zn0.95Co0.05)O:N-600 NWs is shown in Fig. 1. The photoelectrochemical characterization of (Zn0.95Co0.05)O:N-y has been carried out in 0.5 M Na2SO4 as an electrolyte, Pt wire as counter electrode and Ag/AgCl as the reference electrode (+0.197 V vs. NHE), using a scan rate of 10 mV/sec and temperature of 260C in H-type cell, where photoanode and cathode (Pt/C) are separated by Nafion membrane (Dupont). Thus, this study explores (Zn1-xCox)O:N NWs as potential semiconductor material for economic and efficient production of hydrogen via PEC water splitting. The results of the synthesis, microstructural and photoelectrochemical activity of these semiconductor materials will be presented and discussed.

The material design of highly active catalysts for energy storage applications (e.g., metal-air batteries and fuel cells) is crucial for solving global energy problems. The oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is used for charging and discharging rechargeable metal-air batteries, respectively. Although precious metal-based catalysts facilitate these reactions, the kinetics of the OER and ORR are rather sluggish due to their multistep electron transfer. It is therefore important to explore the mechanism of the OER and ORR to discover the key factors for highly active catalysts. As demonstrated by Suntivich et al. (2011)[1],[2], when the number of electrons in the eg orbital is close to unity for transition metals, perovskite oxides exhibit maximum OER and ORR activities. In other words, Mn3+ (t2g 3 eg 1 for both surface and bulk), Co3+ (t2g 5 eg 1 for surface), and Ni3+ (t2g 6 eg 1 for both surface and bulk) become OER and ORR active sites for Mn3+, Co3+ and Ni3+ based compounds . However, LaMnO3 exhibits a significantly lower specific OER activity[3] compared with LaCoO3 and LaNiO3 (~6% of LaNiO3 at 1.8 V vs. RHE). Also, Mn3O4 exhibits a low specific OER activity[4] (40% of Mn2O3 at 1.8 V vs. RHE) similar to LaMnO3. It is therefore important to explore what causes degradation of the OER activity of Mn3+-based (t2g 3 eg 1) compounds to improve their catalytic activity. We attempt to improve the performance of Mn3+-based oxides by controlling the degradation factors of their OER activities. To directly compare the OER activities of Mn3+-based oxides containing more than two cation sites, Mn3+ concentrations at the octahedral site and their initial crystal structures were maintained. Mn3-x Co x O4 (0 ≤ x < 1), a series of tetragonally distorted spinel compounds, satisfies this condition, as their octahedral sites remain occupied by only Mn3+ ions . For Mn3+-based oxides with a single cation site, Mn3+ was directly substituted by Co3+.We systematically studied the OER and ORR performances of Mn3+-based oxides by Co3+-substitution. Nanoparticles were prepared for the evaluation of catalytic activity to minimize the influence of geometric factors. Electrochemical measurements were conducted using a rotating ring disk electrode rotator (RRDE-3A, BAS Inc., Japan) at 1600 rpm, in combination with a bipotentiostat (ALS Co., Ltd, Japan). In addition, a Pt wire counter electrode, and an Hg/HgO reference electrode (International Chemistry Co., Ltd., Japan) filled with 0.10 M KOH (Nacalai Tesque, Inc., Japan) were used. Electrochemical measurements were conducted with O2 saturation (30 min bubbling O2 gas through the solution) at ~25 °C, where the equilibrium potential of the O2/H2O redox couple was fixed at 0.304 V vs. Hg/HgO (or 1.23 V vs. RHE). During OER current density measurements for each sample, the potential of the sample-modified GC was controlled from 0.3-0.9 V vs. Hg/HgO (1.226-1.826 V vs. RHE) at 10 mV/s. For all measurements, the OER current density was iR-corrected (R = ~43 Ω) using the measured solution resistance, and capacitance-corrected by averaging the anodic and cathodic scans to remove the influence of the current related to the formation of an electrical double layer. With an increase in Co content, signicant improvements in the OER and ORR activities were observed for Mn3+-based oxides. For example, Mn2.1Co0.9O4 exhibited a high specific OER activity (1700 % of Mn3O4 at 1.76 V vs. RHE) and a long-term stability over 100 cycles [5]. The OER activities of Mn3+-based oxides increased linearly with the suppression of the Jahn–Teller distortion. We examined whether the electronic state of Mn3+ changes in favor of enhancing the OER and ORR activities. When the Jahn-Teller distortion of Mn3+O6 octahedra is suppressed due to the increase in Co content, the splitting of the Mn3+ eg orbitals becomes smaller and the electron occupying the Mn3+ eg orbital shifts to a higher energy level. Thus, the overlap of the antibonding Mn3+ eg orbitals with the O 2p orbitals of the oxygen adsorbate becomes stronger. The OER and ORR activities of Mn3+-based oxides should therefore be enhanced due to the stronger binding of OER intermediates to the catalytic surface. We therefore conclude that the suppression of Jahn–Teller distortion enhances the OER and ORR activities of Mn3+-based oxides. Our result suggests a future application of Mn3+-based oxides as bifunctional catalysts for metal-air batteries. [1] J. Suntivich et al., Science, 2011, 334, 1383. [2] J. Suntivich et al., Nature Chemistry, 2011, 3, 546. [3] B. Han et al., Phys. Chem. Chem. Phys., 2015, 17, 22576. [4] A. Ramirez et al., J. Phys. Chem. C, 2014, 118, 14073. [5] S. Hirai et al., RSC Adv., 2016, 6, 2019.

Benchmarking of oxygen evolution catalysts on porous nickel supports

Sluggish oxygen evolution kinetics are one of the key limitations of bismuth vanadate (BiVO4 ) photoanodes for efficient photoelectrochemical (PEC) water splitting. To address this issue, we report a vanadium oxide (VOx ) with enriched oxygen vacancies conformally grown on BiVO4 photoanodes by a simple photo-assisted electrodeposition process. The optimized BiVO4 /VOx photoanode exhibits a photocurrent density of 6.29 mA cm-2 at 1.23 V versus the reversible hydrogen electrode under AM 1.5 G illumination, which is ca. 385 % as high as that of its pristine counterpart. A high charge-transfer efficiency of 96 % is achieved and stable PEC water splitting is realized, with a photocurrent retention rate of 88.3 % upon 40 h of testing. The excellent PEC performance is attributed to the presence of oxygen vacancies in VOx that forms undercoordinated sites, which strengthen the adsorption of water molecules onto the active sites and promote charge transfer during the oxygen evolution reaction. This work demonstrates the potential of vanadium-based catalysts for PEC water oxidation.

Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Introduction Pt-M (M = Ni, Co, etc. ) alloy nanoparticles (NPs) have attractive attentions as oxygen reduction reaction (ORR) electrocatalysts used not only in acidic but also in alkaline solution [1]. Although the 3d-transition metal elements M should dissolve into strong acid electrolyte easily, they might remain as (hydro)oxides on the topmost surfaces in alkaline environments [2]. Therefore, ensemble effects of Pt- and M-related surface species of the Pt-M NPs should be considered to develop highly active ORR electrocatalysts in alkaline solution. Also, because such the M elements (Ni and Co) show high oxygen evolution reaction (OER) activity, the Pt-M alloy NPs can be applied to ORR and OER bifunctional catalysts [3] for the metal-air battery. However, effects of the topmost surface structures (atomic arrangements, alloy compositions) of the alloy NPs on ORR and OER properties have been unclear. In this study, we fabricated well-defined Co/Pt(111) bimetallic surface by molecular beam epitaxy and investigated the ORR and OER activity. Experimental All the sample fabrication processes were conducted in ultra-high vacuum (UHV; ~10-8Pa). Pt(111) single-crystal substrate was cleaned by repeated Ar+ sputtering and annealing at 1173 K in UHV. Then, n-monolayer(ML)-thick-Co layers (n = 0.13-2.0) were deposited onto the substrates at a room temperature by using an electron-beam evaporator. Hereafter, the Co-deposited samples were denoted as nML-Co/Pt(111). After the sample transfers from the UHV chamber to an electrochemical system without air exposure, cyclic voltammograms (CVs) were recorded in N2-purged 0.1 M KOH. Subsequently, linear sweep voltammetry (LSV) was performed in O2-saturated 0.1 M KOH at a disk rotation rate of 1600 rpm for ORR and OER activity evaluations by using the rotating disk electrode (RDE) method. All the electrochemical measurements have been conducted at a room temperature. Results Fig.1 (a) shows CVs of the nML-Co/Pt(111) surfaces recorded in N2-purged 0.1 M KOH. The clean Pt(111) (dashed line) shows symmetric features derived from hydrogen- and hydroxyl- adsorption-desorption (Hads&des, OHads&des) reactions at 0.05−0.35 V and 0.6−0.9 V, respectively. In contrast, the nML-Co/Pt(111) show sharp redox peaks at around 0.65 V and 0.5 V, respectively, which correspond to oxidation-reduction reaction between Co(OH)2 and CoOOH [4]. Figs.1 (b) and (c) present the respective LSV curves for ORR and OER of the nML-Co/Pt(111) samples . It can be seen that half wave potentials for ORR (b) shift to higher potentials with increasing Co deposition up to 0.25 ML. However, the diffusion limiting current regions (0.4 V to 0.7 V) become obscure, accompanying the lower-potential shifts for the samples over the 0.5 ML-thick-Co samples. As for OER currents (c), the onset potentials monotonically shift to lower potentials with increasing the Co-thickness, suggesting that the surface Co-related species show much higher OER activity than the clean Pt(111) surface. Activity enhancement factors for ORR and OER of the nML-Co/Pt(111) samples vs. clean Pt(111) are summarized in Fig. 1 (d). ORR activity enhancements were judged by the j k values at 0.9 V that estimated using the Koutecky-Levich equation. As for OER, the enhancement factors were compared based on the electrochemical current densities at OER overpotential (η) of 350 mV. The enhancement factors of the ORR peaked at 0.25 ML-Co, where the factor is ca. 1.7 and, above 0.5 ML, the factors become less than 1 (less-active than clean Pt(111)). In contrast, the OER activity increased with increasing the Co-thicknesses up to 2 ML and the enhancement factor of 10 ML-Co sample is lower than that of 1 ML- and 2 ML-Co, suggesting that the surface Co-related-species influenced by the underneath Pt(111) substrate could contribute to the OER enhancements [5]. The results demonstrate that tuning the surface alloy compositions is a key to develop highly-active Pt-based alloy catalysts not only for ORR but also for OER. Acknowledgments This study was supported by Nippon Sheet Glass Foundation and Advanced Research and Education Center for Steel (ARECS) in Department of Materials Science, Tohoku University.

Photoelectrochemical (PEC) water splitting utilizing semiconductor-based photoelectrodes has gained substantial attention as a promising next generation clean energy technology, capable of directly converting solar energy into chemical fuels such as hydrogen and oxygen.[1,2] Achieving high-efficiency PEC water splitting requires strategic interfacial engineering of both solid-liquid and solid-solid interfaces to enable efficient transfer of photogenerated electrons and holes within semiconductor materials. Recent advancements include the development of p-n integrated PEC cells, featuring p-type photocathodes for the hydrogen evolution reaction (HER) and n-type photoanodes for the oxygen evolution reaction (OER).[2,3] Furthermore, photovoltaic (PV)-assisted PEC tandem cells (PV-PEC tandem cells) have emerged as highly promising devices, employing a visible-light-absorbing semitransparent photoelectrode as the top cell and a PV-grade narrow-bandgap material as the bottom cell to harvest transmitted light passing through the top cell.[3,4] To maximize sunlight utilization, significant progress has been made in developing both p-n PEC tandem cells and PV-PEC tandem cells incorporating visible-light-absorbing semitransparent photoelectrodes optimized for water-splitting reactions. These photoelectrodes must meet several critical criteria: (i) efficient light absorption within the bandgap to generate carriers for water splitting, (ii) adequate light transmission beyond the absorption edge to illuminate a secondary photoelectrode or PV cell, and (iii) robust stability during water-splitting reactions. Addressing these requirements involves optimizing the photoelectrode structure, selecting appropriate electrocatalyst materials, and employing transparent conductive substrates to ensure effective carrier transport throughout the circuit. Electrocatalysts must enhance water-splitting activity without hindering light absorption while minimizing the overpotential for HER and OER. Transparent conductive substrates must provide efficient electrical conductivity and establish a reliable interface between the semiconductor material and the conductive layer.Tantalum nitride (Ta₃N₅) has emerged as a promising candidate for visible-light-absorbing semitransparent photoanodes for OER, with a bandgap energy of 2.06 eV, corresponding to an absorption edge of 600 nm.[3,4] Semitransparent Ta₃N₅ photoanodes have been successfully fabricated on chemically stable substrates such as n-type GaN-coated sapphire.[5] Ta₃N₅ photoanodes modified with ultrathin NiFeO x electrocatalyst layers exhibit an anodic photocurrent density of 7.4 mA cm-2 at 1.23 V vs. the reversible hydrogen electrode under simulated AM 1.5G solar illumination, achieving 60% of the theoretical maximum (12.4 mA cm-2) assuming 100% quantum efficiency for water splitting. When paired with a CuInSe2 (CIS)-based PV cell and a Ni/Pt electrocatalyst for HER in a tandem configuration,[5] the resulting PV-PEC tandem cell achieves a solar-to-hydrogen energy conversion efficiency of 9.0% during the initial stage of the reaction.Engineering solid-solid interfaces is pivotal for elucidating the physicochemical properties of semitransparent photoelectrodes. For instance, Ta3N5 photoanodes fabricated directly on insulating SiO2 substrates generate anodic photocurrents attributed to PEC OER even without a back-contact conducting layer.[6] This behavior arises from the n-type self-conductivity inherent to Ta3N5 thin films, a property linked to their intrinsic semiconductor characteristics. Conversely, for p-type materials, CuBi2O4 thin-film photocathodes fabricated on F-doped SnO₂ (FTO) substrates demonstrate enhanced cathodic photocurrent densities under simulated AM 1.5G solar illumination, facilitated by improved back-contact conductivity of FTO.[7] This presentation will provide a detailed discussion on the interplay between interfacial engineering and the PEC performance of photoelectrodes, highlighting insights into their optimization.References.[1] M. G. Walter et al., Chem. Rev., 2010, 110, 6446-6473.[2] K. Sivula, R. van de Krol, Nat. Rev. Mater., 2016, 1, 15010 (16 pages).[3] Y. Kawase et al., Adv. Energy Sustain. Res., 2021, 2, 2100023 (20 pages).[4] T. Higashi et al., Angew. Chem. Int. Ed., 2019, 58, 2300-2304.[5] T. Higashi et al., Energy Environ. Sci., 2022, 15, 4761-4775.[6] T. Higashi et al., Phys. Chem. Chem. Phys.,2023, 25,20737-20748.[7] T. Higashi et al., J. Mater. Chem. C, 2024, 12, 16443-16458.

Aiming to overcome the low efficiency of the oxygen evolution reaction (OER) at photoanodes, this study developed a novel medium-entropy oxide catalyst for photoelectrochemical water splitting. By in situ growing medium-entropy oxides on a BiVO4 substrate to optimize the electronic structure of the catalyst, the prepared CoFeMnMoOx/BiVO4 composite photoanode exhibits excellent catalytic performance under alkaline conditions, including a remarkable photocurrent density of 5.58 mA cm-2 at 1.23 V vs. RHE, low overpotential and good stability. Additionally, the results from photoelectrochemical catalytic water splitting performance tests and DFT calculations reveal that Fe, Co and Mo sites act as active centers for the OER, the introduction of Mn increases active sites and promotes electron transfer, and the multimetallic synergistic effect enhances the d-p orbital hybridization of Mo-O bonds. After the water oxidation reaction, the lattice oxygen content in CoFeMnMoOx/BiVO4 decreases while the oxygen vacancy content increases, indicating that the lattice oxygen-mediated mechanism dominates the OER reaction pathway. This study not only improves the efficiency of PEC water splitting for hydrogen production but also demonstrates the broad application prospects of medium-entropy oxides in the field of photoelectrochemical water splitting.

Photoelectrochemical (PEC) water splitting provides a promising approach for solving sustainable energy challenges and achieving carbon neutrality goals. The oxygen evolution reaction (OER), a key bottleneck in the PEC water-splitting system occurring at the photoanode/electrolyte interface, plays a fundamental role in sustainable solar fuel production. Proper surface or interface engineering strategies have been proven to be necessary to achieve efficient and stable PEC water oxidation. This review summarizes the recent advances in interface engineering, including junction formation, surface doping, surface passivation or protection, surface sensitization, and OER cocatalyst modification, while highlighting the remarkable research achievements in the field of PEC water splitting. The benefits of each interface engineering strategy and how it enhances the device performance are critically analyzed and compared. Finally, the outlook for the development of interface engineering for efficient PEC water splitting is briefly discussed. This review illustrates the importance of employing rational interface engineering in realizing efficient and stable PEC water splitting devices.

Hydrogen evolution through ecofriendly photoelectrochemical (PEC) water splitting is considered to be one of the most cost-effective and desirable methods for meeting ever-growing energy demands. However, the low photoconversion efficiency limits the practical applicability of PEC water splitting. To develop an efficient photoelectrode, here the morphology of ZnO is tuned from 0D to 3D. It is observed that vertically grown 2D nanosheets outperform other morphologies in PEC water splitting by generating nearly 0.414 mA cm-2 at 0 V vs Ag/AgCl. Furthermore, these perpendicularly developed 2D nanosheets of ZnO are sensitized by metal-free carbon (C) dots to improve the photoconversion efficiency of ZnO. The prepared ZnO/C dots work as an effective photoanode, which can produce a 0.831 mA cm-2 photocurrent density upon application of 0 V vs Ag/AgCl under constant illumination, which is 2 times higher than that of bare ZnO. The enhanced PEC performance of ZnO/C dots is confirmed by the photoconversion efficiency (η). The ZnO/C dots exhibit a 2-fold-higher photoconversion efficiency (η) compared to that of ZnO. Additionally, the enhancement in PEC activity of ZnO/C dots is attributed to the higher carrier concentrations in the heterostructure. Bare ZnO has a 1.77 × 1020 cm-3 carrier density, which becomes 3.70 × 1020 cm-3 after sensitization with C dots. Enhanced carrier density successively leads to higher PEC water splitting efficiency. Band alignments of ZnO and C dots indicate the creation of the type-II heterostructure, which facilitates successful charge transportation among C dots and ZnO, producing a charge-carrier separation. Two-dimensional sheets of ZnO and ZnO/C dots exhibit appreciable stability under continuous illumination for 1 and 2 h, respectively.

In this paper, a high-performance BiVO4 photoanode deposited with serial hole transfer layers was fabricated for photoelectrochemical (PEC) water splitting in order to overcome the shortcomings of pure BiVO4 electrodes in terms of poor charge transport properties and undesirable surface water oxidation kinetics. The hole transfer layer of Fe2O3 was first deposited on the surface of pure BiVO4 to promote the hole transfer from the bulk of the semiconductor to the electrode surface (bulk/surface transfer process), and then the hole transfer layer of NiOOH/FeOOH was deposited on the surface to improve the hole transfer from the electrode surface to the electrolyte (surface/electrolyte transfer process). The results showed a remarkable improvement in PEC water splitting performance for the NiOOH/FeOOH/Fe2O3/BiVO4 photoanode. The photocurrent was up to 2.24 mA cm-2 at 1.23 V vs. RHE, which was about 2.95 times that of the pristine BiVO4 photoanode. Meanwhile, the charge transport efficiencies in the bulk (ηbulk) and the surface (ηsurface) were enhanced by 1.63 and 2.62 times compared to those of the BiVO4 photoanode at 1.23 V vs. RHE, respectively. In addition, the novel photoanode was assembled with a commercial silicon PVC for self-bias PEC water splitting, and a stable photocurrent density of ∼2.60 mA cm-2, corresponding to a ∼3.2% STH conversion efficiency, was achieved spontaneously. Our study provided a more efficient serial hole transfer strategy for achieving a BiVO4 photoanode with enhanced PEC water splitting.

IntroductionDevelopment of Oxygen Evolution Reaction (OER) catalyst is an important research issue for green hydrogen production by water splitting. Perovskite oxides have attracted attention as OER catalysts which can achieve both low cost and high catalytic activity. So far, some hypotheses have been proposed to understand key factors of catalytic activity of perovskite catalyst. Suntivich et al. reported volcano-like relationship between the OER activity and the number of the e g-electron of the B-site transition metal and the highest catalytic activity is achieved when the number of the e g-electron is close to 1.21. Besides, oxygen vacancies act as electrochemically active sites by incorporating reaction intermediates directly2,3. However, it is difficult to distinguish the contribution of electric state and oxygen defects on OER activity because both electronic state and oxygen defect concentration change simultaneously. For instance, the oxygen vacancy formation creates two electrons to maintain charge neutrality (2TMTM ×+OO ×↔2TMTM '+V O ∙∙+1/2 O2(g)).The aim of this study is to clarify the key factors improving OER catalytic activity. For the aim, we controlled the Ni valence state and oxygen defect concentration in layered perovskite oxide La2-x Sr x NiO4+δ by tuning Sr content (x) and the amount of oxygen defect (δ) independently (Fig. 1-a), to reveal the influence of transition metal electronic state and oxygen defects on OER activity. La2-x Sr x NiO4+δ has two types of oxygen defects: oxygen vacancy (V O ∙∙) and unoccupied interstitial site (V i ×). Comparing OER activity of oxygen stoichiometric La2NiO4 (Ni2+), La1.8Sr0.2NiO4 (Ni2.2+) and La1.6Sr0.4NiO4 (Ni2.4+) allowed us to evaluate the influence of Ni valence state on OER activity while no oxygen vacancy and fully unoccupied interstitial sites ([V O ∙∙]=0,[V i ×]=2.0). Additionally, comparing OER activity of La2NiO4.1 (Ni2.2+,[V O ∙∙]=0,[V i ×]=1.9), La1.8Sr0.2NiO4 (Ni2.2+,[V O ∙∙]=0,[V i ×]=2.0) and La1.6Sr0.4NiO3.9 (Ni2.2+,[V O ∙∙]=0.1,[V i ×]=2.0) allowed us to evaluate the influence of oxygen defects on OER activity with negligible effect of Ni valence state.ExperimentalLa2-x Sr x NiO4+δ (x=0, 0.2, 0.4) were synthesized by Pechini method. The oxygen content of La2-x Sr x NiO4+δ was controlled by annealing them under suitable oxygen partial pressure and temperature and then quenched based on oxygen nonstoichiometric data4. Obtained samples were characterized by XRD, XAS and SEM.For the OER test, the catalyst ink was prepared by mixing the catalyst powder, acetylene black, K+-exchanged Nafion and tetrahydrofuran. 6.4 μl of the catalyst ink was sonicated and dropped onto the glassy-carbon rotating disk electrode. The electrochemical measurements were performed in O2-saturated 0.1 M KOH. The disk electrode potential was controlled between 0.3 and 0.9 V versus Hg/HgO reference electrode filled with 0.1 M KOH at a scan rate of 10 mVs-1.Result and DiscussionOxygen stoichiometric La2NiO4 (Ni2+), La1.8Sr0.2NiO4 (Ni2.2+) and La1.6Sr0.4NiO4 (Ni2.4+) showed similar catalytic activity regardless of Ni valence (Fig. 1-b). This implies that the electronic state of Ni has insignificant influence on catalytic activity when the sample has almost no oxygen vacancy. The observed tendency is inconsistent with the Suntivich’s result that the relationship between OER activity and e g filling1.La2NiO4.1 (Ni2.2+,[V O ∙∙]=0,[V i ×]=1.9) and La1.8Sr0.2NiO4 (Ni2.2+,[V O ∙∙]=0,[V i ×]=2.0) which have different amount of unoccupied interstitial sites and almost no oxygen vacancy showed similar OER activity (Fig. 1-c). In contrast, La1.6Sr0.4NiO3.9 (Ni2.2+,[V O ∙∙]=0.1,[V i ×]=2.0) which has oxygen vacancy and fully unoccupied interstitial sites showed much better OER activity than La1.8Sr0.2NiO4 (Ni2.2+,[V O ∙∙]=0,[V i ×]=2.0) which has almost no oxygen vacancy and fully unoccupied interstitial sites (Fig. 1-c). These results indicate that oxygen vacancy has high OER activity as suggested in earlier works2.3, while unoccupied interstitial site which has available space for oxygen intercalation has negligibly low catalytic activity.The difference between oxygen vacancy and interstitial site is the presence of direct bonding with B-site cation. Reaction intermediates in oxygen vacancies can form direct bonding with Ni, while that in the interstitial site cannot. As suggested in the previous studies, a higher hybridization promotes charge transfer between the transition metal and reaction intermediate and effective charge transfer improves OER activity1,5.SummeryIn this work, to reveal the influence of transition metal electronic state and oxygen defects on OER activity, we have synthesized La2-x Sr x NiO4+δ , and controlled Ni valence state and oxygen defect concentration independently by tuning x and δ. It was revealed that oxygen vacancy boosts OER activity while influence of Ni valence state and unoccupied interstitial sites is insignificant.