Photoanodes For Water Oxidation Research Articles

Dual-layer cocatalysts of Co–Pi and Ni(OH)2 on BiVO4 photoanodes for efficient water oxidation

Abstract Slow oxygen evolution kinetics and severe corrosion limit the further application of BiVO4 photoelectrodes in photoelectrochemical water splitting. Here, BiVO4/Co–Pi/Ni(OH)2 photoanodes were synthetized with the aim of improving their photoelectric conversion efficiency and stability. Compared with a pure BiVO4 photoanode, the BiVO4/Co–Pi/Ni(OH)2 photoanode enhanced the photocurrent from 1.1 to 4.5 mA/cm2 at 1.23 V versus the reversible hydrogen electrode. In addition, Ni(OH)2 acted as a corrosion protection layer on the surface of the photoanode, allowing the BiVO4/Co–Pi/Ni(OH)2 photoanode to exhibit excellent stability.

Wafer-Scale Si-Based Metal–Insulator–Semiconductor Photoanodes for Water Oxidation Fabricated Using Thin Film Reactions and Multiple-layer Electrodeposited Catalysts

Wafer-Scale Si-Based Metal–Insulator–Semiconductor Photoanodes for Water Oxidation Fabricated Using Thin Film Reactions and Multiple-layer Electrodeposited Catalysts

Unveiling the effect of the structural transformation of CoZn-MOF on BiVO4 photoanode for efficient photoelectrochemical water oxidation

Photoelectrochemical (PEC) water splitting has been widely investigated for solar-to-hydrogen conversion. However, issues like high charge recombination rate and slow surface water oxidation kinetics severely hinder its (PEC) conversion efficiency. Herein, we constructed MOF-derived CoOOH cocatalyst on BiVO4 photoanode, using a feasible electrochemical activation strategy. The BiVO4-based photoanode obtained shows a high photocurrent density of 3.15 mA/cm2 at 1.23 VRHE and low onset potential. Detailed experiments and theoretical calculations show that during the activation of CoZn-MOFs, there was a partial breakage of 2-methylimidazole (mIM) linker, an increase in the oxidation state of Cobalt ion (Co), and increased O2–. The high PEC performance is mainly attributed to the MOF-derived CoOOH, which provides rich active sites for hole extraction and reduces the overpotential for oxygen evolution reaction. Furthermore, when CoZnNiFe-LDHs were decorated on BiVO4 using the ions exchange method, the photocurrent density of BiVO4/CoZnNiFe-LDHs photoanode got to 4.0 mA/cm2 at 1.23 VRHE, accompanied with high stability. This study provides insights into understanding the key role played by the structural transformation of MOF cocatalyst in PEC water splitting processes.

Hole transfer layer of glucose-derived carbon enabled rapid charge transfer and hole storage for efficient water splitting

Charge recombination at the interface of oxygen evolution catalyst (OEC) and BiVO4-based photoanode is one of the great challenges to achieve efficient photoelectrochemical (PEC) water splitting due to the poor charge transfer and separation efficiency of BiVO4. Herein, a composite photoanode (NiFe/C-Mo:BiVO4) consisting of a molybdenum doped BiVO4 photoanode, an OEC interfacial layer of NiFeOOH and a tailorable hole transfer layer (HTL) made of glucose-derived carbon in between, was designed and fabricated for highly efficient PEC water splitting. The composite NiFe/C-Mo:BiVO4 photoanode exhibited the photocurrent density of 5.62 mA cm−2 at 1.23 VRHE and an outstanding photo-stability. Characterization and photoelectrochemical analyses revealed that the inserted carbon HTL, though not active for surface water oxidation kinetics, greatly suppressed the unwanted recombination of electron-hole pairs and drastically improved the transfer and storage of charge carriers at the OEC/BiVO4 interface. A surface charge separation efficiency of 89 % at 1.23 VRHE and remarkable hole storage properties were obtained. This work afforded a novel approach to the design of low-cost and sustainable HTL integrated photoanodes for efficient PEC water oxidation.

Influence of O–O formation pathways and charge transfer mediator on lipid bilayer membrane-like photoanodes for water oxidation

Inspired by the function of crucial components in photosystem II (PSII), electrochemical and dye-sensitized photoelectrochemical (DSPEC) water oxidation devices were constructed by the self-assembly of well-designed amphipathic Ru(bda)-based catalysts (bda = 2,2ʹ-bipyrdine-6,6ʹ-dicarbonoxyl acid) and aliphatic chain decorated electrode surfaces, forming lipid bilayer membrane (LBM)-like structures. The Ru(bda) catalysts on electrode-supported LBM films demonstrated remarkable water oxidation performance with different O–O formation mechanisms. However, compared to the slow charge transfer process, the O–O formation pathways did not determine the PEC water oxidation efficiency of the dye-sensitized photoanodes, and the different reaction rates for similar catalysts with different catalytic paths did not determine the PEC performance of the DSPECs. Instead, charge transfer plays a decisive role in the PEC water oxidation rate. When an indolo[3,2-b] carbazole derivative was introduced between the Ru(bda) catalysts and aliphatic chain-modified photosensitizer in LBM films, serving as a charge transfer mediator for the tyrosine-histidine pair in PSII, the PEC water oxidation performance of the corresponding photoanodes was dramatically enhanced.

Regulating the binding strength of oxygen intermediates with a conductive π–d conjugated metal-organic polymer CoTAA on BiVO4 photoanode for efficient photoelectrochemical water oxidation

The slow charge transfer and sluggish water oxidation reaction kinetics represent the main obstacles in the advancement of efficient BiVO4 photoanodes for photoelectrochemical (PEC) water splitting. Herein, a conductive π-d conjugated metal–organic polymer composed of 1,4,8,11-tetraaza[14]annulene (TAA) and atomically dispersed Co sites (CoTAA) was developed as an efficient oxygen evolution catalyst (OEC) to overcome these limitations. In CoTAA, the highly delocalized π-d conjugation results in superior electrical conductivity, leading to improved charge transfer. Additionally, X-ray absorption fine structure (XAFS) spectra and theoretical simulations reveal that the atomically dispersed Co sites within CoTAA, configured as Co-N4, modulates the binding strength of water oxidation intermediates (*OH, *O, and *OOH), thereby decreasing the energy barrier and facilitating the water oxidation reaction kinetics. Consequently, when combining with ultrathin FeOOH withdrawing the holes, the optimized CoTAA/FeOOH/BiVO4 photoanode achieves a high photocurrent density of 4.97 mA cm−2 at 1.23 VRHE under one sun irradiation (100 mW cm−2). This photocurrent represents a 2.59-fold enhancement when compared to that of pure BiVO4. These findings demonstrate the tremendous potential of π-d conjugated metal–organic polymers as active OECs in enhancing the PEC performance of water splitting.

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.

Cobalt site coordinated polyterthiophene derivant/hematite hybrid photoanode for light-driven water oxidation

The cobalt site-modified polymer could significantly promote charge transfer and reduce charge recombination of hematite for light-driven 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.

Efficiently catalyzing photo-oxidation of water by surface engineering of bismuth vanadate with nickel molybdenum oxide coverages

Bismuth vanadate with suitable band edges is one of the efficient photocatalysts for water oxidation. Establishing heterojunction can improve electron diffusion lengths and photocatalytic ability of BiVO4. In this work, nickel molybdenum oxide (Ni–Mo–O) and BiVO4 heterojunction is established by the hydrothermal process with different nickel to molybdenum precursor ratios. The Ni–Mo–O/BiVO4 electrode is applied as photoanodes for water oxidation. The growth mechanism of Ni–Mo–O on BiVO4 surface is proposed. The Ni–Mo–O deposition amount is optimized regarding to light absorbance and charger transportation. The largest photocurrent density of 1.72 mA/cm2 at 1.23 VRHE is obtained for the optimal NiMoO4/BiVO4 electrode (NM12) in the electrolyte without hole scavenger, while the pure BiVO4 electrode only shows a photocurrent density of 0.90 mA/cm2. The NM12 electrode even presented an impressive photocurrent density of 5.39 mA/cm2 at 1.23 VRHE in the electrolyte with the hole scavenger, owing to the abundant active sites and higher light absorbance as well as the favorable synergistic effects from its suitable Ni to Mo ratio. The NM12 electrode also shows excellent long-term stability with the photocurrent retention of 83% after illumination for 6500 s. This work opens a blueprint for establishing a novel heterojunction with the adjustable metal ratio and coverage on BiVO4.

Improving the photoelectrochemical stability of a bismuth vanadate photoanode for solar water oxidation

Improving the photoelectrochemical stability of a bismuth vanadate photoanode for solar water oxidation

Solid-state magic: How Bi2O3 and V2O5 combine to create a high-performance photoanode for water oxidation

A simple method to create a BiVO4–V2O5 photoanode with excellent interfacial contact and self-doping with V4+ and oxygen vacancies was proposed in this study. Direct generation of BiVO4 within the film through a solid state reaction between Bi2O3 and V2O5 was achieved, resulting in a highly efficient charge separation and a photocurrent density of 6.6 mA cm−2 at 1.23 V vs. RHE. The bulk charge separation efficiency was 95 %. This study highlights the importance of good interfacial contact for suppressing charge recombination and enabling efficient water oxidation. The developed synthesis method can be a practical approach for fabricating advanced photoelectrodes for sustainable water-splitting technologies, enabling carbon-free fuel production.

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.

Extremely low onset potential of modified Fe2O3 photoanode for water oxidation

The photoelectrochemical (PEC) performance of hematite (α-Fe2O3)-based photoanodes is still far from its theoretical value because they are constrained by serious charge recombination and sluggish water oxidation kinetics. Herein, we developed a heterostructural photoanode by coating boron doped graphite carbon nitride nanolayers onto an Fe2O3 nanoarray (BCN/Fe2O3), followed by modification with NiCo(OH)x cocatalyst. The constructed NiCo(OH)x/BCN/Fe2O3 demonstrates a significantly improved photocurrent density of 2.7 mA cm−2 at 1.23 VRHE (V versus reversible hydrogen electrode), which is 3.2 times higher than that of pristine Fe2O3. Additionally, it exhibits an extremely negative onset potential of 0.60 VRHE. Detailed studies reveal that the BCN/Fe2O3 heterojunction endows a driving force for charge separation and establishes a direct pathway for their transport, while NiCo(OH)x acts as a cocatalyst to accelerate the surface water oxidation reaction.

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.

Reliable bi-functional nickel-phosphate /TiO2 integration enables stable n-GaAs photoanode for water oxidation under alkaline condition

Hydrogen is one of the most widely used essential chemicals worldwide, and it is also employed in the production of many other chemicals, especially carbon-free energy fuels produced via photoelectrochemical (PEC) water splitting. At present, gallium arsenide represents the most efficient photoanode material for PEC water oxidation, but it is known to either be anodically photocorroded or photopassivated by native metal oxides in the competitive reaction, limiting efficiency and stability. Here, we report chemically etched GaAs that is decorated with thin titanium dioxide (~30 nm-thick, crystalline) surface passivation layer along with nickel-phosphate (Ni-Pi) cocatalyst as a surface hole-sink layer. The integration of Ni-Pi bifunctional co-catalyst results in a highly efficient GaAs electrode with a ~ 100 mV cathodic shift of the onset potential. In this work, the electrode also has enhanced photostability under 110 h testing for PEC water oxidation at a steady current density Jph > 25 mA·cm−2. The Et-GaAs/TiO2/Ni-Pi║Ni-Pi tandem configuration results in the best unassisted bias-free water splitting device with the highest Jph (~7.6 mA·cm−2) and a stable solar-to-hydrogen conversion efficiency of 9.5%.

Bifunctional Co-TiO2 co-promoted bulk and interfacial charge transfer of BiVO4 photoanodes for solar water oxidation

The primary causes of photogenerated carrier recombination in BiVO4 photoanodes are the formation of bulk polarons and sluggish water oxidation kinetics. To address these issues effectively, we designed a bifunctional Co-TiO2 to modify the BiVO4 photoanodes. UV–vis DRS and Mott-Schottky results indicate that Co adjusts the band gap of TiO2, and there forms a type-II heterojunction between BiVO4 and Co-TiO2. The experimental tests and theoretical calculations for OER as well as EIS results demonstrate that Co sites in Co-TiO2 provide more reaction sites for the interfacial oxidation reaction, reducing the overpotential. Under the dual action of Co-TiO2, the co-promotion for the migration ability of photogenerated carriers in the bulk and on the surface of BiVO4 photoanode has been realized. This conclusion is supported by analysis of photogenerated charge transfer behavior through transient photocurrent and IMPS. As a result, the photogenerated current density of BiVO4/Co7 %-TiO2 photoanode reaches 4.38 mA cm−2 at 1.23 V vs. RHE under AM 1.5 G illumination, which is four times that of pure BiVO4 photoanodes.

(Invited) Understanding Photoelectrocatalysis on Epitaxial Oxide Surfaces

The intermittent nature of renewable energy sources requires a clean, scalable means of converting and storing energy. One Earth abundant storage option is water electrolysis, storing energy in the bonds of O2 and H2. Photoelectrochemical (PEC) cells based on semiconductor/liquid interfaces can convert sunlight to chemical fuels without external circuitry, such as “splitting” water into O2 and H2 upon illumination. The efficacy of conversion depends in part on the location of the semiconductor band edges, dictating absorption, and also the rectifying properties of semiconductor–electrolyte junctions, which drives the separation of electron–hole pairs.We will present studies of model oxide La(1-x)SrxFeO3 photoelectrodes grown by molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) that display a known crystallographic orientation, surface area, path for charge transport, and strain. Aliovalent doping can tune the transition metal redox properties as well as the material band gap and conductivity, resulting in a rich phase space of possible water-oxidation photoanodes. Comparing the photooxidation of a fast redox couple to the slugging water oxidation reaction demonstrates the potential of electronic states that must be filled prior to oxygen evolution. Measurements of the surface speciation in a humid environment demonstrate that Sr incorporation also promotes hydroxylation, suggesting that water oxidation proceeds through the oxidation of FeIII-OH states, which is facilitated by Sr incorporation and results in an increased photovoltage. Surface-sensitive measurement of the valence band and unoccupied states (via O K-edge X-ray absorption spectroscopy) under humid conditions further support this picture. This fundamental insight builds understanding necessary for the design of active, earth-abundant photocatalysts that can be integrated into PEC devices for efficient conversion of solar energy into chemical fuels.

Photoelectrochemical water oxidation using hematite modified with metal-incorporated graphitic carbon nitride film as a surface passivation and hole transfer overlayer

Hematite (α-Fe2O3) is renowned as a promising photoanode for water oxidation, even though it displays poor photoconversion efficiency. In this study, ∼5 nm-thick graphitic carbon nitride (g-C3N4; CN) and metal-incorporated CN (M-CN; M = Ag, Fe, Co) films are uniformly deposited on hematite via a facile one-step evaporation method. Herein, the Co-CN layer leads to the highest photoelectrochemical activity with hematite-based photoanode. The subsequent loading of Co-CN layer with oxygen evolution catalysts (FeNiOOH and CoOOH) further enhances photocurrent density to ∼3.5 mA cm−2 and oxygen evolution at > 95 % of Faradaic efficiency over 24 h at E = 1.23 V. Detailed analysis based on spectroscopic and electrochemical measurements demonstrate that the primary role of CN layer is improving the charge separation efficiency by passivating the hematite surface. Then the incorporated metals contribute to reducing charge transfer resistance and thereby mediating hole transfer to interfacial water.

Introducing Bidirectional Axial Coordination into BiVO4@Metal Phthalocyanine Core-Shell Photoanodes for Efficient Water Oxidation.

Core-shell photoanodes have shown great potential for photoelectrochemical (PEC) water oxidation. However, the construction of a high-quality interface between the core and shell, as well as a highly catalytic surface, remains a challenge. Herein, guided by computation, we present a BiVO4 photoanode coated with ZnCoFe polyphthalocyanine using pyrazine as a coordination agent. The bidirectional axial coordination of pyrazine plays a dual role by facilitating intimate interfacial contact between BiVO4 and ZnCoFe polyphthalocyanine, as well as regulating the electron density and spin configuration of metal sites in ZnCoFe phthalocyanine, thereby promoting the potential-limiting step of *OOH desorption. The resulting photoanode displayed a high photocurrent density of 5.7±0.1 mA cm-2 at 1.23 VRHE. This study introduces a new approach for constructing core-shell photoanodes, and uncovers the key role of pyrazine axial coordination in modulating the catalytic activity of metal phthalocyanine.