BiVO4 Photoanode Research Articles

Amorphous metal-organic frameworks loaded on BiVO4 photoanodes with unique internal metal-like structure for promoting photoelectrochemical water splitting

Although ferrocene (Fc) based Metal-Organic Frameworks (MOFs) can act as oxygen evolution co-catalysts (OECs) for improvement of catalytic reactivity, the poor conductivity and lack of highly active metal sites limit its further application in photoelectrochemical (PEC) water splitting. Herein, the amorphous NiFc-MOF was grafted on BiVO4 photoanode (BiVO4@aNiFc-MOFs) for efficient PEC water splitting. This novel BiVO4@aNiFc-MOFs exhibits high current density of 4.34 mA cm−2 at 1.23 VRHE and relative low onset potential of 0.223 VRHE. The subsequent characterizations demonstrate that Ni species with metal-like state in bulk of aNiFc-MOFs form strong metal-support interaction with BiVO4, thereby promoting the interfacial charge transfer. Moreover, the surface of aNiFc-MOFs is short-range ordered with abundant coordinatively unsaturated Ni sites, creating a more favorable pathway for oxygen evolution reaction from thermodynamics. This work provides a simple method to design photoanodes with efficient OECs of amorphous MOFs for feasible PEC water splitting application.

Anthraquinone-based polymer modified BiVO4 photoanode with strong electron-withdrawing functional groups for boasting photoelectrochemical water oxidation

The photoelectrochemical (PEC) performance ofBiVO4 is limited by sluggish water oxidation kinetics and severe carrier recombination. Herein, a novel high-performance BiVO4/NiFe-NOAQ photoanode is prepared by a simple one-step hydrothermal method, using BiVO4 and 1-Nitroanthraquinone (NOAQ) as raw materials. The BiVO4/NiFe-NOAQ photoanode has an excellent photocurrent density of 5.675 mA cm−2 at 1.23 VRHE, which is 3.35 times higher than that of the pure BiVO4 (1.693 mA cm−2) photoanode. The BiVO4/NiFe-NOAQ shows a significant improvement in charge separation efficiency (86.12 %) and charge injection efficiency (87.86 %). The improvement is ascribable to the NiFe-NOAQ form a type II heterojunction with BiVO4 to inhibit carrier recombination. More importantly, the kinetic isotope experiment suggests that the proton-coupled electron transfer (PCET) process can enhance the charge transfer of BiVO4/NiFe-NOAQ. The contact angle measurements show that modifying functional groups enhanced the hydrophilicity of BiVO4/NiFe-NOAQ, which can further accelerate the PCET process. The XPS and PL results as well as the tauc plot indicate that the strong electron-withdrawing ability of –NO2 which can promote the extension of π conjugation, results in more π electron delocalization and produces more efficient active sites, thus achieving efficient photoelectrochemical water oxidation.

Nanoplate structured BiVO4 homojunction photoanode for boosting photoelectrochemical water splitting

Modulation of two-dimensional (2D) morphology structure is an effective strategy to improve the electron transport properties of BiVO4 photoanodes. However, up to now, the application of 2D BiVO4 in photoelectrochemical water splitting has been extremely scarce reported. Here, a simple method to prepare nanoplate BiVO4 photoanodes is reported. Mo-BiVO4 nanoplates were grafted on the conventional BiVO4 particles by the mild method. Due to the special structure, the specific surface area of the photoanode is increased, which enhances the light absorption and utilization efficiency. Moreover, the homojunction formed between BiVO4 substrate and Mo-doped BiVO4 nanoplate promotes the separation of photogenerated carriers and the transfer of photogenerated holes to the electrolyte interface. The abundant specific surface area also provides more support for the co-catalyst. As a result, under multiple facilitation, the photocurrent density of the BiVO4/Mo-BiVO4/CoPi photoanode is enhanced to 5.1 mA/cm2 at 1.23 V vs. RHE.

Bifunctional NiCo-LDH cocatalyst with hole extraction and high catalytic activity for boosting photoelectrochemical water oxidation of Nd doped BiVO4 photoanode

The slow charge transfer kinetics and water oxidation dynamics hinder the efficient photoelectrochemical (PEC) water splitting of BiVO4. Here, Nd:BiVO4@NiCo-LDH photoanode was successfully prepared by introducing rare earth element Nd with a special 4 f energy level as a dopant and loading a bifunctional NiCo-LDH cocatalyst. The Nd:BiVO4@NiCo-LDH photoanode exhibits good PEC performance. The photocurrent density was 4.1 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (RHE), which is about 5.1 times more than that of the unmodified BiVO4 electrode, and the onset potential was negatively shifted by 399 mV. Various physical characterizations and photoelectrochemical tests indicate that the Nd:BiVO4@NiCo-LDH photoanode has higher charge transfer rate and lower charge recombination rate than that of BiVO4. The NiCo-LDH catalyst serves as a bimetallic oxygen evolution reaction (OER) cocatalyst, where Co cation has hole extraction ability to reduce carrier recombination, and Ni addition increases the active site. The synergistic effect of the two significantly boosts the PEC water oxidation activity of the photoanode. This work has broad prospects for the construction of high-performance water oxidation systems in various photoanodes.

Decoupled Crystallization and Particle Growth of BiVO4 via Rapid Thermal Process for Enhanced Charge Separation

AbstractBiVO4 is one of the most promising candidates for photoelectrochemical water oxidation. However, the poor crystallinity and short hole diffusion length limit its charge separation. One bottleneck stems from the contradiction between high crystallinity and small particles via conventional furnace heating processes. This paper describes the design and fabrication of BiVO4 photoanodes via rapid thermal process (RTP), rather than furnace heating, to decouple the constraints between nucleation, crystallization, and growth processes of BiVO4. The higher heating ramp rate of RTP compared with furnace heating promotes the fast diffusion of reactant molecules, which elevates the nucleation rate above the particle growth rate of BiVO4, leading to small particles with high crystallinity. Moreover, the ultra‐high heating temperature makes it possible to crystallize the small BiVO4 particle within a short time. Thus, a high crystallinity can be obtained for the RTP‐treated BiVO4 while maintaining small particle size, achieving a charge separation efficiency of up to 82%, 30% higher than that of furnace‐treated BiVO4 photoanode.

Boosting photoelectrochemical water oxidation by self-assembled Co@β-cyclodextrin supramolecules modified on BiVO4 photoanode

Severe charge recombination and poor surface water oxidation kinetics are key restrictions of BiVO4 photoanode for efficient photoelectrochemical (PEC) water oxidation. In order to overcome this issue, we introduce Co@β-cyclodextrin (Co@β-CD) supramolecule as oxidation evolution catalyst (OEC) into BiVO4 photoanode for the first time via a facile two-step immersion method. The optimal Co@β-CD/BiVO4 photoanode displays a remarkable photocurrent density of 5.3 mA cm−2 at 1.23 VRHE, which is ca. 3.5 times as high as that of pristine BiVO4 photoanode. Further analyses indicate that photogenerated holes could quickly transfer from BiVO4 to Co sites through β-CD, thereby suppressing charge recombination. Moreover, due to the spatial confinement effect of β-CD, the dispersed Co sites in Co@β-CD are considered to possess the high catalytic activity in similar with single-atom catalyst, which accelerates the surface water oxidation kinetics. This work not only demonstrates the superiority of supramolecule/semiconductor composite photoanode, but also provides a facile preparation route for the large-scale preparation of high-performance photoanodes.

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.

Supramolecular catalyst with [FeCl4] unit boosting photoelectrochemical seawater splitting via water nucleophilic attack pathway

Propelled by the structure of water oxidation co-catalysts in natural photosynthesis, molecular co-catalysts have long been believed to possess the developable potential in artificial photosynthesis. However, the interfacial complexity between light absorber and molecular co-catalyst limits its structural stability and charge transfer efficiency. To overcome the challenge, a supramolecular scaffold with the [FeCl4] catalytic units is reported, which undergo a water-nucleophilic attack of the water oxidation reaction, while the supramolecular matrix can be in-situ grown on the surface of photoelectrode through a simple chemical polymerization to be a strongly coupled interface. A well-defined BiVO4 photoanode hybridized with [FeCl4] units in polythiophene reaches 4.72 mA cm−2 at 1.23 VRHE, which also exhibits great stability for photoelectrochemical seawater splitting due to the restraint on chlorine evolution reaction by [FeCl4] units and polythiophene. This work provides a novel solution to the challenge of the interface charge transfer of molecular co-catalyst hybridized photoelectrode.

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.

Gradient‐Tuned VO4 Vacancies in BiVO4 Photoanode for Boosting Bulk Hole Transport and Oxygen Evolution Reaction Performance

AbstractAlthough bismuth vanadate (BiVO4) photoanode has been widely used, the solar‐driven oxygen evolution reaction (OER) performance is constrained due to the substantial bulk recombination and poor mobility of charge carriers. Herein, a facile solvothermal post‐treatment approach that employs N,N‐dimethylformamide (DMF) is developed to induce a VO4 vacancy gradient from bulk to surface of BiVO4 film, which not only improves the charge diffusion but also establishes an advantageous upward band gradient to promote the bulk hole transport. Under Air Mass 1.5 Global (AM1.5G) simulated solar illumination, the optimized BiVO4 photoanode with a VO4 vacancy gradient (denoted as B(VO)1‐δ) exhibits excellent OER performance with a charge separation efficiency of 74.4% at 1.23 V versus reversible hydrogen electrode (RHE) and incident‐photon‐to‐current conversion efficiency of 30.7% at 445 nm, 1.2 and 1.4 times higher than that of the pristine BiVO4, respectively. After loading nickel‐iron hydroxyl oxide (NiFeOOH) as a cocatalyst, the photocurrent density of B(VO)1‐δ escalates to 5.92 mA cm−2 in a hole scavenger (Na2SO3) solution and 5.07 mA cm−2 in a potassium borate buffer solution at 1.23 V versus RHE, far superior to the pristine BiVO4. This work highlights that the gradient‐tuned VO4 vacancies can effectively modulate the bulk band structure and charge transfer in BiVO4 photoanode, providing a new strategy for boosting solar water splitting performance.

Efficient BiVO4 Photoanode with an Excellent Hole Transport Layer of CuSCN for Solar Water Oxidation

AbstractBismuth vanadate (BiVO4) is reported as a key material in photoelectrocatalysis owing to high theoretical efficiency, relatively narrow band gap of 2.4 eV, and favorable conduction band edge position for hydrogen evolution. However, the sluggish hole transport dynamics lead to slow photogenerated charge separation and transport efficiencies, which result in charge recombination due to aggregation. Herein, a novel hole transport layer of copper(I) thiocyanate (CuSCN) with the aim of significantly enhancing the efficiency of charge transport and stability of BiVO4 photoanodes is reported. The introduction of the hole transport layer could provide an appropriate intermediate energy level for photogenerated hole transfer and avoid charge recombination and trapping. After a photoassisted electrodeposition process of NiCoFe‐Bi catalysis, the obtained photoanode achieves a photocurrent density of 5.6 mA cm−2 at 1.23 V versus reversible hydrogen electrode under AM 1.5 G simulated solar radiation, and an applied bias photon to current efficiency of 2.31%. With the CuSCN layer, BiVO4 photoanode presented impressive stable photocurrent during 50 h continuous illumination. Meanwhile, the unbiased tandem device of the NiCoFe‐Bi/CuSCN/BiVO4 photoanode and the Si solar cell exhibit a solar‐to‐hydrogen efficiency of 5.75% and excellent stability for 14 h.

Understanding the Internal Conversion Efficiency of BiVO4/SnO2 Photoanodes for Solar Water Splitting: An Experimental and Computational Analysis.

This work aims to understand the spin-coating growth process of BiVO4 photoanodes from a photon absorption and conversion perspective. BiVO4 layers with thicknesses ranging from 7 to 48 nm and the role of a thin (<5 nm) SnO2 hole-blocking layer have been studied. The internal absorbed photon-to-current efficiency (APCE) is found to be nonconstant, following a specific dependence of the internal charge separation and extraction on the increasing thickness. This APCE variation with BiVO4 thickness is key for precise computational simulation of light propagation in BiVO4 based on the transfer matrix method. Results are used for accurate incident photon-to-current efficiency (IPCE) prediction and will help in computational modeling of BiVO4 and other metal oxide photoanodes. This establishes a method to obtain the sample's thickness by knowing its IPCE, accounting for the change in the internal APCE conversion. Moreover, an improvement in fill factor and photogenerated voltage is attributed to the intermediate SnO2 hole-blocking layer, which was shown to have a negligible optical effect but to enhance charge separation and extraction for the lower energetic wavelengths. A Mott-Schottky analysis was used to confirm a photovoltage shift of 90 mV of the flat-band potential.

Hydrogel Enabled Dual‐Shielding Improves Efficiency and Stability of BiVO4 Based Photoanode for Solar Water Splitting

AbstractPhotoelectrochemical solar to hydrogen conversion is a promising approach to solve energy and environmental problems. One of the paramount challenges to overcome is to achieve high solar to hydrogen efficiency while retaining stable service behavior without photocorrosion of the photoanode. In this study, a hydrogel‐enabled dual‐shielding (FeCoOx/PAAM) strategy is reported to improve efficiency and suppress photocorrosion of photoanode during solar water splitting especially in a high‐intensity illumination. Anodic photocorrosion of BiVO4 photoanodes involves the chemical and physical construction changes under the conditions of light field, electric field, and electrolyte. The utilization of FeCoOx/PAAM on BiVO4 inhibits the improvement of oxygen evolution reaction kinetics, bubble dynamics, and blocking ion dissolution diffusion, which effectively suppresses anodic photocorrosion and simultaneously improves efficiency of BiVO4 photoanode. The improved BiVO4/FeCoOx/PAAM photoanode shows a superior photocurrent density of 19.88 mA cm−2 at 1.23 V versus RHE, and a high charge separation efficiency of 98% with enhanced stability over 60 h (300 mW cm−2). This study contributes to developing a universal method of photoanode protection to anti‐photocorrosion, facilitating the development of high‐efficiency, high‐stability, and pollution‐free hydrogen production.

Ligand doping engineering induced robust internal electric field in MOFs/BiVO4 photoanode for water splitting

A robust internal electric field (IEF) within heterojunctions is essential for enhancing the separation and transfer of photogenerated charge carriers during photoelectrochemical (PEC) water splitting. In this contribution, a ligand doping engineering approach is innovatively proposed to design 3,5-Th2-TzH = 3,5-di(thiophen-2-yl)-1H-1,2,4-triazole (CuMTZ)/BiVO4 photoanodes, in which thiophene-containing electron-rich ligands (3,5-Th2-TzH) strengthen the interactions between CuMTZ and BiVO4. This results in the formation of a robust IEF, which serves as a strong driving force for efficient charge carrier transfer and separation, leading to an enhanced photovoltage. Furthermore, the uniform CuMTZ nanolayers act as a cocatalyst that not only increases the number of active reaction sites but also boosts photogenerated holes injection into the electrolyte to participate directly in the water oxidation process. Consequently, CuMTZ/BiVO4 photoanode exhibits a significant 4.26-fold improvement in photocurrent for water oxidation compared to BiVO4. This work highlights the significance of ligand doping engineering in the design of metal-organic framework (MOF)/semiconductor photoanodes for enhancing PEC efficiency.

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.

Synergistic Effect of Co3(HPO4)2(OH)2 Cocatalyst and Al2O3 Passivation Layer on BiVO4 Photoanode for Enhanced Photoelectrochemical Water Oxidation

Bismuth vanadate (BVO) is regarded as an exceptional photoanode material for photoelectrochemical (PEC) water splitting, but it is restricted by the severe photocorrosion and slow water oxidation kinetics. Herein, a synergistic strategy combined with a Co3(HPO4)2(OH)2 (CoPH) cocatalyst and an Al2O3 (ALO) passivation layer was proposed for enhanced PEC performance. The CoPH/ALO/BVO photoanode exhibits an impressive photocurrent density of 4.9 mA cm−2 at 1.23 VRHE and an applied bias photon-to-current efficiency (ABPE) of 1.47% at 0.76 VRHE. This outstanding PEC performance can be ascribed to the suppressed surface charge recombination, facilitated interfacial charge transfer, and accelerated water oxidation kinetics with the introduction of the CoPH cocatalyst and ALO passivation layer. This work provides a novel and synergistic approach to design an efficient and stable photoanode for PEC applications by combining an oxygen evolution cocatalyst and a passivation layer.

HClO‐Mediated Photoelectrochemical Epoxidation of Alkenes with Near 100 % Conversion Rate and Selectivity by Regulating Lattice Chlorine Cycle

AbstractDirectional organic transformation via a green, sustainable catalytic reaction has attracted a lot of attention. Herein, we report a photoelectrochemical approach for highly selective epoxidation of alkenes in a salt solution using Co2(OH)3Cl (CoOCl) as a bridge of photo‐generated charge, where the lattice Cl− of CoOCl can be oxidized to generate HClO by the photo‐generated holes of BiVO4 photoanode and be spontaneously recovered by Cl− of a salt solution, which then oxidizes the alkenes into the corresponding epoxides. As a result, a series of water‐soluble alkenes, including 4‐vinylbenzenesulfonic acid sodium, 2‐methyl‐2‐propene‐1‐sulfonic acid sodium, and 3‐methyl‐3‐buten‐1‐ol can be epoxidized with near 100 % conversion rate and selectivity. Through further inserting a MoOx protection layer between BiVO4 and CoOCl, the stability of CoOCl−MoOx/BiVO4 can be maintained for at least 120 hours. This work opens an avenue for solar‐driven organic epoxidation with a possibility of on‐site reaction around the abundant ocean.

HClO-Mediated Photoelectrochemical Epoxidation of Alkenes with Near 100 % Conversion Rate and Selectivity by Regulating Lattice Chlorine Cycle.

Directional organic transformation via a green, sustainable catalytic reaction has attracted a lot of attention. Herein, we report a photoelectrochemical approach for highly selective epoxidation of alkenes in a salt solution using Co2 (OH)3 Cl (CoOCl) as a bridge of photo-generated charge, where the lattice Cl- of CoOCl can be oxidized to generate HClO by the photo-generated holes of BiVO4 photoanode and be spontaneously recovered by Cl- of a salt solution, which then oxidizes the alkenes into the corresponding epoxides. As a result, a series of water-soluble alkenes, including 4-vinylbenzenesulfonic acid sodium, 2-methyl-2-propene-1-sulfonic acid sodium, and 3-methyl-3-buten-1-ol can be epoxidized with near 100 % conversion rate and selectivity. Through further inserting a MoOx protection layer between BiVO4 and CoOCl, the stability of CoOCl-MoOx /BiVO4 can be maintained for at least 120 hours. This work opens an avenue for solar-driven organic epoxidation with a possibility of on-site reaction around the abundant ocean.

High-performance bismuth vanadate photoanodes cocatalyzed with nitrogen, sulphur co-doped ferrocobalt-metal organic frameworks thin layer for photoelectrochemical water splitting

In this study, we prepare a highly efficient BiVO4 photoanode co-catalyzed with an ultrathin layer of N, S co-doped FeCo-Metal Organic Frameworks (MOFs) for photoelectrochemical water splitting. The introduction of N and S into FeCo-MOFs enhances electron and mass transfer, exposing more catalytic active sites and significantly improving the catalytic performance of N, S co-doped FeCo-based MOFs in water oxidation. The optimized BiVO4/NS-FeCo-MOFs photoanode exhibits impressive results, with a photocurrent density of 5.23 mA cm−2 at 1.23 V vs. Reversible Hydrogen Electrode (RHE) and an incident photon-to-charge conversion efficiency (IPCE) of 74.4 % at 450 nm in a 0.1 M phosphate buffered solution (pH = 7). These values are 4.84 times and 6.2 times higher than those of the original BiVO4 photoanode, respectively. Furthermore, the optimized BiVO4/NS-FeCo-MOFs photoanode demonstrates exceptional long-term stability, maintaining 96 % of the initial current after five hours.

High-performance BiVO4 photoanodes: elucidating the combined effects of Mo-doping and modification with cobalt polyoxometalate

Significant water oxidation reactivity enhancement in BiVO4 photoanodes is achieved by simultaneous bulk doping with molybdenum and surface-modification with a polyoxometalate water oxidation co-catalyst.