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 V_RHE, 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 V_RHE. 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.     

Hierarchical mesoporous SnO2/BiVO4 photoanode decorated with Ag nanorods for efficient photoelectrochemical water splitting

Bismuth vanadate has been extensively investigated as a potential visible light photoanode for PEC water splitting. The performance of BiVO₄ is restricted by fast charge recombination and slow oxygen evolution reaction kinetic. To address these issues, hierarchical SnO₂ (HSN) mesoporous support is developed via a novel sol-electrophoretic approach, and BiVO₄ film is decorated with silver nanorods (Ag NRs). The photocurrent density of HSN/BiVO₄ photoanode is 3.98 mA/cm² at 1.23 V vs. reversible hydrogen electrode (RHE) and onset potential (V_onset) of 0.5 V vs. RHE. The PEC performance is attributed to the appropriate band alignment between SnO₂ and BiVO₄, as well as the hierarchical structure of SnO₂. Ag-HSN/BiVO₄ photoanode shows photocurrent density of 4.30 mA/cm² at 1.23 V vs. RHE and V_onset of 0.28 V vs. RHE. The enhanced photocurrent and negatively shifted V_onset can be attributed to radiative localized surface plasmon resonance decay and catalytic effect of Ag NRs, respectively.     

Nanostructured Ni:BiVO4 photoanode in photoelectrochemical water splitting for hydrogen generation

Photoelectrochemical water splitting using bismuth vanadate (BiVO₄) is drawing attention but on account of presence of high charge recombination and poor water oxidation kinetics its performance is restricted. Present study attempts to understand the role of dopant Ni on BiVO₄ in a) reducing the charge recombination and b) to improve water oxidation kinetics. Ni doped BiVO₄ thin films are prepared via electrodeposition method and photoelectrochemical properties are investigated in 0.1 M phosphate buffer solution with and without sodium sulfite hole scavenger. Photocurrent density of 1.36 mA/cm² at 1.23 V vs. RHE has been obtained using 1.5% Ni doped BiVO₄. This sample also offered lower flat band potential, high open circuit potential and applied bias photon-to-current conversion efficiency. Addition of hole scavenger significantly increases the photoelectrochemical performance. Ni as a dopant therefore can play an important role in not only suppressing the electron-hole pair recombination but also in offering significantly enhanced photoelectrochemical response.     

Photodeposited CoOx as highly active phases to boost water oxidation on BiVO4/WO3 photoanode

Surface decoration of photoanodes with oxygen evolution cocatalysts is an efficient approach to improve the photoelectrochemical water splitting performance. Herein, ultrafine CoO_x was selectively immobilized on the surface of BiVO₄/WO₃ photoanode by using the photogenerated holes to in-situ oxidize Co₄O₄ cubane. The composited photoanode (CoO_x/BiVO₄/WO₃) displayed an enhanced photoelectrochemical (PEC) water oxidation performance, with a photocurrent density of 2.3 mA/cm² at 1.23 V_RHE under the simulated sunlight irradiation, which was 2 times higher than that of bare BiVO₄/WO₃. The characterization results for the morphological, optical and electrochemical properties of the photoelectrodes revealed that, the enhanced PEC performances could be attributed to the improved charge carrier separation/transport behaviors and the promoted water oxidation kinetics when the photoelectrodes were loaded with CoO_x.     

An ultra-thin NiOOH layer loading on BiVO4 photoanode for highly efficient photoelectrochemical water oxidation

Monoclinic bismuth vanadate has been widely used as a promising n-type semiconductor for photoelectrochemical (PEC) water decomposition due to its high reserves, good stability in neutral solutions, and relatively narrow band gap. Here, we developed a simple method to prepare a thin NiOOH layer on the surface of BiVO₄ nanorod arrays. The heterostructured photoanode shows great enhancement for the photocurrent density of 2.7 mA cm⁻² at 1.23 V vs. RHE, which is ~2.3 times higher than that of pristine BiVO₄ electrode, due to NiOOH as an efficient oxygen-releasing catalyst with abundant oxygen vacancies. The NiOOH/BiVO₄ photoanodes are systematically studied with X-ray diffraction, Raman, X-ray photoelectron spectra, scanning electron microscopy, transmission electron microscopy, and UV–vis diffuse-reflectance spectrum. The heterostructured photoanode shows excellent PEC activity, which can provide a promising and easy strategy to prepare such photoanode with high-efficient oxygen evolution co-catalysts.     

Fabrication of In2O3/In2S3 heterostructures for enhanced photoelectrochemical performance

Constructing core/shell heterojunction has always been an effective strategy for photoelectrochemical (PEC) water splitting owing to special morphology characterization and band structure. Herein, we synthesized a series of In₂O₃/In₂S₃ core/shell structure photoanodes via a simple two-step hydrothermal method to improve the PEC performance of In₂O₃. Various methods were employed to investigate the influence of sulfurization time on the morphologies, microstructures, photoelectrochemical properties and band structures of the as-prepared photoanodes. The results indicated that the In₂O₃/In₂S₃-5 possessed stronger visible light absorption, faster charge transfer rate and higher electron carrier density, which resulted in an excellent PEC performance. Under visible light irradiation, the photocurrent density of the In₂O₃/In₂S₃-5 photoanode reached 0.53 mA cm⁻² at 1.23 V vs RHE in 1 M NaOH solution, which was about twice as high as that of the pristine In₂O₃. Furthermore, the onset potential of the In₂O₃/In₂S₃-5 photoanode had an obvious negative shift (~200 mV) when compared to the pure In₂O₃ nanorod photoanode.     

Molybdenum doped bilayer photoanode nanotubes for enhanced photoelectrochemical water splitting

Conversion of solar energy into hydrogen energy via photoelectrochemical (PEC) water splitting is one of the most promising approaches for generation of clean and sustainable hydrogen energy in order to address the alarming global energy crisis and environmental problems. To achieve superior PEC performance and solar to hydrogen efficiency (STH), identification, synthesis, and development of efficient photoelectrocatalysts with suitable band gap and optoelectronic properties along with high PEC activity and durability is highly imperative. With the aim of improving the performance of our previously reported bilayer photoanode of WO₃ and Nb and N co-doped SnO₂ nanotubes i.e. WO₃-(Sn_0.95Nb_0.05)O₂:N NTs, herein, we report a simple and efficient strategy of molybdenum (Mo) doping into the WO₃ lattice to tailor the optoelectronic properties such as band gap, charge transfer resistance, and carrier density, etc. The Mo doped bilayer i.e. (W_0.98Mo_0.02)O_3-(Sn_0.95Nb_0.05)O₂:N revealed a higher light absorption ability with reduced band gap (1.88 eV) in comparison to that of the undoped bilayer (1.94 eV). In addition, Mo incorporation offered improvements in charge carrier density, photocurrent density, with reduction in charge transfer resistance, contributing to a STH (～3.12%), an applied bias photon-to-current efficiency (ABPE ～ 8% at 0.4 V), including a carrier density (N_d ～ 7.26 × 10²² cm^?3) superior to that of the undoped bilayer photoanode (STH ～2%, ABPE ～ 5.76%, and N_d ～5.11 × 10²² cm^?3, respectively). The substitution of Mo⁶⁺ for W⁶⁺ in the monoclinic lattice, forming the W–O–Mo bonds altered the band structure, realizing further enchantments in the PEC reaction and charge transfer kinetics. Additionally, doped bilayer photoanode revealed excellent long term PEC stability under illumination, suggesting its robustness for PEC water splitting. The present work herein provides a simple and effective Mo doping approach for generation of high performance photoanodes for PEC water splitting.     

Efficient suppression of surface charge recombination by CoP-Modified nanoporous BiVO4 for photoelectrochemical water splitting

BiVO₄ is a promising photoanode material for water splitting due to its substantial absorption of solar light as well as favorable band edge positions. However, the poor water oxidation kinetics of BiVO₄ results in its insufficient photocurrent density. Herein, we demonstrate the use of CoP nanoparticles for facile surface modification of nanoporous BiVO₄ photoanode in potassium borate buffer solution (pH 9.0), which can generate a tremendous cathodic shift of ~430 mV in the onset potential for photoelectrochemical water oxidation. In addition, a remarkable photocurrent density of 4.1 mA cm^?2 is achieved at 1.23 V vs. RHE under AM 1.5G illumination. The photoelectrochemical measurement using sodium sulfite as a hole scavenger clearly shows that the greatly improved performances are attributed to the efficient suppression of interfacial charge recombination through loading of CoP catalyst. Moreover, the maximum surface charge injection yield can reach >81% at 1.23 V vs. RHE and the maximum IPCE of CoP/BiVO₄ can reach 75.8% at 420 nm, suggesting the potential application of CoP-modified BiVO₄ photoanode for overall solar water splitting in cost-effective tandem photoelectrochemical cells.     

An efficient tandem photoelectrochemical cell composed of FeOOH/TiO2/BiVO4 and Cu2O for self-driven solar water splitting

An integrated solar water splitting tandem cell without external bias was designed using a FeOOH modified TiO₂/BiVO₄ photoanode as a photoanode and p-Cu₂O as a photocathode in this study. An apparent photocurrent (0.37 mA/cm² at operating voltage of +0.36 V_RHE) for the tandem cell without applied bias was measured, which is corresponding to a photoconversion efficiency of 0.46%. Besides, the photocurrent of FeOOH modified TiO₂/BiVO₄–Cu₂O is much higher than the operating point given by pure BiVO₄ and Cu₂O photocathode (∼0.07 mA/cm² at +0.42 V_RHE). Then we established a FeOOH modified TiO₂/BiVO₄–Cu₂O two-electrode system and measured the current density-voltage curves under AM 1.5G illumination. The unassisted photocurrent density is 0.12 mA/cm⁻² and the corresponding amounts of hydrogen and oxygen evolved by the tandem PEC cell without bias are 2.36 μmol/cm² and 1.09 μmol/cm² after testing for 2.5 h. The photoelectrochemical (PEC) properties of the FeOOH modified TiO₂/BiVO₄ photoanode were further studied to demonstrate the electrons transport process of solar water splitting. This aspect provides a fundamental challenge to establish an unbiased and stabilized photoelectrochemical (PEC) solar water splitting tandem cell with higher solar-to-hydrogen efficiency.     

Promoting photoelectrochemical hydrogen production performance by fabrication of Co1-XS decorating BiVO4 photoanode

Photoelectrochemical (PEC) water splitting is an effective way of converting solar energy into hydrogen (H₂) energy. However, the carriers’ transmission and the reaction kinetics of the photoelectrode are dilatory, which will influence the conversion efficiency of solar energy to H₂. In this work, a novel of BiVO₄/Co_1-XS photoanode was successfully fabricated through the successive ionic layer adsorption reaction. The photocurrent density of optimal sample BiVO₄/Co_1-XS (2.9 mA cm^?2 at 1.23 V_RHE) has reached up to 5 times that of pure BiVO₄, and the applied bias photon to current conversion efficiency increased from 0.04% (BiVO₄) to 0.4% (BiVO₄/Co_1-XS). The superior PEC performance of the BiVO₄/Co_1-XS photoanode is mainly related to the improved conductivities and reaction kinetics. The charge injection efficiency of BiVO₄/Co_1-XS grew to about 80%, and the charge separation efficiency was up to 34%, revealing that the decoration of Co_1-XS could significantly accelerate the transfer speed of photogenerated carriers from the electrode surface to the electrolyte. This work provided an efficient and simple scheme for improving the PEC performance of photoanode, through reasonable design and research.     

Plasmonic Bi nanoparticle decorated BiVO4/rGO as an efficient photoanode for photoelectrochemical water splitting

We report the application of plasmonic Bi nanoparticles supported rGO/BiVO₄ anode for photoelectrochemical (PEC) water splitting. Nearly, 2.5 times higher activity was observed for Bi-rGO/BiVO₄ composite than pristine BiVO₄. Typical results indicated that Bi-rGO/BiVO₄ exhibits the highest current density of 6.05 mA/cm² at 1.23 V, whereas Bi–BiVO₄ showed the current density of only 3.56 mA/cm². This enhancement in PEC activity on introduction of Bi-rGO is due to the surface plasmonic behavior of BiNPs, which improves the absorption of radiation thereby reduces the charge recombination. Further, the composite electrode showed good solar to hydrogen conversion efficiency, appreciable incident photon-to-current efficiency and low charge transfer resistance. Hence, Bi-rGO/BiVO₄ provides an opportunity to realize PEC water splitting.     

Rational design of W-doped BiVO4 photoanode coupled with FeOOH for highly efficient photoelectrochemical catalyzing water oxidation

Developments of promising photocatalyst for PEC water oxidation gain significant interest in the research field of PEC water splitting. The BiVO₄ has been envisioned as suitable photocatalyst material for the PEC water oxidation due to suitable bandgap with favorable band edge positions. Nevertheless, the poor electron-hole separation and low charge transfer efficiency of BiVO₄ yield sluggish surface catalysis reaction. Herein, facile electrodeposition and annealing techniques are proposed to fabricate W-doped BiVO₄ photoanode coupled with FeOOH (W–BiVO₄/FeOOH) for efficient photocatalytic water oxidation. This synthesis is simple, cost-effective and less time consuming. The doping concentration of W and deposition time of FeOOH are optimized to improve photocatalytic ability of BiVO₄. At 1.23 V vs. reversible hydrogen electrode (RHE) under 1 sun illumination, the W–BiVO₄/FeOOH photoanode exhibits a high photocurrent density of 2.2 mA/cm², which is seven folds higher than that of the pristine BiVO₄ photoanode (0.31 mA/cm² 1.23 V vs. RHE). The enhanced photocatalytic ability of W–BiVO₄/FeOOH photoanode is due to the enhanced charge transport properties and synergistic effects of W doping and FeOOH deposition. The excellent long-term stability with the photocurrent density retention of 90% after continuous light illumination for 1000 s is also achieved for the W–BiVO₄/FeOOH photoanode.     

Regulating the hole transfer from CuWO4 photoanode to NiWO4 electrocatalyst for enhanced water oxidation performance

The electrocatalyst coupling with CuWO₄ has resulted in a comparable or worse performance when compared to the bare CuWO₄. This work attempts to address this challenge by coupling CuWO₄ with NiWO₄ electrocatalyst that can form a Type-II heterojunction with a suitable energy-level alignment allowing for effective hole transfer from CuWO₄ to NiWO₄ electrocatalyst. We applied thermal annealing to the WO₃ nanoplates by adding Cu(NO₃)₂ and Ni(NO₃)₂ precursors to obtain the CuWO₄/NiWO₄ composite with common anions. A high surface-to-volume ratio, perfect interface lattice match, suitable energy level alignment, and high electrocatalytic activity were exhibited in the composite. These characteristics led to a 100 mV negative shift on the onset potential compared to the pure CuWO₄ photoanode. Moreover, it featured a 0.7-fold higher photocurrent density than that of the pure CuWO₄ photoanode. Only 9% of photocurrent density decreased after 4 h of photo-irradiation, demonstrating excellent photostability. Our mechanism study demonstrated that NiWO₄ could act as a semiconductor to form a Type-II heterojunction with CuWO₄, promoting hole transfer from the CuWO₄ valence band to the NiWO₄. Meanwhile, the NiWO₄ effectively injects the separated holes into the water solution as a promising electrocatalyst, thus enhancing the overall water splitting performance. This work provides an important design consideration by focusing on the corrected level alignment and lattice match for developing the CuWO₄/electrocatalyst system to work effectively.     

In-situ surface nanoetching WO3 photoanode for enhanced photoelectrochemical performance

Nanoarray films have received great attention in solar water splitting due to their high surface area and excellent photoelectrochemical (PEC) performance. However, it is difficult to further increase the surface area of the nanoarray film. In this work, we demonstrate an in-situ surface nanoetching method (WO₃→WO₃/Bi₂WO₆→WO₃/Bi₂S₃→etching WO₃) to increase surface area of WO₃ nanosheet array film. The characterization results indicate rougher and more uneven surface of the etched WO₃ (E-WO₃) film compared with the pristine WO₃ film. Moreover, the photocurrent density of E-WO₃ film is about 0.40 mA cm^?2 at 1.23 V (vs. RHE) under light illumination without cocatalyst, which is 2.7 times higher than that of pristine WO₃ film. This may be due to an increased surface area of the E-WO₃ photoanode, which provides more active sites for the catalytic reactions and accelerates the charge transfer. This research can provide a simple and effective method to further increase the surface area of the nanoarray photoelectrode.     

Recent trends and outlooks on engineering of BiVO4 photoanodes toward efficient photoelectrochemical water splitting and CO2 reduction: A comprehensive review

Photoelectrochemical (PEC) water splitting, and carbon dioxide (CO₂) utilization devices have attracted immense attention as sustainable technologies for the generation of hydrogen (H₂) fuel and value-added chemicals feedstock. Among numerous semiconductors, bismuth vanadate (BiVO₄) has emerged as a promising photoanode owing to its fascinating features such as high chemical stability, straddling band alignment with water redox levels, eco-friendly, and cost-effectiveness. However, sluggish oxidation kinetics, photo-corrosive nature, low electronic conductivity, and short carrier diffusion length limit its commercialization on the PEC horizon. To mitigate these inadequacies, several strategies have emerged such as novel heterojunctions, doping with unique materials, interface modulation, morphology, facet orientation, co-catalyst loading for surface engineering, etc. to realize the outstanding cost-to-efficiency ratios and long-term stability of PEC devices. The review highlights the recent advancement in BiVO₄-based photoanodes in last five years (2018–2022) and their utilization in the single absorber and unexplored tandem PEC systems towards boosted water splitting and CO₂ reduction. A discussion on theoretical studies of BiVO₄-based PEC systems elucidates the microscopic mechanism of promotion effect of the bulk/interface/surface strategies on surface catalysis as well as interfacial charge transfer in boosting oxidation kinetics. Moreover, this review addresses the versatility of the BiVO₄-based photoanode for the novel yet commercially viable PEC applications. This review will provide a broad avenue in designing highly durable, and scalable BiVO₄-based systems toward various PEC energy conversion devices.     

TiO2 nanoparticles modified with ZnIn2S4 nanosheets and Co-Pi groups: Type II heterojunction and cocatalysts coexisted photoanode for efficient photoelectrochemical water splitting

The optimization of photoelectrode is the key issue for the efficient photoelectrochemical water splitting process. In this work, the TiO₂ photoanode is synthesized and modified with ZnIn₂S₄ nanosheets and Co-Pi cocatalyst (TiO₂/ZnIn₂S₄/Co-Pi) for a favorable photoelectrochemical performance. The synthesis and modification process of the TiO₂ photoanode are optimized. The physical and chemical characterizations indicate that the TiO₂ has a nano-cauliflower-like structure and rutile crystal form modified with a network hexagonal ZnIn₂S₄ nanosheets and amorphous Co-Pi groups. After optimization of the hydrothermal and annealing process, the optimized TiO₂ photoanode manifests a photocurrent density of 1.82 mA cm^?2, 1.73-fold of the pristine TiO₂ photoanode (1.05 mA cm^?2). With the surficial ZnIn₂S₄ and Co-Pi modification, the photocurrent density of the TiO₂/ZnIn₂S₄/Co-Pi photoanode is raised to 5.05 mA cm^?2, 5.32-fold of the optimized TiO₂ photoanode (1.82 mA cm^?2). The applied bias photon-to-current efficiency, the charge separation and injection efficiencies of the TiO₂/ZnIn₂S₄/Co-Pi photoanodes are 8.79, 3.40, and 1.64-folds of the optimized TiO₂ photoanode. Combined the Tauc plot, valence band XPS spectra, EIS and Mott-Schottky analysis, the PEC water splitting mechanism could be that: (i) the type II heterojunction formed by the TiO₂ and ZnIn₂S₄ semiconductors improves the charge separation/injection efficiencies; (ii) the Co-Pi groups facilitate the oxygen evolution kinetics; (iii) the Co-Pi groups and 2D ZnIn₂S₄ nanosheets synergistically enhance the charge separation efficiency. This investigation could offer a prospect of practical implementation for photoelectrochemical water splitting.     

Amorphous CoFeBO nanoparticles as highly active electrocatalysts for efficient water oxidation reaction

The precise control of designing and synthesizing highly efficient water oxidation catalysts is a key role in enhancing the electrochemical processes such as water electrolysis. Herein, we fabricate the ultrafine Fe (variations on Fe amounts, χ_Fe = M_Fe/M_(Co+Fe)) and B-modulated CoFeBO nanoparticles as efficient water oxidation precatalysts. In view of intrinsic amorphous nanostructure, optimal Fe-doping amounts and borate-rich CoFeOOH active nanosheets formed on the surface of CoFeBO nanoparticles during oxygen evolution reaction (OER) process, the CoFeBO (χ_Fe = 0.3) precatalyst has demonstrated active electrochemical water oxidation properties under widen pH value from neutral to alkaline conditions. It exhibits only 263 mV in overpotential to reach a current density of 10 mA/cm² on glassy carbon (GC) electrode with the lowest Tafel slope of 39 mV/dec in 1 M KOH solution, which outperforms those of individual CoBO and FeBO as well as commercial RuO₂ electrocatalyst in the same electrolyte. Moreover, continuous stability tests in 1 M KOH solution by recording potential response at 10 mA/cm² for 28 h and current response at overpotential of 270 mV for another 28 h show no obvious fluctuation. Combining the merits of cost-efficient, scalable synthesis and active water oxidation performances, amorphous CoFeBO nanoparticles could be employed as highly active OER precatalysts for OER.     

Flexible polypyrrole activated micro-porous paper-based photoanode for photoelectrochemical water splitting

We herein demonstrate polypyrrole decorated micro-porous laboratory filter paper (PFP) as photoanode (PA) for efficient and stable water splitting. The straddling band position with water redox and the measured band gap of ~1.98 eV, make these PFP-PAs effective for water splitting reactions. The results manifest excellent photo-anodic PEC activity of these PFP-PAs, yielding a photocurrent density of ~9.5 mA/cm² (at 1.23 V vs. RHE) in a three-electrode configuration. The incident photon-to-current efficiency (IPCE) and applied bias photon-to-current efficiency (ABPE) was measured to be 43.19% and ~1%, respectively. Moreover, the robustness of these flexible PFP-PAs was visualized by the provided stability for more than ~160 min in alkaline conditions. The current study provides a proof-of-concept for the realization of a cost-effective, flexible, and efficient paper-based artificial catalyst (like a natural leaf) for solar-driven water splitting.     

Enhanced performance of NiF2/BiVO4 photoanode for photoelectrochemical water splitting

The serious surface charge recombination and fatigued photogenerated carriers transfer of the BiVO₄ photoanode restrict its photoelectrochemical (PEC) water splitting performance. In this work, nickel fluoride (NiF₂) is applied to revamp pure BiVO₄ photoanode by using a facile electrodeposition method. As a result, the as-prepared NiF₂/BiVO₄ photoanode increases the dramatic photocurrent density by approximately 180% compared with the pristine BiVO₄ photoanode. Furthermore, the correlative photon-to-current conversion efficiency, the charge injection, and the separation efficiency, as well as the hydrogen generation of the composite photoanode have been memorably enhanced due to the synergy of NiF₂ and BiVO₄. This study may furnish a dependable guidance in fabricating the fluoride-based compound/semiconductor composite photoanode system.