Stepping towards Solar Water Splitting: Recent Progress in Bismuth Vanadate Photoanodes

Scaling up BiVO4 Photoanodes on Porous Ti Transport Layers for Solar Hydrogen Production

Commercialization of photoelectrochemical (PEC) water-splitting devices requires the development of large-area, low-cost photoanodes with high efficiency and photostability. Herein, we address these challenges by using scalable fabrication techniques and porous transport layer (PTLs) electrode supports. We demonstrate the deposition of W-doped BiVO4 on Ti PTLs using successive-ionic-layer-adsorption-and-reaction methods followed by boron treatment and chemical bath deposition of NiFeOOH co-catalyst. The use of PTLs that facilitate efficient mass and charge transfer allowed the scaling of the photoanodes (100 cm2 ) while maintaining ~90 % of the performance obtained with 1 cm2 photoanodes for oxygen evolution reaction, that is, 2.10 mA cm-2 at 1.23 V vs. RHE. This is the highest reported performance to date. Integration with a polycrystalline Si PV cell leads to bias-free water splitting with a stable photocurrent of 208 mA for 6 h and 2.2 % solar-to-hydrogen efficiency. Our findings highlight the importance of photoelectrode design towards scalable PEC device development.

Ultra-sensitive photoelectrochemical sensor for copper ion detection based on ITO/BiVO4 photoelectrode

In this work, a photoelectrochemical sensor for Cu2+ trace detection was prepared based on the excellent semiconductor material BiVO4 modified ITO electrode. It was found that Cu2+ formed a doping effect by adsorption on the surface of ITO/BiVO4 electrode, which inhibited the photogenerated electron complex and thus caused an increase in photocurrent. In addition, due to the complexation effect of EDTA on Cu2+, the Cu2+ adsorbed on the electrode surface is desorbed and the photocurrent returns to the blank value, making it possible to reuse electrodes. Under optimal conditions, the linearity range was measured between 0.1 pM and 0.1 μM and the detection limit was 0.063 pM. The PEC sensor exhibits the advantages of high sensitivity and reusability for Cu2+ detection. This provides a novel PEC detection platform for Cu2+ monitoring in environmental water samples.

Recent Progress on Bimetallic-Based Spinels as Electrocatalysts for the Oxygen Evolution Reaction

Electrocatalytic water splitting is a promising and viable technology to produce clean, sustainable, and storable hydrogen as an energy carrier. However, to meet the ever-increasing global energy demand, it is imperative to develop high-performance non-precious metal-based electrocatalysts for the oxygen evolution reaction (OER), as the OER is considered the bottleneck for electrocatalytic water splitting. Spinels, in particular, are considered promising OER electrocatalysts due to their unique properties, precise structures, and compositions. Herein, the recent progress on the application of bimetallic-based spinels (AFe₂ O₄ , ACo₂ O₄ , and AMn₂ O₄ ; where A = Ni, Co, Cu, Mn, and Zn) as electrocatalysts for the OER is presented. The fundamental concepts of the OER are highlighted after which the family of spinels, their general formula, and classifications are introduced. This is followed by an overview of the various classifications of bimetallic-based spinels and their recent developments and applications as OER electrocatalysts, with special emphasis on enhancing strategies that have been formulated to improve the OER performance of these spinels. In conclusion, this review summarizes all studies mentioned therein and provides the challenges and future perspectives for bimetallic-based spinel OER electrocatalysts.

Two-Step Process of a Crystal Facet-Modulated BiVO4 Photoanode for Efficiency Improvement in Photoelectrochemical Hydrogen Evolution

The photoactivity of nanoporous bismuth vanadate (BiVO₄, BVO) photoanodes that were fabricated by a two-step process (electrodeposition and then thermal conversion) in photoelectrochemical (PEC) hydrogen (H₂) evolution can be enhanced about 1.44-fold by improving the constitutive ratio of (111̅), (061), and (242̅) crystal facets. The PEC characterization was carried out to investigate the factors altering the performance, which revealed that the crystal facet modulation could improve the photoactivity of the BVO photoanodes. In addition, the orientation-controlled BVO thin-film electrodes are introduced as evidence that the present crystal facet modulation is the positive effect for BVO photoanodes in PEC. The investigation of energy band structures and interfacial charge carrier dynamics of the BVO photoanodes reveals that the crystal facet modulation could result in a shorter lifetime of charge carrier recombination and larger band bending at the interface between BVO and electrolytes. This outcome could improve the charge separation and charge transfer efficiencies of BVO photoanodes, promoting the efficiency of PEC H₂ evolution. Moreover, this crystal facet modulation can combine with co-catalyst decoration to further improve the solar-to-hydrogen efficiency of BVO photoanodes in PEC. This study presents a potential strategy to promote the PEC activity by crystal facet modulation and important insights into the interfacial charge transfer properties of semiconductor photoelectrodes for the application in solar fuel generation.

Emerging Surface, Bulk, and Interface Engineering Strategies on BiVO4 for Photoelectrochemical Water Splitting

The photoelectrochemical (PEC) cell that collects and stores abundant sunlight to hydrogen fuel promises a clean and renewable pathway for future energy needs and challenges. Monoclinic bismuth vanadate (BiVO₄ ), having an earth-abundancy, nontoxicity, suitable optical absorption, and an ideal n-type band position, has been in the limelight for decades. BiVO₄ is a potential photoanode candidate due to its favorable outstanding features like moderate bandgap, visible light activity, better chemical stability, and cost-effective synthesis methods. However, BiVO₄ suffers from rapid recombination of photogenerated charge carriers that have impeded further improvements of its PEC performances and stability. This review presents a close look at the emerging surface, bulk, and interface engineering strategies on BiVO₄ photoanode. First, an effective approach of surface functionalization via different cocatalysts to improve the surface kinetics of BiVO₄ is discussed. Second, state-of-the-art methodologies such as nanostructuring, defect engineering, and doping to further enhance light absorption and photogenerated charge transport in bulk BiVO₄ are reviewed. Third, interface engineering via heterostructuring to improve charge separation is introduced. Lastly, perspectives on the foremost challenges and some motivating outlooks to encourage the future research progress in this emerging frontier are offered.

Suppressing photoinduced charge recombination at the BiVO4||NiOOH junction by sandwiching an oxygen vacancy layer for efficient photoelectrochemical water oxidation

Nickel oxyhydroxide (NiOOH) is regarded as one of the promising cocatalysts to enhance the catalytic performance of photoanodes but suffers from serious interfacial charge-carrier recombination at the photoanode||NiOOH interface. In this work, surface-engineered BiVO₄ photoanodes are fabricated by sandwiching an oxygen vacancy (O_vac) interlayer between BiVO₄ and NiOOH. The surface O_vac interlayer is introduced on BiVO₄ by a chemical reduction treatment using a mild reducing agent, sodium hypophosphite. The induced O_vac can alleviate the interfacial charge-carrier recombination at the BiVO4||NiOOH junction, resulting in efficient charge separation and transfer efficiencies, while an outer NiOOH layer is coated to prevent the O_vac layer from degradation. As a result, the as-prepared NiOOH-P-BiVO₄ photoanode exhibits a high photocurrent density of 3.2 mA cm^-2 at 1.23 V vs. RHE under the irradiation of 100 mW/cm² AM 1.5G simulated sunlight, in comparison to those of bare BiVO₄, P-BiVO_4, and NiOOH-BiVO₄ photoanodes (1.1, 2.1 and 2.3 mA cm^-2, respectively). In addition to the superior photoactivity, the 5-h amperometric measurements illustrate improved stability of the surface-engineered NiOOH-P-BiVO₄ photoanode. Our work showcases the feasibility of combining cocatalysts with O_vac, for improved photoactivity and stability of photoelectrodes.

Activating a TiO2/BiVO4 Film for Photoelectrochemical Water Splitting by Constructing a Heterojunction Interface with a Uniform Crystal Plane Orientation

The construction of a heterojunction has been considered one of the most effective strategies to improve the photoelectrochemical (PEC) performance of photoanodes; however, most researchers only focus on the design and preparation of a novel and efficient heterojunction photoelectrode, and the investigation on the effect of the heterojunction interface structure on PEC performance is ignored. In this work, a TiO₂/BiVO₄ photoanode with a uniform crystal plane orientation in the heterojunction interface (TiO₂-110/BiVO₄-202) was prepared by an in situ transformation method. We found that the PEC activity of the TiO₂/BiVO₄ photoanode can be activated by constructing such a heterojunction interface. Compared with a TiO₂/BiVO₄ photoanode with a random crystal plane orientation prepared by a simple soaking-calcining method (S-TiO₂/BiVO₄, 0.04 mA/cm² at 1.23 V_RHE), the TiO₂/BiVO₄ photoanode prepared by the in situ transformation method (I-TiO₂/BiVO₄) exhibits a significantly better PEC performance, and the photocurrent density of I-TiO₂/BiVO₄ is about 2.2 mA/cm² at 1.23 V_RHE under visible light irradiation without a cocatalyst. This is mainly attributed to the fact that I-TiO₂/BiVO₄ has a faster electron transfer rate in the heterojunction interface according to the results of PEC analysis. Furthermore, density functional theory (DFT) calculations show that the BiVO₄-202 surface has a higher Fermi energy level, thereby expediting the photogenerated carrier transport in the heterojunction interface. This work corroborates and strengthens the view that the heterojunction interface structure has a significant effect on the PEC performance.

Unique Layer-Doping-Induced Regulation of Charge Behavior in Metal-Free Carbon Nitride Photoanodes for Enhanced Performance

Photoinduced charge carrier behavior is critical in determining photoelectrocatalytic activity. In this study, a unique layer-doped metal-free polymeric carbon nitride (C₃ N₄ ) photoanode is fabricated by using one-pot thermal vapor deposition. With this method, a photoanode consisting of a phosphorus-doped top layer, boron-doped middle layer, and pristine C₃ N₄ bottom layer, was formed as a result of the difference in thermal polymerization kinetics associated with the boron-containing H₃ BO₃ -melamine complex and the phosphorus-containing H₃ PO₄ -dicyandiamide complex. This layer-doping fabrication strategy effectively contributes to the formation of dual junctions that optimizing charge carrier behavior. The ternary-layer C₃ N₄ photoanode exhibits significantly enhanced photoelectrochemical water oxidation activity compared to pristine C₃ N₄ , with a record photocurrent density of 150±10 μA cm^-2 at 1.23 V vs. RHE. This layer-doping strategy provides an effective means for design and fabrication of photoelectrodes for solar water oxidation.

Nanoporous BiVO4 nanoflake array photoanode for efficient photoelectrochemical water splitting

A high-quality nanoporous BiVO₄ nanoflake array photoanode was prepared by using an in situ transformation approach, which exhibited an excellent photoelectrochemical activity.

Photoelectrocatalytic Hydrogen Peroxide Production Using Nanoparticulate WO3 as Photocatalyst and Glycerol or Ethanol as Sacrificial Agents

Photoelectrochemical production of hydrogen peroxide was studied by using a cell functioning with a WO3 photoanode and an air breathing cathode made of carbon cloth with a hydrophobic layer of carbon black. The photoanode functioned in the absence of any sacrificial agent by water splitting, but the produced photocurrent was doubled in the presence of glycerol or ethanol. Hydrogen peroxide production was monitored in all cases, mainly in the presence of glycerol. The presence or absence of the organic fuel affected only the obtained photocurrent. The Faradaic efficiency for hydrogen peroxide production was the same in all cases, mounting up to 74%. The duplication of the photocurrent in the presence of biomass derivatives such as glycerol or ethanol and the fact that WO3 absorbed light in a substantial range of the visible spectrum promotes the presently studied system as a sustainable source of hydrogen peroxide production.