BiVO4 Photoanode For Water Oxidation Research Articles

High‐Temperature One‐Step Synthesis of Efficient Nanostructured Bismuth Vanadate Photoanodes for Water Oxidation

Bismuth vanadate (BiVO4) is a promising photoanode material for photoelectrochemical water splitting due to its well‐suited valence band edge and comparatively narrow band gap. First insights on the high‐temperature and scalable synthesis of efficient nanostructured BiVO4 photoanodes for water oxidation are provided. Nanostructured BiVO4 films with a tunable optical density and porosity ranging from 12% to 80% are synthesized in few seconds by direct deposition of flame‐made BiVO4 nanoparticle aerosols. The impact of the BiVO4 film structural properties on the photooxidation performance has been systematically investigated by a set of electrochemical and physical characterizations indicating key directions for its morphological optimization. It is found that the BiVO4 water oxidization performance is mainly determined by two competitive factors, viz., accessible surface area and carrier conductivity through the grain boundaries. Optimization of these two factors increases the photocurrent densities by more than three times resulting in ≈1.5 mA cm−2 for sulfite oxidation and ≈1 mA cm−2 for water oxidation with a FeOOH/NiOOH co‐catalyst at 1.0 V vs the reversible hydrogen electrode (RHE) under simulated 1 sun illumination. These findings provide novel insights into the structure–activity relationships of high‐temperature synthesized BiVO4 photoanodes for solar‐powered water splitting and introduce a scalable and low‐cost approach for their rapid nanofabrication.

Boosting solar water oxidation activity and stability of BiVO4 photoanode through the Co-catalytic effect of CuCoO2

The poor surface charge separation and transfer properties as well as sluggish water oxidation kinetics jointly limit the performance of BiVO4 photoanode for water oxidation. In this work, p-type CuCoO2 with high-spin Co3+(Oh) was investigated as co-catalyst to synergistically improve the surface charge separation and transfer efficiencies as well as reaction kinetics of BiVO4 film for photoelectrochemical water oxidation. In comparison with the photocurrent on BiVO4 photoanode for water oxidation (1.21 mA/cm2 at 1.23 V vs. RHE), the CuCoO2-coupled BiVO4 (CuCoO2/BiVO4) photoanode exhibits a higher photocurrent density of 3.32 mA/cm2 at 1.23 V vs. RHE under AM 1.5G illumination. In addition, a significant improvement on the reaction stability is achieved on the CuCoO2/BiVO4 photoanode, about ∼79% water oxidation activity is retained on the CuCoO2/BiVO4 photoanode after operating at 0.8 V vs. RHE for 5 h, while only ∼9% activity is retained on the BiVO4 photoanode. The boosted water oxidation activity and stability on CuCoO2/BiVO4 photoanode could be attributed the synergistic effect that originated from CuCoO2-electrocatalysis and BiVO4-photocatalysis in thermodynamics and kinetics. Specifically, p-n heterojunctions are formed in the coupling interface between CuCoO2 (p-type) and BiVO4 (n-type), which thermodynamically improve the surface charge separation and transfer efficiencies of BiVO4 photoanode during water oxidation. Simultaneously, the high-spin Co3+(Oh) of CuCoO2 could act as active sites to accelerate the water oxidation of CuCoO2/BiVO4 photoanode in kinetics. In addition, Cu2+ active sites are formed for water oxidation through the oxidation reaction of photogenerated holes with the Cu+ of CuCoO2.

Single-Source Bismuth (Transition Metal) Polyoxovanadate Precursors for the Scalable Synthesis of Doped BiVO4 Photoanodes.

Single-source precursors are used to produce nanostructured BiVO4 photoanodes for water oxidation in a straightforward and scalable drop-casting synthetic process. Polyoxometallate precursors, which contain both Bi and V, are produced in a one-step reaction from commercially available starting materials. Simple annealing of the molecular precursor produces nanocrystalline BiVO4 films. The precursor can be designed to incorporate a third metal (Co, Ni, Cu, or Zn), enabling the direct formation of doped BiVO4 films. In particular, the Co- and Zn-doped photoanodes show promise for photoelectrochemical water oxidation, with photocurrent densities >1 mA cm-2 at 1.23 V vs reversible hydrogen electrode (RHE). Using this simple synthetic process, a 300 cm2 Co-BiVO4 photoanode is produced, which generates a photocurrent of up to 67 mA at 1.23 V vs RHE and demonstrates the scalability of this approach.