Large area high-performance bismuth vanadate photoanode for efficient solar water splitting

Abstract

Commercial-scale photoelectrochemical (PEC) water splitting devices have been proved to be cost-competitive with fossil-based fuels. Nevertheless, large-scale PEC water splitting cells with high performance have not yet been demonstrated, and controllably scaling up high-performance photoelectrodes still remains challenging. Typically, bismuth vanadate (BiVO₄) has been regarded as one of the most efficient photoanodes for use in PEC water splitting because of its well-suited band structure. However, state-of-the-art PEC performances achieved with BiVO₄ photoanodes to date have generally been tested on small scales (<1 cm²), due to the nonuniformity of the scaled-up photoactive films, increased resistive losses and inhomogeneous potential distribution in scaled-up substrates, non-linear diffusion of reactants and resultants toward scaled-up photoanodes and so on. Here, a uniform scaled-up precursor (BiOI) film was prepared via a spatial-current-density-distribution-regulated electrodeposition method with a tilt-electrode-configuration. The as-prepared BiOI film was covered with a thin vanadium source layer and then converted to BiVO₄ film via a modified thermal-chemical conversion method. The photocurrent density for sulfite oxidation of the 54.32 cm² BiVO₄ photoanode (∼2.55 mA cm⁻² at 1.23 V_RHE) was the highest value yet reported, and it was comparable with that of the 1.66 cm² BiVO₄ photoanode (∼2.85 mA cm⁻² at 1.23 V_RHE). To reduce resistive losses and improve the homogeneity of distributed potential in the FTO substrate, metal grids were deposited on the BiVO₄ photoanode via thermal evaporation. The maximum fill factor of the photocurrent density curve of the scaled-up BiVO₄ photoanode was raised from ∼29 to ∼44.5%. After the uniform coating of an oxygen evolution catalyst (OEC) layer that was formed by connecting tannin acid molecules with metal ions, the photoanode achieved a photocurrent density of ∼2.23 mA cm⁻² at 1.23 V_RHE and ∼0.83% STH conversion efficiency at 0.65 V_RHE. Finally, the stability test was conducted in a designed test system with water circulation to facilitate diffusion of generated oxygen. The scaled-up OEC/BiVO₄ photoanode showed <4% decay after operating under harsh conditions for 5 h.