Photoelectrochemical evaluation of SILAR-deposited nanoporous BiVO4 photoanodes for solar-driven water splitting

Abstract

We report a photoelectrochemical investigation of BiVO 4 photoanodes prepared by successive ionic layer adsorption and reaction (SILAR), a facile method that yields uniform nanoporous films. After characterization of the phase, morphology, composition, and optical properties of the prepared films, the efficiencies of charge separation ( η sep ) and water oxidation ( η ox ) in solar water splitting cells employing these photoanodes were estimated following a previously reported procedure. Unexpected wavelength and illumination direction dependencies were discovered in the derived efficiencies, casting doubt on the validity of the analysis. An alternative approach using a diffusion–reaction model that explicitly considers the efficiency of electron collection resolved the discrepancies and explained the illumination direction dependence of the photocurrent. Electron diffusion lengths ( L n ) of 0.45 μm and 0.55 μm were derived for pristine and cobalt phosphate (Co-Pi) modified BiVO 4 , respectively, which are much shorter than the film thickness of ~2.1 μm. The Co-Pi treatment also increased η ox from 0.86 to ~1, which is the main reason for the overall performance enhancement caused by adding Co-Pi. These findings suggest that there is little scope for improving the performance of SILAR-deposited BiVO 4 photoanodes by further catalyzing water oxidation, but enhanced performance is achievable if electron transport can be improved.