Efficient solar photoelectrolysis by nanoporous Mo:BiVO4 through controlled electron transport

Abstract

A detailed understanding of doping level, electron diffusion length and coefficient, as well as light capture and charge separation efficiencies in nanoporous Mo-doped BiVO₄ (Mo:BiVO₄) photoanodes is obtained using photoelectrochemical techniques. Efficient water oxidation is achieved by doping with 1.8% Mo, resulting in a several-fold enhancement in photooxidation rate versus non-doped BiVO₄. Two techniques are used to study the effect of Mo doping on the electron transport: (1) an analysis of the front/back illumination ratio of incident photon-to-current efficiency and (2) intensity modulated photocurrent spectroscopy. These techniques show that Mo doping improves the diffusion coefficient four-fold and increases the diffusion length to ca. 300 nm (from 10 nm for the non-doped material), which is also the empirically-determined optimal Mo:BiVO₄ film thickness for photoelectrolysis. These films are found to have a 90% charge separation efficiency and an 80% absorbed photon-to-current efficiency, excellent values for metal oxide photoabsorbers. Among the many oxygen evolution catalysts studied, surface modification with iron oxyhydroxide (FeOOH), a simple earth abundant catalyst, dramatically enhances the water oxidation performance of Mo:BiVO₄ to an integrated IPCE of 2.41 mA cm⁻² and a photocurrent density of 2.77 mA cm⁻² in neutral phosphate at 1.23 V vs. RHE.