Hole Collection and Surface Kinetics in Mo-Doped FeVO₄ Photoanodes during Photoelectrochemical Water Oxidation

Abstract

Triclinic FeVO4 is a potential photoanode material for photoelectrochemical (PEC) water oxidation owing to its near-optimum band gap and ease of preparation from abundant elements. However, useful performance is yet to be achieved, apparently due to the short hole diffusion length (Lp), positive flat-band potential (Efb), and high donor density (ND). Other factors, such as hole transfer and surface recombination kinetics, have not been studied in detail. Here, we report pure and Mo-doped FeVO4 photoanodes prepared by a simple drop-casting technique that exhibit water oxidation photocurrents comparable to the highest reported to date for this material. We find that some previous estimates of ND and Lp are likely to be too large due to oversimplified data analysis that neglects changes in the potential drop across the double layer and the roughness and porosity of typical films. Using a more realistic approach, we estimate lower ND of the order of 1020 cm-3 and extremely short Lp (<1 nm) that appears to increase after Mo doping. Fast surface recombination relative to water oxidation limits the photocurrent at potentials <1.6 V versus RHE, with faster recombination observed for Mo-doped FeVO4 due to higher majority carrier and surface state densities. Band unpinning occurs under illumination near the photocurrent onset potential and must be considered when interpreting the potential dependence of the photocurrent to estimate Lp. This work identifies the main factors limiting the water oxidation performance of FeVO4 photoanodes and clarifies the effects of Mo doping. The analytical approach presented here should also be useful for interpreting PEC data obtained from other porous photoelectrodes.