Polaron-Mediated Transport in BiVO4 Photoanodes for Solar Water Oxidation

Hydrogen dopants and oxygen vacancies (OVs) play crucial roles in BiVO4 photoanodes. However, the decisive factor determining the charge transport of the hydrogenated BiVO4, particularly with electron small polaron formation, remains elusive. Here we show a decreased charge transport barrier upon mildly hydrogenating the nanoporous BiVO4 photoanode, as evidenced by the thermally activating photocurrent responses. Monochromatic light photoelectrochemical performance, temperature-dependent conductivity, proton nuclear magnetic resonance, and density functional theory calculation disclose that the external hydrogen atoms occupy the intrinsic OVs in the BiVO4, reducing the hopping activation energy and facilitating electron small polaron transport. The resulting BiVO4 photoanode with NiFeOx cocatalyst achieves an applied-bias photon-to-current efficiency of 1.91% at 0.58 V vs RHE with front-illumination. This study extends the common understanding of the beneficial role in conventional donor density/surface chemisorption mediations of hydrogen doping to now include small polaron hopping.