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 NiFeO_x 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.