High-performance BiVO4 photoanodes: elucidating the combined effects of Mo-doping and modification with cobalt polyoxometalate

Abstract

Bulk doping of BiVO₄ with molybdenum combined with surface modification with a cobalt polyoxometalate water oxidation catalyst (CoPOM = Na₁₀[Co₄(H₂O)₂(PW₉O₃₄)₂]) is reported. The best-performing Mo–BiVO₄/CoPOM photoanode exhibits a photocurrent density of 4.32 mA cm⁻² at 1.23 V vs. RHE under AM 1.5G (1 sun) illumination and an applied-bias photoconversion efficiency (ABPE) of ∼0.73%, which is an improvement by a factor of ∼24 with respect to pristine BiVO₄. Mechanistic analyses are used to prove the contributions of Mo-doping and CoPOM modification. Mo-doping is shown to result in enhanced electronic conductivity and passivation of surface states, whereby these beneficial effects are operative only at relatively high applied bias potentials (>0.9 V vs. RHE), and at lower bias potentials (<0.7 V vs. RHE) they are counterbalanced by strongly detrimental effects related to increased concentration of electron polaronic states induced by the Mo-doping. CoPOM deposition is related to the enhancement of water oxidation catalysis. The molecular CoPOM acts as a pre-catalyst and undergoes (partial) conversion to cobalt oxide under the PEC operating conditions. The study demonstrates CoPOM-derived catalysts as effective water oxidation catalysts at BiVO₄ photoanodes and suggests that further progress in BiVO₄ photoanode development depends on alternative strategies for conductivity enhancement to avoid detrimental polaronic effects associated with the conventional bulk doping of BiVO₄.