High-Performance BiVO4 Photoanodes: Elucidating the Combined Effects of Mo-Doping and Surface-Modification with Polyoxometalate Co-Catalysts

Doping and surface-modification by co-catalysts are well-established strategies for the performance enhancement of bismuth vanadate (BiVO4) photoanodes for photoelectrochemical (PEC) water splitting devices. However, our knowledge of the complex effects of doping and surface modification that govern the PEC performance is still underdeveloped, which makes the rational design of high-performance BiVO4-based photoanodes challenging. Herein, a remarkably effective strategy for enhancing the performance of BiVO4 photoanodes by combining the bulk doping of BiVO4 with molybdenum (Mo) and its surface modification with a well-defined molecular cobalt polyoxometalate water oxidation catalyst (CoPOM = Na10[Co4(H2O)2(PW9O34)2]) via a simple impregnation protocol without any linkers or binders is reported. The best-performing optimized 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 the factor of ~24 with respect to pristine BiVO4. The respective contributions of the Mo-doping and modification with CoPOM to the performance enhancement are disentangled based on detailed mechanistic investigations. The positive effect of Mo-doping is shown to be related to 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. The highly beneficial effect of CoPOM deposition is unambiguously demonstrated to be related to the enhancement of water oxidation catalysis. Notably, the molecular CoPOM acts as a pre-catalyst, as it undergoes (at least partially) conversion to cobalt oxide under the PEC operating conditions. Our work thus establishes CoPOM-derived catalysts as effective water oxidation catalysts at BiVO4 photoanodes, and suggests that further progress in BiVO4 photoanode development depends critically on devising alternative strategies for conductivity enhancement that would avoid the negative polaronic effects associated with the conventional bulk doping of BiVO4.