Unravelling the bulk and interfacial charge transfer effects of molybdenum doping in BiVO4 photoanodes

[Display omitted] •Transparent Mo6+ doped BiVO4 (Mo:BV) photoanodes were synthesized by spin coating.•PEC performances in both sulfite and water oxidation reactions were compared.•Electrochemical surface area (ECSA) almost doubled upon Mo dopant incorporation.•Doping BiVO4 reduced the interfacial charge transfer efficiency in sulfite oxidation.•Mo:BV outperformed in water oxidation due to better conductivity and hole injection. The role of Mo6+ doping on the photoelectrochemical (PEC) performance of BiVO4 photoanodes was investigated both in the presence and in the absence of sulfite as hole scavenger. Optically transparent, flat BiVO4 photoanodes containing different amounts of Mo6+ dopant were synthesized by spin coating. An increase of Mo6+ dopant amount was found to both improve the electron transport in the BiVO4 bulk by increasing its conductivity, as unequivocally ascertained when employing a Ni/Fe oxyhydroxide co-catalyst, and facilitate the charge transfer at the electrode/electrolyte interface in water oxidation, in the absence of hole scavenger. On the other hand, increasing amounts of the Mo6+ dopant in BiVO4 induced an unexpected decrease in PEC performance per unit surface area in sulfite oxidation, resulting from enhanced interfacial charge transfer resistance, as demonstrated by electrochemical impedance spectroscopy. First evidence is thus provided of a different behaviour observed upon Mo6+ doping of BiVO4 depending on the nature of the involved electron donor species, together with an intriguing multifaceted role played by Mo6+ doping in enhancing the PEC performance of modified BiVO4 electrodes.