Unraveling the surface kinetics of Co:BiVO4/CoOx photoanodes for photoelectrochemical water oxidation

BiVO₄ is one of the most promising photoanodes for photoelectrochemical water splitting but their performance is still limited by charge recombination and sluggish surface kinetics. To deal with these limitations, various Co:BiVO₄/CoO_x photoanodes were prepared in this work. Photocurrent density was increased by a factor of 2-3 by optimizing Co-doping. To find out the enhance mechanism, photoinduced absorption spectroscopy (PIAS) was employed to establish the relationship between photocurrent density and concentration of the photogenerated holes on surface. Meanwhile, intensity-modulated photocurrent spectroscopy (IMPS) was applied to measure the rate constant of charge transfer and surface recombination. The improved PEC performance was attributed to the synergy effect of Co-doping that increased the bulk charge separation efficiency and significantly suppressed surface recombination by two orders of magnitude at high potentials. More importantly, our investigation reveals that not all the surface holes could participate in water oxidation reaction and the utilization proportion of them was strongly dependent on Co-doping and potential. Not more than 12 % of the photogenerated holes were used for water oxidation on the pristine BiVO₄ surface over a wide potential range. In contrast, as high as 80 % of the photogenerated holes participated in water oxidation with the assistance of the CoO_x nano-catalysts. It is the critical enhancement mechanism of the Co:BiVO₄/CoO_x photoanodes for photoelectrochemical water oxidation.