Novel highly active and self-healing Co(CO3)xOHy cocatalysts on BiVO4 photoanodes for effective solar water oxidation

Abstract

Recent reports show that Co-Pi cocatalysts on some photoanodes, such as Fe₂O₃ or BiVO₄, accelerate the surface water oxidation rate, acting as a hole extracting layer that improves charge separation or as a “surface passivation agent”. In contrast, on semiconductors such as BiVO₄, deposition of classic Co-based oxygen evolution catalysts (e.g. Co₃O₄, CoOOH) leads to a relatively low improvement of the photoelectrochemical (PEC) performance. This illustrates that even though a species may be an excellent electrocatalyst, it may not be an effective co-catalyst for PEC reactions. Clearly, for different semiconductors there is a need to directly identify and prepare specific, highly active and low-cost cocatalysts for promoting PEC water oxidation. Co(CO₃)_0.5(OH)·0.11H₂O (or in general Co(CO₃)_xOH_y) is usually used as a precursor to synthesize a range of Co-based efficient electrocatalysts. Herein, we for the first time show that Co(CO₃)_xOH_y on BiVO₄ provides an excellent oxygen evolution reaction activity, even close to the so far most efficient dual-layer FeOOH/NiOOH cocatalysts. Decoration of BiVO₄ with Co(CO₃)_xOH_y and the resulting photoanode shows a remarkable enhancement of photocurrent, achieving a photocurrent density of 5.0 mA cm⁻² at 1.23 V_RHE with an onset potential of 0.3 V_RHE, which is one of the highest PEC activities of BiVO₄ reported up to now. The outstanding PEC performance can be attributed to the improvement of charge separation and transfer efficiency caused by a uniform thin coating of Co(CO₃)_xOH_y. The Co(CO₃)_xOH_y exhibits the highest density of active sites, the highest surface area, and the best OER activity among the investigated cobalt-based cocatalysts. Remarkably, the Co(CO₃)_xOH_y also displays a self-healing function in borate buffered electrolytes. Therefore, this work not only describes low-cost but highly active cobalt-based catalysts for PEC solar water oxidation, but additionally shows a self-repair function of this system.