Ultrasmall CoO(OH)x Nanoparticles As a Highly Efficient True Cocatalyst in Porous Photoanodes for Water Splitting

The coupling of light absorbers to cocatalysts with well-designed optical and catalytic properties is of fundamental importance for the development of efficient photoelectrocatalytic devices for solar-driven water splitting. We achieved an effective loading of visible-light-active porous hybrid photoanodes for water photooxidation with ultrasmall (similar to 1-2 nm), highly disordered CoO(OH)(x) nanoparticles using a two-step impregnation method. Under visible light (lambda > 420 nm) irradiation, the resulting photoanodes significantly outperformed photoanodes loaded with conventional cobalt-based cocatalyst (Co-Pi) comprising larger nanoparticles (similar to 5 nm) in terms of both Faradaic efficiency of oxygen evolution (by the factor of 2) and performance stability under long-term irradiation. A combination of STEM, XAS, cyclic voltammetry, and photoelectrochemical techniques was used to elucidate the advantages of using ultrasmall CoO(OH)(x) nanoparticles as cocatalysts. Specifically, due to the high transparency of ultrasmall CoO(OH)(x) nanoparticles in the visible range, higher loading of porous photoanodes with cobalt catalytic sites can be achieved, while the photocurrent losses due to parasitic light absorption by the cocatalyst are minimized. Notably, a significant enhancement in stability of ultrasmall CoO(OH)(x) nanoparticles in borate electrolytes as compared to phosphate electrolytes has been observed. EXAFS data recorded before and after photoelectrocatalysis indicated that the effect of the electrolyte on the stability can be explained by the difference in structural ordering dictated by different interaction of the electrolyte anions with cobalt ions, as corroborated by DFT calculations. This study highlights the strong impact of structural and optical properties of cocatalysts as well as the strong influence of the electrolyte composition on the activity and stability of photoelectrocatalytic systems comprising transition metal oxide electrocatalysts.