Why does NiOOH cocatalyst increase the oxygen evolution activity of a-Fe2O3?

Nickel oxyhydroxide (NiOOH) is known to increase the oxygen evolution reaction (OER) performance of hematite (Fe2O3) photoanodes. In recent experimental studies, it has been reported that the increased OER activity is related to the activation of the hematite (α-Fe2O3) surface by NiOOH rather than the activity of NiOOH itself. In this study, we investigate the reason behind the higher activity and the low overpotentials for NiOOH-Fe2O3 photoanodes using first principles calculations. To study the activity of possible catalytic sites, different geometries with NiOOH as a cluster and as a strip geometry on hematite (110) surfaces are studied. Density functional theory + U calculations are carried out to determine the OER activity at different sites of these structures. The geometry with a continuous strip of NiOOH on hematite is stable and is able to explain the activity. We found that the Ni atoms at the edge sites of the NiOOH cocatalyst are catalytically more active than Ni atoms on the basal plane of the cocatalyst; the calculated overpotentials are as low as 0.39 V.