OERKOP aims to understand and describe the mechanisms underlying the OER for co-catalyst modified hematite photoanodes, by using a correlative operando characterization approach at the nanoscale. OERKOP proposes a comparative study between two archetypal materials used as photoanodes: α-Fe2O3 and BiVO4, both catalytically activated by oxyhydroxide catalysts (M-OOH, with M = Fe, Ni, Co, Cu, Zn). Surface kinetics and time stability during the oxidation will be systematically evaluated. We intend combining multi-scale and multi-selective techniques - scanning transmission X-ray microscopy (STXM) and scanning transmission electron microscopy (STEM). Both techniques will employ a common sample environment in a dedicated photoelectrochemical cell. Complementary DFT calculations will allow dissecting complex effects and determining the reaction priorities of different photocatalytic sites.