Figure 5.

Mechanistic scheme summarizing the metal oxidation observed in our in situ XAS measurements of the Ni₆₅Fe₃₅(O_xH_y) catalyst. The resting state (non-catalytic potential) is observed between 1.0–1.4 V vs. RHE. Above this potential (>1.4 V) the oxidation of Ni²⁺ to Ni^3+/4+ occurs. An oxidation state lower than +4 is usually observed both in Ni(OOH) and in mixed Ni-Fe(O_xH_y) catalysts. In the 1^st metal oxidation step (shown in green), a discharge of an OH⁻ group takes place, which adsorbs to a vacant site and increases the oxidation state by one unit. In this step a radical species may instead be formed (-OH∙) which would not result in a metal oxdation state increase (this is not shown in our cycle however may happen at both Fe and Ni sites). In the 2^nd metal oxidation step, Ni is further oxidized to Ni⁴⁺. However, if there is a consecuetive step where charge re-distribution occurs from O 2p → Ni 3d, the average metal oxidation state is lowered (Ni^+4-δ), and as a consequence the oxygen oxidation state is increased (O^−2+δ). This step according to our O K-edge data is a stable state and is strongly correlated to the metal oxidation step, and occurs prior to O-O bond formation.