Operando Spectroscopic Identification of Active Sites in NiFe Prussian Blue Analogues as Electrocatalysts: Activation of Oxygen Atoms for Oxygen Evolution Reaction

Developing highly efficient oxygen evolution reaction (OER) catalysts and understanding their activity are pivotal for electrochemical conversion technologies. Here, we report NiFe Prussian blue analogue (PBA) as a promising electrocatalyst for OER in alkaline conditions. This material has an impressively low overpotential of 258 mV that reaches a current density of 10 mA cm^-2. Post-mortem characterization showed that the as-prepared catalyst is entirely transformed into amorphous nickel hydroxide after the electrochemical treatment, and Ni(OH)₂ acts as the active species. Operando X-ray spectroscopic studies further found that this in situ generated Ni(OH)₂ displays an unique feature that allows deprotonation under applied potential creating NiOOH_{2- x} that contains Ni⁴⁺ ions. The deprotonation reaction is reversible and potential-dependent, i.e., the amount of Ni⁴⁺ increases with increasing applied potential. Theoretical calculations were used to show that the role of Ni⁴⁺ is to trigger oxidized oxygen ions as electrophilic centers with the subsequent activation of anion redox reactions for OER.