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

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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 NiOOH2- 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.