Distribution of high valence Fe sites in nickel-iron hydroxide catalysts for water oxidation

Nickel-iron (Ni-Fe) hydroxides have received much attention as abundant and efficient electrocatalysts for the oxygen evolution reaction (OER) under alkaline conditions. However, the behavior of Fe dopants during the reaction is still under debate. Herein, we use first-principles calculations to investigate the dehydrogenation of the basal (0001) surface of 25% Fe-doped Ni hydroxide from Ni_3/4Fe_1/4(OH)₂ to Ni_3/4Fe_1/4OOH, which is generally considered to be the active phase. Our calculations show that the high valence Fe ions tend to form domains by undergoing double-exchange processes with the neighboring Ni ions, while the oxidation states of the Ni ions do not increase steadily but fluctuate between Ni²⁺ and Ni³⁺ during the dehydrogenation. The boundaries of domains between high-valence Fe³⁺ and Fe⁴⁺ ions are the most reactive sites for the OER, with overpotentials as low as 0.36 V. This finding not only suggests that the abundant (0001) facet, often considered catalytically inactive in previous studies, can actually make an important contribution to the catalytic performance of nickel-iron hydroxides, but is also relevant to the design of more effective and efficient catalysts for the OER.