Identification of Highly Active Fe Sites in (Ni,Fe)OOH for Electrocatalytic Water Splitting

TLDR

Highly active catalysts for the oxygen evolution reaction (OER) are required for the development of photoelectrochemical devices that generate hydrogen efficiently from water using solar energy. Here, we identify the origin of a 500‑fold OER activity enhancement achieved with mixed (Ni,Fe)oxyhydroxides (Ni₁₋ₓFexOOH) over their pure Ni and Fe parent compounds, producing one of the most active known OER catalysts in alkaline electrolyte. Using computational methods, we establish that the short Fe–O bond octahedral sites in Ni₁₋ₓFexOOH create a structural motif that yields near‑optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. The study shows that Fe³⁺ occupies short‑bond octahedral sites in Ni₁₋ₓFexOOH, creating a motif that gives near‑optimal OER intermediate adsorption and low overpotentials at Fe sites, while Ni sites remain inactive, accounting for the 500‑fold activity boost over pure Ni or Fe oxides.

Abstract

Highly active catalysts for the oxygen evolution reaction (OER) are required for the development of photoelectrochemical devices that generate hydrogen efficiently from water using solar energy. Here, we identify the origin of a 500-fold OER activity enhancement that can be achieved with mixed (Ni,Fe)oxyhydroxides (Ni1–xFexOOH) over their pure Ni and Fe parent compounds, resulting in one of the most active currently known OER catalysts in alkaline electrolyte. Operando X-ray absorption spectroscopy (XAS) using high energy resolution fluorescence detection (HERFD) reveals that Fe3+ in Ni1–xFexOOH occupies octahedral sites with unusually short Fe–O bond distances, induced by edge-sharing with surrounding [NiO6] octahedra. Using computational methods, we establish that this structural motif results in near optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. By contrast, Ni sites in Ni1–xFexOOH are not active sites for the oxidation of water.