Ni-Doped β-FeOOH Hyperfine Nanorods Surface-Modified Ni Species Catalyzing Water Oxidation at Neutral pH: Strained FeOOH Core and Phase-Changing Ni(OH)2

Stacked electrodes of Ni-doped β-FeOOH(Cl) hyperfine nanorods with an average size of φ5 × 16 nm, whose surface was modified with Fe incorporated Ni hydroxide (β-FeOOH:Ni/Ni(OH)2:Fe), exhibit low overpotential for the electrochemical oxygen evolution reaction (OER) by water oxidation in CO2-saturated borate-containing solutions at a neutral pH of 6.9. The OER in a neutral-pH solution is highly desired for a scalable CO2 reduction system that requires simultaneous operation in a membrane-free single electrolyte reactor, essential to solar-driven electrochemical and photocatalytic systems. A starting catalyst material, β-FeOOH:Ni nanorods surface-modified with Ni(OH)2 (β-FeOOH:Ni/Ni(OH)2) are prepared via a simple one-pot room-temperature synthesis. Here, an activation of OER is demonstrated using β-FeOOH:Ni/Ni(OH)2 subjected to a heat treatment, followed by electrolysis treatment under alkaline conditions, to form the new catalyst, β-FeOOH:Ni/Ni(OH)2:Fe. 57Fe Mössbauer spectroscopy and X-ray absorption spectroscopy (XAS) clarified that the 200 °C treatment, a temperature close to the upper temperature limit of the phase transition to α-Fe2O3, led to disorder of the FeO3(OH)3 octahedra in core β-FeOOH. The subsequent alkaline treatment induced a slight shift of the Ni valence from Ni2+ to Ni3+, a shift of β-Ni(OH)2 toward β-NiOOH-like short-range ordering on the surface, and the Ni species modified the local state around Fe ions. Operando XAS further clarified a tangible picture of the phenomenon: the Fe–Fe spacing is shortened, and β-Ni(OH)2 is further shifted toward β-NiOOH by responding to the electrical bias potential for the OER, which shows a reversible tendency in response to the external bias. Electron microscopy, analyses of composition and chemical state, and electrochemical measurements also suggested that the coexistence of Ni and Fe in both the β-FeOOH:Ni and the Ni(OH)2:Fe phases induced by the post treatment would contribute to the highly improved OER activity of the nanorod-stacked β-FeOOH:Ni/Ni(OH)2:Fe electrocatalyst.