Ni-Doped β‑FeOOH\nHyperfine Nanorods Surface-Modified\nwith Ni Species Catalyzing Water Oxidation at Neutral pH: Strained\nFeOOH Core and Phase-Changing Ni(OH)<sub>2</sub>

Stacked electrodes of Ni-doped β-FeOOH(Cl) hyperfine\nnanorods\nwith an average size of φ5 × 16 nm, whose surface was modified\nwith Fe incorporated Ni hydroxide (β-FeOOH:Ni/Ni(OH)<sub>2</sub>:Fe), exhibit low overpotential for the electrochemical oxygen evolution\nreaction (OER) by water oxidation in CO<sub>2</sub>-saturated borate-containing\nsolutions at a neutral pH of 6.9. The OER in a neutral-pH solution\nis highly desired for a scalable CO<sub>2</sub> reduction system that\nrequires simultaneous operation in a membrane-free single electrolyte\nreactor, essential to solar-driven electrochemical and photocatalytic\nsystems. A starting catalyst material, β-FeOOH:Ni nanorods surface-modified\nwith Ni(OH)<sub>2</sub> (β-FeOOH:Ni/Ni(OH)<sub>2</sub>) are\nprepared via a simple one-pot room-temperature synthesis. Here, an\nactivation of OER is demonstrated using β-FeOOH:Ni/Ni(OH)<sub>2</sub> subjected to a heat treatment, followed by electrolysis treatment\nunder alkaline conditions, to form the new catalyst, β-FeOOH:Ni/Ni(OH)<sub>2</sub>:Fe. <sup>57</sup>Fe Mössbauer spectroscopy and X-ray\nabsorption spectroscopy (XAS) clarified that the 200 °C treatment,\na temperature close to the upper temperature limit of the phase transition\nto α-Fe<sub>2</sub>O<sub>3</sub>, led to disorder of the FeO<sub>3</sub>(OH)<sub>3</sub> octahedra in core β-FeOOH. The subsequent\nalkaline treatment induced a slight shift of the Ni valence from Ni<sup>2+</sup> to Ni<sup>3+</sup>, a shift of β-Ni(OH)<sub>2</sub> toward β-NiOOH-like short-range ordering on the surface, and\nthe Ni species modified the local state around Fe ions. <i>Operando</i> XAS further clarified a tangible picture of the phenomenon: the\nFe–Fe spacing is shortened, and β-Ni(OH)<sub>2</sub> is\nfurther shifted toward β-NiOOH by responding to the electrical\nbias potential for the OER, which shows a reversible tendency in response\nto the external bias. Electron microscopy, analyses of composition\nand chemical state, and electrochemical measurements also suggested\nthat the coexistence of Ni and Fe in both the β-FeOOH:Ni and\nthe Ni(OH)<sub>2</sub>:Fe phases induced by the post treatment would\ncontribute to the highly improved OER activity of the nanorod-stacked\nβ-FeOOH:Ni/Ni(OH)<sub>2</sub>:Fe electrocatalyst.