Electrochemical Water Oxidation over Core/Shell structured Hyperfine β-FeOOH (akaganeite) Crystalline Nanorods coated with Bias-Responsive Amorphous Ni(OH)₂

The water oxidation to extract electrons from water molecules for the oxygen evolution reaction (OER) is essential for the development of a sustainable system to synthesize valuable chemicals such as hydrogen and organic compounds from H₂O and CO₂. The catalysts consisting of earth-abundant elements are required for the system integration with minimized cost and total CO₂ emission in its lifecycle.
We have developed a 10 nm-sized highly crystalline red rust catalyst for OER composed of pure β-phase FeOOH(Cl) hyperfine nanorods (an average diameter of 3 nm and a length of 14 nm) synthesized by a facile one-pot process at room temperature and ambient pressure.[1] This one-pot process yields β-FeOOH(Cl) nanorods sizing more than 10 times smaller than those by conventional methods. The process also enables doping with Ni ions in the crystal lattice (β-FeOOH(Cl):Ni) and simultaneous surface-coating with amorphous a-Ni(OH)₂ (a Ni to Fe ratio up to 22 at.%), which forms a core/shell structure.[2] The overpotential for electrochemical OER over anodes stacked with the core/shell β-FeOOH:Ni/a-Ni(OH)₂ was 170 mV, and an OER current of 10 mA/cm² was obtained at an overpotential of 430 mV in a 0.1 M KOH solution. The high current density at low potential compared with many Fe-rich oxide and (oxy)hydroxide electrodes reported previously.
X-ray absorption fine structure analysis (XAS), Mössbauer spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and impedance spectroscopy suggested that surface coating with the a-Ni(OH)₂ lowered the OER overpotential of β-FeOOH(Cl), resulting in reduced total impedance in the electrode. Mössbauer spectroscopy suggested interaction between Fe and Ni species [2], and operando X-ray absorption spectroscopy (XAS) under biased conditions in the aqueous solution revealed a characteristic behavior that does not occur in Fe-Ni mixed oxide systems.[3] The nearest neighbor structure and valence of Fe³⁺ ions did not change under the OER conditions. In contrast, Ni ions showed second nearest neighbor ordering which was assignable to β-Ni(OH)₂, and a fraction of Ni²⁺ ions was partially oxidized to Ni³⁺ at the bias for OER. This is presumably the change at the interface of the β-FeOOH:Ni nanorods and the surface a-Ni(OH)₂. This Ni valence change was reversible, following the sweep of the electrical bias. These findings show an essential role of Fe-Ni interactions in the core/shell β-FeOOH:Ni/a-Ni(OH)₂, accompanied by Ni species' structural and partial valence change under the electrical bias.
Further, after treatment in an alkaline solution of the β-FeOOH:Ni/ a-Ni(OH)₂ stacked electrode operates long-term OER even in a nearly neutral pH solution. The electrode as an anode was series-connected with a Mn-complex catalyst cathode for CO₂ reduction and a Si solar cell in a one-compartment reactor to construct a system mainly consisting of earth-abundant elements.[4] Under a nearly neutral pH solution bubbled with CO₂ (pH 6.9), the system produced CO and achieved solar-to-chemical energy conversion efficiency of 6.6 % in a single electrolyte solution. A long-term OER in the nearly neutral pH solution performed after a specific treatment of β-FeOOH:Ni/ a-Ni(OH)₂ will also be presented.

References
[1] T. M. Suzuki, T. Nonaka, A. Suda, N. Suzuki, Y. Matsuoka, T. Arai, S. Sato and T. Morikawa, Sustain. Energy Fuels, 1 (2017) 636-643.
[2] T. M. Suzuki, T. Nonaka, K. Kitazumi, N. Takahashi, S. Kosaka, Y. Matsuoka, K. Sekizawa, A. Suda and T. Morikawa, Bull. Chem. Soc. Jpn., 91 (2018) 778–786.
[3] T. Morikawa, S. Gul, Y. F. Nishimura, T. M. Suzuki and J. Yano, Chem. Commun., 56 (2020) 5158-5161.
[4] T. Arai, S. Sato, K. Sekizawa, T. M. Suzuki and T. Morikawa, Chem. Commun., 55 (2019) 237-240.