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Tomiko Suzuki1 Takamasa Nonaka1 Kosuke Kitazumi1 Naoko Takahashi1 Satoru Kosaka1 Yoriko Matsuoka1 Keita Sekizawa1 Akihiko Suda1 Takeshi Morikawa1

1, Toyota Central R&D Labs Inc, Nagakute, , Japan

The catalytic oxygen evolution reaction (OER) to extract electrons from water molecules is important for the artificial photosynthesis to generate useful chemicals such as hydrogen and organic compounds [1, 2]. In terms of elements strategy, utilization of abundant element for OER catalysts is remarkably advantageous for future low-costly artificial photosynthetic system. Fe-based OER catalysts, based on the fourth most earth-abundant element, are attractive, but are known to suffer from low OER activity due to limited electrical conductivity and non-ideal electronic structures near the surfaces of these catalysts.
Here, we report a highly crystalline, 10 nm-sized red rust OER catalyst composed of pure β-phase FeOOH(Cl) nanorods (ca. 3 × 13 nm) doped with Ni ions (β-FeOOH(Cl):Ni) [3] and surface-modified with amorphous Ni(OH)2 (a-Ni(OH)2, at a Ni to Fe ratio of 22 at.%), which shows the highest level of performance among Fe-rich oxides and oxyhydroxides. This catalyst can be synthesized by a facile one-pot process at room temperature, and colloidal aqueous solutions of the β-FeOOH(Cl)Ni/a-Ni(OH)2 nanorods are very stable, with no apparent precipitation over a time span of at least one month.
Electrochemical measurements for β-FeOOH:Ni/a-Ni(OH)2 stacked nanorod anodes deposited on carbon paper (CP) were performed in a 3-electrode configuration using a Ag/AgCl reference electrode and a Pt wire counter electrode. The overpotential during the electrochemical OER over the anodes was 170 mV, and an OER current of 10 mA/cm2 was obtained at an overpotential of 430 mV(+1.66 V vs. RHE) in 0.1 M KOH (without subtracting the iR drop). It is suggested that the surface modification with the a-Ni(OH)2 lowered the OER overpotential of β-FeOOH(Cl):Ni, resulting in the very high current density at low potential compared with Fe-rich oxide and oxyhydroxide electrodes reported previously. Mössbauer spectroscopy also suggested electronic interaction between Fe and Ni species, which may be crucial evidence for the enhanced activity in the Fe-rich OER system [4].
The present cost-effective Fe-based OER catalysts can be widely applied to construct artificial photosynthetic systems for solar fuel generation by combination with CO2 reduction catalysts.

References
[1] T. R. Cook, et al., Chem. Rev., 110 (2010) 6474. [2] C. C. L. McCrory, et al., J. Am. Chem. Soc., 136 (2013) 16977. [3] T. M. Suzuki, T. Morikawa, et al., Sustainable Energy Fuels, 1 (2017) 636. [4] T. M. Suzuki, T. Morikawa, et al., Bull. Chem. Soc. Jpn., 91 (2018) 778.

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