Ultrahigh Electrocatalytic Activity of an Iron-Based Bimetallic Oxide for Oxygen Evolution Reaction in Alkaline
Authors: Sugawara Yuuki , Kamata Keigo , Ishikawa Atsushi , Tateyama Yoshitaka , Yamaguchi Takeo 
Published: December 2020

The present environmental issue has been drawn the demand on a breakthrough in efficient energy conversion and storage systems. Solar and wind energies are expected as attractive renewable energy resources, which can be inexhaustibly used to generate electricity, but their intermittency and isolated locations from consuming sites prevent their practical application. Hence, these natural energies are required to be converted into energy carriers to store and transport them to consuming sites. One of promising candidates as such energy carriers is hydrogen. Hydrogen is a clean fuel which does not discharge CO2 and other poisonous emissions and possesses high energy density. To efficiently and sustainably produce hydrogen, alkaline electrolyzers have recently received much interest. Alkaline electrolyzers can produce pure hydrogen from abundant water using electricity and have been considered to be a cost-effective mode for energy conversion, because alkaline conditions allow to employ nonprecious metals as electrocatalysts. However, the sluggish anodic oxygen evolution reaction (OER) (4OH− → O2 + 2H2O + 4e−) of water splitting with a large overpotential is the main concern [1]. Ruthenium oxide (RuO2) and iridium oxide (IrO2) are well-known as state-of-the-art electrocatalysts for OER [2–4], but the high costs of these precious metals limit widespread use. Hence, cost-effective electrocatalysts that possess excellent OER activity are highly desirable.

As alternative OER electrocatalysts, iron (Fe)-based materials have recently attracted much attention [5], because Fe is the most abundant 3d transition metal in the earth's crust [6] which is found to be the fourth place in the Clarke numbers. Although simple Fe oxides are intrinsically inactive for electrochemical OER, its OER activity can be dramatically enhanced by doping and substitution with other metals. Most of previous works studied the OER activities on spinel [7] and perovskite [8]-type oxides due to their good electrochemical activities and facile synthesis, whereas very few reports have investigated other classes of Fe-based metal composite oxides consisting of. e.g., edge- and face-shared polyhedral networks.

We herein present a Fe-based bimetallic oxide with prominent OER activity, comprising a distinct crystalline structure from normal spinel- and perovskite-type oxides. The crucial factor of the excellent OER activity of the Fe-based bimetallic oxide was elucidated through comparison with similar Fe-based bimetallic oxide analog containing alkaline-earth metals as second metal components. It is noteworthy that the OER specific activity per surface areas of the Fe-based oxide particles overcame those of previously reported Fe-based bimetallic oxides comprising alkaline-earth and rare-earth metals as well as even benchmark IrO2. Furthermore, a durability test by potential cycles at OER potential region caused the only negligible loss of OER current, and its crystalline structure and metal compositions were thoroughly retained, demonstrating high stability of the oxide in alkaline OER. Moreover, the prominent OER activity of the Fe-based oxide was elucidated by DFT calculations, suggesting that the OER activity of the oxide was boosted by its balanced structural and electronic effects. Since this Fe-based oxide has many advantages, such as simple synthesis, high abundance, cost effectiveness and environmental friendliness, it is a quite promising OER electrocatalyst for water splitting and can open a new way for development of energy conversion devices.

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Acknowledgement

Part of this presentation is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) and supported by PRESTO program (No. JPMJPR15S3) of the JST. The calculations were carried out on the supercomputers at NIMS, Hokkaido Univ., Kyushu Univ., and the HPCI systems (project ID: hp180115).