Influence of Elemental Compositions and Crystalline Structures on Electrocatalytic Activity of Fe-Based Oxides for Oxygen Evolution Reaction in Alkaline Water Splitting

Abstract

Current energy and environmental issues, alongside the depletion in fossil fuels, have increased the demand for the development and use of sustainable alternative energy resources. Due to their inexhaustibility, renewable energy resources such as solar and wind energies, are highly promising candidates for the construction of sustainable energy systems. To effectively use these intermittent sources in nature, it is essential that it is converted into chemical fuels that can be efficiently stored and transported. One such example of harnessing natural energy is the electrochemical water splitting of naturally abundant water to produce hydrogen, which is a promising carbon-free chemical fuel with a high energy density that, with some further work, can be stored and transported for use on a large scale. Electrochemical water splitting can be carried out using renewable energy generated electricity, making it a clean and cost-effective process. However, the anodic oxygen evolution reaction (OER) in the water splitting process has a large overpotential. Thus, there is a need to develop highly active and inexpensive electrocatalysts for OER to overcome this barrier. Iron (Fe) is an abundant element in the Earth’s crust that is inexpensive and has low toxicity, making it a potential candidate for widespread use in catalysts. Although simple Fe oxides, such as Fe2O3, have intrinsically low OER activity, the activity can be dramatically enhanced by combining the Fe species with other metals, resulting in optimization of the energy levels of the Fe 3d and O 2p bands and modification of the valence state of the Fe cation. Among the OER catalysts, perovskite-type metal oxides with the general formula ABO3, where the A- and B-sites are occupied by alkaline-earth or rare-earth metals and transition metals, respectively, have attracted great interest due to their prominent OER activities, facile synthesis and environmental friendliness. Although several active Fe-based perovskite-type OER catalysts have been previously reported,[1,2] most of these studies focused on the choice of elements for the A-site metal. While the effects of crystalline structures and elemental compositions of catalysts with Fe and A-site metals have not yet been systematically investigated, the knowledge is crucial to understanding the OER process catalyzed by Fe-based oxides and for the efficient design of promising electroactive materials. In this study, a variety of Fe-based oxides with the general formula AxFeyOz were synthesized, and their electrocatalytic OER activities in alkaline media were systematically investigated to elucidate the effects that the structure and composition of the materials have on the OER activity. Common alkaline-earth metals, Ca, Sr, and Ba, were selected as A-site metals, as shown in Figure 1A. Fe-based oxides were synthesized via a carboxylic acid-aided sol–gel method, which is a versatile synthetic tool to easily control the elemental compositions of the A- and B-site metals and has been previously employed to produce excellent electrocatalysts.[3] The structures of the fabricated oxides were characterized by X-ray diffraction, and the Brunauer–Emmet–Teller specific surface areas of the oxides were also measured. Subsequently, the OER activities of the oxides in alkaline medium were evaluated using a rotating-disk electrode and compared. Figure 1B shows the OER specific activities of the Fe-based oxides and reveals that a Ca-containing oxide was found to possess the highest OER specific activity among the synthesized oxides, exhibiting one of the best performances observed when compared to previously reported Fe-based oxides. These findings not only present an excellent electrocatalyst for OER in alkaline media, but also provide new guidelines for the design of metal oxide-type OER electrocatalysts.