Site-Specific Single Atom Incorporation in Cobalt Spinel Oxide for High-Performance Acidic Oxygen Evolution Reaction

The capability to finely control specific surface properties of a catalyst has been recognized as a significant issue in achieving optimized catalytic processes, owing to the dependence of catalytic performance on these surface properties. Spinel oxides with an AB₂O₄ crystal structure have been of particular interest. Depending on the cation selection at the A and B sites, the physicochemical properties of spinel oxides can be precisely modified.<br/>Very recently, Co₃O₄ has emerged as a representative non-noble metal catalyst for the acidic oxygen evolution reaction (OER), providing a potential substitute for the rare metals Ru and Ir. However, with a few exceptions, the incorporation of metal elements in cobalt spinel oxide has been limited to first-row transition metals (e.g., Fe, Mn, Ni, Cu, and Zn). It is highly desirable to design a cobalt spinel oxide system that can withstand harsh OER conditions while maintaining high reactivity through rational metal cation incorporation. Although intriguing, the physicochemical properties of cobalt spinel oxides that incorporate metal cations other than those mentioned above have not been well elucidated. In particular, the early transition metals in the higher periods of the periodic table, such as Hf, Ta, and W, which readily react with water to form heterogeneous oxides, pose a significant challenge for their atomic distribution in Co₃O₄. Due to the synthetic difficulty, little is known about the unique properties exhibited by these metals in the form of single atoms within cobalt spinel oxide.<br/>Herein, we provide unified understanding on the metal cation incorporation in cobalt spinel oxide crystal structure. First, we developed a general synthetic method that enabled us to directly synthesize the versatile metal doped Co₃O₄ from early transition metals to metalloids. Using highly porous metal organic framework (MOF) structure, we could successfully incorporate Hf, Ta, W, Ti, Ga, Ge, and Pd into Co₃O₄ without formation of any hetero metal oxide species. Second, we clarified each metal species has different stabilization sites in cobalt spinel structure. Ta, W,and Ge have a great tendency to be stabilized on the surface of Co₃O₄, which dramatically increases Co²⁺ species. The dopant-rich shell along with high density of surface Co²⁺ then provides active and protective layers for high-performance acidic OER. Ta doped Co₃O₄ has the low overpotential of 378 mV at 10 mA cm^-2, and maintained its activity over 140 hours in acidic electrolytes. According to in-situ XAS and ICP, the protective shell suppresses over-oxidation and dissolution of Co species during the reaction.<br/>In summary, we present a unified picture of the single-atom-doped cobalt spinel oxide crystal structure and a general synthetic principle applicable to a broad range of metal elements beyond the first-row transition metals. Additionally, we elucidate the metal-specific stabilization sites in Co3O4, which exhibit unique surface physicochemical properties. These finely controlled surface properties can enhance catalytic performance and provide durable stability in acidic OER. Our discovery of a general synthesis method for incorporating various metals into cobalt spinel oxide, along with the corresponding control of surface properties, can be applied to various catalytic processes for sustainable energy storage and conversion systems.