Abstract
Spinel-type oxides are technologically important in many fields, including electronics, magnetism, catalysis and electrochemical energy storage and conversion. Typically, these materials are prepared by conventional ceramic routes that are energy consuming and offer limited control over shape and size. Moreover, for mixed-metal oxide spinels (for example, CoxMn3−xO4), the crystallographic phase sensitively correlates with the metal ratio, posing great challenges to synthesize active product with simultaneously tuned phase and composition. Here we report a general synthesis of ultrasmall cobalt manganese spinels with tailored structural symmetry and composition through facile solution-based oxidation–precipitation and insertion–crystallization process at modest condition. As an example application, the nanocrystalline spinels catalyse the oxygen reduction/evolution reactions, showing phase and composition co-dependent performance. Furthermore, the mild synthetic strategy allows the formation of homogeneous and strongly coupled spinel/carbon nanocomposites, which exhibit comparable activity but superior durability to Pt/C and serve as efficient catalysts to build rechargeable Zn–air and Li–air batteries.
Highlights
Spinel-type oxides are technologically important in many fields, including electronics, magnetism, catalysis and electrochemical energy storage and conversion
Spinels are traditionally synthesized through solid-state methods involving grinding and firing the mixtures of the corresponding metal oxides, nitrates or carbonates[5,6], which require elevated temperature and prolonged time to overcome the reaction energy barriers[7]
Based on O K-edge X-ray absorption spectroscopy analysis (Supplementary Fig. 6), we find a moderate affinity between surface metal cation and absorbed oxygen-containing species in both c-CoMn2 and t-CoMn2 spinels, which would facilitate the rate-limiting steps of hydroxide/ peroxide displacement and regeneration during the ORR28
Summary
Spinel-type oxides are technologically important in many fields, including electronics, magnetism, catalysis and electrochemical energy storage and conversion. If the cation Mn3 þ concentration is low enough (r60–65%), Jahn–Teller distortion would not be prominent and the spinels could exist in cubic form[16] It is feasible but remains a great challenge to rationally synthesize Co–Mn spinels with controlled phase over a wide composition range, which is highly desired in structuresensitive applications. To develop efficient mixed 3d–metal–spinel catalysts with the above factors in mind, we describe a unique strategy for the rational synthesis of nanocrystalline Co3 À xMnxO4 (1rxr2) as well as NiCo2O4, FeCo2O4, ZnCo2O4 and ZnMn2O4 This solution-based synthesis involves oxidizing precipitation of Mn/Co salts in air atmosphere and subsequent crystallization into spinels at mild temperature (180 °C). The tetragonal form is extended in a composition range of 1rxr[3], while the cubic
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