Ultrasmall α-MnO₂ with Low Aspect Ratio: Applications to Electrochemical Multivalent-Ion Intercalation Hosts and Aerobic Oxidation Catalysts

Hollandite-type α-MnO₂ exhibits exceptional promise in current industrial applications and in advancing next-generation green energy technologies, such as multivalent (Mg²⁺, Ca²⁺, and Zn²⁺) ion battery cathodes and aerobic oxidation catalysts. Considering the slow diffusion of multivalent cations within α-MnO₂ tunnels and the catalytic activity at edge surfaces, ultrasmall α-MnO₂ particles with a lower aspect ratio are expected to unlock the full potential. In this study, ultrasmall α-MnO₂ (<10 nm) with a low aspect ratio (c/a ≈ 2) is synthesized using a newly developed alcohol solution process. This material demonstrates exceptional performance across various multivalent battery systems, primarily due to the significantly reduced cation diffusion distance. Notably, an ultrasmall α-MnO₂-graphene composite achieves high capacity with low overpotential when paired with an F-free electrolyte in Ca battery. Regarding aerobic oxidation catalysis, the nanosizing of α-MnO₂ has a profound impact on aerobic oxidation catalysis. The increased efficiency of oxidative conversion reactions, such as the oxidation of 1-phenylethanol, is attributed to the greatly expanded active surface area of the catalyst. The versatile functionality of ultrasmall α-MnO₂ underscores its potential to revolutionize energy storage and catalysis, offering broad applicability in next-generation green energy technologies.