One-step transformation of MnO2 into MnO2−x@carbon nanostructures for high-performance supercapacitors using structure-guided combustion waves

Abstract

The manipulation of micro/nanostructured metal oxides is crucial to advancing their diverse applications, including as electrodes in supercapacitors or batteries, catalysts, and pigments. However, controlling the physicochemical properties of metal oxides requires complex procedures with bulky setups that incur high costs and long processing times. Herein, we present a facile one-step manipulation of the reduced states of manganese oxides and the synthesis of carbon coatings surrounding them, using structure-guided combustion waves (SGCWs), which is induced by incomplete combustion through the chemical fuel-wrapped materials. Controlled oxygen release from MnO₂ using SGCWs in air and in an Ar atmosphere enabled direct fabrication of reduced Mn₂O₃/Mn₃O₄/MnO and MnO, respectively. Furthermore, control of the incompletely combusted carbonaceous fuel facilitated the synthesis of carbon coating layers to form Mn₂O₃/Mn₃O₄/MnO@C and MnO@C. These core–shell nanostructures of reduced manganese oxides and carbon layers were applied as supercapacitor electrodes. These electrodes showed better specific capacitance (maximum 438 F g⁻¹ at 10 mV s⁻¹ scan rate for Mn₂O₃/Mn₃O₄/MnO@C) and improved stability in charge–discharge performance compared with bare MnO₂, due to the carbon coatings enhancing electrical conductivity in the percolation network of electrodes and facilitating the reversible redox reaction without degradation during cycling operations. SGCWs are applicable for fast, low-cost, and large-scale fabrication of reduced metal oxides and organic material coatings, which could significantly contribute to electrochemical applications.