Engineering Oxygen Vacancies on Mixed-Valent Mesoporous α-MnO₂ for High-Performance Asymmetric Supercapacitors

Intrinsically poor conductivity and sluggish ion-transfer kinetics limit the further development of electrochemical storage of mesoporous manganese dioxide. In order to overcome the challenge, defect engineering is an effective way to improve electrochemical capability by regulating electronic configuration at the atomic level of manganese dioxide. Herein, we demonstrate effective construction of defects on mesoporous α-MnO₂ through simply controlling the degree of redox reaction process, which could obtain a balance between Mn³⁺/Mn⁴⁺ ratio and oxygen vacancy concentration for efficient supercapacitors. The different structures of α-MnO₂ including the morphology, specific surface area, and composition are successfully constructed by tuning the mole ratio of KMnO₄ to Na₂SO₃. The electrode materials of α-MnO₂-0.25 with an appropriate Mn³⁺/Mn⁴⁺ ratio and abundant oxygen vacancy showed an outstanding specific capacitance of 324 F g^-1 at 0.5 A g^-1, beyond most reported MnO₂-based materials. The asymmetric supercapacitors formed from α-MnO₂-0.25 and activated carbon can present an energy density as high as of 36.33 W h kg^-1 at 200 W kg^-1 and also exhibited good cycle stability over a wide voltage range from 0 to 2.0 voltage (kept at approximately 98% after 10 000 cycles in galvanostatic cycling tests) and nearly 100% Coulombic efficiency. Our strategy lays a foundation for fine regulation of defects to improve charge-transfer kinetics.