MOF-derived manganese oxide/carbon nanocomposites with raised capacitance for stable asymmetric supercapacitor

Owing to immense application potentials in electrochemical energy storages, metal organic framework (MOF)-derived metal oxide/carbon nanocomposites have attracted extensive interest of research. Although thermolysis has been widely employed to convert MOFs into various active materials, a large set of in situ changes in chemical composition, phase(s) and morphology requires delicate control over heating parameters. Through an innovative two-stage process, Mn-MIL-100 is first transformed into MnO@C by annealing at 700 °C under N₂ flow, which is then transformed into Mn₃O₄@C at 200 °C in air, while retaining a high surface area. The appropriate retention of carbon content for Mn₃O₄@C can also be easily obtained with the control of heating time. In contrast, thermolysis of MnO@C at higher temperatures gives rise to manganese oxides with negligible carbon content and a greatly reduced surface area. The optimized Mn₃O₄@C-2 h, derived from MnO@C at 200 °C for 2 hours, showed the highest capacitance, far exceeding that of MnO@C and other derivatives. When combined with graphene oxide (GO) nanosheets to form a flexible Mn₃O₄@C/rGO paper electrode, it demonstrated a capacitance of 328.4 F cm^-3. The Mn₃O₄@C/rGO-based asymmetric supercapacitor thus assembled also shows favorable performance. The present work demonstrates the excellent controllability afforded by the innovative two-stage thermolysis in optimizing the electrochemical performance of MOF-derived active materials as electrode materials in supercapacitors.