Regulating manganese valence in MnO_x/rGO composite for high-performance supercapacitors

Manganese oxides (MnOx) have become an increasingly popular electrode material for supercapacitors due to their low cost, abundance, and environmentally-friendly nature. One important factor that enhances their electrochemical properties is the manganese valence. Mn3+ is particularly useful as it exhibits two redox couples (Mn2+/Mn3+ and Mn3+/Mn4+), which maximize the redox reaction and result in excellent electrochemical performance. This study focused on regulating the manganese valence using a facile pyrolysis method, resulting in a Mn3+-rich MnOx/rGO composite (rGO denotes reduced graphene oxide) at a pyrolysis temperature of 400 degrees C. The composite demonstrated a high specific capacitance of 304 F g(-1). Additionally, asymmetric supercapacitors (MnOx/rGO//activated carbon) were assembled with aqueous and quasi-solid electrolytes respectively, and they demonstrated similar performance. The supercapacitor device also showed good capacitance retention of 82% after 10,000 charge/discharge cycles at a current density of 10 A g(-1). Furthermore, the variation of the energy storage mechanism for MnOx/rGO with the change of manganese valence was discussed. Overall, this work highlights the importance of controllable synthesis of MnOx-based electrode materials for high-performance supercapacitors, and could pave the way for future developments in the field.