Smart confinement of MnO enabling highly reversible Mn(II)/Mn(III) redox for asymmetric supercapacitors

Abstract

The energy density of the intensively-studied aqueous asymmetric supercapacitors (ASCs) is limited by the conventional carbon anodes with low specific capacitance. Low-valence manganese oxides with high pseudocapacitive capacitance that can work at low potential windows show great promise to substitute the capacitive carbon anodes, and yet is severely challenged by the dissolution of Mn(II) in aqueous electrolyte. In this work, we develop a high-performance anode that is free from Mn(II) dissolution through the smart confinement of manganese monoxide into nitrogen-rich carbon nanosheets (MnO@NCN, 12.28 at.% N)with both physically spatial protection and interfacial chemical binding (C–N–Mn and C–O–Mn). The strongly coupled MnO–NCN interface empowers this nanocomposite highly reversible Mn(II)/Mn(III) redox even at a negative potential of −1.2 V (vs. SCE). The MnO@NCN anode attains a high specific capacitance of 562.3 C g−1 and extremely long-term 10,000 cycles with tiny capacitance decay. An ASC consisted of NiCo-layered double hydroxides (NiCo-LDH) and the MnO@NCN anode delivers specific energy as high as 94 Wh kg−1, remarkably outperforming its counterparts built with the commonly-used carbon anodes and the vast majority of ASCs. This work establishes a viable strategy to stabilize manganese-oxide-based electrodes in aqueous energy storage systems.