Double-Carbon Matrix-Supported MnO₂ for High-Voltage Supercapacitors in a Neutral Aqueous System

The low conductivity and poor structural stability of MnO2 nanoparticles have impeded further enhancement in specific energy density for aqueous asymmetric supercapacitors. To address this issue, in this article, carbon nanotubes (CNTs) and mesoporous carbon (meso-C) are merged together, ultrasonically treated with poly(sodium 4-styrenesulfonate) surfactant and then immersed in a KMnO4 solution at room temperature to generate a composite, namely, double-carbon matrix (CNTs and meso-C)-supported K-MnO₂ (K⁺ incorporated state). When this composite was employed as an electrode in the neutral aqueous electrolyte, this material behaved as a redox pseudocapacitor and delivered a maximum specific capacity of 292.5 C g^-1 (∼585 F g^-1). When the composite was used as one electrode and the negative-activated carbon was employed as the other electrode, the as-assembled hybrid asymmetric device in the neutral aqueous system could achieve a specific capacitance of 86.0 F g^-1 within an ultrahigh potential range of 0-2.1 V, breaking through a bondage of 2.0 V. This energy-storage device could deliver 52.7 W h kg^-1, correlating to a power density of 525 W kg^-1. Moreover, the effects of various ratios between CNTs and meso-C on the resulting performance were also investigated and compared.