From hierarchically porous carbon to Mn3O4/Carbon composites for high voltage aqueous supercapacitors

The energy density of aqueous supercapacitors can be enhanced by large capacitance and high operating voltage, which is mainly limited by the narrow stability window of the water. Herein, carbonaceous materials with nitrogen-doping and hierarchical porosity are successfully designed and prepared via a facile sol−gel route accompanied by the cooperative self-assembly of triblock copolymer and melamine-resorcinol-formaldehyde polymer, followed by carbonization and CO2 activation. Based on the as-prepared carbon material, symmetric supercapacitors achieve a high cell voltage of 1.8 V in 1 M Li2SO4, and exhibit a high energy density of 27.8 Wh kg−1 at power density of 0.23 kW kg−1. Taking advantage of the reversible redox reaction of Mn3+/Mn4+ accompanied with the insertion/extraction of Li+ of birnessite-type MnO2, the working potential of positive electrode is further expanded to 0–1.2 V (vs SCE). Then asymmetric supercapacitors with high cell voltage of 2.2 V have been successfully fabricated, exhibiting a higher energy density of 44.5 Wh kg−1 at power density of 0.27 kW kg−1 and an acceptable energy density of 21.8 Wh kg−1 even at a high power density of 5.6 kW kg−1.