Science 350, 1508–1513 (2015)

The ability to quickly charge and discharge electrical energy makes supercapacitors ideal devices for short-term energy storage. However, supercapacitors do not usually possess high specific capacitance (the electric charge stored per unit mass or volume) and thus high energy density. Carbon-based electrode materials have a capacitance of merely around 300 F g−1. Fuqiang Huang and colleagues at Shanghai Institute of Ceramics, Peking University and University of Pennsylvania have now reported a nitrogen-doped porous carbon electrode that possesses a capacitance of 855 F g−1 and a supercapacitor based on the electrode that stores an energy of 41 W h kg−1 or 19.5 W h l−1.

The electrode features nanoporous walls consisting of a few graphene-like sheets in which a certain amount of nitrogen atoms occupy the original carbon lattices due to nitrogen doping. Huang and colleagues showed that those nitrogen atoms, adjacent to defects on the graphene-like sheets, are mainly responsible for the fast redox reaction in electrochemical cells. The electrode is also hydrophilic, meaning that an aqueous electrolyte can easily wet it, lowering the interface resistance and increasing the electrical conductivity. These properties, along with the high surface area of the electrode arising from its porous characteristics, explain the exceptionally high capacitance.