Biomass-derived hierarchical porous carbons: boosting the energy density of supercapacitors via an ionothermal approach

High mass energy density coupled with high power density is highly desired for electrical double-layer capacitors. Usually the capacitive performance is improved by optimizing the pore size and volume distribution. Herein, the authors report an efficient approach to optimize the porous structure through a facile ionothermal carbonization method. A series of hierarchical porous carbons with unique sub-micrometer sized morphology, high surface area and abundant mesopores (e.g. S-BET = 2532 m(2) g(-1) and V-meso = 1.077 cm(3) g(-1)) have been synthesized, using Jujun grass as a nitrogen-containing precursor. The ionic liquid acts not only as a reaction medium for the conversion of biomass to carbon but also as a porogenic agent for inducing mesoporosity. The results indicate that the ionothermal method can balance the micro-and mesoporosity of the optimized porous carbon, making it one of the competent alternatives to the state-of-the-art electrodes for ultra-high energy density supercapacitors. The optimized ionothermal carbon (ITC-JG-900) shows an impressively high specific capacitance of 336 F g(-1) at 1 A g(-1) in 6 M KOH, and even retained a capacitance of 222 F g(-1) at 10 A g(-1), which is 66% of the initial capacitance. The maximum energy density of ITC-JG-900 as a supercapacitor is found to be over 72.7 W h kg(-1) when the power density is 1204 W kg(-1), which is higher than those of most of the equivalent benchmarks tested in aqueous electrolytes.