Hierarchical porous carbon obtained from frozen tofu for efficient energy storage

Frozen tofu, as a carbon and nitrogen source, is converted to N (0.6-6.7 at%) and 0 (3.6-9.5 at%) co-doped porous carbon with a specific surface area of up to 3134 m(2) g(-1) by one-step carbonizationactivation. This hierarchical carbon consists of a high volume of mesopores (1.11 cm(3) g(-1)) and micropores (0.71 cm(3) g(-1)) with a typical pore size distribution between 0.8 and 4 nm. Supercapacitors with the obtained carbon as electrodes exhibit a specific capacitance of 243 F g(-1) (measured at 0.1 A g(-1)) in 1 M sulfuric acid (H2SO4) aqueous electrolyte with a capacitance retention of 93% after 10 000 cycles at 10 A g(-1). The carbon also exhibits a specific capacitance of 170 F g(-1) (measured at 1 A g(-1)) with a good rate capability (135 F g(-1) at 20 A g(-1)) in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) ionic liquid electrolyte, providing an energy density of 72 W h kg(-1) (at an average power density of 889 W kg(-1)). A supercapacitor fabricated from the carbon in the ionic liquid electrolyte can readily power 25 light-emitting diodes in parallel for more than 2 min after being charged for 25 s at a current density of 10 A g(-1). In addition, as an anode for Li-ion batteries, the porous carbon exhibits a high reversible charge capacity of 2120 mA h g(-1) in the first cycle (measured at 0.1 A g(-1)) or 1035 mA h g(-1) after 300 cycles (measured at 1 A g(-1)).