N-doped carbons with hierarchically micro- and mesoporous structure derived from sawdust for high performance supercapacitors

A series of nitrogen-doped carbons were prepared by using biomass waste sawdust as carbon source for supercapacitors. A new nitrogen-doping strategy was proposed to introduce nitrogen element into carbons by using ethylenediamine and carbon as nitrogen source through the polymerization reaction on sawdust under moderate conditions. The raw carbons were further activated with KOH to improve the surface area to increase the electric double layer capacitance while the heteroatoms can improve the pseudocapacitance. The prepared carbons were characterized with different techniques and the electrochemical performances were tested. It was found that the nitrogen-doped carbons activated with a KOH to carbon mass ratio of 5 (NC600-800-5) has the largest specific surface area (1674 m(2)g(-1)) and moderate nitrogen content (2.88 wt%). Compared with the carbon without introduction of nitrogen element (C600-800-5), the capacitive performances of nitrogen-doped carbons were greatly improved, which may be due to the increase of pseudocapacitance and the improvement of wettability introduced by nitrogen element. The largest specific capacitance, 367 F g(-1), was reached at 0.5 A g(-1 )in 6 M KOH solution while the specific capacitance of C600-800-5 is only 279 F g(-1). After 10,000 cycles at 3 A g(-1), the specific capacitance can keep 96.7% of the initial value, suggesting the excellent cycling performance. The energy density can reach 8.7 Wh Kg(-1) at a power density of 251.6 W kg(-1). Thus, it is proved that the nitrogendoping strategy is effective and the introduction of nitrogen element to carbons is particularly important to the improvement of the electrochemical performances.