Nitrogen-doped hierarchical porous carbons prepared via freeze-drying assisted carbonization for high-performance supercapacitors

Functionalized carbon electrodes with hierarchical porous structure play an essential role in the fabrication of high-performance supercapacitors . Here, we report a novel freeze-drying assisted carbonization approach for the preparation of nitrogen-doped hierarchically porous carbons (NHPCs). Using urea as both the pore-forming agent and the nitrogen source, and sodium alginate (SA) as the carbon precursor, we are able to modulate the porosity and N-doping concentration of the NHPCs by simply altering the carbonization temperature and the dosage of urea. The prepared NHPCs exhibit specific surface area as high as 1179 m 2  g −1 with 70% micropore and high N- and O-doping concentration of 6.7 wt% and 13.8 wt%, respectively. Carbon electrodes based on these NHPCs show a high gravimetric capacitance (305 F g −1 at 1 A g −1 ) and an excellent rate ability (retaining 73.8% at 20 A g −1 ) in three-electrode systems. The NHPCs symmetric supercapacitor delivers excellent energy density of up to 23.8 W h kg −1 as well as long-term electrochemical stability (95.8% capacitance retention over 10, 000 cycles). The abundance of the raw materials combined with the simple fabrication procedure makes the SA-derived NHPCs as promising electrode materials for the next generation advanced supercapacitors .