Carbon-based materials are currently considered as promising candidates for electrode material of supercapacitors due to its excellent electric conductivity and desirable specific surface area. The capacitance performance of carbon-based material can be improved from two aspects: firstly, one can enlarge the specific surface area (SSA) to improve the electric double layer capacitance (EDLC); secondly, the pseudo-capacitance behavior of carbon-based materials can be enhanced by doping with other elements like N or compositing with transitional metal oxides.

In the present study, dopamine (DPA) and urea were employed to derive porous carbon spheres by solution route. For the DPA-derived carbon spheres, the nitrogen-doped porous spheres (NPCS) with different activation conditions...[ Read more ]

Carbon-based materials are currently considered as promising candidates for electrode material of supercapacitors due to its excellent electric conductivity and desirable specific surface area. The capacitance performance of carbon-based material can be improved from two aspects: firstly, one can enlarge the specific surface area (SSA) to improve the electric double layer capacitance (EDLC); secondly, the pseudo-capacitance behavior of carbon-based materials can be enhanced by doping with other elements like N or compositing with transitional metal oxides.

In the present study, dopamine (DPA) and urea were employed to derive porous carbon spheres by solution route. For the DPA-derived carbon spheres, the nitrogen-doped porous spheres (NPCS) with different activation conditions were synthesized to compare their electrochemical performances. The results indicate that NPCS-3, obtained with the calcination of nitrogen-doped carbon spheres (NCS) in KOH in the mass ratio of NCS:KOH=3, exhibits the best performance, delivering 38 mAh g^-1 specific discharge capacity after 1000 cycles. XPS analysis demonstrates that the NPCS-3 sample contains more N element, and the doped N may contribute to the enhanced pseudo-capacitance. When the obtained NPCS-3 sample was further activated with CaCl₂, the obtained Ca-NPCS exhibits even higher capacity, with 73 mAh g^-1 at initial cycle and maintaining 53.5 mAh g^-1 after 1000 cycles. BET and XPS were measured to explain the enhanced electrochemical performance.

Concerning the high cost of DPA, urea was used to replace DPA to derive carbon spheres denoted as UPCS. The synthesized carbon spheres are desirable, with perfect sphere-like shape and consistent size. When treated with calcination and activation like NPCS-3, the obtained UPCS delivered comparable electrochemical performance with NPCS-3. Moreover, CaCl2 activation poses no obvious influence on the performance of urea-derived carbon spheres, unlike that of DPA derived spheres.

The electrochemical performance of the prepared samples (NPCS-3, Ca-NPCS, UPCS and Ca-UPCS) was also tested in KOH aqueous electrolyte. The obtained results are consistent with that in organic electrolyte. Furthermore, symmetric and asymmetric supercapacitors were assembled to explore the performance of Ca-NPCS sample. The assembled supercapacitors delivers desirable capacity but unpromising cycling ability.