Nitrogen and phosphorus co-doped cubic ordered mesoporous carbon as a supercapacitor electrode material with extraordinary cyclic stability

Abstract

Heteroatoms and porosity both have different, but definite effects on the electrochemical capacitance of carbon materials. These effects are studied in detail by using cubic ordered mesoporous carbons (OMCs) co-doped with N and P. 3-Dimensional (3D) mesoporous silica, KIT-6, with bicontinuous cubic Ia3d symmetry is utilized as a hard template to synthesize the cubic OMCs. Interestingly, although the porosity parameters e.g. surface area and pore volume do not change much with N doping, a significant increase of these values is observed upon P doping. Moreover, the P content does not affect the N doping characteristics on co-doping of both N and P. When tested as a supercapacitor electrode, the N-OMC, despite its much lower porosity parameters, exhibits a similar specific capacitance to that of the P-OMC. The high specific capacitance of N-OMC arises mainly from the pseudocapacitive effect of doped N species, whereas high porosity parameters are the main reason for the specific capacitance shown by P-OMC. The synergistic contribution of both effects enables the NP co-doped OMC to show the highest specific capacitance of 210 F g⁻¹ at 1.0 A g⁻¹. Moreover, excellent retention of specific capacitance with more than 90% of initial capacitance is observed for NP-OMC at a high current density of 10 A g⁻¹ and also for 3000 charge–discharge cycles. This is mainly because of high-surface area hierarchical porous structures with uniform and ordered mesopores in the cubic OMC, which facilitate the unrestricted movement of electrolyte ions to access the active surfaces, as well as the excellent synergistic effect of co-doping of N and P. This is also supported by electrochemical impedance spectroscopic analysis, which shows negligible mass transfer resistance and internal cell resistance. Overall, the synthesized cubic OMC materials are found to be highly promising as electrodes for supercapacitors and other energy-related applications.