Nitrogen and phosphorus dual-doped hierarchical porous carbon with excellent supercapacitance performance

    The incorporation of heteroatom into well-developed porous carbon materials can significantly improve the capacitive performance. We employ a facile route for the synthesis of nitrogen (N) and phosphorus (P) dually doped carbon materials via hydrothermal carbonization of nitrogen-containing biomass-derived compounds (chitosan) followed by phosphoric acid activation. The obtained carbon possesses well-defined hierarchical porosity (surface area: 639–1142 m²g ^-1 , pore volume: 0.48-0.76 cm³g^-1 ) and large
nitrogen and phosphorus content, depending on synthesis conditions. Due to heteroatom-doping and hierarchical pore size distribution, the specific capacitances of carbon materials reach 312.4 and 204.4 F g^-1 at 0.2 and 10 A g^-1 respectively. This excellent capacitive performance is ascribed to collaborative effect of electrical double layer capacitor and faradaic pseudocapacitor from the redox reactions of the nitrogen and phosphorus species. Moreover, the specific capacitance retains 97% after 2000 cycles indicating a good stability.

     In summary, nitrogen and phosphorus dual-doped porous carbon has been fabricated using chitosan as precursors via hydrothermal carbonization and H ₃ PO ₄ activation. The NPC-700 exhibits excellent electrochemical performance in 6 M KOH, including specific capacitance of 312.4 and 204.4 F g ^-1 at 0.2 and 10 A g ^-1 respectively, and excellent cycling stability (below 3% decay after 2000 cycles) due to hierarchical porous structure and heteroatom doping. Importantly, micropores can accumulate ions for efficient energy storage and mesopores offer facile channels for the ion transport and diffusion. Furthermore, the incorporation of nitrogen and phosphorus into carbon frameworks not only contribute electrical double layer capacitors (carbon with high specific surface area) and pseudocapacitors (redox reaction), but also improve the electrical conductivity and wettability of carbonmaterials. Thus, we believe these carbon materials with nitrogen and phosphorus dual doping porous and hierarchical structure can be widely utilized in other applications, such as catalysis (as a support), gas adsorption and storage, Li-ion batteries and oxygen reduction reaction.

http://dx.doi.org/10.1016/j.electacta.2017.07.077