A dual carbon Na-ion capacitor based on polypyrrole-derived carbon nanoparticles

N/S-co-coped carbon nanoparticles (mean size ∼ 80–90 nm) have been synthesized by a simple strategy based on the preparation of polypyrrole nanospheres by a room temperature oxidative polymerization process followed by their S doping using elemental sulfur. The developed materials combine a highly disordered microstructure with a high level of N- and S-doping and short solid-state diffusion distances. These characteristics endow the materials with a large number of readily accessible adsorption/insertion sites for sodium storage, yielding thus high reversible capacities (of up to 726 mAh g−1 at 0.1 A g−1) that are well retained at high current rates (up to 188 mAh g−1 at 10 A g−1). The material with optimized Na storage performance was tested as the negative electrode in a sodium-ion capacitor, using as the positive electrode highly porous nanoparticles prepared by KHCO3 activation of the same polypyrrole nanospheres (SBET ∼ 2970 m2 g−1). The sodium-ion capacitor with an optimized positive-to-negative electrode mass ratio of 2 delivers as much as 69 Wh kg−1 at a high-power density of 25 kW kg−1, and can be successfully charged and discharged for 10000 cycles experiencing a capacity loss of only 0.0024% per cycle at 2 A g−1.