Sustainable Synthesis of Hierarchically Porous Hollow Carbon Spheres for Enhanced Zinc-Ion Hybrid Supercapacitors

We have developed an environmentally friendly and scalable method for synthesizing hierarchically porous hollow carbon spheres (HCSs) as cathode materials for zinc-ion hybrid supercapacitors (ZISCs). This method involves the sodium chloride (NaCl)-confined pyrolysis of polystyrene@polypyrrole nanoparticles. Unlike traditional methods, this one-pot, chemical-recycling, molten salt-confined pyrolysis process was conducted under an air atmosphere, eliminating the need for expensive inert gas protection. NaCl served as a noncorrosive barrier to isolate air during pyrolysis. The optimized HCSs possessed abundant hierarchical pore structures, high specific surface areas (578.2 m(2) g(-1)), and significant levels of oxygen (2.6 at %) and nitrogen (5.6 at %) doping. The HCSs exhibited a high specific capacitance of 108.7 F g(-1) in a three-electrode system with a 6 M KOH electrolyte. Furthermore, ZISCs fabricated using HCSs as cathodes demonstrated an impressive energy density of 26.2 W h kg(-1) at a power density of 140.1 W kg(-1) in a 2 M ZnSO4 electrolyte. These results highlight environmentally fabricated porous carbon materials and the potential application of HCSs as ideal carbon electrode materials for various electrochemical energy storage devices.