Discarded Polyimide Film-Derived Hierarchical Porous Carbon Boosting the Energy Density of Supercapacitors in Na₂SO₄ and Spiro-(1,1′)-bipyrrolidinium Tetrafluoroborate Electrolytes

Na2SO4 and spiro-(1,1′)-bipyrrolidinium tetrafluoroborate (SBPBF4) are very promising electrolytes with low cost and high conductivity. For matching with two electrolytes to boost the energy density of the supercapacitor, a suitable electrode needs to be fabricated. Herein, porous carbon derived from discarded polyimide films is fabricated via H3PO4-assisted KOH activation. The hierarchical pore framework of porous carbon, which consists of micro–mesopores concentrated at 0.7–1 nm and >2 nm along with abundant cracks in the cross section, increases the effective energy storage sites and facilitates the ion diffusion rate of electrolytes. The high synergy between electrolytes and the architecture of electrodes endows the symmetrical capacitors (SCs) with outstanding performance. The SC in the Na2SO4 electrolyte delivers the operating voltages of 1.8 V and an energy density of 23.2 Wh kg–1 at a power density of 450 W kg–1. In the SBPBF4 electrolyte, the SC exhibits a high withstand voltage of 3.2 V and a conspicuous energy density of 70.2 Wh kg–1 at a power supply of 320 W kg–1. This work constructs a cooperative system of porous carbon electrodes and electrolytes boosting the energy density of supercapacitors and a sustainable strategy for resource utilization of discarded polyimide films.