Soymilk-assisted degradation of polyimide for the preparation of activated carbon and its application in supercapacitors

Activated carbons were obtained by the degradation of polyimide through a combined hydrothermal and physical activation. The addition of soymilk during the hydrothermal process not only significantly enhanced the degradation efficiency, resulting in a yield exceeding 50 %, but also increased the nitrogen-containing functional group of the activated carbon to over 3 at.%. The electrochemical performance of the activated carbon was evaluated using a symmetric supercapacitor. It exhibited significantly higher specific capacitance in acidic and alkaline electrolytes compared to neutral electrolytes, highlighting the critical role of nitrogen and oxygen contents in enhancing the electrochemical performance. The hollow particles with a relatively low specific surface area (∼358 m 2 /g) demonstrated superior comprehensive electrochemical performances to other morphologies, delivering a gravimetric capacitance of 235.29 F/g at 0.4 A/g, volumetric capacitance of 162.35 F/cm 3 at 0.4 A/g, with an excellent capacitance retention of 74 % at 20 A/g. This underscores the importance of rational morphology in optimizing the electrochemical performance of activated carbon. Additionally, the material exhibited excellent long-term stability, achieving a 94.8 % capacitance retention after 10,000 GCD cycles. A considerable energy density of 8.26 Wh/kg is exhibited at a power density of 0.14 kW/kg, which remains at 6.08 Wh/kg at a high power density of 5.21 kW/kg. Furthermore, the supercapacitors based on the PI-converted activated carbon were successfully used as energy sources for ultraviolet light, effectively exciting carbon dots that displayed distinct fluorescence colors. • Soymilk facilitate the hydrothermal degradation of PI foam. • Acidic hydrothermal condition leads to small scale morphology • Ammonia-activated carbons possess high density and nitrogen contents. • Favorable morphology plays significant role in improving electrochemical performance. • Fluorescence is emitted by carbon dots under UV light powered by supercapacitors.