High energy density symmetric supercapacitors coupling redox-active gel polymer electrolytes with N-doped carbons

An adoption of a redox-active species can be instrumental in enhancing the specific capacitance of Electrical Double Layer Capacitors (EDLCs). Nitrogen-self-doped activated carbon (NDAC), derived from jack bean meal, exhibits a consistent distribution of nitrogen throughout a graphitized carbon framework, making it an ideal electrode for EDLCs. To enhance its capacitive properties, we integrated it with the redox additive para-phenylene diamine (PPD) and a gel polymer electrolyte made of poly (vinyl alcohol) and potassium hydroxide (PVA-KOH). Overlapping operative potential range of PPD with NDAC was the selection criterion. Further, the effects of concentration-dependent performance analysis interlinking PPD and PVA-KOH was evaluated via electrochemical studies. In a three-electrode setup, 25 mM PPD increased the specific capacitance six-fold to 835 F g⁻1 at 1 A g⁻1. Dunn's deconvolution analysis revealed a predominantly diffusive charge storage mechanism (75.8%), confirming the redox-active contributions. In a two-electrode symmetric supercapacitor (SSC), a blended gel polymer electrolyte of PVA-KOH and PPD delivered 179.94 F g⁻1 at a current density of 1 A g⁻1, demonstrating 77.5% capacitance retention and 99.4% coulombic efficiency after 10,000 cycles at 40 A g⁻1. Voltage-holding and self-discharge studies confirmed excellent stability, low leakage current, and minimal self-discharge. It was identified that 10 mg of PPD was an ideal concentration, enabling an energy density of 35.99 Wh kg⁻1 at a power density of 900 W kg⁻1. Thus, the results emphasized the synergy between PPD blended gel polymer electrolytes and NDAC as an ideal combination for high-performance supercapacitors.