Impact of carbon dioxide-assisted activation on Millettia pinnata shell-derived porous activated carbon for enhanced performance in symmetric supercapacitors

This paper presents a successful application of the physical activation method to convert agricultural biowaste, specifically Millettia pinnata (MP) shells, into activated carbon (AC) for use in cost-effective and environmentally sustainable high-performance supercapacitors. The AC derived from MP shells exhibited remarkable porosity, with a high surface area of 710 m² g⁻¹ and a total pore volume of 0.143 cm³ g⁻¹, as determined by Brunauer–Emmett–Teller (BET) analysis. Electrochemical assessments revealed that the prepared AC materials demonstrated exceptional electrochemical performance. Experimental results indicate that the AC material achieved a notable specific capacitance of 314.5 F g⁻¹ at 0.5 A g⁻¹ and retains 80 F g⁻¹ at 20 A g⁻¹ in 6.0 M KOH electrolyte. The symmetric supercapacitors constructed using MPAC exhibit an impressive specific capacitance of 120.4 F g⁻¹ at 0.2 A g⁻¹ and energy density of 16.72 W h kg⁻¹ at a power density of 200 W kg⁻¹. Furthermore, these supercapacitors demonstrate good cyclability with capacity retention of 77.1%, maintaining their performance consistently over an extensive span of 5000 charge–discharge cycles at 1 A g⁻¹. This study highlights the promising potential of AC materials derived from MP shells for the development of high-performance supercapacitors, substantiated by strong experimental evidence.