Turning agricultural waste into energy: Red cowpea pod-derived bio-carbon for sustainable high-performance supercapacitor electrode

Driven by the growing sustainability concerns associated with energy storage technology, the development of supercapacitors has been stimulated by the search for eco-friendly alternatives to fossil-fuel derived carbons, with biocarbon emerging as the most promising material to enhance supercapacitor performance. This study details the preparation and characterization of a red cowpea pod-derived biocarbon-based supercapacitor electrode. The preparation involves a two-step process that comprises activation through NaOH treatment and carbonization at different temperatures (500 °C, 700 °C and 900 °C). The physicochemical analysis (Raman spectroscopy, FTIR, XRD, SEM, TEM, XPS and BET) confirmed that the biocarbon treated at lowest carbonization temperature (P-500) showed the ideal characteristics: abundant active sites, high surface functionality, efficient ion transport, high specific surface area and highest energy density. Electrochemical evaluations using cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy revealed that among the samples tested, P-500 demonstrated a high specific capacitance value of 336.72 F/g, along with high power and energy densities of 225 W/kg and 37.88 Wh/kg, respectively at 0.5 A/g. The P-500 material shows exceptional long-term stability of 95.17% of its initial capacitance after 3500 cycles at a high current density of 5 A/g. Furthermore, P-500 showed good charge-discharge symmetry and high reversibility. This study demonstrates the potential of transforming a low-value natural biocarbon into a sustainable and high-performing supercapacitor electrode material.