Heteroatom self-doped hierarchical porous carbon from nitrogen-rich jack bean meal for high-performance supercapacitor and efficient oxygen reduction

Self-doping of nitrogen offers a more uniform and stable integration of nitrogen, enhancing the material’s properties beyond conventional external doping protocols. This study reports the first ever synthesis of nitrogen self-doped carbons (NDCs) obtained from Jack bean meal (JBM). JBM is a byproduct obtained after oil extraction from Jack beans (Canavalia ensiformis) inherently abundant in proteins, minerals, and fiber and a nitrogen-rich entity. Thus, by subjecting JBM via carbonization process, we obtained NDCs. Employing a simplified hydrothermal pyrolysis of hydrochars followed by KOH activation in different proportions, we obtained two structurally different variants, i.e., NDC1 and NDC2. An excess of KOH activation affected the graphitic features of the NDCs which were evinced by X-ray diffraction data. I_D/I_G ratios from Raman spectroscopic analysis confirmed the presence of effective graphitized scaffold of the carbons. NDC2 was found to be rich in proportion of N in the pyridinic form as inferred from X-ray photoelectron spectroscopy (XPS), which enhances the pseudocapacitive behavior. Electron microscopic analysis revealed a homogeneous distribution of porosity throughout the samples while the adsorption isothermal studies revealed a hierarchical porous structure in both the samples, ranging from 1880 to 2930 m² g⁻¹. Three-electrode studies showed predominantly capacitive behavior, which was also evident from Dunn’s deconvolution studies. Interestingly, charge–discharge analysis revealed a higher specific capacitance for NDC2 (493.75 F g⁻¹). At a high current density of 20 A g⁻¹, two-electrode studies were conducted on an all-solid-state symmetric supercapacitor (SSC) which demonstrated exceptional cycling stability for 15,000 cycles, with over 99.5% coulombic efficiency. Voltametric curves recorded on rotating disk electrode (RDE) showed excellent performance for NDC2 (4.14 mA cm⁻²), and minimal Tafel slopes ratified a near-perfect four-electron process.