Superior cycle stability performance of a symmetric coin cell fabricated using KOH activated bio-char derived from agricultural waste – Cajanus cajan stems

Abstract Approximately 25 million tons of stems of Cajanus cajan are available as residue after harvesting the seeds. Currently, most of the stems are burnt or buried and do not have any major applications. The exploitation such unconventional agricultural waste to get novel bio-char and its chemical activation for efficient construction of symmetric supercapacitor device (SSD) is important from economic and conservational outlook. Herein, we demonstrate for the first-time utilization of stems of Cajanus cajan for the fabrication of SSD. Initially, the stems of Cajanus cajan were carbonized (CC) and activated using KOH at two different temperatures 700 & 800 °C (CC-1 & CC-2) in N2 atmosphere. All the carbon materials were characterized by powder X-ray diffraction (P-XRD), Raman spectroscopy, Brunauer-Emmett-Teller (BET) surface area, X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS) analytical techniques. CC-1 & CC-2 exhibited almost the same electrochemical performances were found to be higher than CC in the presence of 1 M KOH solution in a three-electrode system. A symmetric coin cell was fabricated using CC-2, exhibited specific capacitance (Cs), energy density (ED) & power density (PD) of 126 F/g, 30.6 Wh/kg & 1.1 kW/kg respectively. Further, the device exhibited 50,000 cycle stability at high current densities (25 & 50 A/g) with 100% coulombic efficiency. The device was able to power four red colored LEDs connected in series for up to 8 min without any change in luminescence hence showing high potential to be used for supercapacitor applications.