Sequential processing of nitrogen-rich, biowaste-derived carbon quantum dots combined with strontium cobaltite for enhanced supercapacitive performance

Disposal of waste edible oil is a major concern for government regulatory bodies worldwide, with improper disposal leading to the blocking of sewer lines, polluting water bodies, and degrading the environment. In the present work, waste soybean oil is converted into soot and hydrothermally processed to produce nitrogen-rich carbon quantum dots (N-CQDs). The N-CQDs are further treated with strontium cobaltite (SC) prepared via the sol-gel method. A significant difference is observed in the morphology, surface area, and electrochemical performance of the N-CQDs (124 Fg−1), SC particles (106.7 Fg−1), and the combined SC@N-CQDs material (386.5 Fg−1). The bio derived SC@N-CQD sample is used as an electrode in a symmetrical supercapacitor, and the device shows EDLC behavior with co-contribution of minor pseudocapacitance due to the nitrogen functional groups and Co2+/Co3+ states of the SC. The SC@N-CQDs deliver an exceptionally high specific capacitance of 180.24 Fg−1 at 0.5 Ag−1 due to the enhanced surface area offered by the combined material, good conductivity imparted by the N-CQDs, and oxygen vacancies in the SC. The synergy leads to fast ion diffusion between the electrode and the electrolyte interface, and an additional boost is achieved by the pyrrolic-N and pyridinic-N present in the electrode material. The capacitance retention is 80.8% of the initial capacitance after 5000 cycles. The SC@N-CQD supercapacitor provides a high energy density of 7.51 Whkg−1 and can operate a red LED, thus demonstrating the efficient utilization of waste edible oils to yield high-performance material for supercapacitor applications. This study can have a broad environmental impact on the reduction and reuse of biowaste.