Electrochemical Charge Storage of Biochar-Based Electrode Materials for Electrochemical Double-Layer Capacitors (EDLCs)

Abstract

This dissertation explores the electrochemical performance of highly porous Biochar- Polymer Composites. Biochar, derived from biomass, is an attractive alternative to conventional carbon materials due to its renewable nature, affordability, and environmental benefits. Biochar materials with two different physical morphologies, namely flakes as well as granular form were used for synthesizing these composites. Composites with several different Biochar weight ratios in the polymer matrix were tested for their viability as electrochemical double layer capacitor (EDLC or supercapacitor) electrode materials. Two different electrolyte types, such as aqueous 6 Molar Potassium Hydroxide (KOH) as well as 1-butyl-1-methylpyrrolidinium tris (pentafluoroethyl) (BMP-FAP) a Room-Temperature Ionic Liquid (RTIL) was used. Several state of the art electrochemical tools along with other physical characterization tools, which includes, Raman Spectroscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X- Ray Spectroscopy (EDAX), were utilized in order to understand the physical nature as well as the electrochemical response of these composites. Cyclic Voltammetry (CV), Galvanostatic Charge-Discharge (GCD), and Electrochemical Impedance Spectroscopy (EIS) measurements were performed in order to analyze the charge storage capabilities of these composites. Few core observations could be made from our experimental results. The specific capacitances of the i electrodes increased with increasing biochar content and seems to reach a saturation value after a certain point. Our results show that Flake Biochar-Polymer Composite (FBC) in KOH exhibits stable capacitive behavior, with a specific capacitance of 123 F/g at a 2:1 biochar-to-polymer ratio at scan rates of ~ 0.1 mv/s, dominated by double-layer capacitance mechanism. From the EIS measurements, a systematic decrease of the equivalent series resistances (ESR) with increasing Biochar content were seen for all the electrodes. In RTIL (BMP-FAP),a specific capacitance value of ~206 F/g at 0.1 mV/s was obtained using the same FBC ratio. Further, we saw that an energy density of 19.4 Wh/kg, and a power density of 280 W/kg was obtained from electrode with a 2:1 biochar-to-polymer ratio. This increase in the specific capacitances, energy and power density stems from the fact that RTILs have a higher operating voltage window. Some minor ion diffusion limitations was observed in case of RTIL. The Granular Biochar- Polymer Composite (GBC), with its higher specific surface area, showed even better performance in some cases. In KOH, GBC, with 15% biochar achieved 53 F/g, significantly outperforming FBC with a similar biochar content (16% in FBC reached only ~27 F/g). In RTIL, GBC with just 3% biochar reached 57 F/g at 0.1 mV/s, demonstrating efficient charge storage. These findings highlight the potential of biochar-polymer composites as a sustainable and cost-effective alternative to conventional supercapacitor materials. Future work should focus on optimizing biochar content, polymer selection, and electrolyte composition to enhance conductivity, stability, and overall performance.