Sustainable carbon for energy storage applications: investigation on chemical refinements of sorghum biomass for tuneability of carbon structures and supercapacitor performance

Abstract

Biomass-derived carbon is a promising sustainable material for energy storage applications, but the correlation between biomass precursor characteristics and the resultant carbon's features and its electrochemical performance is not yet well defined. This study establishes a link between sorghum biomass characteristics such as lignocellulosic composition and morphology and derived carbon features, particularly its surface area, porosity, and specific capacitance. By systematically adjusting the biomass composition through chemical refinements, we demonstrate that cellulose-rich precursors lead to carbon structures with higher porosity and surface area, resulting in improved electrochemical performance. The carbon produced from water-washed sorghum biomass exhibited a specific capacitance of 54 F g⁻¹, on par with commercial activated carbon. A mild alkali treatment (2% w/v NaOH) of the biomass partially removed non-cellulosic components, enhancing the surface area and capacitance of the resultant hard carbon. Additional refinement through bleaching further removed hemicellulose and lignin, increasing the precursor's crystalline cellulose content. The carbonisation of this cellulosic precursor yielded carbon with higher porosity and surface area. These refinements yielded hard carbon with high specific capacitance values (98 F g⁻¹ for a current density of 0.2 A g⁻¹). Activating the biomass-derived carbon further improved its specific capacitance by 70%, reaching 140 F g⁻¹ at a current density of 0.2 A g⁻¹ while maintaining 100% stability over 5000 cycles. Overall, this research establishes a favourable link between the electrochemical characteristics of biomass-derived hard carbons in supercapacitors and their cellulose content. In contrast, other non-lignocellulosic components, such as hemicellulose and lignin, adversely affect these characteristics. These findings provide groundwork for maximising the potential of carbon materials sourced from biomass by systematically fractionating primary biomass plant cell wall components.