Ionic liquid-assisted hydrothermal valorization and redox site engineering of spruce cone biowaste for high-performance heteroatom-doped and ceria-modified electrodes for sustainable supercapacitor applications

Abstract

Utilizing biowaste to generate precious carbon-based electrodes offers a sustainable and cost-effective approach to advanced energy storage technologies while addressing environmental and waste management issues. In contrast to dry pyrolysis, ionic liquid-assisted hydrothermal carbonization of biomass to produce functional carbon materials has garnered significant attention due to the catalytic carbonization effects and simultaneous tuning of carbon chemistry by the inorganic salt. Herein, we report the carbonization of spruce cone biowaste in the presence of an ionic liquid i.e., 1-butyl-3-methylimidazolium tetrachloroferrate ([Bmim][FeCl₄]). The obtained nanospheres of hydrochar were further heat-treated and modified with ceria (CeO₂) (denoted as Ce-IHSC-x). Experimental results showed the post-heat treatment increased the degree of graphenization and specific surface area (SSA) of hydrochar, while ceria nanoparticles along with contributing to redox active sites, help to preserve the mesoporosity in hydrochar materials. Simultaneously, [Bmim]⁺ and [FeCl₄]⁻ ions from ionic liquids induced Fe and N-species in the hydrochar matrix during the carbonization process, further enhancing the functionality of hydrochar. The Ce-IHSC-x materials exhibited a synergistic combination of high SSA, a well-tuned meso- and microporous structure, and a nitrogen-doped conductive interface. This unique architecture enabled efficient multi-dimensional ion and electron transport during charge–discharge processes. As a result, the optimized Ce-IHSC-x electrode demonstrated a high specific capacitance of 992.7 F g⁻¹ at 0.5 A g⁻¹ and 752.7 F g⁻¹ at 9 A g⁻¹, outperforming many biomass-derived carbon materials. In addition, Ce-IHSC-x adhered good tolerance to long-term cycling with 98.8% and 95.2% retention of coulombic efficiency and specific capacitance after 7000 GCD cycles. This work further highlights the versatility of this carbonization approach combined with redox-active modification, showcasing its effectiveness in enhancing the electrochemical performance of biomass-derived supercapacitor electrodes.