Development of highly functional carbon materials from cassava peel and rice husk for the fabrication of high-performance supercapacitors

Nowadays, energy shortages pose a significant global issue due to the depletion of primary energy resources, which primarily consist of fossil fuels and petroleum. Simultaneously, concerns regarding carbon dioxide emissions from the combustion of fossil fuels and petroleum are escalating. Renewable energies, such as solar energy, wind energy, and hydropower, have garnered attention as viable solutions. They have the potential to mitigate energy shortage problems and are poised to dominate future electrical generation. However, the production of renewable energies requires the utilization of high-performance energy storage devices to effectively store energy for unrestricted usage across various locations and times. Supercapacitors, high-performance energy storage devices, have undergone extensive development to cater to diverse applications. Concurrently, the pursuit of sustainable and environmentally friendly energy storage materials has intensified. Biomass, a renewable and abundant source, holds the potential to transform various waste materials into value-added products. In this work, we demonstrate the fabrication of three types of supercapacitors based on cassava peel-derived carbon dots (CDs) and activated carbon (AC), nickel-doped malachite/rouaite (NiDMR), Ti3C2Tx MXenes, and rice husk-derived AC and porous silicon (P-Si). Firstly, the novel fabrication of cassava-based supercapacitors was described in which CDs and AC were prepared from cassava peel and a quasi-solid polymer electrolyte was prepared for the first time from cassava starch (CS). A specific capacitance (Csp) of 138.2 F g-1 was obtained from the AC electrode in the CS/H2SO4 electrolyte, which was much greater than 95.9 F g-1 of the AC electrode in the PVA/H2SO4 solution. The CDs were employed as additives in electrode and electrolyte. The addition of 10%wt CDs to AC electrode in the pristine CS/H2SO4 electrolyte increased Csp to 239.5 F g-1. When 0.02%wt CDs were added to the CS/H2SO4 electrolyte, a high Csp of 374.6 F g-1 was obtained. Moreover, the fabricated supercapacitor based on cassava exhibited an impressive cycling stability of 93.3% after 10,000 cycles and produced good performance in a wide temperature range from -40 to 50 oC. Based on intensive electrochemical analysis using several models, the CDs were demonstrated to improve the Csp and cycling stability by enhancing the surface capacitance and surface-controlled processes. Secondly, the novel combination of 0D, 1D, and 2D nanomaterials and 3D-nanostructured materials was reported. Specifically, composite materials based on nanoporous AC and nanowire NiDMR were utilized as the electrode materials, whereas MXenes and CDs were employed as the diffusion- and surface-process enhancers, respectively. The AC/NiDMR composite electrode yielded an areal capacitance of 71.2 mF cm-2. Upon adding 10%wt of CDs and MXenes separately, the capacitance was improved by 122.2% and 158.3%, respectively. The highest Csp of 126.9 mF cm-2 was obtained by adding both 5%wt of CDs and 5%wt of MXenes, which corresponds to a 178.2% enhancement compared to the pure AC/NiDMR electrode. Extensive analysis shows the synergistically contributing roles of AC, NiDMR, CDs, and MXenes to the enhanced supercapacitor performance. Thirdly, the promising advantages between the high energy density of lithium-ion batteries and the high power density of supercapacitors are hybridized to construct lithium-ion capacitors (LICs). The high capacity of anode materials is challenging for high-performance LICs. Simultaneously, the quest for sustainable and environmentally friendly energy storage materials has attracted attention. In this work, rice husk was treated with alkaline to separate silica and biochar, followed by magnesiothermic reduction and thermal activation to produce porous silicon (P-Si) and sulfur-doped activated carbon (SAC), respectively. The P-Si had high porosity for accommodating volumetric expansion during cycling, resulting in outstanding rate capability and cycling stability, whereas the SAC had a high surface area with micro- and mesopores (2,511 m2 g-1), a high degree of graphitization, and excellent electrochemical performance. For the fabrication of half-cell lithium-ion batteries, P-Si gave a specific capacity of 1,005.3 mAh g-1 between 0.01-2 V, while SAC provided a specific capacity of 108 mAh g-1 (2.0-4.5 V), and showed a better electrochemical stability than the both commercial silicon and activated carbon. When a full-cell LIC was fabricated, the optimized P-Si//SAC LIC had a high energy density of 107 Wh kg-1 at a power density of 163 W kg-1 and a capacitance retention of 81.3 and 61.8% between a potential range of 2.0-4.5 V after 1,000 and 3,000 charge-discharge cycles, respectively. This study not only turned agricultural waste into valuable, high-performance LICs but also made significant progress toward the creation of environmentally responsible, long-lasting, and sustainable energy storage systems.