A review of nanocomposites/hybrids made from biomass-derived carbons for electrochemical capacitors

Carbon materials derived directly from biomass, as a viable alternative to fossil carbon, are highly desirable for energy storage and conversion applications. However, the performance of pristine biomass-derived carbon electrodes in energy storage applications, e.g., electrochemical capacitors (ECs), is limited because of the low electrical conductivity, the uncontrolled pore hierarchy in terms of meso-macropores, the lack of surface functionality at a high temperature, and the poor structural stability after cycling at a high current rate. Proper pore templating procedures, different activation protocols, and the incorporation of heteroatoms (e.g., N, S, P, B, and F) in the carbon network have been found to improve the electrochemical performance of such biomass-derived carbon electrodes. This performance can be further enhanced using the synergy obtained by adding metal nanoparticles, metal oxides, metal hydroxides, layered double hydroxides, metal chalcogenides, metal organic frameworks, conductive polymers, or other carbon allotropes (e.g., graphene, carbon nanotubes) to the biomassderived carbon. This review discusses recent developments in nanocomposites/hybrids made from biomassderived carbons for use in ECs. More emphasis is placed on understanding the role and mechanisms of heterocomponents in biomass-derived carbon nanocomposites, along with their overall performance comparison in three-electrode cells and the extent of performance matrices in symmetric vis-a`-vis asymmetric/hybrid cells. Finally, challenges in the development of biomass-derived carbon composite electrodes and future directions for research in this area are thoroughly discussed.