Biochar-based composites as electrode active materials in hybrid supercapacitors with particular focus on surface topography and morphology

The development of biochar-based composites from biomass and the prospect of developing carbon-based electrodes have attracted considerable attention within the electrochemistry community. Although functional carbon-based materials (e.g. activated carbon, carbon nanotubes/fibers, and graphene) are conventionally used as an electrode in energy storage systems due to their low potential plateau, acceptable capacity, and stable cycling performance, there still are significant disadvantages associated with these materials reliance on fossil fuels and energy-intensive synthesis conditions which make them environmentally harmful and costly. Hence, the conversion of biomass into biochar-based composites with desirable structure (i.e. heteroatom doped, hierarchical porous, interconnected 3D pore network, few-layer graphene, few-walled carbon nanotubes (CNTs), and olive and circular-shaped structures) has recently been introduced as an alternative to conventional electrode active material and successfully enhanced the energy content and mitigated waste management issues. Herein we review and summarize the various physical and chemical modifications for biochar-based composites and compare their electrochemical behaviors in energy storage systems. To the authors' knowledge, there is presently no available literature that concisely summarizes the surface topography and morphology on nitrogenbased 3D interconnected composites.