Transition metal compounds and their hybrids with carbonaceous materials for electrochemical energy storage applications

This article comprehensively reviews the work done, mostly in the last decade, on transition metal compounds (TMCs), e.g., transition metal sulfides (TMSs), transition metal oxides (TMOs), transition metal hydroxides (TMHs), transition metal nitrides (TMNs), and their nanocomposites with carbonaceous materials like carbon nanotubes (CNTs), graphene nanoplatelets (GNPs), reduced graphene oxides (rGO), and graphitic carbon (GC) for supercapacitor applications. The inclusion of carbonaceous materials (e.g., CNTs, GNPs, GOs, etc.) in the TMCs has been reported to cause the formation of different hierarchical structures like necklaces (3D:Conb@CG), sandwiched (CNT-rGo-Co3S4//N-doped graphene), nano-honeycomb (MCS/GNF), mesoporous composites (MnO2@CNT), coaxial fiber paper (CFASC paper), etc. These hierarchical structures are known to have mesoporosity, high surface area, and conductive channels for faster electronic and ionic diffusion, ultimately which results in the best combinations of electrochemical performance (specific capacitance, cyclic performance, power and energy densities, etc.) of the TMCs composites. Among the various synthesis routes for preparation of TMCs and their composites, the two-step hydrothermal method has been reported to be most feasible in producing binary and ternary metal compounds-based nanocomposites with excellent energy, power densities along with good cyclic stability due to the combined effect of pressure and temperature, which facilitate the fabrication of ordered structure.