Converting coal gasification slag into monolithic supercapacitor electrodes via endogenous Fe-catalyzed growth of N-doped carbon nanotubes

Abstract

Rather than regarding endogenous Fe in coal gasification slag (CGS) as waste, it could be considered as a potential catalyst precursor for converting CGS into functional materials for energy storage applications. Herein, a monolithic carbon-based electrode, denoted as Fe₃C@NCNTs/CGS, is developed by the solidification of CGS powder followed by directly growing high-density N-doped carbon nanotubes (NCNTs) with encapsulated Fe₃C nanoparticles via the chemical vapor deposition (CVD) method using melamine as the sole C and N sources. During the CVD process, the endogenous Fe species within the solidified CGS (S-CGS) substrate can serve as efficient catalysts to catalyze the growth of NCNTs from melamine pyrolysis, avoiding the use of external high-purity metal catalysts. The as-fabricated Fe₃C@NCNTs/CGS electrode is conductive and mechanically strong, and it exhibits a high areal capacitance (C_a) of 1955.9 mF cm⁻² at 1.0 mA cm⁻² and a 57.9% capacitance retention at 30 mA cm⁻². Moreover, a symmetric supercapacitor (SC) assembled with Fe₃C@NCNTs/CGS delivers a C_a of 1066.5 mF cm⁻² at 1.0 mA cm⁻², achieves an energy density of 37.0 µWh cm⁻² at a power density of 254.5 µW cm⁻², and retains 105.2% capacitance with a nearly unit coulombic efficiency at 10 mA cm⁻² after 50 000 cycles. This strategy of converting endogenous metal species in industrial solid waste into highly efficient catalysts provides a promising avenue for the development of low-cost, high-performance monolithic carbon-based electrodes for SCs.