Synthesis of poly(m-phenylenediamine)-coated hexagonal Co₉S₈ for high-performance supercapacitors

Poly(m-phenylenediamine)-coated hexagonal Co₉S₈ was firstly synthesized by the polymerization with self-prepared hexagonal Co(OH)₂ used as cobalt source, and the calcination with sulfur powder providing sulfur source. By adjusting the concentration of m-phenylenediamine in the polymerization process, the coating was achieved with the different coated results, further affecting their supercapacitor behaviors by improving diffusion path for fast electron transfer. The resulting Co₉S₈/PmPD electrodes demonstrate excellent electrochemical performance with high specific capacitance of 950.1 F g⁻¹ at a current density of 0.5 Å g⁻¹, good rate performance of 81.1% capacitance retention as the current density grows from 0.5 to 20 Å g⁻¹, and almost no capacitance loss after 1000 cycles. Moreover, as asymmetric positive electrode material, the Co₉S₈/PmPD-1 shows high specific capacitance of 115.3 F g⁻¹ at 0.5 Å g⁻¹, outstanding rate performance of 78.2% capacitance retention even increased to 20 Å g⁻¹, superior cycle stability of 87.9% capacitance retention after 5000 cycles at 2 Å g⁻¹ and largest energy density of 36.0 W h kg⁻¹ at power density of 374.67 W kg⁻¹. This novel Co₉S₈/PmPD composite suggests a bright prospect for supercapacitor applications.