Highly stable supercapacitors with MOF-derived Co9S8/carbon electrodes for high rate electrochemical energy storage

Co9S8 has received intensive attention as an electrode material for electrical energy storage (EES) systems due to its unique structural features and rich electrochemical properties. However, the instability and inferior rate capability of the Co9S8 electrode material during the charge/discharge process has restricted its applications in supercapacitors (SCs). Here, MOF-derived Co9S8 nanoparticles (NPs) embedded in carbon co-doped with N and S (Co9S8/NS-C) were synthesized as a high rate capability and super stable electrode material for SCs. The Co9S8/NS-C material was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). It was found that the Co9S8/NS-C material possessed a unique nanostructure in which Co9S8 NPs were encapsulated in porous graphitic carbon co-doped with N and S. The N/S co-doped porous graphitic carbon of composite led to improved rate performance by enhancing the stability of the electrode material and shortening the ion diffusion paths due to a synergistic effect. The as-prepared Co9S8/NS-C-1.5 h material exhibited a high specific capacitance of 734 F g(-1) at a current density of 1 A g(-1), excellent rate capability (653 F g(-1) at 10 A g(-1)) and superior cycling stability, i.e., capacitance retention of about 99.8% after 140 000 cycles at a current density of 10 A g(-1). Thus, a new approach to fabricate promising electrode materials for high-performance SCs is presented here.