Novel cuboid-like cobalt nickel phosphate/manganese dioxide/multi-walled carbon nanotubes composites as binder-free electrodes for high-performance supercapacitors

Abstract

Using NiCl2·6H2O, CoCl2·6H2O, Na2HPO4, manganese dioxide (MnO2) and Multi-walled carbon nanotubes (MWCNTs) as the main raw materials, the cuboid-like cobalt nickel phosphate/MnO2/MWCNTs (CNPMM) composites were successfully prepared by a simple hydrothermal method. The morphology, composition and structure of the composites were studied by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectrum, specific surface area analyzer and particle size analysis. SEM images showed that the composite material presented a cuboid-like morphology. The width, length and height of cuboids are 0.3–0.8 µm, 2–3 µm and 8–10 µm, respectively. The special structure and morphology of the sample provide more active sites, making it exhibit superior pseudocapacitance. The electrochemical performance of the three-electrode system supercapacitor was systematically studied by cyclic voltammetry, constant current charge-discharge and electrochemical impedance spectroscopy. When the mass ratio of MnO2 to MWCNTs is 1:1 with the total doping amount of 3.9 wt%, the as-prepared CNPMM1:1 composite shows the best capacitance performance. With 3 mol·L−1 KOH as the electrolyte, it exhibits a specific capacitance as high as 2334 F g−1 at the current density of 1 A g−1. Even at the high current density of 10 A g−1, the specific capacitance can still be maintained at 1672 F g−1, and the corresponding specific capacitance retention of 71.6%, indicating that the composite electrode material has good rate performance. The results show that the cuboid-like CNPMM1:1 composite is a promising electrode material for supercapacitors.