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

Using NiCl2 center dot 6H(2)O, CoCl2 center dot 6H(2)O, 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 mu m, 2-3 mu m and 8-10 mu m, 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 asprepared CNPMM 1a 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.