Electrochemical properties of reduced graphene oxide/Ni_xMn_1-x/2O₂ composites for supercapacitors

As a kind of promising cathode materials of supercapacitor, MnO₂ has higher theoretical capacitance and better cycle stability, but its application is limited by poorer electronic conductivity. The reduced graphene oxide (RGO)/Ni_xMn_1-x/2O₂ composites were synthesized by hydrothermal method. The phase composition, microstructure and electrochemical properties of the prepared RGO/Ni_xMn_1-x/2O₂ composites were characterized by XRD, SEM, TEM, FTIR and electrochemical analysis. The electrochemical tests show that, the capacitor performance of MnO₂ is enhanced by partial substitution of Mn by Ni. The specific capacitance of MnO₂ synthesized by hydrothermal method is 66 F/g (scanning speed is 10 mV/s), when x=0.02, the specific capacitance of Ni_0.02Mn_0.99O₂ is 111 F/g. When the RGO is added, the capacitance performance of RGO/Ni_xMn_1-x/2O₂ composites is further improved. As a result, the specific capacitance of 2wt% RGO/Ni_0.02Mn_0.99O₂ is 136 F/g. The addition of RGO increases the electron mobility of the active materials. Meanwhile, the conductivity is enhanced because of the doping of Ni, which results in the existence of appropriate point defects in the MnO₂ lattice. The super capacitor with RGO/Ni_xMn_1-x/2O₂ as the cathode material has advantages of electrical double-layer capacitor and faradaic pseudocapacitor at the same time. The electrochemical properties of RGO/Ni_xMn_1-x/2O₂ composites are enhanced by the synergistic action of doping of Ni ions and loading of RGO.