Synthesis of cobalt-nickel pyrophosphates/N-doped graphene composites with high rate capability for asymmetric supercapacitor

Transition metal pyrophosphates have recently shown great potential applications in the field of supercapacitor due to their high capacitance and stable structure. However, the poor electric conductivity of these materials has seriously restricted the rate performance of supercapacitor. Here we report a simple synthesis of Co0.4Ni1.6P2O7/N-doped graphene (NG) composites, which possess fine conductive network and satisfied mesopore structure for improving the electrochemical capacitance performances. By this method, the graphene nanosheets could be simultaneously N-doped by the released NH3 gases during the transformation of the precursor Co0.2Ni0.8NH4PO4·H2O to Co0.4Ni1.6P2O7, without introducing any other N-containing resources. The effects of N-doped graphene content on the morphology and the electrochemical properties of the obtained Co0.4Ni1.6P2O7/NG materials were studied. With the increase of N-doped graphene content, the morphology of the composites could be changed from sheet-like agglomerate to flower-like clusters. The optimized composite showed much higher specific capacitance and remarkable rate capability (1473 F g-1 at 1 A g−1 and 1443 F g-1 at 15 A g−1) compared with pure pyrophosphates. By further using the as-prepared material for positive electrode, we assembled a robust asymmetric supercapacitor with excellent energy density and power density as well as long cycling life, which demonstrate its great potential to be used in supercapacitor application.