Bio-inspired synthesis of NaCl-type CoxNi1−xO (0 ≤ x < 1) nanorods on reduced graphene oxide sheets and screening for asymmetric electrochemical capacitors

A bio-inspired approach has enabled the first synthesis of CoxNi1−xO (0 ≤ x < 1) nanorods on reduced graphene oxide (RGO) sheets. The key is the crystallization process from amorphous precursors in a disordered and hydrated state being able to take compositions arbitrarily different from that of the known stable mixed oxide NiCo2O4. This success has permitted further screening of the compositions for electrochemical capacitors. CoxNi1−xO/RGO nanocomposite electrodes achieve a peak specific capacitance when the Co/Ni molar ratio is close to 1. For example, Co0.45Ni0.55O/RGO nanocomposite electrode has exhibited a specific capacitance up to 823.0 F g−1 (based on the total active materials mass) and 909.4 F g−1 (based on the oxide mass) at 1 A g−1, which are among the highest for Co/Ni oxides. Also revealed was their superior cycling stability compared to the Co3O4/RGO and NiO/RGO nanocomposites, with a surprising increase of the specific capacity in the initial 100 cycles before flattening out. In addition, testing of (Co0.45Ni0.55O/RGO)//RGO asymmetric cells yielded an energy density up to 35.3 Wh kg−1 at a cell voltage of 1.5 V, much higher than those of the symmetric cells (Co0.45Ni0.55O/RGO)//(Co0.45Ni0.55O/RGO) (20.2 Wh kg−1) and RGO//RGO (4.5 Wh kg−1). Even at a high power density of 3614.0 W kg−1, the asymmetric cell could still maintain an energy density of 28.0 Wh kg−1. There was only a <4% loss of the initial specific capacitance after 1000 cycles of charge/discharge at 2 A g−1.