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

Abstract

A bio-inspired approach has enabled the first synthesis of Co_xNi_1−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. Co_xNi_1−xO/RGO nanocomposite electrodes achieve a peak specific capacitance when the Co/Ni molar ratio is close to 1. For example, Co_0.45Ni_0.55O/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 (Co_0.45Ni_0.55O/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 (Co_0.45Ni_0.55O/RGO)//(Co_0.45Ni_0.55O/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.