Improving the performance of cobalt–nickel hydroxide-based self-supporting electrodes for supercapacitors using accumulative approaches

In this paper we describe an accumulative approach to move beyond simple incorporation of conductive carbon nanostructures, such as graphene and carbon nanotubes, to improve the performance of metal oxide/hydroxide based electrodes in energy storage applications. In this approach we first synthesize Co–Ni double hydroxides/graphene binary composites through a co-precipitation process. We then assemble these composites into films (∼6 mg cm−2) by integrating with carbon nanotubes that can be used directly as electrodes. Experimental results indicate that the synergistic contributions from nanotubes, graphene and cobalt substitution enabled electrodes with substantially improved energy storage performance metrics. With 50% Co and 50% Ni (i.e. Co0.5Ni0.5(OH)2), the composite exhibited a remarkable maximum specific capacitance of 2360 F g−1 (360 mA h g−1) at 0.5 A g−1 and still maintained a specific capacitance as high as 2030 F g−1 at 20 A g−1 (∼86% retention). More importantly, the double hydroxides exhibited tunable redox behavior that can be controlled by the ratio between cobalt and nickel. These results demonstrate the importance of the rational design of functional composites and the large-scale assembly strategies for fabricating electrodes with improved performance and tunability for energy storage applications.