Ni, Co-double hydroxide wire structures with controllable voids for electrodes of energy-storage devices

We report the facile synthesis of Ni, Co-double hydroxide wire (NCHW)-based electrodes directly grown on a conductive substrate via a hydrothermal process. Various NCHW nanostructures were grown on Ni foam, and the growth was controlled using different compositions of solvents (ethanol and water). With increasing volume ratio of ethanol to water, the density of the wires decreased, and the spatial voids between the wires increased. The formation of large empty spaces improved the electrochemical performance because the exposure of a large surface area of the structure to the electrolyte resulted in a large number of active sites and facile electrolyte penetration into the structure. The different NCHW structures were ascribed to the pivotal role of the solvent in the urea hydrolysis; the solvent triggered the formation of hydroxides during the hydrothermal synthesis. The electrochemical performance of the NCHW electrodes was investigated via galvanostatic charge/discharge tests, cyclic voltammetry, and electrochemical impedance spectroscopy. The highest specific capacitance was 1694.7 m F/cm² at 2 mA/cm², with excellent capacitance retention of 81.5% after 5000 cycles. The superior electrochemical performance of the NCHW electrodes is attributed to the large number of active sites and facile electrolyte diffusion into the structure, due to the well-organized structure with an optimized density of nanowires and large voids between the wires.