Optimizing the structure of Ni–Ni(OH)2/NiO core-shell nanowire electrodes for application in pseudocapacitors: the influence of metallic core, Ni(OH)2/NiO ratio and nanowire length

An experimental analysis was performed to optimize the structure of Ni–Ni(OH)2/NiO core-shell nanowire electrodes for improving their application in pseudocapacitors. Thickness and composition of the active phase shell were compared after thermal treatments at 40, 70 and 300 °C. A Ni(0) core was found by XPS in the electrode treated at 40 °C, it was still present after a temperature increase to 70 °C but with a lower Ni(OH)2/NiO ratio of the active phase shell, and disappeared after annealing at 300 °C, with a corresponding increase of the charge transfer resistance. The capacitance values increased from 40 to 70 °C and drop to a minimum for the electrode treated at 300 °C. A tailoring of the capacitance of the nanowire electrodes by controlling their length was evaluated. A constant capacitance per nanowires length unit was attained between 5 and 21 μm, indicating that the corresponding mass transfer resistance was negligible over the explored range. These results evidence the relevance to tightly control the thickness and ratio between the Ni(OH)2 and NiO in the nanowires shell and provide a guideline to optimize the capacitance of the electrode by controlling the nanowire length. © 2020 Elsevier B.V.