Nanoscale Composition Tuning of Cobalt–Nickel Hydroxide Nanosheets for Multiredox Pseudocapacitors

We demonstrate enhanced electrochemical performance through nanoscale composition tuning of cobalt–nickel hydroxide (Co–Ni(OH)2) films acting as the multiredox electrode for pseudocapacitors. Our electrodes were prepared using a two-step approach: bottom-up synthesis of Co–Ni(OH)2 nanosheet (NS) colloidal solutions and their immobilization on a metal foam substrate. Co–Ni(OH)2 NSs were synthesized by kinetically controlled vapor diffusion of ammonia into a metal precursor solution. A highly stable and chemically uniform Co–Ni(OH)2 NS colloid was synthesized in a liquid medium (water and formamide) to afford Co1–xNixOH2 NSs (0 < x < 1). Horizontally aligned Co–Ni(OH)2 NSs were directly immobilized on the nickel substrate using an electrophoretic deposition (EPD) method, and their electrochemical characteristics were investigated according to the Co/Ni molar ratio in the electrodes. Oxidation potentials of the electrode gradually shifted from 0.04 to 0.31 V with an increase in the Ni ratio of the metal hydroxide NS electrodes. Moreover, we successfully obtained the multiredox Co–Ni(OH)2 electrode by nanoscale composition tuning of Co–Ni(OH)2 NS films, which were accomplished by the preparation of a mixed colloidal solution. The nanoscale multicomposition Co–Ni(OH)2 NS film showed comprehensive redox behavior from the relative contributions of each Co–Ni(OH)2 having a different molar ratio of Co/Ni. Optimized multiredox Co–Ni(OH)2 NS electrodes exhibited a high rate capability of 92% at 50 A g–1 and good cycle stability with 90.5% after 1,000 cycles. This method offers a route to produce highly stable pseudocapacitor electrodes with a wide operating window.