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 Co_1–xNi_xOH₂ 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.