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-x NixOH2 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.