Electrochemical Effect of Interlayer Anions on Ni-Co Layered Double Hydroxides

Owing to their high capacity, layered double hydroxides (LDHs) are promising materials as supercapacitor electrodes for use in alkaline aqueous electrolytes. The interlayer anions in LDHs have a strong effect on the electrochemical properties. However, the relationship between the interlayer anions and electrochemical properties is unclear because the chemical species have not been directly compared. In this study, we investigated the electrochemical and physical properties of Ni-Co (1:1) LDHs containing different interlayer anionic species of nitrate, chloride, and acetate ions. A low number of interlayer anions, specifically nitrate ions, enhanced the capacity of the LDH at a low scan rate owing to the larger number of redox-active divalent cations. In addition, the anion species also determined the rate capability. The charge-rate capability was improved by decreasing the number of anions adsorbed onto the Co cations, particularly chloride ions, which adsorbed less onto the Co. The lower redox potential of Co than that of Ni provided capacity during high-rate charging at a low overpotential. Moreover, the rate capability is determined not by the interplanar spacing from the anion introduced in the pristine material, but by the interplanar spacing after electrochemical operation. These findings provide a design concept for supercapacitors with high capacity and high rate capability.