Cobalt‐Nickel Ultrathin Hexagonal Nanosheets for High‐performance Asymmetric Supercapacitors

Abstract In this work, uniform ultrathin hexagonal nanosheets of Co−Ni bimetallic hydroxides are synthesized using a simple green hydrothermal synthesis method. By tuning a Co/Ni mole ratio of 1 : 2, a special nanosheet structure of Co 0.32 Ni 0.68 (OH) 2 was obtained with high interlayer spacing and large pore size. This nanosheet exhibits an enhanced specific capacity as high as 1021.96 C/g at 0.5 A/g, 12 times higher than Co(OH) 2 (83.23 C/g). The high electrochemical performance is attributed to the interfacial interaction between Co 2+ and Ni 2+ , as well as its special nanosheet structure. The advantages of the composition and structure are further confirmed by density functional theory (DFT) calculations. Besides, the energy storage process was visually observed by in situ Fourier transform infrared (FTIR) spectroscopy. Furthermore, an asymmetric supercapacitor (ASC) is assembled by Co 0.32 Ni 0.68 (OH) 2 and activated carbon electrodes. The ASC delivers a high energy density of 54.97 W h/kg at a power density of 1.68 kW/kg and maintains 33.52 W h/kg at 32.66 kW/kg. These results highlight the promising applications of ultrathin Co 0.32 Ni 0.68 (OH) 2 nanosheets as a high‐performance electrode material in supercapacitors.