In this article, a series of single-phase NixMn1-x(OH)2 hydroxides with x = 1, 3/4, 2/4, 1/4 and 0 have been prepared by coprecipitation reaction. The effect of chemical composition of the NixMn1-x(OH)2 hydroxides on the capacitive characteristics has been systematically investigated. Density functional theory (DFT) calculations indicate that the incorporation of Mn ions into Ni(OH)2 hydroxide can reduce the deprotonation energy and energy gap leading to better electron transport. However, the morphological structure of the hydroxides also becomes larger and coarser with decreasing the x value from 1 to 0, which is unfavorable for the electrochemical performance. Therefore, there is a trade-off between deprotonation energy, band gap, morphology and surface area to achieve high-performance capacitive materials. The results demonstrate Ni3/4Mn1/4(OH)2 hydroxide with the optimal chemical composition shows the best electrochemical performance with a capacitance of 758.7 F g−1 at 1 A g−1 and excellent rate performance of 517.6 F g−1 at 10 A g−1. The asymmetric supercapacitors incorporated with Ni3/4Mn1/4(OH)2 and active carbon (AC) also exhibit a high energy density of 40.7 Wh kg−1 at power density of 800 W kg−1 and remarkable cycling life along with 94.4% retention after 3000 cycles.
