Morphology evolution and electrochemical behavior of NixMn1-x(OH)2 mixed hydroxides as high-performance electrode for supercapacitor

• A series of single-phase Ni x Mn 1-x (OH) 2 (0 ≤ x ≤ 1) have been prepared by coprecipitation reaction. • With the incorporation of Mn ions into Ni(OH) 2 , the deprotonation energy can be reduced. • Ni 3/4 Mn 1/4 (OH) 2 shows the best electrochemical performance with a capacitance of 758.7 F g −1 at 1 A g −1 . • The Ni 3/4 Mn 1/4 (OH) 2 //AC exhibits a high energy density of 40.7 Wh kg −1 at power density of 800 W kg −1 . In this article, a series of single-phase Ni x Mn 1-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 Ni x Mn 1-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 Ni 3/4 Mn 1/4 (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 Ni 3/4 Mn 1/4 (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.