ZnNiCo layered double hydroxide@Mxene with 2D/2D hierarchical structure derived from micro-sized ZIF-8/67 anchored to Mxene for asymmetric supercapacitors

Compared with other power storage technologies such as batteries, supercapacitors can attain higher power density, but the situation of lower energy density requires improvement. In this study, in order to maintain the stability of the template structure, zinc ions in the template of ZnO nanosheets were partially dissolved and combined with 2-methylimidazole (2-MI) to form ZnO@ZIF-8 on the residual ZnO by controlling the addition amount of organic ligands. Then the remaining ZnO was continued to be dissolved with 2-MI, and the released zinc ions and the co-added cobalt ions were further integrated with the organic complex to form ZIF-8/67, which can be anchored to two-dimensional monolayer Mxene. Finally, ZIF-8/67 can be etched with nickel nitrate and cobalt nitrate to obtain ZnNiCo LDH@Mxene (ZNC@M) with 2D/2D hierarchical structure. The specific capacitance of ZNC@M can achieve 2011F g-1 (1 A g-1). Meanwhile, it has excellent rate performance (10 A g-1 maintained at 68.6 %) and cyclic stability (maintained at 92.8 % after 5000 cycles). Potential window and energy density of the asymmetric supercapacitor (ASC) were obviously improved while composites are applied as cathode and activated carbon (AC) as anode, and KOH solutions with potassium ferricyanide as electrolyte. When the power density of the assembled device is 800-8000 W kg- 1, the energy density is maintained from 65.6 Wh kg- 1 to 37.3 Wh kg- 1. In addition, the density of states (DOS), project density of states (PDOS), and energy band changes before and after the addition of Mxene were discussed by using density functional theory (DFT) calculation. The above conclusions confirm that Mxene is beneficial to promote the electron transfer state of ZnNiCo LDH. Therefore, this work demonstrates that the anchoring of micro-sized materials is a powerful approach for the preparation of two-dimensional hierarchical structures.