Zeolite-imidazole framework derived nickel-cobalt hydroxide on ultrathin MXene nanosheets for long life and high performance supercapacitance

Abstract

Zeolite-imidazole frameworks (ZIFs) are the promising materials for supercapacitors (SC), however, the shortcomings of easy agglomeration, low conductivity and low stability limit the further improvement of their electrochemical properties. Hence, in order to produce high-performance SC, ZIFs are usually grown directly on some conductive substrates or post-treated. In this paper, MXene was explored as the substrate and Ni-doped ZIF-67 (denoted as NiCo-ZIF-67) was in situ anchored on negatively charged MXene surface via chemical bonds to supply more available active sites for the enhancement of capacitance. At the same time, the surface functional groups in MXene can also enhance the chemical connection between NiCo-ZIF-67 and MXene to attain higher ion diffusion and charge transfer rates. The size of the NiCoZDH (ZIFs-derived double hydroxide denoted as ZDH) obtained by the alkali treatment of NiCo-ZIF-67 is smaller, which is good for increasing specific surface area. The prepared composite MXene/NiCoZDH electrode shows a larger specific capacitance (877 F g-1) than MXene/NiCo-ZIF-67 (460 F·g-1), NiCoZDH (354 F g-1), NiCo-ZIF-67 (130 F g-1) and MXene (15 F g-1), and superior cycling stability with a capacitance retention of 90.9% after 30000 cycles. Additionally, the assembled asymmetric supercapacitor delivers an energy density of 34 Wh kg-1 at the power density of 748 W kg-1 and maintains 93.3% of the initial capacitance after 30,000 cycles.