ZIF-67 wraps Ni-Mn LDHs nanosheets to enhance the capacitive contribution of supercapacitors

Abstract

Layered double hydroxides (LDHs) have long been a hot research topic in supercapacitors because of their abundant reaction sites. However, LDH nanosheets tend to easily stack together during the synthesis process. Therefore, in this paper, a simple and effective stirring aging method was used to grow ZIF-67 uniformly on the surface of Ni-Mn LDH. The strategy can fully utilize the high specific surface area of ZIF-67 to provide a large number of active sites for Ni-Mn LDHs, which results in a significant increase of the capacitance contribution. The core–shell structure NM LDHs@Z has significant redox peaks and the surface redox reaction. The capacitance contribution of NM LDHs@Z increase from 21.7 % to 91.4 %. But it does not belong battery-type supercapacitors from the CV curves. It is necessary to find out a better characterization parameter to estimate the type of battery or capacitor, instead of the CV and GCD curve shapes. The Ni-Mn LDHs@ZIF-67 composites can achieve a specific capacitance of 1340 F g⁻¹at a current density of 1 A g⁻¹. As the specific current density is increased to 10 A g⁻¹, the capacity retention can reach about 75 %, which is superior to that of the Ni-Mn LDHs (60 %). The hybrid supercapacitor is composed of Ni-Mn LDHs@ZIF-67 composite as the cathode and activated carbon (AC) as the anode with a 128 F g⁻¹ specific capacitance at a 1 A g⁻¹ current density and a 45.8 Wh kg⁻¹ energy density at a 850 W kg⁻¹ power density. The Ni-Mn LDH@ZIF-67 composites has a potential application in hybrid supercapacitors.