Zeolitic-imidazolate framework derived Ni–Co layered double hydroxide hollow microspheres with enhanced pseudocapacitive properties for hybrid supercapacitors

Abstract

With the advantages of electrochemical activity for Ni and Co elements, as well as joint enhanced nature, layered double hydroxides (i.e., Ni–Co LDHs) are believed to be competitive candidates for hybrid supercapacitors (HSCs) but they possess poor structural stability. Here, a series of citrate-intercalated Ni–Co LDH electrode materials are synthesized by hydrolyzing ZIF-67 at room temperature. Due to three-carboxylic coordinate sites with metal hydroxide layers, citrate ions strongly support the layered structure as pillars and offer efficient enlarged interlayer paths for OH⁻ transportation, resulting in an obviously enhanced utilization of the intrinsic properties of Ni–Co LDHs. In combination with the abundant active sites of hollow microsphere morphology, the electrochemical characteristics of Ni and Co elements are further determined, and thus, an evolution from typical pseudocapacitive to battery-type charge storing behaviors is first demonstrated. As a result, the Ni–Co-LDH-C-1:4 with a high Co content exhibits pseudocapacitive characteristics including high specific capacities of 135 and 124 mA h g⁻¹ at 1 and 10 A g⁻¹, and over 100% capacity retention after 10 000 charge–discharge cycles, showing excellent rate capability and cycling stability. Simultaneously, the Ni–Co-LDH-C-4:1 with a high Ni content demonstrates typical battery-type behavior with an obvious charge–discharge plateau and a maximum specific capacity of 255 mA h g⁻¹ (1835 F g⁻¹) at 1 A g⁻¹, and good rate capability with 164 mA h g⁻¹ at 10 A g⁻¹. In addition, the hybrid electrode Ni–Co-LDH-M exhibits pseudocapacitance-like electrochemical characteristics due to the deliberate design of involving multiple compositions of Ni–Co-LDH-Cs, which is an anticipated property for battery-type electrodes to balance the dynamics of capacitive electrodes in HSCs. This method provides a route to design highly stable and activated pseudocapacitive electrodes for future-generation HSC applications.