Improved ionic diffusion and interfacial charge/mass transfer of ZIF-67-derived Ni–Co-LDH electrodes with bare ZIF-residual for enhanced supercapacitor performance

Abstract

MOFs (metal–organic frameworks) are ideal precursors for constructing hierarchical structures utilized for energy storage, gas separation/capture, and waste adsorption. ZIF-67, a widely used MOF, can be converted to a Ni–Co-LDH battery-type electrode in Ni(NO₃)₂ ethanol solution for supercapacitor application. Herein, ZIF-67 was treated with Ni(NO₃)₂ ethanol solution at different soaking time periods. All products exhibited hierarchically porous hollow nanocages and were characterized to illustrate the conversion process and the effects of ZIF-residuals on the electrode performance. The results indicated that the ZIF residuals drastically influenced the ionic diffusion efficiency of electrolyte ions within the electrode and faradaic resistance between the electrolyte/electrode interfaces. The optimized Ni–Co-LDH-4.5 h electrode presented a remarkable charge storage behavior with a specific capacity of 732 C g⁻¹ at the current density of 1 A g⁻¹ and maintained its 68.3% capacitance (500 C g⁻¹) at 25 A g⁻¹. The fabricated all-solid state asymmetric supercapacitor presented a remarkable energy density of 45.33 W h kg⁻¹ at the power density of 395.13 W kg⁻¹. The cycling test indicated that 78.55% of the initial capacitance was retained after 8000 cycles, illustrating the remarkable long-life performance.