Rational design of metal-organic framework (MOF)-based supercapacitor electrodes for enhanced energy storage

Abstract

Metal-organic frameworks (MOFs) are gaining popularity as a new electrode material for supercapacitor applications, due to their large specific surface area, excellent tunability and unique pore characteristics. Yet, the poor structural stability and low electrical conductivity of MOFs usually limit their performance. In this study, the supercapacitor performance of MOF-based materials was enhanced using several approaches, including the template strategy, binder free/metal doping and in situ growth with carbon-based materials. Prior to applying these methods, pristine MOFs and their composite electrodes were prepared using the solvothermal method and then characterized by XRD, SEM/EDX, TEM, BET, TGA, Raman and FTIR. The template strategy transformed double-linker Ni-MOF into NiO and NiO/Ni composite electrodes after annealing in air under different conditions. The electrochemical analysis results measured by CV, GCD and EIS showed that MOF calcined at 400 oC (NiO/Ni-400) delivered the highest capacitance (753 Fg-1 at 1 Ag-1 ) and a high energy/power density. Additionally, NiO/Ni-400 exhibited good cyclic stability, with 90% retention after 1000 cycles. Electrochemical kinetics analysis confirmed the pseudocapacitive behavior of the electrode materials at all scan rates. Zn-doped Ni-MOFs grown directly on nickel foam and used as binder-free electrodes exhibited flower-decorated ball-shaped microstructures, in addition to high surface areas and excellent pore characteristics. The composite containing Zn/Ni (1:2) exhibited the highest specific capacitance (391 Fg-1 at 1 Ag-1 ), with superior rate capability and good cycling stability. This electrode delivered maximum energy and power densities of 12 Wh kg-1 and 2500 Wkg-1 , respectively. The charge storage mechanism was predominantly controlled by the diffusion process, indicating a promising battery-type supercapacitor electrode. In situ growth of ZIF-8-derived ZnO/C on functionalized MWCNT produced a new ZnO/C@MWCNT nanocomposite, which exhibited better storage performance compared to ZnO/C. The remarkable performance of the MOF-based electrodes in this study was attributed to the synergistic effects of the compositing materials, low charge transfer resistance and optimum dopant concentration. Aside from expanding the applicability of MOFs, this research may help to create a new path to bridge the performance gap between supercapacitors and batteries.