Multi-porous electrode derived from zeolitic imidazolate framework-8 coating through continuous two-stage heat treatment for vanadium redox flow batteries

Developing high-performance electrodes is imperative for improving the energy efficiency and power density of vanadium redox flow battery (VRFB). This study developed a multi-porous graphite felt (GF) electrode, comprising micropores, mesopores, and macropores. This was achieved through a continuous two-stage heat treatment based on zeolitic imidazolate framework-8 coating. Cyclic voltammetry and electrochemical impedance spectroscopy results confirmed the effect of the pore size on the electrochemical properties by comparing the single-scale pore structure and multi-porous structure. The electrochemical activities were enhanced both by micropores, which significantly increased the number of active sites, and by mesopores and macropores, which improved the accessibility of vanadium ions to the active sites. Consequently, the multi-porous electrode showed the highest electrochemical activity for vanadium ion redox reactions owing to the synergistic advantages conferred by the presence of different pore sizes. The VRFB with the multi-porous electrode exhibited the highest energy efficiency (70.75 %) at a current density of 200 mA cm−2, which was 12.07 % higher than that of Bare GF. Furthermore, it exhibited a specific discharge capacity of 13.51 Ah L−1at a current density of 250 mA cm−2. The VRFB with multi-porous electrodes showed stable operation for 3300 cycles without a significant decrease in efficiency or modifications to the pore structure. The proposed method can be used to effectively fabricate high-performance multi-porous electrodes applicable to VRFBs and other energy storage systems.