Promoting vanadium redox flow battery performance by ultra-uniform ZrO2@C from metal-organic framework

The application of metal oxide is limited due to low conductivity, weak combination, poor dispersion, and hard nanocrystallization in vanadium redox flow battery. Herein, metal–organic framework (MOF) was firstly employed to prepare metal oxide and porous carbon nanocomposite, which was used to ultra-uniformly decorate graphite felt by in-situ growth. Graphite felt was modified with UiO-66 (Zr-MOF) nanoparticle using hydrothermal synthesis followed by conversion into porous nanocomposite (ZrO2@C) via high-temperature carbonization. ZrO2@C owns large surface area, regular arrangement of ZrO2, and high conductivity, which provides large reaction place, high active site density, and rapid electron transfer for redox reaction. Therefore, ZrO2@C can boost the electrochemical performance of graphite felt for VO2+/VO2+ and V3+/V2+ reactions by promoting diffusion, charge transfer, and electron transport, systematically. The modified flow cell using ZrO2@C/GF has better stability and higher utilization of electrolyte than pristine cell during 500-cycle test. ZrO2@C/GF decreases electrochemical polarization of cell at different current densities. At 200 mA cm−2, voltage efficiency (77.5%) and energy efficiency (75.2%) of modified cell increase by 14.6% and 13.9% respectively compared with pristine cell. The modified cell can operate at a high current density up to 300 mA cm−2 and shows a 62.4% of energy efficiency. The cell performance in this study is superior to that reported in previous metal oxide-related works. This work presents a design allowing MOF-derived catalyst with nano size, high conductivity, good adhesion, and uniform dispersion, simultaneously.