Enhanced electrochemical performance of vanadium redox flow battery electrodes via thermally treated MOF-carbon felt composites

Abstract

This study investigates the enhancement of energy storage capabilities in commercial carbon felts for vanadium redox flow batteries (VRFBs) by strategically synthesizing and modifying metal-organic frameworks (ZIFs) with Fe, Co, Ni, Cu, and Zn metal centers. ZIFs were hydrothermally synthesized and then thermally treated at 450 ◦C (in an oxygen atmosphere) and 900 ◦C (in an inert nitrogen atmosphere). Thermally treated MOFs provide several advantages as electrodes, including enhanced stability within the aggressive VRFB environment, larger porosity, and optimized electrical conductivity. The resulting materials were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), inductively coupled plasma spectroscopy (ICP), and cyclic voltammetry (CV) in three-electrode cells, providing detailed insights into their properties. The incorporation of ZIF-derived materials provided suitable electrochemical kinetics for catalyzing VRFB reactions, as determined by Tafel analysis. Upon identifying materials that exhibit superior electrochemical performance for use as both positive and negative electrodes in a VRFB, these were incorporated into a two-electrode flow cell configuration and compared with a cell assembled using unmodified commercial carbon felt. Electrochemical evaluation revealed significant performance improvements with Zn-modified electrodes, particularly at high current densities. For example, energy efficiency increased from 5 % to 13 % (50–500 mA/cm2 ), and accessible capacity improved from 5 % (50 mA/cm2 ) to 50 % (500 mA/cm2 ), attributed to reduced polarization losses. Likewise, the battery maintained a high coulombic efficiency for 500 cycles. Electrochemical impedance spectroscopy (EIS) further confirmed lower resistive contributions and enhanced electrode-electrolyte interaction, validating ZIF modification as a viable strategy for optimizing VRFB performance.