Oxygen-less electrodes for stability enhancement of vanadium non-aqueous redox flow batteries

Abstract

A meticulous study was conducted to investigate the capacity loss and degradation of non-aqueous vanadium flow batteries. It was observed that the presence of moisture and oxygen in the system triggers a parasitic reaction leading to the formation of irreversible VO(acac)(2), which hampers high-performance operation. The main source of this issue was identified as oxygen-rich electrodes commonly used in these batteries. To address this challenge, a novel approach was employed to modify the surface of carbon felt electrodes through hydrogen reduction thermal treatment, effectively mitigating the concentration of oxygen functional sites. The electrodes of carbon felt were treated at 700 degrees C with hydrogen greatly reduced the oxygen content in the electrodes, leading to increased reaction rate constants and reduced overall system impedance. The thermal treatment under a reducing atmosphere successfully mitigated the degradation mechanisms and extended the device's lifespan. The battery equipped with these modified electrodes demonstrated a storage capacity of 521 mAh l(-1) over 45 charge/discharge cycles, representing the highest reported stability. Moreover, the maximum current density was improved by 60 % compared to the system using pristine electrodes. Morphological analysis was conducted alongside electrochemical testing to evaluate the performance of the modified electrodes. This technique paves the way to a closer level toward pilot-scale commercialization.