Synergistic effects of electrolyte additives and electrode compression on vanadium redox flow battery performance

Redox flow batteries (RFBs) have attracted significant attention as promising renewable energy storage systems due to their scalability, low maintenance costs, and long-life cycling. Among them, Vanadium Redox Flow Batteries (VRFBs) are the most mature technology, offering electrolyte recyclability, good efficiencies, and no cross-contamination. Nevertheless, VRFBs can only operate at a limited range of temperature, as the vanadium species tend to precipitate at different temperatures, leading to severe loss of electrochemical performance. While numerous studies have separately evaluated the effects of electrode compression and electrolyte additives on cell performance, their combined impact remains insufficiently understood. In this work, we investigate the coupled influence of electrode compression and additive-containing electrolytes on the performance and stability of vanadium redox flow batteries, to improve their cyclability and inhibit precipitation of active species at high temperatures. The study offers structural characterization of compressed carbon felt electrodes using SEM/EDX, cyclic voltammetry of additive-containing electrolytes, and cell cycling under distinct compression ratios and temperatures. The results obtained demonstrate that the effect of electrode compression is highly dependent on the electrolyte composition. In particular, the reduced porosity and permeability associated to higher electrode compression significantly change the additive effectiveness on stabilizing the vanadium species. These findings highlight that electrode compression must be optimized in conjunction with electrolyte composition to ensure stable cyclability and performance. Overall, the addition of PVP and (NH 4 ) 2 HPO 4 additives combined with an electrode compression of 22% led to the highest electrochemical performance, at room temperature. At 45 °C, these additives showed some ability to delay the precipitation of vanadium species; however, stable cycling was not achieved, with the performance deterioration attributed to the combined effects of accelerated precipitation and parasitic oxidation reactions.