Numerical insights into the interplay between mass transport and parasitic hydrogen evolution in vanadium redox flow batteries

Efforts to increase power density, primarily motivated by the need to reduce capital cost, have become a central focus in research on redox flow batteries (RFBs). However, the increase in power density intensifies the parasitic hydrogen evolution reaction (HER) at the negative electrode, posing a significant operational challenge for RFBs. Conventional engineering countermeasures frequently involve lowering the charging cut-off voltage to mitigate HER. However, this approach reduces electrolyte utilization, ultimately leading to increased electrolyte costs. To enable more rigorous control strategies and elucidate the mechanistic basis of hydrogen evolution, a three-dimensional model incorporating HER phenomena in vanadium RFBs is developed in this work. The numerical simulation and accompanying experiments indicate that HER displays pronounced spatial heterogeneity across the porous electrode, producing hotspots for gas formation and accumulation, where active-species concentrations are low and under-rib convection is weak. Enhancing mass transport and improving the uniformity of active-species distribution are shown to substantially mitigate HER. We find that increasing the electrolyte flow rate from 1 to 11 mL min−1 cm−2 reduced the hydrogen gas fraction within the electrode from ∼1.2% to ∼0.4% while raising state of charge (SOC) from 0.8 to 0.9, a strategy that suggests simultaneous improvement of SOC and suppression of the HER side reaction is achievable, but this method incurs higher pumping losses. Moreover, HER is sensitive to other operating conditions (e.g., current density and cut-off voltage), which similarly imply trade-offs between instantaneous power density, electrolyte utilization, and parasitic losses. Collectively, the model and supporting experiments provide mechanistic insight into HER behavior during RFB operation, offering guidance to mitigate these trade-offs and minimize parasitic reactions, thereby enhancing overall system efficiency and durability.