Polymeric membranes’ comprehensive assessment under conditions equivalent to real VRFB operation

The growing integration of renewable energy sources into modern power grids demands reliable large-scale energy storage, with vanadium redox flow batteries (VRFBs) emerging as a promising technology due to their scalable design and long operational lifespan. A central challenge in VRFB development is the membrane, which must simultaneously provide high proton conductivity, strong vanadium ion selectivity, chemical stability, and economic viability. Despite extensive literature on VRFB membranes, systematic comparisons under near-practical operating conditions remain scarce. This study evaluates four distinct types of membranes: Nafion-212, Asahi, SPEEK, and PBI through power distribution curves and galvanostatic cycling at current densities spanning 60–120 mA cm-2. The power density and electrolyte flowrate were scaled from a 5 kW pilot energy storage system to maintain close to real conditions of laboratory cell tests. Nafion-212 achieves the highest peak power density of 490 mW cm-2 and leads energy efficiency at 120 mA cm-2 (72.6%), with Asahi following at 72.1% at roughly half the cost. PBI records the highest Coulombic efficiency at every condition (up to 93.3%) and matches Nafion-212 in energy efficiency at moderate loads, while its voltage penalty grows at higher currents. SPEEK consistently underperforms, reaching only 68.6% energy efficiency at 120 mA cm-2. These results provide data-grounded criteria for membrane selection across different VRFB operating regimes.