Ion-exchange membrane impact on preferential water transfer in all-vanadium redox flow battery

Preferential water transfer (PWT) is an important issue in vanadium flow batteries, which affects the cycling time. In this work, we propose a new way to control PWT by membrane modification. To assess PWT quantitatively, we developed an accurate method of water volume determination by optical monitoring of an anolyte. As result, we show that PWT for cation-, anion-exchange, and amphoteric benchmark membranes have opposite directions. Thus, we propose control of PWT by balancing concentrations of cation and anion groups on the polymer membrane surface. We modify Nafion 115 membrane by layer-by-layer modification. We use ATR-FTIR spectroscopy, AFM, and Kelvin probe, to track charged layer formation on the polymer membrane surface. The results of the observation show vanadium permeability drop seven times and proton conductivity decreased to 30 mS cm-1 with three times PWT suppression. We demonstrate a low capacity fade (70% vs 46% after 100 cycles for modified and non-modified membranes), and high energy efficiency (91%) for the two bilayer modified membranes. Thus, surface functionalization of ion-exchange membranes is one of the new ways of PWT control. • Membrane type affects preferential water transfer in VFB. • Amphoteric membrane shows the least preferential water transfer. • Layer-by-layer modification of Nafion control preferential water transfer. • Capacity fade minimized using LbL membrane modification. • Precise method of preferential water transfer tracking was developed.