Lewis acid-mediated hydrogen-bond networks boosting the conductivity and stability of membranes for flow batteries

New-generation iron-titanium flow batteries (ITFBs) with low cost and excellent stability are a promising candidate for kW-level flow battery systems. Their low-oxidizing environment enables low-cost SPEEK membranes to replace expensive Nafion membranes, significantly reducing stack costs. Herein, to improve the conductivity and stability of the SPEEK membrane in ITFBs, a hard Lewis acid (nanometer m-ZrO 2 ) is introduced into the SPEEK membrane. Based on the hard-soft acid-base principle, hard Lewis acids as a mediator can promote the reconfiguration of hydrogen bond. Specifically, we detailed the hydrogen bond formation and ion transport mechanism using X-ray photoelectron spectroscopy (XPS) and 1 H NMR spectra. Mechanistic analysis indicates that the “-OH” groups of ZrO 2 would be protonated in the acidic electrolyte, which builds hydrogen bond networks with H 2 O or “-SO 3 H” groups in SPEEK. These rich hydrogen bond networks enhance proton conductivity via the Grotthuss mechanism. Moreover, the evenly distributed ZrO 2 in the SPEEK membrane disrupts polymer chain continuity, preventing membrane swelling and providing excellent dimensional stability. As a result, the ITFB achieves high coulombic efficiency (CE, 99.9 %) and voltage efficiency (VE, 81.7 %) at 40 mA cm - 2 , surpassing Nafion 115 (VE, 78.94 %). Notably, the ITFB maintains stable VE over 500 cycles. • The SPEEK composite membrane with nanometer ZrO 2 is successfully prepared. • The nanometer ZrO 2 surface holds abundant surface Lewis acid sites. • The “-[OH 2 ] +” groups establish rich continuous hydrogen-bonding networks. • The ZrO 2 breaks polymer chain continuity and prevents membrane swelling. • The voltage efficiency of the ITFB reaches 81.7 % at 40 mA cm - 2 .