Three birds with one stone: Microphase separation induced by densely grafted short chains in ion conducting membranes

Abstract

Widespread implementation of redox flow batteries (RFBs) is restricted by inefficient ion conducting membranes. It is especially challenging for membranes to achieve high ionic conductivity, high ion selectivity, and excellent chemical stability, simultaneously. Herein, we demonstrate a versatile solution to this toughest problem via designing multi-functional sidechain topology and manipulating their arrangement in ion conducting membranes. A series of comb-shaped ion conducting membranes with variable sidechains, along which the number of hydrophilic sites is different, are constructed, and their structure-performance relationships are comprehensively explored. Specifically, in the case of the same number of hydrophilic sites, the membranes with densely grafted short chains (the “stone”) outperform those with loosely grafted long chains in terms of proton conductivity and ion selectivity (the first two “birds”), which is attributed primarily to the interconnected and size-constrained hydrophilic channels formed in the former, as demonstrated by an integrated experimental and simulation study. Furthermore, the former arrangement improves chemical stability of the membranes (the third “bird”) since the size-constrained channels hamper the offensive active species from entering. These benefits result in higher energy efficiencies (EE: 92.7–71.7% at 40–200 mA cm−2) and slower capacity decay rate (CDR: 0.09% per cycle) of vanadium RFBs compared with the membranes with loosely grafted long chains (EE: 90.8–66.7% at 40–200 mA cm−2, CDR: 0.15% per cycle). This work sheds light on the path to comprehensively outstanding ion conducting membranes via molecular engineering, for energy-related devices and beyond.