Hydrolysis of ionic clusters to induce interconnective sieving pores in ion-conductive membranes for vanadium flow batteries

Interconnective sieving pores are fabricated by the hydrolysis of ionic clusters in precursor ion-conductive membranes to provide a versatile strategy for the proton transport-ion selectivity trade-off in vanadium flow batteries. The sulfate ester group has higher acidity than the well-formed hydrophilic ionic clusters in the precursor dense membrane and converts into smaller hydroxyl groups by subsequent hydrolysis, producing numerous angstrom-scale pores (4–8 Å) as independent H+/Vn+ ion sieving units. Meanwhile, the collapse of electrostatic interactions between the precursor sulfate ester groups and the membrane swelling during hydrolysis induced interconnection of the hydrated ionic clusters; therefore, the converted hydroxyl groups tend to disperse in the hydrophilic domains to form the interconnected collection of numerous sieving pores as broad proton conductive channels. By tuning the porous morphology by ion exchange capacity, excellent cell performance is achieved (high energy efficiency of 85.8%, low discharge capacity decay of 0.22% per cycle, and stable operation of 1000 charge–discharge cycles at 100 mA cm−2), which is better than that of Nafion 212 and most contemporary porous membranes.