Ultra-microporous anion conductive membranes for crossover-free pH-neutral aqueous organic flow batteries

Aqueous organic redox flow battery (AORFB) operating in pH-neutral solutions is an attractive electrochemical energy storage device because of its decoupled energy and power, high safety, low corrosivity, and potentially low cost. However, it remains largely unclear how to develop high-performing AORFB membranes that enable fast ion transport while impeding electrolyte cross-contamination. Here we report a series of anion conductive membranes derived from Tröger's Base polymers with intrinsic microporosity as promising contender membranes for pH-neutral AORFBs. The quantity of sub-nanometer-sized pores in these membranes is regulated by tuning chain rigidity, while a similar pore size distribution is retained. We discover that membrane conductivity is determined by the quantity of pores, whereas the energy barrier for ion transport is related to the pore size distribution. Contrasting conventional knowledge, crossover-free operation of pH-neutral AORFB can be accomplished by implementing microporous polymer membranes that enable fast ion conduction but are moderately selective to redox-active electrolytes. The pH-neutral AORFB exhibits a low capacity fade rate of 0.002% per cycle (0.02% per hour) during 500 consecutive charge/discharge cycles at 40 mA cm−2 while maintaining an energy efficiency of up to 80% and a coulombic efficiency of ∼100%. These results demonstrate that ultra-microporous membranes possess great potential for AORFB applications.