Design Rules for Membranes from Polymers of Intrinsic Microporosity for Crossover-free Aqueous Electrochemical Devices

Here, we lay the design rules for linking microporous polymer membrane architecture and pore chemistry to membrane stability, conductivity, and transport selectivity in aqueous electrolytes over a broad range of pH. We tie these attributes to prospects for crossover-free electrochemical cell operation. These guiding principles are applied to two emerging cell chemistries for grid batteries: specifically, Zn–TEMPO-4-sulfate and Zn–K4Fe(CN)6 cells. Key to our success is the placement of ionizable amidoxime functionalities, which are stable at high pH, within the pores of microporous ladder polymer membranes, yielding a family of charge-neutral and cation exchange membranes at low and high pH, respectively—which we call AquaPIMs. Their notably high conductivity (up to 21.5 mS cm−1 in 5.0 M aqueous KOH) and high transport selectivity (up to 104 reduction in active-material permeability through the membrane) suggest exciting opportunities for battery development, even beyond those presently demonstrated. [Display omitted] •Microporous polymer membranes with amidoximes are remarkably stable at high pH•Amidoxime membranes show high conductivity in alkaline electrolytes (21 mS cm−1)•Amidoxime membranes prevent the crossover of a wide range of active materials•A simple figure of merit predicts cell cycle life based on membrane selectivity The energy efficiency and cycle life of electrochemical cells with dissolved active materials are inextricably tied to the stability, conductivity, and transport selectivity of the cell’s membrane. Membrane design rules have been lacking for such cells operating under harsh conditions, such as high alkalinity, due to the lack of selective, stable membranes. Here, we examined several classes of membranes for three aqueous Zn-based cell chemistries. In doing so, we uncovered a simple relationship between the membrane selectivity and the cell’s cycle life, such that it is now possible to predict the lifetime of the cell on the basis of its membrane properties, thus avoiding time- or resource-intensive experimentation in large-format cells. Our work should greatly accelerate the identification of membranes for long-lasting, MW-scale redox-flow, and other low-cost grid batteries, which are required to last 10–20 years. Placement of amidoxime functionalities within the pores of microporous polymer membranes yields a new family of selective membranes for aqueous electrochemical cells—which we call AquaPIMs. At high pH, where amidoximes a