Design Rules for Membranes from Polymers of Intrinsic Microporosity for Crossover-free Aqueous Electrochemical Devices
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 are ionized, AquaPIM membranes feature concomitantly high conductivity and transport selectivity when compared to other membranes, including Nafion. Design rules are laid out, tying membrane architecture and pore chemistry to membrane stability, conductivity, and transport
... nd transport selectivity in aqueous electrolytes over a broad range of pH. These attributes dictate whether it is possible to operate aqueous electrochemical cells without the influence of active-material crossover. HIGHLIGHTS Microporous polymer membranes with amidoximes are remarkably stable at high pH Amidoxime membranes show high conductivity in alkaline electrolytes (21 mS cm À1 ) 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 Baran et al., SUMMARY 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-K 4 Fe(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 À1 in 5.0 M aqueous KOH) and high transport selectivity (up to 10 4 reduction in active-material permeability through the membrane) suggest exciting opportunities for battery development, even beyond those presently demonstrated.