Bound Water Enhances the Ion Selectivity of Highly Charged Polymer Membranes

Electrochemical technologies for water treatment, resource recovery, energy generation, and energy storage rely on charged polymer membranes to selectively transport ions. With the rise of applications involving hypersaline brines, such as management of desalination brine or the recovery of ions from brines, there is an urgent need for membranes that can sustain high conductivity and selectivity under such challenging conditions. Current membranes are constrained by an inherent trade-off between conductivity and selectivity, alongside concerns regarding their high costs. Moreover, a gap in the fundamental understanding of ion transport within charged membranes at high salinities prevents the development of membranes that could meet these stringent requirements efficiently. Here, we present the synthesis of scalable, highly charged membranes that demonstrate high conductivity and selectivity while contacting 1 and 5 molal NaCl solutions. A detailed analysis of the membrane transport properties reveals that the high proportion of bound water in the membranes, enabled by the high charge content and hydrophilic structure of the polymers, enhances both the ion partitioning and diffusion selectivities of the membranes. These structure/property relationships derived from this study offer valuable guidance for designing next-generation membranes that simultaneously achieve exceptional conductivity and selectivity in high-salinity conditions.