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.