Counter-ion Conductivity and Selectivity Trade-Off for Commercial Ion-Exchange Membranes at High Salinities

The growing interest in using ion-exchange membranes
(IEMs) for
high-salinity applications such as brine concentration and produced
water treatment necessitates a better understanding of their properties
under relevant conditions. In this study, we examine the ion transport
properties of 40 different commercial membranes contacted by either
1 or 5 m NaCl solutions. To sample a broad materials
space, we selected 28 membranes marketed toward desalination applications
and 12 membranes marketed toward energy applications. We quantified
the equilibrium ion concentrations, salt permeability coefficients,
and ionic conductivities of these membranes. Using these results,
we derived the effective counter-ion and co-ion diffusion coefficients
in the membranes and calculated the counter-ion conductivity and counter-ion/co-ion
selectivity. There is a clear trade-off between the counter-ion conductivity
and selectivity, with desalination membranes generally exhibiting
combinations of high selectivity/low conductivity and energy IEMs
generally exhibiting combinations of high conductivity/low selectivity.
We decoupled the total selectivity into the partition and diffusion
selectivity and correlated these parameters with membrane structural
properties to establish structure/property relationships. The results
of this study highlight shortcomings in the performance of these membranes
and identify gaps in our fundamental understanding of ion transport
in IEMs at high salinities.