Tracking mobile ion interactions with fixed charge groups in cation exchange membranes using Raman spectroscopy

• Raman spectroscopy was used to observe ion interactions in cation exchange membranes (CEMs). • Two prevalent commercial CEMs exchanged in chloride salt solutions are reported. • Peak shifts are shown to be different in membranes with high and low water content. • Results show polymer structure has strong effects on electrostatic ion interactions. Membrane based electrochemical systems for separations and selectivity are a promising method of obtaining valuable chemicals from complex aqueous streams. A critical component of these systems is the cation exchange membrane (CEM). Traditional CEMs are thin layers of hydrocarbon polymers with fixed negative charge groups. The interactions between fixed charges and mobile positive ions are imperative to understand because they correlate to the CEM’s ability to transport cations, yet there is a lack of understanding regarding these interactions. In this work, we use Raman spectroscopy to study changes of the vibrational modes of the fixed sulfonate groups in two commercial CEM’s exchanged with ten different chloride salts. We also report on the differences in vibrational peak shifts between CEMs with low and high water contents. We find that in the low-hydration state there is a linear correlation between fixed charge Raman peak shift and mobile cation’s ionic radius. In the fully hydrated state, Raman peak shifts indicate the electrostatic interactions between fixed and mobile charges are affected by the spatial distance due to an ion’s hydrated radius. These findings are supported by water uptake and conductivity measurements. This study shows we can utilize rapid, non-invasive Raman spectroscopy to improve the state of understanding of transport in CEMs.