Bimodal Ionic Conduction through Polymer Films due to Nano Confinement

Kinetic and selectivity of ionic conduction through polymer films is a key factor in the function of many systems including membrane technology, energy devices, sensors, and corrosion. Typically, the ionic conduction is accelerated by increasing the ionic strength of the electrolyte. However, counterintuitively, polymer films with very high electrochemical impedance demonstrate the opposite effect, i. e. ionic conduction slows down by increasing the ionic strength of the electrolyte. This bimodal nature of ionic conduction was discovered five decades ago and yet the mechanism has remained poorly understood. This minireview explains the long-standing anomaly through the lens of recent advances in nano-confinement electrochemistry. Emerging evidence shows that changes in solvation shells occurs due to the size of the channel through which ionic diffusion takes place. These solvation shells can significantly affect the mobility of ions, their interactions with functional groups of the polymer, and the overall conductivity of the electrolyte. At dimensions only a few nanometers, partially hydrated ions form stronger bonds with polymer slowing down the ionic conduction. Increasing ionic strength will add ion-pairing interactions further reducing ions mobility. The interplay of solvated ions, polymer chemistry, and ionic content offer an explanation for the anomalous bimodal ionic conduction.