Segregation of Factors Leading to Temperature-Dependent Ion Transport in Mixed Cationic Systems in Cation Exchange Membranes

This study delves into the interplay of temperature, composition, tortuosity, and electrostatic interactions on ion diffusion within cation exchange membranes. It explores the temperature dependence (16-60 °C) of the self-diffusion coefficients (SDCs) of Ba2+ and Eu3+ ions within the Nafion 117 cation exchange membrane, particularly in the presence of Na+ ions. Radiotracer techniques and electrochemical impedance spectroscopy were employed to investigate these SDCs. Significant differences were observed between the SDCs of Ba2+ and Eu3+ ions in both monoionic (only Na+, Ba2+, or Eu3+) and binary (Ba2+-Na+ and Eu3+-Na+) systems. Temperature-induced reduction in water uptake was probed by the gravimetric as well as 3H radiotracer techniques for the first time, and the effect of the tortuosity factor on ion diffusion was understood. This enabled quantification of the electrostatic interaction factor as a function of temperature, revealing a decreasing trend with increasing temperature in both mono- and binary systems. An attempt was also made to correlate the activation energy (Arrhenius-type plots) and the net interaction potential within this temperature range. The findings indicate that ion diffusion is primarily influenced by electrostatic interaction, especially for multivalent ions, while tortuosity plays a negligible role in determining the overall diffusion trend.