Mobility of Cations and Water Molecules in Sulfocation-Exchange Membranes Based on Polyethylene and Sulfonated Grafted Polystyrene

Abstract

The main patterns of the hydration of sulfo groups, the translational mobility of water molecules, alkali metal cations, and ionic conductivity in sulfocation-exchange membranes (MSC) based on polyethylene and sulfonated grafted polystyrene have been investigated using NMR and impedance spectroscopy techniques. It has been shown that at moisture contents λ < 4 (λ is the number of water molecules per sulfo group) the H⁺ counterions in the membranes form diaquahydrogen ions ${{{\text{H}}}_{{\text{5}}}}{\text{O}}_{{\text{2}}}^{ + }{\text{.}}$ In the temperature range below 0°C at λ < 12, water molecules retain high mobility and do not form the ice phase. Water molecules diffusion coefficients (for the H⁺ form, the average diffusion coefficient of water molecules and acidic protons) and first for ion-exchange systems, and the diffusion coefficients of counterions Li⁺, Na⁺, and Cs⁺ have been measured by pulsed field gradient ¹H, ⁷Li, ²³Na, and ¹³³Cs NMR spectroscopy. In MSC membranes in contact with water, the self-diffusion coefficients of cations increase in the Li⁺ < Na⁺ < Cs⁺ series. The cation conductivity values are in the same Li⁺ < Na⁺ < Cs⁺$\ll$ H⁺ sequence. The cation conductivity values calculated from the self-diffusion coefficients based on the Nernst–Einstein equation are essentially higher than the experimental values.