Mobility of Li+, Na+, Cs+ Cations in Sulfocation-Exchange Membranes Based on Polyethylene and Grafted Sulfonated Polystyrene Studied by NMR Relaxation

Abstract

Mobility of alkaline metal cations Li⁺, Na⁺, Cs⁺ in membranes based on polyethylene and sulfonated grafted polystyrene has been investigated by NMR relaxation technique. The kinetic curves of longitude recovery and transverse decay magnetizations of ⁷Li, ²³Na, ¹³³Cs nuclei were recorded. It was indicated that spin relaxation is due to interaction of nuclear quadrupole moment with electric field gradient generated by cation hydrated water molecules and sulfonate groups. The correlation times and activation energies of translational cation mobility have been calculated from spin-lattice (T₁) and spin-spin (Т₂) relaxation temperature dependences. Cation mobility increases in the next sequence Li⁺ < Na⁺ < Cs⁺. Diffusion coefficients calculated from NMR relaxation have been compared with macroscopic diffusion coefficients measured by pulsed field gradient NMR and impedance spectroscopy techniques. On the basis of this comparison, a model of heterogeneous membrane ionic transfer is discussed. It has been shown that membrane conductivity is restricted by ionic transfer in narrow pores with low functional group concentration.