Effect of the Nature of Counterion on Properties of Perfluorosulfonic Acid Membranes with Long and Short Side Chains

The paper presents the results of a study of water uptake, ionic conductivity, and Donnan potential in systems with perfluorosulfonic acid membranes in the H⁺, Li⁺, Na⁺, and K⁺ ionic forms and solutions of inorganic electrolytes. The properties of commercial membranes Aquivion E87-05S and Nafion 212, as well as membranes obtained from dispersions of Nafion 212 in solvents of various nature (N,N-dimethylformamide, 1-methyl-2-pyrrolidone, mixtures of isopropyl alcohol with water in a volume ratio of 80–20) have been studied. The effect of the number of functional groups, the length of the side chain of polymer macromolecules, and the morphology of the polymer in membranes on their equilibrium and transport properties depending on the nature of the counterion has been determined. The effect of relaxation and electrophoretic factors on the transfer of alkali metal ions through the system of pores and channels of perfluorosulfonic acid membranes is discussed. The slope of the concentration dependences of the Donnan potential for all highly hydrated membranes in the H⁺ form has been found to be close to the Nernstian one, while the selectivity to alkali metal ions increases for membranes with the highest ion exchange capacity or the lowest amount of sorbed water and diffusion permeability due to the exclusion of co-ions from the membrane phase.