Size dependent ion-exchange of large mixed-metal complexes into Nafion® membranes

Abstract

Perfluorosulfonate ionomers have been shown to demonstrate a profound affinity for large cationic complexes, and the exchange of these ions may be used to provide insight regarding Nafion^® morphology by contrasting molecular size with existing morphological models. The trimetallic complex, [{(bpy)₂Ru(dpp)}₂RhBr₂]⁵⁺, is readily absorbed by ion-exchange into Na⁺-form Nafion^® membranes under ambient conditions. The dimensions of three different isomers of the trimetallic complex were found to be: 23.6 Å × 13.3 Å × 10.8 Å, 18.9 Å × 18.0 Å × 13.7 Å, and 23.1 Å × 12.0 Å × 11.4 Å, yielding an average molecular volume of 1.2 × 10³ ų. At equilibrium, the partition coefficient for the ion-exchange of the trimetallic complex into Nafion^® from a DMF solution was found to be 5.7 × 10³. Furthermore, the total cationic charge of the exchanged trimetallic complexes was found to counterbalance 86 ± 2% of the anionic SO₃⁻ sites in Nafion^®. The characteristic dimensions of morphological models for the ionic domains in Nafion^® were found to be comparable to the molecular dimensions of the large mixed-metal complexes. Surprisingly, SAXS analysis indicated that the complexes absorbed into the ionic domains of Nafion^® without significantly changing the ionomer morphology. Given the profound affinity for absorption of these large cationic molecules, a more open-channel model for the morphology of perfluorosulfonate ionomers is more reasonable, in agreement with recent experimental findings. In contrast to smaller monometallic complexes, the time dependent uptake of the large trimetallic cations was found to be biexponential. This behavior is attributed to a fast initial ion-exchange process on the surface of the membrane, accompanied by a slower transport-limited ion-exchange for exchange sites that are buried further in the ionomer matrix.