Atomistic MD Study of Nafion Dispersions: Role of Solvent and Counterion in the Aggregate Structure, Ionic Clustering, and Acid Dissociation

A theory-based understanding of perfluorosulfonic acid (PFSA) class of ionomer aggregation in different solvents is crucial for transitioning from empirically guided fabrication of high-performance electrodes of fuel cells and other electrochemical devices. Considering Nafion as a model PFSA ionomer, herein, we report the details of its molecular assembly in various solvents (dielectric constant, epsilon(r) = 2.38-109) using fully atomistic molecular dynamics simulation. In contrast to the long-held view of elongated aggregate structure of Nafion, our study shows that a variety of structures exist depending on solvent polarity. The work revealed the existence and clustering of ion pairs in Nafion dispersions, a phenomenon reported for the first time for PFSA ionomer dispersions. We introduce a new aggregation phase diagram for Nafion in different solvents: (a) type-I aggregate: aggregates with ionomer chains physically cross-linked via ionic clusters in low dielectric solvents [toluene, isopropyl alcohol (IPA), ethanol, glycerol]; (b) type-II aggregate: lamella like two-dimensional (2D)-aggregates self-assembled via weak hydrophobic interactions in intermediate dielectric solvents (formic acid); and (c) type III aggregate: tightly packed elongated aggregates formed through strong hydrophobic interactions in high dielectric solvents (water, formamide). Further, our fully atomistic simulations highlight the significant shortcomings of a prior coarse-grained approach wherein the solvent atoms are not explicitly modeled, resulting in an unrealistic aggregate structure in all solvents. With explicit consideration of the solvent molecules, we uncovered a new phenomenon of microphase segregation of solvents (isopropyl alcohol/water mixture) driven by PFSA's surfactant-like hydrophilic/hydrophobic character.