Dispersion of Nafion Ionomer Aggregates in 1-Propanol/Water Solutions: Effects of Ionomer Concentration, Alcohol Content, and Salt Addition

Abstract

Coarse-grained molecular dynamics simulations using explicit solvent models were performed to understand Nafion ionomer morphology in 1-propanol (NPA)/water solutions under various conditions (ionomer concentration, NPA/water fraction, and salt addition). The self-assembly behavior of ionomers into a cylindrical aggregate with a diameter of ∼2−3 nm was observed. At low ionomer concentration (≤5.0 wt %), the ionomer aggregate becomes smaller in size and thinner with increasing NPA fractions. At high ionomer concentration (7.5 wt %), the size of aggregates in a longitudinal direction increases significantly at high NPA fractions, suggesting that excess addition of NPA does not necessarily contribute to the dispersivity of ionomers, particularly at high ionomer concentration. These nonmonotonic behaviors of ionomer aggregation are controlled by the electrostatic repulsion among the sulfonate groups, which is determined by the balance among the dielectric constants of solvents, the distribution of hydronium ions, and the surface density of sulfonate groups. Upon the addition of salts, the size of aggregates increases significantly, and the formation of a larger disk-shaped aggregate and a secondary aggregate of multiple bundles for low and high NPA contents, respectively, was found.

Engineering Catalyst Layers for Next‐Generation Polymer Electrolyte Fuel Cells: A Review of Design, Materials, and Methods

“…[ 267,269 ] Despite the variation in the literature, consensus has built around the simplified model of binary solvents exhibiting rod‐like nano‐aggregates, on the order of 50 nm, [ 273 ] in which the non‐polar backbones constitute the core of the aggregates while the anionic groups sit at the aggregate−solvent interface. [ 273–275 ]…”

“…Mabuchi et al. [ 274 ] reported that higher ionomer concentration caused the number of Nafion chains in the aggregate and the aggregate size to increase, as shown in Figure 18 . The effect of solvent ratio on aggregate size was also dependent on ionomer concentration, due to competing effects of dielectric constant and hydronium ion distribution.…”

Probing Ionomer Interactions with Electrocatalyst Particles in Solution

“…The contrasting hydrophobic backbone and hydrophilic sidechains of PFSA have competing preferences in solution, and changing solvent type (including the water:propanol ratio, two common ink solvents) causes PFSA to adopt conformations that reflect this. [5][6][7][8][9][10][11][12][13][14] These different conformations affect the self-assembly of ionomer aggregates and the properties of their films post-drying. [15][16] Furthermore, different conformations will alter how the ionomer adsorbs to catalyst particles.…”

The Martini Model in Materials Science

“…Furthermore, Martini has been applied to model ion‐conducting polymeric materials used as ion‐exchange membranes for fuel cell applications. [ 210,220–226 ] Proton‐exchange‐membrane fuel cells use acid polyelectrolytes, such as Nafion, as the membrane material. The membranes are formed by solution‐processing techniques.…”

“…In this context, Mabuchi and co‐workers investigated dilute solutions of Nafion ionomers, the initial stage of solution processing: they studied ionomer self‐assembly in mixtures of 1‐propanol and water and probed the effects of ionomer concentration, alcohol content, and inclusion of salt. [ 223,225 ] Goncalves et al. studied instead how cavities nucleate and grow in a hydrated Nafion membrane subject to mechanical deformation, obtaining a nanoscale view on the mechanical properties of such membranes.…”