Nafion Ionomer Dispersion in Mixtures of 1‐Propanol and Water Based on the Martini Coarse‐Grained Model

Abstract

The self-assembly of Nafion ionomer in a mixture of 1-propanol (NPA) and water was investigated using coarsegrained molecular dynamics simulations. Ionomer formation into cylindrical bundle-like aggregates is observed when the ionomer chain size is sufficiently large. The size of ionomer bundles decreases (the diameter decreased from~2.5 to~1.9 nm) with increasing NPA content, which indicates that the ionomers tend to be more dispersed at higher NPA content. The results of simulations are in good quantitative agreement with the stable size of an ionomer bundle estimated from our free energy calculations as well as the available experimental data. Furthermore, in contrast to the polarizable NPA model, the standard nonpolarizable NPA model shows the opposite trend that the ionomer size increases with increasing NPA content because of a higher localization of hydronium ions at the bundle surface, revealing that the polarization effect plays a significant role in determining the aggregation behaviors. The present study provides insight into the control of ionomer self-assembly toward obtaining targeted structures for specific purposes.

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

“…Aggregate size within ionomer–solvent dispersions has been reported to increase with ionomer concentration because of reduced electrostatic repulsion between sulfonate groups. [ 275 ] In addition, low pH solvents favor side‐chain extension into the solvent, due to electrostatic attraction, reducing particle aggregation, and inducing linear conformation. [ 277 ] Through the use of cryo‐TEM and ultra‐SAXS, surface functionalization has been shown to allow control over agglomerate stabilization.…”

“…[ 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 ]…”

“…Vast pH change with ionomer concentration and counter‐ion may rationalize the conflicting work that proposes differing structures for binary solutions. [ 267,269,273–275 ] Addition of salt (such as NaCl) to give cation‐form Nafion dispersions has been shown to increase aggregate size and domain size in cast films. [ 274,278 ] The increasing tendency to form intra‐ and inter‐chain associations with increasing ion valency cation size, polarity, and hydrophobicity have been shown to tune self‐assembly thermodynamics of PFSA copolymers to control membrane morphology and transport properties.…”

Longitudinal strand ordering leads to shear thinning in Nafion

“…In the case of CG MD Nafion simulations a number of CG based 39,43,46,61 and dissipative particle dynamics 62,63 force fields have been employed, most of them tailor-made for each study. Lately, studies have been published 42,54,64 where the MARTINI force field 65,66 is used for coarse-graining.…”

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.…”