Morphology of supported polymer electrolyte ultra-thin films: a numerical study

TL;DR

This study uses molecular dynamics simulations to explore how substrate surface properties influence the morphology of ultra-thin Nafion films in fuel cell catalyst layers, revealing significant effects on film structure and potential performance implications.

Contribution

It introduces a simulation-based analysis of substrate effects on Nafion film morphology, emphasizing the role of surface hydrophilicity in structural formation.

Findings

Substrate water affinity critically affects ionomer film morphology.

Different substrate hydrophilicity levels lead to heterogeneous film structures.

Implications for fuel cell catalyst layer performance are discussed.

Abstract

Morphology of polymer electrolytes membranes (PEM), e.g., Nafion, inside PEM fuel cell catalyst layers has significant impact on the electrochemical activity and transport phenomena that determine cell performance. In those regions, Nafion can be found as an ultra-thin film, coating the catalyst and the catalyst support surfaces. The impact of the hydrophilic/hydrophobic character of these surfaces on the structural formation of the films has not been sufficiently explored yet. Here, we report about Molecular Dynamics simulation investigation of the substrate effects on the ionomer ultra-thin film morphology at different hydration levels. We use a mean-field-like model we introduced in previous publications for the interaction of the hydrated Nafion ionomer with a substrate, characterized by a tunable degree of hydrophilicity. We show that the affinity of the substrate with water plays a crucial role in the molecular rearrangement of the ionomer film, resulting in completely different morphologies. Detailed structural description in different regions of the film shows evidences of strongly heterogeneous behavior. A qualitative discussion of the implications of our observations on the PEMFC catalyst layer performance is finally proposed.