Understanding the mechanism of nitrogen transport in the perfluorinated sulfonic-acid hydrated membranes via molecular dynamics simulations

Effect of ionic groups on the morphology and transport properties in a novel perfluorinated ionomer containing sulfonic and phosphonic acid groups: a molecular dynamics study

Combining the phosphonic acid group with the sulfonic acid group in PEMs has been shown to be an effective strategy for improving the fuel cell performance. However, the interplay of two different ionic groups and the resulting effect on the membrane properties have not been fully elucidated. Here, we used classical molecular dynamics simulation to investigate the morphologies, transport properties and effects of ionic groups in a novel perfluorinated PEM containing two ionic groups (PFSA-PFPA) in comparison to the corresponding homopolymers. Phase separations between hydrophilic and hydrophobic domains are confirmed in these PEMs and result from the evolution of water clusters formed around the ionic groups. The combination of both ionic groups brings a complicated morphological feature in PFSA-PFPA, with near-cylindrical aqueous domains of large length scales interconnected by tortuous domains of small sizes. And we found that the self-diffusion coefficients of water molecules are strongly related to morphologies, with the water transport in PFSA-PFPA lying between two analogous homopolymers. At the molecular level, we found that the sulfonic and phosphonic acid groups have distinct effects on the coordination behaviors and the dynamics of water molecules and hydronium ions. Strong electrostatic interactions lead to compact coordination structures and sluggish dynamics of hydronium ions around phosphonic acid groups, which determine the morphological evolution and transport properties in PFSA-PFPA. Our study affords insights into the relationship between molecular characteristics and transport properties bridged by phase-separated morphologies in a novel PEM containing both sulfonic acid and phosphonic acid groups, which deepens the understanding of the interplay between two ionic groups and may inspire the rational design of high-performance PEMs.

Innovative Insight into O2/N2 Permeation Behavior through an Ionomer Film in Cathode Catalyst Layers of Polymer Electrolyte Membrane Fuel Cells

It is crucial to clarify the permeation behavior of O₂ through the ionomer film for enhancing local O₂ transport in cathodes of fuel cells. However, all existing studies mainly deal with pure O₂ rather than air. Herein, the permeation behavior of the O₂/N₂ mixture through the ionomer film has been well explored in view of molecular bond length variations by molecular dynamics simulations. The bond lengths for O₂ and N₂ are shortened under a low hydration level when permeating through a dense layer with small free voids while no obvious change occurs at higher hydration. In the bulk ionomer region, O₂ molecules residing in water domains are energetically unstable because the bond lengths deviate far from the equilibrium length; thus, O₂ diffuses through the interfacial or hydrophobic regions. N₂ molecules show similar properties with O₂. This study provides a novel perspective on the permeation behavior of O₂ and N₂ through the ionomer film, which definitely benefits enhancing local O₂ transport.