Impact of Nano- and Mesoscales on Macroscopic Cation Conductivity in Perfluorinated-Sulfonic-Acid Membranes

Abstract

A mean-field local-density theory is outlined for ion transport in perfluorinated-sulfonic-acid (PFSA) membranes. A theory of molecular-level interactions predict nanodomain and macroscale conductivity. The effects of solvation, dielectric saturation, dispersion forces, image charge, finite size, and confinement are included in a physically consistent 3D-model domain geometry. Probability-distribution profiles of aqueous cation concentration at the domain-scale are in agreement with atomistic simulations using no explicit fitting parameters. Measured conductivities of lithium-, sodium-, and proton-form membranes with equivalent weights of 1100, 1000, and 825 g/mol(SO3) validate the macroscale predictions using a single-value mesoscopic fitting parameter. Cation electrostatic interactions with pendant sulfonate groups are the largest source of migration resistance at the domainscale. Tortuosity of ionically conductive domains is the largest source of migration resistance at the macroscale. Our proposed transport model is consistent across multiple lengthscales. We provide a compelling methodology to guide material design and optimize performance in energy-conversion applications of PFSA membranes.

Chemical and Morphological Origins of Improved Ion Conductivity in Perfluoro Ionene Chain Extended Ionomers

“…Hydrophilic domain spacing, ordering, and connectivity are important parameters related to phase separation that determine transport pathways. These morphology features need to be understood to construct relationships between molecular structure and ion conductivity . SAXS can reveal the degree of phase separation in ionomer membranes.…”

“…This suggests subtle chemistry differences could play a role in nanoswelling of domains and overall conductivity. Modeling and simulation studies have shown that electrostatic interactions within polymer-bound anionic groups and cation solvation energy have the greatest impact on charge migration at the molecular and domain scale, and this is influenced by the chemistry and resulting charge delocalization on the side-chain. , Previous work has shown that the proximity of protogenic groups and the charge delocalization on the side-chain are important factors that dictate hydrogen bonding and proton dissociation at low hydration levels . PFICE ionomers have a side-chain chemistry and combination of protogenic groups that facilitates water hydrogen-bonding configurations promoting proton dissociation, as illustrated in Figure .…”

“…These morphology features need to be understood to construct relationships between molecular structure and ion conductivity. 74 SAXS can reveal the degree of phase separation in ionomer membranes. Because ionomers, and many similar charge-containing polymers in general, are relatively disordered, the peaks in SAXS tend to be broad without higher-order peaks visible, indicating only short-range order in these materials.…”

Morphology and Transport of Multivalent Cation-Exchanged Ionomer Membranes Using Perfluorosulfonic Acid–CeZ+ as a Model System

“…The change in the slope of conductivity against hydration was previously reported for proton-form PFSAs, and associated with the ion exchange capacity and more recently modeled to be related to mesoscale network tortuosity. 6,60 Conductivity of an interconnected water domainnetwork as a function of the domain volume fraction is expressed as:…”

OH− and H3O+ Diffusion in Model AEMs and PEMs at Low Hydration: Insights from Ab Initio Molecular Dynamics

“…In recent years, nano-confined structures have been exploited in the study of cost-effective and reliable polymer architectures for electrochemical devices [ 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 ]. Understanding the water structure and the behavior of ions in these confined structures is essential to gain insight into the ion diffusion mechanisms.…”