The Role of Local Structure and Dynamics on Transport in Fluorine-Free Functionalized Ionic Polymers

Karen Winey, University of Pennsylvania (Principal Investigator)

Justin Kennemur, Florida State University (Co-Principal Investigator)

Email Frischknecht, Sandia National Labs (Co-Principal Investigator)

Michael Hickner, Michigan State University (Co-Principal Investigator)

This research project will reveal fundamental scientific principles that promote fast proton, hydroxide, and salt transport in hydrated fluorine-free polymers to advance a variety of energy-related applications.  The primary project objective is to establish new and robust fundamental understanding of novel ion transport mechanisms in fluorine-free polymers, particularly proton and hydroxide transport. The resulting insights will accelerate the rational design of exceptional materials for polymeric membranes in fuel cells and electrolyzers. The polymers have saturated hydrocarbon backbones with pendant functional groups for flexible chain conformations to facilitate nanophase separation into hydrophobic and hydrophilic co-continuous domains. The central hypothesis is that the morphological details of these hydrated polymers and the local structure near the functional groups dictate the ion conductivity and transport mechanisms. The research approach features precise and versatile polymer synthesis, advanced atomistic molecular dynamics (MD) simulations, and a suite of characterization methods that span critical length and time scales including electrochemical impedance spectroscopy (EIS), X-ray scattering, and NMR and FTIR spectroscopies. Ongoing feedback between these activities will accelerate the progress toward unraveling the complexity of transport in ionic polymers.

      The proposed research is organized into three synergistic aims. Aim 1 explores the role of the percolated nanoscale morphology on transport properties. The hypothesis is that the chemistry, composition, and architecture of these fluorine-free polymers are the primary dictators of the hydrated network structures and hence the proton and hydroxide conductivity. Novel polymers will be designed and synthesized based on cyclopentenes and post-polymerization modification methods that modulate backbone stiffness and side-chain flexibility. Structures of hydrated percolated water channels will be generated by classical all-atom MD simulations and rigorously compared to X-ray scattering data. Quantitative descriptions of the simulated and experimental network structures will include channel width distributions and fractal dimensions. The non-classical transport mechanism will be incorporated into the MD simulations to improve comparisons with EIS experiments. Aim 2 investigates the local structure and dynamics near the hydrophilic / hydrophobic interface, with the hypothesis that the hydrogen bonding network of the nanoconfined water impacts proton and hydroxide transport. A quantitative comparison between the measured FTIR spectra with spectra computed from the all-atom MD simulations will be made employing the various polymers from Aim 1. Comparison of water diffusion and rotation in the MD simulations with NMR spectroscopies (pulsed field gradient and relaxometry, respectively) will also be made to understand the influences of polymer chemistry and nanoscale morphology on confined water. Going beyond proton and hydroxide conductivity, additional studies in Aim 3 will expose these membranes to salt solutions to explore how the percolated nanoscale morphology (Aim 1) and local structure and dynamics of water (Aim 2) influence anion and cation transport. The salt concentrations relevant for water treatment and flow battery applications are expected to modify the ionic interactions within the percolated water channels and impact the water structure and dynamics. New insights into salt transport in hydrated channels will be established by leveraging the knowledge across length scales of the tunable polymers in this project.