The Role of Local Structure and Dynamics on Proton and Hydroxide Transport in Ion-Conducting Polymers

Fuel cells are an increasingly important technology for energy conversion, particularly for transportation. Perfluorosulfonic acid (PFSA) polymers have been used for decades as proton exchange membranes (PEMs) within hydrogen fuel cells, because they have excellent proton transport properties along with high chemical stability. Under high humidity or hydrated conditions, the best-performing PEMs exhibit proton conductivities as high as 0.1 – 0.2 S/cm while still maintaining mechanical stability. However, PFSA membranes have a variety of drawbacks that limit their further development and thus impede progress toward a hydrogen economy, as well as hamper progress in a variety of other technologies (e.g., flow batteries, solar fuels, anion exchange membranes). In particular, fluorine adds expense to the polymer due to both its notoriously dangerous chemistry and its detrimental impact on the environment. Moreover, the fluorine chemistry involved with synthesizing PFSA polymers limits the availability of controlled polymer architectures, such that detailed correlations between polymer microstructure, processing and performance are incomplete. These gaps in understanding significantly impede further development of PEMs, and the understanding of anionic exchange membranes (AEMs) which conduct hydroxide ions is even further behind. This project endeavors to build a comprehensive and mechanistic understanding of ion transport in hydrated polymers by using a combination of exquisitely controlled chemistry, extensive atomistic molecular dynamics simulations, and a suite of characterization methods that span the critical length and time scales. Motivated by the critical need for proton and hydroxide conducting membranes for hydrogen technologies, this project will design, synthesize, and study hydrocarbon-based ion exchange membranes to mimic the well-accepted salient features of PFSA membranes. The PEM polymers are amorphous, hydrocarbon-based materials polymerized via ring opening metathesis polymerization (ROMP) with a phenyl sulfonate pendent group on precisely every fifth carbon. The saturated hydrocarbon backbone provides the conformational flexibility for strong nanophase separation, namely sharp hydrophobic/hydrophilic interfaces with the sulfonic acid groups lining the percolated water channels. A valuable, and as yet untapped, attribute of this new ROMP-based polymer is the exceptional control of the polymer microstructure, particularly of the acid content and of the length of the spacer groups between the saturated backbone and the sulfonic acid group. The nature of the interface between the hydrophobic domains and hydrophilic domains is critically important for controlling proton and hydroxide transport. The attributes of the hydrophobic/hydrophilic interface important for ion transport include the following: the chemical composition profiles of the polymer backbone, polymer functional groups and water across the interface; the local roughness of the interface; the interfacial area per functional group; and any correlations between composition and topology at the < 1 nm length scales. In addition, the details of the hydration structures of the water and ions (protons, hydroxides, sulfonates, quaternary ammoniums) impact proton and hydroxide mobility. Interactions between the ions and the polymer backbone may also affect ion mobility, particularly if the polymer backbone atoms are present at the water domain interfaces. Improving proton and hydroxide conductivity requires better fundamental understanding, particularly with respect to the local structure and dynamics. These ionic domain interfacial structures and hydration interactions will ultimately impact the transport across the entire polymer. We have assembled a team of four PIs with the combined skills to establish the role of the local structure and dynamics at the hydrophobic/hydrophilic interface on proton and hydroxide transport in these important materials. Our research requires synthetic control and versatility to modify the molecular structure, which is available in Kennemur's group using ROMP and subsequent functionalization. As recently determined by Frischknecht's all-atom molecular dynamics simulations and corroborated by Winey's X-ray scattering, the linear saturated carbon backbones of these polymers have sufficient flexibility to form well-developed percolated water domains. While Frischknecht and Winey have previously combined simulations, electrochemical impedance spectroscopy, and quasielastic neutron scattering to extract new insights about ion and chain dynamics, Hickner brings essential expertise in IR and NMR spectroscopies that will reveal local structure and dynamics in terms of the fundamental role of water in determining the ion transport in these new polymers. Through a highly coordinated effort, our combination of novel polymer synthesis, advanced simulations and comprehensive characterization will provide new understanding of how protons and hydroxide ions move in hydrated polymers.