(Invited) Using Molecular Dynamics Simulations to Relate Morphology to Water and Ion Dynamics in Hydrated Ion-Conducting Polymers

Abstract

Hydrated ion-containing polymer membranes are used as electrolytes in a variety of applications, including in fuel cells and in alkaline batteries. Achieving high conductivity in hydrated, ion-conducting polymers typically requires the hydrophobic backbone of the polymers to be nano-phase separated from the hydrophilic domains so that the ions (protons or hydroxide ions) have well-defined water domains for efficient transport. The ion transport thus depends strongly on the hydration level and on the detailed morphology of the hydrophilic domains. It is often difficult to determine the morphology of these domains experimentally. I will present results from atomistic molecular dynamics (MD) simulations that reveal the domain morphology in various ionic polymers. Increasing water content generally leads to an increase in the characteristic spacing between hydrophilic domains. However, this swelling can be obscured in X-ray scattering due to a loss of scattering contrast between the hydrophobic and hydrophilic domains. The MD simulations reveal that the systems are nano-phase separated, even when X-ray scattering appears to indicate otherwise. Calculated and measured scattering profiles are in good qualitative agreement. I will discuss several methods to characterize the water domain morphology in more detail, including various measures of pore size distributions, surface areas, and fractal dimension. Water and ion diffusivities calculated from the MD simulations follow the experimentally-measured trends in conductivity and are consistent with changes in the nanoscale morphology with changing water content. I will discuss the implications of the results for the future design of ion-conducting polymers for use as membranes in electrochemical devices.This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. Figure 1