(Invited) Multiscale Molecular Modeling for Polymer Electrolyte Membrane for Fuel Cell

Abstract

Multi-scale “molecular” modeling approach has been developed and applied to polymer electrolyte membrane. The goal of this work is to investigate the possible morphology, structure, dynamics, and mechanical properties of perfluorosulfonic acid (PFSA) membrane with realistic polymer chain lengths (>100 monomer units) by molecular dynamics (MD) simulations. First, we have carried out a series of all-atom (AA) MD simulations of PFSA polymer with a short chain length (10 monomer units) to yield reference structural data for building a coarse-grained (CG) molecular model. Although many phenomenological modeling based on dissipative particle dynamics has been proposed previously, a chemically accurate CG model should be beneficial to investigate the morphology in realistic membranes and to provide a concrete molecular design. Thus, we attempted to construct a predictive CG model for the structure and morphology of PFSA membranes that is compatible with the SDK CG water model. [1] As a first trial, we extended the parameter set in the SDK CG force field to examine a hydrated PFSA membrane based on thermodynamic and structural data from experiments and AA-MD simulations. However, a noticeable degradation of the morphology was detected, which motivated us to improve the structural properties by a combination with the iterative Boltzmann inversion (IBI) approach. The hybrid SDK/IBI model was determined to reasonably reproduce both the thermodynamic and structural properties of the PFSA membrane for all examined water contents. [2] In addition, the obtained CG model demonstrated good transferability. Then, we used the CG-MD to simulate realistic long-chained (including more than 120 monomer units) PFSA membranes. We obtained successfully a well converged structure for this long-chained membrane system. We also developed a reverse-mapping method and successfully used it to produce a well-equilibrated structure at all-atom resolution. We further investigated the mechanical properties of this realistic PSFA membranes at AA resolution. The uniaxial tension tests were conducted to understand the mechanical behavior of hydrated PEM membranes, especially the failure mechanism of the membranes. It was observed that the results (Poisson's ratio, free volume) of the uniaxial deformation were sensitive to the hydration level. This computational methodology will provide more chemically realistic predictions of the PEM membrane properties and give an insight into how to develop the large-scale atomistic PEM model in a multi-scale framework. We will further discuss the effect of equivalent weight (EW) and side chain length on morphology and dynamics of PFSA membranes.[3,4][1] Shinoda et al., Mol. Simul. 33, 27 (2007).[2] Kuo et al., J. Chem. Phys. 147, 094904 (2017).[3] Kuo et al., J. Phys. Chem. C 120, 25832 (2016).[4] Kuo et al., Polymer, 146, 53 (2018).