Abstract
Perfluorosulfonic acid (PFSA) polymer membranes are widely used as proton exchange membranes. Because the structure of the aqueous domain within the PFSA membrane is expected to directly influence proton conductance, many coarse-grained (CG) simulation studies have been performed to investigate the membrane morphology; these studies mostly used phenomenological models, such as dissipative particle dynamics. However, a chemically accurate CG model is required to investigate the morphology in realistic membranes and to provide a concrete molecular design. Here, we attempt to construct a predictive CG model for the structure and morphology of PFSA membranes that is compatible with the Sinoda-DeVane-Klein (SDK) CG water model [Shinoda et al., Mol. Simul. 33, 27 (2007)]. First, we extended the parameter set for the SDK CG force field to examine a hydrated PFSA membrane based on thermodynamic and structural data from experiments and all-atom (AA) molecular dynamics (MD) simulations. However, a noticeable degradation of the morphology motivated us to improve the structural properties by using the iterative Boltzmann inversion (IBI) approach. Thus, we explored a possible combination of the SDK and IBI approaches to describe the nonbonded interaction. The hybrid SDK/IBI model improved the structural issues of SDK, showing a better agreement with AA-MD in the radial distribution functions. 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. In addition, the model demonstrated good transferability and has considerable potential for application to realistic long-chained PFSA membranes.
Highlights
Polymer electrolyte membrane fuel cells (PEMFCs) are an efficient and “green” alternative power source.1,2 The key component of these fuel cells is the proton exchange membrane, which is manufactured from a special ionomer containing both nonpolar repeat units and a small amount of pendant ionized units
We present CG-molecular dynamics (MD) results for hydrated Perfluorosulfonic acid (PFSA) membranes obtained with three different approaches (SDK, iterative Boltzmann inversion (IBI), and hybrid SDK/IBI)
All examined CG models were constructed to be compatible with the SDK-CG water model,59 in which three water molecules are packed into a single CG site
Summary
Polymer electrolyte membrane fuel cells (PEMFCs) are an efficient and “green” alternative power source. The key component of these fuel cells is the proton exchange membrane, which is manufactured from a special ionomer containing both nonpolar repeat units and a small amount of pendant ionized units. The chemical structure of PFSA consists of a hydrophobic polytetrafluoroethylene backbone with hydrophilic sidechains terminated by a sulfonic acid This amphiphilic nature causes micro-phase separation between the hydrophobic and hydrophilic domains in hydrated PFSA membranes. Because the morphology of a hydrated PFSA membrane is expected to directly influence the proton transportation and mechanical characteristics of the membrane, membrane morphology has been extensively studied in various experimental and modeling investigations.. Because the morphology of a hydrated PFSA membrane is expected to directly influence the proton transportation and mechanical characteristics of the membrane, membrane morphology has been extensively studied in various experimental and modeling investigations.2,4–9 Many morphological models, such as the cluster-network model, fibrillar structure model, parallel-cylinder model, and film-like model, have been proposed to describe the water swelling behavior and a)Author to whom correspondence should be addressed: w.shinoda@chembio.
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