Molecular dynamics simulation of the mechanical properties and thermal conductivity of aromatic electrolytes in proton exchange membrane fuel cells

Evaluation of mechanical and thermodynamic properties and understanding of the influence mechanism are crucial for the fuel cell membranes. The molecular dynamics simulation are performed to quantitatively study the variation of the absorption, swelling rates, tensile compression, and thermal conductivity for four membranes containing three typical aromatic polymers, namely sulfonated poly-ether-ether-ketone (PEEK), polysulfone acid-phenol-F (PAEF), phosphoric-pyridine-polyethers (PEP), and typical Nafion 213 (perfluorinated sulfonic acid PFSA). A microscopic model is constructed to establish the relationship between the hydrophobicity of the functional groups, water cluster distribution, and swelling properties. PFSA (Nafion) exhibits the best antiswelling properties. Main chain hydrophobicity and side chain hydrophilicity can enhance water aggregation, which leads to uneven water distribution during water absorption. The membrane swelling rate decreases when the water distribution is inhomogeneous. The sensitivity of the membrane thermal conductivity is more apparent when the water distribution is inhomogeneous. Nanoscale cavities and rearrangements in the molecular chain orientations along the stretching direction are identified based on microscopic morphological analysis. Aromatic polymer films exhibit higher thermal conductivity than that of Nafion, but their large water swelling rates and relatively low tensile-compression properties need to be improved by further research for realizing widescale practical applications.