Molecular Dynamics Study on the Microscopic Mechanism of Mechanical Properties of Nafion Membrane

As the core component of new energy vehicles, the safe operation of polymer electrolyte fuel cells has become more important with the rapid development of the new energy field. Since polymer electrolyte membrane (PEM) works at a hydrated environment, the mechanical properties of its hydrated state must remain stable. At present, studies have shown that the increase of water content will reduce the mechanical properties of materials, such as the decrease of Young's modulus, but the microscopic mechanism has not been elucidated. This presents a challenge to the work to develop novel PEMs. We tried to use the method of coarse-grained (CG) molecular dynamics (MD) simulation to study the microscopic mechanism of the mechanical properties of basic PEM Nafion membrane under different water content conditions. The purpose of this study is to seek to demonstrate the microscopic mechanism by which macroscopic mechanical properties are determined from the MD perspective, thereby providing general guidance for the future development of novel PEMs. More specifically, we tested the Nafion membrane (CG model with 4 beads on the main chain and 2 beads on the side chain) under tensile and compressive stress (strain 0-0.5) at different water contents (2-14). Quantified micro-parameters make it possible to establish macro-micro relationships. Considering that our chosen Nafion is the basic model, it may have good generality to other complex models. At the macroscopic level, our results show that increasing the water content reduces Young's modulus of the material. The decreasing rate of Young's modulus shows a slowing trend. This is in good agreement with previous research. At the microscopic level, our results show that the coordination number between any two types of atoms except water molecules within the same volume decreases with increasing water content, suggesting that the increase in water molecules forces other atoms to away from each other, thereby increasing the spacing between molecules. Therefore, the dominant force on this distance scale, that is, the electrostatic attraction between positive and negative ions is reduced. We have a reason to think that this is the main micromechanical factor to change macroscopic mechanical properties. In addition, the results of polymer chain entanglement parameters show that the degree of entanglement of polymer chains deepened with the increase of water content. The complex entangled structure will increase the difficulty of the polymer deformation. We believe that this is one of the micromechanical factors to decrease the Young's modulus. The study of these mechanisms may help in the development of new Nafion materials.