چكيده به لاتين
Molecular dynamics simulation has become a powerful tool for describing the behavior of membranes in fuel cell systems. The advantages of using this tool to investigate the parameters affecting membrane performance, such as functional groups, humidity of inlet gases, operational temperature, and others, are indispensable as a complement to experimental studies. However, an appropriate strategy for selecting simulation options is crucial for obtaining reliable results. Consequently, before examining the effects of the aforementioned parameters on the performance of poly (ether ether ketone) proton exchange membranes through molecular dynamics, the first part of this research focused on optimizing simulation options, including membrane structure, methods for controlling temperature and pressure in the system, arrangement of ensembles, and ultimately the force field. To achieve these goals, various calculations and tests were conducted, including total solubility parameter, density, glass transition temperature, elastic modulus, thermal and energy profiles, radial distribution function, mean square displacement, self-diffusion coefficient, and ionic conductivity. The investigation of the membrane structure based on structural parameters - density, total solubility parameter, and glass transition temperature - led to the selection of six polymer chains and fifteen monomers (values of 1.261 g.cm-3 for density, 53.35 J0.5.cm-1.5 for total solubility parameter, and 463 Kelvin for glass transition temperature). It is noteworthy that the basis for selecting optimal boxes in this research was the convergence of results with experimental data. In the selection of thermostat/barostat methods, the Berendsen/Berendsen approach yielded results closer to experimental data compared to other methods (11.0 Ų for mean square displacement and 9.48 K² for temperature variance). Furthermore, the results for the self-diffusion coefficient of hydronium ions preferred the use of 3 nanoseconds for both canonical and isothermal-isobaric ensembles (simulation value cm².s⁻¹ compared to experimental data cm².s⁻¹). Ultimately, the superior performance of the Dreiding force field in predicting structural, mechanical, and thermal behavior of membranes and better efficiency of the COMPASS force field for calculating dynamic properties were additional findings. In the second part, as mentioned, parameters affecting membrane performance were examined. Initially, varying degrees of sulfonation (47%, 53%, 60%, and 67%) were investigated. The results indicated that an increase up to 67% enhances the transport properties in the membrane. The results concerning the impact of increased humidity of inlet gases also progressed as expected, with a hydration degree of 12 showing better transport properties compared to 9 and 6.22. It is important to note that excessive increases in sulfonation degree and hydration degree lead to significant reductions in membrane resistance and stability within the system. Finally, the effect of adding carboxylate functional groups to the membrane in the form of side chains was examined at two temperatures (333.15 K and 353.15 K). As anticipated, increases in temperature and percentage of carboxylate groups resulted in enhanced hydronium ion permeability in the membrane while concurrently reducing mechanical and thermal resistance. In conclusion, it can be stated that molecular dynamics, when appropriately selecting simulation options, serves as a suitable tool for investigating ion exchange membranes. In most cases, the choice of method depends on the type of test and the system in question. It is also noteworthy that considering membrane stability is crucial when examining the improvement of transport properties through various factors.
