(475h) Investigating Ion Transport, Mechanical Properties, and Stability of Tetraalkylammonium-Functionalized Polyethylene Using Molecular Dynamics Simulations

Alkaline fuel cells offer several advantages due to their improved oxidation and reduction kinetics that enable the use of less expensive catalysts. Therefore, there is a growing interest in alkaline anion exchange membranes (AAEMs) for their ability to selectively transport OH-. However, these membranes are prone to attack by OH- ions, which can neutralize the positive charge on the polymer, reducing its effectiveness. When designing advanced AAEMs, it is crucial to consider key factors including enhancing processability, improving mechanical properties, increasing OH- ion diffusivity, and minimizing structure degradation. Tetraalkylammonium-functionalized polyethylene (TFP) is a solvent-processible polymer with high hydroxide conductivity. Prior work suggests that increasing the ionic content in TFP results in a higher water uptake, which improves ion conductivity but causes swelling and loss of mechanical integrity of the membrane. In this study, we use atomistic molecular dynamics simulations with the AMBER force field to investigate ion transport, mechanical properties, and stability of AAEMs made from TFP. We generate multiple systems with ion content ranging from 20% to 66%, and water/hydroxide ratio ranging from 0 – 40, to examine the ion diffusivity and polymer flexibility. Mean squared displacement is used to calculate ion diffusivity, while flexibility is evaluated by analyzing stress and strain at the breaking point during uniaxial deformation. To investigate the stability of the polymer, we use the radial distribution function (RDF) to find the local packing of the anion around the polymer; this will inform us of where the ion can potentially attach to and eventually attack different sites in the polymer. Using this information, we calculate the decomposition rates of trimethylammonium groups, under the assumption that the number of occurrences in which hydroxide ions approach the positively charged sites in the polymer correlates to the chance of a decomposition reaction. These findings provide valuable insights that can be used to design and optimize novel AAEMs for alkaline fuel cells and other electrochemical applications.