|M.Sc Student||Pusara Srdjan|
|Subject||Molecular Simulation of Quanternary Ammonium|
Solutions at Low Hydration Levels
|Department||Department of Chemical Engineering||Supervisor||Professor Dario Dekel|
Highly conductive and highly chemically stable anion exchange membranes (AEMs) are the key to achieve high-performance anion exchange membrane fuel cells (AEMFCs). In overcoming the chemical stability challenge, the degradation of the quaternary ammonium cations in the currently developed AEMs is a primary concern. Much experimental and computational work has focused on the stability of various substituted ammonium salts. While cation chemistry dictates the AEM stability, chemical degradation has been recently shown to be significantly influenced by the hydration level at which the AEM operates. At low hydration levels, it is now known that almost every quaternary ammonium may suffer significant decomposition. Understanding the principles governing the chemical degradation at low hydration levels will facilitate the path to overcome the challenge and develop new highly-stable AEMs. In this work, we use molecular dynamics simulations to explore the behavior of three common quaternary ammonium cations with stoichiometric hydroxide concentration and at very low hydration. We find that water preferentially solvates that hydroxide anions, and hence when water is present at sufficient amount (more than 4 water molecules per ion pair), stability of the cations is expected to significantly improve. However, lower amounts of water (microsolvation stage) result in the formation of isolated molecular clusters and ammonium-hydroxide pairing that leads to degradation of the cation. The substituted groups on the ammonium salts affect their interaction with the hydroxide, particularly at low hydration. The composition and size distribution of the water-hydroxide-cation clusters that form are shown to be significantly affected by the cation chemistry. Implications of the observed behavior are discussed in view of recent experimental results on the differences in stability of these cations. This study highlights, for the first time, the crucial importance of studying hydroxide-water-cation interactions at low hydration levels, which in turn may help to elucidate new understanding to crack the cation stability challenge in the AEMFC environment.