| Ab initio Study of Interfacial Structure and Dynamics in Polymer Electrolyte Membranes |
Ata Roudgar, Sudha P. Narasimachary and Michael Eikerling
Department of Chemistry, Simon Fraser University, Burnaby, Canada |
| Transport properties and stability of proton-conducting polymer electrolyte membranes (PEM) depend on chemical architecture, phase separation at the mesoscale, and random morphology at the macroscale. Understanding these relations is vital for the design of advanced PEM. Our calculations focus on the concerted dynamics of flexible charged sidechains, water molecules, and protons inside PEMs. We performed ab initio calculations for a model that consists of a 2D hexagonal array of flexible acidic surface groups (SG) with fixed endpoints and one water molecule per SG. We vary separations dCC of SG and explore corresponding interfacial conformations and the strength of water binding. At small dCC the minimally hydrated is strongly correlated and it weakly interacts with an additional water molecule (< 0.1eV). At a critical SG separation of dCCts~6.7Å we found a transition between highly correlated and cluster-like conformations of surface groups. This transition includes proton transfer. We have identified hydronium translation, surface group rotation, and surface group tilting as collective coordinates that trigger this transition. We explored transition paths by calculating contour plots of the total energy as a function of these three collective coordinates for 3 values of dCC around dCCts~6.7Å. The barrier-energy at dCC=dCCts is 0.55~eV. In order to explore proton exchange between the surface layer and adjacent water molecules we have added an additional layer of 14 water molecules. Our result at dCCts shows that SG weakly interact with second water layer (~0.1 eV/SG). Overall, results of our calculations provide valuable fundamental insights into proton transport mechanisms in PEMs at elevated temperature and minimal hydration that we plan to exploit in view of advanced membrane design. |
 |
|
