Molecular Modeling of Interfacial Proton Transport in Polymer Electrolyte Membranes

The proton conductivity of polymer electrolyte membranes (PEMs) plays a crucial role for the performance of polymer electrolyte fuel cells (PEFCs). High hydration of Nafion-like membranes is crucial to high proton conduction across the PEM, which limits the operation temperature of PEFCs to 100o C) and minimal hydration, interfacial proton transport becomes vital for membrane operation. Along with fuel cell systems, interfacial proton conduction is of utmost importance in biology and materials science; yet experimental findings of ultrafast proton transport at densely packed arrays of anionic surface groups have remained controversial and unexplained. In the main part of this thesis work, ab initio simulations were performed on a minimally hydrated, densely packed array of sulfonic acid surface groups (SGs). This system served as a model to study the mechanism of interfacial proton transport in perfluorosulfonic acid membranes. Specifically, simulations were performed to explore the impact of the density of SGs on the mechanism of interfacial proton transport. Results reveal a mechanism of highly collective proton motion at a critical SG separation of 6.6 Å. The activation free energy of proton translocations exhibits a high sensitivity to the SG density. A spontaneous concerted proton transition was observed with low activation barrier at a surface group separation of 6.8 Å. When protons translocate concertedly, the activation barrier of the transition drops by more than a factor of two to the value of 0.25 eV compared to the case of disconcerted proton transfer. Results show that the hydrogen-bond network with long-range order that forms upon densification of SGs at the interface enables highly effective proton transport under minimal hydration conditions. These results were then incorporated in a soliton theory for describing collective proton transport through minimally hydrated and highly charged interfaces.