Polymer electrolyte fuel cells (PEFCs) are touted as the next generation of energy delivering devices. Within a decade, PEFC-driven powertrains are expected to become a viable alternative to internal combustion engines in vehicles. Moreover, PEFCs could provide power to a plethora of portable and stationary applications. The critical component in a PEFC is the polymer electrolyte membrane (PEM). Current PEMs require a high level of hydration in order to provide sufficient proton conduction. Of particular interest in this field is the synthesis of advanced functional membranes that could attain high proton mobility at minimal hydration and at elevated temperature (> 100ºC). At these temperatures, structural correlations and proton dynamics at acid-functionalized polymer aggregates could be vital for membrane operation. Theoretical predictions should guide the efforts in the design of advanced PEMs. Our model system consists of a minimally hydrated interfacial array of acid-terminated surface groups. The density of these surface groups is the main variable parameter of the model; moreover, we have evaluated different chemical architectures of surface groups. We have employed ab initio calculations based on density functional theory to study interfacial energies, structural correlations and transitions in the hydrogen bonded network of hydronium ions and protogenic surface groups.The first part of this thesis focuses on rationalizing the effect of various parameters of highly acid-functionalized interfaces in PEMs, such as density, chemical architecture, and conformational flexibility of acidic surface groups on interfacial structural correlations and transitions. At high surface group density and under minimal hydration, with one water molecule per surface group, sulfonic acid head groups are perfectly dissociated. They assemble into a highly ordered condensed surface state with two sublattices; one of them is formed by hexagonally ordered sulfonate anions; the other sublattice corresponds to interstitial hydronium ions. Sulfonate anions and hydronium ions form a hydrogen bonded network. The saturation of hydrogen bonds renders the network in a superhydrophobic state. Lowering the surface group density triggers a sequence of transitions to states with decreasing long range order, decreasing the number of interfacial hydrogen bonds and the degree of dissociation. Moreover, the interface becomes hydrophilic. The same sequence of transitions was found for arrays with varying length and chemical structure of surface groups. These findings emphasize the importance of 2D correlation effects at polymer-water interfaces in PEMs. The second part of the thesis focuses on the impact of a second monolayer of water molecules on stability, interfacial structural correlations and and transitions of the hydrated array of acidic surface groups. Upon increasing the surface group spacing, the bonding energy of additional water increases, undergoing a transition from superhydrophobic to hydrophilic wettability. At sufficiently large surface group separation, a hydronium ion is seen to transfer from the minimally hydrated interfacial network to the second water layer, where it is then observed to form a Zundel ion.
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Thesis advisor: Michael, Eikerling
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