(358b) On the Mechanism of Proton Transport in Model Perfluorosulfonic Acid Systems: Ab Initio Molecular Dynamics Simulations

Perfluorsulfonic acid (PFSA) ionomers have drawn great interest in the scientific community due to their use as the electrolyte in proton exchange membrane fuel cells (PEMFCs). PEMFCs have the potential to provide an efficient and clean energy supply through the electrochemical conversion of hydrogen and oxygen into water. However, the mechanisms of proton transport through these membranes are poorly understood, particularly at low hydration levels. As these membranes are hydrated, a phase separation between the hydrophobic polymer backbone and the ionomeric, hydrophilic sidechains occurs, creating a labyrinthine, inhomogeneous system. Due to this complexity, as well as difficulties in ionomer synthesis, elucidating the structure-proton transport relationship has been difficult. A detailed understanding of this highly disordered system is required for the development of ionomers capable of facilitating efficient proton transport.

Experimental and theoretical efforts have shown that, upon hydration, PFSA ionomers swell, creating clusters and/or channels consisting of the aqueous domain and hydrated ions. At low hydration levels, these regions are quite small or narrow, being on the order of one nanometer in diameter. As the disorder of these channels makes them difficult to study theoretically, we have employed carbon nanotubes as scaffolding for model PFSA channels. Carbon nanotubes are very structurally defined on a nanometer length scale and their large variations in diameter provide a rich nanostructured architecture for the investigation of proton transport and diffusion, local environmental effects and protogenic spacing. Initial studies have indicated that side chain flexibility and motion of the sulfonate groups are important to efficient proton conductivity.

Ab Initio molecular dynamics (AIMD) is a computational technique which decouples the light, fast moving electrons from the slower heavier nuclei. The electrons are then treated quantum mechanically, while the nuclei are evolved classically, which saves much computational effort. As such, AIMD does not require empirical parameters, force fields, or assumptions about the mechanisms of proton transport. The former feature is extremely important, as AIMD may reveal surprising reaction dynamics and pathways. The AIMD simulations were performed using the Vienna Ab Initio Simulations Package (VASP). The simulation cells were constructed with periodic boundary conditions along the length of the CNT and 6 Å of vacuum in the perpendicular directions. The geometry was optimized to its minimum energy structure and the system was then heated to 300K using repeated velocity scaling. Once the cells were thermalized, 10 to 20 ps trajectories were obtained in the microcanonical ensemble for the calculation of diffusion coefficients and the analysis of ion formation.