(358c) Energetics of Proton Transfer of Hydrated Perfluorosulfonic Acids

The hydrated proton exchange membrane (PEM) functions as the electrolyte in a fuel cell, effectively separating the anode and cathode and reactant gases. Hence, the conduction of protons is very important to the efficient operation of the device. Several experimental and computational studies have been undertaken to understand the mechanism of proton transport in these phase separated materials. A complete understanding of the mechanism has not been acquired but critical to long range transport is the formation of a continuous network of dynamic hydrogen bonds. In this work, the structure and energetics of proton transfer in minimally hydrated perfluorosulfonic acid (PFSA) membranes is explored through electronic structure calculations of model systems possessing the critical components.

PFSA membranes are the most commonly used electrolyte and electrode separator in PEM fuel cell due to their substantial chemical and mechanical stability. Under operating conditions, the acidic groups (-SO₃H) of the ionomer are hydrated and the protons dissociated generating protonic charge carriers. Proton may be transferred through structural diffusion via the solvating water molecules but also through coupled vehicular diffusion where there is a coupling of proton and water molecule as hydronium ions (H₃O⁺). The hydrogen bonding of the water, water content, and density of the sulfonic acid groups is critical for the formation of the protonic charge carriers and mobility of the protons.

The energetics and associated structures for proton transfer in minimally hydrated PFSA membranes is investigated with electronic structure calculations of model systems consisting of pentafluoroethanesulfonic acid (C₂F₅SO₃H) hydrated with xH₂O (x = 4, 5, 6, 7) molecules or oligomeric fragments consisting of two or more perfluorinated side chains. Ab initio self-consistent field (SCF) molecular orbital calculations were performed using the GAUSSIAN 03 suite of programs. Several different initial configurations were chosen to avoid biasing the resulting fully optimized structure. The optimizations were undertaken without symmetry constraints initially employing Hartree-Fork theory with the 6-31G(d,p) basis set. The resulting equilibrium structures were then further refined with density functional theory with Becke's 3 parameter functional (B3LYP) and the same basis set. In the final optimized structure for each system, the hydrogen bond from the center oxygen (¹O) of each hydronium ion (H₃O⁺) to the oxygen atom (²O) in a neighboring H₂O was selected for transferring the proton between the oxygen atoms. The distance between ¹O and ²O was constrained during the transfer and at least 2 different protons were transferred (separately) at each hydration level. These scanning calculations were performed at the B3LYP/6-31(d,p) level. The results show that the energy barrier of transferring a proton is strongly affected by the structure features of the neighboring waters, similar to what is observed in conversions between Zundel and Eigen ions in bulk water. However, the presence of the sulfonate group(s) tends to break the symmetry of the energy profile. The hydration level was also found to affect the energetic barrier in these systems.