Proton Transport Mechanism of Perfluorosulfonic Acid Membranes

Abstract

An understanding of proton transport within perfluorosulfonic acid (PFSA) membranes is crucial to improve the efficiency of proton exchange membrane fuel cells. Using reactive molecular dynamics simulations, we have examined proton transport in two PFSA materials, Hyflon and the 3M membrane, at three different hydration levels. The interaction between the sulfonate group of the polymer side chains and the hydrated protons was found to have only a small influence on the proton transport dynamics. Instead, proton swapping between sulfonate groups is the primary transport mechanism for the proton transport within the pore. The larger water clusters and more flexible side chain of the 3M membrane allows for an enhancement of this swapping mechanism compared to Hyflon. Membranes that can enhance this mechanism may result in greater proton conductivity.

Impact of Nano- and Mesoscales on Macroscopic Cation Conductivity in Perfluorinated-Sulfonic-Acid Membranes

“…At low water contents, there is insufficient water to hydrate the cations fully, which decreases the solvation energy at the center of the channel, and results in the dominance of electrostatic interactions.Figure S3shows that as water content decreases, the free energy balances from solvation to electrostatic and increases the fraction of cations associated with sulfonate groups. The predicted fraction of cations associated with sulfonate groups shows excellent agreement with atomistic simulations 26,[29][30].…”

“…Figure 4b shows the cation concentration, +¿ ρ ¿ (normalized by the average cation concentration in the unit cell, Figure 6 shows that the radial distribution function (RDF) of the cation with respect to the center of the sulfonate group displays three peaks, also consistent with molecular dynamics simulations. 26 Supporting Information gives details of the RDF calculation. The first peak, located at 2.4 Å, is caused by partially desolvated cations that form contact-ion pairs with the sulfonate groups (Inset a).…”

Effects of water nanochannel diameter on proton transport in proton‐exchange membranes

“…Theoretical studies have been devoted to providing a detailed relationship between the morphological features and proton transport using both classical MD and RMD simulations . Liu et al .…”

“…Theoretical studies have been devoted to providing a detailed relationship between the morphological features and proton transport using both classical MD and RMD simulations. [25][26][27][28][29][30][31][32][33][34][35][36] Liu et al 28 have performed a classical MD simulation and suggested the diffusivity is controlled by the balance between the connectivity and the confinement of water domains in PEMs. Selvan et al 27 has used an analytical model based on the confined random walk simulation approach and demonstrated that accounting for water content, confinement, and connectivity is important and sufficient to understand the water diffusive behavior in PEMs.…”

Dependence of electroosmosis on polymer structure in proton exchange membranes

“…Yan et al (Yan et al, 2008) have studied electroosmotic drag using the classical hydronium cation model in the MD simulations and predicted a value of K drag about 2.5 times greater than the experimental data at 300 K in completely hydrated Nafion membranes. Although few studies incorporate the Grotthuss mechanism into MD simulations to develop reactive MD using, for example, empirical valence bond (EVB) approaches (Feng et al, 2012, Feng and Voth, 2011, Jorn et al, 2012, Petersen and Voth, 2006, Petersen et al, 2005, Tse et al, 2013, Seeliger et al, 2005, Spohr et al, 2002, Savage et al, 2014, they have focused only on the proton diffusion in the equilibrium polymer systems, and therefore the electroosmotic properties have not been fully understood.…”