Molecular Modeling of Proton Transport in the Short-Side-Chain Perfluorosulfonic Acid Ionomer

Abstract

An explanation for the superior proton conductivity of low equivalent weight (EW) short-side-chain (SSC) perfluorosulfonic acid membranes is pursued through the determination of hydrated morphology and hydronium ion diffusion coefficients using classical molecular dynamics (MD) simulations. A unique force field set for the SSC ionomer was derived from torsion profiles determined from ab initio electronic structure calculations of an oligomeric fragment consisting of two side chains. MD simulations were performed on a system consisting of a single macromolecule of the polymer (EW of 580) with the general formula F3C-[CF(OCF2CF2SO3H)-(CF2)7]40-CF3 at hydration levels corresponding to 3, 6, and 13 water molecules per sulfonic acid group. Examination of the hydrated morphology indicates the formation of hydrogen bond "bridges" between distant sulfonate groups without significant bending of the polytetrafluoroethylene backbone. Pair correlation functions of the system identify the presence of ion cages consisting of hydronium ions hydrogen-bonded to three sulfonate groups at the lowest water content. Such structures exhibit very low S-OH3+ separations, well below 4 A and severely inhibit vehicular diffusion of the protons. The number of sulfonate groups in the first solvation shell of a given hydronium ion correlates well with the differences between Nafion and the SSC polymer (Hyflon). The calculated hydronium ion diffusion coefficients of 2.84 x 10-7, 1.36 x 10-6, and 3.47 x 10-6 cm2/s for water contents of 3, 6, and 13, respectively, show only good agreement to experimentally measured values at the lowest water content, underscoring the increasing contribution of proton shuttling or hopping at the higher hydration levels. At the highest water content, the vehicular diffusion accounts for only about 1/5 of the total proton transport similar to that observed in Nafion.

Mesoscopic Simulations of the Hydrated Morphology of the Short-Side-Chain Perfluorosulfonic Acid Ionomer

“…Ab initio molecular dynamics (AIMD) calculations have also been performed to investigate proton transfer and dynamics for model systems exhibiting a high density of perfluorinated sulfonic acid groups (29)(30)(31)(32). On a larger scale are the classical molecular dynamics (MD) simulations on structural correlations and transport properties of PEMs (33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43). These include empirical valence bond (EVB) models of the solvation and transport of hydrated protons (44,45).…”

Water, proton, and oxygen transport in high IEC, short side chain PFSA ionomer membranes: consequences of a frustrated network

“…11 SSC ionomers have also been the subject of several theoretical and molecular-level modelling studies, [12][13][14][15][16][17][18] from which, the effect of backbone flexibility, length of side chain, IEC, and molecular weight on proton transport processes, under varying levels of hydration, has been revealed.…”

“…Much larger water uptakes were observed compared to low IEC Hyflon membranes, and molecular modelling studies of proton transport in SSC PFSA membranes (IEC = 1.72 mmol g À1 ) revealed that the calculated hydronium ion diffusion coefficient increased with water content. 18 Noteworthy, the studies referredt oa b o v ed e s c r i b ep r o p e r t i e s of SSC PFSA ionomer membranes prepared by extrusion; the properties of solution-cast SSC PFSA membranes is sparse, despite the significant effect processing conditions exert on membrane properties, as clearly demonstrated for LSC PFSA ionomer membranes. 1 Recently, we reported fuel cell performances of membrane-electrode-assemblies using high IEC SSC ionomer (1.3, 1.4, 1.5 mmol g À1 ) as the proton conducting medium dispersed in the catalyst layer.…”

Effects of hydration level, temperature, side chain and backbone flexibility of the polymer on the proton transfer in short-side-chain perfluorosulfonic acid membranes at low humidity conditions

“…In addition, classical MD simulations of a SSC PFSA membrane of EW 580 showed that SSC PFSA membranes have a greater number of free hydronium ions at the same hydration level compared with the nafion [59]. This interestingly suggests that the complexity of proton conduction is one of the major determinants of the proton conductivity since, as mentioned previously, Kreuer et al [43] showed that the nafion has higher proton conductivity than the SCC PFSA membranes at low hydration level.…”