Vehicular Diffusion Research Articles

Anion Exchange Membranes with Low Hydration Conditions from an Ab Initio Molecular Dynamics Perspective

Regardless of the initial hydration level, a typical Anion Exchange Membranes (AEMs) fuel cell device would operate ultimately under extremely low hydration values. It is therefore vital to fully explore the hydroxide diffusion in AEMs under such conditions. In this work, we present fully atomistic ab initio molecular dynamics (AIMD) simulations to obtain a molecular-level understanding of the hydroxide solvation complexes and diffusion mechanism in low hydration AEMs. By changing the polymer electrolyte cation spacing and the hydration values we create three different AEMs models. We find that under extreme low hydration conditions, the water molecules attain a non-uniform distribution. We further see that the unique water distribution results in distinct hydroxide diffusion mechanisms varies from a stationary behavior, a vehicular diffusion, and a mixture of structural and vehicular diffusion, depending on the existence of a water oxygen at the hydroxide second solvation shell. Therefore, we find the water density to be a better descriptor than hydration values when exploring AEM under low hydration condition. Furthermore, as the hydroxide transport is essentially different compared to bulk solution, we provide an idealized hydroxide diffusion mechanism for the three diffusion categories. We believe the results presented in this study enable us to define the terms required for achieving high hydroxide conductivity in high-performance AEM fuel cell devices under extremely low hydration conditions.

Atomistic simulation study of the hydrated structure and transport dynamics of a novel multi acid side chain polyelectrolyte membrane

Perfluoroimide acid (PFIA) belongs to a new class of Multi Acid Side Chain (MASC) polyelectrolyte membranes. Classical molecular dynamics simulations were carried out to study the hydrated nanostructure of PFIA and transport of water molecules and hydronium ions at T = 300 K and T = 353 K for a range of hydration levels. The radial distribution functions showed negligible change with temperature. The PFIA chain radius of gyration was minimally influenced by hydration and temperature which makes it suitable for fuel cells. Our simulations showed the formation of a large continuous water phase cluster in PFIA at high hydration levels which has also been observed in conductive probe atomic force microscopy experiments. These large continuous clusters lead to significantly higher vehicular diffusion rates for water molecules and hydronium ions at higher hydration levels. The vehicular diffusivity constants for water molecules and hydronium ions for PFIA were comparable to those for Nafion at both T = 300 K and T = 353 K. The vehicular proton conductivity values for PFIA were observed to be higher than those for Nafion at both T = 300 K and T = 353 K which agrees qualitatively with the experimental trends.

Molecular simulations of poly(2,5‐benzimidazole): Effect of water concentration, phosphoric acid doping, and temperature on hydrogen bonding and vehicular diffusion

AbstractResults of atomistic simulations of neat, hydrated, and phosphoric acid (PA)‐doped poly(2,5‐benzimidazole) (ABPBI) are reported in this article, with an emphasis on the effect of water concentration, PA‐doping level, and temperature on the hydrogen bond network and vehicular diffusion. Analysis of radial distribution functions indicates that varying water concentrations and PA‐doping levels does not have a significant effect on hydrogen bond lengths; however, high temperature causes hydrogen bond lengths to increase reflecting lower bond strength. Calculations of the number of hydrogen bonds indicate those involving PA increase with PA‐doping level, decrease with increasing temperature, and increase with decreasing water content. Sorption calculations using Grand Canonical Monte Carlo indicate the number of water molecules absorbed per 100 ABPBI repeat units is 141.5 at 298 K, in agreement with the experimental value. Vehicular diffusion coefficients in PA‐doped ABPBI decrease in the order: water > PA > ABPBI. Specifically, the vehicular diffusion of PA increases with PA‐doping level, temperature, and water content. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers

Proton conductivity and diffusion in molten phosphinic acid (H3PO2): The last member of the phosphorus oxoacid proton conductor family

From a fundamental point of view, the family of compounds known as phosphorus oxoacids is among the most intriguing proton conducting systems. Two of its members, phosphoric (H3PO4) and phosphonic (H3PO3) acid, show the highest known intrinsic proton conductivities. However, little is known about the last member, phosphinic acid (H3PO2), that completes this series and could provide a fundamental insight into the relationship between proton transport and the other properties of the hydrogen bond network. Here we present the results of a proton conductivity and diffusion study of molten nominally dry phosphinic acid (H3PO2) by 1H PFG-NMR (Pulsed Magnetic Field Gradient Nuclear Magnetic Resonance) and AC impedance spectroscopy. While the general features of proton transport in H3PO2 share some similarities to those of H3PO4 and H3PO3, there are also important differences between these systems. The intrinsic conductivity is found to arise primarily, from the vehicular diffusion of protonic charge carriers that form as a result of the relatively high degree of self-dissociation (~3.5–4.75%). Nevertheless, the experimentally measured diffusion coefficients of the exchangeable protons are ~10% higher than the molecular diffusion coefficients, and assuming that the correlations in the structural diffusion of protonic charge carriers are slightly higher (Haven ratio in the range of ~2–3.5), this results in ~21–38% of the total conductivity coming from structural diffusion.

Insight from molecular modelling: does the polymer side chain length matter for transport properties of perfluorosulfonic acid membranes?

We present a detailed analysis of the nanostructure of the short side chain (SSC) perfluorosulfonic acid membrane and its effect on H(2)O clustering, H(3)O(+) and H(2)O diffusion, and mean residence times of H(2)O near SO(3)(-) groups based on molecular dynamics simulations. We studied a range of hydration levels (λ) at temperatures of 300 and 360 K, and compare the results to our findings in the benchmark Nafion® membrane. The water cluster diameter is nearly the same in the two membranes, while the extent of SO(3)(-) clustering is more in the SSC membrane. The calculated cluster diameter of about 2.4 nm is in excellent agreement with the recently proposed cylindrical water channel model of these membranes. The diffusion coefficients of H(2)O and H(3)O(+) are similar in SSC and Nafion membranes. Raising the temperature of the SSC membrane from 300 to 360 K provides a much bigger increase in proton vehicular diffusion coefficient (by a factor of about 4) than changing the side chain length. H(3)O(+) ions are found to exchange more frequently with SO(3)(-) partners at the higher temperature. Our key findings are that (a) the hydrophobic-hydrophilic separation in the two membranes is surprisingly similar; (b) at all hydration levels studied, the long side chain of Nafion is bent and is effectively equivalent to a short side chain in terms of extension into the water domain; (c) vehicular proton transport occurs mainly between SO(3)(-) groups; and (d) changing the size of the simulation cell does not change the results significantly. The simulations are validated in good agreement with the corresponding experimental values for the simulated membrane density and diffusion coefficients of H(2)O.

Proton transport in water confined in carbon nanotubes: a reactive molecular dynamics study

The effects on the structural and transport properties of a proton in water confined in carbon nanotubes of radii ranging from 5.42 to 10.85 Å were studied by employing a recently devised reactive molecular dynamics (RMD) scheme. The formation of distinct layers was observed in the computed radial density profile of water. Affinity of hydronium ions towards the tube–water interface and its preferential orientation with the oxygen atom protruding towards the wall was observed. The axial water diffusivity was observed to decrease with increasing confinement of water. Analysis of the axial charge diffusivity and its two components (structural and vehicular) was also performed. Confinement was found to have a more significant effect on structural diffusion than on vehicular diffusion. The axial vehicular component of the charge diffusivity in the nanotube of radius 10.85 Å was found to be equal to the value computed in bulk water while structural component was 12% of the value observed in bulk water, which resulted in a total charge diffusivity of 42% of the diffusion in bulk water. The confined geometry affects the system energetically and perturbs the solvation structure around the proton from that found in bulk water. The RMD algorithm, which defines the occurrence of a proton transfer reaction based on the satisfaction of a set of triggers, identified the energetic factor to be greatly responsible for the decreased structural diffusion of a proton.

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

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.

Molecular Dynamics Study of Structure and Transport of Water and Hydronium Ions at the Membrane/Vapor Interface of Nafion

Through the use of molecular dynamics simulation, we examine the structural and transport properties of water and hydronium ions at the interface of a Nafion polymer electrolyte membrane and a vapor phase. The effect of humidity was studied by examining water contents of 5%, 10%, 15%, and 20% by weight. We observe a region of water depletion in the membrane near the vapor interface. We report the vehicular diffusion of hydronium ions and water as components parallel and perpendicular to the interface. In the interfacial region, for hydronium ions, we find that the component of the vehicular diffusivity parallel to the interface is largely unchanged from that in the bulk hydrated membrane, but the component perpendicular to the interface has increased, due to local decrease in density. We find similar behavior with water in the interfacial region. On the basis of these diffusivities, we conclude that there is no observable additional resistance to mass transport of the vehicular component of water and hydronium ions due to the interface. In terms of structure at the interface, we find that there is a decrease in the fraction of fully hydrated hydronium ions. This translates into a lower probability of forming Eigen ions, which are necessary for structural diffusion. Finally, we observe that the hydronium ions display a preferential orientation at the interface with their oxygen atoms exposed to the vapor phase.

Characterization of the Solvation and Transport of the Hydrated Proton in the Perfluorosulfonic Acid Membrane Nafion

The solvation and transport properties of the sulfonate-hydronium ion pair have been studied in hydrated Nafion through molecular dynamics simulation. Explicit proton and charge delocalization of the excess proton transport, via the Grotthuss hopping mechanism, were treated using the self-consistent multistate empirical valence bond (SCI-MS-EVB) method. The nature of the sulfonate-hydronium ion pair was characterized through analysis of free-energy profiles. It was found that, in general, the excess proton is solvated between two water molecules of a Zundel moiety while in the contact ion pair position, but then it transitions to an Eigen-like configuration in the solvent-separated pair position. Furthermore, the positive charge associated with the excess proton passes between the contact and solvent-separated ion pair positions through the Grotthuss mechanism rather than simple vehicular diffusion. The total proton diffusion was decomposed into vehicular and Grotthuss components and were found to be of the same relative magnitude, but with a strong negative correlation resulting in a smaller overall diffusion. Correlated motions between the ion pair were examined through the distinct portion of the van Hove correlation function, and a characteristic time scale of approximately 425 ps was observed. Additionally, the association of the hydrated proton with the hydrophobic polymer backbone suggests its amphiphile-like behavior (see Acc. Chem. Res. 2006, 39, 143; Phys. Rev. 1954, 95, 249; J. Chem. Phys. 2005, 123, 084309).

Effect of monomeric sequence on nanostructure and water dynamics in Nafion 117

We studied the effect of polar/nonpolar sequences on the structure and water transport of Nafion 117 with 20 wt.% water by molecular dynamics (MD) simulations. Although the systems show clear hydrophilic/hydrophobic segregation in all conditions studied, we observe that increased blockiness leads to better phase segregation, with the anionic moieties unevenly distributed in the interface leaving big hydrophobic patches of polymer/water contact. The vehicular diffusion of water and hydronium was not affected by the blockiness.

Nanophase-Segregation and Transport in Nafion 117 from Molecular Dynamics Simulations: Effect of Monomeric Sequence

Nafion polyelectrolyte is widely used in polymer electrolyte membrane fuel cells (PEMFC) due to its high proton conductivity. The properties of hydrated Nafion are attributed to its nanophase-segregated structure in which hydrophilic clusters are embedded in a hydrophobic matrix. However, there has been little characterization of how the monomeric sequence of the Nafion chain affects the nanophase-segregation structure and transport in hydrated Nafion. To study such properties, we carried out molecular dynamics (MD) simulations of Nafion 117 using two extreme monomeric sequences: one very blocky and other very dispersed. Both produce a nanophase-segregated structure with hydrophilic and hydrophobic domains. However, the blocky Nafion leads to a characteristic dimension of phase-segregation that is ∼60% larger than for the dispersed system. We find that the water−polymer interface is heterogeneous, consisting of hydrophilic patches (water contacting sulfonate groups of Nafion) and hydrophobic patches (water contacting fluorocarbon group). The distribution of the hydrophilic and the hydrophobic patches at the interface (i.e., the heterogeneity of interface) is much more segregated for blocky Nafion. This leads to a water diffusion coefficient for the dispersed case that is ∼25% smaller than for the blocky case (0.46 × 10-5 vs 0.59 × 10-5 cm2/s at 300 K). The experimental value (0.50 × 10-5 cm2/s) is within the calculated range. On the other hand, we find that the vehicular diffusion of hydronium is not affected significantly by the monomeric sequence. These results should be useful in optimizing the properties of Nafion and as targets for developing other membranes to replace Nafion in PEMFC and other applications.