Relationships of Acid and water content to proton transport in statistically sulfonated proton exchange membranes: variation of water content via control of relative humidity

An in-depth analysis for proton exchange membranes to examine the effects of acid concentration and effective proton mobility upon proton conductivity as well as their relationship to water content was carried out on two main-chain, statistically sulfonated polymers at 25 degrees C. These polymer systems consisted of poly(ethylenetetrafluoroethylene-graft-polystyrenesulfonic acid) (1) and sulfonated trifluorostyrene (BAM) membrane (2). Nafion (3) was used for comparison. Water content (as represented by Xv, the water volume fraction, where Xv = volume of water in hydrated PEM / volume of hydrated PEM), for each sample was varied by adjusting the relative humidity (RH) of the membrane environment from 50% to 98%. It was found that, at low RH (RH < 70%), the major factor determining proton conductivity is proton mobility. In order to remove the differences in acid strength for the membranes, proton mobility values at infinite dilution (Xv = 1.0) and 25 degrees C were calculated and found to be 2.6 +/- 0.2 x 10-3 (average of 1a-c), 1.6 +/- 0.3 x 10-3 (average of 2a-e), and 2.32 +/- 0.01 x 10-3 cm2 s-1 V-1 (3). These were then compared to the theoretical value for the mobility of a free proton at infinite dilution and to previously reported data. Possible differences in tortuosity and the juxtaposition of acid groups are proposed in order to account for the significant deviations of all samples from the theoretical value.     

Proton transport in a binary biomimetic solution revealed by molecular dynamics simulation

We report the simulation results of the proton transport in a binary mixture of amphiphilic tetramethylurea (TMU) molecules and water. We identify different mechanisms that either facilitate or retard the proton transport. The efficiency of these mechanisms depends on the TMU concentration. The overall picture is more complicated than a recent suggestion that the presence of amphiphilic molecules suppresses the proton mobility by slowing down the reorientation of the surrounding water molecules. It has also been suggested that the hydronium ion induces local water orientational order, which results in an ordered region that has to move along with the proton potentially slowing down the proton transport as suggested by experiment. We find that water-wire like structures formed at low amphiphile concentrations facilitate proton transfer, and reduction of the hydrogen bond connectivity induced at high concentrations retards it.     

Structure and energetics of the hydronium hydration shells

Proton solvation and proton mobility are both subjects of great interest in chemistry and biology. Here we have studied the hydration shells of H3O+ at temperatures ranging from 260 to 340 K using the multistate empirical valence-bond methodology (MS-EVB2). We have calculated the radial distribution functions for the protonium and its solvation shells. Furthermore, we have determined the Gibbs energy and the enthalpy for hydrogen bonds donated or accepted by the first two solvation shells, in comparison to bulk water. We find systematic bond-energy differences that appear to agree with a recent IR study on proton hydration. Implications of our results to various proton mobility mechanisms are discussed.     

Proton transfer and the mobilities of the H+ and OH- ions from studies of a dissociating model for water

Hydrogen (H(+)) and hydroxide (OH(-)) ions in aqueous solution have anomalously large diffusion coefficients, and the mobility of the H(+) ion is nearly twice that of the OH(-) ion. We describe molecular dynamics simulations of a dissociating model for liquid water based on scaling the interatomic potential for water developed by Ojama?e-Shavitt-Singer from ab initio studies at the MP2 level. We use the scaled model to study proton transfer that occurs in the transport of hydrogen and hydroxide ions in acidic and basic solutions containing 215 water molecules. The model supports the Eigen-Zundel-Eigen mechanism of proton transfer in acidic solutions and the transient hyper-coordination of the hydroxide ion in weakly basic solutions at room temperature. The free energy barriers for proton transport are low indicating significant proton delocalization accompanying proton transfer in acidic and basic solutions. The reorientation dynamics of the hydroxide ion suggests changes in the proportions of hyper-coordinated species with temperature. The mobilities of the hydrogen and hydroxide ions and their temperature dependence between 0 and 50 °C are in excellent agreement with experiment and the reasons for the large difference in the mobilities of the two ions are discussed. The model and methods described provide a novel approach to studies of liquid water, proton transfer, and acid-base reactions in aqueous solutions, channels, and interfaces.     

Composite Polymer Electrolytes for Fuel Cell Applications: Filler-Induced Effect on Water Sorption and Transport Properties

Nafion- and sulfonated polysulfone (SPS)- based composite membranes were prepared by incorporation of SnO₂ nanoparticles in a wide range of loading (0 35 wt. %). The composites were investigated by differential scanning calorimetry, dynamic vapor sorption and electrochemical impedance spectroscopy to study the filler effect on water sorption, water mobility, and proton conductivity. A detrimental effect of the filler was observed on water mobility and proton conductivity of Nafion-based membranes. An increase in water mobility and proton conductivity was instead observed in SPS-based samples, particularly at low hydration degree. Analysis of the water sorption isotherms and states of water revealed that the presence of SnO₂ in SPS enhances interconnectivity of hydrophilic domains, while not affecting the Nafion microstructure. These results enable the design of suitable electrolyte materials that operate in proton exchange membrane fuel cell conditions.     

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

The effect of ion exchange capacity (IEC) on the water sorption properties of high IEC, short side chain (SSC) PFSA ionomer membranes, and the relationships between water content, proton conductivity, proton mobility, water permeation, oxygen diffusion, and oxygen permeation are investigated. SSC PFSA ionomer membranes possessing 1.3, 1.4, and 1.5 mmol g(-1) IEC are compared to a series of long side chain (LSC) PFSA ionomer membranes ranging in IEC from 0.9 to 1.13 mmol g(-1). At 25 °C, fully-hydrated SSC ionomer membranes are characterized as possessing higher water contents (56-75 vol%), moderate λ values (15-18), high analytical acid concentrations (2-2.8 M), and moderate conductivity (88-115 mS/cm); but lower than anticipated effective proton mobility. Complementary measurements of water permeability, oxygen diffusion, and oxygen permeability also yield lower than expected values given their much higher water contents. Potential benefits afforded by reducing the side chain length of PFSA ionomer membranes, such as increased crystallinity, higher IEC, and high hydrated acid concentration are offset by a less-developed, frustrated hydrophilic percolation network, which provides a motivation for future improvements of transport properties for this class of material.     

Ab initio molecular-dynamics simulation of aqueous proton solvation and transport revisited

The solvation and transport of the hydrated excess proton is studied using the Car-Parrinello molecular-dynamics (CPMD) simulation method. The simulations were performed using BLYP and HCTH gradient-corrected exchange-correlation energy functionals. The fictitious electronic mass was chosen to be small enough so that the underlying water structural and dynamical properties were converged with respect to this important CPMD simulation parameter. An unphysical overstructuring of liquid water in the CPMD simulations using the BLYP functional resulted in the formation of long-lived hydrogen-bonding structures involving the excess proton and a particular (special) water oxygen. The excess proton was observed to be attracted to the special oxygen through the entire length of the BLYP CPMD simulations. Consequently, the excess proton diffusion was limited by the mobility of the special oxygen in the slowly diffusing water network and, in turn, the excess proton self-diffusion coefficient was found to be significantly below the experimental value. On the other hand, the structural properties of liquid water in the HCTH CPMD simulation were seen to be in better agreement with experiment, although the water and excess proton diffusions were still well below the experimental value.     

Special pair dance and partner selection: elementary steps in proton transport in liquid water

Conditional and time-dependent radial distribution functions reveal the details of the water structure surrounding the hydronium during the proton mobility process. Using this methodology for classical multistate empirical valence bond (MS-EVB) and ab initio molecular dynamics trajectories, as well as quantal MS-EVB trajectories, we supply statistical proof that proton hops in liquid water occur by a transition from the H3O+[3H2O] Eigen-complex, via the H5O2+ Zundel-complex, to a H3O+[3H2O] centered on a neighboring water molecule. In the "resting period" before a transition, there is a distorted hydronium with one of its water ligands at a shorter distance and another at a longer distance than average. The identity of this "special partner" interchanges rapidly within the three first-shell water ligands. This is coupled to cleavage of an acceptor-type hydrogen bond. Just before the transition, a partner is selected by an additional translation of the H3O+ moiety in its direction, possibly enabled by loosening of donor-type hydrogen bonds on the opposite side. We monitor the transition in real time, showing how the average structure is converted to a distorted H5O2+ cation constituting the transitional complex for proton hopping between water molecules.     

Transport properties of composite membranes containing silicon dioxide and Nafion

Silicon dioxide (SiO₂) nanoparticles were incorporated into Nafion 115 membranes using the sol–gel method in order to investigate their effect on water retention/transport, proton concentration, effective proton mobility, and proton conductivity. By adjusting the sol–gel reaction time, Nafion/SiO₂ membranes were fabricated with SiO₂ content ranging from 5.9 to 33.3 wt%. Because the density of the membranes decreased with increasing SiO₂ content and because dimensional changes with swelling in water of the composite membranes were less than that of unmodified Nafion 115 despite having increased water content, the theory that rigid scaffolding is formed inside the membrane is supported. Water content increases with increasing SiO₂ content due to void space formed inside the membrane. This increase in water content dilutes the protons in the membrane leading to lower proton concentration and therefore lower proton conductivity. A decreasing effective proton mobility with increasing SiO₂ content, likely due to an increase in the tortuosity of the proton-conducting pathway, also contributes to the decreasing conductivity. However, as evidenced by the similar water vapour permeance values, the SiO₂ nanoparticles do not increase the effective tortuosity of the water vapour transmission pathways.     

Breakdown of Born‐Oppenheimer Approximation as a Physical Mechanism for Ultrafast Hydrogen Migrations in Strong Laser Driven Molecules

We propose a physical mechanism based on breakdown of the Born‐Oppenheimer approximation to rationalize the ultrafast hydrogen migration in strong laser driven isomerization reactions. A three nuclei (proton, donor, and acceptor) model is employed to develop a three step solution scheme. The proton‐donor Coulomb repulsion is shown to be responsible for the high proton mobility. We identify a proton tunneling process and use the Keldysh‐Faisal‐Reiss theory to calculate the tunneling probability. The effect of laser parameters (intensity, frequency, polarization, and pulse duration) has been studied and found to be consistent with recent experiments.     

Water sorption and diffusion coefficients of protons and water in PVDF-g-PSSA polymer electrolyte membranes

Water sorption properties, proton NMR spectra, and diffusion of water and protons in poly(vinylidene fluoride)-graft-polystyrene sulfonic acid (PVDF-g-PSSA) polymer electrolyte membranes were studied. Sorption curves for the membranes with different degrees of grafting in protonated and Na⁺ form were measured by equilibrating the membranes over saturated salt solutions. The membrane water content was found to be sensitive to changes in relative humidity (RH). The water/sulfonic acid ratio λ for the protonated samples was around 2 at 20% RH and increased to λ ∼ 30 at 100%. Proton NMR, pulsed field gradient proton NMR (PFG-NMR), and impedance measurements were made on membranes with different λ. In the proton NMR spectra only one peak was found, originating from the water in the membrane. The chemical shift of the peak was found to be dependent on the counterion and the water content. The water self-diffusion coefficients D_H2O, measured by PFG-NMR, increased with degree of grafting and water content of the membranes. The proton conductivity and the calculated proton mobility decreased more steeply than the D_H2O with decreasing water content. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2893–2900, 1999     

Structure and Mobility Pattern of Proton-Containing Groups in Hydrates of Titanium,Zirconium and Tin Acid Phosphates; Their Conducting and Ion Exchanging Properties

Abstract

Phosphates of tetravalent elements are practically important for ion exchange, catalysis and conductivity. This study deals with a number of hydrates of titanium, tin and zirconium phosphates. PMR data show that the structure of water molecules in hydrates is slightly distorted, and at temperatures higher than 160 K water has high translation mobility. NMR ³¹P proves HPO² ₄ dissociation to be growing with increase of temperature. Energetic parameters of this process are determined. Close values of anion dissociation enthalpy (0, 16/2/Ev) and obtained activation energy of conductivity for di- and monohydrates (0,17/2/Ev) show tunnel pattern of proton transfer along H-bond direction, This type of correlation was not observed in anhydrous compounds. That can be explained by impossibility of anion proton tunneling because of H-bond weakening. Proton conductivity of acid phosphates was studied. Ten-fold decrease of conductivity at room temperature with the loss of each water molecule proves H₂O participation in proton transport. Mechanism of this process is discussed with the use of NMR data. Dependence of water mobility and conductivity level on the degree of crystallinity is also discussed. With the help of NMR-data processes of ion exchange in tin and zirconium acid phosphates, as well as the state of developed salt forms were studied. Presence of lithium with high mobility in Li₂Sn(PO₄)₂ ·nF₂O was established.     

Structure and dynamics of N,N-diethyl-N-methylammonium triflate ionic liquid, neat and with water, from molecular dynamics simulations

We investigated by means of molecular dynamics simulations the properties (structure, thermodynamics, ion transport, and dynamics) of the protic ionic liquid N,N-diethyl-N-methylammonium triflate (dema:Tfl) and of selected aqueous mixtures of dema:Tfl. This ionic liquid, a good candidate for a water-free proton exchange membrane, is shown to exhibit high ion mobility and conductivity. The radial distribution functions reveal a significant long-range structural correlation. The ammonium cations [dema](+) are found to diffuse slightly faster than the triflate anions [Tfl](-), and both types of ions exhibit enhanced mobility at higher temperatures, leading to higher ionic conductivity. Analysis of the dynamics of ion pairing clearly points to the existence of long-lived contact ion pairs. We also examined the effects of water through characterization of properties of dema:Tfl-water mixtures. Water molecules replace counterions in the coordination shell of both ions, thus weakening their association. As water concentration increases, water molecules start to connect with each other and then form a large network that percolates through the system. Water influences ion dynamics in the mixtures. As the concentration of water increases, both translational and rotational motions of [dema](+) and [Tfl](-) are significantly enhanced. As a result, higher vehicular ionic conductivity is observed with increased hydration level.     

Proton mobility in thin ice films: a revisit

We have examined proton transport through an ice film in the temperature range 73-140 K by initially adding hydronium ions into the interior of the film and then monitoring the build-up of hydronium ion population at the film surface. The result confirms that the proton exhibits limited mobility in the ice film at low temperature, but it becomes highly mobile at temperature above 130 K. Based on this result we suggest an explanation of the anomalous experimental observations in the literature for the proton mobility in ice films.     

Investigation of water mobility and diffusivity in hydrating micronized low-substituted hydroxypropyl cellulose,hydroxypropylmethyl cellulose,and hydroxypropyl cellulose matrix tablets by magnetic resonance imaging (MRI)

The water mobility and diffusivity in the gel-layer of hydrating low-substituted hydroxypropyl cellulose (LH41) tablets with or without a drug were investigated by magnetic resonance imaging (MRI) and compared with those properties in the gel-layer of hydroxypropylmethyl cellulose (HPMC) and hydroxypropyl cellulose (HPC) tablets. For this purpose, a localized image-analysis method was newly developed, and the spin-spin relaxation time (T(2)) and apparent self-diffusion coefficient (ADC) of water in the gel-layer were visualized in one-dimensional maps. Those maps showed that the extent of gel-layer growth in the tablets was in the order of HPC>HPMC>LH41, and there was a water mobility gradient across the gel-layers of all three tablet formulations. The T(2) and ADC in the outer parts of the gel-layers were close to those of free water. In contrast, these values in the inner parts of the gel-layer decreased progressively; suggesting that the water mobility and diffusivity around the core interface were highly restricted. Furthermore, the correlation between the T(2) of (1)H proton in the gel-layer of the tablets and the drug release rate from the tablets was observed.     

IR spectroscopic properties of H(MeOH)n + clusters in the liquid phase: evidence for a proton wire

The long-standing problem of understanding the nature of the "excess proton" in acidified water is simplified by studying the proton in methanol. The 3D network of hydrogen bonds in H(aq) + is reduced to a 1D problem. Infrared spectroscopic characterization of linear chain methanol proton solvates in H(CH3OH)n + for n=2-8 provides insight into some of the puzzling IR spectral features associated with O-H-O vibrations. These include the virtual disappearance of otherwise strong bands from H-bonded methanol molecules adjacent to symmetrical O-H+-O groups. The data indicate that a chain of up to four O--HO bonds either side of this group can act as an electrical wire to separate positive charge. This suggests a refinement of the Grotthuss proton-hopping mechanism for explaining the anomalously high mobility of H+ in H-bonded media.     

Water structure and dynamics at a silica surface: Pake doublets in 1H NMR spectra

Detailed knowledge about the dynamics and structure of liquids in the vicinity of a solid surface is important in several fields of research. In this study a homogeneous model system of colloidal and nonporous silica particles with a narrow particle size distribution was used to examine such properties of adsorbed water and 1-heptanol. Doublet (1)H water resonances ("Pake doublets") indicate a preferred spatial orientation for the water molecules, as well as a lower molecular density in the surface-induced water structures compared to bulk water. These surface-induced structures are found to extend at least 8 nm from the silica surface. T(1) relaxation measurements at several temperatures indicate weaker H-bonding in the adsorbed water compared to bulk water. T(2) relaxation measurements at several temperatures reveal the presence of two water phases and give quantitative information on the mobility of water molecules and proton exchange processes. The presence of 1-heptanol changes the water characteristics, primarily in the water phase closer to the surface, where water molecules experience decreased translational and increased rotational freedom. In the absence of water, adsorbed 1-heptanol forms surface aggregates encompassing several molecular layers, where the first adsorbed layer shows severe restrictions in mobility and subsequent layers are more mobile.     

Effects of dielectric saturation and ionic screening on the proton self-diffusion coefficients in perfluorosulfonic acid membranes

Proton transport in perfluorosulfonic acid (PFSA) membranes is investigated through a statistical mechanical model that includes the effects of the interaction of the tethered sulfonate groups with both the water and solvated protons. We first derive a potential that describes the electrostatic field due to the dissociated sulfonic acid groups by extending the work of Gronbech-Jensen et al. [ Mol. Phys. 92, 941 (1997)] to a finite array of point charges. A highly convergent series is obtained which includes the effects of screening due to the protons. We then investigate the effects of both dielectric saturation and two distinct formulations of ionic screening on the proton self-diffusion coefficient in Nafion membranes over a range of water contents. Our computations show that the two phenomena (i.e., dielectric saturation and ionic screening) under constant temperature conditions result in canceling affects. Our calculations provide a radial dependence of the proton mobility suggesting that the dominant self-diffusion occurs in the central region of the pores, well separated from the sulfonate groups. Through comparison of our calculated diffusion coefficients with the experimental values we derived a slightly smaller average separation distance of the hydronium ion from the sulfonate ions than suggested by either electronic structure calculations or multistate empirical valence bond molecular-dynamics simulations.     

Proton mobility in hydrates of inorganic acids and acid salts

Published data on the mechanisms of hydrogen ion transport in solids and aqueous solutions are described systematically, including defect formation, rotational mobility of proton-containing groups, proton hopping along a hydrogen bond, proton translational mobility, and proton conduction. Resorting to the authors" theoretical results and published data, the main criteria for the selection of systems possessing high proton mobility are formulated.     

Alcohol and proton transport in perfluorinated ionomer membranes for fuel cells

To clarify the transport mechanisms of alcohols and proton in perfluorosulfonated ionomer (PFSI) membranes for fuel cells, four membranes having different equivalent weight (EW) values were examined. Membranes were immersed in methanol, ethanol, and 2-propanol to prepare a total of 12 samples, and membrane swelling, mass (alcohol and proton) transports, and interactions between alcohols and proton were investigated systematically in the fully penetrated state. The membrane expansion fraction theta and alcohol content lambda increased with decreasing the EW value for all the samples. The self-diffusion coefficients (D's) of the alkyl group and of OH (including protons) were measured separately by the pulsed-gradient spin-echo (PGSE)-NMR method and the D's also increased with decreasing the EW value. These results implied that the alcohols penetrate into the hydrophilic regions of the PFSI membranes and diffuse through the space expanded by the alcohols. The ionic cluster regions formed by the alcohols resemble those induced by water in the water swollen membrane, where protons dissociated from sulfonic acid groups transport through the regions together with water molecules. The D values decreased with increasing the molecular weight of alcohols. This trend was supported by activation energies Ea estimated from the Arrhenius plots of D in the temperature range from 30 to -40 degrees C. The PGSE-NMR measurements also revealed that protons move faster than the alkyl groups in the membranes. The proton transport by the Grotthuss (hopping) mechanism was facilitated by the increase of the alcohol content and the decrease of the molecular weight. This result was also supported by the experimental results of proton conductivity kappa and mobility u(H(+)). Density functional theory (DFT) calculations of the interaction energy DeltaE(int) between proton and alcohol (including OH) showed that the /DeltaE(int)/ increases with increasing the molecular weight of alcohols, which is in a inverse relationship with the kappa and u(H(+)) values. The proton transport depends strongly on the DeltaE(int) in the membranes.