Proton NMR method for the quantitative determination of the water content of the polymeric fluorosulfonic acid Nafion-H

Hydrogen-bond network distortion of water in the soft confinement of Nafion membrane

A Compton spectroscopy investigation is carried out in hydrated Nafion membranes, enabling identification of distortions in the hydrogen-bond distribution of the polymer hydrating water by means of the subtle changes reflected by the Compton profiles. Indeed, deformations of the Compton profiles are observed when varying hydration, and two different bonding kinds are associated with the water molecules: at low hydration, water surrounds the sulfonic groups, while on increasing hydration, water molecules occupy the interstitial cavities formed upon swelling of the membrane. The analysis is proposed in terms of averaged OH bond length variation. A sizable contraction of the OH distance is observed at low hydration (∼0.09 Å), while at higher hydration levels, the contraction is smaller (∼0.02 Å) and the OH bond length is closer to bulk water. An evaluation of the electron kinetic energy indicates that the spatial changes associated with the water distribution correspond to a consistent binding energy increase. Distinct temperature dependences of each water population are observed, which can be straightly related to water desorption into ice on cooling below the freezing point.

Molecular and Ionic Diffusion in Ion Exchange Membranes and Biological Systems (Cells and Proteins) Studied by NMR

The results of NMR, and especially pulsed field gradient NMR (PFG NMR) investigations, are summarized. Pulsed field gradient NMR technique makes it possible to investigate directly the partial self-diffusion processes in spatial scales from tenth micron to millimeters. Modern NMR spectrometer diffusive units enable to measure self-diffusion coefficients from 10-13 m2/s to 10-8 m2/s in different materials on 1 H, 2 H, 7 Li, 13 C, 19 F, 23 Na, 31 P, 133 Cs nuclei. PFG NMR became the method of choice for reveals of transport mechanism in polymeric electrolytes for lithium batteries and fuel cells. Second wide field of application this technique is the exchange processes and lateral diffusion in biological cells as well as molecular association of proteins. In this case a permeability, cell size, and associate lifetime could be estimated. The authors have presented the review of their research carried out in Karpov Institute of Physical Chemistry, Moscow, Russia; Institute of Problems of Chemical Physics RAS, Chernogolovka, Russia; Kazan Federal University, Kazan, Russia; Korea University, Seoul, South Korea; Yokohama National University, Yokohama, Japan. The results of water molecule and Li+, Na+, Cs+ cation self-diffusion in Nafion membranes and membranes based on sulfonated polystyrene, water (and water soluble) fullerene derivative permeability in RBC, casein molecule association have being discussed.

Insights into Interfacial Water Structuring at the Nafion Surface by T1-Weighted Magnetic Resonance Imaging

T₁-weighted magnetic resonance images of water in the surroundings of a Nafion surface allowed the identification of the presence of a low-mobility zone (LMZ), 60 μm thick, consisting of water molecules structured in a hydrogen-bonding network, promoted by the presence of the acidic protons on the surface of the sulphonated polymer. In parallel, the exclusion zone (EZ) was assessed by observing in optical microscopy the distribution of microspheres suspended in the medium in contact with the Nafion membrane. It was found that the LMZ and the EZ do not correspond: in fact, the former is thinner and more stable over time than the latter and they behave differently when ions are present in the medium in which Nafion is immersed.

Comprehensive Picture of Water Dynamics in Nafion Membranes at Different Levels of Hydration

¹H NMR spectroscopy is employed to study the long-range (diffusion) and short-range (relaxation) motions of water in the hydrophilic channels of Nafion at different stages of hydration, λ_water. From Bloembergen, Purcell, and Pound analysis of the temperature-dependent ¹H spin-lattice relaxation rates, the coexistence of two motional modes for λ_water < 9 was observed, which can be attributed to the nonfreezing behavior of water molecules. At higher hydration levels, a clear transition between two different motional modes was detected associated with the freezing of bulk water. The hydration-level-dependent diffusion ¹H NMR studies showed a steep increase in diffusion coefficients at 6 ≤ λ_water ≤ 9 followed by a linear increase with the increasing hydration level (λ_water ≥ 12). Taken together the correlation of long- and short-range motion studies allowed us to establish a comprehensive picture of the hydration shell formation for the sulfonic acid groups in Nafion dependent on λ_water that can be divided into three characteristic regions. These include the formation of the first hydration shell at λ_water ≈ 3 (region I), the second hydration shell at λ_water ≈ 8 (region II), and finally (iii) the presence of bulk water above λ_water ≥ 10 (region III).

Porous structure of ion exchange membranes investigated by various techniques

A comparative review of various techniques is provided: mercury intrusion porosimetry, nitrogen sorption porosimetry, differential scanning calorimetry (DSC)-based thermoporosimetry, and standard contact porosimetry (SCP), which allows determining pore volume distribution versus pore radius/water binding energy in ion-exchange membranes (IEMs). IEMs in the swollen state have a labile structure involving micro-, meso- and macropores, whose size is a function of the external water vapor pressure. For such materials, the most appropriate methods for quantifying their porosity are DSC and SCP. Especially significant information is given by the SCP method allowing measuring porosimetric curves in a very large pore size range from 1 to 10⁵nm. Experimental results of water distribution in homogeneous and heterogeneous commercial and modified IEMs are presented. The effect of various factors on water distribution is reviewed, i.e. nature of polymeric matrix and functional groups, method for membrane preparation, membrane ageing. A special attention is given to the effect of membrane modification by embedding nanoparticles in their structure. The porosimetric curves are considered along with the results of electrochemical characterization involving the measurements of membrane conductivity, as well as diffusion and electroosmotic permeability. It is shown that addition of nanoparticles may lead to either increase or decrease of water content in IEMs, different ranges of pore size being affected. Hybrid membranes modified with hydrated zirconium dioxide exhibit much higher permselectivity in comparison with the pristine membranes. The diversity of the responses of membrane properties to their modification allows for formation of membranes suitable for fuel cells, electrodialysis or other applications.

Structure of perfluorinated membranes investigated by method of standard contact porosimetry

The results of investigation of various factors influencing water distribution in perfluorinated membrane structure by method of standard contact porosimetry are summarized. The Nafion membranes (Dupon de Nemoure, USA) and MF-4SK membranes ("Plastpolymer", Russia) were the objects of the research. The influence of production process and conditioning method on porosimetric curves of perfluorinated membrane is discussed. New results related to the porosity of perfluorinated membranes after reinforcing fabric introduction and processing by organic solvents are reported. The role of the modifying components of various nature in the shaping of transport channels in perfluorinated membrane is studied. The influence of polyaniline and hydrogen zirconium phosphate on water distribution in membrane structure is revealed. The correlation between the maximum porosity value of the membrane and its diffusion and electroosmotic permeability, as well as between the fraction of the gel pore volume and membrane selectivity is established. It allows the prediction of possible changes in the structural characteristics and also in the transport properties of the membranes under the influence of the modifying components of different types and various operating conditions.

2H double- and zero-quantum filtered NMR spectroscopy for probing the environments of water in Nafion

Multiple quantum 2H NMR spectroscopy is used to study the structure and dynamics of D2O in Nafion membranes as a function of membrane hydration. By employing both double- and zero-quantum filtered experiments, residual quadrupolar coupling constants and T2 relaxation values are obtained. The residual couplings vary from 240 to 20 Hz and the T2 values range from 20 to 180 ms, with the high hydration values having smaller couplings and longer T2 values. Analysis of the data using a water-exchange model suggests that the changes in parameters arise from a change in the fraction of time water spends in the anisotropic environments and not from changes in the order parameters that characterize the anisotropic sites. It has been found that a two-site model is needed to accurately fit the spectra above a hydration level of 10 D2O per sulfonate, with the second site having a negligible residual quadrupolar coupling. The data supports a model with two different hydration layers at high hydration and can be understood in terms of the recently proposed parallel-channel model for Nafion hydration.

Characterization of water in proton-conducting membranes by deuterium NMR T1 relaxation

(2)H T(1) NMR relaxation was used to characterize the molecular motion of deuterated water ((2)H(2)O) in Aquivion E87-05, Nafion 117, and sulfonated-Radel proton-exchange membranes. The presence of bound water with solid character was confirmed by the dependence of the (2)H T(1) relaxation on the magnetic field of the spectrometer. By comparing the (2)H T(1) relaxation times of the different membranes that were equilibrated in varying humidities, the factors that influence the state of water in the membranes were identified. At low levels of hydration, the molecular motion of (2)H(2)O is affected by the acidity and mobility of the sulfonic acid groups to which the water molecules are coordinated. At higher levels of hydration, the molecular motion of (2)H(2)O is affected by the phase separation of the hydrophilic/hydrophobic domains and the size of the hydrophilic domains.

Moving Beyond Mass-Based Parameters for Conductivity Analysis of Sulfonated Polymers

The proton conductivity of polymer electrolytes is critical for fuel cells and has therefore been studied in significant detail. The conductivity of sulfonated polymers has been linked to material characteristics to elucidate trends. Mass-based measurements based on water uptake and ion exchange capacity are two of the most common material characteristics used to make comparisons between polymer electrolytes, but they have significant limitations when correlated to proton conductivity. These limitations arise in part because different polymers can have significantly different densities and because conduction occurs over length scales more appropriately represented by volume measurements rather than mass. Herein we establish and review volume-related parameters that can be used to compare the proton conductivity of different polymer electrolytes. Morphological effects on proton conductivity are also considered. Finally, the impact of these phenomena on designing next-generation sulfonated polymers for polymer electrolyte membrane fuel cells is discussed.

Theoretical Studies on the Structure of Hydration in Perfluorinated Lithium Salt Membranes

Using an ab initio molecular orbital (MO) method, the normal frequencies are calculated for perfluorinated lithium sulfonate and carboxylate membranes by construction of a cluster model, which severs the ion core from the polymer chain, and then analysis of the experimentally observed infrared (IR) spectra is carried out. During the process of dehydration, small sharp peaks at about 3650 and 3700 cm(-1) appeared on the shoulder of the broad band at about 3500 cm(-1). These sharp peaks are identified as the symmetric and asymmetric stretching modes of the free water molecule. Furthermore, by estimation of the evaporation ratio based on thermochemical analysis, it can be assumed that the first hydration shells are naked in some part of the ion core, thereby allowing evaporation to take place within the external hydration shell during the dehydration process.