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A study of capillary porous structure and sorption properties of nafion proton-exchange membranes swollen in water
Journal of The Electrochemical Society
Abstract
The standard porosimetry method (SPM) was used to study capillary porous and sorption properties of Nafion® proton-exchange membranes. Pore volume-size and surface-size distributions were recorded together with water distributions as a function of capillary pressure and desorption isotherms. The measurements were performed at 20 and 80°C; only a small temperature effect was found for the porous volume structure. Using these data, the charge density at the inner pore surfaces was evaluated. The estimate suggested only a small degree of diffuseness of the proton double layer at the inner pore surface.
... The water content in the membrane is the fraction of the channels that has expanded to accommodate the liquid water [28]. It is determined by the integral pore size distribution function, V(r C ), normalized to the saturation water content for the membrane (λ S ), which is equivalent to the maximum volume of the pores in the membrane [33]: ...
... In the case of Nafion 112 the maximum water content is 0.44 cm 3 cm −3 [33]. ...
... A straight line is fitted to the integral pore size distribution data [33] (V(r C ) versus r C ) of Nafion 112. Using this relation and Eq. ...
A model and an analytical solution for the model are presented for the resistance of the polymer electrolyte membrane of a H2/O2 fuel cell. The solution includes the effect of the humidity of the inlet gases and the gas pressure at the anode and the cathode on the membrane resistance. The accuracy of the solution is verified by comparison with experimental data. The experiments were carried out with a Nafion 112 membrane in a homemade fuel cell test station. The membrane resistances predicted by the model agree well with those obtained during the experiments.
... Kreuer finds that the water distribution forms continuous pathways throughout the membrane. The permeation of the liquid phase explains the high conductivity of Nafion sorption properties of Nafion membranes [34]. They found that the membranes contain a connected system of pores of a wide range of sizes. ...
... Previous work such as that done by Choi et al. [37] [38], and Futerko and Hsing [39], determine sorption in the membrane as a whole in order to quantify sorption isotherms. The present work focuses on the sorption on the scale of micropores [34], in order to help quantify the complex microstructure of Nafion as it sorbs water, in particular the distribution and connectivity of the hydrophilic domain which dramatically impacts the protonic conductivity. We noted earlier that geometric conditions affect the stability of liquid water in hydrophobic confines and we shall examine in particular the stability of water in a single microscopic pore by means of a parametric analysis of the wettability of a Nafion micropore. ...
... To compute the surface density of protons we assume S p = C ¡V T m · S ¢ −1 , whereV T m is the specific molar volume of the membrane, S is the specific pore surface area, and C is a constant we use to vary the value. We take the values,V T m = 537 cm 3 /mol and S = 210 m 2 /cm 3 for Nafion 117 from [38], and [34], respectively; with C = 1, S p = 8.87 × 10 −6 mol/m 2 . To compute G − G 0 we use a value of θ = 98 • taken from Zawodzinski et al. [52], which was measured at p r = 1. ...
Abstract In this work a model for the wetting and swelling of pores with water within a Nafion membrane,is developed. This model is based on minimizing all contributions to the total free energy of the proposed system. Wefind that equilibrium state depends on entropic mixing forces and energetic surface forces. The wetting of the pore relies on
... The clusters, channels, some defects of the structure, spaces between ion-exchange resin particles, the binder and the fabrics of the cloth form a system of pores in an IEMs, whose size varies from a few nm to 1-2 µm [13,[37][38][39]. The investigations of the pore size distribution made using the standard contact porosimetry (Divisek et al. [40]; Berezina et al. [37] and Kononenko et al. [38,39]) and differential scanning calorimetry (DSC)-based thermoporosimetry (Bryk et al. [41]; Berezina et al. [42]; Kononenko et al. [39]) methods have shown that the homogeneous IEMs (namely, perfluorinated MF-4SK membranes (Plastpolymer, Russia) [39,41,42]), as well as Nafion-112, Nafion-115, Nafion-117 membranes [40]) do not contain macropores (the pores, whose effective radius size is larger than 100 nm), while heterogeneous membranes have such pores. It was found [37,39,42] that the pore size distribution of Russian heterogeneous MK-40 and MA-40 membranes has two maxima, the first one is at about 10 nm (which is the range of micro-and mesopores) and the second one relates to macropores with size of about 1000 nm. ...
... The clusters, channels, some defects of the structure, spaces between ion-exchange resin particles, the binder and the fabrics of the cloth form a system of pores in an IEMs, whose size varies from a few nm to 1-2 µm [13,[37][38][39]. The investigations of the pore size distribution made using the standard contact porosimetry (Divisek et al. [40]; Berezina et al. [37] and Kononenko et al. [38,39]) and differential scanning calorimetry (DSC)-based thermoporosimetry (Bryk et al. [41]; Berezina et al. [42]; Kononenko et al. [39]) methods have shown that the homogeneous IEMs (namely, perfluorinated MF-4SK membranes (Plastpolymer, Russia) [39,41,42]), as well as Nafion-112, Nafion-115, Nafion-117 membranes [40]) do not contain macropores (the pores, whose effective radius size is larger than 100 nm), while heterogeneous membranes have such pores. It was found [37,39,42] that the pore size distribution of Russian heterogeneous MK-40 and MA-40 membranes has two maxima, the first one is at about 10 nm (which is the range of micro-and mesopores) and the second one relates to macropores with size of about 1000 nm. ...
Artificial charged membranes, similar to the biological membranes, are self-assembled nanostructured materials constructed from macromolecules. The mutual interactions of parts of macromolecules leads to phase separation and appearance of microheterogeneities within the membrane bulk. On the other hand, these interactions also cause spontaneous microheterogeneity on the membrane surface, to which macroheterogeneous structures can be added at the stage of membrane fabrication. Membrane bulk and surface heterogeneity affect essentially the properties and membrane performance in the applications in the field of separation (water desalination, salt concentration, food processing and other), energy production (fuel cells, reverse electrodialysis), chlorine-alkaline electrolysis, medicine and other. We review the models describing ion transport in ion-exchange membranes and electromembrane systems with an emphasis on the role of micro- and macroheterogeneities in and on the membranes. Irreversible thermodynamics approach, “solution-diffusion” and “pore-flow” models, the multiphase models built within the effective-medium approach are examined as the tools for describing ion transport in the membranes. 2D and 3D models involving or not convective transport in electrodialysis cells are presented and analysed. Some examples are given when specially designed surface heterogeneity on the membrane surface results in enhancement of ion transport in intensive current electrodialysis.
... Besides, it is well known that the properties and water content of the membrane play important roles in the factors dominating the cell performance. Several research efforts have been dedicated to the investigation for water management problems in the membrane [18][19][20][21][22][23][24][25][26][27][28][29][30]. It has been pointed out by Divisek et al. [25] that a dry membrane will swell in water and the membrane volume (or thickness) depends on its water content. ...
... Several research efforts have been dedicated to the investigation for water management problems in the membrane [18][19][20][21][22][23][24][25][26][27][28][29][30]. It has been pointed out by Divisek et al. [25] that a dry membrane will swell in water and the membrane volume (or thickness) depends on its water content. They found experimentally that this property produces a significant effect on the water transport characteristics in the membrane. ...
The water transport in a proton exchange membrane fuel cell (PEMFC) is investigated in this study. A five-layer theoretical model is proposed that includes anode and cathode gas diffusion layers (GDLs), catalyst layers (CLs), and the layer of proton exchange membrane. Especially, the volume of membrane is assumed to be variable with its water content and this effect on water transport is examined. Both steady and transient transport phenomena are considered by changing several crucial system parameters such as the relative humidity of reactant gas, the porosity of GDL, and the membrane thickness. The results show that if the humidification of the reactant gases is sufficient, the water management would be better for larger porosities of GDLs or a thinner membrane, and the resistance and overvoltage of the membrane can be reduced significantly as well. Furthermore, it is found that the membrane swelling effect will increase the water content of the membrane especially in the region close to the cathode interface, and lengthen the response time for a PEMFC to reach steady state as switching between two different operating conditions in comparison with the case ignoring this effect.
... NA is the Avogadro constant and a PEM p = 210 m 2 cm −3 the pore surface area per unit volume of the membrane [137]. Fitting Eq. 44 to published experiments on Nafion 117, NR211 and NR212 at temperatures between 20 • C and 30 • C [12,25,63,82,116,131,132] yields K = 92 and n = 12.8, which is close to the finding of Thampan et al. [28]. ...
Ever since modeling has become a mature part of proton exchange membrane fuel cell (PEMFC) research and development, it has been plagued by significant uncertainty lying in the detailed knowledge of material properties required. Experimental data published on several transport coefficients are scattered over orders of magnitude, even for the most extensively studied materials such as Nafion membranes, for instance. For PEMFC performance models to become predictive, high-quality input data is essential. In this bipartite paper series, we determine the most critical transport parameters for which accurate experimental characterization is required in order to enable performance prediction with sufficient confidence from small to large current densities. In the first part, a macro-homogeneous two-phase membrane-electrode assembly model is furnished with a comprehensive set of material parameterizations from the experimental and modeling literature. The computational model is applied to demonstrate the large spread in performance prediction resulting from experimentally measured or validated material parameterizations alone. The result of this is a ranking list of material properties, sorted by induced spread in the fuel cell performance curve. The three most influential parameters in this list stem from membrane properties: The Fickean diffusivity of dissolved water, the protonic conductivity and the electro-osmotic drag coefficient.
... NA is the Avogadro constant and a PEM p = 210 m 2 cm −3 the pore surface area per unit volume of the membrane [137]. Fitting Eq. 44 to published experiments on Nafion 117, NR211 and NR212 at temperatures between 20 • C and 30 • C [12,25,63,82,116,131,132] yields K = 92 and n = 12.8, which is close to the finding of Thampan et al. [28]. ...
Ever since modeling has become a mature part of proton exchange membrane fuel cell (PEMFC) research and development, it has been plagued by significant uncertainty lying in the detailed knowledge of material properties required. Experimental data published on several transport coefficients are scattered over orders of magnitude, even for the most extensively studied materials such as Nafion membranes, for instance. For PEMFC performance models to become predictive, high-quality input data is essential. In this bipartite paper series, we determine the most critical transport parameters for which accurate experimental characterization is required in order to enable performance prediction with sufficient confidence from small to large current densities. In the first part, a macro-homogeneous two-phase membrane-electrode assembly model is furnished with a comprehensive set of material parameterizations from the experimental and modeling literature. The computational model is applied to demonstrate the large spread in performance prediction resulting from experimentally measured or validated material parameterizations alone. The result of this is a ranking list of material properties, sorted by induced spread in the fuel cell performance curve. The three most influential parameters in this list stem from membrane properties: The Fickean diffusivity of dissolved water, the protonic conductivity and the electro-osmotic drag coefficient.
... Pan et al. [27] studied ion aggregation of pendant-type AEMs via coarse-grained molecular dynamics (CGMD) simulations; the predicted ordered structure of the OHassembly was subsequently confirmed by TEM and SAXS. In contrast, a haploid molecular architecture with cationic groups closely attached to a polymer backbone was found to give rise to poor phase separation between the hydrophobic polymer backbone and the hydrophilic short side chains [11,28,29]. ...
A novel pendant-type cross-linked anion exchange membrane (pc-AEM) was successfully synthesized using a pre-synthesis approach to precisely control the IEC value and the degree of cross-linking. The physical properties of the pc-AEMs and the non-cross-linked pc-AEMs as well as Nafion 117 were determined, and the results were systematically compared. It was found that the synthesized pc-AEMs show much better dimensional retention capacity than the non-cross-linked pc-AEM and Nafion 117. In addition, the mechanical strength of the pc-AEMs was also remarkably enhanced. By increasing the IEC value of the pc-AEMs to the same level of Nafion 117, the highest ionic conductivity of 0.036 S/cm at 80 °C was reached. The remarkable enhancement of conductivity is chiefly attributed to the construction of highly efficient ionic transport channels resulting from the combined pendant-type and cross-linked architectures of the pc-AEMs.
... It was found that no water loss and drop of conductivity until 80 C for cross-linked membrane increases continuously with increasing temperature. The proton conductivity of the cross-linked membrane reached to 5.4 Â 10 À2 S/cm at 80 C. The structure of proton exchange membrane has been extensively studied by many researchers, and it is well known that protons transfer between ionic clusters consisting of polar groups such as sulfonic groups [15][16][17]. The number of ionic clusters increases with the increase of the number of sulfonic groups and water content in membrane [18,19]. ...
... Method of standard porosimetry (MSP) [40] is a measurement technique developed from experimental investigations of cermet in solid oxide fuel cells [87] and Nafion membranes [88]. The schematic of the method is shown in Figure 4. ...
Water transport and the corresponding water management strategy in proton exchange membrane (PEM) fuel cells are quite critical for the improvement of the cell performance. Accuracy modeling of water transport in porous electrodes strongly depends on the appropriate constitutive relationship for capillary pressure which is referred to as pc-s correlation, where pc is the capillary pressure and s is the fraction of saturation in the pores. In the present PEM fuel cell two-phase models, the Leverett-Udell pc-s correlation is widely utilized which is proposed based on fitting the experimental data for packed sands. However, the size and structure of pores for the commercial porous electrodes used in PEM fuel cells differ from those for the packed sands significantly. As a result, the Leverett-Udell correlation should be improper to characterize the two-phase transport in the porous electrodes. In the recent decade, many efforts were devoted to measuring the capillary pressure data and developing new pc-s correlations. The objective of this review is to review the most significant developments in recent years concerning the capillary pressure measurements and the developed pc-s correlations. It is expected that this review will be beneficial to develop the improved PEM fuel cell two-phase model.
... Experimental evidence of membrane structure.-The general structure of the membrane as a function of water content has been the source of many studies including rheological, 22 swelling, 23,24 infrared, 25 porosimetry, 26 and especially X-ray [27][28][29][30][31] and microscopy studies. [32][33][34][35][36] For the most part, all of these have shown that a hydrated membrane contains two phases, an ionic phase which water is associated with, and a matrix or nonionic phase. ...
The previously developed mathematical model, based on our physical model, is validated by comparing simulations to experiments. The mathematical model is placed within a simple pseudo-two-dimensional fuel-cell (FC) model. The comparisons mainly involve the net flux of water per proton in the membrane. In the complete FC model, there is only one fitting parameter, the transport coefficient (effective permeability) of the diffusion media, and it is fit at only one simulation condition for each FC setup. The simulations agree very well with both values and trends taken from various literature sources under many different operating conditions, including pressure, temperature, current density, and inlet humidification and stoichiometry. Furthermore, the model allows for an explanation of those trends. The sensitivity and the details of the mathematical model are also examined. Overall, the mathematical model and the physical model are validated and shown to be generally applicable for describing water transport in a polymer-electrolyte membrane.
... [33][34][35]42 A more feasible technique, which employs the working liquid, i.e., water, as a pore-filling medium and allows discriminating hydrophobic and hydrophilic pores, is the method of standard porosimetry ͑MSP͒. 43,44 It measures the amount of a wetting liquid in a test sample that is kept in equilibrium with standard samples of known porous structure. The experimental information about pore size distributions is provided in the form of capillary isotherms, displaying the saturation of the wetting liquid in the porous medium as a function of the vapor pressure or capillary radius. ...
The complex role of water as reaction product, proton shuttle, and asphyxiant in polymer electrolyte fuel cells is discussed. Water is transported into the fuel cell as part of the reactants and it is also considered to be a product of the overall reaction. Water exists as a liquid in the polymer electrolyte that separates the anode and cathode, and equilibrates with gaseous reactants that flow through the electrodes and supporting structures in the fuel cell. Insufficient water in the polymer electrolyte and catalyst layers causes poor reactivity and transport of proton, and the excess heat produced by insufficient operation hastens the fuel cells degradation. Hydrogen in a polymer electrolyte fuel cell, or methanol in a directed methanol fuel cell is oxidized in the catalyst layer on the anode side of the fuel cell. Water in polymer electrolyte serves as the proton shuttle and improves the mobility of hydrogen ions.
... It is widely used in fuel cell and sensing applications because of its high proton conduction properties. 15,16 Different water uptake characteristics from liquid and vapor phase are observed in Nafion resulting in Schroeder's paradox 17 with water absorption being 2 orders of magnitude slower from vapor than from liquid. 18 The transport of water in Nafion membranes is due to both diffusion and electro-osmotic drag. ...
A layer-by-layer approach was used for the fabrication of multilayer films for ultra high gas barrier applications. The ultra high gas barrier material was designed by incorporating Nafion layer in between bilayers of poly (ethylene imine) and poly (acrylic acid) on a Surlyn substrate. When the barrier film with self assembled Nafion is exposed to the moist environment, Nafion absorbs and desorbs water molecules simultaneously, thereby reducing the ingress of moisture in to the film. In order to study the effect of Nafion, the fabricated barrier materials with and without the presence of Nafion were tested for water vapor barrier properties. The barrier films were further used for encapsulating organic photovoltaic devices and were evaluated for their potential use in barrier applications. The devices encapsulated with the films containing Nafion exhibited better performance when subjected to accelerated aging conditions. Therefore, this study demonstrates the effectiveness of self assembled Nafion in reducing the water vapor permeability by nearly five orders of magnitude and in increasing the lifetimes of organic devices by ̴ 22 times under accelerated weathering conditions.
... 14,40 The pore-size distribution for Nafion has been measured by the methods of standard contact porosimetry. 41,42 In those studies, the distribution included both micropores ͑the channels͒ and macropores ͑the clusters͒. Since we are interested only in the channel-size distribution, only the micropore data are fitted, using a log-normal distribution 43 ...
A mathematical model is developed that is based on our previous physical model. The governing equations are presented for both the vapor- and liquid-equilibrated transport modes as well as when they both occur. Thus, this model bridges the gap between the one-phase and two-phase macroscopic models currently used in the literature. In addition to being able to model such phenomena as Schroeder's paradox, the model incorporates other relatively novel features including the effect of temperature on water uptake by the membrane from water vapor, and its associated effects on transport properties. Just as in the physical model, the mathematical model uses the wealth of knowledge contained in the literature to examine and determine values for the relevant transport and membrane parameters. This also helps in corroborating the physical model. The mathematical model developed is further validated and its results examined in a subsequent paper where it is placed in a simple fuel-cell model. (C) 2004 The Electrochemical Society.
... Nafion was developed by Dupont de Nemours in late 1960s and it showed surprisingly high ionic conductivity and durability. As almost the only commercial proton exchange membrane for decades, a lot of research work has been done to study Nafion's structure [28][29][30][31][32][33][34], proton conducting mechanism [35][36][37][38][39][40][41], physical and chemical properties [42][43][44][45], computational modeling [46][47][48], and composite membranes [49][50][51][52][53][54][55] as well. Currently almost all the commercial PEM fuel cells and stacks are still using Nafion as the PEM material. ...
... It is generally believed that Nafion has a special structure with a hydrophilic ionic region scattering in hydrophobic domains, H 2 O and CH 3 OH diffuse principally via the hydrophilic passage. [32,33] During the Nafion/SiO 2 samples preparation, CH 3 OH percentage, either of CH 3 OH/H 2 O solution or of CH 3 OH/TEOS solution, has great effects in composite membrane preparation. On the one hand, it affects the rate of hydrolysis reaction. ...
A novel technique in controlling the size of SiO2 nano-particles in the preparation of Nafion/SiO2 composite membranes via in situ sol–gel method, as well as the effects of nano-particle size on membrane properties and cell performance, is reported in this paper. Nafion/SiO2 composite membranes containing SiO2 nano-particles with four different diameters (5 ± 0.5, 7 ± 0.5, 10 ± 1, and 15 ± 2 nm) are fabricated by altering the reactant concentrations during in situ sol–gel reaction. Sequentially, size effects of SiO2 nano-particles on membrane properties and cell performance are investigated by SEM/EDAX, TEM, TGA, mechanical tensile, and single cell tests, etc. The results suggest that 10 nm is a critical diameter for SiO2 incorporated into Nafion matrix, exhibiting desirable physico-chemical properties for operation at elevated temperature and low humidity. At 110°C and 59% RH, the output voltage of the cell equipped with Nafion/SiO2 (10 nm) obtains an output voltage of 0.625 V at 600 mA/cm2, which is 50 mV higher than that of unmodified Nafion. Copyright © 2010 John Wiley & Sons, Ltd.
... The proton conductivity of the cross-linked membrane reached to 5.4×10 -2 S/cm at 80 °C. The structure of proton exchange membrane has been extensively studied by many researchers, and it is well known that protons transfer between ionic clusters consisting of polar groups such as sulfonic groups151617. The number of ionic clusters increases with the increase of the number of sulfonic groups and water content in membrane1819. ...
The low cost proton exchange membrane was prepared by cross-linking water soluble sulfonated-sulfinated poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phenylene-oxy-phenylene) (SsPEEK-WC). The prepared cross-linked membrane became insoluble in water, and exhibited high proton conductivity, 2.9 × 10<sup>-2</sup> S/cm at room temperature. The proton conductivity was comparable with that of Nafion<sup>®</sup> 117 membrane (6.2 × 10<sup>-2</sup> S/cm). The methanol permeability of the cross-linked membrane was 1.6 × 10<sup>-7</sup> cm<sup>2</sup>/s, much lower than that of Nafion<sup>®</sup> 117 membrane. © 2009 Professor T. Nejat Veziroglu.
... Thus hydrated protons can freely move through the channels produced by the phase separation, leading to high conductivity of the membrane. However, phase separation simultaneously gives channels for methanol and water molecules to pass through under the driving forces of concentration, pressure gradients and electro-osmosis [2,3]. It has been realized that methanol transported through the PEM would be oxidized at the cathode. ...
The polycation (poly(diallyldimethylammonium chloride),PDDA) modified Au particles were prepared by ethanol reducing. Au particles showed zeta potential of +32 mV and size of about 4 nm. With the coulomb gravitation, the polycation modified Au nano-particles were self-assembled onto the Nation (TM) 212 membrane surface to form methanol blocking proton exchange membranes, and the methanol crossover decreased from 168 mA/cm(2) to 18 mA/cm(2) at the condition of 2 mol/L methanol at 60 degrees C. All the self-assembled single cells have higher open circuit voltage (OCV) and performance than original Nation212 membrane.
... However, it is probably due to the interfacial properties of the membrane, such as the fluorocarbon-rich skin on the surface of Nafion ® [29,30] or the removal of a liquid-vapor meniscus at the membrane surface [31]. Overall, the final picture of a liquid-equilibrated membrane is a porous structure, with average channel and cluster sizes between 1 and 2 nm and 2-4 nm, respectively [32,33]. ...
In this chapter, the various approaches to the macroscopic modeling of transport phenomena in polymerelectrolyte membranes are discussed. This includes general background and modeling methodologies, as well as exploration of the governing equations and some membrane-related topic of interest.
... It is accepted that Na®on TM has a dual structure with a hydrophobic region interspersed with ion-rich hydrophilic domains [8]. The pore sizes of the latter domain are inaccessible by decane (and octane) molecules but accessible by water molecules [9]. It is known that methanol diffuses primarily through the water-rich domains. ...
In order to reduce methanol cross-over to the positive electrode in a direct-methanol fuel cell (DMFC), the concept of modifying the morphology of the proton-conducting membrane is proposed. The method involves using plasma etching and palladium-sputtering on a Nafion™ polymer membrane. Methanol permeability tests were conducted in a specially-designed cell. Plasma etching of Nafion™ membrane increases the roughness of the membrane surface and decreases the methanol permeability. The sputtering of palladium on the plasma-etched Nafion™ further decreases the methanol cross-over. Improved DMFC performance curves are obtained in a single cell which contains the modified Nafion™.
... Thus hydrated protons can freely move through the channels produced by the phase separation, leading to high conductivity of the membrane. However, phase separation simultaneously gives channels for methanol and water molecules to pass through under the driving forces of concentration, pressure gradients and electro-osmosis [2,3]. It has been realized that methanol transported through the PEM would be oxidized at the cathode. ...
Charged Au nanoparticles were prepared by refluxing a solution of hydrogen tetrachloroaurate trihydrate and protective cationic agents, PDDA, in ethanol/water. The Au nanoparticle showed zeta potential of 32mV and size of about 4nm. The charged Au nanparticles were self-assembled onto the Nafion™ 212 membrane surface as methanol barriers and the methanol crossover decreases from 168 to 18mA/cm2 at the condition of 2mol/L methanol, 60°C. All the self-assembled PEMs have higher OCV and performance than original Nafion 212 membrane. And the PEMs self-assembled for 24h have the highest performance due to the decrease of the methanol permeation current density and acceptable membrane area resistance.
... Commercially thermal processed Nafion membranes have been shown to have a wide spectrum of pore sizes ranging from 1-100nm [34], with the average pore size of only 2nm. Therefore zeolite nanocrystals in the 15-50nm size range should fit within the polymer matrix of Nafion without deforming the polymer backbone too much. ...
Proton exchange membrane fuel cells (PEMFCs) have recently received a great deal of attention for their potential as compact, high efficiency power sources for portable, distributed generation, and transportation applications. Unfortunately, current proton exchange membrane (PEM) technology hinders fuel cell performance by limiting fuel cell operation temperature and methanol feed concentration in direct methanol fuel cells (DMFCs). Nafion-zeolite nanocomposite membranes that take advantage of the hydrophilicity, selectivity, and proton conductivity of zeolite nanocrystals have been developed to address these problems. All known zeolite topologies were evaluated as potential additives to Nafion proton exchange membranes. Zeolites Y and beta were determined to have great potential as additives due to their low framework density, three dimensional pore structure, and high hydrophilicity. Zeolite Y nanocrystal syntheses were optimized to enhance yield and produce smaller crystal size. Significant improvement of the acid stability of the zeolite Y nanocrystals was not achieved with both ammonium hexafluorosilicate treatments and direct high silica nanocrystal synthesis. However, control of zeolite Y nanocrystal framework Si/Al ratio was demonstrated in the range of SiO2/Al2O3 = 4.38 to 5.84 by manipulating the tetramethylammonium structure directing agent hydroxide content. Zeolite beta nanocrystals were investigated due to their inherent high silica content and high acid stability. Zeolite beta nanocrystals were hydrothermally synthesized with and without phenethyl (called PE-BEA and BEA respectively) organic functional groups. Sulfonic acid functionalized zeolite beta (SAPE-BEA) was generated by treating the PE-BEA nanocrystals with a concentrated sulfuric acid post synthesis treatment. SAPE-BEA samples demonstrated proton conductivities up to 0.01 S/cm at room temperature under water-saturated conditions using a newly developed characterization technique. With optimization, acid functionalized zeolite materials could possibly perform as competent stand-alone proton conducting materials with the proper engineering. BEA and SAPE-BEA zeolite nanocrystals mixed with suspensions of Nafion were cast into nanocomposite membranes. DMFC membrane electrode assemblies (MEAs) prepared with a 2.5wt% SAPE-BEA nanocomposite membrane delivered twice the peak power of a MEA with a commercial Nafion 117 membrane. Membrane performance improvements of this magnitude could ultimately lead to DMFC cost and size reductions that make the technology commercially viable for a variety of applications.
... Ϫ 0.14452093 ϫ 10 Ϫ7 (T 3 ϩ 6.5459673) ln(T) [15] The oxygen diffusion coefficient is calculated as a function of the binary diffusion coefficients [16] while the binary diffusion coefficients D O2-w and D O2-N2 are calculated from the Slattery and Bird estimation 10 D k,m ϭ ϫ 10.13 [17] Under the assumption that the ORR is irreversible, first order in oxygen and that electroneutrality holds in any representative elementary volume, the cathode transfer current density can be expressed as [18] In Eq. 18, the dependency of j on the proton concentration is included in the reaction rate constant k, defined as the number of kilomoles of reactant consumed per unit volume and time. The surface overpotential is the difference between the potential of the electronically conductive carbon and the potential of the membrane phase. ...
A mathematical model for polymer electrolyte membrane (PEM) fuel cells is developed and rigorous analytical solutions of the model are obtained. The modeling domain consists of the cathode gas channel, gas diffuser, catalyst layer, and the membrane. To account for the composite structure of the gas diffuser and for its gradient in liquid water content, the gas diffuser is modeled as a series of parallel layers with different porosity and tortuosity. Starting from the oxygen transport equations and Ohm's law for proton migration, expressions for the oxygen mass fraction distribution in the gas channel, gas diffuser, and catalyst layer, and current density and membrane phase potential in the catalyst layer and membrane are derived. The solutions are presented in terms of the physical and thermodynamic parameters of the fuel cell. The polarization curve is expressed parametrically as a function of the surface overpotential. Expressions for cathode internal and overall effectiveness factors, active fraction of the catalyst layer. catalyst layer resistance, limiting current density, and the slope of the polarization curve are also presented. Due to the advantage of the closed-form solutions this model can be easily used as a diagnostic tool for a PEM fuel cell operating on H-2 and air.
... Swollen Nafion membranes have been found to contain troughs on the surface. 47 The diffusion coefficient of water through Nafion suggests a dominant pore volume of 1-100 nm. These pores are found in the interior of membrane with a small volume of larger pores associated with a rough outer surface. ...
and batteries.2-4 This study has focused on the application of the Nafion range of cation-exchange membranes in proton exchange membrane fuel cells ~PEMFCs!. In the PEMFC the proton conductivity of the mem- brane is particularly important since it plays a significant role in controlling the performance of the fuel cell. 5,6 Higher levels of pro- ton conductivity allow much higher power densities to be achieved. This is particularly important for automotive applications of PEM- FCs. The two common strategies to improve the conductivity of the membrane are to raise the specific conductivity and to reduce the thickness. There is, however, a practical limit on the thickness since, much below 25 mm, mixing of the hydrogen and air ~or oxygen! reactant gasses due to crossover through the ion-exchange material is too high for pure Nafion membranes and there is a loss of effi- ciency. Reducing the membrane thickness also increases the risks with respect to mechanical properties such as strength, raising con- cerns regarding the durability and ease of handling of the
... Experimental evidence of membrane structure.-The general structure of the membrane as a function of water content has been the source of many studies including rheological, 22 swelling, 23,24 infrared, 25 porosimetry, 26 and especially X-ray [27][28][29][30][31] and microscopy studies. [32][33][34][35][36] For the most part, all of these have shown that a hydrated membrane contains two phases, an ionic phase which water is associated with, and a matrix or nonionic phase. ...
In this paper, a physical model is developed that is semiphenomenological and takes into account Schroeder’s paradox. Using
the wealth of knowledge contained in the literature regarding polymer-electrolyte membranes as a basis, a novel approach is
taken in tying together all of the data into a single coherent theory. This approach involves describing the structural changes
of the membrane due to water content, and casting this in terms of capillary phenomena. By treating the membrane in this fashion,
Schroeder’s paradox can be elucidated. Along with the structural changes, two different transport mechanisms are presented
and discussed. These mechanisms, along with the membrane’s structural changes, comprise the complete physical model of the
membrane. The model is shown to agree qualitatively with different membranes and different membrane forms, and is applicable
to modeling perfluorinated sulfonic acid and similar membranes. It is also the first physically based comprehensive model
of transport in a membrane that includes a physical description of Schroeder’s paradox, and it bridges the gap between the
two types of macroscopic models currently in the literature. © 2003 The Electrochemical Society. All rights reserved.
Proton conductivity through perfluorosulfonate acid (PFSA) polymer electrolyte membranes was investigated using a nanoporous network model, which was developed for the purpose of quantitatively describing transport of charged species through typical PFSA fuel cell membranes. The membrane was modeled as a collection of random fractal nanopores with the anionic groups (i.e.,-SO 3-) assumed to be fixed along the pore wall according to a distribution determined by the equivalent weight and dry membrane density. The transport of the hydronium ions inside the pore was expressed using a simplified Nernst-Einstein equation. Continuum percolation theory and a fractal structural transport model were used to modify the diffusion coefficient and illustrate the transport mechanism. The conductivity of the membrane was deduced in terms of the following quantities: water content, equivalent weight, temperature, and the architecture of the PFSA polymer side chain. Theoretical predictions of the model for varying water content and temperature were compared against experimental data of conductivity for four membranes: Nafion 117 (EW = 1100, a long side chain with a pendant CF 3 group), Membrane C (EW = 900, same side chain as Nafion, but with a shorter backbone repeat unit), a 3M membrane (EW = 1000, long side chain without a pendant CF 3 group), and Dow's XUS 13204.10 (EW = 800, same as the 3M membrane, but with shorter backbone unit and side chain). The theoretical predictions of the model matched the experimental data with reasonable quantitative accuracy in most cases. One of the most efficient alternative clean energy conversion devices for long-term use is the polymer electrolyte membrane fuel cell (PEM-FC). The essential but performance-limiting component of a PEM-FC is the proton exchange membrane. Perfluorosulfonic acid (PFSA) polymer membranes are the most widely adopted in PEM-FC technology; for examples, DuPont's Nafion, Membrane C of Chlorine Engineers Corp., Dow's XUS 13204.10, Aciplex of Asahi Kasei Corp., and membranes developed by 3M Corp., all of which are currently commercially available. 1 Hydrogen gas is commonly used as an energy source in a PEM-FC by splitting the H 2 molecules into pairs of protons and electrons. The hydrated protons move through the membrane, while the electrons pass through an external circuit. The efficiency of the PEM-FC depends strongly on the rate of transport of protons, which is directly related to the morphology of the membrane. The chemical structure of a PFSA polymer combines a hy-drophobic backbone with short hydrophilic side chains terminated with sulfonic acid groups. The chain backbone provides the structural support of the membrane, while the sulfonic acid groups donate protons to water clusters within the hydrated pores of the membrane. During the last two decades, numerous physical and chemical property data of PFSA polymer membranes were accumulated under different operational conditions; however, their morphological characteristics are still subject to debate. Various membrane structural models have been proposed based on their monomeric chemical properties as well as data from neutron and small angle X-ray scattering. In hydrated membranes, the PFSA molecules are generally classified as assuming various types of model morphologies, such as spherical clusters, 2,3 the reverse micelle-channel model, 4 polymer bundles, 5,6 channel networks, 7 layered structures, 8 and parallel cylindrical pores. 9 Of course, the exact nature of the nanoporous morphology is likely highly dependent on the water content. 10 Several of these typical morphologies were compared by Schmidt-Rohr and Chen based upon experimental data from small-angle X-ray scattering data of Nafion, which was compared to simulated structures of the hydrated PFSA environment. 9 These simulations suggested that a Nafion morphology based on a parallel cylindrical pore channel array gave a better match to the X-ray scattering data than the spherical * Electrochemical Society Student Member. z E-mail: bje@utk.edu water cluster model, channel network model, or the polymer-bundle model. The hypothesis of cylindrical conduction channels within PFSA membranes has been used previously to study the transport properties and proton conductivity within these materials. Din and Michaelides performed molecular dynamics simulations to study the movement of proton and water within pores of radii 9.36 and 12.24 Å. 11 They found that the proton and water distributions depended on the water content and wall surface-charge density. Paddison et al. derived a statistical model for a similar system in order to investigate the wall surface-charge density dependence. 12,13 This model was extended to incorporate species transport equations within the pores by Kumar et al.; 14 these researchers were able to predict conductivities of Nafion membranes as functions of pore radii, surface-charge distribution, water content, etc. Although fundamental, these transport equations require the specification of several structural parameters (such as pore radius, charge distribution, pore length, pore geometry, etc.) that cannot easily be obtained directly from experiments. In this work, we exploit some key architectural characteristics of the PFSA macro-molecules, coupled with percolation theory, to calculate the diffusion coefficient that dictates the strength of the conduction effect of the Nernst-Einstein equation under different environmental conditions of water content and temperature. In the model used for conductivity and dry membrane density calculations , we assumed that the PFSA polymer membrane was composed of a nanoporous network of indeterminate morphology. The sulfonic acid groups at the terminal position of the side chains were assumed to be distributed randomly on the inside walls of the pores. For a given PFSA polymer membrane, the free volume of the dry membrane was calculated and assumed to equate to the total dry-channel volume of the nanoporous network. Under exposure to humidified air, water enters into the nanopores and hydrates the protons of the sulfonic acids groups. These pores were assumed to be deformable. The expression of membrane conductivity was based on the Nernst-Einstein equation, which depends directly on the diffusion coefficient of hydronium ions. This coefficient was developed using continuum percolation theory, which also aided the theoretical understanding of the proton transport mechanism. Predictions of this model were compared against experimental data for four different types of PFSA membranes: Nafion 117 (denoted in the following as N), Dow XUS 13204.10 (D), Membrane C (denoted as C, produced by Chlorine Engineers, Japan), and a 3M membrane (3M).) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 160.36.33.249 Downloaded on 2015-07-09 to IP
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 10⁵ nm. 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.
Adsorption on polysaccharide derivatives can be a low-cost procedure of choice in water decontamination for extraction and separation of compounds, and a useful tool for protecting the environment. In current study, polysaccharides-silica composites were synthesized by sol-gel method, physical and chemical adsorption of chitosan and carrageenan by silica surface. Obtained adsorbents were applied to adsorption of highly toxic compounds: cationic and anionic dyes, herbicides and heavy metals.
In this comprehensive review, recent progress and developments on perfluorinated sulfonic-acid (PFSA) membranes have been summarized on many key topics. Although quite well investigated for decades, PFSA ionomers' complex behavior, along with their key role in many emerging technologies, have presented significant scientific challenges but also helped create a unique cross-disciplinary research field to overcome such challenges. Research and progress on PFSAs, especially when considered with their applications, are at the forefront of bridging electrochemistry and polymer (physics), which have also opened up development of state-of-the-art in situ characterization techniques as well as multiphysics computation models. Topics reviewed stem from correlating the various physical (e.g., mechanical) and transport properties with morphology and structure across time and length scales. In addition, topics of recent interest such as structure/transport correlations and modeling, composite PFSA membranes, degradation phenomena, and PFSA thin films are presented. Throughout, the impact of PFSA chemistry and side-chain is also discussed to present a broader perspective.
Water uptake of polymer exchange membranes (PEM) such as Nafion® leads to water channels that are separated from the solid polymeric phase. In this study, the proton mobility in Nafion® and the influence of the water channel morphology on its overall conductivity are characterized by experimental approaches in combination with reported computational results. Using impedance spectroscopy with amplitude and frequency variation, the proton mobility was derived to be independent of the length scale of the proton permeation through fully hydrated Nafion®, suggesting equal direct current (DC) and alternating current (AC) conductivities. The proton conductivities of aqueous solutions with equal proton concentrations as the aqueous phase of fully hydrated Nafion® membranes were measured to be approximately 6.0 ± 0.7 times higher than the conductivity of the membranes. Recently reported resistor network modeling results characterized the influence of the morphology of the aqueous phase on the proton permeation through fully hydrated Nafion® to approximately the same degree. By comparing these experimental and modeled result, the mean proton mobility in the aqueous phase of fully hydrated Nafion® was estimated to equal that in aqueous solutions.
The comprehensive, accessible introduction to fuel cells, their applications, and the challenges they pose Fuel cells-electrochemical energy devices that produce electricity and heat-present a significant opportunity for cleaner, easier, and more practical energy. However, the excitement over fuel cells within the research community has led to such rapid innovation and development that it can be difficult for those not intimately familiar with the science involved to figure out exactly how this new technology can be used. Fuel Cells: Problems and Solutions, Second Edition addresses this issue head on, presenting the most important information about these remarkable power sources in an easy-to-understand way. Comprising four important sections, the book explores: The fundamentals of fuel cells, how they work, their history, and much more The major types of fuel cells, including proton exchange membrane fuel cells (PEMFC), direct liquid fuel cells (DLFC), and many others The scientific and engineering problems related to fuel cell technology The commercialization of fuel cells, including a look at their uses around the world Now in its second edition, this book features fully revised coverage of the modeling of fuel cells and small fuel cells for portable devices, and all-new chapters on the structural and wetting properties of fuel cell components, experimental methods for fuel cell stacks, and nonconventional design principles for fuel cells, bringing the content fully up to date. Designed for advanced undergraduate and graduate students in engineering and chemistry programs, as well as professionals working in related fields, Fuel Cells is a compact and accessible introduction to the exciting world of fuel cells and why they matter.
The electrodes in fuel cells simultaneously realize electric and ionic conductivity. In the case of acidic polymer electrolytes, the electrodes are typically made of composites of carbon-supported catalyst and Nafion® polymer electrolyte binder. In this study, the interaction of the proton conduction, the electron conduction, and the electrochemical hydrogen conversion in such composite electrode materials was examined. Exposed to a hydrogen atmosphere, these composites displayed up to tenfold smaller resistivities for the proton conduction than that of Nafion® membranes. This effect was ascribed to the simultaneously occurring electrochemical hydrogen oxidation and evolution inside the composite samples, which are driven by different proton and electron resistivities. The parasitic electrochemical currents resulting were postulated to occur in the anode of fuel cells with polymer, solid oxide, or liquid alkaline electrolytes, when the ohmic drop of the ion conduction in the anode is higher as the anodic kinetic overvoltage (as illustrated in the graphical abstract). In this case, the parasitic electrochemical currents increase the anodic kinetic overpotential and the ohmic drop in the anode. Thinner fuel cell anodes with smaller ohmic drops for the ion conduction may reduce the parasitic electrochemical currents.
Proton conductivity through perfluorosulfonate acid (PFSA) polymer electrolyte membranes was investigated using a nanoporous network model, which was developed for the purpose of quantitatively describing transport of charged species through typical PFSA fuel cell membranes. The membrane was modeled as a collection of random fractal nanopores with the anionic groups (i.e., –SO3–) assumed to be fixed along the pore wall according to a distribution determined by the equivalent weight and dry membrane density. The transport of the hydronium ions inside the pore was expressed using a simplified Nernst-Einstein equation. Continuum
percolation theory and a fractal structural transport model were used to modify the diffusion coefficient and illustrate the transport mechanism. The conductivity of the membrane was deduced in terms of the following quantities: water content, equivalent weight, temperature, and the architecture of the PFSA polymer side chain. Theoretical predictions of the model for varying water content and temperature were compared against experimental data of conductivity for four membranes: Nafion 117 (EW = 1100, a long side chain with a pendant CF3 group), Membrane C (EW = 900, same side chain as Nafion, but with a shorter backbone repeat unit), a 3M membrane (EW = 1000, long side chain without a pendant CF3 group), and Dow’s XUS 13204.10 (EW = 800, same as the 3M membrane, but with shorter backbone unit and side chain). The theoretical predictions of the model matched the experimental data with reasonable quantitative accuracy in most cases.
The distribution of water fluxes is strongly influenced by the properties of the media adjacent to the cathode catalyst layer (CCL), viz. polymer electrolyte membrane (PEM) and gas diffusion media (GDM). We propose a model that is applied under varying humidification of the cathode feed gas, with water removal in liquid and vapor form. The model warrants the definition of a critical current density up to which water removal out of the CCL could proceed completely via vapor diffusion to the cathode side. Above the critical current density, excessive water generation leads to the build-up of an excess liquid pressure in the CCL, which drives hydraulic fluxes to PEM and GDM sides. Experimental data from literature were analyzed and found to be in good agreement with predictions of the model. This analysis reveals that the effective liquid water permeability of commercial GDMs (that includes an MPL) lies within the same order of magnitude of the liquid water permeability of the PEM.
Water sorption isotherm of catalyst layers in polymer electrolyte fuel cells is analyzed by a mathematical model. The model takes into account multi-layer water adsorption on adsorption sites and capillary condensation in hydrophilic pores. The detailed structures of CLs including the pore size distribution, volume fraction of hydrophilic pores, and ionomer coverage over carbon supports and Pt surface are thoroughly considered. Quantitative estimation of water uptake at different locations in CLs as a function of vapor activity is successfully achieved. The model reveals that water adsorption in CLs firstly takes place on the adsorption sites such as side chains in Nafion (R) ionomer, Pt surface, and surface groups on carbon supports at low vapor activity. The primary pores are filled with capillary condensed water at activity of 0.4. The condensation of water in the secondary pores is observed at activity from 0.9 to 1. The wetting property of the primary pores strongly depends on carbon materials, while surface of the secondary pores is found to show the hydrophobic nature irrespective of carbon materials. The amount and location of water in CLs are correlated with electrochemical double layer and effective proton conductivity.
Charged Au nanoparticles with various diameters, from 2.7 nm to 10.3 nm, were synthesized by using various reducing agents. Then charged Au nanoparticles were self-assembled onto the NafionTM membrane surface as methanol barriers. It was found that the proton exchange membrane self-assembled with 3.9 nm Au particles had the lowest methanol crossover. The proton conductivity decreased with the increase of the particles size until the particles size was more than 6.2 nm and they had little influence on the membrane conductivity. All the self-assembled membranes have higher open circuit voltage (OCV) and performance than original Nafion212TM membrane. And the membranes self-assembled with 3.9 nm poly (diallymethylammonium chloride) (PDDA) modified Au (PDDA-Au) particles had the highest performance due to the reciprocal of the methanol permeation current density and membrane area resistance.
A theory is proposed for the continuum modeling of liquid and gas diffusion through a deformable porous solid. The solid and the voids are locally homogenized to yield a macroscopic solid phase. Liquid saturation of the voids and gas pressure are included as state variables to model diffusion. A model based on the proposed theory is implemented numerically using the finite element method, and tested with three simulations on sand and Nafion.
Nafion/Faujasite zeolite composite membranes have been prepared by solution casting at a zeolite content ranging from 0.98 to 21.4 wt %. The effect of the zeolite loading on the mobility of both liquid and vapor water through the Nafion membrane has been investigated by using two complementary techniques, that is, differential scanning calorimetry and dynamic vapor sorption. The relationship between water mobility, proton conductivity, and direct methanol fuel cell (DMFC) performance of composites is also discussed. The addition of zeolite contributes to the enhancement of the water mobility degree in the composite membrane due to both the surface composition of the additive and the introduction of porosity at the polymer/filler interface. Nafion/zeolite composites having higher proton conductivity and DMFC performance than bare Nafion can thus be fabricated by fine-tuning of the additive content and the membrane morphology.
The great demand for high-temperature operation of polymer electrolyte membrane fuel cells (PEMFCs) has been well answered by short-side-chain perfluorinated sulfonic acid (SSC-PFSA) membranes through a good balance between transport properties and stability. It has been evidenced that fuel cells assembled with SSC-PFSA possess higher and more stable performance at elevated temperature up to 130 degrees C compared to that of fuel cells based on conventional long-side-chain (LSC) PFSA (Nafion (R)) membranes. Moreover, the shorter side-pendent chains and the absence of the ether group and of the tertiary carbon also endow SSC-PFSAs with better durability, making them more suitable for working at harsh conditions in fuel cell systems. This critical review is dedicated to summarizing the properties of SSC-PFSA and providing insight into an understanding of their micro-morphologies, mass diffusion, enhanced proton transportation and their mutual correlation. Diversified measurement techniques applied to investigate the evolution of micro-morphologies, unique diffusion and transportation properties of SSC-PFSAs are reviewed. Despite the higher crystalline and higher water absorption of SSC-PFSAs than those of LSC-PFSAs, the notably less developed and less interconnected ionic clusters in SSC-PFSAs lead to lower mass permeability, and hence the high water uptake is not as well translated into transportation performance as expected. The factors and reasons for the enhanced electrochemical performance of SSC-PFSAs such as higher proton conductivity at elevated temperatures and low humidity conditions are also discussed and understood. Highlights of recent advances in SSC-PFSA-based membranes for fuel cell applications at wider temperature ranges are summarized as general references for researchers to further prompt the development of SSC-PFSAs. The SSC-PFSAs based membranes give a bright future for the next generation of high-temperature PEMFCs.
We present a water balance model of membrane electrode assemblies (MEAs) with ultrathin catalyst layers (UTCLs). The model treats the catalyst layers in an interface approximation and the gas diffusion layers as linear transmission lines of water fluxes. It relates current density, pressure distribution, and water fluxes in the different functional layers of the assembly. The optimal mode of operation of UTCLs is in a fully flooded state. The main challenge for MEAs with UTCLs is efficient liquid water removal, to avoid flooding of the gas diffusion layers. The model provides strategies for increasing the critical current density for the onset of flooding, via liquid permeabilities, vaporization areas, and gas pressure differentials. Finally, we discuss methods to identify regimes of transport via water flux measurements.
The results of studies of the principal components of the membrane-electrode assembly (ion-exchange and capillary membranes, polymer hydrophobization agents) obtained by standard contact porosimetry are considered. The phenomenon of inversion of ion-exchange groups of ionomers that affects the hydrophilic-hydrophobic properties of catalytic layers is described. The effect of the porous structure and wettability of components of the membrane-electrode assembly on the electrochemical characteristics of fuel cells is discussed. The bibliography includes 167 references.
The flux of liquid water through Nafion membranes of different thickness and equivalent weight was measured as a function of hydrostatic pressure and temperature. Hydraulic water transport across Nafion membranes increases with temperature and equivalent weight of the Nafion. Hydraulic permeability increases with temperature due to both decreased water viscosity and increased hydrophilic volume fraction. Convective flow from the applied hydrostatic water pressure is an order of magnitude greater than the estimated diffusive water flux associated with the water activity gradient. Water sorption and hydraulic permeability data predict a hydrophilic pore network with hydrophilic domains 2.5 nm in diameter spaced 5.5 nm apart. The pore network structure from water sorption and hydraulic permeability are consistent with the spacing between hydrophilic domains observed with small angle X-ray scattering experiments.
The intrinsic relationship between proton conductivity, thermal stability and structural symmetries of phosphotungstic acid (HPW)-functionalized mesoporous silica (HPW-meso-silica) membrane was investigated with mesoporous silica from 2D hexagonal p6mm, 3D face-centered cubic (Fm3¯m), body-centered Im3¯m, to cubic bicontinuous Ia3¯d symmetries. HPW-meso-silica nanocomposites with 3D mesostructures display a significantly higher proton conductivity and higher stability as a function of relative humidity in comparison to 2D mesostructures. The best result was obtained with body-centered cubic (Im3¯m)-HPW-meso-silica, showing proton conductivities of 0.061 S cm−1 at 25 °C and 0.14 S cm−1 at 150 °C, respectively, and an activation energy of 10.0 kJ mol−1. At 150 °C, the cell employing a HPW-meso-silica membrane produced a maximum power output of 237 mW cm−2 in a methanol fuel without external humidification. The high proton conductivity and excellent performance of the new methanol fuel cells demonstrate the promise of HPW-meso-silica nanocomposites with 3D mesostructures as a new class of inorganic proton exchange membranes for use in direct methanol fuel cells (DMFCs).
Proton-exchange membrane (PEM) electrolyzers have been a source of interest due to the ability of these electrolyzers to produce product gases at high differential pressures across the membrane electrode assembly (MEA) without the need for external compression. This work studies species transport within the membrane of a high-pressure oxygen-generating water electrolyzer using the dusty-fluid model (DFM) for the case involving liquid water being supplied to the cathode. The model was calibrated against experimental polarization data from Hamilton Sundstrand's high-pressure oxygen generating assembly (HPOGA) at varying differential oxygen pressures. The governing equations were cast in a non-dimensional form to examine the dehydration of the cell with increasing current density and differential pressure expressed in terms of a Damkohler number and the ratio of the membrane diffusion coefficient to the species diffusion coefficient. It was determined that the dehydration of the cell occurred at an approximately constant Damkohler number of 0.196 ± 0.004 regardless of the ratio of the pressure difference. It was also observed that electro-osmotic drag had a strong influence on the dehydration of the cell since the drag coefficient directly and substantially elevated the required water flux for a given current density operation above what was needed based only on the stoichiometry of the chemical reactions.
Advances in the design of materials for energy storage and conversion (i.e., "energy materials") increasingly rely on understanding the dependence of a material's performance and longevity on three-dimensional characteristics of its microstructure. Three-dimensional imaging techniques permit the direct measurement of microstructural properties that significantly influence material function and durability, such as interface area, tortuosity, triple phase boundary length and local curvature. Furthermore, digital representations of imaged microstructures offer realistic domains for modeling. This article reviews state-of-the-art methods, across a spectrum of length scales ranging from atomic to micron, for three-dimensional microstructural imaging of energy materials. The review concludes with an assessment of the continuing role of three-dimensional imaging in the development of novel materials for energy applications.
Electrodialysis is a mature process which is applied since more than 50years on a large industrial scale for the production of potable water from brackish water sources. But more recently electrodialysis in combination with bipolar membranes or with ion-exchange resins has found a large number of new interesting applications in the chemical process industry, in the food and drug industry as well as in waste water treatment and the production of high quality industrial water.In this paper the principle of electrodialysis is described and its advantages and limitations in various applications are pointed out. More recent developments in electrodialysis as well as in related processes such as electrodialytic water dissociation or continuous electrodeionization are discussed and their present and potential future applications are indicated. Research needs for a sustainable growth of electrodialysis and related processes are pointed out.
For the first time the effect of essential shift of the overlimiting state potentials on the current–voltage curves for the composite membranes PANI/MF-4SC has been observed. This effect is explained by morphology changes of initial membrane due to formation of interpolymeric complexes of electron-conducting polyaniline with polymer segments of perfluorinated matrix. The obtained results indicate the dominating role of water in the polarization phenomena.
A new porosimetric method — the Method of Standard Contact Porosimetry (MSP) is described which allows the investigation of the structure and the properties of all kinds of porous materials including soft, frail, amalgamated materials and also of powders. This method is relatively simple, nondestructive, is not connected with the use of mercury and can be applied for measurements in a wide range of pore sizes from about 1 to 3×105 nm. This method is very informative. It has been used for measuring the pore volume and the pore surface area distribution in terms of the pore radii and the pore shapes, the distribution of liquids in porous materials in terms of the liquid-sample free energy and the capillary pressure, and also for the measuring of adsorption isotherms, of structural changes during contraction and swelling of the samples in different liquids, of different properties of multicomponent hydrophilic-hydrophobic systems etc. The results obtained by applying the MSP for investigating different processes in porous systems are discussed. The following processes were investigated; swelling and ion exchange polymeric materials (membranes, conducting polymers); pressing of powdered materials (PVC, Raney silver); the influence of temperature on the porous structure; the influence of pore-forming agents; chemical and electrochemical sintering of catalysts; deposition of solid products in the pore volume of the cathode during reduction of SOCl2 in lithium batteries; structural changes during formation and cycling of lead and silver oxide electrode, etc. The MSP includes different manual operations. In order to avoid them on the base of MSP, an automated standard porosimeter (ASP) was developed which includes a computer, an electronic balance, an automatic manipulator, and a device for a controlled drying of the porous samples.
This dissertation describes studies of the crystal structure of disordered carbons and the electrochemical intercalation of lithium in the disordered carbons. One of the most important applications of carbons is as an electrode material in rechargable lithium-ion (rocking chair) battery systems. These usually use carbon as the anode and thus depend on the related behavior of lithium intercalation in carbons. An important quantity for measuring the performance of such a battery is the maximum reversible capacity, which strongly depends on the carbon crystal structure. In order to study the structure of disordered carbons, we have developed a structural model for disordered carbons and a corresponding automated structure refinement program for X-ray powder diffraction patterns of disordered carbons. These diffraction patterns can be complex to interpret because of the complicated nature of layer stacking in disordered carbons. The structural model used in the refinement program is divided into two cases, the one-layer model (for highly disordered carbons) and the two-layer model (for graphitic carbons). Some of the important parameters of the model are, for example, (1) the probability P of finding a random shift between layers, which is large for disordered carbons like coke and carbon fibers, small for heat treated synthetic graphitic carbons and practically zero for natural graphite; (2) P_{t}, the probability of finding a local 3R stacking fault in graphitic carbons; (3) 1-g (only in the one layer model), the percentage of unorganized carbon in disordered carbons; (4) zeta, a dimensionless parameter for measuring in-plane strain in the carbon layer; (5) the finite size of carbon grains L_{a}, (parallel to the layers) and L_{c}, (perpendicular to the layers), (6) fluctuations in the spacing between adjacent layers; (7) the average lattice constants, c and a; (8) the constant background and other important quantities. The program minimizes the difference between the observed and calculated diffraction profiles in a least -squares sense by optimizing model parameters analogously to the popular Rietveld refinement method. Unlike the Rietveld method, which is designed for crystalline materials, this program allows the quantification of the finite size, strain and disorder present in disordered carbon fibers and cokes. We have used our model and program ^1 to fit over 50 carbons from Canadian, US and Japanese sources. These include cokes, heat treated cokes, fibers, synthetic graphites and mesocarbon etc. The structural data have been correlated to the maximum reversible capacity, x_{max }, of Li/Li_{x}C_6 electrochemical cells to determine how the carbon structure influences the intercalation of lithium. A phenomenological picture which explains the trends in the data has been proposed, which allows us to predict x_ {max} for any carbon, given its structural parameters. We are able to understand qualitatively the variation in x{max} with heat treatment temperature and with the types of disorder present in both hard and soft carbons. A general statement about which classes of carbons are most suitable as anodes in lithium-ion cells has been made in the conclusion and some suggestions for future research directions are given. ftn ^1This program is available from the author.
A new porosimetry method is described which allows the investigation of all kinds of porous materials including soft or frail materials and powders. The method is relatively simple and nondestructive, and can be used for measurements in a wide range of pore sizes from 1 to 10(sup 6) nm. This method can also be used for the evaluation of wetting angles and of the water wettability of multicomponent porous materials. This method is now widely used for investigation of battery components: porous electrodes, membranes, etc.
The pore structure of heterogeneous ion exchange membranes was studied by the mercury penetration method using a commercially available apparatus. Results obtained from the porosymmetrical measurements show that the membranes possess two layers, the upper one being less porous than the lower backing layer. The distribution of the pores in width and length is non-homogeneous. Anion exchange membrane MA-41 has a relatively more homogeneous pore structure.Application of the mercury penetration method makes it possible to quantitatively assess the degree of mechanical destruction of anion exchange membranes during the working period. It was found that membranes operated at the cathode and anode compartments of the electrodialysis unit are destroyed more quickly.
The x-ray interference is calculated for layer lattices in which the phase shifts between consecutive layers and the scattering powers of individual layers do not follow a strictly periodic arrangement. In the second section the scattering power of all layers is assumed to be the same but the phase shifts can take on different values. In the third section neither the scattering powers nor the phase shifts have fixed values but a simplifying assumption is made about the phase shifts according to which distances between neighboring layers can be represented as sums of two distances characteristic of the individual layers. In both these sections a random sequence of the layers is assumed. In the fourth section the problem of arbitrary scattering powers and phase shifts is treated, and furthermore a statistical correlation between neighboring layers is introduced. In the following section the general theory is applied to a specific partially ordered stacking of layers encountered in micas and other similar minerals. The last section treats irregularities in close packed structures of spheres and irregular sequences of layers in graphite.
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