Recent advances in perfluorinated ionomer membranes: structure, properties and applications

j o u r n a l of MEMBRANE SCIENCE ELSEVIER

Journal of MembraneScience 120 (1996) I 33

Review

Recent advances in perfluorinated ionomer membranes: structure, properties and applications Carla Heitner-Wirguin Department ~f lnorganic and Analytical Chemistry The Hebrew Uni~'er~ity~['Jerusalem. Jerusalem 91904, Israel Received 7 August 1995; re,~ised9 May 1996: accepted 10 May 1996

Abstract This paper is a survey of the studies made in recent years on perfluorinated membranes, structure properties and applications. The properties and structure of these membranes were mainly studied by SAXS, SANS, NMR, ESR conductance and IR spectroscopy. Special attention has been dedicated to the solutions of these membranes and recasting of new films and composite ones. Applications in the field of fuel cells, alkali cells, electrolyzers and sensors are described. Keywords: Perfluorinatedmembranes:Nation; Ionomers;Cation exchange membranes:Composite membranes

1. Introduction l. 1. Composition and general properties The interest in ionomers-ion containing polymers started in the forties when the development of organic ion exchange resins began. The properties of these materials are so different from the normal polymers and their applications are so numerous and diversified that their studies still increase continuously. The interaction between the ion and the polymer in the ionomer is so important that these materials may be used as permselective membranes, thermoplastics and films for microencapsulation and coatings. The scope of this review was to study one group of these materials which consists of a linear polymer

of fluorocarbon and a small percentage of pending acid groups (not more than 10 tool%). The acid groups are mainly sulfonic acid or carboxylic acid and in them the hydrogen ion may be partially or totally exchanged by all kinds of cations or cationic groups. As will be described in detail, the polymeric chain is chemically and thermally very stable and very suitable as a backbone for very thin membranes while the acid groups are very appropriate for ion exchange. Dupont de Nemours was the first one in 1962 to develop a perfluorosulfonic membrane which was called commercially, Nation '~~ ~. This membrane consists of a polytetrafluoroethylene backbone and regular spaced long perfluorovinyl ether pendant side

i Nafion.~is a registered trademark of E.I. I)uPontde Nemours.

0376-7388/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S037~'~-7388(96)00155-X

2

C. Heitner-Wirguin / Journal ()f Membrane Science 120 (1996) 1-33

Table 1 From Grot [1]

Monomers for Commercial Membranes

CF 2 = C F - O - C F

2-CF-O-

CF 2 - C F 2 - S O 2 F

CF 3 CF 2 = CF - O - C F 2 - CF - O - CF 2 - CF 2 - CF 2 - S O 2 F

Nation

Dupont

Flemion

A S A H I Glass

ACIPLEX

ASAHI Chemical

CF 3 C F 2 = C F - O- CF 2 - CF - O - CF 2 - CF 3 - C O 2 C H 3 CF 3 C F 2 = C F - O- CF 2 - C F 2 - C F 2 - C O 2 C H 3

Nation

Dupont

ACIPLEX

ASAHI Chem.

Flemion

A S A H I Glass

Monomer for the Experimental D O W Membranes

C F 2 = C F - O - CF 2 - CF 2 - S O 2 F

chains terminated by a sulfonate ionic group. The chemical formula is: ['(CF2-CF2)n-CF-CF2-]m

I

O-CF-CF2-O-CF2-SO-aM +

I CF3

where M + is the counter ion (H +, Li +, Na+). The most frequently used membrane is 0.178 mm thick and the equivalent weight (EW) is 1100 g polymer per equivalent of ionic groups (n ~ 6.5). The molecular weight of the polymer is difficult to determine and industrial laboratories do not publish their results which are apparently changing all the time. A rough estimation gives the molecular weight 105 to 106 (100 < m < 1000). The Nation membrane is used as a separator in chor-alkali production due to its excellent ionic conductivity, chemical resistance and ion selectivity, replacing the old

Experimental D O W M o n o m e r

sodium amalgam or diaphragm cells. The relative permselectivity of cations vs. anions enables the membrane to permit the passage of cations and water while blocking the passage of anions even under high current densities and ion concentration gradients. Thus the use of these membranes allows the production of very concentrated and pure soda with a very high current efficiency (higher than 98%). The initial perfluorosulfonic membranes prepared by DuPont had a very good ionic conductivity, however a poor ionic selectivity. A solution to this problem was found by Asahi Glass Group in Japan which prepared a carboxylic membrane called Flemion ~ 2 containing the same chain as Nation. The Flemion membrane had a selectivity of nearly 100% but poor conductivity. The membrane which is now used in chlor-alkali cells is a multilayer of perfluoro-

2 Flemion ® is a registered trademark of Asahi Glass Group in Japan.

C. Heitner-Wirguin / Journal of Membrane Science 120 (I996) 1-33

sulfonic and carboxylic layers fortified by a network of polytetrafluoroethylene. The perfluorosulfonic membrane was also used in fuel cells for power supplies for satellites launched in the 60s, in electrochemical devices, in metal ion recovery and water electrolysis. Another use of these membranes is as superacids, i.e. acid catalysts in many organic reactions. As all the syntheses of these membranes are industrial processes, they vary from place to place and are constantly being improved and changed, many details are not known about them. The equivalent weight (EW), i.e. the reciprocal of the exchange capaciiy EC (EC = 100/EW) is supplied with every membrane. All these commercial membranes differ by their monomers (Table 1) [1], i.e. the length of the unity of the chain which also determines the amount of ions and type of pendants. In recent years, the Dow Company prepared a membrane very similar to Nation, however, it is still in the experimental stage. The uses of all these membranes are limited due to their high cost resulting from their complicated syntheses. If these ionic polymers are compared to the original polymer it is found that there are large differences in their properties affected mainly by the water content which is also dependent on the cation in the ionic polymer. These membranes contain incompatible components; the fluorocarbon phase and an ionic phase, including the ions and the water molecules in

it. These phases are separated to the limit that the covalent bonds hold them together. The rather special structure proposed for these membranes is of a cluster type containing the aqueous ions imbedded in a continuous fluorocarbon phase. The clusters are interconnected by narrow channels which determine the transport properties (Fig. 1) of the ions and water [2]. The water content of the membrane is of great importance for its properties and is determined by the type of the polymer and the kind of counter ion. More recent applications of these materials were obtained by dissolution of the membranes and preparation of very thin membranes. The low equivalent weight polymers (EW < 900) are soluble in many polar solvents, the high EW membranes EW > 1000 are swollen in solvents but are not soluble. A method was developed and patented by Covich [4] refluxing these membranes with high boiling points solvents such as amides. The solutions, thus obtained, are of a dark color and partially decomposed. In the 80s another method of solubilization of Nation was developed and patented by Grot et al. [5] from DuPont and by Martin [6]. In this method Nation was heated in mixtures of ;,~ater and alcohol at 250°C in an autoclave under pressure. These solutions were used for casting thin fihns and composite films as well as for coating of electrodes and for repairing damaged membranes, The scope of the paper is to describe the recent advances in the field for these membranes. Due to

CLUSTER-NETWORK MODEL

so~

3

so~

SO~

SO~

so; Fig. 1. Cluster-network model (from Gierke and Hsu [2]).

4

C. Heitner-Wirguin/Journal of Membrane Science 120 (1996) 1-33

the enormous number of publications in these fields only those which appeared after 1988 will be discussed here. The numerous applications developed in recent years have also led to a more detailed study of properties and structure of these membranes. Special attention will be given to this subject. The papers which appeared before 1988 are reviewed in books and chapters cited here and provide a good introduction to this field [1-13].

10 5 x

'~"\"

7"~ 10 4 ~

,, +

2. Structure and properties as defined by physical and spectral measurements

2.1. Diffraction and microscopic studies Pineri's group [14] analyzed the shape and intensity of the spectrum of small angle neutron scattering (SANS) of a perfluorinated ionomer membrane containing various amounts of water and proposed a model for the distribution of the hydrated micelles in the perfluorinated matrix. The model for these micelles shows a very locally ordered structure with four first neighbors located at a defined distance and embedded in a completely disordered gas of micelles. This model tits well with the experimental results for sample of membranes with water contents from 6-26% (volume fraction). The radius of the micelles and the distance to the first neighbors have been calculated for each sample and enable the evaluation of the number of charges per micelles in three independent ways. The number of charges increases slightly with the hydration process (from 20-45) and thus keeps constant the specific surface of each charge over the interface with the polymer ( ~ 7 3 ~2). In a paper from Cooper's group [15], a combined method ASAXS (anomalous small-angle X-ray scattering) was employed in order to use the same sample for all the measurements. This method eliminates the chance for a mismatch in morphologies between samples prepared and to have different contrasts for SANS (small angle neutron scattering) and SAXS (small angle X-ray scattering). The authors have studied by these methods, among other ionomers, Nation in the acid form and neutralized by nickel. The results are consistent with the three phase

°

. . . . . . . . Ni 2÷ Notion 10

3

0.1

. . . . . . . .

I

1.0

. . . .

,,,,

10.0

q (nm-') Fig. 2. Scattering patterns for (--) acid Nation and (---) neutralized Nation, both taken at 50 eV below the measured nickel K-edge in the ionomer(from Register and Cooper[15]). model. The Nation in the acid form shows t w o peaks, a crystallite one at 4 ,~ 1 and an ionomer o 1 peak at 22 A - . In Nation, the acid groups are believed to form microdomains similar to the ionic aggregates in neutralized Nation. The nickel neutralized Nation shows a crystallite reflection at a reduced intensity in comparison with the acid, while the ionomer peak is not visible at all (Fig. 2). This is apparently due to an incidental equality of the ionic aggregates and the amorphous polymer electron densities. Lee et al. [16] have used a new combination of small and wide angle neutron scattering, diffraction patterns being measured over a momentum transfer range of 0.03-20 A-~ for three isotopic water mixtures in Nation. It was found that the local water structure is similar to bulk water. From these experiments, the average radius of the aqueous domains was evaluated as 18.5 ]~ with a mean separation of 70 A. The results obtained by this method are also consistent with the water being located in a continuous network. Very recently, a first X-ray scattering study (a wide angle only) was made for the membrane developed by the Dow Company [17]. This note is very preliminary and the only conclusion of it is that there is no crystallinity in the membrane and it has a similar structure to the amorphous PTFE (polytetra-

C. Heitner-Wirguin /Journal (~fMembrane Science 120 ~1996) 1-33

fluoroethylene), which is, as a matter of fact, its backbone. In recent years, structural characterizations of Nation membranes have been made by TEM (transmission electron microscopy). Xue et al. [18] have prepared films of Nation by dissolving the membrane in a mixture of low aliphatic alcohols and water and casting the solutions on clean glass. The films were stained with R u O 4 and were characterized by FTIR (Fourier transform infrared spectroscopy) and XRF (X-ray fluorescence spectroscopy) after being air dried for 24 h and kept in a vacuum oven at 50°C overnight in order to standardize the amount of water. SEM (scanning electron microscopy) measurements have clearly identified three phases: the ionic phase, the organic phase and an interracial phase. The average size of the clusters was found as 25-50 ,~. By XRF and FTIR, it was concluded that the staining reaction involved the oxidation of absorbed water by RuO 4 to give RuO 2 microparticles. Rieberer and Norian [19] characterized some properties of Nation by analytical electron nilcroscopy. Samples were prepared by microtoming sections as well as by dissolution of the membrane and recasting films, no staining, however, was used. As will be shown later by other methods, dissolution of membranes and recasting do change the structure of Nation. TEM measurements show a range of cluster sizes in both samples of 10-50 A similar to the results of SAXS and ED (electron diffraction). Nation shows by electron diffraction measurements a pattern of sharp rings due to crosslinking with well defined bonding distances between the fluorocarbon chains. From X-ray analysis, the ratio of S to Cs can be determined and shows that almost 100% of the sulfonic groups are exchanged by caesium ions.

2.2. Sl~ectroscopic measurements, IR and FTIR studies The first papers on IR measurements have been reviewed in 1986 [7] and deal mainly with the vibrations of the backbones of Nation which are independent of the counter ions and the degree of hydration. Another purpose of these studies was the assignment of the water bands in order to define the type of water molecules and their binding. The main

5

band in the wet sample ( ~ 3520 cm ~) is assigned to water which forms hydrogen bonds, the strength of which are significantly weaker than that of the hydrogen bonds in pure water. It was expected that less extensive hydrogen bonding exists in a small cluster than in pure water. The model proposed in these papers invokes three types of water, not hydrogen bonded water (3714 cm t), partially hydrogen bonded (3668 cm ~) and hydrogen bonded (3524 cm i). The major peak in the wet samples (3524 cm ~) is assigned to completely hydrogen bonded water to either another molecule of water or to a sulfonate group from Nafion. The highest peak is only seen in the driest samples and is attributed to water with both protons surrounded by fluorocarbons not forming any hydrogen bonds. The peak at (3668 cm ~) is assigned to water which is partially bounded to fluorocarbon and has one proton available for hydrogen bonding. In recent ,,'ears, IR and Raman measurements have not been used for the proper elucidation of Nafion structure but only to investigate some special problems. Thus IRRAS (infrared reflectance absorption spectroscopy) [20] has been used to study the thermal stability of Nation films cast on platinum foil substrates. These films were heated to various temperatures (22-300°C) and their IR spectra measured. Spectral changes found up to 200°C were practically nil while after 200°C a significant decrease in the intensity of the S - O vibrations indicated that a substantial loss of sulfonic acid groups takes place in Nation. Bribes et al. [21] have used Raman spectroscopy for assignment of the bonds of industrial perfluorosulfonic membranes immersed in aqueous solutions. The same authors studied also the influence of the interaction of counter ions (cations) with the sulfonic groups. The intensity ratio R of the vibrations lu~S03/IT~(C -O-C) shows a very good linearity for the monovalent ionic radii and also for the divalent ionic radii. Cable and Moore [22] have studied the effect of hydrophobic cations on the SO~ and C - O C symmetric vibrations. These bonds shift systematically with increasing tetrabutylammonium content. If these bonds are compared with those of sodium neutralized Nation it appears that the hydrophobic cations change the chemical nature of the ionic domains. This behavior is caused by a change in

6

C. Heitner-Wirguin/Journal of Membrane Science 120 (1996) 1 33

polarity of the microenvironment surrounding a portion of the sulfonate groups from hydrated, inverted micelle type domains to a more hydrophobic, low dielectric medium. As the content of the hydrophobic ion increases, the ionic clusters become more hydrophobic, dehydrate and the quantity of the other linkages which penetrate into the polar interior of the ionic clusters decreases while the quantity of ether groups surrounded by the less polar regions of the ionomer increases. Another study dealing with effects of counter ions in perfluorinated membranes in water-alcohol solutions was made by Kujawski et al. [23]. FTIR spectra of membranes loaded with alkaline counter ions and equilibrated with different liquid media were recorded by ATR (attenuated total reflectance). The antisymmetric and symmetric vibrations y~ and % of the sulfonic group are very much influenced by the presence of solvents and indicate a change in the local chemical environment. In the dry membrane, the smaller the bare counter ion, the higher the symmetric vibration showing a stronger interaction of sulfonate-counter ion in the ion pair. Equilibrating the membrane with pure solvents, water, ethanol, propanol, Z reaches a constant value of solvated Nation - Li +. All the other perfluorinated membranes reach this value only in water. The affinity of the Nation to alcohols is attributed to its stronger acidity. Maclean et al. [24] have attempted to find out in which phase, big metal complexes are localized. They have measured the resonance - Raman and UV-Vis spectrum of phthalocyanine and tetraphenylporphyrin of cobalt in a Nation membrane. These measurements show that the phthalocyanine is associated with the ionic domains while the porphyrins are associated with the hydrophobic domains. The spectrum of porphyrin changes in Nation and shows a change in geometry and a decreased core size. Phthalocyanine has protonated groups and can therefore be sorbed in ionic domains. Luminescence probes were loaded in Nation [25] and the emission spectra recorded in order to study the polarity of the cluster phase in the membrane. The probes used were pyrene, a cationic dansyl derivative and Ru(bpy)~ +. According to these measurements a fully hydrated Nation membrane (about 40 Wt% H20) shows a microenvironment polarity appreciably lower than bulk water. These probes

show also that the SO3H sites are chemically heterogeneous. The effect of water content and the influence of the type of counter ion on the polarity of the cluster phase was also studied. The cluster phase becomes nonpolar when water is removed and/or the counter ion is exchanged by a hydrophobic one. It was also seen in this study that charged and uncharged probes behave in the same way. Blatt et al. [26] have tried to characterize the different microenvironments of Nafion using pyrene and two charged derivatives as probes incorporated in the membrane. When cations like Pb 2+ are added as a quencher a redistribution of the pyrene probe occurs, i.e. it is displaced from the hydrophilic region (associated to the ether side chains) to a more hydrophobic environment associated to the fluorocarbon backbone. The positively charged derivative is bound to the sulfonic site and is not displaced from the hydrophilic region by Pb ~-+. In the case where pyrene was displaced by Pb 2+, phosphorescence of pyrene is observed. No phosphorescence was observed for the tmpa derivative [trimethyl (pyren-l-yl) ammonium iodide], which is strongly bound to the sulfonate group and is not displaced by pb 2+. Martin et al. [27] have measured the decay rate constants for luminescence of Ru(bipy)~ + in Nation and conclude from these values that the probe ions reside in sites with different local water activities. The binding properties of metals to Nation membranes have also been studied by laser induced fluorescence measurements of europium(III) ions [28]. The hypersensitivity of the D ~ F transitions of europium(III) to the binding site has been analysed as a function of various experimental parameters. The fluorescence spectra and lifetime measurements provide evidence for distinct metal binding sites within the polymer, each of which is sensitive to the conditions of the preparation of the membrane. 2.3. Nuclear magnetic resonance (NMR), electron spin resonance (ESR) and electron nuclear double resonance (ENDOR)

Nafion has three magnetically active nuclei (~ H, 13C, 19F) which have a high enough natural abundance, and may be observed in an unenriched sam-

C. Heitner-Wirguin / Journal of Membrane Science 120 (1996) 1-33

pie. Most of the older studies deal with the H NMR spectrum in order to study water in solid Nafion. Duplessix et al. [29] have studied the NMR spectrum of Nation in the H + form at various water concentrations (from 2.7-20%) and have found in all the samples only one single proton resonance. Till 4.4 wt% H20, the linewidth increases and the chemical shift moves to higher field, after 4.4 wt% water, linewidth and chemical shift are practically independent of concentration. These different changes with concentration imply two types of water absorption, the slight decrease in linewidth in the wettest sample being connected with a significant increase of the proton mobility of the water molecules at higher water contents. McBrierty et al. [30] have also studied the Fe 3+ and Na ÷ exchanged salts and have discussed the glass like behavior at low temperatures. The results indicate a series of heterogeneous sites of water and fast exchange on the NMR time scale between the sites near room temperature. The same authors [31] have also studied the 19F NMR spectrum and through it the motion behavior of the backbone matrix which are constrained by the ion clustering. Spin-diffusion considerations in the Fe 3+ exchanged salt provide some information about the size and nature of 19F regions in the membrane. In a third paper of this group [32] a perfluorocarboxylate membrane was studied by ' H and 19F NMR and it was concluded that the charged end group (carboxylate or sulfonate) has little influence on the structure or behavior of the aqueous phase. The exchanged cations are not coordinated by the charged groups but by water in the

7

aqueous phase. In this context the model proposed by Komoroski et al. [33] and Mauritz et al. [34] has also to be mentioned as it was developed from FTIR and NMR results. Fig. 3 shows the proposed fourstate model where each state depends on the amount of water present in the membrane. As the content of water decreases, the distance between counter ion and sulfonic group is smaller, the hydration shell getting narrower and disappears. In the absence of water, a contact ion pair is obtained. In more recent years, development in NMR equipment has been used to obtain more detailed information about the structure of the membrane. Zawodzinski et al. [35] have made pulse field gradient spin-echo t H NMR measurements and evaluated I H intradiffusion coefficients in a hydrated Nafion membrane (water content was from 2 - 1 4 molecules per sulfonate group). The dependence of membrane water content on water activity enabled the evaluation of the chemical diffusion coefficients from the intradiffusion coefficients measured by NMR. Schlick et al. [36] have measured the 19F NMR spectrum of the acid Nafion membranes and the corresponding solutions. The membranes were swollen by water, formamide, methanol and ethanol. The spectra consist of sharp peaks superimposed on a broad component. These sharp signals were assigned to fluorine nuclei with different shifts in the perfluorinated backbone and in the pendant chains. The spectra of Nafion solutions were compared to those of Nafion membranes and it was found that the spectra of these membranes swollen by all these solvents are intermediate to anhydrous membranes

PRIMARY HYDRATION SHELL

S--

COUNTERION

(I)

(2)

+

+

H20

H20

(3)

(4)

Fig. 3. Four-state model of hydration-mediated equilibrium between unbound and sidechain-associated counter ions in ionomeric membranes (from Komoroski and Mauritz [33]).

8

C Heitner-Wirguin/Journal of Membrane Science 120 (1996) 1-33

and to those in solutions. The effect of addition of solvent to a dry Nation membrane is the same on the JgF NMR spectrum as an increase in temperature. Xu and Pak [37] have measured the proton and deuteron spectra of the perfluorinated membrane produced by Dow Chemical (XUS) and by DuPont (Nation) in order to study the ionic motion in them. In the hydrated Dow membrane two relaxation peaks were found by 1H NMR showing two different environments for protons. The water sorption of Nation is characterized by an inverse proportion dependence of the line width - this is not observed in the Dow membrane. The 2H NMR spectrum of Nation shows a doublet attributed to nuclear quadrupole interaction, while the Dow membrane treated in 100% relative humidity (D20) presents only a single line in the 2 H NMR spectrum. The authors conclude that although the water molecules are more restricted in motion in the Dow membrane due to its structure (existence of narrow-pore breadths) it does not provide strong hydrogen bonding to eliminate the rapid average motion over the interaction of the nuclear quadrupole. In another paper, these authors together with others [38] have studied water in Nation membranes by deuteron and oxygen-17 NMR and dielectric relaxation techniques at low temperatures. From these studies it is clear that the water molecules reside in glassy domains. The activation energy corresponding to deuteron spin-lattice relaxation in Nation, increases with increasing water content. Even fully saturated films do not exhibit behavior consistent with isotropically free water contained in voids. In spite of a high degree of molecular mobility, angular dependent spectra in both as received and stretched samples reflect considerable anisotropy of the host polymer. Electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) have been mostly made by Schlick and co-workers who have done in these years a very detailed and thorough work. These papers deal with introduction into the Nation membrane of paramagnetic cations such as Cu 2+, Ti 3+ and VO 2+ in various solvents and from these ESR and ENDOR measurements the location of these ions, structure and environment have been determined. Two sites of copper were detected by ESR, one bound to the oxygen of water and the second bound through the sulfonic groups to the polymeric

1~03 I

ul

\',/ Cu(~).-

I,.- ~'"

Ul

5~

;/I, ,,\ /

~15o

i

i 'I

~ Cue2)

i, 02

I 0

/ \'\ /

O~

\ o,

1

,

I

I

I ''~.

~

t

Fig. 4. Suggested structure of the dimers, based on experimental ESR data and simulation of the dimer signals. Ligation to the solvent is indicated by broken lines (from Schlick and AlonsoAmigo [39]).

backbone [39,40]. The relative proportions of the two sites are determined by the amount of water in the membrane. When the copper concentration in the solution increases (above 10%) Cu2+-Cu 2+ pairs are observed with a mean distance of 5.5 ,~. The symmetries defined were tetragonal and tetrahedral for Cu 2+ and trigonally and tetragonally distorted octahedral for Ti :~+ (detected in two similar sites to the Cu 2+ sites). The structure of the copper dimer in the Nation membrane has also been investigated in the Nation membrane swollen by water, methanol, DMF, THF and mixtures of water/methanol. The half field shape for the dimers is identical in all the solvents swollen Nation, the intensity of the spectra increasing with the concentration [41]. The intercation distance was evaluated for the dimer in Nafion swollen by water and was found 5.0 + 0.2 ,~, the cations being attached to two sulfonic groups of the network which act as bridging groups (Fig. 4). The clustering process is described as an increase in concentration of the dimers and a decrease in the interdimer distance. The intercation distance in the cluster seems to be equal or larger than this in the dimer. The intercation distance deduced for the isolated ions are about 10 ,~ [42] and are large enough to accommodate cupric ions fully hydrated by water

C. Heitner- Wirguin / Journal of Membrane & ience 120 (1996) 1-33

molecules. These results are in agreement with previous results that only in dimers and in higher aggregates binding between copper ions and sulfonic group exists. Another important result from these papers [43] is that no evidence was found for aggregation of ions in Nation membranes swollen by other solvents or containing more than 20% methanol/water mixtures. Distances between the isolated ions in these solvents are similar to those calculated in water assuming a distribution of the cations arranged in cubic array. It is possible that in these solvent smaller ionic domains exist however they have not been detected. In another study [44,45] Schlick and co-workers have used acetonitrile as the solvent for Nation swelling. The advantage of this solvent is the superhyperfine splitting from the ligands who can give an indication of the number of ligands closest to the central cation. If cupric ions in dry Nation are soaked with CD3CN, the remaining water are preferentially solvated even in a big excess of CD3CN compared to water. If one cycle of drying is performed on a membrane soaked with CH 3CN the water ligands are partially replaced. If two water drying cycles are performed and soaking with CH3CN or CD~CN, four nitrogen ligands are found around the copper ion in a tetrahedrally distorted tetragonal symmetry. No evidence of clustering was found in fully neutralized membranes after one drying cycle and soaking with CH~CN. Till here the binding of copper to the membrane and the structure of the copper ionic species have been investigated. In a more recent paper [46] Cu 2+ was used as a sensitive ESR probe for the penetration of various polar solvents in the Nation membrane and as an indicator for the replacement of one solvent by another. The structure of the solvent around the cation can be evaluated through the ESR parameters. The replacement of one solvent in the membrane by another depends on the polarity of the two solvents. In mixtures of water/acetonitrite (9:1, 1:1 and 1:9 by volume) preferential solvation of the cation by water is observed. The electron nuclear double resonance (ENDOR) method has been used for Ti ~+ ions in Nation in water and methanol [47,48]. This method has a higher resolution than ESR and an increased sensitivity compared to NMR and can thus better identify the

9

nearest environment of the paramagnetic species to about 1 nm. The method of orientation selection E N D O R has been applied to Ti ~ solvation in methanol and in Nation membranes swollen in mixtures of water-methanol, in site 1 of Ti ~+ in methanol an axial g-tensor was measured implying a trigonally distorted octahedral symmetry. The distance of the cation to the methyl protons is in the range 4.1-4.6 ,~ and no orientation selection is possible. In site 2 the g-tensors are different and suggest a tetragonally distorted octahedral symmetry. Barklie et al. [49] have deduced from the dependence of the ESR line width the concentration of VO 2+ that the ionic clusters contain 2 0 - 4 0 SO 3 groups and have a diameter of 3.1-3.9 nm. Martini et al. [50] have used ESR and ENDOR to define the vanadyl ion sorbed and its binding. The temperature dependence of the ESR line shape indicates that the vanadyl ion maintains its square pyramidal structure and a relatively high mobility even at low temperatures. These results prove that the VO 2- ions are located in a ~ater pool which have properties not very different than those of bulk water that is water sorbed in large ionic clusters. A small fraction of these ions aggregated as dimers with an intercationic distance of > 3.5-3.7 ,~. The ENDOR spectra allowed also the determination o1" the distance VO ~ F which was 10-13 A. This big distance again proves that most of the VO 2 ~ ions were located in the ionic clusters of the Nation membrane. Organic probes such as stable nitroxide radicals (5 doxyl-sodium stearate and 16-doxyl-sodium stearate) have also been studied in Nation membranes [51] and Nafion solutions [52] by ESR. In membranes and in ionomer solutions aggregates were detected however these probes drag part of their solvation shell into the polymer (because of their smaller polarity). In a dry Nation membrane neutralized by Ti:; ~, the superoxide radical anion O~ has been detected by ESR [53]. It is assumed that ()~ is formed by electron transfer from Ti ~-. Below 300 K this anion is motionally stable and rigid. Above 31(l K its signal disappears rapidly. The rates of formation of the superoxide are appreciably enhanced by the presence of AI 3- in the cation mixture which reduces the clustering of Ti 3 + in Nation. Analysis of the results of multifrequency ESR and ENDOR have enabled the study of cation solvation,

10

c. Heitner-Wirguin / Journal of Membrane Science 120 (1996) 1-33

formation of dimers and aggregates in the Nation membrane in one solvent or in a mixture of them. From this study [54] it is clear that dimers and aggregates are only formed in water swollen Nafion. In methanol, DMF and THF there is no indication of aggregation. A small amount of dimers of Ti 3+ and VO 2+ were found in these solvents by ENDOR with a distance of Ti 3+ to fluorine of 8 A and the size of the solvent cluster being less than 20 ~,. The structure of the dimers is the same in all solvents however in water swollen Nation 50% of the cation present is in the dimer form while in the Nation swollen by organic solvents its concentration is significantly smaller. ESR and ENDOR experiments have also been made on Cu 2+ in Nation solutions in various solvents and reconstituted membranes [55] at different temperatures. The counter ions dissolved in Nation in mixed solvents are preferentially solvated by methanol. This is seen from the ESR specific parameters and in agreement also with the results of ENDOR. The ESR parameters measured in Nation solutions containing NMF (N-methylformamide) show 14N nuclei ligated to the counter ion. This explains also the improved quality of the reconstituted films obtained by annealing of a Nation solution of a mixture of water ethanol and NMF, the NMF solvent acting as a plasticizer in the regenerated membrane. NMF is retained in the reconstituted film obtained by drying up to 373 K, at higher temperature (493 K) the original membrane is obtained. As it will be shown later, this result is not accepted by all the authors.

2.4. M o s s b a u e r m e a s u r e m e n t s

Although the Mossbauer method has been used very much for the study of the properties of the perfluorinated membranes in the early eighties, this method was much less used in recent years. The use of this method is limited to the divalent and tervalent iron and the tervalent europium ion. The iron ions are not the easiest one to use for structure investigation as the tervalent one is very easily hydrolyzed at rather low pH while the divalent is oxidized quickly to a tervalent one.

Most of the early Mossbauer work and magnetization studies have been done by Pineri and co-workers, summarized in [56-60,63-67] as well as in our laboratories [61,62,68-70]. This method is especially suitable for the characterization of the immediate chemical environment, the interaction of the counter ion with their surroundings and to enable information about the ionic domains in the membrane. A general feature of the Mossbauer spectra of all the ion exchange membranes (Nations included) is the disappearance of the Mossbauer absorption at a determined temperature, i.e. 225 K for Fe 2÷ and 245 K for Fe 3+. These temperatures were related with Tg the glass transition in the early works but however, the Mossbauer method does not enable to assign this to the Tg ionic phase or the matrix. The Mossbauer parameters have been measured for a Nation membrane neutralized by divalent iron as a function of water content at low temperature. Drastic changes in all parameters indicated that the first water molecules are absorbed specifically at the cation sites. In freshly prepared ferric membranes, three species are present: isolated hydrated Fe 3÷ ions (about 12 A each from the o t h e r ) , o x o b r i d g e d ferric d i m e r s [(H20)sFeOFe(H20)5 ] < 4+ and some aggregate ions (clustered in small groups) which have not been well defined. It must be taken into consideration that not much is known about the interior pH in the membrane although if neutralization started from Nation in the acid form, it should be strongly acidic. The reactions in the perfluorocarboxylic acid [60] are somewhat different. Ferric ions cannot be directly introduced into the membrane due to the fact that the carboxylic acid is a weak acid and probably hydrolyzes the tervalent iron species at the surface, due to the much higher pH inside. By aging or oxidizing a ferrous membrane, small groups of hydrated Fe 3+ are formed which can be found together with isolated hydrated Fe 2+. Less water is present in this membrane enabling a stronger interaction between cation and anion. Another study of Mossbauer's spectra of Pineri's group deals with [63-67] precipitated iron particles and the influence of water. In these studies, Nation is used as a model for concentrated aqueous solutions in order to obtain information on ferric hydrolysis in water. From all these papers it is clear that the iron is always in the aqueous phase in various species up to

C. Heimer-Wirguin / Journal of Membrane Science 120 (1996) 1-33

precipitated particles. The type of species is dependent on the water content and on the pH. Nation iron membranes have been soaked in KOH 0.2 M at 20°C and small superparamagnetic iron hydroxide particles of various sizes are formed. The Mossbauer parameters enabled to evaluate the size of these particles. As was mentioned before, the Nafion membrane was used as a model system for concentrated aqueous solution [66]. Mossbauer spectra were measured on a ferric exchanged Nation membrane and according to two different quadrupole doublets observed, the authors conclude that two dimers of Fe are formed a /x oxo bridged (quadrupole splitting QS = 1.6 mm s - l ) and a hydrogen oxide H 3 0 ~ bridged aquo Fe 3+ species (QS = 0.4 m m s i). At high pH the iron hydroxide precipitate can coexist with these two species. In another study of this group [67], the dehydration of the complex Fe3+(H20)6 was studied in a Nation membrane by Mossbauer spectroscopy. Dehydrating the membrane exchanged with Fe 3+ by heating at various temperatures, the following species were detected according to the Mossbauer parameters evaluated: -H +

2Fe(H20)~ + ,,-* ( H 2 0 ) s F e ( H 3 0 2 ) F e ( H 2 0 ) 5 5 + -H

+

H~O

( H 2 0 ) s F e O F e ( n : o ) 5 4+

~ ( ' (H 20)4Ve(OH)2Ve(H 20)~+ ~.O ~ - .~ H , O

~

2Fe e+

Although the Mossbauer parameters of the last stage correspond to divalent iron, it looks strange that at high temperatures and vacuum, tervalent iron is reduced to divalent. A Mossbauer study performed in this Laboratory on Fe 2+ and Eu 3+ hydrated cations in Nafion membranes at low temperatures have shown a very similar behavior to macromolecular systems like hemoglobin, myoglobin and ferritin [68,69]. These Nation membranes equilibrated with iron ions show two types of Mossbauer spectra. The spectrum at 150 K and others below 180 K show a normal spectrum of Fe 2- in a solid environment consisting of a

II

hyperfine doublet with relatively sharp lines corresponding to the elastic or recoilless absorption characteristic for solids. Above 180 K, the spectrum changes dramatically, showing a relatively narrow part similar to this at low temperature and a very broad line with wings extending up to velocities of _+ 15 mm s ~. This spectrum may be decomposed into a narrow and wide compound which are essentially of comparable intensity and must be ascribed to the dynamics of the iron ions in the membrane. At 180 K there is an onset of a bounded diffusional motion of the iron ions together with a marked softening of the membrane. The phase transition at 180 K is apparently connected with the glass transition of the ionic phase• The glass transition temperature T~ differs strongly from that observed in a free aqueous solution, and may be due to the fact that in the membrane the ions are bound to the polymer side chains. The bounded diffusion observed in the membrane above Tg are never seen in tree aqueous solutions. The bounded diffusion causes the appearance of an inelastic line in the Mossbauer spectrum. Litterst et al. [71] have predicted such a spectrum for vinyl ferrocene, however, they have not seen it. A phase change for Fe Nation at 180 K has been observed in the papers of Rodmack et al. [72] for diand tervalent iron in Nation and of Boyle et al. [73] for europium in the Nafion membrane. These spectra indicate that the iron is bound to some part of the polymer, probably to the water molecules and sulfonic groups and thus iron moves together with larger parts of the polymer. The similarity of this spectrum to that of proteins implies that bounded diffusion is a universal feature of macromolecules. In a later study in our Laboratory the influence of the iron ion size on the dynamics in the Nation membrane has been studied [70]. The ions chosen were Fe(H20)~ + and Fe(bipy){ ÷ From this study it has been seen that while Fe(H20){ + Nafion loses ~ 19 molecules of H~O per molecule up to 620 K, Fe(Npy)~ loses only 6 molecules of H 2 0 up to this temperature. The Mossbauer parameters in the two Nafions are identical up to 170 K. In the Fe(H20){ ~ Nation there is an onset of additional motions at 180 K while in the bipyridyl complex this starts only at 220 K, i.e. 40 K higher. It is assumed that the relative strong affinity of the (bipy)~- to the Nation is due to the hydrophobic interaction of this complex

12

C. Heitner-Wirguin/Journal of Membrane Science 120 (1996) 1-33

to the organic chain while the interaction of the Fe(H20)62+ is electrostatic and much weaker. The softening of the membrane pocket is much larger in the Nation Fe(H20) 2+ membrane than in the Fe(bipy)32+ Nation, as can be seen from the onset. Due to the larger size of Fe(bipy)~ +, the slow collective motion evaluated from Mossbauer parameters is much smaller than that of Fe(H20)62+ in the isolated pockets of 10 A diameter. This is also in agreement with the different amounts of water in the two membranes. It is therefore assumed that the size of the complex ions influences strongly the dynamics of these ions in the membrane because of various water content and different binding to the side chain.

3. Ion and water transport in membranes As the practical applications of the perfluorinated membranes are of major importance, water, ion transport and current are very much studied in relation to the structure of the membranes and under the chemical conditions that these membranes will be used. There are, however, some new theoretical studies, too. Yeager's group [74,75] has studied and compared diffusion and water sorption of carboxylated and sulfonated perfluorinated membranes. Cation and water diffusion coefficients are very large in both materials, however, in the carboxylate membrane the diffusion coefficients are even larger and the water sorption is smaller compared to the Nation membrane. This is due to the different clustered morphology of these ion exchange membranes. These authors suppose that the intrusions of fluorocarbon is less frequent in the carboxylate membrane, and therefore, the phase separation is more complete compared to Nation. In the second part of this study, the authors have studied the sorption and transport properties in concentrated solutions of NaOH and NaC1 of a perfluorinated carboxylate and sulfonate membrane. This study was made at rather high temperatures up to 90°C and it was found that the electrolyte uptake was constant with increasing solution concentration while

the water uptake decreased. The diffusion properties of the two membranes are different as may be expected for the difference in acid strengths. In a further study of Yeager's group [76], conductance measurements in an electrolytic membrane cell were made at elevated temperatures and pressures. The solutions studied were concentrated alkali metal hydroxide at temperatures and pressures up to 200°C and 100 bar. The current densities of the membrane were up to 1.5 A cm 2. A tenfold decrease in specific conductance was found when the solution concentration is increased from 5.0 to 10.0 M sodium hydroxide in the temperature range of 100-180°C. The specific conductance increases in diluted solutions due to the sorption of electrolyte in the membrane while in concentrated electrolyte solution the decrease in conductance is not only due to the dehydration of the polymer but also to the ionic interactions and even to the formation of ion pairs when the membrane is in contact with concentrated electrolyte solutions. These results are important for the practical uses of the membranes at high temperatures and pressures. Hsu and Gierke [77], while presenting a theory of ion clustering of Nation, have connected it with the percolative aspects of ion transport. They have derived a semiphenomenological expression for the evaluation of the diameters of the ionic clusters which vary with water content, equivalent weight and type of cation. Their theory predicts also that the short channels connecting two neighboring clusters are thermodynamically stable. Although in other membranes transport is a one-dimensional process (Donnan equilibrium), according to this theory, in Nation it is a three dimensional one controlled by percolation. The experimental data for transport and current efficiency are consistent with the percolative cluster network model but not with the conventional Donnan effect. In a further study of Yeager's group [78], the diffusion of anions such as halides and sulfate in perfluorosulfonated and perfluorocarboxylated membranes was investigated in hot concentrated brine and caustic solutions which are typical to the brine electrolysis cells. The diffusion coefficients evaluated are different from those of the cations previously evaluated and for conventional ion exchange polymers. Different pathways are therefore proposed

C. Heitner-Wirguin / Journal Of Membrane Science 120 (1996) 1-33

for cations and anions in these polymers (not yet defined) and this feature contributes to the high permselectivity in these membranes. Another group who studied practical and theoretical aspects of the perfluorosulfonic membranes properties is the Narebska's one [79-88]. In the first papers they studied electrochemical models of the membranes in order to compute from them the phase composition [79,80]. A multilayer electrochemical model has been found to confirm the experimental results of sorption of external electrolyte and water. From this model the calculations of phase composition of the membranes were evaluated. In another study of this group [81-83,86] the transport across charged membranes and Nation among them has been evaluated by irreversible thermodynamics. Transport of NaC1 and NaOH solutions has been studied and the parameters such as straight resistance coefficients, partial frictions and diffusion indices are evaluated and discussed. If the numerical values of these parameters are compared between NaCI and NaOH, it appears that for N a + - O H - ion pairs the frictional force of cation-anion (usuall~ assumed as zero) cannot be neglected. These interactions influence diffusional and osmotic transport of a solute and water across the membrane. Another specific effect of OH ions is the very low friction of OH ions with water which is very important for membrane permeation. The solutions of phenomenological transport equations have been presented [85] to calculate both the efficiency of energy conversion for the transport of cations and loss of energy originating in the coupling of water and cations flows. This is very important for the processes in electrodialysis or electrochemical cells. The same group has also studied the conductivity of ion exchange membranes (including Nation) and discussed the results on the basis of irreversible thermodynamics [86-88]. They have separated the conductivity coefficients in the conductivity of counter ions against water, the convection conductivity and the conductivity of sorbed electrolyte. These calculations have been made for Nation 120 in contact with NaCI and NaOH solutions and it appears from them that the convection conductivity is very high amounting to about 50~ of the membrane conductivity. They discussed also the mobilities of Na +, Cl- and O H - and water in the Nation membrane in terms of friction

13

coefficients which describe single interactions between components of the system. In recent years, much effort has been made in order to study water, solvent and proton transport in the membranes. Pineri et al. [89] have evaluated the self-diffusion coefficients of water in membranes equilibrated at vapor pressures lower than saturated pressure using far infrared spectroscopy. They have drawn from these measurements the following conclusions: Changes in the water diffusion coefficients are not due to plasticization of the Nation by water but to a change in structure by the swelling of water. The sell' diffusion coefficients evaluated are numerically in good agreement with those determined by other methods (permeation and sorption). Many studies in this field were made by Verbrugge's group, who developed experimental techniques (radiotracers) as well as mathematical models for these purposes. The first system studied by Verbrugge et al. [90,91] was Nation equilibrated with aqueous sulfuric acid solutions. The radiotracer technique developed, enabled to define all the mobile species within the membrane. The concentration of the ions where correlated in a theoretical model with the solvent dipoles and hydration of the mobile ions. From the analytical methods used. membrane partition coefficients were evaluated which are needed lk~r the description of ion and solvent transport within the membrane. From the mathematical model, the ratio of acid in the membrane to this of the reservoir is accurately predicted. The same authors [90] have also siudied the influence of the temperature on the equilibrium and transport properties of Nafion membranes in dilute sulfuric acid. This study was made in the temperature range of 20-90°C and the membrane porosity, proton diffusion coefficient and electrokinetic perineability were determined in order to define polymerelectrolyte fuel cells. Pourcelly et al. [92] have also studied the electrical transport properties of sulfuric acid in Nation 117. The membrane has been confirmed to be very permselective to protons and the transference number of sorbed anions, mainly H S O 4 , is less than 2c/c. Ion and solvent transport in membranes has further been studied theoretically and by radiotracer techniques in Verbrugge's group [93-96]. Behavior of water and other solvents and their properties in

14

c. Heitner-Wirguin / Journal of Membrane Science 120 (1996) 1-33

perfluorinated membranes have recently been much studied [97-101]. Tasaka et al. [97] have studied the water transfer through a series of hydrophilic (among them Nation) and hydrophobic membranes under a temperature gradient. It was found that water is transferred from the cold side to the hot side as the entropy of water transported in the membrane is smaller than the molar entropy of water in the free external water. In a further study of the same group [98], solvent transport was measured under a temperature and osmotic pressure difference in cation exchange membranes, among them Nation and Flemion. The water transport is higher in the H form of the membranes than in the cationic forms as the H + ions can exchange with the hydrogen of the neighboring water molecules and thus contribute to the water transport across the membrane as the proton jumps in conductivity. The direction of the transport in membranes in the lithium form is from the hot side to the cold side, although in thermoosmosis in hydrophilic charged membranes, the transport is towards the hot side. Fuller and Newman [99] have determined experimentally the transport number of water in Nation 117 over a wide range of water contents. The transport number of water decreases slowly with dehydration of the membrane and falls sharply to zero as the concentration of water in the membrane nears zero. The relationship between the transference, the transport number and the electroosmotic drag coefficient (defined as the number of water molecules moving with each hydrogen ion in the absence of concentration gradients) is presented in connection with the water management in solid-polymer-electrolyte fuel cells. Zawodzinski et al. [100,101] have studied the water uptake and transport through Nation 117 membranes in conditions of equal pretreatment in order to get a full understanding of the water dynamics in fuel cells. They have also taken in consideration that the rather hydrophobic membrane surface causes a substantial barrier to the water vapor uptake. The advancing contact angle for a membrane equilibrated with saturated water vapor is particularly high. The hydrophobicity of the membrane surface is of practical importance for fuel cells performance as the only external source of water is vapor carried with the gas streams. Yoshida et al. [102] have investigated the behavior of water in Nation containing various alkali

ions, ammonium and alkylammonium ions by DSC (differential scanning calorimetry). Water molecules in the Nation were divided in three various types, non-freezing, freezing bound and free water, the amount of each type was determined. The permeate flux and the selectivity of water were analyzed depending on the ionic cluster size and the interaction between water and the ions. The size of the cluster, as well as the interaction of water with the ion, affected the permeation of water in water/ethanol mixtures. The permeate flux decreased and the selectivity increased as the interaction of water-alkylammonium ions increased. The opposite was true for the alkaline ions, as the cluster-size swelling with water determined the permeating behavior. Very recently, the proton transport (conduction) was very much studied in Nation [103-106]. Chen and Chou [103] have studied theoretically and experimentally the transport of proton in aqueous and organic aqueous systems. From their study it comes out that the transport is similar in both systems, the only difference being the smaller diffusion coefficient of a proton in the organic-aqueous system than in the aqueous one. Diffusion and proton conductivity data for Nation 117 have been evaluated by Kreuer et al. [105] as a function of temperature and water content. It has been found that these data as well as the enthalpies involved for these processes are similar for the hydrated protonic Nation and acidic aqueous solutions on a molecular scale. The impedance of Nation 117 was measured between 140 K and room temperature by Cappadonia et al. [106] and the activation energies were determined for the membranes after different pretreatments. The conductance between these temperatures shows a change of activation energy similar to acid polyhydrates at the points of phase transition. In water rich Nation the transition is at about 260 K depending on the water content of Nation which is dependent on the pore size and temperature. Above 260 K the conductance and activation energy depend on the water amount which depends on the membrane pretreatment. It is concluded from here that the amount of water has an influence on the proton transfer mechanism in the membrane. As it was pointed out before, the aim of this review cannot be exhaustive as there are too many studies, theoretical and practical, in the last years.

C. Heitner-Wirguin / Journal of Membrane Science 120 (1996) 1-33

Another series of papers are mentioned here [107117], the titles of them already explaining their content. Part of these articles deal with salt uptake and anion transport [107-109], another series deals with mass transfer of various ions and dependence on ionic size and charge [110]. Most of the transport properties were studied by conductivity measurements and impedance [111,112,115] at various temperatures, different ions and solutions (aqueous and methanol [117]). Transport of amino acids were also studied [116]. Cwirko et al. [113,114] have studied theoretical pore models predictions which agree quite well with the experimental results.

4. Properties of solutions of Nation and recast films For various purposes, solutions and films of Nation of different thicknesses were needed. The problem was to obtain a solution of Nafion and from it to prepare thin films and coatings. One of the main purposes was to prepare coatings of electrodes, this has been recently reviewed [118] and will not be covered in this paper. The process of dissolution is a complicated one and has been treated in a series of papers. The second problem was to study the properties of the solutions and films obtained in order to see which properties of the solid are maintained and which are lost in this process. The moment a solution is formed, many ions and molecules can be incorporated in them and then recast composite films may be obtained and used for a variety of purposes. Martin et al. [6] have prepared solutions of Nation from mixtures of 50:50 ethanol-water at 250°C in a high pressure reactor; solutions of 1 wt/vol% of Nation were obtained. A study of Nafion shows that a polymer degradation occurs at 250°C and also at dissolution. Three main properties which are necessary for use of the recast films are however kept: (a) they remain water-insoluble; (b) they retain their ion exchange ability; (c) they strongly retain hydrophobic ions even in the presence of big amounts of hydrophilic ions. As the recast films are rather brittle and cracked, Pineri's group [119,121] and Moore et al. [120] have tried to improve these membranes by adding solvents such as triethyl phosphate (TEP),

15

dimethylformamide (DMF) and other solvents. They got by evaporation at RT (room temperature) uncracked membranes with good mechanical properties after heating at 120°C. In order to analyze the membranes obtained under various conditions, Pineri et al. have made SAXS (small angle X-ray scattering) and WAXS (wide angle X-ray scattering) measurements. It appears from this study that the membrane obtained at higher temperature shows at room temperature a well defined crystallinity inside larger crystallites with a long range order characterized by a repetition order of 200 A. The membrane obtained at RT is a rather dried compact solution with only small entanglements that cannot avoid room temperature dissolutions in various polar solvents. Although the prepared membrane at high temperature has a rather ordered structure there are still structural differences from the original membrane which was dissolved. Similar results were found by Moore et al. [1201. In further work of Pineri's group [121,122], perfluorinated solutions and gels were studied by SANS (small angle neutron scattering), SAXS and conductivity [122,123] measurements. It was found that a hexagonal packing of rod like structures exist in solution and in gel containing a perfluorinated core with the charges on the surface. The diameter of the rods depend more on the surface tension of the solvent than on the dielectric constant. Randolph et al. [124] have studied the techniques of casting membranes and preparing composite perfluorosulfonated membranes by a hot casting method using an aqueous methanol solution with addition of cyclohexanol and DMF. The composite membrane consists of a very thin layer of the membrane material on a porous support and exhibits a high flux and good mechanical strength. Ordinary membranes were cold cast at RT and cured at a higher temperature. Permeation of amino acids was studied and was found higher for the composite membrane than for the original one. A study by Scherer et al. [125,126] has proved that the relative humidity at which the recast films have been dried have a big influence on the thickness, the ion exchange behavior (concerning Ru(blpy)~ and the permselectivity of Nafion toward chloride ions. They have measured SAXS and WAXS for membranes cured under different humidities and these results show that the humidities influ-

16

C. Heimer-Wirguin / Journal of Membrane Science 120 (1996) 1-33

ence the size of ionic clusters, which affect the electrochemical properties of these membranes.

5. Composite membranes: photocatalyticai and other properties With the obtention of Nation solutions and recasting of membranes from them, a new field of applications was developed using these solutions to incorporate various materials such as inorganic and organic semiconductors and to use them in photocatalysis. One of the first groups of semiconductors studied were CdS, CdSe and FeS 2 [127-134]. The main studies in this field have been performed by Bard et al. [127-130,133,134]. They have developed a new technique [127] for the support of highly reactive, small dimension semiconductor particles in a Nation membrane. These particles are metallized and provide a catalytic reduction site which is effective for photoelectrochemical interfaces for photoinduced electron transfer. Photolysing platinized CdS particles in Nation in the presence of a sacrificial electron donor (Na2S), hydrogen gas has been obtained by water reduction. The yield of hydrogen gas was comparable from colloidal and crystal CdS when the same experimental conditions have been used although depending on the crystalline type of the CdS particles. (The cubic J3-CdS are better than the hexagonal a-CdS particles.) Another important factor in this method is the dispersion of platinum on the semiconductors surface. Two different methods could be used, either a homogeneous chemical deposition throughout the membrane or surface coverage by metal vapor deposition. By these methods a good contact metal-semiconductor was obtained and these integrated systems showed prolonged use with good rate of hydrogenation although after a certain time, the rate decreased. The photoactivity can be restored by washing the membrane with boiling water in order to remove the surface coatings. In two further studies, the membranes sorbed with CdS and Pt were studied by SEM and X-ray photoelectron spectroscopy in order to understand and to improve these systems [128,129]. It was found here that while the surface of the hexagonal form of CdS is rich in sulfate, the cubic form has sulfide ions on the surface. Quantum sized cadmium sulfide microcrystal-

lites ( ~ 55 ,~) were incorporated and investigated in a Nation membrane [130]. The luminescence of CdS and CdS-ZnS particles were also studied in Nation membranes in order to elucidate the found synergism in the ZnS-CdS system for photoredox H 2 production [131]. Gopidas and Kamat [132] have prepared quantum size CdSe semiconductor particles in a Nation membrane by exposing a Cd 2+ exchanged membrane to H2Se. The growth of these particles can be controlled by the time of reaction with HzSe or by the concentration of Cd 2+ in the Nation membrane. The position of the emission maximum is dependent on the particle size. Bard et al. [133] have developed an ion dilution technique to prepare ultrasmall CdS and CdSe particles. They diluted the Cd 2+ solution by adding an inert ion, Ca 2+, for example. The ratio of Cd2+/Ca 2+ in the membrane can change the absorption onset for more than 3.5 eV. Bard et al. [134] have also prepared quantum size FeS 2 in a Nation membrane and compared with quantum sized particles of FeS 2 in organic solvents such as MeOH and MeCN. The absorption edge of FeS 2 in Nation is blue shifted by about 0.5 eV compared to the bulk FeS 2 while in the organic solvents the blue shift observed was 1.0 eV. The differences in the blue shift are attributed by the author to the moisture effect on the growth of the particles in the membrane. Among the oxides incorporated in Nation membranes, most studies have been made using TiO 2 [135-140]. Bard et al. have used TiO 2 powders and platinized TiO 2 particles in Nation membranes for a photoinduced oxidation of bromide to bromine. The rates of bromine formation per gram of photosensitive catalyst is greater on TiO~ in Nation membranes than in TiO 2 powders. The photoinduced oxidation of organic molecules has also been studied for a long time. Mau's group [136] has studied among other systems the selective photocatalytic oxidation of mphenoxytoluene on TiO 2 particles supported in Nation membranes. The emission characteristics and catalytic activity of TiO 2 were regenerated by washing the membrane in butanol. In a study in our Laboratory and The Volcani Center [137], a TiO 2 loaded membrane was investigated by TEM, ED (electron diffraction), SEM and EDS (energy dispersive spectrometry). The loading of these small particles was made in situ in the

C. Heimer- Wirguin / Journal q{ Membrane S{'ien~'e 120 (1996) 1-3.?

membrane. The structure of these particles was found to be fine-grained polycrystalline tetragonal TiO 2 of rutile type. Yoneyama and his group studied the photochemical properties of TiO 2 microcrystallites (size quantized) in Nation and clay interlayers [138,139]. The smallest particles were less than 2 nm of diameter and had a band gap energy of 3.95 eV. The photocatalytical activity of these particles in Nation for the decomposition of acetic acid was rather low compared to those in clay interlayers. The authors suggest that the sulfonate group of the Nation decreases this activity being adsorbed on the active sites of the TiO e microcrystals. Similar results were found for Fe20~ particles. The photocatalytic fixation of nitrogen is a very hard task. Tennakone et al. [140] have prepared a Nation membrane in which hydrous iron oxide was incorporated. They have shown that if this membrane is immersed in aerated water at an alkaline pH it fixes nitrogen simultaneously by reductive and oxidative photocatalysis by UV irradiation. This reaction is facilitated by the polymer matrix and yields both NH~ and NO2-/NO ~. If this reaction can be repeated on a macroscopic scale it will be of great importance. The photophysical properties and reactions for uranyl compounds in Nation and Flemion membranes were also investigated, i.e. the energy transfer from UO~ + ion to Eu 3+ [141]. This effect is much more enhanced in Flemion than in Nation. In a study by Mau et al. [142] the photophysical properties of Flemion and Nation were studied and the reason for the differences in these systems discussed. The microenvironments of Flemion are more hydrophobic and rigid than those of Nation and therefore, they are less accessible to oxygen and show phosphorescence at room temperatures. The addition of a heavy atom for the obtention of phosphorescence was not needed in Flemion but was necessary in Nation. The rigidity of the environment is the reason that Pb -~* and I are less diffusing in Flemion than in Nation. Cr ~+ and Ce 4+ were incorporated in Nation and used as catalysts for the oxidation of alcohols [143]. A new class of microcomposite membranes has been studied by Mauritz and co-workers [144-148]. These membranes has been prepared by incorporating by a sol-gel reaction tetraethoxysilane (TEOS)

17

in alcohol-water solutions in Nation and by this procedure an in situ growth of silicon oxide microcluster occurs or an interpenetrating network. The structure of these membranes has been studied by FTIR [145], NMR [145], dielectric relaxation [146], thermal analysis [147] SAXR and SEM [148]. From FTIR and NMR measurements, it was found that the morphology of the inorganic phase is ordered by the two phase structure of the Nation and is preferentially incorporated in the clusters. The hydrolysis of the TEOS is catalyzed by the sulfonic groups. FTIR and NMR have also shown that the inorganic network is not so strongly cross-linked in the membrane as it is in the sol-gel prepared free silica. With increasing uptake of solid, it becomes less connected in the membrane. From the thermal study (TGA and DSC) [147] it was found that the thermochemical degradation begins at around 350°C and then takes place in three stages for filled and untilled membranes. This reinforces the previous results that the structure of Nation does not change with the formation of the silica network. The same results were found from SAXS and SEM measurements [148]. SEM has also indicated that the greatest silicon oxide concentration occurs near the surface of the film and decreases to a minimum in the middle of it. At high silicnn content, SEM shows a briiile surface-attached silica layer. A composite membrane for various electronic devices was prepared by polymerization of pyrrole aniline and similar compounds in Nafion [149-157]. One method of preparation [149] was a photosensitized polymerization of pyrrole by the use of Ru(II)(2,2'-bip5)~ as photosensitizer in aqueous solution in the polymer matrix. By this method, a fine conducting polymer was obtained on insulating materials. Pineri et al. [150] have prepared electronic and ionic conductive polymer composites by electropolymerization of pyrrole, bithiophene or aniline trifluoromethane sulfonate in a Nation type gel. The composites obiained had a high conductivity and good mechanical properties. More rigid composites may be obtained by incorporating finely powdered polypyrrole in the gel. Slight variations of electrosynthesis ol Nation composite films have been prepared by Bidan and Ehui [151]. These membranes have good cation exchange properties with mixed conductivity (ionic and electronic) behavior and faster

18

C. Heitner-Wirguin / Journal of Membrane Science 120 (1996) 1-33

kinetics of redox exchange and can be used in water deionization, batteries, sensors, and SPE (solid polymer electrolyte) for fuel cells. Shimidzu et al. [152] have prepared similar membranes by chemical polymerization. Musiani et al. [153] have prepared a polyaniline Nation membrane and characterized it and its electrocatalytical activity with electrochemical methods such as cyclic voltammetry, chromoamperometry and ac impedance. Electrocatalytic reduction of 02 and oxidation of N2H 4 were achieved on polyaniline Nation electrode. Martin et al. [154] have prepared ultrathin composite polymer films by photochemical synthesis which have many technological applications such as chemical separations, bioreactors and sensors, energy conversion and drug delivery systems. For these purposes, defect free ultrathin films are used on the surface of a microporous matrix. Some films have been found also very good for gas-transport. In a study made by Sata [155-157], anisotropic composite membranes were prepared using cation exchange membranes such as Nation, pyrrole or aniline and ferric ions. One side of the Nation membrane in the ferric ion form was contacted with an aqueous solution of pyrrole, pyrrole polymerized on the surface and produced an isotropic structure, i.e. a thin polymer layer of polypyrrole is formed on the surface of the Nation. This layer does not enable the permeation of polymer ions. This type of composite membranes provides low diffusion rates of electrolytes without increasing the electrical resistance in electrodialysis. During the polymerization Fe 3÷ is reduced to Fe 2+ and this reaction will also contribute to applications of these membranes. For this type of pyrrole polymerization, Nation in copper form was also used. The polymerization with Cu(II) was slower than in the Nafion-Fe 3+ form. The conductivity of the composite membrane was dependent on the immersion period of the film in the aqueous solution. Another series of compounds imbedded in Nation membranes are complexes of metals (Ru, Rh, Pd, Ag, Re and others) [158-169] which may be used mainly as photosensitizers and catalysts. The complex of Ru(bipy)32+ and other Ru compounds were very much studied due to their particular high reactivity of their photoexcited state toward redox

reagents and are therefore used as a probe to detect these reagents. A series of studies of these compounds have been made by Kaneko's group. Ru(bipy) 2+ incorporated in Nation was applied as a luminescence probe for photodetection of oxygen and its quantitative determination in a gas utilizing the luminescence quenching by dioxygen [159]. Electrochemical water oxidation mediated by polynuclear complexes such as [(NH3)sRuORu( NH3)4ORu(NH3) 5]C16 and

[(bipy)2 M @ n

(bipy)2] (C104)3 2 H 2 0

embedded in Nation showed an enhanced oxygen evolution [160]. Kaneko et al. [161] studied also the hydrophobic binding of Ru(bpy)~ + and methyl viologen in Nation aqueous solutions. This study was made by steady-state emission measurements of Ru(bipy)32+ in Nation aqueous solutions and the effects of both quenchers and chemically inert ions. The exchange of MV 2+ with Ru(bipy)~ + and Mg 2+ was studied at various concentrations. The results obtained showed very strong hydrophobic binding of [Rubpy~]Z+AG ° = - 7 . 8 kcal/mol and MV 2+ (AG ° = - 4 . 7 kcal/mol) to the Nation. It seems, however, that no specific hydrophobic regions with well-defined boundaries exist in solution. In a further paper, Rabani et al. [162] continued the study of the hydrophobic interaction and ion exchange of Ru(bpy)3(II), methylviologen and sulfonatopropylviologen in Nation films. The study was made by spectroscopic and electrochemical methods studying the distribution of the solutes between layer and bulk. In two more recent studies of Kaneko et al. [163,164], polynuclear ruthenium complexes were used as heterogeneous catalysts for water oxidation such as Ru-red and Ru-brown in Nation. These two compounds may be reversibly formed: RUI11_ Ru~V_ RuIII Ru-red (neutral or alkaline conditions)

- e~ + e-

RulV_Rulll_Ru

TM

Ru-brown (acidic conditions)

Incorporation of the catalyst in Nation stabilizes it against decomposition [163,164]. Martin et al. [165] have evaluated the diffusion coefficients of

C. Heitne r- Wirguin / Journal ~!['Membrane Science 120 (1996) 1- 3 3

Ru(NH3) 6+ in Nation impregnated Gore Tex membranes (NIGT). Gore Tex membranes are microporous polytetrafluoroethylene filtration membrane, (200 nm mean pore diameter). The diffusion coefficients in these membranes were higher than in the Nation membrane itself. By varying the weight fraction of Nation in the NIGT membrane, the diffusion coefficients of Ru(NH3)~ + and Ru(NH3)~ + can be varied from 3 × 10 s cm2/s to 3 × 10 7 c m : / s . The gain in transport rate in these membranes compared to pure Nation membranes brings with it a decrease in transport selectivity. This is apparently a general effect (according to Martin) that membranes with good diffusion coefficients yield poor selectivities and vice versa membranes with good selectivities have low diffusion coefficients. They call this problem "membrane catch 22" and are looking for a possible solution of it. Very thin Nation membranes (25 nm dry) prepared from DMSO solutions [166] were found to be very inert and showed very large selectivity for oletin separations when they were used in the silver(I) ion form. The unexpected large separation factors for the diene/monoene mixtures can be explained by complexation of dienes to two Ag(I) ions. The photophysics of ReO + in Nation membranes was studied by Gray and Thorp [167] by absorption and emission spectroscopy. From these measurements they concluded that [(ReO2(py)4] + occupies two distinct regions in the membrane, the ion cluster region and the interracial one. Kaneko et al. [168] have s h o w n that [ R e ( b p y ) ( C O ) 3 B r ] and [Re(terpy)(CO)3Br] complexes incorporated in a Nation membrane act as efficient catalysts for the electroreduction of CO~ into formic acid and CO. Terrill et al. [169] have studied the incorporation of a mixed valent osmium bipyridine I I / I I I in Nation. It appears that these complexes are very good electronic conductors due to electron hopping from Os(II) to Os(IlI). Infrared spectra of Os films show that ion pairing between sulfonate and osmium complexes takes place. Honda et al. [170] have prepared a composite membrane Prussian Blue-Nation controlling its structure by monitoring the rate of diffusion in the solid phase by two component ions, Fe :+ and Fe(CN)~ . These membranes as mentioned in this study and by Kaneko et al. [171] have many applica-

19

lions in rechargeable batteries and electrochromic display devices. Many organic dyes absorb radiation over a wide range of the solar spectrum and may thus be used as initiators of photochemical processes and as solar collectors. Methylene Blue (MB) is one of these dyes which has much been studied [I 72-178] and it was found that when MB is incorporated in a membrane it exhibits much stronger properties (fluorescence yield, longer fluorescence lifetime and a blue shift of the absorption spectrum) compared to an aqueous solution. Mau et al. [173] have studied the MB-sensitized photooxidation of anthracene (An) in Nation. It was found that high concentrations of An could be incorporated into a water swollen Nation membrane containing MB (apparently in the hydrophobic region of the membrane). Irradiation of this membrane with light absorbed by MB (,~x 640 nm) resulted in the photooxidation of anthracene. The photooxidation process was monitored by the absorption spectrum showing a decrease of the An absorption maximum at 248 nm and the appearance of the characteristics of the formed anthraquinone. The sensitized photooxidation of the MB is much stronger and faster in Nation than in ethanol, suggesting a nonuniform distribution of MB and an or an enhanced lifetime of the oxidized species. The same authors continued to study the photooxidation of anthracene and its derivatives in Nation by Methylene Blue [174] and compared it with ethanol solutions. Spectroscopic studies have shown that the incorporated chromophores are heterogeneously distributed in micelle like polymer. The uncharged hydrocarbons are located in the non-polar polymer backbone while the positively charged species are located at the interface of aqueous cluster regions. If the photooxidation of An is compared in Nation swollen with H~O or D,O it is much faster in a D~O swollen membrane. The authors attribute it to a more heterogeneous distribution and the behavior of singlet oxygen in the membrane. Mau et al. [175] have also studied the energy transfer of fluorescent dyes in Nation membranes and alcohol. Here, too, a heterogeneous distribution in the micelle-like polymer was found. Mika et al. [176] assigned the band of MB in Nation at 750 nm to the presence of a protonated species (MBH : + ) and calculated the protonation constant and extinction coefficients of MB + and MBH :+. The rate constant of photoreduction

20

C. Heitner-Wirguin / Journal of Membrane Science 120 (1996) 1-33

was low and the back reaction in darkness is inhibited. In a paper of Yoneyama et al. [177] the kinetics of dimerization of Methylene Blue in a Nation film was studied by spectrochemical methods. The dimerization constant of MB and the rate of dimerization evaluated were compared with those obtained in aqueous solutions. It was found that dimerization in Nation is less likely to occur due to the strong interaction between MB and the sulfonate groups of Nation. In a further study of these authors [178], Nation films containing polyaniline and MB have been prepared. The amount of MB can be varied changing the concentration of MB (related to Nation) in the deposition bath of polyaniline. The films obtained showed electrochemical responses of both polyaniline and Methylene Blue. Otsuki and Adachi [179] have investigated the species of MB incorporated in Nation at various degrees of hydration. Protonated charged species have been detected according to the degree of hydration and the concentration of MB. Absorption and emission properties of phenosafranine dyes (in protonated and nonprotonated forms) in a Nation membrane and Nation solution have been studied by Gopidas and Kamat [180]. The nonprotonated form of phenosafranine is photoactive only and undergoes electron transfer with triethylamine by excitation with visible light. Zen and Patonay [181] have shown that Nation is a good substrate for incorporating a near IR (or a fluorescent) pH sensitive dye to determine pH in solution. More work has to be done in order to use it as analytical method. Iyer et al. [182] have studied a laser dye such as 7-amino-4-methyl coumarin and rhodamine 6G in a Nation membrane by absorption and emission spectrometry [182-185]. Both dyes have been found more photostable in Nation than in an aqueous solution. A Nation membrane in which rhodamine 6G was incorporated was used successfully for the determination of hydrogen ion concentrations in the range of 1-5 mol dm -3 in aqueous solutions in the temperature range of 25-100°C [ 185].

6. Use of perfluorinated membranes in industry and research The perfluorinated membranes have many applications, particularly in the field of electrochemistry.

New developments are published the whole time such as in the plating industry, surface treatment of metals, in batteries, sensors, drug release and others. The many uses of these membranes are due to the excellent properties of these membranes such as thermostability and inertness towards corrosive materials and also mechanical properties. Nation, for instance, is only permeable to cations, specifically to protons and water. The easy water transport through the polymer prevents local drying which could occur in some applications. When Nation is swollen to a high degree, the electrical conductivity in the protohated form is high compared to cationic forms of this material. The permeability of polar organic solvents of low molecular weight is lower than that of water. Nation produced by Dupont de Nemours is the most used proton conductor as such or reinforced with other polymers. All these properties enable the use of these materials for many purposes, the only disadvantage of this material being the high price. The most important industrial application and the oldest one of these materials is the electrolysis of sodium chloride solutions. The perfluorinated ion exchange membrane was used as a separator between the half cells in order to eliminate the environmental problems of the older methods (the amalgam process and the old diaphragm process). Today this is the only method used as it was improved to appreciably reduce the energy costs. The principle of a chor-alkali cell is shown schematically in Fig. 5. The function of the separator is to allow the transfer of sodium ions with minimum electrical resistance to prevent mixing of the two gaseous products and the two electrolytes and to minimize the transfer of chloride and hydroxide ions. A small amount of water is also required to be transferred by electroosmosis. Fig. 6 shows a crosssection of a high performance membrane, which satisfies these requirements. This membrane was also reinforced in order to improve the mechanical properties and then the surfaces were improved, first a carboxylic acid barrier layer was added in order to minimize the hydroxyl ion and finally a special coating to the gas release. The membranes which are used in this process have been very much studied and afterwards improved [ 185-191 ]. A rather old application of these membranes was as a separator in hydrogen/oxygen fuel cells who

21

C. Heitner- Wirguin / Journal of Membrane Science 120 11996) 1 33

CI2

t

[

NaCI

IO-i,i %

m

J

-

I~°

a i

DA.O,Y,, ~ANODE *

ANODE

li--

I I CA£OD,

J CATHOLYTE

~e ,,&,

~

CATHODE

t

H20

NaCI 20-25%

Fig. 5. The chlor-alkali electrolyser (from Gavach and Pourcelly

b

[186]). served as power supplies for the satellites developed and launched in the 1960s. This was not then considered a big business, but in recent years has grown as these fuel cells provide clean energy. Dupont and other industries spend large sums of money for the improvement of these materials. Pineri et al. [192] have prepared among other materials a membrane by precipitating locally platinum microparticles on Nation. This membrane was used in an electrolysis cell for more than 15 000 hours of continuous operation without degradation. Scherer et al. [193] have used a partially fluorinated membrane for water electrolysis cells and have shown that under their working condition 80°C and 1 A / c m 2 they are efficient and of economic interest - no degradation was observed after 10 000 hours of continuous operation. Another technology developed in the last years was the solid polymer electrolyte (SPE). The separaSulfonate Polymer

:

Carboxylate Polymer

]0 Gas Release ~ Surface Modification

~o

~0 ~0

- - G a s Release Surface Modification

~o Reinforcing Fiber

!C)

Fig. 6. Cross section through a modern high performance chlor-alkali membrane (from Grot [1]).

Fig. 7. (a) The zero-gap membrane cell. (b) The solid polymer electrolyte (SPE) cell (from Gavach and Pourcelly [I 86]).

tor (a semipermeable membrane) in the electrochemical cell has to be as near as possible to the electrodes in order to decrease the internal ohmic drop. The limiting case is this where there is physical contact between the electrodes and the separator, i.e. the electrodes consisting of a porous electrocatalytic material in order to have a triple contact between the electrode separator and the catholyte or anolyte. This assembly can be obtained in two different ways [1861: 1. The zero gap membrane cells where the electrodes and the separator are the distinct components, i.e. the electrodes are brought into contact with the two faces of the separator (Fig. 7a). 2. The SPE cell is a cell where the electrodes are prepared by deposition on the separator (Fig. 7b). Various cells are therefore dependent on the type and method that these electrodes are created - in all cases Nafion is used as separators. The conductivity of Nation in the protonic form is very high and enables the obtention of high current densities and rather small ohmic drops. As in most of the electro-

22

C. Heimer-Wirguin / Journal of Membrane Science 120 (1996) 1-33

chemical cells, the atmosphere is very corrosive, only noble metals (acid resistant) must be used as electrocatalysts [194-208]. Srinivasan and others [194-199] have used platinum gauze electrodes coated with Nation and studied on them the kinetics of the reduction of oxygen and the oxidation of hydrogen. Millet et al. [195,199,202,208] have prepared a solid polymer electrolyte for water electrolysis using platinum for hydrogen evolution and on the anodic side oxides of iridium and ruthenium. Using rather new loadings of noble metals ( < 1 mg cm -2) with low cell voltage (1.75 V at 1 A cm 2 and 80°C) the system operated for 15 000 hours. These composites apparently operate well for H 2 / O 2 SPE fuel cells. Verbrugge et al. [196] have investigated the properties of the perfluorosulfonic acid membranes for fuel cell electrolytes prepared by Dupont (Nafion) and Dow. It was found that the Dow membrane yields a more easy proton transport due apparently to larger pore breadth. Additional tests on the Dow membranes were made by Eisman [197]. The high voltage at low current densities can lead to high system efficiencies not changing other critical properties needed for fuel cell operation. Holze and Ahn [204] have prepared electrodes for water electrolysis and hydrogen/oxygen fuel cell systems by incorporating the catalyst in PTFE and subsequently, pressed it on the membrane. Savinell et al. [207] have prepared SPE for work up to 200°C. They have obtained materials of high conductivity using Nation with a high boiling point Bronsted base such as phosphoric acid. Gottesfeld and his group [201,205,206] have studied various polymer electrolytes for fuel cell applications, mainly by membrane transport and evaluated parameters such as water sorption characteristics, diffusion coefficients of water and protonic conductivity. Bard [209] has used Nation as support material for semiconductor particles for the photoelectrochemical utilization of solar energy. For other electrochromic applications solid polymer electrolytes such as Nation were also used. Tungsten oxide was used by Murray et al. [210] and Pennisi and Simone [211] for electrochromic devices with Nation. Other devices studied were Methylene Blue in conducting polyaniline matrixes using Nation as an electrolyte anion [212].

Organic electrochemistry was mainly studied by Ogumi's group [213-218] and others [219-221]. Ogumi et al. used in all these papers Nation as a SPE modified for various specific purposes. Thus the proton permselectivity was enhanced by depositing on the SPE surface an ultrathin anionic exchange layer containing pyridine rings by using a glow discharge (plasma) polymerization technique [113]. These treated membranes had very high proton permselectivity, however, the resistance of them were very high. The SPE method (Nation) was used for the hydrogenation of olefinic double bonds [214]. Kolbe reactions of acetate and other organic materials were also found feasible in a Pt-SPE cell [215]. In another study made by Ogumi et al. [216,217] an iron redox couple was incorporated in Pt-Nafion and its behavior towards the oxidation of ascorbic acid studied. SPE was used for electroreduction of substituted aromatic nitro-compounds using C u - P t - N a f ion [218]. The influence of the multiphase structure of Nation on the electroreduction of these compounds was studied. The electrooxidation of methanol was studied on SPE to which Pt, PtSn, PtRu and PtIr was bound [219]. Pt-SPE has a higher catalytic activity than a Pt electrode, however the addition of Sn, Ru or Ir enhances the activity as the oxidation state of Pt is modified by Sn or Ru and to the presence of water and methanol in SPE whose hydrogen bonds are weakened there. Mau et al. [220] have developed perfluorinated membranes as catalyst supports using Nation and porous PTFE. The materials incorporated in these membranes were nickel complexes and platinum and palladium, while the reactions investigated were olefin conversion processes (hydrogenation). The rate of reactions were better than before, the turnover more than 6000 and no poisoning was observed. The membrane with the Ni-complex was used for double bond shift isomerization. In a second paper of the same group [221] Nation supported cationic palladium complexes were studied as catalysts for the dimerization of ethene which improves compared with the reaction in solution. Mau et al. [222] have used also Nation tubes that function as a reaction catalyst and a pervaporation membrane. The purpose of this membrane was to increase the yield of esterification of acetic acid with methanol and n-butanol. Nation removed selectively

C. Heitner- Wirguin / Journal qf Membrane Science 120 ~1996) 1-33

the water from the esterification reaction mixture, the yield was bigger for n-butyl-acetate as the membrane exhibited a greater permselectivity for water over n-butanol compared to water over methanol. Nation in the hydrogen form is a very strong acid (superacid) and as such it is very much used as catalyzer in organic synthesis [223-225]. As Nation is used as particles and not as a membrane, these applications are beyond the scope of this survey. Nation membranes were very much used recently in sensors at room temperature. For oxygen determination these sensors are either of the potentiometric or the amperometric type. The first sensors for oxygen were build of stable zirconia (ZrO2-Y203), however, they had to be used at high temperatures in order to increase the conductivity. Nation membranes work well as proton conductors at room temperature and are therefore more economical in use. Kuwata et al. [226] have used an amperometric sensor for oxygen determination consisting of an electrochemical cell combined with a hydrogen-generation system and a gas diffusion layer. The sensing current under the short circuit condition was found to vary linearly with the partial pressure of the oxygen. Morris et al. [227] developed sensors of potentiometric type for hydrogen, oxygen and water using Nation membranes. SPE was used and a reference electrode of a mixture of Fe -~+ and Fe 3+ sulfate hydrates. The sensors showed a good long term voltage reproducibility and stability, and were used for monitoring hydrogen in metals (steel and zirconium). The authors have found that the equilibrium water concentration in Nation is determined by the activity of water in the gas phase and the diffusion coefficient of water in Nation, it is also a function of the water content. An amperometric sensor for water using Nation has been developed which showed a good reproducibility and responsiveness to changes in the gas humidity. Sima et al. [228] have developed a rather simple oxygen sensor with a Nation membrane as electrolyte and may be developed in any size and shape. This sensor can provide direct information on oxygen permeation which is the main cause for the deterioration of paint films. An electrochemically active membrane was developed based on metallized Nation [229] as a vibration sensor (damper). If vibration in machinery and structures can be controlled, premature failure and noise

23

reduction may be prevented. The "'smart" material which can do it is the metallized Nation membrane. Other types of sensors which are very much used are the optical ones. Bright et al. [230] have studied the transduction mechanism of an optical sensor based on a Nation film impregnated with rhodamine 6G by steady state and frequency domain fluorescence. Sadaoka et al. [231,232] have developed optical humidity sensors using Nation-dye thin films where the dye was crystal violet, pink FGH R or dyes having cyanine or triphenyl structures with a terminal N-phenyl group. Burgess et al. [233] have measured the concentration of sodium hydroxide with a fiber optic sensor containing a renewable reagent. This sensor consists of a weakly bufl'ered acid-base indicator reagent (Bromothymol Blue) delivered continuously to the sensor tip which consists of a length of a Nation hollow fiber membrane, an analyte exchange region and an optical cell. Sodium hydroxide solutions between 0.38-7.2 M were measured with a standard deviation of 1.6¢/c. Medical applications of the Nation membranes and coatings are more and more developed [234]. Nation can be shaped in simple and complex configurations and thus is very important in biosensors and immunodiagnostics. The Nation membranes have shown to be extremely reproducible for glucose sensors in whole blood in vitro and other applications have also been examined. Nation shows also a high degree of biocompatibility and it will perform very well for implantable sensors, ln-vivo animal studies were made in order to develop tests and implants in human beings. Glucose biosensors tk)r use in blood in vitro and biosensors for neurotransmitters have been developed with the use of Nation membranes and will be here surveyed. Fan and Harrison [235] have studied the diffusion coefficients of neutral species like glucose (pH 13), hydroquinone (pH 7.4) and ascorbic acid (pH 1.3) as well as the cation [(trimethylammonium) methyl] ferrocene (pH 71) in Nation membranes. Glucose and ascorbic acid showed effective diffusion coefficients of 3.4 and 22 × 10 s c m 2 / s respectively in Nation films cast from alcoholic solutions at room temperature. If the films are cast from the same solutions at 120°C. diffusion coefficients decrease appreciably. Dimethylformamide (DMF) decrease the constants at RT and also at 120°C. Gottesfeld et al. [236] have devel-

24

C. Heitner-Wirguin / Journal of Membrane Science 120 (1996) 1-33

oped a glucose sensitive electrode composed of glucose oxidase (GOX) embedded in a film of Nation. An amperometric glucose sensor based on G O X / N a t i o n cast on platinum, responds to glucose additions rapidly ( 2 - 4 s) has a long lifetime when stored in a phosphate buffer (55 days). These electrodes exhibit a reduced sensitivity to oxygen, respond linearly to additions of glucose and are not damaged by blood. The Nation confers a permselectivity discriminating against anionic interferents. Wilson et al. [237] have eliminated the acetaminophen interference from an implantable glucose sensor by using a composite membrane consisting of cellulose acetate and Nation. Kaifer et al. [238] have studied the binding of neurotransmitters (dopamine, epinephrine, norepinephrine and serotonin) with a synthetic receptor - cyclobis (paraquatp-phenylene) in aqueous solution by spectroscopical and electrochemical methods. The electrochemical studies of the receptor incorporated in the Nation films cast on the electrode showed that the charge propagation across the Nation is slow, however, the receptor is strongly retained in the matrix. If thicker Nation membranes preloaded with the receptor are used, the neurotransmitters may be extracted from an aqueous solution. These properties are the necessary ones to devise practical sensors for these materials. The group of Cox et al. [239] have studied the neurotransmitters and their properties. Dopamine can be preconcentrated on a Nation membrane. They have found that the uphill transport of dopamine can be promoted by Donnan dialysis. The extension of Donnan dialysis to a compound that can interact with the backbone of the membrane is of analytical importance. Crespi et al. [240] have developed recently a new microbiosensor for detection of neurotransmitters in vitro and in vivo. They used a Nation crown ether coating on a carbon fiber electrode. This film showed a higher sensitivity to dopamine than Nation itself. Cell culture methods were used in order to prove the biocompatibility of Nation compared to other polymers used previously [234]. It was found that the cell growth rates are somewhat slower than in (tissue culture) TC-polystyrene control, however, all the other characteristics are similar. It may thus be concluded that Nation is a very promising material for implantable sensors to be developed.

7. Concluding remarks As already pointed out before, the applications and uses of the perfluorinated membranes increase continuously in recent years. These applications request a very thorough knowledge of the structure and properties of these membranes. It is thus only natural that in the nineties, a lot of work has been done in order to characterize these membranes in the field of transport of ions, water, solvents, water sorption, swelling and other necessary properties. Some of these recent studies will be discussed here. Morris et al. [241] have studied water sorption and transport properties of Nation 117H in order to design from it a humidity sensor. Increasing the water content of this membrane causes a progressive decrease in density and increase in thickness. The diffusion coefficient and the conductivity are also a strong function of the water content. Based on these measurements, these authors have developed a humidity sensor by depositing Nation on a sintered glass. This sensor showed good responsiveness to water vapor pressure changes and has a long term stability. Van Gorkom et al. [242] have studied the properties of water in the protonated form of Nation and Dow membranes by deuterium NMR spectroscopy. These two membranes showed very similar behavior such as the extent of hydrogen bonding to the sulfonic groups. The chemical shift showed the same dependence of the water content in both membranes. Pineri et al. [243] have studied the swelling of perfluorinated membranes as a function of the solvent, the counter ion and the temperature. The donor number of the solvent is relevant to the expansion of the membrane. The influence of the cation can be related to the softness parameter for very polar solvents and their size for the other solvents. Increasing the temperature, the uptake of solvent is also increased, the amount of solvent staying on at room temperature. The dissolution of the membrane depends on the degree of swelling. Martini et al. [244] have investigated the properties of water, large organic molecules or neutral molecules confined in the membrane, by ESR spectroscopy using nitroxide probes. It was found that the behavior of water in the swollen membrane was similar to those in other systems where water is

C. Heimer- Wirguin / Journal o[ Membrane Science 120 (1996) 1-33

finely dispersed on porous solids like silica, aluminas, zeolites and biological systems such as membranes or lipid multilayers. No l i q u i d - s o l i d transition was observed, there was, however, a decrease of the mobility of the liquid which was lower than in bulk water. As there are in the Nation membrane, a variety of adsorption sites, the diffusion coefficients of water vary not only between channels and clusters, but in the cluster system, too. Other solvents, except water, were also investigated such as methanol ethanol and mixed solutions [245-248]. Schlick et al. [245] have studied the behavior of membranes swollen in water, methanol and ethanol and mixed solutions by comparing the esr spectra of cations in these solutions. The correlation time T~ of the cation in membranes swollen by water are all similar and suggest the formation of large water pools around the cations and aggregation of the polymeric materials into micellar structures. For pure alcohol, the ESR spectra are different from solutions indicating by it that the alcohol penetrates into the membrane and forms smaller clusters where the motion of cations is strongly hindered. Comparing ethanol to methanol, it was found that this effect is stronger for ethanol who is a better plasticizer. Iyer et al. [246] have studied methanol and water uptake of Nation 117 by measuring densities, equivalent volumes and thicknesses of some uni- and di-valent cations. They concluded from this study that the water sorption and related volumes correlate with the free energies of hydration of counter ions involved. A very high methane uptake was observed for H +, Li + and Na forms accompanied by appreciable volume and dfickness increase. Cluster size computations have been made, based on swelling data and ion exchange capacity. Miura and Yoshida [247] have studied the structural changes of Nation induced by swelling of ethanol. The molecular motion and structure of samples dried from water swollen and ethanol swollen were measured by dynamic mechanical analysis, SAXS (small angle X-ray scattering analysis) and E P M A (electron probe microanalysis). From D M A (dynamic mechanical analysis) results, it was found that the molecular motion of the ethanol-dry sample is different from that of the water dried sample. The diameter of the ion cluster of the ethanol dried sample was smaller than that of water dried one. From E P M A results, it appears that there is a reorga-

25

nization of the cluster aggregation upon swelling with ethanol. The new model proposed by these authors consists of spherical ion cluster with fluorocarbon shell containing the stressed chain due to the internal stress, caused by ethanol swelling. Tsatsas and Risen [248] have studied the thermal properties and far infrared spectrum of the solution cast Nation films. When these films were heated, an endotherm at about 75°C and another between 105160°C were observed. Changes in the spectrum have been associated with loss of solvent. An exotherm between 150-200°C was observed and assigned to the relaxation which was related to rearrangements and formation of partially crystallized phases to lorm more stable structures. As may be seen from all these studies, very much information exists on the structure and structural changes obtained by changing solvents, cations, organic molecules, physical parameters and other factors. These studies will certainly contribute to numerous new improved applications.

References [I] W,G. Grot. Perfluorinated ion exchange polymers and their use in research and industry, Macromol. Symp., 8 (1994) 161. [2] T.D. Gierke and W.S. Hsu. The cluster-network model of ion clustering in perfluorosulfonated membranes, in A. Eisenberg and H.L. Yeager (Eds.), Perflnorinated lonomer Membranes, ACS Symp. Ser., 180, American Chemical Society, Washington, DC, 1982. [3] A. Eisenberg lEd.), Ions in Polymers, Adv. Chem. Sen, no. 187. American Chemical Society, Washington, DC, 1080. [4] M.J. Covich, US Pat., 4.366.262, 1982. [5] W.G. Grot and C. Chadds, European Pat., 0,li66,369, 1982. [6] C.R. Marlin, T.A. Rhoades and J.A. Ferguson, Dissolution of perfluorinated ion containing polymers. Anal. Chem.. 54 (1982) 161. [7] S.J. Sondheimer, N.Y. Bunce and C.A. Fyfc, Structure and chemistry of Nafion-H: A fluorinated sulfonic acid polymer, J. Macromol. Sci., Rev. Macromol. Chem, Phys., C26 (1986) 353. [8] D.R. Lloyd lEd.), Materials Science of Synthetic Membranes, ACS Syrup. Ser,, 269, American Chemical Society, Washington, DC, 1985. [9] A, Eisenberg and F.E. Bailey (Eds.), Coulombic Interactions in Macromolecular Systems, ACS Symp. Sen, 302, American Chemical Society, Washington, DC. 1986. [10] M. Pineri and A. Eisenberg (Eds.), Structure and Properties of Ionomers, Ser. C: Mathematical and Physical Sciences, Vol. 198, Reidel, Dordreeht. 1987.

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[11] L.L. Ferry, Acidity in Nation, J. Macromol. Sci., Chem., A27 (1990) 1095. [12] Y.E. Kirsh, S.A. Smirnov, Y.M. Popkov and S.F. Timashev, Perfluorinated carbon-chain copolymers with functional groups and cation exchange membranes based on their: synthesis, structure and properties, Russ. Chem. Rev., 59 (1990) 560. [13] K.A. Mauritz, Review and critical analysis of theories of aggregation in ionomers, J. Macromol. Sci., Rev. Macromol. Chem. Phys., C28 (1988) 65. [14] B. Dreyfuss, G. Gebel, P. Aldebert, M. Pineri, M. Escoubes and M. Thomas, Distribution of the "micelles" in hydrated perfluorinated ionomer membranes from SANS experiments, J. Phys. France, 51 (1990) 1341. [15] R.A. Register and S.L. Cooper, Anomalous small angle X-ray scattering from nickel-neutralized ionomers. 2. Semicrystalline polymer matrices, Macromolecules, 25 (1990) 318. [16] E.M. Lee, R.K. Thomas, A.N. Burgess, D.Y. Barnes, A.K. Soper and A.R. Rennil, Local and long range structure of water in a perfluofinated ionomer membrane, Macromolecules, 25 (1992) 3106. [17] G. Xu, X-ray scattering study of new perfluorinated ionomers, Polymer, 25 (1993) 397. [18] T. Xue, Y.S. Trent and K. Osseo-Asare, Characterization of Nation membranes by transmission electron microscopy, J. Membrane Sci., 45 (1989) 261. [19] S. Rieberer and K.H. Norian, Analytical electron microscopy of Nation ion exchange membranes, Ultramicroscopy, 41 (1992) 225. [20] D. Chu, D. Gervasio, M. Razaq and E.B. Yeager, Infrared reflectance absorption spectroscopy (IRRAS). Study of the thermal stability of perfluorinated sulfonic acid ionomers on Pt, J. Appl. Electrochem., 20 (1990) 157. [21] J.L. Bribes, M.EI Boukari and J. Maillols, Application of Raman spectroscopy to industrial membranes Part 2 Perfluorosulfonic membrane. J. Raman Spectrosc., 22 ( 1991 ) 275. [22] K.M. Cable and R.B. Moore, FTIR investigations of ionic interactions in perfluorosulfonate ionomers containing hydrophobic counter ions, Polymer Prepr., Am. Chem. Soc., Div. Polym. Chem., 33 (1992) 1212. [23] W. Kujawski, Q.T. Nguyen and J. Neel, Infrared investigations of sulfonated ionomer membranes. I. Water-alcohol compositions and counter ions effects, J. Appl. Polym. Sci., 44 (1992) 951. [24] A.L. Maclean, R.S. Armstrong and B.J. Kennedy, Nationsupported metal complexes: A Resonance Raman and UVvisible spectroscopy study, J. Raman Spectrosc., 24 (1993) 897. [25] M.N. Szentirmay, N.E. Prieto and C.R. Martin, Luminescence probe studies of ionomers. 1. Steady-state measurements from Nation membrane, J. Phys. Chem., 89 (1985) 3017. [26] E. Blatt, A. Launikonis, A.W.-H. Mau and W.H.F. Sasse, Luminescence probe studies of pyrene and two charged derivatives in Nation, Aust. J. Chem., 40 (1987) 1. [27] J.L. Colon and C.R. Martin, Luminescence probe studies of

[28]

[29]

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