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Ch 6. Structural and Transport Properties of Perfluorinated Ion-Exchange Membranes

Ch 6. Structural and Transport Properties of Perfluorinated Ion-Exchange Membranes

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438



Richard S. Yeo and Howard L. Yeager



component. These membranes are permeable to one kind of ion

while resisting the passage of direct flow of liquids and ions of

opposite charge.

The concept of using an ion-exchange membrane as an "electrolyte" in electrochemical cells was first introduced by Grubb in

1959.5 Since then, extensive research and development programs

have been undertaken by the General Electric Company and

others,6"9 resulting in the present solid polymer electrolyte (SPE)

cells in which Nafion® serves as the sole electrolyte as well as

separator. High voltaic efficiency can be achieved in SPE cells

because of the minimum contact resistance between electrode and

separator.

2. Requirements of High-Performance Membranes



The success of an electrochemical process can depend critically on

the selection of the proper membrane. Yet, general criteria by which

a membrane can be selected do not exist. The essential properties

of the membrane for electrochemical applications include good

permselectivity, high ionic conductivity, no electronic conductivity,

adequate chemical and thermal stability, and sufficient mechanical

strength under operating conditions.

The membrane is a compromise of properties: the requirement

for low internal resistance suggests that the membrane be porous

and thin; good separation dictates that the membrane be of low

permeability and high fixed charge concentration; and adequate

physical strength requires that it be sufficiently thick. At any rate,

membranes can generally be "tailor-made," so that their properties

can be adjusted to yield optimal cell performance and cell life.10

Cell efficiency very often improves at elevated temperatures

because of better conductivity and improved kinetics. However,

hydrocarbon-type membranes are often unstable in such environments. This is particularly true for cells in which oxidizing agents,

such as chlorine or peroxide, are present. The membrane degrades

due to the cleavage of carbon-hydrogen bonds, particularly the

a-hydrogen atom where the functional group is attached.11'12 Perfluorinated materials are better for these electrochemical applications because of their excellent chemical inertness and mechanical

integrity in hot corrosive and oxidative environments.12



Properties of Perfluorinated Ion-Exchange Membranes



439



3. Development of Perfluorinated Ionomer Membranes



The interest in perfluorinated ion-exchange membranes has

increased extensively in recent years because of their industrial

importance. A growing body of research concerning their structure

and properties has developed.13"183 Much of this work has been

summarized in a recent monograph.184 While perfluorinated anionexchange membranes have not yet been produced, several perfluorinated cation-exchange types have been synthesized and fabricated

into membrane form, as listed in Table 1.

The perfluorinated sulfonate (Nafion) polymer was synthesized

and developed by du Pont about 17 years ago.154185186 It was

initially introduced as SPE in fuel cell applications in 1966. It

exhibits excellent stability over conventional cross-linked polystyrene sulfonic acid membranes and becomes the unique candidate

for this type of application. Subsequently, further development has

been made79181 in improving its chemical stability against peroxide

Table 1

Perfluorinated Cation-Exchange Membranes

Structure



Trade name



XF-OCF2CO2H/SO3H



Producer



References



Asahi

Chemical



133

134



XF-O(CF2)MCO2H



Flemion



Asahi

Glass



145, 146"

150-152



XF-OCF2CF2SO3H



Nafion 1000

series



du Pont



49



XF-OCF2CF2SO2NH2



Nafion

Sulfonamide



du Pont



62

187



XF-OCF2CF2CO2H/SO3H



Nafion 901



du Pont



149

169



XF-O(CF2)MCO2H/SO3H



Neosepta-F



Tokuyama

Soda



129

130



XF=-(CF 2 CF 2 ) X -(CFCF 2 ),i



(OCF2CF)W-



1



CF 3



440



Richard S. Yeo and Howard L. Yeager



degradation. Various modifications have been made to a basic

Nafion homogeneous polymer film to produce materials with special

characteristics. Open weave Teflon fabric can be laminated into the

polymer film for increased strength. Also, composite membranes

have been made in which layers of two different equivalent weights

of polymer film are laminated together. Moreover, surface treatment

has been employed to generate membranes in which a thin layer

of weakly acidic sulfonamide exchange sites is formed.187 Thin

amine-treated membranes 29 ' 31 have found applications in chloralkali cells, because improved hydroxyl ion rejection is realized

when this treated surface faces the catholyte.62

Perfluorinated carboxylate membranes were introduced about

seven years ago. These membranes can be synthesized by a variety

of methods or by various chemical conversions from the Nafion

polymer. 129133 ' 150151 Composite membranes which contain both

sulfonate and carboxylate functional groups have also been produced (see Section IV.l for more details). These carboxylate membranes have been widely employed in the advanced membrane

chlor-alkali cells. This major chemical technology is in the process

of being revolutionized by the use of these materials, a remarkable

accomplishment for such a small group of polymers.143

4. Types of Applications

The perfluorinated sulfonic acid (Nafion) membranes have found

a great variety of electrochemical applications. These include the

SPE water electrolyzers,79'80'131'144'181-183 alkaline water electrolyzers,82'142'176 hydrochloric acid electrolyzers,174'175'181 Na 2 SO 4

electrolyzers,80 hydrogen-air fuel cells, 72181 hydrogen-halogen fuel

cells, 174175181 zinc-bromine cells,158 zinc-ferricyanide redox cells,

zinc-ferric redox cells, Donnan dialyzers, 32100 " 102 electrochromic

devices,116 chemically modified electrodes, 26 ' 27 ' 58 ' 89 ' 90126127 ' 157 ionselective electrodes,87 and many others. See References 52, 71, and

181 for detailed discussions.

Potential applications of perfluorinated carboxylate membranes have been focused to date on the chlor-alkali process. It

has been pointed out previously that these polymers in acid form

are not desirable for electrochemical applications because of rather

high resistance.178



Properties of Perfluorinated Ion-Exchange Membranes



441



Perfluorinated ionomer membranes show considerable promise

with respect to their performance characteristics, low resistivity,

high permselectivity, and long-term stability. However, the present

cost of these membranes is more than $300 per square meter. The

relatively high cost limits their application in many electrochemical

cells when cost effectiveness is a major concern.

5. The Scope of this Review



This article aims at describing the microstructure and transport

properties of these polymeric membranes from an electrochemical

point of view. It is intended to provide some direction for the future

development of high-performance membrane cells in industrial

electrolytic or separation processes.

Section II deals with the microstructure of the perfluorinated

ionomers. These structural properties and their effect on transport

properties are currently understood only qualitatively. Although

there are diverse opinions on the detailed morphology of these

polymers, it is generally agreed that microphase separation and ion

clusters do exist. The presence of ion-clustering morphology and

the amount of electrolyte in these membranes can strongly affect

the transport properties. The ion-clustering theory of Eisenberg will

be used to provide a basic understanding about these materials.

Since most electrochemical applications of the materials involve

electrolytes, the solvation phenomena as well as the concentrations

of fixed ions and co-ions in the polymers are of particular interest.

Membrane transport properties in both dilute and concentrated

solution environments are presented in Section III. The membrane

transport properties under industrial electrolysis conditions will be

dealt with in Section IV. For practical cell applications, the conductivity and permeability of the membrane are of great importance.

These properties can significantly affect cell performance. These

subjects are treated in Section V.

II. MICROSTRUCTURE

1. Structural Studies of Ionomers



Perfluorinated ion-exchange membranes belong to a class of

materials called ionomers.188 These perfluorinated ionomers differ



Richard S. Yeo and Howard L. Yeager



442



from conventional ion-exchange membranes in that they are not

cross-linked polyelectrolytes but thermoplastic polymers with pendant acid groups (in concentrations below ~15 mol %) partially or

completely neutralized to form salts. These perfluorinated materials

become soluble when the ionic co-monomer exceeds 15 mol % and

would be considered as polyelectrolytes.188

Research on the structure and properties of solid ioncontaining polymers has been carried out extensively in several

laboratories during the last two decades.189201 The structure of ion

aggregates in these polymers, and the modifications which occur

on solvation are generally not known quantitatively. The reason

for the lack of understanding of the microstructure of ion-containing

polymers lies in their complexity. The microstructure of ioncontaining polymers, particularly those aspects dealing with microphase separation, presents a particularly challenging problem

because none of the usual tools of structure characterization are

readily applicable. Also, results obtained by these techniques are

subject to different interpretations by the various groups working

in the field.



LABILE



SLOWDOWN OF



CROSSLINKS



MOLECULAR



fTHERMOPLASTICrrY)



MOTIONS



1



\



MODULUS



t



VISCOSITY





/



T

9

INCREASE



AGGREGATION



(CRYSTALUNITY?)



\



1



i



NEW

RELAXATION

MECHANISMS



t

MATRIX



ION



INCREASE



INCREASE



NEW

PHASE

SEPARATION



IONIC PHASE

T



I



i



THERMO-



WATER/SOLVENT



RHEOLOGICAL



ABSORPTION



COMPLEXITY



(SELECTIVE

PLASTICIZ'N)



\



9



PERMSELECTIVITY

DIFFUSION

CONTROL

(CHANNELS)



Figure 1. Characteristic ionomer properties and their interrelationships. (Ref. 18; reprinted by permission of the publisher,

The Electrochemical Society, Inc.)



Properties of Perfluorinated Ion-Exchange Membranes



443



It is traditionally considered that for conventional ionic polymers198"200 the ions are dispersed throughout the sample in a very

low state of aggregation, either as ion pairs or ion quartets. The

evidence to support this concept is derived from nuclear magnetic

resonance data,199 viscosity data,198 and dynamic mechanical

measurements.200 As a recent trend, it is widely accepted

that 43,44.46,l



19.163.171.173,183.189-193.196 ^



^



^



^



Q f



^



e m b e d d e d



in the surrounding organic medium can also exist in the polymers,

the most direct evidence coming from small-angle X-ray scattering.

The presence of these large ion aggregates can influence

directly, or indirectly, various properties of these polymers. Such

relationships have recently been demonstrated by Besso and Eisenberg18 and Eisenberg202 and are shown in Fig. 1. In the light of the

importance of ion clustering in determining the structural and

transport properties of the perfluorinated materials, some theoretical aspects of cluster formation will be briefly described in the

following section.

2. Eisenberg's Theory



In 1970, Eisenberg201 set forth an initial theory of ionomer structure

which contains some concepts of general merit. Two basic types of

ionic aggregations have been postulated: small aggregates containing few ion pairs, termed multiplets, and larger aggregates, termed

clusters, which consist of a nonionic backbone material as well as

many ion pairs. The structure of the ionomer can thus be broadly

described as that of a microphase-separated system in which a

matrix of low ion content (due to multiplets) is interspersed with

ion-rich domains (clusters). The bulk of the experimental evidence

has suggested that only multiplet formation occurs at low ion

concentration whereas, beyond a certain ion concentration (which

depends on the material), the multiplets may very well aggregate

to form clusters.

The formation of ionic domains or clusters is regarded as being

a consequence of the thermodynamic incompatability of the ionic

groups with the low dielectric constant organic matrix. The enthalpic advantages of phase separation are obvious but large entropic

forces oppose the process. Eisenberg201 has analyzed the thermodynamics of phase separation in ionomer systems. He has evaluated



444



Richard S. Yeo and Howard L. Yeager



the distances between clusters and the cluster sizes by imposing

various geometries on the cluster. The factors involved in this

computation are: (1) work done to stretch the polymer chains in

cluster formation; (2) electrostatic energy released when clusters

collapse; and (3) the temperature threshold Tc where the elastic

and electrostatic forces just balance each other. A brief description

of his analysis is presented as follows.

The elastic work in cluster formation is related to the energy

changes in two different kinds of chain deformations. The first kind

is the work of stretching a polymer chain from a distance Xo to X,

the distance corresponding to a cluster size of JV. The other kind

is the work of contracting a chain from a distance of Xo to 0.

Accordingly, the elastic work per chain in cluster formation is given

by

Wch = (3kT/4h2)(X2-2X2)



(1)



where h2 = 4a2l2MJMo, the mean square end-to-end distance of

the polymer chain; a2 = chain expansion factor; / = length of

repeat unit; M c /M 0 = degree of polymerization between ionic

groups; X = (NMC/pNA)1/3, the average distance between clusters;

^o = (NOMC/pNA)l/3, the average distance between multiplets

above Tc, p = density of polymer, and k, T, and NA have their

usual meanings.

The electrostatic energy per ion pair released upon cluster

collapse is a function of the geometry of the cluster and the dielectric

constant e of the medium. It is given by

W' = -\e2/er



(2)



where A = the fraction of energy released, e = electron charge, and

r = distance between centers of charge in the ion pair.

The cluster is not infinitely stable. At some temperature Tc the

cluster decomposes, and at that temperature the elastic forces and

the electrostatic forces just balance each other. Since the elastic

force was calculated per chain and the electrostatic force per ion

pair, Wch can be set equal to W at Tc\ thus

\e2/er = (3kTcM0/16a212 MC)[(NMC/ pNA)2/3

-2(N0MJpNA)2/3]



(3)



Properties of Perfluorinated Ion-Exchange Membranes



445



After rearrangement, Eq. (3) yields

N = (pNA/Mc)[162/2McAe2/3/crcM0£r

+ 2(N 0 M c /pN A ) 2/3 ] 3/2



(4)



Since, Tc can be determined experimentally and A calculated for

any particular cluster geometry, JV and X can be calculated from

Eq. (4).

The Eisenberg theory indicates that at low ion concentration,

as the distance between multiplets increases, the interaction between

multiplets is expected to decrease to the point where the elastic

forces become too large to be overcome; below that concentration

clustering would not be expected. This is consistent with the experimental findings. This theory has been applied well to both ethylene

and styrene ionomers which contain carboxylate groups.188 Hashimoto et al43'44'54 have recently shown that the Eisenberg theory

can be used to explain the small-angle X-ray scattering (SAXS)

results on various perfluorinated ionomer membranes.

It should be mentioned that the Eisenberg theory is somewhat

approximate and should be regarded only as demonstrating feasibility rather than predicting true cluster geometries. The theory has

been consulted extensively in many recent models proposed for

Nafion membranes (see Section II.4).

3. Ions in Perfluorinated Ionomers



(i) Ion-Exchange Capacity

The amount of ionic groups in these membranes is conventionally expressed in terms of the equivalent weight (EW) of the

polymer.49 EW is defined as the weight of polymer which will

neutralize one equivalent of base. It is inversely proportional to

the ion-exchange capacity (IEC) according to the following

relationship:

EW=1000/IEC

(5)

where IEC is given in terms of milliequivalents per gram of polymer.

As for the sulfonate membrane, the range of EW of greatest interest

for electrochemical applications is 1100 to 1350, corresponding to

0.741-0.909 meqg"1.



446



Richard S. Yeo and Howard L. Yeager



(11) Ion Clustering



The existence of ion clustering in perfluorinated sulfonate

ionomers was first reported by Yeo and Eisenberg in 1975.171 This

phenomenon has been subsequently studied for perfluorinated sulfonate and carboxylate ionomers by many others. 43 ' 46 ' 56119163183

Experimental evidence to support the conclusion that ion clustering

occurs in these materials includes thermorheological behavior,173

X-ray diffraction results, 43 ' 44 ' 46118139173 IR data,41'55 NMR data,22'74

ESR data, 153 Mossbauer spectroscopic data, 14 ' 56120121172 fluorescence behavior, 8 1 " neutron scattering spectroscopic data, 118119

extended X-ray absorption fine structure (EXAFS) data, 108 electron

microscopic data, 28 ' 46 swelling behavior,177 acidity,183 and transport

properties.46'63'159'163'183

Ion clusters are commonly observed in the ionized forms of

the perfluorinated membranes. The size of the clusters appears to

be larger for sulfonate than for carboxylate membranes. 44 The size

increases in the order H + , Na + , and Cs + and decreases with increasing number of functional groups per chain and with increasing

temperature. 43 As in the case of ethylene ionomers, the perfluorinated carboxylic acid membranes do not form ion clusters, at least

in the dry state.43 The electrostatic interaction may be too weak to

form ionic clusters. These observations are expected according to

the Eisenberg theory (see Section II.2).

Different ionic environments in Nafion have been indicated

from Mossbauer experiments. 14 ' 56120121 Heitner-Wirguin et al14

suggested the existence of clusters and dimers. Most of the ionic

groups are in clusters while the remaining groups are considered

to form multiplets or small ion aggregates. The relative amounts

of ionic groups which are not present in clusters is difficult to

evaluate because of the limitation of available research tools. In

addition, the amount changes with the prehistory of the membrane

and the environment the membrane encounters. Roche et al119 have

made a rough estimate (with an accuracy of ±20%) on Nafion in

the sodium salt form from small-angle neutron scattering results.

They found that the amount of ionic groups in the organic phase

is <60% and <40%, respectively, for N-form and E-form samples

[see Section II.5(i) for definitions].



Properties of Perfluorinated Ion-Exchange Membranes



447



(HI) Ionic Cross-Linking



In contrast to conventional ion-exchange resins or membranes,

there is no cross-linking in these perfluorinated membranes. In

solvents which might dissolve the nonionic prepolymer, the ionic

aggregates are very stable and act as cross-linkage sites. Conversely,

highly polar solvents, which interact with the ionic groups, do not

solvate the polymer. The crystalline domains originating from tetrafluoro ethylene (TFE) backbone behave like cross-linked

points.88177 Also, the clusters of the perfluorinated ionomer membranes never fall apart upon hydration as indicated by SAXS

results.43 Thus, either the ionic regions or the nonionic areas act as

cross-linkings which make the 1100 and higher-EW materials

insoluble, while the 970-EW material, with less TFE content, is too

weak for use in the swollen form and is soluble.26'27'58'89'95126127157

The high-EW membranes absorb a range of solvents to varying

extents177'178'180 and exhibit two solubility parameter values. This

behavior is referred to by Yeo as "dual" cohesive energy densities

of the polymer.177

High temperatures are required to melt the crystalline domains

in the high-EW samples and promote dissolution. Martin et a/.88

have recently found that Nafions with EWs of 1100 and 1200

dissolve in both 50:50 propanol-water and 50:50 ethanol-water,

at 250°C and elevated pressure, because the crystallites of the

materials are eliminated. McCain and Covitch95 have also reported

a similar dissolution technique. The ionic membrane was chemically

converted into the nonionic precursor (sulfonyl fluoride) form prior

to the dissolution process. Due to the nonionic nature of the

precursor, it dissolves under relatively mild conditions. These

dissolution techniques for Nafion polymers provide an important means for preparation of chemically modified

electrodes26'58'89126157 and membranes of any desired geometry.95

4. Structural Models of Nafion



Several structural models of Nafion have been proposed which are

based on various transport and spectroscopic properties of the

polymer.46'60'64'72122 A detailed picture which is consistent with all



448



Richard S. Yeo and Howard L. Yeager



the experimental observations has not yet emerged. Also, the proposed models are either only qualitative or have various adjustable

parameters and assumptions.

In 1977, Gierke proposed a phenomenological cluster network

model. The concept of ion clustering is adopted in this model,

except it is assumed that both the ions and the absorbed solvents

are all in the clusters. This is only an approximation in view of the

more recent spectroscopic observations.41119 The clusters are

assumed to be approximately spherical and the cluster size, for

varying polymer equivalent weight, has been calculated from solvent absorption data. For sodium-form Nation of 1200 EW and a

water content of 9 wt %, the cluster diameter is about 3 mm.

However, based on infrared data, Falk41 has suggested that the

hydrated ion clusters are either much smaller than Gierke estimates,

or more likely are highly nonspherical in shape, with frequent local

intrusions of the fluorocarbon phase. Extensive intrusions of

fluorocarbon material into ion-clustered regions is inferred from

these results.41 The evidence for extensive interaction of the cations

with the fluorocarbon phase has also been indicated by Lee and

Meisel.81

Gierke also considered that these clusters are interconnected

by short, narrow channels in the fluorocarbon backbone network.

The diameter of these channels is about 1 mm estimated from

hydraulic permeability data. He further considered that the Bragg

spacing (~5nm from SAXS data) can represent the distance

between clusters. The cluster-network model is a phenomenological

description. Recently, Hsu and Gierke64 have derived a semiphenomenological expression to correlate the variation of cluster

diameter with water content, equivalent weight, and cation form

of the membrane. They have shown that the short channels are

thermodynamically stable.

Hopfinger and Mauritz60 and Hopfinger61 also presented a

general formalism to describe the structural organization of Nafion

membranes under different physicochemical conditions. It was

assumed that ionic clustering does not exist in the dry polymer.

This assumption is applicable to the perfluorinated carboxylic acid

polymer43 but not the perfluorosulfonate polymers.46 They consider

the balance in energy between the elastic deformation of the matrix

and the various molecular interactions that exist in the polymer.



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