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IV. TRANSPORT PROPERTIES UNDER INDUSTRIAL ELECTROLYSIS CONDITIONS

IV. TRANSPORT PROPERTIES UNDER INDUSTRIAL ELECTROLYSIS CONDITIONS

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Properties of Perfluorinated Ion-Exchange Membranes



471



approach to chlor-alkali manufacture is now economically competitive with the two older technologies of asbestos diaphragm and

mercury cathode cells. However, developing a sound scientific basis

for understanding membrane dynamic properties under industrial

electrolysis conditions still presents a formidable challenge as this

important technology is brought to full commercialization.

The membrane in a chlor-alkali electrolysis cell experiences

rather harsh operating conditions. The anolyte solution consists of

2-5 M NaCl solution which is mildly acidic and is mixed with

evolving chlorine gas. The catholyte solution contains 617 M NaOH (20-45 wt %) and hydrogen gas from the reduction

of water. Cell temperature and current densities are typically in the

ranges of 80-90°C and 0.2-0.5 A cm"2, respectively. While the perfluorinated sulfonate and carboxylate membranes are indefinitely

stable, even under these operating conditions, they are subject to

rather severe physical forces during electrolysis. Thus, electrolyte

and water sorption, transport properties, and even polymer morphology might be expected to differ from those seen under nearequilibrium, dilute-solution conditions. Also, the slow accumulation of brine impurities within the membrane phase affects its

properties. Thus, performance is a function of many variables, some

of which are related to cell operating conditions and others which

are a function of membrane composition.

1. Characteristics of Perfluorinated Chlor-Alkali Membranes

Through the intense research efforts of a small number of companies

over the past 10 to 15 years, several types of high-performance

perfluorinated membranes have been developed for chlor-alkali

production.49-52'62'86'97'98'129'132'133'143'145'146'150'151'211 As discussed in

Section 1.2, the requirements of these membranes are several: good

physical strength, chemical inertness, high rejection of hydroxide

transport, and low electrical resistance. The degree to which a given

material satisfies these requirements generally depends on cell parameters such as brine and caustic concentrations, temperature,

and current density, and no single type has superior performance

over all conditions.

Three types of perfluorinated chlor-alkali membranes are

noteworthy. The first of these, the homogeneous carboxylate films,



472



Richard S. Yeo and Howard L. Yeager



show high current efficiencies, even at the highest concentrations

of caustic. Bilayer or multilayer membranes comprise a second

type. Here each layer has either sulfonate or carboxylate functionality, with perhaps different equivalent weights of polymer in each

layer. For this type, the carboxylate layer is always in contact with

the catholyte for effective hydroxide rejection. A third type is related

to the second in that a carboxylate layer is combined with a sulfonate

layer to yield high current efficiency and strength but low electrical

resistance. Here though, the layer is created by chemical treatment

of a sulfonate film so that the carboxylate layer is now very thin

(on the order of 2-10 ^m). This type shows optimum performance

at somewhat lower caustic concentrations than the first and second

types.

Thus all successful chlor-alkali membranes currently employ

a perfluorocarboxylate polymer to lower the rate of hydroxide ion

transport. The sulfonate portion of some of these membranes is

present mainly to add strength to the thinner carboxylate barrier

layer. Fabric backing is also used in some cases to improve physical

strength.

Homogeneous or bilayer membranes of only sulfonate functionality can yield reasonably high current efficiencies if a high-EW

polymer faces the catholyte. Unfortunately, large electrical resistances also result with such materials. Surface treatment of a sulfonate membrane to yield a layer of sulfonamide exchange sites187

also produces a membrane with improved current efficiency.

However, these sites are slowly hydrolyzed in an operating cell, so

that this approach is not commercially viable.

2. Membrane Permselectivity in a Chlor-Alkali Cell



The ability of these perfluorinated ionomer membranes to limit

migration of hydroxide ion into the anolyte is of course a central

feature of their success in this new technology. However, this

phenomenon is a function of many membrane and cell variables,

and a satisfactory theoretical description of the nature of this

permselectivity has not yet emerged. Certain features of hydroxide

rejection are understood though. First, membrane water content

shows a strong correlation with permselectivity; those membranes

with lower weight percentages of sorbed water under cell operating



473



Properties of Perfluorinated Ion-Exchange Membranes



conditions invariably show higher current efficiencies. Second,

membrane current efficiency shows a pronounced and complicated

dependence on sodium hydroxide catholyte concentration. Finally,

brine concentration, current density, and temperature also influence

membrane performance in a complex manner, but their effects are

generally less pronounced than that of catholyte concentration.

Some of these features are illustrated in Figures 14-18. A rather

typical literature plot of current efficiency vs. sodium hydroxide

concentration for perfluorosulfonate membranes is shown in Fig.

14.62 Nafion 427 is a 1200-EW sulfonate membrane with fabric

reinforcement. Poor hydroxide rejection occurs at catholyte concentrations above 10 wt % but a minimum is seen at higher concentrations, wtih increasing current efficiency from 28 to 40%

caustic (9-14 M). The current efficiency of a 1200-EW

homogeneous perfluorosulfonate film is shown in more detail over

this concentration region in Fig. 15.170 Sodium ion transport number

(fNa+, mol F"1), which is equivalent to caustic current efficiency, is

plotted vs. both brine anolyte and caustic catholyte concentration.

These values were determined using radiotracer techniques, which

have proven to be rapid and accurate methods for the determination

of membrane performance.24137'149'164'167168 A rather sharp

maximum is seen at 14 M NaOH, and the influence of brine con-



20

30

NaOH, WEIGHT %



40



Figure 14. Current efficiency vs. catholyte concentration for

Nafion 427 perfluorosulfonate membrane. (Ref. 62; reprinted by

permission of the publisher, The Electrochemical Society, Inc.)



474



Richard S. Yeo and Howard L. Yeager

Nafion 120



90°C



2 kA/m2



O.8-1



Figure 15. Sodium ion transport number for Nation 120 us. brine anolyte

and caustic catholyte concentrations (Ref. 170).



centration on the maximum is rather minor. Figure 16 shows results

for the same membrane at a higher current density and lower

temperature. These parameters affect the maximum value of fNa+

and the overall shape of the surface.

In Figure 17, sodium ion transport number is plotted vs.

catholyte concentration for a homogeneous perfluorocarboxylate

film. The current efficiency is now higher than 90% over the entire

caustic concentration region studied, although a minimum and

maximum in performance is again observed. These features are

shifted to lower concentration compared to perfluorosulfonate

behavior though. Finally, the performance of a sulfonate-carboxylate bilayer membrane, Nafion 901, is plotted in Fig. 18. For such



Properties of Perfluorinated Ion-Exchange Membranes



Nafion 120 85°C



475



4 kA/m2



Figure 16. Sodium ion transport number for Nafion 120 vs. brine anolyte

and caustic catholyte concentrations (Ref. 170)



a material, the more permselective carboxylate layer faces the

catholyte solution. Performance in this configuration should be very

similar to that of the carboxylate film alone, according to the

treatment by Krishtalik.77 Similarities are evident in Figures 17 and

18, in support of this conclusion.

3. Interpretation of Permselectivity as a Function of Membrane

Properties and Cell Parameters



Several approaches to the understanding of membrane permselectivity can be taken; these include the use of the Nernst-Planck

transport equations,72117'212 irreversible thermodynamics,35 and



476



Richard S. Yeo and Howard L. Yeager



0.8 0



2



4



6



8



10



12



NaOH CONCENTRATION, mol L"1



Figure 17. Sodium ion transport number vs. caustic catholyte

solution for a perfluorinated carboxylate membrane: (•)

anolyte is 5 M NaCl; and (O) anolyte and catholyte are

identical concentrations of NaOH. (Ref. 149; reprinted by

permission of the publisher, The Electrochemical Society,

Inc.)



theories which incorporate morphological and chemical properties

of these perfluorinated ionomers to explain performance. Emphasis

is placed on the latter type here, for these provide some insight

into the molecular basis of permselectivity.

The dependence of current efficiency on polymer structure was

first analyzed by Gierke,45 based on his cluster-channel model

(Section II.4). The current efficiency is given as the ratio of cation

flux to total ionic flux. The flux is determined by the ion mobility

and the ion concentration in the membrane, and the potential

barrier. He calculated the size of the electrostatic potential energy

barrier from the Poisson-Boltzmann equation. This barrier energy

is added to the activation energy for migration. He employed

absolute rate theory to describe the relative reduction in anion

mobility. The model correlates well the increase of current efficiency

with increase of polymer EW by considering the relative mobilities

as a semiempirical and adjustable parameter. Reiss and Bassignana117 have, however, pointed out many shortcomings of Gierke's

approach.

Several new developments in interpreting the transport properties, based on the structural parameters, have been reported by Koh



Properties of Perfluorinated Ion-Exchange Membranes



477



Nafion 901 90°C 2 kA/m2

1.00 n



0.95 -



0.90-



0.85 -



Figure 18. Sodium ion transport number for Nafion 901 vs. brine anolyte and

caustic catholyte concentrations. (Ref. 168; reprinted by permission of the

publisher, The Electrochemical Society, Inc.)



and Silverman,72 Reiss and Bassignana,117 and Yeo.183 These results

generally suggest that the high local concentration of fixed ion (or

surface charge density) in these perfluorinated ionomer membranes

is the primary factor in providing the extraordinary permselectivity

in concentrated solutions.

Mauritz and co-workers60'74'75'93'94 and Hopfinger61 have reported spectroscopic and sorption studies of perfluorosulfonate membranes. Several conclusions drawn in these studies are useful in the

interpretation of current efficiencies as a function of caustic solution

concentration. Sorption measurements of an 1100-EWfilm in equilibrium with NaOH solutions from 7.5 to 18 M show that the



478



Richard S. Yeo and Howard L. Yeager



NaOH/SO3Na ratio remains virtually constant at about 0.27. Membrane water content decreases with increasing solution concentration though, so that a steadily decreasing water-to-ion ratio results.

This ratio indicates that even the primary hydration requirements

of Na+, OH~, and sulfonate ion-exchange sites are not met within

the membrane over most of this concentration range. A continuous

infrared absorbance below 3500 cm"1 is assigned to proton tunneling events in the H3O2 grouping. For a 1500-EW membrane, this

continuous absorbance initially rises with caustic concentration

but goes through a maximum at about 10 M. A sharp minimum in

absorbance is observed at 14 M solution concentration, followed

by an abrupt increase. Thus, the concentration dependence of this

infrared absorbance is notably similar to that of current efficiency

for a perfluorosulfonate film (Figs. 14-16). If the magnitude of

continuous absorption is indicative of the extent of hydroxide

transport by a type of Grotthus chain mechanism, then a qualitative

explanation for the complicated concentration dependence of current efficiency could be provided.

As discussed by Mauritz and Gray,94 the initial rise in infrared

absorbance at lower caustic concentration is simply due to the onset

of NaOH sorption into the polymer. With increasing solution concentration and membrane dehydration, however, proton tunneling

events decrease. In this concentration region, the remaining water

molecules would be present mainly in solvent-separated ion pairs.

Here the electrostatic field of the ion pair would serve to localize

proton position and reduce the frequency of tunneling events. At

the highest external solution concentrations, proton transfer processes are suspected to occur between sulfonate groups and the remaining water molecules in the polymer, yielding the final increase in

absorbance. Thus the complicated concentration dependence of

current efficiency for perfluorosulfonate membranes, referred to

above, may be a reflection of the dependence of various proton

tunneling mechanisms on the amount of hydration water in the

membrane.

An important question is why the perfluorinated carboxylate

polymer presents a much more effective barrier to hydroxide ion

transport compared to the sulfonate analogue, for any caustic

concentration used. There may be several factors involved. First, a

more complete phase separation of the ion-clustered regions for



Properties of Perfluorinated Ion-Exchange Membranes



479



the carboxylate, as inferred from various types of measurements,

would be expected to increase the degree of rejection in terms of

the cluster-network model. Smaller cluster sizes, larger numbers of

interconnecting channels, and less unincorporated exchange sites

and water would all lead to more effective hydroxide blockage.

Also, the lower inherent water content and higher exchange-site

charge density for the carboxylate polymer produce a greater extent

of ion pairing. As concluded from infrared studies, this discourages

proton tunneling events which enhance hydroxide ion migration.

Ultimately though, carboxylate-water interactions would be expected to reduce permselectivity at highest caustic strengths, in analogy

to sulfonate-water interactions. Thus a combination of differences

in morphology and water sorption are seen as central factors in the

relative permselectivities of perfluorinated carboxylate and sulfonate polymer membranes.

Electroosmotic effects also influence current efficiency, not

only in terms of coupling effects on the fluxes of various species

but also in terms of their impact on steady-state membrane water

levels and polymer structure. The effects of electroosmosis on

membrane permselectivity have recently been treated through the

classical Nernst-Planck flux equations,213 and water transport numbers in chlor-alkali cell environments have been reported by several

workers.35"37'73'86149164'213'214 Even with classical approaches, the

relationship between electroosmosis and permselectivity is seen to

be quite complicated.213 Treatments which include molecular transport of water can also affect membrane permselectivity, as seen in

Fig. 17. The different results for the two types of experiments here

can be attributed largely to the effects of osmosis. A slight improvement in current efficiency results when osmosis occurs from anolyte

to catholyte. Another frequently observed consequence of water

transport is higher membrane conductance,133146'214 which is an

important factor in the overall energy efficiency of an operating cell.



V. CONDUCTIVITY AND PERMEABILITY

IN MEMBRANES



The perfluorinated ionomer membranes are widely used as

separators in electrolytic and fuel cells. A primary consideration



480



Richard S. Yeo and Howard L. Yeager



in such applications is the membrane conductivity, because ohmic

losses due to membrane resistance can significantly increase energy

consumption of the electrolytic cell and energy loss of the fuel cell.

Extensive studies of the conductivities of conventional Nafion

membranes in various industrially important electrolytes have been

carried out in several laboratories in recent years. The conductivity

of the carboxylate membranes has been studied only in alkaline

electrolytes because of its primary application in the chlor-alkali

industry.

1. Conductivity in Pure Water—Solid Polymer Electrolyte Systems

In the dry state Nafion behaves like an insulator.63183 However,

the membrane becomes conductive when hydrated. Figure 19 shows

the conductivity of hydrated Nafion,183 along with that of several

polymers containing sulfonic acid groups,8116'215 as a function of

water content. The Nafion polymer becomes conductive when it is

exposed to the atmosphere and absorbs ~6H 2 O/SO^ of moisture.

It has been shown that membranes with this amount of hydration

have sufficient conductivity for use as a semisolid proton conductor

in WO3-based electrochromic displays.116 The water content of the

membrane further increases after immersion in water, resulting in

higher conductivity. This membrane conductivity is regarded as the

intrinsic conductivity because it stems from the strong acidity of

the materials [Section II.5(iv)]. The solid curve is calculated from

the Bruggemann equation216:

K=0.54Ke(l-Vp)15



(11)



where Vp is the volume fraction of polymer and Ke is considered

as the conductivity of sulfuric acid with concentration equal to that

of the sulfonic acid group of the membrane. Equation (11) implies

that K is a strong function of Vp. This is supported by the fact that the

conductivity of the E-form membrane is higher than that of N-form

samples, despite the fact that the latter has a higher value of Ke.lS3

Table 6 compares the conductivity of various polymers and

electrolytes containing sulfonic acid groups. The intrinsic conductivity of Nafion is high and is very similar to other polymers

containing sulfonic acid groups. The activation energy of proton

conduction of Nafion is low in comparison with other polymers,



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