Tải bản đầy đủ - 0 (trang)
III. ION AND WATER DIFFUSION

III. ION AND WATER DIFFUSION

Tải bản đầy đủ - 0trang

460



Richard S. Yeo and Howard L. Yeager



diffusion coefficients in the membrane phase are desirable, not only

to minimize ohmic losses in electrochemical cells, but also to

maximize transport rates for processes driven by concentration

gradients such as Donnan dialysis.32 Solvent transport through the

membrane may or may not be desirable, depending upon the

application. A second goal for the study of diffusion in perfluorinated ionomers is to develop insight into the relationship between

the unusual morphology of these polymers and their transport

properties. In the latter regard, the determination of self-diffusion

coefficients is of particular usefulness. In such experiments the

membrane is in equilibrium with the external solution, so that no

gradients in internal chemical potential exist. Radioactive tracers

can be used to evaluate self-diffusion coefficients which depend

upon membrane composition, but are not coupled to chemical

potential gradients or the fluxes of other species. These values are

therefore more straightforward in their interpretation. Results of

such measurements for perfluorinated sulfonate and carboxylate

membranes are discussed in this section.

1. Diffusional Properties in Dilute Solution Environments



The self-diffusion coefficients of sodium and cesium ions have been

measured for perfluorinated sulfonate (Nafion®) and carboxylate

membranes of similar structure.159163'165 The exchange-site conTable 4

Exchange-Site Concentration and Water Content for Perfluorinated

Ionomer Membranes0

Exchange-site

concentration 6

Ionic

form



mol liter" 1



N-form



E-form



Sulfonate



Na +

Cs +



1.20

1.35



11.9

6.6



18.4

11.3



Carboxylate



Na +

Cs +



1.30

1.37



9.5

5.0



13.9

9.1



Membrane



a

b



Mol

H 2 O/mol

exchange site



From Reference 165.

N-form samples.



Properties of Perfluorinated Ion-Exchange Membranes



461



centrations and water-to-exchange-site ratios for these ionic forms

of the polymers are listed in Table 4. The exchange site concentrations of the two polymers are quite similar, which is useful for the

comparison of their diffusional properties. The water contents

strongly depend on the counterion form and pretreatment [Section

II.5(i)]; the carboxylate polymer sorbs about 20% less water in all

cases. The variability of water content with counterion form is a

reflection of the dynamic character of these ion-clustered, noncross-linked polymers. This is in contrast to conventional crosslinked ion-exchange resins such as the cross-linked polystyrene

sulfonates, where water content is largely independent of counterion

form for a given counterion charge type.205

Self-diffusion coefficients for sodium ion, cesium ion, and water

in these polymers are shown in Fig. 9, as Arrhenius plots from 0

to 40°C.165 Diffusion of sodium ion and water is remarkably similar

10-V



1000/T, K"

Figure 9. Membrane self-diffusion coefficients vs. the reciprocal

of absolute temperature for perfluorocarboxylate (light symbols)

and perfluorosulfonate (dark symbols) polymers: (A, A) H2O,

(O, • ) , Na+ and ( • , • ) Cs+. (Ref. 165; reprinted by permission

of the publisher, The Electrochemical Society, Inc.)



462



Richard S. Yeo and Howard L. Yeager



in the two membranes. Diffusion coefficients are slightly larger for

the carboxylate in each case, even though the membrane always

contains less water per exchange site. In general, these diffusion

coefficients are quite large in relation to the membrane water sorption, about a factor of 10 larger than for polystyrene sulfonate

resins of comparable water contents.206'207 This is a reflection of

the microphase separation of water, exchange sites, and counterions

in these ionomers, which produces highly efficient transport paths.

Average activation energies of diffusion in these membranes are

24.9 kJ mol"1 for the sodium ion and 23.3 kJ moP 1 for water, which

are only marginally higher than their values in pure water, 19.1 and

17.8k.Jmor1, respectively. Therefore a solutionlike diffusion

mechanism for sodium ion and water through a network of ionclustered regions is indicated for both polymers. The self-diffusion

coefficients for cesium ion in these perfluorinated ionomers, which

are smaller than those for sodium ion in all cases, are less easily

interpreted. Normally, cesium ion has a larger diffusion coefficient

than sodium ion in aqueous environments. For example, in water

the ratio of £>Na+ to DCs+ is 0.65, and in an 8% DVB polystyrene

sulfonate resin the ratio is 0.69.206 For these cases equal amounts

of water are present in the diffusing medium though, which is not

the case for these ionomers. The values listed in Table 4 indicate

that the expanded Cs+ form of each membrane has about the same

water content as the normal Na+ form. A comparison of the sodium

ion/cesium ion diffusion coefficient ratios for these cases yields

values of 0.71 and 5.3 for the perfluorocarboxylate and perfluorosulfonate polymers, respectively.165 Thus the apparent anomaly in this

ratio is removed if we compare systems with equal water contents

for the carboxylate, but not for the sulfonate polymers.

Further anomalies are seen in the diffusional behavior of

cesium ion for the perfluorosulfonate membrane. Figure 10 represents the plot of the logarithm of the diffusion coefficient vs. the

function vp/(l-Vp), where Vp is the volume fraction of polymer in

the water-swollen material163 and 1 - Vp is the volume fraction of

water, as calculated from sorption measurements. This plot corresponds to a test of the equation

D=D°exp[-fcV p /(l- Vp)]



(10)



which has been developed by analogy to Cohen and Turnbull's



Properties of Perfluorinated Ion-Exchange Membranes



463



10~5



Figure 10. Logarithm of self-diffusion coefficient vs. polymer-fraction function

for 1200-EW perfluorosulfonate polymer, at 25°C. Na + and Cs + lines without

data points: polystyrene sulfonate behavior. (Refs. 163 and 207; reprinted by

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



free-volume theory of diffusion.207"209 This equation provides an

excellent correlation of the self-diffusion coefficients of sodium ion

and cesium ion in polystyrene sulfonate resins of varying water

content, as shown in Fig. 10.207 The D° intercepts are about one-half

of the solution D° values in water, presumably due to electrostatic

attractions (Wang effect) exchange sites in the polymer phase.

Sodium ion/cesium ion heteroionic forms of the perfluorosulfonate

polymer were used to produce samples of varying Vp, in addition

to the normal and expanded homoionic forms. No correspondence

is seen between the polymer diffusion results for this and polystyrene sulfonates. Most notable perhaps is the complete lack of



464



Richard S. Yeo and Howard L. Yeager



dependence of DCs+ in the perfluorosulfonate membrane on water

content. The activation energy for cesium ion diffusion, about

36 kJ mol"1, is largely independent of water content as well. Water

diffusion again is similar to sodium ion diffusion, this time in terms

of the membrane's water content, with no anomaly seen for the

cesium ion form.

The unusual features of cesium ion diffusion in the perfluorosulfonate polymer are related to the microstructure of these

polymers. The microphase separation into ion-water and fluorocarbon regions is more complete for the perfluorocarboxylate than for

the perfluorosulfonate polymer. For the carboxylate material,

stronger hydrogen bonding in the aqueous phase would result due

to less fluorocarbon interference with extended intermolecular

water interactions. A greater degree of fluorocarbon crystallinity

and a smaller fraction of nonhydrogen bonded water for the carboxylate membrane would also be expected consequences of more

complete phase separation. Spectroscopic studies of the counterion

environments in the perfluorosulfonate polymer provide additional

information of interest. The mid- and far-infrared spectra of alkali

metal in forms of Nafion perfluorosulfonate films indicate that no

ion pairing of counterions to exchange sites occurs, even for

completely dehydrated forms.109 Thus ion pairing is not a relevant

factor in the interpretation of small cesium ion diffusion coefficients. A luminescence quenching study of Ru(bpy)3+ (bpy =

2,2'-bipyridine) in Nafion has been performed.81 The authors conclude that this large, low charge density cation is preferentially

situated in a fluorocarbon environment rather than an aqueous one.

This suggests that the distinction between ion cluster and fluorocarbon may not be as simple as suggested in various structural models,

and that interfacial regions must also be considered.

The less complete degree of phase separation for the sulfonate

polymer may well lead to interfacial regions of partly aqueous

character and relatively high void volume, as depicted in Fig. 2. A

preference of large, low charge density counterions such as

Ru(bpy)3+ and cesium ions for such regions would help to explain

the Cs+ diffusional anomalies discussed earlier. Diffusion along

interfacial boundaries rather than through ion-clustered regions

would result in lower bulk diffusion coefficients for cesium ion

compared to sodium ion. The lack of dependence of DCs+ on water



Properties of Perfluorinated Ion-Exchange Membranes



465



sorption can then be understood by first noting that, as water

sorption increases in Nafion, it enters only into ion-clustered

regions.41 Thus tortuosity would not be reduced for a cation

diffusion in peripheral regions of the aqueous clusters. Although

these interpretations are speculative, it is clear that both spectroscopic and diffusional measurements indicate a more complete

microphase separation into aqueous and fluorocarbon regions for

the carboxylate compared to the sulfonate polymer.

Self-diffusion coefficients of polyvalent cations in these perfluorinated ionomer membranes have not been reported. It can be

inferred from the use of the sulfonate membranes as Donnan

dialysis devices that transport of cations such as Cu(II), Mg(II),

and Al(III) under a concentration gradient is rapid.32100'210 Also,

column chromatographic separation of the alkaline-earth ion is

readily accomplished with a powdered Nafion perfluorosulfonate

polymer, which is again an indication of facile diffusion of these

cations within the polymer phase.160

Recently, several interesting studies of the electrochemical

properties of electrodes coated with thin films of Nafion have been

reported.26'58'89126157 These chemically modified electrodes are prepared using low-EW polymers which are alcohol soluble,26126 or

using a solution of a 1100-EW polymer which has been dissolved

at high pressure and temperature.88 Electrochemical studies for

cations such as the Ru(bpy)3+/2+ couple yielded estimates of ionic

diffusion coefficients in the polymer films. However, results also

indicate that these films are far more porous than conventional

Nafion membranes, so it is not possible to compare values directly

with those discussed above.

2. Diffusion Properties in Concentrated Solution Environments



Industrial applications of perfluorinated ionomer membranes such

as the electrolysis of sodium chloride solution to produce chlorine

and sodium hydroxide often involve the use of highly concentrated

solutions at elevated temperatures. The optimization of these systems depends upon a sound characterization of membrane transport

processes under such conditions. Sodium ion is the major currentcarrying species through the membrane in a chlor-alkali cell, and



466



Richard S. Yeo and Howard L. Yeager



some attention has been focused on its diff usional behavior in these

environments.

The self-diffusion coefficient of sodium ion is plotted in Fig.

11 vs. the reciprocal of absolute temperature for the perfluorosulfonate and perfluorocarboxylate membranes which were discussed in

Section III.I.149168 Here though, the membrane environment consists of concentrated sodium hydroxide solutions of varying concentrations at temperatures of 70-90°C. Under these conditions,



1 0 0 0 / T , K"1

Figure 11. Sodium ion self-diffusion vs. the reciprocal of absolute

temperature for perfluorocarboxylate (light symbols) and perfluorosulfonate (dark symbols) polymers in concentrated NaOH solution

environments. (Refs. 149 and 168; reprinted by permission of the

publisher, The Electrochemical Society, Inc.)



Properties of Perfluorinated Ion-Exchange Membranes



467



major differences between the two types of membrane are also seen,

even for sodium ion diffusion, unlike the results for dilute solution

environments.

In 5 M NaOH, the carboxylate now shows slightly lower values

of the diffusion coefficients compared to the sulfonate membrane.

This pattern is repeated for values measured with 4 M and

5 M NaCl solutions as well.149 At higher NaOH solution concentrations, values for the carboxylate become much smaller, averaging



+

130 _ ACTIVATION ENERGY OF N a

SELF - DIFFUSION,

70° - 9 0 C



120

CARBOXYLATE



110



_o> 100

o

^



90



80

<



*—



SULFONATE



20





10







10

NaOH



CONCENTRATION ,

+



12



14



M



Figure 12. Activation energy of Na diffusion in perfluorinated ionomer

membranes vs. NaOH solution concentration, 70-90°C temperature

interval. (Ref. 168; reprinted by permission of the publisher, The

Electrochemical Society, Inc.)



468



Richard S. Yeo and Howard L. Yeager



about an order of magnitude lower than those for the sulfonate

membrane. This is accompanied by a pronounced increase in the

activation energy of diffusion for the carboxylate polymer, as shown

in Fig. 12. It would appear that the mechanism of sodium ion

diffusion changes for only the carboxylate polymer over this concentration region.

The results of ion and water sorption measurements for the

two polymers under these solution conditions help to explain this

difference. Table 5149 lists the concentrations of various sorbed

species and the mole ratio of water to cation/ anion in the polymer

phase for NaCl and NaOH solution environments. This ratio

decreases both in the polymers and in solution with increasing

concentration. In solution, the ratio varies from 10.8 to 4.0 over

the concentration range of 5-12.5 M NaOH, so that ions in the

polymer phase exist in a significantly less aqueous environment

compared to the solution phase. As noted by Mauritz and coworkers93 for perfluorosulfonate membranes, these water contents

are insufficient to provide even primary hydration spheres for

sodium ions, sorbed anions, and exchange sites, and the likelihood

Table 5

Equilibrium Sorption of Sodium Ion, Anion, and Water in

Perfluorinated Carboxylate and Sulfonate Polymers, 80°Ca

Polymer concentration

(mol dm" 3 )

Polymer type

Carboxylate



Solution



Na +



cr



4.0 M NaCl

5.0 M NaCl



2.15

2.18



0.7

0.7



5.0 M

9.5 hi

11.0 M

12.5 M

Sulfonate



a



NaOH

NaOH

NaOH

NaOH



2.06

3.06

2.74

3.36



4.0 M NaCl

5.0 M NaCl



1.76

1.92



9.5 M NaOH

11.0 M NaOH

12.5 M NaOH



2.61

2.72

2.80



From Reference 149.



OH~



0.6

1.6

1.2

1.9

0.4

0.5

1.2

1.3

1.4



H2O



molH 2 O/

mol Na +



11.6

10.2



5.4

4.7



9.2

8.2

7.0

5.8



4.5

2.7

2.5

1.7



9.9

8.9



5.6

4.6



8.0

7.7

7.6



3.1

2.8

2.7



Properties of Perfluorinated Ion-Exchange Membranes



469



of ion pairs and higher ion multiples increases with decreasing

water levels. The carboxylate polymer generally has even less water

available for ionic hydration than the sulfonate, so that the stability

of such species would be expected to be even stronger. An additional

factor is the relative charge densities of the two types of exchange

sites. While the perfluorosulfonate site is well known to be a very

strong proton donor, the perfluorocarboxylate has a pKa of about

2.149 Thus, it is suspected that strong contact ion pairing in the case

of the carboxylate is responsible for the low sodium ion diffusion

coefficients and high diffusional activation energies. For the sulfonate, sodium ions that function as exchange-site counterions may

be less strongly bound than for the carboxylate case, resulting in

more facile sodium ion self-diffusion in the polymer.

Solution pH can also affect the diffusional properties of perfluorinated ionomer membranes. The perfluorosulfonate exchange

site possesses very little basicity and thus these membranes can be

used even in concentrated acid solutions. However, it has been

shown that modified forms, in which the exchange sites are converted to sulfonamide groups, protonate at solution pH values below

13.161 This protonation severely reduces the ability of sodium ion

to diffuse through the membrane. The exchange-site acidity of the

perfluorocarboxylate membrane is far higher than that of the perfluorosulfonamide of course. Thus no protonation of exchange sites

occurs in neutral or alkaline media. Protonation does occur in

moderately acidic solutions however. Sodium ion diffusion flux in

this membrane in a 5 M NaCl solution environment at 80°C is

plotted vs. solution pH in Fig. 13.149 Also plotted is the membrane

sodium ion concentration, which begins to fall from the neutral

solution value at a solution pH of 2.5. This correlates well with the

calculated solution pH of membrane protonation according to the

pKa of the exchange sites. However, the membrane sodium ion

diffusion flux drops by over an order of magnitude before this

exchange-site protonation is seen. It would appear then that small

amounts of protonation, on the order of 1-5% of membrane

exchange sites, causes a pronounced loss in the ability of the

membranes to transport counterions. Diffusion among clusters is

accomplished through interconnecting regions of lower ion and

water content, according to the cluster-channel model (Section

II.4). If these regions were preferentially protonated, the overall



Richard S. Yeo and Howard L. Yeager



470



CARBOXYLATE FILM, 5M NaC£ f



3



4



5



6



pH (at25°C)

Figure 13. Na+ self-diffusion flux in a perfluorocarboxylate membrane and membrane Na+ concentration vs. solution pH, 5 M NaCl

solution environment, at 80°C. (Ref. 149; reprinted by permission

of the publisher, The Electrochemical Society, Inc.)



effect on sodium ion diffusion might well be what is observed

experimentally. Although this interpretation is only speculative, it

is evident that solution pH is an important consideration in the use

of the perfluorocarboxylate membranes.



IV. TRANSPORT PROPERTIES UNDER INDUSTRIAL

ELECTROLYSIS CONDITIONS

The most important commercial application of perfluorinated

ionomer membranes is currently in the chlor-alkali industry. These

materials are used as permselective separators in brine electrolysis

cells for the production of chlorine and sodium hydroxide. This



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

III. ION AND WATER DIFFUSION

Tải bản đầy đủ ngay(0 tr)

×