Transport Properties of Perfluorosulfonate Polymer Membranes - ACS

4

Transport Properties of Perfluorosulfonate Polymer Membranes HOWARD L. YEAGER

Downloaded by UNIV OF MINNESOTA on June 17, 2013 | http://pubs.acs.org Publication Date: February 4, 1982 | doi: 10.1021/bk-1982-0180.ch004

Department of Chemistry, The University of Calgary, Calgary, Alberta T2N 1N4, Canada

Perfluorinated, high molecular weight sulfonate polymers, such as the Nafion materials (E.I. du Pont de Nemours and Co.), have the high chemical stability and strength to serve as ideal membranes in various separation applications. In addition, homogeneous and uniform membranes of large size can be produced by taking advantage of the thermoplastic characteristics of the polymer in the unhydrolyzed sulfonyl fluoride form (1). The capability of producing membranes of uniform composition and thickness is another important advantage for wide scale industrial application. Finally, these polymers sorb relatively large amounts of water (and other protic solvents) despite the fluorocarbon character of the polymer backbone. This latter feature is related to perhaps the most important characteristic of these materials: cations and water readily diffuse through the polymer, which enables electrolytic communication to be maintained through the membrane phase. It is of course important to characterize the nature of transport processes in perfluorosulfonate polymer membranes in order to optimize their performance in separation systems. The ion-clustered morphology (2) of these polymers is unusual compared to conventional cross-linked sulfonate ion exchange resins, whose transport properties have been reasonably well studied. Therefore i t is expected that differences in transport characteristics w i l l be seen between the two types of polymers. These differences should lend insight into the nature of the ion clustering phenomenon. Dilute solution studies are of particular importance in this regard. Under these conditions, ion-containing polymers exhibit Donnan exclusion of anions, which prevents sorption of electrolytes from the solution (3). Only the cationic exchange counterions are then present in the membrane phase, which helps to simplify the interpretation of the material's transport properties. Experiments performed in concentrated solutions and at elevated temperatures are also necessary, because most applications of ion exchange membranes involve such conditions. It is also important to consider the driving force for transport of 0097-6156/82/0180-0041 $05.75 / 0 © 1982 American Chemical Society In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

42

PERFLUORINATED IONOMER MEMBRANES

s p e c i e s a c r o s s t h e membrane. G r a d i e n t s i n c o n c e n t r a t i o n l e a d t o d i f f u s i o n a l processes, w h i l e e l e c t r i c a l p o t e n t i a l g r a d i e n t s genera t e i o n i c m i g r a t i o n and e l e c t r o o s m o t i c e f f e c t s . The combined e f f e c t s o f these f o r c e s y i e l d the o v e r a l l t r a n s p o r t c h a r a c t e r i s t i c s o f t h e membrane. Experimental r e s u l t s which y i e l d i n s i g h t i n t o the nature of t h e s e p r o c e s s e s i n p e r f l u o r o s u l f o n a t e membranes a r e e m p h a s i z e d i n t h i s chapter. T h i s i n f o r m a t i o n , when c o r r e l a t e d w i t h s t r u c t u r a l s t u d i e s a n d r e s u l t s o f membrane p e r f o r m a n c e i n p r a c t i c a l a p p l i c a t i o n s , should help t o produce a u n i f i e d understanding of t h i s i m p o r t a n t new t y p e o f p o l y m e r membrane.


D i f f u s i o n i n N a f i o n P e r f l u o r o s u l f o n a t e Membranes Membrane D i f f u s i o n i n D i l u t e S o l u t i o n E n v i r o n m e n t s . The measurement o f i o n i c d i f f u s i o n c o e f f i c i e n t s p r o v i d e s u s e f u l i n f o r m a t i o n about t h e n a t u r e o f t r a n s p o r t p r o c e s s e s i n polymer membranes. U s i n g a r a d i o a c t i v e t r a c e r , d i f f u s i o n o f a n i o n i c s p e c i e s c a n be m e a s u r e d w h i l e t h e membrane i s i n e q u i l i b r i u m w i t h the e x t e r n a l s o l u t i o n . This enables the determination of a s e l f d i f f u s i o n c o e f f i c i e n t f o r a polymer phase o f u n i f o r m composition w i t h no g r a d i e n t s i n i o n o r w a t e r s o r p t i o n . I n a d d i t i o n , s e l f d i f f u s i o n c o e f f i c i e n t s a r e more s t r a i g h t f o r w a r d i n t h e i r i n t e r p r e t a t i o n compared t o t h o s e o f e l e c t r o l y t e f l u x e x p e r i m e n t s , w h e r e c a t i o n and a n i o n t r a n s p o r t r a t e s a r e c o u p l e d . T r a c e r s e l f - d i f f u s i o n c o e f f i c i e n t s f o r s o d i u m i o n and c e s i u m i o n h a v e b e e n m e a s u r e d f o r 1200 e q u i v a l e n t w e i g h t N a f i o n membranes ( 4 - 7 ) . R e s u l t s o b t a i n e d a t 25°C a r e l i s t e d i n T a b l e I , a l o n g w i t h Table I .

Medium

Sodium I o n a n d C e s i u m I o n S e l f - D i f f u s i o n C o e f f i c i e n t s , 25°C

D, Na

1200 EW N a f i o n 8.6% D V B - P S S H 0 a

b

2

cm

2

sec

1 D

/ D

Na+ Cs+

ACT>

7

7

5

5.20 χ 1 0 " 1.37 x 1 0 ~ 2.06 χ 10""

8

6

5

18 0.69 0.65

28.3 27. l 19. I

k J mol 1 Çs±

Na+

+

9.44 x 1 0 " 9.44 χ 1 0 " 1.33 χ Ι Ο "

E

C

d

e

a

reference 9 ^ r e f e r e n c e 10 0-40°C 0-25°C e

d

s i m i l a r r e s u l t s f o r a n 8.6% d i v i n y l b e n z e n e c r o s s - l i n k e d p o l y s t y r e n e s u l f o n a t e r e s i n and f o r aqueous s o l u t i o n . Cation diffusion c o e f f i c i e n t s i n t h e s u l f o n a t e i o n exchange r e s i n a r e reduced by

In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

e

66.1 20.0 18.0 e

d

4.

Transport

YEAGER

43

Properties

a b o u t a f a c t o r o f t e n compared t o t h o s e i n aqueous s o l u t i o n , due t o i n c r e a s e d t o r t u o s i t y o f t h e medium. The s l i g h t l y i n c r e a s e d a c t i v a t i o n e n e r g i e s o f d i f f u s i o n can be a s c r i b e d p a r t l y t o t h e same c a u s e and p a r t l y t o e l e c t r o s t a t i c i n t e r a c t i o n s w i t h f i x e d a n i o n i c exchange s i t e s . The r a t i o o f N a and C s d i f f u s i o n c o e f f i c i e n t s r e m a i n t h e same i n b o t h e n v i r o n m e n t s , t h o u g h . T h i s s u g g e s t s t h a t e f f e c t s s u c h a s i o n p a i r i n g , w h i c h w o u l d be d e p e n d e n t on t h e c a t i o n ' s c h a r g e d e n s i t y , a r e n o t a s i g n i f i c a n t f a c t o r i n a f f e c t i n g d i f f u s i o n i n t h e s u l f o n a t e i o n exchange r e s i n . F o r 1200 EW N a f i o n , t h e s o d i u m i o n r e s u l t i s ( c o i n c i d e n t a l l y ) i d e n t i c a l t o t h a t i n t h e s u l f o n a t e r e s i n , b u t t h e c e s i u m i o n v a l u e i s much lower, w i t h an extremely l a r g e a c t i v a t i o n energy o f d i f f u s i o n . This C s a c t i v a t i o n energy i s c l o s e r t o t h a t o f N a d i f f u s i o n i n N a C l c r y s t a l a t a b o u t 600°C, 74 k J m o l ( 8 ) , than a value f o r a s o l u t i o n - l i k e d i f f u s i o n a l process. I o n - p a i r i n g o f cesium i o n t o s u l f o n a t e exchange s i t e s would n o t be s u s p e c t e d as t h e cause o f t h i s d i f f e r e n c e , f o r t h e exchange s i t e charge d e n s i t y on t h e s u l f o n a t e g r o u p s h o u l d b e l o w due t o t h e f l u o r o c a r b o n c o n t e n t o f the polymer. I n o r d e r t o e x p l o r e t h i s anomaly, these d i f f u s i o n c o e f f i c i e n t s w e r e r e m e a s u r e d f o r t h e same s a m p l e o f N a f i o n a f t e r d r y s t o r a g e f o r two y e a r s . A s e c o n d s a m p l e o f N a f i o n , o f r e c e n t m a n u f a c t u r e , was a l s o s t u d i e d ( 6 , 7 ) . R e s u l t s a r e shown i n T a b l e I I and F i g u r e 1. Sodium i o n v a l u e s a r e v i r t u a l l y i d e n t i c a l f o r +

+

+

+


- 1

Table I I .

Ionic Self-Diffusion

Ionic Form

Na

Membrane Sample

+

Cs+

I I I II

0.038 0.520 0.446 0.363

e

a

D

c

i n 1200 EW N a f i o n (7)

40°C

EACT (0-40°C) kJ mol" 1

3.07 2.78 3.18

a

25°C

0°C

I I II a

Coefficients

9.44 11.2 9.83

b

0.520 1.70 2.38 1.88

15.1 14.9 14.8

28.3 29.8 27.3

1.58 3.37 3.01 2.67

66.1 38.9 35.9 35.4

b

a f t e r two y e a r s i n a.s - r e c e i v e d f o r m v a l u e m e a s u r e d a t 5 °C mixed Na -Cs f o r m , i o n i c f r a c t i o n o f Cs+ = 0.14 +

+

Journal of the Electrochemical Society

a l l t h r e e c a s e s , b u t those o f cesium i o n have i n c r e a s e d ( w i t h d e c r e a s e d a c t i v a t i o n e n e r g y o f d i f f u s i o n ) i n t h e aged membrane s a m p l e . R e s u l t s f o r t h e s e c o n d s a m p l e o f N a f i o n show s i m i l a r b e h a v i o r t o aged s a m p l e I f o r b o t h c e s i u m i o n a n d s o d i u m i o n .



44


1

I

I

3.2

3.4

3.6

1000/T,

K'

1

Journal of the Electrochemical Society Figure 1. Logarithm of self-diffusion coefficient vs. reciprocal of absolute temperature for 1200 EW Nafion ( 1 1 ) . Key: Q>> Q Sample 1, 1978; ·, Sample 1, 1980; 3 , f j , Sample 2.


4.

Transport

YEAGER

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Properties

The w a t e r t o exchange s i t e m o l e r a t i o was m e a s u r e d f o r b o t h i o n f o r m s i n t h e aged s a m p l e I and f o r s a m p l e I I , and no change c o u l d be f o u n d f r o m t h e o r i g i n a l v a l u e s o f 11.9 and 6.6 f o r N a and C s forms, r e s p e c t i v e l y . Thus i t a p p e a r s t h a t t h e a g i n g p r o c e s s f o r t h e f i r s t sample i s a c c o m p a n i e d by some k i n d o f m o r p h o l o g i c a l change w h i c h a f f e c t s cesium i o n d i f f u s i o n but n o t sodium i o n d i f f u s i o n . The r e s u l t s f r o m t h e r e c e n t l y p r o d u c e d s e c o n d s a m p l e o f membrane may i n d i c a t e i m p r o v e d a n n e a l i n g o f t h e p o l y m e r t o y i e l d more t i m e i n d e p e n d e n t membrane b e h a v i o r . The i m p o r t a n t f e a t u r e o f t h e s e r e s u l t s l i e s i n t h e d i f f e r e n c e i n r e s p o n s e o f t h e two c a t i o n s t o w h a t e v e r p o l y m e r r e l a x a t i o n process that d i d occur. O r i g i n a l l y , C s d i f f u s e s i n a d i f f e r e n t manner t h a n N a , as c o n c l u d e d by c o m p a r i s o n o f d i f f u s i o n a l a c t i v a t i o n energies. A f t e r aging, the C s activation energy i s c l o s e r t o t h a t expected o f t o r t u o u s polymer d i f f u s i o n , b u t t h e N a / C s d i f f u s i o n c o e f f i c i e n t r a t i o , 6.6, i s s t i l l f a r f r o m t h e e x p e c t e d v a l u e o f 0.7. Thus t h e two c a t i o n s a p p e a r t o h a v e s i m i l a r t r a n s p o r t mechanisms w i t h c e s i u m i o n e n c o u n t e r i n g a more t o r t u o u s d i f f u s i o n a l p a t h w a y . Of c o u r s e , t h e w a t e r c o n t e n t o f t h e p o l y m e r i s a c e n t r a l f a c t o r i n the d i f f u s i o n a l p r o p e r t i e s of a polymer. In o r d e r t o s t u d y w a t e r s o r p t i o n a s a v a r i a b l e , b o t h a s - r e c e i v e d and b o i l e d f o r m s o f 1200 EW w e r e u s e d f o r d i f f u s i o n e x p e r i m e n t s ( 7 ) . I n a d d i t i o n , v a r i o u s N a - C s h e t e r o i o n i c forms o f t h e polymer were p r e p a r e d and t h e i r w a t e r c o n t e n t s d e t e r m i n e d . S i n c e t h e o v e r a l l w a t e r s o r p t i o n d e c r e a s e s s m o o t h l y as t h e C s i o n i c f r a c t i o n i n c r e a s e s , b o t h N a and C s d i f f u s i o n c o u l d be s t u d i e d as a f u n c t i o n o f p o l y m e r w a t e r c o n t e n t . R e s u l t s a r e shown i n F i g u r e 2 ( 7 ) . The f u n c t i o n V / ( 1 - V ) , where V i s the volume f r a c t i o n o f p o l y m e r i n a w a t e r s w o l l e n m a t e r i a l , i s p l o t t e d as t h e a b s c i s s a i n F i g u r e 2. The d e n o m i n a t o r o f t h i s t e r m i s t h e r e f o r e t h e v o l u m e f r a c t i o n o f w a t e r i n t h e membrane, c a l c u l a t e d f r o m s o r p t i o n r e sults. T h i s f u n c t i o n was d e v e l o p e d by Y a s u d a and c o - w o r k e r s t o t r e a t d i f f u s i o n i n v a r i o u s h y d r o p h i l i c polymers (12). T h e i r equation: +

+

+

+


+

+

+

+

+

+

+

+

p

D = D° exp

p

p

[-b V / ( 1 - V p p

(1)

d e s c r i b e s t h e r e l a t i o n s h i p b e t w e e n t h e aqueous and t h e p o l y m e r d i f f u s i o n c o e f f i c i e n t s o f a s p e c i e s , D° and D r e s p e c t i v e l y . The e q u a t i o n i s r e l a t e d t o t h a t d e r i v e d by Cohen and T u r n b u l l f o r t h e d i f f u s i o n c o e f f i c i e n t of a molecule i n a simple l i q u i d D = A exp

(-Yv*/v ) f

(2)

where ν i s a c h a r a c t e r i s t i c v o l u m e f o r d i f f u s i o n o f t h e s p e c i e s , V f i s t h e " f r e e v o l u m e " p e r s o l v e n t m o l e c u l e and γ and A a r e c o n s t a n t s ( 1 3 ) . The c o n s t a n t b i n E q u a t i o n 1 i s r e l a t e d t o t h e e x p o n e n t i n E q u a t i o n 2, w h e r e now v f w o u l d r e p r e s e n t t h e f r e e volume o f w a t e r . E q u a t i o n 1 p r o v i d e s an e x c e l l e n t c o r r e l a t i o n o f




46

I

1

0

1

J

2

I

I

3 4 V p / 1 - Vp

I

I

5

6

Journal of the Electrochemical Society Figure 2. Logarithm of self-diffusion coefficient vs. polymer fraction function for 1200 EW Nafion, 25° C. Na+ and Cs+ lines without data points: polystyrene sul fonate behavior, ( 11 ); Δ, Q, • , denote heteroionic forms.


4.

Transport

YEAGER

47

Properties

the b i n a r y d i f f u s i o n c o e f f i c i e n t s f o r NaCl i n a v a r i e t y of water s w o l l e n polymers, f o r D v a l u e s ranging over f i v e orders of magnitude (12). I n a d d i t i o n , the e q u a t i o n s u c c e s s f u l l y f i t s t r a c e r s e l f - d i f f u s i o n c o e f f i c i e n t s f o r N a and C s i n p o l y s t y r e n e s u l f o n a t e r e s i n s o f v a r y i n g c r o s s - l i n k i n g (and w a t e r c o n t e n t ) ( 1 4 ) . The p r e - e x p o n e n t i a l t e r m i n E q u a t i o n 1 was f o u n d t o depend on t h e e l e c t r o s t a t i c a t t r a c t i o n of the c o u n t e r i o n to f i x e d charge s i t e s i n a d d i t i o n t o D° f o r t h e s e i o n e x c h a n g e p o l y m e r s . Lines which these authors found to d e s c r i b e s e l f - d i f f u s i o n c o e f f i c i e n t s f o r N a and C s a r e shown i n F i g u r e 2, f o r p o l y m e r s w h e r e Vp v a r i e d f r o m 0.85 t o 0.15 ( 1 4 ) . As s e e n i n t h e F i g u r e , d i f f u s i o n r e s u l t s a r e v e r y d i f f e r e n t f o r N a f i o n compared t o c r o s s - l i n k e d p o l y s t y r e n e s u l f o n a t e s . Sodium i o n has a much h i g h e r d i f f u s i o n c o e f f i c i e n t i n N a f i o n f o r a g i v e n water content, which supports the concept of i o n c l u s t e r i n g i n N a f i o n as a m o r p h o l o g y w i t h c o n s i d e r a b l e p h a s e s e p a r a t i o n b e t w e e n f l u o r o c a r b o n and h y d r a t e d c a t i o n s and e x c h a n g e sites. T o r t u o s i t y w o u l d t h u s be r e d u c e d i n N a f i o n , compared t o a c r o s s - l i n k e d p o l y m e r w i t h a more random d i s t r i b u t i o n o f e x c h a n g e sites. This r e s u l t i s a l s o obtained f o r cesium i o n i n the asr e c e i v e d h o m o i o n i c f o r m o f N a f i o n . As w a t e r c o n t e n t i s r a i s e d i n t h e p o l y m e r , by b o i l i n g o r by p a r t i a l e x c h a n g e f o r N a , t h e C s d i f f u s i o n c o e f f i c i e n t remains c o n s t a n t however. A l s o , the i n v e r t e d order of magnitude f o r C s versus N a d i f f u s i o n c o e f f i c i e n t s i s n o t removed a t h i g h e r w a t e r c o n t e n t s , b u t a c t u a l l y becomes more p r o n o u n c e d . The a c t i v a t i o n e n e r g y o f C s i n a s a m p l e which i s l a r g e l y i n the Na f o r m , i n T a b l e I I , shows t h a t t h e mechanism o f C s d i f f u s i o n i s l a r g e l y i n d e p e n d e n t o f membrane w a t e r c o n t e n t and c o u n t e r i o n f o r m . +


+

+

+

+

+

+

+

+

+

+

The s e l f - d i f f u s i o n c o e f f i c i e n t f o r i o d i d e i o n was a l s o m e a s u r e d a t 25°C i n t h e f i r s t s a m p l e o f 1200 EW N a f i o n , b e f o r e a g i n g ( 4 ) . The r e s u l t , 9 x 10"* cm sec" , indicates that this a n i o n i s more m o b i l e t h a n c e s i u m i o n , b u t l e s s t h a n a s o d i u m i o n . T h i s i s a l s o somewhat u n u s u a l . In general, co-ion d i f f u s i o n coeff i c i e n t s a r e s e e n t o be l a r g e r t h a n t h o s e o f c o u n t e r i o n s i n i o n e x c h a n g e r s , b e c a u s e no e l e c t r o s t a t i c a t t r a c t i o n s t o t h e p o l y m e r phase e x i s t . (The membrane c o n c e n t r a t i o n i s o n l y 5 x 10"" mol L " t h o u g h , r e f l e c t i n g t h e e f f e c t s o f Donnan e x c l u s i o n p r o c e s s e s ( 4 ) . ) Water s e l f - d i f f u s i o n c o e f f i c i e n t s have a l s o been d e t e r m i n e d , u s i n g t r i t i u m w a t e r t r a c e r , f o r sample I I ( 7 ) . R e s u l t s a r e g i v e n i n T a b l e I I I and shown i n F i g u r e 2. A l s o shown i n F i g u r e 2 a r e w a t e r d i f f u s i o n c o e f f i c i e n t s f o r s e v e r a l N a - C s h e t e r o i o n i c forms i n o r d e r t o t e s t t h e e f f e c t o f membrane w a t e r c o n t e n t . Values i n Table I I I f o r Na -form Nafion are s i m i l a r to those obtained f o r t h e HT^-form o f 1155 e q u i v a l e n t w e i g h t N a f i o n , w h i c h w e r e d e t e r m i n e d by measurement o f t h e r a t e o f w a t e r s o r p t i o n ( 2 , 1 5 ) . Water d i f f u s i o n c o e f f i c i e n t s a r e s e e n t o f o l l o w a d e p e n d e n c e on t h e Vp f u n c t i o n w h i c h i s s i m i l a r t o t h a t o f s o d i u m i o n . The m a g n i t u d e s of the d i f f u s i o n c o e f f i c i e n t s are q u i t e l a r g e i n r e l a t i o n to the volume f r a c t i o n o f s o r b e d w a t e r . A l s o , a c t i v a t i o n energies of 8

2

1

3

+

+

+

American Chemical Society Library 1155 16th St., N.W. Washington, DC 20036 In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

1

PERFLUORINATED

48 Table I I I .

S e l f - D i f f u s i o n C o e f f i c i e n t s o f Water i n 1200 EW N a f i o n ( 7 ) . D x 10 , 6

Ionic Form

+

Na K+ Cs+

IONOMER MEMBRANES

cm

2

sec

1

25°C

40°C

A C T (5-40°C) kJ mol

2.65 2.15 1.32

3.95

21.4

E

5°C

1.40

0.815

- 1

-

-

2.37

22.0


Journal of the Electrochemical Society +

+

d i f f u s i o n f o r t h e N a - f o r m and C s - f o r m a r e o n l y s l i g h t l y l a r g e r t h a n t h e c o r r e s p o n d i n g v a l u e i n p u r e w a t e r , 17.8 k J m o l " ( 1 6 ) . The r e s u l t s i n d i c a t e t h a t t h e r e i s a h i g h d e g r e e o f p h a s e s e p a r a t i o n b e t w e e n f l u o r o c a r b o n and i o n - c l u s t e r e d r e g i o n s , and t h a t w a t e r d i f f u s i o n among c l u s t e r s i s f a c i l e . T h i s i s t r u e even f o r the C s - f o r m , where the water-exchange s i t e mole r a t i o i s o n l y 6.6. 1

+

A D i f f u s i o n a l Model f o r N a f i o n . S e v e r a l s t r u c t u r a l models of N a f i o n h a v e b e e n p r o p o s e d ; t h e s e h a v e b e e n b a s e d on a v a r i e t y o f t r a n s p o r t and s p e c t r o s c o p i c p r o p e r t i e s o f t h e p o l y m e r ( 1 7 - 2 0 ) . The c l u s t e r - n e t w o r k m o d e l d e v e l o p s t h e c o n c e p t o f s p h e r i c a l i o n i c r e g i o n s s e p a r a t e d by i n t e r - c o n n e c t i n g c h a n n e l s ( 1 7 ) . T h e s e c h a n n e l s a r e s e e n t o h a v e an i m p o r t a n t r o l e i n h y d r o x i d e i o n rejection i n chlor-alkali cells. Rodmacq and c o - w o r k e r s p r o p o s e a t h r e e phase model i n w h i c h f l u o r o c a r b o n m i c r o c r y s t a l l i t e s , i o n w a t e r c l u s t e r s , and a s e c o n d i o n i c r e g i o n o f l o w e r w a t e r c o n t e n t c o e x i s t ( 1 8 ) . F a l k s e e s e v i d e n c e f o r two e n v i r o n m e n t s o f s o r b e d w a t e r i n N a f i o n f r o m i n f r a r e d s p e c t r o s c o p i c s t u d i e s ( 1 9 ) . The f i r s t e n v i r o n m e n t a p p e a r s t o be aqueous i n n a t u r e , w i t h t h e s t r e n g t h of i n t e r m o l e c u l a r hydrogen bonding reduced from t h a t i n pure water. I n the second environment, the water m o l e c u l e s a r e n o t h y d r o g e n bonded and a p p e a r t o be e x p o s e d m a i n l y t o f l u o r o carbon. Extensive i n t r u s i o n s of f l u o r o c a r b o n m a t e r i a l i n t o i o n c l u s t e r e d r e g i o n s i s i n f e r r e d f r o m t h e s e r e s u l t s ( 1 9 ) . L e e and M e i s e l have s t u d i e d t h e m i c r o e n v i r o n m e n t o f the R u ( I I ) - 2 , 2 b i p y r i d i n e c o m p l e x i n N a f i o n , and a l s o f i n d e v i d e n c e f o r e x t e n s i v e i n t e r a c t i o n o f t h i s c a t i o n w i t h f l u o r o c a r b o n phase ( 2 1 ) . A model o f N a f i o n w h i c h i s c o n s i s t e n t w i t h i o n i c d i f f u s i o n a l r e s u l t s and w i t h t h e above o b s e r v a t i o n s h a s b e e n p r o p o s e d ( 7 ) . T h i s approach a l s o d e s c r i b e s t h r e e r e g i o n s i n the polymer, as shown i n F i g u r e 3. R e g i o n A c o n s i s t s o f f l u o r o c a r b o n b a c k b o n e m a t e r i a l , some o f w h i c h i s i n a m i c r o c r y s t a l l i n e f o r m , as de t e c t e d by Rodmacq and c o - w o r k e r s ( 1 8 ) . I o n c l u s t e r s f o r m R e g i o n C, i n w h i c h t h e m a j o r i t y o f s u l f o n a t e exchange s i t e s , c o u n t e r i o n s , and s o r b e d w a t e r e x i s t . The i n t e r f a c i a l R e g i o n Β i s s e e n as one


YEAGER

Transport

Properties


4.

Journal of the Electrochemical Society Figure 3. Three region structural model for Nafion: A, fluorocarbon; B, interfacial zone; C, ionic clusters (11).


49

PERFLUORINATED IONOMER

50

MEMBRANES

of r e l a t i v e l y l a r g e f r a c t i o n a l v o i d volume, c o n t a i n i n g pendant s i d e c h a i n m a t e r i a l , a s m a l l e r amount o f w a t e r , some s u l f o n a t e exchange s i t e s w h i c h have not been i n c o r p o r a t e d i n t o c l u s t e r s , and a c o r r e s p o n d i n g f r a c t i o n o f c o u n t e r i o n s . The r e l a t i v e numbers o f i o n s i n R e g i o n s Β and C w o u l d depend on t h e s i z e , c h a r g e d e n s i t y and h y d r a t i o n e n e r g y o f t h e c a t i o n . I o n s o f l o w c h a r g e d e n s i t y o r l a r g e s i z e , s u c h as C s o r R u ( b p y ) 3 , w o u l d p r e f e r R e g i o n B, w h i l e t h o s e o f l a r g e r c h a r g e d e n s i t y and h y d r a t i o n e n e r g y w o u l d l o c a l i z e i n t h e more aqueous i o n i c c l u s t e r s ( w i t h i n electroneutrality limitations). +


+

2 +

+

In order to i n t e r p r e t the Na and C s d i f f u s i o n a l r e s u l t s i n t e r m s o f t h i s m o d e l , i t i s assumed t h a t b o t h c a t i o n s w o u l d be a b l e t o d i f f u s e r e a d i l y i n b o t h t h e i o n i c c l u s t e r s and i n t e r f a c i a l regions. C e s i u m i o n w o u l d e x p e r i e n c e a more t o r t u o u s d i f f u s i o n p a t h compared t o s o d i u m i o n , and t h u s w o u l d h a v e a s m a l l e r measured s e l f - d i f f u s i o n c o e f f i c i e n t . The i n s e n s i t i v i t y o f t h i s d i f f u s i o n c o e f f i c i e n t t o i n c r e a s i n g w a t e r s o r p t i o n may t h e n be b e c a u s e most o f t h i s w a t e r s e r v e s t o i n c r e a s e t h e s i z e o f i o n i c c l u s t e r s , w h i c h w o u l d h a v e a r e l a t i v e l y m i n o r o v e r a l l e f f e c t on the d i f f u s i o n path l e n g t h . As d e s c r i b e d e a r l i e r , a g i n g o f a s a m p l e o f 1200 EW N a f i o n was a c c o m p a n i e d by an i n c r e a s e o f a f a c t o r o f t h r e e i n t h e d i f f u s i o n c o e f f i c i e n t of C s , w i t h a l a r g e decrease i n a c t i v a t i o n energy of d i f f u s i o n . A l m o s t no change was s e e n i n t h e c o r r e s p o n d i n g v a l u e s f o r Na . I t i s p o s s i b l e t h a t p o r t i o n s of r e g i o n Β were not o r i g i n a l l y as w e l l f o r m e d as shown i n F i g u r e 3, and c o n t a i n e d d i f f u s i o n a l l y i s o l a t e d p o r t i o n s . The a g i n g p r o c e s s w o u l d t h e n h a v e c o n s i s t e d o f a c o n s o l i d a t i o n o f aqueous and f l u o r o c a r b o n p h a s e s . The o r i g i n a l l y i s o l a t e d p o r t i o n s o f t h e i n t e r f a c i a l r e g i o n would y i e l d l a r g e a c t i v a t i o n energies of d i f f u s i o n f o r counterions. D i f f u s i o n o f c e s i u m i o n w o u l d be more s e n s i t i v e t o t h i s i n c o m p l e t e n e s s o f p h a s e s e p a r a t i o n compared t o s o d i u m i o n . The s t u d y o f h e t e r o i o n i c f o r m s o f t h e unaged s a m p l e I w o u l d h a v e helped to r e s o l v e t h i s p o i n t . +

+

Thus t h e m o d e l i n F i g u r e 3 i s c o n s i s t e n t w i t h s p e c t r o s c o p i c and d i f f u s i o n a l r e s u l t s , b u t i s c e r t a i n l y an o v e r s i m p l i f i e d picture nevertheless. Other approaches to the modeling of t r a n s p o r t i n N a f i o n , s u c h as t h e r e c e n t a p p l i c a t i o n o f p e r c o l a t i o n t h e o r y by Hsu and c o - w o r k e r s ( 2 2 ) , may y i e l d f u r t h e r i n s i g h t i n t o the problem. Membrane D i f f u s i o n i n C o n c e n t r a t e d S o l u t i o n E n v i r o n m e n t s . M o s t o f t h e c u r r e n t a p p l i c a t i o n s o f p e r f l u o r o s u l f o n a t e membranes involve electrochemical c e l l s i n which concentrated e l e c t r o l y t e s o l u t i o n s are employed, o f t e n a t e l e v a t e d temperatures. Rela t i v e l y l i t t l e d i f f u s i o n d a t a a r e a v a i l a b l e under these c o n d i t i o n s , a l t h o u g h a l a r g e r amount o f membrane r e s i s t a n c e and o t h e r o p e r a t i n g d a t a have been p u b l i s h e d . Sodium i o n s e l f - d i f f u s i o n c o e f f i c i e n t s h a v e b e e n m e a s u r e d i n v a r i o u s N a f i o n membranes i n c o n c e n t r a t e d NaOH s o l u t i o n s a t e l e v a t e d t e m p e r a t u r e s ( 2 3 ) . This


4.

YEAGER

Transport

Properties

51

e l e c t r o l y t e s y s t e m i s b e i n g s t u d i e d b e c a u s e t h e e m e r g i n g membrane c h l o r - a l k a l i c e l l t e c h n o l o g y i s t h e most i m p o r t a n t c u r r e n t a p p l i c a t i o n o f p e r f l u o r i n a t e d i o n e x c h a n g e membranes. Sodium i o n i s t h e m a j o r c u r r e n t c a r r y i n g s p e c i e s i n t h e membrane p h a s e f o r t h i s a p p l i c a t i o n , and i t s mechanism o f t r a n s p o r t i n N a f i o n u n d e r such c o n d i t i o n s i s o f great i n t e r e s t . Sodium i o n s e l f - d i f f u s i o n c o e f f i c i e n t s i n s e v e r a l N a f i o n membranes a r e p l o t t e d versus the r e c i p r o c a l of absolute temperature i n Figure 4 (23). I n 9.5 M NaOH, c o n s i d e r a b l e e l e c t r o l y t e s o r p t i o n i n t o t h e membrane p h a s e o c c u r s . I n a d d i t i o n , polymer water s o r p t i o n i s r e d u c e d due t o t h e d e h y d r a t i n g e f f e c t o f t h e e x t e r n a l solution. This r e s u l t s i n a reduction i n the N a s e l f - d i f f u s i o n coefficient. F o r e x a m p l e , 1200 EW N a f i o n shows a v a l u e o f 3.45 x 10" c m s e c a t 90°C, a b o u t t h r e e t i m e s s m a l l e r t h a n t h e d i l u t e s o l u t i o n , room t e m p e r a t u r e v a l u e . The 1150 EW m a t e r i a l y i e l d s s l i g h t l y l a r g e r d i f f u s i o n c o e f f i c i e n t s t h a n 1200 EW, due t o t h e g r e a t e r c o n c e n t r a t i o n o f e x c h a n g e s i t e s and l a r g e r w a t e r s o r p t i o n . The 1150 (EDA) membrane i s a l s o a n 1150 EW p o l y m e r f i l m , b u t one surface i s treated w i t h ethylenediamine while the f i l m i s i n the s u l f o n y l f l u o r i d e p r e c u r s o r f o r m . Upon h y d r o l y s i s , e x c h a n g e s i t e s i n a b o u t a 0.04 mm t h i c k l a y e r a r e c o n v e r t e d t o s u l f o n a m i d e g r o u p s . These weakly a c i d i c exchange s i t e s y i e l d improved c u r r e n t e f f i c i e n c y i n a c h l o r - a l k a l i c e l l when t h e t r e a t e d l a y e r o f t h e membrane f a c e s t h e NaOH c a t h o d e s o l u t i o n . The e f f e c t o f t h i s l a y e r i s t o i n c r e a s e t h e a c t i v a t i o n energy o f d i f f u s i o n f o r sodium i o n , a s s e e n i n F i g u r e 4. A f u l l y c o n v e r t e d membrane, l a b e l e d 'EDA , shows t h e e f f e c t more d r a m a t i c a l l y . The a c t i v a t i o n e n e r g i e s o f N a d i f f u s i o n f o r t h e s e membranes a r e : 1150 EW, 10.5 k J m o l " ; 1200 EW, 20.0 k J m o l " ; 1150 (EDA), 28.9 k J m o l " ; and EDA, 50.6 k J m o l . The s u l f o n a m i d e e x c h a n g e s i t e s p r o d u c e a membrane w i t h d e c r e a s e d s o r b e d w a t e r , a n d t h i s a p p e a r s t o b e t h e m a i n f a c t o r f o r d e c r e a s e d N a d i f f u s i o n c o e f f i e n t s and i n c r e a s e d a c t i v a t i o n energies of d i f f u s i o n . Thus h i g h e r c u r r e n t e f f i c i e n c i e s i n o p e r a t i n g c e l l s a r e a c c o m p a n i e d b y h i g h e r membrane v o l t a g e d r o p s a s w e l l f o r t h e s e t y p e s o f membranes. +


7

2

- 1

1

+

1

1

1

- 1

+

D i f f u s i o n a l A r r h e n i u s p l o t s f o r N a f i o n 295 a t t h r e e NaOH c o n c e n t r a t i o n s a r e shown i n F i g u r e 5 ( 2 3 ) . T h i s membrane i s s i m i l a r t o 1150 (EDA), b u t i s b a c k e d w i t h a n o p e n weave T e f l o n f a b r i c f o r added s t r e n g t h . Sodium i o n d i f f u s i o n c o e f f i c i e n t s d r o p r a p i d l y w i t h i n c r e a s i n g c a u s t i c s t r e n g t h and membrane d e h y dration. I n d e e d , a t 25°C i n NaOH s o l u t i o n s o f 10 M o r h i g h e r , t h e s e membranes a r e v i r t u a l n o n c o n d u c t o r s o f i o n s ( 2 3 ) . Activ a t i o n e n e r g i e s o f d i f f u s i o n c a l c u l a t e d f o r t h e 60-90°C t e m p e r a ture i n t e r v a l f o r the p l o t s i n Figure 5 are r e l a t i v e l y constant a t a b o u t 35 k J m o l " . Thus t h e mechanism o f N a d i f f u s i o n a p p e a r s to r e m a i n c o n s t a n t o v e r t h i s s o l u t i o n c o n c e n t r a t i o n range. A n o t h e r f e a t u r e o f t h e 295 membrane i s s e e n i n F i g u r e 6 ( 2 3 ) . H e r e N a s e l f - d i f f u s i o n c o e f f i c i e n t s a r e p l o t t e d v e r s u s NaOH c o n c e n t r a t i o n f r o m d i l u t e t o c o n c e n t r a t e d s o l u t i o n s . The r a p i d d r o p a t h i g h c a u s t i c s t r e n g t h i s a t t r i b u t e d t o membrane 1

+

+



PERFLUORINATED IONOMER M E M B R A N E S



YEAGER

Transport

°C I 2.7

Properties

I 90

I 80 I 2.8

1000/ Figure 5. Arrhenius

I 70 I 2.9

l _ 60 I 3.0

T,°K"'

Journal of the Electrochemical Society plots for Na+ diffusion in Nafion 295, NaOH external solution ( 2 4 ) . Key: Q> 9.5 M ; Q 11.0 Μ ; Δ , 12.5 M .



54


ill

0.01

I

ul

0.10

I

ι ι ι ι ι ni

1.0

I

L

10.0

NaOH CONC., M Journal of the Electrochemical Society Figure 6. Na+ diffusion coefficient in Nafion 295 at 60° C, NaOH external solution; and change in water molarity in NaOH solution ( 2 4 ) .


4.

Transport

YEAGER

Properties

55

dehydration. T h i s e f f e c t i s r e f l e c t e d i n t h e r a p i d d r o p i n NaOH s o l u t i o n water m o l a r i t y i n t h i s concentration r e g i o n , which i s a l s o shown i n t h e F i g u r e . A s i m i l a r d r o p i n membrane d i f f u s i o n c o e f f i c i e n t i s seen i n d i l u t e s o l u t i o n as w e l l . T h i s i s caused by p r o t o n a t i o n o f t h e w e a k l y a c i d i c s u l f o n a m i d e e x c h a n g e s i t e s . Thus membranes o f t h i s t y p e c a n o n l y b e u s e d i n a l k a l i n e m e d i a . I n a d d i t i o n , t h e s e r e s u l t s s u g g e s t t h a t t h e pH g r a d i e n t a c r o s s s u c h membranes i n o p e r a t i n g c h l o r - a l k a l i c e l l s w i l l b e a n i m p o r t a n t f a c t o r i n d e t e r m i n i n g t h e membrane v o l t a g e d r o p . The d i f f u s i o n o f m o l e c u l a r s p e c i e s h a s a l s o b e e n s t u d i e d i n concentrated s o l u t i o n environments (25,26). Yeo and M c B r e e n m e a s u r e d t h e d i f f u s i o n c o e f f i c i e n t s o f H and C l i n 1200 EW N a f i o n membranes immersed i n HC1 s o l u t i o n s , and t h a t o f B r i n HC1 and HBr s o l u t i o n s a s a f u n c t i o n o f e l e c t r o l y t e c o n c e n t r a t i o n and t e m p e r a t u r e ( 2 5 ) . I n c o n c e n t r a t e d HC1 s o l u t i o n s t h e o r d e r o f d i f f u s i o n c o e f f i c i e n t s i s H >Cl2>Br, as expected from molecular size. A c t i v a t i o n e n e r g i e s o f d i f f u s i o n f o r H and C l i n 4.1 M HC1 w e r e f o u n d t o be 21.6 and 23.3 k J mol"" r e s p e c t i v e l y o v e r t h e 25°-50°C t e m p e r a t u r e i n t e r v a l . These v a l u e s a r e v e r y s i m i l a r t o t h o s e f o r w a t e r d i f f u s i o n i n t h e same membrane i n d i l u t e s o l u t i o n , as s e e n i n T a b l e I I I . The a u t h o r s u t i l i z e t h e s e r e s u l t s t o e s t i mate a c o u l o m b i c l o s s o f a b o u t 2% i n a h y d r o g e n - c h l o r i n e f u e l c e l l , due m a i n l y t o c h l o r i n e m i g r a t i o n t h r o u g h t h e membrane. The i n t e r p r e t a t i o n o f B r d i f f u s i o n was c o m p l i c a t e d b y t h e f o r m a t i o n o f B r 3 " and p o s s i b l y o t h e r a n i o n i c b r o m i n e s p e c i e s . W i l l ( 2 6 ) h a s a l s o s t u d i e d b r o m i n e d i f f u s i o n , i n 1200 EW and o t h e r N a f i o n membrane m a t e r i a l s . The e l e c t r o l y t e s u s e d i n t h e s e e x p e r i m e n t s w e r e c o n c e n t r a t e d Z n B r o r NaBr s o l u t i o n s . The B r 3 " i o n i s t h e predominant bromine s p e c i e s i n such media, although m o l e c u l a r B r would appear t o be r e s p o n s i b l e f o r t r a n s p o r t a c r o s s t h e membrane. M e a s u r e d d i f f u s i o n c o e f f i c i e n t s v a r i e d f r o m 1 x 10" t o 5 x 1 0 ~ c m sec"" a t room t e m p e r a t u r e . Values increased w i t h d e c r e a s i n g e q u i v a l e n t w e i g h t o f t h e p o l y m e r and w i t h d e creasing s o l u t i o n concentration. This again supports the view t h a t membrane w a t e r c o n t e n t i s a n i m p o r t a n t f a c t o r i n d e t e r m i n i n g membrane d i f f u s i o n c o e f f i c i e n t s , e v e n f o r n e u t r a l diffusing species. Membrane D i f f u s i o n i n Nonaqueous S o l v e n t E n v i r o n m e n t s . Selfd i f f u s i o n c o e f f i c i e n t s o f N a and C s f o r 1200 EW N a f i o n membranes i n d i l u t e m e t h a n o l and a c e t o n i t r i l e s o l u t i o n s h a v e b e e n m e a s u r e d (5). A r r h e n i u s p l o t s o f t h e s e r e s u l t s a r e shown i n F i g u r e 7 a l o n g w i t h c o r r e s p o n d i n g r e s u l t s f o r aqueous e x p e r i m e n t s ; a c t i v a t i o n energies of d i f f u s i o n are l i s t e d i n Table IV. D i f f u s i o n coeff i c i e n t s o f N a i n m e t h a n o l a n d w a t e r - e q u i l i b r a t e d membranes a r e v e r y s i m i l a r , and t h e a c t i v a t i o n e n e r g y o f d i f f u s i o n f o r t h e methanol system i s o n l y s l i g h t l y h i g h e r than the r e s p e c t i v e v a l u e f o r N a i n p u r e m e t h a n o l s o l v e n t , 12.9 k J m o l ( 2 7 ) . Thus a s o l u t i o n - l i k e d i f f u s i o n mechanism i s i n f e r r e d f o r b o t h s o l v e n t systems. Cesium i o n d i f f u s i o n i n t h e methanol e q u i l i b r a t e d membrane i s much s l o w e r t h a n s o d i u m i o n d i f f u s i o n ; i n f a c t t h e 2

2


2

2

2

2

1

2

2

2

8

7

2

1

+

+

+

+

- 1




56


4.

YEAGER

Transport

Table IV.

A c t i v a t i o n E n e r g i e s o f D i f f u s i o n i n 1200 EW Nafion (5).

E

A kJ mol"

Ion

Solvent

Temp. I n t e r v a l , °C

Na+

H 0 CH 0H CH CN

0-25 0-25 1-25

30.3 15.3 61.3

H 0 CH3OH CH CN

0-25 0-25 25-40

70.3 83.1 83.3

2

3

3

Cs Downloaded by UNIV OF MINNESOTA on June 17, 2013 | http://pubs.acs.org Publication Date: February 4, 1982 | doi: 10.1021/bk-1982-0180.ch004

57

Properties

+

2

3

1

d i f f e r e n c e i s now more p r o n o u n c e d t h a n f o r t h e aqueous c a s e . M e t h a n o l s o r p t i o n i s f a r l e s s w i t h 1200 EW N a f i o n f o r C s - f o r m s a m p l e s compared t o t h e N a - f o r m , w h i c h may f o r m p a r t o f t h e r e a s o n ( 5 ) . F o r membranes w h i c h h a v e b e e n e q u i l i b r a t e d w i t h a n hydrous a c e t o n i t r i l e , d i f f u s i o n c o e f f i c i e n t s f o r both ions a r e d e c r e a s e d f u r t h e r . The a c e t o n i t r i l e v a l u e s a r e e x t r e m e l y s e n s i t i v e t o t r a c e s o f w a t e r i n t h e membrane ( 4 ) . O n l y s m a l l r e s i d u a l amounts i n c r e a s e t h e membrane d i f f u s i o n c o e f f i c i e n t s b y s e v e r a l hundred p e r c e n t . The v e r y s m a l l v a l u e s o f c a t i o n d i f f u s i o n c o e f f i c i e n t s suggest that e i t h e r extensive i o n p a i r i n g predominates f o r t h i s a p r o t i c s o l v e n t , o r t h a t c o m m u n i c a t i o n among c l u s t e r s i s l o s t d u e t o r e l a t i v e l y s m a l l p o l y m e r s w e l l i n g . The s e n s i t i v i t y o f t h e d i f f u s i o n c o e f f i c i e n t s t o s m a l l amounts o f w a t e r w o u l d s u g g e s t t h a t t h e f o r m e r may b e r e s p o n s i b l e . The i n a b i l i t y o f t h e weak L e w i s a c i d a c e t o n i t r i l e m o l e c u l e t o s o l v a t e e x c h a n g e s i t e s would promote s u l f o n a t e - c a t i o n i o n p a i r s , a p r o c e s s w h i c h would be r e v e r s e d b y s m a l l amounts o f s o r b e d w a t e r . The d i f f u s i o n o f S 0 a n d CH3CN i n a c e t o n i t r i l e - e q u i l i b r a t e d forms o f N a f i o n has a l s o been r e p o r t e d (28). A s i m i l a r s e n s i t i v i t y o f d i f f u s i o n r a t e s t o t h e p r e s e n c e o f w a t e r was n o t e d . C a l c u l a t e d d i f f u s i o n c o e f f i c i e n t s were based on s o l u t i o n concent r a t i o n s , a n d t h u s a r e n o t r e a d i l y interprétable i n t e r m s o f membrane p r o p e r t i e s . +

+

2

T r a n s p o r t P r o p e r t i e s under C o n d i t i o n s

o f Current Flow

A p p l i c a t i o n s o f p e r f l u o r o s u l f o n a t e membranes commonly i n v o l v e t h e i r use as separation m a t e r i a l s i n e l e c t r o l y t i c c e l l s , i n which c o n c e n t r a t e d s o l u t i o n s a r e employed. A p r i m a r y c o n s i d e r a t i o n i n s u c h a p p l i c a t i o n s i s t h e c o n d u c t i v i t y o f t h e membrane, b e c a u s e t h e ohmic l o s s d u e t o membrane r e s i s t a n c e c a n s i g n i f i c a n t l y i n c r e a s e energy consumption o f t h e c e l l . The c o n d u c t i v i t i e s o f common N a f i o n membranes h a v e b e e n i n v e s t i g a t e d f o r s e v e r a l


PERFLUORINATED

58

IONOMER M E M B R A N E S

i n d u s t r i a l l y i m p o r t a n t e l e c t r o l y t e environments. F o r example, Yeo a n d c o - w o r k e r s r e p o r t membrane c o n d u c t i v i t i e s i n c o n c e n t r a t e d HC1 ( 2 5 ) a n d i n NaOH and KOH s o l u t i o n s ( 2 9 ) a s a f u n c t i o n o f t e m p e r a t u r e . F o r t h e l a t t e r c a s e , N a f i o n shows l a r g e r c o n d u c t i v i t i e s when e q u i l i b r a t e d w i t h NaOH s o l u t i o n s t h a n w i t h KOH s o l u t i o n s o f e q u a l m o l a r i t y ; a g a i n a c o r r e l a t i o n i s found between membrane c o n d u c t i v i t y a n d w a t e r c o n t e n t . For t h e a p p l i c a t i o n o f t h e s e membranes t o t h e e l e c t r o l y t i c p r o d u c t i o n o f c h l o r i n e - c a u s t i c , other performance c h a r a c t e r i s t i c s i n a d d i t i o n t o membrane c o n d u c t i v i t y a r e o f i n t e r e s t . The s o d i u m i o n t r a n s p o r t number, i n m o l e s N a p e r F a r a d a y o f p a s s e d c u r r e n t , e s t a b l i s h e s t h e c a t h o d e c u r r e n t e f f i c i e n c y o f t h e membrane c e l l . A l s o t h e w a t e r t r a n s p o r t number, e x p r e s s e d a s m o l e s o f w a t e r t r a n s p o r t e d t o t h e NaOH c a t h o l y t e p e r F a r a d a y , a f f e c t s t h e c o n c e n t r a t i o n o f c a u s t i c produced i n t h e c e l l . Sodium i o n a n d w a t e r t r a n s p o r t numbers h a v e b e e n s i m u l t a n e o u s l y d e t e r m i n e d f o r s e v e r a l N a f i o n membranes i n c o n c e n t r a t e d N a C l and NaOH s o l u t i o n environments and e l e v a t e d t e m p e r a t u r e s (30-32). E x p e r i m e n t s were c o n d u c t e d a t h i g h membrane c u r r e n t d e n s i t i e s (2-4 kA n f ) t o duplicate industrial conditions. R e s u l t s o f some o f t h e s e e x p e r i m e n t s a r e shown i n F i g u r e 8, i n w h i c h s o d i u m i o n t r a n s p o r t number i s p l o t t e d v s NaOH c a t h o l y t e c o n c e n t r a t i o n f o r 1100 EW, 1150 EW, and N a f i o n 295 membranes ( 3 0 , 3 1 ) . F o r t h e f i r s t two membranes, t j j + d e c r e a s e s w i t h i n c r e a s i n g NaOH c o n c e n t r a t i o n , a s w o u l d b e e x p e c t e d due t o i n c r e a s i n g e l e c t r o l y t e s o r p t i o n i n t o t h e polymer. I t h a s b e e n f o u n d t h a t u p t a k e o f NaOH i n t o t h e s e memb r a n e s d o e s o c c u r , b u t t h e r e l a t i v e amount o f s o r p t i o n r e m a i n s r e l a t i v e l y constant as s o l u t i o n c o n c e n t r a t i o n i n c r e a s e s (23,33). Membrane w a t e r s o r p t i o n d e c r e a s e s s i g n i f i c a n t l y o v e r t h e same c o n c e n t r a t i o n range however, and so t h e r a t i o o f sodium i o n t o w a t e r s t e a d i l y i n c r e a s e s . M a u r i t z and c o - w o r k e r s p r o p o s e t h a t a t u n n e l i n g process o f t h e form


+

2

a

H Na

•H

OH" ^

^

+

Na 0H"

H 0 2

may e n h a n c e h y d r o x i d e i o n t r a n s p o r t i n e n v i r o n m e n t s o f d e c r e a s e d w a t e r c o n t e n t due t o i n c r e a s e d p o l a r i z a t i o n o f t h e 0-H bond ( 3 3 ) . T h i s w o u l d e x p l a i n t h e d e c r e a s e o f t f l + f o r 1100 EW a n d 1150 EW membranes. The l o w e r e q u i v a l e n t w e i g h t m a t e r i a l shows a s t e e p e r decrease presumably because o f i t s l a r g e r c o n c e n t r a t i o n o f sodium ions. N a f i o n 295 shows a d i f f e r e n t t y p e o f d e p e n d e n c e i n t^ + w i t h i n c r e a s i n g NaOH c o n c e n t r a t i o n . The shape o f t h i s c u r v e i s n o t u n i q u e t o t h i s b i l a y e r membrane, b u t h a s b e e n s e e n i n s i m i l a r N a f i o n p r o d u c t s as w e l l . F i g u r e 9 shows a c o r r e s p o n d i n g p l o t f o r N a f i o n 2 2 7 , w h i c h i s a f a b r i c - b a c k e d m a t e r i a l composed o f 1200 EW polymer w i t h t h e cathode s u r f a c e c o n v e r t e d t o sulfonamide a

a


YEAGER

Transport

Properties


4.

Figure 8. Sodium ion transport number vs. NaOH catholyte molarity for Nafion membranes, 80° C. Key: O, ·, Nafion 295; •, 1150 EW; A, 1100 EW. Anolyte solution is NaOH for light symbols and 5 M NaCl for dark symbols.


59


PERFLUORINATED IONOMER

MEMBRANES

The Electrochemical Society, Inc. Figure 9. Current efficiency vs. NaOH catholyte concentration for Nafion 227 mem brane in a chlor-alkali cell (34). Conditions: current density, 31 A/dm ; tempera ture, 85° C; anolyte concentration, 4.4 Ν NaCl; cell voltage, 4.6 V. 2


4.

YEAGER

Transport

61

Properties

s i t e s ( 3 4 ) . The d e p e n d e n c e o f t j j + on NaOH c o n c e n t r a t i o n has b e e n r e c e n t l y r e p o r t e d f o r p e r f l u o r i n a t e d c a r b o x y l a t e membranes; a minimum a t i n t e r m e d i a t e c o n c e n t r a t i o n f o l l o w e d by a p e a k i n Na b i g h e r c o n c e n t r a t i o n has b e e n o b s e r v e d i n t h e s e c a s e s as w e l l (35-37). S e v e r a l f a c t o r s may be i n v o l v e d i n t h i s r a t h e r c o m p l i c a t e d v a r i a t i o n i n tjja « The common f e a t u r e o f b o t h p e r f l u o r i n a t e d s u l f o n a m i d e and c a r b o x y l a t e p o l y m e r s i s a l o w e r i n h e r e n t w a t e r c o n t e n t compared t o s u l f o n a t e p o l y m e r s . It is p o s s i b l e that w i t h i n c r e a s i n g c a u s t i c s t r e n g t h , the a s s o c i a t e d drop i n water s o r p t i o n would generate e x t e n s i v e i o n p a i r i n g i n t h e membrane. T h i s i n t u r n w o u l d r e d u c e 0-H bond p o l a r i z a t i o n i n r e m a i n i n g w a t e r m o l e c u l e s o f h y d r a t i o n , and l a r g e l y remove t h e h y d r o x i d e i o n t u n n e l i n g mechanism o f t r a n s p o r t ( 3 3 , 3 6 ) . Suhara and Oda s u g g e s t i n s t e a d t h a t as t h e membrane c o n t r a c t s due t o i n c r e a s i n g d e h y d r a t i o n , the l o c a l c o n c e n t r a t i o n of exchange s i t e s i n i o n i c c l u s t e r s increases (35). This i n t u r n r e - e s t a b l i s h e s a Donnan e x c l u s i o n mechanism f o r h y d r o x i d e r e j e c t i o n . I n a n o t h e r a p p r o a c h , K r e s s m a n and Tye p r e d i c t i n g e n e r a l t e r m s t h a t a minimum i n t j j + c a n o c c u r w i t h i n c r e a s i n g s o l u t i o n concentration, i f a s u f f i c i e n t l y large electroosmotic effect i s p r e s e n t , due t o a f r i c t i o n a l i n t e r a c t i o n b e t w e e n w a t e r t r a n s p o r t and h y d r o x i d e i o n m i g r a t i o n ( 3 8 ) . F o r e x p e r i m e n t s w h e r e a n o l y t e and c a t h o l y t e a r e i d e n t i c a l c o n c e n t r a t i o n s o f NaOH, t n 0 v a l u e s d e c r e a s e f r o m a b o u t 3 mol F " t o l e s s t h a n 1 mol F*" f o r s o l u t i o n c o n c e n t r a t i o n s o f 5 M t o 13 M f o r t h e 1150 EW membrane ( 3 0 ) . For N a f i o n 295, t o i e s from 5 t o 1 mol F u n d e r t h e same c o n d i t i o n s (30,32). F o r e x p e r i m e n t s i n w h i c h 5 M N a C l i s u s e d as a n o l y t e , an o s m o t i c component i s a l s o p r e s e n t t o w a t e r t r a n s p o r t . F o r e x a m p l e , t j ^ O r e m a i n s c o n s t a n t a t a b o u t 3 mol F"" f o r NaOH s o l u t i o n c o n c e n t r a t i o n s a b o v e 10 M f o r N a f i o n 295 ( 3 0 ) . Smaller i n c r e a s e s i n t Q a r e o b s e r v e d f o r t h e 1150 EW membrane. As s e e n i n F i g u r e 8, t h i s i s a c c o m p a n i e d by a s h i f t i n t h e t j j + p e a k t o h i g h e r NaOH c o n c e n t r a t i o n . S i m i l a r e f f e c t s were observed w i t h a p e r f l u o r i n a t e d c a r b o x y l a t e membrane, a l t h o u g h t j j o v a l u e s a r e g e n e r a l l y s m a l l e r (36). T h e r e f o r e , w h i l e water t r a n s p o r t i s seen t o i n f l u e n c e t h e i o n i c t r a n s p o r t c h a r a c t e r i s t i c s o f t h e s e memb r a n e s , o t h e r f a c t o r s must be c o n s i d e r e d t o u n d e r s t a n d membrane performance i n these h i g h l y concentrated s o l u t i o n s . a

t

+

a t


+

a

2

1

v

a

r

1

- 1

H 2

1

H 2

a

2

S t u d i e s o f t h e s e p e r f l u o r i n a t e d membranes i n d i l u t e and i n c o n c e n t r a t e d s o l u t i o n e n v i r o n m e n t s s t i l l l e a v e many u n a n s w e r e d q u e s t i o n s a b o u t t h e n a t u r e o f membrane t r a n s p o r t p r o p e r t i e s . However, t h e o b v i o u s i m p o r t a n c e o f t h e s e p o l y m e r s i n membrane s e p a r a t i o n a p p l i c a t i o n s , coupled w i t h the fundamental s i g n i f i c a n c e o f t h e i r i o n c l u s t e r e d m o r p h o l o g y , makes t h e c o n t i n u e d study of these m a t e r i a l s a f r u i t f u l area of r e s e a r c h f o r the future. Literature Cited 1. Price, E.H. "The Commercialization of Ion-Exchange Membranes


62

2. 3. 4.


5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

18.

19. 20.

21. 22. 23.


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YEAGER

Transport Properties

63

Reference 23, reprinted by permission of the publisher, The Electrochemical Society, Inc. Yeo, R.S.; McBreen, J. J . Electrochem. Soc. 1979, 126, 168287. Will, F.G. J. Electrochem. Soc. 1979, 126, 36-42. Calculated from limiting single ion conductivities in Kay, R.L. J . Am. Chem. Soc. 1960, 82, 2099-105; and Vidulich, G.P.; Cunningham, G.P.; Kay, R.L. J. Solution Chem. 1973, 2, 23-35. Kimmerle, F.M.; Breault, R. Can. J. Chem. 1980, 58, 2225-29. Yeo, R.S.; McBreen, J.; Kissel, G . ; Kulesa, F . ; Srinivasan, S. J . Appl. Electrochem. 1980, 10, 741-47. Yeager, H . L . ; O'Dell, B.; Twardowski, Z. J. Electrochem. Soc. 1981, in press. Yeager, H . L . ; unpublished results. Dotson, R . L . ; Lynch, R.W.; Hilliard, G.E. in "Proceedings of the Symposium on Ion Exchange"; Yeo, R.S.; Buck, R.P., Eds., The Electrochemical Society: Pennington, N . J . , 1981. Mauritz, K . A . ; Branchick, K.J.; Gray, C . L . ; Lowry, S.R. Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem. 1980, 20, 122-23. Hora, C . J . ; Maloney, D.E. "Nafion Membranes Structured for High Efficiency Chlor-Alkali Cells", presented at the 152nd National Meeting of The Electrochemical Society, Inc., Atlanta, Ga., October 10-14, 1977. Suhara, M.; Oda, Y. in "Proceedings of the Symposium on Ion Exchange"; Yeo, R.S.; Buck, R.P., Eds., The Electrochemical Society: Pennington, N . J . , 1981. Yeager, H . L . ; Twardowski, Z. "Measurement of Ionic and Water Transport Numbers in a Membrane Chlor-Alkali Cell", presented at the 159th National Meeting, The Electrochemical Society, Minneapolis, Minn., May 10-15, 1981. Seko, M. "Membrane for Chlor-Alkali Electrolysis", presented at the 159th National Meeting, The Electrochemical Society, Minneapolis, Minn., May 10-15, 1981. Kressman, T . R . E . ; Tye, F . L . Trans. Faraday Soc. 1959, 55 1441-50.

RECEIVED August 7, 1981.


Transport Properties of Perfluorosulfonate Polymer Membranes - ACS

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