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Proton and deutron NMR study of PTFE ionomer membranes
SOLID STATE
Solid State Ionics 50 (1992) 339-343 North-Holland
IONICS Proton and deuteron NMR study of PTFE ionomer membranes G. Xu and Y.S. Pak Department of Materials Science, McMaster University, Hamilton, Ontario, Canada LSS 4L 7 Received 9 September 1991 ; accepted for publication 28 October 1991
Proton and deuteron NMR have been conducted to investigate the ionic motion in perfluorinated ionomer membranes from Dow Chemical (XUS) and DuPont (Nafionrt). Two proton relaxation peaks were found in the XUS specimen absorbed with HzO. The major (narrow) peak presented a spin-lattice relaxation time ( T, ) of 107 ms while the minor (broader) one gave much longer Tj. While the former was attributed to the water molecules involved in restricted motion, the latter was expected to be associated with the protons located in the vicinity of the sulfonate groups. Similar to the previous results from the others, only a single peak was detected in Nation R in ~H spectra, indicating that the protons in the different environments were engaging rapid exchange within NMR time scale, In contrast to the inverse proportion dependence of the linewidth on the water sorption in Nation R, the major line of the XUS membrane exhibited insensitive linewidth dependence on the variation of H20 concentration. The difference was attributed to the existence of narrow breaths of the pores in XUS sample, such that free water contribution to the enhancement of proton mobility was limited. The 2H spectra of Nation R were found to possess a doublet, due to nuclear quadrupolar interaction. Dow (XUS) membrane treated in at 100% relative humidity ( R H ) D20 presented a single peak with the linewidth insensitive to the amount of heavy water absorbed. An additional rise emerged on the "shoulder" of this single peak when treated at 33% RH. It is concluded that XUS membrane does not provide strong hydrogen bonding to eliminate the rapid motion average over the nuclear quadrupole interaction.
I. I n t r o d u c t i o n
P e r f l u o r i n a t e d ion e x c h a n g e m e m b r a n e s such as N a f i o n R have received much consideration as electrolyte separator in electrochemical a p p l i c a t i o n s of fuel cell t e c h n o l o g y [la,lb] from w h i c h high e n e r g y e f f i c i e n c y and low e m i s s i o n of harmful pollutants are expected. In these ionomers, p e r f l u o r o c a r b o n sulfonates (-OCF2-CF 2SO3H ) are a t t a c h e d as pendant groups to t h e polytetrafluoroethylene (PTFE) backbones. The t e r m i n a l -SO3H groups p r o v i d e the c a t i o n - e x c h a n g e capacity, and thus ionic conduction. Large c u r r e n t d e n s i t y (-i A / c m 2) and power d e n s i t y (iW/cm2) [2 I have been achieved in their application to the fuel cell development. However, d u r i n g o p e r a t i o n of the fuel cell, the transport of substantial amount of water m o l e c u l e s (H20/H + ratio = 3.5 - 5:1) t h r o u g h the m e m b r a n e does introduce a major w a t e r m a n a g e m e n t p r o b l e m [2,3]. A new series of p e r f l u o r i n a t e d ionomer (XUS) has b e e n i n t r o d u c e d by D o w C h e m i c a l Company. Still h i g h e r current and power d e n s i t i e s yet w i t h less water transport have been reported[4]. Although the exact p r e p a r a t i o n m e t h o d of the m e m b r a n e is proprietary, this p o l y m e r has PTFE like b a c k b o n e s similar to that of N a f i o n R, but
w i t h s h o r t e r s u l f o n a t e side chains [3]. P r e v i o u s study shows by e l e c t r o a n a l y s i s and r a d i o t r a c e r [5] that it has larger porosity, smaller p o r e breaths and similar swelling under water t r e a t m e n t c o m p a r e d w i t h Nafion R. However, the r o l e that water molecules play in the proton t r a n s p o r t a t i o n and m e m b r a n e - p r o t o n i n t e r a c t i o n is still m a r g i n a l l y understood. N u c l e a r m a g n e t i c resonance(NMR) was shown to be a p o w e r f u l t e c h n i q u e in p r o b i n g i o n / a t o m i c d i f f u s i o n and m o l e c u l a r c o n f i g u r a t i o n in solids [6]. W a t e r - i o n o m e r m e m b r a n e i n t e r a c t i o n has b e e n s t u d i e d by NMR in Nafion R in the acid [7] and sodium [8 l form. It was found that the IH r e s o n a n c e was b r o a d e n e d and the c h e m i c a l shifts moved to lower field with decreasing water content. T h e s e results claimed the e x i s t e n c e of h y d r o g e n b o n d i n g and r e s t r i c t e d m o b i l i t y of w a t e r molecules in the membrane. 2H N M R has been e m p l o y e d p r o d u c t i v e l y in the studies of w a t e r p o l y m e r i n t e r a c t i o n in epoxy blends [9], and in p o l y i m i d e films [i0]. The q u a d r u p o l a r m o m e n t of 2H (spin I=l), and the r e s u l t i n g i n t e r a c t i o n w i t h m o l e c u l a r e l e c t r i c field gradient, give a g r e a t e r s e n s i t i v i t y than that of proton to the m o t i o n and c o n f i g u r a t i o n of w a t e r m o l e c u l e s [II]. T h o u g h a n u m b e r of works have b e e n r e p o r t e d on the IH NMR
0167-2738/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
340 for N a f i o n R [ 7 , 8 , 1 2 - 1 4 ] , no p r e s e n t e d on t h e D o w i o n o m e r
G. Iu, ES. Pak / PTFE ionomer membranes NMR data have been m e m b r a n e t h u s far.
It is the m a j o r o b j e c t i v e of t h e p r e s e n t s t u d y to i n v e s t i g a t e p r o t o n - w a t e r i n t e r a c t i o n s t h r o u g h t h e s t u d i e s of IH a n d 2H N M R s p e c t r a of D o w (XUS) ionomer membranes. The lineshapes of the r e s o n a n c e are e x p e c t e d to p r o v i d e i n f o r m a t i o n on the bonding nature between the polymer network a n d t h e a b s o r b e d H20. T h e r e l a t i v e h u m i d i t y (RH) of w a t e r t r e a t m e n t e n v i r o n m e n t is v a r i e d to p r o b e the effects of different absorption concentrations. For c o m p a r i s o n , t h e N M R r e s u l t s of p r o t o n and d e u t e r o n s t u d i e s on N a f i o n R are necessary. The latter is expected to yield information on possible nuclear quadrupolar i n t e r a c t i o n of D20 t h e N a f i o n R a b s o r b s .
2. E x p e r i m e n t a l
3. R e s u l t s and D i s c u s s i o n T h e r e p r e s e n t a t i v e IH a b s o r p t i o n s p e c t r a of D o w (XUS) membrane treated in the 33% and 100% relative humidity, obtained f r o m t h e FID at a t e m p e r a t u r e of 303K, are s h o w n in F i g s . l a and lb. Two distinct proton resonance peaks are exhibited, with a narrow line (major) characterized by a full w i d t h at h a l f m a x i m u m (FWHM) of 1 0 3 ± 5 Hz and a b r o a d e r line (minor) by a F W H M of 2 6 0 ± 5 Hz. The f o r m e r has a s p i n - l a t t i c e r e l a x a t i o n t i m e (TI) of 1 0 7 ± 9 ms, w h i l e t h e l a t t e r i n v o l v e s a T I w h i c h is 6-7 t i m e s longer. M o r e o v e r , t h e c h e m i c a l s h i f t (table I.) of the n a r r o w l i n e is f o u n d to c h a n g e m o r e s u b s t a n t i a l l y than the b r o a d e r one, w i t h c o m p l e t e l y o p p o s i t e b e h a v i o r to that of Nafion R, under the different water concentrations. It is, t h e r e f o r e , demonstrated conclusively that two distinct environments for p r o t o n s e x i s t in the XUS m e m b r a n e . To c o m p a r e the proton results of D o w XUS
Dow developmental fuel cell membrane (XUS 13204.10) and Nafion R perfluorinated membrane (NIl7) w e r e k i n d l y s u p p l i e d by D o w C a n a d a and E.I. du P o n t de N e m o u r s & C o m p a n y r e s p e c t i v e l y . The samples as r e c e i v e d (H20 or D20 ) w e r e rolled tightly into cylinders and p l a c e d into standard 5 m m o.d. N M R tube. Lengths of t h e s a m p l e s w e r e in t h e r a n g e of 2 . 5 - 3 . 0 cm. A f t e r t h e s p e c i m e n s w e r e v a c u u m d r i e d for 2 days, t h e y were conditioned to w a t e r e n v i r o n m e n t (either distilled H20 or 9 9 . 9 % p u r e D20 ) of r e l a t i v e humidity of 100% or 33%, corresponding to approximately 15% or 5% w a t e r c o n c e n t r a t i o n in t h e c a s e of N a f i o n R. S a t u r a t e d M g C I 2 . 6 H 2 0 s o l u t i o n w a s e m p l o y e d to y i e l d 33% RH. T h e N M R t u b e s w e r e t h e n s e a l e d w i t h p l a s t i c c a p s and p a r a f f i n wax. To e l i m i n a t e p o s s i b l e w a t e r c o n d e n s a t i o n on t h e s u r f a c e in t h e c a s e of 100% RH, t r e a t m e n t w a s performed at a slightly lower temperature e n v i r o n m e n t t h a n t h a t of t h e m e a s u r e m e n t . A l l N M R m e a s u r e m e n t s w e r e r e c o r d e d on a B r u k e r AM-500 spectrometer. Proton (deuterium) spectra w e r e a c q u i r e d at 5 0 0 . 1 3 5 (76.774) M H z u s i n g a 5 m m d u a l f r e q u e n c y IH-13C p r o b e . In a d d i t i o n , 2H spectra were obtained with appropriate broadband preamplifier. Sample temperature was maintained at 303 K by a B r u k e r B V T - 1 0 0 v a r i a b l e t e m p e r a t u r e unit. The free induction decay (FID) was processed with Gaussian multiplication for resolution enhancement ( l i n e b r o a d e n i n g 0 - 3 0 Hz, depending on signal to noise ratio of the spectra), followed by Fast Fourier T r a n s f o r m a t i o n . All l i n e w i d t h s are t h e n c o r r e c t e d by s u b t r a c t i n g t h e d i g i t a l line b r o a d e n i n g . S p i n l a t t i c e r e l a x a t i o n t i m e s (TI) w e r e m e a s u r e d w i t h I n v e r s i o n R e c o v e r y (~--T-~/2) p u l s e s e q u e n c e . A l l chemical shifts data (6) for IH and 2H w e r e recorded in p p m r e l a t i v e to t e t r a m e t h y l s i l a n e (TMS). M e a s u r e m e n t s w e r e c a r r i e d out on b o t h D o w polymer and Nafion R specimens.
(a)
4 i
[
13
I t2
lli
i ~0
I 9
I 8
I 7
~ 6
I 5
Chemlcal S h i f t
I 4
(ppm)
(b]
i
13
is
tl
io
g
8
7
6
Chemical S h i f t
5
4
3
2
i
[ppm)
Fig. 1. Proton NMR o f D o w (XUS) membrane treated at 100% RH HsO (a), and 33% RH H20 (b).
G. Xu, KS. Pak / PTFE ionomer membranes
341
J ~4
13
1
1
9
8
7
6
5
4
3
2
1
0
-
CbemlCSl Sh~ft (ppm)
40
Fig. 2. Proton NMR of DuPont (Nation R) membrane treated at 100% RH H20.
ionomer with those of N a f i o n R, similar m e a s u r e m e n t s w e r e c o n d u c t e d on NIl7 m e m b r a n e . At all H20 c o n c e n t r a t i o n s only a single proton r e s o n a n c e p e a k is o b s e r v e d (Fig.2, t r e a t e d at 100% RH), w i t h a T I of 97±5 ms. In addition, the l i n e w i d t h i n c r e a s e d in a sample c o n t a i n i n g less w a t e r (table I.) w h i c h implies that the o v e r a l l m o b i l i t y of p r o t o n s is s i g n i f i c a n t l y e n h a n c e d at h i g h e r w a t e r content. T h e s e are c o n s i s t e n t w i t h the r e s u l t s p u b l i s h e d p r e v i o u s l y [7,8]. It is i n t e r e s t i n g to note that such d e p e n d e n c e of p r o t o n l i n e w i d t h on the H20 s o r p t i o n is not o b s e r v e d for the n a r r o w p e a k of Dow m a t e r i a l (table i.). The cause of this linewidth insensitivity to the water concentration v a r i a t i o n can be e x p l a i n e d by the fact that XUS m e m b r a n e has s h o r t e r p e n d a n t side chains, n a r r o w p o r e breaths, as s u g g e s t e d by V e r b r u g g e g r o u p [5]. The n a r r o w pore b r e a t h s p r o v i d e r e s t r i c t i o n s to the free H20 m o t i o n w i t h i n the pores and thus less e n h a n c e m e n t of the m o b i l i t y is a c h i e v e d by f u r t h e r i n c r e a s i n g w a t e r contents. Such, however, does not n e c e s s a r y imply that the p r o t o n s have
Table 1 FWHM and c h e m i c a l
shifts of IH NMR
Dow(XUS) % RH (H20)
FWHM (Hz)
Chem.shift (ppm)
DuPont(Nafion) FWHM (Hz)
Chem.shift (ppm)
100%
103 260*
7.4866 1.3449"
41
7.0716
33%
112 87*
6.1323 1.3000"
187
8.8861
* minor peak
35
3O
25
2O
15 t0 Cnem%ca] s h ~ f t
5 (pore)
0
-5
-~0
-15
Fig. 3. Deuteron NMR o f D o w (XUS) membrane treated at 33% RH DzO.
s t r o n g h y d r o g e n b o n d i n g w i t h the "lattice" -- the s u r r o u n d i n g s , as is d e m o n s t r a t e d by the d e u t e r o n N M R r e s u l t s in the following. D e u t e r o n NMR s p e c t r a of Dow XUS and D u P o n t N a f i o n R are shown in Figs 3, 4a and 4b. Their l i n e w i d t h s are s u m m a r i z e d in table 2 (note: the l i n e w i d t h of N a f i o n r e p r e s e n t s only half of the d o u b l e t ) . A well d e f i n e d q u a d r u p o l a r s p l i t t i n g is found for N a f i o n R. At 100% r e l a t i v e h u m i d i t y of D20 treatment, the s p l i t t i n g is most e v i d e n t (Fig.4a). The same phenomenon, viz, linewidth i n c r e a s e w i t h d e c r e a s i n g w a t e r c o n t e n t as in the p r o t o n case, is r e s p o n s i b l e for the m e r g e of the s p l i t t i n g p e a k at 33% RH (Fig.4b). This d o u b l e t is r e l a t e d to the n u c l e a r q u a d r u p o l a r i n t e r a c t i o n w i t h the e l e c t r i c field g r a d i e n t of the h y d r o g e n b o n d i n g [ii] in N a f i o n R. The small value of the splitting in frequency indicates that the d e u t e r o n s are e n g a g i n g c e r t a i n m o t i o n p a t t e r n s as the Pake d o u b l e t is a v e r a g e d by a large n u m e r i c a l factor. Such splitting, however, is not o b s e r v e d in D o w m e m b r a n e where, only a s i n g l e p e a k is d i s c o v e r e d when t r e a t e d at 100% RH. An a t t a c h e d small rise a p p e a r s on the "shoulder" (Fig.3) for the s a m p l e t r e a t e d at 33% RH of D20. The l i n e w i d t h (FWHM) of this a d d i t i o n a l "peak" is a p p r o x i m a t e l y equal to that of the main line. From the f r e q u e n c y shift (6) values, it seems that this additional line has the same origin as the s e c o n d a r y p e a k in IH s p e c t r u m of Dow film. Again, the i n s e n s i t i v i t y of the l i n e w i d t h of the m a i n p e a k to D20 c o n t e n t in XUS is observed. The a b s e n c e of the d o u b l e t in m a j o r line of XUS i n d i c a t e s that e f f e c t i v e a v e r a g e of static quadrupole i n t e r a c t i o n s is t a k i n g p l a c e as a r e s u l t of the high speed m o t i o n of D20 m o l e c u l e s . It is thus c o n c l u d e d that few d e u t e r o n s p o s s e s s strong hydrogen bonds to the host polymer network. On the other hand, c o m p a r i s o n of the l i n e w i d t h s to those p a r a m e t e r s m e a s u r e d in liquid D20 c l e a r l y i n d i c a t e s that the heavy w a t e r is not
342
G. Xu, Y.S. Pak / PTFE ionomer membranes
r
J
(a)
(b) 6'0
~.10
2'0
O PP"I
i
-20
-~0
--F-
-60
I;0
;
PPH
-1'00
Fig. 4. Deuteron NMR of DuPont (Nation R) membrane treated at 100% RH D20 (a), and 33% RH D20 (b).
Table 2 F W H M of 2H N M R
% RH (D20)
Dow(XUS) F W H M (Hz)
DuPont(Nafion) F W H M (Hz)
100%
229
447
33%
238
1146
participating in a r a p i d e x c h a n g e m o t i o n w i t h i n N M R t i m e scale, so that o n l y a s i n g l e p e a k is observed in N a f i o n R . S u c h rapid exchange is r e s t r i c t e d , h o w e v e r , in t h e D o w m e m b r a n e s . T h e s e results indicate that XUS is not merely a " g e o m e t r i c a l v a r i a t i o n " of N a f i o n R as s u g g e s t e d by F i g . 1 6 of r e f . [ 5 ] . F u r t h e r s t u d y is n e e d e d to provide a complete description of the interactions.
4.
i s o t r o p i c a l l y free, but u n d e r c e r t a i n r e s t r i c t i o n as s h o w n by p r o t o n NMR. T h e p r e s e n c e of w a t e r d r o p l e t s in m i c r o s c o p i c v o i d s c a n t h u s be r u l e d out. T h e e x i s t e n c e of t h e s e c o n d IH r e l a x a t i o n p e a k in XUS m e m b r a n e p r e s e n t s a m a j o r d i f f e r e n c e of its p r o t o n t r a n s p o r t a t i o n characteristics from t h a t of N a f i o n R- T h e i n s e n s i t i v i t y of t h e c h e m i c a l s h i f t of t h i s b r o a d e r line to w a t e r s o r p t i o n f u r t h e r r e v e a l s t h a t it m u s t be o r i g i n a t e d f r o m a c h e m i c a l l y d i f f e r e n t e n v i r o n m e n t f r o m t h a t of t h e major peak. One expectation w o u l d be t h a t it represents the distinct environment the protons experience in v i c i n i t y of t h e s u l f o n a t e g r o u p s . In fact, the ratio of total number of H20 m o l e c u l e s to t h e n u m b e r of w a t e r s n e a r SO3, w h i c h is about i0 in a 100% RH sample, matches qualitatively the intensity r a t i o of t h e t w o lines. These two types of protons are
Conclusions
T w o r e l a x a t i o n p e a k s w e r e f o u n d by IH N M R in the perfluorinated ionomer membranes absorbed with water from Dow (XUS) representing two distinct environments for p r o t o n s . T h e i n v e r s e p r o p o r t i o n d e p e n d e n c e of the l i n e w i d t h s on t h e w a t e r s o r p t i o n in N a f i o n R w a s not f o u n d in t h e XUS membrane by varying H20 concentration. The difference is u n d e r s t o o d in the c o n t e x t of t h e e x i s t e n c e of n a r r o w p o r e b r e a t h s in XUS s a m p l e . T h e 2H s p e c t r a of N a f i o n R w e r e f o u n d to p o s s e s s a doublet, attributed to nuclear quadrupole interaction. Dow (XUS) m e m b r a n e t r e a t e d in at 100% r e l a t i v e h u m i d i t y (RH) D20 p r e s e n t e d a s i n g l e l i n e in 2H s p e c t r u m . It is t h u s c o n c l u d e d t h a t a l t h o u g h in XUS m e m b r a n e w a t e r m o l e c u l e s are m o r e restricted in m o t i o n d u e to its s t r u c t u r e , it does not provide strong hydrogen bonding to eliminate the rapid average motion over the nuclear quadrupole interaction.
G. Xu, Y.S. Pak / PTFE ionomer membranes Acknowledgement
[5]
The authors are grateful to I.Robson of Dow Canada for the sample d o n a t i o n and helpful discussion. Thanks are also due to B.G.Sayers and D.W.Hughes for their assistance in the NMR measurement. This work is supported in part by the O C M R Grant #MCJ-509.
[6] [7]
[8] [9]
References [i0] [la] A . E i s e n b e r g and H.L.Yeager,ed. P e r f l u o r i n a t e d Ionomer Membranes, ACS Symp. Ser.180 (1982). lib] S.J.Sondheimer, N.J.Bunce and C.A.Fyfe, J.Macromol. Sci-Rev. in Macromol. Chem. and Phys. C26 (1986) 353. [2] A . J . A p p l e b y and F.R.Foulkes, Fuel Cell Handbook, (Van Nostrand 1989). [3] B.C.H.Steele, MRS Bull. June (1989) 19. [4] C h e m i c a l Week, Mar.(1987) 13.
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M . W . V e r b r u g g e and R . F . H i l l , J . E l e c t C h e m . Soc. 137 (1990) 3770. E . N . K a u f m a n n and G.K.Shenoy, e d . , M a t e r i a l s Res. Soc. Symp. Proc.3 (1981) 55. R.Duplessix, M.Escoubes, B.Rodmacq, F.Volino, E.Roche, A . E i s e n b e r g and M.Pineri, ACS Symp. Ser.127 (1980) 469. R.A.Komoroski, Adv. Chem. Ser. 187(1980) 155. L.W.Jelinski, J.J.Dumais, A.L.Cholli, T.S.EIIis and F.E.Karasz, Macromol. 18 (1985) 1091. G . X u , C . C . G r y t e , A . S . N o w i c k , S . Z . L i , Y . S . P a k and S.G.Greenbaum, J.Appl. Phys.66(1989)5290. L.W.Jelinski, in:High Res. NMR Spect.of Syn. P o l y m . i n Bulk,ed. R.A.Komoroski, (VHC Pub. 1986) ch.10. H.W.Starkweather, M a c r o m o l . 1 5 (1982) 320. N.G.Boyle, V . J . M c B r i e r t y and D.C.Douglass, Macromol. 16 (1983) 75. R.C.T.Slade, J . S a r k e r and J.H.Strange, Solid State Ionics 35 (1989) ii.
Solid State Ionics 50 (1992) 339-343 North-Holland
IONICS Proton and deuteron NMR study of PTFE ionomer membranes G. Xu and Y.S. Pak Department of Materials Science, McMaster University, Hamilton, Ontario, Canada LSS 4L 7 Received 9 September 1991 ; accepted for publication 28 October 1991
Proton and deuteron NMR have been conducted to investigate the ionic motion in perfluorinated ionomer membranes from Dow Chemical (XUS) and DuPont (Nafionrt). Two proton relaxation peaks were found in the XUS specimen absorbed with HzO. The major (narrow) peak presented a spin-lattice relaxation time ( T, ) of 107 ms while the minor (broader) one gave much longer Tj. While the former was attributed to the water molecules involved in restricted motion, the latter was expected to be associated with the protons located in the vicinity of the sulfonate groups. Similar to the previous results from the others, only a single peak was detected in Nation R in ~H spectra, indicating that the protons in the different environments were engaging rapid exchange within NMR time scale, In contrast to the inverse proportion dependence of the linewidth on the water sorption in Nation R, the major line of the XUS membrane exhibited insensitive linewidth dependence on the variation of H20 concentration. The difference was attributed to the existence of narrow breaths of the pores in XUS sample, such that free water contribution to the enhancement of proton mobility was limited. The 2H spectra of Nation R were found to possess a doublet, due to nuclear quadrupolar interaction. Dow (XUS) membrane treated in at 100% relative humidity ( R H ) D20 presented a single peak with the linewidth insensitive to the amount of heavy water absorbed. An additional rise emerged on the "shoulder" of this single peak when treated at 33% RH. It is concluded that XUS membrane does not provide strong hydrogen bonding to eliminate the rapid motion average over the nuclear quadrupole interaction.
I. I n t r o d u c t i o n
P e r f l u o r i n a t e d ion e x c h a n g e m e m b r a n e s such as N a f i o n R have received much consideration as electrolyte separator in electrochemical a p p l i c a t i o n s of fuel cell t e c h n o l o g y [la,lb] from w h i c h high e n e r g y e f f i c i e n c y and low e m i s s i o n of harmful pollutants are expected. In these ionomers, p e r f l u o r o c a r b o n sulfonates (-OCF2-CF 2SO3H ) are a t t a c h e d as pendant groups to t h e polytetrafluoroethylene (PTFE) backbones. The t e r m i n a l -SO3H groups p r o v i d e the c a t i o n - e x c h a n g e capacity, and thus ionic conduction. Large c u r r e n t d e n s i t y (-i A / c m 2) and power d e n s i t y (iW/cm2) [2 I have been achieved in their application to the fuel cell development. However, d u r i n g o p e r a t i o n of the fuel cell, the transport of substantial amount of water m o l e c u l e s (H20/H + ratio = 3.5 - 5:1) t h r o u g h the m e m b r a n e does introduce a major w a t e r m a n a g e m e n t p r o b l e m [2,3]. A new series of p e r f l u o r i n a t e d ionomer (XUS) has b e e n i n t r o d u c e d by D o w C h e m i c a l Company. Still h i g h e r current and power d e n s i t i e s yet w i t h less water transport have been reported[4]. Although the exact p r e p a r a t i o n m e t h o d of the m e m b r a n e is proprietary, this p o l y m e r has PTFE like b a c k b o n e s similar to that of N a f i o n R, but
w i t h s h o r t e r s u l f o n a t e side chains [3]. P r e v i o u s study shows by e l e c t r o a n a l y s i s and r a d i o t r a c e r [5] that it has larger porosity, smaller p o r e breaths and similar swelling under water t r e a t m e n t c o m p a r e d w i t h Nafion R. However, the r o l e that water molecules play in the proton t r a n s p o r t a t i o n and m e m b r a n e - p r o t o n i n t e r a c t i o n is still m a r g i n a l l y understood. N u c l e a r m a g n e t i c resonance(NMR) was shown to be a p o w e r f u l t e c h n i q u e in p r o b i n g i o n / a t o m i c d i f f u s i o n and m o l e c u l a r c o n f i g u r a t i o n in solids [6]. W a t e r - i o n o m e r m e m b r a n e i n t e r a c t i o n has b e e n s t u d i e d by NMR in Nafion R in the acid [7] and sodium [8 l form. It was found that the IH r e s o n a n c e was b r o a d e n e d and the c h e m i c a l shifts moved to lower field with decreasing water content. T h e s e results claimed the e x i s t e n c e of h y d r o g e n b o n d i n g and r e s t r i c t e d m o b i l i t y of w a t e r molecules in the membrane. 2H N M R has been e m p l o y e d p r o d u c t i v e l y in the studies of w a t e r p o l y m e r i n t e r a c t i o n in epoxy blends [9], and in p o l y i m i d e films [i0]. The q u a d r u p o l a r m o m e n t of 2H (spin I=l), and the r e s u l t i n g i n t e r a c t i o n w i t h m o l e c u l a r e l e c t r i c field gradient, give a g r e a t e r s e n s i t i v i t y than that of proton to the m o t i o n and c o n f i g u r a t i o n of w a t e r m o l e c u l e s [II]. T h o u g h a n u m b e r of works have b e e n r e p o r t e d on the IH NMR
0167-2738/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
340 for N a f i o n R [ 7 , 8 , 1 2 - 1 4 ] , no p r e s e n t e d on t h e D o w i o n o m e r
G. Iu, ES. Pak / PTFE ionomer membranes NMR data have been m e m b r a n e t h u s far.
It is the m a j o r o b j e c t i v e of t h e p r e s e n t s t u d y to i n v e s t i g a t e p r o t o n - w a t e r i n t e r a c t i o n s t h r o u g h t h e s t u d i e s of IH a n d 2H N M R s p e c t r a of D o w (XUS) ionomer membranes. The lineshapes of the r e s o n a n c e are e x p e c t e d to p r o v i d e i n f o r m a t i o n on the bonding nature between the polymer network a n d t h e a b s o r b e d H20. T h e r e l a t i v e h u m i d i t y (RH) of w a t e r t r e a t m e n t e n v i r o n m e n t is v a r i e d to p r o b e the effects of different absorption concentrations. For c o m p a r i s o n , t h e N M R r e s u l t s of p r o t o n and d e u t e r o n s t u d i e s on N a f i o n R are necessary. The latter is expected to yield information on possible nuclear quadrupolar i n t e r a c t i o n of D20 t h e N a f i o n R a b s o r b s .
2. E x p e r i m e n t a l
3. R e s u l t s and D i s c u s s i o n T h e r e p r e s e n t a t i v e IH a b s o r p t i o n s p e c t r a of D o w (XUS) membrane treated in the 33% and 100% relative humidity, obtained f r o m t h e FID at a t e m p e r a t u r e of 303K, are s h o w n in F i g s . l a and lb. Two distinct proton resonance peaks are exhibited, with a narrow line (major) characterized by a full w i d t h at h a l f m a x i m u m (FWHM) of 1 0 3 ± 5 Hz and a b r o a d e r line (minor) by a F W H M of 2 6 0 ± 5 Hz. The f o r m e r has a s p i n - l a t t i c e r e l a x a t i o n t i m e (TI) of 1 0 7 ± 9 ms, w h i l e t h e l a t t e r i n v o l v e s a T I w h i c h is 6-7 t i m e s longer. M o r e o v e r , t h e c h e m i c a l s h i f t (table I.) of the n a r r o w l i n e is f o u n d to c h a n g e m o r e s u b s t a n t i a l l y than the b r o a d e r one, w i t h c o m p l e t e l y o p p o s i t e b e h a v i o r to that of Nafion R, under the different water concentrations. It is, t h e r e f o r e , demonstrated conclusively that two distinct environments for p r o t o n s e x i s t in the XUS m e m b r a n e . To c o m p a r e the proton results of D o w XUS
Dow developmental fuel cell membrane (XUS 13204.10) and Nafion R perfluorinated membrane (NIl7) w e r e k i n d l y s u p p l i e d by D o w C a n a d a and E.I. du P o n t de N e m o u r s & C o m p a n y r e s p e c t i v e l y . The samples as r e c e i v e d (H20 or D20 ) w e r e rolled tightly into cylinders and p l a c e d into standard 5 m m o.d. N M R tube. Lengths of t h e s a m p l e s w e r e in t h e r a n g e of 2 . 5 - 3 . 0 cm. A f t e r t h e s p e c i m e n s w e r e v a c u u m d r i e d for 2 days, t h e y were conditioned to w a t e r e n v i r o n m e n t (either distilled H20 or 9 9 . 9 % p u r e D20 ) of r e l a t i v e humidity of 100% or 33%, corresponding to approximately 15% or 5% w a t e r c o n c e n t r a t i o n in t h e c a s e of N a f i o n R. S a t u r a t e d M g C I 2 . 6 H 2 0 s o l u t i o n w a s e m p l o y e d to y i e l d 33% RH. T h e N M R t u b e s w e r e t h e n s e a l e d w i t h p l a s t i c c a p s and p a r a f f i n wax. To e l i m i n a t e p o s s i b l e w a t e r c o n d e n s a t i o n on t h e s u r f a c e in t h e c a s e of 100% RH, t r e a t m e n t w a s performed at a slightly lower temperature e n v i r o n m e n t t h a n t h a t of t h e m e a s u r e m e n t . A l l N M R m e a s u r e m e n t s w e r e r e c o r d e d on a B r u k e r AM-500 spectrometer. Proton (deuterium) spectra w e r e a c q u i r e d at 5 0 0 . 1 3 5 (76.774) M H z u s i n g a 5 m m d u a l f r e q u e n c y IH-13C p r o b e . In a d d i t i o n , 2H spectra were obtained with appropriate broadband preamplifier. Sample temperature was maintained at 303 K by a B r u k e r B V T - 1 0 0 v a r i a b l e t e m p e r a t u r e unit. The free induction decay (FID) was processed with Gaussian multiplication for resolution enhancement ( l i n e b r o a d e n i n g 0 - 3 0 Hz, depending on signal to noise ratio of the spectra), followed by Fast Fourier T r a n s f o r m a t i o n . All l i n e w i d t h s are t h e n c o r r e c t e d by s u b t r a c t i n g t h e d i g i t a l line b r o a d e n i n g . S p i n l a t t i c e r e l a x a t i o n t i m e s (TI) w e r e m e a s u r e d w i t h I n v e r s i o n R e c o v e r y (~--T-~/2) p u l s e s e q u e n c e . A l l chemical shifts data (6) for IH and 2H w e r e recorded in p p m r e l a t i v e to t e t r a m e t h y l s i l a n e (TMS). M e a s u r e m e n t s w e r e c a r r i e d out on b o t h D o w polymer and Nafion R specimens.
(a)
4 i
[
13
I t2
lli
i ~0
I 9
I 8
I 7
~ 6
I 5
Chemlcal S h i f t
I 4
(ppm)
(b]
i
13
is
tl
io
g
8
7
6
Chemical S h i f t
5
4
3
2
i
[ppm)
Fig. 1. Proton NMR o f D o w (XUS) membrane treated at 100% RH HsO (a), and 33% RH H20 (b).
G. Xu, KS. Pak / PTFE ionomer membranes
341
J ~4
13
1
1
9
8
7
6
5
4
3
2
1
0
-
CbemlCSl Sh~ft (ppm)
40
Fig. 2. Proton NMR of DuPont (Nation R) membrane treated at 100% RH H20.
ionomer with those of N a f i o n R, similar m e a s u r e m e n t s w e r e c o n d u c t e d on NIl7 m e m b r a n e . At all H20 c o n c e n t r a t i o n s only a single proton r e s o n a n c e p e a k is o b s e r v e d (Fig.2, t r e a t e d at 100% RH), w i t h a T I of 97±5 ms. In addition, the l i n e w i d t h i n c r e a s e d in a sample c o n t a i n i n g less w a t e r (table I.) w h i c h implies that the o v e r a l l m o b i l i t y of p r o t o n s is s i g n i f i c a n t l y e n h a n c e d at h i g h e r w a t e r content. T h e s e are c o n s i s t e n t w i t h the r e s u l t s p u b l i s h e d p r e v i o u s l y [7,8]. It is i n t e r e s t i n g to note that such d e p e n d e n c e of p r o t o n l i n e w i d t h on the H20 s o r p t i o n is not o b s e r v e d for the n a r r o w p e a k of Dow m a t e r i a l (table i.). The cause of this linewidth insensitivity to the water concentration v a r i a t i o n can be e x p l a i n e d by the fact that XUS m e m b r a n e has s h o r t e r p e n d a n t side chains, n a r r o w p o r e breaths, as s u g g e s t e d by V e r b r u g g e g r o u p [5]. The n a r r o w pore b r e a t h s p r o v i d e r e s t r i c t i o n s to the free H20 m o t i o n w i t h i n the pores and thus less e n h a n c e m e n t of the m o b i l i t y is a c h i e v e d by f u r t h e r i n c r e a s i n g w a t e r contents. Such, however, does not n e c e s s a r y imply that the p r o t o n s have
Table 1 FWHM and c h e m i c a l
shifts of IH NMR
Dow(XUS) % RH (H20)
FWHM (Hz)
Chem.shift (ppm)
DuPont(Nafion) FWHM (Hz)
Chem.shift (ppm)
100%
103 260*
7.4866 1.3449"
41
7.0716
33%
112 87*
6.1323 1.3000"
187
8.8861
* minor peak
35
3O
25
2O
15 t0 Cnem%ca] s h ~ f t
5 (pore)
0
-5
-~0
-15
Fig. 3. Deuteron NMR o f D o w (XUS) membrane treated at 33% RH DzO.
s t r o n g h y d r o g e n b o n d i n g w i t h the "lattice" -- the s u r r o u n d i n g s , as is d e m o n s t r a t e d by the d e u t e r o n N M R r e s u l t s in the following. D e u t e r o n NMR s p e c t r a of Dow XUS and D u P o n t N a f i o n R are shown in Figs 3, 4a and 4b. Their l i n e w i d t h s are s u m m a r i z e d in table 2 (note: the l i n e w i d t h of N a f i o n r e p r e s e n t s only half of the d o u b l e t ) . A well d e f i n e d q u a d r u p o l a r s p l i t t i n g is found for N a f i o n R. At 100% r e l a t i v e h u m i d i t y of D20 treatment, the s p l i t t i n g is most e v i d e n t (Fig.4a). The same phenomenon, viz, linewidth i n c r e a s e w i t h d e c r e a s i n g w a t e r c o n t e n t as in the p r o t o n case, is r e s p o n s i b l e for the m e r g e of the s p l i t t i n g p e a k at 33% RH (Fig.4b). This d o u b l e t is r e l a t e d to the n u c l e a r q u a d r u p o l a r i n t e r a c t i o n w i t h the e l e c t r i c field g r a d i e n t of the h y d r o g e n b o n d i n g [ii] in N a f i o n R. The small value of the splitting in frequency indicates that the d e u t e r o n s are e n g a g i n g c e r t a i n m o t i o n p a t t e r n s as the Pake d o u b l e t is a v e r a g e d by a large n u m e r i c a l factor. Such splitting, however, is not o b s e r v e d in D o w m e m b r a n e where, only a s i n g l e p e a k is d i s c o v e r e d when t r e a t e d at 100% RH. An a t t a c h e d small rise a p p e a r s on the "shoulder" (Fig.3) for the s a m p l e t r e a t e d at 33% RH of D20. The l i n e w i d t h (FWHM) of this a d d i t i o n a l "peak" is a p p r o x i m a t e l y equal to that of the main line. From the f r e q u e n c y shift (6) values, it seems that this additional line has the same origin as the s e c o n d a r y p e a k in IH s p e c t r u m of Dow film. Again, the i n s e n s i t i v i t y of the l i n e w i d t h of the m a i n p e a k to D20 c o n t e n t in XUS is observed. The a b s e n c e of the d o u b l e t in m a j o r line of XUS i n d i c a t e s that e f f e c t i v e a v e r a g e of static quadrupole i n t e r a c t i o n s is t a k i n g p l a c e as a r e s u l t of the high speed m o t i o n of D20 m o l e c u l e s . It is thus c o n c l u d e d that few d e u t e r o n s p o s s e s s strong hydrogen bonds to the host polymer network. On the other hand, c o m p a r i s o n of the l i n e w i d t h s to those p a r a m e t e r s m e a s u r e d in liquid D20 c l e a r l y i n d i c a t e s that the heavy w a t e r is not
342
G. Xu, Y.S. Pak / PTFE ionomer membranes
r
J
(a)
(b) 6'0
~.10
2'0
O PP"I
i
-20
-~0
--F-
-60
I;0
;
PPH
-1'00
Fig. 4. Deuteron NMR of DuPont (Nation R) membrane treated at 100% RH D20 (a), and 33% RH D20 (b).
Table 2 F W H M of 2H N M R
% RH (D20)
Dow(XUS) F W H M (Hz)
DuPont(Nafion) F W H M (Hz)
100%
229
447
33%
238
1146
participating in a r a p i d e x c h a n g e m o t i o n w i t h i n N M R t i m e scale, so that o n l y a s i n g l e p e a k is observed in N a f i o n R . S u c h rapid exchange is r e s t r i c t e d , h o w e v e r , in t h e D o w m e m b r a n e s . T h e s e results indicate that XUS is not merely a " g e o m e t r i c a l v a r i a t i o n " of N a f i o n R as s u g g e s t e d by F i g . 1 6 of r e f . [ 5 ] . F u r t h e r s t u d y is n e e d e d to provide a complete description of the interactions.
4.
i s o t r o p i c a l l y free, but u n d e r c e r t a i n r e s t r i c t i o n as s h o w n by p r o t o n NMR. T h e p r e s e n c e of w a t e r d r o p l e t s in m i c r o s c o p i c v o i d s c a n t h u s be r u l e d out. T h e e x i s t e n c e of t h e s e c o n d IH r e l a x a t i o n p e a k in XUS m e m b r a n e p r e s e n t s a m a j o r d i f f e r e n c e of its p r o t o n t r a n s p o r t a t i o n characteristics from t h a t of N a f i o n R- T h e i n s e n s i t i v i t y of t h e c h e m i c a l s h i f t of t h i s b r o a d e r line to w a t e r s o r p t i o n f u r t h e r r e v e a l s t h a t it m u s t be o r i g i n a t e d f r o m a c h e m i c a l l y d i f f e r e n t e n v i r o n m e n t f r o m t h a t of t h e major peak. One expectation w o u l d be t h a t it represents the distinct environment the protons experience in v i c i n i t y of t h e s u l f o n a t e g r o u p s . In fact, the ratio of total number of H20 m o l e c u l e s to t h e n u m b e r of w a t e r s n e a r SO3, w h i c h is about i0 in a 100% RH sample, matches qualitatively the intensity r a t i o of t h e t w o lines. These two types of protons are
Conclusions
T w o r e l a x a t i o n p e a k s w e r e f o u n d by IH N M R in the perfluorinated ionomer membranes absorbed with water from Dow (XUS) representing two distinct environments for p r o t o n s . T h e i n v e r s e p r o p o r t i o n d e p e n d e n c e of the l i n e w i d t h s on t h e w a t e r s o r p t i o n in N a f i o n R w a s not f o u n d in t h e XUS membrane by varying H20 concentration. The difference is u n d e r s t o o d in the c o n t e x t of t h e e x i s t e n c e of n a r r o w p o r e b r e a t h s in XUS s a m p l e . T h e 2H s p e c t r a of N a f i o n R w e r e f o u n d to p o s s e s s a doublet, attributed to nuclear quadrupole interaction. Dow (XUS) m e m b r a n e t r e a t e d in at 100% r e l a t i v e h u m i d i t y (RH) D20 p r e s e n t e d a s i n g l e l i n e in 2H s p e c t r u m . It is t h u s c o n c l u d e d t h a t a l t h o u g h in XUS m e m b r a n e w a t e r m o l e c u l e s are m o r e restricted in m o t i o n d u e to its s t r u c t u r e , it does not provide strong hydrogen bonding to eliminate the rapid average motion over the nuclear quadrupole interaction.
G. Xu, Y.S. Pak / PTFE ionomer membranes Acknowledgement
[5]
The authors are grateful to I.Robson of Dow Canada for the sample d o n a t i o n and helpful discussion. Thanks are also due to B.G.Sayers and D.W.Hughes for their assistance in the NMR measurement. This work is supported in part by the O C M R Grant #MCJ-509.
[6] [7]
[8] [9]
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