J. Phys. Chem. 1991, 95, 6040-6044
6040
rate limitation by internal electron transfer would interrupt
an
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expected increase in kl2 at low ionic strength, provided the internal reaction is less affected by variations in ionic strength.
Elucidation of Site of Reactivity with Electron Acceptors. The preceding analysis demands evidence that the measured kn values correspond to reaction exclusively at the SO heme site on our experimental time scale. Sulfite oxidase is a dimer of two identical subunits, each of which contains a Mo atom present as a molybdopterin cofactor,13,31 the site at which oxidation of sulfite occurs.13 Each subunit also contains the above mentioned cyt 65-like center where reduction of the physiological electron acceptor, cytochrome c, occurs subsequent to an internal charge transfer.13 Cleavage of the holoenzyme into Mo- and Fe-containing domains by trypsin and isolation of the Mo fragment yields a sulfite-reducible molybdoprotein which has lost the ability to reduce cytochrome c, but which retains a level of ferricyanide reductase activity which is virtually unchanged compared to that observed for the holoenzyme.13,14* Against this established background, we tested whether various metal complex electron acceptors catalytically oxidize sulfite in the presence of the molybdoprotein fragment, using voltammetric experiments conducted as described above. No catalytic currents are observed in mixtures of Mo fragment, sulfite, and mediator, for [Co(4,7-Me2-phen)3]2+/3+, [Co(bpy)3]2+/3+, [Ru(NH3)6]2+/3+, or the anionic complex [Co(dps)3]4~/3_ (vide supra). The ability
of the sulfite-reduced fragment to reduce ferricyanide
was con-
firmed both electrochemically and spectrophotometrically, thus demonstrating that the Mo fragment sample was active. From this we infer that the reaction site of these mediators (except [Fe(CN)6]4~/3~) is the enzymic Fe(II) site, as is the case for the physiological acceptor, cytochrome c.13 It should also be noted that no evidence was found for direct electron transfer between the EPG electrode and the Mo fragment, either in the presence or in the absence of sulfite. Acknowledgment. This research was supported in part by grants from the National Science Foundation and the North Carolina Biotechnology Center. Registry No. [Co(3,4,7,8-Me4-phen)3]2+, 47889-06-5; [Co(3,4,7,8Me4-phen)3]3+, 86176-94-5; [Co(4,7-Me2-phen)3]2+, 47872-45-7; [Co(4,7-Me2-phen)3]3+, 62791-75-7; [Co(5,6-Me2-phen)3]2+, 47872-55-9; [Co(5,6-Me2-phen)3]3+, 62869-82-3; [Co(4-Me-phen)3]2+, 80711-11-1;
[Co(4-Me-phen)3]3+, 80711-14-4; [Co(5-Me-phen)3]2+, 47860-25-3; [Co(5-Me-phen)3]2+, 96504-30-2; [Co(phen)3]2+, 16788-34-4; [Co(phen)3]3+, 18581-79-8; [Ru(NH3)6]2+, 19052-44-9; [Ru(NH3)6]3+, 18943-33-4; [Co(5-NH2-phen)3]2+, 113442-71-0; [Co(5-NH2-phen)3]3+, 113461-17-9; [Co(terpy)2]2+, 18308-16-2; [Co(terpy)2]3+, 19137-07-6; Fecytc, 9007-43-6; [Co(bpy)3]2+, 15878-95-2; [Co(bpy)3]3+, 19052-39-2; [Fe(CN)6]3‘, 13408-62-3; [Fe(CN)6]4-, 13408-63-4; TMPD'\ 3452755-4; TMPD°, 100-22-1; SO, 9029-38-3; molybdoheme, 126858-98-8; sulfite, 14265-45-3.
Determination of Water Diffusion Coefficients in Perfluorosulfonate Ionomeric Membranes Thomas A. Zawodzinski, Jr.,* Michal Neeman, Laurel O. Sillerud, and Shimshon Gottesfeld Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received: September 17, 1990; In Final Form: March 18, 1991)
'H NMR measurements of 'H intradiffusion coefficients at 30 °C in hydrated Nafion membranes reported. The dependence of the 'H self-diffusion coefficient on membrane water content was a central part of this investigation. ’H diffusion coefficients ranged from 0.6 X 10"6 to 5.8 X 10"* cm1 2/s for the range of membrane water content 2-14 water molecules per sulfonate. The membrane water content was controlled by isopiestic equilibration of the membrane sample with water vapor above aqueous LiCl solutions of well-defined water activities. The dependence of membrane water content on water activity enables us to estimate “chemical diffusion coefficients’’ from the intradiffusion coefficients measured Pulsed field gradient spin-echo
are
by
NMR.
Introduction Fuel cells using perfluorosulfonate ionomers (e.g. Nafion) as proton (H+) conducting electrolytes are attractive candidates for use in electric vehicles.1 In our laboratory, such fuel cells are constructed by hot-pressing together a “sandwich” consisting of an ionomeric membrane separating a pair of gas diffusion electrodes impregnated with the recast ionomer.2 The only liquid added to this system in order to operate it as a H2/02 fuel cell is distilled water. Water is supplied to the fuel cell by humidifying the gas feed stream. Adequate water content of the ionomeric material is essential to maintain the conductivity of the polymer electrolyte membrane. This has, in turn, a strong effect on cell performance. However, excessive amounts of liquid water could impede mass transport within the electrode structure. As part of a larger effort directed at understanding the role of water (1) Lemons, R. J. Power Sources 1990, 29, 251. (2) (a) Raistrick, I. D. In Diaphragms, Separators, and Ion-Exchange Membranes-, Van Zee, J. W., White, R. E., Kinoshita, K., Burney, H. S., Eds.; The Electrochemical Society: Pennington, NJ, 1985; p 172. (b) Ticianelli, E. A.; Derouin, C. R.; Srinivasan, S. J. Electroananl. Chem. 1988, 251, 275. (c) Gottesfeld, S.; Raistrick, I. D.; Srinivasan, S.; Springer, T. E.; Ticianelli, E.; Derouin, C. R.; Beery, J. G.; Pafford, J.; Sherman, R. J. Presented at the 1988 Fuel Cell Seminar, Long Beach, CA.
0022-3654/91 /2095-6040S02.50/0
content and possible modes of water management in polymer electrolyte fuel cells, we are engaged in a variety of experimental studies of the properties of water in perfluorosulfonate ionomers with the goal of developing models of water distribution across a fuel cell under typical operating conditions. The diffusion coefficient of water in Nafion and related membranes as a function of water content is a necessary input for the analysis of the performance of cells based on these membranes. Nafion-based polymer electrolyte fuel cells (PEFC) are usually operated under conditions of partial hydration of the polymer. Water is lost from the membrane during fabrication of the membrane/electrode assembly and is apparently not fully replaced by humidification of the membrane with water vapor carried into the fuel cell with the reactant gases. The level of hydration could also vary significantly with position in the membrane when the cell is under current. A nonuniform water concentration profile is set up in the membrane by an electroosmotic drag of water from anode to cathode and by the production of water at the cathode. The water concentration gradient set by these two effects is expected to be made less steep due to water “back-diffusion” from cathode to anode. Clarification of the water profile across the ionomeric membrane in a cell under current thus requires knowledge of water diffusion coefficients as a function of water ©1991
American Chemical Society
Perfiuorosulfonate Ionomeric Membranes
The Journal
of Physical Chemistry,
Vol. 95, No. 15, 1991
6041
Figure 1. Schematic illustration of water transport within a PEFC.
content. A schematic description of water currents in the membrane of a fuel cell is given in Figure 1. Diffusion coefficients for sorbed solvent and ions in Nafion have been estimated by using several techniques. Yeo and Eisenberg3 studied the sorption of water by a dry slab of Nafion and estimated the diffusion coefficient of water in the membrane over the temperature range 0-99 °C from the water uptake dynamics. Diffusion coefficients from these measurements increased over the range (1-10) X 10"* cmJ/s with increasing temperature with a reported activation energy of 4.5 kcal/mol. The method used to estimate the diffusion coefficients by Yeo and Eisenberg, taking advantage of the /'/2 dependence of the uptake in the initial portion of the uptake curve, is not necessarily appropriate. The sorption of water takes place in a membrane with varying water content, whereas the diffusion coefficient is not known a priori to be independent of the state of membrane hydration. As Crank has pointed out for the case of variable diffusion coefficients,4 sorption will depend on t'l* no matter what the relationship is between the diffusion coefficient and the concentration of water in the film. Yeager and Steck5 reported diffusion coefficients of water in Nafion 120 membranes containing various alkali-metal cations determined by radiotracer measurements. In these studies, the diffusion coefficient of water was measured only for fully hydrated membranes. The water diffusion coefficient is dependent on the cation present in the membrane, with the value of the diffusion coefficient of water in Na+ form (fully hydrated) Nafion equal to 2.65 X 10"6 cm2/s at 25 °C and the values for K+ and Cs4 forms slightly smaller. More recently, Verbrugge and co-workers have published studies of the transport of methanol and of protons (referred to throughout this manuscript as H+ with no intent to imply specifics of speciation) and other ions through Nafion membranes by radiotracer These experiments were carried out with the techniques.6 membrane exposed to a bulk aqueous acid solution. This situation is not identical with that found in a PEFC in terms of either the hydration state of the membrane or the presence of co-ions in the membrane. We report here some results obtained by pulsed field gradient spin-echo (PGSE) NMR measurements of 'H intradiffusion coefficients in Nafion membranes of various, well-controlled states
of hydration. Experimental Section Nafion 117 membranes were pretreated by boiling in dilute H202 solution, rinsing in boiling water, boiling in dilute sulfuric acid solution, and rinsing in boiling water. Establishment and maintenance of a well-defined water content are prerequisites for measurement of transport properties in membranes of different states of hydration. To control the membrane hydration state, we have employed isopiestic equilibration of membrane samples
Figure 2. Cell used for determination of membrane conductivity: (1) Kel-F block; (2) thumbscrew; (3) open area to allow equilibration; (4) membrane sample; (5) blackened Pt foil; (6) Pt ribbon lead.
with water vapor above aqueous LiCl solutions of known water activity, following Pushpa et al.7 The water content of a membrane sample was set by suspending the membrane following the above pretreatment over an aqueous LiCl solution in a sealed glass jar kept in a temperature bath at 30 ± °C. Equilibrium, as indicated by constant sample weight, was reached under these 1
conditions after 4 days. All membrane weighing steps were carried out by rapidly transferring the membrane to a weighing bottle and weighing by difference. The water content of a given membrane was determined by weighing the equilibrated membrane followed by completely drying the same membrane sample by suspending it over P205 in a sealed jar for several days and reweighing it. This last treatment leads to a membrane water content of (practically) zero based on the following argument. Drying the membrane under vacuum at room temperature for long = (X = the number of water periods leads to a water content of X molecules per sulfonate), as demonstrated by Bunce et al.8 We find that further drying at 105 °C under vacuum of a membrane dried in the latter fashion results in a weight loss corresponding to loss of one water per sulfonate. That all water is lost at 105 °C is consistent with the results reported by LaConti and coworkers.9 This is slightly at odds with data presented by Yeo and Eisenberg,3 which suggests a small amount (roughly 0.2 wt %) of residual water present after drying at 105 °C under vacuum. Drying by suspending the membrane over P2Os leads to weight losses equivalent to those obtained via the 105 °C vacuum treatment. Furthermore, both our vapor-phase isotherms (see below) and maximum uptake data upon immersion in liquid water are in quantitative agreement with those reported by others using other extreme drying protocols. We believe that our low-temperature drying method is to be preferred since exposure of polymer samples in the acid form to elevated temperatures generally leads to some charring of the polymer, with unknown effects on, for example, the ion-exchange capacity of the polymer. Membrane conductivity was determined by using the cell sketched in Figure 2. The “window" structure was employed to 1
allow membrane equilibration in situ. The resistivity of the membrane was measured by using a pair of pressure-attached high surface area Pt electrodes at 5 kHz. Both real and imaginary components of the impedance were measured and compared to ensure that the real Z-axis intercept was closely approximated and that the measurement was free of the effects of lead inductance. For determinations of conductivity of partially hydrated membranes, the cell was suspended in a closed vessel above the equilibration solution. Conductivity measurements on fully hydrated membrane samples were carried out with the cell immersed in liquid water. Temperature was controlled in both cases by
(3) Yeo, S. C.; Eisenberg, A. J. Appl. Polym. Scl. 1977, 21, 875. (4) Crank, J. The Mathematics of Diffusion, 2nd ed.; Clarendon: Oxford,
(7) Pushpa, K. K.; Nandan, D.; Iyer, R. M. J. Chem. Soc., Faraday Trans. 1988. 84, 2047. (8) Bunce, N.; Sondheimer, S.; Fyfe, C. Macromolecules 1986, 19, 333.
1975: Chapter 9.
1
Verbrugge, M. J. Electrochem. Soc. 1989,136, 417. (c) Hill, R.; Verbrugge, M. J. Electrochem. Soc. 1990, 137, 886, 893.
(9) LaConti, A.; Fragala, A.; Boyack, J. In Electrode Materials and Processes for Energy Conversion and Storage; McIntyre, J. D. E., Srinivasan, S., Will, F., Eds.; The Electrochemical Society: Pennington, NJ, 1977; p 354.
(5) Yeager, H.; Steck, A. J. Electrochem. Soc. 1981, 128, 1880. (6) (a) Verbrugge, M.; Hill, R. J. Phys. Chem. 1988, 92, 6778. (b)
6042
The Journal
of Physical
Zawodzinski et al.
Chemistry, Vol. 95, No. 15. 1991
ISOPIESTIC SORPTION CURVE 30*C
Pulsea Gradient Spin Echo tPGSEl
14
12
r
*
&
|
6
"
4
Slow Diffusion Enhanced Pulsed Gradient Spin Echo ISDE-PGSE)
RF Pulses
2
96
180
0-
U_ gr
0
0
0.1
02
0.3
0.4 0.5 0.6 WATER ACTIVITY
0.7
0.8
0.9
gd
A gd
gr
1.0
Figure 3. Isopiestic sorption curve for Nafion 117 membrane. T
=
30
•C.
immersing the entire vessel in a water bath. The cell constant was calculated from the spacing of the electrodes and the thickness of the membrane sample. (The latter was measured with a micrometer in each case.) NMR experiments were carried out using a Bruker AM-400 NMR spectrometer using a Bruker microimaging probe equipped with 50-mm gradient coils and a 5-mm Helmholtz coil insert. Gradient pulses were shaped through a preemphasis network to minimize the effect of eddy currents. The effect of any remaining eddy currents was checked by measuring the signal intensity of a sample of water as a function of time between the end of an applied gradient pulse and the onset of acquisition. For times of 20 ms or longer, the signal intensity was constant. Experiments were carried out with delays between gradient pulses and rf pulses or acquisition of 20 ms or more. Samples were prepared by equilibration and then placed in a homemade multicompartment NMR tube within which equilibration could be maintained. A small Kel-F container with the equilibration solution was screwed onto the bottom of a chamber containing the membrane which was then screwed into a positioning rod. Small bits of threaded rod were used to join the various compartments. A hole drilled through the center of the threaded connector between the chambers containing membrane and solution allowed vapor phase contact. The sample temperature was controlled by means of the Bruker VT1000 probe temperature controller and calibrated by a Luxtron 1000B fiber optic temperature probe. The gradient was calibrated by observing the profile of a calibrated water phantom in the presence of gradients covering the range used experimentally. To extract the diffusion coefficient from the raw experimental data, the observed signal intensity for 16 incremented field gradient strengths was fit to the predicted decay curve using a nonlinear least-squares procedure from SAS software (SAS Institute Inc.) on a micro VAX computer. Results and Discussion
Equilibration of Nafion membranes with aqueous LiCl solutions at 30 °C leads to the water sorption curve shown in Figure 3. The shape of this curve is similar to that obtained by Pushpa et al. at 25 °C. However, a somewhat lower water uptake was observed by us over most of the water activity range. A particularly interesting feature of this curve is the relatively small change in water content over the range of water activity 0.15-0.75. At water activities above 0.75, the water content of the membrane changes much more rapidly with water activity. Though measurements of diffusion coefficients by the PGSE technique have been reported for polymer/penetrant systems,10 to our knowledge none at all have been reported for ion-exchange polymers. In the pulsed field gradient spin-echo (PGSE) NMR experiment," an intradiffusion coefficient for species bearing the detected nucleus is determined from the diffusions! dephasing of (10) Stilbs, P. Prog. Nucl. Magn. Resort. Speclrosc. 1987, 19, 1 and references therein. (11) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288.
(b)
-a-
Figure 4. (a) Pulsed field gradient spin-echo (PGSE) sequence, (b) Slow diffusion enhanced modification of PGSE sequence (SDE-PGSE).
gradient-encoded magnetization. Experimentally, a pair of symmetrically spaced field gradient pulses is placed onto a normal spin-echo sequence, as illustrated in Figure 4a. The predicted dependence of signal attenuation on gradient strength is a
where A(g) is the signal intensity observed with applied gradient g, A(0) is the signal intensity observed in the absence of an applied gradient, y is the nuclear gyromagnetic ratio, D is the intradiffusion coefficient, and 5 and A are as shown in Figure 4a. The pulse spacing and duration can be altered to obtain information
restrictions to diffusion (e.g. pore walls) on the distance scale determined by the experimental time scale and the diffusion on any
coefficient.12
A fundamental requirement of the PGSE technique is that diffusional attenuation be significant relative to relaxation of the observed nuclei. If relaxation is too rapid, determination of diffusion coefficients via the PGSE experiment is difficult or impossible to carry out. Boyle et al.n have indicated, based on their relaxation study of water in Nafion, that PGSE experiments would be impractical for this system. However, the Nafion samples used by Boyle et al. were, as they demonstrated, contaminated with iron, causing significantly shortened relaxation times (7, on the order of 10 ms for 1100 EW Nafion). Other workers have measured proton 7,s and 72s which are significantly longer at room temperature (roughly 100 and 50 ms, respectively).14 We find in our experiments on well-purified, protonic forms of Nafion that proton longitudinal relaxation times vary over the approximate range 80-200 ms for water at various concentrations in Nafion 117 membranes (less water in the membrane leads to shorter relaxation times), while transverse relaxation times are generally about half as long at 25 °C. There is thus an ample time window in which to perform PGSE measurements.
'H diffusion coefficients in pieces of hydrated Nafion 117 membrane were determined for samples with various water contents, as established by the isopiestic technique described above. A typical plot of the observed signal attenuation versus gradient strength (for a sample containing 9 waters per sulfonate) is shown in Figure 5 together with the best-fit curve predicted from eq 1. The measured intradiffusion coefficients are summarized in Table I as a function of water content. 'H diffusion coefficients in Nafion decrease with decreasing water content. No systematic difference (12) Tanner, J. E.; Stejskal. E. O. J. Chem. Phys. 1968, 49, 1768. (13) Boyle, N. G.; McBrierty, V. J.; Douglass, D. C. Macromolecules 1983, 16, 75. (14) (a) Slade, R. C. T.; Hardwick, A.; Dickens, P. G. Solid Slate Ionics 1983, 9/10, 1093. (b) Sivashinsky, N.; Tanny, G. B. J. Appl. Polym. Scl. 1981, 26, 2625.
The Journal
Perfluorosulfonate Ionomeric Membranes
of Physical
Chemistry, Vol. 95, No. 15, 1991
6043
0.12
Jj
0.10
5
|
0.08
5 D
0.06
8
0.04
Q Z
O
E
0
0
10
20
30
MOLES OF WATER PER EQUIVALENT
Figure 6. Specific conductivity of Nafion 117 as a function of hydration state.
TABLE I: ]H Intradiffusion Coefficients, 30 water content
water content
D,
(H2Q/S03H)
10* cm2/s
0.6 ± 0.06“ (SDE4) 1.2 ± 0.08 (SDE) 2.1 ± 0.06 (SDE)
6 9 14
3.7 ± 0.15 4.4 ± 0.07 5.8 ± 0.21
(H2Q/SQ3H) 2 3
4
°C_
D, 10* cm2/s
“
Errors reported here are the standard error of the nonlinear leastsquares fit to experimental data. 4The label SDE indicates that this diffusion coefficient was determined by using the SDE-PGSE technique.
TABLE II: Effect of Gradient Strength
on
D Measured by the
SDE-PGSE Experiment D,
gradient strength, G/cm2
10* cm2/s
0 2.38 4.76
3.6 • 0.3 4.0 ± 0.3 3.9 m 0.2
gradient strength, G/cm2 7.14 9.52
D, cm2/s 4.3 ± 0.2 3.3 • 0.4 106
diffusion coefficients measured for different values of A. In addition to suggesting that there are no restrictions to diffusion apparent on the length scale probed by this experiment ((Dt)'/2 is on the order of a few microns for these experiments), this result enhances our confidence that eddy currents are not significantly affecting the measurement. A modification of the Stejskal-Tanner sequence, referred to here as the slow diffusion enhanced (SDE) PGSE experiment,15 was employed for two purposes: (i) determination of slow diffusion coefficients and (ii) testing for the presence of two distinguishable types of water, one rapidly diffusing and one slowly diffusing. This pulse sequence is shown in Figure 4b. Additional gradient pulses are inserted symmetrically before and after the gradient pulses of the normal PGSE experiment so that the spins precess under the influence of both gradients during time A. Neeman et al.15 have indicated the potential application of the SDE-PGSE technique to enhance the signal attenuation in slowly diffusing systems, in our case equilibrated Nafion samples with low water contents. Diffusion coefficients for membranes with water contents in the range 2-4 H20/S03H were determined by using the SDE-PGSE technique with a single value of gradient gr (except, of course, for the determination of S(0), for which no gradients is observed between the
were used).
H diffusion coefficients were determined for a hydrated membrane sample by using several incremented values of gr. Under the influence of gr, any rapidly diffusing components are more strongly attenuated. A decrease in the measured diffusion coefficient with increasing gr indicates that several water environments with different diffusional character exist in the sample. As shown in Table II, the measured diffusion coefficients are independent of the value of gr, indicating that no dispersion in the diffusion rates of 'H nuclei exist in the sample. Thus, we conclude that no large “pockets” of bulk water (e.g. water-filled (15) (a) Neeman, M.; Freyer, J.; Sillerud, L. J. Magn. Resort. 1990, 90, 303. (b) Neeman, M.; Jarrett, K. A.; Sillerud, L.; Freyer, J. P. Cancer Res., in press.
Figure 7. Comparison of H+ mobility (from protonic conductivity) and 'H intradiffusion coefficient for water in Nafion 117 as a function of extent of hydration.
bubbles in the membrane) exist under the conditions of these experiments. Having described the determination of 'H diffusion coefficients in hydrated Nafion membranes, we must now consider what the diffusion coefficients mean. In a hydrated Nafion membrane, 'H nuclei exchange rapidly between H20 and H+ (we reiterate that the symbol “H+” covers all forms, including aqueous complexes, of H+ in the membrane) environments. The measured 'H diffusion coefficient is a weighted average of the diffusion coefficients for the separate environments. Thus, the diffusion coefficient measured in this work is not exactly the intradiffusion coefficient of water in these membranes. However, the identification of the 'H diffusion coefficient with that of water is reasonable at high water contents since 'H20 is in large excess over 'H+ (28 'H present as H20 to 1 present as H+ when X = 14). At low water contents, the identification of the lH diffusion coefficient with that of water is based on a more subtle argument outlined below. The conductivity of Nafion 117 as a function of membrane water content is shown in Figure 6. The mobility of H+ can be estimated from the conductivity data using the Nemst-Einstein equation. A plot of the H+ diffusion coefficient (from ionic mobility) and the *H diffusion coefficient (from NMR) versus membrane water content is shown in Figure 7. The diffusion coefficients are similar at low water contents, differing increasingly as the water content increases. From this behavior, we infer that transport of H+ by Grotthus hopping probably becomes increasingly significant at high water contents, whereas it seems to be negligible at low water contents. This is not unreasonable, since water in the polymer at low water contents is likely to be solvating H+ and S03" and becomes more bulklike only at high water contents. Falk has shown that H-bonding interactions between water molecules, of importance for proton transport by hopping, are significantly weaker in Nafion even for fully hydrated membranes.16 Indeed, the ratio of H+ mobility in liquid water to the diffusion coefficient of water molecules in water is higher than the corresponding ratio (Figure 7) for H+ and water in the fully hydrated polymer. The NMR spectrum (16) Falk, M. Can. J. Chem. 1980, 58, 1495.
J. Phys. Chem. 1991, 95, 6044-6047
6044
TABLE III: Chemical Diffusion Coefficients water content
water content
Adwn>
(H20/S03H)
106
cm2/s
2
1.3
3
4.2 2.3
4
(H20/S03H)
106
cm2/s 2.0
6 9 14
1.7 1.5
indicates no immobile water at low membrane water contents since no very broad components are observed in the spectrum. Thus, we conclude that, in the extreme of low water content, H20 and H+ probably diffuse by an identical mechanism, i.e. that the mobile species under an electric field is solvated H+, of mobility identical
with that of H20. At such low water levels in the membrane, all ‘H nuclei would thus diffuse together and the measured diffusion coefficient of 'H is, again, a good estimate of the water diffusion coefficient in the membrane. Thus, on the basis of the above arguments for high and for low water contents, we tentatively identify the 'H diffusion coefficient measured here as the intradiffusion coefficient of H20 in the membrane over the entire range of membrane water contents. We are, however, planning experiments employing H2170 to provide definitive water diffusion coefficients in order to further substantiate this point. In a real membrane in which a water concentration gradient exists, such as in an operating fuel cell, diffusion of water through the membrane takes place in a chemical potential gradient. Flux as described by Fick’s law is generally relative to gradient, i.e.
flux
=
a
concentration
Z)chem(dC/dx)
(2)
Conversion of a measured intradiffusion coefficient to a “chemical” diffusion coefficient is carried out using the equation
“chemical” diffusion coefficient, D* is the intradiffusion coefficient, a is the activity of the diffusing species, where
Dchem is the
and C is the concentration of the diffusing species (i.e. the membrane water content). The “chemical” diffusion coefficients are obtained by use of eq 3 and the isopiestic data shown above. The curve shown in Figure 3 is transformed into a plot of In amur vs In Cwater. This curve is next fit to a third-order polynomial, and the derivative term in eq 3 is then directly obtained. The calculated values of Z>chem show a much smaller range of variation with water content than do the self-diffusion coefficients (Table III). This is caused mainly by the much stronger variation of water activity with water content at the lower water contents studied, as can be seen in Figure 3. The large variations of the activity coefficient of water at low water contents provide an additional “drive” for water transport, compensating for the large drop in the value of the intradiffusion coefficients. This report demonstrates the capability of the NMR technique to provide the desirable information of diffusion coefficients of water (*H) in PEFC membranes maintained at a well-defined state of partial hydration. More extensive work is currently in progress in our laboratory to determine diffusion coefficients of water in Nafion and other ionomeric membrane systems under a variety of conditions, most importantly, elevated temperatures. PGSE measurements on hydrated Nafion membranes employing 170 as the probe nucleus will be carried out to directly ascertain the diffusion coefficient of water in the membrane. As suggested above, the results described in this communication are currently being applied in the development of a water transport model for polymer electrolyte fuel cell systems.17
Acknowledgment. This work was supported by the U.S. Department of Energy, Office of Conservation and Renewable Energy. Registry No. H20, 7732-18-5. (17) (a) Springer, T. E.; Gottesfeld, S.; Radzinski, S.; Zawodzinski, T. A. Book of Abstracts, 178th Meeting of the Electrochemical Society, Seattle, WA, Oct 1990; Abstract No. 118. (b) Springer, T. E.; Zawodzinski, T. A.; Gottesfeld, S. J. Electrochem. Soc., in press.
Radical Scavenging In the Sonolysis of Aqueous Solutions of I-,
Br,
and
N3~
Maritza Gutierrez, Amim Henglein,* Hahn-Meitner-Institut Berlin GmbH, Bereich
S, 1000 Berlin 39, FRG
and Fernando Ibanez
Facultad de Quimica, Pontiflcia Universidad Catdlica de Chile, Santiago, Chile (Received: December 28, 1990)
Iodide and bromide solutions are sonolyzed under pH conditions, where reactions of the products, i.e. hydrogen peroxide and iodine (or bromine), do not occur. The total yield of the products as well as the hydrogen yield is independent of solute concentration. The results are understood in terms of the competition of the OH + OH and the OH -I- solute reactions. A local concentration of 4 X 10'3 4M of the OH radicals in an interfacial region between the cavitation bubbles and the liquid is derived from the data obtained. The sonolysis of azide solutions is also investigated. The main product is nitrogen, which is formed in the reaction of N3- with OH radicals in the millimolar concentration range. At higher azide concentrations, additional nitrogen is formed as hydrogen atoms are also scavenged. Ammonia and hydrazine are minor products of the N3" sonolysis.
Introduction The kinetics of chemical reactions that are initiated by ultrasound are rather complex.1-4 The reactions do not take place (1) (a) Henglein, A. Ultrasonics 1987, 25, 6. (b) Advances in Sonochemistry. Mason, T, Ed.; Jai Press; Vol. 3, in press. (2) Mason, T. J.; Lorimer, J. P. Sonochemistry, Theory, Applications and Uses of Ultrasound in Chemistry, Ellis Horwood Ltd.; Chichester, 1988.
0022-3654/91/2095-6044S02.50/0
homogeneously in the solution but in “hot-spots” where cavitation bubbles pulsate or collapse. During the adiabatic compression phase in such gas bubbles, temperatures of several 1000 K are reached and pressures of up to 100 bar. Molecules can be dis(3) Suslick, K. S. Ultrasound, Its Chemical, Physical, and Biological
Effects-, VCH: Weinheim, 1988. (4) Carmichael, A. J.; Mossoba, M.; Riesz, P.; Christman, C. L. Trans. 1986, UFFC-33, 148. ©1991
American Chemical Society
IEEE
