1836
The Journal of Physical Chemistry, Vol. 83, No. 14, 1979
I
I
t
ii I/
The rapid photocurrent decay constitutes a major hindrance for further quantitative work. It would be desirable to capture all available excited electrons or holes irreversibly. Some of the “current-doubling’’ reagents used with semiconductors21may be useful in this regard. Acknowledgment. One of us (H.0.E) thanks the IBM Corporation for a fellowship. We also acknowledge the Research Corporation for partial support of this work a t the California Institute of Technology.
40-
na 20
H. L. Yeager and B. Kipling
References and Notes
-
.-U
-0
0
m
U
V Flgure 4. Peak photocurrent vs. potential. R film electrode in 0.1 M TEAP/CH,CN; X 350 nm: (-) argon saturated; (- -) after bubbling with N20; (. .) after further bubbling with argon.
..
potentials, and removal of the N20 by A.r bubbling reduces them, thus indicating the presence of some photoelectron emission. Protons are not useful scavengers in nonaqueous media because proton reduction severely limits the accessible potential range. An analogous hole scavenging experiment was attempted by adding azide ion; N3- oxidizes irreversibly to NP. However, no significant enhancement of anodic photocurrents was observed in a 10 mM solution of NaN3 in DMF. One can surmise that the surface concentration of azide ion was too low to compete with water or other electron scavengers a t the electrode surface. It is probable that strong adsorption of azide is necessary to obtain a sufficiently high concentration for effective hole scavenging.
(1) H. Gerischer, Ber. Bunsenges. Phys. Chem., 77, 771 (1973). (2) A recent sampling of research groups in this field include: (a) Yu. V. Pleshov and 2. A. Rotenberg, J. Electroanal. Chem., 94, 1 (1978); (b) A. A. Ovchinnikov, V. A. Benderskii, S.D. Babenko, and A. G. Krivenko, ibld., 91, 321 (1978); (c) V. Concialini and 0. Tubertini, ibld., 88, 57 (1978); (d) M. Heyrovsky and F. Pucclarelil, /bid., 75, 353 (1977); (e) T. E. Furtak and J. K. Sass, Surface Scl., 78, 591 (1978); (f) J. H. Rlchardson, S.M. George, J. E. Harrar, and S. P. Perone, J . Phys. Chem., 82, 1818 (1978). (3) Yu. V. Pleskov and 2. A. Rotenberg, Adv. Electrochem. Electrochem. Eng., 11, 1 (1978). (4) A. M. Brodsky and Yu. V. Pleskov, frog. Surface Scl., 2, 1 (1972). (5) G. C. Barker, A. W. Gardner, and D. C. Sammon, J . Electrochem. SOC.,113, 1182 (1966). (6) 2. A. Rotenberg, Yu. A. Prishchepa, and Yu. V. Pleskov, J. Electroanal. Chem., 56, 345 (1974). (7) H. Gerischer, E. Meyer, and J. K. Sass, Ber. Bunsenges. Phys. Chem., 76, 1191 (1972). (8) J. K. Sass, R. K. Sen, E. Meyer, and H. Gerischer, Surface Scl., 44, 515 (1974). (9) J. K. Sass, E. Meyer, and H. Gerischer, Ber. Bunsnges. Phys. Chem., 79, 1077 (1975). (10) E. Meyer, Dissertation, T. U. Munchen, 1973. (11) J. K. Sass, Dissertation, T. U. Berlin, 1973. (12) H. 0. Finkiea, Ph.D. Thesis, California Institute of Technology, 1976. (13) R. A. Holroyd and M. Alien, J . Chem. Phys., 54, 5014 (1971). (14) G. C. Barker, Electrochim. Acta, 13, 1221 (1968). (15) H. Imai and K. Yamashita, Bull. Chem. Soc. Jpn., 42, 578 (1969). (16) R. Memming and G. Kursten, Ber. Bunsenges. Phys. Chem., 76, 4 (1972). (17) M. Heyrovsky, Proc. R . SOC.London, Ser. A , 301, 411 (1967). (18) (a) G. C. Barker, B. Sbinger, and M. J. Williams, J. Ektroanal. Chem., 51, 305 (1974); (b) G. C. Barker, Ber. Bunsenges. Phys. Chem., 75, 728 (1971). (19) S.S.Fratoni, Jr., and S.P. Perone, Anal. Chem., 48, 287 (1976). (20) Yu. V. Pieskov and 2. A. Rotenberg, J. Electroanal. Chem., 20, 1 (1969). (21) E. C. Dutoit, F. Cardon, and W. P. Gomes, Ber. Bunsenges. Phys. Chem., 80, 1285 (1976).
Ionic Diffusion and Ion Clustering in a Perfluorosulfonate Ion-Exchange Membrane H.
L. Yeager“
and B. Kipling
Department of Chemistry, The University of Calgary, Calgary, Alberta T2N 1N4, Canada (Received January 2, 1979) Publication costs assisted by fhe National Research Council of Canada
Tracer self-diffusion coefficients for Na+ and Cs+ have been measured in Nafion perfluorosulfonate ion-exchange membranes for three solvent systems: water, methanol, and acetonitrile. Results indicate that ion clustering exerts a pronounced effect on the diffusional properties of this material. Diffusion coefficients and activation energies of diffusion vary dramatically for small changes in swelling. This is attributed to changes in the ability of counterions to move between clusters. The ionic transport properties of Nafion are therefore very different from poly(styrenesu1fonate) ion-exchange resins of similar swelling.
The Journal of Physical Chemistty, Vol. 83,
Perfluorosulfonate Ion-Exchange Membrane
at least two separate models.lOJ1 The first of theselo has been found to be most applicable to highly swollen p o l y m e r ~ . ~ The ~ ~ J second ~ model, that of Yasuda and co-workers,ll was developed by analogy to free volume theory of transport in glassy systems.13 Fernandez-Prini and Phillip7 have recently shown that this approach fits Na+ and Cs+ diffusion results for several poly(styrenesulfonate) resins over a wide range of fractional water volumes in the polymers. The dependence of diffusion coefficient on this water volume fraction was different for the two ions; an electrostatic effect was also inferred with the Yasuda model. Activation energies for ionic diffusion in these polymers have been found to be slightly higher than comparable solution values, again suggesting hindered diffusion due to electrostatic and obstruction effects. We wish to test these relationships by using an ion-exchange polymer which has a fundamentally different morphology to cross-linked poly(styrenesulfonates), Nafion (Du Pont and Co.) perfluorosulfonic acid resin. Nafion polymers have the general form (CFZCF,), (CFCF,), (OCF,CF)~OCF,CF,SO,-M
TABLE I: Ionic Self-Diffusion Coefficients in Nafion 120 D,cm2 s-' ion t, " C H,O CH ,OH CH,CN Na+ 0 3.07 X 4.69 X 7.88 X 15 3.23 x 10-9 25 9.44 x 10-7 8.26 x 10-7 7.67 x 10-9 40 1.51 X 1.17 X 1.94 X lo-' cs+ o 3.84 x 10-9 5.57 x 1 0 - 1 0 5 1.16 x 10-9 25 5.20 X 1.21 X l o - * 1.07 X lo-'' 2.33 X 10'' 5.39 X lo-'' 40 1.58 X a Value obtained a t t = 1.1"C.
to the diffusion cell and constant cell temperature was obtained by using externally circulated water. Temperature regulation was h0.05 "C in all cases. A solution concentration of 0.05 M was used in most cases. Due to solubility limitations, 0.01 M CsI in acetonitrile was employed. For aqueous experiments and for the Na+methanol system, corrections for film diffusion were necessary. The steady state ionic flux of radioactive tracer can be represented by the equationz4
+
I
CF,
where m and n can be varied to produce materials of different ion exchange capacities and 1 is small. The commercial polymers have very large molecular weights, although they are presumably not cross linked. Water absorption experiments have shown that the material is moderately hydrophilic.14 Recently the physical properties and structure of Ndion have been investigated by Yeo and Eisenberg15 and others.16J7 Results of spectroscopic measurements have also been reported.lSz0 These studies lead to the conclusion that sulfonate exchange sites, counterions, and sorbed water exist in clusters as a separate phase, surrounded by fluorocarbon polymer. Results of low-angle X-ray scattering studies indicate that the Bragg spacing of these clusters is about 50 8,for both cesium ion15 and hydrogen ion16 forms. We have previously reported results of ionic diffusion and ion-exchange selectivity measurements for N a f i ~ n . ~ lDiffusion -~~ coefficients are large in protic solvent systems and very small in aprotic solvents. Also, Nafion shows unusually high selectivity differences for the alkali metal ions. We report here the results of measurements of the temperature dependence of tracer self-diffusion coefficients in Nafion which has been equilibrated with water, methanol, and acetonitrile solvents in order to test the influence of ion clustering on ionic transport properties. Experimental Section Ion-exchange membranes were Ndion 120 (Du Pont and Co.) with nominal capacity of 0.83 mequiv/g and thickness of 0.025 cm. Details of the equilibration of membranes in solvent and measurement of ion-exchange capacities, thicknesses, and water contents have been previously Anhydrous reagent grade chloride salts were used for aqueous experiments; sodium iodide (Fisher Certified) and cesium iodide (Alfa Ventron, 99.9%) were used to prepare nonaqueous solutions, due to their higher solubilities. The purification of methanol and acetonitrile has been described.21r22The radioisotopes z2Naand 137Cs were obtained as carrier-free aqueous solutions and were used as received. The designs of the diffusion cells, radioactivity measurements, and details of procedure for diffusion experiments have been described.21%22 A water jacket was added
No. 14, 1979 1037
Ji
=
-DC
d ( l + 2DCG/DCd)
where D is the diffusion coefficient, C is molar concentration, d is the membrane thickness, and 6 is the thickness of the unstirred liquid films at the membrane surfaces. The barred quantities refer to values within the membrane phase. Equation 1 can be rearranged to
_-1 - d
26 + Ji - E DC
For the above systems, the steady-state flux of tracer through the membrane was measured at several external solution concentrations between 0.2 and 0.01 M with solution stirrers fixed at a speed of 900 rpm. Then -11Ji was plotted vs. 2/DC to yield 6 as the slope. These lines are linear as expected; values for 6 of 15-20 pm in aqueous experiments and 5-15 hm for methanol were obtained. These are typical values for well-stirred systems, and provide corrections of about 15% (Na+-H20),
