3850
J. Phys. Chem. 1986, 90, 3850-3856
Conclusions The combination of results presented here supports the idea that the internal structure of polyelectrolyte coatings on electrodes strongly influences the electrochemical behavior of electroactive counterions incorporated in them. For the particular combination of styrene and (dialkylaminomethy1)styrene monomer units examined in this study, the block copolymers yielded coatings with more desirable properties than did corresponding random copolymers if more than half of the styrene groups present were unsubstituted. However, with copolymers containing a larger proportion of amino groups the difference between block and random copolymers was not significant. Both types of polymer yield highly swollen, partially soluble, and therefore unstable, coatings when more than 70-80% of the styrene groups contain amine substituent groups. The electrochemical behavior exhibited by coatings, as well as their internal structures, was influenced
strongly by the nature of the casting solvent employed. Diffusion coefficients for incorporated Fe(CN)63-anions are notably larger in block than in random copolymer coatings, although neither type of coating exhibits diffusion coefficients nearly as large as those reported recently for a ternary copolymer-homopolymer composite.14 Understanding the reasons for this difference is one objective of continuing studies. Acknowledgment. We are grateful to Prof. J. P. Revel and Pat Koen for assistance with the electron microscopy measurements and to Donald Montgomery and Takeshi Inoue for helpful discussions. This work was supported by the National Science Foundation and the US.Army Research Office. Registry No. I, 3749-75-5; Fe(CN):-, 13408-63-4; Fe(CN),3-, 13408-62-3; C, 7440-44-0; 4-(chloromethyl)styrene, 1592-20-7; diethylamine, 109-89-7; styrene, 100-42-5.
Charge-Transfer Reactions in Pendant Viologen Polymers Coated on Graphite Electrodes and at Electrode/Pendant Viologen Polymer Film Interfaces Noboru Oyama,* Takeo Ohsaka, Hisao Yamamoto, and Masao Kanekot Department of Applied Chemistry for Resources, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan (Received: January 15, 1986: In Final Form: March 12, 1986)
Charge-transfer reactions in poly(styrene-co-chloromethylstyrene)pendant viologens (PMV) with various loadings ( x = 3-34%) of viologen sites coated on graphite electrodes and at electrode/PMV film interfaces were examined by cyclic voltammetry and normal pulse voltammetry. The relevant parameters characterizing these charge-transfer reactions (Le., apparent effective diffusion coefficient Dappfor the homogeneous charge-transport process and standard rate constant ko and cathodic transfer coefficient a, for the heterogeneous electron-transfer reaction) were obtained for the PMV films with various x's. As x was of electroactive viologen sites from 6.7 X increased from 9% to 34%, which corresponds to the concentrations mol cm-), Dappincreased from 2.5 X lo-" to 3.9 X cm2 s-l, ko increased from 3.0 X to 8.9 X to 3.3 X cm s-], and ac was independent of (0.42 f 0.04). The finding of the almost linear dependence of Dappon CMy+ suggests the significant contribution of electron self-exchange between viologen redox couples confined in the polymer chain to the overall charge transport in the PMV films. On the basis of these results, the mechanism of the charge-transport process within PMV films is discussed.
e&$+
a linear dependence on the concentration (C) of electroactive sites Introduction in polymer film^.^^^^'^'^ However, in most cases, such a concenThe mechanism and kinetics of charge-transport processes in thin polymer films on electrodes have become of considerable ( 1 ) (a) Kaufman, F. B.; Engler, E. M. J. Am. Chem. SOC.1979,101,547. interest in the present stage of research in "polymer-coated (b) Kaufman, F. B.; Schroeder, A. H.; Engler, E. M.; Kramer, S . R.; Chamelectrode^".^-^^ Physical models for the mechanisms of charge bers, J. Q.J. Am. Chem. SOC.1980,102,483. (c) Schroeder,A. H.; Kaufman, F. B.; Patel, V.; Engler, E. M. J. Eleciroanal. Chem. 1980, 113, 193. transport in the polymer film, factors affecting their electro(2) (a) Rubinstein, I.; Bard, A. J. J. Am. Chem. SOC.1981,103, 5007. (b) chemical response, and mathematical modeling have been presPeerce, P. J.; Bard, A. J. J. Eleciroanal. Chem. 1980, 112, 97. (c) Peerce, ented and studied. The details of charge transport are hard to P. J.; Bard, A. J. J. Eleciroanal. Chem. 1980, 114, 89. (d) White, H. S . ; ascertain on a molecular level and probably differ for different Leddy, J.; Bard, A. J. J. Am. Chem. SOC.1982, 104, 4811. (e) Martin, C. R.; Rubinstein, I.; Bard, A. J. J. Am. Chem. SOC.1982, 104, 4817. systems. However, in a number of cases the charge transport (3) (a) Oyama, N.; Anson, F. C. J. Am. Chem. SOC.1979,101, 3450. (b) seems to be adequately described phenomenologically by simple Oyama, N.; Anson, F. C. J. Elecirochem. SOC.1980,127,640. (c) Shigehara, Thus, apdiffusion across a concentration gradient.1-5~7a,8a~9a*bJ2 K.; Oyama, N.;Anson, F. C. J. Am. Chem. Sot. 1981,103,2552. (d) Oyama, parent effective diffusion coefficients (Dapp)have been obtained. N.; Anson, F. C. Anal. Chem. 1980, 52, 1192. (e) Buttry, D. A.; Anson, F. C. J. Electroanal. Chem. 1981, 130, 333. (f) Mortimer, R. J.; Anson, F. C. An understanding of the obtained Dapp)sis complicated because, J. Eleciroanal. Chem. 1982, 138, 325. (g) Martigny, P.; Anson, F. C. J. in addition to electron-transfer reactions, we must consider polymer Electroanal. Chem. 1982, 139, 383. (h) Buttry, D. A.; Anson, F. C. J . Am. motion, diffusions of electrolyte and solvent, and changes in Chem. SOC.1982,104,4824. (i) Buttry, D. A,; Anson, F. C. J. Am. Chem. polymer layer structure with oxidation and reduction. In any case, SOC.1983, 105, 685. 6) Anson, F. C.; Ohsaka, T.; SavCant, J. M. J. Phys. Chem. 1983,87,640. (k) Anson, F. C.; Ohsaka, T.; SavCant, J. M. J. Am. charge (ion or electron) transport in redox polymer films is Chem. SOC.1983, 105, 4883. (I) Anson, F. C.; SavBant, J. M.; Shigehara, considered to occur via electron hopping between redox sites K. J. Am. Chem. SOC.1983, 105, 1096. (m) Anson, F. C.; SavBant, J. M.; ( I a,b,2ce,3e.i,4a3e,g,1114.15 and/or the physical diffusion of redox Shigehara, K. J. Electroanal. Chem. 1983, 145, 423. (n) Shigehara, K.; . ' ~ ~when ~ ~redox ~ ~sites ~ ~ ~ Oyama, ~ , ~N.;~Anson, ~ ~ F.~C.~Inorg. ~ ~Chem. ~ ~1981, ~ 20, ~ 518. ~ species t h e m ~ e l ~ e ~ Especially are covalently bonded to polymer chains, an electron-hopping (4) (a) Nowak, R.; Schultz, F. A,; Umaiia, M.; Lam, R.; Murray, R. W. Anal. Chem. 1980, 52, 315. (b) Daum, P.; Lenhard, J. R.; Rolison, D.; charge-transport mechanism seems to be operative. Murray, R. W. J. Am. Chem. SOC.1980, 102, 4649. (c) F a d , J.; Murray, According to Dahms-Ruff's ideaL4J5proposed for charge R. W. J. Phys. Chem. 1981,85,2870. (d) Daum, P.; Murray, R. W. J. Phys. transport in solutions, whenever electron self-exchange makes a Chem. 1981, 85, 389. (e) Facci, J.; Schmehl, R. H.; Murray, R. W. J . Am. Chem. SOC.1982, 104, 4959. (0 Ikeda, T.; Schmehl, R.; Denisevich, P.; significant contribution to the diffusion process, Dappshould exhibit +The Institute of Physical and Chemical Research, Wako-shi, Saitama 351, Japan.
0022-3654/86/2090-3850$01.50/0
Willman, K.; Murray, R. W. J . Am. Chem. SOC.1982, 104, 2683. (g) Schmehl, R. H.; Murray, R. W. J. Electroanal. Chem. 1983, 152, 97. (h) Facci, J.; Murray, R. W. J. Electroanal. Chem. 1981, 124, 339. (i) Kuo, K.; Murray, R. W. J . Electroanal. Chem. 1982, 131, 37.
0 1986 American Chemical Society
Charge-Transfer Reactions in Pendant Viologen Polymers
cycl
CH2
6
c'-
Q
I CI-
cH3 PMV
Figure 1. Structure of poly(styrene-co-chloromethylstyrene) pendant viologens (PMV) used.
tration dependence of Dapphas not been observed. In contrast, the values of Dappwere independent of C or decreased with an A dependence of Dappon C, expected increase in C.3c-tJ34h95a9f-' from Dahms-Ruffs idea,14-15has been observed in the Co( b p ~ ) , ~ + (bpy / + = 2,2'-bipyridine)-Nafion system3' and poly(vio1ogen)-poly(p-styrenesulfonate) and -Nation polymer complex systems.5k The redox behaviors of these systems are not simple because the changing of the concentration of elstroactive species (or sites) in the films causes at the same time the physicochemical change of its surroundings. This is due to the significant electrostatic interaction between electroactive species and the charged sites of the film^.^^,^,^^,^,^^,^^,^^ This problem, in most cases, is unavoidable because of the essential electrostatic interactions that occur in the incorporation of electroactive species (sites) into (5) (a) Oyama, N.; Yamaguchi, S.;Nishiki, Y.; Tokuda, K.; Matsuda, H.; Anson, F. C. J . Electroanal. Chem. 1982,139,371. (b) Oyama, N.; Ohta, N.; Ohnuki, Y.; Sato, K.; Matsuda, H. Nippon Kagaku Kaishi 1983,940.(c) Oyama, N.; Sato, K.; Matsuda, H. J. Electroanal. Chem. 1980,115, 149. (d) Kaneko, M.; Moriya, S.; Yamada, A.; Yamamoto, H.; Oyama, N. Electrochim. Acta 1984,29, 115. (e) Oyama, N.; Oki, N.; Ohno, H.; Ohnuki, Y.; Matsuda, H.; Tsuchida, E. J. Phys. Chem. 1983,87,3642.(f) Oyama, N.; Sato, K.; Yamaguchi, S.; Matsuda, H. Denki Kagaku oyobi Kogyo Butsuri Kagaku 1983,51, 91. (g) Sato, K.; Yamaguchi, S.; Matsuda, H.; Ohsaka, T.; Oyama, N . Bull. Chem. SOC.Jpn. 1983, 56, 2004. (h) Oyama, N.; Ohsaka, T.; Kaneko, M.; Sato, K.; Matsuda, H. J. Am. Chem. Soc. 1983,105, 6003. (i) Oyama, N.; Ohsaka, T.; Ushirogouchi, T. J . Phys. Chem. 1984,88, 5274. (j)Ohsaka, T.; Yamamoto, H.; Kaneko, M.; Yamada, A,; Nakamura, M.; Nakamura, S.; Oyama, N. Bull. Chem. SOC.Jpn. 1984,57, 1844. (k) Ohsaka, T.; Sato, K.; Matsuda, H.; Oyama, N. J . Electrochem. SOC.1985, 132, 1871. (1) Ohsaka, T.; Ushirogouchi, T.; Oyama, N. Bull. Chem. SOC. Jpn. 1985,58,3252. (m) Oyama, N.; Ohsaka, T.; Shimidzu, T. Anal. Chem. 1985,57, 1526. (6) (a) Wrighton, M. S.; Austin, R. G.; Bocarsly, A. B.;Bolts, J. M.; Haas, 0.; Legg, K. D.; Nadjo, L.; Palazzotto, M. C. J. Am. Chem. SOC.1978,100, 1602. (b) Lewis, T. J.; White, H. S.; Wrighton, M. S. J . Am. Chem. SOC. 1984,106,6947. (7) (a) Andrieux, C. P.; SavCant, J. M. J. Electroanal. Chem. 1980,111, 377. (b) Andrieux, C. P.; Dumas-Bouchiat, J. M.; Saveant, J. M. J . Electroanal. Chem. 1982,131, 1. (c) Andrieux, C. P.; Saveant, J. M. J . Electroanal. Chem. 1982,142, 1. (8) (a) Laviron, E. J . Electroanal. Chem. 1980,lZ2, 1. (b) Laviron, E. J. Electroanal. Chem. 1982,131,61. (c) Laviron, E. J. Electroanal. Chem. 1981,122,37. (9) (a) Majda, M.; Faulkner, L. R. J. Electroanal. Chem. 1982,137,149. (b) Majda, M.; Faulkner, L. R. J . Electroanal. Chem. 1984,169,77. (c) Majda, M.; Faulkner, L. R. J. Electroanal. Chem. 1984,169,97. (IO) (a) Lau, A. N. K.; Miller, L. L.; Zinger, B. J. Am. Chem. SOC.1983, 105, 5278. (b) Miller, L. L.; Zinger, B.; Degrand, C. J. Electroanal. Chem. 1984,178,87. (c) Degrand, C.; Miller, L. L. J. Electroanal. Chem. 1982, 132,163. (11) Yap, W. T.; Durst, R. A.; Blubaugh, E. A.; Blubaugh, D. D. J . Electroanal. Chem. 1983,144,69. (12) Doblhofer, K.; Durr, W.; Jauch, M. Electrochim. Acta 1982,27,677. (13) Inzelt, G.; Chamber, J. Q.;Kinstle, J. F.; Day, R.W. J . Am. Chem. SOC.1984,106, 3396. (14) (a) Ruff, I. Electrochim. Acta 1970,15, 1059. (b) Ruff, I.; Friedrich, V. J . Phys. Chem. 1971,75,3297. (c) Ruff, I.; Friedrich, V.; Csillag, K. J. Phys. Chem. 1971,75,3303. (d) Ruff, I.; Friedrich, V.; Csillag, K. J . Phys. Chem. 1972,76, 162. (e) Ruff, I.; Friedrich, V. J . Phys. Chem. 1972,76, 2954. (15) Dahms, H. J. Phys. Chem. 1968,72, 362. (16) Rubinstein, I. J. Electroanal. Chem. 1985,188, 227. (17) Espenscheid, M. W.; Martin, C. R. J. Electroanal. Chem. 1985,188, 73.
The Journal of Physical Chemistry, Vol. 90, No. 16, 1986 3851
polymer domains. Therefore, there are many factors to be considered and the explanation of the observed behaviors seems rather complicated. Murray et al.4c3g have successfully overcome this problem using the redox copolymer containing electroactive Os and Ru sites which are identical in all structural and electrostatic respects but are not electroactive at the same potential. In that case, the Os sites have been diluted with Ru polymer sites. In this study, we prepared the pendant viologen polymers (PMV) with various percent loadings (3-34%) of viologen sites and examined the overall electrode reaction of these PMV films coated on basal-plane pyrolytic graphite (BPG) electrode surfaces. These PMV films were selected because of their chemical stability and uncomplicated electrochemical behavior (so long as the electrode potential is cycled within the range where the valence of the viologen site is changed between dication and monocat i ~ n ) , ~ ~practical J , ~ - ' ~applications of their a n a l o g ~ e ~ , 3 ~ ~ ~ ~ and relatively fast electron self-exchange r e a ~ t i o n . ~ The ) , ~ ~homogeneous charge-transport process within the PMV films was examined by cyclic voltammetry and normal pulse voltammetry (NPV). The apparent effective diffusion coefficients (Dapp)for the homogeneous charge-transport process were obtained for the PMV films with various loadings of viologen sites. The kinetic parameters (standard rate constant ( k " ) and cathodic transfer coefficient (a,))of the heterogeneous electron-transfer process between electrode (BPG) and viologen site in PMV films were also evaluated by NPV. It will be seen that Dapp(and k o ) increases with an increase in concentration of viologen sites in PMV. Further, on the basis of the data obtained, the mechanism of the charge-transport process within PMV films is discussed. Experimental Section Materials. The poly(styrene-co-chloromethylstyrene) pendant viologens (PMV) (Figure 1) were prepared from poly(styreneco-chloromethylstyrene) (CMPS) and N-methyl-4,4'-bipyridinium chloride (MMbpy) in a xylene/ 1-butanol mixture as described e l ~ e w h e r e . Ten ~ ~ ~polymers ~ J ~ ~ loaded from 0% to 34% of viologen units were prepared by changing the relative amount of CMPS and MMbpy, reaction temperature, and/or reaction period: (x, y, z) = (34, 7, 59), (18, 23, 59), (15, 26, 59), (13, 28, 59), (9, 32, 59), (6, 35, 59), (3, 38, 59), (18, 32, 50), (12, 38, 50), and (0, 50, 50) where x, y , and z denote the percent loadings of viologen, chloromethylstyrene, and styrene moieties, respectively. The values of (x, y , z) were estimated on the basis of the elementary analysis of each PMV and the CMPS used in its preparation. The stock solution of each of these PMV polymers was prepared as a methanolic solution, and the concentration was 2.55 mg mL-'. Thus, the molar concentrations of viologen units in the respective PMV stock solutions varied, depending on the percent loading (x) of viologen unit. The basal-plane pyrolytic graphite (BPG) (Union Carbide Co.) disk electrodes (area 0.17 cm2) were prepared and mounted in a glass tube with a heat-shrinkable polyolefin t ~ b e . , ~Fresh , ~ electrode surfaces were produced by cleaving the disk with a scalpel. Aqueous solutions were prepared from doubly distilled water. The supporting electrolyte was 0.2 M KC1 (Wako Pure Chemical Industries, Ltd., Osaka) adjusted to pH 3.0 with HCl. Other chemicals were reagent grade and were used as received. Apparatus and Procedures. Aliquots of the stock solution of each of the PMV's were spread by a microsyringe on the freshly cleaved BPG surface and air-dried to remove methanol. The quantity of the electroactive viologen sites (MV2+) in PMV (18) Oyama, N.; Ohsaka, T.; Sato, K.; Yamamoto, H. Anal. Chem. 1983, 55, 1429.
(19) Akahoshi, H.; Toshima, S.; Itaya, K. J . Phys. Chem. 1981,85, 818. (20) Factor, A,; Rouse, T. 0. J . Electrochem. SOC.1980,127, 1313. (21) (a) Bookbinder, D. C.; Wrighton, M. S. J . Am. Chem. Soc. 1980,102, 5123. (b) Bruce, J. A,; Murahashi, T.; Wrighton, M. S . J . Phys. Chem. 1982, 86, 1552. (22) Abruna, H. D.; Bard, A. J. J . Am. Chem. SOC.1981, 103,6898. (23) Takuma, K.; Sakamoto, T.; Nagamura, T.; Matsuo, T. J . Phys. Chem. 1981,85,619. (24) Curtis, J. C.; Sullivan, B. P.; Meyer, T. J. Znorg. Chem. 1980,19, 3833.
3852 The Journal of Physical Chemistry, Vol. 90, No. 16, 1986
coatings on electrodes was determined coulometrically3bby integrating the current that passed when the electrode potential was stepped from a value where no current was flowing to a value where the MV2+sites were reduced to the corresponding radical monocations (MV”) and by measuring the area of cyclic voltammograms (for the one-electron oxidation-reduction reaction of the MV2+/’+redox couple) obtained at slow potential scan rates of 1-2 mV s-I. These quantities were estimated as the surface concentration (I’MV?+)in units of mol cm-2. The molar Concentration (in units of mol ~ m - of ~ )the MV2+ sites was calculated from the r M V ? + thus obtained by using the thickness of the film under the swelling state once the film is placed in the electrolyte solution. The film thicknesses (4) under the “swelling“ state and the “dry” state were measured with a Surfcom 550A (surface texture measuring instruments, Tokyo Seimitsu Co.). The measurement required a sharp boundary between a clean portion of a glass slide surface and a part covered by the polymer film. Such a boundary step was created as follows: Aliquots of the PMV stock solutions were cast and then spread by a microsyringe on a limited surface (the area is known) of a glass slide, the remaining part of which was previously covered by a silicone film, and then the solvent was evaporated at room temperature. After that, the silicone film was stripped. The thicknesses of the resulting “dry“ films were measured several times with respect to one sample. After that, the PMV-coated glass slide was soaked for 30-180 min in a 0.2 M KCl solution (pH 3.0), and after the water was absorbed on the PMV film surface by filter paper the thicknesses of the film under the “swelling” state were measured several times with one sample. The average value of these measurements was taken as the “dry” or “swelling” thickness. The electrochemical measurements were made with a conventional potentiostatic device with three electrodes. Normal pulse voltammograms and cyclic voltammograms were obtained with homemade instruments and were recorded with an X-Y recorder (Watanabe Co., Tokyo). Positive feedback circuitry was employed to compensate the resistances (these were typically ca. 20-50 Q) associated with the polymer coatings as much as possible. In normal pulse voltammetric experiments, the pulse width of 10-100 ms and the interval of 30-120 s between successive pulses were employed. Thus, it can be safely considered that the depletion layer of reactants produced during the preceding pulse completely disappears before the following pulse starts. Normal pulse voltammetric measurements were confined to times (typically 1-10 ms) sufficiently short to ensure that semiinfinite linear diffusion prevailed.5f-” Nitrogen gas was passed through the solutions to remove the dissolved oxygen before the experiments and over the solutions during the experiments. Potentials were measured and are reported with respect to a sodium chloride saturated calomel electrode (SSCE). Experiments were conducted at ambient laboratory temperature (25 -+ 1 “C). Results and Discussion Changes of 4, I’$$+/I’&$+, and with x . The values of electrochemically detected (observed) surface coverage (I-$$;) were calculated from the integration of the current in cyclic voltammograms obtained at various scan rates. At the scan rates ( u ) slower than 2 mV s-l, the values of I’”,$+ are independent of u and are of almost constant value (2.1 X mol cm-2), which was in agreement with that coulometrically estimated at -0.65 V vs. SSCE, but at scan rates faster than 5 mV s-l the I’$$+ values decreased gradually with an increase in u. Thus, it can be safely considered that the values of r$$+obtained at u < 2 mV s-l reflect almost all the actually electroactive viologen sites in the PMV coating. Note that the I?@$+ (2.1 X 10” mol cm-2) thus obtained is smaller than the amount (6.0 X mol cm-2) of viologen sites in the PMV coating coated originally on electrode surfaces. This indicates that not all the viologen sites in the PMV coating are electroactive. We find that about two-thirds of the quantity of viologen sites were located in unswollen portions of the polymer, where they remained inaccessible to the supporting electrolytes
CMV+
Oyama et al.
0
10
20
30
Lo
x /% Figure 2. Dependences of r$$+/I’b$+,$, and CMy on+the percent of each of loading ( x ) of viologen sites in PMV’s. The amount (rg$i+) the PMV’s coated on electrodes was kept constant (2.9 X g cmT2). The values of r$$+and $ ($wet and $d ) were measured as described in the Experimental Section, and the values were calculated as I‘$v$+/$w,. $dry and $,,.et indicate the “dry” and *wet” thicknesses, respectively, of PMV films. The fractions of styrene groups in PMV’s (Z) are ( 0 ,0) 59% and (A,A) 50%. Supporting electrolyte: 0.2 M KCI (pH 3.0). Widths of error bars indicate uncertainties in the measure$, and CMy+. ments of r$$+/r’E’$+,
*a+
and were thus electroinactive. This explanation seems to be reasonable from the fact that a large portion of the PMV (about 70%) is the hydrophobic moiety composed of styrene and chloromethylstyrene. A similar behavior has been observed for other polymeric viologen ~ o a t i n g s . ~ ~ , ~ 8 * ~ ~ , ~ In Figure 2A are shown the values of I’$$+/rE‘$+ for the PMV films with various loading ratios of viologen sites. Some aspects seen from this figure are (i) the values of I’$$+/I’Etb+ are not unity for the all PMV films used, that is, not all the viologen sites in the PMV polymers originally deposited on electrodes are tend to decrease electroactive, and (ii) the values of r$$+/I’E$+ almost linearly with a decrease in the loading ratio (x) of viologen site. The second point may be ascribed to the change in the degree of the PMV film swelling with the loading ratio of viologen sites. The viologen moieties with positively charged sites are considered to be hydrophilic, and thus as x is increased the degree of the swelling of the PMV film becomes larger. This seems to be supported from the “dry” and “wet (swelling)” thickness measurements of PMV films. The relevant data are shown in Figure 2B. As expected, the degree (estimated as the ratio of to (pdry) of the swelling increases with an increase in x. The wet and dry thicknesses of each of PMV films were found to be proportional to the amount ( w ) of PMV film coated on electrodes in the range to 3 x lo4 g cm-2). The ratio f$wet/+dry examined of w (1 x increased from 1 to 1.28 with an increase in x from 0 to 34. Compared with other polymer films (e.g., protonated poly(4vinylpyridir~e)’~J”’~~*~-~ and p~ly(lysine)’j-~) coated on electrodes, the degree of the swelling of the PMV films used is significantly smaller, and this may probably be due to the hydrophobic portion of poly(styrene) and poly(chloromethy1styrene) chains which is about 66-97% of the whole structure of PMV. It is also apparent from Figure 2B that the dry thicknesses (ddry)tend to increase with increasing x . In this case, the constant amount (5.0 X IOd g) of each PMV film was coated on electrode surfaces. The change
Charge-Transfer Reactions in Pendant Viologen Polymers
The Journal of Physical Chemistry, Vol. 90, No. 16, 1986 3853 T
q
u 0
4 \
7
+
h
$3 u
Y
0
-
5
10
15
(scan rate)"2/ ( r r'') ~ '-s Figure 4. Typical scan rate dependence of cathodic peak current for the one-electron reduction of MV2+ to MV" of PMV's. The cathodic peak currents normalized with respect to the respective C$$+'sof PMV's are represented in the figure. PMV (x, y , z): (A) (34,7, 59), (B) (13, 28, 59), and (C) (9, 32, 59). Experimental conditions are the same as in Figure 3.
-0.5 0 E I V vs. SSCE
Figure 3. Typical cyclic voltammograms obtainec ir the one-electron oxidation-reduction reaction of the PMV's with brious loadings of viologen sites. The amount (rg$+)of each of the PMV's coated on electrodes was kept constant (2.9 X 1W5g cm-2). Supporting electrolyte: 0.2 M KCl (pH 3.0). Scan rate: 5 mV s-'. BPG electrode area: 0.17 cm2. Note that the currents normalized with respect to the respective volume concentrations (CMy+) of PMV's are represented in the figure The values of CMy+ are shown in Figure 2C. PMV (x, y , z): (A) (34, 7, 59), (B)(13, 28, 59h (C) (9, 32, 59), (D)(6, 35, 591, and (E)(3, 38, 59).
of (bdry with x may reflect the change in the morphology of the PMV film in the dry state. The increased partial molar volume of the viologen sites (including the necessary complement of two chloride ions) relative to that for the chloromethylstyrene and styrene moieties could also be responsible for the increase in the dry thickness with x. The volume concentration of the electroactive viol0 en from F&A+ site in PMV coatings can be calculated as roY+/(bwet and (bwet in Figure 2A,B. The values of C$$+ thus obtained are shown in Figure 2C. As can be expected from the results shown in Figure 2A,B, Cy+increases, almost linearly, with an increase in x; that is, G g c h a n g e s from 1.2 X to 3.3 X lo4 mol cm-3 while x changes from 6 to 34. Cyclic Voltammetry of PMV Films. In Figure 3 are shown typical cyclic voltammograms obtained for the PMV's with various loading ratios of viologen units. In each case, the amount of PMV g cm-2) and coated on electrodes was kept constant (2.9 X the potential was scanned between 0 and -0.65 V vs. SSCE. In this potential region, the valence of viologen site changes between +2 and 1. So long as the potential is cycled within this region, both anodic and cathodic peak currents of the cyclic voltammogram remained substantially unchanged even after several hours, showing that the PMV was stable and strongly attached to the BPG electrode. However, when the potential scanning was extended to the region from 0 to -1 .O V vs. SSCE, where the valence of the viologen site changes from +2 to 0, the oxidation peaks corresponding to the oxidations of MV'+ to MV2+ and MVa to MV'+ were not observed and the reduction peaks for the reductions of MV2+ to MV'+ and MV'+ to MVo decreased gradually with the potential cycling. Such a loss of the electroactivity of PMV films may be ascribed to the formation of the electroinactive materials via the dimerization reaction among the reduced viologen sites25,26or an interaction between the reduced viologen sites and the original onesgc Thus, the potentials of the electrode were scanned only over the first wave (i.e., the reduction wave of the
(e$$+)
+
( 2 5 ) Furue, M.; Nozakura, S. Chem. Lett. 1980, 821. (26) Bird, C. L.; Kuhn, A. T. Chem. SOC.Rev. 1981, IO, 49.
' -y-\; Hetkogeneous El= tron- t ransf er Process (ko,dc)
Hanogeneous Charge - transpor t Process (DWP)
Figure 5. Schematic depiction of charge-transfer processes on PMVcoated electrodes. ko and a, are the standard rate constant and the cathodic transfer coefficient, respectively, of the heterogeneous electron-transfer process at the electrode/PMV film interface. D,, is the apparent diffusion coefficient for the diffusion-like charge-transport process within PMV films. MV2+and MV" indicate the dication and radical monocation of viologen sites, respectively, in PMV films. Note that heterogeneous and homogeneous charge-transfer processes are associated with charge-compensating counterion motion.
viologen dication as PMV to the radical monocation) of the two reduction waves. The currents in Figure 3 are normalized with of PMV's. respect to the respective volume concentrations Hence, the considerable differences in peak currents for the respective PMV films can be considered to reflect the differences in the rates of heterogeneous electron-transfer reactions between the electrode and viologen site and/or of the homogeneous charge (electron and/or ion) transport process in different PMV films. For the PMV's with x of 9-34%, the well-defined cathodic and anodic peaks were observed on the cyclic voltammograms. The redox potentials (Eo) for the MV2+/'+ couple in each of the PMV films, which were estimated as the average of the anodic and cathodic peak potentials of the cyclic voltammograms of the respective PMV films, shifted (though slightly) to more negative values with decreasing x (AEO' for PMV's with x = 9 and 34% was 12 mV), reflecting the change of the surroundings about the viologen sites in PMV coatings with x. Figure 4 shows the typical examples representing the scan rate (v) dependence of the cathodic peak current )2;( for the first reduction wave of MV2+ to MV". For all the PMV films examined ( x = 9%, 13%, 15%, 18%, and 34%), plots of vs. u 1 l 2 were nearly linear from 5 to 200 mV s-' and the straight lines drawn by the extrapolation of these plots to u = 0 passed through the origin. These indicate that the charge transport through the PMV films is apparently diffusion-controlled and obeys Fick's law. The different slopes of the straight lines in Figure 4 show the different rates of the charge-transport process in PMV films and thus different effective diffusion coefficients (Dapp)for the diffusion-like charge transport. The relevant quantitative data will be obtained in the next section.
(e;$+)
I:
The Journal of Physical Chemistry, Vol. 90, No. 16, 1986
3854
Oyama et al.
TABLE I: Apparent Diffusion Coefficients for the Charge Transport within PMV Films and Kinetic Parameters for the Heterogeneous Electron Transfer at Electrode/PMV Film Interfaces' samDle no. I
Xl%
VI%
z/%
34 18 I5 13 9 18 12
7 23 26 28 32 32 38
59 59 59 59 59 50 50
7
I
3 4
5 6 7
D,,,lcm2 s-' (3.9 f 0.7) X (2.5 f 0.4) X (1.3 f 0.2) X (5.6 f 0.8) X IO-'' (2.5 f 0.4) x (1.8 f 0.3) X (4.6 f 0.7) X
k"lcm s-I (8.9 f 1.2) x (9.5 1.5) x (7.1 i 1.0) x (4.3 f 0.7) x (3.0 0.5) x (1.7 0.3) x (8.8 f 1.0) X
a, 10-5
10-5 10-5 10-5 10-5
10-4
0.42 0.38 0.39 0.41 0.45
f 0.02
f 0.02 f 0.02 f 0.02 f 0.02 0.44 f 0.02 0.46 f 0.02
y . and z indicate the percent loadings of viologen, chloromethylstyrene, and styrene moieties, respectively, in PMV's (see Figure 1).
97
--
0.5
1.0
T-l/2/(m 9-1l2
Figure 7. Plots of limiting current, iIim,vs. (sampling time)-1/2 for the normal pulse voltammograms shown in Figure 6 .
I -08 I
1
I
I
I
-0 4
-06
E /vvs
I
I
-0.2
I
1 0
SSCE
Figure 6. Typical normal pulse voltammograms for the one-electron reduction of the viologen dication as PMV ( x = 18, y = 32, z = 50) coated on BPG electrode at various sampling times in a 0.2 M KCI solution (pH 3.0). 2.1 X lo4 mol cW3. Electrode area: 0.17 cm2. Sampling times (ms) are given on each voltammogram.
e$$+:
Normal Pulse Voltammetry of PMV Films. Normal pulse voltammetry (NPV) has proved to be useful for the kinetic study of the electrode reaction (see Figure 5) at the polymer-coated e l e ~ t r o d e ; ~ ~Le., - ~ Jboth " the heterogeneous electron-transfer reaction between the electrode and the electroactive site (species) confined in polymer domains and the homogeneous chargetransport process in polymer domains can be quantitatively examined by the analysis of the normal pulse voltammogram as shown below. In Figure 6 are shown typical normal pulse voltammograms for the one-electron reduction of the viologen dication (MV2+) to the corresponding radical monocation MV" as PMV coated on BPG electrodes a t various sampling times in a 0.2 M KCl solution (pH 3.0). These S-shaped voltammograms are similar to those observed for solution-phase redox species at an uncoated electrode2' and for other electroactive polymer-coated electrodes examined p r e v i o u ~ l y . ~Plots ~ - ~ *of~ the cathodic limiting current (jam) of these normal pulse voltammograms against the inverse square root of the sampling time (7)were found to be linear (see Figure 7), as expected for the diffusion-controlled limiting current.5f-km.27Thus, the values of the apparent diffusion coefficients, Daw,for the process of the charge transport within the PMV films were obtained from the slopes of the ilimvs. 7-'12 plots by using the normal pulse voltammetric Cottrell e q ~ a t i o n ~ * ~ ~ ~ (id)&tt = UFACO(D,,,/TT) lI2
for the case of a reversible electrode process. For the case of an irreversible electrode process
with
(1)
(27) (a) Matsuda, H. Bull. Chem. SOC.Jpn. 1980,53, 3439. (b) Yamaguchi, S.; Matsuda, H.; Ohsaka, T.; Oyama, N. Bull. Chem. SOC.Jpn. 1983, 56, 2952. (c) Ohsaka, T.; Oyama, N.; Yamaguchi, S.; Matsuda, H. Bull. Chem. SOC.Jpn. 1981, 54, 2475. (28) For example: (a) Vetter, K. J. Electrochemical Kinetics; Academic: New York, 1967; p 107. (b) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications; Wiley: New York, 1980; Chapter 3.
where (id)& denotes the cathodic limiting diffusion current, n the number of electrons involved in the heterogeneous electrontransfer reaction, F the Faraday constant, A the electrode area, and C' the volume concentration of the electroactive site confined in polymeric coatings. In the present system, n = 1 , A = 0.17 The . data obtained for Dappare summarized cm2, and C' = CM!$+ in Figure 9 and Table I. The discussions about the homogeneous charge-transport process within PMV films on the basis of these data will be made below. As reported p r e v i o u ~ l y , ~ an ~ - ~analysis . ~ * ~ ~of the rising part of the current-potential curves shown in Figure 6 allows us to estimate the kinetic parameters (Le., the standard rate constant ko and the cathodic transfer coefficient cy,) of the heterogeneous electron-transfer reaction. The relevant current-potential relationship for normal pulse voltammograms for the simple electrode process, Ox + ne s Red, has already been derived and is given for the reduction by27a RT x E = E\,2 - - In nF I-x
where E is the electrode potential, i the normal pulse voltammetric current at potential E, Ei12the reversible half-wave potential which can be expressed by E;,2 = Eo' ( R T / n F )In (DRed/DOx)1/2, and
+
(29) Cottrell, F. G. 2.Phys. Chem., Stoechiom. Verwandschaftsl. 1903, 42, 385.
Charge-Transfer Reactions in Pendant Viologen Polymers
The Journal of Physical Chemistry, Vol. 90, No. 16, 1986 3855 1
I
-0.6
I
- 0.5
I
- 0.4
E / V vs.SSCE
Figure 8. Plots of In (x[{1.75 + x2(1+ exp([))2)/{1- x ( l + ex^([)))]'/^) vs. E for the normal pulse voltammograms shown in Figure 6 . Sampling times (ms) are indicated on each straight line. T the sampling time; Dox and DRd are diffusion coefficients of Ox and Red,respectively, and R and T have their usual meanings. Figure 8 shows the typical examples of the plots of In (x[(1.75 x2(1 + exp(f))2)/(1- x(1 + e ~ p ( f ) ) ) ] l /vs. ~ ) E for the reduction of MV2+ to MV'+ as PMV coated on BPG electrodes at various sampling times. These plots correspond to the normal pulse voltammograms shown in Figure 6 . The plots gave the straight lines, the slopes of which were constant at the different sampling times ranging from 2 to 10 ms. Further, the potential ( E * ) at the intersection of these straight lines with the abscissa shifted to more negative values with a decrease in T . Thus, according to eq 3, from the slopes of the straight lines and the E* potentials and k" were estimated by using shown in Figure 8, the values of CY, the known values of E{l2,D, and T . The values of Ef12were estimated as the average of the anodic and cathodic peak potentials of the cyclic voltammograms for the oxidation-reduction of the MV2+/'+ redox couple in PMV coatings on BPG electrodes in a supporting electrolytic solution. Further, with the assumption that DRcd and Do, correspond to Dapp'sfor cathodic and anodic processes, respectively, and Dappfor the cathodic process is approximately equal to Dappfor the anodic process, the values of D were estimated as those of D,, for the cathodic process. The data for Dapp,k", and CY, thus obtained are summarized in Figure 9 and Table I. Both k" and Dappincreased with an For a series of PMV films with z = 59%, as increase in e$$+ were increased from 6.7 X to 3.3 X lo4 mol ~ m - D,~, , cm2 s-l and k" from increased from 2.5 X lo-" to 3.9 X to 8.9 X cm s-l. On the other hand, the CY, value, 3.0 X which is one of the kinetic parameters of the hetero eneous electron-transfer reaction, was almost independent of e,$+ %s and equals 0.42 f 0.04. In the case of the PMV (x = 34, y = 7, and z = 59)-Nafion and -poly@-styrenesulfonate)(PSS) polymeric intermolecular complexes where the constant amount of PMV and the various amounts of Nafion (or PSS) were blended and coated on BPG electrode surfaces, the increase in k" and Dappwith has been also observed.5k When x is the same, increasing e$$+ the values of k" and Dappfor the PMV films with different z's are different. For example, the values of k" and Dappfor the PMV with x = 18, y = 32, and z = 50 are about 2 and 7 times larger than those for the PMV with x = 18, y = 23, and z = 59. These results suggest that the rates of the heterogeneous electron-transfer reaction at the electrode/PMV film interfaces and the homogeneous charge-transport process within the PMV films depend on the structure of the PMV's and more particularly on the morphology of the PMV films swollen in the supporting electrolytic solutions. As found previously for other redox polymer-coated electrode ~ y s t e m s , ~ git-was ~ also found from Figure 9 that there is a linear relationship between log k" and log D, p. Charge-Transport Mechanism in PMV Films. eharge (electron or ion) transport through the solvent-swollen polymer films undergoing electrolysis (Le., oxidation or reduction of incorporated electroactive species (or sites)) is generally believed to occur via an electron-hopping process between redox species and/or the physical diffusion of redox species themselves (which are temporarily confined in polymer domains), la,b,2a,~edb,~,4a,b,$qgi,5,9a.12,14.15
'F---
+
e@$+.
0
18
I 4.0
I 3.5
obsd
- log(cMv 2+/mo1 cm-3) Figure 9. Dependences of Dapp,k o , and cyc on C$$+. The amount (I'E$+) of each of the PMV's coated on electrodes was kept constant (2.9 X g c r d ) . Other experimental conditions and the symbols used are the same as in Figure 2. The percent loadings of viologen sites in PMV's (x) are indicated on each symbol. Widths of error bars indicate uncertainties in the measurements of Dappr k o , and ac.
Both processes will require, for charge neutrality, concurrent uptake of counterions into the polymer matrix or expulsion of co-ions initially present in the film as an ion pair. Thus, the charge transport will be determined by the intrinsic electron-transfer process between adjacent redox species, the physical diffusion of electroactive species themselves, the charge compensating counterion motion which is necessarily coupled to electron transfer, the motion of solvent, and/or the segmental motion of polymeric chain. The charge-transport rate, which will be determined by only the slower of these various processes, is characterized by an effective diffusion coefficient for charge transport, Dapp,and thus the values of D for a number of redox polymer films have been These previous data demonstrate that redox couples (as sites or species) incorporated in polymer and polyelectrolyte coatings on electrodes exhibit a much wider range of effective diffusion coefficients (over several orders of magnitude, 104-10-14 cm2 s-l) than they do when dissolved in fluid solutions. With fixed-site electroactive polymers in a supporting electrolytic solution, electrons are reasonably considered to be transported through redox polymers by hopping between redox ~ ~o, t~h e r ~ , ~ * ~ , g , ~ j , ~ sites.la As pointed out by Buttry and A n ~ o nand in this situation the Dahms-Ruff electron-hopping chargetransport m e ~ h a n i s m , lwhich ~ * ~ ~was originally developed for the charge transport in solutions, can be applied. Since the electron-transfer sites are futed, the effective charge-transport diffusion coefficient, Dam,is composed of contributions from actual diffusion governed by the diffusion coefficient Do and electron transfer (electron self-exchange) and is expressed by the e q ~ a t i o n ~ ~ ~ ~ ' . ' ~ * ' ~ if
Dapp= Do + -k,,62C 4
(8)
where k,, is the second-order electron-exchange rate constant for the redox couple, 6 is the distance between the sites when the electron transfer occurs, and C is the concentration of exchange sites (the sum of the concentrations of the oxidized and reduced forms of the redox couple). Rapid charge transport is therefore
3856
The Journal of Physical Chemistry, Vol. 90, No. 16, 1986
Oyama et al.
above) can thereby also be altered. The PMV system examined here was chosen to modify this condition. As expected, the Dapp increased with an increase in C, although the obtained concentration dependence of Dappis not that expected from the Dahms-Ruff e ~ a t i o n l ~(see , ' ~Figure 10) (Le., the dependence Tul N of Dappon C&!z+ was not completely linear).30 This finding E demonstrates the significant contribution of electron self-exchange \ between viologen redox couples confined in the polymer chain to 3. Q. the overall charge transport in the PMV films. Cl We will speculate that the disrepancy between the obtained C n dependence of Dappand that predicted from eq 8 probably arises 0 because of the additional factors introduced by changing x in a -2 series of the PMV films used. We expected that the changing *9 €13 x leads to only the change of the concentration of the viologen 1 I I 1 0 site in PMV films.31 Actually, however, this is not so. The 0 1 2 3 4 viologen sites are more hydrophilic than the styrene and chloromethylstyrene moieties in PMV polymers. Thus, the larger x, L, obsd the larger the degree of the swelling of the PMV films is. This I O .C 2+ /mol cm-3 is true as shown in Figure 2. As the PMV films are more swollen, the motions of PMV polymer chain itself, solvent, and counterion Figure 10. Plot of DSw vs. CMy+. The percent loadings of viologen sites in PMV's (x) are indicated on each point. The values of Dappand CMy+ are considered to become easier. As a result of easier movement were taken from Figure 9. of the PMV polymer chains, the rate of the viologen site selfdiffusive motion may become larger. The rate of the self-diffusive favored by proper alignment and high density of the exchange motion of the viologen site is expressed as Doin eq 8. The more sites is in the polymer matrix. The concentration dependence (such swelling of the PMV films may also result in the increase in k,,. as that expected from eq 8) of Dapphas not been observed many On the basis of these considerations, it may be thought that Do times. Well-behaved examples for such a concentration depenand k, in eq 8 are functions of C. This reasoning seems to explain dence of Dappare the cases of the transition-metal bipyridyl redox reasonably the actually observed C dependence of Dapp.These polymer films,"qg the C ~ ( b p y ) ~ ~(bpy + / + = 2,2'-bipyridine) conspeculative interpretations need further study. fined in Nafion films,3' and the polymeric intermolecular complexes It is interesting to estimate the k,, value from the slope of the composed of viologen polymer (such as that shown in Figure 1) straight line drawn in the Dappvs. C plot (Figure 10) as a first and Nafion or poly(p-styrenesulfonate).5k In most cases where approximation by assuming that Dappis proportional to C. The the redox species are electrostatically held in a polyelectrolyte film obtained values of k ,are (0.4-2) X lo5 M-] s-l with the ascarrying the opposite ~ h a r g e , ~ ~ , ~the , ~increasing ~ , ~ * ~ C~ in* ~ ~ , ~sumption ~ of 6 = 10 these values are much smaller than the troduces an additional effect: as C i s increased, the degree of the electron self-exchange rate constant24 (8 X lo6 M-' s-l ) of the electrostatic cross-linking between the redox sites (ions) and the monomeric MV2+/'+ redox couple in the ordinary solution. oppositely charged sites of polymer films increases, and as a result, Acknowledgment. This work was partially supported by a the physical diffusion of the redox ion itself, the charge comGrant-in-Aid for Scientific Research (No. 6021 101 1) to N. pensating counterion (or co-ion) motion which is necessarily Oyama from the Ministry of Education, Science, and Culture, coupled to electron transfer, the motion of solvent, and/or the Japan, and the Nissan Science Foundation. segmental motion of polymer chain become slower. This results in the decrease of Dpppwith increasing C. In these cases it is Registry No. Graphite, 7782-42-5; KCI, 7447-40-7. considered that the electron self-exchange reactions still occur. However, the rate-determining step of the overall charge-transport (30) Figure 10 is essentially the same as the log D,,, vs. log in Figure process is not the electron-exchange process but one of the other 9. However, for the discussion of the C dependence of D,,, on the basis of processes mentioned above. Dahms-Ruffs idea,14*1J the plot of Dappvs. e$$+ is preferable (see eq 8). (31) Distances between viologen sites, which are calculated by assuming Thus, we must vary the redox site concentration without the the simple cubic lattice model for the statistical distribution of viologen sites extraneous influences on D,, caused by replacing the redox site in PMV films and by using the CMy+'s (shown in Figure 2C) for the respective with a dissimilar diluent site, since solvent swelling, cross-linking, PMV's, are 17, 20, 21, 24, 29, and 52 A for the PMV's ( z = 59) with x = 34, 18, 15, 13, 9, and 6 , respectively. and other aspects of internal polymer structure (as mentioned
i
A;
