J . Phys. Chem. 1987,91, 1519-1525
1519
Structural Effects on the Electrochemical Behavior of Redox Couples Incorporated in Electrode Coatings Prepared from Copolymers and Composltes Takeshi Inoue and Fred C. Anson* Arthur Amos Noyes Laboratory, Division of Chemistry and Chemical Engineering,t California Institute of Technology, Pasadena, California 91 125 (Received: August 22, 1986)
Five new random vinyl copolymers were prepared and tested as coating materials for binding redox active ions to electrode surfaces. Three of the copolymers were polycationic because of quaternary nitrogen sites. Two uncharged copolymers were used to prepare polycationic coatings by mixing with selected homopolymers, poly(4-vinylpyridine); poly(N-vinyl-2methylimidazole);poly(4-vinylbenzyldiethylamine), that became polycationic by protonation at the pH where they were employed (4.5). Coatings with the most attractive properties for incorporating and retaining Fe(CN)6" anions were prepared from mixtures of the polycationic copolymers with poly(4-vinylpyridine). The presence of alkyl benzyl ether groups in the copolymers was shown to enhance their desirable properties as electrode coatings. Transmission electron micrographs of coatings revealed some correlation between their internal morphologies and their electrochemical behavior.
Polyelectrolytes have gained popularity as coating materials to prepare electrode surfaces with high ion-exchange capacities.'-13 It is also desirable for many applications that redox couples exchanged into the coatings be cycled between oxidation states at high rates. This requires large values of diffusion coefficients for charge propagation by the incorporated ions within the coatings. Recently it was found that unusually large diffusion coefficients for incorporated anions could be obtained with coatings prepared from blends of a particular ternary copolymer with a variety of homopolymer^.^^^'^*'^ In an effort to explore the origin of such attractively large diffusional rates we have examined an additional set of copolymers both alone and in blends with several homopolymers. The interesting behavioral patterns revealed by these studies are the subject of this report.
Experimental Section Materials. Styrene (Matheson) and dimethyIformamide, DMF (Mallinckrodt), were distilled before use. Azobis(isobutyronitnle), AIBN (Polysciences), was recrystallized from methanol. Other commercially available chemicals were used as received. One monomer and the various polymers were prepared as follows: 4-( Viny1benzyl)diethylamine (Monomer I). 4-(Chloromethy1)styrene (70 g, 0.46 mol) was added to 67 g of diethylamine (0.92 mol) in 100 mL of methanol and the resulting mixture was refluxed for 2 h. The solvent was removed by evaporation and 100 mL of 6 M HC1 was added to the residue which was subsequently extracted three times with diethyl ether to remove any unreacted 4-(chloromethy1)styrene. NaOH (50 g) dissolved in 50 mL of H 2 0 was then carefully added to the aqueous layer which was further extracted with ether. The ether extracts were dried with MgS04, and the solvent was evaporated. The product was obtained by distillation under reduced pressure. Yield -84%; bp 88-92 O C (0.45-0.48 mmHg). The N M R spectrum was in accord with the desired structure. Copolymer I . (Structures and compositions of the copolymers synthesized are given in Figure 1.) This random copolymer was obtained by radical polymerization of styrene and 4-(chloromethy1)styrene (Seimi Chem. Co., Tokyo). Appropriate quantities of the two monomers were dissolved in benzene to obtain about 25 mL of a solution 3.0 M in total monomer. AIBN (2 mol %) was added and the mixture was allowed to react for 24 h at 60 O C in a previously degassed reaction ampule. The copolymer was isolated by precipitation with methanol. The product was dissolved in benzene and freeze-dried. Typical yields were 8045%. MW: -25000-30000 daltons (GPC). Elemental analyses of all of the copolymers that were prepared are given in Table I. Copolymer ZZ. Copolymer I (0.4 g, 23 mol 5% styrene) was dissolved in 25 mL of 1:l (by volume) DMFethanol. The solution 'Contribution no. 7459.
0022-3654/87/2091-1519$01.50/0
was heated for 24 h a t 90 OC in a closed tube. Addition of water to the cooled solution produced a precipitate that was suspended and dialyzed against pure water. The product was obtained by freeze-drying the aqueous suspension to remove the water. Yield -90%. (IR showed the presence of new ether (C-O-C) bonding and N M R (CDC13) revealed new ethyl protons (methylene 6 = 3.46, methyl 6 = 1.23). Copolymer ZZZ. Styrene (2.8 g, 27 mmol), monomer I (2.2 g, 12 mmol), and AIBN (9 mg, 0.06 mmol) were dissolved in benzene. The mixture was allowed to react for 50 h at 60 OC. Product isolation followed the procedure employed with copolymer I. Yield: 20-30%. MW: -50000 daltons (GPC). This copolymer (0.52 g, 0.8 mmol) and bromoethane (0.19 g, 1.7 mmol) were dissolved in 8:2 (by weight) DMF:THF and the mixture was sealed into a glass tube that was kept at 80 OC for 24 h. The product was precipitated and purified as with copolymer 11. Typical yield -60-70%. Copolymer ZV. Copolymer I (1.0 g, 23 mol % styrene) was added to triethylamine (0.18 g) in 50 mL of 1:l (by volume) DMF:ethanol. The solution was heated for 40 h at 90 OC in a sealed tube. The product was isolated and purified as with copolymer 11. Yield -96%. Copolymer V. Copolymer I (0.4 g, 17 mol % styrene) was added to triethanolamine (0.62 g) in 25 mL of 1:l (by volume) DMF:ethanol. The mixture was heated for 24 h at 90 OC in a sealed tube. The product was isolated and purified as with copolymer 11. Yield -81%. The homopolymers poly(4-vinylpyridine), PVP, poly(N(1) Reviews: Faulkner, L. R. Cbem. Eng. News 1984, 62, 29. Murray, R. W. In Electroanalytical Chemistry, Vol. 13, Bard, A. J., Ed.; Marcel Dekker: New York, 1984. (2) Oyama, N.; Anson, F. C. J . Electrochem. SOC.1980, 127, 247. (3) Oyama, N.; Shimomura, T.; Shigehara, K.; Anson, F. C. J . Electroanul. Chem. 1980, 112, 271. (4) Rubinstein, I.; Bard, A. J. J . Am. Chem. SOC.1980, 102, 6641. (5) Bookbinder, D. C.; Bruce, J. A,; Dominey, R. N.; Lewis, N . S.; Wrighton, M. S.Proc. Natl. Acad. Sci. U.S.A. 1980, 77, 6280. (6) Shigehara, K.; Oyama, N.; Anson, F. C. J. Am. Cbem. SOC.1981, 103, 2552. (7) Facci, J.; Murray, R. W. J . Phys. Chem. 1981, 85, 2870. (8) Kuo, K.; Murray, R. W. J . Electroanal. Chem. 1982, 131, 37. (9) Mortimer, R. J.; Anson, F. C. J . Electroanal. Chem. 1982, 138, 325. (10) Majda, M.; Faulkner, L. R. J . Electroanal. Chem. 1982, 137, 149. (11) Anson, F. C.; Saveant, J.-M.; Shigehara, K. J. Am. Chem. SOC.1983, 105.,.-1096. ~ -.- -
(12) Montgomery, D. D.; Shigehara, K.; Tsuchida, E.; Anson, F. C. J . Am.
Chem. SOC.1984, 106, 7991.
(13) Espenscheid, M. W.; Martin, C. R. J. Electroanal. Chem. 1985, 188, 7,
I>.
(14) Montgomery, D. D.; Anson, F. C. J . Am. Chem. SOC.1985, 107, 3431. (15) Sharp, M.; Montgomery, D. D.; Anson, F. C. J . Electroanal. Chem. 1985, 194, 247.
0 1987 American Chemical Society
1520 The Journal of Physical Chemistry, Vol. 91, No. 6,1987
Inoue and Anson
TABLE I: Elemental Analysis of Polymers Employed in This Study C H
polymer" I
I1 111 IV V
c1
N
found
calcd
found
calcd
73.04 (11.72 76.18 (10.48 74.14 (9.64 76.58 (9.40 69.58 (8.59
73.42 11.92)b 77.49 10.81) 73.97 9.30) 78.97 9.36) 70.97 8.60)
6.23
6.16
7.21
7.32
1.69
7.95
8.15
8.44
8.10
8.25
found
2.72 (0.35 1.43 (0.17 2.62 (0.32
calcd
2.70 0.34)b 1.46 0.17) 2.44 0.30)
calcd
found 20.13 (3.23 11.05 (1.52
20.37 3.31)b 11.33 1.52)
6.32 (0.77 6.02 (0.74
6.52 0.77) 6.17 0.75)
"Polymer structures are given in Figure I . bThe values in parentheses are the ratios of C, N, and C1 to H. Monomer Units Present in the R m d m Copolymers Prepared in this Study A
C
B
-kCH2CH-k
I C H $H
-k
f.CH2CH+ t
CH2
CH2
Ct
0
I
I
I
'2"5
+ E
SCHZCHJ-
ye
X-
Copo Iy me r I
+
N(C2H40H)3
Composition AO 17 BO83
It
AO 23 6 0 4 5 CO 32
111
A068 DO32
( X - = Br)
IV
AO23 BO I3 CO 47 DO17
V
A017
(x- = CI-) (x- = c l - )
COM E035
Figure 1. Structures of the random copolymers examined in this study.
vinyl-2-methylimidazole), PVI, and poly(4-(vinylbenzyl)diethylamine), PVBDA, were prepared by conventional radical polymerization with AIBN and precipitated. The average molecular weights of the products were determined viscometrically or by gel permeation chromatography using commercially available polystyrene samples as calibration standards. The molecular weights obtained in kilodaltons were as follows: PVP, 760, 287, 203, 41, 13; PVI, 70; PVBDA, 86. Solutions of K,Fe(CN), were prepared from reagent grade salt immediately before use. Supporting electrolyte solutions consisted of 0.1 M sodium acetate adjusted to pH 4.5 with glacial acetic acid. Procedures. Polymer coatings were cast on the surface of glassy carbon electrodes (0.34 cm2) that had been freshly polished (0.3-wm alumina) and subsequently sonicated in pure water, by transferring to the surface 1 WLof 0.5 wt.% solutions of the polymers in methanol (copolymers 111, IV, V) or tetrahydrofuran (copolymers I and 11). Mixed coatings were prepared by applying separate aliquots of solutions of the homopolymer (PVP or PVI in methanol; PVBDA in tetrahydrofuran) and the copolymer to the electrode surface where they were carefully stirred with the tip of the microsyringe employed. The solvent was allowed to evaporate at room temperature for at least 30 min. The solutions of copolymer V in methanol were slightly turbid but yielded satisfactory coatings. Fe(CN):- anions were incorporated into
the coatings by immersion in 0.1 mM solutions of the anion in 0.1 M acetate buffer adjusted to pH 4.5, where the coated electrodes were cycled between -0.2 and +0.5 V until the peak currents had stabilized at their maximum value. The quantities incorporated were determined by coulometric assay immediately after the coated electrode was transferred to a pure supporting electrolyte solution. The charge passed was measured as the electrode potential was scanned at 50 mV s-I from -0.2 to +0.4 V where it was held until the flow of charge had decayed to background levels. Cyclic voltammograms were obtained with conventional procedures and instrumentation. Potentials were measured and are reported with respect to a sodium chloride saturated calomel electrode (SSCE). Solutions were prepared with distilled water that had been further purified by passage through a purification train (Barnsted Nanopure Organo-pure). Experiments were conducted at ambient laboratory temperatures (22 f 2 "C). Diffusion coefficients for Fe(CN)6e within the coatings were evaluated from the slopes of chronocoulometric charge-(time)Il2 plots as previously described.14 The coating thicknesses required in order to calculate diffusion coefficients were estimated, as before,I3 from manual micrometer measurements with electrodes coated with 20 to 100 times more polyelectrolyte than was used in the electrochemical experiments with the assumption that the thickness was a linear function of the quantity of polyelectrolyte employed. Transmission electron micrographs were obtained with a Phillips EM201 instrument. Copper minigrids for the electron microscope were coated with polymers and stained with IrC1,2- as described previo~sly.'~ The best micrographs resulted when about four times more polymer per cm2 was applied to the minigrids than was employed in the electrochemical experiments.
Results Copolymers Tested. Five related vinyl copolymers were examined in this study. The combinations and proportions of the pendant groups present in the random copolymers employed are shown in Figure 1 . Based on our previous r e s ~ l t s ~ ~the , ' ~set ,'~ of functional groups synthesized for this study encompassed both uncharged hydrophobic groups and positively charged hydrophilic groups. Coatings were cast from T H F or methanol solutions of the copolymers alone and from solutions containing mixtures of one of the copolymers and a homopolymer. The three homopolymers tested were poly(4-vinylpyridine) (PVP), poIy(4vinyl-2-methylimidazole) (PVI), and poly(4-(vinylbenzyl)diethylamine) (PBVDA). As before,', the experimental protocol involved casting coatings on polished glassy carbon electrodes, equilibrating the coatings with 0.1 mM Fe(CN)," in 0.1 M acetate buffer (pH 4.5) supporting electrolyte, and measuring the quantity of Fe(CN),+ retained by the coating immediately after its transfer to a pure supporting electrolyte and 4 5 min later. Such experiments provided a measure of the capacity of the coating to incorporate multiply-charged anions and to retain them for extended periods under conditions where they would all be lost from the coating if equilibrium prevailed. The mobilities of the retained ions within the coatings were measured in subsequent potential-step experiments to be described.
Copolymer and Composite Electrodes
w ,
The Journal of Physical Chemistry, Vol. 91, No. 6, 1987 1521
0.5 v
0.5V
Figure 3. cyclic voltammograms for FI?(CN)~~-/~incorporated in coatings prepared from copolymer 111 or copolymer IV alone and in
Figure 2. Cyclic voltammograms for Fe(CN)63-ld incorporated in coatings prepared from pure PVP and from mixtures of PVP with copolymer I or copolymer 11. (A) Copolymer I-PVP mixed coating in 0.1 mM Fe(CN)6d. Scans recorded every 5 min after the coating was transferred to the solution. The peak currents increase with each scan until a maximum is reached. (C) Repeat of A with copolymer 11-PVP mixed coating. (B,D) After the coatings were transferred to pure supporting electrolyte. Outer curve: immediately after transfer; inner curve: 45 min later. (E,F) Pure PVP coating before and after transfer to pure supporting electrolyte (0.1 M acetate buffer, pH 4.5). Scan rate: 100 mV s-l. Initial potential: -0.2 V vs. SSCE. The potential scale is the same for all curves. Each coating contained 11.6 pg cm-2 of PVP and (if present) 13 pg cm-2 of copolymer.
The qualitative differences in the behavior of the various coatings were readily apparent from cyclic voltammograms for the Fe(CN)6'/" couple incorporated in the coatings. Copolymers I and I1 (Figure 1) contain no charged groups and therefore yield coatings that do not incorporate Fe(CN)6". However, coatings prepared from mixtures of these copolymers with PVP exhibit anion-exchange capacities that are greater than those of coatings prepared from the same quantity of pure PVP. These features are evident in the voltammograms shown in Figure 2. Figure 2, A and C, shows the gradual incorporation of Fe(CN)," from a 0.1 mM solution by coatings consisting of a mixture of copolymer I or copolymer I1 with PVP. After the peak currents stopped increasing, the coatings were rinsed and transferred to a pure supporting electrolyte solution where voltammograms were recorded immediately and 45 min later (Figure 2, B and D). A small portion of the incorporated Fe(CN)64-is lost during the rinsing and transfer process as reflected in the smaller peak currents but the portion that remains is retained for extended periods. Repetition of these steps with coatings of pure PVP (Figure 2, E and F) produced smaller, shorter-lived responses primarily because of the greater solubility that resulted in lower stability of PVP coatings. Then gradual loss of PVP from coated electrodes could be observed visually by close inspection of the coating/solution interface. Copolymers 111, IV, and V contain positively charged quaternary ammonium groups so that coatings prepared from these
mixtures with PVP. (A, B) copolymer 111; (C, D) copolymer IV; (E, F) copolymer 111-PVP mixed coating; (G, H) copolymer IV-PVP mixed coating. A, C, E, and G were recorded with coatings equilibrated in 0.1 mM Fe(CN),& solution. B, D, F, and H were recorded after transfer to pure supporting electrolyte. Outer curve: immediately after transfer; inner curve: 45 min later. Each coating contained 1 1.6 pg cm-2 of each component. Other conditions as in Figure 2. copolymers exhibit anion-exchange behavior. A larger fraction of the Fe(CN)6" ions exchanged into such coatings is retained upon transfer to pure supporting electrolyte solutions as can be judged from the cyclic voltammograms for copolymers I11 and IV in Figure 3 (curves A, B, C, and D). Coatings prepared from mixtures of copolymer 111 or IV also exhibit attractive ion-exchange capacities and counterion retention properties that are far superior to those of coatings prepared from the same quantities of pure PVP (Table 11). However, copolymer 11, which carries no positively charged groups, is more effective than copolymer I11 in producing coatings with enhanced ion-exchange capacities when mixed with PVP (see Table I1 and compare Figures 2D and 3F). Coatings prepared from mixtures of copolymer IV or V with PVP exhibited properties that were very similar to tbose reported previously for composite coatings in which a somewhat different copolymer was emp10yed.l~ Quantitative comparisons of the extent of Fe(CN)64-incorporation and its retention in pure supporting electrolyte are given in Table I1 for several pure and mixed coatings. Mixed coatings were also prepared with the homopolymers PVI and PVBDA and with PVP samples of increasing molecular weight. In these mixed coatings, the largest incorporation of Fe(CN)," anions and the best retentions resulted with coatings consisting of mixtures of PVP with copolymer IV or V. Protonated PVI and PVBDA appeared to be lost from mixed coatings more readily than did PVP which decreased the stability of such coatings. Effects of Coating Composition and PVP Molecular Weight. Increasing the molecular weight of the PVP used to prepare mixed coatings generally improved the coating performance but increases above ca. lo5 daltons produced relatively small effects (Figure 4A). The proportions of PVP and copolymer V used to prepare the mixed coatings influenced the total quantity of Fe(CN)64incorporated more than its retention after transfer to pure supporting electrolyte (Figure 4B). The presence of copolymer V in the coating was enough to ensure that about 80% of the Fe-
1522 The Journal of Physical Chemistry, Vol. 91, No. 6. 1987 TABLE 11: Extent of Incorporation and Retention of Fe(CN),& by Polyelectrolyte Coatings on Glassy Carbon Electrodes
I I1
0 0
111 IV V PVP PVI PVBDA I PVPf I1 PVP 111 PVP 1v PVP v PVP v PVI V PVRDA
2.24 1.21 2.02 1.1 0.8 7.04 1.1 1.1 3.3 2.3 3.1 2.8 9.06
+ + + +
+ + +
0 0 0.367 0.261 0.468 0.3 14 0.197 0.106 0.239 0.632 0.264 1.64 1.79 0.728 0.28 1
0.66 0.86 0.88 0.1 1 0.07 0.06 0.09 0.23 0.08 0.53 0.54 0.23 0.12
0.9 I 0.92 0.85 0.5 1 0.75 0.63 0.55 0.70 0.68 0.88 0.83 0.87 0.8 I
"Structures of polymers I-V are given in Figure 1. PVP = poly(4vinylpyridine). (MW = 28.7 X IO4 daltons); PVI = poly(N-vinyl-2methylimidazole) ( M W = 7.0 X IO' daltons). PVBDA = poly(4-(vinylbenzy1)diethylamine) (MW = 8.6 X lo4 daltons). hQuantity of positively charged groups in the polymer deposited on the electrode.
