14016
J. Phys. Chem. 1996, 100, 14016-14021
Reversible Electron Transfer Reaction between Polyaniline and Thiol/Disulfide Couples Tetsu Tatsuma, Hiroshi Matsui, Eiichi Shouji, and Noboru Oyama* Department of Applied Chemistry, Faculty of Technology, Tokyo UniVersity of Agriculture and Technology, Naka-cho, Koganei, Tokyo 184, Japan ReceiVed: March 13, 1996X
Reversible electron transfer was observed between polyaniline (PAn) and thiol/disulfide couples of 2,5-dimercapto-1,3,4-thiadiazole (DMcT), 2-mercapto-5-methyl-1,3,4-thiadiazole, 2-mercaptopyridine, and thiophenol. Thus, PAn can be used as a molecular current collector for those insulating organosulfur compounds, which are promising high-capacity energy storage materials. Among those couples, DMcT exhibits the fastest reversible electron transfer. Electron transfer from other aromatic and aliphatic thiols to oxidized PAn is also observed. Effects of protons on the reactions and reaction kinetics are discussed.
Introduction We have studied organosulfur polymer batteries1-5 as high energy density rechargeable batteries. Organosulfur compounds having two or more mercapto groups undergo oxidative polymerization, and the generated polydisulfide can be reduced to give the parent monomer (eq 1), so that such compounds have attracted considerable attention as high energy density cathode materials for lithium batteries.6 However, those compounds are
nHS-R-SH a H-(-S-R-S-)n-H + 2(n - 1)e- + 2(n - 1)H+ (1) not practical as they are because their redox reaction is normally slow at most electrode materials at room temperature, and they are not electrically conducting. To overcome these problems, we have mixed 2,5-dimercapto-1,3,4,-thiadiazole (DMcT, Figure 1b), an organosulfur compound, with polyaniline (PAn, Figure 1c) at a molecular level. As a result, the composite cathode coupled with a lithium anode exhibited a gravimetric energy density of over 600 (mW h)/(g-cathode).1 We have speculated that PAn, which is a conducting polymer, functions as an electron mediator for DMcT; PAn may work not only as a cathode active material but also as a molecular current collector. As a molecular current collector, PAn should oxidize DMcT in the charging (oxidation) process and reduce oxidized DMcT (DMcT oligomers and polymers, Figure 1a) in the discharging (reduction) process. That is, reduced PAn should donate electrons to the oxidized DMcT, and oxidized PAn should accept electrons from DMcT (Figure 1, reaction A). However, the reversible electron transfer between DMcT and PAn has not yet been certified. In our previous work,8 we found that an electrochemically inactivated PAn, which is in the oxidized and deprotonated form (Figure 1e), can be reactivated by DMcT and that the reactivated PAn is in the reduced form (Figure 1, reaction D). That is, the inactivated PAn can be reduced by DMcT. In the present work, we study the electron transfer from DMcT to the active (protonated) oxidized PAn (Figure 1d) and that from the reduced PAn to the DMcT dimer (bis(2-mercapto1,3,4-thiadiazolyl) 5,5′-disulfide, DMcT2) (Figure 1, reaction A) in propylene carbonate (PC) by spectroelectrochemical means. Although some interaction between PAn and DMcT may play an important role in those reactions, we discuss whether the reversible electron transfer is possible or not from the viewpoint of thermodynamics. 2-Mercapto-5-methyl-1,3,4X
Abstract published in AdVance ACS Abstracts, July 15, 1996.
S0022-3654(96)00774-5 CCC: $12.00
Figure 1. Possible electron transfer reactions of PAn and DMcT.
thiadiazole (McMT) and its dimer (bis(2-methyl-1,3,4-thiadiazolyl) 5,5′-disulfide, McMT2) are also examined. Furthermore, effects of an acid on the electron transfer reactions are investigated because it is known that the electrochemistry of DMcT/DMcT2 and McMT/McMT2 couples is deeply concerned with protonation of these compounds.9 Some other aromatic and aliphatic thiol/disulfide couples are examined as well to learn which couple is most suitable for the reversible electron exchange with PAn and thereby for an organosulfur cathode. Apparent kinetics of these reactions is also discussed. Experimental Section Materials. Partially oxidized and partially deprotonated PAn was obtained from Nitto Denko (Japan). DMcT, 2-mercaptopyridine (PySH), trithiocyanuric acid (TTCA), n-dodecylmercaptan (C12SH), diphenyl disulfide (PhSSPh), and 2,2′dipyridyl disulfide (PySSPy) were purchased from Tokyo Kasei (Japan). McMT, thiophenol (PhSH), and pentaerythritol tetrakis(2-mercaptoacetate) (PTMA) were obtained from Wako Pure Chemical (Japan), Kanto Chemical (Japan), and Nisso Yuka (Japan), respectively. DMcT2 and McMT2 were synthesized and purified as described elsewhere.9 Preparation of Polyaniline-Coated Electrodes. A 25-µL aliquot of N-methyl-2-pyrrolidone (NMP) containing PAn (1 mM as a monomer unit) was cast on an indium-tin oxide-coated glass plate (ITO electrode, 7 × 19 mm, 10 Ω/sq, Matsuzaki Shinku, Japan), and NMP was evaporated under vacuum at 80 © 1996 American Chemical Society
Reversible Electron Transfer Reaction
Figure 2. Visible spectra of the electrochemically oxidized PAn film: immediately (a) or 4 h (b) after the immersion in 0.1 M LiClO4/PC.
°C for 90 min. The thickness of the resulting film was 0.02 µm, unless otherwise noted. Surface coverage (monomer unit) should be 1.9 × 10-8 mol/cm2. Evaluated films were subjected to acid treatment. Instruments. The electrode potential was controlled using a potentiostat (PS-07, Toho Giken, Japan) with a silver wire as a pseudoreference electrode and a platinum wire as a counter electrode. UV-visible spectra were obtained using a UV-vis spectrophotometer (U-Best-55, JASCO, Japan). Procedure. The experimental procedure for reduction of electrochemically oxidized PAn by a thiol is as follows. First a PAn film was electrochemically reduced at -480 mV vs SSCE in 0.1 M LiClO4/PC, and a visible spectrum was obtained. Then the film was oxidized at +720 mV vs SSCE, and a visible spectrum was obtained. The surface density of the electrochemically active sites calculated from the charge passed was ca. 5 × 10-9 mol/cm2 for the PAn-coated electrodes. The PAncoated electrode was then transferred to PC containing a thiol (10 mL). In the course of the soaking, the electrode was transferred to 0.1 M LiClO4/PC containing no thiol, and a visible spectrum was obtained at regular (or irregular) time intervals. After a steady state was reached, the electrochemical activity of the film was examined again in 0.1 M LiClO4/PC. Spectroscopic data for reaction kinetics were obtained in situ in the presence of a thiol. Oxidation processes of the electrochemically reduced PAn by disulfides were examined in a similar procedure to that for the reduction by thiols. All experiments were carried out under nitrogen. Results Spectroscopic Behavior of PAn. Before we studied the electron transfer between PAn and thiol/disulfide couples, the spectroscopic behavior of electrochemically oxidized and reduced PAn in the absence of organosulfur compounds was examined. PAn was electrochemically oxidized at +720 mV vs SSCE in 0.1 M LiClO4/PC and was allowed to stand at open circuit. As shown in Figure 2, the broad absorption band spread over the wavelength range examined (400-800 nm, curve a) gradually decreased, and the broad peak at around 600 nm (curve b) increased instead. The former band corresponds to absorption of protonated quinone-diimine structure (Figure 1d).8,10 The latter peak reflects deprotonated quinone-diimine structure (Figure 1e).8,10 That is, the oxidized PAn is gradually deprotonated in the solution because the pKa value of the oxidized PAn (N sites) is relatively low (2-3)11 and the solution contains no proton sources. Since visible absorption of the reduced PAn is much weaker than that of the oxidized PAn, we can determine whether a PAn film is reduced or oxidized from its visible
J. Phys. Chem., Vol. 100, No. 33, 1996 14017
Figure 3. Visible spectra of a PAn film on the ITO electrode obtained in 0.1 M LiClO4/PC. PAn was electrochemically reduced at -480 mV vs SSCE (a), then oxidized at +720 mV (b), and finally soaked in 7.5 mM DMcT/PC for 2 min (c).
Figure 4. Time courses of absorbance at 600 nm for a PAn film on the ITO electrode in 1 (a) or 0.3 mM (b) DMcT/PC.
spectrum. The kinetics of electron transfer from thiols to the oxidized PAn is monitored in this work at 600 nm, at which absorbance is not decreased (but increased to some extent) by deprotonation; the absorbance is decreased only by reduction. The oxidized PAn was not deprotonated in the presence of 25 mM methanesulfonic acid. As to PAn reduced at -480 mV vs SSCE in 0.1 M LiClO4/ PC, ca. 10% of the PAn was oxidized in 720 min in the presence of 25 mM methanesulfonic acid even under nitrogen. PAn must have been oxidized by residual oxygen. Reduction of the Oxidized PAn by DMcT. Electron transfer from DMcT to the electrochemically oxidized PAn was examined. The experimental procedure is described in the Experimental Section. Visible spectra were obtained for the electrochemically reduced (Figure 3a) and oxidized (Figure 3b) PAn films. Then, the oxidized film was immersed in 7.5 mM DMcT/PC, and the spectrum was obtained 2 min after, in the absence of DMcT (Figure 3c). The potential of the PAn-coated ITO electrode was not controlled during the soaking. The spectrum was unchanged by further immersion in the DMcT solution. As can be seen, the spectrum obtained after the treatment with DMcT (Figure 3c) was similar to that of the electrochemically reduced PAn (Figure 3a). This suggests that the treated PAn is almost the same as the reduced form. PAn was still electrochemically active after the treatment with DMcT. Since the reaction was fast at 7.5 mM, the kinetics was examined at lower DMcT concentrations (1 and 0.3 mM). Absorbance at 600 nm was monitored during the reaction (Figure 4). The reaction was slower at a lower DMcT concentration. The kinetics is discussed below. Electron transfer from DMcT (7.5 mM) to the oxidized PAn was observed even in the presence of 25 mM methanesulfonic acid, though the reaction was a little bit slower than that in its
14018 J. Phys. Chem., Vol. 100, No. 33, 1996
Figure 5. Time courses of absorbance at 600 nm for a PAn film on the ITO electrode in 5 (a) or 0.3 mM (b) DMcT/PC containing 25 mM methanesulfonic acid.
absence. Then, the kinetics was examined at lower DMcT concentrations (5-0.3 mM) in the presence of the acid. As a result, it was found that the PAn reduction was decelerated after half of the oxidized PAn was reduced (Figure 5). This retardation was more prominent at a lower DMcT concentration. Thus, an excess amount of DMcT reduces the oxidized PAn both in the absence and in the presence of the acid. In other words, electron transfer from DMcT to the oxidized PAn is possible. However, the reducing ability of DMcT in the presence of acid is less than that in its absence. Reduction of the Oxidized PAn by McMT. Similar experiments were conducted using McMT in place of DMcT. As a result, it was found that the excess amount of McMT can reduce the electrochemically oxidized PAn. However, the kinetics was much slower than that of DMcT. At 10 and 3 mM McMT, reaction was completed in 30 and 100 min, respectively. In the presence of 25 mM methanesulfonic acid, PAn reduction was retarded after about a half of the oxidized PAn was reduced. Even though the McMT concentration was high (30 mM), the retardation was much more significant than in the case of DMcT; 50% reduction was completed in a few minutes, but a portion of the oxidized PAn (12 h at 5 mM DMcT2) than did the reduction of PAn by monomeric DMcT. Oxidation of the Reduced PAn by McMT Dimer. As to McMT2, oxidation of PAn was possible both in the absence and in the presence (Figure 6) of methanesulfonic acid (25 mM). However, 10-20% of the active PAn was left as the reduced form even after about 30 min had passed. The reaction in the presence of the acid was faster than that in its absence. In either
Tatsuma et al.
Figure 6. Visible spectra of a PAn film on the ITO electrode obtained in 0.1 M LiClO4/PC. PAn was electrochemically oxidized at +720 mV vs SSCE (a), then reduced at -480 mV (b), and finally soaked in 10 mM McMT2/PC containing 25 mM methanesulfonic acid for 30 min (c).
case, reaction was faster than that for DMcT2. PAn that was oxidized by McMT2 in the absence of acid was the deprotonated form. However, the spectrum for the protonated, oxidized PAn was obtained after methanesulfonic acid (final concentration was 25 mM) was added. Electron Transfer between PAn and Other Thiol/Disulfide Couples. Electron transfer reactions to the oxidized PAn from other thiols, PhSH, PySH, TTCA, C12SH, and PTMA, were also examined. All these thiols reduced the oxidized PAn. The kinetics of these reactions is discussed below. We examined electron transfer reactions from the reduced PAn to disulfides, PhSSPh (the oxidized form of PhSH) and PySSPy (the oxidized form of PySH) as well. Those experiments were performed in the presence of 25 mM methanesulfonic acid to maintain the oxidized PAn in the protonated form. Those disulfides oxidized about 80% of the reduced PAn, though the reaction was slow. The kinetics is discussed below. Discussion Reversible Electron Transfer Reaction between PAn and Mercaptothiadiazoles. The results obtained for the electron transfer reactions between PAn and DMcT/DMcT2 and McMT/ McMT2 couples are summarized as follows (eqs 2-5). In the absence of methanesulfonic acid:
PAn(ox) + DMcT f PAn(red) + DMcT2
(2)
In the presence of 25 mM methanesulfonic acid:
PAn(ox) + DMcT a PAn(red) + DMcT2
(3)
In the absence of methanesulfonic acid:
PAn(ox) + McMT a PAn(red) + McMT2
(4)
In the presence of 25 mM methanesulfonic acid:
PAn(ox) + McMT a PAn(red) + McMT2
(5)
That is, reversible electron transfer reaction between PAn and DMcT/DMcT2 is possible in the presence of 25 mM methanesulfonic acid, and that between PAn and McMT/McMT2 is possible both in the presence and in the absence of the acid. Electron Transfer from DMcT to the Oxidized PAn. As mentioned above, electron transfer from DMcT to the oxidized PAn in the presence of methanesulfonic acid was decelerated after half of the oxidized PAn was reduced. However, this retardation was not observed in the absence of acid. Therefore,
Reversible Electron Transfer Reaction
J. Phys. Chem., Vol. 100, No. 33, 1996 14019
Figure 7. Potential diagram of DMcT, McMT, DMcT2, and McMT2 in the presence and absence of 25 mM methanesulfonic acid (MSA). A thermodynamic reduction and oxidation potential should exist in the region indicated with a bar, the top and bottom of which correspond to reduction and oxidation peak potentials observed in cyclic voltammetry for each compound (in 0.1 M LiClO4/PC at 20 mV/s). White bars correspond to a species of low concentration (i.e., peaks were small). Cyclic voltammogram of PAn film (50 mV/s) is also indicated.
the behavior should be related to the protonation of DMcT. Dissociation of one of the two protons of DMcT generates an anion, DMcT-. The energy level of the highest occupied molecular orbital (HOMO) of DMcT- is higher than that of neutral DMcT.9 In cyclic voltammetry using a glassy carbon electrode (20 mV/s), deprotonated DMcT was not observed in the presence of acid, though DMcT was deprotonated to some extent (
