J . Phys. Chem. 1989, 93, 1637-1642 mechanism in harmony with the Stark spectrum30 or the temperature dependence of the charge ~eparati0n.l~ We still find that this route cannot explain the fast charge separation. For the excitonic coupling between P* and BL’HL- a delocalization between the LUMOs of B L and HL is needed. This mechanism becomes particular effective if the energies of BL’HL- and BL* are close in energy. We took the exchange coupling J of the radical pair P’HL- as evidence for orbital delocalization. We established the interrelation, (14), between the exchange-coupling J and the square of the combined electron-transfer coupling V,. This relation may be tested for artificially linked donor-acceptor systems with variable bridges, where the coupling may not be so easily affected by relaxation processes. It seems that we have reached a point where it becomes important to use further improved structural data and to incorporate more elements of the protein surrounding such as water molecules (30) Scherer, P. 0. J.; Fischer, S . F. Chem. Phys. Letf. 1986, 131, 153.
1637
to predict the orbital energy spacings more accurately. Also a self-consistenttreatment of the partial delocalization in conjunction with the nuclear reorgani~ation~’-~* should be included for the description of this class of rapid charge-separation processes. In a forthcoming paper33we will show that the Stark spectrum and the spectrum for linear dichroism can be well described if the orbital energies are properly adjusted. Acknowledgment. We thank Dr. Plato for fruitful discussions that helped us to develop an INDO program similar to the one he is using. We also thank Dr. Deisenhofer, who provided us with the structural data. Finally, we acknowledge support by the Deutsche Forschungsgemeinschaft SFB 143 C2. (31) Fischer, S . F.; Nussbaum, I.; Scherer, P. 0. J. In Antennas and Reaction Centers of Photosynthetic Bacteria; Michel-Beyerle, M. E., Ed.; Springer-Verlag: Berlin, 1985; p 256. (32) Knapp, E. W.; Fischer, S. F. J . Chem. Phys., in press. (33) Scherer, P. 0. J.; Fischer, S. F. Chem. Phys., special issue, edited by D. Wiersma, in preparation.
Doping and Anion-Exchange Thermochemistry of Electrochemically Prepared Polypyrrole Larry S. Curtin, Marc McEllistrem, and William J. Pietro* Department of Chemistry, University of Wisconsin-Madison, (Received: May 31, 1988)
Madison, Wisconsin 53706
Solid-state voltaic cells having silver anodes and polypyrrole cathodes were employed to study the thermochemistryof halogen doping of the electrically conductive polymer, as well as the thermochemistry of dopant exchange with aqueous solvated anions. Moderately large enthalpic driving forces exist for the exchange of perchlorate in polypyrrole for halide in the aqueous phase. In addition, the activities and activity coefficients for halides in halogen-doped polypyrroles have been determined.
Within the past decade, polypyrrole (l),has emerged to be one
._ k
1
of the most important and extensively studied electrically conductive polymers.’ Its convenient electrochemical preparation affords a variety of anionically “doped” materials possessing a wide range of electrical conductivities.2 Moreover, these polymers are electroactive and can be electrochemically “switched” between the oxidized, conductive state and the neutral, insulating state. This property has been exploited to produce novel ion-gate membranes3 and microelectrochemical diode^.^ (1) For comprehensive reviews of the preparation and properties of polypyrrole see: (a) Diaz, A. F.; Kanazawa, K. K. In Extended Linear Chain Compounds; Miller, J., Ed.; Plenum: New York, 1983; pp 417-441. (b) Reynolds, J. R. J . Mol. Electron. 1986, 2, 1-21. (2) (a) Dall’Olio, A.; Drascola, Y.; Varacca, V.; Bocchi, V. C. R. Seances Acad. Sci., Ser. C. 1968, 267, 433-435. (b) Diaz, A. F. Chem. Scr. 1981, 17, 145-148. (c) Murthy, A. S.N.; Shri, P.; Reddy, K. S . J. Mater. Sci. Lett. 1984, 3,745-747. (d) Asavapiriyanont, S.;Chandler, C. K.; Gunwardena, G. A.; Pletcher, D. J. Elecfroanal. Chem. 1984, 177, 229-244. (e) Wernet, W.; Monkenbush, M.; Wegner, G. Macromol. Chem., Rapid Commun. 1984, 5, 157-164. (f) Wynne, K. J.; Street, G. B. Macromolecules 1985, 18, 2361-2368. (g) Warren, L. F.; Anderson, D. P. J . Elecrrochem. SOC.1987, 134, 101-105. (3) Burgmayer, P.; Murray, R. W. J. Am. Chem. SOC. 1982, 104, 6139-61 40.
0022-3654/89/2093-1637$01.50/0
We,5 and others,6 have recently discovered that the anions incorporated into the polypyrrole matrix during electrochemical film growth can be subsequently replaced with a large number of different anions by simply soaking the film in an aqueous or acetonitrile solution of the desired anion. We have demonstrated a potential use for this phenomenon by preparing electrochemically grown poly(pyrro1e iodide), a previously unknown materiaLs The iodide-doped polymer cannot be directly prepared by the conventional electrochemical method since the potential required for the oxidation of pyrrole is higher than the 12/1- redox couple. This anion-exchange phenomenon is thus very useful for the incorporation of potentially interesting electroactive anions into polypyrrole that would be otherwise inaccessible by direct electrochemical preparation:
w
Jn 0sysq.0s2sq
The concurrent incorporation of anions during film growth, as well as the facile anion-exchange process, indicates that the anions are fairly mobile within polypyrrole and only loosely interact with (4) Kittlesen, G. P.; White, H. 1985, 107, 7373-7380.
S.;Wrighton, M.S.J . Am. Chem. SOC.
( 5 ) Curtin, L. S.; Komplin, G. C.; Pietro, W. J. J . Phys. Chem. 1988, 92, 12-13. (6) Schlenoff, J. B.; Chien, J. C. W. J. Am. Chem. Soc. 1987, 109, 6269-6274.
0 1989 American Chemical Society
Curtin et al.
1638 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
the polymer matrix. In this contribution, we report solid-state electrochemical measurements, from which the heats of doping and thermochemistry of anion exchange in polypyrrole films can be derived. Solid-state galvanic cells have previously been employed to study the formation thermodynamics of silver halides,',* nonstoichiometric phases in silver chalcogenides? electron donor-acceptor complexes with iodine,1° and organic and inorganic triiodides.s We now describe the construction and study of solid-state galvanic cells employing doped polypyrrole cathodes. The method described herein is completely general and can be utilized to arrive at similar thermodynamic parameters for many other electrically conductive polymers.
Figure 1. Three-layer solid-state galvanic cell employing polypyrrole calhodes.
Experimental Section
Materials. Pyrrole (Aldrich, 98%) was purified by fractional distillation under reduced pressure and stored in the dark at -15 'C. Tetrabutylammonium perchlorate was purchased from Southwestern Analytical (electrometric grade), recrystallized twice from ethanol, and dried in vacuo, a t 65 OC before use. All inorganic potassium salts were recrystallized twice from triply distilled water and dried in vacuo at 100 "C before use. Acetonitrile (Aldrich, reagent grade) was purified by predrying over Linde 4-.& molecular sieves followed by percolation through a column of freshly activated alumina. All aqueous solutions were prepared in triply distilled, deionized water. y-AgI used in the construction of solid-state galvanic cells was freshly prepared by rapid precipitation from AgNO,(aq) upon addition of KI(aq). This procedure results in predominantly the y form of silver iodide." The Agl was thoroughly washed with triply distilled water, dried a t 120 OC in vacuo, and stored in the dark. Finely powdered silver was prepared by reduction of AgN03(aq) with elemental copper. AgCIO, was prepared by the action of 50% perchloric acid on freshly prepared silver hydroxide. The salt was recrystallized from dilute perchloric acid, dried a t 120 OC in vacuo, and stored in the dark in a desiccator. Preporation of Polypyrrole Films. Polypyrrole films were prepared by the method of Diaz and co-workers.l2 In brief, pyrrole was oxidatively polymerized a t a constant current density of 1 mA/cm' onto a 19.6-cm2 platinum disk electrode from a deoxygenated 1% aqueous/99% acetonitrile solution, 0.20 M in pyrrole, and 0.10 M in n-Bu,NCIO,. The electrode surface was carefully polished to a mirror finish so that the films could be easily removed with a minimal amount of mechanical damage. A 4-cm2 platinum mesh was used as the counter electrode. The potential was typically 1.4 V vs a silver pseudoreference electrode and remained fairly constant throughout the polymerization. Typically 100-200 mg of polypyrrole was deposited in a single film. The films were then washed with acetonitrile in a Soxhlet apparatus for 24 h. The perchlorate anion was then exchanged for Cl-, Br; or 1- by gently swirling the polymer film in a 1.0 M aqueous solution of the potassium salt of the desired anion a t room temperature for 24 h. The films were then Soxhlet extracted for 24 h with water and dried in vacuo at 80 O C overnight. Routine elemental microanalyses were performed by Galbraith Laboratories, Knoxville, TN. Solid-State Galuanic Cells. All solid-state galvanic cells used in this study were prepared in a similar fashion; the poly(pyrrole perchlorate) cell will be described as a representative example. All of the procedures involving the handling of y-AgI or any composition containing y-AgI were performed under subdued light. The cathode was prepared by thoroughly grinding together 100 mg of poly(pyrro1e perchlorate), 250 mg of y-Agl, 50 mg of AgCIO,. and 100 mg of powdered graphite (to increase the electronic conductivity of the mixture) in an agate mortar and (7) Reinhold. H.2.Anorg. Chem. 1928. 171, 181-230. (8) Topi. L. E. Inorg. Chem. 1968, 7, 451454. (9) Kach. W.; Rickert, H.; Schlectriemen. G. Solid Slore Ionics 1983. 9+10, 1397-1204. (IO) McKcchnie, J. S.; Turner. L. D. S.;Vincent, C. A. J . Chem. Ther-
modynnm. 1979, I I , 1189-1195.
( I I ) Takahashi. T.; Kuwabara, K.; Vamamota, 0.3. Electroehem. Sm. 1969, 116, 357. (12) Diar, A. F.; Hall. B. IBM J. Res. De". 1983. 27, 342.
llrY
, , r
~
,,sLi
Figure 2. Detail of cell housing assembly.
pestle, followed by pulverization in a high-speed mechanical shaker. The mixture was then pressed under 5.2 X I@ kPa for 2 min and reground to a fine powder in an agate mortar and pestle. This press and grind procedure was repeated twice. The cathode mixture was then pressed into a pellet I 3 mm in diameter and about 2 mm thick under 2.0 X IO4 kPa. y-AgI (400 mg) was uniformly placed on top of the cathode, and the materials were pressed under 5.2 X IO4 kPa for 5 min, resulting in a composite pellet. The anode consisted of an intimate mixture of 250 mg of finely powdered silver and 500 mg of y-AgI. This mixture was also formed into a composite pellet with pure y-AgI in a manner similar to that described above. The cathode and anode composite pellets were pressed separately, otherwise an electrical short circuit would develop through the pellet press resulting in some degradation of the galvanic cell. The two composite pellets were then placed together in a Teflon cylinder with the two y-AgI sides contacting each other, forming a three-layer solid-state galvanic cell (Figure 1). Contact was made with either side by platinum faced, spring-loaded, brass plungers. This entire assembly was placed in a nitrogen-purged aluminum housing (Figure 2) and submerged in a thermostated water bath. A copper-constantan differential thermocouple was mounted in the housing close to the cell to determine when thermal equilibrium was attained. The cell EMF was monitored as a function of temperature between 0 and 65 OC with a Keithley 617 electrometer having an input impedance of IO" Q. All cables connecting the high-impedance electrometer to the cell were low-noise graphite-sheathed triaxial design, and the cell housing was enclosed in a Faraday cage. The potentials of these cells are somewhat unstable when first prepared, drifting by about IO mV/day. Sufficient stability is eventually obtained after about 1 month a t room temperature. We have found, however, that a high degree of stability can be achieved by annealing the cells a t 80 'C overnight while pressed in the cell holder. Typical EMF drifts after such treatment are less than 100 pV/day. We attribute the initial instability to poor interfacial contact between the various materials within the cell. Annealing under pressure may cause the silver iodide electrolyte to cold-flow. affording enhanced electronic and ionic contact a t the interfaces.
-
Results and Discussion
Composition of Anion-Exchanged Polypyrrole Films. The compositionsof the polypyrrole films were determined by elemental analyses and are presented in Table I. Polypyrrole electrochemically polymerized on platinum always contains a significant amount of oxygen in the material. This has been observed for
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1639
Anion-Exchange Thermochemistry in Polypyrrole TABLE I: Composition of Anion-Exchanged Polypyrrole Film
stoichiometr? anion Clod-
c1B rI-
Cb
H
N
4 4 4 4
3.5 3.6 3.3 3.1
0.98 0.98 1.00 0.96
0' 0.50 0.50 0.45 0.49
Y
2
0.24d 0.23 0.15 0.16
0.00 0.05 0.02
TABLE 11: Linear Least-Squares Parameters for the Temwrature-Deoendent EMF of AelAeII(P.X.). Galvanic Cells"
18.70 f 0.22 294.0 f 0.20 399.6 f 0.08 521.5 f 0.03
Clod-
c1BrI-
-0.236 f 0.007 0.189 f 0.007 0.121 f 0.003 0.060 k 8.3 X lo4
+ (dE/dT)(T - 273) were fit to
"Lines of the form E(T) = E2,,' the temperature-dependent EMFs. bCarbon stoichiometry always defined as 4. CExcessoxygen (not included in anion). dunexchanged sample. polypyrrole films grown under a variety of different conditions. Poly(pyrro1e anion), where the anions are tosylate, sulfate, nitrate, perchlorate, and ferricyanide have been shown to contain between 0.40 and 0.84 mol equiv of excess (not associated with the anion) oxygen per equivalent of pyrrole m0n0mer.l~ In addition, these same investigators have found between 0.24 and 0.54 mol equiv of excess hydrogen. Interestingly, it has recently been demonstrated that poly(pyrro1e tosylate) films grown on pyrolytic carbon electrodes at high voltage conditions (3 V) at 0 O C do not contain significant amounts of excess oxygen, even when grown in open air.2f Wynne and Street have also shown that poly(pyrro1e tosylate) and poly(pyrro1e tetrafluoroborate) are somewhat hygroscopic, slowly adsorbing water from the atmosphere.2f Although this may account for some of the excess hydrogen and oxygen, our samples were carefully dried in a vacuum oven, sealed, and shipped under dry argon, and the elemental analyses were performed with standard inert-atmosphere conditions. We feel that much of the excess hydrogen may be a result of short polypyrrole oligomers existing in the film. The ideal 3:4 hydrogen to carbon ratio will be realized only in the infinite polymer; any short chains will tend to increase this ratio. We discount the possibility that the excess oxygen is due to intercalation of molecular oxygen into the polymer matrix, as has been previously asserted,I3 since intercalated oxygen will certainly be removed under vacuum. We observe no change in the oxygen content of poly(pyrro1e perchlorate) films upon prolonged exposure to dynamic vacuum a t 80 "C. Anion exchange with chloride and bromide is straightforward. Whereas chloride completely replaced perchlorate, bromide was observed to induce only partial replacement (about 70%) under the described conditions. We have previously observed partial replacement of perchlorate in polypyrrole by a number of different anions.5 Elemental analysis shows that polypyrrole incorporates only trace amounts of the cation present in the exchange solution. Exchange with iodide is apparently anomalous. It appears that some of the cationic charge within the polymer is lost during the exchange process. One possible explanation is that polypyrrole in the +0.24 oxidation state is sufficiently oxidizing to convert iodide to molecular iodine, which subsequently dissolves in the aqueous KI solution. The conversion continues until the polypyrrole has been reduced to the +0.18 oxidation state. Further evidence for this hypothesis is derived from the instantaneous formation of the characteristic starch-triiodide complex upon exposure of starch to the supernatant KI solution, after the latter has been exposed to poly(pyrro1e perchlorate). Neither 1.0 M aqueous KI or KI/KC104 solutions form this characteristic complex with starch. Solid-state Galvanic Measurements. Solid-state galvanic cells employing silver anodes and solid-state silver ion conductors have been applied to investigate a wide variety of thermodynamic phenomenz for many decades. Reinhold introduced the first such cell in 1928 to accurately determine the heats of formation of AgCl(s) and AgBr(s).' Solid-state galvanic cells have been more recently used to study the enthalpies and free energies of formation of various inorganic triiodides6 and organic charge-transfer complexes,I0transport processes in cY-Ag,Se: stoichiometries of various (13) Qian, R.; Qiu, J. Polym. J . 1987, 19, 157-172.
0.996 0.995 0.998 0.987
phases in phenothiazinium iodide,I4 and the binding energy of nickel phthaloycanine iodide-a molecular electronic conductor." In this study, solid-state galvanic cells of the form Ag/AgI/ (P-X,), were fabricated, and their EMF/temperature profiles were recorded. E M F is produced in such a cell by the combination of the electrochemical half-reactions nyAg (P-X,),
-
+ nye-; anode half-reaction nyX- + P,; cathode half-reaction
nyAg+
+ nye-
-
and the interfacial reaction nyAg+
+ nyX-
-
nyAgX
The measured cell E M F is thus the potential for the net reaction nyAg
+ (P-X,),
-.+
nyAgX
+ P,
y-AgI is an electronic insulator; however, it is a very good Ag' ion conductor over the temperature range studied and acts as a solid electrolyte in the cell. Because all of the reactants are chemically pure solids in their standard states, the measured cell potential is identically the Eo for reaction 1. In practice, the voltage measurements are made at essentially zero current so that reaction 1 does not actually occur within the cell. In this way, the E M F of the cell may be continuously monitored over very long periods of time with negligible drainage or polarization drop effects. The free energy of reaction 1 is related to its Eo by AGO = -nyFEo
If we define the AGO as the reaction free energy per mole of pyrrole monomer units, then AGO = -yFEo
(2)
The temperature dependence dAGo/dT = -yF d E " / d T and is equal to the negative of the reaction entropy dE ASo=yFdT
(3)
The reaction enthalpy is thus AHo = AGO
+ TAS"
(4)
Hence, by studying the temperature-dependent E M F of the cell, one can derive all three thermodynamic quantities for reaction 1. All solid-state galvanic cells employing polypyrrole cathodes used in this study exhibited a linear E M F dependence on temperature, with correlation coefficients, R , of better than 0.98. These data are shown in Figure 3, and the linear least-squares analyses for all of the systems studied appear in Table 11. Least-squares analyses were performed by using an iterative linear regression algorithm that accounts for uncertainties in both the abscissa and ordinate. Thermodynamic quantities for the corresponding cell reactions are presented in Table 111. It is important to note that the cell potential for the polypyrrole perchlorate cathode is smaller than that of both the poly(pyrro1e (14) Matsumoto, T.; Matsunaga, Y . Bull. Chem. SOC.Jpn. 1981, 54, 648-653. (15) Euler, W. B.; Melton, M. E.; Hoffman, B. M. J . Am. Chem. SOC. 1982, 104, 5966-5971.
1640 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 TABLE 111: Thermodynamic Quantitiesa for Solid-state Galvanic Cell Reactions nyAg AG2980,kJ/mol E298", mV X Y C1Oi
12.80 f 0.4 253.7 f 0.4 402.6 f 0.2 523.0 f 0.05
0.24 0.24 0.15 0.16
c1Br1-
-0.30 -5.89 -5.81 -8.07
+ (PsX,),
f 0.08 f 0.08 f 0.08 f 0.01
-
Curtin et al. nyAgX
+ P,
AH", kJ/mol
AS",eu/mol
f 0.08 f 0.08 f 0.13
-1.31 f 0.04 1.05 0.05 1.75 f 0.04 0.22 f 0.03
-1.96 -4.60 -3.64 -7.78
*
f 0.08
'These quantities are per mole of pyrrole monomer unit. TABLE I V Thermochemical Quantities of Formation of Silver Halides, AgX, at 25 "C X AG,",kJ/mol AH;, kJ/mol ASf", eu/mol CI" -97.4 -1 13.7 -13.2 Bra -85.3 -100.3 -12.3 Ib -66.0 -61.5 3.9 a
10 20
0
30
40
Temperature
50
From ref 7. *From ref 8.
TABLE V: Doping Thermochemistry of Polypyrrole Films with Halogens at 25 "C" halogen AG,", kJ/mol AHd", kJ/mol ASd",eu/mol
60
Cl2(g) Br,(l) Ids)
("C)
-15.6 -6.98 -2.5 1
-20.4 -11.5 -2.05
'As defined by the doping reaction (ny/2)X2
b
30 40
10 20
0
TemDerature
50
60
("C)
/, ,
'
5 2 5 '
m
.-
U
5
-Y
a
523
,
,
,
1
,
10 20 30 40 50 Temperature ("C)
0
> E 524 v
,
I
I
'
m
-
-
-d
t
3 522 t
PPP
521yl 0
,
,
10
,
I
20
,
I
30
Temperature
,
,
,
40
,
4
50
("C)
Figure 3. Temperature-dependent EMF and least-squares linear regression fit for the (a) AglAgIl[P~(C104)~.~41nr (b) AglAgIl(P.Clo,t3),,(4 AglAgIl(P.Bro,,,),, and (d) AglAgII(P.1, 16)n solid-state galvanic cells.
iodide) and poly(pyrro1e bromide) cathodes, and therefore the small amounts of unexchanged perchlorate remaining in the halide films should not significantly affect the measured cell EMFs.
+ P,
-
(PeX,)",
Although the presence of a small quantity of the auxiliary silver salt in the cathode improves voltage stability, the final cell EMFs were found to be independent of the amount of auxiliary silver salt used. For example, the cell potential and temperature dependence for the poly(pyrro1e chloride) cell are not significantly affected by grinding several hundred milligrams of silver chloride into the cathode. Moreover, the cell potential and temperature dependence remain unchanged when the silver salt of an inconsequential anion, such as AgBr, is added to the cathode. These data indicate that anion exchange between polypyrrole and the solid silver salts present in the cathode does not occur in the solid state. Doping Heats. The data in Table 111 can be combined with known heats and free energies of formation for various silver halides to arrive at the doping thermochemistry of polypyrrole with elemental halogens. By summation of reactions of the form nyAg + (P.X,), nyAgX + P, nyAgX
4001
-3.95 -3.60 +0.40
-
-
nyAg
nY + -X2 2
(5)
the thermochemistry of the net reaction (P-X,),
P,
nY + -x2 2
(6)
can be derived. We define reaction 6 as the reverse of the doping reaction. Table IV contains the enthalpies, entropies, and free energies of formation for the various silver halides, as evaluated from previous electrochemical data.'J The corresponding thermochemical parameters for reaction 6 appear in Table V. Again, these thermochemical quantities are defined per mole of pyrrole monomer. The trend in the doping enthalpies appears to follow the relative reactivities of the elemental halogens-chlorine being the most energetic dopant and iodine being the least. The doping entropies for chlorine and bromine are moderately large and negative, in accordance with the incorporation of a high-entropy gaseous or liquid state (Cl,(g) and Br2(l)) into a solid polymer matrix. The doping entropy for iodine is small and positive, reflecting a crystalline solid (12(s))being dispersed into a relatively disordered polymer. The values of the doping free energies indicate that at 25 OC the equilibrium for doping unoxidized polypyrrole with elemental chlorine and bromine would lie substantially to the right, whereas a small but nonnegligible partial pressure of 12(g) should remain in equilibrium with iodindoped polypyrrole. The free energy of doping is directly related to the activity of the halogen in polypyrrole. Consider a hypothetical cell employing
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1641
Anion-Exchange Thermochemistry in Polypyrrole TABLE VI: Activities and Activity Coefficients of Halogens in Halogen-Doped Polypyrrole at 25 OC halogen a' X Y Po.Torr P,.sl,.A,Torr 760 3.0 X CI 4.0 X 0.18 2.2 X 216b 1.5 X lod Br 6.9 X 0.13 5.3 X lo-* 0.14 1.3 X O.3lc 5.4 X IO4 I 1.8 X lo-'
Defined relative to the following standard states: Cl,(g) (1 atm),
Br2(l), 12(s), all at 298 K. bBarin, I.; Knacke, 0. Thermochemical Properties of Inorganic Substances; Heidelberg: Springer-Verlag, 1975; p 114. CVaporpressure of the pure element was evaluated by the relation log P(atm) = -3512.83/T - 2.013 log T + 13.374 (Gillespie, L. J.; Frazer, L. H. D. J . Am. Chem. SOC.1936, 58, 2260-2263).
L
'2
2
Cells of the form AglAgI(2 have been studied by McKechnie and co-workers.1° The derived free energy of the doping reaction 11 is AG298' = -8.86 kJ. Reaction 11 corresponds to the hypo3 -12(s) + Per@) 2(s) (1 1) 2
-
a perfect halide ion conductor, C, corresponding to the doping reaction (6). The form of this cell would be
thetical galvanic cell I,ICIPer,2, which would produce an EMF, E,', of 30.6 mV at 25 'C. This voltage is a direct measure of the difference in chemical potential of iodine in 12(s) and in 2(s). Subtracting eq 7 from eq 12 yields eq 13
X~ICIP,(P.Xy)n
FE,' = MI(I2) - CLI(2)
(12)
F(Es' - EPI') = ~ ~ [ ( p * I y )-n lMI(^)
(13)
and the generated cell potential, EpXo,would be
We now redefine Per.13 as our standard state for iodine: F(Es' - EPI') = ~ ~ [ ( p * I y )-n MI' l
A cell of this type is known as a galvanic cell of the first kind, and it can be demonstrated that the cell E M F is a direct measure of the difference in chemical potential of the halogen in poly(pyrrole halide) and in the element: FEpXo = M X ( X ~- )~x[(P*xy)nI
(7)
We now define X2 as the standard state for the halogen: CLX(X2) = M X 0
Of course, the chemical potential of X in (P-X,), is ~x[(p.Xy)nI = MX'
+ R T ln ax
(8)
where ax is the activity of halogen in the polypyrrole film and AGd' In ax = (9) YRT The activity coefficient can therefore be evaluated since the mole fraction of halogen in polypyrrole,Xx,is known from the elemental analysis:
Activities and activity coefficients for halogens in polypyrrole halides calculated by eq 9 and 10 are presented in Table VI. We may use these activities to calculate the partial pressures of elemental halogen over halogen-doped polypyrrole: ax = P x / P x o where Pxo is the standard-state vapor pressure of the pure halogen. These values are also tabulated in Table VI. Note that the calculated partial pressure of I, over a sample of (P-Io,16),,is, although small, nonnegligible. This may also account for the apparent decrease in the degree of partial oxidation of polypyrrole when Clod- is exchanged with I-. Activity Coefficient of Iodine in Poly(pyrro1e iodide). The activity coefficient is a physically useful quantity for describing the nature of a species in a given environment. The activity coefficient affords a quantitative measure of how physicochemically different a species in its present state is from its standard state (where a = 1). The extremely small values of y for the halogens in poly(pyrro1e halides) reflect a great difference in the physicochemical nature of halogen when doped into polypyrrole relative to 'he elemental form. This is, of course, to be expected and is consistent with the assertion that the halogen exists as an anion in a cationic polymer matrix. It would be more convenient, and indeed more meaningful, to redefine our standard state to a more closely related system. We will choose the perylene-iodine charge-transfer complex (2) as our standard state for iodine, and evaluate the activity of iodine in polypyrrole iodide relative to this new standard state.
and, by substituting with eq 8 In aI(P.Iy) =
F RT
-(E,O
- EPIo)
(14)
The activity and activity coefficient of iodine in (P&16)nrelative to the 2 standard state are presented in Table VI1 along with the activities and activity coefficients of iodine in a number of other charge-transfer complexes and doped polymers as calculated by eq 14. The activity coefficients appearing in Table VI1 reflect the "chemical availability" of iodine in the respective charge-transfer solid relative to 2. For example, the pyrene.14 complex (which should probably be written as pyrenel12.12)is known to be a very weakly associated complex of essentially neutral diatomic iodine and neutral pyrene.16J7 The material has a substantial 12(g)vapor pressure and dissociates readily upon standing in an open vessel at atmospheric pressure. X-ray crystallographic data indicate the pyrene-iodine and the phenothiazine-iodine complexes possess layered structures with molecular iodine intercalated between the 1 a ~ e r s . l2~is much more stable than the pyrene complex, although it too emits 12(g) slowly at atmospheric pressure. This difference in the "chemical availability" of iodine in these complexes is reflected in their respective activity coefficients. In contrast, poly(pyrro1e iodide) has a much lower activity coefficient (0.042) for iodine, indicating either a more tightly bound species or that the iodine exists in equilibrium with substantially less neutral I2 than in the above charge-transfer complexes. Recent electrochemical diffusion studies of Martin and co-workers at Texas A&M demonstrate that anions in polypyrrole are quite mobile.I8 This concurs with our previous observation of rapid anion exchange in polypyrrole films.5 These observations tend to discount the possibility that the anions are tightly bound to the polymer matrix, and we assert that the anomalously low activity coefficient for iodine in poly(pyrro1e iodide) is due to a decreased concentration of I, in equilibrium with the anionic species (relative to 2). We therefore conclude that the iodine in poly(pyrro1e iodide) exists predominately as I-. This is in contrast with iodine-doped polyacetylene, in which the iodine is known to exist primarily as the triiodide, 13-, hence possessing substantially greater I, character, as is also reflected in the activity coefficient for iodine-doped polyacetylene presented in Table VII. Anion-Exchange Thermochemistry. Consider a cell reaction employing a polypyrrole cathode containing anion X:
(16) Kommandeur, J.; Hall, F. R. J . Chem. Phys. 1961, 34, 129-133. (17) Garcia-Echarri, A,; Sarris, C.; Walso De Reca, N. E. An. Asoc. Quim. Argent. 1985, 73, 405-419. (18) Martin, C . , personal communication.
Curtin et al.
1642 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 TABLE VII: Activity Coefficients of Iodine in Iodine-Doped Polymers and Charge-Transfer Solids Relative to the Perylene-Iodine (2) Standard State at 25 O C
complex
EO(I,ICIRI,), mV 2 30.6" Phe11.1~~ 38.0a1 0.6Oa1 pyrene.1,' 33.O4Il1 (CH.1097)" 16 2.4"'" (p'10 16)"
XI 0.75 0.75 0.80 0.065 0.14
a1
1 0.749 3.22 0.91 1 5.88 X lo-'
2-
x-
1.33
CI' Br-
c10,-
1.oo
4.03 13.9 0.042
Pyrene-iodine complex: .$;-
TABLE VIII: Heats of Solution of Silver Salts at 25 'C AHSOLV'.
kJ/mol
AgC104" AgClb' Ag B rb" AgIb"'
kJ/mol
-12.2 55.6 69.0 89.9
5.39 42.2 64.4 88.2
A&oLv"~
eu/mol
+14.1 -10.6 -3.9 -1.20
"From solubility data: Smith, G. F.; Ring, R. J . A m . Chem. SOC. 1937, 56, 1889-1 890. *Evaluated from temperature-dependent solubility products taken from the following sources: (i) Linke, W. F. Solubilities; D. Van Nostrand: New Jersey, 1958; Vol. VI, p 60. (ii) Gledhill, J. P.; Malan, C. McP. Trans. Faraday Soc. 1954, 50, 126-128. (iii) Ruff, 0. 2. Anorg. Chem. 1930, 185, 387-402.
having cell potential Exo. Equation 15 is actually identical with eq I . Consider now a similar cell reaction with a different anion, X'. and a cell
E M F of Ex,'. Normalizing both equations per mole of silver and subtracting eq 16 from eq 15 give 1 I(- :->P, + -(P.Xy),, + AgX'(s) n
Y'
nY
-
I
-,( P.X',,),, nY
+ AgX(s)
+
YI
"These values were calculated from solid-state EMF data taken from the following sources: (i) Reference 10. (ii) Guttmann, F.; Hermann, A. M.; Rembaum, A. J . Electrochem. S O C . 1967, 113, 323-329. (iii) BBniZre, F.; Boils, D.; Cinepa, H.; Franco, J.; LeCorre, A,; Louboutin, J. P. J . Phys. Colloq. 1983, 44(C3), 567-572. (iv) This work Phenothiozine-iodine complex:
AGSOLVO,
TABLE I X Thermodynamics of Anion Exchange in Polypyrrole Films: Z-(aq) + X-(PP) z Z-(PP) X-(aq) (at 25 "C)
(1 7 )
Combining reaction 17 with the aqueous solvation reactions
--
AgX(s) + AgX'(s) Ag+(aq) + X'-(aq) one can arrive at the exchange reaction
IBrI' I-
C104ClodCI-
c1Br-
AGExcO,
kJ/mol -44.7 -43.5 -52.7 +1.25 -8.28 -9.20
~HExc". kJ/mol -26.2 -31.1 -42.2 -4.97 -16.3 -11.1
UEXC',
eu/mol +14.9 +9.9 +8.5 -5.0 -6.4 -1.4
where AC~OLVO(A) is the free energy of aqueous solvation of species A. The entropy ALSEXCO is
Literature values of AGSOLVofor various silver salts are collected in Table VIII. These values have been calculated from solubility data from a number of sources. From the temperature dependence of the solubility one can calculate ASsoLvo and AHsoLv'. These quantities are also presented in Table VIII. The enthalpies, entropies, and free energies of anion exchange between polypyrrole and aqueous solution, as defined by eq 19, have been calculated by using eq 20 and 21 and appear in Table IX, for a number of anion pairs. Exchange free energies and enthalpies between the halogens in polypyrrole are fairly small, all less than 21 kJ/mol. There is also an intriguing trend correlating with the size of the halide-iodide being most enthalpically favored for incorporation into the polymer and chloride being the least. More interestingly, however, is that the perchlorate-doped polypyrrole film possesses relatively large enthalpic driving forces for exchange with halides. This is likely due to differences in aqueous solvation energies of Clod- vs halides and not necessarily attributed to differences in interaction energies within the polymer matrix. Summary Solid-state galvanic cells employing electrically conductive polypyrrole cathodes may be used to accurately determine the doping enthalpies, entropies, and free energies of electrochemically prepared polypyrrole with halogens. When combined with known thermodynamic quantities of aqueous solvation for the silver salts of the incorporated anions, the cell EMFs provide enthalpies, entropies, and free energies for the anion-exchange process. From these data we have determined that there exists a fairly large enthalpic driving force for the exchange of perchlorate in polypyrrole for chloride, bromide, and iodide. Smaller enthalpies were determined for the exchange between halides. Moreover, the cell voltages are directly related to the activities of the dopant species in the polymer matrix. The values for the activities and activity coefficients strongly suggest that the halogen in halogen-doped polypyrrole exists predominantly as halide. This is in contrast to most iodine-doped conductive polymers and charge-transfer complexes in which iodine exists as triiodide or molecular iodine.
(19)
Acknowledgment. Funding for this research was provided by the Wisconsin Alumni Research Foundation. W e are indebted to Glenn Komplin, Harlan Friske, and Allan Behling for their generous assistance in the design and construction of the cell housing used in this study. We also express our gratitude to Dr. Gary Wesenberg for the use of his data analysis, numerical regression, and graphics computer programs and to Professor Art Ellis for many fruitful discussions.
where (PP) indicates that the anion is incorporated in polypyrrole. The free energy AGEXCofor the anion exchange reaction 19 is .?rGEXCo( x-,x'-) = F(Ex,O - E x ' ) + AGsoLvO(AgX) - ACsoLv'(AgX') (20)
Registry No. 2, 2877-00-1; PhenaI,, 16025-81-3; pyreneeI,, 2876-95-1; y-Agl, 7783-96-2: Clod-, 14797-73-0 CI-, 16887-00-6: Br-, 24959-67-9; I-, 20461 -54-5; I,, 7553-56-2; Br,, 7726-95-6; CI,, 7782-50-5; Ag, 7440-22-4: AgCIO,, 7783-93-9; pyrrole, 109-97-7; polypyrrole, 306048 1-0: polyacetylene, 25067-58-7.
-
which may be more conveniently written as X-(PP)
+ X'-(aq)
X-(aq)
+ X'-(PP)
