Electrode potentials of electronically conducting polymer polypyrrole

Elecrmchlmico Acla, Vol. 37, No. Printed in Great Britain.

6, pp. 10754081,

1992 6

0013-4686/92 $5.00 +O.W 1992. persunon PIem plc.

ELECTRODE POTENTIALS OF ELECTRONICALLY CONDUCTING POLYMER POLYPYRROLE QIBING FW and RENYUAN QIAN Institute of Chemistry, Academia Sinica, Beijing 100080, China (Received 16 July 1991; in revised&m

28 Augusf 1991)

Abstraet-The electrode potentials of electronically conducting polymer polypyrrole film-coated Pt electrodes (PPy/Pt) and the effects of the solution pH, electrolyte concentration, temperature, and spontaneous counter anion exchange on the potentials have been studied. The results indicate that the electrode potentials are the genuine responses of the polymeric coating. Thus measurement of the

potentials provides a novel method to investigate the conducting polymer which is usually intractable. PPy/Pt electrodes definitely respond to the pH value of an electrolyte solution in contact but do so quite slowly, due to the slow protonation and deprotonation involved in PPy. The potential-pH diagram shows an inclined line with a slope around -4OmV/pH, depending slightly on the PPy layer thickness. The electrodes are very sensitive to the change of the electrolyte concentration. The slope values of the potential-1ogC lines are related to the solution pH. Nemst response is observed in solutions at ca pH 7, indicative of the use of such electrodes as ion sensors. Key words: conducting polymer, polypyrrole, electrode potential, Nemst response, ion sensor.

INTRODUCTION Recently, electronically conducting polymers have attracted much attention from the community of electrochemists due to their novel electrochemical properties[ l-41. Of these materials, polypyrrole (PPy) is among the most intensively investigated owing to its high stability and ease of tailoring[5] to prepare functional&d polypyrroles. While many articles have been published[l, 3,5] on the electrochemical redox of PPy films formed on Pt electrodes either as modified electrodes for electrocatalysis or as battery electrode materials[Q 7], there are only a few studies on the quasi-equilibrium electrode potentials or rest potentials (after Reck et af.[lO]) of such PPy electrodes[&lO], though an understanding of the nature of this will be of scientific interest as well as of technical importance. The electrochemical redox process of PPy may be expressed as[ll, 121 -Py+-+ee-+-Py-+xX-, Xwhere -Pyrepresents a part of the PPy chain with three to four pyrrole rings. X- is the counter anion compensating the positive charge on the PPy backbone. It has been reported[9] that a PPy(Cl-)/Pt electrode shows a Nemst electrode response to Clions in an aqueous KC1 solution, E = E” - RT/F ln[Cl-]

= E” - 0.059 mV log[Cl-]

at T = 298 K.

*Present address: Department of Physics, University, LinEping, S-581 83, Sweden.

Linkiiping

Genies and Syed[8] observed a changing equilibrium electrode potential with changing pH of the electrolyte solution, contributed to simultaneous proton transport. Reck et al.[lO] carried out similar experiments with a thicker PPy film hoping to coat the substrate Pt or glassy carbon electrode perfectly, and concluded that the potential in an aqueous electrolyte was independent of the anion concentration in the solution as well as of its pH value. Thus they suggested that the results of Genies and Syed[8] could be artifacts due to the porous nature of a thin PPy film, exposing the surface groups of the substrate Pt as the active electrode,

They also suggested the electrode potential of a PPy/substrate electrode would more properly be termed “rest electrode potential”. In this paper, the potentials of PPy film coated platinum electrode (PPy/Pt) have been studied in more detail. We found that the so-called rest electrode potential should be attributed to the behaviour of the PPy layer. Thus the electrode potential will reflect the chemical potential or redox level of PPy chains. Measurement of the potential should shed some light on the structure of the polymer.

EXPERIMENTAL

AND RESULTS

The PPy(N0;) coated Pt electrode was prepared by electrochemical polymerization of pyrrole in an aqueous solution containing 0.1 M pyrrole and 0.1 M NaNO, adjusted to pH 3 with HN03. Constant potential of 0.70 V vs. see was applied until the charge consumed in the electropolymerization was 3.3 or 1075

1076

Q, Pat and R.

33 mC mm-*, corresponding to PPy layers of ca 0.8 or 8 pm in thickness, respectively, calculated from the following equation,

&m)

=

QIAN 0.6

QP22Fd PJtpyrrokj + 0.22McNOrjl104,

where Q is the charge (Ccm-*) consumed during electropolymerization, d the density of a PPy(N0,) film, ie 1.516g~rn-~ (from Ref.[l3]), M molecular weight, and F the Faraday constant. Here the PPy(N0; ) film as-prepared is considered to contain 0.22mole of NO; per mole pyrrole unit, based on elemental analysis. Calculated thickness of a PPy layer of 1 pm corresponds to a Q of 4.1 mC mm-*. Various Q to L ratios have been quoted in previous studies, for examples, including (in mC mm-* pm-‘) 1.5[14, 151, 1.8[16], 2.4[17], 3.7[18, 191, 4.0[20], and 6.0[21]. This variation stems from (1) the coulombic efficiency for polymerization, (2) dopant species and concentration in the as-polymerized PPy, (3) the density of the grown film, which depends on the substrate electrode materials, composition and temperature of the electrolyte solution, and (4) whether a thick or thin film has been grown, The as-prepared PPy/Pt electrodes were immersed in aqueous Kolthoff buffer solutions (0.2 M Na,HPO, + 0.1 M citric acid) containing 1M

-0.4 0

100

200

300

400

500

f

TIME WIN)

Fig. 2. Decay of potential with time of 8pm thick PPy(NO,)/Pt in aqueous Kolthoff buffers containing 1 M NaNO, with, from the bottom upwards, pH = 13, 10, 7, 4 and 1.

NaNO,, and the decay of electrode potentials with time was recorded on an X-Y recorder (Model LZ 3-204, No. 2 Automatic Instrument Manufacturer, Shanghai) as shown in Figs 1 and 2. For reference, the potentials were also measured using a digital voltmeter (Model DS42A, No. 1 Radio Factory, Tianjin) with an input impedance higher than 1000 Mn. Both methods produced the same value of electrode potential. All experiments were carried out in a thermostat of 15.0 f O.l”C if not specifically noted. 1. Effect of solution pH on the electrode potential

0

50

100

150

TIME (hIIN)

Fig. 1. Decay of potential with time of 0.8 pm thick PPy(NO,)/Pt in aqueous electrolyte solutions of Kolthoff buffers containing 1M NaNO, with, from the bottom upwards, pH = 13, 11, 9, 7, 5, 3 and 1.2, and 1 M HNO, . Fig. l(b) is a section of Fig. l(a).

For a 0.8 pm thick PPy(N0; ) film immersed in a buffer solution containing 1 M NaNO,, the decay of the potential of the as-prepared PPy/Pt electrode greatly depends on the pH value of the buffer in an interesting way as shown in Fig. 1. In neutral solutions of pH around 7, the potential changes little with time after an initial decay into more negative values. In strongly acidic solutions the decay curves consist of two parts, the initial decay to some levelling value followed by a step increase of potential. This latter increase of potential comes sooner as the pH of the solution is lower. In basic solutions the decay curves also consist of two parts. The initial drop of potential may be quite large (> 0.4 V) at high pH values, which is then followed by a gradual increase of potential. The latter increase of potential comes sooner as the pH of the solution is higher. As a result, although the potentials of the PPy/Pt electrode immediately after contact with buffered 1 M NaNO, solutions are nearly independent of the pH (Fig. 3a), the Pourbais diagram, ie the potentials approaching equilibrium after 150 min immersion vs. solution pH (Fig. 3b) becomes an inclined line with a slope of -45 mV/pH, similar to those previously reported by Genies et a1.[8] and by Beck et al.[lO] using 0.3 pm or 0.2 pm PPy films respectively except for a slight difference in the value of the slope. For an 8 pm thick PPy(NO<) film, the electrode potential decay follows the general behaviour of the thin fhm described above, but the change is much slower as shown in Fig. 2. The potential of the PPy/Pt

Electronically conducting polymer polypyrrole

o.4L

i-z 0.4

.

8

.

8

0.2 -

l?0

z

0.0

F 2

0

.O .

?I 2 0.2

8

1077

9

-0.2

‘a0

0

o-

O. P

!

PH

Fig. 3. Potential-pH diagram of PPy(NO;)/Pt derived from Fig. 1 for the electrodes (a) immediately after contact with the solutions, and (b) at equilibrium after 1SOmin immersion.

electrode immediately after contact with the buffer solution is almost independent of the pH- value (Fig. 4a), while the equilibrated potential after 600min immersion shows a Pourbais diagram of an inclined line (Fig. 4b) with a slope of -4OmV/pH, similar to the diagram for the 0.8pm PPy lilm (-45 mV/pH, Fig. 3b). This is contrary to the results of Beck et uL[lO] who observed that the Pourbais diagram is a horizontal line for -a 5 pm thick PPy layer. To eliminate the probable influence of the buffer ion species in the electrolyte solution and differences among individual samples (though of the same thickness) on the electrode potential, we re-examined the potential of one l3pm thick PPy film immersed successively in aqueous solutions of 1 M NaNO, adjusted to a wide range of pH values with NaOH or HNO, solutions (non-buffered). At each change of pH of the solution, the electrode underwent continuous but very slow change until reaching a new equilibrated value after several hours immersion. The equilibrated potentials are shown in Fig. 5. The

a

acu 4 ;

0

,,

,.,.,.

l.,

4

10

14

PH Fig. 4. Potential-pH diagram of PPy(NOF )/Pt derived from Fig. 2 for the electrodes (a) immediately aRer contact with the solutions, and (b) at equilibrium a!& 6OOmin immersion.

“2. ”

10

4

14

PH

Fig. 5. Potential-pH diagram of PPy(NO, )/Pt electrode being immersed successfidly in aqueous solutions of 1 M NaNO, adjusted to various pH values with HNO, or NaOH (non-buffered). The thickness of the PPy layer is 8 gm (0) and 0.8 km (0).

change of potential with pH of the solution was found to be nearly reversible (after one cycle, the potential decreased slightly). Similar observations were carried out also for a thinner PPy layer of 0.8 pm in thickness. For both thin and thick PPy layers the Pourbais diagram shows straight lines with negative slopes (Fig. 5). The effects of film thickness are (1) the establishment of a new equilibrium after changing the solution pH is slower for the thicker Glrn, cu 1 h for the 0.8 pm layer and cu 5 h for the 8 pm layer, (2) a thicker flhn shows slightly lower potentials, especially in acidic solutions, and (3) the slopes of Pourbais diagrams are ca 38 mV/pH for an 8 pm ftlm and -42 mV/pH for a O.Sprn film. A comparison of Fig. 5 with Figs 3 and 4 indicates the influence of buffer ion species and sample difference on the potential being very small. As a convincing proof for the solution pH response of the PPy(X-) electrode potential being a genuine one, a free-standing PPy(TsO-) film of good rigidity and strength without a Pt substrate was used. phe PPy(TsO-) film (ca 1Opm thick) was prepared through electrochemical polymerization from an aqueous solution containing pyrrole and sodium tosylate. Details about the preparation will be published elsewhere.] A silver wire was attached to the film by conducting silver paste for electrode potential measurement. In acidic (pH 2) and neutral @H 7) solutions containing 1 M NaN03 (Table 1), the values of electrode potential after 10 h immersion in the solution lie close to those in Fig. 5. But in basic solution (pH 12) the potential is much lower than that in Fig. 5. In aqueous solutions containing 0.2 M sodium tosylate at pH 2-12, all values of electrode potential of the free standing film lie close to those in Fig. 5. Both cases show an increasing potential with decreasing pH close to values of a PPy/Pt electrode, indicating that the electrode potential of PPy/Pt comes from the polymer layer, not from the Pt substrate.

Q. PEI and R. @iN

1078

Table 2. Slopes of the potentiallog[NO, ] plots of Fig. 6

Table 1. Electrode potentials of freestanding PPy(fsO-) films of 10pm thick

in aqueous buffer solutions containing (a) 1 M NaNO, and (h) 0.2 M TsONa after 10 h immersion in the solutions

Solution pH 1.3 2.5 6.5 8.7 9.2 10.5 12.1

Electrode potentials (V vs. see)

Solution pH a. pH2 a. pH7 a. pH 12

0.343 0.180 -0.145

b. pH2 b. pH7 b. pH 12

0.322 0.193 -0.057

2. Nernst response of the electrode potential Nemst response of the electrode potential of PPy(A-)/Pt to A- anions in solution has been reported[9,10,22,23] for PPy layers of 0.5 to several pm in thickness. In the case of our 0.8 pm thick PPy(N0;) film on Pt substrate in aqueous neutral solutions, the Nernst response was also observed, ie 56 mV increase of potential with lo-fold decrease of the NO; concentration in solution. For an 8 pm thick PPy(N0;) film on Pt substrate immersed in aqueous solutions of NaNO, of concentrations 0.3-3 M, adjusted to different pH by adding HNOr or NaOH solution, the relationships of the electrode potentials of PPy(N0; )/Pt to NO, anion concentrations in the solution are shown in Fig. 6. Linear relationships are observed between the potentials and log concentrations in all cases. However, the slope values of the linear plots satisfy a Nemst response to anions NO; in the solution only in neutral aqueous solutions. In aqueous basic and acidic solutions, the slope values are smaller than 50mV per decade decrease of NO; concentration as shown in Table 2. In all cases, the electrode potential follows the change of NO; concentration very fast. A new equilibrium

0.4 -

Slopes (mV/logC)

1.3

-16.1 -36.4 -56.3 -41.0 -36.0 -20.3 - 18.4

ri

after changing the NO; concentration in solution can be achieved within 1 min, compared to slow response of the potential to change of solution pH where several hours will be taken to approach a new equilibrium. 3. Efect on temperature on electrode potential The decreasing electrode potential of PPy(Cl- )/Pt in 1 M KC1 aqueous solution with increasing temperature at 15, 25, 35, and 45°C has been observed previously[9]. We investigated the temperature dependence of the electrode potential of PPy(NO;)/Pt in 1 M NaNOr aqueous solution in a wider temperature range, 345°C as shown in Fig. 7. Apparently the potential increases with increasing temperature below 20°C and then decreases with increasing temperature above 25°C. Even after taking into consideration that the absolute potential of the see reference electrode decreases with rising temperature at a rate of -6 to - 7 mV/lO”C[24], the temperature-dependent potential of PPy(N0; )/Pt still shows the tendency displayed in Fig. 7. According to the Nemst equation, the slope value of the electrode potential-log(concentration) straight line is related to absolute temperature (T) by

slope value (mV/logC) = - 0.198 T. For the PPy(N0; )Pt electrode in aqueous solutions containing 0.1 to 2 M NaNO,, the slope values of potential-1ogC lines at 545°C are shown in Fig. 8. In neutral solutions of pH 6.5 (not-buffered), a linear relation is observed between the slope values and temperatures. The slope of this straight line is

0.3 -

O12.1 -0.1 . . 0.1

0.17 0.3

0.5

1

3

5

CONCENTRATION (M)

Fig. 6. Potential-electrolyte concentration diagram of 8 pm thick PPy(NO;)/Pt electrodes in aqueous NaNO, solutions adjusted to various pH with HNO, or NaOH (nonbuffered). Numerals on the diagram represent the solution pH values.

0

10

20

I

I

30

40

I 50

TEMPERATURE (“C)

Fig. 7. Temperature dependence of the equilibrated electrode potential of PPy(NO, )/Pt in a neutral aqueous solution at pH6.5 (non-buffered). The thickness of the PPy layer is 8pm.

Electronically conducting polymer polypyrrole

0.20

-10 0

I

I

1

I

10

20

30

40

50

TEMPEFLATURE PC)

Fig. 8. Temperature dependence of the slope values of potential-log(electrolyte concentration) lines for PPy(N0; )/Pt electrode in aqueous solutions containing 0.1 to 2 M NaNO, at (a) pH 6.5, (b) 2.5 and(c) 10.5, all adjusted with HNO, or NaOH (non-buffered). The thickness of the PPy layer is 8 pm. -0.224/K, close to the theoretical value, -0.198/K. In acidic solutions of pH 2.5 (adjusted with HNO, solution), a similar slope-temperature line is observed with a slope of -0.231/K. On the other hand, in basic solutions of pH 10.5 (adjusted with NaOH solution), the slope-temperature relationship is no longer linear especially at higher temperatures, indicating probably some structural changes of the PPy chain. 4. Spontaneous counter anion exchange The spontaneous counter anion exchange of X- in PPy(X-) samples with Y- in an aqueous solution of M+ Y- has been studied recently[25-281. It was found that this exchange process does not affect the electronic structure of the conducting polymer[26]. It is of interest to see whether the equilibrium electrode potential would change during this process. Three PPy(N0; )/Pt electrodes with PPy layers of 8 pm in thickness were immersed in 1 M NaN03 aqueous solution (PH 6.5 buffered with 0.08 M NaH,PO,/Na,HPO~) for 10 h to reach an equilibrium potential of E = +0.210 V vs. see. Then they were put into 1 M KCl, KBr, and NaClO, neutral

Fig. 9. Decay of PPy(NO;)/Pt electrode potential during the spontaneous anion exchange of the 8 pm thick PPy layer steeped into 1 M (a) NaClO,, (b) NaBr, or (c) NaCl aqueous solution. The PPy/Pt electrode was immersed in an aqueous solution of 1 M NaNO, for about 10 h to reach at an equilibrated potential before the anion exchange.(the dashed line). All of the solutions have a pH 6.5 buffered with 0.08 M NaH,PO,/Na,HPO,.

aqueous solutions (PH = 6.5 buffered in the same way) respectively. The change of the electrode potentials with time were recorded as shown in Fig. 9. The potential changes reach a new equilibrium within 10min. The equilibrated potentials are related only slightly to the anions present in the solutions (within 30 mv). For PPy(Cl-)/Pt, PPy(Br- )/Pt, and PpY(ClOi )/Pt electrodes, similar phenomena were observed, and the equilibrated electrode potentials are summarized in Table 3.

DI!XUSSION

From the results of this investigation the controversml issue of the origin of potential change of a PPy(X-)/Pt electrode with the solution pH in contact as discussed by Genies and Syed[8] and by Beck et a/.[101 is resolved. The effect of solution pH on the electrode potential and the potential response to the change of X- concentration in solution is a genuine one from the PPy(X-) layer, not the response of the substrate Pt electrode due to the porosity of the

Table 3. Equilibrated electrode potentials of PPy/Pt after spontaneous anion exchange in aqueous solutions (the underlined data are the equilibrated potentials of PPyvc-)* in corresponding NaX solutions before anion exchange) Equilibrated potentials, V vs. sce# Solutions for immersion7 1M 1M 1M 1M

NaCl NaBr NaNO, NaClO,

PPy(Cl- )

PPy(Br- )

PPy(NO, )

PpY(CIo,- )

0.193 0.194 0.200 0.215

0.172 0.176 0.184 0.202

0.190 0.280 0.210 0.215

0.215 0.220 0.210 Q.4J

lPPy(X-) tilms were electrochemically polymerized from aqueous solutions containing 0.1 M pyrrole and 0.1 M NaX under a constant potential of 0.70 V vs. see for Mmin. The films were co 8@1 thick. tAl1 solutions were buffered to a pH 6.5 with 0.08 M NaHsPO,/Nar HPO,. $The potentials approached quilibrium in aboat IO-20 min.

Q. PEI and R. Q~AN

1080

PPy(X-) film. However, as the electrode potential of PPy/Pt equilibrated in an electrolyte solution responds to the change of pH value of the solution very slowly, especially when the PPy layer is thick, it is very likely that an unchanged potential in short time intervals after immersion into solutions of varied pH values will be observed (see Figs 3a and 4a). The protonation and deprotonation of PPy chain in acidic and basic solutions have been proposed[8,29], but were taken into serious consideration only recently[30,31]. Due to H+ produced during the electrochemical polymerization of pyrrole the PPy chain as polymerized has pyrrole units, some but maybe not the whole, protonated and some units lost electrons to the electrode; both will lead to a positively charged chain. The counter anions should be considered to be contributed both by electron transfer (y mol per pyrrole unit) and protonation (JJ, mol per pyrrole unit): Pyrrole units in FPy os- polymerized :

H

_pJ____&____~ H Yx-

H

H

py + H,O + e-+pyH

+ OH-.

No anion X- or NO; is involved in this process, leading to a electrode potential less dependent on the anion concentration in solution (Table 2). In strongly acidic solutions some of the pyrrole units on PPy chains as polymerized are protonated. In another study[32] it has been shown that neutral PPy” after electrochemical reduction is protonated to a certain extent in strong acids. The redox of PPy in a strong acid thus includes (PYHY (YX-) + Ye-+pyH

+YX-

YJ

pi-i29

Pyrrole unks In F?y after deptutorwtion :

processes and consequently slower change of the potentials. In both acidic and basic solutions, the response of a PPy electrode to the anion concentration [x-l in solution will deviate from the Nemst value, having a larger deviation for solutions further away from pH 7 (Table 2), due also to the protonation and chemical compensation of PPy chains. If one denotes the ideal pyrrole unit (the right units are shown in Scheme 1) as pyH, H being the hydrogen to the N atom of the units, the five pyrrole units in Scheme 1 could then be written down in order as (pyH)Y+ (vX_), (PYHH,, Y’ + (AX- ), PYH, PY. and PYH. In basic solutions the redox of PPy includes

+

=4J-p When the PPy film as polymerized is immersed in a strong acid more pyrrole units on the chain will be protonated, leading to an increase of y, and slightly increased conductivity[30], which apparently results in a more positive electrode potential. When the PPy film as polymerized is in a buffer of pH 7, the PPy chain will be deprotonated so that y,+O while y remains unchanged, leading to a decrease of conductivity to around half of the starting film whilst the electrode potential moves to a less positive value. Under this condition a Nernst response of the PPy electrode potential to anion concentration [x-l in solution results. When the PPy film as polymerized is immersed in a highly alkaline solution, say buffers of pH > 9, the PPy chain will be further deprotonated from the N-H group, very probably leading to a quinoid structure, leading to de-doping and a low conductivitiy of 10m5S cm-’ known as chemical compensation. The PPy electrode potential moves much more towards negative. Thus the general increase of the electrode potential of PPy with immersion time in strong acids and its decrease in strong bases shown in Figs 1 and 2 can be attributed to the gradual protonation and de-doping or chemical compensation of the PPy chains. Thicker films involve slower

PYH + YZH- +Y,X- =(PYHH,,(~~+(Y,X- ). The potential-log[X-] relation depends on the protonation-deprotonation equilibrium, that is either doped PPy (y,) or neutral PPy” (yz) is protonated to a greater extent. The result in Table 2 indicates y, > y,, or neutral PPy” is more protonated. This is reasonable as doped PpY has already been positively charged, making the incorporation of proton more difficult. The slow response of PPy/Pt electrode to the change of solution pH in contact indicates that both the protonation and deprotonation processes are slow ones, ion transport probably being the limiting step[l6, 19,251. Several hours are needed to complete the modification of PPy in solutions of different pH for an 8 pm thick film. The fast response of the electrode potential of conducting PPy film to anion concentration in solution manifests facile electron transport in PPy. The Nemst response of the PPy/Pt electrode to anions in the solution in contact presumably comes from the outer surface of the PPy layer. Thus such a PPy/Pt electrode could be used as an ion sensor[22,23] in a wide range of pH values, ie pH 1 to 13, as the response to pH is slow, the potential vs. anion concentration relation follows the Nemst equation in solutions of any pH value in short time interval. This sensor will have poor selectivity to anions because the variation of the type of anion in a solution does not greatly affect the electrode potential of the PPy/Pt (see Table 3) due to the spontaneous anion exchange of PPy, especially when small and monovalent anions are involved[26], in accordance with results by Dong et a1.[23]. For practical applications the PPy/Pt sensor is to be best used for solutions of pH 3-9 and at room temperature. With a thick and tough free-standing PPy film the use of noble metal Pt could be avoided. The only slight deviation of the potential of PPy after spontaneous counter anion exchange supports

Electronically conducting polymer polypyrrole

the conclusion[26] that the counter anions in conducting PPy does not a&ct the electronic state of the conjugated polymer chain. The effect of temperature on the electrode potential of PPy is rather complicated. Both the electrolyte solution and PPy phases are sensitive to temperature change. The decreasing potential with increasing temperature at T > 25°C shown in Fig. 7 is probably due to among others the promoted reaction between the positively charged PPy chains with water inducing the formation of pyHaH, another form of chemical compensation[33]. This reaction is facilitated in basic solutions (see Fig. 8c) and causes irreversible change on PPy chains. It is not clear now why the potentials decrease with decreasing temperature at T < 20°C. Another unsolved issue on such conducting polymer electrodes is whether the inserted anions X- are solvated or stripped off the solvation sheath when inserting into the polymer network. These issues could be addressed only after more extensive studies. CONCLUSION Through measurements of the dependences of the electrode potential of PPy film-coated Pt in aqueous solutions on the solution pH, electrolyte concentration, and temperature, it has been demonstrated that the potentials are genuine responses of the polymer layer, rather than the substrate Pt. These electrode potentials thus naturally reflect the nature and electronic state of the intractable conducting PPy, insoluble and infusible. These methods may be extended to other conducting polymers which are also stable enough for practical purpose but are intractable also. PPy/Pt electrodes respond to the change of solution pH quite slowly owing to the slow protonation and deprotonation processes of the PPy chains involved. The Pourbais diagram shows an inclined line with a slope around -40 mV/pH, depending slightly on the PPy layer thickness. On the contrary, PPyfPt electrodes are very sensitive and respond quickly to the anion concentration of a solution in which they are immersed. The slope values of E-1ogC lines are related to the solution pH due to structural changes of PPy chain under different pH values. A theoretical understanding of the charge transfer process in PPy and the anion and maybe cation mass transport through the polymer matrix deserves further study. REFERENCES 1. A. F. Diaz and J. Bargon, in Handbook of Conducting Polymers, Vol. 2 (Edited by T. A. Skotheim), p. 81. Marcel Dekker, New York (1986).

1081

2. J. GM. Boekris and D. Miller, in Conducting Polymers, Special Appficutions (Edited by L. Aleacor). p. 1. D. Reidel, Dordrecht (1987). 3. A. F. Diaz, J. F. Rubinson and H. B. Mark, Jr. Adu. Polym. Sci. 84, 113 (1988). 4. C. P. Evans, Adu. Electrockem. Sci. Eng. 1, 1 (1990). 5. A. Dexonzier and J. C. Moutet. Act. Chcm. Rcs. 22,249 (1989). 6. A. Mohammadi, 0. IngaaHs and I. Lundstrom, J. electrochem. Sot. 133, 947 (1986). I. R. E. G. Bittihn, F. WcelIler, H. Munstedt, H. Narrman and D. Daegele, Mukromol. Chcm. Symp. 8, 51 (1987). 8. E. M. Genies and A. A. Syed, Synth. Met. 10, 21 (1984/85). 9. R. Qian, Y. Li, B. Yan and H. Zhang, Synth. Met. 28, C51 (1989). 10. F. Beck, J. Jiang, M. Kolberg, H. Krohn and F. Sehloten, 2. Phy&. Chem. Ne&Folge 160, 83 (1988). 11. A. F. Diaz Chem. Scriota 17. 145 11981). 12. A. F. Di&, J. I. Casillo, J.- Logan and W.-Y. Lee, J. electroad. Gem. 129, 115 (1981). 13. R. Qian and J. Qiu, Pofym. J. 19, 157 (1987). 14. Y. Tezuka, K. Aoki and K. Shinozaki, Synth. Met. 30, 369 (1989). 15. S. Basak, K. Rajeshwar and M. Kaneto, Anal. Chem. 62, 1407 (1990). 16. V. Krishna, Y.-H. Ho, S. Basak and K. Rajeshwar, J. Am. them. Sot. 113. 3325W9lb 17. A. F. Diaz and J. I. .Castillo, J. *them. Sot., Chem. Commruz.397 (1980). 18. E. M. Genies and J. M. Pemaut, J. electroamrl. Chcm. 191, 111 (1985). 19. R. M. Penner, L. S. Van Dyke and C. R. Martin, J. phys. Chem. 92, 5274 (1988). 20. J. H. Kaufman, K. K. Kanazawa and G. B. Street, Phys. Rev. Lett. 53, 2461 (1984). 21. 0. Inganiis, E. Erlandsson, C. Nylander and I. Lundstriim, J. Phys. Chem. Solti 4S, 427 (1984). 22. S. Dong, Z. Lu and Z. Sun, Kexw Tongbao 34, 1677 (1989). 23. S. Dong, Z. Sun and Z. Lu, Acta Chimica Sinica 43,337 (1990). 24. D. Dobos, Electrochemical Tables, p. 264. Akademiai, Budapest (1975). 25. J. B. Sohlenoff and J. C. W. Chien, J. Am. them. Sot. 109, 6269 (1987). 26. Q, Pei and R. Qian, Proc. of C-MRS’90 Intematl. Symp., Beijing, June, 1990, Vol. 3, p. 195. Elsevier, Amsterdam (1990). 27. M. Yamaura, K. Sato and T. Hagiwara, Synth. Met. 30, 43 (1990). 28. G. Zotti, G. Schiavon and N. Comisso, Synth. Met. 40, 309 (1991). 29. H. Munstedt, Pofymer 27, 899 (1986). 30. Q. Pei and R. Qian, Synth. Met. 45, 35 (1991). 31. K. L. Tan, B. T. G. Tan, E. T. Kang and K. G. Neoh, J. them. Phys. 94, 5382 (1991). 32. Q. Pei and R. Qian, Synth. Met., submitted. 33. G. Wegner, W. Wemet, D. T. Glatshofer, J. Ulanski, C. Krohnke and M. Mohammadi, Synth. Met. 18, 1 (1987).