potentiometric behavior of diverse polypyrrole-sulphate films electrosynthesized on graphite –...

Potentiometric behavior of diverse polypyrrole-sulphate films electrosynthesized on graphite – epoxy resin composite

T. de J. Licona-Sánchez a, G. A. Álvarez-Romero* a, M. E. Palomar-Pardavéb, C. A. Galán-Vidal a, María Elena Páez Hernández a

a Área Académica de Química, Laboratorio de Química Analítica. Universidad Autónoma del Estado de Hidalgo. Ciudad Universitaria, Carretera Pachuca-Tulancingo

Km. 4.5, C.P. 42184, Mineral de la Reforma, Hidalgo. b Universidad Autónoma Metropolitana-Azcapotzalco. Área de Ciencias de Materiales.

Av. San Pablo 180, Col Reynosa Tamaulipas, C.P. 02200, México, D.F.

In this work, polypyrrole films are electrosynthesized in aqueous media over graphite-epoxy resin composite; sulphate ions used to dope the polymer are obtained from different salts (Na2SO4,Li2SO4, (NH4)2SO4 and K2SO4). Potentiostatic techniques are used to induce the polymerization. Potentiometric response of Ppy-SO4

-2

films was evaluated by calibration curves. Analyzing the slope sign, behavior of the different films can be classified in three responses:anionic, cationic and anionic-cationic. These behaviors were observed for all synthesized films, independently of the sulphate salt used. When analyzing the transients obtained for the differentsalts, similarities where found for those with the same potentiometric behavior, but significative differences were observed when comparing transients of different potentiometric response. It was determined that the cation size of the sulphate salt influences the Ppy-SO4

-2 film response. Boundaries on the potentiostatic synthesis parameters are suggested to induce a specific potentiometric response.

Introduction

One of the most studied and used conducting polymer is polypyrrole (Ppy), due to its good conductivity and stability. Ppy has found multiple applications like batteries, artificial muscles, development highly selectivity chemical sensors, etc (1-6). Ppy films can be obtained by electrochemical oxidation of pyrrole monomer to produce cationic radicals; this induces the insertion of anion species from supporting electrolyte into the polymer matrix by electrostatic attraction (1,5,7). This dynamic movement of the anions makes the Ppy film to form appropriate cavities with anion’s geometry (molecular impression) which restricts diffusion of other species across the polymer (8,9). Many variables may affect the electropolymerization process, for instance: solvent, electrolyte concentration, pH and the imposed potential (10-13). The polymer properties will depend on the electrosynthesis conditions, and related with this, the ion-diffusion characteristics (14-18). Among the wide variety of anions that can be used to dope Ppy films, sulphate ion stands out due to its electrochemical and potentiometric properties that significantly differ from other most common doping anions (14,19-23). Considering this, the

ECS Transactions, 20 (1) 31-40 (2009)10.1149/1.3268370 © The Electrochemical Society

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applications for Ppy-SO4-2 films is considerably wide (1,2). It has been reported that

sulphate ions (SO42-) have low mobility in Ppy polymeric matrix because of its size and

to its double charge (24,25). Differences in anions mobility through polymeric matrixes can be associated to the different interaction strength with conducting polymer chain, this interaction is stronger when ions are with small radios and large charge (5,14,24,25), this means that there is a strong ionic bound between SO4

-2 in solution and positive charge of the polymeric chain, which prevents SO4

-2 interchange between the polymer and the solution (14). It has been reported that cations coming from the salt used as source of SO4

-2 anions have influence in the properties of Ppy-SO4-2 films (14,26). Cation mobility

of Li+, Na+, K+ and Cs+ in Ppy-SO4-2 films depends on the cation radius and its

hydratation sphere, cations with larger radius are less hydrated and therefore more mobile in the polymer (14,27). Doping Ppy with low mobility anions, like sulphate, results in a cationic potentiometric response, this behavior has been demonstrated to depend in the Ppy synthesis conditions, solvent, the anion charge, etc (24,25,28-30).

In this work, the potentiometric behavior of Ppy-SO4-2 films synthesized in a graphite-

epoxy resin composite is studied. Graphite - epoxy resin composite is a more versatile and cheaper material than those used in traditional metallic electrodes, it can be miniaturized and can easily adopt different geometries which makes it a very versatile electrode (31,32). Ppy films are obtained potentiostatically considering different values for the imposed potential (E), time (t), pyrrole concentration and sulphate salt; these parameters influence directly on the Ppy properties and are highly correlated (33-35). In all cases, current transients are compared to determine if there are differences in the electrochemical kinetics of the polymerization, and so in the potentiometric properties. To evaluate the effect of the cation in the potentiometric response of the resulting Ppy-SO4

-2 film, different sulphate salts are used for the electrosynthesis. The Ppy-SO4-2

materials are obtained over graphite - epoxy resin composite, due this material is more versatile than traditional metallic electrodes, because it facilitates miniaturization, adaptability to different geometries and their use in applications diverse, such as the development of miniaturized chemical sensors.

Experimental

Reagents

All reagents used are analytical grade. Na2SO4, Li2SO4, K2SO4 (Aldrich), (NH4)2SO4

(Fluka) are used as support electrolytes. Pyrrole (Py) (Aldrich) is distillated in presence of pure N2. Ultrapure monocrystalline graphite powder 99,999%. Araldit epoxy resin and H.Y hardener. All solutions were prepared with deionised water obtained from a Milli Q (Millipore) system with 18.2 M . Py solutions were bubbled with pure N2 (36).

Instrumentation

A typical electrochemical cell with three electrodes is used. A platinum wire is used as counter electrode, an Ag/AgCl 900200 Orion electrode is used as reference, and thecomposite as work electrode. The composite electrode is prepared by a mixturing the graphite powder and the resin components in a 60:40 proportion relation. The composite is supported using a PTFE tube with an electrical contact. Ppy films are obtained

ECS Transactions, 20 (1) 31-40 (2009)


potenciostatically using a PGSTAT 30 AUTOLAB electrochemical system (Ecochemie).Potentiometric measurements are registered using a Mettler Toledo AG Potentiometer. All potentiometric measurements are referred to an Ag/AgCl saturated electrode.

Synthesis of Ppy films using different sulphate salts

Ppy-SO42- films are electropolymerized by chronamperometry over the exposed

surface of the composite electrode, the potentiostatic variables controlled are: imposed potential (E), time (t), pyrrole concentration [Py] and sulphate salt concentration [SO4

2-].Using the Box-Behnken method a series of experiments are proposed in order tomaximize sensitivity towards SO4

2- ion, using the corresponding sulphate salt as supportelectrolyte. The potentiometric response is evaluated by construction and statistical analysis of calibration curves using standards from the different sulphate salts. In order to optimize the potentiostatic variables used for the Ppy-SO4

-2 electrosynthesis towards a potentiometric anionic response, is used the optimization method Box - Behnken, which is based on a factorial design of second order at three levels (37).

Results and discussion

First experiments are done using the NaSO4 salt. Boundaries for the potentiostaticvariables are established to ensure an adequate synthesis of the polymer, results are shown in table 1. These boundary values are based on the results of some previous experiments:

To establish the working potential (E), a cyclic voltammogram is obtained using Na2SO4 0.1 M and Py 0.1 M in deionized water. The potential scan is set from -1200 to1200 mV (starting from the zero-current potential) and a scan rate of 100 mV/ s, the voltammogram is shown in figure 1. An oxidation wave is observed at potentials above -300 mV, this is associated with the Py oxidation and the progressive polymerization; this suggests that in cronamperometric experiments potentials above -300 mV will be enough to electrosynthesize the Ppy. However, in practice, with potentials between -300 and 200 mV no modification occurs on the electrode’s surface, between 200 and 520 mV only some small cumulus of the Ppy are observed; only with potentials above 520 mV anadequate synthesis is achieved, as it is observed a complete covering of the electrode´ssurface, maybe because at these potentials the applied energy is enough for the formation of the insoluble polymer instead of certain soluble olygomers.

TABLE I. Boundaries for potentiostatic parameters used for the Ppy-SO4-2 electrosynthesis.

Parameter Low HighE (V) 0.52 1.2t (min) 4.0 15[Py] (mol L-1) 0.05 0.4[SO4

2-] (mol L-1) 0.005 0.7



-6.00

-1.00

4.00

9.00

14.00

-1300 -800 -300 200 700 1200

J/m

Acm

-2

t/s

Figure 1. Cyclic voltammogram for the potentiodynamic synthesis of Ppy-SO4-2 using

a composite electrode, [Py] = 0.1 M, [SO42-] = 0.1M, 10 cycles and a scan rate of 100

mV/s.

When evaluating the time for the imposed potential (t), is was observed that lessthan four minutes are not enough for the polymer growth; times higher than 15 minutes have the disadvantage of long stabilization times when potentiometric measurements are carried out, this is attributed to the polymer thickness that difficult the ions diffusion and so equilibrium is established at long times. Then, the used interval for time is between 4 and 15 minutes.

In the case of sulphate concentration, it was observed that using concentrations under 5x10-3 M the polymer synthesis is not favored, whereas using higher concentrations to 0.7 M the sodium salt does not dissolved completely, so it is established an concentration interval between 5x10-3 and 0.7 M.

The same considerations used to establish the sulphate concentration interval are applied to establish the Py concentration; the interval is set between 0.05 and 0.4 M.

Calibration curves are obtained from the experiments suggested by Box-Behnken method, and the minimum square method is used to fit the experimental data. Sensitivity is evaluated through the slope’s value. Analysing the results for all the experiments, according to the Nernst equation the different potentiometric responses can be classified as follows: one corresponding to an anionic sensitivity (negative slope, the response is attribute to ion SO4

2-), another one to a cationic one (positive slope, the response is attributed to counter-ion) and both sensitivities anionic-cationic depending on the the considered concentrations, in figure 2 some representative calibration curves are shown for each case. These different behaviors can be attributed to the change in the morphology as a result of the different conditions of electroshynthesis. Figure 3 shows three current-density transients, one for each type of potentiometric behavior found; significant differences are observed when comparing each other. This suggests that the kinetics of the polymers is different for each case, and so the morphology, resulting in the different potentiometric responses observed. These results are interesting, considering that only cationic responses are reported for Ppy-SO4

-2 polymers (30) which is attributed to low mobility, size and charge of SO4

2- ion (26,27,30). However, in this research it is found that depending on the potentiostatic conditions used for Ppy-SO4

-2 polymerization,other behavior can be found.



60

80

100

120

140

160

180

-6 -5 -4 -3 -2 -1 0

E/m

V (A

g/A

gC

l)

log [SO4-2]

0

5

10

15

20

25

30

35

40

-5.5 -4.5 -3.5 -2.5 -1.5 -0.5 0.5

E/m

V (A

g/A

gC

l)

log [SO4-2]

(a) (b)

50

55

60

65

70

75

80

85

90

-6 -5 -4 -3 -2 -1 0

E/m

V(A

g/A

gC

l)

log [SO4-2]

(c)

Figure 2. Typical calibration curves for Ppy-SO4- 2polymers: (a) anionic response, (b)

cationic response, (c) anionic – cationic response.

11.00

11.50

12.00

12.50

13.00

13.50

14.00

14.50

0 100 200 300 400 500 600 700

J/m

Acm

-2

t/s

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

0 50 100 150 200 250 300 350

J/m

Acm

-2

t/s

(a) (b)



3.00

3.50

4.00

4.50

5.00

5.50

6.00

0 50 100 150 200 250 300

J/m

Acm

-2

t/s

(c)

Figure 3.Representative current-density transients for different Ppy-SO42- (a) Anionic

response, [Py] =0.287 M, [SO42-] =0.546M, E=0.96 V and t=10.80 min (b) Cationic

response [Py] =0.05 M, [SO42-] =0.5M, E=1.0 V and t=6.0 min, and (c) Anionic –

cationic response , [Py] =0.1 M, [SO42-] =0.1M, E=0.8 V and t=5.0 min.

When evaluating the different experiment combinations that the optimization method suggests, electrosynthesis boundaries can be established to induce the different potentiometric responses; this is shown in table 2. For the anionic response a theoretical value of -29.60 mV per decade of SO4

-2 concentration was expected, meanwhile for the cationic response, which corresponds to the diffusion of the Na+ ion, a slope of 59.14 mV per decade of sodium concentration was expected(both data to 25ºC). The maximum experimental slopes found for each type of response are shown in table 3; as it can be seen, in all cases subnernstian slopes were found.

TABLE II. Boundaries where the potentiometric response types can be induced for the Ppy-SO4-2.

Anionic Cationic Anionic-CationicE (V) 0.86 – 0.96 0.80 – 1.00 0.52 – 0.86t (min) 10.8 – 15.0 6.0 – 10.0 4.00 – 15.0[Py] (mol L-1) 0.28 – 0.40 0.05 – 0.4 0.05 – 0.225[SO4

2-] (mol L-1) 0.35 – 0.54 0.005 – 0.5 0.005 – 0.35

TABLE III. Maximum slope values found for Ppy-SO4-2 synthesized with Na2SO4.

Response Slopes maximum (mV per decade)Anionic -16.2Cationic 9.2Anionic - cationic -14.2 y 6.7

In behalf of the results found when using Na2SO4 as sulphate ion source, similar analysis was performed with Ppy-SO4

2- prepared using other sulphates salts (Li2SO4,

K2SO4 y (NH4)2SO4) in order to study the cation influence in the polymers morphology and in the potentiometric response. Figure 4 shows the different calibration curves for each type of potentiometric response with the different sulphate salts. It can be observed very similar behaviors to that found for the sodium salt, but with different slopes magnitudes, as it can be observed in table 4. The ammonium salt shows the highercationic response, this can be attributed to the fact that the cation has a bigger size and so it cannot be easily hydrated, therefore ammonium ions mobility within the polymer matrix is favored, meanwhile the contrary effect is observed for the lithium ion with asmaller size. Figure 5 shows the current-density transients obtained for the different salts,



both cationic and anionic – cationic transients show the same shape compared with those found for the sodium salt, which suggests that in all cases the kinetics of polymerization and nuclei formation are the same. Contrary, transients corresponding to the anionic response differ, which suggesting different kinetics mechanisms.

50

70

90

110

130

150

170

-6 -5 -4 -3 -2 -1 0

E/m

V (A

g/A

gC

l)

log [SO42-]

Li+

NH4+

K+

Na+

40

60

80

100

120

-6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0

E/m

V (A

g/A

gC

l)

log [SO42-]

Li+

Na+

NH4+

K+

(a) (b)

40

60

80

100

120

140

-6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0

E/m

V (A

g/A

gC

l)

log [SO42-]

Li+

Na+

NH4+

K+

(c)

Figure 4. Typical calibration curves corresponding to different sulphate salts (Na+, Li+,NH4

+ and K+) used for Ppy-SO42- electrosynthesis: (a) Anionic response, [Py] =0.287 M,

[SO42-] =0.546M, E=0.96 V and t=10.80 min; (b) Cationic response [Py] =0.05 M, [SO4

2-

] =0.5M, E=1.0 V and t=6.0 min, and (c) Anionic – cationic response, [Py] =0.1 M, [SO4

2-] =0.1M, E=0.8 V and t=5.0 min.

TABLE IV. Maximum slope values (mV per /decade) for the different Ppy-SO42- synthesized with different sulphate

salts.Anionic Cationic Anionic-Cationic

Na2SO4 -16.2 9.2 -14.2 y 6.7Li2SO4 -9.35 7.6 -6.35 y 17.2 K2SO4 -8.82 8.35 -7.1 y 4.75(NH4)2SO4 -10.35 17.61 -15.5 y 13.8



12.0

13.0

14.0

15.0

16.0

17.0

18.0

0 100 200 300 400 500 600

J/m

Acm

-2

t/s

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

0 50 100 150 200 250 300 350

J/m

Acm

-2

t/s

(a) (b)

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0 50 100 150 200 250 300

J/m

Acm

-2

t/s

(C)

Figure 5. Current-density transients for the different Ppy-SO42- electrosynthesized with

different sulphate salts (Na+, Li+, NH4+ and K+): (a) anionic (b) cationic (c) anionic -cationic.

Conclusions

According to the results obtained for the diverse Ppy-SO42- films potentiostatically

growth, different potentiometric responses were observed depending on the electrochemical variables used: anionic, cationic and a anionic – cationic combinationThis is attributed to different polymerization kinetics, as demonstrate with the different current-density transients obtained, therefore there are morphology differences in that have direct influence in the diffusion of ions through the polymer matrix. In all cases Ppy-SO4

2- films showed subnernstian slopes, the anionic had a maximum value of -16.2mV per decade, for when using the sodium salt; while the minimum value found was of -8.82 mV per decade for the Ppy-SO4

2- obtained from the potassium salt. The cation size of the counter ion sulphate salt has some influence in the potentiometric response, the big size cations move easier in the Ppy-SO4

2- matrix, because they are less hydrated, inducing a cationic response. The contrary effect is observed for smaller cations like lithium.

Acknowledgments

Na+

K+

NH4

+

Li+

Na+K

+

NH4

+

Li+

Na+

K+

NH4

+Li

+



TJLS is grateful to CONACYT for the stipend received for her Ph. D. studies. The authors are also grateful to CONACYT and PROMEP for the financial support giventhrough projects 80058 and 46481 respectively.

References

1. A. Ivaska, Electroanalysis 3, 247 (1991).2. M. Josovicz, Analyst., 120, 1019 (1995).3. J. Tamm, A. Hallik, A. Alumaa, and V. Sammelselg, Electrochim. Acta, 42, 2929

(1997).4. G. A. Álvarez Romero, E. Garfias García, M.T. Ramírez Silva, C. Galán Vidal,

M. Romero Romo, M. Palomar Pardavé, Appl. Surf. Sci., 252, 5783 (2006).5. T. A. Skotheim, Handbook of Conducting Polymers, Ed., New York: Marcel

Dekker, (1986).6. R. H. Baugham, Synth. Met., 78, 339 (1996).7. J. Tamm, U. Johanson, M. Marandi, T. Tamm, and L. Tamm, Russ. J.

Electrochem., 40, 3, 344 (2004).8. V. K. Gupta, S.Chandra, R. Mangla, Electrochim. Acta, 47, 1579 (2002).9. A. Michalska, U. Nadrzycka, K. Maksymiuk, Fresenius, J. Anal Chem., 371, 35,

(2001).10. E. Garfias García, M. Romero Romo, M.T. Ramírez Silva, J. Morales, M.

Palomar Pardavé, Electroanal Chem., 613, 67 (2008).11. R. Y. Quian, J. J. Qui, Polymer 19, 57 (1987).12. Y. Li, J. Yang, J. Polym. Sci., 65, 2739 (1997).13. Y. F. Li, J. Electroanal. Chem., 433, 181, (1997).14. J. Tamm, A. Alumaa, A. Hallik, U. Johanson, L. Tamm and T. Tamm.

Electrochem., 38, 2 (2002).15. T. A Skotheim, R. L. Elsenbaumer and J. R. Reynolds, Handbook of Conducting

Polymers, Eds., New York: Marcel Dekker (1998).16. G. Inzelt, Electrochim. Acta, 34, 83 (1989).17. A. R. Hillman, and S. Bruckenstein, J. Chem. Soc., 89, 339 (1993).18. J. Tamm, A. Alumaa, A. Hallik, J. Electroanal. Chem., 414, 149 (1996).19. L. F. Warren and D. P. Anderson, J. Electrochem. Soc. 134, 101 (1987).20. J. O. M. Bockris and F. B. Diniz, Electrochim. Acta, 34, 567 (1989).21. R. Qian and J. Qiu, J. Polym. 19, 157 (1987).22. A. Kassim, F. J. Davis and G. R. Mitchell, Synth. Met. 62, 41 (1994).23. G. Maia, E.A. Ticianelli and F.C. Nart, J. Phys. Chem. 186, 245 (1994).24. J. Tamm, A. Hallik, A. Alumaa and V. Sammelselg, Electrochim. Acta, 42, 2929

(1997).25. J. Tamm, A. Alumaa, A. Hallik and V. Sammelselg, J. Electroanal Chem., 448,

25 (1998).26. K. Naoi, M. Lien and W.H. Smyrl, J. Electrochem. Soc., 138, 440 (1991).27. Y. Li and, R. Qian, J. Electroanal. Chem., 362, 267 (1993).28. J. Tamm, A. Alumaa, A. Hallik, T. Silk, and V. Sammelselg, J. Electroanal

Chem., 414, 149 (1996).29. A. Ersoz, V.G. Gavalas, L. G. Bachas, Anal Bioanal Chem., 372, 786 (2002).30. S. Wencheng, J.O. Iroh, Synth. Met., 95, 159 (1998).31. S. Alegret, J. Alonso, J. Bartrolí, J. M. Paulis, Anal Chim. Acta, 164, 147 (1984).



32. G. A. Álvarez Romero, M. E. Palomar Pardavé, M.T. Ramírez Silva, Anal Bioanal Chem., 387, 1533 (2007).

33. B. R. Scharifker, E. Bockris, (eds), Moderns Aspects of Electrochemistry Plenum Press, Nueva York, (1992).

34. A. S. Liu and M. A. S. Oliveira, J. Braz. Chem. Soc., 18, 1, 143 (2007).35. A. A. Yakovleva, Russ. J. Electrochem., 36, 12, 127 (2000).36. Wang J., Analytical Electrochemistry, Second edition, Wiley-VCH, New York,

(2000).37. S. L. C. Ferreira, R.E. Bruns, H. S. Ferreira, Anal. Chim. Acta, 597, 179 (2007).



potentiometric behavior of diverse polypyrrole-sulphate films electrosynthesized on graphite –...

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