- Email: [email protected]
dodecylbenzenesulphonate films in aqueous and non-aqueous electrolytes
Solid State Ionics 154 – 155 (2002) 331 – 335 www.elsevier.com/locate/ssi
Ion movement in polypyrrole/dodecylbenzenesulphonate films in aqueous and non-aqueous electrolytes K.P. Vidanapathirana a, M.A. Careem a,*, S. Skaarup b, K. West c b
a Department of Physics, University of Peradeniya, Peradeniya, Sri Lanka Department of Chemistry, Technical University of Denmark, Lyngby, Denmark c Riso National Laboratory, Roskilde, Denmark
Accepted 22 February 2002
Abstract The electrochemical characteristics during the redox process of polypyrrole (PPy) films, prepared using dodecylbenzenesulphonate (DBS ) dopant species, have been investigated using a combination of cyclic voltammetry and Electrochemical Quartz Crystal Microbalance (EQCM) measurements. Investigations were carried out using aqueous and non-aqueous electrolytes to study the effect of solvent on the ion movement during redox processes. When PPy films are cycled in aqueous electrolytes transport of both anion and cation occurs during oxidation and reduction. However, when cycled in the nonaqueous electrolyte propylene carbonate (PC) only anion movement takes place. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Conducting polymers; Cyclic voltammetry; Electrochemical quartz crystal microbalance; Polypyrrole; Ion movement
1. Introduction Conducting polymers such as polypyrrole (PPy), polythiophene and polyaniline are very attractive materials because of their unique redox, optical and electrical properties. Such properties have been the basis for the use of these materials in batteries [1], electrochromic devices [2], capacitors [3] and actuators [4]. Charge transport in these materials is controlled by the presence and movement of ions present. Since the ion transport in most conducting polymer films is much
*
Corresponding author. Tel.: +94-8-389-133; fax: +94-8-388018. E-mail address: [email protected] (M.A. Careem).
slower than the electron transport, ion movement governs their redox properties. There can be anion, cation and solvent transportation taking place during the redox processes [5]. The type of ions involved and the solvent amount transported can be obtained from cyclic voltammetry and EQCM measurements. Earlier we have reported that in PPy films polymerized and cycled in the presence of small anions in non-aqueous electrolytes the main mobile species are anions in propylene carbonate and cations in acetonitrile. When larger detergent type anions are used mainly cations present in the electrolyte become mobile but there is also some anion movement [6]. Also in poly-N-methylpyrrole films prepared and cycled in the presence of small anions in aqueous electrolytes anion transport occurs during redox process with water molecules
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 5 3 0 - 1
332
K.P. Vidanapathirana et al. / Solid State Ionics 154 – 155 (2002) 331–335
moving in the opposite direction [7]. Even though there have been a few studies related to the identification of moving ionic species during redox processes in conducting polymers, very little work has been reported regarding the role of the solvent and its transport in the redox processes. In this work, we attempt to study the effect of the solvent in determining the mobile species and the amount of solvent transported along with the ions in PPy films prepared in the presence of large, immobile dodecylbenzenesulphonate (DBS ) anion and cycled in aqueous and non-aqueous NaClO4 electrolytes.
2. Materials and methods PPy films were galvanostatically electro-polymerized on Pt wire electrodes or AT-cut quartz crystal electrodes as described elsewhere [6]. The polymerization electrolyte consisted of 0.05 M sodium dodecylbenzenesulphonate (SDBS) and 0.05 M pyrrole monomer. Films were prepared to have a calculated thickness of 0.25 Am. Ion movements in these films during redox processes were investigated using cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM) techniques. Cycling tests were
done in both NaClO4/H2O and NaClO4/propylene carbonate(PC) electrolytes.
3. Results and discussion A cyclic voltammogram of a PPy/DBS film cycled in aqueous NaClO4 electrolyte is shown in Fig. 1. During synthesis, DBS ions are incorporated into the polymer film to balance the charge on the polymer chain. It is believed that the large amphiphilic DBS anion is trapped inside the polymer matrix and is not directly involved in the redox process. This is partially assigned to the size and partially to the compatibility of the polar end with the charged matrix and of the non-polar end with the neutral (reduced) matrix [8]. Therefore, it is reasonable to assume that only the ions present in the cycling electrolyte move during the redox process. In the reduced state, for the polymer matrix to be neutral, cations(Na + ) from the electrolyte must be inserted. When the polymer is oxidized the cations should be expelled from the film. There are two oxidation peaks visible in the cyclic votammogram, one close to 0.5 V and the other close to 0.2 V, indicting that there are, however, two different pro-
Fig. 1. Cyclic voltammogram of a PPy/DBS film cycled in aqueous 0.5 M NaClO4 electrolyte. Scan rate: 50 mV s
1
.
K.P. Vidanapathirana et al. / Solid State Ionics 154 – 155 (2002) 331–335
cesses taking place during oxidation. In the first process, a partial oxidation of the PPy film takes place with cations (Na + ) being expelled. In the second process, occurring at higher voltages, the PPy film gets highly oxidized and anions (ClO4 ) from the electrolyte are inserted into the film to maintain the charge balance. Thus the redox process is a two step process similar to the ones reported for PPy made with dodecylsulphate [9]. Simultaneous cyclic voltammogram and the corresponding frequency change obtained for a PPy/DBS film prepared on a quartz electrode and cycled in 0.1 M aqueous NaClO4 electrolyte is shown in Fig. 2a. Here the frequency axis shows the deviation of the crystal frequency from its initial value ( f 9 MHz). During the first oxidation process, there is an increase in frequency indicating a decrease in mass of the film occurring up to 0.1 V. This decrease in mass can be attributed to expulsion of cations (Na + ). This has been established earlier also for the case of PPy/DBS films cycled in NaCl aqueous electrolyte [6]. The slope of the graph of mass change (DW) versus number of moles of electrons which participated in the process
333
(Dn) is 248.1 g mol 1 (Fig. 2b) which is much greater than the value of 23 g mol 1 expected for Na + ion movement alone. It seems that water molecules accompany the cations (Na + ). The difference in values of the slopes indicates that 10– 20 water molecules accompany each Na + ion. This is a far larger amount than the one to four water molecules expected to be strongly bound in the solvation shell of Na + [10]. An earlier work has indicated that any solvent inside the polymer matrix can be fully exchanged during cycling [11]. However, in the system under study full exchange of water occurs too but it seems that the water expulsion from the film is not as fast as insertion since there is a considerable difference in the slopes. At more positive potentials ( f 0.25 V), crystal frequency reduces indicating an increase in the polymer mass. This can be attributed to anion (ClO4 ) insertion. The slope of the graph of DW vs. Dn corresponds to this region is 70.59 g mol 1 of electron which is smaller than 99.5 g mol 1 expected for only anion movement. Therefore, in addition to anion insertion, expulsion of water molecules also must take place. From the difference in values for the masses, it
Fig. 2. Mass changes during cycling of a PPy/DBS film prepared on EQCM electrode. Cycling electrolyte: 0.1 M NaClO4, Scan rate: 5 mV s 1. (a) Cyclic voltammogram with frequency change, (b) mass change (DW) vs. number of moles of electrons which participated in the process (Dn).
334
K.P. Vidanapathirana et al. / Solid State Ionics 154 – 155 (2002) 331–335
Fig. 3. Cyclic voltammograms of PPy films made with aqueous SDBS electrolyte and with ClO4 ion in non-aqueous electrolyte (propylene carbonate—PC). Cycling electrolyte: 0.5 M NaClO4/PC. Scan rate: 50 mV s 1.
can be concluded that for each anion inserted one or two water molecules move out of the film. When the scanning potential is reversed, opposite reactions to the ones mentioned above are taking place. At the beginning, mass of the polymer decreases
due to ClO4 anion expulsion with water molecules getting inserted into the film to fill up the space left by the anions. There is no noticeable mass change between the potentials 0.1 to 0.60 V. This may be due to cancellation of mass loss caused by the anion
Fig. 4. Mass changes in a PPy/DBS film deposited on an EQCM electrode and cycled in 0.5 M NaClO4/PC, scan rate: 5 mV s 1. (a) Cyclic voltammogram with frequency change, (b) mass change (DW) vs. number of moles of electrons which participated in the process (Dn).
K.P. Vidanapathirana et al. / Solid State Ionics 154 – 155 (2002) 331–335
ejection or by the mass increase due to water insertion. However, at 0.6 V a dramatic increase in mass of the film occurs corresponding to the large reduction peak in the cyclic voltammogram. The mass increase can be attributed to cation insertion along with a large amount of water molecules. Peres et al. [9] have reported a similar two step redox process for the gravimetric study of PPy with dodecylsulphate anion [9]. Fig. 3 shows the cyclic voltammograms for PPy/ ClO4 and PPy/DBS films cycled in a non-aqueous PC electrolyte containing NaClO4. There is only one set of well-defined oxidation and reduction peaks in the cyclic voltammograms. It has been shown that ClO4 ions get inserted/ejected during oxidation/reduction in PPy/ClO4 films when cycled in PC electrolytes [11]. Therefore, it can be concluded that similar ion movements also occur in the PPy/DBS system when the solvent is propylene carbonate. Fig. 4 shows the simultaneous cyclic voltammogram and the corresponding frequency and mass changes of a PPy/DBS film when cycled in nonaqueous PC electrolytes. The EQCM results indicate a decrease in mass of the film during oxidation. It is difficult to identify the moving ionic species from the large value of the mass change obtained, 520.8 g mol 1. A possible reason could be that the large solvent movement taking place may over shadow the anion movement. Another indication for this large solvent movement was the observed increase of the impedance of the crystal from a value of 350 to 823 V before and after oxidation. This shows that the large solvent insertion may cause swelling of the polymer film causing it to be very soft. The swelling causes the polymer film to loose its rigidity and produces damped oscillation. This shows up as an increase in the impedance of the crystal. In other words, in this situation the film may be too soft for making any reliable EQCM measurements.
4. Conclusions When PPy/DBS films are cycled in NaClO4 aqueous electrolytes, movements of both anions and cat-
335
ions take place as a two-step process during redox reactions. The type of ion involved depends on the potential at which oxidation or reduction occurs. Water transport also takes place during the redox process. While 10 –20 water molecules accompany a cation (Na + ), only one to four water molecules move out when an anion (ClO4 ) get inserted in to the film. However, cycling in non-aqueous PC electrolyte results in only anion movement accompanied by large amount of solvent. According to the results obtained, it can be concluded that the mobile ionic species in PPy/DBS films depends on the type of the solvent used for the cycling electrolyte.
Acknowledgements Financial assistance from International Science Program, Uppsala University, Sweden is highly appreciated. A research grant for the ARTMUS—Artificial muscles project from the THOR—program of the Danish Research Council is gratefully acknowledged.
References [1] A. Mohommadi, O. Inganas, I. Lundstrom, J. Electrochem. Soc. 133 (1986) 947. [2] K. Kobayashi, N. Colaneri, M. Boysel, F. Wudl, A.J. Heeger, J. Chem. Phys. 82 (12) (1985) 5717. [3] A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, J.P. Ferraris, J. Power Sources 47 (1994) 89. [4] Q. Pei, O. Inganas, Synth. Met. 55 – 57 (1993) 3718. [5] H. Yang, J. Kwak, J. Phys. Chem. B 101 (1997) 4656. [6] S. Skaarup, K. West, L.M.W.K. Gunaratne, K.P. Vidanapathirana, M.A. Careem, Solid State Ionics 136 – 137 (2000) 577. [7] K.P. Vidanapathirana, M.A. Careem, in: B.V.R. Chowdari, et al. (Eds.), Solid State Ionics: Science and Technology, World Scientific Publishing, Singapore, 2000, p. 421. [8] M.A. De Paoli, R.C.D. Peres, S. Panero, B. Scorosati, Electrochim. Acta 37 (7) (1992) 1173. [9] R.C.D. Peres, M.A. De Paoli, R.M. Torresi, Synth. Met. 48 (1992) 259. [10] R.A. Robinson, R.H. Stokes, Electrolyte Solutions, Butterworths, London, 1970, p. 331. [11] S. Skaarup, K. West, B. Zachau-Christiansen, M.A. Careem, G.K.R. Senadeera, Solid State Ionics 72 (1994) 108.
Ion movement in polypyrrole/dodecylbenzenesulphonate films in aqueous and non-aqueous electrolytes K.P. Vidanapathirana a, M.A. Careem a,*, S. Skaarup b, K. West c b
a Department of Physics, University of Peradeniya, Peradeniya, Sri Lanka Department of Chemistry, Technical University of Denmark, Lyngby, Denmark c Riso National Laboratory, Roskilde, Denmark
Accepted 22 February 2002
Abstract The electrochemical characteristics during the redox process of polypyrrole (PPy) films, prepared using dodecylbenzenesulphonate (DBS ) dopant species, have been investigated using a combination of cyclic voltammetry and Electrochemical Quartz Crystal Microbalance (EQCM) measurements. Investigations were carried out using aqueous and non-aqueous electrolytes to study the effect of solvent on the ion movement during redox processes. When PPy films are cycled in aqueous electrolytes transport of both anion and cation occurs during oxidation and reduction. However, when cycled in the nonaqueous electrolyte propylene carbonate (PC) only anion movement takes place. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Conducting polymers; Cyclic voltammetry; Electrochemical quartz crystal microbalance; Polypyrrole; Ion movement
1. Introduction Conducting polymers such as polypyrrole (PPy), polythiophene and polyaniline are very attractive materials because of their unique redox, optical and electrical properties. Such properties have been the basis for the use of these materials in batteries [1], electrochromic devices [2], capacitors [3] and actuators [4]. Charge transport in these materials is controlled by the presence and movement of ions present. Since the ion transport in most conducting polymer films is much
*
Corresponding author. Tel.: +94-8-389-133; fax: +94-8-388018. E-mail address: [email protected] (M.A. Careem).
slower than the electron transport, ion movement governs their redox properties. There can be anion, cation and solvent transportation taking place during the redox processes [5]. The type of ions involved and the solvent amount transported can be obtained from cyclic voltammetry and EQCM measurements. Earlier we have reported that in PPy films polymerized and cycled in the presence of small anions in non-aqueous electrolytes the main mobile species are anions in propylene carbonate and cations in acetonitrile. When larger detergent type anions are used mainly cations present in the electrolyte become mobile but there is also some anion movement [6]. Also in poly-N-methylpyrrole films prepared and cycled in the presence of small anions in aqueous electrolytes anion transport occurs during redox process with water molecules
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 5 3 0 - 1
332
K.P. Vidanapathirana et al. / Solid State Ionics 154 – 155 (2002) 331–335
moving in the opposite direction [7]. Even though there have been a few studies related to the identification of moving ionic species during redox processes in conducting polymers, very little work has been reported regarding the role of the solvent and its transport in the redox processes. In this work, we attempt to study the effect of the solvent in determining the mobile species and the amount of solvent transported along with the ions in PPy films prepared in the presence of large, immobile dodecylbenzenesulphonate (DBS ) anion and cycled in aqueous and non-aqueous NaClO4 electrolytes.
2. Materials and methods PPy films were galvanostatically electro-polymerized on Pt wire electrodes or AT-cut quartz crystal electrodes as described elsewhere [6]. The polymerization electrolyte consisted of 0.05 M sodium dodecylbenzenesulphonate (SDBS) and 0.05 M pyrrole monomer. Films were prepared to have a calculated thickness of 0.25 Am. Ion movements in these films during redox processes were investigated using cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM) techniques. Cycling tests were
done in both NaClO4/H2O and NaClO4/propylene carbonate(PC) electrolytes.
3. Results and discussion A cyclic voltammogram of a PPy/DBS film cycled in aqueous NaClO4 electrolyte is shown in Fig. 1. During synthesis, DBS ions are incorporated into the polymer film to balance the charge on the polymer chain. It is believed that the large amphiphilic DBS anion is trapped inside the polymer matrix and is not directly involved in the redox process. This is partially assigned to the size and partially to the compatibility of the polar end with the charged matrix and of the non-polar end with the neutral (reduced) matrix [8]. Therefore, it is reasonable to assume that only the ions present in the cycling electrolyte move during the redox process. In the reduced state, for the polymer matrix to be neutral, cations(Na + ) from the electrolyte must be inserted. When the polymer is oxidized the cations should be expelled from the film. There are two oxidation peaks visible in the cyclic votammogram, one close to 0.5 V and the other close to 0.2 V, indicting that there are, however, two different pro-
Fig. 1. Cyclic voltammogram of a PPy/DBS film cycled in aqueous 0.5 M NaClO4 electrolyte. Scan rate: 50 mV s
1
.
K.P. Vidanapathirana et al. / Solid State Ionics 154 – 155 (2002) 331–335
cesses taking place during oxidation. In the first process, a partial oxidation of the PPy film takes place with cations (Na + ) being expelled. In the second process, occurring at higher voltages, the PPy film gets highly oxidized and anions (ClO4 ) from the electrolyte are inserted into the film to maintain the charge balance. Thus the redox process is a two step process similar to the ones reported for PPy made with dodecylsulphate [9]. Simultaneous cyclic voltammogram and the corresponding frequency change obtained for a PPy/DBS film prepared on a quartz electrode and cycled in 0.1 M aqueous NaClO4 electrolyte is shown in Fig. 2a. Here the frequency axis shows the deviation of the crystal frequency from its initial value ( f 9 MHz). During the first oxidation process, there is an increase in frequency indicating a decrease in mass of the film occurring up to 0.1 V. This decrease in mass can be attributed to expulsion of cations (Na + ). This has been established earlier also for the case of PPy/DBS films cycled in NaCl aqueous electrolyte [6]. The slope of the graph of mass change (DW) versus number of moles of electrons which participated in the process
333
(Dn) is 248.1 g mol 1 (Fig. 2b) which is much greater than the value of 23 g mol 1 expected for Na + ion movement alone. It seems that water molecules accompany the cations (Na + ). The difference in values of the slopes indicates that 10– 20 water molecules accompany each Na + ion. This is a far larger amount than the one to four water molecules expected to be strongly bound in the solvation shell of Na + [10]. An earlier work has indicated that any solvent inside the polymer matrix can be fully exchanged during cycling [11]. However, in the system under study full exchange of water occurs too but it seems that the water expulsion from the film is not as fast as insertion since there is a considerable difference in the slopes. At more positive potentials ( f 0.25 V), crystal frequency reduces indicating an increase in the polymer mass. This can be attributed to anion (ClO4 ) insertion. The slope of the graph of DW vs. Dn corresponds to this region is 70.59 g mol 1 of electron which is smaller than 99.5 g mol 1 expected for only anion movement. Therefore, in addition to anion insertion, expulsion of water molecules also must take place. From the difference in values for the masses, it
Fig. 2. Mass changes during cycling of a PPy/DBS film prepared on EQCM electrode. Cycling electrolyte: 0.1 M NaClO4, Scan rate: 5 mV s 1. (a) Cyclic voltammogram with frequency change, (b) mass change (DW) vs. number of moles of electrons which participated in the process (Dn).
334
K.P. Vidanapathirana et al. / Solid State Ionics 154 – 155 (2002) 331–335
Fig. 3. Cyclic voltammograms of PPy films made with aqueous SDBS electrolyte and with ClO4 ion in non-aqueous electrolyte (propylene carbonate—PC). Cycling electrolyte: 0.5 M NaClO4/PC. Scan rate: 50 mV s 1.
can be concluded that for each anion inserted one or two water molecules move out of the film. When the scanning potential is reversed, opposite reactions to the ones mentioned above are taking place. At the beginning, mass of the polymer decreases
due to ClO4 anion expulsion with water molecules getting inserted into the film to fill up the space left by the anions. There is no noticeable mass change between the potentials 0.1 to 0.60 V. This may be due to cancellation of mass loss caused by the anion
Fig. 4. Mass changes in a PPy/DBS film deposited on an EQCM electrode and cycled in 0.5 M NaClO4/PC, scan rate: 5 mV s 1. (a) Cyclic voltammogram with frequency change, (b) mass change (DW) vs. number of moles of electrons which participated in the process (Dn).
K.P. Vidanapathirana et al. / Solid State Ionics 154 – 155 (2002) 331–335
ejection or by the mass increase due to water insertion. However, at 0.6 V a dramatic increase in mass of the film occurs corresponding to the large reduction peak in the cyclic voltammogram. The mass increase can be attributed to cation insertion along with a large amount of water molecules. Peres et al. [9] have reported a similar two step redox process for the gravimetric study of PPy with dodecylsulphate anion [9]. Fig. 3 shows the cyclic voltammograms for PPy/ ClO4 and PPy/DBS films cycled in a non-aqueous PC electrolyte containing NaClO4. There is only one set of well-defined oxidation and reduction peaks in the cyclic voltammograms. It has been shown that ClO4 ions get inserted/ejected during oxidation/reduction in PPy/ClO4 films when cycled in PC electrolytes [11]. Therefore, it can be concluded that similar ion movements also occur in the PPy/DBS system when the solvent is propylene carbonate. Fig. 4 shows the simultaneous cyclic voltammogram and the corresponding frequency and mass changes of a PPy/DBS film when cycled in nonaqueous PC electrolytes. The EQCM results indicate a decrease in mass of the film during oxidation. It is difficult to identify the moving ionic species from the large value of the mass change obtained, 520.8 g mol 1. A possible reason could be that the large solvent movement taking place may over shadow the anion movement. Another indication for this large solvent movement was the observed increase of the impedance of the crystal from a value of 350 to 823 V before and after oxidation. This shows that the large solvent insertion may cause swelling of the polymer film causing it to be very soft. The swelling causes the polymer film to loose its rigidity and produces damped oscillation. This shows up as an increase in the impedance of the crystal. In other words, in this situation the film may be too soft for making any reliable EQCM measurements.
4. Conclusions When PPy/DBS films are cycled in NaClO4 aqueous electrolytes, movements of both anions and cat-
335
ions take place as a two-step process during redox reactions. The type of ion involved depends on the potential at which oxidation or reduction occurs. Water transport also takes place during the redox process. While 10 –20 water molecules accompany a cation (Na + ), only one to four water molecules move out when an anion (ClO4 ) get inserted in to the film. However, cycling in non-aqueous PC electrolyte results in only anion movement accompanied by large amount of solvent. According to the results obtained, it can be concluded that the mobile ionic species in PPy/DBS films depends on the type of the solvent used for the cycling electrolyte.
Acknowledgements Financial assistance from International Science Program, Uppsala University, Sweden is highly appreciated. A research grant for the ARTMUS—Artificial muscles project from the THOR—program of the Danish Research Council is gratefully acknowledged.
References [1] A. Mohommadi, O. Inganas, I. Lundstrom, J. Electrochem. Soc. 133 (1986) 947. [2] K. Kobayashi, N. Colaneri, M. Boysel, F. Wudl, A.J. Heeger, J. Chem. Phys. 82 (12) (1985) 5717. [3] A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, J.P. Ferraris, J. Power Sources 47 (1994) 89. [4] Q. Pei, O. Inganas, Synth. Met. 55 – 57 (1993) 3718. [5] H. Yang, J. Kwak, J. Phys. Chem. B 101 (1997) 4656. [6] S. Skaarup, K. West, L.M.W.K. Gunaratne, K.P. Vidanapathirana, M.A. Careem, Solid State Ionics 136 – 137 (2000) 577. [7] K.P. Vidanapathirana, M.A. Careem, in: B.V.R. Chowdari, et al. (Eds.), Solid State Ionics: Science and Technology, World Scientific Publishing, Singapore, 2000, p. 421. [8] M.A. De Paoli, R.C.D. Peres, S. Panero, B. Scorosati, Electrochim. Acta 37 (7) (1992) 1173. [9] R.C.D. Peres, M.A. De Paoli, R.M. Torresi, Synth. Met. 48 (1992) 259. [10] R.A. Robinson, R.H. Stokes, Electrolyte Solutions, Butterworths, London, 1970, p. 331. [11] S. Skaarup, K. West, B. Zachau-Christiansen, M.A. Careem, G.K.R. Senadeera, Solid State Ionics 72 (1994) 108.