- Email: [email protected]
Molar enthalpy of the polypyrrole electrochemistry
Journal of
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 562 (2004) 161–165 www.elsevier.com/locate/jelechem
Molar enthalpy of the polypyrrole electrochemistry T.F. Otero a
a,1
, M. Teresa Cortes b, I. Boyano
c,*
Lab. of Electrochemistry, Intelligent Materials and Devices, Universidad Polit ecnica de Cartagena, Paseo Alfonso XIII, 48, Cartagena 30203, Spain b Departamento de Quımica, Universidad de los Andes, Edificio A, tercer piso, Carrera 1 # 18A –10 Bogot a, Colombia c Lab. Ampliaci on de Quimica-Fisica, Universidad del Paıs Vasco, Pso. Manuel de Lardiz abal 3, San Sebasti an 20009/20010, Spain Received 26 March 2003; received in revised form 6 August 2003; accepted 27 August 2003
Abstract The kinetics of the angular movement of a triple-layer muscle working potentiostatically under different temperatures allowed the obtention of both the activation energy and the molar enthalpy of the process. By compaction of a polypyrrole film under constant temperature and at a constant cathodic potential, followed by a subsequent oxidation at a constant anodic potential under different temperatures, the molar enthalpy was obtained by application of the ESCR model. Similar results, 27.1 kJ/mol, from muscles, versus 27.7 kJ/mol, from relaxation experiments, were obtained. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Artificial muscle; Conformational relaxation; Anodic compaction; Cation exchange; Enthalpy
1. Introduction All organic artificial muscles formed by bilayer, or triple layers between electroactive polyconjugated materials and electronic insulators, adherent and flexible polymers, are accepted to work under electrochemical stimulation of the change of volume in the polyconjugated material [1–3]. The conformational movements stimulated by the extraction or injection of electrons from, or towards, the chains transform single bonds into double bonds [4], changing bond angles between monomeric units and promoting conformational movement and generation or destruction of free volume [5,6]. Penetration or expulsion of counterions guarantee both, charge balance inside the material and changes of volume [7]. When these conformational changes and changes of volume reach the polymer–polymer interface of the device, a stress gradient appear, responsible for the macroscopic angular movement of the muscle.
*
Corresponding author. Tel.: +34943015314; fax: +34943212236. E-mail addresses: [email protected] (T.F. Otero), pobbosai@ sc.ehu.es (I. Boyano). 1 Tel.: +34968325519; fax: +34968325932. 0022-0728/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2003.08.029
A second field, where the electrochemically induced conformational movements controls the kinetics of a process, is the oxidation of compacted films. Films of polypyrrole are reduced and compacted by cathodic polarization. When the compacted film was submitted to anodic oxidation by a potential step the attained chronoamperogram does not show the usual fast jump of the current to high anodic values followed by an exponential decay as expected from a diffusion control process. The presence of a nucleation process gives a second maxima, which was described and simulated under influence of different variables by the electrochemically stimulated conformational relaxation (ESCR) model [8–10]. The presence of these maxima also are known in the literature as a memory effect: cathodic polarization at the same potential, for the same time, under constant experimental conditions always gives the same chronoamperogram by a potential step to the same positive potential. Our aim now is to check if studying both systems working under control of conformational movements from their polymeric chains, a triple-layer muscle and a compacted film of polypyrrole, we can quantify some basic energetic magnitude like the molar enthalpy. Those results must be of particular interest for applications as artificial muscles acting as transducers from
162
T.F. Otero et al. / Journal of Electroanalytical Chemistry 562 (2004) 161–165
an electrical energy to a mechanical energy, and also for kinetic aspects related to smart windows, polymeric batteries, electrochemical liberation of chemicals, etc.
2. Experimental methods Polypyrrole films were electropolymerized and checked in a one-compartment electrochemical cell, connected to a PAR M270 potentiostat–galvanostat and controlled by a PC by Research Electrochemistry Software 4.23. Working electrode and counter electrodes were platinum sheets having 1 and 4 cm2 of surface area, respectively. A saturated calomel electrode (SCE) from Crison Instruments was used as the reference electrode. Pyrrole (Janssen) was distilled under vacuum before use and stored under N2 at )10 °C. Acetonitrile (Lab Scan, HPLC grade) and anhydrous lithium perchlorate (Janssen, minimum 99% content) were used as received. All the solutions were deareated by bubbling N2 for 10 min before the current flow. A Sartorius 4504MP8 ultramicrobalance (precision 107 g) was used to determine the polymer weight.
3. Results and discussion Two films of polypyrrole were electrogenerated by consecutive square waves of potential between )322 mV (2 s) and 872 mV (8 s) in 0.2 M pyrrole + 0.2 lithium perchlorate acetonitrile solution (with a 2% volume water content). One of the films was grown to a thickness of 0.22 lm on a platinum electrode having a 1 cm2 surface area by passing 100 mC. The film shows electrochromic properties (yellow in the reduced state and
blue dark after oxidation) until the end of the electrogeneration. Once reduced at )322 mV, rinsed and dried, the coated electrode was weighed and, by difference with the platinum weight, we concluded that 35 lg of polymer were electrogenerated. The second film was grown on a 3 cm2 stainless steel electrode by passing 28 C of charge, using two counterelectrodes of the same material, having the same dimensions on both sides and in parallel to the working electrode. Two films, each on one side of the electrode, having a thickness of 13 lm and weighing 6 mg, after reduction, rinsing and drying, were obtained. Using a double side tape, a triple layer artificial muscle: polypyrrole/tape/polypyrrole was obtained.
4. Molar enthalpies DH involved in the working muscle The movement through 90° of the free end of an electrochemomechanical device based on a triple layer (PPy(ClO 4 )/adhesive film/PPy(ClO4 )) was studied under potentiostatic conditions (Fig. 1). The reference electrode output was sort-circuited with the counterelectrode output being connected to one of the polypyrrole layers. The working electrode output was connected to the second polypyrrole film of the device. A constant anodic potential of 1 V was applied between the two films of polypyrrole. Electrochemomechanical properties were followed in 1 M LiClO4 aqueous solution at different temperatures: 5, 15, 25, 35 and 50 °C. Chronoamperometric responses were obtained during the established movement of the triple layer for every studied temperature (Fig. 2). From there the charge consumed to produce the stated mechanical movement at each temperature is obtained:
Fig. 1. (a) Pictures of movement of an artificial muscle. (b) A sketch of the triple-layer actuator (PPy/adherent film/PPy). The sketch shows a 90° movement related to the vertical position. This actuator moves in movements until 180° in LiClO4 aqueous solution.
T.F. Otero et al. / Journal of Electroanalytical Chemistry 562 (2004) 161–165
Fig. 2. Chronoamperomegrams obtained during a 90° movement of a triple layer (PPy/adherent polymer/PPy) in 1 M LiClO4 aqueous solution. The movement takes place under a constant anodic potential of 1 V.
Q¼
Z i dt;
163
Fig. 3. Arrhenius plot for the movement of 90° of a triple layer (PPy/ adherent polymer/PPy) in 1 M LiClO4 aqueous solution and under a constant anodic potential of 1 V. The slope is given by Ea =R (activation energy/gas constant).
ð1Þ
where Q is the consumed electric charge, i is the current flowing instantaneously in the electromechanical device and t is the differential time. Although the required time to cross-over 90° change with the temperature, the charge consumed remains constant (250 mC), whatever the temperature studied: the change of volume responsible for the movement is a function of the ions incorporated (oxidation) or expelled (reduction), which are under control (Eq. (1)) of the consumed charge, as it was stated previously [11]. This is because the position (cross-over angle) of the triple layer directly depends on the oxidation deep of the polypyrrole films. So, as the movement is the same the consumed charge will be the same although the temperature changes [12]. The activation energy of the involved reaction can be obtained from the chronoamperometric results. Because the angle described is controlled by the consumed charge, the rates of the movement must follow 1 Ea vel ffi ¼ A exp ; ð2Þ t RT 1 Ea ln ¼ ln A ; ð3Þ t RT where t is the required time to cross-over 90°, Ea is the activation energy, R is the gas constant, A is the preexponential Arrhenius factor, and T is the temperature of work. As expected by the fact that the movement rate is under control of the charge consumed per unit of time through Eq. (1), an Arrhenius plot is obtained from the experimental results (Fig. 3). From the slope, the empirical activation energy for the working muscle along the movement of 90° was obtained. The attained activation energy was: 29585.97 J/mol. From this energy, we can calculate the molar enthalpy
(DH ) involved in the electrochemomechanical process at any temperature through DH ¼ Ea RT :
ð4Þ
Considering a device working at ambient temperature, 25 °C (298 K): DH ¼ 29585:97 J=mol ð8:313 J=mol KÞð298 KÞ and DH ¼ 27108:7 J=mol: This molar enthalpy takes into account all the energetic components acting on the device: extraction (from one of the PPy films) or injection (into the second PPy film) of an electron from, or to the polymeric chains; rearrangement of the double bonds and bond angles producing conformational changes; changes in polymer–polymer, polymer–counterions and polymer–solvent interactions along the oxidation (one film) or reduction (the second film) processes from the initial stationary state to the final stationary state moreover diffusion of counterions into the PPy film (oxidation) or towards the solution (reduction).
5. Molar enthalpies, DH by ESCR model Any triple-layer artificial muscle works by electrochemical generation, in one of the PPy films, or destruction, in the second film of free volume by electrochemical stimulation of the conformational movements of the polymeric chains. All these aspects were treated by the ESCR model, which allowed a good description of the electrochemical responses with conducting polymers [8,9]. The model includes a parallel procedure to get the enthalpy consumed per mole of polymeric segments (DH ).
164
T.F. Otero et al. / Journal of Electroanalytical Chemistry 562 (2004) 161–165
Conducting polymers are compacted by reduction at high cathodic potentials. After compaction all the experimental chronoamperograms show a maximum, indicating different regions with conformational relaxation, nucleation, coalescence and diffusion control of the oxidation kinetics [10]. By physico-chemical quantification of those processes an equation was obtained for the evolution of the current after a potential step. By derivation, and equalizing to zero, was obtained [13] that the maximum follows: ln tmax ¼ C DH =RT :
ð5Þ
This equation indicates that performing experimental chronoamperograms between the same potential limits (overpotentials are included in DH ), keeping constant the electrolyte concentration and compacting the film under the same conditions, and changing the temperature of the potentiostatic oxidation every time, DH can be obtained from the slope of the semilogarithmic representation. Following those indications the aboveindicated platinum electrode coated with a polypyrrole film weighing 35 lg, which was immersed in a cell containing deareated 0.1 M LiClO4 acetonitrile solution. There it was submitted to a compaction potential [14,15] of )1200 mV (this is the reason to use acetonitrile solutions, avoiding water discharge) for 60 s. Then the compacted electrode was transferred into a second cell containing the same solution at a fixed temperature. There it was submitted to a potential step from )1200 mV, to an oxidation potential of 100 mV, kept for a time long enough to attain a stationary state of oxidation. In spite of the apparent different potential acting on the muscle and here, both anodic films support similar oxidation potentials of 100 mV when referred to the same reference (SCE) electrode. We mind that the driving 1 V was applied to the muscle between the two constituent polypyrrole films, one acting as the working electrode and the second, acting as the counterelectrode, shortcircuited to the RE output. The equivalent now, with the coated electrode, should be the potential difference between the WE and CE. So the actual potentials of the anodes are similar in both experiments. The influence of both the oxidation potential and the compaction potential (or memory effect) on the molar enthalpy will be considered in subsequent works. The procedure of electrochemical compaction at ambient temperature in one of the experimental cells and the subsequent potentiostatic oxidation under relaxation– nucleation–diffusion control in a second thermostated cell was repeated for different temperatures of oxidation (second cell) ranging between )10 and 30 °C. The experimental chronoamperograms are depicted by Fig. 4. Faster oxidation processes, and shorter tmax were obtained when the thermal energy of the bath was increases, producing faster conformational movements, faster electrochemical processes and faster diffusion of ions.
Fig. 4. Experimental chronoamperograms obtained in 0.1 M LiClO4 acetonitrile solutions using potential steps to 100 mV at different temperatures shown in the figure, after compaction of the film in a second cell at ambient temperature by polarization at )1200 mV for 60 s in the same electrolyte.
Coming back to Eq. (5), R being the gases constant, T is the temperature for the oxidation under conformational relaxation–nucleation–coalescence and diffusion control, tmax are the maximum times on the experimental chronoamperograms, and DH is the molar enthalpy of the overall process. Fig. 5 shows the semilogarithmic plot of the experimental results according to Eq. (5). And from the slope DH can be obtained. Slope R ¼ DH
ð6Þ
Fig. 5. Semilogarithmic representation of tmax vs. 1=T : the experimental values taken from Fig. 2(d). The conformational energy consumed per mole of polymeric segments in the absence of any electric field (DH ) can be obtained from the slope, following Eq. (5) and multiplying by the gas constant.
T.F. Otero et al. / Journal of Electroanalytical Chemistry 562 (2004) 161–165
(3316 (K) 8.31 (J/mol K) ¼ 27.7 kJ/mol). The similitude between enthalpic results obtained from the working conditions of artificial muscles under different temperatures, and those involving specifically conformational relaxation kinetic control, also under different temperatures, indicates, spite the different solvent, the prevalent influence of the conformational changes on the overall energetic influence.
6. Conclusions The importance of conformational relaxation processes in the oxidation of conducting polymer film was underlined by considering the effect of temperature on the rate of both the movement of the muscles under constant potential and conformational changes during a chronoamperometric oxidation after a cathodic compaction. The molar enthalpic increment between two stationary states was similar in both systems, obtained from the rate of the muscle movement, or using the theoretical ESCR model: DH by ESCR model ¼ 27:7 kJ=mol; DH by artificial muscles ¼ 27:1 kJ=mol: The ESCR model allows us, for the first time, to establish quantitative energetic magnitudes related to the electrochemical processes. Deeper studies in this direction would indicate new possibilities for the understanding, quantification and optimization of the multiple properties and applications related to the electrochemistry of conducting polymers, like: artificial
165
muscles, smart windows, all solid polymeric batteries, etc. The way is opened now to more careful studies on how thermodynamic magnitudes are influenced by solvents, experimental potentials, or other physical variables.
References [1] T.F. Otero, E. Angulo, J. Rodrıguez, C. Santamarıa, J. Electroanal. Chem. 341 (1992) 369. [2] T.F. Otero, J.M. Sansi~ nena, Bioelectrochem. Bioeng. 38 (1995) 411. [3] T.F. Otero;, J. Rodrıguez;, E. Angulo, C. Santamarıa, Synth. Met. 55 (1993) 3713. [4] J.L. Bredas, B. Themas, J.G. Flipiat, J.M. Andre, R.R. Chence, Phys. Rev. B 29 (1984) 6761. [5] P. Marque, J. Roncali, J. Phys. Chem. 94 (1990) 8614. [6] T. Yeu, K.M. Yin, J. Carbajal, R.E. White, J. Electrochem. Soc. 138 (1991) 2869. [7] M. Slama, J. Tanguy, Synth. Met. 28 (1989) C171. [8] T.F. Otero, H. Grande, J. Rodrıguez, Phys. Chem. 101 (1997) 3688. [9] T.F. Otero, H. Grande, J. Rodrıguez, J. Electroanal. Chem. 394 (1995) 211. [10] T.F. Otero, H. Grande, J. Electroanal. Chem. 414 (1996) 171. [11] T.F. Otero, in: H.S. Nalwa (Ed.), Handbook of Organic Conductive Molecules and Polymers, vol. 4, Wiley, New York, 1997, pp. 517–594. [12] T.F. Otero, M.T. Cortes, Electrochemical characterization and control of triple layer muscles, in: Proceedings of SPIE, Electroactive Polymer Actuators and Devices (EAPAD), vol. 3987, 2000, pp. 252–260. [13] T.F. Otero, H. Grande, J. Rodrıguez, Synth. Metals 83 (1996) 205. [14] T.F. Otero, H. Grande, Electrochim. Acta 44 (1999) 1983. [15] T.F. Otero, H. Grande, J. Rodrıguez, Synth. Metals 76 (1996) 293.
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 562 (2004) 161–165 www.elsevier.com/locate/jelechem
Molar enthalpy of the polypyrrole electrochemistry T.F. Otero a
a,1
, M. Teresa Cortes b, I. Boyano
c,*
Lab. of Electrochemistry, Intelligent Materials and Devices, Universidad Polit ecnica de Cartagena, Paseo Alfonso XIII, 48, Cartagena 30203, Spain b Departamento de Quımica, Universidad de los Andes, Edificio A, tercer piso, Carrera 1 # 18A –10 Bogot a, Colombia c Lab. Ampliaci on de Quimica-Fisica, Universidad del Paıs Vasco, Pso. Manuel de Lardiz abal 3, San Sebasti an 20009/20010, Spain Received 26 March 2003; received in revised form 6 August 2003; accepted 27 August 2003
Abstract The kinetics of the angular movement of a triple-layer muscle working potentiostatically under different temperatures allowed the obtention of both the activation energy and the molar enthalpy of the process. By compaction of a polypyrrole film under constant temperature and at a constant cathodic potential, followed by a subsequent oxidation at a constant anodic potential under different temperatures, the molar enthalpy was obtained by application of the ESCR model. Similar results, 27.1 kJ/mol, from muscles, versus 27.7 kJ/mol, from relaxation experiments, were obtained. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Artificial muscle; Conformational relaxation; Anodic compaction; Cation exchange; Enthalpy
1. Introduction All organic artificial muscles formed by bilayer, or triple layers between electroactive polyconjugated materials and electronic insulators, adherent and flexible polymers, are accepted to work under electrochemical stimulation of the change of volume in the polyconjugated material [1–3]. The conformational movements stimulated by the extraction or injection of electrons from, or towards, the chains transform single bonds into double bonds [4], changing bond angles between monomeric units and promoting conformational movement and generation or destruction of free volume [5,6]. Penetration or expulsion of counterions guarantee both, charge balance inside the material and changes of volume [7]. When these conformational changes and changes of volume reach the polymer–polymer interface of the device, a stress gradient appear, responsible for the macroscopic angular movement of the muscle.
*
Corresponding author. Tel.: +34943015314; fax: +34943212236. E-mail addresses: [email protected] (T.F. Otero), pobbosai@ sc.ehu.es (I. Boyano). 1 Tel.: +34968325519; fax: +34968325932. 0022-0728/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2003.08.029
A second field, where the electrochemically induced conformational movements controls the kinetics of a process, is the oxidation of compacted films. Films of polypyrrole are reduced and compacted by cathodic polarization. When the compacted film was submitted to anodic oxidation by a potential step the attained chronoamperogram does not show the usual fast jump of the current to high anodic values followed by an exponential decay as expected from a diffusion control process. The presence of a nucleation process gives a second maxima, which was described and simulated under influence of different variables by the electrochemically stimulated conformational relaxation (ESCR) model [8–10]. The presence of these maxima also are known in the literature as a memory effect: cathodic polarization at the same potential, for the same time, under constant experimental conditions always gives the same chronoamperogram by a potential step to the same positive potential. Our aim now is to check if studying both systems working under control of conformational movements from their polymeric chains, a triple-layer muscle and a compacted film of polypyrrole, we can quantify some basic energetic magnitude like the molar enthalpy. Those results must be of particular interest for applications as artificial muscles acting as transducers from
162
T.F. Otero et al. / Journal of Electroanalytical Chemistry 562 (2004) 161–165
an electrical energy to a mechanical energy, and also for kinetic aspects related to smart windows, polymeric batteries, electrochemical liberation of chemicals, etc.
2. Experimental methods Polypyrrole films were electropolymerized and checked in a one-compartment electrochemical cell, connected to a PAR M270 potentiostat–galvanostat and controlled by a PC by Research Electrochemistry Software 4.23. Working electrode and counter electrodes were platinum sheets having 1 and 4 cm2 of surface area, respectively. A saturated calomel electrode (SCE) from Crison Instruments was used as the reference electrode. Pyrrole (Janssen) was distilled under vacuum before use and stored under N2 at )10 °C. Acetonitrile (Lab Scan, HPLC grade) and anhydrous lithium perchlorate (Janssen, minimum 99% content) were used as received. All the solutions were deareated by bubbling N2 for 10 min before the current flow. A Sartorius 4504MP8 ultramicrobalance (precision 107 g) was used to determine the polymer weight.
3. Results and discussion Two films of polypyrrole were electrogenerated by consecutive square waves of potential between )322 mV (2 s) and 872 mV (8 s) in 0.2 M pyrrole + 0.2 lithium perchlorate acetonitrile solution (with a 2% volume water content). One of the films was grown to a thickness of 0.22 lm on a platinum electrode having a 1 cm2 surface area by passing 100 mC. The film shows electrochromic properties (yellow in the reduced state and
blue dark after oxidation) until the end of the electrogeneration. Once reduced at )322 mV, rinsed and dried, the coated electrode was weighed and, by difference with the platinum weight, we concluded that 35 lg of polymer were electrogenerated. The second film was grown on a 3 cm2 stainless steel electrode by passing 28 C of charge, using two counterelectrodes of the same material, having the same dimensions on both sides and in parallel to the working electrode. Two films, each on one side of the electrode, having a thickness of 13 lm and weighing 6 mg, after reduction, rinsing and drying, were obtained. Using a double side tape, a triple layer artificial muscle: polypyrrole/tape/polypyrrole was obtained.
4. Molar enthalpies DH involved in the working muscle The movement through 90° of the free end of an electrochemomechanical device based on a triple layer (PPy(ClO 4 )/adhesive film/PPy(ClO4 )) was studied under potentiostatic conditions (Fig. 1). The reference electrode output was sort-circuited with the counterelectrode output being connected to one of the polypyrrole layers. The working electrode output was connected to the second polypyrrole film of the device. A constant anodic potential of 1 V was applied between the two films of polypyrrole. Electrochemomechanical properties were followed in 1 M LiClO4 aqueous solution at different temperatures: 5, 15, 25, 35 and 50 °C. Chronoamperometric responses were obtained during the established movement of the triple layer for every studied temperature (Fig. 2). From there the charge consumed to produce the stated mechanical movement at each temperature is obtained:
Fig. 1. (a) Pictures of movement of an artificial muscle. (b) A sketch of the triple-layer actuator (PPy/adherent film/PPy). The sketch shows a 90° movement related to the vertical position. This actuator moves in movements until 180° in LiClO4 aqueous solution.
T.F. Otero et al. / Journal of Electroanalytical Chemistry 562 (2004) 161–165
Fig. 2. Chronoamperomegrams obtained during a 90° movement of a triple layer (PPy/adherent polymer/PPy) in 1 M LiClO4 aqueous solution. The movement takes place under a constant anodic potential of 1 V.
Q¼
Z i dt;
163
Fig. 3. Arrhenius plot for the movement of 90° of a triple layer (PPy/ adherent polymer/PPy) in 1 M LiClO4 aqueous solution and under a constant anodic potential of 1 V. The slope is given by Ea =R (activation energy/gas constant).
ð1Þ
where Q is the consumed electric charge, i is the current flowing instantaneously in the electromechanical device and t is the differential time. Although the required time to cross-over 90° change with the temperature, the charge consumed remains constant (250 mC), whatever the temperature studied: the change of volume responsible for the movement is a function of the ions incorporated (oxidation) or expelled (reduction), which are under control (Eq. (1)) of the consumed charge, as it was stated previously [11]. This is because the position (cross-over angle) of the triple layer directly depends on the oxidation deep of the polypyrrole films. So, as the movement is the same the consumed charge will be the same although the temperature changes [12]. The activation energy of the involved reaction can be obtained from the chronoamperometric results. Because the angle described is controlled by the consumed charge, the rates of the movement must follow 1 Ea vel ffi ¼ A exp ; ð2Þ t RT 1 Ea ln ¼ ln A ; ð3Þ t RT where t is the required time to cross-over 90°, Ea is the activation energy, R is the gas constant, A is the preexponential Arrhenius factor, and T is the temperature of work. As expected by the fact that the movement rate is under control of the charge consumed per unit of time through Eq. (1), an Arrhenius plot is obtained from the experimental results (Fig. 3). From the slope, the empirical activation energy for the working muscle along the movement of 90° was obtained. The attained activation energy was: 29585.97 J/mol. From this energy, we can calculate the molar enthalpy
(DH ) involved in the electrochemomechanical process at any temperature through DH ¼ Ea RT :
ð4Þ
Considering a device working at ambient temperature, 25 °C (298 K): DH ¼ 29585:97 J=mol ð8:313 J=mol KÞð298 KÞ and DH ¼ 27108:7 J=mol: This molar enthalpy takes into account all the energetic components acting on the device: extraction (from one of the PPy films) or injection (into the second PPy film) of an electron from, or to the polymeric chains; rearrangement of the double bonds and bond angles producing conformational changes; changes in polymer–polymer, polymer–counterions and polymer–solvent interactions along the oxidation (one film) or reduction (the second film) processes from the initial stationary state to the final stationary state moreover diffusion of counterions into the PPy film (oxidation) or towards the solution (reduction).
5. Molar enthalpies, DH by ESCR model Any triple-layer artificial muscle works by electrochemical generation, in one of the PPy films, or destruction, in the second film of free volume by electrochemical stimulation of the conformational movements of the polymeric chains. All these aspects were treated by the ESCR model, which allowed a good description of the electrochemical responses with conducting polymers [8,9]. The model includes a parallel procedure to get the enthalpy consumed per mole of polymeric segments (DH ).
164
T.F. Otero et al. / Journal of Electroanalytical Chemistry 562 (2004) 161–165
Conducting polymers are compacted by reduction at high cathodic potentials. After compaction all the experimental chronoamperograms show a maximum, indicating different regions with conformational relaxation, nucleation, coalescence and diffusion control of the oxidation kinetics [10]. By physico-chemical quantification of those processes an equation was obtained for the evolution of the current after a potential step. By derivation, and equalizing to zero, was obtained [13] that the maximum follows: ln tmax ¼ C DH =RT :
ð5Þ
This equation indicates that performing experimental chronoamperograms between the same potential limits (overpotentials are included in DH ), keeping constant the electrolyte concentration and compacting the film under the same conditions, and changing the temperature of the potentiostatic oxidation every time, DH can be obtained from the slope of the semilogarithmic representation. Following those indications the aboveindicated platinum electrode coated with a polypyrrole film weighing 35 lg, which was immersed in a cell containing deareated 0.1 M LiClO4 acetonitrile solution. There it was submitted to a compaction potential [14,15] of )1200 mV (this is the reason to use acetonitrile solutions, avoiding water discharge) for 60 s. Then the compacted electrode was transferred into a second cell containing the same solution at a fixed temperature. There it was submitted to a potential step from )1200 mV, to an oxidation potential of 100 mV, kept for a time long enough to attain a stationary state of oxidation. In spite of the apparent different potential acting on the muscle and here, both anodic films support similar oxidation potentials of 100 mV when referred to the same reference (SCE) electrode. We mind that the driving 1 V was applied to the muscle between the two constituent polypyrrole films, one acting as the working electrode and the second, acting as the counterelectrode, shortcircuited to the RE output. The equivalent now, with the coated electrode, should be the potential difference between the WE and CE. So the actual potentials of the anodes are similar in both experiments. The influence of both the oxidation potential and the compaction potential (or memory effect) on the molar enthalpy will be considered in subsequent works. The procedure of electrochemical compaction at ambient temperature in one of the experimental cells and the subsequent potentiostatic oxidation under relaxation– nucleation–diffusion control in a second thermostated cell was repeated for different temperatures of oxidation (second cell) ranging between )10 and 30 °C. The experimental chronoamperograms are depicted by Fig. 4. Faster oxidation processes, and shorter tmax were obtained when the thermal energy of the bath was increases, producing faster conformational movements, faster electrochemical processes and faster diffusion of ions.
Fig. 4. Experimental chronoamperograms obtained in 0.1 M LiClO4 acetonitrile solutions using potential steps to 100 mV at different temperatures shown in the figure, after compaction of the film in a second cell at ambient temperature by polarization at )1200 mV for 60 s in the same electrolyte.
Coming back to Eq. (5), R being the gases constant, T is the temperature for the oxidation under conformational relaxation–nucleation–coalescence and diffusion control, tmax are the maximum times on the experimental chronoamperograms, and DH is the molar enthalpy of the overall process. Fig. 5 shows the semilogarithmic plot of the experimental results according to Eq. (5). And from the slope DH can be obtained. Slope R ¼ DH
ð6Þ
Fig. 5. Semilogarithmic representation of tmax vs. 1=T : the experimental values taken from Fig. 2(d). The conformational energy consumed per mole of polymeric segments in the absence of any electric field (DH ) can be obtained from the slope, following Eq. (5) and multiplying by the gas constant.
T.F. Otero et al. / Journal of Electroanalytical Chemistry 562 (2004) 161–165
(3316 (K) 8.31 (J/mol K) ¼ 27.7 kJ/mol). The similitude between enthalpic results obtained from the working conditions of artificial muscles under different temperatures, and those involving specifically conformational relaxation kinetic control, also under different temperatures, indicates, spite the different solvent, the prevalent influence of the conformational changes on the overall energetic influence.
6. Conclusions The importance of conformational relaxation processes in the oxidation of conducting polymer film was underlined by considering the effect of temperature on the rate of both the movement of the muscles under constant potential and conformational changes during a chronoamperometric oxidation after a cathodic compaction. The molar enthalpic increment between two stationary states was similar in both systems, obtained from the rate of the muscle movement, or using the theoretical ESCR model: DH by ESCR model ¼ 27:7 kJ=mol; DH by artificial muscles ¼ 27:1 kJ=mol: The ESCR model allows us, for the first time, to establish quantitative energetic magnitudes related to the electrochemical processes. Deeper studies in this direction would indicate new possibilities for the understanding, quantification and optimization of the multiple properties and applications related to the electrochemistry of conducting polymers, like: artificial
165
muscles, smart windows, all solid polymeric batteries, etc. The way is opened now to more careful studies on how thermodynamic magnitudes are influenced by solvents, experimental potentials, or other physical variables.
References [1] T.F. Otero, E. Angulo, J. Rodrıguez, C. Santamarıa, J. Electroanal. Chem. 341 (1992) 369. [2] T.F. Otero, J.M. Sansi~ nena, Bioelectrochem. Bioeng. 38 (1995) 411. [3] T.F. Otero;, J. Rodrıguez;, E. Angulo, C. Santamarıa, Synth. Met. 55 (1993) 3713. [4] J.L. Bredas, B. Themas, J.G. Flipiat, J.M. Andre, R.R. Chence, Phys. Rev. B 29 (1984) 6761. [5] P. Marque, J. Roncali, J. Phys. Chem. 94 (1990) 8614. [6] T. Yeu, K.M. Yin, J. Carbajal, R.E. White, J. Electrochem. Soc. 138 (1991) 2869. [7] M. Slama, J. Tanguy, Synth. Met. 28 (1989) C171. [8] T.F. Otero, H. Grande, J. Rodrıguez, Phys. Chem. 101 (1997) 3688. [9] T.F. Otero, H. Grande, J. Rodrıguez, J. Electroanal. Chem. 394 (1995) 211. [10] T.F. Otero, H. Grande, J. Electroanal. Chem. 414 (1996) 171. [11] T.F. Otero, in: H.S. Nalwa (Ed.), Handbook of Organic Conductive Molecules and Polymers, vol. 4, Wiley, New York, 1997, pp. 517–594. [12] T.F. Otero, M.T. Cortes, Electrochemical characterization and control of triple layer muscles, in: Proceedings of SPIE, Electroactive Polymer Actuators and Devices (EAPAD), vol. 3987, 2000, pp. 252–260. [13] T.F. Otero, H. Grande, J. Rodrıguez, Synth. Metals 83 (1996) 205. [14] T.F. Otero, H. Grande, Electrochim. Acta 44 (1999) 1983. [15] T.F. Otero, H. Grande, J. Rodrıguez, Synth. Metals 76 (1996) 293.