Conducting polymers as simultaneous sensor-actuators
Toribio Fernández-Otero*, Gemma Vázquez, Laura Valero
Center for Electrochemistry and Intelligent Materials, Universidad Politécnica de Cartagena, Paseo Alfonso XIII, 52, 30204 Cartagena, Spain
ABSTRACT
Environmental and electrical variables, as temperature, electrolyte concentration or driving current, influence oxidation and reduction oxidation rates of free-standing polypyrrole/DBSA/ClO4
- films. Under flow of a constant current for a constant time, decreasing electrical energies are consumed to oxidize or to reduce the film under increasing temperatures or rising electrolyte concentrations. By consuming the same charge under flow of rising constant currents, the consumed electrical energy increases. As conclusion the consumed electrical energy by flow of constant charges during oxidation, or reduction, of the film is a sensor of the electrochemical cell temperature, the electrolyte concentration or the flowing current. Those sensing capabilities seem to be a general property of the electrochemistry of conducting polymers. Any electrochemical based device, as actuators, polymeric batteries, smart membranes, smart drug delivery devices and others, are expected to sense environmental conditions while working. The sensing abilities of a complex actuator constituted by four polypyrrole films, two acting as electrodes (anodes or cathodes) and the other two as counter electrodes (cathodes or anodes, respectively) are presented. Experimental results are equivalent to sensing charge/discharge processes in all polymeric batteries.
Keywords: actuators, artificial muscles, reactive materials, sensing materials, linear movements, amperometric sensors, concentration sensors, temperature sensors, current sensors, electrical energy.
1. INTRODUCTION Films of conducting polymers behave as reactive electroactive materials in electrolytes: they are oxidized or reduced under flow of anodic or cathodic currents, respectively. The reaction occurs between polymeric chains integrated in a polymer film, balancing counterions that are interchanged between the reactive film and a solution, and electrons interchanged between the reactive film and the metallic contact. During reactions, electrons are injected or extracted from the polymeric chains and counterions are interchanged with the electrolyte for charge balance. Properties such as conductivity, volume, colour, porosity, stored charge and stored chemicals are reactive properties changing in parallel with the counterion content in the material along the reaction1-2.
The chemical nature of the reaction is being used in the design of devices, such as electrochemomechanical actuators, with unique simultaneous actuating and sensing abilities3-5. Any physical or chemical variable acting on the reaction (by improving or retarding the reaction rate) is expected to promote a shift of the electrical signals. So, bending artificial muscles (bilayer6-8 or three-layers9-12) working under constant current, respond at decreasing muscle potentials for rising actuation temperatures or increasing electrolyte concentrations10. When those muscles lift attached weights, the working chemical reaction encounters a resistance, because the volume change associated to the reaction becomes more difficult: rising muscle potentials are obtained for constant driving currents9,13.
Therefore, electro-chemo-mechanical actuators are able to sense: temperature, electrolyte concentration, shifted weight or driving current14. The actuator sense actuating and environmental variables while working. Both, actuating (current) and sensing (potential) signals, were included by the two connecting wires. The question remains if simultaneous actuation and sensing is a general property of the electrochemical reaction, in which case, devices based on any electrochemical property of the conducting polymers (CP) (actuator, battery, smart window, smart membrane, smart drug delivery, etc) should exhibit simultaneous sensing-actuating properties. *[email protected]; phone +34 968 32 55 19; fax +34 968 32 59 31
In this manner, the aim of this paper is to determine the sensing abilities of free-standing films of polypyrrole/DBSA/ClO4
- during electrochemical oxidation/reduction processes. As experimental variables, driving current, electrolyte concentration and working temperature will be explored. Based on the sensing abilities of self-supported films, the sensing capacity of complex devices constituted by four polypyrrole films integrated in two three-layers CP/tape/CP was investigated as a function of the same experimental variables previously studied in self-supported materials.
2. EXPERIMENTAL Pyrrole (Fluka) was purified by distillation under vacuum using a diaphragm vacuum pump MZ 2C SCHOTT and stored under nitrogen atmosphere at -10ºC. Anhydrous lithium perchlorate salt (Fluka) and dodecyl benzenesulfonic acid solution (70 wt % in 2-propanol) (DBSA) (Aldrich) were used as received. Ultrapure water from Millipore Milli-Q equipment was used.
Polypyrrole films were prepared at room temperature (20 ± 2ºC) in dark conditions in a one-compartment electrochemical cell from an aqueous solution of 0.1M LiClO4, 0.1M DBSA and 0.1M pyrrole. The working electrode was an AISI 316 stainless steel sheet, having a surface area of 5 cm2 either side. Deposition was performed on both sides of the electrode. As counter electrodes two larger electrodes (10 cm2) of the same material were used placed symmetrically on both sides of the working electrode in order to obtain a uniform electrical field. A standard Ag/AgCl electrode from Metrohm was used as reference electrode.
Polypyrrole (pPy) was electrogenerated by applying a constant anodic current of 2 mA·cm-2 during 2 hours. The overall charge consumed during the electropolymerization was 72 C. Two separate films coating each of the electrode faces, with a mass of 30 ± 0.2 mg and a thickness of 70 ± 10 μm were obtained. After peeling off the films from the working electrode, they were submerged in de-ionized water for 24 hours to remove DBSA excess from the polymer surface. Films obtained in the mixed media are more flexible and mechanical resistant than those obtained in perchlorate solutions.
The total mass of electropolymerized films was determined by means of a precision balance (±0.1 μg) by weight difference between coated and uncoated electrodes. Film thickness was measured using a COMECTA electronic digital micrometer with a precision of ±1 μm. All electrochemical studies were performed using an Autolab PGSTAT-100 potentiostat/galvanostat controlled by a personal computer using GPES electrochemical software. The electrochemical measurements were carried out in 0.1M LiClO4 aqueous solutions, using the polymer film as the working electrode, a stainless steel counterelectrode and a Ag/AgCl reference electrode, as shown in Figure 1. A Julabo T25 Cryostat/Thermostat (±0.1ºC) was used, to study the influence of the temperature. All the other experiments were performed at 20ºC (room temperature).
Figure 1. Experimental arrangement of the electrochemical equipment; electrical connections with: working electrode, WE, (polypyrrole self-supported film), counter electrode, CE, (steel plate) and reference electrode, RE, (Ag/AgCl) partially located inside the electrochemical cell. The cell contains the electrolytic solution.
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3.1 Construction of pPy triple layers.
For construction of triple-layer 15 it is necessary the electrogeneration of the polypyrrole films6. A charge of 28 Coulombs was consumed to generate on a stainless steel electrode two polypyrrole films coating both electrode faces and weighing 13.25±0.25 mg. The thickness of the films was obtained by the equation:
e = ρ⋅A
m (1)
The obtained thickness was 12.5±1 μm. Triple layers polypyrrole/doubleside tape/polypyrrole having an area of 2 cm×1.5 cm with a finally polypyrrole weight of 12.75±0.35 mg were manufactured. 3.2 Construction of actuators based on pPy.
To construct the actuators based on pPy films it was necessary to cut longitudinally a triple layer into two equal parts (Fig. 2(a)). At top of both triple layers an electric connection using conductive carbon cement and cooper wires were performed (Fig. 2(b)). The two triple layers were put face to face and the extra tapes at the bottom of the layer were adhered (Fig. 2(c)), remaining electrically connected the two internal polypyrrole films (Fig. 2(d)).
(a)
(b)
(c)
(d)
Figure 2. Construction of an actuator based on pPy. The external polypyrrole films were inter-connected to the working electrode (WE) of the potentiostat, and the two internal polypyrrole films acting as counterelectrode (CE)16. In order to follow the potential gradient between polypyrrole films acting as working and counter electrodes the reference electrode output from the potentiostat was short-circuited to the internal films (the counterlectrode). In this way the working potential17 of the device, or muscle potential, can be obtained as potential difference between working and counter electrodes at any time during current flow14. The new actuator was studied in LiClO4 aqueous solution.
Constant electric currents were applied through the system, recording the chronopotentiometric responses at ambient temperature or under different temperatures or different electrolyte concentrations.
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3. RESULTS AND DISCUSSION 3.1 Voltammetric responses.
The electrochemical behavior of the obtained polypyrrole/DBSA/ClO4- film was investigated by cyclic voltammetry in
0.1M LiClO4 aqueous solution at ambient temperature and 6mV/s of scan rate. Figure 3 shows the experimental voltammograms recorded between +0.4 V, as anodic potential limit, and -1.3V as cathodic potential limit. Only one cathodic maximum at -0.840 V and one anodic maximum at 0.150 V are observed on the electrochemical response. A large potential separation between anodic and cathodic peaks is observed, which can be attributed to the time consumed by balancing counterion diffusion throughout a so thick film. The high cathodic potential limit did not appear to lead to film degradation indicating that the hydrogen evolution is inhibited. Consecutive cyclic voltammograms give reproducible voltammetric responses indicating the high stability of the film under those experimental conditions.
The shape of the obtained voltammograms and peak potentials are similar to those reported for thinner PPy-DBS films18,19 in aqueous solutions and quite different to those reported for PPy-ClO4
- films.20 These results indicate that dodecyl benzene sulfonate (DBS-) is the prevailing counterion inside the material. Due to its larger size, the counterion remains trapped promoting an exchange of Li+ cations predominately during oxidation/reduction processes. This interchange will promote the insertion of cations promoting polymeric swelling during reduction and the cations expulsion with polymeric shrinking during oxidation. These volume changes have been confirmed by the sense of the angular movements of a PPy/tape actuator under the flow of anodic and cathodic currents21.
Figure 3. Experimental voltammogram recorded between +0.4 V, as anodic potential limit and –1.3 V as cathodic potential limit being
6 mV/s the scan rate: anodic maximum at 0.150V, and cathodic maximum at –0.850V.
The electrochemical reactions can be written in a simplified form as:
(pPy chain)s (DBS-Li+)n ↔ (pPy chain n+)(DBS-)n + n Li+ + n e- (2)
The reaction occurring from left side to right side is the anodic process in which electrons are extracted from the polymeric chains. In figure 1 these processes correspond to the current maximum at 0.150V. The charge extracted by oxidation of the material can be obtained by integration of this maximum from the line of current at 0.0 mA.
The reaction from the right side towards the left side is the cathodic process, that is, electrons are injected into the polymeric chains. This process is related to the cathodic current maximum at -0.850V and negative currents on the voltammograms. The injected charge during the reduction also can be obtained by integration of this maximum.
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3.2 Sensing abilities under constant current
In order to study the response of the material to different experimental variables a galvanostatic procedure was designed. After stabilization of the initial oxidation state by applying a constant current (-0.01 mA for 350 s) the material film was submitted to three consecutive current square waves (Figure 4). The chronopotentiometric responses obtained during consecutive oxidation/reduction processes were recorded.
_____Applied mA input signal
Experimental mV output signal
Initial polaritazion
Anodic
Cathodic
Rel
ativ
e po
tent
ial /
mV
Time / s
Cur
rent
/ m
A
Figure 4. Scheme of the applied current program used and potential response to the applied current during film oxidation (shift to
positive potentials) or reduction (shift to negative potentials) Figure 5 shows the chronopotentiograms obtained after the PPy film was subjected to different anodic and cathodic constant currents, ranging from ±1.5 to ±24 mA, passing a constant charge of 180 mC (figure 6.a). The measured potential shows that higher currents induce higher initial potential steps. This step is related to the different resistances present in the system: film resistance, interface resistance due to ion interchange, solution resistance or counterion resistance to diffuse inside the film. After this initial change the potential increases or decreases with time, for anodic or cathodic currents, respectively, following the electrodic processes.
Figure 5. Chronopotentiograms obtained when different cathodic and anodic currents, indicated on the figure; are applied according
the experimental methodology described by Figure 2, to a PPy/DBSA/ClO4- film in 0.1 M LiClO4 aqueous solution.
The electrical energy (Ee) consumed by the polymer film during each oxidation/reduction was calculated as:
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∫= dtEiEe ·· (3)
where i is the experimental constant current, E the potential evolution and t the time of current flow. Figure 6.b shows the variation of the electrical energy as a function of the applied current. A linear fit was obtained for both, cathodic and anodic processes. This result indicates that, as it was reported in previous works14 for bilayer or three-layer electrochemomechanical actuators, the sensing ability of the muscle potential related to the current is a general property of the material, and therefore, it could be extended to different electrochemical devices.
0 5 10 15 20 25-200
-150
-100
-50
0
50
100
150
200
Elec
tric
cha
rge
/ m
C
Current / mA
Anodic Cathodic
0 5 10 15 20 25
100
200
300
400
500
600
Elec
tric
al e
nerg
y / J
g-1
Current / mA
Anodic R2 0.998 Cathodic R2 0,989
(a) (b)
Figure 6. a) A constant electrical charge of 180 mC was consumed during the experiments from figure 5. b) Electrical energy consumed per gram by a pPy polymer film during constant current experiments. The evaluations were made in 0.1 M LiClO4 aqueous
solution at room temperature; the weight of the polymer film was 2.45 mg. R2 are the correlation coefficients.
3.3 Temperature sensor
In reaction (2), as for any other chemical or electrochemical process, under constant conditions (electrolyte concentration and oxidation potential) the reaction rate (anodic or cathodic) is expected to increase for rising experimental temperatures. This is equivalent to saying that working at a constant reaction rate, which means under flow of a constant current, the reaction is expected to occur with a lower resistance (that is at lower potentials) for increasing temperatures.
In order to gain more knowledge into the sensing abilities of the electroactive material, the film is submitted to square current waves (±0.8 mA for 60 s every current) at different temperatures (5, 10, 15, 20, 30 and 45 ºC).
Figure 7 shows the obtained chronopotentiograms in which the evolution of the reactive film potential is monitored.
For the anodic processes, decreasing potentials are observed, for the same times of current flow, at increasing temperatures. Rising available thermal energies require lower consumption of electrical energies.
Under constant current the material reaction requires lower potentials at increasing temperatures. By applying cathodic currents, a similar, but inverse, behaviour was observed.
Figure 8 shows the linear increase of the consumed electrical energy for decreasing working temperatures13 .As a partial conclusion the electroactivity of the material, understood as electrochemical reactions of polymeric oxidation or reduction (reaction 2), act as temperature sensors.
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0 100 200 300 400 500 600 700-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
Pote
ntia
l / V
Temperature / ºC
5ºC 10ºC 15ºC 20ºC 30ºC 45ºC
Figure 7. Chronopotentiograms obtained at different indicated temperatures when, +0.8 mA and -0.8 mA are applied to a
PPy/DBSA/ClO4- film during 60 s in 0.1 M LiClO4 aqueous solution.
0 10 20 30 40 502.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
Elec
tric
ene
rgy
/Jg-1
Temperature / ºC
AnodicR2 0.994 Cathodic R2 0.991
Figure 8. Electrical energy consumed per gram by a PPy/DBSA/ClO4
- film at different temperatures by flow of ±0.8 mA in 0.1 M LiClO4 aqueous solution, at room temperature. R2 are the correlation coefficients.
3.4 Concentration sensor
Coming back to reaction (2), under constant conditions (oxidation potential and temperature) the reaction rate (cathodic or anodic) is expected to increase for rising experimental electrolyte concentrations. This is equivalent to saying that working at a constant reaction rate, by flow of a constant current, the reaction is expected to occur with a lower resistance (that is lower potentials) for increasing concentrations of Li+ ions in solution. In order to gain more knowledge into the sensing abilities of the electroactive material, the film is submitted to square current waves (±1 mA for 50 s every current) at electrolyte concentrations (0.07, 0.1, 0.5, 0.8 and 1 M). Figure 9 shows the obtained chronopotentiograms in which the evolution of the reactive film potential is monitored and was referred to a common origin potential 0.0V. For the anodic processes, decreasing potentials are observed, for the same times of current flow, at increasing electrolyte concentrations. Rising available concentrations require lower consumption of electrical energies. Under constant current the constant material reaction rate requires lower potentials at increasing concentrations to occur. By applying cathodic currents, a similar, but inverse, behaviour was observed.
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0 10 20 30 40 50
0.0
0.2
0.4
0.6
0.8
1.0
Pote
ntia
l / V
Time / s
0.07M 0.1M 0.5M 0.8M 1M
0 10 20 30 40 50-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Pote
ntia
l / V
Time / s
Figure 9. Chronopotentiograms obtained from Ppy/DBSA/LiClO4 self-supported films for different LiClO4 concentrations. After state by applying a constant current of -0.01 mA for 350 sstabilization of the initial oxidation , the material film was submitted to square current steps of ±1 mA flowing for 50 s per step.
f ±1 mA in 0.07, 0.1,0.5, 0.8 ,1 M LiClO4 aqueous solution, at room temperature, the weight of the film was 2.54 mg.
The electrical energy per gram by a PPy/DBSA/ClO4- film as a function of the electrolyte concentration is showed in figure 10 by flow o
0.0 0.2 0.4 0.6 0.8 1.012.0
12.5
13.0
13.5
14.0
14.5
15.0
15.5
Elec
tric
ene
rgy
/ Jg
-1
[ LiClO
Anodic___ R2 0,991
Cathodic
4) / M
R2 0,997
, 0.1,0.5, 0.8 ,1 M LiClO4 aqueou erature, the weight of the film was 2.54 mg. R2 are the correlation coefficients
olution of the potential difference between the working and the counter-
Figure 10. Variation of the electrical energy per gram by a PPy/DBSA/ClO4
- film as a function of the electrolyte concentration. by s solution, at room tempflow of ±1 mA in 0.07
3.5 Behavior of devices based on pPy films.
Actuators described in the experimental part and based on pPy/ClO4- films produce a lineal movement by flow of a
constant current in an electrolytic solution. During current flow films acting as anodes are oxidized and swell: anions and water penetrate into polymeric matrix for charge and osmotic pressure balance. Films acting as cathodes are reduced and shrink2,22,23. In parallel to the current flow the evelectrodes (the muscle potential) was recorded.
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The electrochemical reactions taking place in this material can be written as:
the bottom of the device, related to the top, was up to 60% the length of the device under galv
Figure 11. Lineal movement of an actuator based on pPy films during oxidation process.
temperatures (0º, 5º,
or muscle potential, evolves through decreasing potential values for
2O)b]gel+ n(A )aq + mH2O [(pPyc+)(
Where: [c = a + n], [d = b + m].
The involved electrochemical reactions produced an opposite bending movement24-27 between triple layer films. As consequence a lineal movement of the device’s bottom towards the top is produced (figure 2). The relative lineal displacement attained by
anostatic conditions.
(a) (b)
Figure 11 shows a displacement of 38.8% the length of the device under galvanostatic conditions, by flow of ±10 mA during 30 seconds in 1M LiClO4. The actuator was characterize for those reverse lineal movements under different electrolyte concentrations (0.2M, 0.4M, 0.6M, 0.8M and 1M LiClO4) and under different working10º, 15º, 20º, 25º and 30ºC) following the muscle potential and the consumed electrical energies.
A constant current of 10 mA was sent through the device for the same time. For every concentration the movement starts from the same initial position of the device (figure 11a). Under different electrolyte concentrations the evolution of potential was can be observed in figure 12. The consumed charge (Q = i.t) was constant, so the lineal displacement was always the same. The potential of the actuator, increasing electrolyte concentrations (Figure 12).
0 5 10 15 20 25 30
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
0.2M 0.4M 0.6M 0.8M 1M
Pote
ntia
l / V
Time / s0 5 10 15 20 25 30
-2.25
-2.00
-1.75
-1.50
-1.25
-1.00
-0.75
-0.50
-0.25
0.00
Pote
ntia
l / V
Time / s
(a)
Fig of 38.8% the actuator length. (a) Oxidat f 10 mA. (b) Reduction process, or
downward lineal movement, under flow of -10mA.
(b)
ure 12. Evolution of the actuator potential for different LiClO4 concentration at room temperature during a relative displacemention process, or upward lineal movement, under flow o
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Similar results were obtained (Figure 13) for increasing experim
0 5 10 15 20 25 30
ental temperatures.
0
200
400
600
800
1000
1200
Pote
ntia
l / m
V
Te ature / smper
0ºC 5ºC 10ºC 15ºC 20ºC 25ºC 30ºC
0 5 10 15 20 25 30
-1200
-1000
-800
-600
-400
-200
0
(a)
Pote
ntia
l / m
V
Te rature / ºC
Fi t of 38.8% the actuator length: (a) Oxid of 10 mA. (b) Reduction process, or
downward lineal movement, under flow of -10mA.
describe the same lineal movement decreases for
s conclusion the sensing abilities of the complex actuators are similar to those of the basic polymeric material.
mpe
(b)
gure 13. Evolution of the actuator potential for different temperatures in 1M LiClO4 aqueous solution during a relative displacemenation process, or upward lineal movement, under flow
By integration of the experimental chronopotentiograms, the consumed electrical energies were obtained. Figure 14 shows that the lineal evolution of the consumed electrical energies toincreasing electrolyte concentrations and for increasing temperatures.
A
0.2 0.4 0.6 0.8 127
28
29
30
31
32
33
Elec
tric
ene
rgy
/ J.g
-1
[ LiClO4] / M
Anodic R2 0.993
Cathodic R2 0.990
(a)
0 5 10 15 20 25 30
27
28
29
30
31
32
33
34
Elec
tric
al e
nerg
y / J
.g-1
Temper
Anodic R2 0.995
ature /ºC
Cathodic R2 0.993
conditions ±10 mA during 30 second. (a orking in different temperatures in 1M LiClO4 electrolyte. R2 are the correlation coefficients.
(b)
Figure 14. Evolution of electrical energy of the actuator describing a lineal movement of 38.8% the device length under galvanostatic ) Working in different electrolyte concentrations. (b) W
Taking into account that the complex actuator is constituted by four polypyrrole films, working as anodes and cathodes, alternatively, Figures 12 and 13 also can be interpreted as charge/discharge curves of a polymeric
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battery. From that point of view, figures 14 in ose curves to working temperature
4. CONCLUSIONS
e working temperature. The two connecting wires should include both,
complex artificial muscle constituted by four polypyrrole films which consumed lectrical energy is a sensor of the working temperature and the electrolyte concentration. Taking into account that this evice is equivalent to a polymeric battery we can conclude that also polymeric batteries sense experimental variables
rge processes.
uthors acknowledge financial support from Spanish Government, Project MAT2008-06702, Seneca Foundation, roject 08684/PI/08, Consejería de Educación de Murcia, Plan Regional de Ciencia y Tecnología 2007-2010, Mexico overnment Project UAEM-PROMEP 103.5/06/1975 and Conacyt (Mexico) Project 47066.
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Both, oxidation and reduction reactions of the film studied under flow of constant currents act as sensors of either, the flowing current, the working temperature and the electrolyte concentration. The consumed electrical energy is, for every variable, a linear function of the studied variable.
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