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High performance electrochemical pseudocapacitors from ionic liquid assisted electrochemically synthesized p-type conductive polymer
Accepted Manuscript High performance electrochemical pseudocapacitors from ionic liquid assisted electrochemically synthesized p-type conductive polymer A. Ehsani, H. Mohammad Shiri, E. Kowsari, R. Safari, J. Torabian, S. hajghani PII: DOI: Reference:
S0021-9797(16)30901-8 http://dx.doi.org/10.1016/j.jcis.2016.11.024 YJCIS 21752
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
10 September 2016 4 November 2016 7 November 2016
Please cite this article as: A. Ehsani, H. Mohammad Shiri, E. Kowsari, R. Safari, J. Torabian, S. hajghani, High performance electrochemical pseudocapacitors from ionic liquid assisted electrochemically synthesized p-type conductive polymer, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis. 2016.11.024
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High performance electrochemical pseudocapacitors from ionic liquid assisted electrochemically synthesized p-type conductive polymer
A. Ehsani*a, H. Mohammad Shirib, E. Kowsaric, R. Safaria, J. Torabiana, S. hajghanib
a
Department of Chemistry, Faculty of science, University of Qom, Qom, Iran b
c
Department of Chemistry, Payame Noor University, Iran
Department of chemistry, Amirkabir University of Technology, Tehran, Iran
*Corresponding author. E-mail address:[email protected] [email protected]
1
Abstract In this paper firstly, 1-methyl-3-methylimidazolium bromide (MB) as a new high efficient ionic liquid was synthesized using chemical approach and then fabricated POAP/ MB films by electro- polymerization of POAP in the presence of MB to serve as the active electrode for electrochemical supercapacitor. Theoretical study (AIM) and electrochemical analysis have been used for characterization of ionic liquid and POAP/ MB composite film. Different electrochemical methods including galvanostatic charge– discharge experiments, cyclic voltammetry and electrochemical impedance spectroscopy are carried out in order to investigate the performance of the system.
This work
introduces new most efficient materials for electrochemical redox capacitors with advantages including ease synthesis, high active surface area and stability in an aqueous electrolyte.
Keywords: POAP, composite, supercapacitor, electrosynthesis, ionic liquid
2
1. Introduction
The great need of new technologies to energy storage devices caused to attract the attention of researchers to electrochemical capacitors (ECs) .
Electrochemical
supercapacitors are the charge-storage devices having high power density and long cyclic life [1–3]. Carbon materials, metal oxides and conductive polymers are three categories of active materials that use in ECs. The first stores the electrical energy at the double layer interface and the others store the energy by a charge transfer reaction [4-19]. Electronically conducting polymers (ECPs) such as polyaniline (PANI), polypyrrole (PPy)
and
poly(3,4-ethylenedioxythiophene)
(PEDOT)
have
been
applied
in
supercapacitors, due to their excellent electrochemical properties and lower cost than other ECPs. Polyaniline (PANI) is one of the most important conducting polymers because of its unique electrical, optical, and optoelectrical properties, as well as its ease of preparation and its excellent environment stability. Aminophenols are interesting members of the class of substituted anilines. The hydroxyl group in the phenyl ring can be oxidized to quinine and quinine can be reduced again. Poly ortho aminophenol (POAP) gives a surface film of interesting electrochemical and electrochromic properties when it is electropolymerised in acidic solution. This film is electroactive in aqueous and non-aqueous solutions containing protons but no response is observed at pH-value higher than pH 7. The variety of results for conductivity of the POAP film reported in the literature [20-25] show that the electrochemical response of POAP is strongly influenced by the experimental procedure used to produce the polymer film, dopant anions and the
3
purity starting monomer [20-24]. The electrical properties of conductive polymer could be modified by the addition of different organic and inorganic filler. Basically, a typical ionic liquid molecule is formed by a bulky organic cation (i.e. ammonium, imidazolium, pyri-dinium, pyrrolidinium, piperidinium, phosphonium, sulfonium) in combination with a complex anion such as bromide, chloride, methioninate. Simple changes in the cation and anion combinations or the nature of the moieties attached to each ion allow the physical properties of ionic liquids to be tailored for specific applications. The chemistry of ionic liquids has developed dramatically during the last decade. Their generic, yet not universal, properties, such as negligible vapour pressure, nonflammability, chemical and thermal stability, and outstanding solvation ability, enabled rapid advance in numerous applications. In particular, the use of ionic liquids in electrochemical devices such as dye-sensitized solar cells, supercapacitors, lithium batteries, actuators and fuel cells has been studied extensively [26,27]. In the present work, room temperature electrochemically synthesized POAP/ MB electrode is presented as an efficient potential candidate in supercapacitor application. Our goals in this paper were increasing the capacitance of POAP electrode by using 1methyl-3-methylimidazolium bromide (MB) (Scheme 1) to form a composite electrode and moreover increase the cycle ability of the electrode. The capacitive behavior of composite was tested by cyclic voltammetry, galvanostatic charge discharge and impedance spectroscopy techniques.
4
Scheme 1. Molecular structure of ionic liquid (1-methyl-3-butylimidazolium bromide)
5
2. Experimental 2.1. Reagent and materials All the chemical materials used in this work, obtained from Merck Chemical Co., were of analytical grade and used without further purification. Double distilled water was used throughout the experiments. 2.2. Characterization All electrochemical experiments were carried out by a Potentiostat/galvanostat (Ivium V21508, Vertex). A conventional three electrode cell with an Ag/AgCl reference electrode (Argental, 3 M KCl) was used in order to carry out the electropolymerization of the POAP. A platinum wire and a carbon paste electrode was used as the counter and working electrodes respectively.
2.3. Synthesis of ionic liquid(MB) Preparation of ionic liquid 1-methyl-3-butylimidazolium bromide The mixture of the 1methylimidazole (1.58 mL, 0.02 mol) and 1-bromobutane (2.1 mL, 0.02 mol) was stirring at room temperature for 20 min and then was refluxed for 12 h in toluene. The reaction mixture was cooled to room temperature and the resultant viscous 1-methyl-3methylimidazolium bromide was dried in vacuum for 5 h and washed with ether. The crude product was purified by crystallization from acetonitrile to give IL (0.33 g, 75.5%). Spectral
data
synthesis
of
1-methyl-3-butylimidazolium
bromide
(C8H15N2Br) yield: 0.33 g (75.5%); IR (KBr): (cm−1) 3173 (m), 31,220 (m), 2981 (m), 2952 (m), 2894 (m), 1583 (s), 14,752 (s), 1391 (m), 1355 (m), 1183 (s), 1041 (s), 773 (m)
6
cm−1. 1H NMR (acetone-d6): δ=0.90 (t, 3 H, CH2CH2CH2CH3, J(HH)=7.2 Hz), 1.94 (m, 2 H, CH2CH2CH2CH3, 3J(HH)=7.2 Hz), 1.98 (m, 2 H, CH2CH2CH2CH3, 3J(HH)=7.2 Hz), 4.13 (s, 3 H, NCH3), 4.44 (t, 2 H, N CH2CH2CH2CH3, 3 J(HH)=7.2 Hz), 7.98 (d, 2 H, H4,5 (Im)), 10.22 (t, 1 H, H2 (Im)). Anal. Calcd. for C8H15N2Br (219.14): C 43.50, H 6.83, Br 36.45. Found: C 43.12, H 6.62, Br 36.97 [28].
2.4. Synthesis of POAP and POAP/ MI composite POAP/ MB composites were prepared by in a stirring solution containing 0.01 M monomer, 0.5 M HClO4, 0.1 M LiCO4 and 5.0 ×10-3 M sodium dodecyl sulfate (SDS) on the surface of the ionic liquid (3% w/w) assisted carbon paste electrode. POAP electrode was synthesized in same solution without MI in the carbon paste electrode. Electropolymerizations were conducted by 40 consecutive cycles at the sweep rate of 50 mV.s-1 in the potentials between -0.5 to 1.0V. The
mass of POAP
films was
approximated assuming a current efficiency for the electropolymerization process of 100%, using Faraday’s law.
7
2. Results and discussion Firstly, although the focus of the present work has been on the experimental study of the electrochemical systems, the results obtained here may be used to semi-empirical and theoretical (as computational) study charge/energy transfer effects in the used ionic liquid Systems (to better understand transport effects/phenomena in these systems). For example, to conclude our discussion, we have been studied the transport effects in the molecular systems, using theory of atoms- in- molecule (AIM), which is a generalization of quantum theory to proper nano-size systems, describing molecular systems in terms of the topology of the atomic electron density, and its Laplacian, bond critical points and its bond order, atomic electronic energy, and Virial/potential force [29-32]. Based on the QTAIM, the atomic electronic energy, Eelec () , is given by
Eelec () elec () Kelec () where elec ()
(1)
and Kelec () are the total atomic virial and kinetic energies,
respectively. In addition, based on the AIM theory, the electron population of the atomic basin in molecule is given by n (r )dr , where (r ) the is local electron
density. In this work, the electronic wave functions corresponding to the optimized molecular geometries obtained at density functional level of theory (DFT-B3LYP/631G*). In addition, the details of the local intra-molecular electronic charge and energy transfer of the ionic liquid (1-methyl-3-butylimidazolium bromide) system are calculated, using the AIM theory. Samples of the results are reported in Tab. 1 and demonstrated in Fig. 1.
8
Table 1: the average value (in a.u.) of the electron density, (r ) Laplacian of electron density, ,
(r ) , atomic electronic kinetic energy K () , and average local Virial/potential force, v(r ) , for some atomic (or functional group) basins ( ), of the molecular system is studied in this work. 2
N (PY-ring) C (C-N bond) Br
(r )
2 (r )
K ()
(r )
6.896
2.124 102
5.503 101
1.1074 102
5.942
1.172 102
3.717 101
7.527 101
17.834
4.42 101
24.874 101
49.543 102
9
(a)
(d)
(b)
(c)
(e)
(f)
Fig. 1: The contour maps ( (topology of the electronic properties) ) of the local electron density (a),
Laplacian of electron density (b),
electronic kinetic energy (c), virial force (d) and
HOMO/LUMO orbital’s (e/f) of the bi-pyridine (PY) ring, are calculated using AIM theory.
According to our previous works [33, 34] cyclic voltammetry (CV) was used to deposit POAP coatings from ortho aminophenol monomer solution. Fig. 2 shows the typical multi-sweep cyclic voltammograms during electropolymerization in the presence of ionic liquid. As seen in the Fig. 2, OAP is oxidized irreversibly at around 600 mV without corresponding cathodic processes in the reverse scan.
10
Figure 2. Typical multi-sweep cyclic voltammograms during electropolymerization of POAP in the presence of ionic liquid.
To elucidate the effect of ionic liquid on the property of POAP films, electrochemical performance of composite films was evaluated by carrying out CV measurements in acidic solution [35-37]. Figure 3 shows the CVs of POAP and POAP/ MB electrodes in 0.1 M HClO4 solution. As observed, the capacitance of composite electrode is about two times greater than that of POAP electrode, which shows using ionic liquid in POAP electrode, enhances the capacity of the electrode. The synergetic effect resulting from the interactions of POAP and ionic liquid may affect the shape of CV curves. The CV of POAP/ MB electrode exhibits nearly rectangular shapes in potential window that shows the incorporation of MB in POAP matrix increase the capacitance of composite film.
11
Figure 3. Cyclic voltammograms of POAP (1) and POAP/ MB (2) electrodes in acidic solution. The surface morphology of POAP/ MB film was studied by using the fractal concept. It has been reported [22, 23] that for a diffusion controlled redox transition, which occurred via diffusion of an electroreactant species to a target surface, followed by heterogeneous electron transfer, there is a power dependence between the peak current (I pc) in cyclic voltammograms and the corresponding potential sweep rate (ν):
I pc F
(2)
Where is the fractal parameter and F is a proportionality factor.
12
Thus, the fractal parameter can be obtained easily by plotting the peak current against the sweep rate in log-log scale. On the other hand, the fractal parameter is related to the fractal dimension (Df) of the electrode surface as [20-22]:
Df 1 2
(3)
Eq. (3) is applicable to electrochemical methods, as it has been successfully used for calculating the fractal dimension of electrode surface. Based on this information, Fig. 4 represents cyclic voltammograms of POAP/ MB films that were recorded in different potential sweep rates in the range of 10-400 mVs-1. The slope of the straight line of anodic peak current and potential sweep rate in log-log scale lines gives the value of α for composite film . Substituting the value in Eq. (3), gives the fractal dimensions of 2.41 electrosynthesized film. The presented values for fractal dimension confirms porous structure of the electrosynthesized composite film.
13
Figure 4. Cyclic voltammograms of POAP/ MB films that were recorded in different potential sweep rates in the range of 10-400 mVs-1.
Galvanostatic charge/discharge method has been used to highlight the capacitance characteristic of POAP/ MB composite electrode. Figure 5 shows the charge/discharge behavior of POAP and POAP/ MB electrodes in 0.1M HClO4 solution at current of 0.005mA. As it can be seen, a triangular shape between this potential ranges is observed that indicating good columbic efficiency and ideal capacitive behavior of POAP/ MB as electrode for application in supercapacitor. Specific capacitance of the composite film obtained by charge/discharge curves using the following equation [24]:
14
(4) Where I is the current loaded, m is reactive material mass, V is the potential change during discharge process and t is the discharge time. The mass of deposited polymer on electrode was calculated from the charge (Q) passed during characteristic CV of electrode based on Faraday’s law: Q = znF, where Q is the difference of passed charge through polymer electrodes, z is the number of exchanged electrons and n is the number of moles. By substituting the obtained values in equation 5, the SC of POAP/ MB electrode was found to be 489 F.g-1. One of the most important parameters for practical application is cycling stability. Figure 6 shows the stability of POAP and POAP/MB composite electrodes when cycled at a current of 1 mA for 1000 cycles. Stability of electrodes is compared in terms of losing their capacities as stability percentage. The stability of electrode calculated from following equation: Stability = Cn /C1 ×100
(5)
In that Cn is the capacitance of electrode in each cycles and C1 is capacitance of electrode in the first cycle. Results shows, using MB in POAP caused an excellent retention in stability percentage of composite electrode suggesting the good stability toward long time charge-discharge applications. While the POAP electrode loses its stability fast, composite electrode maintains its stability and saves more that 90.6% of its capacitance of the first cycle under consecutive cycles after 1000 cycles.
15
Figure 5. (a) Galvanostatic charge and discharge measurements of POAP and POAP/ MB electrode at current of 0.005 mA in 0.1M HClO4 solution,(b) galvanostatic charge and discharge curves of POAP/MB during 10 consecutive cycles and (c) during100 consecutive charge-discharge.
16
Figure 6. Cycle stability of POAP (1) and POAP/ MB(2) composite electrodes.
Electrochemical impedance spectroscopy (EIS) is one of the best techniques for analyzing the properties of conducting polymer electrodes and it has been broadly discussed in the literature using a variety of theoretical models [36-43]. In the case of electrochemical system, EIS can reveal information regarding processes occurring in the polymer matrix when it is doped. This may include kinetic values of the doping process and parameters of the diffusion of ions into the polymers. EIS was analyzed for POAP and POAP/ MB films in acidic solution of HClO4. Fig. 7 shows the Nyquist diagrams of electrodes in OCP. The plot in Fig. 7 depicts a single semi-circle in the high frequency region and a straight line in the low-frequency region for two spectra. The high-frequency arc is the overall contact impedance generated from the electrical connection between POAP/ MB composites and the backing plate as well as the charge transfer at the contact interface between the electrode and the electrolyte solution [35]. In spite of the similar 17
shape of the impedance spectra, there is an obvious difference between the diameters of the two semi-circles. That is, the diameters of the semicircles decline with presence of ionic liquid in POAP. In other words, the bulk-film transport of electrons and the charge transfer resistance (Rct) of POAP/ MB films are lower than that of the pure POAP films. This means that the composites have faster electron transport in the bulk-film and charge transfer in the parallel POAP film/solution interface compared to that in the originally single POAP film/solution interface. This fact may suggest that the ionic liquid has an obvious improvement effect, which makes the composites have more active sites for faradic reactions and a larger specific capacitance than pure POAP.
Figure 7. Nyquist plots recorded from 10 kHz to 0.01 Hz with ac amplitude of 5 mV for POAP (1) and POAP/ MB (2) electrode. 18
4. Conclusions We
have
demonstrated
a
simple
and
general
strategy,
namely
in
situ
electropolymerization for preparing conducting polymer/ ionic liquid composite films. We have introduced the POAP/ ionic liquid composite electrode to improve the specific capacitance and power characteristic of electrochemical capacitance. The used ionic liquid (MB) has an obvious improvement effect, which makes the composites have more active sites for faradic reaction and larger specific capacitance than pure POAP.
Acknowledgements The authors would like to express their deep gratitude to the Iranian Nano Council for supporting this work.
References [1] B.E. Conway, Electrochemical Supercapacitors, Scientific Fundamentals and Technological Applications, Kulwar Academic/Plenum Publishers, New York, 1997. [2] B.E. Conway, J. Electrochem. Soc. 138 (1991) 1539–1547. [3] K. Kakaei, M.Hamidi, S. Husseindoost, J. Colloid interface. Sci, 479(2016) 121-126. [4] K. Chen, D. Xue, J. Colloid interface. Sci, 416(3)(2014)172-176. [5] K. Chen, D. Xue, J. Colloid interface. Sci, 424(18)(2014)84-89. [6] E. Frackowiak, F. Beguin, Carbon 40 (2002) 1775. E. Frackowiak, F. Beguin, Carbon 39 (2001) 937. [8] N. Salehifar, J. Shabani Shayeh, SO. Ranaei Siadat, K. Niknam, A. Ehsani, S. Kazemi Movahhed, RSC Adv, 5 (2015) 96130 – 96137. 19
[9] J. Shabani-Shayeh, A. Ehsani, MR. Ganjali, P. Norouzi, B. Jaleh, Appl. Surf. Sci, 353(2015) 594. [10] J. Shabani Shayeh, P. Norouzi, M.R. Ganjali, RSC Advances, 5(2015) 20446-20452. [11] J. Shabani-Shayeh, A. Ehsani, A. Nikkar, P. Norouzi, M. R. Ganjali, M. Wojdyla, New. J. Chem, 39( 2015) 9454. J. Power Sources J. Power Sources J. Power Sources [15] K.S. Ryu, K.M. Kim, Y.J. Park, N.G. Park, M.G. Kang, S.H. Chang, Solid State 152 (2002) 861. Ionics, [16] C. Portet, P.L. Taberna, P. Simon, E. Flahaut, C.L. Robert, Electrochim. Acta, 50 (2005) 4174. [17] K. Chen, S. Song, F. Liu, D. Xue, Chem. Soc. Rev, 44(2015) 6230-6257. [18] K. Chen, D. Xue, J. Mater. Chem. A, 4(2016) 7522-7537. [19]J. Shabani, [20] M. Naseri, L. Fotouhi, A. Ehsani, H. Mohammad Shiri, J. Colloid interface. Sci, 484 (2016) 308–313. [21] A. Ehsani, M.G. Mahjani, M. Jafarian, Synth. Met, 162(2012) 199. [22] H. Mohammad Shiri, A. Ehsani, J. Colloid interface. Sci, 484 (2016) 70–76. [23] H. Mohammad Shiri, A. Ehsani , J. Colloid interface. Sci, 5 (2016) 91062. [24] H. Mohammad Shiri, A. Ehsani, J. Colloid interface. Sci, 484 (2016) 70–76. [25] J. Torabian, M. G. Mahjani, H. Mohammad Shiri, A. Ehsani, J. Shabani Shayeh, RSC Adv, 6(2016) , 41045.
20
[26] R. Hagiwara, J.S. Lee, Ionic liquids for electrochemical devices, Electrochem- istry 75 (2007) 23 [27] E. Kowsari, A. Zare, V. Ansari, Int. J. Hydogen Energy, 40 (2015) 13964-13978. [28] I. Yavari, E. Kowsari, Mol Divers, 13 (2009) 519–528. [29] R.F.W. Bader, Atoms in Molecules (Oxford University, U.K., 1995). [30] P. Popelier, Atoms in Molecules (Pearson, U.K., 2000). [31] C.F. Matta, R.J. Boyd, The Quantum Theory of Atoms in Molecules (Wiley, Weinheim, 2007). [32] C.F. Matta and R.J. Boyd, Quantum Biochemistry (Wiley, Weinheim, 2010). [33] A. Ehsani, M G. Mahjani, F. Babaei, H. Mostaanzadeh, RSC Adv, 5 (2015) 30394 – 30404. [34] A. Ehsani, M.G. Mahjani, R. Moshrefi, H. Mostaanzadeh, J. Shabani- Shayeh, RSC Advances, 4(2014 ) 20031-20037. [35] H. Mohammad Shiri, A. Ehsani , J. Shabani Shayeh, RSC Advances, 2015, 5, 91062–91068. [36] A. Ehsani, M.G. Mahjani, S. Adeli, S. Moradkhani, Prog. Org. Coat. 2014, 77, 1674. [37] A. Ehsani, H. Mohammad Shiri, E. Kowsari, R. Safari, J. Torabian, S. Kazemi, J. Colloid interface. Sci, 478 (2016)181-187. [38] A. Ehsani, F. Babaei, H. Mostaanzadeh, J. Braz. Chem. Soc, 2015, 26, 331. [39] A. Ehsani, Prog. Org. Coat, 78(2015)133-139. [40] A. Ehsani, A. Vaziri-Rad, F. Babaei, H. Mohammad Shiri, Electrochim. Acta, 159 (2015) 140–148. 21
[41] M. Naseri, L. Fotouhi, A. Ehsani, S. Dehghanpour, J. Colloid interface. Sci, 484 (2016) 314–319 [42] A. Ehsani, M. G. Mahjani, and M. Jafarian, Turk. J. Chem, 35(2011)735-743. [43] A. Ehsani, M.G. Mahjani, M. Jafarian, A. Naeemy, Electrochim. Acta, 71 (2012) 128-133.
22
tpartsra aciGparG
23
S0021-9797(16)30901-8 http://dx.doi.org/10.1016/j.jcis.2016.11.024 YJCIS 21752
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
10 September 2016 4 November 2016 7 November 2016
Please cite this article as: A. Ehsani, H. Mohammad Shiri, E. Kowsari, R. Safari, J. Torabian, S. hajghani, High performance electrochemical pseudocapacitors from ionic liquid assisted electrochemically synthesized p-type conductive polymer, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis. 2016.11.024
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High performance electrochemical pseudocapacitors from ionic liquid assisted electrochemically synthesized p-type conductive polymer
A. Ehsani*a, H. Mohammad Shirib, E. Kowsaric, R. Safaria, J. Torabiana, S. hajghanib
a
Department of Chemistry, Faculty of science, University of Qom, Qom, Iran b
c
Department of Chemistry, Payame Noor University, Iran
Department of chemistry, Amirkabir University of Technology, Tehran, Iran
*Corresponding author. E-mail address:[email protected] [email protected]
1
Abstract In this paper firstly, 1-methyl-3-methylimidazolium bromide (MB) as a new high efficient ionic liquid was synthesized using chemical approach and then fabricated POAP/ MB films by electro- polymerization of POAP in the presence of MB to serve as the active electrode for electrochemical supercapacitor. Theoretical study (AIM) and electrochemical analysis have been used for characterization of ionic liquid and POAP/ MB composite film. Different electrochemical methods including galvanostatic charge– discharge experiments, cyclic voltammetry and electrochemical impedance spectroscopy are carried out in order to investigate the performance of the system.
This work
introduces new most efficient materials for electrochemical redox capacitors with advantages including ease synthesis, high active surface area and stability in an aqueous electrolyte.
Keywords: POAP, composite, supercapacitor, electrosynthesis, ionic liquid
2
1. Introduction
The great need of new technologies to energy storage devices caused to attract the attention of researchers to electrochemical capacitors (ECs) .
Electrochemical
supercapacitors are the charge-storage devices having high power density and long cyclic life [1–3]. Carbon materials, metal oxides and conductive polymers are three categories of active materials that use in ECs. The first stores the electrical energy at the double layer interface and the others store the energy by a charge transfer reaction [4-19]. Electronically conducting polymers (ECPs) such as polyaniline (PANI), polypyrrole (PPy)
and
poly(3,4-ethylenedioxythiophene)
(PEDOT)
have
been
applied
in
supercapacitors, due to their excellent electrochemical properties and lower cost than other ECPs. Polyaniline (PANI) is one of the most important conducting polymers because of its unique electrical, optical, and optoelectrical properties, as well as its ease of preparation and its excellent environment stability. Aminophenols are interesting members of the class of substituted anilines. The hydroxyl group in the phenyl ring can be oxidized to quinine and quinine can be reduced again. Poly ortho aminophenol (POAP) gives a surface film of interesting electrochemical and electrochromic properties when it is electropolymerised in acidic solution. This film is electroactive in aqueous and non-aqueous solutions containing protons but no response is observed at pH-value higher than pH 7. The variety of results for conductivity of the POAP film reported in the literature [20-25] show that the electrochemical response of POAP is strongly influenced by the experimental procedure used to produce the polymer film, dopant anions and the
3
purity starting monomer [20-24]. The electrical properties of conductive polymer could be modified by the addition of different organic and inorganic filler. Basically, a typical ionic liquid molecule is formed by a bulky organic cation (i.e. ammonium, imidazolium, pyri-dinium, pyrrolidinium, piperidinium, phosphonium, sulfonium) in combination with a complex anion such as bromide, chloride, methioninate. Simple changes in the cation and anion combinations or the nature of the moieties attached to each ion allow the physical properties of ionic liquids to be tailored for specific applications. The chemistry of ionic liquids has developed dramatically during the last decade. Their generic, yet not universal, properties, such as negligible vapour pressure, nonflammability, chemical and thermal stability, and outstanding solvation ability, enabled rapid advance in numerous applications. In particular, the use of ionic liquids in electrochemical devices such as dye-sensitized solar cells, supercapacitors, lithium batteries, actuators and fuel cells has been studied extensively [26,27]. In the present work, room temperature electrochemically synthesized POAP/ MB electrode is presented as an efficient potential candidate in supercapacitor application. Our goals in this paper were increasing the capacitance of POAP electrode by using 1methyl-3-methylimidazolium bromide (MB) (Scheme 1) to form a composite electrode and moreover increase the cycle ability of the electrode. The capacitive behavior of composite was tested by cyclic voltammetry, galvanostatic charge discharge and impedance spectroscopy techniques.
4
Scheme 1. Molecular structure of ionic liquid (1-methyl-3-butylimidazolium bromide)
5
2. Experimental 2.1. Reagent and materials All the chemical materials used in this work, obtained from Merck Chemical Co., were of analytical grade and used without further purification. Double distilled water was used throughout the experiments. 2.2. Characterization All electrochemical experiments were carried out by a Potentiostat/galvanostat (Ivium V21508, Vertex). A conventional three electrode cell with an Ag/AgCl reference electrode (Argental, 3 M KCl) was used in order to carry out the electropolymerization of the POAP. A platinum wire and a carbon paste electrode was used as the counter and working electrodes respectively.
2.3. Synthesis of ionic liquid(MB) Preparation of ionic liquid 1-methyl-3-butylimidazolium bromide The mixture of the 1methylimidazole (1.58 mL, 0.02 mol) and 1-bromobutane (2.1 mL, 0.02 mol) was stirring at room temperature for 20 min and then was refluxed for 12 h in toluene. The reaction mixture was cooled to room temperature and the resultant viscous 1-methyl-3methylimidazolium bromide was dried in vacuum for 5 h and washed with ether. The crude product was purified by crystallization from acetonitrile to give IL (0.33 g, 75.5%). Spectral
data
synthesis
of
1-methyl-3-butylimidazolium
bromide
(C8H15N2Br) yield: 0.33 g (75.5%); IR (KBr): (cm−1) 3173 (m), 31,220 (m), 2981 (m), 2952 (m), 2894 (m), 1583 (s), 14,752 (s), 1391 (m), 1355 (m), 1183 (s), 1041 (s), 773 (m)
6
cm−1. 1H NMR (acetone-d6): δ=0.90 (t, 3 H, CH2CH2CH2CH3, J(HH)=7.2 Hz), 1.94 (m, 2 H, CH2CH2CH2CH3, 3J(HH)=7.2 Hz), 1.98 (m, 2 H, CH2CH2CH2CH3, 3J(HH)=7.2 Hz), 4.13 (s, 3 H, NCH3), 4.44 (t, 2 H, N CH2CH2CH2CH3, 3 J(HH)=7.2 Hz), 7.98 (d, 2 H, H4,5 (Im)), 10.22 (t, 1 H, H2 (Im)). Anal. Calcd. for C8H15N2Br (219.14): C 43.50, H 6.83, Br 36.45. Found: C 43.12, H 6.62, Br 36.97 [28].
2.4. Synthesis of POAP and POAP/ MI composite POAP/ MB composites were prepared by in a stirring solution containing 0.01 M monomer, 0.5 M HClO4, 0.1 M LiCO4 and 5.0 ×10-3 M sodium dodecyl sulfate (SDS) on the surface of the ionic liquid (3% w/w) assisted carbon paste electrode. POAP electrode was synthesized in same solution without MI in the carbon paste electrode. Electropolymerizations were conducted by 40 consecutive cycles at the sweep rate of 50 mV.s-1 in the potentials between -0.5 to 1.0V. The
mass of POAP
films was
approximated assuming a current efficiency for the electropolymerization process of 100%, using Faraday’s law.
7
2. Results and discussion Firstly, although the focus of the present work has been on the experimental study of the electrochemical systems, the results obtained here may be used to semi-empirical and theoretical (as computational) study charge/energy transfer effects in the used ionic liquid Systems (to better understand transport effects/phenomena in these systems). For example, to conclude our discussion, we have been studied the transport effects in the molecular systems, using theory of atoms- in- molecule (AIM), which is a generalization of quantum theory to proper nano-size systems, describing molecular systems in terms of the topology of the atomic electron density, and its Laplacian, bond critical points and its bond order, atomic electronic energy, and Virial/potential force [29-32]. Based on the QTAIM, the atomic electronic energy, Eelec () , is given by
Eelec () elec () Kelec () where elec ()
(1)
and Kelec () are the total atomic virial and kinetic energies,
respectively. In addition, based on the AIM theory, the electron population of the atomic basin in molecule is given by n (r )dr , where (r ) the is local electron
density. In this work, the electronic wave functions corresponding to the optimized molecular geometries obtained at density functional level of theory (DFT-B3LYP/631G*). In addition, the details of the local intra-molecular electronic charge and energy transfer of the ionic liquid (1-methyl-3-butylimidazolium bromide) system are calculated, using the AIM theory. Samples of the results are reported in Tab. 1 and demonstrated in Fig. 1.
8
Table 1: the average value (in a.u.) of the electron density, (r ) Laplacian of electron density, ,
(r ) , atomic electronic kinetic energy K () , and average local Virial/potential force, v(r ) , for some atomic (or functional group) basins ( ), of the molecular system is studied in this work. 2
N (PY-ring) C (C-N bond) Br
(r )
2 (r )
K ()
(r )
6.896
2.124 102
5.503 101
1.1074 102
5.942
1.172 102
3.717 101
7.527 101
17.834
4.42 101
24.874 101
49.543 102
9
(a)
(d)
(b)
(c)
(e)
(f)
Fig. 1: The contour maps ( (topology of the electronic properties) ) of the local electron density (a),
Laplacian of electron density (b),
electronic kinetic energy (c), virial force (d) and
HOMO/LUMO orbital’s (e/f) of the bi-pyridine (PY) ring, are calculated using AIM theory.
According to our previous works [33, 34] cyclic voltammetry (CV) was used to deposit POAP coatings from ortho aminophenol monomer solution. Fig. 2 shows the typical multi-sweep cyclic voltammograms during electropolymerization in the presence of ionic liquid. As seen in the Fig. 2, OAP is oxidized irreversibly at around 600 mV without corresponding cathodic processes in the reverse scan.
10
Figure 2. Typical multi-sweep cyclic voltammograms during electropolymerization of POAP in the presence of ionic liquid.
To elucidate the effect of ionic liquid on the property of POAP films, electrochemical performance of composite films was evaluated by carrying out CV measurements in acidic solution [35-37]. Figure 3 shows the CVs of POAP and POAP/ MB electrodes in 0.1 M HClO4 solution. As observed, the capacitance of composite electrode is about two times greater than that of POAP electrode, which shows using ionic liquid in POAP electrode, enhances the capacity of the electrode. The synergetic effect resulting from the interactions of POAP and ionic liquid may affect the shape of CV curves. The CV of POAP/ MB electrode exhibits nearly rectangular shapes in potential window that shows the incorporation of MB in POAP matrix increase the capacitance of composite film.
11
Figure 3. Cyclic voltammograms of POAP (1) and POAP/ MB (2) electrodes in acidic solution. The surface morphology of POAP/ MB film was studied by using the fractal concept. It has been reported [22, 23] that for a diffusion controlled redox transition, which occurred via diffusion of an electroreactant species to a target surface, followed by heterogeneous electron transfer, there is a power dependence between the peak current (I pc) in cyclic voltammograms and the corresponding potential sweep rate (ν):
I pc F
(2)
Where is the fractal parameter and F is a proportionality factor.
12
Thus, the fractal parameter can be obtained easily by plotting the peak current against the sweep rate in log-log scale. On the other hand, the fractal parameter is related to the fractal dimension (Df) of the electrode surface as [20-22]:
Df 1 2
(3)
Eq. (3) is applicable to electrochemical methods, as it has been successfully used for calculating the fractal dimension of electrode surface. Based on this information, Fig. 4 represents cyclic voltammograms of POAP/ MB films that were recorded in different potential sweep rates in the range of 10-400 mVs-1. The slope of the straight line of anodic peak current and potential sweep rate in log-log scale lines gives the value of α for composite film . Substituting the value in Eq. (3), gives the fractal dimensions of 2.41 electrosynthesized film. The presented values for fractal dimension confirms porous structure of the electrosynthesized composite film.
13
Figure 4. Cyclic voltammograms of POAP/ MB films that were recorded in different potential sweep rates in the range of 10-400 mVs-1.
Galvanostatic charge/discharge method has been used to highlight the capacitance characteristic of POAP/ MB composite electrode. Figure 5 shows the charge/discharge behavior of POAP and POAP/ MB electrodes in 0.1M HClO4 solution at current of 0.005mA. As it can be seen, a triangular shape between this potential ranges is observed that indicating good columbic efficiency and ideal capacitive behavior of POAP/ MB as electrode for application in supercapacitor. Specific capacitance of the composite film obtained by charge/discharge curves using the following equation [24]:
14
(4) Where I is the current loaded, m is reactive material mass, V is the potential change during discharge process and t is the discharge time. The mass of deposited polymer on electrode was calculated from the charge (Q) passed during characteristic CV of electrode based on Faraday’s law: Q = znF, where Q is the difference of passed charge through polymer electrodes, z is the number of exchanged electrons and n is the number of moles. By substituting the obtained values in equation 5, the SC of POAP/ MB electrode was found to be 489 F.g-1. One of the most important parameters for practical application is cycling stability. Figure 6 shows the stability of POAP and POAP/MB composite electrodes when cycled at a current of 1 mA for 1000 cycles. Stability of electrodes is compared in terms of losing their capacities as stability percentage. The stability of electrode calculated from following equation: Stability = Cn /C1 ×100
(5)
In that Cn is the capacitance of electrode in each cycles and C1 is capacitance of electrode in the first cycle. Results shows, using MB in POAP caused an excellent retention in stability percentage of composite electrode suggesting the good stability toward long time charge-discharge applications. While the POAP electrode loses its stability fast, composite electrode maintains its stability and saves more that 90.6% of its capacitance of the first cycle under consecutive cycles after 1000 cycles.
15
Figure 5. (a) Galvanostatic charge and discharge measurements of POAP and POAP/ MB electrode at current of 0.005 mA in 0.1M HClO4 solution,(b) galvanostatic charge and discharge curves of POAP/MB during 10 consecutive cycles and (c) during100 consecutive charge-discharge.
16
Figure 6. Cycle stability of POAP (1) and POAP/ MB(2) composite electrodes.
Electrochemical impedance spectroscopy (EIS) is one of the best techniques for analyzing the properties of conducting polymer electrodes and it has been broadly discussed in the literature using a variety of theoretical models [36-43]. In the case of electrochemical system, EIS can reveal information regarding processes occurring in the polymer matrix when it is doped. This may include kinetic values of the doping process and parameters of the diffusion of ions into the polymers. EIS was analyzed for POAP and POAP/ MB films in acidic solution of HClO4. Fig. 7 shows the Nyquist diagrams of electrodes in OCP. The plot in Fig. 7 depicts a single semi-circle in the high frequency region and a straight line in the low-frequency region for two spectra. The high-frequency arc is the overall contact impedance generated from the electrical connection between POAP/ MB composites and the backing plate as well as the charge transfer at the contact interface between the electrode and the electrolyte solution [35]. In spite of the similar 17
shape of the impedance spectra, there is an obvious difference between the diameters of the two semi-circles. That is, the diameters of the semicircles decline with presence of ionic liquid in POAP. In other words, the bulk-film transport of electrons and the charge transfer resistance (Rct) of POAP/ MB films are lower than that of the pure POAP films. This means that the composites have faster electron transport in the bulk-film and charge transfer in the parallel POAP film/solution interface compared to that in the originally single POAP film/solution interface. This fact may suggest that the ionic liquid has an obvious improvement effect, which makes the composites have more active sites for faradic reactions and a larger specific capacitance than pure POAP.
Figure 7. Nyquist plots recorded from 10 kHz to 0.01 Hz with ac amplitude of 5 mV for POAP (1) and POAP/ MB (2) electrode. 18
4. Conclusions We
have
demonstrated
a
simple
and
general
strategy,
namely
in
situ
electropolymerization for preparing conducting polymer/ ionic liquid composite films. We have introduced the POAP/ ionic liquid composite electrode to improve the specific capacitance and power characteristic of electrochemical capacitance. The used ionic liquid (MB) has an obvious improvement effect, which makes the composites have more active sites for faradic reaction and larger specific capacitance than pure POAP.
Acknowledgements The authors would like to express their deep gratitude to the Iranian Nano Council for supporting this work.
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