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organic three-phase interfacial synthesis of coral-like polypyrrole toward enhanced electrochemical capacitance
Electrochimica Acta 56 (2011) 6049–6054
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Water/ionic liquid/organic three-phase interfacial synthesis of coral-like polypyrrole toward enhanced electrochemical capacitance Linrui Hou a , Changzhou Yuan a,∗ , Diankai Li a , Long Yang a , Laifa Shen b , Fang Zhang b , Xiaogang Zhang b,∗∗ a b
Anhui Key Laboratory of Metal Materials and Processing, School of materials Science & Engineering, Anhui University of technology, Ma‘anshan, 243002, PR China College of Material Science & Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, PR China
a r t i c l e
i n f o
Article history: Received 27 February 2011 Received in revised form 24 April 2011 Accepted 24 April 2011 Available online 4 May 2011 Keywords: Polypyrrole Three-phase interface polymerization Two-phase interface polymerization Ionic liquid Electrochemical capacitance
a b s t r a c t Two interfacial synthesis strategies are proposed to synthesize polypyrrole samples for electrochemical capacitors (ECs). In contrast to water/organic two-phase route, unique water/ionic liquid (IL)/organic three-phase interface strategy is first performed to prepare coral-like polypyrrole with even better electrochemical capacitance, where 1-Ethyl-3-methylimidazolium tetrafluoroborate IL, as a “buffering zone”, is set between the water and organic phases to control the morphology and micro-structure of the polypyrrole phase during polymerization. The polypyrrole synthesized by three-phase interfacial route owns more ordered structure, less charge transfer resistance and better electronic conductivity, compared with two-phase method, and delivers larger specific capacitance, higher rate performance and better electrochemical stability at large current densities in 3 M KCl aqueous electrolyte. Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved.
1. Introduction Electrochemical capacitors (ECs), due to their greater power density than batteries and larger energy density than conventional capacitors, are widely investigated presently [1,2]. They not only enable electric vehicles but also provide back-up for wind and solar energy, which are both essential to meet the challenge of global warming and the finite nature of fossil fuels. Now much effort has been devoted to increase their energy and power density meanwhile, as well as using environmentally friendly materials with low cost. Particularly, conducting polymers (CPs) have been considered as one of the most potential electrode materials for ECs application. Among various CPs, polypyrrole (PPy) is an especially practical material for ECs thanks to its high electronic conductivity, long-term environmental stability, environmental friendliness and relative easy synthesis process [2–7]. For electroactive PPy, a key point to obtain a high and stable specific capacitance (SC) at high rates lies in a pregnant design of the electroactive region [3–7]. For instance, the overoxidation and more –CH2 – formation at its rings during the polymerization should be avoided, and its ordered structure and good electronic conductivity must be guaranteed meanwhile. They greatly influence the electrochemical capacitance of the PPy electrode. Of
∗ Corresponding author. Tel.: +86 555 2311871; fax: +86 555 2311570. ∗∗ Corresponding author. Tel.: +86 025 52112902; fax: +86 025 52112626. E-mail addresses: [email protected] (C. Yuan), [email protected] (X. Zhang).
note, these factors are firmly dependent upon the polymerization process. Therefore, it is of great significance to explore a facile but efficient way to control the PPy polymerization. Recently, an organic-aqueous soft interface has been established as an alternative useful approach to conventional homogeneous synthesis [8–10]. Two-phase interface polymerization was performed commonly [10], unfortunately, three-phase interface synthesis has been scarcely reported up to now. In this work, water/ionic liquid (IL)/organic three-phase interface route was first used to synthesize coral-like PPy sample, where 1-Ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF4 ) IL was set between organic and aqueous phases and played a great role of “buffering zone” to control the PPy polymerization process. In comparison with the two-phase route, coral-like PPy synthesized by the three-phase strategy possessed more ordered structure, less charge transfer resistance and better electronic conductivity, and delivered larger SC, higher rate behavior and better electrochemical stability in 3 M KCl aqueous solution. 2. Experimental 2.1. Interfacial synthesis and characterization of the PPy samples [Emim]BF4 IL and pyrrole monomer were purchased from Aldrich. The typical three-phase synthesis was described as follows. 0.23 mL pyrrole monomer was added into 6 mL CHCl3 to form organic phase, then 7 mL [Emim]BF4 IL was set between 7 mL FeCl3 aqueous solution (0.5 M) and the CHCl3 phase. After
0013-4686/$ – see front matter. Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.04.087
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Fig. 1. FESEM images of the as-synthesized TH-PPy sample.
5 days, the PPy sample was collected, washed and dried at 40 ◦ C in vacuum. For comparison, the two-phase route was also performed, which was same as the three-phase method as mentioned above just with exception of IL phase. The PPy samples synthesized by two-phase and three-phase methods are donated as TW-PPy and TH-PPy, respectively. The samples were examined by powder X-ray diffraction (XRD) (Max 18 XCE), field-emission scanning electron microscope (FESEM, JEOL-6300F), Fourier transform infrared (FT-IR) spectra (360 Nicolet AVATAR FT-IR) and the Brunauer–Emmett–Teller (BET) surface area of the samples was obtained with a Micromeritics ASAP 2010 analyzer. The BET method was used to calculate the specific surface area (SSA) of the samples. 2.2. Electrochemical tests Electrochemical performance of the PPy samples was evaluated in 3 M KCl aqueous solution by cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) tests, which were performed on CHI660C electrochemical workstation at room temperature. Working electrodes were prepared by mixed the electroactive materials with acetylene black (AB) and polytetrafluoroethylene (PTFE) with the weight ratio of 5:1.5:0.5, which was smeared onto the pretreated graphite substrates [9,11]. A platinum plate (ca. 1 cm2 ) and a saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively.
tively. Typical diffraction patterns shown in Fig. 3 clearly indicate that both of the TH-PPy and TW-PPy samples are mainly amorphous structure. Of note, the sharper peaks at 19.1◦ , 26.2◦ and 29.2◦ of the TH-PPy, as indicated by the red ellipses, illustrate that the alignment and orientation of TH-PPy molecular chain are more regular than those of the TW-PPy [4,7,12]. Specifically, the peaks at 2 of 19.1◦ and 26.2◦ should be attributed to pyrrole-counterion or intercounterion interaction scattering and arise from PPy chains close to the interplanar van der Waals distance for aromatic groups, respectively. And the high angle peak at 2 = 29.2◦ indicates order in the polymer backbone. FT-IR spectra of the TH-PPy and the TW-PPy samples are shown in Fig. 4, respectively. The characteristic peaks ranged from 700 to 2000 cm−1 of the TH-PPy and the TW-PPy samples are similar to each other, indicating the two PPy samples have the same conjugated structure [7,13]. However, the TH-PPy presents very weak absorption bands in the region of 2830–2980 cm−1 , compared to the TW-PPy sample, which means the less –CH2 – groups in TH-PPy matrix [7,13]. Therefore, it is confirmed that the [Emim]BF4 IL can regulate and control the PPy polymerization, due to its great role of “buffering zone”, which makes the TH-PPy sample own more ordered structure and produces less nonconducting oligomers –CH2 – groups existing in the TH-PPy matrix. Notably, the less –CH2 – groups in the TH-PPy sample favor for the enhancement of its electronic conductivity, which can be further verified by the following EIS plots. Consequently, the more ordered structure and higher electronic conductivity of the THPPy should greatly benefit for its more desirable electrochemical performance.
3. Results and discussion 3.1. Morphology and structure analysis of the as-synthesized PPy samples Fig. 1 presents the FESEM images of the as-prepared TH-PPy sample. Clearly, the TH-PPy sample exhibits loosed coral reef-like morphology, as seen from Fig. 1a. The higher-magnification FESEM image (Fig. 1b) shows that the as-synthesized TH-PPy possesses a typical emanative coral-like nanostructure. Notably, the PPy synthesized by the two-phase route displays a mixture of microspheres and micro sheets, as shown in Fig. 2a and b, just without the IL phase during the polymerization. And the higher-magnification FESEM presented in Fig. 2c demonstrates that the size of the PPy microspheres is ranged from 1 to 2 m. It demonstrates that the existence of the [Emim]BF4 IL layer greatly influences the PPy polymerization process indeed, which results in the distinct morphologies and micro-structures of the PPy samples. XRD analysis was performed to evaluate the alignment and orientation of the synthesized PPy samples. Fig. 3 shows the XRD diffraction patterns of the TH-PPy and TW-PPy samples, respec-
3.2. Electrochemical properties of the as-prepared PPy samples Fig. 5a depicts the CV curves of the TH-PPy electrode at different scanning rates from 2 to 20 mV/s in 3 M KCl aqueous solution. The electrochemical response currents on the positive sweep of the TH-PPy electrode are nearly mirror-image symmetric to their corresponding counterparts on the negative sweep with respect to the zero-current line in the given potential range from −0.8 to 0.5 V (vs. SCE) at all the scanning rates, indicating their good characteristic supercapacitive feature. Moreover, even when the scanning rate increases to a scan rate of 20 mV/s, the electrochemical response current of the TH-PPy electrode subsequently increases while the CV shape changes little and still keeps quasi-rectangular in shape, indicating its good rate performance. In contrast, the CV plots for the TW-PPy electrode at various scanning rates are also depicted in Fig. 5b. Obviously, the TW-PPy electrode displays good electrochemical capacitance within the electrochemical window between −0.7 and 0.6 V (vs. SCE). However, electrochemical response currents of the TW-PPy are much less than those of the TH-PPy at all the
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Fig. 2. FESEM images of the as-synthesized TW-PPy sample.
same scan rates, it suggests that the TH-PPy electrode can deliver much larger energy storage ability than the TW-PPy electrode. The specific capacitance, capacitive reversibility and power property of an electrode are usually examined by the charge–discharge study under various applied constant current densities. Thereupon, typical CP curves of the TH-PPy and TW-PPy electrodes at various current densities are shown in Fig. 6a and b, respectively. Fig. 6a demonstrates the galvanostatic constant current charge–discharge curves of the TH-PPy at different current densities within an electrochemical window from −0.8 to 0.5 V (vs. SCE). Obviously, the E–t relationships on charge–discharge profiles are all symmetric at various current densities, indicating their good electrochemical capacitance. For comparison, the CP curves of the TW-PPy electrode at various current densities are demonstrated
in Fig. 6b. At the same current densities, the discharge time is even less than that of the TH-PPy, indicating its smaller SC than the TH-PPy electrode. The specific capacitances of the TH-PPy and TW-PPy electrodes were calculated from the CP curves shown in Fig. 6a and b, according to the following equation:
Fig. 3. Typical XRD patterns of the TH-PPy and the TW-PPy samples.
Fig. 4. FT-IR spectra of the TH-PPy and the TW-PPy samples.
SC =
It V
(1)
where SC, I, t and V are the specific capacitance (F/g) of the electrodes, the current density (A/g) used for charge/discharge, the time (s) elapsed for the charge or discharge cycles, and the potential interval (V) of the charge or discharge, respectively. And the SCs, according to Eq. (1), at different current densities are collected and shown in Table 1. Impressively, a SC of 243 F/g can be achieved by the TH-PPy electrode at 0.4 A/g. More importantly, a SC of 197 F/g, ca. 81% of that
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Fig. 5. CV curves of the TH-PPy (a) and the TW-PPy (b) samples at various scanning rates.
at 0.4 A/g, still can be delivered by the TH-PPy electrode even at a larger current density of 2.5 A/g, which reveals that the TH-PPy can deliver larger SCs even at higher current densities. Commonly, the larger the SSA is, the higher SCs are. However, it is not for all the cases. Therefore, electrochemical utilization per surface area, i.e., area specific capacitance, is another very important parameter to evaluate the electrochemical capacitance of any electroactive material. Calculation of the pure electric double-layer capacitance (EDLC) using the BET SSA of an average value of 20 F/cm2 gives the EDLC of ca. 8 F/g for the TH-PPy with an SSA of ca. 41 m2 /g, which is much lower than the corresponding measured SC of 243 F/g. Therefore, it is further demonstrated that the main component of the measured SC is produced from the pseudocapacitive surface redox process and Faradaic pseudocapacitance can be estimated as ca. 235 F/g, i.e., 573 F/cm2 . It suggests its high electrochemical utilization for energy storage. While for the TW-PPy, a SC of 168 F/g is just obtained at a small current density of 0.4 A/g, which is even less
Table 1 Specific capacitances of the TH-PPy and the TW-PPy electrodes at various current densities. I (A/g)
0.4
1
1.5
2.0
2.5
SC (TH-PPy, F/g) SC (TW-PPy, F/g)
243 168
230 151
221 140
215 133
197 127
Fig. 6. CP curves of the TH-PPy (a) and the TW-PPy (b) electrodes at different current densities.
than that of TH-PPy at 2.5 A/g. Considering its SSA of ca. 34 m2 /g, a Faradaic pseudocapacitance of ca. 161 F/g, i.e., 474 F/cm2 , can be calculated for the TW-PPy sample, demonstrating its less surface utilization for energy storage than the TH-PPy electrode. Moreover, just 127 F/g, ca. 76% of that at 0.4 A/g, of the TW-PPy electrode can be obtained when the current density increases to 2.5 A/g. According to the above analysis, it is easy to conclude that the TH-PPy electrode owns greater ability to deliver larger SC and higher electrochemical utilization at higher rates than the TW-PPy electrode, for which the reason is that the TH-PPy sample with more ordered structure, better electronic conductivity and larger SSA, as mentioned above, can promote ions and electrons entering into/ejecting much richer electroactive sites for energy storage and contribute to its high charge–discharge rate performance. Another important parameter, coulombic efficiency (), can be evaluated from Eq. (2): =
tD × 100% tC
(2)
where tD and tC are the time for galvanostatic discharging and charging, respectively. The columbic efficiencies of the two electrodes at various current densities could be calculated based on the charge–discharge curves shown in Fig. 6a and b according to Eq. (2). And the typical coulombic efficiency data are shown in Fig. 7. Evidently, the coulombic efficiency of the TH-PPy electrode at various current densities are all more than 97%, and even up to 100%
L. Hou et al. / Electrochimica Acta 56 (2011) 6049–6054
Fig. 7. Columbic efficiencies of the TH-PPy and the TW-PPy electrodes at various current densities.
when the current densities are not less than 1.5 A/g, revealing its good electrochemical reversibility. While for the TW-PPy electrode, the coulombic efficiency just reaches 97% even when the current is up to 2 A/g, which demonstrates even better electrochemical reversibility of the TH-PPy electrode. Electrochemical impedance spectroscopy was further employed to monitor the electrochemical behaviors of the two PPy electrodes and the impedance characteristics were recorded at a voltage of 0.0 V (vs. SCE). In the Nyquist impedance plots depicted in Fig. 8, the imaginary part of impedance is plotted as a function of its real component in the frequency range from 10 mHz to 105 Hz with an excitation signal of 5 mV. At very high frequencies, from the Nyquist plot, the intercept of the electrode with the real impedance (Z) axis reports the sum of the internal resistance of the PPy samples, electrolyte resistance, and the contact resistance at the interface between electroactive materials and the current collector [14]. Here, due to the same making technique of three-electrode cell for tests and the same electrolyte, that is, the electrolyte resistance and the contact resistance are identical both for the TH-PPy and TW-PPy electrodes. Therefore, it can be considered that the different intercepts reflect the different conductive properties of the
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Fig. 9. Change of specific capacitance of the TH-PPy and the TW-PPy electrodes as a function of the number of charge–discharge cycles at a current density of 2.5 A/g.
two PPy samples. As can be seen from the data in Fig. 8, the TH-PPy electrode owns much better electronic conductivity (0.45 ) than TW-PPy (0.96 ), which is in good agreement with the analysis above. In the high-medium frequency region, the little semi-circles are observed both for the two PPy electrodes. It indicates the existence of certain charge transportation resistance (Rct ). An Rct of ca. 0.5 can be estimated for the TH-PPy electrode, which is larger than ca. 1.1 for the TW-PPy electrode, which reveals that the THPPy owns less charge transportation resistance. Out of question, the less charge transportation resistance and better electron conductivity, to some extent, can be beneficial for its better power property and larger SCs. For further understanding the electrochemical performances, the long-term cycle ability of the electrodes was also evaluated by repeating the charge/discharge test at a current density of 2.5 A/g for 500 cycles. The SC as a function of the cycle number is presented in Fig. 9. After 500 continuous cycle tests, the SC degradation of the TH-PPy is ca. 12%, much less than ca. 30% for the TW-PPy, which demonstrates that the TH-PPy electrode maintains much better electrochemical stability. The improved cycle ability of the TH-PPy electrode should be primarily attributed to its more ordered structure and better electronic conductivity. The more homogeneous stress distribution can occur in the TH-PPy with more ordered structure and better electronic conductivity during oxidation/reduction cycling, which is favorable for enhancing the cycling stability of the TH-PPy sample. 4. Conclusions
Fig. 8. EIS plots of the TH-PPy and the TW-PPy electrodes at 0.0 V (vs. SCE).
The idea of water/ionic liquid/organic three-phase interface strategy to prepare coral-like polypyrrole for electrochemical capacitors has been demonstrated in this work. [Emim]BF4 IL, as a buffering zone, was set between the water and organic phases to control the morphology and mico-structure of the polypyrrole samples during the polymerization process. Physical and electrochemical characterization illustrated that the TH-PPy electrode owned better electronic conductivity, more ordered structure and less –CH2 – formation than the TW-PPy. Owing to these advantages in structure, the TH-PPy electrode delivered a larger SC, higher rate performance, and better electrochemical reversibility and stability than TW-PPy in 3 M KCl aqueous electrolyte. Furthermore, the strategy we first proposed here provides a universal and facile approach to synthesize other CPs, and even their composites with good electrochemical performance for ECs application.
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Acknowledgements This work was financially supported by State Basic Research Program of PRC (973 Program) (no. 2007CB209703), National Natural Science Foundation of PRC (no. 20873064), Natural Science Foundation of Anhui Province (no. 10040606Q07) and 2010 Young Teachers’ Research Foundation of Anhui University of Technology (no. QZ201003). References [1] J. Tollefson, Nature 456 (2008) 436. [2] C.Z. Yuan, B. Gao, L.F. Shen, S.D. Yang, L. Hao, X.J. Lu, F. Zhang, L.J. Zhang, X.G. Zhang, Nanoscale 3 (2011) 529.
[3] J.Y. Huang, K. Wang, Z.X. Wei, J. Mater. Chem. 20 (2010) 1117. [4] J. Wang, Y.L. Xu, F. Yan, L.B. Zhu, J.P. Wang, J. Power Sources 196 (2011) 2373. [5] L.Z. Fan, J. Maier, Electrochem. Commun. 8 (2006) 937. [6] S. Sadki, P. Schottland, N. Brodie, G. Sabouraud, Chem. Soc. Rev. 29 (2000) 283. [7] J.P. Wang, Y.L. Xu, J. Wang, X.F. Du, F. Xiao, J.B. Li, Synth. Met. 160 (2010) 1826. [8] C.Z. Yuan, L.F. Shen, F. Zhang, X.J. Lu, D.K. Li, X.G. Zhang, J. Colloid Interface Sci. 349 (2010) 181. [9] C.Z. Yuan, L.R. Hou, L. Yang, C.G. Fan, D.K. Li, J.M. Li, L.F. Shen, F. Zhang, X.G. Zhang, Mater. Lett. 65 (2011) 374. [10] Y.G. Wang, W. Wu, L. Chen, P. He, C.X. Wang, Y.Y. Xia, Adv. Mater. 20 (2008) 1. [11] C.Z. Yuan, L. Chen, B. Gao, L.H. Su, X.G. Zhang, J. Mater. Chem. 19 (2009) 246. [12] V.T. Truong, B.C. Ennis, M. Forsyth, Polymer 36 (1995) 1933. [13] P. Yang, J. Zhang, Y. Guo, Appl. Surf. Sci. 255 (2009) 6924. [14] C.Z. Yuan, X.G. Zhang, Q.F. Wu, B. Gao, Solid State Ionics 177 (2006) 1237.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Water/ionic liquid/organic three-phase interfacial synthesis of coral-like polypyrrole toward enhanced electrochemical capacitance Linrui Hou a , Changzhou Yuan a,∗ , Diankai Li a , Long Yang a , Laifa Shen b , Fang Zhang b , Xiaogang Zhang b,∗∗ a b
Anhui Key Laboratory of Metal Materials and Processing, School of materials Science & Engineering, Anhui University of technology, Ma‘anshan, 243002, PR China College of Material Science & Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, PR China
a r t i c l e
i n f o
Article history: Received 27 February 2011 Received in revised form 24 April 2011 Accepted 24 April 2011 Available online 4 May 2011 Keywords: Polypyrrole Three-phase interface polymerization Two-phase interface polymerization Ionic liquid Electrochemical capacitance
a b s t r a c t Two interfacial synthesis strategies are proposed to synthesize polypyrrole samples for electrochemical capacitors (ECs). In contrast to water/organic two-phase route, unique water/ionic liquid (IL)/organic three-phase interface strategy is first performed to prepare coral-like polypyrrole with even better electrochemical capacitance, where 1-Ethyl-3-methylimidazolium tetrafluoroborate IL, as a “buffering zone”, is set between the water and organic phases to control the morphology and micro-structure of the polypyrrole phase during polymerization. The polypyrrole synthesized by three-phase interfacial route owns more ordered structure, less charge transfer resistance and better electronic conductivity, compared with two-phase method, and delivers larger specific capacitance, higher rate performance and better electrochemical stability at large current densities in 3 M KCl aqueous electrolyte. Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved.
1. Introduction Electrochemical capacitors (ECs), due to their greater power density than batteries and larger energy density than conventional capacitors, are widely investigated presently [1,2]. They not only enable electric vehicles but also provide back-up for wind and solar energy, which are both essential to meet the challenge of global warming and the finite nature of fossil fuels. Now much effort has been devoted to increase their energy and power density meanwhile, as well as using environmentally friendly materials with low cost. Particularly, conducting polymers (CPs) have been considered as one of the most potential electrode materials for ECs application. Among various CPs, polypyrrole (PPy) is an especially practical material for ECs thanks to its high electronic conductivity, long-term environmental stability, environmental friendliness and relative easy synthesis process [2–7]. For electroactive PPy, a key point to obtain a high and stable specific capacitance (SC) at high rates lies in a pregnant design of the electroactive region [3–7]. For instance, the overoxidation and more –CH2 – formation at its rings during the polymerization should be avoided, and its ordered structure and good electronic conductivity must be guaranteed meanwhile. They greatly influence the electrochemical capacitance of the PPy electrode. Of
∗ Corresponding author. Tel.: +86 555 2311871; fax: +86 555 2311570. ∗∗ Corresponding author. Tel.: +86 025 52112902; fax: +86 025 52112626. E-mail addresses: [email protected] (C. Yuan), [email protected] (X. Zhang).
note, these factors are firmly dependent upon the polymerization process. Therefore, it is of great significance to explore a facile but efficient way to control the PPy polymerization. Recently, an organic-aqueous soft interface has been established as an alternative useful approach to conventional homogeneous synthesis [8–10]. Two-phase interface polymerization was performed commonly [10], unfortunately, three-phase interface synthesis has been scarcely reported up to now. In this work, water/ionic liquid (IL)/organic three-phase interface route was first used to synthesize coral-like PPy sample, where 1-Ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF4 ) IL was set between organic and aqueous phases and played a great role of “buffering zone” to control the PPy polymerization process. In comparison with the two-phase route, coral-like PPy synthesized by the three-phase strategy possessed more ordered structure, less charge transfer resistance and better electronic conductivity, and delivered larger SC, higher rate behavior and better electrochemical stability in 3 M KCl aqueous solution. 2. Experimental 2.1. Interfacial synthesis and characterization of the PPy samples [Emim]BF4 IL and pyrrole monomer were purchased from Aldrich. The typical three-phase synthesis was described as follows. 0.23 mL pyrrole monomer was added into 6 mL CHCl3 to form organic phase, then 7 mL [Emim]BF4 IL was set between 7 mL FeCl3 aqueous solution (0.5 M) and the CHCl3 phase. After
0013-4686/$ – see front matter. Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.04.087
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L. Hou et al. / Electrochimica Acta 56 (2011) 6049–6054
Fig. 1. FESEM images of the as-synthesized TH-PPy sample.
5 days, the PPy sample was collected, washed and dried at 40 ◦ C in vacuum. For comparison, the two-phase route was also performed, which was same as the three-phase method as mentioned above just with exception of IL phase. The PPy samples synthesized by two-phase and three-phase methods are donated as TW-PPy and TH-PPy, respectively. The samples were examined by powder X-ray diffraction (XRD) (Max 18 XCE), field-emission scanning electron microscope (FESEM, JEOL-6300F), Fourier transform infrared (FT-IR) spectra (360 Nicolet AVATAR FT-IR) and the Brunauer–Emmett–Teller (BET) surface area of the samples was obtained with a Micromeritics ASAP 2010 analyzer. The BET method was used to calculate the specific surface area (SSA) of the samples. 2.2. Electrochemical tests Electrochemical performance of the PPy samples was evaluated in 3 M KCl aqueous solution by cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) tests, which were performed on CHI660C electrochemical workstation at room temperature. Working electrodes were prepared by mixed the electroactive materials with acetylene black (AB) and polytetrafluoroethylene (PTFE) with the weight ratio of 5:1.5:0.5, which was smeared onto the pretreated graphite substrates [9,11]. A platinum plate (ca. 1 cm2 ) and a saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively.
tively. Typical diffraction patterns shown in Fig. 3 clearly indicate that both of the TH-PPy and TW-PPy samples are mainly amorphous structure. Of note, the sharper peaks at 19.1◦ , 26.2◦ and 29.2◦ of the TH-PPy, as indicated by the red ellipses, illustrate that the alignment and orientation of TH-PPy molecular chain are more regular than those of the TW-PPy [4,7,12]. Specifically, the peaks at 2 of 19.1◦ and 26.2◦ should be attributed to pyrrole-counterion or intercounterion interaction scattering and arise from PPy chains close to the interplanar van der Waals distance for aromatic groups, respectively. And the high angle peak at 2 = 29.2◦ indicates order in the polymer backbone. FT-IR spectra of the TH-PPy and the TW-PPy samples are shown in Fig. 4, respectively. The characteristic peaks ranged from 700 to 2000 cm−1 of the TH-PPy and the TW-PPy samples are similar to each other, indicating the two PPy samples have the same conjugated structure [7,13]. However, the TH-PPy presents very weak absorption bands in the region of 2830–2980 cm−1 , compared to the TW-PPy sample, which means the less –CH2 – groups in TH-PPy matrix [7,13]. Therefore, it is confirmed that the [Emim]BF4 IL can regulate and control the PPy polymerization, due to its great role of “buffering zone”, which makes the TH-PPy sample own more ordered structure and produces less nonconducting oligomers –CH2 – groups existing in the TH-PPy matrix. Notably, the less –CH2 – groups in the TH-PPy sample favor for the enhancement of its electronic conductivity, which can be further verified by the following EIS plots. Consequently, the more ordered structure and higher electronic conductivity of the THPPy should greatly benefit for its more desirable electrochemical performance.
3. Results and discussion 3.1. Morphology and structure analysis of the as-synthesized PPy samples Fig. 1 presents the FESEM images of the as-prepared TH-PPy sample. Clearly, the TH-PPy sample exhibits loosed coral reef-like morphology, as seen from Fig. 1a. The higher-magnification FESEM image (Fig. 1b) shows that the as-synthesized TH-PPy possesses a typical emanative coral-like nanostructure. Notably, the PPy synthesized by the two-phase route displays a mixture of microspheres and micro sheets, as shown in Fig. 2a and b, just without the IL phase during the polymerization. And the higher-magnification FESEM presented in Fig. 2c demonstrates that the size of the PPy microspheres is ranged from 1 to 2 m. It demonstrates that the existence of the [Emim]BF4 IL layer greatly influences the PPy polymerization process indeed, which results in the distinct morphologies and micro-structures of the PPy samples. XRD analysis was performed to evaluate the alignment and orientation of the synthesized PPy samples. Fig. 3 shows the XRD diffraction patterns of the TH-PPy and TW-PPy samples, respec-
3.2. Electrochemical properties of the as-prepared PPy samples Fig. 5a depicts the CV curves of the TH-PPy electrode at different scanning rates from 2 to 20 mV/s in 3 M KCl aqueous solution. The electrochemical response currents on the positive sweep of the TH-PPy electrode are nearly mirror-image symmetric to their corresponding counterparts on the negative sweep with respect to the zero-current line in the given potential range from −0.8 to 0.5 V (vs. SCE) at all the scanning rates, indicating their good characteristic supercapacitive feature. Moreover, even when the scanning rate increases to a scan rate of 20 mV/s, the electrochemical response current of the TH-PPy electrode subsequently increases while the CV shape changes little and still keeps quasi-rectangular in shape, indicating its good rate performance. In contrast, the CV plots for the TW-PPy electrode at various scanning rates are also depicted in Fig. 5b. Obviously, the TW-PPy electrode displays good electrochemical capacitance within the electrochemical window between −0.7 and 0.6 V (vs. SCE). However, electrochemical response currents of the TW-PPy are much less than those of the TH-PPy at all the
L. Hou et al. / Electrochimica Acta 56 (2011) 6049–6054
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Fig. 2. FESEM images of the as-synthesized TW-PPy sample.
same scan rates, it suggests that the TH-PPy electrode can deliver much larger energy storage ability than the TW-PPy electrode. The specific capacitance, capacitive reversibility and power property of an electrode are usually examined by the charge–discharge study under various applied constant current densities. Thereupon, typical CP curves of the TH-PPy and TW-PPy electrodes at various current densities are shown in Fig. 6a and b, respectively. Fig. 6a demonstrates the galvanostatic constant current charge–discharge curves of the TH-PPy at different current densities within an electrochemical window from −0.8 to 0.5 V (vs. SCE). Obviously, the E–t relationships on charge–discharge profiles are all symmetric at various current densities, indicating their good electrochemical capacitance. For comparison, the CP curves of the TW-PPy electrode at various current densities are demonstrated
in Fig. 6b. At the same current densities, the discharge time is even less than that of the TH-PPy, indicating its smaller SC than the TH-PPy electrode. The specific capacitances of the TH-PPy and TW-PPy electrodes were calculated from the CP curves shown in Fig. 6a and b, according to the following equation:
Fig. 3. Typical XRD patterns of the TH-PPy and the TW-PPy samples.
Fig. 4. FT-IR spectra of the TH-PPy and the TW-PPy samples.
SC =
It V
(1)
where SC, I, t and V are the specific capacitance (F/g) of the electrodes, the current density (A/g) used for charge/discharge, the time (s) elapsed for the charge or discharge cycles, and the potential interval (V) of the charge or discharge, respectively. And the SCs, according to Eq. (1), at different current densities are collected and shown in Table 1. Impressively, a SC of 243 F/g can be achieved by the TH-PPy electrode at 0.4 A/g. More importantly, a SC of 197 F/g, ca. 81% of that
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Fig. 5. CV curves of the TH-PPy (a) and the TW-PPy (b) samples at various scanning rates.
at 0.4 A/g, still can be delivered by the TH-PPy electrode even at a larger current density of 2.5 A/g, which reveals that the TH-PPy can deliver larger SCs even at higher current densities. Commonly, the larger the SSA is, the higher SCs are. However, it is not for all the cases. Therefore, electrochemical utilization per surface area, i.e., area specific capacitance, is another very important parameter to evaluate the electrochemical capacitance of any electroactive material. Calculation of the pure electric double-layer capacitance (EDLC) using the BET SSA of an average value of 20 F/cm2 gives the EDLC of ca. 8 F/g for the TH-PPy with an SSA of ca. 41 m2 /g, which is much lower than the corresponding measured SC of 243 F/g. Therefore, it is further demonstrated that the main component of the measured SC is produced from the pseudocapacitive surface redox process and Faradaic pseudocapacitance can be estimated as ca. 235 F/g, i.e., 573 F/cm2 . It suggests its high electrochemical utilization for energy storage. While for the TW-PPy, a SC of 168 F/g is just obtained at a small current density of 0.4 A/g, which is even less
Table 1 Specific capacitances of the TH-PPy and the TW-PPy electrodes at various current densities. I (A/g)
0.4
1
1.5
2.0
2.5
SC (TH-PPy, F/g) SC (TW-PPy, F/g)
243 168
230 151
221 140
215 133
197 127
Fig. 6. CP curves of the TH-PPy (a) and the TW-PPy (b) electrodes at different current densities.
than that of TH-PPy at 2.5 A/g. Considering its SSA of ca. 34 m2 /g, a Faradaic pseudocapacitance of ca. 161 F/g, i.e., 474 F/cm2 , can be calculated for the TW-PPy sample, demonstrating its less surface utilization for energy storage than the TH-PPy electrode. Moreover, just 127 F/g, ca. 76% of that at 0.4 A/g, of the TW-PPy electrode can be obtained when the current density increases to 2.5 A/g. According to the above analysis, it is easy to conclude that the TH-PPy electrode owns greater ability to deliver larger SC and higher electrochemical utilization at higher rates than the TW-PPy electrode, for which the reason is that the TH-PPy sample with more ordered structure, better electronic conductivity and larger SSA, as mentioned above, can promote ions and electrons entering into/ejecting much richer electroactive sites for energy storage and contribute to its high charge–discharge rate performance. Another important parameter, coulombic efficiency (), can be evaluated from Eq. (2): =
tD × 100% tC
(2)
where tD and tC are the time for galvanostatic discharging and charging, respectively. The columbic efficiencies of the two electrodes at various current densities could be calculated based on the charge–discharge curves shown in Fig. 6a and b according to Eq. (2). And the typical coulombic efficiency data are shown in Fig. 7. Evidently, the coulombic efficiency of the TH-PPy electrode at various current densities are all more than 97%, and even up to 100%
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Fig. 7. Columbic efficiencies of the TH-PPy and the TW-PPy electrodes at various current densities.
when the current densities are not less than 1.5 A/g, revealing its good electrochemical reversibility. While for the TW-PPy electrode, the coulombic efficiency just reaches 97% even when the current is up to 2 A/g, which demonstrates even better electrochemical reversibility of the TH-PPy electrode. Electrochemical impedance spectroscopy was further employed to monitor the electrochemical behaviors of the two PPy electrodes and the impedance characteristics were recorded at a voltage of 0.0 V (vs. SCE). In the Nyquist impedance plots depicted in Fig. 8, the imaginary part of impedance is plotted as a function of its real component in the frequency range from 10 mHz to 105 Hz with an excitation signal of 5 mV. At very high frequencies, from the Nyquist plot, the intercept of the electrode with the real impedance (Z) axis reports the sum of the internal resistance of the PPy samples, electrolyte resistance, and the contact resistance at the interface between electroactive materials and the current collector [14]. Here, due to the same making technique of three-electrode cell for tests and the same electrolyte, that is, the electrolyte resistance and the contact resistance are identical both for the TH-PPy and TW-PPy electrodes. Therefore, it can be considered that the different intercepts reflect the different conductive properties of the
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Fig. 9. Change of specific capacitance of the TH-PPy and the TW-PPy electrodes as a function of the number of charge–discharge cycles at a current density of 2.5 A/g.
two PPy samples. As can be seen from the data in Fig. 8, the TH-PPy electrode owns much better electronic conductivity (0.45 ) than TW-PPy (0.96 ), which is in good agreement with the analysis above. In the high-medium frequency region, the little semi-circles are observed both for the two PPy electrodes. It indicates the existence of certain charge transportation resistance (Rct ). An Rct of ca. 0.5 can be estimated for the TH-PPy electrode, which is larger than ca. 1.1 for the TW-PPy electrode, which reveals that the THPPy owns less charge transportation resistance. Out of question, the less charge transportation resistance and better electron conductivity, to some extent, can be beneficial for its better power property and larger SCs. For further understanding the electrochemical performances, the long-term cycle ability of the electrodes was also evaluated by repeating the charge/discharge test at a current density of 2.5 A/g for 500 cycles. The SC as a function of the cycle number is presented in Fig. 9. After 500 continuous cycle tests, the SC degradation of the TH-PPy is ca. 12%, much less than ca. 30% for the TW-PPy, which demonstrates that the TH-PPy electrode maintains much better electrochemical stability. The improved cycle ability of the TH-PPy electrode should be primarily attributed to its more ordered structure and better electronic conductivity. The more homogeneous stress distribution can occur in the TH-PPy with more ordered structure and better electronic conductivity during oxidation/reduction cycling, which is favorable for enhancing the cycling stability of the TH-PPy sample. 4. Conclusions
Fig. 8. EIS plots of the TH-PPy and the TW-PPy electrodes at 0.0 V (vs. SCE).
The idea of water/ionic liquid/organic three-phase interface strategy to prepare coral-like polypyrrole for electrochemical capacitors has been demonstrated in this work. [Emim]BF4 IL, as a buffering zone, was set between the water and organic phases to control the morphology and mico-structure of the polypyrrole samples during the polymerization process. Physical and electrochemical characterization illustrated that the TH-PPy electrode owned better electronic conductivity, more ordered structure and less –CH2 – formation than the TW-PPy. Owing to these advantages in structure, the TH-PPy electrode delivered a larger SC, higher rate performance, and better electrochemical reversibility and stability than TW-PPy in 3 M KCl aqueous electrolyte. Furthermore, the strategy we first proposed here provides a universal and facile approach to synthesize other CPs, and even their composites with good electrochemical performance for ECs application.
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Acknowledgements This work was financially supported by State Basic Research Program of PRC (973 Program) (no. 2007CB209703), National Natural Science Foundation of PRC (no. 20873064), Natural Science Foundation of Anhui Province (no. 10040606Q07) and 2010 Young Teachers’ Research Foundation of Anhui University of Technology (no. QZ201003). References [1] J. Tollefson, Nature 456 (2008) 436. [2] C.Z. Yuan, B. Gao, L.F. Shen, S.D. Yang, L. Hao, X.J. Lu, F. Zhang, L.J. Zhang, X.G. Zhang, Nanoscale 3 (2011) 529.
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