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Acid Red 27-crosslinked polyaniline with nanofiber structure as electrode material for supercapacitors
Materials Letters 212 (2018) 259–262
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Acid Red 27-crosslinked polyaniline with nanofiber structure as electrode material for supercapacitors Mingwei Shi, Mengdi Bai, Baoming Li ⇑ College of Material Science and Engineering, Fuzhou University, 2 Xue Yuan Road, Fuzhou 350108, PR China
a r t i c l e
i n f o
Article history: Received 29 September 2017 Received in revised form 18 October 2017 Accepted 25 October 2017 Available online 5 November 2017 Keywords: Polymers Nanofiber Microstructure Electronic materials Supercapacitors
a b s t r a c t Acid Red 27-crosslinked PANI (ARCP) was prepared via chemical oxidative polymerization in the presence of Acid Red 27 (AR-27) as crosslinking agent and doping agent. The results showed that in comparison with the microsized bulky structure of uncrosslinked PANI, the nanofiber structure of ARCP was observed and the diameter of ARCP nanofiber decreased from 300 nm to 60 nm with the increase of feeding molar ratio of AR-27 to aniline. ARCP exhibited a large specific capacitance of 463 Fg 1 at 2 Ag 1 and a great electrochemical cycling stability with the specific capacitance retention of 85.5% after 1000 cycles when the feeding molar ratio of AR-27 to aniline was 0.04:1, which was better than uncrosslinked PANI with a specific capacitance of 280 Fg 1 and the specific capacitance retention of 70.8%. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction In recent years, polyaniline (PANI) has been extensively investigated as electrode material for supercapacitor. Unfortunately, PANI underwent large volumetric swelling and shrinking during the process of charge/discharge as a result of ion doping and dedoping, which would lead to the structural breakdown and fast capacitance fading [1]. In order to improve the cycle ability of conducting polymer, many efforts have been made such as the incorporation of aryl sulfonates as large-molecule dopants into the polymer matrix during oxidative polymerization [2]. Anthraquinone derivatives, as the largest group of naturally present quinones, were able to bring about the fast redox reaction in aqueous solutions as pseudocapacitive materials. The results revealed that polypyrrole doped with anthraquinone derivative could exhibit the improved electrochemical performances, which was due to its special structure with high porosity and large specific surface area as well as its high electrical conductivity [2–4]. However, the effect of large anthraquinone derivative containing azo unit and sulfonate group as crosslinking agent and doping agent on the morphology and electrochemical performances of PANI was not investigated in detail so far. In this paper, Acid Red 27 (AR-27), which was an anthraquinone derivative and an azo dye with a large p-conjugated structure and three sulfonate anions, was used as doping agent for ARCP synthesized by chemical oxidation polymerization for the first time. Here, ⇑ Corresponding author. E-mail address: [email protected] (B. Li). https://doi.org/10.1016/j.matlet.2017.10.107 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
AR-27 was not only a doping agent, but also played the role of a crosslinking agent, which controlled the morphology of PANI by adjusting the feeding molar ratio of AR-27 to aniline, leading to the improved electrochemical performance of PANI. 2. Material and methods ARCP was synthesized by chemical oxidative polymerization of aniline in the presence of AR-27 that has been reported elsewhere [5], and the feeding molar ratio of AR-27 (87% purity, Aladdin Reagent Co., Ltd) to aniline (Sinopharm Chemical Reagent Co., Ltd) was varied as 0.01:1, 0.02:1, 0.04:1, 0.06:1 and 0.1:1. For comparison, PANI without AR-27 was synthesized under the same reaction condition. For the convenience of discussion, the resulting ARCP samples were designated as ARCP-1, ARCP-2, ARCP-3, ARCP-4 and ARCP-5 with the increase of feeding molar ratio of AR-27 to aniline. Detailed material and methods were given in ‘‘Supporting Information”. 3. Results and discussion 3.1. UV–Vis spectral analysis UV–Vis absorption spectra of AR-27, PANI and ARCP dispersed well in DMF were presented in Fig. 1. The peaks at 300–350 nm and 600–650 nm were assigned to the p ? p⁄ benzenoid transition and p ? p⁄ quinonoid transition, respectively [6]. When the intensity of absorption peak at 300–350 nm was normalized, it was
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because of the low crosslinking density as shown in Fig. 3. With the increase of feeding molar ratio, the crosslinking density of ARCP enhanced, which caused that the thin fiber could easy to precipitate from the reaction system and the chain segment of PANI arranged closely to form ARCP fiber with the smooth surface [9].
3.3. Electrochemical test
Fig. 1. UV–Vis absorption spectra of AR-27, PANI and ARCP.
obvious that the intensity of absorption peak at 600–650 nm assigned to the p ? p⁄ quinonoid transition became weak with the increase of feeding molar ratio, which demonstrated that ARCP had the higher content of benzenoid structure and the higher doping level than PANI. Moreover, the peak at 520 nm assigned to the n ? p⁄ transition of azo-aromatic chromophore of AR-27 was observed [7], and became strong with the increase of feeding molar ratio in UV–Vis absorption spectra of ARCP, which proved the successful doping of PANI with AR-27.
3.2. Morphology and formation mechanism SEM images of PANI and ARCP were shown in Fig. 2. In comparison with the microsized bulky structure of uncrosslinked PANI, the nanofiber structure of ARCP was observed and the diameter of ARCP nanofiber decreased from 300 nm to 60 nm with the increase of feeding molar ratio. The formation mechanism of ARCP nanofiber was illustrated in Fig. 3. PANI was not only doped but also crosslinked by AR-27 because of its three sulfonate anions [8–10]. Compared with AR27, H2SO4 was only a doping agent and not a crosslinking agent. When the molar ratio of AR-27 to aniline was 0.01:1, the crosslinking density of ARCP was low, so that it could not easy to precipitate from the reaction mixture until the diameter of fiber was about 300 nm. At the same time, the chain segment of PANI would extend out of the fiber surface to form H2SO4-doped PANI cilium
The CV curves of PANI and ARCP with the same mass were shown in Fig. 4(a). It was found that all CV curves showed the typical reduction peaks (R1 and R2) and oxidation peaks (O1 and O2) of PANI, which were attributed to the redox conversions of leucoemeraldine/emeraldine (O1 and R1) and emeraldine/pernigraniline (O2 and R2), respectively [11]. Moreover, the oxidation peaks and the reduction peaks of all ARCP samples moved to the low potential direction in comparison with those of PANI, which illustrated that AR-27 could fully participate in the redox reaction of PANI and promote the charge carrier conduction of ARCP. EIS measurements of PANI and ARCP were presented in terms of Nyquist plots as shown in Fig. 4(b). It was clear that Nyquist plots of PANI and ARCP were all consist of an incomplete semicircle at the high frequency region and a straight line at the low frequency region. The charge-transfer resistance (Rct) at the interface between the electrode and the electrolyte could be quantitatively estimated by the diameter of curvature along the X-axis in the Nyquist plot [12]. The Rct value considerably depended on the conducting ability of active material, and the high conductivity of active material could evidently accelerate the electron transfer at the interface between the electrode and the electrolyte and reduce the Rct value [13]. The Rct values of ARCP estimated from the diameter of curvature were 0.87, 0.76, 0.40, 0.53 and 0.67 X with the increase of feeding molar ratio and the Rct value of uncrosslinked PANI was 1.30 X. The smallest Rct value for ARCP-3 indicated that it possessed good conductivity and best electrochemical performance [13], which might be due to its high doping level. With the further increase of feeding molar ratio, the Rct value of ARCP began to increase, which could be explained by the limited mobility of large AR-27 molecule hindering the further increase of crosslinking density and the extra resistance as a result of the adsorption of excess AR-27 on the surface of ARCP [3,8,14]. The GCD curves of PANI and ARCP at 2 Ag 1 were shown in Fig. 4(c) and the corresponding specific capacitances calculated from the GCD curves were also listed out in Fig. 4(c). It was
Fig. 2. SEM images of PANI and ARCP.
M. Shi et al. / Materials Letters 212 (2018) 259–262
261
Fig. 3. Formation mechanism of ARCP nanofibers.
Fig. 4. (a) CV curves of PANI and ARCP, (b) GCD curves of PANI and ARCP, (c) Nyquist plots of PANI and ARCP and (d) Electrochemical cycling stabilities of PANI and ARCP-3.
observed clearly that the specific capacitance of PANI was 280 Fg 1 and the specific capacitance of ARCP increased firstly and then decreased with the increase of feeding molar ratio. ARCP showed the largest specific capacitance of 463 Fg 1 when the feeding molar ratio of AR-27 to aniline was 0.04:1, which was slightly higher than that of nanostructured PANI reported elsewhere [5,15]. The largest specific capacitance for ARCP-3 could be explained reasonably by its smallest Rct value, appropriate crosslinking density, special nanofiber structure and high doping level.
The electrochemical cycling stabilities of PANI and ARCP-3 were studied by CV test at 100 mVs 1, and the corresponding specific capacitance retentions were shown in Fig. 4(d), where the inset displayed the CV curves of ARCP-3 at 1st cycle and 1000st cycle, respectively. The results showed that the specific capacitances of PANI and ARCP-3 retained 70.8% and 85.5% of their initial specific capacitances after 1000 cycles, respectively, which revealed that the electrochemical cycling stability of ARCP-3 was superior to PANI. The improved electrochemical cycling stability of ARCP-3 was attributed to the introduction of AR-27 with a rigid
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M. Shi et al. / Materials Letters 212 (2018) 259–262
p-conjugated structure and three sulfonate anions, which acted as a crosslinking agent or a support skeleton to effectively prevent PANI from volume expansion or contraction in the process of charge-discharge.
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.matlet.2017.10.107.
4. Conclusions
References
ARCP nanofibers were prepared successfully via chemical oxidative polymerization of aniline in the presence of AR-27 as crosslinking agent and doping agent. The results revealed that AR-27 could control the morphology of ARCP nanofibers by adjusting the feeding molar ratio of AR-27 to aniline, and the resulting ARCP exhibited large specific capacitance and great electrochemical cycling stability. ARCP showed the largest specific capacitance of 463 Fg 1at 2 Ag 1 and the great capacitance retention of 85.5% after 1000 cycles when the feeding molar ratio of AR-27 to aniline was 0.04:1. The reasonable large value of specific capacitance with great electrochemical cycling stability of ARCP certified its efficiency as electrode material for supercapacitors.
[1] C. Pan, H. Gu, L. Dong, J. Power Sources 303 (2016) 175. [2] X.M. Lang, Q.Y. Wan, C.H. Feng, X.J. Yue, W.D. Xu, J. Li, S.S. Fan, Synth. Met. 160 (2010) 1800. [3] E. Hakansson, T. Lin, H. Wang, A. Kaynak, Synth. Met. 156 (2006) 1194. [4] X. Wang, J. Deng, X. Duan, D. Liu, P. Liu, Appl. Energy 153 (2015) 70. [5] H. Xu, H. Jiang, X. Li, G. Wang, RSC Adv. 5 (2015) 76116. [6] C.M.S. Izumi, A.M.D.C. Ferreira, V.R.L. Constantino, M.L.A. Temperini, Macromolecules 40 (2007) 3204. [7] X.B. Chen, J.J. Zhang, H.B. Zhang, Z.H. Jiang, G. Shi, Y.B. Li, Y.L. Song, Dyes Pigm 77 (2008) 223. [8] Y.L. Wang, C. Yang, P. Liu, Chem. Eng. J. 172 (2011) 1137. [9] L.J. Pan, G.H. Yu, D.Y. Zhai, H.R. Lee, W.T. Zhao, N. Liu, H.L. Wang, B.C.K. Tee, Y. Shi, Y. Cui, Z.N. Bao, Proc. Natl. Acad. Sci. U.S.A. 109 (2012) 9287. [10] D. Mahanta, G. Madras, S. Radhakrishnan, S. Patil, J. Phys. Chem. B 112 (2008) 10153. [11] Y.G. Wang, H.Q. Li, Y.Y. Xia, Adv. Mater. 18 (2006) 2619. [12] H. Xu, J.X. Wu, C.L. Li, J.L. Zhang, X.X. Wang, Electrochim. Acta 90 (2013) 393. [13] T. Li, Z. Qin, B. Liang, F. Tian, J. Zhao, N. Liu, M.F. Zhu, Electrochim. Acta 177 (2015) 343. [14] G.R. Mitchell, F.J. Davis, C.H. Legge, Synth. Met. 26 (1988) 247. [15] H. Xu, X. Li, G. Wang, J. Power Sources 294 (2015) 16.
Acknowledgments This work was supported by the National Natural Science Foundation of China (grant no. 61205182).
Appendix A. Supplementary data
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Acid Red 27-crosslinked polyaniline with nanofiber structure as electrode material for supercapacitors Mingwei Shi, Mengdi Bai, Baoming Li ⇑ College of Material Science and Engineering, Fuzhou University, 2 Xue Yuan Road, Fuzhou 350108, PR China
a r t i c l e
i n f o
Article history: Received 29 September 2017 Received in revised form 18 October 2017 Accepted 25 October 2017 Available online 5 November 2017 Keywords: Polymers Nanofiber Microstructure Electronic materials Supercapacitors
a b s t r a c t Acid Red 27-crosslinked PANI (ARCP) was prepared via chemical oxidative polymerization in the presence of Acid Red 27 (AR-27) as crosslinking agent and doping agent. The results showed that in comparison with the microsized bulky structure of uncrosslinked PANI, the nanofiber structure of ARCP was observed and the diameter of ARCP nanofiber decreased from 300 nm to 60 nm with the increase of feeding molar ratio of AR-27 to aniline. ARCP exhibited a large specific capacitance of 463 Fg 1 at 2 Ag 1 and a great electrochemical cycling stability with the specific capacitance retention of 85.5% after 1000 cycles when the feeding molar ratio of AR-27 to aniline was 0.04:1, which was better than uncrosslinked PANI with a specific capacitance of 280 Fg 1 and the specific capacitance retention of 70.8%. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction In recent years, polyaniline (PANI) has been extensively investigated as electrode material for supercapacitor. Unfortunately, PANI underwent large volumetric swelling and shrinking during the process of charge/discharge as a result of ion doping and dedoping, which would lead to the structural breakdown and fast capacitance fading [1]. In order to improve the cycle ability of conducting polymer, many efforts have been made such as the incorporation of aryl sulfonates as large-molecule dopants into the polymer matrix during oxidative polymerization [2]. Anthraquinone derivatives, as the largest group of naturally present quinones, were able to bring about the fast redox reaction in aqueous solutions as pseudocapacitive materials. The results revealed that polypyrrole doped with anthraquinone derivative could exhibit the improved electrochemical performances, which was due to its special structure with high porosity and large specific surface area as well as its high electrical conductivity [2–4]. However, the effect of large anthraquinone derivative containing azo unit and sulfonate group as crosslinking agent and doping agent on the morphology and electrochemical performances of PANI was not investigated in detail so far. In this paper, Acid Red 27 (AR-27), which was an anthraquinone derivative and an azo dye with a large p-conjugated structure and three sulfonate anions, was used as doping agent for ARCP synthesized by chemical oxidation polymerization for the first time. Here, ⇑ Corresponding author. E-mail address: [email protected] (B. Li). https://doi.org/10.1016/j.matlet.2017.10.107 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
AR-27 was not only a doping agent, but also played the role of a crosslinking agent, which controlled the morphology of PANI by adjusting the feeding molar ratio of AR-27 to aniline, leading to the improved electrochemical performance of PANI. 2. Material and methods ARCP was synthesized by chemical oxidative polymerization of aniline in the presence of AR-27 that has been reported elsewhere [5], and the feeding molar ratio of AR-27 (87% purity, Aladdin Reagent Co., Ltd) to aniline (Sinopharm Chemical Reagent Co., Ltd) was varied as 0.01:1, 0.02:1, 0.04:1, 0.06:1 and 0.1:1. For comparison, PANI without AR-27 was synthesized under the same reaction condition. For the convenience of discussion, the resulting ARCP samples were designated as ARCP-1, ARCP-2, ARCP-3, ARCP-4 and ARCP-5 with the increase of feeding molar ratio of AR-27 to aniline. Detailed material and methods were given in ‘‘Supporting Information”. 3. Results and discussion 3.1. UV–Vis spectral analysis UV–Vis absorption spectra of AR-27, PANI and ARCP dispersed well in DMF were presented in Fig. 1. The peaks at 300–350 nm and 600–650 nm were assigned to the p ? p⁄ benzenoid transition and p ? p⁄ quinonoid transition, respectively [6]. When the intensity of absorption peak at 300–350 nm was normalized, it was
260
M. Shi et al. / Materials Letters 212 (2018) 259–262
because of the low crosslinking density as shown in Fig. 3. With the increase of feeding molar ratio, the crosslinking density of ARCP enhanced, which caused that the thin fiber could easy to precipitate from the reaction system and the chain segment of PANI arranged closely to form ARCP fiber with the smooth surface [9].
3.3. Electrochemical test
Fig. 1. UV–Vis absorption spectra of AR-27, PANI and ARCP.
obvious that the intensity of absorption peak at 600–650 nm assigned to the p ? p⁄ quinonoid transition became weak with the increase of feeding molar ratio, which demonstrated that ARCP had the higher content of benzenoid structure and the higher doping level than PANI. Moreover, the peak at 520 nm assigned to the n ? p⁄ transition of azo-aromatic chromophore of AR-27 was observed [7], and became strong with the increase of feeding molar ratio in UV–Vis absorption spectra of ARCP, which proved the successful doping of PANI with AR-27.
3.2. Morphology and formation mechanism SEM images of PANI and ARCP were shown in Fig. 2. In comparison with the microsized bulky structure of uncrosslinked PANI, the nanofiber structure of ARCP was observed and the diameter of ARCP nanofiber decreased from 300 nm to 60 nm with the increase of feeding molar ratio. The formation mechanism of ARCP nanofiber was illustrated in Fig. 3. PANI was not only doped but also crosslinked by AR-27 because of its three sulfonate anions [8–10]. Compared with AR27, H2SO4 was only a doping agent and not a crosslinking agent. When the molar ratio of AR-27 to aniline was 0.01:1, the crosslinking density of ARCP was low, so that it could not easy to precipitate from the reaction mixture until the diameter of fiber was about 300 nm. At the same time, the chain segment of PANI would extend out of the fiber surface to form H2SO4-doped PANI cilium
The CV curves of PANI and ARCP with the same mass were shown in Fig. 4(a). It was found that all CV curves showed the typical reduction peaks (R1 and R2) and oxidation peaks (O1 and O2) of PANI, which were attributed to the redox conversions of leucoemeraldine/emeraldine (O1 and R1) and emeraldine/pernigraniline (O2 and R2), respectively [11]. Moreover, the oxidation peaks and the reduction peaks of all ARCP samples moved to the low potential direction in comparison with those of PANI, which illustrated that AR-27 could fully participate in the redox reaction of PANI and promote the charge carrier conduction of ARCP. EIS measurements of PANI and ARCP were presented in terms of Nyquist plots as shown in Fig. 4(b). It was clear that Nyquist plots of PANI and ARCP were all consist of an incomplete semicircle at the high frequency region and a straight line at the low frequency region. The charge-transfer resistance (Rct) at the interface between the electrode and the electrolyte could be quantitatively estimated by the diameter of curvature along the X-axis in the Nyquist plot [12]. The Rct value considerably depended on the conducting ability of active material, and the high conductivity of active material could evidently accelerate the electron transfer at the interface between the electrode and the electrolyte and reduce the Rct value [13]. The Rct values of ARCP estimated from the diameter of curvature were 0.87, 0.76, 0.40, 0.53 and 0.67 X with the increase of feeding molar ratio and the Rct value of uncrosslinked PANI was 1.30 X. The smallest Rct value for ARCP-3 indicated that it possessed good conductivity and best electrochemical performance [13], which might be due to its high doping level. With the further increase of feeding molar ratio, the Rct value of ARCP began to increase, which could be explained by the limited mobility of large AR-27 molecule hindering the further increase of crosslinking density and the extra resistance as a result of the adsorption of excess AR-27 on the surface of ARCP [3,8,14]. The GCD curves of PANI and ARCP at 2 Ag 1 were shown in Fig. 4(c) and the corresponding specific capacitances calculated from the GCD curves were also listed out in Fig. 4(c). It was
Fig. 2. SEM images of PANI and ARCP.
M. Shi et al. / Materials Letters 212 (2018) 259–262
261
Fig. 3. Formation mechanism of ARCP nanofibers.
Fig. 4. (a) CV curves of PANI and ARCP, (b) GCD curves of PANI and ARCP, (c) Nyquist plots of PANI and ARCP and (d) Electrochemical cycling stabilities of PANI and ARCP-3.
observed clearly that the specific capacitance of PANI was 280 Fg 1 and the specific capacitance of ARCP increased firstly and then decreased with the increase of feeding molar ratio. ARCP showed the largest specific capacitance of 463 Fg 1 when the feeding molar ratio of AR-27 to aniline was 0.04:1, which was slightly higher than that of nanostructured PANI reported elsewhere [5,15]. The largest specific capacitance for ARCP-3 could be explained reasonably by its smallest Rct value, appropriate crosslinking density, special nanofiber structure and high doping level.
The electrochemical cycling stabilities of PANI and ARCP-3 were studied by CV test at 100 mVs 1, and the corresponding specific capacitance retentions were shown in Fig. 4(d), where the inset displayed the CV curves of ARCP-3 at 1st cycle and 1000st cycle, respectively. The results showed that the specific capacitances of PANI and ARCP-3 retained 70.8% and 85.5% of their initial specific capacitances after 1000 cycles, respectively, which revealed that the electrochemical cycling stability of ARCP-3 was superior to PANI. The improved electrochemical cycling stability of ARCP-3 was attributed to the introduction of AR-27 with a rigid
262
M. Shi et al. / Materials Letters 212 (2018) 259–262
p-conjugated structure and three sulfonate anions, which acted as a crosslinking agent or a support skeleton to effectively prevent PANI from volume expansion or contraction in the process of charge-discharge.
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.matlet.2017.10.107.
4. Conclusions
References
ARCP nanofibers were prepared successfully via chemical oxidative polymerization of aniline in the presence of AR-27 as crosslinking agent and doping agent. The results revealed that AR-27 could control the morphology of ARCP nanofibers by adjusting the feeding molar ratio of AR-27 to aniline, and the resulting ARCP exhibited large specific capacitance and great electrochemical cycling stability. ARCP showed the largest specific capacitance of 463 Fg 1at 2 Ag 1 and the great capacitance retention of 85.5% after 1000 cycles when the feeding molar ratio of AR-27 to aniline was 0.04:1. The reasonable large value of specific capacitance with great electrochemical cycling stability of ARCP certified its efficiency as electrode material for supercapacitors.
[1] C. Pan, H. Gu, L. Dong, J. Power Sources 303 (2016) 175. [2] X.M. Lang, Q.Y. Wan, C.H. Feng, X.J. Yue, W.D. Xu, J. Li, S.S. Fan, Synth. Met. 160 (2010) 1800. [3] E. Hakansson, T. Lin, H. Wang, A. Kaynak, Synth. Met. 156 (2006) 1194. [4] X. Wang, J. Deng, X. Duan, D. Liu, P. Liu, Appl. Energy 153 (2015) 70. [5] H. Xu, H. Jiang, X. Li, G. Wang, RSC Adv. 5 (2015) 76116. [6] C.M.S. Izumi, A.M.D.C. Ferreira, V.R.L. Constantino, M.L.A. Temperini, Macromolecules 40 (2007) 3204. [7] X.B. Chen, J.J. Zhang, H.B. Zhang, Z.H. Jiang, G. Shi, Y.B. Li, Y.L. Song, Dyes Pigm 77 (2008) 223. [8] Y.L. Wang, C. Yang, P. Liu, Chem. Eng. J. 172 (2011) 1137. [9] L.J. Pan, G.H. Yu, D.Y. Zhai, H.R. Lee, W.T. Zhao, N. Liu, H.L. Wang, B.C.K. Tee, Y. Shi, Y. Cui, Z.N. Bao, Proc. Natl. Acad. Sci. U.S.A. 109 (2012) 9287. [10] D. Mahanta, G. Madras, S. Radhakrishnan, S. Patil, J. Phys. Chem. B 112 (2008) 10153. [11] Y.G. Wang, H.Q. Li, Y.Y. Xia, Adv. Mater. 18 (2006) 2619. [12] H. Xu, J.X. Wu, C.L. Li, J.L. Zhang, X.X. Wang, Electrochim. Acta 90 (2013) 393. [13] T. Li, Z. Qin, B. Liang, F. Tian, J. Zhao, N. Liu, M.F. Zhu, Electrochim. Acta 177 (2015) 343. [14] G.R. Mitchell, F.J. Davis, C.H. Legge, Synth. Met. 26 (1988) 247. [15] H. Xu, X. Li, G. Wang, J. Power Sources 294 (2015) 16.
Acknowledgments This work was supported by the National Natural Science Foundation of China (grant no. 61205182).
Appendix A. Supplementary data