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Flexible and foldable supercapacitor electrodes from the porous 3D network of cellulose nanofibers, carbon nanotubes and polyaniline
Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Flexible and foldable supercapacitor electrodes from the porous 3D network of cellulose nanofibers, carbon nanotubes and polyaniline Chuang Yang, Dagang Li College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 March 2015 Accepted 20 April 2015
Flexible and foldable supercapacitor electrodes fabricated by an in situ chemical polymerization method coating PANI on the surface and inside network of the CNFs (cellulose nanofibers)/MWCNTs (multi-walled carbon nanotubes) films are reported. Due to the porous 3D network structure of the aerogel-like CNFs/ MWCNTs films which made by an ethanol replacement treatment method, the CNFs/MWCNTs/PANI (polyaniline) electrodes get more PANI nanofibers inside of the porous CNFs/MWCNTs films and obtain a low charge transfer resistance (6.31 Ω) and a excellent specific capacitance (249.7 F/g at 10 mV/s). The lowcost, light-weight and flexible electrode materials may be potential applications for the high-performance flexible all-solid-state supercapacitors. & 2015 Published by Elsevier B.V.
Keywords: Supercapacitor Cellulose nanofibers Carbon nanotubes Polyaniline Electrical properties
1. Introduction In recent years, supercapacitors have attracted great interest for the important applications in the area of electrochemical energy storage because of their high energy density, high power density, long life cycle and environmental friendliness [1–3]. The main supercapacitor electrodes materials that have been widely studied incorporate carbons [4–6], metal oxides [7,8] and conducting polymers [9,10]. Among conducting polymers, polyaniline (PANI) has been considered the most promising material due to its low cost, ease of synthesis, fast electrochemical switching and good environmental stability. Since carbon nanotubes (CNTs) were discovered by Iijima [11] in 1991, the interest in the fundamental properties of them and their exploitation goes on increasing through a vast range of applications [12]. With the development of flexible display devices, much attention has been turned to develop the supercapacitor electrodes which are light-weight, flexible and with high performance [2,7,13,14], so flexible substrates gradually become a hot research as templates for depositing active conducting materials [10,15]. Cellulose nanofibers (CNFs), derived from cellulose the most abundant and sustainable natural polymer may find extensive applications in flexible energy-storage devices because of their high aspect ratios, high surface area, high porosity, excellent mechanical properties, excellent flexibility and more importantly, strongly bind with conductive carbon nanofillers such as graphene and CNTs [16]. In this paper, we report on foldable, flexible, light-weight, aerogel-like 3D CNFs/MWCNTs/PANI film electrodes fabricated by adsorption and in situ chemical polymerization of aniline on
the surface and inside network of the aerogel-like CNFs/ MWCNTs films. CNFs and MWCNTs intertwine with each other as a substrate, which greatly improves the flexibility and mechanical strength of the film electrode. Electrochemical properties of CNFs/MWCNTs/PANI films used as the electrode material of supercapacitors which are free from binder and electric conductors for fabricating supercapacitor electrode has been significantly improved as compared to the CNFs/MWCNTs films. It is a preliminary study on the electrode materials for all-solid-state supercapacitors.
2. Experimental Materials: Bamboo powders from moso bamboo were sieved through a 60 mesh, then air dried and set aside. The MWCNTs powers were purchased from Shenzhen Nanotech Port Co., Ltd. Aniline, ammonium persulfate (APS) and sodium dodecyl benzene sulfonate (SDBS) were purchased from Shanghai Ling Feng Chemical Reagent Co., Ltd. Benzene, ethanol, sodium chlorite, acetic acid, potassium hydroxide, hydrochloric acid and other chemicals were of laboratory grade and used without further purification. Methods: Chemical purification of bamboo cellulose tissues was performed according to the method of the literature [17]. After chemical treatment, we passed the slurry of 1 wt% purified cellulose through a grinder (MKCA6-2, Masuko Sangyo Co., Ltd., Japan) at 1500 rpm [18] to get a CNFs slurry. MWCNTs powers with 0.1 g of SDBS as a dispersant and 2.5 g (0.1 wt%) of CNFs slurry were dispersed and mixed using an ultrasonic, subsequently, the mixture was drained to get a CNFs/MWCNTs filter cake by vacuum
http://dx.doi.org/10.1016/j.matlet.2015.04.096 0167-577X/& 2015 Published by Elsevier B.V.
Please cite this article as: Yang C, Li D. Flexible and foldable supercapacitor electrodes from the porous 3D network of cellulose nanofibers, carbon nanotubes and polyaniline. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.04.096i
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C. Yang, D. Li / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
filtration. Repeatedly soaked ethanol until had replaced all the water, then freeze-dried the cake to get the CNFs/MWCNTs electrode. The CNFs/MWCNTs/PANI electrode was fabricated by coating PANI on the surface and inside network of the CNFs/ MWCNTs film by an in situ chemical polymerization. The no freeze-dried CNFs/MWCNTs film was taken out from ethanol and soaked in a solution of 5 mmol aniline monomer in 50 ml of 1 M HCl for 1 h. Then, 5 mmol of APS dissolved in 50 ml of 1 M HCl solution was added dropwise to the aniline solution. The polymerization was carried out at 0 1C and with stirring for 3 h and then washed with distilled water and ethanol. Finally, the CNFs/ MWCNTs/PANI electrode was obtained after freeze-dried the darkgreen film for 24 h.
Fig. 1. The FTIR spectra of (a) CNFs, (b) PANI, (c) CNFs/MWCNTs electrode and (d) CNFs/MWCNTs/PANI electrode.
3. Results and discussion The FTIR spectrum of CNFs shows a typical IR spectrum of cellulose in Fig. 1(a). It gives seven peaks at 3328, 2893, 1639, 1428, 1367, 1023 and 896 cm 1. The PANI (Fig. 1(b)) spectrum consists of five distinct peaks at 1585, 1495, 1303, 1143 and 826 cm 1. The bands are attributed to the NQQQN stretching, N–B–N stretching (B and Q represent benzenoid and quinoid moieties in the PANI chains), C–N stretching of secondary aromatic amines, C–H bending and N–H wag of the secondary amine [19]. Fig. 1(c) and (d) are the FRIR spectra of CNFs/MWCNTs and CNFs/MWCNTs/PANI composite samples. Compared with the spectrum of CNFs/MWCNTs composite sample, CNFs/MWCNTs/PANI composite sample not only retains the peaks at 2118 and 2384 cm 1 in the CNFs/ MWCNTs spectrum but also exhibits the peaks of PANI sample. The absorption peaks at 1585, 1495 and 1303 cm 1 in the spectrum of PAN are shifted to 1538, 1446 and 1278 cm 1 in the spectrum of CNFs/MWCNTs/PANI composite sample. Fig. 2(a) shows the SEM image of freeze-dried CNFs sample with a diameter in the range from 10 to 30 nm to form a network structure. It can be observed from the SEM image of the CNFs/ MWCNTs film (Fig. 2(b)) that CNFs and MWCNTs intertwined together randomly and constructed a 3D conducting nanoporous network. The porous structure of CNFs/MWCNTs film enables aniline molecules easily infiltrating into the film, which is necessary for an efficient deposition of PANI onto CNFs and MWCNTs surface. The pure PANI nanofibers are presented in Fig. 2(c), they form a random, interconnected, nearly flat web structure of diameters roughly 30–70 nm. Fig. 2(d) is the inner surface of the torn oblique section of the CNFs/MWCNTs/PANI film, it is observed that PANI was formed within the CNFs/MWCNTs film and the ternary film is a porous 3D network structure. The electrochemical measurements of the CNFs/MWCNTs and CNFs/MWCNTs/PANI electrodes were investigated by cyclic voltammetry (CV), galvanostatic charge–discharge (G-CD) and electrochemical impedance spectroscopy (EIS) in a three electrode testing system (CHI 660E electrochemical workstation, Chenhua, Shanghai) with a platinum electrode as counterelectrode and saturated calomel electrode (SCE) as reference
Fig. 2. SEM images of (a) CNFs, (b) CNFs/MWCNTs film, (c) PANI and (d) CNFs/MWCNTs/PANI film.
Please cite this article as: Yang C, Li D. Flexible and foldable supercapacitor electrodes from the porous 3D network of cellulose nanofibers, carbon nanotubes and polyaniline. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.04.096i
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C. Yang, D. Li / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Fig. 3. The electrochemical performances of the two samples: (a) CV curves at 10 mV/s. (b) Galvanostatic charge–discharge curves at 0.2 A/g. (c) Cycle stability of the CNFs/ MWCNTs/PANI electrode at 2 A/g. The inset shows the first 10 cycles of charging–discharging curves (d) EIS measurements at open circuit potential. The inset is the equivalent circuit. (e) Digital image of the flexible CNFs/MWCNTs/PANI film. (f) Digital image which shows that the ternary film was folded into a paper airplane and used as a wire to let a red LED glowed well. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
electrode in 1 M H2SO4 electrolyte solution. Fig. 3(a) shows the CV curves of the CNFs/MWCNTs and CNFs/MWCNTs/PANI electrodes between 0.2 V and 0.8 V at 10 mV/s. Compared to CNFs/ MWCNTs electrode, CNFs/MWCNTs/PANI electrode has a larger integrated CV area and possesses high reversibility. According to R the equation: Cs ¼ ( IdV)/mυΔV, the specific capacitance (Cs) of CNFs/MWCNTs/PANI electrode can reach 249.7 F/g more than that of CNFs/MWCNTs electrode (50 F/g) because of the pseudocapacitances of PANI. G-CD curves (Fig. 3(b)) were measured between 0 V and 0.8 V at a constant current density of 0.2 A/g. The shapes of curves for the two samples are triangular in nature which lead to the ideal capacitive behavior of the material with almost symmetric charge–discharge curve. According to the equation: CS ¼IΔt/(mΔV), the obtained Cs of 207.2 F/g at a discharge current density of 0.2 A/g for the CNFs/ MWCNTs/PANI electrode is approximately 11 times higher than that of the CNFs/MWCNTs electrode (18.5 F/g). Fig. 3(c) shows the long life stability of the supercapacitor. The specific capacitance of the CNFs/ MWCNTs/PANI electrode retained 82.4% of the initial capacitance after 1000 cycles at 2 A/g. The inset of Fig. 3(c) shows typical charge– discharge curves in a continuous operation for the first 10 cycles. EIS
measurements were performed at open circuit potential with an AC voltage amplitude of 5 mV and the frequency range from 100 kHz to 0.01 Hz. In the Nyquist plots (Fig. 3(d)), the diameter of semicircle presented the charge transfer resistance (Rct) of the CNFs/MWCNTs/ PANI electrode (6.31 Ω) is much lower than that of CNFs/MWCNTs electrode (18.9 Ω). And the impedance curves of CNFs/MWCNTs/PANI electrode showed a nearly vertical line in the low frequency region, suggesting a good capacitive performance. As shown in Fig. 3(e), the CNFs/MWCNTs/PANI film can be bent, rolled and even folded. Finally, the CNFs/MWCNTs/PANI film was folded into a paper airplane (Fig. 3 (f)) and used as a wire to light up a red LED.
4. Conclusion In conclusion, we have successfully prepared a foldable supercapacitor electrode made of CNFs, MWCNTs and PANI with high electrochemical performance. Alcohol replacement treatment increased the immersion of aniline monomer, which increased the content of PANI inside the CNFs/MWCNTs film. Due to coating with the PANI, MWCNTs connected together more effectively,
Please cite this article as: Yang C, Li D. Flexible and foldable supercapacitor electrodes from the porous 3D network of cellulose nanofibers, carbon nanotubes and polyaniline. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.04.096i
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made the charge transfer resistance be reduced by 67%. Besides, the CNFs/MWCNTs electrode yielded specific capacitance of 50 F/g, however the electrochemical behavior of CNFs/MWCNTs/PANI was greatly boosted, the specific capacitance could reach 249.7 F/g. In addition, the flexible and foldable CNFs/MWCNTs/PANI electrode materials could be useful to design high-performance flexible allsolid-state supercapacitors.
References [1] [2] [3] [4] [5] [6]
Acknowledgments This work is financially supported by National Natural Science Foundation of China (31170514 and 31370557), Graduate Cultivation Innovative Project of Jiangsu Province (CXZZ11-0525), the Doctoral Program of Higher Education (20113204110011) and the Analysis & Test Center of Nanjing Forestry University. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2015. 04.096.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Yan J, Wang Q, Wei T, Fan Z. Adv Energy Mater 2014;4(4)1300816. Liu L, Niu Z, Zhang L, Zhou W, Chen X, Xie S. Adv Mater 2014;26:4855–62. Long C, Qi D, Wei T, Yan J, Jiang L, Fan Z. Adv Funct Mater 2014;24:3953–61. Zhang Y, Liu C, Wen B, Song X, Li T. Mater Lett 2011;65:49–52. Hulicova J-D, Kodama M, Shiraishi S, Hatori H, Zhu Z-H, Lu G-Q. Adv Funct Mater 2009;19:1800–9. Chen L-F, Huang Z-H, Liang H-W, Gao H-L, Yu S-H. Adv Funct Mater 2014;24:5104–11. Li S, Wen J, Mo X, Long H, Wang H, Wang J, et al. J Power Sources 2014;256:206–11. Yan X, Tong X, Ma L, Tian Y, Cai Y, Gong C, et al. Mater Lett 2014;124:133–6. Hao G-P, Hippauf F, Oschatz M, Wisser F-M, Leifert A, Nickel W, et al. ACS Nano 2014;8:7138–46. Hu W, Chen S, Yang Z, Liu L, Wang H. J Phys Chem B 2011;115:8453–7. Iijima S. Nature 1991;354:56–8. Oueiny C, Berlioz S, Perrin F-X. Prog Polym Sci 2014;39:707–48. Lu X, Yu M, Wang G, Tong Y, Li Y. Energy Environ Sci 2014;7:2160–81. Fei H, Yang C, Bao H, Wang G. J Power Sources 2014;266:488–95. Zheng G, Cui Y, Karabulut E, et al. MRS Bull 2013;38(04):320–5. Li S, Huang D, Zhang B, Xu X, Wang M, Yang G, et al. Adv Energy Mater 2014;4 (10). Chen W, Yu H, Liu Y. Carbohydr Polym 2011;86:453–61. Abe K, Yano H. Cellulose 2010;17:271–7. Ohira M, Sakai T, Takeuchi M, et al. Synth Met 1987;18:347–52.
Please cite this article as: Yang C, Li D. Flexible and foldable supercapacitor electrodes from the porous 3D network of cellulose nanofibers, carbon nanotubes and polyaniline. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.04.096i
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Flexible and foldable supercapacitor electrodes from the porous 3D network of cellulose nanofibers, carbon nanotubes and polyaniline Chuang Yang, Dagang Li College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 March 2015 Accepted 20 April 2015
Flexible and foldable supercapacitor electrodes fabricated by an in situ chemical polymerization method coating PANI on the surface and inside network of the CNFs (cellulose nanofibers)/MWCNTs (multi-walled carbon nanotubes) films are reported. Due to the porous 3D network structure of the aerogel-like CNFs/ MWCNTs films which made by an ethanol replacement treatment method, the CNFs/MWCNTs/PANI (polyaniline) electrodes get more PANI nanofibers inside of the porous CNFs/MWCNTs films and obtain a low charge transfer resistance (6.31 Ω) and a excellent specific capacitance (249.7 F/g at 10 mV/s). The lowcost, light-weight and flexible electrode materials may be potential applications for the high-performance flexible all-solid-state supercapacitors. & 2015 Published by Elsevier B.V.
Keywords: Supercapacitor Cellulose nanofibers Carbon nanotubes Polyaniline Electrical properties
1. Introduction In recent years, supercapacitors have attracted great interest for the important applications in the area of electrochemical energy storage because of their high energy density, high power density, long life cycle and environmental friendliness [1–3]. The main supercapacitor electrodes materials that have been widely studied incorporate carbons [4–6], metal oxides [7,8] and conducting polymers [9,10]. Among conducting polymers, polyaniline (PANI) has been considered the most promising material due to its low cost, ease of synthesis, fast electrochemical switching and good environmental stability. Since carbon nanotubes (CNTs) were discovered by Iijima [11] in 1991, the interest in the fundamental properties of them and their exploitation goes on increasing through a vast range of applications [12]. With the development of flexible display devices, much attention has been turned to develop the supercapacitor electrodes which are light-weight, flexible and with high performance [2,7,13,14], so flexible substrates gradually become a hot research as templates for depositing active conducting materials [10,15]. Cellulose nanofibers (CNFs), derived from cellulose the most abundant and sustainable natural polymer may find extensive applications in flexible energy-storage devices because of their high aspect ratios, high surface area, high porosity, excellent mechanical properties, excellent flexibility and more importantly, strongly bind with conductive carbon nanofillers such as graphene and CNTs [16]. In this paper, we report on foldable, flexible, light-weight, aerogel-like 3D CNFs/MWCNTs/PANI film electrodes fabricated by adsorption and in situ chemical polymerization of aniline on
the surface and inside network of the aerogel-like CNFs/ MWCNTs films. CNFs and MWCNTs intertwine with each other as a substrate, which greatly improves the flexibility and mechanical strength of the film electrode. Electrochemical properties of CNFs/MWCNTs/PANI films used as the electrode material of supercapacitors which are free from binder and electric conductors for fabricating supercapacitor electrode has been significantly improved as compared to the CNFs/MWCNTs films. It is a preliminary study on the electrode materials for all-solid-state supercapacitors.
2. Experimental Materials: Bamboo powders from moso bamboo were sieved through a 60 mesh, then air dried and set aside. The MWCNTs powers were purchased from Shenzhen Nanotech Port Co., Ltd. Aniline, ammonium persulfate (APS) and sodium dodecyl benzene sulfonate (SDBS) were purchased from Shanghai Ling Feng Chemical Reagent Co., Ltd. Benzene, ethanol, sodium chlorite, acetic acid, potassium hydroxide, hydrochloric acid and other chemicals were of laboratory grade and used without further purification. Methods: Chemical purification of bamboo cellulose tissues was performed according to the method of the literature [17]. After chemical treatment, we passed the slurry of 1 wt% purified cellulose through a grinder (MKCA6-2, Masuko Sangyo Co., Ltd., Japan) at 1500 rpm [18] to get a CNFs slurry. MWCNTs powers with 0.1 g of SDBS as a dispersant and 2.5 g (0.1 wt%) of CNFs slurry were dispersed and mixed using an ultrasonic, subsequently, the mixture was drained to get a CNFs/MWCNTs filter cake by vacuum
http://dx.doi.org/10.1016/j.matlet.2015.04.096 0167-577X/& 2015 Published by Elsevier B.V.
Please cite this article as: Yang C, Li D. Flexible and foldable supercapacitor electrodes from the porous 3D network of cellulose nanofibers, carbon nanotubes and polyaniline. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.04.096i
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C. Yang, D. Li / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
filtration. Repeatedly soaked ethanol until had replaced all the water, then freeze-dried the cake to get the CNFs/MWCNTs electrode. The CNFs/MWCNTs/PANI electrode was fabricated by coating PANI on the surface and inside network of the CNFs/ MWCNTs film by an in situ chemical polymerization. The no freeze-dried CNFs/MWCNTs film was taken out from ethanol and soaked in a solution of 5 mmol aniline monomer in 50 ml of 1 M HCl for 1 h. Then, 5 mmol of APS dissolved in 50 ml of 1 M HCl solution was added dropwise to the aniline solution. The polymerization was carried out at 0 1C and with stirring for 3 h and then washed with distilled water and ethanol. Finally, the CNFs/ MWCNTs/PANI electrode was obtained after freeze-dried the darkgreen film for 24 h.
Fig. 1. The FTIR spectra of (a) CNFs, (b) PANI, (c) CNFs/MWCNTs electrode and (d) CNFs/MWCNTs/PANI electrode.
3. Results and discussion The FTIR spectrum of CNFs shows a typical IR spectrum of cellulose in Fig. 1(a). It gives seven peaks at 3328, 2893, 1639, 1428, 1367, 1023 and 896 cm 1. The PANI (Fig. 1(b)) spectrum consists of five distinct peaks at 1585, 1495, 1303, 1143 and 826 cm 1. The bands are attributed to the NQQQN stretching, N–B–N stretching (B and Q represent benzenoid and quinoid moieties in the PANI chains), C–N stretching of secondary aromatic amines, C–H bending and N–H wag of the secondary amine [19]. Fig. 1(c) and (d) are the FRIR spectra of CNFs/MWCNTs and CNFs/MWCNTs/PANI composite samples. Compared with the spectrum of CNFs/MWCNTs composite sample, CNFs/MWCNTs/PANI composite sample not only retains the peaks at 2118 and 2384 cm 1 in the CNFs/ MWCNTs spectrum but also exhibits the peaks of PANI sample. The absorption peaks at 1585, 1495 and 1303 cm 1 in the spectrum of PAN are shifted to 1538, 1446 and 1278 cm 1 in the spectrum of CNFs/MWCNTs/PANI composite sample. Fig. 2(a) shows the SEM image of freeze-dried CNFs sample with a diameter in the range from 10 to 30 nm to form a network structure. It can be observed from the SEM image of the CNFs/ MWCNTs film (Fig. 2(b)) that CNFs and MWCNTs intertwined together randomly and constructed a 3D conducting nanoporous network. The porous structure of CNFs/MWCNTs film enables aniline molecules easily infiltrating into the film, which is necessary for an efficient deposition of PANI onto CNFs and MWCNTs surface. The pure PANI nanofibers are presented in Fig. 2(c), they form a random, interconnected, nearly flat web structure of diameters roughly 30–70 nm. Fig. 2(d) is the inner surface of the torn oblique section of the CNFs/MWCNTs/PANI film, it is observed that PANI was formed within the CNFs/MWCNTs film and the ternary film is a porous 3D network structure. The electrochemical measurements of the CNFs/MWCNTs and CNFs/MWCNTs/PANI electrodes were investigated by cyclic voltammetry (CV), galvanostatic charge–discharge (G-CD) and electrochemical impedance spectroscopy (EIS) in a three electrode testing system (CHI 660E electrochemical workstation, Chenhua, Shanghai) with a platinum electrode as counterelectrode and saturated calomel electrode (SCE) as reference
Fig. 2. SEM images of (a) CNFs, (b) CNFs/MWCNTs film, (c) PANI and (d) CNFs/MWCNTs/PANI film.
Please cite this article as: Yang C, Li D. Flexible and foldable supercapacitor electrodes from the porous 3D network of cellulose nanofibers, carbon nanotubes and polyaniline. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.04.096i
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C. Yang, D. Li / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Fig. 3. The electrochemical performances of the two samples: (a) CV curves at 10 mV/s. (b) Galvanostatic charge–discharge curves at 0.2 A/g. (c) Cycle stability of the CNFs/ MWCNTs/PANI electrode at 2 A/g. The inset shows the first 10 cycles of charging–discharging curves (d) EIS measurements at open circuit potential. The inset is the equivalent circuit. (e) Digital image of the flexible CNFs/MWCNTs/PANI film. (f) Digital image which shows that the ternary film was folded into a paper airplane and used as a wire to let a red LED glowed well. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
electrode in 1 M H2SO4 electrolyte solution. Fig. 3(a) shows the CV curves of the CNFs/MWCNTs and CNFs/MWCNTs/PANI electrodes between 0.2 V and 0.8 V at 10 mV/s. Compared to CNFs/ MWCNTs electrode, CNFs/MWCNTs/PANI electrode has a larger integrated CV area and possesses high reversibility. According to R the equation: Cs ¼ ( IdV)/mυΔV, the specific capacitance (Cs) of CNFs/MWCNTs/PANI electrode can reach 249.7 F/g more than that of CNFs/MWCNTs electrode (50 F/g) because of the pseudocapacitances of PANI. G-CD curves (Fig. 3(b)) were measured between 0 V and 0.8 V at a constant current density of 0.2 A/g. The shapes of curves for the two samples are triangular in nature which lead to the ideal capacitive behavior of the material with almost symmetric charge–discharge curve. According to the equation: CS ¼IΔt/(mΔV), the obtained Cs of 207.2 F/g at a discharge current density of 0.2 A/g for the CNFs/ MWCNTs/PANI electrode is approximately 11 times higher than that of the CNFs/MWCNTs electrode (18.5 F/g). Fig. 3(c) shows the long life stability of the supercapacitor. The specific capacitance of the CNFs/ MWCNTs/PANI electrode retained 82.4% of the initial capacitance after 1000 cycles at 2 A/g. The inset of Fig. 3(c) shows typical charge– discharge curves in a continuous operation for the first 10 cycles. EIS
measurements were performed at open circuit potential with an AC voltage amplitude of 5 mV and the frequency range from 100 kHz to 0.01 Hz. In the Nyquist plots (Fig. 3(d)), the diameter of semicircle presented the charge transfer resistance (Rct) of the CNFs/MWCNTs/ PANI electrode (6.31 Ω) is much lower than that of CNFs/MWCNTs electrode (18.9 Ω). And the impedance curves of CNFs/MWCNTs/PANI electrode showed a nearly vertical line in the low frequency region, suggesting a good capacitive performance. As shown in Fig. 3(e), the CNFs/MWCNTs/PANI film can be bent, rolled and even folded. Finally, the CNFs/MWCNTs/PANI film was folded into a paper airplane (Fig. 3 (f)) and used as a wire to light up a red LED.
4. Conclusion In conclusion, we have successfully prepared a foldable supercapacitor electrode made of CNFs, MWCNTs and PANI with high electrochemical performance. Alcohol replacement treatment increased the immersion of aniline monomer, which increased the content of PANI inside the CNFs/MWCNTs film. Due to coating with the PANI, MWCNTs connected together more effectively,
Please cite this article as: Yang C, Li D. Flexible and foldable supercapacitor electrodes from the porous 3D network of cellulose nanofibers, carbon nanotubes and polyaniline. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.04.096i
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made the charge transfer resistance be reduced by 67%. Besides, the CNFs/MWCNTs electrode yielded specific capacitance of 50 F/g, however the electrochemical behavior of CNFs/MWCNTs/PANI was greatly boosted, the specific capacitance could reach 249.7 F/g. In addition, the flexible and foldable CNFs/MWCNTs/PANI electrode materials could be useful to design high-performance flexible allsolid-state supercapacitors.
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Acknowledgments This work is financially supported by National Natural Science Foundation of China (31170514 and 31370557), Graduate Cultivation Innovative Project of Jiangsu Province (CXZZ11-0525), the Doctoral Program of Higher Education (20113204110011) and the Analysis & Test Center of Nanjing Forestry University. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2015. 04.096.
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Please cite this article as: Yang C, Li D. Flexible and foldable supercapacitor electrodes from the porous 3D network of cellulose nanofibers, carbon nanotubes and polyaniline. Mater Lett (2015), http://dx.doi.org/10.1016/j.matlet.2015.04.096i
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