Kexin Tangabc,
Yuping Li*b,
Hongbin Caoabc,
Feng Duanb,
Jason Zhanga,
Yi Zhangabc and
Yi Wangb
aNational Engineering Research Centre for Distillation Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China
bKey Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China. E-mail: ypli@ipe.ac.cn; Fax: +86-10-82544844-816; Tel: +86-10-82544844-810
cCollaborative Innovation Centre of Chemical Science and Engineering, Tianjin University, Tianjin 300072, PR China
First published on 21st April 2015
We report a ternary composite vanadium/single-walled carbon nanotubes (SWCNTs)/carbon nanofibers (VSCNFs) material using hybrid-electrospinning and carbonizing of polyacrylonitrile, polyvinylpyrrolidone, SWCNTs and vanadyl acetylacetonate. The morphology and structure of the ternary composites are characterized. Its electrochemical properties are measured in a 6 mol L−1 aqueous KOH electrolyte. VSCNFs possesses a hierarchical structure with micropores and mesopores and a specific surface area of 821 m2 g−1, and it also exhibits a reversible specific capacitance of 479 F g−1 at 1 A g−1 and 367.4 F g−1 at 10 A g−1, and retains 94% of its initial capacitance after 5000 cycles (8 A g−1). The results show that simultaneously adding vanadium and SWCNTs can greatly enhance the conductivity, capacitive performance and stability by forming a closer connection with nanofibers through V–N–C, V–O–C and VO bonds. SWNCTs are not only mechanically mixed in CNFs to enhance the extent of graphitization, but also involved in the transmutation process of CNFs graphitization with the participation of vanadium.
Due to the abundant storage, low cost and relative high structure stability, carbon materials are most commonly used in supercapacitors,3,4 such as activated carbon,5 carbon nanofibers,6 carbon aerogels,7 ordered mesoporous carbon,8 carbon nanotubes,9 carbon black10 and graphene,11 especially two or more carbon materials are composited together to achieve sophisticated structure and characteristics,4,12,13 like three-dimensional network,14 where activated carbon (AC), graphene or carbon nanofibers (CNFs) usually acts as carbon skeleton, and carbon nanotubes (CNTs) or carbon black (CB) acts as ancillary elements. For example, single-walled carbon nanotubes (SWCNTs) are sometimes used as additive for CNFs electrode because of its high conductivity and large effective specific surface area,15 and it also can help to build hierarchical pore structure.16
Metal oxides such as TiO2, RuO2, MnO2, and VOx, are widely used to modify carbon materials to get pseudo-capacitance.17,18 RuO2 is reported to be one of the most effective additives, but its high price limited its application.5,19 TiO2 composite carbon nanofibers are more likely used in photocatalysis research.20,21 One of the most development MnO2 modified carbon electrode show relative low capacitance.22 V2O5 (vanadium pentoxide),23–25 VO2 (vanadium dioxide)26,27 and organic vanadium28 are reported to modify porous carbon electrode, while the insulation of these compound weaken carbon nanofibers conductivity and pore structure, but still vanadium compounds might be a suitable additives due to its relatively low cost and abundance in China.29 There are two distinctive formations of vanadium compound inserted in the carbon electrode, namely, amorphous vanadium and crystal vanadium oxide. Some studies show that amorphous vanadium may exhibit more superior capacitive properties compared to crystal vanadium,27,28,30 because amorphous vanadium is likely linked to carbon skeleton with covalent bonds in atom level, and thus enhance the contact of transition metal with ions. In comparison, the crystal vanadium is physically dispersed in the carbon skeleton. Therefore, the research on single-step synthesis and simultaneously incorporating vanadium and conductive additives to form an integrated structure with tunable pore distribution and efficient surface area and good conductivity may be a possible approach to achieve high performance and steady supercapacitor.
Electrospinning technique is a simple and versatile method for producing continuous nanofibers from polymers, composites or ceramics.31 In this work, we have successfully synthesized a composite vanadium/SWCNTs/CNFs electrode for supercapacitors with hybrid electrospinning and carbonization techniques. The fabricated composite CNFs (VSCNFs) have achieved advantages as follows: (1) hierarchical structure, which can enhance ion diffusion within CNFs; (2) unique V–N–C bond to combine carbon skeleton and graphite fragments; (3) high extent of graphitization to improve structure regularity and conductivity; (4) high capacitance, and stable electrochemical cyclic properties. This work provides a performable method of synthesizing stably composited metal–carbon materials and predicts the vanadium and SWCNTs involved structure.
Samples | Pure PAN/PVP | SWCNTs + PAN/PVP | VO(acc)2 + PAN/PVP | VO(acc)2 + SWCNTs + PAN/PVP | ||
---|---|---|---|---|---|---|
10% VO(acc)2 | 15% VO(acc)2 | 20% VO(acc)2 | ||||
Precursor nanofibers | PNFs | SNFs | VNFs | 10VSNFs | 15VSNFs | 20VSNFs |
Carbonized at 800 °C | PCNFs-800 | SCNFs-800 | VCNFs-800 | 10VSCNFs-800 | 15VSCNFs-800 | 20VSCNFs-800 |
Carbonized at 900 °C | PCNFs-900 | SCNFs-900 | VCNFs-900 | 10VSCNFs-900 | 15VSCNFs-900 | 20VSCNFs-900 |
The specific capacitance (F g−1) obtained from CV method is based on eqn (1):
![]() | (1) |
The specific capacitance (F g−1) derived from GCD curves (mean value over discharging time) is calculated from the eqn (2):
![]() | (2) |
Atom | PNFs | PCNFs | SNFs | SCNFs | VNFs | VCNFs | 15VSNFs | 15VSCNFs | ||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
800 | 900 | 800 | 900 | 800 | 900 | 800 | 900 | |||||
C | 67.7 | 81.8 | 86.8 | 67.0 | 85.0 | 90.2 | 69.2 | 78.0 | 81.6 | 67.0 | 74.0 | 82.5 |
N | 22.5 | 12.1 | 5.8 | 22.9 | 7.4 | 3.6 | 18.1 | 6.5 | 4.2 | 19.9 | 6.3 | 4.8 |
O | 9.8 | 6.1 | 7.4 | 10.1 | 7.6 | 6.2 | 11.6 | 10.2 | 8.8 | 11.4 | 11.7 | 4.2 |
V | 0 | 0 | 0 | 0 | 0 | 0 | 1.1 | 5.3 | 5.4 | 1.7 | 8.0 | 8.5 |
Fig. 2 displays the N2 sorption isotherm curves of as-synthesized samples (800 °C). PCNFs electrode exhibits typical type-I Langmuir adsorption behaviour (Fig. 2a) according to IUPAC, and other five samples possess hierarchical structure with micropores and mesopores (Fig. 2b), which can be distinguished through the small adsorption hysteresis during pressure P/P0 from 0.8 to 0.3 as well as the strong adsorption behaviour during low pressure (P/P0 < 0.1). As shown in Table 3, SCNFs (696 m2 g−1) have slightly larger specific surface area (SBET) compared to PCNFs (642 m2 g−1), and the SBET of VCNFs is slightly increased after inserting vanadium, but the micropores volume ratio is decreased, which impairs electrochemical performance,2 and adding vanadium and SWCNTs into PCNFs can hold SBET and micropores volume ratio simultaneously. This can be explained as follows: according to ref. 28, 32 and 39, carbonization process of composite PAN/PVP nanofibers contains two pathways: (I) polymer cross-linking and pyrolysis; (II) competitive reactions between VO(acc)2 and polymer skeleton. Carbonization process of PNFs and SNFs without VO(acc)2 is driven by pathway I, resulting almost pure micropores, meanwhile, VNFs and VSNFs are driven by both pathway I and II, resulting expanded mesopores. On the other hand, the opened SWCNTs with inner diameter less than 2 nm can increase the ratio of micropores and the specific surface area, and the external structure of SWCNTs may also facilitate the generation of mesopores to enlarge the ion channel. However, excessive VO(acc)2 (20 wt%) overly strengthen pathway II, resulting serious SBET and micropores volume ratio decline.
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Fig. 2 Nitrogen isothermal adsorption analysis: (a) nitrogen adsorption isotherm curves and (b) pore size distribution plots of samples carbonized at 800 °C. |
Samples | PCNFs | SCNFs | VCNFs | 10VSCNFs | 15VSCNFs | 20VSCNFs | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Temp. | 800 | 900 | 800 | 900 | 800 | 900 | 800 | 900 | 800 | 900 | 800 | 900 |
a SBET is abbreviation of the specific surface area calculated from Brunauer–Emmett–Teller (BET) method, and Vmic and Vtotal are the effective adsorption volume of micropores and total volume. | ||||||||||||
SBET (m2 g−1) | 642 | 683 | 696 | 730 | 721 | 106 | 817 | 343 | 821 | 168 | 418 | 101 |
Vmic (cm3 g−1) | 0.292 | 0.302 | 0.280 | 0.292 | 0.184 | 0.052 | 0.285 | 0.082 | 0.310 | 0.063 | 0.178 | 0.041 |
Vtotal (cm3 g−1) | 0.321 | 0.327 | 0.370 | 0.388 | 0.308 | 0.194 | 0.395 | 0.269 | 0.470 | 0.218 | 0.327 | 0.183 |
Vmic/Vtotal (%) | 91.0% | 92.4% | 75.7% | 75.3% | 59.7% | 26.8% | 72.2% | 30.5% | 66.0% | 28.9% | 54.4% | 22.4% |
Carbonizing temperature could affect the pore structure of CNFs as well. When raising carbonizing temperature from 800 °C to 900 °C, the SBET of PCNFs and SCNFs increase from 642, 696 to 683, 730 m2 g−1, respectively. However, the SBET of VCNFs, VSCNFs decrease dramatically. This may due to the enhanced reaction of pathway II when raising temperature. These results indicate that raising carbonized temperature is a feasible method to enhance structure property of vanadium-free carbon materials, but to vanadium-composited sample, carbonized temperature varying method is undesirable.
The transmission electron microscopy (TEM) images show the morphology and surface pore structure of PCNFs-800, SCNFs-800 and 15VSCNFs-800 (Fig. 3 and S9†). PCNFs-800 and SCNFs-800 present denser structure than 15VSCNFs-800 although they have similar diameter around 350 nm. The reason is that 15VSCNFs-800 possesses larger ratio of mesopores than PCNFs and SCNFs (Fig. 2b), and it is supposed that the mesopores in 15VSCNFs-800 lead some electron beams generated by laser in TEM to penetrate the edge of CNFs and finally to present looser internal structure. The core of CNFs shows black possibly because those mesopores are arranged irregular and vermicular40,41 (Fig. 3d and S9d†).
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Fig. 3 Transmission electron microscopy images: (a) PCNFs-800, (b) SCNFs-800, (c) 15VSCNFs-800 and (d) high magnification image of 15VSCNFs-800. |
Fig. 4 shows the Raman spectroscopy of the composited CNFs. Two peaks at 1360 cm−1 and 1580 cm−1 are obtained (Fig. 4a), corresponding to the D-band for amorphous carbon and G-band for orderly graphitized carbon, respectively.43,44 The ratio of relative intensity of D-band and G-band (ID/IG), which represents the extent of carbon defects, is summarized in Fig. 4b. For SWCNT-free CNFs, the ID/IG value of VCNFs-800 is much higher than that of PCNFs. On the contrary, no obvious increase of ID/IG is observed after embedding SWCNTs. ID/IG of samples at 900 °C is smaller than that at 800 °C except for 15VSCNFs (Fig. 4b), suggesting that the graphitic sheets should be continuously growing when carbonized temperature is increased, and for 15VSCNFs, the exothermic reactions between VO(acc)2 and PAN/PVP may be accelerated when carbonized temperature and VO(acc)2 content were increased simultaneously. Therefore, adding 15% VO(acc)2 and 10% SWCNTs within electrospun precursors is a highly effective approach to improve the graphitizing extent of composite CNFs.
The effects of SWCNTs content, VO(acc)2 content and carbonizing temperature on crystal structure of CNFs are studied by XRD (Fig. 5). Two diffractive peaks are observed at 2θ ≈ 25° and 43°, which are attributed to graphitic structure (0 0 2) and turbostratic carbon structure (1 0 1), respectively.44 It's obvious that the (0 0 2) peak gets sharper and gradually shifts right (2θ ≈ 26°) when SWCNTs are added into CNFs, confirming that SWCNTs have been successfully loaded. The (0 0 2) peak of CNFs also turns sharper and stronger after loading vanadium; this can be explained that vanadium may participate graphitizing process and enhance the extent of graphitization. Furthermore, vanadium in CNFs may be amorphous as vanadium oxide peaks are failed to be detected.30 The (0 0 2) peak and (1 0 1) peak are both slightly magnified when carbonizing temperature is increased from 800 °C to 900 °C, confirming the graphitization and turbostratic structure are both enhanced. The Raman and XRD results show that SWNCTs are not only mechanically mixed in CNFs to enhance the extent of graphitization, but also involved in the transmutation process of CNFs graphitization with the participation of VO(acc)2.
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Fig. 6 X-ray photoelectron spectroscopy of 15VSCNFs-800: (a) survey scan, and peak curves of (b) C1s, (c) N1s, (d) O1s, and (e) V2p. |
Element | N1s | O1s | V2p3/2 | V2p1/2 | |
---|---|---|---|---|---|
![]() |
Ferrer42 | 399.1 | 532.9 | 516.5 | 524.3 |
This work | 398.9 | 532.9 | 516.4 | 524.4 |
TGA/DSC method is conducted to promote the understanding of the thermal behaviours after inserting vanadium and SWCNTs. As shown in Fig. 7, the sharp exothermic peaks around 300 °C are observed obviously, mainly due to the cyclization and oxidation reaction of PAN and PVP,32 and the small endothermic peak around 480 °C is caused by dehydrogenation and excessive oxidation of PAN, and the wide exothermic peak from 500 °C to 800 °C is about the further dehydrogenation and intensively cyclization of carbon skeleton.32 The decomposition of PVP mainly happens at 600 °C.39 With the increase of vanadium content, the weight loss rate slows down, and SWCNTs barely influence the weight loss (Fig. 7a). The embedded vanadium could influence carbonizing process in two competitive ways: firstly, VO(acc)2 would be connected to carbon skeleton during the pyrolysis process of PAN and PVP, which may produce micropores and mesopores with emitting of N2, CO, CO2, HCN, etc.,32 resulting in accelerating the pyrolysis process (Fig. 7b) and more serious weight loss. Secondly, the replacement of C, N, H atoms with vanadium atom which has higher relative molecular weight would greatly slow down the weight loss. Fig. 7c displays the Raman spectra of 15VSCNFs-800. Four distinguishable variations are observed for the following Raman peaks: 1010 [v(VO)], 749 [vas(V–O)], 501 [vs(V–O)], and 267 cm−1 [v(V–N)], which are in accordance with ref. 48 and 49. Combining with analysis from XPS, this provides the direct information about the new formed V–O–C, V–N–C and V
O bonds.
Based on above discussion, we predict the vanadium-involved graphite skeleton structure (Fig. 7d). According to the atom ratio of 15VSCNFs-800, there are three kinds of possible structure: V–O–C and V–N–C can link graphene fragments into large graphite sheets, or V4+ substitute for carbon atom, or V3+ bond with carbon skeleton margin through (O)′V–O–C form to cause faradic response when applying for capacitive devices. Notably, V–O–C and V–N–C covalent bonds may not only exist between fragments, but also bond between two layers to form multiple layers graphite. This structure can store more electrons when charging the electrode. However, due to the molecular structure of carbon, nitrogen and vanadium, the desultory carbon skeleton would wave naturally; consequently this structure would form much steric hindrance for storing electrons and lead to increase of carbon disorder and amorphous, so SWCNTs (OD < 2 nm) can support between carbon skeleton to separate layer, prompt graphite flake growth and act as conductive tunnel.
The two peaks of VCNFs-800 and 15VSCNFs-800 at reduction potential of −0.9 V and oxidation potential of −0.3 V are possibly corresponding to the redox of vanadium-related groups in composite CNFs,28 suggesting pseudo-capacitive behaviour (Fig. 8e). The CV area at 5 mV s−1 is increased from PCNFs, SCNFs, VCNFs to 15VSCNFs, suggesting both SWCNTs and vanadium can enhance PCFNs capacitive performance, but in different ways: the former enhance conductivity, and the latter introduce pseudo-capacitance. As predicted previously, the integrated 15VSCNFs combine these two key factors, in result of even larger CV area. The capacitive rate performance of 15VSCNFs-800 is not as good as PCNFs and SCNFs, probably due to pseudo-capacitive behaviour with 8% vanadium. However, it has CCV specific capacitance of 264 F g−1 at scan rate of 5 mV s−1, which is much higher than other samples. These results indicate the positive effects of incorporating vanadium and SWCNTs into CNFs on enhancing the conductivity and capacitive performance of CNFs.
The GCD curves of PCNFs, SCNFs, VCNFs and 15VSCNFs at varying current density from 1 to 10 A g−1 are demonstrated in Fig. 9, where the charge–discharge cycling period has been significantly increased by compositing SWCNTs and vanadium in CNFs, and the corresponding GCD curves for CNFs-800 at a constant current density of 2 A g−1 are also obtained (Fig. 9e). PCNFs and SCNFs have almost the same GCD curves, but still the discharging time of SCNFs-800 (206 F g−1) is slightly increased, which is 10.2% higher than that of PCNFs-800 (187 F g−1). Notably, both VCNFs and 15VSCNFs have a turning point at −0.75 V for discharging and another at −0.38 V for charging, corresponding to redox reactions caused by the embedded vanadium(IV), and the CGCD of 15VSCNFs-800 (428 F g−1 at current density of 2 A g−1) is higher than that of VCNFs-800 (305 F g−1 at current density of 2 A g−1) because of the integrated structure formed by inserting vanadium and SWNCTs into CNFs.
A good supercapacitor should be able to perfectly work at high current density. Fig. 9f shows the specific capacitance retention behaviour. 15VSCNFs-800 exhibits the highest specific capacitance of 479 F g−1 at current density of 1 A g−1 and 367.4 F g−1 at current density of 10 A g−1, where 76.7% of initial capacitance at 1 A g−1 is remained at high density of 10 A g−1, and samples of PCNFs-800 and SCNFs-800 keep 88.1% and 87.8% of initial capacitance, respectively.
As shown in Table S3,† in terms of capacitance rate performance, VCNFs electrode is worse than 15VSCNFs. With the same vanadium loading, 10VSCNFs preserve 76.7% capacitance at current density of 10 A g−1 relative to 1 A g−1, however, only 66.3% for VCNFs. When adding initial vanadium loading to 15%, the rate performance is decreased in some cases, but the general capacitance is greater than other samples. Notably, increasing temperature can enhance rate performance, but impair capacitance, probably due the destructive pore structure.
Fig. 10a shows the Nyquist plots for PCNFs-800, SCNFs-800, VCNFs-800 and 15VSCNFs-800 at open-circuit potential. Nyquist plot consists of two parts. The semi-circle part at high frequency represents electron transfer rate, and the line part with a certain slope at low frequency stands for ion diffusion ability.50 The electrode series resistance of PCNFs-800 (35.6 Ω) is much higher than three others by comparing the semi-circle part, and the series resistance decreases slightly with the order of SCNFs-800 (1.8 Ω), VCNFs-800 (0.96 Ω) and 15VSCNFs-800 (0.48 Ω) (inserting plot in Fig. 10a), demonstrating vanadium and SWCNTs synergistically enhance CNFs conductivity by improving the graphene flake integrity. The ideal capacitor behaviour of SCNFs-800 is identified by the dramatically increase of angle from 45° to 80° after loading SWCNTs. Similarly, the slightly grow of slopes of SCNFs-800, VCNFs-800 and 15VSCNFs-800 may confirm that both SWCNTs and vanadium are helpful to build three-dimensional network among composite CNFs skeleton.
GCD over 5000 cycles at current density of 8 A g−1 are performed to evaluate the cyclic performance of composite CNFs electrode. The best performance electrode 15VSCNFs-800 remain 94% of initial capacitance after 5000 cycles (Fig. 10b), suggesting impressive stability. Its charge efficiency (the ratio of charge and discharge amount) also remains 90.5% after cycling, indicating stable series resistance. Notably, this stable and low series resistance can increase energy efficiency without producing wasted heat. The similar charge and discharge curves at the beginning and final cycles also indicate the good stability of 15VSCNFs-800.
From the above discussions, high conductivity with lowest series resistance and good pseudo-capacitive behaviour are confirmed for 15VSCNFs-800, which supports the conclusion that the V–N–C and V–O–C bonds have formed inside graphite sheet fractures, and SWCNTs have contributed to build a unique capacitive structure, finally, this unique integrated structure enhance the stability and electrochemical performance of 15VSCNFs-800, resulting in superior supercapacitor.
Footnote |
† Electronic supplementary information (ESI) available: Table of transitional pre-oxidization procedure, SEM images captured at low magnification, N2 sorption isotherm of samples carbonized at 900 °C, nanofiber diameter distributions, FT-IR spectrum, CV and GCD curves, Nyquist plots of samples carbonized at 900 °C, TEM images, electrode digital images. See DOI: 10.1039/c5ra02237a |
This journal is © The Royal Society of Chemistry 2015 |