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High performance carbon nanotube yarn supercapacitors with a surface-oxidized Cu current collector Daohong Zhang, wu yun long, Ting Li, Yin Huang, Aiqing Zhang, and Menghe Miao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08110 • Publication Date (Web): 02 Nov 2015 Downloaded from http://pubs.acs.org on November 9, 2015

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High performance carbon nanotube yarn supercapacitors with a surface-oxidized Cu current collector Daohong Zhang1*,Yunlong Wu1, Ting Li1,Yin Huang1, Aiqing Zhang1, Menghe Miao2* 1

Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs

Commission & Ministry of Education, South-Central University for Nationalities, Wuhan, Hubei Province, 430074, China. 2

CSIRO Manufacturing, PO Box 21, Belmont, Victoria 3216, Australia.

*Dr D Zhang. E-mail: [email protected] *Dr M Miao. E-mail: [email protected] Keywords: Linear supercapacitor; Carbon nanotube yarn; Current collector; Metal filament; Potential window.


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Abstract Threadlike linear supercapacitors have demonstrated high potential for constructing fabrics to power electronic textiles (eTextiles). To improve the cyclic electrochemical performance and to produce power fabrics large enough for practical applications, current collector has been introduced into the linear supercapcitors to transport charges produced by active materials along the length of supercapacitor with high efficiency. Here, we first screened six candidate metal filaments (Pt, Au, Ag, AuAg, PtCu and Cu) as current collectors for carbon nanotube (CNT) yarn-based linear supercapacitors. Although all the metal filaments significantly improved the electrochemical performance of the linear supercapacitor, two supercapacitors constructed from Cu and PtCu filaments respectively demonstrate far better electrochemical

performance

than

the

other

four

supercapacitors.

Further

investigation shows that the surfaces of the two Cu-containing filaments are oxidized by the surrounding polymer electrolyte in the electrode. While the unoxidized core of the Cu-containing filaments remains to be highly conductive and functions as current collector, the resulting CuO on the surface is an electrochemically active material. The linear supercapacitor architecture incorporating dual active materials CNT+Cu extends the potential window from 1.0 V to 1.4 V, leading to significant improvement to the energy density and power density.



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1. Introduction Highly flexible linear supercapacitors based on carbon nanotubes (CNTs) have attracted increasing attention because of their high power density, long cycling life, short charge/discharge time, low cost and easy fabrication1-3. Integration of linear supercapacitors with wearable electronics or smart textiles leads to the development of many interesting applications4-7. For example, flexible supercapacitors can power on-body sensing systems to support people in various situations and activities, such as monitoring of surrounding conditions and physiological signals in sports, healthcare, rehabilitation, and high-risk environments8-9. Carbon nanomaterials, including carbon nanotubes (1D), graphene (2D), and aggregation of mesoporous carbon (3D), are promising electrode materials due to their extremely large surface area, excellent mechanical and electrical properties and high electrochemical stability10-12. Supercapacitors fabricated from CNT yarns may be woven or knitted, alone or in combination with textile yarns, into fabrics that are strong and comfortable to wear12-16. Recent research has focused on improving the electrochemical performance of carbon nanotube-based nanocomposite electrodes by depositing high performance pseudocapacitive materials including graphenes and nano-structured conducting polymers and transition metal oxides17-26. A common architecture for conventional supercapactors includes a metal sheet (e.g., aluminum foil) as current collector. This architecture has been adopted in developing planar (2D or film) CNT-based supercapacitors, for example, CNT forests grown on metal substrates 27, vertically aligned multiwalled carbon nanotubes (CNTs)


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grown on bulk copper foil substrates 28, and graphene-based carbon nano-fibres grown on thin-sheet sinter-locked Ni-fibre

29

. Zhang, et al, recently reported a platinum

filament core/CNT sheath carbon nanotube yarn electrode architecture for linear supercapacitors 12. The carbon nanotubes form a thin surface layer around the highly conductive metal filament core. The metal core serves as current collector so that charges produced on the active materials along the length of the supercapacitor are transported efficiently, resulting in significant improvement in electrochemical performance and allows the length of the linear supercapacitor to be scaled up dramatically. To further improve the electrochemical efficiency of the metal/CNT core-spun yarn electrode, the architecture of the supercapacitor needs to be optimized. Here, we screened six types of metal filaments to replace the Pt filament in the core-spun yarn. Further investigation has shown that the surfaces of Cu-containing filaments were oxidized by the surrounding polymer electrolyte in the electrode, and the resulting CuO together with the CNT yarn in the two electrodes formed a high efficiency supercapcitor system. 2. Materials and Method Production of as-spun carbon nanotube yarns. CNT forests were grown on silicon wafer substrates bearing a thermal oxide layer and iron catalyst coating using chemical vapour deposition (CVD) of acetylene in helium. The synthesis procedures have been reported previously 30. The resulting CNTs had 7 ± 2 walls with an outer diameter of 10 nm and an inner diameter of 4 nm approximately. The length of the



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CNTs was approximately 350 µm by measuring the height of the forests. The CNT yarns were spun to a twist level of 5,000 turns/m using an Up-spinner 31. Preparation of PVA-H3PO4 gel. 98-99% hydrolyzed polyvinyl alcohol (PVA) with average molecular weight of 57,000-66,000 was supplied by Alfa Aesar. 0.8 g of H3PO4 (analytical grade) was added to 10 ml deionized water with vigorous stirring and then 1 g of PVA powder was added. The solution was heated up steadily to 90°C under vigorous stirring until the solution became clear to form the PVA-H3PO4 gel.

Preparation of linear electrodes and supercapacitors. All metal filaments were supplied by Beijing Doublink Solders Co., Ltd. The diameters of the Cu, Pt, AuAg, Ag, and PtCu filaments are all 25 µm, i.e., approximately the same as the diameter of the CNT yarn. The diameter of the Au filament is 18 µm. A typical two-ply M+CNT yarn electrode based on carbon nanotubes and metal filament is fabricated using the following procedures. One CNT yarn and one metal filament were twisted together at a twist density of 150 T/m to form a M+CNT yarn, where M stands for one of the six metal filaments used in this investigation, Pt, Au, AuAg, Ag, PtCu and Cu. Coating of PVA-H3PO4 gel was carried out using a home-built continuous coating line detailed previously12. Two identical PVA coated M+CNT electrodes were twisted together to form a M+CNT symmetric linear supercapacitor using the twisting device. A pure CNT yarn supercapacitor control was produced in the same way except for that no metal filament was used. Similarly, a Cu supercapacitor control was produced without a CNT yarn. Cu//CNT asymmetric supercapacitor was made from a pure CNT yarn electrode and a Cu filament electrode.


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Characterization. Scanning electron microscopy (SEM) micrographs of the samples were taken using a Hitachi SU8010 field emission scanning electron microscope. The surface compositions of the samples were determined on a Vacuum Generator Mutilab 2000 X-ray photoelectron spectrometer (XPS) using the C1s (284.6 eV) peak of the contamination carbon as internal standard. Cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) were performed on an electrochemical workstation (CompactStat.e10800, Ivium Technologies BV) using a two-electrode configuration. Short length supercapacitors (10-50 mm) were mounted on a paper frame and connected to the instrument for measurement. Long supercapacitors (500 mm) were wrapped on a tube with two ends mounted on a paper frame connector10. Gravimetric capacitances of the supercapacitors were derived using the following formula. C=

௏೎ ଵ ‫ܸ݀)ܸ(ܫ ׬‬ ௪௩(௏೎ ି௏ೌ ) ௏ೌ

(1)

where C is specific capacitance (F/g), w is the mass of active constituents in the working electrode (g), v is the scanning rate (V/s), Vc - Va is the potential window (V), and I(V) is the current density. Mass of the active constituents (CNT) was calculated by subtracting the linear density of the metal filament from the linear density of the M+CNT yarn. Areal capacitances of the supercapacitors were derived using the following formula. C=

ଵ ௌ௩(௏೎ ି௏ೌ

௏

೎ ‫ܸ݀)ܸ(ܫ ׬‬ ) ௏ೌ

(2)

where C is specific capacitance (mF/g2), S is the area of yarn in the working electrode (cm2) calculated from the yarn or metal filament diameter.



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3. Results and Discussion 3.1. Supercapacitor architecture The CNT yarn was fabricated by twisting a continuous CNT web drawn from a solid state multi-walled carbon nanotube forest, which was synthesized using the chemical vapor deposition (CVD) method32-33. SEM images of the as-spun CNT yarn at two magnifications are shown in Fig. 1. The diameter of the pure CNT yarn is approximately 25 µm. The yarn has a porous microstructure, with the carbon nanotubes aligned in the direction of the twist.

Figure 1. SEM micrographs of the pure CNT yarn at (a) low magnification; and (b) high magnification.

The architecture of the resulting linear supercapacitors based on carbon nanotube yarn and metal filament is shown schematically in Figure 2(a). Each electrode of the supercapacitor is formed by twisting together an as-spun CNT yarn and a metal filament. The electrode is then coated with polyvinyl alcohol - phosphoric acid (PVA-H3PO4) gel electrolyte. Two identical electrodes are then plied together to form


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a linear supercapacitor, which is coated with the PVA-H3PO4 electrolyte again to ensure full coverage. The six types of metal filaments were used to produce six types of linear yarn supercapacitors. In addition, a pure CNT yarn supercapacitor (i.e., without plying with any metal filament) was made as a control. SEM images of the Cu filament, Cu+CNT electrode and the resulting supercapacitor are shown in Figure 2(b-e). The final linear supercapacitors have a diameter of about 120±20 µm, which is similar to a very fine count cotton yarn. The average thickness of the electrolyte coating was estimated to be about 17.5±2.5µm by subtraction of the average diameters of coated and uncoated samples.

Figure 2. CNT yarn /metal filament linear supercapacitor. (a) Architecture of the linear supercapacitor; (b) SEM image of bare metal filament Cu; (c) SEM of Cu+CNT electrode; (d-e) SEM of final Cu+CNT linear supercapacitor.



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3.2. Electrochemical characteristics The electrochemical properties of the above seven types of symmetrical linear supercapacitors were characterized by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS). The CV curves of the seven supecapacitors are displayed in two groups. The first group，shown in Figure 3(a), comprises of five types of electrodes, CNT, Pt+CNT, Au+CNT, Ag+CNT and AuAg+CNT. The second group, shown in Figure 3(b), includes two types of electrodes, Cu+CNT and PtCu+CNT. In the positive scan of the CV curves, there was a remarkable oxidation peak at ~0.1 V for all the samples. This is considered to be the oxidation of –OH on the surface of CNT. For samples Pt+CNT and Au+CNT, no other redox peaks were observed, indicating that Pt and Au were stable. A pair of redox peaks around ~0.3 V is observed for samples AuAg+CNT and Ag+CNT, consistent with the redox between Ag and Ag2O. For samples Cu+CNT and PtCu +CNT (Fig 3b), oxidation peaks appeared at ~0.2 V in the positive scan, which are attributed to the oxidation of Cu. This is also consistent with the potential-pH diagram of Cu-H2O system34, which shows the oxidation potential of Cu to be ~ 0.1V at pH =1 (i.e., 1M H3PO4). The incorporation of all of the six types of metal filaments with the CNT yarn increased the current density, as indicated by the areas of their CV windows. This can be attributed to the effect of metal filaments as collectors12.


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Figure 3. Electrochemical properties of symmetrical supercapacitors: (a-b) cyclic voltammographs of supercapacitors at scanning rate of 50 mV/s; (c) Cyclic voltammographs of supercapacitor Cu+CNT at different scanning rates; (d) Galvanostatic charge/discharge curves of supercapacitors at constant current density 3.57A/g; (e) Specific capacitances of all supercapacitors at different scanning rates; (f) Electrochemical impedance spectra (100 Hz -1 MHz). The two supercapacitors in Figure 3(b) (Cu+CNT, PtCu+CNT) showed much higher current density than all those in Figure 3(a). The reason for this will be



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analysed later. CV curves of the best performing electrode, Cu+CNT, at different scanning rates from 10 mV/S to 500 mV/S are presented in Figure 3(c). Figure 3(d) displays the galvanostatic charge-discharge curves of all the seven supercapacitors. The near triangle-shaped charge-discharge curves are typical for all supercapacitors35. In comparison with the pure CNT supercapacitor, the incorporation of metal filaments in CNT supercapacitors prolonged the charge-discharge time. The specific capacitances of the seven supercapacitors are plotted against scanning rate in Fig. 3(e). All the metal+CNT supercapacitors showed significant improvement in specific capacitance over the pure CNT yarn supercapacitor. The metal filament acts as a current collector which accelerates electron transport, thus providing a diffusion channel for electrolyte ions and electrons into the electrodes12, 36. The improvement derived from the metal filaments did not seem to follow the order of the conductivity of the metal filaments. The Cu and PtCu filaments caused the largest improvement. Au, Ag and AuAg filaments, which have similar resistivity as the PtCu and Cu filaments, have led to much smaller improvements. The reason for this difference was further investigated and is presented in the next section. All the seven supercapacitors were characterized using electrochemical impedance spectroscopy (EIS). The Nyquist plots in Fig. 3(f) departed from each other at the low frequency range (higher ends of the curves). The lower ends of the curves in Fig. 3(f) show the impedances of the seven supercapacitors at the high frequency (HF) range. The size of the HF loops may be associated with the contact impedance between current collector and the active material37-38. PtCu+CNT and


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Ag+CNT showed smaller HF loops, followed by Cu+CNT and AuAg while Pt+CNT and Au+CNT showed the largest HF loops. The x-intercept of the Nyquist plots represents the equivalent series resistance (ESR) for two-electrode supercapacitor and the charge transport resistance39. The ESR of the pure CNT supercapacitor was as high as 1300 ohms. Incorporating the metal filament reduced the ESR of the supercapacitors significantly to values between 36 and 91 ohms.

3.3. Surface oxidation of metal filaments The metal+CNT electrodes that had been charged once were carefully disassembled to remove the metal filaments. The disassembled metal filaments were examined under scanning electron microscope (SEM) and analysed by energy dispersive spectroscopy (EDS). The SEM images and the EDS plots are presented in Figures 4 and 5. Pt and Au filaments show smooth surfaces and very low level O element, indicating there was almost no oxidation. The disassembled AuAg and Ag filaments show surface roughness, but very low level O element deposition, indicating that the roughness was due to deposition of other substances from the electrolyte instead of being oxidized. The rough surfaces of PtCu and Cu filaments were accompanied by high contents of oxygen element on the surface (16.93 wt% and 20.76 wt%, respectively), indicating that the filament surfaces were severely oxidized. The XPS spectra taken from these samples (Figure 6) also

indicate high contents of

oxygen element on the surface (33.47 wt% and 50.73 wt%, respectively) of the disassembled PtCu and Cu filaments, compared with low contents of oxygen element (20.08 wt%, 20.84 wt%, 21.36 wt% and 26.85 wt%, respectively) of disassembled Pt,



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Au, AuAg and Ag filaments. Contents difference of oxygen element from XPS and EDS is attributed to that XPS data obtained from whole region on the surface of disassembled filaments and but EDS data obtained from local region, and another reason is that XPS data includes carbon element and EDS data does not include. The high capacitance of the PtCu+CNT and Cu+CNT supercapacitors would have originated from the oxidation of the Cu element under the PVA-H3PO4 gel. The CuO formed on the surface of the Cu and PtCu filaments is known to be an electrochemically active material40-41. So the supercapacitors Cu+CNT and PtCu+CNT contain two types of active materials, CNT and CuO while the un-oxidized core of the metal filaments still performs as current collector.

Figure 4. SEM micrographs of metal filaments from disassembled supercapacitors after one cycle of charge/discharge. (a) Pt; (b) Au; (c) AuAg; (d) Ag; (e) PtCu and (f) Cu.


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Figure 5. Energy dispersive spectra (EDS) on surface of metal filaments from disassembled supercapacitors after one cycle of charge/discharge. (a) Pt; (b) Au; (c) AuAg; (d) Ag; (e) PtCu and (f) Cu.

Figure 6. XPS spectra of metal filaments from disassembled supercapacitors after one cycle of charge/discharge. (a) Whole scanning range; (b) O1s.

3.4. The role of CuO on filament surface The efficiency of the CuO formed on the Cu filament surface was studied by constructing two simple supercapacitors, a symmetric supercapacitor from two Cu filament electrodes (surface oxidized) and an asymmetric supercapacitor consisting of



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a Cu filament (surface oxidized) as one electrode and an as-spun CNT yarn as the other electrode, the latter of which will be called asymmetric supercapacitor Cu//CNT. Clearly, each of the two new supercapacitors is comprised of parts of the complex symmetric supercapacitor Cu+CNT illustrated in Figure 2(a). The two supercapacitors were characterized and compared with the complex symmetric supercapacitor Cu+CNT. Figure 7(a) shows the galvanostatic charge/discharge curves of the symmetric supercapacitor Cu and the asymmetric supercapacitor Cu//CNT. The symmetric supercapacitor Cu had a very short charge/discharge cycle (magnified in the inset). Using a pure CNT electrode to replace one of the Cu filament electrode, which gives the Cu//CNT asymmetric supercapacitor, prolonged the charge/discharge cycle time by two orders of magnitude. The complex symmetric supercapacitor Cu+CNT showed a further significant increase of charge/discharge cycle time over the Cu//CNT asymmetric supercapacitor. This may be attributed to the inclusion of two types of active materials (CNT yarn and CuO) in both electrodes. As the absolute quantity of CuO formed on the Cu filament was difficult to determine, we used surface areal capacitance to compare these supercapacitors, as shown in Figure 7(b). The capacitance of the symmetric supercapacitor Cu was quite small in comparison with the pure CNT symmetric supercapacitor. However, combination of the two types of electrodes into asymmetric supercapacitor Cu//CNT provided areal capacitance four times as much as the sum of the two symmetric supercapacitors formed by single type of active component. Figure 7(b) also shows


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that the areal capacitance of the complex symmetric supercapacitor Cu+CNT was coincidentally about

the same as the sum of the areal capacitances of the two

symmetric supercapacitors CNT and Cu and the asymmetric supercapacitor Cu//CNT. Figure 7(c&d) show the Nyquist plots of the asymmetric supercapacitor Cu//CNT, the two simple symmetric supercapacitors Cu and CNT, and the complex symmetric supercapacitor Cu+CNT.

Symmetric supercapacitor Cu and complex

symmetric supercapacitor Cu+CNT showed much lower ESR than both the symmetric supercapacitor CNT, which does not have a metal current collector in its electrodes and the asymmetric supercapacitor Cu//CNT, which has a metal current collector in one electrode but has no metal current collector in the other electrode.

Figure 7. Electrochemical characteristics of Cu-based symmetric and asymmetric supercapacitors. (a) Galvanostatic charge/discharge curves with current density of 3.57A/g; (b) Areal capacitance; (c) Electrochemical impedance spectra (EIS) (100



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Hz-1 MHz); (d) Enlargement of EIS in high frequency region.

3.5. Extension of potential window Further experiments showed that the electrochemical potential window of the complex symmetric supercapacitor Cu+CNT could be extended from 1.0 V to 1.4 V. Figure 8 (a&b) show the CV and charge/discharge curves of the supercapacitor at 1.0V, 1.2V and 1.4V. The gravimetric capacitance showed a 20% increase as the potential window was extended from 1.0V to 1.4V, as shown in Figure 8(c). The Ragone plot in Figure 8(d) shows the significant increase of energy density and power density due to this extension of potential window, 134% increase in energy density and 729% increase in power density.

Figure 8. Electrochemical properties of complex symmetric supercapacitor Cu+CNT: (a) cyclic voltammographs at scanning rate of 100 mV/s; (b) Galvanostatic


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charge/discharge curves with current density of 3.57A/g; (c) Capacitance for different potential windows; (d) Ragone plot showing the relation between energy density and power density. The symmetric linear supercapacitor Cu+CNT was further tested for performance stability against cyclic charge/discharge. Figure 9(a) shows that the supercapacitor maintained 97% and 91% of its original capacitance after 1000 and 3000 charge/discharge cycles, respectively. Textile materials experience many cycles of bending actions during fabrication and end use. As part of a wearable electronic system, the linear supercapacitors must maintain their electrochemical performance after a large number of folding-unfolding cycles. As shown in Figure 9(b), the capacitance of the Cu+CNT supercapacitor had little change (~2.5%) after being subjected to 1000 cycles of folding and unfolding.

Figure

9.

Capacitance

retention

of

Cu+CNT

supercapacitor.

(a)

cyclic

charge/discharge; (b) cyclic folding-unfolding. 4. CONCLUSION We first screened six candidate metal filaments (Pt, Au, Ag, AuAg, PtCu and Cu) as current collector for carbon nanotube (CNT) yarn-based linear supercapacitors.



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Although all the metal filaments considerably improved the electrochemical performance of the linear supercapacitors, two supercapacitors constructed from Cu and PtCu filaments demonstrated far superior electrochemical performance than the other four. Further investigation has shown that the surfaces of these two Cu-containing filaments were oxidized by the surrounding polymer electrolyte. While the unoxidized core of the filaments remains to be highly conductive and functions as a current collector, the resulting CuO on the filament surface provides an additional active material. The linear supercapacitor architecture based on dual capacitance materials CNT+Cu extends the potential window from 1.0 V to 1.4 V, resulting in dramatic improvements to energy density and power density.

Acknowledgment We gratefully acknowledge the financial support of Hubei Province Natural Science Fund for Distinguished Young Scientists (2014CFA037) and the National Natural Science Foundation of China (21403305). References (1) Meng, Y.; Zhao, Y.; Hu, C.; Cheng, H.; Hu, Y.; Zhang, Z.; Shi, G.; Qu, L., All‐Graphene Core ‐ Sheath Microfibers for All ‐ Solid ‐ State, Stretchable Fibriform Supercapacitors and Wearable Electronic Textiles. Adv. Mater. 2013,25, 2326-2331. (2)De Volder, M. F.; Tawfick, S. H.; Baughman, R. H.; Hart, A. J., Carbon Nanotubes: Present and Future Commercial Applications. Science 2013,339, 535-539. (3)Jha, N.; Ramesh, P.; Bekyarova, E.; Itkis, M. E.; Haddon, R. C., High Energy Density Supercapacitor Based on A Hybrid Carbon Nanotube–Reduced Graphite Oxide Architecture. Adv. Energy Mater. 2012,2, 438-444. (4)Dalton, A. B.; Collins, S.; Munoz, E.; Razal, J. M.; Ebron, V. H.; Ferraris, J. P.; Coleman, J. N.; Kim, B. G.; Baughman, R. H., Super-Tough Carbon-Nanotube Fibres. Nature 2003,423, 703-703. (5)Wang, K.; Meng, Q.; Zhang, Y.; Wei, Z.; Miao, M., High‐Performance Two‐Ply Yarn Supercapacitors Based on Carbon Nanotubes and Polyaniline Nanowire Arrays. Adv. Mater. 2013,25, 1494-1498.


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