Vertically Aligned Carbon Nanotubes on Carbon Nanofibers: A

Dec 31, 2014 - Hierarchical structures enable high-performance power sources. We report here the preparation of vertically aligned carbon nanotubes di...
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Vertically Aligned Carbon Nanotubes on Carbon Nanofibers: A Hierarchical Three-Dimensional Carbon Nanostructure for HighEnergy Flexible Supercapacitors Yongcai Qiu, Guizhu Li, Yuan Hou, Zhenghui Pan, Hongfei Li, Wanfei Li, Meinan Liu, Fangmin Ye, Xiaowei Yang,* and Yuegang Zhang* i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, China S Supporting Information *

ABSTRACT: Hierarchical structures enable high-performance power sources. We report here the preparation of vertically aligned carbon nanotubes directly grown on carbon nanofibers (VACNTs/CNFs) by combining electrospinning with pyrolysis technologies. The structure and morphology of VACNTs/CNFs could be precisely tuned and controlled by adjusting the percentage of reactants. The desired VACNTs/ CNFs could not only possess high electric conductivity for efficient charge transport but could also increase surface area for accessing more electrolyte ions. When using an ionic liquid electrolyte, VACNTs/CNFs-based electric double layer (EDL) flexible supercapacitors can deliver a high specific energy of 70.7 Wh/kg at a current density of 0.5 A/g and at 30 °C, and an ultrahigh-energy density of 98.8 Wh/kg at a current density of 1.0 A/g and at 60 °C. Even after 20 000 charging/discharging cycles, the EDL capacitor still retains 97.0% of the initial capacitance. The excellent performance highlights the important role of the branched VACNTs in storing and accumulating charge and the CNF backbone in transporting charge, thereby boosting both power density and energy density.

Supercapacitors have great potential applicable in portable electronics, stationary power stations, automobile vehicles, hybrid vehicles, and backup power supplies.1−5 Flexible/ bendable supercapacitors can be achieved more readily compared with other energy storage devices due to a very simple charging circuit.6−10 Recently, carbon textiles such as carbon cloth and carbon nanotube (CNT) yarns have been widely used as flexible and binder-free electrodes to directly grow various electrochemically active materials and exhibit good capacitance performance.11−21 However, the discharge voltage is not high enough (4 V), high ionic conductivity (∼11500 μS/cm), and excellent thermal stability. Recently, we showed that chemically converted graphene (CCG) gel films prepared by capillary compression containing the nonvolatile electrolyte EMIMBF4 have a superhigh packing density.36,37 The intersheet spacing creates a continuous ion transportation network and leads to high-energy densities in EDL supercapacitors. The VACNTs/CNFs also show great expectations for high-performance EDL capacitors using the ionic liquid electrolyte, as the surfaces of VACNTs should be accessible to large electrolyte molecules.

of 10 mV/s, and 128.0 F/g at a higher scan rate of 1000 mV/s (Figure 2C). However, the capacitance of CNFs is only 128 F/ g at a scan rate of 10 mV/s and has a rapid decay down to 75.6 F/g at a scan rate of 200 mV/s. Furthermore, the specific energy of the VACNTs/CNFs-based supercapacitor in an aqueous NaOH electrolyte is 7.42 Wh/kg, which is more than 1.5 times higher than that of CNFs-based supercapacitor (4.82 Wh/kg) (Figure 3D). The significantly improved performance could be attributed to the unique structure of VACNTs/CNFs in which the branches of highly conductive CNTs help chargestorage and -accumulation. To further interpret this benefit, the electrochemical impedance spectroscopy (EIS) measurements were carried out at a DC bias of 0.2 V with sinusoidal signal of 10 mV over the frequency range from 200 kHz to 100 mHz (Figure S7, Supporting Information).34 The Nyquist plots of the VACNTs/CNFs- and CNFs-based supercapacitors both show a straight line in the low-frequency region and an arc in the high-frequency region. The x-intercept at the left end of the semicircle represents the equivalent series resistance (ESR) of the capacitor, which is the combination of the contact resistance between electrode material and conductive substrate, the bulk solution resistance of the electrolyte, and the resistance of the electrode material itself. The ESR value of the VACNTs/ CNFs-based supercapacitor is only 0.25 ohm, which is smaller than that of CNFs-based supercapacitor (0.66 ohm). Based on the calculation of the maximum power density, Pmax = V2/ 4(ESR)Mtot (W/kg), where V is the cell voltage, and Mtot is the mass of total active material in the symmetric supercapacitor (including positive and negative electrodes), the achieved maximum power density of the VACNTs/CNFs-based supercapacitor will be at least 2.5 times higher than that of the CNFsbased supercapacitor. In other words, the former can be D

DOI: 10.1021/cm503784x Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 4. Electrochemical characterizations of the VACNTs/CNFs-based EDL capacitors in EMIMBF4 ionic liquid electrolyte. (A) Optical image of a VACNTs/CNFs-based flexible EDL capacitor and a lighted LED indicator. (B) Charge/discharge curves at various current densities. (C) Corresponding specific capacitance and cell capacitance as a function of current densities. (D) IR drop as a function of current densities. (E) Specfic energy density as a function of power density. (F) Discharge curves of the VACNTs/CNFs-based EDL capacitors in an EMIMBF4 ionic liquid electrolyte at different temperatures and at 1 A/g. The VACNTs/CNFs-based EDL capacitors in the EMIMBF4 ionic liquid electrolyte can light up a blue LED indicator (3.2 V).

value is then determined from the initial voltage drop of the discharge curves. By plotting the values of initial voltage drop as a function of current density, the internal resistance of VACNTs/CNFs-based EDL capacitors can be expressed by the following equation: IRdrop (V) = 0.04646 + 0.04582I (Figure 4D). High voltage operation windows and low internal resistance would provide a high maximum discharge power density of about 170 kW/kg for the supercapacitor. Owing to operation at higher voltage, outstanding energy densities of 70.7, 62.5, 43.9, 32.7, 25.8 Wh/kg are obtained with corresponding power densities of 614.4, 1154.3, 2095.7, 4924.7, and 8795.1 W/kg, respectively (Figure 4E). This means that the VACNTs/CNFs-based EDL capacitors have a storage energy density of 32.7 Wh/kg (equivalent to that of a lead-acid battery) and can be fully recharged in less than 1 min. Furthermore, the results demonstrate that the ionic liquidbased EDL capacitor can deliver 10 times higher energy density

An optical image of a fully fabricated EDL capacitor is shown in Figure 4A. After charging, the flexible EDL capacitor can light up a red LED indicator (see movie S1, Supporting Information) or a blue LED indicator (3.2 V). To evaluate the performance of the VACNTs/CNFs-based EDL capacitors at 4 V, galvanostatic cycling was performed at various rates. The capacitance values are calculated from the slope of the discharge curves, based on the equation of C = 2I(Δt/ΔV). As shown in Figure 4B, the VACNTs/CNFs electrode can deliver the specific capacitance of 146.8, 132.1, 94.8, 76.9, and 63.0 F/g, respectively (Figure 3C), at current densities of 0.5, 1, 2, 5, and 10 A/g, corresponding to discharge times of 415, 195, 72, 20, and 8.5 s, respectively. The nonlinear behavior of the discharge curve indicates the existence of a redox process. As also discussed previously, the pseudocapacitance should originate from the surface-oxygenated functionalization of the VACNTs/ CNFs electrodes introduced by HNO3. The internal resistance E

DOI: 10.1021/cm503784x Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials than the aqueous-based EDL capacitor. The achieved energy density of our VACNTs/CNFs-based supercapacitors is significantly higher than the values for other carbon-based supercapacitors that typically range from 3 to 15 Wh/kg and from 20 to 45 Wh/kg with aqueous and organic electrolytes, respectively,2,23,38−41 and is at least on a par with the highest energy density reported for carbon-based EDL capacitors.42,43 Temperature is also an important aspect affecting the performance of the ionic liquid-based EDL capacitors. A temperature increase can decrease the viscosity of ionic liquid and increase the ionic mobility in the electrolyte, which could improve wettability of electrolyte in micropores and boost more efficient double layer formation. Figure 4F plots the galvanostatic discharge curves as a function of temperature (from 30 to 60 °C). Obviously, a higher electrolyte temperature promotes the EDL formation, leading to improved capacitance. For example, the energy density of VACNTs/CNFs-based EDL capacitor can reach up to 98.8 Wh/kg at 1 A/g and at 60 °C, which is 1.58 times higher than that at 30 °C. To further understand the mechanism, Arrhenius theory of dissociation is used to address the role of the temperature. The Arrhenius-type equation can be expressed as ln C = ln A − E/RT, where C is the amount of charges accumulated at the electrode−electrolyte interface, A is the pre-exponential constant, E is the activation energy, T is temperature, and R is the universal gas constant. The inset of Figure 5A shows a plot of ln C as a function of 1/T at 1 A/g. The activation energy, E, can thus be obtained from the slope of the straight line (∼15 kcal/mol). This activation energy is related to the binding energy between the BF4− ions and the EMIM+ ions as well as the mobility of the ions in the electrolyte. A higher temperature apparently facilitates the dissociation and movement of the ions and thus boosts the charge/discharge processes. Good capacitance retention is important for the practical application of supercapacitors. The cycling performance of the optimized EDL capacitor was evaluated for over 20 000 charging/discharging cycles at a current density of 1 A/g (Figure 5A) and at 30 °C. After undergoing a sharp decay of 1.6% for the first 700 cycles, the EDL capacitor retains a constant capacitance value (97% of the initial capacitance) over the remaining cycles, indicating an excellent long-term stability of the EDL capacitor. In addition, the VACNTs/CNFs-based EDL capacitor is lightweight and flexible and can be curved to any angle without damaging the structural integrity of the capacitor (Figure 3A, and movie S1, Supporting Information). Figure 5B shows CV curves of the VACNTs/CNFs-based EDL capacitor at a scan rate of 100 mV/s under different bending conditions. The supercapacitor presents an additional redox process at around 2 V in all CVs, in accordance with discharge curves. Moreover, the additional redox processes are fast and reversible, advantageously introducing additional Faradaic charge storage to the capacitor. When the bending angle was changed from 0 to ∼180°, the capacitor retains above 92% of its original capacitance; even after 200 bending cycles, it maintains above 82% of its original capacitance, indicative of good capacitance retention (Figure 5C). The loss of the capacitance is likely due to a loosening and delamination of the carbon electrode and/or an influence of interfacial contact between the carbon electrode and the electrolyte. This result reveals the great potential of the VACNTs/CNFs-based flexible EDL capacitor device in wearable electronic applications.

Figure 5. Stability tests. (A) Cycling stability over 20 000 cycles at 1 A/g. (B) CV curves of the VACNTs/CNFs-based EDL capacitor under different bending conditions. (C) Capacitance retention of the VACNTs/CNFs-based EDL capacitor under different bending conditions. All bending tests were conducted one after another, and the angle was changed manually.

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CONCLUSIONS In summary, we have achieved a remarkable capacitive performance of a flexible EDL capacitor using a hierarchical three-dimensional VACNTs/CNFs nanostructure. The VACNTs/CNFs material was obtained by using PVP as a solid carbon source and Ni nanoparticles as the catalysts. When assembled into a flexible EDL capacitor using an ionic liquid electrolyte, it can deliver a high specific energy of 70.7 Wh/kg at a current density of 0.5 A/g and at 30 °C, and an ultrahighenergy density of 98.8 Wh/kg at a current density of 1.0 A/g and at 60 °C. Even after 20 000 charging/discharging cycles, the EDL capacitor still retains 97.0% of the initial capacitance. The excellent performance highlights the important role of the branched CNTs in storing and accumulating charge and the backbone of nanofibers in transporting charge, thereby boosting both power density and energy density. F

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ASSOCIATED CONTENT

S Supporting Information *

Additional TGA trace showing the decomposition temperature of PVP; Raman spectra, BET data, and XPS spectra of the VACNTs/CNFs; EIS data. This material is available free of charge via the Internet an http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21303251, 21433013, and 1403287), the Natural Science Foundation of Jiangsu Province, China (BK20140383), China Postdoctoral Science Foundation (2014M550314), and Suzhou Science and Technology Development Program (ZXG2013002).

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DOI: 10.1021/cm503784x Chem. Mater. XXXX, XXX, XXX−XXX