Facile synthesis of hierarchical CuO nanorod arrays

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CERAMICS INTERNATIONAL

Ceramics International 40 (2014) 15973–15979 www.elsevier.com/locate/ceramint

Facile synthesis of hierarchical CuO nanorod arrays on carbon nanofibers for high-performance supercapacitors Seyyed Ebrahim Moosavifarda,n, Javad Shamsib, Saeed Fanic, Saeed Kadkhodazaded a

Young Researchers and Elite Club, Central Tehran Branch, Islamic Azad University, Tehran, Iran b Department of Chemistry, University of Tehran, Tehran, Iran c Department of Chemistry, University of Science and Technology, Tehran, Iran d Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran Received 19 June 2014; received in revised form 17 July 2014; accepted 24 July 2014 Available online 31 July 2014

Abstract A facile low-temperature solution method combined with a post simple annealing treatment has been used to synthesize one-dimensional hierarchical hybrid nanostructure composed of CuO nanorod arrays grown on carbon nanofibers. The electrochemical properties of this hybrid nanostructure have been evaluated as electrode material for supercapacitor. The material exhibits a high capacitance of 398 F g 1 at 1 A g 1 and even shows a capacitance of 153 F g 1 at an extremely high current density of 50 A g 1, with high power density of 10 kW kg 1 in 3 M KOH electrolyte. The electrochemical stability of the material for 5000 continuous cycles demonstrates excellent cycling performance at different current densities. This high performance has been related to the structural features, including shape and size of well defined hierarchical nanorod arrays and robust carbon nanofibers backbone. In view of the high electrochemical performance and the facile and cost-effective synthesis, this hierarchical CNF@CuO NR might hold as a promising electrode material for high-performance supercapacitor applications. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Carbon nanofibers; Hierarchical CuO; Nanorod arrays; Supercapacitor

1. Introduction Due to high power density, long cycle life, short charging time, and minimal safety concerns, supercapacitors (SCs) have attracted increasing research attention [1–3]. Consequently, low cost and high performance electrode materials for SCs have attracted much research effort in recent years. The charge storage mechanism in SCs is based on electrochemical double layer capacitance (EDLC) in carbonaceous materials or fast, reversible Faradaic redox reactions (pseudocapacitance) in conducting polymers and metal oxides [4,5]. Application of carbonaceous materials in SCs has some advantages such as low cost, high electrical conductivity, chemical stability and structural flexibility. But these materials suffer from low specific capacitance and easy agglomeration [6–8]. Transition n

Corresponding author. Tel./fax: þ 98 21 82883455. E-mail address: [email protected] (S.E. Moosavifard).

http://dx.doi.org/10.1016/j.ceramint.2014.07.126 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

metal oxides, due to their potentially higher specific capacitance compared to EDLCs have been extensively investigated as pseudocapacitive materials [9–11]. Among the transition metal oxides, copper oxide (CuO) is particularly attractive because of the abundant resources, non-toxicity and easy preparation in various shapes of nanosized dimensions [12,13]. However, this metal oxide suffers from intrinsically low electrical conductivity. Therefore, novel strategies to improve performance of the electrode materials, including fabrication of nano-engineered structures or utilizing synergistic effects of hybrid materials, are needed [14,15]. There have been some successful attempts to increase electrochemical capacitance performance of CuO using carbonaceous materials. Liu et al. [16] reported a series of uniform composite networks consisting of CuO nanosheets/single-wall carbon nanotubes with maximum specific capacitance of 137.6 F g 1, and CuO NSs/rGO hybrid lamellar film [17] with a specific capacitance of 163.7 F g 1 in KOH electrolyte,

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S.E. Moosavifard et al. / Ceramics International 40 (2014) 15973–15979

Zhao et al. [18] reported hierarchical self-assembly of microscale leaf-like CuO on graphene sheets with a specific capacitance of 331.9 F g 1 in KOH electrolyte. However, to the best of our knowledge, the synthesis and capacitor characteristics of hierarchical CuO nanorod arrays grown on carbon nanofibers (CNFs) remains unreported till now. Herein, a facile low-temperature solution method is combined with a simple post annealing process to synthesize onedimensional (1D) hierarchical hybrid nanostructure composed of CuO nanorods grown on CNFs, denoted as CNF@CuO NR. The electrochemical behavior of the sample as a novel material for supercapacitor electrodes was evaluated using different electrochemical techniques including cyclic voltammetry (CV), chronopotentiometry (CP), and electrochemical impedance spectroscopy (EIS). 2. Experimental All the reagents were of analytical purity and used as received without further purification. 2.1. Synthesis of carbon nanofibers The first step involved the synthesis of Te nanowire templates via a simple hydrothermal method [19]. In a typical synthesis, 0.5 mmol of sodium tellurite (Na2TeO3), 0.6 g polyvinyl pyrrolidone (PVP; MW, 58,000) was dissolved into 15 mL of double-distilled water under vigorous magnetic stirring to form the homogeneous solution at room temperature. Hydrazine hydrate (1 mL, 85% w/w) and 2 mL of an aqueous ammonia solution (25% w/w) were added into the mixed solution. The final solution was clear, which was then transferred into a Teflon-lined autoclave. The Te nanowires were obtained after reaction at 180 1C for 4 h. The product was centrifuged and washed several times with double-distilled water and absolute ethanol. Then the CNFs were prepared by a template-directed hydrothermal carbonization procedure [20]. The obtained Te nanowires were dispersed in 48 mL of double-distilled water. Then 3 g glucose was added into the dark solution, and transferred to a Teflon-lined autoclave. The autoclave was then heated at 180 1C for 16 h to get the carbon nanofibers. The products were dispersed into HCl/H2O2/H2O mixed solution with a volume ratio of 2/5/20 for overnight, In order to remove the Te nanowires [21]. The final product was centrifuged and washed with double-distilled water and absolute ethanol several times. 2.2. Facile synthesis of CNF@CuO NR hybrid structure As shown in Scheme 1, the 1D hierarchical hybrid nanostructure composed of CuO nanorods on CNFs were synthesized through facile solution method combined with a simple thermal treatment. In a typical synthesis, 27 mg of carbon nanofiber was dispersed into 50 mL of ethanol and sonicated for 30 min. 3 mmol of Cu(NO3)2 6H2O was dissolved into 50 mL of DI water to form a transparent solution. The as-prepared solutions were then mixed and after addition

of 24 mmol of urea heated to 80 1C for 6 h. The product was centrifuged and washed with DI water and absolute ethanol several times. After drying, in order to get well defined crystallized CNF@CuO NR hybrid structure, the product was annealed at 250 1C for 2 h with a slow heating rate of 0.5 1C min 1. For the synthesis of pure CuO nanorods (CuO NR), above procedure was repeated without the presence of CNFs. 2.3. Characterization The prepared samples were characterized using X-ray powder diffraction (XRD, Philips X'pert diffractometer with Cu Kα radi-ation (λ¼ 0.154 nm) generated at 40 kV and 30 mA with a step size of 0.041 s 1). X-ray photoelectron spectroscopy (XPS) analyze of the sample was conducted on a VG ESCALAB MKII spectrometer using an Mg Kα X-ray source (1253.6 eV, 120 W) at a constant analyzer. The morphology of the samples was investigated by a Zeiss field-emission scanning electron microscope (FESEM) and a Philips EM 208 transmission electron microscope (TEM). Nitrogen adsorption/desorption was determined at 77 K by the Brunauer–Emmett–Teller (BET) measurements using an ASAP-2010 surface area analyzer. The pore size distribution was also obtained from desorption isotherm using the Barrett– Joyner–Halenda (BJH) method. 2.4. Electrochemical measurements For electrochemical measurements, the electrodes were prepared by mixing active material, acetylene black, and polyvinylidene fluoride (PVDF) with a mass ratio of 85:10:5. A 5% solution of the mixture in acetone was prepared and sprayed on nickel foam (as the current collector) and then dried in 120 1C for 2 h. The prepared electrode was used as the working electrode in a three-electrode configuration, while platinum plate and saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. All potentials were referred to the reference electrode and all electrochemical measurements were performed at room temperature in aqueous 3 M KOH solution on a CHI 660D electrochemical workstation. Specific capacitance of the electrodes was calculated through galvanostatic charge-discharge measurements using the following equation: Csp ¼

IΔt mΔV

ð1Þ

where Csp is the specific capacitance, I is the discharge current–density in A, Δt is the discharge duration in s, m is the loaded mass of the active material in g, and ΔV is the potential range in V. Energy density was derived from the galvanostatic discharge curves using the following equation: ED ¼

C sp ΔV 2 2

ð2Þ

where Csp is specific capacitance in F g 1 and ΔV is the potential range in V. The power density of the electrode was


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Scheme 1. Preparation of 1D hierarchical hybrid nanostructure composed of CuO nanorods on CNFs.

Fig. 1. XRD patterns of CNFs and CNF@CuO NR hybrid structure (a), wide scan XPS spectrum of CNF@CuO NR (b) and high resolution scan of Cu 2p XPS (c), FESEM of CNFs (d) and CNF@CuO NR (e), TEM of CNF@CuO NR hybrid structure (f), FESEM of CuO NR (g).

calculated from the following equation: PD ¼

ED Δt

ð3Þ

where ED is the energy density in W h kg-1, and Δt is the discharge time. 3. Results and discussion 3.1. Characterization Fig. 1a displays the powder X-ray diffraction (PXRD) patterns of CNFs and CNF@CuO NR, showing the positions and relative intensities of the diffraction peaks matched well with standard CuO and carbon patterns. The diffraction peaks with 2θ value of around 261 and 431 were attributed to the (002) and (101) planes of the graphite structure in CNFs (JCPDS no. 13-0148). There were nine diffraction peaks with 2θ values matched well with the growth of the CuO in monoclinic phase (JCPDS 05-0661), indicating that CuO NRs successfully growth on the surface of CNFs. No reflection peaks for impurities such as Cu(OH)2 and Cu2O were observed. Additionally, among all of these diffraction peaks there are two intense peaks located at 2θ values at 35.51 and 38.61 that correspond to the (1–11) and (111) which are

characteristic of the phase pure monoclinic CuO crystallites [22]. Further evidence of the composition and purity of CNF@CuO NR was investigated using X-ray photoelectron spectroscopy (XPS). As shown in Fig. 1b, the full scan spectrum demonstrates the presence of carbon, oxygen and copper. Fig. 1c shows the high-resolution core-level spectrum of Cu 2p peaks over 930–965 eV. The peaks in the Cu 2p spectrum observed at 933.6 and 953.5 eV with splitting of 19.9 eV were attributed to Cu 2p3/2 and Cu 2p1/2, respectively. Additionally, the presence of two satellite peaks positioned at higher binding energies as compared to the main peaks is characteristic of the Cu(II) state and rules out the possibility of the presence of Cu2O phase in the sample [23]. Field emission scanning electron microscopy (FESEM) was employed to explore the morphology of the prepared sample. Fig. 1d, e and g shows the typical FESEM images of the CNFs, CNF@CuO NR hybrid structure and CuO NR, respectively. As can be seen in Fig. 1e and D nanostructure is composed of uniform CuO nanorods grown on the surface of CNF. Furthermore, the nanorods exhibit a hierarchical array feature with empty space among adjacent nanorods. This feature may contribute to the optimization of electrochemical performance by facilitating electrolyte diffusion along the nanostructures. Fig. 1f shows the typical TEM image of the

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Fig. 2. Nitrogen adsorption–desorption isotherms (a), and BJH desorption pore size distribution plot of CNF@CuO NR hybrid structure.

Fig. 3. CVs of bare Ni foam (current collector), CNF, CuO NR, and CNF@CuO NR electrodes at a scan rate of 5 mV s 1 in aqueous 3 M KOH electrolyte (a), CV curves of CNF@CuO NR electrode at various scan rates (inset is relationship between the anodic peak currents and square root of scan rate) (b), galvanostatic charge-discharge curves (c, d), rate capability (e), cycling performance and coulombic efficiency of CNF@CuO NR electrode at different current densities (f).

CNF@CuO NR hybrid structure. It can be clearly observed that CuO nanorods are grown uniformly surrounding the CNF backbone to form the 1D structure. From the SEM and TEM images could be estimated that the length of the nanorods to be around 300 nm while the diameter of the nanorods ranges from several nanometers at the tips to around 15 nm near the bottom of the nanorods. Fig. 2(a and b) shows the corresponding nitrogen adsorption–desorption isotherms and BJH pore size distribution plot of the CNF@CuO NR hybrid structure. The hysteresis loop in the nitrogen adsorption–desorption isotherms, and BJH plot displays the nanoporous structure of sample [24]. The BET surface area of the CNF@CuO NR hybrid structure is 112 m2 g 1. Because of the ion transfer rate in the porous system and the extent of electrode/electrolyte interfacial area, the nanoporous structures with high surface area are very beneficial to supercapacitor performance during the charge– discharge process [25]. As described above, the well defined

CNF@CuO NR hybrid nanostructure can be obtained by this facile and the cost-effective solution route. 3.2. Electrochemical properties To evaluate the electrochemical properties of the as-prepared samples, cyclic voltammetry (CV) and chronopotentiometry (CP) measurements were conducted in a three-electrode cell. Fig. 3a shows the typical CVs of the bare electrode (Ni foam), CNFs, CuO NR and CNF@CuO NR electrodes at a scan rate of 5 mV s 1. The CV curve of CNF electrode exhibits a nearly rectangle-shaped profile without obvious redox peaks, which is the characteristic of an ideal double-layer capacitor. Obvious strong anodic and cathodic peaks within 0.1–0.4 V related to the faradaic redox reactions of Cu þ /Cu2 þ associated with OH anions, clearly reveal the pseudocapacitive characteristics of the CNF@CuO NR and CuO NR electrodes [26,27]. Fig. 3b shows the CV curves of the CNF@CuO NR at various scan rates


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Fig. 4. Ragone plot for CNT@CuO NR electrode at different current densities (a), EIS plot of the CNT@CuO NR electrode at open circuit potential. The inset is an enlarged curve of the high frequency region.

ranged from 5 to 50 mV s 1. It is clearly seen that, with the increase of scan rate, the anodic peaks shift toward the positive potentials and in contrast cathodic peaks shift toward the negative potentials, showing the quasi-reversible nature of the redox reactions. The linear relationship between the square root of the scan rate and the anodic peak currents (inset of Fig. 3b) reveals that the redox reaction kinetics controlled by diffusion of OH anions. In order to evaluate the application of CNF@CuO NR as supercapacitor electrode materials, galvanostatic charge–discharge measurements were performed at current densities between 1 and 50 A g 1. As can be seen in Fig. 3c and d, the obvious nonlinear shape of the charge/discharge curves reveals that the capacitance of the CNF@CuO NR electrode is mainly originated from faradic reactions, which is in agreement with the result of the CV curves. The specific capacitance values calculated from the charge–discharge tests are 398, 371, 320, 254, 198, and 153 F g 1 at current densities of 1, 2, 5, 10, 20, and 50 A g 1, respectively. The specific capacitances of CNF electrode are 82, 78, 68, 55, 44, and 36 F g 1 at current densities of 1, 2, 5, 10, 20, and 50 A g 1, respectively. The specific capacitances of CuO NR electrode are 123, 105, 73, 44, and 16 F g 1 at current densities of 1, 2, 5, 10, and 20 A g 1, respectively. The specific capacitance values of CNF@CuO NR electrode are substantially higher than those of CNF, CuO NR and previous CuO nanostructures [12,13,16– 18,27–38]. In order to give a better representation, the relationships between specific capacitance values and current densities for CNF, CuO NR and CNF@CuO NR electrodes are illustrated in Fig. 3e. As expected, the capacitance decreased with increasing current density due to the limited diffusion on the electrode surface. However, for CNF@CuO NR electrode, after increasing the current density by a factor of 50, still shows a capacitance of 153 F g 1 (about 38% retention). This suggests a very good rate capability for the CNF@CuO NR under very high current density operation conditions (typically here 50 A g 1) which is significant in practical supercapacitor applications. The long-term cycling stability of the CNF@CuO NR electrode was also investigated by continuous charge– discharge measurements over 5000 cycles at different current

densities as shown in Fig. 3f. As can be seen, the specific capacitance (at 2 A g 1) gradually decreases to 341 F g 1 after 2000 cycles corresponding to a capacitance loss of 8.1%. In the next 1000 cycles (at 5 A g 1) the specific capacitance decreased to 85% of its initial value, and after the next 1000 cycles (at 10 A g 1), the sample retained about 86% of its capacitance. After 4000 continuous cycles at successively increased current densities, the capacitance recovered to 347 F g 1 when the current density was turned back to 2 A g 1. The next 1000 cycles exhibited excellent stability with about 96% capacitance retention. These observations demonstrate excellent cycling performance of CNF@CuO NR at different current densities. Furthermore, as can be seen in Fig. 3f, the columbic efficiency of the sample during 5000 continuous cycles indicates excellent reversibility of the material during charge–discharge. Power density (PD) and energy density (ED) of a supercapacitor, determining its operational performance/efficiency, are considered the most important parameters in supercapacitor devices [39]. As shown in Fig. 4a, in order to demonstrate the overall performance of the CNF@CuO NR, the Ragone plot (PD vs. ED) is shown at various current densities. The Ragone plot shows that the ED of the CNF@CuO NR electrode at a PD of 200 W kg 1 is 8.84 Wh kg 1. More significantly, Ragone plot shows that the CNF@CuO NR delivers a high PD of 10 kW kg 1 at an ED of 3.4 Wh kg 1. In order to further characterization of the electrochemical behavior of the CNF@CuO NR electrode, electrochemical impedance spectroscopy (EIS) was performed at open circuit potential and the Nyquist plot is shown in Fig. 4b. Depressed semicircle at the high frequency region (inset of Fig. 4b), corresponded to the charge transfer resistance (Rct) [40], was attributed to the redox reactions of the Cu2 þ /Cu þ redox couples. The straight line in the medium frequency region, corresponded to the Warburg impedance [41], was related to the diffusion of electrolyte along the nanostructures. The steeper line at low frequencies was attributed to the capacitance nature of the electrode (vertical line for an ideal capacitor). Accordingly, the obtained EIS results reveal the small charge transfer resistance and an almost vertical line in

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low frequencies which all show desirable pseudo-capacitance characteristics feature of the prepared CNF@CuO NR as a supercapacitor electrode material. The high electrochemical performance of CNF@CuO NR could be attributed to its structural features such as shape and size of nanorods, and robust CNFs backbone. The well defined hierarchical nanorod arrays lead to the separation of neighboring nanorods from each other and largely increases the amount of electroactive sites accessible by electrolyte. The relatively small diameter of the CuO nanorods could facilitate the electrolyte ion and electron transport in the charge and discharge processes. Furthermore, at high current densities, this feature may contribute to the enhancement of specific capacitance. The CNFs backbone provides a robust support for the CuO nanorods, which could ensure good mechanical adhesion and integration during the repeated charge and discharge processes. These results suggested that the CNF@CuO NR could be considered as a promising electrode material for high-performance supercapacitors. 4. Conclusions A simple strategy was used to synthesize one-dimensional hierarchical hybrid nanostructure composed of CuO nanorods grown on carbon nanofibers with high electrochemical performance including high capacitance (398 F g 1 at 1 A g 1) high-rate capability, high-power density (10 kW kg 1) and remarkable cycling stability at different current densities. This high electrochemical performance was related to the structural features, including shape and size of nanorods and robust CNFs backbone. References [1] G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797–828. [2] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845–854. [3] J. Ji, L.L. Zhang, H. Ji, Y. Li, X. Zhao, X. Bai, X. Fan, F. Zhang, R.S. Ruoff, Nanoporous Ni(OH)2 thin film on 3D ultrathin-graphite foam for asymmetric supercapacitor, ACS Nano 7 (2013) 6237–6243. [4] C. Liu, F. Li, L.P. Ma, H.M. Cheng, Advanced materials for energy storage, Adv. Mater. 22 (2010) E28–E62. [5] L. Dai, D.W. Chang, J.B. Baek, W. Lu, Carbon nanomaterials for advanced energy conversion and storage, Small 8 (2012) 1130–1166. [6] X. Li, B. Wei, Supercapacitors based on nanostructured carbon, Nano Energy 2 (2013) 159–173. [7] Z.S. Wu, G. Zhou, L.C. Yin, W. Ren, F. Li, H.M. Cheng, Graphene/metal oxide composite electrode materials for energy storage, Nano Energy 1 (2012) 107–131. [8] S.P. Lim, N.M. Huang, H.N. Lim, Solvothermal synthesis of SnO2/ graphene nanocomposites for supercapacitor application, Ceram. Int. 39 (2013) 6647–6655. [9] W. Wei, X. Cui, W. Chen, D.G. Ivey, Manganese oxide-based materials as electrochemical supercapacitor electrodes, Chem. Soc. Rev. 40 (2011) 1697–1721. [10] L. Yu, G. Zhang, C. Yuan, X.W. Lou, Hierarchical NiCo2O4@MnO2 core–shell heterostructured nanowire arrays on Ni foam as highperformance supercapacitor electrodes, Chem. Commun. 49 (2013) 137–139.

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