carbon core-branch nanowire arrays as cathode materials for supercapacitor application

carbon core-branch nanowire arrays as cathode materials for supercapacitor application

Materials Letters 134 (2014) 190–193 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet F...

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Materials Letters 134 (2014) 190–193

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Fabrication of cobalt oxide/carbon core-branch nanowire arrays as cathode materials for supercapacitor application Ruiping Zhang a,n, Jun Liu b, Hongge Guo a, Xili Tong c a

Institute of Electronic Information Technology, Taiyuan University of Science and Technology, Taiyuan 030024, China North University of China, Key Lab on Instrumentation Science & Dynamic Measurement of the Ministry, Taiyuan 030051, China c State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, CAS, Taiyuan 030001, China b

art ic l e i nf o

a b s t r a c t

Article history: Received 11 June 2014 Accepted 5 July 2014 Available online 15 July 2014

Porous core-branch nanowire arrays are critical for developing advanced electrochemical devices. In this work, Co3O4/C core-branch nanowire arrays are successfully fabricated by combining hydrothermal synthesis and atomic layer deposition (ALD) methods. Interconnected amorphous carbon nanoflakes are homogeneously coated on the Co3O4 nanowire core forming Co3O4/C core-branch nanowires with diameters of  280 nm. Supercapacitor electrodes based on the Co3O4/C core-branch nanowire arrays are tested. The Co3O4/C core-branch nanowire arrays exhibit excellent electrochemical performances with a high capacitance of 700 F g  1 at 2 A g  1 as well as good cycling stability due to the attractive characteristics such as good conductivity, porous structure and direct growth on the conductive substrates. Our synthetic approach may pave the way for fabrication of other metal oxides/carbon arrays for applications in electrochemical energy storage. & 2014 Elsevier B.V. All rights reserved.

Keywords: Porous materials Deposition Thin films Cobalt oxide Supercapacitors Core-branch

1. Introduction Electrochemical supercapacitors based on nanostructured metal oxides are receiving considerable attention. High performance of supercapacitors relies largely on scrupulous design of nanoarchitectures and smart hybridization of bespoke pseudocapacitive materials. Although RuO2 has been demonstrated with excellent pseudocapacitive performance, the high cost of RuO2 limits its commercial application [1,2]. Therefore, it is highly desirable to search for alternative low-cost transition metal oxides with high capacitances [3]. Co3O4 and Co3O4-based composites have been extensively investigated as active materials for supercapacitors due to their high capacitances, good capability retention, low cost, and high stability [4–6]. The performance of supercapacitors is mainly determined by the electrochemical activity and kinetic feature of the electrode materials. To improve the performance of supercapacitors, it is crucial to enhance the kinetics of ion and electron transport in electrodes and at the electrode/electrolyte interface. To date, great efforts have been dedicated to designing nanostructured Co3O4 nanomaterials (such as nanowire, nanoflake, core/ shell structures) [7–9], and Co3O4-based composites with conductive layers, which can improve the electron transfer leading to

n

Corresponding author. Tel.: þ 86 351 4605282. E-mail address: [email protected] (R. Zhang).

http://dx.doi.org/10.1016/j.matlet.2014.07.054 0167-577X/& 2014 Elsevier B.V. All rights reserved.

good high-rate capability [10,11]. Particularly, several works are focused on Co3O4–C composite materials and enhanced performances have been proven in these systems [7,12]. In addition, there are a few reports about Co3O4/C core/shell nanowire arrays for supercapacitor application. For example, Pan's group constructed cobalt oxide/carbon core/shell nanowires with dense CVD-carbon shell and Co3O4 nanowire and a high specific capacity of 116 mAh g  1 was obtained in this core/shell arrays [7], when they were applied for supercapacitors. Different from the above Co3O4/C core/shell nanowire arrays, herein, we report novel nanostructured Co3O4/C core-branch nanowire arrays, in which carbon nanoflake shell is homogeneously coated on the Co3O4 nanowire core by combining hydrothermal synthesis and atomic layer deposition (ALD) methods. As cathode materials of supercapacitor, the as-prepared Co3O4/C CBNAs show excellent supercapacitor performance with a high capacitance of 700 F g  1 as well as good cycling stability.

2. Experimental The Co3O4/C core-branch nanowire arrays were synthesized as follows. First, the precursor of Co2(OH)2CO3 nanowires were synthesized by a hydrothermal process as described in the reference [13]. Second, the Co2(OH)2CO3 nanowires were coated with Al2O3 of 120 cycles by ALD at 120 1C. Trimethylaluminum Al

R. Zhang et al. / Materials Letters 134 (2014) 190–193

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Fig. 1. SEM and TEM images of (a, b) Co2(OH)2CO3 nanowire arrays and (c, d) Co3O4/C core-branch nanowire arrays. (e) XRD patterns of Co2(OH)2CO3 and Co3O4/C corebranch nanowire arrays. (f) Raman spectrum of Co3O4/C core-branch nanowire arrays.

(CH3)3 and water were used as the aluminum and oxygen sources, respectively. Third, the Co2(OH)2CO3–Al2O3 nanowires were immersed in 0.04 M glucose for 24 h and annealed in argon at 350 1C for 2 h. Finally, the Al2O3 was removed in 1 M KOH forming the Co3O4/C corebranch nanowire arrays. The weight of Co3O4/C core-branch nanowire arrays were  1.7 mg cm  2. The carbon in the core-branch nanowires accounted for  3% analyzed by element analysis and Inductively Coupled Plasma Mass Spectrometry (ICP-MS, Thermo PQ3). The morphologies and structures of the samples were characterized by X-ray diffraction (XRD, RIGAKU D/Max-2550 with Cu Kα radiation), field emission scanning electron microscopy (FESEM, FEI SIRION), transmission electron microscopy (TEM, JEOL JEM200CX) and Raman spectroscopy (LABRAM HR-800). The electrochemical measurements were carried out in a threeelectrode electrochemical cell containing 2 M KOH aqueous solution as the electrolyte, Hg/HgO as the reference electrode and a Pt foil as the counter-electrode. Cyclic voltammetry (CV) and

galvanostatic charge/discharge measurements were performed on CHI660c electrochemical workstation (Chenhua, Shanghai) and Neware program-control test system, respectively. The specific capacitance was calculated using the following equation: C¼

IΔt mΔV

ð1Þ

where C (F g  1) and I (mA) represented specific capacitance and the current applied. ΔV (V), m (mg), and Δt (s) designated the potential drop during discharge, mass of the active materials, and total discharge time, respectively.

3. Results and discussion SEM images of Co2(OH)2CO3 nanowires and Co3O4/C corebranch nanowire arrays are shown in Fig. 1. After hydrothermal

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Fig. 2. Electrochemical properties of Co3O4/C core-branch nanowire arrays: (a) CV curves at a scanning rate of 10 mV s  1; (b) discharge curves at different discharge current densities; (c) corresponding specific capacitances; (d) cycling performance at 2 A g  1.

synthesis, the whole Ni foam substrate is uniformly covered by the Co2(OH)2CO3 nanowires with diameters of  80 nm, which grow almost vertically to the substrate (Fig. 1a). In addition, the Co2 (OH)2CO3 nanowires have smooth texture (Fig. 1b). After ALD and glucose carbonization with annealing process, the Co2(OH)2CO3 nanowires convert into Co3O4 nanowires, which are tightly wrapped by a shell of carbon flakes with thicknesses of  100 nm. The Co3O4/C core-branch nanowire shows a diameter of  280 nm. Fig. 1e shows the XRD patterns of Co2(OH)2CO3 nanowires and Co3O4/C core-branch nanowire arrays. All the peaks of the precursor film could be well indexed to crystalline orthorhombic Co2(OH)2CO3 (JCPDS 48-0083). For the Co3O4/C core-branch nanowire arrays, all the diffraction peaks are attributed to spinel Co3O4 phase (JCPDS 42-1467), indicating that the crystalline Co3O4 has been formed after heat treatment [13]. In addition, no obvious diffraction peaks of carbon are detected, implying the amorphous nature of carbon nanoflake shell. In the Raman spectrum (Fig. 1f), the Co3O4/C core-branch nanowire arrays exhibit four obvious peaks in the ranges of 400 to 800 cm  1, which are typical features of Co3O4 vibration modes. Meanwhile, there are two extra peaks around 1350 and 1583 cm  1 [14], which are the D and G bands of amorphous carbon, respectively, indicating a layer of carbon exist in the core-branch nanowires. The core-branch nanowire arrays show some interesting characteristics, such as high porosity, high surface area and direct growth on conductive substrates. The interconnected carbon flake shell is also able to improve the conductivity of the whole material, which is vital important for electrochemical applications [7,9]. Additionally, the carbon flake shell also has the following

advantages. First, the porous structure of carbon nanoflake makes it easier for the contact between electrolyte and the core active material, without sacrifice the conductivity. Especially, in some situation, the core active material must be exposed, such as the application in oxygen reduction reaction (ORR). Second, carbon material is also a common energy storage material. The open structure and high porosity offered by the flake shell improve the utilization of active material. As a preliminary investigation, we tested the electrochemical properties of the Co3O4/C core-branch nanowire arrays as cathode of supercapacitors. The pseudocapacitive properties of the Co3O4/C core-branch nanowire arrays are investigated by cyclic voltammograms (CV) and galvanostatic charge/discharge tests. Fig. 2a shows the CV curve of the Co3O4/C core-branch nanowire arrays at a scanning rate of 10 mV s  1. A pair of redox peaks is noticed in the CV curve, which corresponds to reaction between Co3O4 and CoOOH. In our case, the capacitance mainly comes from the pseudocapacitive behavior of Co3O4. The capacitance contribution from carbon is negligible. Discharge profiles of the Co3O4/C corebranch nanowire arrays at various discharge current densities and corresponding specific capacitances are shown in Fig. 2b and c. The specific capacitances are calculated by subtracting the discharge time of bare nickel foam at the same current density, respectively. The Co3O4/C core-branch nanowire arrays present a specific capacitance from 700 F g  1 (2 A g  1) to 576 F g  1 (36 A g  1). These values are much higher than those of the single Co3O4 nanowire arrays (323 F g  1 at 2 A g  1 and 248 F g  1 at 40 A g  1) [13] and NiO materials [15,16]. The cycle life of the Co3O4/C core-branch nanowire arrays at 2 A g  1 is presented in

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Fig. 2d. There is an obvious activation process, the specific capacitance increases from 700 F g  1 to 750 F g  1 at the first 600 cycles. After 5000 cycles, the Co3O4/C core-branch nanowire arrays deliver a capacitance of 705 F g  1 with a capacitance retention of 94%. The excellent electrochemical performance indicates that the Co3O4/C core-branch nanowire arrays are a promising cathode material for high-performance supercapacitors. 4. Conclusion Free-standing Co3O4/C core-branch nanowire arrays are prepared by combining hydrothermal synthesis and atomic layer deposition (ALD) methods. Carbon nanoflake shell is well decorated on the Co3O4 nanowire core. As a cathode of supercapacitors, the Co3O4/C core-branch nanowire arrays exhibit outstanding electrochemical performance with high capacitance and good cycle life due to the unique core-branch architecture, and thus proving its potential application in electrochemical energy storage devices. References [1] Liu X, Pickup PG. J Power Sources 2008;176:410–6. [2] Zhang J, Vukmirovic MB, Xu Y, Mavrikakis M, Adzic RR. Angew Chem Int Ed 2005;44:2132–5. [3] Xia X, Zhang Y, Chao D, Guan C, Zhang Y, Li L, et al. Nanoscale 2014;6:5008–48.

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