Home
Search
Collections
Journals
About
Contact us
My IOPscience
NiO/nanoporous graphene composites with excellent supercapacitive performance produced by atomic layer deposition
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 504001 (http://iopscience.iop.org/0957-4484/25/50/504001) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 169.230.243.252 This content was downloaded on 15/12/2014 at 04:50
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 504001 (9pp)
doi:10.1088/0957-4484/25/50/504001
NiO/nanoporous graphene composites with excellent supercapacitive performance produced by atomic layer deposition Caiying Chen1,2, Chaoqiu Chen1, Peipei Huang3,4, Feifei Duan1,2, Shichao Zhao1,3, Ping Li3,4, Jinchuan Fan2, Weiguo Song4 and Yong Qin1 1
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, People’s Republic of China 2 College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, 030024, People’s Republic of China 3 University of Chinese Academy of Sciences, Beijing, 100039, People’s Republic of China 4 Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China E-mail: [email protected] and [email protected] Received 27 June 2014, revised 21 July 2014 Accepted for publication 5 August 2014 Published 26 November 2014 Abstract
Nickel oxide (NiO) is a promising electrode material for supercapacitors because of its low cost and high theoretical specific capacitance of 2573 F g−1. However, the low electronic conductivity and poor cycling stability of NiO limit its practical applications. To overcome these limitations, an efficient atomic layer deposition (ALD) method is demonstrated here for the fabrication of NiO/nanoporous graphene (NG) composites as electrode materials for supercapacitors. ALD allows uniform deposition of NiO nanoparticles with controlled sizes on the surface of NG, thus offering a novel route to design NiO/NG composites for supercapacitor applications with high surface areas and greatly improved electrical conductivity and cycle stability. Electrochemical measurements reveal that the NiO/NG composites obtained by ALD exhibited excellent specific capacitance of up to ∼1005.8 F g−1 per mass of the composite electrode (the specific capacitance value is up to ∼1897.1 F g−1 based on the active mass of NiO), and stable performance after 1500 cycles. Furthermore, electrochemical performance of the NiO/NG composites is found to strongly depend on the size of NiO nanoparticles. S Online supplementary data available from stacks.iop.org/NANO/25/504001/mmedia Keywords: NiO/graphene composite, atomic layer deposition, supercapacitor, nanoporous graphene (Some figures may appear in colour only in the online journal) 1. Introduction
layer capacitors (EDLC) with carbon electrodes based on charge separation at the electrode/electrolyte interface and faradaic pseudocapacitors with metal oxide/hydroxide or conducting polymer electrodes based on the reversible redox reactions at the electrode [1–4]. Each type of electrode material has its own advantages and disadvantages. Carbonbased materials usually have high power capabilities and good conductivity but low energy density, as only the surface of the carbon is accessed [2, 5]. By contrast, metal oxides/
Supercapacitors, also known as ultracapacitors or electrochemical capacitors, have attracted much interest for applications in hybrid electric vehicles, electrical vehicles, portable electronic devices, and backup power because of their high power density, fast charging/discharging rate, and long life cycle [1, 2]. According to the charge–discharge mechanisms, supercapacitors are divided into two types: electric double0957-4484/14/504001+09$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 504001
C Chen et al
hydroxides and conducting polymers can provide much higher energy density than conventional carbon materials since the electrochemical processes occur both on the surface and in the bulk near the surface of these electrodes. However, they usually suffer from low conductivity and large volume change during the charge/discharge processes, leading to low power density and poor stability [6]. As a consequence, numerous efforts have been made to develop pseudocapacitive material/conductive matrix hybrid nanostructures, which have been demonstrated to exhibit improved performance by combining unique properties of individual constituents [7–10]. As a new member of carbon allotropes, graphene is regarded as the ideal matrix for growth of pseudocapacitive material/conductive matrix hybrid nanostructures because of its high theoretical surface area (2630 m2 g−1) and high electrical conductivity [11]. However, the experimentally accessible surface area of graphene materials is far below its theoretical value due to the strong aggregation tendency of graphene sheets [12]. To fully utilize and further explore new functions of graphene materials, great effort has been focused on the preparation of porous graphene materials and various methods have been developed such as ice-segregationinduced self-assembly, [13] template-directed CVD [11], and templated assembly method [10]. Among these porous graphene materials, nanoporous graphene foam reported recently by Zhao’s group possesses unique features including controlled pore size, high surface area, ultra-large pore volume, and easy functionalization with metal oxide nanomaterials [14], making it an excellent scaffold for fabrication of hybrid electrodes. Nickel oxide (NiO), as one of the most important transition metal oxides, is of particular interest in view of its high theoretical specific capacitance of 2573 F g−1, environmental benignity and low cost [15]. However, the low electronic conductivity and poor long-term stability of NiO limit its practical applications. To overcome these problems, extensive research has been launched into the development of NiO–graphene composites, which can improve the conductivity of the composites and shorten the electron and ion diffusion pathways due to the aforementioned advantages of graphene [3, 4, 16–25]. Xia et al reported a specific capacitance of 400 and 324 F g−1 for graphene sheet/porous NiO hybrid at 2 and 40 A g−1, respectively [3]. Su et al fabricated a petal-like graphene nanosheet/NiO composite by the microwave-assisted method which showed a specific capacitance of 799 F g−1 at a constant current density of 0.3 A g−1 in 6 M KOH electrolyte [20]. Most recently, Wu et al described the synthesis of a novel 3D NiO/ultrathin derived graphene hybrid with a specific capacitance of 425 F g−1 at a constant current density of 2 A g−1 [16]. Unfortunately, in all these cases the observed specific capacitance is much less than the theoretical value of 2573 F g−1, as the hybrid materials are mostly composed of a random aggregation of NiO-anchored graphene or 3D NiO macrostructure grown on graphene, resulting in low surface areas and relatively long electron or ion diffusion pathways. Therefore, it is desirable to develop a new approach to the synthesis of NiO/graphene composite materials with
controllable decoration of NiO on nanoporous graphene surface at atomic scale, whilst preserving the characteristic properties of graphene. Atomic layer deposition (ALD), a unique film deposition technique, has been proved to be a promising technology for surface modification and fabrication of complex nanostructured materials targeting applications in energy conversion and storage [26–28]. This technology may offer a promising solution for the NiO deposition because of its simplicity, reproducibility and the high conformality of the obtained films [28]. More importantly, the self-limiting character of ALD enables controlled uniform deposition on porous, high surface area supports [29]. These characteristics, combined to the low synthesis temperature and the versatility in term of materials that can be deposited, make ALD a highly suitable technique for the synthesis of NiO/nanoporous graphene for improved supercapacitor performance and the study of size–activity relationships for the electrode materials. However, uniform oxide deposition on graphene by ALD is expected to be difficult due to the lack of dangling bonds in the graphene plane, as in the case of the carbon nanotube [30]. Recently, our group developed a simple and effective route for the synthesis of NiO nanoparticles with good crystallinity and uniform dispersion on pristine CNTs by ALD using O3 as both oxidizing agent for the CNTs and oxygen source for the NiO growth [31]. Herein, we applied the ALD approach to the deposition of NiO nanoparticles of various sizes on the nanoporous graphene (NG) and evaluated the composite performance for use in supercapacitors. We have systematically studied the structural, electrical, and electrochemical properties of the produced NiO/NG composite electrodes as a function of the NiO nanoparticle size. The interconnected conductive network of the NG ensures high electron conductivity. The nanoscale size of the NiO offers a short ion diffusion pathway. The NiO/NG composite obtained after 500 ALD cycles exhibited the best performance with the specific capacitance of up to 1005.8 F g−1 at current densities of 1 A g−1, based on the total mass of the composite electrode. (The specific capacitance value is up to ∼1897.1 F g−1, based on the active mass of NiO).
2. Experimental 2.1. Preparation of graphene oxide (GO) aqueous suspension
Graphite oxide was synthesized from graphite purchased from Alfa Aesar, using the Staudenmaier method. Briefly, 5.0 g of graphite was dispersed into a mixture solution of concentrated H2SO4 (90 mL) and fuming HNO3 (45 mL) cooled within an ice-water bath through vigorous magnetic stirring for 15 min. Then, 55 g of KClO3 was added very slowly into the mixture. All the operations were carried out very carefully in a fume hood to avoid the explosion due to the release of chlorine dioxide gas. After stirring at 25 °C for 96 h, the mixture was poured into 4 L of deionized water, and filtered to obtain graphite oxides. The obtained graphite 2
Nanotechnology 25 (2014) 504001
C Chen et al
Figure 1. TEM and Magnified TEM images of (a), (b) NG, (c), (d) 300-NiO/NG, (e), (f) 500-NiO/NG, and (g), (h) 800-NiO/NG. The insets of (e) and (f) present the corresponding SAED pattern and HRTEM image of 500-NiO/NG, respectively.
oxides were dispersed into 2 L of deionized water and sonicated for 5 h, followed by filtration to remove the underoxidized graphite. The filtrate was dried in a freeze drier, yielding graphene oxides. The as-made graphene oxides were then dispersed into deionized water, and sonicated for 2 h to form homogeneous graphene oxide/H2O suspension (1.0 mg mL−1).
2.3. Synthesis of NiO/nanoporous graphene composite
The ALD NiO process was carried out in a hot-wall closed chamber-type ALD reactor. Nickelocene (NiCp2) and O3 were used as precursors. Prior to ALD, the nanoporous graphene was dispersed in ethanol by ultrasonic agitation and then dropped onto a quartz wafer. After being air-dried, the NiO nanoparticles were deposited by sequential exposure of the nanoporous graphene to O3 and NiCp2. The deposition temperature was maintained at 300 °C. The temperature of the NiCp2 source was kept at 80 °C. For convenience, the products obtained by applying different ALD cycles of NiO deposition were denoted as n-NiO/NG, where n designates the number of ALD cycles for NiO deposition.
2.2. Preparation of nanoporous graphene
Nanoporous graphene was prepared by the method of Zhao’s group [14]. Briefly, 1.0 g of Pluronic F 108 and 1.0 g of 1,3,5trimethylbenzene were mixed into 30 mL of 2 M HCl aqueous and kept stirring for 6 h at 25 °C. Then, 1.0 g of tetraethyl orthosilicate was added dropwise into the suspension under vigorous stirring. After reacting for 6 h, 0.5 g of dimethoxydimethylsilane was added into the suspension and the reaction was continued for another 48 h to obtain hollow siliceous spheres. The resulting mixture was dialyzed in deionized water for 48 h. The dialyzed suspension was diluted to 60 mL by deionized water and mixed with 600 mL of the above prepared graphene oxide/H2O suspension (1.0 mg mL−1). The whole system was stirred for 12 h at room temperature. Then, the solid precipitate was collected by centrifugation at 4500 rpm and dried in a freeze drier. The dried precipitate was calcined at 900 °C for 2 h under argon atmosphere. The sample was then washed twice with an HF solution (5 wt %) in order to remove the hollow siliceous sphere and obtain nanoporous graphene.
2.4. Characterization
X-ray diffraction (XRD) experiments were performed using a Bruker D8 Advanced X-Ray Diffractometer. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) measurements were performed using a JEOL-2100 F microscope. X-ray photoelectron spectroscopy (XPS) data were acquired using a Kratos AXIS 165 multitechnique electron spectrometer with a monochromatic Al Kα (1486.6 eV) source. Raman experiments were performed using a DXR Raman Microscope (Thermo Fisher Scientific Inc.) with a laser wavelength of 532 nm. N2 adsorption and desorption experiments were performed on an Autosorb-1 system at 77 K. The specific surface area was determined by using the Brunauer–Emmett–Teller (BET) method. The pore size distribution was 3
Nanotechnology 25 (2014) 504001
C Chen et al
Figure 2. (a) XRD patterns of NG and NiO/NG composites. (b) TGA
curves of NiO/NG composites in air.
obtained by the Barrett–Joyner–Halenda (BJH) method. The weight content of the NiO in the NiO/nanoporous graphene composites were estimated using thermogravimetric analysis (TGA) in an oxidizing atmosphere of air. Such measurements were carried out on a Netzsch5 TG/DTA thermoanalyzer. TGA measurements were performed on the samples by heating them from room temperature to 950 °C at a 5 °C min−1 ramping rate. Air was blown over the samples at 10 mL min−1. 2.5. Electrochemical tests
Figure 3. (a) Raman spectra and (b) XPS survey of NG and 500-
NiO/NG. (c) High-resolution of C 1s spectrum of NG.
Electrochemical performances of the NiO/nanoporous graphene composites were carried out on a CHI760d electrochemical workstation using a three-electrode electrochemical cell containing 2 M KOH aqueous solution as the electrolyte. The working electrode was made by mixing 80 wt% active material, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride in N-methyl pyrrolidone with magnetic stirring for 12 h. The slurry was then coated onto nickel foam (1.0 cm × 1.0 cm) and dried at 80 °C in a vacuum for 12 h to make the working electrodes. Each electrode contained about 6 mg of electroactive material. A platinum electrode and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. The cyclic voltammetry
(CV) measurements and galvanostatic charge–discharge tests were carried out. Electrochemical impedance spectroscopy (EIS) tests were made with a superimposed 5 mV sinusoidal voltage in the frequency range of 100 kHz–0.01 Hz. The specific capacitance is calculated according to equation (1), C=
IΔt , MΔV
(1)
where C (F g−1) is the specific capacitance; I (A) and Δt (s) denote the applied current and discharge time, respectively; m 4
Nanotechnology 25 (2014) 504001
C Chen et al
Figure 4. Cyclic voltammograms of (a) NG, (b) 300-NiO/NG, (c) 500-NiO/NG, and (d) 800-NiO/NG.
(g) represents the mass of the active materials and V (V) is the potential-drop during discharge.
are the corresponding high-magnification images of these NiO/NG composites. From these TEM images, it can be seen that NiO/NG composites with dense and uniform NiO nanoparticles were obtained. Moreover, the size of the NiO nanoparticles can be easily adjusted by varying the number of ALD cycles because of the self-limited reactions in the ALD technique [32]. The average NiO nanoparticles’ diameters measured by TEM are 3.9, 5.4 and 8.7 nm after 300, 500 and 800 cycles, respectively, in agreement with our previous reports [31]. The corresponding selected area electron diffraction (SAED, inset in figure 1(e)) shows typical ring patterns, indicating the crystalline nature of the NiO nanoparticles. The high-resolution TEM (HRTEM) images show the well-defined crystalline lattice spacing of 0.241 and 0.209 nm, which can be indexed as (111) and (200) crystallographic planes of the cubic NiO (inset in figure 1(f)), respectively. Figure 2(a) shows the XRD patterns of the NG, 300-NiO/ NG, 500-NiO/NG, and 800-NiO/NG, respectively. The NG shows two significant diffraction peaks at 2θ = 26.2 and 44.3° attributed to the (002) and (101) reflections of graphitic carbon, respectively (JCPDS 75-1621) [33], while the peak from graphene oxide (2θ = 12.5°) is very weak, which implies that GO has been reduced to graphene during the calcination process under Ar atmosphere. In addition to the characteristic peaks from graphene, the NiO/NG composites present three new diffraction peaks. They coincide with the (111),
3. Results and discussion 3.1. Materials characterizations
NG was selected as the substrate for NiO deposition because of its nanoporous structure, high surface area and good electrical conductivity. The morphology and structure of the NG and NiO/NG composites were examined by TEM, as shown in figure 1. The porous structure throughout the entire NG can be clearly observed (figure 1(a)); the structure is constructed by graphene sheets (figure 1(b)). The obtained NG exhibits a high surface area of 651 m2 g−1 and has a narrow pore size distribution centered at 32.5 nm (see supplementary information figure 1), quite close to the particle diameter of the silica sphere hard templates (see supplementary information figure 2). These results are in agreement with the literature [14]. Figures 1(c), (e), and (g) are TEM images of the NiO/NG composites prepared after 300, 500 and 800 cycles ALD of NiO deposition, respectively. These images clearly show that the nanoporous structure of NG substrate was preserved during the ALD process of NiO deposition. Their specific surface areas calculated from N2 adsorption isotherms are 235.2, 144.5, and 49.5 m2 g−1, respectively (see supplementary information figure 3). Figures 1(d), (f) and (h) 5
Nanotechnology 25 (2014) 504001
C Chen et al
Figure 5. Discharge profiles of (a) NG, (b) 300-NiO/NG, (c) 500-NiO/NG, and (d) 800-NiO/NG and changes in the specific capacitance of
the produced electrode samples for: (e) NiO/NG composites and (f) contribution of NiO in these composites as a function of current density.
(200) and (220) planes in the standard NiO pattern (JCPDS 65-5745). Moreover, the characteristic peaks of NiO become stronger due to the increase of the NiO contents when the number of ALD cycles increases. In order to determine the content of NiO in the NiO/NG composites, thermogravimetric analyses for NiO/NG composites were carried out under air. Figure 2(b) shows the TGA curves of NiO/NG composites. The amount of NiO in the 300-NiO/NG, 500-NiO/NG and 800-NiO/NG are about 39.4 wt%, 51.3 wt% and 62.1 wt%, respectively, according to the mass loss of GN in these NiO/ NG composites. Raman spectroscopy is a nondestructive and widely used method to characterize the purity and the degree of the
disorder in the graphitic materials [34]. Figure 3(a) shows the Raman spectra of NG and 500-NiO/NG composite. It can be clearly seen that there are two broad peaks at 1344 and 1585 cm−1 in both samples, corresponding to the D and G bands of graphene, respectively. The G band represents the in-plane bond-stretching motion of the pairs of C sp2 atoms (the E2g phonons); whereas the D band corresponds to the breathing modes of rings or κ-point phonons of A1g symmetry. In addition, a broad 2D peak (ca. 2700 cm−1) in the Raman spectra, which is the most prominent feature of graphene, is also observed in the Raman spectra of the NG and 500-NiO/NG composite. The appreciable D-peak signal and weak 2D peak signal imply a large amount of defects in the 6
Nanotechnology 25 (2014) 504001
C Chen et al
centered at 852.9 and 870.5 eV are observed in the XPS survey, corresponding to Ni2p3/2 and Ni2p1/2 spin–orbits, respectively [36]. These results are consistent with the TEM, XRD and Raman results. 3.2. Electrochemical performances
Owing to the large surface area, excellent electrical properties of NG substrate and small particle size of NiO, these obtained NiO/NG composites might be advantageous for electrochemical energy storage. The electrochemical performance of NG and NiO/NG composites was evaluated as capacitive electrodes using a conventional three-electrode system with SCE as the reference electrode and Pt electrode as the counter electrode in an aqueous electrolyte solution of 2 M KOH. Figure 4 shows the CV curves of the NG and NiO/NG composites electrodes at different scan rates. The CV curve of NG is not a perfect rectangle and two redox couples can be observed, indicating that the charge-storage mechanism is different from that of the electric double-layer capacitance. These two redox couples may be attributed to the reversible reaction of NiO/NiOOH formed on the nickel foam surface and the Faraday reaction of OH- with the residual function groups in the NG [3, 23]. For all the NiO/NG composites, only one redox couple is observed in all the CV curves. The anodic peak is due to the oxidation of NiO to NiOOH and the cathodic peak is for the reverse process, as shown in equation (2) [37]. NiO + OH − ↔ NiOOH + e−
(2)
Importantly, the area surrounded by the CV curve is dramatically enhanced after NiO was deposited on the NG. The NiO/NG composites all exhibit a much stronger redox couple with current intensities about one order of magnitude larger than those of NG. These results indicate that the observed capacities of NiO/NG composites have mainly originated from the pseudocapacitance of the electrochemically active NiO nanoparticles based on the reversible redox mechanism. This, in turn, suggests that the introduction of NiO nanoparticles contributes to the increase in the capacitance. In addition, the number of ALD cycles for NiO deposition has an important effect on the electrochemical performance of the NiO/NG composites (figures 4(b)–(d)). With an increasing ALD cycle number (from 300 to 800), the current intensities of redox coupling in NiO/NG composites initially increased and subsequently decreased. The 500-NiO/ NG composite showed higher specific capacitance than 300NiO/NG and 800-NiO/NG composites. The capacitive performance was further investigated with galvanostatic charge/discharge cycling experiments between 0 and 0.5 V at different current densities. As displayed in figure 5, the galvanostatic charge/discharge measurements reveal similar trends. On the basis of the discharging curve line, the specific capacitance of NG, 300-NiO/NG, 500-NiO/ NG, and 800-NiO/NG electrodes was calculated to be 50.6, 480.3, 1005.8, and 693.5 F g−1 at 1 A g−1, respectively, further suggesting that the electrochemical performance of NiO/ NG composites is strongly dependent on the size of the NiO
Figure 6. (a) Cycling performance of the 500-NiO/NG hybrid
electrode at a current density of 2 A g−1. (b) Nyquist plots of the produced electrode samples.
NG [14, 35]. These defect sites can react with ALD precursors to afford active oxide growth [30]. From the Raman spectra of the 500-NiO/NG composite, besides the G band and D band, a broad peak at about 514 cm−1 is also observed, which can be attributed to the stretching mode of NiO [19]. X-ray photoelectron spectroscopy (XPS) measurements were performed to determine the composition and chemical state of elements in the NG and 500-NiO/NG composite. As shown in figure 3(b), the XPS survey suggests that NG contains C as the main elements. In addition, a weak peak at 531.8 is observed, indicating the presence of residual oxygencontaining group in NG [19]. The C1s peak in the XPS spectra of the NG can be fitted into two peaks (figure 3(c)), corresponding to carbon atoms in two different functional groups: non-oxygenated carbon (C–C or C = C, 284.6 eV) and carbon in C–O bonds (286.1 eV) [3, 19]. The intensity of C–O bonds is much lower than that of C–C or C = C, further indicating that GO has been reduced to graphene during the calcination process under the Ar atmosphere. After exposure to 500 ALD cycles of NiO deposition, two new peaks 7
Nanotechnology 25 (2014) 504001
C Chen et al
Table 1. Summary of electrochemical measurements reported in recent papers for NiO/graphene electrodes.
Samples
Specific capacitance (F g−1)
Ref (year)
2
400
3 (2011)
30% KOH
0.1
220
self-assembly
6 M KOH
0.2
525
17 (2011) 4 (2011)
solvothermal
6 M KOH
1
576
homogeneous coprecipitation
6 M KOH
0.2
700
one-pot green method
6 M KOH
0.38
428
hydrothermal
6 M KOH
1
429
microwave-assisted hydrothermal
6 M KOH
0.3
799
hydrothermal
6 M KOH
1
274
Nanocasting and chemical bath deposition solvothermal
5 M KOH
2
425
6 M KOH
1
1077
ALD
2 M KOH
1
1005.8
Preparation method
Electrolyte
electrophoretic deposition and chemical-bath deposition. self-assembly
1 M KOH
Monolayer graphene/NiO nanosheets Reduced graphene oxide–NiO Reduced graphene oxide–NiO
Graphene Sheet/Porous NiO Graphene/NiO sandwich
Flowerlike NiO/Reduced Graphene Oxide Graphene porous NiO nanocomposite Graphene nanosheets/NiO composite NiO nanoflakes/graphene 3D-NiO/graphene NiO/reduced graphene oxide 500-NiO/NG
Current density (A g−1)
23 (2012) 24 (2012) 21 (2013) 18 (2013) 20 (2013) 19 (2013) 16 (2014) 38 (2014) This work
lower than that of the NG electrode, which is unexpected as the metal oxide has significantly lower conductivity than the graphene. The high Rs of NG could be due to its oxidized nature and the high density of defects within NG. The low Rs of the NiO/NG composites might suggest that the growth of NiO on NG can increase the number of charge carriers at the NiO/NG interface [1]. In the low frequency area, the slope of the curve shows the Warburg impedance, which represents the electrolyte diffusion in the porous electrode and ion diffusion in the electrode. 300-NG and 500-NiO/NG are straighter along the imaginary axis, indicating their lower diffusion resistance. Table 1 compares the electrochemical performance of the obtained 500-NiO/NG composite against several NiO/graphene composites in the literature. Note that the test conditions vary widely throughout the literature. Apparently, the specific capacitance of 500-NiO/NG prepared by ALD technology is higher. These results confirm the excellent promise of ALD techniques in supercapacitor applications.
nanoparticles. This behavior can be explained by the chargestorage mechanism of NiO, which is based on the fast and reversible redox reaction at or near the surface of NiO. The highest specific capacitance of 500-NiO/NG is probably due to the best matching of NiO content and shorter ion/electron transport length, which stands in correlation to the NiO size. Furthermore, the capacitance of the NiO nanoparticles alone in 500-NiO/NG electrode approaches 1897.1 F g−1 (figure 5(f)). This value has achieved 74% of the theoretical specific capacitance of NiO. Figures 5(e) and (f) show capacitance retention of the electrodes at increasing current density. The 500-NiO/NG electrode exhibited specific capacitance of 906.6, 836.1, 705.6 and 595.1 F g−1 at 2, 4, 8 and 10 A g−1, respectively, which were also higher than those obtained from 300-NiO/NG and 800-NiO/NG electrodes. The cycling stability of the 500-NiO/NG electrode was examined over a large number of charge/discharge cycles at the current density of 2 A g−1, as shown in figure 6(a). The 500-NiO/NG electrode exhibited good stability, maintaining 94% of its initial capacitance after 1500 cycles. Electrochemical impedance spectroscopy (EIS) was employed to examine the electrochemical response of these electrodes, as shown in figure 6(b). In the high frequency area, the intersection of the curve at real part Z′ (which is equal to equivalent series resistance (Rs)) indicates the bulk resistance of the electrochemical system. It can be seen that the Rs for all of the samples is very low, ranging from as little as 0.6 to 0.9 Ω, probably due to the interconnected network of high conductive graphene with these electrodes. It should be noted that the Rs of the NiO/NG composite electrodes is
4. Conclusions This study demonstrates that ALD is capable of controlling the deposition of NiO on nanoporous graphene (NG), resulting in NiO/NG composites with 3D nanoporous structures and high electrical conductivity. We have systematically studied the structural, electrical, and electrochemical properties of the produced NiO/NG composite electrodes as a function of the NiO nanoparticle size. The interconnected 8
Nanotechnology 25 (2014) 504001
C Chen et al
[12] Han S, Wu D, Li S, Zhang F and Feng X 2014 Adv. Mater. 26 849–64 [13] Vickery J L, Patil A J and Mann S 2009 Adv. Mater. 21 2180–4 [14] Huang X, Qian K, Yang J, Zhang J, Li L, Yu C and Zhao D 2012 Adv. Mater. 24 4419–23 [15] Ding S, Zhu T, Chen J S, Wang Z, Yuan C and Lou X W 2011 J. Mater. Chem. 21 6602–6 [16] Wu C, Deng S, Wang H, Sun Y, Liu J and Yan H 2014 ACS Appl. Mater. Interfaces 6 1106–12 [17] Lv W, Sun F, Tang D-M, Fang H-T, Liu C, Yang Q-H and Cheng H-M 2011 J. Mater. Chem. 21 9014–9 [18] Jiang Y, Chen D, Song J, Jiao Z, Ma Q, Zhang H, Cheng L, Zhao B and Chu Y 2013 Electrochim. Acta 91 173–8 [19] Zhu Y-G, Cao G-S, Sun C-Y, Xie J, Liu S-Y, Zhu T-J, Zhao X B and Yang H Y 2013 RSC Adv. 3 19409–15 [20] Su X, Chai H, Jia D, Bao S, Zhou W and Zhou M 2013 New J. Chem. 37 439–43 [21] Li W, Bu Y, Jin H, Wang J, Zhang W, Wang S and Wang J 2013 Energy Fuels 27 6304–10 [22] Ge C, Hou Z, He B, Zeng F, Cao J, Liu Y and Kuang Y 2012 J. Sol-Gel Sci. Technol. 63 146–52 [23] Yang Y-Y, Hu Z-A, Zhang Z-Y, Zhang F-H, Zhang Y-J, Liang P-J, Zhang H-Y and Wu H-Y 2012 Mater. Chem. Phys. 133 363–8 [24] Zhu X, Dai H, Hu J, Ding L and Jiang L 2012 J. Power Sources 203 243–9 [25] Wu M-S, Lin Y-P, Lin C-H and Lee J-T 2012 J. Mater. Chem. 22 2442–8 [26] Luo J, Liu J, Zeng Z, Ng C F, Ma L, Zhang H, Lin J, Shen Z and Fan H J 2013 Nano Lett. 13 6136–43 [27] Liu M, Li X, Karuturi S K, Tok A I Y and Fan H J 2012 Nanoscale 4 1522–8 [28] Marichy C, Bechelany M and Pinna N 2012 Adv. Mater. 24 1017–32 [29] Wegener S L, Marks T J and Stair P C 2011 Acc. Chem. Res. 45 206–14 [30] Wang X, Tabakman S M and Dai H 2008 J. Am. Chem. Soc. 130 8152–3 [31] Tong X, Qin Y, Guo X, Moutanabbir O, Ao X, Pippel E, Zhang L and Knez M 2012 Small 8 3390–5 [32] George S M 2010 Chem. Rev. 110 111–31 [33] Chen S, Duan J, Tang Y and Zhang Qiao S 2013 Chem. Eur. J. 19 7118–24 [34] Tao L, Zai J, Wang K, Wan Y, Zhang H, Yu C, Xiao Y and Qian X 2012 RSC Adv. 2 3410–5 [35] Guo H-L, Wang X-F, Qian Q-Y, Wang F-B and Xia X-H 2009 ACS Nano 3 2653–9 [36] Spinner N and Mustain W E 2011 Electrochim. Acta 56 5656–66 [37] Cao C-Y, Guo W, Cui Z-M, Song W-G and Cai W 2011 J. Mater. Chem. 21 3204–9 [38] Lee G, Cheng Y, Varanasi C V and Liu J 2014 J. Phys. Chem. C 118 2281–6
conductive network of the NG ensures high electron conductivity. The nanoscale size of the NiO offers a short ion diffusion pathway. Electrochemical measurements reveal that the NiO/NG composites obtained by ALD exhibited excellent specific capacitance of up to ∼1005.8 F g−1 per mass of the composite electrode (the specific capacitance value is up to ∼1897.1 F g−1 based on the active mass of NiO), and stable performance after 1500 cycles. Furthermore, electrochemical performance of the NiO/NG composites is found to strongly depend on the size of the NiO nanoparticles. This synthesis method can be further extended to fabricate other hybrid pseudocapacitive materials such as mixed metal oxides/graphene composites and binary metal oxide/graphene composites which are expected to provide improved performance.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21173248, 21203229), the Hundred Talent Program of the Chinese Academy of Sciences, and the Hundred Talent Program of Shanxi Province.
References [1] Boukhalfa S, Evanoff K and Yushin G 2012 Energy Environ. Sci. 5 6872–9 [2] Lin Y, Wang X, Qian G and Watkins J J 2014 Chem. Mater. 26 2128–37 [3] Xia X, Tu J, Mai Y, Chen R, Wang X, Gu C and Zhao X 2011 Chem. Eur. J. 17 10898–905 [4] Zhao B, Song J, Liu P, Xu W, Fang T, Jiao Z, Zhang H and Jiang Y 2011 J. Mater. Chem. 21 18792–8 [5] Zhai Y, Dou Y, Zhao D, Fulvio P F, Mayes R T and Dai S 2011 Adv. Mater. 23 4828–50 [6] Wang G, Zhang L and Zhang J 2012 Chem. Soc. Rev. 41 797–828 [7] Ji J, Zhang L L, Ji H, Li Y, Zhao X, Bai X, Fan X, Zhang F and Ruoff R S 2013 ACS Nano 7 6237–43 [8] Qu Q, Yang S and Feng X 2011 Adv. Mater. 23 5574–80 [9] Yan J, Fan Z, Sun W, Ning G, Wei T, Zhang Q, Zhang R, Zhi L and Wei F 2012 Adv. Func. Mater. 22 2632–41 [10] Meng Y, Wang K, Zhang Y and Wei Z 2013 Adv. Mater. 25 6985–90 [11] Cao X, Shi Y, Shi W, Lu G, Huang X, Yan Q, Zhang Q and Zhang H 2011 Small 7 3163–8
9
Search
Collections
Journals
About
Contact us
My IOPscience
NiO/nanoporous graphene composites with excellent supercapacitive performance produced by atomic layer deposition
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 504001 (http://iopscience.iop.org/0957-4484/25/50/504001) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 169.230.243.252 This content was downloaded on 15/12/2014 at 04:50
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 504001 (9pp)
doi:10.1088/0957-4484/25/50/504001
NiO/nanoporous graphene composites with excellent supercapacitive performance produced by atomic layer deposition Caiying Chen1,2, Chaoqiu Chen1, Peipei Huang3,4, Feifei Duan1,2, Shichao Zhao1,3, Ping Li3,4, Jinchuan Fan2, Weiguo Song4 and Yong Qin1 1
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, People’s Republic of China 2 College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, 030024, People’s Republic of China 3 University of Chinese Academy of Sciences, Beijing, 100039, People’s Republic of China 4 Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China E-mail: [email protected] and [email protected] Received 27 June 2014, revised 21 July 2014 Accepted for publication 5 August 2014 Published 26 November 2014 Abstract
Nickel oxide (NiO) is a promising electrode material for supercapacitors because of its low cost and high theoretical specific capacitance of 2573 F g−1. However, the low electronic conductivity and poor cycling stability of NiO limit its practical applications. To overcome these limitations, an efficient atomic layer deposition (ALD) method is demonstrated here for the fabrication of NiO/nanoporous graphene (NG) composites as electrode materials for supercapacitors. ALD allows uniform deposition of NiO nanoparticles with controlled sizes on the surface of NG, thus offering a novel route to design NiO/NG composites for supercapacitor applications with high surface areas and greatly improved electrical conductivity and cycle stability. Electrochemical measurements reveal that the NiO/NG composites obtained by ALD exhibited excellent specific capacitance of up to ∼1005.8 F g−1 per mass of the composite electrode (the specific capacitance value is up to ∼1897.1 F g−1 based on the active mass of NiO), and stable performance after 1500 cycles. Furthermore, electrochemical performance of the NiO/NG composites is found to strongly depend on the size of NiO nanoparticles. S Online supplementary data available from stacks.iop.org/NANO/25/504001/mmedia Keywords: NiO/graphene composite, atomic layer deposition, supercapacitor, nanoporous graphene (Some figures may appear in colour only in the online journal) 1. Introduction
layer capacitors (EDLC) with carbon electrodes based on charge separation at the electrode/electrolyte interface and faradaic pseudocapacitors with metal oxide/hydroxide or conducting polymer electrodes based on the reversible redox reactions at the electrode [1–4]. Each type of electrode material has its own advantages and disadvantages. Carbonbased materials usually have high power capabilities and good conductivity but low energy density, as only the surface of the carbon is accessed [2, 5]. By contrast, metal oxides/
Supercapacitors, also known as ultracapacitors or electrochemical capacitors, have attracted much interest for applications in hybrid electric vehicles, electrical vehicles, portable electronic devices, and backup power because of their high power density, fast charging/discharging rate, and long life cycle [1, 2]. According to the charge–discharge mechanisms, supercapacitors are divided into two types: electric double0957-4484/14/504001+09$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 504001
C Chen et al
hydroxides and conducting polymers can provide much higher energy density than conventional carbon materials since the electrochemical processes occur both on the surface and in the bulk near the surface of these electrodes. However, they usually suffer from low conductivity and large volume change during the charge/discharge processes, leading to low power density and poor stability [6]. As a consequence, numerous efforts have been made to develop pseudocapacitive material/conductive matrix hybrid nanostructures, which have been demonstrated to exhibit improved performance by combining unique properties of individual constituents [7–10]. As a new member of carbon allotropes, graphene is regarded as the ideal matrix for growth of pseudocapacitive material/conductive matrix hybrid nanostructures because of its high theoretical surface area (2630 m2 g−1) and high electrical conductivity [11]. However, the experimentally accessible surface area of graphene materials is far below its theoretical value due to the strong aggregation tendency of graphene sheets [12]. To fully utilize and further explore new functions of graphene materials, great effort has been focused on the preparation of porous graphene materials and various methods have been developed such as ice-segregationinduced self-assembly, [13] template-directed CVD [11], and templated assembly method [10]. Among these porous graphene materials, nanoporous graphene foam reported recently by Zhao’s group possesses unique features including controlled pore size, high surface area, ultra-large pore volume, and easy functionalization with metal oxide nanomaterials [14], making it an excellent scaffold for fabrication of hybrid electrodes. Nickel oxide (NiO), as one of the most important transition metal oxides, is of particular interest in view of its high theoretical specific capacitance of 2573 F g−1, environmental benignity and low cost [15]. However, the low electronic conductivity and poor long-term stability of NiO limit its practical applications. To overcome these problems, extensive research has been launched into the development of NiO–graphene composites, which can improve the conductivity of the composites and shorten the electron and ion diffusion pathways due to the aforementioned advantages of graphene [3, 4, 16–25]. Xia et al reported a specific capacitance of 400 and 324 F g−1 for graphene sheet/porous NiO hybrid at 2 and 40 A g−1, respectively [3]. Su et al fabricated a petal-like graphene nanosheet/NiO composite by the microwave-assisted method which showed a specific capacitance of 799 F g−1 at a constant current density of 0.3 A g−1 in 6 M KOH electrolyte [20]. Most recently, Wu et al described the synthesis of a novel 3D NiO/ultrathin derived graphene hybrid with a specific capacitance of 425 F g−1 at a constant current density of 2 A g−1 [16]. Unfortunately, in all these cases the observed specific capacitance is much less than the theoretical value of 2573 F g−1, as the hybrid materials are mostly composed of a random aggregation of NiO-anchored graphene or 3D NiO macrostructure grown on graphene, resulting in low surface areas and relatively long electron or ion diffusion pathways. Therefore, it is desirable to develop a new approach to the synthesis of NiO/graphene composite materials with
controllable decoration of NiO on nanoporous graphene surface at atomic scale, whilst preserving the characteristic properties of graphene. Atomic layer deposition (ALD), a unique film deposition technique, has been proved to be a promising technology for surface modification and fabrication of complex nanostructured materials targeting applications in energy conversion and storage [26–28]. This technology may offer a promising solution for the NiO deposition because of its simplicity, reproducibility and the high conformality of the obtained films [28]. More importantly, the self-limiting character of ALD enables controlled uniform deposition on porous, high surface area supports [29]. These characteristics, combined to the low synthesis temperature and the versatility in term of materials that can be deposited, make ALD a highly suitable technique for the synthesis of NiO/nanoporous graphene for improved supercapacitor performance and the study of size–activity relationships for the electrode materials. However, uniform oxide deposition on graphene by ALD is expected to be difficult due to the lack of dangling bonds in the graphene plane, as in the case of the carbon nanotube [30]. Recently, our group developed a simple and effective route for the synthesis of NiO nanoparticles with good crystallinity and uniform dispersion on pristine CNTs by ALD using O3 as both oxidizing agent for the CNTs and oxygen source for the NiO growth [31]. Herein, we applied the ALD approach to the deposition of NiO nanoparticles of various sizes on the nanoporous graphene (NG) and evaluated the composite performance for use in supercapacitors. We have systematically studied the structural, electrical, and electrochemical properties of the produced NiO/NG composite electrodes as a function of the NiO nanoparticle size. The interconnected conductive network of the NG ensures high electron conductivity. The nanoscale size of the NiO offers a short ion diffusion pathway. The NiO/NG composite obtained after 500 ALD cycles exhibited the best performance with the specific capacitance of up to 1005.8 F g−1 at current densities of 1 A g−1, based on the total mass of the composite electrode. (The specific capacitance value is up to ∼1897.1 F g−1, based on the active mass of NiO).
2. Experimental 2.1. Preparation of graphene oxide (GO) aqueous suspension
Graphite oxide was synthesized from graphite purchased from Alfa Aesar, using the Staudenmaier method. Briefly, 5.0 g of graphite was dispersed into a mixture solution of concentrated H2SO4 (90 mL) and fuming HNO3 (45 mL) cooled within an ice-water bath through vigorous magnetic stirring for 15 min. Then, 55 g of KClO3 was added very slowly into the mixture. All the operations were carried out very carefully in a fume hood to avoid the explosion due to the release of chlorine dioxide gas. After stirring at 25 °C for 96 h, the mixture was poured into 4 L of deionized water, and filtered to obtain graphite oxides. The obtained graphite 2
Nanotechnology 25 (2014) 504001
C Chen et al
Figure 1. TEM and Magnified TEM images of (a), (b) NG, (c), (d) 300-NiO/NG, (e), (f) 500-NiO/NG, and (g), (h) 800-NiO/NG. The insets of (e) and (f) present the corresponding SAED pattern and HRTEM image of 500-NiO/NG, respectively.
oxides were dispersed into 2 L of deionized water and sonicated for 5 h, followed by filtration to remove the underoxidized graphite. The filtrate was dried in a freeze drier, yielding graphene oxides. The as-made graphene oxides were then dispersed into deionized water, and sonicated for 2 h to form homogeneous graphene oxide/H2O suspension (1.0 mg mL−1).
2.3. Synthesis of NiO/nanoporous graphene composite
The ALD NiO process was carried out in a hot-wall closed chamber-type ALD reactor. Nickelocene (NiCp2) and O3 were used as precursors. Prior to ALD, the nanoporous graphene was dispersed in ethanol by ultrasonic agitation and then dropped onto a quartz wafer. After being air-dried, the NiO nanoparticles were deposited by sequential exposure of the nanoporous graphene to O3 and NiCp2. The deposition temperature was maintained at 300 °C. The temperature of the NiCp2 source was kept at 80 °C. For convenience, the products obtained by applying different ALD cycles of NiO deposition were denoted as n-NiO/NG, where n designates the number of ALD cycles for NiO deposition.
2.2. Preparation of nanoporous graphene
Nanoporous graphene was prepared by the method of Zhao’s group [14]. Briefly, 1.0 g of Pluronic F 108 and 1.0 g of 1,3,5trimethylbenzene were mixed into 30 mL of 2 M HCl aqueous and kept stirring for 6 h at 25 °C. Then, 1.0 g of tetraethyl orthosilicate was added dropwise into the suspension under vigorous stirring. After reacting for 6 h, 0.5 g of dimethoxydimethylsilane was added into the suspension and the reaction was continued for another 48 h to obtain hollow siliceous spheres. The resulting mixture was dialyzed in deionized water for 48 h. The dialyzed suspension was diluted to 60 mL by deionized water and mixed with 600 mL of the above prepared graphene oxide/H2O suspension (1.0 mg mL−1). The whole system was stirred for 12 h at room temperature. Then, the solid precipitate was collected by centrifugation at 4500 rpm and dried in a freeze drier. The dried precipitate was calcined at 900 °C for 2 h under argon atmosphere. The sample was then washed twice with an HF solution (5 wt %) in order to remove the hollow siliceous sphere and obtain nanoporous graphene.
2.4. Characterization
X-ray diffraction (XRD) experiments were performed using a Bruker D8 Advanced X-Ray Diffractometer. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) measurements were performed using a JEOL-2100 F microscope. X-ray photoelectron spectroscopy (XPS) data were acquired using a Kratos AXIS 165 multitechnique electron spectrometer with a monochromatic Al Kα (1486.6 eV) source. Raman experiments were performed using a DXR Raman Microscope (Thermo Fisher Scientific Inc.) with a laser wavelength of 532 nm. N2 adsorption and desorption experiments were performed on an Autosorb-1 system at 77 K. The specific surface area was determined by using the Brunauer–Emmett–Teller (BET) method. The pore size distribution was 3
Nanotechnology 25 (2014) 504001
C Chen et al
Figure 2. (a) XRD patterns of NG and NiO/NG composites. (b) TGA
curves of NiO/NG composites in air.
obtained by the Barrett–Joyner–Halenda (BJH) method. The weight content of the NiO in the NiO/nanoporous graphene composites were estimated using thermogravimetric analysis (TGA) in an oxidizing atmosphere of air. Such measurements were carried out on a Netzsch5 TG/DTA thermoanalyzer. TGA measurements were performed on the samples by heating them from room temperature to 950 °C at a 5 °C min−1 ramping rate. Air was blown over the samples at 10 mL min−1. 2.5. Electrochemical tests
Figure 3. (a) Raman spectra and (b) XPS survey of NG and 500-
NiO/NG. (c) High-resolution of C 1s spectrum of NG.
Electrochemical performances of the NiO/nanoporous graphene composites were carried out on a CHI760d electrochemical workstation using a three-electrode electrochemical cell containing 2 M KOH aqueous solution as the electrolyte. The working electrode was made by mixing 80 wt% active material, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride in N-methyl pyrrolidone with magnetic stirring for 12 h. The slurry was then coated onto nickel foam (1.0 cm × 1.0 cm) and dried at 80 °C in a vacuum for 12 h to make the working electrodes. Each electrode contained about 6 mg of electroactive material. A platinum electrode and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. The cyclic voltammetry
(CV) measurements and galvanostatic charge–discharge tests were carried out. Electrochemical impedance spectroscopy (EIS) tests were made with a superimposed 5 mV sinusoidal voltage in the frequency range of 100 kHz–0.01 Hz. The specific capacitance is calculated according to equation (1), C=
IΔt , MΔV
(1)
where C (F g−1) is the specific capacitance; I (A) and Δt (s) denote the applied current and discharge time, respectively; m 4
Nanotechnology 25 (2014) 504001
C Chen et al
Figure 4. Cyclic voltammograms of (a) NG, (b) 300-NiO/NG, (c) 500-NiO/NG, and (d) 800-NiO/NG.
(g) represents the mass of the active materials and V (V) is the potential-drop during discharge.
are the corresponding high-magnification images of these NiO/NG composites. From these TEM images, it can be seen that NiO/NG composites with dense and uniform NiO nanoparticles were obtained. Moreover, the size of the NiO nanoparticles can be easily adjusted by varying the number of ALD cycles because of the self-limited reactions in the ALD technique [32]. The average NiO nanoparticles’ diameters measured by TEM are 3.9, 5.4 and 8.7 nm after 300, 500 and 800 cycles, respectively, in agreement with our previous reports [31]. The corresponding selected area electron diffraction (SAED, inset in figure 1(e)) shows typical ring patterns, indicating the crystalline nature of the NiO nanoparticles. The high-resolution TEM (HRTEM) images show the well-defined crystalline lattice spacing of 0.241 and 0.209 nm, which can be indexed as (111) and (200) crystallographic planes of the cubic NiO (inset in figure 1(f)), respectively. Figure 2(a) shows the XRD patterns of the NG, 300-NiO/ NG, 500-NiO/NG, and 800-NiO/NG, respectively. The NG shows two significant diffraction peaks at 2θ = 26.2 and 44.3° attributed to the (002) and (101) reflections of graphitic carbon, respectively (JCPDS 75-1621) [33], while the peak from graphene oxide (2θ = 12.5°) is very weak, which implies that GO has been reduced to graphene during the calcination process under Ar atmosphere. In addition to the characteristic peaks from graphene, the NiO/NG composites present three new diffraction peaks. They coincide with the (111),
3. Results and discussion 3.1. Materials characterizations
NG was selected as the substrate for NiO deposition because of its nanoporous structure, high surface area and good electrical conductivity. The morphology and structure of the NG and NiO/NG composites were examined by TEM, as shown in figure 1. The porous structure throughout the entire NG can be clearly observed (figure 1(a)); the structure is constructed by graphene sheets (figure 1(b)). The obtained NG exhibits a high surface area of 651 m2 g−1 and has a narrow pore size distribution centered at 32.5 nm (see supplementary information figure 1), quite close to the particle diameter of the silica sphere hard templates (see supplementary information figure 2). These results are in agreement with the literature [14]. Figures 1(c), (e), and (g) are TEM images of the NiO/NG composites prepared after 300, 500 and 800 cycles ALD of NiO deposition, respectively. These images clearly show that the nanoporous structure of NG substrate was preserved during the ALD process of NiO deposition. Their specific surface areas calculated from N2 adsorption isotherms are 235.2, 144.5, and 49.5 m2 g−1, respectively (see supplementary information figure 3). Figures 1(d), (f) and (h) 5
Nanotechnology 25 (2014) 504001
C Chen et al
Figure 5. Discharge profiles of (a) NG, (b) 300-NiO/NG, (c) 500-NiO/NG, and (d) 800-NiO/NG and changes in the specific capacitance of
the produced electrode samples for: (e) NiO/NG composites and (f) contribution of NiO in these composites as a function of current density.
(200) and (220) planes in the standard NiO pattern (JCPDS 65-5745). Moreover, the characteristic peaks of NiO become stronger due to the increase of the NiO contents when the number of ALD cycles increases. In order to determine the content of NiO in the NiO/NG composites, thermogravimetric analyses for NiO/NG composites were carried out under air. Figure 2(b) shows the TGA curves of NiO/NG composites. The amount of NiO in the 300-NiO/NG, 500-NiO/NG and 800-NiO/NG are about 39.4 wt%, 51.3 wt% and 62.1 wt%, respectively, according to the mass loss of GN in these NiO/ NG composites. Raman spectroscopy is a nondestructive and widely used method to characterize the purity and the degree of the
disorder in the graphitic materials [34]. Figure 3(a) shows the Raman spectra of NG and 500-NiO/NG composite. It can be clearly seen that there are two broad peaks at 1344 and 1585 cm−1 in both samples, corresponding to the D and G bands of graphene, respectively. The G band represents the in-plane bond-stretching motion of the pairs of C sp2 atoms (the E2g phonons); whereas the D band corresponds to the breathing modes of rings or κ-point phonons of A1g symmetry. In addition, a broad 2D peak (ca. 2700 cm−1) in the Raman spectra, which is the most prominent feature of graphene, is also observed in the Raman spectra of the NG and 500-NiO/NG composite. The appreciable D-peak signal and weak 2D peak signal imply a large amount of defects in the 6
Nanotechnology 25 (2014) 504001
C Chen et al
centered at 852.9 and 870.5 eV are observed in the XPS survey, corresponding to Ni2p3/2 and Ni2p1/2 spin–orbits, respectively [36]. These results are consistent with the TEM, XRD and Raman results. 3.2. Electrochemical performances
Owing to the large surface area, excellent electrical properties of NG substrate and small particle size of NiO, these obtained NiO/NG composites might be advantageous for electrochemical energy storage. The electrochemical performance of NG and NiO/NG composites was evaluated as capacitive electrodes using a conventional three-electrode system with SCE as the reference electrode and Pt electrode as the counter electrode in an aqueous electrolyte solution of 2 M KOH. Figure 4 shows the CV curves of the NG and NiO/NG composites electrodes at different scan rates. The CV curve of NG is not a perfect rectangle and two redox couples can be observed, indicating that the charge-storage mechanism is different from that of the electric double-layer capacitance. These two redox couples may be attributed to the reversible reaction of NiO/NiOOH formed on the nickel foam surface and the Faraday reaction of OH- with the residual function groups in the NG [3, 23]. For all the NiO/NG composites, only one redox couple is observed in all the CV curves. The anodic peak is due to the oxidation of NiO to NiOOH and the cathodic peak is for the reverse process, as shown in equation (2) [37]. NiO + OH − ↔ NiOOH + e−
(2)
Importantly, the area surrounded by the CV curve is dramatically enhanced after NiO was deposited on the NG. The NiO/NG composites all exhibit a much stronger redox couple with current intensities about one order of magnitude larger than those of NG. These results indicate that the observed capacities of NiO/NG composites have mainly originated from the pseudocapacitance of the electrochemically active NiO nanoparticles based on the reversible redox mechanism. This, in turn, suggests that the introduction of NiO nanoparticles contributes to the increase in the capacitance. In addition, the number of ALD cycles for NiO deposition has an important effect on the electrochemical performance of the NiO/NG composites (figures 4(b)–(d)). With an increasing ALD cycle number (from 300 to 800), the current intensities of redox coupling in NiO/NG composites initially increased and subsequently decreased. The 500-NiO/ NG composite showed higher specific capacitance than 300NiO/NG and 800-NiO/NG composites. The capacitive performance was further investigated with galvanostatic charge/discharge cycling experiments between 0 and 0.5 V at different current densities. As displayed in figure 5, the galvanostatic charge/discharge measurements reveal similar trends. On the basis of the discharging curve line, the specific capacitance of NG, 300-NiO/NG, 500-NiO/ NG, and 800-NiO/NG electrodes was calculated to be 50.6, 480.3, 1005.8, and 693.5 F g−1 at 1 A g−1, respectively, further suggesting that the electrochemical performance of NiO/ NG composites is strongly dependent on the size of the NiO
Figure 6. (a) Cycling performance of the 500-NiO/NG hybrid
electrode at a current density of 2 A g−1. (b) Nyquist plots of the produced electrode samples.
NG [14, 35]. These defect sites can react with ALD precursors to afford active oxide growth [30]. From the Raman spectra of the 500-NiO/NG composite, besides the G band and D band, a broad peak at about 514 cm−1 is also observed, which can be attributed to the stretching mode of NiO [19]. X-ray photoelectron spectroscopy (XPS) measurements were performed to determine the composition and chemical state of elements in the NG and 500-NiO/NG composite. As shown in figure 3(b), the XPS survey suggests that NG contains C as the main elements. In addition, a weak peak at 531.8 is observed, indicating the presence of residual oxygencontaining group in NG [19]. The C1s peak in the XPS spectra of the NG can be fitted into two peaks (figure 3(c)), corresponding to carbon atoms in two different functional groups: non-oxygenated carbon (C–C or C = C, 284.6 eV) and carbon in C–O bonds (286.1 eV) [3, 19]. The intensity of C–O bonds is much lower than that of C–C or C = C, further indicating that GO has been reduced to graphene during the calcination process under the Ar atmosphere. After exposure to 500 ALD cycles of NiO deposition, two new peaks 7
Nanotechnology 25 (2014) 504001
C Chen et al
Table 1. Summary of electrochemical measurements reported in recent papers for NiO/graphene electrodes.
Samples
Specific capacitance (F g−1)
Ref (year)
2
400
3 (2011)
30% KOH
0.1
220
self-assembly
6 M KOH
0.2
525
17 (2011) 4 (2011)
solvothermal
6 M KOH
1
576
homogeneous coprecipitation
6 M KOH
0.2
700
one-pot green method
6 M KOH
0.38
428
hydrothermal
6 M KOH
1
429
microwave-assisted hydrothermal
6 M KOH
0.3
799
hydrothermal
6 M KOH
1
274
Nanocasting and chemical bath deposition solvothermal
5 M KOH
2
425
6 M KOH
1
1077
ALD
2 M KOH
1
1005.8
Preparation method
Electrolyte
electrophoretic deposition and chemical-bath deposition. self-assembly
1 M KOH
Monolayer graphene/NiO nanosheets Reduced graphene oxide–NiO Reduced graphene oxide–NiO
Graphene Sheet/Porous NiO Graphene/NiO sandwich
Flowerlike NiO/Reduced Graphene Oxide Graphene porous NiO nanocomposite Graphene nanosheets/NiO composite NiO nanoflakes/graphene 3D-NiO/graphene NiO/reduced graphene oxide 500-NiO/NG
Current density (A g−1)
23 (2012) 24 (2012) 21 (2013) 18 (2013) 20 (2013) 19 (2013) 16 (2014) 38 (2014) This work
lower than that of the NG electrode, which is unexpected as the metal oxide has significantly lower conductivity than the graphene. The high Rs of NG could be due to its oxidized nature and the high density of defects within NG. The low Rs of the NiO/NG composites might suggest that the growth of NiO on NG can increase the number of charge carriers at the NiO/NG interface [1]. In the low frequency area, the slope of the curve shows the Warburg impedance, which represents the electrolyte diffusion in the porous electrode and ion diffusion in the electrode. 300-NG and 500-NiO/NG are straighter along the imaginary axis, indicating their lower diffusion resistance. Table 1 compares the electrochemical performance of the obtained 500-NiO/NG composite against several NiO/graphene composites in the literature. Note that the test conditions vary widely throughout the literature. Apparently, the specific capacitance of 500-NiO/NG prepared by ALD technology is higher. These results confirm the excellent promise of ALD techniques in supercapacitor applications.
nanoparticles. This behavior can be explained by the chargestorage mechanism of NiO, which is based on the fast and reversible redox reaction at or near the surface of NiO. The highest specific capacitance of 500-NiO/NG is probably due to the best matching of NiO content and shorter ion/electron transport length, which stands in correlation to the NiO size. Furthermore, the capacitance of the NiO nanoparticles alone in 500-NiO/NG electrode approaches 1897.1 F g−1 (figure 5(f)). This value has achieved 74% of the theoretical specific capacitance of NiO. Figures 5(e) and (f) show capacitance retention of the electrodes at increasing current density. The 500-NiO/NG electrode exhibited specific capacitance of 906.6, 836.1, 705.6 and 595.1 F g−1 at 2, 4, 8 and 10 A g−1, respectively, which were also higher than those obtained from 300-NiO/NG and 800-NiO/NG electrodes. The cycling stability of the 500-NiO/NG electrode was examined over a large number of charge/discharge cycles at the current density of 2 A g−1, as shown in figure 6(a). The 500-NiO/NG electrode exhibited good stability, maintaining 94% of its initial capacitance after 1500 cycles. Electrochemical impedance spectroscopy (EIS) was employed to examine the electrochemical response of these electrodes, as shown in figure 6(b). In the high frequency area, the intersection of the curve at real part Z′ (which is equal to equivalent series resistance (Rs)) indicates the bulk resistance of the electrochemical system. It can be seen that the Rs for all of the samples is very low, ranging from as little as 0.6 to 0.9 Ω, probably due to the interconnected network of high conductive graphene with these electrodes. It should be noted that the Rs of the NiO/NG composite electrodes is
4. Conclusions This study demonstrates that ALD is capable of controlling the deposition of NiO on nanoporous graphene (NG), resulting in NiO/NG composites with 3D nanoporous structures and high electrical conductivity. We have systematically studied the structural, electrical, and electrochemical properties of the produced NiO/NG composite electrodes as a function of the NiO nanoparticle size. The interconnected 8
Nanotechnology 25 (2014) 504001
C Chen et al
[12] Han S, Wu D, Li S, Zhang F and Feng X 2014 Adv. Mater. 26 849–64 [13] Vickery J L, Patil A J and Mann S 2009 Adv. Mater. 21 2180–4 [14] Huang X, Qian K, Yang J, Zhang J, Li L, Yu C and Zhao D 2012 Adv. Mater. 24 4419–23 [15] Ding S, Zhu T, Chen J S, Wang Z, Yuan C and Lou X W 2011 J. Mater. Chem. 21 6602–6 [16] Wu C, Deng S, Wang H, Sun Y, Liu J and Yan H 2014 ACS Appl. Mater. Interfaces 6 1106–12 [17] Lv W, Sun F, Tang D-M, Fang H-T, Liu C, Yang Q-H and Cheng H-M 2011 J. Mater. Chem. 21 9014–9 [18] Jiang Y, Chen D, Song J, Jiao Z, Ma Q, Zhang H, Cheng L, Zhao B and Chu Y 2013 Electrochim. Acta 91 173–8 [19] Zhu Y-G, Cao G-S, Sun C-Y, Xie J, Liu S-Y, Zhu T-J, Zhao X B and Yang H Y 2013 RSC Adv. 3 19409–15 [20] Su X, Chai H, Jia D, Bao S, Zhou W and Zhou M 2013 New J. Chem. 37 439–43 [21] Li W, Bu Y, Jin H, Wang J, Zhang W, Wang S and Wang J 2013 Energy Fuels 27 6304–10 [22] Ge C, Hou Z, He B, Zeng F, Cao J, Liu Y and Kuang Y 2012 J. Sol-Gel Sci. Technol. 63 146–52 [23] Yang Y-Y, Hu Z-A, Zhang Z-Y, Zhang F-H, Zhang Y-J, Liang P-J, Zhang H-Y and Wu H-Y 2012 Mater. Chem. Phys. 133 363–8 [24] Zhu X, Dai H, Hu J, Ding L and Jiang L 2012 J. Power Sources 203 243–9 [25] Wu M-S, Lin Y-P, Lin C-H and Lee J-T 2012 J. Mater. Chem. 22 2442–8 [26] Luo J, Liu J, Zeng Z, Ng C F, Ma L, Zhang H, Lin J, Shen Z and Fan H J 2013 Nano Lett. 13 6136–43 [27] Liu M, Li X, Karuturi S K, Tok A I Y and Fan H J 2012 Nanoscale 4 1522–8 [28] Marichy C, Bechelany M and Pinna N 2012 Adv. Mater. 24 1017–32 [29] Wegener S L, Marks T J and Stair P C 2011 Acc. Chem. Res. 45 206–14 [30] Wang X, Tabakman S M and Dai H 2008 J. Am. Chem. Soc. 130 8152–3 [31] Tong X, Qin Y, Guo X, Moutanabbir O, Ao X, Pippel E, Zhang L and Knez M 2012 Small 8 3390–5 [32] George S M 2010 Chem. Rev. 110 111–31 [33] Chen S, Duan J, Tang Y and Zhang Qiao S 2013 Chem. Eur. J. 19 7118–24 [34] Tao L, Zai J, Wang K, Wan Y, Zhang H, Yu C, Xiao Y and Qian X 2012 RSC Adv. 2 3410–5 [35] Guo H-L, Wang X-F, Qian Q-Y, Wang F-B and Xia X-H 2009 ACS Nano 3 2653–9 [36] Spinner N and Mustain W E 2011 Electrochim. Acta 56 5656–66 [37] Cao C-Y, Guo W, Cui Z-M, Song W-G and Cai W 2011 J. Mater. Chem. 21 3204–9 [38] Lee G, Cheng Y, Varanasi C V and Liu J 2014 J. Phys. Chem. C 118 2281–6
conductive network of the NG ensures high electron conductivity. The nanoscale size of the NiO offers a short ion diffusion pathway. Electrochemical measurements reveal that the NiO/NG composites obtained by ALD exhibited excellent specific capacitance of up to ∼1005.8 F g−1 per mass of the composite electrode (the specific capacitance value is up to ∼1897.1 F g−1 based on the active mass of NiO), and stable performance after 1500 cycles. Furthermore, electrochemical performance of the NiO/NG composites is found to strongly depend on the size of the NiO nanoparticles. This synthesis method can be further extended to fabricate other hybrid pseudocapacitive materials such as mixed metal oxides/graphene composites and binary metal oxide/graphene composites which are expected to provide improved performance.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21173248, 21203229), the Hundred Talent Program of the Chinese Academy of Sciences, and the Hundred Talent Program of Shanxi Province.
References [1] Boukhalfa S, Evanoff K and Yushin G 2012 Energy Environ. Sci. 5 6872–9 [2] Lin Y, Wang X, Qian G and Watkins J J 2014 Chem. Mater. 26 2128–37 [3] Xia X, Tu J, Mai Y, Chen R, Wang X, Gu C and Zhao X 2011 Chem. Eur. J. 17 10898–905 [4] Zhao B, Song J, Liu P, Xu W, Fang T, Jiao Z, Zhang H and Jiang Y 2011 J. Mater. Chem. 21 18792–8 [5] Zhai Y, Dou Y, Zhao D, Fulvio P F, Mayes R T and Dai S 2011 Adv. Mater. 23 4828–50 [6] Wang G, Zhang L and Zhang J 2012 Chem. Soc. Rev. 41 797–828 [7] Ji J, Zhang L L, Ji H, Li Y, Zhao X, Bai X, Fan X, Zhang F and Ruoff R S 2013 ACS Nano 7 6237–43 [8] Qu Q, Yang S and Feng X 2011 Adv. Mater. 23 5574–80 [9] Yan J, Fan Z, Sun W, Ning G, Wei T, Zhang Q, Zhang R, Zhi L and Wei F 2012 Adv. Func. Mater. 22 2632–41 [10] Meng Y, Wang K, Zhang Y and Wei Z 2013 Adv. Mater. 25 6985–90 [11] Cao X, Shi Y, Shi W, Lu G, Huang X, Yan Q, Zhang Q and Zhang H 2011 Small 7 3163–8
9
