- Email: [email protected]
Co3O4-doped two-dimensional carbon nanosheet as an electrode material for high-performance asymmetric supercapacitors
Journal Pre-proof Co3O4-doped two-dimensional carbon nanosheet as an electrode material for highperformance asymmetric supercapacitors Xueying Yang, Chenglong Cai, Yongjin Zou, Cuili Xiang, Hailiang Chu, Erhu Yan, Shujun Qiu, Lixian Sun, Fen Xu, Xuebu Hu PII:
S0013-4686(20)30002-5
DOI:
https://doi.org/10.1016/j.electacta.2020.135611
Reference:
EA 135611
To appear in:
Electrochimica Acta
Received Date: 20 November 2019 Revised Date:
30 December 2019
Accepted Date: 1 January 2020
Please cite this article as: X. Yang, C. Cai, Y. Zou, C. Xiang, H. Chu, E. Yan, S. Qiu, L. Sun, F. Xu, X. Hu, Co3O4-doped two-dimensional carbon nanosheet as an electrode material for highperformance asymmetric supercapacitors, Electrochimica Acta (2020), doi: https://doi.org/10.1016/ j.electacta.2020.135611. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Credit author statement Author contributions Xueying Yang：Experiment, Ideas. Chenglong Cai: Experiment, Writing - Original Draft Yongjin Zou: Writing- Review & Editing Cuili Xiang: Writing - Review & Editing, Supervision Hailiang Chu: Data analyses Erhu Yan: Data analyses Shujun Qiu: Data analyses Lixian Sun: Data analyses Fen Xu: Data analyses Xuebu Hu: Review & Editing, Revision, Supervision
Co3O4-doped two-dimensional carbon nanosheet as an electrode material for high-performance asymmetric supercapacitors
Xueying Yanga, Chenglong Caia, Yongjin Zoua, Cuili Xianga,*, Hailiang Chua, Erhu Yana, Shujun Qiua, Lixian Suna, Fen Xua, Xuebu Hub,*
a
Department of Materials Science and Engineering, Guilin University of Electronic
Technology, Guilin 541004, P.R. China b
Chongqing University of Technology, College of Chemistry & Chemical Engineering,
Chongqing, 400054, P. R. China
*Corresponding author Dr. C. Xiang Material Science and Engineering, Guilin University of Electronic Technology, 1# Jinji Road, Guilin, 541004 (China) E-mail address: [email protected]
Dr. X. Hu Chongqing University of Technology, College of Chemistry & Chemical Engineering, Chongqing, 400054, P. R. China E-mail address: [email protected]
Abstract Transition metal oxide-doped two-dimensional (2D) carbon nanosheets are attractive for the preparation of supercapacitors. However, the synthesis of the composites involves complex steps and multiple reagents. Herein, we demonstrate a simple and effective method for the fabrication of 2D carbon nanosheets with highly dispersed cobalt oxide nanoparticles on their surface. Gelatin and melamine were used as the carbon sources, while cobalt acetate (Co(CH3COO)2) was used as the source of Co3O4. After the hydrothermal reaction and carbonization, Co3O4 was well anchored on the surface of the 2D carbon nanosheets. Interestingly, the morphology could be tuned by varying the amount of cobalt acetate added to the gelatin and melamine co-polymer. A 2D carbon nanosheet formed when more than 1 g of cobalt acetate was used along with 1 g of gelatin and 0.3 g of melamine has excellent electrochemical properties. When used in an asymmetric supercapacitor, high energy density, and long stability could be achieved, which is promising for the development of high-performance supercapacitors.
Keywords: Composites; Electrical properties; Transition metal oxides; Batteries
Highlights •
an efficient and environmentally benign carbonization protocol was established for the preparation of GM-C@Co3O4
•
The morphology of the composite can be tuned by change the concentration of cobalt acetate in the precursor
•
GM-C@Co3O4 composite exhibits superior electrochemical properties and stability
•
GM-C@Co3O4 shows great promise as an electrode material for supercapacitors
1. Introduction With the growth of economy around the world, the demand for energy is increasing rapidly [1-3]. Although mankind has benefitted tremendously from traditional fossil energies, they inevitably cause greenhouse effect and pollution [4]. During the past decades, renewable energies such as wind energy, solar energy, and tidal energy have been booming, as they are considered to be effective ways to resolve the energy crisis [5-7]. However, these energy sources need to be stored in efficient devices, such as lithium-ion batteries or supercapacitors [8, 9]. Nowadays, increased attention has been paid to supercapacitors, because of their long service life, high power density, and environment benignity [10]. In general, supercapacitors can be classified into electrical double layer capacitors (EDLCs) and battery-type capacitor. The former rely on electrostatic forces to store charges on carbon materials. Therefore, their energy storage capability is high dependent on the surface area and pore structure of the carbon materials [11-13]. The latter however rely on Faradic reactions to store charge. Conducting polymer-transition metal oxide composites are preferred materials for pseudo-capacitors, because they can store significantly more charges than the materials of EDLCs. However, their poor conductivity and gradually deteriorating stability are still obstacles to their widespread application [14, 15]. Therefore, the rational design of a composite material consisting of a carbon substrate and pseudo-capacitive materials is highly desired [16, 17].
Cobalt-based oxides are prominent representatives of low-cost transition metal oxides that present high theoretical specific capacitances and rich valence changes [18-21]. However, more studies are required for engineering carbon materials with cobalt oxide particles anchored on their surface [22, 23]. Firstly, the interfacial resistance between the two materials must be sufficiently small for the smooth flow of electrons between them. Secondly, cobalt oxide particles must be tightly immobilized on the surface of the carbon substrate so as to decrease their possible aggregation during the charge–discharge processes. So far, great efforts have been made to resolve these issues. For instance, Bao et al. dispersed Co3O4 on a three-dimensional graphene framework, and the composite facilitated a capacitance of 1765 F g−1 at 1 A g−1 [24]. Young et al. prepared Co3O4-doped porous carbon by the carbonization of a metal organic framework and cobalt salt. The obtained composite showed an areal capacitance of 1.22 F· cm−2 at 0.5 mA cm−2 [25]. The structure and morphology of the carbon material have great impact on their electrochemical performance. Materials with a zero-dimensional structure can provide a large surface area with exposed surface atom, which accelerates the electron transfer between the material and electrolyte [26-28]. For example, Du et al. reported porous carbon spheres and nanosheets, which exhibited an exceptional specific capacitance of 196.5 F g−1 at a surface area of 1230 m2 g−1 [27]. Pu et al. controlled the microstructure of a two-dimensional (2D) porous cobalt oxalate thin sheet, which showed a specific capacitance of 1.631 F g−1 at 1.2 mA cm−2 [7]. Essentially, 2D materials are suitable for
energy storage application, because they possess unique properties. However, the in situ formation of 2D materials from a bulk material has been rarely reported. In this work, we demonstrate an efficient and environmentally benign synthetic route for the preparation of Co3O4 nanoparticle-anchored porous 2D carbon sheets using gelatin and melamine as the carbon sources. Gelatin has abundant functional groups that can chelate with metal ions [29, 30], while melamine is a nitrogen-rich polymer that can facilitate adequate N-doping of the carbon backbone. Furthermore, cobalt acetate not only introduces cobalt into the carbon nanosheet but also tunes the morphology of the composite. The 2D Co3O4-doped carbon nanosheets were simply prepared through the carbonization of the C precursors. The resulting product has outstanding electrical conductivity and excellent electrochemical activity, which is favorable for high-performance supercapacitors.
2. Experimental Section 2.1 Materials Cobalt acetate (Co(CH3COO)2, purity ≥99.5 %), gelatin, and melamine (C3H6N6, purity ≥99.0 %) were purchased from the Xilong Scientific company (China). Other chemicals were of analytical grade and used as received. 2.2 Synthesis of gelatin and melamine co-polymer-derived carbon (GM-C)@Co3O4 Gelatin (1 g) was added to distilled water (25 ml) at 60 °C and stirred at a moderate speed until the solution became clear. Then, 0.3 g of melamine was added to the gelatin
solution under stirring. Subsequently, 1 g of cobalt acetate was dissolved in the above mixture under constant stirring for 1 h. Then, the solution was allowed to stand for 24 h at room temperature. The solution was then freeze-dried over 72 h and the obtained solid was carbonized at 700 °C under N2 for 2 h. Finally, the product was heated to 250 °C under O2 for 1 h to oxidize the reduced Co particles. The final powder is denoted as GM-C@Co3O4. For comparison, porous carbon derived from the gelatin and melamine co-polymer (GM-C) was prepared under identical condition without the addition of metal salt. Co3O4 purchased from commercial sources was used as control. The carbon composites with different amounts of cobalt acetate (0.3, 0.5, and 1.2 g) added to the precursor were also prepared. The corresponding products are denoted as [email protected],
[email protected],
and
[email protected],
respectively.
GM-C@Co3O4 refers to the sample prepared with 1 g of cobalt acetate, unless otherwise stated. The preparation of GM-C@Co3O4 is illustrated in Fig. 1. 2.3 Structure and morphology characterization Structure and morphology of the prepared carbon materials were characterized by transmission electron microscopy (TEM), selected-area electron diffraction (SAED), high-resolution TEM (HR-TEM), and high angle annular dark-field scanning TEM (HAADF-STEM) performed on an FEI Tecnai F30 instrument equipped with an energy-dispersive X-ray spectrometer (EDS). The conductivity of the samples was measured with a Keithley 2602 digital electrometer. Other instrumental information and the corresponding experimental conditions used for characterizing the materials by
techniques
including
Fourier
transform
infrared
(FTIR)
spectroscopy,
thermogravimetric (TG) analysis, scanning electron microscopy (SEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy can be found in our previous work [31]. 2.4 Electrochemical measurements The electrochemical tests were performed on an IM6e electrochemical workstation (Zahner-Elektrik, Kronach, Germany). Pt foil was used as the auxiliary electrode and a Hg/HgO electrode was used as the reference electrode. The preparation of the working electrode and configuration of the electrochemical test are same as those reported previously [32]. The equation for the calculation of the specific capacitance, loading masses of the positive and negative electrodes of the asymmetric supercapacitor (ASC), energy density, and power density can be found in a previous report [33].
3. Results and Discussion 3.1. Characterization of the materials Melamine has abundant –NH2 groups, which can conjugate with the –COO− groups in gelatin to form salt linkages. Furthermore, Co2+ ions from cobalt acetate chelate to the rest of the –NH2 groups [34], ensuring the homogeneous dispersion of metal oxide nanoparticles on the carbon substrate. Co(CH3COO)2 was chosen as the Co source in this study, because it can be thermally decomposed to generate a large number of gases, which potentially facilitate the formation of a porous structure of carbon.
Based on this concept, the GM-C@Co3O4 composite was synthesized. Furthermore, as gelatin and melamine are readily available commercially, the composite can be prepared in a large scale. Fig. 2a shows the TG curves of gelatin-melamine (GM) and GM-Co(CH3COO)2 complexes. The TG curve of the GM can be divided into two parts. The weight loss in the first part located at approximately 140–200 °C is attributed to the evaporation of water. The weight loss in the second part at ~300 °C represents the decomposition of GM into carbon residues. In the TG curve of GM-Co(CH3COO)2, apart from the two stages of weight loss observed for GM, two additional stages of weight loss can be observed, owing to the decomposition of Co(CH3COO)2 [35]. The crystal phases of GM-C@Co3O4 were identified by XRD (Fig. 2b). The diffraction peaks of GM-C@Co3O4 located at 19.0°, 31.3°, 38.5°, 44.8°, 55.6°, 59.3°, 65.2° correspond to the (111), (222), (311), (400), (511), and (440) crystal planes of FCC-Co3O4, respectively (JCPDS card No. 42-1467). The XRD data confirms the successful formation of Co3O4 nanocrystals. For comparison, the XRD pattern of GM-C (without Co3O4 nanoparticles) is also shown. It has two broad peaks at 25° and 44°, corresponding to the (002) and (101) planes of amorphous carbon, respectively [26]. The microstructure and morphology of the different samples prepared were investigated by SEM and TEM. Fig. 3 shows the typical SEM images of the composites with different cobalt contents. The gelatin and melamine co-polymer-derived carbon (GM-C) only shows a flat and smooth surface (Fig. 3a). However, the obtained sample
obtained with the addition of 0.3 g of cobalt acetate to the co-polymer is composed of small debris (Fig. 3b). When the cobalt content was increased further to 0.5 g, the entire sample was broken further (Fig. 3c). When the amount of cobalt acetate was increased to 1 g, rough and wrinkled 2D nanosheets were obtained (Fig. 3d–f). Increasing the amount of Co(CH3COO)2 further to 1.2 g in the precursor did not cause any change in the morphology of the composite (Fig. S1). Most probably, the adsorption equilibrium of Co2+ was attained. Therefore, the carbon layer was peeled from the bulk phase. The TEM images show that the Co3O4 nanoparticles, ~10 nm in size, are highly dispersed on the 2D nanosheets (Fig. 4a,b). The HRTEM image shows a single particle of Co3O4 confined on the surface of carbon (Fig. 4c). The carbon layer observed clearly around the Co3O4 nanoparticle can be the graphitic layer of carbon, which was formed due to the catalytic effect of the transition metal, Co [31]. The Co3O4 nanoparticle is uniformly and stably immobilized on 2D carbon nanosheets. This configuration has two advantages. On one hand, it leads to improved stability of the Co3O4 nanoparticle, and the volume change of the Co3O4 nanoparticle during the charge–discharge can be potentially inhibited. On the other hand, the active surface of the Co3O4 nanoparticle can be directly exposed to the electrolyte, which ensures maximum utilization the properties of the Co3O4 nanoparticle. The elemental distribution in GM-C@Co3O4 was investigated by HAADF–STEM–EDS (Fig. 4d–h). The EDS maps of the elements indicate that C, N, O, and Co are evenly distributed on the entire 2D nanosheet, confirming that there is no aggregation of Co nanoparticles.
The spectroscopic properties of GM-C@Co3O4 were evaluated by FT-IR and Raman spectroscopy. Fig. 5a shows the FTIR spectra of melamine, GM-Co(CH3COO)2, and GM-C@Co3O4. In the spectrum of melamine, the bands appearing at 3469 and 3418 cm−1 can be ascribed to the –NH2 antisymmetric vibration, and those at 1652, 1551, 1435, and 1025 cm−1 correspond to the bending vibration of –NH, stretching vibration of C=N, and the twisting vibration of –NH, respectively. Apart from the peaks of melamine, an additional peak can be observed at 1395 cm−1 in the spectrum of GM-Co(CH3COO)2, which represents the C=N stretching vibration, indicating that melamine was successfully conjugated with gelatin. The FTIR spectrum of GM-C@Co3O4 has a strong peak at 1410 cm−1, which can be assigned to C–N stretching [29], indicating that N atoms reside in the carbon backbone after carbonization. The N doping of the carbon substrate not only increases the wettability of the material, but also facilitates Faradic reaction and pseudo-capacitance. Further, the two peaks at 552 and 659 cm−1 can be indexed to the vibrations of Co–O. Fig. 5b presents the Raman spectra of GM-C and GM-C@Co3O4. The D peak is located at ~1350 cm−1, while the G peak is found at ~1580 cm−1. The characteristic D and G peaks of carbon materials represent the defects and sp2-bonded graphitic carbons, respectively. The intensity ratio of the D and G bands, ID/IG, of GM-C@Co3O4 is lower than that of GM-C, indicating that the graphitic degree was improved after the doping of Co3O4. A higher graphitic degree would lead to higher conductivity, which is favorable for electron transfer in the composite [36]. Further, the electrical conductivities of
GM-C@Co3O4 and GM-C were measured by the four-probe method, and were found to be 1.9×104 and 2.7×103 S/m, respectively. GM-C@Co3O4 displayed much higher conductivity than GM-C, which is consistent with the Raman results. XPS was used to investigate the chemical states of various elements in the sample. As revealed by the XPS survey spectra, GM-C@Co3O4 contains four elements, C, N, O, and Co; the corresponding peaks are located at 285 eV (C 1s) , 399 eV (N 1s), 532 eV (O 1s), and 781 eV (Co 2p) (Fig. 6a). The high-resolution C 1s XP spectrum could be deconvoluted into three peaks (Fig. 6b), corresponding to C=C (284.0 eV), C=N/C–O (284.7 eV), and C–N (285.5 eV) bonds [37, 38]. The N 1s peak could be deconvoluted into three peaks at 399.0, 400.1, and 401.0 eV, (Fig. 6c), representing pyridinic-N, pyrrolic-N, and graphitic-N, respectively [39] . The pyridinic-N and pyrrolic-N are likely to improve the charge–discharge capacity through the redox reaction on the surface, while the graphitic-N can increase the ionic conductivity of the material [40]. Fig. 6d shows two intense peaks of Co at 779.1 and 793.4 eV, assigned to Co 2p3/2 and Co 2p1/2 of Co3+ in GM-C@Co3O4. The peaks at 780.9 and 798.2 eV represent the 2p3/2 and 2p1/2 core levels of Co2+ [41]. These characteristics correspond to the phase of Co3O4, and the results agree with the XRD results. Fig. 7a shows the N2 adsorption-desorption isotherms of the samples.
GM-C
shows type-I characteristic without a hysteresis loop, thus indicating the existence of micropores and macropores. By contrast, the curves of GM-C@Co3O4 demonstrate typical type-IV characteristic with obvious hysteresis loops [42], indicating that
GM-C@Co3O4 contains micropores and mesopores. The pore size distribution of GM-C@Co3O4 is mainly centered at ~3.6 nm (Fig. 7b). The specific surface areas of GM-C and GM-C@Co3O4 are 156.4 and 240.5 m2 g−1, respectively. The result indicates that the use of cobalt acetate during the synthesis of a carbon material can increase its specific surface area.
3.2 Electrochemical properties Fig. 8a shows the cyclic voltammetry (CV) curves of GM-C, Co3O4 and GM-C@Co3O4 at a scan rate of 5 mV s−1. The CV curve of GM-C@Co3O4 shows symmetrical redox peaks, indicating the obvious electrochemical capacitance characteristic associated with the fast and reversible Faradic redox reactions [43]. The reactions are presented below [44, 45]. + + → 3 + (1) + → + + (2) The CV curves of GM-C also shows some Faradic reactions, but they occur to a much lower extent than that of GM-C@Co3O4, and they originate from the background current of the nickel foam [46]. The CV curve area of Co3O4 are also lower than that of GM-C@Co3O4 due to the poor conductivity. The internal resistance of GM-C@Co3O4 is supposed to be lower than that of GM-C. To verify this hypothesis, electrochemical impedance spectra (EIS) were recorded. Obviously, the impedance curves contain a straight line and a semicircle in the
low- and high-frequency regions, respectively (Fig. 8b). In the high-frequency region, the radius of the arc represents the charge-transfer resistance (Rct). Based on the equivalent circuit (Fig. S3), the calculated Rct values of GM-C@Co3O4, GM and Co3O4 are 0.2, 0.45 and 0.4 ohm, respectively. The smaller charge-transfer resistance of GM-C@Co3O4 than that of Co3O4 and GM-C indicates the fastest charge transfer in GM-C@Co3O4. Moreover, in the low-frequency region, the curves are almost vertical to the X axis, indicating a lesser diffusive resistance of the materials. Further, the galvanostatic charge–discharge (GCD) curves are presented in Fig. 8c. The specific capacitances of GM-C, Co3O4 and GM-C@Co3O4 are 55, 27 and 1503 F g−1, respectively. GM-C@Co3O4 demonstrates an ultrahigh specific capacitance even when compared with those reported recently for other carbon materials (Table S1), indicating that the electrochemical performance of Co3O4 is fully utilized by the 2D nanosheets. The intimate contact and confinement of Co3O4 on the carbon nanosheet enable the fast electron transfer between the two materials. Furthermore, the surface of Co3O4 was exposed to the electrolyte, which guaranteed rapid Faradic reactions. The uniform N-doping of the carbon nanosheets not only improves the wettability of the materials, but also provides pseudo-capacitance. When combined together, these effects inevitably increase the electrochemical performance of GM-C@Co3O4. The GCD curves of GM-C@Co3O4 composites prepared with different amounts of Co(CH3COO)2 were also recorded (Fig. S2). It can be observed that GM-C@Co3O4 displays the longest discharge time under the same condition. The capacitances of [email protected],
[email protected], GM-C@Co3O4, and [email protected] are 1210, 1305, 1503, and 1407 F g−1, respectively. As expected, GM-C@Co3O4 shows a higher capacitance than those of [email protected] and [email protected], indicating the superiority of the 2D carbon nanosheets. Although [email protected] also has 2D morphology, Co3O4 already fully occupied the surface of the carbon nanosheets. Furthermore, increasing the amount of Co(CH3COO)2 only led to the aggregation of the Co3O4 particles. Further, the CV curves of GM-C@Co3O4 were recorded at various scan rates ranging from 5 to 100 mV s−1 (Fig. 8d). The shape of the redox peaks is well maintained up to 100 mV s−1, indicating rapid electron transfer. The GCD curve of GM-C@Co3O4 was also recorded (Fig. 8e). GM@Co3O4 exhibits pseudo-capacitance with capacitances of 1503 F g−1 at 1 A g−1, 1457 F g−1 at 5 A g−1, 1401 F g−1 at 10 A g−1, and 1300 F g−1 at 20 A g−1, respectively, as shown in Fig. 8f. The rate performance of GM-C@Co3O4 is remarkable, and 86.5% of the capacitance at 1 A g–1 could be retained at 20 A g–1.
3.3 Electrochemical characterization of the proposed ASC, GM-C@Co3O4//GM-C We tested an ASC built using GM-C@Co3O4 as the positive electrode and GM-C as the negative electrode. As the specific capacitances of these two electrodes are different, the mass ratio of the two electrodes was calculated according to Eq. (3) [45]:
×∆
= ×∆
(3)
where, , , , , ∆ , and ∆ represent the masses of the positive and negative material, specific capacitances of mass of the positive and negative material,
and the voltage window of the positive and negative material, respectively. By calculating the window voltage and specific capacitance of the GM-C@Co3O4 and GM-C electrodes from the CV curves in Fig. 9a, the GM-C@Co3O4 / GM-C mass ratio was found to be 0.61:1. In Fig. 9b, the curves of the GM-C@Co3O4//GM-C ASC at different potential windows are obviously rectangular with broad redox peaks located at ~0.6 V and ~0.5 V. This result is ascribed to the fast ion transport and kinetics of Faradic reactions. As no obvious polarization of the CV curves was observed up to 1.5 V, the range of 0–1.5 V was chosen as the ASC potential window for further measurements. Fig. 9c shows the change in the CV curves of the ASC at scan rates ranging from 5 to 100 mV s−1. Clearly, the shape of the CV curves is well maintained, indicating fast electron transfer between the two electrodes. The GCD curves of the ASC are approximately linear (Fig. 9d). The specific capacitances of the ASC are 155, 147, 143, 136, and 131 F g−1 at 2, 4, 8, 10, and 15 A g−1, respectively. The curves indicate that 83.2 % of the specific capacitance at 1 A g−1 is still retained at 15 A g−1, indicating the rate capability. Fig. 9e shows the Ragone plot of GM-C@Co3O4//GM-C ASC. This device shows the highest energy density of 48.4 Wh kg−1 at a power density of 1502 W kg−1. Energy density of up to 41.3 Wh kg−1 could be retained at a power density of 11263 W kg−1. The energy density of GM-C@Co3O4//GM-C ASC is comparable to those recently reported for materials such as Co3O4/PANI (41.5 Wh kg−1 at 800 W kg−1) [47], CoOx//graphene (44.06 Wh kg−1 at 800 W kg−1), CoMoO4@Co3O4/OMEP//AC (45.98 Wh kg−1 at 1647.5 W kg−1)
[48], Co-M@SrGO//AC (23.3 Wh kg−1 at 2300 W kg−1) [49], CoMoO4/Co3O4//AC (44.27 Wh kg−1 at 637 W kg−1)[50], and CNMnL-m//AC(38.4 Wh kg−1 at 800 W kg−1) [51]. Notably, the ASC retains 83.3% of the capacity in a 5000-cycle test at 1 A g−1 (Fig. 9f), revealing its excellent stability.
4. Conclusions We demonstrated a simple and effective way to prepare 2D material carbon nanosheets with Co3O4 nanoparticles dispersed on the surface. The GM-C@Co3O4 composite shows excellent electrochemical performance owing to the uniform distribution and confinement of Co3O4 nanoparticles on the carbon nanosheets as well as N-doping. The GM-C@Co3O4 composite exhibits a high specific capacitance of 1503 F g−1 at 1 A g−1. The assembled GM-C@Co3O4//GM-C ASC operates excellently in the potential window of 0–1.5 V, with an energy density of 48.4 Wh kg−1 at a power density of 1502 W kg−1. Owing to the easy preparation and abundant sources of the raw materials, GM-C@Co3O4 can be produced in a large scale, and it can be potentially applied in energy storage or other electrochemical applications.
Acknowledgements We appreciate the funding support from NSFC (No. 51861005 and 51861004), Innovation Project of Guangxi Graduate Education (YCSW2019149), and Guangxi Natural Science Foundation (2017AD23029).
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Fig. 1. Schematic of the preparation of GM-C@Co3O4.
40
(511)
(440)
(400)
JCPDS card No. 42-1467
(101)
60
(222)
(311)
GM-C GM-C@Co3O4
(002)
80
Intensity/a.u.
Weight loss/%
b
GM GM-Co(CH3COO)2
(111)
a 100
20 200
400 600 Temperature/°C
800
20
40 60 2 Theta/degree
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Fig. 2. (a) TG curves of GM and GM-Co(CH3COO)2. (b) XRD patterns of GM-C@Co3O4 and GM-C.
Fig. 3. SEM images of (a) GM-C, (b) [email protected], (c) [email protected], and (d–f) GM-C@Co3O4 at different magnifications.
Fig. 4. TEM images (a,b) and an HRTEM image (c) of GM-C@Co3O4. HAADF STEM image (d) and EDS elemental maps of Co (e), N (f), Ni (g), and O (h) in GM-C@Co3O4.
a
b
GM-C@Co3O4 659 552
Intensity/a.u.
Intensity/a.u.
1410 GM-Co(CH3COO)2 1395
ID/IG=1.099
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GM-C@Co3O4
Melamine
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1652
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Fig. 5. (a) FTIR spectra of melamine, GM-Co(CH3COO)2, and GM-C@Co3O4. (b) Raman spectra of GM-C and GM-C@Co3O4.
a
C1s
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405
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Fig. 6. XPS survey spectrum of GM-C@Co3O4 (a). High-resolution C 1s (b), N 1s (c),
a
GM-C GM-C@Co3O4
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GM-C GM-C@Co3O4
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-1
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b
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and Co 2p XPS spectra (d).
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0.08 0.04 0.00
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Fig. 7. (a) N2 adsorption–desorption isotherms and (b) pore size distributions of GM-C and GM-C@Co3O4.
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GM-C GM-C@Co3O4
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Fig. 8. Electrochemical characterization of GM-C,GM-C@Co3O4 and Co3O4: (a) CV curve, (b) EIS curve, and (c) GCD curve. (d) CV curve of GM-C@Co3O4 at various scan rates. (e) GCD curves of GM-C@Co3O4 at various current densities. (f) Specific capacitance vs. current density of GM-C@Co3O4.
6
a
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Fig. 9. Electrochemical characterization of GM-C@Co3O4//GM-C ASC: (a) CV curve of the positive and negative electrodes of the ASC, (b) CV curves over different potential windows, (c) CV curves at various scan rates, (d) GCD curves at various current densities, (e) Ragone plot, and (f) Cycling test.
Declaration of interest statement There are no conflicts of interest to declare in this manuscript.
S0013-4686(20)30002-5
DOI:
https://doi.org/10.1016/j.electacta.2020.135611
Reference:
EA 135611
To appear in:
Electrochimica Acta
Received Date: 20 November 2019 Revised Date:
30 December 2019
Accepted Date: 1 January 2020
Please cite this article as: X. Yang, C. Cai, Y. Zou, C. Xiang, H. Chu, E. Yan, S. Qiu, L. Sun, F. Xu, X. Hu, Co3O4-doped two-dimensional carbon nanosheet as an electrode material for highperformance asymmetric supercapacitors, Electrochimica Acta (2020), doi: https://doi.org/10.1016/ j.electacta.2020.135611. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Credit author statement Author contributions Xueying Yang：Experiment, Ideas. Chenglong Cai: Experiment, Writing - Original Draft Yongjin Zou: Writing- Review & Editing Cuili Xiang: Writing - Review & Editing, Supervision Hailiang Chu: Data analyses Erhu Yan: Data analyses Shujun Qiu: Data analyses Lixian Sun: Data analyses Fen Xu: Data analyses Xuebu Hu: Review & Editing, Revision, Supervision
Co3O4-doped two-dimensional carbon nanosheet as an electrode material for high-performance asymmetric supercapacitors
Xueying Yanga, Chenglong Caia, Yongjin Zoua, Cuili Xianga,*, Hailiang Chua, Erhu Yana, Shujun Qiua, Lixian Suna, Fen Xua, Xuebu Hub,*
a
Department of Materials Science and Engineering, Guilin University of Electronic
Technology, Guilin 541004, P.R. China b
Chongqing University of Technology, College of Chemistry & Chemical Engineering,
Chongqing, 400054, P. R. China
*Corresponding author Dr. C. Xiang Material Science and Engineering, Guilin University of Electronic Technology, 1# Jinji Road, Guilin, 541004 (China) E-mail address: [email protected]
Dr. X. Hu Chongqing University of Technology, College of Chemistry & Chemical Engineering, Chongqing, 400054, P. R. China E-mail address: [email protected]
Abstract Transition metal oxide-doped two-dimensional (2D) carbon nanosheets are attractive for the preparation of supercapacitors. However, the synthesis of the composites involves complex steps and multiple reagents. Herein, we demonstrate a simple and effective method for the fabrication of 2D carbon nanosheets with highly dispersed cobalt oxide nanoparticles on their surface. Gelatin and melamine were used as the carbon sources, while cobalt acetate (Co(CH3COO)2) was used as the source of Co3O4. After the hydrothermal reaction and carbonization, Co3O4 was well anchored on the surface of the 2D carbon nanosheets. Interestingly, the morphology could be tuned by varying the amount of cobalt acetate added to the gelatin and melamine co-polymer. A 2D carbon nanosheet formed when more than 1 g of cobalt acetate was used along with 1 g of gelatin and 0.3 g of melamine has excellent electrochemical properties. When used in an asymmetric supercapacitor, high energy density, and long stability could be achieved, which is promising for the development of high-performance supercapacitors.
Keywords: Composites; Electrical properties; Transition metal oxides; Batteries
Highlights •
an efficient and environmentally benign carbonization protocol was established for the preparation of GM-C@Co3O4
•
The morphology of the composite can be tuned by change the concentration of cobalt acetate in the precursor
•
GM-C@Co3O4 composite exhibits superior electrochemical properties and stability
•
GM-C@Co3O4 shows great promise as an electrode material for supercapacitors
1. Introduction With the growth of economy around the world, the demand for energy is increasing rapidly [1-3]. Although mankind has benefitted tremendously from traditional fossil energies, they inevitably cause greenhouse effect and pollution [4]. During the past decades, renewable energies such as wind energy, solar energy, and tidal energy have been booming, as they are considered to be effective ways to resolve the energy crisis [5-7]. However, these energy sources need to be stored in efficient devices, such as lithium-ion batteries or supercapacitors [8, 9]. Nowadays, increased attention has been paid to supercapacitors, because of their long service life, high power density, and environment benignity [10]. In general, supercapacitors can be classified into electrical double layer capacitors (EDLCs) and battery-type capacitor. The former rely on electrostatic forces to store charges on carbon materials. Therefore, their energy storage capability is high dependent on the surface area and pore structure of the carbon materials [11-13]. The latter however rely on Faradic reactions to store charge. Conducting polymer-transition metal oxide composites are preferred materials for pseudo-capacitors, because they can store significantly more charges than the materials of EDLCs. However, their poor conductivity and gradually deteriorating stability are still obstacles to their widespread application [14, 15]. Therefore, the rational design of a composite material consisting of a carbon substrate and pseudo-capacitive materials is highly desired [16, 17].
Cobalt-based oxides are prominent representatives of low-cost transition metal oxides that present high theoretical specific capacitances and rich valence changes [18-21]. However, more studies are required for engineering carbon materials with cobalt oxide particles anchored on their surface [22, 23]. Firstly, the interfacial resistance between the two materials must be sufficiently small for the smooth flow of electrons between them. Secondly, cobalt oxide particles must be tightly immobilized on the surface of the carbon substrate so as to decrease their possible aggregation during the charge–discharge processes. So far, great efforts have been made to resolve these issues. For instance, Bao et al. dispersed Co3O4 on a three-dimensional graphene framework, and the composite facilitated a capacitance of 1765 F g−1 at 1 A g−1 [24]. Young et al. prepared Co3O4-doped porous carbon by the carbonization of a metal organic framework and cobalt salt. The obtained composite showed an areal capacitance of 1.22 F· cm−2 at 0.5 mA cm−2 [25]. The structure and morphology of the carbon material have great impact on their electrochemical performance. Materials with a zero-dimensional structure can provide a large surface area with exposed surface atom, which accelerates the electron transfer between the material and electrolyte [26-28]. For example, Du et al. reported porous carbon spheres and nanosheets, which exhibited an exceptional specific capacitance of 196.5 F g−1 at a surface area of 1230 m2 g−1 [27]. Pu et al. controlled the microstructure of a two-dimensional (2D) porous cobalt oxalate thin sheet, which showed a specific capacitance of 1.631 F g−1 at 1.2 mA cm−2 [7]. Essentially, 2D materials are suitable for
energy storage application, because they possess unique properties. However, the in situ formation of 2D materials from a bulk material has been rarely reported. In this work, we demonstrate an efficient and environmentally benign synthetic route for the preparation of Co3O4 nanoparticle-anchored porous 2D carbon sheets using gelatin and melamine as the carbon sources. Gelatin has abundant functional groups that can chelate with metal ions [29, 30], while melamine is a nitrogen-rich polymer that can facilitate adequate N-doping of the carbon backbone. Furthermore, cobalt acetate not only introduces cobalt into the carbon nanosheet but also tunes the morphology of the composite. The 2D Co3O4-doped carbon nanosheets were simply prepared through the carbonization of the C precursors. The resulting product has outstanding electrical conductivity and excellent electrochemical activity, which is favorable for high-performance supercapacitors.
2. Experimental Section 2.1 Materials Cobalt acetate (Co(CH3COO)2, purity ≥99.5 %), gelatin, and melamine (C3H6N6, purity ≥99.0 %) were purchased from the Xilong Scientific company (China). Other chemicals were of analytical grade and used as received. 2.2 Synthesis of gelatin and melamine co-polymer-derived carbon (GM-C)@Co3O4 Gelatin (1 g) was added to distilled water (25 ml) at 60 °C and stirred at a moderate speed until the solution became clear. Then, 0.3 g of melamine was added to the gelatin
solution under stirring. Subsequently, 1 g of cobalt acetate was dissolved in the above mixture under constant stirring for 1 h. Then, the solution was allowed to stand for 24 h at room temperature. The solution was then freeze-dried over 72 h and the obtained solid was carbonized at 700 °C under N2 for 2 h. Finally, the product was heated to 250 °C under O2 for 1 h to oxidize the reduced Co particles. The final powder is denoted as GM-C@Co3O4. For comparison, porous carbon derived from the gelatin and melamine co-polymer (GM-C) was prepared under identical condition without the addition of metal salt. Co3O4 purchased from commercial sources was used as control. The carbon composites with different amounts of cobalt acetate (0.3, 0.5, and 1.2 g) added to the precursor were also prepared. The corresponding products are denoted as [email protected],
[email protected],
and
[email protected],
respectively.
GM-C@Co3O4 refers to the sample prepared with 1 g of cobalt acetate, unless otherwise stated. The preparation of GM-C@Co3O4 is illustrated in Fig. 1. 2.3 Structure and morphology characterization Structure and morphology of the prepared carbon materials were characterized by transmission electron microscopy (TEM), selected-area electron diffraction (SAED), high-resolution TEM (HR-TEM), and high angle annular dark-field scanning TEM (HAADF-STEM) performed on an FEI Tecnai F30 instrument equipped with an energy-dispersive X-ray spectrometer (EDS). The conductivity of the samples was measured with a Keithley 2602 digital electrometer. Other instrumental information and the corresponding experimental conditions used for characterizing the materials by
techniques
including
Fourier
transform
infrared
(FTIR)
spectroscopy,
thermogravimetric (TG) analysis, scanning electron microscopy (SEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy can be found in our previous work [31]. 2.4 Electrochemical measurements The electrochemical tests were performed on an IM6e electrochemical workstation (Zahner-Elektrik, Kronach, Germany). Pt foil was used as the auxiliary electrode and a Hg/HgO electrode was used as the reference electrode. The preparation of the working electrode and configuration of the electrochemical test are same as those reported previously [32]. The equation for the calculation of the specific capacitance, loading masses of the positive and negative electrodes of the asymmetric supercapacitor (ASC), energy density, and power density can be found in a previous report [33].
3. Results and Discussion 3.1. Characterization of the materials Melamine has abundant –NH2 groups, which can conjugate with the –COO− groups in gelatin to form salt linkages. Furthermore, Co2+ ions from cobalt acetate chelate to the rest of the –NH2 groups [34], ensuring the homogeneous dispersion of metal oxide nanoparticles on the carbon substrate. Co(CH3COO)2 was chosen as the Co source in this study, because it can be thermally decomposed to generate a large number of gases, which potentially facilitate the formation of a porous structure of carbon.
Based on this concept, the GM-C@Co3O4 composite was synthesized. Furthermore, as gelatin and melamine are readily available commercially, the composite can be prepared in a large scale. Fig. 2a shows the TG curves of gelatin-melamine (GM) and GM-Co(CH3COO)2 complexes. The TG curve of the GM can be divided into two parts. The weight loss in the first part located at approximately 140–200 °C is attributed to the evaporation of water. The weight loss in the second part at ~300 °C represents the decomposition of GM into carbon residues. In the TG curve of GM-Co(CH3COO)2, apart from the two stages of weight loss observed for GM, two additional stages of weight loss can be observed, owing to the decomposition of Co(CH3COO)2 [35]. The crystal phases of GM-C@Co3O4 were identified by XRD (Fig. 2b). The diffraction peaks of GM-C@Co3O4 located at 19.0°, 31.3°, 38.5°, 44.8°, 55.6°, 59.3°, 65.2° correspond to the (111), (222), (311), (400), (511), and (440) crystal planes of FCC-Co3O4, respectively (JCPDS card No. 42-1467). The XRD data confirms the successful formation of Co3O4 nanocrystals. For comparison, the XRD pattern of GM-C (without Co3O4 nanoparticles) is also shown. It has two broad peaks at 25° and 44°, corresponding to the (002) and (101) planes of amorphous carbon, respectively [26]. The microstructure and morphology of the different samples prepared were investigated by SEM and TEM. Fig. 3 shows the typical SEM images of the composites with different cobalt contents. The gelatin and melamine co-polymer-derived carbon (GM-C) only shows a flat and smooth surface (Fig. 3a). However, the obtained sample
obtained with the addition of 0.3 g of cobalt acetate to the co-polymer is composed of small debris (Fig. 3b). When the cobalt content was increased further to 0.5 g, the entire sample was broken further (Fig. 3c). When the amount of cobalt acetate was increased to 1 g, rough and wrinkled 2D nanosheets were obtained (Fig. 3d–f). Increasing the amount of Co(CH3COO)2 further to 1.2 g in the precursor did not cause any change in the morphology of the composite (Fig. S1). Most probably, the adsorption equilibrium of Co2+ was attained. Therefore, the carbon layer was peeled from the bulk phase. The TEM images show that the Co3O4 nanoparticles, ~10 nm in size, are highly dispersed on the 2D nanosheets (Fig. 4a,b). The HRTEM image shows a single particle of Co3O4 confined on the surface of carbon (Fig. 4c). The carbon layer observed clearly around the Co3O4 nanoparticle can be the graphitic layer of carbon, which was formed due to the catalytic effect of the transition metal, Co [31]. The Co3O4 nanoparticle is uniformly and stably immobilized on 2D carbon nanosheets. This configuration has two advantages. On one hand, it leads to improved stability of the Co3O4 nanoparticle, and the volume change of the Co3O4 nanoparticle during the charge–discharge can be potentially inhibited. On the other hand, the active surface of the Co3O4 nanoparticle can be directly exposed to the electrolyte, which ensures maximum utilization the properties of the Co3O4 nanoparticle. The elemental distribution in GM-C@Co3O4 was investigated by HAADF–STEM–EDS (Fig. 4d–h). The EDS maps of the elements indicate that C, N, O, and Co are evenly distributed on the entire 2D nanosheet, confirming that there is no aggregation of Co nanoparticles.
The spectroscopic properties of GM-C@Co3O4 were evaluated by FT-IR and Raman spectroscopy. Fig. 5a shows the FTIR spectra of melamine, GM-Co(CH3COO)2, and GM-C@Co3O4. In the spectrum of melamine, the bands appearing at 3469 and 3418 cm−1 can be ascribed to the –NH2 antisymmetric vibration, and those at 1652, 1551, 1435, and 1025 cm−1 correspond to the bending vibration of –NH, stretching vibration of C=N, and the twisting vibration of –NH, respectively. Apart from the peaks of melamine, an additional peak can be observed at 1395 cm−1 in the spectrum of GM-Co(CH3COO)2, which represents the C=N stretching vibration, indicating that melamine was successfully conjugated with gelatin. The FTIR spectrum of GM-C@Co3O4 has a strong peak at 1410 cm−1, which can be assigned to C–N stretching [29], indicating that N atoms reside in the carbon backbone after carbonization. The N doping of the carbon substrate not only increases the wettability of the material, but also facilitates Faradic reaction and pseudo-capacitance. Further, the two peaks at 552 and 659 cm−1 can be indexed to the vibrations of Co–O. Fig. 5b presents the Raman spectra of GM-C and GM-C@Co3O4. The D peak is located at ~1350 cm−1, while the G peak is found at ~1580 cm−1. The characteristic D and G peaks of carbon materials represent the defects and sp2-bonded graphitic carbons, respectively. The intensity ratio of the D and G bands, ID/IG, of GM-C@Co3O4 is lower than that of GM-C, indicating that the graphitic degree was improved after the doping of Co3O4. A higher graphitic degree would lead to higher conductivity, which is favorable for electron transfer in the composite [36]. Further, the electrical conductivities of
GM-C@Co3O4 and GM-C were measured by the four-probe method, and were found to be 1.9×104 and 2.7×103 S/m, respectively. GM-C@Co3O4 displayed much higher conductivity than GM-C, which is consistent with the Raman results. XPS was used to investigate the chemical states of various elements in the sample. As revealed by the XPS survey spectra, GM-C@Co3O4 contains four elements, C, N, O, and Co; the corresponding peaks are located at 285 eV (C 1s) , 399 eV (N 1s), 532 eV (O 1s), and 781 eV (Co 2p) (Fig. 6a). The high-resolution C 1s XP spectrum could be deconvoluted into three peaks (Fig. 6b), corresponding to C=C (284.0 eV), C=N/C–O (284.7 eV), and C–N (285.5 eV) bonds [37, 38]. The N 1s peak could be deconvoluted into three peaks at 399.0, 400.1, and 401.0 eV, (Fig. 6c), representing pyridinic-N, pyrrolic-N, and graphitic-N, respectively [39] . The pyridinic-N and pyrrolic-N are likely to improve the charge–discharge capacity through the redox reaction on the surface, while the graphitic-N can increase the ionic conductivity of the material [40]. Fig. 6d shows two intense peaks of Co at 779.1 and 793.4 eV, assigned to Co 2p3/2 and Co 2p1/2 of Co3+ in GM-C@Co3O4. The peaks at 780.9 and 798.2 eV represent the 2p3/2 and 2p1/2 core levels of Co2+ [41]. These characteristics correspond to the phase of Co3O4, and the results agree with the XRD results. Fig. 7a shows the N2 adsorption-desorption isotherms of the samples.
GM-C
shows type-I characteristic without a hysteresis loop, thus indicating the existence of micropores and macropores. By contrast, the curves of GM-C@Co3O4 demonstrate typical type-IV characteristic with obvious hysteresis loops [42], indicating that
GM-C@Co3O4 contains micropores and mesopores. The pore size distribution of GM-C@Co3O4 is mainly centered at ~3.6 nm (Fig. 7b). The specific surface areas of GM-C and GM-C@Co3O4 are 156.4 and 240.5 m2 g−1, respectively. The result indicates that the use of cobalt acetate during the synthesis of a carbon material can increase its specific surface area.
3.2 Electrochemical properties Fig. 8a shows the cyclic voltammetry (CV) curves of GM-C, Co3O4 and GM-C@Co3O4 at a scan rate of 5 mV s−1. The CV curve of GM-C@Co3O4 shows symmetrical redox peaks, indicating the obvious electrochemical capacitance characteristic associated with the fast and reversible Faradic redox reactions [43]. The reactions are presented below [44, 45]. + + → 3 + (1) + → + + (2) The CV curves of GM-C also shows some Faradic reactions, but they occur to a much lower extent than that of GM-C@Co3O4, and they originate from the background current of the nickel foam [46]. The CV curve area of Co3O4 are also lower than that of GM-C@Co3O4 due to the poor conductivity. The internal resistance of GM-C@Co3O4 is supposed to be lower than that of GM-C. To verify this hypothesis, electrochemical impedance spectra (EIS) were recorded. Obviously, the impedance curves contain a straight line and a semicircle in the
low- and high-frequency regions, respectively (Fig. 8b). In the high-frequency region, the radius of the arc represents the charge-transfer resistance (Rct). Based on the equivalent circuit (Fig. S3), the calculated Rct values of GM-C@Co3O4, GM and Co3O4 are 0.2, 0.45 and 0.4 ohm, respectively. The smaller charge-transfer resistance of GM-C@Co3O4 than that of Co3O4 and GM-C indicates the fastest charge transfer in GM-C@Co3O4. Moreover, in the low-frequency region, the curves are almost vertical to the X axis, indicating a lesser diffusive resistance of the materials. Further, the galvanostatic charge–discharge (GCD) curves are presented in Fig. 8c. The specific capacitances of GM-C, Co3O4 and GM-C@Co3O4 are 55, 27 and 1503 F g−1, respectively. GM-C@Co3O4 demonstrates an ultrahigh specific capacitance even when compared with those reported recently for other carbon materials (Table S1), indicating that the electrochemical performance of Co3O4 is fully utilized by the 2D nanosheets. The intimate contact and confinement of Co3O4 on the carbon nanosheet enable the fast electron transfer between the two materials. Furthermore, the surface of Co3O4 was exposed to the electrolyte, which guaranteed rapid Faradic reactions. The uniform N-doping of the carbon nanosheets not only improves the wettability of the materials, but also provides pseudo-capacitance. When combined together, these effects inevitably increase the electrochemical performance of GM-C@Co3O4. The GCD curves of GM-C@Co3O4 composites prepared with different amounts of Co(CH3COO)2 were also recorded (Fig. S2). It can be observed that GM-C@Co3O4 displays the longest discharge time under the same condition. The capacitances of [email protected],
[email protected], GM-C@Co3O4, and [email protected] are 1210, 1305, 1503, and 1407 F g−1, respectively. As expected, GM-C@Co3O4 shows a higher capacitance than those of [email protected] and [email protected], indicating the superiority of the 2D carbon nanosheets. Although [email protected] also has 2D morphology, Co3O4 already fully occupied the surface of the carbon nanosheets. Furthermore, increasing the amount of Co(CH3COO)2 only led to the aggregation of the Co3O4 particles. Further, the CV curves of GM-C@Co3O4 were recorded at various scan rates ranging from 5 to 100 mV s−1 (Fig. 8d). The shape of the redox peaks is well maintained up to 100 mV s−1, indicating rapid electron transfer. The GCD curve of GM-C@Co3O4 was also recorded (Fig. 8e). GM@Co3O4 exhibits pseudo-capacitance with capacitances of 1503 F g−1 at 1 A g−1, 1457 F g−1 at 5 A g−1, 1401 F g−1 at 10 A g−1, and 1300 F g−1 at 20 A g−1, respectively, as shown in Fig. 8f. The rate performance of GM-C@Co3O4 is remarkable, and 86.5% of the capacitance at 1 A g–1 could be retained at 20 A g–1.
3.3 Electrochemical characterization of the proposed ASC, GM-C@Co3O4//GM-C We tested an ASC built using GM-C@Co3O4 as the positive electrode and GM-C as the negative electrode. As the specific capacitances of these two electrodes are different, the mass ratio of the two electrodes was calculated according to Eq. (3) [45]:
×∆
= ×∆
(3)
where, , , , , ∆ , and ∆ represent the masses of the positive and negative material, specific capacitances of mass of the positive and negative material,
and the voltage window of the positive and negative material, respectively. By calculating the window voltage and specific capacitance of the GM-C@Co3O4 and GM-C electrodes from the CV curves in Fig. 9a, the GM-C@Co3O4 / GM-C mass ratio was found to be 0.61:1. In Fig. 9b, the curves of the GM-C@Co3O4//GM-C ASC at different potential windows are obviously rectangular with broad redox peaks located at ~0.6 V and ~0.5 V. This result is ascribed to the fast ion transport and kinetics of Faradic reactions. As no obvious polarization of the CV curves was observed up to 1.5 V, the range of 0–1.5 V was chosen as the ASC potential window for further measurements. Fig. 9c shows the change in the CV curves of the ASC at scan rates ranging from 5 to 100 mV s−1. Clearly, the shape of the CV curves is well maintained, indicating fast electron transfer between the two electrodes. The GCD curves of the ASC are approximately linear (Fig. 9d). The specific capacitances of the ASC are 155, 147, 143, 136, and 131 F g−1 at 2, 4, 8, 10, and 15 A g−1, respectively. The curves indicate that 83.2 % of the specific capacitance at 1 A g−1 is still retained at 15 A g−1, indicating the rate capability. Fig. 9e shows the Ragone plot of GM-C@Co3O4//GM-C ASC. This device shows the highest energy density of 48.4 Wh kg−1 at a power density of 1502 W kg−1. Energy density of up to 41.3 Wh kg−1 could be retained at a power density of 11263 W kg−1. The energy density of GM-C@Co3O4//GM-C ASC is comparable to those recently reported for materials such as Co3O4/PANI (41.5 Wh kg−1 at 800 W kg−1) [47], CoOx//graphene (44.06 Wh kg−1 at 800 W kg−1), CoMoO4@Co3O4/OMEP//AC (45.98 Wh kg−1 at 1647.5 W kg−1)
[48], Co-M@SrGO//AC (23.3 Wh kg−1 at 2300 W kg−1) [49], CoMoO4/Co3O4//AC (44.27 Wh kg−1 at 637 W kg−1)[50], and CNMnL-m//AC(38.4 Wh kg−1 at 800 W kg−1) [51]. Notably, the ASC retains 83.3% of the capacity in a 5000-cycle test at 1 A g−1 (Fig. 9f), revealing its excellent stability.
4. Conclusions We demonstrated a simple and effective way to prepare 2D material carbon nanosheets with Co3O4 nanoparticles dispersed on the surface. The GM-C@Co3O4 composite shows excellent electrochemical performance owing to the uniform distribution and confinement of Co3O4 nanoparticles on the carbon nanosheets as well as N-doping. The GM-C@Co3O4 composite exhibits a high specific capacitance of 1503 F g−1 at 1 A g−1. The assembled GM-C@Co3O4//GM-C ASC operates excellently in the potential window of 0–1.5 V, with an energy density of 48.4 Wh kg−1 at a power density of 1502 W kg−1. Owing to the easy preparation and abundant sources of the raw materials, GM-C@Co3O4 can be produced in a large scale, and it can be potentially applied in energy storage or other electrochemical applications.
Acknowledgements We appreciate the funding support from NSFC (No. 51861005 and 51861004), Innovation Project of Guangxi Graduate Education (YCSW2019149), and Guangxi Natural Science Foundation (2017AD23029).
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Fig. 1. Schematic of the preparation of GM-C@Co3O4.
40
(511)
(440)
(400)
JCPDS card No. 42-1467
(101)
60
(222)
(311)
GM-C GM-C@Co3O4
(002)
80
Intensity/a.u.
Weight loss/%
b
GM GM-Co(CH3COO)2
(111)
a 100
20 200
400 600 Temperature/°C
800
20
40 60 2 Theta/degree
80
Fig. 2. (a) TG curves of GM and GM-Co(CH3COO)2. (b) XRD patterns of GM-C@Co3O4 and GM-C.
Fig. 3. SEM images of (a) GM-C, (b) [email protected], (c) [email protected], and (d–f) GM-C@Co3O4 at different magnifications.
Fig. 4. TEM images (a,b) and an HRTEM image (c) of GM-C@Co3O4. HAADF STEM image (d) and EDS elemental maps of Co (e), N (f), Ni (g), and O (h) in GM-C@Co3O4.
a
b
GM-C@Co3O4 659 552
Intensity/a.u.
Intensity/a.u.
1410 GM-Co(CH3COO)2 1395
ID/IG=1.099
ID/IG=1.046
GM-C
GM-C@Co3O4
Melamine
3467 3416
4000
1652
3000 2000 -1 Wavenumber/cm
1435 1551
1025
1000
700
1400 2100 -1 Raman shift/cm
2800
3500
Fig. 5. (a) FTIR spectra of melamine, GM-Co(CH3COO)2, and GM-C@Co3O4. (b) Raman spectra of GM-C and GM-C@Co3O4.
a
C1s
b
C1s
Intensity/a.u.
Intensity/a.u.
C=N/C-O
Co2p
O1s N1s
0
200
400 600 Binding energy/eV
800
C-N
C-C/C=C
1000
280
285
290
295
Binding energy/eV
c
d
2p1/2
Co2p
N1s
Intensity/a.u.
Intensity/a.u.
Pyrrolic-N
Pyridinic-N
Quaternary-N
395
400
405
2p1/2 2p3/2
2p3/2
Sat.
Sat.
410
780
790
Binding energy/eV
800
810
Binding energy/eV
Fig. 6. XPS survey spectrum of GM-C@Co3O4 (a). High-resolution C 1s (b), N 1s (c),
a
GM-C GM-C@Co3O4
-1
GM-C GM-C@Co3O4
0.12
-1
150
-3
3
b
0.16 (dV/dD)/cm g nm
200
-1
Quanity absorbed/cm g STP
and Co 2p XPS spectra (d).
100
50
0.08 0.04 0.00
0.00
0.25 0.50 0.75 -1 Relative pressure/P P0
1.00
0
5
10 15 20 Pore diameter/nm
25
30
Fig. 7. (a) N2 adsorption–desorption isotherms and (b) pore size distributions of GM-C and GM-C@Co3O4.
5
b
4
20
GM-C GM-C@Co3O4
-Z"/ohm
Current density/A g-1
a Co3O4
0
-20
3
GM-C GM-C@Co3O4 Co3O4
2 1 0
0.1
0.2 0.3 0.4 E vs (Hg/HgO)/V
0.5
0
c
0.5
200
Co3O4
0.4 0.3 0.2
100
2 Z'/ohm
3
4
5 mV s-1 10 mV s-1 20 mV s-1 50 mV s-1 80 mV s-1 100 mV s-1
0 -100 -200
0.1 300
600
900 Time/s
e
1200
1 A g-1 5 A g-1 10 A g-1 20 A g-1
0.5 0.4
0.1
1500
0.3 0.2
0.2 0.3 0.4 E vs (Hg/HgO)/V
0.5
5 10 15 Current density/A g-1
20
f Specific capacity/F g-1
0
E vs (Hg/HgO)/V
d
−1
Current density/A g−
E vs (Hg/HgO)/V
GM-C GM-C@Co3O4
1
1500
1200
900
0.1 0
300
600 900 Time/s
1200
1500
0
Fig. 8. Electrochemical characterization of GM-C,GM-C@Co3O4 and Co3O4: (a) CV curve, (b) EIS curve, and (c) GCD curve. (d) CV curve of GM-C@Co3O4 at various scan rates. (e) GCD curves of GM-C@Co3O4 at various current densities. (f) Specific capacitance vs. current density of GM-C@Co3O4.
6
a
b Current density (A g− 1)
Current density/A g-1
40
20
0
-20
1.0 V 1.2 V 1.4 V 1.5 V
4 2 0 -2
-40 32
c
Current density/A g-1
24 16 8
-0.6
-0.3 0.0 0.3 E vs (Hg/HgO)/V
0.6
5 mV s-1 10 mV s-1 20 mV s-1 50 mV s-1 80 mV s-1 100 mV s-1
0.0
0 -8
0.4 0.8 1.2 E vs (Hg/HgO)/V
d
1.5 E vs (Hg/HgO)/V
-0.9
1.6 2 A g-1 4 A g-1 8 A g-1 10 A g-1 15 A g-1
1.2 0.9 0.6 0.3
-16
0.0 0.0
50
0.3
0.6 0.9 E vs (Hg/HgO)/V
1.2
0
1.5
45
120 180 Time/s
240
300
1000
2000 3000 Cycle numbers
4000
5000
f
100
Capacitance retention/%
Energy density/Wh kg-1
e
60
80
60
40
40 0
3000
6000
9000
Power density/W kg-1
12000
0
Fig. 9. Electrochemical characterization of GM-C@Co3O4//GM-C ASC: (a) CV curve of the positive and negative electrodes of the ASC, (b) CV curves over different potential windows, (c) CV curves at various scan rates, (d) GCD curves at various current densities, (e) Ragone plot, and (f) Cycling test.
Declaration of interest statement There are no conflicts of interest to declare in this manuscript.