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Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance
international journal of hydrogen energy xxx (xxxx) xxx
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Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance Haicheng Xuan*, Hongsheng Li, Jing Yang, Xiaohong Liang, Zhigao Xie, Peide Han, Yucheng Wu College of Materials Science and Engineering, Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, People’s Republic of China
highlights
graphical abstract
A core-shell CoMoO4@CoS composite was prepared by an efficient two-step approach. The
unique
morphology
structure
contribute
to
and the
enhanced SCs performance. The
CoMoO4@CoS
largest
specific
exhibits
a
capacitance
of
3380.3 F g1 (1 A g1). The ASC displays an outstanding energy density (59.2 W h kg1) and power density.
article info
abstract
Article history:
Engineering multicomponent active materials as an advanced electrode with the rational
Received 31 October 2019
designed core-shell structure is an effective way to enhance the electrochemical perfor-
Received in revised form
mances for supercapacitors. Herein, three-dimensional self-supported hierarchical
17 December 2019
CoMoO4@CoS core-shell heterostructures supported on reduced graphene oxide/Ni foam
Accepted 24 December 2019
have been rationally designed and prepared via a facile approach. The unique structure
Available online xxx
and the synergistic effects between two different materials, as well as excellent electronic conductivity of the reduced graphene oxide, contribute to the increased electrochemically
Keywords:
active site and enhanced capacitance. The core-shell CoMoO4@CoS composite displays the
Supercapacitor
superior specific capacitance of 3380.3 F g1 (1 A g1) in the three-electrode system and
Core-shell architecture
81.1% retention of the initial capacitance even after 6000 cycles. Moreover, an asymmetric
CoMoO4@CoS composite
device was successfully prepared using CoMoO4@CoS and activated carbon as positive/
Asymmetric supercapacitors
negative electrodes. It is worth mentioning that the device delivered the high energy
Energy density
density of 59.2 W h kg1 at the power density of 799.8 W kg1 and the excellent cycle
* Corresponding author. E-mail address: [email protected] (H. Xuan). https://doi.org/10.1016/j.ijhydene.2019.12.178 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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performance (about 91.5% capacitance retention over 6000 cycles). These results indicate that the core-shell CoMoO4@CoS composites offers the novelty strategy for preparation of electrodes for energy conversion and storage devices. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction There is an ever urgent need to find and develop suitable pollution-free energy substitutes to address the rapid depletion of fossil fuels and increasing environmental crisis in recent years [1]. Although convention new energy such as solar energy and wind energy have long been considered as promising alternatives, their flexibility and reliability are still far from satisfactory to meet the expected demand of realistic energy storage device [2,3]. As a result, the development of high performance energy storage device is crucial to ensure the consistent energy supply and allow greater penetration of renewable energy into practical applications [4]. Supercapacitors (SCs), as one of the energy storage devices, have received increasing attention for their desirable properties including remarkable energy density, outstanding cycling stability and reliable security compared with metal-air batteries, Li-ion batteries and other commercial batteries [5,6]. Meanwhile, with the characteristics of complementary to rechargeable batteries and conventional capacitors, SCs have been adopted in a lot of fields, for example the hybrid electric vehicles, short-term power sources, and consumer electronics [7]. According to the charge storage mechanisms and performances of electrode material, it can be generally categorized into electrical double-layer capacitors (EDLC) and pseudo-capacitors [8,9]. EDLCs are largely based upon carbonaceous materials, which have been extensive applied on the commercial areas because of its high conductivity, stable mechanical and chemical stability, and smaller maintenance costs [10]. Even so, the relatively low theoretical specific capacitance influences its further development in SCs energy storage system [11]. For the pseudo-capacitors, the energy storage is decided by reversible and redox reactions occurred on the electrode surface. In particular, pseudocapacitors possess higher specific capacitance than many traditional batteries due to the abundant redox valence states during the Faradaic reaction and electrochemical processes [8,12]. Unfortunately, the characteristic of the low conductivity and poor structure stability of electrode materials for pseudocapacitors is still a big problem which needs to be solved urgently. Consequently, the big challenge is further to explore and develop advanced SCs materials with satisfactory capacitance and cyclic ability to consistently improve the electrochemical performance of SCs and satisfy the increasing energy needs [13]. To date, various metal oxides ranging from RuO2, Fe2O3, MnO2 to Co3O4 have been investigated as typical electrodes in SCs studies [14]. However, these single metal oxides often suffer from a rapid decrease in specific capacitance for SCs [15]. Moreover, some drawbacks such as easy flaking off from the substrate and low electrical conductivity because of crystal expansion/ shrinkage in the charge/discharge cycling process, suggest that their electrochemical performances far below an ideal state [16].
Recently, numerous research works have concentrated on binary metal oxides, such as NiCo2O4, MnCo2O4, NiMoO4, CoMoO4, MgMoO4 and CaMoO4, owing to their multiple oxidation states and fascinating electrochemical properties for energy storage [17e21]. Among these various binary transition metal oxides, CoMoO4 is causing much attention on account of the mature production technology and great synergic effects. In the CoMoO4 material, Co element shows the rich oxidation valence and displays high capacitance, Mo element presents multiple oxidation states and exhibits high conductivity, and both elements can also be mutual catalysts to each other, resulting in high electrochemical performances for SCs [22]. In the earlier work, Long et al. reported the tunable fabrication of CoMoO4 nanosheets on carbon cloth substrates as advanced electrodes [23], and the capacitance of the composite was almost 1234 F g1, which is better than many other binary metal oxides. Despite CoMoO4 electrode material possesses such high electrochemical performance, it is still confronted with practical ability below theoretical worth and poor structural stability which results in structure collapse in the long cycling. An effective approach to overcome these shortfalls is to construct multi-dimensional hierarchical core-shell architectures with enhanced ions and electronic transfer kinetics as well as structural stability. Three-dimensional (3D) hierarchical composite exhibits outstanding structural advantages such as suitable ion transmission distance, ample surface active sites, and enlarged surface area. In recent work, Li et al. synthesized coreshell CoMoO4@Ni(OH)2 composite through the multistep high temperature hydrothermal method, which delivered the high capacitance of 1812 F g1 under the 2 mA cm2 as well as the great cyclic performance [24]. Wang et al. prepared heterostructured core-shell CoMoO4@MnO2 using the hydrothermal method and the composite displayed the specific capacitance of 1800 F g1 at 1 A g1 [25]. Zhang et al. reported CoMoO4@NiMoO4 core-shell nanosheet arrays with an enhanced specific capacitance of 1639.8 F g1 at 10 mA cm2 and a good cycling performance with a 95% retention rate [26]. Hence, it can find out that the electrochemical performance of the CoMoO4-based composites can be significantly improved by constructing core-shell structures. Recently, transition metal sulfides have caused extensive research as the SCs electrodes due to their unique advantages when compared with respective metal oxides [27,28]. Among these transition metal sulfides, cobalt sulfide (CoS) exhibits the wide range of applications as the efficient electrodes materials owing to its good conductivity, natural abundance and versatility. Thus, construction of CoMoO4@CoS core-shell nano-architecture with the reasonable design would be an effective approach to improve the SCs properties. Moreover, it is well known that the reduced graphene oxide (rGO) is an excellent platform for the load of electrode materials because of its larger specific surface area and excellent rate and surface functional properties [29]. Hence, it can be able to design the 3D hierarchical CoMoO4@CoS core-shell composites based
Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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on rGO to further improve the performance of electrode materials. Herein, a unique three-dimensional (3D) hierarchical coreshell CoMoO4@CoS composite supported on rGO/Ni foam (rGO/NF) is reasonable synthesized via a facile approach. CoMoO4 nanosheets arrays (core) were directly grown on rGO by a hydrothermal growth approach, then the shell of CoS nanoparticles was electrodeposited on the surface of CoMoO4 to form core-shell structure composites. The produced 3D hierarchical core-shell composite shows the ultrahigh specific capacitance of 3380.3 F g1 (1 A g1), as well as the high capacity retention of 81.1% through 6000 cycles at 10 A g1. Moreover, a CoMoO4@CoS//AC asymmetric device was assembled, which exhibited the high energy density and the pleasurable cycling stability.
Experiment Synthesis of CoMoO4 nanosheets on rGO/NF The Graphene oxide (GO) and rGO on Ni foam (rGO/NF) were prepared though the modified Hummers and chemical deposition method, which had been mentioned in our previous work [30]. For the preparation of CoMoO4 nanosheets, 1 mmol of Na2MoO4$2H2O and 1 mmol of Co(NO3)2$6H2O were dissolved into 60 mL deionized water with the constant magnetic stirring. Then the solution and the as-prepared rGO/NF substrate were transferred into a 100 mL Teflon autoclave and heated at 160 C for 5 h. Finally, the obtained samples was washed by deionized water and ethanol and dried at 60 C for 12 h. The mass loading of the CoMoO4 nanosheets on NF@rGO is about 1.0 mg cm2.
Electrodeposition of CoS on CoMoO4 nanosheets arrays The electrode of CoMoO4@CoS composites was synthesized by using the electrodeposition way to deposit the CoS shell on the CoMoO4 core composites. The electrodeposition processes were performed on a CHI 660E electrochemical analyzer using the three-electrode electrochemical system, where the CoMoO4, platinum plate and Ag/AgCl were served as the working electrode, counter electrode and reference electrode, respectively. Electrodeposition solution was consisted of 2 mmol Co(NO3)2$6H2O, 20 mmol thiourea and 60 mL deionized water. Among them, Co(NO3)2$6H2O and thiourea were the sources of Co and S elements, respectively. The whole electrochemical deposition was carried out with the voltage range of 1.2 Ve0.2 V under the scan rate of 5 mV s1 for several cycles to tune the deposited mass of CoS. After rinsed and dried, the core-shell CoMoO4@CoS composites coated on rGO/NF were obtained. The electrodeposition mass loading of CoS was about 0.5 mg cm2. For comparison, a bare CoS was synthesized on the rGO/NF using the same electrodeposition method. The mass loading of CoS on the rGO/NF was about 1.5 mg cm2.
Material characterizations The crystal structure of the synthesized composites was characterized via X-ray diffraction (XRD) with Cu-Ka radiation
3
(10 e80 ). X-ray photoelectron spectroscopy (XPS) was adopted to detect the composition and valence for the as-prepared electrodes. The microstructures and morphologies of the composites were obtained through transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Electrochemical performance tests were carried out by electrochemical workstation in 2 M KOH electrolyte with conventional three-electrode cell configuration. Here the CoMoO4@CoS composites, Hg/HgO electrode, and platinum plate were used as the working electrode, reference electrode, and counter electrodes, respectively. In addition, the specific capacitance (Cs) can be calculated through the following equation [31,32]: Cs ¼
I Dt m DV
(1)
in which the m (g), I (A), Dt (s), and DV (V) indicate the mass of CoMoO4@CoS composites, the discharge current, the discharge time and the potential drop during the discharging, respectively.
Fabrication of ASC device The asymmetric supercapacitor (ASC) was assembled by the CoMoO4@CoS and active carbon (AC) as the positive/negative electrodes. The electrochemical tests of the ASC device were performed under the 2 M KOH solution with a two-electrode cell. The negative electrode was made through grinding 80% AC and 10% polyvinylidene fluoride (PVDF), 10% acetylene black. Thus, the weight ratio of CoMoO4@CoS and AC electrodes is optimized according to the charge balance equation qþ ¼ q [33]. The mass ratio between the cathode and anode can be derived from the equation (2) [34]: Mþ C DV ¼ M Cþ DVþ
(2)
where M, C, DV represent the mass of active materials (AC, CoMoO4@CoS), specific capacitance, and potential window of all samples. Hence, the optimum mass rate of CoMoO4@CoS and AC was controlled to be ~0.16. The energy density (E) and power density (P) of the assembled ASC device can be determined from Eq. (3) and Eq. (4): 1 E ¼ Cd DV2 2 P¼
E Dt
(3)
(4)
where the Cd, DV, and Dt are the specific capacitance, working potential windows and discharge time of the ASC device, respectively.
Results and discussion Characterization of morphology and structure All morphologies and microstructures of active materials were observed via the SEM images (Fig. 1). Fig. 1a and b shows that the surface of rGO is uniformly covered with CoMoO4 nanosheets by a facile in situ hydrothermal reaction. The
Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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nanosheets of the CoMoO4 grow mostly perpendicular to the substrate and extend outwards to the radial direction. In the hydrothermal synthesis process, the Mo6þ and Co2þ react together to form the CoMoO4 nanoparticles which would be served as source of metal cation grafted on the substrate of NF/rGO. Afterward, the adjacent CoMoO4 particles gradually aggregate and grow along a certain crystal orientation, resulting in the cross-linked nanosheet structure on NF/rGO [35]. Fig. 1c and d shows that CoS ultrathin nanoflakes are attached on the surface of rGO in an amorphous flocculent shape, and a uniform and irregular network structure is formed on the surface of the NF/rGO. The SEM images of the as-prepared CoMoO4@CoS composite is illustrated in Fig. 1e and f. After the electrodeposition reaction, the surface of the CoMoO4 nanosheets were covered by the CoS ultrathin nanoflakes, forming the core-shell nanostructure. The magnified SEM image in Fig. 1f depicts that CoS interconnect with CoMoO4 nanosheets to generate the highly porous nanostructures, which can provide the much active sites and high conductivity and enhance the contact surface area during the Faraday reaction, benefiting to the performances of the SCs electrodes. The EDX mapping for the CoMoO4@CoS composite is provided in Fig. S2. The elemental mapping of the CoMoO4@CoS hybrid indicates the presence of the Co, Mo, O and S elements
and these elements are evenly distributed over the composite. The structure and morphology of the CoMoO4@CoS were further investigated through TEM and HRTEM images. CoMoO4@CoS and rGO was separated from the NF by the ultrasound machine and dispersed into a suspension in solution. A few drops were placed on an electron microscope copper wire covered with an ultrathin carbon film via the dropper. After drying, the sample used for TEM observation can be obtained. The TEM pictures of CoMoO4@CoS show that the composite is composed with nanosheets and flocculent structures (Fig. 2a and b). The flocculent substance is attached to the nanosheets, which is same as the SEM results (Fig. 1f), and the thickness of nanosheets and floccus are about 5 nm and 2 nm, respectively. Fig. 2c displays the representative HRTEM of the typical core-shell composite structure, where the core of CoMoO4 is surrounded with the ultrathin flocculent CoS shell. A selected-area electron diffraction pattern of the composite is presented in the inset image. The existences of the visible rings confirm the polycrystalline feature for the CoMoO4@CoS composite. In Fig. 2d, the distance of the adjacent lattice fringes are determined to be 0.16 and 0.27 nm, corresponding to the (061) and (022) planes of CoMoO4, while the interplanar distances of 0.29 nm and 0.19 nm are indexed to (100) and (102) planes of CoS, demonstrating that the coreshell composite is comprised of CoMoO4 and CoS composites.
Fig. 1 e SEM images of (a, b) CoMoO4, (c, d) CoS and (e, f) CoMoO4@CoS with different magnification. Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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Fig. 2 e (a and b) TEM and (c and d) HRTEM images of the as-prepared CoMoO4@CoS.
Moreover, the interconnections between the CoMoO4 and CoS can provide powerful synergistic effect and larger active surface area which would improve the electrochemical properties of the as-fabricated composites. The crystallographic nature of CoMoO4, CoS and CoMoO4@CoS are revealed through XRD and depicted in Fig. 3a. The strong peaks located in 44.6 , 51.9 and 76.3 are assigned to the Ni foam. The prominent peaks at 2q ¼ 23.3 , 26.5 , 33.7 , 40.2 , 47.0 and 60.4 are corresponding to the (021), (002), (222), (003), (241) and (424) planes of the cubic CoMoO4 (JCPDS No. 21-0868), respectively [36]. Similarly, the as-prepared CoS sample exhibits diffraction peaks at 29.8 , 47.5 and 62.7 , corresponding to the (100), (102) and (103) planes of CoS (JCPDS No. 01-1279), indicating that it is successfully prepared by the electrodeposition method. For the pattern of CoMoO4@CoS composite, the diffraction peaks correspond well to the standard pattern of CoMoO4 (JCPDS No. 21-0868), suggesting the successful synthesis of the CoMoO4 core. However, no strong peaks for the CoS can be obtained from the XRD of CoMoO4@CoS composite, which is probably due to the relative small quality and the weak crystallize of CoS [27,37]. The diffraction peaks become weaker and broader width after electrodeposition progress which would be due to the lower crystallinity of the core-shell composite. Furthermore, the existence of the CoS shell material can be further confirmed via the following XPS results.
The XPS test was measured to obtain the valence state and composition information of CoMoO4@CoS composite. The survey spectrum of the composite is illustrated in Fig. 3b. The peaks situated at 780 eV, 230 eV, 531 eV, 162 eV and 285 eV demonstrates the presence of Co 2p, Mo 3d, C 1s, S 2p and O 1s, which is consistent with the results of EDS. The spectrum of Co 2p in Fig. 3c displays two strong peaks in 780.8 and 796.9 eV, corresponding to Co 2p3/2 and Co 2p1/2, respectively, indicating the Co2þ oxidation state. Meanwhile, the main peak at 780.8 eV with the lower binding energy and a shake-up feature in the high binding energy are obtained, corresponding to the feature of Co2þ in CoS [37e39]. In Fig. 3d, two peaks located at 234.2 and 231.1eV are part of Mo 3d3/2 and Mo 3d5/2, respectively, the width distance of 3.1 eV between the two peaks further demonstrates the presence of Mo6þ oxidation state [40,41]. Determined through the valence state, the cobalt and molybdenum exist as a compound of CoMoO4 due to that it can be generated by the facile non-redox process of MoO2 4 and Co2þ, which is in accordance with a analysis from XRD pattern [23,42]. The O 1s XPS spectrum is consist of two obviously peaks (Fig. 3e). The peak at 530 eV represents the metal-oxygen bonds, and the peak at 531.3 eV suggests the presence of low coordination oxygen ions at the surface [43]. Furthermore, Fig. 3f depicts a spectrum of S 2p, in which characteristic doublets at 162.7 and 164.1 eV correspond to S 2p1/2 and S 2p3/2, respectively, suggesting that most of the S
Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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elements exist in the form of S2 [44]. The observation of a peak at 170.8 eV would be related to sulfur oxides caused by some surface oxidation [45]. The XPS spectrum demonstrates clearly that CoS was successfully grown on the surface of CoMoO4 nanosheets by the simple electrodeposition process, which is consistent with the XRD analysis.
Co3O4 þ H2O þ OH 4 3CoOOH þ e
(6)
CoOOH þ OH 4 CoO2 þ H2O þ e
(7)
CoS þ OH 4 CoSOH þ e
(8)
CoSOH þ OH 4 CoSO þ H2O þ e
(9)
Electrochemical performances In order to investigate the advantages of the designed coreshell structure for energy storage applications, a series of electrochemical tests were performed with the prepared composites as working electrodes. The typical cyclic voltammetry (CV) curves of the CoMoO4, CoS and CoMoO4@CoS samples were collected under the scan rate of 10 mV s1 (Fig. 4a). Visibly, all the CV curves show the couple welldefined redox peaks, indicating a pseudocapacitance feature of all composites. The CoMoO4@CoS sample possesses higher redox peak and a larger CV area than that of CoMoO4 and CoS, which indicates the higher charge storage capacity of the composite. This result proves that the deposited CoS shell material can further improve the electrochemical activity of CoMoO4, which can be due to the synergic effect of CoS and CoMoO4. Fig. 4b depicts CV curves of the core-shell CoMoO4@CoS composite tested from 0 V to 0.7 V with various scan rates (5, 10, 20, 30 and 50 mV s1). Each CV curve has a couple of well-defined redox peak, which illustrates that a Faradaic capacitive behavior is the major factor for the capacitive characteristics. The faradaic redox reactions of the core-shell CoMoO4@CoS can be described as [46]: 3Co(OH)3 4 Co3O4 þ 4H2O þ OH þ 2e
(5)
Specifically, the obtained capacitance of the composite originates from the quasi-reversible electron transfer kinetics, which is primarily attributed to the reversible Co2þ/Co3þ redox reactions [47,48]. As a transit ion metal element Mo, it does not directly participate in the redox reaction, but improves the electrochemical performance for the samples by increasing the conductivity of the composite [49]. It is found that the oxidation and reduction peaks of the composite shift to the right and left with the scan rate increases, respectively, which would be mainly caused by the increased internal diffusion resistance. The electrochemical reaction kinetics can be qualitatively analyzed according to the relationship between the sweep rates (v) and electrochemical response currents (i) from the CV curves [50]: i ¼ avb
(10)
In which a and b are both volatile variables. The value of the b can be determined by the slop of the log (i) versus log (v) plot and varies from 0.5 to 1.0. It is well known that the b value in the diffusion-controlled process is close to 0.5, while b value approaches 1.0 in the surface capacitance dominated process. A measured value of b is 0.57 for anodic peaks of CoMoO4@CoS electrode. In addition, a linear relationship between i and v1/2 is shown in Fig. S3, proving that diffusion-controlled reaction
Fig. 3 e (a) XRD patterns of the different samples, (b) XPS survey, (c) Co 2p, (d) Mo 3d, (e) O 1s, and (f) S 2p for the CoMoO4@CoS electrode. Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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Fig. 4 e (a) Comparative CV curves of the as-obtained samples at 30 mV s¡1. (b) CV curves of CoMoO4@CoS at various scan rates. (c) Comparison of GCD curves of the as-obtained samples at 1 A g¡1. (d) GCD curves of CoMoO4@CoS at different current densities. (e) Specific capacitance of different electrodes at different current densities. (f) EIS Nyquist plots.
is the dominated storage mechanism of the CoMoO4@CoS composite. A comparison of galvanostatic charge-discharge (GCD) curves of the CoMoO4, CoS and CoMoO4@CoS electrodes is displayed in Fig. 4c at the same current density of 1 A g1. The core-shell CoMoO4@CoS composite (1676 s) shows the longest discharge time among all the electrodes at the current density of 1 A g1, meaning that it could store more charges and exhibit a biggest specific capacitance value. According to the equation (1), a maximum specific capacitance up to 3380.3 F g1 is achieved for CoMoO4@CoS electrode at the current density of 1 A g1, which is not only higher than that of the CoMoO4 (1100.4 F g1) and CoS (1896.2 F g1) electrodes but also better than those of recently reported similar composites applied for SCs applications (Table S1). Fig. 4d depicts GCD curves of the CoMoO4@CoS electrode under various current
densities with the potential window of 0e0.5 V. With the increase of current density, GCD curves are symmetric and do not show a graphic deviation, demonstrating the reversible electrochemical characteristics and excellent rate capability. The obvious discharge platforms further indicate the pseudocapacitive characteristic of the CoMoO4@CoS electrode, which is the same as CV analysis. The relationships between the current density and specific capacitance of CoMoO4, CoS and CoMoO4@CoS are presented in Fig. 4e. The CoMoO4@CoS electrode exhibits the specific capacitance values of 3380.3, 3312.2, 3250.1, 3107.4, 2981.8 and 2820.2 F g1 at 1, 2, 3, 5, 7 and 10 A g1, respectively, which is superior than CoMoO4 and CoS electrodes at the same current density. In addition, the CoMoO4@CoS electrode shows a good rate capability of about 83.4% with the current density increasing from 1 to 10 A g1,
Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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which is obviously enhanced compared with that of CoMoO4 (68.9%) and CoS (77.8%), indicating the excellent rate capability for energy storage devices. The Nyquist plots of electrochemical impedance spectroscopy (EIS) for CoS, CoMoO4 and CoMoO4@CoS samples were measured between 0.01 Hz and 100 kHz, as shown in Fig. 4f. It is obvious that the shape of the impedance spectra is basically the same, composing of semicircles in a high frequency section and sloped lines in the low frequency area. The sloped lines represent Warburg impedance (Zw) for composites, reflecting the electrolyte diffusion efficiency. The CoMoO4@CoS composite has larger slopes of straight line than those of CoMoO4 and CoS electrodes, certificating the minimal diffusion resistance of electrolyte in the core-shell structure composite. The charge transfer resistance (Rct) can be obtained through the diameter of the semicircular in the Nyquist plot at the high frequency region, which mainly derives from the electron transfer between the electrolyte and electrode. The Rct value of the CoMoO4@CoS electrode is about 0.19 U, which is significantly lower than that of the CoMoO4 (0.54 U) and CoS (0.72 U) electrodes, which means that the CoMoO4@CoS electrode possesses the highest electrical conductivity. The equivalent series resistance (Rs), which originates from the intrinsic resistance and ionic resistance of the composites, is determined from the intersection with the horizontal axis. The values of Rs are 0.75, 1.10, and 0.72 U for CoMoO4, CoS and CoMoO4@CoS samples, respectively. The small impedance value of CoMoO4@CoS electrode allows more electrode materials and electrolyte ions available for the electrochemical reaction process, resulting in the improved capacitance. The cyclic performance is a significant factor in evaluating the performance of electrodes for SCs. The CoMoO4, CoS and CoMoO4@CoS composites were tested through 6000 continuous GCD cycles at the current density of 10 A g1 (Fig. 5). The specific capacitance of the core-shell CoMoO4@CoS electrode shows outstanding cycle life (81.1% of the initial value) and retains the value of 2287.4 F g1 through 6000 cycles. The retention rate is higher than that of CoMoO4 (70.7%) and CoS (60.9%) electrodes, indicating an improved cycling stability after compounding with CoS. As analyzed above, the CoMoO4@CoS electrode has an exceptional SCs performance compared to the pure CoMoO4 and CoS electrodes. The remarkable SCs performances would be originated from the following points. Firstly, the composite grown directly on NF/rGO could effectively reduce the “dead surface” which is generally produced in traditional slurrycoating. At the same time, the rGO, with a characteristic of ideal chemical and mechanical stability, as a robust current collector, would significantly improve the electrochemical performance of the composites by supplying more ion/electron paths for insertion/extraction as well as protecting the composites from corrosion by the electrolyte during the electronic reaction processes [51]. Secondly, the core-shell nanostructure of the CoMoO4@CoS plays an important role in promoting the redox reactions by expanding the space for transmitting ions from the electrolyte, which could not only increase the utilization of electrode materials by facilitating the contact between electrode materials with electrolyte but also reduce the volume change during long-term cycling.
Fig. 5 e Comparison of cycling performance at a constant current density of 10 A g¡1.
Thirdly, CoS is directly grown on CoMoO4 by electrochemical deposition, which tightly combination of CoMoO4 and CoS could take much advantage of multifunctional and synergistic effects and show better electrochemical behavior. Lastly, the smaller EIS Rct, and Zw indicate that the electrolyte and electroactive material generate very low electrical resistance during the high-speed electrochemical processes, resulting in the faster reaction kinetics and higher conductivity. Overall, the core-shell CoMoO4@CoS nanostructure with these advantages possesses better electrochemical performance than pure CoMoO4 and CoS electrodes, making it more suitable in practical applications. The effects of electrodeposition quality of CoS on the electrochemical performance for CoMoO4@CoS composites were also studied. The deposited mass of CoS was changed by varying the CV cycles during the electrodeposition progress. The electrochemical performances for all samples are shown in Fig. S1, and the detailed analysis are supplied in supplementary materials. Obviously, the specific capacitance of these CoMoO4@CoS composites increases first and then decreases with the increase mass of CoS. The core-shell CoMoO4@CoS electrodes with the CoS mass of 0.5 mg cm2 shows clearly superior electrochemical properties compared with other samples, confirming that 0.5 mg cm2 is the optimum mass in these composites. Further increase the mass loading of CoS, the CoMoO4 nanosheets would be covered by a thick layer of the CoS shell, leading to the difficulties in electron transfer and mass transport. Therefore, the CoMoO4@CoS composite with the CoS mass of 0.5 mg cm2 was selected for the systematic study.
Preparation of the asymmetric supercapacitor An asymmetric supercapacitor (ASC) was assembled by employing CoMoO4@CoS and AC as positive and negative electrodes. According to charge balance theory [52], the weight ratio of CoMoO4@CoS and AC is calculated to be 8:1. Thus the mass loading of AC is about 8.8 mg cm2. The CV curves of the CoMoO4@CoS composite (0e0.7 V) and AC (-1 - 0 V) measured
Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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under a three electrode system at 10 mV s1 are employed to confirm the operating potential window (Fig. 6a). Fig. 6b shows the CV curves of the CoMoO4@CoS//AC with various potential windows at 10 mV s1. As can be seen, the shape of the CV curves remains stable without obvious polarization when the operating voltage window of the CoMoO4@CoS//AC is increased to 1.7 V, indicating that the optimum operating potential window for the as-fabricated ASC is 0e1.7 V. The CV curves of CoMoO4@CoS//AC with different scan rates demonstrate that the capacitance is contributed via both pseudo-capacitance and EDLC features (Fig. 6c). The CV curves exhibit no obvious change with the scan rate increasing,
9
suggesting the excellent rate ability of the device. Fig. 6d displays the GCD curves of the ASC at various current densities. The specific capacitance of the ASC was obtained by the GCD curves and the equation (1), in which m represents the weight of positive and negative electrodes. The specific capacitance of the ASC is found to be 189.5, 166.6, 151.2, 144.9, 136.5, 129.3, 119.4 and 108.7 F g1 under the current densities of 0.5, 1, 2, 3, 4, 5, 7 and 10 A g1, respectively (Fig. 6e). The decrease of the capacitance at high current densities would be due to the active materials cannot fully participate in the electrochemical redox reaction [35,53]. The cyclic performance of the CoMoO4@CoS//AC measured at the 2 A g1 is presented in
Fig. 6 e (a) CV curves comparison of AC and CoMoO4@CoS at a scan rate of 10 mV s¡1 in a three-electrode system. (bec) CV curves of the CoMoO4@CoS//AC ASC at different operation voltages and scan rates. (dee) GCD curves and specific capacitance of the CoMoO4@CoS//AC ACS at different current densities and (f) the cycling performance of the ACS after the 6000th cycles at 3 A g¡1. Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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Fig. 7 e Ragone plot related to energy and power densities vs discharge time of the assembled CoMoO4@CoS//AC ASC.
substrate via a facile approach. The tightly contact between the two promising pseudo-capacitance materials of CoMoO4 and CoS can take much advantage of synergistic and multifunctional effects. The CoMoO4@CoS electrode displays the excellent SCs performances, involving the excellent specific capacitance of as high as 3380.3 F g1 (1 A g1) and good capacitance retention about 81.1% after 6000 cycles. In addition, the CoMoO4@CoS//AC device delivers large specific energy density of 59.2 W h kg1 under the power density of 799.8 W kg1 and the high cycle performance (the capacitance loss of only around 8.5% even after 6000 cycles). The outstanding electrochemical properties of the core-shell CoMoO4@CoS composite indicate that the method of combining CoMoO4 with promising CoS supplies an effective approach to prepare coreshell composites and can be applied in other energyrelated fields.
Table 1 e Comparison of the as-assembled ASCs with those reported in the literatures. Positive electrode //negative electrode
Electrolyte
Energy density (W h kg1)
Power density (W kg1)
Cycle ability
Ref.
Ni3S2@CoS//AC CoMoO4/Co1-xS//AC MnCo2O4@CoS//AC CoMoO4//AC Cu2O/CuMoO4//AC NiMoO4@CoMoO4//AC CoMoO4@CoS//AC
2 M KOH 3 M KOH PVA/KOH 2 M KOH 2 M KOH PVA/KOH 2 M KOH
28.3 39.8 55.1 46.7 75.1 49.3 59.2
134.4 804.5 477.3 800 420 630 799.8
95.5%//2000 86.4%//4000 97.3%//6000 94.0%//3000 86.6%//3000 92.8%//5000 91.5%//6000
[37] [53] [56] [58] [59] [60] This work
Fig. 6f. It can be observed that the specific capacitance of the CoMoO4@CoS//AC device is reduced very slowly and still remains 91.5% of its initial value through 6000 charge and discharge cycles, suggesting the excellent long-term cycling performance of the ASC device. In addition, the P and E of the CoMoO4@CoS//AC devices are calculated according to formulas (2) and (3), as presented in Fig. 7. The ASC device provides the largest energy density of 59.2 W h kg1 under the power density of 799.8 W kg1 with the corresponding discharge time of 266.6 s, and even keeps 38.6 W h kg1 at the high power density of 7996.3 W kg1. The values of the device are better than many similar ASC devices, such as CoMoO4/Co1-xS//AC (39.8 W h kg1 at 804.5 W kg1) [53], CoMoO4/Co9S8//AC (42 W h kg1 at 181.13 W kg1) [54], CoMoO4 @Co3O4//CNTs (45.9 W h kg1 at 1647 W kg1) [55], MnCo2O4@CoS//AC (55.1 W h kg1 at 477.3 W kg1) [56] and NiCo2S4/Co9S8@AC (49.6 W h kg1 at 123 W kg1) [57]. The comparison of the electrochemical performance of the ASC device is listed under Table 1. The above attractive results demonstrate that the as-obtained CoMoO4@CoS composites possess outstanding electrochemical performances, making it become a new type of energy storage material with great research prospects in practical applications.
Conclusions In summary, the hierarchical core-shell CoMoO4@CoS composites have been successfully synthesized on rGO/NF
Acknowledgements This work is supported by the Natural Science Foundation of Shanxi Province, China (Grant No. 201801D121100), the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi (OIT), the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (STIP) (Grant No. 201802033), and the Collaborative Innovation Center for Shanxi Advanced Permanent Magnetic Materials and Technology (2016-06).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.178.
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Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
Available online at www.sciencedirect.com
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Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance Haicheng Xuan*, Hongsheng Li, Jing Yang, Xiaohong Liang, Zhigao Xie, Peide Han, Yucheng Wu College of Materials Science and Engineering, Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, People’s Republic of China
highlights
graphical abstract
A core-shell CoMoO4@CoS composite was prepared by an efficient two-step approach. The
unique
morphology
structure
contribute
to
and the
enhanced SCs performance. The
CoMoO4@CoS
largest
specific
exhibits
a
capacitance
of
3380.3 F g1 (1 A g1). The ASC displays an outstanding energy density (59.2 W h kg1) and power density.
article info
abstract
Article history:
Engineering multicomponent active materials as an advanced electrode with the rational
Received 31 October 2019
designed core-shell structure is an effective way to enhance the electrochemical perfor-
Received in revised form
mances for supercapacitors. Herein, three-dimensional self-supported hierarchical
17 December 2019
CoMoO4@CoS core-shell heterostructures supported on reduced graphene oxide/Ni foam
Accepted 24 December 2019
have been rationally designed and prepared via a facile approach. The unique structure
Available online xxx
and the synergistic effects between two different materials, as well as excellent electronic conductivity of the reduced graphene oxide, contribute to the increased electrochemically
Keywords:
active site and enhanced capacitance. The core-shell CoMoO4@CoS composite displays the
Supercapacitor
superior specific capacitance of 3380.3 F g1 (1 A g1) in the three-electrode system and
Core-shell architecture
81.1% retention of the initial capacitance even after 6000 cycles. Moreover, an asymmetric
CoMoO4@CoS composite
device was successfully prepared using CoMoO4@CoS and activated carbon as positive/
Asymmetric supercapacitors
negative electrodes. It is worth mentioning that the device delivered the high energy
Energy density
density of 59.2 W h kg1 at the power density of 799.8 W kg1 and the excellent cycle
* Corresponding author. E-mail address: [email protected] (H. Xuan). https://doi.org/10.1016/j.ijhydene.2019.12.178 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
2
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performance (about 91.5% capacitance retention over 6000 cycles). These results indicate that the core-shell CoMoO4@CoS composites offers the novelty strategy for preparation of electrodes for energy conversion and storage devices. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction There is an ever urgent need to find and develop suitable pollution-free energy substitutes to address the rapid depletion of fossil fuels and increasing environmental crisis in recent years [1]. Although convention new energy such as solar energy and wind energy have long been considered as promising alternatives, their flexibility and reliability are still far from satisfactory to meet the expected demand of realistic energy storage device [2,3]. As a result, the development of high performance energy storage device is crucial to ensure the consistent energy supply and allow greater penetration of renewable energy into practical applications [4]. Supercapacitors (SCs), as one of the energy storage devices, have received increasing attention for their desirable properties including remarkable energy density, outstanding cycling stability and reliable security compared with metal-air batteries, Li-ion batteries and other commercial batteries [5,6]. Meanwhile, with the characteristics of complementary to rechargeable batteries and conventional capacitors, SCs have been adopted in a lot of fields, for example the hybrid electric vehicles, short-term power sources, and consumer electronics [7]. According to the charge storage mechanisms and performances of electrode material, it can be generally categorized into electrical double-layer capacitors (EDLC) and pseudo-capacitors [8,9]. EDLCs are largely based upon carbonaceous materials, which have been extensive applied on the commercial areas because of its high conductivity, stable mechanical and chemical stability, and smaller maintenance costs [10]. Even so, the relatively low theoretical specific capacitance influences its further development in SCs energy storage system [11]. For the pseudo-capacitors, the energy storage is decided by reversible and redox reactions occurred on the electrode surface. In particular, pseudocapacitors possess higher specific capacitance than many traditional batteries due to the abundant redox valence states during the Faradaic reaction and electrochemical processes [8,12]. Unfortunately, the characteristic of the low conductivity and poor structure stability of electrode materials for pseudocapacitors is still a big problem which needs to be solved urgently. Consequently, the big challenge is further to explore and develop advanced SCs materials with satisfactory capacitance and cyclic ability to consistently improve the electrochemical performance of SCs and satisfy the increasing energy needs [13]. To date, various metal oxides ranging from RuO2, Fe2O3, MnO2 to Co3O4 have been investigated as typical electrodes in SCs studies [14]. However, these single metal oxides often suffer from a rapid decrease in specific capacitance for SCs [15]. Moreover, some drawbacks such as easy flaking off from the substrate and low electrical conductivity because of crystal expansion/ shrinkage in the charge/discharge cycling process, suggest that their electrochemical performances far below an ideal state [16].
Recently, numerous research works have concentrated on binary metal oxides, such as NiCo2O4, MnCo2O4, NiMoO4, CoMoO4, MgMoO4 and CaMoO4, owing to their multiple oxidation states and fascinating electrochemical properties for energy storage [17e21]. Among these various binary transition metal oxides, CoMoO4 is causing much attention on account of the mature production technology and great synergic effects. In the CoMoO4 material, Co element shows the rich oxidation valence and displays high capacitance, Mo element presents multiple oxidation states and exhibits high conductivity, and both elements can also be mutual catalysts to each other, resulting in high electrochemical performances for SCs [22]. In the earlier work, Long et al. reported the tunable fabrication of CoMoO4 nanosheets on carbon cloth substrates as advanced electrodes [23], and the capacitance of the composite was almost 1234 F g1, which is better than many other binary metal oxides. Despite CoMoO4 electrode material possesses such high electrochemical performance, it is still confronted with practical ability below theoretical worth and poor structural stability which results in structure collapse in the long cycling. An effective approach to overcome these shortfalls is to construct multi-dimensional hierarchical core-shell architectures with enhanced ions and electronic transfer kinetics as well as structural stability. Three-dimensional (3D) hierarchical composite exhibits outstanding structural advantages such as suitable ion transmission distance, ample surface active sites, and enlarged surface area. In recent work, Li et al. synthesized coreshell CoMoO4@Ni(OH)2 composite through the multistep high temperature hydrothermal method, which delivered the high capacitance of 1812 F g1 under the 2 mA cm2 as well as the great cyclic performance [24]. Wang et al. prepared heterostructured core-shell CoMoO4@MnO2 using the hydrothermal method and the composite displayed the specific capacitance of 1800 F g1 at 1 A g1 [25]. Zhang et al. reported CoMoO4@NiMoO4 core-shell nanosheet arrays with an enhanced specific capacitance of 1639.8 F g1 at 10 mA cm2 and a good cycling performance with a 95% retention rate [26]. Hence, it can find out that the electrochemical performance of the CoMoO4-based composites can be significantly improved by constructing core-shell structures. Recently, transition metal sulfides have caused extensive research as the SCs electrodes due to their unique advantages when compared with respective metal oxides [27,28]. Among these transition metal sulfides, cobalt sulfide (CoS) exhibits the wide range of applications as the efficient electrodes materials owing to its good conductivity, natural abundance and versatility. Thus, construction of CoMoO4@CoS core-shell nano-architecture with the reasonable design would be an effective approach to improve the SCs properties. Moreover, it is well known that the reduced graphene oxide (rGO) is an excellent platform for the load of electrode materials because of its larger specific surface area and excellent rate and surface functional properties [29]. Hence, it can be able to design the 3D hierarchical CoMoO4@CoS core-shell composites based
Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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on rGO to further improve the performance of electrode materials. Herein, a unique three-dimensional (3D) hierarchical coreshell CoMoO4@CoS composite supported on rGO/Ni foam (rGO/NF) is reasonable synthesized via a facile approach. CoMoO4 nanosheets arrays (core) were directly grown on rGO by a hydrothermal growth approach, then the shell of CoS nanoparticles was electrodeposited on the surface of CoMoO4 to form core-shell structure composites. The produced 3D hierarchical core-shell composite shows the ultrahigh specific capacitance of 3380.3 F g1 (1 A g1), as well as the high capacity retention of 81.1% through 6000 cycles at 10 A g1. Moreover, a CoMoO4@CoS//AC asymmetric device was assembled, which exhibited the high energy density and the pleasurable cycling stability.
Experiment Synthesis of CoMoO4 nanosheets on rGO/NF The Graphene oxide (GO) and rGO on Ni foam (rGO/NF) were prepared though the modified Hummers and chemical deposition method, which had been mentioned in our previous work [30]. For the preparation of CoMoO4 nanosheets, 1 mmol of Na2MoO4$2H2O and 1 mmol of Co(NO3)2$6H2O were dissolved into 60 mL deionized water with the constant magnetic stirring. Then the solution and the as-prepared rGO/NF substrate were transferred into a 100 mL Teflon autoclave and heated at 160 C for 5 h. Finally, the obtained samples was washed by deionized water and ethanol and dried at 60 C for 12 h. The mass loading of the CoMoO4 nanosheets on NF@rGO is about 1.0 mg cm2.
Electrodeposition of CoS on CoMoO4 nanosheets arrays The electrode of CoMoO4@CoS composites was synthesized by using the electrodeposition way to deposit the CoS shell on the CoMoO4 core composites. The electrodeposition processes were performed on a CHI 660E electrochemical analyzer using the three-electrode electrochemical system, where the CoMoO4, platinum plate and Ag/AgCl were served as the working electrode, counter electrode and reference electrode, respectively. Electrodeposition solution was consisted of 2 mmol Co(NO3)2$6H2O, 20 mmol thiourea and 60 mL deionized water. Among them, Co(NO3)2$6H2O and thiourea were the sources of Co and S elements, respectively. The whole electrochemical deposition was carried out with the voltage range of 1.2 Ve0.2 V under the scan rate of 5 mV s1 for several cycles to tune the deposited mass of CoS. After rinsed and dried, the core-shell CoMoO4@CoS composites coated on rGO/NF were obtained. The electrodeposition mass loading of CoS was about 0.5 mg cm2. For comparison, a bare CoS was synthesized on the rGO/NF using the same electrodeposition method. The mass loading of CoS on the rGO/NF was about 1.5 mg cm2.
Material characterizations The crystal structure of the synthesized composites was characterized via X-ray diffraction (XRD) with Cu-Ka radiation
3
(10 e80 ). X-ray photoelectron spectroscopy (XPS) was adopted to detect the composition and valence for the as-prepared electrodes. The microstructures and morphologies of the composites were obtained through transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Electrochemical performance tests were carried out by electrochemical workstation in 2 M KOH electrolyte with conventional three-electrode cell configuration. Here the CoMoO4@CoS composites, Hg/HgO electrode, and platinum plate were used as the working electrode, reference electrode, and counter electrodes, respectively. In addition, the specific capacitance (Cs) can be calculated through the following equation [31,32]: Cs ¼
I Dt m DV
(1)
in which the m (g), I (A), Dt (s), and DV (V) indicate the mass of CoMoO4@CoS composites, the discharge current, the discharge time and the potential drop during the discharging, respectively.
Fabrication of ASC device The asymmetric supercapacitor (ASC) was assembled by the CoMoO4@CoS and active carbon (AC) as the positive/negative electrodes. The electrochemical tests of the ASC device were performed under the 2 M KOH solution with a two-electrode cell. The negative electrode was made through grinding 80% AC and 10% polyvinylidene fluoride (PVDF), 10% acetylene black. Thus, the weight ratio of CoMoO4@CoS and AC electrodes is optimized according to the charge balance equation qþ ¼ q [33]. The mass ratio between the cathode and anode can be derived from the equation (2) [34]: Mþ C DV ¼ M Cþ DVþ
(2)
where M, C, DV represent the mass of active materials (AC, CoMoO4@CoS), specific capacitance, and potential window of all samples. Hence, the optimum mass rate of CoMoO4@CoS and AC was controlled to be ~0.16. The energy density (E) and power density (P) of the assembled ASC device can be determined from Eq. (3) and Eq. (4): 1 E ¼ Cd DV2 2 P¼
E Dt
(3)
(4)
where the Cd, DV, and Dt are the specific capacitance, working potential windows and discharge time of the ASC device, respectively.
Results and discussion Characterization of morphology and structure All morphologies and microstructures of active materials were observed via the SEM images (Fig. 1). Fig. 1a and b shows that the surface of rGO is uniformly covered with CoMoO4 nanosheets by a facile in situ hydrothermal reaction. The
Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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nanosheets of the CoMoO4 grow mostly perpendicular to the substrate and extend outwards to the radial direction. In the hydrothermal synthesis process, the Mo6þ and Co2þ react together to form the CoMoO4 nanoparticles which would be served as source of metal cation grafted on the substrate of NF/rGO. Afterward, the adjacent CoMoO4 particles gradually aggregate and grow along a certain crystal orientation, resulting in the cross-linked nanosheet structure on NF/rGO [35]. Fig. 1c and d shows that CoS ultrathin nanoflakes are attached on the surface of rGO in an amorphous flocculent shape, and a uniform and irregular network structure is formed on the surface of the NF/rGO. The SEM images of the as-prepared CoMoO4@CoS composite is illustrated in Fig. 1e and f. After the electrodeposition reaction, the surface of the CoMoO4 nanosheets were covered by the CoS ultrathin nanoflakes, forming the core-shell nanostructure. The magnified SEM image in Fig. 1f depicts that CoS interconnect with CoMoO4 nanosheets to generate the highly porous nanostructures, which can provide the much active sites and high conductivity and enhance the contact surface area during the Faraday reaction, benefiting to the performances of the SCs electrodes. The EDX mapping for the CoMoO4@CoS composite is provided in Fig. S2. The elemental mapping of the CoMoO4@CoS hybrid indicates the presence of the Co, Mo, O and S elements
and these elements are evenly distributed over the composite. The structure and morphology of the CoMoO4@CoS were further investigated through TEM and HRTEM images. CoMoO4@CoS and rGO was separated from the NF by the ultrasound machine and dispersed into a suspension in solution. A few drops were placed on an electron microscope copper wire covered with an ultrathin carbon film via the dropper. After drying, the sample used for TEM observation can be obtained. The TEM pictures of CoMoO4@CoS show that the composite is composed with nanosheets and flocculent structures (Fig. 2a and b). The flocculent substance is attached to the nanosheets, which is same as the SEM results (Fig. 1f), and the thickness of nanosheets and floccus are about 5 nm and 2 nm, respectively. Fig. 2c displays the representative HRTEM of the typical core-shell composite structure, where the core of CoMoO4 is surrounded with the ultrathin flocculent CoS shell. A selected-area electron diffraction pattern of the composite is presented in the inset image. The existences of the visible rings confirm the polycrystalline feature for the CoMoO4@CoS composite. In Fig. 2d, the distance of the adjacent lattice fringes are determined to be 0.16 and 0.27 nm, corresponding to the (061) and (022) planes of CoMoO4, while the interplanar distances of 0.29 nm and 0.19 nm are indexed to (100) and (102) planes of CoS, demonstrating that the coreshell composite is comprised of CoMoO4 and CoS composites.
Fig. 1 e SEM images of (a, b) CoMoO4, (c, d) CoS and (e, f) CoMoO4@CoS with different magnification. Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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Fig. 2 e (a and b) TEM and (c and d) HRTEM images of the as-prepared CoMoO4@CoS.
Moreover, the interconnections between the CoMoO4 and CoS can provide powerful synergistic effect and larger active surface area which would improve the electrochemical properties of the as-fabricated composites. The crystallographic nature of CoMoO4, CoS and CoMoO4@CoS are revealed through XRD and depicted in Fig. 3a. The strong peaks located in 44.6 , 51.9 and 76.3 are assigned to the Ni foam. The prominent peaks at 2q ¼ 23.3 , 26.5 , 33.7 , 40.2 , 47.0 and 60.4 are corresponding to the (021), (002), (222), (003), (241) and (424) planes of the cubic CoMoO4 (JCPDS No. 21-0868), respectively [36]. Similarly, the as-prepared CoS sample exhibits diffraction peaks at 29.8 , 47.5 and 62.7 , corresponding to the (100), (102) and (103) planes of CoS (JCPDS No. 01-1279), indicating that it is successfully prepared by the electrodeposition method. For the pattern of CoMoO4@CoS composite, the diffraction peaks correspond well to the standard pattern of CoMoO4 (JCPDS No. 21-0868), suggesting the successful synthesis of the CoMoO4 core. However, no strong peaks for the CoS can be obtained from the XRD of CoMoO4@CoS composite, which is probably due to the relative small quality and the weak crystallize of CoS [27,37]. The diffraction peaks become weaker and broader width after electrodeposition progress which would be due to the lower crystallinity of the core-shell composite. Furthermore, the existence of the CoS shell material can be further confirmed via the following XPS results.
The XPS test was measured to obtain the valence state and composition information of CoMoO4@CoS composite. The survey spectrum of the composite is illustrated in Fig. 3b. The peaks situated at 780 eV, 230 eV, 531 eV, 162 eV and 285 eV demonstrates the presence of Co 2p, Mo 3d, C 1s, S 2p and O 1s, which is consistent with the results of EDS. The spectrum of Co 2p in Fig. 3c displays two strong peaks in 780.8 and 796.9 eV, corresponding to Co 2p3/2 and Co 2p1/2, respectively, indicating the Co2þ oxidation state. Meanwhile, the main peak at 780.8 eV with the lower binding energy and a shake-up feature in the high binding energy are obtained, corresponding to the feature of Co2þ in CoS [37e39]. In Fig. 3d, two peaks located at 234.2 and 231.1eV are part of Mo 3d3/2 and Mo 3d5/2, respectively, the width distance of 3.1 eV between the two peaks further demonstrates the presence of Mo6þ oxidation state [40,41]. Determined through the valence state, the cobalt and molybdenum exist as a compound of CoMoO4 due to that it can be generated by the facile non-redox process of MoO2 4 and Co2þ, which is in accordance with a analysis from XRD pattern [23,42]. The O 1s XPS spectrum is consist of two obviously peaks (Fig. 3e). The peak at 530 eV represents the metal-oxygen bonds, and the peak at 531.3 eV suggests the presence of low coordination oxygen ions at the surface [43]. Furthermore, Fig. 3f depicts a spectrum of S 2p, in which characteristic doublets at 162.7 and 164.1 eV correspond to S 2p1/2 and S 2p3/2, respectively, suggesting that most of the S
Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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elements exist in the form of S2 [44]. The observation of a peak at 170.8 eV would be related to sulfur oxides caused by some surface oxidation [45]. The XPS spectrum demonstrates clearly that CoS was successfully grown on the surface of CoMoO4 nanosheets by the simple electrodeposition process, which is consistent with the XRD analysis.
Co3O4 þ H2O þ OH 4 3CoOOH þ e
(6)
CoOOH þ OH 4 CoO2 þ H2O þ e
(7)
CoS þ OH 4 CoSOH þ e
(8)
CoSOH þ OH 4 CoSO þ H2O þ e
(9)
Electrochemical performances In order to investigate the advantages of the designed coreshell structure for energy storage applications, a series of electrochemical tests were performed with the prepared composites as working electrodes. The typical cyclic voltammetry (CV) curves of the CoMoO4, CoS and CoMoO4@CoS samples were collected under the scan rate of 10 mV s1 (Fig. 4a). Visibly, all the CV curves show the couple welldefined redox peaks, indicating a pseudocapacitance feature of all composites. The CoMoO4@CoS sample possesses higher redox peak and a larger CV area than that of CoMoO4 and CoS, which indicates the higher charge storage capacity of the composite. This result proves that the deposited CoS shell material can further improve the electrochemical activity of CoMoO4, which can be due to the synergic effect of CoS and CoMoO4. Fig. 4b depicts CV curves of the core-shell CoMoO4@CoS composite tested from 0 V to 0.7 V with various scan rates (5, 10, 20, 30 and 50 mV s1). Each CV curve has a couple of well-defined redox peak, which illustrates that a Faradaic capacitive behavior is the major factor for the capacitive characteristics. The faradaic redox reactions of the core-shell CoMoO4@CoS can be described as [46]: 3Co(OH)3 4 Co3O4 þ 4H2O þ OH þ 2e
(5)
Specifically, the obtained capacitance of the composite originates from the quasi-reversible electron transfer kinetics, which is primarily attributed to the reversible Co2þ/Co3þ redox reactions [47,48]. As a transit ion metal element Mo, it does not directly participate in the redox reaction, but improves the electrochemical performance for the samples by increasing the conductivity of the composite [49]. It is found that the oxidation and reduction peaks of the composite shift to the right and left with the scan rate increases, respectively, which would be mainly caused by the increased internal diffusion resistance. The electrochemical reaction kinetics can be qualitatively analyzed according to the relationship between the sweep rates (v) and electrochemical response currents (i) from the CV curves [50]: i ¼ avb
(10)
In which a and b are both volatile variables. The value of the b can be determined by the slop of the log (i) versus log (v) plot and varies from 0.5 to 1.0. It is well known that the b value in the diffusion-controlled process is close to 0.5, while b value approaches 1.0 in the surface capacitance dominated process. A measured value of b is 0.57 for anodic peaks of CoMoO4@CoS electrode. In addition, a linear relationship between i and v1/2 is shown in Fig. S3, proving that diffusion-controlled reaction
Fig. 3 e (a) XRD patterns of the different samples, (b) XPS survey, (c) Co 2p, (d) Mo 3d, (e) O 1s, and (f) S 2p for the CoMoO4@CoS electrode. Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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Fig. 4 e (a) Comparative CV curves of the as-obtained samples at 30 mV s¡1. (b) CV curves of CoMoO4@CoS at various scan rates. (c) Comparison of GCD curves of the as-obtained samples at 1 A g¡1. (d) GCD curves of CoMoO4@CoS at different current densities. (e) Specific capacitance of different electrodes at different current densities. (f) EIS Nyquist plots.
is the dominated storage mechanism of the CoMoO4@CoS composite. A comparison of galvanostatic charge-discharge (GCD) curves of the CoMoO4, CoS and CoMoO4@CoS electrodes is displayed in Fig. 4c at the same current density of 1 A g1. The core-shell CoMoO4@CoS composite (1676 s) shows the longest discharge time among all the electrodes at the current density of 1 A g1, meaning that it could store more charges and exhibit a biggest specific capacitance value. According to the equation (1), a maximum specific capacitance up to 3380.3 F g1 is achieved for CoMoO4@CoS electrode at the current density of 1 A g1, which is not only higher than that of the CoMoO4 (1100.4 F g1) and CoS (1896.2 F g1) electrodes but also better than those of recently reported similar composites applied for SCs applications (Table S1). Fig. 4d depicts GCD curves of the CoMoO4@CoS electrode under various current
densities with the potential window of 0e0.5 V. With the increase of current density, GCD curves are symmetric and do not show a graphic deviation, demonstrating the reversible electrochemical characteristics and excellent rate capability. The obvious discharge platforms further indicate the pseudocapacitive characteristic of the CoMoO4@CoS electrode, which is the same as CV analysis. The relationships between the current density and specific capacitance of CoMoO4, CoS and CoMoO4@CoS are presented in Fig. 4e. The CoMoO4@CoS electrode exhibits the specific capacitance values of 3380.3, 3312.2, 3250.1, 3107.4, 2981.8 and 2820.2 F g1 at 1, 2, 3, 5, 7 and 10 A g1, respectively, which is superior than CoMoO4 and CoS electrodes at the same current density. In addition, the CoMoO4@CoS electrode shows a good rate capability of about 83.4% with the current density increasing from 1 to 10 A g1,
Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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which is obviously enhanced compared with that of CoMoO4 (68.9%) and CoS (77.8%), indicating the excellent rate capability for energy storage devices. The Nyquist plots of electrochemical impedance spectroscopy (EIS) for CoS, CoMoO4 and CoMoO4@CoS samples were measured between 0.01 Hz and 100 kHz, as shown in Fig. 4f. It is obvious that the shape of the impedance spectra is basically the same, composing of semicircles in a high frequency section and sloped lines in the low frequency area. The sloped lines represent Warburg impedance (Zw) for composites, reflecting the electrolyte diffusion efficiency. The CoMoO4@CoS composite has larger slopes of straight line than those of CoMoO4 and CoS electrodes, certificating the minimal diffusion resistance of electrolyte in the core-shell structure composite. The charge transfer resistance (Rct) can be obtained through the diameter of the semicircular in the Nyquist plot at the high frequency region, which mainly derives from the electron transfer between the electrolyte and electrode. The Rct value of the CoMoO4@CoS electrode is about 0.19 U, which is significantly lower than that of the CoMoO4 (0.54 U) and CoS (0.72 U) electrodes, which means that the CoMoO4@CoS electrode possesses the highest electrical conductivity. The equivalent series resistance (Rs), which originates from the intrinsic resistance and ionic resistance of the composites, is determined from the intersection with the horizontal axis. The values of Rs are 0.75, 1.10, and 0.72 U for CoMoO4, CoS and CoMoO4@CoS samples, respectively. The small impedance value of CoMoO4@CoS electrode allows more electrode materials and electrolyte ions available for the electrochemical reaction process, resulting in the improved capacitance. The cyclic performance is a significant factor in evaluating the performance of electrodes for SCs. The CoMoO4, CoS and CoMoO4@CoS composites were tested through 6000 continuous GCD cycles at the current density of 10 A g1 (Fig. 5). The specific capacitance of the core-shell CoMoO4@CoS electrode shows outstanding cycle life (81.1% of the initial value) and retains the value of 2287.4 F g1 through 6000 cycles. The retention rate is higher than that of CoMoO4 (70.7%) and CoS (60.9%) electrodes, indicating an improved cycling stability after compounding with CoS. As analyzed above, the CoMoO4@CoS electrode has an exceptional SCs performance compared to the pure CoMoO4 and CoS electrodes. The remarkable SCs performances would be originated from the following points. Firstly, the composite grown directly on NF/rGO could effectively reduce the “dead surface” which is generally produced in traditional slurrycoating. At the same time, the rGO, with a characteristic of ideal chemical and mechanical stability, as a robust current collector, would significantly improve the electrochemical performance of the composites by supplying more ion/electron paths for insertion/extraction as well as protecting the composites from corrosion by the electrolyte during the electronic reaction processes [51]. Secondly, the core-shell nanostructure of the CoMoO4@CoS plays an important role in promoting the redox reactions by expanding the space for transmitting ions from the electrolyte, which could not only increase the utilization of electrode materials by facilitating the contact between electrode materials with electrolyte but also reduce the volume change during long-term cycling.
Fig. 5 e Comparison of cycling performance at a constant current density of 10 A g¡1.
Thirdly, CoS is directly grown on CoMoO4 by electrochemical deposition, which tightly combination of CoMoO4 and CoS could take much advantage of multifunctional and synergistic effects and show better electrochemical behavior. Lastly, the smaller EIS Rct, and Zw indicate that the electrolyte and electroactive material generate very low electrical resistance during the high-speed electrochemical processes, resulting in the faster reaction kinetics and higher conductivity. Overall, the core-shell CoMoO4@CoS nanostructure with these advantages possesses better electrochemical performance than pure CoMoO4 and CoS electrodes, making it more suitable in practical applications. The effects of electrodeposition quality of CoS on the electrochemical performance for CoMoO4@CoS composites were also studied. The deposited mass of CoS was changed by varying the CV cycles during the electrodeposition progress. The electrochemical performances for all samples are shown in Fig. S1, and the detailed analysis are supplied in supplementary materials. Obviously, the specific capacitance of these CoMoO4@CoS composites increases first and then decreases with the increase mass of CoS. The core-shell CoMoO4@CoS electrodes with the CoS mass of 0.5 mg cm2 shows clearly superior electrochemical properties compared with other samples, confirming that 0.5 mg cm2 is the optimum mass in these composites. Further increase the mass loading of CoS, the CoMoO4 nanosheets would be covered by a thick layer of the CoS shell, leading to the difficulties in electron transfer and mass transport. Therefore, the CoMoO4@CoS composite with the CoS mass of 0.5 mg cm2 was selected for the systematic study.
Preparation of the asymmetric supercapacitor An asymmetric supercapacitor (ASC) was assembled by employing CoMoO4@CoS and AC as positive and negative electrodes. According to charge balance theory [52], the weight ratio of CoMoO4@CoS and AC is calculated to be 8:1. Thus the mass loading of AC is about 8.8 mg cm2. The CV curves of the CoMoO4@CoS composite (0e0.7 V) and AC (-1 - 0 V) measured
Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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under a three electrode system at 10 mV s1 are employed to confirm the operating potential window (Fig. 6a). Fig. 6b shows the CV curves of the CoMoO4@CoS//AC with various potential windows at 10 mV s1. As can be seen, the shape of the CV curves remains stable without obvious polarization when the operating voltage window of the CoMoO4@CoS//AC is increased to 1.7 V, indicating that the optimum operating potential window for the as-fabricated ASC is 0e1.7 V. The CV curves of CoMoO4@CoS//AC with different scan rates demonstrate that the capacitance is contributed via both pseudo-capacitance and EDLC features (Fig. 6c). The CV curves exhibit no obvious change with the scan rate increasing,
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suggesting the excellent rate ability of the device. Fig. 6d displays the GCD curves of the ASC at various current densities. The specific capacitance of the ASC was obtained by the GCD curves and the equation (1), in which m represents the weight of positive and negative electrodes. The specific capacitance of the ASC is found to be 189.5, 166.6, 151.2, 144.9, 136.5, 129.3, 119.4 and 108.7 F g1 under the current densities of 0.5, 1, 2, 3, 4, 5, 7 and 10 A g1, respectively (Fig. 6e). The decrease of the capacitance at high current densities would be due to the active materials cannot fully participate in the electrochemical redox reaction [35,53]. The cyclic performance of the CoMoO4@CoS//AC measured at the 2 A g1 is presented in
Fig. 6 e (a) CV curves comparison of AC and CoMoO4@CoS at a scan rate of 10 mV s¡1 in a three-electrode system. (bec) CV curves of the CoMoO4@CoS//AC ASC at different operation voltages and scan rates. (dee) GCD curves and specific capacitance of the CoMoO4@CoS//AC ACS at different current densities and (f) the cycling performance of the ACS after the 6000th cycles at 3 A g¡1. Please cite this article as: Xuan H et al., Rational design of hierarchical core-shell structured CoMoO4@CoS composites on reduced graphene oxide for supercapacitors with enhanced electrochemical performance, International Journal of Hydrogen Energy, https:// doi.org/10.1016/j.ijhydene.2019.12.178
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Fig. 7 e Ragone plot related to energy and power densities vs discharge time of the assembled CoMoO4@CoS//AC ASC.
substrate via a facile approach. The tightly contact between the two promising pseudo-capacitance materials of CoMoO4 and CoS can take much advantage of synergistic and multifunctional effects. The CoMoO4@CoS electrode displays the excellent SCs performances, involving the excellent specific capacitance of as high as 3380.3 F g1 (1 A g1) and good capacitance retention about 81.1% after 6000 cycles. In addition, the CoMoO4@CoS//AC device delivers large specific energy density of 59.2 W h kg1 under the power density of 799.8 W kg1 and the high cycle performance (the capacitance loss of only around 8.5% even after 6000 cycles). The outstanding electrochemical properties of the core-shell CoMoO4@CoS composite indicate that the method of combining CoMoO4 with promising CoS supplies an effective approach to prepare coreshell composites and can be applied in other energyrelated fields.
Table 1 e Comparison of the as-assembled ASCs with those reported in the literatures. Positive electrode //negative electrode
Electrolyte
Energy density (W h kg1)
Power density (W kg1)
Cycle ability
Ref.
Ni3S2@CoS//AC CoMoO4/Co1-xS//AC MnCo2O4@CoS//AC CoMoO4//AC Cu2O/CuMoO4//AC NiMoO4@CoMoO4//AC CoMoO4@CoS//AC
2 M KOH 3 M KOH PVA/KOH 2 M KOH 2 M KOH PVA/KOH 2 M KOH
28.3 39.8 55.1 46.7 75.1 49.3 59.2
134.4 804.5 477.3 800 420 630 799.8
95.5%//2000 86.4%//4000 97.3%//6000 94.0%//3000 86.6%//3000 92.8%//5000 91.5%//6000
[37] [53] [56] [58] [59] [60] This work
Fig. 6f. It can be observed that the specific capacitance of the CoMoO4@CoS//AC device is reduced very slowly and still remains 91.5% of its initial value through 6000 charge and discharge cycles, suggesting the excellent long-term cycling performance of the ASC device. In addition, the P and E of the CoMoO4@CoS//AC devices are calculated according to formulas (2) and (3), as presented in Fig. 7. The ASC device provides the largest energy density of 59.2 W h kg1 under the power density of 799.8 W kg1 with the corresponding discharge time of 266.6 s, and even keeps 38.6 W h kg1 at the high power density of 7996.3 W kg1. The values of the device are better than many similar ASC devices, such as CoMoO4/Co1-xS//AC (39.8 W h kg1 at 804.5 W kg1) [53], CoMoO4/Co9S8//AC (42 W h kg1 at 181.13 W kg1) [54], CoMoO4 @Co3O4//CNTs (45.9 W h kg1 at 1647 W kg1) [55], MnCo2O4@CoS//AC (55.1 W h kg1 at 477.3 W kg1) [56] and NiCo2S4/Co9S8@AC (49.6 W h kg1 at 123 W kg1) [57]. The comparison of the electrochemical performance of the ASC device is listed under Table 1. The above attractive results demonstrate that the as-obtained CoMoO4@CoS composites possess outstanding electrochemical performances, making it become a new type of energy storage material with great research prospects in practical applications.
Conclusions In summary, the hierarchical core-shell CoMoO4@CoS composites have been successfully synthesized on rGO/NF
Acknowledgements This work is supported by the Natural Science Foundation of Shanxi Province, China (Grant No. 201801D121100), the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi (OIT), the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (STIP) (Grant No. 201802033), and the Collaborative Innovation Center for Shanxi Advanced Permanent Magnetic Materials and Technology (2016-06).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.178.
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