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Ni3S4 nanosheet-on-nanorod arrays
Journal of Alloys and Compounds 814 (2020) 152269
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Morphology evolution and electrochemical properties of hierarchical MoS2/Co3S4/Ni3S4 nanosheet-on-nanorod arrays Ying Liu, Shuangyan Lin*, Zhikun Xu**, Lin Li*** Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin, 150025, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2019 Received in revised form 11 September 2019 Accepted 12 September 2019 Available online 16 September 2019
Reasonable design of multi-compositions electrode material with hierarchical structure can effectively enhance the electrochemical performance of electrode. Here, the hierarchical structure of MoS2/Co3S4/ Ni3S4 (CMS) nanosheet-on-nanorod arrays have been synthesized through a simple in situ hydrothermal. The growth process is proposed according to the morphology evolution on hydrothermal time. The hierarchical structure obtained through optimized time enhances the accessible surface area for electrolyte, and multi-compositions provide rich redox reactions. Therefore, the hierarchical CMS electrode exhibits a high areal capacitance of 3.94 F cm2 at 5 mA cm2, and prominent cycle stability with 91.8% retention after 5000 cycles. These results demonstrate that the CMS electrode developed in this study has good potential for high-performance supercapacitors. © 2019 Elsevier B.V. All rights reserved.
Keywords: MoS2 Co3S4 Ni3S4 Metal sulfide Supercapacitor
1. Introduction With the deterioration of the environmental pollution and the increase in energy demand, clean energy storage devices have attracted extensive research interest. As a potential candidate, supercapacitors have gained much attentions in view of their promising features such as excellent power density, outstanding rate performance and cycle stability [1e7]. As supercapacitors store energy through physical adsorption of charges and/or faradic reactions on the surface of electrode materials, the choice of electrode materials and the design of their structure are the key factors to affect the performance of supercapacitors. Thus, it is currently the major target to improve the electrochemical performance of electrode by reasonably designing the component and structure. As advanced pseudocapacitive materials, transition metal sulfides have attracted extensive research by virtue of their high redox activity and high electrical conductivity. Specifically, Co3S4 and Ni3S4 have been widely studied as low-cost electroactive materials with high faradic activity [8,9]. Recent reports have indicated that the design and synthesis of hybrid metal sulfides are effective
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail address: [email protected] (S. Lin). https://doi.org/10.1016/j.jallcom.2019.152269 0925-8388/© 2019 Elsevier B.V. All rights reserved.
strategies to further enhance the electrochemical performance of corresponding single component [10]. Among various structures, the hierarchical arrays can provide the shortened paths for electrons/ions migration and expose the abundant electroactive sites, offering the maximal capacitive performances [11]. For examples, the hierarchical hollow Ni3S2/NiS@Ni3S4 exhibits a specific capacity of 1031.3 C g1 at 2 A g1 [12]; the hierarchical Ni@rGO-Co3S4 exhibits a specific capacitance of 1369 F g1 at 1.5 A g1 [13]; the Co3S4@Ni3S4 nanowires deliver an areal capacitance of 3.6 F cm1 at 0.8 mA cm2 and 80% capacitance retention after 5000 cycles [10]. Requirements for practical applications demand further improvement of the cycling stability. It is proved that the MoS2 can improve capacitance and cycle stability of electrodes [14e17]. For examples, the MoS2eCo3S4 exhibits a superior performance with specific capacitance of 1369 F g1 and capacitance retention of 83% after 10000 cycles [18]. The Ni3S4@MoS2 behaves a high capacitance of 1440.9 F g1 at 2 A g1 and excellent cycle stability with 90.7% capacitance retention after 3000 cycles [19]. The above works have confirmed that the hybrid electrode of MoS2 and Co3S4 (Ni3S4) can optimize redox activity and cycle stability, however, hierarchical arrays of MoS2/Co3S4/Ni3S4 have not be reported. Herein, the hierarchical MoS2/Co3S4/Ni3S4 (CMS) nanosheet-onnanorod arrays were directly grown on Ni foam through a simple in situ hydrothermal method. Ni foam not only acts as Ni source for formation of electroactive Ni3S4, but also as a substrate to ensure
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high electrical conductivity of electrode. The effect of hydrothermal time on the morphology and electrochemical properties has been investigated. Among the four electrodes prepared with different hydrothermal time, CMS-20 (20 refers to the hydrothermal reaction time of 20 h) electrode exhibits outstanding performance including high areal capacitance, good rate capability and excellent cycleability. This work could spark extensive interests in the preparation of high-performance hybrid metal sulfide electrode materials by simple method.
2. Experimental section Ni foam (1.5 1 0.1 cm3) was sequentially washed in 3 M HCl, deionized water and ethanol for each 20 min under ultrasonication. All reagents used were at analytical grade without further purification.
2.1. Synthesis of hierarchical CMS on Ni foam CMS was prepared using a simple in-situ hydrothermal method. Typically, at room temperature, 0.241 g Na2MoO4$2H2O, 0.291 g Co(NO3)2$6H2O and 0.012 g thioacetamide (TAA) were successively added to 20 ml deionized water with stirring to form a lavender solution, which was transferred into a 50 mL Teflon-lined stainlesssteel autoclave with three piece of washed Ni foams, and the autoclave was maintained at 160 C for a certain time. After being cooled to room temperature, the CMS on Ni foam was obtained after washed with deionized water for several times and dried at 60 C in a vacuum oven. The products obtained with the hydrothermal reaction time of 4, 12, 20 and 28 h are named as CMS-4, CMS-12, CMS-20 and CMS-28, respectively.
2.2. Characterization The crystal structure was recorded by X-ray diffraction (XRD) using Cu Ka radiation (l ¼ 1.5418 Å) with the reflection mode on a Rigaku D/max2600 X-ray diffractometer. Chemical states and compositions of as-prepared sample was analyzed by XPS (PHI 5700 ESCA System, American). The morphologies of the CMS were investigated by scanning electron microscopy (SEM; SU70, Hitachi, Japan). The microstructures, EDS mapping of the CMS-20 were characterized by Transmission electron microscopy (TEM, FEI, Tecnai TF20).
2.3. Electrochemical measurements Electrochemical performances of the obtained electrodes were measured in 2 M KOH aqueous solution by a three-electrode system on an electrochemical workstation (VMP3) at ambient temperature. CMS on Ni foam (1 1 cm2) was used as the binder-free working electrode, a platinum plate as the counter electrode and Ag/AgCl as the reference electrode. The cyclic voltammogram (CV) curves were recorded within a potential range from 0 V to 0.55 V (vs. Ag/AgCl), and the galvanostatic charge-discharge (GCD) behavior was evaluated at different current densities (5e50 mA cm2) in a potential of 0e0.45 V (vs. Ag/AgCl). Electrochemical impedance spectroscopy (EIS) measurements were performed by applying an alternating voltage of 10 mV over a frequency range of 100 mHz to 100 kHz. The cycle stability was evaluated by GCD measurement at 20 mA cm2 for 5000 cycles. The areal capacitances were calculated according to the following equation:
Ca ¼
It SV
(1)
where Ca (F cm2) is the areal capacitance, S (cm2) is the area of the electrode, V (V) is the potential window of discharge, I (mA) is the discharge current, t (s) is the discharge time. 3. Results and discussion 3.1. Structural characterization The crystal phases of samples were confirmed by XRD as showed in Fig. S1a. The diffraction peaks marked "▽" can be attributed to the metallic Ni foam. For CMS-20, diffraction peaks can be assigned to MoS2 phase (JCPDS No. 2e132), Co3S4 phase (JCPDS No. 2e825) and Ni3S4 phase (JCPDS No. 47e1739), respectively, which indicates that CMS-20 is comprised of MoS2, Co3S4 and Ni3S4 phases. The morphologies of the CMS samples were analyzed by SEM and TEM. The Fig. 1a and b show the SEM images of CMS-20 at different magnifications, which reveal that the surface of Ni foam is covered by nanorod-like CMS-20, the surface of nanorods is rough and their diameter is approximately 150 nm. The CMS-20 nanorods interlace with each other on the Ni foam to form abundant open space, which facilitates the transport of electrolyte ions. The detailed morphology and microstructure of the CMS-20 were characterized by the TEM (Fig. 1c). The surfaces of nanorods are covered by very thin nanosheets, forming a hierarchical nanosheets-on-nanorods structure, which would provide more active sites for electrochemical reactions occurring at the electrode/ electrolyte interfaces [20]. The HRTEM images of CMS-20 were presented in Fig. 1def with well-defined lattice fringes. The lattice spacings of 0.62, 0.29 and 0.23 nm correspond to the (002) plane of MoS2 (JCPDS No. 2e132), the (311) plane of Ni3S4 (JCPDS No. 47e1739), and the (400) plane of Co3S4 phase (JCPDS No. 2e825), respectively, confirming the chemical composition of the CMS-20. In addition, elemental mapping was performed to illustrate the element distribution and structural properties of the CMS-20 (Fig. 1gek). It is observed that the Co, Mo and S elements are all evenly distributed throughout the nanorod. Differently from that, Ni shows high content in the edges of the nanorod. To investigate the structure evolution, CMS-x (x ¼ 4, 12, 20 and 28) were also synthesized by adjusting the reaction time from 4 to 28 h. CMS samples were grown on the Ni foam through in-situ strategy using Ni foam as the nickel source and substrate, Na2MoO4$2H2O as the molybdenum source, Co(NO3)2$6H2O as the cobalt source and TAA as the sulfur source. Based on the SEM images of CMS-x (Fig. S2-3), the possible formation mechanism of the hierarchical CMS arrays is illustrated in Fig. 2. At the beginning, the surface of the Ni foam is covered by a thin layer of small nanosheets at the hydrothermal reaction of 4 h that act as the “seeds” for the following polymerization and transform into nanorods in the next 8 h reaction time [21,22]. As the reaction time increases to 20 h, thin nanosheets are formed on the surface of the nanorods, forming hierarchical nanosheets-on-nanorods arrays. The formation of the hierarchical arrays is mainly due to the dissolution-recrystallization process occurring at the surface of the nanorods [23,24]. As the reaction time further prolongs to 28 h, the nanorods become thicker with the diameters increasing from about 150 to 300 nm, which will reduce the specific surface area and further electrochemical active area. To further characterize the near surface chemical composition and chemical state, CMS-20 scraped from Ni foam was measured by XPS. As showed in Fig. S1b, the XPS spectrum shows the presence of Ni, Co, Mo and S elements in CMS-20. The Ni 2p spectrum (Fig. 3a)
Y. Liu et al. / Journal of Alloys and Compounds 814 (2020) 152269
3
Fig. 1. (a, b) SEM images, (c, d) TEM and HRTEM images of CMS-20. (e, f) The local magnification of labeled region in (d). (hek) Selected area elemental mapping for the (g) image of CMS-20.
Fig. 2. Schematic illustration of the structure evolution of MoS2/Co3S4/Ni3S4 (CMS) on Ni foam.
can be well fitted to 2p3/2 (located at 855.6 eV) and 2p1/2 (located at 873.2 eV) and satellite peaks (identified as “Sat.”, at 861.3 and 879.4 eV), which indicates the existence of Ni2þ and Ni3þ [25]. The high-resolution Co 2p spectrum (Fig. 3b) is split into Co 2p1/2 (796.1 ev) and Co 2p3/2 (780.2 eV) accompanied with shakeup satellites, which could be assigned to Co2þ and Co3þ [26,27]. In the Mo 3d region (Fig. 3c), the binding energies of 229.2 and 232.4 eV are attributed to the Mo4þ 3d5/2 and Mo4þ 3d3/2, respectively, while the high binding energy peak at 235.5 eV is corresponded to the high state Mo6þ that may result from oxidation of sample in air [28]. In the S 2p region (Fig. 3d), the binding energies at 162.5 (S 2p3/2) and 163.7 eV (S 2p1/2) are ascribable to NieS and CoeS bonds. The high binder energy at 167.6 eV is the satellite peak [10,29]. XPS analysis
as well as XRD and TEM results demonstrate the presence of MoS2, Co3S4 and Ni3S4 in CMS-20.
3.2. Electrochemical performances To evaluate the electrochemical properties, the CMS arrays grown on Ni foam as binder-free electrodes were studied in a threeelectrode system. Fig. 4a reveals the CV curves of four CMS electrodes present a pair of redox peaks, indicating that the capacitance characteristics are mainly derived from the Faraday reversible reactions. Ni3S4, Co3S4 and MoS2 are typical pseudocapacitive materials, all of which provide capacitance in alkaline electrolyte [10,18,19,30e32]. The possible redox reactions of the CMS
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Fig. 3. High-resolution XPS spectra of (a) Ni 2p, (b) Co 2p, (c) Mo 3d and (d) S 2p for CMS-20.
Fig. 4. (a) CV curves at a scan rate of 20 mV s1, (b) GCD curves at a current density of 5 mA cm2, and (c) Areal capacitances at different current densities for four CMS electrodes. (d) Comparison of the total stored charge of four CMS electrodes at a scan rate of 5 mA cm2 and 20 mA cm2. (e) Cyclic stability of CMS-20 electrode at a current density of 20 mA cm2.
electrodes in KOH electrolyte could be expressed in equations (2)e(5) [10,33]:
Co3 S4 þ OH 4Co3 S4 OH þ e
(2)
Co3 S4 OH þ OH 4Co3 S4 O þ H2 O þ e
(3)
Ni3 S4 þ OH 4Ni3 S4 OH þ e
(4)
MoS2 þ OH 4MoS2 OH þ e
(5)
Y. Liu et al. / Journal of Alloys and Compounds 814 (2020) 152269
The CMS-20 electrode possesses the largest CV curve enclosed area among four samples, which indicates the electrode has the highest areal capacitance [34]. The CV curves of four CMS electrodes at different scan rates were shown in Fig. S4. With the increase of scan rate, the shape of CV curves of CMS-20 can well maintain, meaning the excellent rate capability. GCD curves of four CMS electrodes at the current density of 5 mA cm2 (Fig. 4b) show the CMS-20 electrode has longest discharge time, which indicates the highest areal capacitance that is in accordance with the CV analysis. For comparison, the GCD curves at different current densities of four CMS electrodes are shown in the Fig. S5. Moreover, the symmetric characteristic of GCD curves during the successive charge/discharge process indicates the good coulombic efficiency. According to GCD curves, the areal capacitances of four CMS electrodes are calculated and plotted as a function of the current density in Fig. 4c. It is remarkable that the CMS-20 electrode shows a high areal capacitance of 3.94 F cm2 at 5 mA cm2, which is higher than CMS-4 (1.54 F cm2), CMS-12 (3.19 F cm2) and CMS-28 (2.77 F cm2). The areal capacitance of CMS-20 is also obvious higher than the related electrodes, such as Co3S4@Ni3S4 electrode (3.6 F cm2 at 0.8 mA cm2) [10], Co3S4/NiS electrode (1.81 F cm2 at 4 mA cm2) [35], MoS2/CoS2 electrode (0.142 F cm2 at 1 mA cm2) [36], Ni foam/graphene/Co3S4 electrode (0.525 F cm2 at 7.5 mA cm2) [37], Co3S4/CoMo2S4 electrode (2.9 F cm2 at 2 mA cm2) [38], C@MoS2/Ni3S4 electrode (z1.47 F cm2 at 3 mA cm2) [39]. With the current density increasing to 50 mA cm2, the CMS-20 electrode retains 78.7% of capacitance (3.10 F cm2), which is higher than CMS-4 (69.4%), CMS-12 (73.7%) and CMS-28 (77.2%), indicating excellent rate capability. Additional, the electrochemically active surface areas of four CMS samples were evaluated by electrochemical double layer capacitance (Cdl) [40]. As shown in Fig. S6, the CMS-20 electrode exhibits the largest Cdl (7.15 mF cm2) among all four samples, which indicates that CMS-20 electrode has the largest electrochemically active surface area, ensuring the highest areal capacitance among four samples. To unravel the possible origination of the excellent rate performance of CMS-20, electrochemical kinetics of CMS samples were evaluated by Trasatti analysis [41]. The method assumes that the charge storage process is the combination of surface capacitive effect and diffusion-controlled process. The relationship of the total stored charge (q) and the capacitive charge (q∞) can be calculated by the following equation [42]:
qðvÞ ¼ q∞ þ kv1=2
(6)
where k is a constant, q∞ is the capacitive charge, n is the scan rate, and kv1/2 represents the diffusion-controlled charge. The q∞ can be extrapolated by plotting the v1/2 vs. q. Comparison of q and q∞ of the four CMS electrodes are shown in Fig. 4d and S7, the results show that the CMS-20 electrode shows the maximal q and the highest proportion of q∞ among four electrodes. The maximal q indicates the highest capacitance, the highest proportion of q∞ makes the CMS-20 electrode the excellent rate performance. The EIS of four CMS electrodes were measured in the frequency region from 100 kHz to 0.01 Hz, and the Nyquist plots are shown in Fig. S8a. All CMS electrodes show straightline in low frequency region, which indicates the capacitive nature of the electrodes. The slope of CMS-20 electrode is larger than other three samples, indicating the lower diffusion resistance. All the series resistance (Rs) of CMS electrodes are less than 1 U, especially, Rs of CMS-20 electrode is 0.4 U. The low Rs is mainly due to the firm interfacial connection of CMS on conductive Ni foam as result of direct growth of CMS on conductive Ni foam. The low Rs and diffusion resistance
5
make CMS-20 electrode a preferable faradic electrode with excellent rate performance. Furthermore, the cycle stability of the electrode is an important parameter to evaluate its practical applications in supercapacitors. The cycle performance of CMS-20 electrode was measured at 20 mA cm2 (Fig. 4e). The capacitance retention rises obviously at first, and then decreases to a stable state. The initial rise may be due to the activation process. After 5000 GCD cycles, the CMS-20 electrode retains 91.8% initial capacitance, which reveals a long-term electrochemical stability. Compared with Co3S4- based and Ni3S4- based materials (Table S1), the cycle performance is still outstanding. The similar GCD curves for the first and last five cycles and the small differences resistance before and after the stability test (Fig. S8b) also confirm the excellent cycle stability of the electrode. The excellent cycle performance is mainly attributed to strong contact between nanorods and Ni foam as well as the space between adjacent nanorods that can withstand volume change during redox processes. For hybrid CMS electrode, Mo, Co and Ni elements have various valence states, so all components of MoS2, Co3S4 and Ni3S4 contribute to the total capacitance. In general, Ni3S4 and Co3S4 contributes more capacitance than MoS2 [18,19,30e32]. Previous reports have proved that the MoS2 can improve capacitance and cycle stability of electrodes [14e19]. The outstanding supercapacitive properties of the CMS-20 binder-free electrode can be mainly ascribed to the following features: 1) The synergistic effect between MoS2, Co3S4 and Ni3S4 provides richer redox chemistry than those of the corresponding individual compounds. 2) The hierarchical nanosheet-on-nanorod arrays provide abundant accessible surface area for electrolyte ions. The interconnected CMS nanorods facilitate the penetration of OH and buffer volume changes during charge/discharge. Thus, the individual components are fully utilized to ensure maximal capacitive performance. 3) The strong interface between CMS and Ni foam accelerate the transport of electrons from nanorods to highly conductive Ni foam. 4. Conclusions In summary, the hierarchical CMS nanosheet-on-nanorod arrays were synthesized by a simple in-situ hydrothermal process using Ni foam as Ni source and current collector. A possible growth process was proposed based on the structure evolution, which shows that small nanosheets formed on Ni foam firstly turn into nanorods and then the nanorods covered with thin nanosheets as the hydrothermal time increases. The optimal reaction time can make the hierarchical CMS nanosheet-on-nanorod arrays exhibit the high electrochemical properties, i.e. the CMS-20 binder-free electrode exhibits a high areal capacitance of 3.94 F cm2 at 5 mA cm2 and good cycle stability with 91.8% capacitance retention after 5000 cycles. Therefore, the work provides a low-cost and facile method to prepare hierarchical transition metal sulfides electrode for supercapacitor. Acknowledgments This research work was supported by the Natural Science Foundation of Heilongjiang Province (Nos. YQ2019E031, QC2017077), the National Natural Science Foundation of China (No. 61605036). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152269.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Morphology evolution and electrochemical properties of hierarchical MoS2/Co3S4/Ni3S4 nanosheet-on-nanorod arrays Ying Liu, Shuangyan Lin*, Zhikun Xu**, Lin Li*** Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin, 150025, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2019 Received in revised form 11 September 2019 Accepted 12 September 2019 Available online 16 September 2019
Reasonable design of multi-compositions electrode material with hierarchical structure can effectively enhance the electrochemical performance of electrode. Here, the hierarchical structure of MoS2/Co3S4/ Ni3S4 (CMS) nanosheet-on-nanorod arrays have been synthesized through a simple in situ hydrothermal. The growth process is proposed according to the morphology evolution on hydrothermal time. The hierarchical structure obtained through optimized time enhances the accessible surface area for electrolyte, and multi-compositions provide rich redox reactions. Therefore, the hierarchical CMS electrode exhibits a high areal capacitance of 3.94 F cm2 at 5 mA cm2, and prominent cycle stability with 91.8% retention after 5000 cycles. These results demonstrate that the CMS electrode developed in this study has good potential for high-performance supercapacitors. © 2019 Elsevier B.V. All rights reserved.
Keywords: MoS2 Co3S4 Ni3S4 Metal sulfide Supercapacitor
1. Introduction With the deterioration of the environmental pollution and the increase in energy demand, clean energy storage devices have attracted extensive research interest. As a potential candidate, supercapacitors have gained much attentions in view of their promising features such as excellent power density, outstanding rate performance and cycle stability [1e7]. As supercapacitors store energy through physical adsorption of charges and/or faradic reactions on the surface of electrode materials, the choice of electrode materials and the design of their structure are the key factors to affect the performance of supercapacitors. Thus, it is currently the major target to improve the electrochemical performance of electrode by reasonably designing the component and structure. As advanced pseudocapacitive materials, transition metal sulfides have attracted extensive research by virtue of their high redox activity and high electrical conductivity. Specifically, Co3S4 and Ni3S4 have been widely studied as low-cost electroactive materials with high faradic activity [8,9]. Recent reports have indicated that the design and synthesis of hybrid metal sulfides are effective
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail address: [email protected] (S. Lin). https://doi.org/10.1016/j.jallcom.2019.152269 0925-8388/© 2019 Elsevier B.V. All rights reserved.
strategies to further enhance the electrochemical performance of corresponding single component [10]. Among various structures, the hierarchical arrays can provide the shortened paths for electrons/ions migration and expose the abundant electroactive sites, offering the maximal capacitive performances [11]. For examples, the hierarchical hollow Ni3S2/NiS@Ni3S4 exhibits a specific capacity of 1031.3 C g1 at 2 A g1 [12]; the hierarchical Ni@rGO-Co3S4 exhibits a specific capacitance of 1369 F g1 at 1.5 A g1 [13]; the Co3S4@Ni3S4 nanowires deliver an areal capacitance of 3.6 F cm1 at 0.8 mA cm2 and 80% capacitance retention after 5000 cycles [10]. Requirements for practical applications demand further improvement of the cycling stability. It is proved that the MoS2 can improve capacitance and cycle stability of electrodes [14e17]. For examples, the MoS2eCo3S4 exhibits a superior performance with specific capacitance of 1369 F g1 and capacitance retention of 83% after 10000 cycles [18]. The Ni3S4@MoS2 behaves a high capacitance of 1440.9 F g1 at 2 A g1 and excellent cycle stability with 90.7% capacitance retention after 3000 cycles [19]. The above works have confirmed that the hybrid electrode of MoS2 and Co3S4 (Ni3S4) can optimize redox activity and cycle stability, however, hierarchical arrays of MoS2/Co3S4/Ni3S4 have not be reported. Herein, the hierarchical MoS2/Co3S4/Ni3S4 (CMS) nanosheet-onnanorod arrays were directly grown on Ni foam through a simple in situ hydrothermal method. Ni foam not only acts as Ni source for formation of electroactive Ni3S4, but also as a substrate to ensure
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high electrical conductivity of electrode. The effect of hydrothermal time on the morphology and electrochemical properties has been investigated. Among the four electrodes prepared with different hydrothermal time, CMS-20 (20 refers to the hydrothermal reaction time of 20 h) electrode exhibits outstanding performance including high areal capacitance, good rate capability and excellent cycleability. This work could spark extensive interests in the preparation of high-performance hybrid metal sulfide electrode materials by simple method.
2. Experimental section Ni foam (1.5 1 0.1 cm3) was sequentially washed in 3 M HCl, deionized water and ethanol for each 20 min under ultrasonication. All reagents used were at analytical grade without further purification.
2.1. Synthesis of hierarchical CMS on Ni foam CMS was prepared using a simple in-situ hydrothermal method. Typically, at room temperature, 0.241 g Na2MoO4$2H2O, 0.291 g Co(NO3)2$6H2O and 0.012 g thioacetamide (TAA) were successively added to 20 ml deionized water with stirring to form a lavender solution, which was transferred into a 50 mL Teflon-lined stainlesssteel autoclave with three piece of washed Ni foams, and the autoclave was maintained at 160 C for a certain time. After being cooled to room temperature, the CMS on Ni foam was obtained after washed with deionized water for several times and dried at 60 C in a vacuum oven. The products obtained with the hydrothermal reaction time of 4, 12, 20 and 28 h are named as CMS-4, CMS-12, CMS-20 and CMS-28, respectively.
2.2. Characterization The crystal structure was recorded by X-ray diffraction (XRD) using Cu Ka radiation (l ¼ 1.5418 Å) with the reflection mode on a Rigaku D/max2600 X-ray diffractometer. Chemical states and compositions of as-prepared sample was analyzed by XPS (PHI 5700 ESCA System, American). The morphologies of the CMS were investigated by scanning electron microscopy (SEM; SU70, Hitachi, Japan). The microstructures, EDS mapping of the CMS-20 were characterized by Transmission electron microscopy (TEM, FEI, Tecnai TF20).
2.3. Electrochemical measurements Electrochemical performances of the obtained electrodes were measured in 2 M KOH aqueous solution by a three-electrode system on an electrochemical workstation (VMP3) at ambient temperature. CMS on Ni foam (1 1 cm2) was used as the binder-free working electrode, a platinum plate as the counter electrode and Ag/AgCl as the reference electrode. The cyclic voltammogram (CV) curves were recorded within a potential range from 0 V to 0.55 V (vs. Ag/AgCl), and the galvanostatic charge-discharge (GCD) behavior was evaluated at different current densities (5e50 mA cm2) in a potential of 0e0.45 V (vs. Ag/AgCl). Electrochemical impedance spectroscopy (EIS) measurements were performed by applying an alternating voltage of 10 mV over a frequency range of 100 mHz to 100 kHz. The cycle stability was evaluated by GCD measurement at 20 mA cm2 for 5000 cycles. The areal capacitances were calculated according to the following equation:
Ca ¼
It SV
(1)
where Ca (F cm2) is the areal capacitance, S (cm2) is the area of the electrode, V (V) is the potential window of discharge, I (mA) is the discharge current, t (s) is the discharge time. 3. Results and discussion 3.1. Structural characterization The crystal phases of samples were confirmed by XRD as showed in Fig. S1a. The diffraction peaks marked "▽" can be attributed to the metallic Ni foam. For CMS-20, diffraction peaks can be assigned to MoS2 phase (JCPDS No. 2e132), Co3S4 phase (JCPDS No. 2e825) and Ni3S4 phase (JCPDS No. 47e1739), respectively, which indicates that CMS-20 is comprised of MoS2, Co3S4 and Ni3S4 phases. The morphologies of the CMS samples were analyzed by SEM and TEM. The Fig. 1a and b show the SEM images of CMS-20 at different magnifications, which reveal that the surface of Ni foam is covered by nanorod-like CMS-20, the surface of nanorods is rough and their diameter is approximately 150 nm. The CMS-20 nanorods interlace with each other on the Ni foam to form abundant open space, which facilitates the transport of electrolyte ions. The detailed morphology and microstructure of the CMS-20 were characterized by the TEM (Fig. 1c). The surfaces of nanorods are covered by very thin nanosheets, forming a hierarchical nanosheets-on-nanorods structure, which would provide more active sites for electrochemical reactions occurring at the electrode/ electrolyte interfaces [20]. The HRTEM images of CMS-20 were presented in Fig. 1def with well-defined lattice fringes. The lattice spacings of 0.62, 0.29 and 0.23 nm correspond to the (002) plane of MoS2 (JCPDS No. 2e132), the (311) plane of Ni3S4 (JCPDS No. 47e1739), and the (400) plane of Co3S4 phase (JCPDS No. 2e825), respectively, confirming the chemical composition of the CMS-20. In addition, elemental mapping was performed to illustrate the element distribution and structural properties of the CMS-20 (Fig. 1gek). It is observed that the Co, Mo and S elements are all evenly distributed throughout the nanorod. Differently from that, Ni shows high content in the edges of the nanorod. To investigate the structure evolution, CMS-x (x ¼ 4, 12, 20 and 28) were also synthesized by adjusting the reaction time from 4 to 28 h. CMS samples were grown on the Ni foam through in-situ strategy using Ni foam as the nickel source and substrate, Na2MoO4$2H2O as the molybdenum source, Co(NO3)2$6H2O as the cobalt source and TAA as the sulfur source. Based on the SEM images of CMS-x (Fig. S2-3), the possible formation mechanism of the hierarchical CMS arrays is illustrated in Fig. 2. At the beginning, the surface of the Ni foam is covered by a thin layer of small nanosheets at the hydrothermal reaction of 4 h that act as the “seeds” for the following polymerization and transform into nanorods in the next 8 h reaction time [21,22]. As the reaction time increases to 20 h, thin nanosheets are formed on the surface of the nanorods, forming hierarchical nanosheets-on-nanorods arrays. The formation of the hierarchical arrays is mainly due to the dissolution-recrystallization process occurring at the surface of the nanorods [23,24]. As the reaction time further prolongs to 28 h, the nanorods become thicker with the diameters increasing from about 150 to 300 nm, which will reduce the specific surface area and further electrochemical active area. To further characterize the near surface chemical composition and chemical state, CMS-20 scraped from Ni foam was measured by XPS. As showed in Fig. S1b, the XPS spectrum shows the presence of Ni, Co, Mo and S elements in CMS-20. The Ni 2p spectrum (Fig. 3a)
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Fig. 1. (a, b) SEM images, (c, d) TEM and HRTEM images of CMS-20. (e, f) The local magnification of labeled region in (d). (hek) Selected area elemental mapping for the (g) image of CMS-20.
Fig. 2. Schematic illustration of the structure evolution of MoS2/Co3S4/Ni3S4 (CMS) on Ni foam.
can be well fitted to 2p3/2 (located at 855.6 eV) and 2p1/2 (located at 873.2 eV) and satellite peaks (identified as “Sat.”, at 861.3 and 879.4 eV), which indicates the existence of Ni2þ and Ni3þ [25]. The high-resolution Co 2p spectrum (Fig. 3b) is split into Co 2p1/2 (796.1 ev) and Co 2p3/2 (780.2 eV) accompanied with shakeup satellites, which could be assigned to Co2þ and Co3þ [26,27]. In the Mo 3d region (Fig. 3c), the binding energies of 229.2 and 232.4 eV are attributed to the Mo4þ 3d5/2 and Mo4þ 3d3/2, respectively, while the high binding energy peak at 235.5 eV is corresponded to the high state Mo6þ that may result from oxidation of sample in air [28]. In the S 2p region (Fig. 3d), the binding energies at 162.5 (S 2p3/2) and 163.7 eV (S 2p1/2) are ascribable to NieS and CoeS bonds. The high binder energy at 167.6 eV is the satellite peak [10,29]. XPS analysis
as well as XRD and TEM results demonstrate the presence of MoS2, Co3S4 and Ni3S4 in CMS-20.
3.2. Electrochemical performances To evaluate the electrochemical properties, the CMS arrays grown on Ni foam as binder-free electrodes were studied in a threeelectrode system. Fig. 4a reveals the CV curves of four CMS electrodes present a pair of redox peaks, indicating that the capacitance characteristics are mainly derived from the Faraday reversible reactions. Ni3S4, Co3S4 and MoS2 are typical pseudocapacitive materials, all of which provide capacitance in alkaline electrolyte [10,18,19,30e32]. The possible redox reactions of the CMS
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Fig. 3. High-resolution XPS spectra of (a) Ni 2p, (b) Co 2p, (c) Mo 3d and (d) S 2p for CMS-20.
Fig. 4. (a) CV curves at a scan rate of 20 mV s1, (b) GCD curves at a current density of 5 mA cm2, and (c) Areal capacitances at different current densities for four CMS electrodes. (d) Comparison of the total stored charge of four CMS electrodes at a scan rate of 5 mA cm2 and 20 mA cm2. (e) Cyclic stability of CMS-20 electrode at a current density of 20 mA cm2.
electrodes in KOH electrolyte could be expressed in equations (2)e(5) [10,33]:
Co3 S4 þ OH 4Co3 S4 OH þ e
(2)
Co3 S4 OH þ OH 4Co3 S4 O þ H2 O þ e
(3)
Ni3 S4 þ OH 4Ni3 S4 OH þ e
(4)
MoS2 þ OH 4MoS2 OH þ e
(5)
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The CMS-20 electrode possesses the largest CV curve enclosed area among four samples, which indicates the electrode has the highest areal capacitance [34]. The CV curves of four CMS electrodes at different scan rates were shown in Fig. S4. With the increase of scan rate, the shape of CV curves of CMS-20 can well maintain, meaning the excellent rate capability. GCD curves of four CMS electrodes at the current density of 5 mA cm2 (Fig. 4b) show the CMS-20 electrode has longest discharge time, which indicates the highest areal capacitance that is in accordance with the CV analysis. For comparison, the GCD curves at different current densities of four CMS electrodes are shown in the Fig. S5. Moreover, the symmetric characteristic of GCD curves during the successive charge/discharge process indicates the good coulombic efficiency. According to GCD curves, the areal capacitances of four CMS electrodes are calculated and plotted as a function of the current density in Fig. 4c. It is remarkable that the CMS-20 electrode shows a high areal capacitance of 3.94 F cm2 at 5 mA cm2, which is higher than CMS-4 (1.54 F cm2), CMS-12 (3.19 F cm2) and CMS-28 (2.77 F cm2). The areal capacitance of CMS-20 is also obvious higher than the related electrodes, such as Co3S4@Ni3S4 electrode (3.6 F cm2 at 0.8 mA cm2) [10], Co3S4/NiS electrode (1.81 F cm2 at 4 mA cm2) [35], MoS2/CoS2 electrode (0.142 F cm2 at 1 mA cm2) [36], Ni foam/graphene/Co3S4 electrode (0.525 F cm2 at 7.5 mA cm2) [37], Co3S4/CoMo2S4 electrode (2.9 F cm2 at 2 mA cm2) [38], C@MoS2/Ni3S4 electrode (z1.47 F cm2 at 3 mA cm2) [39]. With the current density increasing to 50 mA cm2, the CMS-20 electrode retains 78.7% of capacitance (3.10 F cm2), which is higher than CMS-4 (69.4%), CMS-12 (73.7%) and CMS-28 (77.2%), indicating excellent rate capability. Additional, the electrochemically active surface areas of four CMS samples were evaluated by electrochemical double layer capacitance (Cdl) [40]. As shown in Fig. S6, the CMS-20 electrode exhibits the largest Cdl (7.15 mF cm2) among all four samples, which indicates that CMS-20 electrode has the largest electrochemically active surface area, ensuring the highest areal capacitance among four samples. To unravel the possible origination of the excellent rate performance of CMS-20, electrochemical kinetics of CMS samples were evaluated by Trasatti analysis [41]. The method assumes that the charge storage process is the combination of surface capacitive effect and diffusion-controlled process. The relationship of the total stored charge (q) and the capacitive charge (q∞) can be calculated by the following equation [42]:
qðvÞ ¼ q∞ þ kv1=2
(6)
where k is a constant, q∞ is the capacitive charge, n is the scan rate, and kv1/2 represents the diffusion-controlled charge. The q∞ can be extrapolated by plotting the v1/2 vs. q. Comparison of q and q∞ of the four CMS electrodes are shown in Fig. 4d and S7, the results show that the CMS-20 electrode shows the maximal q and the highest proportion of q∞ among four electrodes. The maximal q indicates the highest capacitance, the highest proportion of q∞ makes the CMS-20 electrode the excellent rate performance. The EIS of four CMS electrodes were measured in the frequency region from 100 kHz to 0.01 Hz, and the Nyquist plots are shown in Fig. S8a. All CMS electrodes show straightline in low frequency region, which indicates the capacitive nature of the electrodes. The slope of CMS-20 electrode is larger than other three samples, indicating the lower diffusion resistance. All the series resistance (Rs) of CMS electrodes are less than 1 U, especially, Rs of CMS-20 electrode is 0.4 U. The low Rs is mainly due to the firm interfacial connection of CMS on conductive Ni foam as result of direct growth of CMS on conductive Ni foam. The low Rs and diffusion resistance
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make CMS-20 electrode a preferable faradic electrode with excellent rate performance. Furthermore, the cycle stability of the electrode is an important parameter to evaluate its practical applications in supercapacitors. The cycle performance of CMS-20 electrode was measured at 20 mA cm2 (Fig. 4e). The capacitance retention rises obviously at first, and then decreases to a stable state. The initial rise may be due to the activation process. After 5000 GCD cycles, the CMS-20 electrode retains 91.8% initial capacitance, which reveals a long-term electrochemical stability. Compared with Co3S4- based and Ni3S4- based materials (Table S1), the cycle performance is still outstanding. The similar GCD curves for the first and last five cycles and the small differences resistance before and after the stability test (Fig. S8b) also confirm the excellent cycle stability of the electrode. The excellent cycle performance is mainly attributed to strong contact between nanorods and Ni foam as well as the space between adjacent nanorods that can withstand volume change during redox processes. For hybrid CMS electrode, Mo, Co and Ni elements have various valence states, so all components of MoS2, Co3S4 and Ni3S4 contribute to the total capacitance. In general, Ni3S4 and Co3S4 contributes more capacitance than MoS2 [18,19,30e32]. Previous reports have proved that the MoS2 can improve capacitance and cycle stability of electrodes [14e19]. The outstanding supercapacitive properties of the CMS-20 binder-free electrode can be mainly ascribed to the following features: 1) The synergistic effect between MoS2, Co3S4 and Ni3S4 provides richer redox chemistry than those of the corresponding individual compounds. 2) The hierarchical nanosheet-on-nanorod arrays provide abundant accessible surface area for electrolyte ions. The interconnected CMS nanorods facilitate the penetration of OH and buffer volume changes during charge/discharge. Thus, the individual components are fully utilized to ensure maximal capacitive performance. 3) The strong interface between CMS and Ni foam accelerate the transport of electrons from nanorods to highly conductive Ni foam. 4. Conclusions In summary, the hierarchical CMS nanosheet-on-nanorod arrays were synthesized by a simple in-situ hydrothermal process using Ni foam as Ni source and current collector. A possible growth process was proposed based on the structure evolution, which shows that small nanosheets formed on Ni foam firstly turn into nanorods and then the nanorods covered with thin nanosheets as the hydrothermal time increases. The optimal reaction time can make the hierarchical CMS nanosheet-on-nanorod arrays exhibit the high electrochemical properties, i.e. the CMS-20 binder-free electrode exhibits a high areal capacitance of 3.94 F cm2 at 5 mA cm2 and good cycle stability with 91.8% capacitance retention after 5000 cycles. Therefore, the work provides a low-cost and facile method to prepare hierarchical transition metal sulfides electrode for supercapacitor. Acknowledgments This research work was supported by the Natural Science Foundation of Heilongjiang Province (Nos. YQ2019E031, QC2017077), the National Natural Science Foundation of China (No. 61605036). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152269.
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