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High-performance asymmetric supercapacitor based on 1T-MoS2 and MgAl-Layered double hydroxides
Electrochimica Acta 330 (2020) 135195
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
High-performance asymmetric supercapacitor based on 1T-MoS2 and MgAl-Layered double hydroxides Yongping Gao a, Zhengnan Wei c, Jing Xu b, * a
College of Science and Technology, Xinyang College, Xinyang, 464000, PR China College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang, 464000, PR China c Postdoctor Scientific Research Station of Shengli Petroleum Administration, SINOPEC, Dongying, 257000, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 September 2019 Received in revised form 30 October 2019 Accepted 2 November 2019 Available online 6 November 2019
In this work, carbon fiber clothes (CFC) supported 1T phase MoS2 anode material is synthesized through a one-pot hydrothermal method. The CFC-MoS2 composites has high specific capacitance of 1485.08 F g1 at the current density of 2 A g1, and the 54% of initial capacitance can be remained at the current density reached to 15 A g1. The 87.5% of initial capacitance can be harvested even at the ceaseless cycled for 10,000 cycles at 4 A g1. In addition, the MgAl layered double hydroxides and graphene oxide (MgAl-LDH-GO) cathode composite material is synthesized. The MgAl-LDH-GO cathode exhibits high specific capacitance (1208.50 F g1 at 1 A g1) and long cycle life (76.9% retention during 10,000 cycles). Finally, we successfully assemble the CFC-MoS2//MgAl-LDH-GO asymmetric supercapacitor. It has high energy density of 326.54 Wh kg1 at power density of 1500 W kg1, and the energy density can be remained at 279.56 Wh kg1 even the power density reach to 12 kW kg1. This excellent performance satisfied the current demand of the high power and energy density. Thus, the lightweight (<0.25 mg/cm2), ultrathin (<0.8 mm), and the favourable electrochemical performance make it suitable for the next generation energy storage device. © 2019 Elsevier Ltd. All rights reserved.
Keywords: 1T phase MoS2 CFC-MoS2//MgAl-LDH-GO Asymmetric supercapacitory
1. Introduction The lightweight and intelligent electronic equipment are developing at top speed. Supercapacitors exhibit the high power capability, excellent energy density, good operational safety, and long cycling stability [1e3], and thus has been widely regarded as an important class of energy storage devices. Traditional nanostructure materials unable to meet the demand of electrode materials under this tendency. Based on this consideration, we designed a lightweight electrode material composed of carbon fiber cloth supported 1T phase MoS2 and used for supercapacitors. As a star-material, MoS2 has aroused much more concern and widely used for supercapacitors. It has a layered structure and composed of three atom layers (SeMoeS) stacked together through the weak van der Waals interactions, which can enable the rapid intercalation of ions without the volume change [4e11]. Compared with 2H phase, the hydrophilic 1T phase MoS2 can increase the conductivity by 100 times, and shows excellent electrochemical performance
* Corresponding author. E-mail address: [email protected] (J. Xu). https://doi.org/10.1016/j.electacta.2019.135195 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
[12e16]. However, because of the its own flaws on electrical conductivity (5 1028 S m1), pure 1T-MoS2 can’t obtain high capacitance and good cycling performance. To address this problem, carbon fiber clothes (CFC) supported 1T-MoS2 (CFCeMoS2) anode material was synthesized through a one-pot hydrothermal method. The obtained CFC-MoS2 avoiding the energy attenuation and stability losing because of its highly utilization and slightly volume changed caused by the restacking of the sheets [17]. The porous honeycomb structure offered guarantee that the ion can transfer easily between electrode and electrolyte, making the electrode get the high capacitance and obtained the excellent rate capability due to the low transfer resistance. And the synergic effects bring by the interfacial coupling between the MoS2 nanosheet and CFC improved the capacitance and cycling life of the electrode materials. In this work, the obtained CFC-MoS2 harvested meritorious properties and exhibited a rather high specific capacitance of 1485.08 F g1 at the current density of 2 A g1. Even at the high current density of 15 A g1, 807.63 F g1 of the capacitance can be obtained. The 87.5% of initial capacitance can be harvested even ceaseless cycled it for 10,000 cycles at 4 A g1. Otherwise, the obtained CFC-MoS2 (0.25 mg per square centimeter) far lighter than
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other electrode materials based on carbon fiber clothes or nickel foam. The CFC-MoS2 can restore to its original state and remain its electrochemical properties after bending and distorting. The lightweight and excellent electrochemical properties shows latent capacity for commercial value in energy filed. As one of the multimetal clay materials, Layered double hydroxides (LDH) is consisted of positive host layers and intercalated anions/water [18]. LDH aroused much more concern in energy storage attribute to their controllable component, unique layered structure and high ion conductivity [18e22]. In current work, we also designed an MgAl based LDH and GO (MgAl-LDH-GO) composite cathode material used for supercapacitors. It exhibited a high specific capacitance of 1208.50 F g1 at 1A g1. The detailed synthesis method and steps are shown in the Scheme 1. Finally, the designed CFC-MoS2//MgAl-LDH-GO asymmetric supercapacitor displayed a high energy density of 326.54 Wh Kg1 at a power density of 1.5 kW kg1, and the high energy density of 279.56 Wh Kg1 can be obtained at the power density of 12 kW kg1. When uninterrupted cycled for 10,000 cycles at 2A g1, there are only 23.2% of the energy decrease. 2. Experimental section 2.1. Synthesis of CFC-MoS2 Before use, the CFC was treated with concentrated HNO3/H2SO4 mixed solution (volume ratio 3/1) at 100 C for 4 h. Then washed several times and dried at 60 C for 10 h. With a sample hydrothermal method, moderate Na2MoO4_2H2O and L-cysteine (concentration ratio 1/10) was dissolved in 80 mL deionized water under magnetic stirring for 30 min. Then, the homogeneous
flavescens solution was transferred to a 100 mL Teflon-lined stainless autoclave with a piece of processed CFC (3 cm*6 cm) immersed and at a 200 C degree heated for 24 h. The active materials loading weight of the MoS2 anode material is about 1.2 mg/ cm2. 2.2. Synthesis of MgAl-LDH-GO Graphite oxide (GO) was first prepared according to a modified Hummers’ method. The MgAl-LDH-GO was assembled by a typical coprecipitation method [18]. Firstly, 50 mg of GO was dispersed into 100 mL deionized water and exfoliated by sonication for 1 h. Then, 0.2 g of citric acid (CA) was dissolved in the above GO suspension for another 0.5 h. Secondly, a mixed buffered solution (100 mL) containing 32 mmol NaOH and 10 mmol Na2CO3 was added into above GO suspension under magnetic stirring to modulate the pH to 10.0 and kept for 10 min. Thirdly, 5 mmol MgSO4 and 2.5 mmol KAlSO4 were dissolved into 50 mL DI water and then transferred into CA-GO suspension. Meanwhile, the GO/ MgAl hybrid solution keeping constant pH at 10.0 by instilling buffered solution. Finally, the mixed solution was under magnetic stirring at 60 C for 4 h. Then the obtained material were further annealed under a flow of nitrogen at 900 C for 2 h. The active materials loading weight of MgAl-LDH-GO cathode is about 3 mg/ cm2. 2.3. Structural characterization The structural information and phase purity of the as-prepared samples were characterized by XRD (RigakuD/Maxr-A X-ray diffractometer (Japan) with graphite monochromatized high-
Scheme 1. Schematic illustration of CFC-MoS2 anode, MgAl-LDH-GO cathode and the assembled asymmetric supercapacitor.
Y. Gao et al. / Electrochimica Acta 330 (2020) 135195
intensity Cu Ka radiation (l ¼ 1.54,178 Å), Raman spectroscopy (Renishaw Raman spectrometer (model 1000) with a 200 mW argon-ion laser at an excitation wavelength of 514.5 nm) and XPS (PerkinElmer PHI 5600 XPS system from a monochromated aluminum anode X-ray source). Thermogravimetric analysis (TGA) was conducted on a SDTQ 6000 instrument at a ramping rate of 10 C min1 under an air flow. The morphology and microstructure of the samples were further investigated by SEM (Hitachi S-4800) and TEM (JEM 2100).
2.4. Electrochemical measurements The electrochemical properties of the individual electrode materials were carried out under a traditional three-electrode system by using CHI 660E electrochemical workstation. A platinum foil and a saturated calomel electrode were used as counter electrode and reference electrode, respectively. Electrochemical impedance spectroscopy (EIS) measurements were performed on a RSTAT 5000 electrochemical workstation with a frequency range of 0.01e100,000 Hz. All the measurements were performed in 2 M KOH aqueous solution. The specific capacitance (Cm) of the electrode can be calculated as the following equation: Cm ¼ (I *dt)/ (m*dV). Where I is the discharging current, t is the discharging times, m is the mass of active material, and V is discharging voltage.
2.5. Assemble of asymmetric supercapacitor The aqueous device was constructed with the CFC-MoS2 anode and MgAl-LDH-GO cathode in opposition. A MgAl-LDH-GO electrode was prepared by mixing of 80 wt% active material, 10 wt% carbon black and 10 wt% PTFE, then press on nickel foam, and the After dried completely, the electrode was bridged to binder-free CFC-MoS2 electrode surface to assemble full cell with 2 M KOH electrolyte solution. The anode and cathode were pressed together and separated by a cellulose membrane. To build all-solid-state supercapacitor, the two electrodes coated with PVA-KOH gel electrolyte which played the role of electrolyte and membrane were prepared and then oppressed together, airing at room temperature. The preparation process of PVA-KOH gel electrolyte as follows: appropriate PVA (z3 g) and 3 g KOH were vigorously stirring dissolved in 30 mL distilled water for 1 h at 80 C, and then obtained the uniform sol solution. The estimation of specific capacitance of materials based on the GCD curves are based on the equation given below:
C¼
I$Dt m$DV
where m is active material mass and DV corresponds to the potential window, I is the charge-discharge current, Dt represents the discharge time in the equation of GCD test. According to the capacitance of CFC-MoS2 and MgAl-LDH-GO electrode obtained from three-electrode studies, the mass ratio of þ anode and cathode is 0.8 ðm m ¼
C Cþ
Vþ D DVþ Þ. The energy density (E)
and power density (P) were calculated based on the following equations:
ð I Vdt E¼
M
and P ¼
E dt
Where M (mg) is the total mass of both anode and cathode electrode, V (V) is the voltage window of the asymmetric supercapacitor, and t (s) is discharging times.
3
3. Results and discussion 3.1. Characterizations and electrochemical performance of the CFCMoS2 The SEM image in Fig. S1 indicated that the clean CFC posses a one-dimensional uniform linear structure with a smooth surface. After hydrothermal process, MoS2 nanosheet encased the surface of CFC successfully. As shown in Fig. 1, the MoS2 nanosheet mass existence and linked together constitutes a flower-like structure in a higher concentration (0.6 mmol L1). As the concentration decreases, the thickness of MoS2 nanoshet become thinner, and evenly distributed on the surface of CFC. When the reactant concentration at 0.3 mmol L1, the MoS2 slice growth orderly and closely with CFC. The 3D porous honeycomb structure avoid overlapping-induced and the specific surface area loss, and thus shorten the ion diffusion pathways [23]. The cycling life can guaranteed thanks to the unique structure evade the volume expansion during charging and discharging. The TEM image shown in Fig. S2 also expose the thin sheet of MoS2, the high-magnification TEM image inserted in Fig. S2 expose the (003) lattice of MoS2 with a characteristic lattice fringe corresponding to 0.701 nm. For further study the structure and phase composition of CFCMoS2, XRD, XPS and Raman measurements were carried out. Fig. 2a shows the XRD pattern of CFC-MoS2, a gentle peak of clean CFC appears at 2q z 25.04 . The (001) peak of 1T-MoS2 at 7.6 dominates the restacked nanosheets in the XRD pattern. It manifest that an additional separation between layers during restacking and the monolayer nature of the obtained MoS2 nanosheets. In addition, the broader peak of (001) and (002) indicate that the nanosheets are randomly arranged in the restacking process. Fig. 2b shows XPS spectra of CFC and CFC-MoS2. The existence of N element proves functionalized of CFC. CFC-MoS2 contain abundant Mo and S elements, indicated MoS2 was prepared successfully. The Mo 3d spectrum was shown in Fig. 2c, the two obvious characteristic peaks arising from Mo 3d5/2 and Mo 3d3/2 orbitals observed at 227.9 and 231.3 eV are related to the presence of 1T-MoS2. The two small peaks located at 229.5 and 232 eV can attributed to the exist of 2HeMoS2. Furthermore, the calculated 1T phase concentration is 81% by deconvolution of the Mo 3d spectrum, which is 4 times larger than the content of 2H phase. As known, the 1T MoS2 phase is hydrophilic and 107 times more conductive than the semiconducting 2H phase. Thus, we can entirely believe that the prepared CFC-MoS2 had excellent electrical conductivity. All of the above characteristics reveal that the obtained MoS2 are mainly constitute by aimlessly arranged monolayer 1T phase MoS2 nanoflake. The Raman spectra of CFC-MoS2 shown in Fig. 2d, the obvious spectrum of carbon can be observation. The high intensity ratio of ID/IG indicates the high graphitization degree of the carbon species. Fig. 3a shows the CV curves of CFC-MoS2 electrode at a sweep potential window of 1.4 Ve0 V at different scan rates. The electrode emerged typical pseudo-capacitance behavior from 10 to 50 mV s1. The redox peaks with symmetric feature illustrating high reversibility during charging and discharging process. The peak current gradually increases with the increase of scan rate, indicating that the rapid diffusion-controlled kinetics of ion transport in the electrode and electrolyte interface. Fig. 3b shows galvanostatic charge/discharge (GCD) curves of CFC-MoS2 electrode at different current densities. The platform in GCD curves during charging and discharging indicated that the obtained electrode material conform to redox reaction mechanism, which is coordinate with CV curves. The specific capacitance shown in Fig. 2c is calculated from GCD curves. The specific capacitance is 1485.08, 1147.23, 887.58, 825.66, 810.22 and 807.63 F g1 at the current density of 2, 4, 6, 8, 10 and 15 A g1, respectively. The 54% initial
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Fig. 1. SEM images of CFC-MoS2 nanostructure at different concentration: (aec) 0.6 mmol L1, (def) 0.4 mmol L1, (gei) 0.3 mmol L1.
capacitance is reserved at the current density increased from 2 to 15 A g1 implying the high-rate capability. Fig. 3d exhibits stability performance of CFC-MoS2 electrode at increased current densities. After continuous 300 cycles at diversified density and reduced back to original current density at 4 A g1, about 90.72% initial capacitance can be obtained and follow-up for another 100 cycles without distinct decrease. The 87.5% of initial capacitance can be harvested even cycled it for 10,000 cycles at 4 A g1 (Fig. 3e). The excellent electrochemistry performances are higher than other reported MoS2 based electrode materials (Table S1). The electrochemical impedance spectroscopy (EIS) measurements shown in Fig. 3f, the resistance of CFC-MoS2 is about 0.70 U, suggesting that the high charge transfer and ion diffusion rate. Arguably, the attractive performance of CFC-MoS2 is related to the lower equivalent resistance. A possible reason is that the uniformly distribute porous structure improved the utilization of 1T phase MoS2 in the electrochemical reactions process. Moreover, the well-designed nanoscale thickness honeycomb structure obviously decrease the electrolyte ions and electrons diffusion path lengths, and thus improving the electrolyte ions diffusion and migration rate in the charge/discharge process. Furthermore, the voids interspersed between the nanosheet can facilitate the electrolyte ions
transportation rate and avoid volume change in the charge/ discharge process. In addition, the carbon fiber cloth ensured the excellent distribution of MoS2, making them fully electrochemically accessible. Therefore, the carbon fiber clothes supported 1T phase MoS2 shows an excellent electrochemical performance.
3.2. Characterizations and electrochemical performance of the MgAl-LDH-GO According to SEM image in Fig. 4a, MgAl-LDH and GO sheet are vertical bridged together. The interlaced nanosheet structure can be seen through TEM image as shown in Fig. 4b. From the lattice fringes in HRTEM image (Fig. 4c), interplanar distance of 0.58 nm can be attributed to the (006) plan of MgAl-LDH. Calcined LDHs have memory effect which can regenerate the original layered structure once contact with an anion in aqueous solution, or simply ambient moisture [24,25]. Fig. 4d illustrate the XRD pattern of GO and MgAl-LDH-GO. As shown in Fig. 4d, the LDH phase is transformed into an Mg(Al)O solid solution, with XRD patterns showing an MgO crystalline phase after calcination [26]. The XPS spectrum was used for exploring the constituent of MgAl-LDH-GO composite. In Fig. 4e, the obvious peaks of Mg and Al elements illustrate that
Y. Gao et al. / Electrochimica Acta 330 (2020) 135195
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Fig. 2. (a)XRD pattern of CFC-MoS2, (b)XPS spectrum of clean CFC and CFC-MoS2, (c)XPS survey scan of Mo3d, (d)Raman spectra of CFC-MoS2.
the positive introduction of Mg and Al to GO nanosheet. From the Raman spectrum in Fig. 4f, the laigh ratio of ID/IG illustrates less defects remaine in the graphitic structure. On the basis of N2 adsorption/desorption measurement (Fig. S3), the adsorption curve is A type, with the surface area about 43.13 m2 g1. It is worth noting that the mesopores were concentrate upon 1.5e2 nm (Fig. 3b). The appropriate pore size and surface area signifying that the electronic can intercalated LDH layer efficiency. The TGA (Fig. S4) testing were detected the composition of MgAl-LDH-GO in air atmosphere under 900 C. The 63.33 wt% lost show that 36.67 wt% of GO are contained. The electrochemical performances were shown in Fig. 5. The CV curves of the obtained MgAl-LDH-GO composite electrode with a electrochemical windows from 0 to 0.9 V at different scan rates (10e50 mV s1) were shown in Fig. 5a. The typical redox peaks implied that the capacitance characteristics are mainly from Faradaic pseudocapacitance instead of the electric double layer capacitors (EDLC). These shined redox peaks can be interpreted as the reversible chemical reactions of M-O/M-O-OH (M ¼ Mg and Al). Because of the electron hopping resistance to ion and electron transfer, the redox peaks skews slightly with the augment of the scan rate. For further research the MgAl-LDH-GO electrode, the GCD measurements were also executed at a series current density with the potential window of 0e0.5 V. It can be seen that there are
two obvious voltage plateaus in the charge and discharge curves in Fig. 5b, which are matched with the reduction peakes in the CV curves. The Fig. 5c shows the specific capacitance of 1208.50, 1135.24, 995.43, 882.22 and 679.50 F g1 at 1, 2, 3, 5 and 10 A g1, respectively. The transport kinetics during electrochemical reaction process was tested by EIS measurements as shown in Fig. 5e. The MgAl-LDH-GO electrode shows low electrochemical resistance (Rs) about 0.24 U, lower than GO, exhibiting that MgAl-LDH-GO has low charge transfer resistance and high ion diffusion rate. The excellent electrochemical performance can be owing to the rapid ion transfer between the porous electrode material as the picture shown in Fig. 5f.
3.3. The assembly and performance of the 1T-MoS2@CFC//MgAlLDH-GO asymmetric supercapacitor To further exploration the application value, the asymmetric supercapacitor was building by connect with MgAl-LDH-GO cathode and CFC-MoS2 anode as shown in Fig. 6a. For achieving the best possible comprehensive electrochemical performance, the charge balance between anode and cathode is crucial. According to the capacitance of CFC-MoS2 and MgAl-LDH-GO electrode obtained from three-electrode studies, the mass ratio of anode and cathode is 0.8. Fig. 6b shows the CV curves of the asymmetric supercapacitor
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Fig. 3. Electrochemical performance of CFC-MoS2. (a) CV curves. (b) GCD curves. (c) Specific capacitance at different scan rete. (d) Rate performance and cycling stability. (e) Cycling performance. (f) Nyquist plot.
Fig. 4. (aec) SEM, TEM and HRTEM of MgAl-LDH-GO. (def) XRD, XPS and Raman spectra of MgAl- LDH-GO.
at the scan rate from 20 to 10 mV s1. There are two redox peaks in all CV curves indicating that the redox reactions occur in the electrode and electrolyte interface. The discharge curves and the corresponding specific capacitance at different current densities
are shown in Fig. 6c and d, respectively. The asymmetric supercapacitor delivers the capacitance of 643.66, 519.43, 505.64, 481.11 and 447.87 F g1 at the current density of 1, 2, 3, 5 and 10 A g1, respectively. In addition, the 76.8% of initial specific capacitance can
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Fig. 5. The electrochemical performance of MgAl-LDH-GO. a) The CV curves at different scan rate. b) GCD curves at various current density. c) The specific capacitance at different current density. d) The cycling life at the scan rate of 20 mV s1. e) The EIS spectral of MgAl-LDH-GO and GO. f) The schematic of ion transfer in the porous MgAl-LDH-GO.
Fig. 6. a) Fabricated CFC-MoS2//MgAl-LDH-GO asymmetric supercapacitor. b) CV curves, c) galvanostatic discharge curves, d) specific capacitance at the current density from 1 to 10 A g1, e) cycling performance and the Nyquist plots of asymmetric supercapacitor.
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electrodes. The benign application further proof the CFC-MoS2 can be put into practice. Therefore, we can confident that this remarkable material opening a window to assemble high performance supercapacitor and the other energy storage devices. Acknowledgments This work is supported by the training plan of young backbone teachers in Colleges and universities of Henan Province (2019GGJS287), Key Scientific Research Projects of Henan Province (19A150045), Key Project of Xinyang College (2019-XJLYB-009). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.135195. References Fig. 7. Ragone plots of the as-assembled CFC-MoS2//MgAl-LDH-GO asymmetric supercapacitor and comparision with other MoS2 based materials.
be retained after 10,000 cycles at the current density of 2 A g1 (Fig. 6e). This high electrode capacitance can be attributed to the low polarization resistance value and the high charge transfer rate of the prepared materials. According to the Nyquist plot of asymmetric shown in Fig. 6f, the asymmetric capacitor shows a low electrochemical resistance (Rs) of 0.86 U. The low charge transfer resistance and high ion diffusion rate are in agreement with the remarkable performance especially at high rates. The energy and power densities of the CFC-MoS2//MgAl-LDH-GO asymmetric supercapacitor was shown in the Ragone plot (Fig. 7). The asymmetric supercapacitor shows an energy density of 326.54 Wh Kg1 at a power density of 1500 W kg1. Even at the power density of 12 kW kg1, it also remains an energy density of 279.56 Wh Kg1, which is higher than others MoS2 based supercapacitors [17,27]. Accordingly, the stable architecture, enhanced electrochemical kinetics and fast ion transfer caused the CFC-MoS2//MgAl-LDH-GO asymmetric supercapacitor get the high energy and power densities with excellent cycling stability. Furthermore, the simple synthetic method and lower manufacturing costs help it applied to the practical production. 4. Conclusion In conclusion, the lightweight carbon fiber clothe supported 1T phase MoS2 anode was synthesized by a simple one-pot hydrothermal method. The excellent lightweight properties with high specific capacitance (1485.08 F g1 at 2A g1) and cycling performance (87.5% retention during 10,000 cycles) indicate that it can be used for the wearable and intelligent electronic equipment. The prepared CFC-MoS2 and MgAl-LDH-GO as anode and cathode to assemble CFC-MoS2//MgAl-LDH-GO asymmetric supercapacitor. The designed asymmetric supercapacitor exhibits a high energy density up to 279.56 Wh Kg1 at a very high power density of 12 kW kg1, and the excellent cyclic performance. Moreover, the all-solid supercapacitor are assembled by using the same
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
High-performance asymmetric supercapacitor based on 1T-MoS2 and MgAl-Layered double hydroxides Yongping Gao a, Zhengnan Wei c, Jing Xu b, * a
College of Science and Technology, Xinyang College, Xinyang, 464000, PR China College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang, 464000, PR China c Postdoctor Scientific Research Station of Shengli Petroleum Administration, SINOPEC, Dongying, 257000, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 September 2019 Received in revised form 30 October 2019 Accepted 2 November 2019 Available online 6 November 2019
In this work, carbon fiber clothes (CFC) supported 1T phase MoS2 anode material is synthesized through a one-pot hydrothermal method. The CFC-MoS2 composites has high specific capacitance of 1485.08 F g1 at the current density of 2 A g1, and the 54% of initial capacitance can be remained at the current density reached to 15 A g1. The 87.5% of initial capacitance can be harvested even at the ceaseless cycled for 10,000 cycles at 4 A g1. In addition, the MgAl layered double hydroxides and graphene oxide (MgAl-LDH-GO) cathode composite material is synthesized. The MgAl-LDH-GO cathode exhibits high specific capacitance (1208.50 F g1 at 1 A g1) and long cycle life (76.9% retention during 10,000 cycles). Finally, we successfully assemble the CFC-MoS2//MgAl-LDH-GO asymmetric supercapacitor. It has high energy density of 326.54 Wh kg1 at power density of 1500 W kg1, and the energy density can be remained at 279.56 Wh kg1 even the power density reach to 12 kW kg1. This excellent performance satisfied the current demand of the high power and energy density. Thus, the lightweight (<0.25 mg/cm2), ultrathin (<0.8 mm), and the favourable electrochemical performance make it suitable for the next generation energy storage device. © 2019 Elsevier Ltd. All rights reserved.
Keywords: 1T phase MoS2 CFC-MoS2//MgAl-LDH-GO Asymmetric supercapacitory
1. Introduction The lightweight and intelligent electronic equipment are developing at top speed. Supercapacitors exhibit the high power capability, excellent energy density, good operational safety, and long cycling stability [1e3], and thus has been widely regarded as an important class of energy storage devices. Traditional nanostructure materials unable to meet the demand of electrode materials under this tendency. Based on this consideration, we designed a lightweight electrode material composed of carbon fiber cloth supported 1T phase MoS2 and used for supercapacitors. As a star-material, MoS2 has aroused much more concern and widely used for supercapacitors. It has a layered structure and composed of three atom layers (SeMoeS) stacked together through the weak van der Waals interactions, which can enable the rapid intercalation of ions without the volume change [4e11]. Compared with 2H phase, the hydrophilic 1T phase MoS2 can increase the conductivity by 100 times, and shows excellent electrochemical performance
* Corresponding author. E-mail address: [email protected] (J. Xu). https://doi.org/10.1016/j.electacta.2019.135195 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
[12e16]. However, because of the its own flaws on electrical conductivity (5 1028 S m1), pure 1T-MoS2 can’t obtain high capacitance and good cycling performance. To address this problem, carbon fiber clothes (CFC) supported 1T-MoS2 (CFCeMoS2) anode material was synthesized through a one-pot hydrothermal method. The obtained CFC-MoS2 avoiding the energy attenuation and stability losing because of its highly utilization and slightly volume changed caused by the restacking of the sheets [17]. The porous honeycomb structure offered guarantee that the ion can transfer easily between electrode and electrolyte, making the electrode get the high capacitance and obtained the excellent rate capability due to the low transfer resistance. And the synergic effects bring by the interfacial coupling between the MoS2 nanosheet and CFC improved the capacitance and cycling life of the electrode materials. In this work, the obtained CFC-MoS2 harvested meritorious properties and exhibited a rather high specific capacitance of 1485.08 F g1 at the current density of 2 A g1. Even at the high current density of 15 A g1, 807.63 F g1 of the capacitance can be obtained. The 87.5% of initial capacitance can be harvested even ceaseless cycled it for 10,000 cycles at 4 A g1. Otherwise, the obtained CFC-MoS2 (0.25 mg per square centimeter) far lighter than
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other electrode materials based on carbon fiber clothes or nickel foam. The CFC-MoS2 can restore to its original state and remain its electrochemical properties after bending and distorting. The lightweight and excellent electrochemical properties shows latent capacity for commercial value in energy filed. As one of the multimetal clay materials, Layered double hydroxides (LDH) is consisted of positive host layers and intercalated anions/water [18]. LDH aroused much more concern in energy storage attribute to their controllable component, unique layered structure and high ion conductivity [18e22]. In current work, we also designed an MgAl based LDH and GO (MgAl-LDH-GO) composite cathode material used for supercapacitors. It exhibited a high specific capacitance of 1208.50 F g1 at 1A g1. The detailed synthesis method and steps are shown in the Scheme 1. Finally, the designed CFC-MoS2//MgAl-LDH-GO asymmetric supercapacitor displayed a high energy density of 326.54 Wh Kg1 at a power density of 1.5 kW kg1, and the high energy density of 279.56 Wh Kg1 can be obtained at the power density of 12 kW kg1. When uninterrupted cycled for 10,000 cycles at 2A g1, there are only 23.2% of the energy decrease. 2. Experimental section 2.1. Synthesis of CFC-MoS2 Before use, the CFC was treated with concentrated HNO3/H2SO4 mixed solution (volume ratio 3/1) at 100 C for 4 h. Then washed several times and dried at 60 C for 10 h. With a sample hydrothermal method, moderate Na2MoO4_2H2O and L-cysteine (concentration ratio 1/10) was dissolved in 80 mL deionized water under magnetic stirring for 30 min. Then, the homogeneous
flavescens solution was transferred to a 100 mL Teflon-lined stainless autoclave with a piece of processed CFC (3 cm*6 cm) immersed and at a 200 C degree heated for 24 h. The active materials loading weight of the MoS2 anode material is about 1.2 mg/ cm2. 2.2. Synthesis of MgAl-LDH-GO Graphite oxide (GO) was first prepared according to a modified Hummers’ method. The MgAl-LDH-GO was assembled by a typical coprecipitation method [18]. Firstly, 50 mg of GO was dispersed into 100 mL deionized water and exfoliated by sonication for 1 h. Then, 0.2 g of citric acid (CA) was dissolved in the above GO suspension for another 0.5 h. Secondly, a mixed buffered solution (100 mL) containing 32 mmol NaOH and 10 mmol Na2CO3 was added into above GO suspension under magnetic stirring to modulate the pH to 10.0 and kept for 10 min. Thirdly, 5 mmol MgSO4 and 2.5 mmol KAlSO4 were dissolved into 50 mL DI water and then transferred into CA-GO suspension. Meanwhile, the GO/ MgAl hybrid solution keeping constant pH at 10.0 by instilling buffered solution. Finally, the mixed solution was under magnetic stirring at 60 C for 4 h. Then the obtained material were further annealed under a flow of nitrogen at 900 C for 2 h. The active materials loading weight of MgAl-LDH-GO cathode is about 3 mg/ cm2. 2.3. Structural characterization The structural information and phase purity of the as-prepared samples were characterized by XRD (RigakuD/Maxr-A X-ray diffractometer (Japan) with graphite monochromatized high-
Scheme 1. Schematic illustration of CFC-MoS2 anode, MgAl-LDH-GO cathode and the assembled asymmetric supercapacitor.
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intensity Cu Ka radiation (l ¼ 1.54,178 Å), Raman spectroscopy (Renishaw Raman spectrometer (model 1000) with a 200 mW argon-ion laser at an excitation wavelength of 514.5 nm) and XPS (PerkinElmer PHI 5600 XPS system from a monochromated aluminum anode X-ray source). Thermogravimetric analysis (TGA) was conducted on a SDTQ 6000 instrument at a ramping rate of 10 C min1 under an air flow. The morphology and microstructure of the samples were further investigated by SEM (Hitachi S-4800) and TEM (JEM 2100).
2.4. Electrochemical measurements The electrochemical properties of the individual electrode materials were carried out under a traditional three-electrode system by using CHI 660E electrochemical workstation. A platinum foil and a saturated calomel electrode were used as counter electrode and reference electrode, respectively. Electrochemical impedance spectroscopy (EIS) measurements were performed on a RSTAT 5000 electrochemical workstation with a frequency range of 0.01e100,000 Hz. All the measurements were performed in 2 M KOH aqueous solution. The specific capacitance (Cm) of the electrode can be calculated as the following equation: Cm ¼ (I *dt)/ (m*dV). Where I is the discharging current, t is the discharging times, m is the mass of active material, and V is discharging voltage.
2.5. Assemble of asymmetric supercapacitor The aqueous device was constructed with the CFC-MoS2 anode and MgAl-LDH-GO cathode in opposition. A MgAl-LDH-GO electrode was prepared by mixing of 80 wt% active material, 10 wt% carbon black and 10 wt% PTFE, then press on nickel foam, and the After dried completely, the electrode was bridged to binder-free CFC-MoS2 electrode surface to assemble full cell with 2 M KOH electrolyte solution. The anode and cathode were pressed together and separated by a cellulose membrane. To build all-solid-state supercapacitor, the two electrodes coated with PVA-KOH gel electrolyte which played the role of electrolyte and membrane were prepared and then oppressed together, airing at room temperature. The preparation process of PVA-KOH gel electrolyte as follows: appropriate PVA (z3 g) and 3 g KOH were vigorously stirring dissolved in 30 mL distilled water for 1 h at 80 C, and then obtained the uniform sol solution. The estimation of specific capacitance of materials based on the GCD curves are based on the equation given below:
C¼
I$Dt m$DV
where m is active material mass and DV corresponds to the potential window, I is the charge-discharge current, Dt represents the discharge time in the equation of GCD test. According to the capacitance of CFC-MoS2 and MgAl-LDH-GO electrode obtained from three-electrode studies, the mass ratio of þ anode and cathode is 0.8 ðm m ¼
C Cþ
Vþ D DVþ Þ. The energy density (E)
and power density (P) were calculated based on the following equations:
ð I Vdt E¼
M
and P ¼
E dt
Where M (mg) is the total mass of both anode and cathode electrode, V (V) is the voltage window of the asymmetric supercapacitor, and t (s) is discharging times.
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3. Results and discussion 3.1. Characterizations and electrochemical performance of the CFCMoS2 The SEM image in Fig. S1 indicated that the clean CFC posses a one-dimensional uniform linear structure with a smooth surface. After hydrothermal process, MoS2 nanosheet encased the surface of CFC successfully. As shown in Fig. 1, the MoS2 nanosheet mass existence and linked together constitutes a flower-like structure in a higher concentration (0.6 mmol L1). As the concentration decreases, the thickness of MoS2 nanoshet become thinner, and evenly distributed on the surface of CFC. When the reactant concentration at 0.3 mmol L1, the MoS2 slice growth orderly and closely with CFC. The 3D porous honeycomb structure avoid overlapping-induced and the specific surface area loss, and thus shorten the ion diffusion pathways [23]. The cycling life can guaranteed thanks to the unique structure evade the volume expansion during charging and discharging. The TEM image shown in Fig. S2 also expose the thin sheet of MoS2, the high-magnification TEM image inserted in Fig. S2 expose the (003) lattice of MoS2 with a characteristic lattice fringe corresponding to 0.701 nm. For further study the structure and phase composition of CFCMoS2, XRD, XPS and Raman measurements were carried out. Fig. 2a shows the XRD pattern of CFC-MoS2, a gentle peak of clean CFC appears at 2q z 25.04 . The (001) peak of 1T-MoS2 at 7.6 dominates the restacked nanosheets in the XRD pattern. It manifest that an additional separation between layers during restacking and the monolayer nature of the obtained MoS2 nanosheets. In addition, the broader peak of (001) and (002) indicate that the nanosheets are randomly arranged in the restacking process. Fig. 2b shows XPS spectra of CFC and CFC-MoS2. The existence of N element proves functionalized of CFC. CFC-MoS2 contain abundant Mo and S elements, indicated MoS2 was prepared successfully. The Mo 3d spectrum was shown in Fig. 2c, the two obvious characteristic peaks arising from Mo 3d5/2 and Mo 3d3/2 orbitals observed at 227.9 and 231.3 eV are related to the presence of 1T-MoS2. The two small peaks located at 229.5 and 232 eV can attributed to the exist of 2HeMoS2. Furthermore, the calculated 1T phase concentration is 81% by deconvolution of the Mo 3d spectrum, which is 4 times larger than the content of 2H phase. As known, the 1T MoS2 phase is hydrophilic and 107 times more conductive than the semiconducting 2H phase. Thus, we can entirely believe that the prepared CFC-MoS2 had excellent electrical conductivity. All of the above characteristics reveal that the obtained MoS2 are mainly constitute by aimlessly arranged monolayer 1T phase MoS2 nanoflake. The Raman spectra of CFC-MoS2 shown in Fig. 2d, the obvious spectrum of carbon can be observation. The high intensity ratio of ID/IG indicates the high graphitization degree of the carbon species. Fig. 3a shows the CV curves of CFC-MoS2 electrode at a sweep potential window of 1.4 Ve0 V at different scan rates. The electrode emerged typical pseudo-capacitance behavior from 10 to 50 mV s1. The redox peaks with symmetric feature illustrating high reversibility during charging and discharging process. The peak current gradually increases with the increase of scan rate, indicating that the rapid diffusion-controlled kinetics of ion transport in the electrode and electrolyte interface. Fig. 3b shows galvanostatic charge/discharge (GCD) curves of CFC-MoS2 electrode at different current densities. The platform in GCD curves during charging and discharging indicated that the obtained electrode material conform to redox reaction mechanism, which is coordinate with CV curves. The specific capacitance shown in Fig. 2c is calculated from GCD curves. The specific capacitance is 1485.08, 1147.23, 887.58, 825.66, 810.22 and 807.63 F g1 at the current density of 2, 4, 6, 8, 10 and 15 A g1, respectively. The 54% initial
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Fig. 1. SEM images of CFC-MoS2 nanostructure at different concentration: (aec) 0.6 mmol L1, (def) 0.4 mmol L1, (gei) 0.3 mmol L1.
capacitance is reserved at the current density increased from 2 to 15 A g1 implying the high-rate capability. Fig. 3d exhibits stability performance of CFC-MoS2 electrode at increased current densities. After continuous 300 cycles at diversified density and reduced back to original current density at 4 A g1, about 90.72% initial capacitance can be obtained and follow-up for another 100 cycles without distinct decrease. The 87.5% of initial capacitance can be harvested even cycled it for 10,000 cycles at 4 A g1 (Fig. 3e). The excellent electrochemistry performances are higher than other reported MoS2 based electrode materials (Table S1). The electrochemical impedance spectroscopy (EIS) measurements shown in Fig. 3f, the resistance of CFC-MoS2 is about 0.70 U, suggesting that the high charge transfer and ion diffusion rate. Arguably, the attractive performance of CFC-MoS2 is related to the lower equivalent resistance. A possible reason is that the uniformly distribute porous structure improved the utilization of 1T phase MoS2 in the electrochemical reactions process. Moreover, the well-designed nanoscale thickness honeycomb structure obviously decrease the electrolyte ions and electrons diffusion path lengths, and thus improving the electrolyte ions diffusion and migration rate in the charge/discharge process. Furthermore, the voids interspersed between the nanosheet can facilitate the electrolyte ions
transportation rate and avoid volume change in the charge/ discharge process. In addition, the carbon fiber cloth ensured the excellent distribution of MoS2, making them fully electrochemically accessible. Therefore, the carbon fiber clothes supported 1T phase MoS2 shows an excellent electrochemical performance.
3.2. Characterizations and electrochemical performance of the MgAl-LDH-GO According to SEM image in Fig. 4a, MgAl-LDH and GO sheet are vertical bridged together. The interlaced nanosheet structure can be seen through TEM image as shown in Fig. 4b. From the lattice fringes in HRTEM image (Fig. 4c), interplanar distance of 0.58 nm can be attributed to the (006) plan of MgAl-LDH. Calcined LDHs have memory effect which can regenerate the original layered structure once contact with an anion in aqueous solution, or simply ambient moisture [24,25]. Fig. 4d illustrate the XRD pattern of GO and MgAl-LDH-GO. As shown in Fig. 4d, the LDH phase is transformed into an Mg(Al)O solid solution, with XRD patterns showing an MgO crystalline phase after calcination [26]. The XPS spectrum was used for exploring the constituent of MgAl-LDH-GO composite. In Fig. 4e, the obvious peaks of Mg and Al elements illustrate that
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Fig. 2. (a)XRD pattern of CFC-MoS2, (b)XPS spectrum of clean CFC and CFC-MoS2, (c)XPS survey scan of Mo3d, (d)Raman spectra of CFC-MoS2.
the positive introduction of Mg and Al to GO nanosheet. From the Raman spectrum in Fig. 4f, the laigh ratio of ID/IG illustrates less defects remaine in the graphitic structure. On the basis of N2 adsorption/desorption measurement (Fig. S3), the adsorption curve is A type, with the surface area about 43.13 m2 g1. It is worth noting that the mesopores were concentrate upon 1.5e2 nm (Fig. 3b). The appropriate pore size and surface area signifying that the electronic can intercalated LDH layer efficiency. The TGA (Fig. S4) testing were detected the composition of MgAl-LDH-GO in air atmosphere under 900 C. The 63.33 wt% lost show that 36.67 wt% of GO are contained. The electrochemical performances were shown in Fig. 5. The CV curves of the obtained MgAl-LDH-GO composite electrode with a electrochemical windows from 0 to 0.9 V at different scan rates (10e50 mV s1) were shown in Fig. 5a. The typical redox peaks implied that the capacitance characteristics are mainly from Faradaic pseudocapacitance instead of the electric double layer capacitors (EDLC). These shined redox peaks can be interpreted as the reversible chemical reactions of M-O/M-O-OH (M ¼ Mg and Al). Because of the electron hopping resistance to ion and electron transfer, the redox peaks skews slightly with the augment of the scan rate. For further research the MgAl-LDH-GO electrode, the GCD measurements were also executed at a series current density with the potential window of 0e0.5 V. It can be seen that there are
two obvious voltage plateaus in the charge and discharge curves in Fig. 5b, which are matched with the reduction peakes in the CV curves. The Fig. 5c shows the specific capacitance of 1208.50, 1135.24, 995.43, 882.22 and 679.50 F g1 at 1, 2, 3, 5 and 10 A g1, respectively. The transport kinetics during electrochemical reaction process was tested by EIS measurements as shown in Fig. 5e. The MgAl-LDH-GO electrode shows low electrochemical resistance (Rs) about 0.24 U, lower than GO, exhibiting that MgAl-LDH-GO has low charge transfer resistance and high ion diffusion rate. The excellent electrochemical performance can be owing to the rapid ion transfer between the porous electrode material as the picture shown in Fig. 5f.
3.3. The assembly and performance of the 1T-MoS2@CFC//MgAlLDH-GO asymmetric supercapacitor To further exploration the application value, the asymmetric supercapacitor was building by connect with MgAl-LDH-GO cathode and CFC-MoS2 anode as shown in Fig. 6a. For achieving the best possible comprehensive electrochemical performance, the charge balance between anode and cathode is crucial. According to the capacitance of CFC-MoS2 and MgAl-LDH-GO electrode obtained from three-electrode studies, the mass ratio of anode and cathode is 0.8. Fig. 6b shows the CV curves of the asymmetric supercapacitor
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Fig. 3. Electrochemical performance of CFC-MoS2. (a) CV curves. (b) GCD curves. (c) Specific capacitance at different scan rete. (d) Rate performance and cycling stability. (e) Cycling performance. (f) Nyquist plot.
Fig. 4. (aec) SEM, TEM and HRTEM of MgAl-LDH-GO. (def) XRD, XPS and Raman spectra of MgAl- LDH-GO.
at the scan rate from 20 to 10 mV s1. There are two redox peaks in all CV curves indicating that the redox reactions occur in the electrode and electrolyte interface. The discharge curves and the corresponding specific capacitance at different current densities
are shown in Fig. 6c and d, respectively. The asymmetric supercapacitor delivers the capacitance of 643.66, 519.43, 505.64, 481.11 and 447.87 F g1 at the current density of 1, 2, 3, 5 and 10 A g1, respectively. In addition, the 76.8% of initial specific capacitance can
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Fig. 5. The electrochemical performance of MgAl-LDH-GO. a) The CV curves at different scan rate. b) GCD curves at various current density. c) The specific capacitance at different current density. d) The cycling life at the scan rate of 20 mV s1. e) The EIS spectral of MgAl-LDH-GO and GO. f) The schematic of ion transfer in the porous MgAl-LDH-GO.
Fig. 6. a) Fabricated CFC-MoS2//MgAl-LDH-GO asymmetric supercapacitor. b) CV curves, c) galvanostatic discharge curves, d) specific capacitance at the current density from 1 to 10 A g1, e) cycling performance and the Nyquist plots of asymmetric supercapacitor.
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electrodes. The benign application further proof the CFC-MoS2 can be put into practice. Therefore, we can confident that this remarkable material opening a window to assemble high performance supercapacitor and the other energy storage devices. Acknowledgments This work is supported by the training plan of young backbone teachers in Colleges and universities of Henan Province (2019GGJS287), Key Scientific Research Projects of Henan Province (19A150045), Key Project of Xinyang College (2019-XJLYB-009). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.135195. References Fig. 7. Ragone plots of the as-assembled CFC-MoS2//MgAl-LDH-GO asymmetric supercapacitor and comparision with other MoS2 based materials.
be retained after 10,000 cycles at the current density of 2 A g1 (Fig. 6e). This high electrode capacitance can be attributed to the low polarization resistance value and the high charge transfer rate of the prepared materials. According to the Nyquist plot of asymmetric shown in Fig. 6f, the asymmetric capacitor shows a low electrochemical resistance (Rs) of 0.86 U. The low charge transfer resistance and high ion diffusion rate are in agreement with the remarkable performance especially at high rates. The energy and power densities of the CFC-MoS2//MgAl-LDH-GO asymmetric supercapacitor was shown in the Ragone plot (Fig. 7). The asymmetric supercapacitor shows an energy density of 326.54 Wh Kg1 at a power density of 1500 W kg1. Even at the power density of 12 kW kg1, it also remains an energy density of 279.56 Wh Kg1, which is higher than others MoS2 based supercapacitors [17,27]. Accordingly, the stable architecture, enhanced electrochemical kinetics and fast ion transfer caused the CFC-MoS2//MgAl-LDH-GO asymmetric supercapacitor get the high energy and power densities with excellent cycling stability. Furthermore, the simple synthetic method and lower manufacturing costs help it applied to the practical production. 4. Conclusion In conclusion, the lightweight carbon fiber clothe supported 1T phase MoS2 anode was synthesized by a simple one-pot hydrothermal method. The excellent lightweight properties with high specific capacitance (1485.08 F g1 at 2A g1) and cycling performance (87.5% retention during 10,000 cycles) indicate that it can be used for the wearable and intelligent electronic equipment. The prepared CFC-MoS2 and MgAl-LDH-GO as anode and cathode to assemble CFC-MoS2//MgAl-LDH-GO asymmetric supercapacitor. The designed asymmetric supercapacitor exhibits a high energy density up to 279.56 Wh Kg1 at a very high power density of 12 kW kg1, and the excellent cyclic performance. Moreover, the all-solid supercapacitor are assembled by using the same
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