Enhanced Electrochemical Performance of Self-Assembled Nanoflowers of MoS2 Nanosheets as Supercapacitor Electrode Materials.

As supercapacitor electrode materials, their structures, including specific surface area, instability, and interconnection, determine the electrochemical performances (specific capacitance, cycle stability, and rate performance). In this study, 1T-MoS2 nanosheets were self-assembled into nanoflowers via a one-pot facile hydrothermal reaction. The nanoflowers retain the excellent electrical conductive performance and the feature of inherent high specific surface area of the nanosheets. For the sheets are interconnected to each other in flower structure, the structure is more stable and the charges are more easily transferred. Thus, compared to the nanosheet electrode, the nanoflower electrode shows the remarkable advantage when used as the electrode of the energy-storage device, whether it is 1T phase or 2H phase in KCl or in KOH. When measured at 0.5 A g-1 in KOH electrolyte, the MoS2 nanoflower electrode exhibits a high specific capacitance of 1120 F g-1. At the same time, when cycling 2000 times at a current density of 10 A g-1, the capacitance retention ratio can reach up to 96%.

Introduction

Supercapacitors have been attracting much scientific and technical interest owing to their advantages of high charge/discharge rate, excellent cycle stability, and outstanding energy density compared to traditional electrochemical energy-accumulation devices like dielectric capacitors and batteries,[1] which are regarded as the inevitable part in addressing the shortage of energy and environmental problems along with fossil energy.[2,3] The leading challenge toward practical application is to improve the economics, societal acceptance, and energy density of current supercapacitors. Meanwhile, their native features of excellent power density and cyclic stability are maintained.[4] As a result, many researchers devoted efforts to eco-efficient, environmentally friendly, and crust-rich element-based electrodes, such as Fe-, Mn-, C-, and S-containing electrodes,[5−11] instead of the rare and precious metal-containing electrodes, such as Ru and Pt, and toxic Ni- and Co-containing electrodes.[12−17] After porous structure optimization, morphology control, and surface modification, carbon materials currently can alone exhibit a specific capacitance as high as 473 F g–1 at 0.5 A g–1 with a mass loading of 3 mg cm–2.[18] The high capacitance basically originates from the charge accumulation at the interface between the electrode and the electrolyte as electrochemical double-layer capacitors (EDLCs), as well as the faradic redox reaction on the doped heteroatom (mainly N and O) as pseudocapacitance.[19] Nanostructured MnO2–carbon nanotube–sponge hybrid electrodes have been synthesized successfully by Chen and colleagues.[20] At 1 mV s–1, the electrode possesses a high specific capacitance of 1230 F g–1 (based on the mass of MnO2) with a lower mass loading of less than 0.1 mg cm–2. After cycling 100 000 times at 10 V s–1, they show the excellent cyclic stability with the capacitance retention ratio of 98%, which indicates that MnO2 is a competitive alternative to RuO2 in future capacitors. Studies on other naturally abundant alternative materials, such as MoS2, TiO2, Fe2O3, V2O5, polyaniline, polythiophene, polypyrrole, and their derivatives, have also undergone great breakthroughs.[21−25]

For the sake of enhancing the electrochemical energy-storage capacity of a capacitor, it is not enough only to handle the electrode materials, although it is the main factor and we have also gained a lot by optimizing its structure, morphology, and surface.[26−28] Increasingly more researchers treat all electrode components, including electrode materials and current collectors, as a whole to make the electrochemical performances better. It is widely accepted that the assembly configuration of these components heavily determines the performances of the capacitors. Many kinds of advanced nanostructured electrode materials, including nanosheets, nanorods and nanowires, are directly grown on the metal foam current collectors to integrate the electrodes and reduce the entire internal resistance of the cell.[29−35] Layered MoS2–graphene material is fabricated successfully via a modified solution-phase method.[36] The electrode possesses a maximum specific capacitance of 243 F g–1 at 1 A g–1. At the same time, the energy density and power density are 73.5 Wh kg–1 and 19.8 kW kg–1, respectively. Fast supercapacitors were perpared using binder-free electrodes of MoS2 nanosheets grown on plasma pyrolyzed cellulose microfiber paper.[37] The electrodes can possess a power density of 12.05 W cm g–3 at 15 V s–1 in organic electrolyte. The MoS2 nanosheets and carbon cloth composite was synthesized by a traditional hydrothermal process. The electrode can reach up to the high specific capacitance of 151.1 F g–1 and possess outstanding capacitance retain ratio of 86.1% after 2000 cycles.[38] The MoS2/Bi2S3 composite electrode was synthesized via a simple hydrothermal way. It reaches a high specific capacitance of up to 120 F g–1 at 1 A g–1 and capacitance retention ratio of 87.7% after 2000 cycles.[39] Carbon–MoS2 yolk–shell microsphere electrode was synthesized via a simple hydrothermal method possess. It can reach up to the high specific capacitance of 122.6 F g–1 at 1 A g–1 and energy density of 17.03 Wh kg–1 at a power density of 500.1 W kg–1.[40] The three-dimensional flowerlike MoS2 nanosheets and two-dimensional interconnected carbon nanosheet nanocomposite was synthesized through a simple hydrothermal way. It reaches a specific capacitance of up to 381 F g–1 at 1 A g–1 and an outstanding capacitance retention ratio of 92% after 3000 cycles in KOH electrolyte.[41] MoS2/CNT nanocomposite was prepared via a simple hydrothermal method. It shows the specific capacitance of 74.05 F g–1 at 2 A g–1. At the same time, the electrode possesses the high capacitance retain ratio of 80.8% after 1000 cycles.[42] The Ag NPs/MoS2 nanocomposites prepared by a self-assembly method show excellent electrochemical performances.[43]

In this paper, we will report that besides nanostructuring to increase the accessible interface, self-assembly of these nanostructured materials into a firm structure is also beneficial for improving the electrochemical performances of a supercapacitor electrode. The assembly can enhance the probability that the nanostructured electrode materials are active and conductive to the collector and reduce the internal resistance. Because the pasting method can be further used on the self-assembled nanostructured materials to prepare electrodes, a large mass loading of active materials can be achieved. As a proof of concept, MoS2 nanosheets, including both 1T and 2H phase, were self-assembled into nanoflowers by the help of NaCl additive. After assembly, the specific capacitance of the nanoflowers increased significantly from 135 to 316 F g–1 in 3 M KCl electrolyte at 10 A g–1. As the battery-type charge storage mode can be activated in alkaline electrolytes, the specific capacitance of the 1T-MoS2 nanoflowers could reach up to 1120 F g–1 in 3 M KOH electrolyte at 0.5 A g–1.

Results and Discussion

Characterization

The prepared products were first observed through a field emission scanning electron microscope, and the typical photographs are exhibited in Figure . In Figure a–c, without addition of NaCl, we obtained millions of wrinkled MoS2 nanosheets that possess a diameter of 120 nm and thickness of 2.6 nm. They are cluttered together, leaving millions of voids among them because of the wrinkled structure.

Figure 1

FESEM images of MoS2 nanosheets (a–c) and MoS2 nanoflowers (d–f) at different magnifications.

The NaCl additive can change the product morphology dramatically, as shown in Figure d–f. Although MoS2 nanosheets with similar size can also be observed in the product, they are assembled into individual spherical particles with a diameter of ∼800 nm, which seem like rose flowers. The nanosheets are like petals. The self-assembly process of the nanosheets can also be confirmed by the transmission electron microscopy (TEM) images shown in Figure . In Figure a, the TEM image of the product without NaCl shows a relative evenly distributed mass-density contrast, which indicates randomly scattered nanosheets.

Figure 2

TEM images of MoS2 nanosheets (a) and MoS2 nanoflowers (c) (inset: the corresponding selected area electron diffraction patterns); (b, d) the corresponding high-resolution TEM images.

The dark lines correspond to the wrinkles of the sheets. However, in Figure c, the TEM image of the product with NaCl includes several radial mass-density contrast zones where the darkness gradually decays from the center to outside. The kind of mass-density contrast image is consistent with the spherical rose flower morphology observed by FESEM in Figure d–f. Of course, the nanosheet components can also be clearly observed especially in the edge area.

However, the addition of NaCl in the solution doesn’t change their crystallography and phase any more. As shown in Figure b and d, both the MoS2 nanosheets and MoS2 nanoflowers show identical spacing of interference fringe and identical diameters of the Debye–Scherrer ring, which indicates they have an identical crystallographic structure. That is further confirmed by their X-ray diffraction (XRD) patterns shown in Figure S1. They possess a similar XRD pattern in the 2θ range of 5–80°. The diffraction peaks at 2θ = 14.1, 32.9, 35.9, 39.5, and 58.8° could be unambiguously assigned to the (002), (100), (102), (103), and (110) crystal planes of JCPDS card #37-1492, respectively.[44] The phase structures were investigated by Raman spectroscopy that is an effective means to differentiate 1T-MoS2 and 2H-MoS2 on account of its sensitivity to the symmetry of the sulfur in the matrix.[45,46] As shown in Figure a, we see obvious Raman shifts of both spectra at 146, 219, and 333 cm–1, which coincided with the phonon modes in 1T-MoS2 and depressed typical Raman shifts at 378.3 and 401.1 cm–1 for 1E2g and A1g of 2H-MoS2, respectively.[47][47]

Figure 3

(a) Raman spectra of MoS2 nanoflowers and MoS2 nanosheets; X-ray photoelectron spectroscopy (XPS) images of MoS2 nanoflowers and MoS2 nanosheets: (b) survey spectra; (c) high-resolution XPS images from Mo 3d region of MoS2 nanoflowers; (d) high-resolution XPS images from Mo 3d region of MoS2 nanosheets. Contributions from 1T and 2H phase components in the Mo 3d spectrum and S 2s spectrum are distinguished through the blue and green curves, separately.

It implies that both the MoS2 nanoflowers and MoS2 nanosheets contain two components: 1T and 2H phase MoS2. Calculated through the Raman shift difference between A1g and 1E2g modes, we obtained the amount of the monolayers of MoS2 nanosheets. It is the same in both samples, about 22.8 and 24.3 cm–1, corresponding to three and four monolayers.[48,49] The phase compositions were further identified by XPS. The XPS survey spectra of MoS2 nanoflowers and MoS2 nanosheets are exhibited in Figure b. The spectra are similar to each other, and both of them contain two predominant Mo and S peaks. As is well known, the 1T phase could cause ∼1 eV chemical shift to lower binding energy on Mo 3d peaks and S 2p peaks.[50] The high-resolution XPS images of Mo 3d peaks and S 2p peaks of each sample were further studied, and the relevant spectra are exhibited in Figures c and d and S2, respectively. As shown in Figure c,d, the Mo4+ 3d5/2 and 3d3/2 peaks of MoS2 nanoflowers and those of MoS2 nanosheets are at the same binding energy position, which are 228.2 and 231.3 eV, respectively. In fact, both the 3d5/2 and 3d3/2 peaks contain two parts. One is contributed by 1T-MoS2 component located at 228.0 and 231.1 eV, and the other one is contributed by the 2H-MoS2 component located at 229.0 and 232.1 eV.[51] So the high-resolution X-ray photoelectron spectrum can be fitted by these two-component peaks, and the component content can be calculated by the fitting results. The relative content of the 1T phase in MoS2 nanoflowers is about ∼78.0%, which is also comparable to that of MoS2 nanosheets (75.3%). The multiple peaks fitting results in S 2p peaks in Figure S2 are also consistent with this conclusion. On the basis of N2 adsorption–desorption isotherms, as displayed in Figure S3, the self-assembly process did not even change the specific surface area of the sample. The Brunauer–Emmett–Teller (BET) surface area of the nanoflowers is about 54.7 m2 g–1, which is slightly lower compared to that of the nanosheets (59.8 m2 g–1).

On account of all of the characterization results, it could be summarized that the addition of NaCl in the solution only changes the nucleation mechanism and does not affect the growth. After hydrothermal reaction, the solution and the autoclave wall are carefully observed by the naked eye. As shown in Figure S4, this solution with NaCl is very clear and the corresponding autoclave is covered by a thick layer of black product. To collect the product, a chemical spoon was used to peel them from the wall. However, as none of NaCl is added in the hydrothermal reaction solution, most of the product is suspended in the solution after reaction. And the autoclave wall also becomes cleaner. We speculate that the nanosheets are generated from homogeneous nucleation. Each sheet grows from an unattached nucleus away from a surface. After adding NaCl in the solution, homogeneous nucleation becomes even harder. Most of nanosheets have to nucleate on the nucleus at the wall. For many sheets shear one nucleus, the nanosheets are self-assembled to a spherical rose flower. In the following electrochemical testing, it will be found that the self-assembly unites the sheet together. So every nanosheet is interconnected. After pasted on the collector, they have high probability to connect with the current collector and keep active. Thus, the self-assembly process significantly improves the electrochemical performances of the electrode materials.

Electrochemical Analysis

The electrochemical performances of the MoS2 nanoflower and MoS2 nanosheet electrodes with mass loading of 2 mg cm–2 as supercapacitor were measured through cyclic voltammogram (CV), galvanostatic charge–discharge curves (GCD), and electrochemical impedance spectroscopy measurements with three-electrode equipment, which consists of saturated calomel electrode (SCE) and Pt plate as the reference and counter electrodes, separately. The typical CV curves of MoS2 nanoflowers and MoS2 nanosheets measured at 20 mV s–1 in the potential window of −1.05 to −0.3 V in 3 M KCl electrolyte are exhibited in Figure a. Apparently, all obtained CV curves in KCl present a typical horizontal straight line without any obvious redox peaks. It implies that MoS2 can act as an EDLC and store charge by adsorbing them on the electrode–electrolyte interface.[52] However, the CV curve encircled area of MoS2 nanoflowers is dramatically enlarged compared to that of MoS2 nanosheets, which means a higher specific capacitance of MoS2 nanoflowers. CV curves of MoS2 nanoflowers at different scan rates (Figure b) retain their quasi-rectangular shapes as scan rates increase from 5 to 200 mV s–1. The specific capacitance of MoS2 nanoflower and MoS2 nanosheet electrodes was further confirmed through GCD measurement. The typical GCD curves of the two electrodes at 10 A g–1 shown in Figure c present classical quasi-triangular shapes owing to the EDLC mechanism of charge accumulation with voltage. At the same time, the MoS2 nanoflower electrode possesses a longer discharging time, indicating a higher special capacitance than the MoS2 nanosheet electrode, matching well with the results shown in the CV curves in Figure a. All of the GCD curves of MoS2 nanoflower electrode at different current densities are shown in Figure d. The calculated specific capacitances are summarized in Figure e. The MoS2 nanoflower electrode presents a high specific capacitance of 483 F g–1 at 0.5 A g–1 in KCl. As the current density increases 40 times to 20 A g–1, the electrode retains 305 F g–1 and obtains the capacitance retain ratio of 62.3%. The specific capacitance retention of the MoS2 nanosheet electrode is slightly higher, which is about 71.2%. However, the specific capacitance is only 120 F g–1 at 20 A g–1, which is lower than that of nanoflower electrode. The cyclability of both electrodes in KCl electrolyte was measured at 10 A g–1 over 2000 cycles. The MoS2 nanoflower and MoS2 nanosheet electrodes showed capacity ratios of 94% and 87% after 2000 cycles, respectively, as shown in Figure f, implying the MoS2 nanoflower electrode possesses excellent cycle stability.

Figure 4

(a) Comparison of CV curves of MoS2 nanoflowers and MoS2 nanosheets at 20 mV s–1 in KCl; (b) CV curves of MoS2 nanoflowers at different scanning rates; (c) comparison of GCD curves of MoS2 nanoflowers and MoS2 nanosheets at 10 A g–1; (d) GCD curves of MoS2 nanoflowers at different current densities from 0.5 to 20 A g–1; (e) specific capacitance of MoS2 nanoflower and MoS2 nanosheet electrodes at different current densities of 0.5–20 A g–1; and (f) cycling performance of MoS2 nanoflower and MoS2 nanosheet electrodes measured at 10 A g–1 for 2000 cycles.

In this work, we investigate the electrochemical performances of MoS2 nanoflowers and MoS2 nanosheets in 3 M KOH. The typical CV curves of MoS2 nanoflowers and MoS2 nanosheets measured at 20 mV s–1 in a potential window ranging from 0 to 0.75 V are exhibited in Figure a. Apparent reversible redox peaks at 0.50/0.36 and 0.44/0.35 V are seen in both CV curves, which suggest the battery-type charge storage mechanism. The area encircled by the CV curve of MoS2 nanoflower electrode is dramatically enlarged compared to the MoS2 nanosheet electrode, which indicates that MoS2 nanoflower electrode possesses a higher specific capacitance. The resulting CVs of the MoS2 nanoflower and MoS2 nanosheet electrodes at different scan rates from 5 to 200 mV s–1 are exhibited in Figures b and S5a. The specific capacitance as a function of scan rate was calculated from the obtained CV curves, as displayed in Figure S5b.

Figure 5

(a) Comparison of CV curves of MoS2 nanoflowers and MoS2 nanosheets at 20 mV s–1 in KOH; (b) CV curves of MoS2 nanoflowers at different scanning rates; (c) comparison of GCD curves of MoS2 nanoflowers and MoS2 nanosheets at 10 A g–1; (d) GCD curves of MoS2 nanoflowers at different current densities of 0.5–20 A g–1; (e) specific capacitance of MoS2 nanoflower and MoS2 nanosheet electrodes at different current densities of 0.5–20 A g–1; and (f) cycling performance of MoS2 nanoflower and MoS2 nanosheet electrodes measured at 10 A g–1 for 2000 cycles.

It shows that the specific capacitance of the MoS2 nanoflower and MoS2 nanosheet electrodes decreases gradually with increasing scan rate. It has been reported that water bilayers which absorbed on MoS2 monolayers could take part in charge storage. The charge storage mechanism is that the proton of water molecule absorbed on MoS2 could enter the water bilayers when charging in the alkaline electrolytes.[53] Benefiting from the battery-type charge storage, the specific capacitance of both electrodes increases dramatically as they are tested in KOH contrary to KCl. The specific capacitances of MoS2 nanoflowers and MoS2 nanosheets are 1346 and 434 F g–1 at 5 mV s–1, respectively. However, when the scan rate reaches up to 200 mV s–1, the specific capacitances of MoS2 nanoflowers and MoS2 nanosheets are 642 and 209 F g–1, respectively.

The excellent electrochemical performances reflected in CV experiment could be verified by the GCD measurements in KOH. Figure c shows the comparative GCD curves of MoS2 nanoflower and MoS2 nanosheet electrodes at 10 A g–1, which further supports the higher special capacitance of MoS2 nanoflower electrode owing to its longer discharging time, matching well with the result of the CV curves in Figure a. Meanwhile, all of the GCD curves display well-defined voltage plateaus, indicating battery behavior and accordance with redox peaks obtained from CV curves. In Figures d and S8, there are GCD measurements on both electrodes at different current densities. It is observed that both charging and discharging time durations decrease with increasing current density from 0.5 to 20 A g–1. The calculated specific capacitances of both electrodes at each current density are summarized in Figure e. The specific capacitances of the MoS2 nanoflower and MoS2 nanosheet electrodes are calculated to be 1120 and 380 F g–1 at the current density of 0.5 A g–1, and still maintain at 648 and 262 F g–1, respectively, when the current density increases by as much as 40 times (20 A g–1), which indicate that 57.9% of the initial specific capacitance of MoS2 nanoflower electrode is retained, compared to 68.0% for MoS2 nanosheet electrode. Both of them present an outstanding rate capability. Although the specific capacitance of MoS2 nanoflower electrode declines more dramatically than MoS2 nanosheet electrode with scan rate, it possesses threefold specific capacitance as much as MoS2 nanosheet electrode at the highest current density of 20 A g–1. In fact, the MoS2 nanoflower electrode presents the highest specific capacitance for pure MoS2 nanosheets, as shown in Figure (51,54) Even as the current density further increases to 150 A g–1, it still retains a specific capacitance of 300 F g–1, as shown in Figure S6, which is still higher than most of carbonous supercapacitor.[55−58] The cycling stability of the MoS2 nanoflower and MoS2 nanosheet electrodes was examined by repeated charge–discharge at 10 A g–1 for 2000 cycles. In Figure f, the MoS2 nanoflower and MoS2 nanosheet electrodes retained 96% and 91% of their initial specific capacitance, respectively. It means that the self-assembly can endow the electrodes a more excellent cycling stability.

Figure 6

Comparison of capacities at different rates for our pure MoS2 electrode materials and other pure electrode materials reported in different electrolytes.

To assess the practical application of MoS2 electrode, a hybrid supercapacitor equipment was prepared using the MoS2 as positive electrode and AC as negative electrode in 3 M KOH aqueous electrolyte (denoted as MoS2//AC), as schematically exhibited in Figure S7a. The CV graph of the equipment in Figure S7b exhibits close EDLC properties, indicating a capacitor behavior. And the CV curves did not show obvious distortion when the scan rate reaches up to 400 mV s–1, demonstrating a good rate character. When measured at different current densities with a potential window of 1.7 V, the GCD curves show analogous triangular shape, as shown in Figure S7c. In Figure S7d, there are curves of specific capacitance calculated through GCD measurements. The device presents an outstanding high value of 78.82 F g–1 at 0.5 A g–1. At the same time, the corresponding energy density and power density are 31.64 Wh kg–1 and 425 W kg–1. The cycle test of the device was performed through GCD measurements at 2 A g–1 up to 10 000 cycles, and the result that includes capacitance and coulombic efficiency is shown in Figure S7e. A small section of GCD curve close to the end is shown in the inset. The specific capacity and coulombic efficiency maintain a relatively stable stage after initial hundreds of cycles. In particular, the GCD curves still remain in their original shape after 10 000 cycles, and the specific capacitance retention ratio of the device reaches up to 95.4%.

Conclusions

In this study, 1T-MoS2 nanosheets and nanoflowers self-assembled by these nanosheets were fabricated via a one-pot facial hydrothermal reaction. The self-assembly enhances the interconnection and stability of these sheets. When both materials are used as electrochemical supercapacitor electrode materials, the electron transferring in the nanoflowers is easier than that in the nanosheets, as the conductive 1T-MoS2 nanosheets are connected with each other in the nanoflowers. Thus, the MoS2 nanoflower electrode shows excellent electrochemical properties in both KCl (where MoS2 acts as EDLCs) and KOH (where MoS2 acts as batteries). In KCl, the specific capacitances of the nanoflower electrode at 0.5 and 20 A g–1 are 483 and 305 F g–1, which are only 169 and 120 F g–1 for the nanosheet electrode, respectively. The electrode possesses excellent cyclic stability with the capacitance retention ratio of 94% over 2000 cycles. The specific capacitance of the nanoflower electrode further increased to 1120 F g–1 in KOH. The experiments in this paper indicate that assembling the electrode material into a stable structure is of great help in improving its electrochemical performance.

Experimental Section

Reagents and Materials

Molybdenum(VI) oxide powder (CAS: 1311-27-5); thioacetamide (CAS: 66-55-5); urea (CSA: 57-13-6); and sodium chloride (CAS: 7647-14-5).

Methods

First, 12 mg of MoO3 and 10 mL of deionized water were mixed and stirred violently for 10 min. Second, 14 mg of thioacetamide, 0.12 g of urea, and 0.68 g of NaCl were mixed in the above-prepared solution in proper sequence and stirred intensively for another 2 h to form a homogeneous solution. Third, a Teflon-lined stainless steel autoclave was loaded into an electric oven and heated for 12 h at 200 °C. Fourth, the autoclave was removed from the oven to terminate the reaction. Finally, through centrifuging, thorough washing, and freeze drying, the prepared black precipitate material was obtained. For comparison, the MoS2 nanosheets were also prepared via identical procedure without NaCl additive.

The crystallography was investigated by XRD. The morphology was characterized by FESEM and TEM. The BET and Barrett–Joyner–Halenda surface area were measured by Micromeritics ASAP 2020 analyzer. The Raman and XPS images were obtained on WITec CRM200 confocal Raman microscopy system and an ESCALAB Mk II, respectively.

Electrochemical Measurements

Supercapacitor tests were carried out using an electrochemical workstation (Princeton Applied Research) in a three-electrode system at room temperature, containing MoS2, SCE, and Pt electrodes as the working electrode, reference electrode, and counter electrode, respectively, and the 3 M KCl and KOH aqueous solution as the electrolytes. Electrodes were first cycled for 20 cycles before measurement to stabilize the electrochemical response. The slurry was the mixture that consists of the active material, acetylene black, and poly(vinylidene fluoride) binder in a weight ratio of 80:15:5 with N-methyl-2-pyrrolidinone as solvent. Then, the slurry was carefully coated onto a clean nickel foam (1 × 1 cm2) substrate with a mass loading of 2.0 mg cm–2. Finally, the electrodes were dried at 80 °C for 12 h in vacuum. The cyclic voltammograms (CVs) and galvanostatic charge–discharge (GCD) curves were characterized with a CHI-660B electrochemical workstation with 3 M KCl and KOH aqueous solutions as electrolytes.