The in-situ synthesis of MnO2@MoS2/PPy ternary composite was shown in Fig. 1a, MoS2 sheet was successfully prepared by hydrothermal reaction after keeping the temperature at 220℃. In this system, H8MoN2O4 and CH4N2S provided the source of Mo and S, respectively. During the experimental process, CH4N2S was converted to hydrogen sulfide, ammonia and carbon dioxide in hydrothermal solution. More importantly, due to the reductive character, H2S could react with H8MoN2O4, reducing the chemical valence of molybdenum ion from +6 to +4. The synthesis of MoS2 sheet follows equations (2) and (3):
Schematic diagram of synthetic progress of MnO2@MoS2/PPy ternary composite
Then MnO2 was formed in-situ growth on as-prepared MoS2 sheet by hydrothermal method (Fig. 1b). The MnSO4 and KMnO4 reacted could be described as Equation (4):
The growth of PPy on the 2D sheet surfaces was exhibited in Fig. 1c. Under the oxidation of APS, pyrrole monomers was supported on MnO2@MoS2 binary complex sheets through in-situ oxidation polymerization at room temperature. The strong coordination interaction generated between N groups and Mo ion. The MnO2 particles, in coordination with the MoS2, formed a rigid support and space for ion permeation. Therefore, charge exchange efficiency and stability of the PPy was improved. The method of in-situ grown provided a strong connection for the ternary structure, as well the possibility of rapid electrolyte penetration.
The morphological characteristics of MnO2@MoS2/PPy composite was monitored in SEM, TEM and energy dispersive X-ray spectroscopy (EDS). The results of Fig. 2a revealed that PPy was closely deposited on the surface of MnO2@MoS2 sheet. The TEM image (Fig. 2b) of ternary hybrid further presented the uniform loading between MoS2, MnO2 and PPy spherical particles. Evidently, MoS2 was achieved with a suitable layer structure, owning the smooth edge of the film and natural wrinkle after hydrothermal. As marked in the green lines, the high-resolution TEM (HR-TEM) image in inset (b) clearly indicated the lattice fringe spacing of 0.61 nm which could be ascribed to (002) plane of MoS2. The characteristic of MnO2@MoS2 sheet provided rational electrochemically active surface sites for PPy covering. Furthermore, the EDS elemental mappings of N, C, O, S and Mo elements, which were taken from the enclosed area in Fig. 2a, confirmed the composite structure (Fig. 2c). The elements of N and C suggested the presence of the PPy. The S, Mo, O and Mn as the major elements were corresponding to the MoS2 and MnO2 which were coexistence in composite. Elemental mapping explicitly reflected enrichment of nanoparticles. A dense PPy was successfully dispersed on MnO2@MoS2 sheet. The nanosheets of ternary hybrid would contribute to increasing the surface area of the active material to improve electrochemical performance. In experiments, it was possible that the ternary structure of MnO2@MoS2/PPy led to a synergistic effect that further promoted the rapid transport of ions and electrons in the electrolyte for supercapacitor application. The result of elemental mapping obtained was consistent with SEM and TEM, confirming the experience as well.
The SEM (a) and TEM (b) of sphere MnO2@MoS2/PPy composite and EDS elemental mappings of different elements of S, Mo, C, N, O and Mn (c)
The XRD spectra of MnO2@MoS2/PPy composite was represented in Fig. 3. Main diffraction peaks for the as-prepared composite could be indexed to α-MnO2 (JCPDS No. 44-0141) and MoS2 (JCPDS No.37-1492). Narrow peak at 2θ=37.5° was an evidence of high α-MnO2 crystallinity. The X-ray diffraction data indicated that the interplanar spacing of MoS2 was about 6.1 Å, which was consistent with the value of the HR-TEM image (inset in Fig. 2b). A typical broad peak at 2θ=25° was formed because of PPy which provided abundant conductivity. The broad and strong peak of PPy in spectra was related to the fact that structural characterization with a mass of PPy covered on MnO2@MoS2 sheet. No obvious impurity peak was detected, indicating that the high purity MnO2
@MoS2/PPy composite was produced. It was obviously observed that the combination of ternary composite was more balanced. In brief, the result of the XRD demonstrated that PPy was attached onto MnO2@MoS2 sheets successfully.
The XRD spectra of MnO2@MoS2/ PPy composite powders (a), FT-IR spectrum of MnO2, MnO2/MoS2 and MnO2@MoS2/PPy composite (b), XPS survey of MnO2@MoS2/ PPy (c) and XPS spectrum of Mn 2p, O 1s, C 1s, N 1s, Mo 3d and S 2p (d)–(i)
The FT-IR spectra exhibited more structural information at the molecular level for MnO2, MnO2@MoS2 and MnO2@MoS2/PPy composite (Fig. 3). The MnO2@MoS2/ PPy contained the majority of characteristic bands with PPy. The characteristic peak at 470 cm-1 was assigned to vibration of Mo-S band. Remarkably, Mn-O vibration was identified at approximately 540 cm–1. Besides, the peak at 1184 cm–1 was allocated to the C-C breathing vibration and the bands at 963 cm–1 might be related to the presence of polymerized pyrrole. The characteristic peak appeared at 1628 cm–1 because of the vibration of C=C/C-C as well. The peak at 1038 cm–1 was caused by N-H in-plane vibration bands of the pyrrole ring. And that the observed peak at 1554 cm–1 corresponded to typical pyrrole rings vibration. Another important factor was that the band at 1295 cm–1 and 1474 cm-1, assigned to the =C-H out-of-plane vibration, which was different from PPy alone at 1480 and 1559 cm–1, respectively. Hence, the predominant phenomenon here was that wave moved into lower numbers. Our observation of the difference peak position was between the sample and the pure PPy reported in the literature. One possibility would be that a mutual interaction between PPy and MoS2. All the information of FT-IR spectra further demonstrated that a MnO2@MoS2/PPy hybrid composite was formed.
The XPS in Fig. 3c–3i further confirmed the elemental component and mutual synergies of MnO2@MoS2/ PPy composite. Fig. 3e and 3d clearly showed Mn 2p and O 1s. In Fig. 3f, the peaks of C 1s could divide into three peaks at C-C, C-N and C=O which proved the presence of PPy in the compound product. It was worth noting that N 1 s peak of N-Mo at 401.6 eV (Fig. 3g) which had coordination interactions with Mo 3 p peak at 395.2 and 412.8 eV. This apparent synergism was due to the fact that few PPy was suitable for the appropriate content of Mo and the limited penetration depth of X-ray source. Expect Mo4+ 3d5/2 and Mo4+ 3d3/2 at 228.9 eV and 323.2 eV, part of Mo5+ and Mo6+ appeared, as a consequence of PPy oxidation. Additionally, peaks at 163.1eV and 161.8 eV belonged to the S 2p1/2 and S 2p3/2. According to the comparison with Mo 3d peaks of previous literature, compared with the semiconducting 2H phase, sheet MoS2 likely had metallic 1T phase with better conductivity after chemical exfoliation.
The Brunauer-Emmett-Teller (BET) surface areas and pore size distribution were relevant to the specific capacitance. Fig. 4 exhibited a N2 adsorption-desorption isotherm (Fig. 4a) and pore size distribution (Fig. 4b) of MnO2, MnO2@MoS2, MnO2@MoS2/PPy. Nitrogen adsorption-desorption isotherms exhibited type IV isotherms with H3-type hysteresis loops. The observation confirmed that the mesopores existed in all samples which were essentially significant to contribute to capacitance. According to BET analysis, the total specific surface area of synthesized composites was higher than reported in pure MnO2 (42.1 m2/g), MoS2 (40.6 m2/g) and original PPy (41 m2/g). Surface area of MnO2@MoS2/PPy and MnO2@MoS2 were 196 m2/g and 147 m2/g, respectively. Obviously, PPy molecules reduced stacking of MoS2 sheets and led to higher surface area. Moreover, obtained pore-size distribution (Fig. 4b) indicated that there was distribution within the diameter range both of mesoporous and micropore pore. Meanwhile, pore size distribution of MnO2@MoS2 nanocomposites showed that mainly micropores were larger compared with MnO2@MoS2. The larger micropores were peobably formed by the cover of PPy. Larger pores offered lower resistant for ions, leading to increase of the transport and diffusion of electrolyte ions during the rapid charge/discharge process. The MnO2@ MoS2/PPy composite with higher surface area and larger porous structures, was potential for the pseudocapacity applications.
The N2 adsorption/desorption isotherms of MnO2, MnO2@MoS2, MnO2@MoS2/PPy at 77 K (a) and pore size distribution (b)
Flexible supercapacitor device was prepared to explore the performance of MnO2@MoS/PPy used for electrode application. Figure 5a showed the schematic diagram of flexible material and digital photographs of MnO2@ MoS2/PPy flexible supercapacitor, owing obvious flexibility. It was necessary to point out that the MnO2@MoS2/PPy had no binder such as carbon black additive or Ni collectors because PPy served as a binder. The MnO2@MoS2/PPy could be designed to flexible belt and bent highly in supercapacitor device. This material reduced the weight of the device, so the flexible supercapacitor device displayed outstanding performance.
Schematic diagram of flexible material and digital photographs of flexible supercapacitor (a), CV curves of MnO2, PPy, MoS2, MnO2@MoS2 and MnO2@MoS2 /PPy hybrid composites at scan rates of 100 mV/s (b), charging/discharging curves of MnO2, PPy, MoS2, MnO2@MoS2 and MnO2@MoS2 /PPy hybrid composites the composites at 1 A/g (c), specific capacitances of MnO2, PPy, MoS2, MnO2@MoS2 and MnO2@MoS2/PPy hybrid composites at various scan rates (d), charging/discharging curves of the MnO2@MoS2/ PPy hybrid composites at different current density (e), nyquist plots of the EIS for MnO2, PPy, MoS2, MnO2@MoS2 and MnO2@MoS2/PPy hybrid composite electrodes (f) and cycling performance at a constant current density at 1 A/g (g)
The CVs measurements were tested for MnO2, MoS2, PPy, MnO2@MoS2, MnO2@MoS2/PPy at scan rates of 100 mV/s for electrochemical performance (Fig. 5b). The pure MnO2 showed poor conductivity because of distorted CV shape. On the contrary, hybrid with nearly rectangular shape had a larger area to show better capacitance behavior. In addition, the current density of the MnO2@MoS2/PPy electrode was much higher compared with the other two current densities under the same sweep. The charge storage mechanism of MnO2 was (MnO2)surface+Na++e–$ \leftrightarrow $(MnO2–Na+)surface and charge- discharge possibly achieved through intercalation/ deintercalation of Na+. Moreover, MoS2 had a large surface area, which could assist in efficient charge storage electrolyte and electron transport.
The galvanostatic charge-discharge behaviors were performed on pristine and MnO2@MoS2 and MnO2@ MoS2/PPy at the 1 A/g in Fig. 5c. At the same applied current density, the charging and discharging time were measured. As a result, the sample of MnO2@MoS2/PPy showed the highest values of specific capacitance of 450 F/g and lower IR drop, compared with the results of monolithic composites and MnO2@MoS2. Similarly, ternary composites had a high specific capacitance and excellent rate capability compared with the binary complex. The PPy properly mixed nanocomposites resulting in a strong synergistic effect on MnO2@MoS2. The PPy structure was uniformly grown around the monolayer MoS2 structure, offering a substantial electrochemically active surface area for charge transfer. And it also reduced ion diffusion length during the charge/discharge process. At the same time, the sheet structure ensured that the electrolyte ions effectively entered the active material, greatly reducing the dead volume. Moreover, galvanostatic charge/discharge curves at various current densities were presented in Fig. 5e at 0.0–1.0 V in a voltage window. Value of ternary composite showed a satisfactory rate performance. Apparently, MnO2/MoS2@ PPy ternary composite expressed a wonderful specific capacitance (1050 F/g) at a discharge current of 0.5 A/g and 42 F/g at a discharge current density of 10 A/g. The balance between MnO2/MoS2 and PPy maximized the capacitance and rate performance.
Figure 5d shows specific capacitance of the sample at different scan rates. Specific capacitance value was calculated through the CV curve integral. Note that the specific capacitance of the MnO2@MoS2/PPy ternary composite was the highest of all. The specific capacitance of ternary composite was two or three times of a single substance and better than binary complex. Particularly, the maximum specific capacitance of MnO2@MoS2/PPy ternary composite improved to about 600 F/g at the scan rate of 10 mV/s. The higher specific capacitance of ternary composite was the result of easier charge transfer and an additional contribution of MoS2 as a substrate.
The synergistic effect on the ternary MnO2@MoS2/PPy hybrid composite electrode was further investigated by EIS measurements. Figure 5f shows the resulting Nyquist plots of the EIS spectra for the MnO2@MoS2/PPy composite electrodes, PPy/MoS2, PPy, MnO2 and MoS2. The MnO2@MoS2/PPy electrode had a smaller the equivalent series resistance Rs and charge-transfer resistance Rct. This suggested that the MnO2@MoS2/PPy complex had a significant synergistic effect, as compared with the purity components. The lower resistance of the MnO2@MoS2/ PPy electrode would exhibit a faster Faraday reaction, which gives rise to better rate capability. Furthermore, the finite slope of the straight line represented the diffusive resistance of the electrolyte in the electrode pores and cation diffusion in the host materials.
The long-term operating stability (Fig. 5g) was carried out to evaluate the performance of the charge and discharge stability. During the charge-discharge cycle, the polymer might degrade because of expansion and contraction. Hence the pure conductive polymer generally possessed a poor long-term stability. A surprising finding of our study was that MnO2@MoS2/PPy ternary composite was extraordinary stable after long-term cycling. After 1000 cycles, the capacitance of MnO2@MoS2/PPy maintained 90% of its initial value while pure PPy only remained about 40% of its initial capacitance. It was known that the dissolution of active material and the mechanical failure of the electrode would lead to the smaller specific capacitance. However, this composite structure would inhibit the dissolution and agglomeration of the active substance during the cycle test. Excellent synergetic interaction between PPy and MnO2/MoS2 should be the reason for significantly improving cycle stability. On the other hand, the MoS2 served as substrates at the bottom of the compound and was a beneficial role for stability. As a result, ternary composite material had almost no significant loss compared with the initial value. These results suggested the advantages of ternary composites complied the requirements of high capacitance, excellent rate capability and long cycle life.