The Co-based organic skeleton was combined with graphene oxide (GO) as a precursor, Ni2+ hydrolysis etching was introduced, and finally, NiCo-LDH was obtained. The final composite electrode material NiCo2S4/rGO was obtained by high-temperature vulcanization. The introduction of MOFs and rGO significantly increased the specific surface area of the material and made it have excellent electrochemical properties. The specific capacitance of the composite NiCo2S4/20rGO reaches an astonishing 2452.65 F g− 1 when the current density is 1 A g− 1. In addition, at a large current of 10 A g− 1, the specific capacitance of the material can also reach 1250 F g− 1, and after a long cycle of 5000 cycles at such a current density, the capacity remains at the original 73.2%. With NiCo2S4/20rGO electrode material as the positive electrode and activated carbon as the negative electrode, the hybrid supercapacitor is assembled. At an energy density of 56.9 Wh kg− 1, its power density reaches an excellent 799 W kg− 1, and it still has a capacity retention rate of 74% at a current density of 10 A g− 1. The excellent properties of composites demonstrated in this work open up new possibilities for high-quality energy storage devices.
Research Article
NiCo2S4/rGO composite electrode material derived from CO-based MOFs for high-efficiency hybrid supercapacitors
https://doi.org/10.21203/rs.3.rs-3648438/v1
This work is licensed under a CC BY 4.0 License
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Supercapacitor
Metal sulfide
NiCo2S4
Co-MOF
rGO
With the progress of society and the development of The Times, people are faced with a variety of problems such as fossil fuel shortage, environmental pollution and global warming, which lead to the increasing demand for sustainable renewable sources and green energy storage technology [1–4]. Supercapacitors have gradually entered people's field of vision because of their high-power density, long charge-discharge cycle life, short charging time and long storage time, and are widely used in automotive, electric power, communication, aerospace and other fields [5–8]. However, due to their low energy density, improving the electrode material that plays an important role in the device has become a key issue for supercapacitors [9–12].
At present, transition metal sulfides have attracted the attention of many scholars because of their high specific capacity and good electrical conductivity. In addition, transition metal sulfide can be prepared by hydrothermal method, stripping method, chemical vapor deposition method and other methods, such preparation method is simple and low cost, so the field of reindustry has also been favored by people [13–15]. In recent years, scholars in the field of supercapacitors for transition metal sulfide as electrode materials have endless articles. For example, Sajjad Hasan et al. prepared MoS2-doped CuCo2S4 nanocomposites by hydrothermal method, and electrochemical data showed that the CuCo2S4-MoS2 electrode had a specific capacitance of 820 F g− 1 at a current density of 0.5 A g− 1 [16]. Muhammad Imran Rafiq et al. reported a hollow NiCo2S4 eccentric sphere supported by carbonized wood (CW) in a channel by a formless anion exchange method. Electrochemical data showed that the material had a specific capacitance of 1472.1 F g− 1 at 1 A g− 1 [17]. Based on the research of many articles, it can be seen that transition metal sulfides have a certain potential as electrode materials for supercapacitors. However, the transition metal sulfide as electrode material affects its overall performance because of its poor conductivity and low energy density [18, 19]. Therefore, the modification and modification on this basis are essential. In recent years, MOFs (Metal-organic Frameworks), as a new electrode material, have attracted wide attention in the field of supercapacitors [20–23]. This material has high porosity and good chemical stability, and it is precisely because of this property that it is often used as a template for nanomaterials, which plays an important role in improving the specific capacity and energy density of materials [24, 25]. For example, Li et al. prepared NiCo-S electrode material on nickel foam using a two-step hydrothermal method with a specific capacitance of 2081 F g− 1 at 1 A g− 1, using a binary metal-organic skeleton as a sacrifice template [26]. Using ZIF-67 as a sacrificial template, Liu et al. successfully prepared NiCo2S4 hollow nanomagnets assembled with ultra-thin nanosheets by a simple solid-state chemical vulcanization method, with a specific capacitance of 1232 F g− 1 at 2 A g− 1 [27]. Wang et al. used a one-step solvothermal method to prepare 3D hollow cage CuCo2S4 through the vulcanization and cation exchange reaction of Co-based MOF. With a large specific surface area and abundant active sites, CuCo2S4 has a high specific capacitance of 1096.27 F g− 1 at a current density of 0.5 A g− 1 [28]. Therefore, the correct and reasonable introduction of MOF as a sacrifice template in the preparation of materials is of great benefit to the properties of materials. Two-dimensional graphene is widely regarded as a good composite carrier because of its large specific surface area and high electrical conductivity [29–32]. The composite material of transition metal sulfide and graphene solves the shortcomings of low conductivity and poor stability of transition metal sulfide, increases the specific surface area of the material, improves the conductivity of the material, and makes the composite material itself have more excellent performance. For example, Wu et al. prepared a Zn-Co nanosphere structure connected to a reduced graphene oxide sheet by hydrothermal method based on a metal-organic skeleton with a bimetallic structure, with an ultra-high specific capacitance of 2925 F g− 1 at a current density of 0.5 A g− 1 [33]. Zeinab Karimzah et al have prepared reduced graphene oxide-based Co-MOF at ambient temperature by a fast and simple cold plasma method. Co-MOF@rGO electrode has a specific capacitance of 967.68 F g− 1 at a current density of 1 A g− 1 [34]. Dai et al. reported that NiCoP/rGO hollow nanocomposites were prepared by in-situ growth of cobalt-based MOF on reduced graphene oxide (RGO) by ion etching and high-temperature phosphorylation, which had an ultra-high specific capacitance of 2580 F g− 1 at a current density of 1 A g− 1 [35]. Based on literature reports, reduced graphene oxide and MOF-based transition metal sulfide composites have a good prospect for the development of electrode materials for supercapacitors.
Based on literature reports, reduced graphene oxide and MOF-based transition metal sulfide composites have a good prospect for the development of electrode materials for supercapacitors. Then NiCo2S4/10rGO, NiCo2S4/20rGO and NiCo2S4/30rGO were prepared by changing the content of rGO to explore the influence of different content of rGO on NiCo2S4. In addition, the electrochemical properties of NiCo2S4 monomer and its composites were simply compared. It was found that compared with the monomer NiCo2S4, the specific surface area and electrical conductivity of the material were greatly improved by the introduction of Co-MOF and rGO. Based on these, the performance of the composite material was significantly improved. To fully understand the prepared composite nanomaterials, some related materials and key materials were characterized by XRD, TEM, SEM, EDS element mapping and XPS. We found that NiCo2S4/20rGO composite nanomaterial stood out, with an ultra-high specific capacitance of 2452.65 F g− 1 at 1 A g-1 current density, and the assembled HSC device achieved an excellent power density of 799 W kg− 1 at an energy density of 56.9 Wh kg− 1. The excellent electrochemical performance of NiCo2S4/20rGO composite nanomaterials also reflects that it has considerable application prospects in the electrode materials of supercapacitors.
2.1 Chemical
All reagents are analytical grade and do not require further purification. C4H6N2 (2-methylimidazole), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), potassium hydroxide (KOH), activated carbon, nickel nitrate hexahydrate (Ni(NO3)2·6H2O), sulfur powder purchased from Shanghai Aladdin Co., LTD., China. Deionized water, methanol and anhydrous ethanol are provided by Tianjin Baishi Chemical Co., LTD. Foam nickel (NF) is purchased from Hefei Kejing Company. GO is provided by Kaina Carbon New Materials LTD.
2.2 Synthesis of ZIF-67/GO
Weigh 10 mg, 20 mg and 30 mg of GO and add them into 3 parts of 20 ml methanol solution and stir for 15 min. When the stirring is finished, it is ultrasonic for 10 min, and the whole process is repeated three times. Weigh 4 mmol of Co(NO3)2·6H2O and add it into 40 ml methanol solution and stir for 20 min. Then, the stirred and ultrasonic GO methanol solution was slowly poured into the Co (NO3)2·6H2O methanol solution and then stirred for 20 min. Then weigh and add 24 mmol of dimethylimidazole into 140 ml methanol solution and stir for 20 min. Finally, GO and Co(NO3)2·6H2O methanol solution were poured into dimethylimidazole methanol solution, stirring for 30 min and standing for 24 h. Finally, GO and Co(NO3)2·6H2O methanol solution were poured into dimethylimidazole methanol solution, stirring for 30 min and standing for 24 h. The mixed solution was poured off the supernatant to get a purple substance, washed with alcohol 5 times, and the product was placed in a constant temperature drying oven at 60 ℃ for 24 h. The obtained sample was named ZIF-67/GO, in which the ZIF-67 with different GO content was named ZIF-67/GO, ZIF-67/10GO, ZIF-67/20GO and ZIF-67/30GO respectively.
2.3 Synthesis of NiCo2S4/rGO
Weigh 100 mg of ZIF-67/GO and 200 mg of Ni(NO3)2·6H2O and dissolve it in 30 ml of alcohol. The solution was then transferred to a drying oven at 60 ℃ for 18 h, and the obtained product was centrifuged with alcohol 5 times, and finally dried to obtain a sample named NiCo-LDH/10GO. NiCo-LDH/10GO and sulfur powder reagent were weighed and their mass ratio was fixed at 1:3. The two were poured into a crucible and ground several times before being transferred to a porcelain boat. In the argon atmosphere, the black powder was first ground at 2 ℃ per minute to 350 ℃, and then at 5 ℃ per minute to 450 ℃, and then named sample NiCo2S4/10rGO. Similarly, NiCo2S4, NiCo2S4/20rGO and NiCo2S4/30rGO were also obtained by the above experimental steps.
2.4. Materials characterization
X-ray diffraction (XRD; D8 Advance) characterized the phase structure and chemical composition of the sample in the range of 10°-80°. Scanning electron microscopy (SEM; JSM-6701F) to characterize the morphology and structure of the materials. Transmission electron microscopy (TEM; JMS-2010) for a more detailed and in-depth characterization of the material. X-ray photoelectron spectroscopy (XPS; BIL-KV201) was used to analyze the composition of the sample. EDS was used to analyze the composition and distribution of elements in specific regions of the sample. N2 adsorption/desorption isotherms were measured by Micromeritics ASAP 2020s to investigate the specific surface area and pore size distribution.
2.5. Electrochemical characterization
In the three-electrode system, 50 ml 3 mol/L KOH solution was used as the electrolyte. Platinum electrodes and Ag/AgCl electrode are used as reference electrodes. The working electrode is based on our prepared nanomaterial (2 mg), conductive carbon black, polyvinylidene fluoride (PVDF) with a mass ratio of 8:1:1 and 1-methyl-2-pyrrolidone (NMP) mixed and evenly coated in clean and dry nickel foam (NF; 1.0×1.5 cm2), dried at 60 ℃ for 12 h and pressed at 10 MPa for a few seconds. The linear cyclic voltammetry (CV), constant current charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) tests of all materials were performed on the electrochemical workstation (CS350H). For the assembled hybrid supercapacitor, the positive material and the negative material are determined by the charge balance formula [36].
Figure 1 shows the preparation process of NiCo2S4/rGO nanocomposites. First, Co-based MOFs grown in situ on sheet graphene were prepared. Ni2+ is introduced to hydrolyze and produce H+ into Co MOF, which destroys the coordination bonds of Co2+ and 2-MIM in MOF. This causes the Co2+ inside to diffuse outwards, but in the process of diffusion, O2 and NO3− in the solution will oxidize it into partial Co2+/Co3+. At the same time, the outer Ni2+ will also spread into the MOF. In this process, the two combine and precipitate to form NiCo-LDH [37]. However, due to the different diffusion rates of the two metal ions, a hollow NiCo-LDH structure is formed (Kirkendall effect) [38]. NiCo2S4/rGO nanocomposites can be obtained by grinding NiCo-LDH and sulfur powder through high-temperature vulcanization.
Figure 2 is a detailed analysis of the crystal structure of the material by X-ray diffraction. We can see that the diffraction peak of graphene oxide is at 11.4° corresponding to its (220) crystal face [39]. In addition, the diffraction peaks of NiCo2S4 nanomaterial are 16.3°, 26.8°, 31.5°, 38.3°, 50.4° and 55.3° corresponding to (110), (220), (311), (400), (511) and (440) crystal planes, respectively. It corresponds exactly to the standard card (JCPDS No. 20–0782) [40]. It can be found from the XRD pattern that NiCo2S4/rGO improves significantly with the increase of rGO content when it is about 25.7° compared with NiCo2S4. This is because GO has been reduced to rGO during the process of high-temperature vulcanization of the sample, and the diffraction peak of rGO is 25.7°, which leads to this result, which also proves the existence of rGO in the composite material [41]. In summary, it can be found that we have successfully prepared NiCo2S4/rGO nanocomposites with good crystallinity.
Figure 3 shows the X-ray photoelectron spectroscopy (XPS) characterization of NiCo2S4/20rGO in order to analyze the chemical composition and valence states of the material. Figure 3(a) is an XPS of S 2p with two satellite peaks at 169 ev and 170.1 ev. Among them, the satellite peak at 169 ev may be from the sulfur oxide caused by O2 oxidation, and the satellite peak at 170.1 ev. This may be from the C-S bond formed by the combination of the C element and S element in rGO [42]. And there are also two peaks at 162.7 ev and 161.5 ev corresponding to S 2p3/2 and S 2p1/2 respectively, which are the peaks shown by S2−. Figure 3(b) is the XPS diagram of the C 1s, where the binding energies at 284.8 ev, 285.4 ev, 286.6 ev, and 288.9 ev correspond to the C-C, C-O-C, C-O, and O = C-O bonds, respectively. Figure 3(c) is the XPS diagram of Ni 2p, where we can find that there are two spin double orbital peaks, Ni2+ and Ni3+. Ni2+ corresponds to two diffraction peaks at 857.5 ev (Ni 2p3/2) and 876.5 ev (Ni 2p1/2). Ni3+ corresponds to two diffraction peaks at 853.5 ev (Ni 2p3/2) and 874.1 ev (Ni 2p1/2). In addition, it also has two satellite peaks at 862.0 ev and 881.2 ev. Figure 3(d) is an XPS diagram of Co 2p, which also has two satellite peaks at 786 ev and 803.2 ev. In addition, 778.7 ev (Co 2p3/2) and 794.2 ev (Co 2p1/2) are the two peaks shown by Co3+, and 782.5 ev (Co 2p3/2) and 798 ev (Co 2p1/2) are the two peaks shown by Co2+ [43].
Figure 4 shows the morphologies of Co-MOF/GO, NiCo-LDH/GO and NiCo2S4/20rGO by scanning electron microscopy. Figure 4(a) shows how large or small, regular dodecahedral Co-MOF nanomaterials are tightly attached to sheets of graphene oxide with surface folds. Figure 4(b) shows that hollow nanospheres (NiCo-LDH) with a surface full of folds after Ni2+ hydrolytic etching are well-grown on the surface of flake graphene oxide. Figure 4(c) and Fig. 4(d) are the morphologies of NiCo2S4/20rGO at different magnifications. From Fig. 4(d), we can see that NiCo2S4 is well grown on and around the surface of redoxed graphene in a more macroscopic way, which indicates that the composite effect of the two materials is successful. From Fig. 4(c), we can more clearly and carefully find the scattered distribution of NiCo2S4 particles on the surface of redox graphene. From the image, we can also find that due to high-temperature vulcanization, NiCo2S4 particles of different sizes and NiCo2S4 particles with any hollow structure are produced. This structure also contributes to good electrolyte penetration, thus improving material properties.
Figure 5 shows the morphological structure of NiCo2S4/20rGO by transmission electron microscope. From TEM, we can learn more about the structure of the material. Figure 5(a,b) both show flecked rGO and NiCo2S4 nanoparticles growing around rGO. However, due to the large thickness of the sheet rGO in the figure, the NiCo2S4 nanoparticles growing on its surface cannot be projected. In general, the transmission results were consistent with SEM. Figure 5(c) is the selected electron diffraction of NiCo2S4/20rGO, whose bright diffraction rings show the polycrystalline properties of the material. Figure 5 (d) shows the lattice fringes of the material. The spacing of three groups of lattice fringes of the material is 0.17 nm, 0.27 nm and 0.3 nm, corresponding to the crystal faces (311), (511) and (440), respectively. The results shown in the images are fully combined with their XRD. Figure 5(e) is a TEM image of a NiCo2S4/20rGO selected specifically, corresponding to an element map of the material. Figure 5(f-i) is an element mapping of NiCo2S4/20rGO, corresponding to S, C, Co, and Ni elements respectively. The element composition here also fully corresponds to the results of sample characterization, which further explains the high purity of NiCo2S4/20rGO nanocomposites.
In order to study the electrochemical properties of the material, we assembled the material in a three-electrode system, using 3 M KOH as the electrolyte, and investigated its CV, GCD, EIS and other related properties. Figure 6 (a) shows the CV curves of NiCo2S4 and three NiCo2S4/rGO composites at a scanning rate of 50 mv s− 1 and a voltage window of -0.2-0.7 V. From the figure, we can find that the REDOX peak of each material is obvious, which is a typical characteristic of pseudocapacitive materials. In addition, all materials have large voltage Windows, among which NiCo2S4/20rGO has the largest CV curve area. This also shows that NiCo2S4/20rGO has the best electrochemical performance among composites and has a strong charge storage capacity. Figure 6 (b) is the CV curve of NiCo2S4 material at a window of -0.2-0.7 V and scanning rate from 5–50 mv s− 1. It can be seen from the figure that with the increase in scanning rate, the oxidation peak and reduction peak move towards the correct and more negative potential respectively. This is due to the polarization reaction as the scanning rate increases. Figure 6 (c) shows that each material has a distinct charge-discharge platform, which corresponds exactly to the REDOX peak of the CV curve. In addition, GCD data can be obtained, the capacities of NiCo2S4, NiCo2S4/10rGO, NiCo2S4/20rGO and NiCo2S4/30rGO are 1843 F g− 1, 2086 F g− 1, 2452.65 F g− 1 and 2187 F g− 1 at the same voltage window and 1 A g− 1 current density, respectively. Figure 6 (d) shows NiCo2S4 at A voltage window of 0-0.4 V, from 1 A g− 1, 2 A g− 1, 3 A g− 1, 5 A g− 1, and 10 A g− 1 current densities. Its capacities are 2452.65 F g− 1, 2011.86 F g− 1, 1775.95 F g− 1, 1532.5 F g− 1 and 1246.93 F g− 1, respectively. Figure 6(e) shows that the capacity retention rates of NiCo2S4, NiCo2S4/10rGO, NiCo2S4/20rGO and NiCo2S4/20rGO are 50.8%, 46.8%, 50.8% and 69.5%, respectively, at the current density of 1 A g− 1-10 A g− 1 for four materials. It can be found from the data that the overall power retention of the material is ok, and the power of the NiCo2S4/20rGO material with the best capacity needs to be improved. Figure 6(f) depicts the material retention rate of 73.2% after 5000 cycles of NiCo2S4/20rGO at a high current density of 10 A g− 1. As the number of cycles increases, its retention rate decreases, possibly due to the loss of electrode material. Figure 6(g) depicts the original impedance and fitted impedance of NiCo2S4 monomers and NiCo2S4/20rGO. The middle illustration is the equivalent circuit diagram at fitting. Rs = 0.51 and Rct = 0.21 of NiCo2S4 were obtained by fitting impedance. Rs = 0.35 and Rct = 0.15 for NiCo2S4/20rGO. In contrast, we can see that for monomers, composites have smaller semicircles in the high-frequency region and higher slopes in the low-frequency region. This also shows that NiCo2S4 has low charge transfer resistance and stronger ion diffusion ability after combining rGO. Figure 6(h) is the logarithm of the scan rate and peak current of the CV curve of NiCo2S4/20rGO at three electrodes. Through fitting, the b value of the anode is 0.54 and that of the cathode is 0.6, which indicates that the material is mainly controlled by the surface capacitance behavior in the process of charge storage. Figure 6 (i) further describes the relationship between capacitance contribution and scan rate. As can be seen from the figure, the diffusion control behavior decreases with the increase of the scanning rate.
Figure 7(a) is a hybrid supercapacitor composed of NiCo2S4/20rGO material as a positive electrode and AC as a negative electrode. The purpose is to study the practical application of the device. According to formula (1), the active mass of the positive electrode material is 2 mg, and that of the negative electrode material is 5.8 mg. Figure 7(b) shows the CV curves of AC and NiCo2S4/20rGO at a sweep speed of 50 mv s− 1 in a three-electrode system. As can be seen from the figure, the voltage window of AC is -1-0 V, and the voltage window of NiCo2S4/20rGO is -0.2-0.7 V. Figure 7(c) shows CV curves of the NiCo2S4/20rGO//AC device at different voltage Windows at 50 mv s− 1 sweep speed. When the voltage window is 0-1.4 V, it can be seen from the image that the material does not undergo a polarization reaction. When the voltage window is 0-1.8 V, the corresponding CV image is obviously deformed. When the window is 0-1.6 V, it is the best suitable window for the HSC device. In Fig. 7(d), it can be seen that the CV area gradually increases during the scanning rate from 5–50 mv s− 1, but the image shape remains basically unchanged. This indicates that NiCo2S4/20rGO//AC devices have stable charge transfer and good capacitance characteristics. Figure 7 (e) records the specific capacity of the HSC device from 1 A g− 1-10 A g− 1. It can also be seen in the figure that the five groups of graphs are highly approximate, which also reflects the high reversibility of the electrochemical reaction process of the material. Figure 7(f) shows the specific capacities of NiCo2S4/20rGO//AC devices at current densities of 1 A g− 1-10 A g− 1 corresponding to 160 F g− 1, 120 F g− 1, 106 F g− 1, 92 F g− 1, and 73 F g− 1, respectively. It can also be seen from the magnification curve that the material has a good capacity retention rate. Figure 7(g) records that the device achieves an excellent power density of 799 W kg− 1 at an energy density of 56.9 Wh kg− 1, and an excellent power density of 7983 W kg− 1 at an energy density of 26.1 Wh kg− 1. Table 1 corresponding to Fig. 7(g) more intuitively shows the excellent properties of this work compared to other transition metal sulfides. Figure 7(h) reflects the material's nearly 100% coulomb efficiency and 74% capacity retention after 10,000 cycles at a high current density of 10 A g− 1. This fully demonstrates the excellent cyclic properties and research prospects of the material.
| Materials | Energy density (Wh kg− 1) | Power density (W kg− 1) | Ref. |
|---|---|---|---|
| NiS//AC | 38 | 1500 | [44] |
| NiCo2S4@GO//AC | 26.9 | 658 | [45] |
| NiS2/CoS2/NC-500//AC | 53.93 | 800 | [46] |
| RGO/ NiCo2S4// RGO | 40.3 | 375 | [47] |
| NiCo2S4//AC | 39.3 | 749.6 | [48] |
| NiCo2S4/CC//CNF | 41.28 | 1564 | [49] |
| NiS/rGO//IHPC | 56.1 | 880 | [50] |
| NiCo2S4/20rGO//AC | 56.9 | 799 | This work |
In conclusion, NiCo2S4/20rGO nanocomposites were successfully prepared by Ni2+ etching and vulcanization calcination using redox-graphene as the base and Co-based MOF as the template. The high porosity of MOF, combined with the large specific surface area of rGO and the synergistic effect of the material, greatly alleviates the shortcomings of NiCo2S4 material and improves its electrochemical performance. The NiCo2S4/20rGO has an extremely high specific capacitance (2452.65 F g− 1 at 1 A g− 1) and good cycle stability (5000 cycles at 10 A g− 1, holding rate of 73.2%) under three-electrode testing. When assembled into HSC devices, the power density reached an excellent 799 W kg− 1 at an energy density of 56.9 Wh kg− 1. In conclusion, this study provides a good method for the design of energy storage materials.
CRediT authorship contribution statement
Ji-wei Zhao: Conceptualization, Writing-Review＆Editing. Zhi-qiang Wei: Methodology. Can Wang: Disposal data. Mei-pan Zhou: Investigation. Cheng-gong Lu: Validation.
Declaration of competing interest
The authors declare no competing financial interest.
Funding
Hong Liu First-class Disciplines Development Program of Lanzhou University of Technology.
Availability of data and materials
The corresponding author will provide data and materials supporting the present results upon reasonable request.
Ethical Approval
There are no ethical implications in this study
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No competing interests reported.