Co9S8 is a highly promising electrochemically active material for energy devices; its rational design and manufacture for further enhancing the electrochemical activity and durability are still challenging. Herein, Co9S8@CNT compounds are synthesized by one-step pyrolysis, which self-assembled the monomolecular precursor and carbon nanotubes (CNTs). The CNTs effectively improve the electrical conductivity of the materials and availability of the catalytically active sites, which means that the electrochemical ability of Co9S8@CNT is better than that of individual Co9S8 and CNTs. The onset potential of Co9S8@CNT is 132 mV, which has greatly decreased. At the mass current density of 10 mA mg−1, the overpotential is 337 mV, and the Tafel slope is 49.8 mV dec−1. The addition of CNTs makes up for the deficiency of low electrical conductivity of the CoSx. Furthermore, the three-dimensional (3D) structure of the composite improves its electrocatalytically active surface area, and the electrocatalytic ability has been improved efficiently, owing to the increased number of catalytic sites on the surface.
Among the electrocatalysts of chalcogenide, Co9S8 has been widely used in energy storage and water electrolysis due to its excellent electrocatalytic performance.1–6 Up until now, there were various morphologies of the Co9S8 catalytic material, such as granular, two-dimensional sheet, core–shell structure composite, and three-dimensional structure composite.7–11 As for the electrocatalytic hydrogen production, compared to the CoSx family with the conventional solvothermal method, Co9S8 nanosheets prepared by monomolecular precursor body heat hydrolysis exhibit higher electrocatalytic performance. Moreover, the Co9S8 nanosheets themselves stack easily, which leads to less exposure to catalytically active centers, and affects their electrical conductivity. If these deficiencies are improved, further improvement in the electrocatalytic performance looks promising.
In recent years, carbon nanomaterials such as graphene and mesoporous carbon, especially carbon nanotubes (CNTs), have attracted extensive attention of scientists due to their large surface area, high conductivity, and many surface adsorption sites.12–17 Carbon-based composite materials have gradually become one of the research focuses. These carbon-based, composite functional nanomaterials are mostly used in catalysis, energy storage and conversion, sensing, etc.18–21 For functional nanomaterials with special structures prepared by monomolecule precursor pyrolysis, the addition of carbon materials improves the electrical conductivity, and it also has scope for improving the electrical conductivity of other transition metal sulfide materials. In addition, more catalytically active sites are exposed due to the large surface area of carbon materials, thereby effectively improving the catalytic performance.22
There are many methods to prepare a hybrid of cobalt sulfide and carbon-based materials, such as ball milling,23,24 hydrothermal,25,26 solvothermal,27,28 and Sol-gel29.30 The process of preparing materials is complicated, and heterostructure products are difficult to be prepared by these methods.22,31–33 Therefore, it is of important practical significance to develop a convenient and fast method to prepare heterojunction materials. The pyrolysis of monomolecular precursors in an organic system is applied to precisely regulate the crystalline phase, composition, and morphology of the target product via tuning the experimental parameter.7 What is more, one-step pyrolysis of a monomolecule precursor saves the tedious pretreatment process, simplifies the reaction route, and improves the material properties effectively. This method is expected to be an effective strategy for the rational design of interfacially modified cobalt sulfide and carbon-based materials.34–36
In this work, Co9S8 nanosheets were loaded on CNTs by one-step synthesis to form Co9S8@CNT composite materials for the electrocatalytic hydrogen evolution reaction. The Co9S8@CNT composites have a three-dimensional (3D) structure. Compared to the Co9S8 nanosheets obtained by direct pyrolysis (denoted as DR-Co9S8), the composition of Co9S8 was not changed by this method. The addition of CNTs shortens the charge transfer path at the interface and improves the conductivity of the material. On the other hand, the large electrocatalytically active surface area of CNTs also provides more reactant loading sites for the growth of Co9S8, making it grow along the tube wall, and increasing the surface area of Co9S8 nanosheets, which expose more active sites. The overpotential, Tafel slope, and charge transfer resistance of the composites are significantly reduced, and the electrocatalytic performance is obviously improved.
Synthesis of the precursor, cobalt dibutyldithiocarbamate (Co(dbdc)2): NaOH (2.64 g) and n-dibutylamine (11 ml) were added into methanol (80 ml) to form a uniform solution, which cooled in an ice water bath. Next, CS2 (3.96 ml) was added dropwise into the above mixture. Then, the yellow mixture was mixed with an aqueous solution of CoSO4 (80 ml) containing CoSO4·7H2O (9.5 g) and stirred vigorously for 10 h, and the whole reaction process was carried out under the protection of nitrogen. The product was separated by filtering, washed with redistilled water three times, and dried for 8 h under a vacuum at 30 °C.
Synthesis of Co9S8@CNT composites: The above precursor Co(dbdc)2 (60 mg) was weighed and transferred to a three-neck flask, and then oleylamine (5.77 g), triphenyl phosphine (1.5 g), and carbon nanotube (1 mg) were added sequentially, continuously fed with N2 for 20 min to exhaust the air in the device, the temperature of the reaction system was raised to 220 °C, kept for 7 h, the reaction was stopped after the reaction was complete, cooled to room temperature naturally, the products were transferred to ethanol for precipitation, then washed 3 times with n-heptane and chloroform, respectively, and collected by centrifugation, and the collected product was placed in a vacuum drying oven and dried at 30 °C for 8 h to obtain a black powder, denoted as Co9S8@CNT.
The crystalline phase analyses of the as-prepared product were characterized using an x-ray diffractometer (XRD, Bruker D8 Advance with Cu Kα, λ = 1.5418 Å). The morphology analyses of the products were made by using a transmission electron microscope (TEM, FEI Tecnai G2 F20 S-TWIN) operating at 200 kV. X-ray photoelectron spectra (XPS) were obtained on a PHI Quantera SXM spectrometer with an Al Kα = 1486.6 eV excitation source, where binding energies were calibrated by referencing the C1s peak (284.8 eV) to reduce the sample charge effect.
All the electrocatalytic properties of electrode materials were evaluated with a three-electrode configuration of the electrochemical workstation. The electrochemical measurements were performed on a CHI760E electrochemical working station (CH Instruments, Chenhua Inc. Shanghai, China). All potentials measured in this work were converted into 0 potential vs reversible hydrogen electrode (RHE), which yields the relation E(RHE) = E(SCE) + 0.24 + 0.059 pH (V). For linear-sweep voltammetry (LSV) measurements, the scan rate was set as 2 mV·s–1. Electrochemical impedance spectra (EIS) were recorded with the initial value of −0.46 V vs RHE, and a scanning frequency range 100 kHz to 0.1 Hz.
RESULTS AND DISCUSSION
The synthesis strategy of Co9S8@CNTs is depicted in Scheme ; the in situ pyrolysis method follows the self-assembly process. In the previous research, regular triangular nanosheets are obtained by the pyrolysis method of monomolecular precursors; this regular morphology is derived from the formation, aggregation, growth, and self-assembly of nanocrystals during the pyrolysis process.7 After the CNTs are added, there is not just one precursor in the system; the CNTs participate in all of the reaction processes of the formation, growth, and self-assembly of nanocrystals. Some of the crystal nuclei are assembled into small triangles according to the original growth mode, while the others grow along the tube wall as irregular nanosheets and finally form three-dimensional composite materials.
The crystallographic structures are displayed in Fig. 1. As a contrast, the XRD patterns of DR-Co9S8 (Co9S8 nanosheets obtained by direct pyrolysis), ST-CoS (CoS nanosheets prepared by the solvothermal method), and ST-CoS2 (CoS2 nanosheets prepared by the solvothermal method) are also shown together.7 There is a typical diffraction peak of Co9S8@CNT at 2θ values of 26.2°, which is well indexed to the (111) crystal planes of CNT (JCPDs card No. 75-444). There are no other miscellaneous peaks, indicating the high purity of the material. What is more, the peaks of CO (101) at 47.2° and CoS2 (311) at 54.9° are more obvious, suggesting that the Co9S8 in the composite material is still a heterostructure comprised of Co and CoS2.
Figure 2 shows the TEM images of the Co9S8@CNT composite material and the comparative materials. The morphology of CNTs can be clearly visible, and the CNTs are decorated with some triangular or uneven attachments in Fig. 2(a). The DR-Co9S8 material is synthesized by pyrolysis; it shows a stack of triangular Co9S8 nanosheets in Fig. 2(b), and Figs. 2(c) and 2(d) are CoSx nanosheets prepared by the solvothermal method. It can be seen that the CoS nanosheets forms flower shapes in the morphology; it is heavily stacked, thus forming relatively thick nanomaterials. CoS2 nanosheets form large single-layer sheets, but they are prone to curling, and the morphology is irregular. It suggests that the solvent will affect the morphology of nanomaterials. Combined with XRD data, it can be concluded that the Co9S8 nanosheets of a monomolecular precursor synthesized by in situ pyrolysis and CNTs are efficiently assembled. Moreover, along the axial direction [the red dotted arrow in Fig. 2(a)] of the CNTs, most of the Co9S8 nanosheets still maintain the original trianglular shape, and some form irregular nanosheets, which grow in a disordered fashion. In relation to the surface smoothness, the radial direction is smaller than the axial one. Compared to the CoSx nanosheets synthesized by the solvothermal method, the Co9S8 component in the composite material Co9S8@CNT, synthesized by the pyrolysis method, has more regular morphology, more dispersed structure, and smaller size.
The elemental compositions and chemical states of the Co9S8@CNTcomposite material are demonstrated by the XPS spectra in Fig. 3. Compared to the peak positions of DR-Co9S8, the peak positions of Co2+ at 780.8 eV and Co0 at 778.4 eV, and the corresponding satellite peak positions have not changed.7,37–39 In Fig. 3(b), the peak position of S corresponds to the Co9S8 composite, where the peak positions at 162.2, 161.4, and 167.4 eV are identified as S22−, S2−, and S6+, respectively.40 The difference is that the 2p peak areas of Co2+ and S6+ is larger than those before composite, while the 2p peak areas of S22− and S2− are smaller than before. And the C1s peak of Co9S8@CNT is more obvious; the peak at 284.4 eV corresponds to C–C. Because the CNTs used in the experiment are surface oxidized, there are C=O groups that provide site points for the adsorption and nucleation of cobalt ions on the surface. And the peak at 287.9 eV is attributed to C=O.41 It shows that although the existing form of Co and S in the Co9S8@CNT composite material has not changed, the degree of oxidation on the surface is relatively large, and the phenomenon that non-metal exists in the form of high valence is more obvious.42,43 The original defect structure has not been changed, but the number of defective sites is different. This result is also consistent with the result of the TEM image.
The electrocatalytic performance of the as-prepared composite material is investigated in Fig. 4; it is found that the electrocatalytic performance of Co9S8@CNT is outstanding compared to other materials. At the same mass current density of 10 mA mg−1, the overpotential of the Co9S8@CNT composite is 337 mV, while the overpotential of CNTs is more than 550 mV. The onset potential of 132 mV for Co9S8@CNT is also significantly lower, that of 228 mV for DR-Co9S8. At the same termination voltage of −0.38 V, the mass current density of the CNTs reaches 72 mA mg−1 [Fig. 4(a)]. This result demonstrates that the addition of CNTs not only improves the electrical conductivity of the material, but also enhances the electrocatalytic activity.
For acid electrolytes, the hydrogen evolution reaction (HER) includes three possible processes: the Volmer step, the Heyrovsky step, and the Tafel step; the catalytic reaction mechanism can be judged by the Tafel slope.44,45 The smaller the Tafel slope, the faster the hydrogen evolution reaction kinetics. The fitted Tafel curves are shown in Fig. 4(b); the results show that the Tafel slope of the Co9S8@CNT is 49.8 mV dec−1, which is the smallest compared to those of the other three materials. It suggests that the electrocatalytic desorption process is the rate determining step of the material used in the electrocatalytic hydrogen evolution reaction, and the catalytic reaction mechanism is Volmer–Heyrovsky. The reaction mechanism is the same as that for the DR-Co9S8 prepared by pyrolysis, indicating that the addition of CNTs does not change the kinetic mechanism of the reaction, but accelerates the rate of the electrocatalytic hydrogen evolution reaction to a certain extent. Under the same conditions, the Tafel slope of pure CNT is relatively large, indicating that the catalytic reactions are relatively slow, and it also indicates that pure carbon materials have poor performance when used in electrocatalytic hydrogen evolution reactions.
The exchange current density calculated by the Tafel equation also evaluates the electrocatalytic activitythe greater the exchange current density, the smaller the external voltage required, indicating that the driving force required for the electrode reaction is smaller.46 The exchange current density of the Co9S8@CNT is 5.0 × 10−2 mA cm−2, which is higher than that of the precursor material, 2.39 × 10−2 mA cm−2, indicating that the composite material catalyzes the electrode reaction at a faster rate compared to that without adding CNTs material, this result is consistent with the Tafel slope.
Electrochemical impedance spectroscopy (EIS) can further reflect the kinetic characteristics of charge transfer, in which the smaller the charge transfer resistance, the faster the electron migration speed between the interfaces.47,48 The charge transfer resistances Rct of materials are exhibited in Fig. 4(c); the impedance spectra show semi-circles in the high-frequency region, indicating charge transfer on the surface of the catalyst. It the resistance of Co9S8@CNT is calculated as 40 Ω, which is much smaller than that of CNTs. This result indicates that the addition of carbon materials can speed up the charge transfer, thereby accelerating the electrocatalytic reaction kinetics.
The electrochemical stability is another important parameter for evaluating the electrocatalytic performance. At the same experimental conditions, the long-term stability of Co9S8@CNT and DR-Co9S8 is examined by potentiostatic electrolysis in Fig. 4(d). Both the materials are tested for up to 16 h; it can been seen that the degradation of the current density tends to be stable after a short time of activation, On the whole, the current density of Co9S8@CNT is higher than that of DR-Co9S8 and even increases slightly, which indicates a more excellent electrocatalytic performance, although the data of DR-Co9S8 show less change and are more stable.
The electrochemically active surface area is another criterion used for assessing the catalytic activities of the electrocatalyst, while the double-layer capacitances value directly reflects the surface area.49,50 Cyclic voltammetry is usually carried out at various scan rates in a narrow non-Faraday reaction part for calculating the capacitances. Here, the electrocatalytically active surface area of Co9S8@CNT and DR-Co9S8 is tested by cyclic voltammetry (Fig. 5), it has been found that the electrocatalytically active surface area of the Co9S8@CNT composite material is about 3.8 mF cm−2, which is larger than that of DR-Co9S8, which is about 2.0 mF cm−2, indicating that the addition of CNTs helps increase the active area of the material, improves the current density of the material, and exposes more catalytically active sites, thereby improving the electrocatalytic performance of the material.
To summarize, Co9S8@CNT compounds are synthesized by one-step pyrolysis, which self-assembled the monomolecular precursor and CNTs. The Co9S8 nanosheets are grown by the CNT carriers to form a composite material with a 3D-structure. The Co9S8 nanosheets in the Co9S8@CNT composite are still heterogeneous structures, comprised of Co and CoS2. And its morphology is more regular, the structure is more dispersed, and the size is smaller. The addition of CNTs does not change the defect structure, and the defective sites in the material still exist. The Co9S8@CNT materials exhibit excellent electrocatalytic performance. The onset potential of Co9S8@CNT is 132 mV. At the mass current density of 10 mA mg−1, the overpotential is 337 mV, the Tafel slope is 49.8 mV dec−1, and the charge transfer resistance Rct is 40 Ω. The addition of CNTs makes up for the deficiency of low electrical conductivity of the CoSx. What is more, the three-dimensional (3D) structure of the composite improves its electrocatalytically active surface area, and the electrocatalytic ability has been improved, owing to the increased number of catalytic sites on the surface, which greatly promotes the electrocatalytic performance.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 22279118 and 22279117), the Henan Science and Technology Open Cooperation Project (Grant No. 172106000067), the Key Scientific and Technological Project of Henan Province (Grant No. 212102210490), and the Key Scientific Research Project of Colleges and Universities in Henan Province (Grant No. 22A140032).
Conflict of Interest
The authors have no conflicts to disclose.
Xianghong Ge: Investigation (equal); Methodology (equal); Supervision (equal); Writing – original draft (equal); Writing – review & editing (equal). Xinwen Zhang: Data curation (equal); Investigation (equal). Xingxing Ding: Investigation (equal); Writing – review & editing (equal). Ruofan Shen: Data curation (equal); Investigation (equal). Yanyan Liu: Conceptualization (equal); Investigation (equal); Supervision (equal). Xianli Wu: Data curation (equal); Funding acquisition (equal). Erjun Liang: Conceptualization (equal); Methodology (equal).
The data that support the findings of this study are available within the article.