Gas–solid two-phase flow (GSF) mechanochemical synthesis of dual-metal–organic frameworks and research on electrochemical properties

Abstract

As an alternative approach for conventional mechanochemical synthesis, a novel gas–solid two-phase flow (GSF) synthetic technique for the mechanochemical synthesis of dual metal–organic frameworks (DMOFs) was reported for the first time. The prepared CoMn₂(BTC)₂ was characterized by FT-IR, DTA, TG/DTG, and XRD studies. The results indicated that CoMn₂(BTC)₂ (BTC = 1,3,5-benzenetricarboxylate) was successfully synthesized after 10 min at a rate of 60 kg h⁻¹. CoMn₂O₄ microspheres were also prepared via the CoMn₂(BTC)₂ precursor method and characterized using FT-IR, XPS, XRD, SEM, EDS, and BET methods. The electrochemical properties of the as-prepared CoMn₂O₄ were investigated, and the GSF results showed that the microsphere electrodes of CoMn₂O₄ had a high specific capacitance (969 F g⁻¹) at a current density of 1 A g⁻¹ in 3 M aqueous KOH solution.

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Introduction

Bimetallic oxides have been extensively studied and are considered a candidate for lithium-ion battery (LIB) anode materials because of their higher theoretical specific capacity than graphite, low cost, and environmental benignity. In particular, CoMn₂O₄ has the advantages of cobalt- and manganese-based metal oxides and superior application in the field of LIBs.^1,2

The precursor method was one of the main methods to prepare CoMn₂O₄. Usually, sodium hydroxide or acid citrate is used to provide coordination sites, and cobalt and manganese are added to form a complex at high temperature and high pressure by the solvothermal method. Then, the obtained complex is burned to prepare the target product. However, this traditional method requires the extensive use of hostile organic solvents, thereby increasing environmental burden. Moreover, it is impossible to achieve the process conditions of batch preparation.

In this study, we used dual metal–organic frameworks (DMOFs) [CoMn₂(BTC)₂] as the precursor of CoMn₂O₄.^3–12 Before mechanochemical synthesis,^13–17 DMOFs were also prepared through the conventional solvohermal method. Nowadays, MOFs are realized by mechanical ball milling and twin-screw extrusion. However, these synthetic techniques suffer from many drawbacks, such as time and energy consumption.^18–20 Thus, the development of a novel mechanochemical synthetic technique is highly desirable.

This work explored the use of a gas–solid two-phase flow (GSF) (Fig. 1) for the mechanochemical synthesis of DMOFs (Fig. S5† shows the reaction process flow chart of the GSF). The results indicated that CoMn₂(BTC)₂ (BTC = 1,3,5-benzenetricarboxylate) was successfully synthesized. CoMn₂(BTC)₂ was also used as a precursor to prepare CoMn₂O₄ microspheres, and their electrochemical properties were studied.

Fig. 1 The reaction process flow chart of the GSF.

Experimental

Chemicals and materials

Cobaltous nitrate (Co(NO₃)₂·6H₂O, AR, 98.0%) and manganous acetate (Mn(CH₃COO)₂, AR, 99%) were purchased from Kelong (Chengdu, China). 1,3,5-Trimesic acid (H₃BTC), was purchased from Aladdin (Shanghai, China). Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet-5700 FTIR spectrometer using pressed KBr pellets to test the chemical bonding of the samples from 4000 to 400 cm⁻¹. X-ray photoelectron spectroscopy (XPS) experiments were carried out using an XPS-7000 spectrometer with Al Kα radiation. Thermogravimetric analysis (TGA) was carried out on a TGA 4000 (Perkin Elmer Co., Ltd) with the temperature range from room temperature to 600 °C in air. Differential thermal analysis (DTA) curves were recorded on a WCR-1B instrument in flowing air at a heating rate of 10 °C min⁻¹. Field-emission scanning electron microscopy (FESEM) measurements were performed on an Ultra 55 microscope (ZEISS Company, The German) with an acceleration voltage of 15.0 kV with energy dispersive spectroscopy (EDS) detectors. XRD was performed on a Philips X'Pert Pro X-ray diffractometer (PANalytical, Holland). The N₂ adsorption–desorption tests were performed on a Micromeritics ASAP 2460 instrument.

Synthesis of CoMn₂(BTC)₂

CoMn₂(BTC)₂ was prepared as follows. Co(NO₃)₂·6H₂O (1.309 kg), Mn(CH₃COO)₂ (0.805 kg) and H₃BTC (1.38 kg) were mixed, and the mixture was transferred into an impact chamber at the rate of 1 kg min⁻¹ (60 kg h⁻¹). Then, the materials were accelerated to supersonic velocity with compressed air (1.5 MPa).²¹ The products were collected after different reaction times. Here, 1.68 kg of the purple crystal were collected after 10 min. Yields: 91% based on H₃BTC.

Precursor method for the synthesis of CoMn₂O₄ microspheres

CoMn₂O₄ was prepared as shown in Scheme 1. The as-prepared CoMn₂(BTC)₂ was placed in a tube furnace and heated to 600 °C at a temperature increase rate of 10 °C min⁻¹ in air, and maintained at this temperature for 12 h, then cooled down to room temperature, and the CoMn₂O₄ microspheres were obtained.^22–25

Scheme 1 Preparation of CoMn₂O₄ with a controlled composition and size.

Results and discussion

Characterization

Fig. 2 shows the FT-IR spectrum of CoMn₂O₄. Similar to the results reported in the literature, the peaks at 3412 and 1628 cm⁻¹ originated from a hydroxyl group in the form of physically adsorbed water.^26–28 The two peaks at 611 and 495 cm⁻¹ corresponded to the Mn–O and the Co–O bonds.

Fig. 2 Fourier transform infrared (FT-IR) spectra of CoMn₂O₄.

Fig. 3a illustrates the XPS survey.²⁹Fig. 3b shows that the O 1s peak was at 529.8 and 528.8 eV. A hybrid of the two peaks with Gaussian fitting was used to obtain the amount of points for calculation of the two peaks. The peaks at 529.8 and 528.8 eV represented the Mn–O–Mn and the Mn–OH bonds, respectively. Cobalt oxide was negligible because the quantity of the doped cobalt was low. The Mn–O–Mn and the Mn–OH bonds were related to MnO₂ and Mn₂O₃, respectively. As such, the manganese oxide (Fig. 3c) in CoMn₂O₄ was mixed with MnO₂ and Mn₂O₃ at 29.81% and 70.19%, respectively, on the basis of the peak separation results. The valence of manganese oxide was less than quadrivalent, and its oxidation state was 2.6. Fig. 3d shows the Co 2p_1/2 and Co 2p_3/2 peaks at 780.5 and 795.9 eV, which was consistent with the Co 2p orbital peak of Co₃O₄. Thus, Co, existing in the form of a Co₃O₄ spinel structure, can cause lattice distortion in the manganese oxide mixture and increase surface spacing, making it easier to conduct ions into the manganese oxide.³⁰

Fig. 3 (a) High-resolution XPS spectra of (b) deconvoluted O 1s, (c) Co 2p, and (d) Mn 2p of CoMn₂O₄.

Fig. S6 and S7† show the XRD patterns of the product collected at different times and after TGA, respectively. As illustrated in Fig. 4, the XRD pattern can be readily assigned to the body-centered tetragonal CoMn₂O₄ (JCPDS card no. 77-0471), which showed good properties.³¹ The absence of the characterization peaks from residues or other contaminants indicated the high purity of the products.

Fig. 4 XRD pattern of CoMn₂O₄ and the standard diffraction pattern of CoMn₂O₄ (JCPDS card no. 77-0471).

The electrochemical performance of active materials is greatly related to the morphology and assembled structures. Consequently, the formation of nanosheet assembled microspheres is distinctly important to the electrochemical performance of CoMn₂O₄. The hybrids were investigated by field-emission scanning electron microscopy to obtain the microscopic structure of CoMn₂O₄, explore its application value, and further illustrate CoMn₂O₄ synthesis. Fig. 5a shows the typical SEM image of the obtained CoMn₂O₄ in the form of a regular microsphere. As shown in Fig. 5b, the microsphere showed plate-like particles when we further enlarged the surface of CoMn₂O₄. The surface of the CoMn₂O₄ microsphere was smooth and had many micropores. Meanwhile, the chemical analysis conducted via EDS revealed the compositional signals for Co, Mn, and O, as confirmed by the elemental mapping images (Fig. 5c–f).³²

Fig. 5 (a and b) SEM images of CoMn₂O₄, (c–e) elemental mapping images, and (f) corresponding elemental contents measured by energy dispersive spectroscopy.

Nitrogen adsorption/desorption isotherms were obtained at 77 K to study the textural properties of CoMn₂O₄. Because of its spherical structure, its specific surface area was large. Based on the nitrogen adsorption–desorption curves, the BET surface area of CoMn₂O₄ (ref. 33) (Fig. 6) was 63.3 m² g⁻¹ (Fig. S8† shows the BET surface area of CoMn₂(BTC)₂).

Fig. 6 N₂ adsorption/desorption isotherms of CoMn₂O₄.

Supercapacitive properties

The electrochemical properties of the as-carbonized materials were evaluated in the context of supercapacitors. For a three-electrode system in 3 M aqueous KOH solution, a Ag/AgCl electrode (0.22233 V) and Pt plates were used as reference and counter electrodes, respectively. A free-standing CoMn₂O₄ film with a thickness of about 0.2 mm served directly as the working electrode and was prepared by pressing the CoMn₂O₄ film (2.65 mg) onto a piece of copper foam (1 cm × 1 cm). Cyclic voltammetry (CV) and chronopotentiometry measurements were conducted at various scan rates (0.0–0.8 V) and current densities (1–20 A g⁻¹), respectively. Benefiting from the porous microsphere structures, the CoMn₂O₄ electrodes were subjected to CV tests to evaluate their electrochemical behavior in 3 M aqueous KOH solution, as shown in Fig. 7.

Fig. 7 Cyclic voltammetry curve of CoMn₂O₄ electrodes obtained using the three-electrode method at a voltage scan rate of 50 mV s⁻¹.

The CV curves were measured at scan rates between 5 and 100 mV s⁻¹ within a potential window of 0.0–0.8 V (vs. Ag/AgCl), as shown in Fig. 8. Typical cathodic and anodic peaks were observed during the redox reaction between CoMn₂O₄ and MnO–CoO. The well-developed CV curve of CoMn₂O₄ exhibited the largest integral area, demonstrating that CoMn₂O₄ possessed the best supercapacitive behavior among the three electrodes. Meanwhile, two pairs of redox peaks were associated with the Faraday effect at 0.38 and 0.45 V and 0.08 and 0.58 V of the CV curve. The quasiregular rectangle shapes also confirmed the excellent supercapacitive nature of CoMn₂O₄.

Fig. 8 Cyclic voltammetry curve of supercapacitors with CoMn₂O₄ as the electrode at scan rates of (a) 100, (b) 50, (c) 20, (d) 10, and (e) 5 mV s⁻¹.

The supercapacitive properties³⁴ of CoMn₂O₄ were used for the subsequent method. Specific capacitance was calculated from various current densities by using the following equation:

where I (A) represents the discharge current, Δt (s) refers to the discharge time within the potential change ΔV (V), and m is the active material weight in the working electrode. All electrochemical measurements were carried out using a CHI660C (Shanghai, China) electrochemical working station.

Fig. 9 shows the results of the constant current charge and discharge test of the CoMn₂O₄ supercapacitor in a voltage window of 1 V. The charge and discharge currents were 20, 5, 4, 2, and 1 A g⁻¹. The supercapacitors with CoMn₂O₄ electrodes can be used as charge storage devices. When the current decreased, the specific capacity of CoMn₂O₄ ultracapacitors increased because the electrode process was limited by the diffusion and transfer rates of the charge and ion. The electrode material cannot be fully utilized, and its capacity was reduced when the diffusion and transfer rates of the charge and ion were not as fast as the rate of the charge flow. For comparison with different methods reported before, the results are listed in Table 1. A maximum specific capacitance of 969 F g⁻¹ was obtained after charging and discharging at a current density of 1 A g⁻¹.

Fig. 9 Constant current charge/discharge curves of supercapacitors with CoMn₂O₄ as the electrode at different current densities.

Table 1 Specific capacitance of CoMn₂O₄ for different methods

Sample	Current density	Specific capacitance	Ref.
CoMn2O4 microspheres	1 A g^−1	969 F g^−1	This work
CoMn2O4 spinel material	1 A g^−1	188 F g^−1	Ren L. et al.^35
CoMn2O4 electrode	5 V s^−1	700 F g^−1	Vigneshwaran P. et al.^36

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Conclusion

The GSF method provided a new route for the mechanochemical synthesis of DMOFs, and is the successor and developed model of traditional solid phase synthesis methods. The GSF method overcame the defects in traditional solid-phase synthesis and achieved more efficient preparation. Thus, the GSF can be a novel and effective technique for the mechanochemical synthesis of DMOFs under solvent-free conditions. CoMn₂(BTC)₂ was successfully synthesized to confirm the feasibility of this method.^35–40 CoMn₂O₄ microspheres were successfully prepared via the CoMn₂(BTC)₂ precursor method, and further exploration was carried out in the application. The results showed that CoMn₂O₄ had good capacitive behavior when used as a supercapacitor electrode. A specific capacitance of 969 F g⁻¹ was obtained after charging and discharging at a current density of 1 A g⁻¹.^41–45

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

We are grateful for financial support from the National Natural Science Foundation of China (project no. 51972278), Major Frontier Innovation Project of Science and Technology Department of Sichuan Province (project no. 2019YJ0447), and Project of State Key Laboratory of Environment-friendly Energy Materials, Southwest University of Science and Technology (19 fksy04).

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Footnote

† Electronic supplementary information (ESI) available: The prepared CoMn₂(BTC)₂ was characterized by FT-IR, DTA, TG/DTG, and XRD studies (Fig. S1–S4). See DOI: 10.1039/d0na00749h

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