CoMoO₄ Nanoflowers Doped with La Element for Advanced Electrode Materials

Abstract

La-CoMoO₄ was prepared as the electrode material for supercapacitors using the freeze-drying method. The physical and structural properties of the prepared electrode La-CoMoO₄ were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). We further investigated the electrochemical performance of La-CoMoO₄ electrode materials through cyclic voltammetry, constant current charge–discharge, and electrochemical impedance spectroscopy. The research results indicate that compared with CoMoO₄ material (1400 F/g), La-CoMoO₄ material has a high specific capacitance of 2248 F/g at a current density of 1 A/g. In addition, La-CoMoO₄ has a high stability, with a capacitance retention rate of up to 99.2% after 5500 cycles. Finally, supercapacitor devices using La-CoMoO₄ material as the positive electrode have a high energy density of 55 Wh/Kg (power density of 1000 W/Kg), making them a promising electrode material.

1. Introduction

With the evolution of the times, human demand for energy is increasing day by day. With the advancement of industrialization, resources are becoming rapidly depleted, and environmental problems are becoming increasingly serious [1,2,3,4]. As an economical and clean energy source, electricity can be applied in many situations, but it has the disadvantage of being difficult to store. Therefore, people need to search for low-cost and high-efficiency energy storage devices [5,6,7,8]. Supercapacitors are considered one of the most promising energy storage devices in the energy field due to their high power density, fast charging and discharging, green environmental protection, and long service life [9,10,11,12,13,14,15]. The low energy density has limited the development and application of capacitors and is also the main factor affecting their performance [16,17,18,19]. According to the energy density E = 1/2CV², it can be found that the specific capacitance (C) and voltage window (V) directly affect the energy density. Therefore, we can develop excellent electrode materials through experimental development to improve the specific capacitance (C) and enhance the energy density of supercapacitors [20,21,22,23]. In addition to developing excellent electrode materials, asymmetric supercapacitors can also be assembled by using two different electrode materials, which can expand the voltage window and thereby improve the energy density [24,25,26,27]. At present, the optimization of electrode materials has become a worldwide research hotspot.

Cobalt molybdate, as an electrode material for capacitors, has a high conductivity, cycling stability, redox activity, and good rate performance [28,29,30]. It is a promising electrode material, but it is not as good at capacitance. To overcome these problems, researchers have made many attempts, such as designing nanostructures, doping elements, etc., to improve the performance of the products. The current main research focus is on creating nanomaterials with distinctive structures. Wang et al. [31]. successfully synthesized a CoMoO₄ nanowire array using the hydrothermal method. The material has a specific capacity of 940 F/g at a current density of 1 A/g. Using this material as the positive electrode and activated carbon (AC) as the negative electrode, this asymmetric supercapacitor device has a maximum voltage of 1.6 V, energy density of 46.7 Wh/kg, and power density of 8000 W/kg. In addition, element doping also greatly improves the performance of materials. Doping rare earth elements can alter the binding strength of electrode materials, the local environment of the lattice, and the valence state of cations, thereby introducing defects and increasing oxygen vacancies in electrode materials. The introduction of defects can change the disordered state inside atoms, adjust the size of ion transport channels, improve conductivity, and oxygen-rich vacancies can act as active sites to absorb more OH⁻ and accelerate surface reaction kinetics, thereby improving electrochemical performance. This has a positive significance in solving the problem of slow reaction kinetics and increasing material specific capacity [32,33]. Theerthagiri et al. [34] doped a series of Co₃O₄ materials with rare earth metals (La, Nd, Gd, Sm) and found that compared to pure Co₃O₄ materials, the specific capacitance of the products increased from 656.2 F/g to 2193 F/g, while also exhibiting good cycling stability. After 5000 cycles, the capacitance retention rate was 93.18%.

This article uses a simple and low-cost freeze-drying method to modify CoMoO₄ materials with rare earth element La, and successfully prepares La-CoMoO₄ nanoflower structures with large specific surface areas. The materials are characterized by various characterization methods, and the results show that La-CoMoO₄ materials have a unique three-dimensional flower-like structure. Through electrochemical performance testing, La-CoMoO₄ material has a high specific capacitance of 2248 F/g at a current density of 1 A/g. After 5500 cycles, the capacitance retention rate reached 99.2%. We further assembled the supercapacitor device using La-CoMoO₄ material as the positive electrode and CNTs as the negative electrode. The device has a high specific capacity of 149 F/g at a current density of 1 A/g and a high energy density of 55 Wh/Kg (power density of 1000 W/Kg), and it exhibits a superior electrochemical performance compared to the original CoMoO₄ material. This article provides a reference value for the application of rare earth elements in electrode materials.

2. Materials and Methods

2.1. Preparation of CoMoO₄ Material

Add 1.5 mmol of Co(NO₃)₂·6H₂O and 3 mmol of Na₂MoO₄·7H₂O into 30 mL of deionized water, stir the solution evenly, put 1 cm² of foam nickel into the beaker, and continue stirring for 2 h. Put the stirred solution into a high-pressure reactor for hydrothermal reaction, adjust the temperature to 60 °C, and adjust the time to 24 h. Then, remove the product and repeatedly clean it with deionized water. Freeze-dry the cleaned sample for 24 h, and finally calcine it in the air at 400 °C and 1 °C/min for 5 h to obtain CoMoO₄ nanoflowers.

2.2. Preparation of La-CoMoO₄ Material

Repeat the step of preparing CoMoO₄ in step 1.1, add lanthanum acetate solution to the above solution, continue to stir evenly, put 1 cm² foam nickel into the beaker, and continue to stir for 2 h. Put the stirred solution into a high-pressure reactor for hydrothermal reaction, adjust the temperature to 60 °C, and adjust the time to 24 h. Then, remove the product and repeatedly clean it with deionized water. Freeze-dry the cleaned sample for 24 h, and finally calcine it in the air at 400 °C and 1 °C/min for 5 h to obtain La-CoMoO₄ nanoflowers (Figure 1).

2.3. Preparation of CNTs Electrode Materials

References on the preparation of carbon nanotubes [35,36].

2.4. Assembly of Components

A supercapacitor was assembled using La-CoMoO₄ nanoflower structure material as the positive electrode, carbon nanotubes (CNTs) as the negative electrode, and KOH as the electrolyte. The area of the positive and negative electrode materials is 1 cm². Mix 2.8 g KOH and 3 g PVA into 5 mL deionized water to prepare the gel electrolyte. Dip the electrode materials and separator into the electrolyte mentioned above, take them out after 5 min, assemble them, and place them in the air for curing. Dry the device under vacuum conditions for 24 h to obtain La-CoMoO₄/Ni//CNTs/Ni asymmetric devices, as shown in Figure 2.

2.5. Material Characterization

Observe the morphological characteristics of the sample material using scanning electron microscopy (SEM) and transmission electron microscopy (TEM, JEOL JEM-2100 F). The cyclic voltammetry (CV) test is conducted within the potential range of −0.2 to 0.6 V (relative to SCE). The constant current charging/discharging test is conducted at various current densities within the potential range of 0–0.5 V (relative to SCE). An electrochemical impedance spectroscopy (ElS) measurement is performed by applying an alternating voltage with an amplitude of 5 mV and a frequency range of 0.1 Hz to 100 kHz at an open circuit potential. Element distribution and energy dispersive X-ray spectroscopy tests were conducted on Hitachi S-4800 scanning electron microscopy, transmission electron microscopy (TEM), and high-resolution HRTEM. The structures of the samples were characterized by X-ray diffraction (XRD, Rigaku D/max-rB, CuKa radiation, =0.1542 nm, 40 kV, 100 mA). An FD-1C-5 instrument is required in the material preparation process. Electrochemical testing was conducted on the CHI660E electrochemical workstation at Shanghai Chenhua, and the relevant calculation formulas involved in the article are as follows:

$$q=Cs\times \mathsf{\Delta }v\times m$$

(1)

$$m+/m-=c- \times \mathsf{\Delta }v-/c+ \times \mathsf{\Delta }v+$$

(2)

$$Cs=I\mathsf{\Delta }t/m\mathsf{\Delta }v$$

(3)

$$p=3600E/\mathsf{\Delta }t$$

(4)

Cs (specific capacitance, F/g), I (discharge current, A), ΔT (discharge time, s), ΔV (voltage difference, v), m (mass of active material, g).

3. Results and Discussion

We conducted scanning tests on the experimentally prepared La-CoMoO₄ nanoflower material and observed its microstructure. Figure 3a shows the SEM image of the foam Ni substrate, which shows that a large number of closely arranged La-CoMoO₄ materials are attached to the Ni substrate. We observed these materials at a low magnification and found that these tightly arranged La-CoMoO₄ materials exhibit a nanoflower-like structure, as shown in Figure 3b. To observe the microstructure of La-CoMoO₄ nanoflower material more clearly, it was further observed at a high magnification, as shown in Figure 3c. The figure shows that the nanoflower is composed of a large number of sheet-like structures growing in different directions, interweaving to form a nanoflower structure. We further conducted SEM mapping tests on the selected area in Figure 3c, as shown in Figure 3f, which showed the following four elements: Co, Mo, O, and La. The microstructure of the material plays an important role in its electrochemical performance, and we further conducted TEM testing on the material. As shown in Figure 3d, the TEM image of La-CoMoO₄ material shows that the flower-like structure is composed of ultra-thin nanosheets, which is consistent with the SEM test results in the above figure. Figure 3e shows a high-power TEM image of the La-CoMoO₄ flower-like structure material, with a lattice spacing of 0.336 nm, corresponding to the (002) lattice plane of La-CoMoO₄. The inset in the figure shows the selected electron diffraction pattern, which shows that the material is polycrystalline. We also conducted TEM mapping tests on the prepared materials, as shown in Figure 3g, indicating that the prepared materials contain the following four elements: Co, Mo, O, and La.

Figure 4a shows the elemental analysis test of La-CoMoO₄ material. It can be seen from the figure that the flower-shaped structure material contains four elements, La, Co, Mo, O, and no other elements. Figure 4b shows the XRD pattern of La-CoMoO₄ material, where CoMoO₄ is a monoclinic phase of CoMoO₄ (PDF, card No. 21-0868). The spectrum contains several weak diffraction peaks, namely CoMoO₄·0.9H₂O. After doping with the rare earth element La, the diffraction peak of the product La-CoMoO₄ shifts slightly to the left, and the intensity slightly decreases. This is because the doping element reduces the crystallinity and disorder of the product, causing lattice distortion. The above experimental tests indicate that the La-CoMoO₄ material has been successfully prepared.

To further investigate the effect of La doping on the morphology of materials, the specific surface areas of the experimentally prepared CoMoO₄ and La-CoMoO₄ materials were further analyzed through N² adsorption–desorption curves and pore size distribution curves. As shown in Figure 5a, the test results show that the specific surface area of the CoMoO₄ flower-like structure material is 98.82 m²/g, and the pore size distribution is around 27 nm. Figure 5b shows that the specific surface area of the La-CoMoO₄ flower-like structure material is 126.7 m²/g, with a pore size distribution of around 27 nm. After comparison, it was found that the pore size of the material did not change significantly before and after doping, while the specific surface area of the material increased after doping. La-CoMoO₄ material has abundant pores and a large specific surface area, providing more electrochemical reaction sites for the reaction, allowing the electrolyte to be fully distributed on the electrode surface, shortening the ion transport path, and thereby accelerating the electrochemical rate and improving the electrochemical performance.

We further investigated the surface chemical composition and the percentage of samples obtained by elemental composition. The samples were characterized and analyzed by XPS test method, and the presence of Mo3d, O 1s, Co2p and La 3d peaks is shown in Figure 6.

We conducted electrochemical testing of Ni, CoMoO₄, and La-CoMoO₄ flower-shaped nanomaterials. As shown in Figure 7a, at a scanning rate of 5 mV/s, the CV curves of Ni, CoMoO₄, and La-CoMoO₄ samples are determined by the size of the sample enclosure area. Among the three samples, La-CoMoO₄ has the largest area enclosed by the CV curve, indicating the best electrochemical performance. The area enclosed by foam nickel is almost zero, so the influence on the capacitive performance of the material is negligible. Figure 7b shows the charge–discharge curves of three materials. La-CoMoO₄ nano-cosmetic material has a longer discharge time compared to Ni and CoMoO₄. Based on Figure 7b, we further obtained the specific capacitances of the three samples, which are 40, 1400, and 2248 F/g. After comparing the specific capacitances of the three samples, it can be seen that the La-CoMoO₄ flower-shaped nanostructure material has a better capacitance performance, as shown in Figure 7c. We selected the La-CoMoO₄ flower-shaped nanomaterial with the best electrochemical performance among the three materials for further electrochemical testing. Figure 7d shows the CV curves of La-CoMoO₄ material at different scanning speeds. With the increase in scanning speed, the peak current increases, and the area enclosed by the curve gradually increases, indicating that the reaction process accelerates and the charge storage capacity increases. As the scanning rate increases, there is no significant change in the curve shape, indicating that the La-CoMoO₄ material has a good stability. Figure 7e shows the charge–discharge curves at densities of 1, 3, 5, 8, 10, and 12 A/g under different currents. According to the formula, the corresponding specific capacities are calculated to be 2248, 2160, 1930, 1860, 1780, and 1698 F/g, as shown in Figure 7f.

We further conducted cyclic stability tests on La-CoMoO₄ material at a current density of 1 A/g. After 5500 cycles, the specific capacity decreased from 2248 F/g to 2229 F/g, with a specific capacity retention rate of 99.2%. The material exhibited an excellent cyclic stability performance, as shown in Figure 8a. Figure 8b shows the impedance comparison between the first and 5500th cycles of the La-CoMoO₄ nanoflower material. It can be seen that the diffusion resistance slightly increases after 5500 cycles, which is due to the detachment of the active material and its inability to fully reflect with the electrolyte after 5500 cycles.

Figure 9a shows the change in the specific capacity of La-CoMoO₄ material after continuously changing the current density and returning to the initial current. When the initial current is 8 A/g, the specific capacity is 1860 F/g. After multiple changes in current density and returning to the initial current of 8 A/g, the specific capacity is 1856 F/g and the capacitance retention rate is 99.8%. There is no significant attenuation in the specific capacity, and the material exhibits a good rate performance and stability. Figure 9b shows the schematic diagram of the reaction between the nanoflower-structure electrode material and ions and electrons. The La-CoMoO₄ electrode material has an excellent electrochemical performance for the following reasons: foam nickel itself has good conductivity, and porous foam nickel is conducive to the diffusion of electrolyte on its surface; La-CoMoO₄ is directly grown on the foam nickel substrate, reducing the influence of adhesive on conductivity; the flower-like structure increases the specific surface area of the electrode material, which is conducive to full contact between ion electrons and the electrode material and improves the progress of chemical reactions. We will compare the specific capacity of the prepared La-CoMoO₄ material with other works, as shown in

Table 1. Comparison of electrode material properties with the literature.

Electrode Material	Current Density [1 A/g]	Specific Capacitance [F/g]	Number of Cycles	Capacity Retention Rate (%)	Ref
KCu7S4@NiMoO4	1	1194.6	10,000	92.3%	[34]
NiMoO4@Mg-Co(OH)F/NF	1	1630	6000	86%	[35]
NiCo2S4@NiMoO4	5	1447	10,000	88%	[36]
CNTs/NiMoO4	20	727.2	2000	86.8%	[37]
La-CoMoO4	1	2248	5500	99.2%	this work

.

Figure 10a shows the cyclic voltammetry curves of the La-CoMoO₄ positive electrode and CNTs negative electrode under a three-electrode testing system with a scanning rate of 5 mV/s. The potential window of La-CoMoO₄ is −0.2–0.6 V, and that of carbon nanotubes is −1.0–0.0 V. The voltage window of the asymmetric supercapacitor device assembled with two materials is the difference between the positive and negative voltage windows, so the theoretical voltage window of the asymmetric device assembled with two materials is −1.0–0.6 V. We placed the experimentally assembled La-CoMoO₄/Ni//CNTs/Ni supercapacitor device in a two-electrode system and conducted performance tests at a scanning rate of 5 mV/s. Figure 10b shows the cyclic voltammetry test curves of the device at different voltages of 0.8, 1.0, 1.2, 1.4, and 1.6 V and a scanning rate of 5 mV/s. It can be seen from the figure that there is no significant change in the cyclic voltammetry curve when the voltage window is 0–1.6. Further experiments were conducted to test the cyclic voltammetry curves of La-CoMoO₄/Ni//CNTs/Ni devices at different scanning rates of 10, 30, 50, 80, and 100 mV/s with a voltage window of 1.6 V, as shown in Figure 10c. As the scanning rate gradually increases, the peak current gradually increases, and the area enclosed by the curve also gradually increases, indicating that the material has a good capacitive performance. When the scanning rate increased to 100 mV/s, the curve shape did not change, indicating that the material has good stability. Next, we will conduct charge–discharge tests on the device, as shown in Figure 10d. The curve has good symmetry, proving that the device has good stability within the test voltage range. Figure 10e shows the specific capacities of the device calculated based on the GCD curve at different currents of 1, 3, 5, 8, 10, and 12 A/g, with capacities of 149, 125, 113, 109, 102, and 99 F/g. Figure 10f shows the energy comparison of La-CoMoO₄/Ni//CNTs/Ni devices. When the current density is 1 A/g, the power density of the device is 1000 W/kg and the energy density is 55 Wh/kg. Compared with the performance of devices in other studies, it has a better performance [38,39,40,41].

4. Conclusions

A La-CoMoO₄ nanoflower structure was synthesized using the freeze-drying method, and the microstructure and elemental composition of the material were analyzed by SEM, TEM, XRD, and EDS. The electrochemical properties of CoMoO₄ nanoflowers and La-CoMoO₄ nanoflower materials were tested and analyzed using CV, GCD, and EIS. The experimental results show that the specific capacitance of the original CoMoO₄ nanoflower material is 1400 F/g, and the specific capacitance of the doped La-CoMoO₄ nanoflower material is 2248 F/g. We conducted cycling stability tests on La-CoMoO₄ nanoflowers, and after 5500 cycles at a current density of 1 A/g, the capacitance retention rate was 99.2%. The energy density of the asymmetric supercapacitor device assembled with La-CoMoO₄ nanoflowers as the positive electrode is 55 Wh/Kg (power density 1000 W/Kg), and La-CoMoO₄ nanoflowers are valuable electrode materials for applications.