Study on the performance of composite supercapacitors based on carbon materials and manganese dioxide

1. Introduction

Supercapacitors are a new type of energy storage device between batteries and traditional capacitors. Due to their high energy density and excellent cycle charge and discharge performance, they have broad application prospects in high-power energy storage, electric vehicles and uninterruptible power supplies. At present, the main research directions of supercapacitors are the selection, preparation and assembly process of electrode materials, while the key technology of supercapacitors - research on electrode materials - mainly focuses on metal oxides, activated carbon materials and conductive polymers and mixtures of the above three materials [1]. Because these materials have large specific surface areas or are easy to produce Faraday pseudocapacitance, capacitors composed of these materials as electrodes have high specific capacitance and power density. Electrodes composed of new carbon materials - carbon nanotubes and transition metal oxides (such as manganese dioxide, nickel oxide, etc.) have shown good prospects in the application of supercapacitor electrodes due to their high specific capacitance, high power density and simple preparation method.

2. Experiment

2.1 Preparation of electrode materials

2.1.1 Activation of carbon nanotubes

Carbon nanotubes are generally extremely hydrophobic macromolecules composed of numerous aromatic delocalized carbon atoms. They are almost insoluble in any solvent and tend to aggregate into bundles in solution. These characteristics limit the excellent properties of carbon nanotubes as electrode materials. Therefore, before preparing electrodes, the activation treatment of materials is very important. In this paper, multi-walled carbon nanotubes are immersed in a strongly oxidizing 20% ​​nitric acid solution for 25 minutes, which can fully infiltrate the carbon nanotube array, so that ions are embedded in the carbon layer, and a pore structure that is conducive to capacitance is generated. It can also dissolve the iron, nickel catalyst and other acid-soluble impurities dissolved in the tube due to the precursor treatment.

The activated carbon nanotubes were gradually titrated with 6 mol/L KOH solution to a pH of 7, and dried in a drying oven at 150° C. to a constant weight for later use.

2.1.2 Preparation of amorphous manganese dioxide powder

There are many methods for preparing manganese dioxide, including sol-gel method, electrochemical deposition method, thermal decomposition method, liquid phase coprecipitation method and low temperature solid phase reaction method. Among them, the liquid phase method for preparing manganese oxide is the main method used by people. It has the advantages of simple equipment, high purity and controllable preparation process factors, but the prepared particles are easy to agglomerate [3]. This paper adopts the method of high temperature thermal decomposition of potassium permanganate to prepare manganese dioxide, by heating KMnO4 powder at 550°C in a closed manner for 2h until the reaction is sufficient. After analysis, the product obtained by thermal decomposition is amorphous/crystalline manganese dioxide, which has a high specific surface area and can be used as an electrode material for supercapacitors.

2.2 Preparation of carbon-carbon composite electrodes

2.2.1 Preparation of activated carbon/carbon nanotube composite electrodes

With acetylene black as the conductive agent, activated carbon and activated carbon nanotubes are prepared in three different ratios, so that the mass ratio of activated carbon in the electrode is 30%, 45%, and 60%, respectively, and the mass ratio of acetylene black is 10%. After grinding with an agate mortar, the mixture is evenly mixed, and isopropanol solution is added to fully wet the powder to form a slurry, and an appropriate amount of PTFE is added as a binder. At room temperature of 25°C, the mixture is evenly mixed with ultrasound for 20 minutes, and then placed in a drying oven and dried at 100°C until the slurry is semi-dry. Then, the material is pressed into a 0.2mm thick film using a film preparation process, and then the film is pressed onto a metal tantalum foil current collector at a pressure of 10MPa. The film electrodes made of the above three different ratios are numbered A, B, and C for testing.

2.2.2 Preparation of activated carbon/manganese dioxide composite electrode

In the same way as described in 2.2.1, 60% by mass of activated carbon powder was mixed with the above amorphous manganese dioxide, and the film was pressed onto a tantalum foil current collector. The electrode was numbered D for testing.

3 Performance test of composite electrode

3.1 Cyclic voltammetry test

A three-electrode system was used, i.e., the above electrode was used as the working electrode, mercurous chloride was used as the reference electrode, and a large-area platinum black electrode was used as the auxiliary electrode. The electrolyte was a 6 mol/L KOH solution, the scanning speed was 2 mV/S, and the potential range was -0.4 V to +0.6 V.

3.2 AC impedance spectroscopy test

The composite electrode was used as a diaphragm and a 6 mol/L KOH solution as an electrolyte to form a simulated symmetrical experimental capacitor. The impedance spectrum test was performed using a 2mV sinusoidal AC signal as the test signal. The frequency range of the test signal was 0.01Hz~100kHz.

3.3 Constant current charge and discharge test

The above experimental capacitor was charged and discharged at a constant current of 2 mA, with a potential range of 0V~0.9V, and the charge and discharge performance of the experimental capacitor composed of the composite electrode was measured. The specific capacity of the composite electrode material can be obtained by the following formula:

Among them, Cp is the specific capacitance of the supercapacitor, in F/g; I is the charge and discharge current, in A; ΔV is the potential difference during the discharge process, in V; Δt is the time difference during the discharge process, in s; m is the mass sum of the electrode active materials on the two symmetrical electrodes, in g.

3.4 Physical properties test of electrode materials

[page] The microstructure and surface morphology of the above-mentioned electrode materials were tested using a JEOL scanning electron microscope, aiming to test the porosity characteristics and electrical conductivity of the electrode materials from a microscopic perspective.

4 Results and discussion

4.1 Physical properties of electrode materials

Figure 1 (a) and (b) are scanning electron micrographs of carbon nanotubes before and after activation. It can be seen that after nitric acid activation, the carbon nanotubes have a short-range network structure that they did not have before activation. The long-range chain structure before activation is interrupted, and the outer tube wall becomes rougher than before activation, forming a better intertwined structure, which is more conducive to the adsorption and desorption of electrolyte ions and increases the corresponding specific surface area [4]. At the same time, the acidification process also connects a wealth of active functional groups to the carbon nanotubes, such as hydroxyl, carboxyl and carbonyl. The attachment of these active functional groups is conducive to improving the conductivity of carbon nanotubes.

Figure 2 is a SEM image of the activated carbon/manganese dioxide composite electrode. As can be seen from the figure, the electrode has a loose surface pore structure, and this unique structure can provide a good environment for the double layer reaction and Faraday pseudocapacitance reaction occurring on the electrode surface.

4.2 Capacitive characteristics of composite electrodes

Figure 3 is the cyclic voltammetry curve of the composite electrode. It can be seen that the composite electrode exhibits good reversibility and has obvious capacitance characteristics. Comparing electrodes A, B and C, it can be seen that as the carbon nanotube content increases, the area enclosed by the cyclic voltammetry curve gradually decreases, and the capacitance also gradually decreases. This is because although carbon nanotubes are a material with a high specific surface area, in double-layer capacitors, carbon materials mainly complete the energy storage process by reversibly adsorbing electrolyte ions on the electrode surface to form a double layer. The thickness of the double layer depends on the ion radius and the concentration of the electrolyte. The inner diameter of carbon nanotubes is generally between 20nm and 60nm. The smaller inner diameter within this range is difficult for electrolyte ions to enter, so the micropores only contribute to the specific surface area of ​​the material and do not have the desired effect on improving the capacitance [5]. The pore size of activated carbon is larger than that of carbon nanotubes, which is conducive to the adsorption and desorption of electrolyte ions on its surface. Therefore, as the activated carbon content increases, the area covered by the cyclic voltammetry curve of the composite electrode also increases accordingly.

Comparing electrodes C and D, it can be seen that when the mass fractions of manganese dioxide and carbon nanotubes are both 30%, the area enclosed by the cyclic voltammetry curve of the composite electrode composed of activated carbon/manganese dioxide is larger than that of the composite electrode composed of activated carbon/carbon nanotubes. In addition, no redox peak appears within the range of the scanning CV curve, which indicates that the redox reaction proceeds uniformly within the scanning potential. At the same time, the double-layer capacitance formed on the activated carbon/manganese dioxide composite electrode and the Faraday pseudocapacitance generated by the redox reaction of the active substances at the electrode-electrolyte interface are combined to increase the specific capacitance of the electrode [6].

[page] Figure 4 shows the discharge curves of each electrode at a constant current of 2 mA. It can be seen that each electrode has good linear discharge properties. The specific capacitances of the four electrode materials A, B, C and D are calculated by formula (1) to be 51.3 F/g, 56.2 F/g, 93.2 F/g and 126 F/g, respectively. From the analysis in the previous section, it can be seen that the combined effect of the double layer and Faraday pseudocapacitance increases the capacity of the activated carbon/manganese dioxide composite electrode, and also shows good discharge characteristics. In terms of contribution to capacitance, manganese dioxide is better than carbon nanotubes.

4.3 Impedance characteristics of composite electrodes

Figure 5 is the AC impedance spectrum of the composite electrode. The internal resistance of the composite electrode is mainly composed of the contact resistance, electronic resistance and ionic resistance between the electrode material and the tantalum foil current collector. The starting point of the impedance circle in the high-frequency region reflects the size of the equivalent series resistance of the capacitor, and the radius of the impedance circle reflects the transfer resistance to some extent. As can be seen from Figure 5, the D electrode exhibits good impedance characteristics. This is because the activated carbon matrix provides a well-conductive network for the manganese dioxide deposited thereon. When the content of manganese dioxide is 30% (mass fraction), the equivalent series resistance of the capacitor is 0.405Ω. At the same time, for an ideal electrode, the complex plane of impedance should be a straight line perpendicular to the real axis. Although obvious capacitance characteristics are seen in the low-frequency region for the D electrode, it still deviates from the characteristics of the ideal capacitor. This is due to the uneven distribution of the pore size of the activated carbon. The penetration of the 2mV AC signal at the same frequency is different. The electrolyte ions are easier to penetrate into the large pores, while it is more difficult to penetrate into the small pores and micropores, resulting in frequency dispersion, which is also the main reason why the impedance behavior of the electrode deviates from the ideal straight line in the low-frequency range.

In addition, it can be seen from Figure 5 that when the content of carbon nanotubes in the electrode increases, the transfer resistance shows a decreasing trend. This is because carbon nanotubes have incomparable advantages over activated carbon in terms of conductivity: carbon nanotubes can be regarded as hexagonal graphite layers curled 360° in space. As the content in the electrode increases, the degree of interweaving and winding of carbon nanotubes increases, providing a good conductive channel for electrolyte ions. Therefore, as the content increases, the radius of the impedance circle decreases, making the impedance characteristics of the composite electrode material better.

5 Conclusion

Supercapacitors based on carbon materials have high specific capacity and high power characteristics. By exploring the ratio of electrode materials, it was found that when the composite electrode is composed of 30% manganese dioxide, 60% activated carbon powder and 10% acetylene black conductive agent, if 6 mol/L KOH solution is used as the electrolyte, the specific capacity of the electrode reaches 126F/g, the internal resistance is 0.405Ω, and it has good cyclic voltammetric characteristics and charge-discharge characteristics, meeting the requirements of high-power discharge. In addition, the conductivity of carbon nanotubes is better than that of activated carbon powder. The increase in the content of carbon nanotubes in the composite electrode improves the impedance characteristics of the electrode. However, due to the large proportion of micropores, part of the surface area does not participate in the double-layer reaction, which is a useless surface area in the actual sense, thereby reducing the capacitance. Therefore, it is concluded that the supercapacitor composed of the above optimized ratio is a new type of energy storage device with excellent performance, which can play a good role in power release in the field of pulse power supply and electric vehicles.

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