Before reading this article, please click on "Follow" to make it easy for you to review a series of high-quality articles from previous issues at any time, and to facilitate discussion and sharing. Thank you very much for your support!Wen|AfangEdit|AfangprefaceSupercapacitors are an emerging energy storage device, widely used in many fields such as automobiles, electronic devices, communication products, etc

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preface

Supercapacitors are an emerging energy storage device, widely used in many fields such as automobiles, electronic devices, communication products, etc. in recent years due to their high energy density, power density, fast charging and discharging ability, and long cycle life.

According to the mechanism of energy storage, supercapacitors can be divided into double layer supercapacitors (EDLCS) and Faraday Gu capacitor supercapacitors. In EDLCS, energy storage is mainly generated through electron and ion transport at the interface between the electrode material and the electrolyte. The structural characteristics of the electrode material determine its performance. As a special 1D carbon material, carbon nanofibers are a typical EDLCS electrode material with excellent conductivity Large specific surface area and aspect ratio.

Kim et al. mixed polypropylene and polyphenylene by spinning and activated at 800C to obtain porous carbon nanofibers with a specific surface area of 1220m/g. The microporous and mesoporous pore volumes were 0.71c/g and 0.2cm/g, respectively. In a 30% KOH aqueous solution, the specific capacitance reached 178F/g. Although pure carbon materials have excellent conductivity and long cycle life, they are limited by specific capacitance and energy density and cannot meet people's growing needs.

Faraday pseudocapacitor supercapacitor based on Transition metal oxides and hydroxides can have a specific capacitance at least one order of magnitude higher than that of pure carbon material EDLCS due to Redox. Huang et al. use foam nickel as the substrate, hydrothermally load A1 Ni double hydroxide, and its specific capacitance is up to 2123F/g at 1A/g, but Transition metal oxides and oxides are prone to volume expansion during Redox, It manifests as poor cyclic stability.

Therefore, electrode materials combining electric double-layer capacitance and pseudo capacitance have become a major concern, and their electrochemical performance will be greatly improved. In this study, carbon nanofibers obtained from the pre oxidation stabilization and carbonization of pure nanofiber films prepared by Electrospinning will be used as the substrate, and Fe2O3 will be loaded on carbon nanofibers by electrodeposition, Obtain uniformly deposited and appropriately sized Fe2O3 loaded carbon nanofibers (Fe2O3/CNF) composite materials as electrode materials for supercapacitors, and conduct electrochemical characterization.

1. Main raw material polypropylene powder (PAN, molecular weight 150000)

SigmaAldrich; N-N dimethylamine (DMF), oxalic acid ((NH4) 2C204: H20), ferric chloride (FeCl36H20), all analytically pure, Tianjin Tianli Chemical Reagent Co., Ltd; The experimental water is all deionized water, self-made.

2. Preparation of gray nano scale dimension counting thin films

Weigh a certain amount of PAN powder and DMF solution (the mass ratio of the two is 1:10), mix them in an oil bath pan at 70C, stir them at constant temperature, and then prepare transparent spinning solution through homogeneous dispersion. Take 10mL of PAN powder and use a high-voltage Electrospinning device for spinning. The process parameters of Electrospinning are set as follows: the temperature is 35C, the power supply voltage is 25kV, and the propulsion speed of the syringe is 1mL/h.

Place the prepared Electrospinning film on the high-temperature quartz plate, and conduct pre oxidation treatment in the air atmosphere of the tubular furnace. The temperature rise program is: 1 C/min to 280 C, constant temperature 2. After the pre oxidation is completed, use the sexual gas as the protective gas, 5 C/min to 800 C, and constant temperature 30 min for carbonization to obtain carbon nanofibers.

3. Electrodeposition of Fc20

The preparation of composite thin films involves stirring 0.45g FeCl36H0 and 0.7g (NH4) C04.6H20 into a mixed solution of 5LDMF and water (DMF: water=3:1) to prepare an electrodeposition solution. Using an electrochemical workstation (CHI660E type, Shanghai Chenhua Technology Co., Ltd.), a platinum electrode is used as the counter electrode, and 2cX2m carbon nanofiber film is used as the working electrode. The electrodeposition is then carried out, which is placed in a tube furnace and calcined at 350C to obtain Fe2O3; Loaded carbon nanofiber Fe2O3/ For comparative research, CNF-3 thin film was obtained by adjusting DMF: water ratio to 1:1 and following the above steps to obtain Fe2O3/CNF-1 thin film.

4. Preparation of thin film electrodes

The carbon nanofiber is directly sliced, about 3mg is taken, evenly spread between two foam nickel discs with a diameter of 1. c, and then the nickel strip is placed as a conductor. The electrode sheet is obtained after being taken out after being kept under 10MPa for 5min by a Tablet press. The electrochemical test is carried out after being fully soaked in 6mol/LKOH solution. This flexible film material avoids the drawbacks of adding adhesive and conductive agent in the process of preparing electrodes for powder materials.

5. Test characterization

Observing the morphology of the prepared carbon nanofibers using desktop scanning electron microscopy (SEM, PheomPro type, FEI company); The sample was analyzed by Diffractometer (XRD, MiiFlez600): the pore structure of the sample was determined by adsorption instrument (SI-21, Quantachromme) at 77K, using high-purity nitrogen as adsorbent.

Electrochemical performance test: using a three electrode system, taking the Saturated calomel electrode as the reference electrode, the platinum electrode as the counter electrode, and the electrode sheet prepared by the active substance as the working electrode, the cyclic voltammetry, constant current charge discharge and AC impedance tests are carried out through the electrochemical workstation; using the LANHE blue battery tester (CT2001A) to test the cycle life of the electrode materials.

1. Structural characterization analysis of carbon nanofibers

Figure 1 shows the SEM images of CNF, Fe2O3/CNF-1, and Fe2O3/CNF-3. From the images, it can be seen that continuous and smooth carbon nanofibers are arranged in a staggered manner in CNF, while blocky Fe2O3 can be seen in Fe2O3/CNF-1; Uneven deposition on carbon nanofibers and agglomeration phenomenon also occur.

It is evident from Fe2O3/CNF-3 that each carbon nanofiber is uniformly loaded with small Fe2O3 nanoparticles, and the smooth surface of the carbon fiber becomes rough after loading Fe2O3. Compared with Fe2O3/CNF-1 and Fe2O3/CNF-3, it is evident that an increase in DMF concentration in the electrodeposition solution can effectively inhibit the growth of Fe2O3, resulting in the formation of smaller sized nanoparticles uniformly loaded on the carbon nanofibers.

Figure 2 (a) shows the XRD spectra of CNF and Fe2O3/CNF-3. It can be seen from the figure that CNF has a wide peak of carbon at 20=26, indicating its amorphous structure. The characteristic peak of Fe2O3 can be clearly seen from the figure, indicating that Fe2O3 has been successfully loaded on carbon nanofibers. Figure 2 (b) shows the pore size distribution of CNF and Fe2O3/CNF-3. It can be seen from the figure that Fe2O3/CNF-3 has more abundant pore structures between 220n compared to CNF, which are conducive to the entry and exit of ions, And then reduce the resistance during the transmission process.

2. Electrochemical performance analysis

To investigate the electrochemical performance of electrodeposited Fe2O3 carbon nanofibers, constant current charge discharge, cyclic voltammetry, and alternating current were performed

The results of current impedance and cycle life test are shown in Figure 3. It can be seen from Figure 3 (a) that the constant current charge discharge curve of CNF is an early regular Isosceles triangle between -0.9-0.1V in the voltage window, which is a typical feature of the electric double layer capacitance formed by carbon materials. The charge discharge time of Fe2O3/CNF-3 is longer than that of CNF, and the mass specific capacitance of the material is calculated according to Formula (1).

According to equation (1), the specific capacitance of CNF is 87.7F/g, while the specific capacitance of Fe2O3/CNF-3 reaches 330.1F/g. The prepared Fe2O3/CNF-3 is 3.76 times the specific capacitance of CNF.

From Figure 3 (b), it can be seen that the cyclic voltammetry curve of CNF is a regular rectangle with no redox peak; The cyclic voltammetry curve of Fe2O3/CNF3 also presents a relatively regular rectangle, with a hump like oxidation peak at -0.8~-0.4V, indicating that Fe2O3/CNF3 has both double layer capacitance and pseudocapacitance energy storage mechanisms.

Using Zview fitting software to establish a suitable equivalent circuit for impedance spectrum analysis, as shown in the lower right corner illustration of Figure 3 (c). It can be seen from Figure 3 (c) that both CNF and Fe2O3/CNF-3 electrodes have a semicircle in the high-frequency region and a straight line in the low-frequency region, representing charge transfer and ion transfer processes, respectively.

The Fe2O3/CNF-3 electrode exhibits a relatively small semicircle compared to the CNF electrode, indicating that the charge transfer resistance (R=0.19992) of the Fe2O3/CNF-3 electrode is smaller. The Fe2O3/CNF-3 electrode exhibits a straight line more perpendicular to the axis in the low-frequency region compared to the CNF electrode, indicating that the Fe2O3/CNF-3 electrode has better capacitive performance and lower ion diffusion resistance, which is consistent with the conclusion of pore size distribution.

Figure 4 shows the constant current charge discharge curves, mass specific capacitance curves, and cyclic voltammetry curves of Fe2O3/CNF-3 at different electrical densities (120A/g) and scanning rates (5-100mV/s). It can be seen from the figure that the calculated specific capacitance of 1A/g2A/g, 5A/g, 10A/g, and 20Ag are 330.1F/g, 260.8F/g, 211Fg, 180F/g, and 158Fg, respectively. The sample can retain 47.86% at 20A/g, indicating good rate performance.

The cyclic voltammetry curves of Fe2O3 at different scanning rates (5-100mV/s) are approximately rectangular, neither ideal double layer capacitance nor pseudocapacitance behavior. This is because in Fe2O3/CNF-3, the double layer capacitance is mainly provided by CNF, and Fe2O3 can provide a certain amount of pseudocapacitance. The electrochemical mechanism of pseudocapacitance provided by Fe2O3 is as follows: when Fe2O3/CNF-3 is immersed in KOH electrolyte, cations (Kt) can enter interlayer, channel, and pore structures, This leads to the reaction between F3+and the electrolyte, causing Fe2O3 to generate pseudocapacitance, where the insertion and exit of K play a dominant role.

The cycling life of electrode materials is also an important indicator to measure material performance. Figure 5 shows the cycling life of CNF and Fe2O3/CNF-3 under constant current charging and discharging at 2Ag. It can be seen from the figure that Fe2O3/CNF-3 and CNF can maintain a specific capacitance of 91.69% and 92.65% after 5000 cycles, respectively. Fe2O3/CNF-3 can still maintain a specific capacitance of 91.45% after 8000 cycles, indicating that both materials have excellent cycling stability, However, the Fe2O3/CNF-3 curve fluctuates significantly and the specific capacitance retention is slightly lower than that of CNF, due to the volume expansion and contraction of metal oxide materials during the charging and discharging process.

Compared to pure carbon nanofibers, the specific capacitance of Fe2O3/CNF-3 is significantly increased, which is attributed to the synergistic effect of carbon nanofibers and Fe2O3 (see Figure 6 for the schematic diagram of composite material preparation). Carbon nanofibers can serve as both the core of conduction and the outer layer of Fe2O3; The rapid Redox of nanoparticles can transport electrons, and also act as a buffer substrate to slow the volume expansion and contraction of Fe2O3 in the chemical reduction reaction process, so as to achieve the purpose of extending the cycle life. In addition, Fe2O3/CNF-3 composite films can significantly shorten the ion transport distance, increase the contact area, reduce the electron transport resistance, and significantly improve the electrochemical performance of pure carbon nanofibers.

Fe2O3 loaded carbon nanofiber electrode materials with excellent performance were prepared by adjusting the concentration ratio of DMF to water in the electrodeposition solution using the electrodeposition method. The experimental results showed that increasing the concentration of DMF can inhibit the deposition of Fe2O3 and reduce the size of Fe2O3.

When DMF: water=3:1, Fe2O3 loaded carbon nanofibers with excellent morphology were obtained. Electrodes were prepared using Fe2O3/CNF-3, and the addition of Fe2O3 introduced pseudocapacitance and rich mesoporous structure into the pure carbon material, greatly improving the specific capacitance of the pure carbon material. At the same time, the excellent cycling stability of the carbon material effectively avoided the disadvantage of metal oxide decay, The specific capacitance of the Fe2O3/CNF-3 electrode, which combines two excellent properties, is 3.76 times that of the CNF electrode prepared under the same conditions. After 8000 charges and discharges, the specific capacitance can still be retained by 91.45%, indicating good stability.

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