Before reading this article, please click "Follow", so that you can consult a series of high-quality articles in previous issues at any time, and at the same time facilitate discussion and sharing, thank you very much for your support!
Text | Afang
Edit | Afang
preface
Supercapacitor is an emerging energy storage device, which has been widely used in many fields such as automobiles, electronic devices, and communication products in recent years due to its high energy density, power density, fast charge and discharge ability and long cycle life.
According to the mechanism of energy storage, supercapacitors can be divided into electric double-layer supercapacitors (EDLCS) and Faraday Gu capacitor supercapacitors in EDLCS, energy storage is mainly generated by electrode material and electrolyte contact interface electron and ion transport, the structural characteristics of the electrode material determine if its performance, carbon nanofiber as a special 1D carbon material, is a typical EDLCS electrode material, such as excellent conductivity, large specific surface area and aspect ratio.
Kim et al. mixed polypropylene and polybenzomide spinning, activated at 800C to obtain porous carbon nanofibers with a specific surface area of 1220m/g, its microporous pore volume is 0.71c/g, mesoporous capacity is 0.2cm/g, and the specific capacitance reaches 178F/g in 30% KOH aqueous solution, although the pure carbon material has excellent conductivity and long cycle life, but is limited by the specific capacitance and energy density, can not meet people's growing demand.
Faraday pseudocapacitor supercapacitor based on transition metal oxides and hydroxides due to redox reaction, its specific capacitance can be at least an order of magnitude higher than pure carbon material EDLCS, Huang et al. with nickel foam as the base, hydrothermal load A1-Ni double-layer hydroxide, at 1A/g its specific capacitance is as high as 2123F/g, but transition metal oxides and oxides are prone to volume expansion during redox reaction, manifested by poor cycle stability.
Therefore, the electrode material combining electric double layer capacitor and pseudocapacitor has become the main focus, and its electrochemical performance will be greatly improved, in this study, the simple nanofiber film prepared by electrospinning method is pre-oxidized stable and carbonized to obtain carbon nanofibers obtained by pre-oxidation stabilization and carbonization, and Fe2O3 is loaded on carbon nanofibers by electrodeposition method, and Fe2O3 loaded carbon nanofibers (Fe2O3/CNF) with suitable size are obtained by adjusting the ratio of N-N dimethylmethylamine to water Composites are used as electrode materials for supercapacitors and are electrochemically characterized.
One. Experimental part
1. Main raw material polypropylene eye powder (PAN, molecular weight of 150000)
Sigma Aldrich; N-N diylmethylamine (DMF), oxalic acid ((NH4)2C204:H20), ferric chloride (FeCl36H20), all are pure for analysis, Tianjin Tianli Chemical Reagent Co., Ltd.; The experimental water was all deionized water, homemade.
2. Preparation of gray nanometer dimension
Weigh a certain amount of PAN powder and DMF liquid (the mass ratio of the two is 1:10) mixed, put in an oil bath at 70C constant temperature stirring, and then prepare transparent spinning liquid by homogeneous dispersion, take 10mL using high-pressure electrospinning device for spinning, electrospinning process parameters are set to: temperature is 35C, power supply voltage is 25kV, syringe advancement speed is 1mL/h.
The prepared electrospinning film is placed on a high-temperature quartz plate, and the heating procedure is pre-oxidized under the air atmosphere in the tube furnace: 1C/min to 280C, constant temperature 2 to be pre-oxidized after the completion of sexual gas gas as protective gas, 5C/min to 800C, constant temperature 30min for carbonization, carbon nanofibers can be obtained.
3. Electrodeposition Fc20
The preparation of composite film was weighed 0.45g FeCl36H0 and 0.7g (NH4)C04.6H20 agitated in 5LDMF and water mixed solution (DMF: water = 3:1) to prepare electrodeposition solution, using electrochemical workstation (CHI660E type, Shanghai Chenhua Technology Co., Ltd.), platinum electrode as the counterelectrode, 2cX2m carbon nanofiber film as the working electrode, electrodeposition, and then placed in a tubular furnace, roasted at 350C to obtain Fe2O3; Loaded carbon nanofiber Fe2O3;/CNF-3 film, for comparative study, DMF:water = 1:1 was adjusted, and Fe2O3/CNF-1 film was obtained after the above steps.
4. Preparation of thin film electrodes
The carbon nanofiber is directly sliced, take about 3mg, evenly spread in the middle of two nickel foam discs with a diameter of 1.c, and then put the nickel sheet strip as a wire, use a tablet press to hold at 10MPa for 5min and then take out to obtain the electrode sheet, and conduct electrochemical tests after full infiltration in 6mol/LKOH solution, this flexible film material avoids the drawbacks of adding binders and conductive agents in the preparation of electrodes for powder materials.
5. Test characterization
Benchtop scanning electron microscopy (SEM, Pheom Pro, FEI) was used to observe the morphology of the prepared carbon nanofibers. The sample was analyzed by X-ray diffractometer (XRD, MiiFlez600 type): the pore structure of the sample was determined by adsorption meter (SI-21 type, American Quanta chromme) under 77K conditions, with high-purity nitrogen as adsorbent.
Electrochemical performance test: using three-electrode system, saturated calomel electrode as reference electrode, platinum electrode as counter electrode, electrode sheet prepared by active material as working electrode, cyclic voltammetry, constant current charge and discharge and AC impedance test through electrochemical workstation: LANHE blue electric battery tester (CT2001A type) for cycle life test of electrode material.
Two. Results and discussion
1. Structural characterization analysis of carbon nanofibers
Figure 1 is shown in the SEM diagram of CNF, Fe2O3/CNF-1 and Fe2O3/CNF-3, from which it can be seen that continuous and smooth carbon nanofibers can be seen in the CNF staggered arrangement, and block Fe2O3 can be seen in Fe2O3/CNF-1; Unevenly deposited on carbon nanofibers, agglomeration also occurs.
Fe2O3/CNF-3 clearly shows that each carbon nanofiber is uniformly loaded with fine Fe2O3 nanoparticles, and the smooth carbon fiber surface becomes rough after loading Fe2O3, compared with Fe2O3/CNF-1 and Fe2O3/CNF-3, it can be clearly seen that the increase of DMF concentration in the electrodeposition solution can well inhibit the growth of Fe2O3, thereby forming smaller nanoparticles uniformly loaded on carbon nanofibers.
Figure 2 (a) is the XRD spectrum 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 that it is an amorphous structure, from the figure you can clearly see the characteristic peak of Fe2O3, indicating that Fe2O3 has been successfully loaded on carbon nanofibers, Figure 2 (b) is the pore size distribution of CNF and Fe2O3/CNF-3, it can be seen that Fe2O3/CNF-3 has a richer pore structure than CNF between 220n. These mesoporous structures facilitate the entry and exit of ions, thereby reducing resistance during transport.
2. Electrochemical performance analysis
In order to explore the electrochemical performance of electrodeposited Fe2O3 carbon nanofibers, constant current charge and discharge, cyclic voltammetry and intercourse were carried out
Flow impedance and cycle life test, the results are shown in Figure 3, from Figure 3 (a It can be seen that the constant current charge and discharge curve of CNF is an isosceles triangle with a voltage window of -0.9-0.1V, which is a typical feature of electric double layer capacitance formed by carbon materials, Fe2O3/CNF-3 charge and discharge time is longer than CNF, and the mass ratio capacitance of the material is calculated according to equation (1).
According to Equation (1), the specific capacitance of CNF is 87.7F/g, while the specific capacitance of Fe2O3/CNF-3 is 330.1F/g, and the Fe2O3/CNF-3 prepared is 3.76 times that of CNF.
It can be seen from Figure 3(b) that the cyclic voltammetry curve of CNF is a regular rectangle, and no redox peak appears; The cyclic voltammetry curve of Fe2O3/CNF3 also presents a relatively regular rectangle, and a hump-like oxidation peak occurs at -0.8~-0.4V, indicating that Fe2O3/CNF-3 has two energy storage mechanisms: electric double layer capacitance and pseudo-capacitor.
The impedance spectrum is analyzed by establishing a suitable equivalent circuit by Zview fitting software, see the illustration in the lower right corner of Figure 3(c), and it can be seen from Figure 3(c) that the CNF and Fe2O3/CNF-3 electrodes have a semicircle in the high frequency region and a straight line in the low frequency region, which indicates the charge transfer and ion transport processes, respectively.
Fe2O3/CNF-3 electrode exhibits a relatively small semicircle relative to the CNF electrode, which indicates that the charge transfer resistance (R=0.19992) of Fe2O3/CNF-3 electrode is smaller, and Fe2O3/CNF-3 electrode exhibits a straight line more perpendicular to the axis than the CNF electrode in the low frequency region, which indicates that Fe2O3/CNF-3 electrode has better capacitance performance and lower ion diffusion resistance, which is consistent with the pore size distribution conclusion.
Figure 4 is Fe2O3/CNF-3 under different electrical densities (120A/g) constant current charge and discharge curve, mass ratio capacitance diagram and different scanning rates (5~100mV/s) cyclic voltammetry curve, it can be seen from the figure, after calculation, the specific capacitance of 1A/g2A/g, 5A/g, 10A/g, 20Ag is calculated to be 330.1F/g, 260.8F/g, 211Fg, 180F/g, 158Fg, the sample at 20A/ g can retain 47.86%, indicating good rate performance.
The cyclic voltammetry curve of Fe2O3 at different scanning rates (5~100mV/s) is approximately rectangular, which is neither the ideal electric double layer capacitance behavior nor the pseudocapacitance behavior, this is because in Fe2O3 / CNF-3, mainly the electric double layer capacitor provided by CNF, Fe2O3 can provide a certain pseudocapacitance, and the electrochemical mechanism of the pseudocapacitor provided by Fe2O3 is as follows: when Fe2O3/CNF-3 is immersed in KOH electrolyte, cations (Kt) can enter the interlayer, The channel and hole structure causes F3+ to react with the electrolyte, causing Fe2O3 to produce pseudocapacitance, in which the embedding and output of K play a dominant role.
The cycle life of electrode materials is also an important indicator to measure the performance of materials, Figure 5 is CNF and Fe2O3/CNF.3 under 2Ag constant current charge and discharge cycle life diagram, it can be seen from the figure, Fe2O3/CNF-3 and CNF after 5000 cycles after the specific capacitance can maintain 91.69% and 92.65%, Fe2O3/CNF-3 after 8000 cycles can still maintain 91.45% specific capacitance, It shows that both materials have excellent cycle stability, but the Fe2O3/CNF-3 curve fluctuates greatly and the specific capacitance retention is slightly lower than CNF, which is due to the volume expansion and contraction of metal oxide materials during charging and discharging.
Compared with 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 preparation of composite materials) Carbon nanofibers can be used as the core of conduction and are the outer layer of Fe2O3. In addition, Fe2O3/CNF-3 composite film 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.
Three. conclusion
Fe2O3-supported carbon nanofiber electrode materials with excellent performance were prepared by adjusting the concentration ratio of DMF to water in electrodeposition solution by electrodeposition method, and the experimental results show that the increase of DMF concentration can inhibit the deposition of Fe2O3 and reduce the size of Fe2O3.
When DMF: water = 3:1, Fe2O3 loaded carbon nanofibers with excellent morphology are obtained, and the electrode prepared by Fe2O3/CNF-3, due to the addition of Fe2O3, introduces pseudocapacitance and rich mesoporous structure for pure carbon materials, which greatly improves the specific capacitance of pure carbon materials, and the excellent cycle stability of carbon materials effectively avoids the shortcomings of easy attenuation of metal oxides, combined with two Fe2O3/ The specific capacitance of CNF-3 electrode is 3.76 times that of CNF electrode prepared under the same conditions, and after 8000 times of charge and discharge, the specific capacitance can still retain 91.45%, which has good stability.