The electrospinning technique produces continuous fibers of various structures and compositions with diameters ranging from a few micrometers to nanometers. For this, we have fabricated inorganic manganese oxide (MnOx) nanofibers (NFs) with different compositions by electrospinning technique for supercapacitor applications. In this study, we prepared Mn(OAc)2/poly(vinylpyrrolidone) (PVP) composite electrospun NFs using the electrospinning technique.
The as-prepared inorganic NFs were annealed at different temperatures to remove the polymer matrix and resulted in the MnOx NFs of different composition. Interestingly, we found that the capacitance of MnOx NFs annealed at 500 oC is the highest among all samples with a mixed phase of Mn2O3 and Mn3O4. Schematic illustration for the fabrication of the electrospun NFs and subsequent calcination process for hybrid MnOx NFs supercapacitors.
The inset graphs show the diameter distribution of each NFs. f) Plot of mean diameter obtained from more than 50 individual SEM images as a function of calcination temperature. The standard deviations are given as error bars. a) XRD patterns of the MnOx NFs after calcination at 400-700 oC compared to the patterns of pure Mn3O4 and Mn2O3.
Introduction
Supercapacitor
- Principle of supercapacitor
- Manganese oxide as active material of supercapacitor
- Effect of electrode structure
Batteries consist of the material of the negative and positive electrodes, the electrolyte, which enables the transfer of ions between the two electrodes, and the clamps, which enable the flow of current from the battery to perform work.7, 21-22. One is electrochemical double-layer capacitors (EDLCs) and the other is pseudocapacitors. 6 EDLCs store charges by reversible absorption of ions at the electrode-electrolyte interface, and they usually use carbon-based materials with a large surface area as active materials (Figure 2 a ). 24-27 EDLCs they can offer very high power density and good cycling stability due to their fast electrochemical process. In the case of EDLCs, the specific capacitance depends on the electrode materials, such as carbon-based nanomaterials, and the electrolytes must be carefully selected to increase the operating voltage. higher than organic electrolytes.16 In addition, it enables very fast energy density and higher power.
On the other hand, the typical conducting polymers are polyanilines, polypyrroles and other π-conjugated polymers.40-41 The specific capacitance of pseudocapacitive materials is higher than that of carbon-based materials which are active materials of EDLCs. Among pseudo-capacitive materials, RuO2 is an active material that has been widely studied due to its high theoretical specific capacitance (1358 F/g) and good electrical conductivity (3 × 102 S/cm). Among many transition metal oxides, MnOx is clearly noteworthy, exhibiting a fine specific capacitance (200-600 F/g) and reversible charge-discharge characteristic with its natural abundance and low cost.
MnOx has been studied as an active material for a long time because it is clearly remarkable in showing fine specific capacitance (F/g) and reversible charge-discharge function with its natural abundance and low cost. Recent trends in supercapacitors involved the development of large surface area electrode to improve the performance of the capacitance and conductivity. Among many nanostructures, 3D porous electrodes are known to exhibit a superior capacitance due to increased specific surface area and improved access of electrolytes to the surface of porous electrode materials.
This electrode showed a high specific capacitance of ~1145 F/g for MnO2.55 and the MnO2-CNT sponge supercapacitor reported by Alshareef et al.
Electrospinning
- Parameter of electrospinning…
- Control of structure of electrospun NFs
- Preparation of electrospun inorganic NFs
- Electrospun fibers for supercapacitor
The stirring instability is mainly due to the electrostatic interactions between the external electric field and the surface charges on the jet. The formation of fibers is mainly achieved by stretching and accelerating the liquid filament in the instability region. Due to the diversity of electrospun NFs in terms of materials, structures, architectures, and functionalities, the use of the electrospinning method has increased for the production of NFs and fabrics and membranes made from them.
Note that the path of the beam shown in B has been traced to improve visibility.67 Copyright Elsevier Science, 2001. The diameter and morphology of electrospun NFs are influenced by processing parameters such as the intrinsic properties of the solution and the operating conditions. Second, the operational conditions are the applied voltage, the distance between the nozzle and the ground substrate, and the pumping speed of the polymer solution.
In addition, environmental conditions including ambient humidity and temperature have an effect on the morphology and diameter of electrospun NFs.61-63, 76. The diameter of electrospun fibers can be adjusted by changing parameters such as polymer concentration , the electrical conductivity of the solution, the applied voltage and the pumping speed. In this study, the diameter depends significantly on the interplay between surface tension and electrostatic repulsions.70 The equilibrium point can be related to the feed rate, electric field strength, and surface tension of the polymer solution.
For example, electrospun fibers of ribbon-like structures with rectangular cross-section were reported by Reneker's group instead of the usual round shape.78-80 In this paper, electrospun fibers with ribbon-like structure were obtained by rapid evaporation of the solvent and then residual solvent disappeared by diffusion through the skin. Thus, the shape of electrospun fibers could be changed by controlling the solvent in solution. NFs with a core/shell structure could be produced by co-electrospinning two different polymer solutions via two coaxial capillaries (Figure 9).81 In order to produce the continuous and uniform fibers with a core/shell structure, it is important that two solutions must be immiscible .
This study showed that when there were equal amounts of the two polymers in solution, the fibers became highly porous structures. It is possible to adjust the pore size and density of electrospun fibers by changing the processing parameters. The diameter of the electrospun inorganic fibers was usually several hundred nanometers, and it is possible to reduce the diameter of the fibers by adjusting the parameters of the electrospinning process.
It can improve the specific capacitance at high current density due to the faster transport of ions.90 In addition, to improve the capacitance of carbon electrodes, some studies reported that the composite consisted of electrospun carbon mat and other components such as silver,91 nickel,92 or CNTs93 . The porous structure provides a large surface area and high porosity, which leads to an effective permeability of the electrolyte and a good surface activity.
Experiments
Preparation of electrospun MnO x NFs
Electrochemical measurements
Characterizations
Results and Discussion
Characterization of electrospun MnO x NFs
Furthermore, the electron diffraction pattern of a selected area of MnOx also exhibited a lattice fringe of 0.384 nm, which is associated with the (121) peak of Mn2O3 (Figure 15b). We also confirmed that Mn and O were homogeneously distributed within the electrospun MnOx NFs by EDS (Figure 15c). Overall, HRTEM image and the electron diffraction pattern confirmed the presence of mixed phase of Mn3O4 and Mn2O3 in the MnOx NFs calcined at 500 °C, in good agreement with the result of XRD.
Electrochemical performance of electrospun MnO x NFs
The trends for the galvanostatic and CV measurements agree well with each other, although the values for the latter are higher. The value obtained from galvanostatic measurements is comparable to the value (highest capacitance of 230.5 F/g at a scan rate of 25 mV/s at calcination temperature of 300 °C) published in a recent study by Lin et al., in which supercapacitor properties of different phases of MnOx films coated by a sol-gel process and annealed at C was evaluated.96 In contrast, the MnOx NFs prepared in this study exhibited significantly higher capacitance than the MnOx film prepared by a sol-gel process due to their unique 3D internal structures. Our study also suggested that electrospinning allows the creation of 3D electrodes characterized by both increased surface area and efficient and fast ion transport between the electrode and the electrolyte, improving the electrochemical properties of the supercapacitor.
The kinetics and interfacial resistance that are critical in evaluating the electrochemical reactions can be studied by electrochemical impedance spectroscopy (EIS). This result indicates that the mixed phase of Mn3O4 and Mn2O3 exhibits a lower charge transfer resistance, which is responsible for the improved electrochemical performance of the supercapacitors. In addition to the above observation, we argue that the improved supercapacitive performance stems from the balance between the conductivity and capacitance of two different phases of MnOx NFs, such as Mn3O4 and Mn2O3.
It is known that the conductivity of Mn2O3 is relatively lower than that of Mn3O4, while the capacitance of the former is relatively higher than that of the latter. Thus, the fine balance between the conductivity and the capacitance of the two different phases of MnOx NFs can result in the increased capacitance of the mixed phases of MnOx NFs. electrode is still needed, the results presented here highlight the potential of electrospinning to produce electroactive 3D electrodes. The inset graphs show the diameter distribution of each NF. f) Plot of average diameter obtained from more than 50 individual SEM images as a function of calcination temperature.
TGA plots were measured in (a - c) air and (d) N2 atmosphere at a heating rate of 5 ºC/min. a) XRD patterns of the MnOx NFs after calcination at 400-700 oC compared to the patterns of pure Mn3O4 and Mn2O3.
Conclusion
Huang, Z.-M.; Zhang, Y.-Z.; Kotaki, M.; Ramakrishna, S., A review of polymer nanofibers by electrospinning and their applications in nanocomposites.
