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Facile fabrication of coaxial-cable like Mn2O3 nanofiber by electrospinning: Application as electrode material for supercapacitor
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Short Communication
Facile fabrication of coaxial-cable like Mn2 O3 nanofiber by electrospinning: Application as electrode material for supercapacitor Jiyuan Liang a,∗, Ling-Tao Bu a,b, Wei-Guo Cao b,∗, Teng Chen b, Yuan-Cheng Cao a,∗ a b
Key Laboratory of Optoelectronic Chemical Materials and Devices, Ministry of Education, Jianghan University, Wuhan 430056, China Shandong Tianbao Chemical Corporation, Pingyi, Shandong 273300, China
a r t i c l e
i n f o
Article history: Received 29 January 2016 Revised 15 April 2016 Accepted 7 June 2016 Available online xxx Keywords: Electrospinning Mn2 O3 nanofiber Coaxial-cable Supercapacitor
a b s t r a c t Coaxial-cable like Mn2 O3 nanofibers are fabricated by a facile and cost-effective single-nozzle electrospinning technique and subsequent calcination. The morphology, microstructure, crystal structure, composition and specific surface area are characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and N2 adsorption–desorption. A possible formation mechanism of the coaxial-cable like Mn2 O3 nanofiber has also been proposed. Due to the large specific surface area and porous structure, the synthesized coaxial-cable like Mn2 O3 nanofiber is employed as the electrode for supercapacitor in 6 M KOH aqueous condition. The specific capacitance is up to 216 F/g at 0.5 A/g and the electrode also exhibited excellent cycling stability of 93% capacitance retention after 10 0 0 cycles. The encouraging results show the potential of the coaxial-cable like Mn2 O3 nanofiber as supercapacitor electrode material. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Nowadays, one-dimensional micro/nano structure materials, such as nanowires, nanotubes, nanorods and nanofibers have attracted great attention because of their distinctive geometries and special physical/chemical properties [1,2]. Many methods have been reported for fabricating these structure materials. For example, hydrothermal method [3], self-assembly method [4], template method [5] and electrospinning [6,7]. Among these methods, electrospinning technique is believed to be a convenient, cost-effective and versatile method for preparing nanofibers, such as polymeric [8,9], inorganic [10–12] and metallic nanofibers [13] with various structures and morphologies. It is a process that uses an electric field to control the formation of nanofibers [14]. So far, many solid nanofibers and hollow nanofibers have been fabricated by single nozzle electrospinning [1,15,16]. However, 1D nanomaterials with the complex tubular structures, such as tube-in-tube and wirein-tube, are usually prepared by coaxial-electrospinning technique [17,18]. Unfortunately, coaxial electrospinning technology should precisely control the solution viscosity, interfacial tension of core solution and shell solutions. Evidently, coaxial electrospinning is very cumbersome and thus not feasible for large scale production.
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Corresponding authors. Tel.: +8615195987523. E-mail addresses: [email protected] (J. Liang), [email protected] (W.-G. Cao), [email protected] (Y.-C. Cao).
Hence, the exploration of facile process for the fabrication of 1D nanostructure materials with complex structure (such as coaxialcable like) still remains great challenge. The ever increasing energy demands and the limited sources of fossil fuels have led to the development of energy storage devices. Supercapacitor as a new kind energy storage device, with high power density, fast charging/discharging and long lifespan, has attracted great research interests in recent years. According to the storage mechanism, supercapacitors can be classified into two kinds: electrical double-layer capacitor (EDLC) and pseudocapacitor. EDLC usually uses high surface area carbonaceous materials as the electrode materials [19,20]. However, a low specific capacitance of only around 10 0–20 0 F/g restricts its wide application. Transition metal oxides or conducting polymers are considered to be the most promising pseudocapacitor candidate materials [21,22]. Pseudocapacitor, which relies on faradaic process associated with surface or near-surface redox reaction, can provide higher specific capacitance than EDLC [23]. Among the various transition metal oxides, manganese oxides have been widely used in the field of catalysts, energy, and waste remove because of their low cost, availability in abundance, and environmental benignity [24,25]. The chemical valence of manganese includes 2+, 3+, 4+ and 6+. Amongst them, Mn2 O3 exhibits distinctive chemical and physical properties [26]. Recently, Mn2 O3 nanomaterials have attracted great attention as the electrode materials for Li-ion battery [27–30]. However, the Mn2 O3 nanomaterial as the electrode material for supercapacitor, especially in alkaline electrolyte, is received much less attention.
http://dx.doi.org/10.1016/j.jtice.2016.06.005 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: J. Liang et al., Facile fabrication of coaxial-cable like Mn2 O3 nanofiber by electrospinning: Application as electrode material for supercapacitor, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.06.005
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Furthermore, it is well known that the behavior of nanomaterials strongly depends on their structure, shape and size, which greatly influence their performances. Thus it is also necessary and important to study the capacitance performance of various structure Mn2 O3 . Pseudocapacitance is an interfacial phenomenon related to the morphology of the electroactive material, it is expected that the coaxial-cable like structure of a material enhances the electrochemical properties by reducing the diffusion path and by providing a large electroactive sites for ions. To date, Mn2 O3 materials with different structures have been prepared by different approaches including solvent-thermal method [29], thermal decomposition [31], template method [32] and electrospinning [33]. Just as we discussed above, electrospinning technique is a very facile and novel method to prepare 1D nanomaterials. Recently, Yang et al. prepared hollow Mn2 O3 nanofibers through single nozzle electrospinning [26]. In this paper, we prepared a complex coaxial-cable like Mn2 O3 nanofiber via single-nozzle electrospinning method. Compared with coaxial electrospinning technique, the preparation process is simple and without choosing any special electrospinning solution. Based on the various characterization datum, the possible formation mechanism is also proposed. The performance of coaxial-cable like Mn2 O3 nanofiber as the electrode materials for supercapacitor in alkaline aqueous condition is also studied. 2. Experimental 2.1. Materials Polyvinylpyrrolidone (PVP, Mw = 130 0,0 0 0) was purchased from Sigma Aldrich. Manganese acetate tetrahydrate (Mn(CH3 COO)2 4H2 O, Mn(Ac)2 4H2 O) and acetic acid (CH3 COOH, HAc) were purchased from Sinopharm Chemical Reagent Co., Ltd. All the above chemicals were analytical grade and used as received without further purification. 2.2. Synthesis of coaxial-cable structured Mn2 O3 nanofibers In a typical synthesis, 1.5 g PVP powder was added to 10 mL absolute ethanol solution and magnetic stirred for 1 h to generate a homogeneous solution. Subsequently, 0.005 moL Mn(CH3 COO)2 ·4H2 O was dissolved in the solution of 5 mL acetic acid and 2 mL distilled water, and then mixed with the PVP solution. The mixture solution was continuously stirred for another 1 h. Finally, the spinning precursor solutions were transferred to a hypodermic syringe. The positive terminal of a variable high-voltage power supply was connected to the needle tip of the syringe. A grounded iron drum, covered with an aluminum foil, served as the counter electrode. In electrospinning conditions, a flow rate was fixed at 0.2 mL/h. The distance between the needle tip and collector was 20 cm, and the voltage was set at 1 kV/cm. After electrospinning, the obtained nanofibers were dried initially for 6 h at 80 °C under vacuum and then calcined in the air atmosphere at a rate of 1 °C/min and maintained for 6 h at 700 °C to get Mn2 O3 nanofibers. For comparison, the pure Mn2 O3 particles were synthesized by direct annealing the Mn(Ac)2 powders at 700 °C for 5 h in the air. 2.3. Characterizations The samples were characterized by X-ray powder diffraction (XRD, hilips X’Pro X-ray diffractometer), field emission scanning electron microscopy (FESEM, Hitach S-4800), high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F), and Fourier transform infrared spectroscopy (FT-IR; Bruker Vertex 70). Thermogravimetric-differential scanning calorimetry (TG-DSC) Please
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analysis was performed on the thermogravimetric analyzer (TA Instruments Q50) at a heating rate of 2 °C/min under air atmosphere. The specific surface area and pore size distribution were also evaluated from nitrogen sorption isotherms measured by an analyzer (Tristar II, Micromeritics) at 77 K. Electrochemical tests were carried out on an electrochemical workstation (CHI 660D) in a three electrode system in 6 M KOH solution, with Pt wire serving as the counter electrode and Hg/HgO as the reference electrode. The active material was mixed with acetylene black and polyvinylidene fluoride with a mass ratio of 85:10:5. The mixture was drop-cast onto a graphite electrode of 1 cm × 1 cm area and dried in a vacuum oven at 80 °C overnight. The specific capacitance from galvanostatic charge–discharge curve was calculated according to the following equation:
Cm = C/m = (I × t)/(V × m) where I is the discharge current (A), m is the mass of active materials (g), t is the discharge time (s), and V is the range of potential window (V). 3. Results and discussion Fig. 1a shows the SEM image of Mn(Ac)2 /PVP nanofiber. It can be seen that the nanofibers are randomly distributed on the substrate and the diameter is around 250 nm. Fig. 1b exhibits the typical SEM image of Mn2 O3 nanofibers. It is to be noted that nanofiber structure can be still retained despite the Mn2 O3 nanofibers are broken. What’s more, the diameter of Mn2 O3 nanofiber is shrunk to 200 nm due to the decomposition of PVP after calcination at 700 °C. The inner structure can be clearly seen from the fracture surface. The zoom-in image (Fig. 1c) shows that the average diameter of Mn2 O3 nanofiber core and shell are 50 nm and 200 nm, respectively. The coaxial-cable architecture of Mn2 O3 is further demonstrated by TEM image. Obviously, the interior of Mn2 O3 nanofibers shows coaxial-cable like structure with a wire located in the center of hollow nanofiber. A probable formation mechanism is proposed and discussed in details in a latter section. Fig. 1d inset shows the HRTEM image of Mn2 O3 nanofibers shell. The lattice fringes can be clearly observed, indicating the good crystalline of the as-prepared samples. The measured inter-planar distance is 0.38 nm, corresponding to the (211) crystal facet. Hence, the coaxial-cable like Mn2 O3 nanofibers are successfully prepared by single nozzle electrospinning. TG-DSC techniques were used to study the thermal properties of Mn(Ac)2 /PVP. Fig. 2 shows the TG-DSC curves of Mn(Ac)2 /PVP composite fiber between room temperature and 800 °C. TG result indicates that there are three steps weight loss. The first 10% weight is lost below 220 °C, which results from the evaporation of water, ethanol and acetic acid. The second 40% weight loss between 220 °C and 370 °C, which is assigned to the decomposition of the Mn(Ac)2 and PVP. The third weight loss can be interpreted as the complete decomposition of the metal precursor and PVP polymer. It is worth noting that there is no more weight loss in the temperature range between 450 °C and 800 °C, indicating the complete decomposition of the metal precursor and PVP. The products further investigated their crystalline structure with XRD characterizations. Fig. 3 represents the XRD patterns of Mn(Ac)2 /PVP composite fibers, Mn2 O3 particles and coaxial cable structure Mn2 O3 nanofibers. As shown in Fig. 3a, the as-prepared composite fibers shows typical amorphous structure and without any peaks being detected. After being calcined at 700 °C, as shown in Fig. 3c, the crystallization of Mn2 O3 (JCPDS card no. 41-1442) phase is formed at 2θ = 23.33, 33.18, 35.93 degree. The XRD result also shows that Mn2 O3 particles can be obtained by calcining Mn(Ac)2 powder in air at 700 °C. Furthermore, according to the
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Fig. 1. (a) SEM images of Mn(Ac)2 /PVP composite and (b) coaxial-cable like Mn2 O3 nanofiber; (c) Zoom in SEM image of the area indicated in Fig. 1b; (d) TEM image of coaxial-cable like Mn2 O3 nanofiber, inset is the high resolution TEM image.
Fig. 2. TG-DSC curves of Mn(Ac)2 /PVP composite fiber in the air.
Fig. 4. (a) FT-IR spectra of PVP powders, (b) as-prepared Mn(Ac)2 /PVP composite fibers, and (c) coaxial-cable like Mn2 O3 nanofiber.
Fig. 3. (a) XRD patterns of Mn2 O3 /PVP composite fibers; (b) Mn2 O3 particles; (c) coaxial-cable like Mn2 O3 nanofibers.
Scherrer equation D = Kλ/β cosθ [34], where D is the average particle size (nm), K is Scherrer constant (K = 0.89), λ is wavelength of X-ray radiation (λ = 0.15418 nm), θ (rad) is characteristic X-ray diffraction peak and β (rad) is the line width at half-maximum Please
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height, the particle size of coaxial cable like Mn2 O3 nanofibers and Mn2 O3 particle is 39 and 44 nm, respectively. Surface functional group distribution was determined by FTIR spectroscopy. Fig. 4 shows the FT-IR spectrums of Mn2 O3 , Mn(Ac)2 /PVP nanofiber and pure PVP powder. For PVP, the characteristic stretching band at 1663 cm−1 is attributed to C=O group. The band at 2993 cm−1 can be attributed to the CH2 stretching vibration of PVP [35]. After being calcined at 700 °C, it is noted that the bands corresponding to organic components are disappeared, indicating that the PVP is removed completely. At the same time, some new bands appeared, such as the bands at 682 cm−1 , 580 cm−1 and 522 cm−1 of Mn2 O3 nanofibers corresponding to the Mn–O stretching vibration [36]. The bands between 3200 and 3700 cm−1 could be ascribed to the O–H stretching vibration of H2 O absorbed by the sample. Notably, there always exist one peak at 2362 cm−1 , which should be assigned to the trace of adsorbed or atmospheric CO2 in the air [37]. These results illustrate that pure inorganic Mn2 O3 nanofiber could be obtained at 700 °C. N2 adsorption/desorption isotherms of coaxial-cable structured Mn2 O3 nanofibers and Mn2 O3 particles are depicted in Fig. 5 to
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Fig. 5. (a) Nitrogen adsorption/desorption isotherm of coaxial-cable like Mn2 O3 nanofibers and Mn2 O3 particles.(b) Pore size distribution of coaxial-cable like Mn2 O3 nanofibers and Mn2 O3 particles.
Calcination COx NOx Calcination
Mn(Ac) 2/PVP composite nanofiber
Mn2O3/C composite nanofiber Mn2O3 particle
Mn2O3 nanofiber Fig. 6. The formation mechanism of coaxial-cable like Mn2 O3 nanofibers.
study the porosity and textural properties. The isotherm of coaxialcable like Mn2 O3 nanofibers is typical IV type curve with a hysteresis loop, which indicates the pore size distribution in the mesoporous region. The specific surface area of Mn2 O3 nanofiber and Mn2 O3 particles are 16 and 3 m2 /g, respectively. The pore size distribution of Mn2 O3 nanofibers shows a relative wide range of mesopores. The porous structure contained small mesopores (4 nm) and large mesopores (30 nm) in the holes of the porous nanotube walls. It can be conjectured that the smaller mesopores are mainly endowed by the surface holes of coaxial-cable like Mn2 O3 nanofibers, whereas the larger mesopores are ascribed to the cavity of coaxial-cable like Mn2 O3 nanofibers. However, the pore size distributions in Mn2 O3 particle are mainly micropores. One thus expects poor capacitive performances for the Mn2 O3 particles. Based on the above characterization discussion, the proposed formation mechanism of coaxial-cable like Mn2 O3 nanofiber is illustrated in Fig. 6. The slow heating rate and the constant temperature and fiber diameter are crucial factors to the coaxial-cable like structure formation. In short, during calcination process, Mn(Ac)2 is decomposed into Mn2 O3 particle in company with the carbonation of PVP. With the temperature increasing, gas will be released from the decomposition of PVP, such as carbon–oxygen compounds (COx ) and nitrogen-oxide compounds (NOx ), which would push the Mn2 O3 particles toward the fiber surface. If the diameter of Please
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nanofiber is larger than the move length of Mn2 O3 particle, some Mn2 O3 particles will leave in the center of Mn2 O3 fiber. As the calcination goes on, Mn2 O3 particles will connect with each other as a result of the Ostwald ripening, and finally formation of coaxialcable like structure. Unlike the often cumbersome coaxial electrospinning technique for fabrication of nanofibers with complex tubular structure, the present synthesis approach is novel, simple, and suitable for large scale production. The electrochemistry properties of the Mn2 O3 nanofibers were further evaluated. Fig. 7 shows the cyclic voltammogram (CV) curves of coaxial-cable like Mn2 O3 nanofibers and Mn2 O3 particles electrodes at different scan rates ranging from 5 to 100 mV/s within the potential window of 0–0.4 V(versus SCE) in 6 M KOH solution. It is obvious that all of the CV curves exhibit two significant redox peaks, suggesting that the specific capacitance primarily originate from the pseudocapacitive capacitance based on a redox mechanism, which is very different from that of electrical double layer capacitors that usually produce a CV curve close to an ideal rectangular shape. Moreover, for most MnOx electrodes in neutral electrolyte, the CV curves usually exhibit rectangular shapes and no clear redox peaks can be found [38,39]. However, herein, Mn2 O3 electrode exhibits a pair of redox peaks in the alkaline solution. The electrochemical reaction of Mn2 O3 in alkaline solution has not been discussed much in the literature according to our knowledge. Teressa Nathan et al. considered these redox peaks corresponding
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Fig. 7. (a) Cyclic voltammograms of coaxial-cable like Mn2 O3 nanofibers and (b) Mn2 O3 particles electrodes in 6 M KOH.
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Fig. 8. (a) Chronopotentiometry curves of coaxial-cable like Mn2 O3 nanofibers and (b) Mn2 O3 particles electrodes; (c) Dependence of specific capacitances on the different current density; (d) Cycling stability tested at 0.5 A/g.
to the OH− chemisorption/intercalation into Mn2 O3 structure according to the following equation [40]:
Mn[Mn]O3 + OH− ↔ Mn4+ [Mn3+ ]OH− O3 +e− With increasing the scan rate, the redox peaks become broader and gradually shift to the both ends of potential window, indicating fast redox reaction occurring at electroactive material/electrolyte interfaces. Apparently, at the same scan rate condition, the area integrated within the current-potential curves of coaxial-cable like Mn2 O3 nanofiber is larger than that of Mn2 O3 particles, indicating that much more capacitance can be generated. It should be attributed to higher specific surface area of coaxial-cable like Mn2 O3 nanofibers, which can adsorb more OH− ions on the electrode surface and/or intercalate and deintercalate OH− ions. Fig. 8a and b show the chronopotentiometry (CP) curves of coaxial-cable like Mn2 O3 nanofibers and Mn2 O3 particles electrodes at different current densities in the potential window of 0 –0.4 V. Clearly, the nonlinear charge–discharge curves further verify the pseudocapacitance of Mn2 O3 electrode materials. The discharge time of coaxial-cable like Mn2 O3 nanofibers is longer than that of Mn2 O3 particles, which means that coaxial-cable like Mn2 O3 nanofiber has a larger specific capacitance. The correspondPlease
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ing specific capacitance is calculated based on the galvanostatic charge/discharge method, and the correlation between the specific capacitance and the various current densities for different electrodes is presented in Fig. 8c. The corresponding specific capacitance is 216 and 70 F/g at a current density of 0.5 A/g for the coaxial-cable like Mn2 O3 nanofiber and Mn2 O3 particle, respectively. The capacitance of coaxial-cable like Mn2 O3 electrode can still retain 52% when the current density increases to 5 A/g (from 216 to 112 F/g). The specific capacitance decreases gradually with increasing current density, which can be attributed to the limited ion diffusion and electron transfer within the electrode. At low current density/scan rate, the ions from the electrolyte have enough time to diffuse into all the sites which lead to the higher capacitance. On the other hand, at high current density/scan rate, the ions from electrolyte confront the difficulty to access all the available site in the active electrode due to their partial rate of movement in the electrolyte. The specific capacitance of coaxial-cable like Mn2 O3 nanofiber is larger than that of Mn2 O3 particles. The results maybe attribute to the regular hollow structure with larger specific surface area and pore volume, which provides short and effective diffusion channels for the electrolyte ions (OH− ). Furthermore, there are more electrolytes in the inner void space of Mn2 O3 nanofibers, which guarantees a steady supply of electrolyte ions
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electrospin-
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J. Liang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–7 Table 1 Comparison of the specific capacitance of various MnOx compiles from the literature.
Mn2 O3 nanofibers Mn2 O3 nanospheres Mn2 O3 sub-micron powder Mn3 O4 nanoparticles Porous nanostructured MnO2 α -MnO2 nanorod Birnessite-type MnO2 Coaxial cable like Mn2 O3 nanofiber
Electrolyte
Specific capacitance
Reference
0.5 M Na2 SO4 6 M KOH 0.5 M Na2 SO4 6 M KOH 1 M Na2 SO4 0.1 M Na2 SO4 1.0 M Na2 SO4 6 M KOH
140.1 F/g (10 mV/s) 100 F/g (5 mV/s) 88 F/g(0.5 A/g) 94 F/g(0.5 mA/cm2 ) 168 F/g(1 mV/s) 166.2 F/g (0.2 A/g) 210 F/g(0.2 A/g) 216 F/g(0.5 A/g)
[44] [40] [45] [46] [47] [48] [49] This work
as well as high current density of reaction [41,42]. Unfortunately, Mn2 O3 particles, due to lower specific surface area, contains lower number of electroactive sites, and the irregular pores induce random scattering of OH− , which enhance the diffusion resistance and decrease the diffusion kinetics of these ions inside the sample. It should be stressed that most researches combine MnOx with carbon materials to improve their performance in the previous works [3,38,43]. However, the content of MnOx in the composite is low (less than 50%). Thus the whole specific capacitance is also contributed by carbon materials. On the contrary, the active materials present here are pure Mn2 O3 materials and not the Mn2 O3 – carbon composites. Table 1 shows the comparison of various MnOx in electrochemical capacitive performance. For a fair comparison, the listed electrode active materials are only MnOx materials and not the MnOx –carbon composites. It can be seen that the specific capacitance of coaxial-cable like Mn2 O3 nanofiber is much higher than that of Mn2 O3 nanosphere [40], Mn2 O3 nanofibers [44], Mn2 O3 powders [45] and other manganese oxides [46–49]. It is also expected that the specific capacitance of the coaxial-cable structured Mn2 O3 nanofibers could be further enhanced by optimizing the experimental conditions, such as calcination temperature, post treatment and so on. Long cycle-life is another important quality required for supercapacitor practical applications. To evaluate the cycle stability of the two different structural Mn2 O3 electrodes, the galvanostatic charge–discharge cycling is performed at a current density of 0.5 A/g. As we show in Fig. 8d, after 10 0 0 cycles, the capacity decay of coaxial-line Mn2 O3 electrode is only 7%, indicating the good cycle durability. This behavior can be attributed to the high specific surface area along with high pore volume present in the special structure, which is beneficial for electrolyte transporting during the charge–discharge process. However, the capacity retention of Mn2 O3 particle electrode is only 40% over 10 0 0 cycles, showing a poor cycle performance. These results demonstrate that such a coaxial-cable architecture is suitable for construction of supercapacitor electrodes with excellent electrochemical performance. 4. Conclusion In summary, coaxial-cable structured Mn2 O3 nanofiber has been successfully fabricated through a simple single nozzle electrospinning process. The XRD and FT-IR results confirm that coaxialcable like Mn2 O3 nanofibers can be prepared after calcining composite fiber at 700 °C. Furthermore, the present process is a very simple and cost effective way for synthesis of one dimensional coaxial-cable like metal oxide materials. As electrodes, coaxialcable like Mn2 O3 nanofibers showed a high specific capacitance of 216 F/g at a current density of 0.5 A/g and extraordinary cyclic stability. The excellent performance is attributed to their high specific surface area and special coaxial-cable like structure, which is beneficial for electrolyte transport. The coaxial-cable structured Mn2 O3 nanofibers are promising candidates for electrochemical energy storage and conversion. Please
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Acknowledgment This work is supported by the National Natural Science Foundation of China (No. 21406116), Jiangsu Province Innovation for Ph.D Candidate (CXLX13_028). This work also is supported by the Scientific Research Initial funding for the advanced talent of Jianghan University (08010 0 01, 06660 0 01), Basic Research Project of Wuhan City (2015011701011593) and 4th Yellow Crane Talent Project of Wuhan City and National High Technology Research and Development Program of China (863 Program: 2015AA033406).
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J. Liang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–7 [19] Liang JY, Chen SL, Xie MJ, Wang YZ, Guo XK, Guo XF, et al. Expeditious fabrication of flower-like hierarchical mesoporous carbon superstructures as supercapacitor electrode materials. J Mater Chem A 2014;2:16884–91. [20] Titirici M-M, White RJ, Brun N, Budarin VL, Su DS, del Monte F, et al. Sustainable carbon materials. Chem Soc Rev 2015;44:250–90. [21] Wang G, Zhang L, Zhang J. A review of electrode materials for electrochemical supercapacitors. Chem Soc Rev 2012;41:797–828. [22] Snook GA, Kao P, Best AS. Conducting-polymer-based supercapacitor devices and electrodes. J Power Sour 2011;196:1–12. [23] Liu R, Duay J, Lee SB. Heterogeneous nanostructured electrode materials for electrochemical energy storage. Chem Commun 2011;47:1384–404. [24] Indra A, Menezes PW, Zaharieva I, Baktash E, Pfrommer J, Schwarze M, et al. Active mixed-valent MnOx water oxidation catalysts through partial oxidation (Corrosion) of nanostructured MnO particles. Angewandte Chem Int Ed 2013;52:13206–10. [25] Guo J, Liu Q, Wang C, Zachariah MR. Interdispersed amorphous MnOx–carbon nanocomposites with superior electrochemical performance as lithium-storage material. Adv Funct Mater 2012;22:803–11. [26] Yang G, Yan W, Wang J, Yang H. Fabrication and formation mechanism of Mn2 O3 hollow nanofibers by single-spinneret electrospinning. CrystEngComm 2014;16:6907–13. [27] Wang Y, Wang Y, Jia D, Peng Z, Xia Y, Zheng G. All-nanowire based Li-ion full cells using homologous Mn2 O3 and LiMn2 O4 . Nano Lett 2014;14:1080–4. [28] Zhang Y, Yan Y, Wang X, Li G, Deng D, Jiang L, et al. Facile synthesis of porous Mn2 O3 nanoplates and their electrochemical behavior as anode materials for lithium ion batteries. Chem- Eur J 2014;20:6126–30. [29] Su H, Xu Y-F, Feng S-C, Wu Z-G, Sun X-P, Shen C-H, et al. Hierarchical Mn2 O3 hollow microspheres as anode material of lithium ion battery and its conversion reaction mechanism investigated by XANES. ACS Appl Mater Interfaces 2015;7:8488–94. [30] Sekhar BC, Kalaiselvi N. Pristine hollow microspheres of Mn2 O3 as a potential anode for lithium-ion batteries. CrystEngComm 2015;17:5038–45. [31] Lei S, Tang K, Fang Z, Liu Q, Zheng H. Preparation of α -Mn2 O3 and MnO from thermal decomposition of MnCO3 and control of morphology. Mater Lett 2006;60:53–6. [32] Tholkappiyan R, Naveen AN, Vishista K. Novel: Eggshell membrane template assisted synthesis of Mn2 O3 electrode material for supercapacitor applications. In: SOLID STATE PHYSICS: Proceedings of the 59th DAE Solid State Physics Symposium 2014. AIP Publishing; 2015. [33] Liang J, Yang J, Cao W, Guo X, Guo X, Ding W. Preparation of coaxial-line and hollow Mn2 O3 Nanofibers by single-nozzle electrospinning and their catalytic performances for thermal decomposition of ammonium perchlorate. J Nanosci Nanotechnol 2015;15:7229–34.
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fabrication
of
coaxial-cable
like
Mn2 O3
nanofiber
by
electrospin-
ning: Application as electrode material for supercapacitor, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.06.005
JID: JTICE
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Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2016) 1–7
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Short Communication
Facile fabrication of coaxial-cable like Mn2 O3 nanofiber by electrospinning: Application as electrode material for supercapacitor Jiyuan Liang a,∗, Ling-Tao Bu a,b, Wei-Guo Cao b,∗, Teng Chen b, Yuan-Cheng Cao a,∗ a b
Key Laboratory of Optoelectronic Chemical Materials and Devices, Ministry of Education, Jianghan University, Wuhan 430056, China Shandong Tianbao Chemical Corporation, Pingyi, Shandong 273300, China
a r t i c l e
i n f o
Article history: Received 29 January 2016 Revised 15 April 2016 Accepted 7 June 2016 Available online xxx Keywords: Electrospinning Mn2 O3 nanofiber Coaxial-cable Supercapacitor
a b s t r a c t Coaxial-cable like Mn2 O3 nanofibers are fabricated by a facile and cost-effective single-nozzle electrospinning technique and subsequent calcination. The morphology, microstructure, crystal structure, composition and specific surface area are characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and N2 adsorption–desorption. A possible formation mechanism of the coaxial-cable like Mn2 O3 nanofiber has also been proposed. Due to the large specific surface area and porous structure, the synthesized coaxial-cable like Mn2 O3 nanofiber is employed as the electrode for supercapacitor in 6 M KOH aqueous condition. The specific capacitance is up to 216 F/g at 0.5 A/g and the electrode also exhibited excellent cycling stability of 93% capacitance retention after 10 0 0 cycles. The encouraging results show the potential of the coaxial-cable like Mn2 O3 nanofiber as supercapacitor electrode material. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Nowadays, one-dimensional micro/nano structure materials, such as nanowires, nanotubes, nanorods and nanofibers have attracted great attention because of their distinctive geometries and special physical/chemical properties [1,2]. Many methods have been reported for fabricating these structure materials. For example, hydrothermal method [3], self-assembly method [4], template method [5] and electrospinning [6,7]. Among these methods, electrospinning technique is believed to be a convenient, cost-effective and versatile method for preparing nanofibers, such as polymeric [8,9], inorganic [10–12] and metallic nanofibers [13] with various structures and morphologies. It is a process that uses an electric field to control the formation of nanofibers [14]. So far, many solid nanofibers and hollow nanofibers have been fabricated by single nozzle electrospinning [1,15,16]. However, 1D nanomaterials with the complex tubular structures, such as tube-in-tube and wirein-tube, are usually prepared by coaxial-electrospinning technique [17,18]. Unfortunately, coaxial electrospinning technology should precisely control the solution viscosity, interfacial tension of core solution and shell solutions. Evidently, coaxial electrospinning is very cumbersome and thus not feasible for large scale production.
∗
Corresponding authors. Tel.: +8615195987523. E-mail addresses: [email protected] (J. Liang), [email protected] (W.-G. Cao), [email protected] (Y.-C. Cao).
Hence, the exploration of facile process for the fabrication of 1D nanostructure materials with complex structure (such as coaxialcable like) still remains great challenge. The ever increasing energy demands and the limited sources of fossil fuels have led to the development of energy storage devices. Supercapacitor as a new kind energy storage device, with high power density, fast charging/discharging and long lifespan, has attracted great research interests in recent years. According to the storage mechanism, supercapacitors can be classified into two kinds: electrical double-layer capacitor (EDLC) and pseudocapacitor. EDLC usually uses high surface area carbonaceous materials as the electrode materials [19,20]. However, a low specific capacitance of only around 10 0–20 0 F/g restricts its wide application. Transition metal oxides or conducting polymers are considered to be the most promising pseudocapacitor candidate materials [21,22]. Pseudocapacitor, which relies on faradaic process associated with surface or near-surface redox reaction, can provide higher specific capacitance than EDLC [23]. Among the various transition metal oxides, manganese oxides have been widely used in the field of catalysts, energy, and waste remove because of their low cost, availability in abundance, and environmental benignity [24,25]. The chemical valence of manganese includes 2+, 3+, 4+ and 6+. Amongst them, Mn2 O3 exhibits distinctive chemical and physical properties [26]. Recently, Mn2 O3 nanomaterials have attracted great attention as the electrode materials for Li-ion battery [27–30]. However, the Mn2 O3 nanomaterial as the electrode material for supercapacitor, especially in alkaline electrolyte, is received much less attention.
http://dx.doi.org/10.1016/j.jtice.2016.06.005 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: J. Liang et al., Facile fabrication of coaxial-cable like Mn2 O3 nanofiber by electrospinning: Application as electrode material for supercapacitor, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.06.005
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Furthermore, it is well known that the behavior of nanomaterials strongly depends on their structure, shape and size, which greatly influence their performances. Thus it is also necessary and important to study the capacitance performance of various structure Mn2 O3 . Pseudocapacitance is an interfacial phenomenon related to the morphology of the electroactive material, it is expected that the coaxial-cable like structure of a material enhances the electrochemical properties by reducing the diffusion path and by providing a large electroactive sites for ions. To date, Mn2 O3 materials with different structures have been prepared by different approaches including solvent-thermal method [29], thermal decomposition [31], template method [32] and electrospinning [33]. Just as we discussed above, electrospinning technique is a very facile and novel method to prepare 1D nanomaterials. Recently, Yang et al. prepared hollow Mn2 O3 nanofibers through single nozzle electrospinning [26]. In this paper, we prepared a complex coaxial-cable like Mn2 O3 nanofiber via single-nozzle electrospinning method. Compared with coaxial electrospinning technique, the preparation process is simple and without choosing any special electrospinning solution. Based on the various characterization datum, the possible formation mechanism is also proposed. The performance of coaxial-cable like Mn2 O3 nanofiber as the electrode materials for supercapacitor in alkaline aqueous condition is also studied. 2. Experimental 2.1. Materials Polyvinylpyrrolidone (PVP, Mw = 130 0,0 0 0) was purchased from Sigma Aldrich. Manganese acetate tetrahydrate (Mn(CH3 COO)2 4H2 O, Mn(Ac)2 4H2 O) and acetic acid (CH3 COOH, HAc) were purchased from Sinopharm Chemical Reagent Co., Ltd. All the above chemicals were analytical grade and used as received without further purification. 2.2. Synthesis of coaxial-cable structured Mn2 O3 nanofibers In a typical synthesis, 1.5 g PVP powder was added to 10 mL absolute ethanol solution and magnetic stirred for 1 h to generate a homogeneous solution. Subsequently, 0.005 moL Mn(CH3 COO)2 ·4H2 O was dissolved in the solution of 5 mL acetic acid and 2 mL distilled water, and then mixed with the PVP solution. The mixture solution was continuously stirred for another 1 h. Finally, the spinning precursor solutions were transferred to a hypodermic syringe. The positive terminal of a variable high-voltage power supply was connected to the needle tip of the syringe. A grounded iron drum, covered with an aluminum foil, served as the counter electrode. In electrospinning conditions, a flow rate was fixed at 0.2 mL/h. The distance between the needle tip and collector was 20 cm, and the voltage was set at 1 kV/cm. After electrospinning, the obtained nanofibers were dried initially for 6 h at 80 °C under vacuum and then calcined in the air atmosphere at a rate of 1 °C/min and maintained for 6 h at 700 °C to get Mn2 O3 nanofibers. For comparison, the pure Mn2 O3 particles were synthesized by direct annealing the Mn(Ac)2 powders at 700 °C for 5 h in the air. 2.3. Characterizations The samples were characterized by X-ray powder diffraction (XRD, hilips X’Pro X-ray diffractometer), field emission scanning electron microscopy (FESEM, Hitach S-4800), high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F), and Fourier transform infrared spectroscopy (FT-IR; Bruker Vertex 70). Thermogravimetric-differential scanning calorimetry (TG-DSC) Please
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analysis was performed on the thermogravimetric analyzer (TA Instruments Q50) at a heating rate of 2 °C/min under air atmosphere. The specific surface area and pore size distribution were also evaluated from nitrogen sorption isotherms measured by an analyzer (Tristar II, Micromeritics) at 77 K. Electrochemical tests were carried out on an electrochemical workstation (CHI 660D) in a three electrode system in 6 M KOH solution, with Pt wire serving as the counter electrode and Hg/HgO as the reference electrode. The active material was mixed with acetylene black and polyvinylidene fluoride with a mass ratio of 85:10:5. The mixture was drop-cast onto a graphite electrode of 1 cm × 1 cm area and dried in a vacuum oven at 80 °C overnight. The specific capacitance from galvanostatic charge–discharge curve was calculated according to the following equation:
Cm = C/m = (I × t)/(V × m) where I is the discharge current (A), m is the mass of active materials (g), t is the discharge time (s), and V is the range of potential window (V). 3. Results and discussion Fig. 1a shows the SEM image of Mn(Ac)2 /PVP nanofiber. It can be seen that the nanofibers are randomly distributed on the substrate and the diameter is around 250 nm. Fig. 1b exhibits the typical SEM image of Mn2 O3 nanofibers. It is to be noted that nanofiber structure can be still retained despite the Mn2 O3 nanofibers are broken. What’s more, the diameter of Mn2 O3 nanofiber is shrunk to 200 nm due to the decomposition of PVP after calcination at 700 °C. The inner structure can be clearly seen from the fracture surface. The zoom-in image (Fig. 1c) shows that the average diameter of Mn2 O3 nanofiber core and shell are 50 nm and 200 nm, respectively. The coaxial-cable architecture of Mn2 O3 is further demonstrated by TEM image. Obviously, the interior of Mn2 O3 nanofibers shows coaxial-cable like structure with a wire located in the center of hollow nanofiber. A probable formation mechanism is proposed and discussed in details in a latter section. Fig. 1d inset shows the HRTEM image of Mn2 O3 nanofibers shell. The lattice fringes can be clearly observed, indicating the good crystalline of the as-prepared samples. The measured inter-planar distance is 0.38 nm, corresponding to the (211) crystal facet. Hence, the coaxial-cable like Mn2 O3 nanofibers are successfully prepared by single nozzle electrospinning. TG-DSC techniques were used to study the thermal properties of Mn(Ac)2 /PVP. Fig. 2 shows the TG-DSC curves of Mn(Ac)2 /PVP composite fiber between room temperature and 800 °C. TG result indicates that there are three steps weight loss. The first 10% weight is lost below 220 °C, which results from the evaporation of water, ethanol and acetic acid. The second 40% weight loss between 220 °C and 370 °C, which is assigned to the decomposition of the Mn(Ac)2 and PVP. The third weight loss can be interpreted as the complete decomposition of the metal precursor and PVP polymer. It is worth noting that there is no more weight loss in the temperature range between 450 °C and 800 °C, indicating the complete decomposition of the metal precursor and PVP. The products further investigated their crystalline structure with XRD characterizations. Fig. 3 represents the XRD patterns of Mn(Ac)2 /PVP composite fibers, Mn2 O3 particles and coaxial cable structure Mn2 O3 nanofibers. As shown in Fig. 3a, the as-prepared composite fibers shows typical amorphous structure and without any peaks being detected. After being calcined at 700 °C, as shown in Fig. 3c, the crystallization of Mn2 O3 (JCPDS card no. 41-1442) phase is formed at 2θ = 23.33, 33.18, 35.93 degree. The XRD result also shows that Mn2 O3 particles can be obtained by calcining Mn(Ac)2 powder in air at 700 °C. Furthermore, according to the
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electrospin-
ning: Application as electrode material for supercapacitor, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.06.005
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Fig. 1. (a) SEM images of Mn(Ac)2 /PVP composite and (b) coaxial-cable like Mn2 O3 nanofiber; (c) Zoom in SEM image of the area indicated in Fig. 1b; (d) TEM image of coaxial-cable like Mn2 O3 nanofiber, inset is the high resolution TEM image.
Fig. 2. TG-DSC curves of Mn(Ac)2 /PVP composite fiber in the air.
Fig. 4. (a) FT-IR spectra of PVP powders, (b) as-prepared Mn(Ac)2 /PVP composite fibers, and (c) coaxial-cable like Mn2 O3 nanofiber.
Fig. 3. (a) XRD patterns of Mn2 O3 /PVP composite fibers; (b) Mn2 O3 particles; (c) coaxial-cable like Mn2 O3 nanofibers.
Scherrer equation D = Kλ/β cosθ [34], where D is the average particle size (nm), K is Scherrer constant (K = 0.89), λ is wavelength of X-ray radiation (λ = 0.15418 nm), θ (rad) is characteristic X-ray diffraction peak and β (rad) is the line width at half-maximum Please
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height, the particle size of coaxial cable like Mn2 O3 nanofibers and Mn2 O3 particle is 39 and 44 nm, respectively. Surface functional group distribution was determined by FTIR spectroscopy. Fig. 4 shows the FT-IR spectrums of Mn2 O3 , Mn(Ac)2 /PVP nanofiber and pure PVP powder. For PVP, the characteristic stretching band at 1663 cm−1 is attributed to C=O group. The band at 2993 cm−1 can be attributed to the CH2 stretching vibration of PVP [35]. After being calcined at 700 °C, it is noted that the bands corresponding to organic components are disappeared, indicating that the PVP is removed completely. At the same time, some new bands appeared, such as the bands at 682 cm−1 , 580 cm−1 and 522 cm−1 of Mn2 O3 nanofibers corresponding to the Mn–O stretching vibration [36]. The bands between 3200 and 3700 cm−1 could be ascribed to the O–H stretching vibration of H2 O absorbed by the sample. Notably, there always exist one peak at 2362 cm−1 , which should be assigned to the trace of adsorbed or atmospheric CO2 in the air [37]. These results illustrate that pure inorganic Mn2 O3 nanofiber could be obtained at 700 °C. N2 adsorption/desorption isotherms of coaxial-cable structured Mn2 O3 nanofibers and Mn2 O3 particles are depicted in Fig. 5 to
fabrication
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coaxial-cable
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Mn2 O3
nanofiber
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electrospin-
ning: Application as electrode material for supercapacitor, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.06.005
ARTICLE IN PRESS
JID: JTICE 4
[m5G;June 23, 2016;12:3]
J. Liang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–7
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Fig. 5. (a) Nitrogen adsorption/desorption isotherm of coaxial-cable like Mn2 O3 nanofibers and Mn2 O3 particles.(b) Pore size distribution of coaxial-cable like Mn2 O3 nanofibers and Mn2 O3 particles.
Calcination COx NOx Calcination
Mn(Ac) 2/PVP composite nanofiber
Mn2O3/C composite nanofiber Mn2O3 particle
Mn2O3 nanofiber Fig. 6. The formation mechanism of coaxial-cable like Mn2 O3 nanofibers.
study the porosity and textural properties. The isotherm of coaxialcable like Mn2 O3 nanofibers is typical IV type curve with a hysteresis loop, which indicates the pore size distribution in the mesoporous region. The specific surface area of Mn2 O3 nanofiber and Mn2 O3 particles are 16 and 3 m2 /g, respectively. The pore size distribution of Mn2 O3 nanofibers shows a relative wide range of mesopores. The porous structure contained small mesopores (4 nm) and large mesopores (30 nm) in the holes of the porous nanotube walls. It can be conjectured that the smaller mesopores are mainly endowed by the surface holes of coaxial-cable like Mn2 O3 nanofibers, whereas the larger mesopores are ascribed to the cavity of coaxial-cable like Mn2 O3 nanofibers. However, the pore size distributions in Mn2 O3 particle are mainly micropores. One thus expects poor capacitive performances for the Mn2 O3 particles. Based on the above characterization discussion, the proposed formation mechanism of coaxial-cable like Mn2 O3 nanofiber is illustrated in Fig. 6. The slow heating rate and the constant temperature and fiber diameter are crucial factors to the coaxial-cable like structure formation. In short, during calcination process, Mn(Ac)2 is decomposed into Mn2 O3 particle in company with the carbonation of PVP. With the temperature increasing, gas will be released from the decomposition of PVP, such as carbon–oxygen compounds (COx ) and nitrogen-oxide compounds (NOx ), which would push the Mn2 O3 particles toward the fiber surface. If the diameter of Please
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nanofiber is larger than the move length of Mn2 O3 particle, some Mn2 O3 particles will leave in the center of Mn2 O3 fiber. As the calcination goes on, Mn2 O3 particles will connect with each other as a result of the Ostwald ripening, and finally formation of coaxialcable like structure. Unlike the often cumbersome coaxial electrospinning technique for fabrication of nanofibers with complex tubular structure, the present synthesis approach is novel, simple, and suitable for large scale production. The electrochemistry properties of the Mn2 O3 nanofibers were further evaluated. Fig. 7 shows the cyclic voltammogram (CV) curves of coaxial-cable like Mn2 O3 nanofibers and Mn2 O3 particles electrodes at different scan rates ranging from 5 to 100 mV/s within the potential window of 0–0.4 V(versus SCE) in 6 M KOH solution. It is obvious that all of the CV curves exhibit two significant redox peaks, suggesting that the specific capacitance primarily originate from the pseudocapacitive capacitance based on a redox mechanism, which is very different from that of electrical double layer capacitors that usually produce a CV curve close to an ideal rectangular shape. Moreover, for most MnOx electrodes in neutral electrolyte, the CV curves usually exhibit rectangular shapes and no clear redox peaks can be found [38,39]. However, herein, Mn2 O3 electrode exhibits a pair of redox peaks in the alkaline solution. The electrochemical reaction of Mn2 O3 in alkaline solution has not been discussed much in the literature according to our knowledge. Teressa Nathan et al. considered these redox peaks corresponding
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J. Liang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–7
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b
Fig. 7. (a) Cyclic voltammograms of coaxial-cable like Mn2 O3 nanofibers and (b) Mn2 O3 particles electrodes in 6 M KOH.
a
b
c
d
Fig. 8. (a) Chronopotentiometry curves of coaxial-cable like Mn2 O3 nanofibers and (b) Mn2 O3 particles electrodes; (c) Dependence of specific capacitances on the different current density; (d) Cycling stability tested at 0.5 A/g.
to the OH− chemisorption/intercalation into Mn2 O3 structure according to the following equation [40]:
Mn[Mn]O3 + OH− ↔ Mn4+ [Mn3+ ]OH− O3 +e− With increasing the scan rate, the redox peaks become broader and gradually shift to the both ends of potential window, indicating fast redox reaction occurring at electroactive material/electrolyte interfaces. Apparently, at the same scan rate condition, the area integrated within the current-potential curves of coaxial-cable like Mn2 O3 nanofiber is larger than that of Mn2 O3 particles, indicating that much more capacitance can be generated. It should be attributed to higher specific surface area of coaxial-cable like Mn2 O3 nanofibers, which can adsorb more OH− ions on the electrode surface and/or intercalate and deintercalate OH− ions. Fig. 8a and b show the chronopotentiometry (CP) curves of coaxial-cable like Mn2 O3 nanofibers and Mn2 O3 particles electrodes at different current densities in the potential window of 0 –0.4 V. Clearly, the nonlinear charge–discharge curves further verify the pseudocapacitance of Mn2 O3 electrode materials. The discharge time of coaxial-cable like Mn2 O3 nanofibers is longer than that of Mn2 O3 particles, which means that coaxial-cable like Mn2 O3 nanofiber has a larger specific capacitance. The correspondPlease
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ing specific capacitance is calculated based on the galvanostatic charge/discharge method, and the correlation between the specific capacitance and the various current densities for different electrodes is presented in Fig. 8c. The corresponding specific capacitance is 216 and 70 F/g at a current density of 0.5 A/g for the coaxial-cable like Mn2 O3 nanofiber and Mn2 O3 particle, respectively. The capacitance of coaxial-cable like Mn2 O3 electrode can still retain 52% when the current density increases to 5 A/g (from 216 to 112 F/g). The specific capacitance decreases gradually with increasing current density, which can be attributed to the limited ion diffusion and electron transfer within the electrode. At low current density/scan rate, the ions from the electrolyte have enough time to diffuse into all the sites which lead to the higher capacitance. On the other hand, at high current density/scan rate, the ions from electrolyte confront the difficulty to access all the available site in the active electrode due to their partial rate of movement in the electrolyte. The specific capacitance of coaxial-cable like Mn2 O3 nanofiber is larger than that of Mn2 O3 particles. The results maybe attribute to the regular hollow structure with larger specific surface area and pore volume, which provides short and effective diffusion channels for the electrolyte ions (OH− ). Furthermore, there are more electrolytes in the inner void space of Mn2 O3 nanofibers, which guarantees a steady supply of electrolyte ions
fabrication
of
coaxial-cable
like
Mn2 O3
nanofiber
by
electrospin-
ning: Application as electrode material for supercapacitor, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.06.005
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J. Liang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–7 Table 1 Comparison of the specific capacitance of various MnOx compiles from the literature.
Mn2 O3 nanofibers Mn2 O3 nanospheres Mn2 O3 sub-micron powder Mn3 O4 nanoparticles Porous nanostructured MnO2 α -MnO2 nanorod Birnessite-type MnO2 Coaxial cable like Mn2 O3 nanofiber
Electrolyte
Specific capacitance
Reference
0.5 M Na2 SO4 6 M KOH 0.5 M Na2 SO4 6 M KOH 1 M Na2 SO4 0.1 M Na2 SO4 1.0 M Na2 SO4 6 M KOH
140.1 F/g (10 mV/s) 100 F/g (5 mV/s) 88 F/g(0.5 A/g) 94 F/g(0.5 mA/cm2 ) 168 F/g(1 mV/s) 166.2 F/g (0.2 A/g) 210 F/g(0.2 A/g) 216 F/g(0.5 A/g)
[44] [40] [45] [46] [47] [48] [49] This work
as well as high current density of reaction [41,42]. Unfortunately, Mn2 O3 particles, due to lower specific surface area, contains lower number of electroactive sites, and the irregular pores induce random scattering of OH− , which enhance the diffusion resistance and decrease the diffusion kinetics of these ions inside the sample. It should be stressed that most researches combine MnOx with carbon materials to improve their performance in the previous works [3,38,43]. However, the content of MnOx in the composite is low (less than 50%). Thus the whole specific capacitance is also contributed by carbon materials. On the contrary, the active materials present here are pure Mn2 O3 materials and not the Mn2 O3 – carbon composites. Table 1 shows the comparison of various MnOx in electrochemical capacitive performance. For a fair comparison, the listed electrode active materials are only MnOx materials and not the MnOx –carbon composites. It can be seen that the specific capacitance of coaxial-cable like Mn2 O3 nanofiber is much higher than that of Mn2 O3 nanosphere [40], Mn2 O3 nanofibers [44], Mn2 O3 powders [45] and other manganese oxides [46–49]. It is also expected that the specific capacitance of the coaxial-cable structured Mn2 O3 nanofibers could be further enhanced by optimizing the experimental conditions, such as calcination temperature, post treatment and so on. Long cycle-life is another important quality required for supercapacitor practical applications. To evaluate the cycle stability of the two different structural Mn2 O3 electrodes, the galvanostatic charge–discharge cycling is performed at a current density of 0.5 A/g. As we show in Fig. 8d, after 10 0 0 cycles, the capacity decay of coaxial-line Mn2 O3 electrode is only 7%, indicating the good cycle durability. This behavior can be attributed to the high specific surface area along with high pore volume present in the special structure, which is beneficial for electrolyte transporting during the charge–discharge process. However, the capacity retention of Mn2 O3 particle electrode is only 40% over 10 0 0 cycles, showing a poor cycle performance. These results demonstrate that such a coaxial-cable architecture is suitable for construction of supercapacitor electrodes with excellent electrochemical performance. 4. Conclusion In summary, coaxial-cable structured Mn2 O3 nanofiber has been successfully fabricated through a simple single nozzle electrospinning process. The XRD and FT-IR results confirm that coaxialcable like Mn2 O3 nanofibers can be prepared after calcining composite fiber at 700 °C. Furthermore, the present process is a very simple and cost effective way for synthesis of one dimensional coaxial-cable like metal oxide materials. As electrodes, coaxialcable like Mn2 O3 nanofibers showed a high specific capacitance of 216 F/g at a current density of 0.5 A/g and extraordinary cyclic stability. The excellent performance is attributed to their high specific surface area and special coaxial-cable like structure, which is beneficial for electrolyte transport. The coaxial-cable structured Mn2 O3 nanofibers are promising candidates for electrochemical energy storage and conversion. Please
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Acknowledgment This work is supported by the National Natural Science Foundation of China (No. 21406116), Jiangsu Province Innovation for Ph.D Candidate (CXLX13_028). This work also is supported by the Scientific Research Initial funding for the advanced talent of Jianghan University (08010 0 01, 06660 0 01), Basic Research Project of Wuhan City (2015011701011593) and 4th Yellow Crane Talent Project of Wuhan City and National High Technology Research and Development Program of China (863 Program: 2015AA033406).
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nanofiber
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electrospin-
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fabrication
of
coaxial-cable
like
Mn2 O3
nanofiber
by
electrospin-
ning: Application as electrode material for supercapacitor, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.06.005