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Fabrication and electrochemical properties of porous VN hollow nanofibers
Journal of Alloys and Compounds 651 (2015) 785e792
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Fabrication and electrochemical properties of porous VN hollow nanofibers Jingxin Zhao a, b, Bao Liu a, Shan Xu a, *, Juan Yang c, Yun Lu d a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China b College of Physics and Electronic Information Engineering, Qinghai University for Nationalities, Xining 811600, PR China c Laboratory of Clean Energy Chemistry and Materials, Lanzhou Institute of Chemical Physics, Chinese of Academy of Sciences, Lanzhou 730000, PR China d Graduate School & Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba-shi, Chiba 263-8522, Japan
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Article history: Received 23 January 2015 Received in revised form 8 June 2015 Accepted 15 June 2015 Available online 12 August 2015
Porous vanadium nitride (VN) hollow fibers are successfully synthesized using low-cost starting materials by electrospinning combined with subsequent annealing. In this approach, precursor composite fibers are initially synthesized in an electrospinning precursor solution and then calcined at high temperature levels and an appropriate heating rate to form V2O5 hollow fibers. In the subsequent annealing under NH3 atmosphere, V2O5 hollow fibers act as a template for VN hollow fibers. The as-made VN hollow fibers retain their 1D texture, and their side walls consist of numerous porous nanoparticles. The electrochemical performance of VN hollow nanofibers is investigated, and the specific capacitance is 115 F/g at a current density of 1 A/g in 2 M KOH electrolyte. Meanwhile VN hollow fibers display typical pseudocapacitance properties. The capacitance fade of VN hollow nanofibers during cycling is also studied. © 2015 Elsevier B.V. All rights reserved.
Keywords: Vanadium nitride hollow fibers Electrospinning Pseudocapacitors
1. Introduction Supercapacitors as a charge storage device have attracted considerable interest because of their high power density [1,2], excellent reversibility [3], and potential applications in numerous fields, including power back-up devices, portable electronic devices, emergency doors of aircrafts, and hybrid electrical vehicle systems [4e8]. Supercapacitors mainly consist of two types based on charge storage mechanism, namely, (i) conventional electrochemical electric double layer capacitors (EDLC), which utilize charge storage by the formation of a double layer at the electrode/ electrolyte interface, and (ii) pseudocapacitors, which utilize charge transfer by pseudocapacitance arising from reversible faradaic redox reactions between the electrode surface and the electrolyte. For EDLC, many electrode materials with high specific surface area, including activated carbon, mesoporous materials, have been extensively studied by researchers for a long time. Many disadvantages, such as low average specific capacitance value and instability in aqueous electrolyte media, have also been exposed
* Corresponding author. E-mail address: [email protected] (S. Xu). http://dx.doi.org/10.1016/j.jallcom.2015.06.111 0925-8388/© 2015 Elsevier B.V. All rights reserved.
[9,10]. Therefore, increasing research attention was focused on developing pseudocapacitors. Numerous materials can be used in pseudocapacitors, and the most common materials include conducting polymers (polyaniline, polypyrrole, and polythiophene) [11,12], transition metal oxides (RuO2, MnO2, and NiO) [13,14], and nanotextured carbons with surface functionalities [15,16]. Among transition metal oxides, hydrous RuO2 is currently a benchmark pseudocapacitive material, with capacitances reaching as high as 1000 F/g. However, the high cost of ruthenium limits its utilization in supercapacitors [4,8]. Therefore, the current research mainly aims to find an economical alternative to ruthenium oxide. Based on this consideration, transition metal nitrides (TiN, MoN2, and VN, among others) can be used because of their low cost, oxidative and corrosion resistance, and good electrical conductivity [17,18]. Among these materials, VN exhibits stability in aqueous electrolytes and can be synthesized with high surface areas [14]. Moreover, VN is a hard refractory material with metallic properties, with good metallic conductivity and heat stability [19]. Thus, researchers have focused on the development of VN material [4,8,13]. VN is currently considered as one of the most promising candidates for supercapacitors. VN nanomaterials can be fabricated through different methods, such as temperature-programmed ammonia reduction of V2O5 [8],
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two-step ammonolysis reaction of VCl4 in anhydrous chloroform [13], microwave plasma torch [19], chemical vapor deposition [20], and magnetron sputtering [21]. However, these methods often suffer from particle agglomeration, troublesome processing steps, low yields, high costs, and so on. Therefore, a simple method to prepare VN nanomaterials should be developed. Recently, electrospinning has been widely used as a convenient and facile method to prepare various solid fibers with diameters that range from tens of nanometers to several micrometers. This method is perhaps one of the simplest and most versatile processes for generating VN nanofibers. The most advantageous feature of this approach lies in its ability to generate mats or uniaxially aligned arrays of nanofibers with well-controlled compositions and morphology [22,23]. To our knowledge, electrospinning of hollow VN nanofibers has yet to be reported. In this paper, we report the preparation of porous VN hollow fibers by electrospinning, subsequent annealing, and nitride process. The influence of nitride temperature on structure and electrochemical performance was investigated in detail. Electrochemical characterization results show that these porous textured nitrides exhibit acceptable rate capabilities in 2 M KOH aqueous electrolytes. This observed performance of VN is linked to its crystalline structure and the composition of the surface layer. Furthermore, the capacitance fade of VN hollow nanofibers during cycling was also discussed. 2. Experimental 2.1. Materials Polyvinylpyrrolidone (PVP, K88-96, Mw ¼ 1 300 000) was purchased from Aladdin Reagent Co., Ltd. Oxalic acid dihydrate (C2H2O4$2H2O) (Sinopharm Chemical Reagent Co., Ltd) and ammonium metavanadate (NH4VO3) (Shenyang Xingshun Chemical Co., Ltd) were used as received. Ethanol (C2H5OH, 99.7%; Tianjin Rionlon BoHua Medical Chemistry Co., Ltd.) was utilized without any further modifications. Other reagents were commercially available and were of analytical reagent grade. Twice-distilled water with a resistance of approximately 18 MU cm1 was used throughout the experiment.
The morphology and microstructures of the samples were analyzed by field emission scanning electron microscopy (JSM-6701F) and transmission electron microscopy (TEM; JEM-1200EX). X-ray photoelectron spectroscopy (XPS) data were obtained using a PerkineElmer PHI-5702 multifunctional X-ray photoelectron spectroscope (Physical Electronics, USA) and an Al-Ka radiation of 1486.6 eV as excitation source. All XPS spectra were calibrated using Au 4f7/2 at 84.0 eV. The electrochemical measurements of each as-prepared electrode were carried out using an electrochemical working station (CHI660D; Shanghai, China) in a half-cell setup configuration at RT. Three-electrode cells were assembled to assess the electrochemical properties of VN. Working electrodes were prepared according to the method reported in literature [24]. An 80 wt.% VN powder was mixed with 7.5 wt.% acetylene black (>99.9%) and 7.5 wt.% conducting graphite in an agate mortar until a homogeneous black powder was obtained. To this mixture, 5 wt.% of poly(tetrafluoroethylene) was added along with several drops of ethanol. After the solvent was briefly allowed to evaporate, the resulting paste was pressed at 10 MPa to nickel gauze with nickel wire to form an electrical connection. The electrode assembly was dried at 80 C for 16 h in air. Each VN electrode contained about 5 mg of electroactive material and possessed a geometric surface area of about 1 cm2. A platinum gauze electrode and a saturated calomel electrode (SCE) served as counter and reference electrodes, respectively. The corresponding specific capacitance was calculated from the slope of each discharge curve according to the equation C ¼ (I Dt)/ (DV m), where C is the specific capacitance, I is the constant discharge current, Dt is the discharge time, DV is the voltage difference in discharge, and m is the mass of VN samples coated on each work electrode.
3. Results and discussion The XRD patterns of V2O5 samples and samples nitrided at
2.2. Preparation of VN hollow fibers C2H2O4$2H2O (2.72 g) and NH4VO3 (1.25 g) were added to a mixture of 10 mL of C2H5OH and 10 mL of water. The solution was stirred for clarification, and then 1.1 g of PVP was added into the above solution to increase viscidity. Subsequently, the mixture was magnetically stirred for several hours at room temperature to form a homogeneous precursor solution. In a typical electrospinning process, the as-obtained spinnable sol was loaded into a plastic syringe equipped with a 7# stainless steel needle. A high voltage of 15 kV was supplied by a direct-current power supply. The distance between the tip of the needle and collector was 15 cm. For the following thermolysis process, as-spun nanofibers were placed in a muffle furnace and calcined at 400 C for 15 min with a heating rate of 0.5 C min1 to remove PVP and obtain V2O5 hollow fibers. To obtain VN fibers, we heated V2O5 samples up to 400 C, 600 C, and 800 C for 1 h with a heating rate of 2.0 C min1 and cooled the furnace naturally under NH3 flow overnight. 2.3. Characterization X-ray diffraction (XRD) measurements were conducted on an Xray diffractometer using Cu-Ka radiation (XRD, Panalytical X' Pert Pro) from 10 to 90 to analyze the crystal structures of the samples.
Fig. 1. (a) XRD patterns of V2O5, VO1.27, VN-600, and VN-800, and the schematic of (b) V2O5 (orthorhombic) and (c) VN (cubic) crystal structures.
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Fig. 2. SEM images of VO1.27 (a) and VN prepared at different temperature levels: (b) 600 C and (c) 800 C. Insets show hollow fibrous structures.
different temperature levels are shown in Fig. 1(a). For precursor fibers annealed at 400 C, the XRD patterns show evident diffraction peaks at 2q values of 15.38 , 20.17, 21.67, 26.17, 31.11, 34.27, 41.02 , 47.31, 51.21, 55.57, 62.15 , and 72.35 , which correspond to (020), (001), (011), (110), (031), (130), (002), (060), (200), (201), (170), and (260) crystal planes of cubic V2O5 with spinel structure (JCPDS NO. 85-0601), respectively. The average V2O5 crystallite size as calculated from Scherrer formula is about 32.31 nm. For samples nitrided at 400 C, VO1.27 instead of VN (JCPDS NO. 15-0629) is formed. These results demonstrate that the V2O5 fibers are insufficient for nitriding at this relatively low temperature. After the temperature was increased to 600 C, five strong diffraction peaks are observed at 2q values of 37.88 , 43.95 , 64.05 , 76.88 , and 80.93 . These diffraction peaks can be ascribed to the crystal planes of (111), (200), (220), (311), and (222) of cubic VN (JCPDS NO. 781315). No diffraction peaks attributed to VOx can be detected, which indicates that the product is pure VN. Further increase in temperature to 800 C results in increased intensities of all diffraction peaks. Compared with the sample nitrided at 600 C, the diffraction
peaks exhibited certain shifts from 37.88 , 43.95 , 64.05 , 76.88 , and 80.93 e37.65 , 43.73 , 63.65 , 76.44 , and 80.49 , respectively. The average VN crystallite size as calculated from Scherrer formula based on the (200) peaks are about 14.58 and 29.18 nm for VN-600 and VN-800 (samples nitrided at 600 C and 800 C are denoted as VN-600 and VN-800, respectively), respectively. To determine the phase composition of V2O5 and VN, we performed Rietveld refinement to analyze the XRD data. The lattice parameter values of V2O5 were calculated by Rietveld refinement [see the Supporting information, SI, Fig. S1(a)] and found to be a ¼ 3.564 Å, b ¼ 11.519 Å, and c ¼ 4.373 Å. Rietveld refinement [Fig. S1(b)] yielded calculated VN cell parameters of a ¼ b ¼ c ¼ 4.1370 Å, which are consistent with previous report [25]. The crystal structure for nanostructured V2O5 and VN are derived from the Rietveld refined parameters and are given in Fig. 1(b) and (c). As shown in Fig. 1(b), the crystalline peaks of V2O5 are indexed according to the orthorhombic structure with the Pmn21 space group. In addition, VN synthesized at 800 C exhibited a cubic structure with the space group Fm3m [Fig. 1(c)].
Fig. 3. Low- (a) and high-magnification (b), and high-resolution (c) TEM images of VN fibers prepared at 600 C and its corresponding SAED (d).
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Fig. 4. Pore size distribution of (a) V2O5, (b) VO1.27, (c) VN-600, and (d) VN-800, as determined by BJH method.
The SEM images of precursor fibers and the resulting V2O5 samples are shown in Fig. S2 (Supporting information, SI). Fig. S2(a) shows that precursor fibers exhibit ultra-fine fiber morphology and fiber surfaces appear to be smooth because of amorphous nature [23,26]. The average length reaches tens of micrometers, with an average diameter of approximately 770 nm. Compared with precursor fibers, V2O5 still retains its fibrous morphology, but the average diameter decreases to about 640 nm, and the fiber surface becomes rougher [Fig. S2(b)]. The former finding may be due to the removal of PVP and the decomposition of inorganic salts, whereas the latter is attributed to the growth of V2O5 nanocrystals at high temperature. The high-magnification SEM images in the inset of
Fig. S2(b) show that the V2O5 fibers possess an evident opening, suggesting that V2O5 fibers may own a hollow structure. The microstructures of V2O5 fibers were further investigated through TEM. As shown in Fig. S2(c), the intense contrast between the dark edges and the light center further demonstrates the hollowness of the obtained V2O5 fibers. This observation is consistent with the SEM results. High-magnification TEM images [insert of Fig. S2(c)] clearly show that the wall of the hollow fiber is constructed by the agglomeration of irregular nanoparticles. Fine lattice fringes shown in HRTEM [Fig. S2(d)] indicate a good crystallite structure. The spacing of adjacent lattice planes is 0.24 nm, which is consistent with the interplanar spacing of the (130) plane of V2O5. The well-
Fig. 5. Diagram of the formation of VN hollow fibers.
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resolved fringe shown in selected area electron diffraction (SAED) patterns [inset of Fig. S2(d)] demonstrates the local single crystallinity of the V2O5 hollow fibers. The effect of nitriding temperature on the morphology of the sample was investigated by SEM. As shown in Fig. 2, all samples retained their 1D morphology after nitriding at 800 C, indicating that the samples possess high thermal stability. From Fig. 2, we can observe that the diameter of the sample decreases and the surface becomes rougher with the increase in nitriding temperature. The former observation can be attributed to the growth of nanocrystalline VN at high temperature, whereas the latter results from the increase in VN crystalline size with increasing nitriding temperature. In addition, the cross-section SEM images in the insert of Fig. 2 demonstrate that fibers nitrided at different temperature levels invariably exhibit openings, indicating the hollow structure of VO1.27 and VN. The TEM images of VN samples nitrided at 600 C are shown in Fig. 3. Fig. 3(a) shows that the intense contrast between the dark edges and the light center demonstrates that the obtained VN fibers are hollow and corresponds with the SEM results [Fig. 2(b)]. Furthermore, the wall of VN hollow fibers is composed of nanoparticles with porous structure, as shown in Fig. 3(b), which may be accessible to electrolytes and result in better capacitance performance. The HRTEM image recorded on the wall of hollow fibers is shown in Fig. 3(c), in which the well-defined lattice fringes with 0.21 nm spacing is consistent with the interplanar distance of (200) crystal planes of VN. The SAED pattern [Fig. 3(d)] shows evident diffraction rings combined with spots, thereby suggesting the polycrystalline nature of VN fibers. The pore size distribution of V2O5, VO1.27, VN-600, and VN-800 was determined by nitrogen sorption measurement (Fig. 4). The measured BrunauereEmmetteTeller (BET) surface area of V2O5 [Fig. 4(a)] was 11.0 m2/g, with an average pore size of 87 nm, which possesses a macroporous structure. Meanwhile, the surface areas of VO1.27, VN-600, and VN-800 were 17.4, 26.7, and 14.02 m2/g, with average pore sizes of 23, 44, and 9 nm, respectively. This pore structure change may be caused by the etching effect of NH3, which is beneficial to enhancing specific capacitance [27]. The possible formation mechanism of hollow V2O5 and VN hollow fibers could be explained as follows (Fig. 5). In the process of preparing the precursor solution, NH4VO3, C2H2O4$2H2O, and PVP were completely dissolved in H2O and C2H5OH to form a blue viscous sol. (NH4)2[(VO)2(C2O4)3] was formed due to the reaction that occurred between NH4VO3 and C2H2O4$2H2O. During electrospinning, (NH4)2[(VO)2(C2O4)3] nanocrystals began to grow accompanying the rapid evaporation of solvent. Meanwhile, PVP functioned as a template for the growth of attached (NH4)2[(VO)2(C2O4)3] nanoparticles, resulting in the formation of (NH4)2[(VO)2(C2O4)3]/PVP composite fibers. In addition, the evaporation of solvents would result in the formation of a gel layer on the surface of precursor fibers. This layer importantly functions in maintaining the fibrous texture during the subsequent heat treatment. During this process, precursor fibers acted as a template for V2O5 hollow fibers and could be consumed when the transformation from precursor fibers to V2O5. The transformation process would start at the surface of precursor fibers. Precursor fibers would continue to be consumed, and transformation would continue toward the interior of precursor fibers [28]. Two opposite effects can result in the transformation process and exert considerable influence on the final morphology of inorganic fibers. One such effect is the diffusion of gas decomposed from PVP, which leads to the movement of nanoparticles from the interior of the composite fibers to the exterior of composite fibers forming the hollow structure. The other effect is the shrinkage of composite fibers that results from the movement of nanoparticles toward the
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interior of composite fibers, leading to the formation of solid fibers. In this work, the rate of gas diffusion from PVP decomposition is faster than that of diffusion among nanoparticles. Thus, the inside pressure of precursor fibers would become increasingly larger in comparison with the pressure during PVP decomposition, inducing nanoparticles to move and assemble to the surface of precursor fibers and resulting in the formation of V2O5 hollow fibers. In the subsequent annealing under NH3 atmosphere, V2O5 hollow fibers act as templates for VN hollow fibers and could be consumed during the transformation from V2O5 to VN. In addition, V2O5 hollow fibers begin to react with NH3 to form VN hollow fibers when NH3 flows into V2O5 hollow fibers. The surface of VN fibers is seen to be composed of numerous nanoparticles, among which numerous pores appeared. Given that the cell parameters of V2O5 (a ¼ 3.564 Å, b ¼ 11.519 Å, and c ¼ 4.373 Å) exceed those of VN (a ¼ b ¼ c ¼ 4.137 Å), the transformation from V2O5 to VN would result in the shrinkage of fiber diameter, as well as the presence of numerous pores in the hollow fiber wall. The electrochemical performance of VN hollow fibers was evaluated by cyclic voltammetry (CV) and galvanostatic charge/ discharge (GCD) measurements in a three-electrode configuration. All tests were conducted in 2 M KOH electrolytes. The suitable voltage windows are defined as 1.2 V to 0.2 V [27]. GCD (Fig. 6) yield capacitances of 9, 115, and 70 F/g for supercapacitor samples obtained by annealing at 400 , 600 , and 800 C, respectively, were measured at a constant current density of 1 A/g. This result may be related to the electrochemical impedance of the VN material, and the specific reason could be explained in the next paragraph. The CV curves of the VN-600 electrode at different scan rates (2, 10, 50, 100 and 200 mV/s) between 1.2 and 0.2 V (vs. SCE) in 2 M KOH aqueous electrolyte [27] are shown in Fig. 7(a). The VN-600 electrode deviates from idealized double layer behavior with a pair of broad, superimposed, and reversible faradaic surface redox reactions, and behaves as a pseudocapacitor. Moreover, evident distortion in CV curves was not observed with increasing sweep rate, suggesting a highly reversible system. The excellent CV curves reveal an extremely rapid current response to voltage reversal at each end potential [28]. The charge/discharge measurements at different current densities were recorded to understand the rate capability of VN. The image in Fig. 7(b) shows the GCD curves of the VN-600 electrode at different current densities from 5 A/g to 50 A/ g. The shape of GCD curves shows characteristic pseudocapacitance, which agrees with the result derived from the CV curves. The value of specific capacitance for VN-600 electrode at different current densities (5, 7, 10, 20, 30, and 50 A/g) was calculated to be
Fig. 6. GCD curves of VO1.27, VN-600, and VN-800 electrodes at a current density of 1 A/g in 2 M KOH electrolyte.
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90, 87.5, 85, 55, 48, 40, and 35 F/g, respectively, and the reaction mechanism of VN in KOH electrolyte was VN þ OH / VN//OH. According to BET analysis results, the pore size of VN was bigger than that of V2O5 (Fig. 4). Larger pore size is beneficial for the insertion and adjection of K ions. In consequence, the capacitance of VN was larger than V2O5 (Fig. S5). Electrochemical impedance spectroscopy (EIS) measurements were performed on the VO1.27, VN-600, and VN-800 electrodes in 2 M KOH aqueous electrolyte, and the Nyquist plot is shown in
Fig. 8. CV curves of VO1.27, VN-600, and VN-800 electrodes at a current density of 1 A/g in 2 M KOH electrolyte.
Dischange capacity(F/g)
Fig. 7(c). Evidently, both impedance spectra were nearly similar, with one semicircle component at high frequency and then with a linear component at low frequency. From the point intersecting with the real axis in the high frequency range, the internal resistance (which is equal to Rb) of the electrode material includes the total resistances of the ionic resistance of the electrolyte, the intrinsic resistance of the active material, and the contact resistance at the active material/current collector interface. Given that the electrochemical process occurring on the exterior surface of electrodes can be sensed at high frequencies, the semicircle is suggested to represent faradaic charge-transfer resistance (Rct) at the interface between the current collector and VN as well as that within the VN material. Therefore, the semicircle may be due to both faradaic reactions. At lower frequencies, a straight sloping line represents the diffusive resistance (Warburg impedence) of the electrolyte in electrode pores and proton diffusion in the host material. As shown by the Nyquist plot, the VN-600 electrode exhibits smaller Rb (0.740 U) than the VN-800 electrode (0.940 U), whereas the VN-600 and VO1.27 electrodes possess similar small Rb values (0.79 U). In addition, we measured the conductivity of VO1.27, VN-600, VN-800, and V2O5, and the numerical values of conductivity were 0.17, 0.19, 0.069, and 0.00017 S/m, respectively. Considering the close relationship of conductivity and internal resistance (Rb), and on the basis of conductivity data, we determined that the VO1.27 and VN-600 exhibited similar conductivities,
Fig. 7. Electrochemical properties of VN hollow fibers calcined at 600 C under NH3 atmosphere: (a) CV curves at different scan rates, (b) GCD curves at different current densities, and (c) Nyquist plot.
120 110 100 90 80 70 60 50 40 30 20 10 0
0
200
400 600 Number of cycles
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1000
Fig. 9. Cycling curve of the VN-600 electrode at a current density of 5 A/g in 2 M KOH electrolyte. The inset shows the first five charge/discharge curves.
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which explain the similar Rb values. The conductivity of VN-800 was lower than that of VN-600; thus, VN-800 possessed a larger Rb than VN-600. We also tested the conductivity of V2O5 and found that the numerical value was lower than those of VO1.27, VN-600, and VN-800. Moreover, the semicircle associated with the surface properties of porous electrode corresponded to pseudo chargetransfer resistance (Rct). The Rct of VN-600 and VN-800 electrodes were considerably smaller than that of the VO1.27 electrode. These observations are strongly related to the increase in total capacitance. In addition, all Nyquist plots exhibited a Warburg angle exceeding 45 , thus indicating the suitability of VN-600 as electrode material in supercapacitors [29,30]. The equivalent circuit of VN-600 for impedance fit was studied in SI. The CV curves of the VN-600 electrode at scan rate of 100 mV/s at different temperature levels (400 C, 600 C, and 800 C) between 1.2 and 0.2 V (vs. SCE) in 2 M KOH aqueous electrolyte are shown in Fig. 8. All VN-600 electrodes deviate from ideal double-layer behavior through a pair of broad, superimposed, and reversible faradaic surface redox reactions. The electrode behaves as a pseudocapacitor. The specific capacity of VN-600 is the largest in the CV curves. Capacitance was calculated from CV curves acR cording to the formula, C ¼ ð Idf=2mvDVÞ, where I is the current density, DV is the potential window, m is the mass of the electrode material, and V is the scan velocity. In comparison with values in Ref. [27], the capacitance calculated from CV curves at a scan rate of 100 mV/s was larger at approximately 250 F/g. The cycle life of the VN-600 electrode at a current density of 5 A/ g in 2 M KOH electrolyte is depicted in Fig. 9. As shown in Fig. 8, the specific capacitance of VN-600 electrodes decreases gradually with the increase in cycle number. Approximately 54% of the original capacitance is retained after 1000 cycles. To understand the capacitance fade during cycling, we tested the surface characteristic of the sample before and after cycling measurement by using XPS. The XPS spectra of VN-600 before and after cycling are exhibited in Fig. 10. As shown in Fig. 10(a) and (b), a
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clear O 1s line and V 2p3 line confirms that a thin oxide layer exists on the surface of VN-600 and V in the samples. A slightly enhanced O 1s line indicates that a relatively thick oxide layer exists on the surface of VN-600 after cycling [Fig. 10(c)] compared with before cycling. The oxygen signal can be fitted with two peaks. The main component (530.4 eV) is assigned to oxygen in vanadium oxide, whereas the second peak centered at 532.1 eV is typical for a hydroxyl group (eOH) bonded to VN-600 [8]. Three peaks based on values in literature were required to fit the V 2p3 peak [8]. As shown in Fig. 10(d), the signal at 513.9 eV corresponds to V in VN. In addition, the two other peaks at 514.0 and 516.4 eV are ascribed to the V3þ and V5þ oxidation states of V in surface oxides. Therefore, we can conclude that the capacitance fade of active VN-600 originated from the oxidation of VN. Moreover, Figs. S3(a) and S3(b) show the N 1s XPS spectra of nanocrystalline VN before and after the cycling test. As shown in Fig. S3(a), the peak at 397.1 eV belongs to VN, and two peaks in the N 1s XPS spectra are observed at 400.1 and 397.1 eV in Fig. S3(b), respectively. Binding energies of 400.1 eV may be ascribed to the NeO bond obtained in the cycling test, and another peak at 397.1 eV closely approximates the value of 397.4 eV for VN [31]. XRD (Fig. S4, SI) was conducted to further investigate the capacitance fade of the sample during cycling and to verify the surface characteristic of the sample. In contrast to the peaks of VN600 prior to cycling, the peaks associated with planes of (111), (220), and (222) disappear, and diffraction peaks attributed to the crystal planes of (200) and (311) show a certain increase in intensity after cycling. In addition, two new diffraction peaks corresponding to the planes of (110) and (200) of V2O5 appeared, indicating the oxidation of VN to V2O5 during cycling. Owing to the relatively low content, the diffraction peaks corresponding to V2O3 does not appear in XRD patterns. Furthermore, CV and EIS measurements suggest that the existence of V2O5 results in increased resistance. On the basis of the above discussion, the capacitance fade of VN hollow nanofibers during cycling can be attributed to the presence of V2O5, which possesses higher resistance.
Fig. 10. XPS spectra and curve fitting of (a) O 1s, (b) V 2p3, and (c) O 1s, (d) V 2p3 spectra of VN-600 fibers before and after 500 cycles.
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4. Conclusion In summary, porous VN hollow fibers are successfully fabricated using a two-step annealing of electrospun precursor fibers. The crystal structure, morphology, and electrochemical properties of VN hollow fibers are investigated. The results indicate that the phase structure of hollow nanofibers belongs to cubic structure and that the wall of hollow fibers consists of numerous nanoparticles exhibiting porous structure. The maximum specific capacitance of 115 F/g is achieved at a current density of 1 A/g in 2 M KOH electrolyte. In addition, the capacitance fade of VN hollow nanofibers is related to the presence of V2O5 with higher resistance during cycling. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2015.06.111. References [1] [2] [3] [4] [5] [6] [7] [8]
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[9] O. Barbieri, M. Hahn, A. Herzog, R. Kotz, Carbon 43 (2005) 1303e1310. [10] G. Gryglewicz, J. Machnikowski, E.L. Grabowska, E. Frackowiak, J. Electrochem. Soc. 142 (1995) 2699e2703. guin, C.V. Guterl, J. Mater. Chem. 21 [11] C.M. Ghimbeu, E.R. Pinero, P. Fioux, F. Be (2011) 13268e13275. [12] G.A. Snook, P. Kao, A.S. Best, J. Power Sources 196 (2011) 1e12. [13] B.D. Choi, G.E. Blomgren, P.N. Kumta, Adv. Mater. 18 (2006) 1178e1182. [14] P. Pande, P.G. Rasmussen, L.T. Thompson, J. Power Sources 207 (2012) 212e215. guin, Adv. Funct. Mater. 18 (2006) 1877e1882. [15] E.R. Pinero, F. Leroux, F. Be [16] G. Lota, K. Lota, E. Frackowiak, Electrochem. Commun. 9 (2007) 1828e1832. [17] D. Choi, P.N. Kumpta, J. Electrochem. Soc. 153 (2006) 2298e2303. [18] T.C. Liu, W.G. Pell, B.E. Conway, S.L. Roberson, J. Electrochem. Soc. 145 (1998) 1882e1888. [19] Y.C. Hong, D.H. Shin, H.S. Uhm, Mater. Chem. Phys. 101 (2007) 35e40. [20] A. Newport, C.J. Carmalt, I.P. Parkin, S.A. O'Neill, Eur. J. Inorg. Chem. 21 (2004) 4286e4290. [21] T. Suszko, W. Gulbinski, A. Urbanowicz, W. Gulbinski, Mater. Lett. 65 (2011) 2146e2148. [22] J.X. Zhao, Y.L. Cheng, X.B. Yan, D.F. Sun, F.L. Zhu, Q.J. Xue, CrystEngComm 14 (2012) 5879e5885. [23] Y.L. Cheng, B.L. Zou, J.L. Yang, C.J. Wang, Y.J. Liu, X.Z. Fan, L. Zhu, Y. Wang, H.M. Ma, X.Q. Cao, CrystEngComm 13 (2011) 2268e2272. [24] J.W. Lang, L.B. Kong, M. Liu, Y.C. Luo, L. Kang, J. Electrochem. Soc. 157 (2010) 1341. [25] N.S. Gajbhiye, R.S. Ningthoujam, Mater. Res. Bull. 41 (2006) 1612e1621. [26] Z.Y. Zhang, X.H. Li, C.H. Wang, L.M. Wei, Y.C. Liu, C.L. Shao, J. Phys. Chem. C 113 (2009) 19397e19403. [27] X.H. Zhou, C.Q. Shang, L. Gu, S.M. Dong, X. Chen, P.X. Han, L.F. Li, J.H. Yao, Z.H. Liu, H.X. Xu, Y.W. Zhu, G.L. Cui, ACS Appl. Mater. Inter. 3 (2011) 3058e3063. [28] H. Pang, Q.Y. Lu, Y.C. Li, F. Gao, Chem. Commun. 48 (2009) 7542e7544. [29] J. Yan, Z.J. Fan, T. Wei, J. Cheng, B. Shao, K. Wang, L.P. Song, M.L. Zhang, J. Power Sources 194 (2009) 1202e1207. [30] M.W. Xu, D.D. Zhao, S.J. Bao, H.L. Li, J. Solid State Electrochem. 11 (2007) 1101e1107. [31] S.S. Zhang, K. Xu, T.R. Jow, Electrochim. Acta 49 (2004) 1057e1061.
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Fabrication and electrochemical properties of porous VN hollow nanofibers Jingxin Zhao a, b, Bao Liu a, Shan Xu a, *, Juan Yang c, Yun Lu d a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China b College of Physics and Electronic Information Engineering, Qinghai University for Nationalities, Xining 811600, PR China c Laboratory of Clean Energy Chemistry and Materials, Lanzhou Institute of Chemical Physics, Chinese of Academy of Sciences, Lanzhou 730000, PR China d Graduate School & Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba-shi, Chiba 263-8522, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 January 2015 Received in revised form 8 June 2015 Accepted 15 June 2015 Available online 12 August 2015
Porous vanadium nitride (VN) hollow fibers are successfully synthesized using low-cost starting materials by electrospinning combined with subsequent annealing. In this approach, precursor composite fibers are initially synthesized in an electrospinning precursor solution and then calcined at high temperature levels and an appropriate heating rate to form V2O5 hollow fibers. In the subsequent annealing under NH3 atmosphere, V2O5 hollow fibers act as a template for VN hollow fibers. The as-made VN hollow fibers retain their 1D texture, and their side walls consist of numerous porous nanoparticles. The electrochemical performance of VN hollow nanofibers is investigated, and the specific capacitance is 115 F/g at a current density of 1 A/g in 2 M KOH electrolyte. Meanwhile VN hollow fibers display typical pseudocapacitance properties. The capacitance fade of VN hollow nanofibers during cycling is also studied. © 2015 Elsevier B.V. All rights reserved.
Keywords: Vanadium nitride hollow fibers Electrospinning Pseudocapacitors
1. Introduction Supercapacitors as a charge storage device have attracted considerable interest because of their high power density [1,2], excellent reversibility [3], and potential applications in numerous fields, including power back-up devices, portable electronic devices, emergency doors of aircrafts, and hybrid electrical vehicle systems [4e8]. Supercapacitors mainly consist of two types based on charge storage mechanism, namely, (i) conventional electrochemical electric double layer capacitors (EDLC), which utilize charge storage by the formation of a double layer at the electrode/ electrolyte interface, and (ii) pseudocapacitors, which utilize charge transfer by pseudocapacitance arising from reversible faradaic redox reactions between the electrode surface and the electrolyte. For EDLC, many electrode materials with high specific surface area, including activated carbon, mesoporous materials, have been extensively studied by researchers for a long time. Many disadvantages, such as low average specific capacitance value and instability in aqueous electrolyte media, have also been exposed
* Corresponding author. E-mail address: [email protected] (S. Xu). http://dx.doi.org/10.1016/j.jallcom.2015.06.111 0925-8388/© 2015 Elsevier B.V. All rights reserved.
[9,10]. Therefore, increasing research attention was focused on developing pseudocapacitors. Numerous materials can be used in pseudocapacitors, and the most common materials include conducting polymers (polyaniline, polypyrrole, and polythiophene) [11,12], transition metal oxides (RuO2, MnO2, and NiO) [13,14], and nanotextured carbons with surface functionalities [15,16]. Among transition metal oxides, hydrous RuO2 is currently a benchmark pseudocapacitive material, with capacitances reaching as high as 1000 F/g. However, the high cost of ruthenium limits its utilization in supercapacitors [4,8]. Therefore, the current research mainly aims to find an economical alternative to ruthenium oxide. Based on this consideration, transition metal nitrides (TiN, MoN2, and VN, among others) can be used because of their low cost, oxidative and corrosion resistance, and good electrical conductivity [17,18]. Among these materials, VN exhibits stability in aqueous electrolytes and can be synthesized with high surface areas [14]. Moreover, VN is a hard refractory material with metallic properties, with good metallic conductivity and heat stability [19]. Thus, researchers have focused on the development of VN material [4,8,13]. VN is currently considered as one of the most promising candidates for supercapacitors. VN nanomaterials can be fabricated through different methods, such as temperature-programmed ammonia reduction of V2O5 [8],
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two-step ammonolysis reaction of VCl4 in anhydrous chloroform [13], microwave plasma torch [19], chemical vapor deposition [20], and magnetron sputtering [21]. However, these methods often suffer from particle agglomeration, troublesome processing steps, low yields, high costs, and so on. Therefore, a simple method to prepare VN nanomaterials should be developed. Recently, electrospinning has been widely used as a convenient and facile method to prepare various solid fibers with diameters that range from tens of nanometers to several micrometers. This method is perhaps one of the simplest and most versatile processes for generating VN nanofibers. The most advantageous feature of this approach lies in its ability to generate mats or uniaxially aligned arrays of nanofibers with well-controlled compositions and morphology [22,23]. To our knowledge, electrospinning of hollow VN nanofibers has yet to be reported. In this paper, we report the preparation of porous VN hollow fibers by electrospinning, subsequent annealing, and nitride process. The influence of nitride temperature on structure and electrochemical performance was investigated in detail. Electrochemical characterization results show that these porous textured nitrides exhibit acceptable rate capabilities in 2 M KOH aqueous electrolytes. This observed performance of VN is linked to its crystalline structure and the composition of the surface layer. Furthermore, the capacitance fade of VN hollow nanofibers during cycling was also discussed. 2. Experimental 2.1. Materials Polyvinylpyrrolidone (PVP, K88-96, Mw ¼ 1 300 000) was purchased from Aladdin Reagent Co., Ltd. Oxalic acid dihydrate (C2H2O4$2H2O) (Sinopharm Chemical Reagent Co., Ltd) and ammonium metavanadate (NH4VO3) (Shenyang Xingshun Chemical Co., Ltd) were used as received. Ethanol (C2H5OH, 99.7%; Tianjin Rionlon BoHua Medical Chemistry Co., Ltd.) was utilized without any further modifications. Other reagents were commercially available and were of analytical reagent grade. Twice-distilled water with a resistance of approximately 18 MU cm1 was used throughout the experiment.
The morphology and microstructures of the samples were analyzed by field emission scanning electron microscopy (JSM-6701F) and transmission electron microscopy (TEM; JEM-1200EX). X-ray photoelectron spectroscopy (XPS) data were obtained using a PerkineElmer PHI-5702 multifunctional X-ray photoelectron spectroscope (Physical Electronics, USA) and an Al-Ka radiation of 1486.6 eV as excitation source. All XPS spectra were calibrated using Au 4f7/2 at 84.0 eV. The electrochemical measurements of each as-prepared electrode were carried out using an electrochemical working station (CHI660D; Shanghai, China) in a half-cell setup configuration at RT. Three-electrode cells were assembled to assess the electrochemical properties of VN. Working electrodes were prepared according to the method reported in literature [24]. An 80 wt.% VN powder was mixed with 7.5 wt.% acetylene black (>99.9%) and 7.5 wt.% conducting graphite in an agate mortar until a homogeneous black powder was obtained. To this mixture, 5 wt.% of poly(tetrafluoroethylene) was added along with several drops of ethanol. After the solvent was briefly allowed to evaporate, the resulting paste was pressed at 10 MPa to nickel gauze with nickel wire to form an electrical connection. The electrode assembly was dried at 80 C for 16 h in air. Each VN electrode contained about 5 mg of electroactive material and possessed a geometric surface area of about 1 cm2. A platinum gauze electrode and a saturated calomel electrode (SCE) served as counter and reference electrodes, respectively. The corresponding specific capacitance was calculated from the slope of each discharge curve according to the equation C ¼ (I Dt)/ (DV m), where C is the specific capacitance, I is the constant discharge current, Dt is the discharge time, DV is the voltage difference in discharge, and m is the mass of VN samples coated on each work electrode.
3. Results and discussion The XRD patterns of V2O5 samples and samples nitrided at
2.2. Preparation of VN hollow fibers C2H2O4$2H2O (2.72 g) and NH4VO3 (1.25 g) were added to a mixture of 10 mL of C2H5OH and 10 mL of water. The solution was stirred for clarification, and then 1.1 g of PVP was added into the above solution to increase viscidity. Subsequently, the mixture was magnetically stirred for several hours at room temperature to form a homogeneous precursor solution. In a typical electrospinning process, the as-obtained spinnable sol was loaded into a plastic syringe equipped with a 7# stainless steel needle. A high voltage of 15 kV was supplied by a direct-current power supply. The distance between the tip of the needle and collector was 15 cm. For the following thermolysis process, as-spun nanofibers were placed in a muffle furnace and calcined at 400 C for 15 min with a heating rate of 0.5 C min1 to remove PVP and obtain V2O5 hollow fibers. To obtain VN fibers, we heated V2O5 samples up to 400 C, 600 C, and 800 C for 1 h with a heating rate of 2.0 C min1 and cooled the furnace naturally under NH3 flow overnight. 2.3. Characterization X-ray diffraction (XRD) measurements were conducted on an Xray diffractometer using Cu-Ka radiation (XRD, Panalytical X' Pert Pro) from 10 to 90 to analyze the crystal structures of the samples.
Fig. 1. (a) XRD patterns of V2O5, VO1.27, VN-600, and VN-800, and the schematic of (b) V2O5 (orthorhombic) and (c) VN (cubic) crystal structures.
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Fig. 2. SEM images of VO1.27 (a) and VN prepared at different temperature levels: (b) 600 C and (c) 800 C. Insets show hollow fibrous structures.
different temperature levels are shown in Fig. 1(a). For precursor fibers annealed at 400 C, the XRD patterns show evident diffraction peaks at 2q values of 15.38 , 20.17, 21.67, 26.17, 31.11, 34.27, 41.02 , 47.31, 51.21, 55.57, 62.15 , and 72.35 , which correspond to (020), (001), (011), (110), (031), (130), (002), (060), (200), (201), (170), and (260) crystal planes of cubic V2O5 with spinel structure (JCPDS NO. 85-0601), respectively. The average V2O5 crystallite size as calculated from Scherrer formula is about 32.31 nm. For samples nitrided at 400 C, VO1.27 instead of VN (JCPDS NO. 15-0629) is formed. These results demonstrate that the V2O5 fibers are insufficient for nitriding at this relatively low temperature. After the temperature was increased to 600 C, five strong diffraction peaks are observed at 2q values of 37.88 , 43.95 , 64.05 , 76.88 , and 80.93 . These diffraction peaks can be ascribed to the crystal planes of (111), (200), (220), (311), and (222) of cubic VN (JCPDS NO. 781315). No diffraction peaks attributed to VOx can be detected, which indicates that the product is pure VN. Further increase in temperature to 800 C results in increased intensities of all diffraction peaks. Compared with the sample nitrided at 600 C, the diffraction
peaks exhibited certain shifts from 37.88 , 43.95 , 64.05 , 76.88 , and 80.93 e37.65 , 43.73 , 63.65 , 76.44 , and 80.49 , respectively. The average VN crystallite size as calculated from Scherrer formula based on the (200) peaks are about 14.58 and 29.18 nm for VN-600 and VN-800 (samples nitrided at 600 C and 800 C are denoted as VN-600 and VN-800, respectively), respectively. To determine the phase composition of V2O5 and VN, we performed Rietveld refinement to analyze the XRD data. The lattice parameter values of V2O5 were calculated by Rietveld refinement [see the Supporting information, SI, Fig. S1(a)] and found to be a ¼ 3.564 Å, b ¼ 11.519 Å, and c ¼ 4.373 Å. Rietveld refinement [Fig. S1(b)] yielded calculated VN cell parameters of a ¼ b ¼ c ¼ 4.1370 Å, which are consistent with previous report [25]. The crystal structure for nanostructured V2O5 and VN are derived from the Rietveld refined parameters and are given in Fig. 1(b) and (c). As shown in Fig. 1(b), the crystalline peaks of V2O5 are indexed according to the orthorhombic structure with the Pmn21 space group. In addition, VN synthesized at 800 C exhibited a cubic structure with the space group Fm3m [Fig. 1(c)].
Fig. 3. Low- (a) and high-magnification (b), and high-resolution (c) TEM images of VN fibers prepared at 600 C and its corresponding SAED (d).
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Fig. 4. Pore size distribution of (a) V2O5, (b) VO1.27, (c) VN-600, and (d) VN-800, as determined by BJH method.
The SEM images of precursor fibers and the resulting V2O5 samples are shown in Fig. S2 (Supporting information, SI). Fig. S2(a) shows that precursor fibers exhibit ultra-fine fiber morphology and fiber surfaces appear to be smooth because of amorphous nature [23,26]. The average length reaches tens of micrometers, with an average diameter of approximately 770 nm. Compared with precursor fibers, V2O5 still retains its fibrous morphology, but the average diameter decreases to about 640 nm, and the fiber surface becomes rougher [Fig. S2(b)]. The former finding may be due to the removal of PVP and the decomposition of inorganic salts, whereas the latter is attributed to the growth of V2O5 nanocrystals at high temperature. The high-magnification SEM images in the inset of
Fig. S2(b) show that the V2O5 fibers possess an evident opening, suggesting that V2O5 fibers may own a hollow structure. The microstructures of V2O5 fibers were further investigated through TEM. As shown in Fig. S2(c), the intense contrast between the dark edges and the light center further demonstrates the hollowness of the obtained V2O5 fibers. This observation is consistent with the SEM results. High-magnification TEM images [insert of Fig. S2(c)] clearly show that the wall of the hollow fiber is constructed by the agglomeration of irregular nanoparticles. Fine lattice fringes shown in HRTEM [Fig. S2(d)] indicate a good crystallite structure. The spacing of adjacent lattice planes is 0.24 nm, which is consistent with the interplanar spacing of the (130) plane of V2O5. The well-
Fig. 5. Diagram of the formation of VN hollow fibers.
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resolved fringe shown in selected area electron diffraction (SAED) patterns [inset of Fig. S2(d)] demonstrates the local single crystallinity of the V2O5 hollow fibers. The effect of nitriding temperature on the morphology of the sample was investigated by SEM. As shown in Fig. 2, all samples retained their 1D morphology after nitriding at 800 C, indicating that the samples possess high thermal stability. From Fig. 2, we can observe that the diameter of the sample decreases and the surface becomes rougher with the increase in nitriding temperature. The former observation can be attributed to the growth of nanocrystalline VN at high temperature, whereas the latter results from the increase in VN crystalline size with increasing nitriding temperature. In addition, the cross-section SEM images in the insert of Fig. 2 demonstrate that fibers nitrided at different temperature levels invariably exhibit openings, indicating the hollow structure of VO1.27 and VN. The TEM images of VN samples nitrided at 600 C are shown in Fig. 3. Fig. 3(a) shows that the intense contrast between the dark edges and the light center demonstrates that the obtained VN fibers are hollow and corresponds with the SEM results [Fig. 2(b)]. Furthermore, the wall of VN hollow fibers is composed of nanoparticles with porous structure, as shown in Fig. 3(b), which may be accessible to electrolytes and result in better capacitance performance. The HRTEM image recorded on the wall of hollow fibers is shown in Fig. 3(c), in which the well-defined lattice fringes with 0.21 nm spacing is consistent with the interplanar distance of (200) crystal planes of VN. The SAED pattern [Fig. 3(d)] shows evident diffraction rings combined with spots, thereby suggesting the polycrystalline nature of VN fibers. The pore size distribution of V2O5, VO1.27, VN-600, and VN-800 was determined by nitrogen sorption measurement (Fig. 4). The measured BrunauereEmmetteTeller (BET) surface area of V2O5 [Fig. 4(a)] was 11.0 m2/g, with an average pore size of 87 nm, which possesses a macroporous structure. Meanwhile, the surface areas of VO1.27, VN-600, and VN-800 were 17.4, 26.7, and 14.02 m2/g, with average pore sizes of 23, 44, and 9 nm, respectively. This pore structure change may be caused by the etching effect of NH3, which is beneficial to enhancing specific capacitance [27]. The possible formation mechanism of hollow V2O5 and VN hollow fibers could be explained as follows (Fig. 5). In the process of preparing the precursor solution, NH4VO3, C2H2O4$2H2O, and PVP were completely dissolved in H2O and C2H5OH to form a blue viscous sol. (NH4)2[(VO)2(C2O4)3] was formed due to the reaction that occurred between NH4VO3 and C2H2O4$2H2O. During electrospinning, (NH4)2[(VO)2(C2O4)3] nanocrystals began to grow accompanying the rapid evaporation of solvent. Meanwhile, PVP functioned as a template for the growth of attached (NH4)2[(VO)2(C2O4)3] nanoparticles, resulting in the formation of (NH4)2[(VO)2(C2O4)3]/PVP composite fibers. In addition, the evaporation of solvents would result in the formation of a gel layer on the surface of precursor fibers. This layer importantly functions in maintaining the fibrous texture during the subsequent heat treatment. During this process, precursor fibers acted as a template for V2O5 hollow fibers and could be consumed when the transformation from precursor fibers to V2O5. The transformation process would start at the surface of precursor fibers. Precursor fibers would continue to be consumed, and transformation would continue toward the interior of precursor fibers [28]. Two opposite effects can result in the transformation process and exert considerable influence on the final morphology of inorganic fibers. One such effect is the diffusion of gas decomposed from PVP, which leads to the movement of nanoparticles from the interior of the composite fibers to the exterior of composite fibers forming the hollow structure. The other effect is the shrinkage of composite fibers that results from the movement of nanoparticles toward the
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interior of composite fibers, leading to the formation of solid fibers. In this work, the rate of gas diffusion from PVP decomposition is faster than that of diffusion among nanoparticles. Thus, the inside pressure of precursor fibers would become increasingly larger in comparison with the pressure during PVP decomposition, inducing nanoparticles to move and assemble to the surface of precursor fibers and resulting in the formation of V2O5 hollow fibers. In the subsequent annealing under NH3 atmosphere, V2O5 hollow fibers act as templates for VN hollow fibers and could be consumed during the transformation from V2O5 to VN. In addition, V2O5 hollow fibers begin to react with NH3 to form VN hollow fibers when NH3 flows into V2O5 hollow fibers. The surface of VN fibers is seen to be composed of numerous nanoparticles, among which numerous pores appeared. Given that the cell parameters of V2O5 (a ¼ 3.564 Å, b ¼ 11.519 Å, and c ¼ 4.373 Å) exceed those of VN (a ¼ b ¼ c ¼ 4.137 Å), the transformation from V2O5 to VN would result in the shrinkage of fiber diameter, as well as the presence of numerous pores in the hollow fiber wall. The electrochemical performance of VN hollow fibers was evaluated by cyclic voltammetry (CV) and galvanostatic charge/ discharge (GCD) measurements in a three-electrode configuration. All tests were conducted in 2 M KOH electrolytes. The suitable voltage windows are defined as 1.2 V to 0.2 V [27]. GCD (Fig. 6) yield capacitances of 9, 115, and 70 F/g for supercapacitor samples obtained by annealing at 400 , 600 , and 800 C, respectively, were measured at a constant current density of 1 A/g. This result may be related to the electrochemical impedance of the VN material, and the specific reason could be explained in the next paragraph. The CV curves of the VN-600 electrode at different scan rates (2, 10, 50, 100 and 200 mV/s) between 1.2 and 0.2 V (vs. SCE) in 2 M KOH aqueous electrolyte [27] are shown in Fig. 7(a). The VN-600 electrode deviates from idealized double layer behavior with a pair of broad, superimposed, and reversible faradaic surface redox reactions, and behaves as a pseudocapacitor. Moreover, evident distortion in CV curves was not observed with increasing sweep rate, suggesting a highly reversible system. The excellent CV curves reveal an extremely rapid current response to voltage reversal at each end potential [28]. The charge/discharge measurements at different current densities were recorded to understand the rate capability of VN. The image in Fig. 7(b) shows the GCD curves of the VN-600 electrode at different current densities from 5 A/g to 50 A/ g. The shape of GCD curves shows characteristic pseudocapacitance, which agrees with the result derived from the CV curves. The value of specific capacitance for VN-600 electrode at different current densities (5, 7, 10, 20, 30, and 50 A/g) was calculated to be
Fig. 6. GCD curves of VO1.27, VN-600, and VN-800 electrodes at a current density of 1 A/g in 2 M KOH electrolyte.
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90, 87.5, 85, 55, 48, 40, and 35 F/g, respectively, and the reaction mechanism of VN in KOH electrolyte was VN þ OH / VN//OH. According to BET analysis results, the pore size of VN was bigger than that of V2O5 (Fig. 4). Larger pore size is beneficial for the insertion and adjection of K ions. In consequence, the capacitance of VN was larger than V2O5 (Fig. S5). Electrochemical impedance spectroscopy (EIS) measurements were performed on the VO1.27, VN-600, and VN-800 electrodes in 2 M KOH aqueous electrolyte, and the Nyquist plot is shown in
Fig. 8. CV curves of VO1.27, VN-600, and VN-800 electrodes at a current density of 1 A/g in 2 M KOH electrolyte.
Dischange capacity(F/g)
Fig. 7(c). Evidently, both impedance spectra were nearly similar, with one semicircle component at high frequency and then with a linear component at low frequency. From the point intersecting with the real axis in the high frequency range, the internal resistance (which is equal to Rb) of the electrode material includes the total resistances of the ionic resistance of the electrolyte, the intrinsic resistance of the active material, and the contact resistance at the active material/current collector interface. Given that the electrochemical process occurring on the exterior surface of electrodes can be sensed at high frequencies, the semicircle is suggested to represent faradaic charge-transfer resistance (Rct) at the interface between the current collector and VN as well as that within the VN material. Therefore, the semicircle may be due to both faradaic reactions. At lower frequencies, a straight sloping line represents the diffusive resistance (Warburg impedence) of the electrolyte in electrode pores and proton diffusion in the host material. As shown by the Nyquist plot, the VN-600 electrode exhibits smaller Rb (0.740 U) than the VN-800 electrode (0.940 U), whereas the VN-600 and VO1.27 electrodes possess similar small Rb values (0.79 U). In addition, we measured the conductivity of VO1.27, VN-600, VN-800, and V2O5, and the numerical values of conductivity were 0.17, 0.19, 0.069, and 0.00017 S/m, respectively. Considering the close relationship of conductivity and internal resistance (Rb), and on the basis of conductivity data, we determined that the VO1.27 and VN-600 exhibited similar conductivities,
Fig. 7. Electrochemical properties of VN hollow fibers calcined at 600 C under NH3 atmosphere: (a) CV curves at different scan rates, (b) GCD curves at different current densities, and (c) Nyquist plot.
120 110 100 90 80 70 60 50 40 30 20 10 0
0
200
400 600 Number of cycles
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Fig. 9. Cycling curve of the VN-600 electrode at a current density of 5 A/g in 2 M KOH electrolyte. The inset shows the first five charge/discharge curves.
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which explain the similar Rb values. The conductivity of VN-800 was lower than that of VN-600; thus, VN-800 possessed a larger Rb than VN-600. We also tested the conductivity of V2O5 and found that the numerical value was lower than those of VO1.27, VN-600, and VN-800. Moreover, the semicircle associated with the surface properties of porous electrode corresponded to pseudo chargetransfer resistance (Rct). The Rct of VN-600 and VN-800 electrodes were considerably smaller than that of the VO1.27 electrode. These observations are strongly related to the increase in total capacitance. In addition, all Nyquist plots exhibited a Warburg angle exceeding 45 , thus indicating the suitability of VN-600 as electrode material in supercapacitors [29,30]. The equivalent circuit of VN-600 for impedance fit was studied in SI. The CV curves of the VN-600 electrode at scan rate of 100 mV/s at different temperature levels (400 C, 600 C, and 800 C) between 1.2 and 0.2 V (vs. SCE) in 2 M KOH aqueous electrolyte are shown in Fig. 8. All VN-600 electrodes deviate from ideal double-layer behavior through a pair of broad, superimposed, and reversible faradaic surface redox reactions. The electrode behaves as a pseudocapacitor. The specific capacity of VN-600 is the largest in the CV curves. Capacitance was calculated from CV curves acR cording to the formula, C ¼ ð Idf=2mvDVÞ, where I is the current density, DV is the potential window, m is the mass of the electrode material, and V is the scan velocity. In comparison with values in Ref. [27], the capacitance calculated from CV curves at a scan rate of 100 mV/s was larger at approximately 250 F/g. The cycle life of the VN-600 electrode at a current density of 5 A/ g in 2 M KOH electrolyte is depicted in Fig. 9. As shown in Fig. 8, the specific capacitance of VN-600 electrodes decreases gradually with the increase in cycle number. Approximately 54% of the original capacitance is retained after 1000 cycles. To understand the capacitance fade during cycling, we tested the surface characteristic of the sample before and after cycling measurement by using XPS. The XPS spectra of VN-600 before and after cycling are exhibited in Fig. 10. As shown in Fig. 10(a) and (b), a
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clear O 1s line and V 2p3 line confirms that a thin oxide layer exists on the surface of VN-600 and V in the samples. A slightly enhanced O 1s line indicates that a relatively thick oxide layer exists on the surface of VN-600 after cycling [Fig. 10(c)] compared with before cycling. The oxygen signal can be fitted with two peaks. The main component (530.4 eV) is assigned to oxygen in vanadium oxide, whereas the second peak centered at 532.1 eV is typical for a hydroxyl group (eOH) bonded to VN-600 [8]. Three peaks based on values in literature were required to fit the V 2p3 peak [8]. As shown in Fig. 10(d), the signal at 513.9 eV corresponds to V in VN. In addition, the two other peaks at 514.0 and 516.4 eV are ascribed to the V3þ and V5þ oxidation states of V in surface oxides. Therefore, we can conclude that the capacitance fade of active VN-600 originated from the oxidation of VN. Moreover, Figs. S3(a) and S3(b) show the N 1s XPS spectra of nanocrystalline VN before and after the cycling test. As shown in Fig. S3(a), the peak at 397.1 eV belongs to VN, and two peaks in the N 1s XPS spectra are observed at 400.1 and 397.1 eV in Fig. S3(b), respectively. Binding energies of 400.1 eV may be ascribed to the NeO bond obtained in the cycling test, and another peak at 397.1 eV closely approximates the value of 397.4 eV for VN [31]. XRD (Fig. S4, SI) was conducted to further investigate the capacitance fade of the sample during cycling and to verify the surface characteristic of the sample. In contrast to the peaks of VN600 prior to cycling, the peaks associated with planes of (111), (220), and (222) disappear, and diffraction peaks attributed to the crystal planes of (200) and (311) show a certain increase in intensity after cycling. In addition, two new diffraction peaks corresponding to the planes of (110) and (200) of V2O5 appeared, indicating the oxidation of VN to V2O5 during cycling. Owing to the relatively low content, the diffraction peaks corresponding to V2O3 does not appear in XRD patterns. Furthermore, CV and EIS measurements suggest that the existence of V2O5 results in increased resistance. On the basis of the above discussion, the capacitance fade of VN hollow nanofibers during cycling can be attributed to the presence of V2O5, which possesses higher resistance.
Fig. 10. XPS spectra and curve fitting of (a) O 1s, (b) V 2p3, and (c) O 1s, (d) V 2p3 spectra of VN-600 fibers before and after 500 cycles.
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4. Conclusion In summary, porous VN hollow fibers are successfully fabricated using a two-step annealing of electrospun precursor fibers. The crystal structure, morphology, and electrochemical properties of VN hollow fibers are investigated. The results indicate that the phase structure of hollow nanofibers belongs to cubic structure and that the wall of hollow fibers consists of numerous nanoparticles exhibiting porous structure. The maximum specific capacitance of 115 F/g is achieved at a current density of 1 A/g in 2 M KOH electrolyte. In addition, the capacitance fade of VN hollow nanofibers is related to the presence of V2O5 with higher resistance during cycling. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2015.06.111. References [1] [2] [3] [4] [5] [6] [7] [8]
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