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Preparation of NiO nanofibers by electrospinning and their application for electro-catalytic oxidation of ethylene glycol
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
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Preparation of NiO nanofibers by electrospinning and their application for electro-catalytic oxidation of ethylene glycol Sayed Reza Hosseini*, Shahram Ghasemi, Mina Kamali-Rousta, Seyed Reza Nabavi Nanochemistry Research Laboratory, Faculty of Chemistry, University of Mazandaran, 47416-95447, Babolsar, Iran
article info
abstract
Article history:
In this work, nickel oxide nanofibers (NiO-NFs) are produced by electrospinning method
Received 15 July 2016
and calcination in air for 5 h. The thermal gravimetric analysis (TGA), field-emission
Received in revised form
scanning electron microscopy (FE-SEM), fourier transform infrared (FT-IR), X-ray diffrac-
11 September 2016
tion (XRD), energy dispersive X-ray spectroscopy (EDS), and electrochemical impedance
Accepted 15 September 2016
spectroscopy (EIS) are used for characterization of the NiO-NFs. The FE-SEM images of the
Available online xxx
precursor indicate that a large quantity of nanofibers with diameters ranging from 100 to 150 nm with tens of micrometers in length can be acquired. Also, the results show that
Keywords:
rough NiO-NFs having large specific surface area are composed of the small nanoparticles.
Electrospinning
After calcination, the nanofibers are composed of cubic structure. The EIS study shows that
Nickel oxide nanofibers
the value of charge-transfer resistance of the NiO-NFs modified carbon paste electrode
Ethylene glycol
(NiO-NFs/CPE) is much smaller than that CPE, indicating a faster electron-transfer process.
Electro-catalysis
The NiO-NFs are used as potential catalysts for electro-catalytic oxidation of ethylene glycol (EG) in 0.2 M NaOH solution. The results demonstrate that the NiO-NFs/CPE reveals good electro-catalytic activity towards EG oxidation, showing a suitable stability and robustness. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays, nanostructured materials of mutable morphologies and aspect ratios especially one-dimensional (1D) nanostructures such as fibers, wires, rods, tubes, and belts gained increasing attraction because their electrical properties depend on directionality [1e5]. Lithographic [6] and wet chemical techniques [7,8] are suitable methods for the synthesis of 1D nanostructure on a laboratory scale. However, restricted lengths of the nanostructures concluded these
routes are limited to several microns. The versatile electrospinning method is fairly simple and cost-effective technique for creating 1D nanostructures and it is scalable to yield nanostructures at an industrial level. The electro-spun nanofibers have great porosity and surface-to-volume ratio [9,10]. A suitable post-electrospinning treatment produces continuous nanofibers of the matching materials, if the polymer solution comprises metal ions to form metal oxides [11,12]. Direct alcohol fuel cells (DAFCs), as alternative power sources; attract significant attentions owing to their portable
* Corresponding author. Fax: þ98 1135302350. E-mail address: [email protected] (S.R. Hosseini). http://dx.doi.org/10.1016/j.ijhydene.2016.09.116 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Hosseini SR, et al., Preparation of NiO nanofibers by electrospinning and their application for electrocatalytic oxidation of ethylene glycol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.116
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applications [13,14]. Among numerous types of alcohols, ethylene glycol (EG) is one of the most promising candidates as it can be produce straightly from biomass [15]. The EG is mostly prepared from heterogeneous hydrogenation of cellulose derivatives [16]. This diol has great water solubility, low flammability and little volatility [17]. Moreover, EG has larger theoretical energy density (7.56 kWh L1) and higher boiling point (198 C) than methanol [18]. Pt-based catalysts were widely applied in DAFCs [19e26]. Also, Pd-based catalysts are known as suitable alternatives to Pt-based catalysts due to their good electro-catalytic properties [27e36]. Although the noble metals exhibit high electro-catalytic activities towards electro-oxidation of EG, the electrode surfaces are simply fouled by chemisorbed intermediates. Furthermore, high cost and low abundance of the electrode materials may restrict their marketable applications. Discovering novel anode catalysts is extremely desire which indicates increasing benefits in the progress of noble metals-free catalysts in current years. Also, it should be noted that oxidation of the EG in alkaline media is much easier than acidic solutions [37,38]. Besides the precious metals, in particular Ni which is less costly and more plentiful on earth than Pt and Pd, has been noted to be extremely active in alkaline media. In contrast to Ni, which is unstable and simply oxidized in air and solution, its oxide (or hydroxide) is fairly stable [39e41]. Previously, NiO nanofibers were obtained by calcining the electro-spun composite fibers of poly (styreneco-acrylonitrile) and nickel (II) acetate tetra hydrate. The electro-spun NiO nanofiber webs were used for thermistor applications [42]. Till now, there have been some reports on the application of Ni oxide modified electrodes for EG electrooxidation. Although Ni oxide-based electrodes were used in the EG oxidation, their application is extremely limited to some conducting polymers which were made in presence of sodium dodecyl sulfate; such as poly (o-aminophenol) [43], poly (4-aminoacetanilide) [44], poly (N,N-dimethyl aniline) [45] and poly (m-toluidine)/Triton X-100 film [46]. Recently, we have introduced an electro-spun CuO nanoparticles/CPE and its application for hydrazine hydrate electro-oxidation [47]. The polyvinyl alcohol/copper acetate (PVA/Cu(OAc)2) nanofibers with diameter ranging from 200 to 300 nm were obtained and after calcination, the fibers didn't remain as continuous structures and altered to CuO nanoparticles. In present work, PVA/Ni(OAc)2 composite nanofibers were formulated by electrospinning process and the nickel oxides nanofibers (NiO-NFs) were achieved by following thermal treatment procedure. As far as we know, application of the electro-spun NiO-NFs for electro-catalysis of EG oxidation has been not reported. Encouraged by the great length-to-diameter ratio and high surface-to-volume ratio of 1D nanostructure which is satisfied by electrospinning technique, the electro-catalytic properties of the prepared NiO-NFs/CPE towards EG oxidation were tested in alkaline medium.
electrode (CPE) with NiO-NFs was performed according to our previous work [47] with some modification. Briefly, a combination of NiO-NFs, graphite powder and paraffin oil in a ratio of 20:56:24 (w/w %) was blended until a homogeneously wetted paste was achieved. The NiO-NFs/CPE was further electrochemically conditioned by potential cycling between 0.1 and 0.7 V in 0.2 M NaOH solution at y ¼ 50 mV s1 for 100 cycles. TGA (STA504, BAHR Co., Germany) was performed on a thermo-analyzer in temperature ranging from 25 C to 600 C with a heating rate of 10 C min1 in air. Electrospinning was carried out in a home-made setup (Fanavaran Nanomeghyas Co., Iran) with a motor-controlled rotating drum as a collector. The materials, solution preparation, electrospinning process, construction of the NiO-NFs/CPE and instrumental were described in Supplementary information.
Results and discussion Physical characterization Morphological features of the as-fabricated samples are analyzed through FE-SEM. Fig. 1 shows FE-SEM image of the electro-spun PVA/Ni(OAc)2 nanofibers, indicating a large quantity of nanofibers with diameters ranging from 100 to 150 nm. The electro-spun nanofibers have approximately uniform surfaces, thickness and bead-free structures. This figure evidently reveals the morphology of the NFs which are randomly orientate due to bending instability associated with a spinning jet. The PVA/Ni(OAc)2 nanofibers obtained by electrospinning should be calcined because large amounts of organic polymer and solvent are contained in these fibers. The corresponding thermal behavior of the PVA/Ni(OAc)2 fibers is shown in Fig. 2A. When the temperature exceeds at about 500 C, no more weight loss occurs, indicating whole removal
Experimental Ni(CH3COO)2$H2O (>99%, Riedel-deHaen) and EG (>99%, Merck) were used as received. Modification of carbon paste
Fig. 1 e FE-SEM image of the PVA/Ni(OAc)2 nanofibers.
Please cite this article in press as: Hosseini SR, et al., Preparation of NiO nanofibers by electrospinning and their application for electrocatalytic oxidation of ethylene glycol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.116
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
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Fig. 3 e (A) FE-SEM image and (B) EDS of the NiO-NFs. Fig. 2 e (A) TGA of the PVA/Ni(OAc)2 nanofibers. (B) FT-IR spectrum of the NiO-NFs.
of the organic species. After 600 C, there is no change in weight loss, indicating the creation of pure inorganic oxides. Therefore, calcination temperature at about 600 C is employed. The FT-IR spectrum of the NiO-NFs is taken in a region between 4000 and 400 cm1 by using KBr pellet technique (Fig. 2B). The absence of characteristics peaks of PVA [48] and presence of only NieO (metal-oxygen covalent bond) stretching vibration peak at about 455 cm1 reveal the complete elimination of polymeric template and formation of uncontaminated metal oxides. The broad peak at about 3427 cm1 is attributed to the symmetric stretching vibration of OH groups (water absorbed by the fibers sample or KBr). The peaks corresponding to the adsorbed gases such as dioxygen (1135 and 1460 cm1) and carbon dioxide (1658 cm1) are also observed in the spectrum. The surface of nanofibers turns rough during calcination and PVA template vanishes after thermal treatment. Fig. 3 shows the FE-SEM image of NiO-NFs. It is observed that the surface of nanofibers are been rough and they are composed
of closely packed nanoparticles as primary building blocks. After calcination treatment, the resultant NiO-NFs inherits the 1D shape from the precursor fibers and diameters of the NFs become non-uniform. However, the surface are much rougher which are derived from decomposition of the organic components such as PVA, acetate groups and the crystallization of the PVA/Ni(OAc)2 precursors. The EDS spectrum of NiO-NFs shows the peaks corresponding to 86.2 wt % Ni and 13.8 wt % O elements. The presence of Ni and O peaks and absence of C peak in the spectrum can be pointed to removal of the organic components and formation of pure NiO. Powder XRD pattern of the NiO-NFs is displayed in Fig. 4. The shrill peaks in the pattern reveals the crystalline characteristic of the NiO-NFs. The complete elimination of organic materials and the simultaneous conversion to NiO take place at calcination temperature. All the diffraction peaks can be assigned to cubic structural of NiO. The peaks at 2q positions about 37.2, 43.3, 62.7, 75.3 and 79.4 correspond to 111, 200, 220, 311 and 222 reflections of cubic NiO phase, respectively. Absence of any other peaks in the XRD pattern implies the NiO-NFs have good phase purity. Lack of broad
Please cite this article in press as: Hosseini SR, et al., Preparation of NiO nanofibers by electrospinning and their application for electrocatalytic oxidation of ethylene glycol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.116
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Fig. 4 e XRD pattern of the electrospun NiO-NFs.
peaks around at 2q ¼ 20 confirms that the semi-crystalline PVA template [49] thoroughly is decomposed. Moreover, average crystallite size of the NiO-NFs according to DebyeScherer's equation [50] is evaluated at about 60 nm from (200) reflection peak.
EIS study A typical impedance spectrum (Nyquist plot) involves a semicircle quota at higher frequencies (limited electrontransfer process) and a linear part at lower frequency (diffusion-limited process). Diameter of the semicircle equals the electron-transfer resistance which controls the electron transfer kinetics of the redox probe at the interface. Thus, the resistance describes the interface properties. Nyquist plots for the CPE (a) and NiO-NFs/CPE (b) acquired in the presence of 1.0 mM [Fe(CN)6]3/4 (1:1) þ 0.1 M KCl solution at open circuit condition are presented in Fig. 5. It can be observed from this figure that with introduction of the NiO-NFs into the CPE (plot b), semicircle part is almost eliminated due to significant decrease in the charge transfer resistance and its diagram is comprised of only a linear part. It means that the electron transfer resistance becomes unimportant in relation to Warburg impedance. The reason may be featured to nanometer sizes of the materials which bring in great surface areas and active sites.
Fig. 5 e EIS obtained at open circuit potential for CPE (a) and þ 0.1 M KCl NiO-NFs/CPE (b) in 1.0 mM Fe(CN)3¡/4¡ 6 solution.
an electro-catalytic activity (an increment in the anodic peak current followed a decrease in cathodic peak current (comparison of curves c and d)). This behavior is typical of that assumed for mediated oxidation of EG as previously was reported in literature [43e46]:
Electro-catalytic oxidation of EG As is well-known, EG does not suffer oxidation prior to discharge of supporting electrolyte at the CPE in NaOH solution in potential range between 0.1 and 0.7 V due to large overpotential (comparison of curves a and b). Therefore, CPE has no catalytic activity, hence its surface is modified with suitable nano-scale catalysts (i.e., NiO-NFs). The electro-catalytic activity of the NiO-NFs/CPE for EG oxidation is examined by cyclic voltammetry (CV) in 0.2 M NaOH solution containing 30 mM EG at y ¼ 20 mV s1 (Fig. 6). The NiO-NFs/CPE exhibits
Fig. 6 e Electrochemical response of the CPE in 0.2 M NaOH solution at y ¼ 20 mV s¡1 (a) in the absence and (b) presence of 30 mM EG. CVs of the NiO-NFs/CPE; (c) in the absence and (d) presence of EG at the same conditions.
Please cite this article in press as: Hosseini SR, et al., Preparation of NiO nanofibers by electrospinning and their application for electrocatalytic oxidation of ethylene glycol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.116
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It was generally agreed that nickel oxides in an alkaline media catalyze the EG oxidation through an overall eight electron process for creating oxalate anions as final product: C2 H6 O2 þ 10OH /C2 O4 2 þ 8H2 O þ 8e
(3)
It should be mentioned that reliable comparison between Ni-based catalysts reported in literature and our electro-spun NiO-NFs is difficult. That is due to different factors such as concentration of supporting electrolyte, potential scan rate, and EG concentration which affect activity of the electrocatalysts towards EG oxidation. However, we have tried to prepare a rough comparison between the our electrochemical data with some previous researches for Ni modified electrodes (Table 1). In comparison with previous works, it seems that NiO-NFs/CPE can acts as a comparable catalyst in the EG oxidation and shows a good performance. Among nano-sized ceramic architectures, nanofibers due to their high aspect ratios are more attractive over their nano-particulate counterparts. The technique used for preparation of the NiO-NFs is simple and environmental friendly. Furthermore, it has economic cost and is scalable at industrial levels. The catalyst layer (polymer þ nickel prepared in the previous works) was deposited as a thin film on the electrode surface (surface modification); meanwhile the NiO-NFs/CPE is centered based on bulk modification (a rapid method for generating a renewable and reproducible surface). Also, the materials chosen to be catalyst in previously published works, are formed from many materials which leads to many variables that are difficult to control and consequently to obtain reproducible results in electro-catalytic oxidation of EG in NaOH solution. However, the proposed electro-catalyst needs to further improvements in the catalytic activity and stability to realize the practical applications.
Effects of EG concentration and potential sweep rates Fig. 7A discloses the effect of EG concentration on the NiONFs/CPE in 0.2 M NaOH solution. As can be seen, EG oxidation can be catalyzed at the modified electrode surface. In addition, the anodic peak current density (jp) rises with increasing concentration up to 55 mM. While the EG concentration exceeds from this limit, no more significant current increasing occurs. Also, the CVs are recorded at different potential scan rates in 0.2 M NaOH solution containing 30 mM EG
Fig. 7 e (A) Electrochemical responses of the NiO-NFs/CPE in 0.2 M NaOH solution with different EG concentrations at y ¼ 20 mV s¡1. (a) 0, (b) 10, (c) 15, (d) 25, (e) 30, (f) 40, (g) 60, (h) 85 and (i) 115 mM. Inset of plots: jp as a function of EG concentration. (B) CVs of the NiO-NFs/CPE in 30 mM EG þ 0.2 M NaOH solution at different potential scan rates: (a) 5, (b) 10, (c) 20, (d) 30, (e) 50, (f) 80, (g) 100, (h) 200, (i) 400, (j) 600 and (k) 800 mV s¡1. Inset of plot: jp vs. y1/2.
Table 1 e Comparison of the electrochemical data of some modified CPEs in the electro-catalytic oxidation of EG in NaOH solution at y ¼ 20 mV s¡1. Modifier Ni/SDS-POAP Ni/SDS-PPAA Ni/SDS-PDMAN Ni/TX-100-PMT NiO-NFs
NaOH con./mol L1
EG con./mol L1
Ep/V
jp/mA cm2
Reference
0.1 0.1 1.0 0.1 0.2
0.28 0.25 0.20 0.06 0.03
0.80 0.70 0.85 0.67 0.63
11.1 5.9 83.1 10.5 10.1
[43] [44] [45] [46] This work
Please cite this article in press as: Hosseini SR, et al., Preparation of NiO nanofibers by electrospinning and their application for electrocatalytic oxidation of ethylene glycol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.116
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Fig. 8 e Iet transient of the NiO-NFs/CPE for 30 mM EG oxidation in 0.2 M NaOH at 0.63 V vs. AgjAgCljKCl (3 M). (B) CVs of the NiO-NFs/CPE in 0.2 M NaOH solution in the presence of 30 mM EG at y ¼ 20 mV s¡1; 1st (a) and 100th CV (b). (C) CVs of the NiO-NFs/CPE; 1st day (a) and 21st day (b). Other conditions are the same of part B.
(Fig. 7B). It can be seen that a linear relationship is obtained when jp values are plotted against to the square root of potential scan rates (y1/2). This behavior displays that the electrooxidation of EG is a diffusion controlled process.
(Fig. 8C). The electrode response retains 97% and 95% of its initial response, respectively.
Conclusion Stability of the NiO-NFs/CPE Practically, long-term stability of the electrode is very important. Chronoamperometry method is usually applied for study of activity and stability. A chronoamperogram with a large time window for the modified electrode is obtained at 0.63 V in presence of 0.03 M EG in 0.2 M NaOH solution (Fig. 8A). As can be seen, the decrease in current is relatively slow and, when the time is above 100 s, the current reaches a relatively stable value, which is still about 60% of the initial current. It is obvious that the NiO-NFs/CPE exhibits a high stability toward EG oxidation. The long-term stability of the NiO-NFs/CPE is confirmed by measuring its response for EG oxidation after 100 CVs in the presence of 0.03 M EG þ 0.2 M NaOH (Fig. 8B) and 3 weeks of storage in laboratory atmosphere condition
The PVA/Ni(OAc)2 nanofibers were successfully prepared without occurrence of beads and defects by electrospinning process and the NiO-NFs were obtained after calcination. Structures and morphology of the materials were characterized by TGA, FE-SEM, FT-IR, XRD and EDS analysis. The FESEM images showed that NiO-NFs were formed by calcination in air for 5 h. Moreover, after calcination at about 600 C, polymeric template of the NFs thoroughly was decomposed and their surface turned into harsh. The XRD pattern of the calcined sample showed that the precursor converted into pure NiO cubic structure. The result of EIS showed that the charge transfer resistance was decreased, when the carbon paste was spiked with NiO-NFs. The electro-catalytic performance of the low cost and renewable bulk modified electrode (i.e. electro-spun NiO-NFs/CPE) for EG oxidation was
Please cite this article in press as: Hosseini SR, et al., Preparation of NiO nanofibers by electrospinning and their application for electrocatalytic oxidation of ethylene glycol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.116
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e8
investigated in the electro-catalysis of EG oxidation in alkaline solution. The modified electrode exhibited good electrocatalytic activity towards EG oxidation, demonstrating a reasonable stability and durability.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.09.116.
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Please cite this article in press as: Hosseini SR, et al., Preparation of NiO nanofibers by electrospinning and their application for electrocatalytic oxidation of ethylene glycol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.116
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Preparation of NiO nanofibers by electrospinning and their application for electro-catalytic oxidation of ethylene glycol Sayed Reza Hosseini*, Shahram Ghasemi, Mina Kamali-Rousta, Seyed Reza Nabavi Nanochemistry Research Laboratory, Faculty of Chemistry, University of Mazandaran, 47416-95447, Babolsar, Iran
article info
abstract
Article history:
In this work, nickel oxide nanofibers (NiO-NFs) are produced by electrospinning method
Received 15 July 2016
and calcination in air for 5 h. The thermal gravimetric analysis (TGA), field-emission
Received in revised form
scanning electron microscopy (FE-SEM), fourier transform infrared (FT-IR), X-ray diffrac-
11 September 2016
tion (XRD), energy dispersive X-ray spectroscopy (EDS), and electrochemical impedance
Accepted 15 September 2016
spectroscopy (EIS) are used for characterization of the NiO-NFs. The FE-SEM images of the
Available online xxx
precursor indicate that a large quantity of nanofibers with diameters ranging from 100 to 150 nm with tens of micrometers in length can be acquired. Also, the results show that
Keywords:
rough NiO-NFs having large specific surface area are composed of the small nanoparticles.
Electrospinning
After calcination, the nanofibers are composed of cubic structure. The EIS study shows that
Nickel oxide nanofibers
the value of charge-transfer resistance of the NiO-NFs modified carbon paste electrode
Ethylene glycol
(NiO-NFs/CPE) is much smaller than that CPE, indicating a faster electron-transfer process.
Electro-catalysis
The NiO-NFs are used as potential catalysts for electro-catalytic oxidation of ethylene glycol (EG) in 0.2 M NaOH solution. The results demonstrate that the NiO-NFs/CPE reveals good electro-catalytic activity towards EG oxidation, showing a suitable stability and robustness. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays, nanostructured materials of mutable morphologies and aspect ratios especially one-dimensional (1D) nanostructures such as fibers, wires, rods, tubes, and belts gained increasing attraction because their electrical properties depend on directionality [1e5]. Lithographic [6] and wet chemical techniques [7,8] are suitable methods for the synthesis of 1D nanostructure on a laboratory scale. However, restricted lengths of the nanostructures concluded these
routes are limited to several microns. The versatile electrospinning method is fairly simple and cost-effective technique for creating 1D nanostructures and it is scalable to yield nanostructures at an industrial level. The electro-spun nanofibers have great porosity and surface-to-volume ratio [9,10]. A suitable post-electrospinning treatment produces continuous nanofibers of the matching materials, if the polymer solution comprises metal ions to form metal oxides [11,12]. Direct alcohol fuel cells (DAFCs), as alternative power sources; attract significant attentions owing to their portable
* Corresponding author. Fax: þ98 1135302350. E-mail address: [email protected] (S.R. Hosseini). http://dx.doi.org/10.1016/j.ijhydene.2016.09.116 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Hosseini SR, et al., Preparation of NiO nanofibers by electrospinning and their application for electrocatalytic oxidation of ethylene glycol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.116
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applications [13,14]. Among numerous types of alcohols, ethylene glycol (EG) is one of the most promising candidates as it can be produce straightly from biomass [15]. The EG is mostly prepared from heterogeneous hydrogenation of cellulose derivatives [16]. This diol has great water solubility, low flammability and little volatility [17]. Moreover, EG has larger theoretical energy density (7.56 kWh L1) and higher boiling point (198 C) than methanol [18]. Pt-based catalysts were widely applied in DAFCs [19e26]. Also, Pd-based catalysts are known as suitable alternatives to Pt-based catalysts due to their good electro-catalytic properties [27e36]. Although the noble metals exhibit high electro-catalytic activities towards electro-oxidation of EG, the electrode surfaces are simply fouled by chemisorbed intermediates. Furthermore, high cost and low abundance of the electrode materials may restrict their marketable applications. Discovering novel anode catalysts is extremely desire which indicates increasing benefits in the progress of noble metals-free catalysts in current years. Also, it should be noted that oxidation of the EG in alkaline media is much easier than acidic solutions [37,38]. Besides the precious metals, in particular Ni which is less costly and more plentiful on earth than Pt and Pd, has been noted to be extremely active in alkaline media. In contrast to Ni, which is unstable and simply oxidized in air and solution, its oxide (or hydroxide) is fairly stable [39e41]. Previously, NiO nanofibers were obtained by calcining the electro-spun composite fibers of poly (styreneco-acrylonitrile) and nickel (II) acetate tetra hydrate. The electro-spun NiO nanofiber webs were used for thermistor applications [42]. Till now, there have been some reports on the application of Ni oxide modified electrodes for EG electrooxidation. Although Ni oxide-based electrodes were used in the EG oxidation, their application is extremely limited to some conducting polymers which were made in presence of sodium dodecyl sulfate; such as poly (o-aminophenol) [43], poly (4-aminoacetanilide) [44], poly (N,N-dimethyl aniline) [45] and poly (m-toluidine)/Triton X-100 film [46]. Recently, we have introduced an electro-spun CuO nanoparticles/CPE and its application for hydrazine hydrate electro-oxidation [47]. The polyvinyl alcohol/copper acetate (PVA/Cu(OAc)2) nanofibers with diameter ranging from 200 to 300 nm were obtained and after calcination, the fibers didn't remain as continuous structures and altered to CuO nanoparticles. In present work, PVA/Ni(OAc)2 composite nanofibers were formulated by electrospinning process and the nickel oxides nanofibers (NiO-NFs) were achieved by following thermal treatment procedure. As far as we know, application of the electro-spun NiO-NFs for electro-catalysis of EG oxidation has been not reported. Encouraged by the great length-to-diameter ratio and high surface-to-volume ratio of 1D nanostructure which is satisfied by electrospinning technique, the electro-catalytic properties of the prepared NiO-NFs/CPE towards EG oxidation were tested in alkaline medium.
electrode (CPE) with NiO-NFs was performed according to our previous work [47] with some modification. Briefly, a combination of NiO-NFs, graphite powder and paraffin oil in a ratio of 20:56:24 (w/w %) was blended until a homogeneously wetted paste was achieved. The NiO-NFs/CPE was further electrochemically conditioned by potential cycling between 0.1 and 0.7 V in 0.2 M NaOH solution at y ¼ 50 mV s1 for 100 cycles. TGA (STA504, BAHR Co., Germany) was performed on a thermo-analyzer in temperature ranging from 25 C to 600 C with a heating rate of 10 C min1 in air. Electrospinning was carried out in a home-made setup (Fanavaran Nanomeghyas Co., Iran) with a motor-controlled rotating drum as a collector. The materials, solution preparation, electrospinning process, construction of the NiO-NFs/CPE and instrumental were described in Supplementary information.
Results and discussion Physical characterization Morphological features of the as-fabricated samples are analyzed through FE-SEM. Fig. 1 shows FE-SEM image of the electro-spun PVA/Ni(OAc)2 nanofibers, indicating a large quantity of nanofibers with diameters ranging from 100 to 150 nm. The electro-spun nanofibers have approximately uniform surfaces, thickness and bead-free structures. This figure evidently reveals the morphology of the NFs which are randomly orientate due to bending instability associated with a spinning jet. The PVA/Ni(OAc)2 nanofibers obtained by electrospinning should be calcined because large amounts of organic polymer and solvent are contained in these fibers. The corresponding thermal behavior of the PVA/Ni(OAc)2 fibers is shown in Fig. 2A. When the temperature exceeds at about 500 C, no more weight loss occurs, indicating whole removal
Experimental Ni(CH3COO)2$H2O (>99%, Riedel-deHaen) and EG (>99%, Merck) were used as received. Modification of carbon paste
Fig. 1 e FE-SEM image of the PVA/Ni(OAc)2 nanofibers.
Please cite this article in press as: Hosseini SR, et al., Preparation of NiO nanofibers by electrospinning and their application for electrocatalytic oxidation of ethylene glycol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.116
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Fig. 3 e (A) FE-SEM image and (B) EDS of the NiO-NFs. Fig. 2 e (A) TGA of the PVA/Ni(OAc)2 nanofibers. (B) FT-IR spectrum of the NiO-NFs.
of the organic species. After 600 C, there is no change in weight loss, indicating the creation of pure inorganic oxides. Therefore, calcination temperature at about 600 C is employed. The FT-IR spectrum of the NiO-NFs is taken in a region between 4000 and 400 cm1 by using KBr pellet technique (Fig. 2B). The absence of characteristics peaks of PVA [48] and presence of only NieO (metal-oxygen covalent bond) stretching vibration peak at about 455 cm1 reveal the complete elimination of polymeric template and formation of uncontaminated metal oxides. The broad peak at about 3427 cm1 is attributed to the symmetric stretching vibration of OH groups (water absorbed by the fibers sample or KBr). The peaks corresponding to the adsorbed gases such as dioxygen (1135 and 1460 cm1) and carbon dioxide (1658 cm1) are also observed in the spectrum. The surface of nanofibers turns rough during calcination and PVA template vanishes after thermal treatment. Fig. 3 shows the FE-SEM image of NiO-NFs. It is observed that the surface of nanofibers are been rough and they are composed
of closely packed nanoparticles as primary building blocks. After calcination treatment, the resultant NiO-NFs inherits the 1D shape from the precursor fibers and diameters of the NFs become non-uniform. However, the surface are much rougher which are derived from decomposition of the organic components such as PVA, acetate groups and the crystallization of the PVA/Ni(OAc)2 precursors. The EDS spectrum of NiO-NFs shows the peaks corresponding to 86.2 wt % Ni and 13.8 wt % O elements. The presence of Ni and O peaks and absence of C peak in the spectrum can be pointed to removal of the organic components and formation of pure NiO. Powder XRD pattern of the NiO-NFs is displayed in Fig. 4. The shrill peaks in the pattern reveals the crystalline characteristic of the NiO-NFs. The complete elimination of organic materials and the simultaneous conversion to NiO take place at calcination temperature. All the diffraction peaks can be assigned to cubic structural of NiO. The peaks at 2q positions about 37.2, 43.3, 62.7, 75.3 and 79.4 correspond to 111, 200, 220, 311 and 222 reflections of cubic NiO phase, respectively. Absence of any other peaks in the XRD pattern implies the NiO-NFs have good phase purity. Lack of broad
Please cite this article in press as: Hosseini SR, et al., Preparation of NiO nanofibers by electrospinning and their application for electrocatalytic oxidation of ethylene glycol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.116
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Fig. 4 e XRD pattern of the electrospun NiO-NFs.
peaks around at 2q ¼ 20 confirms that the semi-crystalline PVA template [49] thoroughly is decomposed. Moreover, average crystallite size of the NiO-NFs according to DebyeScherer's equation [50] is evaluated at about 60 nm from (200) reflection peak.
EIS study A typical impedance spectrum (Nyquist plot) involves a semicircle quota at higher frequencies (limited electrontransfer process) and a linear part at lower frequency (diffusion-limited process). Diameter of the semicircle equals the electron-transfer resistance which controls the electron transfer kinetics of the redox probe at the interface. Thus, the resistance describes the interface properties. Nyquist plots for the CPE (a) and NiO-NFs/CPE (b) acquired in the presence of 1.0 mM [Fe(CN)6]3/4 (1:1) þ 0.1 M KCl solution at open circuit condition are presented in Fig. 5. It can be observed from this figure that with introduction of the NiO-NFs into the CPE (plot b), semicircle part is almost eliminated due to significant decrease in the charge transfer resistance and its diagram is comprised of only a linear part. It means that the electron transfer resistance becomes unimportant in relation to Warburg impedance. The reason may be featured to nanometer sizes of the materials which bring in great surface areas and active sites.
Fig. 5 e EIS obtained at open circuit potential for CPE (a) and þ 0.1 M KCl NiO-NFs/CPE (b) in 1.0 mM Fe(CN)3¡/4¡ 6 solution.
an electro-catalytic activity (an increment in the anodic peak current followed a decrease in cathodic peak current (comparison of curves c and d)). This behavior is typical of that assumed for mediated oxidation of EG as previously was reported in literature [43e46]:
Electro-catalytic oxidation of EG As is well-known, EG does not suffer oxidation prior to discharge of supporting electrolyte at the CPE in NaOH solution in potential range between 0.1 and 0.7 V due to large overpotential (comparison of curves a and b). Therefore, CPE has no catalytic activity, hence its surface is modified with suitable nano-scale catalysts (i.e., NiO-NFs). The electro-catalytic activity of the NiO-NFs/CPE for EG oxidation is examined by cyclic voltammetry (CV) in 0.2 M NaOH solution containing 30 mM EG at y ¼ 20 mV s1 (Fig. 6). The NiO-NFs/CPE exhibits
Fig. 6 e Electrochemical response of the CPE in 0.2 M NaOH solution at y ¼ 20 mV s¡1 (a) in the absence and (b) presence of 30 mM EG. CVs of the NiO-NFs/CPE; (c) in the absence and (d) presence of EG at the same conditions.
Please cite this article in press as: Hosseini SR, et al., Preparation of NiO nanofibers by electrospinning and their application for electrocatalytic oxidation of ethylene glycol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.116
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It was generally agreed that nickel oxides in an alkaline media catalyze the EG oxidation through an overall eight electron process for creating oxalate anions as final product: C2 H6 O2 þ 10OH /C2 O4 2 þ 8H2 O þ 8e
(3)
It should be mentioned that reliable comparison between Ni-based catalysts reported in literature and our electro-spun NiO-NFs is difficult. That is due to different factors such as concentration of supporting electrolyte, potential scan rate, and EG concentration which affect activity of the electrocatalysts towards EG oxidation. However, we have tried to prepare a rough comparison between the our electrochemical data with some previous researches for Ni modified electrodes (Table 1). In comparison with previous works, it seems that NiO-NFs/CPE can acts as a comparable catalyst in the EG oxidation and shows a good performance. Among nano-sized ceramic architectures, nanofibers due to their high aspect ratios are more attractive over their nano-particulate counterparts. The technique used for preparation of the NiO-NFs is simple and environmental friendly. Furthermore, it has economic cost and is scalable at industrial levels. The catalyst layer (polymer þ nickel prepared in the previous works) was deposited as a thin film on the electrode surface (surface modification); meanwhile the NiO-NFs/CPE is centered based on bulk modification (a rapid method for generating a renewable and reproducible surface). Also, the materials chosen to be catalyst in previously published works, are formed from many materials which leads to many variables that are difficult to control and consequently to obtain reproducible results in electro-catalytic oxidation of EG in NaOH solution. However, the proposed electro-catalyst needs to further improvements in the catalytic activity and stability to realize the practical applications.
Effects of EG concentration and potential sweep rates Fig. 7A discloses the effect of EG concentration on the NiONFs/CPE in 0.2 M NaOH solution. As can be seen, EG oxidation can be catalyzed at the modified electrode surface. In addition, the anodic peak current density (jp) rises with increasing concentration up to 55 mM. While the EG concentration exceeds from this limit, no more significant current increasing occurs. Also, the CVs are recorded at different potential scan rates in 0.2 M NaOH solution containing 30 mM EG
Fig. 7 e (A) Electrochemical responses of the NiO-NFs/CPE in 0.2 M NaOH solution with different EG concentrations at y ¼ 20 mV s¡1. (a) 0, (b) 10, (c) 15, (d) 25, (e) 30, (f) 40, (g) 60, (h) 85 and (i) 115 mM. Inset of plots: jp as a function of EG concentration. (B) CVs of the NiO-NFs/CPE in 30 mM EG þ 0.2 M NaOH solution at different potential scan rates: (a) 5, (b) 10, (c) 20, (d) 30, (e) 50, (f) 80, (g) 100, (h) 200, (i) 400, (j) 600 and (k) 800 mV s¡1. Inset of plot: jp vs. y1/2.
Table 1 e Comparison of the electrochemical data of some modified CPEs in the electro-catalytic oxidation of EG in NaOH solution at y ¼ 20 mV s¡1. Modifier Ni/SDS-POAP Ni/SDS-PPAA Ni/SDS-PDMAN Ni/TX-100-PMT NiO-NFs
NaOH con./mol L1
EG con./mol L1
Ep/V
jp/mA cm2
Reference
0.1 0.1 1.0 0.1 0.2
0.28 0.25 0.20 0.06 0.03
0.80 0.70 0.85 0.67 0.63
11.1 5.9 83.1 10.5 10.1
[43] [44] [45] [46] This work
Please cite this article in press as: Hosseini SR, et al., Preparation of NiO nanofibers by electrospinning and their application for electrocatalytic oxidation of ethylene glycol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.116
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Fig. 8 e Iet transient of the NiO-NFs/CPE for 30 mM EG oxidation in 0.2 M NaOH at 0.63 V vs. AgjAgCljKCl (3 M). (B) CVs of the NiO-NFs/CPE in 0.2 M NaOH solution in the presence of 30 mM EG at y ¼ 20 mV s¡1; 1st (a) and 100th CV (b). (C) CVs of the NiO-NFs/CPE; 1st day (a) and 21st day (b). Other conditions are the same of part B.
(Fig. 7B). It can be seen that a linear relationship is obtained when jp values are plotted against to the square root of potential scan rates (y1/2). This behavior displays that the electrooxidation of EG is a diffusion controlled process.
(Fig. 8C). The electrode response retains 97% and 95% of its initial response, respectively.
Conclusion Stability of the NiO-NFs/CPE Practically, long-term stability of the electrode is very important. Chronoamperometry method is usually applied for study of activity and stability. A chronoamperogram with a large time window for the modified electrode is obtained at 0.63 V in presence of 0.03 M EG in 0.2 M NaOH solution (Fig. 8A). As can be seen, the decrease in current is relatively slow and, when the time is above 100 s, the current reaches a relatively stable value, which is still about 60% of the initial current. It is obvious that the NiO-NFs/CPE exhibits a high stability toward EG oxidation. The long-term stability of the NiO-NFs/CPE is confirmed by measuring its response for EG oxidation after 100 CVs in the presence of 0.03 M EG þ 0.2 M NaOH (Fig. 8B) and 3 weeks of storage in laboratory atmosphere condition
The PVA/Ni(OAc)2 nanofibers were successfully prepared without occurrence of beads and defects by electrospinning process and the NiO-NFs were obtained after calcination. Structures and morphology of the materials were characterized by TGA, FE-SEM, FT-IR, XRD and EDS analysis. The FESEM images showed that NiO-NFs were formed by calcination in air for 5 h. Moreover, after calcination at about 600 C, polymeric template of the NFs thoroughly was decomposed and their surface turned into harsh. The XRD pattern of the calcined sample showed that the precursor converted into pure NiO cubic structure. The result of EIS showed that the charge transfer resistance was decreased, when the carbon paste was spiked with NiO-NFs. The electro-catalytic performance of the low cost and renewable bulk modified electrode (i.e. electro-spun NiO-NFs/CPE) for EG oxidation was
Please cite this article in press as: Hosseini SR, et al., Preparation of NiO nanofibers by electrospinning and their application for electrocatalytic oxidation of ethylene glycol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.116
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investigated in the electro-catalysis of EG oxidation in alkaline solution. The modified electrode exhibited good electrocatalytic activity towards EG oxidation, demonstrating a reasonable stability and durability.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.09.116.
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Please cite this article in press as: Hosseini SR, et al., Preparation of NiO nanofibers by electrospinning and their application for electrocatalytic oxidation of ethylene glycol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.116
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Please cite this article in press as: Hosseini SR, et al., Preparation of NiO nanofibers by electrospinning and their application for electrocatalytic oxidation of ethylene glycol, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.116