Fabrication of electrospun nanofiber catalysts and ammonia borane hydrogen release efficiency

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 e1 0

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Fabrication of electrospun nanofiber catalysts and ammonia borane hydrogen release efficiency Bilge Cos‚kuner Filiz, Aysel Kantu¨rk Figen* _ Department of Chemical Engineering, Yildiz Technical University, Istanbul, Turkey

article info

abstract

Article history:

In the present study, Electrospun nanofibers (Co-NF, Ni-NF, and Cu-NF) were used as effi-

Received 16 December 2015

cient catalysts for hydrogen release through ammonia borane (NH3BH3) hydrolysis. Poly(-

Received in revised form

vinyl alcohol) solutions containing metal acetates provided solegel materials for

28 March 2016

electrospinning. The electrospun fibers were fabricated by a low cost and facile technique.

Accepted 28 March 2016

The effects of different process parameters such as solution preparation and electro-

Available online xxx

spinning conditions that directly influence the texture, crystalline phase, and chemical properties of electrospun nanofibers fibers were studied. The nanofibers were characterized

Keywords:

by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), BrunauereEm-

Hydrogen

metteTeller surface area (BET), and scanning electron microscope (SEM). The metal oxide-

Ammonia borane

loaded nanofibers with average diameters from approximately 187 to 452 nm showed cat-

Electrospun

alytic activity toward increasing NH3BH3 hydrogen release efficacy, with activation energies

Nanofiber catalysts

as low as 41.59 kJ mol
1 (Co-NF), 35.54 kJ mol
1 (Ni-NF), and 36.70 kJ mol
1 (Cu-NF). Based on recyclability tests, Co-NF metal oxide nanofiber catalysts showed unaltered catalytic activity toward hydrogen release and good chemical stability after ten successive cycles. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The electrospinning solegel method is a convenient technique for fabricating fibers with unique properties such as controlled diameter, very large surface area, flexibility in surface functionalities, and superior mechanical properties [1]. Electrospinning is an unequaled approach that uses electrostatic forces to produce fine fibers from polymer solutions/melts ranging in diameter from nanometers to micrometers. Synthetic and natural polymers such as polyglycolide (polyglycolic acid, PGA), polyvinyl alcohol (PVA), silk fibroin, gelatin, and cellulose are used to form fine nanofibers for use in a wide of

applications, for example in nanocatalysis, tissue engineering scaffolds, protective clothing, optical electronics, filtration, defense and security, and environmental engineering, as well as those important to applications in biomedical, pharmaceutical, healthcare, and biotechnology fields [2]. Electrospinning is finding increasing importance in the hydrogen energy system. Electrospun fiber catalysts, having smaller pores and higher surface area than regular catalysts, have been successfully applied in hydrolysis reactions of metal hydrides to supply pure hydrogen gas to a fuel cell. Hydrogen release from ammonia borane (NH3BH3, AB), an amminetrihydroboron, is one of the future strategies for energy applications due to its high hydrogen content (19.6 wt%)

* Corresponding author. Department of Chemical Engineering, Yildiz Technical University, Istanbul, 34210, Turkey. Tel.: þ90 2123834774; fax: þ90 2123834725. E-mail addresses: [email protected], [email protected] (A. Kantu¨rk Figen). http://dx.doi.org/10.1016/j.ijhydene.2016.03.182 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Cos‚kuner Filiz B, Kantu¨rk Figen A, Fabrication of electrospun nanofiber catalysts and ammonia borane hydrogen release efficiency, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.182

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and a low molecular weight (30.9 g mol
1) [3,4]. The hydrolysis reaction has received considerable attention since 2006, with special focus on catalytic materials [5]. Noble and non-noble metals [6], metal oxides [7], bimetallic nanoparticles [8], and porous powders [9] have been produced by several techniques, including chemical reduction, wet impregnation [10], solegel [11], and precipitation [12] due to their high catalytic activities and large surface areas; these catalytic materials are of special interest for hydrogen applications [13]. Also, oxide forms of metals are more stable against atmospheric degradation and humidity compared with metallic forms. To obtain high catalytic activity, the recommendation is to use metals in oxide forms, or doped on various supports, or embedded in polymeric mediums [14]. It is also possible to use metal oxides in combination with elemental boron to increase their activity [15]. Beyond that, the conventional catalyst preparation techniques are utilized by electrospinning to control the surface area, size, and morphology of catalysts to obtain the required acceleration of hydrogen release. Cu0/S-doped TiO2 nanoparticles decorated with carbon nanofibers as a photo catalyst for the hydrolysis of AB under visible light were produced by electrospinning; reportedly, the nanofiber catalyst exhibited good photo catalytic activity [16]. Co-B nano flake-like structures supported over nanoporous TiO2 was synthesized in two steps using electrospinning along with chemical reduction and was tested in AB hydrolysis. The synthesized nanocatalyst showed a low effective activation energy and high maximum rate [17]. A new system of ionic liquid with metal complex, chemically crosslinked, electrospun nanofibers based on polyvinylidene fluoride (PVDF) was developed for hydrogen generation by hydrolysis of NaBH4. The cross-linked nanofibers retained their morphology even after thermal treatment and have potential as a catalyst in energy-related applications [18]. Cerium-nickeleloaded titanium nanofibers have been fabricated for use in catalytic applications for hydrogen production; it has been shown that addition of cerium along with nickel significantly enhanced the catalytic activity, but that excessive cerium-loading had a negative effect on the NaBH4 hydrolysis [7]. A synchronous etchingepitaxial growth approach has been developed for the fabrication of facet-coupling NaTaO3/Ta2O5 hetero-structured nanofibers for use as a photo catalyst for hydrogen production from pure water and 20 vol% methanol aqueous solutions. In the absence of any catalysts this system can exhibit a photocatalytic activity for hydrogen production as high as 1579 mmol h
1 g
1 [19]. A silica-supported palladium (Pd/SiO2) nanofiber catalyst with an average diameter of 500 nm has shown 93.48% efficiency in a hydrogenation reaction for acrylic acid; this could be promising for a wide range of applications in the catalyst industry [20]. It has been reported that chemically stable bimetallic NiCu nanorods incorporated with carbon nanofibers show superior catalytic activity toward hydrogen release from AB, with a low activation energy of about 28.9 kJ mol
1 [21]. Bimetallic Pd-doped Co nanofibers have been successfully prepared by electrospinning to improve the stoichiometric hydrogen release performance from AB via hydrolysis and photo hydrolysis, the same efficiency over several successive test [22]. To best of our knowledge, there are only limited reports of nanofiber catalysts for NH3BH3 hydrogen release; fabrication

of the effective metal-loaded nanofiber might be a successful strategy to obtain rapid hydrogen release with low activation energy. The present study investigates that approach by fabrication of electrospun nanofiber catalysts (Co-NF, Ni-NF, Cu-NF); their NH3BH3 hydrogen-release efficiency is demonstrated and opens a new perspective on a different class of catalysts.

Experimental Materials Poly(vinyl alcohol) (PVA, MW 85,000e124,000, 99% hydrolyzed) was purchased from Aldrich. Cobalt(II)acetate tetrahydrate (Co(C2H3O2)2$4H2O), nickel(II)acetate tetrahydrate (Ni(C2H3O2)2$4H2O), and copper(II)acetate tetrahydrate (Cu(C2H3O2)2$4H2O) from Aldrich were used as received. Homemade NH3BH3 was used as a hydrogen storage carrier for hydrogen release prepared by the one-pot chemical reaction described in our previous work [23].

Fabrication of electrospun nanofiber catalysts PVA fibers and electrospun fiber catalysts were prepared by electrospinning by forcing the polymer solution through a spinneret by an electrical field. The fibers were subsequently dried and then calcinated, resulting in nanometer-sized fibers. In the present study, we first prepared PVA solutions for fabrication of PVA fibers with different process parameters such as solution concentration, preparation conditions, electrospinning conditions, collection distance, polymer flow rates (see Table 1). PVA solutions with different concentrations of 5e15 wt% were prepared by dissolving a weighted amount of polymer in deionized water at 80  Ce90  C and gently stirring for 5 h. To obtain completely homogeneous and spinnable solutions, the PVA solutions underwent magnetic stirring for another 24 h at room temperature before being used in the electrospinning process. The electrospinning of the as-prepared PVA solutions was carried out by loading the solutions in a 10-ml plastic syringe. A single-nozzle basic-level electrospinning machine, (Inovenso Co.) was used to charge the solution across the needle and to supply the variable high voltage for electrospinning. A flat metal plate covered with aluminum foil was used to collect the fibers. After electrospinning under different conditions, the resulting PVA fibers were characterized; the results are shown in Table 1. After electrospinning, the fiber mats were kept in an oven at 110  C to remove any residual water. To fabricate electrospun fiber catalysts, PVA-metal acetate solutions were prepared by dissolving the metal acetate (Co, Ni, Cu) in PVA solution, following by stirring at 90  C for 2 h. The resulting metal acetate-containing polymer solutions were used for electrospinning; 20, 25, or 30 kV volts were applied between the spinneret and the drum collector. The flow rate of the polymer solution was adjusted to 1 ml h
1 using a pump; the tip-to-collector distance varied from 7.5 cm to 15 cm to determine its effect of nanofiber formation. The nanomats were dried for 110  C for 6 h; the Co, Ni, Cu composite nanomats were then subjected to heat treatment to

Please cite this article in press as: Cos‚kuner Filiz B, Kantu¨rk Figen A, Fabrication of electrospun nanofiber catalysts and ammonia borane hydrogen release efficiency, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.182

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Table 1 e Optimization of fiber formation conditions such as solution preparation and electrospinning conditions. Set

1

2

Solution preparation conditions

Electrospinning conditions

Temperature,  C

Stirring time, h

Spinning voltage, kV

Collection distance, cm

Flow rate, ml h
1

5 wt. % PVA

85

5

7.5 10 15

85

5

15 wt. % PVA

85

5

0.5 0.7 0.8 1.0 0.5 0.7 0.8 1.0 0.5 0.7 0.8 1.0

No

10 wt. % PVA

20 25 30 40 20 25 30 40 20 25 30 40

5 wt. % PVA

90

5

25

7.5

1.0

YES

5 wt. % PVA þ5 wt. % Co

90

2

20

7.5 10 15

1

No

25

7.5 10 15 7.5 10 15 7.5 10 15

1

YES No

1

No

7.5 10 15 7.5 10 15 7.5 10 15

1

YES No

1

No

1

No

7.5 10 15 7.5 10 15

1

YES No

1

No

30

3

5 wt. % PVA þ5 wt. % Ni

90

2

20

25

30

4

Fiber formation

Concentration

5 wt. % PVA þ5 wt. % Cu

90

2

20

25

30

remove the polymer matrix and to obtain the metal oxide-NF catalysts. To obtain a smooth fiber catalyst, heating/cooling treatment was applied using combinations of different modes: isothermal heating (1  C min
1 up to 450  C), nonisothermal heating (waiting 4 h at 450  C), and isothermal cooling (cooling to room temperature at 1  C min
1). Texture, crystalline phase, and chemical properties of the obtained metal oxide-NF catalysts after the heating/cooling treatment were recorded with scanning electron microscope (SEM, Fig. 1), BrunauereEmmetteTeller surface analysis (BET, Fig. 2 and Table, 2), Fourier Transform Infrared Spectroscopy (FT-IR, Fig. 3), X-ray diffraction (XRD, Fig. 4 and Table 2) techniques.

7.5 10 15 7.5 10 15

1

SEM techniques. Crystalline structures of the samples were determined by an XRD (Philips Panalytical X'Pert-Pro) diffractometer with CuKa radiation at operating parameters of 40 mA and 45 kV, with step size 0.02 and speed 1 /min. Phase identification of solids was determined according to the inorganic crystal structure database (ICSD). Chemical bonds of samples were identified by FT-IR/ATR (Perkin Elmer Spectrum One) in the spectral range 4000 to 650 cm
1 at ambient temperature; the resolution used was 4 cm
1. The specific surface area of the samples was measured using BET surface area analysis (Quantachrome) under N2 adsorptive gas with multipoint modes. The morphology of samples was characterized using SEM (JEOL JSM 5410 LV).

Characterization of texture, crystalline phase and chemical properties

Hydrogen release from ammonia borane

Texture, crystalline phase, and chemical properties of the obtained materials were studied using XRD, FT-IR, BET, and

Hydrogen release from NH3BH3 was performed in a glass microreactor and monitored by a typical water-filled gas

Please cite this article in press as: Cos‚kuner Filiz B, Kantu¨rk Figen A, Fabrication of electrospun nanofiber catalysts and ammonia borane hydrogen release efficiency, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.182

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Fig. 1 e SEM images of metal oxide-NF catalysts: Co-NF, 40,000X (a), 60,000X (b); Ni-NF, 40,000X (c), 60,000X (d); Cu-NF, 40,000X (e), 60,000X (f).

burette system with a cooling jacket at every milliliter. The metal oxide-NF catalysts (5 mg) were treated with 1.2 mmol NH3BH3 to release a maximum of 78 ml hydrogen gas. After no hydrogen gas was observed, the amount of water vapor was estimated and deducted from the total gas obtained. We made several runs under the same conditions; hydrogen release was performed various temperature (22e80  C) to determine the activation energies (Ea) based on power law reaction models and the Arrhenius equation (Figs. 5 and 6).

Recyclability tests

Fig. 2 e BET Isotherms of metal-NF catalysts.

The ten-cycle recyclability of the metal oxide-NF catalysts was investigated at 60  C with 5 mg catalyst. The catalyst recyclability data were used to calculate the turnover frequency (TOF,

Please cite this article in press as: Cos‚kuner Filiz B, Kantu¨rk Figen A, Fabrication of electrospun nanofiber catalysts and ammonia borane hydrogen release efficiency, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.182

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Fig. 3 e FT-IR spectrums of PVA/metal composites mats and metal oxide-NF catalysts.

Table 2 e Texture and crystalline phase properties of metal oxide-NF catalysts. Catalyst

Co-NF Ni-NF Cu-NF

Crystalline phase properties Oxide phase/JCPDS

Crystal system

CoO (JCPDS:01-048-1719) Co3O4 (JCPDS:00-009-0418) NiO (JCPDS:01-089-7390) CuO (JCPDS: 00-005-0661) Cu4O3 (JCPDS:01-083-1665)

Cubic Cubic Rhombohedral Monoclinic Tetragonal

Texture properties

Crystalline Semi-quantatif Average fiber BET area, size, nm amount, % diameter, nm m2 g
1 32.40 33.72 45.70 36.27 32.14

27.2 72.8 100 84 16

Pore volume, cm3 g
1

Pore diameter,  A

286

6.50

0.045576

280.4751

187 452

11.26 1.29

0.045561 0.021893

162.0523 678.3784

mole H2 mole
1 catalyst min
1) for metal oxide-NFecatalyzed NH3BH3 hydrolysis reactions. In recyclability experiments, after the first hydrogen generation reaction was completed, the catalyst was kept in the reaction solution under ambient conditions and another 1.2 mmol NH3BH3 was added to the residual solution and the reaction monitored. This was repeated ten times to determine the recyclability; in addition, the hydrogen generation performance was evaluated (Fig. 7).

Results and discussion

Fig. 4 e XRD patterns of metal-NF catalysts.

In the present study, the solution parameters such as preparation temperature and concentration, as well as spinning

Please cite this article in press as: Cos‚kuner Filiz B, Kantu¨rk Figen A, Fabrication of electrospun nanofiber catalysts and ammonia borane hydrogen release efficiency, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.182

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Fig. 5 e Hydrogen release vol. from NH3BH3 in the presence of metal oxide-NF catalysts against time at different temperatures.

Fig. 6 e Arrhenius plots for the zero order metal oxide-NF catalyzed NH3BH3 hydrogen release reaction.

parameters such as spinning voltage, collection distance, and flow rate were optimized for fabrication of PVA fiber and metal oxide-NF (Table 1). Characterization of PVA fiber (Fig. S1) and metal oxideeNF catalysts was performed by FT-IR, SEM, BET, and XRD; see Figs. 1e4 and Table 2.

Characterization results of PVA fiber The PVA solution used as a polymer matrix in the metal oxideNF catalysts and the operational conditions were carefully examined by performing a series of electrospinning runs at varying solution concentration or viscosity, spinning voltage, collection distance, flow rate, and solution preparation temperatures, all of which have significant impact on the fiber formation (Table 1).

Fig. 7 e Recyclability tests results: Hydrogen release vol. from NH3BH3 in the presence of metal oxide-NF catalysts against time at 60  C, inset plot TOF values against time for every cycle.

Our relevant findings were that PVA solutions prepared at 85  C at any concentration and different spinning parameter were not able to produce any fibrous structure using electrospinning. At a lower preparation temperature, the solution leaks out from the syringe tip. A slight increase of preparation temperature results in the disappearance of droplets, and the fiber appeared in the flat metal plate. To solve the above problem and produce a homogeneous and spinnable PVA solution, we prepared a 5 wt% PVA solution at 90  C; the resultant solution was subjected to a high spinning voltage (25 kV), a high flow rate (1 ml h
1), and collection distance (7.5 cm);

Please cite this article in press as: Cos‚kuner Filiz B, Kantu¨rk Figen A, Fabrication of electrospun nanofiber catalysts and ammonia borane hydrogen release efficiency, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.182

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this solution tended to form fibers. Fig. S1 shows SEM images and the related FT-IR spectra of the PVA fibers. PVA fibers with diameters of 172 ± 29 nm were obtained from the 5 wt% PVA under the spinning voltage 25 kV, flow rate 1 ml h
1, and distance 7.5 cm. Morphological changes could be observed with increasing PVA concentration. The formation of beads only occurred when the polymer concentration was further increased. The shape of beads also changed with increasing polymer concentration. The success of the electrospinning process depended on the distribution of nanofiber diameters, solution properties, and electrospinning parameters [24]. Ro sic et al. prepared PVA solutions by dissolving PVA in distillated water in 2e14 wt% PVA concentration and spun them at 15 kV and a 15-cm collection distance. Those authors found that concentrations below 4 wt% PVA resulted in the formation of beads, and above 12 wt%, the PVA solution had a very high viscosity and could not be formed into nanofibers [24]. Many reports have shown the effect of the process parameters on the morphology of the fibers. For instance, Jia et al. produced PVA nanofibers with an average diameter in the ranging from 100 nm to 1000 nm based on the electrospinning conditions [25]. Supaphol and Chuangchote investigated PVA fiber mats using various PVA concentrations (6e10 wt% PVA) and process parameters; the average diameters of as-spun PVA fibers ranged between 85 and 647 nm [26]. Ding et al. successfully prepared PVA fibers with diameters 50e250 nm with electrospinning parameters in the range 7e19 kV and 4e12 cm collection distance [27]. In FT-IR spectra of PVA nanofibers, the very strong peak of hydroxyl groups at 3295 cm
1, CH2 asymmetric stretching at 2940 cm
1, terminal polyvinyl groups at 1088 cm
1, presence of CeH alkyl groups at 2908 cm
1, -CH3 and -CH bending bands at 1421 and 1322 cm
1, and an -OH vibration band at 847 cm
1 were detected [28].

Texture, crystalline phase and chemical properties of metal oxide-NF catalysts According to the characterization of our PVA fibers, 5 wt% PVA concentration was the optimal polymer matrix for preparing the PVA/metal acetate composite solution. To obtain the composite solution, metal acetate powders were added to the PVA solution. The operational conditions were studied by performing a series of electrospinning runs at several different parameters, such as spinning voltage, collection distance, and flow rate that can directly affect the electrospun fiber formation (Table 1). PVA/metal acetate composite solutions were prepared by heating at 90  C for 2 h, mixing, and subjecting to electrospinning at 20 kV, 25 kV, 30 kV; 7.5, 10, 15 cm collection distance; and 1 ml h
1 constant flow rate. The electrostatic forces under optimized conditions (25 kV, 7.5 cm and 1 mlh
1) were applied to fabricate a smooth mat from all PVA/metal acetate composite solutions. Metal oxide NF catalysts were fabricated based on electrospinning of 5 wt% PVA solution containing 5 wt% metals (Co, Ni, Cu). After heating, the amount of the metal oxide supported on the NF was approximately 30 wt% for all three metal oxides and had a different structure and crystallinity. Texture, crystalline phase, and chemical properties were

7

identified using SEM (Fig. 1), BET (Fig. 2 and Table 2), FT-IR (Fig. 3), and XRD (Fig. 4 and Table 2). (1) Texture properties: Texture properties of electrospun fibers are listed in Table 2. SEM images at 40,000 and 60,000 magnifications of the electrospun catalyst fibers are shown in Fig. 1. The fibrous structures were not broken down after the combined heating/cooling treatment. The nanofibers of the metal oxide-NF catalyst with surface and diameters in the range of 187e452 nm were obtained after a heating/cooling treatment that removed the PVA and acetate groups. The surface of Co-NF and Cu-NF catalysts were smooth as a result of the amorphous nature of the PVA and Co, Cu acetated composites. The Ni-NF catalyst contained some beads on the surface; the surface became rougher because of the decomposition of the PVA and the development of NiO crystals. The BET surface area is an important parameter for metal oxideNF catalysts. The Ni-NF catalyst with 11.26 m2 g
1 showed the highest BET surface area which is attributable to its minimal pore diameter and bumpy surface (Fig. 2). (2) Chemical Properties of Electrospun Fibers: FT-IR spectra of the composite fiber (dried at 110  C) and electrospun catalyst fibers (heating/cooling treatment of composite fiber) are shown in Fig. 3. Peaks associated with the PVA and acetate structure are numbered 1 through 8. The peaks at 679 cm
1 (1), 1026 cm
1 (2), 1058 cm
1 (3), 1331 cm
1 (4), 1394 cm
1 (5), 2927 cm
1(7), and 3290 cm
1 (8) were assigned to the -CO2 sym. bending, -CH3 rocking, -CH3 rocking, -CH3 sym. bending, eCO2 sym. stretching, -CH3 sym. stretching, -OH sym. Stretching, all features of the acetate structure. The FT-IR spectra of PVA/ metal composites mats include metal acetate (1550 cm
1, 6) and an organic matrix after drying at 110  C [29]. As observed in FT-IR spectra of PVA/metal composites mats after the heating/cooling treatment, the PVA decomposed and the metal acetate peaks were weakened or disappeared, showing that all organic molecules were completely removed from the PVA acetate fibers after calcination. In addition, metal oxide vibrations always disappeared below 650 cm
1 [28]. (3) Crystalline Phase: The XRD patterns and crystalline phase properties are shown in Fig. 4 and Table 2, respectively. The pattern of the metal oxide-NF catalysts indicates that the oxide structures were formed after the PVA was removed from the fiber mats; FT-IR results confirmed that the products are pure metal oxide fibers. The Scherrer equation was applied to calculate the crystalline size of the metal oxide particles, which were in the range 32e46 nm. The sizes of cubic CoO and cubic Co3O4 nanoparticles were about 32.40 nm and 33.72 nm, respectively. These results confirmed that the electrospinning process generates nano-sized metal oxide catalytic materials. According to the quantitative Rietveld analysis results, Co-NF contained 27.2% CoO and 72.3% Co3O4, and also that Cu-NF included 84% CuO and 16% Cu4O3.

Hydrogen release from NH3BH3 To examine the effect of temperature on the hydrogen release characteristics, the temperatures were increased from 22  C to 80  C. Fig. 5 shows the time course of hydrogen release from aqueous AB in presence of Co, Ni, Cu nanofiber catalysts in the experiments; the amount of each catalyst was maintained at

Please cite this article in press as: Cos‚kuner Filiz B, Kantu¨rk Figen A, Fabrication of electrospun nanofiber catalysts and ammonia borane hydrogen release efficiency, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.182

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5 mg in the 1.2 M NH3BH3 and 10 ml water. As shown in Fig. 5, the Co-NF, Cu-NF catalysts could completely hydrolyze the NH3BH3, except for the Ni-NF catalyst at low temperatures. Better activity of the nanofibers could be obtained at higher temperatures (up to 22  C). The reaction time, in terms of higher reaction rate, decreased as the temperature increased. With an increase in solution temperature from 22 to 80  C, the hydrogen release time of the CoeNF catalyst decreased about 15-fold. H2 generation rates (ml H2 min
1 g
1 catalyst) were calculated from the slope in volume/time curves for all the temperatures (Table 3). The hydrogen release rate of the CoeNF catalyst was higher than that of the Ni-NF and Cu-NF catalysts; the hydrogen release performance of the CoeNF catalyst increased dramatically from 334 to 5798 ml H2 min
1 g
1 catalyst directly depending on the temperature. The activity of a catalyst is largely dependent upon the morphology, support, and metal type. In this study, the catalytic activity seemed to be independent of the specific surface area; the metal support content of the catalyst had the most important effect on the activity. The maximum specific surface area of 11.26 m2 g
1 was observed in the Ni-NF catalyst, whereas the Co-NF catalyst had the maximum hydrogen generation rate. This showed that the metal support content of the nanofiber catalyst determined the catalytic activity for hydrolysis of NH3BH3. The catalysts containing non-noble metals such as Co, Ni, and Cu have been used for practical applications in hydrogen storage systems. Among them, Co-based catalysts were found to have the highest catalytic activity due to much greater structural distortion capability and therefore a much higher concentration of active sites for the reaction [30]. It has been suggested that using metals precursors in an oxidized form for hydrogen generation and active phases on the catalyst surface such as CoO, Co3O4, NiO, CuO, and Cu3O4 will affect the catalytic activity. The crystallinity and morphology of oxides will also change the activity level of the catalyst [31]. It has been

Table 3 e H2 Generation rate of metal oxide-NF catalysts at various temperatures. Catalyst

Co-NF Ni-NF Cu-NF

H2 generation rate, ml H2 min
1 g
1 catalyst 22  C

40  C

60  C

80  C

333.90 38.64 81.34

1035.66 96.16 114.96

2307.60 200.20 169.72

5797.60 499.62 1240.58

reported that the catalytically active phase formed from Co3O4 showed higher activity than that formed from CoO by a direct measurement of the heat evolved during the sodium and potassium borohydride catalytic hydrolysis [32]. According to the hydrogen generation profiles in our study, the NiO NF catalyst had lower catalytic activity due to aggregation during the hydrolysis reaction as compared with the Co-, Ni-, and Cubased NF catalysts. Due to the ease of agglomeration of copper oxideys particles during the hydrolysis reaction, cobalt oxides were more active than copper oxides [33]. Ni and Cu nanoparticles have also been shown to have an agglomeration problem under harsh reaction conditions [21]. Because of that problem, Niand Cu-based catalysts exhibit lower release of hydrogen; their hydrogen release rate was lower than that of the Cobased catalyst. In our study, we observed the same aggregation problem for Ni-NF and Cu-NF catalysts, which would affect the recyclability results. In addition to the measurements stated previously, the pH values of aqueous 1.2-mmol NH3BH3 solutions were measured as approximately 9.0; this is similar to value of 9.1 reported by Chandra et al. [34]. After 10 hydrolysis cycles, pH values of the solutions were 9e9.5; the pH of the solution was also independent of the concentration of residual materials. The activation energy for catalytic hydrogen release of aqueous AB was calculated using the rate law and Arrhenius equation. Zero-, first-, and second-order power law reaction models were applied, which indicated the hydrogen release rate is independent of the AB concentration: it showed zeroorder kinetics as in the previously reported study [15]. Fig. 5 shows the Arrhenius plot of ln(k) versus the reciprocal absolute temperature: the slope of the straight line gives the Ea values for hydrogen release in the zero-order reaction. The apparent Ea was 41.59 kJ mol
1, 35.54 kJ mol
1, and 36.70 kJ mol
1 for Co-NF, Ni-NF, and Cu-NF, respectively (Table 4). Catalytic activities of the electrospun metal oxide nanofibers can be compared with previous reports on the metal and metal oxide catalysts for hydrogen release. The Ea was lower for both metal oxide nanofiber catalysts than that reported for Co/Y-Al2O3 (62 kJ mol
1) [4], Ni powder (70 kJ mol
1) [35], and Ni@Ru core@shell NPs (44 kJ mol
1) [35]; in contrast, Ea for the nanofiber catalysts was relatively higher than the results for NiCu-nanorods@Carbon nanofibers (28.90 kJ mol
1) [21], CoPd nanoparticles (NPs) (27.5 kJ mol
1) [36], and Co-B nano flakelike structure supported over nanoporous TiO2 nanofibers

Table 4 e Comparison of activation energy values for catalyzed hydrogen release from NH3BH3 in the presence of metal oxide-NF catalysts and various catalysts. Catalyst Co-B nanoflakes-like structure supported over nanoporous TiO2 nanofibers CuO doped in TiO2 nanofibers NiCu nanorods@ C nanofibers PAN/PdePt composite nanofiber membrane (PAN)/Ag/Pd composite nanofibers Co-NF Ni-NF Cu-NF

TOF values

Ea, kJ mol
1

References

e

16.54

[6]

e e 51.9 mol H2 min
1 (mol PdePt alloy)
1 60.28 mol H2 min
1 mol
1 Pd 147 mol H2 min
1 mol
1 Co 8 mol H2 min
1 mol
1 Ni 11 mol H2 min
1 mol
1 Cu

44.8 28.90 e e 41.59 35.54 36.70

[14] [21] [38] [39] In this study

Please cite this article in press as: Cos‚kuner Filiz B, Kantu¨rk Figen A, Fabrication of electrospun nanofiber catalysts and ammonia borane hydrogen release efficiency, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.182

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 e1 0

(16.54 kJ mol
1) [6]. Finally, NieRu alloy NPs (37 kJ mol
1) [37] and Cu0.2Ni0.8/MCM-41 (38 kJ mol
1) [3], Co0.75B0.25 (40.85 kJ mol
1) [11], Cu0.75B0.25 (48.74 kJ mol
1) [11], Ni0.75B0.25 (43.19 kJ mol
1) [11], and Co-B (47.5 kJ mol
1) [15] catalysts had apparent activation energies for catalytic hydrogen similar found in the present study.

Recyclability tests Recyclability tests were performed to test the stability of the catalysts and to determine the changing rate of hydrogen production after several cycles. Fig. 7 shows the generated hydrogen volume after using the introduced nanofibers for 10 cycles; the inset shows the plot of TOF values versus time. These recyclability tests showed that 3 mol of hydrogen were released after all cycles with a change of hydrogen release times; the introduced nanofibers showed catalytic activity that remained unaffected after multiple uses. After the first 6 cycles, the reaction completion time reached 8.55 min for the Co-NF catalyst, with an initial 147 mol H2 min
1 mol
1 Co TOF value. In presence of Ni-NF catalysts, the hydrolysis reaction time increased from 67 min to 124 min in inverse relation to TOF values from 8 to 4 mol H2 min
1 mol
1 Ni. For Cu-NF catalyst, the hydrolysis reaction was completed within a range 63e319 min and the TOF value decreased from 11 to 2 mol H2 min
1 mol
1 Cu after the 10th run. Based on the TOF values of the nanofiber metal oxide catalyst at the end of the 10th run, Co-NF catalysts showed the best recyclability results and preserved activity. The hydrogen generation rates were calculated before and after the 10th cycles for each nanofiber; it decreased about 43% for Co-NF, 44% for Ni-NF, and 82% for Cu-NF metal oxide catalysts. The accessibility of active sites decreases as a resulting of passivation of the catalyst surface by an increasing amount of byproduct after the each run, thereby reducing the hydrogen generation rate and TOF values. The TOF values of electrospun metal oxide nanofibers are comparable with the values found in the literature for hydrogen release from NH3BH3. Additionally, because Ni-NF and Cu-NF have lower initial TOF values, Co-NF is comparable with previously reported results. The reported TOF values of noble and non-noble metal nanofiber composites, such as PAN/PdePt composite nanofiber membrane (51.9 mol H2 min
1 (mol PdePt alloy)
1) [38], and (PAN)/Ag/Pd composite nanofibers (60.28 mol H2 min
1 mol
1 Pd) [39] were lower than our values for Co-NF (147 mol H2 min
1 mol
1 Co).

Conclusion Electrospun nanofibers of Co oxides (as CoO and Co3O4), Ni oxide (as NiO), and Cu oxide (as CuO and Cu4O3) with diameters of 187e452 nm and crystalline size of 32e46 nm were used to catalyze hydrogen release from AB The fibers were prepared by electrospinning of PVA (5 wt%)/metal acetate composites at spinning voltage 25 kV, flow rate 1 ml h
1, and collection distance 7.5 cm. An initial drying followed by heating/cooling treatment were required to fabricate smooth nanofibers. Our results demonstrate that the Co-NF catalyst exhibits higher catalytic efficacy for hydrogen release from

9

NH3BH3 than the Ni- and Cu-based metal oxide-NF catalysts. In the present study, the Ea values of nanofiber catalysts prepared by electrospinning were lower than most of the metal oxide-supported non-precious metal catalysts reported in literature to date. These results imply that the hydrogen release from AB could be improved with an electrospun metal oxide nanofiber as the catalyst. Overall, the hydrolysis findings showed that the Co-NF catalyst was smooth and nanocrystalline, was easily re-used as a result of the presence of elemental cobalt, and that the catalytic activity does not decrease after the 10 successive cycles.

Acknowledgments The authors would like to thank the Yildiz Technical University Research Foundation (Project no: 2013-07-01-GEP01) for its financial support.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.03.182.

references

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Please cite this article in press as: Cos‚kuner Filiz B, Kantu¨rk Figen A, Fabrication of electrospun nanofiber catalysts and ammonia borane hydrogen release efficiency, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.03.182