Hydrogen generation from hydrolysis of sodium borohydride using nanostructured NiB catalysts

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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Hydrogen generation from hydrolysis of sodium borohydride using nanostructured NieB catalysts Yan Wang a,b,*, Yunshu Lu a, Dan Wang a, Shiwei Wu c,**, Zhongqiu Cao a, Ke Zhang a, Hongxin Liu d, Shigang Xin c a

Institute of Catalysis for Energy and Environment, College of Chemistry and Chemical Engineering, Shenyang Normal University, Shenyang 110034, PR China b Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Collaborative Innovation Center of Chemical Science and Engineering, College of Chemistry, Nankai University, Tianjin 300071, PR China c Experimental Center, Shenyang Normal University, Shenyang 110034, PR China d College of Life Science, Shenyang Normal University, Shenyang 110034, PR China

article info

abstract

Article history:

The nanostructured NieB catalysts were synthesized on the Cu sheet by electroless plating

Received 12 January 2016

at ambient temperature. The catalytic performance for hydrogen generation from hydro-

Received in revised form

lysis of sodium borohydride solution has been investigated. The concentration of the

23 May 2016

reducing agent was shown to have a significant influence on the hydrogen generation

Accepted 29 May 2016

performance. The NieB catalyst deposited with 0.08 g L
1 reducing agent showed excellent

Available online xxx

catalytic activity due to the smaller particle or crystal size and the higher surface roughness. The highest hydrogen generation rate could reach 4991.8 mL min
1 g
1 and the lower

Keywords:

activation energy was 36.3 kJ mol
1. The NieB catalyst also displayed high stability. The

Electroless plating

catalytic activity could retain about 80.5% of its initial value after 5 cycles.

NieB catalyst

© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Sodium borohydride Hydrogen generation

Introduction Sodium borohydride (NaBH4) has drawn much attention because of its high gravimetric hydrogen storage capacity (10.8 wt%) and non-toxicity [1]. NaBH4 can be employed as an ideal hydrogen source for proton exchange membrane fuel cell (PEMFC) [2,3]. It can release pure hydrogen from NaBH4 via hydrolysis according to the following reaction:

NaBH4 (s) þ 2H2O (l) / NaBO2 (aq) þ 4H2 (g).

(1)

To accelerate the reaction rate of hydrolysis of NaBH4, the catalysts are the key factor [4e6]. So far now, many catalysts have been reported for hydrogen generation from the hydrolysis of NaBH4. Among them, noble metal (such as Pt and Ru) based catalysts have shown higher catalytic activity [7e10]. However, it is well known that the noble metals are very expensive and restrict their practical application. Therefore, it

* Corresponding author. Institute of Catalysis for Energy and Environment, College of Chemistry and Chemical Engineering, Shenyang Normal University, Shenyang 110034, PR China. Tel.: þ86 24 86576903; fax: þ86 24 86593072. ** Corresponding author. Experimental Center, Shenyang Normal University, Shenyang 110034, PR China. Tel.: þ86 24 86576903; fax: þ86 24 86593072. E-mail addresses: [email protected] (Y. Wang), [email protected] (S. Wu). http://dx.doi.org/10.1016/j.ijhydene.2016.05.258 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wang Y, et al., Hydrogen generation from hydrolysis of sodium borohydride using nanostructured NieB catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.258

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is necessary to develop inexpensive and highly efficient catalysts. Recently, many investigations have been carried out to develop non-noble metal based catalysts, such as Co and Nibased catalysts, including Ni [11,12], Co [13], CoeP [14e18], NieB [19,20], CoeB [21], CoeNieP [22,23], CoeWeP [24], NieCoeB [25], CoePeB [26] and CoeNiePeB [27]. It has been found that the combination of a metal (such as Ni or Co) with a metalloid atom (such as B or P) can improve the catalytic activity because of the change of electronic states of the active metals. Among the non-noble metal catalysts, there are many reports on CoeB and CoeP catalysts for the hydrolysis of NaBH4. Compared with CoeB and CoeP catalysts, NieB catalysts have potential advantages because Ni is considered as a cheap and environmentally friendly element. Lee et al. [28] have demonstrated that the activation energy of NieB/Ni foam as the catalyst for hydrolysis of NaBH4 decreases in comparison with other Ni composites. Zhang et al. [19] have studied the amorphous NieB catalysts prepared by complexing reduction route. They also have pointed out the influence of reaction conditions (such as the NaBH4 concentration and NaOH content) on the catalyst property for NaBH4 hydrolysis. However, there is limited study on NieB as catalysts for hydrogen generation from hydrolysis of NaBH4. Moreover, electroless plating is a chemical reduction process has been widely used in the preparation of functional coating on numerous substrate materials. It depends on the catalytic reduction of a metallic ion in an aqueous solution containing a reducing agent and the subsequent deposition of the metal or metallic alloy without the use of electrical energy. Nevertheless, the preparation of NieB catalysts by electroless plating method has not yet been systematically studied. Herein, we reported nanostructured NieB catalysts synthesized on the Cu sheet by electroless plating method at ambient temperature for the first time. The concentration of the reducing agent on the effect of the catalytic activity has been systematically investigated. The as-prepared NieB catalysts exhibit excellent catalytic activity and high stability for the hydrolysis of NaBH4.

was dipped in hot alkaline solution for 3 min, and then etched in Cu eroded solution for 3 min to remove the greasy dirt and other impurities. 2) The processed copper sheet piece was sensitized, activated and washed with distilled water and absolute ethyl. 3) The copper sheet piece was dried and then transferred to electroless plating bath for 5 min to form NieB plating catalysts. 4) The deposited NieB catalysts on the Cu sheet were taken out from the coating bath solution. 5) The asprepared catalysts were washed with redistilled water and absolute ethanol for three times and dried in vacuum atmosphere at 25  C for 24 h and weighted to determine the weight of the deposited NieB catalysts.

Experimental

Hydrogen generation testing

Preparation of NieB catalysts

For catalytic activity measurements, an alkaline-stabilized solution of 5 wt% NaBH4 was prepared by addition of 1 wt% NaOH (purchased from Sinopharm Chemical Reagent Co., Ltd, AR), which was placed into a three-necked round-bottom flask, and then a certain amount of NieB catalyst was added into the solution. The flask was immersed in a water bath to maintain the temperature without any stirring. The outlet tube exhaust was placed under an inverted, water-filled and graduated burette which was situated in a water-filled vessel for recording the H2 volume. The hydrogen generation test was carried out at 30  C and the volume of the released hydrogen was measured by the water displacement method [30]. The hydrogen generation rate was calculated on the basis of the amount of NieB catalyst, excluding the weight of the Cu sheet.

The nanostructured NieB catalysts were prepared on the Cu sheet by electroless plating at ambient temperature (25  C), in which 0.1 mol L
1 NiCl2$6H2O (purchased from Sinopharm Chemical Reagent Co., Ltd, AR) was used as the precursor, 0.6 mol L
1 NH2CH2COOH (purchased from Sinopharm Chemical Reagent Co., Ltd, AR) was used as the complexing agent and NaBH4 (Sigma Aldrich, 95% purity) was the reducing agent. In order to optimize the reaction conditions and catalytic activity, various concentrations of the reducing agent are applied, such as 0.04, 0.06, 0.08 and 0.10 g L
1. After the pH value adjusted to 13, electroless plating was performed for 5 min. The operation performed is as follows: 1) Copper sheet

Catalyst characterization The as-deposited NieB catalysts were characterized by X-ray diffraction (XRD, Rigaku-Dmax 2500, Cu Ka radiation, l ¼ 1.54178
A) and scanning electron microscopy (SEM, Hitachi S-4800) with energy-dispersive spectroscopy analysis (EDS). The surface roughness was determined by atomic force microscopy (AFM, Bruker Dimension icon). Surface electronic states and composition of the catalysts were studied by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD multi-technique electron spectrometer), and the corresponding spectra were processed and fitted by XPSPEAK41 software using Lorenzian-Gaussian curve profile and Shirley baseline. Inductively coupled plasma-optical emission spectroscopy (ICP-OES, ICP-9000, Thermo Jarrell-ASH Corp.) analysis was carried out to determine the chemical composition of the different NieB catalysts. According to the ASTM D3359
02 standard test method for measuring adhesion, tape test [29] was employed to evaluate the bonding strengths of the NieB plating to the substrates. The test was carried out by scratching 1 mm-wide parallel lines vertically and horizontally on the specimens with a special steel knife. Then the scratched specimen was adhered tightly with the tape for about 70 s. Finally, the tape was peeled rapidly in a direction parallel to the plating surface. When the plating is broken and peeled from the substrate, it can be deduced that the plating exhibits a poor bonding strength, whereas, the plating shows a good bonding strength when merely scratched marks are revealed on the surface.

Please cite this article in press as: Wang Y, et al., Hydrogen generation from hydrolysis of sodium borohydride using nanostructured NieB catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.258

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Results and discussion Effects of the concentrations of the reducing agent in NieB bath To determine the chemical composition of the different catalysts, the as-prepared NieB catalysts on Cu sheet catalysts with various reducing agent concentrations are characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES) technique. The results are shown in Table 1. It can be found that the weight ratio of B and Ni increases with increase of the concentrations from 0.04 to 0.08 g L
1. In contrast, the weight ratio of B and Ni decreases with further increasing the concentration to 0.10 g L
1. That is to say, when the reducing agent concentration is 0.08 g L
1, the B content is high. The results imply that the elemental chemical composition of the NieB catalyst is sensitive to the reducing agent concentration. In addition, according to the weight of the NieB/Cu sheet and the copper sheet prior to plating, the loading content of different NieB catalysts on the Cu sheets with various reducing agent concentrations from 0.04 to 0.10 g L
1 is determined to be 4.8 wt%, 3.8 wt%, 1.6 wt% and 6.0 wt%, respectively. Fig. 1 shows the SEM images of the NieB catalysts deposited on the Cu sheet from NieB bath with various concentrations of the reducing agent. As shown in Fig. 1, the morphologies of the as-prepared NieB catalysts display nanospheres with different size distribution. Obviously, when the concentration of reducing agent is 0.08 g L
1, the spherical particles of the NieB catalyst are relatively uniformly distributed on the surface and the size of the particles is smallest. The corresponding particle size distributions are presented in Fig. 2aed. It can be seen that the average size of the NieB sample is about 83.6 nm (Fig. 2a) with the reducing agent concentration of 0.04 g L
1. With the increase of the reducing agent concentration from 0.06 to 0.08 g L
1, the average size of the NieB sample increases (Fig. 2b, c). When the concentration of reducing agent is 0.08 g L
1, the average size reaches the minimum value of about 40.9 nm (Fig. 2c). Subsequently, further increasing the reducing agent concentration to 0.10 g L
1, the average size of the NieB sample drastically increases to 70.7 nm (Fig. 2d) and the bigger irregular spherical particles structure is visible (Fig. 1d). Therefore, according to the results of SEM images (Fig. 1aed) and the corresponding diagrams of particle size distributions (Fig. 2aed), it can be justified that the as-prepared NieB catalysts are nanostructured. In Fig. 2e we present the XRD patterns of the NieB catalysts deposited on the Cu sheet from NieB bath with various

Table 1 e Chemical composition of the different NieB catalysts by ICP-OES analysis. Concentration (g L
1) 0.04 0.06 0.08 0.10

Ni (wt%)

B (wt%)

Weight ratio (B/Ni)

Atomic ratio (Ni/B)

96.25 89.39 88.72 90.92

3.75 10.61 11.28 9.08

0.039 0.119 0.127 0.100

5.01 1.55 1.45 1.84

3

concentrations of the reducing agent. When the concentration of the reducing agent is 0.04 g L
1, the sharp diffraction peaks around 43.3 , 50.4 and 74.1 can be assigned to the Cu substrate (see JCPDS No. 04-0836). The three small peaks at 38.3 , 65.3 and 78.4 can be indexed as Ni3B (see JCPDS No. 19-0834). Meanwhile, the strong diffraction peak at 44.8 and weak peaks near 52.2 and 76.8 can be indexed as metallic Ni (see JCPDS No. 03-1051), corresponding to diffractions of Ni crystal face (111), (200), and (220), respectively. But it must be pointed out that the relative intensities of (200) and (220) diffraction lines for Ni phase of all the as-prepared catalysts are much lower than the standard value in JCPDS card. Karbasi et al. [31] have pointed out that the preferred orientation through an (hkl) plane can be selected by values of relative TC  25%. The texture coefficient (TC) of crystal faces can be calculated according to the following equation:
IðhklÞ I0ðhklÞ
 100% TCðhklÞ ¼ P IðhklÞ I0ðhklÞ

(2)

where I(hkl) and I0(hkl) are the diffraction intensities of the (hkl) plane measured for our Ni phase and the standard Ni powder sample from the JCPDS date, respectively. The calculated TC(hkl) values of the Ni phase are listed in Table 2. It can be seen that the value of TC(111) is much higher than that of TC(200) or TC(220), which indicates that (111) direction is the preferred orientation of Ni phase for the deposited NieB catalysts with various concentrations. Hence, the intensities diffraction of peaks near 52.2 and 76.8 seem to be very weak. The results are coincident with the results reported by Hang et al. [32] and Zhang et al. [33,34]. In addition, the average crystal size of the metallic Ni phase is about 41.8 nm calculated from the Scherrer's equation. When the concentration of the reducing agent increases from 0.06 to 0.08 g L
1, the peak intensity ascribed to the metallic Ni and Ni3B phase decreases gradually. The corresponding average crystal size of the metallic Ni phase has changed from 29.9 to 21.4 nm. Further increasing the concentration of the reducing agent to 0.10 g L
1, there is no further decrease of the peak intensity of metallic Ni and Ni3B phase and the average crystal size of the metallic Ni phase is 41.1 nm. These changing trends are consistent with the results of the particle size distributions (see Fig. 2aed). Fig. 2f shows the hydrogen generation kinetics from alkaline NaBH4 solution catalyzed by the NieB catalysts deposited with various concentrations of the reducing agent. It can be seen that the hydrogen generation rate increases with the increase of the reducing agent concentration from 0.04 to 0.08 g L
1, which demonstrates an enhancement of hydrolysis reaction by reducing agent. When the reducing agent concentration is 0.08 g L
1, the hydrogen generation rate reaches a maximum value of 4991.8 mL min
1 g
1. On the contrary, the hydrogen generation rate decreases when the reducing agent concentration further increases to 0.10 g L
1. Eom et al. [35] have reported that smaller particles of catalysts can exhibit a higher catalytic activity in alkaline NaBH4 solution. Thus, the smaller particles size (see Figs. 1c and 2c) or average crystal size (see Fig. 2e) achieved in NieB catalyst deposited at the reducing agent concentration of 0.08 g L
1 might be one of the reasons for the high catalytic activity of NieB catalyst.

Please cite this article in press as: Wang Y, et al., Hydrogen generation from hydrolysis of sodium borohydride using nanostructured NieB catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.258

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Fig. 1 e SEM images of the NieB catalysts deposited on the Cu sheet with various concentrations of the reducing agent: (a) 0.04, (b) 0.06, (c) 0.08 and (d) 0.10 g L¡1, respectively.

Fig. 2 e Particle size distributions (aed) and XRD diffraction patterns (e) for NieB catalysts deposited on the Cu sheet with various concentrations of the reducing agent: (a) 0.04, (b) 0.06, (c) 0.08 and (d) 0.10 g L¡1, respectively; (f) hydrogen generation kinetics from alkaline NaBH4 solution catalyzed by the as-prepared NieB catalysts.

Table 2 e Texture coefficients (TC) of different crystal faces of metallic Ni phase for the different NieB catalysts. Concentration (g L
1) 0.04 0.06 0.08 0.10

TC(111)

TC(200)

TC(220)

84.7% 83.7% 72.4% 81.8%

9.1% 10.2% 14.4% 12.2%

6.2% 6.1% 13.2% 6.0%

In order to further know whether Ni and B are existent and uniformly distributed on surface of the as-prepared NieB catalysts, elemental distribution of Ni and B is characterized by EDS mapping, as shown in Fig. 3(a1ea4) and (b1eb4), respectively. It can be seen that elements of Ni and B are observed clearly and uniformly distributed. Simultaneously, the elemental B in Fig. 3b3 is relatively more than that in Fig. 3(b1, b2, and b4). The results of chemical composition of the different NieB catalysts are listed in Table 3 by EDS analysis. It can be determined that the NieB catalyst exhibits higher B content with the reducing agent concentration of

Please cite this article in press as: Wang Y, et al., Hydrogen generation from hydrolysis of sodium borohydride using nanostructured NieB catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.258

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Fig. 3 e EDS mappings of Ni (a1ea4) and B (b1eb4) for the NieB catalysts deposited with reducing agent concentration of (a1, b1) 0.04, (a2, b2) 0.06, (a3, b3) 0.08, and (a4, b4) 0.10 g L¡1.

Table 3 e Chemical composition of the different NieB catalysts by EDS analysis. Concentration (g L
1) 0.04 0.06 0.08 0.10


1

Ni (wt%)

B (wt%)

Weight ratio (B/Ni)

Atomic ratio (Ni/B)

93.68 92.88 92.03 93.28

6.32 7.12 7.97 6.72

0.067 0.077 0.087 0.072

2.73 2.40 2.12 2.55

0.08 g L , which is consistent with the result of ICP-OES analysis. Hence, it indicates that the surface of the catalyst is enriched with B compared with the other concentration conditions. To deep characterize the NieB catalysts deposited on the Cu sheet with various concentrations of the reducing agent, it

is necessary to gain insight into the surface topographies. Fig. 4aed shows the atomic force microscopy (AFM) images in three dimensions. It can be seen that the surface topographies of the catalysts are fluctuated and composed of pyramid-like or mountain-like structures. In addition, the crests or troughs in between the pyramids or mountains are different with the change of the reducing agent concentration. In order to indicate the surface morphology change quantitatively, the average roughness (Ra) analysis of the catalysts is performed. Ra can be obtained from AFM images according to the Eq. (3): Ra ¼

N   1 X Zj  N j¼1

(3)

where Zj is the height value of the AFM topography image, and N is the number of points within the image. The curve of the average roughness versus the reducing agent concentration can be found in Fig. 4e. Obviously, the

Fig. 4 e AFM images of the NieB catalysts deposited on the Cu sheet with various concentrations of the reducing agent: (a) 0.04, (b) 0.06, (c) 0.08 and (d) 0.10 g L¡1, respectively; (e) the curve of the average roughness versus the reducing agent concentration. Please cite this article in press as: Wang Y, et al., Hydrogen generation from hydrolysis of sodium borohydride using nanostructured NieB catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.258

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surface roughness of the NieB catalyst is increased with the increase of the reducing agent concentration from 0.04 to 0.08 g L
1, and then decreases with further increasing the concentration to 0.10 g L
1. When the reducing agent concentration is 0.08 g L
1, the highest Ra is attained to be about 72.9 nm. The higher average roughness can lead to the a large number of defects, such as vacancies, dislocations, grain boundaries, phase boundaries and so on, which can provide more active sites on the surface of catalyst. All of these features are beneficial for catalytic hydrolysis reaction of NaBH4 [36].

Effect of hydrolysis solution temperature Activation energy (Ea) is an important parameter concerning the catalytic hydrolysis of NaBH4 solution. To determine the value of activation energy, the hydrolytic reactions are performed at different solution temperature. Fig. 5a shows the temperature-dependent hydrogen generation properties of the NieB catalyst (deposited with 0.08 g L
1 reducing agent) from 30 to 60  C. As expected, the hydrogen generation rate is increased with the increase of the solution temperature. On the basis of the slope of the fitted kinetics line and the gas state equation, the reaction rates (mL min
1 g
1) and rate constant k (mol min
1 g
1) at different temperatures T (K) can be obtained. The detailed information is summarized in Table 4. Arrhenius plot of ln k versus 1/T has been shown in Fig. 5b. According to the slope of the Arrhenius plot and the Arrhenius equation [37], the activation energy is calculated to be 36.3 kJ mol
1. Table 5 shows summarized dates of the activation energies of NaBH4 hydrolysis catalyzed by the other catalysts. Compared with the values of activation energies reported, it can be found that the activation energy of the asprepared NieB catalyst is lower than that of NieRu/50WX8 catalyst [8], CoeNieP catalyst [22], CoeNieP/PdeTiO2 catalyst [23], NieB catalyst [19], Ni0.75B0.25 catalyst [20], Ni67.4B32.6 catalyst [38] and NieB/Ni foam catalyst [28]. Consequently, the result illustrates that the as-prepared NieB catalyst exhibits high catalytic activity for hydrogen generation from hydrolysis of NaBH4.

Stability of the NieB catalysts The stability of the catalyst is essential for the practical application in the hydrogen generation system. In the cycling

Table 4 e Hydrogen generation properties at different solution temperatures for the hydrolysis of NaBH4 catalyzed by NieB catalyst deposited on the Cu sheet with 0.08 g L¡1 reducing agent. T ( C) r (mL min
1 g
1) k (mol min
1 g
1) 1/T (1/K) 30 40 50 60

4991.8 7583.5 11827.4 20313.8

0.200 0.294 0.444 0.740

0.00330 0.00319 0.00310 0.00300

lnk
1.609
1.223
0.810
0.300

Table 5 e Values of activation energies for hydrolysis of NaBH4 catalyzed by various catalysts. Catalyst NieRu/50WX8 CoeNieP CoeNieP/PdeTiO2 NieB Ni0.75B0.25 Ni67.4B32.6 NieB/Ni foam NieB

Ea (kJ mol
1)

Ref.

52.7 38 57.0 64.90 43.19 45 61.8 36.3

[8] [22] [23] [19] [20] [38] [28] This work

test, the catalyst is separated from the solution after the full hydrolysis reaction, washed with distilled water and absolute ethyl alcohol, dried in atmosphere for 24 h and then weighed and reserved to reuse. The weight of NieB catalyst remaining on Cu sheet following each cycle is plotted in Fig. 6. As shown in Fig. 6, the weight of NieB catalyst decreases by 5.4% after 5 cycles. Fig. 7a shows the SEM image of the NieB catalyst deposited with 0.08 g L
1 reducing agent after 5 cycles. Compared with the SEM image before cycling test, it can be found that the particles become agglomerates and large. Although the agglomeration of particles lowers the effective surface area and causes the degeneration of the catalytic activity, the active materials still load on the supporting Cu sheet, according to the histogram of hydrogen generation rate versus number of cycles for the catalyst (Fig. 7b). It can be observed that the hydrogen generation rate still reaches 4017.3 mL min
1 g
1 after 5 cycles, which demonstrates that the catalyst deactivates slightly after the hydrolysis reaction. However, the catalytic activity retains about 80.5% of its initial value after 5 cycles. In contrast, recently, there are other

Fig. 5 e (a) Effect of solution temperature on the hydrogen generation from NaBH4 solution catalyzed by NieB catalyst deposited on the Cu sheet with 0.08 g L¡1 reducing agent, and (b) the corresponding Arrhenius plot of lnk versus 1/T. Please cite this article in press as: Wang Y, et al., Hydrogen generation from hydrolysis of sodium borohydride using nanostructured NieB catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.258

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Fig. 6 e Curves of number of cycles vs the weight of the NieB catalyst.

catalysts reported about the stability in the literature: The reported Co/Ni catalyst loses most of its initial activity after 3 cycles [39]; CoeP catalyst retains 60% of the initial activity after 5 cycles [22]; CoeWeP/Cu sheet catalyst loses 49% of its

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initial activity [16]; and the activity of Co/PCM catalyst loses around 32% after 2 cycles [40]. Therefore, the result of this work illustrates that the as-prepared NieB catalyst is relatively stable in hydrolysis of NaBH4. To evaluate the bonding strength of the NieB plating to the Cu substrate, tape test was employed according to the ASTM D3359
02 standard test method for measuring adhesion [29]. Fig. 8 shows the optical images of bonding strength of the NieB plating to substrate before cycling and after 5 cycles. As shown in Fig. 8, whether it is before cycling or after 5 cycles, merely scratched marks can be observed on the surface of cross-cut area of the NieB plating. The edges of cuts are completely smooth and none of the squares of the lattice is detached, which means that the NieB plating shows a good bonding strength. Hence, it can be concluded that the decrease of the weight of NieB catalyst may be not caused by the bonding strength. For the as-prepared NieB catalyst (deposited with 0.08 g L
1 reducing agent) before cycling and after 5 cycles, XPS spectra of the Ni 2p 3/2 and B 1s level photoemission signals are presented in Fig. 9. As shown in Fig. 9a, the peaks located at the binding energies of 852.2 and 855.7 eV before cycling can be assigned to metallic Ni (Ni0) and oxidized Ni (Ni(OH)2), respectively [41e43]. The peaks due to oxidized Ni might be

Fig. 7 e (a) SEM image of the NieB catalyst deposited with 0.08 g L¡1 reducing agent after 5 cycles, and (b) the histogram of hydrogen generation rate versus number of cycles.

Fig. 8 e Optical images showing bonding strength of the NieB plating to substrate: (a) before cycling, and (b) after 5 cycles, respectively. Please cite this article in press as: Wang Y, et al., Hydrogen generation from hydrolysis of sodium borohydride using nanostructured NieB catalysts, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.258

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Fig. 9 e XPS spectra of (a) Ni 2p 3/2 and (b) B 1s level for the as-prepared NieB catalyst (deposited with 0.08 g L¡1 reducing agent) before cycling and after 5 cycles, respectively.

Table 6 e Atomic ratios of Ni and B determined by XPS before cycling and after 5 cycles.

Before cycling After 5 cycles

Ni (at.%)

B (at.%)

Atomic ratio (Ni/B)

59.29 68.58

40.71 31.42

1.46 2.18

formed during the catalyst preparation reaction between NaBH4 and metal salts [44]. In the B 1s level (Fig. 9b) before cycling, the peak at the binding energy of 187.8 eV should be assigned to the elemental boron, and the peak at 191.7 eV can be assigned oxidized boron (BO
2 ) [25,26,42,45]. After 5 cycles, compared with the Ni 2p 3/2 spectra before cycling, it can be observed that the intensities of peaks signifying metallic Ni and oxidized Ni decrease obviously, and the intensities of peaks representing the elemental boron and oxidized boron are also weakened. However, whether it is before cycling or after 5 cycles, it can be found that a positive shift for elemental boron is evident comparing with pure boron (187.1 eV), which indicates that partial electrons transferred from elemental boron to the metal Ni in the NieB catalyst [46]. Li et al. [25] have reported that the increase of the boron content on surface would contribute more electrons to metallic Ni in their corresponding alloys. That is to say, high boron contents on the surface of the NieB catalyst are advantageous for enhancing the catalytic activity. This proves that in NieB catalyst before cycling, Ni may exchange electrons more easily than the NieB catalyst after 5 cycles, which could be connected to the B-enrichment in NieB catalyst before cycling as seen by the compositional analysis of XPS spectra (Table 6). Thus, the result illustrates that the reason for the slight decrease of catalytic activity is due to the decrease of boron contents after 5 cycles based on the preceding XPS spectra analysis. Moreover, it's worth noting that the decrease of the intensity of Ni 2p 3/2 and B 1s may be related with the leaching of the catalyst during the reaction (see Fig. 6).

Conclusions In summary, the nanostructured NieB were synthesized on the Cu sheet by electroless plating at ambient temperature

and the catalytic performance for hydrogen generation from hydrolysis of NaBH4 solution is studied. The concentration of the reducing agent was shown to have a significant influence on the hydrogen generation performance. Compared with the as-prepared NieB catalysts, the NieB catalyst deposited with 0.08 g L
1 reducing agent showed excellent catalytic activity for the hydrolysis of NaBH4, which may be due to the smaller particle or crystal size and the higher surface roughness. The highest hydrogen generation rate could reach 4991.8 mL min
1 g
1. The lower activation energy was 36.3 kJ mol
1. The NieB catalyst also displayed high stability and the catalytic activity could retain about 80.5% of its initial value after 5 cycles.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (51501118), the General Scientific Research Project of Education Office of Liaoning Province (L2015500), School-based Program of Shenyang Normal University (L201503) and Science and Technology Project of Shenyang (F16-205-1-17).

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