Hydrogen generation from alkaline NaBH4 solution using nanostructured Co–Ni–P catalysts

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Hydrogen generation from alkaline NaBH4 solution using nanostructured CoeNieP catalysts Yan Wang a,b,*, Guode Li c,**, Shiwei Wu c, Yongsheng Wei d, Wei Meng a, Yuan Xie a, Ying Cui a, Xin Lian a, Yongge Chen a, Xinyu Zhang a 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 Hydrogen Technology Research Center, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, PR China

article info

abstract

Article history:

In the present study, nanostructured CoeNieP catalysts have been successfully prepared

Received 16 March 2017

on Cu sheet by electroless plating method. The morphologies of CoeNieP catalysts are

Received in revised form

composed of football-like, granular, mockstrawberry-like and shuttle-like shapes by tuning

2 May 2017

the depositional pH value. The as-deposited mockstrawberry-like CoeNieP catalyst ex-

Accepted 3 May 2017

hibits an enhanced catalytic activity in the hydrolysis of NaBH4 solution. The hydrogen

Available online xxx

generation rate and activation energy are 2172.4 mL min
1 g
1 and 53.5 kJ mol
1, respectively. It can be inferred that the activity of catalysts is the result of the synergistic effects

Keywords:

of the surface roughness, the particle size and microscopic architectures. Furthermore, the

Sodium borohydride

stability of mockstrawberry-like CoeNieP catalyst has been discussed, and the hydrogen

Electroless plating

generation rate remains about 81.4% of the initial value after 5 cycles.

CoeNieP catalyst

© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Hydrogen generation

Introduction It is well known that hydrogen is considered to be a much promising fuel source due to its high calorific value and zero pollutant emission. For proton exchange membrane fuel cell (PEMFC), hydrogen can produce energy in the form of the anodic fuel, giving water as the only by-product [1]. Nevertheless, the broad application of hydrogen is severely limited

by the lack of the convenient, efficient hydrogen generation and storage methods. Recently, hydrogen storage materials with the high capacities, including ammonia borane and alkaline or alkaline earth metal hydrides (e. g. sodium borohydride, lithium borohydride, and lithium aluminium hydride), have been developed. Among these chemical hydrides, sodium borohydride (NaBH4) has received great interest in the PEMFC because of its many advantages, such as a high theoretical hydrogen capacity (10.8 wt%), a low price and a high

* Corresponding author. Institute of Catalysis for Energy and Environment, College of Chemistry and Chemical Engineering, Shenyang Normal University, Shenyang 110034, PR China. Fax: þ86 24 86593072. ** Corresponding author. Experimental Center, Shenyang Normal University, Shenyang 110034, PR China. Fax: þ86 24 86505871. E-mail addresses: [email protected] (Y. Wang), [email protected] (G. Li). http://dx.doi.org/10.1016/j.ijhydene.2017.05.034 0360-3199/© 2017 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 alkaline NaBH4 solution using nanostructured CoeNieP catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.034

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stability in alkaline solution [1e3]. Moreover, the stored hydrogen can be released by the presence of catalysts for hydrolysis of the alkaline NaBH4 solution at room temperature according to the following process: NaBH4 þ 2H2 O/NaBO2 þ 4H2

(1)

Nowadays, various catalysts have been prepared and verified to be able to accelerate the hydrogen generation rate for the above-mentioned NaBH4 hydrolysis process, including noble metal-based catalysts (e.g. Pd [4], Ru [5,6], PtRu/LiCoO2 [7], Pt/Co3O4 [8], Pt/Ni [9,10]), and non noble metal-based catalysts (e.g. Ni or Cu [11], Co [12e15], CoeP [16], CoeB [17,18], CueB [17], CoeNieB [19], NieB [20], CoeNieP [21]). Although noble metal-based catalysts have exhibited remarkable catalytic activity, high cost and limited abundances of these noble metals have hindered their wide applications. For these reasons, non noble metal-based catalysts have been gradually developed and showed promising catalytic activities, especially in the aspect of Co- and Ni-based catalysts. Moreover, the activities can be further improved by doping P or B to transition metals (e.g. Co or Ni). The corresponding catalysts can be prepared in powder or coating form. However, powder catalysts have some disadvantages such as difficult to separate and easy to aggregate. In contrast, catalysts prepared in the coating form (such as CoeP/Cu sheet [22], CoeB/Cu sheet [23], CoeNieB/Cu sheet [19], CoeP/Ni foam [24] and CoeNieP/ Cu foam [25]) can avoid above-mentioned problems, be easily separated, collected and recycled. Meanwhile, the hydrogen generation rate for NaBH4 hydrolysis catalyzed by coating catalysts can be controlled by changing the contact area. In this paper, nanostructured CoeNieP catalysts have been successfully prepared on Cu sheet by electroless plating method, which has been characterized by XRD, SEM, EDS, AFM and XPS technique. Compared with those common Coebased coating catalysts, our prepared CoeNieP catalysts show novel morphologies, including football-like, granular, mockstrawberry-like and shuttle-like shapes. These nanostructured CoeNieP materials have not been reported as catalysts for NaBH4 hydrolysis. What is more important is that the mockstrawberry-like CoeNieP catalyst exhibits a relatively enhanced catalytic activity and stability for NaBH4 hydrolysis reaction in contrast to the previous noble metal catalysts [26].

Experimental Material preparation Copper (Cu) sheet with an exposed surface area of 24 cm2 was chosen as the substrate for CoeNieP thin film catalysts. Cobalt chloride hexahydrate (CoCl2$6H2O, AR) and Nickel chloride hexahydrate (NiCl2$6H2O, AR) were used as the precursor for the catalyst preparation. Glycine (NH2CH2COOH, AR) was used as the complexing agent. Sodium hypophosphite (NaH2PO2, AR) was used as the reducing agent. NaBH4 (95%, Sigma Aldrich) and sodium hydroxide (NaOH, AR) were used for the hydrolysis process. Ethanol absolute (CH3CH2OH, AR)

and deionized water were also used. All of the chemicals except NaBH4 were purchased from Sinopharm Chemical Reagent Co., Ltd. The CoeNieP catalysts were prepared by electroless plating method. Before electroless plating, the pretreatment process of Cu sheet is as follows: 1) The Cu sheet was dipped in hot alkaline solution for 3 min. 2) The obtained Cu sheet was directly etched in Cu eroded solution for 3 min. 3) The Cu sheet removed the greasy dirt and other impurities was sensitized in the SnCl2/HCl system for 3 min. 4) The sensitized Cu sheet was activated in the PdCl2/HCl system for 2 min. 5) The activated Cu sheet was washed three times with deionized water and ethanol absolute. Subsequently, 0.1 mol L
1 CoCl2$6H2O, 0.1 mol L
1 NiCl2$6H2O and 0.6 mol L
1 NH2CH2COOH were dissolved in deionized water under stirring and 0.8 mol L
1 NaH2PO2 was added to the solution. The pH value of the solution was adjusted to 11.0, 11.5, 12.0 and 12.5 with NaOH. The solution was heated and maintained to 78  C. Then, the pretreated Cu sheet was transferred to the solution for 10 min to obtain the deposited CoeNieP catalyst and taken out from the solution. Finally, the Cu sheet was washed with deionized water and ethanol absolute and dried in vacuum atmosphere at 25  C for 24 h. To determine the weight of the as-obtained CoeNieP catalyst, the Cu sheet was weighted before and after electroless plating.

Material characterization Powder X-ray diffraction (XRD) patterns were taken on a RigakueDmax 2500 X-ray diffractometer with Cu Ka radiation (l ¼ 1.54178
A). Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were characterized on Hitachi Se4800 scanning electron microscope equipped with an energy dispersive spectroscopy analysis system. Atomic force microscopy (AFM) was obtained by using Bruker Dimension Icon. X-ray photoelectron spectroscopy (XPS) were studied by Kratos Axis Ultra DLD multi-technique electron spectrometer.

Catalyst activity measurement An alkaline solution of NaBH4 (5 wt %) and NaOH (1 wt %) was dissolved in 10 mL of water in a three-necked round-bottom flask, which was immersed in a 30  C water bath to maintain the temperature with no stirring. Then a certain amount of CoeNieP catalyst was added to this NaBH4 solution. The volume of the hydrogen gas generated was measured by the water displacement method using an inverted, water-filled and graduated burette [27]. Based on the weight of CoeNieP catalyst (m) except the Cu sheet, the hydrogen generation rate (r) was determined from the slope of the plot of the generated hydrogen volume (V) versus time (min) in the linear region according to the following equation: rðthe hydrogen generation rate; ¼

tðtime;

mL$min
1 $ g
1 Þ

Vðthe generated hydrogen; mLÞ minÞ  mðthe weight of Co
Ni
P catalyst;

(2) gÞ

Please cite this article in press as: Wang Y, et al., Hydrogen generation from alkaline NaBH4 solution using nanostructured CoeNieP catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.034

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Fig. 1 e SEM images of CoeNieP catalysts deposited at different pH value: (a, e) 11.0, (b, f) 11.5, (c, g) 12.0 and (d, h) 12.5; (i) the simple schematic diagram of the morphology evolution for CoeNieP catalysts deposited at different pH value.

Results and discussion Catalyst characterization The surface morphologies of as-prepared CoeNieP catalysts on the Cu sheet at different pH value have been shown in Fig. 1(a-h), including the low-magnification (Fig. 1(a-d)) and high-magnification (Fig. 1(e-h)) SEM images. It can be found that the catalyst shows a football-like morphology at the pH value of 11.0 as demonstrated in Fig. 1(a, e). With the increase of pH value to 11.5, the surface morphologies of catalysts change into granular shape and lots of particles get together and display irregular morphology in Fig. 1(b, f). When the pH value is increased to 12.0, mockstrawberry-like CoeNieP catalysts made up of lots of particles can be observed in Fig. 1(c, g) and the diameter is about 50 nm. The novel mockstrawberrylike nanostructure will contribute to providing more active sites on the catalyst surface and producing high specific surface area [28,29], which is beneficial to enhance the catalytic activity. However, further increasing the pH value to 12.5, the

shuttle-like nanostructure is appeared in Fig. 1(d, h) and the size of particles is obviously larger than that prepared at pH value of 12.0. A simple schematic diagram of the morphology evolution is represented in Fig. 1(i) with different pH value. Fig. 2(a) shows EDS spectra of the mockstrawberry-like CoeNieP catalysts, and Co, Ni and P can be detected clearly, which can also be verified by the EDS mappings in Fig. 2(b, c and d), respectively. It can be seen that elements of Co, Ni and P are observed to be distributed on the catalyst surface. Fig. 3 shows the XRD patterns of CoeNieP catalysts deposited at different pH value. From the XRD patterns, it can be found that the peaks corresponding to the Cu substrate (JCPDS No. 65e9026) at 2q ¼ 43.3 , 50.5 and 74.2 are obvious at different pH value from 11.0 to 12.5. Additionally, a closedpacked hexagonal Co phase are detected for the CoeNieP catalysts at 2q ¼ 41.7 and 44.8 . It should be noted that the intensity of the diffraction peaks ascribed to the (100) and (002) plane of Co phase increases gradually at the increase of pH value from 11.0 to 12.0, then the intensity reduces after increasing the pH value to 12.5. Meanwhile, the small peaks around 38.4 , 76.1 and 78.1 are assigned to Ni12P5 phase for

Fig. 2 e EDS spectra (a) and EDS mappings of elements of Co (b), Ni (c) and P (d) for the mockstrawberryelike CoeNieP catalyst, respectively. Please cite this article in press as: Wang Y, et al., Hydrogen generation from alkaline NaBH4 solution using nanostructured CoeNieP catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.034

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the CoeNieP catalyst deposited at the pH value of 12.0. Moreover, the pattern of detectable Co3O4 phase presents the small peaks at 2q ¼ 31.3 and 65.2 , which may be attributed to the surface of Co phase oxidized by oxygen in the atmosphere during the catalyst preparation and storage process. In order to further characterize the CoeNieP catalysts deposited at different pH value, the atomic force microscopy (AFM) has been shown in three dimensions in Fig. 4. It can be seen that the different CoeNieP catalysts display different surface topographies, including the mountain-like structures or pyramid-like structures. The position and height of crests or troughs in between the pyramids or mountains are different with the change of the pH value. On the basis of AFM images, the average roughness (Ra) for the as-prepared CoeNieP catalysts can be obtained according to the following equation: Ra ¼ Fig. 3 e XRD patterns of CoeNieP catalysts deposited at different pH value.

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 average

Fig. 4 e AFM images in three dimensions of CoeNieP catalysts deposited at different pH value: (a) 11.0, (b) 11.5, (c) 12.0 and (d) 12.5; According twoedimensional AFM images at the pH value of (e) 12.0 and (f) 12.5, respectively. Please cite this article in press as: Wang Y, et al., Hydrogen generation from alkaline NaBH4 solution using nanostructured CoeNieP catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.034

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the pH value of 12.0. The reason may be the novel mockstrawberryelike nanostructure made up of lots of particles based on the SEM images (Fig. 1(c, g)) and the smaller average particle size according to the AFM image in two dimensions (Fig. 4(e)), providing high specific surface area [28,29]. Moreover, according to the previous report, the larger surface roughness on the catalyst surface is beneficial for catalytic reaction [30]. That is to say, the hydrogen generation rate should be highest because of the highest surface roughness of 64.6, when the pH value is 12.5 based on the result of AFM analysis. However, this result is not inconsistent with the veritable experimental results provided by Fig. 5. Thus, it may be inferred that the surface roughness is not the only one factor to decide the activity of the CoeNieP catalysts. Comprehensive analysis shows that the catalytic property is the result of the synergistic effects of the surface roughness, the particle size and microscopic architectures. Fig. 5 e Catalytic activity of the bare Cu sheet and the CoeNieP catalysts deposited at different pH value from 11.0 to 12.5 for the hydrolysis of NaBH4 solution. roughness of the CoeNieP catalysts is 41.4, 44.5, 58.5 and 64.6, corresponding to the pH value of 11.0, 11.5, 12.0 and 12.5, respectively. It can be illustrated that the average roughness increases from 41.4 to 64.6 with the increase of the pH value from 11.0 to 12.5. Fig. 4e and f shows the AFM topography image in two dimensions for the CoeNieP catalysts deposited at pH value of 12.0 and 12.5, respectively. Obviously, the particle size of catalysts deposited at pH value of 12.0 is smaller than that of the catalysts deposited at pH value of 12.5.

Effect of pH value in CoeNieP bath Fig. 5 gives out the effect of bare Cu sheet and pH value of plating bath on the catalytic performance for the hydrolysis of NaBH4 catalyzed by the as-prepared CoeNieP catalysts. It can be found that bare Cu plate has no any catalytic activity for hydrolysis of NaBH4. The hydrogen generation rate increases gradually with the pH value of plating bath increasing from 11.0 to 12.0. Nevertheless, further increase in the pH value to 12.5, the hydrogen generation rate decreases. A maximum hydrogen generation rate of 2172.4 mL min
1 g
1 is reached at

Effect of solution temperature Fig. 6a presents the effect of hydrolysis temperature on the hydrogen generation rate from the NaBH4 solution catalyzed by the mockstrawberryelike CoeNieP catalysts. The hydrolytic reactions have been carried out at 25, 30, 35 and 40  C, respectively. It is quite clear that the hydrogen generation rate is significantly increased with the increase of the hydrolysis temperature. The activation energy (Ea) for the hydrolytic reaction of NaBH4 solution can be estimated according to the following equation (i.e., Arrhenius equation): lnk ¼ lnA


Ea 1 $ R T

(4)

where k is rate constant at various temperatures, A is the preeexponential factor, R is the gas constant, T is the absolute temperature of the hydrolytic reaction. The Arrhenius plot of lnk versus 1/T is drawn using Eq. (4) and given in Fig. 6b. Based on the fitted result from the Arrhenius slope, Ea for the hydrolysis of NaBH4 reaction catalyzed by the mockstrawberryelike CoeNieP catalysts is calculated to be 53.5 kJ mol
1. H2 generation rate and Ea value for hydrolysis of NaBH4 catalyzed by the as-prepared CoeNieP catalyst and other previously reported catalysts are summarized in Table 2. It can be found that the Ea value in the present work is

Fig. 6 e (a) Effect of hydrolysis temperature on the hydrogen generation rate from the NaBH4 solution catalyzed by the mockstrawberryelike CoeNieP catalyst; (b) the corresponding Arrhenius plot of ln k versus 1/T. Please cite this article in press as: Wang Y, et al., Hydrogen generation from alkaline NaBH4 solution using nanostructured CoeNieP catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.034

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Table 1 e Catalyst systems, synthetic temperature, H2 generation rate and Ea value for hydrolysis of NaBH4 catalyzed by previously reported catalysts in the literature. Catalyst CoeWeP/Cu sheet CoeNiePeB CoeNieP/Cu sheet Co/C CoeB CoeP/Cu foam CoeCueB NieCoeP/geAl2O3 CoeNieMoeP/g eAl2O3 NieRu/50WX8 CoeNieP/Cu sheet CoeMneB/Ni foam Co@AHs RueIRAe400 CoeNieP/PdeTiO2 CoeP/Cu sheet NieCoeB CoeB Ru/LiCoO2 RueSZ

Synthetic method

Synthetic temperature  ( C)

H2 generation rate (mL min
1 g
1)

Ea (kJ mol
1)

Electrodeposition Chemical reduction Electroplating Impregnationechemical reduction Chemical reduction Electroless plating Chemical reduction Electroless plating Electroless deposition

27 300 25 200e500 60 50 400 85 90

5000 e 3636 e 1640 1490 1280 e 10125

22.8 29 38 44.1 44.5 46.8 49.6 52.1 52.4

[16] [38] [25] [39] [44] [45] [40] [41] [42]

Electroless deposition Electroless plating Electroplating Chemical reduction Incipient wetness Electroless deposition Electroless plating Chemical reduction Chemical reduction Microwaveeassisted polyol process Solegel method

e 78 40 25 50 e 60 575 120 100 625

400 2172.4 e e 606 460 3330 2608 2750 e 9100

52.7 53.5 55 55.6 56.0 57.0 60.2 62 64.9 68 76

[43] This work [31] [32] [36] [21] [33] [34] [35] [37] [26]

Table 2 e Atomic ratios of Co, Ni and P determined by XPS.

Before cycling After 5 cycles

Co (at. %)

Ni (at. %)

P (at. %)

93.04 86.54

5.06 12.2

1.90 1.26

lower than that of reported catalysts [21,31e35], even some noble metalebased catalysts [26,36,37], but still higher than that of some other catalysts [16,25,38e45]. The H2 generation rate of the as-prepared CoeNieP catalyst is obviously higher than that of some Coebased catalysts [21,40,44,45], NieRu/ 50WX8 [43] and RueIRAe400 [36], except for CoeNieMoeP/ geAl2O3 [42], CoeWeP/Cu sheet [16], CoeP/Cu sheet [33] and RueSZ [26] catalysts. Hence, to some extent, the results indicated that the as-obtained CoeNieP catalyst in this work shows an enhanced catalytic activity for hydrogen generation from hydrolysis of NaBH4.

Ref.

Stability of the CoeNieP catalyst For the practical application of PEMFC, it is very important to enhance the stability and reusability of aseprepared catalyst system during the hydrogen generation process. In the recycling experiments, the used catalyst is taken out from the hydrolysis solution after the end of hydrogen generation, washed with deionized water and ethanol absolute, dried in a vacuum, weighed, and then reused to catalyze the following hydrolysis reaction. Fig. 7a displays the curve of number of cycles versus the weight of the mockstrawberryelike CoeNieP catalyst remained on Cu sheet after each cycle. As shown in Fig. 7a, it can be seen that the weight of CoeNieP catalyst reduces by 6.6% after 5 cycles. Fig. 7b shows the attenuation situation of the catalytic activity for the mockstrawberryelike CoeNieP catalyst after multiecycle tests. Obviously, the hydrogen generation rate is slightly attenuated after 5 cycles. This phenomenon may be caused by morphology changes and the decrease of active sites in the catalyst [46e48]. Fig. 7c shows the SEM images of the

Fig. 7 e (a) Curve of number of cycles versus the weight of the mockstrawberryelike CoeNieP catalyst; (b) the corresponding recycling experiments in the hydrolysis of NaBH4; (c) SEM image of the catalyst after 5 cycles. Please cite this article in press as: Wang Y, et al., Hydrogen generation from alkaline NaBH4 solution using nanostructured CoeNieP catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.034

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Fig. 8 e XPS spectra of (a) Co 2p3/2, (b) Ni 2p3/2 and (c) P 2p level for the aseprepared mockstrawberryelike CoeNieP catalyst before cycling and after 5 cycles, respectively.

mockstrawberryelike CoeNieP catalyst after 5 cycles. The SEM images confirms that the surface morphology is different between before cycling (Fig. 1c, g) and after 5 cycles. The mockstrawberryelike architectures almost disappear and the boundary of the surface seems even a bit fuzzy after 5 cycles. Although this phenomenon might decrease the active sites or surface area of the catalyst, thereby giving rise to the attenuation of the hydrogen generation rate, the active components still load on the surface of Cu sheet substrate according to the dates of the recycling experiments in Fig. 7b. It can be found that about 81.4% of the initial hydrogen generation rate remains constant after 5 cycles. For comparing, the stability and reusability of other catalysts reported in the literature have been presented as follow: The catalytic activity for electrodeposited CoeWeP/Cu sheet catalyst retains 51% of its initial value after 5 cycles [16]; The catalytic activity for the electrodeposited Co/PCM catalyst retains 68% of reactivity after 2 cycles [47]; The catalytic activity for the Co/Ni deactivates rapidly after 3 cycles [48]; The catalytic activity for the electrodeposited CoeP/Cu catalyst sheet decreased by 32% after the first cycle [22]. Hence, it is worth noting that the obtained mockstrawberryelike CoeNieP catalyst is relatively stable for the NaBH4 hydrolysis. Table 1. Fig. 8 illustrates the XPS spectra of Co 2p 3/2, Ni 2p 3/2 and P 2p level photoemission signals of the mockstrawberryelike CoeNieP catalyst before cycling and after 5 cycles. As shown in Fig. 8a, the XPS peak located at the binding energy of 778.1 eV before cycling can be assigned to metallic Co (Co0) [49], The peaks located at 780.9 eV and 786.1 eV stand for oxidized Co (Co2þ, CoO) and oxidized Co (Co3þ, Co2O3), respectively [24], which might be caused by combining Co with oxygen in atmospheres during the preparation or storage process of the CoeNieP catalysts. The aboveementioned results are in good agreement with the XRD results seen from Fig. 3. Moreover, the Ni 2p 3/2 spectra in Fig. 8b shows two peak at 856.0 and 860.9 eV before cycling, which is assigned to oxidized Ni. In the P 2p level (Fig. 8c), only one peak at 132.9 eV representing oxidized P (P5þ) is observed before cycling. As reported by Huang et al. [50], it can be concluded that the formation of nickel phosphate (Ni12P5) on the surface observed in XRD pattern in Fig. 2a is correlated with the peaks at 856.0 and 860.9 eV in the Ni 2p3/2 spectra and the peak at 132.9 eV in the P 2p spectra. After 5 cycles, the peaks signifying metallic

Co (Co 0), oxidized Co (Co2þ, Co3þ) and oxidized Ni decrease obviously in amplitude for Co 2p 3/2 and Ni 2p 3/2 spectra. Meanwhile, the peak attributed to oxidized P (P5þ) also decreases significantly after 5 cycles. It has been reported that P is beneficial to create a large number of active sites of metallic Co for catalytic hydrolysis reaction [51,52]. Therefore, it can be inferred from the present results that the slight attenuation of the catalytic activity after 5 cycles is due to the decrease of the amount of the P, which is proved by the compositional analysis of XPS spectra (Table 2). In addition, the decrease of oxidized Ni and P representing the formation of Ni12P5 may be another reason, because Ni12P5 is considered as to be an efficient catalyst for hydrogen generation [50].

Conclusions In this work, CoeNieP catalysts with different morphologies have been successfully prepared on Cu sheet by electroless plating method, including footballelike, granular, mockstrawberryelike and shuttleelike shapes. During the catalytic hydrolysis of NaBH4 solution, the asedeposited mockstrawberryelike CoeNieP catalyst exhibits an enhanced catalytic activity. The hydrogen generation rate is 2172.4 mL min
1 g
1. The activation energy is calculated to be 53.5 kJ mol
1. The result shows that the activity of catalysts is the result of the synergistic effects of the surface roughness, the particle size and microscopy. After 5 cycles, the catalytic activity could retain about 81.4% of the initial value. Compared with the reported catalyst, our obtained mockstrawberryelike CoeNieP catalyst is relatively stable for the NaBH4 hydrolysis.

Acknowledgments This research was supported by the fund of the National Natural Science Foundation of China (51501118, 21606115), Natural Science Foundation (20170540814) of Liaoning Province, the General Scientific Research Project of Education Office of Liaoning Province (L2015500) and the Science and Technology Project of Shenyang (F16e205e1e17).

Please cite this article in press as: Wang Y, et al., Hydrogen generation from alkaline NaBH4 solution using nanostructured CoeNieP catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.034

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Please cite this article in press as: Wang Y, et al., Hydrogen generation from alkaline NaBH4 solution using nanostructured CoeNieP catalysts, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.034