Novel Ni–Co–B hollow nanospheres promote hydrogen generation from the hydrolysis of sodium borohydride

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 8 ) 1 e1 0

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/he

Novel NieCoeB hollow nanospheres promote hydrogen generation from the hydrolysis of sodium borohydride Jie Guo, Yongjiang Hou*, Bo Li, Yulei Liu School of Environmental Science and Engineering, Hebei University of Science and Technology, Shi Jiazhuang, 050018, China

article info

abstract

Article history:

NieCoeB hollow nanospheres were synthesized by the galvanic replacement reaction

Received 11 February 2018

using a CoeB amorphous alloy and a NiCl2 solution as the template and additional reagent,

Received in revised form

respectively. The NieCoeB hollow nanospheres that were synthesized in 60 min (NieCoe

15 June 2018

B-60) showed the best catalytic activity at 303 K, with a hydrogen production rate of 6400

Accepted 19 June 2018

mLhydrogenmin
1g
1 catalyst and activation energy of 33.1 kJ/mol for the NaBH4 hydrolysis re-

Available online xxx

action. The high catalytic activity was attributed to the high surface area of the hollow structure and the electronic effect. The transfer of an electron from B to Co resulted in

Keywords:

higher electron density at Co sites. It was also found that Ni was dispersed on the CoeB

NieCoeB hollow nanospheres

alloy surface as result of the galvanic replacement reaction. This, in turn, facilitated an

Galvanic replacement reaction

efficient hydrolysis reaction to enhance the hydrogen production rate. The parameters that

NaBH4

influenced the hydrolysis of NaBH4 over NieCoeB hollow nanospheres (e.g., NaOH con-

Hydrogen generation

centration, reaction temperature, and catalyst loading) were investigated. The reusability test results show that the catalyst is active, even after the fifth run. Thus, the NieCoeB hollow nanospheres are a practical material for the generation of hydrogen from chemical hydrides. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Hydrogen is universally regarded as a clean energy source. It requires only oxygen for combustion, and water is the only product [1,2]. When hydrogen fuel is used in engines, it can reduce the amount of greenhouse gases, smog, and acid rain. In practical applications, however, the storage and transportation of hydrogen fuel remain major issues. Since the early 2000s, various strategies for hydrogen storage and transport have been investigated. One strategy is the

exploration of chemical hydrogen-storage materials, such as NaBH4, KBH4 and NH3BH3 [3e6]. These materials have large storage capacities for hydrogen and release pure hydrogen gas via hydrolysis reactions in the presence of catalysts at room temperature. Among various chemical hydrides, NaBH4 is considered the most promising hydrogen storage material because it produces clean H2 gas, at a very high rate at room temperature, and nontoxic hydrolysis byproducts. It produces 4 mol of H2 in the presence of a catalyst, as shown in equation (1) [7e9], and half of the produced hydrogen originates from water.

* Corresponding author. E-mail address: [email protected] (Y. Hou). https://doi.org/10.1016/j.ijhydene.2018.06.117 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Guo J, et al., Novel NieCoeB hollow nanospheres promote hydrogen generation from the hydrolysis of sodium borohydride, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.117

2

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 8 ) 1 e1 0

catalyst

NaBH4 þ ð2 þxÞH2 O ƒ! NaBO2 $xH2 O þ4H2 DH ¼
248:9 kj=mol (1) Numerous studies have focused on the development of an appropriate catalyst. Noble metal-based catalysts, using Ru and Pt, have been reported to promote high catalytic activity for the hydrolysis of NaBH4 [10e13]. However, they have many limitations in real applications due to their high cost and short lifetime. Among the various non-noble metal catalysts, Co-based catalysts are the most attractive due to their good catalytic activities and low cost [14e22]. Chemical reduction methods are widely used for the synthesis of Co-based alloys. However, the exothermicity during the reduction of metal ions leads to the agglomeration of particles, which decreases the effective surface area and number of active sites on the material. Hence, supports, such as copper, clay, or graphene, are used to increase the surface area, improve nanoparticle dispersion on the surface, thus preventing agglomeration. Wei et al. demonstrated the formation of a furrow-like CoeNieB catalyst on a Cu sheet [23]. Saha et al. used a graphene-supported Co-Ni catalyst [24], and Tian et al. designed an attapulgite clay-supported CoeB catalyst for hydrogen generation [25]. However, metal leaching from the surface of the support cannot be avoided, and metal leaching is not suitable for large-scale production. Another way to increase the catalytic activities is by including other metals, such as Ni, W or Zn [21,22,26,27]. Among them, Co- and Ni-based catalysts are inexpensive materials with high catalytic activity. Fernandes et al. and Wu et al. indicated that the presence of mixed CoeNi catalysts with B atoms resulted in an enhanced catalytic effect with respect to CoeB powders [21,22]. Rahul et al. reported Ni as a co-catalyst in NieCoeB alloys [28]. However, NieCoeB alloys are still synthesized by a traditional chemical reduction of Ni and Co metal ions [21,29]. One strategy to improve the catalytic activity of these catalysts is to reduce the catalyst particle size, thereby increasing the surface area and number of active sites. However, smaller nanoparticles are more susceptible to agglomeration. Thus, the specific surface area is generally less than 100 m2 g
1 cat. Another strategy is based on the synergetic effect. The optimal molar ratio of Ni:Co is beneficial to the hydrolysis reaction. The catalysts synthesized by chemical reduction method are limited inhomogeneous mixing Co and Ni not reaching atomic cluster level. Thus, the distribution and dispersion of Ni and Co in a NieCoeB alloy cannot be adjusted. Few studies have investigated the mechanism of the NieCoeB alloy synergetic effect in the hydrolysis of NaBH4, but this mechanism needs further clarification. Hollow structures could increase the surface area of catalysts. Versatile and simple hollow nanostructures can be prepared by the galvanic replacement reaction [30,31]. Currently, this technique has been used to prepare binary noble-metal catalysts [32,33] or catalysts that include combinations of transition and noble metals [34,35]. In this paper, hollow CoeNieB nanosphere catalysts for NaBH4 hydrolysis were first synthesized by the galvanic replacement reaction using a CoeB amorphous alloy and a NiCl2 solution as the

template and additional reagent, respectively. The method effectively increased the surface area of the catalysts, and it produced well-dispersed Ni atomic clusters, with optimal content, on the surface of the catalyst. The morphology, BET surface area, electronic interaction and catalytic performance of the products were investigated. The catalytic activity was correlated with the duration of replacement reactions. When the replacement time was 60 min, the as-prepared hollow nanosphere CoeNieB catalyst presented a higher surface area and superior catalytic activity towards hydrogen generation than other samples, which shows great potential for its use in NaBH4 hydrolysis. Additionally, the mechanism of the hydrolysis of NaBH4, based on the synergetic effect of NieCoeB alloys, was proposed.

Experimental Catalyst preparation The CoeB sample was prepared by reducing 0.5 mol/L cobalt chloride [CoCl2$6H2O] (Aladdin Reagent Co., Shanghai, China) and 0.5 mol/L tartaric acid (Aladdin Reagent Co., Shanghai, China) mixed solution (10 mL each) with potassium borohydride (KBH4) (Chuandong Chemical Co., Ltd., Chongqing, China). The metal to KBH4 molar ratio was 1:4 to ensure the complete reduction of Co ions. The temperature of the flask was maintained at 273 K during the reaction to prevent a vigorous reaction. After completion of the reaction, the product was filtered and washed several times with deionized water and ethanol. For the synthesis of NieCoeB hollow nanospheres, the CoeB amorphous alloy was prepared using a similar synthesis procedure to the CoeB sample. The CoeB amorphous alloy was mixed with 10 mL of 0.5 mol/L nickel chloride [NiCl2$6H2O] (Aladdin Reagent Co., Shanghai, China) solution and sonicated for 30, 60 and 90 min; the samples were marked as NieCoeB-30, NieCoeB-60 and NieCoeB-90, respectively. Finally, the as-obtained NieCoeB samples were washed with deionized water and ethanol, dried in a vacuum oven, and weighed.

Catalyst characterization The morphology of CoeB and NieCoeB catalysts was characterized by transmission electron microscopy (TEM, JEOL, JEM-2100). The quantitative chemical composition of catalysts was measured by energy-dispersive X-ray spectroscopy (EDX, Genesis Spectrum, 200 kV). The structure of the catalysts was analyzed by X-ray diffraction (XRD, Rigaku, D/max 2500PC) with Cu Ka radiation (g ¼ 1.5418 Å) in the 2q range of 20e80. Xray photoelectron spectroscopy (XPS, Perkin, PHI-1600 ESCA) measurements were recorded with a spectrophotometer using an Mg X-ray (hn ¼ 1253.6 eV) source for excitation. The binding energy (BE) values were calibrated using C 1s ¼ 284.6 eV as a reference. Hydrogen temperatureprogrammed desorption (H2-TPD) measurements were performed on a TP-5076 instrument (Tianjin Xianquan Instrument Co. Ltd., China). The BET surface area was measured using a surface area analyzer (Quantachrome Instruments, Autosord-IQ).

Please cite this article in press as: Guo J, et al., Novel NieCoeB hollow nanospheres promote hydrogen generation from the hydrolysis of sodium borohydride, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.117

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 8 ) 1 e1 0

Hydrogen generation tests The catalytic performance of the materials in the hydrolytic dehydrogenation of NaBH4 was determined at 303 K NaBH4 (1 wt.%) and NaOH (2 wt.%) were mixed in a reactor containing 10 mL of water and 10 mg of catalyst. Magnetic stirring was maintained throughout the reaction. The generated H2 was collected in a graduated jar and the volume determined using the water-replacement method (i.e., change of the height of water with time). To study the effect of the reaction

3

temperature on the reaction rate, NaBH4 hydrolysis was performed at different temperatures (298, 303, 308, and 313 K).

Results and discussion Fig. 1 shows the TEM micrographs of the CoeB and NieCoeB samples. As shown in Fig. 1a, the unmodified CoeB alloy was composed of nanoparticles. The particles were aggregated, and the specific surface area was 65.9 m2 g
1. Upon the

Fig. 1 e TEM images of samples (a) CoeB (b) NieCoeB-30, (c) NieCoeB-60, (d) NieCoeB-60 at higher magnification, (e) NieCoeB-90. Please cite this article in press as: Guo J, et al., Novel NieCoeB hollow nanospheres promote hydrogen generation from the hydrolysis of sodium borohydride, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.117

4

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 8 ) 1 e1 0

Intensity (a.u.)

The surface electronic interaction between the atoms in the compound could be characterized by XPS spectra, which are shown in Fig. 3. The XPS spectra in Fig. 3a demonstrate that the Ni species in the NieCoeB-60 sample (cNi ¼ 0.54) were

Ni-Co-B-90

Ni2p

855.5 eV

Ni-Co-B- 60

Intensity (a.u.)

Ni-Co-B- 30 Co-B

20

30

40 50 60 2 Theta (degree)

70

80

851.7 eV

Ni-Co-B-60

a

Fig. 2 e XRD patterns of CoeB and NieCoeB samples.

890

NiðaqÞ þ 2e
/NiðsÞ
0:25V vs:SHE

(2)

þ CoðsÞ /Co2þ ðaqÞ þ 2e
0:28V vs:SHE

(3)

2þ

Co þ NiðaqÞ /Ni þ Co2þ ðaqÞ

870

865

860

Co2p

855

850

845

781.7 eV

Ni-Co-B-60 777.7eV

Co-B 777.2 eV

b

(4)

As a result, Co atoms were oxidized and incorporated into the solution, leading to the formation of loose NieCoeB-30 in the sample. Simultaneously, the electrons migrated quickly to the surface of the nanoparticle and were captured by Ni2þ ions, generating Ni atoms on the CoeB surface via reduction. The specific surface area of NieCoeB-30 in Fig. 1b is 84.8 m2 g
1. Then, continuous dissolution of Co from the template resulted in the transformation of the nanoparticle into a nanostructure characterized by a hollow interior (NieCoeB-60). The NieCoeB-60 sample shown in Fig. 1c and d, with a larger specific surface area of 159.6 m2 g
1, was composed of hollow nanospheres. Excessive dealloying caused the hollow nanospheres to collapse, and the NieCoeB90 sample was composed of fragments, as shown in Fig. 1e, with a surface area of 103.9 m2 g
1. The EDX spectrum (Fig. 1S) shows that both Ni and Co elements exist in the NieCoeB samples. The Ni/(Ni þ Co) molar ratios for the NieCoeB-30, NieCoeB-60, and NieCoeB-90 samples were 0.39, 0.54, and 0.64, respectively. Fig. 2 shows the XRD spectra of the CoeB and NieCoeB samples; there is only one broad peak at a 2q value around 45 , which is characteristic of a typical amorphous structure and consistent with the results found for other NieCoeB amorphous alloys [37,38].

875

Binding energy (eV)

815

810

805

800

795

790

785

780

775

770

Binding energy (eV)

B1s

Ni-Co-B-60 188.4 eV

Intensity (a.u.)

2þ

880

Intensity (a.u.)

addition of an aqueous NiCl2 solution to an aqueous suspension of CoeB nanoparticles, the galvanic replacement reaction started immediately with the release of energy. Based on the standard reduction potentials (
0.28 V vs. SHE for the Co/ CoCl2 pair and
0.25 V vs. SHE for the Ni/NiCl2 pair [36]), galvanic replacement should occur according to the following equations.

885

192.2 eV

Co-B

188.2 eV

c 196

194

192

190

188

186

Binding energy (eV) Fig. 3 e XPS spectra of (a) Ni2p, (b) Co2p and (c) B1s states.

Please cite this article in press as: Guo J, et al., Novel NieCoeB hollow nanospheres promote hydrogen generation from the hydrolysis of sodium borohydride, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.117

5

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 8 ) 1 e1 0

528 590

TCD Singal(a.u.)

504

566

Ni-Co-B-90

Ni-Co-B-60

596 502

509

300

400

500

Ni-Co-B-30

605

Co-B

600

700

800

900

Temperature (K) Fig. 4 e H2-TPD spectra of CoeB and NieCoeB samples.

250 200

H2 generation volume/ml

present in both elemental and oxidized states, corresponding to binding energies (BE) of 851.7 eV and 855.5 eV, respectively. The oxidized state resulted from partial oxidation reactions during the sample preparation before the XPS measurement [39]. The feature peak of Co2p3/2 for elemental Co of the NieCoeB-60 sample was 777.7 eV, which is shown in Fig. 3b. The BE value of metallic Co in the NieCoeB-60 was 0.5 eV and was negatively shifted when compared to that of CoeB. To make a comparative study, XPS analysis was also carried out for NieCoeB-30 and NieCoeB-90 catalysts (Fig. S2). Compared to the BE peak of elemental Co in CoeB, a negative shift by 0.4 eV (777.8 eV for NieCoeB-30 and NieCoeB-90) was observed for both catalysts. Similarly, two peaks were also observed in Fig. 3c for the B1s state, with BEs of 188.2 and 192.2 eV, indicating the presence of elemental and oxidized boron in CoeB [40]. In comparison with that of pure amorphous B (187.1 eV), the BE of elementary B in NieCoeB-60 (188.4 eV), NieCoeB-30 (188.4 eV) and NieCoeB-90 (188.2 eV) were positively changed, which indicated the electrons of element B transferred into the empty d orbits of metal Co and/ or Ni, resulting in a lack of electrons in element B but enriched electrons in Co (or Ni in NieCoeB samples). This correlates well with the trend shown by amorphous metal borides [41,42]. Moreover, in both CoeNieB catalysts, a negative shift in the BE peak of elemental Co indicates the presence of higher electron density on cobalt sites compared to CoeB. One of the crucial requirements for a good hydrogen generation catalyst is the presence of high electron density at the catalytically active sites, which can facilitate the hydrolysis of NaBH4 [43]. From the above XPS results, it is clear that Ni plays a key role in increasing the electron density at Co sites to make them more active for hydrolysis of sodium borohydride. Fig. 4 shows the H2-TPD spectra of all samples. CoeB exhibited two peaks, at around 509 K and 605 K, indicating the presence of two adsorbing sites on the surface of the catalyst. NieCoeB-30 also exhibited two peaks, but the higher temperature peak shifted to 596 K. The higher temperature peak of NieCoeB-60 shifted to 566 K. As the time of the replacement reaction is extended, the distribution of the active sites tends to become more uniform. NieCoeB-60 has more enhanced and uniform catalytic active sites than the

150 100

Ni-Co-B-30 Ni-Co-B-60 Ni-Co-B-90 Co-B

50 0

0

2

4

6

8

10

12

Time/min

Fig. 5 e Catalytic hydrogen generation from the hydrolysis of a mixed solution of 1 wt.% NaBH4 þ 2 wt.% NaOH at 303 K. Comparative study of four catalysts (i) CoeB, (ii) NieCoeB-30, (iii) NieCoeB-60 and (iv) NieCoeB-90.

other NieCoeB samples. With increasing duration of the replacement reaction, the peak area of NieCoeB-90 decreased significantly. Fig. 5 shows the comparative H2-generation capability of four different catalysts, i.e., CoeB, NieCoeB-30, NieCoeB-60 and NieCoeB-90 at 303 K. The H2-generation rate of all catalysts first increased and reached a maximum before decreasing, revealing a reaction rate other than zero-order. NieCoeB-60 had the largest value of a maximum hydrogen generation rate of 6400 mLhydrogen min
1 g
1catalyst. This value is larger than those of other catalysts in previous reports, such as CoeB particle (850 mLhydrogen min
1 g
1catalyst) [42], claysupported CoeB catalyst (1270 mLhydrogen min
1 g
1catalyst) [44], heat-treatment NieCoeB catalysts (708 mLhydrogen min
1 g
1catalyst) [22], and NieCo/r-GO nanoparticle catalysts (1280 mLhydrogen min
1 g
1catalyst) [17]. The H2-generation rates for the rest samples are presented in Table 1. Currently, it is generally accepted that metal (M)-catalyzed hydrolysis of NaBH4 involves the dissociative chemisorption of BH
4 on the catalyst surface as the first kinetic step [27,28,45e47]. The mechanism proposed by Holbrook and Twist [48] suggests that hydrogen was generated from both the borohydride and water. We propose a schematic of the NaBH4 hydrolysis mechanism and hydrogen spillover from Ni

Table 1 e Maximum hydrogen generation rate obtained from the hydrolysis of alkaline NaBH4. Catalyst Co-B Ni-Co-B-30 Ni-Co-B-60 Ni-Co-B-90

Maximum hydrogen generation rate (mL min
1g
1 catalyst) 2770 5000 6400 3920

Hydrolysis was carried out using NaBH4 (1 wt. %) and NaOH (2 wt. %) solution by 10 mg catalyst.

Please cite this article in press as: Guo J, et al., Novel NieCoeB hollow nanospheres promote hydrogen generation from the hydrolysis of sodium borohydride, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.117

6

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 8 ) 1 e1 0

on NieCoeB-60 catalyst, as presented in Fig. 6. In the first step, BH
4 species are adsorbed on electron-enriched Co active sites
of NieCoeB-60. The BH
4 then dissociates to form Co-BH3 and
Co-H intermediates. Co-BH3 subsequently reacts with H2O, possibly via a BH3 intermediate, to generate Co-H and BH3 ðOHÞ
, as presented in Fig. 6 and described by equations (a) to (d). The former undergoes stepwise replacement of B-H bonds by B-OH
bonds and finally yields Co-ðOHÞ
3 , as presented in Fig. 6 and described by equations (e) to (g). The latter combines with two Co-H molecules to afford H2 and to regenerate the active sites. Obviously, the adsorption of borohydride is enhanced by electron-enriched Co active sites, which improves the reaction rate. However, the adsorption of H2 was also simultaneously enhanced. During the hydrolysis reaction, H2 agglomeration and coverage of the active sites could block the formation of new active sites for continual

adsorption of BH
4 and H2 Ospecies. With the appropriate galvanic replacement treatment, i.e., 60 min, the surface of the hollow NieCoeB-60 sample became homogenous and highly dispersed, with a comparable number of Ni-atom clusters. This formation could prompt hydrogen spillover from the catalyst and release Co active sites, as presented in Fig. 6 and described by equations (h) to (j). The possible reasons were consistent with other studies [28]. All the above catalytic hydrolysis processes involve not only surface reactions but also the diffusion and release of hydrogen in the metal catalysts. NieCoeB-60 has well-dispersed Ni-atom clusters, with optimal content, on the surface of CoeB prepared by the galvanic replacement method, which could produce a better synergetic effect than catalysts produced by the chemical reduction of Co and Ni ions. However, excessive galvanic replacement treatment, i.e., 90 min, led to the

Fig. 6 e Schematic of the NaBH4 hydrolysis mechanism and hydrogen spillover from Ni on the NieCoeB-60 catalyst. Please cite this article in press as: Guo J, et al., Novel NieCoeB hollow nanospheres promote hydrogen generation from the hydrolysis of sodium borohydride, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.117

7

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 8 ) 1 e1 0

decomposition of hollow NieCoeB nanospheres and the aggregation of crystalline Ni particles on the surface. This resulted in fewer available Co sites for adsorption of borohydride and lower overall activity. The hydrolysis kinetics of NaBH4 are dependent on catalyst performance, and it is affected by other factors, such as NaOH concentration and reaction temperature [49]. Fig. 7a shows the effects of NaOH concentration on the rate of reaction at 303 K. The results indicated that when the amount of NaOH was highest (8 wt.%), the hydrolysis rate was slow. The gradual decrease in the rate of the reaction with increasing NaOH concentration might be because excess NaOH produces more Naþ, which is converted to NaBO2 species during the hydrolysis process, hindering the active sites of the catalyst and slowing the reaction. Moreover, the high viscosity and stability of NaBH4 at high pH imparted the rate adversity [45]. Next, we analyzed the effect of temperature on the hydrolysis of NaBH4. Temperature plays a vital role in many catalytic

reactions, and high reaction rates are generated at optimal temperatures due the increased number of molecular collisions [50,51]. Fig. 7b shows the hydrogen generation rates at temperatures ranging from 298 to 313 K, at which the rate of hydrolysis of NaBH4 increases with increasing temperature. Kinetic studies at different temperatures were also carried out using the optimized solution (1 wt.% NaBH4þ2 wt.% NaOH). Fig. 7c represents the kinetic plots of hydrogen generation ranging from 298 to 313 K. To evaluate the catalytic activity, the initial hydrogen generation rates k (mol min
1 g
1) were used to determine the activation energy, as shown in equation (5) [52]: lnk ¼ lnk0
ðEa =RTÞ

(5)
1


1

where k0 is the rate constant (mol min g ), R is the gas constant (8.3143 Jmol-1 K
1), Ea is the activation energy (kJ mol
1), and T is the reaction temperature (K). Fig. 5d displays an Arrhenius plot, in which ln k is plotted against the

250 H2 generation volume/ml

a

200 150 100

2% NaOH 4% NaOH 6% NaOH 8% NaOH

50 0

0

2

4

6

8

Time/min

b

-1.0

H2 generation volume/ml

200

c -1

Ea=33.1 KJ mol

150

-1.2

50

. . . .

ln K

   

100

-1.4

-1.6 0

0

1

2

3

4

Time / min

5

6

7

0.00320

0.00324

0.00328

0.00332

0.00336

1/T K-1

Fig. 7 e (a) Catalytic hydrogen generation from the hydrolysis of a mixed solution of 1 wt.% NaBH4 þ NaOH extent (2 wt.%, 4 wt.%, 6 wt%, and 8 wt.%) at 303 K for NieCoeB-60. (b) Temperature-dependent comparative catalytic activity of NieCoeB60 at 298 K, 303 K, 308 K and 313 K. (c) Arrhenius plot ln K vs. the reciprocal absolute temperature 1/T in the temperature range 298e313 K (c). Please cite this article in press as: Guo J, et al., Novel NieCoeB hollow nanospheres promote hydrogen generation from the hydrolysis of sodium borohydride, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.117

8

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 8 ) 1 e1 0

Table 2 e The comparison of morphology, BET, HGR, reusability and the activation energy of our catalyst and other catalysts reported in the literature. Catalyst

Morphology

BET Surface m2 g
1 cat

Remaining activity

Activation Energy (kJ mol
1)

Hydrogen generation rate (mL min
1 g
1 cat )

Reference

particle particle particle particle particle particle hollow Rod-like particle hollow

17.07 e 38.29 89.6 e e 100.7 e 77 159.6

e 95% after 3 cycles e e e 53.5% after 5 cycles e 68.5 after 9 cycles 70% after 5 cycles 88.3% after 5 cycles

e 43.1 49.11 51.6 57.8 55.12 45.5 56.32 44 33.1

708 e 3976 12503 2073.1 1280 10140 1270 6294 6400

[22] [28] [43] [53] [54] [17] [55] [44] [41] This work

heat-treated Ni-Co-B Co-B@Ni/RGO Plasma treated Co-B-P Co-B/TiO2 Co-B/Carbon Ni-Co/r-GO thermal-treated Co-B clay-supported Co-B CoB/Ag-TiO2 Ni-Co-B hollow nanospheres

Not reported or no detailed data are available.

reciprocal of absolute temperature (1/T). From the slope of the straight line, the activation energy was calculated to be 33.1 kJ/ mol. Hydrogen generation properties for the hydrolysis of NaBH4, catalyzed by various reported catalysts, are displayed in Table 2. The obtained Ea value of the NieCoeB-60 catalyst is

H2 generation volume/ml

200

a

150

100

5 mg 10 mg 20 mg

50

0

0

2

4

6

8

b

1200

-1

-1

H2 generation rate （mL min g ）

Time / min

relatively lower than that of previously reported catalysts, such as CoeB@Ni/RGO [28], plasma-treated CoeBeP [43], and NieCo/RGO catalyst [17]. It is worth noting that the hydrogen generation rate of the NieCoeB-60 catalyst is significantly higher than that of most Coebased catalysts listed in Table 2. Hence, the result indicates that the as-obtained hollow nanosphere NieCoeB catalyst in the present work shows high catalytic activity for hydrogen generation from NaBH4 solution. Moreover, compared with the catalysts in Table 2, our preparation method does not require catalyst supports, such as carbon, TiO2 or RGO. NieCoeB hollow nanospheres have lower activation energies for hydrogen production and higher stability, without thermal or plasma treatment. Therefore, the high catalytic activity, low cost and simple preparation method are very attractive in the development of hydrogen generation from the hydrolysis of sodium borohydride. Finally, to examine the effects of catalyst-loading on the hydrogen generation rate, 5, 10, and 20 mg of the catalysts were evaluated. Fig. 8a shows the hydrogen generation rate measured using the optimized catalyst. As observed in Fig. 8a, with increasing amounts of catalyst, the H2 production rate also increased, implying that the hydrogen generation rate can be determined by controlling the catalyst-loading in the reactor. Moreover, we have investigated the recyclability of the catalysts. The results in Fig. 8b show successful hydrolysis in five successive cycles, while maintaining 88.3% activity, demonstrating the robustness and reusability of the catalysts.

900

Conclusions 600

300

0

1

2

3 4 Number of cycles

5

Fig. 8 e (a) Hydrogen generation rate of NieCoeB-60. (a) Comparative study at three different amounts of catalyst. (b) Recyclability test of catalyst (10 mg) in five successive cycles.

In the present study, NieCoeB catalysts were synthesized by the galvanic replacement reaction. The NieCoeB catalyst prepared in 60 min (NieCoeB-60) exhibited significantly better catalytic activity in the NaBH4 hydrolysis reaction compared to those prepared in 30 and 90 min. The activation energy of the NieCoeB-60 catalyst was 33.1 kJ/mol. This value is remarkably lower than those of many other catalysts described in previous reports. The optimized hydrogen generation performance of the NieCoeB-60 can be explained by the hollow nanosphere structure, the electron effect of the Co active sites, and hydrogen spillover onto Ni. The hydrogen generation rate of NieCoeB-60 was highest when the

Please cite this article in press as: Guo J, et al., Novel NieCoeB hollow nanospheres promote hydrogen generation from the hydrolysis of sodium borohydride, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.117

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 8 ) 1 e1 0

concentration of NaOH was 2 wt.%. In addition, the hydrogen generation rate increased exponentially with temperature. The reusability tests for the NieCoeB-60 catalyst confirmed that most of the original activity was preserved after five consecutive runs, which shows great potential for applications based on the production of energy from H2.

Acknowledgments The authors gratefully acknowledge the Natural Science Foundation of Hebei Province (B2018208188) and the Science and Technology Projects for Returned Scholars of Hebei Province, China (No.CL201610).

Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.06.117.

references

[1] Lee H, Lee J-W, Kim D-Y, Park J, Seo Y-T, Zeng H, et al. Nature 2005;434:743e6. [2] Barreto L, Makihira A, Riahi K. The hydrogen economy in the 21st century: asustainable development scenario. Int J Hydrogen Energy 2003;28(3):267e84. [3] Chou C-C, Lee D-J, Chen B-H. Hydrogen production from hydrolysis of ammonia borane with limited water supply. Int J Hydrogen Energy 2012;37(20):15681e90. [4] Patel N, Fernandes R, Bazzanella N, Miotello A. Enhanced hydrogen production by hydrolysis of NaBH4 using “CoeB nanoparticles supported on Carbon film” catalyst synthesized by pulsed laser deposition. Catal Today 2011;170(1):20e6. [5] Ingersoll JC, Mani N, Thenmozhiyal JC, Muthaiah A. Catalytic hydrolysis of sodium borohydride by a novel nickel-cobaltboride catalyst. J Power Sources 2007;173(1):450e7. [6] Ouyang LZ, Zhong H, Li ZM, Cao ZJ, Wang H, Liu JW, et al. Low-cost method for sodium borohydride regeneration and the energy efficiency of its hydrolysis and regeneration process. J Power Sources 2014;269:768e72. [7] Kojima Y, Haga T. Recycling process of sodium metaborate to sodium borohydride. Int J Hydrogen Energy 2003;28(9):989e93. [8] Akdim O, Demirci UB, Miele P. More reactive cobalt chloridein the hydrolysis of sodium borohydride. Int J Hydrogen Energy 2009;34(23):9444e9. [9] Senoh H, Siroma Z, Fujiwara N, Yasuda K. A fundamental study on electrochemical hydrogen generation from borohydrides. J Power Sources 2008;185(1):1e5. [10] Zhang JS, Delgass WN, Fisher TS, Gore JP. Kinetics of Ru catalyzed sodium borohydride hydrolysis. J Power Sources 2007;164:772e81. [11] Hsueh CL, Chen CY, Ku JR, Tsai SF, Hsu YY, Tsau F, et al. Simple and fast fabrication of polymer template-Ru composite as a catalyst for hydrogen generation from alkaline NaBH4 solution. J Power Sources 2008;177:485e92. € Kılınc¸ D, Saka C. Hydrogen generation from [12] S‚ahin O, hydrolysis of sodium borohydride with a novel palladium metal complex catalyst. J Energy Inst 2016;89(2):182e9.

9

[13] Liu Z, Guo B, Chan SH, Tang EH, Hong L. Pt and Ru dispersed on LiCoO2 for hydrogen generation from sodium borohydride solutions. J Power Sources 2008;176:306e11. [14] Gupta S, Patel N, Fernandes R, Kothari DC, Miotello A. Mesoporous CoeB nanocatalyst for efficient hydrogen production by hydrolysis of sodium borohydride. Int J Hydrogen Energy 2013;38:14685e92. [15] Liu BH, Li Q. A highly active CoeB catalyst for hydrogen generation from sodium borohydride hydrolysis. Int J Hydrogen Energy 2008;33(24):7385e91. [16] Liu BH, Li ZP, Suda S. Nickel- and cobalt-based catalysts for hydrogen generation by hydrolysis of borohydride. J Alloy Comp 2006;415:288. [17] Chou C-C, Hsieh C-H, Chen B-H. Hydrogen generation from catalytic hydrolysis of sodium borohydride using bimetallic NieCo nanoparticles on reduced graphene oxide ascatalysts. Energy 2015;90:1973e82. [18] Park D, Kim T. Electroless plated Co-Ni-P-B/Ni foam catalyst for hydrogen generation from sodium borohydride. J Nanosci Nanotechnol 2016;16(2):1740e3. [19] Guo Y, Feng Q, Ma J. The hydrogen generation from alkaline NaBH4 solution by using electroplated amorphous Co-Ni-P film catalysts. Appl Surf Sci 2013;273:253e6. € Kılınc¸ D, Saka C. Bimetallic Co-Ni based complex [20] S‚ahin O, catalyst for hydrogen production by catalytic hydrolysis of sodium borohydride with an alternative approach. J Energy Inst 2016;89:617e26. [21] Fernandes R, Patel N, Miotello A, Filippi M. Studies on catalytic behavior of CoeNieB in hydrogen production by hydrolysis of NaBH4. J Mol Catal A Chem 2009;298:1e6. [22] Wu C, Bai Y, Liu DX, Wu F, Pang ML, Yi BL. NieCoeB catalyst promoted hydrogen generation by hydrolyzing NaBH4 solution for in situ hydrogen supply of portable fuel cells. Catal Today 2011;170:33e9. [23] Wei YS, Meng W, Wang Y, Gao YX, Qi KZ, Zhang K. Fast hydrogen generation from NaBH4 hydrolysis catalyzed by nanostructured Co-Ni-B catalysts. Int J Hydrogen Energy 2012;37:1568e76. [24] Saha S, Ganguly S, Banerjee D, Kargupta K. Novel bimetallic graphene-cobalt-nickel(G-Co-Ni) nano-ensemble electrocatalyst for enhanced borohydride oxidation. Int J Hydrogen Energy 2015;40:1760e73. [25] Tian Hongjing, Guo Qingjie, Xu Dongyan. Hydrogen generation from catalytic hydrolysis of alkaline sodium borohydride solution using attapulgite clay-supported CoeB catalyst. J Power Sources 2010;195:2136e42. [26] Xiang CL, Jiang DD, She Z, Zou YJ, Chu HL, Qiu SJ, et al. Hydrogen generation by hydrolysis of alkaline sodium borohydride using a cobalt-zinc-boron/graphene nanocomposite treated with sodium hydroxide. Int J Hydrogen Energy 2015;40:4111e8. [27] Dai HB, Liang Y, Wang P. Effect of trapped hydrogen on the induction period of cobaltetungsten-boron/nickel foam catalyst in catalytic hydrolysis reaction of sodium borohydride. Catal Today 2011;170:27e32. [28] Rahul K, Fernandes Diana M, Catarina D, Ventura J, Cristina F. Synthesis of novel CoeB@Ni/RGO nanocomposite: a cost effective catalyst for improved hydrogen generation with enhanced electrochemical activity. Int J Hydrogen Energy 2016;41:11498e509. [29] Kalidindi SB, Vernekar AA, Jagirdar BR. Co-Co2B, Ni-Ni3B, CoNi-B nanocomposites catalyzed ammonia-borane methanolysis for hydrogen generation. Phys Chem Chem Phys 2009;11:770e5. [30] Xia XH, Wang Y, Ruditskiy A, Xia YN. 25th Anniversary article: galvanic replacement: a simple and versatile route to hollow nanostructures with tunable and well-controlled properties. Adv Mater 2013;25:6313e33.

Please cite this article in press as: Guo J, et al., Novel NieCoeB hollow nanospheres promote hydrogen generation from the hydrolysis of sodium borohydride, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.117

10

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 8 ) 1 e1 0

[31] Oh Myoung Hwan, Taekyung Yu, Yu Seung-Ho. Galvanic replacement reactions in metal oxide nanocrystals. Science 2013;340:964e8. [32] Sun Y, Xia Y. Mechanistic study on the replacement reaction between silver nanostructures and chloroauric acid in aqueous medium. J Am Chem Soc 2004;126:3892e901. [33] Lim B, Jiang M, Camargo PHC, Cho EC, Tao J, Lu X, et al. Pd-Pt bimetallic nanodendrites with high activity for oxygen reduction. Science 2009;324:1302e5. [34] Lu X, Au L, McLellan J, Li Z-Y, Marquez M, Xia Y. Fabrication of cubic nanocages and nanoframes by dealloying Au/Ag alloy nanoboxes with an aqueous etchant based on Fe(NO3)3 or NH4OH. Nano Lett 2007;7:1764e9. [35] Hu YJ, Shao Q, Wu P, Zhang H, Cai CX. Synthesis of hollow mesoporous PteNi nanosphere for highly active electrocatalysis toward the methanol oxidation reaction. Electrochem Commun 2012;18:96e9. [36] Bard AJ, Parsons R, Jordan J. Standard potentials in aqueous solution. Taylor & Francis; 1985. [37] Narayanan Sankara TSN, Stephan A, Guruskanthan S. Electroless NieCoeB ternary alloy deposits: preparation and characteristics. Surf Coating Technol 2004;179:56e62. [38] S-l Wang. Electroless deposition of NieCoeB alloy films and influence of heat treatment on the structure and the magnetic performances of the film. Thin Solid Films 2007;515:8419e23. [39] Chen Y. Chemical preparation and characterization of metalemetalloid ultrafine amorphous alloy particles. Catal Today 1998;44:3e16. [40] Li HX, Wu YD, Zhang J, Dai WL, Qiao MH. Liquid phase acetonitrile hydrogenation to ethylamine over a highly active and selective NieCoeB amorphous alloy catalyst. Appl Catal A Gen 2004;275:199e206. [41] Shen XC, Wang Q, Wu QQ, Guo SQ, Zhang ZY, Sun ZY, et al. CoB supported on Ag-activated TiO2 as a highly active catalyst for hydrolysis of alkaline NaBH4 solution. Energy 2015;90:464e74. [42] Patel N, Fernandes R, Miotello A. Promoting effect of transition metal-doped CoeB alloy catalysts for hydrogen production by hydrolysis of alkaline NaBH4 solution. J Catal 2010;271:315e24. € Karakas‚ DE, Kaya M, Saka C. The effects of plasma [43] S‚ahin O, treatment on electrochemical activity of CoeBeP catalyst for

[44]

[45]

[46]

[47]

[48] [49] [50]

[51]

[52]

[53]

[54]

[55]

hydrogen production by hydrolysis of NaBH4. J Energy Inst 2017;90(3):466e75. Tian HJ, Guo QG, Xu DY. Hydrogen generation from catalytic hydrolysis of alkaline sodium borohydride solution using attapulgite clay-supported CoeB catalyst. J Power Sources 2010;195:2136e42. Retnamma R, Novais AQ, Rangel CM. Kinetics of hydrolysis of sodium borohydride for hydrogen production in fuel cell applications: a review. Int J Hydrogen Energy 2011;36:9772e90. Netskina OV, Kochubey DI, Prosvirin IP, Kellerman DG, Simagina IV, Komova OV. Role of the electronic state of rhodium in sodium borohydride hydrolysis. J Mol Catal A Chem 2014;390:125e32. Dai HB, Liang Y, Ma LP, Wang P. New insights into catalytic hydrious kinetics of sodium borohydride from MichaelisMenten model. J Phys Chem C 2008;112:15886e92. Holbrook KA, Twist PJ. Hydrolysis of the borohydride ion catalysed by metal-boron alloys. J Chem Soc A 1971;15:890e4. Nie M, Zou YC, Huang YM, Wang JQ. Ni-Fe-B catalysts for NaBH4 hydrolysis. Int J Hydrogen Energy 2012;37:1568e76. Saha S, Basak V, Dasgupta A, Ganguly S, Banerjee D, Kargupta K. Graphene supported bimetallic G-Co-Pt nanohybrid catalyst for enhanced and cost effective hydrogen generation. Int J Hydrogen Energy 2014;39:11566e77. Dong H, Yang HX, Ai XP, Cha CS. Hydrogen production from catalytic hydrolysis of sodium borohydride solution using nickel boride catalyst. Int J Hydrogen Energy 2003;28(10):1095e100. Jeong SU, Kim RK, Cho EA, Kim HJ, Nam SW, Oh I, et al. A study on hydrogen generation from NaBH4 solution using the high performance CoeB catalyst. J Power Sources 2005;144:129e34. Lu YC, Chen MS, Chen YW. Hydrogen generation by sodium borohydride hydrolysis on nanosized CoB catalysts supported on TiO2, Al2O3 and CeO2. Int J Hydrogen Energy 2012;37:4254e8. Zhao J, Ma H, Chen J. Improved hydrogen generation from alkaline NaBH4 solution using carbon-supported CoeB as catalysts. Int J Hydrogen Energy 2007;32:4711e6. Ma H, Ji WQ, Zhao JZ, Liang J, Chen J. Preparation, characterization and catalytic NaBH4 hydrolysis of CoeB hollow spheres. J Alloy Comp 2009;474:584e9.

Please cite this article in press as: Guo J, et al., Novel NieCoeB hollow nanospheres promote hydrogen generation from the hydrolysis of sodium borohydride, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.117