Microstructure and hydrogen storage properties of porous Ni@Mg

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 3 6 ( 2 0 1 1 ) 1 4 4 8 4 e1 4 4 8 7

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Microstructure and hydrogen storage properties of porous Ni@Mg Y.J. Chai a,*, Z.Y. Liu a, H.N. Gao a, Z.Y. Zhao a, N. Wang b, D.L. Hou c,** a

School of Chemistry and Materials Science, Hebei Normal University, Hebei Shijiazhuang 050016, China Experimental Center, Hebei Normal University, Hebei Shijiazhuang 050016, China c School of Physics Science and Information Engineering, Hebei Normal University, Hebei Shijiazhuang 050016, China b

article info

abstract

Article history:

A nanoporous Ni layer on the surface of Mg was prepared utilizing the reduction reaction of

Received 20 June 2011

Ni2þ by Mg in NiCl2.12H2O solution. XRD result showed that the product was composed of

Received in revised form

cubic Ni, hexagonal Mg and little Mg(OH)2. SEM and TEM results suggested that a nano-

25 July 2011

porous Ni layer microencapsulated Mg and formed Ni@Mg. The specific surface area of

Accepted 7 August 2011

Ni@Mg formed in this reaction was 65.3 m2/g. Hydrogen uptake increased as the temper-

Available online 3 September 2011

ature increased. Moreover, Ni@Mg showed a reversible hydriding-dehydriding process without formation of the stable hydrides, which indicated that the nanoporous Ni layer on

Keywords:

the surface of Mg showed an obvious physisorption characteristics rather than hydrogen

Hydrogen storage

storage behavior of Mg.

Ni@Mg

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Surface modification

reserved.

Reduction reaction

1.

Introduction

A porous Ni layer formed on the surface of the hydrogen storage alloy by surface treatment using acid, alkaline and salt, which effectively improves the reaction kinetics and the electrochemical properties of hydrogen storage alloys [1e6]. It was reported that the dissolution of rare earth elements and Mn lead to the formation of porous Ni layer, which increased the specific surface area and catalyzed the electrochemical hydrogen absorption/desorption reaction. However, no obvious effect on the hydrogen storage capacity was observed [1]. Ni formed a thin concentrated layer in subsurface treated by LiOH and enabled a faster hydrogenationedehydrogenation reaction for LaNi5 [3]. The surface of F-treated alloys formed LaF3 and Ni-enriched two layers. H2 molecules still easily penetrated through the porous and full

of microcracks layers to dissociate into H atoms instantly and form hydride [4]. Replacing Ni2þ with the active composition formed a porous Ni layer on AB5-type alloy surface after surface treatment using a weak acid solution containing Ni2þ [6], which improved the hydrogen absorption/desorption capacity and the reactivity of the alloys, but no details were described in that work. In the previous reports, lots of work was focused on the investigation of the formed fresh Ni layer on the effect of hydrogen storage and the electrochemical performance of the bulk alloy, little concern cared for the investigation of the formed porous surface layer on the hydrogen storage properties. To understand the effect of the porous Ni layer Mg was immersed in Ni-containing solution and a nanoporous Ni layer on Mg surface (Ni@Mg) was obtained in the present work. The microstructure and hydrogen storage properties were also discussed.

* Corresponding author. Tel.: þ86 311 86268175. ** Corresponding author. E-mail addresses: [email protected] (Y.J. Chai), [email protected] (D.L. Hou). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.08.024

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 3 6 ( 2 0 1 1 ) 1 4 4 8 4 e1 4 4 8 7

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examine the specific area, Brunauer-Emmet-Teller (BET) measurement was applied at 77 K on NOVA 4000, Quantachrome Corporation. The hydriding/dehydriding behavior of w0.5 g Ni@Mg was measured by Sievert’s type gas system. Firstly, it was degassed at 200  C and 1  104 Pa for at least 2 h to remove the impurities. Then, the stainless chamber with the volume of 6 cm3 was cooled down to the assigned temperature. The hydrogen gas with purity 99.999% was allowed to the chamber during the hydriding/dehydriding process.

Fig. 1 e XRD pattern of porous Ni@Mg.

2.

Experimental

In this experiment, 0.1 g NiCl2.12H2O was dispersed into deionized water, and then NH3_H2O was slowly added dropwise into the solution under stirring until the coordination compound solution was obtained. 0.1 g Mg with purity of 99.99% purchased from Aladdin-reagent Company was dispersed into Ni-containing solution under slowly stirring for several minutes. At the beginning, the reduction reaction was slow. However, once the reaction occurred, amount of black Ni produced and deposited on the surface of Mg to form Ni@Mg. Meanwhile, Mg(OH)2 formed. Because Ni has intrinsic magnetic property Ni@Mg powders were adhered to a magnet and Mg(OH)2 moved to other side of beaker. Then the magnet with powders was repeatedly taken out to another beaker with acetone and the product was rapidly rinsed several times to move the hydroxide. It should be noted that this process must be as quick as possible to reduce the formation of hydroxide. The crystal structure of this sample was carried out by D8 ADVANCE Bruke X-ray diffractometer with Cu Ka radiation in the range from 10 to 80 with 0.03 min1. The morphology of Ni@Mg was investigated by means of HITACHI S-4800 fieldemission scanning electron microscope (FESEM) and transmission electron microscopy (TEM) image on a HITACHI H-600 microscope at an acceleration voltage of 80 kV. In order to

3.

Results and discussion

3.1.

Structure and morphology of Ni@Mg

Fig. 1 shows XRD pattern of Ni@Mg. It can be seen that Ni@Mg was composed of Ni, Mg and a little amount of Mg(OH)2. The sharp peaks of 2q ¼ 32.1 , 34.4 , 36.6 are assigned to hexagonal Mg with P63/mmc space group (ICDD-JCPDS card no. 653365), which corresponds to (100), (002) and (101) planes, respectively. The peaks of 2q ¼ 44.6 , 51.7 , 76.5 are regarded as (111), (200) and (220) planes for cubic Ni with Fm-3m space group (ICDD-JCPDS card no. 65-2865). The broad peaks of 21.5 and 58.9 are assigned to Mg(OH)2. The content of Ni, Mg and Mg(OH)2 calculated by EVA software are 31.5%, 63.2% and 5.3%, respectively. This indicates that only the oxidationereduction reaction occurs between Ni2þand Mg. In this chemical process, partial Ni2þ was reduced to Ni by Mg and they microencapsulated Mg to form Ni@Mg, as shown in Fig. 2(a). TEM results (Fig. 2(b)) showed that the size of the formed Ni was w20 nm and the thickness of a Ni layer was 30e40 nm. Although Mg(OH)2 were separated from Ni@Mg in the preparation process, it was unavoidable to exist in the production due to the existence of OH ion. The chemical reaction proceed in the solution is written as following: NH3 $H2 O/NH4þ þ OH

(1)

2þ Mg þ NiðNH3 Þ2þ n /MgðNH3 Þn þ Ni

(2)

Mg2þ þ 2OH /MgðOHÞ2

(3)

Fig. 3 shows N2 adsorption isotherms of porous Ni@Mg. The isotherm of this sample was typical of a type IV isotherm

Fig. 2 e SEM (a) and TEM (b) images of porous Ni@Mg.

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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 3 6 ( 2 0 1 1 ) 1 4 4 8 4 e1 4 4 8 7

100

0.6

0.5

Hydrogen content (wt%)

80

3 cm /g STP

100 200 300

60 0

50

100

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40

0.4

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0.8

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1

2

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5

Equilibrium pressure (MPa)

Fig. 3 e N2 adsorption isotherms of porous Ni@Mg.

Fig. 5 e Hydriding/dehydriding curves of porous Ni@Mg.

according to the IUPAC classification, which indicated that porous/interstices formed in Ni@Mg. The electrode potential 2þ of f2þ Mg=Mg and fNi=Ni is 2.37 V and 0.23 V, respectively. Therefore, Ni and Mg can form the microbattery in the solution to decompose H2O. The produced gas permeates Ni layer and generates a nanostructure porous/interstices Ni layer. The specific surface area, the average pore size and the pore volume of Ni@Mg based on the calculation of BJH data was 65.3 m2/g, 18.1 nm and 0.15 cc/g, respectively, which is slightly higher than other porous nanocrystalline materials such as Mg72Li28, MgH-Ni, Mm(Ni-Co-Al-Mn)4.76 [6e8].

distinguish if it is the hydrogenation process of Mg or porous Ni layer, as shown in Fig. 5. It can be seen that the hydrogen uptake linearly increased and it remained 0.38wt％ at 100  C under the pressure of 5 MPa. Reducing hydrogen pressure, the hydrogen content decreased, but the desorpted curve did not completely follow the absorption one. It was w68％H2 could be released, strongly suggesting a predominantly physically adsorbed/desorpted process. Increasing temperature, the hydrogen uptake was w0.5wt％ at 200  C and 0.57 wt％ at 300  C, meanwhile, 86％of the stored hydrogen was released. Generally, H2 firstly decomposed at the surface of hydrogen storage alloy and then diffused to alloy matrix phase [5]. Moreover, Mg can form hydride by decreasing the particles size or adding catalyst [13,14]. However, XRD result after hydriding showed that no MgeH hydride was detected even if the size of Mg was close to w50 nm, as shown in Fig. 6. TiO2, ZnO et al. nanostructured materials owing to the large specific surface area can absorb amounts of hydrogen onto the pores/ interstices by van der Waals forces, chemisorption or lattice defects [15,16]. In this work, the obtained specific surface area (65.3 m2/g) mainly comes from the nanoporous Ni layer. Because no hydride was detected and hydrogen was released with decreasing pressure, it is believed that H2/H was mainly stored in nanostructured porous Ni layer by physisorption.

3.2.

Hydrogen storage properties of Ni@Mg

Fig. 4 shows the kinetics curve for Ni@Mg at 200  C under w5 MPa. It took 2000 s to reach the maximum hydrogen content of 0.5wt％ for Ni@Mg suggesting a sluggish kinetics in comparison with other Mg-containing materials [9,10]. Pure Mg absorbs hydrogen at higher than 300  C with the equilibrium hydrogen pressure of w1 MPa [11,12], hence pressure-composition-isothermal (PCI) curve was consecutively measured at different temperatures in order to 0.6

Hydrogen content ( wt%)

0.5

0.4

0.3

0.2

0.1

0.0 -500

0

500

1000

1500

2000

2500

3000

3500

4000

Time (s)

Fig. 4 e Kinetics curve of porous Ni@Mg.

Fig. 6 e XRD pattern of porous Ni@Mg after hydriding.

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 3 6 ( 2 0 1 1 ) 1 4 4 8 4 e1 4 4 8 7

Increasing temperature, Ni-H band plus OH-H band formed by electrostatic interaction and finally led to the increase of the hydrogen uptake compared with that at 100  C. These bands broke at higher temperature with decreasing pressure. Thus, w20％of hydrogen uptake was released at 200e300  C than at 100  C. In short, the existence of the porous Ni layer in the hydrogen storage materials not only dissociates H2 but also physically absorb amount of hydrogen, which depends on the specific surface area of the porous layer to a certain degree.

4.

Conclusion

[2]

[3]

[4]

[5] [6]

In summary, Ni2þ was reduced by Mg and formed the product of Ni@Mg. The obtained Ni with the size of w20 nm encapsulated Mg and formed the thickness of the layer was 30e40 nm. XRD result showed that the layer was composed of Ni, Mg and Mg(OH)2 with the ratio of 31.5%, 63.2% and 5.3%, respectively. The specific surface area, the average pore size and the pore volume of the layer were 65.3 m2/g, 18.1 nm and 0.15 cc/g, respectively. The hydrogen uptake was w0.38 wt% under 5 MPa hydrogen pressure at 100  C, which indicated an obvious physisorption process rather than a hydride formation.

[7]

[8]

[9] [10]

[11]

Acknowledgment

[12] [13]

We acknowledge financial support from the National Natural Science Foundation of China (50901030) and Doctoral Science Foundation of Hebei Normal University (L2010B20).

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