Structure, morphology and electrocatalytic characteristics of nickel powders treated by mechanical milling

international journal of hydrogen energy 33 (2008) 6351–6356

Available at www.sciencedirect.com

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

Structure, morphology and electrocatalytic characteristics of nickel powders treated by mechanical milling Xiangyu Zhao, Yi Ding, Liqun Ma*, Xiaodong Shen, Suyuan Xu College of Materials Science and Engineering, Nanjing University of Technology, 5 Xinmofan Road, Nanjing, Jiangsu Province 210009, China

article info

abstract

Article history:

The structure, morphology and electrocatalytic characteristics of nickel powders treated by

Received 20 February 2008

mechanical milling have been studied. The milling process is controlled by the processes of

Received in revised form

cold welding and fracture. During the cold welding dominant stage, severe deformation of

10 July 2008

the particles occurs. The nickel particles are flattened and cold welded together, leading

Accepted 30 July 2008

to the formation of a lamellar structure and an increase of the average particle size. During

Available online 18 September 2008

the fracture dominant stage, the average particle size decreases and quasi-spherical particles form. Nanocrystalline structure is obtained when the milling time reaches 5 h.

Keywords:

The energy produced by mechanical milling is mostly introduced into refining the grain

Nickel

size during the milling period of 5 h and expanding the unit cell during the milling period

Mechanical milling

from 5 to 40 h. The formation of nanocrystalline structure with a proper density of dislo-

Nanostructure

cations by mechanical milling can substantially improve the electrocatalytic activity of the

Dislocation

nickel powders for hydrogen adsorption.

Lattice strain

ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

Cyclic voltammetry

1.

Introduction

Metallic nickel is widely used in nickel–cadmium (Ni–Cd) and nickel–metal hydride (Ni–MH) battery systems due to its excellent electrical conductivity, corrosion resistance and electrocatalytic properties. In recent years, for its environmental advantage and good electrode performance [1–3], the Ni–MH battery plays a dominant role in the application of electric vehicles (EVs) [4,5]. Hydrogen storage alloys, as the main material of the negative electrode of the Ni–MH battery, can be classified as AB5-type alloys [6], AB2-type alloys [7], AB-type alloys [8], AB3-type alloys [9], A2B7-type alloys [10], Mg-based alloys [11] and V-based solid solution alloys [12]. Nickel is extensively used as an alloying element for promoting the hydrogen dissociation from water, and it plays an important role on the kinetic performance of hydrogen storage alloys.

reserved.

Iwakura et al. [13] and Ramya et al. [14] claimed that appropriate content of nickel could improve the high rate chargeability and dischargeability of the Ni–MH battery. Li et al. [15] proposed that Ni-riched layer which formed after surface treatment improved the surface catalysis and the discharge ability of the Ni–MH battery. During the preparation of the negative electrode of the Ni–MH battery, foam nickel and nickel powders were used as the current collector and the conductive agent, respectively [16– 18]. It can be seen that nickel is a very important element for the preparation and application of the Ni–MH battery. As one of the common methods for preparing nonequilibrium materials, mechanical milling (MM) is a process of milling powder materials such as pure metals, intermetallic compounds, and prealloyed powders to get uniform distribution of composition [19]. It is often used due to its advantages such as simplicity, low-cost, and facility to produce non-equilibrium

* Corresponding author. Tel.: þ86 25 83587243; fax: þ86 25 83240205. E-mail address: [email protected] (L. Ma). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.07.117

6352

international journal of hydrogen energy 33 (2008) 6351–6356

2.

Experimental

Nickel powders with an average particle size of 30 mm and a purity of 99.9 at % were used as raw material for mechanical milling. They were introduced into a stainless steel container with stainless steel balls (10 and 6 mm in diameter) under argon atmosphere. The ball to powder ratio was 20:1. The milling was performed in a planetary mill with a rotation speed of 300 rpm and the milling times were 5, 10, 20 and 40 h. The result of X-ray fluorescence spectroscopy showed that after 40 h of milling the contamination by Fe, Cr, and Mn from the milling tools was less than 0.2 at %. The structure of the as-milled powders was studied by Xray diffraction (XRD) in a Thermo ARL X’ TRA diffractometer equipped with Cu Ka radiation. The diffraction patterns were analyzed using the MDI Jade 5.0 software (Materials Data Inc., Livermore, California, USA). The morphology of the powders was analyzed by a JSM-5900 scanning electron microscopy (SEM). The average particle size of the powders was characterized by laser diffraction technique using a Mastersizer 2000 particle size analyzer. Nickel electrodes were prepared by pressing 0.4 g of the asmilled powders into a pellet with 10 mm in diameter and 0.6– 0.8 mm thick under a pressure of 20 MPa for the electrochemical measurements. Cyclic voltammetry (CV) studies were carried out at a sweep rate of 10 mV/s from 1.3 to 0 V (vs. Hg/HgO) in a halfcell consisting of a Ni(OH)2/NiOOH counter electrode and a Hg/ HgO reference electrode in a 6 M KOH solution on a CHI 660B electrochemical workstation at 298 K.

11 10 9 8

Volume (%)

structures and a great number of different materials [20]. During the MM process, intensive deformation is introduced into powder materials, leading to various crystal defects such as dislocations, vacancies, stacking faults, and a large amount of grain boundaries which generate severe structural and morphological changes [19,21]. Nanocrystalline and other nonequilibrium phases can often be obtained by MM. For nanocrystalline materials, ultrafine grains give rise to a great amount of interfaces and a substantial fraction of atoms are located on the interfaces. These interfaces have become an important structural component, which is the main difference between nanocrystalline materials and polycrystalline materials. Therefore the kinetic hydrogenation properties of the nanocrystalline materials are better than those of the polycrystalline materials [22–24]. In this work, nickel powders with nanocrystalline structure are prepared by mechanical milling. Changes of powder morphology, particle size, phase structure, grain size, lattice parameters and lattice strain with milling time are investigated. The electrocatalytic activity of the nickel electrodes prepared by the nickel powders for hydrogen adsorption is also studied.

Non-milled Milled 5 h Milled 10 h Milled 20 h Milled 40 h

7 6 5 4 3 2 1 0

0.1

1

10

100

1000

Particle size (um) Fig. 1 – Particle size distributions of nickel powders for different milling time.

sizes are 30.0, 33.9, 46.9, 26.2 and 18.2 mm for the milling time of 0, 5, 10, 20 and 40 h, respectively. Fig. 2 shows the change of the morphology of the powders milled at several times. It can be seen that the morphology changes greatly after the mechanical milling. It is well known that cold welding and fracture are the two essential processes involved in the milling process [25]. When the milling time is 5 h, cave generates in the particles as shown in Fig. 2(b), indicating the occurrence of severe deformation of the particles. When the milling time is 10 h, the particles are repeatedly flattened, and then cold welded together to form a lamellar structure shown in Fig. 2(c) and the average particle size increases to 46.9 mm. The increase of the particle size can be explained by the dominant cold welding process. When the milling time increases to 20 h, the fracture process plays a dominant stage and there is no apparent lamellar structure, and the average particle size decreases to 26.2 mm. When the milling time reaches 40 h, the shape of the particles becomes quasi-spherical and the lamellar structure disappears. Fig. 3 shows the evolution of the average particle size as a function of milling time. Two stages, i.e., cold welding dominant stage and fracture dominant stage, can be differentiated depending on milling time. In the cold welding dominant stage, the average particle size increases from 30 to 46.9 mm by increasing the milling time from 0 to 10 h. In the fracture dominant stage, the average particle size decreases from 46.9 to 18.2 mm as the milling time increases from 10 to 40 h. Therefore, particle size and morphology of nickel powders are determined by the competition between cold welding and fracture. In addition, the particle size distribution tends to be uniform with the increase of milling time as shown in Fig. 1.

3.

Results and discussion

3.2.

3.1.

Morphology and particle size analysis

X-ray diffraction reflections are broadened due to instrumental effects, small grain size and lattice strain [19]. After being subtracted the instrumental broadening determined from a silicon standard sample, the total broadening can be

Fig. 1 shows the distributions of the particle size of the nickel powders as a function of the milling time. The average particle

Structural analysis

international journal of hydrogen energy 33 (2008) 6351–6356

6353

Fig. 2 – SEM micrographs for the morphology of the milled nickel powders: (a) Non-milled, (b) Milled for 5 h, (c) Milled for 10 h, (d) Milled for 20 h and (e) Milled for 40 h.

written as [19,26,27]: 0:9l þ hsin q (1) Bcos q ¼ d where B is the width at half the maximum intensity of the diffraction curve, q is the Bragg angle, l is the X-ray

Average particle size (µm)

50

40

30

20

Cold welding Fracture dominant dominant stage stage

10

0

0

10

20

30

40

Milling time (h) Fig. 3 – Change of particle size with milling time. The dash line is the trendline of the change.

wavelength, d is the grain size and h is the lattice strain. The values of d and h can be calculated. Fig. 4 shows the evolution of XRD patterns of nickel powders as a function of the milling time. The diffraction peaks corresponding to Ni phase (PDF card no. 65-2865) with a cubic structure, and the micro-structural parameters are listed in Table 1. With the increase of milling time, the diffraction peaks shift to the low-angle direction and the peak widths tend to be broad as shown in Fig. 4. Fig. 5 shows the changes of the unit cell volume, grain size and lattice strain with different milling time. After the ball milling, the grain size decreases from 141.7 to 9.4 nm, the lattice strain increases from 0.04 to 0.96% and the unit cell volume increases from 43.7 ˚ 3. During the milling period of 5 h, the energy introto 44.9 A duced by ball milling makes drastic decrease of the grain size of nickel corresponding to substantial increase of lattice strain. It is well known that the milling may lead to several modifications: atomic site interchange [28], vacancies [23] and dislocations [20]. The atomic site interchange cannot occur here due to the milling of ‘‘pure’’ nickel powders. During the whole milling process of the nickel powders, the unit cells expand continuously, therefore the formation of vacancies contradictorily be used as a proof to explain the present results. Hence dislocations are considered to be responsible

6354

international journal of hydrogen energy 33 (2008) 6351–6356

(111)

(111)

1 2 3 4

(200) (220)

Intensity (arb.unit)

Intensity (arb.unit)

Non-milled

Milled 5 h Milled 10 h Milled 20 h Milled 40 h

(200) (220) 1 2 3 4

40

50

60

70

80

40

50

60

70

80

2 theta (degree)

2 theta (degree)

Fig. 4 – Evolution of XRD patterns of nickel powders as a function of the milling time.

3.3.

Electrocatalytic characteristic

To determine the electrocatalytic activity of the nickel electrodes prepared by the as-milled nickel powders for hydrogen adsorption, the nickel electrodes are all fully activated after five cycles. The formation of the anodic peaks indicates that previous chemical adsorption occurs on the surface of the

nickel powders. The formation of the anodic peak can be described by reactions as follows [33]: H2 O þ e / H þ OH

(2)

Ni þ H / NiHads

(3)

NiHads þ OH / Ni þ H2 O þ e

(4)

Unit cell volume (Å3)

46

44

150

Grain size (nm)

for the present results. Lattice strain caused by MM is commonly attributed to plastic deformation, i.e., the generation and movement of dislocations [29]. As shown in Fig. 2, severe deformation occurs during the milling, indicating that the density of the dislocations increases. Fecht [30] claimed that the generation and movement of dislocations could decrease grain size. Rawers and Cook [31] showed that the strain on the nano-grain boundary could extend into nanograin expanding the lattice. Therefore two stages of the function of the lattice strain, as shown in Fig. 6, can be observed in the milling process of the nickel powders. The first stage is the milling period of 5 h. High lattice strain corresponding to a high density of dislocations forms and it is mostly introduced into decreasing the grain size. Then the nanocrystalline structure is obtained. The second stage is the milling period between 5 and 40 h. The grain size decreases slowly and tends to be a constant. A similar result has been reported earlier [32]. However, the unit cell volume increases with a relative high rate. This is because the increased dislocations or lattice strain are mostly introduced into expanding the unit cell in this stage.

100

50

0

Table 1 – Micro-structural parameters of the nickel powders as a function of the milling time Samples

˚ )a a (A

˚ 3)b V (A

d (nm)

h (%)

Non-milled Milled 5 h Milled 10 h Milled 20 h Milled 40 h

3.5223 3.5233 3.5353 3.5514 3.5559

43.7 43.8 44.2 44.8 44.9

141.7 14.4 11.0 10.3 9.4

0.04 0.62 0.81 0.87 0.96

a lattice constant. b unit cell volume.

Lattice strain (%)

1.0 0.8 0.6 0.4 0.2 0.0

0

10

20

30

40

Milling time (h) Fig. 5 – Changes of the unit cell volume, grain size and lattice strain with the milling time. The dash lines are trendlines of the changes.

international journal of hydrogen energy 33 (2008) 6351–6356

140

Table 2 – Electrochemical parameters of nickel electrodes obtained from the curves of cyclic voltammetry

120

Samples

100 80 60 44 40

Unit cell volume (Å3)

Grain size (nm)

46

6355

20 0

0.0

0.2

0.4

0.6

0.8

1.0

Lattice strain (%) Fig. 6 – Effect of the lattice strain on the grain size and the unit cell volume. The dash lines are the trendlines of the evolutions.

where Hads represents the hydrogen atoms adsorbed on the nickel surface. Fig. 7 shows the CV curves of the nickel electrodes in a 6 M KOH solution at a scan rate of 10 mV/s. The values of anodic peak current and anodic peak potential are listed in Table 2. It can be seen that the anodic peak current Ia increases from 36.4 to 70.9 mA with the increase of the milling time from 0 to 5 h and decreases to 39.6 mA when the milling time is 40 h. Further, it is clear that the anodic peak area related to the electric charge drastically increases at short milling time and then decreases with long milling time. The anodic peak area and the anodic peak current reveal the hydrogen adsorption ability. Additionally, with the increase of the milling time, the anodic peak potential E shifts to the positive direction, indicating that the relatively steady Ni–Hads bond forms after MM. It is interesting that two slight anodic peaks exhibited by the non-milled nickel appear in the CV curve. This may be explained by two types of Ni– Hads bond with different bond energies form on the surface of the non-milled nickel, leading to the oxidation of hydrogen occurs at different potential regions. Hitz and Lasia [34]

0.10 10 mV/s

0.05

Current (A)

0.00 -0.05 -0.10 -0.15 -0.20 0.0

Non-milled Milled 5 h Milled 10 h Milled 20 h Milled 40 h -0.2

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

Potential (V vs.Hg/HgO) Fig. 7 – Cyclic voltammetry curves of the nickel electrodes in 6 M KOH.

Non-milled Milled 5 h Milled 10 h Milled 20 h Milled 40 h

Ia (mA)

E (V)

36.4 70.9 48.4 43.7 39.6

0.878 0.764 0.771 0.773 0.757

proposed that the main factor influencing the electrode activity seemed to be the real surface area. However, according to the particle size distributions of the nickel powders as shown in Fig. 1, this cannot explain the present results because the evolution of the real surface area of the nickel powders is not corresponding to the changes of the anodic peak area and Ia here. During the milling period of 5 h, the nanocrystalline structure forms and the anodic peak area and Ia increase drastically. The electrocatalytic activity of the nanostructure for hydrogen adsorption is better as compared to that of the polycrystalline structure. When the milling time is longer, the grain size decreases slowly and tends to be constant (Fig. 6) while the anodic peak area and Ia decrease greatly, i.e., the electrocatalytic activity of the nickel electrodes becomes lower. The change of the density of the dislocations should be the cause for the change of the electrocatalytic activity for hydrogen adsorption. A proper density of the dislocations in the nickel powders with nanostructure should improve the electrocatalytic activity while an excessive density of the dislocations introduces a detrimental effect here.

4.

Conclusions

The structure, morphology and electrocatalytic characteristics of the nickel powders treated by mechanical milling (MM) have been studied. The milling process is controlled by the processes of cold welding and fracture. During the cold welding dominant stage, severe deformation occurs. The nickel particles are flattened and cold welded together, leading to the formation of a lamellar structure. The average particle size increases to 46.9 mm when the milling time is 10 h. During the fracture dominant stage, quasi-spherical particles with an average size of 18.2 mm form. Nanocrystalline structure forms during the milling. When the milling time is 40 h, the grain size decreases to 9.4 nm, while the unit cell volume and the lattice strain ˚ 3 and 0.96%, respectively. The energy increase to 44.9 A produced by MM is thus mostly introduced into refining the grain size during the milling period of 5 h and expanding the unit cell during the milling period from 5 to 40 h. The formation of nanocrystalline structure with a proper density of dislocations by MM can substantially improve the electrocatalytic activity of the nickel powders for hydrogen adsorption while an excessive density of dislocations may introduce a detrimental effect. The nanocrystalline nickel powders with a proper density of dislocations should be a better catalyst for hydrogen adsorption as compared to the polycrystalline one.

6356

international journal of hydrogen energy 33 (2008) 6351–6356

references

[1] Shaju KM, Kumar VG, Rodrigues S, Munichandraiah N, Shukla AK. Effect of morphology on the performance of metal-hydride electrodes. J Appl Electrochem 2000;30(3): 347–57. [2] Chen WX. Cyclic voltammetry and electrochemical impedance of MmNi3.6Co0.7Mn0.4Al0.3 alloy electrode before and after treatment with a hot alkaline solution containing reducing agent. J Power Sources 2000;90(2):201–5. [3] Kohno T, Yoshida H, Kawashima F, Inaba T, Sakai I, Yamamoto M, et al. Hydrogen storage properties of new ternary system alloys: La2MgNi9, La5Mg2Ni23, La3MgNi14. J Alloys Compd 2000;311(2):L5–7. [4] Ovshinsky SR, Fetcenko MA. Development of high catalytic activity disordered hydrogen-storage alloys for electrochemical application in nickel–metal hydride batteries. Appl Phys A 2001;72(2):239–44. [5] Feng F, Northwood DO. Effect of surface modification on the performance of negative electrodes in Ni/MH batteries. Int J Hydrogen Energy 2004;29(9):955–60. [6] Szajek A, Jurczyk M, Rajewski W. The electronic and electrochemical properties of the LaNi5, LaNi4Al and LaNi3AlCo systems. J Alloys Compd 2000;307:290–6. [7] Kim DM, Jang KJ, Lee JY. A review on the development of AB2-type Zr-based Laves phase hydrogen storage alloys for Ni–MH rechargeable batteries in the Korea Advanced Institute of Science and Technology. J Alloys Compd 1999; 293–295:583–92. [8] Drenchev B, Spassov T. Electrochemical hydriding of amorphous and nanocrystalline TiNi-based alloys. J Alloys Compd 2007;441(1–2):197–201. [9] Liao B, Lei YQ, Chen LX, Lu GL, Pan HG, Wang QD. The effect of Al substitution for Ni on the structure and electrochemical properties of AB3-type La2Mg(Ni1xAlx)9 (x ¼ 0–0.05) alloys. J Alloys Compd 2005;404–-406:665–8. [10] Zhang FL, Luo YC, Wang DH, Yan RX, Kang L, Chen JH. Structure and electrochemical properties of La2xMgxNi7. 0 (x ¼ 0.3–0.6) hydrogen storage alloys. J Alloys Compd 2007; 439(1–2):181–8. [11] Rongeat C, Grosjean MH, Ruggeri S, Dehmas M, Bourlot S, Marcotte S, et al. Evalution of different approaches for improving the cycle life of MgNi-based electrodes for Ni–MH batteries. J Power Sources 2006;158(1):747–53. [12] Tsukahara M, Takahashi K, Mishima T, Sakai T, Miyamura H, Kuriyama N, et al. Metal hydride electrodes based on solid solution type alloy TiV3Nix (0  x  0.75). J Alloys Compd 1995; 226(1–2):203–7. [13] Iwakura Ch, Miyamoto M, Inoue H, Matsuoka M, Fukumoto Y. Kinetics of the hydrogen evolution reaction in stoichiometricand non-stoichiometric hydrogen storage alloys for nickel-hydrogen batteries. J Electroanal Chem 1996; 411(1–2):109–13. [14] Ramya K, Rajalakshmi N, Sridhar P, Sivasankar B. Electrochemical studies on the effect of nickel substitution in TiMn2 alloys. J Alloys Compd 2003;352(1–2):315–24.

[15] Li ZP, Higuchi E, Liu BH, Suda S. Electrochemical properties and characteristics of a fluorinated AB2-alloy. J Alloys Compd 1999;293–295:593–600. [16] Liu PS, Chen H, Liang KM, Gu SR, Yu Q, Li TF, et al. Relationship between apparent electrical-conductivity and preparation conditions for nickel foam. J Appl Electrochem 2000;30(16):1183–6. [17] Zhao XY, Ding Y, Yang M, Ma LQ. Effect of surface treatment on electrochemical properties of MmNi3.8Co0.75Mn0.4Al0.2 hydrogen storage alloy. Int J Hydrogen Energy 2008;33(1):81–6. [18] Wang MH, Zhang LZ, Zhang Y, Sun LX, Tan ZC, Xu F, et al. The effects of partial substitution of Fe and Co for Ni in the Mg1.75Al0.25Ni electrode alloy on electrochemical performances. Int J Hydrogen Energy 2006;31(5):621–5. [19] Suryanarayana C. Mechanical alloying and milling. Prog Mater Sci 2001;46(1–2):1–184. [20] Koch CC. Synthesis of nanostructured materials by mechanical milling: problems and opportunities. Nanostruct Mater 1997;9(1–8):13–22. [21] Guerrero-Paz J, Jaramillo-Vigueras D. Nanometric grain formation in ductile powders by low-energy ball milling. Nanostruct Mater 1999;11(8):1123–32. [22] Suryanarayana C. Nanocrystalline materials. Int Mater Rev 1995;40(2):41–64. [23] Zhou LZ, Guo JT, Jiang T D, Wang SH. Investigation of defects in a mechanically alloyed nanocrystalline NiAl alloy by positron annihilation spectroscopy. J Mater Sci Lett 1998;17(2):137–9. [24] Ares JR, Cuevas F, Percheron-Gue0 gan A. Mechanical milling and subsequent annealing effects on the microstructural and hydrogenation properties of multisubstituted LaNi5 alloy. Acta Mater 2005;53(7):2157–67. [25] Zhou JB, Rao KP. Structure and morphology evolution during mechanical alloying of Ti–Al–Si powder systems. J Alloys Compd 2004;384(1–2):125–30. [26] Cullity BD. Elements of X-ray diffraction. Massachusetts: Addison-Wesly publication company; 1978. [27] Aymard L, Delahaye-Vidal A, Portemer F, Disma F. Study of the formation reactions of silver–palladium alloys by grinding and post-milling isothermal annealing. J Alloys Compd 1996;238(1–2):116–27. [28] Liang G, Huot J, Schulz R. Hydrogen storage properties of the mechanically alloyed LaNi5-based materials. J Alloys Compd 2001;320(1):133–9. [29] Mohamed FA. A dislocation model for the minimum grain size obtainable by milling. Acta Mater 2003;51(14):4107–19. [30] Fecht HJ. Nanostructure formation by mechanical attrition. Nanostruct Mater 1995;6(1–4):33–42. [31] Rawers J, Cook D. Influence of attrition milling on nano-grain boundaries. Nanostruct Mater 1999;11(3):331–42. [32] Oleszak D, Shingu PH. Nanocrystalline metals prepared by low energy ball milling. J Appl Phys 1996;79(6):2975–80. [33] Gao XP, Liu J, Ye SH, Song DY, Zhang YS. Hydrogen adsorption of metal nickel and hydrogen storage alloy electrodes. J Alloys Compd 1997;253–254:515–9. [34] Hitz C, Lasia A. Experimental study and modeling of impedance of the her on porous Ni electrodes. J Electroanal Chem 2001;500(1):213–22.