Hydrides of substituted derivatives based on the YNi3 compound ...

Materials Science, Vol. 43, No. 4, 2007

HYDRIDES OF SUBSTITUTED DERIVATIVES BASED ON THE YNi3 COMPOUND V. V. Berezovets’, R. V. Denys, O. B. Ryabov, and I. Yu. Zavalii

UDC 546.3–19′11

The crystallographic characteristics of YNi3 – x Mnx pseudobinary compounds are determined by the methods of X-ray diffraction analysis. It is shown that the partial substitution of nickel for manganese occurs mainly in the layers with CaCu5 -type structure but not in the layers with Mg Zn2-type structure, as was reported for A3 B8 X compounds. All synthesized samples are efficient absorbers of hydrogen and their hydrogen-absorption capacity monotonically decreases as the concentration of the alloying element increases. At the same time, this type of alloying substantially decreases the absorption–desorption pressure of hydrogen, which makes the developed alloys quite promising as hydrogen-storage materials. All investigated materials have good charging–discharging characteristics in KOH solutions. The best electrochemical parameters are observed for the Y Ni2.67 Mn0.33 alloy (Cmax = 305 mA ⋅ h / g).

At present, the so-called AB5 -type alloys, where A is a mixture of rare-earth metals (La, Ce, Pr, and Nd) and B is nickel partially replaced with other metals, are the main commercial materials used in manufacturing nickel–metal-hydride batteries [1]. The number of investigations aimed at the elevation of efficiency and reducing the costs of production of electrode materials is permanently increasing. As one of the directions of these investigations, one can mention the search of efficient electrodes among AB2 -type alloys (as a rule, based on Zr and Ti) characterized by high discharge capacitances but not used in the industry due to their slow activation [2]. At the same time, it is also possible to study AB3 compounds, where A is a rare-earth metal (REM) and B is a transition metal (Fe, Co, or Ni). The structure of these compounds can be regarded as hybrid because their elementary cells are formed by layers with structures of different types, namely, AB5 and AB2 (Fig. 1). As a final result, this gives [ AB5 + 2 ( AB2 ) = 3 ( AB3 ) ] [3]. The structures of hydrides of the AB3 compounds were investigated for various representatives, including Ho Ni3 [4], Er Co3 , and Y Co3 [5]. The formation of these hydrides is accompanied by the anisotropic expansion of the elementary cell. At the same time, the hydride of the CeNi3 compound is responsible for the anomalous anisotropic expansion of the lattice (∼ 30%) in the [ 00 z ] direction, whereas in the basal plane, the cell even undergoes contraction [6]. The hydrogen-absorption properties of the AB3 compounds (where A is Dy, Ho, Er, Tb, or Gd and B is Fe or Co) are investigated in [7]. It is shown that the hydrogen-absorption capacity of AB3 -type alloys is higher than for the AB5 -type alloys. Hence, these alloys are quite promising for manufacturing negative electrodes of the nickel–metal-hydride batteries. Among the REM Ni3 compounds (where REM is La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, or Y), YNi3 is characterized by the highest discharge capacitance [8]. Note that two types of hydrides are formed on the basis of this compound: lower ( YNi3 H1.6 ) and higher ( YNi3 H4.4 ). The YNi3 -base compounds in which Ni is partially replaced with Fe, Co, or Mn form a single hydride [9]. After a partial substitution of Ni atoms with larger atoms, the pressure of hydrogen required for the formation of hydride decreases, which may promote the choice of a sorbent of hydrogen with optimal parameters, the increase in the discharge capacitance of the electrode material, etc. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, Lviv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 4, pp. 47 – 52, July – August, 2007. Original article submitted February 11, 2007. 1068–820X/07/4304–0499

© 2007

Springer Science+Business Media, Inc.

499

500

V. V. BEREZOVETS’, R. V. DENYS, O. B. RYABOV,

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I. YU. Z AVALII

Fig. 1. Projection of the structure of AB3 compounds (of the PuNi3 structural type [14]). In the investigation of the influence of the substitution of components on the properties of AB3 -type compounds, our main attention is given to the hydrogen-absorption and electrochemical properties of the compounds based on YNi3 and alloyed with Mn. Experimental Procedure The alloys were synthesized from compact metals (Y, Ni, Mn with contents of the main component not lower than 99.9 at.%) by electric-arc melting on a copper water-cooled bottom in an atmosphere of argon. The samples of alloys were subjected to homogenizing annealing in evacuated quartz ampoules. The phase-structural analysis of alloys and their hydrides was carried out according to the data of powder diffraction obtained by using a DRON-3.0 diffractometer in the Cu Kα-radiation. The crystal structure of the original alloys and their hydrides was determined more precisely by the Rietveld method [10] with the help of the GSAS program [11]. The hydrogen-absorption capacity of the synthesized alloys was measured by the standard manometric method in a constant volume [12]. Prior to hydrogenation, the samples were activated by heating to 300–400°C in a vacuum of 1–10 Pa. For the electrochemical testing of metal hydride (MH) electrodes, we used glass threeelectrode cells. A cell of this sort has special compartments containing a working MH electrode, an auxiliary platinum electrode (platinum wire), and a reference electrode (silver–silver-chloride (Ag / AgCl) electrode, ϕ0 = 0.222 V). The volume of the cell is filled with an electrolyte (freshly prepared 6M KOH solution). The MH electrodes were originally made of alloys or their hydrides whose powders were screened through a sieve with a mesh size of 80 µm and mixed with a copper powder (in the 1 : 1 ratio). The obtained mixtures were compacted in the form of pellets with a mass of ∼ 0.2 g, a diameter of 12 mm, and a thickness of ∼ 1 mm by pressing in a nickel net under a load of 5000 kg / cm2 . The MH electrodes made of the original alloys were activated by repeating the cycles of charging and discharging.

HYDRIDES OF SUBSTITUTED DERIVATIVES BASED ON THE Y Ni3 COMPOUND

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Table 1. Crystallographic Parameters of the AB3 Phases in Y Ni3 – x Mnx Lattice constants, Å Alloy

V, Å

3

Vf.u. , Å

a

c

YNi3

4.9749(1)

24.439(1)

523.84(3)

58.20

YNi2.933 Mn0.067

4.9754(1)

24.4284(9)

523.71(2)

58.19

YNi2.867 Mn0.133

4.9844(3)

24.413(2)

525.28(5)

58.36

YNi2.8 Mn0.2

4.9918(3)

24.413(2)

526.82(5)

58.53

YNi2.733 Mn0.267

5.0006(1)

24.414(1)

528.72(3)

58.75

YNi2.667 Mn0.333 (1)

5.0105(1)

24.439(1)

531.33(3)

59.04

YNi2.667 Mn0.333 (2)

5.0182(1)

24.4447(7)

533.11(2)

59.23

YNi2.6 Mn0.4

5.0220(2)

24.454(1)

534.12(4)

59.35

YNi2.5 Mn0.5

5.0406(3)

24.479(2)

538.63(7)

59.85

YNi2.4 Mn0.6

5.0550(3)

24.511(2)

542.41(6)

60.27

YNi2.333 Mn0.667

5.0650(1)

24.5267(9)

544.91(3)

60.55

YNi2.267 Mn0.733

5.0682(2)

24.555(2)

546.23(5)

60.69

YNi2.167 Mn0.833

5.0818(3)

24.590(2)

549.97(6)

61.11

3

Comment: (1) annealing at 900°C for 150 h; (2) annealing at 700°C for 500 h.

Results of Investigation and Discussion Results of the X-ray phase diffraction analysis of the original pseudobinary alloys [with the following composition: Y Ni3 – x Mnx (0 ≤ x ≤ 0.833)] are presented in Table 1. The major part of the synthesized alloys was annealed at 800°C for 200 h. The main component of the alloys is the AB3 phase with insignificant admixtures of the AB5 , AB2 , and A2 B7 phases. The AB3 compounds crystallize in the following two structural types: Pu Ni3 (trigonal) and Ce Ni3 (hexagonal). Thus, in the family of compounds with Pu Ni3 structural type, we can mention the Gd Ni3 , Ho Ni3 [13], Er Co3 [5], La Ni3 , and Y Ni3 [8] compounds. The Y Ni3 compound crystallizes in the Pu Ni3 structural type (space group R – 3m) (Fig. 1). For the metals alloyed with Mn, we observe no changes in the structural type as the content of the substituting metal increases. According to the data of X-ray phase diffraction analysis, the solid solution based on Y Ni3 exists up to the following composition: Y Ni2.167 Mn0.833 . As expected, as a result of the substitution of Ni atoms for larger Mn atoms, the volume of the elementary cell increases. For all alloys, we obtained hydrides. They were also studied by the X-ray phase diffraction analysis. The original phases and hydrides were investigated more precisely by the Rietveld complete-profile method with the help of the GSAS program (Fig. 2 and Tables 2 and 3).

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(a)

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(b)

Fig. 2. Improved X-ray diffraction patterns of the YNi2.8 Mn0.2 parent alloy (a) and its YNi2.8 Mn0.2 H 4.06 hydride (b) with PuNi3 structural type.

Table 2. Crystallographic Parameters of the YNi2.8 Mn0.2 Compound Atom

CSP1

x /a

y /b

z /c

SOF

Uiso , Å2

Y1

3a

0

0

0

1.0

0.834

Y2

6c

0

0

0.861903

1.0

0.834

Ni1

3b

0

0

1 /2

1.0

0.489

Ni2

6c

0

0

0.665424

0.7

0.489

Ni3

18h

0.832277

0.167723

0.583688

1.0

0.489

Mn2

3b

0

0

0.665424

0.3

0.489

Comment: 1. CSP stands for the correct system of points, x / a, y / b, and z / c are the crystallographic positions of atoms, SOF is the site occupation factor, and Uiso is the thermal vibration of atoms. Space group: R – 3m (No.166); a = 4.9918(3) Å, c = 24.413(2) Å; R are the improvement factors: Rwp = 2.70%, Rp = 3.08%, and χ2 = 2.42.

The process of substitution of Ni atoms for Mn atoms occurs mainly in the layers of AB5 . The indicated behavior differs the investigated alloys from the alloys of R – Ni– Al systems (where R stands for Pr, Nd, Sm, Gd, Tb, Ho, Dy, and Er) in which, as a result of the substitution of Ni atoms for Al atoms, we observe the formation of a ternary phase (of the Ce3 Co8 Si structural type, i.e., the superstructure for Ce Ni3 ). In these phases, the substituted atoms are ordered only inside the AB2 layer (position 2a) [15]. As a result of hydrogenation of alloys from the Y Ni3 – x Mnx series, the elementary cell isotropically expands with preservation of the original symmetry of the metal matrix (Table 4). For all alloys except Y Ni2.933 Mn0.067 , we obtain single-phase hydrides. The Y Ni3 and Y Ni2.933 Mn0.067 alloys form lower hydrides characterized by the anisotropic expansion of the cell in the direction of the z-axis. A similar expansion is observed for the lower hydrides of the Ho Ni3 and Ce Ni3 compounds [4, 6]. As the amount of the substituting component increases, the hydrogen-absorption capacity of the material almost linearly decreases (Fig. 3).


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Fig. 3. Dependence of the hydrogen-absorption capacity of Y Ni3 – x Mnx alloys on the content of manganese. Table 3. Crystallographic Parameters of the YNi2.8 Mn0.2 Hydride Atom

CSP

x /a

y /b

z /c

SOF

Uiso , Å2

Y1

3a

0

0

0

1.0

0.834

Y2

6c

0

0

0.863986

1.0

0.834

Ni1

3b

0

0

1 /2

1.0

0.489

Ni2

6c

0

0

0.668853

0.7

0.489

Ni3

18h

0.839077

0.160924

0.586386

1.0

0.489

Mn2

3b

0

0

0.668853

0.3

0.489

Comment: Space group: R – 3m (No. 166); a = 5.2871(2) Å, c = 26.628(1) Å; R are the improvement factors: Rwp = 2.37%; Rp = 2.09%, and χ2 = 1.95.

We checked the investigated alloys as negative electrodes of nickel–metal-hydride chemical current sources and tested them in the galvanostatic mode at a current density of 100 A / kg. The discharge characteristics obtained as a result are presented in Table 5. Examples of discharge curves for the Y (Ni, Mn)3 materials are presented in Fig. 4. As follows from Fig. 3, even in the case of substitution of an insignificant amount of nickel in the Y Ni3 compound, the discharge capacitance ( C d) increases. The discharge capacitance of the electrodes based on the Y Ni3 – x Mnx phases attains its maximum for x = 0.33 and fairly rapidly decreases as the amount of manganese increases further (Fig. 5). The indicated changes in the discharge capacitance are caused by the superposition of two factors. Thus, on the one hand, the absorption capacity of the material decreases as nickel is substituted for manganese (Fig. 3) but, on the other hand, for small amounts of manganese, the indicated decrease is overcompensated by the lowering of the absorption–desorption plateau leading to the increase both in the amount of hydrogen participating in the electrochemical process and in the discharge capacitance of the material from 183 to 305 mA⋅ h / g. The maximum discharge capacitance of Y Ni3 samples (183 mA ⋅ h / g) constitutes only 39% of the theoretically predicted value. Note that the value obtained in the present work is close to the value established in [8].

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Fig. 4. Discharge curves of the compounds based on Y Ni3 . Table 4. Structural Data of the Hydrides Based on Y Ni3 – x Mnx ∆ a / aimc

∆ c /aimc

∆ V /Vimc ,

CH, wt.%

a, Å

YNi3 H4.4 [8]

1.65

5.283

26.82

648.3

6.1

9.7

23.6

YNi3 H1.61

1.65

4.987

26.82

577.7

0.2

9.7

10.2

YNi2.933 Mn0.067 H4.3

1.62

5.2817(2)

26.754(2)

646.34(6)

6.15

9.52

23.4

YNi2.933 Mn0.067 H4.3 – x1

1.62

5.045(2)

26.61(2)

586.5(5)

1.4

8.9

11.9

YNi2.867 Mn0.133 H4.23

1.60

5.2800(8)

26.670(5)

643.9(2)

5.93

9.24

22.6

YNi2.8 Mn0.2 H4.06

1.53

5.2871(2)

26.628(1)

644.61(4)

5.91

9.0

22.3

YNi2.733 Mn0.267 H3.45

1.47

5.2111(8)

26.541(4)

624.2(2)

4.20

8.71

18.1

YNi2.667 Mn0.333 H3.83

1.45

5.2797(3)

26.636(2)

643.02(6)

5.37

8.98

21.0

YNi2.5 Mn0.5 H3.83

1.45

5.3122(3)

26.598(2)

650.02(8)

5.38

8.65

20.7

YNi2.4 Mn0.6 H3.73

1.42

5.3070(2)

26.4994(9)

646.34(3)

4.98

8.11

19.2

YNi2.333 Mn0.667 H3.77

1.44

5.3268(3)

26.487(2)

650.89(7)

5.16

7.99

19.6

YNi2.167 Mn0.833 H3.73

1.43

5.3495(3)

26.502(2)

656.79(7)

5.26

7.77

19.4

Hydride

c, Å

3

V, Å

%

%

Comment: 1 designates a lower hydride which formed as a result of the partial desorption of hydrogen from a sample during holding in air.


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Fig. 5. Dependence of the discharge capacitance of electrodes based on Y Ni3 – x Mnx on the content of manganese. Table 5. Characteristics of Electrodes on the Base of Alloys with an AB3 Composition Alloy

Ctheor , mA⋅ h / g

Cmax , mA⋅ h / g

Cmax / Ctheor , %

YNi3

458

183

39

YNi2.933 Mn0.067

450

233

51

YNi2.867 Mn0.133

444

264

59

YNi2.8 Mn0.2

425

273

64

YNi2.733 Mn0.267

364

288

79

YNi2.667 Mn0.333

402

305

75

YNi2.6 Mn0.4

380

276

72

YNi2.5 Mn0.5

402

217

53

YNi2.4 Mn0.6

394

183

46

YNi2.333 Mn0.667

400

162

40

YNi2.267 Mn0.733

411

94

22

YNi2.167 Mn0.833

397

69

17

As a result of the analysis of the electrochemical properties of alloys from the Y Ni3 – x Mnx series up to the chemical composition Y Ni2.167 Mn0.833 , inclusively, it was discovered that the Y Ni2.667 Mn0.333 alloy ( Cmax = 305 mA ⋅ h / g ) is the best electrode material among the investigated materials. The values close to the values obtained for the indicated alloy ( Cmax = 303 mA ⋅ h / g ) were also observed for a similar alloy with aluminum. We studied the cyclic stability of the Y Ni2.667 Mn0.333 alloy (Fig. 6) whose discharge capacitance remains stable for 200 cycles (in a 6M KOH solution), which is the best result for the electrode materials based on the AB3 phases [16].

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Fig. 6. Cyclic stability of the electrode based on Y Ni2.667 Mn0.333 . CONCLUSIONS We study the structure and hydrogen-absorption capacities of pseudobinary compounds with the following chemical composition: Y Ni3 – x Mnx . It is shown that Ni atoms are substituted for Mn atoms mainly in the layers with Ca Cu5 structural type. A continuous substitution solid solution is observed up to the Y Ni2.167 Mn0.833 composition. The hydrogen-absorption capacity of Y Ni3 – x Mnx decreases from 1.65 to 1.43 wt.% as the level of substitution with manganese increases. The hydrogenation of the Y Ni3 – x Mnx compounds leads to the isotropic expansion of the crystal lattice. The charging–discharging characteristics of the electrodes based on Y Ni3 – x Mnx alloys are investigated. It is shown that the Y Ni2.167 Mn0.833 alloy is characterized by the maximum discharge capacitance (305 mA ⋅ h / g) decreasing by 10% after 200 cycles of charging and discharging. The present work was financially supported by the INTAS (Grant 05-100005-7671). REFERENCES 1. www.cobasys.com, www.duracell.com. 2. O. A. Petrii, S. Ya. Vasina, and I. I. Korobov, “Electrochemistry of hydride-forming compounds and alloys,” Usp. Khim., 65, No. 3, 195–210 (1996). 3. A. F. Wells, Structural Inorganic Chemistry, Clarendon Press, Oxford (1982). 4. Y. E. Filinchuk, D. Sheptyakov, and K. Yvon, “Directional metal-hydrogen bonding in interstitial hydrides. 2. Structural study of Ho Ni3 D x ( x = 0, 1.3, 1.8 ),” J. Alloys Comp., 413, 106–111 (2005). 5. M. I. Bartashevich, A. N. Pirogov, V. I. Voronin, T. Goto, et al., “Crystal structure of γ-phase R Co3 H∼4 hydrides,” J. Alloys Comp., 231, 104–107 (1995). 6. V. A. Yartys, O. Isnard, A. B. Riabov, and L. G. Akselrud, “Unusual effects on hydrogenation: anomalous expansion and volume contraction,” J. Alloys Comp., 356–357, 109–113 (2003). 7. C. A. Bechman, A. Goudy, T. Takeshita, et al., “Solubility of hydrogen in intermetallics containing rare-earth and 3d transition metals,” Inorg. Chem., 15, 2184–2192 (1976). 8. X. Zhang, W. Yin, Y. Chai, and M. Zhao, “Structure and electrochemical characteristics of Re Ni3 alloy,” Mat. Sci. Eng., B., 117, 123–128 (2005). 9. V. V. Burnasheva and B. P. Tarasov, “Effect of the partial replacement of nickel or yttrium by other metals on hydrogen absorption by yttrium–nickel ( Y Ni3 ) compounds,” Russ. J. Inorg. Chem., 29, 651–655 (1984). 10. R. Young (editor), Rietveld Method, Oxford University Press (2000). 11. A. C. Larson, R. B. Von Dreele, M. Lujan, Jr., Neutron Scattering Center, MS-H805, Los Alamos National Laboratory (2000). 12. V. A. Yartys’, I. Yu. Zavalii, and M. V. Lotots’kyi, “Low-pressure hydrogen absorbers based on Zr–V and Zr–V–Fe alloys modified by oxide admixtures” Koord. Khim.,18, No. 4, 409–423 (1992).


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13. Y. V. Burnasheva and V. P. Tarasov, “The absorption of hydrogen by the intermetallic compounds RNi3 , where R represents a lanthanide of the yttrium subgroup,” Russ. J. Inorg. Chem., 27, 1077–1079 (1982). 14. J. H. N. Van Vucht and N. V. Philips, “Note on the structures of GdFe3 , GdNi3 , GdCo3 and the corresponding yttrium compounds,” J. Less-Common Met., 10, 146–147 (1966). 15.E. I. Gladyshevskii and O. I. Bodak, Crystallochemistry of Intermetallic Compounds of Rare-Earth Metals [in Russian], Vyshcha Shkola, Lvov (1982). 16. R. Baddour–Hadjean, J. Pereira–Ramos, M. Latroche, A. Percheron–Guegan, “New ternary intermetallic compounds belonging to the R–Y–Ni (R = La, Ce) system as negative electrodes for Ni–MH batteries,” J. Alloys Comp., 330–332, 782–786 (2002).