Hydrogen storage properties of the MmNi4.6Sn0.4 system

htt. J. Hydrogen Energy. Vol. 17, No. 8, pp. 603-606. 1992. Prinlcd in Great Britain.

0360-3199/92 $5.00 + 0.00 Pergamon Press Ltd. © 1992 International Association for Hydrogen Energy.

HYDROGEN STORAGE PROPERTIES OF THE

MmNi4.6Sno.4SYSTEM

M. N. MUNGOLE,*R. BALASUBRAMANIAM,*K. N. RAI? and K. P. SINGH~ *Department of Metallurgical Engineering, Indian Institute of Technology, Kanpur, India ")'Department of Metallurgical Engineering and Materials Science Program, Indian Institute of Technology, Kanpur, India (Received for publication 6 April 1992)

Abstract--The hydrogen absorption and desorption characteristics of cast and annealed MmNi4.6Sn04 intermetallic have been investigated in the temperature range 278-308 K. Hydride formation pressure is in the range of 4 - 1 0 atm in the above temperature range. The ease of hydriding of MmNia.6Sn0.4 has been related to an increase in the unit cell volume of MmNi5 with Sn substitution. The enthalpies of hydride formation for the as-cast and annealed conditions are -26.33 kJ tool ~ H2 and -25.53 kJ mol-~ H2, respectively. The entropies of hydride formation are -108.21 J K t mol ~ H2 and -105.49 J K ~mol -I H2 for the as-cast and annealed intermetallic, respectively. Annealing markedly flattened the plateau pressure and increased the hysteresis at any fixed temperature. Hysteresis reduced with increasing temperature for both the cast and annealed conditions. The hydrogen storage capacity (i.e. the maximum H/M) was limited to 0.6.

INTRODUCTION

EXPERIMENTAL METHOD

The intermetallic LaNi5 is a promising material for rechargable hydrogen storage. It shows a relatively low hydride plateau pressure at room temperature, low hysteresis losses and easy activation [ 1 - 3 ]. However, due to the high cost of lanthanum it is replaced in LaNi5 with a cheaper rare-earth (RE) mixture called mischmetal [ 4 - 6 ] . The composition of mischmetal (Mm) varies depending upon the source, and its constituents are La, Ce, Pr, Nd and Sm. The main impurity element in mischmetal is generally iron. MmNis, however, does not show very favourable hydrogen storage properties. It exhibits very high hydriding (60 atm [6]) and dehydriding (20 atm [6]) pressures, and a large hysteresis of about 40 atm between the absorption and desorption pressures [6]. In order to overcome these problems, modifications have been made to the compound MmNi5 by partially substituting either Mm with Ca[4, 7] and T i [ 7 ] , or Ni with AI[4, 8], Mn[4, 6, 9], C u [ 4 ] , C r [ l l ] or Si[12]. These modifications have resulted in easier activation, lowering of the room temperature hydride plateau pressure and reduction of hysteresis losses [4, 6], and thus their possible use as hydrogen storage materials. (where the Ni is partially replaced with Sn) is investigated for its hydriding behaviour, using Indian mischmetal. The effect of annealing on the microstructure, hydride plateau pressure, hysteresis and the enthalpy of hydride formation is reported.

The mischmetal used in this study was procured from M/S Raw Flint Pvt Ltd, Rajkote, India. The composition of mischmetal was determined analytically and it contained (in wt%) 43% Ce, 23% La, 18% Nd, 5% Pr, 3% Sm and 8% Fe. It was mixed with high purity (99.99%) Ni and Sn to obtain MmNi5 and MmNia.6Sn04 intermetallics. Buttons weighing 20 g were prepared by arc melting in an argon atmosphere and subsequently melted three times for better homogeneity. One half of each batch was heat treated at 1000°C for 24 h under 10 -2 torr pressure. The bulk samples of the as-cast and annealed MmNi46Sn04 were first activated at room temperature using a compression and evacuation cycle (10 min at 30 atm pressure hydrogen gas). They were found to be fully activated in 15 cycles. The pressure-composition isotherms of these samples were obtained in a locally fabricated pressure system [ 13]. The microstructure of the samples, in both the as-cast and annealed conditions, was obtained using a JSM 840A scanning electron microscope. The microhardness of these intermetallics was measured as a function of thermal history. Precise lattice parameters were determined from the powder diffraction pattern of the samples mixed with annealed Cu powder (standard) using a Rich Siefert 2002D X-ray diffractometer. The radiation used in the diffractometer was CrK,. RESULTS AND DISCUSSION The scanning electron micrographs of MmNi5 and MmNi4.6Sno 4 samples in the as-cast condition are shown in

:~Retired professor, I. I. T.. Kanpur, India. 603

604

M. N. MUNGOLE

Fig. la and b, respectively. It is seen that the MmNi5 intermetallic exhibits macrosegregation. The segregation morphology, however, is altered by the partial substitution of Ni by Sn (Fig. lb). On the other hand, the micrographs of the annealed samples (Fig. lc and d), show that the morphology of the macrosegregates has been modified in MmNis. The segregate morphology is not altered on annealing in MmNi46Sn04. The microhardnesses of these samples are listed in Table 1. The partial substitution of Ni with Sn in MmNi5 results in a sharp increase in the microhardness for the as-cast condition. Annealing decreases the hardness of both the intermetallics with the effect being more drastic for MmNi4 ~Sn04. Pressure-composition isotherms (PCT) of hydrogen absorption and desorption for the activated MmNi46Sn. 4 in the as-cast and annealed conditions are shown in Figs 2 and 3, respectively. The maximum hydrogen storage capacity of MmNi~6Sn04 is limited to about 0.6. A pronounced decrease in the maximum H/M with temperature is observed for the cast intermetallic, whereas there is no such change for the annealed sample. The plateau pressure increases with temperature for both the as-cast and annealed samples. The high hydride formation plateau pressure (60 atm) [6] for MmNi5 has been significantly lowered with the partial substitution of Ni by Sn in MmNis. The plateau pressure of MmNi4~,Sn04 is between 4 and 10 atm in the temperature range 2 7 8 - 3 0 8 K (Figs 2 and 3). The X-ray diffraction analysis indicated that the partial substitution of Ni by Sn in MmNi5 resulted in an increase in the unit cell volume (Voc) of MmNi~ (Table 2). There is a corresponding increase in the interstitial volume and this is related to the increased hydride stability, i.e. lowering of the plateau pressure. This correlation between hydriding stability and unit cell volume has been established for other RENi5 [ 14, 15] and RECo5 [ 14, 15] compounds and also holds true for the MmNi5 system where Ni is partially substituted by AI [4], Mn [4, 16] and Cu [4]. The PCT isotherms for the as-cast sample (Fig. 2) indicate that the plateau pressure region is not fiat but shows a sloping tendency. This is due to the inhomogeneity in the sample [4] (Fig. 1). The annealing treatment enhances homogeneity and therefore the slopes of the plateau pressure region are reduced (Fig. 3). This is similar to the annealing effects of RENi5 intermetallics observed by other investigators [4, 6]. The enthalpies of hydriding were determined from the absorption isotherm for both the as-cast and annealed samples from the Van't Hoff plots presented in Fig. 4. These enthalpies are - 2 6 . 3 3 kJ mol ~ H2 and - 2 5 . 5 3 kJ mol ~ H2 for the as-cast and annealed samples, respectively. These values are similar to the enthalpy of hydriding for the MmNi5 system ( - 2 6 . 3 7 ld mol ~ H2 [6]). The entropies of hydride formation were - 108.21 J K ~ m o l t He and - 1 0 5 . 4 9 J K ~ mol ' H2 for the as-cast with annealed samples, respectively. These values along with their error limits are given in Table 3. The MmNi46Sr~j.6 system also exhibits hysteresis during hydride formation and decomposition (Figs 2 and 3), namely, the hydride formation pressure (P3 is higher

et al.

Fig. 1. (a) Microstructure of the as-cast MmNi5 intermetallic. (b) Microstructure of the as-cast MmNia6Sno.4 intermetallic. (c) Microstructure of annealed MmNi5 intermetallic. (d) Microstructure of annealed MmNi46Sn0.4 intermetallic.

MmNi4.6Sn0.a SYSTEM

605

Table 1. Microhardness of the alloys 14

System

--

Microhardness (VHN)

MmNi5 MmNi46Sn04

As-cast

Annealed

687.88 862.59

650.64 652.36

O

278K

a

288

K

.

.

.

.

.

12

"

./.-'°t

10

'~ 8,

¢

_/._~/J

¢' )

/

/

///

/

.

/

than the hydride decomposition pressure (Pd). This represents a loss in useful available energy which is given by JRT ln(pf/pa) mol ~H. A convenient method of representing hysteresis is by considering the hysteresis factor (Po/pf) [ 17]. This is graphically presented as a function cf temperature for the as-cast and annealed MmNia6Sn04 in Fig. 5. The hysteresis factor was obtained at H/M = 0.3. The results indicate that the annealing treatment has effectively increased hysteresis at all temperatures. The hysteresis losses decrease with increase in temperature. A similar effect of annealing and temperature on hysteresis has been observed in the LaNi5 system [18]. A detailed analysis of hysteresis in MmNi~_xM~ system will appear later.

2

.

.4

( Annectted 0l 0

i

i 0.1

I 0.2

,

1

I 0.3

i

I 0.4

0.5

HIM

Fig.

3. Pressure-composition, absorption and desorption isotherms for annealed MmNi4.6Sno.4 intermetallic.

CONCLUSIONS

12

//

/

7/

(1) Hydriding of MmNi5 is facilitated by substituting Ni with Sn and this can be related to an increase in unit cell volume with Sn substitution. (2) The enthalpies of hydriding are comparable to that for MmNis. (3) Annealing does not drastically effect hydriding properties, but leads to increased hysteresis losses. (4) Hysteresis decreases with increase in temperature.

"

/I

/i

10

Acknowledgement--The authors are grateful to the Department of

p-

Non-Conventional Energy Sources, New Delhi for financial support.

/e,./ ..-" ..yY / m

_

2

.

f /

k" 0 0 Fig.

r

.

.

Table 2. Lattice parameters and unit cell volume of MmNi5 and MmNi4.oSno 4 intermetallics

.

MmNi4.6Sn0.t, (As cast) I ~ I 0.1 0.2

i

I 0.3 H/M

~

I 0.4

i

L 0.5

]

0.6

2. Pressure-composition, absorption and desorption isotherms for the as-cast MmNi46Sn04 intermetallic.

System

ao(,~)

co(~,)

V,,(J,,) 3

MmNi5 MmNi46Sno.~

4.9335 4.9695

3.9898 4.0296

84.0992 86.1823

M . N . MUNGOLE et al.

606

1.0

3.C

0.8 2.5

f

J

'Io 0.

0.6 ¢1 CI IL

2.C

It

,," o.,~ 1.5

1.0 3.1

I

I 3.2

o

As

c,

Annealed

I

cast

I 3.3

~

Mm NI4"6 $n0"4

0.2

[ 3.1,

I

11Tx1000

I 3.5

I

I 3.6

I

0.0 3.7

K-1

~

-C

Z

I 278

I

l 288

Temperature

Fig. 4. Van't Hoff plots for as-cast and annealed MmNi46Sn04 intermetallics,

AS COS! Annealed

I

] 298

, K

Fig. 5. Hysteresis factor of as-cast and annealed MmNi4.6Sno.4 intermetallics.

Table 3. Enthalpy (AH) and entropy (,5S) of hydride formation for MmNi5 and MmNi46Sno 4

System MmNi4 6Sno., as-cast MmNi4 6Sn0.1, annealed MmNi5

AH kJ mol i H2

~S J K -I mol H~

-26.33 + 0.14 -25.53 + 0.14 -26.37 [6]

- 108.21 + 0.70 -105.49 .4- 0.68 --

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