Thermodynamic Consideration on Multi-Step Hydrogenation

of Mg17Al12 Assisted by Phase Separation

Masashi Sato and Toshiro Kuji

Department of Applied Chemistry, School of Engineering, Tokai University, Hiratsuka 259-1292, Japan

The multi-step hydrogenation and dehydrogenation on the pressure-composition-temperature relations for the Mg17Al12-H system wasthermodynamically discussed on the based upon the enthalpy for Mg-Al alloy recombination during corresponding reaction with hydrogen.Pressure-composition-temperature diagrams were measured Mg17Al12-H system by means of the volumetric technique in the temperature rangeof 598–648 K. The isotherms show two step hydrogenation and dehydrogenation indicating a disproportionate formation of MgH2 via Mg2Al3intermetallic phase. The decomposition of Mg from the intermetallic phase causes thermodynamic destabilisation for the formation of Mghydrides. The molar enthalpies of hydride formation for the investigated systems are: �Hf ¼ �38:5� 0:9 kJ (molH)�1 for the Mg-H system;�Hf ¼ �36� 1 kJ (molH)�1 for the first-step hydrogenation in Mg17Al12; �Hf ¼ �31� 1 kJ (molH)�1 for the second-step hydrogenation inMg17Al12. [doi:10.2320/matertrans.M2011123]

(Received April 22, 2011; Accepted June 30, 2011; Published August 25, 2011)

Keywords: metal hydride, gas-solid reaction, thermodynamics, hydrogen storage, magnesium

1. Introduction

Magnesium hydride, MgH2, is potentially high capacityhydrogen absorbers since the Mg absorbs considerableamount of hydrogen around 7.6 mass%.1,2) However, theformation or decomposition of MgH2 takes place at relativelyhigh temperature above 573 K, which does not satisfy therequirement in an actual application use.3) As mentionedabove, the Mg hydride is thermodynamically very stable andthe kinetics on hydrogenation-dehydrogenation is signifi-cantly slow.4–7)

To control those properties of Mg hydride, two aspectsshould be paid from the view of ‘‘kinetics’’ and ‘‘equi-librium’’. On the first view, the addition of catalyst seemsto be the most probable solution for the enhancement ofhydrogenation-dehydrogenation kinetics.8,9) The transitionmetals or metal oxides play the role of pathway for electronexchange between Mg and hydrogen molecules upon thehydrogen charge-discharge reactions. The second viewprovides the modification of the thermodynamic propertiesusing Mg-based alloys or compounds. The properties of themetal hydrides are traditionally controlled by the nature andrelative amount of the constituent elements.10) However, thisstraight forward way does not affect the thermodynamicproperties of Mg-based hydrides in most cases, because thethermodynamic stability of Mg-based alloy is dominated bythe strong Mg-H bonding.11,12)

From the thermodynamic point of view, stability of Mg-based alloys gives spontaneously the results that hydrogenabsorption-desorption pressures becomes lower. Even ifthe attempt from the kinetics view succeeds the decreasingtemperature for hydrogen charge-discharge reaction to Mg,the Mg-based hydrides are not convenient as hydrogenstorage media since the hydrogen supplying pressure istoo low less than 10�6 MPa at 373 K. The combinationof both approaches is of importance towards to hydrogenapplications.

Previously reported5,6) Mg17Al12 intermetallic compounduptakes hydrogen accompanied with two-step disproportio-nations from the parent compound as:

Mg17Al12 þ 9H2 $ 9MgH2 þ 4Mg2Al3 ð1ÞMg2Al3 þ 2H2 $ 2MgH2 þ 3Al ð2Þ

It should be emphasised that only decomposed Mg from thecompounds absorbs hydrogen during the reactions shownabove. Consequently, the P-C isotherms for Mg17Al17-Hsystems show two pressure plateaux, and both plateaux aresignificantly higher than that of Mg-H systems. This impliesthe fact that the thermodynamic destabilisation of the MgH2

takes place in the reactions of (1) and (2). Andreasen pointedout that endothermic reaction from the decomposition ofintermetallic contributes to the enthalpy changes on thehydrogenation-dehydrogenation, implying that the stabilityof MgH2 becomes lower.13)

The present study is focused on the thermodynamicdestabilisation of MgH2 with a phase separation of Mg17Al12

intermetallic compound. The influence of a phase separationon the thermodynamic stability of MgH2 was quantitativelydiscussed. The considerable differences between single-stepand multi-step reactions were demonstrated. The idea ofthermodynamics for such reaction, ‘‘hydrogen storage alloywith phase separation’’, will be proposed.

2. Experimental Procedures

2.1 Preparation of sampleThe Mg17Al12 intermetallic compound was prepared by a

flexible shaking-type high energy ball milling (Super-MisuniNEV 8) under Ar atmosphere with a rotation speed of720 rpm for 5 h. A stoichiometric mixture of startingmaterials from Mg (powder with <180 mm, >99:9% puritygrade), Al (powder 53–106 mm, >99:99% purity grade) wereloaded into the SUS304 steel vial with fourteen ballstogether. The inner volume of the vial and the size of ballwere, respectively, 100 mm3 and 12.7 mm diameter. Theweight ratio of the ball to the sample was 73 : 1. On themilling processes, the temperature around the vial wascontrolled by water-cooling system at 293 K.

The formation of the Mg17Al12 with the Mn-type structurewas confirmed by powder X-ray diffraction using CuK�1

Materials Transactions, Vol. 52, No. 9 (2011) pp. 1773 to 1776#2011 The Japan Institute of Metals

radiation. No significant impurity phases were found. Therefined unit cell parameter for Mg17Al12 was a =1.0559(4) nm. The value agrees very well with the referencedata.14) Stoichiometric composition was determined byElectron Probe Micro Analyser. Any significant contamina-tive impurities from the vial or balls were confirmed to benegligibly small.

2.2 Pressure-composition isotherms measurementPressure-Composition-Temperature (P-C-T) relations

were volumetrically determined by a Sieverts’ method at598 K, 623 K and 648 K. The samples were activated at 623 Kfor 24 h under hydrogen atmosphere at pressure 6.0 MPa, andsubsequently several hydrogenation-dehydrogenation cycleswere made prior to the P-C-T studies. Hydrogen gas of99.99999% (7N) grade purity was used.

3. Results and Discussion

3.1 P-C-T isothermsFigure 1 shows P-C-T diagram for the Mg17Al12-H

systems at 598–648 K. The presence of clear paleaux couldbe reproducibly seen approximately in the range of 5 �H/Mg17Al12 � 17 and 19 � H/Mg17Al12 � 32. The reac-tions on the plateaux are identified as corresponding toeq. (1) for the first plateau and eq. (2) for the secondplateau. The curves finally saturate in the vicinity ofH/Mg17Al12 ¼ 34. Such reversible two plateau behaviouris in good agreement with previous data.5,6)

The stability of hydrides is closely related to the plateaupressures. Two plateaux in Mg17Al12-H systems showsignificantly higher pressures than that in Mg-H system asshown in Fig. 2. The maximum hydrogen capacity wasnormalised as 0 < r < 1. These pressures change in thesequence of Mg < first plateau in Mg17Al12 < secondplateau in Mg17Al12. The value of chemical potential of Hin metal can be expressed by 1=2RT lnPH2

, where PH2the

hydrogen dissociate pressure, T the temperature and R the gasconstant. The chemical potential for the Mg17Al12-H systemat 598 K is 1.9 kJ (molH)�1 on the first plateau and3.4 kJ (molH)�1 on the second plateau higher than for theMg-H system. As mentioned earlier, only Mg absorbshydrogen in the system Mg17Al12-H. The changes in theplateau pressure indicate the thermodynamic destabilisationof Mg hydrides.

The destabilised Mg hydrides are reported for MgCu2-Hand Mg2Al3-H systems.15–17) Common feature of thoseis disproportionation reaction upon the hydrogenation-dehydrogenations. Possibly, such behaviour is the only theway to destabilise Mg hydrides thermodynamically.

The plateau behaviour on the second plateau for theMg17Al12-H systems seems to be rather of difference than thebehaviour for the Mg2Al3-H systems,16,17) although bothreactions can be commonly described as eq. (2). The secondplateau pressure for the Mg17Al12-H system shows 0.95 MPaat 598 K, while the plateau pressure on the Mg2Al3-Hsystems equals to approximately 0.7 MPa.17) It suggests thereaction (2) may not be simple from the thermodynamic pointof view, when going through the reaction (1). This will bediscussed as related to the thermodynamic properties later inthe part of this paper.

3.2 Thermodynamics for Mg17Al12-H systemsBased on the temperature dependence of the isotherms

presented in Fig. 1, the molar enthalpies �Hf for the hydrideformation were determined from the van’t Hoff plots of PH2

versus 1=T in the plateaux (see Fig. 3). The calculated valuesare summarised in Table 1. The values of enthalpies for theMg17Al12-H system are significantly higher than of Mg-Hsystem. Such lower thermodynamic stability of the systemMg17Al12-H is in good agreement with the measuredisotherms (see Fig. 2).

Overall reaction of the Mg17Al12-H system can beseparately described with the enthalpy changes as followings:

00.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5 648 K abs 648 K des 623 K abs 623 K des 598 K abs 598 K des

Equ

ilibr

ium

pre

ssur

e, P

H2

/ MP

a

H / Mg17

Al12

5 10 15 20 25 30 35

Fig. 1 Pressure-Composition-Temperature (P-C-T) diagrams for

Mg17Al12-H systems at 598–648 K. Hydrogen absorption (abs) and

desorption (des) curves are presented.

0.00.0

0.5

1.0

1.5598 K

Mg17

Al12

-H System Mg-H System

1.9 kJ/molH

3.4 kJ/molH

Equ

ilibr

ium

pre

ssur

e, P

H2

/ MP

a

r0.5 1.0

Fig. 2 Hydrogen absorption in P-C-T curves for Mg-H and Mg17Al12-H

systems at 598 K. Hydrogen concentration in the systems is normalised as

0 < r < 1.

1774 M. Sato and T. Kuji

Mg17Al12 ¼ 17Mgþ 12Al �HEndothMg17Al12

ð3Þ

2Mgþ 3Al ¼ Mg2Al3 �HExothMg2Al3

ð4Þ

Mgþ H2 ¼ MgH2 �HExothMgH2

ð5Þ

Mg2Al3 ¼ 2Mgþ 3Al �HEndothMg2Al3

ð6Þ

The combination of eqs. (3), (4) and (5) or eqs. (5) and (6)describes the reaction on the first plateau or the secondplateau for the Mg17Al12-H system, respectively, as:

Mg17Al12 þ 9H2 ¼ 9MgH2 þ 4Mg2Al3 �H1stf ; where

�H1stf ¼ �HEndoth

Mg17Al12þ 4�HExoth

Mg2Al3þ 9�HExoth

MgH2ð7Þ

Mg2Al3 þ 2H2 ¼ 2MgH2 þ 3Al �H2ndf ; where

�H2ndf ¼ �HEndoth

Mg2Al3þ 2�HExoth

MgH2ð8Þ

From eqs. (7) and (8), the calculated values of enthalpychanges for the first plateau �H1st

f and the second plateau�H2nd

f are, respectively, �35:3� 1:8 and �35:4� 1:2 kJ(molH)�1, in which the values of �H in eqs. (3) to (6) aretaken from Table 1 and the literature data18) as �HEndoth

Mg17Al12¼

þ107� 11 kJ (molMg17Al12)�1, �HExoth

Mg2Al3¼ �12:5� 1:3 kJ

(molMg2Al3)�1, �HEndoth

Mg2Al3¼ þ12:5� 1:3 kJ (molMg2Al3)

�1,�HExoth

MgH2¼ �77:0� 0:9 kJ (molH2

)�1. Obtained �H1stf is in

well agree to the experimental data (�36� 1 kJ (molH)�1),which is consist with the result by Andreasen.13) However,the trends of behaviour for �H2nd

f change does not agree withthe experimental data (�31� 1 kJ (molH)�1) at all. Thisleads to the question arising that the idea of endothermic con-tribution from the phase separation may be more complex.

The chemical potential of H in metal, ��H, can beexpressed by ��H ¼ ���H þ �id

H þ �exH , where ���H is the

reference, �idH is the ideal and �ex

H is the excess contributions.As a zeroth approximation, �id

H and �exH depends on only H

concentration in host metal.19) Because only Mg absorbshydrogen in whole reactions, the �id

H and �exH should be

independent of the root of hydrogen uptake even for thesingle step or multi step reactions. Thus, the deferencebetween the first plateau and the second plateau may beassociated with the reference part ���H.

Figure 4 illustrates the schematic isotherms for two-stepphase separation type reaction as a function of normalised Hper formula unit of metal r. For two reaction steps, the insetaxis are individually given as the re-normalised r0 and r00. The���H refers to the infinite dilute region of H in metal,19) whichcan be defined as r0 ! 0 for the first and r00 ! 0 for thesecond steps. It should be noted that the reference points areof difference between the first and the second reactionsagainst real scale r.

In both reactions, the pressure plateaux may occur in thestructure transformation type from Mg (P63/mmc) to MgH2

(P42/mmm). In this case, the enthalpy change could be givenby Rudman.20)

�Habf ¼

1

b0 � a0

Z b0

a0�H�;r

0!0H dr0 þ

1

b0 � a0

Z b0

a0�Hex

H dr0

ð9Þ

�Hcdf ¼

1

d0 � c0

Z d0

c0�H�;r

00!0H dr00 þ

1

d0 � c0

Z d0

c0�Hex

H dr00

ð10Þ

When the values of enthalpy change were calculated foreqs. (7) and (8), which was presented earlier in this part, thereference enthalpy was taken the same value for bothreactions with the r! 0 (¼ r0 ! 0). As discussed above,however, it should be rejected for eq. (8). Based on eq. (10),the value of the enthalpy change for the second plateaudescribing in reaction (8) was calculated again, with thereference enthalpy taken from the value �H1st

f ¼ �35:3�1:8 kJ (molH)�1 (¼ r00 ! 0). The obtained �H2nd

H ¼�32:3� 2:1 kJ (molH)�1 excellently agrees with comparing

1.510-1

100

101

Mg17

Al12

(abs-2nd) Mg

17Al

12(des-2nd)

Mg17

Al12

(abs-1st) Mg

17Al

12(des-1st)

Mg(abs) Mg(des)

Equ

ilibr

ium

pre

ssur

e, P

H2

/ MP

a

1000 / T / K-1

1.6 1.7

Fig. 3 van’t Hoff plots on the plateau pressures for Mg-H and Mg17Al12-H

systems.

Table 1 Plateau pressures and molar enthalpies of hydride formation for

Mg and Mg17Al12-H systems.

Sample PH2at 598 K/MPa �Hf/kJ (molH)�1

Mg 0.26 �38:5� 0:9

Mg17Al12

0.52 (1st plateau) �36� 1

0.92 (2nd plateau) �31� 1

10

vv

vv

b'a'

d'c'

10 r '

10 r ''

dcba

Che

mic

al p

oten

tial,

Δ μH (

a.u.

)

r

Fig. 4 Schematic isotherm for multi-step hydrogen absorption or desorp-

tion. Insets present the renormalized figures for the first-step and the

second-step reactions.

Thermodynamic Consideration on Multi-Step Hydrogenation of Mg17Al12 Assisted by Phase Separation 1775

to the experimental value �31� 1 kJ (molH)�1. On the basisof these considerations, we suggest that in the multi-disproportionation the thermodynamic references should betaken as the different points in the whole reactions.

4. Conclusions

The present study shows a multi-step hydrogenation for thesystem Mg17Al12. A beneficial advantage of hydrogenstorage alloy with phase separation is to decrease thermody-namic stability of Mg hydrides. Hydrogenantion drives twosteps formation of MgH2 with phase separation from theintermetallic Mg17Al12 phase, and the thermodynamicstability of MgH2 becomes lower than that of pure Mg-Hsystem. For the first-step disproportionate hydrogenation,the thermodynamic behaviour can be well explained by theonly endothermic contribution with a phase separation. Thesecond-step hydrogenation is more complex, and the refer-ence part of a chemical potential in the solid phase should bereconsidered again with the endothermic contribution.

Acknowledgements

This work was supported under contract 08007857-0 withthe New Energy and Industrial Technology DevelopmentOrganization (NEDO), Japan, by its Fuel Cell and HydrogenTechnology Development Department. The authors thankMr. Shintaro Asakura and Mr. Kenji Sakuragi for helpingthe P-C-T measurements.

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