Hydrogenation properties of Mg-Al alloys

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http://dx.doi.org/10.1016/j.ijhydene.2008.09.095http://dx.doi.org/10.1016/j.ijhydene.2008.09.095


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Hydrogenation properties of Mg-Al alloys

Anders Andreasen ,1

Materials Research Department, National Laboratory for Sustainable Energy, Ris

DTU, Fredriksborgvej 399, DK-4000 Roskilde, Denmark.

Abstract

In this paper the properties of Mg-Al alloys in relation to hydrogen storage are re-viewed. The main topics of this paper are materials preparation, hydrogen capacity,

thermodynamics of hydride formation, and the kinetics of hydride formation anddecomposition.

Hydrogenation of Mg-Al leads to disproportionation with the formation of mag-nesium hydride and metallic aluminum as the final product. Experimental evidencerenders this process reversible. It is observed that the enthalpy of hydride formationof magnesium is lowered upon alloying with Al due to a slightly endothermic dispro-portionation reaction. Further, it is found that the kinetics of hydrogenation, as welldehydrogenation, may be significantly improved by alloying compared to pure Mg.The expense of these improvements of the hydrogenation/dehydrogenation proper-ties is a lower gravimetric hydrogen density in the hydrogenated product.

Key words: Metal hydrides, Magnesium, Aluminum, Kinetics, Thermodynamicproperties

1 Introduction

Hydrogen storage in magnesium has been the subject of an intense research

effort during the last 30 years (see e.g. [1,2] and references therein), mainlydue to its high theoretical gravimetric hydrogen density ofm(H2) = 7.6 wt.%.However, due to slow kinetics [3] and a high thermodynamic stability [4,5]of MgH2, heating to above 573 K is required in order to release hydrogen atconditions relevant for practical applications. This is the ultimate show stopper

Email address: [email protected] (Anders Andreasen).1 Present address: Basic Research, Process Development, MAN Diesel A/S, Tegl-holmsgade 41, DK-2450 Copenhagen SV, Denmark.

Preprint submitted to International Journal of Hydrogen Energy25 September 2008


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for the usage of magnesium as a hydrogen storage medium in combination witha Polymer Electrolyte Membrane (PEM) fuel cell, operating at significantlylower temperatures, in future mobile applications.

A variety of alloying elements have been explored in order to bring down

the hydrogen desorption temperature [58], although with somewhat limitedsuccess. Mg2Ni being one of the most successful attempts [911] still requiremore than 523 K in order to release hydrogen at 1 bar. Thus, hydrogen storageapplications based on magnesium seems to be limited to either traditionalcombustion engines or fuel cells working at higher temperatures than the PEMe.g. for stationary applications.

Besides the high gravimetric hydrogen capacity one of the great advantages ofmagnesium compared to traditional hydrides such as the AB (e.g. TiFe), AB2(e.g. TiCr2) and AB5 (e.g. LaNi5, CaNi5) mainly being based on transition

metals and rare earth elements, is its abundance and its price. Magnesium isthe 8. most abundant element in the earths crust [12] and one cubic meter ofseawater contains 1.3 kg Mg. The price of Mg is approx. 3.5 $/kg in comparisonwith a price of 4-15 $/kg for Ni, Ti, V and Cr and a price of 350 $/kg for La[13]. Thus, in terms of price pr. stored kg of H2 is superior to most traditionalhydrides.

When alloying with Al the low price is retained (Al price approx. 1.4 $/kg[13]) and Al adds improved heat transfer to the hydride bed [14], which isessential for fast dehydrogenation. In addition to this it has generally beenfound that the thermodynamics and kinetics of Mg-Al compared to Mg areimproved along with the resistance towards oxygen contamination.

In the present paper the hydrogen storage properties of Mg-Al alloys arereviewed. The focus of this review will be materials preparation, hydrogencapacity, thermodynamics of hydride formation, and kinetics of hydrogena-tion/dehydrogenation.

2 Mg-Al phase diagram and stable alloy phases

The phase diagram of Mg-Al includes at least four stable phases [15,16]:the fcc solid solution of magnesium in aluminum, (Al), the hcp solid solu-tion of aluminum in magnesium, (Mg), the -phase (Mg2Al3) [17] and the-phase (Mg17Al12 or Mg58Al42). Furthermore, a line compound, [18], alsodenoted R at approx. 56-58 at.% aluminum is known to exist [15,19]. Severalmetastable and high-temperature phases have been proposed e.g. , MgAl2,, [15,19,20,16].

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The crystallographic information and composition ranges of the well estab-lished phases in Mg-Al system are summarized in Table 1. The unique struc-tures of the different stable Mg-Al phases are visualized by their calculatedX-ray diffraction patterns in Fig. 1.

3 Alloy preparation

A mandatory requisite when studying the hydrogenation behavior of Mg-Alis the preparation of an alloy with the desired composition. Several differentmagnesium-aluminum alloys are commercially available with applications inautomobile industry, aviation, aircrafts etc. [21]. However, these are usuallyeither of the (Mg) structure (with an Al content of up to approx. 10 wt.%)with additional alloying elements of e.g. Zn and Mn (Examples are AM60,

AZ31 [22], AZ80, and AZ91) or the (Al) structure with a small amount ofmagnesium e.g. alloy 5052, 5056, and 5056 with additional alloying elementsof e.g. Cr. No commercial alloys with the , , or structure are available (tothe best of our knowledge). Thus alloys other than (Mg) and (Al) must beprepared ad hoc. For this purpose either melting of the elements or mechanicalalloying are applied.

Ball milling (mechanical alloying) has previously been applied for preparing ofMg-Al alloys. Bouaricha et al. [23] prepared alloys with an Al content rangingfrom 10 at.% up to 80 at.%. For the lowest Al content both hcp Mg and

fcc Al were present for shorter milling times, whereas the latter disappearedfor longer milling times during formation of the -phase. Increasing the Alcontent up to 42 at.% resulted in a single phase (), a further increase led to theformation of-phase, and finally the fcc solid solution of Mg in Al were formedfor the highest Al content. Crivello et al. did a similar excercise when preparingalloys with an Al content ranging from 30 to 52.5 at.% when studying thehomogeneity range of the -phase [24]. Other examples of preparation of Mg-Al alloys can be found in [25,26]. Bulk mechanical alloying (BMA) has beenapplied in a few studies [2628]. Compared to the ball milling process BMAapparently offers a shorter processing time [26].

Arc melting has been applied in previous works to prepare an alloy with 30at.% Al containing both (Mg) and phase [29] and an alloy with a composi-tion of the starting materials corresponding to phase [30]. Induction meltinghas been used in order to prepare an alloy containing both and phase [31]as well as pure phase [32,33]. When preparing the alloy by either inductionor arc melting the volatility of magnesium should be taken into account. Al-though the melting points of Mg and Al are almost the same a huge differencein vapor pressure exists (Mg: 9 102 bar @ 923 K, Al: 1 105 bar @ 933K [12]). The mole ratio of Mg/Al in the starting materials can be increased

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slightly above the desired ratio in the alloy in order to compensate for theevaporation of magnesium [33]. This shortcoming of the melting techniquescan be avoided by using the mechanical alloying techniques.

4 General hydrogenation/dehydrogenation behavior

A number of studies on the hydrogenation/dehydrogenation behavior of Mg-Alalloys (ranging from hcp solid solution of Al to true alloys) have been published[14,22,23,26,27,2938]. It is generally observed that upon hydrogenation theMg-Al alloy disproportionates under formation of MgH2 and Al according tothe overall scheme below

MgxAly + xH2 xMgH2 + yAl (1)

During dehydrogenation Mg and Al reacts and an Mg-Al alloy is recovered[23,27] as illustrated in Fig. 2. In the hydrogenated state only MgH2 and Alis present, after dehydrogenation diffraction peaks from MgH2 and Al areabsent and only those corresponding to an Mg-Al alloy (mainly -phase inthis case) are present. Due to incomplete hydrogenation the reaction product,in addition to MgH2 and Al, may also contain a small amount of Mg dissolved

in the fcc-Al [33].

The reason for the disproportionation of Mg-Al alloys during hydrogenationmay be explained by a relatively high thermodynamic stability of MgH2 com-pared to that of the alloy [36,39] combined with a relatively low stability of aMg-Al-H compound [40,41]. Formation of AlH3 will not occur during hydro-genation of Mg-Al since the temperature applied to reach acceptable kineticsfor MgH2 exceeds the decomposition temperature of AlH3 (H= 30 kJ/molH2, Tdec

2 423 K) unless a very high hydrogen pressure is applied [12,42].

Magnesium alanate (Mg(AlH4)2), belonging to the class of so-called complex

hydrides, have recently received a great deal of attention due to its high hy-drogen density [4345]. Although the dehydrogenation product is an Mg-Alalloy [44,46] reversibility viz. formation of Mg(AlH4)2 from an Mg-Al alloyhave only been observed using severe reaction conditions under a hydrogenplasma [34]. Hence, it will not be treated in more detail in the present paper.

2 Tdec is defined as the temperature needed in order to reach a desorption pressureof 1 bar H2

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The theoretical gravimetric hydrogen storage capacity, m(H2), of Mg-Al al-loys can be easily estimated from reaction Eq. 1 in wt.% as

m(H2) =xMH2

xMH2 + xMMg + yMAl100 (2)

where MH2, MMg, and MAl are the molar mass of hydrogen, magnesium andaluminum, respectively. Table 2 summarizes calculated storage capacities ofthe stable Mg-Al phases.

5 Pressure composition isotherms

Relatively few pressure composition isotherms (PCI) have been reported forthe Mg-Al-H system [14,23,26,27,35,36,47]. As seen from Fig. 3 two distinctplateaus may be observed [27,36]. This may be rationalized in the followingway. Upon hydrogenation Mg is depleted from the Mg-Al alloy which, in thecase of magnesium content exceeding that of the -phase, results in the trans-

formation of-phase into -phase. This corresponds to the first plateau. Thesecond plateau corresponds to depletion of Mg from the -phase effectivelyleading to the formation of (Al). As observed from Fig. 3 the length of thefirst plateau is dependent on the magnesium content. When the magnesiumcontent is decreased, approaching that of the -phase, the length of the firstplateau decreases and eventually it will diminish. From the existence of the twoplateaus it can be inferred that i) the hydrogenation (and dehydrogenation)is at least a two step process (when -phase is present) and ii) the enthalpy oftransforming -phase to (Al) exceeds that of the to -phase transformation.The thermodynamics of the Mg-Al system will be treated in more detail inthe following section. Two distinct plateaus have also been observed for the

-phase [36] which has been assigned to partial rearrangement of Mg and Alat lower temperatures (below 623 K). In other studies only a single plateauis observed even when two plateaus should be expected [26] or the PCI isunder heavy influence by sloping making the distinction less obvious [14,23].Some possible reasons for this behavior may be e.g. a magnesium contentlower than anticipated (close to or lower than -phase), or that the individ-ual data points resembling the PCI represents partial equilibrium rather thantrue equilibrium. In Table 3 the experimentally observed plateau pressures aresummarized.

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6 Thermodynamics of the Mg-Al-H system

6.1 Analysis using Mg-Al alloy formation enthalpy

For hydrogenation of pure -phase the reaction can essentially be viewed as aone step process

Mg2Al3 + 2H2 2MgH2 + 3Al (3)

with the enthalpy of formation of the hydride (per mole H2) given as

Hf() = Hf(MgH2) 1

2Halloy() (4)

with the heat of formation of the alloy given per formula unit (f.u.). Thus, ifboth Hf(MgH2) and Halloy() are known Hf() can be calculated. Thevalue for Halloy() given in [48] calculated by the Miedema model [4954]is -13 kJ/mol f.u. By fitting the comprehensive compilation of experimentallydetermined plateau pressures published by Zeng et al. [4] the formation en-thalpy of pure MgH2 is found to be -77.5 kJ/mol H2. Applying these valuesHf() is calculated to -71 kJ/mol H2.

For pure -phase the hydrogenation reaction is essentially a two step process.Step 1 is

Mg17Al12 + 9H2 9MgH2 + 4Mg2Al3 (5)

where the above step can be decomposed into

Mg17Al12 + 9H2 17Mg + 12Al (6)

8Mg + 12Al 4Mg2Al3 (7)

9Mg + 9H2 9MgH2 (8)

with the enthalpy of formation given as

Hf() = Hf(MgH2) 1

9Halloy()

+4

9Halloy() (9)

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The second step in the hydrogenation of the -phase is equivalent with Eq.3 and the enthalpy of formation is given by Eq. 4. Using Halloy() = 84kJ/mol f.u. (-2.9 kJ/mol atom) given in [25] calculated by the Miedema modelHf() is calculated to -74 kJ/mol H2.

Thus, from the above analysis it is seen that the addition of aluminum leads toa reduced enthalpy of formation of the hydride i.e. a destabilization of MgH2,realized by an exothermic formation of Mg-Al during the endothermic dehy-drogenation of MgH2 + Al. In terms of thermodynamics the hydrogenationbehavior of Mg-Al alloys is apparently quite similar to those of Mg2Cu [35,5557] which also disproportionates during hydrogenation and similarities withthe Mg2Si system is also noted [58].

6.2 Vant Hoff analysis

Corresponding plateau pressures and temperatures are related through theVant Hoff equation [1,35,5962]

ln

pH2p

=

HfRT

SfR

(10)

where Hf and Sf are the change in enthalpy and entropy, respectively,upon hydrogenation.

The reported plateau pressures in Table 3 have been used to construct VantHoff plots in Fig. 4. The data has been divided in two groups; one for databelonging to hydrogenation/dehydrogenation of-phase (1. plateau) and onecorresponding to -phase (2. plateau). It has not been possible to group intoboth absorption and desorption due to the sparse nature of the data. Fits areincluded as obtained from fitting Eq. 10 to the data. For comparison a VantHoff plot is also included for pure Mg obtained by fitting Hf and Sf inEq. 10 to the comprehensive compilation of experimentally determined plateaupressures published by Zeng et al. [4]. Hf and Sf for Mg-Al and pure MgH2are listed in Table 4 along with values for a few other Mg based alloys.

In general, as seen from Fig. 4, alloying of Mg with Al leads to higher plateaupressures i.e. a lower temperature is required to obtain an equilibrium pressureof 1 bar H2 than for pure MgH2. From Table 4 it is found that the hydride for-mation enthalpy for -phase is improved compared to both pure Mg, Mg2FeH6,and even Mg2Ni, which is quite interesting since Mg2Ni is usually viewed asthe optimal destabilization of Mg in relation to hydrogen storage applications.On the other hand, hardly any change in Hf() compared to Hf(MgH2)is seen.

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Listed in Table 4 are also temperatures required to obtain an equilibriumpressure of 1 bar H2 estimated by rearranging Eq. 10

Tdec =HfSf

(11)

From the listed Tdec it is observed that the decomposition temperature ofMgH2 + Al (-phase) is lowered approx. 50 K compared to pure Mg and it iseven 20 K lower than for Mg2Ni. Nevertheless, the thermodynamic parametersfor Mg-Al are somewhat uncertain due to the scatter in the plateau pressuresreported (some of which may be due to hysteresis), and should preferablybe verified by additional measurements of the plateau pressure at differenttemperatures, in order to draw more solid conclusions.

7 Hydrogenation/dehydrogenation kinetics

7.1 General experience

Mintz et al. [63] studied the effect of dilute group IIIA element (Al, Ga, andIn) additions on the apparent activation energy of hydrogenation of Mg. Itwas observed that small amounts of Al (


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be found in the fact that hydrogen diffusivity in Al (at 300 K) is similar toMg or slightly improved [3,64,65] whereas hydrogen diffusion in both Al andMg is significantly better than for MgH2 [66]. Further, the formation of a sta-ble surface hydride film blocking hydrogen diffusion [1,6770] is less probabledue to the creation of Al grains during hydrogenation of Mg-Al facilitating

hydrogen diffusion in analogy with MgH2/Mg2Cu [55,7174].

7.2 Kinetic analysis

Only few studies report temperature dependent kinetic data of hydrogena-tion/dehydrogenation of Mg-Al alloys [30,32,63]. In the following the hydro-genation data reported by Wang et al. [32] for pure -phase is analyzed. TheJohnson-Mehl-Avrami (JMA) approach [75,76] is used. In the JMA formalismthe formed phase fraction is given as

(t) = 1 exp((kt)) (12)

where t is time, k is a temperature dependent rate constant, and is referredto as the Avrami exponent from which information about the growth dimen-sionality as well as the rate limiting step can be deduced. Eq. 12 has provenvery versatile in explaining the kinetics of hydrogenation/dehydrogenation ofa variety of metal-hydrogen systems see e.g. [30,74,77]. Eq. 12 is fitted to thedata in [32] and the resulting fit is shown in Fig. 7. The fitted constants are

summarized in Table 5. In order to provide rate constant for both constantT and driving force, they have been corrected by a (1

PplP

) term where Pis the actual pressure and Ppl is the plateau pressure calculated by the VantHoff equation.

As seen from Fig. 7 the JMA fit provides an excellent fit to the experimentaldata. In the fit the maximum hydrogen uptake was assumed to be 2.5 wt.%,despite the fact that this is lower than the theoretical uptake by the -phase.As seen from Table 5 the value of is quite close to 1/2, which may be inter-preted as a one dimensional growth process with diffusion being rate limiting[55]. The fact that a diffusional process is rate limiting was also proposed byBouaricha et al. [23] and Mintz et al. [63] and the kinetics share many simi-larities with those reported for Mg/Mg2Cu by Karty et al. [55]. Applying theJMA model to the data in Fig. 5 reported by Bouaricha et al. [23] returns abest fit value of of 0.47, 0.46, 0.52, and 0.49 for Mg, Mg90Al10, Mg75Al25, andMg58Al42, respectively. Thus, the results on hydrogenation are quite consistentand apparently the rate-limiting step does not change with composition.

The rate constant data in Fig. 5 has been fitted by an Arrhenius law and resultis shown in Fig. 8. The slope of the linear regression provides the apparent

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activation energy of the hydrogenation process and a value of 81 kJ/mol isfound. It is interesting to note that this value is slightly higher than thatfound for hydrogen sticking/dissociation on magnesium thin films [78] of 72kJ/mol but slightly lower than the values of 90-140 kJ/mol usually reportedfor activated magnesium powder samples [36,55,79,70].

The dehydrogenation kinetics of an air exposed non-activated Mg-Al alloy wasprobed in a recent study [30]. An apparent activation energy of 160 kJ/molwas found by a JMA analysis similar to the one outlined above. This value is inthe range of that generally experienced for dehydrogenation of activated MgH2of 120160 kJ/mol [80,81], although in the high end. However, it is interestingto note that the sample has a significantly lower apparent activation energythan that experienced for magnesium contaminated with a surface oxide layerof 300 kJ/mol [8082]. This suggests that the presence of aluminum createsa hydride system less prone to hampered kinetics due to oxygen contamina-

tion. Several reasons for this behavior are possible i) A lower terminal oxidethickness for Al compared to Mg [83,84] ii) The possibility of an amorphousoxide layer [85] with improved diffusion properties [86] compared to a crys-talline oxide iii) The possible creation of a less compact composite oxide layerwith co-existence of MgO, Al2O3, and metallic Al [85] with similar propertiesas that found for Mg2Ni, and Mg2Cu [55,87].

A closer analysis (not shown) of the dehydrogenation data in [30] reveals tobe in the range of 2.53. According to [55] this may imply a 3-dimensional in-terface transformation process (=3, constant nuclei number), a 2-dimensionalinterface transformation process (=3, constant nucleation rate), or a 3-dimensional

diffusion process (=5/2), respectively, being rate-limiting. Thus, not conclu-sive at all. An analysis of the dehydrogenation data of Bouaricha et al. [23]for Mg, Mg90Al10, Mg75Al25, and Mg58Al42 gives values of of 2.28, 2.45, 1.94,and 0.74, respectively. For the lower contents of Al these results are quite con-sistent with that of [30], although for higher content (similar to that in [30])there is obviously a discrepancy.

8 Summary

The hydrogen storage properties of magnesium can be effectively modifiedby alloying with aluminum. The main conclusions from this review are listedbelow.

Capacity. The theoretical hydrogen capacity of pure magnesium of 7.6 wt.%H2 is effectively lowered when alloying with aluminum due to the fact thathydrogen is bonded as MgH2. Unless severe reaction conditions are appliedAl does not take up hydrogen. The hydrogen content of the three stable

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Mg-Al phases are 4.44, 3.17, and 3.02 wt.% H2 for the -phase, the -phase,and the -phase respectively.

Thermodynamics. Compared to pure magnesium the plateau pressure ex-perienced for Mg-Al alloys are generally higher which is explained by areduction (less exothermic) in the enthalpy of hydride formation due to

an endothermic disproportionation of the Mg-Al prior to MgH2 formation.Based on a compilation of available experimentally determined plateau pres-sures of the Mg-Al-H system the enthalpy of hydride formation is approxi-mated to be -62.7 kJ/mol H2 (compared to -77.5 kJ/mol H2 for pure Mg).This is even lower than the enthalpy of hydride formation of -64.5 kJ/molH2 Mg2Ni. However, additional data are required for verification.

Kinetics. The kinetics generally found to be improved upon addition of Al tothe Mg/MgH2 system both regarding hydrogenation and dehydrogenation.For hydrogenation the process appears to be limited by a diffusion processand the apparent activation energy is indicated to be lower than that usually

reported for pure Mg. For dehydrogenation the current available data doesnot allow conclusions to be made about the rate limiting step.

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30 40 50 60 70Diffraction angle 2

Intensity[a.u.]

phase: Mg17

Al12

phase: Mg42Al58

phase: Mg2Al

3

Fig. 1. Calculated diffraction patterns of the , and -phase of Mg-Al. The patternshave been produced with GSAS/EXPGUI [88,89] with crystallographic informationfrom ref. [20]

I

ntensity[a.u.]

20 30 40 50 60 70 80

Diffraction angle 2 [o]

MgH2

(110)

MgH2

(101)

MgH2

(200)

MgH2

(211)

MgH2

(220)

MgH2

(002)

MgH2

(310)

MgH2

(112)

MgH2

(301)

MgH2

(202)

Al(111)

Al(200)

Al(311)

Al(220)

Fig. 2. X-ray diffraction patterns of an Mg-Al alloy in the hydrogenated state (top)and dehydrogenated state (bottom). Data are from ref. [30].

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Fig. 3. Absorption PCI for Mg-Al alloys at 623 K for -phase (open) and Mg50Al50(filled). The data are adapted from [27].

1.4 1.5 1.6 1.7 1.8 1.9 2

1000/T [K-1

]

-2

-1

0

1

2

3

4

5

ln(pH2

/po)[--]

1. Plateau Mg-Al-H2. Plateau Mg-Al-H1. plateau (fit)

2. plateau (fit)Mg/MgH

2

1. Plateau: H=-77.7 kJ/mol, S=-144 J/(mol K)2. Plateau: H=-62.7 kJ/mol, S=-123 J/(mol K)

Fig. 4. Vant Hoff plot for Mg-Al/H from experiments with fits included. Data forpure Mg (full line) is included for comparison [4].

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0 200 400 600 800 1000Time [s]

0

0.2

0.4

0.6

0.8

1

PhasefractionMgH2[

--]

MgMg:Al (90:10)Mg:Al (75:25)Mg:Al (58:42)

Fig. 5. Hydrogenation kinetics of pure un-milled Mg compared with Mg-Al com-pounds with varying Al content ball milled for 20 h. Hydrogenation performed atT = 673 K and a hydrogen pressure of 38 bar. Data extracted from ref. [23].

0 300 600 900 1200 1500Time [s]

-7

-6

-5

-4

-3

-2

-1

0

Massloss[wt.%]

Mg:Al (92:8)Mg

Fig. 6. Dehydrogenation kinetics of pure Mg and Mg:Al (92:8) ball milled for 20 hobtained by heating from RT at t = 100 s to T = 573 K at t = 400 s. From t = 400s the temperature is maintained at approx. 573 K. Data extracted from ref. [40].

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0 500 1000 1500 2000Time [s]

0.0

0.5

1.0

1.5

2.0

2.5

Hydrogenuptake[wt.

%]

T=473 K

T=493 K

T=513 K

T=553 K

T=573 K

Fig. 7. Experimental observations of hydrogen uptake in -phase extracted from[32] (full lines). The fitted curves using the JMA approach are also shown (dottedlines). Hydrogenation was conducted at pH2 = 40 bar.

1.7 1.8 1.9 2 2.1 2.2

1000/T [K-1

]

-10

-9

-8

-7

-6

-5

lnk[--]

Fig. 8. Arrhenius plot of rate constants obtained from a JMA fit to the hydrogena-tion data in Figure 7 (circles) and linear regression (full line).

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Table 1Stable Mg-Al phases. Information extracted from ref. [15,16,20].

Phase Composition range [at.% Mg] Sg. Lattice parameters [A]

(Al) 0-18.6 Fm3m a = 4.05-4.22

(Mg2Al3) 38.5-40.3 Fd3m a = 28.22-28.16

42 R3 a = 12.82

c = 21.75

(Mg17Al12) 45-60.5 I43m a = 10.47-10.61

(Mg) 89-100 P63/mmc a = 3.16-3.20

c = 5.16-5.21

Table 2

MgxAly phases and their theoretical gravimetric hydrogen density calculated withEq. 2.

Phase Mg concentration [at.% ] x y m(H2) [wt.%]

(Mg) 89-100 0.89-1 0.11-0 6.877.66

(Mg17Al12) 58.6 17 12 4.44

R 42 42 58 3.17

(Mg2Al3) 40 2 3 3.02

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Table 3Reported plateau pressures for Mg-Al PCI. ppl,i is the mid-plateau pressure ofplateau i. If only ppl,2 is given this corresponds to a PCI with only one distinct

plateau except for [26] where the single plateau has been assumed to represent ppl,1. or refers to approx. stoichiometric alloys, whereas + refers to alloys with acomposition in between the two e.g. Mg50Al50.

Temp. [K] Alloy ppl,1 [bar] ppl,2 [bar] Reference Abs/Des

523 0.47 [26] Des

523 0.55 [26] Abs

553 + 2.06 3.32 [14] N/A

608 8.47 11.97 [36] Des

623 12.13 15.98 [36] Des623 7.9 12.1 [27] Abs

623 + 7.3 11.3 [27] Abs

623 9.4 14 [23] Abs

623 11.8 20 [23] Des

648 22.22 [36] Des

683 47.47 [36] Des

Table 4Hydride formation enthalpies and entropies and corresponding decomposition tem-peratures for different magnesium based hydrogen storage systems. Values for Mg-Alare from fitting the Vant Hoff equation to the data provided in Fig. 4. Values forMg-Ni and Mg-Fe are from ref. [68] and values for Mg are from ref. [4].

Alloy system H [kJ/mol H2] S [J/(mol K)] Tdec [K]

(Mg-Al)/H -77.7 -144 540

(Mg-Al)/H -62.7 -123 510

MgH2 -77.5 -139 557

Mg2NiH4 -64.5 -122 528

Mg2FeH6 -79.2 -137 578

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Table 5Kinetic parameters obtained by fiiting the JMA equation to the kinetic data in Fig.7.

T [K] k [105 s1] []

473 9.9 0.56

493 16.3 0.47

513 40.5 0.37

553 121.4 0.37

573 391.1 0.42

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Hydrogenation properties of Mg-Al alloys