Effect of LaCl3 addition on the hydrogen storage properties of MgH2

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Energy 79 (2015) 177e182

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Energy

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Effect of LaCl3 addition on the hydrogen storage properties of MgH2

M. Ismail*

School of Ocean Engineering, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia

a r t i c l e i n f o

Article history:Received 3 April 2014Received in revised form22 October 2014Accepted 2 November 2014Available online 26 November 2014

Keywords:Hydrogen storageMagnesium hydrideLanthanum chlorideCatalytic effect

* Tel.: þ60 9 6683487; fax: þ60 9 6683991.E-mail address: [email protected].

http://dx.doi.org/10.1016/j.energy.2014.11.0010360-5442/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

In this study, the effect of LaCl3 on the hydrogen storage properties of MgH2 prepared by ball millingwas investigated for the first time. It was found that the MgH2 þ 10 wt.% LaCl3 sample started todecompose at around 300 �C, which was 50 �C lower than in as-milled MgH2. For desorption kinetics, theLaCl3-doped MgH2 composite sample released about 4.2 wt.% hydrogen at 320 �C after 5 min dehy-drogenation, while the as-milled MgH2 only released about 0.2 wt.% hydrogen for the same temperatureand time. Meanwhile, a hydrogen absorption capacity of 5.1 wt.% was reached at 300 �C in 2 min for theLaCl3-doped MgH2 sample. In contrast, the ball-milled MgH2 only absorbed 3.8 wt.% hydrogen at 300 �Cin 2 min. The activation energy of dehydrogenation was 166.0 kJ/mol for the as-milled MgH2 and143.0 kJ/mol for the 10 wt.% LaCl3-added MgH2, indicating that the LaCl3 additive decreased the acti-vation energy for the hydrogen desorption of the MgH2. The improved hydrogen storage properties of theMgH2 in the presence of LaCl3 is believed to be due to the catalytic effects of the LaeMg alloy and MgCl2that were formed in situ during the heating process.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Compared to gaseous and liquid hydrogen storage, solid-statehydrogen storage attracts attention due to its advantages such ashigh gravimetric hydrogen capacity, safety and space for storage,and processing convenience [1]. Although solid-state storage ispromising as a potential energy carrier for the future, the problemis finding the materials that can achieve the target by DOE(Department of Energy) such as release and absorb >6.5 wt.% H2,reversibility, and the ability to operate at moderate temperatures[2]. Among solid-state hydrogen storage materials, growing inter-est has been shown in MgH2 of the metal hydride family due to itslarge gravimetric density (7.6 wt.% H2), abundant resources, lowcost and good reversibility. However, the disadvantages of MgH2are that it is too stable, leading to desorption temperatures that aretoo high. Furthermore, the kinetics of the hydrogen uptake andrelease in Mg are poor. Many studies have been conducted toimprove the hydrogen storage properties of MgH2 through the useof a catalyst [3e10], the use of ball milling to produce smallerparticles [11e13] and the combination of MgH2 with other metal/complex hydrides (destabilisation systems) [14e27]. Among these,the use of catalysts has been shown to play a vital role in reducingthe decomposition temperature, enhancing the sorption kinetics,

and improving the reversibility of MgH2. Among catalysts, metalhalide- andmetal oxide-based such as TiF3 and Nb2O5 are found themost promising catalyst. The significant improvement of MgH2sorption properties in the MgH2/TiF3 [28] and MgH2/Nb2O5 [29] isdue to the catalytic effects of in-situ-generated TiH and NbH2

species which were formed during the dehydrogenation/hydroge-nation process.

Rare-earth chlorides as catalysts have beenwidely applied in thelight complex metal hydrides such as NaAlH4 [30e32] and LiAlH4

[33,34]. However, the role of rare-earth chlorides as a catalyst in thehydrogen storage capability of MgH2 has not yet been extensivelyexplored, to the best of the author's knowledge. According to Sunet al. [31], by using a series of rare-earth chlorides, namely, SmCl3,CeCl3, NdCl3, GdCl3, LaCl3, and ErCl3, as dopants, the dehydridingrate of doped NaAlH4 can be considerably improved. The highestcatalytic efficiency was found for SmCl3 and CeCl3, followed byNdCl3, GdCl3, LaCl3, and ErCl3. Sun et al. used LaCl3 to furtherexplore the mechanism of rare-earth elements and claimed that Lawas first hydrogenised as LaH2 and then formed some kinds ofLaeAl alloy (e.g., La3Al11) when the temperature was raised to acertain level. Moreover, the reactions that occurred between LaCl3and NaAlH4 were found to enhance the dehydrogenation kinetics ofthe whole system. Therefore, it is reasonable to hypothesise thatLaCl3 would show great potential as a catalyst to advance MgH2hydrogen storage performance. However, the effects of the LaCl3additive on MgH2 have not been reported so far, to the best of theauthor's knowledge.

Fig. 1. TPD patterns for the dehydrogenation of the as-received MgH2, as-milled MgH2

and MgH2 doped with 10 wt.% LaCl3.

M. Ismail / Energy 79 (2015) 177e182178

In this work, LaCl3 was used as a catalyst precursor to study theeffect on the hydrogen storage properties of MgH2 prepared by ballmilling. The aim of this study is to further investigate the additionof a different type of catalyst and examine the difference in the waythey take effect, and therefore, gain deeper understanding on themodification of hydrogen storage properties of the MgH2. Thepossible catalyst mechanism that is supported by the results isdiscussed accordingly.

2. Experimental details

Ball milling of MgH2 and LaCl3 powders was performed in aplanetary ball mill for 1 h, bymilling for 0.5 h, resting for 6 min, andthen milling for another 0.5 h in a different direction at the rate of400 rpm. Handling of the samples was conducted in an MBraunUnilab glovebox filled with a high-purity Ar atmosphere. About400 mg of MgH2 (�95% pure; Sigma Aldrich) was mixed with10 wt.% of LaCl3 (�98% pure; Sigma Aldrich). The samples were putinto a sealed stainless steel vial together with hardened stainlesssteel balls. The ratio of the weight of the balls to the weight of thepowder was 30:1.

The measurements for dehydrogenation and rehydrogenationwere performed in a Sievert-type PCT (Pressure-compositiontemperature) apparatus (Advanced Materials Corporation) or alsoknown as GRC (Gas Reaction Controller) apparatus. The GRC per-forms quantitative analysis of the gasesolid reaction. It admits acontrolled amount of gas into the reaction chamber that holds aspecimen and monitors the pressure of the gas while the temper-ature of the chamber is held constant or slowly changed. Theamount of gas absorbed by the specimen is determined by calcu-lating the amount of the remaining gas. The instrument is con-nected to a computer and controlled by software (GrcLV), whichperforms in fully automatic operations. About 100mg of the samplewas loaded into a sample vessel. This apparatus can operate at up to200 atm and 900 �C. For desorption purposes, all the samples wereheated under a controlled vacuum of 0.1 atm. The heating rate forthe desorption experiment was 5 �C/min. Rehydrogenation studieswere carried out after the first complete dehydrogenation, and thesamples were kept at 300 �C under 30 atm hydrogen pressure for1 h in order to reabsorb hydrogen. All the temperature andhydrogen absorb/desorbs measurements reported in this paperwere correct within ±2 �C and ±0.1 wt.%. The measurements fordehydrogenation and rehydrogenation were repeated for threetimes and the average was used as a final result.

The phase structures of the samples, before and after desorption,as well as after rehydrogenation, were determined by X-raydiffraction (Rigaku MiniFlex II diffractometer with Cu Ka radiation).XRD (X-ray diffraction) is a non-destructive analyticalmethodwhichcan yield the unique fingerprint of Bragg reflections associated witha crystal structure. The nature of the powders, whether crystalline oramorphous, can be determined using XRD. A crystalline powder is amaterial that has an internal structure in which the atoms are ar-ranged in an orderly three-dimensional configuration. An amor-phous powder is a non-crystallinematerial that has no definite orderor crystallinity. X-rays with a similar wavelength to the distancesbetween planes of the crystal structure can be reflected such that theangle of reflection is equal to the angle of incidence. This is called‘diffraction’ and can be described by Bragg's law:

2d sin q ¼ nl (1)

where d is the interplanar spacing, q the Bragg angle, n is the orderof reflection, and l is the X-ray wavelength. When Bragg's law issatisfied, constructive interference of diffracted X-ray beams oc-curs, and a ‘Bragg reflection’will be detected by a detector scanning

at this angle. The position of these reflections is related to the inter-layer spacings of atoms in the crystal structure. Before measure-ment, a small amount of sample was spread uniformly on thesample holder, which was wrapped with plastic wrap to preventoxidation. q�2q scans were carried out over diffraction angles from20� to 80� with a speed of 2.00�/min.

DSC (Differential scanning calorimetry) analysis of the dehy-drogenation process was carried out on a Mettler Toledo TGA(Thermogravimetry analysis)/DSC 1, with temperature range fromroom temperature to 1200 �C and gas flow between 0 and 200 mL/min, with the capability to switch up to 4 gases such as air, argon,nitrogen, and oxygen. DSC is another type of thermoanalyticaltechnique inwhich the difference in the amount of heat required toincrease the temperature of a sample is recorded. DSC can be usedto determine the thermodynamics properties data such as on en-tropy and enthalpy. Approximately 2e6 mg of sample was loadedinto an alumina crucible in the glovebox. The crucible was thenplaced in a sealed glass bottle in order to prevent oxidation duringtransportation from the glovebox to the DSC apparatus. An emptyalumina crucible was used as the reference material. The sampleswere heated from room temperature to 500 �C under a 1 atmflowing argon atmosphere, and different heating rates were used.

3. Results and discussion

Fig. 1 shows the TPD (temperature-programmed desorption)results for the as-received MgH2, the as-milled MgH2 and the MgH2with 10 wt.% LaCl3 added. The as-received MgH2 started to releasehydrogen at about 410 �C, and desorbed about 7.2 wt.% hydrogen(7.6 wt.% H2 was theoretically released). After milling, the onsetdesorption temperature of the MgH2 was reduced to about 350 �C,indicating that the milling process also influenced the onsetdecomposition temperature of the MgH2. The as-milled MgH2

released about 7.4 wt.% hydrogen after 420 �C. After adding 10 wt.%LaCl3, the onset decomposition temperature of theMgH2 decreasedto about 300 �C and the full dehydrogenationwas completed below375 �C, which was 50 and 110 �C lower than for the as-milled andas-received MgH2, respectively. The total amount of hydrogenrelease was about 6.7 wt.%.

To further examine the effects of doping MgH2 with LaCl3, theisothermal dehydrogenation kinetics curve of the LaCl3-doped

Fig. 2. Isothermal desorption kinetics curves for the as-milled MgH2 and the MgH2

doped with 10 wt.% LaCl3 at 320 �C under vacuum.Fig. 4. DSC traces of the as-milled MgH2 and the MgH2þ10 wt.% LaCl3 (Heating rate:25 �C/min, argon flow: 30 ml/min).

M. Ismail / Energy 79 (2015) 177e182 179

MgH2 composite at 320 �C under vacuum (after rehydrogenationunder 30 atm H2 at 300 �C) was collected, as shown in Fig. 2. Theisothermal dehydrogenation of MgH2 was also examined for com-parison purposes under the same conditions. After 5 min dehy-drogenation, the LaCl3-doped MgH2 composite sample releasedabout 4.2 wt% hydrogen at 320 �C, but the MgH2 sample onlyreleased about 0.2 wt.% hydrogen at the same time and tempera-ture. Saturation of the dehydrogenation process for the LaCl3-doped MgH2 composite sample at 320 �C was achieved within15 min. This result indicates that the dehydrogenation kinetics ofthe MgH2 was significantly improved after doping with LaCl3.

In order to investigate the isothermal rehydrogenation kineticsof the LaCl3-doped MgH2 composite, the rehydrogenation of thedehydrogenated samples was performed under 30 atm of H2 at300 �C, as shown in Fig. 3. The undoped MgH2 was also examinedfor comparison. The results show that adding LaCl3 caused theMgH2 to absorb as much as 5.1 wt.% hydrogen within 2 min at

Fig. 3. Isothermal absorption kinetics measurement of the as-milled MgH2 and theMgH2 doped with 10 wt.% LaCl3 at 300 �C under 30 atm hydrogen pressure.

300 �C, while the MgH2 only absorbed about 3.8 wt.% hydrogenafter the same period of time. This result suggests that the LaCl3additive also improved the rehydrogenation kinetics of the MgH2.

The thermal properties of the LaCl3-doped MgH2 sample werefurther investigated by DSC, as shown in Fig. 4 (25 �C/min heatingrate). For comparison, the as-milled MgH2 was also included in theDSC investigation. The DSC curve of the as-milled MgH2 and theLaCl3-doped MgH2 sample displayed only one strong endothermicpeak at approximately 442.562 �C and 380.254 �C, respectively,corresponding to the decomposition of the MgH2. The notablereduction of the peak temperature in the DSC results reveals thatthe dehydrogenation properties of MgH2 were significantlyimproved by adding LaCl3. However, the onset decompositiontemperatures of the samples in the DSCwere slightly higher than inthe TPD (Fig. 1). These differences may have resulted from the factthat the dehydrogenation was conducted under different heatingrates and there were different heating atmospheres in the twotypes of measurements, as explained in our previous papers[35e38].

The improvement of the decomposition temperature andsorption kinetic is related to the energy barrier for H2 release fromMgH2. In the present study, the activation energy for decomposi-tion of the MgH2 was reduced by adding LaCl3. To calculate theactivation energy of the as-milledMgH2 and the LaCl3-addedMgH2,the Kissinger plot was used. The plot was obtained from the Kis-singer equation [39] as follows:

lnhb.T2p

i¼ �EA

�RTp þ A (2)

where b is the heating rate, Tp is the peak temperature in the DSCcurve, R is the gas constant and A is a linear constant. Thus, theactivation energy, EA, can be obtained from the slope in a plot of ln[b/Tp2] versus 1000/Tp. Fig. 5(a) and (b) show the DSC traces for theas-milled MgH2 and LaCl3-added MgH2 composite at differentheating rates.

From a Kissinger plot of the DSC data (Fig. 6), the apparentactivation energy for the LaCl3-added MgH2 composite is found tobe 143.0 kJ/mol, which is much lower than the activation energy ofthe decomposition of the as-milled MgH2 (166.0 kJ/mol). Thisreduction indicates that the apparent activation energy for

Fig. 5. DSC traces at different heating rates for (a) as-milled MgH2 and (b)MgH2þ10 wt.% LaCl3.

Fig. 6. Kissinger's plot of the dehydrogenation for the 10 wt.% LaCl3-doped MgH2

composite as compared with the as-milled MgH2.

Fig. 7. XRD patterns of the 10 wt.% LaCl3-doped MgH2 (a) after milling, (b) afterdehydrogenation, and (c) after rehydrogenation.

M. Ismail / Energy 79 (2015) 177e182180

decomposition of hydrogen from the MgH2 was reduced by dopingwith the LaCl3.

In order to clarify the phase structure of the LaCl3-doped MgH2sample after 1 h milling, after dehydrogenation at 450 �C, and afterrehydrogenation at 300 �C under 3 MPa hydrogen pressure, XRDwas used, as shown in Fig. 7. After the ball milling processes(Fig. 7(a)), the main phases presented were the formations of theparentmaterials, MgH2 and LaCl3. No new compoundswere formedfrom the mixtures. After dehydrogenation at 450 �C, the XRDpattern of Fig. 7(b) reveals that there were distinct peaks of Mg,which indicates that the dehydrogenation process of MgH2 wascompleted. In addition, the peaks for LaCl3 disappear, and somenew peaks corresponding to an LaeMg alloy (Mg3La) andMgCl2 areobserved, suggesting that the reaction of MgH2 with LaCl3 mayhave occurred during the heating process as follows:

9MgH2 þ 2LaCl3/3MgCl2 þ 2Mg3Laþ 9H2 (3)

The standard Gibbs free energy, DG�f of MgH2, LaCl3, MgCl2 and

Mg3La is �35.98 [40], �708.9 [41], -592.12 [40] and �79.65 kJ/mol[42], respectively; thus, the total change DG associated with thereaction in equation (2) will be �194.04 kJ/mol of MgH2. This

confirms the possibility of the reaction in equation (2) from thethermodynamic potentials. In the XRD pattern of the rehydro-genated sample (Fig. 7(c)), the characteristic diffraction peaks of Mgdisappear and the characteristic diffraction peaks of MgH2 appear,indicating that Mg was largely transformed into MgH2 during therehydrogenation process. In addition, the MgCl2 and Mg3La peaksremained unchanged after the rehydrogenation process.

From the above analyses, the improved sorption properties ofMgH2 by doping with LaCl3 could be explained by a number ofreasons. The formation of the LaeMg alloy, Mg3La, that resultedfrom the reaction of theMgH2 and LaCl3 (Eq. (3)) during the heatingprocess may play an important role in the enhancement of MgH2sorption. It is well known that the dehydrogenation product in thelight metal hydride-catalyst system could act as a real catalyst tofacilitate the de/rehydrogenation step. These products could createsurface activation and form a large number of nucleation sites atthe surface of the MgH2 matrix. It is also believed that the finely

M. Ismail / Energy 79 (2015) 177e182 181

dispersed dehydrogenated products may contribute to kinetic de/absorption improvement by serving as the active sites for nucle-ation and creating the dehydrogenation product by shortening thediffusion distance of the reaction ions [43,44]. Furthermore, thefunction of Cl� may also introduce an extra catalytic effect on MgH2sorption properties. As discussed in the literature [45,46], the cat-alytic effect of a metal halide on the hydrogen sorption of MgH2could also be simultaneously influenced by several factors, such asthe formation of MgF2 and the catalytic influence of transitionmetal halides (with different levels of metal oxidation state). Basedon this, in this study, the chlorine-based product, MgCl2, may havealso introduced an extra catalytic effect on MgH2 sorption proper-ties, as proved in previous reports [7,47]. The catalytic effect ofMgCl2 may further combine with the catalytic function of theLaeMg alloy species to generate a synergetic effect. In addition, thereaction in equation (3) could generate clean surfaces (withoutMgO at the MgH2 surface as proved by the XRD results in Fig. 7)and, subsequently, increase the surface reactivity and the decom-position reaction. However, further work is necessary to clarify theexact role of the LaCl3 addition in MgH2 by observation methodssuch as transmission electron microscopy.

4. Conclusion

In summary, LaCl3 showed a good catalytic effect, giving MgH2both a significantly decreased decomposition temperature andenhanced sorption kinetics. The addition of 10 wt.% LaCl3 led to therelease of hydrogen at about 300 �C, decreasing the decompositiontemperature by 50 �C compared to the as-milledMgH2. Furthermore,the kinetic desorption results showed that the added MgH2 releasedabout 4.2wt.% hydrogenwithin 5min at 320 �C,while theMgH2 onlyreleased 0.2 wt.% hydrogen within the same time and temperatureparameters. Meanwhile, a hydrogen absorption capacity of 5.1 wt.%was reached at 300 �C in 2 min for the LaCl3-doped MgH2 sample. Incontrast, the ball-milled MgH2 only absorbed 3.8 wt.% hydrogen at300 �C in 2 min. The apparent activation energy for hydrogendesorption was decreased from 166.0 kJ/mol for the as-milled MgH2to 143.0 kJ/mol by the addition of 10 wt.% LaCl3. This indicates thatthe catalytic effect due to the addition of LaCl3 significantlydecreased the activation energy for the hydrogen desorption ofMgH2. Based on the results, it is believed that the significant effects ofthe LaCl3 on the hydrogen storage properties of MgH2 were due tothe catalytic effects of the LaeMg alloy andMgCl2 that formed in situduring the heating process.

Acknowledgements

The author thanks the Universiti Malaysia Terengganu forproviding the facilities to carry out this project. The author alsoacknowledges the Malaysian Government for financial supportthrough the Fundamental Research Grant Scheme (59295).

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