Effect of different additives on the hydrogen storage properties of the MgH₂-LiAlH₄ destabilized system

Abstract

The hydrogen storage properties of the MgH₂-LiAlH₄ (4 : 1) composite system with and without additives were studied. 5 wt.% of TiF₃, NbF₅, NiF₂, CrF₂, YF₃, TiCl₃·1/3AlCl₃, HfCl₄, LaCl₃, CeCl₃, and NdCl₃, respectively, was added to the MgH₂-LiAlH₄ (4 : 1) mixture, and their catalytic effect was investigated. Temperature programmed desorption results show that addition of metal halides to the MgH₂-LiAlH₄ (4 : 1) composite system improves the onset desorption temperature. The hydrogen desorption properties of metal halide-doped MgH₂-LiAlH₄ (4 : 1) composites were also improved as compared to the undoped MgH₂-LiAlH₄ (4 : 1) composite. Furthermore, the activation energy and the change in enthalpy in the doped and undoped composite were measured by differential scanning calorimetry. In addition, the reaction pathway of the MgH₂-LiAlH₄ (4 : 1) composite system and the mechanisms that work in this composite during the de/re-hydrogenation process were determined by X-ray diffraction.

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1. Introduction

As one of the ideal candidates as an energy carrier for both mobile and stationary applications, hydrogen is also considered as a material that can avert adverse effects on the environment and reduce dependence on imported oil for countries without natural resources.¹ There are many approaches to store hydrogen, including high pressure, cryogenics, and chemical compounds that reversibly release H₂ upon heating (solid state storage). Storing hydrogen in solid state form offers several benefits over a pressurized gas or a cryogenic liquid form. Among the solid-state hydrogen storage materials, much work has been focused on MgH₂, due to its high hydrogen capacity (7.6 wt%), with the added advantages of low cost,^2,3 and superior reversibility.⁴ However, MgH₂ only starts to desorb hydrogen above 300 °C,⁵ and has slow desorption kinetics.⁶ These disadvantages have been overcome by reducing the particle size,⁷ doping with catalysts,^8–10 and reacting with other metal hydrides (destabilization concept).^11–16 The destabilization concept has been extensively investigated as an approach aimed at modifying the thermodynamics and kinetics of the hydrogen sorption reaction. Vajo et al.¹⁷ destabilised MgH₂ by adding Si. The results indicated that the MgH₂/Si system could be practical for hydrogen storage at reduced temperature. However, the formation of Mg₂Si would reduce the gravimetric hydrogen density because Si cannot be hydrogenated, so this intermediate phase seems hard to make reversible. Zhang et al.¹¹ showed that MgH₂ can be destabilized effectively by LiAlH₄. They found that the reaction enthalpy of the MgH₂-relevant decomposition in MgH₂-LiAlH₄ composites (1 : 1, 1 : 2, and 4 : 1 in mole ratio) was reduced by 31, 27.4, and 15 kJ mol⁻¹ H₂ compared to as-milled pristine MgH₂ (76 kJ mol⁻¹ H₂). According to Zhang et al., the hydrogen desorption is observed to take place in two stages: the first stage is the two-step decomposition of LiAlH₄ as shown in eqn (1) and (2).

3LiAlH₄ → Li₃AlH₆ + 2Al + 3H₂ (1)

Li₃AlH₆ → 3LiH + Al + 3/2H₂ (2)

During the second stage, the yielded LiH and Al phases decompose the MgH₂ to form Li_0.92Mg_4.08 and Mg₁₇Al₁₂ phases, as shown in eqn (3) and (4).

4.08MgH₂ + 0.92LiH → Li_0.92Mg_4.08 + 4.54H₂ (3)

17MgH₂ + 12Al → Al₁₂Mg₁₇ + 17H₂ (4)

The hydrogen absorption at 400 °C under 4.0 MPa hydrogen involves two reactions, as shown in eqn (5) and (6).

Li_0.92Mg_4.08 + 4.5H2 →? 4.08MgH₂ + 0.92LiH (5)

Al₁₂Mg₁₇ + (17 − 2y)H₂ → yMg₂Al₃ + (17 − 2y)MgH₂ + (12 − 3y)Al (6)

Recently, Chen et al.¹³ confirmed the two-stage nature of the hydrogen desorption in LiAlH₄ + xMgH₂ (x = 1, 2.5, and 4). They found that the two-stage decomposition was similar to what is described by eqn (1), (2), (3), and (4), but from the X-ray diffraction (XRD) pattern, the rehydrogenated composites showed the absence of an Al₃Mg₂ phase, which indicates that Mg₁₇Al₁₂ is fully transformed into MgH₂ and Al as shown in reaction (7):

Al₁₂Mg₁₇ + 17H₂ → 17MgH₂ + 12Al (7)

Chen et al. found that eqn (7) could be fully reversible without formation of Mg₂Al₃ if a high pressure of hydrogen (>10 MPa) was applied during the rehydrogenation process. Both articles claimed that the destabilization of MgH₂ by the LiAlH₄ resulted from the formation of Al₁₂Mg₁₇ and Li_0.92Mg_4.08. Although the hydrogen storage properties of MgH₂ were improved after combination with LiAlH₄, it still doesn't fulfil the requirements for practical application as a suitable hydrogen storage material. Therefore, it is of importance to find a catalyst or additive that can improve the hydrogen storage properties of the MgH₂-LiAlH₄ composite system.

In this paper we report the hydrogen storage properties of MgH₂-LiAlH₄ (4 : 1) composite system with additives including 5 wt.% of TiF₃, NbF₅, NiF₂, CrF₂, YF₃, TiCl₃·1/3(AlCl₃), HfCl₄, LaCl₃, CeCl₃, and NdCl₃, respectively, and investigated the additives' catalytic effects.

2. Experimental details

MgH₂ (hydrogen storage grade), LiAlH₄ (≥95%), TiF₃, NbF₅ (98%), NiF₂, CrF₂ (98%), YF₃ (99.9%), TiCl₃ 1/3(AlCl₃), and HfCl₄ (98%) were purchased from Sigma Aldrich. LaCl₃ (99.9%), CeCl₃ (99.5%), and NdCl₃ (99.9%) were obtained from Alfa Aesar. Ball milling (BM) of MgH₂ and LiAlH₄ powders in the mole ratio of 4 : 1 was performed in a planetary ball mill (QM-3SP2) for 18 min at the rate of 400 rpm. For simplicity, the mixture of MgH₂ and LiAlH₄ with molar ratio of 4 : 1 will be referred to as MgH₂-LiAlH₄ composite. Handling of the samples was conducted in an MBraun Unilab glove box filled with high-purity Ar atmosphere. Samples were put into a sealed stainless steel vial together with hardened stainless steel balls. The ratio of the weight of balls to the weight of powder was 30 : 1. 5 wt.% of TiF₃, NbF₅, NiF₂, CrF₂, YF₃, TiCl₃·1/3AlCl₃, HfCl₄, LaCl₃, CeCl₃, and NdCl₃ was respectively mixed with MgH₂-LiAlH₄ under the same conditions to investigate their catalytic effects. Pure MgH₂ and LiAlH₄ were also prepared under the same conditions for comparison purposes.

The temperature programmed desorption (TPD) and re/de-hydrogenation kinetics experiments were performed in a Sieverts-type pressure-composition-temperature (PCT) apparatus (Advanced Materials Corporation). The sample was loaded into a sample vessel in the glove box. For the TPD experiment, all the samples were heated in a vacuum chamber, and the amount of desorbed hydrogen was measured to determine the lowest decomposition temperature. The heating rate for the TPD experiment was 5 °C/min, and samples were heated from room temperature to 450 °C. The re/de-hydrogenation kinetics measurements were performed at the desired temperature with initial hydrogen pressures of 3.0 MPa and 0.001 MPa, respectively.

The phase structures of the samples before and after desorption, as well as after rehydrogenation, were determined by X-ray diffraction (XRD, GBC MMA X-ray diffractometer with Cu K_α radiation). Before measurement, a small amount of sample was spread uniformly on the sample holder, which was wrapped with a plastic wrap to prevent oxidation. θ-2θ scans were carried out over diffraction angles from 25° to 80° with a speed of 2.00°/min.

Differential scanning calorimetry (DSC) analysis of the dehydrogenation process was carried out on a Mettler Toledo TGA/DSC 1. About 2–6 mg of sample was loaded into an alumina crucible in the glove box. The crucible was then placed in a sealed glass bottle in order to prevent oxidation during transportation from the glove box to the DSC apparatus. An empty alumina crucible was used as the reference material. The samples were heated from room temperature to 500 °C under an argon flow of 30 ml/min, and different heating rates were used.

3. Results and discussion

Fig. 1 displays the TPD performance of the MgH₂-LiAlH₄ composite system. The as-milled LiAlH₄ and as-milled MgH₂ were also included for comparison purposes. The as-milled MgH₂ starts to release hydrogen at about 330 °C and desorbs about 7.1 wt.% H₂ after 420 °C. Meanwhile, the as-milled LiAlH₄ starts to desorb hydrogen at about 142 °C and about 173 °C for the first and second stage, respectively. For the MgH₂-LiAlH₄ composite, there are two significant stages of dehydrogenation that occur during the heating process. The first stage, which takes place within the temperature range from 130 to 250 °C, is attributed to the two-step decomposition of LiAlH₄, as indicated in eqn (1) and (2). The second dehydrogenation stage, starting at about 270 °C and completed at about 360 °C, can be attributed to the MgH₂-relevant decomposition. These two stages of dehydrogenation are of the same order as reported by Zhang et al.¹¹ and Chen et al.¹³

Fig. 1 TPD curves of the as-milled MgH₂, the as-milled LiAlH₄, and the MgH₂-LiAlH₄ composite.

In order to investigate the reaction progress and mechanism, XRD measurements were performed on the MgH₂-LiAlH₄ composite after dehydrogenation at 250 °C and 400 °C, as shown in Fig. 2. The as-milled MgH₂-LiAlH₄ is also included in this figure. From Fig. 2(a), it can be seen that MgH₂ and LiAlH₄ phases are present in the as-milled MgH₂-LiAlH₄ composite. Fig. 2(b) indicates the presence of LiH and Al phases beside the MgH₂ after dehydrogenation at 250 °C. The fact that no LiAlH₄ phase was found indicates that the reactions in eqn (1) and (2) had been completed at this stage. After dehydrogenation at 400 °C, the XRD pattern of Fig. 2(c) reveals that the intermediate phases Li_0.92Mg_4.08 and Al₁₂Mg₁₇ were eventually formed in the composite system besides Mg. These results confirmed that the hydrogen released in the second stage is from the MgH₂-relevant decomposition through the reactions in eqn (3) and (4), and the decomposition of the excess MgH₂(Eqn 8), which occurs at a temperature 60 °C lower than the decomposition temperature of the pure as-milled MgH₂. In this study, it was found that the decomposition temperatures of the first and second stage dehydrogenation are quite similar to what was reported by Zhang et al.,¹¹ but slightly higher compared to those reported by Chen et al.¹³ This difference may due to the different duration of the milling process, as Chen et al. reported that the sample was ball milled for 10 h, as compared to 18 min in this study.

MgH₂ → Mg + H₂ (8)

Fig. 2 XRD patterns of the MgH₂-LiAlH₄ composite after 18 min ball milling (a), and after dehydrogenation at (b) 250 °C and (c) 400 °C.

Fig. 3(a) and (b) shows the influence of catalytic additives on the MgH₂-LiAlH₄ composite decomposition temperature as measured by temperature-programmed-desorption (TPD). The undoped MgH₂-LiAlH₄ is also included for comparison. Among the metal fluorides used in this study (Fig. 3(a)), TiF₃ exhibits a strong catalytic influence on MgH₂-LiAlH₄ decomposition, followed by NiF₂, CrF₂, NbF₅, and YF₃. The TiF₃-doped MgH₂-LiAlH₄ composite sample starts to release hydrogen at about 70 °C and about 180 °C for the first and second stage, respectively, which represents respective reductions of about 60 °C and about 90 °C compared with undoped MgH₂-LiAlH₄. Meanwhile, among the metal chloride additives (Fig. 3(b)), TiCl₃·1/3AlCl₃ shows the best catalytic effect. It seems that the catalytic effect of TiCl₃·1/3AlCl₃ on the decomposition temperature of MgH₂-LiAlH₄ composite is similar to that of TiF₃, in which the decomposition temperature is reduced by about 60 °C and about 90 °C for the first and second stage, respectively, compared with undoped MgH₂-LiAlH₄. From the results, one finds also that apart from TiF₃ and TiCl₃·1/3AlCl₃, all the other metal halide additives yielded no significant change in the second dehydrogenation stage of the MgH₂-LiAlH₄ system.

Fig. 3 TPD curves of (a) the metal fluoride-added MgH₂-LiAlH₄ and (b) the metal chloride-added MgH₂-LiAlH₄ composites.

Fig. 4(a) and (b) compares the isothermal dehydrogenation kinetics of MgH₂-LiAlH₄ composite with and without metal halides at 320 °C. The dehydrogenation of MgH₂ was also examined for comparison under the same conditions. At 320 °C, the pure MgH₂ releases about 3.4 wt.% hydrogen after 60 min. Mixed with LiAlH₄, the dehydrogenation kinetics of MgH₂ was improved. The composite releases about 4.6 wt.% hydrogen after 40 min of dehydrogenation. With the addition of 5 wt.% metal halide, the results show that all the metal halides improved the dehydrogenation kinetics of MgH₂-LiAlH₄ compared with undoped MgH₂-LiAlH₄. The titanium based metal halide-added MgH₂-LiAlH₄ showed the best improvement, so that saturation of the dehydrogenation process for the TiF₃-added MgH₂-LiAlH₄ sample can be achieved within 10 min, and within 20 min for the TiCl₃·1/3AlCl₃-added MgH₂-LiAlH₄ sample.

Fig. 4 Isothermal dehydrogenation kinetics at 320 °C of (a) the metal fluoride-added MgH₂-LiAlH₄ and (b) the metal chloride-added MgH₂-LiAlH₄ composites.

Fig. 5 presents the DSC traces of the MgH₂-LiAlH₄ composite and the as-milled MgH₂, which is also shown for comparison purposes. For the MgH₂-LiAlH₄ composite, the first exothermic peak at 140 °C is due to the presence of surface hydroxyl impurities in the LiAlH₄ powder, as reported in our previous papers,^18–20 and the first endothermic peak at 170 °C corresponds to the melting of LiAlH₄.²¹ The second exothermic peak at 180 °C corresponds to the decomposition of liquid LiAlH₄ (first step decomposition, eqn (1)), and the second endothermic peak at 225 °C is assigned to the decomposition of Li₃AlH₆ (second step decomposition, eqn (2)). The last endothermic peak at about 365 °C is due to the decomposition of MgH₂, which occurs at a temperature 55 °C lower than that of the pure as-milled MgH₂ (peak at about 420 °C). This decrease in the hydrogen release temperature is correlated with the results observed in the PCT measurement (Fig. 1).

Fig. 5 DSC traces of the MgH₂-LiAlH₄ composite and the as-milled MgH₂.

Fig. 6 and 7 show the DSC curves for all metal halide-added MgH₂-LiAlH₄ samples. Apart from the YF₃-added MgH₂-LiAlH₄ sample, the number of thermal events in all the metal halide-added MgH₂-LiAlH₄ samples is quite different from what occurs in the undoped MgH₂-LiAlH₄. These metal halide-doped MgH₂-LiAlH₄ samples showed one exothermic peak and two endothermic peaks. The exothermic peak and the first endothermic peak correspond to the decomposition of LiAlH₄ and Li₃AlH₄, respectively. These results show that LiAlH₄ decomposes without melting under the catalytic effects of TiF₃, NiF₂, CrF₂, NbF₅, TiCl₃·1/3AlCl₃, HfCl₄, LaCl₃, CeCl₃, and NdCl₃, agreeing well with the results on the 0.5 h ball-milled 4 mol% TiF₃-doped LiAlH₄ reported by Liu et al.²² and our previously reported NbF₅-doped LiAlH₄.¹⁸ For all metal halide-added MgH₂-LiAlH₄ samples, the second endothermic event, corresponding to the decomposition of MgH₂, is similar to what occurs in the undoped LiAlH₄-MgH₂ sample. However, the broad peak in the TiF₃ and TiCl₃·1/3AlCl₃-added MgH₂-LiAlH₄ composites indicates faster dehydrogenation kinetics.

Fig. 6 DSC traces of (a) the undoped MgH₂-LiAlH₄ and the MgH₂-LiAlH₄ with added (b) TiF₃, (c) NbF₅, (d) NiF₂, (e) CrF₂, and (f) YF₃.

Fig. 7 DSC traces of (a) the undoped MgH₂-LiAlH₄ and the MgH₂-LiAlH₄ with added (b) TiCl₃·1/3AlCl₃, (c) HfCl₄, (d) LaCl₃, (e) CeCl₃, and (f) NdCl₃.

We obtained the activation energy required for the MgH₂-relevant decomposition using the Kissinger equation,²³ which is usually utilized in thermal analysis as follows:

ln[β/T_p²] = −E_A/RT_p + A (9)

where β is the heating rate, T_p is the peak temperature in the DSC curve, R is the gas constant, and A is a linear constant. Thus, the activation energy, E_A, can be obtained from the slope in a plot of ln[β /T_p²] versus 1000/T_p. Fig. 8 shows DSC traces for the as-milled MgH₂ and MgH₂-LiAlH₄ composite at different heating rates (β = 10, 15, and 20 °C min⁻¹, respectively). From a Kissinger plot of the DSC data (inset of Fig. 8) the apparent activation energy for the MgH₂-relevant decomposition in the MgH₂-LiAlH₄ composite is found to be 126 kJ/mol, which is much lower than the activation energy of the decomposition of as-milled MgH₂ (162 kJ/mol). This reduction indicates that the apparent activation energy for decomposition of hydrogen from MgH₂ was reduced by adding LiAlH₄. Table 1 shows the apparent activation energy measured by the Kissinger method for selected metal halide-added MgH₂-LiAlH₄ and undoped MgH₂-LiAlH₄ composites, as well as for as-milled MgH₂. The table show that, titanium-based metal halides, TiF₃ and TiCl₃·1/3AlCl₃, exhibit the best additives in reducing the activation energy for H-desorption in the MgH₂-LiAlH₄ composites. The apparent activation energies calculated were found to be 83 and 98 kJ/mol for the hydride decomposition of TiF₃ and TiCl₃·1/3AlCl₃-added MgH₂-LiAlH₄ composites, respectively.

Fig. 8 DSC traces of the as-milled MgH₂ and the MgH₂-LiAlH₄ at different heating rates. The inset plot is the Kissinger's analysis for as-milled MgH₂ and MgH₂-LiAlH₄ composite.

Table 1 The apparent activation energy and the enthalpy changes for as-milled MgH₂, MgH₂-LiAlH₄ composite, and selected metal halide-added MgH₂-LiAlH₄

	Non-catalysed		4MgH2-LiAlH4 catalysed with metal halide
	As-milled MgH2	4MgH2-LiAlH4	TiF3	NbF5	NiF2	TiCl3·1/3AlCl3	HfCl4	LaCl3
Activation energy (kJ/mol)	162	126	83	110	120	98	106	123
Enthalpy (kJmol^−1 H2)	76	61	59	62	63	60	62	64

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To determine the enthalpy (ΔH_dec) of MgH₂ decomposition, the DSC curves were analysed by STARe software. From the integrated peak areas, the hydrogen desorption enthalpy was obtained. For the as-milled MgH₂, the hydrogen desorption enthalpy can be calculated as 75.7 kJ mol⁻¹ H₂. This value is almost the same as the theoretical value (76 kJ mol⁻¹ H₂). By the same methods, the reaction enthalpies of the MgH₂-LiAlH₄ composite and the metal halide-added MgH₂-LiAlH₄ samples were determined. The enthalpy changes for selected samples are listed in Table 1. For the MgH₂-LiAlH₄ composite, the enthalpy change calculated from the DSC curves is 61 kJ mol⁻¹ H₂, which is lower than the overall decomposition enthalpy of pure MgH₂ (75.7 kJ mol⁻¹ H₂). This result indicates that the presence of LiAlH₄ destabilizes MgH₂. The enthalpy change in the MgH₂-LiAlH₄ composite system is similar to that reported by Zhang et al.¹¹ (61 kJ mol⁻¹ H₂). After the addition of metal halide, the enthalpy of hydrogen desorption from MgH₂-LiAlH₄ was similar to that of undoped MgH₂-LiAlH₄. Although the enthalpy reaction of these metal halide-doped MgH₂-LiAlH₄ composites remains unchanged, the destabilized complex hydride composites, MgH₂-LiAlH₄-5 wt.% metal halides, have better hydrogen storage behaviour with improved hydrogen desorption and faster desorption kinetics.

In order to investigate the reversibility of MgH₂-LiAlH₄ composite and metal halide-added MgH₂-LiAlH₄, the rehydrogenation of the dehydrogenated samples was performed under ∼3 MPa of H₂ at 320 °C. The MgH₂ system was also examined for comparison. However, the rehydrogenation measurements revealed that the MgH₂-LiAlH₄ (4 : 1) composite did not show any improvement in kinetics compared with the MgH₂, as shown in Fig. 9. After 5 min, about 5.0 wt.% hydrogen was absorbed for the MgH₂ while the MgH₂-LiAlH₄ composite just absorbed about 3.6 wt.% hydrogen after the same time. With addition of the titanium-based metal halides, the MgH₂-LiAlH₄ samples also did not show any improvement. Both the TiF₃ and TiCl₃·1/3AlCl₃-added MgH₂-LiAlH₄ sample absorbed just about 3.3 wt.% hydrogen after 5 min rehydrogenation.

Fig. 9 Isothermal rehydrogenation kinetics of the MgH₂, the MgH₂-LiAlH₄ composite and the titanium-based metal halide-added MgH₂-LiAlH₄ at 320 °C and under 3 Mpa.

In order to determine the rehydrogenation products, XRD measurements were carried out on the rehydrogenated undoped MgH₂-LiAlH₄ sample, as shown in Fig. 10. From the pattern, it can be seen that the peaks correspond to MgH₂, Al, and LiH, along with a small peak for Al₃Mg₂ that appears after rehydrogenation. The disappearance of the Al₁₂Mg₁₇ and Li_0.92Mg_4.08 peaks and the appearance of an Al₃Mg₂ peak in the rehydrogenated sample confirm that reactions (5) and (6) have occurred during the rehydrogenation process.

Fig. 10 XRD patterns of the undoped MgH₂-LiAlH₄ composite after rehydrogenation at 320 °C and under 3 MPa.

To understand the possible mechanism behind the metal halide effects on the enhancement of MgH₂-LiAlH₄ composite, X-ray diffraction analysis was carried out on the TiCl₃·1/3AlCl₃-and TiF₃-added MgH₂-LiAlH₄ composite. Fig. 11(a) and (b) shows the XRD patterns of the TiCl₃·1/3AlCl₃ and TiF₃-added MgH₂-LiAlH₄ composites after 18 min ball milling and after dehydrogenation at 400 °C. For both samples, after 18 min ball milling, MgH₂ phases are detected along with small peaks of LiAlH₄, Li₃AlH₆, and Al (with Li₃AlH₆ and Al peaks overlapping in the XRD pattern). As compared with the peaks of LiAlH₄ in the undoped MgH₂-LiAlH₄ sample (Fig. 2(a)), the diffraction peaks of LiAlH₄ in the TiCl₃·1/3AlCl₃ and TiF₃-added MgH₂-LiAlH₄ samples become weaker. The appearance of Li₃AlH₆ and Al indicates that LiAlH₄ has already partly decomposed into Li₃AlH₆ and Al (eqn (1)) after 18 min ball milling in the presence of TiCl₃·1/3AlCl₃ or TiF₃. After dehydrogenation at 400 °C, as compared with the undoped MgH₂-LiAlH₄ sample (Fig. 2(c)), the new phases Al₃Ti, LiCl, and LiF were formed. The formation of Al₃Ti and LiCl (Fig. 11(a)), and Al₃Ti and LiF (Fig. 11(b)) may be due to the reaction of LiAlH₄ with TiCl₃·1/3AlCl₃ or TiF₃ during the ball milling or the dehydrogenation process. According to Resan et al.²⁴ and Balema et al.²⁵, doping LiAlH₄ with Al₃Ti improved the dehydrogenation behaviour. Meanwhile, a study by Yin et al.²⁶ revealed that it was the F that actively played the catalytic role in enhancement of NaAlH₄-TiF₃ system. We assume that the catalytic effect of TiF₃ on the dehydrogenation process of LiAlH₄ could be similar to its effect on NaAlH₄, as investigated by Liu et al.²² Furthermore, according to,^27–29 the catalytic effect of a Ti-containing phase and the active function of the F anion have also been proved to be significant in improving the hydrogen sorption properties of MgH₂. Therefore, Al₃Ti and LiF are believed to act as the actual catalysts in the TiCl₃·1/3AlCl₃ and TiF₃-added MgH₂-LiAlH₄ composites, which may promote the interaction of LiAlH₄ and MgH₂, and accelerate the hydrogen desorption process of the MgH₂-LiAlH₄ composite system.

Fig. 11 XRD patterns of the MgH₂-LiAlH₄ with addition of (a) 5 wt% TiCl₃·1/3AlCl₃ and (b) 5 wt.% TiF₃, after 18 min ball milling and after dehydrogenation at 400 °C.

4. Conclusion

In the present study, the hydrogen storage properties of the MgH₂-LiAlH₄ (4 : 1) composite system with and without additives were investigated. It was found that the destabilization of MgH₂ by the LiAlH₄ resulted from the formation of Al₁₂Mg₁₇ and Li_0.92Mg_4.08 during the dehydrogenation process, furthermore improves the dehydrogenation properties of MgH₂. The dehydrogenation process in the MgH₂-LiAlH₄ composite can be divided into two stages: the first stage is the two-step decomposition of LiAlH₄. In the second stage, the yielded LiH and Al phases decompose the MgH₂ to form Li_0.92Mg_4.08 and Al₁₂Mg₁₇ phases accompanied with the self-decomposition of the excessive MgH₂. Among the additives examined, the titanium-based metal halides, TiF₃ and TiCl₃·1/3AlCl₃, exhibit the best improvement in reducing the dehydrogenation temperature and enhancing the dehydrogenation rate. From the Kissinger plot, the activation energy for H-desorption is reduced from 126 kJ/mol for MgH₂-LiAlH₄ composite to 83 kJ/mol and 98 kJ/mol after addition of TiF₃ and TiCl₃·1/3AlCl₃, respectively. DSC measurements indicate that the enthalpy change in the MgH₂-LiAlH₄ composite system was unaffected by the addition of metal halides. It is believed that the formation of Ti-containing and F-containing species during the ball milling or the dehydrogenation process may be actually responsible for the catalytic effects and thus further improve the dehydrogenation of the TiF₃ and TiCl₃·1/3AlCl₃-added MgH₂-LiAlH₄ composite system.

Acknowledgements

The authors thank the University of Wollongong for financial support for this research. M. Ismail acknowledges the Ministry of Higher Education Malaysia for a PhD scholarship. Many thanks also go to Dr T. Silver for critical reading of the manuscript.

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