Functions of MgH2 on Hydrogen Storage Properties of Na3AlH6

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Functions of MgH on Hydrogen Storage Properties of NaAlH-LiBH Composite Muhammad Firdaus Ashraf Abdul Halim Yap, and Mohammad Ismail J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07934 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on October 4, 2018

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The Journal of Physical Chemistry

Functions of MgH2 on Hydrogen Storage Properties of Na3AlH6-LiBH4 Composite M. F. A. A. Halim Yap and M. Ismail* School of Ocean Engineering, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia *Corresponding author: Tel: +609-6683487; Fax: +609- 6683991 E-mail address: [email protected]

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Abstracts The hydrogen storage properties of a ternary composite system developed from a combination of MgH2, Na3AlH6 and LiBH4 in a molar ratio of 1:1:4 were systematically investigated. X-ray diffraction (XRD) results found that MgH2-Na3AlH6-LiBH4 (1:1:4) composite was converted to MgH2/Li3AlH6/NaBH4 after 6 h of milling. The temperatureprogrammed desorption results showed that the ternary composite exhibited four main dehydrogenation stages with the total hydrogen capacity of 10.5 wt%. Upon dehydriding range of 175–200 °C, Li3AlH6 decomposed to form LiH and Al. Then, MgH2 started to decompose and reacted with LiH and Al to form Li-Mg and Mg-Al compounds at approximately 250 °C. Subsequently, NaBH4 started to release hydrogen at 310 °C through the formation of Mg-Al-B alloys. The absorption and desorption kinetics of the ternary composite also remarkably improved. Calculation based on the Kissinger plot showed that the values of apparent activation energy of the decomposition of MgH2 and NaBH4 in the ternary composite were 127 kJ/mol and 138 kJ/mol, respectively. It was believed that the observation of Li-Mg, Mg-Al and Mg-Al-B alloys from the XRD analysis after the dehydrogenation process possibly provided synergistic effect in facilitating the nucleation growth and diminished the energy barrier. Thus, this leads to the favourable operating temperature with high capacity hydrogen release and kinetic enhancement of the MgH2Na3AlH6-LiBH4 (1:1:4) composite system.


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1.

Introduction

In decades, hydrogen has been considered as one of the interesting energy vectors due to its benefits to fulfil the global energy requirements. However, one of the biggest challenges towards hydrogen technology development is to determine the possible method in terms of safety and efficiency to store hydrogen. Current methods of hydrogen storage have been proposed, namely high pressure gas cylinders,1 liquid hydrogen2-4 and storage in solid-state.57

Among them, storing hydrogen in solid form is the most attractive one and investigation on

the group of solid form materials has received a great attention from researchers.8-16 Solidstate hydride material offers a very high purity of hydrogen released and can be used directly to power a fuel cell.17 On the other hand, owing to its high gravimetric and volumetric hydrogen capacity, hydride material becomes an ideal material for hydrogen storage. Hydride materials such as lithium borohydride (LiBH4), magnesium hydride (MgH2) and sodium aluminium hexahydride (Na3AlH6) have high gravimetric hydrogen capacity at 18.5 wt%,18 7.6 wt%19 and 3.0 wt%,20 respectively. Unfortunately, the major problems of these hydrides are slow rates of hydrogen release and strong bonding of hydrogen, which requires high temperature.21 These drawbacks hinder them from being considered as on-board storage materials for practical usage. To overcome the problems, materials with better performance of hydrogen storage properties, such as high gravimetric capacity and faster sorption kinetics, as well as the ability to operate under mild temperature need to be acquired. Hence, strategies to modify hydrogen storage performance such as nanoconfinement,22-23 catalysed reactions,24-26 dual-tuning the thermodynamic and kinetics by plasma milling27-33 and combination with other metal hydrides34-35 were applied as reported. To date, massive effort has been made and investigations on complex hydride materials and their combinations with other hydride materials have been done. Recent work36 3 ACS Paragon Plus Environment


showed that the onset desorption temperature of MgH2 destabilised with Na3AlH6 composites was found to be at 170 °C, which was 55 °C lower compared to Na3AlH6 alone. The formation of Mg17Al12 and NaMgH3 was believed to act as active species, thus helped to reduce the dehydrogenation temperatures of the MgH2-Na3AlH6 (in mole ratio of 4:1) composite system. On the other hand, the addition of MgH2 to LiBH4 increased the reversibility rate of LiBH4 with a reduction of reaction enthalpy as reported by Vajo et al..37 Based on their work, it was concluded that the formation of MgB2 in LiBH4-MgH2 composite may play a critical role in stabilising the dehydrogenated state of LiBH4. Meanwhile, a work done by Niemann et al.38 on LiNH2-MgH2-LiBH4 (2:1:1) composite showed that the samples with MgH2 started to decompose at a low temperature of 150 °C. Gao et al.39 studied Ca(BH4)2-LiBH4-MgH2 (1:2:2) and found that the onset desorption temperature of the ternary composite system was approximately 75 °C due to the formation of CaMgH3.72 and released about 8.1 wt% of hydrogen. Furthermore, Zhou et al.40 showed that the hydrogen capacity of approximately 8.0 wt% could be stored in LiBH4CaH2-MgH2 (6:1:3) composite below 400 °C compared to 6LiBH4-CaH2 and LiBH4 samples. Based on the literature discussed, it is reasonable to conclude that the remarkable results could be due to the newly produced Mg after heating, thus providing better hydrogen storage properties such as low operating temperature with high capacity of hydrogen release and faster hydrogen sorption. Therefore, it is interesting to investigate the mutual interaction between MgH2, Na3AlH6 and LiBH4 components in order to develop a high capacity reactive hydride composite. To the best of the author’s knowledge, no literature has discussed the hydrogen storage properties of MgH2-Na3AlH6-LiBH4 (in mole ratio of 1:1:4) composite. Recently, previous work41 showed that Li3AlH6 and NaBH4 were formed after 6 h of milling between 4 ACS Paragon Plus Environment

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Na3AlH6 and LiBH4 components. It was believed that the metathesis reaction had occurred during the ball milling process towards Na3AlH6-LiBH4 (in mole ratio of 1:4) composite. The composite was found to release hydrogen in three stages with the total capacity of hydrogen release of 9.4 wt% due to the formation of AlB2. However, the reversible capacity of hydrogen still needs to be further improved to meet the requirement for hydrogen technology. For the present work, the de/rehydrogenation performance of a ternary hydride, MgH2-Na3AlH6-LiBH4 (1:1:4) was systematically determined by using the Sieverts-type pressure-composition-temperature (PCT) and differential scanning calorimetry (DSC). The structural characteristics of the composites were elucidated in detail by X-ray diffraction (XRD) and Fourier transform infrared (FTIR). Meanwhile, scanning electron microscope (SEM) was applied to determine the surface morphology of the samples. The roles played by MgH2 on Na3AlH6-4LiBH4 composite were then discussed.

2.

Experimental details

Commercial magnesium hydride (MgH2), sodium hydride (NaH), sodium aluminium hydride (NaAlH4), lithium borohydride (LiBH4) and sodium borohydride (NaBH4) were purchased from Sigma-Aldrich without any pre-treatment. In this research, all experimental procedures were performed in an atmosphere argon-filled glove box (MBraunUnilab glove box). Prior to analysis, the preparation of Na3AlH6 composite was conducted as reported in a previous work.42 Then, the composites of Na3AlH6, MgH2 and LiBH4 were prepared by ball milling in a planetary ball mill (NQM-0.4) for 6 h with the ratio of the weight of the balls to the weight of the powder of 40:1. The temperature-programmed desorption (TPD) and the re/dehydrogenation kinetics measurement were performed in a Sieverts-type pressure-composition-temperature (PCT) 5 ACS Paragon Plus Environment


apparatus (Advanced Materials Corporation). The sample vessel with the amount of 70 mg of freshly ball-milled composites was subjected to TPD experiments under vacuum condition and heated in the range of 25 °C to 550 °C at a heating rate of 5 °C/min. To determine the re/dehydrogenation kinetics, the experiments were conducted under selected temperature and hydrogen pressure. Mettler Toledo TGA/DSC 1 apparatus was used in the DSC analysis to determine the thermal properties of the samples. Approximately 5 mg of the samples were loaded into an alumina crucible and heated from room temperature to 600 °C. The samples were characterised at several heating rates under an argon atmosphere. Meanwhile, in order to characterise surface morphology, the samples were prepared on carbon tape and then coated with gold spray. A scanning electron microscope (SEM; JEOL JSM-6360LA) was used. XRD analysis of the samples after ball milling and decomposition of the samples at each stage, as well as after rehydrogenation, were conducted by using a Rigaku MiniFlex Xray diffractometer with Cu-Kα radiation. The measurement was performed at the speed of 2.00 °/min and 1.00 °/min, respectively, over the diffraction angles of 20° to 80°. In addition, FTIR analysis was conducted by using an IRTracer-100 spectrophotometer. The FTIR curve was taken at a spectral resolution of 4 cm-1 for 40 scans from 800 cm-1 to 3000 cm-1.

3.

Results and Discussion

3.1

Dehydrogenation temperature

The TPD measurement was done on the milled Li3AlH6, milled MgH2, milled NaBH4, Na3AlH6-4LiBH4 and MgH2-Na3AlH6-4LiBH4 composites as shown in Figure 1. From the desorption curve, it can be seen that the milled Li3AlH6 begins to decompose hydrogen at 180 °C. In addition, the milled MgH2 starts to desorb hydrogen at 350 °C. Furthermore, the 6 ACS Paragon Plus Environment

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dehydriding reaction of the milled NaBH4 occurs at about 480 °C. For Na3AlH6-4LiBH4 composite, the dehydrogenation process was represented in three major stages with the total hydrogen release of 9.4 wt% as reported in our previous work.41 The first stage occurred at about 175 °C to 185 °C due to the decomposition of Li3AlH6-relevant stage. Meanwhile, the second and third stages that took place at 380 °C and 430 °C, respectively, originated from the H2-desorption of the NaBH4-relevant stage.

Fig. 1–TPD pattern of the milled Li3AlH6, the milled MgH2, the milled NaBH4, the Na3AlH64LiBH4 and the MgH2-Na3AlH6-4LiBH4 composites. (I, II, III and IV indicates the first, second, third and fourth dehydrogenation stages, respectively).

Moreover, the TPD curves revealed that after adding MgH2 to Na3AlH6-4LiBH4 composite, the dehydriding reaction occurred in four stages with the total hydrogen release of 10.5 wt%, which was about 1.1 wt% higher compared to Na3AlH6-4LiBH4 composite alone. In detail, the decomposition process of MgH2-Na3AlH6-4LiBH4 composite took place at a temperature range of 175 °C to 185 °C for the first dehydrogenation stage and continued to decompose at about 250 °C (second dehydrogenation stage). Then, further heating up to 310 °C led to the 7 ACS Paragon Plus Environment


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third dehydrogenation stage and it was completed at approximately 405 °C. The final dehydrogenation stage was found to occur at 420 °C. These results suggested that the capacity of hydrogen release of Na3AlH6-4LiBH4 composite had increased due to the addition of MgH2 component.

3.2

Structural characteristics analysis

To further understand the role of MgH2 in Na3AlH6-4LiBH4 composite after milling and dehydrogenation stages, the reaction mechanism of the composite was determined by using the XRD analysis. In Figure 2 (a), the characteristic peaks that corresponded to MgH2 along with the metathesis product between Na3AlH6 and LiBH4, namely Li3AlH6 and NaBH4 phases were detected after 6 h of milling. The formation of Li3AlH6 and NaBH4 showed that the metathesis reaction between Na3AlH6 and LiBH4 occurred during the ball milling process as reported in a previous work.41 After hydrogen desorption at 220 °C (Figure 2(b)), the peaks of LiH and Al were formed with regard to the decomposition of Li3AlH6 as follows: Li3AlH6 → 3LiH + Al + 3/2H2

(1)

The result proved that the first dehydrogenation stage in TPD measurement (Figure 1) corresponded to the decomposition of Li3AlH6 component. A total of 5.6 wt%43 hydrogen are predicted to be released originating from the reaction (1) using following equations44: ρm(H2) = [xMH2 / (xMH2 + xMm)] x 100

(2)

where MH2 and Mm are the molar mass of hydrogen and molar mass of metal, respectively. Obviously, the experimental capacity of hydrogen is lower than that calculated hydrogen release. The difference may be due to the dehydrogenation of intermediate decomposition product, LiH occurs at high temperatures (above 600 °C),45 while the dehydrogenation process of this stage is only up to 220 °C. Therefore, indirectly limiting the hydrogen release 8 ACS Paragon Plus Environment

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of the decomposition of Li3AlH6 in this present work. In addition, the phases of MgH2 and NaBH4 were clearly observed, suggesting that these two components did not decompose at this stage. However, it was believed that MgH2 started to decompose and reacted with LiH and Al after the composite was heated up to 310 °C through the formation of intermediate composites, Li3Mg7 and Mg2Al3 as detected by the XRD analysis (Figure 2(c)). The results confirmed that the hydrogen release at the second stage of the TPD measurement (Figure 1) originated from MgH2 and the reactions are as follows: 7MgH2 + 3LiH → Li3Mg7 + 8.5H2

(3)

2MgH2 + 3Al → Mg2Al3 + 2H2

(4)

Meanwhile, along with the NaBH4 peaks, excessive LiH and Al were also observed in the XRD pattern at this stage. However, the measured dehydrogenation amount at this stage as presented in Figure 1 is lower compared with the calculated value for reaction (3) and (4), 8.2 wt% and 3.0 wt%44 respectively. This discrepancy probably due to the destabilization effect among the reactions has occurred in this present work and were decomposed at the same time, while the individual Mg-Al or Mg-Li composite is decompose separately. Meanwhile, after the dehydrogenation process to 415 °C (Figure 2(d)), the new peaks of Mg1-xAlxB2 were observed, proposing that NaBH4 started to decompose at this stage. The formation of Mg1xAlxB2

was also discussed in the work by Mao et al.46 on the LiAlH4-MgH2-LiBH4 (1:1:1)

system. The peaks of Li3Mg7, Mg2Al3, LiH, Al and excessive NaBH4 were also detected at this stage. After continuing with heating up to 530 °C (Figure 2(e)), the peaks of Na were observed, while no characteristic peaks of NaBH4 appeared in the XRD pattern. Thus, it is recommended that NaBH4 has fully decomposed as can be seen in Equation (4). The appearance of Na peaks after the decomposition of the NaBH4 in this present work are in a good agreement with previous work47 and other reported literatures.48-50 Moreover, the 9 ACS Paragon Plus Environment


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amorphous state of boron was undetected by the XRD experiments as discussed in our previous work41 and other researchers’ work.50-51 In addition, the theoretical dehydrogenation capacity based on the following reaction is calculated to be 10.6 wt%, which is higher compared to the hydrogen release in this present work (6.5 wt%). The reduction of the capacity of hydrogen release could be due to the NaBH4 has already decompose at temperature of about 310 °C through the formation of Mg-Al-B alloys as discussed in Figure 2 (d)). NaBH4 → Na + B + 2H2

(4)

Fig. 2- XRD patterns after 6 h ball milling (a), after dehydrogenation stage at 220 °C (b), at 310 °C (c), at 415 °C (d) and at 530 °C (e) of the MgH2-Na3AlH6-4LiBH4 composites. 3.3

Thermal properties

To better understand the behaviour of the thermal properties of MgH2 added with Na3AlH64LiBH4 composite, the DSC analysis was applied. The thermal properties of the milled NaBH4, milled MgH2 and Na3AlH6-4LiBH4 composite are shown in Figure 3. It was 10 ACS Paragon Plus Environment

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observed that the DSC results of milled NaBH4 exhibited one endothermic peak and one exothermic peak. The endothermic peak at 510 °C corresponded to the melting of NaBH4 as discussed in our previous work41 and other reported literature,52 whereas the exothermic peak at 535 °C was assigned to the decomposition of NaBH4. For the milled MgH2, it was observed that an endothermic peak at 430 °C appeared, which originated from the dehydrogenation process of MgH2. Meanwhile, three endothermic peaks of the decomposition of Na3AlH6-4LiBH4 composite were observed at 210 °C, 455 °C and 480 °C as reported in our previous work.41 In addition, as can be seen in the figure, four endothermic peaks are present in the MgH2-Na3AlH6-4LiBH4 composite, which show good agreement with the dehydrogenation steps in the TPD results as shown in Figure 1. The endothermic peaks at 180 °C and 280 °C were ascribed to the decomposition of Li3AlH6 and MgH2 in the composite, respectively. Meanwhile, the two endothermic peaks at 430 °C and 460 °C were assigned to the decomposition of NaBH4-relevant in the composite. However, the results were found to be slightly different between the dehydrogenation temperature of TPD measurement (Figure 1) and DSC analysis. This phenomena could be due to the different experimental conditions as mentioned in the experimental parts and discussed in our previous work.53



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Fig. 3- DSC curves of the milled NaBH4, the milled MgH2, the Na3AlH6-4LiBH4 and MgH2Na3AlH6-4LiBH4 composites at a heating rate of 20 °C/min. 3.4

Activation energy

In order to further examine the kinetic enhancement of MgH2-Na3AlH6-4LiBH4 composite, the apparent activation energy (Ea) was determined. The Ea of the ternary composite was calculated by using the Kissinger analysis54 as follows: ln [β/Tp2] = -Ea/RTp + A

(6)

Where, Ea is the apparent activation energy for hydrogen desorption, β is the heating rate, Tp is the peak temperature, R is the gas constant and A is a linear constant. Ea can be obtained from the slope in a plot of ln (β/Tp2) versus 1000/Tp. In this work, the DSC measurement was conducted at various heating rates to calculate the Ea for the MgH2 and NaBH4 decomposition stages in MgH2-Na3AlH6-4LiBH4 composite as shown in Figure 4.


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Fig. 4- DSC traces of MgH2 (a) and NaBH4 (b) stage in MgH2-Na3AlH6-4LiBH4 composite at different heating rates.

The Kissinger plots of MgH2 and NaBH4-relevant decomposition in the ternary composite are shown in Figure 5. From the graph, it can be seen that the Ea values of the MgH2 and NaBH4relevant decomposition are 127 kJ/mol and 138 kJ/mol, respectively. Apparently, it is approximately 33 kJ/mol lower compared to the Ea of NaBH4-relevant decomposition in Na3AlH6-4LiBH4 as calculated in our previous work41 indicating that the presence of MgH2 was destabilize NaBH4 through the formation of Mg1-xAlxB2 as discussed in Figure 2 (d). The obtained results also showed that the Ea of the MgH2-relevant decomposition in this ternary composite was significantly reduced compared to milled MgH2 (158 kJ/mol).42 As comparison, the apparent activation energy in this work is quite higher than reported work for CeH2.73-MgH2-Ni nanocomposites (63 kJ/mol).55 Nevertheless, these results indicate that the presence of MgH2 reduces hydrogen barriers, thus promoting better hydrogen storage performance of this ternary composite.



Fig. 5-Kissinger’s analysis of the MgH2- and NaBH4-relevant decomposition in MgH2Na3AlH6-4LiBH4 composites.

3.5

Surface morphology

The SEM images of surface morphology of the milled Li3AlH6, milled NaBH4, milled MgH2, Na3AlH6-4LiBH4 and MgH2-Na3AlH6-4LiBH4 composites are shown in Figure 6. The milled Li3AlH6 sample (Figure 6(a)) consisted of large regular particles, whereas the particles of milled NaBH4 (Figure 6(b)) were quite agglomerated. For milled MgH2 sample (Figure 6(c)), the average size of the particles was 1 µm due to the agglomeration of irregular particles that occurred following the milling process. Meanwhile, for Na3AlH6-4LiBH4 composite (Figure 6(d)) after 6 h of milling, the particles had a disordered surface. Interestingly, it is apparent that the Na3AlH6-4LiBH4 composite is determined to be smaller and less agglomerated after the addition of MgH2 (Figure 6(e)). The SEM images show that the introduction of MgH2 has been proven to enhance the performance of Na3AlH6-4LiBH4 hydrogen storage properties. The results are supported by Li et al.,56 where the significant reduction in the particle size


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with high surface defect density could enhance diffusion, thus improving hydrogen de/absorption kinetics.

Fig. 6- SEM images of the milled Li3AlH6 (a), the milled NaBH4 (b), the milled MgH2, the Na3AlH6-4LiBH4 (d) and the MgH2-Na3AlH6-4LiBH4 (e) composites.

3.6

Hydorgen absorption/desorption kinetic properties of the MgH2-added samples

To further characterise the reversibility of Na3AlH6-4LiBH4 composite after the addition of MgH2, rehydrogenation process was conducted on the completely dehydrogenated samples under 33 atm of hydrogen pressure at 320 °C. For comparison purposes, the rehydrogenation process of Na3AlH6-4LiBH4 composite was also measured. Figure 7 shows that about 4.0 wt% of hydrogen can be absorbed by the dehydrogenated MgH2-Na3AlH6-4LiBH4 composite after 60 min of rehydrogenation process. Compared to Na3AlH6-4LiBH4 composite, only 2.8 15 ACS Paragon Plus Environment


wt% of hydrogen can be absorbed within the same time period. These results indicated that the absorption kinetics of Na3AlH6-4LiBH4 composite were effectively enhanced after the addition of MgH2.

Fig. 7- Hydrogen absorption behaviours the MgH2-Na3AlH6-4LiBH4 composites at 320 °C under 33 atm.

The isothermal desorption curves of Na3AlH6-4LiBH4 composite with and without MgH2 are shown in Figure 8. Based on the figure, the MgH2-Na3AlH6-4LiBH4 composite could desorb hydrogen of about 1.9 wt% within 120 min, whereas the Na3AlH6-4LiBH4 composite could only desorb hydrogen of approximately 1.5 wt% under the same conditions. These obtained results show that the performance of dehydrogenation kinetics of the ternary composite system is much better compared to Na3AlH6-4LiBH4 composite. Interestingly, about 1.5 wt% of hydrogen was desorbed from the MgH2-Na3AlH6-4LiBH4 composite after 30 min at 320 °C, whereas it released only 0.8 wt% of hydrogen for Na3AlH6-4LiBH4 within the same time of period. This clearly demonstrates that MgH2 component indeed facilitates the hydrogen desorption of the ternary system. Moreover, previous work57 had showed that the individual 16 ACS Paragon Plus Environment

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MgH2 can desorb about 0.9 wt% of hydrogen within 30 min at 320 °C. This fact indicates that the MgH2 may mainly responsible for desorption kinetics enhancement due to the its theoretical capacity of hydrogen release (7.6 wt%), which is contribute to the increasing of capacity hydrogen release. Therefore, it is noteworthy that mutual interactions has occur between the three components during the isothermal process and thus leading to the enhancement of hydrogen desorption behaviours of the ternary composite system.

Fig. 8- Hydrogen desorption behaviours the MgH2-Na3AlH6-4LiBH4 composites at 320 °C under vacuum.

3.7

Reaction mechanism of the hydrogenated MgH2-added sample

To understand the chemical reaction of the hydrogenated samples, the XRD results were collected as shown in Figure 9. After the rehydrogenation process of MgH2-Na3AlH6-4LIBH4 composite at 320 °C under 33 atm, minor peaks assigned to MgH2 were detected. In addition, the characteristic peaks, which corresponded to LiH and Al, were also observed. However, the peaks of Li3Mg7 and Mg2Al3 could not be detected from the XRD pattern after the process. These results suggest the reactions of the reversibility of MgH2 as follows: 17 ACS Paragon Plus Environment


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2Li3Mg7 + 17H2 → 14MgH2 + 6LiH

(7)

Mg2Al3 + 2H2 → 2MgH2 + 3Al

(8)

These results are in good agreement with the work done by Mao et al.46 where Li3Mg7 can be associated with MgH2 and LiH. On the other hand, Mg2Al3 could also be transformed into MgH2 and Al as discussed by Bouaricha et al..58 Meanwhile, Mg1-xAlxB2 phases could still be observed in the composite even after rehydrogenation. Similar results were also obtained in a previous work.59

Fig. 9- XRD pattern of the MgH2-Na3AlH6-4LiBH4 composite after rehydrogenation at 320 °C under 33 atm.

Based on the XRD analysis after the rehydrogenation process, the phases of either NaBH4 or LiBH4 were not detected and only MgH2 phases were identified. Therefore, further analysis by using FTIR analysis was done to determine the reversibility of either NaBH4 or LiBH4. The FTIR spectra of the milled NaBH4 and LiBH4 were also collected as shown in Figure 10. From the figure, the FTIR analysis of MgH2-Na3AlH6-4LiBH4 composite shows several peaks at around 2395 cm-1, 2300 cm-1, 2223 cm-1 and 1100 cm-1 after the rehydrogenation process. The peaks around 2300 cm-1 and 1100 cm-1 were believed to originate from NaBH4 18 ACS Paragon Plus Environment

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as reported in our previous study.41 However, these peaks’ positions were also considered as the confirmation of LiBH4, since the peaks were attributed to the B-H group.60 As discussed by Pan et al.,61 the intensities at 2381 cm-1, 2293 cm-1 and 2225 cm-1 corresponded to the B-H vibration of LiBH4. Moreover, the work done by Zhou et al.40 also confirmed the formation of Li-B-H phase through the observation of typical B-H absorbances at 2382 cm-1, 2291 cm-1, 2223 cm-1 and 1123 cm-1. Therefore, it is noteworthy to assume that the hydrogenated samples mainly consist of MgH2, NaBH4 and LiBH4, thus leading to better performance of ab/desorption kinetics of the ternary system.

Fig. 10- FTIR spectra of the LiBH4, NaBH4 and MgH2-Na3AlH6-4LiBH4 composite after rehydrogenation process under 320 °C. These results display that the formation of Mg-Al, Li-Mg and Mg-Al-B alloys might promote better performance of MgH2-added Na3AlH6-4LiBH4 composite. This could be supported by other researchers’ works such as Li et al.,62 where the presence of Mg1-xAlxB2 in Al-added MgH2-2LiBH4 composite through the reaction of LiBH4 and Mg-Al alloys contributed to the enhancement of the reversibility performance of MgH2-2LiBH4 composite. The mixture composite also started to release hydrogen at about 530 K; about 80 K lower than the onset desorption temperature of MgH2-2LiBH4. In addition, the formation of Mg-Al, Li-Mg and 19 ACS Paragon Plus Environment


Mg-Al-B alloys also facilitated the diffusion of hydrogen through the reaction barriers in the de/hydrogenation processes of LiBH4 and LiAlH4-added MgH2 composite, thus improving their hydrogen storage properties as reported by Zhang et al..63 They also found that the formation of these alloys helped to increase the capacity of hydrogen absorption/desorption cycles of the composite. Therefore, it is worth noting that the presence of Mg-Al, Li-Mg and Mg-Al-B alloys in this work plays a crucial role in reducing the dehydrogenation temperature with high capacity of hydrogen release, as well as accelerates the re/dehydrogenation kinetics of the present ternary composite.

4.

Conclusion

In summary, the introduction of MgH2 to the Na3AlH6-4LiBH4 composite system promotes better hydrogenation properties compared to the individual MgH2, Na3AlH6 and LiBH4 components. It was found that MgH2-Na3AlH6-LiBH4 (1:1:4) was readily converted to MgH2/Li3AlH6/NaBH4 via the metathesis reaction during the ball milling process. From the TPD graph, the composite began to release hydrogen at 180 °C through the decomposition of Li3AlH6 to form LiH and Al. Further heating up to 250 °C led to the decomposition of MgH2, which then reacted with LiH and Al to form Li3Mg7 and Mg2Al3 as the intermediate compounds. After heating to 415 °C, NaBH4 started to decompose through the formation of Mg1-xAlxB2. Then, excessive NaBH4 was fully decomposed at 530 °C. These results showed that the onset desorption temperature of MgH2 and NaBH4 in this composite was reduced to 100 °C and 170 °C in comparison to the milled MgH2 and NaBH4, respectively. Meanwhile, high capacity of hydrogen absorb and desorb could be obtained by this ternary composite. From the Kissinger analysis, the Ea values for hydrogen desorption were found to be 127 kJ/mol and 138 kJmol for MgH2 and NaBH4-relevant decomposition stage in the composite, respectively. The synergistic role played by the formation of Mg-Al, Li-Mg and Mg-Al-B 20 ACS Paragon Plus Environment

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alloys during the heating process was believed to enhance the thermodynamic reactions by changing the de/rehydrogenation pathway. As a result, favourable operating temperatures with high capacity of hydrogen release and kinetics enhancement were achieved for the MgH2-Na3AlH6-4LiBH4 composite.

Acknowledgement The authors would like to thank Universiti Malaysia Terengganu for the facilities provided. This study was financially supported by Universiti Malaysia Terengganu through the Talent and Publication Enhancement-Research Grant (TAPE-RG) (VOT 55134). F. A. Halim Yap is thankful for his BUMT scholarship by the Universiti Malaysia Terengganu.

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Functions of MgH2 on Hydrogen Storage Properties of Na3AlH6

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