Catalytic effects of MgFe2O4 addition on the dehydrogenation properties of LiAlH4

Catalytic effects of MgFe2O4 addition on the dehydrogenation properties of LiAlH4

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Catalytic effects of MgFe2O4 addition on the dehydrogenation properties of LiAlH4 N.A. Ali a, N.H. Idris a, N.A. Sazelee a, M.S. Yahya a, F.A. Halim Yap b, M. Ismail a,* a

Energy Storage Research Group, Faculty of Ocean Engineering Technology and Informatics, Universiti Malaysia Terengganu, 21030, Kuala Nerus, Terengganu, Malaysia b Faculty of Innovative Design and Technology, Universiti Sultan Zainal Abidin, Gong Badak Campus, 21300, Kuala Nerus, Terengganu, Malaysia

highlights  The H2 storage properties of LiAlH4/MgFe2O4 have been studied for the first time.  Desorption properties of LiAlH4 are improved after adding with MgFe2O4.  LiAlH4//MgFe2O4 composite could desorb 3.5 wt% H2 within 30 min at 90  C.  The formation of new species of Fe, LiFeO2 and MgO phases plays a catalytic role.

article info

abstract

Article history:

Study on the catalytic roles of MgFe2O4 on the dehydrogenation performance of LiAlH4 was

Received 31 July 2019

carried out for the first time. Notable improvement on the dehydrogenation of LiAlH4

Received in revised form

eMgFe2O4 compound was observed. The initial decomposition temperatures for the cata-

4 September 2019

lyzed LiAlH4 were decreased to 95  C and 145  C for the first and second stage reactions,

Accepted 10 September 2019

which were 48  C and 28  C lower than the milled LiAlH4. As for the desorption kinetics

Available online xxx

performance, the MgFe2O4 doped-LiAlH4 sample was able to desorb faster with a value of 3.5 wt% of hydrogen in 30 min (90  C) while the undoped LiAlH4 was only able to desorb

Keywords:

0.1 wt% of hydrogen. The activation energy determined from the Kissinger analysis for the

Hydrogen storage

first two desorption reactions were 73 kJ/mol and 97 kJ/mol; which were 31 and 17 kJ/mol

Lithium aluminium hydride

lower as compared to the milled LiAlH4. The X-ray diffraction result suggested that the

Catalyst

MgFe2O4 had promoted significant improvements by reducing the LiAlH4 decomposition

Magnesium iron oxide

temperature and faster desorption kinetics through the formation of active species of Fe, LiFeO2 and MgO that were formed during the heating process. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The need for developing a new environmentally friendly energy carrier has become more significant and is corresponding to the rapid growth of energy demand. Since hydrogen is the

richest element in the universe and considered as a clean energy carrier, hydrogen is often described as a future fuel to substitute the fossil fuel [1]. It is renewable and possesses high energy substances than any other fuel [2]. For instance, hydrogen holds three times higher energy content (120 MJ kg1) compared to gasoline (44 MJ kg1) [3]. One of the

* Corresponding author. E-mail address: [email protected] (M. Ismail). https://doi.org/10.1016/j.ijhydene.2019.09.083 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Ali NA et al., Catalytic effects of MgFe2O4 addition on the dehydrogenation properties of LiAlH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.083

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key issues that are needed to be resolved is the way to store hydrogen. At present, hydrogen can be stored in several states which are high-pressure gas, liquid hydrogen in tanks and solid-state in hydrides [4]. Hydrogen storage via the solid-state is the most preferred due to its safety and systematic way to store energy for onboard applications [5,6]. Among the materials studied for the solid-state hydrogen storage, light complex hydrides such as LiAlH4 and NaAlH4 are described as the most potential candidates that offer high gravimetric as well as high volumetric hydrogen capacity [7]. In this context, LiAlH4 is preferred due to its high hydrogen content which is 10.5 wt% [8]. However, this gas will be released by LiAlH4 in 3 steps as demonstrated in the following reaction [9]: 3LiAlH4 / Li3AlH6 þ 2Al þ 3H2

(1)

the pristine LiAlH4. Moreover, a study by Zhai et al. [37] revealed that the initial dehydrogenation temperature was greatly reduced with the aid of MnFe2O4 as the catalyst. Hydrogen was started to release at 80  C compared to the pure LiAlH4 which was released at 150  C. At present, no research has been done on the roles of MgFe2O4 on the dehydrogenation performance of LiAlH4. Our previous study had demonstrated an excellent hydrogenation performance of MgH2 and NaBH4 when doping with MgFe2O4 [38,39]. It is predicted that the addition of MgFe2O4 will also provide a synergetic effect on the hydrogen storage performance of LiAlH4. Hence, it is interesting to explore the catalytic effect of MgFe2O4 on the dehydrogenation performance of LiAlH4. The possible catalytic mechanisms on the dehydrogenation performance of LiAlH4 are elucidated in this paper.

(5.3 wt% H2, 150  Ce175  C) Li3AlH6 þ 2Al / LiH þ 3Al þ 3/2H2

(2)

(2.6 wt% H2, 180  Ce220  C) 3LiH þ 3Al / 3LiAl þ 3/2 H2

(3)

(2.6 wt% H2, >400  C) In spite of its high hydrogen capacity, the performances of LiAlH4 are restricted by its slow sorption kinetics and irreversibility. Furthermore, the thermal decomposition of LiAlH4 as in equation (3) is also considered as unsuited for practical applications [10]. Therefore, in order to overcome the drawbacks of LiAlH4, a large number of researches were carried out like by catalysts doping [11e13] from different materials such as metals [14e16], Ti-based additives [17e22], metal oxide [23e25] and metal halides [26e32]. Previous scrutinies demonstrated that additives that were based on ternary metal oxides had significantly improved the dehydrogenation performances of LiAlH4. For instance, Li et al. [33] showed that with 3 mol of NiFe2O4, the onset temperature for the LiAlH4 decomposition was lowered to 61  C, which was 94  C lower than the pure LiAlH4. A new by-product was found after the desorption process and its phase had provided an essential role on the amelioration of the desorption performance of LiAlH4. In another study, an introduction of 2 mol of CoFe2O4 had reduced the initial decomposition temperature of the CoFe2O4-doped LiAlH4 sample to 65  C and 135  C for the first and second dehydrogenation stages, which were 90  C and 60  C lower than the pure LiAlH4 [34]. In addition, from the XRD result, the formations of LiFeO2, LiAlO2, Fe0.98O, and Al0.52Co0.48 after the desorption process were confirmed. This finely dispersed reaction product ameliorates the dehydrogenation characteristics of the LiAlH4 by acting as the active centre for the nucleation of the dehydrogenated product. In terms of activation energy, Li et al. [35] proved that the activation energy was significantly reduced with the addition of NiCo2O4. The activation energies calculated based on the Kissinger equation for the pure LiAlH4 were 116.2 kJ/mol and 133 kJ/mol for the first and second decomposition stages [36]. Meanwhile, the activation energies for the LiAlH4eNiCo2O4 sample were 79.12 kJ/mol and 95.11 kJ/mol, which were 37.08 kJ/mol and 37.89 kJ/mol lower compared to

Experimental details LiAlH4 powders (95% purity) supplied by Sigma Aldrich were used without any purification. MgFe2O4 was synthesized from the hydrothermal method as presented in our previous studies [38,40]. 400 mg of LiAlH4 was milled together with 10 wt% of MgFe2O4. The mixture was ball-milled for 1 h in a planetary ball miller (NQM-04) at a rotation speed of 400 rpm. LiAlH4 without the catalyst was also milled under the same environment for comparison purposes. The samples preparation was done in an inert atmosphere glovebox (MBraun Unilab) to prevent from oxidation and humidity. The onset decomposition temperature and sorption kinetics were examined using the Sievert-type apparatus (Advanced Materials Corporation) as explained in our previous studies [41,42]. For the onset decomposition temperature measurement, all samples were heated in a vacuum chamber from 25  C to 250  C at a heating rate of 5  C/min. For the desorption kinetics experiment, all samples were heated at a constant temperature of 90  C under 1.0 atm of hydrogen pressure. The phase structure characterization was carried out using the Rigaku MiniFlex X-ray diffraction (XRD) apparatus equipped with Cu Ka radiation. The XRD measurements were collected at 2q in the range of 20 e80 at 2.00 /min. The IR Tracer-100 Shimadzu was used to record the Fourier transform infrared (FTIR) spectrum for the both doped and undoped samples. The characterization of the samples was in the range of 800e2000 cm1. For the surface morphology study, the scanning electron microscopy (SEM) (JEOL JSM -6350LA) was used. Differential scanning calorimetry characterizations were performed using the DSC/TGA 1 to examine the thermal properties of the undoped and doped samples. The samples were heated from ambient temperature to 250  C with different heating rates; 15, 20, 25 and 30  C/min. In addition, the samples were heated under an argon flow of 50 ml/min.

Result and discussions Fig. 1 presents the TPD curve of the pure LiAlH4, milled LiAlH4, and LiAlH4 þ 10 wt% MgFe2O4. The dehydrogenation process

Please cite this article as: Ali NA et al., Catalytic effects of MgFe2O4 addition on the dehydrogenation properties of LiAlH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.083

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of LiAlH4 happens in two steps with a total amount of 7.4 wt% of hydrogen. Before the milling procedure, the initial decomposition temperatures of the pure LiAlH4 are 145  C and 175  C. After the 1 h of milling, the desorption temperatures of LiAlH4 have lessened to 143  C and 173  C. Meanwhile, with the introduction of 10 wt% of MgFe2O4, the decomposition temperatures of the doped LiAlH4 are considerably reduced to 95  C and 145  C. Comparing to the pure LiAlH4, notable improvement on the decomposition temperatures is shown by the LiAlH4e MgFe2O4 sample. However, slight decrement in the total amount of hydrogen released can be seen for the MgFe2O4edoped LiAlH4 sample which is caused by the deadweight of the dopant that does not contain hydrogen. The catalytic roles of MgFe2O4 on the performance of LiAlH4 was further studied by conducting the desorption kinetics experiment under a low-temperature environment as presented in Fig. 2. The kinetics behaviour of both doped and undoped LiAlH4 was included for comparison. The isothermal desorption kinetics of the LiAlH4 þ 10 wt% MgFe2O4 sample was conducted at 90  C. From the curves, it can be seen that within 20 min, only 0.1 wt% and 0.2 wt% of hydrogen was desorbed by the pure and milled LiAlH4, respectively. The low amount of released hydrogen indicates the sluggish desorption kinetics of LiAlH4. Meanwhile, when 10 wt% of MgFe2O4 was added, the isothermal desorption property of LiAlH4 has shown a notable improvement where the doped composite was able to desorb 2.0 wt% of hydrogen under the same condition. The isothermal desorption kinetics of the LiAlH4 þ 10 wt% MgFe2O4 sample has demonstrated the dehydrogenation rate that is 20 times faster compared to the undoped LiAlH4 for the first 20 min. The improvement in kinetics performance may result from the particles size reduction throughout the milling process, the formation of active materials and defect creation [43e45]. To further study the thermal behaviour, DSC experiments were conducted. Fig. 3 shows the DSC profile of the MgFe2O4 doped-LiAlH4 and undoped sample for the heating rate of 30  C/min. Both the doped and the undoped LiAlH4 samples display two distinct exothermic and endothermic peaks. For the milled LiAlH4, the first exothermic peak (154  C) is

correlated to LiAlH4 that interacts with the surface hydroxyl impurities while the first endothermic peak (182  C) describes its melting process [46]. Meanwhile, the second exothermic peak (198  C) is ascribed to the dehydrogenation step of liquid LiAlH4. As for the second endothermic peak (213  C), it is correlated to the decomposition of Li3AlH6. The MgFe2O4 doped-LiAlH4 sample shows similar DSC traces with the milled LiAlH4 which also display two endothermic and two exothermic peaks but occurring at a lower temperature. The first exothermic reaction occurs at 125  C while the first endothermic reaction occurs at 148  C. Meanwhile, the second exothermic reaction that assigned to the dehydrogenation of liquid LiAlH4 as in equation (1) occurs at 177  C and the second endothermic reaction that referring to the dehydrogenation of Li3AlH6 as in equation (2) occurs at 217  C. In terms of activation energy (EA), EA is the minimum energy that is needed to initiate any chemical reaction [47]. In

Fig. 1 e Decomposition temperature profile for the pure LiAlH4, milled LiAlH4 and MgFe2O4-doped LiAlH4 sample.

Fig. 3 e DSC curves of the milled LiAlH4 and LiAlH4 þ 10 wt % MgFe2O4 at 30  C/min of heating rate.

Fig. 2 e Dehydriding kinetics curves of the pure LiAlH4, milled LiAlH4 and the MgFe2O4-doped LiAlH4 sample at 90  C and under 1.0 atm of pressure.

Please cite this article as: Ali NA et al., Catalytic effects of MgFe2O4 addition on the dehydrogenation properties of LiAlH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.083

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this study, the activation energy refers to the energy needed for LiAlH4 to start a desorption process. Normally, the improvement on the initial decomposition temperature and the dehydriding kinetics performance are corresponding to an energy barrier for hydrogen to be released from LiAlH4. Equation (4) presents the Kissinger analysis that was used to determine the activation energy. ln[b/T2p] ¼ -EA/RTpþ A

(4)

Where b is corresponding to the heating rate of the DSC curve, Tp is corresponding to the temperature from the DSC curve, R is the gas constant and A is a linear constant. In this work, Tp was obtained from the DSC profiles of the doped and un-doped samples at various heating rates as shown in Fig. 4. Then, the activation energy can be calculated by referring to the slope value of the Kissinger plot, ln [b/Tp2] versus 1000/Tp as displayed in Fig. 5. The Kissinger analysis was applied to the second exothermic and second endothermic reactions. For the milled LiAlH4, it is found that the activation energy that was obtained for the first two-step reactions are 104 kJ/mol and 112 kJ/mol, respectively. Meanwhile, when 10 wt% of MgFe2O4 was added, the apparent activation energy has significantly reduced to 73 kJ/mol and 97 kJ/mol, respectively. These values are 31 kJ/ mol and 15 kJ/mol lower than the milled LiAlH4. Therefore, from these results, it reveals that the use of MgFe2O4 as a catalyst has notably lowered the activation energy of LiAlH4 and has enhanced the desorption kinetics performances of LiAlH4 þ 10 wt% MgFe2O4 sample. Information on the surface morphology and size of the particles were examined using the SEM apparatus. The SEM images representing the pure LiAlH4, milled LiAlH4, and LiAlH4 þ 10 wt% MgFe2O4 are shown as in Fig. 6. From the figure, it is noticeable that the pure LiAlH4 exhibits large and uniform particles size distribution [48]. The estimated particles size were in the range of 15 mme50 mm with irregular rod-shaped particles. After 1 h of milling, the particles size of LiAlH4 were reduced but with some agglomerations and inhomogeneous in shape. Meanwhile, the addition of 10 wt% MgFe2O4

shows that the morphology of the sample has greatly improved, less agglomeration with smaller particle sizes compared to the pure and milled LiAlH4. Smaller particle sizes provide larger surface defects and create more grain boundaries which as a result, enhancement on the dehydrogenation performance of LiAlH4 is achieved [37]. This phenomenon is in a good agreement with previous studies that demonstrated the reduction of particles size of LiAlH4 due to catalyst doping [49,50]. In this context, the doped catalyst will act as a dispersing agent that will prevent the agglomeration of the sample [51]. Therefore, it can be claimed that MgFe2O4 is a catalyst that is functional to reduce the particle sizes of the composite. To further explore the mechanism of the samples during the milling process, the phase structure of the samples was verified using the XRD method. The XRD spectra for the pure LiAlH4, milled LiAlH4, and LiAlH4 þ 10 wt% MgFe2O4 samples are shown as in Fig. 7. As for the pure LiAlH4, the XRD pattern displays that all peaks are correlated to the peaks of LiAlH4 which demonstrating the high purity of LiAlH4. Similarly, after the milling process (1 h), the XRD pattern only shows the LiAlH4 peaks. As reported in previous studies, LiAlH4 will remain stable during the milling process [26,34,52] and will not decompose which is similar to current finding. Furthermore, the LiAlH4 peaks still remained for the doped sample as in Fig. 7(c). However, it should be noted that the peaks of MgFe2O4 were not able to be detected by the XRD. It may due to the amount of MgFe2O4 in LiAlH4 þ 10 wt% MgFe2O4 sample is too low to be detected. This condition is similar to previous studies that showed the phase of the catalyst that is cannot be discovered after the milling process [17,19]. Fig. 8 shows the FTIR spectra of the pure LiAlH4, milled LiAlH4, and MgFe2O4-doped LiAlH4 sample. FTIR measurements of the samples were performed in order to verify the presence of Li3AlH6. Two regions of active IR peaks that are corresponding to the AleH bonds are detected in the pure LiAlH4, milled LiAlH4 and LiAlH4 þ 10 wt% MgFe2O4 samples. The peaks at the range of 1600e1800 cm1 are referred to the [AlH4]- stretching mode while the peaks at the range of 800e900 cm1 are correlated to the [AlH4]- bending mode.

Fig. 4 e DSC traces at heating rates of 15, 20, 25, and 30  C/min of the (a) milled LiAlH4 and (b) MgFe2O4-doped LiAlH4 sample. Please cite this article as: Ali NA et al., Catalytic effects of MgFe2O4 addition on the dehydrogenation properties of LiAlH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.083

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Fig. 5 e The Kissinger plot of (a) first peak and (b) second peak for the milled LiAlH4 and LiAlH4 þ 10 wt.% MgFe2O4 composite.

After the addition of MgFe2O4, the AleH stretching mode of Li3AlH6 is detected at around 1400 cm1. This result has suggested that after 1 h of milling with the catalyst, LiAlH4 has decomposed into Li3AlH6 and Al. To further investigate the structure of the phases and the catalytic mechanism of the dehydrogenated sample, the dehydrogenated sample of LiAlH4 þ 10 wt% MgFe2O4 was

examined using the XRD and is presented as in Fig. 9. Referring to the XRD pattern, the dehydrogenated sample displays the dehydrogenation products which are LiH and Al; without LiAlH4 and Li3AlH6 peaks. This result demonstrates that the first two dehydrogenation steps of LiAlH4 have completed when heated up to 250  C. Besides LiH and Al, there are also new peaks of Fe, LiFeO2 and MgO species after the

Fig. 6 e SEM images of the (a) pure LiAlH4 (b) milled LiAlH4 and (c) LiAlH4 þ 10 wt% MgFe2O4. Please cite this article as: Ali NA et al., Catalytic effects of MgFe2O4 addition on the dehydrogenation properties of LiAlH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.083

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Fig. 7 e XRD spectra of the (a) pure LiAlH4 (b) milled LiAlH4 and (c) LiAlH4 þ 10 wt% MgFe2O4.

dehydrogenation process. Based on this XRD result, the possible overall reaction of LiAlH4 and MgFe2O4 that has occurred during the heating process can be presented as follows: 4LiAlH4 þ 2MgFe2O4 / LiH þ 4Al þ Fe þ 3LiFeO2 þ 2MgO þ15/ (5) 2H2 Fe is well known as a promising additive for LiAlH4 [53]. It is reasonable to state that the in situ formation of Fe after the dehydrogenation process may actually responsible for the notable improvement on the dehydrogenation performance of LiAlH4. This is because the formation of fresh and fine Fe metals that is in contact with hydrogen atoms may help the dissociation process of the dehydrogenation step of LiAlH4 and thus improved the dehydrogenation rate. Moreover, the in situ LiFeO2 formed may also promote an auxiliary catalytic role on the dehydrogenation performance of LiAlH4. These

Fig. 9 e XRD pattern for the dehydrogenated LiAlH4 þ 10 wt % MgFe2O4 sample at 250  C.

findings are in a good agreement with the numerous exploration that studies the effect of transition metal oxide on the improvement of LiAlH4 [33,54]. Other than that, previous studies showed that MgO had improved the sorption kinetics performance and lowered the activation energy of MgH2 [55e57]. Therefore, it is believed that the MgO phase may also provide a significant effect that will help to improve the dehydrogenation performance of LiAlH4. Through the analyses, it can be hypothesized that the newly formed species of Fe, LiFeO2 and MgO work synergistically to enhance the kinetics performance of LiAlH4. This is because these products could generate surface activation and provide a high number of nucleation sites on the surface of LiAlH4 matrix as hydrogen diffusion channel and contribute to the improvement of the kinetics desorption performance by reducing the diffusion length of the reaction ions [34]. It is reasonable to clarify those in situ formations of Fe, LiFeO2 and MgO phases are synergistically contributing to the improvement of the desorption kinetics performance of LiAlH4.

Conclusion

Fig. 8 e IR spectra of the (a) pure LiAlH4 (b) milled LiAlH4 and (c) LiAlH4 þ 10 wt% MgFe2O4.

In conclusion, the addition of MgFe2O4 as the catalyst provides auxiliary improvements on the dehydrogenation performance of LiAlH4. The initial decomposition temperatures of 10 wt% MgFe2O4 doped LiAlH4 were reduced to 95  C and 145  C for the two-step decomposition reactions. In contrast, the milled LiAlH4 was started to decompose at 143  C and 173  C, respectively. Moreover, in terms of desorption kinetic, the MgFe2O4-doped LiAlH4 sample showed 20 times faster on the desorption kinetics performance as compared to the undoped LiAlH4. In addition, the LiAlH4 þ 10 wt% MgFe2O4 sample had successfully desorbed 2.0 wt% of hydrogen within 20 min at 90  C while the undoped LiAlH4 only desorbed 0.1 wt% of hydrogen within the same duration of time. Furthermore, the activation energy determined from the Kissinger plot for the milled LiAlH4 was 104 kJ/mol and 112 kJ/mol. However, for the doped sample, the activation energies were reduced to 73 kJ/

Please cite this article as: Ali NA et al., Catalytic effects of MgFe2O4 addition on the dehydrogenation properties of LiAlH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.083

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mol and 97 kJ/mol, respectively. From the XRD spectra of the dehydrogenated sample, the in situ formed of Fe, LiFeO2 and MgO species were believed to boost the dehydrogenation performance of LiAlH4. Therefore, it is reasonable to deduce that MgFe2O4 is a favourable dopant that improves the dehydrogenation performance of LiAlH4.

Acknowledgements This research was supported by the Universiti Malaysia Terengganu (UMT) through the Golden Goose Research Grant (GGRG) (VOT 55190). The authors also like to acknowledge UMT for the best facilities to perform this research.

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Please cite this article as: Ali NA et al., Catalytic effects of MgFe2O4 addition on the dehydrogenation properties of LiAlH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.083