Study of the Hydrogen Storage Properties and Catalytic Mechanism of a MgH2-Na3AlH6 System Incorporating FeCl3.

In this work, the catalytic effects of FeCl3 toward the hydrogen storage properties of the MgH2-Na3AlH6 composite were investigated for the first time. The temperature-programed desorption results indicated that the onset temperature of the hydrogen release of a 10 wt % FeCl3-doped MgH2-Na3AlH6 composite was ∼30 °C lower than that of the undoped MgH2-Na3AlH6 composite. The addition of FeCl3 into the MgH2-Na3AlH6 composite resulted in improved absorption and desorption kinetics performance. The absorption/desorption kinetics measurements at 320 °C (under 33 and 1 atm hydrogen pressure, respectively) indicated that within 10 min, the doped sample absorbed ∼4.0 wt % and desorbed ∼1.5 wt % hydrogen. By comparison, the undoped sample absorbed only ∼2.1 wt % and desorbed only ∼0.6 wt % hydrogen under the same conditions and time. Comparably, the apparent activation energy value of the doped composite is 128 kJ/mol, which is 12 kJ/mol lower than that of the undoped composite (140 kJ/mol). The formation of the new species of MgCl2 and Fe in the doped composite was detected from X-ray diffraction analysis after heating and absorption processes. These two components were believed to play a vital role in reducing the decomposition temperature and kinetics enhancement of the MgH2-Na3AlH6 composite.

Introduction

The development of efficient and safe hydrogen storage technology is required to commercialize the hydrogen economy. So far, several states of storing hydrogen have been explored, namely, gaseous state,[1−3] liquid state,[4−6] and solid state.[7−14] Each of these kinds of storage has its own benefits. However, storing hydrogen in the solid state has more benefits, particularly regarding its safety and high volumetric hydrogen capacity. In recent years, a Mg-based hydride material, MgH2, has been promptly promoted in numerous studies because of its high hydrogen release (7.6 wt %), good reversibility,[15−18] and highest energy density (9 MJ/kg).[19] Besides, NaAlH4 is also a potential candidate to store hydrogen in the solid state due to its high theoretical hydrogen capacity (7.4 wt %).[20] However, the high temperature of hydrogen release (up to 400 °C for MgH2 and up to 200 °C for NaAlH4)[21−23] and poor sorption kinetics have impeded the practical use of MgH2 and NaAlH4 as the hydrogen storage medium.[24−26] To deal with these issues, a number of methods like enhancing the kinetics performance and lowering the decomposition temperature by doping with a potential catalyst[27−40] and improving the surface properties using the ball milling method[41,42] have been widely studied by many researchers.

Apart from the stated methods, reacting with other hydrides is one of the alternative methods that have been applied in solid-state hydrogen storage research to obtain better hydrogen storage performance. In recent years, this type of method has grown rapidly in finding potential solid-state hydrogen storage materials.[43−48] Previous research indicated that decomposition temperatures of as-milled MgH2 and as-milled Na3AlH6 were reduced to approximately 55 °C after the reaction of MgH2 and Na3AlH6.[49]

By the combination of the two hydrides, the enthalpy reaction can be improved, but it still cannot meet the practical application of hydrogen storage as a suitable requirement. Thus, a catalyst is used to enhance the sorption properties of the destabilized MgH2–Na3AlH6 system. Our previous study demonstrated that the catalyst based on the metal fluorides had significantly improved hydrogen storage properties of the MgH2–Na3AlH6 system.[50] The TiF3-doped MgH2–Na3AlH6 sample began releasing hydrogen at 140 °C, which is 30 °C lower than the undoped MgH2–Na3AlH6. The reaction mechanism analysis indicated that the formation of NaF, AlF3, and Al3Ti plays a dominant role by serving as an active mechanism for nucleation and growth of dehydrogenated products.

Motivated by our previous research, another catalyst from a different metal group, namely, iron chloride (FeCl3), was introduced to study its effect on the MgH2–Na3AlH6 composite. To the best of our knowledge, no report has been claimed on the application of FeCl3 as the catalyst for the hydrogen storage properties of the MgH2–Na3AlH6 system to date. The effect of FeCl3 on the Li–N–H system reported by Zhang et al.[51] demonstrated that the dehydrogenation peak and termination temperature of the doped 1 mol % FeCl3 sample had been reduced, and the apparent activation energy was reduced by approximately 14.93 kJ/mol. Hence, it is interesting for this research to explore the effect of FeCl3 on the hydrogen storage properties of the MgH2–Na3AlH6 composite and gain an understanding of the nature and catalytic mechanism of the catalyst in the system. Hydrogenation properties and thermal properties were studied by pressure composition temperature (PCT) and differential scanning calorimetry (DSC), respectively. Meanwhile, the surface morphology of the sample was determined by scanning electron microscopy (SEM), and the structural characterization of the samples was determined by X-ray diffraction (XRD).

Results and Discussion

Characterization of Na3AlH6

The XRD characterization of the NaH–NaAlH4 (2:1) composite after milling for 20 h is displayed in Figure . The XRD pattern indicates that only Na3AlH6 peaks were present, whereas the peaks of NaH and NaAlH4 were absent, indicating a complete transformation, which is represented by eq

Figure 1

XRD profile of the NaH–NaAlH4 (2:1) composite milled for 20 h.

The metastable β-Na3AlH6 peaks were also detected after the process of ball milling. It is expected that the polymorphic transformation of Na3AlH6 and β-Na3AlH6 has partially occurred as reported in previous work.[52]

Dehydrogenation Temperature

Figure illustrates the TPD performance of the as-milled MgH2, as-milled Na3AlH6, MgH2–Na3AlH6 composite, and MgH2–Na3AlH6–10 wt % FeCl3. As displayed in Figure , the as-milled MgH2 and Na3AlH6 exhibit the same decomposition process, which is only one dehydrogenation step. The dehydrogenation process of each one of the samples starts at around 350 and 230 °C. By comparing the decomposition properties, the MgH2–Na3AlH6 composite with and without a catalyst has three dehydrogenation steps. These properties could have corresponded to the decomposition of Na3AlH6 and MgH2 during the heating process. For the composite without a catalyst, the first dehydrogenation process had started at 170 °C and released 1.0 wt % hydrogen after heating at 220 °C. Next, the dehydrogenation process in the second stage occurred within 270–350 °C. Then, the third dehydrogenation process with a total hydrogen release of 5.9 wt % occurred at 375 °C. Meanwhile, the onset temperature for the composite with FeCl3 is 140 °C, which results in the reduction of the decomposition temperature compared to the pristine MgH2–Na3AlH6 composite. With further heating, the dehydrogenation process for the second stage takes place at 270–350 °C, and the third stage occurs at 360–450 °C.

Figure 2

TPD profile of the as-milled MgH2, as-milled Na3AlH6, MgH2–Na3AlH6, and MgH2–Na3AlH6–10 wt % FeCl3 composites.

Sorption Kinetics Properties

The rehydrogenation of the dehydrogenated samples of MgH2–Na3AlH6 with a 10 wt % FeCl3 catalyst was conducted for the reversibility property. Figure displays the rehydrogenation kinetics profile of the studied materials at 33 atm hydrogen pressure and an operating temperature of 320 °C. For a duration of 60 min, approximately 4.2 wt % hydrogen was absorbed by the catalyzed composite, whereas it was approximately 3.1 wt % for the pristine composite under a similar test condition. These results indicate that the absorption properties and the rehydrogenation process of the MgH2–Na3AlH6 system were enhanced by the addition of FeCl3.

Figure 3

Rehydrogenation kinetics under a constant temperature of the composites at 320 °C and under 33 atm.

The catalytic effect of FeCl3 on the dehydrogenation properties of materials was explored at 1 atm in 60 min and an operating temperature of 320 °C, as illustrated in Figure . For comparison, the undoped composite was characterized under the same condition. It can be observed that in 60 min, the catalyzed composite released approximately 1.7 wt % hydrogen at 320 °C. Conversely, the composite without the catalyst released 0.85 wt % hydrogen. These results indicate that the addition of FeCl3 contributed to improving the desorption kinetics. The comparison of the hydrogen storage properties of 4MgH2–Na3AlH6 doped with different catalysts is shown in Table . Clearly, the onset dehydrogenation temperature of the FeCl3-doped MgH2–Na3AlH4 sample is lower than that of the SrTiO3-doped MgH2–Na3AlH4 sample. For the rehydrogenation kinetic performance, the FeCl3-doped MgH2–Na3AlH4 sample is better than the TiF3- and SrTiO3-doped MgH2–Na3AlH4 samples. In addition, compared with other Mg–Al–H systems, such as Mg(AlH4)2 that was synthesized by high-energy ball milling of Mg(AlH4)2(Et2O) in a specially designed jar,[53] the MgH2–Na3AlH4 system showed a slightly higher onset decomposition temperature. According to Pang et al.,[53] the as-synthesized Mg(AlH4)2 nanorods start to decompose at about 130 °C and 9.0 wt % hydrogen capacity was released within a two-step reaction.

Figure 4

Dehydrogenation kinetics under a constant temperature of the materials at 320 °C and 1 atm hydrogen pressure.

Table 1

Comparison of the Hydrogen Storage Properties of 4MgH2–Na3AlH6 Doped with Different Catalysts

system	dehydrogenation temperature (°C)	rehydrogenation time (min)	hydrogen absorb (wt %)
4MgH2–Na3AlH6 + TiF3[50]	140.0	10.0	3.3
4MgH2–Na3AlH6 + SrTiO3[54]	145.0	10.0	3.7
4MgH2–Na3AlH6 + FeCl3 (this work)	140.0	10.0	4.0

Kinetic models can be used to further analyze the behavior of the sorption kinetics of the composite. In this study, the absorption and desorption behavior of the composites have been calculated using two kinetics models: (i) contracting volume and (ii) Johnson–Mehl–Avrami (JMA). The models are considered because the experimental data can be fitted to the models, and they are relatively accurate, as mentioned by Pang and Li.[55] The experimental data and the kinetics equation can be used to deduce the rate-limiting step of the kinetics process. The best linear plot from the models represents the rate-limiting step of the sorption behavior.

The calculation of the kinetic models based on equations in Table S1 (Supporting Information) is performed for the sorption kinetics operated at 320 °C and is illustrated in Figure . The calculations for both cases were done for a hydrogen capacity range of 0–80%.[56] The result indicates that the rate-limiting step of the absorption and desorption of the doped composite at 320 °C is the diffusion of 3D growth that is regulated by reducing the interface velocity.

Figure 5

Calculation result of different kinetics equations of the MgH2–Na3AlH6–10 wt % FeCl3 composite based on Table for (a) absorption at 320 °C and (b) desorption at 320 °C.

Thermal Properties

DSC curves of the doped and undoped MgH2–Na3AlH6 composites at 20 °C/min (heating rate) are plotted in Figure . Two endothermic peaks of the undoped composite were observed at temperatures of 250 °C (first peak) and 390 °C (second peak). These endothermic peaks were attributed to the decomposition process of Na3AlH6[57] and MgH2,[25] respectively. For the doped composite, two decomposition peaks were observed in which the peaks were decomposed at a lower temperature as compared to that for the undoped composite. The first and second peaks were decomposed at 240 and 365 °C, respectively. There is a reduction of around 10 and 25 °C in the decomposition temperature compared to that of the undoped composite. These findings are compatible with the TPD outcomes described in Figure but at a higher temperature. The disparity in the measurement condition between the two methods might be the reason for this phenomenon, as discussed in previous studies.[58,59]

Figure 6

DSC profiles at 20 °C/min for the MgH2–Na3AlH6 and MgH2–Na3AlH6–10 wt % FeCl3 samples.

Apparent Activation Energy

To determine the effect of the introduction of FeCl3 on the kinetic characteristic of the MgH2–Na3AlH6 composite system, the apparent activation energy for hydrogen release from the MgH2–Na3AlH6–10 wt % FeCl3 sample was investigated. The apparent activation energy of the undoped composite was also measured for comparison. A Kissinger plot was developed based on the Kissinger equation[60] presented as follows to determine the value of activation energywhere β is the heating rate, TP is the temperature of the peak in the DSC curve, R is given as the gas constant, and A is the linear constant. Meanwhile, in a graph of ln[β/TP2] against 1000/TP, the apparent activation energy, EA, can be calculated from the slope. Figure a,b indicates the DSC curves for the undoped and doped samples at different heating rates.

Figure 7

DSC curves of (a) the MgH2–Na3AlH6 composite and (b) the MgH2–Na3AlH6–10 wt % FeCl3 composite at different heating ramps (15, 20, 25, and 30 °C/min).

The apparent activation energy for the decomposition of MgH2 (second stage) of the undoped composite is 140 kJ/mol, based on the Kissinger analysis illustrated in Figure . By contrast, the apparent activation energy for the decomposition of MgH2 (second stage) of the doped composite was calculated to be 128 kJ/mol. The value for the reduction is approximately 12 kJ/mol. These values indicate that the FeCl3 additive played a crucial role in decreasing the activation energy of the 4MgH2–Na3AlH6 composite.

Figure 8

Kissinger analysis of (a) the MgH2–Na3AlH6 and (b) MgH2–Na3AlH6–10 wt % FeCl3 composites.

Scanning Electron Microscopy Analysis

Figure illustrates the morphologies of the doped and undoped MgH2–Na3AlH6 composites with FeCl3. Morphologies of pure MgH2, milled MgH2, milled Na3AlH6, and pure FeCl3 have been added for comparison purposes. The pure particle image of MgH2 reveals an angular thin shape that is larger than 50 μm (Figure a). After milling for 1 h, particle sizes were reduced and less homogeneous for MgH2, as illustrated in Figure b. Figure c presents the SEM image of milled Na3AlH6 in which the particle is deposited in a coral-like shape. Meanwhile, the SEM image of the as-received FeCl3 is presented in Figure d. The particles’ sizes are larger than 1 μm without any further purification. Additionally, the MgH2–Na3AlH6 composite shows a decrease in particle sizes, as illustrated in Figure e. Following the addition of FeCl3 to the composite MgH2–Na3AlH6 (Figure f), the particle sizes were decreased relative to the undoped composite. Smaller particle sizes can improve the absorption performance of the hydrogen because they increase the total particle reaction surface area and minimize the hydrogen diffusion length.[61]

Figure 9

Surface morphology of pure MgH2 (a), milled MgH2 (b), milled Na3AlH6 (c), pure FeCl3 (d), MgH2–Na3AlH6 composite (e), and MgH2–Na3AlH6–10 wt % FeCl3 composite (f).

Reaction Mechanism Analysis

The XRD analysis results at various stages of dehydrogenation for the undoped MgH2–Na3AlH6 composite are illustrated in Figure . After a 60 min process of ball milling, peaks of MgH2, Na3AlH6, and metastable β-Na3AlH6 appeared (Figure a). New hydride phases in the form of perovskite, NaMgH3 and MgH2, that did not react were observed at the first stage of the 230 °C dehydrogenation process (Figure b). This phenomenon occurred because the Na3AlH6 phase disappeared after the process of heating. Moreover, there were a few Al peaks that were clearly observed after the desorption process at 230 °C. The formation of NaMgH3 and Al peaks was due to the decomposition of Na3AlH6 that had reacted with MgH2, as displayed in eq

Figure 10

XRD profiles of the MgH2–Na3AlH6 sample (a) after 1 h of milling and after desorption at (b) 230 °C, (c) 375 °C, and (d) 450 °C.

In Figure c, when the MgH2–Na3AlH6 composite was heated to 375 °C, the NaH phase was detected. However, the intermediate peaks of Mg17Al12 and Mg dominated the XRD phase. Furthermore, phases of MgH2 and NaMgH3 disappeared. The observation of the diffraction peaks indicated that the desorption of hydrogen at 375 °C corresponded to MgH2 whose decomposition and reaction with Al are presented in eqs and 5, respectively. Meanwhile, the decomposition of NaMgH3 is represented by eq .

When the process temperature was increased up to 450 °C, as displayed in Figure d, the peak of Na was detected. It was believed that the NaH phase had fully dehydrogenated at this stage, as illustrated in eq

To clarify the impact of FeCl3 on the destabilized MgH2–Na3AlH6 system, the XRD measurements were also conducted at various dehydrogenation stages, as illustrated in Figure . The peaks of MgH2, Na3AlH6, metastable β-Na3AlH6, and FeCl3 were detected after 60 min of ball milling, as illustrated in Figure a. When the FeCl3-doped MgH2–Na3AlH6 sample was heated at 220 °C, new phases, Fe and MgCl2, were detected from the XRD pattern (Figure b). Additionally, MgH2, NaMgH3, and Al species also appeared, which are liable for improving the desorption temperature of the MgH2–Na3AlH6 system. Additional phases were detected after the heating process at 350 and 420 °C and are depicted in Figure c,d, respectively. These results indicate that new species of MgCl2 and Fe that formed and acted as the active species were due to the reaction between FeCl3 and Mg components, as illustrated in eq

Figure 11

XRD profiles of the MgH2–Na3AlH6–10 wt % FeCl3 sample (a) after 1 h of milling and after desorption at (b) 220 °C, (c) 350 °C, and (d) 420 °C.

The XRD analysis was run for 4MgH2–Na3AlH6 and the destabilized MgH2–Na3AlH6–10 wt % FeCl3 system to examine the reaction mechanism after the absorption process. These measurements were carried out under 33 atm H2 pressure at 320 °C, as depicted in Figure . Figure a indicates that NaMgH3, Al3Mg2, MgH2, Al, and MgO phases can be identified in the MgH2–Na3AlH6 sample. After addition of FeCl3 (Figure b), Fe and MgCl2 were detected in the doped composite. The diffraction peaks were previously observed in the destabilized MgH2–Na3AlH6–10 wt % FeCl3 system after dehydrogenation. The peaks of Mg17Al12 and Mg disappeared after the absorption process for undoped and doped samples. These results indicated full conversion of MgH2, as illustrated in eq

Figure 12

XRD profiles of the (a) MgH2–Na3AlH6 and (b) the MgH2–Na3AlH6–10 wt % FeCl3 composites after absorption at 320 °C.

The formation of Fe and MgCl2 species during the desorption process with the addition of FeCl3 could play a significant role in enhancing the hydrogen sorption performances of the MgH2–Na3AlH6 composite. Fe is well known for being a promising catalyst for MgH2.[62−64] The in situ formed Fe may interact with molecules of hydrogen and cause hydrogen molecules to dissociate, subsequently boosting the re/dehydrogenation kinetics. Meanwhile, the catalytic effect of Cl-containing species (MgCl2) may also have an impact on the sorption kinetics. MgCl2 plays a crucial part in ameliorating the rehydrogenation kinetics of MgH2 doped with NiCl2 and CoCl2, as reported by Mao et al.[65] Additionally, the desorption and absorption kinetics can be enhanced by shortening the diffusion distance of reaction ions from MgCl2 and like an active site for the products of the nucleation and desorption. Thus, the newly developed products, MgCl2 and Fe, have a promising catalytic impact on enhancing the hydrogen sorption performances of the destabilized MgH2–Na3AlH6 system. Therefore, these new species also served as active sites for the dehydrogenated products in nucleation and growth, thus enhancing the hydrogen sorption properties of the MgH2–Na3AlH6 system.

Conclusions

In summary, doping with the FeCl3 catalyst increased the efficiency of the MgH2–Na3AlH6 composite in hydrogen storage. The FeCl3-doped MgH2–Na3AlH6 composite starts to release hydrogen at approximately 140 °C, which is approximately 30 °C lower than the onset dehydrogenation temperature of the undoped 4MgH2–Na3AlH6 composite. Additionally, the absorption and desorption kinetics of the MgH2–Na3AlH6 composite were reinforced by the addition of FeCl3. The apparent activation energy for MgH2-relevant decomposition in the MgH2–Na3AlH6 composite was decreased from 140 to 128 kJ/mol utilizing FeCl3 from the Kissinger plot. For the SEM images, the doped composite displayed smaller sizes of particles compared to those of the undoped composite. These improvements were made possible by the formation of catalytic species, Fe and MgCl2, during the heating processes. It is rational to assume that the development of such active species increased the interaction between MgH2 and Na3AlH6, thus further enhancing the efficiency of the MgH2–Na3AlH6 system in hydrogen storage.

Experimental Details

Starting materials, magnesium hydride (MgH2), sodium hydride (NaH), sodium aluminum hydride (NaAlH4), and iron(III) chloride (FeCl3) were purchased from Sigma-Aldrich with nearly 100% purity. Na3AlH6 was synthesized through the mechanochemical reaction by mixing NaH and NaAlH4 at a molar ratio of 2:1.[66] The composite made of MgH2 and Na3AlH6 with a mole ratio of 4:1 (denoted as MgH2–Na3AlH6) and the FeCl3 catalyst were milled in a planetary ball mill (NQM-04) for 1 h at a rotation speed of 400 rpm. The sample was placed into a sealed stainless steel vial together with hardened stainless steel balls. The ball-to-powder ratio in terms of weight was 40:1. To avoid the exposure of samples to moisture, all sample preparations were conducted in a glovebox (MBraun Unilab) under an inert gas atmosphere (argon).

For the temperature-programed desorption (TPD) measurement, a sample of approximately 60 mg was heated from room temperature to 450 °C at a 5 °C/min heating rate in a Sieverts-type PCT apparatus. Moreover, the absorption and desorption kinetics were investigated at a constant temperature of 320 °C under 33 and 1 atm hydrogen pressure, respectively. For the evaluation of the thermal properties of samples, the DSC measurement was conducted using a Mettler Toledo TGA/DCS 1. Around 5 mg of samples were heated from room temperature to 450 °C under an argon flow and different heating rates (15, 20, 25, and 30 °C/min) were used.

A scanning electron microscope (JEOLJSM-6360LA) was used to determine the morphology of the as-received MgH2, as-milled MgH2, synthesized Na3AlH6, and as-milled MgH2–Na3AlH6 composite with and without FeCl3. Meanwhile, reaction mechanisms of the FeCl3-doped MgH2–Na3AlH6 composite after the milling and decomposition and after the absorption processes were evaluated by an X-ray diffractometer (Rigaku MiniFlex). The scans were carried out over diffraction angles from 20 to 80° at a speed of 2.00°/min.