A study on the effects of K₂ZrF₆ as an additive on the microstructure and hydrogen storage properties of MgH₂

Abstract

In this work, the hydrogenation properties of MgH₂-doped K₂ZrF₆ with X wt% (X = 5, 10, 15 and 20) have been investigated for the first time. Analysis indicated that MgH₂ doped with 10 wt% K₂ZrF₆ is the best composite for the improvement of the hydrogen storage properties of MgH₂. The results showed that the onset desorption temperature after the addition of 10 wt% K₂ZrF₆ was 250 °C, which experienced the reduction of 100 °C and 200 °C from 350 °C and 450 °C for as-milled and as-received MgH₂, respectively. The re/dehydrogenation kinetics also significantly improved compared to the un-doped MgH₂. The results of the Arrhenius plot exhibited that the activation energy for the hydrogen desorption of MgH₂ was reduced from 164 kJ mol⁻¹ to 80 kJ mol⁻¹ after the addition of 10 wt% K₂ZrF₆. Moreover, the X-ray diffraction spectra displayed the formation of new phases of KH and ZrH₂ by the doping of K₂ZrF₆ with MgH₂ after the dehydrogenation and rehydrogenation processes. These two compounds are believed to act as the active species and play a catalytic role in improving the hydrogen storage properties. It is, therefore, concluded that this newly developed catalytic doping system works well in improving the hydrogen sorption properties of MgH₂.

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1. Introduction

Even though hydrogen is an ideal energy carrier, there are still extensive technological challenges that limit its uses as a fuel. Hydrogen can be used as a fuel in PEMFC (proton exchange membrane fuel cell), which is a very promising power source for electric vehicles (EV).¹ According to the U.S. DOE's 2010 target, large volumetric (>62 kg m⁻³) and gravimetric (>6.5 wt%) densities are strictly needed for on-board hydrogen storage in fuel cell vehicles. To accomplish the DOE's targets, a high-performance hydrogen storage system is desired. High-pressure and cryogenic hydrogen storage systems are impractical for vehicular applications due to safety concerns and volumetric constraints. In the recent years, attention has been focused on solid hydrogen storage materials due to their significant advantages related to safety, energy efficiency and cost effectiveness.² To date, many types of solid-state hydrogen storage materials, such as metal hydrides,^3–6 complex hydrides,^7–13 carbon materials,^14,15 and metal–organics frameworks (MOFs),^16,17 have been explored. Among all of these materials, metal hydrides have attracted intensive research interest because of their high hydrogen storage capacities and moderate working temperatures. Among the various metal hydrides, MgH₂ is highly recommended due to its high hydrogen capacity with a theoretical value of 7.6 wt%, high reversibility, low cost^18–20 and reserves in the earth's crust.²¹ Therefore, MgH₂ is viewed as one of the most promising hydrogen storage materials. Unfortunately, the practical application of MgH₂ is greatly limited by its slow kinetics and high operation temperature.²² Its working temperature is above 300 °C, and the de/re-hydrogenation kinetics are slow; these issues limit its practical application for on-board hydrogen storage. To overcome these problems, considerable effort has been made in the past decade to improve its hydrogen storage properties, such as combining with other materials (destabilization of the system),^23–30 improving the surface and kinetics by ball milling,³¹ and doping with catalysts.^32–36 Among them, the introduction of catalysts or additives into MgH₂ by ball milling method has shown a significant effect on the hydrogen storage properties of MgH₂.

Another promising additive for MgH₂ is K₂ZrF₆. It is believed that upon heating, K₂ZrF₆ and MgH₂ will react by an in situ reaction to form active species that enhance the hydrogen sorption properties of MgH₂. Xiao et al.³⁷ demonstrated the superior effects of KH for improving the dehydrogenation properties of NaAlH₄. It has also been confirmed that ZrH₂ can provide favourable effects for enhancing the dehydrogenation properties of MgH₂.³⁸ Therefore, it is reasonable to believe that K₂ZrF₆ shows great potential as an additive to advance the hydrogen storage properties of MgH₂ by combining together these in situ metal hydride positive factors.

In this work, K₂ZrF₆ was introduced to examine its improvement of the hydrogen storage performance of MgH₂ obtained by ball milling. To the best of the our knowledge, no studies have reported the effects of K₂ZrF₆ on the microstructure and hydrogen storage properties of MgH₂. The hydrogen storage properties and reaction mechanisms of MgH₂ and K₂ZrF₆ were investigated in experiments using a Sievert-type pressure-composition-temperature (PCT) apparatus, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The possible catalysis mechanism supported by the results of the experiment was then discussed.

2. Experimental details

Pure MgH₂ (hydrogen storage grade with 98% purity) and K₂ZrF₆ (97% purity) were purchased from Sigma Aldrich and were used as received without further purification. Approximately 200 mg of MgH₂ doped with 20 mg of K₂ZrF₆ was loaded into a sealed stainless steel vial together with four hardened stainless steel balls in a MBraun Unilab glove box in an argon atmosphere. For simplicity, the mixture of MgH₂ with 10 wt% of K₂ZrF₆ will be referred to as MgH₂-10 wt% K₂ZrF₆ sample. The sample was then milled in a planetary ball mill (NQM-0.4) for 1 h, first by milling for 0.25 h, resting for 2 min, and then repeating the milling for another 2 cycles in a different direction at the rate of 400 rpm. The as-received MgH₂ was also prepared under the same conditions for comparison purposes. The de/rehydrogenation experiments were performed in a Sievert-type pressure-composition-temperature (PCT) apparatus (Advanced Materials Corporation). The composite was loaded into a sample vessel in the glove box. For the temperature-programmed-desorption (TPD) experiment, all the composites 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⁻¹, and the samples were heated from room temperature to 450 °C. The re/dehydrogenation kinetics measurements were performed at the desired temperatures with initial hydrogen pressures of 33.00 atm and 1.00 atm, respectively.

XRD analysis was performed using a Rigaku MiniFlex X-ray diffractometer with Cu Kα radiation scans carried out over diffraction angles from 20° to 80° at a speed of 2° min⁻¹. Prior to the measurement, a small amount of sample was uniformly placed on the sample holder, which was wrapped with plastic wrap to avoid oxidation. Moreover, the surface morphology of the composite was observed using a scanning electron microscope (SEM; JEOL JSM-6360LA) by setting the samples on carbon tape and then coating the samples with gold spray under vacuum.

3. Results and discussion

3.1 Dehydrogenation temperature

Fig. 1 shows the TPD performances for the dehydrogenation of as-received MgH₂, as-milled MgH₂ and MgH₂ doped with 5 wt%, 10 wt%, 15 wt% and 20 wt% K₂ZrF₆. From the TPD patterns, it is evident that doping with a 5 wt% to 20 wt% of K₂ZrF₆ to MgH₂ resulted in a drop of the onset desorption temperature compared to un-doped MgH₂. The as-received MgH₂ started to dehydrogenate H₂ at a temperature of 450 °C, and the total dehydrogenation capacity was 7.5 wt% H₂. The onset desorption temperature of MgH₂ after milling slightly dropped to about 350 °C, showing that the process of milling also affected the onset desorption temperature.³¹ After milling, the curve shows that there was no reduction in the hydrogen desorption capacity of MgH₂. The addition of K₂ZrF₆ greatly improved the onset desorption temperature of MgH₂. MgH₂ doped with 5 wt% K₂ZrF₆ started to decompose at about 260 °C with a total dehydrogenation capacity of 6.6 wt% H₂. For the 10 wt% doped sample, the dehydrogenation occurred at 250 °C, which represents a decrease in the onset desorption temperature of about 100 °C from 350 °C for as-milled MgH₂ with a total dehydrogenation capacity of 6.6 wt% H₂, which is the same as the hydrogen desorption capacity of the 5 wt% doped sample. Further increasing the doping amounts to 15 wt% and 20 wt% reduced the desorption onset temperatures to about 280 °C and 290 °C, but the amount of hydrogen released slightly dropped to about 5.6 wt% and 5.3 wt% H₂, respectively. This phenomena may be due to the excessive catalytic effects brought about by the relatively high levels of added K₂ZrF₆, and this result is almost the same as that reported in our previous paper.¹⁹

Fig. 1 TPD patterns for the dehydrogenation of as-received MgH₂, as-milled MgH₂ and MgH₂ doped with 5 wt%, 10 wt%, 15 wt% and 20 wt% K₂ZrF₆.

3.2 Re/dehydrogenation kinetics

Fig. 2 depicts the hydrogen absorption rate of MgH₂ after ball milling for an hour, and samples of MgH₂ doped with K₂ZrF₆, which were also milled for an hour at 300 °C under 33.0 atm. The results were compared, and it was found that MgH₂ that has been ball milled for an hour can absorb about 3.3 wt% of H₂ in 5 min, and approximately 4.0 wt% of H₂ in 60 min. On the other hand, after adding 10 wt% K₂ZrF₆ to MgH₂, it was found that a slight improvement was attained. About 3.5 wt% of H₂ was absorbed in 5 min and approximately 4.1 wt% of H₂ can be absorbed in 60 min. Moreover, for the 5 wt%, 15 wt% and 20 wt% K₂ZrF₆-doped MgH₂, the hydrogen absorbed as much as 3.2 wt%, 3.3 wt% and 3.0 wt% H₂ within 5 min. Fig. 3 displays that the MgH₂ samples doped with 5 wt%, 10 wt%, 15 wt% and 20 wt% K₂ZrF₆ released about 3.7 wt%, 4.1 wt%, 4.0 wt% and 3.6 wt% hydrogen at 300 °C in 60 min, respectively, whereas the un-doped MgH₂ sample desorbed only 0.3 wt% hydrogen in the same period of time. The doped composites displayed an enhancement in terms of desorption kinetics. The results of the desorption onset temperature and isothermal re/dehydrogenation kinetics indicated that the sample of 10 wt% K₂ZrF₆ shows the best performance in improving the hydrogen storage properties of MgH₂, and thus can be considered to be the best sample of MgH₂ doped K₂ZrF₆. Therefore, the MgH₂ doped with 10 wt% K₂ZrF₆ was chosen for further analysis.

Fig. 2 Isothermal absorption kinetics measurements of as-milled MgH₂ and MgH₂ doped with 5 wt%, 10 wt%, 15 wt% and 20 wt% K₂ZrF₆ at 300 °C.

Fig. 3 Isothermal desorption kinetics curves for as-milled MgH₂ and MgH₂ doped with 5 wt%, 10 wt%, 15 wt% and 20 wt% K₂ZrF₆ at 300 °C.

To further analyse the hydrogen desorption kinetics of the MgH₂-10 wt% K₂ZrF₆ sample, isothermal dehydrogenation measurements were performed at different temperatures. Fig. 4 compares the isothermal dehydrogenation kinetics between the as-milled MgH₂ and MgH₂-10 wt% K₂ZrF₆ sample at 300 °C, 320 °C and 350 °C. The dehydrogenation kinetics test was carried out after the rehydrogenation kinetics process under 33.0 atm of H₂ at 300 °C, 320 °C and 350 °C. The result was as expected, as the dehydrogenation rate for the as-milled MgH₂ was lower than that for the doped MgH₂ at 350 °C (Fig. 4c and f). After the addition of K₂ZrF₆, the desorption kinetics increased significantly, as the 10 wt% K₂ZrF₆-doped MgH₂ released 4.5 wt% of H₂ within 10 min at 350 °C. Under the same conditions, it was shown that the amount of hydrogen released from the as-milled MgH₂ was less than 0.5 wt%, and only 2 wt% even after 20 min. Further investigation of the 10 wt% K₂ZrF₆-doped MgH₂ at 300 °C and 320 °C showed similar results (Fig. 4((a, d) and (b, e))), where doped MgH₂ revealed better desorption kinetics than the un-doped MgH₂. This improvement is due to the energy barrier, where the activation energy for the release of hydrogen from MgH₂ is reduced by the addition of K₂ZrF₆. The activation energy can be calculated using the Arrhenius equation as follows:

k = k₀ exp(−E_A/RT) (1)

where k is the rate of hydrogenation, k₀ is a temperature independent coefficient, E_A is the apparent activation energy for hydride decomposition, R is the gas constant and T is the absolute temperature. By plotting the graph of ln(k) vs. 1/T, as indicated in Fig. 5, the apparent activation energy, E_A, for H₂ to be released from the as-milled and K₂ZrF₆-doped MgH₂ was revealed. From these calculations, the activation energies for the as-milled MgH₂ and K₂ZrF₆-doped MgH₂ were 80 kJ mol⁻¹ and 164 kJ mol⁻¹, respectively. There was an improvement of 84 kJ mol⁻¹, according to the result. Based on the lowering of the activation energy, the decomposition of MgH₂ was substantially improved.

Fig. 4 Isothermal dehydrogenation curves of as-milled MgH₂ and 10 wt% K₂ZrF₆-doped MgH₂ at 300 °C (a, d), 320 °C (b, e) and 350 °C (c, f).

Fig. 5 Arrhenius plots of ln(k) vs. 1/T for as-milled MgH₂ and MgH₂ + 10 wt% K₂ZrF₆.

The cycling performance of MgH₂ doped with K₂ZrF₆ composite was further investigated. As shown in Fig. 6, a temperature of 300 °C was applied in life cycle studies of the rehydrogenation kinetics of the MgH₂ + 10 wt% K₂ZrF₆ composite. The absorption kinetics continued to be good, even after the 10th cycle, with a hydrogen capacity of about 3.9 wt%. As shown in Fig. 7, the life cycle desorption kinetics of MgH₂ + 10 wt% K₂ZrF₆ at 300 °C showed almost no decrease in hydrogen storage capacity within 60 min even after 10 cycles, maintaining a capacity of about 4.1 wt%. The result shows that K₂ZrF₆ is a good additive for the cycle life of MgH₂.

Fig. 6 Isothermal rehydrogenation kinetics of the MgH₂ + 10 wt% K₂ZrF₆ composite in the 1st, 5th and 10th cycles.

Fig. 7 Isothermal dehydrogenation kinetics of the MgH₂ + 10 wt% K₂ZrF₆ composite in the 1st, 5th and 10th cycles.

3.3 Scanning electron microscopy

Fig. 8 illustrates the grain size of the as-received MgH₂, as-milled MgH₂, and K₂ZrF₆-doped MgH₂. The SEM examination revealed that the grain size before ball milling (Fig. 8(a)) was the largest compared to the as-milled MgH₂ (Fig. 8(b)). Moreover, as can be seen from Fig. 8(c), the grain size of K₂ZrF₆-doped MgH₂ was smaller than the as-milled MgH₂. The significant decrease in the grain size of doped MgH₂ increased the surface area of contact of MgH₂. As the surface area of contact increases, the rate of reaction at which MgH₂ releases and absorbs H₂ will increase. Mustafa and Ismail³⁹ reported that a smaller particle size will improve the hydrogen re/desorption, because it reduces the diffusion length of hydrogen and makes the particles' reactive surfaces larger. Hence, the activation energy and re/desorption will be improved.

Fig. 8 SEM images for as-received MgH₂ (a), as-milled MgH₂ (b) and MgH₂ doped with 10 wt% K₂ZrF₆ (c).

3.4 X-ray diffraction

To determine the chemical composition of K₂ZrF₆-doped MgH₂ after milling, XRD scans were obtained after the dehydrogenation and re-hydrogenation processes. Fig. 9 depicts the XRD patterns of MgH₂-10 wt% K₂ZrF₆ after 1 h ball milling, after dehydrogenation at 450 °C and after rehydrogenation at 300 °C. From the pattern, after 1 h ball milling (Fig. 9(a)), only MgH₂ peaks were detected and no K₂ZrF₆-containing phases existed. The absence of the peaks of K₂ZrF₆ might be due to its transformation into an amorphous state during the milling, or the amount of K₂ZrF₆ might be too little to be detected by the matrix of XRD. After dehydrogenation at 450 °C (Fig. 9(b)), distinct peaks of Mg were present, which indicates that the dehydrogenation process was complete. Minor peaks of KH and ZrH₂ were present after the dehydrogenation process, as they are the active species in the combination of MgH₂ and K₂ZrF₆ after ball milling or heating processes. The spectra of re-hydrogenated K₂ZrF₆-doped MgH₂ (Fig. 9(c)) represent major peaks of Mg, where peaks of MgH₂ were less and the active species of KH and ZrH₂ were still present. From these results, it can be determined that MgH₂ is not fully reversible because the peaks of Mg are still present in the rehydrogenation phase.

Fig. 9 X-ray diffraction patterns of MgH₂-10 wt% K₂ZrF₆ (a) after milling, (b) after dehydrogenation, and (c) after rehydrogenation.

In order to verify the phase structure of the doped samples in more detail, XRD measurements were obtained using the 20 wt% K₂ZrF₆ added MgH₂ sample. After increasing the amount of catalyst to 20 wt%, the peaks for the KH and ZrH₂ species increased, as shown in Fig. 10. Due to its amorphous state, the peaks for the MgF₂ species could not be observed in the XRD pattern. This finding corroborates with the study by Sun et al.⁴⁰ that Ag–MgF₂ cermet films consist of a mainly amorphous MgF₂ matrix with embedded fcc-Ag nanocrystalline grains. Moreover, the absence of the Mg peak indicated that by increasing the amount of catalyst to 20 wt%, MgH₂ can be entirely reversible, as can be seen in Fig. 9. The reaction between MgH₂ and K₂ZrF₆ in the dehydrogenation stage can be represented by the following equation:

6MgH₂ + 2K₂ZrF₆ → 6MgF₂ + 4KH + 2ZrH₂ + 2H₂ (2)

Fig. 10 X-ray diffraction patterns of MgH₂-20 wt% K₂ZrF₆ (a) after milling, (b) after dehydrogenation and (c) after rehydrogenation.

The values for the standard Gibbs free energy, ΔG^°_f, of MgH₂, MgF₂, KH and ZrH₂ are −35.98, −1071.10, −34.06, and −128.87 kJ mol⁻¹,⁴¹ respectively. In addition, the standard Gibbs free energy, ΔG^°_f of K₂ZrF₆ is −2456 kJ mol⁻¹.⁴² Therefore, the total change in ΔG° will be −1691.26 kJ mol⁻¹ for MgH₂. This result confirms the possibility of reaction (2) from the thermodynamic potentials. The standard-state free energy of reaction, ΔG°, can be calculated as follows:

ΔG° = ∑ΔG^°_{f products} − ∑ΔG^°_{f reactants} (3)

The results of this study prove that the formation of active species of KH and ZrH₂ during the de/rehydrogenation process from the reaction of MgH₂ and K₂ZrF₆ may play an important role to enhance the sorption properties of MgH₂. This discovery verifies the work by Xiao et al.³⁷ that the addition of KH can effectively improve the dehydrogenation properties of the second step reaction of a NaAlH₄ system. In addition to that, Luo et al.⁴³ verified that the addition of a catalytic amount (<4 mol%) of KH significantly increased the hydrogen absorption rate of the desorbed material, and had a less significant effect on the kinetics of hydrogen desorption, as also shown by Wang et al.⁴⁴ On the other hand, many findings have exposed that mixing MgH₂ with ZrH₂ improves hydrogenation behaviour.^45,46 For example, Kyoi et al.³⁸ claimed that mixing MgH₂ with ZrH₂ to form face centred cubic (FCC) hydrides reduced hydrogen-desorption temperatures by 170 K compared to MgH₂. In addition, among the unstable reaction products, ZrH₂ has a very stable phase as reported by Jat et al.,⁴⁵ as well as excellent stability under irradiation conditions.⁴⁶ Therefore, it is sensible to assume that this newly developed product of KH and ZrH₂ acts as a real catalyst to enhance the hydrogen sorption properties of MgH₂ by serving as an active site for the nucleation and growth of dehydrogenation products.

4. Conclusion

In summary, the improvement of the hydrogen desorption properties of MgH₂ have been verified after adding the catalyst K₂ZrF₆. Among the different amounts of K₂ZrF₆ added, 10 wt% K₂ZrF₆ shows the best performance in improving the hydrogenation properties of MgH₂. The TPD result exhibited that the MgH₂-10 wt% K₂ZrF₆ sample started to release H₂ at about 250 °C, where the onset desorption temperature was reduced by approximately 100 °C and 200 °C compared to the as-milled and as-received MgH₂, respectively. The dehydrogenation and rehydrogenation kinetics of the doped MgH₂ also improved significantly compared to the undoped MgH₂. From the Arrhenius plot, the results showed that the activation energy of MgH₂ after doping with 10 wt% of K₂ZrF₆ was reduced by approximately 84 kJ mol⁻¹. Nevertheless, the hydrogen release capacity decreased slightly due to the dead weight of K₂ZrF₆ that contained no hydrogen. In addition, new phases of KH and ZrH₂ formed by the doping of K₂ZrF₆ with MgH₂ during the heating process are believed to act as the active species. From all the obtained results, it can be concluded that the products formed from the addition of K₂ZrF₆ play a catalytic role in improving the hydrogen storage properties of MgH₂.

Acknowledgements

The authors would like to acknowledge Universiti Malaysia Terengganu for providing the facilities to carry out this project. This work was financially supported by the Fundamental Research Grant Scheme (FRGS 59295). F. A. Halim Yap and N. S. Mustafa are thankful to the Ministry of Education Malaysia for the MyBrain15 scholarship.

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