Improved hydrogen storage properties of MgH₂ by addition of Co₂NiO nanoparticles

Abstract

A sample of MgH₂ and 10 wt% Co₂NiO was prepared by the ball milling technique. The hydrogen storage properties and reaction mechanism of the sample were examined. The temperature-programmed desorption result shows that the addition of 10 wt% Co₂NiO to MgH₂ exhibited a lower onset desorption temperature of 300 °C, which was decreased to 117 and 70 °C compared to as-received and as-milled MgH₂, respectively. The de/rehydrogenation kinetics of MgH₂ + 10 wt% Co₂NiO showed improvement compared to un-doped MgH₂. The results of the Kissinger plot shows that the activation energy for the hydrogen desorption of MgH₂ was reduced to about 65.0 and 15.0 kJ mol⁻¹ compared to as-received and as-milled MgH₂, respectively. Meanwhile, the X-ray diffraction analysis shows the formation of a new phase of Mg–Co alloy and Co_1.29Ni_1.71O₄ after the dehydrogenation and rehydrogenation process. It is reasonable to conclude that the Co₂NiO additive plays a catalytic role through the formation of active species of Mg–Co alloy and Co_1.29Ni_1.71O₄ during the heating process, thus improving the hydrogen storage properties of MgH₂.

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1. Introduction

The U.S Department of Energy has established a minimum target of 5.5 wt% ab/desorption hydrogen storage capacity for the year 2015.¹ MgH₂ can be seen as a promising material among metal hydrides due to its high gravimetric hydrogen storage (7.6 wt%), good reversibility, and low cost.² However, MgH₂ has deficiencies, including high decomposition temperature and slow desorption/absorption kinetics.³ In order to make MgH₂ feasible as a potential material for solid state hydrogen storage, different approaches have been used including improving the surface and kinetics by ball milling,⁴ adding a catalyst,^5,6 and using the destabilization concept.^7–12 The addition of catalysts by ball milling into MgH₂ has produced a significant effect on hydrogen sorption properties,^5,6 thus it has caught the interest of many researchers. Various additives have been doped into MgH₂ such as metal,^13–15 alloy,^16,17 metal oxide,^18–21 metal halide,^5,22–27 carbon materials,^28–30 and metal hydrides.^31,32

The addition of transition metal oxides has shown promising results in the ab/desorption kinetics of MgH₂. This enhancement could be associated with a highly effective catalysis shown by the transition of metal compounds due to the high affinity of metal cation transition towards hydrogen.^33–37 Besides that, transition metal oxides also can act as a good catalyst by facilitating the dissociation of hydrogen molecules and the recombination of hydrogen atom towards the molecular state.³⁸ It is believed that the additive oxide phase facilitates to overcome the MgO layer formed on the surface of MgH₂ particles, thus improving the reaction kinetics.^39,40

To the best of the author's knowledge, no reported study has used Co₂NiO nanoparticle as an additive for the hydrogen storage properties of MgH₂. The selection of Co₂NiO as an additive was driven from several studies, which used Ni- and Co-based additives. Ni- and Co-based additives have been given considerable attraction due to their ability to destabilize the dehydrogenation of MgH₂.^41,42 Mao et al. reported that the dehydrogenation temperature and the sorption kinetics of MgH₂ were improved by doping with NiCl₂ or CoCl₂. Their results suggest that all the dehydrogenation products MgCl₂, Mg₂Ni and Mg₂Co may play an important role in improving the hydrogen sorption kinetic properties of MgH₂, while MgCl₂ may play a dominant role.³⁸ A recent study by Cabo et al. showed that the addition of mesoporous Ni- and Co based oxides reduced the onset dehydrogenation temperature and lowered the activation energy of MgH₂.⁴³ Meanwhile, Mandzhukova et al. reported that the addition of 10 wt% nickel cobaltite, NiCo₂O₄ improved the dehydrogenation kinetics of MgH₂.^44,45

In this paper, the additive effect of Co₂NiO nanoparticle on the hydrogen storage properties of MgH₂ was investigated. The aim of this study is to combine the in situ active species factors together thus enhancing the de/hydrogenation properties of MgH₂. The hydrogen storage properties and reaction mechanism of MgH₂ and Co₂NiO were investigated using a Sieverts-type pressure-composition-temperature (PCT) apparatus, X-ray diffraction (XRD), differential scanning calorimetry (DSC), and scanning electron microscope (SEM).

2. Experimental details

MgH₂ (hydrogen storage grade, 98% purity) and Co₂NiO (<150 nm size particle, 99% trace material basis) were purchased from Alfa Aesar and Sigma-Aldrich, respectively and were used as received without further purification. Sample handling was performed in an MBraun Unilab glove box filled with high-purity Ar atmosphere. Approximately 200 mg MgH₂ was added with 10 wt% Co₂NiO. This mixture was loaded into a sealed stainless steel vial together with four hardened stainless balls. As-milled MgH₂ sample under the same condition was prepared for comparison. Ball milling was performed in a planetary ball mill (NQM-0.4) for 1 h by milling for 1/4 h, resting for 2 min, and then milling for another 1/4 h in a different direction for 3 cycles at the rate of 400 rpm. The ratio of the weight of the balls to the weight of the powder was 80 : 1.

The temperature-programmed desorption (TPD) and re/dehydrogenation kinetics experiments were performed in a Sieverts-type pressure-composition-temperature (PCT) apparatus (Advanced Materials Corporation). The sample was loaded into a sample vessel in the glove box. For the TPD experiment, the samples were heated in a vacuum chamber from room temperature to 450 °C with a heating rate of 5 °C min⁻¹ and the lowest decomposition temperature determined by the amount of desorbed hydrogen. The re/dehydrogenation kinetics experiments were performed at temperature of 320 °C with hydrogen pressure of 30 atm and 1 atm, respectively.

The phase structure of as-milled samples, before and after desorption and after rehydrogenation were measured using Rigaku MiniFlex II Diffractometer with Cu Kα radiation. Before the measurement, a small amount of sample was spread uniformly in the sample holder and wrapped with a plastic wrap to prevent oxidation. θ–2θ scans were carried out over the diffraction angles from 20° to 80° at a speed of 2.00° min⁻¹. Meanwhile, the microstructure of the samples were characterized using a scanning electron microscope (SEM; JEOL JSM-6360LA) by preparing the samples on the surface of carbon tape and then coating it with gold spray under vacuumed condition. All samples were also prepared in the glove box in order to minimize oxidation. A DSC analysis was carried out using a Mettler Toledo TGA/DSC 1. About 5–6 mg weight of the sample was loaded in an alumina crucible in the glove box. The crucible was then placed in a sealed glass bottle in order to prevent oxidation during transportation from the glove box to the DSC apparatus. The samples were heated from room temperature to 480–550 °C under Ar atmosphere with different heating rates. An empty alumina crucible was used for reference.

3. Results and discussion

The dehydrogenation behavior of as-received MgH₂, as-milled MgH₂, and the MgH₂ + 10 wt% Co₂NiO composite was investigated using a Sieverts-type PCT apparatus. Fig. 1 presents the temperature-programmed-desorption (TPD) performances of as-received MgH₂, as-milled MgH₂, and the MgH₂ doped with 10 wt% Co₂NiO. From the TPD curves, it can be seen clearly that the addition of 10 wt% of Co₂NiO decreased the onset decomposition temperature compared to as-milled and as-received MgH₂. The onset decomposition of as-received and as-milled MgH₂ was about 417 and 370 °C with a total dehydrogenation capacity of 6.2 and 5.4 wt% H₂ at 450 °C, respectively. These results show that the ball milling process also has an influence in reducing the onset desorption temperature of MgH₂ due to the larger surface area in magnesium surfaces. Therefore, a higher diffusion hydrogen absorption can be achieved.^4,19 However, the as-milled MgH₂ showed slightly lower hydrogen desorption capacity compared to as-received MgH₂, and this could happen due to the released hydrogen during ball milling process. MgH₂ + 10 wt% Co₂NiO had a positive effect on the onset desorption temperature of MgH₂, since the desorption started at around 300 °C with a total dehydrogenation capacity of 5.3 wt% hydrogen at 450 °C, which was lowered by 117 °C and 70 °C compared to the as-received and as-milled MgH₂. These results suggest that the Co₂NiO component in the doped MgH₂ could play a catalytic role and thus, improving the onset temperature of MgH₂.

Fig. 1 TPD curves for dehydrogenation of the as-received, as-milled and MgH₂ + 10 wt% Co₂NiO composite.

Fig. 2 shows the isothermal desorption kinetics curve for as-milled MgH₂ and MgH₂ + 10 wt% Co₂NiO composite measured at 320 °C under 1.0 atm pressure. The result shows that the sample doped with 10 wt% Co₂NiO released about 2.5 wt% hydrogen in 6 min. In contrast, almost no hydrogen was released in the same period by as-milled MgH₂. Therefore, it can be assumed that Co₂NiO also had significant effect on improving the dehydrogenation kinetics of MgH₂. Fig. 3 shows the isothermal rehydrogenation kinetics for as-milled MgH₂ and MgH₂ + 10 wt% Co₂NiO samples. The samples were soaked at a constant temperature of 320 °C and under 30.0 atm hydrogen pressure. The hydrogen absorbed by MgH₂ doped with 10 wt% Co₂NiO samples reached about 2.5 wt% hydrogen within 1.7 min, with a total hydrogen absorption of 3.75 wt%. Meanwhile, as-milled MgH₂ took about 3.4 min to absorb the same amount of hydrogen with a total hydrogen absorption of 3.78 wt%. It can be seen that the doped sample showed better rehydrogenation kinetics than as-milled MgH₂. Taken together, these results suggest that the rehydrogenation kinetics of MgH₂ can also be improved by doping it with Co₂NiO.

Fig. 2 Isothermal dehydrogenation kinetics of the as-milled MgH₂ and MgH₂ + 10 wt% Co₂NiO composite at 320 °C.

Fig. 3 Isothermal rehydrogenation kinetics of the as-milled MgH₂ and doped MgH₂ + 10 wt% Co₂NiO composite at 320 °C and under 30.0 atm hydrogen pressure.

Fig. 4 representatives the cyclic performance of MgH₂ + 10 wt% Co₂NiO sample at temperature of 320 °C and under 30 atm hydrogen pressure. The hydrogen absorption kinetics of the sample shows some degradation after prolonged cyclic. However, the degradation was involving small values. After the 10^th cycle, the absorption continued to be good with hydrogen capacity of 3.44 wt%. Meanwhile, the cycle performances for the desorption kinetics of MgH₂ + 10 wt% Co₂NiO sample also shows good performances even after 10^th cycle as shown in Fig. 5 with the desorption kinetics of the sample in 60 minutes for 10^th cycle was 3.75 wt%. The hydrogen desorption also degrades after prolonged time with involving small amount hydrogen. This result shows that the good cyclic performances of MgH₂ can be achieved by doping with Co₂NiO.

Fig. 4 Isothermal rehydrogenation kinetics of the MgH₂ + 10 wt% Co₂NiO composite for the ten cycles.

Fig. 5 Isothermal dehydrogenation kinetics of the MgH₂ + 10 wt% Co₂NiO composite for the ten cycles.

Fig. 6 shows the recyclability capacity of the MgH₂ + 10 wt% Co₂NiO composite after 60 min absorb/desorb for the ten cycles. From the graph, it can be seen that for all cycles, the hydrogen desorption have slightly higher capacity compared to hydrogen absorption. It can be seen that the amount of hydrogen absorbed and desorbed degrades after prolonged cyclic. For hydrogen desorption, shows only slight degradation observed in hydrogen capacity of the 5^th to 10^th cycles compared to amount degrades for the first four cycles.

Fig. 6 Recyclability capacity of the MgH₂ + 10 wt% Co₂NiO composite after 60 min absorb/desorb for the ten cycles.

Fig. 7 presents the DSC curves of the MgH₂ + 10 wt% Co₂NiO sample at a heating rate of 15 °C min⁻¹. As-received MgH₂ and as-milled MgH₂ were included for comparison purposes. The DSC curve of as-received MgH₂ showed only one strong endothermic peak at approximately 454.06 °C, which corresponds to the decomposition of MgH₂. Meanwhile, the DSC curves for as-milled MgH₂ and MgH₂ + 10 wt% Co₂NiO had strong endothermic peaks at 418.72 °C and 412.66 °C, respectively. The noticeable reduction in the peak temperatures of the samples based on the DSC results revealed that dehydrogenation was improved by adding Co₂NiO. However, it could be seen that the onset decomposition temperature of the samples in DSC were slightly higher than those in TPD (Fig. 1), which could be due to the differences in heating rates and atmospheres between DSC and PCT measurements, as discussed in our previous papers.^46–49 TPD measurement was conducted from 1.0 atm vacuum with a 5 °C min⁻¹ heating rate, while DSC measurement was run under 1.0 atm argon flow with heating rate of 15 °C min⁻¹.

Fig. 7 DSC traces of as-received MgH₂, as-milled MgH₂, and MgH₂ + 10 wt% Co₂NiO at heating rate of 15 °C min⁻¹.

The Kissinger equation⁵⁰ was used to calculate the activation energy (E_A) of MgH₂ + 10 wt% Co₂NiO at different heating rates. The activation energy of as-received and as-milled MgH₂ was included for comparison purposes. Fig. 8(a)–(c) shows the DSC curves at different heating rates for as-received MgH₂, as-milled MgH₂, and MgH₂ + 10 wt% Co₂NiO, respectively. From the Kissinger equation:

ln[β/T_p²] = −E_A/RT_p + A (1)

where β is the heating rate, T_p is the peak temperature in the DSC curve, R is the gas constant, and A is a linear constant. Thus, the activation energy, E_A, can be obtained from the slope of the plot, ln[β/T_p²] versus 1000/T_p. Fig. 8(d) shows the Kissinger plot for the dehydrogenation of MgH₂ + 10 wt% Co₂NiO composite compared to as-received MgH₂ and as-milled MgH₂. The apparent activation energy for MgH₂ + 10 wt% Co₂NiO composite estimated from the Kissinger analysis was found to be 118.0 kJ mol⁻¹, which was lower than that of as-received and as-milled MgH₂ (183.0 and 133.0 kJ mol⁻¹, respectively). These values show that improved dehydrogenation behavior and reduced activation energy can be achieved by doping Co₂NiO with MgH₂.

Fig. 8 DSC traces of (a) as-received MgH₂, (b) as-milled MgH₂, (c) MgH₂ + 10 wt% Co₂NiO composite at different heating rates of 15–30 °C min⁻¹ and (d) the Kissinger's analysis for (a) as-received MgH₂, (b) as-milled MgH₂, and (c) MgH₂ + 10 wt% Co₂NiO composite.

Fig. 9(a)–(c) show the SEM images of as-received MgH₂, as-milled MgH₂, and MgH₂ + 10 wt% Co₂NiO, respectively. In Fig. 9(a), it can be seen that as-received particles of MgH₂ were angular shaped with sizes larger than 100 μm. Fig. 9(b) shows a sample subjected to 1 h ball milling process, which caused a drastic reduction in MgH₂ particles with non-homogenous particles size. Besides that, some agglomeration was detected in the sample. After introducing Co₂NiO through the ball milling process, the size of the particles became smaller compared to as-milled MgH₂ sample (Fig. 9(c)). The hardness of the Co₂NiO helped in breaking MgH₂ particles into smaller sizes, increasing the specific surface area and reducing the diffusion length of the hydrogen, thus achieving the minimum onset desorption temperature.^24,51

Fig. 9 The SEM images of the (a) as-received MgH₂, (b) as-milled MgH₂ and (c) MgH₂ + 10 wt% Co₂NiO.

To have a better understanding on the reaction process and mechanism of this sample, XRD measurements were performed on MgH₂ + 10 wt% Co₂NiO samples after 1 h of milling, after dehydrogenation at 250 °C, after dehydrogenation at 450 °C, and after rehydrogenation at 320 °C under 30.0 atm hydrogen pressure, as shown in Fig. 10. After 1 h of milling, the MgH₂ and Co₂NiO peaks were seen to dominate the XRD pattern with few Mg peaks. Mg peaks appeared at this stage confirmed there was hydrogen released during ball milling. After heating to 250 °C, it was seen that the same peaks appeared, such as those in ball milling. Further heating to 450 °C caused MgH₂ and Co₂NiO to disappear. The dehydrogenation of MgH₂ can be confirmed with distinct Mg peaks, and the transformation of MgH₂ into Mg can be represented as follows:

MgH₂ → Mg + H₂ (2)

Fig. 10 XRD patterns of MgH₂ doped 10 wt% Co₂NiO (a) after 1 h ball milling, (b) after dehydrogenation at 250 °C, (c) after dehydrogenation at 450 °C and (d) after rehydrogenation at 320 °C.

MgO peak was also detected in the dehydrogenation spectra due to slight oxygen contamination when transferring the samples to the XRD instrument. In addition, new Mg–Co alloy and Co_1.29Ni_1.71O₄ peaks could also be seen. The presence of Mg–Co alloy and Co_1.29Ni_1.71O₄ might have appeared during the dehydrogenation process. XRD examination of the dehydrogenated MgH₂ doped with Co₂NiO sample verified the formation of Mg–Co alloy and Co_1.29Ni_1.71O₄ peaks and these peaks remained unchanged after rehydrogenation at 320 °C. Hence, we speculate that the Mg–Co alloy and Co_1.29Ni_1.71O₄ species could have acted as the real catalysts. In addition, it was seen that Mg was largely transformed into MgH₂ after the rehydrogenation process.

In order to investigate the Co₂NiO containing phase after dehydrogenation in detail, we prepared a MgH₂ + 50 wt% Co₂NiO sample, as it was difficult to analyse the phase composition with 10 wt% Co₂NiO by XRD. Fig. 11 shows the XRD patterns of MgH₂ doped with 50 wt% Co₂NiO sample (a) after 1 h ball milling, (b) after dehydrogenation at 250 °C and (c) after dehydrogenation at 450 °C. After increasing the amount of Co₂NiO to 50 wt%, the Co₂NiO peaks increased compared to as-milled MgH₂ + 10 wt% Co₂NiO sample as shown in Fig. 10(a). Mg peaks were detected in the XRD spectra indicating that hydrogen was released during ball milling. After heating to temperature 250 °C, the same Mg, MgH₂ and Co₂NiO peaks were seen with few Mg–Co alloy and Co_1.29Ni_1.71O₄ peaks. The formation of Mg–Co alloy and Co_1.29Ni_1.71O₄ at 250 °C indicates that these active species were formed during the heating process. Further temperature increment to 450 °C saw the disappearance of MgH₂ and Co₂NiO peaks with more frequent appearance of Mg–Co alloy and Co_1.29Ni_1.71O₄ peaks. As compared to the XRD pattern of MgH₂ + 10 wt% Co₂NiO sample (Fig. 10), the formation of Mg–Co alloy and Co_1.29Ni_1.71O₄ were more discernible after 50 wt% Co₂NiO was added.

Fig. 11 XRD patterns of MgH₂ doped 50 wt% Co₂NiO (a) after 1 h ball milling, (b) after dehydrogenation at 250 °C and (c) after dehydrogenation at 450 °C.

From the results, we speculate that the formation of Mg–Co alloy and Co_1.29Ni_1.71O₄, which resulted from the reaction of MgH₂ and Co₂NiO during de/hydrogenation process, could play an important role in the enhancement of hydrogen sorption. The formation of Mg–Co alloy in this study is mutual with the findings by Mao et al.³⁸ They suggested that Mg–Co alloy formed in the MgH₂–CoCl₂ system could act as a catalyst besides MgCl₂ alloy, thus improving the de/hydrogenation of MgH₂ through the additive. It can be speculated that, this additive reduces the barrier of nucleation, and thus hydrogen desorption takes place at a lower driving force and the transition metal helps in facilitating the dissociation of hydrogen molecules and the recombination of hydrogen atom towards the molecular state.¹⁴ In addition, it is believed that the newly formed ball-milled or dehydrogenation product in the complex hydride catalyst system could act as a real catalyst to facilitate the de/rehydrogenation steps.⁵² In this study, Co–Ni oxide species with an altered valance state, Co_1.29Ni_1.71O₄, could have acted as a real catalyst because they could create surface activation and form a large amount of nucleation sites at the surface of the MgH₂ matrix. It is also speculated that these finely dispersed ball-milled products could serve as the active sites for nucleation by shortening the diffusion distance of the reaction ions.⁵²

4. Conclusion

In summary, the hydrogen sorption properties of MgH₂ improved after doping it with 10 wt% Co₂NiO. MgH₂ + 10 wt% Co₂NiO composite has a lower onset dehydrogenation temperature of 300 °C, which was decreased to about 117 °C and 70 °C compared to as-received and as-milled MgH₂, respectively. In terms of dehydrogenation kinetics, MgH₂ + 10 wt% Co₂NiO sample released about 2.5 wt% hydrogen in 6 min at 320 °C, while as-milled MgH₂ sample showed no hydrogen release under the same condition. Meanwhile, for absorption kinetics, MgH₂ + 10 wt% Co₂NiO sample absorbed about 2.5 wt% hydrogen within 1.7 min at 320 °C, but as-milled MgH₂ sample took about 3.4 min to absorb the same amount of hydrogen. The results from the Kissinger plot showed that the activation energy for hydrogen desorption of MgH₂ was reduced compared to those of as-received and as-milled MgH₂ from 183.0 and 133.0 kJ mol⁻¹ to 118.0 kJ mol⁻¹, respectively, after the addition of 10 wt% Co₂NiO. It is reasonable to conclude that, in this study, the Co₂NiO additive doped with MgH₂ played a catalytic role through the formation of active species of Mg–Co alloy and Co_1.29Ni_1.71O₄ during the heating process. This newly developed product acted as a real catalyst in improving the hydrogen storage properties of MgH₂.

Acknowledgements

The authors would like to thank the Universiti Malaysia Terengganu for providing the facilities to carry out this project and the Malaysian Ministry of Education for the research grant under Fundamental Research Grant Scheme (FRGS 59362). N. Juahir and N. S. Mustafa are grateful to the Malaysian Ministry of Education for a MyBrain15 scholarship.

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