N. Juahira,
N. S. Mustafaa,
A. M. Sininb and
M. Ismail*a
aSchool of Ocean Engineering, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia. E-mail: mohammadismail@umt.edu.my; Fax: +60-9-6683991; Tel: +60-9-6683487
bCentre for Foundation and Liberal Education, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia
First published on 8th July 2015
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 Co1.29Ni1.71O4 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 Co1.29Ni1.71O4 during the heating process, thus improving the hydrogen storage properties of MgH2.
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).
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.
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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.
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Fig. 2 Isothermal dehydrogenation kinetics of the as-milled MgH2 and MgH2 + 10 wt% Co2NiO composite at 320 °C. |
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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 10th 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 10th cycle as shown in Fig. 5 with the desorption kinetics of the sample in 60 minutes for 10th 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.
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Fig. 4 Isothermal rehydrogenation kinetics of the MgH2 + 10 wt% Co2NiO composite for the ten cycles. |
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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 5th to 10th cycles compared to amount degrades for the first four cycles.
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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.
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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 (EA) 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/Tp2] = −EA/RTp + A | (1) |
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
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) |
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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 Co1.29Ni1.71O4 peaks could also be seen. The presence of Mg–Co alloy and Co1.29Ni1.71O4 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 Co1.29Ni1.71O4 peaks and these peaks remained unchanged after rehydrogenation at 320 °C. Hence, we speculate that the Mg–Co alloy and Co1.29Ni1.71O4 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 Co1.29Ni1.71O4 peaks. The formation of Mg–Co alloy and Co1.29Ni1.71O4 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 Co1.29Ni1.71O4 peaks. As compared to the XRD pattern of MgH2 + 10 wt% Co2NiO sample (Fig. 10), the formation of Mg–Co alloy and Co1.29Ni1.71O4 were more discernible after 50 wt% Co2NiO was added.
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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 Co1.29Ni1.71O4, 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, Co1.29Ni1.71O4, 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
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