Nanolayer-like-shaped MgFe₂O₄ synthesised via a simple hydrothermal method and its catalytic effect on the hydrogen storage properties of MgH₂

Abstract

In this study, the effect of nanolayer-like-shaped MgFe₂O₄ that is synthesised via a simple hydrothermal method on the performance of MgH₂ for hydrogen storage is studied. MgH₂ + 10 wt% MgFe₂O₄ is prepared by using the ball milling method. The MgFe₂O₄-doped MgH₂ sample started to release H₂ at approximately 250 °C, 90 °C and 170 °C lower than the milled and pure MgH₂ respectively. At 320 °C, the isothermal desorption kinetic study has shown that the doped sample has desorbed approximately 4.8 wt% H₂ in 10 min while the milled MgH₂ desorbed less than 1.0 wt% H₂. For isothermal absorption kinetics, the doped sample can absorb approximately 5.5 wt% H₂ in 10 min at 200 °C. Meanwhile, the undoped sample absorbs only 4.0 wt% H₂ in the same condition. The activation energy of 10 wt% MgFe₂O₄-doped MgH₂ composite is 99.9 kJ mol⁻¹, which shows a reduction of 33.1 kJ mol⁻¹ compared to the milled MgH₂ (133.0 kJ mol⁻¹). X-ray diffraction spectra display the formation of new species which are Fe and MgO after dehydrogenation, and these new species are believed to act as the real catalyst that plays a crucial role in improving the sorption performance of the MgFe₂O₄-doped MgH₂ system by providing a synergetic catalytic effect.

---

1. Introduction

To prepare for the future and ensure global environmental viability, energy systems have to be reliable, clean, low cost, environmentally friendly and flexible. Humanity is expected to use 40 TW of power (40 billion of kW) in the future. To satisfy this demand, different sources of renewable energy, such as hydrogen, are needed. Sustainable hydrogen is an ideal clean energy carrier because there is no carbon dioxide or other greenhouse gas emission at the end-user level. Commonly, there are 3 forms of storing hydrogen which is high-pressure gas, cryogenic liquid hydrogen in tanks (stored at 21.2 K) and as solid state hydrogen storage by either reacting with chemical compounds or absorbing. Among these approaches, solid-state hydrogen storage has higher potential for higher hydrogen density and may yield greater utility towards the practical implementation of hydrogen storage. Among the various materials for solid-state hydrogen storage, MgH₂ considered as one of the most potential material due to its high hydrogen storage capacity (7.6 wt%), excellent reversibility and low cost.¹ However, MgH₂ is restricted by the decomposition temperature, which is high with slow sorption kinetics and is excessively stable thermodynamically.² Many research have been conducted to overcome these disadvantages by altering the thermodynamics and improve the kinetic properties by producing nanostructures^3,4 and utilizing catalysts such as carbon-based materials,^5,6 metals,^7–10 metal hydrides,^11,12 metal oxides,^13–18 metal halides,^19–21 and nanosized alloys.^22–24

Previous research has proved that catalyst based on ternary metal oxides greatly improved the hydrogen storage performance of MgH₂.^15,25–35 Zhang et al.²⁵ demonstrate that ferrite nanoparticles (MnFe₂O₄, ZnFe₂O₄, Mn_0.5Zn_0.5Fe₂O₄ and CoFe₂O₄) can greatly lower the decomposition temperature of MgH₂. CoFe₂O₄ provides the best catalytic effect compared with other ferrites. New by-products are found after the dehydrogenation process, and its phase shows a great catalytic effect on the properties of hydrogen storage of MgH₂. Furthermore, Li et al.²⁶ shows a significant improvement in the desorption performance of MgH₂ when catalysed with MnFe₂O₄. X-ray photoelectron spectroscopy and X-ray diffraction (XRD) tests show that Fe_0.872O and Mg₂MnO₄ phases take part as significant role in enhancing the dehydriding performance of MgH₂. Meanwhile, we showed in our previous study that MnFe₂O₄ synthesised via a simple hydrothermal method provides a remarkable effect in improving the hydrogen storage performance of MgH₂.²⁷ Interestingly, our result showed that Fe metal formed after dehydrogenation instead of Fe_0.872O species, as claimed by Li et al.²⁶ This variation paved the way for the debate on how ternary metal oxides, particularly ferrites, work as catalysts in improving the hydrogen sorption performance of MgH₂. Moreover, the difference in the synthesis method of the catalysts may also provide a different effect in the catalytic role.

Inspired by the role of active species that formed during the heating process in the MgH₂-ternary metal oxides catalyst system, it is quite interesting to investigate the use of other ferrites (e.g. MgFe₂O₄) as catalysts to improve the hydrogen sorption performance of MgH₂. Therefore, in this work, MgFe₂O₄ was synthesised by using a simple hydrothermal method, and its catalytic effects on the hydrogen sorption performance of MgH₂ were systematically studied. To the best of authors' knowledge, this paper is the first to study the hydrogen sorption performance of MgH₂ catalysed with MgFe₂O₄. The possible catalysis mechanisms of MgFe₂O₄ in the sorption performances of MgH₂ are also discussed in this paper.

2. Experimental details

The nanolayer-like-shaped MgFe₂O₄ was synthesised via a hydrothermal method. In a typical synthesis, a stoichiometric amount of Mg(NO₃)₂·6H₂O (Sigma-Aldrich) and Fe(NO₃)₃·9H₂O (Sigma-Aldrich) were dissolved in 50 ml distilled water. A total of 10 ml of H₄N₂·H₂O (Sigma-Aldrich) was added dropwise to the above solution to attain the resultant pH of >9. The mixture was then transferred into a sealed Teflon lined stainless-steel autoclave (125 ml capacity) and heated for 12 h at 180 °C. The final product was washed several times with deionised water and dried overnight at 60 °C under vacuum. A total of 10 wt% of as-prepared MgFe₂O₄ was mixed with 300 mg of MgH₂ (95% pure; Sigma-Aldrich) and undergo intensive ball milling for 1 h in a planetary ball mill at the rate of 400 rpm. For comparison, pure MgH₂, and MgH₂ added with 10 wt% Fe (Alfa Aesar) and 10 wt% MgO (R&M Chemicals), respectively were also prepared under the same conditions. All preparations, including loading and weighing, were conducted in an argon atmosphere glove box (MBraun Unilab).

The onset decomposition temperature and sorption kinetic measurement for doped and undoped samples were characterised by using Sievert-type pressure–composition–temperature apparatus (Advanced Materials Corporation). For onset decomposition temperature measurement, the samples were heated from room temperature to 450 °C at a heating rate of 5 °C min⁻¹ in vacuum chamber. Meanwhile, the sorption kinetics was conducted under 1.0 atm at 320 °C for desorption kinetic measurement and under 33.0 atm at 200 °C for absorption kinetic measurement. The thermal properties of the doped and undoped samples were performed using differential scanning calorimeter (DSC)/thermogravimetric analysis from Metler Toledo. With a flow of 50 ml min⁻¹ argon, the samples were heated with 15, 20, 25 and 30 °C min⁻¹ heating rate from 25 °C to 500 °C.

The phase composition of the samples was analysed by XRD via a Rigaku MiniFlex X-ray diffraction apparatus equipped with Cu Kα radiation. Data were collected in the 2θ range 20° to 80° at 2° min⁻¹. The morphologies of the samples were observed by scanning electron microscopy (SEM) (JEOL JSM-6350LA). Fourier transform infrared (FTIR) spectrometry was recorded on an IR Shimadzu Tracer-100 between 400 and 2000 cm⁻¹. Raman spectra were recorded on Renishaw Raman spectroscopy (532 nm radiation) extended with 0.1% power laser measurement at room temperature.

3. Results and discussion

Before milling with MgH₂, the phase structure of MgFe₂O₄ was confirmed by XRD, as shown in Fig. 1(a). The crystallographic planes of (220), (311), (400), (422), (511), (440) and (533) correspond to the diffraction peaks at 2θ of 30.1°, 35.4°, 43.1°, 53.5°, 57.0°, 62.6° and 74.1°, respectively. This result is in good agreement with the standard cubic spinel structure (JCPDS 71-1232). No other peaks were detected in the sample. The average crystallite size of MgFe₂O₄ was approximately 19 nm, as determined by using Scherrer's formula:

L = kλ/B cos θ, (1)

where L is the average crystallite size (nm), θ is the angle of diffraction, k is Scherrer's constant (k = 0.94), λ is the X-ray wavelength (0.15405 nm) and B is the full width at half maximum of the diffraction peak in radian (FWHM). The SEM image (Fig. 1(b)) reveals that the MgFe₂O₄ forms a large layer with a nanosized thickness. From the FTIR spectrum (Fig. 1(c)), two typical peaks of MgFe₂O₄ were observed at the low wavenumber, thus indicating the formation of spinel ferrite structure.^36,37 The peak at 417 cm⁻¹ can be ascribed to the Fe–O vibration in the octahedral site, and the peak at 538 cm⁻¹ can be assigned to the Fe–O vibration in the tetrahedral and octahedral sites. Furthermore, the Raman peaks (Fig. 1(d)) at 475 and 694 cm⁻¹ can be assigned to the typical characteristic peaks of MgFe₂O₄,³⁸ whereas the peak at 283 cm⁻¹ corresponding to the stretching vibration of the Mg–O chemical bond.³⁹ The XRD, FTIR and Raman spectroscopy results confirm that pure MgFe₂O₄ was successfully synthesised by the hydrothermal method.

Fig. 1 (a) XRD pattern, (b) SEM image, (c) FTIR spectra and (d) Raman spectra of MgFe₂O₄.

Fig. 2(a) shows the onset decomposition temperature results for the pure MgH₂, milled MgH₂ and MgH₂ with 10 wt% MgFe₂O₄. Before milling, pure MgH₂ started to desorb hydrogen at approximately 420 °C. The total amount of hydrogen desorbed is approximately 7.0 wt%. After milling for 1 h, the onset decomposition temperature of MgH₂ was decreased to approximately 340 °C. This phenomenon demonstrate that the sorption performance of MgH₂ also influenced by the milling process. From the curve, it can be seen that after milling, the amount of hydrogen desorb of MgH₂ slightly decreases. This might be ascribed to the hydrogen released from MgH₂ during the milling process. After doping with 10 wt% of MgFe₂O₄, it is clear that the onset decomposition temperature of the MgH₂ was dramatically reduced to 250 °C, 90 °C and 170 °C lower than that for the milled and pure MgH₂, respectively. However, the hydrogen desorption capacity decrease slightly to approximately 6.5 wt% because the dopant used in this study, namely, MgFe₂O₄, does not contain hydrogen.⁴⁰ From Fig. 2(a), it can be concluded that MgFe₂O₄ additive plays a positive role in decreasing the decomposition temperature of MgH₂.

Fig. 2 (a) Decomposition temperature profile of pure and milled MgH₂, and 10 wt% MgFe₂O₄-doped MgH₂ sample, (b) measurement of isothermal absorption kinetics for the milled MgH₂ and 10 wt% MgFe₂O₄-doped MgH₂ sample at 200 °C under 33.0 atm H₂ pressures, and (c) measurement of isothermal desorption kinetics for the milled MgH₂ and 10 wt% MgFe₂O₄-doped MgH₂ sample at 320 °C under 1.0 atm.

To further examine the sorption properties of the MgFe₂O₄-doped MgH₂ sample, the isothermal absorption kinetic was studied. The amount of hydrogen absorbed from the milled MgH₂ and the MgFe₂O₄-doped MgH₂ sample was measured under 33.0 atm H₂ and at constant temperature of 200 °C, as shown in Fig. 2(b). The MgFe₂O₄-doped MgH₂ sample shows better absorption kinetics than undoped MgH₂. For the MgFe₂O₄-doped MgH₂ sample, the amount of 5.5 wt% H₂ was absorbed in 10 min, whereas the milled MgH₂ only absorbed approximately 4.0 wt% H₂ within the same time. From the result, it clearly shows that the addition of MgFe₂O₄ enhanced the rehydrogenation kinetics of MgH₂.

For further studies on the catalytic effect of MgFe₂O₄ on the sorption kinetic of MgH₂, isothermal desorption kinetic was performed under 1.0 atm at 320 °C. As shown in Fig. 2(c), the MgFe₂O₄-doped MgH₂ sample shows significant enhancement compared with the milled MgH₂. The results shows that the undoped MgH₂ released less than 1.0 wt% H₂ after 10 min, whereas the doped sample can released approximately 4.8 wt% H₂ under the same condition. In contrast, it can be seen that MgFe₂O₄ also plays a significant role in enhancing the dehydrogenation kinetic of MgH₂.

The catalytic effect of MgFe₂O₄ was further studied with the cycling performances of MgFe₂O₄-doped MgH₂ system. Fig. 3(a) presents the isothermal absorption kinetics of the 10 wt% MgFe₂O₄ doped with MgH₂ at 320 °C under a hydrogen pressure of 33.0 atm over 10 cycles. From the result, it can be seen that after the ten cycles, the absorption kinetics show a small reduction in the hydrogen capacity. After completing the 10^th cycle, the system is able to absorb 5.6 wt% of hydrogen in 60 minutes. The result shows that the doped system displays good absorption properties even after 10 cycles. As for the desorption kinetics, Fig. 3(b) shows the isothermal desorption kinetics for 10 cycles that was carried out at 320 °C and under 1.0 atm of pressure. Like the absorption kinetics, a small hydrogen capacity degradation is shown after completing the 10^th cycle. The doped system possesses a good performance after completing the 10^th cycle as it is able to desorb about 5.5 wt% of hydrogen within 60 minutes. These results demonstrated that MgFe₂O₄ plays a vital catalytic role for the cycle life of MgH₂.

Fig. 3 (a) Measurement of isothermal absorption kinetics for the 10 wt% MgFe₂O₄-doped MgH₂ sample in the 2^nd, 4^th, 6^th, 8^th and 10^th cycle at 320 °C under 33.0 atm H₂ pressures and (b) measurement of isothermal desorption kinetics for the 10 wt% MgFe₂O₄-doped MgH₂ sample in the 2^nd, 4^th, 6^th, 8^th and 10^th cycle at 320 °C under 1.0 atm.

The thermal properties of the 10 wt% MgFe₂O₄-doped MgH₂ and undoped MgH₂ sample were further studied by DSC at heating rate of 30 °C min⁻¹ and under a flow of 50 ml min⁻¹ argon (Fig. 4). Obviously, the DSC trace for the pure MgH₂ showed one endothermic peak at approximately 482.9 °C. This strong endothermic peak related to the released of hydrogen from the MgH₂. Similar to the pure MgH₂, DSC traces of the milled MgH₂ and MgFe₂O₄-doped MgH₂ showed only one strong endothermic peak at 438.8 °C and 393.3 °C respectively. The peaks correlated to the decomposition of MgH₂ but at lower temperatures.

Fig. 4 DSC curves of pure MgH₂, milled MgH₂, and 10 wt% MgFe₂O₄-doped MgH₂.

The improvement in desorption behaviour is correlated with the kinetic barrier of the hydrogen desorbed from the MgH₂. By doping MgH₂ with MgFe₂O₄, low value of activation energy for released hydrogen is obtained. Kissinger analysis⁴¹ (eqn (2)) was conducted to determine the activation energy of doped and undoped MgH₂ samples.

ln[β/T_p²] = −E_A/RT_p + A, (2)

where β is the heating rate, E_A is the activation energy, R is the gas constant, T_p is the peak temperature of DSC curves and A is the linear constant. Fig. 5 illustrates the curves of DSC for the milled MgH₂ and MgH₂ doping with 10 wt% MgFe₂O₄ samples at heating rates of 15, 20, 25, and 30 °C min⁻¹. From the Kissinger plot of the DSC data (Fig. 5(c)), it can be perceived that the activation energy, E_A, for the MgFe₂O₄-doped MgH₂ composite is 99.9 kJ mol⁻¹, which is decrease 33.1 kJ mol⁻¹ compared with the milled MgH₂ (133.0 kJ mol⁻¹). The result indicates that addition of MgFe₂O₄ reduces the decomposition activation energy and boost the desorption performances of MgH₂.

Fig. 5 DSC curves at heating rates of 15, 20, 25, and 30 °C min⁻¹ for (a) milled MgH₂, (b) MgH₂ +10 wt% MgFe₂O₄ and (c) the Kissinger plot of decomposition for 10 wt% MgFe₂O₄-doped MgH₂ composite and milled MgH₂.

Fig. 6 presents the microstructures of the pure and milled MgH₂, and MgFe₂O₄-doped MgH₂. From the images, it can be seen clearly that the particle size of the pure MgH₂ is around 50–100 μm (Fig. 6(a)). Fig. 6(b) shows the image of the MgH₂ after 1 h ball milling. The size of the milled MgH₂ was decreased dramatically compared to the pure MgH₂. However, the image shows agglomeration and inconsistent particle sizes. Fig. 6(c) shows that the particle size of 10 wt% MgFe₂O₄-doped MgH₂ was the smallest and had less agglomeration than the pure and milled MgH₂. Smallest particle size gives a larger region of contact to the MgH₂, thus resulting in the higher rate of reaction of MgH₂.

Fig. 6 SEM images for the pure MgH₂ (a), milled MgH₂ (b) and MgH₂ + 10 wt% MgFe₂O₄ (c).

To investigate the phase structure, XRD measurement was performed on the 10 wt% MgFe₂O₄-doped MgH₂ sample, as shown in Fig. 7. From Fig. 7(a), it can be observed that the MgH₂ and MgFe₂O₄ phases are present in the as-milled MgFe₂O₄-doped MgH₂ sample. No additional peaks were found from the spectra. After dehydrogenation at 450 °C (Fig. 7(b)), the XRD pattern showed that the MgH₂ was completely dehydrogenated to Mg. This result demonstrates that the decomposition of MgH₂ was completed after heating for up to 450 °C. Furthermore, a small peak of MgO and Fe formed after the desorption process, thus demonstrate that the partial reaction of MgH₂ with MgFe₂O₄ may occur during the heating process as follows:

4MgH₂ + MgFe₂O₄ → Mg + 4MgO + 2Fe + 4H₂ (3)

Fig. 7 XRD spectra of the MgH₂ + 10 wt% MgFe₂O₄ (a) after ball milling for 1 h, (b) after desorption under 1.0 atm at 450 °C and (c) after absorption under 33.0 atm at 320 °C.

The standard Gibbs Free energy, , of MgH₂, MgFe₂O₄ and MgO are −35.9824, −1317.1232 and −569.024 kJ mol⁻¹, respectively.⁴² The total change ΔG correlated with the reaction in eqn (3) is −815.0168 kJ mol⁻¹. These values can confirm the possibility of the reaction in eqn (3) from thermodynamic potentials. Fig. 7(c) shows the XRD patterns for the rehydrogenated sample under 33.0 atm H₂ at 320 °C. The result illustrates that the phase of Mg was fully converted into MgH₂. Furthermore, the peak of Fe and MgO still remained after undergo rehydrogenation.

From the above analyses, the improvements in the sorption properties of MgH₂ doped with 10 wt% MgFe₂O₄ may be resulted from the formations of Fe and MgO. Fe and MgO may act as the real catalysts that play a vital role in the improvements of MgH₂ sorptions. Therefore, to verify the effect of Fe and MgO on MgH₂, samples of 10 wt% MgO-doped MgH₂ and 10 wt% Fe-doped MgH₂ were prepared and the TPD profiles for the dehydrogenations were shown as in Fig. 8. It is clearly seen that the decomposition temperature of MgH₂ are reduced with the addition of MgO or Fe as compared to the pure and milled MgH₂. However, the performance of these MgO and Fe are not significant as the sample of 10 wt% of MgFe₂O₄ doped with MgH₂. This demonstrated that the in situ generated MgO and Fe from the reaction of MgH₂ + 10 wt% of MgFe₂O₄ may play a significant role that introduce a synergetic catalytic effect that cause a significant improvement on the dehydrogenation performances of MgH₂ doped with 10 wt% of MgFe₂O₄. In addition, the in situ formed Fe and MgO in the MgH₂ + 10 wt% of MgFe₂O₄ sample are speculated to have a higher degree of dispersion and more compact phase segregation as compared to the milled MgH₂ + 10 wt% Fe and milled MgH₂ + 10 wt% MgO. This would be likely to lead the improvement of the sorption kinetics.

Fig. 8 Decomposition temperature profile of pure and milled MgH₂, 10 wt% MgO-doped MgH₂, 10 wt% Fe-doped MgH₂ and 10 wt% MgFe₂O₄-doped MgH₂ sample.

From the result obtained, we postulate that formation of fresh and fine MgO and Fe species which resulted from the reaction between MgH₂ and MgFe₂O₄ during the dehydrogenation process may play significant role in improving the sorption performances of MgH₂. Numerous studies have claimed that the newly active species formed during the de/absorption process may play as a real catalyst in the enhancement of MgH₂ sorption.^43,44 Many works have proven that Fe is an excellent catalyst for sorption performance in MgH₂.^7–9 It is believed that the presence of fresh and fine Fe could interact with H₂ molecules, thus possibly leading to the dissociation of H₂ molecules and the improvement of the de/rehydrogenation kinetic. Furthermore, previous studies have shown that the sorption performance in MgH₂ can be enhanced with the addition of MgO. Ares-Fernández and Aguey-Zinsou⁴⁵ claimed that the addition of MgO to MgH₂ during the milling process led to an enhancement of sorption kinetics because of the high electronegativity MgO. In another study, the same group also claimed that during the milling process, MgO may act as a process control agent that can lead to the reduction of the particle agglomeration of MgH₂ by an optimal breakage rate, thus aiding the high stability of these particles and evading the use of cold welding.⁴⁶ Shan et al.¹⁵ also revealed that MgO has a great catalytic effect on the MgH₂ sorption performance. Their study showed that during the heating process in CoFe₂O₄-doped MgH₂ composite system, MgO is formed. The catalytic effect of MgO could work together with the catalytic role of the Fe metal to create a synergetic effect. Therefore, the in situ active species of Fe and MgO may actually act as real catalysts and further enhance the hydrogen sorption performance of MgH₂. However, further studies are needed to elucidate more details on the exact role of MgFe₂O₄ addition in MgH₂.

4. Conclusion

In this study, nanolayer-like-shaped MgFe₂O₄ was successfully synthesised through a rapid, simple hydrothermal method. The addition of 10 wt% as-synthesised MgFe₂O₄ to MgH₂ reduces the onset decomposition temperature and enhances sorption kinetics. The MgFe₂O₄-doped MgH₂ sample has started to release H₂ at approximately 250 °C, 90 °C and 170 °C lower than milled and pure MgH₂ respectively. In a duration of 10 min, the isothermal desorption kinetic study showed that the doped sample can release approximately 4.8 wt% H₂ at 320 °C while the milled MgH₂ only desorbed less than 1.0 wt% H₂ under the same condition. For isothermal absorption kinetics, the doped sample can absorb approximately 5.5 wt% H₂ in 10 min at 200 °C. By contrast, the milled MgH₂ sample absorbed only 4.0 wt% H₂ in the same condition. From the Kissinger analysis, the apparent activation energies, E_A, for the MgFe₂O₄-doped MgH₂ sample were calculated to be 99.9 kJ mol⁻¹, which is decreased by 33.1 kJ mol⁻¹ compared with the milled MgH₂ (133.0 kJ mol⁻¹). The XRD exploration displays the formation of new species of Fe and MgO after the dehydrogenation process, and these species remained unchanged after rehydrogenation. It is believed that the new species of Fe and MgO play a synergistic role in enhancing the hydrogen storage performances of MgH₂.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the Ministry of Higher Education of Malaysia for financial support through the Fundamental Research Grant Scheme (FRGS 59362). The authors also would like to give appreciation to Universiti Malaysia Terengganu for the good facilities to accomplish this project.

References

1. I. P. Jain, C. Lal and A. Jain, Int. J. Hydrogen Energy, 2010, 35, 5133–5144.
2. L. Ouyang, J. Tang, Y. Zhao, H. Wang, X. Yao, J. Liu, J. Zou and M. Zhu, Sci. Rep., 2015, 5, 10776.
3. M. Polanski, J. Bystrzycki and T. Plocinski, Int. J. Hydrogen Energy, 2008, 33, 1859–1867.
4. J. Huot, G. Liang, S. Boily, A. Van Neste and R. Schulz, J. Alloys Compd., 1999, 293–295, 495–500.
5. N. N. Sulaiman and M. Ismail, Dalton Trans., 2016, 45, 19380–19388.
6. M. A. Lillo-Ródenas, Z. X. Guo, K. F. Aguey-Zinsou, D. Cazorla-Amorós and A. Linares-Solano, Carbon, 2008, 46, 126–137.
7. N. Hanada, T. Ichikawa and H. Fujii, J. Phys. Chem. B, 2005, 109, 7188–7194.
8. H. Chen, H. Yu, Q. Zhang, B. Liu, P. Liu, X. Zhou, Z. Han and S. Zhou, J. Power Sources, 2016, 322, 179–186.
9. G. Liang, J. Huot, S. Boily, A. Van Neste and R. Schulz, J. Alloys Compd., 1999, 292, 247–252.
10. S. A. Shevlin and Z. X. Guo, J. Phys. Chem. C, 2013, 117, 10883–10891.
11. J. Lu, Y. J. Choi, Z. Z. Fang, H. Y. Sohn and E. Ronnebro, J. Am. Chem. Soc., 2009, 131, 15843–15852.
12. N. Patelli, M. Calizzi, A. Migliori, V. Morandi and L. Pasquini, J. Phys. Chem. C, 2017, 121, 11166–11177.
13. N. S. Mustafa and M. Ismail, J. Alloys Compd., 2017, 695, 2532–2538.
14. D. Pukazhselvan, N. Nasani, P. Correia, E. Carbó-Argibay, G. Otero-Irurueta, D. G. Stroppa and D. P. Fagg, J. Power Sources, 2017, 362, 174–183.
15. J. Shan, P. Li, Q. Wan, F. Zhai, J. Zhang, Z. Li, Z. Liu, A. A. Volinsky and X. Qu, J. Power Sources, 2014, 268, 778–786.
16. Z. Jalil, A. Rahwanto, F. Mulana and M. Mustanir, Int. J. Technol., 2016, 7, 1301–1306.
17. T. Ma, S. Isobe, Y. Wang, N. Hashimoto and S. Ohnuki, J. Phys. Chem. C, 2013, 117, 10302–10307.
18. M. S. Yahya and M. Ismail, Int. J. Hydrogen Energy, 2018, 43, 6244–6255.
19. N. N. Sulaiman, N. S. Mustafa and M. Ismail, Dalton Trans., 2016, 45, 7085–7093.
20. L. P. Ma, X. D. Kang, H. B. Dai, Y. Liang, Z. Z. Fang, P. J. Wang, P. Wang and H. M. Cheng, Acta Mater., 2009, 57, 2250–2258.
21. N. N. Sulaiman, N. Juahir, N. S. Mustafa, F. A. Halim Yap and M. Ismail, J. Energy Chem., 2016, 25, 832–839.
22. D. Wu, L. Ouyang, C. Wu, H. Wang, J. Liu, L. Sun and M. Zhu, J. Alloys Compd., 2015, 642, 180–184.
23. X. B. Yu, Y. H. Guo, H. Yang, Z. Wu, D. M. Grant and G. S. Walker, J. Phys. Chem. C, 2009, 113, 5324–5328.
24. X. B. Yu, Z. X. Yang, H. K. Liu, D. M. Grant and G. S. Walker, Int. J. Hydrogen Energy, 2010, 35, 6338–6344.
25. J. Zhang, J. Shan, P. Li, F. Zhai, Q. Wan, Z. Liu and X. Qu, J. Alloys Compd., 2015, 643, 174–180.
26. P. Li, Q. Wan, Z. Li, F. Zhai, Y. Li, L. Cui, X. Qu and A. A. Volinsky, J. Power Sources, 2013, 239, 201–206.
27. N. H. Idris, N. S. Mustafa and M. Ismail, Int. J. Hydrogen Energy, 2017, 42, 21114–21120.
28. N. Juahir, N. S. Mustafa, A. Sinin and M. Ismail, RSC Adv., 2015, 5, 60983–60989.
29. Q. Wan, P. Li, J. Shan, F. Zhai, Z. Li and X. Qu, J. Phys. Chem. C, 2015, 119, 2925–2934.
30. T. Zhang, S. Isobe, A. Jain, Y. Wang, S. Yamaguchi, H. Miyaoka, T. Ichikawa, Y. Kojima and N. Hashimoto, J. Alloys Compd., 2017, 711, 400–405.
31. T. Zhang, S. Isobe, Y. Wang, H. Oka, N. Hashimoto and S. Ohnuki, J. Mater. Chem. A, 2014, 2, 4361–4365.
32. G. Xu, N. Shen, L. Chen, Y. Chen and W. Zhang, Mater. Res. Bull., 2017, 89, 197–203.
33. N. S. Mustafa, N. N. Sulaiman and M. Ismail, RSC Adv., 2016, 6, 110004–110010.
34. M. Baricco, M. W. Rahman, S. Livraghi, A. Castellero, S. Enzo and E. Giamello, J. Alloys Compd., 2012, 536(supplement 1), S216–S221.
35. M. S. Yahya and M. Ismail, J. Energy Chem., 2018 DOI:org/10.1016/j.jechem.2017.10.020.
36. A. Pradeep, P. Priyadharsini and G. Chandrasekaran, J. Magn. Magn. Mater., 2008, 320, 2774–2779.
37. K. B. Modi, M. C. Chhantbar and H. H. Joshi, Ceram. Int., 2006, 32, 111–114.
38. Y. Yin, W. Liu, N. Huo and S. Yang, ACS Sustainable Chem. Eng., 2017, 5, 563–570.
39. Z. Yan, J. Gao, Y. Li, M. Zhang and M. Guo, RSC Adv., 2015, 5, 92778–92787.
40. Y. Liu, C. Liang, H. Zhou, M. Gao, H. Pan and Q. Wang, Chem. Commun., 2011, 47, 1740–1742.
41. H. E. Kissinger, Anal. Chem., 1957, 29, 1702–1706.
42. http://www.chemistry-reference.com/StandardThermodynamicValues.pdf.
43. Y. Cheng, W. Zhang, J. Liu, K. Cheng and Z. Zhao, Int. J. Hydrogen Energy, 2017, 42, 356–365.
44. M. Ismail, N. S. Mustafa, N. Juahir and F. A. H. Yap, Mater. Chem. Phys., 2016, 170, 77–82.
45. J.-R. Ares-Fernández and K.-F. Aguey-Zinsou, Catalysts, 2012, 2, 330.
46. K. F. Aguey-Zinsou, J. R. Ares Fernandez, T. Klassen and R. Bormann, Mater. Res. Bull., 2006, 41, 1118–1126.

---