Effect of CeH_2.73-CeO₂ Composites on the Desorption Properties of Mg₂NiH₄

Introduction

With the advantages of abundant natural resources and no pollution to the environment, hydrogen has been widely considered as an ideal carbon-free energy carrier. Hydrogen energy storage technology is a prerequisite for the large-scale utilization of hydrogen energy. Light-weight solid-state hydrogen storage materials have been considered to be ideal candidates for hydrogen storage because of the high hydrogen storage density and security consideration (Mohtadi and Orimo, 2016; Wang et al., 2016). Among the existing solid-state hydrogen storage materials, Mg-based hydrogen storage materials are widely studied and considered as promising solid-state hydrogen storage materials due to the high hydrogen storage capacity, abundance on the earth and low production cost (Ouyang et al., 2013, 2017; Rusman and Dahari, 2016; Shao et al., 2018).

MgH₂ and Mg₂NiH₄ are two typical Mg-based hydrogen storage materials with hydrogen densities of 7.6 wt and 3.6 wt%, respectively (Reilly and Wiswall, 1968; Bogdanović, 1984; Liu et al., 2016, 2019; Zhan et al., 2016; Chen et al., 2018; Ding et al., 2019). Nevertheless, the stable thermodynamics and slow kinetics of hydrogen storage properties of Mg-based materials lead to harsh conditions for dehydrogenation (Bogdanović et al., 2010; Jain et al., 2010). The hydrogen desorption temperatures of MgH₂ and Mg₂NiH₄ are usually as high as 250–350C. Many efforts have been made by researchers to improve the kinetics and reduce the desorption temperatures of MgH₂ and Mg₂NiH₄. The commonly conducted methods include mechanical alloying, doping catalysts, nanostructuring, surface modification and so on (Terashita et al., 1999; Lin et al., 2012a, 2016; Shao et al., 2012; Eleskandarany et al., 2016; Zhang et al., 2017; Khan et al., 2018; Xu et al., 2019).

Doping catalysts by ball milling is a commonly-used and efficient method to enhance the hydrogenation/dehydrogenation kinetics of Mg-based materials (Ouyang et al., 2014a,b; Wang and Wang, 2017). To date, many additives have been explored, including oxides (Barkhordarian et al., 2003), transition metals (Hanada et al., 2005), hydrides (Ma et al., 2017), carbon-based materials (Lototskyy et al., 2013) and so on. It has been demonstrated that compounds with higher valences show higher catalytic effect on the hydrogen storage performances of Mg-based materials than that of the lower valence compounds (Bobet et al., 2002). Because of the unique 4f electron of the Ce element, Ce-based compounds are widely used in the catalysis field (Trovarelli, 1996). Long et al. (2013) reported Mg-Ce oxide powders produced by an arc plasma evaporation method. As a result, enthalpies of hydrogenation and dehydrogenation for Mg/MgH₂ reduce to −71.0 and 75.4 kJ/mol H₂, respectively. Moreover, the hydrogenation activation energy is reduced to only 47.75 kJ/mol. The composite can absorb 4.07 wt%-H at 323 K in 10 h. Their study indicates that minor addition of Ce oxide can remarkably improve the hydrogenation kinetics of Mg/MgH₂. We previously reported that CeF₄ was an efficient catalyst to enhance the hydrogen storage properties of MgH₂ (Lin et al., 2015), which can lead to reduced dehydrogenation temperature and activation energy because of the formation of new Mg-Ce-F species on the surface of MgH₂. Gulicovski (Gulicovski et al., 2012) et al. prepared MgH₂-CeO₂ composite by ball milling of MgH₂ and nano-CeO₂ particles. The dehydrogenation activation energy is reduced to 60 ± 10 kJ/mol, indicating that the activation energy is sufficiently decreased by the catalytic effect of vacant CeO₂ particles. We also developed a new symbiotic CeH_2.73/CeO₂ nano-catalyst, which was in-situ produced by controlling hydrogenation and oxidation treatments upon the amorphous Mg-Ce-Ni alloys (Lin et al., 2014), leading to significantly improved dehydrogenation performance of MgH₂-based composite. Moreover, in situ TEM and DFT study show that the remarkable catalysis effect is attributed to the spontaneous hydrogen release effect at the interface between cerium oxide and hydride. Because the composite contains only major MgH₂ but also minor Mg₂NiH₄, the effect of CeH_2.73/CeO₂ composite on the dehydrogenation properties of Mg₂NiH₄ has not been well-understood.

In order to clarify the effect of cerium hydride/oxide composites on the dehydrogenation properties of Mg₂NiH₄, in the present study, a series of CeH_2.73/CeO₂ composites with different ratios of cerium oxide and hydride were synthesized from pure cerium hydride via controlled oxidation treatments in the air at room temperature, and then they were doped into Mg₂NiH₄ by ball milling. The dehydrogenation properties of cerium hydride and cerium oxide-doped Mg₂NiH₄ were studied, and the initial dehydrogenation temperature and activation energy were characterized by TG-DSC and a Kissinger's method. Moreover, the phase transitions during dehydrogenation were analyzed through in situ XRD experiments.

Experimental Details

Materials

The Mg₂NiH₄ used in this experiment was prepared by a method of hydrogenation combustion synthesis method (HCS), which was carried out in Prof. Yunfeng Zhu's group in Nanjing Tech University (Gu et al., 2009; Zhu et al., 2017). CeH_2.73 (purity >99%) was purchased from Hunan Research Institute of Non-ferrous metals. About 0.1 g CeH_2.73 powder was located in a plastic bottle of 3 ml, in the Ar glove-box. Then it was transferred into the small side box of glove-box. The door of side box was then open to let air in for different time: 0.5, 1.5, 3, 10, and 60 min. The oxidized CeH_2.73 became CeH_2.73/CeO₂ composites, and 2 mol% of CeH_2.73/CeO₂ composites were then doped into Mg₂NiH₄ by ball milling at 400 rpm for 4 h.

Characterizations

The phase structures were analyzed by X-ray diffraction (XRD), and the phase transition during dehydrogenation was analyzed by in situ XRD analysis. The XRD analysis was carried out by an X-ray diffractometer apparatus (UItima IV, Rigaku, Japan) with Cu-K α (λ = 0.15405 nm). The tube voltage and tube current were 40 kV and 40 mA, respectively. Thermogravimetry (TG) and Differential Scanning Calorimetery (DSC) were used to study the hydrogen desorption behaviors at different heating rates on a METTLER TGA/DSC 3⁺ synchronous thermal analyzer. The microstructures of the Mg₂NiH₄-Ce_2.73/CeO₂ composites were observed by backscattered electron imaging using scanning electron microscope (SEM) and transition electronic microscopy (TEM).

Results and Discussion

Preparation of the CeH_2.73/CeO₂ Catalysts

Six sets of CeH_2.73/CeO₂ composites prepared by oxidation treatments for different durations were analyzed by XRD. The CeH_2.73/CeO₂ composites are marked as S1, S2, S3, S4, S5, and S6, according to the oxidation duration of CeH_2.73 of 0, 0.5, 1.5, 3, 10, and 60 min, respectively. The diffraction patterns are shown in Figure 1. With the increase of oxidation time, CeH_2.73 gradually transforms into CeO₂. After oxidation of 60 min, no CeH_2.73 has left. The obtained XRD patterns were refined by a Jade 6.0 software and the phase contents were calculated by the K value (RIR) method. The relative contents of the CeO₂ phase as increase of the oxidation time are shown in Figure 2. The oxidation data can be fitted by the Avrami–Erofeev equation deduced from the nucleation and growth process:

where α is the ratio of reacted material to total material, m and B are constants. The fitted m is 1.085, which is very close to 1.07, indicating the oxidation of CeH_2.73 at room temperature is a three-dimensional interface reaction process (Lin et al., 2012b). For the S4 sample, the relative content of CeH_2.73 is 54.2 wt%, and the relative content of CeO₂ phase is 45.8 wt%, indicating almost the same molar ratio of cerium hydride and oxide when the oxidation time is around 3 min. After the oxidation treatments, the samples were doped into Mg₂NiH₄ by ball milling.

Figure 1. XRD images of six groups of CeH_2.73/CeO₂ composite catalysts obtained by oxidation of CeH_2.73 for different durations: (A) 0 min, S1, (B) 0.5 min, S2, (C) 1.5 min, S3, (D) 3 min, S4, (E) 10 min, S5, (F) 60 min, S6.

Figure 2. Relative content of the CeO₂ composites phase as increase of oxidation time.

In order to understand the morphologies of the CeH_2.73/CeO₂ composite doped Mg₂NiH₄, back scattering scanning electron microscopy (BSEM) was carried out. Because the S1–S4 sample will continue to oxidize in the air, the S5 doped Mg₂NiH₄ sample was selected as the experimental material. The morphologies of the as-prepared CeH_2.73/CeO₂ composite (S5 sample) and the ball-milled Mg₂NiH₄ + S5 sample are shown in Figure 3. It could be clearly seen that the particle size of the as-prepared CeH_2.73/CeO₂ composite is about 200–400 nm. After ball milling, the particle size of the CeH_2.73/CeO₂ composite is greatly reduced to below 30–100 nm. Figure 3B shows that the CeH_2.73/CeO₂ composites, which are brighter particles, are uniformly distributed in the Mg₂NiH₄ matrix. The homogeneous CeH_2.73/CeO₂ composites could be beneficial for catalyzing hydrogen storage properties for the Mg-based materials (Hong et al., 2009; Shao et al., 2011; Li et al., 2018).

Figure 3. BSEM image of (A) the as-oxidized CeH_2.73/CeO₂ composite (S5) and (B) the ball-milled Mg₂NiH₄ + S5 sample.

Dehydrogenation of Mg₂NiH₄ + CeH_2.73/CeO₂ Composites

The decomposition behaviors of the ball-milled Mg₂NiH₄+CeH_2.73/CeO₂ composites are studied by TG-DSC synchronous thermal analyzer as shown in Figure 4. The DSC study of Mg₂NiH₄ + CeH_2.73/CeO₂ composites are at a heating rate of 20 K/min, showing the dehydrogenation initial temperature of Mg₂NiH₄ decreases after addition of CeH_2.73/CeO₂ composites. Among the six sets of catalysts, the S4 sample obtained by oxidation of CeH_2.73 for 3 min, exhibits the highest catalytic effect on reducing the dehydrogenation temperature of Mg₂NiH₄. The dehydrogenation temperature is decreased to 267°C, which is about 17°C lower than the that of as-milled Mg₂NiH₄ (284°C).

Figure 4. (A) DSC and (B) TG curves of the ball-milled Mg₂NiH₄-CeH_2.73/CeO₂ composites materials, at a heating rate of 20 K/min.

To further elucidate the dehydrogenation activation energy of the Mg₂NiH₄-CeH_2.73/CeO₂ composites, DSC experiments for the Mg₂NiH₄ + CeH_2.73/CeO₂ composites at heating rates of 10 K/min and 50 K/min were carried out (results not shown). The activation energy of the dehydrogenation process was calculated by using the Kissinger's method (Kissinger, 1957),

where E_des is the dehydrogenation activation energy, T_m, β, R, and C are the peak temperature, heating rate of DSC experiments, gas constant and another constant, respectively. The relation between T_m and β is linearly fitted as plotted in Figure 5, and the activation energy was summarized in Table 1. Results show that the S4 sample by oxidation of CeH_2.73 for 3 min exhibits the best catalytic effect on reducing the dehydrogenation activation energy of Mg₂NiH₄ (from 76.3 kJ/mol to 62.6 kJ/mol). Zhang (Zhang et al., 2018) et al. suggested that alloying Mg₂Ni with Ti, V, Fe, or Si could reduce the hydrogenation and dehydrogenation activation energies to 60–70 kJ/mol. Our study suggests that the CeH_2.73/CeO₂ composites are also efficient to reduce the dehydrogenation activation energy of Mg₂NiH₄.

Figure 5. Kissinger's Plot of Mg₂NiH₄-CeH_2.73/CeO₂ composite system.

Table 1. Initial temperature, peak temperature and activation energy of dehydrogenation for the ball-milled Mg₂NiH₄ + CeH_2.73/CeO₂ composites (20 K/min).

Combining XRD and TG-DSC study, it indicates clearly that the initial dehydrogenation temperature and activation energy of Mg₂NiH₄ are maximally reduced as the cerium hydride and cerium oxide are in the same amount. These results well-accord with our previous finding on the effect of CeH_2.73/CeO₂ composites on the dehydrogenation properties of MgH₂ (Lin et al., 2014), which means the CeH_2.73/CeO₂ composite with the same molar ratio of cerium hydride and oxide could be a good catalyst for the dehydrogenation properties of Mg-based hydrogen storage materials, including both MgH₂ and Mg₂NiH₄.

Microstructures of the Mg₂NiH₄ + CeH_2.73/CeO₂ Composite

The morphology and microstructure of Mg₂NiH₄ and as-oxidized 10 min CeH_2.73/CeO₂ composite (S5) are shown in Figure 6a. From the HRTEM images in Figure 6b, it could be found that the composite matrix is Mg₂NiH₄, which lattice fringe corresponds to the lattice plane of (111) and (220). Particle sizes of the CeO₂ nanoparticles are in a large range from 50–200 to 5–10 nm, which have been clearly shown in Figures 6a,d, respectively. Moreover, it could be seen that the in-situ generated CeO₂ particles are embedded homogeneously in the Mg₂NiH₄ matrix after ball milling. The interfaces between Mg₂NiH₄ and CeO₂ composite, as shown in Figures 6c,d, are contacted closely with each other. The homogeneously distributed catalysts can lead to high catalysis effect, and thus be beneficial for the hydrogenation/dehydrogenation properties of Mg₂NiH₄. Because of the too small amount of CeH_2.73, we could not clarify their microstructures in the TEM study.

Figure 6. (a) TEM and (b–d) HRTEM images of the Mg₂NiH₄ + S5 composite. (b) shows the Mg₂NiH₄ matrix, (c) shows the magnified interface between Mg₂NiH₄ and CeO₂, (d) shows a CeO₂ particle with size of 5–10 nm.

In situ XRD of Dehydrogenation

The phase transition of the Mg₂NiH₄ + S5 sample during dehydrogenation was further studied by in situ XRD. The obtained in situ XRD patterns are shown in Figure 7. The XRD pattern of Mg₂NiH₄ + S5 sample at room temperature contains the diffraction peaks of Mg₂NiH₄ and CeO₂ because the XRD intensity of CeH_2.73 is too weak. The diffraction peaks of CeO₂ do not obviously change during the whole dehydrogenation process, indicating that the CeO₂ phase might act as a catalyst of Mg₂NiH₄ dehydrogenation. When the temperature was raised to 250°C, the diffraction peak of Mg₂NiH_0.3/Mg₂Ni phase appears, which means the dehydrogenation of Mg₂NiH₄ starts. As temperature increases, the intensity of Mg₂NiH_0.3/Mg₂Ni phase gradually enhances, and that of the Mg₂NiH₄ peaks decreases. As temperature reaches about 330°C, the diffraction peaks of CeO₂ remains unchanged and no MgO is found.

Figure 7. In situ XRD patterns of dehydrogenation of the ball-milled Mg₂NiH₄ + S5.

The in situ XRD patterns are then refined using Jade 6.0, and the relative contents of phases are calculated by the RIR method. At 270°C, the relative content of Mg₂NiH_0.3 phase is 47.1 wt%, and the relative content of Mg₂NiH₄ phase is 52.9 wt%. After heating to 290°C, the relative content of the two phases changes rapidly, and the relative content of Mg₂NiH_0.3 phase increases to 81.8 wt% and Mg₂NiH₄ The relative content of the phase was reduced to 18.2 wt%. Subsequently, the rate of phase change during the heating process was significantly slowed down. The relative content of Mg₂NiH_0.3 phase was 89.1 wt% at 400°C, and the relative content of Mg₂NiH₄ phase reduces to about 10.9 wt%.

Conclusion

In summary, CeH_2.73/CeO₂ composites with different proportions of cerium hydride and oxide are synthesized from pure cerium hydride via oxidation treatments in the air at room temperature. Oxidation time of 3 min leads to formation of CeH_2.73/CeO₂ composite with the same molar ratio of cerium hydride and oxide, which maximally reduces the initial dehydrogenation temperature and activation energy of Mg₂NiH₄. The CeH_2.73/CeO₂ composite with the same molar ratio is a good catalyst for reducing dehydrogenation temperatures of Mg-based materials.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation, to any qualified researcher.

Author Contributions

KW and DC: methodology and writing - original draft. KS and TX: methodology and formal analysis. PZ: resources and supervision. WL: supervision and project administration. H-JL: conceptualization, resources, writing - review and editing, supervision, and funding acquisition.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We thank Prof. Yunfeng Zhu for providing the Mg₂NiH₄. Financial supports from National Natural Science Foundation of China (No. 51601090), Guangdong Basic and Applied Basic Research Foundation (No. 2019A1515011985) and the Fundamental Research Funds for the Central Universities (No. 21619415) are appreciated.

References