Edited by: Yongfeng Liu, Zhejiang University, China
Reviewed by: Yao Zhang, Southeast University, China; Rapee Utke, Suranaree University of Technology, Thailand
This article was submitted to Inorganic Chemistry, a section of the journal Frontiers in Chemistry
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
A series of CeH2.73/CeO2 composites with different ratios of hydride and oxide phases are prepared from the pure cerium hydride via oxidation treatments in the air at room temperature, and they are subsequently doped into Mg2NiH4 by ball milling. The desorption properties of the as-prepared Mg2NiH4+CeH2.73/CeO2 composites are studied by thermogravimetry and differential scanning calorimetery. Microstructures are studied by scanning electron microscopy and transmission electron microscopy, and the phase transitions during dehydrogenation are analyzed through
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,
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,
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.,
In order to clarify the effect of cerium hydride/oxide composites on the dehydrogenation properties of Mg2NiH4, in the present study, a series of CeH2.73/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
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.,
The phase structures were analyzed by X-ray diffraction (XRD), and the phase transition during dehydrogenation was analyzed by
Six sets of CeH2.73/CeO2 composites prepared by oxidation treatments for different durations were analyzed by XRD. The CeH2.73/CeO2 composites are marked as S1, S2, S3, S4, S5, and S6, according to the oxidation duration of CeH2.73 of 0, 0.5, 1.5, 3, 10, and 60 min, respectively. The diffraction patterns are shown in
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 CeH2.73 at room temperature is a three-dimensional interface reaction process (Lin et al.,
XRD images of six groups of CeH2.73/CeO2 composite catalysts obtained by oxidation of CeH2.73 for different durations:
Relative content of the CeO2 composites phase as increase of oxidation time.
In order to understand the morphologies of the CeH2.73/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 CeH2.73/CeO2 composite (S5 sample) and the ball-milled Mg2NiH4 + S5 sample are shown in
BSEM image of
The decomposition behaviors of the ball-milled Mg2NiH4+CeH2.73/CeO2 composites are studied by TG-DSC synchronous thermal analyzer as shown in
To further elucidate the dehydrogenation activation energy of the Mg2NiH4-CeH2.73/CeO2 composites, DSC experiments for the Mg2NiH4 + CeH2.73/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,
where
Kissinger's Plot of Mg2NiH4-CeH2.73/CeO2 composite system.
Initial temperature, peak temperature and activation energy of dehydrogenation for the ball-milled Mg2NiH4 + CeH2.73/CeO2 composites (20 K/min).
Mg2NiH4 | 284 | 320 | 76.3 |
Mg2NiH4 + S1 | 283 | 317 | 82.4 |
Mg2NiH4+ S2 | 283 | 323 | 77.8 |
Mg2NiH4+ S3 | 276 | 315 | 66.9 |
Mg2NiH4+ S4 | 267 | 308 | 62.6 |
Mg2NiH4+ S5 | 276 | 317 | 75.7 |
Mg2NiH4+ S6 | 275 | 314 | 82.3 |
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 CeH2.73/CeO2 composites on the dehydrogenation properties of MgH2 (Lin et al.,
The morphology and microstructure of Mg2NiH4 and as-oxidized 10 min CeH2.73/CeO2 composite (S5) are shown in
The phase transition of the Mg2NiH4 + S5 sample during dehydrogenation was further studied by
The
In summary, CeH2.73/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 CeH2.73/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 CeH2.73/CeO2 composite with the same molar ratio is a good catalyst for reducing dehydrogenation temperatures of Mg-based materials.
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation, to any qualified researcher.
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.
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.
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.