A novel Ce_0.8Fe_0.1Zr_0.1O₂ solid solution with high catalytic activity for hydrogen storage in MgH₂

Abstract

The effect of the solid solution Ce_0.8Fe_0.1Zr_0.1O₂, successfully prepared by a hydrothermal synthesis method, on the hydrogen sorption properties of MgH₂ is systemically investigated. The Ce_0.8Fe_0.1Zr_0.1O₂-modified MgH₂ composite exhibits remarkable hydrogen kinetics properties and thermodynamics behavior compared to those of as-milled MgH₂, with a reduction in the initial desorption temperature of approximately 82 K. With respect to the hydrogen kinetics, the Ce_0.8Fe_0.1Zr_0.1O₂-added sample could uptake approximately 5.3 wt% H₂ at 473 K in 2500 s, whereas only 1.5 wt% hydrogen could be absorbed by pristine MgH₂ in the same conditions. Furthermore, about 4.5 wt% of hydrogen could be desorbed by Ce_0.8Fe_0.1Zr_0.1O₂-doped MgH₂ composite at 623 K, which was 2 wt% higher than the as-milled MgH₂ sample over the same period of time. The decomposition apparent activation energy for MgH₂–Ce_0.8Fe_0.1Zr_0.1O₂ is reduced to 84.3 kJ mol⁻¹, which is about 77 kJ mol⁻¹ lower than that of pristine MgH₂. It is believed that the notable improvement in the hydrogen sorption kinetics is due to the in situ-formed active species of CeH_2.51 and MgO as well as the abundant oxygen vacancies, which play a vital role in catalyzing the hydrogen sorption performance of MgH₂.

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1. Introduction

Hydrogen, as a green fuel, is seen as one of the most promising energy carriers due to its high energy density, cost-effectiveness, and non-pollution, which could also efficiently solve the major issues in regard to fossil fuels: urban air pollution and climate change impact.^1,2 To make full use of hydrogen energy sources and realize hydrogen economy, many factors, including production, distribution, transportation, and storage need to be considered. The key to the widespread utilization of hydrogen is to develop safe and efficient storage materials for hydrogen. It is generally accepted that solid-state hydrogen storage materials act like a sponge with the ability to absorb and release hydrogen, which make them propitious candidates for hydrogen storage. Among the solid-state hydrogen storage materials, magnesium has received much attention owing to its high hydrogen capacity, abundant availability, and low cost. However, to make MgH₂ a benign and viable hydrogen storage material, two critical barriers need to be overcome. It is well known that MgH₂ possesses superior stability and relatively high temperatures are needed to decompose the Mg–H bond to release H₂.

Numerous methods have been optimized to overcome the disadvantages of MgH₂ by ball-milling, nano-structuring, alloying, catalyst doping, etc. Among them, ball-milling various metal catalysts with MgH₂ is considered an efficient way to significantly improve the hydrogen storage properties of MgH₂. One of the well-known ergastic additives is the rare earth material ceria (Ce). A large number of experiments have been launched to study the catalytic effect of Ce-based materials. Ismail et al.³ reported that doping with CeCl₃ could significantly reduce the initial decomposition temperature and enhance the sorption kinetics of MgH₂. Some research showed that the peak temperature and the decomposition apparent activation energy for the hydrogen desorption of Mg–20Ni–CeO₂ decreased to 318.9 °C and 72.7 kJ mol⁻¹ due to the presence of CeO_2.⁴ Leng et al.⁵ systemically investigated the influence of Ce on TiFe_0.9Mn_0.1 alloy and found that the addition of Ce significantly enhanced the hydrogen storage properties of TiFe_0.9Mn_0.1 alloy on account of its high dispersion on the TiFe_0.9Mn_0.1 matrix.

Fe has also been widely proved to be an effective catalyst that can promote remarkable improvement of the hydrogen absorption and desorption performances of MgH₂.⁶ Chen et al.⁷ prepared nanosheet Fe through a wet-chemical ball milling method which contributes to the enhancement of the hydrogen storage properties of MgH₂. Wang and Yan et al.⁸ have shown the improved kinetics results of MgH₂ catalyzed by FeB/CNTs; the composites could absorb about 6.2 wt% of hydrogen at 150 °C. Song et al.⁹ prepared MgH₂ catalyzed by 10 wt% Fe₂O₃ through a mechanical milling method. They reported that 10 wt% Fe₂O₃ shows far better hydrogen/dehydrogen storage behaviors compared to pristine MgH₂.

Furthermore, recent studies have demonstrated that oxygen vacancies (O_vac) are greatly important active sites for hydrogen storage which can capture H₂ molecules and help the diffusion of H atoms during the hydrogen absorption/desorption processes.¹⁰ The enhancement of hydrogen dynamics and thermodynamics properties is mainly ascribed to the abundant oxygen vacancies, as confirmed by Zhou et al.¹¹ An abundance of oxygen vacancies can be achieved by the addition of Zr into CeO₂.^12,13 Moreover, extensive research has revealed that the density of oxygen vacancies on transition doped Ce–Zr oxides is far better than on single Ce–Zr oxides;^14,15 O_vac defects can be produced on the CeO₂ surface by Ce³⁺–Ce⁴⁺ transfer owing to the transition metal dopant.

Inspired by these studies, a Ce–Fe–Zr oxide solid solution catalyst with high activity and O_vac defects was designed and the influence of the Ce–Fe–Zr solid solution on the hydrogen storage properties of MgH₂ was systemically investigated. Moreover, the catalytic mechanism of the synthesized Ce–Fe–Zr solid solution in MgH₂ is explained, which will provide novel strategies for the design of multiple catalysts and enrich the hydrogen storage field.

2. Experimental section

2.1 Synthesis of Ce_0.8Fe_0.1Zr_0.1O₂ catalyst

Ce(NO₃)₃·6H₂O (99.9%), ZrOCl₂ (99.9%) and Fe(NO₃)₃·6H₂O (99.9%) were purchased from Aladdin Chemical Reagent Co. Ltd and used as reactants without any purification. The above reactants in a mass ratio of 8 : 1 : 1 were dissolved in a solvent composed of CTAB with NH₃·H₂O added dropwise into the mixed solution to adjust the pH to 9–11. The mixture was stirred in the reactor for 1–2 hours and then stood at room temperature for 12 hours. After standing, the solution was filtered and washed several times with deionized water, then put into an oven at 60 °C to dry for 12 h. After drying, the sample was calcined in a muffle furnace and raised to the target temperature of 550 °C at a rate of 1 °C min⁻¹. After holding at the target temperature for a period, the solid solution of cerium zirconium was obtained.

2.2 Synthesis of MgH₂–Ce_0.8Fe_0.1Zr_0.1O₂ composites

MgH₂ was prepared from commercial Mg powder (98%) through hydrogen combustion. The Mg powder was purchased from Aladdin Chemical Reagent Co. Ltd and hydrogenated at 400 °C at 4 MPa for 10 h. Pure MgH₂ was successfully obtained by repeating the above procedure five times.

The doped MgH₂–Ce_0.8Fe_0.1Zr_0.1O₂ sample was fabricated through a mechanical ball-milling method by milling MgH₂ and Ce_0.8Fe_0.1Zr_0.1O₂ in a mass ratio of 5 : 1. An increased milling temperature was inhibited by controlling the mill direction, starting in one direction for 15 min, switching to the opposite direction for 15 min and then pausing for 10 min. The ball-to-sample weight ratio was 20 : 1. To prevent the sample from contacting oxygen and vapor, the operation was conducted in an Ar-filled glove box.

2.3 Characterization

X-ray diffraction (XRD) was conducted on a SmartLab high resolution X-ray diffractometer (Rigaku) with Cu Kα radiation at 40 kV and 40 mA. The scanning speed was 4° min⁻¹ in the range of 10° to 80°. Scanning electron microscopy (SEM) was employed to observe the micro-structure and morphology of the formed sample. The element distribution and active oxygen species on the surface of the catalyst were determined by X-ray photoelectron spectroscopy (XPS).

The measurements of the initial desorption temperature and hydrogen absorption/desorption kinetics were performed on a Sieverts-type pressure-composition-temperature (PCT) apparatus (GRINM Co., China). During the initial desorption temperature test, the sample was heated from room temperature to 1000 K at 0.01 MPa with a heating rate of 10 K min⁻¹ in a vacuum chamber. For the hydrogen absorption kinetics tests, the sample was heated to 473 K and 523 K under 3.0 MPa of pressure, while for the hydrogen desorption kinetics tests, the sample was heated to 598 K and 623 K under a pressure of 0.001 MPa. The thermal behaviors of the as-milled MgH₂ and Ce_0.8Fe_0.1Zr_0.1O₂-catalyzed MgH₂ were examined by differential scanning calorimetry (Mettler Toledo, TGA/DSC 1). All the samples were heated under an Ar atmosphere from room temperature to 500 °C at heating rates of 5 K min⁻¹, 10 K min⁻¹, 15 K min⁻¹, and 20 K min⁻¹.

3. Results and discussion

3.1 Characteristics of the prepared Ce_0.8Fe_0.1Zr_0.1O₂

The XRD pattern of the synthesized Ce_0.8Fe_0.1Zr_0.1O₂ in Fig. 1 obviously shows strong peaks at 2θ = 28.86°, 33.52°, 47.78°, 56.66°, 59.63°, 70.35°, 77.56°, and 78.19° which correspond well to the standard card (PDF# 34-0394). All the strong peaks are shifted to the left compared to the standard card (PDF# 34-0394), indicating the successful synthesis of the solid solution Ce_0.8Fe_0.1Zr_0.1O₂ catalyst.

Fig. 1 XRD pattern of Ce_0.8Fe_0.1Zr_0.1O₂ catalyst.

3.2 TPD curves of MgH₂–Ce_0.8Fe_0.1Zr_0.1O₂ and MgH₂ samples

Fig. 2 depicts the temperature programmed desorption (TPD) patterns for the hydrogen desorption process of ball-milled MgH₂ and Ce_0.8Fe_0.1Zr_0.1O₂-catalyzed MgH₂ materials. The ball-milled pristine MgH₂ begins to release hydrogen at about 643 K, with a maximum hydrogen capacity of about 6.5 wt% of hydrogen reached by 800 K. Moreover, Ce_0.8Fe_0.1Zr_0.1O₂ contributes to the hydrogen desorption of MgH₂ in a relatively lower temperature range than that of pure MgH₂. An obvious feature seen in Fig. 2 is that Ce_0.8Fe_0.1Zr_0.1O₂-catalyzed MgH₂ starts to release hydrogen at a lower temperature, approximately 561 K, achieving a full hydrogen capacity of about 5.5 wt% H₂ at 690 K. Furthermore, the modified sample completed dehydrogenation at 710 K. In contrast, the saturation of the desorption process for pure MgH₂ is at 830 K. According to the TPD curves, the desorption rate and temperature determined for Ce_0.8Fe_0.1Zr_0.1O₂-catalyzed MgH₂ are superior to those of MgH₂ without the catalyst. The onset temperature reduction (82 K) on the doped sample signifies that Ce_0.8Fe_0.1Zr_0.1O₂ is a promising catalyst in the MgH₂ system which helps lower the desorption temperature and rate.

Fig. 2 TPD curves of ball-milled MgH₂ and Ce_0.8Fe_0.1Zr_0.1O₂-catalyzed MgH₂ at the heating rate of 10 K min⁻¹.

3.3 Sorption kinetics of MgH₂–Ce_0.8Fe_0.1Zr_0.1O₂ and MgH₂ samples

Further investigation of the catalytic activity of Ce_0.8Fe_0.1Zr_0.1O₂ on the hydrogen desorption properties of MgH₂ was performed by hydrogen sorption release. Fig. 3 exhibits the hydrogen desorption properties for the milled MgH₂ and the modified sample at 598 K and 623 K. It is obvious that the ball-milled MgH₂ shows sluggish desorption kinetic properties compared to those with catalyst addition at the investigated temperature. Almost no hydrogen is released by MgH₂ at the temperature of 598 K. At this temperature, the Ce_0.8Fe_0.1Zr_0.1O₂-doped MgH₂ yields a significant increase to 2.5 wt% hydrogen at 598 K within 2000 s. When the temperature shows a sharp shift to 623 K, both the hydrogen desorption properties and desorption rate are highly improved. The Ce_0.8Fe_0.1Zr_0.1O₂-added sample results in the liberation of approximately 4.5 wt% hydrogen at 623 K after 2000 s of dehydrogenation, whereas the as-milled MgH₂ sample desorbs less than 2.5 wt% hydrogen over the same period. It is evident that Ce_0.8Fe_0.1Zr_0.1O₂ plays a significant catalytic role in enhancing the desorption behavior of MgH₂.

Fig. 3 Desorption kinetics curves of ball-milled MgH₂ at 598 K (a) and 623 K (b) and Ce_0.8Fe_0.1Zr_0.1O₂ catalyzed MgH₂ at 598 K (c) and 623 K (d).

The excellent hydrogen release kinetics of MgH₂ are related to the energy barrier of the desorption process on MgH₂ and the apparent energy (E_a) is a key parameter that reflects the improved desorption kinetics. To determine the E_a for the desorption stage, the Kissinger method was used. The equation is as follows:

where α is the different rise rates (K min⁻¹), T_m is the peak temperature for the different desorption rates (K), and R is the gas constant of 8.314 J (mol K)⁻¹. Fig. 4 displays the DSC curves of MgH₂ and Ce_0.8Fe_0.1Zr_0.1O₂-catalyzed MgH₂ composite at various heating rates. It can be noted that the slopes for the undoped and doped samples are −19.39 and −10.14, respectively, and the value of the activation energy for the milled MgH₂ is estimated to be 161.2 kJ mol⁻¹, while the value of the Ce_0.8Fe_0.1Zr_0.1O₂-doped sample is 84.3 kJ mol⁻¹, much lower than that of the milled MgH₂. In addition, it should be mentioned that this E_a value is lower than or close to those of other MgH₂-catalyst composites reported recently, including o-Nb₂O₅ (101.0 ± 5 kJ mol⁻¹), BaFe₁₂O₁₉ (115 kJ mol⁻¹), MoO₃ (114.7 kJ mol⁻¹), FeCoNi@GS (85.14 kJ mol⁻¹), MgNiO₂ (108.0 kJ mol⁻¹), Ti containing phase(s) (110.9 kJ mol⁻¹) and LaFeO₃ (107.0 kJ mol⁻¹).^16–22 The value of E_a obtained from the Kissinger equation shows that the kinetics barrier during the desorption process decreased in the presence of Ce_0.8Fe_0.1Zr_0.1O₂, thus promoting the hydrogen desorption properties of MgH₂.

Fig. 4 DSC curves of MgH₂ (a) and Ce_0.8Fe_0.1Zr_0.1O₂-catalyzed MgH₂ (b) at different heating rates; (c) Kissinger's plots for milled MgH₂ and MgH₂–Ce_0.8Fe_0.1Zr_0.1O₂ composite.

To further clarify the effect of the synthesized Ce_0.8Fe_0.1Zr_0.1O₂ on the hydrogen sorption performance of pristine MgH₂, the isothermal hydrogen absorption was systematically investigated. Fig. 5 collates the findings of the isothermal rehydrogenation kinetics at 473 K and 523 K in the presence of 3.0 MPa hydrogen pressure which demonstrate that the sample of Ce_0.8Fe_0.1Zr_0.1O₂ takes in hydrogen faster than pure MgH₂ and the MgH₂–Ce_0.8Fe_0.1Zr_0.1O₂ sample has the superior hydrogen absorption kinetics rate. At 473 K, only 1.5 wt% of hydrogen could be absorbed by pristine MgH₂ in 2500 s. For the Ce_0.8Fe_0.1Zr_0.1O₂-modified MgH₂ sample, the hydrogen absorption capacity is about three times higher than that of pure MgH₂, with 5.3 wt% of hydrogen adsorbed. Raising the temperature to 598 K leads to a significant increase in hydrogen capacity as well as in the hydrogen rate compared to those presented by the other sample at the identical temperature. The ball-milled MgH₂ presents a slight increase when the temperature rises from 473 K to 523 K, absorbing about 4.4 wt% hydrogen after 2500 s at 523 K. The hydrogen capacity for the modified Ce_0.8Fe_0.1Zr_0.1O₂ sample achieves 5.6 wt% hydrogen in the same conditions. The results signify that Ce_0.8Fe_0.1Zr_0.1O₂ is a striking catalyst that endows the MgH₂–Ce_0.8Fe_0.1Zr_0.1O₂ sample with excellent hydrogen absorption behavior. Many researchers have reported a similar phenomenon where increased temperature exhibits a promising effect on the hydrogen storage capacity and hydrogen storage rate of solid-state hydrogen storage materials.²³

Fig. 5 Absorption kinetics curves of ball-milled samples at different temperatures under 3.0 MPa H₂:MgH₂ at 473 K (a) and 523 K (b) and Ce_0.8Fe_0.1Zr_0.1O₂-catalyzed MgH₂ at 473 K (c) and 523 K (d).

In order to verify the cause of the improvement of the hydrogen absorption kinetics for MgH₂ attributed to the addition of Ce_0.8Fe_0.1Zr_0.1O₂, the hydrogenation mechanism was investigated by comparing the hydrogen absorption rate curves with the rate equations for MgH₂–Ce_0.8Fe_0.1Zr_0.1O₂ and MgH₂ composites. The Avrami–Erofeev equation (eqn (2)) is usually employed to fit the hydrogenation absorption process and gives strong insight into the nucleation and growth processes.

α = 1 − exp(−kt^m) (2)

Here, α is the reacted fraction, k is the rate constant, and m is the order of the reaction. Based on the obtained results, it is evident that there exists a distinct difference between the hydrogen sorption kinetics and the fitting curves result for MgH₂. The same phenomenon does not appear in the Ce_0.8Fe_0.1Zr_0.1O₂-catalyzed MgH₂. It is obvious that for the Ce_0.8Fe_0.1Zr_0.1O₂-catalyzed MgH₂ composites, the correlation and fitting degree of both curves agree very well. The fitting lines in Fig. 6 depict that the reaction mechanism of MgH₂ catalyzed by the Ce_0.8Fe_0.1Zr_0.1O₂ composite is subject to nucleation and growth processes. The value of m gives a specific explanation of the rate-controlling step for the hydrogenation diffusion process. It is reported that an m value approaching 0.620 belongs to a one-dimensional diffusion process, whereas a value approximating 1.070 could be ascribed to a three-dimensional diffusional process; a one-dimensional diffusion process is beneficial to hydrogen adsorption.²⁴ According to the fitted curves, the m value for as-milled MgH₂ is equal to 0.91, which is close to 1.070, indicating that the hydrogen sorption process for pure MgH₂ is a three-dimensional diffusion process. For the MgH₂ catalyzed by Ce_0.8Fe_0.1Zr_0.1O₂, the value of m (0.36) approaches 0.620, which indicates one-dimensional diffusion. Therefore, the rate-controlling step for the hydrogenation process changes from a three-dimensional diffusion process to a one-dimensional diffusion process due to the addition of Ce_0.8Fe_0.1Zr_0.1O₂.

Fig. 6 Fitted hydrogenation kinetic curves of MgH₂ and Ce_0.8Fe_0.1Zr_0.1O₂-catalyzed MgH₂ composite at 523 K under the pressure of 3 MPa.

3.4 XRD analysis

To further investigate the reaction mechanism of the improved hydrogenation/dehydrogenation kinetics and thermodynamics of Ce_0.8Fe_0.1Zr_0.1O₂-catalyzed MgH₂, XRD was employed to clarify the phase components of MgH₂–Ce_0.8Fe_0.1Zr_0.1O₂ composites at different stages; the XRD patterns are presented in Fig. 7. At the ball-milled stage in Fig. 7a, the dominant diffraction peak is indexed to MgH₂ and some new peaks relating to CeH_2.51 could be detected. Additionally, a diffraction peak at 2θ = 42.5° ascribed to MgO could be found. The appearance of MgO may be due to slight oxygen contamination during XRD. The newly formed phase gives an indication that there has been a chemical reaction between Ce_0.8Fe_0.1Zr_0.1O₂ and MgH₂ during the ball-milling process. After the composites were hydrogenated at 3.0 MPa (Fig. 7b), all the diffraction peaks become sharper and narrower compared to those of the ball-milled stage and the diffraction peaks for MgH₂ and CeH_2.51 remain. In the XRD pattern of the dehydrogenated sample at 623 K (Fig. 7c), the characteristic diffraction peaks for MgH₂ vanish, while those for Mg are obvious, indicating that MgH₂ has been largely transformed to Mg during the dehydrogenation process. Fe and Zr cannot be detected in the XRD patterns, due to the small amounts or the formation of solid solution, and the obtained results correspond well to the XRD analysis. It is obvious that the diffraction peaks for CeH_2.51 and MgO remain present through the whole hydrogenation/dehydrogenation process. It has been confirmed by numerous studies that CeH_2.51 is beneficial to the enhancement of MgH₂ hydrogen uptake and release.²⁵

Fig. 7 XRD patterns of Ce_0.8Fe_0.1Zr_0.1O₂-catalyzed MgH₂ at different stages: (a) after ball-milling, (b) hydrogenation at 3.0 MPa and (c) dehydrogenation at 623 K.

3.5 XPS analysis

To explain the reason for the improvement of the hydrogen performance by Ce_0.8Fe_0.1Zr_0.1O₂-modified MgH₂, the chemical states of key elements which play vital roles in the enhancement by Ce_0.8Fe_0.1Zr_0.1O₂ are investigated. The Ce 3d and O 1s spectra for Ce_0.8Fe_0.1Zr_0.1O₂ are presented with that of pure CeO₂ as a comparison. The O 1s spectrum (Fig. 8A) can be divided into three main peaks: the one centered at 529.5 eV, which is related to lattice oxygen O_α(O²⁻), and two others centered at 530.4 and 532.8 eV, which are ascribed to surface chemically adsorbed oxygen O_β(O⁻, O₂⁻).²⁶ It is obvious that the amount of lattice oxygen O_α on the surface of Ce_0.8Fe_0.1Zr_0.1O₂ is higher than that in pure CeO₂, indicating that more oxygen vacancies exist in the Fe and Zr doped CeO₂.

Fig. 8 (A) O 1s XPS spectra of (a) CeO₂ and (b) Ce_0.8Fe_0.1Zr_0.1O₂; (B) Ce 3d XPS spectra of (a) CeO₂ and (b) Ce_0.8Fe_0.1Zr_0.1O₂.

According to Fig. 8B, the complex spectrum of Ce 3d can be decomposed into eight peaks, labeled as V (882.4 eV), V′ (885.0 eV), V′′ (889.2 eV), V′′′ (898.1 eV), U (900.9 eV), U′ (903.3 eV), U′′ (907.6 eV), and U′′′ (916.7 eV). The split peaks labeled V, V′′, V′′′, U, U′′, and U′′′ are ascribed to Ce⁴⁺ species, while the peaks located at V′ and U′ correspond to Ce³⁺ species.²⁷ The primary chemical valence state on the surface of the sample is Ce³⁺. It can be clearly seen in Fig. 8B that the surface Ce³⁺ amount is higher on Ce_0.8Fe_0.1Zr_0.1O₂ than on pure CeO₂. Huang²⁸ and Weng et al.²⁹ confirmed that the improved catalytic activity could be due to the formation of more Ce³⁺ accompanied by oxygen defects. According to the obtained results of the XPS analysis, the Fe and Zr doped CeO₂ possesses more oxygen vacancies and Ce³⁺ on the Ce_0.8Fe_0.1Zr_0.1O₂ surface.

Based on the XRD and XPS analyses, the catalytic mechanism of Ce_0.8Fe_0.1Zr_0.1O₂ that improves the hydrogenation/dehydrogenation properties of MgH₂ can be summarized. The generation of CeH_2.51 and MgO derived from the reaction between Ce_0.8Fe_0.1Zr_0.1O₂ and MgH₂ may play a vital role in altering the hydrogenation/dehydrogenation behaviors of MgH₂. It has been reported by numerous researchers that the in situ-formed light rare earth hydride CeH_2.51 could serve as the real catalyst promoting the hydrogenation/dehydrogenation procedures.^30,31 The light rare earth hydrides generate the activation of the surface of MgH₂ and lead to more nucleation sites on the substrate of MgH₂, also providing H diffusion channels and easily facilitating the hydrogen absorption and desorption processes.³² In addition, the produced MgO also brings a positive catalytic effect for the improvement of MgH₂. Ismail et al.³³ showed that MgO could work with Fe, Mg–Cu alloy as an active species to catalyze the hydrogen storage properties of MgH₂ in CuFe₂O₄-doped MgH₂ composite. MgO also played a crucial role in improving the sorption performance of the MgH₂–MgFe₂O₄ system through the synergetic catalytic effect.³⁴ Also, its high electronegativity promotes the improvement of the kinetics of magnesium-based hydrides.³⁵ Furthermore, the addition of Fe and Zr promotes more oxygen vacancies and Ce³⁺ on the surface of Ce_0.8Fe_0.1Zr_0.1O₂ and multi-valence catalysts and abundant oxygen vacancies have shown great potential in enhancing the de/hydrogenation kinetics of MgH₂.^11,36 Therefore, the great improvement in the hydrogenation kinetics is ascribed to the in situ-formed active species CeH_2.51 and MgO and the abundant oxygen vacancy defects, which may be the major factors boosting the hydrogen sorption performance of MgH₂.

4. Conclusions

The solid solution Ce_0.8Fe_0.1Zr_0.1O₂ was successfully prepared by hydrothermal synthesis method and exhibited surprisingly high catalytic activity, leading to the MgH₂–Ce_0.8Fe_0.1Zr_0.1O₂ composite reflecting excellent hydrogen sorption kinetics. The Ce_0.8Fe_0.1Zr_0.1O₂-doped MgH₂ presented striking hydrogen storage properties as it starts to liberate H₂ at 561 K in the dehydrogenation process, 82 K lower than the pristine MgH₂. With respect to hydrogen adsorption, the Ce_0.8Fe_0.1Zr_0.1O₂-added sample could uptake approximately 5.3 wt% H₂ at 473 K, while only 1.5 wt% hydrogen could be absorbed by pristine MgH₂ in the same conditions. The dehydrogenation properties of MgH₂ are also improved, with a desorption amount of 4.5 wt% hydrogen at 623 K, where the as-milled MgH₂ sample desorbs less than 2.5 wt% hydrogen over the same period of time. The Kissinger plot shows that the apparent activation energy for MgH₂ is reduced from 161.2 kJ mol⁻¹ to 84.3 kJ mol⁻¹ due to the presence of the solid solution Ce_0.8Fe_0.1Zr_0.1O₂-modified sample. It is believed that the great improvement in hydrogen sorption kinetics is due to the in situ-formed active species of CeH_2.51 and MgO and the abundant oxygen vacancies, which play vital roles in boosting the hydrogen sorption performance of MgH₂.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was funded by Natural Science Foundation of Hebei Province of China (E2019415036; B2021415002); Science and Technology Project of Hebei Education Department (BJ2020043; QN2020142); Hebei University of Environmental Engineering (Top-notch Talents Cultivation Program for Young Science and Technology 2020ZRBJ01); Doctoral Foundation of Hebei University of Environmental Engineering (201805).

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