Metal carbonates-induced solution-free dehydrogenation of alkaline earth metal hydrides at room temperature

https://doi.org/10.1016/j.jssc.2020.121485Get rights and content

Highlights

  • Metal carbonates can induce the dehydrogenation of MgH2 and CaH2 at room temperature.

  • Hydrogen yield depends on the nature of metal carbonate and duration of ball milling.

  • Hydrogen yield also depends on rate of ball milling and reaction mixture composition.

  • For MgCO3-added MgH2 and CaH2, hydrogen yields are achieved as high as 94% and 90%.

  • The work opens a new way to achieve the dehydrogenation of MgH2 and CaH2.

Abstract

We report an efficient, convenient, solution-free method to achieve the room-temperature dehydrogenation of alkaline earth metal hydrides (MgH2 and CaH2). It is found, for the first time, that metal carbonates, which are readily available and cheap, can efficiently induce the hydrogen desorption of alkaline earth metal hydrides at room temperature under mechanical ball milling conditions. We demonstrate that the hydrogen yield and purity mainly depend on the nature of metal carbonates, rate and duration of ball milling, and reaction mixture composition. The best performances are obtained with the systems of MgCO3-MgH2 and MgCO3-CaH2 milled at 350 ​rpm for 12 ​h, with the COx-free hydrogen yield of 94% and 90%. This work opens a new way to achieve the efficient dehydrogenation of alkaline earth metal hydrides at room temperature.

Graphical abstract

It is found, for the first time, that metal carbonates can efficiently induce the hydrogen desorption of alkaline earth metal hydrides (MgH2 and CaH2) at room temperature under mechanical ball milling condition.

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Introduction

Hydrogen has been considered to be a promising energy source for the future. In order to achieve its use for vehicles, personal electronics, and other portable power applications, various efficient methods for production, storage and use of hydrogen should be developed [1,2]. Highly efficient and convenient hydrogen desorption technology is still the main challenge for today’s development of hydrogen economy [3,4]. On the one hand, achieving room-temperature hydrogen desorption will reduce production costs and unnecessary energy consumption. On the other hand, the hydrogen generating at room temperature can be directly applied to hand-portable proton exchange membrane fuel cells (PEMFC).

Alkaline earth metal hydrides, especially for magnesium hydride (MgH2) and calcium hydride (CaH2), have attracted worldwide attention recently because of the potential to release hydrogen with a high theoretical capacity of H2 and low cost [[5], [6], [7], [8]]. The dehydrogenation temperature of pure MgH2 or CaH2 is above 300 ​°C, which means that the hydrogen from the heated MgH2 or CaH2 can not be directly used by PEMFC [[9], [10], [11]]. Most efforts have been made to achieve the hydrogen desorption of MgH2 or CaH2 at room temperature by hydrolysis [[12], [13], [14], [15], [16], [17]]. Although the room-temperature hydrogen desorption of alkaline earth metal hydrides by hydrolysis is convenient, some disadvantages of this method are found. The byproduct of water-insoluble hydroxides upon hydrolysis usually results in poor hydrogen generation rate and incomplete hydrolysis. The hydrogen yield can be significantly improved when the hydride reacts with the acid solutions, but equipment will be corroded by an acidic solution, which is not beneficial for the practical application. At the same time, the dehydrogenation from hydride hydrolysis lacks control of the rate of hydrogen generation, which causes the inability to meet the requirements of partial applications [18,19]. Therefore, there is a necessity for the development of alternative methods by which efficient, convenient and solution-free hydrogen desorption of alkaline earth metal hydrides can be achieved at room temperature.

As far as we know, only three efforts have achieved the hydrogen desorption of alkaline earth metal hydrides at room temperature by solution-free methods (nano miniaturization, or mechanical deformation) [[20], [21], [22]]. Roman Nevshupa et al. reported that dehydrogenation of MgH2 could be achieved by mechanical deformation of MgH2 pellets in vacuum at room temperature [20]. Ares et al. found that Pd-added MgH2 films desorb hydrogen under air exposure because of thermodynamic reasons [21]. Qu et al. demonstrated that sandwich-like Pd–Mg–Pd films prepared by magnetron sputtering could absorb hydrogen entirely and dehydrogenate rapidly in the air at room temperature [22]. However, a complicated process is needed and the hydrogen yield is not satisfying for the three solution-free room-temperature hydrogen desorption techniques.

Recently, the reactions of CO2 with metal hydrides have been extensively researched [[23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]]. It was found that CO2 can improve the dehydrogenation properties of some hydrides [[31], [32], [33]]. Xiong et al. found the hydrogen desorption kinetics of BH3NH3 can be enhanced by the interaction with CO2. Our previous investigations prove that metal hydrides can react with CO2 producing H2 at room temperature. CO2 presents on earth in all three aggregate states (in the gas state, in solution, or in solid state): in gas-state in the air, in the dissolved form in the water, and in the solid phase stabilized in carbonate rocks [34,35]. CO2 and carbonates (CO32−) have similar redox properties. Metal carbonates are cheap, readily available and ample [[29], [30], [31]]. Therefore, it is interesting to examine whether carbonates can enhance the dehydrogenation properties of metal hydrides.

In this paper, we report an efficient, convenient, solution-free method to achieve the room-temperature dehydrogenation of alkaline earth metal hydrides (MgH2 and CaH2). It is found, for the first time, that metal carbonates can efficiently induce the hydrogen desorption of alkaline earth metal hydrides at room temperature under mechanical ball milling conditions. The influences of metal carbonate additives and ball milling conditions on the hydrogen desorption performance of alkaline earth metal hydrides are discussed in detail. The best performances are obtained with the systems of MgCO3-MgH2 and MgCO3-CaH2 milled at 350 ​rpm for 12 ​h, with the COx-free hydrogen yield of 94% and 90%.

Section snippets

The hydrogen desorption reaction of the metal carbonate with alkaline earth metal hydrides

Experiments were performed using commercial anhydrous metal carbonates and hydrides. MgCO3 (purity 98%), CaCO3 (purity 99.95%), Li2CO3 (purity 99%), Na2CO3 (purity 99.8%), and CaH2 (purity 98%) were purchased from J&K Chemical Ltd., China. MgH2 (purity 98%) was purchased from Langfang serribeda technology co. Ltd., China. Mechanochemical hydrogen desorption reactions between carbonates and alkaline earth metal hydrides were performed at room temperature as follows. For the mechanochemical

Mechanochemical hydrogen desorption reaction of carbonates with alkaline earth metal hydrides

To elucidate the dependence of gaseous product composition on ball milling rate, MgH2 and MgCO3 with a mole ratio of 2 ​mol/mol were ball milled at 200, 350, and 450 ​rpm for 12 ​h. Fig. 1a depicts the chromatograms acquired for the gas products. The response of the TCD to hydrogen (0.3 ​min) is apparent in the figure; the CO and CO2 peaks are not visible in the chromatogram. As can be seen from Fig. 1a, the peak area of hydrogen increases with the increase of ball milling speed from 200 to

Conclusions

It is found, for the first time, that metal carbonates, which are easily available and cheap, can efficiently induce the hydrogen desorption of alkaline earth metal hydrides (MgH2 and CaH2) at room temperature under mechanical ball milling condition. The hydrogen yield and purity mainly depend on the nature of metal carbonates, rate and duration of ball milling, and reaction mixture composition. Among all investigated carbonate-alkaline earth metal hydride systems, the MgCO3-MgH2 and MgCO3-CaH2

CRediT authorship contribution statement

Song Zhang: Methodology, Software, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization. Lu Wang: Investigation, Writing - original draft, Validation. Yun-Long Tai: Data curation, Formal analysis. Juan Zhao: Formal analysis. Wei Zhu: Software. Bao-Xia Dong: Writing - original draft, Writing - review & editing, Funding acquisition.

Declaration of competing interest

There are no conflicts of interest to declare. All the authors do not have any possible conflicts of interest.

Acknowledgments

This work is financially supported by the NNSF of China (Nos. 21671169 and 21573192), SRF of SEM for ROCS, RFDP (No. 20133250120008), Six Talent Peaks Project in Jiangsu Province (No. 2015-XNY-011), the Foundation from the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the High-Level Personnel Support Program of Yang-Zhou University.

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