Hydrogen desorption/absorption properties of Li–Ca–N–H system
Introduction
The hydrogen storage technologies for realizing the hydrogen energy society in near future are currently under development all over the world. Particularly, hydrogen storage materials composed of light elements have been considered as one of the suitable candidates for on-board hydrogen tank application since its gravimetric hydrogen density can be expected to be high. For example, carbon based materials [1], lithium borohydride [2], complex aluminum hydrides [3], imide–amide systems and so on, have attracted much attention in these years.
Recently, metal–N–H systems have been paid considerable attention as a new family of hydrogen storage materials since the Chen's report in 2002 that lithium nitride (Li3N) reversibly absorbs/desorbs a large amount of hydrogen (∼9.3 mass%), where the reaction is expressed by the following two-step reactions [4]:Li3N + 2H2 ↔ Li2NH + LiH + H2 ↔ LiNH2 + 2LiHThe large standard enthalpy change (ΔH ∼ 148 kJ/mol H2 [5]) of the first reaction indicates that a very high temperature over 430 °C needs for completing the hydrogen desorption from Li2NH (lithium imide) to Li3N [6]. However, the second step reaction has much lower ΔH ∼ 45 kJ/mol H2 and still a large amount of hydrogen capacity (6.5 mass%). Therefore, the second reaction can be suitable for on-board uses. In fact, Ichikawa et al. [7] have examined the hydrogen storage properties of the ball-milled mixture of lithium hydride (LiH) and lithium amide (LiNH2) with a small amount of titanium chloride (TiCl3) as a catalyst, and the results obtained showed that a large amount of hydrogen (5.5–6.0 mass%) is reversibly desorbed/absorbed at 150–250 °C according to the following reaction:LiNH2 + LiH ↔ Li2NH + H2Furthermore, they have experimentally demonstrated that the hydrogen desorption reaction (2) proceeds by the following two elementary reactions mediated by ammonia (NH3) [5]:2LiNH2 → Li2NH + NH3andLiH+NH3 → LiNH2 + H2Here, it is to be noted that the existence of the above two reactions has already been described in the previous reports [8], [9].
The above reaction (4) suggests that it is possible to synthesize LiNH2 by ball-milling LiH under a gaseous NH3 atmosphere at room temperature [5]. Indeed, since the reaction (4) is an exothermic [5] and ultra-fast [8] reaction, the LiNH2 product can be easily synthesized by the above reactive ball-milling method. Furthermore, other alkali or alkaline earth metal amides M(NH2)x (such as NaNH2, Mg(NH2)2 and Ca(NH2)2) were also synthesized by ball-milling their hydrides MHx (such as NaH, MgH2 and CaH2) under the NH3 atmosphere. Their decomposition properties were investigated in detail [10], [11], [12]. Those properties were useful for designing new metal–N–H systems for the hydrogen storage, because LiNH2 or LiH in reaction (2) can be replaced with other metal hydrides or amides, respectively. Actually, Leng et al. have reported the ball-milled mixture of magnesium amide (Mg(NH2)2) and LiH with the 3:8 molar ratio as a novel H-storage system. In this composite system, a large amount of H2 (>6 mass%) was reversibly absorbed/desorbed at 150–200 °C [13].
In addition, Xiong et al. [14] have reported that the mixture of LiNH2 and calcium hydride (CaH2) with the 2:1 molar ratio desorbs hydrogen from 70 °C and takes a hydrogen desorption peak at 206 °C at the heating rate of 2 °C/min in the temperature-programmed desorption (TPD) profile. Furthermore, Hino et al. [12] have examined the thermal desorption mass spectra (TDMS) of the calcium amide (Ca(NH2)2) in details and clarified that Ca(NH2)2 decomposes into the calcium imide (CaNH) and emits NH3 at lower temperatures than any of LiNH2 and Mg(NH2)2 decompositions.
In this paper, we examined the hydrogen storage properties of Li–Ca–N–H systems by analogy with the Li–N–H and Li–Mg–N–H systems. For that, two kinds of the ball-milled mixtures of Ca(NH2)2 and 2LiH (Ca(NH2)2–2LiH), and CaH2 and 2LiNH2 (CaH2–2LiNH2) were prepared and examined the hydrogen desorption and absorption characteristics.
Section snippets
Experimental details
The starting materials, LiH (95 mass% purity), CaH2 (99.99 mass% purity) and LiNH2 (95 mass% purity) were purchased from Sigma–Aldrich Co. In this work, Ca(NH2)2 was mechanochemically synthesized by ball-milling CaH2 in a pure NH3 atmosphere of 0.6 MPa [10]. The mechanical milling was performed for 10 h at room temperature by using a planetary ball-mill apparatus (Fritsch P5) at 250 rpm. Each mixture of Ca(NH2)2–2LiH and CaH2–2LiNH2 without any catalyst was mechanically ball-milled under 1 MPa argon
Results and discussion
As described in Section 1, we found that the hydrogen desorption reaction between LiNH2 and LiH was controlled by the two-step elementary reactions mediated by NH3. Similarly, it can be expected that the reaction between Ca(NH2)2 and LiH, or CaH2 and LiNH2 proceeds by a molecule-solid reaction mediated NH3 as well.
Then, both the mixtures of Ca(NH2)2 + 2LiH, and CaH2 + 2LiNH2 were mechanically ball-milled under an Ar atmosphere of 1 MPa for 2 h and their hydrogen desorption properties were examined.
Summary
The H-storage properties for both the ball-milled composites of Ca(NH2)2 and 2LiH, and 2LiNH2 and CaH2 were examined. The results obtained are summarized as follows:
- (1)
Both the dehydrogenated states were confirmed to form the mixture of Li2NH and CaNH phases after dehydrogenation at 200 °C under vacuum for 8 h, which were transformed into an “unknown imide phase” after heating up to 400 °C.
- (2)
The TDMS profiles indicated that the peak temperatures due to hydrogen desorption are 200 and 220 °C for the CaH2
Acknowledgement
This work was supported by the project “Development for Safe Utilization and Infrastructure of Hydrogen Industrial Technology” of NEDO, Japan.
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