3.1. First Dehydrogenation/Hydrogenation Properties
DTA curves and associated MS traces measured for the additive-free sample Mg–Li as well as the K
2Mn(NH
2)
4-containing samples (
Table 1) are shown in
Figure 1A–C. The Mg–Li sample exhibits an endothermic peak at 200 °C with a shoulder at 190 °C, which is in agreement with the two-step dehydrogenation pathway of Mg(NH
2)
2 + 2LiH [
45]. At this temperature, mainly H
2 is released together with a small amount of NH
3. Above 225 °C, the release of NH
3 becomes dominant. Compared to the pristine system, the dehydrogenation reaction of the system containing 1 mol% K
2Mn(NH
2)
4 starts at a lower temperature (i.e., 110 °C), whereas the maximum of the dehydrogenation peak at 200 °C remains unchanged. When the additive amount is increased to 5 mol% (Mg–Li–5KMN), both dehydrogenation onset and peak temperature decrease to 80 °C and 172 °C, respectively. For both samples, Mg–Li–1KMN and Mg–Li–5KMN, the release of NH
3 below 200 °C is below the detection limit. However, increasing the amount of additives beyond 5 mol% (Mg–Li–35KMN) leads to significant NH
3 release, which starts at around 175 °C. Despite the fact that the dehydrogenation peak temperature of Mg–Li–35KMN is the lowest (143 °C), the H
2 peak area is also notably smaller in respect to those of the other samples. This is clearly due to the high amount of additive contained in this sample (~49 wt.%). Due to the pour amount of released H
2 and high amount of released NH
3, the sample Mg–Li–35KMN was not further investigated. DTA and MS results indicate that Mg–Li–5KMN has the optimum properties (among the investigated systems), since it presents the lowest H
2 release onset temperature (80 °C) with no NH
3 release until 200 °C.
In order to understand the reaction pathway of Mg–Li–5KMN, its dehydrogenation/rehydrogenation reactions were investigated via in-situ SR-PXD technique. The results of this investigation (PXD patterns as the function of temperature) are presented in the two-dimensional contour plot of
Figure 2. The diffraction pattern acquired at RT (after ball milling) shows the existence of tetragonal Mg(NH
2)
2, cubic LiH, MgO and Mn
4N. Besides that, no K
2Mn(NH
2)
4 reflections are present. During the first dehydrogenation (
Figure 2A), the reflections of cubic KH appear at 85 °C. At the same temperature, peaks belonging to orthorhombic Li
2Mg(NH)
2 are also observed. As the temperature increases, the intensity of the Mg(NH
2)
2 peaks diminishes and the intensity of the Li
2Mg(NH)
2 peaks increases. The continuous shift of the diffraction peaks towards lower 2θ angles, observed upon heating, is due to thermal expansion. Through the first hydrogenation attempt (
Figure 2B), Li
2Mg(NH)
2 peak intensities start to decrease at 110 °C and a broad peak of Mg(NH
2)
2 appears at about 2θ = 9°. Mg(NH
2)
2 and LiH suddenly recrystallize at 165 °C (sharp Mg(NH
2)
2 and LiH peaks appear). The absence of peaks ascribable to K
2Mn(NH
2)
4 through all the first cycles indicates that during ball milling, K
2Mn(NH
2)
4 transforms into Mn
4N and a potassium compound with amorphous or nanocrystalline features. Therefore, K
2Mn(NH
2)
4 must be considered as a precursor for the formation of Mn
4N and KH.
Rietvelt refinement performed on the diffraction patterns acquired after the first dehydrogenation/rehydrogenation cycle (
Figure S2) reveals that the specimen is composed of Mg(NH
2)
2 (60.6 wt.%), LiH (29.8 wt.%), KH (3.1 wt.%), MgO (3.5 wt.%), and Mn
4N (3 wt.%). Since K
2Mn(NH
2)
4 was not present in the in-situ SR-PXD data (
Figure 2), we expect that it decomposes into KH and Mn
4N with NH
3 and H
2 release as shown in reaction 2;
In the literature, some metal nitrides (TaN and TiN) are known to catalytically enhance dehydrogenation of the 2LiNH
2 + MgH
2 system [
20]. Besides that, the Mn
4N + LiH system is known to be an effective catalyst for NH
3 synthesis [
46]. Therefore, we firstly studied the possible effect of Mn
4N on the reaction kinetics and thermal behavior of the Mg(NH
2)
2 + 2LiH system. However, we could not observe any beneficial effect from Mn
4N either in DSC analysis or in the volumetric H
2 release curves (
Figures S3 and S4). Secondly, we investigated the effect of pure KH additions, which is also known to be an effective additive to reduce dehydrogenation peak temperature and improve reaction kinetics of the Mg(NH
2)
2 + 2LiH system [
21,
27]. In order to investigate the effect of KH on the hydrogen sorption properties, three additional samples were prepared with increasing the amount of KH (Mg–Li–7KH, Mg–Li–15KH and Mg–Li–30KH). These samples were prepared at the same milling conditions as Mg–Li–5KMN. Volumetric H
2 release curves of the first dehydrogenation of the prepared samples are presented in
Figure 3A. Upon dehydrogenation, Mg–Li shows a release of hydrogen equal to 4.1 wt.%. The same amount was released in the Mg–Li, Mg–Li–7KH systems. The sample Mg–Li–5KMN has slightly lower H
2 capacity (3.8 wt.%). The amount of released hydrogen decreases to 3.5 wt.% and 3.2 wt.% for the samples Mg–Li–15KH and Mg–Li–30KH, respectively. In order to compare the kinetic behavior of all investigated samples, the measured H
2 capacity was normalized (
Figure S5). Apparently, the kinetic behaviors of all additive-containing samples are very similar and they are comparably faster than the pristine Mg–Li. The slow dehydrogenation kinetics obtained from the Mg–Li sample at 180 °C is not surprising, since the corresponding peak temperature observed in the DSC analysis measured under 1 bar of H
2 is as high as 216 °C (
Figure 1A).
The measured rehydrogenation kinetics of all the investigated samples are presented in
Figure 3B. The rehydrogenation kinetics of Mg–Li are slower than those of KH-containing samples and those containing K
2Mn(NH
2)
4. Interestingly, the rehydrogenation rate of Mg–Li–5KMN noticeably increases toward the end of the hydrogenation. In fact, the last 1 wt.% of H
2 is loaded in only 2 min. This rehydrogenation rate (at this stage of hydrogenation) is four times faster than that of Mg–Li–7KH. This result suggests that the presence of K
2Mn(NH
2)
4 and the in-situ dual formation of KH and Mn
4N accelerated the diffusive processes commonly found in the last stage of hydrogenation processes owing to the formation of low H
2 diffusion coefficients of the hydride phases [
44].
3.2. Dehydrogenation/Rehydrogenation Properties upon Cycling
This particular feature (rehydrogenation rate increases toward the end of the reaction) is preserved upon the following two dehydrogenation/rehydrogenation cycles (
Figure 4). Interestingly, the first hydrogenation is faster than the second, which was observed before with a similar system [
36]. Usually, hydrogenation of materials are slower at the later stage [
47,
48]. In practical applications, fast kinetics are favored [
49,
50]. In the case of K
2Mn(NH
2)
4-containing samples, the fast hydrogenation kinetics at later reaction stages represent an advantage over other hydrogen storage systems, with which full saturation is hardly reached.
The volumetric measurements of the H
2 dehydrogenation for the 1st and 25th cycles of Mg–Li and Mg–Li–5KMN are presented in
Figure 5A. Mg–Li–5KMN sample releases 3.8 wt.% of H
2 within 3 hours in the first dehydrogenation. The amount of released H
2 reaches 4.2 wt.% in the second dehydrogenation, due to a longer measurement time. For this system, no noticeable loss in H
2 capacity was observed for 25 cycles. In contrast, the amount of released H
2 for Mg–Li after 25 cycles decreased from 4.2 wt.% to 2.2 wt.%. It is clear that repeated dehydrogenation/rehydrogenation cycles lead to slower reaction kinetics for both samples. It was already reported that for the amide-hydride systems, high operating temperatures cause the agglomeration of dehydrogenation/rehydrogenation products and segregation of reactants and K-based additives [
51]. Although reaction kinetics of Mg–Li–5KMN slow down upon cycling, the positive effect of the presence of additives on the reversibility of the sample maintains within the measured 25 cycles. In-situ SR-PXD contour plot of the Mg–Li–5KMN sample at the 25th dehydrogenation (
Figure 5B) reveals that identified phases are the same as in the first dehydrogenation (
Figure 2A). Thus, once formed, KH and Mn
4N additives remain stable.
3.3. Apparent Activation Energies and Rate Constants
The Kissinger method was applied to calculate the apparent activation energies (
Ea) and frequency factor (
A) from DSC curves for Mg–Li and Mg–Li–5KMN samples (
Figure 6A). Considering the complexity of the reaction, mainly for the material with the additive, the temperatures at the observed maximum reaction rate were taken for the calculation of these parameters (
Figures S6 and S7). It is worthy to remind that the dehydrogenation peak temperature of Mg–Li–5KMN was 30 °C lower than that of the pristine Mg–Li (
Figure 1A), and reaction kinetics were comparably faster (
Figure 3). However, the
Ea as well as
A values are higher for the Mg–Li–5KMN than for Mg–Li, i.e., 196 ± 5 kJ/mol H
2 and 2.7 × 10
22, 161 ± 5 kJ/mol H
2 and 3.5 × 10
17, respectively. It is reported that the reaction pathway of KH-added Mg(NH
2)
2+2LiH is different under argon and hydrogen atmosphere [
27,
52]. Dehydrogenation under hydrogen instead of argon alters the dehydrogenation products, which results in an increase of
Ea in the case of Mg–Li–5KMN with respect to Mg–Li. This behavior is different from the reports in the literatures, where
Ea is lowered with the addition of K-compounds [
34,
53]. In order to understand these contradictory findings, rate constants (
k) were calculated (
Figure 6B,
Table S1) by the Arrhenius expression
k = A·exp[−
Ea/RT] (1/s) at the cycling temperature of 180 °C. Then,
k was multiplied with the H
2 capacities obtained from
Figure 3A in order to obtain reaction rates (
Figure 6B). Despite the fact that Mg–Li–5KMN has a higher
Ea value in respect to Mg–Li, the calculated kinetic constant (
Table S1) and reaction rate indicate faster kinetic behavior for Li–5KMN at 180 °C. These outcomes are in agreement with the observed kinetic behavior (
Figure 3). It suggests that the notable increase of the frequency factor for Mg–Li–5KMN prompts a more efficient interaction between reactants at the interphases, resulting in faster kinetic behavior.