## 3 Results and Discussion
### Desorption Kinetics Using Sievert's Type Measurements  
The ramping desorption kinetics (1 \({}^{\circ}\)C/min) of the base material 2 LiNH\({}_{2}\)-MgH\({}_{2}\) and 2 wt. % Ni-added 2 LiNH\({}_{2}\)-MgH\({}_{2}\) compound are given in Figure 5. As expected, Ni addition did not alter the overall desorption capacity of 2 LiNH\({}_{2}\)-MgH\({}_{2}\) since the final desorption temperature was 325 \({}^{\circ}\)C. However, Ni addition showed enhanced kinetics and higher desorption capacity up to 250 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Reaction Kinetics (wt. \%/min)** & **On-Set Temperature (\({}^{\circ}\)C)** \\ \hline TiF\({}_{3}\) & 0.5816 & nanoCo & 121.96 \\ nanoNi & 0.5330 & TiF\({}_{3}\) & 125.99 \\ nanoTi & 0.5312 & nanoTi & 136.65 \\ nanoCo & 0.5255 & nanoFe & 142.86 \\ nanoFe & 0.5113 & nanoNi & 149.30 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Reaction kinetics and on-set decomposition temperature of 2 LiNH\({}_{2}\)-nanoMgH\({}_{2}\) with different catalysts. The absorption kinetics of 2 wt. % Ni added 2 LiNH\({}_{2}\)-MgH\({}_{2}\) and the base material 2 LiNH\({}_{2}\)-MgH\({}_{2}\) at 180, 200, and 220 \({}^{\circ}\)C are given in Figure 6 The catalyst-added compound performed best in terms of kinetics at 200 \({}^{\circ}\)C, reaching 2.6 wt. % capacity in 60 min, whereas the base material can only reach a capacity of 1.75 wt. % in 60 min, as shown in Figure 6. The absorption kinetics of 2 wt. % Ni added 2 LiNH\({}_{2}\)-MgH\({}_{2}\) at 180, 200, and 220 \({}^{\circ}\)C up to 5 h is also given in Figure S2 (see Supplementary Material). The ultimate capacity of 2 wt. % Ni-added 2 LiNH\({}_{2}\)-MgH\({}_{2}\) is around 3.5 wt. % at 200 \({}^{\circ}\)C (Figure S2), which is lower than the theoretical and reported values earlier [41]. This discrepancy is due to differences in the material preparation procedure, slightly different compound ratios (LiNH\({}_{2}\) to MgH\({}_{2}\) ratio is 2:1 in this study as compared to 2:1.1 in Ref [41]), NH\({}_{3}\) emission, and the self-decomposition of Mg(NH\({}_{2}\))\({}_{2}\) to Mg\({}_{3}\)N\({}_{2}\) at elevated temperatures. Among these, the self-decomposition of Mg(NH\({}_{2}\))\({}_{2}\) to Mg\({}_{3}\)N\({}_{2}\) is considered to be the main reason, since every absorption cycle preceded by the evacuation of the 2 LiNH\({}_{2}\)-MgH\({}_{2}\) compound at 325 \({}^{\circ}\)C for 60 min to make sure for the complete desorption of the sample. Moreover, the self-decomposition of Mg(NH\({}_{2}\))\({}_{2}\) to Mg\({}_{3}\)N\({}_{2}\) was also confirmed with the XRD measurements, as explained in Section 3.3. On the other hand, [PERSON] et al. showed a 25% capacity loss after 270 cycles where 7% of the capacity loss was attributed to NH\({}_{3}\) emission that increases with higher desorption temperatures [41]. Since the desorption temperature in this study was much higher (325 \({}^{\circ}\)C compared to 240 \({}^{\circ}\)C in Ref [41]), the main reasons for the apparent capacity loss are twofold: self-decomposition of Mg(NH\({}_{2}\))\({}_{2}\) to Mg\({}_{3}\)N\({}_{2}\), and NH\({}_{3}\) emission. Therefore, high temperatures should be avoided in utilization of the 2 LiNH\({}_{2}\)-MgH\({}_{2}\) compound. To better understand the capacity loss due to self-decomposition of Mg(NH\({}_{2}\))\({}_{2}\) to Mg\({}_{3}\)N\({}_{2}\) and NH\({}_{3}\) emission, further investigations are underway by utilizing gas chromatography and a residual gas analyzer coupled with a quadruple mass-spectrometer. It is well known that the addition of transition metal catalysts to complex hydrides does not change the thermodynamic properties (i.e., enthalpy of formation) of the base material, but enhances the kinetic properties of the base material by lowering the activation energy [55]. The exact mechanism for the improved kinetics by Ni addition is not well understood currently.