## 3 Results and Discussion  
### Thermogravimetric Analysis (TGA) and Thermal Programmed Desorption (TPD)  
TGA and TPD analyses facilitate the rapid screening of the complex hydride materials in a relatively short duration and gives invaluable information on the desorption characteristics such as gravimetric storage capacity and hydrogen decomposition temperature. To determine the optimal catalyst concentration, we focused on the nanoNi catalyst, which showed enhanced performance in various complex hydrides as discussed in the Introduction. It is highly desirable to use a minimum amount of catalyst to limit the cost of the complex hydride. As shown in Figure S1 (see supplementary information), 2 wt. % nanoNi showed the best desorption performance at temperatures up to 300 \({}^{\circ}\)C. Temperatures higher than 300 \({}^{\circ}\)C are not practical for the mobile applications; therefore, 2 wt. % nanoNi doping is the optimal catalyst loading for the 2 LiNH\({}_{2}\)-nanoMgH\({}_{2}\). The concentration of 2 wt. % nanoNi catalyst was also investigated for the LiNH\({}_{2}\)-nanoMgH\({}_{2}\) system, and the TGA and TPD results are given in Figures 1 and 2, respectively. Considering the TGA results of LiNH\({}_{2}\)-nanoMgH\({}_{2}\) given in Figure 1, the addition of 2 wt. % nanoNi catalyst improved the hydrogen desorption up until around 200 \({}^{\circ}\)C, but hampered the ultimate gravimetric capacity (i.e., capacity at 375 \({}^{\circ}\)C) of the LiNH\({}_{2}\)-nanoMgH\({}_{2}\) system. This result is in line with TPD studies. According to Figure 2, the hydrogen signal of the 2 wt. % nanoNi-doped LiNH\({}_{2}\)-nanoMgH\({}_{2}\) increases up until 200 \({}^{\circ}\)C during the TPD experiments, then sharply decreases with increasing temperatures. From the practical application point of view, high temperature performance of the complex hydrides is not critical for the mobile applications; therefore, it is concluded that nanoNi doping is desirable for the LiNH\({}_{2}\)-nanoMgH\({}_{2}\) system. Regarding the TGA of 2 LiNH\({}_{2}\)-nanoMgH\({}_{2}\) material given in Figure 1, the 2 wt. % nano-Ni doping enhances the gravimetric capacity of the base material at all temperatures studied. The TPD  
Figure 1: Thermogravimetric analysis (TGA) profiles of pristine and Ni-doped LiNH\({}_{2}\)-nanoMgH\({}_{2}\) with 1:1 and 2:1 ratios. Figure 2: Thermal programmed desorption (TPD) profiles of pristine and 2 wt. % nanoNi-doped LiNH\({}_{2}\)-nanoMgH\({}_{2}\) with 1:1 and 2:1 ratios. results given in Figure 2, however, show stronger hydrogen desorption signal for 2 wt. % nanoNi added material compared to the base material, which is further evidence for higher H\({}_{2}\) desorption from 2 wt. % nanoNi-doped material. Comparing gravimetric capacities of LiNH\({}_{2}\)-MgH\({}_{2}\) with the molar ratios of 1:1 and 2:1 given in Figures 1 and 2, the 2 LiNH\({}_{2}\)-nanoMgH\({}_{2}\) compound is superior to LiNH\({}_{2}\)-nanoMgH\({}_{2}\), hence 2 LiNH\({}_{2}\)-nanoMgH\({}_{2}\) is selected for further investigation. A closer look into Figures 1 and 2 on the thermogravimetric and thermal programmed desorption profiles of 2 wt. % nanoNi-doped LiNH\({}_{2}\)-nanoMgH\({}_{2}\) with 1:1 and 2:1 ratios are discussed here. The TGA of the 2 wt. % doped 1:1 sample shows the gaseous weight loss close to 5% with two desorption steps, whereas the 2 wt. % nanoNi-doped 2:1 sample shows double the weight loss capacity (\(\sim\)10%) with single major decomposition step and inflections at higher temperatures (\(\sim\)325 \({}^{\circ}\)C). This is confirmed from the TPD profile (Figure 2) of 2 wt. % doped 2:1 compound, where the total effective desorption or decomposition attributed in a single broader step when compared to TPD profiles of 2 wt. % doped 1:1 LiNH\({}_{2}\)-nanoMgH\({}_{2}\) compound, where there are two sharp decomposition steps below 225 \({}^{\circ}\)C. We have also demonstrated TGA with other nickel concentrations, and found that lower nanoNi catalyst concentration of 2 wt.