## 3 Results
#### 3.1.3 Hydrogen Storage Properties  
The kinetics of the first hydrogenation (activation) under 2 MPa of hydrogen at room temperature of the Ti\({}_{1.56}\)V\({}_{0.18}\)(FeV)\({}_{0.18}\)Cr\({}_{1.08}\) + 4%Zr\({}_{7}\)Ni\({}_{10}\) and Ti\({}_{1.56}\)V\({}_{0.36}\)Cr\({}_{1.08}\) + 4%Zr\({}_{7}\)Ni\({}_{10}\) alloys are presented in Figure 4. The activation is similar for both of the alloys, the only difference being a slightly longer incubation time for the ferrovanadium-containing sample. After the incubation time, the absorption proceeds quite rapidly and full capacity is reached in less than 300 s for both pure vanadium and \begin{table}
\begin{tabular}{c c c c c} \hline
**Sample** & **Crystal Structure** & **Lattice Parameter (nm)** & **Crystallite Size (nm)** & **Micro-Strain (\%)** \\ \hline As-cast & BCC & 0.3100 (1) & 14 (1) & 0.4 (1) \\ Hydridded & FCC & 0.4326 (3) & 13 (2) & 1.4 (1) \\ Desorbed & BCC & 0.3089 (3) & 8 (1) & 1.2 (2) \\ \hline \end{tabular}
\end{table} 
Table 2: Crystal parameters of the co-melt Ti\({}_{1.56}\)V\({}_{0.18}\)(FeV)\({}_{0.18}\)Cr\({}_{1.08}\) + 4% Zr\({}_{7}\)Ni\({}_{10}\). Figure 3: XRD pattern of the co-melt Ti\({}_{1.56}\)V\({}_{0.18}\)(FeV)\({}_{0.18}\)Cr\({}_{1.08}\) + 4%Zr\({}_{7}\)Ni\({}_{10}\) alloy in as cast, fully hydride, and desorbed state. vanadium substituted alloys. The maximum capacity is close to 3.2 wt % of hydrogen, which is slightly higher than other Ti-V based materials that was reported in the literature [24; 25; 26; 27]. After activation, the samples were desorbed and the pressure-composition isotherm was registered. However, we found out that there was an important reduction of capacity that makes the measurement practically impossible. For this reason, after activation at room temperature, the temperature was increased while keeping the sample under high hydrogen pressure in order to prevent it to desorb. Thereafter, the desorption curve was registered. This procedure was repeated for each isotherm, meaning that every isotherm was measured using a freshly hydrided sample. Figure 5 gathers the Pressure Composition desorption isotherms (PCT) that were recorded at 150, 180, 200, and 230 \({}^{\circ}\)C. For each temperature, a few measurements were performed with similar results. As the temperature increases, so does the equilibrium pressure. It should be mentioned that the plateau recorded are for the transition from the dihydride to the monohydride. The monohydride plateau is too stable to be recorded with our apparatus. The fact that the plateaus are sloped is probably due to the solid solution nature of the as-cast material. As there is an important reduction of capacity after the first cycle, this may explain why the capacities of various isotherms changed. Further work on cycling properties of this material is needed. This is also the reason why we considered the middle of each plateau as the plateau pressure and not the hydrogen pressure at a given hydrogen concentration. The fact that the capacity increases with temperature is puzzling. One reason may be the lowering of the first plateau that makes the transition from the first to second plateau even more sloping, thus making the measurement of total capacity of the second plateau more difficult. From these pressures, a Van't Hoff plot was obtained and is reported in Figure 5. The formation enthalpy of the hydride has been determined to be \(-59\pm 3\) kJ\(\cdot\)(mol\(\cdot\)H\({}_{2}\))\({}^{-1}\) and its associated entropy \(-166\pm 8\) J\(\cdot\)(mol\(\cdot\)H\({}_{2}\))\({}^{-1}\)\(\cdot\)K\({}^{-1}\). The errors have been calculated using the linear fit of the Van't Hoff plot. As a comparison, the formation enthalpy of the Ti\({}_{1.56}\)V\({}_{0.36}\)Cr\({}_{1.08}\) + 4% Zr\({}_{7}\)Ni\({}_{10}\) hydride has been determined to be \(-61\) kJ\(\cdot\)(mol\(\cdot\)H\({}_{2}\))\({}^{-1}\) and its associated entropy \(-169\) J\(\cdot\)(mol\(\cdot\)H\({}_{2}\))\({}^{-1}\)\(\cdot\)K\({}^{-1}\)[21].