Hydriding and dehydriding properties of Ca–Si–X

We have already reported that CaSi reversibly absorbs and desorbs hydrogen. The maximum hydrogen content and the enthalpy of hydride formation of CaSi were 1.9 wt% (CaSiH1.3) and −62 kJ/mol·H2, respectively. However, further improvement of CaSi is required for its practical use as a hydrogen storage material. Therefore, we have investigated the effectiveness of partial substitution for Ca or Si. In order to select the substitutent X, the stability of Ca–Si–X hydride has been theoretically studied using the first-principles calculations by the ultrasoft pseudopotential method based on the density functional theory. As a result, it was demonstrated that the hydrogen content is improved by the partial substitution of Al, Ge, Mg, Sr, etc. Encouraged by this theoretical result, we have conducted an experiment to investigate the hydriding and dehydriding properties of Ca–Si–X. In the case of X = Al, the experimental result was in close agreement with the theoretical calculation result. Therefore, we confirmed the effectiveness of the calculation in investigating the properties of Ca–Si–X.