Dependence of constituent elements of AB₅ type metal hydrides on hydrogenation degradation by CO₂ poisoning

LaNi₅-based AB₅ type alloy has high tolerance to CO₂ poisoning for hydrogen purification and storage from 20 to 25% CO₂ mixed gas. To elucidate the CO₂ poisoning factors of AB₅ type alloys, which are LaNi₅, CaNi₅, LaCo₅, and MmNi_4.025Co_0.4Mn_0.275Al_0.3 (Mm-Ni), the dependence of the constituent elements has been investigated on hydrogenation degradation by CO₂ poisoning. The tendency of CO₂ poisoning magnitude is CaNi₅ < LaNi₅ << Mm-Ni < LaCo₅, which was evaluated by the hydrogenation rate and capacity under CO₂ partial pressure and after CO₂ exposure. The Ni element of B site in CaNi₅ and LaNi₅ is an important role to maintain higher tolerance of CO₂ poisoning compared to Co element in LaCo₅. Moreover, the element of A site effects on CO₂ poisoning magnitude in AB₅ type alloy. The experimental tendency of CO₂ poisoning magnitude is consistent with the theoretical CO₂ adsorption energy on the (101¯0) surface plane of -1.39, -2.05, and -2.68 eV for CaNi₅, LaNi₅, and LaCo₅, respectively. CO₂ adsorbs on B site with electron charge transfer from AB₅ alloys to carbon. Not only Ni element in B site but also Ca element in A site decreases the energy of CO₂ adsorption on B site in AB₅ type alloys.