Anti-oxidation effect of chromium addition for TiFe hydrogen storage alloys.

Anti-oxidation properties of Cr substitution were investigated by comparing TiFe and TiFe 0.9 Cr 0.1. Kinetic measurements reveal that the degradation rate of TiFe 0.9 Cr 0.1 is slower than that of TiFe, indicating that Cr substitution drastically enhances the anti-oxidation properties of the mother alloy TiFe. In order to understand the mechanism of this effect, X-ray photoelectron spectroscopy (XPS) was used to identify the surface chemical state of these alloys, which indicates that Cr plays quite an important role in the suppression of surface oxidation. It is clarified that the deactivation of TiFe is caused by the oxidation of Ti on the surface and, as a result, the obstruction of hydrogen penetration. On the contrary, with the substitution of Cr for Fe in TiFe 0.9 Cr 0.1 , the Ti oxidation becomes more difficult even though the Cr was totally oxidized, which makes it easier to keep the initial activated state of hydrogenation (Ti with a low oxidation state). It also further demonstrated that Cr acts as a sacrificial element, undergoing complete oxidation to protect Ti from oxidation, and assists in keeping metallic Ti0, which is crucial for maintaining the hydrogen absorption kinetics. Therefore, the Cr substitution for Fe significantly improves the anti-oxidation properties of TiFe-based alloys. • Cr substitution improves the anti-oxidation properties of TiFe alloy. • The oxide layer composition changed when Fe was substituted by Cr in TiFe alloy. • Cr acted as a sacrificial element to prevent the oxidation of Ti. • Ti oxidation is the primary cause of the deactivation of TiFe alloy. [ABSTRACT FROM AUTHOR]