In order to use FeTi for hydrogen storage material, We need two activation steps. The first step is heat treatment, to remove the oxide film of FeTi, obstructing chemisorption of hydrogen. And the second step is the iteration of hydrogen absorption-desorption cycle to increase surface area by creation of new surfaces by cracks. But, there is no exact mechanism of the activation of FeTi, and there are many arguments to that. It is especially interesting how the surface changes during heat treatment and which phase chemisorbs hydrogen after heat treatment, and this is widely disputed.
To find out, in this study, we observed the hydrogen absorption curve of FeTi, mixed with metal powders after heat treatment and slow rate period vs. heat treatment time for a few temperatures. As a result, the phase chemisorbing hydrogen after heat treatment is thought to be Fe clusters which are created by the surface segregation during heat treatment.
However, Fe clusters are contaminated by oxygen which exists in hydrogen and activation is delayed. Namely, there are two processes during heat treatment, the segregation process and the contamination process. For this reason, when heat treatment of FeTi mixed with Cu or Ni powder, whose oxide stability is lower than that of Fe, is carried out the activation never occurs. When heat treatment of FeTi mixed with Al, Si, Mn, Mg powder, whose oxide stability is higher than that of Fe, is carried out the activation is improved (especially in case of Mn, Mg powder which chemisorbs hydrogen).
