Surface properties and their consequences on the hydrogen sorption characteristics of certain materials

Abstract

Surface analyses (X-ray photoelectron spectroscopy, X-ray-induced Auger electron spectroscopy and Auger electron spectroscopy) results of some hydrogen storage intertermetallic compounds of the type AB, AB,, A,B and the hydride, A,BH, (A= Ti, La, Ca or Mg; B = Fe, Ni or Cu) show the presence of numerous oxidation (aerial) products, such as oxides, hydroxides and carbonates. In general, the hydroxide and/or carbonate species are present in small amounts and persist only on the outer passivated surface. However, there are cases where they seem to be predominant, at least in the top few layers. Therefore such species are also expected to play an important role in the surface chemistry of the alloys and hydrides similar to that observed with surface oxides. Accordingly, the activation procedure varies for the different materials. Upon activation the surface may be described as a supported metal system which in turn increases the activity thus explaining the rapid kinetics of the hydriding/dehydriding reactions upon cycling processes. The easy activation of certain alloys could also be explained on the basis of the reducibility and the thickness of the surface oxide layers. In other words, it could be demonstrated based on their oxidation resistance and the segregation behaviour of the metals. The specific reactivity of different systems are related to the segregation, oxidation, and reduction behaviours. Surface enrichment of iron in the TiFe system is demonstrated for the first time.