AbstractThe inﬂuence of heating/cycling on MgH2 samples milled together with 2 mol% of Nb2O5 was investigated. X-ray diﬀraction (XRD) shows a reduction of the Nb2O5 to metallic niobium and a possible MgNb2O3.67 phase, along with an increased formation of MgO compared
to a pure MgH2 sample. It was shown that the additive during the heating process is preventing the MgH2 phase from grain growth. Using X-ray photoelectron spectroscopy metallic niobium and niobium oxides were observed emerging to the surface only after cycling. All reported reactions had taken place already during the ﬁrst cycle. Using annealing experiments at diﬀerent temperatures, samples of the same composition were generated but with diﬀerent XRD crystal sizes. These samples showed similar sorption kinetics, which indicates that the prevention of grain growth by the Nb2O5 additive can only have a minor eﬀect on the kinetics for this range of crystal sizes. Since the Nb2O5 is reduced during the heating, a possible catalytic eﬀect has to be due to metallic Nb or those of its oxides with oxygen deﬁciency, like MgNb2O3.67, formed during the ﬁrst cycle. However, other processes like particle size reduction or a decrease of agglomeration by the additive during milling also have to be taken into account.