First hydrogenation kinetics of Zr and Mn doped TiFe alloy after air exposure and reactivation by mechanical treatment
Introduction
Iron titanium alloy (TiFe) is considered as an attractive intermetallic compound (IMC) for practical hydrogen storage applications (e.g. naval [1] or stationary applications [2]) due to its reasonable hydrogen storage capacity (around 1.9 wt%), low cost, safety and good hydrogen reversibility at ambient temperature [[3], [4], [5]]. A FeTi–H solid solution (α) phase along with two different hydride phases: FeTiH (β) and FeTiH2 (γ) are formed during the hydrogenation of TiFe [3]. The first hydrogenation, also called activation, of TiFe prepared by conventional methods such as arc-melting, induction melting, etc. is difficult and energy-intensive [[6], [7], [8]]. Reilly et al. repeatedly heated the as-cast TiFe alloy at 400 °C under vacuum and exposed it under 6.5 MPa of H2 pressure to begin the first hydrogenation process [3]. To explain the slow first hydrogenation, it has been suggested that a surface oxide layer of TiO2 and/or Fe2O3 is formed during synthesis [9]. The oxide layer blocks the metal-hydrogen (M − H) electron interactions and consequently prevents the hydrogen absorption. Several approaches have been considered for making the activation process faster. Mechanical processes such as ball milling, cold rolling, etc. have been used for the synthesis as well as modification of hydrogen storage materials for decades [10,11]. These processes can introduce non-equilibrium phase, nanoscale structure, and active sites such as defect or grain boundaries that can facilitate the hydrogenation kinetics. On the other hand, partial substitution of the main components of the alloy with some other elements have also been considered as an alternative approach to improve the hydrogen storage properties of metal alloys [12,13]. Presence of a third component such as (e.g. Mn [12], Zr [14], V [15], Ni [16] etc.) in TiFe could improve the activation kinetics. Recently, effect of other additives such as Zr7Ni10, ZrMn2, Co, Ce and Nb etc. on hydrogenation of TiFe alloy were also studied [5,[17], [18], [19]]. Surface modification by deposition of a Pd layer [20,21], n-TiFe layer formation [22,23], Ar+ and H2+ ion implementation [24] were also tried out to improve the activation of TiFe alloy.
In previous studies, it has been observed that the TiFe alloy could be easily activated by adding a small amount of Zr and Mn [18,25]. However, for practical applications, the alloy material should also be resistant towards the surface oxidation due to air exposure. Therefore, we investigated the effect of air exposure on TiFe alloy doped with zirconium and manganese. The TiFe +4 wt% Zr + 2 wt% Mn alloy was synthesized from industrial grade metals, crushed in air and exposed to the air for several days. Afterwards, the effect of air exposure on the first hydrogenation was investigated. To reactivate the alloy after air exposure, it was mechanically treated using cold rolling or ball milling as these processes are easy and cost-effective for treatments of metal [26].
Section snippets
Experimental
The Zr (4 wt%) and Mn (2 wt%) doped TiFe alloys were synthesized by induction melting using the industrial grade elements Fe (ASTM 1005) and Ti (ASTM B265 grade 1) as raw materials. Commercial-purity Zr 702 alloy and electrolytic Mn flakes were used as additives. Elemental composition of these additives is shown in Table 1. The as-synthesized alloy was crushed in air using a mortar and pestle and kept in air for seven and thirty days before the first hydrogenation tests.
A modified version of
Activation kinetics
The first hydrogenation curves of the alloy before and after air exposure are shown in Fig. 1. It is clearly seen that the air exposure strongly affects the first hydrogenation kinetics. The as-crushed alloy started absorbing hydrogen after an incubation time of a few minutes and absorbed a maximum of 1.8 wt% of hydrogen. The incubation time increased to ~4 h when the alloy was exposed to air for 7 days. This observable incubation time is probably due to the oxidation of alloy surfaces during
Discussion
The inertness of the TiFe + 4 wt% Zr + 2 wt% Mn alloy after air exposure could be explained by the formation of stable oxide (TiO2, Fe2O3, FeO) layers on the alloy surface [8,32]. After ball milling, the air exposed alloy, hydrogenation kinetics were improved. The faster hydrogenation kinetics is mainly due to the formation of smaller particles (Fig. 5), reduction of crystallite sizes and formation of grain boundaries after ball milling [33,34]. As can be seen from Fig. 1, the capacity remained
Conclusion
In this investigation, effect of air exposure on hydrogen storage properties of TiFe + 4 wt% Zr + 2 wt% Mn alloy was studied. It has been observed that, after 7 days of air exposure, the first hydrogenation kinetics became sluggish with an initial incubation time. A 30 days air exposure made the alloy totally inactive towards hydrogenation. Ball milling efficiently decreases both particle and crystallite size without changing the alloy composition. Ball milling facilitates the first
Acknowledgements
JM is thankful to Fonds de recherche Nature et technologies Québec (FRQNT) for postdoctoral scholarship.
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