Catalytic effect of transition metals on hydrogen sorption in nanocrystalline ball milled MgH2–Tm (Tm=Ti, V, Mn, Fe and Ni) systems

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Abstract

Intensive mechanical milling was used to make MgH2–Tm (Tm=3d-transition elements Ti, V, Mn, Fe, Ni) nanocomposite powders. The hydrogen storage properties of these composite powders were evaluated. The five 3d-elements Ti, V, Mn, Fe and Ni showed different catalytic effects on the reaction kinetics of Mg–H system. Desorption was most rapid for MgH2–V, followed by MgH2–Ti, MgH2–Fe, MgH2–Ni and MgH2–Mn at low temperatures. The composites containing Ti exhibited the most rapid absorption kinetics, followed in order by Mg–V, Mg–Fe, Mg–Mn and Mg–Ni. Formation enthalpy and entropy of magnesium hydride were not altered by milling with transition metals, while the activation energy of desorption for magnesium hydride was reduced drastically.

Introduction

Magnesium hydride is considered as potential hydrogen storage material for vehicular application because of its high capacity and low cost. However, the slow reaction kinetics of the Mg–H system at low temperatures limits the practical application of MgH2. Numerous works have been done since the 1970s in order to find a suitable alloy, which could absorb hydrogen at room temperature and desorb hydrogen at a temperature low enough to use exhaust–gas waste heat [1], [2], [3]. It was found that mixing magnesium with catalytic transition elements, such as Ni, Co, Ti, Fe or intermetallic compounds e.g. LaNi5, FeTi1.2, ZrFe1.4Cr0.6 [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14] could improve the hydriding and dehydriding kinetics of Mg efficiently at high temperatures (>573K). Our recent works [15] showed that the mechanically milled MgH2–V nanocomposite could desorb hydrogen at 473K and re-absorb hydrogen at room temperature. In the present work we extend the study to other 3d-transitional metals Ti, Mn, Fe and Ni, to investigate and compare their different effects on the hydrogen absorption and desorption in the Mg–H system.

Section snippets

Experimental

Pure MgH2 from Th. Goldschmidt AG (95%MgH2, 5%Mg) was mixed with titanium, vanadium, manganese, iron and nickel powders (99% pure) in the desired composition and then mechanically milled under argon by using a Spex 8000 ball mill. A hardened steel crucible and three steel balls of 12.7 mm in diameter were used for milling. The ball to powder weight ratio was 10:1. A small amount of powders was removed at regular intervals for monitoring the structural changes. All the handlings were performed

Phase structure after mechanical milling and hydrogen absorption/desorption

Mechanical milling of magnesium hydride with 3d transition metals leads to various products, depending on the relative affinity of the 3d-metal with hydrogen. Fig. 1 shows the X-ray spectra of MgH2+5at%Tm (Tm=Ti, V, Mn, Fe, Ni) composites after 20 h of milling. A fraction of γ-MgH2 phase was formed, as reported previously, on milling pure MgH2 [16]. A very stable TiH2 phase was formed in the MgH2+5at%Ti system by reaction of MgH2 with Ti. This reaction also happened when milling MgH2 with V

Conclusions

The MgH2–Tm (Ti, V, Mn, Fe, Ni) nanocomposites prepared by mechanical milling were investigated for hydrogen storage. The titanium and vanadium are better catalysts than Ni for hydrogen absorption and desorption. The composite with Ti or V additives showed very rapid desorption kinetics above 523K and absorption kinetics at temperature as low as 302K. The 3d-metal additives could drastically reduce the activation energy of hydrogen desorption. However, the thermodynamic properties of MgH2 were

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