Catalytic effects of various forms of nickel on the synthesis rate and hydrogen desorption properties of nanocrystalline magnesium hydride (MgH 2) synthesized by controlled reactive mechanical milling (CRMM)

Abstract

Nanocrystalline MgH 2 having the grain (crystallite) size of ∼11–12 nm was synthesized in a magneto-mill by controlled reactive mechanical milling (CRMM) of pure Mg powder under hydrogen in the presence of 0.5 and 2 wt.% of various forms of nickel powder made by the nickel carbonyl decomposition process. These powder additives were designated as micro-Ni, submicro-Ni, and nano-Ni, based on their predominant size and morphology. It is found that the strongest catalytic effect during reactive milling is provided by the nano-Ni which greatly improves the rate of hydrogen absorption by Mg and its conversion into the MgH 2 hydride. The addition of barely 0.5 wt.% of nano-Ni increases about two-fold the rate of hydrogen absorption by Mg as compared to the undoped Mg. Furthermore, a very fast hydrogen absorption rate during reactive milling is obtained with the addition of 2 wt.% of the nano-Ni such that ∼6 wt.% H 2 is already absorbed after ∼15 h of CRMM. The hydrogen desorption properties of MgH 2 mechano-synthesized for 75 h with 2 wt.% are also greatly improved. The DSC onset and peak desorption temperatures are reduced by ∼30–50 °C with respect to the DSC desorption temperatures of the synthesized undoped MgH 2 powder. It is found that the desorption kinetics measured in a Sieverts-type apparatus at the technological conditions of 0.1 MPa hydrogen pressure and no preliminary activation by cycling become very fast which is reflected in the reduction of the activation energy for desorption by ∼60 kJ/mol as compared to the synthesized MgH 2 without additives. In the first desorption run, immediately after reactive synthesis, the MgH 2 synthesized for 75 h in the presence of 2 wt.% of micro-and submicro-Ni desorbs ∼3 wt.% H 2 at 275 °C after ∼3600 s. At 300 °C the 2 wt.% micro-and submicro-Ni doped synthesized mixture desorbs ∼5 wt.% H 2 in ∼1000 s and ∼6 wt.% H 2 in ∼3000 s immediately after milling.