Role of the Added Ni in Hydrogen-Storage Reactions of MgH2-Zn(BH4)2-Tm (Ni, Ti, or Fe) Alloys

Abstract

In the present work, Zn(BH₄)₂, Ni, Ti, and/or Fe were doped to MgH₂ to improve the hydrogen absorption and release features. Samples with compositions of 95 w/o MgH₂ + 2.5 w/o Zn(BH₄)₂ + 2.5 w/o Ni (named MZN), 90 w/o MgH₂ + 5 w/o Zn(BH₄)₂ + 2.5 w/o Ni + 2.5 w/o Ti (named MZNT), and 90 w/o MgH₂ + 1.67 w/o Zn(BH₄)₂ + 5 w/o Ni + 1.67 w/o Ti + 1.67 w/o Fe (named MZNTF) were prepared by grinding in a planetary ball mill in a hydrogen atmosphere. The percentages of the additives were less than 10 w/o to increase hydrogen absorbing and releasing rates without a major sacrifice of the hydrogen-storage capacity. The hydrogen absorption and release features of the prepared samples were examined and in particular, the role of the added Ni in hydrogen-storage reactions of these alloys was studied. MZNTF had the effective hydrogen storage capacity (the quantity of hydrogen absorbed for 60 min) of near 5 w/o (4.98 w/o) at the third cycle (NC = 3). MZNTF had the highest initial releasing rate and the largest quantity of hydrogen released for 60 min, Q_d (60 min), at 593 K under 1.0 bar hydrogen at the first cycle, followed in a descending order by MZNT and MZN. Ni formed Mg₂Ni by a reaction with Mg after hydrogen absorption-release cycling. Mg₂Ni and Mg are known to absorb and release hydrogen under similar temperature and hydrogen pressure conditions and Mg₂Ni is known to have higher hydrogen absorption and release rates than Mg. The formed Mg₂Ni phase is believed to contribute more strongly to the increases of the initial releasing rates and the Q_d (60 min) by its hydride releasing hydrogen and slightly possibly providing paths for the hydrogen released from neighboring MgH₂, compared with the Zn formed after cycling and the TiH_1.924 formed after milling in hydrogen and remaining un-decomposed during cycling.

DOI: http://dx.doi.org/10.5755/j01.ms.24.4.19051