Role of the Added Ni in Hydrogen-Storage Reactions of MgH2-Zn(BH4)2-Tm (Ni, Ti, or Fe) Alloys
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, Qd (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 Qd (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 TiH1.924 formed after milling in hydrogen and remaining un-decomposed during cycling.
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