A comparison of TiF3 and NbF5 catalytic effects on hydrogen absorption and desorption kinetics of a ball-milled Mg85Zn5Ni10 alloy

Abstract

In this investigation, the as-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) composites were successfully produced via ball milling. The different influences between the catalysts TiF₃ and NbF₅ on the hydrogen storage behavior and microstructure of the composites were investigated by XRD, SEM, TEM and hydrogen absorption/desorption tests. The as-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys contain the major phase Mg, the secondary phase Mg₂Ni, a small amount of MgZn₂, TiF₃ and NbF₅. After hydrogenation, MgH₂ and Mg₂NiH₄ are formed, which convert back into Mg and Mg₂Ni after dehydrogenation indicating that MgZn₂ and the catalysts TiF₃ and NbF₅ do not react with hydrogen. Compared with NbF₅ catalyzed alloy, the TiF₃ catalyzed alloy has a faster hydrogen absorption/desorption kinetics. On the basis of Arrhenius equation, the dehydrogenation activation energy values of the as-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys are 75.514 and 82.367 kJ mol⁻¹ H₂, respectively, while the value of ball-milled Mg₈₅Zn₅Ni₁₀ alloy is 109.830 kJ mol⁻¹ H₂. As a result, both TiF₃ and NbF₅ can significantly ameliorate the hydrogen storage thermodynamics. TiF₃ shows better catalytic influence on hydrogen storage property of Mg₈₅Zn₅Ni₁₀ than NbF₅.