A comparative study of NbF5 catalytic effects on hydrogenation/dehydrogenation kinetics of Mg-Zn-Ni and Mg-Cu-Ni systems

Highlights: • NbF₅ decomposes in Mg₈₅Cu₅Ni₁₀ system, but not in Mg₈₅Zn₅Ni₁₀ system. • The stable MgZn₂ and transformation between Mg₂Cu and MgCu₂ may explain the decomposition of NbF₅ in Mg-Cu-Ni systems. • NbF₅ can ameliorate the hydriding/dehydriding kinetics of Mg₈₅Zn₅Ni₁₀ and Mg₈₅Cu₅Ni₁₀ systems significantly. In this study, Mg₈₅Zn₅Ni₁₀/Mg₈₅Cu₅Ni₁₀ – x wt% NbF₅ (x = 0, 2, 4) alloys were synthetized via vacuum induction smelting and high energy ball milling. The different NbF₅ catalytic effects and mechanisms on microstructure, hydriding/dehydriding kinetics of Mg₈₅Zn₅Ni₁₀/Mg₈₅Cu₅Ni₁₀ alloys were discussed in detail. The results prove that NbF₅ promotes the formation of cracks and pulverization on the alloy particles and to producing more numbers of grain boundaries, amorphous-nanocrystalline and dislocations which are beneficial for H₂ diffusion. The dehydrogenation activation energies are reduced to 82.367 and 87.076 from 109.830 and 116.767 kJ/mol respectively for Mg₈₅Zn₅Ni₁₀ and Mg₈₅Cu₅Ni₁₀ by 4 wt% NbF₅. NbF₅ decomposes in Mg₈₅Cu₅Ni₁₀ system, but not in Mg₈₅Zn₅Ni₁₀ system. The stable MgZn₂ and transformation between Mg₂Cu and MgCu₂ may explain the decomposition of NbF₅ in Mg-Cu-Ni systems. NbF₅ can ameliorate the hydriding/dehydriding kinetics of Mg₈₅Zn₅Ni₁₀ and Mg₈₅Cu₅Ni₁₀ systems significantly. NbF₅ does not change the thermodynamic properties of Mg₈₅Zn₅Ni₁₀/Mg₈₅Cu₅Ni₁₀ alloys.