Crystalline Al_1 − xTi_x phases in the hydrogen cycled NaAlH₄ + 0.02TiCl₃ system

Abstract

The hydrogen (H) cycled planetary milled (PM) NaAlH₄ + 0.02TiCl₃ system has been studied by high resolution synchrotron X-ray diffraction and transmission electron microscopy during the first 10 H cycles. After the first H absorption, we observe the formation of four nanoscopic crystalline (c-) Ti-containing phases embedded on the NaAlH₄ surface, i.e. Al₂Ti, Al₃Ti, Al₈₂Ti₁₈ and Al₈₉Ti₁₁, with 100% of the originally added Ti atoms accounted for. Al₂Ti and Al₃Ti are observed morphologically as a mechanical couple on the NaAlH₄ surface, with a moderately strained interface. Electron diffraction shows that the Al₈₂Ti₁₈ phase retains some ordering from the L1₂ structure type, with the observation of forbidden (100) ordering reflections in the fcc Al₈₂Ti₁₈ lattice. After 2 H cycles the NaAlH₄ + 0.02TiCl₃ system displays only two crystalline Ti-containing phases, Al₃Ti and Al₈₉Ti₁₁. After 10 H cycles, the Al₈₉Ti₁₁ is completely converted to Al₈₅Ti_15. Al₈₉Ti₁₁, Al₈₅Ti₁₅ and Al₃Ti do not display any ordering reflections, and they are modeled in the A1 structure type. Quantitative phase analysis indicates that the Al₃Ti proportion continues to increase with further H cycles. The formation of Ti-poor Al_{1 −  x} Ti _x (x < 0.25) phases in later H cycles is detrimental to hydrogenation kinetics, compared to the starting Ti-richer near-surface Al₂Ti/NaAlH₄ interface present during the first absorption of hydrogen.

Keywords:

• hydrogen storage
• complex hydrides
• nanoparticles
• Al_{1− x} Ti _x alloys

Acknowledgements

This work was supported by the Synchrotron Program of the Research Council of Norway. MPP thanks the staff of the Swiss–Norwegian Beam Line for providing experimental assistance and logistics during long-term attachments.