Direct observation on activation mechanism of V-based hydrogen storage alloys containing secondary C15 Laves phase

V-based alloys are candidates for high-capacity hydrogen storage. However, harsh activation condition is generally required before their normal operation, and this blocks practical application. The activation process can be greatly improved or even eliminated by introducing Laves phase into the V-based BCC matrix. However, the mechanism is still not clear and lacks direct observed evidence. In this work, by inducing ZrV 2 C15 Laves phase, V-xZr (x = 3, 6, 9, 12 at.%) alloys are successfully activated at 25 ºC under 3 MPa-H 2 by one-time hydrogenation. By XRD and TOF-SIMS analysis, direct evidence of the activation mechanism is obtained. It is found that V-xZr alloys consist of a multi-phase structure of V(Zr) BCC solid solution matrix phase surrounded by network of Zr-rich phases, including not only ZrV 2 C15 phase but also Zr(V) solid solution and a ZrV phase. In particular, the ZrV phase is a new phase first found in this work with fcc lattice. During activation, the complex of Zr-rich phases absorbs hydrogen first, and the resulting volume expansion leads to formation of microcrack, forming preferentially within the interface between ZrV 2 and ZrV or Zr(V) phases but not in ZrV 2 phase, which makes fresh surface inside the alloy exposed to hydrogen. Also, it is proven that ZrV 2 can work as fast hydrogen diffusion path for the V-based main phase. Therefore, the activation enhancement in the V-xZr alloys should be attributed to both microcrack formation and fast hydrogen diffusion related to the complex of Zr-rich phases.