Micron-/nano-scale hierarchical structures and hydrogen storage mechanisms in a cast vanadium-based multicomponent alloy

Abstract

Multicomponent vanadium-based alloys (MVAs), often considered as conventional coarse-grained alloys, have been extensively studied in past decades as important metal hydride electrodes and solid state hydrogen storage materials. A micron-scale microstructure composed of a V-based main phase and a TiNi-based secondary phase has been often used to explain the electrochemical performance of MVAs, where the micron-scale TiNi-based secondary phase with a three-dimensional network is considered as a catalyst for electrochemical reaction and/or a current collector. However, the atomic-scale microstructure of MVAs has been largely unknown to date. Here, using advanced aberration corrected electron microscopy, we have found micron-/nano-scale hierarchical structures in an as-cast MVA, V0.35Cr0.1Ti0.25Ni0.3. The micron-scale TiNi-phase contains VCr nanoprecipitates whereas the micron-scale VCr-phase contains TiNi nanoprecipitates and Ni-rich nanoclusters. In addition, we have found that the nanoprecipitates plays an essential role in the hydrogen storage, namely, TiNi nanoprecipitates with a diameter of approx. 30 nm absorb hydrogen faster than their VCr-matrix. Our studies revise conventional understanding on the microstructures in MVAs and the hydrogen storage mechanism, which may guide the development of nanostructured hydrogen storage materials for practical applications