Development of Nb-Ti-Co alloy for high-performance hydrogen separating membrane

Abstract

The compositional window leading to the crystallization of primary α-Nb phase and the eutectic (α-Nb + TiCo) phase in Nb-Ti-Co alloys is of high technical relevance due to its favourable properties with respect to hydrogen permeation. To this end, a new compositional window for hydrogen-permeable Nb-Ti-Co alloys is discussed and established for the first time in this study. It is found that improving the hydrogen permeability and increasing the durability are in contradiction with each other, and are closely related to the alloy composition. Optimization of the design of these alloys is therefore a compromise between above-mentioned two parameters. As-cast Nb45Ti25Co30 shows a good balance between permeability and durability. Secondly, Nb-Ti-Co amorphous (a-) membranes have been successfully prepared by the melt-spinning technique. The importance of the annealing condition in determining crystallization behaviour, hydrogen permeability and durability is brought out. Subsequent annealing decreases the hydrogen permeation flux of the a- membrane, but significantly increases its thermal stability and durability, compared with its amorphous state, which is attributed to the microstructural change from amorphous to a duplex structure. Typically, a- Nb30Ti35Co35 membrane after annealing at 1000 °C for 12 h shows a high hydrogen permeation flux of 15.55 cc H2 cm−2 min−1 and excellent durability of ~ 112 h at 400 °C. Thirdly, the permeability of crystallized (c-) Nb30Ti35Co35 membrane obtained by annealing at 1000 °C for 12 h towards hydrogen was tested and evaluated under the mixed atmosphere of H2/CO or H2/CO2. This membrane possesses a relative stable H2 flux and good durability without corrosion/chemical reaction-induced failure. The present work demonstrates that Nb-Ti-Co amorphous alloys followed by proper annealing are an attractive candidate material for hydrogen separation membrane.