Enhanced initial hydrogenation of TiFe-based hydrogen storage alloys containing C

Abstract

The hydrogen storage properties of TiFe-based AB-type alloys containing up to 6 at% C were evaluated, with particular emphasis on the effect on the initial hydrogenation properties. The addition of C formed substoichiometric TiC1-x particles with a dendritic morphology as the secondary phase in the TiFe-phase matrix. Interestingly, the initial hydrogenation of the alloys with TiC particles was significantly enhanced at 50 degrees C, whereas the alloy without C was not hydrogenated. Moreover, the initial hydrogenation was kinetically accelerated with increasing loading of TiC particles. The hydrogen storage capacity tended to increase with increasing TiC content in the pressure-composition-isotherm curves of the alloys measured after initial hydrogenation. No significant degradation in the hydrogen storage capacity of the alloys containing C was observed during the hydrogen sorption cycles. The initial hydrogenation started around the TiC particles, where primary cracks were caused by volume expansion induced by the formation of hydride phases. The TiC particles provide pathways for hydrogen diffusion into the TiFe matrix along the interphase boundaries between these particles and the matrix, and through the particles with vacant C sites during the initial hydrogenation. The study shows that the addition of carbon, a common impurity in raw materials for the fabrication of TiFe-based hydrogen storage alloys, is very effective in enhancing the initial hydrogenation, which is a chronic problem for these alloys.