Improvement of hydrogen absorption and desorption properties of TiFe-based alloys by adding yttrium

Abstract

Hydrogen storage by metal hydride has broad prospects for development. In this paper, we study the effect of a trace amount of Y element substituting Ti on the hydrogen storage properties and microstructure of Ti1.1Fe0.8Mn0.2 alloy in detail. The alloys with the composition of Ti1.1-xYxFe0.8Mn0.2 (x = 0, 0.02, 0.04, 0.06, 0.08) were prepared by vacuum high-frequency electromagnetic induction melting furnace. The phase composition, microstructure and morphology of the alloys were characterized by X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy. In the microscopic morphology, the doping of Y makes the elongated second phase originally existing in the alloy uniformly disperse. At the same time, the solubility of Y in the TiFe phase is limited. When x = 0.02, the alloy is composed of pure TiFe phase. With the gradual increase of the Y content, the excess Y exists in the form of a precipitation phase. The addition of Y improves the activation property of the alloy and significantly enhances the hydrogen absorption capacity from 1.23 wt% to 1.84 wt% at 303 K. Using the van't hoff equation to calculate the enthalpy change (ΔH) and entropy change (ΔS) of the alloy during hydrogen absorption and desorption, it is found that the addition of Y results in the increase of ΔH (−28.7→−24.1 kJ/mol) and the decline in ΔS (−84.0→−93.9 J/mol/K) during hydrogen absorption, reduced energy barriers to overcome for hydrogenation reactions. Among the experimental specimens, the Ti1.08Y0.02Fe0.8Mn0.2 alloy has excellent performances at room temperature and can quickly absorb H2 to a capacity of 1.84 wt% which is close to the theoretical value of the TiFe-based hydrogen storage alloy (1.86 wt%).