TEM investigation of hydrogenation mechanism in Ceria added TiFe hydrogen storage alloy

Abstract

TiFe alloys, a type of AB-class hydrogen storage material, have attracted considerable attention due to their low cost and high hydrogen storage capacity. To promote hydrogen adsorption and diffusion at the metal-hydrogen interface, the addition of rare earth elements (REEs) such as cerium (Ce), known for their high hydrogen affinity, has emerged as a promising strategy to enhance the initial hydrogenation performance of AB-type alloys. Despite numerous reports indicating the positive effects of this approach on hydrogen storage properties, the specific role of REEs during the initial hydrogen absorption process remains unclear, mainly because most existing studies have proposed potential mechanisms based on indirect evidence. In this study, we investigated the microstructural evolution induced by hydrogenation in TiFe alloys with added ceria (CeO₂), utilizing SEM, SPM, and TEM analyses, and proposed a mechanism for the initial hydrogenation process. The alloy was charged with hydrogen at 30°C under a pressure of 30 bar. Surface and cross-sectional analyses were conducted using SEM-FIB (Helios G4, FEI), and in-situ topography observations were performed with SPM (NX-10, Park Systems). These observations revealed that hydrogenation led to progressive expansion and cracking around the ceria particles within the alloy matrix. To further elucidate the underlying cause, precise microstructural analyses were carried out using TEM (Titan 80-300, FEI). TEM lamellae prepared by SEM-FIB were transferred to the TEM under vacuum conditions using a vacuum transfer TEM holder (648 Double-tilt holder, Gatan) to minimize atmospheric exposure. TEM results confirmed that the initially HCP-structured ceria underwent a phase transition to an FCC-structured cerium hydride upon hydrogenation. This phase transformation caused an approximately 26.7% lattice volume expansion, generating deformation stresses in the surrounding matrix and leading to surface crack formation. The TEM findings provided a detailed explanation for the structural evolution observed in SEM and in-situ SPM studies, offering a comprehensive understanding of the initial hydrogenation process involving ceria in hydrogen storage alloys.