The mechanistic role of Ti4Fe2O1-x phases in the activation of TiFe alloys for hydrogen storage

TiFe alloy is considered an excellent candidate for stationary hydrogen storage owing to its superior hydrogen storage properties. However, the requisite for activation at high temperatures has hindered the practical application of TiFe alloy for hydrogen storage. This work investigates the activation of TiFe alloys with different Ti/Fe ratios. Results show that the TiFe0.90 alloy can be activated at room temperature and moderate hydrogen pressure (80 bar) compared to the poor activation kinetics of equimolar TiFe alloy. Microstructural characterization and surface analysis indicate that a higher content of Ti4Fe2O1-x phase formed in the TiFe0.9 alloy than in the TiFe0.95 and TiFe alloys. The embedded Ti4Fe2O1-x phase in the TiFe matrix plays a critical role in the overall activation processes, which serves as a conduit for hydrogen to interact with TiFe when it cracks during the initial stage of hydrogenation. The activation behavior is described in the following steps: (1) hydrogen absorption by the Ti4Fe2O1-x oxide on the surfaces of the TiFe matrix; (2) the Ti4Fe2O1-x cracks; (3) cracks extend into the TiFe matrix; (4) TiFe absorbs hydrogen. Understanding the activation behavior is critical to designing TiFe-based alloys with improved hydrogen storage properties.