Mechanism and microstructural evolution of TiCrVFe hydrogen storage alloys upon de-/hydrogenation

Abstract

TiCrVFe alloys have attracted significant attention because of their fast de-/hydrogenation rates and high reversible hydrogen capacities at moderate temperatures, but few studies have reported their de-/hydrogenation kinetics and thermodynamics. In this work, TiCrVFe alloys with different Ti/Cr ratios were prepared by arc melting, and their microstructural evolution and de-/hydrogenation mechanisms were investigated. XRD analysis confirmed that the crystal structure of TiCrVFe alloy transformed from BCC to FCC after hydrogenation, with lattice constants increasing from 3.0327 Å to 4.2714 Å. Combined the XRD with SEM-EDS analysis of the alloy after de-/hydrogenation tests, the hydrogenation expansion destroyed partial crystal lattice of the alloy and resulted in the generation of numerous defects and dislocations. This explains the slight fading of the cyclic capacity after several cycles. It was found that the hydrogen absorption kinetics of TiCrVFe alloys were controlled by 3D diffusion mechanisms, with an activation energy of −18.41 kJ/mol in the temperature range of 3–65 °C. The enthalpy change of the alloy (Ti27Cr27V40Fe6) was determined to be 45.68 kJ/mol upon hydrogenation and 51.33 kJ/mol upon dehydrogenation by the PCT curves and Van’t Hoff equation. The enthalpy change of the Ti27Cr27V40Fe6 hydride upon dehydrogenation was consistent with the heat change in its DSC result. The results obtained in this work are useful for the application of TiCrVFe alloys as H2 storage materials in fuel cell vehicles or devices.