Chemical characterization of the deactivation and protection of FeTi thin films using complementary non-destructive techniques

Abstract

FeTi is an excellent hydrogen storage material; however, it deactivates after numerous charge-discharge cycles. This study investigates the mechanism for deactivation as well as the use of a protective palladium coating to eliminate deactivation using a non-destructive technique which samples the integrity of the FeTi below the palladium coating. Specifically, conversion electron Mössbauer spectroscopy (CEMS), X-ray photoelectron spectroscopy (XPS) and “static” secondary ion mass spectrometry (SIMS) were used to analyze the topmost 10nm, 2nm and monolayer respectively of a 10nm FeTi specimen after various annealings and reduction treatments. A 5 nm palladium coating was applied to an identical specimen and analyzed with CEMS after similar treatments. Analyses of the uncoated specimen revealed that titanium, initially in FeTi, reacted with O 2-H 2O impurities in the H 2 charging gas to produce TiO 2 at the outermost surface while forming iron domains in the near-surface region. During annealing at 450 K for 3 h, a solid state reaction was also noted to occur in which Fe 2O 3 was reduced by adjacent titanium in FeTi. This solid state reaction parallels the gas phase reaction for initial activation of FeTi after exposure to air. Analyses of the palladium-coated specimen could only be performed using CEMS because of its unique ability to probe below the coating at the Pd-FeTi interface, a region which is below the sampling depths of both XPS and static SIMS. The palladium coating provided an effective oxygen barrier which prevented deactivation and maintained the integrity of FeTi.