Activation of erbium films for hydrogen storage

Abstract

Hydriding of metals can be routinely performed at high temperature in a rich hydrogen atmosphere. Prior to the hydrogen loading process, a thermal activation procedure is required to promote facile hydrogen sorption into the metal. Despite the wide spread utilization of this activation procedure, little is known about the chemical and electronic changes that occur during activation and how this thermal pretreatment leads to increased rates of hydrogen uptake. This study utilized variable kinetic energy X-ray photoelectron spectroscopy to interrogate the changes during in situ thermal annealing of erbium films, with results confirmed by time-of-flight secondary ion mass spectrometry and low energy ion scattering. Activation can be identified by a large increase in photoemission between the valence band edge and the Fermi level and appears to occur over a two stage process. The first stage involves desorption of contaminants and recrystallization of the oxide, initially impeding hydrogen loading. Further heating overcomes the first stage and leads to degradation of the passive surface oxide leading to a bulk film more accessible for hydrogen loading.

The influence of vacuum annealing on the uranium–hydrogen reaction

“…The partial reduction of the oxide increases the surface reactivity and permeability of the oxide to hydrogen [12]. The formation of the UO x C y phase increases H 2 dissociation upon and subsequent transport through the oxide by improving conduction of electrons to the surface [13].…”

3He bubble evolution in ErT2: A survey of experimental results

“…An ''activation'' step is needed in order to get the H:Er reaction to happen [19]; however, the activation step is not associated with an energy barrier to the reaction that needs to be overcome. Instead, it has been shown that upon heating erbium films, oxygen is driven into the bulk of the erbium leaving an oxygen deficient Er 2 O 3Àx surface that reacts with and permits the diffusion of hydrogen [33,37]. Parish et al showed that oxygen is a very common contaminant in erbium films, forming dispersed erbium oxide nanoinclusions and larger macroscopic particles [24].…”

“…If the air exposed erbium film is then hydrided, the oxide thickness increases, depending on the hydrogen gas purity [36]. While the oxide surface is predominantly Er 2 O 3 , the presence of hydroxides has also been detected using XPS [33,37]. Even though the heat of formation for ErH 2 is negative (À2.30 eV/mole) [34], the reaction does not proceed because the erbium oxide layer prevents hydrogen diffusion into the bulk and the H:Er reaction.…”

The Influence of Vacuum Annealing on the Nucleation and Growth Kinetics of Uranium Hydride

“…This is in agreement with the observations of Swissa et al [11] and Brumbach el al. [7] who reported thermally driven redistributions of the oxygen from the oxide over-layer into the bulk metal for uranium and erbium respectively. Shown in Figure 4 are the O1s and C1s regions of XPS spectra obtained from as-polished and annealed samples.…”

“…Recently, vacuum annealing has been shown to promote the surface reactivity of some rare earth metals towards hydrogen adsorption owing to the desorption of surface contaminants and modifications to the oxide over-layer [5][6][7], therefore, a similar enhanced surface reactivity with uranium could be expected [8].…”