Oxidation State and Local Structure of Ti-Based Additives in the Reactive Hydride Composite 2LiBH₄ + MgH₂

Abstract

Nowadays, the technological utilization of reactive hydride composites (RHC) as hydrogen storage materials is limited by their reaction kinetics. However, addition of transition-metal-based additives, for instance titanium isopropoxide (Ti-iso), to the 2LiBH4+MgH2 system, results in a significant improvement of sorption kinetics. In this work, the evolution of chemical state and local structure of the Ti-based additive has been investigated by means of X-ray absorption (XAS) and photoemission (XPS) spectroscopy. X-ray absorption near-edge structure (XANES) as well as extended X-ray absorption fine structure (EXAFS) analysis have been undertaken at the Ti K-edge. The measurements reveal the formation of a highly dispersed and disordered TiO2-like phase during ball milling. During first desorption reduced titanium oxide and titanium boride are formed and remain stable upon cycling. The surface analysis performed by XPS shows that the reduction processes of the Ti-based additive during first desorption is coupled to the migration of the Ti species from the surface to the bulk of the material. Several factors, related to favoring heterogeneous nucleation of MgB2 and the increase of interfacial area through grain refinement are proposed as potential driving force, among other effects, for the observed kinetic improvement.

Combined x-ray photoelectron spectroscopy and scanning electron microscopy studies of the LiBH4–MgH2 reactive hydride composite with and without a Ti-based additive

“…In previous works [20][21][22] it has been also demonstrated that the Ti based additive is present after ball milling as Ti oxide like species whereas upon cycling the additive migrates to the bulk as a mix of TiB 2 and Ti 2 O 3 species. It has been pointed out that the Ti-boride active phase may play its main role at the LiBH 4 -Mg interface in the bulk where it is promoting the heterogeneous nucleation of the MgB 2 .…”

“…These active species have been unequivocally detected by previous XAS and TEM studies. [19][20][21] Figure 6 also shows the Ti 2p photoelectron peaks for the same samples after one hour Ar + sputtering treatment. The removal of material, by long time sputtering, shows the presence of the Ti phases in the cycled samples in the subsurface region.…”

“…15,16 The characterization of the additives in the initial and cycled composites was a first step toward understanding the function of additives and the mechanism of H 2 sorption reactions. [18][19][20][21] It has been demonstrated that additives evolve during milling and/or first desorption to form metal boride nanoparticles. These particles act as nucleation sites for the heterogeneous nucleation of the MgB 2 phase what makes the kinetic faster and reduce strongly the incubation period for the second desorption step.…”

“…At the same time the additive species migrate to the bulk where they favor heterogeneous reactions at the interface of reactants. 20 Additionally a recent exhaustive microstructural characterization for the 2LiBH 4 -MgH 2 RHC system with and without the Ti-iso additive 22 has shown that microstructural effects, related to the high dispersion of titanium-based additives, result in a distinct grain refinement of MgB 2 and an increase in the number of reaction sites, causing acceleration of desorption and absorption reactions.…”

“…Data are compared to previous results by X-ray absorption spectroscopy ͑XAS͒, x-ray diffraction ͑XRD͒, transmission electron microscopy ͑TEM͒, selected-area electron diffraction ͑SAED͒, and electron energy loss spectroscopy ͑EELS͒ in Refs. [18][19][20][21][22]. The new results in this paper will contribute to the understanding of the reaction mechanism and the influence of the additives on the kinetics.…”