Article pubs.acs.org/JPCC
Effects of Titanium-Containing Additives on the Dehydrogenation Properties of LiAlH4: A Computational and Experimental Study Jennifer L. Wohlwend,*,†,‡ Placidus B. Amama,†,§ Patrick J. Shamberger,† Vikas Varshney,†,‡ Ajit K. Roy,† and Timothy S. Fisher†,⊥ †
Thermal Sciences and Materials Branch, Materials and Manufacturing Directorate, Air Force Research Laboratory, Dayton, Ohio 45433, United States ‡ Universal Technology Corporation, Dayton, Ohio 45432, United States § University of Dayton Research Institute, University of Dayton, Dayton, Ohio 45469, United States ⊥ School of Mechanical Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *
ABSTRACT: Metal hydrides are attractive materials for use in thermal storage systems to manage excessive transient heat loads and for hydrogen storage applications. This paper presents a combined computational and experimental investigation of the influence of Ti, TiO2, and TiCl3 additives on the dehydrogenation properties of milled LiAlH4. Density functional theory (DFT) is used to predict the effect of Ti-containing additives on the electronic structure of the region surrounding the additive after its adsorption on the LiAlH4 (010) surface. The electron distribution and charge transfer within the LiAlH4/additive system is evaluated. Electronic structure calculations predict covalent-like bonding between the Ti atom of the adsorbate and surrounding H atoms. The hydrogen (H) binding energy associated with the removal of the first H from the modified LiAlH4 surface is calculated and compared with experimental dehydration activation energies. It is seen that the weaker H binding corresponds to the larger amount of charge transferred from the Ti atom to adjacent H atoms. A reduction in charge transfer between the Al atom and surrounding H atoms is also observed when compared to charge transfer in the unmodified LiAlH4 surface. This reduction in charge transfer between Al−H weakens the covalent bond within the [AlH4]− tetrahedron, which in turn reduces the dehydrogenation temperature exhibited by LiAlH4 when Ti-containing additives are used.
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INTRODUCTION Thermal energy storage (TES) devices play an important role in the thermal management of transient heat loads. Conventional TES systems rely on the enthalpy of fusion of phase change materials (PCMs) such as paraffin wax to absorb and store excess thermal load.1,2 However, these systems generally suffer from low thermal conductivity, leading to low heat absorption rates and only moderate thermal storage density on a mass or volume basis. An alternative method of TES is thermo-chemical storage, in which materials absorb and release heat during a reversible endothermic/exothermic reaction. Thermo-chemical storage materials can exhibit an order of magnitude greater thermal storage density (on a mass basis) when compared to PCMs.3,4 Prototype thermo-chemical storage systems based on the highly endothermic (exothermic) dehydrogenation (hydrogenation) reaction of metal hydrides have already been demonstrated.3,5,6 However, these initial systems are limited in their utility due to their reliance on low H2-content Ni-based hydrides with resulting low thermal © 2012 American Chemical Society
storage density (e.g., Ca0.2M0.8Ni5H6, contains 300 °C for MgH2).3 In addition to TES systems, metal hydrides are attractive for use in high capacity hydrogen storage devices because of their ability to store hydrogen in compact volumes.8 For both applications, it is desirable to maximize the hydrogen content of the hydride and enhance the kinetics of dehydrogenation so that the H2 release occurs at a working temperature (
