Title

Nanoengineering-Enabled Solid-State Hydrogen Uptake and Release in the LiBH4+MgH2 System

Date of Completion

January 2011

Keywords

Engineering, Chemical|Nanotechnology|Energy|Engineering, Materials Science

Degree

Ph.D.

Abstract

LiBH4, a potential hydrogen storage material with the highest reversible capacity for vehicle applications, has always been hydrogenated and dehydrogenated at high temperature (above 400°C) because of its stable chemical bonds. The addition of MgH2 in LiBH4 can improve the dehydrogenation thermodynamics. For the LiBH4+MgH2 system, although the hydrogen exchange reactions can be reversible and the released/absorbed hydrogen reaches 9 wt%, the temperatures of the dehydrogenation/hydrogenation are high (above the melting temperature ∼280°C of LiBH4). Moreover, the slow kinetics of the LiBH4+MgH2 system is still not improved effectively. Both thermodynamic and kinetic issues hinder the application of the LiBH4+MgH1 system as onboard hydrogen storage materials.^ We have proposed nanoengineering approaches to develop LiBH4+MgH 2 hydrogen storage materials with favorable thermodynamic and kinetic properties. Multiple nanoengineering approaches have been investigated. First, long-term high energy ball milling is performed to reduce the particle size of hydrides to 200-300 nm, which enables the reversible hydrogenation and dehydrogenation reactions to happen in the solid state (below 280°C). The hydrogenation kinetics of the LiH+MgB2 mixture is found to be diffusion-controlled. The enhancement mechanism and a two-step ion-exchange reaction pathway for hydrogenation/dehydrogenation have been established through XRD, FTIR, TEM, and NMR analysis. The addition of V in the LiBH4+MgH 2 system through ball milling can improve the dchydriding performance. Second, nanoporous carbon aerogels (CAs) have been used as scaffolds to further reduce the particle size of LiBH4 to 10-100 nm through infiltration into CAs of the LiBH4 solution in the anhydrous tetrahydrofuran solvent. Due to the nano-confinement effect, the dehydrogenation temperature of the infiltrated LiBH4 is reduced to 80°C and the completion of H2 release is at 275°C, without the emission of borane. Third, nanoscale, amorphous LiBH4 powder with sizes from 20 to 50 nm has been synthesized via a solvent evaporation process without the aid of nanoscaffolds. The onset temperature of releasing H2 of nanoscale LiBH4 is reduced to ∼32°C. The amount of H2 released in the solid state reaches 3.5 wt%, which is substantially higher than that released by bulk LiBH4. The nanoscale LiBH 4 enhances the dehydrogenation kinetics and alters the rate-limiting step.^