Hydrogen for Fuel Cells from NanoMushrooms Made of NaAlH4

Hydrides are used to deliver hydrogen for fuel cells for automotive applications. The shape (or morphology of those hydrides) determine the speed of hydrogen gas delivery (or kinetics). Nanostructuring is a key strategy in improving speed of hydrogen desorption reaction. Study of those nanostructures is made possible by synchrotron x-ray scattering performed at the Advanced Photon Source. X-rays are used to determine shape and size of nanoconfined hydrides by measuring mass fractal dimension (D). Initially, NaAlH4 powders are surface fractals. After infiltration into porous alumina, these powders are mass fractals.

Narase Gowda S., Ilavsky J., Gold S.A., Dobbins T., Ultra Small Angle X-ray Scattering (USAXS) Studies of Morphological Changes in NaAlH4," Materials Challenges in Energy, Edited by Wicks G.G., Simon J., Zidan R. et al., 224, 51-60 (2011).

Dobbins, T.A., Bruster E.B., Oteri, E.U., Ilavsky J.,Ultrasmall-Angle X-ray Scattering (USAXS) Studies of Morphological Trends in High Energy Milled NaAlH4 Powders," Journal of Alloys and Compounds , 446-447, 248-54 (2007).

Kinetic limitations improved when Thermodynamic Destabilized Hydrides are doped with TiCl3

Kinetic limitations improved when Thermodynamic Destabilized Hydrides are doped with TiCl3

Thermodynamic destabilization occurs with formation of a stable alloy or compound comprised of the destabilizer atoms/ions and the H- coordinated cation on the hydride. Our 3D imaging work reveals that 4 mol % TiCl3 lowers kinetic barriers in MgH2-destabilized LiBH4 by increasing interphase volume (relative to bulk volume). In related work, we showed that TiN can serve as a destabilizer for NaAlH4.

Dobbins T., NaraseGowda S., Butler L., Study of the Morphological Changes in MgH2 Destabilized LiBH4 Systems using Computed X-ray Microtomography," Materials (Open Access), 6[10], 1740-51 (2012).

Dobbins T., Abrecht M., Uprety Y., Moore K., An X-ray Photoemission Electron Microscopy Study of the Formation of Ti-Al Phases in 4 mol % TiCl2 Catalyzed NaAlH4 during High Energy Ball Milling," Nanotechnology, 20, 204014 (2009).

NaAlH4 nanoconfined in Fe-BTC MOF shows significant Hydrogen Mobility at 77oC

NaAlH4 nanoconfined in Fe-BTC MOF shows significant Hydrogen Mobility at 77oC

Nanostructuring of hydrides is made possible by infiltration into Fe-BTC metal organic frameworks (MOFs) having a pore size of 2-5nm. The effect of nanostructuring on hydrogen mobility is examined using quasi-elastic neutron scattering (QENS). While a graduate student, Shathabish NaraseGowda measured nanoconfined NaAlH4 and observed 20% H atoms showing mobility at 350K and 400K. This measurement provides direct evidence that nanostructuring improves long-range mobility of hydrogen at lower temperatures. The possibility for tunable hydrogen desorption temperature via morphology control is near.

Solid State diffusion is enhanced in a-LiAlH4 when mixed with TiH2

Solid State diffusion is enhanced in a-LiAlH4 when mixed with TiH2

Mass transfer limitations inhibit H2 desorption from LiAlH4 at temperatures lower than melting. Recent work has shown that without melting, desorption does not occur for pure LiAlH4 heated repeatedly to just below melting at 150oC. However, after heating to above the melting temperature to 200oC, hydrogen is fully desorbed in 105s. Our work has examined enhancing atomic mobility with amorphous LiAlH4 (which is liquid-like on the molecular scale). We destabilize the a-LiAlH4 using TiH2 to yield Al (crystalline) and 4.8 wt % H2. Step 1 yields 3.0 wt % H2 at 77oC. Step 2 yields 1.8 wt. % H2 at 130oC.

Dobbins T., Ukpai W., Kinetics of H2 Desorption from TiN-Destabilized NaAlH4," ”, Materials Challenges in Energy Materials Challenges in Energy, Edited by Wicks G.G., Simon J., Zidan R. et al., 224, 99-106 (2011).

Diffusion limits H adatom adsorption into Mg to form MgH2

Diffusion limits H adatom adsorption into Mg to form MgH2

Our recently collected X-ray reflectivity film thickness data from Pd/Mg multilayers shows that a surface MgH2 film of 14.4nm forms and prohibits further hydrogenation. Since this layer is impermeable to H the hydrogenation of Mg requires a surface "activation“ (i.e. heating the Mg under vacuum conditions to remove it). Our future work will examine approaches to prevent the formation of this "native hydride" layer.

Dobbins T.A., Kamineni V., Lvov Y.M., Nanoarchitecture of Protective Coatings for the Air Sensitive Metal Hydrides," Material Matters (publication of the Sigma-Aldrich Corporation), , invited (2007).

Kamineni V., Lvov Y.M., Dobbins T.A., Layer-by-Layer Nanoassembly Using Formamide as a Working Solvent," Langmuir, 23, 7423-27 (2007).