Alanes

Aluminum hydride, AlH₃, is a fascinating material that has recently attracted attention for its potential as a hydrogen storage medium for low temperature fuel cells. It has a volumetric hydrogen capacity (0.148 g/mL), twice that of liquid hydrogen (0.07 g/mL) at 20 K, and a gravimetric hydrogen storage capacity exceeding 10 wt %. AlH₃ is stable at room temperature, despite having an equilibrium hydrogen pressure of approximately 7 kbar at 298 K. The stability is generally attributed to a surface oxide layer, which acts as a kinetic barrier to decomposition and protects AlH₃ from the environment. Under this project, three aluminum hydride phases (a-AlH₃, b-AlH₃, and g-AlH₃) were prepared by organometallic synthesis.

Figure 1. Crystallographic structures of a-AlH₃ (R-3c), b-AlH₃ (Fd-3m), and g-AlH₃ (Pnnm) determined from x-ray and neutron powder diffraction.

The crystallographic structures of a-AlH₃, b-AlH₃, and g-AlH₃ (Figure 1) were determined using x-ray and neutron powder diffraction. Although the structure of the a phase is well known, the structures of the b and g phases were recently determined through collaboration with IFE (Norway). a-AlH₃ has a hexagonal unit cell (R-3c space group) and exhibits corner-connected AlH6 octahedra. b-AlH₃ crystallizes in a cubic structure (Fd-3m space group) and also exhibits corner-connected octahedra with large 4 Å channels penetrating the structure. g-AlH₃ has an orthorhombic structure (Pnnm space group) and exhibits both corner and edge-sharing octahedra. AlH₃ was also prepared in two amorphous solvated phases of AlH₃•Et₂O and AlH₃•THF.

The temperature-dependent rate constants were determined by measuring the isothermal hydrogen evolution between 30°C and 140°C (Figure 2). Fractional decomposition curves showed good fits using both the second and third-order Avrami-Erofeyev (A-E) equations. This suggests that the kinetics of the aluminum hydride polymorphs are controlled by nucleation and growth. Figure 2 also shows that the decomposition of the a phase is slower at room temperatures than b and g phases, but exhibits similar rates at temperatures greater than 100°C. Therefore, the a phase was down-selected for further studies. In general, because of its high hydrogen storage capacity and fast kinetics around 100°C, AlH₃ is a good candidate material for storing hydrogen. However, the conventional organometallic synthesis is an energy intensive and costly procedure, and AlH₃ is not a reversible hydride at moderate hydrogen pressures. Therefore, the utility of this material will depend on the development of new techniques to regenerate AlH₃ from spent Al powder in a cost-effective manner.

Figure 2. Hydrogen discharge rates (based on 100 kg AlH₃) as a function of temperature. The dotted line represents the DOE full flow target for a 50 kW fuel cell.

Table I shows nine DOE storage targets. AlH₃ can meet the first five targets, and work is underway in developing novel regeneration routes. The goal is to regenerate AlH₃ with an energy input of less than 120 kJ/mol-H₂. Once this goal is attained, durability and operability demonstration tests are planned.
Table I:

DOE 2010 Hydrogen Storage Targets	Project Status
Gravimetric system capacity (2 kWh/kg)	3.3 kWh/kg (material)
Volumetric system capacity (1.5 kWh/L)	4.5 kWh/L (material)
Operating temperature (85°C)	(110-115)°C
Operating Pressure ([4-100] atm)	4 atm
Fuel flow rate (.02[g/s]/kW)	0.02 [g/s]/kWh
Health and Safety	Ongoing
Energy Efficiency (Regeneration)	Challenging
Durability/Operability	TBD
Refueling time (180 s)	Challenging
Costs ($4/kWh)	Challenging

A summary of the participants and the R&D activities in the Aluminum Hydride project is given in Table II. The project consists of 7 organizations within the MHCoE with expertise in experimental studies, theory and modeling, and process diagnostics. The team is currently working on three major areas: 1) synthesis, 2) material properties and 3) regeneration.

Table II. Participants and research activities in Project D, Aluminum Hydride

Aluminum Hydride Synthesis
BNL	Jim Wegrzyn, Jason Graetz	Aluminum hydride synthesis
U. Hawaii	Craig Jensen	Novel synthesis processes
Aluminum Hydride Material Properties and Kinetics
BNL	Jim Wegrzyn, Jason Graetz	Decomposition kinetics
SNL	TBD	Thermal management fuel tank modeling
U. Illinois	Ian Robertson	TEM oxide coating studies
JPL	Bob Bowman	NMR material studies
Aluminum Hydride Regeneration
BNL	Jim Reilly, Jason Graetz	Solvent phase regeneration
SRNL	Ragaiy Zidan	Electro-chemical regeneration
ORNL	Gilbert Brown	Alane chemistry studies

- Decomposition curves fit to A-E equation:
[-ln(1-a)]1/2 = kt

• Rate limited by nucleation & growth of Al phase
• T≥100 C rates similar for a, b and g-AlH₃
• g and b phases decompose more rapidly than a at <100C
• Down select a-AlH₃ due to low temperature stability

Recent Publications:

[1] J. Graetz and J.J. Reilly, "Kinetically Stabilized Hydrogen Storage Materials" Scripta Materialia, in press (2006).

[2] H.W. Brinks, C. Brown, C.M. Jensen, J. Graetz, J.J. Reilly, B.C. Hauback, "The crystal structure of a-AlD3" J. Alloys Compd., in press (2006).

[3] H.W. Brinks, W. Langley, C.M. Jensen, J. Graetz, J.J. Reilly, B.C. Hauback, "Synthesis and crystal structure of a-AlD₃" J. Alloys Compd., in press (2006).

[4] S. Chaudhuri, J. Graetz A. Ignatov, J. J. Reilly and J. T. Muckerman, "Understanding the role of Ti in reversible hydrogen storage as sodium alanate: A combined experimental and first-principles theoretical approach" J. Amer. Chem. Soc. 128 11404 (2006).

[5] S.-J. Wong, R.C. Bowman, J. Graetz and J.J. Reilly, "Solid State NMR Studies of the Aluminum Hydride Phases", Mat. Res. Soc. Conf. Proc. 927 (2006).

[6] J. Graetz, J. Reilly, G. Sandrock, J. Johnson, W.-M. Zhou, and J. Wegrzyn, "Aluminum hydride, AlH₃, as a hydrogen storage compound" TMS Proceedings Advanced Materials for Energy Conversion 111 p. 57 (2006).

[7] J. Graetz and J. J. Reilly, "Thermodynamics of the a, b and g polymorphs of AlH₃" J. Alloys Comp., 424 262 (2006).

[8] G. Sandrock, J. Reilly, J. Graetz, W.-M. Zhou, J. Johnson and J. Wegrzyn, "Alkali metal hydride doping of a-AlH₃ for enhanced H₂ desorption kinetics" J. Alloys Comp., 421 185 (2006).

Recent Presentations:

[1] "Kinetics and Thermodynamics of the Aluminum Hydride Polymorphs", International Symposium on Metal-Hydrogen Systems, (invited) 2006.

[2] "Thermodynamics and Kinetics of the aluminum hydride polymorphs", Spring Meeting of the Materials Research Society, 2006.

[3] "Bonding and Local Atomic Structure of Ti in Complex Metal Hydrides", Spring Meeting of the Materials Research Society, 2006.