Project Lead, Jim Wegrzyn,
Brookhaven National Laboratory
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.
|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|
|Refueling time (180 s)||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
 J. Graetz and J.J. Reilly, "Kinetically Stabilized Hydrogen Storage Materials" Scripta Materialia, in press (2006).
 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).
 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).
 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).
 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).
 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).
 J. Graetz and J. J. Reilly, "Thermodynamics of the a, b and g polymorphs of AlH3" J. Alloys Comp., 424 262 (2006).
 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).
 "Kinetics and Thermodynamics of the Aluminum Hydride Polymorphs", International Symposium on Metal-Hydrogen Systems, (invited) 2006.
 "Thermodynamics and Kinetics of the aluminum hydride polymorphs", Spring Meeting of the Materials Research Society, 2006.
 "Bonding and Local Atomic Structure of Ti in Complex Metal Hydrides", Spring Meeting of the Materials Research Society, 2006.