Darlene Slattery, Ph'D'

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Alanates as Hydrogen Storage Compounds

• Darlene Slattery, Ph.D.
• Florida Solar Energy Center
• 1679 Clearlake Road
• Cocoa, FL 32922
• Phone [phone] Fax [phone]

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What are Alanates?

• Complex Hydrides containing AlH4-
• NaAlH4 LiAlH4 Mg(AlH4)2
• Ca(AlH4)2 KAlH4 Ti(AlH4)4

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• Hydrogen atoms arranged tetrahedrally around Al
• Hydrogens retain significant hydride or
  electron-rich character
• 18 previously reported alanates
• see hydpark.ca.sandia.gov

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• A.E. Finholt, A.C. Bond Jr., H.I. Schlesinger J.
  Am. Chem. Soc. 69, 1199-1203 (1947).
• 4 LiH AlCl3 ether LiAlH4 3 LiCl
• Also noted evidence for NaAlH4 and Ca(AlH4)
• Wiberg (Angew.Chem., 65, 16 (1953)) reports
  Mg(AlH4)2

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Improved Synthesis

• E. Ashby, G. Brendel, H. Redman, Inorg. Chem., 2,
  499 (1963). Quant. Yield in THF, 140C, 5000 psi
  H2
• NaH Al 3/2 H2 NaAlH4
• Na Al 2 H2 NaAlH4
• T. Dymova, N. Eliseeva, S. Bakum, Yu. M.
  Dergachev, Dokl. Akad. Nauk SSSR, 215, 256
  (1974). melt, 270-280C, 2539 psi
• Na Al 2 H2 NaAlH4

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Mechanical Alloying

• T.N. Dymova, et al. Russ. J. Coord. Chem.,
  19(1993) 607.
• MH AlH3 MAlH4
• Ball to powder ratio, 201
• Vibration time of 3 hours

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Complex Hydride Uses

• Reducing agent in synthetic chemistry
• O
• CH3C OH CH3CH2OH
• FeCl3 3 LiAlH4 3 LiCl Fe.3Al 6 H2

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Hydrogen Storage?

• Hydride H2 Content
• LiAlH4 10.5
• NaAlH4 7.5
• KAlH4 5.7
• Be(AlH4)2 11.3
• Mg(AlH4)2 9.3
• Ca(AlH4)2 7.7
• Ti(AlH4)4 9.3

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Decomposition Reaction

• NaAlH4 1/3 Na3AlH6 2/3 Al H2
• Na3AlH6 3 NaH Al 3/2
  H2
• Dymova, et al. described multi-step decomp. but
  practical reversibility not demonstrated prior to
  1995

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Reversibility

• B. Bogdanovic, M. Schwickardi, U.S. Patent
  6,106,801, 2000.
• NaAlH4 Ti(OBu)4 Ti-NaAlH4
• NaAlH4 TiCl4 Ti-NaAlH4

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• NaAlH4 1/3 Na3AlH6 2/3 Al H2
• Na3AlH6 3 NaH Al 3/2
  H2

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Advantages of Ti Doping

• Reversibility
• Reversible content of doped 3.1 - 4.2 wt
  undoped 0.5 0.8 wt
• Improved H2 desorption rate
• Higher cycle stability
• Reduction in dehydriding T by 50 OC

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C.M. Jensen et al., Int. J. Hydrogen Energy, 24
(1999) 461-465. Dry doped with Ti(OBu)4
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J. Alloys and Compounds, 285 (1999) 119-122.
H/M wt.
temperature, oC
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Zr as a Dopant

• Zr inferior to Ti for NaAlH4 to Na3AlH6
• Zr superior for Na3AlH6 to NaH and Al
• Doping with both Zr and Ti improves overall
  dehydriding reaction

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Difficulties Resulting from Doping Procedures

• Use of alkoxides contaminates desorbed H2
• Weight penalty
• Oxygen from decomp. of alkoxide contaminates
  active material
• K.J. Gross, G.J. Thomas, C.M. Jensen, J. Alloys
  and Compounds, 330-332 (2002) 683-690.

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Doping with TiCl4

• TiCl4 4 NaAlH4 Ti 4 NaCl
  4 Al 8 H2
• For every mole of TiCl4 added, 4 moles of NaAlH4
  consumed
• 4 mole of NaCl contributing to weight

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Doping with Ti

• Ball milling of elemental Ti and NaAlH4
• Kinetics of initial release better than ball
  milling alone
• Reduction in decomp. T lt for TiCl4
• Rehydrides at 120 oC and 800 psi
• Subsequent dehydriding maintains content but
  kinetics poor

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Doping with Carbon

• A. Zaluska et al., J. Alloys and Compounds, 298
  (2000) 125-134.
• NaAlH4 ball milled with 10 wt C
• Kinetics improved over other dopants
• Rate increases with subsequent cycles
• Second step of decomp also affected but slower
  than first step
• Rehydrog occurs under practical conditions

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Nature of Catalyst

• K.J. Gross, et al., J. Alloys and Compounds, 297
  (2000) 270-281.
• Surface localized species
• V.P. Balema et al., J. Alloys and Compounds, 329
  (2001), 108-114
• Ti 4 Al Al3Ti Al

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• D. Sun et al., J. Alloys and Compounds, 337
  (2002) L8-L11.
• Lattice Expansion Substitution

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Other Alanates LiAlH4

• Chen, et al., J. Phys. Chem. B, 105 (2001)
  11214-11220.
• Showed that vibrating-mill technique produces
  Ti-doped LiAlH4 and Li3AlH6 powders with
  nanocrystallites
• 1st decomp step at 100 oC releases 5.3 wt
• 2nd decomp step at 135 oC releases 2.6 wt

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LiAlH4 - continued

• T.N. Dymova, et al., Russian Journal of
  Coordination Chem. 21(3) (1995)165-172.
• Melting during decomp allows rearrangement from
  tetrahedral to octahedral
• AlLi intermetallic compound forms

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Other Alanates Na2LiAlH6

• Huot, et al., J. Alloys and Compounds, 383 (1999)
  304-306.
• NaH LiH NaAlH4 MA Na2LiAlH6
• B. Bogdanovic, M. Schwickardi J. Alloys and
  Compounds, 253-254 (1997) 1-9.
• LiAlH4 NaH toluene/H2 Na2LiAlH6

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Other Alanates - KAlH4

• Morioka, et al., J. Alloy and Compounds, 353
  (2003) 310-314.
• KAlH4 300 1/3 K3AlH6 2/3 Al H2
• K3AlH6 340 3 KH Al 3/2 H2
• Reversible without a catalyst

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Other Alanates Mg(AlH4)2

• M. Fichtner, O. Fuhr J. Alloys and Compounds, 345
  (2002) 286-296.
• 2 LiAlH4 MgX2 THF Mg(AlH4)2 2 LiX
• where X Br or Cl
• Forms as solvent adduct
• M. Fichtner, et al., In press
• Decomp. at 163 oC, rehydriding not studied

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Mg(AlH4)2 Cont.

• Dymova et al., Russian J. of Coordination Chem.
  25(5), (1999) 312-315
• Does not form intermediate complex hydrides
• Mg(AlH4)2 135-155 C MgH2 2 Al 3 H2
• MgH2 2 Al 300 C 0.43 Al0.93 Mg0.07
  2.58 Al0.62Mg0.38 H2

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Engineering Considerations

• G. Sandrock, et al., J. Alloys and Compounds,
  330-332 (2002) 696-701
• Contamination of resulting H2
• Weight penalty from dopants
• Self-heating exceeds m.p.
• Volume changes lower than conventional
• Desorption kinetics (down to r.t.)

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• Future Work Needed
• B. Bogdanovic, G. Sandrock, MRS Bulletin, Sept.
  2002, 712-716.

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ID and Mechanism of Ti Catalyst

• XRD and microscopic analyses have failed
• amorphous
• too fine
• part of alanate lattice

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• Optimization of Catalyst

• Concentration on Ti, Zr, Fe, C
• Is there something better
• Can decomp of Na3AlH6 be improved

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H2 Capacity and Cyclic Retention

• Achieving full cyclic potential
• H2 content to fixed amount of Al
• More cyclic data needed
• Additional techniques

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Engineering Data

• What configuration should be used for alanate
  beds
• Thermal conductivities
• Expansion behavior
• Potential reactions with container materials
• Need to quantify safety aspects

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Thermodynamic Tailoring

• Substitution in intermetallics
• Li substituted for Na
• Response to catalysis
• Increasing plateau pressure w/o losing hydrogen
  capacity

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Improved Synthesis

• Only LiAlH4 available in industrial quantities
• NaAlH4 expensive
• Other alanates not commercially available
• Best method for catalyzing

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Other Complex Hydrides

• Borohydrides, BH4-
• Beryllium hydrides, BeH3-
• Transition metal complex hydrides
• FeH64-
• PdH22-
• PtH42-

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Conclusions

• Prior to Bogdanovics discovery, complex hydrides
  thought to be irreversible
• NaAlH4 has been studied extensively
• Data for other complex hydrides still limited
• Much work remains before DOE goals are achieved