Storage breakout session: an overview of current R

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Storage breakout session an overview of current
RD status

• George Thomas
• Consultant, Sandia National Laboratories
• FreedomCAR Storage Tech Team Member
• NSF Workshop
• Arlington, VA
• June 27-29, 2004

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Hydrogen can be stored in many different forms

• high pressure gas
• ambient temperature or low temperature
• liquid
• ambient pressure or high pressure
• solid
• adsorbed (surface) or absorbed (bulk)
• molecular or atomic
• direct or complex
• reversible or non-reversible
• (Note some chemical hydrides may be liquid at
  ambient temperature. This may be an advantage!)

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Current storage development efforts focussed in
these broad areas

• reversible metal hydrides
• complex hydrides
• nitrogen systems
• carbon adsorption
• metal-organic frameworks (MOF)
• clathrates
• chemical hydrides (non-reversible)

absorbed H (bulk)
adsorbed H (surface)
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Reversible hydrides

• have been studied for over 30 years
• have a wide range properties
• short course in hydride thermodynamics
• hydrides absorb hydrogen exothermically
• hydrides release hydrogen endothermically
• hydrogen overpressure in equilibrium with solid
• P exp(-?H/RT ?S/R) or lnP -?H/RT
  lnPTinf
• d(lnP)/d(1/T) ?H/R
• ?Hheat of formation, Rgas constant, Tabs.
  temp.
• P is called the plateau pressure
• so plot of lnP vs 1/T is a straight line (vant
  Hoff plot)
• slope of plot gives you heat of formation

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vant Hoff Plot
-50 C
200 C
0 C
100 C
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Complex hydrides

• Complex hydrides consist of a HM complex with
  additional bonding element(s)
• hydrogen complexes include
• (AlH4) (alanates)
• (BH4)
• H with Group VIII elements.
• advantages
• can have lower formation energy.
• can have high H/M.
• Issues
• decomposition, kinetics
• 173 complex hydrides listed on
  hydpark.ca.sandia.gov

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Total hydrogen content of some alanates
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Issues with complex hydrides

• Reversibility
• role of catalyst or dopant.
• Thermodynamics
• pressure, temperature.
• Kinetics
• long-range transport of heavy species.
• Capacity
• only NaAlH4 has been studied in detail to date.
• theoretical reversible capacity 5.5 wt.
• 4-4.5 wt. demonstrated
• role of catalyst/dopant still not understood
• mechanisms of release and rehydriding not
  understood

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nitrogen systems

• Li3N system (Chen, et. al., Nature 420, 302,
  2002)
• 6.3 wt. (?) , reversibility at 250 C
• Li3N 2H2 Li2NH LiH
• Li2NH LiH H2 LiNH2 2LiH (Li3NH4)
  5.1 wt
• both reactions would yield 10.8 wt
• Fujii, et. al. (National ACS meeting, FUEL 0123)
• modified system to achieve 10.4 wt.
• reversibility at much lower temperatures
• this work needs to be verified

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amino-borane
ammonia-borane complex (A. T-Raissi, 2002 APR.
Golden, CO) H3BNH3(l) H2BNH2(s) H2(g) 6,49
wt. ?H-21.7 kJ/mol xH2BNH2(s) (H2BNH2)x(
s) (polymerizes) (H2BNH2)x(s) (HBNH)x(s)
xH2(g) 6.94 wt. 13.43 wt.
total (HBMNH)x borazine others BN H2
(gt500 C)
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Carbon materials

• Hydrogen adsorbs on carbon surfaces.
• liquid hydrogen density on surface.
• van der Waals bonding (6 kJ/mol).
• very high surface area needed to achieve
  sufficiently high packing density.
• There are many unique carbon structures with high
  surface area
• fullerenes.
• activated carbon.
• nanotubes.
• . . .

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Many carbon materials have been studied as
potential hydrogen storage media

• Material Limitation
• activated carbon low volumetric density
• carbon foam low volumetric density
• carbon aerogels low volumetric density
• fullerenes high temperature release
• (H-C bond)
• graphite fibers low capacity
• (no intercalation)

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Single wall nanotubes have been studied for about
10 years

• Potential for high hydrogen capacity.
• single wall structure
• multiple adsorption sites
• high packing density if aligned
• estimated capacity 6 wt..
• Issues
• variability in results
• processing uncertainties
• release temperatures
• synthesis of large quantities with high purity

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nanoporous inorganic-organic compounds
Synthetic inorganic-organic compound with ZnO4
tetrahedral clusters linked by C6H4-C-O2
struts (Li, Nature, 1999). 1.29 nm spacing
between centers of adjacent clusters.
Benzene-silica hybrid material with 3.8 nm pore
diameter (Inagaki, Nature, 2002).
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Volumetric density is a key issue with all
adsorbed hydrogen systems
Example ZnO4 tetrahedral clusters linked by
C6H4-C-O2 struts 1.29 nm spacing between
centers of adjacent clusters.
700 bar compressed gas
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Clathrates

• clathrate hydrates
• inclusion compounds with water and hydrophobic
  molecules CH4, H2S, CO2, H2
• H2 conditions are extreme
• gt3 kbar (44,000 psi) at -10 C
• gt8 kbar (125,000 psi) at 25 C
• 1.9 wt. theoretical capacity
• need to explore other
• clathrate-forming compounds
• for hydrogen storage

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Reversible hydrogen systems have similar process
chain as gasoline
HYDROCARBON FUEL
REVERSIBLE HYDROGEN SYSTEM
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Chemical hydrogen storage nonreversible
hydrides have more steps in the fuel process
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Chemical hydrogen storage could replace
transport/distribution of hydrogen
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Chemical hydrides some examples

• hydrolysis of metal hydrides
• MHx xH2O ? M(OH)x xH2O
• organic hydrides
• e.g., methylcyclohexane-toluene,
  decaline-napthalene
• FT paraffins
• CnH2n2 ? (n1)H2 nC
• requires heat input, e.g., FT gasoline 23.5
  kJ/mole H2
• hydride slurries
• e.g., MgH2 slurry
• amino-boranes, amino-alanes
• e.g., NH3BH3 , NH3AlH3

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Problems with chemical hydrides

• Issues common to all systems
• recovery, reprocesing infrastructure
• energy cost of rerocessing
• Specific issues
• hydrolysis
• weight, volume of reactants and products limit
  energy density
• e.g., AlH3 H20 ? Al(OH)3 3H2
• 10 wt 11 wt 7.5 wt
• reaction requires Al/oxide mixture resulting in
  an
• overall weight density of 2.6 wt

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some specific issues with chemical hydride systems

• organic hydrides
• catalysts and their lifetime
• high temperature endothermic
• requires separation of products
• amino-boranes
• intermediate reactions, polymerization
• toxicity, safety
• FT-paraffins
• C byproduct
• slurries
• complex systems with low overall energy density
• energy losses

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Hydrogen Densities of Materials
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Hydrogen Densities of Materials
H/C 1
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Novel materials discussed at the Hydrogen Storage
Workshop August 2002
Crystalline Nanoporous Materials Self-Assembled
Nanocomposites Inorganic Organic Compounds BN
Nanotubes Hydrogenated Amorphous
Carbon Mesoporous materials Advanced
Hydrides Bulk Amorphous Materials (BAMs) Nanosize
powders Iron Hydrolysis Hydride
Alcoholysis Polymer Microspheres Metallic Hydrogen
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DOE BES proposal callNovel Materials for
Hydrogen Storage

• complex hydrides
• nanostructured materials
• other materials
• theory, modeling and simulation
• novel analytical and characterization tools

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Potential topics for our consideration

• new materials for hydrogen storage
• innovative materials approaches leading to high
  hydrogen capacities
• new synthesis methods, e.g., physical, chemical,
  metallurgical, thin film deposition
• catalysts
• mechanistic understanding of material behavior
• quantitative, qualitative materials
  characterization
• thermodynamics, kinetics
• modeling of material properties and behavior
• from ab-initio to phenomenological
• the role of catalysts and dopants

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Potential topics for our consideration(continued)

• material properties
• thermal conductivity
• effects of gaseous impurities
• phase, volume changes
• reactivity (e.g., to air, water)
• system modeling
• round-trip energy costs for chemical hydrides
• thermal requirements for filling solid-state
  systems
• integration with fuel cell

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