Hydrogen Storage in NHxBHx Materials' Effect of Nanoporous Templates on H2 Release

1
Hydrogen Storage in NHxBHx Materials. Effect of
Nanoporous Templates on H2 Release

• Integrated theory/experimental program to develop
  a fundamental understanding of the H2 release and
  uptake
• Maciej Gutowski
• Tom Autrey
• Pacific Northwest National Laboratory
• Chemical Sciences Division
• Richland, WA 99352

2
Goal and Approach

• Design a new material that would
• Have improved hydrogen capacity in comparison
  with NaAlH4 or irreversible chemical hydrides
• Avoid formation of thermodynamically very stable
  B-O bonds (as from hydrolysis of NaBH4)
  reversibility problem
• Construction principle
• Physisorption not promising we explore
  chemically bound H
• H2 will probably break heterolytically (H2 ? H-
  H)
• New materials should have alternating electron
  rich and electron deficient sites capable of
  binding H and H-, respectively
• For example nitrogen compounds are electron
  rich, boron (aluminum) compounds are electron
  deficient? NHxBHx and NHxAlHx compounds.

3
Why NHxBHx are promising?

• Mass weight percent of H in NHxBHx is 24.4 and
  19.5 for x4 and 3, respectively (hydrogen is
  stored on both the N and B site)
• BN is isoelectronic with CC ? NH4BH4 and NH3BH3
  are analogous to CH4 and C2H6, respectively, but
  B and N differ in electronegativity ? the BN
  compounds are solids ? they meet the volumetric
  requirements.
• The NHxBHx are stable in water and air
• Elements B and N commonly available.
• How close are the H2 release steps from
  thermoneutrality?

4
Materials for H2 Storage
Ref A. Züttel, Materials for hydrogen storage,
Materials Today, Septemper (2003), pp. 18-27
5
Step-by-Step Decomposition of NHxBHxTheoretical
Estimates of Reaction Energies
(NHBH)n ? nBN(s) nH2 -9.2

• No thermodynamic sink for this hydrogen
  release
• Need for catalysts for hydrogen release/uptake
• Theoretical ca. 6 wt of H2 per step

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Mesoporous Scaffolds Control Reactivity and
Selectivity from Ammonia-BoraneNH3BH3(s) ?
(NH2BH2)n nH2 ? (NHBH)n nH2
50-60 C SBA-15
Extent of Reaction
80, 85 C Neat
Time (min)

• Kinetics have sigmoidal time dependence ?
  Autocatalytic
• Preliminary findings suggest nanoporous substrate
  lowers barrier to initiation
• Nanoporous substrate removes unwanted side
  products

7
Fundamental Issues Dihydrogen Bonding
Catalysis for B-N formation

• Initiation and Growth of Dehydropolymerization
  reaction
• What is mechanism for H2 release and uptake from
  NHxBHx?
• Whats the role of dihydrogen bonding?
• What are reaction pathways and the barriers for
  the two distinct steps, initiation and
  polymerization?
• Whats the branching ration between
  intramolecular vs. intermolecular pathways for
  consecutive steps of hydrogen release?
• Interfaces and Catalysis
• How does the interface with a template affect the
  reactivity and selectivity?
• What are effects of templates other than silica
  (TiO2, BN nanotubes)?
• What are catalytic sites for B, N bond formation
  (and scission) that leads to concurrent release
  (and uptake) of hydrogen?

8
Future Work

• Theory
• Kinetic barriers for all steps of hydrogen
  releaseuptake
• Time evolution of hydrogen rich materials as a
  function of temperature and hydrogen gas pressure
  through molecular dynamics simulations
• Effect of zero-point motion and tunneling on
  hydrogen binding/release and diffusion (Feynman
  path integral simulations)
• Thermodynamics and kinetics of chemically
  modified NHxBHx materials (CBN hydrides)
• Experiment
• Mechanistic studies of hydrogen release/uptake as
  a function of chemical composition of the support
  and its nano-geometry (functionalized SiO2, TiO2,
  and BN nanotubes)
• NMR, IR, and DSC experiments to provide further
  insight atomic structure, thermodynamics, and
  kinetics of hydrogen releaseuptake at
  interfaces.
• Catalytic activity of transition metals in the
  process of hydrogen release uptake.

9
Summary

• NHxBHx compounds display favorable properties for
  hydrogen storage
• They meet or exceed the requirements formulated
  by DOE and automotive industry
• No thermodynamic sink upon hydrogen release or
  uptake
• Air and water stable
• B and N elements commonly available
• Nanoporous templates provide an additional handle
  to control hydrogen release
• Need for nano science approach and novel
  catalysts to further tune thermodynamics and
  kinetics