Alcohol Dehydrogenation Catalysts Bound to Fuel Cell Electrodes

1
Alcohol Dehydrogenation Catalysts Bound to Fuel
Cell Electrodes

• Tova Sardot and Dr. Eric Kelson
• Sigma Xi Symposium 2005
• California State University, Northridge

2
Hydrogen Fuel Cells
Anode 2H2 gt 4H 4e- Cathode
O2 4H 4e- gt 2H2O Net 2H2
O2 gt 2H2O
3
Challenges for Fuel Cells

• Pressurized hydrogen gas is hazardous
  (Flammability, etc.)
• On-demand hydrogen production is inefficient.
  (High cost of Pt reformer catalysts)
  (Pollutants can still form)

4
Research Rationale

• Employ organic alcohol fuels (Avoids
  flammability and storage issues)
• Dr. Kelsons group has developed Ru catalysts
  for harvesting hydrogen gas from alcohols.
• Techniques needed to immobilize catalysts on
  fuel cell electrodes.

5
Research Objectives

• Develop electrode coatings that bind metal
  catalysts for fuel cell applications.
• Specifically

Paint on Nafion cation exchange resin.
Electropolymerized vinylpyridines.
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Main Catalyst
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Cationic Catalyst Models
More simple than actual catalysts. RuIII/RuII
redox couples better behaved. Charge useful for
binding.
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Cyclovoltammetry
Voltage applied to electrode varied linearly
with time as current is simultaneous
measured. Surge of current with increasing or
decreasing voltage represents oxidation or
reduction, respectively. Average of peak
voltages represents potential of RuIII/RuII
redox couple.
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Nafion Coatings

• Nafion is a polymer with sulfonic (SO3-)
  groups attached to Teflon chains.
• Functions as a strong proton donor.
• Cationic compounds could exchange for protons
  within Nafion.
• Nafion can be painted onto electrode and then
  dipped into catalyst solution.
• Bound catalyst can be measured
  electrochemically through its RuIII/RuII couple.

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Binding Complexes in Nafion
Blank Nafion
Complex 2 in Nafion
Nafion soaked in 2 or 3 solution exhibits clear
RuIII/RuII signal.
Indicates that complex bound in Nafion
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Binding Complexes in Nafion
For 2 in Nafion (0.033 M Na2SO4)
Linear relationship indicative of bound complex.
Catalyst 1 did not bind in Nafion in spite of
sulfonate groups that should have protonated
it.
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Polyvinylpyridine Layers

• 2-Vinylpyridine reported to electropolymerize
  in pH4 electrolytes at -1.3 V (Ag/AgCl).

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Polyvinylpyridine Layers

• Resulting polypyridine is partially protonated
  Protonated groups can bind anions.
  Remaining pyridine groups can bind to Ru.

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Polyvinylpyridine Layers

• 2-Vinylpyridine electropolymerizes at -1.3 V
  (Ag/AgCl) onto Au electrodes at pH4.
• Coating durations of 1 second optimal.

Tested through electrochemistry of
RuIII/RuII couple of 2
Enough to begin distorting signal. Signal
still clear.
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2 in Polyvinylpyridine
200 mV shift in RuIII/RuII potential due to
coating
Without layer
With layer
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2 in Polyvinylpyridine

• RuIII/RuII potential restored when layer
  physically removed.
• 200 mV potential shift also observed when
  2-vinylpyridine added to 2 in solution.
• Behavior suggests 2 must bind to layer
  pyridines to transfer electrons.

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2 in Polyvinylpyridine

• Current versus scan rate behavior indicates 2
  binds reversibly and rapidly.
• Nevertheless, 2 appears to bind to layer to
  transfer electrons.

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Conclusions

• Nafion electrode coatings are easily formed by
  solution application.
• Cationic complexes 2 and 3 bind in Nafion but
  1 does not.
• 2-Vinylpyridine and electropolymerize into
  electrode coatings.
• Complex 2 reversibly binds to
  2-polyvinylpyridine for electron transfer to
  electrode.

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Future Directions

• Explore possible binding of complexes 1 and 3
  to polyvinylpyridines.
• Survey effects of polyvinylpyridine
  modifications to encourage binding.
• Synthetically incorporate catalysts directly
  into polymer chain.

20
Acknowledgements

• JPL-NASA Pair Program
• Dr. Carol Shubin
• Dr. Eric Kelson