Transcript Alcohol Dehydrogenation Catalysts Bound to Fuel Cell Electrodes

Alcohol Dehydrogenation
Catalysts Bound to
Fuel Cell Electrodes
Tova Sardot and Dr. Eric Kelson
Sigma Xi Symposium 2005
California State University, Northridge
Hydrogen Fuel Cells
Anode:
2H2 –––> 4H+ + 4eCathode:
O2 + 4H+ + 4e- –––> 2H2O
Net:
2H2 + O2 –––> 2H2O
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)
Research Rationale
 Employ organic alcohol fuels
(Avoids flammability and storage issues)
 Dr. Kelson’s group has developed Ru
catalysts for harvesting hydrogen gas from
alcohols.
 Techniques needed to immobilize catalysts
on fuel cell electrodes.
Research Objectives
 Develop electrode coatings that bind metal
catalysts for fuel cell applications.
 Specifically:
• Paint on Nafion cation exchange resin.
• Electropolymerized vinylpyridines.
Main Catalyst
OH
R1
CH
O
[Ru]
R2
R1
C
+ H2(g)
R2
Pendant
Base
N
[1]
O
N
N
Ru H
N
O
H
N
O
Trpy
Bound
Water
(Trpy)Ru(PyrrolO)2(OH2)
Cationic Catalyst Models
• More simple than actual catalysts.
• RuIII/RuII redox couples better behaved.
• Charge useful for binding.
+
N
+
O
N
H
N
O
RuII
N
NH
H
N
O
RuII
N
NH
N
[2]
(Bpy)2Ru(PyrrolO)2(OH2)+
O
N
[3]
(Bpy)2Ru(PyO)2(OH2)+
Cyclovoltammetry
RuII
(Start)
C
u
rr
e
n
t
RuIII
Voltage (mV) versus (Ag/AgCl) Reference
• 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.
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.
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
Binding Complexes in Nafion
For 2 in Nafion (0.033 M Na2SO4)
12
Current (microA)
10
8
6
4
2
0
0
100
200
300
400
500
600
Scan Rate (mV/s)
• Linear relationship indicative of bound complex.
• Catalyst 1 did not bind in Nafion in spite of
sulfonate groups that should have protonated it.
Polyvinylpyridine Layers
 2-Vinylpyridine reported to electropolymerize
in pH=4 electrolytes at -1.3 V (Ag/AgCl).
N
-1.3 V (Ag/AgCl)
pH=4 electrolyte
N
N
N
Polyvinylpyridine
(Pyridine rings attached to polyethylene chain)
Polyvinylpyridine Layers
 Resulting polypyridine is partially protonated
• Protonated groups can bind anions.
N
NH
[Ru]
Electrostatic
Attraction
• Remaining pyridine groups can bind to Ru.
N
N
[Ru]
Pyridine
Coordination
Polyvinylpyridine Layers
 2-Vinylpyridine electropolymerizes at -1.3
V (Ag/AgCl) onto Au electrodes at pH=4.
 Coating durations of 1 second optimal.
• Tested through electrochemistry of
RuIII/RuII couple of 2:
• Enough to begin distorting signal.
• Signal still clear.
2 in Polyvinylpyridine
200 mV shift in RuIII/RuII potential due to coating
Without layer:
With layer:
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.
2 in Polyvinylpyridine
 Current versus scan rate behavior
indicates 2 binds reversibly and rapidly.
5
Current (microA)
4
3
2
1
0
0
5
10
15
20
25
30
35
Square Root Scan Rate
 Nevertheless, 2 appears to bind to layer to
transfer electrons.
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.
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.
Acknowledgements
JPL-NASA Pair Program
Dr. Carol Shubin
Dr. Eric Kelson