Alcohol Dehydrogenation Alcohol Dehydrogenation Catalysts Bound to Catalysts Bound to Fuel Cell ElectrodesFuel Cell Electrodes

Tova Sardot and Dr. Eric KelsonTova Sardot and Dr. Eric Kelson

Sigma Xi Symposium 2005Sigma Xi Symposium 2005

California State University, NorthridgeCalifornia State University, Northridge

Hydrogen Fuel CellsHydrogen Fuel Cells

Anode: 2H2 –––> 4H+ + 4e-

Cathode: O2 + 4H+ + 4e- –––> 2H2O

Net: 2H2 + O2 –––> 2H2O

Challenges for Fuel CellsChallenges for Fuel Cells

Pressurized hydrogen gas is hazardousPressurized hydrogen gas is hazardous (Flammability, etc.) (Flammability, etc.)

On-demand hydrogen production is On-demand hydrogen production is inefficient. inefficient. (High cost of Pt reformer catalysts) (High cost of Pt reformer catalysts) (Pollutants can still form) (Pollutants can still form)

Research RationaleResearch Rationale

Employ organic alcohol fuelsEmploy organic alcohol fuels (Avoids flammability and storage issues) (Avoids flammability and storage issues)

Dr. Kelson’s group has developed Ru Dr. Kelson’s group has developed Ru catalysts for harvesting hydrogen gas from catalysts for harvesting hydrogen gas from alcohols. alcohols.

Techniques needed to immobilize catalysts Techniques needed to immobilize catalysts on fuel cell electrodes. on fuel cell electrodes.

Research ObjectivesResearch Objectives

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

Specifically:Specifically:

• • Paint on Nafion cation exchange resin.Paint on Nafion cation exchange resin.• • Electropolymerized vinylpyridines. Electropolymerized vinylpyridines.

Main CatalystMain CatalystOH

R1CH

R2

O

R1C

R2

[Ru]+ H2(g)

NN

NRu

OH

H

N O

N O

[1]

(Trpy)Ru(PyrrolO)2(OH2)

PendantBase

Trpy

BoundWater

Cationic Catalyst ModelsCationic Catalyst Models

• • More simple than actual catalysts.More simple than actual catalysts.• • RuRuIIIIII/Ru/RuIIII redox couples better behaved. redox couples better behaved.• • Charge useful for binding. Charge useful for binding.

NN

RuII O

N

N

N

H

O

+

H

(Bpy)2Ru(PyO)2(OH2)+

[3]

NN

RuII O

N

N

HN O

+

H

(Bpy)2Ru(PyrrolO)2(OH2)+

[2]

CyclovoltammetryCyclovoltammetry

• 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.

RuII

RuIII(Start)

Voltage (mV) versus (Ag/AgCl) Reference

C u r r e n t

Nafion CoatingsNafion Coatings

Nafion is a polymer with sulfonic (SONafion is a polymer with sulfonic (SO33--) )

groups attached to Teflon chains. groups attached to Teflon chains. Functions as a strong proton donor.Functions as a strong proton donor. Cationic compounds could exchange for protons Cationic compounds could exchange for protons

within Nafion. within Nafion. Nafion can be painted onto electrode and then Nafion can be painted onto electrode and then

dipped into catalyst solution. dipped into catalyst solution. Bound catalyst can be measured Bound catalyst can be measured

electrochemically through its Ru electrochemically through its RuIIIIII/Ru/RuIIII couple. couple.

Binding Complexes in NafionBinding Complexes in Nafion

Blank NafionBlank Nafion Complex Complex 22 in Nafion in Nafion

Nafion soaked in Nafion soaked in 22 or or 33 solution exhibits clear solution exhibits clear RuRuIIIIII/Ru/RuIIII signal. signal.

Indicates that complex bound in NafionIndicates that complex bound in Nafion

Binding Complexes in NafionBinding Complexes in NafionFor For 22 in Nafion (0.033 M Na in Nafion (0.033 M Na22SOSO44))

• • Linear relationship indicative of bound complex.Linear relationship indicative of bound complex.

• • Catalyst Catalyst 11 did not bind in Nafion in spite of did not bind in Nafion in spite of sulfonate groups that should have protonated it.sulfonate groups that should have protonated it.

0

2

4

6

8

10

12

0 100 200 300 400 500 600

Cu

rre

nt

(mic

roA

)

Scan Rate (mV/s)

Polyvinylpyridine LayersPolyvinylpyridine Layers

2-Vinylpyridine reported to electropolymerize2-Vinylpyridine reported to electropolymerize in pH=4 electrolytes at -1.3 V (Ag/AgCl). in pH=4 electrolytes at -1.3 V (Ag/AgCl).

N N N N

Polyvinylpyridine(Pyridine rings attached to polyethylene chain)

-1.3 V (Ag/AgCl)

pH=4 electrolyte

Polyvinylpyridine LayersPolyvinylpyridine Layers

Resulting polypyridine is partially protonatedResulting polypyridine is partially protonated• Protonated groups can bind anions.• Protonated groups can bind anions.

• Remaining pyridine groups can bind to Ru.• Remaining pyridine groups can bind to Ru.

N NH[Ru]

Electrostatic Attraction

N NPyridine

Coordination

[Ru]

Polyvinylpyridine LayersPolyvinylpyridine Layers

2-Vinylpyridine electropolymerizes at -1.3 2-Vinylpyridine electropolymerizes at -1.3 V (Ag/AgCl) onto Au electrodes at pH=4.V (Ag/AgCl) onto Au electrodes at pH=4.

Coating durations of 1 second optimal.Coating durations of 1 second optimal.

• • Tested through electrochemistry of Tested through electrochemistry of Ru RuIIIIII/Ru/RuIIII couple of couple of 2:2:

• • Enough to begin distorting signal.Enough to begin distorting signal.• • Signal still clear. Signal still clear.

2 in Polyvinylpyridine2 in Polyvinylpyridine200 mV shift in Ru200 mV shift in RuIIIIII/Ru/RuIIII potential due to coating potential due to coating

Without layer:Without layer:

With layer:With layer:

2 in Polyvinylpyridine2 in Polyvinylpyridine

RuRuIIIIII/Ru/RuIIII potential restored when layer potential restored when layer physically removed. physically removed.

200 mV potential shift also observed when 200 mV potential shift also observed when

2-vinylpyridine added to 2-vinylpyridine added to 22 in solution. in solution.

Behavior suggests Behavior suggests 22 must bind to layer must bind to layer pyridines to transfer electrons. pyridines to transfer electrons.

2 in Polyvinylpyridine2 in Polyvinylpyridine Current versus scan rate behavior Current versus scan rate behavior

indicates indicates 22 binds reversibly and rapidly. binds reversibly and rapidly.

Nevertheless, Nevertheless, 22 appears to bind to layer to appears to bind to layer to transfer electrons. transfer electrons.

0

1

2

3

4

5

0 5 10 15 20 25 30 35

Cu

rre

nt

(mic

roA

)

Square Root Scan Rate

ConclusionsConclusions Nafion electrode coatings are easily Nafion electrode coatings are easily

formed by solution application. formed by solution application. Cationic complexes Cationic complexes 22 and and 33 bind in Nafion bind in Nafion

but but 11 does not. does not. 2-Vinylpyridine and electropolymerize into 2-Vinylpyridine and electropolymerize into

electrode coatings. electrode coatings. Complex Complex 22 reversibly binds to reversibly binds to

2-polyvinylpyridine for electron transfer 2-polyvinylpyridine for electron transfer to electrode. to electrode.

Future DirectionsFuture Directions

Explore possible binding of complexes Explore possible binding of complexes 11 and and 33 to polyvinylpyridines. to polyvinylpyridines.

Survey effects of polyvinylpyridine Survey effects of polyvinylpyridine modifications to encourage binding. modifications to encourage binding.

Synthetically incorporate catalysts directly Synthetically incorporate catalysts directly into polymer chain. into polymer chain.

AcknowledgementsAcknowledgements

JPL-NASA Pair ProgramJPL-NASA Pair Program

Dr. Carol ShubinDr. Carol Shubin

Dr. Eric KelsonDr. Eric Kelson