Electrocatalytic and Photolytic Studies in Proton Exchange Membrane Applications

1
Electrocatalytic and Photolytic Studies in Proton
Exchange Membrane Applications

• Brian Seger
• 12/12/07
• Advisor Professor Kamat

2
The Potential of Fuel Cells

• Currently most of our power comes from
  nonrenewable resources.
• Many of these fuels also produce CO2, which
  contributes to global warming.

• Environmentally, fuel cells are much better than
  other sources such as the internal combustion
  engine.
• Fuel cells are inherently more efficient than
  current power sources.

Larminie and Dicks, 2003, Wiley
3
Basic Principles of Fuel Cells

• Below is a typical fuel cell diagram

4
Fuel Cell Issues

• The theoretical voltage difference between the
  reactants and the products is 1.23 V.
• Experimentally efficiency losses drastically
  reduce this voltage.

• Platinum is used as a catalyst, and Nafion is
  typically used as a membrane, both of which are
  very expensive.
• Side reactions degrade the lifetime of the fuel
  cell.

Bernardi et. al. 1992, JES
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My Objectives

• To design and develop new electrocatalysts.
• To elucidate the proton exchange properties of a
  Nafion membrane using dyes as probes.
• To integrate the above concepts to generate
  hydrogen using a photocatalyst/ proton exchange
  membrane assembly.

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What We Want in an Electrocatalyst

• Effective catalyst for oxygen reduction/hydrogen
  oxidation.
• Smaller catalyst particles have a higher surface
  area to mass ratio.
• Disperse particles to prevent competition for
  reactants.
• Catalyst-Support system which has no side
  reactions.

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Oxide Electrocatalysts

• Oxides are typically inert, thus they mitigate
  degradation reactions.
• SiO2 has been investigated, and TiO2, WO3, and
  SnO2 will be investigated in the future.
• A simple NaBH4 reduction process allowed us to
  deposit Pt nanoparticles on silica.

TEM image of 21 PtSiO2 by mass
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Tafel Plot Analysis

• To determine oxygen reduction kinetics Tafel
  plots were carried out.
• The catalyst with the smallest overpotential has
  the best kinetics.
• Unsupported Pt nanoparticles overpotential was
  bigger than all the Pt oxides.

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Fuel Cell Data Operation

• Full fuel cells were produced and tested.
• 1-1 Pt-SiO2 and 2-1 Pt-SiO2 both gave better
  results than the standard.
• These results were taken at 40C with 1 atmosphere
  of back pressure.

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Graphene as an Electrocatalyst

• Graphene is basically a carbon nanotube opened up
  and flattened out.
• By attaching a long alkyl chain, such as
  octadecylamine (ODA), graphene can be solublized.
• When the graphene issues are optimized, the
  catalyst will go through the same analyzation
  process as the silica.

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Probing the Properties of Nafion

• Nafion has a hydrophobic backbone, but
  hydrophilic pores.
• The pores can absorb up to 22 water molecules per
  sulfonate group.

• Almost any positive ion can attach to the
  sulfonate groups.
• Nafion is a great ionic conductor.
• When organic dyes attach, they are almost
  impossible to remove.

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Methylene Blue (MB) Properties

• Methylene Blue, (MB) changes from a 2 charge to
  a 3 charge as the pH is decreased.
• Each form of MB has a distinct wavelength.
• This also works with Phenosafarin, and Nile Blue.

H
H
Methylene Blue pH7
Methylene Blue pHlt0
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Inserting the Dye into Nafion

• A protonated Nafion strip can be inserted into MB
  and the Nafion will soak up the dye.
• Only 0.01 of sites taken up by dye.
• The spectrum of the membrane in water shows the
  dye exists in both forms, e.g. the MB2 and
  MBH3.

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Losses Attributed to Membrane

• Other cations such as Na can attach to the
  sulfonate groups as well.
• Zawodzinski et. al. found water uptake increases
  approximately exponentially with relative
  humidity.

• They also found the relationship
• Due to gas diffusion issues a membrane cant be
  run in water in a fuel cell.

Zawodzinski et. al. 1993, JES
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Attached H vs. H in Solution

• We wanted to see what happened when the Nafion
  was put in acid.
• There were 2 contributions

• One from the H attached to the sulfonate group

• One from the H in solution

-
-
SO
SO
3
3
H

H

SO
-
SO
-
-
3
-
3
SO
H

H

SO
H

H

3
3


-
-
-
-
H
H
SO
SO
MB
SO
SO
3
3
3
3
H

H



H
H
H
H
H

H

H
H
-
-
SO
SO
3
3
-
-
H

H

SO
SO
-
-
SO
SO
3
3


3
H
H
3
H

H

-
-
SO
SO
SO
-
SO
-
3
3
3
3
H

H

H

H

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Reversibility of MB Protonation states

• A protonated MB-Nafion membrane was put in
  alternating sodium and proton ion solutions.
• Every 30s a spectrum was taken to denote the
  changes in absorption.

• As the Na replaces the H in the membrane the
  dye changes protonation forms.
• When H is added again the dye reverses
  protonation states.
• This shows the reversibility of the sites as
  well as the durability of the dye.

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Running the Fuel Cell

• Below is a cartoon and the spectra from the dye.

Seger et al., Langmuir, 2007

• We used a passive methanol fuel cell.
• This shows that it is possible to probe an
  in-situ fuel cell with the dye.

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The Different Protonation States of MB

• The Methylene Blue reaction is as followed

• And the equilibrium coefficient is

• Using absorbance to determine MB via Beers Law
  we get the following

• Using the above equation we determined the acid
  concentration the dye sees to be 2.4M H.

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From Spectra to Acid

• Spectra of the dye was taken at different
  humidities.
• From the equation for the methylene blue
  equilibrium spectra can be converted to acid
  concentration

• This technique will be used to study and optimize
  humidity effects in our fuel cells.

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Generating Hydrogen via Photolysis

• The photo splitting of water reaction is shown
  below.

e-
e-
e-
Overall Stochiometry
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Design of Photolysis Device

• The photo water-splitter will be designed very
  similar to the fuel cell.
• The cathode and membrane will remain unchanged.

• A light harvesting semiconductor (TiO2) will be
  used on the cathode.
• TiO2 has the right semiconductor properties

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Summary

• Oxides and carbon nanostructures are showing
  promise for supports in fuel cells.
• Inserting a dye into Nafion lets us probe many
  properties of the membrane.
• The knowledge gained from the support studies and
  membrane studies will help us build a photo
  water-splitter.
• Thanks to
• Dr. Kamat
• Dr. Vinodgopal
• Dr. Kongkanand
• Army Fuel Cell Grant

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• Questions ?

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Electrochemical Analysis

• Hydrogen desorption in the cyclic voltammogram
  was used to determine ECSA.

Hydrogen Desorption
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Tafel Background
At high overpotentials
Reaction Based
Diffusion Based
Together
Reaction Based
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XRD results
Haddon, 2006, Nanoletters
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Pt-SiO2 Particles
TEM image of 2-1 Pt-SiO2 by mass catalyst
particles
FESEM image of 2-1 Pt-SiO2 deposited on carbon
Toray paper
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Platinum Lattice Fringes
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Electrochemical Analysis

• Hydrogen desorption in the cyclic voltammogram
  was used to determine ECSA.

Hydrogen Desorption
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TEM Images
0.5-1 Pt-SiO2 TEM
10-1 Pt-SiO2 TEM
2-1 Pt-SiO2 TEM
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SEM Images
FESEM image of Pt deposited on carbon Toray paper
FESEM image of 2-1 Pt-SiO2 deposited on carbon
Toray paper
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Hydrogen Peroxide Side Reactions
H2O2
OH Radicals
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Carbon Degradation Reaction
Water
CO2
4e- 4H
v
Carbon Black
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Adding Octadecylamine

• Even though the sheets arent aligned like a
  crystal, they still aggregate.
• Recently Haddons group attached octadecylamine
  to better disperse graphene in organic solutions
  (Niyogi et. al., JACS, 2006).

• Adding SOCl2 helps in the kinetics of attaching
  the octadecylamine

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• _at_ 0 RH we have LogHy-intercept
• At this point the dye sees no water, so its
  effective acid concentration is the H near it
  divided by a minimal volume.

• By replacing the Na counterions with H, we
  increase the H near the dye, thus increase the
  y-intercept.

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Temperature Change

• As the figure below shows, increasing the
  temperature does increase the slope.

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Looking at the equations

• Below is the theoretical equation for H

• Below is the empirical equation from the graph

• Combining the 2 equations, we get

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Effective Diffusivity
1
CH0

• Ficks Law of Diffusion

1/2
CHC0
0
Nafion
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Initial
2 Days Later
Time
Nafion
Nafion
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e-
O2
H2
hn
H
H2O
4H
e-
hn
Conduction Band
H/H2
0.0
2H2
2 H2O O2
Pt catalyst
OH-/O2
1.2
h
Valence Band
4OH-
Semiconductor
Voltage, vs. NHE