Karen Swider-Lyons and Peter BouwmanNaval Research Laboratory

Washington, DC

Wojtek Dmowski University of Tennessee

Knoxville, TN

Karen Swider-Lyons and Peter BouwmanNaval Research Laboratory

Washington, DC

Wojtek Dmowski University of Tennessee

Knoxville, TN

Low-Platinum Nanostructured Catalysts for Fuel Cells

Low-Platinum Nanostructured Catalysts for Fuel Cells

U.S. Transportation Oil Demand

0

2

4

6

8

10

12

14

1970 1980 1990 2000 2010 2020

GAP

Source: Transportation Energy Data Book: Edition 19, DOE/ORNL-6958, September 1999, andEIA Annual Energy Outlook 2000, DOE/EIA-0383(2000), December 1999

Mill

ion

s o

f B

arre

ls p

er D

ay

Domestic Oil Production

Heavy Trucks

Automobiles

Light Trucks

Pas

sen

ger

Veh

icle

s

Strategies needed to close the production/utilization gapUse fuel cells to increase efficiency and decrease fuel consumption

Electricity from electrochemistryElectrochemical conversion process

Oxygen and hydrogen combined on catalysts to produce water, electricity and heat

electrolyte

anode

cathode

O2

H+

O2 + 2 H+ + 2 e- = H2O

H2 = 2 H+ + 2 e-

load

e-

e-

H2H2

H2O

H2OO2

H2O

H2O

H2O H+

H+12

Proton exchange membrane fuel cellPerfluorosulfonic acid (Nafion® polymer) membrane

Pt catalysts

Resistive losses due to materials and

inefficient reactions.

Hydrocarbons as hydrogen source

Logistics fuel• Richest source of hydrogen• Must be reformed to hydrogen• Sulfur, nitrogen and CO2 may be sequestered

refinery

Gasoline/diesel

CH3-(CH2)x-CH3

S

OIL

H2

H2

H2H2 H2

H2

H2

H2

reformer

Hydrogen from water electrolysis is expensive dueto high materials and energy costs of electrolyzers

Storage tanks

SO2

CO2

Fuel cell

Environmental Issues with Platinum

From: The Lonmin Group web siteFrom: The Lonmin Group web site

Mining Pt creates a lot of waste!

In fuel cell era, attention toward environmentally friendly mining and recycling

EPA Advantages of low Pt catalystsLowering Pt will lower the cost of fuel cells

Introduce fuel cells more broadly to consumer market Lower fuel consumption

Less Platinum used Less Pt mined and recycled

Less chemical waste

Lowering Pt will lower the cost of fuel cells Introduce fuel cells more broadly to consumer market

Lower fuel consumption

Less Platinum used Less Pt mined and recycled

Less chemical waste

A possible solution to the problem

Traditional approachMake and test new platinum

“alloys”Change catalyst microstructure

Our approachDesign phase-segregated,

mixed conducting nanocomposites for RAPID TRANSPORT of chemical species

Alloy compositions

Transition metals

nanocomposite phases

?

?

Nanocomposite phases are still largely unexplored -due to difficulty in their characterization?

• Focus on lowering Pt in fuel cell cathode

• Cathode has most Pt because slow oxygen reduction kinetics poor Pt stability and ripening over time.

• Focus on lowering Pt in fuel cell cathode

• Cathode has most Pt because slow oxygen reduction kinetics poor Pt stability and ripening over time.

Support Pt on a metal oxide andimprove opportunity for* proton mobility to Pt sites* chem/phys attraction of O2

* metal-support interactions with Pt

Support Pt on a metal oxide andimprove opportunity for* proton mobility to Pt sites* chem/phys attraction of O2

* metal-support interactions with Pt

Low Pt catalysts for fuel cells

O2 + 4 H+ + 4e- 2 H2O Pt supported on MOx•H2O supported on carbon

Pt?

MOxe-

carbon

H+

OO

H+

H+

NafionH+

O2

O

H2O

Catalyst development via electrochemical and structural analysis

Pt-MOx systems1. Pt-FePOx•xH2O - hydrous iron phosphate• Iron phosphate used as an anti-corrosion

additive in paint

• FePOx is a partial oxidation catalyst

• Under intense scrutiny as a Li-ion battery cathode

2. Pt-SnOx•xH2O - hydrous tin oxide• Prior ORR literature shows promise for

anhydrous Pt-SnOx

• Tin hydrates are corrosion resistant

1. Pt-FePOx•xH2O - hydrous iron phosphate• Iron phosphate used as an anti-corrosion

additive in paint

• FePOx is a partial oxidation catalyst

• Under intense scrutiny as a Li-ion battery cathode

2. Pt-SnOx•xH2O - hydrous tin oxide• Prior ORR literature shows promise for

anhydrous Pt-SnOx

• Tin hydrates are corrosion resistant

Hydrous oxides are excellent for proton conduction

Selected oxides have other ideal catalytic properties (e.g. partial oxidation)

Materials have open framework structures

“Open Framework Inorganic Materials”A. K. Cheetham G. Férey, T. Loiseau, Angew. Chem. 1999, v. 38 p. 3286.

Example:Microporous AlPO4

Pt-FePOx/Vulcan C as ORR catalysts

Pt-FePO4 catalysts have higher ORR activity than Pt/carbon standard

Use rotating disk electrode experiments to compare oxygen reduction activity of new catalysts to standard catalysts

Critical factors:Electrode preparation& Testing conditions0.1 M HClO4

1600 rpm

60 °C

Compare to theoretical values for Pt

Use rotating disk electrode experiments to compare oxygen reduction activity of new catalysts to standard catalysts

Critical factors:Electrode preparation& Testing conditions0.1 M HClO4

1600 rpm

60 °C

Compare to theoretical values for Pt

9% Pt-FePOx•yH2O + 50% VC

20%Pt-VC

FePOx Cyclic Voltammetry

FePOx has no activity for the ORR

9 wt %Pt-FePOx is highly active for the ORR

FePOx has no activity for the ORR

9 wt %Pt-FePOx is highly active for the ORR

Potential vs RHE [V]0.0 0.2 0.4 0.6 0.8 1.0

Current Density [mA.cm

-2]

-2.5-2.0-1.5-1.0-0.50.0

OxygenArgon

400 rpm

1600 rpm

FePOx/VC 9%Pt-FePOx/VC

0.1 M HClO4

1600 rpm

60 °C

Potential vs RHE [V]

0.0 0.2 0.4 0.6 0.8 1.0

Current Density [mA.cm

-2]

-6

-4

-2

0

0.1mg(Pt).cm-2

Air

Argon

Oxygen

200

300

400

500

600

700

800

900

1000

10 20 30 40 50 60

2 theta

Structure of the active Pt-FeO

sample is glassy in conventional electron microscopy and X-ray diffraction

ConventionalX-ray diffraction

TEM

Structure of Pt-FePOx Electrocatalysts

Atomic pair distribution function (PDF)

X-ray or neutron scattering is Fourier-transformed to give distribution of inter-atomic distances in a real space.

∫∞

−+=0

20 )sin(]1)([2

1)( QdqQrQS

rr

πρρ

High energy, high intensity sources are essential to minimize errors and improve statistics.

PDF analysis

S(Q

)

XRD largely amorphous Short-range order

sStructure of Pt-FePOx with PDF analysis

• Iron phosphate (berlinite) – a-quartz

structure

• Iron phosphate (berlinite) – a-quartz

structure

O

Fe

P PDF analysis of high-energy XRD shows ordered medium

range structure• Microporous structure

facilitates high protonic conduction

• Pt serves as a glass-modifier and opens pores for access

• Fe2+/Fe3+ mixed valence states for high catalytic activity

Next steps…

Reduce particle size of oxides to improve electrical properties of FePOx

VC impregnated with Pt-FePOx

Solution-filtered Pt-FePOx

Effect of particle size

Smaller (nano)particle size leads to:

• Higher electronic conductivity (tunneling from carbon)

• Higher surface area

(more sites for catalysis)

Smaller (nano)particle size leads to:

• Higher electronic conductivity (tunneling from carbon)

• Higher surface area

(more sites for catalysis)

Pt-FePOx mixed with Vulcan carbonvs. Pt-FePOx impregnated on Vulcan carbon

•Fuel cells are efficient fuel conversion systems that may lead to significant fuel savings - less pollution

•Lower greenhouse gases at a central fuel reforming site and hydrogen stored in fuel tanks

•Nanomaterials may be useful routes to lowering Pt content of fuel cells and lowering their cost

•New analytical techniques may be needed for the accurate study of new nanomaterials

•Fuel cells are efficient fuel conversion systems that may lead to significant fuel savings - less pollution

•Lower greenhouse gases at a central fuel reforming site and hydrogen stored in fuel tanks

•Nanomaterials may be useful routes to lowering Pt content of fuel cells and lowering their cost

•New analytical techniques may be needed for the accurate study of new nanomaterials

Summary and Outlook

Acknowledgements

Department of Energy, EERE

Office of Naval Research

The synchrotron experiments were carried out at the NSLS - Brookhaven National Lab

Department of Energy, EERE

Office of Naval Research

The synchrotron experiments were carried out at the NSLS - Brookhaven National Lab