Project: LOW PLATINUM LOADING CATALYSTS

Principal Investigator: Radoslav Adzic

Research Associates: Kotaro Sasaki, Tao Huang

With contributions from Jia Wang, Miomir Vukmirovic and Junliang Zhang (student SUSB)

Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973-5000

(This presentation does not contain any proprietary or confidential information.)

Philadelphia, May 24-27, 2004

OBJECTIVES

To assist the DOE in developing of fuel cell technologies by providing low-platinum-loading electrocatalysts.

• To demonstrate the possibility of synthesizing novel electrocatalysts for O2 reduction with a monolayer level Pt loadings.

• To further characterize of the PtRu20 electrocatalyst for H2/CO oxidation and long term tests.

• To gain understanding of the activity of Pt monolayer and the PtRu20electrocatalysts.

PROJECT SAFETY

• All the work on this project is performed within the controls identified in the Experimental Safety Review (ESR) Form for this Project.

• Personnel have all the training identified by ESR.

• CO sensor installed at the CO tolerance experiment. Hazard evaluation of this experiment was performed.

•For the work at synchrotron, the safety procedures and the training requirements of NSLS are followed.

BUDGET

TOTAL FUNDING FOR THE PROJECT (FY 02-04): $624.000

FUNDING IN FY 04: $250.000

TECHNICAL BARRIERS AND TARGETS

The DOE’s Technical Targets for Fuel Cell Stack Systems Operating on Hydrogen (Gasoline Reformate)

year 2003 2005 2010

precious metal loading g/kW <2.0 0.6 0.2

durability hours >2000 >2000 >5000

CO tolerance (2% air bleed) ppm 50 500 1000

APPROACH

Development of low-Pt-loading electrocatalysts by placing a submonolayer-to-monolayer of Pt on nanoparticles of suitable metals or alloys to obtain electrocatalysts with the following characteristics:

• ultimately reduced Pt loading

• enhanced activity of Pt

• complete utilization of Pt

Two methods for Pt monolayer deposition were developed:

1. Electroless (spontaneous ) Pt deposition on Ru.

2. Pt deposition by replacing a UPD metal adlayer.

PROJECT TIMELINE

START June 2001

Proof of principle, characterization of a Pt submonolayer anode electrocatalyst.

June 2002 June 2003

Optimization, MEA tests, O2reduction electrocatalysts.

Pt loading reduced to 1/10 of the standard value.

1000 h MEA tests completed with PtRu20.

O2 reduction Pt monolayer electrocatalysts demonstrated.

Electrocatalyst obtained that does not show any loss in activity in a 870 h test.

June 2004

Optimization of O2reduction monolayer electrocatalysts, MEA tests.

Anticipated Pt loading 1/5 of the current standard.

June 2005

TECHNICAL ACCOMPLISHMENTS

ANODEStability tests at LANL (F. Uribe) show no loss of voltage after 870 h for the PtRu20

electrocatalyst with 18 µg Pt/cm2 (20% Ru; 2% Pt, 3% air bleed), and small losses after 1000 h with 18 µg Pt/cm2 (10% Ru; 1% Pt, 4% air-bleed) and very small losses in a 600 h test with 19 µg Pt/cm2 (2% air-bleed) of combined CO/H2 and H2 operation.

The DOE durability target of 2000h for 2005 can be reached with this electrocatalyst.

The DOE target for 2005 for noble metals of 0.6 g/kW (0.3 g/kW for anode) is met for Pt: only 0.063 g Pt/kW is necessary. If Ru is counted, 0.630 g total metal is needed.

CATHODEA Pt monolayer on C-supported metal or metal alloy nanoparticles can be an active

catalyst for O2 reduction.

The Pt mass-specific activity of Pt/Pd/C is 5-8 times higher than that of Pt(10%)/C.The (Pt + Pd) mass activity is 2.5 times higher. Fuel cell tests (F. Uribe) are quite promising.

A PdCo/C electrocatalyst was synthesized. Its activity is comparable to that of Pt.

A Pt/AuNi/C electrocatalyst was synthesized whose activity is similar to that of Pt.

LONG-TERM FUEL CELL TESTS AT LANL (F. Uribe)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 200 400 600 800 1000

V A

Cel

l Vol

tage

/ V

Current / A

cm-2

t i m e / hours

Cell=50 cm2 ; T= 80 C;A: 0.19 mg /cm2(10%Ru, 1%Pt) C: 0.23 mg Pt /cm2(20% Pt/C, ETEK) Total run time = 1000 hours at constant current. 710 hours of operation with clean H2 and 290 hours with H2 + 50 ppm CO + 4 % air bleed.

Voltage losses after 600 hr:* with neat H2: 20 mV

(0.71-0.69 V)* with H2+CO+2% air: 20 mV

(0.66-0.64 V)

Cell 50 cm2 cell / T= 80 CA: 0.19 mg BNL/cm2 (10% Ru; 1% Pt)C: 0.22 mg Pt/cm2 (ETEK)H2 471 hr; H2 + CO 50 ppm+2% air bleed, 129 hr

No voltage losses after 868 hr:

initial V final V* with H2: 0.717 0.717* with H2+CO+3% air:

0.697 0.701Cell 50 cm2 cell / T= 80 CA: 0.20 mg BNL/cm2 (20% Ru; 2% Pt)C: 0.24 mg Pt/cm2 (ETEK)Running Mode: 20 A currenta) H2 at @ 1.3 stoichb) H2 at @ 1.3 stoich + CO 50 ppm+3% air bleedAir flow: constant @ 2100 sccm

18 µg Pt/cm2 (20% Ru; 2% Pt)19 µg Pt/cm2 (10% Ru; 1% Pt)17 µg Pt/cm2 (10% Ru; 1% Pt)

Voltage losses after 1000 hr:* with neat H2: 40 mV

with H2+CO+4% air: 60 mV

In addition to CO tolerance, the very strong surface segregation of Pt is a key factor in its stability.

CO- σ electron

from CO to Pt-Back donation of Pt d electron to CO 2π*

PtStrong CO adsorption

PtRu20-Lower d-electron density-Lower d-band center εd(Nørskov’s model)Back donation decreasesWeaker CO adsorption

ELECTRONIC EFFECTS vs. BIFUNCTIONAL MECHANISM IN COTOLERANCE OF THE PtRu20 ELECTROCATALYST

εfεd

energy

dos

CO 2π∗

CO 5σ

back

don

atio

n

εd

Fermi level

0

0.4

0.8

1.2

-20 -10 0 10 20 30 40

PtRu20

Pt foilnorm

aliz

ed a

bsor

ptio

n

energy, eV

relative to L3 Pt edge

0

0.4

0.8

1.2

-20 -10 0 10 20 30 40

PtRu20

Pt foilnorm

aliz

ed a

bsor

ptio

n

energy, eVrelative to L2 Pt edge

d band vacancyPtRu20 0.345 (0.41V)

Pt foil 0.30

Conclusion: Both the electronic effects and the “bifunctional “mechanism are operative for this electrocatalyst.

Pt

Ru0

100

200

300

400

500

600

700

0 50 100 150 200 250 300 350

I, µ

A

t, min

PtRu20

1 nmol Pt & 20 nmol Ru

( 1 µg Pt/cm 2, 10 µg Ru/cm2)

A catalyst: Pt2Ru

3

4 nmol Pt & 6 nmol Ru

( 4 µg Pt/cm 2, 3 µg Ru/cm2)

997 ppm CO/H2

0.5 M H2SO

4

60 oC2500 rpm0.05 V(RHE)

INHIBITION OF O2 REDUCTION ON Pt BY ANION ADSORPTION

The kinetic currents are calculated as a function of E and the anion adsorption isotherm, θA(E) usingjk(E) = -j0 (1 - γAθA(E))m

exp(-2.3(E – E0 – εAθA(E))/b),where j0 and b are the intrinsic kinetic parameters, γA is the geometric blocking factor, and εA is the electronic effect of adsorbed anions.•The best fits yielded the intrinsic Tafel slope in the range –118 to –130 mV/dec.•In addition to site blocking, both OH and bisulfate have a negative electronic effect on ORR kinetics, with the effect of the latter being much stronger.• The deviation of the apparent Tafel slope in HClO4 from its intrinsic value can be fully accounted for by the site blocking and electronic effects of adsorbed OH ions, which vary with coverage over the mixed kinetic-diffusion controlled region.Wang et al. J. Phys. Chem., in press.

No OH, HSO4

-

O2 REDUCTION ON Pt1ML/Pd(111)

0.0 0.2 0.4 0.6 0.8 1.0 1.2-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.0

1.5

3.0

4.5

6.0

P t / Pd(111)rpm

360030252500

20251600

1225900

625

400

225

I disk

/ m

A

E / V (R H E )

2025P t / Pd(111)

1600

1225

900

625

400

225

rpm

I ring /

µA

0.00 0.06 0.12 0.18 0.240.0

0.8

1.6

2.4

3.2

4.0

-1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5

0.6

0.7

0.8

0.9

1.0

E / V

0.88V0.87V0.86V0.85V0.84V0.81V0.78V0.20V

I-1 /

mA-1

ω-1/2 / s1/2

log j / mA.cm-2

E / V

RH

E

-90.2 mv / dec

0.0 0.2 0.4 0.6 0.8 1.0 1.2-8

-6

-4

-2

0

Pt / Pd(111)Pt(111)

Pd(111)

j / m

A cm

-2

E / V RHEThe reduced coverage of PtOH appears to be the cause of enhanced activity.

0.2 0.4 0.6 0.8 1.0

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

Cu / Pd (111)

j / m

A.cm

-2

E / V RHE PtML/Pd(111)

250 nmx

250 nm

ACTIVITY OF Pt MONOLAYERS AS A FUNCTION OF THEFRACTIONAL FILLING OF THE d-BAND OF SUBSTRATES

0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05

0.60

0.65

0.70

0.75

0.80

0.85

PtML/Au(111)

PtML/Pd(111)

Pt(111)

PtML/Rh(111)PtML/Ir(111)

PtML/Ru(0001)

E 1/2 /

V

fractional d-band filling DFT calculations by M. Mavrikakis, U. Wisconsin.

Pd(10%) / Vulcan XC-72 commercial

HRTEM OF Pd NANOPARTICLES ON C

20 nm

5 nm

0

5

10

15

20

25

30

35

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57

Cou

nts

Particle size / nm

0.0 0.2 0.4 0.6 0.8 1.0 1.2

-6

-4

-2

0

PtML/Pd20/C

PtML/Pd10/C

Pt10/CPd10/C

1600rpm

j / m

A.c

m-2

E / V RHE

O2 REDUCTION ON Pt/Pd/C

0.0 0.4 0.8 1.2

-1.0

-0.5

0.0

0.5

1.0

Cu/Pd10/C

j / m

A c

m-2

E / V RHE

0.0 0.2 0.4 0.6 0.8 1.0 1.2

-1.5

-1.0

-0.5

0.0

0.0

0.4

0.8

1.2

1.6

2.0

PtML/ Pd / C

rpm

3600302525002025

1600

1225900

625

400

225100

I disk

/ m

A

E / V (RHE)

3600

2500

1600

900

400

100

rpm

PtML/Pd / C

I ring /

µA

0.0 0.1 0.2 0.30.0

0.5

1.0

1.5

2.0

0.0 0.5 1.0 1.5 2.0

0.675

0.750

0.825

0.900

PtML/Pd20/C

0.9000.896

0.8900.8820.8700.8550.8300.560

E / V

1/j

/ mA-1

cm2

ω-1/2 / s1/2

E / V

RH

E

log jk / mAcm-2

-95.8 mv / dec

0

0.4

0.8

1.2

11540 11560 11580 11600 11620 11640

Pt/Pd/C at 0.47 V

Pt foilnorm

aliz

ed a

bsor

ptio

n

energy, eV

IN SITU XANES MEASUREMENTS WITH Pt/Pd/C

XANES reveals a small change in the Pt d band.

0

0 .4

0 .8

1 .2

1 1 5 5 0 1 1 5 6 0 1 1 5 7 0 1 1 5 8 0 1 1 5 9 0 1 1 6 0 0

0 .4 7 V0 .8 2 V0 .9 2 V1 .1 2 V

norm

aliz

ed a

bsor

ptio

n

e n e rg y / e V

∆I

∆I 0.

47 V

P t /Cb

0

0 .4

0 .8

1 .2

1 1 5 5 0 1 1 5 6 0 1 1 5 7 0 1 1 5 8 0 1 1 5 9 0 1 1 6 0 0

0 .4 7 V0 .7 7 V0 .9 7 V1 .1 7 V

norm

aliz

ed a

bsor

ptio

n

e n e rg y / e V

P t /P d1 0

/Ca

0 .8

1

1 .2

1 .4

1 .6

1 .8

2

0 .4 0 .6 0 .8 1 1 .2

P t /CP t/P d

1 0/C

∆l/∆

l 0.47

V

E / V (R H E )

c

0.0 0.4 0.8 1.2-1.5

-1.0

-0.5

0.0

0.5

1.0

Pt10/CPt/Pd10/C

j / m

A cm

-2

E / V RHE

Voltammetry and XANES show delayed Pt oxidation at high potentials in comparison with Pt/C.

0

1

2

3

4

0.85V

0.80V

PtML/Pd20/C

PtML/Pd10/C

Pt10/C

PtML/Pd20/C

PtML/Pd10/C

Pt10/C

j / m

A.c

m-2.µ

g-1m

etal

0

5

10

15

20

25

0.85V

0.80V

PtML/Pd20/C

PtML/Pd10/C

Pt10/C

PtML/Pd20/C

PtML/Pd10/C

Pt10/C

j / m

A.c

m-2.µ

g-1Pt

Pt and (Pt + Pd) MASS-SPECIFIC ACTIVITY OF PtML/Pd/C FOR O2 REDUCTION

Pt Pt + Pd

FUEL CELL TESTS OF Pt/Pd/C AT LANL (F. Uribe)

Performance of Pt-Pd/C (4% Pt-20% Pd) cathode catalyst at 80 °C. Membrane: Nafion® N1135.

Anode loadings in mg Pt/cm2: Cell a: 0.22 ; Cell b: 0.18 ; Cell c: 0.17.

O2 REDUCTION ON PdCo/C

-2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4

-2.4

-2.2

-2.0

-1.8

-1.6

-1.4

Pd/Pt

Pd/Ru

Pd/Ir

Pd/Rh

Pd/AuAdo

sorp

tion

ener

gy, e

V/O

2

d-band center, eV

Pd

DFT calculation by P. Liu, BNL

0.0 0.4 0.8 1.2

-1.6

-1.2

-0.8

-0.4

0.0

0.4 0.8 1.2

-1.2

-0.8

-0.4

0.0

rpm

100

2500

1600

900

400

I D / m

A

ED / V RHE

0

2

4

6

8

Pt/AuNi3/C2500

1600

900

400I Rin

g/µA

1600rpm

PtAuNi3/C

Pt10

/C

E/V RHEI D/m

A

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.070

1

2

3

4

5

6

7

8

-0.6 0.0 0.6 1.2

0.3

0.6

0.9

1.2

0.90V

0.89V

0.88V

0.86V

0.84V0.80V0.40V

-I-1 m

A-1

ω-1/2/ rpm-1/2

-92mv/dec

0

1

2

3

4

PtAuNi3/C

PtAuNi3/C

Pt10/C

Pt10/C

j / m

A.cm

-2.µ

g-1Pt

O2 REDUCTION ON Pt/AuNi/C

Further reduction of Au and the use of an immiscible AuMLNi alloy seem possible.

INTERACTIONS AND COLLABORATIONS

1. Los Alamos National Laboratory Dr. Francisco Uribe – long-term fuel cell tests of electrocatalysts.

2. Plug Power, visit, discussions.

3. Interest expressed in the PtRu20 electrocatalyst and collaboration.

Publications from collaborations:K. Sasaki, J.X. Wang, M. Balasubramanian, J. McBreen, F. Uribe, R.R. Adzic, Ultra-low Platinum Content Fuel Cell Anode Electrocatalyst with a Long-term Performance Stability,Electrochim. Acta, in press.

K. Sasaki, Y. Mo, J.X. Wang, M. Balasubramanian, F. Uribe,J. McBreen, R.R. Adzic, Pt submonolayers on metal nanoparticles – novel electrocatalysts for H2 oxidation and O2 Reduction, Electrochim. Acta, 48 (2003) 3841.

J.X. Wang, N.M. Markovic, R.R. Adzic, Kinetic Analysis of O2 reduction on Pt(111) in Acid Solutions: Intrinsic Kinetic Parameters and Anion Adsorption Effects, J. Phys. Chem. in press.

Responses to Previous Year Reviewers’ Comments

Q. Distinction from Wieckowski’s catalyst not clear.

A. His: Ru on Pt for methanol oxidation; ours: Pt on Ru for H2/CO oxidation.

Q. Not clear how structure/phase behavior (of CO) is exploited to design practical catalysts.

A. Knowing adsorbate’s mobility, lateral interactions and adsorption sites can help in designing electrocatalysts.

Q. Cathode materials of higher importance and needs to be expanded.

A. The work on cathode materials has been expanded.

FUTURE WORK

O2 reduction1. Further development of a Pt/Pd/C electrocatalyst.

Tests at LANL.

2. Further development of immiscible Au-non-noble metal alloy nanoparticles as support for Pt.

3. Multi-metal monolayers to reduce PtOHcoverage and to modify the electronic properties of Pt.

4. Non-noble metal alloys as support for Pt.

H2 oxidation

Pt

OH

Pd

M

1. Pt submonolayers on non-noble metal alloy nanoparticles.