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Low Pt Loading Fuel Cell Electrocatalysts

Radoslav Adzic (P.I.), Junliang Zhang, Kotaro Sasaki, Miomir Vukmirovic, Jia Wang, Minhua Shao

Chemistry Department Brookhaven National Laboratory, Upton, NY 11973-5000

(This presentation does not contain any proprietary or confidential information.) Project ID # FC09

DOE Hydrogen Program Review, May 16-19, 2006

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Overview

• Project start date: 06.02.• Project end date: Multi-year• Percent complete

Barriers addressed

1. Precious metal loading

2. Electrocatalysts’ Activity

3. Electrocatalysts’ Durability

Target 2010: 0.3 g/kW

Target 2010: 5000hr

• Total project funding: $1314K • DOE share: $1314K• Funding received in FY05:

$330K• Funding for FY06: $360K

Budget

Timeline

Collaborations• Los Alamos National Laboratory (Fuel cell tests - F. Uribe, P. Zelenay)• Battelle Memorial Institute (CRADA Scale-up of synthesis - J. Sayre and A. Kawczak ) • 3M (PdCo catalyst, exploratory activities – R. Atanasoski)• Plug Power – Test of the PtRu20 anode catalyst: 2390 hr showed a small loss in

activity (B. Do).• General Motors Co. planning stage (F. Wagner)

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

--- During the current year• To demonstrate the stability of Pt mixed metal monolayer (Pt80Ir20/Pd/C) and

Pt/Au/Ni/C electrocatalysts in fuel cell tests.

• To explore a novel class of electrocatalysts for O2 reduction consisting of Pt monolayer on noble metal -non-noble metal core-shell nanoparticles.

• Further studies of the electrocatalysts with low, or no Pt.

• Scale – up of synthesis based on displacement of a Cu monolayer.

• Addressing the problem of Pt dissolution under potential cycling regimes.

• Testing the effects of Au clusters on Pt stability.

Objectives

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Why Pt monolayer catalysts?

• Complete Pt utilization (of all atoms that are not blocked by Nafion®)

• Ultimate reduction of Pt loading

• Increased activity

(Six patents pending)

APPROACH

For 3-5nm nanoparticles ~25% of atomsare on the surface,

~75% are not available for catalysis.

1. Pt on Pd nanoparticles

Improved activity

Improved durability

2. Mixed-metal Pt on Pd nanoparticles

Even higher activity

Durability tests at LANL

and Battelle

3. Pt on noble/non-noble core-shell nanoparticlesYet higher activityper mass of noble metal

Durability to be tested

Pt

Pd

Pt

PdIr

PtAuNi

Three types of Pt electrocatalysts:

Pt monolayer electrocatalysts

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Compared to commercial MEA baselines (0.6 mg Pt/Ru/cm2): Higher than neat H2 baseline; lower than reformate baseline.

Total run time: 2394 hours

0.022 mgPt/cm2

Long-term test (one segment) of the anode PtRu20electrocatalyst at PLUG POWER

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Segregation at elevated T

ML of Cu by UPD

Cu displaced by Pt

Cu ML Pt ML, or mixed Pt-M ML

1-2 ML of Au, Pd, or PtCo or Ni

New class of catalysts: Pt monolayer on noble metal - non-noble metal core-shell nanoparticles

New class of electrocatalysts that facilitate a further decrease of noble metal content while possessing a very high activity.

Au, Pd, or

Pt

Synthetic route

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O2 Reduction Kinetics and Mass Activities of PtML/PtCo5/C and PtML/PdCo5/C

0.4 0.6 0.8 1.0

-6

-4

-2

0

Pt/C PtML/PdCo5/C

1600 rpm

j / m

A.c

m-2

E/V vs RHE

0.4 0.6 0.8 1.0

-6

-4

-2

0

Pt/C PtML/PtCo5/C

1600 rpm

j / m

A.c

m-2

E/V vs RHE 0.8V 0.85v0

5

10

15

20

-j k mA

/μg

Pt

Pt10/C PtML/AuNi10/C PtML/PdCo5/C PtML/PtCo5/C

Total noble metal mass activity

Pt mass activity

0.8V 0.85v0

1

2

3

4

5

6

-j k mA

/μg

nobl

e m

etal

Pt10/C PtML/AuNi10/C PtML/PdCo5/C PtML/PtCo5/C

Pt

Pd

2- to 5-fold increase in noble metal mass activity with Pt on core-shell nanoparticles

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-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0 0.2 0.4 0.6 0.8 1 1.2

Pt/NbO2/C, 5 μg Pt/cm2

E1/2

= 863 mV

commercial Pt/C, 5 μg Pt/cm2

E1/2

= 778 mV

commercial Pt/C, 10 μg Pt/cm2

E1/2

= 815 mV

I, m

A

E, V(RHE)

0.1 M HClO4 +O

2

1600 rpm, 10 mV/s

O2 Reduction Kinetics and Mass Activities of PtML/NbO2/C and Pt/C/C

PtML/NbO2/C has 3 times higher mass activity than the commercial Pt/C

0

0.2

0.4

0.6

0.8

0.4 0.6 0.8 1 1.2

Pt/NbO2/C

Pt/C

(Δμ

- Δμ 0.

42V)/Δ

μ 0.42

V

E , V(RHE)

relative changes in Pt L3 edge

XANES shows a small oxidation of Pt/NbO2/C compared to Pt/C.

Pt is not observed by XRD in Pt/NbO2/C

0

1

2

3

4

j k, mA

μg-1

0.80 V

Pt/NbO2/C

Pt/C

0.85V

Pt/NbO2/C

Pt/C

Pt/NbO2/C

5 μg Pt/cm2

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Formation of Au clusters on Pt electrodes

0.0 0.2 0.4 0.6 0.8 1.0

-80

-40

0

40

80 Pt(111); Au / Pt(111)

j / μ

Acm

-2

E / V RHE

0.1M HClO4

50mV/s

0.0 0.4 0.8 1.2-2

-1

0

1

j / m

Acm

-2

E / V RHE

Pt/C Au/Pt/C

0.1M HClO450 mV/s

PtAu

2D Au deposited on Pt(111); 3D formed after several cycles to 1.2V in O2 saturated solution

115 nm x 115 nm 120 nm x 120 nm

No effect on H ads. Inhibition of OH ads.

Model of Au clusters on Pt nanoparticle

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0.0 0.2 0.4 0.6 0.8 1.0 1.2

-6

-4

-2

0

Pt (111)

j / m

Acm

-2

E / V RHE

Au2/3ML/Pt (111)

rpm1600

0.0 0.2 0.4 0.6 0.8 1.0 1.2

-6

-4

-2

0

j / m

Acm

-2

E / V RHE

1600rpm

Au2/3ML/Pt/C Pt/C

Effect of submonolayer of Au clusters on O2 reduction on Pt(111) and Pt/C

Small inhibition of O2 reduction by Au, not in proportion to Au coverage

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0.0 0.4 0.8 1.2

-6

-4

-2

0

j / m

Acm

-2

E / V RHE

rpm 1600

Pt/C intial 30,000 cycles

30,000 cycles from 0.6 to 1.1V at 25°C in O2-saturated solution

0.0 0.4 0.8

-0.6

-0.3

0.0

0.3

j / m

Acm

-2E / V RHE

Pt10/C20mV/s, 0.1MHClO4

Initial After 30,000 cycles

Effect of potential cycling on the activity of Pt/C in the ORR

A negative shift of 40mV in E1/2 and a loss of surface area of 45%

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Stabilization of Pt/C by a submonolayer of Au clusters under a potential cycling regime

0.0 0.4 0.8 1.2

-6

-4

-2

0

j / m

Acm

-2

E / V RHE

rpm 1600

Au2/3ML/Pt/C intial 30,000 cycles

30,000 cycles from 0.6 to 1.1V at 25°C in O2-saturated solution

0.0 0.4 0.8 1.2-2

-1

0

1

intial After 30,000 cycles

j / m

Acm

-2

E / V RHE

Au0.67ML/Pt10/C50mV/s, 0.1M HClO4

A negligible shift in E1/2 and a loss of surface area

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0.0 0.4 0.8 1.2

-6

-4

-2

0 rpm 1600

j / m

Acm

-2

E / V RHE

PtML/Pd/C initial 30,000 cycles

0.0 0.4 0.8 1.2

-6

-4

-2

0 rpm 1600

j / m

Acm

-2

E / V RHE

Au2/3ML/PtML/Pd/C

initial 30,000 cycles

30,000 cycles from 0.6 to 1.1V at 25°C in O2-saturated solution

Stabilization of PtML/Pd/C by a submonolayer of Au nanoparticles under a potential cycling regime

A negative shift of 26mV in E1/2 A negligible shift in E1/2

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Pd3Fe

Compressive strain-induced downshift of the d - band center decreases the activity of Pd and the blocking effect of OH, O2, O2

-, H2O2.

The optimal Pd-Pd distance is 0.273 nm.

The activity of Pd3Fe/C is as high as that of commercial Pt/C.

Pd3Fe alloy O2 reduction electrocatalyst

-2.4 -2.2 -2.0 -1.8

-2.2

-2.0

-1.8

-1.6 Pd skin on PdFe

Pd skin on Pd3Fe

Pd3Fe

4.7%compressed Pd

2.3%compressed Pd

Ado

sorp

tion

ener

gy (e

V/O

2)

εd-εf (eV)

Pd

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Comparison between PtML/Pd3Fe/C, PtML/Pd/C and Pt/C

0

2

4

6

8

0

2

4

6

8

0.85 V

Pt/C PtML/Pd/C PtML/Pd3Fe/C

j k / m

A μg

-1

0.8 V

Pd/C particle size of 9 nm.

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

-6

-4

-2

0 Pt/C (Pt: 51 nmol cm-2) Pt/Pd/C (Pd: 51 nmol cm-2; Pt: 5.2 nmol cm-2) Pt/Pd

3Fe/C (Pd: 51 nmol cm-2; Pt: 8.67 nmol cm-2)

j / m

A c

m-2

E / V vs RHE

Total noble metal mass activity is about 5 times that of the commercial Pt catalyst.

Total noble mass activity

Fine tuning of the Pt-substrate interaction: Very high activity of PtML/Pd3Fe/C

E1/2 = 0.880 V

PtML/Pd3Fe/C has 0.14g/kW, which meets 0.15g/kW,i.e., 1/2 of 0.3g/kW, the DOE’s target for 2010

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XANES data indicates pronounced electronic effects in Pd-Co and an increased stability of Pd, its activity being close to that of Pt(10%)/C.

BNL#101-103 are from one sample obtained by different acid wash procedure.

Pd2Co

MeOH Concentration (M)0 1 2 3 4 5 6

i @ 0

.4 V

(mA

/cm

2 )

0

20

40

60

80

100

120

140

160BNL #101BNL #102BNL #103Chalcogenide

Time (h)0 50 100 150 200

Cur

rent

Den

sity

(A/c

m2 )

0.00

0.05

0.10

0.15

0.20

DMFC, 0.5 M, 80°C, 0.3 V

Fuel cell methanol tolerance test by P. Zelenay et al., LANL

Current Density (A/cm2)0.00 0.05 0.10 0.15 0.20 0.25 0.30

Cel

l Vol

tage

(V)

0.0

0.2

0.4

0.6

0.8

1.0BNL #101BNL #102BNL #103Chalcogenide

Methanol tolerance of the Pd2Co electrocatalyst

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Dual-Pathway Kinetic Equation for the Hydrogen Oxidation Reaction on Pt Electrodes

The Butler-Volmer equation, jk = j0 , appears to be inappropriate for describing the kinetics of the H2 oxidation reaction (HOR) on a Pt electrode (1,2).

1. Gasteiger, et al., Power Sources, 127 (2004), 162.2. Chen, Kucernak, J. Phys. Chem. B 108 (2004) 13984.3. J. X. Wang, T. E. Springer, R. R. Adzic, J. Electrochem.

Soc. Submitted.

)( /3.2/3.2 bb ee ηη −−

Measured (symbols, from ref [2]) and fitted (solid lines using the Eq.) polarization curves for the HOR on Pt microelectrodes

)()1( )2/(2/0

/0

RTFRTFH

RTFTk eejejj βηηβη +−− −+−=

Based on the Tafel-Heyrovsky-Volmer mechanism, we derived a new equation, ref. 3, to describe the HOR on Pt over the entire potential region:

Calculated kinetic current (solid line), components, jTV (dashed line) and jHV (dotted line), the kinetic current from the Butler-Volmer equation (dotted-dashed line).

A fast, inverse exponential rising of kinetic current at small overpotentials through the Tafel-Volmer pathway, jTV, and a gradual rise at η > 50 mV through the Heyrovsky-Volmerpathway, jHV. The anode behavior in a PEM fuel cell can now be well understood. New basis for fuel cell modeling, optimization, and diagnosis.

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Future WorkRemainder of FY2006

• PtML/Pd/C electrocatalyst- Post fuel cell tests Z-contrast TEM, XANES.

• Pd2Co electrocatalyst- Stability studies; and fuel cell tests.

• Mixed-metal Pt monolayer electrocatalysts- Stability studies. Effects of Au clusters in MEA’s under potential cycling regimes.

FY 2007

• Stabilization effects of Au clusters:- RDE and MEA studies under potential cycling conditions with Pt/Pd/C; Pt/PdCo5/C.

• Pt/AuNi/C electrocatalyst- Segregation of Pt, Au; Stability tests; Fuel cell tests.

• Further reduction of Pd content: Pt monolayers on core-shell nanoparticles- Basic in situ surface science and electrochemical studies; stability and fuel cell tests.

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BNL ANODE PtRu20/C electrocatalyst:Durability and CO tolerance confirmed in a 2400h test at Plug Power. The performance of 0.063 gPt/kW meets the 2010 target of 0.15g/kW, i.e.,1/2 of 0.5g/kW (Ru not counted).

BNL CATHODE electrocatalysts:1. Novel class of electrocatalysts developed: Pt monolayer on

noble metal - non-noble metal core-shell nanoparticles. The performance of 0.14g/kW meets the target for 2010 of 0.15g/kW.

2. MEA test of Pt/Au/Ni/C failed, attributable to inadequate synthesis.

3. Scale up of catalysts synthesis to 1g/batch accomplished.

4. Au cluster submonolayer has a pronounced stabilization effect on Pt under potential cycling regime. It is a highly promising solution to that problem.

5. The Pd2Co and Pd3Fe have activities similar to that of Pt; Methanol tolerance of Pd2Co confirmed in a fuel cell test at LANL.

6. NbO2 is a promising support for Pt monolayer catalysts.

Summary: Pt monolayer electrocatalysts

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1. Include repeated potential cycling tests as part of durability screening of promising candidates… --- The tests have been started.

2. Do small-scale development of the processes needed to make 50cm2 MEA and, eventually, short-stack quantities of catalysts through the UPD-replacement process. ---The synthesis has been scaled up to make 1 g of the catalyst per day.

3. Should try to reduce the amount of Pd sooner in timeline (2 reviewers) -- Pt monolayer on noble metal -non-noble metal core-shell nanoparticles provide such a possibility.

4. A more appropriate industry collaboration would be with companies that make catalysts,--- BNL is open for collaboration.

Responses to Previous Year Reviewers’ Comments

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Publications

1. J. Zhang, F.H.B. Lima, M. H. Shao, K. Sasaki, J.X. Wang, J. Hanson, R. R. Adzic, Platinum monolayer on non-noble metal - noble metal core-shell nanoparticles electrocatalysts for O2 reduction, J. Phys. Chem. B, 109 (2005) 22701-22704.

2. M.H. Shao, P. Liu, R.R. Adzic, Superoxide is the intermediate in the oxygen reduction reaction on platinum electrode. J. Am. Chem. Soc. (submitted).

3. J. Zhang, M. B Vukmirovic, K. Sasaki, F. Uribe, R. R. Adzic, Platinum monolayerelectrocatalysts for oxygen reduction: effect of substrates, and long-term stability, J. Serb. Chem. Soc., 70 (2005) 513-525 (75th Anniversary issue ).

4. J. X. Wang, F. A. Uribe, and R. R. Adzic, Parameterizing H2/air-PEMFC polarization curves, and quantifying cell performance and major voltage losses”, Electrochem. Solid-State Lett. (submitted).

5. M.H. Shao, K. Sasaki, R.R. Adzic, Pd-Fe nanoparticles as electrocatalysts for oxygen reduction. J. Am. Chem. Soc. 128 (2006) 3536.

Presentations

Five papers at national and three at international meetings,

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• Stability of Au clusters on Pt surfaces

• Stability of surface segregation induced by annealing

• The extent of mixing in bimetallic systems at 80-100°C

• Electronic effects vs. strain effects

Critical Assumptions and Issues