Non-Pt Catalysts for PEFCs

Xiaoping Wang, Deborah Myers, and Nancy KariukiArgonne National Laboratory

May 16-19, 2006

This presentation does not contain any proprietary or confidential informationProject ID # FCP 29

Work sponsored by U.S. Department of Energy, Hydrogen, Fuel Cells and Infrastructure Technologies Program

2

Overview

TimelineProject start date: October FY’03Project end date: Open Percentage complete: n/a

BudgetTotal funding: $750 KFunding for FY’05: $150 KFunding for FY’06: $300 K

BarriersBarriers addressedB. CostC. Electrode performance

PartnersLos Alamos National LaboratoryProf. John Regalbuto, University of Illinois at ChicagoCollaborations with BES-funded groups on characterizationRegularly providing updates and soliciting feedback from FreedomCAR Fuel Cell Technical Team

3

Objective

Develop a non-platinum cathode electrocatalyst for polymer electrolyte fuel cells to meet DOE targets– Promotes the direct four-electron transfer with high electrocatalytic

activity (comparable to that of Pt) (0.44 A/mgPt or 720 μA/cm2 @0.9V)• O2 reduction reaction (ORR) in acidic media (e.g, in PEFC)– Two-electron transfer

O2 + 2H+ + 2e− = H2O2

– Four-electron transferO2 + 4H+ + 4e− = 2 H2O

– Four-electron process is desirable due to its higher efficiency and non-corrosive product

– Chemically compatible with the acidic polymer electrolyte (5000h @80oC, < 40% electrochemical area loss)

– Low cost ($8/KW, 0.3 mg PGM/cm2)

4

Approach

Bi-metallic systems (e.g., base metal, noble metal)– Surface segregation of minor noble metal component to form protective layer– Base metal component chosen to modify d-band center of noble metal

making it more “Pt-like”– Choice of bi-metallic systems is based on the surface segregation energies

and d-bend center shift (Source: A.V. Ruban, H.L. Skriver, J.K. Nørskov, Phys. Rev. B, 59 (1999)15990)

– Examples: Bi-metallics of iridium, rhodium, and palladium– Alternative supports to modify electronic properties of small metal particles

(e.g., titania)

Metal centers attached to electron-conducting polymer backbones– Allows easy control of spacing between metal centers– Electron conductor in close proximity to reaction site can promote high

catalyst utilization

5

The d-band centers of candidate noble metals can be shifted to desired value by alloying with base metals

There is a relationship between the d-band center of the metal and its ORR activity - Nørskov-Hammer theory and results of LBNL groupPt3Co has high ORR activity and, thus, a desirable d-band center (LBNL)

Pt Pd Rh Ir

d-ba

nd c

ente

r ene

rgy

Pt

Pt3Co

Pd

Pd-base metal alloys

Rh

Rh-base metal alloys

IrIr-base metal alloy

Incr

easi

ng b

indi

ng e

nerg

y

6

Progress vs. FY ’06 Milestones

Synthesize and determine the ORR activity of two metal center-polymeric, two bimetallic, and one ACNT systems (06/06)– Two metal center-polymeric systems

• Synthesized and tested two metal center moieties • Identification of electron conducting polymers as backbone is underway

– Two bimetallic systems• Competed one Ir-based bimetallic system for material synthesis (nine

compositions) and testing, including materials characterization• Synthesized one Pd-based bimetallic system (five compositions); testing and

characterization is underway• Finished three more Au-based bimetallic systems

– One Aligned Carbon Nanotube system• Tested ORR activity

Determine the long-term stability of the electrocatalyst with thehighest ORR activity (09/06)

7

Solid solutions of Ir-BM nanoparticles have been formed on Vulcan carbon

0

200

400

600

800

1000

1200

30 40 50 60 70 80

2 Theta

Cou

nts

1 NM:0 BM

1 NM:1 BM

1 NM:3 BM

1 NM:9 BM

0 NM:1 BM0 Ir:1 BM

1 Ir:9 BM

1 Ir:3 BM

1 Ir:1 BM

1 Ir:0 BM

1 Ir:3 BM

20 nm20 nm

3 Ir:1 BM

XRD shows that a solid solution is formed at 400oCTEM shows catalysts to have 2- to 10-nm diameter particles

8

The Ir to base metal ratio affects the ORR activity

Ir to BM molar ratio

-140

-120

-100

-80

-60

-40

-20

0.0(1:0) (9:1) (3:1) (1:1) (46:54) (38:62) (3:7) (1:3) (1:4) (1:9) (0:1)

Cur

rent

(µA

)

Neg. scanPos. scan

Current at 0.75 V vs. SHE, 400oC heat-treated

9

-3.0E-04

-2.5E-04

-2.0E-04

-1.5E-04

-1.0E-04

-5.0E-05

0.0E+00(1:0) (9:1) (3:1) (1:1) (1:3) (1:9) (0:1)

i/A a

t 0.7

5 V

vs S

HE

Neg. scan, Day 1Pos. scan, Day 1

Ir to BM molar ratio1:0 3:19:0 1:1 1:91:3 0:1

Cur

rent

at 0

.75

V vs

. SH

E /A

Acid treated Ir:BM catalyst showed improved ORR activity

Acid treated

20 nm50 nm50 nm

TEM image indicates that acid treatment did not change particle sizeEDX results shows molar ratio of Ir to BM stayed the same after acid treatment

-1.2E-03

-1.0E-03

-8.0E-04

-6.0E-04

-4.0E-04

-2.0E-04

0.0E+000.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

E/V vs. SHE

i/A Ir

BM

3 Ir:1 BM

10

ORR current decreased as the heat treatment temperature increased (Ir:BM)

-1.40E-03

-1.20E-03

-1.00E-03

-8.00E-04

-6.00E-04

-4.00E-04

-2.00E-04

0.00E+00

2.00E-04

0.1 0.3 0.5 0.7 0.9 1.1

i/A

(38:62)Ir:Ru/C, day 1, 400C(38:62)Ir:Ru/C, day 1, 600C(38:62)Ir:Ru/C, day 1, 800C

-1.40E-03

-1.20E-03

-1.00E-03

-8.00E-04

-6.00E-04

-4.00E-04

-2.00E-04

0.00E+00

2.00E-04

0.1 0.3 0.5 0.7 0.9 1.1

i/A

(38:62)Ir:Ru/C, day 1, 400C(38:62)Ir:Ru/C, day 1, 600C(38:62)Ir:Ru/C, day 1, 800C

38Ir:62BM, Room Temperature, 10 mV/s, 1600 rpm

Potential (V vs. SHE)

Cur

rent

(A)

Post-depositionheat-treatment temp.

Red – 800°C

Pink – 600°C

Blue – 400°C

11

Coarsening of Ir:BM particles occurred at higher heat treatment temperatures

50 nm

800oC

20 nm

600oC

50 nm

400oC

400

450

500

550

600

650

700

750

800

850

900

30 40 50 60 70 80

2 Theta

Cou

nts

400°C600°C

12

Nanoparticles of Pd-base metal alloy have been formed on Vulcan carbon

Cu9Pd1 baseline substracted

0

10

20

30

40

50

60

0 100 200 300 400 500 600 700 800 900

Sample exit temperature (oC)

TCD

sig

nal

Cu9Pd1 baseline substracted

0

10

20

30

40

50

60

TCD

sig

nal

1Pd:9BM

• TPR to determine the condition that yields reduced form of the metal

0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

7 0 0

3 0 4 0 5 0 6 0 7 0 8 02 T h e ta

Cou

nts

1Pd:1BM

9Pd:1BM

1Pd:3BM

3Pd:1BM

1Pd:9BM

• XRD indicates alloy formation

20 nm

50 nm

Bimodal size distribution:– 2-5 nm and 10-20 nm– 10-20 nm particles more

evenly distributed

0 100 200 300 400 500 600 700 800Temperature (oC)

13

Base metal increased ORR activity of palladium

-70.0

-60.0

-50.0

-40.0

-30.0

-20.0

-10.0

0.00.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Potential vs. SHE (V)C

urre

nt (m

A/m

g m

etal

)

9Pd 1BM

3Pd 1BM

1Pd 1BM

1Pd 0BM

1Pd 3BM

1Pd 9BM

14

Base metals enhance the ORR activity of Ir and Pd, but further improvement is needed

-8.E-02

-7.E-02

-6.E-02

-5.E-02

-4.E-02

-3.E-02

-2.E-02

-1.E-02

0.E+000.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Potential vs. SHE (Volts)

Cur

rent

(A/m

g m

etal

)

PdCu

Pd

-8.E-02

-7.E-02

-6.E-02

-5.E-02

-4.E-02

-3.E-02

-2.E-02

-1.E-02

0.E+000.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Potential vs. SHE (Volts)

Cur

rent

(A/m

g m

etal

)

Pd-BM

Pd

-8.E-02

-7.E-02

-6.E-02

-5.E-02

-4.E-02

-3.E-02

-2.E-02

-1.E-02

0.E+000.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Potential vs. SHE (Volts)

Cur

rent

(A/m

g m

etal

)

-8.E-02

-7.E-02

-6.E-02

-5.E-02

-4.E-02

-3.E-02

-2.E-02

-1.E-02

0.E+000.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Potential vs. SHE (Volts)

Cur

rent

(A/m

g m

etal

)

PdCu

Pd

-8.E-02

-7.E-02

-6.E-02

-5.E-02

-4.E-02

-3.E-02

-2.E-02

-1.E-02

0.E+000.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Potential vs. SHE (Volts)

Cur

rent

(A/m

g m

etal

)

-8.E-02

-7.E-02

-6.E-02

-5.E-02

-4.E-02

-3.E-02

-2.E-02

-1.E-02

0.E+000.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Potential vs. SHE (Volts)

Cur

rent

(A/m

g m

etal

)

Pd-BM

Pd

-1.5E-03

-1.2E-03

-9.0E-04

-6.0E-04

-3.0E-04

0.0E+000.2 0.4 0.6 0.8 1.0

Potential (V vs. SHE)

Cur

rent

(am

ps/m

g m

etal

)

BM

Ir3Ir:1BM

Ir-based Pd-basedSupported metallic system Ir Ir-BM BM Pd

8.4 mA

Pd-BM’

20 μA 65 μA 3 μAORR mass activityat 0.8 V per mg metal

36.6 mA 440 mA(110 mA)

DOE 2010 target0.9 V

15

Nanocrystalline cores encapsulated in a monolayer of organic molecular or surfactant shell have several advantages– Size and surface properties of nanoparticles are controllable

– Organic shell protects nanoparticles from agglomeration• Stable and highly monodisperse nanoparticles

– Interparticle interactions of the shell molecules confers spatial controllability in the assembly of the particles on support materials

– Deliberate tailoring of the surface of the metallic core with suitable shell structures enables the morphology of the bimetallic composition to be very well controlled

– Large volumes of pre-engineered nanoparticles can be efficiently produced

New synthetic approach for desired structure/morphology of bimetallics --- Metal core-organic shell approach

16

A procedure has been devised for bimetallic system

Step 1: Seed preparation

Δ

Stirring/N2 Capped base metal seed nanoparticles

Organic capping agent

Metal Precursor(e.g. Base metal(acac)3)Octyl ether Δ

Reduction

Δ

Ir on base metalCore-shell nanoparticles

Second metal precursor(e.g. Ir(acac)3)Octyl ether

Step 2: Seed growth

Δ

Stirring/N2 Reduction

Δ

Seed nanoparticles

Δ

Organic capping agent

Step 3: Assemble on support and remove the shell for activation

support Ir on base metal Core-shell Nanoparticles on support

Ir on base metal Core-shell nanoparticles Δ

O2/N2 H2/N2

Δ

17

TEM shows nanoparticle size and monodispersity control---better shaped and more uniform particles than traditional method

Particle size ~ 2 nm

BM seed processed at 120oC, 35 min

Ir@BM processed at 110oC for 40 min

Particle size ~ 3 nm

TEM of Ir on BM reveals size growth associated with overdeposition of Ir on BM nanoparticles used as seeds EDX analysis confirmed bimetallic composition consistent with synthetic feed amounts of metal precursors

Co-impregnation 400oC/regen

50 nm

18

ORR activity of cobalt metal centers attached to polymer backbones begins at 0.6 V

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

Potential (volts vs. SHE)

Cur

rent

(mA

)Deaerated Solution

Oxygen-Saturated

19

Summary

Both supported Ir- and Pd-based bimetallic systems showed an ORR activity better than supported Ir and Pd alone

– ORR started at potential of 0.85 V and 0.93 V, respectively for Ir-and Pd- based system

XRD and TEM analysis indicates alloy formation and nanosizeparticles for both systems

ORR activity was strongly influenced by type and composition of bimetallic system and catalyst preparation and processing conditions (e.g., acid treatment and heat treatment temperature)

New synthetic approach is being explored to obtain desired structure/morphology

20

Future work

Perform elevated temperature RRDE experiment to determine O2reduction mechanism (2e- or 4e- transfer) for Ir-based system

For Pd-based system– ORR activity testing and TEM work for other compositions prepared

Prepare and test the ORR activity of additional bimetallic systems identified

Surface and bulk characterization to verify the desired catalystcomposition/structure, particle size, and electronic properties by using various techniques such as XRD, TEM, SEM, FTIR of adsorbed CO, and TPD of CO

Explore different synthesis methods and temperature treatments

Perform electrochemical and chemical stability studies

Fabricate and test a membrane-electrode assembly using newly-developed electrocatalysts

21

Acknowledgments

J. Vaughey and T. Cruse for XRD

J. Mawdsley for TEM

M. Ferrandon for TPR and TPD

J. Yang and D.J. Liu for preparing ACNT system

22

Publications

“Polymer Electrolyte Fuel Cell Cathode Electrocatalysts”, Xiaoping Wang, Romesh Kumar, and Deborah J. Myers, Poster and Abstract, 2005 Fuel Cell Seminar, Palm Springs, CA, November 14-18, 2005

23

Response to FY ’05 Reviewers’ Comments“The work on bimetallic( Au-based), transition metal carbides and nitrides, and metal centers attached to electron-conducting polymer backbone is too broad area for such relatively small project”

– We have focused our effort on bimetallic systems

“Need to pull in other characterization techniques to better understand behavior of bimetallic system ( microscopy, etc.).”

– We are characterizing catalysts by TEM, XRD, EDX, CO Temperature Programmed Desorption

“See no evidence that the claimed LANL collaboration is underway”

– Once material shows adequate activity in screening tests, LANL will fabricate MEAs

“The reason for the choice of the new catalysts was not evident”

“Screening large number of non-Pt catalysts based on Au is no guarantee that one of the proposed catalyst formulations will work”

– We have elaborated on the rationale behind the choice of bimetallic systems in this presentation