New Electrocatalysts For Fuel Cells

Objective: Reduction of precious metal loading Principal Investigator: Philip N. Ross, Jr.Staff Scientist: Nenad M. MarkovicPost Doctoral Fellow: Vojislav Stamenkovic

Visiting Scientists: Matthias Arenz (Humboldt Fellow)

Berislav Blizanac (Belgrade)

A research program conducted at the Lawrence Berkeley National Laboratory for the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Advanced Transportation Technologies of the U.S. Department of Energy under contract No. DE-AC03-76SF00098

LBNL Materials-by-Design Approach

Single CrystalsModel Catalysts

Pure Metals Alloys

Macroscopic & Microscopic Information

Surface Structures and composition

vs.Kinetics

Reaction Mechanisms

Surface Structuresvs.

KineticsReaction Mechanisms

Taylor made surfaces

Synthesis ofNanoclusters

Commercial Catalyst

Test inFuel Cells

PrototypeCatalyst

Model Systems

MethodologiesReal Catalysts

Ex-Situ

TEMXPS

AES

LEIS

LEED

10 nm

In-Situ RRDE

FTIR

SXS Kinetics

Collaborations

IndustryNafionfilm

Glassy-Carbon(RDE)

Catalysts

GM, Rochester, NY, USA IFC, South Windsor, CT, USA 3M, Minneapolis, MN, USA

Universities and Institutes Modification o

Al acac

CH3

Al CH3acac

Al

acac

CH3

Alacac

CH3

AlCH3

acac

AlCH3

acac

Max-Planck-Institut fuer Kohlenforschung, Muelheim/Ruhr, Germany

Texas Tech University, Lubbock, TX, USA University of Eindhoven, Holland University of Wales, UK University of Bonn, Germany University of Liverpool, UK

Publications (Since 10/2001)Refereed Journals and Refereed Conference Proceedings:

1. U.A. Paulus, A. Wokaun, G.G. Scherer, T.J. Schmidt, V. Stamenkovic, V. Radmilovic, N.M. Markovic, and P.N. Ross, “Oxygen Reduction on Carbon Supported Pt-Ni and Pt-Co Alloy Catalysts”, J. Phys. Chem. B 106 (2002) 4181.2. Stamenkovic V. Schmidt TJ. Ross PN. Markovic NM. “Surface composition effects in electrocatalysis: Kinetics of oxygen reduction on well-defined PtNi and PtCo alloy surfaces.” Journal of Physical Chemistry B. 106(46):11970-11979, 2002 Nov 21. 3. Tripkovic AV. Popovic KD. Grgur BN. Blizanac B. Ross PN. Markovic NM. “Methanol electrooxidation on supported Pt andPtRu catalysts in acid and alkaline solutions.” Electrochimica Acta. 47(22-23):3707-3714, 2002 Aug 30. 4. Schmidt TJ. Stamenkovic V. Arenz M. Markovic NM. Ross PN. “Oxygen electrocatalysis in alkaline electrolyte: Pt(hkl), Au(hkl) and the effect of Pd-modification.” Electrochimica Acta. 47(22-23):3765-3776, 2002 Aug 30. 5. Paulus UA. Wokaun A. Scherer GG. Schmidt TJ. Stamenkovic V. Markovic NM. Ross PN. “Oxygen reduction on high surface area Pt-based alloy catalysts in comparison to well defined smooth bulk alloy electrodes.” Electrochimica Acta. 47(22-23):3787-3798, 2002 Aug 30. 6. Schmidt TJ. Markovic NM. Stamenkovic V. Ross PN. “Surface characterization and electrochemical behavior of well-defined Pt-Pd{111} single-crystal surfaces: A comparative study using Pt{111} and palladium-modified Pt{111} electrodes.” Langmuir.18(18):6969-6975, 2002 Sep 3.7. Schmidt TJ. Ross PN. Markovic NM. “Temperature dependent surface electrochemistry on Pt single crystals in alkaline electrolytes Part 2. The hydrogen evolution/oxidation reaction.” Journal of Electroanalytical Chemistry. 524(Special Issue):252-260, 2002 May 3.8. Arenz M. Stamenkovic V. Schmidt TJ. Wandelt K. Ross PN. Markovic NM. “CO adsorption and kinetics on well-characterized Pd films on Pt(111) in alkaline solutions.” Surface Science. 506(3):287-296, 2002 May 20.9. Paulus UA. Wokaun A. Scherer GG. Schmidt TJ. Stamenkovic V. Radmilovic V. Markovic NM. Ross PN. “Oxygen reduction on carbon-supported Pt-Ni and Pt-Co alloy catalysts.” Journal of Physical Chemistry B. 106(16):4181-4191, 2002 Apr 25. 10. Schmidt TJ. Ross PN. Markovic NM. “Temperature-dependent surface electrochemistry on Pt single crystals in alkaline electrolyte: Part 1: CO oxidation.” Journal of Physical Chemistry B. 105(48):12082-12086, 2001 Dec 6. 11. Schmidt TJ. Stamenkovic V. Attard GA. Markovic NM. Ross PN. “On the behavior of Pt(111)-Bi in acid and alkaline electrolytes.” Langmuir.17(24):7613-7619, 2001 Nov 27.

Research Plan: 2002

Unified concept for both anode and cathode catalysts utilizing PGM-based bimetallic nanoparticles with “grape” structure (PGM skin with base metal core)

Choice of skin and core metals different for anode and cathodePGM/base metal combinations selected based on existing electronic theory and synthesized in UHV

Pursue new synthetic chemistry to synthesizenanoparticles with the “grape” structure Currently focusing on Re as metal core with Pt and Pd as PGMPt and Pd monolayers on Re(0001) model systemRe colloidal chemistry

Optimization of AuPd anode catalyst for HT membranes

Computational screening of non-PGM catalyst concepts using newly developed (under BES funding)ab initio theory of the ORR

Pt Pt3Ni Pt3Co

0.1M HClO4 @ 0.85V

i k /

mA

cm-2

2

4

6

8

10

12

14

16 Sputtered SurfaceAnnealed Surface 333K

Pt3Re

ORR activity

E / E0

0.4 0.6 0.8

Inte

nsity

/ ar

b.un

itsIn

tens

ity /

arb.

units

Ni 0.37

Pt 0.71

LEISSCo 0.37

Top View

Annealed Surface

Sputtered Surface

Segregation Effect: Platinum Skin vs. Bulk Alloy Surfaces

Platinum Skin Effect: Bimetallic Nanoparticle

Higher intrinsic activity (per unit area) Substitution of “buried” Pt atoms in particle core by base metal atoms

Pt Ni

Pt Surface enrichment

Experimental ProcedurePt3Co ; Pt3Ni

Sputtered Surfaces298K

CV stability

0.0<E<1.0V

Hold @ 1.0V; 30 min

Hold @ 1.2V; 60 min

ORR : stabilityactivity

ORR : stabilityactivity

CV stability

333K

Before Treatment

After Treatment

After Treatment

ORR : stabilityactivity

Before Treatment0.0<E<1.0V

0.0<E<1.0V

0.0<E<1.2V

0.0<E<1.2V

Stability: Pt3Co and Pt3Ni Surfaces

E [VRHE]

0.0 0.4 0.8

i [µA

/cm

2 ]

333K

i [µA

/cm

2 ]

Before Treatment After Treatment

298 KPt3Co

0.1 M HClO4

i k/ m

Acm

-2

0

2

4

6

8

10

Before TreatmentAfter Treatment

Pt3Co Pt3Ni

0.1 M HClO4 @ 0.85 V333 K

333 K298 K

298 K

Conclusions

Surface composition is stablebetween 0.0 < E< 1.2 V !

ORR activity remains the samebetween 0.0 < E< 1.2 V !

Mechanism of the ORR at metal electrodes

Pt

C

I

E( eV)E0

‘ SHE

Rate limiting step in electrochemical reduction of O2 is 1st electron transfer

O2 + 1 e- → (O2-)sol Outer Sphere (E0

‘=-0.3 V)O2 + 1 e- → (O2

-)ads Inner Sphere (E0‘ + ∆Gad/F)

Addition of first electron needed to break O-O bond

O2– adsorption strength related to the electronic

properties of the electrode material

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8-4

-2

0

2

4

6

8

Ead=-0.87 eV

(O2- )-Pt

Pote

ntia

l Ene

rgy

(eV

)

(O2-)-Pt Distance (angstroms)

O-O1.3511 O-O1.4659 O-O1.6659 O-O2.8659 O-O3.0659

Activation Barrier 0.46 eV

1.4659

1.9795

Pt

(-0.43 e) (-0.43 e)

(-0.91 e) (-0.91 e)

Pt

(-0.14 e)

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8-1

0

1

2

3

4

5

6

7

8

9

Ead= 0.24 eV

(O2-)-Au

Pote

ntia

l Ene

rgy

(eV

)(O2

-)-Au Distance (angstroms)

O-O1.1569 O-O1.3511 O-O1.3569 O-O1.5569 O-O3.1569 O-O3.3569 O-O3.5569

Au

(-0.74 e) (-0.74 e)

(+0.47 e)

No bound state of O2 –

Activation barrier 2.7 eV

(O2 - )–Me Potential Energy Curves vs. O-O bond length

(+0.82 e)

Pt 5d partially filledExtra electron of O2

- lift 1π close to Pt 5d -----Stronger interaction

O2- 2π above Pt 5d

------charge transfer to Pt

Correlation Diagram of the Molecular Orbitals of (O2-)-Pt

Pt 5dxz, O2- 2π*

Pt 5dxy, O2- 2π

Pt 5dy2, O2- 5σ

A

B

C

Correlation Diagram of Molecular Orbitals of (O2-)-Au

Au 6s, O2- 2π

Au 5dxy, O2- 2π

A

B

Polarization effect

Au 5d completely filled ----No charge transfer between Pt and (O2

-)Orbital with different symmetry do not interactWeak Interaction due to polarization

----Au6s and (O2-) π levels

The Volcano Relation in ORR Kinetics

Θad is mostly OHad not (O2-)ad

H2O = OHad + H+ + e-

orO2

- + 2 H+ + e- = 2 OHad

Exponential term (O2-) Pre-exponential term (1 - Θad)Pt

Pd

CuAg

Au

)/exp()/exp()1( *2

RTGRTFEnFKci xadO Θ∆−−Θ−= β

Log

k

Ni

∆Gintermediate(O2- or OH)

Pt at the Top of the Volcano

• Interaction of the electrode with O2- requires partially filled d-orbitals with large radial extent

Group 1B, 2B, 3B etc. metals have closed d-shells Of Group VIII metals, d-orbitals in first row (3d9-n)

do not have sufficient radial extent The 5d9-n orbitals are the best for forming long bonds

• Interaction of the electrode with OHad must be relatively weak of the Group VII metals, Pt has the weakest interaction with OHad

ORR: Pt(111)-Pdi D

[mA

cm-2

]

-6

-4

-2

0

I R [ µ

A]

0

20

I R [µ

A]

0

100

E [VRHE]

0.0 0.2 0.4 0.6 0.8 1.0

i D [m

Acm

-2]

-4

-2

0

0.1 M KOH1600 rpm

I R [µ

A]

0

100

i D [m

Acm

-2]

-6

-4

-2

0

Pt(111)-PdPt(111)293 K

0.1 M HClO4

2500 rpm

0.05M H2SO4

2500 rpm

a)

b)

c)

amount Pd / ML0.0 0.5 1.0 1.5 2.0

-I /m

Acm

-2

0

1

2

3

4 @ 0.9V

“Vulcano Plot”

Electronic Effect

ORR: Re(0001)-Pd

E [VRHE]0.0 0.4 0.8

i [m

A/c

m2 ]

-6

-4

-2

0

Re (0001) - Pd

298 K, 2500 rpm

0.05 M H2SO4

0.1 M KOH

Surface Alloys Thin Metal Filmsi k[

mA

/cm

2 ]

0

5

10

15

20

25

30

0.1M KOH @ 0.85V

295K

Au(111)-Pd

50 %

Pd

Au(100)-Pd

50 %

Pd

Re(0001)-Pd Pt(111)-1MLPd Pt(111)-xMLPd

Pd ML film on Re(0001) has Ag-liked-band Density of States (DOS)

andActivity for ORR is shifted towardsthat for Ag(111)

electronic modificationshifts frequency

preferential oxidationof high frequency band

two adsorption bands

Re(0001)-Pd 0.1 M HClO4 CO sat. sol.

wave number [cm-1]

18001900200021002200

wave number [cm-1]

2300240025002600

2065 cm-1 1917 cm-1

2062 cm-11938 cm-1

Pt(111)-54%Pd

0.40 V

0.60 V

0.50 V

0.70 V

0.10 V

0.20 V

0.30 V

0.00 V

2x4.5 min dep.

2x4.5 + 7 min dep.

before transfer

E/Eo

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Inte

nsity

[a.u

.]

Re(0001)-Pd

LEISS 1 keV Ne+

Inte

nsity

[a.u

.]In

tens

ity [a

.u.]

In situ FTIR:

Pd Re electronic modificationshifts oxidation potentialby ca. – 0.2 V

CO oxidation: Pt(111)-Pd vs. Re(0001)-Pd

Research Plan: 2003-2004

Unified concept for both anode and cathode catalysts utilizing PGM-based bimetallic nanoparticles with “grape” structure (PGM skin with base metal core)Choice of skin and core metals different for anode and cathodePGM/base metal combinations selected based on existing electronic theory and synthesized in UHV

Pursue new synthetic chemistry to synthesizenanoparticles with the “grape” structure Continue focus on Re as metal core with Pt and Pd as PGMPt and Pd monolayers on Re(0001) as model systemsBegin evaluation of Re-rich supported Pt-Re catalyst for ORR

(if stable this catalyst could reduce Pt loading by a factor of 4)

Optimization of AuPd anode catalyst for HT membranes

Computational screening of non-PGM catalyst concepts using newly developed (under BES funding)ab initio theory of the ORR