Argonne National Laboratory is managed by The University of Chicago for the U.S. Department of Energy

N.M. Marković

Thanks to: V. Stamenkovic, D. Strmcnik, D. Tripkovic, D. Van der Vliet,C. Lucas, General Motors and DOE

2

Elec

trod

e

+-

-

Electrolyte

H2O

inner Helmholtz plane (IHP)

outer Helmholtz plane (OHP)

Nor

mal

wat

er s

truc

ture

Fully solvated ions

Specifically adsorbed ion+

+

+

+

e-

e-

H2O

H2O

Theory

Structure

Chemical Nature

Stability

Catalytic Activity

Composition

nFEΔGads −=

Understanding the surface electrochemistry

3

Use and develop state-of-the-art surface sensitive probes, spectroscopes and EC

Atomic/molecular structuresElectronic structuresChemical natureCompositionSize and shapeKinetics

STRUCTURE/FUNCTION RELATIONSHIPS AND STABILITY

REAL NANOPARTICLES

MODEL NANOPARTICLES

SINGLE CRYSTALS

Approach

4

1. Single crystal surfaces

2. High surface area catalysts

Adsorption of spectator species on Pt(hkl)ORR on Pt(hkl)

Cathode reaction in PEMFC

Monometallic surfaces: structure sensitivity

Bimetallic surfaces:

Polycrystalline Pt3M (M = Ni, Co, Fe, Mn, Cr, V, T)

Electronic (ligand) effectsGeometric effects

ORR on PtMonometallic surfaces: particle size effects

Bimetallic surfacesTailoring catalytic properties

Single crystal Pt3Ni(hkl) surfaces

Electrocatalytic trendsStability

5

ΔGad term

O2 adsorption strength is uniquely related to theelectronic properties of the electrode material

Pt – O2- : ΔGad= -0.87 eV

Au - O2- : ΔGad= 0.24 eV

Oxygen Reduction Reaction

Rotating ring disk method Pt(111)0.1M KOH

Θad is mostly spectators, not O2ad

(1-Θad) term

Effects availability of metal sitesand ΔGad

i = n F k (1-Θ) exp (-γΔG/RT)

6

Structure sensitive kinetics due tostructure sensitive adsorption of Hupd , OHad and anions

ORR kinetics on Pt(hkl) is mainly determined by the (1-Θad) term

“Serial” reaction pathway:

Activation energy (@Er : ΔG~40 kJ/mol) is independent of pH and surface structure

Pt(111) Pt(100) Pt(110)

E [VRHE]0.0 0.2 0.4 0.6 0.8 1.0

I [m

A]

-1.0

-0.5

0.0

I [μ A

]

0

50

100

(a) 0.05 M H2SO4, 50 mV/s, 900 rpm

E [VRHE]0.0 0.2 0.4 0.6 0.8 1.0

I [m

A]

-1.0

-0.5

0.0

I [μ A

]

0

5

10

15

(b) 0.1 M KOH, 50 mV/s, 900 rpm

Hupd

Hopd

HupdHads

Structure sensitivity

Surface Science Reports 45 (2002) 117

O2 O2,ad H2O2,ad H2Ok2 k3

k1

i = n F k (1-Θ) exp (-ΔG/RT)

7

(111)

(100)

Small particles are more oxophilic

pztc increases by increasing the particle size

Particle size effect

JECS 127 (2005) 6819

8

Col 24 vs Col 12

0.6

0.7

0.8

Particle Size [nm]1 10 100

0.0

2.0

4.0 ORR @ 0.85 VRHE

0.1M HClO4

30nm

5nm2nm

1nm333 K

298 K OHad

ORR

Reactivity

3M NSTF catalysts

a)

J.Phys.Chem. B 109 (2005)14433

i = n F k (1-Θ) exp (-γΔG/RT)

9

Pt3M alloys: UHV characterization

Pt - skin2.

Annealed Surface

AES 3 keV

Auger Electron Energy [eV]200 400 600 800

dN(E

)/dE

(arb

. uni

ts)

Fe598

Fe651

Fe703

Pt158

Pt237 Pt251

Pt390

(a)

Pt3M1.

Sputtered Surface

Pt

M SideView

UHV

Sputtered Surface

Annealed Surface

Pt3Fe

E / E0

0.2 0.4 0.6 0.8

LEISS Ne+ 1 keV

PtFe

Sputtered Surface75% Pt, 25% Fe

(b)

Pt3Fe

UPS 90 eV

Binding Energy [eV]02468

Inte

nsity

[arb

. uni

ts]

Pt - poly

Pt3Fe - skinannealed

Pt3Fesputtered

(c)

Pt3Fe

d-ba

nd c

ente

r [e

V]

2.4

2.6

2.8

3.0

3.2

3.4

2.4

2.6

2.8

3.0

3.2

3.4

PtNiCoFeMnCrTiV

PtNiCoFeTiV

Sputtered* surfaces

Pt-skin surfaces

Alloying component

(d)

Annealed Surface:Pt-skin = 100% Pt

SURFACE PROPERTIES

Chemical natureO and C free surfaces

Compositionannealing; pure Pt (“Pt-skin”)spattering: bulk terminated

Electronic structurePt < bulk term. < Pt-skin εd vs. M linear

JECS 128 (2006) 8813

10

Stability

AES 3 keV

Auger Electron Energy [eV]200 400 600 800

dN(E

)/dE

(arb

. uni

ts)

Pt237

Pt390

Pt158

Fe703Fe651

Fe598C271O518

E/E0

0.2 0.4 0.6 0.8

Inte

nsity

[arb

. uni

ts]

LEIS Ne+ 1 keV

Pt 0.71Fe 0.32

Top view Side view

(a)

(b)

-Pt -M

Pt3M

Pt-skeleton = 100% Pt

Pt3Fe-sputtered

Pt-skeleton

Pt3Fe-sputtered

Pt-skeleton

Annealed surface (Pt-skin) is stable: oscillatory segregation profile ?????

Sputtered surface is unstable: “Pt-skeleton”bulk terminated concentration profile“rough” surface

TiV

Pt-skin

Nature Materials 6 (2007) 214

11

Act

ivity

impr

ovem

ent f

acto

r vs

. Pt-

poly

d-band center [eV]2.63.03.4

Pt3TiPt3V

Pt3Fe

Pt3CoPt3Ni

Pt-polyPt-skin surfacesPt-skeleton surfaces

1

2

3

(1−Θad) term

ΔGad termand

(1−Θad) term

d-ba

nd c

ente

r [e

V]

2.4

2.6

2.8

3.0

3.2

3.4

2.4

2.6

2.8

3.0

3.2

3.4

PtNiCoFeMnCrTiV

PtNiCoFeTiV

Sputtered* surfaces

Pt-skin surfaces

Alloying component

(d)

Nature Materials 6 (2007) 214

J. Chem Phys. 123 (2005) 204717

Angew.Chem 45 (2006) 2897 (in colaboration with Norskov’s group)

i = n F k (1-Θ)exp (-γΔG/RT)Pt-skinPt-skeleton

Act

ivity

impr

ovem

ent f

acto

r vs

. Pt-

poly

d-band center [eV]2.63.03.4

Pt3TiPt3V

Pt3Fe

Pt3CoPt3Ni

Pt-polyPt-skin surfacesPt-skeleton surfaces

1

2

3

(1−Θad) term

ΔGad termand

(1−Θad) term

12

Pt3Ni(hkl)- Ex-situ

Science 315 (2007) 493

13

Pt3Ni(111)- In-situ

E [V] vs. RHE

0.0 0.2 0.4 0.6 0.8 1.0

Inte

nsity

[arb

. uni

ts]

6.0

6.5

7.0

Pt3Ni(111)

i [μA

/cm

2 ]

E [V] vs. RHE

Atomic layer1 2 3 4 5 6 7

Pt [a

t. %

]

0

20

40

60

80

100

Ni [

at. %

]

100

80

60

20

0

40 Segregation oscillatory profile: “Pt-skin”

Atomic structure: (1x1)

Pt3Ni(111)

0.0 0.2 0.4 0.6 0.8 1.0

Rel

ativ

e E

xpan

sion

[%]

-1.5

-1.0

-0.5

0.0

Pt3Ni(111)

Pt(111)

a)

14

Pt3(111) system: ORR

Pt3Ni(111)

i [μA

/cm

2 ]

60

30

0

-60

-30

E [V] vs. RHE0.0 0.2 0.4 0.6 0.8 1.0

i [m

A/c

m2 ]

-6

-4

-2

0

IIIIII

H2O

2 [%

]

Pt poly

0.1 HClO4

60oC1600 rpm

050

Pt-skin 15 times (!!!) more active

Pt3Ni(111)

Oxide formation and hydrogen adsorption are inhibited on Pt-skin

( ) ⎟⎠⎞

⎜⎝⎛ Δ−

Θ−= Θ

RTGKi x

ad exp1

O2 O2,ad HO2,-ad OH-

HO2-

4e-

2e-2e-

εd : Pt3Ni(111) < Pt(111)

ΘOH: Pt3Ni(111) < Pt(111)

i = n F k (1-Θ) exp (-γΔG/RT)

15

Spec

ific

Act

ivity

: ik [μA

/cm

2 ] 0

.1M

HC

lO4 a

t 0.

9 V

vs.

RH

E

0

5

10

15

Pt3Ni(111) Pt3Ni(100) Pt3Ni(110)Pt(111) Pt(100) Pt(110)

d-band center [eV] -3.1 -2.8 -3.2 -2.9 -2.7 -2.5

Surface Morphology (111) (100) (110)

|Δd[111]| = 0.3 eV

|Δd[110]| = 0.2 eV|Δd[100]| = 0.3 eV

Pt3(hkl) systems: ORR

ORR on Pt3Ni(111) is the highest that has ever been observed on cathode catalysts !

Science 315 (2007) 493

16

Pt-skin octahedral nanoparticles: Monte Carlo

Segregation profile obtained from MC simulation

Octahedral particles are thermodynamically stable

17

Summary and Targets

d-band center [eV]2.63.03.4

Spec

ific

Act

ivity

: ik @

0.9

V [m

A/c

m2 re

al]

0

1

2

3

4

5

Pt3TiPt3V

Pt3Fe

Pt3CoPt3Ni

Pt-polyPt-skin surfacesPt-skeleton surfaces

Act

ivity

impr

ovem

ent f

acto

r vs

. Pt-

poly

1

2

3

[ ]

2.63.03.4

17

18

19

Target Activity Pt3Ni(111)

(a)

10-fold higher of Pt(111) and 90-fold higher of state-of-the art Pt/C3 m2/gPt will exceed 4xPt mass activity target

a)

Elec

trod

e

+-

-

Electrolyte

H2O

inner Helmholtz plane (IHP)

outer Helmholtz plane (OHP)

Nor

mal

wat

er s

truc

ture

Fully solvated ions

Specifically adsorbed ion+

+

+

+

e-

e-

H2O

H2O

18

ELEC

TRO

CH

EMIC

AL

CO

MM

UN

ITY

INDUSTRY

UNIVERSITIES

NATIONALLABs

STU

DEN

TS/ V

ISIT

OR

S/ ID

EAS

IDEA

S/ F

EED

BA

CK

“ARGONNE” INTERFACE

FUNDAMENTAL PRINCIPLES

ARGONNE RESEARCH FACILITIES

EXCHANGE OF IDEAS