The 4th SUNBEAM Workshop

Structural and Electronic properties of Structural and Electronic properties of platinum platinum nanoparticlesnanoparticles studied bystudied by

in situ xin situ x--ray diffraction and ray diffraction and in situ xin situ x--ray absorption spectroscopyray absorption spectroscopy

Hideto ImaiHideto ImaiFundamental and Environmental Fundamental and Environmental

Research LaboratoriesResearch Laboratories

NEC CorporationNEC Corporation

Micro Fuel CellsMicro Fuel Cells　　 Portable batteries in the Ubiquitous Networking SocietyPortable batteries in the Ubiquitous Networking Society

Fuel (Methanol or hydrogen)

Potential successor to Potential successor to Li Li -- ion batteriesion batteries

Higher energy capacityHigher energy capacityLonger usage periodLonger usage periodSmaller sizeSmaller size

Instantly rechargeable

CO2

Cathode catalysts OO22+ 4H+ 4H++ + 4e+ 4e-- →→ 2H2H22OO

CHCH33OH + HOH + H22O O →→COCO22 + 6H+ 6H++ + 6e+ 6e--

e-

H+

Anode catalysts

Instantly rechargeableProton conducting

membrane

Air (O2) H2OFuel cell-powered laptop computer

(A prototype)

Technological hurdlesTechnological hurdles 　　

Energy Loss at the cathode electrodeEnergy Loss at the cathode electrode

Cathode ReactionCathode ReactionOOxygen xygen RReduction eduction RReactioneaction

OO22+ 4H+ 4H++ + 4e+ 4e-- = 2H= 2H22OOEE00 = 1.23 V= 1.23 V

Catalytic activity of the current Catalytic activity of the current catalyst (Pt) is not enoughcatalyst (Pt) is not enough

Elucidation of ORR mechanismElucidation of ORR mechanismSurface morphologySurface morphologyIntermediate speciesIntermediate speciesElectronic state of platinumElectronic state of platinum

Anode

Cathode

1.2

1.0

0.8

0.6

0.4

0.2

0.0Po

tent

ial (

V)

1.00.50.0Current density (A/cm2)

Cell Voltage(PEFCs) Cell Voltage

(DMFCs)

CHCH33OH + HOH + H22O O →→COCO22 + 6H+ 6H++ + 6e+ 6e--

Voltage lossat cathode

HH2 2 →→ 2H2H+ + + 2e+ 2e--

Current vs. Voltage (Polarization) curve for PEFCs and DMFCs

Stability of platinum in aqueous systems Stability of platinum in aqueous systems 　　

-1.0x10-2

-0.5

0.0

0.5

1.0

Cur

rent

(A)

1.51.00.50.0Potential (V vs. RHE)

-1.0x10-2

-0.5

0.0

0.5

1.0

Cur

rent

(A)

1.51.00.50.0Potential (V vs. RHE)

22HH22OO →→ OO22+ 4H+ 4H++ + 4e+ 4e--

Oxide formation

Hydrogen adsorption

Double layer

22HH++ + 2e+ 2e-- →→ HH2 2

Pt-H Pt Pt-OH?Pt-O?

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

Pote

ntia

l (V

)

14121086420pH

PtPtO or Pt(OH)2

PtO2

PtO3

O2+4H++4e-=2H2O

2H++2e-=H2

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

Pote

ntia

l (V

)

14121086420pH

PtPtO or Pt(OH)2

PtO2

PtO3

O2+4H++4e-=2H2O

2H++2e-=H2

Fuel cell operating region

The potential - pH equilibrium diagram for the system platinum - water

Cyclic voltammogram for carbon supported platinum nanoparticles in 0.5 M H2SO4

In situ XRD and XAFS measurementsIn situ XRD and XAFS measurements　　

Platinum Platinum nanoparticlesnanoparticles (supported on a carbon)(supported on a carbon)ca. 3 nm, 50 wt% ca. 3 nm, 50 wt% -- Pt loading Pt loading

Potential controlPotential controlHome made three electrode electrochemical cellHome made three electrode electrochemical cell0.5M H0.5M H22SOSO44 electrolyteelectrolytePotentiostaticPotentiostatic modemode

In situ XRD BL16XUIn situ XRD BL16XUPhoton energy 30 Photon energy 30 keV keV ( ( λ λ = 0.041nm= 0.041nm ))Transmission mode, Imaging plateTransmission mode, Imaging plate

In situ XAFS BL16B2In situ XAFS BL16B2LL33 and and LL22 absorption edgesabsorption edgesTransmission modeTransmission mode

In situ In situ xx--ray diffractionray diffraction　　

10x106

8

6

4

2

0

Inte

nsity

(a.u

.)

222018161412108 2θ (degree)

(111

)

(200

)

(220

) (311

)

(222

)PtO

PtO

0.0 V vs. RHE 0.4 V 0.7 V 0.9 V 1.0 V 1.2 V 1.4 V Pt (bulk) PtO (bulk)

10x106

8

6

4

2

0In

tens

ity (a

.u.)

1210 2θ (degree)

(111

)

(200

)

PtO

0.0V0.7V, 0.9V

1.0V

1.2V

1.4V

0.0 V vs. RHE 0.4 V 0.7 V 0.9 V 1.0 V 1.2 V 1.4 V Pt (bulk) PtO (bulk)

The potential dependence of x-ray diffraction patterns for carbon supported platinum nanoparticles

In situ In situ xx--ray absorption spectroscopyray absorption spectroscopy　　

L3 absorption edge L2 absorption edge

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0A

bsor

ptio

n (a

.u.)

13.4013.3513.3013.2513.20Photon Energy (keV)

0.0V vs. RHE 0.5V 0.7V 0.8V 0.9V 1.0V 1.1V 1.2V 1.4V

1.5

1.0

0.5

0.0

Abs

orpt

ion

(a.u

.)

11.711.611.5Photon Energy (keV)

0.0V vs. RHE 0.5V 0.7V 0.8V 0.9V 1.0V 1.1V 1.2V 1.4V

The potential dependence of XAS spectra at the L3 absorption edge

The potential dependence of XAS spectra at the L2 absorption edge

Fourier transform of Pt LFourier transform of Pt L33 EXAFSEXAFS

0.2

0.1

0.0

Four

ier T

rans

form

543210 R (Å)

Pt-Pt

Pt-O

0.0V vs. RHE 0.5V 0.7V 0.8V 0.9V 1.0V 1.1V 1.2V 1.4V

30

20

10

0

Four

ier T

rans

form

4321 R (Å)

Pt-Pt

Pt-O

0.0V vs. RHE 0.5V 0.7V 0.8V 0.9V 1.0V 1.1V 1.2V 1.4V

0.2

0.1

0.0Fo

urie

r Tra

nsfo

rm543210

R (Å)

Pt-Pt

Pt-O

0.0V vs. RHE 0.5V 0.7V 0.8V 0.9V 1.0V 1.1V 1.2V 1.4V

The k3-weighted Fourier transform of Pt L3EXAFS

The k-weighted Fourier transform of Pt L3EXAFS

Structural models for the platinum Structural models for the platinum nanoparticlenanoparticle surfacesurface

cf. Pt-O distance 2.58Å (bulk PtO2)0.5 ~ 0.7 V Clean Pt surface (Double layer region) Surface reconstruction

hollow site Pt-O~2.06Å

0.8 ~1.0 V Irreversible OH (or atomic O) adsorption

1.1 ~ 1.4 V Amorphous like PtOx(several layers)

bridge site Pt-OH~ 2.26Å

on-top site Pt-OH~ 2.02Å

XANES spectraXANES spectra

1.5

1.0

0.5

0.0

Abs

orpt

ion

(a.u

.)

11.5911.5811.5711.5611.55Photon Energy (keV)

0.0V vs. RHE 0.5V 0.7V 0.8V 0.9V 1.0V 1.1V 1.2V 1.4V

1.2

1.0

0.8

0.6

0.4

0.2

0.0A

bsor

ptio

n (a

.u.)

13.3013.2913.2813.2713.2613.25Photon Energy (keV)

0.0V vs. RHE 0.5V 0.7V 0.8V 0.9V 1.0V 1.1V 1.2V 1.4V

The potential dependence of XANES spectra at the L3 absorption edge

The potential dependence of XANES spectra at the L2 absorption edge

LL33 and and LL22 absorptions in platinumabsorptions in platinum

Conduction band

5d 5/2

5d 3/2

Spin-orbit coupling1.6 eV

L3

EF

EThe fractional change in the number of unoccupied d states from that of bulk Pt

fd =∆ hT

hTPt=

(∆ A3 + 1.11∆Α2)

(A3 + 1.11Α2)Pt

hT = ( 1 + fd ) hTPt

Ai : edge area for the i th edge∆ Ai = Aisample - AiPthT : total number of unoccupied d states∆hT = hTsample – hTPthTPt = 0.30 (tight-binding +

spin-orbit coupling)

2p 3/2L2

2p 1/2

Electronic states Electronic states Unoccupied states in platinum 5Unoccupied states in platinum 5dd orbitalsorbitals

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0Q

(C)

1.00.50.0Potential (V vs. RHE)

Hydrogen adsorption

1.06V

1.52V

Surface oxidation

0.02V

Pt-H PtOx

Pt-OH

0.45

0.40

0.35

0.30Pt 5

d u

nocc

upie

d st

ates

1.51.00.50.0Potential (vs. RHE)

8x10-3

6

4

2

0

-2

-4C

urrent (A)

bulk platinum

The total charge due to the hydrogen adsorption and the surface oxide formation estimated from the cyclic voltammogram

The potential dependence of unoccupied states in platinum 5d orbitals

SummarySummaryIn situ XRD and XAFS studies clearly showed the followings :

In the oxygen reduction reaction potential regionIn the oxygen reduction reaction potential region……..

Surface reconstruction, adsorption of oxygen species, Surface reconstruction, adsorption of oxygen species, and formation of platinum oxidesand formation of platinum oxides

Irreversible OH Irreversible OH adsorptionadsorption　　(0.8 ~ 1.0V)(0.8 ~ 1.0V)PtPt--OH at onOH at on--top sitetop sitePtPt--O at 3 fold or 4 fold hollow sitesO at 3 fold or 4 fold hollow sites

Surface oxide formation (1.1 ~ 1.4 V)Surface oxide formation (1.1 ~ 1.4 V)amorphous like amorphous like PtOPtOx x (several layers)(several layers)

Charge transfer from platinumCharge transfer from platinumThe number of unoccupied states in Pt 5The number of unoccupied states in Pt 5dd orbitalsorbitals linearly linearly depends on the potential and the coverage of oxygen speciesdepends on the potential and the coverage of oxygen species

The formation of surface oxygen species may significantly reducethe catalytic activity of platinum on oxygen reduction reaction