Electrode Dynamics at Platinum-Water Interface

Osamu Sugino

ISSP, University of Tokyo

Metal/water interface

• Hydrophibic/hydrophobic– wet/repel

• Redox reaction– rusting

• Catalysis– fuel cell reaction– electrolysis

Response to external field: water

• Large dipole moment– free rotation– screening

• H-Bond network– 0.2eV (90% ionic, 10% covalent)– retardation of ~ ps

– H3O+ diffusion (Grothus)

+0.35

−0.7r=78!

Response to ext. field: interface

• H-bond network disturbed– water-metal interaction ~0.5eV

• Contact layer formed– less mobile but not icy– dipole layer

• potential drop: bias voltage• inner Helmholtz layer

V

Response to ext. field: reaction

• Large field and dense surface charge

• Chemical reaction (redox)– electron transfer– reactive species formed

e−

e−

e−

First-Principles MD simulation

• Electrode dynamics @ anode in acid

e−

e−

e−

reservoirpH=0~1

H+H+H

H

H H+

Modeling

•MD (classical nuclei and adiabatic electrons)•32 H2O + 36 Pt•Direct simulation of ~10 ps

•DFT for electrons•Bias up to ~ −0.8 V vs. SHE

To apply bias

•Put excess e−

•Water screens within several ps•analyze the contact layer•see the reaction H3O++ e− H(ad)+H2O

e−

+++++

+

+ +

+

－－

+

+

－

metal－

+

+

+－

－ +

－

DFT

water: r=78ions: Poisson-Boltzmann

Continuum theory

Effective Screening Medium

r

M.Otani. and O.S., PRB 73, 115407 (‘06)

Embed interface slab in classical medium

water

+++++

+－

+ + +－

－

－ －

－+

+－

－

+

+

+－

－+

－

DFT Continuum theory

)(),( rr eI )(rc•Kohn-Sham

)(rr

)()()()(4

)(rrrrV

rcIeH

r

•Poisson

)0(,,),()( crHcc rVr

•Poisson-Boltzmann continuum

)()()()()(2

1 2 rrrVrVr mmmeXCHm

r

watermetal

Large-scale simulation

• Supercomputers

• Simplest ESM modeling– Capacitor model– Classical ions (electrolyte ions) not included

Pt(111)/water interface

Pt

Contact layer

bulk water

Oxygen distribution function

Pt

Contact layer

bulk water

Contact layer formation

1 e− / 40 Pt 1 e− / 12 Pt

Distribution function f(z)

water density larger by 20 %

Top view

• last 2 ps

2D H-bond network

Summary of the structure

• Contact layer– One molecular layer thick (~3 Å)– ‘Bulky’ water: z > 3Å– Water density depends on the bias

• H-bond network– 2D network at the contact layer

• Screening of water (εr~10)

– Surface electrons are densely induced

H3O+ accepts an electron

Reaction H3O++e-H(ad)+H2O

Red: positiveBlue: negative

relative tochargein the bulk

Population

Adiabatic picture on charge transfer

Adiabatic picture on charge transfer

Level crossing

5d

Orbital energy Total energy

H(ad)+

H2 O

H3 O

++e

−

V

H3O+ LUMO

Restructuring afterwards“Reorganization”

After H adsorption

H2O with O-down appears butunfavorable electrostatically

Reorientation hampered by H-bond network

Jumping reorientation motion

0.0ps 1.8ps

H/Pt(111) at aqueous condition

Migrates almost freely (1.7 ps)

Summary

• New first-principles simulation of the biased metal/water interface

• Microscopic details on Helmholtz layer and reaction dynamics

• Water assists the reaction on Pt

• A step towards microscopic understanding of electrochemistry

Thank you!

Acknowledgment

ES and ISSP Supercomputers

Collaborators

Minoru Otani (ISSP)

T. Ikeshoji (AIST), Y. Morikawa and I. Hamada (Osaka U.), Y. Okamoto (NEC)

H/Pt(111) at vacuum

H is trapped at on-top site

Kallen et al. PRB (2001)

DOS projected to the H3O+ orbital

Transfer from 5d band to this orbital

遷移金属と水の相互作用 (UHV)

ロジウム / 水 相互作用IRAS 等による構造決定 ( 吉信研 ) 水の吸着エネルギー DFT 計算

By S. Meng PRB (2004)

遷移金属 / 水界面＝接触層形成

V=−0.23V vs Vpzc

V=+0.52V vs Vpzc

酸素 up 構造

酸素 down 構造

M.F.Toney Nature (’94)

目的• 電位がかかった金属 / 水界面の構造

– 水和構造の解明• 接触層と水素結合網の形成

– 電気二重層の解明• 電位と水の応答

• 高速な化学反応（化学・電気エネルギー変換）– 水素発生、酸素発生のメカニズム– なぜ白金か？水の役割は？

第一原理計算

液体水＝分子動力学計算長い緩和時間→数 ps

ＣＰＵ 1-2 週間＝ 1ps

•3 layer of Pt(111)•12 Pt for each layer

323

32 H2O + H

電場をかける＝表面に過剰電子を配置

Water conduction band

Watervalence band

Pt

Put excess electrons

水の分極と遮蔽

Water conduction band

Watervalence band

Pt

イオン分布の変化→コンデンサモデル

Water conduction band

Watervalence band

Pt

conductor

Capacitor model to mimic

role of the ions in solution

Effective Screening Medium method

r=M.Otani. and O.S., PRB 73, 115407 (‘06)

Embed slab in

dielectric continuu

m

Total energy expression

Poisson equation:

Kohn-Sham equation:

Non-repeated slab embedded in a dielectric continuum

水の構造

• 負の電位を印加（負の表面電荷）– 接触層の形成は？– 水素結合網の形成は？

• ESM-FPMD (STATE) シミュレーション

Contact water layer

0 2 4 6 80

5

10

150.25<n<0.69

H d

istr

ibu

tion

z (A)

hydrogen

0 2 4 6 80

5

10

150.25<n<0.69

O d

istr

ub

utio

n

z (A)

oxygen

−0.04 e/Pt−0.5 V

2D H-bond network in the contact layer−0.08 e/Pt

−1.0 V

化学反応性のシミュレーション

• ヒドロニウムイオンの導入

• 表面からの引力• 接触層へ到達• 電子移動 & プロトン移動反応→水素吸着

– H3O++e−→H2O+H(ad)

• 水素の表面拡散→会合脱離– 2H(ad)→H2

Snapshot

0.0ps ~ 0.5ps ~ 1.4ps ~1.5ps

Reaction intermediate

Excess charge & Dipole moment & Pt-H distance

Reaction intermediate

4-fold coordinated H3+!

Reaction intermediate

The Volmer step

Electronic structure

How does the electron transfer?

Population (isosurface)

： Population

Population analysis

Excess electrons

Electron deficit

+0.35

−0.70

DOS projected to the H3O+ orbital

Transfer from 5d band to this orbital

After the reaction

Water-assisted efficient diffusion of

H

水が反応を促進している

1. Proton-relay via H-bond network• H+ efficiently reaches the contact layer

and the reaction site

2. Polarization of water (ε=10-20)• Large surface electron density prompts

reduction reactions

3. Water-assisted fast surface diffusion

これからの課題• ESM の改良

– イオンによる遮蔽効果• 酸素極での反応

– 多数の経路• 白金の特異性

– 卑金属、酸化物• 非断熱計算

– TDDFT

• 大規模化・超並列化＝ metal O(N) 法

http://www.lsbu.ac.uk/water/hbond.html

Electrode Dynamics

• Non-equilibrium response of water to– existence of metal surface– application of bias potential