electrode dynamics at platinum-water interface osamu sugino issp, university of tokyo
TRANSCRIPT
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