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 –H 3 O + diffusion (Grothus) −0.7  r=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−e− e−e−

First-Principles MD simulation Electrode anode in acid e−e− e−e− e−e− reservoir pH=0~1 H+H+ H+H+ H H H H+H+

Modeling MD (classical nuclei and adiabatic electrons) 32 H 2 O + 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 H 3 O + + e −  H(ad)+H 2 O e−e−

－ － + + － metal － － － + － DFT water:  r =78 ions: Poisson-Boltzmann Continuum theory Effective Screening Medium M.Otani. and O.S., PRB 73, (‘06) Embed interface slab in classical medium water

－ － － － － － + + － － － － + － DFT Continuum theory Kohn-Sham Poisson Poisson-Boltzmann continuum 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 (ε r ~10) –Surface electrons are densely induced

H 3 O + accepts an electron

Reaction H 3 O + +e -  H(ad)+H 2 O Red: positive Blue: negative relative to charge in the bulk Population

Adiabatic picture on charge transfer

Level crossing 5d Orbital energy Total energy H(ad)+H 2 O H 3 O + +e − V H 3 O + LUMO

Restructuring afterwards “Reorganization” After H adsorption H 2 O with O-down appears but unfavorable electrostatically Reorientation hampered by H- bond network

Jumping reorientation motion 0.0ps1.8ps

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 H 3 O + 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 V pzc V=+0.52V vs V pzc 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 32 H 2 O + H

電場をかける＝表面に過剰電子を配置 Water conduction band Water valence band Pt Put excess electrons

水の分極と遮蔽 Water conduction band Water valence band Pt

イオン分布の変化 → コンデンサモデ ル Water conduction band Water valence band Pt conductor Capacitor model to mimic role of the ions in solution

Effective Screening Medium method r=r= M.Otani. and O.S., PRB 73, (‘06) Embed slab in dielectric continuum

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 hydrogenoxygen −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 –H 3 O + +e − →H 2 O+H(ad) 水素の表面拡散 → 会合脱離 –2H(ad)→H 2

Snapshot

Reaction intermediate

Excess charge & Dipole moment & Pt-H distance

Reaction intermediate 4-fold coordinated H 3 + !

Reaction intermediate

The Volmer step

Electronic structure How does the electron transfer?

 Population (isosurface) ： Population Population analysis Excess electrons Electron deficit − 0.70

DOS projected to the H 3 O + 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

ater/hbond.html

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