1. Electrode Dynamics at Platinum-Water Interface Osamu Sugino ISSP, University of Tokyo

2. Metal/water interface Hydrophibic/hydrophobic –wet/repel Redox reaction –rusting Catalysis –fuel cell reaction –electrolysis

3. 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) +0.35 −0.7  r=78!

4. 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

5. 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−

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

7. 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

8. 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−

9. ++++++++++ + + + + － － + + － metal － + + + － － + － DFT water:  r =78 ions: Poisson-Boltzmann Continuum theory Effective Screening Medium M.Otani. and O.S., PRB 73, 115407 (‘06) Embed interface slab in classical medium water

10. ++++++++++ + － + + + － － － － － + + － － + + + － － + － DFT Continuum theory Kohn-Sham Poisson Poisson-Boltzmann continuum watermetal

11. Large-scale simulation Supercomputers Simplest ESM modeling –Capacitor model –Classical ions (electrolyte ions) not included

12. Pt(111)/water interface Pt Contact layer bulk water

13. Oxygen distribution function Pt Contact layer bulk water

14. Contact layer formation 1 e − / 40 Pt 1 e − / 12 Pt

15. Distribution function f(z) water density larger by 20 %

16. Top view last 2 ps 2D H-bond network

17. 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

18. H 3 O + accepts an electron

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

20. Adiabatic picture on charge transfer

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

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

24. Jumping reorientation motion 0.0ps1.8ps

25. H/Pt(111) at aqueous condition Migrates almost freely (1.7 ps)

26. 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

27. Thank you! Acknowledgment ES and ISSP Supercomputers Collaborators Minoru Otani (ISSP) T. Ikeshoji (AIST), Y. Morikawa and I. Hamada (Osaka U.), Y. Okamoto (NEC)

29. H/Pt(111) at vacuum H is trapped at on-top site Kallen et al. PRB (2001)

30. DOS projected to the H 3 O + orbital Transfer from 5d band to this orbital

32. 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)

33. 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)

34. 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？

35. 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

36. 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

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

38. 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

39. Effective Screening Medium method r=r= M.Otani. and O.S., PRB 73, 115407 (‘06) Embed slab in dielectric continuum

40. Total energy expression Poisson equation: Kohn-Sham equation: Non-repeated slab embedded in a dielectric continuum

41. 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

42. Contact water layer hydrogenoxygen −0.04 e/Pt −0.5 V

43. 2D H-bond network in the contact layer −0.08 e/Pt −1.0 V

44. 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

45. Snapshot

47. Reaction intermediate

48. Excess charge & Dipole moment & Pt-H distance

49. Reaction intermediate 4-fold coordinated H 3 + !

50. Reaction intermediate

51. The Volmer step

52. Electronic structure How does the electron transfer?

54.  Population (isosurface) ： Population Population analysis Excess electrons Electron deficit + 0.35 − 0.70

55. DOS projected to the H 3 O + orbital Transfer from 5d band to this orbital

56. After the reaction Water-assisted efficient diffusion of H

57. 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

58. 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

59. http://www.lsbu.ac.uk/w ater/hbond.html

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