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Nanocluster calculations for fuel cell catalysis and molecular electronics

Nanocluster calculations for fuel cell catalysis and molecular electronics. PASI Workshop 2004 Caltech Materials and Process Simulation Center. Timo Jacob. January 13, 2004. Contents. Fuel Cells: Modeling of the cathode reaction Surface model

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Nanocluster calculations for fuel cell catalysis and molecular electronics

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  1. Nanocluster calculations for fuel cell catalysis and molecular electronics PASI Workshop 2004 Caltech Materials and Process Simulation Center Timo Jacob January 13, 2004

  2. Contents Fuel Cells: Modeling of the cathode reaction Surface model Adsorption and dissociation of various compounds Reaction mechanisms Pt/Ni and Pt/Co alloys to enhance reaction rate Molecular Electronics Traditional disulfide compound Search for a new compound with better properties

  3. 1. Fuel Cells: Modeling of the cathode reaction

  4. Main Goal Fuel cell automotives

  5. How it works

  6. Goals 2 H+ + 1/2 O2 + 2 e- H2O Goals: • Understand the reaction mechanism (multi-step process that is difficult to model) • Replace or reduce Pt as catalyst material • Understand influence of environment(Temp, E-Field, …) • Understand influence ofInterface Surface-simulation with • Periodic systems • Finite cluster models Highly accurate ab initio quantum mechanical methods (DFT)

  7. Finite cluster models • Vary size and number of layers • Determine best model for Pt(111) Pt3 Pt8 Pt6 Pt12 Pt6.3 Pt8.4 Pt5.10.5 Pt12.7 Pt9.10.9 1. D. H. Parker et al., Surf. Sci. 217, 342 (1989) 2. A. Eichler et al., Phys. Rev. B62, 4744 (2000)

  8. Model cluster Reasons for choosing Pt14.13.8 First layer is large enough to simulate surface reactions Size of the cluster is about 15 Å, comparable to the dimensions (nm) of the real Pt nano- particles at the cathode Use this model to study O, H, O2, OH, OOH, H2O on Pt(111)

  9. O2 bridge position Ebind=11.29kcal/mol Ediss=30.80 kcal/mol

  10. O2 fcc position (1) Ebind=7.21 kcal/mol Ediss=23.86 kcal/mol

  11. O2 fcc position (2) Ebind=1.33 kcal/mol Ediss=5.06 kcal/mol O2 tilted

  12. OH dissociation Ebind=47.45 kcal/mol Ediss=54.33 kcal/molEdiss=43.70 kcal/mol

  13. OO-H dissociation of OOH (1) Ebind=23.85 kcal/mol Ediss=23.45 kcal/mol

  14. OO-H dissociation of OOH (2) Ebind=17.30 kcal/mol Ediss=8.29 kcal/mol

  15. O-OH diss.of OOH Ebind=23.85 kcal/mol Ediss=30.01 kcal/molEdiss=17.13 kcal/mol

  16. H-OH dissociation of water Ebind=13.90 kcal/mol Ediss=29.73 kcal/mol

  17. Heats of Formation

  18. Mechanism (1)

  19. Mechanism (2)

  20. ReaxFF for Pt Atoms Molecular conformations Pt-clusters Electrons Bond formation MESO MD QC ReaxFF Energy (kcal/mol) ReaxFF QC Empirical force fields ab initio, DFT, HF Cluster description

  21. Pt Particle Ionomer Film of thickness dfilm Carbon Primary Particle (d=40 nm) Outlook Goals: 1. Investigate Nanophase-segregated structure 2. How does the Nafion chain and water molecules change with time (Dynamics)? 3. What are the influences of the interface on the Cathode Reaction? How to control these properties Pt

  22. Bimetallic alloy systemsPt/Ni, Pt/Co

  23. Bimetallic electrodes: Pt-Ni, Pt-Co Kinetic current densities at different potentials (normalized to electrode area) Electrochim. Acta 47, 3787 (2002) Enhancement factor per unit Pt surface atom: > 2 for Pt3Co bulk, 2-5 for supported PtCo

  24. Pt3Ni (bulk) Determine the structure of Pt/Ni alloy: • Pt3Fe, Pt3Ti, and Pt3Cu fcc-like structure • Experimentally observed is acomposition of 75% Pt and25% Ni (Co) • Ni is homologous to Pt similar behavior • Ni has a fcc bulk structure (as Pt) • fcc-unicellcorner atoms replaced by Ni (Co)  

  25. Pt3Ni (bulk) Periodic calculations: • Lattice constant: 7.36 bohr (Pt: 7.55 bohr, Ni: 6.6 bohr) • coh. Energy: Ecoh (Pt3Ni)=22.63 eV [Ecoh (Pt4)=23.36 eV, Ecoh (Ni4)=17.76 eV]  Pt dominates the unit cell structure Taking this as an initial guess for the surface structure

  26. Pt/Ni (surface) Periodic slab-calculations:  nearly no relaxation effects in bulk and on surface

  27. Pt3Ni (surface) Cluster calculations: Pt5Ni1 E=0 eV DE=0.76 eV (17,50 kcal/mol) S=3 S=3 • Higher number of Pt-Ni bonds rather than Pt-Pt bonds • Edge position rather than tip position

  28. Pt3Ni (surface) Pt4Ni2 D E = 0.20 eV (4.62) DE = 0.44 eV (10.21) DE = 0 eV DE = 0.23 eV (5.23) S=3 S=3 S=3 S=3 • 2 Ni atoms prefer larger distance rather than adjacent sites • Ni positively charged by ~0.2-0.3 e-

  29. Pt3Ni (surface) Pt3Ni3 DE= 0.30 eV (6.91) DE = 0.05 eV (1.12) DE = 0.02 eV (0.48)DE = 0 eV S=3 S=3 S=3 S=3 • Prefer • larger distance • 4-fold coordinated sites (bonds to 4 Pt-atoms)

  30. Pt/Ni: Cluster composition • Cluster composition studies similar as with Pt3Ni Changing the composition in all different layers

  31. Pt3Ni cluster summary • On the surface nearly no relaxation or reconstruction (see periodic calculations) • On terrace sites Ni prefers bulk structure (each Ni-atom is surrounded by 6 Pt-atoms)Spin density ~1.5, ~0.5 e- positively charged  3d84s2 electronic configuration • On steps or kinks Ni prefers the step edge (4-fold coordinated) sites rather than tip sites (neighbored by 3 or 2 Pt-atoms) • 4-fold coordinated sites:Spin density ~1.5, ~0.5 e- positively charged 3d84s2 electronic configuration • 2-fold coordinated sites:Spin density ~1 (d8.5 electronic configuration)Charge ~0.5 e- positively charged

  32. Adsorption of O and H on Pt3Ni • Adsorption on 1 layer and 3 layer clusters at all different sites (bonds to only Pt or bonds to Pt and Ni)

  33. H adsorption O adsorption

  34. Pt3Co bimetallic alloy Similar Approach as for Pt3Ni • Periodic calculations of fcc-like unit cell, where the corner atoms are replaced by Co 3:1 ratio between Pt and Co • Periodic calculations: • a0=3.86Å(Pt: 3.99 Å, Co: 2.51/4.07 Å, Pt3Ni=3.89 Å) • Ecoh (Pt3Co)=28.05 eV [Ecoh (Pt4)=23.36 eV, Ecoh (Co4)=17.56 eV, Ecoh (Ni4)=17.76 eV, Ecoh (Pt3Ni)=22.63 eV ]

  35. Pt3Co surface relaxation • Using the obtained geometry (atomic distance) as initial structure for periodic calculations study relaxation and reconstruction

  36. Pt3Co relaxation/segregation • Experimental (Gauthier et al. 1992): Pt80Co20(111): • LEED studies with cleaning and an procedures to get layer-wise composition • 1. layer: x1Pt=100 at.%2. layer: x2Pt= 48 at.%3. layer: x3Pt= 89 at.% Our results: Pt75Co25(111): • 1. layer: 2. layer: 3. layer: Δd12=2.192Å dbulk=2.226Å Δd23=2.057Å Δd12=2.11/2.38Å (2.192) dbulk=2.23Å (2.226) Δd23=2.09/2.13Å (2.057)

  37. Pt3Co surface relaxation • Study relaxation and segregation with different sized unit cells (2x2, 2x3, 3x3) by changingthe composition anddistribution of Pt and Co 2.76 A 2.72 A 2.18 A 2.25 A 2.25 A 2.21 A 2.19 A 2.27 A 2.23 A (fixed) 2.23 A (fixed) 2.73 A (fixed) 2.73 A (fixed)

  38. Pt3Co segregation • 2x2 unit cell with 5-layers • Fixed geometries • Relaxed geometries 100-50-75-50-100 75-75-75-75-75 75-75-25-100-100 50-100-75-100-50 Exp: (100-48-89) E=0eV ΔE=0.44eV ΔE=1.09eV ΔE=2.56eV ΔE=0.70eV ΔE=0.62eV ΔE=1.10eV E=0eV

  39. Pt3Co segregation • 2x3 unit cell with 5-layers 100-50-83-50-100 83-66-83-66-83 100-50-83-50-100 Co Pt Exp: (100-48-89) E=0 eV ΔE=1.09eV ΔE=0.52eV 83-66-83-66-83 100-50-83-50-100 100-50-83-50-100 100-50-83-50-100 ΔE=1.27eV ΔE=0.17eV ΔE=0.16eV ΔE=0.18eV

  40. Molecular Electronics- Alligator Clip Molecules

  41. Molecular switch compound

  42. Source (Pt) Source (Pt) Drain (Pt) Drain (Pt) Dielectric (SiO2) Dielectric (SiO2) Gate (poly silicon) Gate (poly silicon) + S + O O O N N S O O S S O O O O O O S S N N + + – 4PF 6 Mono-disulfide [2]Rotaxane bis-disulfide [2]Rotaxane Energy One side van der Waals force, Ill-defined barrier width, Likely higher barrier Both sides chemical bonds, Thinner barrier, Lower barrier height

  43. + S + O O O N N S O O S S O O O O O O S S N N + + – 4PF 6 3-terminal single-molecule devices: the role of contact symmetry @ gate voltage 2.55V @ gate voltage 2.4V nA nA Lower current, asymmetric I-V Higher current, symmetric I-V gate voltage (V) gate voltage (V) source to drain voltage (mV) source to drain voltage (mV) dI/dV vs. junction bias and gate bias symmetric dI/dV plot vs junction bias Bis-disulfide [2]Rotaxane Mono-disulfide [2]Rotaxane

  44. Which model? • Calculating various structures using Pt12 • Afterwards calculating the correct structures and energetics with Pt35

  45. Working Plan Actual compound:3-methyl-1,2-dithiolane (MDTL) • Plan: • Understand the structure and energetics of 3-methyl-1,2-dithiolane on Pt clusters • Establish correlation between Ebind and conductivity for known species • Apply this knowledge to propose new alligator clip compounds • Calculate structure and energetics for all possible candidates • Calculate I/V characteristic for the most promising species Etot= -953.624 61 h S=0 (singlet-state)

  46. Gas-phase MDTL

  47. Actual Alligator Clip Compound Structure Determination: Etot= -2383.582 5 hE=22.90 kcal/molS=5 (as bare Pt12) Etot= -2383.619 0 hE=0 kcal/molS=5 Etot= -2383.603 2 h E=9.91 kcal/molS=5 Binding energy: Etot= -2383.582 5 h Ebind= -32.44 kcal/mol S=11 (as bare Pt35) Etot= -2383.615 7 hE=2.07 kcal/molS=5

  48. Alternative Compounds (1) 123-Triazole Imidazole Purine 2,4-Diazapentane Etot= -242.222 0 hS=0 Etot= -411.930 7 hS=0 Etot= -227.371 9 h S=0 Etot= -226.215 1 h S=0 3-Methyl,1,2-Oxathiolane 5-Methyl,1,2-Oxathiolane Methanol Methyl-phosphino Etot= -630.628 2 h S=0 Etot= -630.623 6 hS=0 Etot= -115.721 8 hS=0 Etot= -381.826 7 h S=1/2

  49. Alternative Compounds (2) PCH3 Phospholane

  50. 123-Triazole and Imidazole Etot= -1672.188 7 hE=4.33 kcal/molS=5 (as bare Pt12) Etot= -1672.195 6 hE=0 kcal/molS=5 (as bare Pt12) Etot= -1672.188 7 h S=5 (as bare Pt12) Etot= -4413.825 6 h Ebind= -17.69 kcal/mol S=11 (as bare Pt35)

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