Masoud Aryanpour & Varun Rai Ab initio MD Simulation of Oxygen Electroreduction on Pt(111) in the Presence of Protonated Water Masoud Aryanpour & Varun Rai ME346 Project Presentation

ME346 Project Presentation Outline Motivation Ab initio MD Simulation details Current status / Work-in-progress ME346 Project Presentation

Polymer Electrolyte Membrane Fuel Cells Quiet operation Low operating temperatures No emissions No moving parts ME346 Project Presentation

Polymer Electrolyte Membrane Fuel Cells H2 + ½ O2 → H2O Proton Exchange Membrane (PEM): only protons can go across Catalysts (Pt) used to enhance reaction rates Electrochemical reactions (accompanied with charge transfer) ME346 Project Presentation

What Limits PEMFC Performance? Voltage limited by finite reaction rates at cathode (Activation losses) Even at very low current densities substantial drop in cell voltage Effect remains throughout operative range Better understanding of reaction kinetics at cathode important ME346 Project Presentation

Where does It All Happen? Most of the activation losses in PEM Fuel Cells occur at the catalyst layer in cathode due to slow electroreduction of oxygen ME346 Project Presentation

Where does It All Happen? O2 + 4H+ + 4e- → 2H2O Gas diffusion layer O2 Carbon particles (50-100 nm) Catalyst (Pt) (2-5 nm) H+ Membrane (50-150 mm) Membrane, electrode/ catalyst and gas (H2 or O2) need to ‘get together’ 3-phase region Rate of reactions scales with area of 3-phase region ME346 Project Presentation

Electroreduction of Oxygen Focus of this project O2 + 4H+ + 4e- → 2H2O ME346 Project Presentation

ME346 Project Presentation Need for Ab initio MD (1) Classical MD uses “pre-defined” interatomic potentials Complications for systems with different types of atoms Electronic structure and hence, bonding pattern changes during the course of simulation : crucial for studying “chemically complex” systems involving bond breaking/formation Classical MD not appropriate for such systems ME346 Project Presentation

ME346 Project Presentation Need for Ab initio MD (2) In ab initio methods, forces acting on the nuclei are computed from electronic structure calculations Calculations are performed “on-the-fly” as the molecular dynamics trajectory is generated Electronic variables treated as active degrees of freedom instead of integrating them out (Classical MD) Possible to handle “chemically complex” systems, given a suitable approximation of the many-electron problem (approximate solution of the Schrodinger’s equation) ME346 Project Presentation

Applications of Ab initio MD Solids, Polymers, and Materials Semiconductor industry Graphitization on flat and stepped diamond(111) surfaces Clusters, Fullerenes, and Nanotubes Matter at Extreme Conditions Virtual matter laboratory Glasses and Amorphous Solids Surfaces, Interfaces, and Adsorbates Relevant to our project ME346 Project Presentation

Project Objectives Review Two-fold purpose 1. Get a deeper understanding of ab initio methods Wide range of important problems can be attacked similarly 2. Apply the knowledge to study electroreduction of O2 on Pt(111) “Billion Dollar” question in the PEMFC industry ME346 Project Presentation

Ab initio MD: Steps Involved Setting up the model geometry Calculation of the optimized wavefunction Relaxing the system Relax Reoptimize wavefunction Re-relax Actual simulation Analysis of results ME346 Project Presentation

ME346 Project Presentation Model Details (1) System: 3x4 Pt(111) slab, one layer O2 molecule Hydronium complex (3 water molecules + proton) Super cell: Orthorombic: 9.59 x 8.30 x 13.0 A Boundary conditions: Periodic Ab initio method: Car-Parrinello MD (CPMD code) ME346 Project Presentation

System Model: Geometry Pt O H Initially, O2 parallel to Pt(111) surface at 3.5 A Plane of the hydronium complex perpendicular to O2 axis Lowest H atom ~ 2.5 A from O2 center ME346 Project Presentation

ME346 Project Presentation Model Geometry Pt O H ME346 Project Presentation

ME346 Project Presentation Simulation Details Car-Parinnello Molecular Dynamics Canonical Ensemble: Nose-Hover Thermostat Temperature: 350K (typical PEM condition) Time step: 6 a.u. (~ 0.121 fs) Simulation time: ~ 1 ps System: Linux + Xeon 2.4 GHz Elapsed time: ~ 8 days ME346 Project Presentation

ME346 Project Presentation Time for a Movie! ME346 Project Presentation

ME346 Project Presentation Analysis of Results Monitor evolution of the Kohn-Sham energy of the system (equivalent of potential energy of Classical MD) Qualitative picture of intermediates formed Possible quantitative information about activation energy of steps involved Monitor reaction coordinates: O-O, O-H, and Pt-O distances: Quantitative information about the intermediates and their structure ME346 Project Presentation

Results(1): Reaction Coordinates Wang et al., J. Phys. Chem. B, 2004 3 layers of Pt TM pseudo-potentials ME346 Project 1 layer of Pt VDB pseudo-potentials ME346 Project Presentation

Results (2): Kohn-Sham Energy Wang et al., J. Phys. Chem. B, 2004 3 layers of Pt TM pseudo-potentials ME346 Project 1 layer of Pt VDB pseudo-potentials ME346 Project Presentation

ME346 Project Presentation Conclusions Used ab initio MD to study the electroreduction of O2 on Pt(111) Results: Adsorption energy (3.5 eV ) O-O and O-Pt distance after adsroption: ~3 A and ~1.6 A, respectively Good qualitative agreement with literature Changing initial conditions might show the effect of hydronium complex ME346 Project Presentation

ME346 Project Presentation We are Done! Thanks. Questions? ME346 Project Presentation