Molecular Modeling of Structure and Dynamics in Fuel Cell Membranes A. Roudgar, Sudha N.P. and M.H. Eikerling Department of Chemistry, Simon Fraser University, Burnaby, Canada,V5A 1S6 I. Introduction III. Computational simulation of arrays of the simplest and Proper understanding of the relations between structure formation and mobility is critical for the development of highly performing proton conducting membranes for fuel cells. It is, however, impossible to study the shortest sidechain (CF3SO3H) complete scale of structural details in real membranes with quantum mechanical approaches (DFT and AIMD). Feasible routes are to utilize combinations of quantum mechanical and classical approaches or to Part 1: Geometry Optimization consider small substructures of the membrane. Here we apply ab-initio approaches to simplified model Computational details Top view Side view systems. The objective is to understand co-operative phenomena in proton transport and explore effects of length, chemical structure and arrangements of polymeric side chains. • Two-dimensional hexagonal array with fixed positions of carbon atoms. • 3 sidechains + 3 water molecules per unit cell Architecture of Membranes • Vienna Ab-initio Simulation Package (VASP) Nature of backbone Chemical architecture of the side chains • Only Γ point is considered in total energy Fixed carbons calculation • Projected Augmented Wave (PAW) pseudopotential with cut-off energy Ecut=400 eV • PW-91 Functional S-PBI butane PS-g-mac PSSA(21) (graft polymer) Binding energy as a function of sidechain - sidechain distance PAN-g-macPSSA graft copolymers (32) Conductivity 0.01 Scm-1(80°C) Conductivity 0.08 Scm -1 (graft polymer). Conductivity 0.1 Scm-1 A C-C distance of d=6.18Å corresponds to the largest binding energy - fully dissociated array. Distance between side Length of the side chain chains The transition between fully dissociated and fully non- dissociated array occurs at d=7.2Å. In similar calculations for CH3SO3H the transition between fully-dissociated and fully non-dissociated S-PPBP Conductivity 0.001 Scm-1 array occurs at d=6.7Å (weaker acid). Partially sulfonated styrene ethylene. Conductivity 0.002 We expect a high probability of proton transfer in the Scm-1 when x=9. Top-view Top-view region of d~7.2Å, where the difference in energies is small. Morphology of Nafion The ionomer consists of an hydrophobic backbone with side chains that are terminated by acid groups. Good proton conductivity of the membrane is due a spontaneous “nanophase segregation” in the presence of water. Dissociated acid Non dissociated acid Part 2: Ab-initio Molecular Dynamics Computational details • Two dimensional hexagonal arrays with C-C fixed distance d=7.2 • 3 sidechains + 3 water molecules per unit cell • Constant temperature T=300K • Nose-Hoover thermostat with Nose mass Q=0.05 • PW-91 Functional t=0 t=2.1 ps In initial configuration At t>0.5ps the acid head (t=0) all acids groups groups start to approach are non-dissociated each. Local clusters are formed. A partially dissociated state develops. The complexity and large number of involved atoms demand simple but reliable models for computational simulation of such a system. At t>4.1ps the system evolves towards a transition state. t=5.7 ps The potential energy drops. II. Model System and Approaches Acid groups become fully dissociated The energy of the new structure is 1eV lower than the initial (non-dissociated) configuration Step 1: We consider a two- dimensional regular array of sidechains anchored to a substrate. IV. Conclusion We study effects of molecular structure on proton, solvent and polymer dynamics in PEMs. Compare the dynamics of the Our model consists of a minimally hydrated 2-D array of sidechains with fixed end points. sidechains with and without the We perform full quantum mechanical calculations using VASP. T substrate (frequency spectra). Total energy calculation as a function of C-C distance was performed. Step 2: We remove Upon increasing the C-C distance, a transition from dissociated to non-dissociated state occurs. the substrate and fix We have performed a molecular dynamics simulation for 3(CF2SO3H + H2O) at fixed C-C distance d=7.2Å. Our the positions of the results show that a transition occurs at t=4.1ps and a new and more stable structure is formed at t=5ps. endpoint atoms at their initial position. Acknowledgement We gratefully acknowledge the funding of this work by NSERC. References Important characteristics of model system •Carmen Chuy, Jianfu Ding,Edward Swanson, Steven Holdcroft,Jackie Horsfall,and Keith V. Lovell, JECS,150(5) E271-E279(2003). • Length of sidechains. • Nature of acid groups. •M.Eikerling, A.A.Kornyshev, Journal of Electroanalytical Chemistry,502(2001),1-14. • Distance between sidechains. • Number of acid groups on sidechain. •K.D.Kreuer, Journal of Membrane Science,185 (2001),29-39. • Chemical structure of sidechains • Water content. •E.Spohr, P.Commer, and A.A.Kornyshev, J.Phys.Chem.B 2002,106,10560-10569. •M.Eikerling, A.A.Kornyshev, and U.Stimming, J.Phys.Chem.B 1997,101,10807-10820.