Molecular Modeling of Structure and Dynamics in Fuel Cell Membranes

1. 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 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 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 consider small substructures of the membrane. Here we apply ab-initio approaches to simplified model 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. III. Computational simulation of arrays of the simplest and shortest sidechain (CF3SO3H) Part 1: Geometry Optimization • Computational details • Two-dimensional hexagonal array with fixed positions of carbon atoms. • 3 sidechains + 3 water molecules per unit cell • Vienna Ab-initio Simulation Package (VASP) • Only Γ point is considered in total energy calculation • Projected Augmented Wave (PAW) pseudopotential with cut-off energy Ecut=400 eV • PW-91 Functional Top view Side view Architecture of Membranes Nature of backbone Chemical architecture of the side chains Fixed carbons Binding energy as a function of sidechain - sidechain distance PS-g-mac PSSA(21) (graft polymer) Conductivity 0.08 Scm -1 S-PBI butane PAN-g-macPSSA graft copolymers (32) (graft polymer). Conductivity 0.1 Scm-1 Conductivity 0.01 Scm-1(80°C) • A C-C distance of d=6.18Å corresponds to the largest binding energy - fully dissociated array. • 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 array occurs at d=6.7Å (weaker acid). • We expect a high probability of proton transfer in the region of d~7.2Å, where the difference in energies is small. Distance between side chains Length of the side chain S-PPBP Conductivity 0.001 Scm-1 Partially sulfonated styrene ethylene. Conductivity 0.002 Scm-1 when x=9. Top-view Top-view 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 At t>0.5ps the acid head groups start to approach each. Local clusters are formed. A partially dissociated state develops. In initial configuration(t=0) all acids groups are non-dissociated The complexity and large number of involved atoms demand simple but reliable models for computational simulation of such a system. t=5.7 ps • At t>4.1ps the system evolves towards a transition state. • The potential energy drops. • Acid groups become fully dissociated • The energy of the new structure is 1eV lower than the initial (non-dissociated) configuration II.Model System and Approaches 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. • Our model consists of a minimally hydrated 2-D array of sidechains with fixed end points. • We perform full quantum mechanical calculations using VASP. • Total energy calculation as a function of C-C distance was performed. • Upon increasing the C-C distance, a transition from dissociated to non-dissociated state occurs. • We have performed a molecular dynamics simulation for 3(CF2SO3H + H2O) at fixed C-C distance d=7.2Å. Our results show that a transition occurs at t=4.1ps and a new and more stable structure is formed at t=5ps. Compare the dynamics of thesidechains with and without the substrate (frequency spectra). T Step 2: We remove the substrate and fix the positions of the endpoint atoms at their initial position. Acknowledgement We gratefully acknowledge the funding of this work by NSERC. • References • Carmen Chuy, Jianfu Ding,Edward Swanson, Steven Holdcroft,Jackie Horsfall,and Keith V. Lovell, JECS,150(5) • E271-E279(2003). • M.Eikerling, A.A.Kornyshev, Journal of Electroanalytical Chemistry,502(2001),1-14. • K.D.Kreuer, Journal of Membrane Science,185 (2001),29-39. • 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. Important characteristics of model system • Length of sidechains. • Distance between sidechains. • Chemical structure of sidechains • Nature of acid groups. • Number of acid groups on sidechain. • Water content.