1. Simulation of Hydrated Polyelectrolyte Layers as Model Systems for Proton Transport in Fuel Cell Membranes Ata Roudgar, S. P. Narasimachary and Michael Eikerling Department of Chemistry, Simon Fraser University, Burnaby, BC, Canada Acknowledgements The authors thank the funding of this work by NSERC. Simon Fraser University References 2. Model of Hydrated Interfaces inside PEMs 1. Structural Views of the Membrane Anode: H 2  2H + + 2e - Cathode : ½ O 2 + 2H + + 2e -  H 2 O Total: H 2 + ½ O 2  H 2 O 4.Conclusions 3.Results Principal Layout of a PEM Fuel Cell Effective properties (proton conductivity, water transport, stability) hydrophobic phase hydrophilic phase Primary chemical structure backbones side chains acid groups Molecular interactions (polymer/ion/solvent), persistence length Self-organization into aggregates and dissociation Secondary structure aggregates array of side chains water structure “Rescaled” interactions (fluctuating sidechains, mobile protons, water) Heterogeneous PEM random phase separation connectivity swelling Evolution of PEM Morphology and Properties Focus on Interfacial Mechanisms of PT Insight in view of fundamental understanding and design: Objectives  Correlations and mechanisms of proton transport in interfacial layer  Is good proton conductivity possible with minimal hydration? Assumptions:  decoupling of aggregate and side chain dynamics  map random array of surface groups onto 2D array  terminating C-atoms fixed at lattice positions  remove supporting aggregate from simulation Feasible model of hydrated interfacial layer 2. Computational Details Side view fixed positions Top view Unit cell: Ab-initio calculations based on DFT (VASP)  formation energy as a function of d CC  effect of side chain modification  binding energy of extra water molecule  energy for creating water defect Computational resources: Linux clusters PEMFC (our group), BUGABOO (SFU), WESTGRID (BC, AB) 2D hexagonal array of surface groupsd CC Hydrated fibrillar aggregates L. Rubatat, G. Gebel, and O. Diat, Macromolecules 37, 7772 (2004). G. GEBEL, 1989 Structure formation, transport mechanisms MEMBRANE DESIGN Formation energy as a function of sidechain separation for regular array of triflic acid, CF3-SO3-H independenthighly correlated C. Chuy, J. Ding, E. Swanson, S. Holdcroft, J. Horsfall, and K.V. Lovell, J. Electrochem. Soc. 150, E271-E279 (2003). M. Eikerling and A.A. Kornyshev, J. Electroanal. Chem. 502, 1-14 (2001). K.D. Kreuer, J. Membrane Sci. 185, 29- 39 (2001). E. Spohr, P. Commer, and A.A. Kornyshev, J. Phys.Chem. B 106, 10560-10569 (2002). M. Eikerling, A.A. Kornyshev, and U. Stimming, J. Phys.Chem.B 101, 10807-10820 (1997). M. Eikerling, S.J. Paddison, L.R. Pratt, and T.A. Zawodzinski, Chem. Phys. Lett. 368, 108 (2003). Fully dissociated “upright” structure Non-dissociated “tilted” structure Binding energy of additional water molecule  Contour plot for 10x10 grid in xy-plane  Identify favorable positions of extra-H2O  Full optimization and calculation of binding energy Contour plot for d CC = 6.3Å Creation of a Water Defect Energy for removal of one water molecule from the unit cell Sharp transition from weak to strong binding at ~ 7 Å Strong fluctuations expected in this region! Correlations in interfacial layer are strong function of sidechain seperation Transition between upright (“stiff”) and tilted (“flexible”) configurations Extra water molecule: sharp transition from weak to strong binding Water defect: minimally hydrated array is rather stable Side chain separation is key parameter – perspectives for design… Experimental evaluation of interfacial mechanisms is feasible The small binding energy of an extra water and large require energy to remove one water molecule shows that the minimally hydrated systems are very stable and will persist at T>400K. d cc =10.4Å Transition from “upright” to “tilted” structure occurs at d CC = 6.5Å upon increasing C-C distance Current work: establish reaction coordinates and reaction pathways and calculate the corresponding activation energy (using the method of “Transition Path Sampling”) d cc =8.1Å Fully-dissociated “tilted” structure  Highest formation energy E = -2.78 eV corresponds to d CC = 6.2Å (“upright” structure).  Transition between fully dissociated, partially dissociated and non-dissociated states occurs in “tilted” structure.  Distinct DFT implementations gave similar results.  The same structures and transitions were found for CH 3 -SO 3 -H (weaker acid). Numerical values are slightly different. The transition between fully-dissociated and fully non- dissociated states occurs at e.g. at d CC = 6.7Å. At d CC = 7.5Å, the number of H-bonds drops to 7; inter-unit-cell H-bonds are broken and formation of clusters of surface groups commences. Number of H-bonds as a function of C-C distance