Acknowledgements

1. Cathode Cathode Anode Anode + + O O , air , air H H , fuel , fuel 2 2 2 2 H2O CL CL GDL GDL CC CC CC GDL GDL CL CL PEM Anode :H2 2H+ + 2e- Cathode : ½ O2 + 2H+ + 2e-  H2O Total : H2 + ½ O2  H2O Simulation of Hydrated Polyelectrolyte Layers as Model Systems for Proton Transport in Fuel Cell MembranesAta Roudgar, S. P. Narasimachary and Michael EikerlingDepartment of Chemistry, Simon Fraser University, Burnaby, BC, Canada Simon Fraser University 3. Results Formation energy as a function of sidechain separation for regular array of triflic acid, CF3-SO3-H 1. Structural Views of the Membrane highly correlated independent Fully dissociated “upright” structure Non-dissociated “tilted” structure Principal Layout of a PEM Fuel Cell Hydrated fibrillar aggregates G. GEBEL, 1989 dcc=10.4Å • Highest formation energy E = -2.78 eV corresponds to dCC = 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 CH3-SO3-H (weaker acid). Numerical values are slightly different. The transition between fully-dissociated and fully non-dissociated states occurs at e.g. at dCC = 6.7Å. dcc=8.1Å Transition from “upright” to “tilted” structure occurs at dCC = 6.5Åupon increasing C-C distance L. Rubatat, G. Gebel, and O. Diat, Macromolecules 37, 7772 (2004). Structure formation, transport mechanisms MEMBRANE DESIGN Current work: establish reaction coordinates and reaction pathways and calculate the corresponding activation energy (using the method of “Transition Path Sampling”) Fully-dissociated “tilted” structure Evolution of PEM Morphology and Properties • Primary chemical structure • backbones • side chains • acid groups • Secondary structure • aggregates • array of side chains • water structure • Heterogeneous PEM • random phase separation • connectivity • swelling Number of H-bonds as a function of C-C distance At dCC = 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. hydrophobic phase Self-organization into aggregates and dissociation Contour plot for dCC = 6.3Å hydrophilic phase 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 Molecular interactions (polymer/ion/solvent), persistence length “Rescaled” interactions (fluctuating sidechains, mobile protons, water) Effective properties (proton conductivity, water transport, stability) 2. Model of Hydrated Interfaces inside PEMs • Sharp transition from weak to strong binding at ~ 7 Å • Strong fluctuations expected in this region! Focus on Interfacial Mechanisms of PT Energy for removal of one water molecule from the unit cell Creation of a Water Defect Insight in view of fundamental understanding and design: Feasible model of hydrated interfacial layer 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. 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 4. Conclusions 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 Side view 2. Computational Details 2D hexagonal array of surface groups dCC Unit cell: • Ab-initio calculations based on DFT (VASP) • formation energy as a function of dCC • 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) fixed positions Top view References • 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). dCC Acknowledgements The authors thank the funding of this work by NSERC.