Structure and Dynamics at a Dense Array of Hydrated Surface Groups: A Model for Interfacial Proton Transport in Fuel Cell Membranes Ata Roudgar, Sudha P. Narasimachary and Michael Eikerling Department of Chemistry, Simon Fraser University, Burnaby, BC, Canada Simon Fraser University 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 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 Acknowledgements The authors gratefully acknowledge the funding of this work by NSERC. 4.Conclusions 3.Results Formation energy as a function of sidechain separation for regular array of triflic acid, CF 3 -SO 3 -H independenthighly correlated A. Roudgar, S. Narasimachary and M. Eikerling, J. Phys. Chem. B 110, (2006). 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, (2001). E. Spohr, P. Commer, and A.A. Kornyshev, J. Phys.Chem. B 106, (2002). M. Eikerling, A.A. Kornyshev, and U. Stimming, J. Phys.Chem.B 101, (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 Correlations in interfacial layer are strong function of sidechain density. Transition between upright (“stiff”) and tilted (“flexible”) configurations at d CC  7Å involves hydronium motion, sidechain rotation, and sidechain tilting. Extra water molecule: sharp transition at d CC  7Å from weak to strong binding. Sidechain separation is key parameter – critical value: d CC  7Å Reducing interfacial dynamics to the evolution of 3 collective coordinates enabled determination of transition path (activation energy 0.35 eV). This path will be used to initiate Transition Path Sampling. d cc =10.4Å d cc =8.1Å Fully-dissociated “tilted” structure The tilted structure can be found in 3 different states: - fully dissociated - partially dissociated - non-dissociated The same results was found with the BLYP functional. Similar calculations were performed for CH 3 -SO 3 -H with the transition between fully- dissociated and fully non- dissociated arrays at d CC = 6.7Å (weaker acid) Upon increasing sidechain there is a transition from “upright” to “tilted” structure occurs at d CC = 6.5Å The activation energy and reaction coordinates is being calculated in our group by “Transition Path Sampling” method Binding energy of additional water molecule Contour plot of binding energies for 10x10 grid in xy-plane Identify favorable positions of extra-H 2 O Full optimization for this position - determine binding energy Contour plot for Energy to remove water molecule from unit cell (creation of a water defect). Transition from weak to strong binding at Strong fluctuations expected in this region! Zundel-ion formation (H 5 O 2 + ) 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. At d CC =7.5Å the number of H-bonds drops to 7, intra-unit-cell H-Bonds are broken and cluster formation of surface groups occurs. The largest formation energy E = eV at d CC = 6.2 Å corresponds to the fully dissociated upright structure. Number of H-bond and tilting angle as a function of sidechain separation Point 1Point 2Point 3Point 4 Transition involves - hydronium motion (r) - sidechain tilting (Φ) - sidechain rotation (θ) Preliminary results for transition between upright and tilted structure Three collective coordinates: hydronium motion r, sidechain rotation θ and sidechain tilting Φ. Regular 10x10x10 grid of points is generated. Each point represents one configuration of the these three CCs. At each of these positions a geometry optimization including all remaining degrees of freedom is performed. The path which contains the minimum configuration energy is identified (as shown). We can assign movements to elements on the graph. upright structuretilted structure Configuration energy calculation The transition involves H-bond breaking-forming. Number of H-bonds along reaction path Φ r θ Top view Side view  In order to find out the initial path we perform a MD from the saddle points with random velocities.  A correct choice of a set of velocities will provide a complete dynamic path from upright to tilted structure.  The activation energy and the reaction rate can be calculated using the initial by applying Transition Path Sampling method. References