Ab initio Calculations of Interfacial Structure and Dynamics in Fuel Cell Membranes Ata Roudgar, Sudha P. Narasimachary and Michael Eikerling Department of Chemistry Simon Fraser University, Burnaby, BC Canada 2. Model of Hydrated Interfaces inside PEMs 1. Introduction 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 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 3. Stable Structural Conformation Side view fixed carbon positions 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 2D hexagonal array of surface groups d CC Upon increasing sidechain there is a transition from “upright” to “tilted” structure occurs at d CC = 6.5Å independenthighly correlated Formation energy as a function of sidechain separation for regular array of Triflic acid, CF 3 -SO 3 -H Collective Coordinates and Minimum Reaction Path 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) Three collective coordinates: hydronium motion r, surface group rotation  and surface group tilting .  r   Correlations in interfacial layer are strong function of sidechain density.  Transition between upright (“stiff”) and tilted (“flexible”) configurations at d CC = 6.5Å involves hydronium motion, sidechain rotation, and sidechain tilting.  Reducing interfacial dynamics to the evolution of 3 collective coordinates enabled determination of transition path (activation energy 0.55 eV).  The binding energy of second shell becomes weak at small d cc  No proton transfer from interface to bulk is expected. A. Roudgar, S. Narasimachary and M. Eikerling, J. Phys. Chem. B 110, (2006). A. Roudgar, S.P. Narasimachary, M. Eikerling.Chem. Phys. Lett. 457, 337 (2008) M. Eikerling and A.A. Kornyshev, J. Electroanal. Chem. 502, 1-14 (2001). K.D. Kreuer, J. Membrane Sci. 185, (2001). C. Chuy, J. Ding, E. Swanson, S. Holdcroft, J. Horsfall, and K.V. Lovell, J. Electrochem. Soc. 150, E271-E279 (2003). 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). References 6. Conclusions  The tilted structure can be found in 3 different states: - fully dissociated - partially dissociated - non-dissociated  The largest formation energy E = eV at d CC = 6.2 Å corresponds to the upright structure. D E= 0.55eV 4. Proton Transform Mechanism at Interface Computational details Understanding the effect of chemical architecture, phase separation, and random morphology on transport properties and stability of polymer electrolyte membranes (PEM) is vital for the design of advanced proton conductors for polymer electrolyte fuel cells.  Low temperature (T<100˚C), high degree of hydration, proton transfer in bulk, high conductivity  High temperature (T>100˚C), low degree of hydration, proton transfer at interface, conductivity? Evolution of PEM Morphology and Properties Car-Parrinello Molecular Dynamics (CPMD) using functional BLYP Upright Tilted Side view Top view 5. Proton Transform from Interface to Bulk  The optimum density for one layer of water is calculated by varying the density of water layer. The average hydrogen bond length, = 2.92 Å Initialization of the second hydration shell  The hydrogen bonds form in between water layer and oxygen atoms of Triflic acid  We calculated the binding energy between first and second hydration shells: E bin = E SG+wl – E SG – E wl  The binding energy between first and second hydration shells as a function of d CC shows that for small d CC the second shell do not interact with minimally hydration  Hydrophobic?  For large d CC the interaction between first and second shell binding energy is increased  proton transform is more probable Upright conformation  With this density we could make the second hydration shell consist of 14 water molecules. The surface group separation correspond to optimum density of water layer is d CC =7.07Å Optimize geometry of minimally hydration and second hydration shell Frequency spectrum using AIMD Simulation  Car-Parrinello NVT simulation at T = 300K for upright conformation  Simulation time = 60ps  The frequency spectrum is calculated as a Fourier transform of velocity correlation function: The fluctuations of sidechain rotation and sidechain tilting are responsible for proton transfer. Low frequencies ≈ 100cm -1 are responsible for proton transfer.  