1. The total energy as a function of collective coordinates, ,  for upright and tilted structures are plotted. Two types of frequency spectrum are calculated: 1.Coupled system: a geometry optimization is performed, collective coordinates are fully coupled with other degrees of freedom 2.Uncouple system: only a wavefunction optimization has been performed, no coupling with other degrees of freedom are considered The results are well described with a Morse potential of form The frequency of normal modes for both sidechain rotation and tilting are calculated from parameters D and . Interfacial Structure and Dynamics in Fuel Cell Membranes Ata Roudgar, Sudha P. Narasimachary and Michael Eikerling Hydronium motion Sidechain tilting Sidechain rotating 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 2. 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 Structure Collective coordinate Freq. of couple system (cm -1 ) Freq. of uncouple system (cm -1 ) Frequency of normal mode (cm -1 ) upright Sidechain tilting, θ 77.8284.76 120- 150 tilted59.6891.68 upright Sidechain rotation, φ 95.87217.44 tilted74.59227.55 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). 1 2 3 Frequency Spectrum obtained from AIMD Simulation Three collective coordinates: hydronium motion r, sidechain rotation  and sidechain tilting .  r   Car-Parrinello NVT simulation at T = 300K  Simulation time = 20ps  The frequency spectrum is calculated as a Fourier transform of velocity correlation function: Frequency of Normal Modes using Morse Potential Fit  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 frequency of normal modes for sidechain rotation and tilting are calculated using Morse potential fit and compared with frequency spectrum from AIMD simulation. A. Roudgar, S. Narasimachary and M. Eikerling, J. Phys. Chem. B 110, 20469 (2006). M. Eikerling and A.A. Kornyshev, J. Electroanal. Chem. 502, 1-14 (2001). K.D. Kreuer, J. Membrane Sci. 185, 29- 39 (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, 10560-10569 (2002). M. Eikerling, A.A. Kornyshev, and U. Stimming, J. Phys.Chem.B 101, 10807-10820 (1997). References 4. Conclusions The largest formation energy E = -2.78 eV at d CC = 6.2 Å corresponds to the upright structure. The tilted structure can be found in 3 different states: - fully dissociated - partially dissociated - non-dissociated The fluctuations of sidechain rotation and sidechain tilting are responsible for proton transfer. There is a weak coupling between collective coordinates and the rest of the degrees of freedom. Low frequencies ≈ 100cm -1 are responsible for proton transfer. Barrier energy = 0.55eV 2. Mechanism and Dynamics 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 Configuration energy as a function of  and  for couple system Upright Tilted Side view Top view Hydrogen bond breaking occurs