Ab initio Calculations of Interfacial Structure and Dynamics

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Ab initio Calculations of Interfacial Structure and Dynamics in Fuel Cell Membranes Ata Roudgar,

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 4. Proton Transform Mechanism at Interface 1. Introduction 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. v Low temperature (T<100˚C), high degree of hydration, proton transfer in bulk, high conductivity v High temperature (T>100˚C), low degree of hydration, proton transfer at interface, conductivity? Car-Parrinello Molecular Dynamics (CPMD) using functional BLYP Collective Coordinates and Minimum Reaction Path Three collective coordinates: hydronium motion r, surface group rotation j and surface group tilting q. Evolution of PEM Morphology and Properties Primary chemical structure • backbones • side chains • acid groups Secondary structure • aggregates • array of side chains • water structure r q Heterogeneous PEM • random phase separation • connectivity • swelling hydrophobic phase j Side view Top view Regular 10 x 10 grid of points is generated. Each point represents one configuration of these three CCs. 1 2 3 At each of these positions a geometry optimization including all remaining degrees of freedom is performed. Self-organization into aggregates and dissociation Molecular interactions (polymer/ion/solvent), persistence length DE= 0. 55 e. V hydrophilic phase “Rescaled” interactions (fluctuating sidechains, mobile protons, water) Effective properties (proton conductivity, water transport, stability) The path which contains the minimum configuration energy is identified (as shown). Frequency spectrum using AIMD Simulation v Car-Parrinello NVT simulation at T = 300 K for upright conformation j v Simulation time = 60 ps v The frequency spectrum is calculated as a Fourier transform 2. Model of Hydrated Interfaces inside PEMs of velocity correlation function: q Focus on Interfacial Mechanisms of PT Insight in view of fundamental understanding and design: Feasible model of hydrated interfacial layer • The fluctuations of sidechain rotation and sidechain tilting are responsible for proton transfer. • Low frequencies ≈ 100 cm-1 are responsible for proton transfer. 5. Proton Transform from Interface to Bulk Initialization of the second hydration shell Ø With this density we could make the second Ø The optimum density for one layer of water is hydration shell consist of 14 water molecules. calculated by varying the density of water layer. The average hydrogen bond length, <d. O…O> = 2. 92 Å The surface group separation correspond to optimum density of water layer is d. CC=7. 07Å Objectives Assumptions: Correlations and mechanisms of decoupling of aggregate and side chain dynamics proton transport in interfacial layer Is good proton conductivity possible with minimal hydration? map random array of surface groups onto 2 D array terminating C-atoms fixed at lattice positions Optimize geometry of minimally hydration and second hydration shell Upright conformation remove supporting aggregate from simulation 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: 3. Stable Structural Conformation Computational details Ebin = ESG+wl – ESG – Ewl Formation energy as a function of sidechain separation for regular array of Triflic acid, CF 3 -SO 3 -H highly correlated Ab initio calculations based on DFT independent (VASP) 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 formation energy as a function of d. CC effect of side chain modification binding energy of extra water molecule 6. Conclusions energy for creating water defect 2 D hexagonal array of surface groups Correlations in interfacial layer are strong function of sidechain density. Unit cell: Transition between upright (“stiff”) and tilted (“flexible”) configurations at d CC = 6. 5Å involves hydronium motion, sidechain rotation, and sidechain tilting. Side view Reducing interfacial dynamics to the evolution of 3 collective coordinates enabled Upon increasing sidechain there is a transition from “upright” to “tilted” structure occurs at d. CC = 6. 5Å d. CC Upright determination of transition path (activation energy 0. 55 e. V). The binding energy of second shell becomes weak at small d cc No proton transfer from Tilted interface to bulk is expected. fixed carbon positions Ø The tilted structure can be found in 3 different states: - fully dissociated - partially dissociated - non-dissociated Ø The largest formation energy E = -2. 78 e. V at d. CC = 6. 2 Å corresponds to the upright structure. References • A. Roudgar, S. Narasimachary and M. Eikerling, J. Phys. Chem. B 110, 20469 (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, 29 - 39 (2001). • C. Chuy, J. Ding, E. Swanson, S. Holdcroft, J. Horsfall, and K. V. Lovell, J. Electrochem. Soc. 150, E 271 -E 279 (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).