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

1
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
1. Introduction
4. Proton Transform Mechanism at Interface
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.
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.

• Low temperature (Tlt100C), high degree of
  hydration, proton transfer in bulk, high
  conductivity

• High temperature (Tgt100C), low degree of
  hydration, proton transfer at interface,
  conductivity?

DE 0.55eV
Evolution of PEM Morphology and Properties
q
Side view
Top view
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).
2
1
3
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

j
2. Model of Hydrated Interfaces inside PEMs
Focus on Interfacial Mechanisms of PT
q
Insight in view of fundamental understanding and
design

• The fluctuations of sidechain rotation and
  sidechain tilting are responsible for proton
  transfer.
• Low frequencies 100cm-1 are responsible for
  proton transfer.

Feasible model of hydrated interfacial layer
5. Proton Transform from Interface to Bulk
Initialization of the second hydration shell

• 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 dCC7.07Å

• The optimum density for one layer of water is
  calculated by varying the density of water
  layer. The average hydrogen bond length,
  ltdOOgt 2.92 Å

• 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

Upright conformation
Optimize geometry of minimally hydration and
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
• Ebin ESGwl
  ESG Ewl
• The binding energy between first and second
  hydration shells as a function of dCC shows
  that for small dCC the second shell do not
  interact with minimally hydration ?
  Hydrophobic?
• For large dCC the interaction between first and
  second shell binding energy is increased ?
  proton transform is more probable

3. Stable Structural Conformation
Formation energy as a function of sidechain
separation for regular array of Triflic acid,
CF3-SO3-H
Computational details
independent
highly correlated

• 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

6. Conclusions

• Correlations in interfacial layer are strong
  function of sidechain density.
• Transition between upright (stiff) and tilted
  (flexible) configurations at dCC 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 dcc ? No proton transfer from
  interface to bulk is expected.

2D hexagonal array of surface groups
Unit cell
Side view
Upon increasing sidechain there is a transition
from upright to tilted structure occurs at
dCC 6.5Å
dCC
Upright
Tilted
fixed carbon positions

• The tilted structure can be found in 3 different
  states
• - fully dissociated
• - partially dissociated
• - non-dissociated
• The largest formation energy E -2.78 eV at
  dCC 6.2 Å corresponds to the upright
  structure.

References

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  .Chem. Phys. Lett. 457, 337 (2008)
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