Simulation%20of%20Hydrated%20Polyelectrolyte%20Layers%20as%20Model%20Systems%20for%20Proton%20Transport%20in%20Fuel%20Cell%20Membranes%20%20Ata%20Roudgar,%20S.%20P.%20Narasimachary%20and%20Michael%20Eikerling%20%20Department%20of%20Chemistry,%20Simon%20Fraser%20University,%20Burnaby,%20BC,%20Canada

1
Simulation of Hydrated Polyelectrolyte Layers as
Model Systems for Proton Transport in Fuel Cell
Membranes Ata Roudgar, S. P. Narasimachary and
Michael Eikerling Department of Chemistry,
Simon Fraser University, Burnaby, BC, Canada
Simon Fraser University
3. Results
Formation energy as a function of sidechain
separation for regular array of triflic acid,
CF3-SO3-H
1. Structural Views of the Membrane
independent
highly correlated
Fully dissociated upright structure
Non-dissociated tilted structure
Principal Layout of a PEM Fuel Cell
Hydrated fibrillar aggregates
G. GEBEL, 1989
dcc10.4Å

• Highest formation energy E -2.78 eV
  corresponds to dCC 6.2Å (upright structure).
• Transition between fully dissociated, partially
  dissociated and non-dissociated states occurs in
  tilted structure.
• Distinct DFT implementations gave similar
  results.
• The same structures and transitions were found
  for CH3-SO3-H (weaker acid). Numerical values are
  slightly different. The transition between
  fully-dissociated and fully non-dissociated
  states occurs at e.g. at dCC 6.7Å.

dcc8.1Å
Transition from upright to tilted structure
occurs at dCC 6.5Å upon increasing C-C distance
L. Rubatat, G. Gebel, and O. Diat,
Macromolecules 37, 7772 (2004).
Structure formation, transport mechanisms MEMBRANE
DESIGN
Current work establish reaction coordinates and
reaction pathways and calculate the corresponding
activation energy (using the method of
Transition Path Sampling)
Fully-dissociated tilted structure
Evolution of PEM Morphology and Properties

• Primary chemical structure
• backbones
• side chains
• acid groups

• Secondary structure
• aggregates
• array of side chains
• water structure

• Heterogeneous PEM
• random phase separation
• connectivity
• swelling

Number of H-bonds as a function of C-C distance
At dCC 7.5Å, the number of H-bonds drops to 7
inter-unit-cell H-bonds are broken and formation
of clusters of surface groups commences.
hydrophobic phase
Self-organization into aggregates and dissociation
Contour plot for dCC 6.3Å
hydrophilic phase
Binding energy of additional water molecule

• Contour plot for 10x10 grid in xy-plane
• Identify favorable positions of extra-H2O
• Full optimization and calculation of binding
  energy

Molecular interactions (polymer/ion/solvent),
persistence length

• Effective properties
• (proton conductivity,
• water transport, stability)

Rescaled interactions (fluctuating
sidechains, mobile protons, water)
2. Model of Hydrated Interfaces inside PEMs

• Sharp transition from weak to strong binding at
  7 Å
• Strong fluctuations expected in this region!

Focus on Interfacial Mechanisms of PT
Energy for removal of one water molecule from the
unit cell
Creation of a Water Defect
Insight in view of fundamental understanding and
design
Feasible model of hydrated interfacial layer
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 Tgt400K.

• 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

4. Conclusions
Correlations in interfacial layer are strong
function of sidechain seperation Transition
between upright (stiff) and tilted (flexible)
configurations Extra water molecule sharp
transition from weak to strong binding Water
defect minimally hydrated array is rather
stable Side chain separation is key parameter
perspectives for design Experimental evaluation
of interfacial mechanisms is feasible
Side view
2. Computational Details
2D hexagonal array of surface groups
dCC
Unit cell
fixed positions

• 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
• Computational resources Linux clusters
• PEMFC (our group), BUGABOO (SFU),
• WESTGRID (BC, AB)

Top view
References

• 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, 29- 39 (2001).
• 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).
• M. Eikerling, S.J. Paddison, L.R. Pratt, and
  T.A. Zawodzinski, Chem. Phys. Lett. 368, 108
  (2003).

dCC
Acknowledgements
The authors thank the funding of this work by
NSERC.