Ata Roudgar, Sudha P. Narasimachary and Michael Eikerling
Department of Chemistry, Simon Fraser University, Burnaby, BC, Canada
Preliminary results for transition between upright and tilted structure
Structural Views of the Membrane
Formation energy as a function of sidechain separation for regular array of triflic acid, CF 3-SO3-H
Principal Layout of a PEM Fuel Cell
Hydrated fibrillar aggregates
G. GEBEL, 1989
- hydronium motion (r)
- sidechain tilting () )
- sidechain rotation () )
sidechain there is
a transition from
upright to tilted
at dCC = 6.5
Non-dissociated tilted structure
Structure and Dynamics at a Dense Array of Hydrated Surface Groups: A Model for
Interfacial Proton Transport in Fuel Cell Membranes
The activation energy and reaction
coordinates is being calculated in our group
by Transition Path Sampling method
The tilted structure can be
found in 3 different states:
2H + 2e
Cathode : O2 + 2H+ + 2e-
H2 + O2
The largest formation energy
E = -2.78 eV at dCC = 6.2
CC GDL CL PEM CL GDL CC
L. Rubatat, G. Gebel, and O. Diat,
Macromolecules 37, 7772 (2004).
Fully-dissociated tilted structure
random phase separation
Three collective coordinates: hydronium motion r,
sidechain rotation ) and sidechain tilting ) .
The same results was found
with the BLYP functional.
Structure formation, transport mechanisms
array of side chains
Configuration energy calculation
- partially dissociated
corresponds to the fully
dissociated upright structure.
Evolution of PEM Morphology and Properties
Primary chemical structure
- fully dissociated
Regular 10x10x10 grid of points is generated. Each point
represents one configuration of the these three CCs.
Similar calculations were
performed for CH3-SO3-H with
the transition between fullydissociated and fully nondissociated arrays at dCC = 6.7
Number of H-bond and tilting angle as a function of sidechain separation
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).
At dCC=7.5 the number of H-bonds
We can assign movements to elements on the graph.
drops to 7, intra-unit-cell H-Bonds are
broken and cluster formation of
surface groups occurs.
mobile protons, water)
water transport, stability)
Number of H-bonds along reaction path
The transition involves H-bond breaking-forming.
In order to find out the initial path we perform a MD from the
Model of Hydrated Interfaces inside PEMs
Binding energy of additional water molecule
A correct choice of a set of velocities will provide a complete
Focus on Interfacial Mechanisms of PT
Insight in view of fundamental
understanding and design:
saddle points with random velocities.
Contour plot of binding energies for 10x10 grid in xy-plane
Identify favorable positions of extra-H2O
Feasible model of hydrated interfacial layer
Full optimization for this position - determine binding energy
dynamic path from upright to tilted structure.
Contour plot for
d CC 6.3
The activation energy and the reaction rate can be calculated
using the initial by applying Transition Path Sampling method.
Correlations in interfacial layer are strong function of sidechain density.
Correlations and mechanisms of
decoupling of aggregate and side chain dynamics
proton transport in interfacial layer
Is good proton conductivity possible
with minimal hydration?
7 involves hydronium motion, sidechain rotation, and sidechain tilting.
map random array of surface groups onto 2D
Extra water molecule: sharp transition at dCC 7 from weak to strong binding.
terminating C-atoms fixed at lattice positions
Energy to remove water molecule from unit cell
(creation of a water defect).
remove supporting aggregate from simulation
2D hexagonal array of surface groups
Transition between upright (stiff) and tilted (flexible) configurations at d CC
Reducing interfacial dynamics to the evolution of 3 collective coordinates enabled
determination of transition path (activation energy 0.35 eV).
3 SO3H + H2O
Ab-initio calculations based on DFT
This path will be used to initiate Transition Path Sampling.
Transition from weak to strong binding at d CC ~ 7
Strong fluctuations expected in this region!
formation energy as a function of dCC
effect of side chain modification
binding energy of extra water molecule
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 T>400K.
energy for creating water defect
Computational resources: Linux clusters
PEMFC (our group), BUGABOO (SFU),
WESTGRID (BC, AB)
Sidechain separation is key parameter critical value: d CC 7
A. Roudgar, S. Narasimachary and M. Eikerling, J. Phys. Chem. B 110, 20469 (2006).
C. Chuy, J. Ding, E. Swanson, S. Holdcroft, J. Horsfall, and K.V. Lovell, J. Electrochem. Soc. 150, E271-E279
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).
Acknowledgements The authors gratefully acknowledge the funding of this work by NSERC.
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