. 2. Ata Roudgar, Sudha P. Narasimachary and

. 2. Ata Roudgar, Sudha P. Narasimachary and

.

2.

Ata Roudgar, Sudha P. Narasimachary and Michael Eikerling

Simon Fraser
University

Department of Chemistry, Simon Fraser University, Burnaby, BC, Canada

Preliminary results for transition between upright and tilted structure

3. Results
Structural Views of the Membrane

Formation energy as a function of sidechain separation for regular array of triflic acid, CF 3-SO3-H

highly correlated

Principal Layout of a PEM Fuel Cell

Hydrated fibrillar aggregates

independent

Fully dissociated
upright structure

G. GEBEL, 1989

Cathode

Anode

Transition involves
- hydronium motion (r)
- sidechain tilting () )
- sidechain rotation () )
Upon increasing
sidechain there is
a transition from
upright to tilted
structure occurs
at dCC = 6.5

Non-dissociated tilted structure

0.4
=1
d cc

.

Structure and Dynamics at a Dense Array of Hydrated Surface Groups: A Model for
Interfacial Proton Transport in Fuel Cell Membranes

)
r

+
The activation energy and reaction
coordinates is being calculated in our group
by Transition Path Sampling method

O2, air

H2, fuel

Side view

Top view

)

The tilted structure can be
found in 3 different states:

Anode

:

H2

2H + 2e
+

Cathode : O2 + 2H+ + 2e-

H2O

Total

H2O

:

H2 + O2

The largest formation energy
E = -2.78 eV at dCC = 6.2

H2O

CC GDL CL PEM CL GDL CC

L. Rubatat, G. Gebel, and O. Diat,
Macromolecules 37, 7772 (2004).

-

- non-dissociated
dcc=8.1

MEMBRANE DESIGN

Fully-dissociated tilted structure

Heterogeneous PEM
random phase separation
connectivity
swelling
hydrophobic phase

Three collective coordinates: hydronium motion r,
sidechain rotation ) and sidechain tilting ) .

The same results was found
with the BLYP functional.

Structure formation, transport mechanisms

Secondary structure
aggregates
array of side chains
water structure

Configuration energy calculation

- partially dissociated

corresponds to the fully
dissociated upright structure.

Evolution of PEM Morphology and Properties
Primary chemical structure
backbones
side chains
acid groups

- 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
(weaker acid)

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).
upright structure

At dCC=7.5 the number of H-bonds

Point 1

tilted structure

Point 2

We can assign movements to elements on the graph.

Point 3

Point 4

drops to 7, intra-unit-cell H-Bonds are
broken and cluster formation of
surface groups occurs.
Self-organization
into aggregates
and dissociation

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

hydrophilic phase

Rescaled interactions
(fluctuating sidechains,
mobile protons, water)

Effective properties
(proton conductivity,
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.

4. Conclusions
Correlations in interfacial layer are strong function of sidechain density.
Zundel-ion formation
(H5O2+)

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?

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.

array
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

Computational Details
2D hexagonal array of surface groups

Transition between upright (stiff) and tilted (flexible) configurations at d CC

Side view

Reducing interfacial dynamics to the evolution of 3 collective coordinates enabled
determination of transition path (activation energy 0.35 eV).

dCC

__

3
x
CF
Unit cell:
3 SO3H + H2O

Ab-initio calculations based on DFT
(VASP)

This path will be used to initiate Transition Path Sampling.
fixed positions

Top view

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

Sidechain separation is key parameter critical value: d CC 7

References
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
(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).

Acknowledgements The authors gratefully acknowledge the funding of this work by NSERC.

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