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

AcknowledgementsThe authors thank the funding of this work by NSERC.

Simon Fraser University

References

2. Model of Hydrated Interfaces inside PEMs

1. Structural Views of the Membrane

PEM

H2, fuel

Anode

CLGDLCC

H2, fuel

Anode

CLGDL

O2, air

Cathode

CLGDL

+

CC

O2, air

Cathode

CLGDL

+

CCH2O

Anode : H2 2H+ + 2e-

Cathode : ½ O2 + 2H+ + 2e- H2O

Total : H2 + ½ O2 H2O

4. Conclusions

3. Results

Principal Layout of a PEM Fuel Cell

Effective properties (proton conductivity, water transport, stability)

hydrophobic phase

hydrophilic phase

Primary chemical structure• backbones• side chains • acid groups

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

Self-organizationinto aggregatesand dissociation

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

“Rescaled” interactions (fluctuating sidechains,mobile protons, water)

Heterogeneous PEM• random phase separation• connectivity• swelling

Evolution of PEM Morphology and Properties

Focus on Interfacial Mechanisms of PT

Insight in view of fundamental

understanding and design:

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

Feasible model of hydrated interfacial layer

2. Computational DetailsSide view

fixed positions

Top view

__3 3 23 x CF SO H + H O Unit cell:

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)

2D hexagonal array of surface groups dCC

dCC

Hydrated fibrillar aggregates

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

G. GEBEL, 1989

Structure formation, transport mechanisms

MEMBRANE DESIGN

Formation energy as a function of sidechain separation for regular array of triflic acid, CF3-SO3-H

independent highly correlated

• 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).

Fully dissociated “upright” structure

Non-dissociated “tilted” structure

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

Contour plot for dCC = 6.3Å

Creation of a Water Defect

Energy for removal of one water molecule from the unit cell

• Sharp transition from weak to strong binding at ~ 7 Å

• Strong fluctuations expected in this region!

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

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.

dcc =10.4Å

Transition from “upright” to “tilted” structure occurs at dCC = 6.5Å upon increasing C-C distance

Current work: establish reaction coordinates and reaction pathways and calculate the corresponding activation energy (using the method of “Transition Path Sampling”)

dcc=8.1Å

Fully-dissociated “tilted” structure

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.

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.

ucf total CC totalE E d E

Number of H-bonds as a function of C-C distance

Zundel-ion formation(H5O2

+)Zundel-ion formation

(H5O2+)