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

Ata Roudgar, Sudha P. Narasimachary and Michael Eikerling Department of Chemistry, Simon Fraser University, Burnaby, BC, Canada

Simon Fraser University

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

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

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

4. Conclusions

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

independent highly correlated

• 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).

Fully dissociated “upright” structure

Non-dissociated “tilted” structure

Correlations in interfacial layer are strong function of sidechain density.

Transition between upright (“stiff”) and tilted (“flexible”) configurations at dCC

7Å involves hydronium motion, sidechain rotation, and sidechain tilting.

Extra water molecule: sharp transition at dCC 7Å from weak to strong binding.

Sidechain separation is key parameter – critical value: dCC 7Å

Reducing interfacial dynamics to the evolution of 3 collective coordinates enabled

determination of transition path (activation energy 0.35 eV).

This path will be used to initiate Transition Path Sampling.

dcc =10.4Å

dcc=8.1Å

Fully-dissociated “tilted” structure

The tilted structure can be found in 3 different states:

- fully dissociated

- partially dissociated

- non-dissociated

The same results was found with the BLYP functional.

Similar calculations were performed for CH3-SO3-H with the transition between fully-dissociated and fully non-dissociated arrays at dCC = 6.7Å (weaker acid)

Upon increasing sidechain there is a transition from “upright” to “tilted” structure occurs at dCC = 6.5Å

The activation energy and reaction coordinates is being calculated in our group

by “Transition Path Sampling” method

Binding energy of additional water molecule

Contour plot of binding energies for 10x10 grid in xy-plane

Identify favorable positions of extra-H2O

Full optimization for this position - determine binding energy

Contour plot for

Energy to remove water molecule from unit cell

(creation of a water defect).

d 6.3CC Å

Transition from weak to strong binding at

Strong fluctuations expected in this region!

Zundel-ion formation(H5O2

+)

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.

At dCC=7.5Å the number of H-bonds

drops to 7, intra-unit-cell H-Bonds are

broken and cluster formation of

surface groups occurs.

The largest formation energy

E = -2.78 eV at dCC = 6.2 Å

corresponds to the fully

dissociated upright structure.

Number of H-bond and tilting angle as a function of sidechain separation

Point 1 Point 2 Point 3 Point 4

Transition involves - hydronium motion (r) - sidechain tilting (Φ) - sidechain rotation (θ)

Preliminary results for transition between upright and tilted structure

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

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).

We can assign movements to elements on the graph. upright structure tilted structure

Configuration energy calculation

The transition involves H-bond breaking-forming.

Number of H-bonds along reaction path

Φ

r

θ

Top view Side view

In order to find out the initial path we perform a MD from the

saddle points with random velocities.

A correct choice of a set of velocities will provide a complete

dynamic path from upright to tilted structure.

The activation energy and the reaction rate can be calculated

using the initial by applying Transition Path Sampling method.

d ~ 7CC Å References