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
