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+)