Molecular Modeling of Structure and Dynamics

1
Molecular Modeling of Structure and Dynamics in
Fuel Cell Membranes A. Roudgar, Sudha N.P. and
M.H. Eikerling Department of Chemistry, Simon
Fraser University, Burnaby, Canada,V5A 1S6

• I. Introduction
• Proper understanding of the relations
  between structure formation and mobility is
  critical for the development of highly performing
  proton conducting membranes for fuel cells. It
  is, however, impossible to study the complete
  scale of structural details in real membranes
  with quantum mechanical approaches (DFT and
  AIMD). Feasible routes are to utilize
  combinations of quantum mechanical and classical
  approaches or to consider small substructures of
  the membrane. Here we apply ab-initio approaches
  to simplified model systems. The objective is to
  understand co-operative phenomena in proton
  transport and explore effects of length, chemical
  structure and arrangements of polymeric side
  chains.

III. Computational simulation of arrays of the
simplest and shortest sidechain (CF3SO3H)
Part 1 Geometry Optimization

• Computational details
• Two-dimensional hexagonal array with fixed
  positions of carbon atoms.
• 3 sidechains 3 water molecules per unit cell
• Vienna Ab-initio Simulation Package (VASP)
• Only G point is considered in total energy
  calculation
• Projected Augmented Wave (PAW) pseudopotential
  with cut-off energy Ecut400 eV
• PW-91 Functional

Top view
Side view
Architecture of Membranes
Nature of backbone
Fixed carbons
Chemical architecture of the side chains
Binding energy as a function of sidechain -
sidechain distance
PS-g-mac PSSA(21) (graft polymer) Conductivity
0.08 Scm -1
S-PBI butane
PAN-g-macPSSA graft copolymers (32) (graft
polymer). Conductivity 0.1 Scm-1
Conductivity 0.01 Scm-1(80C)

• A C-C distance of d6.18Å corresponds to the
  largest
• binding energy - fully dissociated array.
• The transition between fully dissociated and
  fully non-
• dissociated array occurs at d7.2Å.
• In similar calculations for CH3SO3H the
  transition
• between fully-dissociated and fully
  non-dissociated
• array occurs at d6.7Å (weaker acid).
• We expect a high probability of proton transfer
  in the
• region of d7.2Å, where the difference in
  energies is small.

Distance between side chains
Length of the side chain
S-PPBP Conductivity ?0.001 Scm-1
Partially sulfonated styrene ethylene.
Conductivity 0.002 Scm-1 when x9.
Top-view
Top-view
Morphology of Nafion

• The ionomer consists of an hydrophobic backbone
  with side chains that are terminated by acid
  groups. Good proton conductivity of the membrane
  is due a spontaneous nanophase segregation in
  the presence of water.

Non dissociated acid
Dissociated acid
Part 2 Ab-initio Molecular Dynamics

• Computational details
• Two dimensional hexagonal arrays with C-C fixed
  distance d7.2
• 3 sidechains 3 water molecules per unit cell
• Constant temperature T300K
• Nose-Hoover thermostat with Nose mass Q0.05
• PW-91 Functional

t0
t2.1 ps
At tgt0.5ps the acid head groups start to
approach each. Local clusters are formed. A
partially dissociated state develops.
In initial configuration(t0) all acids groups
are non-dissociated
The complexity and large number of involved atoms
demand simple but reliable models for
computational simulation of such a system.
t5.7 ps

• At tgt4.1ps the system evolves towards a
  transition state.
• The potential energy drops.
• Acid groups become fully dissociated
• The energy of the new structure is 1eV lower
  than the initial (non-dissociated)
  configuration

II. Model System and Approaches
Step 1 We consider a two-dimensional regular
array of sidechains anchored to a substrate.

• IV. Conclusion
• We study effects of molecular structure on
  proton, solvent and polymer dynamics in PEMs.
• Our model consists of a minimally hydrated 2-D
  array of sidechains with fixed end points.
• We perform full quantum mechanical calculations
  using VASP.
• Total energy calculation as a function of C-C
  distance was performed.
• Upon increasing the C-C distance, a transition
  from dissociated to non-dissociated state occurs.
  
• We have performed a molecular dynamics
  simulation for 3(CF2SO3H H2O) at fixed C-C
  distance d7.2Å. Our results show that a
  transition occurs at t4.1ps and a new and more
  stable structure is formed at t5ps.

Compare the dynamics of thesidechains with and
without the substrate (frequency spectra).
T
Step 2 We remove the substrate and fix the
positions of the endpoint atoms at their initial
position.
Acknowledgement We gratefully acknowledge
the funding of this work by NSERC.

• References
• Carmen Chuy, Jianfu Ding,Edward Swanson, Steven
  Holdcroft,Jackie Horsfall,and Keith V. Lovell,
  JECS,150(5)
• E271-E279(2003).
• M.Eikerling, A.A.Kornyshev, Journal of
  Electroanalytical Chemistry,502(2001),1-14.
• K.D.Kreuer, Journal of Membrane Science,185
  (2001),29-39.
• E.Spohr, P.Commer, and A.A.Kornyshev,
  J.Phys.Chem.B 2002,106,10560-10569.
• M.Eikerling, A.A.Kornyshev, and U.Stimming,
  J.Phys.Chem.B 1997,101,10807-10820.

Important characteristics of model system

• Length of sidechains.
• Distance between sidechains.
• Chemical structure of sidechains

• Nature of acid groups.
• Number of acid groups on sidechain.
• Water content.