Prof. Heinz Pitsch - Institut für Regelungstechnik (IRT) der RWTH ...

Seminarankündigung
im Rahmen des
Sonderforschungsbereich 686
Prof. Heinz Pitsch
Center for Turbulence Research
Stanford University, USA
hält am
Dienstag, 17. Juli 2007 um 14.00 Uhr
im
Seminarraum des Instituts für Technische Verbrennung
Raum 221
einen Vortrag zum Thema
„Multi-Scale Simulation of transport and
chemistry in Polymer Electrolyte Membrane
Fuel Cells“

Seminarankündigung
im Rahmen des
Sonderforschungsbereich 686
Prof. Heinz Pitsch
Abstract
Computational chemistry presently is a rapidly evolving field, because of recent improvements in
computational power and theoretical developments, and the great potential of computations to further the
understanding of chemical processes and the interactions of chemistry and transport phenomena. These
computational approaches include quantum chemistry simulations, molecular dynamics simulations, and
dynamic Monte Carlo simulations (DMC).Here we will present the development of a computational chemistry
based multi-scale model for computational fluid dynamics simulations of polymer electrolyte membrane
(PEM) fuel cells. The multi-scale model is based on DMC simulations of the chemistry on the electrocatalyst
surfaces. Several advancements in numerical techniques for computational chemistry simulations and their
applications to real systems will be presented for the example of the PEM fuel cell cathode. Transition
probabilities required for these simulations are determined from quantum chemical simulations. For
electrochemical simulations, the local reaction center theory by Anderson is used. We present an efficient
mathematical framework to determine the potential-dependent transition states of electron transfer reactions by
quantum calculations. This method leads to fast convergence, reliability, and robustness of the located
transition states for more complex systems with a larger number of degrees of freedom, and makes these
computations cost-efficient enough to study a large number of individual reactions. As an example, adsorbent
interactions relevant for electrochemical steps of the oxygen reduction reactions are discussed. Because of the
possible importance of such adsorbent interactions and other non-linear local chemical effects, DMC methods
are expected to describe the chemical behavior more accurately than environment-averaged methods. In PEM
fuel cells, carbon-particle supported platinum nano-particles are often used as electrocatalyst. These Ptparticles
can be approximated to be of cubo-octahedral form. The specific topology of these particles can lead
to important features associated with the complex surface structure. Specifically, the edge/corner sites can
behave differently from sites located on the faces. Environment-averaged approaches, such as the mean-field
approximation, often fail to accommodate the details of such local phenomena. DMC is computationally much
more demanding than conventional approaches, and several different DMC simulations algorithms have been
proposed in the past. An example is the popular Variable Step Size Method (VSSM). VSSM has the advantage
that the computational cost of a single time step is independent of the lattice size for problems with timeindependent
rate parameters, but scales with the square of the number of lattice sites otherwise. Another
method, the First Reaction Method (FRM), can be applied for time-varying rate coefficients, but the
computational cost per time step depends still linearly on the logarithm of the number of lattice sites. Here we
present a new DMC algorithm that can be applied for time-varying rate coefficients, and which has a
computational cost per time step that is independent of the lattice size. To demonstrate the capabilities of the
new method, DMC simulations of cyclic voltammetry of PEM fuel cell electrochemistry will be presented and
compared with experimental observations. Finally, the integration of the DMC simulation technology into a
multi-scale model will be presented. The model describes the interaction of surface chemistry with gas
diffusion in thin electrolyte layers surrounding the platinum particles on the nano-scale and the transport in the
porous material of the catalyst layer on the mirco-scale. Simulation results will be compared with experimental
data of a fuel cell using single crystal Pt electrodes.
Gäste sind herzlich willkommen