(431c) Reactive Molecular Dynamics Applied to Proton Transport in Fuel Cells

Our goal is to develop and apply a coarse-grained description of proton transport that is based on quantum mechanical calculations and correctly reproduces the macroscopic reaction rates. Our approach is to modify the classical equilibrium molecular dynamics simulation algorithm using existing non-reactive potentials. The advantage of this is that we use a computationally efficient, non-reactive potential that already exists and have been optimized. For water, we use the TIP3P potential, which does not explicitly allow a hydronium ion to transfer a proton to a water molecule. The reactive algorithm has three steps. The first step is the reaction trigger. We examine every hydronium ion in the simulation at each time step to determine whether it is in a state where it would transfer a proton. The determination of this state is based upon a set of reaction triggers, some of which are geometric and others energetic. The geometric triggers are based upon quantum mechanical configurations. The energetic limit is tied to the activation energy. If a hydronium ion satisfies all of the geometric and energetic triggers, the reaction instantly takes place. The second step of the reactive algorithm is the instantaneous reaction. In this case, the reaction involves replacing the bond-stretching (harmonic) interaction of the O of the reactant H₃O⁺ and the proton to be transferred with a non-bonded (Lennard-Jones) interaction. The reverse switch is made for the reactant H₂O and the proton to be transferred. The proton to be transferred is moved along the OO axis such that the ratio of O^*-H (where O^* is in H₃O⁺) and O-H distances are the same in the product molecules as they were in the reactant molecules. The third and final step of this procedure is local equilibration, which is performed to exactly satisfy the target, ΔH_r heat of the reaction. In this reaction, ΔH_r, is zero because the reactants and products of the reaction are identical. Once all three steps of the reactive algorithm are complete, we continue the classical simulation using the non-reactive potential. Using this procedure, we can match all four elements that uniquely define a macroscopic description of the reaction: the stoichiometry, frequency prefactor k_o in the rate constant, activation energy E_a and ΔH_r. The matching of the stoichiometry is mandated through the reaction trigger. We can adjust the numerical values of the limits on the geometric triggers to control k_o and the limit on the energetic trigger to control E_a. We have developed and implemented this algorithm, which is simple and efficient. It automatically responds to the environment in the bulk water and hydrated Nafion in the fuel cell.

Acknowledgments: The work is supported by a grant from the U. S. Department of Energy BES under the contract number DE-FG02-05ER15723. This research used resources of the Center for Computational Sciences at Oak Ridge National Laboratory, which is supported by the Office of Science of the DOE under Contract DE-AC05-00OR22725.