Estimation of the Cathodic Overpotential for the Double Layer Structure in Pemfcs Using MD

Abstract

In this work a molecular dynamics (MD) based model is employed to describe and obtain the over-potential in the EDL cathodic structure consisting of the platinum(100)-water-oxygen interface, for Proton Exchange Membrane Fuel Cells (PEMFCs). This approach aims to be integrated in a multiscale model [1], describing the kinetics in the cathodic electrode through density functional theory (DFT) and the macroscopic behavior through a pseudo 2D model for PEMFCs reported in previous works [1]. In the framework of MD simulations, it is required to define an interaction potential model that describes the size, the charge polarization and the chemical characteristics of the electrochemical interface. In this work TIP3P model is used to describe interaction among water molecules, whereas interaction among platinum, water and oxygen is modeled with Lennard-Jones 12-6 model including electrostatic interactions. MD simulations are performed in LAMMPS with a rigid wall of platinum and periodic boundary conditions in x and y directions, using a NPT ensemble to obtain the equilibrium configuration of the system at 1 bar and 353.15K The MD simulation allows to obtain the density distribution functions of water molecules and oxygen atoms perpendicular to X-Y plane. Then, the density distribution profiles are used to calculate the charge distribution through formal charge of water and oxygen. Finally the Poisson equation is employed to get the electrostatic potential across the interface and an associated electrostatic over-potential. Also several simulations are carried out at different cathodic current densities. The obtained water density distribution at OCV is validated according to the results of previous works [2-4]. [1] S. Castaneda Ramirez, a. E. Perez Mendoza, és R. E. Ribadeneira Paz, ECS Trans., 66, 19–39 (2015). [2] X. Xia, L. Perera, U. Essmann, és M. L. Berkowitz, Surf. Sci., 335, 401–415 (1995). [3] K. Foster, K. Raghavan, és M. Berkowitz, Chem. Phys. Lett., 162, 32–38 (1989). [4]E. Spohr, J. Phys. Chem., 93, 6171–6180 (1989). [5] K. Heizinger, Pure and appl Chem. 63, 1733-1742 (1991). [6] D. A. Rose, I. Benjamin. J. Phys. Chem 95 (1991).