A multi-scale dynamic mechanistic model for the transient analysis of PEFCs

Electrochemical impedance spectroscopy is an experimental technique widely used for the transient analysis of PEFCs. Experimental results are usually analyzed using equivalent circuit models, which have to be fitted to each operation point and do not offer direct access to the internal physical parameters of the electrodes. In this paper, a multiscale mechanistic Membrane-Electrode Assembly model is proposed, based on irreversible thermodynamics and electrodynamics, and dependent on the internal physical parameters, such as the effective catalyst surface, the electrical permittivity of the materials, the reaction kinetics, and the diffusion coefficients of the reactant species. This model results from the coupling of a microscale transport phenomena description through the electrode and the electrolyte thickness, a spatially distributed microscale model of the reactant diffusion through the Nafion (R) layer covering the Pt/C particles, and a spatially distributed nanoscale description of the Nafion (R)-Pt/C interface. This interfacial model is based on an internal description of the double layer dynamics, coupling the transport phenomena in the diffuse layer with the electrochemical reactions and water adsorption in the inner layer. The system dynamics can be simulated and are dependent on the current, the temperature, the reactant pressures, and the Pt/Nafion (R) loadings on the electrodes. The first simulations carried out are in good qualitative agreement with experimental results, giving access to the contributions of the different phenomena and to the sensitivity of the operating conditions.