A Multi-Scale Dynamic Mechanistic Model for Transient Analysis of PEFCs

In this paper we propose a new knowledge model to understand the dynamic behavior of a Membrane-Electrodes Assembly (MEA) of a Polymer Electrolyte Fuel Cell. The model, multiscale and mechanistic, is based on irreversible thermodynamics and electrodynamics, and depends on the internal physical parameters such as the specific catalytic area, the electric permittivity of the materials, the reaction kinetics and the diffusion coefficients of the reactant species. The model results of the coupling of a microscale transport phenomena description of charges (electrons and protons) through the electrode and the electrolyte thickness, a spatially distributed microscale model of the reactant diffusion (hydrogen and oxygen) through the Nafion layer covering the Pt/C particles, and a spatially distributed nanoscale description of the Nafion-Pt interface. This interfacial model is based on an internal description of the electrochemical double layer dynamics, coupling the transport phenomena in the diffuse layer and the electrochemical reactions and water adsorption in the compact layer. This multiscale model introduces many couplings between different physical domains, such as diffusionmigration transport and electrochemistry. A distributed-parameter Bond Graph description of the model has allowed to formulate it in a modular way, so that it could be adapted to other electrochemical systems (solid oxide fuel cells, Lithium-ion batteries...) or to be coupled witrh other physico-chemical phenomena (ageing mechanisms...). The numerical resolution allows to simulate the MEA dynamics, depending on current, temperature, reactant pressures and structural composition of the electrodes (Pt and Nafion loadings). Particularly, the model allows evaluating the sensitivity of impedance spectra to operating conditions and electrodes compositions, and it gives access to the contributions of the different phenomena and constitutive layers. A qualitative experimental validation of the model was carried out on a specific bench and on single cells with different platinum loadings.