Rodrigo Ferreira de Morais

Multi-scale Modeling-based Prediction of PEM Fuel Cells MEA Durability under Automotive Operating Conditions

The MEA durability in state-of-art PEMFC is one of the main shortcomings limiting the large-scale development and commercialization of this zero-emission power technology. It is largely observed that the micro-structural properties of the MEA evolve during PEMFC automotive-like operating conditions and translate into the cell potential degradation. Interpretation of the impact of the operation mode on the cell potential degradation mode in relation with the micro-structural experimental observations is difficult because of the strong coupling between the different underlying physicochemical phenomena (Figure 1). From this, the development of physical-based modeling tools is essential for the engineering community not only to elucidate the MEA degradation and failure mechanisms but also to predict the MEA durability in function of its initial material properties and operating conditions. Within this context, we have recently developed at CEA/LCPEM a multiphysics-multiscale model of the electrochemical processes taking place in Ptand Pt-M alloy-based PEMFC MEA, with M being a transition element [1-9]. This approach, dedicated to the understanding of the detailed mechanisms underlying the MEA degradation under automotive operating conditions (Figure 2), scales up ab initio data (obtained from DFT and MD simulations) into coupled/interacting elementary kinetic models –without using Butler-Volmer equations [5]of: 1) the cathodic catalyst oxidation/dissolution and electrochemical ripening [1-2, 7-8], 2) the dissolved metal ions diffusion/electro-migration/re-crystallization in the ionomer [1-2], 3) the cathodic carbon catalyst-support corrosion and the induced Pt coarsening [3-4], 4) the membrane chemical degradation, and 5) the detailed CO contamination kinetics on the anodic catalyst [9]. These aging models are coupled with non-equilibrium thermodynamics mechanistic descriptions of the MEA physicochemistry at the spatial nanoscale (detailed DFTbased HOR and ORR pathways, steric and electrochemical double layer effects...) and microscale (ionic and reactants transfers, water transport). From the algorithmic point of view the global model combines inhouse developed Kinetic Monte Carlo (e.g. for the catalyst degradation description, including the change of the catalyst nanomorphology -reconstruction-) and CFD (for transport phenomena) codes. By accounting for the numerical feedback between the calculated instantaneous local conditions (local reactant species concentrations...) and the degradation phenomena, the model allows predicting the evolution of the materials aging and its transient impact on the MEA performance degradation (cell potential evolution with the current as an input). In this paper we report new results obtained with this model applied to a Pt-based MEA including: the prediction of the impact of different current operation modes (OCV, steady-state and cycled current) on the MEA cell potential degradation and durability (defined as the time where the cell potential goes abruptly to zero) at different electrodes RH and temperatures. The interplaying between the different aging phenomena in relation with reactants humidification conditions is discussed. the impact of long-term (>500h) CO anodic injection on the membrane chemical degradation (this extends our recent result on the use of CO anode contamination as a mitigation method of the cathode carbon corrosion under power-cycled conditions [4, 9]). All the simulations are discussed in comparison with experiments carried out with dedicated single-cells and with TEM/HR-TEM observations as well as EPR and XPS micro-structural characterizations before and after operation in co-flow and counter flow operating single cells. Acknowledgments. This work is funded by the French National Research Agency (ANR) through the program “PAN-H”, within the context of the project “MAFALDA”. References [1] A. A. Franco, M. Tembely, J. Electrochem. Soc., 154 (7) B712 (2007). [2] A. A. Franco, ECS Trans., 6 (10) 1 (2007). [3] A. A. Franco, M. Gerard, J. Electrochem. Soc., 155 (4) B367 (2008). [4] A.A. Franco et al., ECS Trans., 13 (15) 35 (2008). [5] A. A. Franco et al., J. Electrochem. Soc., 153 (6) A1053 (2006). [6] A. A. Franco et al., Fuel Cells, 7, 99 (2007). [7] A. A. Franco et al., ECS Trans., 13 (17), 29 (2008). [8] A.A. Franco et al, J. Electrochem. Soc., 156 B410 (2009). [9] A.A. Franco et al., Electrochim.Acta, in press (2009).

Molecular Mechanics Models for the Image Charge, a Comment on "Including Image Charge Effects in the Molecular Dynamics Simulations of Molecules on Metal Surfaces"

We re‐investigate the image charge model of Iori and Corni (Iori and Corni, J. Comput. Chem. 2008, 29, 1656). We find that a simple symmetrization of their model allows to obtain quantitatively correct results for the electrostatic interaction of a water molecule with a metallic surface. This symmetrization reduces the magnitude of the electrostatic interaction to less than 10% of the total interaction energy.

Force Field for Water over Pt(111): Development, Assessment, and Comparison

Metal/water interfaces are key in many natural and industrial processes, such as corrosion, atmospheric, or environmental chemistry. Even today, the only practical approach to simulate large interfaces between a metal and water is to perform force-field simulations. In this work, we propose a novel force field, GAL17, to describe the interaction of water and a Pt(111) surface. GAL17 builds on three terms: (i) a standard Lennard-Jones potential for the bonding interaction between the surface and water, (ii) a Gaussian term to improve the surface corrugation, and (iii) two terms describing the angular dependence of the interaction energy. The 12 parameters of this force field are fitted against a set of 210 adsorption geometries of water on Pt(111). The performance of GAL17 is compared to several other approaches that have not been validated against extensive first-principles computations yet. Their respective accuracy is evaluated on an extended set of 802 adsorption geometries of H2O on Pt(111), 52 geometries derived from icelike layers, and an MD simulation of an interface between a c(4 × 6) Pt(111) surface and a water layer of 14 Å thickness. The newly developed GAL17 force field provides a significant improvement over previously existing force fields for Pt(111)/H2O interactions. Its well-balanced performance suggests that it is an ideal candidate to generate relevant geometries for the metal/water interface, paving the way to a representative sampling of the equilibrium distribution at the interface and to predict solvation free energies at the solid/liquid interface.