Alejandro A. Franco

Modeling Chemical Degradation of a Polymer Electrolyte Membrane and its Impact on Fuel Cell Performance

We present here an elementary kinetic model of the electrochemical phenomena occurring in a polymer electrolyte membrane fuel cell (PEFC). Chemical degradation of the Nafion® membrane is described through a combination of OH radical formation via Fentons reaction and a radical mechanism of side chain decomposition. At the electrodes, the model is coupled to a description of the hydrogen oxidation reaction, the oxygen reduction reaction, and the hydrogen peroxide formation reaction at the anodic side. We establish a first feedback between the evolution of structural properties of the membrane and its transport parameters (i.e., proton conductivity) for a fully humidified fuel cell. We assess the influence of the chemical membrane degradation on the cell performance and on macroscopic properties of the membrane (i.e., sulfonate site concentration).

Metallofullerenes as fuel cell electrocatalysts: a theoretical investigation of adsorbates on C59Pt.

Nano-structured electrode degradation in state-of-the-art polymer electrolyte membrane fuel cells (PEMFCs) is one of the main shortcomings that limit the large-scale development and commercialization of this technology. During normal operating conditions of the fuel cell, the PEMFC lifetime tends to be limited by coarsening of the cathodes Pt-based catalyst and by corrosion of the cathodes carbon black support. Because of their chemical properties, metallofullerenes such as C(59)Pt may be more electrochemically stable than the Pt/C mixture. In this paper we investigate, by theoretical methods, the stability of oxygen reduction reaction (ORR) adsorbates on the metallofullerene C(59)Pt and evaluate its potential as a PEMFC fuel cell catalyst.

New insights in the electrocatalytic proton reduction and hydrogen oxidation by bioinspired catalysts: a DFT investigation.

In this paper, we present a DFT study of the proton reduction mechanism catalyzed by the complex [Ni(P₂(H)N₂(H))₂](2+), bioinspired from the hydrogenases. A detailed analysis of the reactive isomers is discussed together with the localizations of the transitions states and energy minima. The reactive catalytic species is a biprotonated Ni(0) complex that can show different conformations and that can be protonated on different sites. The energies of the different conformations and biprotonated species have been calculated and discussed. Energy barriers for two different reaction mechanisms have been identified in solvent and in gas phase. Frequency calculations have been performed to check the nature of the energy minima and for the calculations of entropic energetic terms and zero point energies. We show that only one conformation is mostly reactive. All the others species are nonreactive in their original form, and they have to pass through conformational barriers in order to transform in the reactive species.

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).

Fullerene-Based Materials as Catalysts for Fuel Cells

Nano-structured catalyst degradation in state-of-the-art polymer electrolyte membrane fuel cells (PEMFCs) is one of the main shortcomings that limit the large-scale development and commercialization of this technology. During normal operating conditions of the fuel cell, the PEMFC lifetime tends to be limited by coarsening of the cathodes Pt-based catalyst and by corrosion of the cathodes carbon black support. Because of their chemical properties, metallofullerenes, such as C59Pt or PtC60, may be more electrochemically stable than the Pt/C mixture. In this paper we investigate, by theoretical methods, the stability of oxygen reduction reaction (ORR) adsorbates on these metallofullerenes.