A 1D Model for the Chemical Degradation of Nafion

Kurzfassung

Today the use of polymer electrolyte membrane (PEM) fuel cells in mobile and stationary applications represents a promising alternative to fossil energies. However, there are a number of challenges for technology introduction, including optimization of water and air management, cost reduction, and most importantly, increase of durability and reliability. Experimental activities provide evidence concerning the behavior of fuel cells under specific operating conditions. However, it is always difficult to derive conclusions about the origin of the phenomena one can observe due to the complexity of this system. Therefore, over the last years, Franco et al. and Bessler et al. developed novel modeling approaches of physical electrochemistry based on non-equilibrium thermodynamics and elementary kinetics [1-3]. By modeling the vicinity of the catalyst surface where electrochemical reactions occur, and by calculating the potential in each electrode, the models permit to calculate at a given working current the corresponding cell potential. Our work presented here comes in the continuity of our previous work [4]. We are modeling the feedback between chemical degradation of the membrane and transport properties. For that, we simulate the permeation of oxygen through the membrane leading to the formation of water peroxide at the interface anode/membrane. This peroxide can initiate the formation of aggressive radicals in the membrane which attack the membrane and lead to a modification of transport parameters. Because of the lack of kinetic and thermodynamic data for the elementary reactions, these values were fitted to experimental measurements. This was performed for the chemical production rate of H2O2 at the anode as well as for the degradation of the membrane side-chain. The simulations allow drawing conclusions regarding the localization of degradation in the membrane thickness by analyzing the sulfonate group concentration profile. Furthermore, the impact of the concentration of Fenton ions on the global degradation of the membrane was investigated.