Modeling the dynamic effects of catalyst poisoning and mixed potential formation in a DMFC

The present model is developed to investigate the coupled reaction mechanisms in a DMFC and therein associated voltage losses in the catalyst layers. The model accounts for the crossover of both, methanol from anode to cathode and oxygen from cathode to anode. The reactant crossover results in parasitic internal currents that are finally responsible for high overpotentials in both electrodes, so-called mixed potentials. They are most evident at no-load condition in the open circuit voltage. A simplified and general reaction mechanism for the methanol oxidation reaction (MOR) was selected, that accounts for the coverage of active sites by intermediate species occurring during the MOR. The model describes a complete five-layer membrane electrode assembly (MEA), with gas diffusion layers, catalyst layers and membrane. The analysis of the performance losses by means of this model are mainly focused on the electrochemical processes. Therefore, the gas diffusion electrode is modeled as a simplified homogeneous active layer. The simulation of the anode potential relaxation after current interruption shows an undershoot behavior like it was measured in the experiment. The model gives an explanation of this phenomenon by the transients of reactant crossover in combination with the change of CO and OH coverages on Pt and Ru, respectively.