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Abstract:
The Direct Methanol Fuel Cell (DMFC) is a promising technology as mobile power supply. But methanol is problematic as it permeates through the cell membrane (crossover) and is oxidised with oxygen at the cathode, leading to a significantly reduced cell voltage and fuel efficiency. Also notable amounts of water pass the membrane. This can result in a flooding of the cathode leading to reduced cell power or even breakdown of the cell.
To understand the internal physico-chemical phenomena, a rigorous dynamic process model is formulated. Key element with respect to mass transport is the membrane. It is a cation exchange polymer with nanopores inside which protons, water and methanol are mobile. In the DMFC the membrane is in contact with a liquid phase (water-methanol solution) on the anode side, and with a gas phase (air) on the cathode side. The common materials (e.g. NAFION by DuPont) exhibit significant differences in their water uptake from liquid and gas phase, and the mass transport parameters show a strong dependence on the water content. Therefore the complex swelling behaviour and the phase equilibria on both sides have to be accounted for.
In the presented model mass transport is described using the generalised Maxwell-Stefan equations (dusty fluid model) and an activity model for the mobile components inside the membrane material based on a Flory-Huggins approach. This activity model is also used for the anode side phase equilibrium, whereas for the cathode side phase equilibrium an empirical approach is used based on literature data. Only the binary diffusivities in the membrane and the rate constants of the electrochemical reactions are free parameters, all other model parameters are taken from the literature. Simulation results show good agreement with experimental methanol and water crossover fluxes obtained from a fully automated DMFC miniplant and own DMFCs.
