MPI für Dynamik komplexer technischer Systeme / Physical and Chemical Process Engineering |
|Experimental and Model-based Analysis of the Dynamic Operating Behaviour of the Direct Methanol Fuel Cell (DMFC)|
|Authors:||Schultz, T.; Hertel, C.; Sundmacher, K.|
|Publisher:||EFCE - European Federation of Chemical Engineering|
|Place of Publication:||Toulouse, France|
|Date of Publication (YYYY-MM-DD):||2005|
|Title of Proceedings:||7th European Symposium on Electrochemcial Engineering : Multiple Faces of Electrochemical Engineering|
|Name of Conference/Meeting:||7th EEE : 7th European Symposium on Electrochemical Engineering|
|Place of Conference/Meeting:||Toulouse, France|
|(Start) Date of Conference/Meeting|
|End Date of Conference/Meeting |
|Review Status:||not specified|
|Abstract / Description:||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 which also leads to reduced cell power or even breakdown. Finally, the electrochemical reactions taking place at both electrodes are fairly slow. Therefore also here significant performance losses are originating.
In this contribution, a rigorous one-dimensional dynamic mathematical process model of a single cell DMFC will be presented . It accounts for all key mass, energy and charge transport phenomena as well as for the special nature of the electrochemical reactions. Mass transport is described by applying the generalised Maxwell Stefan equations. Special emphasis in this model is placed on a realistic description of the processes inside the polymer electrolyte membrane (PEM). The typical state-of-the-art PEM materials, sulfonated fluorpolymers like NAFION® by DuPont, Flemion® by Asahi Glass, GoreSelect® by Gore etc., exhibit significant swelling in water and methanol, leading to physical and transport properties which strongly depend on the actual water and methanol content. Therefore within the PEM the activities of the mobile species are calculated using a Flory-Huggins approach. With this model, also changes in the local degree of swelling can be described and are taken into account. The model is implemented applying a finite volumes approach.
In addition to the modeling, experiments have been carried out. The dynamic response of a single cell DMFC was studied in a fully automated miniplant, which also enables to determine the methanol and water crossover fluxes from anode to cathode. In the experiments, the methanol feed concentration and the cell current were changed stepwise, while the cell voltage was recorded. For several operating conditions, quite unexpected responses can be observed, especially cell voltage overshoots. The main reasons for this behaviour are assumed to be the methanol transport inside the PEM and the kinetics of the multi-step electrochemical reactions on both electrodes.
Simulations using variants of the mathematical model are presented for similar step changes and the results are compared to the experimental findings. A thorough analysis reveals information about the governing physico-chemical phenomena inside the DMFC and gives hints for further model refinement.
|External Publication Status:||published|
|Communicated by:||Kai Sundmacher|
|Affiliations:||MPI für Dynamik komplexer technischer Systeme/Physical and Chemical Process Engineering|
|External Affiliations:||Otto-von-Guericke-Universität Magdeburg
Fakultät für Verfahrens- und Systemtechnik
Lehrstuhl für Systemverfahrenstechnik