1. Institut für Technische Thermodynamik Dr. W. Schnurnberger Modeling Direct-Methanol-Fuel Cells: taking a look behind experimental current-voltage characteristics * Anette Siebke, Birgit Thoben and Werner Schnurnberger 1. Modeling or numerical fitting 2. Modular DMFC Model 3. Model limits and boundary conditions 4. Results and sensitivity 5. Reality check * Fuel Cell Research Symposium: Modelling and Experimental Validation ETH Zürich, March 18-19, 2004

2. Institut für Technische Thermodynamik Dr. W. Schnurnberger V = E - V act - V ohm - V conc V = E - [ V 0 + V a (1-e -c 1 i )] - [iR ohm ] - [i (c 2 i/i max ) c 3] V (T; p i, ; i ;) Air 95 0 C 50 0 C Oxygen 95 0 C 50 0 C

3. Institut für Technische Thermodynamik Dr. W. Schnurnberger SOFC Variation of Oxygen partial pressure (single cell test)

4. Institut für Technische Thermodynamik Dr. W. Schnurnberger PEFC in situ analytical tools: Electrochemical Impedance Spectra (EIS) EIS: Bode diagramm PEFC ( E-TEK Electrodes, Nafion 117), at 80°C, pH 2 =pO 2 =1bar, at different current densities / cell voltages O E=1024 mV; i= 0 mA/cm 2 E= 841 mV; i= 45 mA/cm 2 E= 597 mV; i= 392 mA/cm 2 + E= 317 mV; i= 761 mA/cm 2

5. Institut für Technische Thermodynamik Dr. W. Schnurnberger Diffusion Membrane Anode Cathode

6. Institut für Technische Thermodynamik Dr. W. Schnurnberger Air H2H2 Local Current Density [mA/cm²] 800.0 -- 850.0 700.0 -- 750.0 650.0 -- 700.0 600.0 -- 650.0 550.0 -- 600.0 500.0 -- 550.0 450.0 -- 500.0 400.0 -- 450.0 350.0 -- 400.0 300.0 -- 350.0 250.0 -- 300.0 200.0 -- 250.0 100.0 -- 150.0 0.0 -- 50.0 Porous Flow Field 80°C i av = 380 mA/cm 2 Pressure 2 bar abs H 2, air: rel. humidity 80% Air Stoichiometry =1,8

7. Institut für Technische Thermodynamik Dr. W. Schnurnberger Overview over Modeling Activities in PEM Fuel Cells kinetics of the electrochemical reactions (methanol oxidation; Kauranen, Divisek) membrane mass transport (mainly Nafion: Meier, Eikerling) transport processes in reaction layers (PEFC cathode: Broka, Springer DMFC anode: Baxter, Nordlund) flow field in gas distributor and GDL (mainly 1+2D models for the cathode: Van Nguyen, Kulikovski, Wieser) cell models focussing on different aspects

8. Institut für Technische Thermodynamik Dr. W. Schnurnberger Modeling of Processes within the l-DMFC From cell performance, no clear conclusions can be drawn concerning the processes within the cell, due to considerable voltage losses in both anode and cathode, and the strong coupling by membrane mass transport. The main goal is to use modeling as a means to explain and quantify the influence of single effects on overall cell performance. For this purpose, a detailed model is needed considering physical and electrochemical phenomena within the multi-layer structure.

9. Institut für Technische Thermodynamik Dr. W. Schnurnberger Developed Model of the l-DMFC The cell model comprises submodels for species transport and electrochemical reaction within single elements of the multi-layer structure. The detailed one-dimensional discretized submodels are coupled both by flow variables and potentials. Cell potential results from the sum of the potential differences in the electric circuit Flooding of the cathode reaction layer may occur depending on the overall water balance.

10. Institut für Technische Thermodynamik Dr. W. Schnurnberger Water Management of the Cathode The evaporating water flux is limited both by the available amount of water and the water saturation pressure. Excess water forms a liquid film and is removed by surface diffusion -> the thickness of the film is determined by the amount of liquid to be removed. oxygen transport is modeled 1+1dim: gas transport (dusty gas model) in z- direction and Fickian diffusion through water film in -direction.

11. Institut für Technische Thermodynamik Dr. W. Schnurnberger DMFC: local effective reaction rate of ORR including reaction kinetics, O 2, H + and H 2 O transport

12. Institut für Technische Thermodynamik Dr. W. Schnurnberger

13. Institut für Technische Thermodynamik Dr. W. Schnurnberger

14. Institut für Technische Thermodynamik Dr. W. Schnurnberger Modeling and Experimental Results

15. Institut für Technische Thermodynamik Dr. W. Schnurnberger No change in: -operating conditions -membrane and backings -structure of reaction layers -overall catalyst loading

16. Institut für Technische Thermodynamik Dr. W. Schnurnberger This explains the performance: At 70°C humidification of the cathode does not impact cell performance since the current density is already limited by the anode. Whereas the anode potential is strongly temperature dependent, the potential of the cathode does hardly change with temperature.

17. Institut für Technische Thermodynamik Dr. W. Schnurnberger Conclusions and visions Sensitivity analysis vs absolute U(i) Characteristics Modular structure of Models - improved transparency of results Boundary conditions and simplifications of the model influence significantly the numerical results! Help needed: physical input parameters and structural information - Diffusivity and transport coefficients (surface diffusion) - Effective porosity and tortuosity data - wetting angle of contact = f(U)? - Structure of the reaction interface (ionomer) and three phase space (boundary) - Kinetic data: rate constants, exchange current densities and activation enthalpies The two tier society of input parameters: set of constant parameters = independent of current density which parameters are sensitive to the current density?