Two-dimensional numerical simulation of a DMFC

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Two-dimensional numerical simulation of a DMFC (Direct Methanol Fuel Cell)

Collaborator: J. Fuhrmann , K. Gärtner

Cooperation with: J. Divisek (Forschungszentrum Jülich GmbH, IWV-3)

Supported by: IWV-3 Jülich

Description: During the second year of the project we have focused on:
- support and adaptation for use at Forschungszentrum Jülich GmbH, IWV-3,
- the extension of the CH₃OH kinetics to Pt/Ru catalysts,
- a model taking into account mixed wettability by negative capillary pressures,
- some code extensions for fuel cells using H₂ rich gas mixtures,
- the energy balance equation and temperature dependence of the parameters,
- the fit of experimental data.

Short general description of a DMFC:

$\minipage{0.65\textwidth}\begin{figure} \ProjektEPSbildNocap {0.95\textwidth}{jfb_fg3_2001_dmfc_fig1.eps} \end{figure}\endminipage$ Methanol (0.5 ... 2.0 mol) dissolved in fluid water enters through channels of the diffusion layer (anode, left). At the PtRu catalyst grains CO₂, H⁺, e^- are produced. The membrane transports H⁺, the skeleton transports e^- to contacts and at the Pt catalyst grains the H⁺, e^-, and O₂ react to water. This water must be removed and competes with the O₂ diffusing from the cathodic (right) channel to the reaction zone.

The following table summarizes the different variables present at the different locations (gases: red, true fluids: blue, solved tracers: green, electrochemical potentials: yellow, temperature: black, the immobile variables describe the fraction of the occupied catalytic sites by the species produced in different electrochemical reaction steps):

Layer	Mobile Species	Immob.	Composition
An. diff.	H₂O; CO₂, CH₃OH; H₂O, CO₂, CH₃OH; e^-; T	0	graphite, teflon
An. react.	H₂O; CO₂, CH₃OH; H₂O, CO₂, CH₃OH; e^-, H⁺; T	8 or 10	graphite, teflon, nafion, Pt, PtRu
Membrane	H₂O; CO₂, CH₃OH; H⁺; T	0	nafion
Cath. rea.	H₂O; CO₂, CH₃OH; H₂O, N₂, O₂; e^-, H⁺; T	8+4	graphite, teflon, nafion, Pt
Cath. diff.	H₂O; CO₂, CH₃OH; H₂O, N₂, O₂; e^-; T	0	graphite, teflon

Due to methanol transport through the membrane, a parasitic reaction of methanol takes place at the cathodic reaction layer. Depending on the width of the reaction zone (and the concentration profile of CH₃OH and O₂), the reaction may be mainly chemical or electro-chemical. This parasitic reaction is responsible for the non-monotonic dependence of the performance of the cell on the methanol concentration: measurements and computations show a decreasing performance from 0.5 to 2.0 mol/l methanol concentration:

$\begin{figure} \ProjektEPSbildNocap {0.7\textwidth}{jfb_fg3_2001_dmfc_fig2.eps} \end{figure}$

An ideal transport assumption (no methanol permeation) would yield increasing performance with concentration.
Further experimental data are needed especially to improve the water transport models.
To improve the overview and interaction of the different (DMFC-related) research communities in Germany and to present information on simulation possibilities, a workshop focusing on all processes around the membrane was organized at WIAS (November 23/24; 30 participants, 14 talks, 3h round table discussion, see also: http://www.wias-berlin.de/dmfc/workshop/workshop-prog.html http://www.wias-berlin.de/dmfc/workshop/workshop-prog.html ). Some further details on the model equations can be found in a talk given by J. Divisek at the Annual Congress of DECHEMA ([1]).

References:

J. DIVISEK, R. JUNG, K. GÄRTNER, J. FUHRMANN, Numerical simulation of Direct Methanol Fuel Cells (DMFC), in: Proceedings of 3rd European Congress of Chemical Engineering, Nuremberg, June 26-28 2001, Gesellschaft für Chemische Technik und Biotechnologie e.V., Frankfurt/M., 2001.
URL: http://www.dechema.de/veranstaltung/ecce/cd/pages/498.htm .
H. DOHLE, J. DIVISEK, R. JUNG, Process engineering of the Direct Methanol Fuel Cell, J. Power Sources, 86 (2000), pp. 469-477.
A.A. KULIKOVSKY, J. DIVISEK, A.A. KORNYSHEV, Two-dimensional simulation of Direct Methanol Fuel Cell: A new (embedded) type of current collectors, J. Electrochemical Soc., 147 (2000), pp. 953-959.