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Fig. 3 Printing Method between gas and water transport. The pretreatment of the carbon cloth is necessary to avoid loosing too much catalytic material in the roughness of the paper and improves the gas and water transport properties of the carbon cloth with regard to unhydrophobed cloth or cloth thatwas directly hydrophobed in PTFE suspension.

The third very new technique to produce PEFC electrodes is an printing method. The catalyst mixture is deposit onto an electrostatic load charged roll. From this roll the catalyst mixture is rolled onto the polymer electrolyte directly, that is like a real printing process. A schematic picture is shown in Fig. 3. With this very new technique it is possible to prepare thin layers of catalyst which allow a very good contact to the electrolyte. We did not investigate the electrochemical performance in full fuel cells yet.

Test facility

As a result of our research we found, that fuel cell conditions and periphery has a major impact on the fuel cell performance. Therefore it will be described more detailed, to enable a comparison of our results to measurements of other labs. All measurements are performed with pure hydrogen (2.3 bar) and pure oxygen (2.5 bar). Hydrogen is humidified at 80°C and keeping the gas at this temperature until it reaches the fuel cell. Oxygen passes through the humidifier at 20°C an then heated up to 30°C before it enters the cell. The hydrogen flow is „dead end" whereas the oxygen flow is adopted to the certain electrodes and its characteristics. Fuel cell temperature itself is 80°C. The electrode area is about 25 cm2. As „standard EME unit" we use a E-Tek electrode on Nafion 117 - without impregnating and hot pressing to minimize the influencing parameters for the sake of system verification. The characterization methods are described in the second paper of DLR at this meeting.

Results

We decided to use a commercial E-Tek electrode (0,4 mg Pt/cm5,20% Pt/C) as a standard reference for comparison reasons. The electrode is used as anode and cathode and is simply laid on Nafion 117 and fixed in the test facility. This was done to minimize the influencing parameters such as impregnating with Nafion suspension or hot pressing. The resulting reference I/U curve is shown in Fig. 5. Preparing the electrodes by supplying the powder mixture vertically via a line funnel under gravity influence resulted in electrodes. It can be seen clearly that this electrode is much to thick to allow effective gas transport. This was confirmed in the electrochemical fuel cell measurements. In the characteristics the hinderance for gas diffusion can clearly be seen.

Changing the rolling parameters did not influence the thickness of the electrode. We attribute this to the fact that the amount of powder retracted into the calandar has its minimum value determined by the retraction angle which is set by the roller diameter and its material. Preparing the electrodes

Fig. 5 characteristics of rolled and commercial electrodes

(E-Tek curve on Nafion 117 without impregnation and hot pressing to minimise the influencing parameters)

current density (mA/cm2)

Fig. 5 characteristics of rolled and commercial electrodes

(E-Tek curve on Nafion 117 without impregnation and hot pressing to minimise the influencing parameters)

in a second manner resulted as dipicted in Fig. 4. The thickness of the reaction layer is in the range of the commercial standard electrode. The power output is essentially the same as with the commercial electrode. The measurements are carried out without hotpressing the electrode membrane structures before. The exchange current-density can be calculated to be in the order of ¡o = 4 10"4 A/cm2 from the characteristics. First experiments of long time behavior shows no degradation effect during a period of 320 hours. [4]

Conclusions

We developed a rolling process which allows the production of electrodes to be used in PEFC which have the same power density characteristic as commercial electrodes. This process offers a possibility to produce electrodes on a large scale base with reproducible characteristics. After having optimized the rolling process to the special requirements of PEFC electrodes, the task is now to include advanced electrode preparation steps into this production process as there is enlargement of the three-dimensional reaction area.

In contrast to the well-known process of impregnating electrodes with solved Nafion we will try to mix solid electrolyte powder into the powder which is rolled onto the support [5]. This will avoid hazardous solvents in the before mentioned process.

A second new technique is developed to prepare layers of catalysts directly onto the polymer membrane. Now we have to investigate the electrochemical behavior of these catalyst layers and the electrochemical performance in a fuel cell.

Acknowledgements

The authors thank their collegues for experimental support, mainly Dr. N. Wagner for the electrochemical measurement (EIS) and Dr. M. von Bradke for the SEM pictures. The authors gratefully acknowledge the financial support of the State of Baden Württemberg.

Literature References

[ I ] Preparation of pours electrodes and laminated electrode-membrane structures for polymer electrolyte fuel cells; K. Bolwin, E. Gülzow, D. Bevers, W. Schnurnberger; Solid State Ionics, 77 (1995) 324-330

[2] Apparatus and method for a plastic-bound activated carbon layer for thin gas-diffusion electrodes; H. Sauer (VARTA AG); Ger. Offen. 2 941 774 (CI H01M4/88); Apr. 30, 1981, Appl. Oct. 16,1976; 9pp

[3] Porous gas electrodes; A.Winsel (VARTAAG); Ger. Offen. DE 3 342 969 (CI. C25B11/06), Jun. 5, 1985, Appl. 28, 1983; 12pp

[4] Characterization and basic research investigations at PEFC electrodes and MEA; M. Schulze, N. Wagner, G. Steinhilber, E. Gülzow, M. Wöhr, K. Bolwin; Proc. Fuel Cell Seminar Orlando (1996)

[5] Preparation and Characterization of Nation powders for low cost PEFC electrode production; D. Bevers, G. Bacsur, N. Wagner, K. Bolwin; Powder Technology, 84 (1995) 269-276

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