Cell Current ampcm2

Figure 2. Effect of thickness of NAFION™ 1100 equivalent weight membrane upon predicted performance; 80°C, Anode: H2, 2.5 atm, 95 % RH, 1.5 stoich., Cathode: Air, 2.5 atm, 50% RH, 2.0 stoich.

Net water transport measurements are usually not definitive. It is difficult to properly represent irreversible processes at the membrane interfaces. In the first case with an inlet cathode relative humidity of 100%, anode inlet was controlled at 80%. Measured and calculated net water transport numbers agreed well and varied from -2.5 to 0.0 moles H20/H\ indicating water transport from the cathode to the anode. For an anode relative humidity of 100%, less than 0.1 moles H20/H* was measured in either direction when the current density is varied from 0.1 to 1.5 amp/cm2. The model predicts -1.1 to +0.3 moles H20/H+ . This difference could be explained by the interface containing higher resistance to water transport than included in the model.

Predicted Performance

The impact of thickness upon performance was explored for NAFION™ 1100 equivalent weight. Fig. (2) illustrates the decrease in performance which arises from the combined effects of higher resistance which is made worse by slower diffusion of water in the direction cathode-to-anode. The high current data tend to indicate where mass transfer limitations become predominant and performance drops rapidly. As this limit is approached, the model is highly unstable.

Other parameters to be explored at the meeting include anode and cathode humidification, and the influence of cell temperature and pressure.

Conclusion

A through-the-electrode model, based upon independently measured or calculated parameters, was combined with down-the-channel subroutines to obtain improved predictions of PEM cell performance. Modeling the down-the-channel vector is necessary to obtain an improved cell representation when water accumulation and transport are important. After model validation with experimental data, predictions were made for parameters dependent upon water balance in the flow channels and transport through the membrane.

Acknowledgment

This work was performed as part of the Electrochemical Engine Project supported by General Motors and the US Department of Energy, Office of Transportation Technology.

References

1. K. R. Weisbrod, S. A. Grot and N. E. Vanderborgh, "Through-the-Electrode Model of a Proton Exchange Membrane Fuel Cell with Independendy Measured Parameters," Extended Abstract for paper presented at the 188th Meeting of the Electrochemical Society, Chicago, 111., October 8-13, 1995.

2. T. A Zawodzinki, T. E. Springer, J. Davey, R. Jestel, C. Lopez, J. Valerio and S. Gottesfeld, J. Electrochem. Soc. 140, 1981 (1993).

3. A. Parthasarathy, S. Srinivasan and A. J. Appleby, J. Electrochem. Soc. 139,2530 (1992).

4. A. Parthasarathy, S. Srinivasan, A. J. Appleby and C. R. Martin J. Electrochem. Soc. 139, 2856 (1992).

5. T. V Nguyen and R. E White, J. Electrochem. Soc. 140, 2178 (1993).

6. DOE/GM Phase I Final Report, Research and Development of Proton Exchange Membrane Fuel Cell System for Transportation Applications, (Contract DE-AC02-90CH10435).

7. S. A. Grot, J. C. Hedstom and N. E. Vanderborgh, Paper W3.5, Materials Research Society Spring Meeting, San Francisco, Ca, April 17-21 (1995).

8. M. S Wilson, J. A Valerio and S. Gottesfeld, Eleclrochim. Acta, 40, p 355 (1995).

9. M. Thomas, Mescoubes, and M Pineri, "Pervaporation and Permeation of Water Through a Perfluorinated Ionomer Membrane: Determination of the Permeant Concentration Profiles to Explain the Transport Properties," Proceedings of Third International Conference on Pervaporation Processes in the Chemical Industry, Nancy, France, September 1988.

NOVEL, LOW-COST SEPARATOR PLATES AND FLOW-FIELD ELEMENTS FOR USE IN PEM FUEL CELLS

DJ. Edlund Northwest Power Systems, LLC Bend, OR

PEM fuel cells offer promise for a wide range of applications including vehicular (e.g., automotive) and stationary power generation. The performance and cost targets that must be met for PEM technology to be commercially successful varies to some degree with the application. However, in general the cost of PEM fuel cell stacks must be reduced substantially if they are to see widespread use for electrical power generation.

A significant contribution to the manufactured cost of PEM fuel cells is the machined carbon plates that traditionally serve as bipolar separator plates and flow-field elements. In addition, carbon separator plates are inherently brittle and suffer from breakage due to shock, vibration, and improper handling. Alternatives to machined carbon bipolar plates that have been disclosed include woven wire mesh or expanded metal mesh composed of corrosion resistant metals (1,2); molded composites consisting of powdered carbon (graphite) suspended in a polymer matrix (3); flexible graphite sheets that contain grooves much like the machined carbon plates (4); and flexible graphite stenciled with discontinuous flow channels (5). These alternative approaches suffer from drawbacks including relatively high electrical resistance, poor performance, and/or high manufacturing costs.

Northwest Power Systems is developing a separator plate (called a bifurcated separator device) that promises to offer low electrical resistance, low manufacturing cost, compact size, and excellent durability. Our approach is to separate the functions of current collection and gas flow distribution and then to design two separate elements to achieve the desired performance characteristics. Thus, the bifurcated separator plate consists of two discrete members; the current collector and the flow-field element. For instance, current collection is achieved using a thin graphite-clad metal sheet, either perforated or slotted. Gas flow distribution is achieved using a mesh fabricated from plastic or metal that has the desired chemical durability within the PEM environment. Combining these two separate elements into a single device yields a bifurcated separator device that performs as a bipolar separator plate.

An advantage of this alternative approach to making bipolar plates is that the current collector and the flow-field element can be independently optimized for performance, durability, and low cost. The result is a non-brittle and thin device that has low electrical resistance, good chemical resistance, and is inexpensive to manufacture. The chemical resistance of the bifurcated separator device is a direct result of the graphite cladding employed with the current collector and the use of chemically robust plastic or metal mesh for the flow-field element.

Manufacturing the bifurcated separator device is simple-no machining is required. The components are punched from sheet stock and pressed into the final shape. In volume, manufacturing costs are expected to be <$l/ft2.

This poster will present 1) the key features of the bifurcated separator plate; 2) performance data obtained with prototype bifurcated separator plates; and 3) manufacturing economics.

References

1. A. Leonida, "Hydrogen/Oxygen SPE Electrochemical Devices For Zero G Applications," Proceedings of the European Space Power Conference, Madrid, Spain, pp. 227r231, October 2-6,1989

2. J.F. McElroy, "SPE Regenerative Hydrogen/Oxygen Fuel Cells For Extraterrestrial Surface Applications," 24th IECEC, Volume 3, pp. 1631-1636, August 1989

3. RJ. Lawrence, "Low Cost Bipolar Current Collector-Separator For Electrochemical Cells," U.S. Patent No. 4,214,969, July 29,1980

4. D.P. Wilkinson, G.J. Lamont, H.H. Voss, and C. Schwab, "Embossed Fluid Flow Field Plate For Electrochemical Fuel Cells," WO 95/16287, June 15, 1995

5. K.B. Washington, D.P. Wilkinson, and H.H. Voss, "Laminated Fluid Flow Field Assembly For Electrochemical Fuel Cells," U.S. Patent No. 5,300,370, April 5,1994

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