Approaches

Cathode and anode polarizations are major voltage loss contributors in a carbonate fuel cell. According to literature (2), the anode polarization is mainly caused by diffusional mass transfer limitation in the pores. The cathode polarization is mixed activation and concentration polarization. Gas accessibility limitation caused by current collector geometry (3) also contribute to the concentration polarization. A mathematical model has recently been used to estimate electrode polarization as a function of electrode porosity, thickness and current collector geometry. Performance data of several 250cm2 cells were used for model verification. The estimated cathode polarization is about 70 mV for ERC's baseline cathode and cathode current collector, using 5NRS oxidant (12.5%02,19.1%C02) at 75% C02 utilization and 160 mA/cm2. The modeling results also indicated that gas-phase diffusional polarization (both in-plane and through-plane, as shown in Figure 1, is an important contributor. This conclusion is also verified by diagnostic testing using He carrier gas (3). Therefore, using thinner cathode and adjusting current collector geometry to increase gas accessibility are expected to reduce cathode polarization. Furthermore, the anode polarization was estimated to be >30 mV, using SRNG fuel (73% H2, 18%C02 and 9% H20) at 75% fuel utilization and 160 mA/cm2. Therefore, there may be an additional opportunity of reducing anode polarization by increasing gas accessibility in the porous anode structure. Based on this model simulation, a new current collector design was developed to enhance gas accessibility.

A thinner matrix allows reduction of ionic resistance and increase of power density. However, it requires a high strength to withstand the stress associated with thin component design for compact cells. An advanced robust matrix was developed recently (4). It lias significandy increased strength compared to the baseline. With such a matrix, the use of thin components becomes feasible.

Another approach for increasing power density is to use a monolithic cell design. In this design the bipolar plate is dimpled (Figure 2). The active components are manufactured as thin tapes. The perforated anode and cathode current collectors are also dimpled. Since the green tapes are pliable, the components in this assembly are in intimate contact. Therefore, the monolithic design allows using thin flexible components and less number of cell hardware. Due to the monolithic structure, additional active area is available for current generation. Because of the higher stress associated with the thin-component monolithic design, the use of the robust matrix discussed above is essential.

cell testing

Several thin-component 250cm2 single cells (planar or monolithic) incorporating advanced matrix and current collector design have been operated. Using a thin robust matrix has reduced cell ohmic resistance by more than 15 mV at 160 mA/cm2. Figure 3 shows a comparison between constant utilization polarization of planar thin- and normal-thickness matrix designs, showing about 25 mV performance improvement at 160 ma/cm2 under ERC's standard SRNG/5NRS condition. Using the advanced anode current collector design, about 20-25 mV performance improvement was recorded at 160 mA/cm2 under same test condition. However, using the advanced current collector design on the cathode side, only a few mV performance improvement can be realized for the normal thickness cathode, predictable by the model simulation. Therefore, for achieving additional cathode performance, a thinner cathode may be desired. In summary, at least 40mV performance improvement can be achieved with the above approaches. Figure 4 shows much increased performance with monolithic cell design, indicating the potential of 400 mA/cm2 operation. This enhanced performance can be attributed to not only the reduced mass transfer resistance (thinner cathode and special current collector design) but also the additional active area for power generation.

conclusion

Several approaches has been evaluated to enhance performance and power density of carbonate fuel cells. Diffusional mass transfer resistance can be reduced by using tliinner electrodes and by enhancing gas accessibility. Thin robust matrix can be used for reducing ionic resistance without compromising gas sealing efficiency. At least 40 mV performance enhancement can be expected by using the above approaches in combination. These improvements need to be translated in commercial-scale hardware.

acknowledgement

This effort was supported by the U.S. Department of Energy under Contracts DE-FC21-95MC31184, DE-FG05-93ER81510 and DE-FG05-93ER81512.

references

1. A.L. Leo, "Santa Clara Demonstration Status," Proc. Advanced Fuel Cells '96 Review Meeting, Morgantown Energy Technology Center, WV, August, 1996.

2. C. Yuh and J.R. Selman, "Polarization of Molten Carbonate Fuel Cell Electrodes, II. Characterization by AC Impedance and Response to Current Interruption," J. Electrochem. Soc., Vol. 138, No. 2, pp. 3649-3656, December, 1991.

3. H.R. Kunz and L.A. Murphy, "The Effect of Oxidant Composition on the Performance of Molten Carbonate Fuel Cells," J. Electrochem. Soc., Vol. 135, No. 5, pp. 1124-1131, May, 1988.

4. C. Yuh, C. Huang and R. Johnsen, "Carbonate Fuel Cell Matrix Strengthening," Proc. Advanced Fuel Cells '96 Review Meeting, Morgantown Energy Technology Center, WV, August, 1996.

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