Performance of the PAFC

Cell performance for any fuel cell is a function of pressure, temperature, reactant gas composition and utilisation. It is well known that an increase in the cell operating pressure enhances the performance of all fuel cells, including PAFCs. It was shown in Chapter 2, Section 2.5.4, that for a reversible fuel cell the increase in voltage resulting from a change in system pressure from Pi to P2 is given by the formula 

The data suggests that equation 7.13 is reasonably valid for a temperature range of 180°C 7 250°C. It is apparent from this equation that each degree increase in cell temperature should produce a performance increase of 1.15 mV. Other data indicate that the coefficient for equation 7.13 may actually be in the range of 0.55 to 0.75, rather than 1.15. 

As mentioned in Section 7.2, fuel and oxidant utilisations are important operating parameters for fuel cells such as the PAFC. In a fuel gas that is obtained, for example, by steam reforming of natural gas (see next chapter) the carbon dioxide and umeacted hydrocarbons (e.g. methane) are electrochemically inert and act as diluents. Because the anode reaction is nearly reversible, the fuel composition and hydrogen utilisation generally do not strongly influence cell performance. The RT term in equation 7.4 is clearly lower than for the MCFC and SOFC. Further discussion is given in Hirschenhofer et al. (1998). 

As with the platinum anode catalyst in the PEM fuel cell, the anode of the PAFC may be poisoned by carbon monoxide in the fuel gas. The CO occupies catalyst sites. Such CO is produced by steam reforming and for the PAFC the level that the anode can tolerate is dependent on the temperature of the cell. The higher the temperature, the greater is the tolerance for CO. The absorption of CO on the anode electrocatalyst is reversible and CO will be desorbed if the temperature is raised. Any CO has some effect on the PAFC performance, but the effect is not nearly so important as in Ihe PEMFC. At a working temperature above 190°C, a CO level of up to 1% is acceptable, but some quote a level of 0.5% as the target. The methods used to reduce the CO levels are discussed in the next chapter, especially in Section 8.4.9. 

Sulphur in the fuel stream, usually present as H2S, will similarly poison the anode of a PAFC. State-of-the-art PAFC stacks are able to tolerate around 50 ppm of sulphur in the fuel. Sulphur poisoning does not affect the cathode, and poisoned anodes can be reactivated by increasing the temperature or by polarisation at high potentials (i.e. operating cathode potentials). 

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