Operational Fuel Cell Voltages

We have seen in Chapter 2 that the theoretical value of the open circuit voltage (OCV) of a hydrogen fuel cell is given by the formula 

Fuel Cell Systems Explained, Second Edition James Larminie and Andrew Dicks 2003 John Wiley Sons, Ltd ISBN 0-470-84857-X 

In this chapter, we consider what causes the voltage to fall below the reversible value, and what can be done to improve the situation. 

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The standard Nemst potential (E°) is the ideal cell voltage at standard conditions. It does not include losses that are found in an operating fuel cell. Thus, it can be thought of as the open circuit voltage. 

Plot of fuel cell voltage versus current density showing the transition between two principal states of operation corresponding to ideally wetted conditions with primary pores filled and secondary pores available for gas diffusion and fully flooded conditions. In the depicted case, the transition involves a bistability. 

Figure 21. Long-term test of the performance stability of the PtRu2o electrocatalyst in an operating fuel cell. The fuel cell voltage at constant current of 0.4 A cm is given as a function of time for the electrode of 50 cm with an anode containing to 0.18 mg Ru cm and 0.018 mg Pt cm / (approximately 1/10 of the standard Pt loading) and a standard air cathode with a Pt/C electrocatalyst. The fuel was clean H2 or H2 with 50 ppm of CO and 3% air temperature 80 C.

The working voltage of an operating fuel cell is even lower, the reasons being (i) the internal ohmic resistance of the eell and (ii) the shift of potential of the electrodes occurring when current flows (also called electrode polarization) caused by slowness or lack of reversibility of the electrode reactions. The effects of polarization can be reduced by the use of suitable catalysts applied to the electrode surface that accelerate the electrode reactions. 

The fuel cell (HT-PEFC) is operated at 453 K with an air ratio of 2 and a hydrogen utilization of 90%, which is rather challenging for reformate operation. The cell voltage is 650 mV. 

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