Effect of Operative Parameters on the Polarization Curve

The operative parameters which can be regulated to optimize the stack performance are MEA humidification, reactant pressure, stack temperature, and stoichiometric ratio. While the role of membrane humidification, already partially discussed in Sect. 3.2, is closely considered in Sect. 4.5 and in case studies (Chaps. 6 and 7), the influence of the other parameters is examined here with reference to the stack of Fig. 3.5. These effects have already been described from a thermodynamic point of view (see Sect. 3.1), while kinetic implications are considered in this section for their importance in determining the stack efficiency. 

As an example. Fig. 3.6 shows the effect of the relative air pressure on the stack voltage for two different current values, 12 and 25 A [45]. 

During the above experiment the pressure difference between anode and cathode is maintained under 40 kPa, according to instructions of the stack 

A higher polarization curve when cell operating pressure is increased can be expected on the base of the Nernst equation (3.15), but the concomitant increase of / o, due to the higher concentration of reactant gases on electrodes, with the consequent improvement of the hydrogen/oxygen reaction rate, has to be also considered [34]. 

The effect of stack temperature on the polarization curve is shown in Fig. 3.7, for the temperature range 303-333 K. 

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