*2.1. Ideal Voltage*

The theoretical maximum voltage of a PEMFC under reference conditions *E*25*C*,1atm can be calculated with the Gibbs free energy Δ*G* as well as the Faraday constant *F* and the number of electrons involved *n* [11]:

$$E\_{25C,14\text{atm}} = -\frac{\Delta G}{nF} = \frac{237340}{2 \times 96485} = 1.23\text{ V}.\tag{2}$$

Variations in temperature *T* and partial pressure of reactants *pi* can be accounted for by using the Nernst-Equation [11]:

$$E\_{T,P} = -\left(\frac{\Delta H}{nF} - \frac{T\Delta S}{nF}\right) + \frac{RT}{nF} \ln\left[\frac{p\_{\text{H}\_2} p\_{\text{O}\_2}^{0.5}}{p\_{\text{H}\_2\text{O}}}\right].\tag{3}$$

Neglecting influences of changing enthalpy Δ*H* and entropy Δ*S*, as well as assuming the product water to be in the liquid phase, the ideal cell voltage *ET*,*<sup>P</sup>* can be expressed as follows [11]:

$$E\_{T,p} = \; 1.482 - 0.000845 \; T + 0.0000431 \; T \; \ln(p\_{\rm Pl\_2} p\_{\rm O\_2}^{0.5}) \;. \tag{4}$$
