Open Circuit Voltage and the Nernst Equation

E° = the equilibrium voltage with all species at their standard states R = the gas constant (8.3144 J/Kmol) 

The reaction quotient, Q, is the ratio (product of all product activities)/ (product of all reactant activities), with each activity raised to the power of that species stoichiometric coefficient. 

For the PEM fuel cell reaction, the Nernst equation takes the form  

229 V at standard conditions. Product water is assumed to be produced in the liquid form. 

In practice, the measured OCV is always lower than the theoretical OCV. The major contributions to this difference include mixed cathode potential due to PtOx formation on the catalyst surface, and fuel crossover from the 

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