[Schniz2]

I am trying to create a model of a fuel cell which is operating at less than 100 degrees C and 1 atmosphere. The temperature variation of the fuel cell is relatively small, from about 55-85 degrees C

The Nernst equation which specifies the open circuit potential of the cell contains a fraction with the partial pressures of the reactants (hydrogen and oxygen) and the product (liquid water). I am confused as to what the partial pressure of a liquid would be (not even sure it makes sense) and how it could be calculated or at least approximated? Some texts assume it to be 1 atmosphere but i have found no justification for this assumption.

Thanks.

[DevaDevil]

the fraction in the Nernst equation refers to "activities", which is indeed partial pressures in case of gases, and concentrations in case of salts.
In case you use liquid water as your base electrolyte, the activity of water is by definition 1.

In case you use wetted gases as your reactants (oxygen and hydrogen streams, with high humidity), then the water will be predominantly in vapor form present, and thus have a partial pressure.

[Schniz2]

Excellent explanation, exactly what i needed to know.
Thanks!