Ecell∘ is dependent on the equilibrium constant, K, which is not dependent on the pressure of any gas involved in the cell. Relation between Electrochemical Cell Potential and Gibbs free Energy change is given by; ΔG=−nFE where Delta G is Gibbs Free energy change , n is the number of electrons involved in the reaction and F is Faraday and E is the Cell potential.
We know that,
dG=VdP−SdT
At constant pressure dP=0
⇒dG=−SdT
S=−dTdGΔS=−(dTΔ(dG))P
⇒dTdEcell∘=nFΔS
A cell is operating reversibly if the cell potential is exactly balanced by an opposing source of potential difference.