A positive overall cell potential indicates that reaction will be spontaneous. There is a connection between the Gibbs free energy change (ΔG) and the overall potential:
ΔG = -nFE
where: n - the number of changed electrons, F - Faraday constant, E - the overall potential.
In order for a process to be spontaneous, the Gibbs free energy change has to be negative, and the above relation indicated that the overall potential has to be positive (since the number of electrons and Faraday constant have positive value).