For the feasibility of a redox reaction in a cell, the emf should be 

Any redox reaction would occur spontaneously if the free energy change $\left(\mathrm{\Delta G}\right)$ is negative. 

${\mathrm{\Delta G}}^{\mathrm{o}}=-{\mathrm{nFE}}^{\mathrm{o}}$

Where n is the number of electrons involved, F is the value of Faraday and ${\mathrm{E}}^{\mathrm{o}}$ is  the cell emf. ${\mathrm{\Delta G}}^{\mathrm{o}}$ can be negative if ${\mathrm{E}}^{\mathrm{o}}$ is positive.