Thermodynamic efficiency is given by the ratio of the Gibbs function change to the Enthalpy change in the overall cell reaction.

$\mathrm{\eta }=\frac{∆\mathrm{G}}{∆\mathrm{H}}=\frac{60}{100}$

$\therefore ∆\mathrm{G}=\frac{-240\times 1000\times 60}{100}\left(\mathrm{as} ∆\mathrm{H}=-240\mathrm{kJ}/\mathrm{mol}\right)$

$\therefore ∆\mathrm{G}=-\mathrm{nFE}°=144000$

(Note: As n factor is 2)

$\mathrm{E}°=\frac{144000}{2\times 96500}=0.746\mathrm{V}$