at cathode $\left[\mathrm{Al}^{3+} \rightarrow \mathrm{Al}+3 \mathrm{e}^{-}\right] \times 4$

Net reaction

$4 \mathrm{Al}+6O^{2-} \rightarrow 3O_{2}+4 \mathrm{Al}$

$\frac 43 A l+2O^{2} \rightarrow O_{2}+\frac 43 A l$

$n=\frac {12}3=4$

$\Delta G^{\circ}=-n F E^{\circ}$

$\Delta \mathrm{G}^{\circ}=+960 \mathrm{kJ} \mathrm{mol}^{-1}$

$=960 \times 1000 \mathrm{J} \mathrm{mol}^{-1}$

$n=4$

$\mathrm{F}=96500$ Coulomb

$960 \times 1000=-4 \times 96500 \times E^{\circ}$

$E^{\circ}=-\frac{960000}{4 \times 96500}$

$=-2.48 \mathrm{V}$

potential difference $\approx 2.5 \mathrm{V}$