Gibbs Energy Questions

The Gibbs energy for the decomposition of ${\text{Al}}_2{\text{O}}_3$ at $500^{\circ }\text{C}$ is as follows:$\frac{2}{3}{\text{Al}}_2{\text{O}}_3\to \frac{4}{3}\text{Al}+{\text{O}}_2,{\Delta }_{r}G=+966{\text{kJ mol}}^{-1}$ The potential difference needed for electrolytic reduction of ${\text{Al}}_2{\text{O}}_3$ at $500^{\circ }\text{C}$ is at least: