For a feasible cell reaction, the standard cell potential ($E^{\circ}_{cell}$) must be positive. The given potentials are standard oxidation potentials (as inferred from the subscript notation): $E^{\circ}_{Co^{2+}/Co^{3+}} = -1.81\text{ V}$, so the standard reduction potential is $E^{\circ}_{Co^{3+}/Co^{2+}} = +1.81\text{ V}$. $E^{\circ}_{Al/Al^{3+}} = +1.66\text{ V}$, so the standard reduction potential is $E^{\circ}_{Al^{3+}/Al} = -1.66\text{ V}$. Since $E^{\circ}_{Co^{3+}/Co^{2+}} > E^{\circ}_{Al^{3+}/Al}$, the $Co^{3+}/Co^{2+}$ half-cell has a higher tendency to get reduced and will act as the cathode, while the $Al/Al^{3+}$ half-cell will act as the anode. The standard EMF of the cell is: $E^{\circ}_{cell} = E^{\circ}_{cathode} - E^{\circ}_{anode}$ $E^{\circ}_{cell} = 1.81\text{ V} - (-1.66\text{ V}) = 1.81 + 1.66 = +3.47\text{ V}$