Normal Aluminium electrode coupled with normal hydrogen electrode gives an emf of 1.66 V. So the standard electrode potential of Aluminium is, 

Al is above hydrogen in the electrochemical series, therefore ${\mathrm{Al}}^{3+}$ has lesser reduction tendency as compared with ${\mathrm{H}}^{+}$. Hence, hydrogen electrode acts as cathode when coupled with aluminium electrode. 

$\begin{array}{l}{\mathrm{E}}_{\mathrm{cell}}^{\mathrm{o}}={\mathrm{E}}_{{\mathrm{H}}^{+}/{\mathrm{H}}_2}^{\mathrm{o}}-{\mathrm{E}}_{{\mathrm{Al}}^{3+}/\mathrm{Al}}^{\mathrm{o}}\\ \therefore 1.66\mathrm{V}=0.0\mathrm{V}-{\mathrm{E}}_{{\mathrm{Al}}^{3+}/\mathrm{Al}}^{\mathrm{o}}\\ {\mathrm{E}}_{{\mathrm{Al}}^{3+}/\mathrm{Al}}^{\mathrm{o}}=-1.66\mathrm{V}.\end{array}$