In a purely capacitive circuit, where there is no resistance, the current i flowing through it deposits a charge dq on the capacitor in time that is
$dq = idt $
$\Rightarrow i=\frac{dq}{dt}$
Now using Kirchoff's law, if total charge deposited on the capacitor is $q$ then
$\frac{q}{C}=E_{0} \sin \omega t $
$\Rightarrow q=C E_{0} \sin \omega t $
$\Rightarrow i=\frac{d q}{d t}=C E_{0} \omega \cos \omega t=i_{0} \cos \omega t$ where
$i_{0}=\frac{E_{0}}{\left(1 / C_{\omega}\right)}$ = peak current
$\therefore i=i_{0} \cos \omega t=i_{0} \sin \left(\omega+\frac{\pi}{2}\right) t$
and $E=E_{0} \sin \omega t$
Hence current is leading the voltage in phase by $\frac{\pi}{2}$
