The potential at the centre of a spherical shell can be written as the sum of potential due to two identical hemispheres Vtotal=Vhemisphere1+Vhemisphere2 ⟹Vtotal=2 Vhemisphere ⟹ $\frac{\sigma R }{\epsilon_0}$ =2Vhemisphere ⟹Vhemisphere= $\frac{\sigma R }{2\epsilon_0}$