A sieve electrode for separating ions

A sieve electrode 10 for separating ions, for use in water purification systems based on electrodeionization, the sieve electrode comprises: a porous substrate 11 having pores of a size between 0.01 and 1 μm; and a graphene nanolayer 12 deposited on the substrate. The graphene nanolayer comprises structural defects comprising at least multiple vacancy defects having a diameter between 0.01 and 1.0 μm, wherein the total area of the defects is between 0.01 and 2% of the area of said graphene nanolayer. The substrate may be polymeric, e.g. polysulfone or polycarbonate, or ceramic, e.g. aluminium oxide or zirconium dioxide. A filtration device 20 is also described, which comprises: a compartment 22 comprising a liquid inlet 23 and a liquid outlet 24, wherein the compartment has at least one side wall open; at least one sieve electrode is fixed at the at least one open side wall of the compartment; wherein the electrical conductor 13 of the sieve electrode(s) forms an electrically conductive gasket that surrounds the compartment to seal the compartment at the open side wall to which the sieve electrode is fixed so that the liquid flows out from the compartment through the graphene nanolayer of the sieve electrode.