A capacitive deionization apparatus may include at least one pair of porous electrodes and a spacer structure disposed between the at least one pair of electrodes. The at least one pair of porous electrodes may include an electrode material having a surface area for the electrostatic adsorption of feed ions. The spacer structure may include an electrically-insulating material with an ion exchange group on the surface thereof. The spacer structure provides a path for flowing a fluid therethrough.