An apparatus to remove ions including a plurality of capacitive electrode stacks. Each capacitive electrode stack may have: a plurality of first electrodes including a plurality of first current collectors; a plurality of second electrodes including a plurality of second current collectors; and a spacer between the first and second electrodes to allow water to flow in between the electrodes. The second current collectors of a first of the plurality of capacitive electrode stacks may be connected to the first current collectors of a second of the plurality of capacitive electrode stacks.