A number of apertures are defined within a wall of a chamber defined to maintain an electrolyte solution. A cation exchange membrane is disposed within the chamber over the number of apertures. The electrolyte solution pressure within the chamber causes the cation exchange membrane to extend through the apertures beyond an outer surface of the chamber. A cathode is disposed within the chamber. The cathode is maintained at a negative bias voltage relative to a top surface of a wafer to be planarized. When the top surface of the wafer is brought into proximity of the cation exchange membrane extending through the apertures, and a deionized water layer is disposed between the top surface of the wafer and the cation exchange membrane, a cathode half-cell is established such that metal cations are liberated from the top surface of the wafer and plated on the cathode in the chamber.