An apparatus and method for planarizing a surface of a substrate using a chamber separated into two parts by a membrane, and two separate electrolytes is provided. The embodiments of the present invention generally provide an electrochemical mechanical polishing system that reduces the number of defects found on the substrate surface after polishing. An exemplary electrochemical apparatus includes a physical barrier that prevents any trapped gas or gas generated during processing from residing in areas that can cause defects on the substrate. The process can be aided by the addition of various chemical components to the electrolyte that tend to reduce the gas generation at the cathode surface during the ECMP anodic dissolution process.