Electrochemical Cell Our electrochemical system consists of an AFM nanoelectrode tip immersed in a filled electrochemical cell and connected to an external potentiostat, which controls and monitors potential and current. At positive potentials (tip vs. substrate), we expect the oxygen evolution reaction to occur at the tip and either copper reduction or hydrogen evolution at the sample. The electrochemical cell was homemade, and consisted of Teflon (R), made leak tight by an O-ring. Top-contacts were made by either attaching a wire connection directly to the sample edge, or by using a spring-loaded pin outside of the O-ring.