Microscopic, spatially controlled, and highly efficient bipolar electrochemistry can be performed on an electrically-floating macroscopic conductive substrate using a tool we call the Scanning Bipolar Cell (SBC). The operating principle for the SBC is that current follows the path of least resistance. A high ohmic potential drop can be generated in the electrolyte adjacent to the conductive substrate by using a moderate conductivity electrolyte with a microjet cell geometry, inducing localized charge transfer on the substrate beneath the microjet. The equal and opposite redox chemistry necessary for sustained bipolar electrochemistry is spread over the macroscopic far-field of the floating conductive substrate. We combine experiments and finite element simulations to demonstrate this system using reversible copper redox chemistry. Bipolar electrochemical coupling to the surface can be highly efficient in the SBC and is governed by the balance of interfacial charge transfer and solution ohmic resistances, as characterized by the Wagner number of the system. Bipolar electrochemistry-spatially segregated, equal and opposite reduction and oxidation on an electrically-floating conductor-is a widely recognized phenomenon that is being reinvigorated as a useful tool for new kinds of electrochemical applications.1-11 The driving force for bipolar electrochemistry is the ohmic potential variation in solution that forms during the passage of current in an electrochemical cell. When there is an appreciable ohmic potential drop through solution, and a conductor is in that potential gradient, the path of least resistance for current flow can sometimes be through the conductor via bipolar electrochemistry.Electrodeposition processes are among the most common domains for this new breed of engineering-oriented bipolar electrochemistry applications. For example, copper interconnects can be grown between two electrically isolated copper posts without any direct electrical contact by placing them in a solution with high ohmic resistance.1,2 This generates a large electric field that creates sufficient voltage across each post to drive copper reduction and the growth of material between them (since it is bipolar, there is an equal but opposite oxidation also occurring). Similarly, one can also use the equal and opposite nature of bipolar electrochemistry to make Janus type conducting particles that are differentially decorated based on the induced bias across the dimensions of the particle.3 Bipolar electrochemistry that employs a reversible electrodeposition/etching chemistry can cause a free floating conductor of the active material to etch on one side and deposits an equal amount on the other side, resulting in a propagating chemical wave that moves in the direction of the solution potential gradient. 4 Applications for surface modification and electroanalytics have also recently been explored using bipolar electrochemistry. For example, bipolar electrochemistry induced by the solution potential gradient across a conductive subs...