| Bipolar electrochemistry can overcome a key limitation in traditional electrochemistry, the need to electrically connect to the substrate. Wiring to a substrate is especially difficult for micro/nano-manufactured systems or complex three-dimensional geometries. Bipolar electrochemistry involves spatially segregated, equal and opposite reduction and oxidation on an electrically floating substrate. We show that spatial and temporal control of bipolar electrochemical couples enables patterned electrodeposition (or etching) by "remote control" without wiring to the substrate. The driving force for bipolar electrochemistry is the potential variation in solution when a current is passed; solution potential variation creates regions of differing electrochemical behavior (oxidation or reduction) on a single conducting substrate. Bipolar electrochemistry has intricate mating between the thermodynamics of the bipolar electrochemical reactions, their charge transfer kinetics, and ionic migration through the electrochemical cell. We characterize local bipolar electrodeposition beneath the tip of a rastering microjet anode configuration we call a scanning bipolar cell (SBC). The fundamental interactions of thermodynamics, kinetics, and transport in the SBC are first explored using local cupric ion and nickel ion reduction to metal beneath the microjet, with concomitant oxidation of the copper substrate as the bipolar counter reaction. The bipolar current efficiency (BCE, the fraction of the applied current that flows through the unwired substrate) can be near unity under certain electrolyte and operating conditions. Advanced electrolyte design enables a wide variety of local electrodeposition chemistries (Ni, Cu, Au, and Ag) on an inert gold substrate; the bipolar counter reaction (ascorbic acid or ferrous ion oxidation) involves a far-field oxidation that leaves the gold substrate unchanged. Simple linearized scaling arguments capture the relationship between tool and electrolyte traits, enabling a 10X improvement in spatial resolution, down to the motion control limit of our device. But, scaling arguments over-predict the BCE compared to experimental values. Electroanalytical measurements of the bipolar electrolytes provide fundamental data that underpins more sophisticated simulations. Finite element method computations are used to explain where simple scaling relationships breakdown. The impact of local bipolar electrodeposition in printed circuit board repair, additive manufacturing, and electroanalytical research is discussed. |
