Scaling Relationships for Designing Local Bipolar Electrochemical Systems
Authors: Braun Trevor M , Schwartz Daniel T. 
Published: February 2020

Bipolar electrochemistry has recently been utilized for many new electrochemical applications from electroanalytics to electrodeposition(1). Bipolar electrochemistry is defined as spatially segregated, equal and opposite reduction and oxidation on an electrically-floating conductor. Passage of current in an electrochemical cell produces an ohmic potential drop through the electrolyte solution. When this potential drop is substantial, and a conductor is in that potential gradient, the path of least resistance for current flow can be through the conductor, inducing bipolar electrochemical reactions.
In previous work, we have shown that a rastering microjet electrode system can create a highly localized potential distribution that is useful for localized bipolar electrodeposition(2, 3). The microjet configuration we use is called the scanning bipolar cell (SBC), where a feeder anode is located inside the microjet and a feeder cathode is placed in the far-field electrolyte pool outside the microjet. In the SBC configuration, potential gradients are high right beneath the microjet. If charge transfer resistances are modest, reduction occurs on the conducting substrate and an equal and opposite oxidation takes place in the far-field of the conductor. In short, no direct electrical connection is made to the conducting substrate, but electrodeposition and patterning occurs when the SBC is rastered over the conductor. The SBC has been used to electrodeposit Cu, Ni, Ag, Au, and various transition metal alloys (CuNi, FeNi, CoNi) on copper and gold substrates.
In this work, we use finite element method (FEM) computations (axisymmetric geometry shown in Figure 1a) to explore the effects of substrate and microjet geometries and compare to experimental demonstrations. A secondary current distribution is appropriate for SBC computations because the concentration is substantially uniform (limiting current densities exceed 10 A cm-2 in the microjet configuration used here). Computed current density profiles for varying SBC operating conditions shown in Figure 1b demonstrate current localization at the conductive substrate. For a secondary current system, the dimensionless Wagner number relates charge transfer resistances to ohmic resistances of the cell. Bipolar electrochemical systems are engineered to have small Wagner numbers (small charge transfer resistances and large ohmic drops) so that the preferred current pathway is through the conductive electrode. Computations show that ohmic resistance is dominated by the region in the annular gap and charge transfer resistance can be approximated using Tafel kinetics for the local reaction beneath the nozzle. These approximations are for an axisymmetric system on a macroscopically large substrate compared to the microjet. When substrate dimensions are not axisymmetric, or approach dimensions of the microjet, the far-field reaction kinetics can substantially affect this scaling relationship. In addition to substrate scaling, we explore how imperfect nozzle geometries affect ohmic drop and the impact of the feeder cathode on the bipolar behavior.
Scale-down of the SBC into nanoscopic dimensions is mainly limited by fabrication and motion control for the microjet. However, given the similarity of the SBC to scanning ion conductance microscopy, which has motion-control and pipette dimensions at sub-micron resolutions, we are confident practical sub-micron systems can be achieved (4).

References

[1] G. Loget, et al., Accounts Chem. Res., 46, 2513 (2013)
[2] T. M. Braun, et al., J. Electrochem. Soc.,
162, D180 (2015)
[3] T. M. Braun, et al., ChemElectroChem, doi:10.1002/celc.201500356 (2015)
[3] R. Thakar, et al., Analyst,
138, 5973 (2013).

Figure 1