Abstract
Bipolar electrochemistry involves spatial separation of charge balanced reduction and oxidation reactions on an electrically floating electrode, a result of intricate coupling of the work piece with the ohmic drop in the electrochemical cell and to the thermodynamics and kinetics of the respective bipolar reactions. When paired with a rastering microjet electrode, in a scanning bipolar cell (SBC), local electrodeposition and patterning of metals beneath the microjet can be realized without direct electrical connections to the workpiece. Here, we expand on prior research detailing electrolyte design guidelines for electrodeposition and patterning with the SBC, focusing on the relationship between kinetics and thermodynamics of the respective bipolar reactions. The kinetic reversibility or irreversibility of the desired deposition reaction influences the range of possible effective bipolar counter reactions. For kinetically irreversible deposition systems (i.e., nickel), a wider thermodynamic window is available for selection of the counter reaction. For kinetically reversible systems (i.e., copper or silver) that can be easily etched, tight thermodynamic windows with a small downhill driving force for spontaneous reduction are required to prevent metal patterns from electrochemical dissolution. Furthermore, additives used for the bipolar counter reaction can influence not only the kinetics of deposition, but also the morphology and microstructure of the deposit. Cyclic voltammetry measurements help elucidate secondary parasitic reduction reactions occurring during bipolar nickel deposition and describe the thermodynamic relationship of both irreversible and reversible bipolar couples. Finally, finite element method simulations explore the influence of bipolar electrode area on current efficiency and connect experimental observations of pattern etching to thermodynamic and kinetic relationships.
Highlights
Bipolar electrochemistry—a phenomenon involving spatially segregated, equal and opposite reduction and oxidation charge transfer reactions on an electrically floating electrode—has recently proven valuable for a range of applications where traditional electrochemical methods are inadequate
In the case of a rastering electrode (i.e., scanning bipolar cell (SBC) microjet nozzle), the local reduction and oxidation regions move with the nozzle, exposing previously deposited material to anodic environments capable of etching the material
The SBC provides a unique platform for electrochemical materials patterning on conducting substrates without direct electrical connections
Summary
Bipolar electrochemistry—a phenomenon involving spatially segregated, equal and opposite reduction and oxidation charge transfer reactions on an electrically floating electrode—has recently proven valuable for a range of applications where traditional electrochemical methods are inadequate. Complete understanding of the intricate coupling between the kinetics of the bipolar redox reactions, their thermodynamic relationship, and ionic/electronic transport through the cell is critical to designing effective bipolar electrochemical systems. Precise control of both reduction and oxidation reactions on an electrode free of direct electrical contact has generated several new bipolar electrochemical applications, ranging from electroanalytical chemistry to material fabrication. Bipolar electrochemistry is useful in device fabrication, including deposition of non-line-of-sight interconnects between electrically isolated posts (Bradley et al, 1997, 1999), production of anisotropic functionalized microparticles (Loget et al, 2012; Tiewcharoen et al, 2017), and growth of single metal nanowires (Wood and Zhang, 2015)
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