Modeling Faradaic Reactions and Electrokinetic Phenomena at a Nanochannel-Confined Bipolar Electrode

Abstract

We present the most comprehensive two-dimensional numerical model to date for a nanoconfined bipolar electrochemical system. By accounting for the compact Stern layer and resolving the diffuse part of the electrical double layer at the bipolar electrode (BPE) surface and channel walls, our model captures the impact of surface polarization and ionic chargescreening effects on the heterogeneous charge-transfer kinetics, as well as nonlinear electrokinetic transport phenomena such as induced-charge electroosmosis and concentration polarization. We employ the Poisson− Nernst−Planck and Stokes flow system of equations, unified with generalized Frumkin−Butler−Volmer reaction kinetics, to describe water electrolysis reactions and the resulting transport of ions and dissolved gases in the confined BPE system. Our results demonstrate that under a sufficiently large applied electric field, the rapid reaction kinetics on our Pt BPE dynamically transition from charge-transfer-limited to mass-transfer-limited temporal regimes as regions depleted of redox species form and propagate outward from the respective BPE poles. This phenomenon was visualized experimentally with a pH-sensitive fluorescein dye and showed excellent phenomenological agreement with our numerical calculations, providing a foundation for further understanding and developing bipolar electrochemical processes in confined geometries. We introduce two prospective applications arising from our work: (1) a hybrid hydrodynamic/electrochemical peristaltic pump and (2) deducing information about chemical kinetics through simulation.

A synthetic chemist's guide to electroanalytical tools for studying reaction mechanisms

“…83 Specifically, the supporting CsF electrolyte (and F – source) could be used at concentrations of only 1 mM, whilst typically 100 mM or more supporting electrolyte is often required. Conducting a reaction in a low supporting electrolyte concentration such as this could provide alternative options in both organic synthesis and analysis 85…”

Recent Advancements in Bipolar Electrochemical Methods of Analysis

“…Pennathur and coworkers devised a series of experiments and computational models to study the charging and discharging behavior of a BPE confined within a nanochannel. 108,125,126 First, the authors modeled the charging of the BPE, the electrode polarization, charge transfer kinetics, and mass transport that occur under an applied potential. 125 Lastly, the group corroborated their experimental results by modeling the discharging behavior of a nano-confined BPE upon removal of an applied external electric field.…”

“…108,125,126 First, the authors modeled the charging of the BPE, the electrode polarization, charge transfer kinetics, and mass transport that occur under an applied potential. 125 Lastly, the group corroborated their experimental results by modeling the discharging behavior of a nano-confined BPE upon removal of an applied external electric field. 108 It was found that after the applied voltage is removed, the EDL continues to keep the BPE polarized, while counterions are attracted to the poles to re-establish neutrality.…”

Regulating the aggregation of colloidal particles in an electro-osmotic micropump

“…29,30 Similar non-linear electrokinetic transport phenomena have also been reported by using isolated symmetrical sharp corners 31,32 or bipolar electrodes inside a straight microchannel. [33][34][35] Other approaches to transporting particles through or trapping them in/near pores use linear electro-osmotic effects in combination with a secondary driving mechanism. For example, Cevheri and Yoda 36,37 have reported that CPs were concentrated at the microchannel wall and form bands when the pressure driven flow opposes electro-osmotic flow.…”