(68g) Electrochemical Analysis of Redox Electrolytes Using Microelectrode Voltammetry Modeling

Voltammetry is a ubiquitous electroanalytical method that can be used to probe environmental processes (e.g., sensing contaminants in water [1]) and to aid in the development of sustainable electrochemical technologies [2,3]. Although most commonly performed using a macroelectrode (radius ~mm), voltammetry conducted with a microelectrode (radius ~μm) enables interrogation of electrolyte compositions inaccessible to macroelectrodes, as the smaller radius minimizes distortions and enables near-steady-state measurements [4]. These characteristics mean that microelectrodes hold unique promise for electrolyte examination, potentially enabling in situ or operando analyses of various electrolytes [1,2,4]. Methodologies aimed to evaluate the behavior of redox-active species leverage well-established expressions [2-5], and while physically grounded, they do not account for conditions where non-idealities (e.g., complex modes of mass transport) impact the current response [3]. In these instances, more extensive modeling, often framed in cylindrical coordinates [5,6], is needed to estimate the features of interest (e.g., bulk concentrations) with sufficient accuracy. Mathematical treatment in cylindrical coordinates usually results in expressions of limited adoptability [6], but by performing a coordinate transform to pose the problem in oblate spheroidal coordinates, solutions are obtainable with greater ease [7,8]. Although this change has simplified modeling of microelectrode voltammograms, to the best of our knowledge, existing treatments in both coordinate systems are only applicable to a restricted set of conditions (e.g., equal diffusion coefficients, no bulk product present) rarely encountered in real electrolytes [6-8], limiting their ability to study relevant electrochemical and environmental processes.

To extend the applicability of this approach, we use oblate spheroidal coordinates to derive steady-state and transient microelectrode voltammogram models able to account for different electron transfer kinetics, dissimilar diffusion coefficients, and the presence of both reactant and product in the bulk solution (shown in the abstract figure). In this presentation, we will first discuss the derivation of the closed form steady-state solutions and their evaluation using cases previously described in literature [6,7]. We will then describe the development of transient models capable of simulating voltammograms over a broader range of conditions (e.g., varying scan rates), which, in turn, are validated using COMSOL^®. Finally, we will explore the implications of the overall framework towards more robustly evaluating electrolytes both in the environment and in burgeoning sustainable technologies.

Acknowledgments: This work was funded by the National Science Foundation under Award Number 1805566. B.J.N. and K.M.T. both gratefully acknowledge the National Science Foundation Graduate Research Fellowship Program under Grant Number 1122374. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

References:

(1) Dean, S. N.; Shriver-Lake, L. C.; Stenger, D. A.; Erickson, J. S.; Golden, J. P.; Trammell, S. A. Machine Learning Techniques for Chemical Identification Using Cyclic Square Wave Voltammetry. Sensors (Switzerland). 2019, 19, 2392. https://doi.org/10.3390/s19102392.

(2) Stolze, C.; Meurer, J. P.; Hager, M. D.; Schubert, U. S. An Amperometric, Temperature-Independent, and Calibration-Free Method for the Real-Time State-of-Charge Monitoring of Redox Flow Battery Electrolytes. Chem. Mater. 2019, 31 (15), 5363–5369. https://doi.org/10.1021/acs.chemmater.9b02376.

(3) Gunasekara, I.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. M. Microelectrode Diagnostics of Lithium-Air Batteries. J. Electrochem. Soc. 2014, 161 (3), A381–A392. https://doi.org/10.1149/2.073403jes.

(4) Kowalski, J. A.; Fenton Jr., A. M.; Neyhouse, B. J.; Brushett, F. R. A Method for Evaluating Soluble Redox Couple Stability Using Microelectrode Voltammetry. J. Electrochem. Soc. 2020, 167 (16), 160513. https://doi.org/10.1149/1945-7111/abb7e9.

(5) Compton, R. G.; Banks, C. E. Understanding Voltammetry, 2nd ed.; Imperial College Press: London, 2011.

(6) Bond, A. M.; Oldham, K. B.; Zoski, C. G. Theory of electrochemical processes at an inlaid disc microelectrode under steady-state conditions. J. Electroanal. Chem. 1988, 245, 71–104. https://doi.org/10.1016/0022-0728(88)80060-3.

(7) Birke, R. L. Steady state concentrations and currents on an oblate spheroid microelectrode. J. Electroanal. Chem. 1989, 274, 297–304. https://doi.org/10.1016/0022-0728(89)87052-4.

(8) Qian, W.; Jin, B.; Diao, G.; Zhang, Z.; Shi, H. Application of a finite analytic numerical method. Part 2. Digital simulation of charge transfer to an oblate hemispheroid microelectrode and experiment verification. J. Electroanal. Chem. 1996, 414 (1), 1–10. https://doi.org/10.1016/0022-0728(96)04647-5.