(602d) Resolving Delocalized Redox-Mediated Reactions on Chemically Inert Conductive Surfaces through Zero Resistance Ammeter

Electrochemical reactors utilize electrode potential as a potent and tunable driving force for heterogeneous redox reactions, enabling applications that generate desirable chemical products, store excess electrical energy, refine critical materials, and measure compositional changes at solid-electrolyte interfaces. Additionally, electrochemical theory, mechanisms, and tools are increasingly offering insight into heterogeneous thermochemical redox reactions.¹ Redox-mediated systems take advantage of this connection to spatially and temporally decouple a redox reaction of interest from an electrochemical reactor. In these devices, a redox-mediator is activated within an electrochemical reactor, where it can then be transferred to an external vessel to drive a reaction with a solid active material or on the surface of a heterogeneous catalyst. Recent literature indicates that this approach can increase the capacity of redox flow batteries,² facilitate difficult-to-perform electrochemical transformations,³ and afford flexibility in system operations.⁴ While promising, numerous questions remain regarding the underlying dynamics of this technology concept. One such line of inquiry pertains to impact conductive supports and additives have on the rate and dynamics of the “off-electrode” redox reaction.⁵ When in close electrical contact, mediated reactions on the conductive surface are akin to galvanic corrosion couples, where two metals undergoing corrosion experience different rates according to their individual stabilities.⁶ Similar physical principles are anticipated to influence devices that employ redox mediators to react with composite active materials or catalysts, including lithium-ion flow batteries, metal-air batteries, and hydrogen electrolyzers.

In this work, we employ zero resistance ammeter (ZRA) techniques to study the mediated oxidation of zinc via a redox-active organic molecule, a reaction of relevance to mediated metal-air batteries. In these ZRA experiments, a zinc electrode is electronically connected to a chemically “inert” (glassy carbon or platinum) electrode through a low series resistance pathway on the potentiostat, allowing passive detection of the current exchanging between them and the shared potential of the electrodes (vs. an Hg/HgO reference). Through comparison to control experiments, the rate of mediator reduction on the chemically-inert surface can be isolated and continuously monitored. By invoking mixed potential theory, we are able to interpret the current and potential dynamics observed in the ZRA experiments and predict the impact of varying mediator concentration, transport rates, and relative surface area. Such observations provide initial insights and formalisms to understand the impact of “inert” conductive surfaces in redox-mediated systems, offering a path toward designing better devices.

Acknowledgements

N.J.M gratefully acknowledges the NSF Graduate Research Fellowship Program under Grant Number 2141064. 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 NSF.

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