Redox-Mediated Processes Toward Modular Electrochemical Systems

Abstract

Electrochemical technologies offer an attractive path toward a sustainable future where conventional methods of storing energy or producing critical materials are increasingly coupled to renewable electricity generation. To enable such a future, it is imperative that we have strong foundational understanding of electrochemical reactions that are useful to our needs. Redox flow batteries (RFBs) have emerged as a promising architecture for large scale storage of electricity to bridge the gap when renewable generation is unavailable. These devices operate by storing charge in the form of redox-active species that are dissolved into an electrolyte, and subsequently passed through an electrochemical cell to either store or release electrical energy. An extension of the concept of RFBs toward more general applications is to use the dissolved redox-active species to drive a reaction with another material, either to increase the energy storage density through an electrochemically active charge-dense material, or to drive a useful chemical reaction. This extension is termed a redox-mediated (RM) process, and inherits many of the complexities and intricacies of conventional electrochemical technologies, specifically that of RFB-type devices. The subject of this thesis is the development of knowledge and techniques for studying RM processes toward practical embodiments. While technical implementations of this concept are still nascent, many promising early results have been found in devices that use redox-mediated reactions to store electricity. Despite this, progress is frequently hindered by a lack of foundational knowledge from which to ideate better systems, and techniques to experimentally determine underlying physics. First, I establish the development of the RM concept over the past years as primarily through proof-of-concept electrochemical reactors which mimic RFBs. Second, we establish that the underlying nature of some RM reactions can be quantified and understood through corrosion principles, which guide our intuition for selecting chemistries and operating conditions. Third, I demonstrate that the behavior of many desirable RM chemistries is intrinsically coupled to passivation phenomena, and that this must be accounted for in reaction design. Fourth and finally, I provide experimental and practical guidance for researchers in this field, coupled with the design of some apparatus and techniques useful for characterizing RM reactions in specific and electrochemical processes in general. This body of work is broadly intended to advance understanding of electrochemically active interfaces and enable technology concepts which promote a sustainable future.