Family Tree for Aqueous Organic Redox Couples for Redox Flow Battery Electrolytes: A Conceptual Review

Abstract

Redox flow batteries (RFBs) are an increasingly attractive option for renewable energy storage, thus providing flexibility for the supply of electrical energy. In recent years, research in this type of battery storage has been shifted from metal-ion based electrolytes to soluble organic redox-active compounds. Aqueous-based organic electrolytes are considered as more promising electrolytes to achieve “green”, safe, and low-cost energy storage. Many organic compounds and their derivatives have recently been intensively examined for application to redox flow batteries. This work presents an up-to-date overview of the redox organic compound groups tested for application in aqueous RFB. In the initial part, the most relevant requirements for technical electrolytes are described and discussed. The importance of supporting electrolytes selection, the limits for the aqueous system, and potential synthetic strategies for redox molecules are highlighted. The different organic redox couples described in the literature are grouped in a “family tree” for organic redox couples. This article is designed to be an introduction to the field of organic redox flow batteries and aims to provide an overview of current achievements as well as helping synthetic chemists to understand the basic concepts of the technical requirements for next-generation energy storage materials.

Sulfonated Cellulose Membranes for Energy Storage Applications

“…The main drawbacks of vanadium redox flow batteries in terms of scalability is the high price of vanadium oxide [42] and the potential toxicity if released in the environment.…”

“…There is an ongoing search for organic redox species that would be suitable for employment in redox flow batteries [43] , with the aim of reducing cost and environmental impact compared to inorganic systems [42] . Several promising candidate organic redox couples exist [42] , but there remain serious challenges to be overcome before they can compete with VRFBs in terms of large scale applicability. Specifically, crossover and stability of the active species are common drawbacks of organic redox flow batteries.…”

A Neutral‐pH Aqueous Redox Flow Battery Based on Sustainable Organic Electrolytes

“…Furthermore, molecular engineering strategies enable the structural tailoring of organic electrolytes to modulate the redox potential and solubility and enhance kinetics and stability under operating conditions [11–15] . Ferrocene, quinone, viologen, phenazine, phenothiazine, 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO), azobenzene, alloxazines, and redox‐active coordination polymers [16] are possible candidates as electrolytes for AORFBs [17,18] …”

“…[11][12][13][14][15] Ferrocene, quinone, viologen, phenazine, phenothiazine, 2,2,6,6tetramethyl-1-piperidinyloxy (TEMPO), azobenzene, alloxazines, and redox-active coordination polymers [16] are possible candidates as electrolytes for AORFBs. [17,18] In addition to vanadium cost, the use of highly concentrated sulfuric acid as a supporting electrolyte on VRFBs can cause problems due to its corrosive nature, leading to the degradation of the battery components. [19] Some AORFBs also use corrosive alkaline and acidic solutions that, besides deterioration of the system, jeopardize the stability of the redox-active materials giving rise to irreversible capacity loss.…”

Potential application of metallacarboranes as an internal reference: an electrochemical comparative study to ferrocene

“…However, there is a highly reversible redox active electrolyte of great significance in aqueous media, namely the ferricyanide to ferrocyanide couple. 11 It is noteworthy that all the internal standards described are complexes in which one of their electroactive pairs fulfills the 18e − rule and are octahedrally coordinated to avoid, as much as possible, a preceding and following interaction of the metal with the solvent so that it cannot significantly alter the formal potential ( E 0 ′) value. Indeed, all these participate in outer sphere electron transfer mechanisms.…”