Redox-mediated Processes

In 2006, the concept of redox targeting was introduced in which solubilized redox-active molecules are used to mediate reversible electron transfer reactions. The aim was to address conductivity concerns rising from insulating lithium-insertion material (e.g. LiFePO₄particles) by using freely diffusing molecules as relays between the current collector and the solid electroactive material. The solubilized molecules were selected such that the standard potential window matches the Fermi level of the solid material to allow spontaneous and reversible charge transfer at the solid–liquid interface. In 2013, the concept was further developed to enhance the energy density of a flow battery. The redox-mediated flow battery includes both solid redox materials (redox booster stored in the tank) and solubilized redox species (redox mediators) to store and carry the charge of the battery, respectively. The mediators are reversibly oxidized or reduced inside the flow cell and transported into the charge storage tanks, where they will drive mediated-electron transfer reactions to charge and discharge the redox boosters. High-energy density solid redox materials can be used to boost the relatively low energy density of a conventional flow battery. In this chapter, we will first discuss the fundamental theory behind redox-mediated processes. Then, we will provide a complete and detailed literature review on reported systems emerging from this concept in the field of redox-flow battery going from redox solid booster to dual-circuit flow battery.