Redox-Mediated Electrified Chemistry for Carbon-Neutral Energy Applications

Redox reaction involving charge transfer at the electrode-electrolyte interface represents an essential process for various electrochemical energy conversion and storage applications, such as fuel cells, electrolyzers and batteries, etc. As a result, the operation (<i>i.e.</i>, cell voltage, current density, number of charges, etc.) of the above devices is inherently dictated and constrained by the redox reactions at the electrode-electrolyte interface. The redox-mediated process — a chemical reaction between an electrolyte-borne redox species electrochemically generated on electrode and a material (generally insoluble in electrolyte) away from the electrode, provides additional flexibility in circumventing the constraints intrinsically confronted by the conventional electrochemical devices. One example is the redox targeting of energy storage materials for flow batteries. The redox-mediated reactions of high-capacity solid material stored in the tank with redox electrolyte flowing through it considerably boost the energy density of redox-flow battery without compromising its salient features of operation flexibility and scalability. Another example is the redox-mediated water electrolysis for spatially decoupled hydrogen production. The electrochemical-chemical cycle enables continuous reaction between an electrolyte-borne redox mediator and an HER or OER catalyst loaded in a fixed-bed reactor spatially separated from the cell, which is believed to be advantageous to enhanced safety and on-demand hydrogen production. So, with the assistance of redox mediators shuttling between the electrode compartment and reactor tank, the border of the conventional electrochemical reaction is spatially extended beyond the electrode compartment, which endows the system with intriguing features for various innovative energy applications.