Mediator Design for Coupled Electrochemical-Chemical Reaction

Electrochemical synthesis exploits electrode-supplied electrons or holes as tunable redox equivalents. However, conventional electrocatalytic processes require electron transfer, substrate activation, and subsequent chemical conversion to all occur within the same electrode/electrolyte interfacial region. This interface-confined paradigm increasingly restricts complex molecular synthesis, as multistep transformations require precise kinetic coupling unsustainable at a single interface. Coupled electrochemical-chemical reaction (CECR) systems overcome this limitation by employing redox mediators that functionally decouple electrode electron input from downstream substrate conversion. In these systems, the electrode generates, regenerates, or modulates active mediator states that relay charge, direct radical reactivity, or serve as electrophilic/nucleophilic units, forming target products via chemical steps away from the electrode surface. This review proposes a mediator-centric paradigm for the rational design of CECR. We first examine the prerequisites for efficient CECR operation from thermodynamic, kinetic, and mediator-design perspectives, focusing on driving force redistribution, rate-space coordination, and the required properties of mediator molecules. Subsequently, based on the dominant role of the mediator in key transformations, we classify CECR mechanisms into three primary modes: electron-transfer mediation, radical-relay processes, and electrophilic/nucleophilic activation. This review aims to shift CECR from empirical reaction discovery toward mechanism-guided electrosynthesis, enabling the preparation of high-value chemicals.