Bulk water redox chemistry enables radical-mediated C–C coupling in CO2 electroreduction

Electrocatalysis has long been viewed as an interfacial process, so the role of bulk water redox chemistry remains largely unexplored. Here we demonstrate that electrolyte-induced water redox radicals activate reactants in bulk solution for electrocatalysis at electrified interfaces. Using formate as a model electrolyte, we show that hydrogen-bond disruption in bulk water generates redox radicals, which oxidize formate to generate C 1 intermediates, as revealed by electron paramagnetic resonance, high-resolution mass spectrometry and Raman spectroscopy. Formate concentration-dependent hydrogen-bond restructuring drives the sequential generation of water-derived radicals and reactive intermediates, establishing bulk water as an active redox mediator rather than a passive solvent. In situ electrochemical studies demonstrate that these C 1 intermediates migrate to the Cu cathode surface, enabling C–C coupling via radical-mediated pathways. This work challenges the conventional view of electrocatalysis as solely an interfacial process and offers an alternative design principle for efficient electrochemical synthesis through tailored electrolyte engineering.