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 C₁ 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 C₁ 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.