Hydrogen Bond Network Descriptors Dictate Reactivity and Selectivity of CO2 Electroreduction.

The hydrogen bond network (HBN) of water plays a key role in the electrocatalytic CO2 reduction reaction (CO2RR). However, a systematic understanding of the HBN and the establishment of predictive descriptors remain lacking, which hinders the rational design of effective electrochemical systems. In this work, two descriptors, hydrogen bond (HB) strength and HB number, are introduced to characterize the flexibility and connectivity of the HBN, respectively. By adding a series of urea derivatives to the electrolyte, both HB strength and HB number can be continuously tuned, enabling precise modulation of HBN flexibility and connectivity. By only varying the HBN properties, the current density, Faradaic efficiency for multicarbon products (FEC2+), and the FEC2H4/FEC2H5OH ratio can be modulated from 0.38 to 0.81 A cm-2, 48.2% to 90.5%, and 1.04 to 2.45, respectively. A direct and predictive link between HBN properties and CO2RR performance is established. An optimal HBN stabilizes the key *CO intermediate through water-mediated hydrogen bonding, thereby enhancing C-C coupling. A highly connected network favors ethylene production, while a flexible HBN promotes ethanol formation. Consequently, the reactivity, FEC2+, and the ethylene/ethanol ratio can all be tuned by adjusting the properties of the HBN.