Disordered interfacial H₂O promotes electrochemical C-C coupling

There is growing interest in the conversion of CO₂ and CO into energy-dense multi-carbon products to help mitigate climate change, but guiding selectivity remains challenging due to competing pathways. Here we show that tuning the structure of interfacial water using highly concentrated NaClO₄ electrolytes enhances CO electroreduction to C₂H₄. Increasing the NaClO₄ concentration from 0.01 to 10 molal increased the CO reduction rate 18-fold, achieving a Faradaic efficiency of 91% for multi-carbon products at -1.43 V versus the normal hydrogen electrode. Temperature-dependent CO reduction, combined with surface-enhanced Raman spectroscopy, revealed that changes in the interfacial H₂O structure correspond to variations in the apparent activation enthalpy and entropy for the reduction of CO to C₂H₄. At higher ionic strength, increases in activation entropy were linked to disrupted hydrogen bonding and the emergence of non-hydrogen-bonded water modes, suggesting that disordered interfacial H₂O layers facilitate CO reduction to C₂H₄. These findings offer valuable insights into how manipulating the structure of interfacial water can enhance the reduction of CO to multi-carbon products.