Mechanistic Insights into the Roles of Electrolyte Additives in Enhancing CO₂ Electroreduction Efficiency

The rational modulation of the competition between CO₂ reduction reaction (CO₂RR) and hydrogen evolution reaction (HER) through electrolyte engineering has emerged as a pivotal strategy to enhance CO₂RR efficiency. Although it has been established that microstructures at the electrochemical interface play a key role in facilitating efficient CO₂-to-CO conversion during CO₂RR, the fundamental understanding of electrolyte-mediated interfacial mechanisms remains elusive. In this work, we propose an environmentally friendly additive-assisted electrolyte engineering strategy to regulate CO₂RR. The results demonstrate that incorporating 40 vol % polyethylene glycol (PEG) into a 0.1 M NaHCO₃ electrolyte enables a CO Faradaic efficiency of 92% on a Au electrode, significantly surpassing the 70% achieved without additives. Further, we scrutinize the origin of the enhancement of the Faradaic efficiency of CO₂RR by comparative studies of PEG with its monomer EG and PEGDME as electrolyte additives. The results of nuclear magnetic resonance, electrochemical impedance spectroscopy, Raman spectroscopy, and computational simulation reveal that the strong solvation of PEG and PEGDME with Na⁺ is the primary factor responsible for the improved CO₂RR efficiency while the reconstruction of the hydrogen-bond network is the secondary factor. Our work provides the underlying mechanism and a simple but practical approach for rational electrolyte engineering toward efficient CO₂ reduction.