Suppressing Co-Ion Generation <i>via</i> Cationic Proton Donors to Amplify Driving Forces for Electrochemical CO <sub>2</sub> Reduction
Wenxiao Guo, Beichen Liu, Matthew A. Gebbie
Abstract
Interfacial microenvironments define reaction pathways for electrocatalytic processes through a combination of electric field gradients and proton activity. Nonaqueous ionic liquid electrolytes have been shown to sustain enhanced interfacial electric fields at intermediate ion concentration regimes of around 1 M, creating local environments that promote CO 2 electroreduction. Notably, water at low concentrations absorbed by nonaqueous electrolytes is usually assumed to be the proton donor for CO 2 reduction. Consumption of protons causes proton donors to become more negative by one unit of charge, which significantly modifies the local concentration of charged species and hence should strongly impact local electric fields. Yet, how the coupling between proton donation and changing interfacial electric fields influences electrocatalytic processes in nonaqueous electrolytes remains largely unexplored. In this work, we show that the high activity of 1,3-dialkylimidazolium ionic liquids for CO 2 reduction in acetonitrile-based electrolytes stems from the ability to act as cationic proton donors that release neutral conjugate bases. Using in situ electrochemical surface-enhanced Raman spectroscopy, we find that the formation of neutral conjugate bases from imidazolium cations preserves local electric field strengths at electrode–electrolyte interfaces, providing a powerful strategy to maintain an active local microenvironment for CO 2 reduction. In contrast, conditions where water behaves as the primary proton donor generate [OH] − anions as negative “co-ions” in the electric double layer, which weakens the interfacial electric field and significantly compromises the steady-state CO 2 reduction activity. Our study highlights that electrochemical driving forces are highly sensitive to the charge state of both reactant and product species and illustrates that the generation of interfacial co-ions plays a key role in determining electrochemical driving forces.