Suppressing Co-ion Generation via Cationic Proton Donors to Amplify Driving Forces for Electrochemical CO2 Reduction

Interfacial microenvironments critically define reaction pathways for electrocatalytic processes through a combination of electric field gradients and proton activity. Non-aqueous ionic liquid electrolytes have been shown to sustain enhanced interfacial electric field gradients at intermediate ion concentration regimes of around 1 M, creating local environments that promote CO2 electroreduction. Notably, water at low concentrations absorbed by non-aqueous electrolytes is usually assumed to be the proton donor for CO2 reduction. Consumption of protons causes proton donors to become more negative by one unit 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 non-aqueous electrolytes remains largely unexplored. In this work, we show that the high activity of 1,3-dialkylimidazolium ionic liquids for CO2 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 CO2 reduction. In contrast, conditions where water behaves as the primary proton donor generates [OH]- anions as negative “co-ions” in the electric double layer, which weakens the interfacial electric field and significantly compromises the steady-state CO2 reduction activity. Our study highlights that electrochemical driving forces are highly sensitive to the charge state of both reactant and product species and highlights the fact that the generation of interfacial co-ions plays a key role in determining electrochemical driving forces.