Organic Films Amplify Cation Effects in Heterogeneous Electrochemical CO2 Reduction

Electrochemical CO₂ reduction (CO₂R) is a method to recycle waste gas from point sources or the atmosphere to renewably make industrially relevant products. Copper is a unique catalyst for CO₂R because of its ability to produce valuable C₂₊ products, albeit with poor selectivity. One strategy to improve performance is to use certain organic additive films that are electrochemically deposited on the electrode, which can dramatically boost the selectivity towards carbon-coupled products. Additionally, the electrolyte cations have a significant role in the performance of CO₂R, and mixing these cations yields a range of results. We investigate how N-tolyl pyridinium additive impacts the cation effects in Cu-catalyzed CO₂R. Electrochemical techniques, including chronoamperometry (CA) and modified pulsed voltammetry (mPV), along with attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) can provide insight into how cation size affects the electrochemical environment. Electrolysis data show that doping small amounts of Cs⁺ into Li⁺ electrolyte makes a noticeable impact on the performance of CO₂R towards C₂₊ products, suggesting a cation competition for accumulation on the cathode surface. To further probe the extent of cation effects, we performed experiments using Cs⁺ with both NH₄⁺ and NEt₄⁺, which reveal that reactivity, along with hydration shell, of the cation is significant. With this mixing strategy, we find that selectivity and activity plateau at different Cs⁺ compositions for bare Cu and additive-modified Cu, emphasizing how the additive creates a unique electrochemical environment close to the electrode surface. Analysis of mPV and ATR-SEIRAS data suggests that the local electric field at the Cu cathode does not change with the presence of the additive, which narrows down the difference in performance between bare and additive-modified Cu to physical, or nonelectric field, effects. These results emphasize how transport effects in electrochemical processes are equally important to consider along with electric field effects when explaining reactivity trends.