(67b) Effect of Pressure and Temperature on Carbon Dioxide Reduction at a Plasmonically Active Silver Cathode

CO₂ can be electrochemically converted to renewable fuels and chemical precursors, preventing CO₂ emissions and the need for sequestration. The primary challenge in CO₂ reduction is developing a catalyst that is both selective and efficient. While plasmon-enhanced electrochemical reduction of CO₂ has been shown to improve in both selectivity and efficiency, we seek a better understanding of the plasmonic mechanisms that drive these changes to design more effective catalysts.

We used electron-beam evaporation to create plasmonically active thin-film silver cathodes. We investigated changes in product distribution and activity from 14 to 32°C and from 0.2 to 1 atm CO₂ partial pressure (P_CO2) across a range of applied potentials in both the dark and the light. Precise product quantification was achieved by an in-line gas chromatograph for gaseous products and by ex situ NMR for liquid products.

At a given applied overpotential, the total current density increased with increasing P_CO2 in both the dark and the light. However, there were significant differences in the Tafel behavior between dark and illuminated conditions, showing that the light does not merely shift the dark activity to lower overpotentials but changes the selectivity. The reduction of CO₂ to CO was found to have first-order behavior with respect to P_CO2 at all applied potentials in both the dark and the light, likely indicating no change in the rate-determining step upon illumination. The investigation of product distribution with temperature demonstrated that the selectivity changes observed upon illumination are not caused by local heating of the cathode surface. While we continue to search for the plasmonic mechanisms responsible for these changes, our results have helped to eliminate several possible pathways.

This work was supported by the National Science Foundation (CBET-1653430, DGE 1106400); the Joint Center for Artificial Photosynthesis (DE-SC0004993); and the Molecular Foundry (DE-AC02- 05CH11231).