Understanding the Temperature Effect on Carbon-Carbon Coupling during CO₂ and CO Electroreduction in Zero-Gap Electrolyzers^†

Cu-catalyzed electrochemical CO₂ reduction reaction (CO₂RR) and CO reduction reaction (CORR) are of great interest due to their potential to produce carbon-neutral and value-added multicarbon (C₂₊) chemicals. In practice, CO₂RR and CORR are typically operated at industrially relevant current densities, making the process exothermal. Although the increased operation temperature is known to affect the performance of CO₂RR and CORR, the relationship between temperatures and kinetic parameters was not clearly elaborated, particularly in zero-gap reactors. In this study, we detail the effect of the temperature on Cu-catalyzed CO₂RR and CORR. Our electrochemical and operando spectroscopic studies show that high temperatures increase the activity of CO₂RR to CO and CORR to C₂H₄ by enhancing the mass transfer of CO₂ and CO. As the rates of these two processes are highly influenced by reactant diffusion, elevating the operating temperature results in high local CO₂ and CO availability to accelerate product formation. Consequently, the *CO coverage in both cases increases at higher temperatures. However, under CO₂RR conditions, *CO desorption is more favorable than carbon-carbon (C—C) coupling thermodynamically at high temperatures, causing the reduction in the Faradaic efficiency (FE) of C₂H₄. In CORR, the high-temperature-augmented CO diffusion overcomes the unfavorable adsorption thermodynamics, increasing the probability of C—C coupling.