Mechanistic Insights into the Formation of CO and HCOOH During Electrochemical CO₂ Reduction Reaction

Since the Industrial Revolution, human energy consumption has relied heavily on coal, fossil oil, and natural gas. Nevertheless, the depletion of these fossil fuels is inevitable. Furthermore, the combustion of carbon-based fossil fuels has significantly elevated atmospheric CO₂ levels, contributing to global warming. To mitigate this issue, various renewable energy production methods have been proposed and developed^1,2. Among these approaches, the electrochemical CO₂ reduction reaction (EC-CO₂RR) has emerged as a promising strategy for converting CO₂ into valuable fuels^3,4. However, EC-CO₂RR is hindered by several challenges including activity and selectivity. For instance, the mechanistic insights into the formation of carbon monoxide (CO) and formic acid (HCOOH) are not clear during EC-CO₂RR. Understanding the surface-adsorbed species for the formation of CO and HCOOH is crucial for the development of advanced electrocatalysts with higher selectivity⁵.
In this study, we investigate the reaction mechanism over a series of AgHg alloy catalysts, utilizing a combination of in-situ X-ray absorption spectroscopy (XAS), Raman spectroscopy, Fourier-transform infrared spectroscopy, online gas chromatography, and liquid chromatography. Our findings indicate that AgHg alloys exhibit various structures, which convert CO₂ molecules into CO and HCOOH during EC-CO₂RR. The formation of HCOOH increases with rising Hg composition, peaking at 35%, after which it declines with further Hg incorporation. In-situ XAS analysis revealed that both Ag and Hg are the active sites during EC-CO₂RR. Furthermore, in-situ Raman spectroscopy results of AgHg alloys show distinct CO- and HCOOH-related peaks during the reaction. Notably, some of the surface-adsorbed CO were found to be inactive in subsequent reaction steps. Moreover, the partial positive charge on Hg promotes OCO^- adsorption, a key processing in the CO₂-to-HCOOH conversion process. These mechanistic insights into CO and HCOOH formation offer the design principles for the development of electrocatalysts with high selectivity during EC-CO₂RR.

References
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