Competitive CO₂ Electrochemical Reduction and Hydrogen Evolution at Sn Electrode through Constant-Potential First-Principles Study and Microkinetic Modeling

The CO2 electrochemical reduction (CO2ER) to formic acid offers a potential solution to a carbon-neutral energy cycle. However, this process is mainly limited by sluggish kinetics and low selectivity for HCOOH/HCOO– over H2 on Sn-based catalysts. Despite extensive research, the exact reaction mechanism remains contentious. Using constant-potential first-principles study and microkinetic modeling, we identify a reaction pathway involving bent CO2_v* intermediate on Sn(200), which is clearly distinct from previous studies by the constant-charge approach. Following this pathway, the simulated CO2ER/hydrogen evolution reaction polarization curves, as well as Faradaic efficiency on Sn(200), exhibit good agreement with experimental findings. The potential-dependent selectivity switch from HCOOH/HCOO– to H2 is attributed to the changing stability of CO2_v* with applied potential. Both excessively stable and overly unstable CO2_v* intermediates are detrimental to HCOOH formation. Moreover, we find that enhancing the adsorption of HCOO* can boost the current density and selectivity for HCOOH/HCOO– production on Sn(200). This work highlights the importance of precisely describing the electrochemical interface by the constant-potential approach in elucidating the CO2ER mechanism, and the insights gained can potentially be used to develop improved electrocatalysts for the CO2ER and other important reactions of technological interest.