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 CO_{2_}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.