Deciphering the Multifunctions of Bicarbonate in Acidic CO₂ Electrochemical Reduction With Multiscale Simulations

Identifying key factors governing the selectivity of the electrochemical CO₂ reduction reaction (eCO₂RR) is challenging and requires simultaneous consideration of microscopic reaction mechanisms at active sites and macroscopic mass-transport effects in microenvironments. In this study, we developed a multiscale simulation framework that integrates potential-dependent density functional theory calculations, microkinetic modeling (MKM), and a continuum transport model to elucidate the multifunctionality of bicarbonate (HCO₃ ^-) in acidic eCO₂RR, thereby identifying three potential regions based on local pH evolution. At low potentials, the hydrogen evolution reaction (HER) dominates, with bicarbonate buffering the local microenvironment and stabilizing the local pH at the CO₂ hydration equilibrium. At intermediate potentials, bicarbonate stabilizes the local CO₂ concentration, with bulk transport dominant. At high potentials, bicarbonate becomes the dominant proton donor for HER because the local microenvironment becomes alkaline and CO₂ is consumed by hydroxide, resulting in a sharp decrease in CO selectivity. Our mechanistic analysis establishes bicarbonate as a multifunctional species that both buffers the local pH and directly participates in the reaction, with the prevailing role dictated by the local pH and applied potentials. This work provides a mechanistic understanding of how buffering species couple mass transport and reaction pathways, offering viable design principles for microenvironment engineering in eCO₂RR.