Electrochemical CO₂ reduction (CO₂R) has emerged as a promising approach for creating a sustainable carbon cycle, driven by the urgent need to address climate change and the growing interest in CO₂ sequestration and conversion. Coupled with renewable energy sources, CO₂R offers a pathway to store intermittent renewable resources at ambient conditions, produce valuable fuels and chemicals, and reduce CO₂ emissions. Gold-based materials stand out as highly active and selective catalysts for CO production, a crucial component of syngas. Therefore, understanding the reaction mechanism on gold catalysts is vital for designing improved materials, forming the core focus of this combined theoretical and experimental study. While the general mechanism of CO₂R to CO via *COOH and *CO intermediates is accepted, the rate-limiting step remains a subject of considerable debate. Based on recent Tafel analysis and kinetic isotope effect (KIE) studies, three different steps have been proposed as rate-limiting: electron transfer to CO₂ during adsorption, proton transfer to *CO₂ to form *COOH, and electron transfer to *COOH to form *CO. This paper aims to resolve this controversy by providing experimental and theoretical evidence supporting field-driven CO₂ adsorption as the limiting step. Previous kinetic models for CO₂ reduction to CO, primarily focusing on silver, have been developed using ab initio reaction energetics. However, these models have employed diverse methodologies and assumptions, leading to discrepancies in the identified rate-limiting step. Furthermore, the importance of mass transport phenomena has been emphasized, but its incorporation into kinetic models often neglects the impact of the charged double layer. Recent studies have highlighted the significant influence of the electric double-layer field on the energetics of critical reaction steps. This work demonstrates the critical role of the double layer in influencing both kinetics and mass transport by affecting the pH and reactant concentrations at the reaction plane.

Several studies have investigated the mechanism of CO2 reduction on gold, leading to varying conclusions about the rate-limiting step. Wuttig et al. (2016) observed no significant kinetic isotope effect (KIE), suggesting CO2 adsorption as the rate-limiting step. Conversely, Dunwell et al. (2017) proposed that the formation of *COOH or the subsequent conversion to *CO could be rate-limiting. Other studies have incorporated mass transport considerations into kinetic models but often neglected the influence of the electric double layer. Chen et al. (2016) highlighted the importance of the electric field in electrochemical CO2 reduction, suggesting that it could significantly influence reaction energetics. However, a unified multiscale modeling approach that incorporates both reaction kinetics and mass transport, explicitly considering double-layer effects, has been lacking. This gap in the literature provides the motivation for the current research.

This research employed a combined experimental and theoretical approach to investigate the CO2 reduction reaction on gold. **Experimental CO2R:** The electrochemical experiments were performed using a custom-built electrochemical cell with a polycrystalline gold working electrode, a platinum counter electrode, and various ion-exchange membranes depending on the electrolyte pH (anion-exchange membranes near neutral pH, bipolar membranes at pH 3.0, and proton-exchange membranes at pH 1.0). Buffered and unbuffered electrolytes (KHCO3, KH2PO4, KClO4, HClO4) were used to assess the influence of pH. CO2 gas was continuously bubbled through the electrolyte. Gas products (CO and H2) were analyzed using online gas chromatography. Experiments at pH 3.0 used short durations to minimize pH variations. The boundary layer thickness was determined using the diffusion-limited current for ferricyanide reduction. **Multiscale Modeling:** A novel multiscale model was developed to integrate ab initio reaction kinetics with mass transport simulations, explicitly including the electric double layer. * **Ab initio Calculations:** Density functional theory (DFT) calculations were performed to determine surface-charge density (σ)-dependent reaction thermodynamics for key intermediates (*CO2, *COOH, *CO) on a (211) gold facet, which is representative of undercoordinated sites. A continuum solvent model and planar countercharge representation were used. The computational hydrogen electrode (CHE) approach was employed to relate reaction energies to the applied cell voltage. * **Microkinetic Model:** A mean-field microkinetic model was constructed based on the ab initio-derived reaction energetics. This model accounted for the surface-charge-dependent formation energies and included buffer reactions (considering both proton and hydroxide-driven reactions). The HER was not explicitly included due to its significantly lower rate compared to CO production at pH 6.8. * **Mass Transport Model:** A detailed continuum mass transport model was developed to simulate the diffusion, migration, and reactions of various species within the boundary layer. This model explicitly incorporated the electric double layer structure, cation-cation repulsion, finite ion size effects, and buffer equilibria. The Modified Poisson-Boltzmann approach was used to account for ion crowding effects, particularly considering the size of potassium cations. Robin boundary conditions were used to relate the surface-charge density in the DFT calculations to the applied cell voltage. * **Coupled Model:** The microkinetic and mass transport models were coupled through a flux boundary condition at the reaction plane. The resulting equations were solved self-consistently using a newly developed program package called CatINT.

The experimental results revealed that the CO production rate was invariant with pH on an absolute potential scale (SHE), indicating that CO2 adsorption is the rate-limiting step. In contrast, the H2 production rate exhibited a clear pH dependence. The multiscale model successfully reproduced the experimental CO polarization curve, including the Tafel slope. The model analysis revealed three distinct rate-limiting regimes: * **Low overpotentials:** The *COOH to *CO conversion step is rate-limiting, exhibiting a Tafel slope of approximately 42 mV/dec. * **Intermediate overpotentials:** CO2 adsorption is rate-limiting, with a Tafel slope around 101 mV/dec, consistent with the experimental range of 120–150 mV/dec. The Tafel slope in this region originates from the potential dependence of the surface-charge density and the resulting stabilization of the dipolar *CO2 state. * **High overpotentials:** CO2 mass transport limits the reaction rate. The model also showed that the *CO coverage remained negligible throughout the potential range considered, consistent with experimental observations. Analysis of the degree of rate control (DRC) confirmed the rate-limiting steps identified above. Cation exchange experiments showed a significant effect on CO production rate, consistent with the model's prediction that the rate is sensitive to the interfacial electric field. The model also predicted the potential-dependent local pH at the electroneutrality plane, showing agreement with previous ATR-SEIRAS measurements. Finally, the model indicated that bicarbonate buffer does not contribute to the supply of reactive CO2 species under stationary conditions.

This study successfully resolved a long-standing debate on the rate-limiting step in electrochemical CO2 reduction on gold. By combining comprehensive experimental data and a novel multiscale modeling approach that incorporates the crucial role of the electric double layer, the researchers demonstrated that CO2 adsorption is the rate-limiting step under most relevant conditions. The model elucidated the dependence of the Tafel slope on the potential-dependent surface charge and the stabilization of the dipolar *CO2 state. The findings highlight the importance of considering both kinetics and mass transport, along with double-layer effects, in understanding and optimizing electrochemical reactions. The results provide valuable insights into the design of more efficient and selective CO2 reduction catalysts.

This research provided a comprehensive understanding of the electrochemical CO2 reduction on gold, resolving a long-standing controversy about the rate-limiting step. The study successfully integrated experimental results and a novel multiscale model to highlight the significant role of the double layer in determining reaction kinetics and mass transport. This work provides valuable insights for the design and optimization of CO2 reduction catalysts, emphasizing the importance of surface charging and the need for more sophisticated models to account for double-layer effects. Future research could explore the effect of different gold surface structures and electrolyte compositions on the reaction mechanism and explore strategies to enhance CO2 adsorption by manipulating double-layer properties.

While the multiscale model successfully reproduced the experimental data and provided insights into the reaction mechanism, certain limitations should be noted. The model made certain assumptions, such as the choice of the (211) facet as a representative active site and the simplification of the double layer structure. While the HER was not included explicitly due to its low rate relative to CO production at pH 6.8, its effect on local pH changes under different conditions might not be completely negligible. Further refinement of the model and additional experimental validation could improve accuracy and enhance the understanding of the system under various conditions. The use of a simplified continuum double-layer model could also be considered a limitation, particularly at high current densities.