Electrochemical CO reduction (COER) offers a sustainable route to produce fuels and chemicals using renewable energy. Copper-based catalysts are effective for generating multi-carbon (C₂₊) products, but their performance needs improvement for commercial viability. Understanding the reaction mechanism, particularly identifying the rate-determining step (RDS), is crucial for optimizing activity and selectivity. Three potential RDS mechanisms have been proposed: *CO-CO(g)* coupling, *CO-*CO coupling, and *CO* protonation to *CO(H)₂*. Previous studies have provided inconclusive results, with controversies stemming from incomplete dynamic analysis, insufficient experimental design, and flawed data interpretation. This study aims to comprehensively analyze theoretical derivations and kinetic experimental results to definitively identify the RDS for COER to C₂₊ products. The importance of this lies in the potential to significantly enhance the efficiency and economic feasibility of COER as a means to store renewable energy in chemical bonds. By pinpointing the RDS, targeted catalyst and process improvements can be developed to increase yield and reduce energy consumption.

Numerous mechanistic studies on COER have explored intermediate species, reaction pathways, RDSs, and electrolyte influences. However, the identification of the RDS remains contentious. Density functional theory (DFT) calculations have suggested three potential RDS mechanisms: *CO-CO(g)* coupling, *CO-*CO coupling, and *CO* protonation. Li et al. favored *CO-*CO coupling based on a Tafel slope of 118 mV-dec, a CO reaction order of 0 at higher CO pressure, and pH-independent C₂₊ current density. Kastlunger et al. further supported this with pH-independent activity in alkaline and acidic conditions. However, inconsistencies arose at lower CO pressures, prompting the need for further investigation to resolve these discrepancies and provide conclusive experimental evidence.

This study combines theoretical kinetic derivations with experimental analysis to determine the RDS. Possible RDSs, including *CO-*CO coupling and *CO* protonation, were considered, and theoretical rate expressions were derived for each mechanism. Polycrystalline copper, deposited on Si(100) wafers via magnetron sputtering, served as the model catalyst. Its polycrystalline nature enhances the generalizability of the findings to commercially relevant catalysts. Electrochemical activity measurements were conducted in a custom three-electrode cell with a high electrolyte flow rate to minimize CO transfer limitations. The experiments involved varying the electrolyte pH (using 0.1 M KOH, 0.1 M KHCO₃, 0.1 M KH₂PO₄, and 0.05 M K₂SO₄), conducting kinetic isotope effect (KIE) studies using H₂O and D₂O electrolytes, and adjusting the CO partial pressure (0.02 to 1 bar). The resulting current densities for C₂₊ products, as well as individual C₂₊ components (C₂H₄, ethanol, n-propanol, acetate) and other products (CH₄, H₂) were analyzed. Commercial copper nanoparticles and oxide-derived copper catalysts were also tested to ensure the robustness of the findings. XPS, XRD, and SEM were employed to characterize the catalysts.

The experimental results showed: 1. **pH Independence:** The C₂₊ product current density remained relatively constant across different pH values (pH 2–13), indicating a reaction order of 0 or negative for protons. 2. **Kinetic Isotope Effect (KIE):** No significant difference in C₂₊ current densities was observed between H₂O and D₂O electrolytes, suggesting that H₂O is not involved in the RDS or preceding steps. 3. **CO Partial Pressure Dependence:** The C₂₊ product activity increased with CO partial pressure up to 0.5 bar, after which it plateaued. Nonlinear fitting of the data, using the theoretically derived rate expressions, revealed that only the *CO-*CO coupling mechanism (Step A2) accurately predicted the observed trends. This conclusion holds true for both the model catalyst and commercial copper nanoparticles and oxide-derived copper catalysts. Analyses of the individual C₂₊ products show that while C₂H₄ closely follows the identified mechanism, other products (ethanol, acetate, n-propanol) show more complex behaviors.

The experimental findings strongly support *CO-*CO coupling as the RDS for C₂₊ product formation during COER on copper catalysts. The pH independence, lack of KIE, and the characteristic CO partial pressure dependence all align with the theoretical predictions for this mechanism and not others. The consistent results obtained across different copper catalysts demonstrate the generality of the findings. Promoting C-C coupling should be the key focus for enhancing C₂₊ production in COER. The observed variations in behavior between C₂H₄ and other C₂₊ products suggest potential differences in RDS for specific products, warranting further investigation.

This study provides definitive experimental evidence for *CO-*CO coupling as the rate-determining step for C₂₊ product formation in electrochemical CO reduction on copper. The combination of theoretical derivations and comprehensive kinetic experiments, including pH dependence, KIE, and CO partial pressure studies, has resolved a long-standing controversy in the field. The findings highlight the importance of promoting C-C coupling for improving the efficiency of C₂₊ production. Future research should focus on investigating the RDS for specific C₂₊ products and exploring C₃₊ product formation.

The study primarily focuses on C₂H₄ due to its high production yield. Although the overall C₂₊ production follows the proposed mechanism, the individual behavior of other C₂₊ products may differ, necessitating further investigation. While efforts were made to control for anion effects by keeping cation concentrations constant, their influence cannot be entirely ruled out. Future experiments could employ even more elaborate controls to fully eliminate this potential confound.