Unraveling the rate-determining step of C₂₊ products during electrochemical CO reduction

The electrochemical reduction of CO (COER) is attractive for producing sustainable fuels and chemicals, yet the mechanism governing multi‑carbon (C2+) product formation remains debated—specifically whether the rate‑determining step (RDS) is C–C coupling or CO hydrogenation. This study combines theoretical kinetic derivations with experiments probing pH dependence, kinetic isotope effects (KIE), and CO partial pressure to identify the RDS on copper catalysts. C2+ activities are invariant with pH changes or deuterated electrolytes, indicating protons or H2O do not participate in or precede the RDS. The CO partial pressure dependence matches the theoretically derived expression for adsorbed CO dimerization (*CO–*CO coupling) as the RDS, transitioning from first order at low PCO to zero order at high PCO. The same RDS is observed on commercial Cu nanoparticles and oxide‑derived Cu. These results provide key kinetic evidence that the dimerization of two adsorbed CO species governs C2+ formation and offer a framework to enhance multi‑carbon product production.