Isotopic labelling of water reveals the hydrogen transfer route in electrochemical CO2 reduction

Understanding the hydrogenation pathway in electrochemical CO2 reduction is important for controlling product selectivity. The Eley–Rideal mechanism involving proton-coupled electron transfer directly from solvent water is often considered to be the primary hydrogen transfer route. However, in principle, hydrogenation can also occur via the Langmuir–Hinshelwood mechanism using surface-adsorbed *H. Here, by performing CO2 reduction with Cu in H2O–D2O mixtures, we present evidence that the Langmuir–Hinshelwood mechanism is probably the dominant hydrogenation route. From this, we estimate the extent to which each mechanism contributes towards the formation of six important CO2 reduction products. Through computational simulations, we find that the formation of C–H bonds and O–H bonds is governed by the Langmuir–Hinshelwood and Eley–Rideal mechanism, respectively. We also show that promoting the Eley–Rideal pathway could be crucial towards selective multicarbon product formation and suppressing hydrogen evolution. These findings introduce important considerations for the theoretical modelling of CO2 reduction pathways and electrocatalyst design. CO2 electroreduction to higher-value carbons can occur through adsorbed hydrogen or through proton-coupled electron transfer from water. Understanding the impact of each route on product selectivity is challenging. Now H/D isotopic labelling reveals the contribution of each mechanism towards product formation and shows that adsorbed hydrogen dominates the reaction.