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

Understanding the hydrogenation pathway in electrochemical CO 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 CO reduction with Cu in HO-DO 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 CO 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 CO reduction pathways and electrocatalyst design.