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

Abstract

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.