The electrochemical reduction of CO2 presents an attractive opportunity to not only valorize CO2 as a feedstock for chemical products, but also to provide a means to effectively store renewable electricity in the form of chemical bonds. The recent surge of experimental and computational studies of electrochemical CO2 reduction (ECR) has brought about significant scientific and technological advances. Yet, considerable gaps in our understanding of and control over the reaction mechanism persist, in particular for the formation of products. Moreover, while theoretical and computational studies have proposed many candidate reaction pathways, comprehensively reconciling these models with experimental observations remains challenging and elusive. The conventional electrocatalytic analysis of catalyst activity and selectivity generally relies on steady-state measurements. In a departure from this convention, we show in this study that time-resolved measurements (i.e., chronoamperometry) provide a powerful diagnostic tool to gain valuable insights into the complex interplay of electrochemical reactions, chemical reactions, and mass transport. Specifically, we show that the initial ECR reaction sequence involves a chemical reaction interposed between two charge transfer reactions (“ECE”). We hope that the methods and insights presented in this work will inspire future studies to exploit chronoamperometric analysis to resolve outstanding questions in ECR and other multi-step electrochemical reaction pathways.

The electrochemical reduction of CO 2 presents an attractive opportunity to not only valorize CO 2 as a feedstock for chemical products but also to provide a means to effectively store renewable electricity in the form of chemical bonds. The recent surge of experimental and computational studies of electrochemical CO 2 reduction (ECR) has brought about significant scientific and technological advances. Yet, considerable gaps in our understanding of and control over the reaction mechanism persist, in particular for the formation of products. Moreover, while theoretical and computational studies have proposed many candidate reaction pathways, comprehensively reconciling these models with experimental observations remains challenging and elusive. The conventional electrochemical analysis of catalyst activity and selectivity generally relies on steady-state measurements. In a departure from this convention, we show in this study that time-resolved measurements (i.e., chronoamperometry) provide a powerful diagnostic tool to gain valuable insights into the complex interplay of electrochemical reactions, chemical reactions, and mass transport. We show that the initial stages of the ECR reaction show signatures of an electrochemical reaction followed by a homogeneous chemical reaction. These signatures have important mechanistic implications and inform dominant reaction pathways, specifically for the sequential electron and proton transfer steps leading to the formation of formate intermediates (*COOH − ). We hope that the methods and insights presented in this work will inspire future studies to exploit chronoamperometric analysis to resolve outstanding questions in ECR and other multi-step electrochemical reaction pathways.