Mechanistic Insights into the Formation of CO and C2 Products in Electrochemical CO2 Reduction-The Role of Sequential Charge Transfer and Chemical Reactions

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 conven-tional 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.

Automated summary

Electrochemical reduction of CO2 is a promising method to utilize CO2 as a feedstock and store renewable electricity. However, there are gaps in understanding the reaction mechanism and controlling the formation of products. In this study, chronoamperometry is used as a diagnostic tool to gain insights into the complex interplay of electrochemical reactions, chemical reactions, and mass transport. The findings provide important mechanistic implications and information on dominant reaction pathways for CO2 reduction.