Role of oxygen-bound reaction intermediates in selective electrochemical CO2 reduction

The electrochemical CO2 reduction reaction (CRR) is intrinsically complex given the multiple possible reaction pathways and end products. Consequently, selectivity is a persistent challenge for the design and operation of CRR electrocatalysts. A detailed understanding of key elementary steps and surface-bound species involved in the C-1-C-3 pathways is important for directing the reaction to a target product. However, there has been limited success in fully explaining the selectivity of CO2 reduction, such as the competing production of oxygen-free hydrocarbons and oxygen-containing alcohols. Recently, oxygen-bound intermediates have been identified as essential species to help explain the full reaction roadmap for CO2 reduction. This Review explores the important role of oxygen-bound intermediates in affecting CRR selectivity to the many reduction products, ranging from two electron products to higher reduced products. These oxygen-bound intermediates have a big influence on addressing mechanistic aspects of competing reaction pathways, based on extensive analysis of adsorption behaviour, reaction thermodynamics and reaction kinetics. Considering available theoretical calculations, electrochemical measurements and operando spectroscopy observations, we highlight the preferred reaction pathways to certain products when regulated by oxygen-bound species. The geometries of these oxygen-bound intermediates and their binding on a catalyst surface dictate the breakage or preservation of C-O bonds, which has a significant effect on directing selectivity toward a final product. Based on this mechanistic evaluation, we summarize practical techniques for probing the evolution of intermediates and propose possible strategies for promoting the selectivity of electrocatalysts.