Mechanistic understanding of the electrocatalytic CO₂ reduction reaction – New developments based on advanced instrumental techniques

Advanced instrumental techniques have played a crucial role in providing experimental evidence of reaction intermediates and kinetic information needed to correlate reactivity and product selectivity with the intrinsic properties of electrocatalysts used in the electrochemical CO₂ reduction reaction (eCO₂RR). In this review article, new developments in instrumental techniques for the mechanistic study of the eCO₂RR under in situ or operando conditions are surveyed. Initially, the reaction pathways for the eCO₂RR proposed in the literature are introduced to provide a basic understanding of the eCO₂RR mechanisms and also to demonstrate the complexity of this reaction. Next, commonly used theoretical approaches for mechanistic studies of the eCO₂RR including electrokinetic studies and theoretical calculation/modelling are outlined. The advantages and limitations associated with these approaches are highlighted to emphasise the importance of undertaking mechanistic studies under in situ or operando conditions using spectroscopic and electrochemical techniques. The main focus of this review is to summarise the research progress and discuss the advantages and limitations associated with each technique. Finally, in the Summary and Future Outlook Section, recommendations on possible future directions for the field are given.