The rational design of efficient artificial photosynthetic components requires thorough understandings towards (photo)electrochemical properties and kinetic processes at the solid/liquid interface. Electrochemical scanning probe microscopy (EC-SPM), which enables the high-spatial resolution imaging in an electrolyte environment, becomes an indispensable experimental technique for operando studies of (photo)electrochemistry. This review summarizes the latest results of relevant EC-SPM techniques to study the interfacial properties of electrocatalysts and photoelectrodes. Covered methods include atomic force microscopy, Kelvin probe force microscopy, conductive atomic force microscopy, scanning tunneling microscopy, scanning electrochemical microscopy, and other advanced SPM-based operando techniques. Finally, we offer some perspectives on the future outlook in this fascinating research area.

Electrochemical CO₂ reduction (CO₂R) represents a sustainable way to store intermittent renewable energies and produce carbon-neutral fuels, yet the energy efficiency remains a huge bottleneck owning to its sluggish kinetics and complex reaction pathways. Highly active, selective, and robust electrocatalysts are strongly demanded to accelerate CO₂ conversion and deploy this technology to practical applications. In this review, we focus on single-atom catalysts (SACs), a unique category of electrocatalysts with atomically dispersed metal active sites, which have shown distinctive performances in CO₂R and offer an ideal platform for in-depth mechanistic studies at the atomic level. Despite various SACs with attractive CO₂R performances have been reported, the relationship between electronic/geometric structure of SACs and the corresponding electrocatalytic performance still needs to be discussed with caution. Here we take a broad overview on the recent progress in understanding the structure–function correlation of SACs in CO₂R, with the purpose of providing deep insights and guiding the future rational design of SACs. First, we provide the fundamental understandings of CO₂R on SACs, following different reaction pathways. Then, we describe the progresses in the development of well-defined SACs and the mechanistic studies on the influences from particular structural parameters, such as first-shell and second-sphere coordination, conductive supports and interface with a secondary catalyst. Finally, some perspectives are highlighted on the path towards efficient CO₂R on SACs.