Abstract: Electrochemical CO₂ reduction technology could solve the CO₂-induced climate warming by electrochemically converting atmospheric CO₂ back into fuel, essentially recycling it and building a low carbon emission economy. However, the electrochemical CO₂ reduction reaction (CO₂RR) poses a significant challenge due to the highly stable and linear CO₂ molecules, in addition to a proton-coupled multi-electron transfer process. Thus, highly active catalysts, placed on activity bolstering materials, and permeable electrodes are crucial for CO₂RR. Single-atom catalysts (SACs) have recently garnered increasing interest in the electrocatalysis community due to their potentially high mass efficiency and cost benefits (every atom is an active center, resulting in nearly 100% utilization) and adjustable selectivity (higher uniformity of the active sites compared to nanoparticles). However, preserving the accessibility and activity of the SACs inside the electrode poses major materials development and electrode design challenges. A conventional layered structure SAC electrode typically consists of a gas diffusion layer (GDL), a microporous layer (MPL) and a SAC catalyst layer (SACCL), fabricated by using a powder bonding process. However, this process usually encounters issues such as delamination and instability of SACs due to the weak binder-catalyst-support interface. Conversely, the free-standing SAC electrode design has the potential to overcome these issues by eliminating the GDL, MPL, and need of a binder, in contrast to the powder bonding process. This work first reviews the latest developments in experimental and modeling studies of powdered SAC electrode by the traditional powder bonding process. Next, it examines the development towards the free-standing SAC electrode for high-performance electrochemical reduction of CO₂. The synthesis-structure-fabrication-performance relationships of SAC-based materials and associated electrodes are analyzed. Furthermore, the article presents future challenges and perspectives for high-performance SAC electrodes for CO₂RR.

Key words: Single-atom catalyst, Free-standing electrodes, Carbon dioxide, Electrochemical reduction