Catalytic transformation of CO₂ into high-value chemicals and fuels has been considered as one of the most forceful approaches because it offers an alternative to fossil energy and the benefit of transforming and utilizing the excessive CO₂ emission on a grand scale. However, owing to the high chemical inertness of CO₂ and the complexity of reaction systems, developing efficient catalysts for selective CO₂ conversion remains challenging. Single-atom catalysts (SACs), which offer maximum atomic utilization, well-defined active sites, and controllable supports, which have presented growing significance for CO₂ utilization with multiple methods have been developed enhance performances their performance. Herein, this review comprehensively summarizes the latest breakthroughs in microenvironment engineering of isolated atomic sites through a systematic comparison in terms of design principles, synthetic methods, characterization techniques, and the theoretical understanding of correlations between structure and performance of state-of-the-art SACs in hydrogen production and CO₂ conversion. Subsequently, recent progress in several typical hydrogen production and CO₂ conversion are investigated to receive in-depth understanding of the catalytic mechanisms over finely-modulated SACs. Finally, challenges and future perspectives for the design of SACs are presented. This review will offer new insights into developing SAC materials for hydrogen production and CO₂ conversion and will ultimately contribute to achieving carbon neutrality.

The oxygen evolution reaction (OER) is critical for sustainable energy technologies, including proton exchange membrane water electrolyzers (PEMWEs) and metal-air batteries. However, its implementation in acidic media remains constrained by sluggish kinetics, high energy barriers, and reliance on scarce noble-metal catalysts. Cobalt-based single-atom catalysts (Co-SACs) have emerged as a breakthrough solution, combining exceptional catalytic activity, stability, and atomic utilization efficiency. Its superior acidic OER performance stems from the electronic structure of low-spin Co³⁺ centers, which optimize t_2g–π orbital interactions with oxygen intermediates. This configuration promotes efficient surface reconstruction and thermodynamically favorable adsorption of OER species, accelerating reaction kinetics. Tailored coordination environments, engineered via supports like nitrogen-doped carbons, graphene, or metal oxides, can further modulate Co electronic and spin states, enhancing activity and durability. This review systematically analyzes advancements in Co-SAC design, elucidating correlations between atomic coordination, electronic properties, and catalytic mechanisms. Advanced synthesis methods and characterization tools are evaluated to discuss structure-activity relationships of Co-SAC. Finally, we address current challenges and future research directions that involve computational modeling, multi-metallic SAC architectures, and operando techniques to guide the rational design of high-performance Co-SACs. Addressing these challenges will accelerate the commercialization of PEMWEs for cost-effective green hydrogen production.