Currently, more than 86% of global energy consumption is still mainly dependent on traditional fossil fuels, which causes resource scarcity and even emission of high amounts of carbon dioxide (CO 2 ), resulting in a severe "Greenhouse effect." Considering this situation, the concept of "carbon neutrality" has been put forward by 125 countries one after another. To achieve the goals of "carbon neutrality," two main strategies to reduce CO 2 emissions and develop sustainable clean energy can be adopted. Notably, these are crucial for the synthesis of advanced single-atom catalysts (SACs) for energyrelated applications. In this review, we highlight unique SACs for conversion of CO 2 into high-efficiency carbon energy, for example, through photocatalytic, electrocatalytic, and thermal catalytic hydrogenation technologies, to convert CO 2 into hydrocarbon fuels (CO, CH 4 , HCOOH, CH 3 OH, and multicarbon [C 2+ ] products). In addition, we introduce advanced energy conversion technologies and devices to replace traditional polluting fossil fuels, such as photocatalytic and electrocatalytic water splitting to produce hydrogen energy and a high-efficiency oxygen reduction reaction (ORR) for fuel cells. Impressively, several representative examples of SACs (including d-, ds-, p-, and f-blocks) for CO 2 conversion, water splitting to H 2 , and ORR are discussed to describe synthesis methods, characterization, and corresponding catalytic activity. Finally, this review concludes with a description of the challenges and outlooks for future applications of SACs in contributing toward carbon neutrality.carbon neutrality, CO 2 reduction reaction, single-atom catalysts, sustainable clean energy
| INTRODUCTIONRapid global industrial revolution with increasing combustion of fossil fuels has led to a substantial increase in greenhouse gas emissions, and the Earth's ecological environment is being irreversibly damaged. In fact, the concentration of carbon dioxide (CO 2 ) has increased drastically in the atmosphere from around 280 ppm (in the early 1800s) to 410 ppm (currently). This has led to a drastic increase in global temperatures and corresponding sea-level rise, as shown in Figure 1. 1 Furthermore, this has also led to disruption in the balance of the carbon cycle. As early as 1960, Syukuro Manabe identified this serious problem and established