Molecular and single-atom catalysts based on earth-abundant transition metals for the electroreduction of CO 2 to C 1

The electrochemical reduction of carbon dioxide driven by renewable energy sources represents a crucial pathway for CO 2 conversion and utilization, offering a viable approach for achieving sustainable carbon neutralization. Recent advancements in the design and comprehension of catalytic materials and electrolyte systems have steadily enhanced the performance of CO 2 electroreduction (CO 2 ERR). Among the promising materials for CO 2 electroreduction, transition metals, such as manganese, nickel, iron, and cobalt, have gained prominence due to their availability in contrast to expensive noble metals. This review delves into the more recent advances on the utilization of molecular catalysts and single-atom catalysts (SACs) for the conversion of CO 2 into carbon monoxide and other C 1 molecules. The utilization of catalysts based on organometallic molecular complexes has demonstrated notable efficiency. This approach enables the fine-tuning of the electrical and coordination characteristics of the metal center, thereby enhancing metal utilization, reducing poisoning events, and allowing for selectivity control. While much of the research has traditionally focused on homogeneous CO 2 electroreduction, recent years have seen increasing attention towards the heterogenization of molecular catalysts. The main organometallic complexes presented in this review are cobalt and iron porphyrin, cobalt phtalocyanine and manganese bipyridine, anchored on carbon-based electrodes. Moreover, single-atom catalysts have emerged as promising candidates for driving CO 2 ERR, due to their exceptional performance. Among these catalysts, the single-atom Metal-Nitrogen-Carbon (M-N-C) structure stands out as particularly promising. Additionally, the incorporation of nitrogen, sulfur, and oxygen to dope the carbon support can ensure a uniform distribution of the atomic metal within the catalyst. Here, iron, cobalt and nickel SACs are outlined, presenting diverse support materials and operating conditions. Both SACs and molecular catalysts have demonstrated commendable Faradaic efficiencies, indicating their capability to convert CO 2 into CO with a high degree of selectivity. These findings suggest that SACs, with their single-atom configurations, and molecular catalysts, with their tailored molecular structures, offer viable and comparable routes for advancing the CO 2 ERR.