The growing consumption of fossil fuels has significantly increased the concentration of carbon dioxide (CO2) in the atmosphere and raised the severe issue of global warming. (1) To tackle this issue, a wide variety of promising strategies have been developed for the capture and utilization of CO2 and, to this end, the reduction of net emissions. (2−4) In particular, the electrochemical conversion of CO2 into various value-added chemicals or fuels has attracted wide research interest due to its flexible compatibility with sustainable energy power and high conversion efficiency of this strategy. (5−7) To date, extensive studies have focused on the electrochemical reduction of CO2 in aqueous solutions at room temperature, which is, however, substantially confronted with sluggish diffusion rates, high-cost membranes, and poor product selectivity. (8,9) Instead, the approach based on solid oxide electrolytic cells (SOECs) that work at high temperatures (e.g., above 700–900 °C) is more efficient for the reduction of CO2. (10,11) This is owing to the fact that both mass transfer and reaction kinetics are accelerated, leading to the production of CO, (12) syngas (CO and H2), (13) or methane (CH4). (14) However, the operation of SOECs at the high electrolysis temperature presents a long-standing challenge in electrode coarsening and, thus, inferior system durability. (11,15) Besides, the general use of high-purity CO2 requires the additional energy and costs for extensive capture and transportation from dilute sources (e.g., flue gas from power plants or refineries). (16,17) Therefore, it is highly desirable to develop efficient and reliable strategies for integrated CO2 capture and utilization that can be directly employed in heavy-emission plants.

Molten-salt based electrolytic cells have received widespread attention because they work at relatively lower temperatures than SOECs, with the use of molten salts as the electrolyte. For example, by using the molten CsH2PO4 salt with superconductivity as the electrolyte, CO2 reduction can be performed at a low temperature of 350 °C. (18) Furthermore, molten salts possess advantages of a wide electrochemical window, high ionic conductivity, and excellent durability. (19−21) In recent years, extensive studies have focused on the electrolysis of carbon dioxide by molten salts to prepare a variety of high-value carbon materials, such as amorphous carbon, (22) graphene, (23) carbon nanotubes, (24) carbon spheres, (25) etc., which have great potential for applications as anode materials for batteries. (26) In addition, molten salt electrolysis can also convert CO2 into clean fuels, such as carbon monoxide, (27) syngas (a mixture of H2 and CO), (28) methane, (29) and long-chain hydrocarbon fuels. (30,31) However, current molten salt electrolysis is mainly based on a single cell made up of magnesium oxide or corundum crucibles. The disadvantage of such an electrolytic cell is that the gaseous products from the reactions taking place on both the cathode and anode are released together and with the remaining CO2. (28,32,33) In this case, further separation of gaseous products is needed, which significantly increases the operation cost. (34,35) In addition, the electrolysis system has been mainly based on the one-pot method in which the molten salts have to be replaced with new ones after each run. (36,37) This leads to a discontinuous process of the capture and conversion of CO2 and thus a limited CO2 conversion efficiency and eventual difficulty in practical applications. To date, the anode catalysts used for the molten salt electrolysis mainly include the iron-based, nickel-based, tin-based materials, or their alloys. The cathode catalysts are mainly commercial materials, such as iron, nickel, and titanium. (8,33,38) There are also a few studies using metal oxides and liquid metals as cathodes to promote the carbon dioxide reduction by the in situ formed nanoparticles. (39,40) However, the research about the catalysts used in molten salt electrolysis has still been limited, and it is still highly desirable to develop highly active, selective, and stable catalysts for CO2 conversion in the molten salt system. Therefore, challenges remain to achieve continuous and efficient electrochemical reduction of CO2 from molten salt systems.

Figure 1. (a) Schematic illustration of the homemade reactor and corresponding working mechanism. (b) Electrolysis voltage during the electrolysis. (c) Corresponding concentrations of reductive gaseous products.