Natural Convection in Molten Salt Electrochemistry

Molten salt electrochemistry has been widely used in many fields, especially for metal extraction/refinement. The understanding of mass transfer in molten salts under harsh operation conditions is of great importance to reveal reaction mechanisms and advance fine technologies. It has been generally assumed that natural convection is negligible in stagnant molten salt electrochemistry. Herein, we report an abnormal natural convection in molten LiCl-KCl, with the arising time from 2.37 s at 873 K to 10.13 s at 673 K. Using the concentration correction factor, the derived thickness of the natural convection-diffusion layer (δ_conv.) was found to be ranging from 128 to 163 μm, much thinner than those in aqueous solutions (∼200 μm). The simulations showed that the notable natural convection resulted from convection-diffusion layer (CDL) convection dominated over the density-driven convection even at high redox concentrations, implying the severe vibration of molten salt systems. To suppress the intense natural convection, we predicted that the use of microelectrodes (with radii less than 23.2 μm for δ_conv. = 150 μm) would be a promising tool, regardless of their inferior stabilities in high-temperature molten salts at this stage. These innovative findings offer insights into the impact of natural convection on mass transfer in molten salts that have not been previously revealed.