(Invited) Carbon Dioxide Solvation and Transport in Molten Carbonates

Abstract

Molten carbonates have properties of interest for practical use as electrolytes: they are highly conductive materials that remain liquid over a wide range of temperature and pressure and can dissolve very efficiently volatiles like water and carbon dioxide. Molten carbonates are nowadays successfully used as electrolytes in Molten Carbonate Fuel Cells (MCFC), with systems reaching 60 MW. They are also good candidates for electroreduction of CO2 as potential Carbon Capture and Storage (CCS) devices, for which there are already small scale attemps. Understanding the reactivity, speciation and transport of CO2 in molten carbonates is crucial for optimising the design of these devices. Here, we present atomistic simulations of molten carbonates and of CO2 dissolved in them. We show that CO2 interacts in a very specific way with the carbonate anions, forming transiently a new species, the pyrocarbonate anion C2O5 2-. This property explains the very high solubility of CO2 in molten carbonates compared to other molten salts. We also demonstrate that through the equilibrium between CO2 and C2O5 2-, the identity of CO2 is quickly lost through O2- exchanges. The transport of CO2 in molten carbonates thus occurs through a Grotthus type mechanism, leading to CO2 diffusion being three times faster that anion or cation diffusion. We will further discuss some consequences of these findings for CO2 electrolysis in molten carbonates.