First-Principles Modeling of Transport Mechanisms in Carbonate–Hydroxide Electrolytes

We performed ab initio molecular dynamics simulations of a molten [Li_0.6K_0.4]₃CO₃OH electrolyte containing dissolved CO₂ and confirmed the presence of pyrocarbonate, bicarbonate, and water along with the constituent ions and molecular CO₂. Our calculations indicate kinetics-driven formation of pyrocarbonate whereas bicarbonate and water are thermodynamically favored. Our results also demonstrate the presence of water at higher concentrations (double or more) than that of CO₂, which reinforces the conclusions in our earlier work [AIChE J. 2020, e16988] based on chemical reaction equilibrium simulations. Structural analysis indicates a larger distortion in water geometry, due to its higher polarizability compared to the nonpolar CO₂, explaining the higher reactivity and smaller average lifetime of H₂O in the melt. The computed lifetime distributions of the reaction products reveal that the bicarbonate ion lives the shortest among all the species present in the system. It initiates a sequence of successive proton exchange events; such sequences of exchanges along a hydrogen-bonded network gives the Grotthuss mechanism for proton transport in liquid water. The estimated proton diffusion, based on a random walk model, is about 30 times faster than the hydroxide diffusion obtained from classical molecular dynamics simulations. We believe that the presence of proton transfer events in the system has a large impact on the overall ion dynamics and electrical conductivity of the medium.