First-Principles Determination of Transference Numbers in Cryolitic Melts

The charge and electron-transport properties of molten ionic systems are among the most relevant properties to consider in the control of several electrochemical processes. First-principles-based equilibrium molecular dynamics (EMD) can provide reliable predictions of both total and partial charge-transport properties. In this work, we calculate the charge-transport properties of the electrolytic bath (Na 3 AlF 6 –AlF 3 –Al 2 O 3 ) of the Hall–Héroult electrolysis cells. We predict both individual and collective charge-transport properties (total and partial conductivities and self-diffusion coefficients) for 11 different compositions typical of industrial conditions via a series of EMD simulations. The predicted total and partial ionic conductivities and their composition dependence are compared to available experimental data. A good agreement is obtained for all studied compositions. From a more fundamental point of view, the microscopic aspect of the charge-transport properties of cryolitic melts is discussed through its correlation with the local structure of different melts. Deviations between the calculated partial conductivities and those derived via the Nernst–Einstein approximation can be explained by the presence of strong short-range ordering within the melts.