Investigation on adsorption of sodium fluoro-aluminates on graphite by density functional theory

Abstract

The carbonous materials have been widely used as electrodes in the current aluminum industry. Cathode wear has been an unavoidable problem and limits the lifetime of electrolysis cells in aluminum production, which is mainly due to the reaction between molten cryolite salts and the graphite surface, formation of an aluminum carbide layer on the surface of the cathode, sodium penetration, and abrasion of molten salts. However, the mechanism of cathode wear is still not fully understood, and not clarified at the atom level. In this paper, density functional theory based calculations were performed to investigate the reactivity properties between fluoro-aluminates and graphite surfaces. The adsorption energy and optimized structures of three typical fluoro-aluminates on various sites were obtained, indicating that chemical adsorption has occurred, and Na2AlF5 was easier than Na3AlF6 and NaAlF4 to adsorb on the graphite surface. The adsorption sites only cause a slight difference in adsorption energy since the size of fluoro-aluminates is larger than the distance between various sites. Moreover, the charge density differences, density of states, and partial charge density were calculated to illustrate the electron transfer between fluoro-aluminates and the graphite cathode. During the adsorption of fluoro-aluminates on the graphite surface, electrons would transfer from sodium to the graphite cathode. Meanwhile, the electrons of carbons are attracted by fluorine atoms. However, there is no electrons transfer directly between the aluminum atom and the graphite surface.