X-ray photoelectron spectroscopy study of sodium reactions in carbon cathode blocks of aluminium oxide reduction cells

Aluminium oxide reduction was performed in a laboratory electrolysis cell with different industrial carbon cathode blocks (semi-graphitic, graphitic, and graphitized blocks). During electrolysis, sodium species migrate from the bath into the carbon cathode. Consequences of this migration include expansion of the blocks—the so-called sodium swelling—that may lead to failure of the cell. Characterisation of the blocks by XPS indicated that in addition to ionic sodium species (e.g. NaF and NaHCO3), two different types of metallic sodium were present in the cathodes. One type of metallic sodium is associated with a degradation of the graphitic structure, suggesting that this sodium is intercalated between the graphene layers, whereas the other type of metallic sodium was most probably present in micropores. Both types of metallic sodium were detected in semi-graphitic blocks while only the “micropore” sodium was found in graphitic and graphitized blocks. The metallic sodium was remarkably stable in the laboratory atmosphere, probably due to the fact that, after electrolysis, the entire porosity of the carbon cathode is filled with penetrated bath. This limits the access of oxygen and humidity to the metallic sodium.