Aluminum production is made by alumina electrolysis (Al₂O₃) in a molten mixture of cryolite (Na₃AlF₆-AlF₃-CaF₂-Al₂O₃) at high temperature (960°C), between a liquid aluminum cathode and a carbon reactive anode:

2Al₂O_3(cry) + 3C_(s) → 4Al_(l) + 3CO_2(g)

As the production of one ton of aluminum causes emission of 1.2 tons of CO₂ [1], an inert anode is under development to form O₂.

Anodes made of iron and nickel mixed spinel oxides have been studied since the 80's. As spinel oxides have no sufficient electrical conductivity, addition of a metallic phase was proposed in the 80's, creating the so-called CERMET (CERamic – METal), with a metallic phase (Ni, Cu and Fe) and a conductive oxide phase (Ni_xFe_3-xO₄ & Ni_1-xFe_xO). In the spinel structure, tetrahedral and octahedral sites are occupied by ions of different valences (eg: Fe²⁺/Fe³⁺), creating an electrical conduction thanks to an electron hopping phenomenon.

CERMET anodes were tested at 0.8A/cm² for different durations (from 30min to 8h) and analyzed by SEM, coupled with the energy-dispersive x-ray spectroscopy (EDX). First experiments revealed both chemical and electrochemical degradation: dissolution of the oxide phase and oxidation of the metallic phase.

To investigate this complex system, experiments without polarization were firstly performed. Results showed that the dissolution of oxide phase is due to a significant substitution phenomenon between Al³⁺ in the electrolyte and Fe³⁺ in the spinel phase [2], leading the formation of a soluble aluminate Ni_xFe_3-x-yAl_yO₄. To study the Al³⁺ impact on the aluminate formation, LiF-CaF₂ solvent was selected and Al³⁺ ions were added as a solute at different concentrations. It was demonstrated that the degradation rate of the oxide phase increases with the Al³⁺content and was estimated by measuring the amount of dissolved oxide phase (Optical Microscopy) vs the immersion duration in the electrolyte.

Then, studies under galvanostatic electrolysis were carried out and new phases were identified: FeF₂, Cu₂O and Ni_1-x-yFe_xCu_yO monoxide.

Thanks to these observations and thermodynamic calculations, a global degradation mechanism of the anode was proposed.

[1] K.C. Grjotheim, C. Krohn, M. Malinovsky, K. Matiasovsky, J. Thonstad, "Aluminium electrolysis: the chemistry of the Hall-Heroult process", Aluminium Verlag, Düsseldorf, 1977.
[2] B. Gillot, A. Rousset, "On the limit of Aluminum substitution in Fe₃O₄ and γ-Fe₂O₃", Phys. Stat. Sol, 1990.