Anodic Overpotential on Metallic Oxygen Evolving Anodes for Aluminum Electrolysis in Low Temperature Electrolyte of KF-Naf-AlF₃-Al₂O₃

Abstract

To eliminate climate gas emissions from aluminum electrolysis, significant research and development efforts focus on non-consumable oxygen evolving anodes to replace the carbon anodes used in the Hall Héroult process. Modifying the cryolite-based electrolyte currently used, by partly replacing Na with K in the molten fluoride, opens up a range of metallic alloys as feasible materials for inert anodes. In this work, the performance of Cu-Ni-Fe anodes, with or without surface treatment, is benchmarked against Pt and the pure components of the alloy.Anodic overpotential using an inert anode with low-temperature electrolyte, 750–800 ⁰C, for aluminum reduction should be low to compensate for ~1.10 V higher reversible potential for O₂ evolution than in conventional Hall-Héroult process at 960 ⁰C. In this study, voltammetry methods were applied in a low-temperature electrolyte mixture of KF-NaF-AlF₃-Al₂O₃ at 800 ⁰C. Oxygen evolution overpotential on pure Pt anode was measured using steady-state polarization. Tafel plots were obtained, and kinetic parameters were derived in the form of Tafel coefficients and exchange current density. A theoretical treatment for anodic process on Pt electrode with a two-step electron transfer mechanism is developed.Some anodic polarization curves were obtained for pure metals of Cu, Ni, Fe, and alloy of Cu-Ni-Fe in KF-NaF-AlF₃-Al₂O₃ at 800 ⁰C. Prior to use, some anodes of Cu-Ni-Fe alloy were oxidized in air at 800 ⁰C for 8 h and found to perform relatively better than an untreated alloy of the same composition. Anodes following electrochemical measurements were characterized by scanning electron microscope (SEM) coupled energy dispersive spectroscopy (EDS).