Sodium and fluoride ions are not discharged during the electrolysis of aluminium oxide because they have a higher reduction potential compared to aluminium. This means that it requires more energy to reduce sodium and fluoride ions than to reduce aluminium ions. As a result, the aluminium ions are preferentially discharged at the cathode during the electrolysis process, while the sodium and fluoride ions remain in the electrolyte. Additionally, the production of sodium or fluoride metal would not be economically viable in the context of aluminium production.
