A study of the passivating fluorocarbon(CF_x) film during electrochemical formation/co-evolution of CF₄ in aluminum electrolysis

In aluminum electrolysis cells, the emissions of perfluorocarbons (PFCs) are not only associated with the high-voltage anode effect (cell voltage > 8 V), but continuous emissions of PFCs have also been detected during cell voltage below 8 V, especially in high-amperage prebake cells. These PFCs are referred to as low-voltage anode effect PFCs (LVAE-PFCs). In this study, the influence of current density on the formation of fluorocarbon (CF_x) film was investigated on an industrial carbon anode with a bottom area of 50 cm² in a high-temperature see-through aluminum electrolysis cell operating at 940 °C. The formation of CF_x films on localized areas of the anode under normal electrolysis conditions was confirmed through observations. The quantity of peeled CF_x films and the value of the back electromotive force gradually increase with increasing current density. Simultaneously, the thermal decomposition of the CF_x films results in the generation of PFCs. Under conditions of sufficient alumina concentration, the peeling of the CF_x films contributes to a reduction in the cell voltage. X-ray photoelectron spectroscopy analysis revealed that regions with high current density on the carbon anode provide favorable conditions for the formation of CF_x components with high resistance characteristics, inevitably leading to an increase in cell voltage.