Anodic Behaviour of Ni₄₂Fe₃₈Cu₂₀ Electrode in Molten Fluoride Salts

The anodic behaviour of Ni42Fe38Cu20 alloy at different sodium-potassium-cryolite-based electrolyte compositions was examined by chronopotentiometry, chronoamperometry, and linear sweep voltammetry at 1098 K for different electrolyte compositions. Steady-state anodic polarisation curves show that the anodic overpotential in electrolytes with molar ratio CR = (NaF+KF)/AlF3 = 1.4 is significantly less than in electrolyte with CR = 1.3. Short-term galvanostatic polarisation curves indicate that the potential difference between the anode and reference electrode (Al) was stable in electrolytes with CR = 1.4 and potassium ratio KR = KF/(KF + NaF) = 0.3 and CR = 1.4 KR = 0.35. The linear sweep voltammetry method indicates the presence of a diffusion-controlled reaction at a potential below the onset of oxygen evolution, which can be attributed to oxide formation. Tafel curves indicate the presence of the reactions below the potential of oxygen evolution but show a similar overpotential level for the oxygen-evolving reaction as for carbon anodes. Furthermore, the effect of pre-oxidation and influence of consistent polarisation on the anode performance was explored. Pre-oxidised anodes have better stability and lower exchange current density in similar experimental conditions compared to untreated anodes. It was also observed that leaving anodes unpolarised could lead to their instability and fluoridation of both types of anodes, with less severe effect on the pre-oxidised anode.