Voltammetric investigations on the transition between dissolution, passivation and deposition characteristics of Ni, Cu and their alloys in fluorine based ionic liquid

Highlights: → Comparison on the voltammetry of Ni, Cu and their alloys in TEA.3HF ionic liquid. → Ni and its alloys in this medium exhibit passivity and irreversibility. → Reversible behavior of Cu in this ionic liquid suggests that the former can function as a reference electrode. → Relative stability of the electrodes in solvents/fluoride medium decreases in the order: Ni > Monel > Ni-Cu alloy > Cu. → Relative solubility of metal fluoride films in the three solvents increases as: PC < sulfolane < AN. - Abstract: Multisweep cyclic voltammetric (CV) responses of nickel, copper, Monel and nickel-copper alloy had been extensively studied and compared in a variety of non-aqueous solvents such as acetonitrile (AN), propylene carbonate (PC) and sulfolane containing triethylamine trishydrogen fluoride (TEA.3HF) ionic liquid. The quantity of dissolution as well as surface morphological transformation on the electrode surfaces as a result of anodic polarization were investigated using atomic absorption spectroscopy (AAS) and scanning electron microscopy (SEM) respectively. The nature of crystallites formed on the polarized electrode was characterized using X-ray diffraction (XRD). The voltammetric study clearly indicates that Ni, Monel and Ni-Cu alloy are passive and stable in neat TEA.3HF medium in the recorded potential region of CV. Surface morphology of Ni after polarization, reveals the generation of pits, whereas the evolution of small crystallites of CuF2 are noted on the polarized alloy material, as evidenced by SEM pictures. Copper electrode shows reversible voltammetric characteristics with high charge recovery ratio (qc/qa) suggesting that in this medium, Cu can certainly serve as reference electrode. Addition of water in TEA.3HF medium increases the solubility and stability of these metal fluoride film. In solvents such as PC, AN and sulfolane containing TEA.3HF, Ni and their alloys exhibit remarkable passivity and the charge recovery ratio decreases to some extent for Cu. In TEA.3HF/AN medium, the dissolution of Cu is very high. The present investigation suggests that the relative stability of all the four electrodes in neat TEA.3HF and solvents containing 0.1 M TEA.3HF decreases in the order: Ni > Monel > Ni-Cu alloy > Cu and relative solubility of metal fluoride films in the three solvents increases in the order: PC < sulfolane < AN.