Electrochemical behavior of Dy2O3 on a MCE and liquid Sn electrode
The cyclic voltammogram for the MCE electrode in the purified CaCl2 melt is shown by the black dashed line in Fig. 4. It can be seen that there are no electrochemically active impurities in the melt after the pre-electrolysis. The redox wave at E’ and the peak E represent the reduction of Ca2+ and the oxidation of metallic Ca, respectively. The solid red line in Fig. 4 is the cyclic voltammograms of the MCE with 0.01 wt% Dy2O3 loaded between a vertex potential of 0 and -1.6 vs Pt. Two pair of redox peaks were observed.
Two corresponding redox peaks are located at -1.38 V and -1.18 V vs Pt (B’ and B). Two other (less obvious) redox peaks are at -0.85 V and -0.76 V vs Pt, labeled A’ and A. The cathodic current peak A’ is thought to be caused by the reduction Dy(III) to Dy(II). Hence, according to the above analysis, the electrode reactions are:
Fig. 2 The EDX mappings of Cl, Ca, O and Dy of Dy2O3 after quenching
Next, a liquid Sn electrode was employed as the working electrode to study the electrochemical behavior of Dy2O3 in molten CaCl2.The image shown in Fig. 5 is the liquid tin electrode used in this experiment. Dotted and solid curves in Fig. 5 designate the CVs before and after the addition of Dy2O3, respectively. In both cases, the anodic peak at about 0.17 V vs Pt corresponds to the dissolution of the Sn electrode. In addition, the current starts to increase at about -0.98 V vs Pt in the
Fig. 3 SEM images of Dy2O3 a as starting material; b after two hours in molten CaCl2 at 1173 K (900 °C)
Fig. 4 Cyclic voltammograms in CaCl2 melt on a MCE (S = 9.3 10-2 m-2) with and without 0.01 wt% Dy2O3, Temperature: 1173 K (900 ° C), scan rate: 50 mVs-1
Fig. 5 Cyclic
voltammograms of electrode in CaCl2 melt obtained on a liquid Sn electrode (S = 1.96 10-3 m-2) with and without 0.01 wt% Dy2O3. Temperature: 1173 K (900 °C), scan rate:
cathodic direction. The reduction peak A’ in the solid curve at about -0.65 V vs Pt corresponds to the reduction of Dy(III) ions to Dy at the liquid Sn electrode.
Thus, it can be inferred that the number of electrons exchanged should be three for the Dy(III) reducing on a liquid tin electrode. The electrode reaction can be written as:
From what has been discussed above, two completely different mechanisms were observed. There are two reduction steps for Dy2O3 on a MCE but only a single step on a liquid Sn electrode. One possible explanation for the difference is that compared to the solid cathode, the metal element can be dissolved in a liquid cathode, which causes a change of the activity and deposition potential of the metal element at liquid cathodes .