Anodic Behavior of Carbon Electrodes in CaO ‐ CaCl2 Melts at 1123 K

Abstract

The anodic process on glassy carbon electrodes in molten CaCl 2 containing CaO was investigated at 1123 K by cyclic voltammetry and convolution voltammetry. It was found that the oxidation sequence occurs through a consecutive two-step electrochemical reaction: O 2Ϫ (diss) ϩ xC r C x O(ads) ϩ 2e and O 2Ϫ (diss) ϩ C x O(ads) r CO 2 (gas) ϩ (x Ϫ 1)C ϩ 2e. Both steps were found to be irreversible. Experimental data indicate that the first electrochemical step is preceded by the dissociation of a complex. The diffusion coefficient of the oxygen-containing species, the charge-transfer coefficient, and the charge-transfer rate constant were found equal to 2.0 ϫ 10 Ϫ6 cm 2 s Ϫ1 , 0.43, and 1.8 ϫ 10 Ϫ5 cm s Ϫ1 , respectively.

Influences of graphite anode area on electrolysis of solid metal oxides in molten salts

“…1a, b, the insets are the enlarged portions of the respective CVs between 0 and 1.30 V, showing three small and irreversible oxidation waves, A2, A3 and A4. These features are not identical with previous observations on glassy carbon electrodes [14], but may be attributed to the trace amount of O 2− ions resulting from hydrolysis of CaCl 2 during thermal drying in air. To further confirm this attribution, CaO was purposely added to the molten salts, and CVs were recorded as shown in Fig.…”

“…2a. Of these, A4 and A3 were more dominant in response to increasing the potential scan rate, seemingly corresponding to reactions (4) and (5), respectively, according to the literature [14]. It was observed that at high-potential scan rates and low CaO concentrations, a reduction wave C4 appeared as shown in Fig.…”

“…At the lowest concentration of added CaO (3.3×10 −5 mol cm −3 which is the same as that reported in ref. [14]), four irreversible oxidation waves appeared on the CVs as shown in Fig. 2a.…”

“…An early study of the anodic behaviour of the O 2− ion was performed on a glassy carbon working electrode in molten CaCl 2 [14]. Two irreversible oxidation current waves were observed on the cyclic voltammograms (CVs).…”

“…At potentials about 0.15 V more positive than the current peak potential for reaction (5), a fast increasing current appeared on the CVs. This oxidation current obviously defines the positive limit of the potential window of the molten salt and was thought to have resulted from Cl − ion oxidation on glassy carbon [14,15]. However, thermodynamically, the following two reactions differ by 1.607 V in cell voltage at 850°C.…”