Mechanism of Graphite Electrode Fluorinated in 2.4NaF/AlF₃–Al₂O₃Melt at 1373 K

Abstract

The electrochemical behavior of a graphite anode in 2.4NaF/AlF 3 -0.05wt%Al 2 O 3 molten salt was investigated via cyclic voltammetry (CV) and chrono-electrochemical techniques. Four oxidation current peaks, located at 1.42, 2.5, 3.1, and 3.94 V vs. Al/Al 3+ , were observed in CV curves, which may be attributed to the formation of CO 2 , CF 4 , graphite fluoride ((CF) n ), and F 2 , respectively. The anode effect occurred when the potential exceeded 3.5 V. Meanwhile, the mechanism of the electrode reaction of CO 2 and (CF) n formation was confirmed to be irreversible charge transfer followed by a chemical reaction. Therefore, we confirmed that the passivation of the graphite anode was due to the formation of (CF) n on the electrode surface, blocking the mass transfer. According to the number of the charge transfer and the characteristics of the electrode reactions, we discuss detailed reaction pathways for CF 4 and (CF) n formation. In addition, the presence of the small quantity of C 2 F 6 in the anode gas is discussed.In the primary aluminum production by the Hall-Héroult process, cryolite-alumina molten salt and a carbon anode are employed. The cell reaction under normal conditions isThe exit gas from the electrolysis cell consists of a mixture of CO 2 and CO, and the secondary reactions between CO 2 and aluminum yield CO. 1 However, a cell malfunction, known as the anode effect (AE), can easily occur when the alumina content in the electrolyte is less than 1 wt%. The anode effect results in not only the generation of perfluorocarbons (PFCs) CF 4 and C 2 F 6 but also a high consumption of energy for a higher cell voltage. 2 Owing to the high global warming potentials of CF 4 and C 2 F 6 3 and the fact that the primary aluminum industry is the major anthropogenic source of CF 4 and C 2 F 6 , 4 the Environmental Protection Agency and the primary aluminum producers have established the Voluntary Aluminum Industrial Partnership (VAIP) with the goal of substantially reducing PFC emission. 5 To gain a better understanding of the mechanism of CF 4 and C 2 F 6 generation, the anodic behaviors of carbonaceous electrodes have been widely studied in recent decades. The traditional view mentioned that fluorine ions react with carbon anodes to directly generate CF 4 and C 2 F 6 during the anode effect. 2,6,7 The formation of CF 4 and C 2 F 6 can be expressed by the following reactions:Moreover, CF 4 and C 2 F 6 coming from the decomposition of COF 2 has been suggested. 8-11 COF 2 is considered an intermediate product prior to the anode effect, then, it decomposes to form CF 4 and C 2 F 6 . The reactions are described as 2Na 3 AlF 6 + Al 2 O 3 + 3C = 3COF 2 + 6NaF + 4Al E 1373K = 1.81 V [4] 2COF 2 = CO 2 + CF 4 lg K 1373K = −0.176 [5] 3COF 2 + 2C = 3CO + C 2 F 6 lg K 1373K = −1.351 [6]

Anodic Passivation of a Graphite Electrode in LiF–KF Melt at 773 K

“…30,31 On the other hand, the O 1s envelope can be deconvoluted into four components (shown in Figure 7b), located at 531. 15 30,32 It is clearly confirmed that organic oxygen (O in carboxyl, carbonyl, or alcoxyl groups) originates from the contaminants on the surface of the graphite electrode.…”

“…To determine the anode effect occurring in their systems, [6][7][8][9][10][11][12][13][14][15][16][17] researchers have widely studied the anodic process of graphite electrodes in KF-2HF melts and cryolite system molten salts. Graphite fluoride, (CF x ) n (0.5 ≤ x ≤ 1), is reported to be formed on the carbon anode in KF-2HF melts at temperatures no greater than 393 K, [6][7][8][9] and gaseous CF 4 and C 2 F 6 have been found during the anode effect in cryolite system molten salts at approximately 1233 K. [10][11][12][13][14][15][16][17] In KF-2HF melts, the (CF x ) n , which have high resistivity and poor wettability, cause graphite anodes to be passivated, resulting in a high anodic overvoltage (approximately 3 V). [6][7][8][9] In cryolite system molten salts, the formation mechanism of CF 4 and C 2 F 6 is still in dispute.…”

“…11 A few other researchers also suggest that F − anions react with carbon to form graphite fluoride, and simultaneously, the graphite fluoride decomposes into amorphous carbon, CF 4 , and C 2 F 6 at a high operational temperature. [12][13][14][15] Although the fluorination mechanism of a carbonaceous anode in fluoride melt is still not clarified, those studies are significant because of their industrial application.…”

Thiophenic Sulfur Transformation in a Carbon Anode during the Aluminum Electrolysis Process

“…However, additional gas may be present. The anodic reaction of aluminum electrolysis process was composed by five step-by-step reactions (eqs –). ,, After the O 2– lost its electrons and before it transformed into CO 2 /CO, O atoms existed on the contact surface of anode–molten salt in the form of elemental O gas adsorbed by the carbon chains of anode (C x O). Given that the electrolysis process is continuous, the contact surface of the anode–molten salt could be seen as an O-rich environment where O is produced and consumed constantly.…”

“…During the aluminum electrolysis process (11931233 K), the temperature of carbon anode usually ranged from 773 to 1233 K. − The lowest temperature exists on the top of the anode, away from the Al 2 O 3 –Na 3 AlF 6 molten salt. Meanwhile, the bottom is immersed in the molten salt and experiences the higher temperature.…”