Chem. Res. Chin. Univ., 2014, 30(5), 800—805 doi: 10.1007/s40242-014-4137-4 ——————————— *Corresponding author. E-mail: popescuamj@yahoo.com Received April 16, 2014; accepted May 8, 2014. Surpported by the Research Program of the Ilie Murgulescu Institute of Physical Chemistry of the Romanian Academy. © Jilin University, The Editorial Department of Chemical Research in Chinese Universities and Springer-Verlag GmbH Oxygen-evolving SnO 2 -based Ceramic Anodes in Aluminium Electrolysis POPESCU Ana-Maria * Ilie Murgulescu Institute of Physical Chemistry of the Romanian Academy, 060021-Bucharest, Romania Abstract This study deals with SnO 2 -based ceramic anodes doped with Sb 2 O 3 and CuO, aiming at contributing new data regarding their electrochemical behavior in cryolite melts. The performances of the anodes were evaluated by anodic polarization, cyclic voltammetry, and current efficiency and corrosion measurements. The investigation proves that the anodic process of SnO 2 -based inert anodes occurs at a low overvoltage and the oxygen discharge takes place in one step with an exchange of two electrons. The current efficiency and corrosion were proved to be dependent on the electrolysis parameters and composition of electrolysis bath. For a long term electrolysis, the dissolution of the anode in the cryolite-alumina melt produced small aluminium contamination(ca. 0.2%, mass fraction). Keywords Inert anode; Aluminium electrolysis; Polarization; Voltammetry 1 Introduction The Hall-Heroult process has some inherent weaknesses, and the most obvious one is the consumable carbon anode that needs to be changed frequently, which causes the emission of greenhouse gases CO 2 , CF 4 and C 2 F 6 and the sulphur gases SO 2 , COS, and H 2 S [1] . During the last 33 years, great efforts have been made to develop a so-called inert anode, i.e., a non-consumable, oxygen-evolving anode. Apart from saving carbon and being more environmentally friendly, inert anodes might also allow for a more compact cell design, with possible savings in capital and energy [2] . Candidate anode materials are ceramics, cermets and metals [3—7] . Inert anodes can probably be operated at a substantially shorter anode-cathode distance (ACD) than carbon anodes and the total cell voltage has been estimated to be lowered by 20% by means of inert anodes in aluminium cells [6,7] . For this and other reasons the study of inert anodes for aluminium electrolysis is of considerable interest. As the principal requirements for inert anodes are good elec- tronic conductivity and chemical stability vs. the electrolyte and the oxygen gas, the materials that meet these requirements and generate much interest are certain ceramic oxides [8—10] . One of such materials is SnO 2 , which is thus chosen for the present work because of its good electrical conductivity, easily prepa- ration and acceptable corrosion resistance [8,9,11] . The experimental data discussed in this paper were re- ferred to anodic overvoltage, cyclic voltammetry and current efficiency, correlated structural changes in SnO 2 -based inert anode during the aluminium electrolysis. 2 Experimental The electrolysis decomposition of alumina dissolved in NaF-AlF 3 melts, with the use of inert anodes, has the overall reaction: Al 2 O 3 =2Al+3/2O 2 (1) while in the conventional Hall-Heroult process for aluminium production, with the use of consumable carbon anodes, the overall reaction is Al 2 O 3 +3/2C=2Al+3/2CO 2 (2) The reversible electromotive force(EMF) of this process at 1000 °C is –1.169 V with respect to –2.196 V for reaction (1). This large difference in EMF between those two processes is partly offset by higher overvoltage on carbon(0.4—0.6 V) than on inert anodes(0.1—0.15 V) at a normal current density of 0.7—0.8 A/cm 2[1,12,13] . The used composition of the tin oxide based anodes was 96% SnO 2 +2% Sb 2 O 3 +2% CuO as recom- mended by former studies [11,14—16] . The dopant CuO increases the density of the SnO 2 -based ceramic, while Sb 2 O 3 increases the electrical conductivity. The oxide mixtures were obtained by wet homogenization of the corresponding powders with a particle size of less than 60 μm. Pellets were obtained by pressing oxide mixtures in a double-action cylindrical die at 50 mPa. The pellets were heated to 1200—1300 °C at a rate of 10 °C/min and after cooling at a rate of 20 °C/min, the plate surfaces were polished and gold/copper plated to ensure a good electrical contact with the platinum wire. The ceramic mate- rial-electric cable junction was so achieved as to ensure a pure- ly ohmic contact and a perfectly uniform distribution of the electric field [17] . Structural, ceramic and electrical characteris- tics of the studied anode are shown in Table 1. Details about the working conditions and apparatus are given in previous pa- pers [18—21] , but the electrolysis cell is presented in Fig.1. The decomposition potential and the anodic overvoltage were measured via a steady state technique [20] .