On the development of metallic inert anode for molten CaCl2–CaO System

Some fundamental aspects related to inert anode development in molten CaCl2–CaO were investigated based on thermodynamic analysis, electrochemistry of metals and solubility of oxide measurements. The Gibbs free energy change of several key anodic reactions including electro-stripping of metals, electro-formation of metallic oxides, electro-dissolution of metallic oxides as well as oxygen and chlorine evolution was calculated and documented, for the first time, as a reference to develop metallic inert anode in chloride based melts. The anodic behaviors of typical metals (Ni, Fe, Co, Mo, Cu, Ag, and Pt) in the melt were investigated. The results confirmed the thermodynamic stability order of metals in the melts and revealed that surface oxide formation can increase the stability of the electrodes in CaO containing melt. Furthermore, solubility of several oxides (NiO, Fe2O3, Cr2O3, Co3O4, NiFe2O4) in pure CaCl2 or CaCl2–CaO melts was measured to evaluate the stability of oxide coating or a cermet inert anode in the melt. It was found that the solubility of NiO decreased with increasing CaO concentration, while that of Fe2O3 increased. Ni coated with NiO film had much higher stability during anodic polarization.

Research highlights
► Stability of metals and oxides in molten CaCl2 and CaCl2–CaO was evaluated for the first time.
► An electrode–metal diagram was constructed to guide the development of inert anode in the melt.
► Solubility of typical oxides and compounds in the melts were measured for the first time.
► Two kinds of dissolution mechanism of oxides in the melt were revealed.
► A concept of oxide coated metallic inert anode in the melt was demonstrated by Ni/NiO film anode.