(Digital Presentation) Anodic Formation of Oxide Film on MoSi2 Containing Al or Nb in LiCl-KCl Molten Salt
Authors: Irie Shunsuke , Morishige Taiki , Takenaka Toshihide , Matsumoto Toshiharu , Hirose Masanori , Nagayasu Katsushi 
Published: November 2022

Li and Mg metals are produced by electrolysis in a molten chloride bath. Currently carbon-based material is used as anode. However, carbon-based anode reacts electrochemically with oxide ions dissolved in the bath, which causes its undesirable consumption. In our laboratory, a new type of inert anode by forming a thin and dense oxide film has been studied. It was shown that a SiO2 film was formed on MoSi2 anode in molten LiCl-KCl melt with Li2O, and that the dissolution of MoSi2 was inhibited by the formation of oxide film. However, sufficient electrolytic current was hardly applied. In this study, the modification of the anodic properties of MoSi2 has been attempted by the addition of Nb or Al in MoSi2.

A MoSi2 rod containing Nb(0.1~1.0mol%) or Al(0.1~0.5mol%) was prepared by sintering from MoSi2, Nb and Al powder. The prepared rod was pre-oxidized (1573K, 4h) to form a SiO2 film in advance, and then used as working electrode. An Al plate and Ag/AgCl were used as counter and reference electrode, respectively. The potential of the Ag/AgCl electrode was calibrated against the Li metal deposition potential measured by cyclic voltammetry. Electrolysis was conducted at 700K in an Ar atmosphere. Anodic behavior of MoSi2 was investigated by cyclic voltammetry, and potentio-static electrolysis was performed. The weight change in MoSi2 anode was measured, and the surface and cross section of the MoSi2 electrode were observed and analyzed by SEM-EDX and XRD.

The shape of the cyclic voltammogram did not change significantly by the addition of Nb or Al in the MoSi2. Anodic peaks were seen around 2.5V (vs. Li / Li+, the same hereinafter) and around 3.6V. Above 3.6V, anodic current continued to flow with gas bubble formation. The current at 3.6V in the cyclic voltammogram changed with the added amount of Nb or Al as shown in Fig.1, which suggests the Nb or Al addition strongly influenced the anodic property of MoSi2.

Current during potentio-static electrolysis at 4.2V increased with current oscillation. It was seen that a black film peeled periodically, and the current oscillation seemed to link the peeling. The quantity of electricity increased with Ni or Al addition in MoSi2. An oxide layer was observed on the MoSi2 surface after potentio-static electrolysis, and a dense layer was formed on MoSi2 containing 0.5~1.0mol% Nb. The weight of MoSi2 decreased by potentio-static electrolysis, and Nb or Al addition in MoSi2 clearly inhibited the weight change. The addition of 0.5~1.0mol%-Nb seemed suitable from the improvement of the anodic property of MoSi2.

It was indicated that Nb or Al addition in MoSi2 influenced its anodic property, and that the addition inhibited its anodic consumption in molten LiCl-KCl. However, it was shown that the formation and peeling of the surface film occurred repeatedly, so the formation of a durable surface film should be necessary.

The results of this research were obtained as a result of the commissioned work (JPNP 14004) of the New Energy and Industrial Technology Development Organization (NEDO).

Figure 1