Anode Properties of MoSi2 Containing Nb and Si in MgCl2-NaCl-CaCl2 and Influence of Electrolytic Potential on It
Authors: Suehiro Riku , Takenaka Toshihide , Matsumoto Toshiharu , Nagayasu Katsushi 
Published: December 2024

Introduction
Mg metal is produced by electrolysis in a molten chloride bath. 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 the anodic behavior of MoSi2 in molten chloride containing O2- has been investigated in expectation of the protection by formed oxide film on it. A SiO2 film was formed on the MoSi2 anode in molten MgCl2-NaCl-CaCl2 containing MgO, and the dissolution of MoSi2 was suppressed by the formed oxide film. It was also shown that the addition of a small amount of Nb or Si improved the anodic properties of MoSi2, but sufficient electrolytic current was hardly applied so far. In this study, the effect of a simultaneous addition of Nb and Si in MoSi2 was studied for the improvement in its anodic property.
Experimental method
MoSi2 with a small amount of Nb and Si was prepared by arc melting, and then shaped cylindrically to use as working electrode. All the electrodes in this study contained 0.6 at% Nb based on our previous study. A Cu plate and a Ag/AgCl couple were used as counter and reference electrode, respectively. The potential of the Ag/AgCl electrode was calibrated with the Mg metal deposition potential measured by cyclic voltammetry. Electrolysis was conducted at 700K in an Ar atmosphere. The anodic behavior of MoSi2 was investigated by cyclic voltammetry, and potentio-static electrolysis was performed at 2.7V (vs. Mg/Mg2+, the same hereinafter). 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. Assuming the three anodic reactions on the MoSi2, such as oxide film formation, Mo elution and the other reactions, their current contributions were estimated from the weight change and average film thickness.
Result and Discussion
The shape of the cyclic voltammogram did not change significantly by the addition of Nb and Si in the MoSi2. A sharp anodic current peak was observed in the cyclic voltammogram around 2.0V regardless of Nb and Si addition, and an anodic current bump was observed above 2.5V. It was considered that the peak around 2.0V corresponded to the oxide film formation, and the fluctuation in the current of this peak changed with the amount of Si addition. The current bump around 2.5V is thought due to a gas generation, and its current increased with the Si addition. Figure 1 shows the influence of Nb and/or Si addition in MoSi2 on the current during the potentio-static electrolysis at 2.7V. Gas bubbles were observed immediately after the start of electrolysis regardless of the Nb and/or Si addition, and the blue fog was observed with an initial current attenuation. The current after the initial attenuation showed gradual change, and was strongly affected by the Si addition. The current became maximum at the electrode with 1.0 at% Si, and the smaller and lager Si addition seemed to cause the decrease in current. An oxide film was formed on the MoSi2 surface more uniformly at the electrode with Si than at the electrode without Si. The current contributions depended on the Si content, and it was shown that the most of current spent the reaction other than the film formation and Mo elution at the electrode with 1.0 at% Si. Since the reaction other than the film formation and Mo elution was thought a gas generation reaction, the MoSi2 seemed to behave as inert anode. From the results in this study, it is concluded that the 1.0 at% addition of Si with 0.6 at% Nb to MoSi2 is appropriate for the improvement of its anode properties.
Acknowledgment
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