Anodic MnO2 deposition on Titanium Electrode Coated with Thermally Prepared Manganese Oxide
Authors: Yoshida Yuuki , Kawaguchi Kenji , Morimitsu Masatsugu 
Published: February 2020

Zinc electrowinning uses an acidic zinc sulfate solution prepared from zinc ore, which contains some other metal ions such as Mn2+. The solution after zinc electrowinning goes back to the solution preparation process, in which the zinc concentration is recovered to the appropriate level, while that of Mn2+ is increased, if no Mn2+ is consumed by anodic deposition in electrowinning cell, which is possible with amorphous RuO2-Ta2O5/Ti anodes [1]. Therefore, the situation that Mn2+ concentration becomes high enough to recover Mn2+ as manganese oxide is expected, and it would resemble the commercial production process of EMD (Electrolytic Manganese Dioxide) that is famous as the positive electrode material of some batteries. For such a novel manganese recovering system in zinc electrowinning, the anode for deposition of MnO2 from zinc electrowinning solution is needed, with which MnO2 is deposited on and harvested from the anode that should be durable for the deposition and harvesting cycle. In this study, we prepared the titanium electrode covered with manganese oxide as the catalytic layer for MnO2 deposition from zinc electrowinning solution, and the obtained electrode was characterized by XRD, SEM, and EDX. The polarization measurement and continuous electrolysis of the anode in Mn2+ containing solutions were also performed to examine the obtained product on the anode and the current efficiency.
The manganese oxide coated titanium electrode was prepared by thermal decomposition of the precursor solution containing Mn2+ painted on a titanium substrate which had been degreased in acetone and etched in 10% oxalic acid. The obtained electrode was analyzed by XRD, SEM, and EDX. The anodic polarization of the anode in H2SO4 solution with and without Mn2+ was examined by cyclic voltammetry with a conventional three-electrode cell equipped with a platinum plate counter electrode and an Ag/AgCl reference electrode in saturated KCl solution. Constant current electrolysis was also performed to obtain MnO2 on the anode, and the weight of the anode before and after the electrolysis was measured to know the amount of the product which was characterized by XRD. The electrolytic solution was 2 mol/L H2SO4 + MnSO4 and used at a temperature from 40 oC to 75 oC.
The catalytic layer prepared by thermal decomposition was found to be Mn2O3 by XRD measurements. Constant current electrolysis with the anode was carried out under different conditions, in which the typical examples are 40 o C or 75 o C as the electrolyte temperature with electrolysis at 5 mA/cm2 for 30 min. For these conditions, the product on the anode was obtained with the increase of the anode’s weight by electrolysis, and the XRD results showed the diffraction pattern corresponding to MnO2, although the diffraction peak intensity was weak for the product obtained at 40 oC compared to that at 75 oC. The current efficiency for MnO2 deposition was calculated with the assumption that no oxygen evolution occurs, and the results were 17.5% at 40 oC and 70.6% at 75 oC. It was also found that the electrolyte was transparent before constant current electrolysis, which was unchanged by the electrolysis at 75 oC, while that became dark red at 40 oC, implying that Mn3
+ is generated at 40 oC and the oxidation of Mn2+ to MnO2 is not completed. More detailed results on the effects of the electrolysis conditions on the obtained product and the current efficiency will be shown in this paper.

This work was financially supported by “Kyoto Super Cluster Program” of Japan Science and Technology Agency (JST).

Reference
[1] T. Zhang, Ph.D. Thesis, Doshisha University (2015).