XAFS Analysis of Sulfuric Acid Solution Related to the Effect of Te and Sb in Suppressing the Disproportionation Reaction of Mn3+ in Mn/Ti Redox Flow Cells

In sulfuric acid (H2SO4) aqueous solutions containing manganese (Mn) and titanium (Ti) ions, which were developed to be used for the positive electrolyte in redox flow (RF) batteries, the addition of tellurium (Te) or antimony (Sb) has been found very effective in suppressing the MnO2 precipitation in the charging process from Mn2+ to Mn3+. To investigate its mechanism, x-ray absorption near edge structure (XANES) measurements were carried out for cations in the solution samples at various state of charges (SOCs). In the case of the Te-added solutions, the change rate of Mn ion valence over the SOC was found to very low at the SOCs lower than 30 %. It was also indicated that the valence change of Te ions from Te4+ to Te6+ proceeded at the SOCs below 30 %. On the other hand, no significant change was observed in the valence of Ti at any SOCs. Similar trends were confirmed in the Sb-added solution samples. In conclusion, the supplied charges were considered to be used to increase not only the valence of Mn ions but also that of Te or Sb ions, which resulted in the suppression of the MnO2 precipitation.