XAFS Analysis of Sulfuric Acid Solution Related to the Effect of Bi in Suppressing the Disproportionation Reaction of Mn3+ in Mn/Ti Redox Flow Cells (Supporting Information)

In sulfuric acid (H2SO4) aqueous solutions containing manganese (Mn) and titanium (Ti) ions, which are used for the positive electrolyte in redox flow (RF) batteries, the addition of a small quantity of bismuth (Bi) has been found to be very effective in suppressing the MnO2 precipitation in the charging process from Mn2+ to Mn3+. The effect of the addition of Bi on the structural change in these solutions has been analyzed using x-ray absorption fine structure (XAFS) techniques. X-ray absorption near edge structure (XANES) spectra indicated that some Mn4+ ions were generated only when Bi was not added. It was also confirmed that the Mn-O octahedral coordination was maintained at a high state of charge (SOC). On the other hand, no change was observed in the valence of Ti and Bi ions even at high SOCs. Extended x-ray absorption fine structure (EXAFS) measurements combined with fitting analysis clarified that the addition of Bi resulted in the generation of Mn-O-Bi coordination at the SOCs of 70 and 90 %. Bi-O-Mn coordination may suppress the MnO2 precipitation at a high SOC.