Effect of Ti(IV) Ion on Mn(III) Stability in Ti/Mn Electrolyte for Redox Flow Battery

Abstract

The mixed electrolyte of titanium and manganese is one of the promising candidates as low-cost electrolytes for redox flow batteries. In this study, the effect of Ti4+ ion in the positive electrolyte was examined in detail. With an increase of Ti4+ ion concentration, the formation reaction of MnO2 particle was slowed down significantly, which means the stability of Mn3+ ion is enhanced. Moreover, the particle size of MnO2 precipitate became smaller in electrolytes containing certain amount of Ti4+ ion. These MnO2 particles were small enough to be flowed in the electrolyte and to be reduced to Mn2+ ion again. Ti4+ ion also affected the crystal structures of MnO2, limiting to gamma-MnO2 which is assumed to be reactive. In cyclic voltammetry study, the higher Ti4+ ion concentration was, the better reversibility of Mn redox reaction was observed. The ion interaction between Ti4+ and Mn3+ was confirmed by UV-visible spectroscopy.

Redox Reaction in Ti−Mn Redox Flow Battery Studied by X‐ray Absorption Spectroscopy

“…On the other hand, the Ti 4+ ions are mostly reduced to Ti 3+ in the anolyte with SOC of 80% by the charge process. Generally, the oxidation reactions are considered as follows [11–15,43] …”

“…Generally, the oxidation reactions are considered as follows. [11][12][13][14][15]43] Negative Electrode:…”

“…However, the material cost of vanadium is problematic to spread the V RFBs. On the other hand, Ti−Mn RFBs have been studied in order to further improve the energy density and reduce the material costs [11–15] . An important subject for the practical use of the Ti−Mn RFBs is to improve the stability of the catholyte.…”

“…An important subject for the practical use of the Ti−Mn RFBs is to improve the stability of the catholyte. When the state of charge (SOC) is high, precipitates are created in the catholyte, which decrease the energy density [11–15] . Thus, stabilization of the catholyte such as addition of some additives has been attempted.…”

“…In parallel, the chemical state and redox reaction of the electrolytes of Ti−Mn RFBs should be clarified by precise analyses, which lead to establishment of strategies to develop novel electrolytes with high stability. The catholyte/anolyte of Ti−Mn RFBs is generally made with aqueous solutions of MnSO 4 ⋅ n H 2 O, TiOSO 4 ⋅ n H 2 O, and H 2 SO 4 [11–15] . The formal valence of Ti and that of Mn in the initial state are thought to be Ti 4+ and Mn 2+ , respectively.…”