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

We performed X‐ray absorption studies for the electrolytes of a Ti−Mn redox flow battery (RFB) to understand the redox reaction of the Ti/Mn ions and formation of precipitates in charged catholyte, because suppression of the disproportionation reaction is a key to improve the cyclability of Ti−Mn RFB and enhance the energy density. Hard X‐ray absorption spectroscopy with a high transmittance and soft X‐ray absorption spectroscopy to directly observe the 3d orbitals were complementarily employed. Moreover, the Ti/Mn 3d electronic structure for each precipitate and solution in the charged catholyte was investigated by using scanning transmission X‐ray microscopy: the valence of Mn in the precipitate is mostly attributed to 4+, and the solution includes only Mn(2+). This charge disproportionation reaction should occur after the Mn ions in the catholyte should be oxidized from Mn(2+) to Mn(3+) by charge.