Local structures of titanium-ion complexes in redox flow battery electrolytes as revealed by X-ray scattering with difference analysis

We report an investigation into the local structure of Ti4+ ions in sulfuric acid solutions as electrolytes for Ti-based redox flow batteries using X-ray total scattering with difference analysis. In the X-ray scattering experiments, we analyzed two concentrated aqueous solutions, one containing 1.5 M TiOSO4 and 2.0 M H2SO4 (termed the Ti-SO4 system) and the other containing 3.5 M H2SO4 (termed the H2SO4 system), which have the same SO42− concentration, and then subtracted the observed X-ray interference function (i(Q)) for the H2SO4 system from that of the Ti-SO4 system to obtain the difference interference function (Δi(Q)). Using Fourier transformation of Δi(Q), we successfully obtained the difference distribution function (Δg(r)), which provides information on the local structure in the first solvation sphere of the Ti ions. The obtained Δg(r) function was interpreted using two possible structural models, i.e., mononuclear and multinuclear Ti-ion complexes. Consequently, we concluded that Ti ions are six-coordinated by O atoms (SO4− and/or H2O) to form octahedral complexes and that mononuclear and multinuclear complexes coexist in the Ti-SO4 system.