Structural study on Ti-ion complexes in concentrated aqueous electrolytes: Raman spectroscopy and high-energy X-ray total scattering

Abstract

Abstract We report a structural study on the characterization of specific structures of Ti-ion complexes in the ion-concentrated aqueous sulfuric acid solutions, which are promising new-type redox flow battery electrolytes. On the basis of the quantitative analysis of Raman spectra, we successfully determined the coordination number of SO4 ions (nSO4) within the Ti-ion complexes in solutions. The resulting nSO4 values depended on the Ti ion and SO4 concentrations (cTi and cSO4, respectively), which implied the formation of multinuclear Ti-ion complexes in the highly concentrated solutions. High-energy X-ray total scattering (HEXTS) measurements indicated that Ti4+ ions formed nanoscale aggregates that are based on the multinuclear complexes, e.g., chain-like -Ti-O-Ti-O– structures; indeed, characteristic small-angle X-ray scattering (SAXS) was observed for high cSO4 solutions. We determined that the SAXS peak decreased in intensity when Ti4+ electrochemically reduced to Ti3+ in solutions, and the extent of decrease was considerably more at higher Ti3+ concentrations. The local structures of such Ti-ion complexes were evaluated on the basis of the radial distribution function G(r), and we discuss the charge effect of Ti ions on the complex structures at the molecular level. We concluded that the reductive reaction of Ti4+ to Ti3+ triggers a change in the solution structure, i.e., nanoscale Ti4+ aggregates were disrupted and yielded mononuclear Ti3+ complexes.