Structural analysis of sulfuric acid solutions containing Ti and Mn using x-ray diffraction, x-ray absorption fine...

Structural analyses have been performed for sulfuric acid (H₂SO₄) aqueous solutions containing Ti and/or Mn. These solutions are used as the positive electrolyte in the Ti-Mn redox flow (RF) battery, in which it had been found that adding Ti⁴⁺ in the positive electrolyte is very effective to reduce the MnO₂ precipitation at a high state of charge. X-ray diffraction and x-ray absorption fine structure (XAFS) measurements were employed in order to obtain total correlation functions, T(r)'s, and coordination numbers around Mn and Ti in the solutions, respectively. The T(r)'s showed some peculiar peaks that were assigned to correspond to S-O, O-O, Mn-O, and Ti-O pairs in the solutions. The XAFS analysis demonstrated that both Mn and Ti have 6-coordinating oxygen atoms in the solutions. The classical molecular dynamics simulation was also carried out to obtain structural models of the solutions. By tuning the Born-Mayer type potential parameters, the T(r)'s calculated from the models showed good agreement with the experimental ones. Regarding the coordination number, the 6-coordinated Mn-O was reproduced successfully, while we need further investigation to find parameters that can reproduce the 6-coordinated Ti-O in the solutions. The simulation results also indicated the existence of Ti-SO₄ bonds, which should promote the H⁺ dissociation from HSO₄^- and increase the H⁺ concentration in the solutions. This may be effective to suppress the MnO₂ precipitation at a high state of charge in the RF battery.