Structural analysis of sulfuric acid solutions containing Ti and Mn using x-ray diffraction, x-ray absorption fine structure, and molecular dynamics simulation

Abstract

Structural analyses have been performed for sulfuric acid (HSO) 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 Ti in the positive electrolyte is very effective to reduce the MnO 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-SO bonds, which should promote the H dissociation from HSO and increase the H concentration in the solutions. This may be effective to suppress the MnO precipitation at a high state of charge in the RF battery.

Structure of Sulfuric Acid Solutions Using Pair Distribution Function Analysis

“…The peak positions and intensity of the short-range sulfate structure depends on the concentration and molar fraction of the related deprotonation of H 2 SO 4 species concentration (S–O ∼ 1.48 Å and O–O interactions within the sulfate group ∼2.44 Å). IR and Raman spectroscopy measurements show that at low concentrations, the dominate species is SO 4 2– , with increasing fractions of HSO 4 – and H 2 SO 4 species at higher acid concentrations. ,,, The sulfate structure was found to be retained in sulfuric acid aqueous solutions, and the subtle shifts in the S–O peak position is suggestive of changes in protonated sulfate species with the increase in acid concentrations.…”

“…− and H 2 SO 4 species at higher acid concentrations. 2,22,32,42 The sulfate structure was found to be retained in sulfuric acid aqueous solutions, and the subtle shifts in the S−O peak position is suggestive of changes in protonated sulfate species with the increase in acid concentrations.…”

“…The vast utilization of sulfuric acid in everyday life and its environmental impact motivates our understanding of both its chemical and physical properties. For example, sulfuric acid is a key component in lead-based batteries for traditional automotive applications and could also play a role in stationary storage with increasing sources of wind and solar energy. , In these systems, the acid changes concentration during operation and is often the bottleneck for discharge, where local SO 4 2– availability limits PbSO 4 growth, and during charge, where the concentration dependence of Pb 2+ solubility limits the dissolution of PbSO 4 . Understanding the structure of the acid in the changing battery environment is a crucial first step to gain insights into the process of crystal growth/dissolution during battery cycling.…”

“…Change in the water coordination number of sulfate ions with acid concentration , (top, averages between bisulfate and sulfate are given for some theoretical studies). Phase diagram of sulfuric acid and water based on figures in Beyer et al and Maynard-Casely et al .…”

“…Experiments detailing the two-step dissociation process have given insights into the ratios of anionic species present in solutions, , with theoretical calculations suggesting a range of hydration numbers for each species . Studies targeting the structure of the solvation shell include NMR, theoretical calculations, , and scattering techniques extended X-ray absorption fine structure (EXAFS) , and pair distribution function (PDF). , These studies identified a decrease in the number of water molecules hydrogen-bonded to the sulfate species with increasing acidic conditions, as shown in Figure . The overall hydration number varied with the model.…”