Molecular interactions in concentrated lithium sulfate solutions and their effect on electrochemical dissolution of...

The electrochemical oxidation of an iron electrode to form dissolved Fe²⁺ was studied in a range of aqueous concentrated Li₂SO₄ solutions from 0.1 m to 2.5 m. The aim was to provide experimental understanding of molecular interactions in concentrated aqueous electrolytes and to determine whether these confer performance advantages for the all-iron redox flow cell anode reaction. Mass transport properties of the electrolyte solutions were probed through measurement of conductivity and determination of diffusion coefficients of dissolved probe species. Both methods showed an inhibition in species mobility above ca. 1 m, consistent with an increase in solution viscosity, loss of free water due to ion solvation and formation of ion pairs. Infra-red (IR) spectroscopy of the solutions showed a shift in wavenumber and asymmetry of the sulfate stretching peak as ion concentration increased, consistent with increasing ion-ion interactions. Distinctive changes to water IR bands were related to Li⁺ solvation and formation of solvent separated ion pairs. Cyclic voltammetry was used to study Fe oxidation and dissolution in the electrolytes. Up until 1.8 m there was an increase in oxidation current as electrolyte concentration was increased, suggesting that use of concentrated electrolytes may be advantageous for this reaction. However above 1.8 m there was a suppression of current, confirmed by Raman spectroscopy to be caused by precipitation of FeSO₄ on the electrode as the saturation limit was reached. In situ IR spectroelectrochemistry was used to investigate the rate of Fe dissolution and the nature of solution molecular interactions as the reaction proceeded. An increase in SO₄^2- at the electrode surface was indicative of Fe dissolution producing Fe²⁺, with the anion concentration increase proposed as balancing the local solution charge as the dissolved cation concentration increases.