Electrochemical dissolution of Fe in concentrated aqueous electrolyte

Abstract

All-iron aqueous redox flow batteries provide a low-cost, safe solution for energy storage by utilising the Fe(II)/Fe(0) couple (Fe0 → Fe2+ + 2e-) at the anode and the Fe(III)/Fe(II) couple (Fe3+ + e- → Fe2+) at the cathode. While the simplicity of this battery design is attractive, several fundamental challenges must be overcome to allow full exploitation. These include slow kinetics for the Fe2+/Fe0 plating and stripping reaction leading to decreased coulombic efficiency and competing H2O reduction at the Fe electrode leading to harmful H2 generation.

In this thesis the Fe2+/Fe0 redox response, measured using cyclic voltammetry, varies with electrolyte concentration (from 0.1 M to 2.5 M Li2SO4). At lower concentrations (from 0.1 M to 1.8 M), the iron dissolution rate increases with the electrolyte concentration and the reaction is rapid. At high concentrations (2.0 M and above), the CV results found that the current drops and the iron dissolution reaction stops.

To understand these results, the viscosity, conductivity, and infrared spectra of lithium sulfate solutions were measured with different concentrations. The viscosity increases with increasing concentration from 0.1 to 1.8 M. Also, the conductivity increases as the concentration increases from 0.1 to 1.8 M. When concentrations exceed 1.8 M, the viscosity increases more significantly and there is no further increase in conductivity values. Turning to the infrared result, there are changes in the structure of water in the electrolyte with increasing concentration. From 0.1 to 1.8 M, ions are more independent of each other. When concentrations reach higher than 1.8 M, an ionic interaction occurs between sulfate and lithium, which causes sulfate to lose its symmetry, which is reflected in the IR spectra. Changes also occur in the hydrogen bonds of water as a result of the trapping of water molecules by electrolyte ions. These results together suggest that the slower oxidation of iron at higher concentrations could be due to increased solution viscosity or decreased conductivity due to ions interacting with each other through ion-pairing.

Raman spectroscopy of the iron electrode after oxidation shows formation of iron sulfate surface films in Li2SO4, which indicates that electrode passivating reactions within the aqueous electrolyte are taking place at 2.0 M and above. This result is consistent with the CV results where the dissolution of iron stopped, and the current became passivated. This passivation is due to the formation of the iron sulfate film.

In-situ IR spectroelectrochemistry data shows changes to water and electrolyte structure at potentials at which Fe dissolution takes place. At low concentrations, the sulfate peak is more symmetrical, and sulfate does not lose its symmetry, and therefore each ion dissolves separately and does not interact much with other ions. At high concentrations, the IR results show changes in the asymmetric sulfate stretch due to the loss of sulfate symmetry and the appearance of a new peak in the symmetric stretch region. This indicates the presence of interactions between ions in the solution and the formation of ion pairs.

The effect of adding 4.5 M MgCl2 to different concentrations of Li2SO4 was also investigated to improve the dissolution efficiency of the iron electrode. The results of cyclic voltammetry showed that the dissolution of iron became faster and the deposition of a layer of iron sulphate on the electrode was prevented as the Raman spectra showed few vibrational peaks of sulfate. The IR absorption results show the sulfate peak loses its symmetry at low concentrations. This is because the increase in the number of ions in the solution leads to increased interaction of sulfate with surrounding ions, which leads to broadening and splitting of the peak.

Finally, the redox response of Fe2+/Fe0 in LiTFSI from 0.1 M to 15 m was studied. The reaction current decreased and the dissolution rate of iron decreased due to the large size of TFSI anions and the high viscosity of the solution. However, no passivation layer was formed on the surface with adsorption of TFSI- anions on the surface according to the Raman and IR results.