Fundamental Studies on Enhanced Redox Performance of Alkaline Iron Batteries by Silicate Additives

Iron oxides or hydroxides are highly desirable “green” battery electrodes owing to their abundance, multiple oxidation states, low cost, and environmental beneficence. However, their low storage capacity and poor cycling life have been lingering concerns due to the substantial hydrogen evolution on charging and the formation of inactive spinel phase on discharging. Herein, we report a fundamental physicochemical study of the redox behavior of iron (II, III) oxide materials containing goethite (α-FeOOH) and magnetite (Fe₃O₄) in an alkaline solution. Also, an unprecedented discovery of how the low concentration of sodium silicate (Na₂SiO₃) electrolyte additive improves the conversion of Fe(OH)₂ into FeOOH through a one-electron-transfer reaction by mitigating the formation of Fe₃O₄.<br/>The phase information is provided by <i>in operando</i> synchrotron X-ray diffraction (XRD) during the redox cycling. X-ray photoelectron spectroscopy (XPS) and electrochemical measurements revealed the iron-oxide/silicate interaction that introduces diffusion limitation of oxygen species on the surface of the electrode, thus favoring the FeOOH formation. As per the classical molecular dynamics (CMD) simulations, the addition of Na₂SiO₃ results in increased Na ions in the solvation shell. Theoretical calculations (DFT) also supported that the (SiO₃)* adsorption is more energetically favored than the (OH)* on the Fe(OH)₂ surface. Moreover, this new battery chemistry enables the high capacity and reversibility of the iron electrode, which could contribute to developing new alkaline battery systems built on non-critical materials for sustainable energy storage.