Iron(III) Chloro Complexation at the Air–Aqueous FeCl3 Interface via Second Harmonic Generation Spectroscopy

Renewable energy sources are an attractive solution for reducing greenhouse gas emissions; yet, these renewable energies require energy storage. Among large-scale energy storage systems, the iron redox flow battery is a candidate because of its low cost, long lifetime, and safety. Aqueous interfaces are the gateway to many electrochemical reactions, including those that involve soluble Fe(III). However, interfacial iron complexation and iron surface prevalence are not well understood, especially under inherently acidic conditions of FeCl3 solutions. In this study, we investigate interfacial Fe(III) species using second harmonic generation (SHG) combined with surface tension and UV/visible absorption. Surface-selective techniques such as SHG have been widely applied to unveil the unique surface properties within the air–aqueous interface. Here, we perform SHG with excitation at 760, 800, and 860 nm wavelengths. For 800 and 860 nm, the kinetics of interfacial FeCl3 molecules follows the Frumkin–Fowler–Guggenheim adsorption model. For 760 nm, we observe two different interfacial concentration regimes marked by two distinctly different SHG trends. Below a 3.0 mol/kg water concentration of FeCl3, nonresonant behavior is observed, which is similar to that from an aqueous sodium iodide surface but much larger in magnitude than aqueous NaCl and NaBr solution surfaces. Above 3.0 mol/kg water, a dramatic increase in the SHG slope is observed. Through evaluation of centrosymmetry and SHG resonance, we propose the existence of the neutral [FeCl3(H2O)3] complex at the air–aqueous interface.