Electrochemical Reaction of Aqueous Iron Sulfate Solutions Studied by Fe L‑Edge Soft X‑ray Absorption Spectroscopy

Change in valence of Fe ions in aqueous iron sulfate solutions at different potentials has been studied by Fe L-edge soft X-ray absorption spectroscopy (XAS) in transmission mode. Each XAS spectrum is measured at a constant potential by using a liquid cell with built-in electrodes. A nonlinear oxidation of Fe(II) ions to Fe(III) ions is observed when the potential is increased from 0.0 to 0.9 V. Two processes are found in the oxidation: one is a simple oxidation process and the other is a process involving the sulfate ions. The potential peak in the latter process is changed with different scanning rates because the sulfate ions affect electrode kinetic parameters and diffusion coefficients. The reduction of Fe(III) ions to Fe(II) ions shows a linear profile when the potential is decreased from 0.9 to −0.4 V. The mechanism of these Fe redox processes is discussed by correlating the XAS results with cyclic voltammetry results at different scanning rates.