EIS Study on Competitive Adsorption at the TiO₂–Solution Interface

Understanding the competition and configuration of ionic adsorption on the electrode surface is crucial for electrocatalytic reactions and corrosion science. Here, we demonstrate how EIS can be employed to identify the competitive adsorption of phosphate and chloride ions on the surface of a Ti/TiO₂ electrode in a complex solution system. By controlling the concentration of phosphate ions in a high-concentration chloride ion (100 mM) and different pH (5.0, 7.0, and 9.0), we clearly observe a transition in the adsorbed species from initially chloride-dominated to phosphate-dominated on the electrode surface and the transformation of the phosphate adsorption mode. This competitive transition of adsorption species is evident in the low-concentration region of the impedance-concentration curves up to approximately 5 mM P concentration. Furthermore, the impedance-concentration curves under neutral and alkaline conditions show an additional similar switching in the high P concentration region compared with acidic conditions, indicating a shift from bidentate to monodentate phosphate adsorption on the electrode surface. This study provides a method for detecting the adsorption behavior of precursors in subsequent corrosion science.