Adsorption Mechanism of Phosphate Anions at Pt–H ₂ O Electrochemical Interfaces via In Situ SHG Spectroscopy

Delving into the electrode-poisoning effect of anions is paramount in understanding the intricate pathways of electrocatalytic reactions. Although phosphate anions, HnPO₄(3-n)-, are the most prevalent electrolytes employed in state-of-the-art energy storage systems, their adsorption mechanisms at metal surfaces remain poorly understood. Here, by integrating in situ second-harmonic generation (SHG) spectroscopy with electrochemistry in interfacial analysis, we comprehensively illustrate that we can quantitatively detect the presence of adsorbates, including Hads, OHads/Oads, and (HnPO₄)ads, on a charged Pt(poly) surface. Our investigation highlights the dominant electronic contribution of Pt-H dipoles to promote the pH-independent SHG increase during hydrogen underpotential deposition (HUPD). In contrast, SHG response during the chemisorption of oxygenated species (OUPD) is largely induced by pH-sensitive surface chemistry. Concomitantly, our results demonstrate a substantial harmony with the reversibility characteristics of Pt's electrochemical processes. Through SHG control experiments with other anions (ClO₄⁻ and HSO₄⁻/SO₄²⁻), we evidence that H₂PO₄⁻ is a nonspecifically adsorbed species without precedent. Notably, we reveal that the relative adsorption strength and sluggish kinetics of the phosphate family feature a descending series corresponding to increasing protonation: HPO₄²⁻ and PO₄³⁻ > H₂PO₄⁻. Furthermore, a competition between OH⁻ and PO₄³⁻ for adsorption sites in highly basic media (pH > 11) is unambiguously observed. Accordingly, OH⁻ inhibition on Pt significantly alters the interfacial electronic structure at the Pt surface between acidic and strongly alkaline conditions. Our work, therefore, leads to an in-depth comprehension of adsorbate-induced surface restructuring.