Adsorption Mechanism of Phosphate Anions at Pt-H₂O 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, H_nPO₄^(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 H_ads, OH_ads/O_ads, and (H_nPO₄)_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 (H_UPD). In contrast, SHG response during the chemisorption of oxygenated species (O_UPD) 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^-/SO₄^2-), 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: HPO₄^2- and PO₄^3- > H₂PO₄^-. Furthermore, a competition between OH^- and PO₄^3- 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.