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, H n PO 4 (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 n PO 4 ) 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 (ClO 4 – and HSO 4 – /SO 4 2– ), we evidence that H 2 PO 4 – 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: HPO 4 2– and PO 4 3– > H 2 PO 4 – . Furthermore, a competition between OH – and PO 4 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.