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.
Pham et al. (Wed,) studied this question.
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