RESUMEN
The surface oxidation states of the metal electrodes affect the activity, selectivity, and stability of the electrocatalysts. Oxide formation and reduction on such electrodes must be comprehensively understood to achieve next-generation electrocatalysts with outstanding performance and stability. Herein, the initial electrochemical oxidation of Pt(111) in alkaline media containing hydrophilic and hydrophobic cations is investigated by X-ray crystal truncation rod (CTR) scattering, infrared (IR) spectroscopy, and nanoparticle-based surface-enhanced Raman spectroscopy (SERS). Structural determination using X-ray CTR revealed surface buckling and Pt extraction at the initial stage of surface oxidation, depending on the cationic species. Vibrational spectroscopy is performed to identify the potential- and cation-dependent formation of three oxide species (IR-active OHad, Raman-active OHad/Oad(H2O), and Raman-active Oad). Hydrophilic alkali metal cations (Li+) inhibit surface roughening via irreversible oxide formation. Hydrophilic Li+ can strongly stabilize IR-active OHad, hindering the extraction of Pt surface atoms. Interestingly, bulky hydrophobic cations such as tetramethylammonium (TMA+) cation also reduce the extent of irreversible oxidation despite the absence of IR-active OHad. Hydrophobic TMA+ inhibits the formation of Raman-active OHad/Oad(H2O) associated with Pt extraction. In contrast, the moderate hydrophilicity of K+ has no protective effect against irreversible oxidation. Moderate hydrophilicity enables the coadsorption of Raman-active OHad/Oad(H2O) and Raman-active Oad. The electrostatic repulsion between Raman-active OHad/Oad(H2O) and neighboring Raman-active Oad promotes Pt extraction. These results provide insights into controlling the surface structures of electrocatalysts using cationic species during the oxide formation and reduction processes.
RESUMEN
In alkaline solutions, interfacial cations affect the hydrogen evolution reaction (HER) activity of platinum electrodes. However, the effects of cations on the HER activity have not been previously investigated based on interfacial structures. In situ surface X-ray diffraction was performed on Pt(110), of which the HER activity is the highest in the low-index planes of Pt, at hydrogen evolution potentials in alkaline solutions, and revealed the interfacial structure of alkali metal cations (Li+ and Cs+). The interfacial structure of the Pt(110) electrode changed reversibly depending on the electrode potential. In the LiOH solution, where the HER activity was higher, the densely packed water layer in the electrical double layer acted as a hydrogen supplier. In the CsOH solution, where the HER activity was lower, the Cs+ cations were aligned in the missing rows of the 1 × 2 reconstructed Pt(110) surface, suggesting that the Cs+ hindered water from accessing the surface, resulting in a lower HER activity.