Most of the electrochemistry processes occur within the thin layer of electrolyte at the electrolyte/electrode interfaces, commonly denoted as the electrical double layer (EDL). Although some classic continuum theories about EDL have been established and widely accepted over the past century, limited experimental information is available regarding the molecular-level details at such solid/liquid interfaces. We have developed in-situ liquid cells which allow us to study such solid/liquid interfaces by means of soft x-ray absorption spectroscopy [1,2]. X-ray absorption is an element- and chemical-specific characterization method. Because the fluorescence x-ray photon has much larger mean free path in condensed matters than the secondary electrons, by comparing the total fluorescence yield (TFY) and total electron yield (TEY) XAS spectra, we can extract useful information about the compositional, structural or chemical differences between the bulk and the interfacial electrolyte. Under different bias, by modulating the incident x-ray, the TEY signal current becomes alternating and can be separated from the dominant faradaic current so that we can obtain interface-sensitive TEY signal under electrochemical conditions.

With this in-situ and operando XAS technique, we investigated the gold/water interface [1] and platinum/sulfuric acid solution interface [3]. Combined with first-principle simulations, it was found that at gold/water interface, the interfacial water layer has more broken hydrogen bonds compared to bulk water. However, the electronic coupling between the LUMO orbitals of interfacial water molecules and the gold substrate greatly suppressed the spectral feature that is related to broken hydrogen bonds in the O K-edge XAS spectra. Under different bias, the polar water molecules respond to the external electrical field and reorient at the gold electrode surface, which significantly changes the amount of distorted or broken hydrogen bonds. First-principle simulations were able to corroborate the experimental results and qualitatively reproduce the change in the x-ray absorption spectra at different bias.

In the platinum/sulfuric acid system, the charged solute species, such as SO₄­^2- ions, hydronium ions, introduce extra complexity at the surface under different bias. Using the in-situ/operando XAS technique, we observed reversible spectral change in O K-edge spectra, which is likely due to sulfate adsorption/desorption at the platinum surface within the oxidative potential window based on first-principle simulation results. However, little oxide/hydroxide species could be identified, contradicting to the widely-accepted models of platinum oxide/hydroxide intermediates.

The studies on gold/water and platinum/sulfuric acid solution interfaces demonstrated the capabilities of the in-situ/operando XAS technique, which provide exciting new opportunities for mechanistic investigations of many important electrochemical systems.

[1]. J. J. Velasco-Velez, C. H. Wu, T. A. Pascal, L. F. Wan, J.-H. Guo, D. Prendergast, and M. B. Salmeron, Science, 346, 831-834 (2014).

[2]. C. H. Wu, R. S. Weatherup, M. B. Salmeron, Phys. Chem. Chem. Phys., 17, 30229 (2015).

[3]. C. H. Wu et al., in preparation.