X-Ray Study of Electrochemical Stern Layer: Ordering and Layering
Authors: Liu Yihua , Kawaguchi Tomoya , Pierce Michael S , Komanicky Vladimir , You Hoydoo 
Published: February 2020

Electrochemical double layers (EDL) form at the interfacial region and have been described by Gouy and Chapman (GC) model[1] or by modified versions of GC model. However, the GC model is suspected to break down at large overpotentials, because it predicts the unlimited rise in differential capacitance. Stern suggested long ago that there should be a layer with a finite ion density, known as ‘Stern layer’[2]. However, direct observation of Stern layer has long been remained elusive to date.
In this talk, we present synchrotron x-ray observation[3] of Stern layer on Pt(111) surface at −850 mV (vs. Ag/AgCl) in alkali fluoride and chloride electrolytes. First, by a direct inversion method[4] for normalized crystal truncation rods, the formation of three sharp layers and one diffuse layer was observed. The diffuse layer spreads over one nanometer, consistent with the Cs+ diffuse layer predicted in GC model. The strong sharp layer among three layers is a Cs+ layer near the Helmholtz plane at 3.5 Å from the Pt(111) surface. Second, in-plane diffraction was used to show that the sharp layers form a (2×2) lattice with one Cs+ per unit cell (Fig. 1). The (2×2) nature of unit cell was used in identifying the two weak water layers. Combination of the x-ray results for the structures normal and parallel to the surface yielded a three dimensional model of Stern layer (Fig. 1). The Cs+ layer is close to Pt(111) surface layer leaving no space for hydrating water molecule. However, three water molecules occupy the empty (2×2) sites, therefore, providing a lateral hydration shell. By adding the three more water molecules above each Cs+, they complete the hydration shell with the Cs+-water distance of 3.1 Å that is in good agreement with known hydration shell distance.
Additional evidence for the existence of dense Stern layer comes from potential jump experiments. [3] In a series of jump experiments with Li, Na, K, and Cs chloride electrolytes, the mass dependence was observed and explained with a simple model of diffusion. The diffusion of the layers were observed because of the high sensitivity of CTR to layers. By combining all aspects of the x-ray studies, we unambiguously identify the Stern layer and its structure.
[1] Bard, A. J.; Faulkner, L. R., Electrochemical methods: fundamentals and applications. 2nd ed.; John Wiley: New York, 2001.
[2] Stern, O., The theory of the electrolytic double layer. Zeitschrift Für Elektrochemie
1924,
30, 508.
[3] Liu, Y.; Kawaguchi, T.; Pierce, M.S.; Komanicky, V.; You, H., Layering and Ordering in Electrochemical Double Layers, J Phys Chem Lett, 2018, 9, 1265.
[4] Kawaguchi, T.; Liu, Y.; Pierce, M. S.; Komanicky, V.; You, H., Direct determination of the one-dimensional interphase structure by crystal truncation rod analysis, J. Appl. Cryst., 2018, 51, 679.
Fig. 1. The (2x2) model of Stern layer. The structure is composed of one Cs+ and two water layers with Cs+ and 6H2O. The perspective (a), top (b), and side views (c,d) are shown. Platinum atoms of the working electrode, Cs+ ions, in-plane water molecules, and the top water molecules are shown by grey, blue, red, and pink balls, respectively.

Figure 1