Dynamic floating solid layers of hydrated ions at electrochemical Stern layer of the solid-liquid interfaces: X-ray crystal truncation rod (CTR) studies

X-ray crystal truncation rod studies show highly dynamic floating solid-like overlayers of hydrated ions formed at electrified solid-metal interfaces. Electrochemical double layers (EDL) is traditionally described as diffuse distributions of cations and anions, proposed in Gouy and Chapman (GC) model [1] and further developed extensively over decades largely based on voltammetry measurements. However, the GC model is suspected to break down at large potentials, 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]. While it is generally accepted that Gouy-Chapman model or its modified versions describe well electrochemical interphase structures, recent studies of Pt(111) surface in CsF solution using a model-independent direct inversion method [3] have shown that Stern layer forms over the large double-layer potential range on Pt(111) surface. [4] In this talk, we present observation of dynamic behavior of Electrochemical Stern layers under electrochemical potential waves. The hydrated ions have enough masses to ‘push’ the metal surface layer with significant recoil forces at the time of departures. The measurements were repeated with Li, Na, K, and Cs to show the presence of the Stern layers regardless of the ions using the consistent behavior of the recoil forces.

[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] Kawaguchi, T. et al., J. Appl. Cryst., 2018 51, 679.

[4] 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.