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X-Ray Crystal Truncation Rod Studies of Electrochemical Double Layers

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 synchrotron x-ray observation of Electrochemical Stern layers on the surfaces of RuO2 single crystals in 0.1 M CsF electrolyte. It is of fundamental importance to study if Stern layer forms on electrode surfaces other than Pt(111). In this case, we will examine RuO2 surfaces and compare the results to that of Pt(111) surface. RuO2 is chosen for several reasons. First, it is a well-studied system because of its oxygen evolution reactions and we are familiar with the structure because of our previous studies.[5,6] Second, it is an oxide with oxygen terminated while Pt is a pure metal. Third, the surfaces of RuO2 are squares or rectangles while that Pt(111) surface is triangles. The Stern layers formed at the interfaces of RuO2 (110) and (100) are compared to the previously reported Stern layer on Pt(111). [4] While the Cs+ density profiles at the potentials close to hydrogen evolution reactions are similar, the hydration layers intervening the surface and the Cs+ layer are significantly denser on RuO2 surfaces than that on Pt(111) surface, reflecting the oxygen termination of RuO2 surfaces. The overall similarities between Stern layers on ruthenium surfaces and platinum surface suggest the universal presence of Stern layers in all well-defined solid-electrolyte interfaces.
 [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.
 [5] Y.S. Chu, T.E. Lister, W.G. Cullen, H. You, Z. Nagy, Physical Review Letters 2001, 86, 3364.
 [6] Rao, R.R. et al., Journal of Physical Chemistry C, 2018, 122, 17802