  Surface X-ray scattering studies of electrochemical Stern layer are reported.
The Stern layers formed at the interfaces of RuO2 (110) and (100) in 0.1 M CsF
electrolyte are compared to the previously reported Stern layer on Pt(111) [Liu
et al., J. Phys. Chem. Lett., 9 (2018) 1265]. While the Cs+ density profiles at
the potentials close to hydrogen evolution reaction are similar, the hydration
layers intervening the surface and the Cs+ layer on RuO2 surfaces are
significantly denser than the hydration layer on Pt(111) surface possibly due
to the oxygen termination of RuO2 surfaces. We also discuss in-plane ordering
in the Stern layer on Pt(111) surface.
