Electrochemical Kinetics: a Surface Science-Supported Picture of Hydrogen Electrochemistry on Ru(0001) and Pt/Ru(0001)

AbstractIn this short review, we compare the kinetics of hydrogen desorption in vacuum to those involved in the electrochemical hydrogen evolution/oxidation reactions (HER/HOR) at two types of atomically smooth model surfaces: bare Ru(0001) and the same surface covered by a 1.1 atomic layer thick Pt film. Low/high H2 (D2) desorption rates at room temperature in vacuum quantitatively correspond to low/high exchange current densities for the HOR/HER in electrochemistry. In view of the “volcano plot” concept, these represent two surfaces that adsorb hydrogen atoms, Had, too strongly and too weakly, respectively. Atomically smooth, vacuum annealed model surfaces are the closest approximation to the idealized slab geometries typically studied by density functional theory (DFT). A predictive volcano plot based on DFT-based adsorption energies for the Had intermediates agrees well with the experiments if two things are considered: (i) the steady-state coverage of Had intermediates and (ii) local variations in film thickness. The sluggish HER/HOR kinetics of Ru(0001) allows for excellent visibility of cyclic voltammetry (CV) features even in H2-saturated solution. The CV switches between a Had- and a OHad-/Oad-dominated regime, but the presence of H2 in the electrolyte increases the Had-dominated potential window by a factor of two. Whereas in plain electrolyte two electrochemical adsorption processes compete in forming adlayers, it is one electrochemical and one chemical one in the case of H2-saturated electrolyte. We demonstrate and quantitatively explain that dissociative H2 adsorption is more important than H+ discharge for Had formation in the low potential regime on Ru(0001). Graphical AbstractLeft: Cyclic voltammograms of Ru(0001) in 0.1 M HClO4, with and without H2 present in solution. Left inset: atomic resolution scanning tunnelling microscope (STM) images of Ru(0001). Centre: volcano plot showing the theoretically predicted hydrogen evolution/oxidation (HER/HOR) current densities on Ru(0001), with variable Had coverage in the adlayer. These values are compared with the exchange current densities for the HER/HOR on Pt(111) and pseudomorphic overlayers of Pt on Ru(0001), where the Pt overlayer thickness is indicated. Temperature programmed desorption (TPD) spectra of D2 on Ru(0001) (upper left) and Pt/Ru(0001) (upper right) are shown along with the respective adsorbate coverage obtained at 300 K. Right: STM image of Pt/Ru(0001). The line indicated how the Pt overlayer thickness varies across the surface, as illustrated at the bottom.

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