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

摘要

In 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 H-2 (D-2) 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, H-ad, 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 H-ad intermediates agrees well with the experiments if two things are considered: (i) the steady-state coverage of H-ad 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 H-2-saturated solution. The CV switches between a H-ad-and a OHad-/O-ad-dominated regime, but the presence of H-2 in the electrolyte increases the H-ad-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 H-2-saturated electrolyte. We demonstrate and quantitatively explain that dissociative H-2 adsorption is more important than H+ discharge for H-ad formation in the low potential regime on Ru(0001).