Comparison of hydrogen electroadsorption from the electrolyte with hydrogen adsorption from the gas phase

This paper focuses on fundamental aspects associated with H adsorption at the solid/liquid and solid/gas interfaces and compares H electroadsorption from the electrolyte with dissociative H chemisorption from the gas phase. At the solid/liquid interface, two distinguishable electroadsorbed H species are observed, the underpotential deposited H (H-UPD) and the overpotential deposited H (H-OPD), and their roles in the hydrogen evolution reaction (HER) and H absorption are discussed. At the solid/gas interface, there is only one distinguishable H species, chemisorbed H (H-chem), which can undergo interfacial transfer into the metal. Three distinct mechanisms of H electroabsorption into the metal electrode are discussed in relation to the adsorption sites of H-UPD and H-OPD. The paper discusses thermodynamic methodology used in determination of Delta G(ads)degrees (H-UPD), Delta S(ads)degrees (H-UPD), and Delta H(ads)degrees (H-UPD) for H-UPD and compares the value of Delta H(ads)degrees (H-UPD) with Delta H(ads)degrees (H-chem). The authors demonstrate a new theoretical formalism which is applied to the determination of the M-H-UPD bond energy (E(M-HUPD)). New data demonstrate that H-UPD and H-chem are bonded to Rh with the same energy; this points to the same binding mechanism and the same adsorption sites of H-UPD and H-chem. The chemical potentials of H-chem, H-UPD, H-OPD, and subsurface H, H-ss, are defined, and the chemical-potential gradient of H associated with its interfacial transfer across the liquid/solid or gas/solid interfaces is formulated in terms of the surface H coverage, theta(H), and the lattice occupancy fraction X(H).