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 and the overpotential deposited H , 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 , 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 and . The paper discusses thermodynamic methodology used in determination of , and for and compares the value of with . The authors demonstrate a new theoretical formalism which is applied to the determination of the bond energy . New data demonstrate that and are bonded to Rh with the same energy; this points to the same binding mechanism and the same adsorption sites of and . The chemical potentials of , and subsurface H, , 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, θH, and the lattice occupancy fraction .