Electrochemical Hydrogen Adsorption and Absorption. Part 1: Under-potential Deposition of Hydrogen

Abstract

The review article presents an atomic-level discussion of the fundamental steps involved in hydrogen electro-adsorption on transition-metal electrodes. It also discusses the basic events involved in hydrogen physisorption and chemisorption under low-pressure gas-phase conditions in order to identify characteristics that are unique to the electrochemical environment. The electrochemical environment that comprises the solid electrode, the liquid electrolyte solution, and the electrostatic field creates favorable conditions for the electro-adsorption of two different hydrogen species, the under-potential and over-potential adsorbed hydrogen (HUPD and HOPD). The electro-adsorption of HUPD and HOPD occurs in two different potential ranges. The electro-adsorption of HUPD is characteristic of Pt group metals only, while the electro-adsorption of HOPD takes place at all electrode materials at which H2(g) can be electrolytically generated. The existence of HUPD and HOPD is supported by spectroscopic and thermodynamic results. A thermodynamic analysis of the electro-adsorption of HUPD results in the determination of thermodynamic state functions that govern the process, namely the standard Gibbs energy, enthalpy and entropy of electro-adsorption, $$ {\Delta_{{\rm{ec - ads}}}}G^\circ \left( {{{\hbox{H}}_{\rm{UPD}}}} \right) $$ , $$ {\Delta_{{\rm{ec - ads}}}}H^\circ \left( {{{\hbox{H}}_{\rm{UPD}}}} \right) $$ , and $$ {\Delta_{{\rm{ec - ads}}}}S^\circ \left( {{{\hbox{H}}_{\rm{UPD}}}} \right) $$ , the Gibbs energy of later interactions, $$ \omega \left( {{{\hbox{H}}_{\rm{UPD}}}} \right) $$ , and the M–HUPD surface bond energy, $$ E^\circ \left( {{\hbox{M}} - {{\hbox{H}}_{\rm{UPD}}}} \right) $$ . Thermodynamic data for the electro-adsorption of HUPD on polycrystalline electrodes (Pt(poly) and Rh(poly)) and on Pt(111) in several aqueous electrolyte (H2SO4, HClO4, and NaOH) solutions are reported. Their analysis and the analysis of $$ {\Delta_{{\rm{ec - ads}}}}H^\circ \left( {{{\hbox{H}}_{\rm{UPD}}}} \right) $$ and $$ E^\circ \left( {{\hbox{M}} - {{\hbox{H}}_{\rm{UPD}}}} \right) $$ allow us to compare on a thermodynamic basis HUPD to Hchem, and to identify surface adsorption sites possibly occupied by HUPD and HOPD adatoms.