The possible existence of subsurface H-atom adsorbates and H2 electrochemical evolution reaction intermediates on platinum in acid solutions

Abstract

The possible existence of H-subsurface states on Pt electrodes in acid solutions was studied using Pt electrodes of different topographies which were characterized by ex-situ scanning tunnelling microscopy (STM). The H-adatom electrodesorption current peak at ca. 0.22 V versus RHE resulting from combined voltammetric and potentiostatic potential routines applied in the H-adatom potential range was assigned to H-subsurface states. Seemingly, the complementary electroreduction occurs at potentials slightly more positive than the hydrogen evolution reaction threshold potential and the reactant, in this case, appears to be related to the hydrogen evolution reaction. The charge density involved in these current contributions is only a small fraction of the H-adatom monolayer charge density. The formation of H-subsurface states appears to be favoured for those Pt surfaces exhibiting a larger contribution of weak H-atom electrosorption. A reasonable correlation between electrochemical and chemisorption data on H on Pt surfaces with different topographies and crystallographic orientations can be established. This fact provides further support for the existence of adsorbed H atom precursor states related to the formation of subsurface H adatoms and to the hydrogen evolution reaction on Pt in acid solutions.