Hydrogen electrochemistry on platinum low-index single-crystal surfaces in alkaline solution

The results of a study of the hydrogen evolution reaction (HER) and the hydrogen oxidation reaction (HOR) on the three low-index faces of Pt in alkaline solution are presented. The study features a new method for the use of Pt single crystals in a rotating disk electrode (RDE) configuration. At low negative overpotentials, the order of activity for the HER increased in the sequence (111) < (100) < (110). At low positive overpotentials, the order of activity for the HOR increased in the sequence (111) ≈ (100) ≪ (110). These differences in activity with crystal face are attributed to different states of adsorbed hydrogen and to different effects of these states on the mechanism of the hydrogen reaction. Two different types of adsorbed hydrogen are observed on Pt(hkl) surfaces. A high binding energy state, often referred to as underpotential deposited hydrogen, H_upd, has an inhibiting (site blocking) effect on the rate of the HER and HOR. A low binding energy state is a reaction intermediate at low overpotentials in both the HOR as well as the HER, and is most prevalent on the (110) surface. At high positive overpotentials, in the potential region where adsorption of hydroxy species (OH_ad) occurs, the effects of surface crystallography on the HOR is attributed to the structural sensitivity of the adsorption of OH_ad on Pt(hkl), with OH_ad having an inhibiting effect on the HOR, the inhibition decreasing in the sequence (100) ≪ (110) < (111).