HYDROGEN ELECTROCHEMISTRY ON PLATINUM LOW-INDEX SINGLE-CRYSTAL SURFACES IN ALKALINE-SOLUTION

The results of a study of the hydrogen evolution reaction (HER) and th e hydrogen oxidation reaction (HOR) on the three low-index faces of Pt in alkaline solution are presented. The study features a new method f or the use of Pt single crystals in a rotating disk electrode (RDE) co nfiguration. 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 seq uence (111)approximate to(100)much less than(110). These differences i n activity with crystal face are attributed to different states of ads orbed hydrogen and to different effects of these states on the mechani sm of the hydrogen reaction. Two different types of adsorbed hydrogen are observed on Pt(hkl) surfaces. A high binding energy state, often r eferred to as underpotential deposited hydrogen, H-upd, has an inhibit ing (site blocking) effect on the rate of the HER and HOR. A low bindi ng energy state is a reaction intermediate at low overpotentials in bo th the HOR as well as the HER, and is most prevalent on the (110) surf ace. At high positive overpotentials, in the potential region where ad sorption of hydroxy species (OHad) occurs, the effects of surface crys tallography on the HOR is attributed to the structural sensitivity of the adsorption of OHad on Pt(hkl), with OHad having an inhibiting effe ct on the HOR, the inhibition decreasing in the sequence (100)much les s than(110)<(111).