Structure-relationships in electrocatalysis: oxygen reduction and hydrogen oxidation reactions on Pt(111) and Pt(100) in solutions containing chloride ions

Kinetics of the oxygen reduction reaction (orr) and the hydrogen evolution–oxidation reactions (her/hor) were studied on the Pt(111) and Pt(100) surfaces in 0.05 M H2SO4 containing Cl−. The orr is strongly inhibited on the (100) surface modified by adsorbed Cl− (Clad), and it occurs as a 3.5e− reduction via solution phase peroxide formation. In the hydrogen adsorption (Hupd) potential region, the orr is even more inhibited, and corresponds only to a 2 e− reduction at the negative potential limit where the electrode is covered by one monolayer of Hupd and some (unknown) amount of Clad. On the Pt(111)Clad surface, the orr is inhibited relatively little (in addition to that caused by strong bisulfate anion adsorption on this surface), and the reaction pathway is the same as in Cl− free solution. The kinetics of the hor on Pt(111) are the same in pure solution and in a solution containing Cl−, since Clad does not affect platinum sites required for the breaking of the HH bond. A relatively large inhibition of the hor is observed on the (100) surface, implying that strongly adsorbed Clad is present on the surface even near 0 V.