O2 surface concentration change and its implication on oxygen reduction mechanism and kinetics at platinum in acidic media.

O 2 concentration near Pt surface during oxygen reduction reaction (ORR) in 0.1 M HClO 4 has been monitored by rotating ring-disk electrodes system. At 0.8 V < E < 1.0 V (vs. RHE), O 2 concentration near Pt surface increases with potential accompanying with the decrease of ORR current at the disk electrode; O 2 concentration in the negative-going scan is larger than that at the same potential in the positive-going scan, while ORR current shows the opposite trend at ω > 400 rpm. At E > 0.8 V accumulation of O ad |OH ad at Pt disk electrode with ORR time is evident, revealing that O ad |OH ad formation rate is faster than that for the removal of OH ad to H 2 O under such conditions. At relatively lower rotation speed and faster scan rate, the cathodic current during ORR in the negative-going scan can be larger than that in the positive-going scan with a current peak at ca. 0.8 V, which is attributed to the superimposition of ORR current increase due to change of O 2 concentration near the surface and the additional reduction of O ad |OH ad formed from decomposed O 2 at higher potentials. [ABSTRACT FROM AUTHOR]