61
and rotating ring-disk electrode, taking into consideration the surface properties,
allowed one to propose the course of the reaction.
The criteria that follow from the scheme provide a means for elucidating the
mechanism of O 2 reduction and explain some previously unanswered issues.
The general scheme is given in Scheme 6.1. It has been written for alkaline solutions, but it can be easily transformed, mutatis mutandis, into an analogous scheme
for acid solutions. The ki are overall rate constants for the i-th step, and the subscripts sa, a, b, and * denote strongly adsorbed, weakly adsorbed, bulk, and the
vicinity of the disk electrode species, respectively. The scheme is a generalization of
the schemes proposed earlier and, as before, consists of two major paths by which
O 2 reduction can proceed. It treats, however, the direct path in a more complex manner (cf. the rate constants k 3 and k 13 ), explicitly includes a superoxide ion, and allows
the possibility for the weakly adsorbed intermediates from the series path to undergo
surface diffusion and form their strongly bound counterparts in the direct path.
Clouser and Yeager [12] and Sepa et al. [13] proposed that the proton transfer
occurs simultaneously with the charge-transfer and proposed the mechanism:
O
O ads
2
2
⇔ ,
(6.9)
O
H e
product s
ads
2,
+
+ →
( )
+
−
(6.10)
According to them, the experimentally determined Tafel slope of −0.060 V dec
−1
(−2.3 RT/F), in the range of low current densities, arose from oxygen-containing
species (PtOH) formed in reaction of Pt with H 2 O and adsorbed under Temkin
O 2,b
O 2,a
O sa
O 2 *
HO
–
2 *
HO
–
2, a
O
–
2, a
HO
–
2, b
O
–
2sa
HO
–
2sa
OH
–
O
–
sa
dif
k 21
k 22
k 20
k –20
k 1
k 10
k 2
k 23
k 12
k 4
k 13
k 5
k 6
k –5
k –25
k 25
k 24
k 3
k –2
k 11
k 10
k 13
dif
Scheme 6.1 General scheme of the ORR in alkaline solution. (Adapted from [11] with
permission)
6.1 Oxygen Reduction Reaction