Electrocatalysis of Oxygen Reduction in Polymer Electrolyte Fuel Cells: A Brief History and a Critical Examination of Present Theory and Diagnostics - Fuel Cell Catalysis: A Surface Science Approach - page 14

Figure 1.3 Pt catalyst coverage by chemisorbed oxygen species (for Pt/C in a PEFC cathode
at 25 8C): (a) 0.85 V, 3 h of exposure to N 2 ,4%O 2 in N 2 and air, and 30 min exposure to O 2 ;
(b) 0.95 V, 3 h exposure to N 2 and 30 min exposure to O 2 . From Paik et al. [2004].
Considering the standard potential of 1.23 V associated with Reaction (1.4a), it can be
understood why a dioxygen molecule can be more reactive in the formation of OH ads
by water oxidation (1.4) than the purely anodic discharge of water driven by a potential
of 0.85 V in an oxygen-free atmosphere. Interestingly, dissociative chemisorption of
O 2 from the gas phase at Pt-group metal surfaces is reported to be strongly accelerated
in the presence of water vapor [Weaver, 2002], suggesting a surface oxidation process
involving dioxygen and surface water molecules, identical or similar to the Reaction
(1.4). As Fig. 1.3 suggests, Reaction (1.4) apparently takes place at cathode potentials