Experimental results for the ORR (oxygen reduction reaction) in perchloric acid for ultra low loading Pt/C
electrodes have been fitted to a number of different ORR mechanisms. These were accomplished as a
function of temperature (280–330 K), oxygen partial pressure(0:01 < po2
p0
o2
< 1) and potential (0.3–1.0V vs.
RHE). A reaction exponent for oxygen of 1 0.1 across the potential range 0.3–0.85V vs. RHE is confirmed.
From the experimental results it is clear that the surface becomes increasingly blocked towards the ORR
as overpotential increases (i.e. as the potential decreases from 0.6 to 0.3 V vs. RHE). The double trap model
[J.X. Wang, J. Zhang, R.R. Adzic, J. Phys. Chem. A, 111 (2007) 12,702] fails to account for this observation,
although we have produced a modified version to include the formation of OOHad intermediates. These
intermediates block the electrode at larger overpotentials and lead to a decrease in electrocatalyst
performance compared to a Tafel type approximation. Furthermore these intermediates can lead to the
formation of hydrogen peroxide at large overpotentials, an experimental observation which is currently
poorly described by models.
The decreased activity at large overpotentials suggests that blocking of active catalyst sites may be as
important to catalyst activity in an operating fuel cell as the absolute performance of the electrode in the
low overpotential region as typically measured on an RDE. It may also offer an explanation to the
increased losses seen in fuel cell electrodes at lower catalyst loadings – i.e. the loses, which are typically
ascribed to increased mass transport loses, may instead result from decreased electrocatalytic
performance at high overpotentials.