Performance measurements and modelling of the ORR on fuel cell electrocatalysts – the modified double trap model

Abstract

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 < p o 2 p o 2 0 < 1 ) and potential (0.3–1.0 V vs. RHE). A reaction exponent for oxygen of 1 ± 0.1 across the potential range 0.3–0.85 V 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 OOH ad 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.