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 20

Figure 1.6 The variation of log J ORR with V cath expected based on (1.5), with the coverage by
surface oxygen derived from voltammetry [Uribe et al., 1992].
has indeed been observed near ambient conditions in a number of investigations at
relevant model Pt/electrolyte interfaces (see, e.g., Parthasarathy et al., 1992a, b).
In this regard, the skepticism expressed by Neyerlin et al. [2006] about the accuracy
of mass transport corrections in evaluating ORR kinetic currents, and hence about the
Tafel slope reported for the ORR at Pt at cathode potentials below 0.75 V, may be
justified for high surface area electrodes, where the rate of the surface process is sig-
nificantly “amplified” compared with the rate of mass transport processes. However,
this criticism does not hold to the same degree for model systems of much lower Pt
surface area per geometric electrode area. Consequently, the 120 mV/decade slope
consistently reported for mass-transport-corrected ORR currents measured near
room temperature at potentials under 0.75 V at RDEs, can be considered as established
data that, with high probability, fully applies to the dispersed Pt metal catalyst in the
PEFC. The fuel-cell-relevant potential region (see Fig. 1.5), falls fully within one
branch of the complete behavior described by Fig. 1.6, and consequently this low,
practically unchanged, value of the Tafel slope measured throughout this limited
potential range (see Fig. 1.5) does not challenge the general analysis based on the
expression (1.6).
Is there a way to verify experimentally not only that the measured low Tafel slope of
60 mV/decade in the fuel-cell-relevant potential range is fully explainable by an
“intrinsic” slope of 120 mV/decade for ORR at Pt metal, but also that this intrinsic
Tafel slope can be experimentally verified within the fuel-cell-relevant potential
range? In fact, an elegant response to this challenge has recently been provided by
impedance spectra measured for the ORR process at Pt/C in a PEFC cathode