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 19

Figure 1.5 The slope of E cath versus log J orr through the fuel-cell-relevant potential range has
an apparently constant value near RT/F (measured current density, here designated i,is
corrected for hydrogen crossover current, designated i x , and the measured cell voltage is
ir-corrected to provide the cathode potential E ) [Neyerlin et al., 2006].
decade of current density has indeed been experimentally established for the process in
the PEFC cathode between 0.90 and 0.75 V, as shown in Fig. 1.5 and can obviously be
used as some “pragmatic parameter” to describe the PEFC cathode behavior.
However, to link it with ORR kinetics at the Pt/hydrated ionomer interface, an analysis
along the lines of the expressions (1.5) and (1.6) is crucial. For one thing, if the appar-
ent Tafel slope is still to provide, as it should, some information on the nature of the
slow step in the ORR process, the intrinsic value b int has to be extracted first from the
combined two effects of cathode overpotential as described in (1.6).
To provide quantitative support for the interpretation of the observed Tafel slope in
terms of the expression (1.6), one needs to show quantitatively that the experimentally
observed behavior, i.e., the apparent slope of about 60 mV/decade in the cathode
potential range 0.90 - 0.75 V, can indeed be the result of an intrinsic slope b int
of 120 mV/decade for ORR at a Pt metal surface, modified for the actual,
Pt/Pt-OH ads mixed surface by the documented variations of u ox with E. This was
indeed shown by Uribe et al. [1992] using a u ox versus E dependence based on
reported voltammetric data [Conway et al., 1990] and an assumption that one electron
is passed per surface site in forming an ORR-blocking, chemisorbed oxygen species.
The result of that treatment, shown in Fig. 1.6, clearly demonstrates that the obser-
vation of a 60 mV/decade Tafel slope for the PEFC cathode between 0.90 and
0.75 V can be effectively explained by the two combined effects of a change in cath-
ode overpotential as in Equation (1.6). Figure 1.6 also shows that, according to
Equation (1.6), the apparent and intrinsic Tafel slopes become identical at high cath-
ode overpotentials, where the coverage by chemisorbed oxygen is negligible and
where the second term on the right-hand side of (1.6) goes to zero. Such a rise of
the Tafel slope, from near 60 to near 120 mV/decade with increasing overpotential,