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 12

et al., 2004] came the 2006 ORR experimental paper [Neyerlin et al., 2006] and pro-
vided apparent support for an ORR reaction order of 0.5 with respect to O 2 from
measurements done on Pt catalyst in the PEFC cathode. This combination of reports
from experimental ORR studies [Neyerlin et al., 2006] and from a theoretical study
of the ORR in acid electrolytes [Nørskov et al., 2004], thus seemed to provide sig-
nificant evidence against the assignment of the Reaction (1.2) as the rate-determin-
ingstepintheORR.
The apparent challenges presented by these recent readings of experimental ORR
results, winning early support from a DFT study, have left some important questions to
answer:
† What is behind the apparent disagreements between Tafel slopes and reaction
orders reported from recent investigations of the ORR at PEFC cathode catalysts
and the slopes and reaction orders measured earlier for model systems of low Pt
surface area? Is the ORR process at a dispersed Pt catalyst possibly different in
nature from the ORR process at low-surface-area Pt?
† Can an ORR mechanism at Pt metal in an acid electrolyte with the Reaction (1.2)
as the first and rate-limiting step be defended in light of the recently reported
apparent Tafel slope and reaction order for ORR in the PEFC cathode?
† With the remaining question marks regarding the mechanism of the ORR and the
nature of the slow step in the ORR, does the viability of the simple yardstick
(M22OH or M22O bond strength), offered for predicting the rate of the complex
ORR process [Nørskov et al., 2004] remain unscathed? Or does its predictive
power possibly depend strongly on the rate-limiting step being reduction of sur-
face oxygen intermediates [Nørskov et al., 2004], rather than the Reaction (1.2)?
One point that should be raised before trying to answer the above questions is the
actual significance of reconciliation between new information and understanding at
the atomic and molecular level, as provided by DFT calculations, and “old-fashioned”
macroscopic parameters such as the Tafel slope and the reaction order. Arguments
have been made, including at the Leiden meeting, to the effect that such macroscopic
parameters should not be considered legitimate diagnostic tools in the age of advanced
calculations of interfacial electronic structures. The experience of the present author
in critically reviewing very recent ORR work, both theoretical and experimental,
has led to a renewed conviction that those “old-fashioned” parameters have significant
value and should, in fact, be considered carefully in theoretical work done at the
atomic/molecular level.
1.4 DOCUMENTED INFORMATION ON THE ACTUAL NATURE
OF THE SURFACE OF A Pt CATALYST IN THE CATHODE
OF AN OPERATING FUEL CELL
Proper answers to the above questions can be provided once it is realized that the
surface of the catalyst in a fuel cell cathode using a Pt or Pt/C catalyst cannot be