Recent Developments in the Electrocatalysis of the O Reduction Reaction - Fuel Cell Catalysis: A Surface Science Approach - page 303

TABLE 9.4 A Comparison of Surface Area and the Catalytic Activity Data for
Pt/C and Au/Pt/C Before and After 30,000 Potential Cycles from 0.6 to 1.1 V under
the Oxidizing Conditions of the O 2 Reduction Reaction. Reprinted with Permission
from Zhang et al. [2007b]

Kinetic Data Half-Wave

Potential at
1600 rpm (V)

Kinetic Current
Density at 0.85 V
(mA/cm 2 )

Specific Kinetic
Current Density at
0.85 V (A/m Pt )

Pt Dispersion
(m 2 /g Pt )

Catalyst

Pt/C initial 65.5 0.841 4.56 5.80

Pt/C after
30,000 cycles

35.5 0.802 1.60 3.72

Au/Pt/C initial 63.1 0.838 4.23 5.64

Au/Pt/C after
30,000 cycles

60.6 0.833 4.10 5.69

the adverse environmental effects of using fossil fuels. A major obstacle to this appli-
cation has been observed in recent studies that recorded a substantial loss of Pt surface
area over time—and it is Pt that is the electrocatalyst for the cathodic reduction of
oxygen in PEMFCs during the stop-and-go driving of an electric car. Such regimes
generate large electrode potential excursions (typically 0.6 - 1.0 V), resulting in Pt
dissolution, which is thus a major challenge for fuel cell vehicles and the science
of electrocatalysis.
Zhang and co-workers demonstrated that Pt oxygen reduction fuel cell electrocata-
lysts can be stabilized against dissolution under potential cycling regimes by modify-
ing the Pt nanoparticles with small Au clusters [Zhang J et al., 2007b]. The stabilizing
effect of Au clusters was assessed in an accelerated stability test by continuously
applying linear potential sweeps from 0.6 to 1.1 V. After 30,000 cycles, Pt/C suffered
a loss of 39 mV half-wave potential and 45% in active surface area under the same
conditions (Fig. 9.25). The same experiment with the Au/Pt/C electrode at 60 8C
showed negligible change in the Pt surface area and electrocatalytic activity. In situ
XANES and voltammetry data suggest that the Au clusters confer stability by raising
the oxidation potential of Pt [Zhang et al., 2007b]. Possibly, Au atoms block the kink
and step sites of Pt where dissolution starts. The same effect was observed with the
aforementioned Pt monolayer-on-Pd nanoparticle electrocatalysts (Table 9.4). This
finding has the potential to solve one of the major problems for fuel cell application
in transportation. Apart from fuel cells, the ability of Au clusters to alter the properties
of other metals may have other scientific and technological applications, although
further research is needed to explore such effects.

ACKNOWLEDGMENTS

Work at BNL was supported by the US Department of Energy, Divisions of Chemical and
Material Sciences, under Contract DE-AC02-98CH10886. Work at ORNL was conducted
at the Center for Nanophase Materials Sciences and was sponsored by the Division of