Pt Skin
on AuCu Intermetallic Substrate: A Strategy
to Maximize Pt Utilization for Fuel Cells
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Abstract
The
dependence on Pt catalysts has been a major issue of proton-exchange
membrane (PEM) fuel cells. Strategies to maximize the Pt utilization
in catalysts include two main approaches: to put Pt atoms only at
the catalyst surface and to further enhance the surface-specific catalytic
activity (SA) of Pt. Thus far there has been no practical design that
combines these two features into one single catalyst. Here we report
a combined computational and experimental study on the design and
implementation of Pt-skin catalysts with significantly improved SA
toward the oxygen reduction reaction (ORR). Through screening, using
density functional theory (DFT) calculations, a Pt-skin structure
on AuCu(111) substrate, consisting of 1.5 monolayers of Pt, is found
to have an appropriately weakened oxygen affinity, in comparison to
that on Pt(111), which would be ideal for ORR catalysis. Such a structure
is then realized by substituting the Cu atoms in three surface layers
of AuCu intermetallic nanoparticles (AuCu <i>i</i>NPs) with
Pt. The resulting Pt-skinned catalyst (denoted as Pt<sup>S</sup>AuCu <i>i</i>NPs) has been characterized in depth using synchrotron
XRD, XPS, HRTEM, and HAADF-STEM/EDX, such that the Pt-skin structure
is unambiguously identified. The thickness of the Pt skin was determined
to be less than two atomic layers. Finally the catalytic activity
of Pt<sup>S</sup>AuCu <i>i</i>NPs toward the ORR was measured
via rotating disk electrode (RDE) voltammetry through which it was
established that the SA was more than 2 times that of a commercial
Pt/C catalyst. Taking into account the ultralow Pt loading in Pt<sup>S</sup>AuCu <i>i</i>NPs, the mass-specific catalytic activity
(MA) was determined to be 0.56 A/mg<sub>Pt</sub>@0.9 V, a value that
is well beyond the DOE 2017 target for ORR catalysts (0.44 A/mg<sub>Pt</sub>@0.9 V). These findings provide a strategic design and a
realizable approach to high-performance and Pt-efficient catalysts
for fuel cells