119
as the core has been reported recently [32]. Synthesis of Pd x Au-Pt core–shell aerogels involves first synthesis of Pd x Au alloyed aerogels with controlled composition,
serving as core substrates, using a spontaneous gelation method. After supercritical
drying, the resulting Pd x Au aerogels exhibit an extensive 3D porous network structure composed of nanowires with an average diameter of about 4.5 nm. These
nanowires fuse and interconnect randomly, growing into the backbones of the
aerogels, thus resulting in a broad pore size distribution with open pores. Lattice
fringes observed in high resolution (HR) indicate a lattice expansion induced by Au
alloying.
Alloying of the aerogel structures was further confirmed by element mapping
based on energy dispersive X-ray spectroscopy (EDXS) in scanning TEM (STEM)
mode, where Au and Pd are distributed throughout the network backbones
(Fig. 8.12). A Pt shell on the as-prepared Pd x Au aerogel core was obtained by galvanic displacement of UPD of Cu by Pt. The core–shell aerogels inherit the winding
topography of the wire-based backbones and remains the 3D network structure
(Fig. 8.12a–c). As indicated in circles in Fig. 8.12d, twined crystal structures were
frequently observed throughout the winding backbones, which usually impose a
compressive strain on the Pt and could be beneficial for the ORR (Fig. 8.13).
This core–shell synthesis was further extended to Pd x M-Pt (M = Ni, Co and Cu)
core–shell aerogels. The metal–metal bond at the core–shell interface allows for a
facile manipulation of the electronic properties of the Pt shell by tuning the core
substrates and leads to a variation of the ORR activity with different core substrates.
A novel, indirect activity descriptor was proposed based on the core substrates to
elucidate the activity variation of the core–shell aerogels. Studies showed that both
the mass and specific activities of the core–shell aerogels exhibit a volcano-type
relationship as a function of the lattice parameter (a) of the core substrates. This
work highlights the great potential of pure metallic core–shell aerogels as highly
efficient electrocatalysts through structural engineering. Furthermore, the proposed
core-based indirect activity descriptor provides new possible strategies for the
design of next-generation core–shell electrocatalysts.
8.1.3.5 Gold Clusters and Alloys, Promotion, and Stability Enhancing
Effects
It is well established that binary Pt and Pd alloys are better catalysts for the ORR
than pure metals. Using cores for a Pt shell in core–shell catalysts appears an
attractive possibility for improving them. A major obstacle to this application has
been observed in recent studies that recorded a substantial loss of Pt surface area
over time. Since Pt is the electrocatalyst for the cathodic reduction of oxygen in
PEMFCs during the stop-and-go driving of an electric car, the regime that generate
large electrode potential excursions (typically 0.6–1.0 V), resulting in Pt dissolution. This is a major challenge for fuel cell technology and the science of
electrocatalysis.
8.1 Oxygen Reduction Reaction (ORR)
as the core has been reported recently [32]. Synthesis of Pd x Au-Pt core–shell aerogels involves first synthesis of Pd x Au alloyed aerogels with controlled composition,
serving as core substrates, using a spontaneous gelation method. After supercritical
drying, the resulting Pd x Au aerogels exhibit an extensive 3D porous network structure composed of nanowires with an average diameter of about 4.5 nm. These
nanowires fuse and interconnect randomly, growing into the backbones of the
aerogels, thus resulting in a broad pore size distribution with open pores. Lattice
fringes observed in high resolution (HR) indicate a lattice expansion induced by Au
alloying.
Alloying of the aerogel structures was further confirmed by element mapping
based on energy dispersive X-ray spectroscopy (EDXS) in scanning TEM (STEM)
mode, where Au and Pd are distributed throughout the network backbones
(Fig. 8.12). A Pt shell on the as-prepared Pd x Au aerogel core was obtained by galvanic displacement of UPD of Cu by Pt. The core–shell aerogels inherit the winding
topography of the wire-based backbones and remains the 3D network structure
(Fig. 8.12a–c). As indicated in circles in Fig. 8.12d, twined crystal structures were
frequently observed throughout the winding backbones, which usually impose a
compressive strain on the Pt and could be beneficial for the ORR (Fig. 8.13).
This core–shell synthesis was further extended to Pd x M-Pt (M = Ni, Co and Cu)
core–shell aerogels. The metal–metal bond at the core–shell interface allows for a
facile manipulation of the electronic properties of the Pt shell by tuning the core
substrates and leads to a variation of the ORR activity with different core substrates.
A novel, indirect activity descriptor was proposed based on the core substrates to
elucidate the activity variation of the core–shell aerogels. Studies showed that both
the mass and specific activities of the core–shell aerogels exhibit a volcano-type
relationship as a function of the lattice parameter (a) of the core substrates. This
work highlights the great potential of pure metallic core–shell aerogels as highly
efficient electrocatalysts through structural engineering. Furthermore, the proposed
core-based indirect activity descriptor provides new possible strategies for the
design of next-generation core–shell electrocatalysts.
8.1.3.5 Gold Clusters and Alloys, Promotion, and Stability Enhancing
Effects
It is well established that binary Pt and Pd alloys are better catalysts for the ORR
than pure metals. Using cores for a Pt shell in core–shell catalysts appears an
attractive possibility for improving them. A major obstacle to this application has
been observed in recent studies that recorded a substantial loss of Pt surface area
over time. Since Pt is the electrocatalyst for the cathodic reduction of oxygen in
PEMFCs during the stop-and-go driving of an electric car, the regime that generate
large electrode potential excursions (typically 0.6–1.0 V), resulting in Pt dissolution. This is a major challenge for fuel cell technology and the science of
electrocatalysis.
8.1 Oxygen Reduction Reaction (ORR)