109
The unique features of Pt ML electrocatalysts enable a wide selection of substrates
to attain an electrocatalyst having low noble-metal content with enhanced catalytic
activity and stability. A desired substrate should be able to affect the Pt monolayer
by causing a weakening of the Pt-OH bond, a reduction of the OH coverage, and a
delay of PtOH formation to more positive potentials than on pure Pt. The followings
are some examples of the design of substrates to achieve the above goals.
Varying the Composition and Structure The composition of the substrate has a
significant impact on the catalytic activity of Pt ML , through strain effect and electronic coupling. Pd has been found the most active metal as the substrate. To further
reduce the content of noble-metal and/or to enhance the catalytic activity and stability, nonnoble metal, for example Co, Ni, Fe, can be incorporated into the core as
well as the components with higher dissolution potential, Au and Ir, for example. In
addition to the random mix, a core-shell structure with stable shell is another effective way to preclude the dissolution of nonnoble metal core and/or to mediate the
activity over a less reactive core.
For example, the addition of a small content of Au to Pd, that is a Pd 9 Au 1 alloy
core, achieves a remarkable enhancement of the stability while retaining the ORR
activity as high as that on Pt ML /Pd/C [13] (See Sect. 8.1.3.5). MEA fuel-cell tests
showed that Pt ML /Pd 9 Au/C remains active even after 200,000 potential cycles. In
addition to alloys, highly stable, inexpensive intermetallic compounds can also be
attractive candidates as the supports for Pt ML . Pt ML electrocatalyst with intermetallic
PdPb core exhibits an ORR activity superior to Pt/C [14]. Surface and subsurface
modifications by monolayer or submonolayer of various metals can be effective.
Surface modifications by Au clusters and 2D and 3D islands all provide possibilities
for improving Pt monolayer electrocatalysts.
Multimetallic alloy nanoparticle cores, obtained by in situ decomposition of a
Prussian blue analogue, for example Pt ML /AuNi 0.5 Fe, show a remarkable Pt mass
activity as 1.38 A/mg Pt and all-noble-metal activity as 0.18 A/mg Pt + Au [15] (Fig. 8.7).
The interaction of Pt ML with the stratified structure of the core containing 3–5 atomic
layers of Au, plays an essential role in determining the activity and stability of the
catalyst (insignificant loss after 15,000 triangular-potential cycles) and the highelectrochemical stability of the gold shell, which precludes the exposure of the relatively active inner-core materials to the electrolyte.
The high-temperature-annealed Pd 3 Fe(111) alloy core, that is Pd/Pd 3 Fe, has a
top layer with the structure same as Pd(111) but different electronic properties, (i.e.
a − 0.25 eV downshift of the d-band center compared to Pd(111)) [16]. It is
considerably more active than Pd(111), and the observed enhancement of the ORR
activity of Pt ML /Pd/Pd 3 Fe originates mainly from the destabilization of OH binding,
leading to a decreased Pt-OH coverage on the Pt surface.
The IrNi core–shell nanoparticle core has a two-layer Ir shell around inner Ni
core fabricated via thermally induced segregation [18]. The Ir shell completely
protects the Ni atoms in the core from oxidation or dissolution under elevated
potentials after 5000 potential cycles. It is evident that Ir shell completely protects
Ni core from oxidation or dissolution in acid electrolyte under elevated potentials.
8.1 Oxygen Reduction Reaction (ORR)
The unique features of Pt ML electrocatalysts enable a wide selection of substrates
to attain an electrocatalyst having low noble-metal content with enhanced catalytic
activity and stability. A desired substrate should be able to affect the Pt monolayer
by causing a weakening of the Pt-OH bond, a reduction of the OH coverage, and a
delay of PtOH formation to more positive potentials than on pure Pt. The followings
are some examples of the design of substrates to achieve the above goals.
Varying the Composition and Structure The composition of the substrate has a
significant impact on the catalytic activity of Pt ML , through strain effect and electronic coupling. Pd has been found the most active metal as the substrate. To further
reduce the content of noble-metal and/or to enhance the catalytic activity and stability, nonnoble metal, for example Co, Ni, Fe, can be incorporated into the core as
well as the components with higher dissolution potential, Au and Ir, for example. In
addition to the random mix, a core-shell structure with stable shell is another effective way to preclude the dissolution of nonnoble metal core and/or to mediate the
activity over a less reactive core.
For example, the addition of a small content of Au to Pd, that is a Pd 9 Au 1 alloy
core, achieves a remarkable enhancement of the stability while retaining the ORR
activity as high as that on Pt ML /Pd/C [13] (See Sect. 8.1.3.5). MEA fuel-cell tests
showed that Pt ML /Pd 9 Au/C remains active even after 200,000 potential cycles. In
addition to alloys, highly stable, inexpensive intermetallic compounds can also be
attractive candidates as the supports for Pt ML . Pt ML electrocatalyst with intermetallic
PdPb core exhibits an ORR activity superior to Pt/C [14]. Surface and subsurface
modifications by monolayer or submonolayer of various metals can be effective.
Surface modifications by Au clusters and 2D and 3D islands all provide possibilities
for improving Pt monolayer electrocatalysts.
Multimetallic alloy nanoparticle cores, obtained by in situ decomposition of a
Prussian blue analogue, for example Pt ML /AuNi 0.5 Fe, show a remarkable Pt mass
activity as 1.38 A/mg Pt and all-noble-metal activity as 0.18 A/mg Pt + Au [15] (Fig. 8.7).
The interaction of Pt ML with the stratified structure of the core containing 3–5 atomic
layers of Au, plays an essential role in determining the activity and stability of the
catalyst (insignificant loss after 15,000 triangular-potential cycles) and the highelectrochemical stability of the gold shell, which precludes the exposure of the relatively active inner-core materials to the electrolyte.
The high-temperature-annealed Pd 3 Fe(111) alloy core, that is Pd/Pd 3 Fe, has a
top layer with the structure same as Pd(111) but different electronic properties, (i.e.
a − 0.25 eV downshift of the d-band center compared to Pd(111)) [16]. It is
considerably more active than Pd(111), and the observed enhancement of the ORR
activity of Pt ML /Pd/Pd 3 Fe originates mainly from the destabilization of OH binding,
leading to a decreased Pt-OH coverage on the Pt surface.
The IrNi core–shell nanoparticle core has a two-layer Ir shell around inner Ni
core fabricated via thermally induced segregation [18]. The Ir shell completely
protects the Ni atoms in the core from oxidation or dissolution under elevated
potentials after 5000 potential cycles. It is evident that Ir shell completely protects
Ni core from oxidation or dissolution in acid electrolyte under elevated potentials.
8.1 Oxygen Reduction Reaction (ORR)