124
Other alloys: Platinum monolayer shell placed on an iridium−rhenium nanoparticle core or platinum and palladium bilayer. These catalysts showed specific electrocatalytic activity for oxygen reduction reaction comparable to that of platinum.
The activities of Pt ML /Pd ML /Ir 2 Re 1 , Pt ML /Pd 2 layers/Ir 2 Re 1 , and Pt ML /Pd 2layers/
Ir 7 Re 3 catalysts were, in fact, better than that of conventional platinum electrocatalysts, and their mass activities exceeded the 2015 DOE target Pd/IrNi cores [9, 34].
8.1.3.6 Intermetallic Compounds as Cores
Structurally ordered intermetallic phases generally exhibit a higher stability and
also higher activity for various reactions than disordered alloys. Thus, recently
within the effort to develop fuel cell electrocatalysts, these compounds attracted a
considerable research interest.
It has been shown previously that intermetallic nanoparticles can be effective
electrocatalysts for oxidation of small organic molecules. Using such intermetallic
particles as support for a Pt monolayer, given their high thermodynamic stability,
can have three beneficial consequences: (i) obtaining a stable core, that is increasing
catalyst stability, (ii) increasing catalyst activity, and (iii) decreasing the price with
low Pt content and high activity for oxygen reduction.
Several selected intermetallic nanoparticles, including PtPb, PdPb, and PdFe
were tested. Single phase PtPb nanoparticles with Pt:Pb molar ratio 1:1, domain
size 18 nm, were synthesized by co-reduction of Pt(acac) 2 and Pb(2-ethylhexanoate)
by sodium napthalide in tetrahydrofuran (THF), at room temperature. The PdFe
phase was synthesized by reduction of FeCl 2 .4H 2 O and PdCl 2 with NaBH 4 .
The initial voltammetry of the PdPb nanoparticles highlighted the dissolution of
Pb, which subsided after several cycles. This finding indicated the presence of some
Pb in the surface layer. The Pt/Pd monolayer was prepared by the galvanic replacement with Pt atoms of an underpotentially deposited Cu.
Table 8.3 Platinum mass activity, electrochemical surface area, specific activity, and particle size
for electrocatalysts before and after potential cycling tests. Adapted from reference [33]
Sample
Pt mass activity
ECSA
Specific activity
Size
[mA mg
−1
]
[cm
−2 mg
−1
]
[mAcm
−2
]
[nm]
Pt initial
0.13
0.90
0.14
3.5
Pt 6 × 10
4
0.05
0.27
0.19
Pt ML /Pd
0.30
1.20
0.25
4.0
Initial
Pt ML /Pd 6 × 10
4
0.24
0.93
0.26
–
Pt ML /Pd 1× 10
5
0.19
0.70
0.27
3.6
Pt ML /Pd 9 Au 1
0.30
0.60
0.5
5.0
Initial
Pt ML /Pd 9 Au 1 2 × 10
5
0.20
0.20
1.0
3.8
8 Catalytic Properties of Pt Monolayer Electrocatalysts
Other alloys: Platinum monolayer shell placed on an iridium−rhenium nanoparticle core or platinum and palladium bilayer. These catalysts showed specific electrocatalytic activity for oxygen reduction reaction comparable to that of platinum.
The activities of Pt ML /Pd ML /Ir 2 Re 1 , Pt ML /Pd 2 layers/Ir 2 Re 1 , and Pt ML /Pd 2layers/
Ir 7 Re 3 catalysts were, in fact, better than that of conventional platinum electrocatalysts, and their mass activities exceeded the 2015 DOE target Pd/IrNi cores [9, 34].
8.1.3.6 Intermetallic Compounds as Cores
Structurally ordered intermetallic phases generally exhibit a higher stability and
also higher activity for various reactions than disordered alloys. Thus, recently
within the effort to develop fuel cell electrocatalysts, these compounds attracted a
considerable research interest.
It has been shown previously that intermetallic nanoparticles can be effective
electrocatalysts for oxidation of small organic molecules. Using such intermetallic
particles as support for a Pt monolayer, given their high thermodynamic stability,
can have three beneficial consequences: (i) obtaining a stable core, that is increasing
catalyst stability, (ii) increasing catalyst activity, and (iii) decreasing the price with
low Pt content and high activity for oxygen reduction.
Several selected intermetallic nanoparticles, including PtPb, PdPb, and PdFe
were tested. Single phase PtPb nanoparticles with Pt:Pb molar ratio 1:1, domain
size 18 nm, were synthesized by co-reduction of Pt(acac) 2 and Pb(2-ethylhexanoate)
by sodium napthalide in tetrahydrofuran (THF), at room temperature. The PdFe
phase was synthesized by reduction of FeCl 2 .4H 2 O and PdCl 2 with NaBH 4 .
The initial voltammetry of the PdPb nanoparticles highlighted the dissolution of
Pb, which subsided after several cycles. This finding indicated the presence of some
Pb in the surface layer. The Pt/Pd monolayer was prepared by the galvanic replacement with Pt atoms of an underpotentially deposited Cu.
Table 8.3 Platinum mass activity, electrochemical surface area, specific activity, and particle size
for electrocatalysts before and after potential cycling tests. Adapted from reference [33]
Sample
Pt mass activity
ECSA
Specific activity
Size
[mA mg
−1
]
[cm
−2 mg
−1
]
[mAcm
−2
]
[nm]
Pt initial
0.13
0.90
0.14
3.5
Pt 6 × 10
4
0.05
0.27
0.19
Pt ML /Pd
0.30
1.20
0.25
4.0
Initial
Pt ML /Pd 6 × 10
4
0.24
0.93
0.26
–
Pt ML /Pd 1× 10
5
0.19
0.70
0.27
3.6
Pt ML /Pd 9 Au 1
0.30
0.60
0.5
5.0
Initial
Pt ML /Pd 9 Au 1 2 × 10
5
0.20
0.20
1.0
3.8
8 Catalytic Properties of Pt Monolayer Electrocatalysts