63
electrochemical oxidation of carbon support leads to degradation in the fuel cell
performance:
C H O CO
H
e
VRHE
+
→
+
+
=
(
)
+
−
2
2
4
4
0 118
E
.
(6.11)
Numerous studies reported that Pt and Pd alloys, with transition metals as Co,
Ni, Cr, and Fe, showed the enhancement factor up to 10 for the ORR if compared
with pure metal (Table 6.2).
Catalysts were carbon-supported alloys, sputtered alloy films, or metallurgically
prepared bulk alloys. The enhancement of the oxygen reduction rate was ascribed to
the: (i) the inhibition of the OH adsorption on Pt surface by the alloying metal [17],
(ii) the geometric modification of Pt [18], and (iii) extensive surface roughening
[19]. In addition, repulsive electronic interactions lowered the strength of adsorbed
oxygen species. The particle size effect on oxygen reduction was not unambiguously determined as far as the different Pt alloys are concerned. The electrochemical
results correlated with in situ X-ray adsorption spectroscopy (See Sect. 7.4.2) data
show that the alloys have higher Pt 5d-orbital vacancies and shorter Pt-Pt distances,
and that inhibits the chemisorptions of OH on the platinum facilitating faster oxygen reduction.
6.1.1 The d-Band Theory of Surface Reactivity
A major development in the theoretical treatment of surface reactivity took place in
the last decade and a half. Theoretical treatment of the atomic-level factors that
determine catalytic activity and selectivity became possible. In the d band theory of
metal surfaces, developed by Hammer and Nørskov, the electronic states in the
entire valence band of a metal surface are regarded responsible for the reactivity of
the surface [20]. For the transition and noble metals, the contribution from the metal
sp states is dominant but relatively constant. The coupling between an adsorbate and
the metal d-states is mainly responsible for the variation in the interaction energy on
different metals.
Table 6.2 Structural and electrochemical characteristics of the Pd/C and Pd-Fe/C electrocatalysts
for the ORR in 0.1 mol dm
−3 HClO 4
Sample
Particle size
(nm)
Pd-Pd bond
distance (nm)
Electrochemical area (cm
2
mg
−1
Pt )
j k at 0.85 V
(mA cm
−2 )
Pd/C
10.2
0.2753
446
0.131
Pd 4 Fe/C 9.6
0.2735
501
0.500
Pd 3 Fe/C 9.7
0.2730
726
0.791
Pd 2 Fe/C 7.6
0.2742
874
0.346
PdFe/C 7.7
0.2743
721
0.420
From Ref. [16]
6.1 Oxygen Reduction Reaction
electrochemical oxidation of carbon support leads to degradation in the fuel cell
performance:
C H O CO
H
e
VRHE
+
→
+
+
=
(
)
+
−
2
2
4
4
0 118
E
.
(6.11)
Numerous studies reported that Pt and Pd alloys, with transition metals as Co,
Ni, Cr, and Fe, showed the enhancement factor up to 10 for the ORR if compared
with pure metal (Table 6.2).
Catalysts were carbon-supported alloys, sputtered alloy films, or metallurgically
prepared bulk alloys. The enhancement of the oxygen reduction rate was ascribed to
the: (i) the inhibition of the OH adsorption on Pt surface by the alloying metal [17],
(ii) the geometric modification of Pt [18], and (iii) extensive surface roughening
[19]. In addition, repulsive electronic interactions lowered the strength of adsorbed
oxygen species. The particle size effect on oxygen reduction was not unambiguously determined as far as the different Pt alloys are concerned. The electrochemical
results correlated with in situ X-ray adsorption spectroscopy (See Sect. 7.4.2) data
show that the alloys have higher Pt 5d-orbital vacancies and shorter Pt-Pt distances,
and that inhibits the chemisorptions of OH on the platinum facilitating faster oxygen reduction.
6.1.1 The d-Band Theory of Surface Reactivity
A major development in the theoretical treatment of surface reactivity took place in
the last decade and a half. Theoretical treatment of the atomic-level factors that
determine catalytic activity and selectivity became possible. In the d band theory of
metal surfaces, developed by Hammer and Nørskov, the electronic states in the
entire valence band of a metal surface are regarded responsible for the reactivity of
the surface [20]. For the transition and noble metals, the contribution from the metal
sp states is dominant but relatively constant. The coupling between an adsorbate and
the metal d-states is mainly responsible for the variation in the interaction energy on
different metals.
Table 6.2 Structural and electrochemical characteristics of the Pd/C and Pd-Fe/C electrocatalysts
for the ORR in 0.1 mol dm
−3 HClO 4
Sample
Particle size
(nm)
Pd-Pd bond
distance (nm)
Electrochemical area (cm
2
mg
−1
Pt )
j k at 0.85 V
(mA cm
−2 )
Pd/C
10.2
0.2753
446
0.131
Pd 4 Fe/C 9.6
0.2735
501
0.500
Pd 3 Fe/C 9.7
0.2730
726
0.791
Pd 2 Fe/C 7.6
0.2742
874
0.346
PdFe/C 7.7
0.2743
721
0.420
From Ref. [16]
6.1 Oxygen Reduction Reaction