4 Catalyst Materials for Oxygen Reduction Reaction
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of Pt atoms. This structure is also conducive to mass transfer. In addition, during the
etching process, Pt atoms will rearrange and present the surface of the Pt skin, which
is not only conducive to improving the catalytic performance but also improving
the durability and stability. Nanoframe or nanocage formation through etching can
greatly improve the catalytic efficiency and reduce the cost of commercialization.
4.3 Pt-Cocatalyst System
Other substances added to the catalyst are not active or less active, but can change
some properties of the catalyst, such as chemical composition, ionic valence, acidity
and alkalinity, surface structure, and grain size, which can improve activity, selectivity, anti-toxicity, or stability. This effect is usually called “synergistic effect,” and
it is generally called “one plus one greater than two.” For a Pt-based ORR catalyst, if
the addition of a foreign substance can promote the activity or stability, we can call
the foreign substance a synergistic component or co-catalyst to form a Pt-cocatalyst.
At present, there are mainly three types of promoters that have a “synergistic effect”
on Pt: (1) transition metal oxides, (2) transition metal carbides, and (3) graphene and
doped graphene.
4.3.1 Transition Metal Oxide ORR Co-Catalyst
The research on the Pt-cocatalyst system can be traced back to 1974, when Tseung
et al. [205] found that antimony-doped SnO 2 in 85% H3PO 4 aqueous solution could
significantly enhance the oxygen reduction performance of Pt electrocatalyst. They
believe that the antimony-doped SnO 2
s effect on ORR activity of Pt is a so-called
“oxygen overflow,” that is, the oxygen molecules adsorbed on the Pt active site
can be transferred to the antimony-doped SnO 2 surface, forming continuous oxygen
transport and proximity storage effect, while greatly improves the oxygen reduction kinetics. Later, we found that many oxides can improve the ORR activity of
Pt, including CeO 2 , WO 3 , NiO x , TiO 2 , MnO 2 , NbO 2 , and Sn 0.96 Sb 0.04 O 2−δ , etc.
[140, 205–208]. These oxides can not only improve the ORR activity of Pt, but also
some of them can improve the alcohol resistance and anti-toxicity of Pt. However,
researchers have found that most oxides have poor electrochemical stability under
acidic conditions, making the “synergistic effect” unsustainable.
Among many oxides, due to its high electrochemical stability in acidity, there
have been many studies using TiO 2 as a co-catalyst for Pt, but because the outer
electrons are involved in bonding, which result in its interaction with Pt is limited.
In addition, the band gap of TiO 2 is very large, and its conductivity is very poor,
which is very unfavorable as a co-catalyst for Pt-based ORR catalysts. Based on
this, researchers have developed a hypoxic type of Magnéli phase Ti n O 2n−1 (3 ≤ n
≤ 10) as a co-catalyst for Pt-based ORR catalysts [140, 209]. This material possess
119
of Pt atoms. This structure is also conducive to mass transfer. In addition, during the
etching process, Pt atoms will rearrange and present the surface of the Pt skin, which
is not only conducive to improving the catalytic performance but also improving
the durability and stability. Nanoframe or nanocage formation through etching can
greatly improve the catalytic efficiency and reduce the cost of commercialization.
4.3 Pt-Cocatalyst System
Other substances added to the catalyst are not active or less active, but can change
some properties of the catalyst, such as chemical composition, ionic valence, acidity
and alkalinity, surface structure, and grain size, which can improve activity, selectivity, anti-toxicity, or stability. This effect is usually called “synergistic effect,” and
it is generally called “one plus one greater than two.” For a Pt-based ORR catalyst, if
the addition of a foreign substance can promote the activity or stability, we can call
the foreign substance a synergistic component or co-catalyst to form a Pt-cocatalyst.
At present, there are mainly three types of promoters that have a “synergistic effect”
on Pt: (1) transition metal oxides, (2) transition metal carbides, and (3) graphene and
doped graphene.
4.3.1 Transition Metal Oxide ORR Co-Catalyst
The research on the Pt-cocatalyst system can be traced back to 1974, when Tseung
et al. [205] found that antimony-doped SnO 2 in 85% H3PO 4 aqueous solution could
significantly enhance the oxygen reduction performance of Pt electrocatalyst. They
believe that the antimony-doped SnO 2
s effect on ORR activity of Pt is a so-called
“oxygen overflow,” that is, the oxygen molecules adsorbed on the Pt active site
can be transferred to the antimony-doped SnO 2 surface, forming continuous oxygen
transport and proximity storage effect, while greatly improves the oxygen reduction kinetics. Later, we found that many oxides can improve the ORR activity of
Pt, including CeO 2 , WO 3 , NiO x , TiO 2 , MnO 2 , NbO 2 , and Sn 0.96 Sb 0.04 O 2−δ , etc.
[140, 205–208]. These oxides can not only improve the ORR activity of Pt, but also
some of them can improve the alcohol resistance and anti-toxicity of Pt. However,
researchers have found that most oxides have poor electrochemical stability under
acidic conditions, making the “synergistic effect” unsustainable.
Among many oxides, due to its high electrochemical stability in acidity, there
have been many studies using TiO 2 as a co-catalyst for Pt, but because the outer
electrons are involved in bonding, which result in its interaction with Pt is limited.
In addition, the band gap of TiO 2 is very large, and its conductivity is very poor,
which is very unfavorable as a co-catalyst for Pt-based ORR catalysts. Based on
this, researchers have developed a hypoxic type of Magnéli phase Ti n O 2n−1 (3 ≤ n
≤ 10) as a co-catalyst for Pt-based ORR catalysts [140, 209]. This material possess