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Y. Li et al.
When graphene and doped graphene support Pt-based ORR catalysts, due
to their high specific surface area, high electrical conductivity, and high chemical/electrochemical stability, they are an ideal carrier for Pt-based nanoparticles,
which was proved by experimental and theoretical calculations [168, 169]. The effect
of the electronic coupling effect between Pt-based nanoparticles and graphene and
doped graphene on the ORR activity of Pt-based nanoparticles further increases its
application prospect in the design of low-Pt ORR catalysts in the future.
4.4 Non-Platinum Catalyst
Pt-based catalysts are currently the best ORR catalyst. However, due to the expensive
price and scarcity of Pt, it is difficult to use them on a large scale as ORR catalysts in
fuel cells. Pd is considered to be an ORR catalyst that can replace Pt. Compared with
Pt, Pd is a cheaper metal and has richer reserves on the earth. Although the ORR
activity of Pd is less than that of Pt, its anti-toxicity performance is better and has
better cathodic selectivity, because it is not active in acidic medium for the oxidation
of methanol and its intermediates, etc. [170, 171]. In alkaline media, Ag can be used
as a cathodic oxygen reduction catalyst [172, 173]. Ag is a cheaper metal and its
reserves on earth are much larger than those of Pt and Pd. In addition, other metal
materials have also been used for research because of their ORR activity, such as
gold [174], nano-gold [175], and ruthenium [176].
Since Jasinski [177] discovered that cobalt phthalocyanine (CoPC) has catalytic
oxygen reduction performance in 1964, a large number of researchers have researched
to find new and effective cheap transition metal-based oxygen reduction catalysts
[178–181]. This type of catalyst is also called non-precious metal catalyst. In addition, some oxides are also active for oxygen reduction, such as Mn 2 O 3 [182],
Mn 3 O 4 [182], MnOOH [183], Co 3 O 4 [158], Fe 3 O 4 [184], MnCo 2 O 4 [159]. Moreover, sandwich-structured polypyrrole and CoFe 2 O 4 nanoparticles [185] have been
reported as oxygen reduction catalysts.
4.4.1 Pd-Based ORR Catalyst
The current price of palladium (Pd) metal is only half or less of Pt. Because the
ORR performance of Pd in alkaline is comparable to that of Pt, currently, Pd-based
non-platinum electrocatalysts have been widely used in cathode reactions of alkaline
alcohol fuel cell. In acidic media, the ORR activity of Pd is much worse than that
of Pt, which is not enough for direct application to proton exchange membrane fuel
cells. The current research focuses on the structural modification, electronic property
adjustment and carrier enhancement of Pd to increase its ORR activity. There have
been many significant advances in related research and it is expected to be applied
and promoted in proton exchange membrane fuel cells.
Y. Li et al.
When graphene and doped graphene support Pt-based ORR catalysts, due
to their high specific surface area, high electrical conductivity, and high chemical/electrochemical stability, they are an ideal carrier for Pt-based nanoparticles,
which was proved by experimental and theoretical calculations [168, 169]. The effect
of the electronic coupling effect between Pt-based nanoparticles and graphene and
doped graphene on the ORR activity of Pt-based nanoparticles further increases its
application prospect in the design of low-Pt ORR catalysts in the future.
4.4 Non-Platinum Catalyst
Pt-based catalysts are currently the best ORR catalyst. However, due to the expensive
price and scarcity of Pt, it is difficult to use them on a large scale as ORR catalysts in
fuel cells. Pd is considered to be an ORR catalyst that can replace Pt. Compared with
Pt, Pd is a cheaper metal and has richer reserves on the earth. Although the ORR
activity of Pd is less than that of Pt, its anti-toxicity performance is better and has
better cathodic selectivity, because it is not active in acidic medium for the oxidation
of methanol and its intermediates, etc. [170, 171]. In alkaline media, Ag can be used
as a cathodic oxygen reduction catalyst [172, 173]. Ag is a cheaper metal and its
reserves on earth are much larger than those of Pt and Pd. In addition, other metal
materials have also been used for research because of their ORR activity, such as
gold [174], nano-gold [175], and ruthenium [176].
Since Jasinski [177] discovered that cobalt phthalocyanine (CoPC) has catalytic
oxygen reduction performance in 1964, a large number of researchers have researched
to find new and effective cheap transition metal-based oxygen reduction catalysts
[178–181]. This type of catalyst is also called non-precious metal catalyst. In addition, some oxides are also active for oxygen reduction, such as Mn 2 O 3 [182],
Mn 3 O 4 [182], MnOOH [183], Co 3 O 4 [158], Fe 3 O 4 [184], MnCo 2 O 4 [159]. Moreover, sandwich-structured polypyrrole and CoFe 2 O 4 nanoparticles [185] have been
reported as oxygen reduction catalysts.
4.4.1 Pd-Based ORR Catalyst
The current price of palladium (Pd) metal is only half or less of Pt. Because the
ORR performance of Pd in alkaline is comparable to that of Pt, currently, Pd-based
non-platinum electrocatalysts have been widely used in cathode reactions of alkaline
alcohol fuel cell. In acidic media, the ORR activity of Pd is much worse than that
of Pt, which is not enough for direct application to proton exchange membrane fuel
cells. The current research focuses on the structural modification, electronic property
adjustment and carrier enhancement of Pd to increase its ORR activity. There have
been many significant advances in related research and it is expected to be applied
and promoted in proton exchange membrane fuel cells.