4 Catalyst Materials for Oxygen Reduction Reaction
157
it often the rate-determining step of ORR. Compared with three different types of
doped N atoms, graphite nitrogen can maximally reduce the energy barrier of the first
electron transfer in the ORR reaction process, and has the highest selectivity for the
four-electron process of ORR, so the graphite nitrogen in N-graphene may have the
highest ORR activity [394]. He et al. [378] synthesized N-graphene, which is mainly
graphite type nitrogen doped. It is one of the best non-metallic ORR catalysts.
4.6.5 Other Impurity Atoms Doped Graphene
When B atom is doped into the skeleton of graphene, B atom has one less valence
electron than C atom, and it will show strong electron absorption characteristics.
When graphene doped with B atom (B-graphene) is used as non-metallic ORR catalyst, the borate covalent bond formed by combining with oxygen and the BO 3 –G
formed by combining with graphene can effectively break the O = O covalent bond,
which is the key step for the reduction of O 2 to H 2 O [395].
Sheng et al. [396] prepared B-graphene with boron doping of 3.2% by heat treatment of graphene with boron oxide. In alkaline medium, this B-graphene showed
very good ORR activity, and its peak potential was - 0.05v (vs. Ag / AgCl), which
was a direct four-electron reaction process. Similar to N-graphene, B-graphene also
shows cathode selectivity, good anti-toxic ability, and good stability.
Tam et al. [396] adding glucose, boric acid, and graphene mixture to prepare
boron-doped graphene hydrogels by hydrothermal method. They can be used as HER,
OER, ORR three functional electrocatalysts and applied to zinc air batteries. The
flexible solid zinc air battery can achieve 1.40 V open-circuit voltage, and the peak
power density is 112 mW cm
−2 , which is 10 mA cm
−2 as water electrolysis. Requires
only 1.61 V. Zhang et al. [397] prepared 1.32% P-graphene with triphenylphosphine
(TPP) and GO by heat treatment. P-graphene doped into graphene exists in P–C bond
and P-O bond. This P-graphene shows very good ORR activity, and its peak potential
is -0.07 V (vs. Ag/AgCl). Different from N-graphene, P-graphene is a mixing process
of two electrons and four electrons. Similar to N-graphene, P-graphene also shows
cathode selectivity, good anti-toxic ability, and good stability. Unlike B, N, and C,
which form the SP
2 hybrid orbital plane, P is located in the third period, and its
covalent radius is larger than C, so the SP
3 orbital configuration is formed. The part
of P covalently bound with C is oxidized to form tetrahedral C 3 PO, C 2 PO 2 , and
CPO 3 . The partially oxidized P doping causes the charge redistribution of graphene.
The O atom with the highest electronegativity will first polarize the P atom, and then
bridge the P atom to attract electrons from the C atom, making the C atom adjacent
to the P atom generate positive charge, becoming the active site of O 2 adsorption and
weakening the O–O bond, and then promote the reduction of O 2 to OH
− [398] by
attracting electrons from the positive electrode.
Yang et al. [399] prepared S-graphene with sulfur content of 1.30% by using
GO and BDS (benzyldisulfide) as precursors. This S-graphene has the same ORR
157
it often the rate-determining step of ORR. Compared with three different types of
doped N atoms, graphite nitrogen can maximally reduce the energy barrier of the first
electron transfer in the ORR reaction process, and has the highest selectivity for the
four-electron process of ORR, so the graphite nitrogen in N-graphene may have the
highest ORR activity [394]. He et al. [378] synthesized N-graphene, which is mainly
graphite type nitrogen doped. It is one of the best non-metallic ORR catalysts.
4.6.5 Other Impurity Atoms Doped Graphene
When B atom is doped into the skeleton of graphene, B atom has one less valence
electron than C atom, and it will show strong electron absorption characteristics.
When graphene doped with B atom (B-graphene) is used as non-metallic ORR catalyst, the borate covalent bond formed by combining with oxygen and the BO 3 –G
formed by combining with graphene can effectively break the O = O covalent bond,
which is the key step for the reduction of O 2 to H 2 O [395].
Sheng et al. [396] prepared B-graphene with boron doping of 3.2% by heat treatment of graphene with boron oxide. In alkaline medium, this B-graphene showed
very good ORR activity, and its peak potential was - 0.05v (vs. Ag / AgCl), which
was a direct four-electron reaction process. Similar to N-graphene, B-graphene also
shows cathode selectivity, good anti-toxic ability, and good stability.
Tam et al. [396] adding glucose, boric acid, and graphene mixture to prepare
boron-doped graphene hydrogels by hydrothermal method. They can be used as HER,
OER, ORR three functional electrocatalysts and applied to zinc air batteries. The
flexible solid zinc air battery can achieve 1.40 V open-circuit voltage, and the peak
power density is 112 mW cm
−2 , which is 10 mA cm
−2 as water electrolysis. Requires
only 1.61 V. Zhang et al. [397] prepared 1.32% P-graphene with triphenylphosphine
(TPP) and GO by heat treatment. P-graphene doped into graphene exists in P–C bond
and P-O bond. This P-graphene shows very good ORR activity, and its peak potential
is -0.07 V (vs. Ag/AgCl). Different from N-graphene, P-graphene is a mixing process
of two electrons and four electrons. Similar to N-graphene, P-graphene also shows
cathode selectivity, good anti-toxic ability, and good stability. Unlike B, N, and C,
which form the SP
2 hybrid orbital plane, P is located in the third period, and its
covalent radius is larger than C, so the SP
3 orbital configuration is formed. The part
of P covalently bound with C is oxidized to form tetrahedral C 3 PO, C 2 PO 2 , and
CPO 3 . The partially oxidized P doping causes the charge redistribution of graphene.
The O atom with the highest electronegativity will first polarize the P atom, and then
bridge the P atom to attract electrons from the C atom, making the C atom adjacent
to the P atom generate positive charge, becoming the active site of O 2 adsorption and
weakening the O–O bond, and then promote the reduction of O 2 to OH
− [398] by
attracting electrons from the positive electrode.
Yang et al. [399] prepared S-graphene with sulfur content of 1.30% by using
GO and BDS (benzyldisulfide) as precursors. This S-graphene has the same ORR