116
Y. Li et al.
the fuel cell at a voltage range of 0.6 to 1.0 V at 80 °C and found that the activity of
Pt ML /Pd 9 Au 1 /C core–shell structure only decreased by 8%, which is far less than the
40% reduction target set by DOE for 30,000 cycles. As a comparison, the initial ORR
activity of Pt ML /Pd/C core–shell structure is equivalent to that of Pt ML /Pd 9 Au 1 /C, but
under the same stability test conditions, the activity decreased by 37%, which is much
greater than Pt ML /Pd 9 Au 1 /C. Under the same stability test conditions, commercial
Pt/C dropped by 61% after 50 000 cycles.
They further calculated the stability of the system using density functional theory
and found that during the separation and stratification process, Au atoms would
preferentially remain at the defect sites left by the surface Pt, and then inhibit the
further dissolution of Pd and Pt [195]. As a result, its stability has been greatly
improved.
(2) Pt-enriched surface layer mode
The Pt-enrich core–shell structure in the surface layer means that Pt atoms are
mainly distributed on the surface layer of the nanoparticles, and there are only a
small number of Pt atoms inside. This type of core–shell structure can be prepared
by heat-treating a PtM (M = Fe, Co, Ni, Cu, etc.) alloy in a specific atmosphere (CO,
NO, O 2 , H 2 , etc.) [196]. During the heat treatment of PtM in these atmospheres, these
gases can induce the Pt atoms to the surface of the alloy nanoparticles. Different PtM
alloys use different gases for the surface heat treatment of Pt atom.
For example, heat treatment of PtCo alloy in CO atmosphere can induce most
Pt atoms to the surface of catalyst particles [197]. In addition, a M@Pt core–shell
ORR catalyst with a Pt-rich surface can be prepared by adding a precursor capable of
decomposing the above-mentioned gases during the preparation of the PtM alloy. For
example, in a special high-temperature solvent and protected by nitrogen, thermal
decomposition of Pt(acac) 2 and Co 2 (CO) 8 can prepare M@Pt core–shell catalyst
with a Pt-rich surface [198, 199].
Wang et al. [200] first took H 2 PtCl 6 ·6H 2 O and CoCl 2 ·6H 2 O as precursors, and
mixed them with carbon powder (Vulcan XC-72) in the liquid phase, and then reduced
them in a H 2 /N 2 atmosphere at 150°C in a H 2 atmosphere. Finally, heat treatment was
performed at 400 °Cand 700 °C for 2 h to obtain Pt 3 Co/C-400 and Pt 3 Co/C-700 with
a size of about 5 nm. These Pt 3 Co/C-400 and Pt 3 Co/C-700 have a core–shell structure
after heat treatment at 400 °C and 700 °C in a H 2 atmosphere. The thickness of the
outer layer is 0.5 nm, which is the 2–3 atomic layer of Pt-rich shell. Compared with
commercial Pt/C, the ORR half-wave potential (E 1/2 ) of Pt 3 Co/C-700 is positively
shifted by approximately 7 mV, and its mass activity at 0.9 V (vs. RHE) reaches
520 mA mg
−1
Pt which is nearly 9 times that of commercial Pt/C (60 mA mg −1Pt).
In addition, Pt 3 Co/C-700 also showed better stability than Pt/C. After 5 000 cycles
of stability tests, its half-wave potential was negatively shifted by less than 10 mV.
Huang Xiaoqing et al. [201] synthesized a PtPb-Pt core–shell nanoplate with a Pt
atomic shell layer of about 1 nm thick, and the two exposed surfaces are both Pt(110)
crystal planes. Biaxial compressive stress was found on the Pt(110) crystal plane.
ORR test results show that the specific activity and mass activity of the catalyst
Y. Li et al.
the fuel cell at a voltage range of 0.6 to 1.0 V at 80 °C and found that the activity of
Pt ML /Pd 9 Au 1 /C core–shell structure only decreased by 8%, which is far less than the
40% reduction target set by DOE for 30,000 cycles. As a comparison, the initial ORR
activity of Pt ML /Pd/C core–shell structure is equivalent to that of Pt ML /Pd 9 Au 1 /C, but
under the same stability test conditions, the activity decreased by 37%, which is much
greater than Pt ML /Pd 9 Au 1 /C. Under the same stability test conditions, commercial
Pt/C dropped by 61% after 50 000 cycles.
They further calculated the stability of the system using density functional theory
and found that during the separation and stratification process, Au atoms would
preferentially remain at the defect sites left by the surface Pt, and then inhibit the
further dissolution of Pd and Pt [195]. As a result, its stability has been greatly
improved.
(2) Pt-enriched surface layer mode
The Pt-enrich core–shell structure in the surface layer means that Pt atoms are
mainly distributed on the surface layer of the nanoparticles, and there are only a
small number of Pt atoms inside. This type of core–shell structure can be prepared
by heat-treating a PtM (M = Fe, Co, Ni, Cu, etc.) alloy in a specific atmosphere (CO,
NO, O 2 , H 2 , etc.) [196]. During the heat treatment of PtM in these atmospheres, these
gases can induce the Pt atoms to the surface of the alloy nanoparticles. Different PtM
alloys use different gases for the surface heat treatment of Pt atom.
For example, heat treatment of PtCo alloy in CO atmosphere can induce most
Pt atoms to the surface of catalyst particles [197]. In addition, a M@Pt core–shell
ORR catalyst with a Pt-rich surface can be prepared by adding a precursor capable of
decomposing the above-mentioned gases during the preparation of the PtM alloy. For
example, in a special high-temperature solvent and protected by nitrogen, thermal
decomposition of Pt(acac) 2 and Co 2 (CO) 8 can prepare M@Pt core–shell catalyst
with a Pt-rich surface [198, 199].
Wang et al. [200] first took H 2 PtCl 6 ·6H 2 O and CoCl 2 ·6H 2 O as precursors, and
mixed them with carbon powder (Vulcan XC-72) in the liquid phase, and then reduced
them in a H 2 /N 2 atmosphere at 150°C in a H 2 atmosphere. Finally, heat treatment was
performed at 400 °Cand 700 °C for 2 h to obtain Pt 3 Co/C-400 and Pt 3 Co/C-700 with
a size of about 5 nm. These Pt 3 Co/C-400 and Pt 3 Co/C-700 have a core–shell structure
after heat treatment at 400 °C and 700 °C in a H 2 atmosphere. The thickness of the
outer layer is 0.5 nm, which is the 2–3 atomic layer of Pt-rich shell. Compared with
commercial Pt/C, the ORR half-wave potential (E 1/2 ) of Pt 3 Co/C-700 is positively
shifted by approximately 7 mV, and its mass activity at 0.9 V (vs. RHE) reaches
520 mA mg
−1
Pt which is nearly 9 times that of commercial Pt/C (60 mA mg −1Pt).
In addition, Pt 3 Co/C-700 also showed better stability than Pt/C. After 5 000 cycles
of stability tests, its half-wave potential was negatively shifted by less than 10 mV.
Huang Xiaoqing et al. [201] synthesized a PtPb-Pt core–shell nanoplate with a Pt
atomic shell layer of about 1 nm thick, and the two exposed surfaces are both Pt(110)
crystal planes. Biaxial compressive stress was found on the Pt(110) crystal plane.
ORR test results show that the specific activity and mass activity of the catalyst