130
core. The prepared Pt ML PdNiN/C catalyst retains 89% of the initial electrochemical
surface area after 50,000 cycles between potentials 0.6 and 1.0 V. Correlation
between the electron energy-loss spectroscopy and X-ray absorption spectroscopy
analyses with electrochemical measurements shows that the significant improvement
of stability of the Pt ML PdNiN/C catalyst is caused by nitrogen doping while reducing
the total precious metal loading [38].
Nitrogen has one of the strongest covalent bonds; thus it is very stable and inert
under normal conditions. Yet nitrogen reacts with selected elements, forming
compounds with a variety of intriguing properties. Although transition metal nitrides
are known for their electrochemical stability, they lack the desired catalytic activity
so as to be used as PEMFC catalysts. However, using a low content of Pt and NH 3
as a reactive environment, PtNi nanoparticles can undergo profound structural and
chemical changes forming Ni 4 N in the core and a thin Pt layer as shell.
DFT calculations indicate that N atoms are in the core–shell structured catalyst.
The N and the core–shell structure in the PtNiN core–shell nanoparticles increases
the Pt ORR activity and provides a stabilizing effect under high oxidizing conditions suppressing the dissolution during potential cycling (Fig. 8.20).
Efforts to design/synthesize catalysts capable to alleviate problems of instability
and low activity of Pt catalysts for cathodes continue. It has been known that Pt
alloys (mostly with transition metals as Co, Ni, and Fe) are more active for the ORR
than pure Pt. However, the Pt loading in these alloys is still high, and moreover, the
nonnoble metals in these alloys are etched easily in the acid environment of the
PEMFC. Therefore, it is necessary to design catalysts that have higher ORR activity,
lower Pt loading, and better stability.
Recently, a new route to the development of PtNiN core-shell catalysts consisting of inexpensive Ni nitride cores covered with very thin Pt shells; despite substantial reduction in Pt loading, the catalysts retained high ORR activity and stability
[39, 40]. The nitride core improves the performance of the Pt shell by inducing both
geometric and electronic effects. The synthesis procedure is facile and applicable to
large- scale synthesis (Fig. 8.21).
The carbon-supported PtFe core-shell nanoparticles were prepared by using
K 2 PtCl 4 and FeCl 6 ·6H 2 O salts with a 1:3 molar ratio mixed with high-area Vulcan
XC72R carbon black to obtain a loading of 30 wt.% total metal. After sonication,
the salts were reduced by NaBH 4 . The mixture obtained was washed and dried,
followed by annealing at 510 °C under NH 3 gas for 2 h. Fe precipitates were
dissolved in 0.1 M H 2 SO 4 . The carbon-supported PtCo core-shell nanoparticles
were prepared in the same manner, except for using Co(NO 3 ) 2 ·6H 2 O salt. The mean
mole ratios of both Pt to Fe (x Pt /x Fe ) and Pt to Co (x Pt /x Co ) were determined to be 1.
Figure 8.22a and c show a high-angle annular dark-field (HAADF) image of a
single PtFeN nanoparticle and comparison of the EELS intensities from the Pt
M-edge (2122 eV) and Fe L-edge (708 eV) respectively, the latter obtained by moving the electron probe along the line indicated in Fig. 8.22a (note the arbitrary scales
of the Y axes). The EELS line profile shows that Pt is enriched (Fe is depleted) at
both edges of the nanoparticle. The thickness of the Pt shells is around 0.5–1 nm,
8 Catalytic Properties of Pt Monolayer Electrocatalysts
core. The prepared Pt ML PdNiN/C catalyst retains 89% of the initial electrochemical
surface area after 50,000 cycles between potentials 0.6 and 1.0 V. Correlation
between the electron energy-loss spectroscopy and X-ray absorption spectroscopy
analyses with electrochemical measurements shows that the significant improvement
of stability of the Pt ML PdNiN/C catalyst is caused by nitrogen doping while reducing
the total precious metal loading [38].
Nitrogen has one of the strongest covalent bonds; thus it is very stable and inert
under normal conditions. Yet nitrogen reacts with selected elements, forming
compounds with a variety of intriguing properties. Although transition metal nitrides
are known for their electrochemical stability, they lack the desired catalytic activity
so as to be used as PEMFC catalysts. However, using a low content of Pt and NH 3
as a reactive environment, PtNi nanoparticles can undergo profound structural and
chemical changes forming Ni 4 N in the core and a thin Pt layer as shell.
DFT calculations indicate that N atoms are in the core–shell structured catalyst.
The N and the core–shell structure in the PtNiN core–shell nanoparticles increases
the Pt ORR activity and provides a stabilizing effect under high oxidizing conditions suppressing the dissolution during potential cycling (Fig. 8.20).
Efforts to design/synthesize catalysts capable to alleviate problems of instability
and low activity of Pt catalysts for cathodes continue. It has been known that Pt
alloys (mostly with transition metals as Co, Ni, and Fe) are more active for the ORR
than pure Pt. However, the Pt loading in these alloys is still high, and moreover, the
nonnoble metals in these alloys are etched easily in the acid environment of the
PEMFC. Therefore, it is necessary to design catalysts that have higher ORR activity,
lower Pt loading, and better stability.
Recently, a new route to the development of PtNiN core-shell catalysts consisting of inexpensive Ni nitride cores covered with very thin Pt shells; despite substantial reduction in Pt loading, the catalysts retained high ORR activity and stability
[39, 40]. The nitride core improves the performance of the Pt shell by inducing both
geometric and electronic effects. The synthesis procedure is facile and applicable to
large- scale synthesis (Fig. 8.21).
The carbon-supported PtFe core-shell nanoparticles were prepared by using
K 2 PtCl 4 and FeCl 6 ·6H 2 O salts with a 1:3 molar ratio mixed with high-area Vulcan
XC72R carbon black to obtain a loading of 30 wt.% total metal. After sonication,
the salts were reduced by NaBH 4 . The mixture obtained was washed and dried,
followed by annealing at 510 °C under NH 3 gas for 2 h. Fe precipitates were
dissolved in 0.1 M H 2 SO 4 . The carbon-supported PtCo core-shell nanoparticles
were prepared in the same manner, except for using Co(NO 3 ) 2 ·6H 2 O salt. The mean
mole ratios of both Pt to Fe (x Pt /x Fe ) and Pt to Co (x Pt /x Co ) were determined to be 1.
Figure 8.22a and c show a high-angle annular dark-field (HAADF) image of a
single PtFeN nanoparticle and comparison of the EELS intensities from the Pt
M-edge (2122 eV) and Fe L-edge (708 eV) respectively, the latter obtained by moving the electron probe along the line indicated in Fig. 8.22a (note the arbitrary scales
of the Y axes). The EELS line profile shows that Pt is enriched (Fe is depleted) at
both edges of the nanoparticle. The thickness of the Pt shells is around 0.5–1 nm,
8 Catalytic Properties of Pt Monolayer Electrocatalysts