133
desorption peaks. In contrast, commercial Pt/C showed a negative shift of 40 mV in
half-wave potential and a considerable loss in surface area, approximately 45%. The
same stability tests were performed on PtFeN/C and PtNiN/C, and the decreases in
half-wave potential were 11 and 5 mV, respectively. The results demonstrated that
nitride cores improved the stability of the core-shell catalysts. DFT study indicates
that the presence of N atoms in cores facilitates the segregation of Pt atoms from
inner shells to defective sites on surfaces, leading to the filling of the sites and consequently improving the stability of the electrocatalysts. Such an action can prevent
further dissolution of Pt and transition metals under the polarization curves for the
ORR on PtFeN/C and commercial Pt/C electrocatalysts.
The order of increase in the ORR activity for nitride-based cores is
PtNiN/C > PtFeN/C > PtCoN/C > Pt/C. From the (111) diffraction profiles, the
changes in lattice constant a relative to bulk Pt a Pt can be estimated according to
(a – a Pt )/a Pt ; these were − 0.5, −2.3, and − 6.4% for PtNiN/C, PtFeN/C, and
PtCoN/C, respectively. The lattice parameters for the catalysts are smaller than that
of Pt, and so Pt atoms are compressively strained. An intriguing feature is that the
order of the decrease in Pt-Pt distance observed is PtNiN/C < PtFeN/C < PtCoN/C,
which is in the reverse sequence of the increase in the ORR activity. Apparently, the
strain at the shell surface has the same tendency since the Pt shells on PtMN/C are
very thin.
Figure 8.23 shows the ORR-specific activities for PtNiN/C, PtFeN/C, PtCoN/C,
and Pt/C plotted as a function of the strain determined by the XRD profiles. It has
been known that the ORR activity of Pt shell (or Pt monolayer) can be enhanced or
decreased by the configuration of the substrate-induced strain in combination with
the electronic (ligand) effect. Strain-induced d-band center shifts apparently are a
major factor determining the catalysts’ activity. It has been demonstrated that with
an increase in compressed strain in a Pt surface, the ORR activity is enhanced by
downshifting d-band center and therefore lowering the binding energy of
intermediate oxygenated adsorbates; however, beyond a certain strain, the binding
becomes too weak and the ORR starts to decline because breaking the O-O bond is
more difficult. The NiN core gives a slight but moderate compressive strain to the Pt
shell, thereby attaining the highest ORR activity among the catalysts, whereas the
FeN and CoN cores impart comparatively higher compressive forces to the Pt surfaces, resulting in lower activities than the NiN core. The electronic effects enhancement mechanism between the overlayer and its supporting substrate requires further
studies. The electronic effect on the ORR activity for the present nitride-based systems was calculated using DFT techniques [39]. Pure Pt binds oxygen too strongly,
while the nitride cores with nonprecious Fe and Co metals introduce too much
strain. The nitriding of Ni tunes the Pt-Pt bond length and therefore the ORR activity, being able to promote the dissociation of O 2 and remove O efficiently under the
ORR conditions at low overpotentials. Furthermore, the bulk strain from the synchrotron-based XRD of PtNiN/C, PtFeN/C, and PtCoN/C (−0.5, −2.3, and − 6.4%,
respectively), verified as qualitatively, was in excellent agreement with the surface
strain from the DFT data (−3.7, −4.4, and − 5.0%, respectively). Overall, both the
8.1 Oxygen Reduction Reaction (ORR)
desorption peaks. In contrast, commercial Pt/C showed a negative shift of 40 mV in
half-wave potential and a considerable loss in surface area, approximately 45%. The
same stability tests were performed on PtFeN/C and PtNiN/C, and the decreases in
half-wave potential were 11 and 5 mV, respectively. The results demonstrated that
nitride cores improved the stability of the core-shell catalysts. DFT study indicates
that the presence of N atoms in cores facilitates the segregation of Pt atoms from
inner shells to defective sites on surfaces, leading to the filling of the sites and consequently improving the stability of the electrocatalysts. Such an action can prevent
further dissolution of Pt and transition metals under the polarization curves for the
ORR on PtFeN/C and commercial Pt/C electrocatalysts.
The order of increase in the ORR activity for nitride-based cores is
PtNiN/C > PtFeN/C > PtCoN/C > Pt/C. From the (111) diffraction profiles, the
changes in lattice constant a relative to bulk Pt a Pt can be estimated according to
(a – a Pt )/a Pt ; these were − 0.5, −2.3, and − 6.4% for PtNiN/C, PtFeN/C, and
PtCoN/C, respectively. The lattice parameters for the catalysts are smaller than that
of Pt, and so Pt atoms are compressively strained. An intriguing feature is that the
order of the decrease in Pt-Pt distance observed is PtNiN/C < PtFeN/C < PtCoN/C,
which is in the reverse sequence of the increase in the ORR activity. Apparently, the
strain at the shell surface has the same tendency since the Pt shells on PtMN/C are
very thin.
Figure 8.23 shows the ORR-specific activities for PtNiN/C, PtFeN/C, PtCoN/C,
and Pt/C plotted as a function of the strain determined by the XRD profiles. It has
been known that the ORR activity of Pt shell (or Pt monolayer) can be enhanced or
decreased by the configuration of the substrate-induced strain in combination with
the electronic (ligand) effect. Strain-induced d-band center shifts apparently are a
major factor determining the catalysts’ activity. It has been demonstrated that with
an increase in compressed strain in a Pt surface, the ORR activity is enhanced by
downshifting d-band center and therefore lowering the binding energy of
intermediate oxygenated adsorbates; however, beyond a certain strain, the binding
becomes too weak and the ORR starts to decline because breaking the O-O bond is
more difficult. The NiN core gives a slight but moderate compressive strain to the Pt
shell, thereby attaining the highest ORR activity among the catalysts, whereas the
FeN and CoN cores impart comparatively higher compressive forces to the Pt surfaces, resulting in lower activities than the NiN core. The electronic effects enhancement mechanism between the overlayer and its supporting substrate requires further
studies. The electronic effect on the ORR activity for the present nitride-based systems was calculated using DFT techniques [39]. Pure Pt binds oxygen too strongly,
while the nitride cores with nonprecious Fe and Co metals introduce too much
strain. The nitriding of Ni tunes the Pt-Pt bond length and therefore the ORR activity, being able to promote the dissociation of O 2 and remove O efficiently under the
ORR conditions at low overpotentials. Furthermore, the bulk strain from the synchrotron-based XRD of PtNiN/C, PtFeN/C, and PtCoN/C (−0.5, −2.3, and − 6.4%,
respectively), verified as qualitatively, was in excellent agreement with the surface
strain from the DFT data (−3.7, −4.4, and − 5.0%, respectively). Overall, both the
8.1 Oxygen Reduction Reaction (ORR)