1 3
Topics in Current Chemistry (2019) 377:4
In addition to noble-metal catalysts, heteroatom-doped carbons [69–71], carbon-supported noble-metal-free nanoparticles [72, 73], and N-coordinated noblemetal-free atoms on carbons (M–N–C) [74, 75] have attracted great interest as
ORR catalysts in fuel cells and metal–air batteries due to abundant sources of
their precursors, easy preparation, and good ORR performance. Among these
materials, N-doped carbon-anchored single M (M–N–C, M = Fe/Co) atoms
showed the best ORR catalytic activity [76]. Current approaches to synthesizing M–N–C catalysts generally involve the pyrolysis of precursors containing
C, N, and metal sources, in which metal nanoparticles are produced simultaneously. The ORR activity of these metal nanoparticles is much poorer than those
of single-atom counterparts, leading to unsatisfactory ORR performance in the
final catalysts. Recent works have focused extensively on optimizing carbon-supported single-atom Fe/Co catalysts. For example, Yin et al. reported that singleatom Co catalysts prepared by directly pyrolyzing bimetallic MOFs showed much
better ORR performance than commercial Pt/C catalysts in alkaline conditions
[63]. Sa et al. put forward a silica-protective-layer-assisted approach for preparation of single-atom Fe catalysts exhibiting ORR performance comparable to
that of Pt/C in alkaline conditions, but the power density of the anion exchange
membrane fuel cell was still poorer than that of Pt/C [49]. Compared to alkaline
conditions, the development of low-cost electrocatalysts for ORR in acidic media
is more cost-effective. Li et al. proposed a simple and scalable atomic isolation
strategy for synthesizing single-atom Fe catalysts [77]. As shown in Fig. 6a, Zn
acted as “inert” atoms to spatially isolate Fe, preferentially generating highly
active single-atom Fe (Fe–N x ) sites while suppressing the formation of large Febased particles. Aberration-corrected TEM characterization results demonstrated
that the final single-atom Fe catalyst had a density of ~ 30 ± 3 atoms per 100 nm
2
(Fig. 6b), which is six times that of the traditional Fe–N–C catalyst. As a result,
the single-atom Fe catalyst showed outstanding ORR performance comparable
to commercial Pt/C catalysts in acidic media (Fig. 6c). After the ORR test, the
density of single Fe atoms remained nearly unchanged, as revealed by aberrationcorrected TEM characterization. This result provides direct evidence of the good
stability of single-atom Fe catalysts. In addition, the method is quite versatile,
as atomic Fe sites can be anchored on many different supports, and many compounds (LiCl, NaCl, KCl, etc.) can act as atom-isolating agents [77].
It is important to note that many strategies have been proposed for the preparation of single-atom Fe/Co catalysts, but significant differences in performance
have been found among these methods based on rotating disk electrode (RDE)
testing. For the materials themselves, they may possess different microstructures
and chemical environments surrounding single metal atoms, resulting in completely different ORR performance. In addition, catalyst loading has a considerable impact on ORR performance—for example, higher loading usually leads to
better RDE data [20]. However, excessively high loading in fuel cells can result
in undesirable output power owing to poor mass transport [78]. Thus, a uniform
standard is needed to evaluate ORR performance with the RDE technique and
even fuel cells.
Reprinted from the journal
137
Topics in Current Chemistry (2019) 377:4
In addition to noble-metal catalysts, heteroatom-doped carbons [69–71], carbon-supported noble-metal-free nanoparticles [72, 73], and N-coordinated noblemetal-free atoms on carbons (M–N–C) [74, 75] have attracted great interest as
ORR catalysts in fuel cells and metal–air batteries due to abundant sources of
their precursors, easy preparation, and good ORR performance. Among these
materials, N-doped carbon-anchored single M (M–N–C, M = Fe/Co) atoms
showed the best ORR catalytic activity [76]. Current approaches to synthesizing M–N–C catalysts generally involve the pyrolysis of precursors containing
C, N, and metal sources, in which metal nanoparticles are produced simultaneously. The ORR activity of these metal nanoparticles is much poorer than those
of single-atom counterparts, leading to unsatisfactory ORR performance in the
final catalysts. Recent works have focused extensively on optimizing carbon-supported single-atom Fe/Co catalysts. For example, Yin et al. reported that singleatom Co catalysts prepared by directly pyrolyzing bimetallic MOFs showed much
better ORR performance than commercial Pt/C catalysts in alkaline conditions
[63]. Sa et al. put forward a silica-protective-layer-assisted approach for preparation of single-atom Fe catalysts exhibiting ORR performance comparable to
that of Pt/C in alkaline conditions, but the power density of the anion exchange
membrane fuel cell was still poorer than that of Pt/C [49]. Compared to alkaline
conditions, the development of low-cost electrocatalysts for ORR in acidic media
is more cost-effective. Li et al. proposed a simple and scalable atomic isolation
strategy for synthesizing single-atom Fe catalysts [77]. As shown in Fig. 6a, Zn
acted as “inert” atoms to spatially isolate Fe, preferentially generating highly
active single-atom Fe (Fe–N x ) sites while suppressing the formation of large Febased particles. Aberration-corrected TEM characterization results demonstrated
that the final single-atom Fe catalyst had a density of ~ 30 ± 3 atoms per 100 nm
2
(Fig. 6b), which is six times that of the traditional Fe–N–C catalyst. As a result,
the single-atom Fe catalyst showed outstanding ORR performance comparable
to commercial Pt/C catalysts in acidic media (Fig. 6c). After the ORR test, the
density of single Fe atoms remained nearly unchanged, as revealed by aberrationcorrected TEM characterization. This result provides direct evidence of the good
stability of single-atom Fe catalysts. In addition, the method is quite versatile,
as atomic Fe sites can be anchored on many different supports, and many compounds (LiCl, NaCl, KCl, etc.) can act as atom-isolating agents [77].
It is important to note that many strategies have been proposed for the preparation of single-atom Fe/Co catalysts, but significant differences in performance
have been found among these methods based on rotating disk electrode (RDE)
testing. For the materials themselves, they may possess different microstructures
and chemical environments surrounding single metal atoms, resulting in completely different ORR performance. In addition, catalyst loading has a considerable impact on ORR performance—for example, higher loading usually leads to
better RDE data [20]. However, excessively high loading in fuel cells can result
in undesirable output power owing to poor mass transport [78]. Thus, a uniform
standard is needed to evaluate ORR performance with the RDE technique and
even fuel cells.
Reprinted from the journal
137