Metal–Organic Framework-Derived FeCo-N-Doped Hollow Porous Carbon Nanocubes for Electrocatalysis in Acidic and Alkaline Media

Article abstract of DOI:10.1002/cssc.201700864

Metal–organic frameworks (MOFs) are ideal precursors/ templates for porous carbons with homogeneous doping of active components for energy storage and conversion applications. Herein, metalloporphyrinic MOFs, PCN-224-FeCo, with adjustable molar ratio of Fe^II/Co^II alternatively residing inside the porphyrin center, were employed as precursors to afford FeCo-N-doped porous carbon (denoted as FeCo-NPC) by pyrolysis. Thanks to the hollow porous structure, the synergetic effect between highly dispersed FeN_x and CoN_x active sites accompanied with a high degree of graphitization, the optimized FeCo₂-NPC-900 obtained by pyrolysis at 900 °C exhibits more positive half-wave potential, higher diffusion-limited current density, and better stability than the state-of-the-art Pt/C, under both alkaline and acidic media. More importantly, the current synthetic approach based on MOFs offers a rational strategy to structure- and composition-controlled porous carbons for efficient electrocatalysis.

Full text of DOI:10.1002/cssc.201700864

DOI: 10.1002/cssc.201700864
Communications
Very Important Paper
Metal–Organic Framework-Derived FeCo-N-Doped Hollow
Porous Carbon Nanocubes for Electrocatalysis in Acidic
and Alkaline Media
+
+
[a]
Xinzuo Fang , Long Jiao , Shu-Hong Yu, and Hai-Long Jiang*
Metal–organic frameworks (MOFs) are ideal precursors/
templates for porous carbons with homogeneous doping of
active components for energy storage and conversion applica-
tions. Herein, metalloporphyrinic MOFs, PCN-224-FeCo, with
materials in basic solution only, M-N-doped porous carbons
(M-NPC materials, M=Fe, Co) involving FeN or CoN species
x
x
possess considerable ORR performance in both alkaline and
acidic solutions, making it one of the most promising and uni-
II
II
[4,5]
adjustable molar ratio of Fe /Co alternatively residing inside
the porphyrin center, were employed as precursors to afford
FeCo-N-doped porous carbon (denoted as FeCo-NPC) by pyrol-
ysis. Thanks to the hollow porous structure, the synergetic
effect between highly dispersed FeN and CoN active sites ac-
versal systems for ORR.
Interestingly, although there have
been only two reports on Fe,Co-codoped NPC (denoted as
FeCo-NPC) electrocatalysts recently, they unambiguously pres-
ent superior ORR performance compared to their Fe-NPC and
Co-NPC counterparts, benefiting from the synergetic effect be-
x
x
companied with a high degree of graphitization, the optimized
tween highly active FeN species and the enhanced corrosion
x
[5a,b]
FeCo -NPC-900 obtained by pyrolysis at 9008C exhibits more
resistance endowed by cobalt species.
2
positive half-wave potential, higher diffusion-limited current
density, and better stability than the state-of-the-art Pt/C,
under both alkaline and acidic media. More importantly, the
current synthetic approach based on MOFs offers a rational
strategy to structure- and composition-controlled porous car-
bons for efficient electrocatalysis.
Currently, most of M-NPC catalysts have been prepared by
pyrolysis of the mixture of metal salt and nitrogen precursors
including polypyrrole (PPy), polyaniline (PANI), cyanamide (CA),
[5c,6]
etc.
Nevertheless, the unpredictable existence form of
metal ions and nitrogen, inevitable agglomeration, and uncon-
trollable microstructure during pyrolysis remain as bottlenecks
and might hamper the performance of these catalysts. Metallo-
porphyrin and its analogues, a definite structure with isolated
metal ions locked by nitrogen, are ideal precursors to give Fe/
With increasing energy demand and environmental concerns,
highly efficient electrochemical energy-storage and conversion
devices, such as fuel cells and rechargeable metal–air batteries,
have been considered as promising power sources. Although
tremendous progress has been made, the sluggish kinetics of
the oxygen reduction reaction (ORR) at the cathode limits the
[5a,7]
Co-NPC catalysts with highly dispersed active sites.
Howev-
er, the metalloporphyrin molecules are prone to agglomerate
randomly, making it hard to realize the high dispersion and ac-
cessibility of active sites in resultant pyrolysis products. Most
recently, Feng and co-workers fabricated porous metallopor-
phyrinic polymers to greatly improve the porosity and ORR ac-
tivity of resultant carbon involving alternative FeN and CoN
[
1]
development of these devices. Presently, platinum and its de-
rivatives are the most efficient catalysts for ORR. However, the
commercialization of Pt suffers from high cost, limited availa-
x
x
species. Unfortunately, the metalloporphyrin motifs are hardly
organized in an ordered way in porous polymers owing to
their amorphous nature and the difficulty to precisely control
[2]
bility, poor stability, and susceptibility to methanol crossover.
In this regard, great effort has been made to explore Pt substi-
tutes, among which heteroatom-doped (e.g., B, N, P, and S)
[5a]
the dispersion of Fe/Co-N active sites. In view of this, it is
x
[
3]
porous carbons are the most promising. The ORR per-
formance of porous carbons strongly relies on the precursor
type, dopant components, carbon support morphology, pore
highly desired to develop crystalline metalloporphyrinic frame-
works as precursors to afford highly porous carbon doping
with homogeneous FeN and CoN active species.
x
x
[
3–5]
structure, and electronic conductivity, etc.
In contrast to the
In this context, metal-organic frameworks (MOFs), a family of
crystalline porous materials constructed by the coordination of
metal ions and versatile organic linkers, should be ideal candi-
ORR activity over base metal oxides and/or metal-free carbon
+
+
[8]
[
a] X. Fang, L. Jiao, Prof. Dr. S.-H. Yu, Prof. Dr. H.-L. Jiang
Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key
Laboratory of Soft Matter Chemistry, Collaborative Innovation Center of
Suzhou Nano Science and Technology, Department of Chemistry
University of Science and Technology of China
dates. The pyrolysis of MOFs is able to provide porous car-
bons with high surface area and homogeneous heteroatom
[9]
dopants. Most of these reports are based on zeolitic type of
MOFs (ZIFs) as they provide high N contents and/or CoN
x
[9a–i]
Hefei, Anhui 230026 (P. R. China)
E-mail: jianglab@ustc.edu.cn
active sites in the products.
To our knowledge, there has
been only one report on the pyrolysis of porphyrinic MOF,
which involves single metal ions in the porphyrin center, to
Homepage: http://staff.ustc.edu.cn/~jianglab/
+
[
] These authors contributed equally to this work.
[10]
afford porous carbon for ORR application. Actually, the por-
phyrin centers can be occupied by different metal ions in
a single MOF to give a multivariate MOF, in which different
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under https://doi.org/10.1002/
cssc.201700864.
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metalloporphyrin motifs are well organized and dispersed.
Upon pyrolysis, MN species with different metals, possessing
x
particular activity and functions, would be stabilized inside the
carbon matrix to guarantee their high dispersion and display
synergetic effect in catalysis. The high porosity of the resultant
carbon, inherited from MOFs, would enable the active sites ac-
cessible to the electrolyte and substrate, thus leading to effi-
cient ORR catalysis.
Considering this, a representative porphyrinic MOF, PCN-
[
11]
2
24, with high stability and mesopores based on six-con-
nected Zr6 cluster and tetrakis(4-carboxyphenyl)porphyrin
H TCPP) ligand, was synthesized, followed by the porphyrin
(
2
II
II
metalation with Fe and Co in different ratios to give PCN-
24-FeCo (x is the feeding ratio of Co/Fe) in cubic shape.
2
x
Figure 1. SEM images of (a) PCN-224-FeCo
(
(
images and corresponding C, N, Fe, and Co elemental mappings for FeCo2-
NPC-900.
2
and (b) FeCo
a) and (b): enlarged single particle). (c) TEM, (d) high-resolution TEM
HRTEM), and (e) high-angle annular dark-field scanning TEM (HAADF-STEM)
2
-NPC-900 (inset in
Upon pyrolysis and acid etching, PCN-224-FeCo was converted
x
to N-doped porous carbon nanocubes in a hollow structure in-
volving highly dispersed FeN and CoN species, denoted as
x
x
FeCo -NPC-T (T represents pyrolysis temperature) (Scheme 1).
x
pores (Figures 1b and S5). TEM observation gives the sharp
contrast between the inner cavity and outer carbon shell, indi-
cating the hollow structure of FeCo -NPC-900, which can be
2
also proved by the SEM image of the cracked hollow carbons
(
Figures 1c and S6). The formation of the hollow structure
[12]
might be owed to the well-known Kirkendall-like effect. Spe-
cifically, in the pyrolysis process, the Co and Fe species in PCN-
2
24-FeCo tend to flow outward at a high rate because of their
2
large diffusion coefficients and then the pyrolysis product from
organic ligands are graphitized around the outer Co/Fe spe-
cies. When the carbon shell surpasses the critical thickness that
can make it stable, then the structure collapse can be well in-
hibited, giving rise to the hollow structure of porous carbon.
The thickness of the carbon shell is about 100 nm and no par-
Scheme 1. Illustration of the stepwise fabrication of hollow porous FeCo
NPC-T nanocubes with varying Co/Fe molar ratios at different pyrolysis
temperatures.
x
-
ticles related to ZrO , Co, or Fe can be observed, aligning well
2
The hollow porous structure of FeCo -NPC-T guarantees the ac-
with the powder X-ray diffraction (PXRD) results, which verify
their complete removal by HF etching (Figures 1c and S7).
Owing to the catalytic graphitization by Fe/Co species during
pyrolysis, the HRTEM image clearly presents the lattice fringes
with an interplanar space of 0.34 nm corresponding to the
(002) plane of carbon (Figure 1d), which is in consistent with
the peak at 26.08 in PXRD patterns (Figure S7). The graphitiza-
x
cessibility of active sites and efficient mass transfer. The coexis-
tence of FeN and CoN species effectively integrates highly
x
x
active catalytic centers and high graphitization of the carbon
network. All characteristics above synergistically boost the ORR
activity, and as a result, the optimized FeCo -NPC-900 compo-
2
site exhibits superior electrocatalytic ORR performance com-
pared to the state-of-the-art Pt/C in both alkaline and acidic
solutions, including catalytic activity, long-term stability, and
methanol tolerance capability.
tion degree of FeCo -NPC-900 can be further characterized by
2
Raman scattering spectra. The D and G bands at 1332 and
ꢀ
1
2
1584 cm correspond to the disordered and sp -hybridized
graphitic carbons, respectively, and the pretty low intensity
ratio (1.02) of I /I manifests the higher graphitization degree
The PCN-224 was chosen as it holds a 3D porous structure,
very high surface area, and most importantly, the empty por-
D
G
[
11]
phyrin centers ready to be metalated. By postsynthetic met-
alation, the optimized PCN-224-FeCo2 (a representative
sample), in cubic shape with an average size of about 800 nm
of FeCo -NPC-900 than Fe-NPC-900, corresponding to the
2
sharp peaks in PXRD results and demonstrating the better
effect of Co on graphitization than Fe (Figures S7 and S8). Ele-
mental mapping suggests the coexistence of Fe, Co, N, and C
elements and that Fe, Co, and N homogenously distribute
throughout the carbon shell matrix and this further confirms
2
ꢀ1
and a high BET surface area of 2100 m g was obtained (Fig-
ures 1a and S1–S3), which was then transformed to FeCo2-
NPC-T (T=800, 900, 1000) at different temperatures deter-
mined by thermogravimetric analysis in N2 atmosphere, fol-
lowed by HF etching (Figure S4). Taking the resultant FeCo2-
NPC-900 as an example, it inherited the cubic morphology and
the hollow structure of FeCo -NPC-900 (Figure 1e).
2
To further probe the chemical composition and MꢀN interac-
tion in FeCo -NPC-900, X-ray photoelectron spectroscopy (XPS)
2
similar size from the parent PCN-224-FeCo and exhibits a high
was used. As shown in Figure 2a, the XPS survey spectrum
clearly portrays the existence of C, N, O, Fe, and Co. The high-
2
2
ꢀ1
BET surface area (1240 m g ) with approximately 3 nm meso-
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2 2
Figure 2. (a) XPS survey spectrum of FeCo -NPC-900. High-resolution XPS spectra of (b) N1s, (c) Fe2p, and (d) Co2p of FeCo -NPC-900.
resolution N1s spectrum fitted into five subpeaks reveals the
presence of five types of N species: pyridinic N, graphitic N,
of the reported non-noble metal catalysts (Figure 3b, Table S2).
Additionally, the LSV curve of Pt/C was tested three times to
confirm the reliability of the data (Figure S9). In contrast, the
monometallic Fe-NPC-900 and Co-NPC-900 present inferior ac-
tivity, suggesting synergetic promotion effect between FeNx
and CoN active sites in FeCo -NPC-900 (Figure 3b). Moreover,
pyrrolic N, MꢀN , and oxidized N (Figure 2b). To further con-
x
firm the chemical interaction between metal and nitrogen, the
high-resolution XPS spectra of Fe2p and Co2p were investigat-
ed. They exhibit broad and weak peaks, possibly owing to the
very low Fe/Co contents. The broad peaks centered at 710.8
x
2
the kinetic current density of FeCo -NPC-900 was calculated to
2
ꢀ
2
and 780.6 eV are, respectively, assignable to Fe3p
and
/2
be 7.80 mAcm at 0.85 V, which is also higher than that of
Pt/C, whereas the Fe-NPC-900 and Co-NPC-900 exhibit lower
kinetic current density, which further confirms the synergetic
3
[
5a,7b]
Co3p3/2 (Figure 2c,d).
The peak at 713.4 eV in the Fe2p
spectrum originated from chemical bonding between Fe and
[
4e,5a]
N confirms the existence of FeꢀN species.
Similarly, the
effect (Table S3). Furthermore, the FeCo -NC-900-mix sample
x
2
peak identified at 781.7 eV indicated the existence of CoꢀN ,
obtained from the physical mixture of TCPP-Fe and TCPP-Co
was also tested and exhibited inferior activity for ORR (Fig-
x
[
4c,5b]
attributed to bonding between Co and N.
To identify the
real content of Fe and Co, inductively coupled plasma atomic
emission spectrometry (ICP-AES) was performed. It can be seen
ure S10), confirming the superiority of FeCo -NPC-900 derived
2
from PCN-224-FeCo . Control experiments for samples with
2
that the actual Fe and Co contents in FeCo -NPC-900 are 0.43
various Fe/Co ratios at different pyrolysis temperatures demon-
strate that the 1:2 Fe/Co feeding molar ratio and 9008C condi-
tions are optimal parameters for achieving the best ORR activi-
2
and 0.37 wt%, respectively (Table S1).The XPS and ICP-AES re-
sults above highlight the presence of Fe/CoꢀN bond as poten-
tial ORR active sites in FeCo -NPC-900.
ty (Figures S9 and S10). Therefore, FeCo -NPC-900 will be inves-
2
2
Encouraged by the results above, the ORR activity of the
tigated in detail as a representative sample hereafter.
FeCo -NPC-900 was first evaluated in alkaline solution using ro-
To assess the ORR electron-transfer mechanism for FeCo2-
NPC-900, RDE measurements were conducted at various rota-
tion rates from 400 to 2400 rpm, showing well-defined curves
of diffusion-limited current density and increasing current den-
sities with increasing rotation speed (Figure 3c). The electron-
transfer number was calculated to be 3.9 according to the
x
tating disk electrode (RDE). The cyclic voltammetry (CV) curves
for all samples exhibit a well-defined cathodic peak at 0.8–
1
.0 V in O -saturated 0.1m KOH, revealing their electrocatalytic
2
activities (Figure 3a). The presence of the highest reduction
peak centered at 0.9 V for FeCo -NPC-900 predicts its superior
2
ꢀ
ORR activity among all FeCo -NPC-900 samples. Linear sweep
Koutecky–Levich (K–L) equation, illustrating the ideal 4 e
x
voltammetry (LSV) curves performed on a RDE at a scan rate of
oxygen reduction process (Figure 3c, inset). The Tafel slope
ꢀ
1
ꢀ1
5
mVs show that FeCo -NPC-900 possesses prominent cata-
was calculated to be ꢀ78 mVdec , much lower than
2
ꢀ
1
lytic activity with values of onset potential of 0.97 V, half-wave
potential of 0.87 V, and diffusion-limited current density of
ꢀ86 mVdec for Pt/C, signifying its more favorable kinetic be-
havior (Figure S13). The Nyquist plots indicate that FeCo -NPC-
2
ꢀ
2
ꢀ
5.5 mAcm , which are better than those of Pt/C and most
900 possesses comparable charge-transfer resistance to Co-
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Figure 3. (a) CV curves and (b) LSV curves for different samples in O
NPC-900 in O
saturated 0.1m HClO
evaluation for FeCo -NPC-900 and the Pt/C in 0.1m KOH solution.
2
2
-saturated 0.1m KOH solution; (c) LSV curves at different RDE rotation rates for FeCo -
2
-saturated 0.1m KOH solution (inset: K–L plots and electron transfer numbers at different potentials); (d) LSV curves for different samples in O
2
-
4
solution; (e) LSV curves of FeCo -NPC-900 before and after 10000 cycles in O -saturated 0.1m KOH solution and (f) methanol-tolerance
2
2
2
NPC-900, but significantly smaller than Fe-NPC-900 (Fig-
ure S14), reflecting that the Co species mainly contributes to
its high graphitization and conductivity, which is highly consis-
tent with PXRD and Raman spectra results indicated above
sponding Fe (or Co)-NPC-T (Figures 3d, S16, and S17), as well
as most of non-noble metal catalysts previously reported
(Table S4). To confirm the ORR performance of Pt/C, the LSV
curves tested three times show the high reliability of the data
(Figure S18). The K–L plots from RDE polarization curves
between 0.2 to 0.6 V exhibit good parallel straight lines, and
(Figures S7 and S8).
Although it is more challenging to obtain an efficient ORR
catalyst in acidic solution, thanks to the FeN and CoN species
the transferred electron number per O molecule was calculat-
x
x
2
ꢀ
involved, FeCo -NPC-900 also exhibits excellent ORR per-
ed to be 3.9, suggesting a 4 e ORR pathway even in acidic
2
formance, surpassing its counterparts, in 0.1m HClO solution.
The CV curve indicates a significant reduction process with
a pronounced cathodic ORR peak at approximately 0.6 V in O2-
solution (Figure S19). The lower Tafel slope of FeCo -NPC-900
4
2
ꢀ
1
(ꢀ71 mVdec )
and
larger
kinetic
current
density
ꢀ
2
(11.48 mAcm ) than Pt/C further reflect its more favorable ki-
netic behavior (Figure S20, Table S3). Notably, although much
effort has been made, there are very limited catalysts outper-
forming Pt/C for ORR in both alkaline and acidic solutions
saturated 0.1m HClO (Figure S15). Remarkably, it shows more
4
positive half-wave potential (0.74 V) and much higher diffu-
ꢀ
2
sion-limiting current (ꢀ0.56 mAcm ), than Pt/C and the corre-
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(
Table S5), which further highlights the great advantages of
Experimental Section
FeCo -NPC-900 in ORR catalysis. To further correlate the ORR
2
ꢀ
Preparation of catalysts
activity with the formation of FeN and CoN , SCN ion (with
x
x
a high affinity to MꢀN sites) was used to poison MꢀN sites.
x
x
As shown in Figure S21, the E
and E of FeCo -NPC-900
Synthesis of organic ligand: The tetrakis(4-carboxyphenyl)por-
onset
1/2
2
phyrin (H TCPP) was synthesized based on previous reports with
negatively shift by 45 and 84 mV, respectively, after the addi-
2
[11]
ꢀ
minor modifications.
Typically, pyrrole (3.0 g, 0.043 mol) and
tion SCN . These experimental studies clearly elucidate that
methyl p-formylbenzoate (6.9 g, 0.042 mol) were added into
a 250 mL three-necked flask with propionic acid (100 mL). The so-
lution was refluxed at 1608C for 12 h and then cooled down to
room temperature. The precipitate was obtained by filtration,
washing with ethanol, ethyl acetate and tetrahydrofuran, and final-
ly drying in vacuum overnight. Then the obtained precipitate
(1.95 g) was dissolved with THF (60 mL) and MeOH (60 mL) in
a three-necked flask, then a solution of KOH (6.82 g) dissolved in
FeN and CoN sites are the origin of the ORR activity of FeCo -
x
x
2
NPC-900.
In addition to the high activity, excellent stability and metha-
nol tolerance are also necessary for an ideal ORR catalyst. The
polarization curve recorded after 10000 cycles of continuous
CV scanning for FeCo -NPC-900 shows negligible difference
2
from the initial one in both 0.1m KOH and 0.1m HClO (Figur-
4
H O (60 mL) was added. This mixture solution was refluxed at 808C
2
es 3e and S22), exemplifying its outstanding stability, in refer-
ence to the activity decline (an approximate 65 and 43 mV
negative shift of the half-wave potential in alkaline and acid
solutions, respectively) of the commercial Pt/C catalyst (Fig-
ure S23). Chronoamperometry tests were also conducted and
demonstrate good activity during 20000 s of continuous oper-
ation (Figure S24), further showing great stability. Moreover,
almost no variation in the current can be observed upon
for 12 h and subsequently cooled down to room temperature. Ad-
ditional H O was added to make sure the solid was completely dis-
2
solved and then the solution was acidified with 1m HCl until no
more purple precipitate generated. The purple solid was filtrated,
washed with water, and dried in vacuum.
Synthesis of PCN-224: PCN-224 was prepared based on the re-
[11]
ported procedure with minor modifications.
Typically, ZrCl4
(
120 mg), H TCPP (50 mg), and benzoic acid (1200 mg) in DMF
2
methanol addition for FeCo -NPC-900 in both solutions, in
2
(8 mL) were ultrasonically dissolved in a 20 mL Pyrex vial. The mix-
stark contrast to the abrupt change for the Pt/C catalyst (Figur-
ture was heated in 1208C oven for 24 h. After cooling down to
room temperature, cubic dark purple crystals were harvested by fil-
tration.
es 3 f and S25). The results demonstrate that FeCo -NPC-900
2
has strong tolerance against the crossover effect, making it
a promising catalyst for direct methanol fuel cells. The excel-
lent stability can be attributed to the highly graphitized nano-
structures, and strong bonding between Fe/Co and N, as well
Synthesis of PCN-224-FeCo : Typically, PCN-224 (200 mg) with
x
a desired molar ratio of FeCl
·4H O and CoCl ·6H O (1.50 mmol
2 2 2
2
[
5a]
FeCl ·4H O and 0.75 mmol CoCl ·6H O for PCN-224-FeCo0.5
;
as highly dispersed Fe/Co-N active sites.
2
2
2
2
x
1
.125 mmol FeCl ·4H O and 1.125 mmol CoCl ·6H O for PCN-224-
2 2 2 2
In summary, starting with a porphyrinic metal–organic
FeCo ; 0.75 mmol FeCl ·4H O and 1.50 mmol CoCl ·6H O for PCN-
1
2
2
2
2
framework (MOF), PCN-224-FeCo , involving tunable molar
x
2
24-FeCo ) in DMF were heated at 1208C with stirring for 12 h.
II
II
2
ratio of Fe and Co alternatively residing inside the porphyrin
centers, as a single precursor (i.e., in the absence of additional
metal species or nitrogen/carbon sources), hollow porous
After that, the mixture was centrifuged. The liquid was decanted
and the remaining solid was washed twice with fresh DMF and
twice with acetone. The acetone was decanted, and the sample
was dried in an oven. PCN-224-Fe and PCN-224-Co were synthe-
carbon nanocubes enriched with uniformly dispersed FeN and
x
CoN active sites were successfully prepared in a controlled
sized through a similar procedure by using only FeCl
2.25 mmol) or CoCl ·6H O (2.25 mmol), respectively.
·4H O
2 2
x
(
manner. The crystalline porous structure of PCN-224-FeCo ef-
2
2
x
fectively prevents the agglomeration of metalloporphyrin
motifs and allows homogeneous dispersion of active sites. As
far as we know, this is the first work on the simultaneous gen-
eration of FeN and CoN active species from crystalline precur-
Synthesis of FeCo -NPC-900: Typically, PCN-224-FeCox powder
x
ꢀ
1
(100 mg) was heated to 9008C with a heating rate of 58Cmin
and pyrolyzed at 9008C for 2 h under flowing N atmosphere, and
2
x
x
then cooled to room temperature naturally to obtain porous
carbon composite materials. Then, the involved metal or metal
oxide nanoparticles in the resultant products were etched twice
with HF solution at 808C for 12 h, and washed thoroughly with
sors. Benefiting from the unique hollow porous nanostructure,
along with the synergetic effect between the highly dispersed
FeN (offering superior activity) and CoN (contributing to high
x
x
lots of water to yield FeCo -NPC-900 catalysts. Finally, the products
were dried in vacuum at 1208C overnight prior to use.
graphitization) sites, the optimized FeCo -NPC-900 catalyst ex-
x
2
hibits excellent oxygen reduction reaction (ORR) electrocatalyt-
ic activity, favorable reaction kinetics, long-term stability, and
superior methanol tolerance under both alkaline and acidic
conditions, surpassing the state-of-the-art Pt/C and most of
the non-noble metal electrocatalysts. The facile, rational, and
controllable strategy demonstrated herein opens up a new
avenue for the fabrication of high-performance non-noble
metal electrocatalysts, on the basis of crystalline MOFs with
tunable and modifiable organic linkers.
Synthesis of FeCo -NC-900-mix: The FeCo -NC-900-mix was ob-
2
2
tained through the similar procedure with FeCo -NPC-900 started
2
from the physical mixture of TCPP-Fe and TCPP-Co.
Electrochemical performance evaluation
Synthesis and electrochemical measurements of working elec-
trode: Electrochemical measurements were performed with a CHI
7
60E electrochemical analyzer (CH Instruments, Inc., Shanghai) and
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a rotating disk electrode (RDE) (Pine Instruments, Grove city, PA).
All electrochemical measurements were conducted in a typical
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each experiment. A flow of O was maintained over the electrolyte
2
2
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alyst ink was prepared by dispersing the catalyst (2 mg) into etha-
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[
ꢀ
2
ꢁ
0.30 mgcm ). The LSV measurements of the catalysts were de-
termined using a RDE (Pine Instruments, Grove city, PA). The scan
rate of the working electrode was 5 mVs with varying rotating
speed from 400 to 2400 rpm. The number of electrons transferred
ꢀ1
(n) during ORR was calculated by Koutecky–Levich equation, at var-
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j
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2
=3 ꢀ1=6
B ¼ 0:62 n F C ðD Þ
n
0
0
[
where j is the measured current density; j and j are the kinetic
k
L
and diffusion-limiting current densities, respectively; w is the angu-
lar velocity of the disk (w=2pN, N is the linear rotation speed); n
represents the overall number of electrons transferred in oxygen
reduction; F is the Faraday constant (F=96485 Cmol ); C is the
bulk concentration of O (1.2ꢁ10 molcm ); D is the diffusion
[
ꢀ1
0
ꢀ6
ꢀ3
2
0
ꢀ
5
2
ꢀ1
coefficient of O in 0.1m KOH and 0.1m HClO (1.9ꢁ10 cm s ),
and n is the kinematic viscosity of the electrolyte (0.01 cm s ).
2
4
2
ꢀ1
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Acknowledgements
This work is supported by the NSFC (21371162, 21673213 and
21521001), the National Research Fund for Fundamental Key
2
014, 7, 442; e) W. Zhang, Z.-Y. Wu, H.-L. Jiang, S.-H. Yu, J. Am. Chem.
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Conflict of interest
The authors declare no conflict of interest.
Keywords: doping
·
electrocatalysis
·
metal–organic
frameworks · oxygen reduction reaction · porous carbon
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Accepted manuscript online: July 3, 2017
Mꢂllen, Angew. Chem. Int. Ed. 2015, 54, 15191; Angew. Chem. 2015, 127,
Version of record online: && &&, 0000
&
ChemSusChem 2017, 10, 1 – 7
www.chemsuschem.org
6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

COMMUNICATIONS
Double-sided cubes: Porphyrinic
metal–organic frameworks are em-
ployed to produce hollow porous
carbon nanocubes with homogeneous
FeN and CoN dopants by pyrolysis.
X. Fang, L. Jiao, S.-H. Yu, H.-L. Jiang*
& – &&
&
Metal–Organic Framework-Derived
FeCo-N-Doped Hollow Porous Carbon
Nanocubes for Electrocatalysis in
Acidic and Alkaline Media
x
x
Thanks to this particular structure, high
surface area and synergetic contribu-
tions between FeN and CoN , the opti-
x
x
mized FeCo -NPC-900 exhibits excellent
2
activity, long-term stability, and MeOH
tolerance, surpassing the Pt/C, toward
oxygen reduction electrocatalysis in
both acidic and alkaline media.
ChemSusChem 2017, 10, 1 – 7
www.chemsuschem.org
7
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ