Communications
(
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
ChemSusChem 2017, 10, 1 – 7
5
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&
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