Hierarchically porous Fe-N-C nanospindles derived from a porphyrinic coordination network for oxygen reduction reaction

Article abstract of DOI:10.1039/c8cy00168e

We report the size and morphology-controlled synthesis of PCN-222 crystals, a class of zirconium-based iron porphyrinic coordination networks, and their pyrolytic derivation of hierarchically porous Fe-N-C electrocatalysts for the oxygen reduction reaction (ORR). By optimizing precursor concentrations, nanometer-scaled and spindle-shaped PCN-222 crystals are obtained through a solvothermal method. A NaCl salt-assisted pyrolysis is employed to convert the PCN-222 crystals to an Fe-N-C catalyst, so as to inherit the nanospindle morphology and the Fe-N coordination structure in the PCN precursor, and at the same time to increase the graphitization degree. A post-activation procedure using concentrated sulfuric acid is further designed to induce a controlled NaCl removal and carbon etching process that optimizes the porous structures and surface properties of the final nanospindle Fe-N-C catalyst. The reaction between the concentrated sulfuric acid and NaCl produces heat to activate acid etching of carbon, and on the other hand consumes the acid to prevent the carbon from over-etching. The unique nanospindle shape and hierarchically porous structure allow efficient exposure of the high-density active sites and their accessibility to reactants, while the high degree of graphitization ensures efficient conduction of electrons involved in the electrocatalytic reaction and good stability of the carbon matrix during the electrocatalytic processes. Based on the results, the as-prepared Fe-N-C catalyst exhibits remarkable ORR performance, with a half-wave potential that is ca. 30 mV more positive than that of the state-of-the-art Pt catalyst, and excellent stability in alkaline solution, as well as an ORR activity comparable to Pt in acid solution.

Full text of DOI:10.1039/c8cy00168e

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ISSN 2044-4753
PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
rsc.li/catalysis

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Hierarchically porous Fe-N-C nanospindles derived from
porphyrinic coordination network for Oxygen Reduction Reaction
Received 00th January 20xx,
Accepted 00th January 20xx
Xing Hua, Jin Luo, Chencheng Shen and Shengli Chen*
DOI: 10.1039/x0xx00000x
We report a size and morphology-controlled synthesis of PCN-222 crystals, a class of zirconium-based iron porphyrinic
coordination network, and their pyrolytic derivation of hierarchically porous Fe-N-C electrocatalysts for the oxygen
reduction reaction (ORR). By optimizing the precursors concentrations, nanometer-scaled and spindle-shaped PCN-222
crystals are obtained through a solvo-thermal method. A NaCl salt-assisted pyrolysis is employed to convert the PCN-222
crystals to Fe-N-C catalyst, so as to inherit the nanospindle morphology and the Fe-N coordination structure in the PCN
precursor, and in the meantime to increase the graphitization degree. A post-activation procedure using concentrated
sulfuric acid is further designed to exert a controlled NaCl removal and carbon etching process that optimizes the porous
structures and surface properties of the final nanospindle Fe-N-C catalyst. The reaction between the concentrated sulfuric
acid and NaCl produces heat to activate acid etching of carbon, and on the other hand consumes the acid to prevent the
carbon from over-etching. The unique nanospindle shape and hierarchically porous structure allow efficient exposure of
the high-density active sites and their accessibility to reactants; while the high graphitization ensures efficient conduction
of electrons involved in the electrocatalytic reaction and good stability of the carbon matrix during the electrocatalytic
processes. In results, the as-prepared Fe-N-C catalyst exhibits remarkable ORR performance, with a half-wave potential
which is ca. 30 mV more positive than that of the state-of-the-art Pt catalyst and excellent stability in alkaline solution, as
well as an ORR activity comparable to Pt in acid solution.
www.rsc.org/
transition metal ions and organic linkers with high specific surface
area, robust porosity and diverse structure have becoming
Introduction
2
7-30
attractive precursors
for porous Fe–N–C catalysts. Among
The oxygen reduction reaction plays a vital role in fuel cells due to
1
-3
various MOFs, zeolitic imidazolate frameworks (ZIFs) have been
widely applied to synthesize various M-N-C catalysts during the past
its sluggish kinetics. Great efforts have been devoted to explore
durable, high-performance and earth-abundant ORR catalysts to
3
1-33
4
-10
years, mainly due to their N-rich porous structure.
However,
leading to Fe–N–C
catalysts with limited number of Fe-N coordination moieties, which
2,15,16 are proven to be highly active for ORR. Recently, Fe porphyrinic
replace scarce Pt based catalysts.
To date, Fe/N co-doped
3
4-36
ZIFs lack N directly coordinated with Fe,
carbons (Fe-N-C) have been attracted much attention owing to their
1
1-14
high activity and stability.
need to be enhanced in order to match those of Pt catalysts.
Despite that the nature of their catalytic sites is still controversial,
However, their performances still
1
37,38
1
7- MOFs
have attracted increasing attention owing to that they
sites. In particular, zirconium-based
1
9
possess abundant Fe-N
4
it is widely believed that high specific surface area and porous
porphyrinic porous coordination networks (PCN), built of Fe-TCPP
(TCPP = tetrakis(4-carboxyphenyl)porphyrin) ligand and Zr clusters
structure can promote the ORR performance for much faster mass
20-22
transport and more accessible reactive sites.
Nevertheless, Fe-
(
Fig. 1a), have demonstrated exceptional stability in electrolyte
N-C catalysts are usually synthesized via pyrolysis of composites
3
9,40
23-26
environment.
4
Due to the presence of Fe-N structure, pristine
containing Fe, N and C at high temperatures,
which makes it
PCNs such as PCN-222 and PCN-223 have been explored as
difficult to control the porous structure and active sites distribution.
Consequently, exploring a highly ordered and porous precursor has
been an effective way to achieve excellent electrocatalytic
performance.
4
1,42
electrocatalysts for ORR,
but they exhibit very limited activity,
probably due to poor electric conductivity.
In this study, we explore pyrolytic boosting of the ORR
performance of PCN-based materials. As far as we know, direct
pyrolysis of PCNs have been rarely reported. Besides, so-far the
porphyrinic MOFs have been mostly reported in the form of bulk
Metal-organic frameworks (MOFs) consisting of well-defined
4
3,44
crystals.
The high-temperature pyrolysis would result in bulk
carbons with very limited exposure of effective catalytic sites.
Therefore, this work first aims to synthesize porphyrinic MOF
precursors of nanoscale dimensions. We have managed to
synthesize nanometer-scaled PCN-222 crystals of spindle
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shapes.
Fig. 1 (a) Schematic illustration of the formation and structure of PCN-222 (Zr: glaucous, C: gray, O: red, N: blue, Fe: lavender); (b) Synthetic
procedure of the hierarchically porous nanospindles Fe-N-C catalyst (PCNNS-Phen-NaCl-H SO
2
4
)
Besides, a NaCl salt-assisted pyrolytic approach is employed to
prevent the aggregation of the small PCN-222 crystals in the high-
temperature processes and to allow the nanospindles morphology
to be inherited by the pyrolytic catalyst. Finally, a post-treatment
procedure with concentrated sulfuric acid is specifically designed
to realize mesoporous structuration of the pyrolytic product,
leading to a hierarchically porous Fe-N-C nanospindles catalyst (Fig.
(Phen) (50 mg) and 2.5 g NaCl were mixed in 50 ml ethanol with
continuous agitation under 80 °C. The dried brown powder was
then heat-treated under argon atmosphere at 800 °C for 1h. After
cooling to room temperature, the samples were activated by 1.5
ml concentrated sulfuric acid for 20 min, and then diluted with 50
ml deionized water. The final sample was obtained by suction
filtration and washed with deionized water. For comparison,
1
b) with excellent ORR performance.
2 4
catalysts were also prepared with 0.5 M H SO solution instead of
concentrated sulfuric acid was used in the post-treatment process
of NaCl removal, or with NaCl being absent in the pyrolytic
Experimental section
precursor. The post-treatment with 0.5 M H
for 12 h at 80 °C.
2 4
SO was performed
Materials preparation
Structure and morphology characterization
Synthesis of PCN-222 crystals. The bulk PCN-222 crystal was
Powder X-ray diffraction (XRD) measurements were characterized
using a Rigaku SmartLab powder diffractometer with a CuKa
radiation source. The morphology of samples was obtained with a
scanning electron microscope (SEM, Zeiss Sigma and Transmission
Electron Microscopy (TEM, JEM-2100F). The Raman spectra were
taken on a Confocal Raman Microspectroscopy (Renishaw inVia,
Renishaw, 532 nm excitation wavelength), and X-ray
photoelectron spectroscopy analyses were investigated by a
synthesized by modifying the commonly used solvo-thermal
3
9,45
method.
DMF)
carboxyphenyl)porphyrin Fe(II) (FeTCPP) (55 mg, 0.065 mmol),
zirconyl chloride octahydrate (ZrOCl ·8H O) (150 mg, 0.265 mmol),
Typically, a mixture of 10 ml N,N-dimethyl-formamide
(
solution containing 5,10,15,20-Tetrakis(4-
2
2
and benzoic acid (1.5 g, 3.6 mmol) was ultrasonically blended to
form the precursor solution, which was transformed to 20 ml
Teflon-lined steel autoclave and then heated at 120 °C in an oven
for 12 h. After being cooled to room temperature, the dark purple
crystals were harvested by filtration and washed with DMF for 3
times. The samples were then soaked in acetone for 2 days and
finally washed with fresh acetone for 3 times. After the removal of
acetone by centrifugation, the sample was dried at 80 °C under
vacuum for 12 h. In order to obtain smaller PCN-222 crystals,
precursor solutions containing the same species but differently
diluted concentrations were prepared and the same synthesis
procedures were performed (see the section of Results and
discussions for details).
2
Kratos (Ltd. XSAM-800). The N adsorption-desorption isotherms
were performed on an ASAP2020 Surface Area and Porosity
Analyzer (Micromeritics, USA).
Electrochemical measurements
All the electrochemical measurements were performed with a
BioLogic electrochemical workstation (SP-300) in a conventional
three-electrode system with a catalyst-modified glassy carbon
(GC) rotating disk electrode (RDE) as the working electrode, a Pt
foil as the counter electrode, and a saturated calomel electrode
(SCE) as the reference electrode for measurements in 0.1 M HClO4
electrolytes and an Hg/HgO electrode as the reference electrode
for measurements in 0.1 M KOH electrolytes respectively, which
Synthesis of Fe-N-C catalysts from PCN-222 crystals. The
above-prepared PCN-222 crystals (80 mg), 1, 10-phenanthroline
2
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were separated from the working electrode by salt bridge with a
Luggin capillary. The working electrode was prepared as follows: 5
mg catalyst was dispersed in 1 mL mixture of 5 wt% Nafion and
isopropanol (volume ratio: 1:49); 18 μL of the catalyst ink was
then dropped onto the GC disk electrode (5 mm diameter) to form
relatively concentrated precursor solution (0.065 mmol FeTCPP,
0.265 mmol ZrOCl ·8H O and 3.6 mmol benzoic acid) was used,
2
2
rod-shaped PCN-222 bulk crystals (PCNMR) of ca. 15 µm lengths
and 5-10 µm diameters (Fig. 2a and Fig. S1a) were produced.
Diluting the solution to ca. 1/3 of those concentrations led to the
formation of PCN-222 nanospindles (PCNNS) 300-500 nm in length
and 100-250 nm in width (Fig. 2b and Fig. S1b). When further
diluting the solution to ca. 1/5 of those concentrations, the PCN-
222 nanoparticles (PCNNP) with irregular shapes and further
decreased dimensions, e.g., nanorods of 100-200 nm lengths and
50 nm widths and nanoparticles of 50-100 nm diameters, were
obtained (Fig. 2c and Fig. S1c).
-2
a thin catalyst layer, with a catalyst loading of 0.450 mg cm . For
comparison, 20 wt% Pt/C catalyst (Johnson-Matthey) was used in
-2
a similar way with a loading of 15 µg Pt cm . All the measured
potentials were converted to the reversible hydrogen electrode
(
RHE). The calibration of the equilibrium potential of the SCE and
the Hg/HgO electrode to the RHE scale was performed in the
hydrogen-saturated electrolyte with a Pt/C-loaded GC electrode as
the working electrode. The steady-state polarization curves and
cyclic voltammograms (CV) were used to evaluate the
electrocatalytic performance of the catalysts for ORR. The steady-
state polarization curves were measured using RDE with an
electrode rotating speed of 1600 rpm and potential scanning rate
These PCN-222 crystals all exhibited X-ray diffraction (XRD)
peaks at 2.4°, 4.8°, 7.1°, and 9.8°, agreeing well with the simulated
3
9,45
XRD pattern of PCN-222 (Fig. 2d),
indicating that the PCN-222
crystals preserved the phase purity when the dimensions go into
nanometer scales. However, the relative intensity of peak at 2.4°
decreased upon downsizing. The porosity of the prepared PCN-
222 crystals was investigated by measuring the N2 adsorption-
desorption behaviours. They exhibited typical type IV isotherms
-1
of 5 mV s in O -saturated solution of 0.1 M KOH or 0.1 M HClO4
2
electrolytes. The CV measurements were performed with
-1
potential scanning rate of 50 mV s in both O - and Ar-saturated
2
electrolytes.
with a steep increase around P/P =0.3, indicating the mesoporous
0
The kinetic current density (j ), which describes the intrinsic
characteristics. The surface area and pore volume were measured
K
2
-1
3
-1
catalytic activity of the catalyst at a certain potential, was
obtained by the mass-transport correction based on the
Koutecky–Levich equation:
to be 2065 m g and 1.105 cm g for PCNMR, agreeing with that
3
9,45
reported in the literatures.
The surface area and pore volume
2
-1
3
-1
decreased to ca. 1739 m g and 0.718 cm g for PCNNS, and
2
-1
3
-1
ꢀ
ꢃ ꢀꢄ
1304 m g and 0.625 cm g for PCNNP (Fig. 2e). Two types of
pores, 1.7 nm and 3.3 nm in sizes respectively, were identified in
the prepared PCN-222 crystals (Fig. 2f), which can be assigned to
triangular microchannels and hexagonal mesochannels
ꢀꢁ
ꢂ
ꢀꢄ ꢅ ꢀ
where j and j are the measured current density and diffusion-
L
limiting current density on the polarization curve respectively.
The rotating ring-disk electrode (RRDE) tests were employed
to detect the H O yields, with a ring electrode potential of 1.3 V.
3
9,45
respectively.
The amounts of mesopores of 3.3 nm decreased
with downsizing. The change in XRD and pore-size distributions
indicated that the growth of mesoscale pores was hindered upon
decreasing the precursor concentrations, leading to the decrease
of the specific surface area.
2
2
The H O yield and the effective electron-transfer number of ORR
2
2
(
n) were calculated using the following equations:
ꢉꢊ⁄ꢋ
H O ꢇ%ꢈ ꢂ 200 ꢃ
ꢆ
ꢆ
ꢉꢌꢍ ꢎ ꢉ ⁄ꢋ
ꢊ
ꢉꢌꢍ
ꢏ ꢂ 4 ꢃ
ꢉꢌꢍ ꢎ ꢉ ⁄ꢋ
ꢊ
where I and I are the ring and disk currents respectively; N is the
R
D
ring collecting efficiency of the Pt ring, which was calibrated to be
.241 using 10 mM K [Fe(CN) ] in 0.1 M KNO .
0
3
6
3
The accelerated stability tests were performed in the O2-
saturated 0.1 M KOH and 0.1 M HClO4 electrolytes at room
temperature by cycling the potential between 0.6 V and 1.0 V with
-1
a potential scanning rate of 50 mV s .
Fig. 2 TEM images of the prepared PCN-222 crystals: (a) PCMMR,
(
b) PCNNS and (c) PCNNP. (d) XRD patterns of the prepared PCN-
22 crystals. (e) N adsorption-desorption isotherms and (f) pore-
Results and discussions
2
2
Morphology control of PCN-222 crystals via varying the precursor
size distributions of PCN-222 crystals.
concentrations. Fig. 1a illustrates the formation and structure of
6
the PCN-222, which consists of D4h-symmetric Zr cluster with
Fe-N-C catalysts derived from PCN-222 crystals. The high
density and uniform distribution of Fe-N coordination structure
and high porosity make the PCN-222 an ideal self-templating
precursor for preparing Fe-N-C catalyst with high density of active
sites. We take several further strategies to realize the advantage
of PCN-222. Besides downsizing the PCN-222 crystals into
eight edges connecting with carboxylates in FeTCPP ligands. The
PCN-222 crystals can be formed using solvothermal reaction of
2 2
FeTCPP, ZrOCl ·8H O and benzoic acid in DMF solution. We found
that the concentration of the precursors crucially affects the
morphologies and dimensions of the PCN crystals. When a
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-
1
-1
nanometer scales, they were blended with NaCl powders before
pyrolysis so that the carbonization process take place under the
confinement of NaCl, which can prevent the PCN-222 nanocrystals
from aggregating and the pyrolytic intermediates from volatilizing,
in the meantime increase the graphitization of the pyrolytic
peaks, namely, D (around 1348 cm ) and G (around 1600 cm )
were observed (Fig. S3b), which should be associated with the
defects and graphitized carbons respectively. The intensity ratio,
I
G
/I
material. The I
Phen-NaCl-H SO
NaCl-H SO respectively, indicating the PCNNS-Phen-NaCl-H
D
, is an indicator of the relative graphitization degree of the
/I values were 1.31, 1.27 and 1.16 for PCNNS-
PCNMR-Phen-NaCl-H SO and PCNNP-Phen-
SO
G
D
46
carbon matrix. In order to further increase the N content, 1, 10-
phenanthroline was dispersed in the cavities of PCN-222 by
thoroughly blending in ethanol. Finally, a controlled NaCl-removal
and catalyst activation step using concentrated sulfuric acid was
designed to refine the porous structure of the pyrolytic catalyst.
The reaction between NaCl and concentrated sulfuric acid on one
hand produces heat to motivate acid etching, on the other hand
consumes concentrated sulfuric acid to prevent over-etching. We
denote the catalysts of thus obtained from PCNMR, PCNNS and
PCNNP as PCNMR-Phen-NaCl-H SO , PCNNS-Phen-NaCl-H SO , and
2
4
,
2
4
2
4
2
4
had the highest graphitization degree, which would be beneficial
to the electron transfer during the ORR process. When the
preparation was conducted in the absence of NaCl, much lower
I
G D
values were observed (Fig. S4), which demonstrate the role of
/I
NaCl in promoting the graphitization of the pyrolytic carbon
4
6
material.
The N adsorption/desorption on the derived Fe-N-C catalysts
2
also exhibited type IV isotherms (Fig. 4a), but with
adsorption/desorption behavior different from their PCN
precursors in high pressure region, where a deep increase
occurred at P/P0 of ca. 0.9. This usually is an indicator of the
2
4
2
4
PCNNP-Phen-NaCl-H SO respectively.
2
4
As seen from the TEM images in Fig. 3 and SEM images in Fig.
S2, the PCNMR-Phen-NaCl-H SO4 and PCNNS-Phen-NaCl-H SO4
2
2
inherited the morphologies of their precursor PCN crystals, but the
surface became much rougher. The TEM images showed that the
rough surface of the PCNMR-Phen-NaCl-H SO virtually consisted
presence of macropores. The BET surface area derived from the N
2
2
-1
2
-1
adsorption/desorption isotherms were 554 m g , 516 m g and
2
-1
g
424 m
respectively for PCNNS-Phen-NaCl-H SO , PCNMR-
2 4
2
4
of large numbers of small nanoparticles. Different from the PCNNS
Phen-NaCl-H SO and PCNNP-Phen-NaCl-H SO . The higher surface
2
4
2
4
which contained solid spindles, the PCNNS-Phen-NaCl-H SO4
area of PCNNS-Phen-NaCl-H SO4 should related to its unique
2
2
catalyst consisted of spindles which were composed of a large
number of aggregated nanoparticles with diameters of 10-20 nm,
structure of nanoparticle-aggregated nanospindle (Fig. 3b). The
primary spindle morphology would prevent them from
agglomeration; while the secondary nanoparticle-aggregated
structure could provide mesopores. The Fe-N-C catalysts treated
with concentrated sulfuric acid exhibited mesopores with a
relatively sharp distribution of 3-4 nm and that with a broad size
distribution spanning from 10 nm to 90 nm (Fig. 4b).
exhibiting highly porous structure. The PCNNP-Phen-NaCl-H SO4
2
also showed irregular particle morphology, the sizes became much
smaller than the PCNNP precursor (Fig. 3c and Fig. S2c).
2
Fig. 4 (a) N adsorption–desorption isotherms and (b) pore-size
distributions of the Fe-N-C catalysts.
X-ray photoelectron spectroscopy (XPS) survey was performed
to elucidate the surface chemical composition of Fe-N-C series. All
the samples were shown to contain C, N, O, Zr and very low
content of Fe (less than 0.2 at%). As shown in the high resolution
Fe 2p spectra of the catalysts (Fig. 5b), very weak Fe signal could
be observed. It is known that even trace amounts of Fe embedded
in carbon frameworks have a significant effect on the ORR
Fig. 3 TEM images of the pyrolytic Fe-N-C catalysts: (a) PCNMR-
Phen-NaCl-H
2 4 2 4
SO , (b) PCNNS-Phen-NaCl-H SO , (c) PCNNP-Phen-
NaCl-H SO and (d) PCNNS-Phen-NaCl (see texts).
2
4
The XRD patterns of the calcined Fe-N-C catalysts exhibited
broad peaks at 2θ values of 24°which can be assigned to the (002)
planes of the graphite carbon; while no characteristic XRD peaks
of Iron species could be observed (Fig. S3a). This should be due to
that the Fe contents were very low in the final catalysts after the
acid treatment. The remaining Fe species in the final catalysts
should be well distributed by coordinating with N and didn’t
aggregated to separated phases. The Raman spectra was used to
investigate the carbon structure of the Fe-N-C catalysts. Two
4
7,48
activity.
be deconvoluted into five types of nitrogen, corresponding to
pyridinic N (398.4 eV) , Fe-N (399.3 eV), pyrrolic N (400.1 eV),
graphitic N (401.2 eV), and oxidized N (402-405 eV) respectively
The high-resolution N 1s spectra of the catalysts could
x
4
9,50
(Fig. 5c).
Combined with the XRD results, the Fe ions are
believed to be coordinated with N atoms to form catalytically
active sites. Fig. 5d shows the contents of different nitrogen
4
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5
2,55
species. Among the three Fe-N-C catalysts, PCNNS-Phen-NaCl-
SO had the highest content of Fe-N , graphitic N and pyridinic
N, which have been believed to be responsible for the ORR activity
producing porous structures and catalytically active sites.
A
H
2
4
x
drawback of this high-temperature activation method is the
difficulty in controlling etching degree. The over-etching may
result in catalysts with low conductivity and stability, as well as
large portion of microscale pores that could bring about mass
1
5, 51
of Fe-N-C catalysts.
53,54
transfer problem.
In comparison, the present concentrated-
sulfuric-acid post-activation procedure is more facile and
controllable. The optimized porous structure can be effectively
obtained by adjusting the amount of concentrated sulfuric acid
and reaction time.
Oxygen reduction characteristics of the Fe-N-C catalysts. The
electrocatalytic characteristics of the synthesized Fe-N-C catalysts
for ORR were first examined by cyclic voltammetry (CV) and RDE
measurements in 0.1 M KOH. Featureless double-layer charging
current was observed in Ar-saturated solution, while obvious
cathodic ORR peaks were seen in O - saturated 0.1 M KOH (Fig.
2
S5). Among the prepared Fe-N-C catalysts, PCNNS-Phen-NaCl-
H SO showed the most positive peak of 0.837 V. Fig. 6a displays
2
4
the steady-state polarization curves for ORR. The PCNNS-Phen-
NaCl-H SO showed a half-wave potential (E1/2) at 0.87 V, which
2
4
were ca. 20 mV and 30 mV more positive than PCNNP-Phen-NaCl
and PCNMR-Phen-NaCl-H SO respectively, and ca. 30 mV more
2
4
positive than the commercial 20 wt% Pt/C. As shown in Fig. 6b, the
kinetic current density at 0.8 V for PCNNS-Phen-NaCl-H SO was
2
4
-
2
4
3.69 mA cm , much larger than that of PCNMR-Phen-NaCl-H SO
2 4
Fig. 5 (a) XPS Survey spectra of Fe-N-C catalysts, high-resolution
b) Fe 2p and (c) N 1s XPS spectra. (d) Contents of different N
species.
-2
-2
(
14.96 mA cm ), PCNNP-Phen-NaCl-H SO (29.62 mA cm ),
2 4
(
-2
-2
PCNNS-Phen-NaCl (31.79 mA cm ) and Pt/C (15.85 mA cm ). In
addition, this value is substantially superior to that of many
1
5-17,51
reported Fe-N-C catalysts.
As illustrated in Fig. 6d, the
As a comparison, Fe-N-C catalyst was prepared through
pyrolysis of PCNNS with the concentrated H SO being replaced
with 0.5 M H SO solution in the final NaCl removal step. The
corresponding Tafel slope in the potential range 0.95-0.90 V for
2
4
-1
the prepared Fe-N-C catalysts were all ca. 56±1 mV dec ,
2
4
suggesting a similar ORR mechanism on these catalysts. The lower
corresponding catalyst of PCNNS-Phen-NaCl showed a similar
spindle morphology, but with much smaller nanoparticles
embedded in the bulk spindles (Fig. 3d and Fig. S2d). Comparing to
-1
Tafel slope of Fe-N-C than that of Pt/C (69 mV dec ) suggested a
facile ORR kinetics in alkaline media.
PCNNS-Phen-NaCl-H
XRD patterns, Raman spectra and N
isotherm (Fig. S3 and Fig. 4a), but lower ratio of I
2
SO
4
, the PCNNS-Phen-NaCl exhibited similar
adsorption/desorption
/I (1.14) and
ratio of the PCNNS-
4
might be due to that the concentrated sulfuric
2
G
D
2
-1
BET surface area (436 m g ). The higher I
Phen-NaCl-H SO
G D
/I
2
acid could remove the surface defects on the pyrolytic product. As
seen from the pore distribution results in Fig. 4b, the PCNNS-Phen-
NaCl possessed much less mesopores and nearly no large pores
2 4
above 10 nm which were seen for PCNNS-Phen-NaCl-H SO . This
should be the reason why it showed lower BET surface area. This
suggested the activation of concentrated sulfuric acid promoted
the formation of adequate mesopores and hierarchical porous
structure with large surface area. We think that the reaction
between NaCl and concentrated sulfuric acid produced heat to
activate the acid etching of carbon; while on the other hand the
reaction also consumed the acid to prevent the carbon from over-
etching.
It is worth mentioning that the treatment under NH
3
/air
atmosphere has been a widely used activation method in
Fig. 6 ORR polarization curves (a) and the corresponding kinetic
52-56
synthesizing porous Fe/N/C catalysts.
3
The reaction of NH and
2 2
current densities at 0.80 V (b), H O yield and effective electron
air with carbon at high temperatures during pyrolysis would
invoke carbon etching and N insertion simultaneously, thus
transfer number (c) and Tafel plots (d) for the Fe-N-C catalysts and
Pt/C (20 wt%) obtained at an electrode rotating speed of 1600
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1
rpm and a scan rate of 5 mV s in O
solution.
2
-saturated 0.1 M KOH
confirmed the beneficial effect of NaCl in promoting the ORR
performance. To further verify the importance of the reaction
between NaCl and concentrated sulfuric acid in the catalyst
preparation, we prepared catalyst sample by performing the
concentrated-sulfuric-acid activation after the removal of NaCl. As
-
The RRDE measurements were conducted to study the 4e
selectivity of ORR. As shown in Fig. S6, ring currents on the Fe-N-C
catalysts due to peroxide oxidation similarly occurred at potentials
below 0.8 V, while the onset potentials for ORR were ca. 0.98
V. The ring currents were perhaps produced due to the
shown in Fig. S7, the resulted catalyst (PCNNS-Phen-NaCl-H
exhibited much inferior activity.
2 4
SO -II)
The catalytic performance of the prepared Fe-N-C catalysts in
.1 M HClO was also investigated. Similar to that in 0.1 M KOH,
PCNNS-Phen-NaCl-H SO exhibited the best ORR activity (Fig. S8
1
7
outersphere electron transfer on carbon. In comparison, the ring
currents on Pt/C were much smaller, forming two peaks in
different potential regions, which should correspond to the
peroxide formation on Pt and carbon support respectively. The
absence of the ring current at the more positive potential region
for Fe-N-C catalysts suggested that the ORR at the active sites of
Fe-N-C catalysts proceeded predominantly through 4-electron
pathway. In addition, among the Fe-N-C catalysts, the PCNNS-
0
4
2
4
and Fig. 8), with the most positive cathodic ORR peak on CV and
the most positive E1/2 on the polarization curve. The kinetic
current density at 0.8 V given by the PCNNS-Phen-NaCl-H SO was
2
4
-
2
ca. 1.69 mA cm , which was among the best of that reported in
5
1,59
the literatures.
In addition, it exhibited a Tafel slope of 66 mV
-
1
dec , similar to that of Pt/C. As shown in Fig. S9, the PCNNP-Phen-
NaCl-H SO gave the smallest ring current associated with H O
2
Phen-NaCl-H SO
produced the smallest ring current,
2
4
2
4
2
corresponding to a reduced H O2 yield (below 2.3%). The
2
production. The corresponding value of the effective electron
-
corresponding value of electron transfer number was estimated to
be larger than 3.95.
transfer number was larger than 3.72, confirming a dominant 4e
reaction pathway.
The durability of the best Fe-N-C catalyst, PCNNS-Phen-NaCl-
H SO , was evaluated by potential cycling between 0.6 and 1.0 V in
2
4
0
.1 M KOH (Fig. 7). The polarization curves nearly unchanged after
ca. 5000 cycles. After 8000 cycles, a slightly negative shift of ca.
.0 mV in E1/2 was seen, whereas a shift of ca. 64 mV occurred for
8
Pt/C. The relatively poor stability of Pt in alkaline solutions due to
the easiness of oxidation and dissolution has been reported in the
57,58
literatures.
Fig. 7 ORR polarization curves of the PCNNS-Phen-NaCl-H SO4
2
catalysts and Pt/C (20 wt%) obtained before and after potential
Fig. 8 ORR polarization curves (a) and the corresponding kinetic
current densities at 0.80 V (b), H O yield and effective electron
-1
cycling in O -saturated 0.1 M KOH with a scan rate of 50 mV s .
2
2
2
transfer number (c) and Tafel plots (d) for the Fe-N-C catalysts and
The high activity and stability of the PCNNS-Phen-NaCl-H SO4
2
Pt/C (20 wt%) obtained at an electrode rotating speed of 1600
catalyst should be related to its unique structure and surface
chemistry. The nanospindles possessed not only mesopores of ca.
-1
rpm and a scan rate of 5 mV s in O
2 4
-saturated 0.1 M HClO
solution.
3
.6 nm sizes inherited from the PCN precursors, but also large
pores with sizes from 10-100 nm due to the nanoparticle-
aggregation structure (Fig. 4b) produced by acid activation. The
hierarchical porous structure offered large specific surface area,
while the unique nanospindle shape should reduce the tendency
of agglomeration and produce voids beneficial to mass transport.
Besides, the NaCl-assisted pyrolysis increased the graphitization of
the carbon matrix of the catalyst.
The durability of the PCNNS-Phen-NaCl-H
considerably inferior in 0.1 M HClO than that in alkaline medium
Fig. 9). A ca. 29 mV negative shift of E1/2 was observed after 3000
cycles. This, however, was comparable to that of the best reported
2 4
SO catalyst was
4
(
1
3,35,60
Fe-N-C catalysts.
In contrast, the Pt/C showed much superior
durability in acid solution (13 mV negative shift in E1/2).
We also investigated the ORR activity of the sample prepared
in absence of NaCl (denoted as PCNNS-Phen-H SO , Fig. S7). The
2
4
half-wave potential of the sample was only 0.83 V, which was ca.
4
0 mV more negative than the PCNNS-Phen-NaCl-H SO . This
2 4
6
| J. Name., 2012, 00, 1-3
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C Pa l tea al ys se i sd So c ni eo nt c ae d &j u Ts et cmh an rog l ion gs y
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ARTICLE
6
7
8
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2
1
1
1
1
2
1
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PCN-222 nanospindles crystals can be synthesized via controlling
the concentration of the precursor solution in a solvo-thermal
method. A highly active and stable Fe-N-C catalyst has been
successfully fabricated through NaCl-assisted pyrolysis of the PCN-
7
936.
1
1
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prevent the nanospindles from agglomerating at high-
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and increase the graphitization of the pyrolytic carbon materials.
The concentrated-sulfuric-acid activation can effectively optimize
the porous structure, resulting in hierarchical porosity. The
hierarchical porosity and the nanospindle shape should
synergistically bring about high accessibility of the active sites and
mass transport of reactants and products. In results, the catalyst
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1
1
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Table of Contents
NaCl-assisted pyrolysis of Fe porphyrinic coordination network combined with concentrated-sulfuric-acid post-activation result in
hierarchically porous Fe-N-C nanospindles catalyst.
8
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