Ultrahigh pressure synthesis of highly efficient FeN:X/C electrocatalysts for the oxygen reduction reaction

Article abstract of DOI:10.1039/c7ta05334g

Ultrahigh pressure (UHP) was employed for the first time as a green method for the synthesis of highly efficient Fe, N co-doped carbon-based (FeN_x/C) electrocatalysts for the oxygen reduction reaction (ORR). Compared with traditional pyrolysis under atmospheric conditions, the synthesis of FeN_x/C catalysts under UHP could be done efficiently with much less consumption of time, energy and chemicals. The observed highly efficient synthesis and high ORR activity of such catalysts could be due to the fast heating system (12 °C per second) under UHP, which leads to highly efficient doping of heteroatoms on carbon with much less consumption of chemicals and energy; the UHP-induced high graphitization degree of the carbon support and the selective formation of highly active sites of pyridinic N and Fe-N_x for the ORR also contribute in part to the high ORR catalytic activity of the catalyst. The work presented here paves a new way for the green, environmentally friendly synthesis of heteroatom-doped highly efficient catalysts for energy or chemical processes.

Full text of DOI:10.1039/c7ta05334g

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Kun Chang, Zhaorong Chang et al.
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Ultrahigh pressure (UHP) method was employed to rapidly synthesize (5 min) a high-efficiency ORR FeNx/C catalyst for the first time.
1

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ARTICLE
Ultrahigh Pressure Synthesis of Highly Efficient FeN /C
x
Electrocatalysts for Oxygen Reduction Reaction
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
c
a
a,d
a,d
a
a,*
Xin Guo, Xiaopeng Jia, Ping Song, Jing Liu, Erling Li, Mingbo Ruan and Weilin Xu
DOI: 10.1039/x0xx00000x
Ultrahigh pressure (UHP) was employed for the first time as a green method for the synthesis of highly efficient Fe, N co-
doped carbon-based (FeN /C) electrocatalysts for oxygen reduction reaction (ORR). Compared with traditional pyrolysis
under atmospheric condition, the synthesis of FeN /C catalysts under UHP could be done efficiently with much less
x
www.rsc.org/
x
consumption of time, energy and chemicals. The observed highly efficient synthesis and high ORR activity of such catalysts
could be due to the fast heating system (12°C/second) under UHP, which leads to highly efficient doping of heteroatoms
on carbon with much less consumption of chemicals and energy; the UHP-induced high graphitization degree of carbon
x
support and the selective formation of highly active sites of pyridinic N and Fe-N for ORR also contribute in part to the
high ORR catalytic activityof the catalyst. The work presented here paves a new way for the green, environmentally
friendly synthesis of heteroatoms-doped highly efficient catalysts for energy or chemical process.
possesses some unique advantages in preparation and performance
improvement of materials. For instance, UHP can effectively
modulate phase transformations, morphology, lattice structure,
1
. Introduction
With increasingly severe energy crisis and environmental pollution,
fuel cells have attracted extensive interest due to their advantages
in sustainable energy conversion, wherein the oxygen reduction
reaction (ORR) is a critical process at the cathode of polymer
electrolyte fuel cells (PEFCs) for direct conversion of the chemical
energy into electricity. Hence, ORR electrocatalysts with high
efficiency are essential for large-scale applications in fuel cells. To
date, Pt-based materials are known as the most efficient ORR
microstructure, etc., influencing optical, electrical, and thermal
12,13
performances in materials.
In this work, to deal with the problems in traditional synthesis of
heteroatom-doped catalysts, for the first time, UHP technique was
1
,2
adopted for highly efficient synthesis of FeN
x
/C with high ORR
2
,14,15
3
,4
electrocatalytic activity.
Such high efficient synthesis can be
done in short time (down to 5min) with little consumption of
chemicals (such the nitrogen precursor) and energy (electricity) due
to the fast and high efficient doping of heteroatoms. In this way,
5
-7
catalysts;
however, Pt-based materials suffer from many
problems in commercial application including prohibitive cost, finite
reserves, low durability and CO poisoning effects of Pt.
8
,9
the obtained FeN /C catalysts with high ORR electrocatalytic activity
x
possess one of the best price/performance ratio and make the
method the most environmentally friendly.
Consequently, it is critical to develop energetically high-efficient
nonprecious metal catalysts for ORR to achieve the substitution of
Pt-based catalysts. In recent decade, the nonprecious metal
catalysts, especially, Fe,
FeN /C) with excellent electrocatalytic performance for ORR have
been well realized to be promising candidates to Pt-based ones.
N co-doped carbon-based catalysts
(
x
10,11
However, these catalysts were usually synthesized through a
pyrolysis at high temperature under atmospheric condition with
large consumption of chemicals and electricity. Obviously, such
environmentally unfriendly synthesis protocol is time-, energy-, and
chemical-consuming. On the other hand, due to the specialty of
sealed ultrahigh pressure (UHP) environment, the UHP technique
a.
State Key Laboratory of Electroanalytical Chemistry & Jilin Province Key
Laboratory of Low Carbon Chemical Power, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences. E-mail: weilinxu@ciac.ac.cn
Department of Materials Science and Engineering,Changchun University of
b.
Science and Technology, 7989 Weixing Road, Changchun, 130022, P. R. China.
State Key Laboratory of Superhard Materials, Jilin University, 2699 Qianjin Street,
c.
Changchun, 130012, P. R. China.
University of Chinese Academy of Sciences,Beijing, 100049, P. R. China
d.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Fig. 1 Scheme for UHP synthesis:(a) precursor processing and (b) UHP
synthesis at controllable high temperature and the schematic diagram of
pressure cavity in cubic anvil high-pressure apparatus, and assembled
chamber (1-steel plate, 2-graphite heater, 3-dolomitic cylinder, 4-
molybdenum box, 5-sample, 6-pyrophyllite).
content was obtained from ICP-MS (ICAP-6000, Thermo Fisher
Scientific). The Brunauer-Emmett-Teller (BET) surface areas and
2
pore volumes were obtained from 77K N sorption isotherms using
ASAP 2020 instrument.
The ORR activity of as-prepared catalysts was evaluated by
voltamperometry on a glassy carbon electrode. Fabrication of the
working electrodes was done by pasting catalyst inks on a glassy
carbon rotating disk electrode (4mm in diameter). Its apparent
2
. Experimental Procedures
2
.1 Materials
The carbon black BP2000 (BP) was purchased from Asian-Pacific
Specialty Chemicals Kuala Lumpur. Urea (CH O, >99%), ferric
chloride hexahydrate (FeCl ·6H O, >99%) and Potassium hydroxide
KOH, >85%) were purchased from Sinopharm Chemical Reagent
2
surface area (0.1256cm ) was used to normalize the ORR activity of
the catalysts. The ink was formed by mixing 5mg of the catalysts,
4 2
N
5
0μL of a 5wt.% Nafion solution in alcohol, and 950μL of ethanol in
3
2
a plastic vial under ultra-sonication. A 10-μL ink was dropped on the
surface of the glassy carbon rotating disk electrode, yielding an
(
Co. LTD. Nafion solution (5wt.%) was obtained from Sigma-Aldrich.
All the chemicals were used as delivered without further treatment.
Deionized water with the specific resistance of 18.2mΩ·cm was
obtained by reversed osmosis followed by ion-exchange and
filtration. Rotating ring-disk electrode of glassy carbon (RRDE, 4mm
in diameter) was purchased from CH Instruments, Inc, USA.
-
2
approximate catalyst loading of 0.39mg·cm . For comparison, a
commercially available platinum/carbon (Pt/C) catalyst, nominally
2
0wt.% on carbon black from E-TEK was used. The platinum-based
ink was obtained by mixing 1mg catalyst, 50μL of a 5wt.% Nafion
solution in alcohol, 950μL of ethanol. Then, a 15-μL platinum ink
was dropped on the glassy carbon rotating disk electrode, yielding
2
.2 Preparation of Fe, N co-doped carbon materials
Firstly, the N (urea) and Fe (FeCl ·6H
homogeneously with carbon support (BP2000 (BP)) by refluxing for were a platinum wire and a SCE electrode (Fig. S2), respectively. The
hours at 80°C, as shown in Figure 1a; secondly, for the UHP potential of the electrode was controlled by an EG&G (model 273)
experiments (Fig. 1b, Fig. S1), the obtained mixture containing potentiostat/galvanostat system. Cyclic voltammetry was
-2
an approximate loading of 0.015mg or 24ug Pt cm . The electrolyte
3
2
O) precursors were mixed was 0.1M KOH solution; the counter and the reference electrodes
7
-1
carbon and chemical precursors was pressed tightly to be a φ performed from 0.2 to -1.2V at 50mV·s after purging the
0.5×2 mm cylinder by cold compression, and then was assembled electrolyte with O2 or N2 gas for 30min. Voltamperometry
1
into the sample chamber of cubic anvil high-pressure measurements were performed by using a RRDE at a rotating speed
apparatus(SPD 6×1200), on which a graphite box was used as the of 1600rpm in an O saturated electrolyte from 0.2 to -1.2V (vs SCE)
2
heating unit, following by preparation at 760°C and under the at a sweep rate of 5mV/s. The measured potentials vs SCE were
pressure of 0.85-3.5GPa for 5min. The sample chamber was sealed converted to a reversible hydrogen electrode (RHE) scale by the
by the UHP exerted from six directions and then heated up rapidly Nernst equation.
with rate of 12°C/second as shown in Fig. 1b. The pressure was
For the calculation of H
2
O
2
yields of as-prepared catalysts in 0.1M
¯%
calibrated by the change in resistance of standard substances. The
synthesis temperature was measured by Pt RH/Pt Rh6
KOH, based on both ring and disk currents from RRDE, the HO
2
a
generated from ORR and the electron transfer number (n) were
estimated by the following equations:
thermocouple placing near the sample. Finally,the as-prepared
catalysts were cracked and ground thoroughly for further the
electrochemical and physical characterization.
-
ꢁ
ꢀ /ꢂ
HO
2
%=200×
(1)
ꢀꢃꢄꢀꢁ/ꢂ
ꢀ
ꢃ
n=4×
(2)
During the pyrolysis in such sealed chamber, nothing will be
leaked out and all the components from the pyrolysis of chemical
ꢀꢃꢄꢀꢁ/ꢂ
where i is the disk current density, i is the ring current density
D
R
precursors will be trapped inside. In this way, a tiny amount of and N is the current collection efficiency of the Pt ring disk. N is 0.37
precursor can induce highly efficient doping of heteroatoms in short from the reduction of K Fe[CN] . All the current densities have
3
6
time and then the consumption of chemicals and energy can be already been normalized to the electrode surface area.
reduced hugely, making it a green and environmentally friendly
method to replace traditional pyrolysis, in which the gas flow under
3
. Results and discussion
atmospheric condition usually leads to a huge consumption or
wasting of large amount of chemicals and energy.
To assess the ORR activities of the catalysts obtained under
different conditions, linear sweep voltammetry (LSV) tests were
2.3 Physical and electrochemical characterization of catalysts
performed on a rotating ring disk electrode(RRDE) in O
.1 M KOH. Based on the half-wave potential (E1/2) obtained from
LSV curves, the synthesis process was optimized by varying the
2
-saturated
The phase of as-prepared catalysts was identified by X-ray
diffraction (XRD, D/MAX 2500) with Cu-Kα radiation. X-ray
0
photoelectron spectroscopic (XPS) measurements were performed synthesis temperature (Fig. S3), time (Fig. S4), weight ratio between
on AXIS Ultra DLD (Kratos company) using a monochromic Al X-ray carbon and urea (Fig. S5), and pressure (Fig. 2a). The optimal
source. The microstructures were obtained using a high-resolution catalyst obtained (under the optimal synthesis conditions: pressure
=
1GPa, pyrolysis temperature = 760 °C and time = 5 min, mass ratio
between BP and urea=1:0.5) finally with Fe 4.7wt% (ICP-MS) and N
.67 wt%(elementary analysis) was named as BP-NFe. For
transmission electron microscope (HRTEM, JEOL JEM-2100FS). The
final N content was determined by element analysis (EA) using
varioEL cube (elementaranalysensystemeGmbh). The final Fe
1
comparison, the BP, BP-Fe (without urea) and BP-N (without
2
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·6H
ARTICLE
FeCl
pyrolysis method with gas flow under atmospheric condition was indeed can improve the ORR performance of the catalysts, probably
adopted to obtain another FeN /C (named as P-BP-NFe) at 760°C for due to its significant effect on the doping efficiency of heteroatoms
min under atmospheric pressure based on the same amount of and the microstructure of active sites on carbon. Significantly, as
raw materials as that for the synthesis of optimal BP-NFe under shown in Fig. S6, the ORR performance of the obtained optimal BP-
3
2
O) were also obtained by UHP method. The traditional
The above results indicate that the UHP in appropriate range
x
5
UHP.
Specially, as for the pressure-dependence of ORR performance of
catalysts, as shown in Fig. 2b, the half-wave potential (E1/2
obtained from LSV curves increases initially with the pressure
increase, while after a maximum at 1GPa, the E1/2 decreases
inversely with the further increase of pressure. The effect of
pressure on ORR performance of the catalysts also could be
revealed by the pressure-dependent variation of the diffusion-
limiting current as shown in Fig. 2c. Similarly, the limiting current
NFe is even better than the commercial Pt/C with Pt 20 wt%
indicated by the huge difference of E1/2 (0.87 V for BP-NFe vs 0.81V
for Pt/C) and on the same level as other reported best Fe-N-based
)
1
7-23
catalysts (Table S1) in alkaline.
As for the advantages of the UHP
method over traditional pyrolysis under atmospheric conditions, as
shown in Fig. 2, the ORR catalytic activity of optimal BP-NFe is much
higher than that of P-BP-NFe, which was obtained with the
consumption of the same amount of chemicals as but much more
energy than that for BP-NFe obtained from UHP.
(
Ilimit) increases first with the increase of pressure probably due to
To deeply understand the high ORR catalytic activity of the
the content increase of N doped on carbon, after a maximum at 1 optimal BP-NFe, the ORR catalytic activity of BP, BP-Fe (without
GPa, the limiting current decreases inversely with the pressure urea) and BP-N (without Fe) obtained by UHP method were also
although the content of N doped on carbon increases (Fig. 2b). The studied. As shown in Figure 2d, the pure carbon BP shows a very
negative shift of E1/2 or the decrease of limiting current after the low ORR activity. While after the sole doping of N- (BP-N) or Fe-(BP-
maximum probably could be attributed to the compact structure Fe) on carbon, the ORR activity was improved more or less
formed at higher pressure, such compact structure decreases the indicated by the increase of onset potential and limiting current. As
accessibility of oxygen to the Fe, N-based active sites, and then expected, after the co-doping of N and Fe on carbon, the ORR
leads to the decrease of both E1/2 and Ilimit although the total performance on BP-NFe was improved significantly due to the well-
2
4,25
content of N doped on carbon increases continuously with the known synergistic effect between N and Fe.
pressure. Such results indicate that there is an optimal content of N transfer numbers (n) and H yields for these catalysts were also
plotted in Fig. 2e,f based on RRDE data. It shows that the H yield
The electron
2 2
O
1
6
doped on carbon or an optimal pressure in such UHP experiments.
2 2
O
Moreover, as shown in Fig. 2a,b,c, the P-BP-NFe possesses the on the optimal BP-NFe is the lowest (<10%) among these catalysts
lowest content of N doped, lowest E1/2 or ORR activity but with in a wide potential range while with the largest n of ~3.9, very close
largest limiting current. This fact indicates that the amount of to that of Pt/C (n~4). This result indicates that the BP-NFe catalyst
exposed active sites on P-BP-NFe is much larger than that on UHP- in alkaline medium is a direct four-electron process with water as
26
based catalysts although the content of N doped on P-BP-NFe is the main product from ORR.
lower than that on UHP-based catalysts. It further indicates that the
intrinsic activity of the exposed active sites on BP-NFe is much
higher than those on P-BP-NFe.
Fig.3 (a) TEM and (b) HRTEM images of BP-NFe. The white circles indicate
the distribution of Fe nanoparticles in (a). (c) Nitrogen adsorption-
desorption isotherms, the inset is pore-size distributions; (d) Synthesis
pressure dependence of the Brunauer-Emmett-Teller (BET) surface area.
To further understand the origin of high ORR activity and the
saturated 0.1M KOH with a scan rate of 5mV/s and a rotation effect of UHP on BP-NFe, the sample was characterized with
Fig. 2 (a) LSV curves of P-BP-NFe and the FeN
UHP in O
x
/C obtained by varying the
2
speed of 1600rpm. (b) Synthesis pressure dependences of E1/2 and the different techniques. Fig. 3a,b show the typical TEM images of BP-
content of N-doped on carbon. (c) Synthesis pressure dependences of the
NFe. The iron nanoparticles and the amorphous porous carbon
limiting current (Ilimit). The green points in (b,c) are the values for P-BP-NFe
structure with thin-layer graphene-like nanosheets can be seen
and BP-NFe catalysts. (d) LSV curves for BP, BP-N, BP-Fe and BP-NFe. (e) and
clearly. Along with the XRD analysis(Fig. S7), interestingly, it was
(
f) are the potential-dependent number of electrons and H O yields on
2 2
2
found that the iron nanoparticles on BP-NFe mainly exist in the
these catalysts. The loadings of catalysts are 0.39mg/cm .
form of Fe
3
O
4
nanoparticles (Fig. 3b). The pressure-dependent
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porous nature of catalysts was further assessed by nitrogen shown in Fig. S8 and Table S2, one can see that the UHP can
adsorption-desorption analysis as shown in Fig. 3c,d. Fig. 3c shows enhance the formation of Fe-N on carbon support, consistent with
x
clearly that the average size of mesopores decreases and the the conclusion made from N-based component.
percentage of micropores increases gradually with the pressure
Fig. 4c shows the Raman spectra of the P-BP-NFe and BP-NFe
increase. Correspondingly, the BET surface area also decreases
gradually with the increase of pressure as shown in Fig. 3d.
Obviously, the observed decreases of pore size and BET surface
area at higher pressure could be attributed to the UHP-induced
formation compact structure. Such compact structure can prohibit
the accessibility of oxygen to the active sites, just like that shown in
Fig. 2.
catalysts, moreover, Raman spectra of all catalysts prepared is also
summarized in Fig. S9, including the P-BP-NFe and BP-NFe with
synthesis pressure from 0.85 to 3.5 GPa. It shows clearly that the
UHP can lead to the graphitization of carbon to generate graphene-
G
like nanosheets with the intensity increase of I , consistent with the
observation from the HRTEM image (Fig. 3b). No significant shifts or
line broadening suggests that the carbon structure is mostly
Furthermore, high-resolution XPS was employed to investigate retained after the UHP process. It is well known that the carbon
the chemical states of doped N and Fe atoms. As shown in Fig. 4a, graphitization in the catalysts can enhance the electronic
2
,32
the doped N atoms mainly exist in four different bonding states. conductivity and corrosion resistance in electrocatalysis,
which
The peaks at 398.3, 400.1, and 401.1eV correspond to pyridinic then also contribute in part to the observed high ORR performance.
2
7
(
33.04%), pyrrolic (26.64%) and graphitic (8.82%) N, respectively,
The durability or stability of the optimal catalyst was assessed
based on the US Department of Energy’s accelerated durability test
protocol by cycling the catalyst between 0.6 and 1.15V at 200 mV/s
2
6
while the peak at 399.5 eV is indexed to the Fe-N bond (31.50%).
Generally, the active centers based on Fe-N bond can highly
activate the ORR process by the significant decrease of oxygen
2
in O saturated 0.1M KOH. As shown in Fig. 4d, the BP-NFe shows a
2
8-30
adsorption energy and extension of the O-O bond.
Such
slight decrease of limiting current and a 14mV negative shift of E1/2
but without negative shift of onset potential after 10000 cycles. The
slight decrease of limiting current probably could be attributed to
the loss of catalyst from the electrode surface, while the slight
decrease of E1/2 represents a deterioration of ORR activity for BP-
NFe, which may be ascribed to the collapsing of local N- or Fe-based
active sites, further impacting the diffusion-limiting current. For
comparison, the durability commercial Pt/C was also tested. As
shown in Fig. S10, due to the migration/aggregation of Pt
nanoparticles caused by continuous potential cycling and
subsequent loss of the specific catalytic activity, the Pt/C displays a
much larger negative shift (24 mV) of E1/2 after 5000 cycles. Such
difference thus indicates an excellent long-term operation stability
of BP-NFe during the ORR process. As expected, such catalyst also
observation can explain the observed synergistic effect between
doped N and Fe during the ORR process as shown in Fig. 2. In
addition, the existence of high contents of pyridinic N also can
partially explain the observed high ORR activity since it has been
known that pyridinic N is the main active component with highest
ORR activity among different N forms in N-doped metal free
24,31
catalysts.
By comparing the N-based chemical states between
BP-NFe and P-BP-NFe, as shown in Fig. S8 and Table S2, it can be
seen that the UHP can enhance the formation of highly active sites
of Fe-N
x
(from 23.58% to 31.50%) and pyridinic N (from 20.75% to
3
3.04%), confirming the significant role of UHP for the selective
formation of highly active sites.
16,33
shows no response to CO and methanol (Fig. S11).
4
. Conclusions
In summary, for the first time, ultrahigh pressure (UHP)
synthesis was adopted as a green synthesis of highly efficient
x
FeN /C electrocatalysts for ORR with much less consumption
of chemicals and energy compared with tradition atmospheric
pyrolysis. The obtained optimal catalyst BP-NFe with a
remarkable durability shows superhigh ORR performance. It
was further found that the UHP can efficiently enhance the
doping efficiency of heteroatoms on carbon, the graphitization
of carbon, and the selective formation of highly active sites of
x
pyridinic N and Fe-N for ORR. The UHP-based synthesis
protocol with much less consumption of chemicals and energy
makes the obtained optimal catalyst possess the best
performance/price ratio. Such work for the "green" synthesis
of highly efficient catalysts opens a new direction for us to
explore highly efficient function materials for energy or
industrial process.
Fig.4 High-resolution XPS spectra of N 1s (a) and Fe 2p (b) of the BP-NFe
catalyst. (c) Raman spectra of the P-BP-NFe and the BP-NFe catalysts. (d)
The durability of BP-NFe in O -saturated 0.1M KOH.
2
The Fe 2p spectrum of BP-NFe shown in Fig. 4b could be divided
into two peaks at 710.4 and 711.3 eV, which can be assigned to the
2
+
3+
binding energies of the 2p3/2 orbitals of Fe and Fe species,
confirming the observation of Fe from the HRTEM (Fig. 3b) and
XRD (Fig. S7). Furthermore, from the atomic ratio of Fe /Fe =2 in
3 4
O
3
+
2+
Acknowledgements
2
+
3+
Fe
content of Fe in the form of Fe-N
be about 6.9%. By comparing it with that (1.0%) on P-BP-NFe as
3 4
O
and the contents of Fe and Fe shown in Table S2, the This work was supported by National Basic Research Program
2
+
x
in the BP-NFe was deduced to of China (973 Program, 2014CB932700), National Natural
Science Foundation of China (U1601211, 21633008, 21433003,
4
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