Air plasma etching towards rich active sites in Fe/N-porous carbon for the oxygen reduction reaction with superior catalytic performance

Article abstract of DOI:10.1039/c7ta05035f

Herein, an electrocatalyst consisting of iron and nitrogen co-doped porous carbon (Fe-N/C) was prepared by catalytic carbonization of chitin with the assistance of FeCl₃ and ZnCl₂. The catalytic activity of Fe-N/C towards the oxygen reduction reaction (ORR) in both acidic and alkaline electrolytes was significantly enhanced by air-plasma etching for only 120 s, showing a four-electron ORR process with an onset potential and limiting current comparable to those of Pt catalysts. This performance enhancement originated from the removal of less stable sp³ and amorphous sp² carbons which would expose more active catalytic FeN₄ centers, as well as the transformation of a small fraction of iron-based nanoparticles into FeN₄ species.

Full text of DOI:10.1039/c7ta05035f

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Zhong, J. Chen, P. Zhang, X. Ren, Y. Li, H. Mi, L. Yao and L. Sun, J. Mater. Chem. A, 2017, DOI:
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Volume
4
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January 2016 Pages 1–330
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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Air plasma etching towards rich active sites in Fe/N-porous carbon
for oxygen reduction reaction with superior catalytic performance
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a,b
a
a
a
Wenhua Zhong , Jiaxiang Chen , Peixin Zhang* , Libo Deng* , Lei Yao , Xiangzhong Ren ,
a
a
a
Yongliang Li , Hongwei Mi and Lingna Sun
DOI: 10.1039/x0xx00000x
www.rsc.org/
activity, particularly due to their advantages over other non-Pt
Herein, an electrocatalyst consisting of iron and nitrogen co-doped
porous carbon (Fe-N/C) was prepared by catalytic carbonization of
chitin with the assistance of FeCl and ZnCl . The catalytic activity
4-7
electrocatalysts in acidic media.
The catalytic efficiency relies heavily on the diffusion and
contact of oxygen with the catalytic centres as well as the
electron transfer within the catalysts and across the
electrolyte-catalyst interface. Particularly, it has been
demonstrated recently that active catalytic sites mainly form
3
2
of Fe-N/C towards oxygen reduction reaction (ORR) in both acidic
and alkaline electrolytes was significantly enhanced by air-plasma
etching for only 120 s, showing a four-electron ORR process with
onset potential and limiting current comparable to Pt catalyst.
This performance enhancement was originated from the removal
at the edges of graphitic sheets, which exist predominantly in
8-10
micropores.
As such, open structures that allow readily
3
2
of less stable sp and amorphous sp carbons which would expose
more active catalytic center FeN , as well as the transformation of
a small fraction of iron-based nanoparticles into FeN species.
access of reactants to the catalytic centres is crucial for
efficient ORR process. Traditional methods of preparing Fe-N/C
electrocatalysts usually involve pyrolysis of precursors
4
4
Platinum-loaded carbons (Pt/C) have long been considered as consisting of Fe, N and C elements at high temperatures. Only
the benchmark electrocatalysts for cathodic oxygen reduction a small fraction of the active sites are exposed while the rests
reaction (ORR) which is a rate-controlling step in fuel cells and are embedded in the matrix of Fe-N/C and thus poor
1
-3
metal-air batteries. However, the high cost, poor durability, performance is obtained in the pyrolyzed carbons.
11-13
To
and intolerance to fuel crossover for Pt-based catalysts have address this issue, numerous methods have been developed,
hindered the widespread application of these energy- such as using the versatile matrix material graphene to load
conversion devices. It is thus vital to develop cost-effective active centers and controlling the pore structure of catalysts
1
1, 14, 15
electrocatalysts with high performance to replace Pt-based using template synthesis.
However, most of those
catalysts. Recently, iron and nitrogen co-doped carbon (Fe-N/C) methods are tedious, costly and difficult to scale up.
has been receiving tremendous interests due to their high ORR
Herein, we report a facile method to enhance the
electrocatalytic performance of an Fe-N/C electrocatalyst
through plasma etching. Chitin which is the second most
abundant and nitrogen-rich biopolymer was used as the
precursor to prepare the carbon matrix. To obtain a high
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surface area and
a
high degree of graphitization electrocatalyst (the samples were denoted as Fe-N/C-x, where
simultaneously, the precursor was carbonized with the x refers to the plasma treatment time). Scanning electron
assistance of both FeCl and ZnCl , which resulted in Fe and N microscopy images illustrated that the surface of as-prepared
2
3
co-doped porous carbon after acid leaching. The sample was carbon monolith before plasma etching is intact and smooth
further etched by air plasma which exposes the catalytic active (Fig. 1e). After treating by air plasma for 120 s (i.e. Fe-N/C-120),
Fe-N (pyridinic N) species, through which excellent catalytic the surface becomes rougher and ordered trenches can be
performances towards ORR in both acidic and alkaline media clearly seen from Fig. 1f (SEM images for samples treated for
were obtained.
The precursor was first pre-oxidized in air and Fe-N/C-120 (Fig. S3) suggested that more defects are created
impregnated with FeCl and ZnCl prior to the high- upon plasma etching, which is consistent with SEM
different durations are shown in Fig. S2). HRTEM images for
3
2
temperature treatment. Coordination of Fe and Zn with the observation. It is also noted the content of iron-containing
polar groups of chitin enables a high capacity for adsorption of nanoparticles is decreased after plasma etching.
3 2
FeCl and ZnCl by chitin, which facilitates the catalytic
graphitization and creation of porous structure upon
carbonization at 800 °C (Experimental details are described in
the electronic supplementary information (ESI)). The
subsequent acid leaching removed most of the metal
compounds on the surface, leaving a low content of
nanoparticles which were encapsulated tightly in the carbon
matrix, as can be seen from the transmission electron
microscopy (TEM) image shown in Fig. 1a. High resolution TEM
(HRTEM) images revealed the carbon is partially graphitized
with a low structural order, and the average diameter of the
nanoparticles is ~7 nm (Fig. 1b). Elemental mapping through X-
ray energy dispersive spectroscopy (EDS) suggests the
uniform distribution of carbon, oxygen and nitrogen in the
pyrolyzed carbon (Fig. S1 in the supporting information) and
the nanoparticles shown in Fig. 1c are mainly consisted of iron,
oxygen and nitrogen (Fig. 1d). The Fe species that survived
after acid leaching are uniformly distributed in the
carbon/nitrogen matrix, which are believed to contribute to
Fig. 1 (a), (b) and (c): TEM images of Fe-N/C catalyst and (d) EDS elemental mapping
corresponding to the area in (c); SEM images for (e) Fe-N/C-0 and (f) Fe-N/C-120.
highly active FeN
4
active sites. However, most of the active
Plasma etching also induced changes in other aspects: (1)
sites are embedded in the matrix of Fe-N/C, which are not XRD characterization (Fig. 2a) showed that the as-pyrolyzed
easily accessible for the reactants, which is reflected by a low carbon is crystalline (strong reflection at 26 ) which endows a
electrocatalytic performance. Re-annealing the acid-leached high conductivity to the carbon. Upon plasma etching, the
Fe-N/C at 800 C did not improve the catalytic performance as crystallinity further increased slightly due to removal of less
significant as Dodelet showed. Thus air-plasma treatment stable sp carbon and amorphous sp carbon; (2) Raman
°
°
1
6
3
2
-
was applied to tune the surface properties of the spectroscopy tests disclosed that the D-band (at ~1340 cm )
1
2
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-1
becomes stronger relative to the G-band (at ~1586 cm ) as the and chemical state upon plasma etching. The full spectra are
irradiation time increases to 120 s but becomes slightly weaker shown in Fig. S5 and the compositions are listed in Table S1. It
as the etching time is extended (Fig. 2b), suggesting that more is noted the content of oxygen increased significantly after
defects were created upon plasma etching, which agrees well treating with plasma, from 11.43at% for Fe-N/C-0 to 22.00at%
with the TEM observation shown in Fig. S3. We speculate that for Fe-N/C-120 but decreased slightly to 19.7% by further
topological defects such as carbon vacancies, Stone-Wales extending the etching time to 240 s (Fig. S6). The increase of
defects, edges of planes and open ends are introduced to the oxygen content would increase the hydrophilicity of the
1
7, 18
Fe-N/C catalysts after the air plasma etching.
(3) Fourier carbon matrix, facilitate the diffusion and bonding of oxygen
transform infrared (FTIR) spectra showed that the with the carbon surface and consequently improve the
1
concentration of hydrophilic groups in the Fe-N/C-0 such as catalytic efficiency. In the meanwhile, the content of carbon
9
-1
3
the absorption at 3433 cm corresponding to OH group is dropped steadily due to the loss of less stable sp and
2
obviously enhanced upon plasma etching (Fig. 2c); (4) The amorphous sp carbons (Fig. S6).
Brunauer–Emmett–Teller (BET, Fig. 2d) surface area increased
In the high resolution Fe 2p XPS spectrum of Fe-N/C-0,
2
from 1730 m g for Fe-N/C-0 to 1963 m g for Fe-N/C-120 the peaks at 711.0 and 724.3eV (Fig. S7) were assigned to Fe
-1
2
-1
2
-1
but decreased to 1780 m
g
for Fe-N/C-240, and the 2p3/2 and Fe 2p1/2, respectively, suggesting the existence of
corresponding pore volume (Fig. S4) also increased slightly iron oxides. The atomic content of Fe was 0.84% for Fe-N/C-0
3
from 0.92 cm g to 1.05 cm g as determined by nitrogen and remained almost constant (within experimental error)
-1
3
-1
adsorption/desorption measurements. Moreover, the rich after plasma etching.
micropores as disclosed by the adsorption/desorption
The high-resolution N1s spectra of all Fe-N/C catalysts can
isotherms in these catalysts are crucial for catalytic activity due be deconvoluted into three peaks with binding energies of
to that it is the micropore that mainly hosts the catalytic active 398.6, 400.1, and 401.0 eV, corresponding to pyridinic N,
9
centers in N-doped carbons.
pyrrolic N, and graphitic N, respectively (Fig. 3a). The peak at
98.6 eV also includes the contribution from nitrogen bound
to iron (Fe-N) due to the small difference between binding
3
1
1, 20
energies of Fe-N and pyridinic N.
The central Fe ion would
coordinate with four pyridinic N that are located at the edges
of graphitic domains. The total nitrogen species accounted for
7.15at% in Fe-N/C-0 which is remarkably higher than those
2
1, 22
2
obtained by direct pyrolysis or by ZnCl activation of chitin.
Such a high doping level is attributed to the coordination
between N and Fe which would prevent the evaporation of N
species during pyrolysis. The total content of nitrogen species
only varied slightly upon irradiation to plasma. However, the
relative composition of N species changed significantly, i.e. the
fraction of pyridinic N (including Fe-N) increased steadily with
the increase of etching time, from 20.0% for Fe-N/C-0 to 28.8%
for Fe-N/C-120 (Fig. 3b) whereas the pyrrolic N decreased from
Fig. 2 Characterization of the effects of plasma etching on structure of Fe-N/C: (a) XRD
patterns, (b) Raman spectra, (c) FT-IR spectra, and (d) N
for the Fe-N/C electrocatalysts.
2
adsorption-desorption curves
X-ray photoelectron spectroscopy (XPS) measurements
were performed to further analyze the change of composition
4
8.3% to 37.5%. It is now generally recognized that the Fe-N
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(pyridinic N) are the catalytic active centers for ORR. Hence, nanoparticles by plasma, which would migrate into the
the increase of these nitrogen species might enhance the adjacent micropore to coordinate with the pyridinic N at the
catalytic performance which will be discussed later.
edges of graphitic domains. This change is also consistent with
the increase of pyridinic N revealed by XPS, which confirmed
that more active sites are exposed upon plasma etching.
Such modifications in chemical state and microstructure
of Fe-N/C induced by plasma etching might pose profound
effects on its electrocatalytic activity. The performance of
these catalysts in an acid electrolyte HClO
assessed by cyclic voltammetry (CV) and the curves are shown
in Fig. S8a. A single cathodic peak resulting from O reduction
4
(0.1 M) was first
2
can be seen in the O
2
-saturated electrolyte whereas no such
reduction peak was observed when the electrolyte was N
2
-
saturated. With the increase of plasma etching time, the ORR
peak becomes stronger and the peak potential shifted
positively to 0.694 V at 120 s (Fig. S8b). These results clearly
suggest that the catalytic activity is enhanced by plasma
etching. However, extending the etching time to 240 s
degraded to performance slightly compared to the one etched
for 120 s, due possibly to the slight but noticeable drop of the
surface area, the lower density of defects and the lower
content of oxygen, which are in turn due possibly to collapse
of micropores, over-etching of the crystalline carbon and
evaporation of less stable oxygen-containing groups. It is
shown later that the change of these structural parameters is
in excellent agreement with the performance order among our
plasma-etched Fe-N/C catalysts. The optimal etching time is
Fig. 3 (a-b) XPS spectra and (c-d) Mössbauer absorption spectra for Fe-N/C-0 and Fe-
N/C-120.
The chemical state of iron in the Fe-N-C was further
5
7
characterized using Fe Mössbauer spectroscopy (Fig. 3c). The
Mössbauer parameters of each component are shown in Table
S2. Three doublets (D1, D2 and D3) with unique isomer shift
Q
(δiso) and quadrupole splitting (ΔE ), which correspond to
2
+
Fe /Fe species are clearly identified. No elemental Fe (i.e.
3+
singlet peak) was detected from the spectrum for Fe-N/C-0. D1
−
1
−1
Q
with an δiso of 0.42 mm s and ΔE of 0.9 mm s is assigned to
II
a four-fold nitrogen coordination of Fe in low-spin (Fe N
II
4
, LS),
and would contribute to the catalytic performance in acid
1
20 s in this case.
2
3, 24
electrolytes.
However, a recent study showed that the
The electrocatalytic activity was further evaluated from
−
1
doublet D2 which has an δiso of 1.23 mm s and ΔE
Q
of 2.13 linear-sweep voltammetry (LSV) performed on a rotating disk
−
1
II
-
2
mm s and is associated with high-spin Fe N
4
, is the most electrode (RDE) at the same catalyst loading (0.14mg cm ).
The doublet D3 is related to surface-
oxidized and nitrogen-rich iron nanoparticles and is less
1
6, 25, 26
active site for ORR.
The onset potential (determined as the potential at 5% of the
2
8
limiting diffusion current from the LSV curve ) also shifted
positively with the increase of plasma etching time, which was
found to be 0.745 V, 0.746 V, 0.750 V, and 0.812 V for Fe-N/C-
2
7
catalytic active. It is noted that after treating with plasma for
1
20 s, the fraction of D1 among the total iron species remains
almost constant whereas the D2 component increases
significantly with the expense of D3 (Fig. 3d). This change
might be due to the sputtering of Fe atoms from the
0
, Fe-N/C-30, Fe-N/C-60, and Fe-N/C-120 (Fig. 4a), respectively.
It should be noted that even better performances can be
obtained for these materials when higher loadings of catalysts
4
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−
2
(
loading ∼80 µg cm ), (d) the H
O
2 2
yields as a function of potential, (e) Durability
on the electrode are used for testing. The onset potential for
evaluation at 0.3 and a rotation rate of 1600 rpm for Pt/C and (f)
V
-2
Chronoamperometric responses on addition of 1M methanol after about 300 s.
Fe-N/C-120 at a loading of 0.60mg cm was 0.88 V which is
close to that of the commercial Pt/C and comparable with the
leading results reported in literatures for Fe- and Co- doped
To gain further insight into the ORR on the catalysts, the
catalytic efficiency was also analysed using rotating-ring disk
electrode (RRDE, Fig. 4c). The corresponding ring current was
simultaneously measured with a Pt ring electrode for detecting
peroxide species formed at the disk electrode. The n values
1
.
3, 29, 30
carbons in an acidic electrolyte, (Table S3)
The current
density for these catalysts also exhibited a similar order, i.e.
-2
the current at 0.3 V (J0.30) increased from 1.96 mA cm for Fe-
-2
N/C-0 to 3.56 mA cm for Fe-N/C-120 ( parameters listed
-2
2 2
and the corresponding H O yield can be also determined from
Table S4) and the limiting current increased from 2.55 mA cm
-
2
the disk and ring currents using Eq. S4 and Eq. S5, respectively.
It was found that in the potential range of 0.2 V to 0.8 V, the n
values for Fe-N/C-120 are in the range of 3.90-3.93 (Fig. 4d).
The trend of the n value calculated from RRDE tests are in
good agreement with those determined from the K-L plots (Fig.
to 3.86 mA cm . Furthermore, the electron transfer number (n)
determined at 0.3 V using the Koutecky–Levich (K–L) equations
(
Eqs. S1-S3 in) increased from 2.73 for Fe-N/C-0 to 3.18, 3.55,
and 3.77 for Fe-N/C-30, Fe-N/C-60, and Fe-N/C-120,
respectively (Fig. 4b and Fig. S9). These results suggest that the
ORR occurs on the as-prepared Fe-N-C catalysts through a
combination of two-electron and four-electron processes.
After etching with air plasma for 120s, the catalysts can
promote the ORR predominately through a four-electron
pathway. These results thus confirmed that an outstanding
catalytic performance can be achieved through plasma etching
for just 120 s.
S9). The corresponding H
to be 4.71-3.40%, which is comparable with that of 20% Pt/C
3.18-1.39%).
Long-term durability and tolerance to the methanol
2 2
O yields on Fe-N/C-120 were found
(
crossover effect are crucial for practical applications of
electrocatalysts. The durability of Fe-N/C-120 and 20% Pt/C
was evaluated by measuring the current loss after prolonged
2 4
operation at a fixed potential in O -saturated 0.1 M HClO
solution for 10000 s (Fig. 4e). It can be seen that Fe-N/C-120
retained 81% of its initial current whereas the current
retention for Pt/C is only 37% after continuous operation for
1
0000 s in 0.1 M HClO
4
solution, suggesting that the durability
of Fe-N/C-120 is superior to the Pt/C catalyst in acid solution
and suggests that the FeN
4
sites in the Fe−N/C catalyst are
5
quite stable in an acidic environment. To further assess the
methanol-tolerance performance,
introduced during the measurement of chronoamperometric
response at 300s in 0.1 M HClO solution (Fig. 4f). After the
1
M
methanol was
4
introduction of 1M methanol, only a slight disturbance in
current density was observed for Fe-N/C-120, whereas an
obvious reduction in current density was found for the Pt/C
catalyst due to methanol oxidation. Therefore, Fe-N/C-120
exhibits excellent tolerance to methanol oxidation, which
holds great promise for direct methanol fuel-cell technology.
Fig. 4 Electrocatalytic performance evaluation for the electrocatalysts: (a) LSV curves
−
2
−1
(
(
loading ∼0.14 mg cm , scan rate 10 mV s and rotating rate 1600 rpm), (b) K-L plots ,
−2
c) LSV curves measured using an RRDE for Fe-N/C-120 (loading ∼0.6 mg cm ) and Pt/C
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As demonstrated above, plasma etching resulted in a cm (loading
Journal Name
-2
−2
0.6 mg cm ). These values are even superior to
∼
higher content of FeN and also led to a higher crystallinity, those for the benchmark Pt/C catalyst.
4
more defects, a higher content of oxygen-containing groups
and a higher surface area in the Fe-N/C catalyst, all of which
might improve the electrocatalytic performance. To get insight
into the contribution of these aspects to the performance
enhancement, the precursor chitin was activated by ZnCl
2
without using FeCl , and the activated carbon was also treated
3
by air plasma and the electrocatalytic performance was Fig. 5 A schematic diagram for the plasma etching process, the possible structure
evaluated (Fig. S10). Interestingly, the performance of the model for nitrogen species in Fe-N/C catalysts and the catalytic ORR process.
ZnCl
2
-activated carbon after plasma etching was only improved
In summary, an Fe-N/C catalyst with excellent
performances in both acidic and alkaline media has been
-2
slightly, with the limiting current increased from 0.79 mA cm
-2
to 0.99 mA cm , i.e. the catalytic performance is obviously prepared by treating chitin-derived carbon with air plasma for
inferior to that of the Fe-N/C. Moreover, the extent of just 120 s. Structural characterizations suggested that the
performance enhancement upon plasma etching for the Fe- performance enhancement is associated with the removal of
free carbon is also less pronounced than that for the Fe-N/C less stable carbons that exposes the active catalytic centers
4
catalyst despite that plasma induced similar effects on FeN , coupled with the transformation of iron-based
structure of these carbons. Therefore, the performance nanoparticles into Fe-N species. The method of achieving high
3
enhancement is mainly attributed to the removal of sp and catalytic performances by plasma etching could extend to
2
amorphous sp carbons which would expose the micropores other materials and can be scaled up for mass production due
that accommodate the highly active catalytic center FeN
4
(the to the fast processing and low cost, and would pave the way
proposed mechanism is shown in Fig. 5). Sputtering of iron for practical applications of Fe-N/C catalysts.
atoms from the nanoparticles induced by plasma and the
This work was supported by the National Natural Science
subsequent migration into micropores and creation of more Foundation of China (Grants #51374146, 51602202), the
high-spin Fe-N species would further improve the catalytic Natural Science Foundation of Guangdong (2015A030310087),
performance. In contrast, unblocking the micropores in Fe-free and Shenzhen Government's Plan of Science and Technology
chitin-derived carbon by plasma is less efficient for (Grants JCYJ20150324141711596 and JCYJ2016042211201273
performance improvement due to the lack of Fe-N species.
A parallel investigation of the performance for these
9).
The authors declare no competing financial interests.
catalysts in a KOH solution (0.1 M) disclosed similar behaviors, References
i.e. the as-prepared Fe-N/C without plasma etching showed a
poor catalytic efficiency which was enhanced remarkably upon
etching for 120s (Figs. S11-S13). Moreover, the performance in
KOH is even better than in the acidic electrolyte. In the KOH
solution, ORR occurs on Fe-N/C-120 through a four-electron
process with an even higher n value (n = 3.9), the onset
potential was 1.041 V and the limiting current was 6.29 mA
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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA05035F
TOC
Plasma etching removed less stable carbons and exposed the active sites in Fe-N/C catalysts and
resulted in excellent performances towards oxygen reduction reaction in both acidic and alkaline
media.