Mesoporous nitrogen-rich carbon materials as catalysts for the oxygen reduction reaction in alkaline solution.

Abstract

ORR MNC, FTW! Mesoporous nitrogen-rich carbon (MNC) materials are synthesized by using polymer-loaded SBA-15 pyrolyzed at different temperatures. The activity and stability of the catalysts in the oxygen reduction reaction (ORR) are investigated by using cyclic voltammetry and rotating-disk electrode measurements. The MNC material pyrolyzed at 800 °C exhibits a high electrocatalytic activity towards the ORR in alkaline medium.

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DOI: 10.1002/cssc.201100284
Mesoporous Nitrogen-Rich Carbon Materials as Catalysts for the Oxygen
Reduction Reaction in Alkaline Solution
Tharamani C. Nagaiah,*[a]Ankur Bordoloi,[b]Miguel D. S?nchez,[b, c]Martin Muhler,*[b]and
Wolfgang Schuhmann[a]
The electrochemical reduction of molecular oxygen is highly
relevant for devices such as metal–air batteries, fuel cells, and
air-breathing cathodes in industrial electrocatalytic processes.
One of the most important applications of the oxygen reduc-
tion reaction (ORR) is the cathodic reaction in fuel cells. Poor
kinetics for the ORR in fuel cell cathodes and the high costs of
platinum-based catalysts are the major factors retarding the
implementation of low-temperature fuel cells. In order to be
competitive in mainstream commercial markets, fuel cells have
to become more cost-effective and highly durable, and many
research efforts have therefore been devoted to the search for
less-expensive and highly stable cathode catalysts.
Carbon materials have been suggested as promising non-
precious metal catalysts for the ORR.[1]Although the detailed
mechanism of the ORR at carbon-based materials remains un-
clear, the specific properties of the exposed carbon surfaces
were found to have a substantial influence on the ORR kinet-
ics. Hence, the modification of carbon electrode surfaces for
enhancing the electrocatalytic activity has become a highly in-
teresting research topic. For example, nitrogen-functionalized
carbon nanotubes,[2,3]mesoporous carbon treated with ammo-
nia,[4]and nitrogen-doped graphitic carbon[5]have been ap-
plied as active catalysts for the ORR or can be used as catalyst
support. The presence of either pyridinic or pyrrolic/pyridinic[6]
and/or quaternary[7]nitrogen species is supposed to be re-
sponsible for the enhanced ORR activity. In many of these ma-
terials these functional groups are generated via chemical pre-
treatment with reactive species such as HNO3, NH3, or HCN.[8]
More importantly, these processes suffer from several disad-
vantages, such as requiring multistep procedures to introduce
the nitrogen functional groups, leading to a poor control over
the chemical homogeneity and reproducibility, long prepara-
tion times, and sometimes deterioration of the structure of the
materials. Consequently, the development of metal-free highly
active catalysts with similar properties based on facile, ver-
satile, inexpensive, and reproducible syntheses is of high
importance.
Mesoporous carbon nitride is a material with unique struc-
tural properties, such as a high specific surface area and pore
volume, tunable pore sizes, relatively high inertness, and resis-
tance to high pressure and temperatures.[9]Particularly, it has
a graphite-like structure with pyridine-like nitrogen atoms.
Here, we explored related and previously synthesized, but
structurally less-well-defined mesoporous nitrogen-rich carbon
(MNC) materials[9d,e]as metal-free catalysts for the ORR. The
fabrication procedure of such materials can be easily scaled up
for mass production, and there is no need for a post-synthesis
introduction of functional nitrogen groups to these MNC mate-
rials, as in the case of nitrogen-modified carbon nanotubes or
other previously reported materials.[2–4]The MNC materials ex-
hibit excellent catalytic ORR activity in alkaline medium with
lower overpotential and better long-term stability than a com-
mercial Pt/C catalyst and similar nitrogen-doped carbon mate-
rials. To the best of our knowledge, MNC materials have not
been previously investigated as electrocatalysts for the ORR.
The MNC materials were synthesized by pyrolyzing polymer-
ized ethylenediamine nanocasted into a SBA-15 hard template
based on a procedure from Ref. [9d] at temperatures of 400
(MNC-400), 600 (MNC-600), and 8008C (MNC-800), followed by
the dissolution of the silica framework. The morphology and
mesoporosity of the MNC catalysts were not destroyed with in-
creasing pyrolysis temperature, as confirmed by scanning elec-
tron microscopy (SEM, Figure S1, Supporting Information).
High-resolution transmission electron microscopy (HRTEM)
images show the well-ordered structure of MNC-800, as evi-
dent by the regular linear array of mesopores (Figure 1b) and
X-ray diffraction results (XRD; Figure 2). The wide-angle diffrac-
tion pattern of the MNC materials (Figure 2, inset) exhibits
a broad peak around 268, originating from the (002) plane of
graphitic carbon. A broad peak around 438 appears in the
spectra of the MNC-600 and the MNC-800 catalysts, due to the
(100) reflection of graphitic carbon. Small-angle XRD measure-
ments (Figure 2) reveal a peak in the range between 0.5 and
68, which can be indexed to the (100) reflection of the hexago-
nal p6mm space group. A very small peak was additionally ob-
served in both MNC-600 and MNC-800, which is supposed to
be due to the related (110) plane. These observations are in
good agreement with the XRD pattern of the pure hexagonally
ordered SBA-15 material (Figure 2) and previously reported re-
sults,[10]indicating that the MNC materials have a well-ordered
[a] Dr. T. C. Nagaiah, Prof. Dr. W. Schuhmann
Analytische Chemie-Elektroanalytik & Sensorik
and Center for Electrochemical Sciences-CES
Ruhr-University Bochum
44780 Bochum (Germany)
Fax: (+49)234-32-14683
E-mail: tharamani.chikka-nagaiah@rub.de
[b] Dr. A. Bordoloi, Dr. M. D. S?nchez, Prof. Dr. M. Muhler
Laboratory of Industrial Chemistry
Ruhr-University Bochum
44780 Bochum (Germany)
Fax: (+49)234-32-14115
E-mail: muhler@techem.rub.de
[c] Dr. M. D. S?nchez
Instituto de Fisica del Sur and Departmento de Fisica
Universidad Nacional del Sur-CONICET
8000 Bahia Blance (Argentina)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201100284.
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two-dimensional mesoporous structure with a hexagonal
porous network.
The N2physisorption isotherms of MNC-600 and MNC-800
exhibit type IV shapes (Figure S2, Supporting Information),
with a H1 hysteresis loop with capillary condensation at a rela-
tive pressure p/po?0.4–0.8, characteristic for ordered mesopo-
rous materials. MNC-600 has a maximum Barrett–Joyner–Ha-
lenda (BJH) pore diameter of 3.8 nm and a specific Brunauer–
Emmett–Teller (BET) surface area of 473 m2g?1. Interestingly,
MNC-800 shows a higher surface area of 517 m2g?1with a simi-
lar pore diameter of 3.8 nm. The pore volume is found to be
higher in case of MNC-800 (0.6 mLg?1) compared to MNC-600
(0.4 mLg?1), which could be due to the formation of a higher
number of micropores at the higher pyrolysis temperature.
To investigate the electrocatalytic activity of the MNC cata-
lysts and to compare the effect of the pyrolysis temperature
on the ORR, cyclic voltammetry (CV) and rotating disc elec-
trode (RDE) measurements were carried out in a potential
window from 0.4 to ?0.6 V at a scan rate of 5 mVs?1vs.
a double-junction Ag/AgCl/3m KCl reference electrode. A first
set of experiments was performed in O2-free 1m NaOH solu-
tion obtained by purging with Ar for 30 min before each mea-
surement. In the absence of O2, the voltammograms
did not show any characteristic oxidation/reduction
peaks. However, the capacitive current increased sub-
stantially as compared to a bare glassy carbon elec-
trode due to the increase in surface area. Significant
changes can be observed in the CVs after saturation
of the 1m NaOH solution with O2, achieved by purg-
ing with O2 for 20 min before each measurement
and above the electrolyte solution during the mea-
surements. The voltammograms show the increase in
the current density (jk) for the ORR (Figure S3; Sup-
porting Information), confirming that the prepared
MNC materials are electrocatalytically active for the
ORR. A remarkable increase in the catalytic activity
with increasing pyrolysis temperature was observed
with respect to both the reduction current and the
shift in the onset potential for the ORR.
RDE measurements were performed in order to
further quantify the ORR activities and compare them
with that of a commercial Pt/C (20% Pt) catalyst. Figure 3a
shows the ORR polarization curves in the presence of O2. The
significant changes in jkand the shift of the ORR onset poten-
tial for the MNC samples pyrolyzed at different temperatures
are clearly visible. Further, with increase in pyrolysis tempera-
Figure 1. a) Schematic ideal structure of mesoporous carbon nitride, adapted from
Ref. [9d]. b) HRTEM image of the MNC catalyst, pyrolyzed at 8008C.
Figure 2. Small- and wide-angle (inset) X-ray diffraction patterns of the MNC
catalysts, pyrolyzed at different temperatures.
Figure 3. a) Polarization curves of MNC catalysts pyrolyzed at different tem-
peratures in O2-saturated 1m NaOH with a rotation rate of 900 rpm at
a scan rate of 5 mVs?1, CE: Pt grid, RE: Ag/AgCl/3m KCl (inset: magnification
of the potential region of the ORR onset). b) Chronoamperometric measure-
ments of MNC-800 at ?0.3 V (inset: bar diagram showing the relative de-
crease of the current density with time).
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ture, the onset potentials shifted towards anodic potentials
from ?0.19 V (MNC-400) to ?0.13 V (MNC-600) and +0.03 V
for MNC-800 at a current density of jK=?0.06 mAcm?2. Inter-
estingly, jkalso increases from ?1.18 mAcm?2(MNC-400) to
?3.5 mAcm?2(MNC-800) at a potential of ?0.6 V with increas-
ing pyrolysis temperature, which is supposed to be due to the
not completely formed mesoporous structure of MNC-400 (Fig-
ure S1A, Supporting Information). Surprisingly, the commercial
Pt/C catalyst has an onset potential of 0.02 V, but shows a sig-
nificant lower ORR activity evident from its lower jk of
?1.3 mAcm?2. Evidently, this observation is either due to an
active site limitation of the current density for the Pt/C modi-
fied electrode in alkaline solution or to a lower average
number of electrons transferred per O2molecule. The increase
of the current density at lower potentials as well as the less
steep current decrease in the kinetic region of the polarization
curve of the MNC materials is due to the mesoporosity of the
material, which is increasing the probability of re-adsorption of
potentially primarily formed H2O2and consecutive electron-
transfer steps.
Despite the fact that the analysis of the kinetic parameters
for the ORR using Koutecky–Levich (KL) plots (Figure S5, Sup-
porting Information) is only considered to be a rough estima-
tion for mesoporous materials, the number of electrons trans-
ferred during the oxygen reduction was estimated to be 3.8
for MNC-800.
The XPS C1s spectra of the MNC-400, MNC-600, and MNC-
800 samples are shown in Figure 4a. The spectra were decon-
voluted into seven components. The two main peaks at 284.5
and 285.3 eV were assigned to sp2-hybridized graphite-like
carbon (C?Csp2) and sp3-hybridized diamond-like carbon
(C?Csp3), respectively, overlapping with sp2carbon bound to
Figure 4. Deconvoluted XPS spectra of a) C1s, and b) N1s, and c,d) bar diagrams representing the variation of the atomic surface concentration of different
carbon species and nitrogen functional groups, respectively, for the different MNC catalysts.
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nitrogen (N?Csp2). The peaks centered at 286.1, 287.4, and
288.7 eV were attributed to surface oxygen groups (designated
as C?O, C=O and ?COO, respectively), and the additional fea-
tures at 290.1 and 291.7 eV are satellites of sp2graphite-like
carbon in good agreement with literature.[11–13]
The N1s region (Figure 4b) exhibits three main contribu-
tions: the peak at the lowest binding energy (N1) shifting from
398.4 eV for MNC-400 to 398.0 eV for MNC-800 originates from
pyridinic nitrogen. The peak at 399.3 eV (N2) is assigned to pyr-
rolic nitrogen, and the peak at 400.6 eV (N3) indicates the pres-
ence of quaternary nitrogen.[11–13]The minor N1s peak at
402.4 eV (N4) originates from the N-oxide of pyridinic nitro-
gen.[11,12]These findings further support the initial assumption
that the material is structurally much less ideal than carbon ni-
tride, providing a variety of different potential adsorption sites
for O2and the reaction intermediates.
The surface atomic concentration ratios were derived from
the XPS spectra as shown in Figure 4c and 4d. With increasing
pyrolysis temperature, a noticeable decrease of the sp3carbon
concentration associated with an increase in the sp2carbon
concentration was observed (Figure 4c). This observation is in
good agreement with the XRD results, demonstrating a well-
ordered graphite-like surface structure for MNC-600 and MNC-
800. The signal at 285.3 eV (Figure 4A) is mainly due to carbon
atoms bound to nitrogen atoms (N?C sp2). The surface con-
centrations of the different nitrogen species are shown in Fig-
ure 4d, in which an appreciable decrease of all nitrogen sur-
face species can be observed. In addition, the MNC materials
were characterized by13CNMR spectroscopy in order to identi-
fy the nature and environment of the carbon and nitrogen
atoms. As seen from Figure S6 (Supporting Information), the
MAS spectra exhibit one broad asymmetric peak: the larger
sharp peak at 123.95 ppm can be attributed to sp2carbon
atoms directly bound to carbon, whereas the broad shoulder
at 135–160 ppm can be assigned to sp2carbon atoms bound
to nitrogen.[9d]
The durability of the catalysts and the long-term stability of
the electrocatalytic activity for the ORR are of major concern in
the development of fuel cells. In order to evaluate the stability
of the electrocatalytic properties of MNC-800, chronoampero-
metric measurements in O2-saturated 1m NaOH were per-
formed while simultaneously rotating the electrode. A constant
potential of ?0.3 V and a rotation rate of 900 rpm were ap-
plied, and the current density was plotted as a function of
time (Figure 3b). After an initial decay of the oxygen reduction
current density, a constant value is reached after about 60 min
at about 80% of the initial activity, demonstrating a good
long-term stability of its electrocatalytic properties on the
timescale of hours in the alkaline medium.
The following properties of MNC-800 are considered to be
relevant for its observed enhanced ORR electrocatalytic activi-
ty: The surface area and pore volume increase with increasing
pyrolysis temperature as derived from the N2physisorption iso-
therm (Figure S2, Table S1, Supporting Information), which are
assumed to lead to improved mass transfer for MNC-800 as
compared with MNC-600. The presence of the nitrogen func-
tional groups and the graphitic nature of the catalysts are
both considered essential for high ORR activity. Our results
imply that also the higher degree of graphitization (Figure 4c)
contributes to the higher activity of MNC-800.[5,14]This is fur-
ther supported by the oxidation of the MNC materials studied
by thermogravimetric analysis in the presence of air. As can be
seen in Figure S7 (Supporting Information), the oxidation of
MNC-400 started below 4008C, which is due to the presence
of more amorphous carbon in the catalyst. However, for MNC-
800 the highest oxidation onset temperature was found indi-
cating a higher degree of graphitization in agreement with the
XPS results.
In summary, mesoporous nitrogen-rich carbon materials syn-
thesized by pyrolysis at 8008C exhibit an interestingly high
electrocatalytic activity in the ORR, and they may be further ex-
ploited as potentially efficient and inexpensive metal-free ORR
catalysts with good long-term stability in alkaline solution.
Experimental Section
Experimental details are provided in the Supporting Information.
Acknowledgements
T.C.N. and A.B. thank the Alexander-von-Humboldt Foundation
for postdoctoral fellowships. The authors are grateful to Dr. Va-
santhakumar G. Ramu for useful discussions, and to the EU and
the state of North-Rhine-Westphalia for financial support in the
framework of the Hightech.NRW project CES.
Keywords: electrocatalytic activity · mesoporous nitrogen-rich
carbon materials · oxygen reduction reaction · RDE · XPS
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Received: June 7, 2011
Revised: October 11, 2011
Published online on March 16, 2012
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