Protic ionic liquids and salts as versatile carbon precursors

Article abstract of DOI:10.1021/ja411981c

Instead of traditional polymer precursors and complex procedures, easily prepared and widely obtainable nitrogen-containing protic ionic liquids and salts were explored as novel, small-molecule precursors to prepare carbon materials (CMs) via direct carbonization without other treatments. Depending on the precursor structure, the resultant CMs can be readily obtained with a relative yield of up to 95.3%, a high specific surface area of up to 1380 m ²/g, or a high N content of up to 11.1 wt%, as well as a high degree of graphitization and high conductivity (even higher than that of graphite). One of the carbons, which possesses a high surface area and a high content of pyridinic N, exhibits excellent electrocatalytic activity toward the oxygen reduction reaction in an alkaline medium, as revealed by an onset potential, half-wave potential, and kinetic current density comparable to those of commercial 20 wt% Pt/C. These low-cost and versatile precursors are expected to be important building blocks for CMs.

Full text of DOI:10.1021/ja411981c

Communication
pubs.acs.org/JACS
Protic Ionic Liquids and Salts as Versatile Carbon Precursors
Shiguo Zhang, Muhammed Shah Miran, Ai Ikoma, Kaoru Dokko, and Masayoshi Watanabe*
Department of Chemistry and Biotechnology, Yokohama National University, Hodogaya-ku, Yokohama 240-8501, Japan
S
* Supporting Information
applications. The graphitization and N content are also
ABSTRACT: Instead of traditional polymer precursors
and complex procedures, easily prepared and widely
obtainable nitrogen-containing protic ionic liquids and
salts were explored as novel, small-molecule precursors to
prepare carbon materials (CMs) via direct carbonization
without other treatments. Depending on the precursor
structure, the resultant CMs can be readily obtained with a
relative yield of up to 95.3%, a high specific surface area of
up to 1380 m2/g, or a high N content of up to 11.1 wt%, as
well as a high degree of graphitization and high
conductivity (even higher than that of graphite). One of
the carbons, which possesses a high surface area and a high
content of pyridinic N, exhibits excellent electrocatalytic
activity toward the oxygen reduction reaction in an alkaline
medium, as revealed by an onset potential, half-wave
potential, and kinetic current density comparable to those
of commercial 20 wt% Pt/C. These low-cost and versatile
precursors are expected to be important building blocks
for CMs.
somewhat insufficient because of the inherent nature of the
limited CM precursors.2b,4 (c) Small molecules with high
volatility such as acetonitrile and benzene can also be used as
precursors to prepare CMs, particularly nanostructured CMs
such as carbon nanotubes, while harsh conditions such as
chemical vapor deposition (CVD) are crucial.8 (d) Recently,
ionic liquids (ILs) were shown to be excellent precursors due to
their negligible volatility, high thermal stability, and molecular
tunability,9 which facilitate the carbonization process, resulting in
CMs with a high N content, conductivity,9,10 and potential for
applications in catalysis and electrochemistry.11 Unfortunately,
only very few cyano/nitrile-containing aprotic ILs could yield
CMs.9a,12 The multistep and time-consuming synthesis, rigorous
purification, metal impurities (e.g., Li, Na, or Ag, possibly from
metathesis), and high cost may complicate their manufacture and
hinder large-scale production.
Herein, we report that protic ILs and salts (PILs/PSs) can be
used as novel, low-molecular weight organic precursors for the
direct synthesis of CMs (Figure 1a). In contrast to conventional
precursors, PILs/PSs possess negligible volatility, comparable to
that of aprotic ILs, but are easily obtained and widely available.
The structural diversity of PILs/PSs also enables us to investigate
the correlation between the precursor structures and the
properties of the final CMs, which allows the synthesis of
specific CMs with controlled structure and properties.
arbon materials (CMs), which are prepared by the
C
carbonization of C-containing precursors, have been
extensively used in diverse areas such as environmental
treatment, catalysis, gas capture/storage, and energy conver-
sion/storage.1 To satisfy such applications, great effort has been
focused on the discovery of precursors and appropriate methods
to obtain CMs with a controlled structure, regulated texture,
graphitic framework, large specific surface area and pore volume,
and desirable heteroatom doping.2 Particularly, the precursors
play a crucial role, and their chemical composition is often
reflected in the type and amount of heteroatoms found in the
final CMs. For example, one can easily incorporate dopants, such
as nitrogen,3 into the CM structure by choosing appropriate
precursor molecules. Despite works related to the synthesis, the
study of organic precursors has been less emphasized in the
past,2b,4 mainly because most organic compounds are evaporated
or decomposed into gaseous products before/during high-
temperature carbonization. The current precursors available are
therefore rather limited to only a few sources: (a) The most
popular precursors are low-vapor-pressure natural or synthetic
polymers such as polyacrylonitrile,5 and phenolic resins,6 which
pose certain difficulties because of their limited solubility and
complicated synthesis. (b) Alternatively, synthetic polymers can
be synthesized in situ from polymerizable monomers such as
furfuryl alcohol,2 and acrylonitrile.7 However, a specific
compound, a prepolymerization step prior to carbonization,
and additional acid or metal catalysts are necessary. The slow
procedures and metal impurities are unfavorable for actual
Figure 1. (a) Representation of the PILs/PSs-based strategy. (b)
Structures of the N-containing bases being investigated. (c) Thermal
gravimetric analysis curves of typical PILs/PSs.
Received: August 10, 2013
Published: January 23, 2014
© 2014 American Chemical Society
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All PILs/PSs were synthesized by stoichiometric neutraliza-
tion of the respective N-containing bases with H2SO4 and
subsequent removal of the solvent. The resulting products were
subjected to direct carbonization at 1000 °C. Thermal
gravimetric analysis of the PILs/PSs is shown in Figure 1c.
Clearly, all the parent bases were completely evaporated or
decomposed below 400 °C, as expected. In contrast, the
corresponding PILs/PSs yielded CM residues, with absolute
(oven) and relative (oven/theoretical) yields up to 46.0%
([Dpa][HSO4]) and 95.3% ([Phen][2HSO4], Table S1,
Supporting Information [SI]), respectively. The CM yield
appears to vary significantly with the nature of the base. Bases
containing the thermally stable benzene moieties gave much
higher yields than aliphatic amines and heterocycles, even those
containing cross-linkable moieties (CN, CC). This is not
unexpected, considering that, in polymer precursors, the
benzene-containing polymers/blocks are always original CM
sources upon carbonization, while the aliphatic polymers/blocks
act as sacrificial components and/or templates to generate
porosity via alkyl chain cleavage. From the results of the diverse
PILs/PSs listed in Table S1, we can conclude that most N-
containing bases could be protonated to behave as potential CM
precursors, even those with a very low molecular weight and a
low boiling point, for which low yields but high surface areas were
obtained (e.g., [Dema][HSO4] and [Aan][HSO4], Table S1).
The only exception we found are PILs/PSs with a very high N
content (usually N/C > 1), which were unable to produce CMs.
Alternatively, they were first transformed into polymeric carbon
nitrides around 550 °C and then decomposed above 800 °C
(Figure S3: [taPyim][H2SO4], N/C = 1.5).13
Figure 2. (a) XPS spectra of [3-CNPy][HSO4]-based carbon materials.
(b) XRD patterns, (c) N2 sorption isotherms, and (d) relative
conductivity of the CMs derived from PILs/PSs. 1, [3-CNPy][HSO4];
2, [oBPy][2HSO4]; 3, [CNC2phIm][HSO4]; 4, [Phen][2HSO4]; 5,
[BeIm][HSO4]; 6, [Dpa][HSO4]; 7, [VIm][HSO4]; 8, [Dema]-
[HSO4]. Red dashed circle = estimated conductivity of CM 2 from
the general relationship between N content and conductivity. (Blue
circle) [EMIm][DCA]-based CM and (blue line) graphite. High-
resolution transmission electron microscopy images of CMs derived
from (e) [3-CNPy][HSO4] and (f) [CNC2phIm][HSO4].
N-doped CMs are currently attracting much attention because
conjugation between the N lone-pair and the π-system of the C
lattice can dramatically alter or improve the physicochemical
properties of the CMs such as the conductivity, basicity, oxidative
stability, and catalytic activity.3,14,15 In situ preparation from N-
containing precursors offers the advantage of easier control of the
incorporated N, which is usually more homogeneously
distributed throughout the material compared to those obtained
by the post-treatment method that requires additional complex
and time-consuming steps.14 The unique compositions of the
PILs/PSs herein happen to allow the fabrication of intrinsic N-
doped CMs. As confirmed by elemental analysis, all the PILs/PSs
can yield N-containing CMs, and the N content varies
significantly, depending on the precursors, between ∼2.1 and
11.1 wt% at 1000 °C (Table S1). In particular, the CM derived
from [3-CNPy][HSO4] exhibits a very high N content of 11.1 wt
%. To our knowledge, such a high content has never been
achieved in a one-step synthesis of N-doped CMs from a single
organic source, although 12.0 wt% could be obtained by co-
carbonization of natural N-rich compounds.11a Considering that
[3-CNPy][HSO4] has a N/C molar ratio as low as 0.33, about
half that of [EMIm][DCA] (0.63),9b the high N content in the
resulting CM is surprising and indicates that at least 30% of the N
atoms originally present in the precursor were finally
incorporated within the CM architecture. Although no exact
correlation of the N content between the PILs/PSs and the final
CMs was found, the high N content in the CMs derived from
both [3-CNPy][HSO4] and [oBPy][2HSO4] (9.5 wt% of N in
CM and incorporation of 50% of the N atoms of the precursor)
suggests that not only the nitrile group, as reported,9 but also the
pyridine moiety behave as sources of N and may improve the
final N fixation. The lower N content in the CMs from aliphatic
amine-based precursors further confirmed this point, due to the
facile decomposition of pendant alkylamino groups (e.g., via
Hofmann elimination) that are covalently bound to the CM with
a weak C−N bonding energy.
Further investigation by X-ray photoelectron spectroscopy
(XPS) indicated that N atoms are structurally integrated into the
CM matrix. As shown in Figure 2a, the N environments in the
CM derived from [3-CNPy][HSO4] are dominated by pyridinic
N (398.3 eV) and quaternary N (400.8 eV). The slight impurity
at higher energy (404.0 eV) is probably oxidized N species.4b
Besides the N, C, and O detected by XPS, all the CMs also
contain a small amount of sulfur (Table S2), ranging from 0.4 to
2.3 at.%. The S2p peak can be resolved into four compositions at
binding energies of 163.8, 165.0, 166.4, and 168.6 eV (inset in
Figure 2a). The former two peaks correspond to the S2p3/2 and
S2p1/2 of thiophene, while the weak peaks at higher binding
energy are minor contributions from oxidized sulfur species.16
This simple method indeed provides a facile route to synthesize
N and S bifunctional CM for potential applications.17 The
obtained CMs vary from nonporous to porous, depending on the
precursor structures (Figure 2c). Although all CMs are
microporous (Figure 2c), some of them exhibit high specific
surface area (SBET). For example, the [Aan][HSO4]-based CM
possesses a SBET as high as 1380 m2/g. As shown in Figure 2c, its
N2 sorption shows a typical Type I isotherm with two sharp
uptakes at low (p/p0 < 0.15) and high (p/p0 < 0.9) relative
pressure, respectively, indicating that it is a microporous material
with some small macropores. Taking into account that highly
porous CMs can only be achieved via a nanocasting method or
activation, this facile one-step method is of great significance.
X-ray diffraction (XRD) was performed on all the CMs
(Figure 2b). Judging from the Bragg reflection of the (002) peak,
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the CMs with higher N content, e.g., [3-CNPy][HSO4],
[Phen][2HSO4], and [CNC2phIm][HSO4]-based CMs, display
a more positive shift of the diffraction position, higher intensity,
and narrower line width and thus are more graphitic (Table S2).
In particular, the [CNC2phIm][HSO4]-based CM shows a high-
intensity (002) diffraction peak centered at 26.2°, along with
clearly observable (100), (004), and (110) reflections character-
istic for graphitic structures. The calculated interlayer distance
(d002) of 0.339 nm is very close to that of graphite.18 Such a high
degree of graphitization has never been reported by one-step
carbonization.19 A crude estimate of the lateral size (La) of the
nanographitic domains based on the width of the (100) peak by
the Scherrer equation indicated that they did not exceed 1.6 nm.
Likewise, the mean stack height (Lc) estimated by the (002) peak
was ∼1.8 nm, indicating that single crystallites were composed of
approximately five π-stacked nanographene sheets.20 High-
resolution transmission electron microscopy provided further
evidence that these CMs are made up of bent graphite-like sheets,
which are uniformly present throughout the entire CM structure
(Figure 2e,f). The most remarkable feature is the somewhat “one
direction” arrangement of the graphene layers in the
[CNC2phIm][HSO4]-based CM due to its high degree of
graphitization, while in the [3-CNPy][HSO4]-based CM, they
are much more randomly oriented (arrows in Figure 2). It is
important to note that graphite-like microstructures were
obtained under mild conditions without additional graphitization
treatments such as complex graphite-like precursors,2b,4b
catalytic graphitization,21 CVD,22 or very high temperature.23
The high crystallinity obtained here for the highly N-doped CMs
is unexpected since it is generally accepted that the incorporation
of a large number of N atoms in a C lattice results in the
amorphization of the graphitic network, which may lead to a bent
CM structure with a relatively long interlayer distance.17,25
Indeed, CMs with a high N content ([3-CNPy][HSO4],
[oBPy][2HSO4], [CNC2phIm][HSO4], and [Phen][2HSO4])
appear to show much wider Raman G and D bands than those
with a lower N content, suggesting that N-doping imparts a new
structural irregularity to the hexagonal rings and, consequently,
many defects in the graphene layers (Figure S6 and Table S4), as
expected. However, XPS spectra revealed that the CMs from [3-
CNPy][HSO4], [CNC2phIm][HSO4], and [Phen][2HSO4]
have a relatively higher pyridinic N content but a lower
quaternary N content than others (Figure S6). It is known that
pyridinic N is planar while quaternary N has an “out-of-plane”
bonding configuration.24 Thus, the above three CMs with a high
pyridinic N content, owing to their planar structure, possess
short interlayer distances. In contrast, the [oBPy][2HSO4]-
based CM, with a high N content, shows a weak, broad (002)
signal, which is common in most PILs/PSs-based CMs (Figure
2b) and often related to an amorphous nature, as confirmed by
electron diffraction measurements (Figure S7).19 This can be
explained by its relatively high content of three-dimensional
quaternary N, which results in curvature and local distortion of
the graphene layer and thus a loss of packing density (Figure S6).
Considering both the high degree of graphitization and the
high N content, these CMs are expected to be highly conductive.
The electrical conductivities of all the CMs were measured under
the same conditions (see SI). As shown in Figure 2d, the
conductivity is interestingly found to be roughly proportional to
the N content in CMs (orange region in Figure 2d), due to the
doping effect of electron-rich N that induced a donor state above
the Fermi energy level and increased its electron density.15,25,26
For example, CMs 1−4, having high N content, generally exhibit
higher conductivity than CMs 5−8. Graphitization can further
improves the conductivity, as revealed by [CNC2phIm][HSO4]-
based CM, which has the highest degree of graphitization and
thus a conductivity comparable to graphite. In particular, the [3-
CNPy][HSO4]-based CM, possessing the highest N content and
relatively high degree of graphitization, is even much more
conductive than graphite. This result is very similar to that seen
with [EMIm][DCA]-based CM (Figure 2d).9b In contrast,
[oBPy][2HSO4]-based CM (2), in spite of its high N content
(9.5 wt%), exhibits a much lower conductivity than expected
(Figure 2d), which is certainly ascribed to its low degree of
graphitization as revealed by XRD.
Because carbonization is a complicated process wherein
multiple chemical reactions occur simultaneously,27 the detailed
carbonization process of PILs/PSs may differ substantially
depending on their respective structure, producing distinct
CMs in terms of yield, N content, porosity, and graphitic
structure, as stated above. In an attempt to clarify the
carbonization mechanism, [CNC2phIm][HSO4], which con-
tains various distinct but common moieties, was selected as a
typical precursor, and its products obtained at various temper-
atures were carefully investigated (see SI). It was found that
during carbonization, the thermally unstable moieties such as
alkyl chains, cyano functional groups, anions and heterocyclic
imidazolium rings were first decomposed at a low temperature,
and the in situ-formed fragments further underwent a number of
transformations (e.g., cross-linking polymerization and recombi-
nation) and partially condensed and/or fused with the thermally
stable domains, e.g., benzene, to form N-containing polycyclic
aromatic compounds and finally transform into N-doped CMs
via denitrogenation, desulfuration, and dehydrogenation. It
should be noted that the high proton transfer energy from the
strong sulfuric acid to the nitrogenous base is also essential to
improve the thermal stability of the PILs/PSs under the harsh
carbonization to achieve high CM yield.28 In combination with
the detailed discussion about the CM nature, it was speculated
that PILs/PSs with heterocycles, cyano groups, and benzene
groups (or fused aromatic structures) are liable to yield CMs with
a high N content and a high degree of graphitization (and thus a
high conductivity). In contrast, those with aliphatic moieties
usually led to highly porous CMs, wherein the pore was formed
at the expense of the anions and the thermally unstable moieties
in the cations. The incorporation of structural N may additionally
improve the expected functional N species.
The as-prepared N-doped porous CM from [Aan][HSO4] (A-
NPC) exhibited excellent electrocatalytic activity toward the
oxygen reduction reaction (ORR), as evaluated by cyclic
voltammetry (CV) and rotating ring-disk electrode (RRDE)
measurements. As shown in Figure 3, a quasi-rectangular
voltammogram without any significant redox peak within the
potential range from −1.0 to +0.2 V was observed in the N2-
saturated 0.1 M KOH solution, as a result of the typical
supercapacitance of highly porous CMs.2c,11a,17 In contrast, a
well-defined cathodic peak at −0.220 V vs Ag/AgCl (−0.162 V
for commercial 20 wt% Pt/C) emerges in the CV as the
electrolyte is saturated with O2. Further experiments by RRDE
revealed that the ring current for A-NPC from peroxide oxidation
was very small (Figure 3). The calculated peroxide yield from the
RRDE data was <15%, and the average electron transfer number
(n) is >3.7 for A-NPC over the range from −1.0 to −0.2 V
(Figure S15), confirming that the ORR proceeds through a
mainly four-electron reduction pathway. Moreover, A-NPC
exhibits an onset potential and a half-wave potential at −0.078
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ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
mwatanab@ynu.ac.jp
■
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to Health and Environmental Issues; Elsevier: Amsterdam, Netherlands,
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by Japan Science and Technology
Agency−Advanced Low Carbon Technology Research and
Development Program of Japan. We thank Mr. Masashi Kondoh
for measurement of TEM.
1693
dx.doi.org/10.1021/ja411981c | J. Am. Chem. Soc. 2014, 136, 1690−1693