Phosphorus functionalization for the rapid preparation of highly nanoporous submicron-diameter carbon fibers by electrospinning of lignin solutions

Article abstract of DOI:10.1039/c7ta08788h

This work presents a fast and versatile method to prepare carbon fibers from lignin. It involves the production of submicron-sized phosphorus-functionalized lignin fibers in only one step by electrospinning of lignin/H₃PO₄ solutions. The phosphorus functionalities enable shortening of the conventional stabilization process from more than 90 h to only 2 h thus avoiding fiber fusion or even stabilizing the lignin fibers in an inert atmosphere. The incorporation of H₃PO₄ into the initial lignin solution produces more oxidized spun lignin fibers, due to the reaction of phosphoric acid with the dissolved lignin, generating phosphate (and/or polyphosphate) esters throughout the structure of lignin fibers. These phosphate groups seem to be responsible for the production of cross-linking reactions during the stabilization step that are, in this case, very active and effective in increasing the glass transition temperature of the lignin fibers, reducing the time needed for the stabilization step and improving this process. Moreover, they promote the chemical activation of lignin fibers and greatly increase their oxidation resistance, avoiding their complete combustion during carbonization under a low concentration of O₂ at temperatures as high as 900 °C. The resulting carbon fibers gather different interesting properties, such as sub-micron diameters (≤1 μm), large surface area (≈2000 m² g^-1), relatively high performance in relation to their mechanical properties for functional applications and a rich variety of uniformly distributed O and P surface functionalities, which make them very attractive for heterogeneous catalysis, adsorption and electrochemical applications.

Full text of DOI:10.1039/c7ta08788h

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
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Phosphorus functionalization for the rapid preparation of highly
nanoporous submicron-diameter carbon fibers by electrospinning
of lignin solutions
Received 00th January 20xx,
Accepted 00th January 20xx
F. J. García-Mateosa, R. Berenguera, M. J. Valero-Romeroa, J. Rodríguez-Mirasol*,a, T. Corderoa.
DOI: 10.1039/x0xx00000x
This work presents a fast and versatile method to prepare carbon fibers from lignin. It involves the production of
submicron-sized phosphorus-functionalized lignin fibers in only one step by electrospinning of lignin/H3PO4 solutions. The
phosphorus functionalities enable to shorten the conventional stabilization process to avoid fiber fusion from more than
90 h to only 2 h or even to stabilize the lignin fibers in inert atmosphere. The incorporation of H3PO4 to the initial lignin
solution produces more oxidized spun lignin fibers, due to the reaction of phosphoric acid with the dissolved lignin,
generating phosphate (and/or polyphosphate) esters throughout the structure of lignin fibers. These phosphate groups
seem to be the responsible for the production of cross-linking reactions during the stabilization step that are, in this case,
very active and effective in increasing the glass transition temperature of the lignin fibers, reducing the time needed for
the stabilization step and improving this process. Moreover, they promote the chemical activation of lignin fibers and
greatly increase their oxidation resistance, avoiding their complete combustion during carbonization under low
concentration of O2 at temperatures as high as 900 °C. The resulting carbon fibers gather different interesting properties,
such as sub-micron diameters (≤ 1 μm), large surface area (≈2000 m2 g-1), relatively high performance in relation to their
www.rsc.org/
mechanical properties for functional applications and
a rich variety of uniformly-distributed O and P surface
functionalities, which make them very attractive for heterogeneous catalysis, adsorption and electrochemical applications.
co-products from lignocellulosic biomass-derived industries,
such as biorefineries and pulp and paper mills, could suppose a
significant opportunity to minimize their related costs and
Introduction
Porous carbon fibers are relevant materials for many
functional applications of industrial and technological
interest.1-4 Especially, they receive great attention in catalysis,
adsorption, gas storage and/or separation processes,
electrochemical devices for energy storage and conversion,
etc.1-4 In spite of such potential applicability, the real and
feasible wider utilization of porous CFs demands a reduction of
their production costs and/or the development of unique
properties or crucial advantages to improve their present
competitiveness and performance.5,6
environmental impacts.7,8
On the other hand, the preparation of carbon fibers by means
of a simple and versatile technique, such as electrospinning,
entails an advantageous and promising approach.14,15 It solves
the most important problems associated to conventional melt-
extrusion, since it allows the preparation of continuous fibers
with considerably smaller diameters (micro- and submicro-
scale) at room temperature, directly in a single step. In this
sense, we previously presented the preparation of carbon
fibers by electrospinning of Alcell® lignin without the aid of
blending polymeric additives and/or chemical (purification,
modification) treatments, thus, considerably reducing the
manufacture complexity, time and costs.9,10
With respect to cost, there is an increased interest and
remarkable scientific efforts are currently being focused, even
for functional applications, on the replacement of more
expensive petroleum-based precursors or different synthetic
polymers, and their associated processing, by a cheaper and
sustainable manufacture from renewable and low-cost
polymeric precursors.5,6 The development of carbon fibers
with novel and useful properties from lignin has received
special attention,5-13 given that it is the second most abundant
polymer in nature. In addition, the development of high-value
After decade-long investigation, however, lignin-based carbon
fibers exhibiting competitive performance or unique
properties for specific applications are challenging. This usually
makes necessary the utilization of additional activation and/or
functionalization post-treatments that, apart from slowing
down, making complex and raising the costs of the process,
may compromise the fibrous structure.11-13 On the other hand,
the production of fibrous carbon materials from lignin is
greatly limited by the need of a long oxidative stabilization
treatment (more than 3 days), given that lignin shows a glass
transition temperature (Tg) far below the temperature
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required for carbonization/activation.9,10,16 In this sense, to
develop a next-generation of lignin-based carbon fibers, with
novel and advanced properties as well as faster production,
the design of strategies to control both structural and chemical
properties of lignin polymer as well as its thermochemical
reactivity will be crucial.
Experimental section
Preparation of electrospun phosphorus-functionalized carbon
fibers
Pure lignin fibers were prepared by electrospinning of Alcell®
lignin-ethanol solutions.9,10 For the H3PO4/lignin fibers,
spinnable H3PO4(85 wt%)/lignin/ethanol solutions (0.3/1/1
weight ratios) were used.22 Solutions were heated at 60
°C and
shaken at 200 rpm during 12 h, obtaining a viscosity (350 –
450 cPs) suitable for electrospinning. The lignin solution flow
rate was 3 cm3 h-1 for the inner capillary tip, whereas pure
ethanol was fed at 0.3 cm3 h-1 by the outermost one. The tip-
to-collector distance was 30 cm and the electrical potential
difference was 24 kV (the collector was at -12 kV and the tips
at +12 kV). Thermo-stabilization and carbonization were
Phosphoric acid has been widely used as chemical agent to
prepare activated carbon materials from different precursors,
including lignin.17-19 Thus, the effect of phosphoric acid on the
textural properties of the final activated carbon has been
widely covered in literature. In particular, the effects of the
impregnation ratio and those of the activation temperature
and time have been reported in several works.18,20 Fierro et al.
found that the impregnation of Kraft lignin with H3PO4 at
H3PO4/Lignin weight ratios between 0.7 and 1.7 produces
microporous activated carbons and the maximum
development of porosity was found at an activation
carried
out
in
a
horizontal
tubular
furnace
(150 cm3 gas (STM) min-1) under different conditions described
in the text and figures. Particularly, the effect of heating
variables during stabilization, and that of the atmosphere used
in the stabilization and carbonization steps have been studied.
Finally, some of the resulting carbonized fibers were analyzed
by different techniques.
temperature of 550-600 °
C. On the other hand, Rosas et al.20
studied the effect of H3PO4 on the preparation of carbon fibers
from hemp and reported a remarkable development of the
mesoporosity without significant damage of the fibers at
impregnation ratio higher than 1. The reaction of phosphoric
acid with lignocellulosic materials has been also studied.20,21 In
the case of lignin, phosphoric acid produces de dehydration of
the lignin structure and promotes cross-linking reactions to
form phosphate and polyphosphate ester-like bonds. In this
sense, the incorporation of H3PO4 to the lignin solution fibers
precursor would favor the cross-linking reactions necessary for
stabilization of the fibers, avoiding their fusion during the
carbonization process and, at the same time, it would promote
the chemical activation upon heating, enhancing the final
carbon fibers properties.
For comparison purposes, equivalent powdery carbon
materials were also prepared from lignin powder without the
electrospinning step. For such
a
purpose, H3PO4
(85 wt %)/lignin/ethanol solutions (n/1/1 weight ratios, with
n = 0; 0.3; and also 0.8), identical to those used for
electrospinning, were dried for 24 h at 60 °C. Next, the
stabilization and carbonization conditions were similar to
those utilized for the fibers. The P-containing carbonized fibers
and powders were washed with distilled water at 60
remove weakly-anchored P species, and dried overnight at
80 C prior to characterization.
°C to
°
In this paper, it is presented a new method to prepare low-cost
lignin-based carbon fibers in a remarkably short time. The
method involves the electrospinning of lignin/H3PO4 solutions
into submicro-sized phosphorus-functionalized lignin fibers
that can be stabilized at much faster heating rates (up to 60
times), in less than 2 h, and under a wider variety of
atmospheres. Furthermore, the method entails the
functionalized fibers to undergo a high development of
porosity directly during their carbonization at high
temperatures by different activation mechanisms acting
together. Interestingly, the stabilized phosphorus-lignin fibers,
apart from being chemical activated during the carbonization
The as-spun, stabilized and carbonized fibers are referred to as
AF, SF and CF for the pure lignin fibers, respectively. In the
case of the preparation of carbon fibers by chemical activation
with H3PO4 (impregnation ratio of 0.3), the fibers are referred
to as PAF, PSF and PCF. In the case of stabilized fibers, the
heating rate (0.08; 1 or 3 °
C min-1) or the particular use of N2
atmosphere is also reflected in the nomenclature. For
carbonized fibers, the stabilization conditions (without P:
C min-1 up to 200
C + 1 h at 200
0.08 °
up to 200
°
C + 60 h at 200
°C; with P: 1 °
C min-1
C) are not specified and only the
°
°
particular use of N2 atmosphere for stabilization (N2-PCF) or
the oxidizing one for carbonization (PCF-3%O2) are considered
in the nomenclature. The carbonized lignin powders are
designated as AC (in reference to conventional activated
carbons (ACs)), followed by a number (n) indicating the
aforementioned H3PO4 weight ratio (n = 0; 03 for 0.3 or 08 for
0.8) in the H3PO4/lignin/ethanol (n/1/1) solutions.
step, can be activated at temperatures as high as 900 °C under
a low concentration of O2, obtaining in this case an extra
activation due to the partial gasification of the produced
carbonaceous fibers. As a result, the obtained carbon fibers
gather
a unique combination of textural and chemical
properties, i.e. a well-developed microporosity and high
surface area (around 2000 m2 g-1) in very fine fibers (mainly
400 nm - 1 μm diameter), together with a rich variety of P and
Characterization of activated carbon fibers
SEM and TEM images were obtained by a JEOL JSM-6490LV
microscope (at 25 kV voltage) and a Philips CM200 microscope
(at an accelerating voltage of 200 kV), respectively. Raman
spectra were recorded with a RENISHAW micro-Raman system
using an Ar+ laser at 514 nm as the excitation source with a
O
surface functionalities. In addition, the phosphorus-
functionalization enhances the oxidation resistance and acidity
of these carbon fibers. All these properties have been found
very attractive for their potential application in heterogeneous
catalysis, adsorption and electrochemistry.
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spectral resolution of 2 cm-1. The textural properties were carbon fibers were manually compressed in between two
characterized by physical adsorption of gases (N2 at -196
and CO2 at 0 C) in a Micromeritics ASAP2020. Samples were
previously outgassed for 8 h at 150 C under vacuum. The
°
C
stainless steel meshes (current collectors).
°
°
Result and discussion
mechanical properties were measured as non-aligned multi-
fiber carbon meshes, in a DEBEN Microstest (2KN Tensile).
Temperature-programmed desorption (TPD) experiments were
performed in a tubular vertical furnace and the amounts of CO
and CO2 evolving groups were monitored by NDIR analyzers
(Siemens; ULTRAMAT 22 model). X-ray photoelectron
spectroscopy (XPS) data were registered by a 5700C model
Physical Electronics apparatus with Mg Kα radiation
(1253.6 eV). For the analysis of XPS peaks, the maximum signal
was set at 284.5 eV. The fitting of the XPS peaks was done by
least squares using Gaussian–Lorentzian peak shapes. For
thermogravimetric analysis (TG), a CI Electronics MK2 balance
and samples between 10 and 15 mg were used.
Preparation of electrospun phosphorus-functionalized carbon
fibers
Phosphoric acid-containing lignin fibers were prepared by
electrospinning of H3PO4/lignin/ethanol (0.3/1/1 wt ratio)
solutions in ethanol. The as-spun H3PO4/lignin fibers show
diameters ranging from 400 nm to 3 μm (PAF in Figure 1),
mostly ≤ 1 μm. Since the presence of H3PO4 increases the
viscosity and electrical conductivity of the lignin/ethanol
solutions, promoting whipping instability,14,15 more curly fibers
were observed when compared with H3PO4-free ones (AF and
PAF in Figure 1).
The electrospun fibers were subjected to a thermo-oxidative
stabilization step prior to carbonization in order to increase
the glass transition temperature of lignin and thus prevent
fibers from partial or total fusion. The heat-treatment was
carried out under air or inert (N2) atmosphere
(150 cm3(STP) min-1) and different heating rates up to a
The catalytic decomposition of 2-propanol was studied in a
fixed bed reactor as previously reported,23 with 70 mg of
carbon fibers, a 2-propanol partial pressure of 0.018 atm in
inert atmosphere and a space time of 0.055 g s µmol-1. The
concentrations of 2-propanol and products were analyzed by
on line gas chromatography (490 micro-GC equipped with
PPQ, 5A molsieve and Wax columns, Agilent). γ-Al2O3 (Puralox
150/200 from Sasol) was used as a reference acid catalyst. The
electrochemical characterization was carried out in a standard
three-electrode cell in different electrolytes by using a Pt wire
as counter electrode and a Ag/AgCl/Cl‒(sat.) electrode as the
reference one (Electrochemical Analyzer 600D Series, CH
Instrument Inc.). To be used as binderless electrodes, the
temperature of 200 °C, and this final temperature was kept for
different times (1 or 60 hours). As shown in Figure 1, in the
absence of H3PO4 the stabilization must be necessary slow
C min-1 to 200
(0.08 ° °C + 60 h at 200 °C) (SF-0.08-200-60) to
maintain a high glass transition temperature, preventing fibers
from fusion.16
Figure 1. Effect of the heating rate (HR) on the morphology (SEM images) of the air-stabilized lignin fibers with (PSF-HR) and
without (SF-HR) H3PO4. This Figure also shows the images of the corresponding as-spun fibers (PAF and AF, respectively)
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Hence, faster heating rates such as 1
°
C min-1 produce Figure 2 shows the O1s, C1s and P2p spectra of as spun and
complete fusion of pure lignin fibers (SF-1-200-1 in Figure 1). stabilized lignin fibers without and with phosphoric acid. In the case
By contrast, and apart from a conventional long stabilizations of as spun lignin fibers, a band centered at about 532.6 eV can be
process (PSF-0.08-200-60 in Figure 1), the presence of H3PO4 observed in the O1s spectrum, corresponding to oxygen as in C-OH
allows using heating rates as high as 1-3 °
C min-1 without and C-O-C bonds (Figures 2b and d). However, the band is more
producing partial fusion or swelling of the fibers (PSF-1-200-1 intense and narrower for the phosphoric acid-containing lignin
and PSF-3-200-1 in Figure 1). Thus, electrospun H3PO4/lignin fibers (PAF in Figure 2d), which can be ascribed to the presence of
fibers can be stabilized in only 1 h and 45 min, whereas the oxygen in C-O-P bonds, as in phosphate and polyphosphates surface
pure lignin fibers require more than 90 h, what entails a groups.24 Stabilization in air of lignin fibers caused a significant
remarkable time reduction (50-60 times) of the modification of the profiles for the O1s and C1s spectra
thermostabilization treatment.
(SF-0.08-200-60 in Figure 2a and b), widening the O1s band at lower
Furthermore, the stabilized fibers showed similar diameter to binding energies, at about 531 eV (Figure 2b), as a consequence of
that of the as-spun ones and the incorporation of H3PO4 formation of oxygen as in C=O bond.
increased the stabilization yields from 76% for SF-0.08-200-60
Table 1. Weight surface concentration by XPS of as spun and
stabilized lignin fibers
Weight surface concentration by
to 83 wt % for PSF-0.08-200-60.
Table 1 shows the weight surface concentration obtained from
XPS analysis for lignin-based fibers before and after
stabilization in inert or oxidizing atmospheres, where an
increase in the oxygen content after stabilization in air is
observed for both prepared fibers. The increase in the oxygen
content after stabilization step is expected to be low due to
the high oxygen content of the raw lignin. Nevertheless, this
increase in the oxygen content is notably lower in the case of
the lignin fibers prepared with phosphoric acid, given that the
oxygen content was already increased during the preparation
of these P-containing fibers (PAF).
XPS (%)
O
Samples
C
P
--
AF
73.4
68.0
83.6
64.5
64.1
63.4
26.6
32.0
16.4
28.2
29.5
28.3
SF-0.08-200-60
SF-0.08-200-60-N2
PAF
--
--
7.3
6.4
8.3
PSF-1-200-1
PSF-1-200-1-N2
Figure 2. XPS spectra of C1s, O1s and P2p core-levels of as-spun and stabilized fibers under oxidizing and inert atmosphere. (a, b)
pure lignin fibers, (c – e) phosphorus lignin fibers.
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In the case of phosphorus-containing lignin fibers, the oxidative appreciated after stabilization of the phosphorus-containing lignin
stabilization process did not produce such a notable change on fibers (PSF-1-200-1 in Figure 2e), which evidences slight
these profiles, leading only to a slight reduction of the band modifications of the state of P in the lignin structure after the
observed at 532.6 eV and to the appearance of a small tail at stabilization treatment, producing some phosphorus in a more
530.9 eV (Figure 2d), which can be associated to oxygen as in C=O reduced state as in C-PO bonds, as a consequence of dehydration
bond or even as in P=O one. For the case of stabilization of lignin and cross-linking reactions.
fibers, a noticeable band (or a shoulder of the C-C band located at These results are in agreement with other studies. The increase in
284.5 eV) can be observed in the C1s spectrum from approximately the oxygen content during oxidative stabilization of lignin fibers is
287.5 to 290.8 eV (Figure 2a), which indicates the presence of C=O related to the incorporation of mainly carbonyl and carboxylic
and/or O-C=O bonds. However, this small band at 288.2 eV is not surface structures at low temperature, which can be incorporated
clearly observed after stabilization of P-containing lignin fibers into anhydride and ester groups,9,10,16 generating cross-links within
(Figure 2c), suggesting that the small tail of the O1s spectrum for the lignin fibers structure. In the case of phosphorus containing
this sample, at about 530.9 eV, may correspond to P=O bond lignin fibers, the increase of oxygen content after stabilization is
instead of C=O one. In this sense, Figure 2e presents the P2p lower than that observed for the pure lignin fibers (Table 1).
spectra for PAF and PSF-1-200-1 that show a band with a main peak Figure 3 and 4 show, respectively, a scheme of the Alcell lignin
at a binding energy around 134 eV, characteristic of pentavalent structure adapted from the lignin structure proposes by Adler29 and
tetracoordinated phosphorus, as in phosphates and/or the scheme of the structure that we propose for this lignin after
polyphosphates.20,21,25-28 A slightly shift of this band to lower reaction with H3PO4 (with approximately the same chemical
binding energy and a small reduction of the band area can be composition of PAF).
Figure 3. Alcell lignin structure adapted from the lignin structure proposed by Adler29
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Figure 4. Alcell lignin structure, proposed in this work, after reaction with H3PO4 (C = 71.6 %at, O = 25.6 %at, P = 2.8 %at).
The incorporation of H3PO4 to the initial lignin solution produces With the aim to study if phosphate and polyphosphate esters,
more oxidized spun lignin fibers (PAF), but of the whole lignin fiber generated during reaction of lignin with H3PO4, are enough to
matrix in this case, due to the reaction of phosphoric acid with the prevent the fusion of phosphorus-lignin fibers the stabilization step
dissolved lignin, generating phosphate (and/or polyphosphate) has been carried out under inert atmosphere (N2). Under this inert
esters throughout the structure of lignin fibers.20,21 These atmosphere, a complete fusion of the pure lignin fibers is produced,
phosphate groups have been found to be very active for surface even at low heating rates (Figure 5a), indicating that the
carbon oxidation, even at room temperature24,26 and thus seems to stabilization of pure lignin fibers necessarily requires oxidation of
be the responsible for the production of cross-linking reactions that the carbonaceous fiber matrix in order to increase the glass
are, in this case, very active and effective in increasing the glass transition temperature and prevent fiber from fusion.16 A clear
transition temperature of the lignin fibers, reducing the time decrease in the oxygen content is observed when the lignin fibers
needed for the stabilization step and improving this process. On the are stabilized under inert atmosphere (Table 1). The XPS analysis of
contrary, the oxidative stabilization of the lignin fibers in the the lignin fibers stabilized in N2 (SF-0.08-200-60-N2) reflects a
absence of phosphoric acid takes place on the surface of the fibers decrease the C–O bond contribution (a clear shortening in the
and, in this case, oxygen has to diffuse to the inner part, the core, of intensity of the band at 286.0 eV in C1s spectrum, Figure 2a) and a
the lignin fiber matrix to form cross-linkage that favour higher glass clear reduction of the whole O1s band spectrum (Figure 2b). These
transition temperatures to prevent fibers from fusion during the results confirm that the heat treatment of pure lignin fibers in N2
carbonization/activation process, which is only possible with much atmosphere only produces the release of oxygen functional groups
slower heating rates.
(mainly by decomposition of certain oxygenated compounds),
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900 °C under inert conditions with a 37 % yield (SF-N2 in Figure 6a)
(27 % total yield, considering stabilization + carbonization).9,10 On
the other hand, the presence of P species in the stabilized lignin
fibers increases the carbonization yield up to 48 % (40 % total yield)
(PSF-N2), by favoring the restriction of tars formation during the
carbonization process and dehydration reactions produced at high
carbonization temperatures, increasing the carbon content.20,30
Regarding the oxidation or combustion behavior, while pure lignin
fibers suffer from total combustion at 700 °C in an atmosphere of
3% of O2 (SF-3%O2), a 18 % of the initial lignin weight remains as
carbon fibers after heating up to 900 °C when phosphorus is
present (15 % total yield, PSF-3%O2). It has been found that during
the relatively high-temperature (700-900 °C) carbonization process
of phosphoric acid-treated lignocellulosic materials different
phosphorus groups (C-O-PO, C-PO type) remain thermally stable
bonded to the surface of the final carbon matrix, being these P-
surface groups responsible of the high oxidation resistance and acid
character of the carbon surface.20,24,30 Thus, phosphorus carbon
fibers can be produced at 900 °C even in an oxidative atmosphere
(O2 3%v), due to the presence of phosphorus surfaces groups that
are incorporated to the carbon fiber matrix during the
carbonization step.
Figure 5. SEM images of stabilized lignin fibers, SF and PSF,
under inert atmosphere
without generating oxidation or cross-linking reactions, avoiding
any possibility of stabilization and producing the fusion of the fibers
during the subsequent carbonization step, for the production of
carbon fibres (Figure 5a). XPS characterization of phosphorus
containing lignin derived fibers shows almost no differences
(Table 1, Figure 2c-e), having the as-spun and the air and inert
stabilized phosphorus-lignin fibers practically identical surface
composition. Therefore, the presence of thermally stable
phosphate and/or polyphosphate esters allows also the stabilization
of electrospun lignin fibers even under inert atmosphere (N2) at
relatively high heating rates (1 or 3 °C min-1, Figure 5b). Thus, these
phosphate and polyphosphate species generate cross-links with the
lignin structure without the need to incorporate oxygen from the
gas phase.
The stabilized materials were heat-treated at 10 °C min-1 up to
900 °C under inert (N2) or oxidizing (3 vol % O2 in N2) atmospheres.
The influence of phosphorus-functionalization and the atmosphere
on the weight loss during carbonization was studied by
thermogravimetric analysis (Figure 6a). In agreement with our
previous works, stabilized pure lignin fibers can be carbonized at
As a consequence of such versatile methodology, due to the
presence of phosphorus groups in the lignin structure, the thermal
processing of as-spun fibers into lignin-based carbon fibres can be
carried out through not only the conventional 2 steps-route,
stabilization (air) + carbonization (N2) (route a, Scheme 1), but also
under 2 new combinations (routes b and c, Scheme 1), as well as
directly in a single step under inert (route d, Scheme 1) or oxidizing
atmosphere (route e, Scheme 1). This allows to produce carbon
fibers with a wider variety of properties and, in the case of a single
step (route e, Scheme 1), with a notably process time reduction, in
only 2 h 40 min (total thermal processing).
Figure 6. (a) Weight-loss of air-thermostabilized lignin fibers (SF and PSF correspond to SF-0.08 and PSF-1 samples, respectively)
during heating up to 900 ºC under inert (N2) or oxidizing (3 vol % O2) conditions. (b,c,d) Different TEM images of PCF sample; and
(e) N2 adsorption-desorption isotherms (-196 ºC) of different carbon fibers.
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Scheme 1. Scheme of the phosphorus carbon fibers preparation by electrospinning of lignin solution, representing the two- and
single-step thermal routes in which the stabilization and carbonization of the fibers could be carried out under different or
similar atmosphere conditions, respectively.
Textural properties.
oxygen incorporation during air stabilization seems to be of
The phosphorus carbon fibres display quite similar diameters paramount importance for the chemical activation of the
to those of the as-spun and stabilized ones, mainly ranging fibers. It has been reported that the phosphate- and/or
between 400 nm (Figure 6b) to 1 μm (Figure 6c), and an polyphosphate-carbon structures may favour oxygen spill-
apparently smooth surface with no microscopic defects, even over24,34,35 by adsorption of molecular oxygen on the P surface
at ending zones (Figure 6d). Textural characterization points complexes and migration of activated oxygen species from
out that the carbonization of pure lignin fibers (CF) produces a these P groups to the carbonaceous matrix. After the oxidative
well-defined microporous structure, as deduced from the type- stabilization step, this activated oxygen in the fiber
I shape of the N2 adsorption isotherm (Figure 6e), and a high carbonaceous matrix may partially gasify the structure of the
specific surface area, of around 850 m2·g-1 (Table 2). In fibers during the carbonization process, developing a larger
particular, the higher CO2-mircopore volume, if we compared porosity and surface area in the final carbon fiber than if no
it with that derived from the adsorption of nitrogen (VDR(CO2) oxygen is used during the stabilization process. In the case of
> Vt (N2)) indicates the presence of narrow micropores phosphorus-containing lignin fibers stabilized in inert
(diameters below 0.7 nm).31 This large development of atmosphere and under the experimental conditions used in
microporosity has already been ascribed to the high oxygen this study (relatively low H3PO4/lignin mass ratio), the oxygen
content of lignin,9,10 which may be a key factor during supplied by the phosphate (and/or polyphosphate) ester
devolatilization in the carbonization process, namely, the groups to the lignin fibers matrix appears to acct just
oxygen-driven inherent activation of lignin fibers. The producing cross-linking reactions during the stabilization
carbonization of phosphorus electro-spun fibers (PCF) results process to avoid fusion of the fibers and it does not seems to
in carbon fibers with a substantially higher surface area (ca. be available during the further carbonization step to activate
1350 m2·g-1) and microporosity, with a higher proportion of the carbonaceous fiber matrix, with no further added
wider micropores (Vt(N2) > VDR(CO2)) (see Table 2). These development of porosity of the final carbon fibers.
effects on the porous texture may be attributed to the H3PO4- On the other hand, the carbonization of stabilized P-
chemical activation process upon heating,17,20,30,32,33 although functionalized fibers under an O2-containing atmosphere up to
the atmosphere of the stabilization process plays a key factor temperatures as high as 900 °C (Figure 6a) further induces an
in the development of porosity in the final carbon fibers. additional porosity development (PCF-3%O2 in Figure 6e and
Actually, for phosphorus-containing carbon fibers derived from Table 2). This O2 activation at high temperatures produces
phosphorus-lignin fibers that have been stabilized under N2 phosphorus carbon fibers with apparent surface areas of ca.
atmosphere (N2-PCF; see Table 2) no effect on the activation of 2000 m2·g-1 (3-fold increase compared to pure lignin derived
the lignin fibers has been detected (similar porous texture carbon fibers), a wider microporosity and the presence of
observed for CF and N2-PCF; Table 2), which suggests that mesopores (around a 10 % of total pore volume; see Table 2).
8 | J. Mater. Chem. A, 2017, 00, 1-3
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Table 2. Porous texture parameters (calculated as reported elsewhere10) and mass surface composition for the studied lignin-
based carbon fibers. For comparison purposes, the table also includes the data of activated carbons (AC) powders prepared from
lignin:H3PO4:ethanol precursor solutions, without the electrospinning step.
N2 adsorption
CO2 adsorption
XPS mass surface concentration
[d]
[e]
[f]
[a]
ABET
m2 g-1
At[b]
Vt[c]
Vmeso
ADR
VDR
C
%wt
O
%wt
P
%wt
Sample
cm3 g-1
m2 g-1
cm3 g-1
m2 g-1
0.4
6
cm3 g-1
AC0
86
0.033
0.282
0.507
0.329
0.580
0.342
0.849
0.005
0.009
0.002
0.006
0.009
0.005
0.107
514
825
0.206
0.331
0.338
0.401
0.412
0.271
0.409
92.1
87.4
87.8
96.3
90.5
90.3
81.2
7.9
10.2
9.4
0.0
2.4
2.8
0.0
1.8
3.4
2.8
AC03
AC08
CF
719
1177
850
18
7
844
1001
1027
675
3.7
PCF
1348
869
25
6
7.7
N2-PCF
PCF-3%O2
6.2
2000
112
1021
16.0
[a] ABET is the apparent surface area. [b] At is the external surface area. [c] Vt is the micropore volume. [d] Vmeso is the
mesopore volume. [e] ADR is the narrow micropore surface area. [f] VDR is the narrow micropore volume.
It has been reported that stable C-O-P type surface groups of the
decompose to C’-P type surface groups at temperatures higher narrowing of D band could point out a lower contribution of
than 700-750 C, with formation of gaseous CO (C’ represents structural disorder in phosphorus carbon fibers. On the
P functionalization. Nevertheless, the noticeable
°
a new surface carbon site), and that in the presence of contrary, the shift of G band towards larger wavenumbers, as
molecular oxygen this C’-P surface groups are (re)oxidized to well as the increase in the G band width and in the ID/IG ratio
C’-O-P, where the P-surface groups act as inhibitors for the observed for the highly porous phosphorus carbon fibers
surface carbon gasification at temperatures lower than 700- (PCF-3%O2) are all Raman features characteristic of a less
750
and as
temperatures higher than 700-750
°
C (enhancing the oxidation resistance of the carbon fiber) ordered structure.38,39 This is in accordance with their much
selective gasification agent or catalyst at more developed microporosity (see Table 2).
C (production of surface
a
°
C’’-P and gaseous C’O and again (re)oxidation to C’’’-O-P).24
The robustness of the carbon fibers is an important property
even for functional applications. The degree of structural
ordering in the phosphorus carbon fibers was followed by
Raman spectroscopy. In spite of the higher porosity of these
phosphorus carbon fibres, the quite similar spectroscopic
features obtained for PCF and PCF-3%O2 samples compared to
that of CF (prepared from pure lignin) (Figure 7 and Table 3)
suggests that the P functionalities may induced or promote
certain structural ordering.36,37 The Raman spectra of the
carbon fibres (Figure 7) show two main broad and overlapping
bands, at about 1350 and 1600 cm-1, the former being
associated to the E2g vibration mode of the ideal graphitic
lattice (G band), and the later characteristic for disordered
carbon (D band). 9,10,38,39
These spectra were deconvoluted as reported so far,10,39 and
the spectral parameters are collected in Table 3. The Raman
shift (
and G bands and the intensity ratio (ID/IG) of the fibers
carbonized at 900 C under inert atmosphere are independent
ν) of G band, the full width at half maximum (Δν) of D
Figure 7. Raman spectra of various lignin-based carbon fibers
°
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Table
3.
Raman parameters determined from the functionalities on phosphorus carbon fibers was further
deconvolution of first-order Raman spectra of different lignin- investigated from the XPS spectra and TPD profiles (Figure 8).
based carbon fibers.
The C1s region of XPS spectra was deconvoluted into 4
contributions (see an example in Figure 8a). Apart from a main
contribution due to the aromatic -C-C- bonds formed upon
G (cm-1)
Δ
ν
G (cm-1)
67.3
Δν
G (cm-1)
Sample
ID/IG
0.87
0.88
0.89
0.94
ν
carbonization, the phosphorus carbon fibers present
a
CF
1599
1600
1600
1607
182.8
153.9
153.9
144.3
remarkable contribution of single-bonded -C-O- and/or -C-P-
species (285.8 eV), as well as some -C=O and –COOH and/or –
COOR species (at 287.9 and 289 eV, respectively).44 On the
other hand, the XPS-P2p spectrum of phosphorus carbon fibers
(Figure 8b) shows a broad band centered at ca. 133 eV,
characteristic of strongly-attached P-species in activated
carbons prepared at high temperatures.17,20,30,32,33 This
spectrum can be deconvoluted into 3 doublets corresponding
PCF
67.3
N2-PCF
PCF-3%O2
57.7
71.6
Mechanical properties are also of great importance even for
functional application carbon fibers. Table 4 shows different values
of Young’s Modulus and tensile strength of non-porous and
activated carbon fibers compared with the carbon fibers prepared
in this work. In this sense Dallmeyer et al.40 found that the inter-
fiber bonding improve the mechanical properties of the carbon
fibers, reporting tensile strength and Young’s modulus values of
75 MPa and 4 GPa, respectively, for bonding fibers carbonized at
1000 °C with a relatively poor development of porosity (BET surface
area of 400 m2·g-1). On the other hand, Uraki et al.41 studied the
effect of the surface area on the tensile strength of carbon fibers
obtained from softwood acetic acid lignin. They found a decrease in
tensile strength value from 150 to 100 MPa when the carbonized
fibers were activated under steam flow for different times,
obtaining BET surfaces areas from 370 to 1930 m2·g-1. Tensile
strength values of 250-300 MPa and Young’s modulus values of 2-
4 GPa have been obtained for the carbon fibers prepared in this
work. In spite of these values are lower than those reported for
non-porous carbon fibers obtained from kraft lignin for structural
applications,42,43 the small carbon fibers diameters obtained by
electrospinning of lignin solution make the carbon materials thus
prepared more mechanically resistant than other carbon fibers
prepared from lignin or other lignocellulosic precursor. In this
sense, it is noteworthy to highlight that these mechanical
properties are not sufficient for structural applications. However,
they are unusually high in such porous materials of small diameter
and enough suitable for functional application such as in adsorption
and heterogeneous catalysis and as electrodes, etc.
to
3
different surface groups of
P
functionalities:
polyphosphates and/or phosphates (referred to as –C–O–P–
species: (CO)3PO, (CO)2PO2 and (CO)PO3)) at 134.0 eV;
–C–P–O– species (C–PO3 and/or C2–PO2 groups) at 133.2 eV;
and the major contribution, –C3PO groups, at 132.0 eV.24
From the TPD profiles of Figures 8c and 8d it can be clearly
discerned that the amount of CO-evolving groups (Figure 8c) in
highly activated phosphorus carbon fibers is considerably
much higher than that of CO2-evolving ones (Figure 8d). In
particular, the CO profiles of PCF fibers are characterized by a
sharp rise from 500
decomposition of phenol, ether and/or carbonyl groups. The
TPD CO evolution from 700 and up to 950 or 1000 C,
presenting a maximum at 860 C, for different carbons
°C to 700 °C, which can be attributed to
°
°
prepared by chemical activation of lignocellulosic materials
with phosphoric acid has been attributed to the
decomposition of C-O-P types bond of C-O-PO3, generating
C’-PO3 surface groups, where C’ refers a new surface carbon
atom.24,32
Table 4. Young’s Modulus and tensile strength values for carbon
fibers prepared in this work and for other carbon fibers reported
in the literature.
Sample
ABET
(m2 g-1)
Tensile strength
(MPa)
Young’s
Modulus
(GPa)
Ref.
This
work
This
855
303
4
CF
PCF
1345
411
435
456
370
1390
--*
248
74
2
4
work
Chemical properties.
BF-1000
BF-800
BF-600
SAL-ACF
SAL-ACF-80
HWKL
Alcell
40
40
40
41
41
42
42
43
As shown in Table 2, the incorporation of P to the lignin-based
carbon fibers increases the surface oxygen content by XPS
from 4 wt % for CF to 8 wt % for PCF and from 6.2 wt% for
N2-PCF to 16 wt % for PCF-3%O2 when carbonization is carried
out under inert or oxidizing atmosphere, respectively. On the
other hand, the amount of surface phosphorus on the P-
containing carbon fibers was found to vary from 1.8 to 3.5 wt
% P by XPS (Table 2). The mass surface concentration of P
determined by XPS of the PCF sample (1.8 wt% P) is quite
similar to that obtained by Inductive Coupled Plasma (ICP)
(1.5 wt% P), indicating that P is well distributed along the
surface (internal and external) of the final carbon fiber. The
nature and relative contribution of the different O and P
76
2
58
2
150
100
422
388
605
--
--
40
40
61
--*
Lignin
--*
*non-porous carbon fibers
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Figure 8. (a) C1s and (b) P2p XPS deconvoluted spectra of PCF-3%O2 fibers; (c) CO and (d) CO2 evolution during TPD experiments
of CFs with and without P.
This higher content in phenols, ethers and/or carbonyl groups of the hereby reported new method for production of functional
and –C–O–P– surface species greatly agrees with the carbon fibers.
functionalities distribution observed by XPS (Figure 8a and 8b) The promotion of stabilization is quite advantageous to shorten the
and it is also found in other less oxidized samples (PCF and carbon fibers production time. This is simply achieved by adding a
N2-PCF) (Figures not shown). CF (without P) presents a much small amount of H3PO4, which reacts with lignin to generate
–
lower evolution of CO and CO2. C-O-P bonds as in phosphate (and/or polyphosphate) ester groups.
The profiles for these two gases indicate that these sample Previous strategies on optimizing the stabilization of fibers from
mainly contain quinone- and/or carbonyl-like surface groups lignin or other biopolymers have been focused on new extraction
evolving as CO above 700
chemistry characteristic of carbon materials prepared at high the incorporation of polymer additives,43,46-48 what usually entails
temperatures under inert atmosphere. more complex and expensive procedures. Other approaches were
°
C (Figure 8c),45 i.e., a poor surface routes, physical and/or chemical modification treatments and/or
The distribution of phosphorus species on the different carbon based on stabilizers or precursors different to H3PO4 or lignin,
fibers was analyzed by X-ray energy dispersive spectroscopy (XEDS) respectively.49,50
(Figure 9). CF displays a negligible blank P signal (Figure 9a). Activated and functionalized carbon fibers can be directly prepared,
However, the much higher intensity and density of P signals on the without
additional
impregnation/drying
and/or
post-
selected area together with the increased and constant intensity of functionalization steps. In order to achieve very high specific
P signals across the width of PCF (Figure 9b) indicate that the surface areas (2000 m2 g-1) and a large amount of functional groups,
phosphorus functionalities are uniformly distributed throughout conventional carbon fibers would necessarily require of additional
this P-containing sample.
chemical (NaOH, KOH, H3PO4, ZnCl2, etc.) that in many cases
damages the morphology and structure of the fibers and/or
physical (CO2 or steam water at 800-900 °C) activation
Properties for interfacial applications
The functionalization of submicron lignin fibers with phosphorus processes,1,11-13,51,52 and liquid- or gas-phase oxidation post-
species plays a decisive role in the length reduction and versatility treatments.34
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In order to emphasis the extent of activation produced by sites, whereas it decomposes through dehydrogenation over
phosphoric acid in the electrospun lignin fibers, powdery activated basic sites, yielding acetone (Ac).
carbons with different impregnation ratios were prepared, for Figure 10a represents the 2-propanol conversion as a function
comparison purposes, without the aid of electrospinning (see of temperature over various lignin-based carbon fibers with
experimental section). In agreement with this result, considerably and without P used as catalysts. The conversion increases with
much higher activating agent contents (from 10 to 20 times) are the reaction temperature for all the catalysts. In particular,
usually necessary to produce the chemical activation degree whereas CF shows total conversion at 600
observed for the phosphorus carbon fibers reported in the carbon fibers catalysts exhibit conversions of 100 % between
present work (see Table 2),1,17,32,33,51,52 which is attributed to the 300 and 370
and their activity increases with the
°C, the P-containing
°C
uniform distribution of the reactant phosphorus species through phosphorus surface content (N2-PCF ≈ PCF-3%O2 > PCF).
the entire fibers matrix obtained by the methodology proposed in The selectivity of the P-containing carbon fibers towards the
this work.
The benefits of the aforementioned textural and chemical is higher than 90 % above 200
characteristics of phosphorus carbon fibers were analyzed for opposite, for CF the selectivity changes towards Ac (100%)
practical application. Firstly, it is well-known that the large surface- even at conversions as low as 15 % (at 415 C). The high
production of PP increases rapidly with temperature, so that it
°C (Figure 10b). On the
°
to-volume ratio and endless form of fibrous structures promote conversions and selectivities to PP found for the P-containing
excellent mass transfer properties, low pressure drop, ease of carbon fibers maybe attributed to the presence of Brönsted
handling and reduced inter-particle boundaries that, compared to acid sites of surface P groups,23,53,54 highlighting their acidic
other morphologies (powder, granular, pellet, etc.), offer several character. On the contrary, the production of acetone is
advantages for heterogeneous catalytic reactions, adsorption and indicative of the presence of basic sites on CF.
1-4
separation processes, electrochemical applications, etc
.
The catalytic performance of the lignin-based phosphorus
-Al2O3
In catalysis. The catalytic behavior of the different carbon carbon fibers was compared to that of commercial
γ
fibers was analyzed for the 2-propanol decomposition (powder), which is among the most studied acid catalysts.
reaction. This is a test reaction for the evaluation of the acid Phosphorus carbon fibers present almost comparable catalytic
and basic properties of heterogeneous catalysts.23,53,54 It is activity for 2-propanol dehydration (Figure 10a), as well as
generally accepted that the reaction conversion increases with similar selectivity towards PP (Figure 10b) to that of
the surface acidity (or basicity) of the catalyst and that catalyst. Nevertheless, it is necessary to increase the
2-propanol mainly dehydrates to propylene (PP) over acid temperature at least 30 C to get the same conversion.
γ-Al2O3
°
Figure 9. Two (1,2) and one (3) dimensional distributions of P and other species (O, C) on different lignin-based carbon fibers, as
determined by XEDS.
12 | J. Mater. Chem. A, 2017, 00, 1-3
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Figure 10. (a) Evolution of 2-propanol (2-PrOH) conversion and (b) selectivity to propylene (SPP) and acetone (SAc) as a function of
temperature for various lignin-based carbon fibers and a commercial reference material ( -Al2O3) used as catalysts (Reaction
γ
conditions: P2-PrOH = 0.018 atm in He; Space time (W/F)2-PrOH= 0.055 g s µmol-1). (c) Non-isothermal oxidation profiles in air of
lignin-based carbon fibers with and without P and. (d) Steady-state voltammograms of PCF in different electrolytes
(concentration = 0.5 M; scan rate = 10 mV s-1).
On the other hand, beside surface acidity, a high oxidation mesopores and wider micropores (Table 2) may enormously
resistance is also a required property of carbon materials for enhance the accessibility and fast diffusion of molecules to the
their potential use as catalysts and catalyst supports in several inner microporous structure.58,59 The benefits of these
reactions involving oxygen at moderate temperatures (200- properties were confirmed for the adsorption of phenol in
400
catalysts under oxygen atmosphere becomes crucial. 180 mg·g-1 for PCF (at 25
°
C).1,2,17,23,30,32,33,53,54 Thus the thermal stability of the aqueous solutions, obtaining adsorption capacities of around
C and 25 mg L-1). These values are
°
Figure 10c shows the non-isothermal air oxidation profiles of higher than those found, at the same adsorption conditions,
lignin-based carbon fibers with and without P. Although CF for activated carbons prepared by conventional physical and
show a remarkable oxidation resistance by itselves, the fibers chemical activation.19,60
start to lose weight (oxidize) slowly from 350-400 °C to 500 °C, In electrochemical applications. As electrode materials,
temperature at which they quickly combust. The P-containing carbons are generally required to show a high conductivity and
carbon fibers, however, do not suffer from any weight-loss surface area, as well as adequate chemical and
until 525-550 °C, showing an increased oxidation resistance electrochemical stability in different electrolytes. The basic
induced by the high thermal stability of the P surface groups in electrochemical features of the phosphorus carbon fibers were
the presence of molecular oxygen, already reported in the tested, directly as binderless electrodes, by conventional cyclic
literature.20,24,25,30
voltammetry in a three-electrode cell. The straight shape and
In adsorption. Because of their high surface area and versatile large current densities observed in the voltammograms of the
physic-chemical properties, porous carbons constitute one of self-standing PCF (Figure 10d) clearly reflect their suitable
the most efficient and broadly used materials as adsorbents.55- conductivity and great potentiality for electrochemical
59 The fine diameter (≤ 1 μm) of the P-containing carbon fibers applications.
prepared in this work,56,57 together with their network of
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Another feature of upmost importance is the electro-chemical implies the electrospinning of H3PO4/lignin solutions to
stability. It is related to the range of electrode potentials produce in only one-step submicron-diameter P-functionalized
(potential window (ΔE)) at which neither the electrode nor the lignin fibers, in which the phosphorus functionalities produce
electrolyte suffer from electrochemical decomposition.
unique effects on their thermal processing, enabling to shorten
As observed in Figure 10d, the phosphorus carbon fibers (see the conventional stabilization process to avoid fiber fusion
as an example the PCF fibers) show no oxidation currents up to from more than 90 h to only 2 h or even to stabilize the lignin
0.90 V, 0.30 V or 0.80 V in the acid, basic or neutral fibers in inert atmosphere. The incorporation of H3PO4 to the
electrolytes, respectively.
initial lignin solution produces more oxidized spun lignin fibers,
This result points out that PCF presents a high electro- due to the reaction of phosphoric acid with the dissolved
oxidation resistance, i.e. they can operate safely up to very lignin, generating phosphate (and/or polyphosphate) esters
high potentials, stressing their potential application in throughout the structure of the lignin fibers. These phosphate
supercapacitors, fuel cells, (bio)sensors, electrosynthesis, groups seems to be the responsible for the production of
etc.61-63 On the other hand, PCF starts to display reduction cross-linking reactions during the stabilization step that are, in
currents at very low potentials on the negative side (below this case, very active and effective in increasing the glass
-0.7 V, -1.35 V or -1.1 V in the acid, basic or neutral transition temperature of the lignin fibers, reducing the time
electrolytes, respectively), at which the electrochemical H needed for the stabilization step and improving this process.
storage and/or H2 evolution occurs.64,65 The observed Moreover, they promote the chemical activation of lignin
enhanced electro-oxidation resistance66 and low potentials for fibers and greatly increase their oxidation resistance, avoiding
the electrolyte decomposition67 have been attributed to the their complete combustion during carbonization under low
presence of surface P functionalities in carbons.
As consequence, PCF exhibits wide stability potential resulting carbon fibers gather different interesting properties,
windows: ΔE = 1.60 V, 1.65 V or 1.90 V in H2SO4, NaOH and such as sub-micron diameters (≤ 1 μm), large surface area
Na2SO4, respectively, which are probably among the widest
2000 m2 g-1), relatively high performance in relation to their
ever reported for aqueous electrolytes (ΔE values for porous mechanical properties for functional applications and a rich
carbons are usually around 1.00 V). Such high variety of uniformly-distributed O and P surface functionalities,
concentration of O2 at temperatures as high as 900 °C. The
a
(
≈
a
electrochemical stability induced by P functionalities has been which make them very attractive for heterogeneous catalysis,
found beneficial for energy storage in supercapacitors.67,68 adsorption and electrochemical applications.
Moreover, the high surface area and microporosity of PCF,
together with its small diameter and the presence of
Conflicts of interest
mesopores are well-known properties of carbons for an
optimum ion adsorption and fast diffusion, respectively, for
this application.63,69 Although a detailed study is not the aim of
the present work, the energy density of these materials in a
supercapacitor (SC) device can be predicted from the equation
E =0.5·C·V2; where C is the cell capacitance and V is the
There are no conflicts to declare.
Acknowledgements
This work was supported by the Spanish Ministry of Economy,
Industry and Competitiveness (MINECO) and FEDER under
CTQ2015-68654-R project. F.J.G.M. and R.B. acknowledge the
assistance of the MINECO for PTA2015-11464-I and IJCI-2014-
20012 contract, respectively.
operating voltage. Considering that in
(2-electrodes cell) the maximum capacitance is a fourth of that
calculated by using 3-electrode cell (from the
a symmetric SC
a
voltammograms of Figure 10d, these values are around 150-
200 F g-1) and that the operating voltage can be optimized to
approach the aforementioned stability potential windows
(1.90-1.60 V), symmetric SCs using PCF electrodes could
potentially display energy densities of 15-20 W h kg-1. These
energy densities are 2-5 times higher than those shown by
commercial activated carbons in organic electrolytes and
similar to those of hybrid Li-ion capacitors. Hence, taking into
account the excellent performance of lignin-based carbon
fibers as electrodes for supercapacitors,69 the obtained PCF,
which exhibits considerably better stability and larger surface
area, constitute a promising low-cost electrode candidate for
energy storage.
References
1
A. Linares-Solano, D. Cazorla-Amorós. Handbook of
Advanced Ceramics (Ed.: S. Somiya) Academic Press: Elsevier
Inc., 2013. pp. 155-169.
2
3
4
5
6
7
Y. Matatov-Meytal, M. Sheintuch. Appl. Catal. A Gen., 2002,
231, 1-16.
V. Thavasi, G. Singh, S. Ramakrishna. Energy Environ. Sci.,
2008, 1, 205-221.
S. Agarwal, A. Greiner, F.H. Wendorff. Prog. Polym. Sci.,
2013, 38, 963-991.
D.A. Baker, T.G. Rials. J. Appl. Polym. Sci., 2013, 130, 713-
728.
E. Frank, L.M. Steudle, D. Ingildeev, J.M. Spörl, M.R.
Buchmeiser. Angew. Chem. Int. Ed., 2014, 53, 5262-5298.
A. J. Ragauskas, G. T. Beckham, M. J. Biddy, R. Chandra, F.
Chen, M.F. Davis, B.H. Davison, R.A. Dixon, P. Gilna, M.
Keller, P. Langan, A.K. Naskar, J.N. Saddler, T.J. Tschaplinski,
G.A. Tuskan, C.E. Wyman. Science, 2014, 344, 1246843.
Conclusions
A new versatile route to prepare advanced functional carbon
fibers in remarkably short time, from an abundant and
renewable polymer like lignin, has been reported. The route
14 | J. Mater. Chem. A, 2017, 00, 1-3
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Journal of Materials Chemistry A
FULL PAPER
8
9
J.M. Rosas, R. Berenguer, M.J. Valero-Romero, J. Rodríguez- 41 Y. Uraki, A. Nakatani, S. Kubo, Y. Sano. J. Wood Sci., 2001, 47
Mirasol, T. Cordero. Front. Mater., 2014, 1:29, 1-17. 465-469.
M. Lallave, J. Bedia, R. Ruiz-Rosas, J. Rodríguez-Mirasol, T. 42 J.F. Kadla, S. Kubo, R.A. Venditti, R.D. Gilbert, A.L. Cornpere,
,
Cordero, J.C. Otero, M. Márquez, A. Barrero, I.G. Loscertales. W. Griffith. Carbon., 2002, 40, 2913-2920.
Adv. Mater., 2007, 19, 4292-4296. 43 S. Kubo, J.F. Kadla. J. Polym. Environ., 2005, 13, 97-105.
10 R. Ruiz-Rosas, J. Bedia, M. Lallave, I.G. Loscertales, A. 44 H.P. Bohem. Carbon, 2002, 40, 145-149.
Barrero, J. Rodríguez-Mirasol, T. Cordero. Carbon, 2010, 48
696-705.
11 S. Kubo, Y. Uraki, Y. Sano. Carbon., 1998, 36, 1119-1124.
,
45 J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. Orfao.
Carbon, 1999, 37, 1379-1389.
46 S. Hu, Y.-L. Hsieh. J. Mater. Chem. A., 2013, 1, 11279-11288.
12 Q. Shen, T. Zhang,W.X. Zhang, S. Chen, M. Mezgebe. J. Appl. 47 W. Qin, J.F. Kadla. J. Appl. Polym. Sci., 2012, 126, E203-E213.
Polym. Sci., 2011, 121, 989-994.
13 J. Lin, S. Kubo, T. Yamada, K. Koda, Y. Uraki. BioResources,
48 E. Sjöholm, G. Gellerstedt, R. Drougge, I. Norberg. WO
2013/112100 A1, 2013.
49 A. Poeppel, E. Frank. EP 2644758 A1, 2013.
2010,
7, 5634-5646.
14 S. Ramakrishna, K. Fujihara, W.-E. Teo, T. Yong, Z. Ma, R. 50 E.C. Chenevey, R.M. Kimmel. US 4002426 A, 1977.
51 H. Sakurai, M. Kitahara, M. Hirata, T. Sawaki. EP 1666649 A1,
2006.
Ramaseshan. Mater. Today, 2006,
9
, 40-50.
15 A. Greiner, J.H. Wendorff. Angew. Chem. Int. Ed., 2007, 46,
5670-5703.
16 J.L. Braun, K. Holtman, J.F. Kadla. Carbon., 2005, 43
394.
52 A. Linares-Solano, M.A. Lillo-Ródenas, J.P. Marco-Lozar, M.
Kunowsky, A.J. Romero-Anaya. Int. J. Ener. Environ. Econ.,
2012, 20, 59-91.
, 385-
17 J. Bedia, R. Barrionuevo, J. Rodríguez-Mirasol, T. Cordero. 53 J. Bedia, J.M. Rosas, J. Márquez, J. Rodríguez-Mirasol, T.
Appl. Catal. B: Env., 2011, 103, 302-310. Cordero. Carbon., 2009, 47, 286-294.
18 V. Fierro, V. Torné-Fernández, A. Celzard. Microporous 54 J. Bedia, R. Ruiz-Rosas, J. Rodríguez-Mirasol, T. Cordero. J.
Mesoporous Mater. 2006, 92, 243-250. Catal., 2010, 271, 33.
19 E. González-Serrano, T. Cordero, J. Rodríguez-Mirasol, L. 55 C. Moreno-Castilla. Carbon., 2004, 42, 83-94.
Cotoruelo, J.J. Rodríguez. Water Res., 2004, 38, 3043-3050.
20 J.M. Rosas, J. Bedia, J. Rodríguez-Mirasol, T. Cordero. Fuel.,
2009, 88, 19-26.
56 J.L. Shmidt, A.V. Pimenov, A.I. Lieberman, H.Y. Cheh. Sep. Sci.
Technol., 1997, 32, 2105-2114.
57 M. Li, Y. Luo, R.C. Wu, H.Y. Liang, S.F. Wang. Advanced
Materials Research, 2012, 356-360, 1524-1527.
21 M. Jagtoyen, F. Derbyshire. Carbon., 1998, 36, 1085-1097.
22 F.J. García Mateos, T. Cordero-Lanzac, R. Berenguer, E. 58 C.T. Hsieh, H. Teng. Carbon., 2000, 38, 863-869.
Morallón, D. Cazorla-Amorós, J. Rodríguez-Mirasol, T. 59 A.M. Redding, F.S. Cannon. Water Res., 2014, 56, 214-224.
Cordero. Appl. Catal. B Environ., 2017, 211, 18-30.
23 J. Bedia, J.M. Rosas, D. Vera, J. Rodríguez-Mirasol, T.
Cordero. Catal. Today, 2010, 158, 89-96
60 N. Tancredi, N. Medero, F. Möller, J. Píriz, C. Plada, T.
Cordero. J. Colloid Interface Sci., 2004, 279, 357-363.
61 B.C.H. Steele, A. Heinzel. Nature, 2001, 414, 345-352.
24 M.J. Valero-Romero, F.J. García-Mateos, J. Rodríguez- 62 A.P.F. Turner. Chem. Soc. Rev., 2013, 42, 3184-3196.
Mirasol, T. Cordero. Fuel Process. Technol., 2017, 157, 116- 63 P. Simon, Y. Gogotsi. Nat. Mater., 2008, , 845-854.
7
64 K. Jurewicz, E. Frackowiak, F. Béguin. Appl. Phys. A., 2004, 78,
981-987.
26 A.M. Puziy, O.I. Poddubnaya, A. Martinez-Alonso, F. Suárez- 65 S. Leyva-García, E. Morallón, D. Cazorla-Amorós, F. Béguin, D.
126.
25 X. Wu, L.R. Radovic. Carbon. 2006, 44, 141-151.
García, J.M.D. Tascon. Carbon., 2005, 43, 2857-2868.
27 R. Fu, L. Liu, W. Huang, P. Sun. J. Appl Polymer Sci., 2002, 57
2253-2264.
28 A.M. Puziy, O.I. Poddubnaya, A.M. Ziatdinov. Appl Surf Sci.,
2006, 252, 8036-8038.
29 E. Adler. Wood Sci. Technol., 1977, 11, 169-218.
Lozano-Castelló. Carbon., 2014, 69, 401-408.
,
66 R. Berenguer, R. Ruiz-Rosas, A. Gallardo, D. Cazorla-Amorós,
E. Morallón, H. Nishihara, T. Kyotani, J. Rodríguez-Mirasol, T.
Cordero. Carbon., 2015, 95, 681-689.
67 C.C. Huang, T. Sun, D. Hulicova-Jurcakova. ChemSusChem.,
2013, 6, 2330-2339.
30 J.M. Rosas, R. Ruiz-Rosas, J. Rodríguez-Mirasol, T. Cordero. 68 D. Hulicova-Jurcakova, A.M. Puziy, O.I. Poddubnaya, F.
Carbon, 2012, 50, 1523-1537.
31 J. Garrido, A. Linares-Solano, J.M. Martín-Martínez, M.
Suárez-García, J.M.D. Tascón, G.Q. Lu. J. Am. Chem. Soc.,
2009, 131, 5026-5027.
Molina Sabio, F. Rodríguez-Reinoso, R. Torregrosa. Langmuir, 69 R. Berenguer, F.J. García-Mateos, R. Ruiz-Rosas, D. Cazorla-
1987,
3
, 76-81.
Amorós, E. Morallón, J. Rodríguez-Mirasol, T. Cordero. Green
Chem., 2016, 18, 1506.
32 J.M. Rosas, J. Bedia, J. Rodríguez-Mirasol, T. Cordero. Ind.
Eng. Chem. Res., 2008, 47, 1288-1296.
33 J. Bedia, R. Ruiz-Rosas, J. Rodríguez-Mirasol, T. Cordero.
AIChE, 2010, 56, 1557-1568.
34 L.T. Weng, P. Ruiz, B. Delmon. Stud. Surf. Sci. Catal., 1992,
72, 399-413.
35 G. Mul, J.P.A. Neeft, F. Kapteijn, J.A. Moulijn. Carbon., 1998,
36, 1269-1276.
36 X. Yan, Y. Liu, X. Fan, X. Jia, Y. Yu, X. Yang. J. Power Sources,
2014, 248, 745-751.
37 Z. Zhou, K. Liu, C. Lai, L. Zhang, J. Li, H. Hou, D.H. Reneker, H.
Fong. Polym., 2010, 51, 2360-2367.
38 J. Rodríguez-Mirasol, T. Cordero, J.J. Rodríguez. Carbon.,
1996, 34, 43-52.
39 A. Sadezky, H. Muckenhuber, H. Grothe, R. Niessner, U.
Poschl. Carbon, 2005, 43, 1731-1742.
40 I. Dallmeyer, L.T. Lin, Y. Li, K. Ko, J.F. Kadla. Macromol.
Mater. Eng., 2014, 299, 540-551.
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/C7TA08788H
TABLE OF CONTENT
Phosphorus functionalization for the rapid preparation of highly
nanoporous submicron-diameter carbon fibers by electrospinning of
lignin solutions
F. J. García-Mateos, R. Berenguer, M. J. Valero-Romero, J. Rodríguez-Mirasol, T. Cordero.
Rapid preparation of low-cost carbon fibers by electrospinning of lignin solutions thanks to the
functionalization with H3PO4.