Cobalt-incorporated, nitrogen-doped carbon nanofibers as effective non-precious catalyst for methanol electrooxidation in alkaline medium

Article abstract of DOI:10.1016/j.apcata.2015.03.042

Among the nanosupports, carbon nanofibers distinctly enhance the performance of the functional electrocatalysts because the adsorption capacity and low resistance electron transfer due to the large axial ratio. Moreover, nitrogen doping shows positive influence toward electrooxidation ability. In this study, Co-incorporated and nitrogen-doped carbon nanofibers are introduced as effective non-precious electrocatalyst for methanol oxidation in the alkaline medium. The introduced NFs have been prepared using facile, simple, high yield, low cost, and effective technique; electrospinning. Typically, calcination of cobalt acetate/urea/poly(vinyl alcohol) electrospun mats at 850 °C in argon atmosphere leads to produce the introduced nanofibers. The structure, composition and morphology were characterized by FT-IR, XRD, EDX, FE-SEM and TEM techniques. The electrocatalytic activity of the introduced nanofibers toward methanol oxidation was evaluated by cyclic voltammetry (CV). Study the influence of the nitrogen content could be investigated by adjusting the urea content in the original electrospun nanofiber mats. The results indicated that the activity of the catalyst increases with increasing urea content in the electrospun solution up to 4%. A maximum current density was 100.84 mA cm^-2 for Co/N(4%)-CNFs that was higher than the undoped ones (63.56 mA cm^-2). Moreover, good chemical stability was observed due to covering the metals NPs by carbon shells. Overall, nitrogen doping enhances electrocatalytic activity of Co/CNFs toward methanol oxidation in alkaline media.

Full text of DOI:10.1016/j.apcata.2015.03.042

Applied Catalysis A: General 498 (2015) 230–240
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Cobalt-incorporated, nitrogen-doped carbon nanofibers as effective
non-precious catalyst for methanol electrooxidation in
alkaline medium
Badr M. Thamera, Mohamed H. El-Newehya,b,∗∗, Salem S. Al-Deyaba,
Mohammad Ali Abdelkareemd, Hak Yong Kimc, Nasser A.M. Barakatc,d,∗
a Petrochemical Research Chair, Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia
b Department of Chemistry, Faculty of Science, Tanta University, Tanta 31527, Egypt
c Organic Materials and Fiber Engineering Department, College of Engineering, Chonbuk National University, Jeonju, South Korea
d Minia University, Faculty of Engineering, Chemical Engineering Department, El Minia, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Among the nanosupports, carbon nanofibers distinctly enhance the performance of the functional elec-
trocatalysts because the adsorption capacity and low resistance electron transfer due to the large axial
ratio. Moreover, nitrogen doping shows positive influence toward electrooxidation ability. In this study,
Co-incorporated and nitrogen-doped carbon nanofibers are introduced as effective non-precious elec-
trocatalyst for methanol oxidation in the alkaline medium. The introduced NFs have been prepared using
facile, simple, high yield, low cost, and effective technique; electrospinning. Typically, calcination of
cobalt acetate/urea/poly(vinyl alcohol) electrospun mats at 850 ◦C in argon atmosphere leads to produce
the introduced nanofibers. The structure, composition and morphology were characterized by FT-IR,
XRD, EDX, FE-SEM and TEM techniques. The electrocatalytic activity of the introduced nanofibers toward
methanol oxidation was evaluated by cyclic voltammetry (CV). Study the influence of the nitrogen con-
tent could be investigated by adjusting the urea content in the original electrospun nanofiber mats. The
results indicated that the activity of the catalyst increases with increasing urea content in the electrospun
solution up to 4%. A maximum current density was 100.84 mA cm−2 for Co/N(4%)-CNFs that was higher
than the undoped ones (63.56 mA cm−2). Moreover, good chemical stability was observed due to covering
the metals NPs by carbon shells. Overall, nitrogen doping enhances electrocatalytic activity of Co/CNFs
Received 24 November 2014
Received in revised form 11 March 2015
Accepted 26 March 2015
Available online 2 April 2015
Keywords:
Electrospinning
Methanol oxidation
Fuel cells
Nitrogen-doping
Cobalt
toward methanol oxidation in alkaline media.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
utilizing precious metals electrodes (e.g. Pt) is facing wide com-
mercial application for not only DMFCs but also all the other fuel
Development a future energy system from sustainable energy
sources as an alternative for fossil fuels is considered a key objec-
tive of modern research. Direct methanol fuel cells (DMFCs) are
the most promising sustainable energy devices because methanol
is an inexpensive, readily available, and easily stored and trans-
ported liquid fuel [1,2]. Unfortunately, high production cost due to
cells. Accordingly, more efforts are being done to develop low
cost materials having good chemical stability and high catalytic
activity to replace platinum [3–5]. In the DMFCs, methanol is oxi-
dized to carbon dioxide at the anode according to the reaction:
CH3OH + H2O = CO2 + 6H+ + 6e. The reaction is considered to be a
combination of adsorption and electrochemical reaction on the
anode surface [6,7]. Accordingly, because of the adsorption capac-
ity of carbon, the later has been incorporated in many recently
reported electrocatalytic materials, not only for the DMFCs but
also other kinds of fuel cells [8–12]. Among the utilized carbon
nanostructural supports, the large axial ratio provides the carbon
nanofibers additional feature due to the low electron transfer com-
pared to other structures [13,14].
∗
Corresponding author at: Organic Materials and Fiber Engineering Department,
College of Engineering, Chonbuk National University, Jeonju, South Korea.
Tel.: +82 63 2702363; fax: +82 63 270 4248.
∗∗
Corresponding author at: Petrochemical Research Chair, Department of Chem-
istry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia.
Tel.: +966 14675899; fax: +966 14676992.
Transition metals nanostructures and their alloys have been
studied as an electroactive material for methanol oxidation and
E-mail addresses: melnewehy@ksu.edu.sa (M.H. El-Newehy), nasser@jbnu.ac.kr
(N.A.M. Barakat).
http://dx.doi.org/10.1016/j.apcata.2015.03.042
0926-860X/© 2015 Elsevier B.V. All rights reserved.

B.M. Thamer et al. / Applied Catalysis A: General 498 (2015) 230–240
231
potentially cheaper than platinum [15–18]. However, the high
chemical activity is a forcible constraint facing wide utilizing of
this new class of materials. Among the utilized transition metals,
cobalt gained the lowest attention due to the low electrocatalytic
activity of the pristine cobalt. Instead, it is used as a co-catalyst
to annihilate the Pt poisoning. Pt–Co alloys have been found to be
excellent CO-tolerant catalysts [19]. However, some recent reports
have introduced cobalt as a main catalyst [13,20,21]. Compared
to nanoparticulate morphology, nanofibers do have low specific
surface area which can be considered as negative characteristic
in catalytic applications however the large axial ratio proved that
it has distinct positive impact in the electrocatalytic oxidation of
methanol [22,23]. Currently, many studies are focusing on improv-
ing the catalytic activity and stability through incorporation of
heteroatoms such as nitrogen into the carbon nanostructure sup-
ports. Nitrogen-doped carbon nanostructure owns high surface
nucleation sites which allows the anchorage and high dispersion
of the catalyst nanoparticles on the support surface material that
results in high interaction between nitrogen-doped carbonaceous
support and the catalytic metals [24–26]. On the other hand, nitro-
gen doping improves the durability of the resultant carbon-support
catalysts because of the enhanced ␲ bonding [27,28], and the basic
property [29] due to the strong electron donor behavior of nitro-
gen. Nitrogen-doped carbon nanotubes/nanofibers (N-CNT/N-CNF)
showed enhanced catalytic activity toward oxygen reduction reac-
tion (ORR) [30–32]. Most of the previous studies were used nitrogen
doped carbon nanostructure with noble metals or their alloys as
catalysts (e.g. Pt, Ru, etc.) for methanol oxidation [33]. Recently, the
same authors studied the influence of nitrogen doping on the elec-
trocatalytic activity of nickel-doped carbon nanofibers; the results
indicated that nitrogen doping has featured influence [34]. In liter-
ature, there are several methods have been introduced to synthesis
the nanofibers, however among these reported techniques, electro-
spinning have drawn the most attention due to several advantages
including simplicity, high yield, low cost and easy morphology con-
trol [35–38].
stirred for 5 h at 50 ◦C. The viscous gels were electrospun to produce
nanofibers mats. The procedure can be summarized as following;
the sol–gel of PVA/CoAc or PVA/CoAc/urea was placed in 20 mL plas-
tic syringe attached to the digital hypodermic syringe and fed into
the stainless steel needle at a flow rate of 0.9 mL/h, which was main-
tained by the digital hypodermic syringe pump. A collecting drum
was covered with a polyethylene sheet. The applied voltage was
17–20 kV and TCD was 15 cm. The electrospinning process was car-
ried out at room temperature and humidity was ≥35%. The collected
NFs mats were peeled from surface of polyethylene sheet. The NFs
mats were dried for 24 h at 80 ◦C under vacuum. Later on, the col-
lected nanofibers mats were calcined in an argon atmosphere at
850 ◦C with a heating rate of 2.83 ◦C min−1 for 5 h. Nanofiber mats
were left inside the furnace until cooled to room temperature and
evolved argon gas in the reaction medium to prevent oxidation.
2.3. Preparation of working electrode for methanol oxidation
It was carried out by mixing 2 mg of the functional material,
20 ␮L Nafion 117 (5 wt%) and 420 ␮L isopropanol. The slurry was
sonicated for 30 min at room temperature. A 15 ␮L from the pre-
pared slurry was poured on the active area of the glassy carbon
electrode (0.07 cm2 area) which was then subjected to drying pro-
cess at 80 ◦C for 20 min. Before poured the slurry solution on
the glassy carbon electrode, it should be cleaned by acetone and
distillated water then polished by adding one drop of diamond
suspension to smooth emery paper and moved the glassy carbon
electrode on it until became like mirror and then cleaned by ace-
tone and distilled water and again cleaned by acetone and distilled
water.
2.4. Characterization
The surface morphology of samples was studied by FESEM
(JEOL-JSM-2100F, Japan) equipped. Thermogravimetric analyses
(TGA) for the prepared electrospun mats were performed on TA
Instruments (Q500 TGA) to study thermal properties of nanofibers
mats. The crystallinity and structure of the as-prepared catalyst
was studied by X-ray diffractometer (XRD, Bruker D8 DISCOVER),
FT-IR (Bruker Optic Tensor 27, Germany), respectively. Normal and
high resolution images of prepared catalysts were observed by
transmission electron microscope (TEM, JEOL-JEM-2100, Japan).
The electrocatalytic activities of prepared catalysts as anode (work-
ing electrode) for methanol electro-oxidation were performed by
Versa STAT 4 (USA) electrochemical analyzer and a conventional
three electrodes electrochemical cell. An Ag/AgCl electrode and Pt
wire were used as the reference and auxiliary (counter) electrode,
respectively.
In this work, cobalt-decorated and nitrogen-doped carbon
nanofibers (Co/N-CNFs) are introduced as anode for methanol
oxidation. The introduced catalysts was fabricated by using elec-
trospinning technique and followed by carbonization process at
850 ◦C. The experimental results revealed good electrocatalytic
activity and non-observable influence on the good stability of Co/N-
CNFs toward methanol oxidation in alkaline medium especially at
high nitrogen content.
2. Experimental
2.1. Materials
Cobalt(II) acetate tetrahydrate (CoAc·H2O, 98%), poly(vinyl alco-
hol) (PVA, MW = 88 K g mol−1), urea (99%), methanol, potassium
hydroxide, isopropyl alcohol, Nafion 117 (10 wt%), diamond sus-
pension and acetone were purchased from Sigma–Aldrich Co. Those
materials were utilized without any further purification. Distilled
water was used as solvent.
3. Results and discussion
3.1. Characterization of Co/N-CNFs catalyst
FESEM instrument was used to study the changes in morphol-
ogy of fibers caused by addition of urea with different concentration
before and after calcination and determine the average diame-
ters of nanofibers. Fig. 1 shows FE-SEM images of the electrospun
PVA/CoAc nanofiber mats without and with different concentration
of urea before calcination. As shown in this figure, the obtained
nanofibers are relatively smooth and the diameter of nanofibers
increase with increase the concentration of urea. This may be
attributed to the increasing of viscosity of the precursor solution
due to the urea interaction with PVA polymer which demonstrated
by FT-IR in Fig. 5. Moreover, as shown in Fig. 1, NFs have a random
orientation because of the bending instability associated with the
2.2. Preparation of N-CoCNFs catalysts
PVA (10%, w/w) solution was prepared at 70 ◦C with a mechan-
ical stirring for 2 h to obtain clear solution. Cobalt acetate
tetrahydrate Co(Ac·4H2O) aqueous solution (20%, w/w) was pre-
pared by sonication for 15 min at room temperature using
ultrasonication. The sol–gel was prepared through mixing PVA and
CoAc·4H2O solutions in a weight ratio of 4:1 and then different
concentrations of urea (0, 1, 2, 3, 4, 5%, w/w) were added and

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B.M. Thamer et al. / Applied Catalysis A: General 498 (2015) 230–240
Fig. 1. SEM images for the PVA/CoAc nanofibers mats with different urea content: (A) 0.0%, (B) 1.0%, (C) 2.0%, (D) 3.0%, (E) 4.0%, and (F) 5.0%.
spinning jet. After calcination process, the nanofibrous morphol-
ogy of nanofibers was slightly changed and the average diameters
of the fibers became smaller (Fig. 2). This can be attributed to the
low thermal decomposition rate of the polymer upon calcination
of the electrospun nanofiber mats in an argon atmosphere which
leads to preserving the nanofibrous morphology [39,40].
Fig. 3 displays the effect of urea content in the original electro-
spun solution on the average diameter of the obtained electrospun
nanofibers and the produced ones after the calcination process. As
shown, increasing the urea content leads to increase the average
diameter which can be attributed, as aforementioned, to increase
the viscosity of the electrospun solution with increasing the urea
content.
75.9◦ and 92.2◦ correspond to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1)
planes of the face-centered cubic crystalline cobalt. According to
the XRD database, these peaks indicate formation of pristine Co.
No other peaks can be observed in the spectra (except graphite
peak) which indicate that the sample is free from cobalt oxide or
any other crystalline materials.
The average grain size in the formed cobalt nanoparticles can
be estimated by Scherrer’s equation (Eq. (1)) through analysis of
the (1 1 1) cobalt peaks using the calculated full width half max
(FWHM).
kꢂ
ꢁ =
(1)
ˇ cos ꢀ
To investigated the structure of the obtained nanofibers and Co
NPs size after the calcination process, XRD analysis was carried out.
As shown in Fig. 4, the strong diffraction peaks at 2ꢀ = ∼44.3◦, 51.6◦,
K is constant (0.9), ꢂ is the X-ray wavelength, ˇ is the full width half
maximum (FWHM), ꢀ is the Bragg angle and ꢁ is the mean size of the
crystalline domains. In general, the average particles size of cobalt

B.M. Thamer et al. / Applied Catalysis A: General 498 (2015) 230–240
233
Fig. 2. FE-SEM images of the prepared electrospun mats after calcination in Ar atmosphere at 850 ◦C: (A) 0.0%, (B) 1.0%, (C) 2.0%, (D) 3.0%, (E) 4.0%, and (F) 5.0% urea.
in Co/N-C◦NFs was approximately 23 nm. Additionally, the peak at
of nitrogen undoped and doped cobalt carbon nanofibers after
˚
2ꢀ of 26.3 corresponds to an experimental d spacing of 3.37 A indi-
the calcination process. The results shown in this figure con-
firmed the doped and undoped of Co/CNFs was free from oxygen.
Two and three main components are detected in the Co/CNFs
and Co/N-CNFs namely: carbon, cobalt, and nitrogen with a dif-
ferent ratio, respectively. Nitrogen was detected in all samples
containing urea and its ratio increases with increasing urea con-
centration in electrospun solution. The ratio of nitrogen increase
with increasing urea concentration in precursors and reached to
6.77%.
cating presence of graphite-like carbon (d (0 0 2), JCPDS; 41-1487).
Moreover, the broad peak at 10◦ for the sample having 4 wt% urea
(d spectrum) can be assigned to formation of some sheets from
graphene oxide; [0 0 2] crystal plane [41].
It is known that XRD can only investigate the crystalline mate-
rial so nitrogen cannot be detected in the XRD spectra. EDX is
an analytical technique used to determine amount and composi-
tion of element in sample. Fig. 5 shows the elemental composition

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B.M. Thamer et al. / Applied Catalysis A: General 498 (2015) 230–240
Fig. 3. Influence of the urea content on the average diameter of the electrospun and
Fig. 4. XRD spectra of Co/CNFs (a) and Co/N-CNFs obtained from electrospun solu-
sintered nanofibers.
tions having urea content of 1% (b), 2% (c), 3% (d), 4% (e) and 5% (f).
Fig. 5. EDX analyses for Co/CNFs (a) and Co/N-CNFs obtained from electrospun solutions having urea content of 1% (b), 2% (c), 3% (d), 4% (e) and 5% (f).

B.M. Thamer et al. / Applied Catalysis A: General 498 (2015) 230–240
235
Fig. 6. FT-IR spectra for the prepared nanofibers before (A) and after (B) calcination process: (a) 0%, (b) 1%, (c) 2%, (d) 3%, (e) 4% and (f) 5% urea.
The structure and interaction between the components of the
prepared nanofibers were demonstrated by FT-IR. Fig. 6 shows FT-
IR spectra of the electrospun nanofibers mats before and after the
calcination process. As shown in Fig. 6A the characteristic absorp-
tion peaks were observed at ∼3325, 2944 and 2910 cm−1 due to
resulted secondary amide group. It is noteworthy mentioning that,
the single band at 3435 cm−1 appeared obviously when the con-
centration of urea more than 2% (w/w). Fig. 6B shows the FT-IR
spectrum of PVA/CoAc (with and without doping) after calcina-
tion. As shown in this figure, the functional groups of PVA and urea
disappeared that indicates the degradation of polymer result from
sintered at high temperature. However, the observed broad band
at 3440 cm−1 and weak band at 1630 cm−1 can be attributed to
stretching and bending vibrations of water molecules [43].
O
H and C H symmetric and asymmetric stretching vibration;
respectively. Other strong bands at ∼1418, 1096, 1045 cm−1 cor-
respond to C H bending, C O stretching vibration, and the weak
band appeared at 1716 cm−1
C O result from acetate group of
cobalt acetate and some ester formation in the product [42]. In
Fig. 6A spectra b to f, new band can be observed at 2200, 1625 cm−1
due to C N stretching and N H bending, respectively. In addition,
new single band appeared at ∼3435 cm−1 due to N H stretch-
ing that indicated the urea interaction with fibers composite and
TGA analysis was used to study thermal degradation of elec-
trospun nanofibers under nitrogen atmosphere with heating rate
of 10 ◦C min−1. Fig. 7 shows the TGA with the first derivative of
PVA/CoAc and PVA/CoAc/urea (3%) composite. Actually, abnormal
decomposition of the acetate anion led to form reducing gases

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B.M. Thamer et al. / Applied Catalysis A: General 498 (2015) 230–240
Fig. 7. TGA analyses of PVA/CoAc electrospun nanofibers mats: (A) urea free and (B) urea 3%.
(CO and H2), these gases are responsible about formation of the pure
cobalt. Typically, formation of pure cobalt is conducted according
to the following equations [40]:
be assigned to the liberation of physical (moisture) and chemical
(combined water molecules with the CoAc) water from the sample,
respectively. Second peak appeared at 215 and 237 ◦C for undoped
and doped fibers, respectively can be attributed to degradation
of PVA backbone. Considering that the high thermal stability of
metal carbonates, the peaks at 361 and 387 ◦C can be assigned
to the decomposition of the synthesized carbonate (Eq. (3)) into
monoxide. The formed cobalt monoxide starts to be reduced by
carbon monoxide gas which is produced by the decomposition
of the acetate at the last peaks at temperatures 487 and 524 ◦C
for undoped and doped fibers [13,39,44,45]. It is noteworthy
mentioning that the doping PVA/CoAc electrospun nanofibers by
urea reveals more thermal stability than the undoped ones. This
may assigned to effect of derivatives of urea during carboniza-
tion process that caused partial cross-linked for PVA at initial
stage.
Co(CH3COO)2·4H2O → Co(OH)(CH3COO) + 3H2O + CH3COOH
(2)
Co(OH)(CH3COO) → 0.5CoO + 0.5CoCO3 + 0.5H2O
+ 0.5CH3COCH3
(3)
CoCO3 → CoO + CO2
(4)
(5)
CoO + CO → Co + CO2
Degradation of PVA/CoAc achieved in four steps. As shown in
Fig. 7 the first and second step at 42–45 ◦C and 139–175 ◦C can

B.M. Thamer et al. / Applied Catalysis A: General 498 (2015) 230–240
237
Fig. 8. Low and high magnifications TEM images of the undoped Co/CNFs (A and B) and nitrogen doped-Co/CNFs mats (3%) (C and D) after calcination.
Fig. 8 shows the TEM images of the nitrogen-free and -doped
Co/CNFs. A and C images show the obtained powder consist of car-
bon nanofibers matrix with metallic NPs incorporated. The high
magnification HR-TEM images indicate that the presence of paral-
lel atomic planes and appear excellent crystallinity for the graphite
layer (Fig. 8B and D). This result confirms FE-SEM result. Moreover,
the metallic nanoparticles are enclosed in graphite sheath which
can work as protector to save the particles from corrosion during
exploiting the introduced nanofibers as electrocatalyst.
KOH aqueous solutions. As shown, the introduced electrocatalyst
showed a good activity toward methanol oxidation. Interestingly,
the oxidation process is observed during the positive potential.
Moreover, the onset potential (0.35 mV) for the solution contain
methanol is shifted to less value as compared with methanol-free
1 M KOH (0.6 V). Carbon monoxide is an intermediate compound
in the methanol electrooxidation, it accumulates on the surface of
the electrode till further oxidation step to carbon dioxide. Usually,
adsorption of CO appears to take place with the formation of islands
of adsorbate [50]; electroactivity appears to be restricted to the out-
sides of these islands. Accordingly, good catalytic activity is related
with the rate of CO removal. Oxidation of the CO leads to appear a
peak in the voltammogram at ∼550 mV (vs. NHE) [51]. Generally,
above 700 mV, the surface is almost free of adsorped CO because
of complete oxidation [52]. As shown in the obtained results for
The insets in Fig. 8A and C display the particle size distribu-
tions for the metallic nanoparticles incorporated in the carbon
nanofibers obtained from nitrogen-free and nitrogen-doped (3%
urea) Co/CNFs, respectively. As shown, nitrogen doping leads to
produce Co/CNFs having smaller size Co nanoparticles (average
diameter ∼17 nm) compared to the undoped nanofibers (average
diameter ∼37 nm). Moreover, the doping results in good distribu-
tion of the metallic nanoparticles as shown in the images. Beside
the metal alkoxides, metal acetates have been widely utilized as
metal precursors as these promised salts have good tendency for
polycondensation to form with the proper polymers electrospin-
able sol–gels [44,46–49]. Therefore, the change in the particle size
distribution upon nitrogen doping can be attributed to the nega-
tive influence of nitrogen on the cobalt acetate polycondensation.
In other words, nitrogen doping decreases the polycondensation of
the acetate molecules which results in decrease the particle size.
X-ray photoelectron spectroscopy (XPS) can provide the
chemical-state information of the detected elements. Fig. 9 displays
the obtained XPS results for the nanofibers obtained from electro-
spun solution having 2 wt% urea. As shown in the figure and the
corresponding inset, nitrogen could be detected (N1S at binding
energy of 395.4 eV) which confirms the doping of the introduced
nanofibers by nitrogen and simultaneously supports the EDX and
FTIR results.
3.2. Electrochemistry measurement
Fig. 10 displays the electroactivity of N-doped Co/CNFs (urea
content 4%) in 1 M KOH (methanol free) and 5 M methanol in 1 M
Fig. 9. XPS analysis for the introduced N-doped Co/C NFs obtained from electrospun
solution having 2 wt% urea. The inset displays the spectrum at N1S region.

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B.M. Thamer et al. / Applied Catalysis A: General 498 (2015) 230–240
Fig. 10. Consecutive cyclic voltammogram of Co/N-CNFs (urea content 4%) in 1 M
KOH at a scan rate of 100 mV s−1
Fig. 11. Study the influence of nitrogen doping on electrocatalytic activity of
Co/CNFs toward methanol oxidation at 3 M methanol + 1 M KOH.
.
the introduced electrocatalyst, the aforementioned CO oxidation
peak is not observed which indicates either instant oxidation of
CO and/or absence of CO as an intermediate compound. In such
a case, the electrooxidation of the methanol on the surface of the
introduced catalyst can be explained as carrying out according to
this route [52]. Due to polarity of methanol and others intermedi-
ates is higher than polarity of CO, the rate adsorption of methanol
and other active intermediates on surface catalyst is faster than
adsorption of CO. The high adsorption of methanol on surface cat-
alyst is related to presence of nitrogen inside CNFs which increases
its polarity and enhances the reactivity of catalyst.
Fig. 12 and the corresponding inset which displays the relationship
between the methanol concentration and the current density at
the anodic peak, the current density increases with increase the
methanol concentration up to 2.0 M. Later on, more increase in
the methanol concentration leads to sharply decreasing in current
density.
3.2.3. Influence of the scan rate
The effect of scan rate on the behavior of methanol oxidation at
N (4%)–Co/CNFs was investigated at various scan rate (10, 50, 100,
150, 200, 300, 400, 500, 700, 1000 mV s−1) and in 3.0 M methanol as
shown in Fig. 13. It can be clearly seen that a peak potential appears
at the low scan rate (10 mV s−1; the red arrow) and shifts positively
with the increase in the scan rate. This peak can be assigned to CO
3.2.1. Influence of nitrogen doping
The electrocatalytic activity of nitrogen-free cobalt CNFs and
N-doped (1, 2, 3, 4 and 5%) cobalt CNFs has been studied by
cyclic voltammetry (CV) technique. It was measured at poten-
tial range between −0.2 and 1.0 V vs. Ag/AgCl and scans rate
50 mV s−1 at 25 ◦C in 1.0 M KOH and 3.0 M methanol. As shown
in Fig. 11, the electrocatalytic activity of nitrogen-containing NFs
increased in term of current density. A maximum current den-
sity was 100.84 mA cm−2 for N (4%)–Co/CNFs which is higher than
nitrogen-free cobalt CNFs (63.56 mA cm−2). The observed high per-
formance may be attributed to increasing the conductivity of CNFs
and the lone pair electrons in the nitrogen atoms with regard to the
conjugated system of a graphite-like hexagonal framework which
enhance the electrochemical oxidation of methanol. Moreover, the
nitrogen doping in the graphite frameworks could improve the
wettability of the interface electrode/electrolyte [53].
3.2.2. Optimum methanol concentration
One of the objectives of methanol cells is reducing the volume of
fuel by increasing the concentration of methanol but it influences
on catalytic activity of the catalyst through decrease of current
density. Sample content 4% urea was selected to study the effect
of methanol concentration on the electrocatalytic activity because
it showed a best catalytic activity toward methanol oxidation as
compared to other samples. Fig. 12 displays the anode polariza-
tion at different methanol concentration at a scan rate of 50 mV s−1
and applied cell potentials ranging from −0.2 to 1.0 V. As shown in
Fig. 12. Cyclic voltammograms of the N (4%)–Co/CNFs at a scan rate of 50 mV s−1
,
25 ◦C, and 1 M KOH at different concentration of methanol.

B.M. Thamer et al. / Applied Catalysis A: General 498 (2015) 230–240
239
carbonaceous species such as COads and CHOads. As shown in
Fig. 14, nitrogen doping did not affect the stability of the intro-
duced nanofibers as both N-free and N-doped Co/C NFs reveal good
stability. It is noteworthy mentioning that the steady current den-
sity of the N-doped Co/CNFs after 1000 s remained 3.96 mA cm−2
which was better than the N-free Co/CNFs (2.94 mA C m−2). It is
noteworthy mentioning that the observed good stability can be
attributed to sheathing of the metallic nanoparticles by a graphite
shell as shown in TEM image (Fig. 8).
4. Conclusion
Cobalt nanoparticles-incorporated nitrogen-doped carbon
nanofibers can be synthesized by facile, simple, high yield, low
cost, and effective technique; electrospinning. The incorporation of
nitrogen in the carbon nanofibers by adding urea to precursor lead
to specific anchoring sites for Co NPs along the longitudinal axis
after calcination process. The introduced Co/N-CNFs reveal good
electrocatalytic activity toward methanol oxidation in alkaline
medium due to the influence of the electronic structure of nitrogen
that enhances the electronic properties of the carbon nanofibers.
Moreover, Co/N-CNFs show good stability toward methanol
oxidation for long time. This work is anticipated to open a new
possibility to rely on cobalt/N-CNFs as an effective non-precious
electrocatalyst for oxidation of small organic compounds.
Fig. 13. The linear sweep curves for methanol oxidation on N (4%)–Co/CNFs at
different scan rate in 1.0 M KOH containing 3.0 M methanol at 25 ◦C.
Acknowledgement
This project was supported by NSTIP strategic technologies pro-
grams, number (11-ENE 1917-02) in the Kingdom of Saudi Arabia
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