Synthesis of boron/nitrogen substituted carbons for aqueous asymmetric capacitors

Article abstract of DOI:10.1016/j.electacta.2011.03.112

Boron/nitrogen substituted carbons were synthesized by co-pyrolysis of polyborazylene/coal tar pitch blends to yield a carbon with a boron and nitrogen content of 14 at% and 10 at%, respectively. The presence of heteroatoms in these carbons shifted the hydrogen evolution overpotential to -1.4 V vs Ag/AgCl in aqueous electrolytes, providing a large electrochemical potential window (～2.4 V) as well as a specific capacitance of 0.6 F/m². An asymmetric capacitor was fabricated using the as-prepared low surface area carbon as the negative electrode along with a redox active manganese dioxide as the positive electrode. The energy density of the capacitor exceeded 10 Wh/kg at a power density of 1 kW/kg and had a cycle life greater than 1000 cycles.

Full text of DOI:10.1016/j.electacta.2011.03.112

Electrochimica Acta 56 (2011) 5369–5375
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Synthesis of boron/nitrogen substituted carbons for aqueous asymmetric
capacitors
Timothy Tomkoa, Ramakrishnan Rajagopalanb,∗, Parvana Aksoyc, Henry C. Foleyd
a Energy and Mineral Engineering, The Pennsylvania State University, University Park, PA 16802, United States
b Materials Research Institute, The Pennsylvania State University, 270 MRL Bldg., University Park, PA 16802, United States
c Energy Institute, The Pennsylvania State University, University Park, PA 16802, United States
d Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Boron/nitrogen substituted carbons were synthesized by co-pyrolysis of polyborazylene/coal tar pitch
blends to yield a carbon with a boron and nitrogen content of 14 at% and 10 at%, respectively. The presence
of heteroatoms in these carbons shifted the hydrogen evolution overpotential to −1.4 V vs Ag/AgCl in
aqueous electrolytes, providing a large electrochemical potential window (∼2.4 V) as well as a specific
capacitance of 0.6 F/m2. An asymmetric capacitor was fabricated using the as-prepared low surface area
carbon as the negative electrode along with a redox active manganese dioxide as the positive electrode.
The energy density of the capacitor exceeded 10 Wh/kg at a power density of 1 kW/kg and had a cycle
life greater than 1000 cycles.
Received 16 October 2010
Received in revised form 26 January 2011
Accepted 18 March 2011
Available online 2 April 2011
Keywords:
Asymmetric electrochemical capacitor
Boron/nitrogen carbon
Coal tar pitch
© 2011 Elsevier Ltd. All rights reserved.
Manganese dioxide
Polyborazylene
1. Introduction
functional groups such as pyridonic groups can result in a pseudo-
capacitive response in aqueous electrolytes [11,12]. Beguin et al.
Carbon has been used extensively as an electrode material in
symmetric and asymmetric electrochemical capacitors for both
aqueous and non-aqueous electrolytes [1–4]. The advantages of
using carbon as an electrode material in electrochemical capacitors
include: excellent cycle life, large coulombic efficiencies and high
power densities. Traditionally, carbons with a high surface area and
mean pore size close to 1–3 nm have been used to make symmetric
capacitors with energy densities in the order of 5 Wh/kg [5,6]. The
narrow pore size distribution of these carbons can also lead to elec-
trochemical hydrogen storage making them feasible to be used as
negative electrodes in aqueous asymmetric electrochemical capac-
itors with energy densities greater than 20 Wh/kg and cell voltage
as high as 2.0 V [7–9].
The specific capacitance of the carbon in aqueous electrolytes
can be improved further by incorporating heteroatoms such as oxy-
gen, nitrogen and boron in the carbon framework due to favorable
redox reactions. It was shown that addition of oxygen functional
groups such as quinone/hydroquinone groups increases the spe-
cific capacitance of the carbon significantly [10]. Similarly, nitrogen
showed that the decoration of carbon nanotubes with nitrogen
functional groups can lead to very high specific capacitance per
unit area [12]. More recently, pseudocapacitive responses in boron
substituted carbons have also been demonstrated. The mechanism
of improved capacitances in heteroatom doped carbons can be
attributed to several reasons including faradaic reactions, improved
wettability, space charge layer capacitance and conductivity of the
material [13,14]. Kyotani et al. recently showed that the improved
specific capacitance was primarily due to faradaic reactions by
evaluating the performance of boron and nitrogen doped carbons
deposited on an anodized alumina template [15].
There have been very few studies on the electrochemical activ-
ity of boron/nitrogen substituted carbons (BCN). The presence of
both boron and nitrogen in the carbon domains can result in a
significant increase of the interfacial capacitance due to possible
redox reactions. Recently, the electrochemical properties of high
surface area BCN were investigated and it was shown that the spe-
cific capacitance of these carbons can be as high as 300 F/g [16].
In this investigation, we report a novel synthetic route to produce
BCN materials with boron and nitrogen content as high as 14 at%.
The synthesized carbons show strong electroadsorption of protons
and are very promising candidates for use as negative electrodes in
aqueous asymmetric capacitors.
∗
Corresponding author. Tel.: +1 814 863 1880; fax: +1 814 863 8561.
E-mail address: rur12@psu.edu (R. Rajagopalan).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.03.112

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T. Tomko et al. / Electrochimica Acta 56 (2011) 5369–5375
2. Experimental
2.1. Synthesis and characterization of electrode materials
Coal tar pitch (CTP) was dissolved in THF and the soluble fraction
of the pitch was recovered through filtration followed by evapora-
tion of excess THF. The filtrate was then mixed with polyborazylene,
1:1 (w/w), and co-dissolved in THF. The blend was co-precipitated
in n-pentane, filtered and pyrolyzed at 800 ◦C under an argon atmo-
sphere. The pyrolyzed carbon was then washed, filtered and dried
to constant weight [17,18]. CTP and polyborazylene were obtained
from Koppers Inc. and Boroscience Inc., respectively.
Synthesis of high surface area MnO2 was adapted from a pro-
cedure described by Subramanian et al. [19]. To produce the
high surface area manganese dioxide (MnO2), 0.5 g of potas-
sium permanganate (KMnO4) was dissolved in 10 mL of water
in a small beaker, while 50 mg of Triton X-100 surfactant, 20 mL
of hexane, and 5 mL of methanol were mixed in a separate
beaker. The surfactant mixture was added in a dropwise fashion
to the aqueous KMnO4. The resultant mixture was stirred and
ultrasonicated for 10 min. The solution was filtered and rinsed
thoroughly several times to ensure no surfactant remained in the
sample. The solid residue was then dried at 100 ◦C to constant
weight.
In order to compare the electrochemical performance of the syn-
thesized carbons, high surface area activated carbon (AC) derived
from the same source (CTP) with a surface area of 1700 m2/g was
prepared using the method described previously [17,18].
XPS measurements (Kratos Analytical Axis ultra Instrument,
Chestnut Ridge, NY) were done to determine the amount of car-
bon, nitrogen, oxygen and boron present in the BCN sample.
Powder X-ray diffraction was used to confirm the composition
of the synthesized BCN using a Scintag X2 powder Diffractome-
ter using a CuK␣ radiation. Acquisition conditions were 35 kV
and 30 mA.
Fig. 1. XRD of synthesized BCN powder.
electrolyte and tested using cyclic voltammetry, electrochemical
impedance spectroscopy and constant current charge/discharge
cycling. Cyclic voltammetry was done using a scan range of 0–1 V
for the symmetric AC capacitor and 0.1–2.4 V for BCN/MnO2 at a
scan rate of 10 mV/s for 100 cycles. The mass of AC, MnO2 and BCN
electrodes were 2 mg, 2 mg and 7 mg, respectively. The EIS mea-
surements were done using a model 5210 Lock-in amplifier coupled
with 263A Potentiostat/Galvanostat. An ac perturbation of 10 mV
was applied at open circuit voltage conditions. The frequency of
the ac perturbation was varied from 105 Hz to 10−3 Hz. The equiv-
alent series resistance (ESR) of the cell was computed using the
impedance data measured at 100 Hz. The obtained Nyquist plot
was also fitted to an equivalent circuit model using ZView soft-
ware. Galvanostatic charge/discharge cycling was done by applying
a load current density ranging from 70 mA/g to 1.4 A/g for symmet-
ric AC and 130 mA/g to 2.5 A/g for BCN/MnO2 and the cycle life
was determined up to 1000 cycles. Specific capacitances, energy
densities and power densities were computed from the discharge
curves.
2.2. Electrochemical characterization
2.2.1. Three-electrode electrochemical testing
The electrodes were prepared by combining 0.085 g of active
material (BCN or AC), 0.01 g Teflon binder and 0.005 g of acety-
lene black. The resultant powder was then dispersed in 1 mL of
THF and the solution was ultrasonicated for 20 min. 2 mg of the
resultant slurry was applied to 1 cm2 of carbon fiber mat (sup-
plied by Technical Fibres Inc.) and blown dry to constant weight.
All electrochemical measurements were done using a 263A Poten-
tiostat/Galvanostat. The active electrode materials were tested
individually as the working electrodes using a platinum wire as
a counter electrode and Ag/AgCl as a reference electrode in aque-
ous 2 M calcium chloride (CaCl2). The pH of the aqueous electrolyte
solution was adjusted to 7 using dilute acetic acid and the cyclic
voltammograms were measured at a scan rate of 10 mV/s using
a scan range of −1 V to 0.8 V for AC and −1.4 V to 1 V for BCN vs
Ag/AgCl, respectively.
3. Results
3.1. Characterization of synthesized BCN
XRD of BCN prepared by pyrolysis of a polyborazylene/CTP blend
showed two broad peaks positioned at 26◦ and 42◦ corresponding
to 0 0 2 and 2 0 0 reflections, respectively (Fig. 1). The d-spacing
˚
of the 0 0 2 plane was 3.45 A indicating highly disordered carbon
with Lc in the order of 1–2 nm. The elemental composition of BCN
was determined using XPS as shown in Table 1. The stoichiomet-
ric ratio of the synthesized compound was B1.4C5.1NO2 with boron
Table 1
Elemental composition and binding energy, as determined by XPS, of BCN powders.
2.2.2. Two electrode measurements
Element
B1s
Composition (at%)
14.24
Components
Binding energy (eV)
Electrode materials namely AC and BCN were prepared simi-
lar to the 3-electrode measurements. 0.07 g of manganese oxide,
0.01 g of Teflon binder, and 0.02 g of acetylene black were used to
make the MnO2 electrode. Prior to assembly of the cell, the elec-
trode materials, as well as a Celgard 3501 membrane, were soaked
in aqueous 2 M CaCl2 electrolyte, under vacuum, for 30 min. Tanta-
lum foils were used as the current collectors for both the electrodes.
The two-electrode capacitor was assembled together such that AC
or BCN was used as the negative electrode and MnO2 was used
as the positive electrode. The cell was then soaked in the aqueous
B–N or B–C
B–O
191.0
192.4
C1s
N1s
O1s
53.61
10.46
21.22
C–C
C–N
285
286.2
287
C
O
B–N or pyridinic N
C–N
Quaternary N
398.5
399.4
400
C
O
532

T. Tomko et al. / Electrochimica Acta 56 (2011) 5369–5375
5371
Fig. 2. Deconvoluted high resolution XPS spectrum of BCN: (a) B1s, (b) C1s, (c) N1s, and (d) O1s.
and nitrogen content as high as 14.24 at% and 10.46 at%, respec-
3.2. Three-electrode measurements
tively. Fig. 2a–d shows the deconvoluted B1s, C1s, N1s and O1s
XPS high resolution spectrum. The B1s spectrum was deconvoluted
into two peaks at 191.0 eV and 192.4 eV [20,21]. The origin of the
191.0 eV peak could be either due to B–N or B–C and the pres-
ence of a peak at 192.4 eV shows that some of the boron species
have been oxidized. N1s was curvefitted into three components
centered at 398.5 eV, 399.4 eV and 400.4 eV. The 398.5 eV could
be assigned to either B–N or the presence of pyridinic N groups
[22]. The peaks at higher binding energies (>399 eV) indicate that
the nitrogen was also bonded with carbon. The deconvoluted C1s
spectrum show peaks at 285 eV, 286.2 eV and 287 eV corresponding
to C–C in polyaromatic domains, C–N and C O, respectively [23].
O1s spectrum was fitted to a single peak at 532.6 eV assigned to
Cyclic voltammetry was performed on AC and BCN in aqueous
CaCl2 at 10 mV/s (Fig. 3). The specific capacitance of the electrode
was calculated from the following:
Qa + |Qc|
C =
(1)
2Vꢀm
where Qa and Qc are the anodic and cathodic charges respectively
in coulombs, V is the scan range in volts, ꢀ is the scan rate in V/s,
and m is the mass of the active material in grams.
The electrochemical potential window was determined based
on the hydrogen evolution potential for negative electrode mate-
rials (BCN and AC) as the lower limit, while the upper limit was
determined by a steep increase in the anodic current due to oxygen
C
O [24].

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T. Tomko et al. / Electrochimica Acta 56 (2011) 5369–5375
Fig. 4. Two-electrode cyclic voltammograms of symmetric AC and asymmetric
Fig. 3. Three-electrode cyclic voltammogram of BCN and AC at a scan rate of 10 mV/s
BCN/MnO2 capacitors at a scan rate of 10 mV/s in 2 M aqueous CaCl2 electrolyte.
using Ag/AgCl as a reference electrode in aqueous 2 M CaCl2 electrolyte.
gas evolution. The AC was cycled between −1 V to 0.8 V vs Ag/AgCl
and showed a specific capacitance of 75 F/g. The as-synthesized BCN
showed an increased potential range of −1.4 V to 1 V vs Ag/AgCl
and a specific capacitance of 30 F/g. Table 2 compares the specific
capacitance per unit area for three different electrode materials.
High surface area AC showed a specific capacitance of 0.04 F/m2,
which was indicative of primarily electrostatic interaction due
to double layer capacitance [25]. BCN showed a specific capac-
itance of 0.60 F/m2, which was substantially higher than double
layer response suggesting possible pseudocapacitive contributions.
In comparison, redox active MnO2 had a specific capacitance of
2.3 F/m2, clearly indicating a pseudocapacitive response.
3.3. Two-electrode measurements
Based on the 3-electrode measurements, mass ratio of
BCN/MnO2 was determined by balancing the charge on each elec-
trode. Fig. 4 shows the two-electrode cyclic voltammograms of a
symmetric AC cell and an asymmetric BCN/MnO2 performed at
a scan rate of 10 mV/s. The specific capacitance was measured as
follows:
Fig. 5. Nyquist plot and equivalent circuit diagram of symmetric AC and asymmetric
BCN/MnO2 systems.
the grain boundary of the electrode materials (charge transfer/grain
boundary resistance). C1 represents the capacitance at each grain
boundary layer, while CPE1 is a constant phase element accounting
for the double layer/pseudocapacitive response at each electrode.
The values of R1 and C2 are high for BCN/MnO2 due to a large ionic
resistance of the low surface area BCN and charge transfer resis-
tance from both BCN and MnO2 (Table 3). The CPE element was
obtained from the fit and its contribution to the overall impedance
is as follows:
I
C =
(2)
ꢀm
where I is the anodic or cathodic current in amperes, ꢀ is the scan
rate in V/s, and m is the total active mass of both the electrodes in
grams.
The gravimetric cell capacitance of both symmetric AC and
asymmetricBCN/MnO2 wereverysimilar(∼20 F/g). However, there
was a dramatic increase in the cell voltage with the asymmetric
BCN/MnO2 system. The cell voltage can be increased by almost 2.5
times the symmetric AC capacitor.
The ESR values of the symmetric AC and asymmetric BCN/MnO2
capacitors were 12 ꢁ and 15 ꢁ, respectively (Fig. 5). An equivalent
circuit was fitted to the impedance spectra as shown in Fig. 5. R1 cor-
responds to the sum total of all the series resistances (electronic and
solution resistance). R2 corresponds to the resistive contributions at
1
Z =
(3)
T(jω)P
where Z is the impedance in ohm, T and P are the CPE parameters,
respectively, and ω is the angular frequency in rad/s
Table 3
Fitted parameters of the equivalent circuit model for the symmetric AC and asym-
metric BCN/MnO2 systems.
Table 2
Specific capacitance (per unit mass and per unit area) of BCN, AC and MnO2.
System
R1 (ꢁ)
R2 (ꢁ)
C1 (␮F)
CPE1
Electrodes
A (m2 g−1
)
C (F g−1
)
C (F m−2
)
T
P
AC
BCN
MnO2
1700
50
130
75
30
300
0.04
0.60
2.30
AC
BCN/MnO2
1.56
1.26
10.85
16.90
5.47
10.12
0.06
0.06
0.98
0.84

T. Tomko et al. / Electrochimica Acta 56 (2011) 5369–5375
5373
Fig. 6. Constant current charge/discharge curves at a load current density of 250 mA/g (a) asymmetric BCN/MnO2 and (b) symmetric AC.
As shown in Table 3, the value of P for AC was 0.98 indicating
a highly capacitive behavior while that of BCN/MnO2 was 0.84 due
to its faradaic response.
The constant current charge discharge curves for both
BCN/MnO2 and symmetric AC shows a linear profile indicating a
capacitive response (Fig. 6a and b). The specific capacitance was
calculated from the discharge curve as follows:
purification. Polyborazylene was used as the boron/nitrogen pre-
cursor and has been shown to be effective at producing BN when
pyrolyzed at high temperatures (800–1200 ◦C) [26–28]. It is also
well known that coal tar pitch is a graphitizing precursor which
forms polyaromatic mesophases consisting of planar aromatic car-
bons when pyrolyzed between 500 and 600 ◦C [29]. Co-pyrolysis of
both these precursors would ideally lead to the incorporation of a
significant amount of BN in aromatic domains. By mixing the two
precursors (1:1, w/w), BCN were produced with more than 10 at%
and 14 at% of B and N, respectively. The presence of boron in the
precursor helps to stabilize nitrogen atoms in BCN during pyrolysis,
allowing for high nitrogen substitution in the carbon. A significant
amount of oxygen is seen in BCN even after high temperature pyrol-
ysis. There are very few reports in the literature that demonstrate
the incorporation of such high concentrations of heteroatoms in
carbon [30,31].
i × ꢂt
C =
(4)
ꢂV
where i is the specific current density in A/g, ꢂt is the discharge
time in seconds, and ꢂV is the potential drop during discharge in
volts.
The symmetric AC was charged up to 1.0 V while the asymmetric
BCN/MnO2 could be charged to 2.4 V at a load current density of
250 mA/g.
The cycle life performance of the asymmetric BCN/MnO2
was tested over
a period of 1000 cycles. Constant current
charge/discharge was performed at a load current density of
250 mA/g (Fig. 7). The specific capacitance of the cell remained
stable at approximately 14.5 F/g throughout the constant current
testing.
The Ragone plot (Fig. 8) shows that the BCN/MnO2 system had an
energy density of 11.5 Wh/kg while the symmetric AC system was
considerably lower at 2.5 Wh/kg. Similarly, the power density of
BCN/MnO2 was also slightly greater than symmetric AC. At 1 kW/kg,
BCN/MnO2 had an energy density of 10 Wh/kg which is almost five
times greater than symmetric AC.
4. Discussion
This paper describes a simple approach to make electrode
materials for electrochemical capacitors using pyrolysis of a
boron/nitrogen containing polymer blended with CTP. The energy
densities of the fabricated capacitors are significantly higher than
an activated carbon based double layer capacitor. The method does
not require any post treatment/processing such as activation or
Fig. 7. Cycle life performance of the BCN/MnO2 capacitor tested using constant
current charge/discharge at a load current density of 250 mA/g.

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T. Tomko et al. / Electrochimica Acta 56 (2011) 5369–5375
and shifting the hydrogen evolution potential. The large over-
potential can also be attributed to the presence of stable redox
functional groups that act as a barrier for adsorption/desorption
processes inhibiting hydrogen evolution reactions [38]. This phe-
nomenon has been successfully used to extend the cell voltage
of the fabricated asymmetric capacitor with excellent cyclabil-
ity. We envision that improving the surface area of the electrode
will substantially increase the energy density of the asymmetric
capacitors.
5. Conclusion
This paper describes a co-pyrolysis approach to develop BCN
electrodes for ultracapacitor applications. The synthesized BCN
shows stable pseudocapacitive behavior capable of increasing the
overpotential of hydrogen evolution reaction to almost −1.4 V vs
Ag/AgCl. The asymmetric capacitor fabricated using BCN/MnO2
showed an energy density of 10 Wh/kg stable up to 1000 cycles,
which is five times more than the symmetric AC system studied. The
synthesized BCN could offer significant advantages in terms of vol-
umetric capacitances as well as cost and purity compared to double
layer activated carbon electrodes used currently in ultracapacitor
technologies.
Fig. 8. Ragone plot showing the performance of symmetric AC and asymmetric
BCN/MnO2.
Recently, Konno et al. showed that it is possible to synthesize
BCN with boron content of 5–10% and a surface area of 400 m2/g
[16]. It was possible to obtain a specific capacitance of 300 F/g
(0.75 F/m2) in sulfuric acid and 0.3 F/m2 in neutral electrolytes,
respectively. Our results are in agreement with their observa-
tions as shown by a specific capacitance of 0.6 F/m2 in neutral
electrolytes. BCN also display a larger electrochemical window
ranging from −1.4 V to 1.0 V vs Ag/AgCl as shown in Fig. 3. Such
a large electrochemical window in an aqueous system has only
been demonstrated in boron doped diamond or nitrogen doped
tetrahedral carbon thin films [32–34]. The increased electrochem-
ical window is due to the overpotential that can be applied due to
the pseudocapacitive reactions. The key active sites for hydrogen
adsorption/redox reactions are polyaromatic carbon, pyridonic N,
C–O and B–C in the carbon framework. The possible reactions are
as follows:
Acknowledgements
The authors acknowledge Consortium of Premium Carbon Prod-
ucts from Coal (CPCPC) for providing the funding for this project
(Subcontract No. 3556-TPSU-DOE-1874) and Materials Research
Institute at the Pennsylvania State University for providing access
to characterization facilities.
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