Band gap modified boron doped NiO/Fe3O4 nanostructure as the positive electrode for high energy asymmetric supercapacitors

Article abstract of DOI:10.1039/c5ra20928e

A boron doped NiO/Fe₃O₄ nanostructure was successfully synthesized by a facile one-step hydrothermal method. The boron doping was confirmed from the decreased band gap and increased electrical conductivity of the NiO/Fe₃O₄ composite. The Nyquist plot of the multimetal oxide was fitted with ZView software for detailed understanding of the effect of concentration of different metal oxides, boron doping and extensive charge-discharge cycles on the electrochemical properties of electrode materials. Very high specific capacitance of ～1467 F g^-1 was achieved as the synergistic effect of low activation energy and short ion diffusion path of the electrode materials. An asymmetric supercapacitor (ASS) was fabricated with the NiO/Fe₃O₄ composite and thermally reduced graphene oxide as the positive and negative electrode, respectively. The ASS showed a large specific capacitance of ～377 F g^-1 at a current density of 3 A g^-1. Furthermore, the ASS showed a large energy density of ～102.6 W h kg^-1 and huge power density of ～6300 W kg^-1 and remained ～82% stable even after 10 000 charge-discharge cycles. Therefore, a facile hydrothermal method was demonstrated to enhance the electrochemical properties of a multimetal oxide by boron doping for the development of next generation energy storage devices.

Full text of DOI:10.1039/c5ra20928e

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Band gap modified boron doped NiO/Fe3O4 nanostructure as the
positive electrode for high energy asymmetric supercapacitor
Sanjit Saha,a,b Milan Jana,a,b Partha Khanra,c Pranab Samanta,a,b Hyeyoung Koo,d Naresh Chandra
Murmu,b and Tapas Kuila a*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The boron doped NiO/Fe3O4 nanostructure was successfully synthesized by facile one-step hydrothermal method. The
boron doping was confirmed from the decreased band gap energy and increased electrical conductivity of the NiO/Fe3O4
composite. The nyquist plot of the multi metal oxide was fitted with ZView software for the detailed understanding of the
effect of concentration of different metal oxide, boron doping and extensive charge-discharge cycles on the
electrochemical properties of the electrode materials. Very high specific capacitance of ~1467 F g-1 was achieved as the
synergistic effect of low activation energy and short ion diffusion path of the electrode materials. Asymmetric
supercapacitor (ASS) was fabricated by NiO/Fe3O4 composite and thermally reduced graphene oxide as positive and
negative electrode, respectively. The ASS showed large specific capacitance of ~377 F g-1 at a current density of 3 A g-1.
Furthermore, the ASS showed a large energy density of ~102.6 W h kg-1, huge power density of ~6300 W kg-1 and remained
~82% stable even after 10,000 charge discharge cycles. Therefore, a facile hydrothermal method was demonstrated to
enhance the electrochemical properties of the multi metal oxide by boron doping for the development of next generation
energy storage device.
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tube, reduced graphene oxide (RGO) and CVD grown graphene are
the proper choice as the negative electrode materials in
supercapacitor device due to their electrochemical double layer
capacitance (EDLC) charge storage mechanism.9,12,13 On the other
hand, conductive polymers and different transition metal
oxides/hydroxides with rich pseudo-capacitance are widely used as
positive electrode materials to enlarge the operating voltage as well
as the energy storage capacity.1,11,14
Introduction
The development of next-generation economical, flexible, light-
weight and renewable energy storage system is strongly required to
meet the future demand of the portable and flexible electronics in
modern society.1-3 In this realm, electrochemical supercapacitors
(ESs) have received considerable attention as high-performance
energy storage devices since they can provide better capacitance,
elevated power density, faster charge-discharge cycles, and long
cycle life as compared to traditional lead-acid batteries or lithium-
ion batteries.1,4,5 However, the low energy density is the major
limitation of the commercially available ESs.6-8 The use of
asymmetric configuration by utilizing both faradic and non-faradic
charge storage mechanism can be a green and non hazardous
solution to improve the operating voltage as well as the energy
density of the ESs.4,9-12 Carbonaceous materials like carbon nano
The fast redox reactions due to the surface faradic process at an
appropriate potential is the origin of the pseudocapacitance.11 The
smaller size and higher surface area of the electrode materials
effectively reduce the diffusion path of electrolyte ions, facilitating
ionic motion and improves the pseudo activity.11 The porous
materials can simultaneously adsorb and retain electrolyte ions
resulting good faradic reaction even in the high current density.15-17
However, the single-component nano-materials as electrodes show
limitations due to the low electrical conductivity, rate capability and
life stability.11,14,15 An efficient approach to increase the rate
capability of metal oxide/hydroxide positive electrode materials is
to form hetero structure by the incorporation of one or two metal
ions into their lattices.11,15,16 The presence of hetero structure
creates abundant structural defects and multiple accessible electro-
active sites enhancing the redox reaction.11,16 The nano-structured
binary Ni-Fe oxide is one of the most promising hetero structure for
supercapacitor applications due to their large theoretical
capacitance value, deep average discharging profile and rich redox
reactions.11,17,18 The inter conversion between Fe(II) and Fe(III)
provides redox capacitance for Fe3O4 in an aqueous electrolyte.18
a.Surface Engineering & Tribology Division, CSIR-Central Mechanical Engineering
Research Institute, Durgapur -713209, India
b.Academy of Scientific and Innovative Research (AcSIR), CSIR-CMERI Campus,
Durgapur-713209
c. Soft Innovative Materials Research Centre, Korea Institute of Science and
Technology (KIST), Jeonbuk 565905, South Korea
d.Soft Innovative Materials Research Centre, Institute of Advanced Composite
Materials, Korea Institute of Science and Technology (KIST), Jeonbuk 565905,
South Korea
† Correspondence to Tapas Kuila. Tel.: +91-9647205077; Fax: 91-343-2548204
E-mail address: tkuila@gmail.com (Tapas Kuila)
Electronic Supplementary Information (ESI) available: XRD, FE-SEM, TEM, UV-vis
spectroscopy, CV of NiO/Fe3O4 composite and the relation between diffusion time
constant and activation energy. See DOI: 10.1039/x0xx00000x
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NiO contributes to the pseudocapacitance based on the surface Korea. Nickel foam was purchased from Shanghai Winfay New
reversible redox mechanism17
Material Co., Ltd., China.
NiO + OH− ↔NiOOH + e−
2.2 Preparation of boron doped NiO/Fe3O4 composites and TRGO
NiO and Fe3O4 are the semiconducting materials with band gap NiSO4 (~2 g) and FeSO4 (~2 g) were dissolved in ~60 ml of distilled
energy of 3.6 - 4 eV and 1.92 - 2.87 eV, respectively.19,20 The hetero water. The solution was taken into a 100 ml teflon lined autoclave.
structure of two different metal oxides is advantageous due to the About 20 ml of ammonia solution was added and the autoclave was
size confinement of the highly enhanced electron-electron kept at 160 °C for 5 h. The product was collected by vacuum
interactions.21 The creation of defects or dislocations are highly filtration (with distilled water and ethanol) and dried inside a
expected due to the different lattice structure of the participating vacuum oven at 50 °C. The sample was designated as FN. Another
semiconductors which can create localized states by trapping the two samples, F1N2 and F2N1 were prepared by varying the
charge carriers and control the overall electrical characteristics of concentration of NiSO4 and FeSO4. About 2 g of NiSO4 and 1 g of
the system.22 Furthermore, quantum interference effect can be FeSO4 were taken for the preparation of F1N2. Similarly, about 1 g
reduced between the thin layer of the semiconducting hetero NiSO4 and 2 g FeSO4 were taken for the preparation of F2N1. F1N2B
structure allowing enhanced ion exchange at the grain boundary.22 was prepared by adding 0.5 g boric acid to the autoclave keeping
The energy separation of two different band structures can create other parameters constant (same that of F1N2). Another three
barrier and as a result the charge carriers can be confined allowing samples F1N2B1, F1N2B2 and F1N2B3 were prepared by increasing
quasi-equilibrium potential state within the electrode.12,22
the concentration of boric acid. About 1, 1.5 and 2 g of boric acid
was taken for F1N2B1, F1N2B2 and F1N2B3, respectively keeping
other parameters constant. GO was prepared from natural graphite
flake by modified Hummer’s method.12 Thermally reduced GO
(TRGO) was prepared through the thermal reduction of GO (~100
mg) at 500 °C for ~15 min (under Ar gas atmosphere).
However, the large band gap or high electrical resistivity of NiO
limits its applications. Therefore, superior electrochemical activity
needs control on the electrical properties through proper doping of
the materials.12,23-26 In addition, control doping significantly
increases the carrier concentration by creating free charge
carriers.25 Saha et al. have shown that the band gap modification of 2.3 Structural and morphological characterization
the insulating boron nitride is highly effective for electrochemical or X-ray diffraction (XRD) studies of the composite were carried
supercapacitor applications.12 Alver et al. have explained the effect out at room temperature on a D/Max 2500 V/PC (Rigaku
of boron doping on the band structure and electrical conductivity of Corporation, Tokyo, Japan) at a scan rate of 1° min-1 (Cu Kα
the NiO.25 Chi et al. have shown that the supercapacitor radiation, λ = 0.15418 nm). The morphology of the NiO/Fe3O4
performance of MnO2 increases after boron doping.26
was measured by field emission scanning electron microscopy
(FE-SEM) using Ʃigma HD, Carl Zeiss, Germany and
transmission electron microscopy (TEM) was carried out using
a JEOLTEM 2100 FS instrument (Japan) at 200 kV. X-ray
photoelectron spectroscopy (XPS) was carried out by using a K-
alpha X-ray photoelectron spectrometer, Thermal scientific
TM.
On the basis of the above considerations, one-step hydrothermal
synthesis of boron doped NiO/Fe3O4 nano composite has been
demonstrated. Nickel sulphate, Iron sulphate and boric acid were
used as the source of nickel, iron and boron, respectively. The
electrical conductivity of the NiO/Fe3O4 nanocomposite increases
significantly due to the boron doping. The variation in the
morphology of the composite can be observed with different ratio
2.4 Optical, Electrical And Electrochemical Properties
UV-vis spectroscopy was measured by Agilent Cary 60
of nickel sulphate and iron sulphate precursor. Furthermore, the spectrophotometer. Band gap was calculated according to the
n
supercapacitive performance varied with the morphology of the
nano structure and better results was obtained after boron doping.
The asymmetric supercapacitor (ASS) cell was designed by using
NiO/Fe3O4 composite as positive and thermally reduced graphene
oxide (TRGO) as the negative electrode materials
Tauc relationship:
,
where
α
is the
α
h
ν
= B
(
h
ν
− Εg
)
absorption coefficient, hν is the energy of photon, B is a
proportionality constant, Eg represents the optical band gap,
and n is the parameter of specific electronic transition within
the band due to the light absorption (n = 1/2, 2 for direct or
indirect transition respectively).12,19,27
The electrical conductivity (σ) was measured using a four
probe set up with a KEITHLEY delta system consisting of an AC
and DC current source (model: 6221) and a Nanovoltmeter
Experimental
2.1 Reagents
1
1
(model: 2182A) following the formula:
,
σ
=
=
ρ
4.532× R× d
Natural graphite flakes were purchased from Sigma-Aldrich.
Hydrochloric acid, sulphuric acid, potassium permanganate,
hydrogen peroxide, nickel sulphate, iron sulphate, boric acid,
where, ρ is the resistivity of the material. R and d are the
resistance and thickness of the prepared pellet.12,18,28 The high
and low current ranges during the measurement were
ammonia solution, N, N-dimethyl formamide (DMF) and potassium recorded in the range of 20×10-6 to -20×10-6 A, and the delta
voltage step was fixed at 100 mV s-1 for all the samples.
hydroxide were purchased from Merck, Mumbai, India. Conducting
The electrochemical properties were recorded with a PARSTAT
4000 (Princeton Applied Research, USA) electrochemical
carbon black (EC-600JD, purity: >95%) and polyvinylidene fluoride
(PVDF) were purchased from Akzo Nobel Amides Co., Ltd, South
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workstation using 1 M Na2SO4 as the electrolyte. Specific
capacitance was calculated from the CV curves using the
The cactus like structure started to distort with increasing the
concentration of Fe3O4 (FN) and completely destroyed for
F2N1. Figure S2(a-c) of the supporting information represent
the high magnifying FE-SEM images of F1N2, FN and F2N1. The
Fe3O4 particles are totally embedded onto the NiO structure
equation:
, where CCV is the specific
CCV
=
(
ΙdV
)
mv
∫
capacitance (F g-1 ), I is the response current density (A g-2 ), V
is the potential window, υ is the scan rate (mV s−1 ), and m is
the deposited mass on the electrode.12,29-31 The specific for F1N2. The increase in particle size and quite
capacitance can be calculated in terms of the discharge time
Ιꢀt
specific capacitance (F g-1 ), I is the discharging current (A g-1 ),
V is the potential window (V) and m is the deposited mass on
the electrode.12,30,31 The mass balance of the electrodes for
asymmetric configuration, was done according to the relation:
inhomogeneous particle size distribution of NiO and Fe3O4 can
was noticed in FN. The arbitrarily distributed Fe3O4 and NiO
particles of F2N1 can be seen from the Figure 1c.
according to the equation:
, where CCD is the
CCD
=
mV
m+ C− × ꢀΕ−
m− C+ × ꢀΕ+
, where m+ and m- are the mass of the positive
=
and negative electrodes, C+ and ΔE+ are the specific
capacitance and potential window (calculated from the three
electrode measurement) for positive electrode, C- and ΔE- are
the specific capacitance and potential window of the negative
electrode measured at the same scan rate.12,32,33 The energy
density (ED) and power density (PD) can be calculated
CV 2
E
according to the formula,
and
, Where
D ꢀΤ
C is the specific capacitance, V is the operating voltage, ΔT is
ED
=
PD =
2
the discharge time.12,33,34
Results and discussion
The formation of NiO and Fe3O4 can be explained by the
following reaction mechanism:
-2
NiSO4 → Ni+2 + SO4
+
Ni+2 + 2NH3 + 2H2O ↔ Ni(OH)2 + 2NH4
Figure 1. Low resolution FE-SEM image of (a) F1N2, (b) FN and
(c) F2N1.
Ni(OH)2 → NiO + H2O
-2
FeSO4 → Fe+2 + SO4
Fe+2 + 3NH3 + 3H2O ↔ Fe(OH)3 + 3NH4
+
Figure 2a shows the comparison of three electrode CV of
F1N2, FN and F2N1. The increase in the current response was
noticed with increasing concentration of NiO. The maximum
specific capacitance of F2N1, FN and F1N2 are calculated as
500, 583 and 666 F g-1, respectively. The increase in specific
capacitance with increasing NiO has been explained by the
Nyquist plot analysis. Figure 2b shows the Nyquist plots of
F1N2, FN and F2N1. The solution resistance of the composites
were comparatively large. The low electrical conductivity of
metal oxides may be the reason of such high solution
resistance. The slight increase in solution resistance was
noticed with the increase of NiO concentration in the
composite. The inclination of the Warburg region towards the
imaginary axis represented good electrolyte accessibility due
to the porous materials or fascinating diffusion path. The FE-
SEM images showed that the particle size of NiO/Fe3O4
composites decreased with increasing NiO concentration. It is
expected that the surface area of the composite increased
with decreasing particle size. The Nyquist analysis also showed
steeper Warburg region for the composites with higher
content of NiO.18,35 The slope of the Warburg region was very
low for the F2N indicating poor porosity as well as low
capacitance value. The supercapacitor performance of solo
NiO and Fe3O4 were tested and compared with the NiO/Fe3O4
Fe(OH)3 → Fe3O4 + H2O
Nickel hydroxide and iron hydroxides were formed in presence
of NiSO4, FeSO4 and aqueous ammonia solution. However, the
hydrothermal process involves high temperature and pressure
results in the formation of multi metal oxides of iron and
nickel. The hydrothermal reaction was carried out only for 5 h
to avoid the formation of spinel phase rather than the single
phase metal oxide composite. Extended duration of
hydrothermal reaction or high temperature annealing can
result in the formation of spinel phase. Figure S1 of the
supporting information represents the XRD pattern of F1N2,
FN and F2N1. The peaks corresponding to both cubic NiO and
Fe3O4 are present in all the three composites.17-20 The relative
intensity of the NiO peaks as compared to Fe3O4 increased
with increasing the concentration of NiSO4 in the precursor.
The amount of NiO in comparison to Fe3O4 increased from
F2N1 to FN and F1N2. The FE-SEM image analysis was carried
out for the detailed morphological study of the multi metal
composites. The FE-SEM images show that the concentration
of nickel oxide plays an important role on the morphology of
the composite. Figure 1(a-c) represent the low resolution FE-
SEM images of F1N2, FN and F2N1. The composite has cactus
like porous structure with higher concentration of NiO (F1N2).
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composite as shown in Fig. S3 of the supporting information. UV-visible absorption spectroscopy of F1N2 and boron doped
The current responses of the solo metal oxides were poor as composites was carried out to visualize the effect of boron on
compared to the NiO/Fe3O4 composite. The specific the electronic property of the multi metal oxide (Figure S6 of
capacitance was calculated at a scan rate of 10 mV s-1 and the supporting information). The significant increase in absorbance
results showed prominent increase in specific capacitance for intensity noticed with the insertion of boron (for F1N2B1). The
the composite (583 F g-1). The specific capacitances of solo absorbance increased for F1N2B2 over F1N2B1 but the
NiO, and Fe3O4 were found to be 143 and 294 F g-1, intensity decreased with further doping. The (αhν)2 vs. hν plot
respectively.
of F1N2 and the boron doped composites is shown in Figure
3a. The band gap can be directly calculated from the
intersection of the slope to the X-axis.12,19,27 F1N2 showed a
wide band gap of 3.93 eV. Boron doping significantly reduced
the band gap of the multi metal oxide. The calculated band
gap of F1N2B1 and F1N2B2 were 3.63 and 3.31 eV,
respectively. The band gap was increased further in F1N2B3
(3.41 eV) due to B doping into the lattices. The variation of the
optical band gap values of multi metal composites can be
attributed to the creation of line and/or planar defects due to
the boron doping in the crystal structure.23,25 The doping of
boron produced free electrons in the conduction band of the
hetero structure multi metal oxide resulting the decrease in
band gap energy. The increase in band gap energy ascribed to
the excess doping over the optimum level with the higher
concentration of boron. The Fermi level energy increased with
increasing the doping concentration of electrons resulting the
shifting of absorption edge to the higher energies.36 This
Figure 2. (a) CV and (b) EIS of F1N1, FN and F2N1.
36
phenomenon is called Burstein–Moss effect. The electrical
conductivity increased abruptly due to the boron doping. The
variation of electrical conductivity of the F1N2 and boron
doped composites are shown in Figure 3b. The un-doped F1N2
showed very low electrical conductivity of 0.52 S m-1 at room
temperature. The electrical conductivity of the multi metal
oxide was enhanced by ~50 times with B doping. F1N2B1 and
F1N2B2 displayed the electrical conductivity of 25.8 and 43.1 S
m-1 at room temperature. The decrease in electrical
conductivity (37.1 S m-1) noticed for F1N2B3 after further
doping. The decrease in electrical conductivity of F1N2B3 may
be attributed to the poor carrier mobility at very high
concentration of boron.25 The electrical conductivity increased
with temperature for all the composites indicating pure
Figure 3.(a) (αhν)2 vs hν plot, (b) variation of electrical semiconducting nature. Figure 3c shows the linear fitted plot
conductivity with temperature and (c) Arrhenius plot of F1N2, of ln(σ) vs. 1/T. The activation energy was calculated from the
slope of the linear fit.12,37,38 The activation energy of F2N1,
F1N2, F1N2B1 and F1N2B2 were 0.29, 0.279, 0.232 and 0.255
F1N2B1, F1N2B2 and F1N2B3.
Figure S4 of the supporting information shows the CV plots of eV, respectively. The Nyquist analysis was carried out for the
F1N2, FN and F2N1, respectively. The current response of CV proper understanding of the effect of surface morphology,
increased for all the three composites. The retention of the boron doping and increased electrical conductivity on the
specific capacitance of the composites has been compared in electrochemical properties of the multi metal oxides.
Figure S5 of the supporting information. The retention is very The Nyquist result was fitted with ZView software to find out
low for F2N1 and increased with NiO concentration. The good the effect of boron doping on the diffusion path and the
capacitance value may be attributed to the synergistic effect of diffusion resistance. A perfect semicircle of Nyquist plot is
faradic reaction from NiO and Fe3O4. However, the capacitance resulted for an electrode involving only EDLC
value was quite low as compared to the theoretical value and mechanism.12,18,39 Warburg region is resulted from the pseudo
the retention is only about ~ 52% at a scan rate of 200 mV s-1. reaction.12,18,37 Figure 4 shows the Z-View fitted results of
On the basis of the above discussion F1N2 was the best F2N1, F1N2, F1N2B1 and F1N2B2. The Warburg impedance
composite for supercapacitor application and the (W) represents the route of ion diffusion between the
electrical/electrochemical properties of F1N2 was further electrode and electrolyte interfaces. W contains three parts:
improved by boron doping.
Ohmic resistance (W-R), diffusion time constant (W-T), and
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Warburg exponent (W-P).34,40-42 The origin of the solution doping. The formation of free electron elevated the diffusion
resistance is the electrode and electrolyte contact and it of charge carrier in the multi metal oxide. The low W-R value
depends on the electrical conductivity of the electrode of F1N2B1 as compared to the F1N2 also supported the
materials.12,34 Table 1 shows the ZView fitted result of F2N1, formation of shorter ion diffusion path.18,35 The low Warburg
F1N2, F1N2B1 and F1N2B2. Both F2N1 and F1N2 were the resistance and diffusion time constant are possibly attributed
undoped composite and no significant variation of solution to the decrease in activation energy of the multi metal oxide.
and faradic resistance were noticed. The solution resistance The Arrhenius equation plays a governing role on the chemical
decreased significantly with the insertion of boron. The kinetics.38 The diffusion rate is related to the activation energy
solution resistance of F2N1, F1N2, F1N2B1 and F1N2B2 were and the lattice parameter of the materials. The correlation
9.6, 9.4, 5.2 and 1.4 Ω, respectively. The decrease in solution between the activation energy and the electrochemical
resistance may be attributed to the generation of free electron diffusion has been explained elaborately in the supporting
and increased electrical conductivity of the boron doped information. The decrease in activation energy of the multi
composites. The faradic resistance also decreased after boron metal composite after boron doping indicated the enhanced
doping. The diffusion path as well as the diffusion time is an ion diffusion through electrode materialds.38 Furthermore, the
important parameter for the electrode material for enriched Warburg resistance arises from the ionic blocking at the
supercapacitor performance.39
interface of the grain boundary of the electrode materials.38
The simultaneous decrease in activation energy and W-R
suggested enhanced ionic exchange at the interface of the
metal oxide grain boundary as the synergistic effect of
increasing concentration of NiO and boron doping. The FE-SEM
and TEM images of F1N2B2 show that the morphology of the
composite remained undistorted after boron doping (Figure S
7a & b of the supporting information). The extensive increase
in current response of the multi metal oxide was noticed after
the modification of the band gap by boron doping (Figure 5a).
The specific capacitance of F1N2, F1N2B1 and F1N2B2 at 10
mV s-1 scan rate was measured as 1184, 1467 and 1293 F g-1,
respectively. The successive increase and decrease in specific
capacitance with different concentration of boron doping
shows consistency with the band gap and electrical
conductivity analysis.
Figure 4. ZView fitted Nyquist plots of (a) F2N1, (b) F1N2, (c)
F1N2B1 and (d) F1N2B2. (e) Corresponding equivalent circuit.
Table 1. ZView fitted result of F2N1, F1N2, F1N2B1 and
F1N2B2
Rs
RF
3.9
W-R
55.9
18.9
5.7
W-T
F2N1
F1N2
9.6
9.4
5.2
1.4
0.0068
0.0048
0.0037
0.0022
3.9
F1N2B1
F1N2B2
0.3
0.29
1.9
The diffusion current constant is directly related to the
diffusion path.39 The diffusion current constant of F2N1 and
F1N2 are 0.0068 and 0.0048, respectively. The decrease in
diffusion path may be attributed to the synergistic effect of
high surface area and better incorporation of NiO and Fe3O4
particles in F1N2. The diffusion current constant further
decreased after boron doping. The low diffusion time constant
of F1N2B1 as compared to the F1N2 may be attributed to the
creation of huge defect sights and free electrons after boron
Figure 5. (a) CV plot of F1N2, F1N2B1, F1N2B2 and F1N2B3. (b)
XPS survey plot of F1N2B2 and (c) B1s spectrum of F1N2B2.
The CD plot shows the F1N2B2 has IR drop from 0.4 to 0.3 V
(Figure S8a of the supporting information). The maximum
specific capacitance of 1305 F g-1 was achieved at a current
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density of 3.5 A g-1. F1N2B2 showed ~ 70% retention in specific the higher scan rates. The CD was carried out with different
capacitance at a high current density of 8 A g-1 (Figure S8b of current density from 3 to 9 A g-1 and the discharging profile of
the supporting information). Figure 5b shows the XPS survey the CD shows almost zero IR drop. The maximum specific
spectra of F1N2B2. The peaks at 711, 724.5, 855.6 and 873.1 capacitance calculated from the CV was 390 F g-1 at a scan rate
eV were attributed to the Fe 2p3/2, Fe 2p1/2, Ni 2p3/2 and Ni of 10 mV s-1 and the ASS showed good capacitance retention
2p1/2, respectively.18,43 The Ni 2p3/2 peak position was of ~ 60 % at a very high scan rate of 200 mV s-1 (Figure S9a of
different as compared to the metallic Ni (852.7 eV). The energy supporting information). The capacitive performance of ASS
difference between Ni 2p3/2 and Ni 2p1/2 core levels (~17.5 was studied from the discharging time as shown in Figure S9b
eV) was also differed from that of the metallic Ni (17.27 eV).43 of supporting information. The ASS exhibited a specific
The positive shift and the increase in the core level energy capacitance of 377 and 157 F g-1 at a current density of 3 and 9
difference may be attributed to the +2 valance state of Ni in A g-1, respectively.
NiO.43 The B1s peak appeared at 187.3 eV. The positive shift of
the elemental B1s peak as compared to the pure B (187.1 eV)
indicates that boron donated its partial electron to the alloying
metals.44 Figure 5c shows the high resolution B1s spectra of
F1N2B2. The B1s spectra were deconvoluted in two separate
peaks. The peaks at 187.2 and 189.1 eV may be attributed to
B-Ni and B-Fe respectively.45,46 The most intense peak of B-Ni
indicates the Bond formation between B and Ni was large as
compare to the B-Fe. The B doping ratio was estimated from
the XPS peak table analysis. The atomic percentage of boron
was calculated as 6.13, 9.71 and 10.6 % for F1N2B1, F1N2B2
and F1N2B3, respectively.
Figure 7. (a) Ragone plot and (b) stability plot of ASS.
Figure 7a shows the Ragone plot of the ASS cell. The large
potential window and high specific capacitance value of ASS
cell provided a very high energy density of 102.6 W h kg-1.
Furthermore, the ASS was able to deliver a huge power density
of ~6300 W kg-1. The stability of the ASS cell was tested up to
10,000 cycles. The initial increase in specific capacitance from
500 to 1500 cycles (108 %) may be attributed to the wetting of
the electrolyte and enhanced electrode electrolyte contact.
The ASS was 100 % stable up to 7000 cycles. The stability
decreased to ~82% after 9000 CD cycles and remained
constant over 10,000 cycles. The degradation of the electrolyte
may be the reason of the deteriorated capacitance.
Figure 6. (a) Compare three electrode CV of F1N2B2 and TRGO,
(b) CV of ASS, (c) CV of ASS at different scan rate and (d) CD of
ASS at different current density.
The specific capacitance of the multi metal oxide composite
increased appreciably after boron doping, but still suffered
from IR drop of ~0.1 V and small potential window. The
asymmetric cell was designed to overcome these limitations.
TRGO was used as the negative electrode and the mass
deposition ratio of TRGO and F1N2B2 was 1: 52. The CV of Figure 8. ZView fitted nyquist plot of ASS after (a) 100, (b) 1500
TRGO, FIN2B2 and ASS has been compared in Figure 6 (a&b). and (c) 10,000 CD cycles.
The CV of ASS at different scan rate is shown in Figure 6c. The
CV nature was nearly rectangular and remained undistorted at
6 | J. Name., 2012, 00, 1-3
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Table 2. ZView fitted result of ASS@100 cycle, ASS@1500 cycle the large energy density of 102.6 W h kg-1 and huge power
and ASS@10000 cycle
Rs
density of ~6300 W kg-1 ensured the boron doped NiO/Fe3O4
as the potential electrode material for new generation energy
storage device.
RF
W-R
9.9
W-T
ASS@100 Cycle
1.2
1.1
5.5
0.2
0.019
ASS@1500
Cycle
0.2
0.5
7.5
9.9
0.017
0.039
Acknowledgements
ASS@10000
Cycle
Authors (a&b) are thankful to the Director of CSIR-CMERI. Authors
are also thankful to the Department of Science and Technology,
New Delhi, India, for the financial support from the DST-INSPIRE
Faculty Scheme - INSPIRE Programme (IFA12CH-47) and Council of
Scientific and Industrial Research, New Delhi, India, for funding
MEGA Institutional project (ESC0112/RP-II). Authors (c&d) would
also like to acknowledge the financial support from the R&D
Convergence Program of MSIP (Ministry of Science, ICT and Future
Planning) of Republic of Korea (Grant CAP-13-2-ETRI).
The Nyquist results of ASS after 100, 1500 and 10,000 CD
cycles were fitted with Z-View for the detailed understanding
of the variation of capacitance during stability test (Figure 8).
The Z-View fitted results were summarized in Table 2. The ASS
showed very low solution resistance of ~1.2 ohm and almost
zero faradic resistance, during the initial cycles. These
parameters remained unchanged after 1500 cycles. The
increased specific capacitance of ASS after 1500 cycles was
attributed to the improve Warburg impedance. The wetting of
the electrode materials decreased the diffusion time constant
and Warburg resistance allowing good ion exchange between
the electrode materials and electrolyte. The solution and
faradic resistance increased after 10,000 cycles possibly due to
the degradation of the electrolyte. The increase in the
diffusion time constant indicated the relatively poor ion
exchange between the electrode materials and electrolyte
after 10,000 cycles.
Notes and references
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Boron doped NiO/Fe3O4 was successfully synthesized by simple
one-step hydrothermal method. The increasing concentration
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