Highly oxygen deficient, bimodal mesoporous silica based supercapacitor with enhanced charge storage characteristics

Article abstract of DOI:10.1016/j.electacta.2018.12.033

The present study elaborates facile approach to generate active sites in cashew nut shaped silica (SiO₂). These active sites are attributed to the high concentration of oxygen vacancies and bimodal mesoporosity in silica owing to etching and calcination treatment. In the etched calcined silica (ECS), mesopores act as buffered spaces, whereas, OVs provide high carrier/donor density (3 ×10²⁴cm^?3). High density of carriers/donor reduces the distance between active sites (2.5 nm) further enhancing the rate of electron transfer. Consequent to the unique combination of OVs and bimodal mesoporosity, ECS exhibits high electrochemically accessible surface area (3170 m² g^?1) and excellent charge storage in ECS||ECS cell (～337 F g^?1 at 1 A g^?1). In addition, the symmetric cell (ECS||ECS) delivers maximum energy density of 46.86 Wh Kg^?1 at power density of 537.59 W kg^?1 with respectable capacitance retention (111% after 10,000 cycles). Remarkably, the solid state flexible device unveiled energy density of 2.16 Wh Kg^?1 at 166.05 W kg^?1 even under the bent state retaining 165% of its capacitance up till 3000 cycles. This work essentially highlights the synergism between mesoporosity and oxygen vacancies on the charge storage characteristics of silica.

Full text of DOI:10.1016/j.electacta.2018.12.033

Accepted Manuscript
Highly oxygen deficient, bimodal mesoporous silica based supercapacitor with
enhanced charge storage characteristics
Akanksha Joshi, Shubra Lalwani, Gurmeet Singh, Raj Kishore Sharma
PII:
S0013-4686(18)32731-2
https://doi.org/10.1016/j.electacta.2018.12.033
EA 33248
DOI:
Reference:
To appear in: Electrochimica Acta
Received Date: 12 August 2018
Revised Date: 13 November 2018
Accepted Date: 4 December 2018
Please cite this article as: A. Joshi, S. Lalwani, G. Singh, R.K. Sharma, Highly oxygen deficient, bimodal
mesoporous silica based supercapacitor with enhanced charge storage characteristics, Electrochimica
Acta (2019), doi: https://doi.org/10.1016/j.electacta.2018.12.033.
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ACCEPTED MANUSCRIPT
Highly Oxygen Deficient, Bimodal Mesoporous Silica based Supercapacitor
with enhanced Charge Storage Characteristics
Akanksha Joshi, Shubra Lalwani, Gurmeet Singh* and Raj Kishore Sharma**
Department of Chemistry, University of Delhi, Delhi-110007, INDIA
Abstract
The present study elaborates facile approach to generate active sites in cashew nut shaped
silica (SiO2). These active sites are attributed to the high concentration of oxygen vacancies
and bimodal mesoporosity in silica owing to etching and calcination treatment. In the etched
calcined silica (ECS), mesopores act as buffered spaces, whereas, OVs provide high
carrier/donor density (3× 10ꢀꢁcm-3). High density of carriers/donor reduces the distance
between active sites (2.5 nm) further enhancing the rate of electron transfer. Consequent to
the unique combination of OVs and bimodal mesoporosity, ECS exhibits high
electrochemically accessible surface area (3170 m2 g-1) and excellent charge storage in
ECS||ECS cell (~337 F g-1 at 1 A g-1). In addition, the symmetric cell (ECS||ECS) delivers
maximum energy density of 46.86 Wh Kg-1 at power density of 537.59 W Kg-1 with
respectable capacitance retention (111 % after 10,000 cycles). Remarkably, the solid state
flexible device unveiled energy density of 2.16 Wh Kg-1 at 166.05 W Kg-1 even under the
bent state retaining 165 % of its capacitance up till 3000 cycles. This work essentially
highlights the synergism between mesoporosity and oxygen vacancies on the charge storage
characteristics of silica.
Abbreviations: as-synthesized silica (AS), etched silica (ES), etched calcined silica (ECS),
oxygen vacancies (OVs)
Keywords:
Cashew nut shaped silica, Oxygen Vacancies, Bimodal mesoporous,
Supercapacitor, Carrier/donor density
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**Corresponding author: Raj K. Sharma (drrajksharma@yahoo.co.in)
*Co-corresponding author: Gurmeet Singh (gurmeet123@yahoo.co.in)
Tel. No.:+91-11-27666616
Research highlights
•
•
•
High carrier/donor density (3× 10ꢀꢁcm-3) in oxygen deficient cashew nut shape silica.
Minimal distance (2.5 nm) between active sites ameliorates the current density.
Bimodal mesoporosity and OVs endows cashew nut shape silica with high specific
capacitance of 337 F g-1 at 1 A g -1.
•
Flexible solid state device exhibits good capacitive behaviour even in deformed state.
1. Introduction
The alarming issues of environment [1,2] and clean energy demand have triggered
researchers towards designing advanced green energy storage devices. Supercapacitors (SCs),
a new class of energy storage devices, meet the demands of advanced applications requiring
high power delivery [3]. SCs have been considered as eco-friendly energy storage devices
with commendable features like high power density, fast charge–discharge characteristics,
peak operating voltage, long cycle life and reliability [4–7].
In order to realize good performance, there should be a balance between specific surface
area and conductivity of the material. Use of porous/large surface area materials is one of the
key approach for shortening the diffusion pathway of electrolytic ions. To enhance
charge/discharge rate, several conductive agents are incorporated into electrode material
[8,9]. However, effective volume density of the electro-active material decreases by addition
of conductive agents [10]. Thus, to boost the conductivity as well as porosity, modification in
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composition and structure of the electrode material is a wise strategy. Silicon, a
semiconducting material [11] has recently gathered attention in the area of energy storage due
to its high abundance in earth crust, easy availability, minimal environmental impact, high
thermal stability [12] and unique electronic and chemical properties. Porous silica (derivative
of silicon) offers desirable properties such as high surface area, low toxicity [13], high
differential capacitance (upto 180 µF cm-2) [14], hydrophilicity [15], and excellent chemical
stability [16] enduring harsh environments like high temperature and strong acids/bases.
Although silica has been explored in Li ion batteries [17–19], but the studies on utilization of
silicon [20–23] and porous silica [24–28] as an electrode in supercapacitors are lacking. A
vast array of morphologies [29–32] and the presence of large number of hydrophilic surface
groups (e.g. OH) [33,34] make silica appealing as an energy storage material. In earlier
studies, improvement in capacitance of carbon electrodes using SiO2 nanoparticle [35] as
fillers has also been reported by Leonard et al. Other reports in continuation endorse the role
of silica particles in polyaniline matrix in achieving better cycling stability [36]. Even though
there are numerous advantages associated with silica, yet low electronic conductivity [37] i.e.
its insulating nature (wide band gap) limits its application as a capacitive electrode. Hence, it
is extremely desirable to improve the conductivity of silica electrode. To eliminate this issue,
we have introduced intrinsic defects into the structure of silica. Electrochemical and
electronic properties of metal oxides are strongly influenced by the intrinsic defects [38,39].
Introduction of oxygen vacancies (OVs), one of the intrinsic defects is considered as an n–
type dopant. These dopants improve the kinetics of charge storage and conductivity of the
material by shifting Fermi level close to the conduction band [40]. Since, the formation
energy of oxygen vacancy is small, it can be easily introduced as anion vacancy into the
metal oxide [41].
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Different from previous studies, here silica is synthesized in cashew nut shaped morphology.
This morphology provides more active edge sites for the participation in redox reactions. To
further obtain the porosity in ionic accessible range, etching is carried out. Generally, etching
deals at high temperature using corrosive and hazardous etchants (e.g. HF) [42]. We have
adopted etching based on the spontaneous dissolution and regrowth of silica using NaBH4
(etching agent) and PVP (surface protecting agent) [43]. The abovementioned process does
not require harsh conditions and can be performed in the absence of inert atmosphere, thus
turning out to be completely facile, green and economical. Interestingly oxygen vacancies
(OVs) with the aid of NaBH4 were introduced on the surface accompanied large size
mesopores. Mesoporous structure helps in providing a large accessible surface area for ion
transport/charge storage and also act as a buffering reservoirs [44–46] whereas oxygen
vacancies (OVs) build the localized states below the conduction band so that valence band
can easily overlap with the conduction band [47]. This in turn improves the conductivity of
silica with enhancement in the active sites for redox reactions. Further, calcination treatment
enriched the silica with small size mesopores. Thus, the final product exhibits bimodal
mesoporosity with ample amount of oxygen vacant sites. Bimodal porosity is shown to have
significant role in improving the ion-transport kinetics [48] as it increases the
electrochemically active surface area of the silica by nearly four times than the BET surface
area. In the present study, we have elucidated the synergistic effect of bimodal porosity and
oxygen vacancy on the electrochemical performance of silica electrodes. Oxygen deficiency
in silica results in a huge improvement in number of carrier/donor densities that in turn
increases the rate of electron transfer. The deduction of oxygen from the SiO2 structure
generates enormous free electrons that direct to the transition in behaviour from insulating to
semiconductive [49]. This also lessen the potential barrier exist in between Fermi level of
silica electrode and redox potential of electrolyte [50]. The oxygen deficient mesoporous
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cashew nut shaped silica electrodes acquires maximum energy density of 46.86 Wh Kg-1 at
power density of 537.59 W Kg-1. The mesoporosity in the structure helps to attain excellent
cycling stability upto 10000 cycles by acting as buffered spaces. For portable application, a
solid state and flexible device is much needed. Here, we have fabricated flexible solid state
device to meet the growing demands of wearable and miniaturized electronic devices. The
assembled flexible solid state device shows a very little change in the capacitive behaviour
under normal to bend conditions and exhibiting energy density of 2.16 Wh Kg-1 at 166.05 W
Kg-1. The uniqueness of our work is the utilization of facile and economical method in
development of silica as a charge storage material by using synergism of mesoporosity and
oxygen deficiency. The above characteristics motivate to explore silica as an alternative to
carbon and metal oxide in the development of flexible, light weight and environment friendly
supercapacitor. To the best of our knowledge, no such work based on the Oxygen deficient
and bimodal mesoporous silica as a charge storage material with such high electrochemical
performance has been reported so far.
2. Experimental section
2.1.
Synthesis of SiO2
SiO2 was prepared by a surfactant assisted sol-gel method in a Stober’s solution. The
molar ratio of the TEOS (tetra ethyl orthosilicate): CTAB (cetyl trimethyl ammonium
bromide): NH3 (aq. ammonia): C2H5OH (ethyl alcohol): H2O reaction mixture was 1.0:
0.38: 4.8: 39.1: 514.33. Initially, the mixture of TEOS, ethanol, CTAB and water was
stirred for 3 h followed by the addition of ammonia. Afterwards, it was continuously
stirred and heated to 70 ºC for 9 h. As obtained white product was centrifuged at 5000
rpm for 10 min and washed several times with ethanol and water. This solid product is
named as as-synthesized silica (AS).
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2.2.
Synthesis of Etched SiO2
Firstly, AS (0.3 g) is dispersed in 150 ml water and then it was further ultrasonically
dispersed with 0.29 g of PVP (polyvinylpyrollidone) in 15 ml of water. To this solution,
0.6g etchant NaBH4 (sodium borohydride) was added under vigorous stirring at 56 °C
for 5 hrs. After etching, product etched silica (ES) was washed using centrifugation and
was further calcined in air at 550 °C for 5 h for effective removal of CTAB [51]. The
resultant product is named as etched calcined silica (ECS). The overall procedure is
schematically shown in Fig. 1.
Fig. 1. Schematic showing the procedure involved in the synthesis of etched-calcined silica
(ECS).
2.3. Characterisation
Structural framework of the powder sample was analysed using a BRUKER, TENSOR
27 FTIR spectrometer. X-ray diffraction pattern was recorded (2 to 80º) using XRD
BRUKER D8 advance with Cu kα 1.54 Å . Raman spectra was measured on RENISHAW
INVIA REFLEX MICRO-RAMAN spectrometer. The surface composition was
estimated by Thermo Gravimetric Analysis (LINSEIS for air and PERKIN-ELMER for
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nitrogen) and X-ray photoelectron spectroscopy (XPS, AXIS ULTRA 165 with C 1s
peak at 284.8 used as standard). The overall composition was evaluated by EDAX
(Energy Dispersive Spectroscopy). The band gap was determined using Tauc plot via
optical (UV-Vis, UV-2600 SHIMADZU) spectrophotometer. Conductivity measurement
was carried out at Keithley 4200-SCS at room temperature. Surface area was examined
by BET technique (Quantachrome ASiQwin). High resolution transmission electron
microscopy (HRTEM, Phillips Technai T-300 microscope) and field-emission scanning
electron microscope (FESEM, ZEISS GEMINI) were employed to study the
morphological feature of the samples. Cyclic Voltammetry (CV) at different scan rates
and Electrochemical Impedance spectroscopy (EIS) was carried out in the frequency
range of 10 mHz-100 kHz by CHI 604D electrochemical analyzer. Galvanostatic
Charge-Discharge (GCD) and cycling measurements were recorded using Potentiostat
Galvanostat EIS Analyzer PARSTAT 4000.
2.4. Cell Fabrication
3.0 mg of ECS (20 % acetylene black) was ultrasonically dispersed with 3 µL of Nafion
binder in isopropyl alcohol. This mixture was deposited onto 1.0 cm2 area of the polished
graphite sheet and was dried overnight at 80 ºC. Mass loading was found to be 0.7 mg
cm-2. ECS||ECS cell was fabricated by sandwiching an ion conducting separator
membrane between two electrodes.
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3. Results and discussion
Fig. 2. Structure and compositional analysis of silica: (a) Raman spectra of AS and ECS. (b)
TGA analysis of ECS in air and nitrogen. (c) XPS spectra of Si 2p (ECS). (d) XPS
spectra of O 1s (ECS).
Fig. 2a shows the characteristic Raman peaks of silica [52–54]. Raman spectra of the ECS
shows broadened peaks as compared to the AS. It is known that phonon confinement effect
(if crystallite grain size < 10 nm) and change in oxygen stoichiometry are responsible for the
peak broadening and red shift of Raman spectra [55]. Here, the possibility of phonon
confinement is ruled out as the size of grain is larger than 10 nm (supported by TEM results
(Fig. S1)). Therefore, the above observation is directing towards the existence of oxygen
vacancies (OVs). In order to ensure the oxygen vacancies (OVs), TGA in Fig. 2b is analysed.
ECS thermograms in air and Nitrogen ambient confirm the presence of oxygen vacancies. It
is observed that there is a less weight loss in air as compared to nitrogen. Approximately
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17.25 % oxygen vacancy is concluded from TGA analysis that warrants the change in
composition from SiO2 to SiO1.65. Si 2p XPS spectra (Fig. 2c) of ECS with peak at about
104.4 eV also suggest the change in composition of SiO2 [15].The O 1s (Fig. 2d) binding
energies 532.93 eV, 534.04 eV and 537.56 represent O2-, OH- and chemisorbed water
respectively [56]. The calculated surface composition of the ECS from XPS is SiO1.69
changes from SiO2.19 of AS (Table. S1). Both TGA and XPS results are close to each other
and hence confirming the compositional change in silica. Further amorphous nature and
structural framework of silica is also confirmed by FTIR and XRD analysis (Fig. S2a and b).
FESEM micrographs in (Fig. 3a, b and c) depict cashew nut shaped morphology of
as-synthesized silica (AS), etched silica (ES) and etched-calcined silica (ECS) respectively.
Fig. 3. FESEM micrographs of silica: (a) as-synthesized silica (AS). (b & d) etched silica
(ES). (c & e) etched-calcined silica (ECS).
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Fig. 3d, high resolution FESEM micrograph of ES illustrates the surface roughness on
the cashew nut shaped structure after etching treatment. During etching process, reduction
occurs and creates oxygen vacancies (OVs) that perturb the arrangement of Si-O units on the
surface [57] leading to a defected surface. After calcination, some small pores are also
generated (Fig. 3e) which results in an overall bimodal mesoporous structure with oxygen
vacant sites at the surface of ECS. This defect induced surface with abundant oxygen
vacancies (OVs) behaves as a quantum well in which the electron hopping take place [58].
TEM micrographs (Fig. 4a) of AS depicts that the SiO2 particles as seen in FESEM
are ~ 2ꢀm in length. The high resolution TEM micrograph of AS in Fig. 4b suggests that
there is a negligible/inaccessible porosity. After etching with NaBH4, the TEM micrograph of
ES (Fig. 4c) reveals rough surface owing to the regrowth of silica. Zhang et al. 2009 [43]
proposed that two separate processes are involved in etching, one is dissolution of SiO2
owing to high basicity of NaBH4 solution and other is regrowth of silica on increasing
concentration of BOꢀꢂions (released on reaction of NaBH4 with water). Retention of surface
morphology even after etching process is due to the PVP which serves as a support to
preserve morphology. The role of PVP molecule is to block the outward dissolution of
silicate [59].
TEM findings (Fig. 4d) indicate occurrence of strong chemical etching that creates
surface defects resulting in pores. Pores of ~12 nm are evidently seen at the surface of etched
cashew nut structures. Etching not only introduces mesopores in the structure but also alter
the surface stoichiometry of oxygen. Post etching calcination treatment decorated the
morphology with smaller pores of dia. 6-7 nm, which are clearly visible in the TEM
micrograph of ECS (Fig. 4e and f). Successive etching and calcination treatments resulted in
bimodal porosity and oxygen vacancy in silica particles without affecting their size and
shape. TEM micrographs suggest that the pores are interconnected and form channels to
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provide swift passage to the ionic charges and electronic transfer [60]. TEM and FESEM
results are further confirmed by BET measurement (Fig. S3a and b) which proves the
existence of bimodal mesoporous structure with high BET surface area ~ 878 m2 g-1.
Fig. 4. TEM micrographs of silica: (a & b) as-synthesized silica (AS). (c & d) etched silica
(ES). (e & f) etched-calcined silica (ECS).
The electrochemical performance of silica electrodes was studied using a three-
electrode cell in 1.0 M H2SO4 electrolyte, with Ag/AgCl as the reference and Pt as the
counter electrodes, respectively. Fig. 5a shows the voltammograms of AS, ES and ECS
measured at 5 mV s-1. Enclosed area in the voltammogram obtained from ECS working
electrode is greater than ES and AS. The large area under the voltammetric loop i.e. high
charge storage in ECS demonstrate the combined advantages of bimodal porosity and oxygen
vacancies into the structure. These results display ECS as best performer among these
electrodes (AS, ES and ECS) for charge storage. Nyquist plot in Fig. 5b, suggests that the
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ECS electrode is more capacitive compared to other electrodes whereas, Bode angle of ECS
(inset Fig. 5b) is slightly larger than ES. To further understand the reason of more capacitive
nature of ECS electrode, Mott–Schottky plot (C-2 vs. V, where C= -1/2πfZ’’) was analysed at
a frequency of 10 kHz. In Fig. 5c, ECS electrode shows the positive slope indicating the n–
type dopant. The slope of Mott–Schottky plot is used to estimate the carrier/donor densities
using equation [61].
ꢂꢐ
ꢋ
ꢌ
ꢃꢄ = ꢅꢆ ꢈꢈ ꢉ ꢊꢄꢅ ꢉꢏ
(1)
ꢀ
ꢍ
ꢄꢎ
ꢇ
ꢇ
Where, Nd is the carrier concentration or donor density, e0 charge of electron, ꢑ is the
dielectric constant of SiO2 (3.9) [62], ꢑꢒ is permittivity of vacuum, C is the capacitance and
V is the applied potential. The carrier/donor density calculated from the above equation is 3.0
× 10ꢀꢁ cm-3 for ECS suggest good electronic conductivity, since the electrical conductivity
is directly depends on the carrier/donor density [63]. As the donor density increases, current
density should also increase because the tunnelling probability of electron increases.
Tunnelling probability depends on the distance between the active sites and this distance can
be calculated from the expression:
ꢐ
ꢋ/ꢖ
ꢕ
ꢓ =
(2)
ꢔ
To calculate the distance between active site in the thin film of geometrical area 1cm2,
the actual thickness of film 2000 Å is also considered. Therefore, for this the carrier density
calculated is 6×1019 cm-3 and distance between the active sites is estimated to be 2.5 nm that
is much less hence escalating the rate capability [58]. Rate of electron transfer also enhances
as it is an exponentially decreasing function of tunnelling distance [58]. To discover the
impact of OVs on the band gap of ECS, UV-vis spectra (Fig. 5d) is recorded. The direct band
gap energy for ECS is reduced to 2.1 eV compared to the previously reported value [64] for
SiO2 due to the generation of defect related energy levels as well as the bimodal mesoporous
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structure. Hence, it can be inferred that oxygen vacancies not only improves the rate of
electron transfer but also contribute to the conductivity of ECS electrode via reducing the
band gap.
Fig. 5. (a) Cyclic voltammograms of AS, ES and ECS. (b) Nyquist plot of AS, ES and ECS
(inset shows Bode plot). (c) Mott–Schottky plot of ECS. (d) UV-visible spectra ( the
inset displays Tauc plot) of ECS.
To evaluate the synergistic effect of bimodal porosity and oxygen vacancies over ECS
electrode, electrochemical measurements were carried out. Fig. S4a shows the
voltammogram of ECS electrode at different scan rates. The specific capacitance calculated
at 5 mV s-1 in Fig. S4b (Supporting Information) show a high value of charge stored (705.0 F
g-1). Furthermore, the ECS electrode demonstrates an excellent cyclic stability with more than
112 % retention after 5000 cycles due to the high carrier concentration that improves the rate
capability (Fig. S5).
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To further understand the dependence of charge storage on scan rate, Trasatti plot is
analysed [65,66]. According to Trasatti, the total charge stored (qt) is the sum of the
contribution of charge provided by inner (qi) and outer surface (qo) of the electrode. Due to
the faster kinetics, the amount of charge stored in the outer surface is diffusion limited and it
is calculated by linear fit of q* vs ʋ-1/2 when the scan rate i.e. ʋ =∞ (Fig. 6a). Similarly, the
total charge stored is evaluated by making linear fit in the curve of 1/q* vs ʋ1/2 at ʋ = 0 (Fig.
6b) as here diffusion of ion is unlimited. The obtained charge at the outer surface (qo) is 59.47
C g-1 and the total amount of charge stored (qt) is 227 C g-1. The value obtained for qo/qt
(25.99) illustrate higher inner active surface contribution compared to outer active surface
due to high ionic mobility and swift pathway for charge carriers in the wide mesopores.
In order to identify the mechanism behind the charge stored in ECS electrode, kinetics
of charge storage at different potential is studied. Commonly, the total amount of charge
stored originates from intercalation/de-intercalation and capacitive mechanism [67]. This is
characterised by studying the cyclic voltammogram at different scan rates using Power’s law
[68].
ꢝ
ꢘ ꢚ
ꢗ ꢙ = ꢛꢜ
(3)
Here, a and b are the adjustable parameters and υ is the scan rate (V s-1). Value of b
can be determined from the slope of log i vs log υ. There are two conditions i.e. for b = 0.5
major contribution arises from intercalation/de-intercalation of ions However, for b = 1
capacitive process dominates. Fig. 6c illustrates the plot of log i vs log υ at different potential
and depicts the value of b closer to 0.5 at each potential (Fig. 6d). This elaborates charge
storage mechanism in ECS is intercalation/de-intercalation (charge storage is diffusive).
Large mesopores of 10 to 18 nm are suitable for the intercalation mechanism. They offer
large surface area and in turn decrease the current density per unit surface area i.e. reducing
the electrode polarization and improving charge transfer [69].
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The current contribution from capacitive and intercalation mechanism can be
determined using the following equation [70].
ꢋ
ꢍ
ꢘ ꢚ
ꢘ ꢚ
ꢗ ꢙ = ꢞꢐ ꢜ ꢟ ꢞꢀ ꢘꢜꢚ
(4)
Plotting the i(V)/υ1/2 vs υ1/2 determines k1 and k2 at each fixed potential. The value of
k1(υ) and k2(υ)1/2 corresponds to current contribution from capacitive and diffusion controlled
intercalation mechanism respectively. At lower scan rates contribution from intercalation is
maximum whereas on further increasing the scan rate intercalative contribution decreases
leading to the enhancement in capacitive current (inset Fig. 6d).
Fig. 6. (a & b) Trasatti plot of ECS. (c) Plot of log (i) vs log (υ) at different potentials. (d)
dependence of slope ‘b’ as a function of different potentials ( the inset shows variation
of Icap/Iint with different scan rate at 0.6 V).
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Intercept of high frequency Nyquist plot (Fig. S6a) on the real axis depicts the
magnitude of solution resistance (Rs) of 2.9 Ω (inset Fig. S6a) [71]. The smaller value of Rs is
due to the good contact between electrode and electrolyte owed to the high surface area. In
the Bode plot, observed phase angle for ECS ~ -59º (Fig. S6b) at 0.01 Hz suggest effective
ion diffusion in mesopores at mid frequency region. This deviation from ideal capacitive
behaviour is attributed to broad pore size distribution in mesoporous range. At the same
frequency, penetration of ac signal varies for different pore dimensions. This leads to
frequency dispersion in which low frequency impedance behaviour is shifted from the
theoretical vertical line [72,73].
Specific capacitance increases considerably with a decrease in the frequency at the
low-frequency region (inset Fig. S6b). From this capacitance value, electrochemically active
surface area i.e. the accessible surface area of the electrode is estimated to be 3170 m2 g-1
which is much higher than the BET surface area. This large difference is ascribed to the
reason that the BET measurement does not include interlayer surface area. Specifically, the
availability of this high electrochemical surface area with less effective distance for
electrolytic ion and electron transport is favourable for intercalation of electrolytic ions.
A shorter Warburg region (17.78 Hz-3.61 Hz) (Figure S6a Supporting Information)
corresponds to high ionic diffusion in ECS [74]. Ionic diffusion can be evaluated from the
plot of real part of impedance and reciprocal root square of lower angle frequency shows a
straight line (Figure S6c Supporting Information).
High value of diffusion coefficient (1.05 × 10-9 cm2 s-1) validates less inhibition in
movement of ions across the wider pores that are estimated by the BJH adsorption value of
average pore diameter of 7.4 nm. The credit for high ionic mobility goes to internal voids and
reduced tunnelling distance between the active sites.
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GCD measurements (Fig. 7a) of ECS||ECS cell are performed at increasing current
densities. The GCD characteristics demonstrate isosceles triangular shape indicating nearly
identical charge and discharge. The slight deviation from isosceles triangular character is due
to the contribution from surface redox reaction of hydroxyl group [21]. The specific
capacitance is evaluated from GCD measurements and found to be 337.0, 218.0, 142.0, 98.0,
70.0 and 52.0 F g-1 at current density of 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 A g-1 respectively. High
capacitance with low ESR of 0.05 Ω (inset Fig. 7a) indicates towards the less consumption of
energy during charging/discharging [75].
Fig. 7. (a) Galvanostatic charge/discharge measurement of ECS||ECS (inset shows plot of IR
drop vs. current density). (b) Ragone plot of ECS||ECS with literature reported values.
(c) Cyclic stability of ECS||ECS at 100 mV s-1.
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Etchant NaBH4 reduces few of silica to silicon thus boost the conductivity without
removing SiO2 core [76]. This lowers the density of silica and thus increases specific
capacitance of unit mass. The gravimetric capacitance of ECS is relatively high compared to
previously reported values for porous silica and silicon [22–26].
Fig. 7b, illustrates the performance of the ECS||ECS cell. It is inferred that ECS
possess a high energy density of 46.86 Wh Kg-1 at 537.59 W Kg-1 at a current density of 1 A
g-1. Fig. 7c, shows 111 % of capacitance retention upto 10000 cycles. This implies, ECS is an
excellent material for supercapacitor device. The reason behind this appreciable cycle
stability may be mesoporous structure with hydrophilic surface functionality as it allows the
wettability of the electrode/electrolyte interface and also the large number of carriers that
enhances the rate of transfer of electron. Considering the specific capacitance and energy
density, the performance of present work is compared with the other silica based electrodes
and electrolyte membrane in Table 1.
Table 1.
Electrode
Electrolyte
Specific
Energy
Reference
Capacitance (F g-1) density
Graphene coated 1-ethyl-3-
-
4.8-4.9 Wh Kg-
23
Porous silicon
SiO2 aerogel
methylimidazolium
tetrafluoroborate
(EMIBF4)
1 at 1000 W
Kg-1
1 M Et4NBF4/PC
62.5 F g-1
318 F g-1
13.5 Wh Kg-1
at 1200 W Kg-
1
24
27
Reduced
graphene oxide-
silica in
1 M H2SO4
17 Wh Kg-1 at
100 W Kg-1
polyaniline
10% silica doped 1 M H2SO4
nanopolyaniline
368 F g-1
37.4 Wh Kg-1
at 430 W Kg-1
36
77
Stainless steel
foil
silicotungstic
acid(SiWA)-
H3PO4-
23 ꢀF cm-2
-
PVA/SiO2
18

ACCEPTED MANUSCRIPT
SiO2@C/TiO2
MWCNT
1 M KOH
1018 F g-1
-
78
79
SiO2, PVDF and
imidazolium ionic
liquid
71.7 F g-1
32.2 Wh Kg-1
at 900 W Kg-1
Oxygen
1 M H2SO4
337 F g-1
46.86 Wh Kg-1
at 537.59 W
Kg-1
(This
work)
deficient,bimodal
mesoporous
silica
Fig. 8. Electrochemical performance of flexible solid state ECS||ECS device: (a) Cyclic
voltammograms. (b) Nyquist plot. (c) Galvanostatic charge/discharge characteristics.
(d) Cycling stability at 100 mV s-1.
To broaden the utility of ECS||ECS device, we have fabricated flexible solid state symmetric
device using active area of 1cm × 1cm with PVA/H2SO4 [80] (gel electrolyte) and flexible
graphite (substrate). Fig. 8a shows the comparison of CV voltammogram under normal and
bend conditions, revealing no change in the shape of curve that confirms the structural
19

ACCEPTED MANUSCRIPT
integrity of the device. Further, Nyquist plot (Fig. 8b) shows slight influence of mechanical
deformation on the device with a minimal increase of solution resistance. However, in both
normal and bent conditions straight line nearly parallel to imaginary axis depicts good
capacitive behaviour of the device. GCD characteristics in Fig. 8c demonstrate the similar
charge discharge behaviour in both conditions. It can be clearly seen that mechanical
distortion has no major effect on capacitive performance of the device and shows an energy
density of 2.16 Wh Kg-1 at power density of 0.16 KW Kg-1 comparable to other solid state
devices [81–83]. Cycle life is another parameter to evaluate the performance of device.
Interestingly, solid state flexible device shows capacitance retention of 165% after 3000
cycles (Fig. 8d) indicating excellent stability of device. The high mechanical and electrical
strength is attributed to the interconnected mesoporous structure and abundant oxygen
vacancies.
4. Conclusions
In summary, the etched-calcined silica (ECS) enriched with abundant oxygen vacant sites
and mesopores is employed as a charge storage material. ECS along with oxygen vacant
sites acquires large electrochemically active surface area (3170 m2 g-1) to ensure the
maximum utilization. These oxygen vacant sites with distinguishable interconnected
pores provide less inhibition in diffusion of electrolytic ions by enlarging the
electrode/electrolyte interfacial area. Oxygen vacancies (OVs) in silica ameliorate the rate
of electron transfer and decrease the band gap by increasing the carrier concentration
(3× 10ꢀꢁ cm-3) and reducing the distance between active sites (2.5 nm). The oxygen
deficient ECS cell shows a high performance of 337 F g-1 at 1 A g-1 that is mainly
attributed to bimodal mesoporosity and the high concentration of carriers. In addition,
more than 110% capacitance is retained up to 10000 cycles that are mainly attributed to
the mesoporous structure. Furthermore, solid state performance of flexible device of silica
20

ACCEPTED MANUSCRIPT
elucidated it as an excellent material for portable and flexible electronic device. Hence,
this work proposes a wise strategy to make silica based materials capacitive by
introducing OVs and simultaneously inculcating bimodal mesopores in the structure. This
study might lead to further progress in research of silica as a charge storage material.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Akanksha Joshi is grateful to Junior Research Fellowship award (09/045 (1422)/2016-EMR-
I) from Council of scientific industrial research (CSIR), India. Shubra Lalwani also
acknowledges CSIR India for award of Senior Research fellowship (09/045/(1365)/2015-
EMR-I ).
References
[1] H. Wang, H. Dai, Strongly coupled inorganic-nano-carbon hybrid materials for energy
storage, Chem. Soc. Rev. 42 (2013) 3088–3113.
[2] E.A. Rosa, T. Dietz, T. Human drivers of national greenhouse-gas emissions, Nature
Clim. Change 2 (2012) 581–586.
[3] A. Nozad Golikand, M. Bagherzadeh, Z. Shirazi, Evaluation of the Polyaniline Based
Nanocomposite Modified with Graphene Nanosheet, Carbon Nanotube, and Pt
Nanoparticle as a Material for Supercapacitor, Electrochimica Acta 247 (2017) 116–124.
[4] N. Padmanathan, S. Selladurai, K.M. Razeeb, Ultra-fast rate capability of a symmetric
supercapacitor with a hierarchical Co3O4 nanowire/nanoflower hybrid structure in
nonaqueous electrolyte, RSC Adv. 5 (2015) 12700–12709.
21

ACCEPTED MANUSCRIPT
[5] A. N. Naveen, S. Selladurai, Novel low temperature synthesis and electrochemical
characterization of mesoporous nickel cobaltite-reduced graphene oxide (RGO)
composite for supercapacitor application, Electrochimica Acta 173 (2015) 290–301.
[6] J, Zhu, Y. Xu, J. Hu, L. Wei, J. Liu, M. Zheng, Facile synthesis of MnO2 grown on
nitrogen-doped carbon nanotubes for asymmetric supercapacitors with enhanced
electrochemical performance, J. Power Sources 393 (2018) 135–144.
[7] T. Alamro, M. K. Ram, Polyethylenedioxythiophene and molybdenum disulfide
nanocomposite electrodes for supercapacitor applications, Electrochimica Acta 235
(2017) 623–631.
[8] V. Sahu, S. Shekhar, P. Ahuja, G. Gupta, S.K. Singh, R.K. Sharma, G. Singh, Synthesis
of hydrophilic carbon black; role of hydrophilicity in maintaining the hydration level and
protonic conduction, RSC Adv. 3 (2013) 3917–3924.
[9] V. Sahu, M. Mishra, G. Gupta, G. Singh, R.K. Sharma, Turning Hazardous Diesel Soot
into High Performance Carbon/MnO2 Supercapacitive Energy Storage Material, ACS
Sustainable Chem. Eng. 5 (2017) 450–459.
[10] H. Sun, Y. Zhao, K. Mølhave, M. Zhang, J. Zhang, Simultaneous modulation of surface
composition, oxygen vacancies and assembly in hierarchical Co3O4 mesoporous
nanostructures for lithium storage and electrocatalytic oxygen evolution, Nanoscale 9
(2017) 14431–14441.
[11] Y. Cui, C.M. Lieber, Functional Nanoscale Electronic Devices Assembled Using Silicon
Nanowire Building Blocks, Science 291 (2001) 851–853.
[12] S. Cao, Z. Zhao, X. Jin, W. Sheng, S. Li, Y. Ge, M. Dong, W. Wu, L. Fang, Unique
double-shelled hollow silica microspheres: template-guided self-assembly, tunable pore
22

ACCEPTED MANUSCRIPT
size, high thermal stability, and their application in removal of neutral red, J. Mater.
Chem. 21 (2011) 19124–19131.
[13] L. Zhang, S.Z. Qiao, Y.G. Jin, Z.G. Chen, H.C. Gu, G.Q. Lu, Magnetic Hollow Spheres
of Periodic Mesoporous Organosilica and Fe3O4 Nanocrystals: Fabrication and Structure
Control, Adv. Mater. 20 (2008) 805–809.
[14] S.Y. Kim, B.H. Kim, Silica decorated on porous activated carbon nanofiber composites
for high-performance supercapacitors, J. Power Sources 328 (2016) 219–227.
[15] P.C. Carman, Constitution of colloidal silica, Trans. Faraday Soc. 36 (1940) 964–973.
[16] H. Peng, M. Jain, D.E. Peterson, Y. Zhu, Q. Jia, Composite Carbon Nanotube/Silica
Fibers with Improved Mechanical Strengths and Electrical Conductivities, Small 2008, 4,
1964–1967.
[17] A. Magasinski, P. Dixon, B. Hertzberg, A. Kvit, J. Ayala, G. Yushin, High-performance
lithium-ion anodes using a hierarchical bottom-up approach, Nat. Mater. 9 (2010) 353–
358.
[18] H. Kim, M. Seo, M.H. Park, J. Cho, A Critical Size of Silicon Nano Anodes for
Lithium Rechargeable Batteries, Angew. Chem. Int. Ed. 49 (2010) 2146–2149.
[19] Q. Liu, Z. Cui, R. Zou, J. Zhang, K. Xu, J. Hu, Surface Coating Constraint Induced
Anisotropic Swelling of Silicon in Si–Void@SiOx Nanowire Anode for Lithium Ion
Batteries, Small 13 (2017) 1603754.
[20] M.P. Liu, C.H. Li, H.B. Du, X.Z. You, Facile preparation of silicon hollow spheres and
their use in electrochemical capacitive energy storage, Chem. Commun. 48 (2012) 4950–
4952.
[21] J.F. Wang, K.X. Wang, F.H. Du, X.X. Guo, Y.M. Jiang, J.S. Chen, Amorphous silicon
with high specific surface area prepared by a sodiothermic reduction method for
supercapacitors, Chem. Commun. 49 (2013) 5007–5009.
23

ACCEPTED MANUSCRIPT
[22] S.E. Rowlands, R.J. Lantham, W.S. Schlindwein, Supercapacitor devices using porous
silicon electrodes, Ionics 5 (1999) 144–149.
[23] L. Oakes, A. Westover, J.W. Mares, S. Chatterjee, W.R. Erwin, R. Bardhan, S.M. Weiss,
C.L. Pint, Surface engineered porous silicon for stable, high performance
electrochemical supercapacitors, Scientific Reports
:10.1038/srep03020.
3
(2013) 3020 DOI
[24] X. Du, C. Wang, T. Li, M. Chen, Studies on the performances of silica aerogel
electrodes for the application of supercapacitor, Ionics 15 (2009) 561–565.
[25] G.H. Jeong, H.M. Lee, J.G. Kang, H. Lee, C.K. Kim, J.H. Lee, J.H. Kim, S.W. Kim,
ZrO2–SiO2 Nanosheets with Ultrasmall WO3 Nanoparticles and Their Enhanced
Pseudocapacitance and Stability, ACS Appl. Mater. Interfaces 6 (2014) 20171–20178.
[26] J. Zhi, S. Deng, Y. Wang, A. Hu, Highly Ordered Metal Oxide Nanorods inside
Mesoporous Silica Supported Carbon Nanomembranes: High Performance Electrode
Materials for Symmetrical Supercapacitor Devices, J. Phys. Chem. C 119 (2015) 8530–
8536.
[27] U. Male, S. Uppugalla, P. Srinivasan, Effect of reduced graphene oxide–silica composite
in polyaniline: electrode material for high-performance supercapacitor, J. Solid State
Electrochem. 19 (2015) 3381– 3388.
[28] J. Zhao, Y. Zhang, T. Wang, P. Li, C. Wei, H. Pang, Reed Leaves as a Sustainable Silica
Source for 3D Mesoporous Nickel (Cobalt) Silicate Architectures Assembled into
Ultrathin Nanoflakes for High Performance Supercapacitors, Adv. Mater. Interfaces 2
(2015) 1400377 DOI: 10.1002/admi.201400377.
[29] M.H. Yu, H.N. Wang, X.F. Zhou, P. Yuan, C.Z. Yu, One Template Synthesis of
Raspberry-like Hierarchical Siliceous Hollow Spheres, J. Am. Chem. Soc. 129 (2007)
14576–14577.
24

ACCEPTED MANUSCRIPT
[30] F. Caruso, R.A. Caruso, H. Möhwald, Nanoengineering of Inorganic and Hybrid Hollow
Spheres by Colloidal Templating, Science 282 (1998) 1111–1114.
[31] J. Liu, S.B. Hartono, Y.G. Jin, Z. Li, G.Q. Lu, S.Z. Qiao, A facile vesicle template route
to multi-shelled mesoporous silica hollow nanospheres, J. Mater. Chem. 20 (2010) 4595–
4601.
[32] J.G. Wang, F. Li, H.J. Zhou, P.C. Sun, D.T. Ding, T.H. Chen, Silica Hollow Spheres
with Ordered and Radially Oriented Amino-Functionalized Mesochannels, Chem. Mater.
21 (2009) 612–620.
[33] V. Sahu, S. Shekhar, R.K. Sharma, G. Singh, Ultrahigh Performance Supercapacitor
from Lacey Reduced Graphene Oxide Nanoribbons, ACS Appl. Mater. Interfaces 7
(2015) 3110–3116.
[34] V. Sahu, S. Goel, R.K. Sharma, G. Singh, Zinc oxide nanoring embedded lacey
graphene nanoribbons in symmetric/asymmetric electrochemical capacitive energy
storage, Nanoscale 7 (2015) 20642–20651.
[35] K.C. Leonard, W.E. Suyama, M.A. Anderson, Improvement of electrochemical
capacitor electrodes using SiO2 nanoparticles, Electrochimica Acta 56 (2011) 10137–
10144.
[36] H. Wei, H. Gu, J. Guo, S. Wei, J. Liu, Z. Guo, Silica Doped Nanopolyaniline with
Endured Electrochemical Energy Storage and the Magnetic Field Effects, J. Phys. Chem.
C 117 (2013) 13000–13010.
[37] H. Jain, A. S. Nowick, Electrical conductivity of synthetic and natural quartz crystals,
J. Appl. Phys. 53 (1982) 477.
[38] S. Balendhran, J. Deng, J. Z. Ou, S. Walia, J. Scott, J. Tang, K.L. Wang, M.R. Field, S.
Russo, S. Zhuiykov, M.S. Strano, N. Medhekar, S. Sriram, M. Bhaskaran, K.K. Zadeh,
25

ACCEPTED MANUSCRIPT
Enhanced Charge Carrier Mobility in Two Dimensional High Dielectric Molybdenum
Oxide, Adv. Mater. 25 (2013) 109–114.
[39] H.S. Kim, J.B. Cook, H. Lin, J.S. Ko, S.H. Tolbert, V. Ozolins, B. Dunn, Oxygen
vacancies enhance pseudocapacitive charge storage properties of MoO3-x, Nature Mater.
16 (2016) 454–460.
[40] D. Yan, W. Wang, X. Luo, C. Chen, Y. Zeng, Z. Zhu, NiCo2O4 with oxygen vacancies
as better performance electrode material for supercapacitor, Chem. Eng. Journal 334
(2018) 864–872.
[41] Y. Liu, C. Xiao, Z. Li, Y. Xie, Vacancy Engineering for Tuning Electron and Phonon
Structures of Two Dimensional Materials, Adv. Energy Mater. 6 (2016) 1600436.
[42] D. Chen, L. Li, F. Tang, S. Qi, Facile and Scalable Synthesis of Tailored Silica
“Nanorattle” Structures, Adv. Mater. 21 (2009) 3804-3807.
[43] T. Zhang, Q. Zhang, J. Ge, J. Goe, M. Sun, Y. Yan, Y.S. Liu, C. Chang, J. Guo, Y. Yin,
A Self-Templated Route to Hollow Silica Microspheres, J. Phys. Chem. C 113 (2009)
3168-3175.
[44] Z.S. Wu, Y. Sun, Y.Z. Tan, S. Yang, X. Feng, K. Müllen, Three-Dimensional Graphene-
Based Macro- and Mesoporous Frameworks for High-Performance Electrochemical
Capacitive Energy Storage, J. Am. Chem. Soc. 134 (2012) 19532–19535.
[45] Q. Zhang, L. Li, Y. Wang, Y. Chen, F. He, S. Gai, P. Yang, Uniform fibrous-structured
hollow mesoporous carbon spheres for high-performance supercapacitor electrodes,
Electrochimica Acta (176) 2015 542–547.
[46] C.T. Hsu, C.C. Hu, Synthesis and characterization of mesoporous spinel NiCo2O4 using
surfactant-assembled dispersion for asymmetric supercapacitors, J. Power Sources 242
(2013) 662–671
26

ACCEPTED MANUSCRIPT
[47] Z. Zheng, B. Huang, X. Meng, J. Wang, S. Wang, Z. Lou, Z. Wang, X. Qin, X. Zhang,
Y. Dai, Metallic zinc- assisted synthesis of Ti3+ self-doped TiO2 with tunable phase
composition and visible-light photocatalytic activity, Chem. Comm. 49 (2013) 868–870.
[48] W. Tian, Q. Gao, W. Qian, Interlinked Porous Carbon Nanoflakes Derived from
Hydrolyzate Residue during Cellulosic Bioethanol Production for Ultrahigh-Rate
Supercapacitors in Nonaqueous Electrolytes, ACS Sustainable Chem. Eng. 5 (2017)
1297–1305.
[49] H. Cheng, M. Klapproth, A. Sagaltchik, S. Li, A. Thomas, Ordered mesoporous WO2.83
:
selective reduction synthesis, exceptional localized surface plasmon resonance and
enhanced hydrogen evolution reaction activity, J. Mater. Chem. A 6 (2018) 2249–2256.
[50] S.A. Ansari, M.M. Khan, S. Kalathil, A. Nisar, J. Lee, M.H. Cho, Oxygen vacancy
induced band gap narrowing of ZnO nanostructures by an electrochemically active
biofilm, Nanoscale 5 (2013) 9238–9246.
[51] X. Fang, C. Chen, Z. Liu, P. Liu, N. Zheng, A cationic surfactant assisted selective
etching strategy to hollow mesoporous silica spheres, Nanoscale 3 (2011) 1632–1639.
[52] F. Dai, J. Zai, R. Yi, M.L. Gordin, H. Sohn, S. Chen, D. Wang, Bottom-up synthesis of
high surface area mesoporous crystalline silicon and evaluation of its hydrogen evolution
performance, Nature Communications (2014) DOI: 10.1038/ncomms4605.
[53] J. Mitra, M. Ghosh, R.K. Bordia, A. Sharma, Photoluminescent electrospun submicron
fibers of hybrid organosiloxane and derived silica, RSC Adv. 3 (2013) 7591–7600.
[54] L.Y. Yang, H.Z. Li, J. Liu, Z.Q. Sun, S.S. Tang, M. Lei, Dual yolk-shell structure of
carbon and silica-coated silicon for high performance lithium-ion batteries, Scientific
Reports DOI: 10.1038/srep10908.
27

ACCEPTED MANUSCRIPT
[55] A. Naldoni, M. Allieta, S. Santangelo, M. Marelli, F. Fabbri, S. Cappelli, C.L. Bianchi,
R. Psaro, V.D. Santo, Effect of Nature and Location of Defects on Bandgap Narrowing
in Black TiO2 Nanoparticles, J. Am. Chem. Soc. 134 (2012) 7600–7603.
[56] E. McCafferty, J.P. Wightman, Determination of the concentration of surface hydroxyl
groups on metal oxide films by a quantitative XPS method, Surf. Interface Anal. 26
(1998) 549–564.
[57] Z. Sun, C. Yanga, G. Liu, H. Lua, R. Zhang, L. Wang, H. Wang, Largely enhanced
electrochemical performance in MoO3-x nanobelts formed by a “sauna reaction”:
Importance of oxygen vacancies, Electrochimica Acta 239 (2017) 16–24.
[58] B. Roh, D.D. Macdonald, Effect of oxygen vacancies in anodic titanium oxide films on
the kinetics of the oxygen electrode reaction, Russ. J. Electrochem. 43 (2007) 125–135.
[59] Q. Yu, J. Hui, P. Wang, X. Wang, Anion-Exchange-Driven Disassembly of a
SiO2/CTAB Composite Mesophase: the Formation of Hollow Mesoporous Silica
Spheres, Inorg. Chem. 51 (2012) 9539–9543.
[60] Z. Huang, Z. Zhang, X. Qi, X. Ren, G. Xu, P. Wan, X. Sun, H. Zhang, Wall-like
hierarchical metal oxide nanosheet arrays grown on carbon cloth for excellent
supercapacitor electrodes, Nanoscale 8 (2016) 13273–13279.
[61] X. Lu, Y. Zeng, M. Yu, T. Zhai, C. Liang, S. Xie, M.S. Balogun, Y. Tong,
Oxygen Deficient Hematite Nanorods as High Performance and Novel Negative
Electrodes for Flexible Asymmetric Supercapacitors, Adv. Mater. 26 (2014) 3148–3155.
[62] J. McPherson, J.Y. Kim, A. Shanware, H. Mogul, Thermochemical description of
dielectric breakdown in high dielectric constant materials, Appl. Phys. Lett. 2003, 82,
2121.
[63] X.Y. Cao, X. Xing, N. Zhang, H. Gao, M.Y. Zhang, Y.C. Shang, X.T. Zhang,
Quantitative investigation on the effect of hydrogenation on the performance of
28

ACCEPTED MANUSCRIPT
MnO2/H-TiO2 composite electrodes for supercapacitors, J. Mater. Chem. A 3 (2015)
3785–3793.
[64] T.H. DiStefano, D. Eastman, The band edge of amorphous SiO2 by photoinjection and
photoconductivity measurements, Solid State Commun. 9 (1971) 2259–2261.
[65] S. Ardizzone, G. Fregonara, S. Trasatti, “Inner” and “outer” active surface of RuO2
electrodes, Electrochim. Acta 35 (1990) 263–267.
[66] R.K. Sharma, L. Zhai, Multiwall carbon nanotube supported poly(3,4-
ethylenedioxythiophene)/manganese oxide nano-composite electrode for super-
capacitors, Electrochim. Acta 54 (2009) 7148–7155.
[67] B. Senthilkumar, K.V. Sankar, L. Vasylechko, Y.S. Lee, R.K. Selvan, Synthesis and
electrochemical performances of maricite-NaMPO4 (M = Ni, Co, Mn) electrodes for
hybrid supercapacitors, RSC Adv. 4 (2014) 53192–53200.
[68] K.V. Sankar, S. Surendran, K. Pandi, A.M. Allin, V.D. Nithya, Y.S. Lee, R.K. Selvan,
Studies on the electrochemical intercalation/de-intercalation mechanism of NiMn2O4 for
high stable pseudocapacitor electrodes, RSC Adv. 5 (2015) 27649–27656.
[69] I. Moriguchi, R. Hidaka, H. Yamada, T. Kudo, H. Murakami, N.A. Nakashima,
Nanocomposite of TiO2 and Carbon Nanotubes as a High Rate Li Intercalation
Electrode Material, Adv. Mater. 18 (2006) 69–73.
[70] M. Sathiya, A.S. Prakash, K. Ramesh, J.M. Tarascon, A.K. Shukla, V2O5-Anchored
Carbon Nanotubes for Enhanced Electrochemical Energy Storage, Mesoporous, J. Am.
Chem. Soc. 133 (2011) 1629–16299.
[71] H. Zhu, Y. Zheng, Mesoporous Co3O4 anchored on the graphitic carbon nitride for
enhanced performance supercapacitor, Electrochimica Acta 265 (2018) 372–378.
29