High voltage supercapacitors based on carbon-grafted NiO nanowires interfaced with an aprotic ionic liquid

Article abstract of DOI:10.1039/c5cc01479d

The report provides a preliminary assessment of the charge storage prerogatives of an asymmetric electrochemical capacitor employing a carbon-grafted NiO electrode interfaced with 1-ethyl-3-methyl imidazoliumdicyanamide as an ionic liquid electrolyte. This configuration has been demonstrated to be potentially exploited for developing hybrid supercapacitors providing as high energy density as 21 W h Kg^-1.

Full text of DOI:10.1039/c5cc01479d

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High voltage supercapacitors based on carbon-grafted
NiO nanowires interfaced with an aprotic ionic liquid
Cite this: DOI: 10.1039/x0xx00000x
a
a
b
b
Anjali Paravannoor, Shantikumar V.Nair, Praveen Pattathil, Michele Manca,
a
Avinash Balakrishnan *
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
6
The report provides a preliminary assessment of the charge ionic dimensions . It could be attributed to the formation of ultraꢀthin
storage prerogatives of an asymmetric electrochemical EDLs, mainly caused by the absence of solvation shells around RTILs,
capacitor employing
a
carbon-grafted NiO electrode which also enhances the accessibility of ions to infiltrate nano/subꢀnano
7,8
interfaced with 1-ethyl-3-methyl imidazoliumdicyanamide as porous electrodes . Increasing specific capacitance through bringing in
ionic liquid electrolyte. This configuration has been the faradaic mechanisms was also shown to be a reliable strategy to
demonstrated to be potentially exploited for developing increase the specific capacitance and in turn the energy density values.
hybrid supercapacitors providing an as high energy density Metal oxides such as RuO
2
, MnO
have been used as pseudocapacitor electrode
materials. Among these materials, RuO exhibits higher specific
2 3 4
, NiO, Co O , metal sulfides, and
-
1
9−15
as 21 WhKg .
conducting polymers
2
Electric doubleꢀlayer capacitors (EDLCs or supercapacitors) have been
extensively explored as promising energy storage devices in which the
charge/energy is stored through ion rearrangement in the interfacial
layer between the electrode and electrolyte. Substantial efforts have
been dedicated to improve the energy density of EDLCs through design
of nanostructured electrodes with a high specific surface area and
understanding the interplay between various phenomena defining the
capacitance in acidic electrolytes. However, the toxicity and low
abundance have necessitated the need to develop other inexpensive
materials of comparable performance. NiO is a likely replacement for
RuO
2
due to its high theoretical specific capacitance, high chemical and
16
thermal stability and lower cost.
Starting from these remarks, the prospects of using an IL electrolyte in
NiO based hybrid supercapacitor shave been preliminarily
demonstrated in the present work. A typical aprotic IL, namely 1ꢀethylꢀ
1
performance of these devices.
For EDLCs, room temperature ionic liquids (RTILs) are widely
considered the electrolyte of the future. RTILs are solventless
electrolytes with many properties that make them attractive for
electrochemical energy storage: high chemical and thermal stability,
negligible vapor pressure, a broad electrochemical stability potential
window and an immense parameter space in terms of ion selection and
3
ꢀmethyl imidazoliumꢀdicyanamide has been examined wherein a
highly porous nickel oxide nanowire/carbon network with a pore size
roughly close to the anionic radii has been used as anode. It has been
assumed that in a similar system dicyanamide ions would form a
17
monolayer surrounded by the charged electrode surfaces .
NiO/carbon composite nanowires were synthesized by following the
2,3
resulting properties. . They are displaced to form alternating layers of
18,19
synthetic route provided in our recently patented procedure.
Briefly,
cations and anions aligned near a charged surface with the peak
0
.7M of nickel chloride and 0.7M of ammonium oxalate ((NH
were individually dissolved in distilled water and blended under
rigorous stirring conditions at room temperature. mL of
4 2 2 4
) C O )
4
densities decomposing to the corresponding bulk values . These
alternating layers are accountable for a number of unforeseen
electrochemical phenomena at small scales. Larger specific
capacitances have been recently achieved through the use of high
surface area activated carbon electrodes in combination with different
1
triethanolamine (TEA) was added to this mixture under stirring. The
resultant solution was transferred to a 50 mL Teflon tank and
autoclaved at 120ºC for 10 h, resulting in a pale green color precipitate.
The precipitate was centrifuged and washed with ethanol and dried in
air to obtain nickel oxalate nanowires. Camphor blocks were mixed
with the nanowire dispersion in methanol in different weight ratio and
pyrolyzed in air. In order to synthesize pristine NiO nanowires the
5
ionic liquid electrolytes.
However it has been shown that not all micropores generated in carbon
preparation can be available for EDL formation as in the case of
5
aqueous electrolytes . Largeot et al. observed the “anomalous” increase
of capacitance when the pore size of an electrode comes close to the
nickel oxalate nanowires were annealed at 300ºC. Details of the phase,
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morphological and electrochemical characterizations are given in the
Electronic Supplementary Information section.
TEA plays a fundamental role in this synthetic route since it acts like a
soft template which modulates the degree of solution supersaturation
and governs the lattice development mode of the growing nanocrystals
by dynamically adhering to specific crystal facets of the nickel oxalate
nuclei. A schematic illustration showing the formation mechanism of
the nanowires is given in Fig SI_1a. The final step of the synthesis
involves the formation of continuous nickel oxalate nanowires due to
Ostwald ripening of the pristine nanocrystals. An increase in reaction
time leads to the formation of bundles of nanowires SI_1b). The
pyrolytic decomposition of camphor with nickel oxalate results in its
dissociation along with the formation of a graphitic layer over the Fig. 2 CV curves of pure NiO (a) and NC samples (b) at 100 mV/s; CV of NC_50
at different scan rates; discharge profiles of NC_50 at different currents (c) and
growing NiO nanclusters. In addition to this, in presence of CO and H
2
st
th
comparison between 1 and 2000 cycles (d).
a partial reduction reaction occurs, leading to the formation of Ni seeds,
which can facilitate dispersion of the carbon and serve as catalysts for
nucleation sites, thus resulting in the formation of an interconnected
The patterns showed an I
D
/I
G
ratio of 0.56. (see Fig.1b). The peak at
ꢀ
1
23
5
00cm corresponds to NiO in the case of NC samples. Fig 1c and 1d
20
graphitic network. Depending on the average amount of carbon in the
electrode, which has been calculated from CHN analysis, the samples
were designated as NC_10, NC_20, NC_50 and NC_60, where N, C
and number refer to NiO nanowires, carbon and amount of carbon,
respectively.
show the TEM and HRꢀTEM images of the nanowire samples. The
presence of carbon as a coating is evident from the phase contrast in Fig
1
c. Fig. 1d depicts the high magnification examination of these samples
revealing the presence of carbon being grafted on the surface wherein
the interplanar spacing was measured to be 0.33 nm. On the NiO
nanowires it results to be 0.24 nm along the lateral growth direction and
both of them are consistent with the identification of (111) plane of as
revealed from XRD pattern.
Fig. 1(a) shows typical XRD patterns of the NC nanowires. The
diffraction peaks indexed corresponded to cubic phases of NiO (JCPDS
(Joint Committee on Powder Diffraction Standards) 78ꢀ0429) along
with traces of metallic nickel (JCPDS 04ꢀ0850) and graphitic carbon
Surface area of the pristine NiO as well as of all the NC samples was
(
JCPDS 75ꢀ2078). The XRD patterns of the nickel oxalate nanowires
JCPDS 01ꢀ0299) and pristine NiO nanowires pyrolyzed without
calculated from a BET analysis: it has been estimated to be around 74
(
2
ꢀ1
(
for NiO), 89 (for NC_10), 92 (for NC_20), 101 (NC_50) and 91 m g
camphor are given in the supporting information (see SI_2a, b).
(
for NC_60) with an average pore size ranging from 0.4 to 0.8 nm (see
Fig 1 (b) represents the Raman spectra of pure carbon sourced from the
24
Fig. SI_3).
21
pyrolysis of camphor and the NC nanowires showing an enhance
intensity of G band. Here CDC and CGN represent camphor derived
carbon and carbon grafted NiO, respectively. Two sharp peaks with
The here referred nanostructured films were thus immersed into an
EMIꢀDCA solution and subjected to a set of exhaustive electrochemical
analysis (experimental details are given in ESI). Fig. 2 displays
ꢀ
1
ꢀ1
intensity maxima at 1345 cm and 1585 cm correspond to the D and
ꢀ
1
representative cyclic voltammograms (CV) (at 100 mV s scan rate) of
both pure NiO (Fig. 2a) and NC electrodes with different carbon
contents (Fig. 2b) in a coin cell configuration. The presence of redox
peaks is reasonably attributable to the absorption and desorption of
dicyanamide ions from the electrolyte.
22
G bands of graphite phase.
We have also investigated the redox activity of NiO in EMIꢀDCA in a
threeꢀelectrodes set up. CV measured at a 10 mV/s scan rate in
narrower electrochemical window vs. SCE is shown in Fig SI_4b. It
clearly highlights the role of redox reactions in determining the
electrochemical capacitance of the NiO electrode: both reduction (at
around 0.2V) and oxidation (at around 0.9V) peaks (marked with
arrows in the figure) are clearly observable. Here the capacitance is
mainly based on the redox reaction because the shape of the CVs is
distinguished from that of electric doubleꢀlayer capacitance, which is
25
normally close to an ideal rectangular. XPS analysis of the same
electrode in the pristine, charged and discharged state has been also
26
provided to support this. See Fig. SI_4a. Analytical data indicate that
NiO is essentially divalent before charging; upon charging it seems to
be consistent with the presence of both divalent and trivalent Ni ions,
confirming the establishment of a faradaic contribution to the
charge/discharge phenomena at NiO/electrolyte interface.
Fig. 1.a) XRD spectrum of an NC_50 sample; b) Raman spectra of camphorꢀ
derived carbon (CDC) and carbonꢀgrafted nickel oxide (CGN); c) TEM and d)
HRꢀTEM images of GCN
2
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Based on these facts a possible redox process is been hypothesized as determines the high frequency offꢀset of the EIS spectrum .
following:
Pure NiO electrode exhibits a huge charge transfer resistance which
(1) completely overwhelms any capacitive effect. These results confirm
2 2
NiO + N(CN) ꢁ ↔ NiO·N(CN) + eꢁ
NC_50 electrodes revealed the best charge storage performance as a that introduction of graphitic carbon dramatically facilitates the electron
consequence of an optimum balance between high surface area and transfer and induces the formation of a doubleꢀlayer capacitor at the
27
good electronic conductivity. CV plots of NC_50 with three different interface. NC_50 has been confirmed to provide the lowest series
ꢀ1
scan rates (100, 200 and 500 mVs ) are shown in Fig 2c. As expected, resistance among all the here tested electrode compositions.
at higher scan rates a less prominent redox pattern can be observed. The charge/discharge prerogatives of the here referred carbonꢀgrafted
We thus used the voltammetric sweep rate dependence to quantitatively NiO electrodes can be reasonably understood by assuming the validity
29
determine the capacitive and faradaic contributions to the current of the following assumed conditions : (1) the structure of
28
response (see Supporting Info). Fig. 2d shows the galvanostatic electrode/ionic liquid interface is to obey the Gouy–Chapman–Stern
30
response of NC_50 at different discharging currents of 10, 30, 70 and theory; (2) the EDLC in this electrode/ionic liquid interface is
00 mA. The observed nonꢀlinearity in the discharge curve can be composed of more than one layer of ions with the same/or different
attributed to the pseudocapacitance arising out of the redox reaction in signs (i.e., cations or/and anions) arranged in proportion to the charge
1
27
the given voltage range .As the current density increases the voltage density of the electrode. The first layer of the ions attached to the
31–32
drop at the onset of the discharge profile becomes more prominent electrode surface is called the compact layer (or Helmholtz layer);
indicating the larger IR drop. Fig. 2e represents the discharge profiles (3) the overall capacitance (Cdl) of such a EDLC in an ionic liquid is
of the system at 1st and 2000th cycles at 20 mA indicating a high cyclic approximately equal to the inner or compact layer capacitance (Cc) and
stability with a fade of less than 5%.
the diffuse layer capacitance (Cd) is negligible. According to this
The most relevant figures of merits, such as specific capacitance Cs, model, EMI ions are arranged to form a charged layer in which ethyl
3
3
power density (P) and energy density (E) were then calculated chains result to be preferentially aligned towards the C/NiO electrode.
9
according to DIN IEC 62391ꢀ1 (equation S1, S2 and S3) . The During the charging process they come to be preferentially adsorbed on
maximum capacitance value was calculated to be 1.6 F. The maximum the electrode surface due to their high hydrophobicity, whereas the
ꢀ
1
ꢀ
2
energy and power density values were calculated to be 21 WhKg and charged moieties (imidazole rings and N(CN) ) could reside away from
ꢀ
1
ꢀ
8.3 kWKg respectively.
the electrode surface. Accordingly, DCA ions, which trigger the
Electrochemical impedance spectroscopy (EIS) measurementswere pseudocapacitive reaction, are likely chemisorbed on the nanostructured
carried out to determine the charge kinetic properties ofthe device electrode during upon discharging, as schematically shown in Fig 4.
associated with the above supercapacitive parameters.Fig. 3a and 3b
show the Nyquist plot of the asꢀprepared coin cells based on pure NiO,
NC50 and NC60 in the frequency range 100 kHz–0.1 Hz at open circuit
potential. The EIS spectra of a pseudocapacitor device usually divided
into three segments following three processes; (i) the overall series
resistance at high frequency (>1 kHz); (ii) capacitive effects at high and
intermediate frequencies (>0.1 kHz); and (iii) Warburg diffusion
resulting from the frequency dependence of ion diffusion/ transport in
the electrolyte at low frequencies (0.01ꢀ0.1 Hz). To elucidate the
individual processes and associated impedance parameters the electrical
equivalent circuit sketched in Fig 3b has been used, which comprises
the series resistance Rs, the electrical double layer capacitance (Cdl), the
charge transfer impedance (Rct), frequency dependent Warburg
w s
impedance (Z ). R is a combination of (i) electrolyte resistance, (ii)
Fig. 4 Schematic diagram showing the formation of double layer at the electrode
intrinsic resistance of the electroactive material, and (iii) the contact electrolyte interface
resistance between the active material and the current collector that
In summary, the pseudocapacitive behavior of carbonꢀgrafted NiOꢀ
NWs electrodes in an EMIꢀDCAꢀbased electrolyte has been assessed
and their electrochemical properties have been investigated through CV
and EIS analysis. Using aprotic IL electrolytes may therefore lead to the
development of robust EDLCs which promise to conveniently
conjugate power and energy density requirements in a wide range of
practical applications.
Notes and references
a
Nanosolar Division, Amrita Centre for Nanosciences and Molecular
Medicine, Ponekkara, Kochi, India. Fax: +91ꢀ484ꢀ2802020; Tel: +91ꢀ
484ꢀ2802020; Eꢀmail: avinash.balakrishnan@gmail.com
Fig.3Nyquist plots measured at 0.1V for NiO, NC_50 and NC_60 electrodes. The
equivalent electrical circuit used to analyze these data is shown in the inset
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b
Center for Biomolecular Nanotechnologies (CBN)ꢀ Fondazione Istituto
Italiano di Tecnologia (IIT), via Barsanti snc, I73010, Arnesano (Lecce),
Italy
†
Electronic Supplementary Information (ESI) available: experimental
details, XRD and XPS spectra, BET analysis, electrochemical analysis.
See DOI: 10.1039/c000000x/
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