Abstract
A nickel hydroxide (Ni(OH)2)-modified reduced graphene
oxide (rGO) foam electrode for supercapacitor application is presented
in this study. The electrode was made without binder through a one step
process. In order to compare the supercapacitor performance of as
synthesized graphene foam electrode, rGO and rGO:carbon black (CB)
standard electrodes with PVDF-HFP binder were also synthesized . All
electrodes were then characterized structurally (XPS, EDS, RAMAN, XRD,
and FT-IR) and morphologically (SEM). Ni-containing electrodes were
found in the α-Ni(OH)2:rGO structure. NirGO3 has 800
Fg-1 capacitance, while rGO:CB has 900. Thus NirGO3
has the best capacitive performance among binder-free electrodes,
comparable to rGO:CB.
Keywords: foam electrode, supercapacitor, self-templated,
nickel hydroxide
- IntroductionSupercapacitors (SCs), as environmentally friendly and renewable
energy storage technology, have recently become popular in the
scientific community. When compared to conventional batteries, their
relatively long lifetime and higher power density makes them good
candidates for electrical application. A typical supercapacitor
contains positive and negative electrodes separated by an
ion-conducting electrolyte [1]. As a critical component of SCs,
the electrodes are prepared by the slurry casting method that needs
adding some high conducting material and/or a binder (generally a
polymer). The function of conductive (and porous) additives is to
improve the interaction at the current collector and the electrode
material interface to reduce the series resistance of the device.
However, additives and insulating binders have some disadvantages,
such as aggregation or clogging pores of active material resulting in
loss of volume and relatively lower volumetric and gravimetric
capacitance of electrodes [2, 3]. In SC applications, carbonaceous
materials, conductive polymers, and metal
oxides/hydroxides have
generally been used as electrode materials. Particularly,
graphene-based composites have commonly been used in those
applications due to their large surface area (theoretical surface area
up to 2630 m2 g−1), high
electronic conductivity, mechanical flexibility and excellent thermal
stability [4-7]. However, π-π interactions and weak van der Waals
forces lead graphene nanosheets to aggregate to be stacked back into
graphite, resulting in decreased accessible surface area and a limited
electrolyte ion diffusion rate. To overcome this problem, some other
carbon-based materials (carbon black (CB), carbon nanotube (CNT),
etc.) or metal oxide/hydroxides
nanoparticles (Ni(OH)2,
Fe2O3, MnO2, etc.)
have been inserted into the electrode structure [8-11]. Although
high capacitance values (maximum theoretical capacitance: 2082 F
g−1) can be achieved with electrodes prepared, a
decrease in conductivity may occur due to the introduction of
impurities into the structure and the increase in the distances
between the lamellas [8,
9]. From this perspective, binder-free, lightweight graphene foams
(GF) with porous structure and high surface area have recently emerged
as an essential development [12-14] . 3D GF is a hierarchical
material with a porous structure exhibiting suitable properties such
as a highly conductive path, extensive surface area, improved
charge-carrying mobility, and lower resistance [15]. Electrons can
be transferred efficiently in the system with low resistance because
of the ideal interconnected network and 3D macrostructure.
Moreover, it is possible to obtain more functional structures for
various applications by making GF composites with different
nanoparticles or semiconductor metal oxides/hydroxides[16-22]. In
the literature, the most viable and efficient methods proposed for the
preparation of 3D GF and GF-based hybrid structures are hydrothermal,
chemical vapor deposition, template, freeze drying, and
electrochemical preparation. [23-26]. For instant, it was reported
that an Au and rGO based 3D composite material was coated on a carbon
electrode surface by electrochemical deposition [27]. This method
formed a three-dimensional structure with an interconnected network
having relatively larger surface area and higher conductivity. It was
also determined that the layer distances of the rGO nanolayer were
increased by the presence of Au nanoparticles, providing a
biocompatible interface. In another study, the GF-TiO2composite was obtained by a procedure called one-step-hydrothermal
method at 180 °C in an autoclave for wastewater treatment. Wang and
co-workers reported that the hybrid nanosheet they prepared exhibited
much superior performance for removing methylene blue (MB) and
chromium (VI) than individual pure graphene foam or pure
TiO2 sheets [28]. Similar structures containing
Ni, NiO, and Ni(OH)2 have been obtained to be used in
supercapacitor applications.
Huang et al. reported a graphene/NiO-based film through various
preparation processes. The prepared graphene/NiO film achieved a
specific 540 F g−1 at 2 A g−1 and
kept 80% of initial capacitance for 2000 CDC [29]. On the other
hand, Pore et al. also proposed another method to obtain a
graphene/NiO foam. In that procedure, the electrode material was
prepared with a hydrothermal method by annealing at 400oC. A specific capacitance of 727.1 F
g−1 at 1mA cm−2 was achieved with
a stability over 80% after about 9000 cycles [3][26]. In
addition, an electrode based on Ni(OH)2 for
supercapacitor application was reported by Viswanathan and
co-workers[8]. As well known, Ni(OH)2 has good
solubility in acids such as H2SO4. A
chemical reduction method was achieved to obtain insoluble
rGO/Ni(OH)2 structure showing a specific capacitance
value over 130 F g−1 in 1M
H2SO4 electrolyte.
This current study developed a self-templated foam based on Ni and
rGO, and its structural analyses was carried out. The prepared foams
in different weight ratios of Ni and rGO were directly grown on the
current collector. These electrodes were electrochemically
characterized as supercapacitor electrodes in aqueous electrolytes and
compared with standard rGO and rGO:CB reference electrodes.
- Experimental Section
- Preparation of electrode material and fabrication of PSC
The Hummer‘s method was modified to synthesize GO efficiently and a
chemical reduction method was applied to obtain rGO [30]. In order
to determine effects of both foam structure and Nickel derivative, three
types of electrode composition were prepared. The standard electrode
(rGO) was obtained by mixing rGO and binder PVDF at a ratio of 95:5
(w/w) in acetone. The standard secondary electrode (rGO:CB) was obtained
by mixing rGO, CB and binder PVDF in the ratio of 80:15:5 (w/w) in
acetone. Both reference electrodes were coated on current collector (1*1
cm2) by drop casting method. Initially, rGO powder (10
mg) and nickel nitrate hexahydrate in the various weight ratio of 1:3,
1:6 and 1:12 was added into 2.5 mL ethylene glycol, and then the
mixtures were sonicated for 3 h (presented weight ratios of starting
materials are optimized values). Subsequently, the resulting dispersions
were cast onto a 250 °C pre-heated graphite sheet (the active material
is 1.0-1.3 mg in 1*1 cm2 surface area). Adding
reactant dispersion onto the hot graphite sheet causes NOx gas’s
evolution, and binder-free NirGO foam forms on the current collector.
Consequently, these electrodes were used as working electrode and
characterised electrochemically in the electrochemical cell system to
determine their capacitive performances.
Structural Characterization of Electrode Materials Methods
X-Ray Diffraction (XRD, Rigaku Ultima IV), depth profiling Raman
spectroscopy (XploRA Raman Microscope), Fourier-transform infrared
spectroscopy (FT-IR, Nicolet iS50 FTIR), energy dispersive spectroscopy
(EDS, JEOL JSM-7100-F), and X-ray photoelectron spectroscopy (XPS)
(Thermo Scientific K-Alpha), methods were used for the structural
characterization of rGO and NirGO composites. Moreover, scanning
electron microscopy (SEM, JEOL JSM-7100-F) analyses were carried out to
take micrographs of all electrodes for morphological analyses.
XRD patterns were recorded to analyse phase compositions of NirGO3 and
rGO with Cu Kα radiation (λ = 1.54 Å), while step size and scan rate are
0.02° and 2°/min, respectively, in a 2θ range from 3° to 95.
Furthermore, Raman spectra of rGO and NirGO3 were used to determine the
intensity and position changes in the D and G bands of rGO and the Raman
shift peaks of Ni(OH)2 in the 400-2500
cm−1. Furthermore, the phase of
Ni(OH)2 and its interaction with rGO through the
functional groups was also clarified by carrying out FT-IR analyses in
the range from 400 to 4000 cm−1.
SEM analyses of rGO, rGO-CB and NirGO3 electrode materials were
performed to examine their morphology. The 250X and 50kX magnitude
micrographs were recorded with secondary electrons detector (SE). In
addition, EDS analyses were used to determine the composite electrodes’
chemical composition (amount of C, Ni and O). XPS analysis was carried
out for a detailed characterization of the composition. In the complete
spectrum of the XPS survey, elements of Ni, C, and O were detected
clearly. Moreover, their chemical states were confirmed by the
deconvolution of Cs1, Os1, and Ni 2p.
Electrochemical Characterization of Prepared Supercapacitors
As it was mentioned above, CV, EIS and CDC techniques were carried out
for determining the capacitive performance of SC cells by using a CH
Instrument CHI 660B electrochemical workstation. Three NirGO electrodes
with different weight ratios and two rGO and rGO: CB reference
electrodes were performed in the three-electrode system. In the
electrochemical cell, an aqueous solution of 1M
H2SO4 was used as the electrolyte, while
platinum (Pt) wire and Ag/AgCl were used counter as and reference
electrodes, respectively. As stated in section 2.1., rGO:Binder was used
as the standard electrode structure. In addition, a second standard
electrode with a higher surface area was prepared by adding CB to the
rGO:Binder mixture.
It was aimed to compare the capacitive performances of NirGO-based
foamed electrodes and the standard electrodes according to their CV
analysis results. All voltammograms were recorded between -0.4 V and
+0.6 V at the potential window.
The specific capacitance (\(C_{\text{sp}})\) of NirGO1, NirGO3 and
NirGO6-based electrodes were calculated by using the equation below
[31].
\begin{equation}
C_{\text{sp}}=\frac{i}{\text{ms}}\nonumber \\
\end{equation}In the equation, \(i,\) \(m,\) and \(s\) represent the average current,
the average mass of electrode materials (binder and additive), and the
scan rate (\(\Delta V/\Delta t\)), respectively.
With EIS analysis, it is possible to determine the resistance across the
entire device, electrode (related to its bulk resistance), and
electrode-electrolyte interface (related to the charge transfer). All
spectra were obtained with the measurements taken in the 10 MHz-100 kHz.
The bulk (Rb) and the charge transfer
(Rct) resistance values were calculated from these
spectra.
The galvanostatic CDC curves of all electrodes were recorded between
-0.4V and +0.6 V potential range under 1 mA current. From these graphs,
the specific capacitances (\(\complement\), F g-1)
were calculated for each electrode using the formula below.
\begin{equation}
\complement=\frac{I*t}{m*V}\nonumber \\
\end{equation}In the equation, I , the discharge current (A), is multiplied by∆t , the discharge time (s). This value is then divided by the
product of ∆V, the potential operating window (V ), andm, the mass of the electrode material (g ). Besides, CDC
plots were also used to calculate the energy density (E , Wh
kg−1) and the power density (P , W
kg−1) by using the following equations:
\(E=\frac{1}{2}C_{\text{sp}}V^{2}\)
\begin{equation}
P=\frac{E}{\text{Δt}}\nonumber \\
\end{equation}In the first equation, the calculated specific capacitance,Csp (F g−1) is
multiplied by the potential operating window, V (V), while in the
second equation, the calculated Energy Density, E , is divided by
the discharging time, Δt (s) represent.
Results and discussion
Structural Characterization of Electrode material
Raman spectroscopy, X-ray diffraction, FT-IR, XPS and EDX analyses were
used to elucidate the structure of the composite materials. Due to the
best overall capacitive performance achieved by NirGO3, some structural
and morphological analyses were reported and evaluated for that active
material.
The Raman spectra exhibited in Fig. 1(a) belong to rGO and NirGO3
electrode materials. In the spectra of rGO and NirGO3, the D band
corresponding to the defects and the disorders, while the G band
corresponding to the vibration of carbon atoms
(sp2-bonded) in the hexagonal lattice were observed at
1320 and 1596 cm−1, respectively [32].
Furthermore, peaks at around 545 and 781 cm−1, which
correspond to the Ni—OH bond, were also observed in the NirGO3
spectrum. These results indicated that the composite contains
oxide/hydroxide forms of Ni rather than
Ni(NO3)2 [33].
Fig. 1(b) shows the XRD graph of reference rGO gave a compatible pattern
with the literature [31, 34]. The broad peak of (002) graphitic
crystal planes in rGO was observed between 20-30°, while the peak
belonging to the (100) plane, π-π stacking of graphene, appeared at
44.2° as evidence for the formation of rGO.
Ni(OH)2 has two
phases, α and β, in the layered hexagonal structure. Although the
structure occurred in the β-phase form has high crystallinity and more
stability, the large surface area of the α-phase shows better
capacitive performance due to its low crystallinity. In the pattern of
NirGO, four characteristic peaks appeared at 10.95°, 33.76°, 45.21° and
59.08° belonging to (003), (016), (018), and (110) planes, respectively,
belong to the α-phase of Ni(OH)2. However, the peak
which must be observed at 20.59° originating from the (006) plane of
Ni(OH)2 could not be detected because the graphitic
crystal peak in the same region covers it. Furthermore, the
disappearance of the sharp peak of the (100) graphene plane and the
observed broad peaks of α-Ni(OH)2 indicate that the
obtained composite has a highly amorphous character [33, 35].
Increasing the amorphous form in the structure provides higher
capacitance with Ni-based composite electrodes. It has also been
previously reported that the α-crystal phase gives better capacitive
performance [36]. Therefore, it can be stated that the prepared
material is suitable for supercapacitor applications in terms of its
crystal structure.
Fig. 1(c) illustrates the FT-IR spectra of rGO and NirGO3. rGO presents
several oxygen functionalities remaining in rGO [37]. In addition,
both electrode materials show very similarly shaped FT-IR spectra, as
they have almost the same functional groups except for the Ni—O—H
structure. Due to the hydrogen bond-free nature of
β-Ni(OH)2, gives a sharp vibration peak at about 3645
cm-─1 corresponding to stretching vibration of O—H.
However, for the FT-IR spectrum of α-Ni(OH)2, this peak
is either unobservable or rarely detectable as a shoulder due to the
high rate of hydrogen bond signal [38, 39]. In the spectrum of
NirGO3, there is a relatively broader O—H band, between 3100
cm− 1 and 3650 cm−1, attributed to
stretching vibration belong to hydroxyl groups of
α-Ni(OH)2 with hydrogen-bonded and H2O
intercalated in between rGO layers[26]. In addition, a noticeable
redshift was observed for FT-IR peaks of NirGO3, particularly in the
strong vibrations of the C—O, C=O and —OH bond, compared to that of
rGO. On the other hand, in the rGO‘s spectrum, the peaks at 1557 (C—C
stretch) and 1627 cm−1 (C=C stretch) are fused in the
FT-IR spectrum NirGO, forming a broad peak appeared around 1614
cm−1 due to the interaction between rGO and
Ni(OH)2. The peaks at about 1384, 1081, and 1042
cm−1 belong to C—C, C—O, and C—O—C bonds,
respectively[26, 35]. These results also support the Raman analyses.
From the SEM images of rGO, rGO-CB and NirGO3 electrode materials, it
was determined that the porous structure of NirGO3 was more evident than
that of rGO (Fig. 2(a)-(f)). Although it could not be determined
numerically, the pore size distribution of NirGO3 material was more
uniform than that of other materials. As a result, it can be proposed
that the foam (or sponge) structured morphology is obtained, and the
surface area is increased while preparing the electrode material
composed from NirGO3. On the other hand, looking at the 50kX image of
the rGO electrode material modified with CB, it could be noticed that
the morphology was similar to that of rGO. In 250X extended images,
however, it was clear that the CB containing sample was more porous than
pure rGO. This is because CB is deposited between the rGO sheets,
reducing π-π stacking. In the lower (NirGO1) and higher (NirGO6)
concentrations of Ni(NO3)2, organic and
inorganic phases were separated, and the formation of an ideal foam
structure did not occur (Fig. S1(a-d)). In conclusion, SEM analyses show
that NirGO3 material has a morphologically significant advantage in
capacitor applications.