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\begin{document}

\title{One-step Preparation of Binder-free Nickel Containing Graphene Foam
Electrode for Supercapacitors}



\author[1]{Sirin SIYAHJANI GULTEKIN}%
\author[2]{Aziz Ahmad KARIMI}%
\author[3]{Mustafa CAN}%
\author[3]{Serafettin DEMIC}%
\affil[1]{Canakkale Onsekiz Mart University}%
\affil[2]{Ege University Solar Energy Institute}%
\affil[3]{Izmir Katip Celebi University}%


\vspace{-1em}



  
  \date{May 30, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
A nickel hydroxide (Ni(OH) \textsubscript{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 \selectlanguage{greek}α-\selectlanguage{english}Ni(OH)2:rGO structure. NirGO3 has 800 Fg
\textsuperscript{-1} capacitance, while rGO:CB has 900. Thus NirGO3 has
the best capacitive performance among binder-free electrodes, comparable
to rGO:CB.%
\end{abstract}%



\sloppy


\textbf{One-step Preparation of Binder-free Nickel Containing Graphene
Foam Electrode for Supercapacitors}

Sirin SIYAHJANI GULTEKIN\textsuperscript{a,b}*, Aziz Ahmad
KARIMI\textsuperscript{b,c}, Mustafa CAN\textsuperscript{c}, Serafettin
DEMIC\textsuperscript{c}

\textsuperscript{a} Canakkale Onsekiz Mart University, Faculty of
Engineering, Department of Chemical Engineering, Canakkale, Turkey

\textsuperscript{b}Ege University Solar Energy Institute, Bornova Turkey

\textsuperscript{c} Izmir Katip Celebi University, Faculty of
Engineering and Architecture, Department of Materials Science and
Engineering, Izmir, Turkey

\textbf{Abstract}

A nickel hydroxide (Ni(OH)\textsubscript{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 \selectlanguage{greek}α-\selectlanguage{english}Ni(OH)2:rGO structure. NirGO3 has 800
Fg\textsuperscript{-1} capacitance, while rGO:CB has 900. Thus NirGO3
has the best capacitive performance among binder-free electrodes,
comparable to rGO:CB.

\textbf{Keywords:} foam electrode, supercapacitor, self-templated,
nickel hydroxide

\begin{enumerate}
\tightlist
\item
  \textbf{Introduction}Supercapacitors (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 m\textsuperscript{2} g\textsuperscript{-1}), high
  electronic conductivity, mechanical flexibility and excellent thermal
  stability {[}4-7{]}. However, \selectlanguage{greek}π-π \selectlanguage{english}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)\textsubscript{2}, Fe\textsubscript{2}O\textsubscript{3},
  MnO\textsubscript{2}, etc.) have been inserted into the electrode
  structure {[}8-11{]}. Although high capacitance values (maximum
  theoretical capacitance: 2082 F g\textsuperscript{-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-TiO\textsubscript{2}composite was obtained by a procedure
  called one-step-hydrothermal method at 180 \selectlanguage{ngerman}°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 TiO\textsubscript{2} sheets {[}28{]}. Similar
  structures containing Ni, NiO, and Ni(OH)\textsubscript{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\selectlanguage{english}\textsuperscript{-1} at 2 A g\selectlanguage{english}\textsuperscript{-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 400\textsuperscript{o}C. A
  specific capacitance of 727.1 F g\selectlanguage{english}\textsuperscript{-1} at 1mA
  cm\selectlanguage{english}\textsuperscript{-2} was achieved with a stability over 80\% after
  about 9000 cycles {[}3{]}{[}26{]}. In addition, an electrode based on
  Ni(OH)\textsubscript{2} for supercapacitor application was reported by
  Viswanathan and co-workers{[}8{]}. As well known,
  Ni(OH)\textsubscript{2} has good solubility in acids such as
  H\textsubscript{2}SO\textsubscript{4}. A chemical reduction method was
  achieved to obtain insoluble rGO/Ni(OH)\textsubscript{2} structure
  showing a specific capacitance value over 130 F g\selectlanguage{english}\textsuperscript{-1}
  in 1M H\textsubscript{2}SO\textsubscript{4} 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.
\item
  \textbf{Experimental Section}
\item
  \textbf{Preparation of electrode material and fabrication of PSC}
\end{enumerate}

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
cm\textsuperscript{2}) 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 degC pre-heated graphite sheet (the active material
is 1.0-1.3 mg in 1*1 cm\textsuperscript{2} 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.

\textbf{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 K\selectlanguage{greek}α \selectlanguage{english}radiation (\selectlanguage{greek}λ \selectlanguage{english}= 1.54 \selectlanguage{ngerman}Å), while step size and scan rate are
0.02° and 2°/min, respectively, in a 2\selectlanguage{greek}θ \selectlanguage{english}range from 3\selectlanguage{ngerman}° 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)\textsubscript{2} in the 400-2500
cm\selectlanguage{english}\textsuperscript{-1}. Furthermore, the phase of
Ni(OH)\textsubscript{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\selectlanguage{english}\textsuperscript{-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.

\textbf{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
H\textsubscript{2}SO\textsubscript{4} 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 (R\textsubscript{b}) and the charge transfer
(R\textsubscript{ct}) 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\textsuperscript{-1})
were calculated for each electrode using the formula below.

\begin{equation}
\complement=\frac{I*t}{m*V}\nonumber \\
 \end{equation}

In the equation, \emph{I} , the discharge current (A), is multiplied
by\emph{[?]t} , the discharge time (s). This value is then divided by the
product of \emph{[?]V,} the potential operating window (\emph{V} ),
and\emph{m,} the mass of the electrode material (\emph{g} ). Besides,
CDC plots were also used to calculate the energy density (\emph{E} , Wh
kg\textsuperscript{-1}) and the power density (\emph{P} , W
kg\textsuperscript{-1}) by using the following equations:

\(E=\frac{1}{2}C_{\text{sp}}V^{2}\)

\begin{equation}
P=\selectlanguage{greek}\frac{E}{\text{Δt}}\selectlanguage{english}\nonumber \\
 \end{equation}

In the first equation, the calculated specific
capacitance,\emph{C\textsubscript{sp}} (\emph{F} g\textsuperscript{-1})
is multiplied by the potential operating window, \emph{V} (V), while in
the second equation, the calculated Energy Density, \emph{E} , is
divided by the discharging time, \selectlanguage{greek}\emph{Δt}\selectlanguage{english} (s) represent.

\textbf{Results and discussion}

\textbf{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
(sp\textsuperscript{2}-bonded) in the hexagonal lattice were observed at
1320 and 1596 cm\textsuperscript{-1}, respectively {[}32{]}.
Furthermore, peaks at around 545 and 781 cm\textsuperscript{-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(NO\textsubscript{3})\textsubscript{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\selectlanguage{ngerman}°, while the peak
belonging to the (100) plane, \selectlanguage{greek}π-π \selectlanguage{english}stacking of graphene, appeared at
44.2\selectlanguage{ngerman}° as evidence for the formation of rGO. Ni(OH)\textsubscript{2} has
two phases, \selectlanguage{greek}α \selectlanguage{english}and \selectlanguage{greek}β, \selectlanguage{english}in the layered hexagonal structure. Although the
structure occurred in the \selectlanguage{greek}β-\selectlanguage{english}phase form has high crystallinity and more
stability, the large surface area of the \selectlanguage{greek}α-\selectlanguage{english}phase shows better
capacitive performance due to its low crystallinity. In the pattern of
NirGO, four characteristic peaks appeared at 10.95\selectlanguage{ngerman}°, 33.76°, 45.21° and
59.08° belonging to (003), (016), (018), and (110) planes, respectively,
belong to the \selectlanguage{greek}α-\selectlanguage{english}phase of Ni(OH)\textsubscript{2}. However, the peak
which must be observed at 20.59\selectlanguage{ngerman}° originating from the (006) plane of
Ni(OH)\textsubscript{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 \selectlanguage{greek}α-\selectlanguage{english}Ni(OH)\textsubscript{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 \selectlanguage{greek}α-\selectlanguage{english}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
\selectlanguage{greek}β-\selectlanguage{english}Ni(OH)\textsubscript{2}, gives a sharp vibration peak at about 3645
cm\textsuperscript{--1} corresponding to stretching vibration of O---H.
However, for the FT-IR spectrum of \selectlanguage{greek}α-\selectlanguage{english}Ni(OH)\textsubscript{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\textsuperscript{- 1} and 3650 cm\textsuperscript{-1,} attributed to
stretching vibration belong to hydroxyl groups of
\selectlanguage{greek}α-\selectlanguage{english}Ni(OH)\textsubscript{2} with hydrogen-bonded and H\textsubscript{2}O
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\textsuperscript{-1} (C=C stretch) are fused in the
FT-IR spectrum NirGO, forming a broad peak appeared around 1614
cm\textsuperscript{-1} due to the interaction between rGO and
Ni(OH)\textsubscript{2}. The peaks at about 1384, 1081, and 1042
cm\textsuperscript{-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 \selectlanguage{greek}π-π \selectlanguage{english}stacking. In the lower (NirGO1) and higher (NirGO6)
concentrations of Ni(NO\textsubscript{3})\textsubscript{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.

\textbf{Hosted file}

\verb`image1.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Hosted file}

\verb`image2.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Hosted file}

\verb`image3.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Fig. 1.} (a) Raman shift spectra, (b) XRD patterns (\# refers to
rGO crystalline signals, * refers to Ni(OH)\textsubscript{2} crystalline
signals) and (c) FT-IR spectra of electrode materials.

In the EDS spectrum (Fig. 2(g)) of Ni-rGO, as expected, there is a Ni
atom in the structure accompanied by C and O. Although
Ni(NO\textsubscript{3})\textsubscript{2} was used as the reactant at the
beginning of the reaction, the absence of the signal of the N atom in
the spectrum indicates that the starting material is fully reacted, and
as a result of this process, oxide/hydroxide derivatives of Ni are
eventually formed.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Fig. 2.} (a) 50kX, (b) 250X micrographs of rGO, (c) 50kX and (d)
250X micrographs of rGO:CB, (e) 50kX and (f) 250X micrographs of NirGO3
materials and (g) EDS spectrum of NirGO3.

In the Cs1 spectrum of the XPS survey (Fig. 3(b)), peaks at 285.00,
286.67, and 288.90 eV were related to C=C, C---OH and C=O bonds,
respectively. The peaks belonging to O=C/O---Ni and HO---C were observed
at 533.04 and 531.56 eV in the Os1 spectrum (Fig. 3(c)), respectively.
Furthermore, in the Ni2p spectrum (Fig. 3(d)), two typical peaks at
855.80 and 873.40 eV are referred to as Ni2p\textsubscript{3/2} and
Ni2p\textsubscript{1/2}, respectively. Spin-energy separation was
calculated as 17.60 eV as the individual indicator for Ni
(OH)\textsubscript{2} phase (Fig. 3(a)). Besides, satellite signals are
defined as two extra peaks at 861.50 and 879.00 eV, which close the
Ni2p\textsubscript{3/2} and Ni2p\textsubscript{1/2}. XPS results agree
with XRD and FT-IR analyses and indicate that the electrode material of
NirGO3 is consisting of rGO and Ni(OH)\textsubscript{2}.

\textbf{Hosted file}

\verb`image6.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Hosted file}

\verb`image7.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Hosted file}

\verb`image8.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Hosted file}

\verb`image9.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Fig. 3.} XPS analysis of NirGO3 material: (a) complete XPS
survey spectrum, deconvoluted spectra of (b) C1s, (c) O1s, and (d) Ni2p.

\textbf{Electrochemical performance of supercapacitor cells}

\textbf{Cyclic voltammetry}

As seen from the voltammograms, the capacitive performance of the
reference rGO electrode (containing binder) was limited because of its
relatively high resistance and low surface area (Fig. 4a). When CB was
added to the mixture, a decrease in the capacitance was observed because
of the increase in the surface area of the electrode. Besides, as it is
well known, the square-like shape of the voltammogram indicates an ideal
capacitive behaviour in a device. However, the rGO-CB graph was not
square, indicating high resistance related to the binder. When the
voltammograms of the binder-free NirGO foamed electrodes are examined,
it is noticed that the graphs are relatively closer to the square-like
shape than those of rGO and rGO: CB. The oxidation peak of
Ni(OH)\textsubscript{2} at 0.3 V, which was expected in NirGO
voltammograms, could not be observed very clearly. This is due to a
phenomenon called the surface-based faradaic process. In this process, a
capacitive faradaic activity is exhibited on the surface thanks to the
delocalized electrons on the electrode. Therefore, square-like
voltammograms were obtained at {[}40{]}. Since there was no binder in
these electrodes, the resistance in the electrode was relatively lower.
In addition, the square-like graphs showed that the pore sizes and ion
sizes were compatible.

The capacitive performance of NirGO3 is comparable to that of rGO:CB
based electrode (900 F g\textsuperscript{-1}). The results regarding
porosity and surface areas were in agreement with the results obtained
from SEM studies. Furthermore, voltammograms were recorded at different
scanning rates (Fig. 4(b), (c) and Fig. S2) to compare pore size-ion
diameter match and resistances at the electrolyte-electrode interface.
When the graphs of rGO:CB and NirGO3, which showed the relatively best
performance, were examined, it was observed that the voltammograms of
NirGO3 obtained at different scan rates were less distorted than those
of rGO:CB as the speed increased (Fig. 4(b)). In the device prepared
with NirGO3 electrode material, this shows that ions move more
efficiently during entry and exit into the pores than in devices
prepared with other electrode materials.

\textbf{Hosted file}

\verb`image10.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Hosted file}

\verb`image11.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Hosted file}

\verb`image12.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Hosted file}

\verb`image13.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Fig. 4.} (a) Comparison of cyclic voltammograms of all
electrodes (scan rate: 50 mV s\textsuperscript{-1}). Cyclic
Voltammograms of rGO:CB, (b) and NirGO3 at various scan rates (c).
Variation of specific capacitance calculated from CV with scan rate for
all electrodes (d). specific capacitance vs scan rate graphs of all
electrodes.

\textbf{3.2.2. Electrochemical impedance spectroscopy (EIS)}

In the EIS spectrum, the bulk resistance (R\textsubscript{b}) is related
to the interruption on the real axis at high frequency. At the same
time, the semicircle at the mid-frequency zone corresponds to the charge
transfer resistance (R\textsubscript{ct}) at the electrolyte/electrode
interface, which is strongly related to the capability of ion transfer
into the pores of an electrode (Table 1). As seen in Fig. 5, the device
based on NirGO3 has the lowest bulk resistance (2 ) and the charge
transfer resistance (1 Ohm) compared to other devices. This is because
the \selectlanguage{greek}π-π \selectlanguage{english}agglomeration in bare rGO is less in NirGO-based electrodes.
During the formation of the foam structure, this agglomeration was
prevented, and thus lower resistance was obtained. Although CB prevents
this agglomeration, there is an increase in bulk resistance due to the
usage of a dielectric binder. Among the NirGOs, the highest resistance
was observed in NirGO6. As seen in the SEM images, the reason is that as
the Ni ratio increases, the regular agglomeration and the formation of
porous structure between the rGO layers deteriorates (Fig. S1).
Therefore, the bulk resistance (11 ) at the electrode and the charge
transfer resistance (4 ) between the electrolyte and the electrode also
increases.

\textbf{3.2.3. Galvanostatic charge-discharge (CDC) characteristics}

The capacitance of each electrode materials was also calculated by using
the galvanostatic CDC curves (Fig. 6 (a)). Calculated capacitance of
rGO, rGO:CB, NirGO1, NirGO3, and NirGO6, were 606, 1000, 714, 833, and
270 Fg\textsuperscript{-1}, respectively (Table 1). When the IR
(current-resistance) drops in each electrode are examined, it is seen
that they are very close to each other but slightly higher than that of
NirGO6 (0.3 Ohm). This shows that the total resistances in the devices
are close to each other. Then, CDC curves were obtained for all
electrode materials at different currents between 1-5 mA. In this way,
charge transfer and interaction between electrode and electrolyte were
investigated. All devices retained their capacitive behaviour even
though their capacitance values decreased with the current increase.

\textbf{Hosted file}

\verb`image14.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Fig. 5.} Electrochemical impedance spectra of electrodes (f=10
mHz-100kHz).

\textbf{Table 1.} Calculated electrochemical parameters of all
electrodes by using EIS and CDC results.\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
\begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
\textbf{Device}\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
\textbf{R\textsubscript{b}} \textbf{()}\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
\textbf{R\textsubscript{ct}} \textbf{()}\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
\textbf{C\textsubscript{SP} EIS} \textbf{(F
g\textsuperscript{--1})}\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
\textbf{C\textsubscript{SP}} \textbf{CDC} \textbf{(F
g\textsuperscript{-1})}\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
\textbf{Energy Density\textsuperscript{1}} \textbf{(Wh
kg\textsuperscript{-1})}\strut
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedright\strut
\textbf{Power Density\textsuperscript{1}} \textbf{(W
kg\textsuperscript{-1})}\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{rGO}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
12.0\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
3.0\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
476\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
606\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
83.3\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
500.0\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{rGO-CB}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
3.5\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
3.0\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
800\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1000\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
136.1\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
490.0\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{NirGO1}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
4.0\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
2.0\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
628\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
714\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
97.0\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
499.0\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{NirGO3}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1.5\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1.5\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
888\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
833\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
113.3\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
509.9\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\textbf{NirGO6}\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
10.0\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
4.0\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
272\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
270\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
35.8\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1795\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\textsuperscript{1}Calculated from CDC data (applied current: 1A)

Furthermore, looking at the discharge current vs. specific capacitance
plot (Fig. 6(d)), the capacitance drop due to the current increase in
foamed electrodes is slightly less than those in rGO and rGO:CB. This
may be due to the incompatibility of the pore size of the rGO and rGO:CB
materials and the ion diameters in the electrolyte. Also, when
considering the decrease in capacitance due to the increase in current,
it is clear that the curve of NirGO3 is more linear, and there is less
drop in the rGO:CB and NirGO3, in which NirGO3 shows higher performance
than the other electrodes. This is because although CB increases the
surface area of the electrode material, it causes an irregularity in
the pore structure due to its structure. The accumulation and release of
ions in the electrolyte decrease at high currents. In addition, specific
energy and power values of the electrodes at different currents were
determined from CDC graphs to obtain the Ragone Plot. The Ragone plot,
which has power density and energy density axes, helps to illustrate the
electrochemical performance of the device. In studies conducted at 5
different current values between 1-5 A g\textsuperscript{-1}, the
energy density values of standard electrodes decrease dramatically with
the increase of current density, although their power densities
increase. However, the decrease of energy density in foamed electrodes
is less than those of reference electrodes. It was observed that the
foamed electrodes retain their specific energy more than those of rGO
and rGO:CB at high specific power values (Fig. 6(e)). As mentioned
above, the ion paths are short and favourable for the diffusion of ions
throughout the foam-structured Ni-based electrodes {[}36{]}.

\textbf{Hosted file}

\verb`image15.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Hosted file}

\verb`image16.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Hosted file}

\verb`image17.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Hosted file}

\verb`image18.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Hosted file}

\verb`image19.emf`  available at \url{https://authorea.com/users/657349/articles/662186-one-step-preparation-of-binder-free-nickel-containing-graphene-foam-electrode-for-supercapacitors}

\textbf{Fig. 6.} (a) Galvanostatic CDC curves of all electrodes (applied
current: 1 mA), (b) CDC curves of rGO:CB electrode under different
currents, (c) CDC curves of NirGO3 electrode under different currents,
(d) specific capacitance (F g\textsuperscript{-1}) vs. discharge current
(1-5 A g\textsuperscript{-1}) and (e) energy density (Wh
kg\textsuperscript{-1}) vs. power density (W kg\textsuperscript{-1})
plots for all electrodes.

\textbf{Conclusion}

Composite electrodes with three Ni/rGO ratios were made without a binder
in one step. The structural characterisation of NirGO1, NirGO3, and
NirGO6 electrodes revealed that the nickel metal present in the
electrodes, \selectlanguage{greek}α-\selectlanguage{english}Ni(OH)2, demonstrated improved capacitive behavior. The
study found that the capacitance values of binder-free NirGO1, NirGO3,
and NirGO6 were 720, 800, and 310 F g\textsuperscript{-1} respectively
in 1M H\textsubscript{2}SO\textsubscript{4} electrode solution using a
three-electrode method. Unlike the reference electrode rGO:CB, its
capacitance value was computed as 900 F g\textsuperscript{-1}. According
to capacitance tests NirGO3 current decay was less than rGO:CB owing to
homogeneous pore structure of NirGO3.

\selectlanguage{ngerman}\textbf{CRediT authors´ contribution statement}

\textbf{Sirin SIYAHJANI GULTEKIN:} Conceptualization, Methodology,
Investigation, Writing- Original draft preparation, \textbf{Aziz Ahmad
KARIMI:} Validation, Investigation, Visualization, \textbf{Mustafa
CAN:}Resources, Writing - Review \& Editing, \textbf{Serafettin
DEMIC:}Resources, Supervision, Writing - Review \& Editing

\textbf{Acknowledgements}

Authors acknowledge to Presidency of Turkish Republic Department of
Strategy and Budget for the financial support Project Grant Nr:
2016K12-2841. Authors also acknowledge TÜB\selectlanguage{polish}İ\selectlanguage{english}TAK-BIDEB 2218 Program for
financial support of Sirin Siyahjani GULTEKIN with Grant Nr.: 118C565.

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