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\DOI{10.5802/crchim.424}
\datereceived{2025-05-01}
\daterevised{2025-09-12}
\dateaccepted{2025-09-18}
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\COI{The authors do not work for, advise, own shares in, or receive
funds from any organization that could benefit from this article, and
have declared no affiliation other than their research organizations.}

%\dateposted{2026-04-23}

\dateposted{2026-08-17}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\title{Recycling graphite waste for high-performance nanofluids:
synthesis and property analysis}

\alttitle{Recyclage des d\'{e}chets de graphite pour des nanofluides
\`{a} haute performance : synth\`{e}se et analyse des
propri\'{e}t\'{e}s}

\author{\firstname{Mourad} \lastname{Makhlouf}\CDRorcid{0000-0002-6157-5073}\IsCorresp}
\address{Scientific and Technological Research Directorate-Cherchell
Academy DPHB, Tipaza, Algeria}
\address{Laboratory of Energy Processes and Nanotechnology, 
University of Blida-Algeria, Algeria}
\email[M. Makhlouf]{makhlouf\_mourad@univ-blida.dz}

\author{\firstname{Hichem} \lastname{Hachemi}\CDRorcid{0000-0002-0341-4538}} 
\addressSameAs{1}{Scientific and Technological Research Directorate-Cherchell
Academy DPHB, Tipaza, Algeria}

\author{\firstname{Meriem} \lastname{Boutamine}\CDRorcid{0000-0002-0908-2896}} 
\addressSameAs{1}{Scientific and Technological Research Directorate-Cherchell
Academy DPHB, Tipaza, Algeria}

\author{\firstname{Zoubir}\nobreakauthor\lastname{Benmaamar}\CDRorcid{0000-0001-9469-7301}} 
\addressSameAs{2}{Laboratory of Energy Processes and Nanotechnology,
University of Blida-Algeria, Algeria}

\author{\firstname{Didier} \lastname{Villemin}\CDRorcid{0000-0002-6266-3817}} 
\address{Laboratory of Molecular and Thio-organic Chemistry, UMR CNRS
6507, INC3M, FR 3038, ENSICAEN and Research Center, University of Caen, France}

\shortrunauthors

\keywords{\kwd{Graphene}
\kwd{Recycling}
\kwd{Graphite}
\kwd{Electrochemical exfoliation}
\kwd{Nanofluid}}

\altkeywords{\kwd{Graph\`{e}ne}
\kwd{Recyclage}
\kwd{Graphite}
\kwd{Exfoliation \'{e}lectrochimique}
\kwd{Nanofluide}}

\begin{abstract}
Graphene, a remarkable material with extraordinary properties, has
revolutionary potential for various technological applications.
However, conventional methods of graphene production often involve
energy-intensive and polluting processes. To address this challenge, we
propose a sustainable approach that combines the production of
high-quality graphene from recycled battery waste and its integration
into nanofluids. Thanks to a simple and scalable electrochemical
exfoliation technique, it is possible to obtain graphene with superior
properties. The present study focuses on the large-scale preparation
of graphene by recycling graphite from energy storage devices using a
simple and inexpensive electrochemical technique. Characterization of
the obtained materials was carried out by Raman spectroscopy, X-ray
diffraction, specific surface area analysis, and scanning electron
microscopy. The viscosity measurement and analysis of a
graphene--water nanofluid were carried out at different
temperatures and volume fractions. All viscosity measurements were
carried out using a capillary viscometer at temperatures between 25 and
65 \textdegree C. The nanofluid showed increasing viscosity with
increasing nanoparticle concentration and decreasing viscosity with
increasing temperature.
\end{abstract}

\begin{altabstract}
Le graph\`{e}ne, un mat\'{e}riau remarquable dot\'{e} de
propri\'{e}t\'{e}s extraordinaires, poss\`{e}de un potentiel
r\'{e}volutionnaire pour diverses applications technologiques.
Cependant, les m\'{e}thodes conventionnelles de production du
graph\`{e}ne impliquent souvent des processus gourmands en \'{e}nergie
et polluants. Pour relever ce d\'{e}fi, nous proposons une approche
durable qui combine la production de graph\`{e}ne de haute qualit\'{e}
\`{a} partir de d\'{e}chets de batteries recycl\'{e}es et son
int\'{e}gration dans des nanofluides. Gr\^{a}ce \`{a} une technique
simple et \'{e}chelonnable d'exfoliation \'{e}lectrochimique, il est
possible d'obtenir du graph\`{e}ne aux propri\'{e}t\'{e}s
sup\'{e}rieures. La pr\'{e}sente \'{e}tude se concentre sur la
pr\'{e}paration \`{a} grande \'{e}chelle du graph\`{e}ne par le
recyclage du graphite provenant de dispositifs de stockage
d'\'{e}nergie, en utilisant une technique \'{e}lectrochimique simple et
peu co\^{u}teuse. La caract\'{e}risation des mat\'{e}riaux obtenus a
\'{e}t\'{e} r\'{e}alis\'{e}e par spectroscopie Raman, diffraction des
rayons X, analyse de la surface sp\'{e}cifique et microscopie
\'{e}lectronique \`{a} balayage. La mesure de la viscosit\'{e} et
l'analyse d'un nanofluide \`{a} base de graph\`{e}ne et d'eau ont
\'{e}t\'{e} effectu\'{e}es \`{a} diff\'{e}rentes temp\'{e}ratures et
fractions volumiques. Toutes les mesures de viscosit\'{e} ont
\'{e}t\'{e} r\'{e}alis\'{e}es \`{a} l'aide d'un viscosim\`{e}tre
capillaire \`{a} des temp\'{e}ratures comprises entre 25 et
65 \textdegree C. Le nanofluide a montr\'{e} une viscosit\'{e} croissante
avec l'augmentation de la concentration en nanoparticules et une
viscosit\'{e} d\'{e}croissante avec l'augmentation de la
temp\'{e}rature.
\end{altabstract}

\editornote{Publication delayed at the request of the author.}
\alteditornote{Publication retard\'ee \`a la demande de l'auteur.}

%\input{CR-pagedemetas}

\maketitle

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}

Graphene has demonstrated excellent mechanical~\cite{1},
thermal~\cite{2}, optical, and electronic properties~\cite{3}, offering
the potential for a range of applications in energy storage~\cite{4},
catalysis~\cite{5}, sensing~\cite{6}, molecular separation~\cite{7},
and protective composite coatings~\cite{8}. However, producing graphene
in an environmentally friendly and cost-effective way \mbox{remains} a
significant challenge~\cite{9}. One \mbox{promising} solution is the
electrochemical exfoliation of graphite, which requires an electrolyte
containing intercalation species, a graphite electrode as a source of
graphene, and a power supply~\cite{10}. One approach that addresses
these concerns is the utilization of recycled graphite waste as a
sustainable and cost-effective source of graphene~\cite{11}. Other
research has explored using graphene and graphene oxide, synthesized
economically via the modified Hummers method, in battery
manufacturing~\cite{12,13,14}. 

Efficient graphite exfoliation into graphene depends on both anion
intercalation and water \mbox{oxidation}. Sulfate anions, due to their
strong binding energy and reversible behavior, facilitate the entry of
water into the graphite, which makes the exfoliation more
effective~\cite{15}. Gupta et~al.~\cite{16} demonstrated that the
materials' electrical properties enable an open-circuit voltage of
around 250 mV and a maximum power of 22~${\upmu}$W. This study suggests
that graphene oxide could be a cost-effective material for producing
compact energy devices, highlighting its potential in the field of
microbatteries. Commonly used anionic intercalation species include
sulfate (SO\tsub{4}\tsup{2\tminus})~\cite{17},
perchlorate (ClO\tsub{4}\tsup{\tminus})~\cite{18},
trifluoroborate
(BF\tsub{4}\tsup{\tminus})~\cite{19}, and
hexafluorophosphate
(PF\tsub{6}\tsup{\tminus})~\cite{20}. Sulfate is
one of the most commonly used anions. It is highly effective in
facilitating the exfoliation process in aqueous solutions of acidic or
neutral inorganic salts, including those containing sulfuric acid and
ammonium sulfate~\cite{21}. Graphene has emerged as a potential leader
in the development of high-performance nanofluids thanks to its unique
mechanical, thermal, and electrical properties~\cite{22,23}. Even at
low concentrations, graphene sheets are known to enhance thermal
conductivity, enabling the development of high-performance cooling
technologies, particularly for electronic devices~\cite{24,25}.
Furthermore, graphene can stabilize nanofluids by forming a protective
layer around most of the released nanoparticles~\cite{26}. 

Nanofluids have found numerous scientific and industrial
applications~\cite{27,28}. However, due to their highly complex nature,
predicting the thermophysical properties of these fluids has become a
major research challenge~\cite{29,30}. Some studies on the rheological
behavior of nanofluids have focused on determining whether they are
Newtonian or non-Newtonian fluids~\cite{31}. However, many factors
influence the viscosity of nanofluids, such as temperature, pH, volume
fraction, particle size, particle size distribution, zeta potential,
and base fluid~\cite{32,33,34}. The influence of nanofluid viscosity on
nanoparticle loading has been extensively studied~\cite{35,36,37}. The
viscosity of nanofluids containing various types of nanoparticles, such
as metals, oxides, and carbon nanotubes, has been studied as a function
of nanoparticle concentration~\cite{38,39}. Despite extensive
experimental studies on the effect of nanoparticle loading on nanofluid
viscosity, there is no universal equation that can predict this
property with any great accuracy~\cite{40,41}. Furthermore, almost all
studies on nanofluid viscosity have demonstrated a positive correlation
between viscosity and increasing nanoparticle volume fraction~\cite{42,43}. 
However, this has not been observed in all nanofluids, except for
those based on carbon nanotubes, which exhibit an inverse relationship
between viscosity and particle loading~\cite{44}. Moreover, Nadooshan
et~al.'s comprehensive study of the rheological behavior of nanofluids
concluded that most nanofluids exhibit Newtonian behavior at low volume
fractions and non-Newtonian behavior at high nanoparticle volume
fractions~\cite{45}. Additionally, an increase in volume fraction has
been shown to lead to nanoparticle aggregation and consequently an
increase in fluid viscosity~\cite{46, 47}. This increase in viscosity
is due to the increase in the surface/volume ratio during aggregate
formation~\cite{48, 49}. The effect of aggregation on the viscosity of
Al\tsub{2}O\tsub{3}--water nanofluids was studied,
and the results confirmed that relative viscosity increases with
increasing aggregate formation. The results also showed that the
nanofluid's volume fraction does not directly affect viscosity and that
an increase in particle loading leads to aggregate formation.
Consequently, viscosity increases with aggregation size~\cite{50}.
However, research into the effect of temperature on the viscosity of
nanofluids has not resulted in a universal formula describing the
viscosity behavior of these complex fluids as a function of
temperature~\cite{51, 52}. This may be because other factors, such as
the type of base fluid, volume fraction, and particle size, affect the
viscosity~\cite{53}. 

Consequently, relative viscosity ($\mu_{\mathrm{rel}} =
{\mu}_{\mathrm{nanofluid}}/\mu_{\mathrm{basefluid}}$) has been found to
be more advantageous than absolute viscosity, as this makes it easier
to understand the temperature dependence of viscosity~\cite{54}.
Relative viscosity remained virtually stable with increasing
temperature for low to moderate particle loading for almost all
nanofluid types. However, at high nanoparticle concentrations, relative
viscosity began to grow with temperature~\cite{55}. A few studies have
shown hysteresis of nanofluid relative viscosity with temperature,
where relative viscosity increases and decreases with
temperature~\cite{56}. This behavior was observed in the study by
Namburu et~al.~\cite{57}. Other researchers have concluded that
relative viscosity decreases with increasing temperature. The study by
Li et~al.~\cite{58} on zinc oxide nanoparticles dispersed in ethylene
glycol (ZnO-EG nanofluids) demonstrates a clear \mbox{correlation} 
\mbox{between}
nanoparticle size, volume {fraction,} and the resulting relative
viscosity of the suspension. Most studies on the influence of
nanoparticle size have shown a decrease in viscosity as the particle
size increases. Viscosity of the water--graphene nanofluid increases
with nanoparticle concentration and decreases with temperature. Surface
tension also decreases in both cases~\cite{59,60}. Other studies~\cite{61}
have demonstrated a direct relationship between viscosity and
nanoparticle size. Ahammed et~al.\ have explored how varying volume
concentration and temperature influences the viscosity and surface
tension of a water--graphene nanofluid. Their results demonstrate that
viscosity is more sensitive to volume concentration than to
temperature, indicating its potential as an effective coolant for
real-time thermal applications~\cite{62}. The novelty of our study lies
in its sustainable and economical approach to producing high-quality
graphene. Rather than using conventional methods that are
energy-intensive and polluting, our research proposes recycling
graphite from battery waste using a simple, low-cost, electrochemical
exfoliation technique.

\section{Experimental section}\label{sec2}

\subsection{Synthesis of graphene}\label{sec21}

High-quality, large-area graphene sheets were produced by synthesizing
graphene through electrochemical exfoliation of graphite rods from used
electric batteries. These rods act as electrodes, serving as both the
anode and the cathode in an aqueous solution of sulfuric acid
(H\tsub{2}SO\tsub{4}). Applying a constant direct
current voltage of 10~V to the electrodes initiated an electrochemical
reaction that intercalated sulfate ions into the graphite layers. This
intercalation process expanded the graphite structure, resulting in its
exfoliation into graphene sheets. The process typically required 1.5~h
to completely consume the graphite anode. The aqueous electrolyte of
graphene was then subjected to centrifugation at  10\,000~tr/min for
5~min and the pellet washed multiple times with distilled water to
remove residual acid and impurities. A 1:10 (w/v) solid-to-liquid ratio
was used during the washing steps. After undergoing a half-hour
sonication process, the graphene was dried under vacuum for 24~h at 
333~K~\cite{8}.

\subsection{Characterization of graphene}\label{sec22}

The microscopic morphology of the samples was characterized using
scanning electron microscopy (SEM on a Quanta FEG 250). The materials'
structures were characterized by Fourier transform infrared
spectroscopy (FT-IR on a JASCO V 770 spectra analyzer), X-ray
diffraction (XRD on a D8 Advance Eco Bruker diffractometer, which
operated using a copper tube at $\lambda= 1.54~\AA{}$). Raman
spectroscopy was also used (inVia from Renishaw, with monochromatic
Ar\tsup{+} laser radiation at a wavelength of 514.5~nm). A
BET analyzer (Gemini VII 4.00) was used to determine the materials'
specific surface area, pore size, and volume. The specific surface area
was determined by relating the amount of nitrogen adsorbed to the
nitrogen pressure. These parameters were calculated using the
Brunauer--Emmett--Teller (BET) and Barrett--Joyner--Halenda (BJH)
methods. \vspace*{-3pt}

\subsection{Preparation of nanofluid}\label{sec23}

Nanofluids were prepared by dispersing the exfoliated graphene in
deionized water (as the base fluid), achieving a uniform and
homogeneous dispersion. The density of graphene is
2.1~g/cm\tsup{3}. In its natural state, graphene exists as
sheets with a two-dimensional structure. It has excellent mechanical,
thermal, and electrical properties. However, its dispersion is complex
due to its large specific surface area. The use of high-power
ultrasonic equipment enabled the dispersion of graphene. The results
show the consistency and stability of the nanofluid. Five samples of
graphene--deionized water were prepared with different concentrations
(0.001, 0.002, 0.003, 0.004, and 0.005~g/L). All prepared samples were
then subjected to ultrasonication (200~W, 20 to 40~kHz) for
approximately 2~h at room temperature to ensure homogeneity and
stability. To ensure the stability and reproducibility of the viscosity
measurements, samples stored for extended periods were re-dispersed via
ultrasonication for 30 to 60~min prior to analysis. \vspace*{-3pt}

\section{Results and discussion}\label{sec3}

Scanning electron microscopy (SEM) is an excellent technique for
studying the micro- and/or nanostructure, topography, and distribution
of different phases in samples. Figure~\ref{fig1} shows images of
graphene at \mbox{different} magnifications: 200, 12\,000 and
24\,000${\times}$.

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}SEM of graphene~\cite{10} ({\textcopyright} 2022
IOP Publishing. Reproduced with permission. All rights reserved).}
\vspace*{2pt}
\end{figure*}


The SEM images in Figure~\ref{fig1} confirm the exfoliation of the
graphite into multi-layered graphene sheets, which are entangled due to
their overlap. They also show a smooth structure for the synthesized
graphene.


The FTIR spectrum of graphene (Figure~\ref{fig2}) shows a broad band
centered around 3500--2800~cm\tsup{\tminus 1}, which is
characteristic of O--H stretching vibrations. This indicates the
presence of adsorbed water molecules and hydroxyl groups~\cite{62}. The
band at 1559~cm\tsup{\tminus 1} is assigned to the
stretching and bending modes of water molecules. The peak at
1624~cm\tsup{\tminus 1} corresponds to the aromatic C${=}$C
sp\tsup{2} stretching vibration. The band at 1014
cm\tsup{\tminus 1} corresponds to the C--O of the alkyl
groups. 

\begin{figure}
\includegraphics{fig02}
\vspace*{-2pt}
\caption{\label{fig2}FT-IR of graphene.}
\vspace*{-8pt}
\end{figure}

X-ray diffraction (XRD) is the gold standard for characterizing
crystalline materials. The XRD patterns of graphite and synthesized
graphene are shown in Figure~\ref{fig3}. 

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}The XRD of (a) graphite and (b) graphene
(reproduced with permission from~\cite{8}).}
\vspace*{3pt}
\end{figure*}

Analysis by XRD enables the determination of crystal lattice
parameters, including interplanar spacing. The XRD pattern provides
crucial \mbox{information} on the degree of exfoliation, the number of layers,
and the extent of structural order or disorder in graphite-derived
carbon materials. The most striking feature of the graphite XRD pattern
(Figure~\ref{fig3}a) is the sharp, intense peak at approximately 
$2\theta = 26.5$\textdegree. The (002) interlayer spacing of graphite
is usually around 0.335~nm. The sharpness of this peak indicates a high
degree of crystallinity and long-range order in the stacked layers.
Following electrochemical exfoliation to produce graphene, the XRD
pattern exhibits a sharp peak at $2\theta = 23$\textdegree, which
corresponds to the (002) interlayer spacing of the graphene sheets. An
additional peak at $2\theta = 44.9$\textdegree~is attributed to the
(100) planes~\cite{63}. The decreased intensity of all diffraction
peaks indicates a decrease in the crystallinity of graphite after
exfoliation. The increased interlayer distance (d) in the synthesized
graphene indicates the intercalation of oxygen-containing functional
groups between the layers, confirming the successful formation of
graphene~\cite{64}. These results demonstrate the structural
rearrangements from graphite to graphene.\looseness=-1

Raman spectroscopy is a powerful, non-destructive technique used to
analyze the molecular composition and structural properties of
materials. It is widely used to characterize sp\tsup{2} and
sp\tsup{3} hybridized carbon atoms. The Raman spectra of
graphene typically exhibit three distinctive peaks known as the D, G,
and 2D bands. The G band arises from the in-plane vibration of
sp\tsup{2} hybridized carbon atoms in the graphitic lattice
and is sensitive to strain and defects. The D band is associated with
structural defects and disorder in the sp\tsup{2} carbon
network. The~intensity ratio of the D and G bands is often used as a
quantitative measure of disorder, providing insight into the quality of
graphene. Figure~\ref{fig4} shows the Raman spectrum of graphite, with
the characteristic D and G bands appearing at
1349~cm\tsup{\tminus 1} and 1573~cm\tsup{\tminus
1}, respectively. Additionally, a 2D band is observed at
2663~cm\tsup{\tminus 1}, arising from a double resonance
process involving two phonons~\cite{65,66}.

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}Raman spectrum of graphite (reproduced
from~\cite{67} under license 
\href{https://creativecommons.org/licenses/by-nc-sa/4.0/}{CC BY-NC-SA 4.0}).}
\vspace*{-3pt}
\end{figure}

Figure~\ref{fig5} shows the Raman spectrum of graphene, which exhibits
four prominent peaks in the 1300--3300~cm\tsup{\tminus 1}
region at 1328 and 1574~cm\tsup{\tminus 1}. The peak at
1328~cm\tsup{\tminus 1} is attributed to the D band, the one
centered at 1574~cm\tsup{\tminus 1} to the G band.

\begin{figure}
\includegraphics{fig05}
\caption{\label{fig5}Raman spectrum of graphene (reproduced
from~\cite{67} under license 
\href{https://creativecommons.org/licenses/by-nc-sa/4.0/}{CC BY-NC-SA 4.0}).}
\vspace*{-3pt}
\end{figure}

The $I_{\mathrm{D}}/I_{\mathrm{G}}$ intensity ratio of the D and G
bands is a quantitative measure of defect density in the graphene
lattice. An increase in disorder within the graphene structure leads to
a corresponding increase in the $I_{\mathrm{D}}/I_{\mathrm{G}}$ ratio
due to increased elastic scattering from defects. However, in highly
amorphous carbon materials, the $I_{\mathrm{D}}/I_{\mathrm{G}}$ ratio
tends to decrease. The tabulated data below (Table~\ref{tab1})
summarize the Raman analysis results for graphite and graphene. 

\begin{table*}
\caption{\label{tab1}Raman analysis values of
different materials}
\begin{tabular}{cccccc}
\thead
\xmorerows{1}{Sample} & D & D & G & G & 
\xmorerows{1}{$I_{\mathrm{D}}/I_{\mathrm{G}}$} \\
& Position (cm\tsup{\tminus 1}) &  Intensity (a.u.) & 
Position (cm\tsup{\tminus 1}) &  Intensity (a.u.) & \\
\endthead
Graphite  & 1328 & 4411 & 1573 & 8565 & 0.52 \\
Graphene & 1322 & 2427 & 1573 & 2260 & 1.07 
\botline
\end{tabular}
\end{table*}

The $I_{\mathrm{D}}/I_{\mathrm{G}}$ ratio was found to be 0.52 for
graphite and 1.07 for graphene. This increase suggests a reduction in
sp\tsup{2} domain size and the incorporation of
oxygen-containing functional groups during exfoliation, resulting in
greater disorder at carbon edges due to extensive oxidation~\cite{44}.
To quantify the specific surface area of graphene, nitrogen
(N\tsub{2}) adsorption--desorption isotherms were measured and
are shown in Figure~\ref{fig6}. 

\begin{figure}
\includegraphics{fig06}
\caption{\label{fig6}Nitrogen adsorption--desorption isotherm of graphene.}
\vspace*{-2pt}
\end{figure}

According to the IUPAC classification, the isotherm of graphene is
classified as type IV, indicating capillary condensation within the
mesopores. The increase in adsorbed volume within the relative pressure
range of 0.7--1.0 is attributed to capillary condensation, which is
accompanied by a hysteresis loop. The specific surface area, pore
volume, and pore size distribution were determined using the BET and
BJH methods and are summarized in Table~\ref{tab2}.


\begin{table}
\caption{\label{tab2}BET and BJH analysis results}
\tabcolsep=2.5pt
\begin{tabular}{cccccc}
\thead
\xmorerows{1}{Sample} &  
\parbox[t]{1cm}{\centering  $S_{\mathrm{BET}}$ (m\tsup{2}/g)} &
\parbox[t]{1.2cm}{\centering  $S_{\mathrm{Langmuir}}$ (m\tsup{2}/g)}& 
\parbox[t]{.7cm}{\centering $S_{\mathrm{BJH}}$ (m/g)}  & 
\parbox[t]{1.1cm}{\centering  $V_{\mathrm{P}}$ (cm\tsup{3}/g)}  & 
\parbox[t]{.6cm}{\centering  $D_{\mathrm{P}}$ (\AA{})}\vspace*{2pt}  \\
\endthead
Graphene & 300.312  & 771.768  & 354.89 & 0.588  & 65.05
\botline
\end{tabular}
\tabnote{$V_{\mathrm{P}}$: Pore volume (cm\tsup{3}/g); 
$D_{\mathrm{P}}$: Pore diameter (\AA{}).}
\vspace*{-6pt}
\end{table}

The specific surface area of graphene usually ranges from 200 to 
2500~m\tsup{2}/g, depending on the preparation method and
the extent of exfoliation. Our graphene sample exhibited an
$S_{\mathrm{BET}}$ of 300.3126~m\tsup{2}/g, which is
attributed to the presence of larger, stacked graphene sheets. The
higher Langmuir surface area of 771.768~m\tsup{2}/g
highlights the sensitivity of the method to smaller surfaces. Using the
BJH method, we determined a surface area of 
354.89~m\tsup{2}/g, which suggests that there is significant
stacking of graphene sheets. The BJH method is particularly sensitive
to larger pores. The pore volume (VP) was found to be
0.588~cm\tsup{3}/g, which also indicates substantial
stacking of graphene layers. 

\section{Viscosity measurements}\label{sec4}

Viscosity was measured using a Cannon--Fenske capillary viscometer. The
viscosity of the graphene-based nanofluids was determined at the lowest
concentrations. A thermostatic bath was used to control the ambient
temperature. The viscosity of all nanofluids was measured at
temperatures of 25, 35, 45, 55, and 65~\textdegree C. 

The experimental flow time was converted into kinematic viscosity
(${\nu}$) using the viscometer constant (${C}$), according to the
following equation: 
{$$
{\nu} = C\times t_{\mathrm{f}}
$$}\unskip
where ${\nu}$ is the kinematic viscosity (expressed in centistokes
cSt), ${C}$ is the calibration constant of the specific capillary used
(cSt/s), and $t_{\mathrm{f}}$ is the efflux time (s) required for the
meniscus to pass between the two calibrated marks. Furthermore, the
dynamic viscosity ($\mu$), which represents the fluid's internal
resistance to shear, was derived by considering the suspension's
density (${\rho}$): $\mu =\nu{\cdot}\rho_{\mathrm{bulk}}$. In this
study, the bulk density (${\rho}_{\mathrm{bulk}}$) of the nanofluids
(expressed in g/cm\tsup{3}) was measured at each temperature
point to account for the thermal expansion of the base liquid and the
presence of the graphene nanosheets.

The results of the viscosity measurements are shown in Figure~\ref{fig7}.

\begin{figure}
\includegraphics{fig07}
\caption{\label{fig7}Viscosity measurements of graphene samples.}
\end{figure}

\begin{table*}
\caption{\label{tab3}Comparative synthesis of results for
graphene-based nanofluids: Impact of concentration and temperature}
\begin{tabular}{cccccc}
\thead
\parbox[t]{2cm}{\centering  Concentration  range} & 
\parbox[t]{2cm}{\centering Temperature  range (K)} & 
Key results/remarks & Reference \vspace*{2pt}\\
\endthead
0.03 wt\% & 298--348 & \parbox[t]{8.2cm}{\raggedright  Viscosity lower than
that of the base fluid and decreases from 217.4 to 40.6 cP as the
temperature increases from 298 to 348 K.} & \cite{68}   \\
0.5--1 wt\% & 278--298 &
\parbox[t]{8.2cm}{\raggedright$\mu_{\mathrm{nanofluid}}$ decreased with
rising temperature but increased with the mass fraction of graphene
nanoparticles. $\mu_{\mathrm{rel}}$ ranged from 1.24 to 2.35.}
&\cite{69}   \\
0.025--0.1 wt\% & 293--333 & \parbox[t]{8.2cm}{\raggedright${\mu}$
decreases 4--44\% with rising $T$ and increases with the concentration
of graphene nanoparticles. $\mu_{\mathrm{rel}}$ increases with rising
$T$.} &\cite{70}   \\
0.025--0.1 wt\% & 293--333 & \parbox[t]{8.2cm}{\raggedright Viscosity decreases for higher
temperatures and increases for higher concentrations of graphene
nanoparticles (+44\% compared to the viscosity of the base fluid for
0.1 wt\% graphene nanoparticles).} &\cite{71}   \\
0.05--0.1 wt\% & 293--333 & \parbox[t]{8.2cm}{\raggedright Viscosity increases with concentration and
decreases with temperature.} &\cite{72}   \\
0.001--0.005~g/L & 298--333 & \parbox[t]{8.2cm}{\raggedright A decrease
in viscosity was observed with increasing temperature. Conversely,
viscosity increased with the graphene nanoparticle concentration.} &
This work\vspace*{2pt}
\botline
\end{tabular}
\end{table*}

Figure~\ref{fig7} shows how the viscosity of five graphene-based
nanofluids changes with temperature at different concentrations. The
measurements were carried out at five specific temperatures: 25, 35,
45, 55, and 65~\textdegree C. As expected, viscosity decreases with
increasing temperature. Conversely, viscosity increases with the
concentration of graphene nanoparticles. Increasing temperature
promotes the thermal agitation of molecules, thereby increasing their
kinetic energy. This weakens the intermolecular attractive forces,
thereby reducing the fluid's internal resistance to flow.

A comparative summary of the results obtained for graphene-based
nanofluids is provided in Table~\ref{tab3}.

This table shows that the viscosity behavior of graphene-based
nanofluids can be predicted and is governed by well-established
physical laws, even when different base fluids and concentrations are
used. Our results are in perfect agreement with those in the scientific
literature, which lends credibility and importance to our contribution
to the field.

\section{Conclusion}\label{sec5}

We present a method of synthesizing graphene through the
electrochemical exfoliation of graphite derived from recycled
batteries. Sulfuric acid and water were used as electrolytes to enhance
exfoliation. Graphene was then dispersed into individual sheets of
graphene using ultrasonic treatment in distilled water. The synthesized
graphene was characterized using various techniques, confirming the
successful production of high-quality graphene. This study focused on
measuring the viscosity and conducting a behavioral analysis of
graphene-based nanofluids. The viscosity of the nanofluids was measured
at different volume fractions ranging from 0.001 to 0.005. All
viscosity measurements were performed using a capillary viscometer at
temperatures ranging from 25 to 65~\textdegree C. The studied
nanofluids exhibited an increase in viscosity with nanoparticle loading
and a decrease with temperature.

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