# From sugarcane bagasse pulp into electroactive materials: Carboxymethyl cellulose/reduced graphene oxide/nickel oxide composites as high-performance supercapacitor components

Mohamed A Abdelfadeel, Sherief A Al Kiey, Peter Hesemann, Ahmad Mehdi, Samir Kamel, Ahmed K El-Ziaty, Naglaa Salem El-Sayed

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Mohamed A Abdelfadeel, Sherief A Al Kiey, Peter Hesemann, Ahmad Mehdi, Samir Kamel, et al. From sugarcane bagasse pulp into electroactive materials: Carboxymethyl cellulose/reduced graphene oxide/nickel oxide composites as high-performance supercapacitor components. International Journal of Biological Macromolecules, 2025, 316, pp.144703. 10.1016/j.ijbiomac.2025.144703. hal-05100474

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# From sugarcane bagasse pulp into electroactive materials: Carboxymethyl cellulose/reduced graphene oxide/nickel oxide composites as high-performance supercapacitor components

Mohamed A. Abdelfadeel<sup>a,b</sup>, Sherief A. Al Kiey<sup>c,d</sup>, Peter Hesemann<sup>a,e,*</sup>, Ahmad Mehdi<sup>a</sup>, Samir Kamel<sup>f</sup>, Ahmed K. El-Ziaty<sup>b</sup>, Naglaa Salem El-Sayed<sup>f,**</sup>

## ARTICLE INFO

### Keywords:

Biosourced composites

rGO

Nickel oxide nanoparticles

Energy storage

Capacitance

## ABSTRACT

We report bio-sourced electrically conductive materials derived from sugarcane bagasse cellulose. First, the cellulose was chemically transformed into carboxymethyl cellulose (CMC). Acrylamide (AM) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) were co-polymerized with CMC, and the resulting material was cross-linked with *N*,*N*'-methylenebisacrylamide (MBA) to improve its mechanical properties. To further enhance the electrical performance, the materials were modified with reduced graphene oxide (rGO), nickel oxide (NiO) nanoparticles and rGO@NiO composites. The nanocomposites were then used as electrodes for solid-state supercapacitors. To evaluate the electrochemical properties of these materials, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were performed. These results demonstrate enhanced electrochemical performance of the composite material modified with both rGO and NiO NPs. CV profiles exhibited well-defined redox peaks indicative of reversible faradaic processes, confirming high pseudocapacitance contributions. The optimized sample X<sub>GNi2.5</sub> exhibited a high specific capacitance of 497.8 F g<sup>-1</sup> at 0.5 A g<sup>-1</sup>, outstanding cycling stability with 91 % capacitance retention after 5000 cycles, and a maximum energy density of 44.2 Wh kg<sup>-1</sup> at a power density of 226.2 W kg<sup>-1</sup>. EIS revealed low charge transfer resistance and efficient ion transport. These results underscore the potential of CMC/rGO/NiO composites as sustainable, high-performance electrode materials for next-generation supercapacitors.

## 1. Introduction

Supercapacitors are emerging energy storage devices that operate through electrostatic charge separation mechanism via a rapid charge-discharge mechanism, delivering high specific energy and exceptional specific power density. Compared to conventional batteries, supercapacitors offer notable advantages such as an extended cycle life and efficient self-discharge, along with their low-cost, simple fabrication steps and maintenance, underscoring their potential for a wide range of energy storage applications [1–4]. Supercapacitors bridge the gap between traditional capacitors, renowned for their high-power density yet

limited energy storage capacity, and batteries, which offer superior energy density but suffer from slow charge-discharge rates. This unique combination makes supercapacitors ideally suited for numerous applications where both rapid energy delivery and significant energy storage are essential [5]. Therefore, it is essential to develop highly electroactive electrode materials that address the inherent trade-offs in current energy storage systems. The rapid charging–discharging capabilities of supercapacitors, while advantageous for power delivery, often come with bulky mass, limiting their application in advanced portable electronic devices. Overcoming these constraints through innovative material design will enhance the integration of supercapacitors into modern,

<footnote><sup>a</sup> ICGM, University of Montpellier-CNRS-ENSCM, Montpellier, France</footnote>

<footnote><sup>b</sup> Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt</footnote>

<footnote><sup>c</sup> Electrochemistry and Corrosion Lab, Physical Chemistry Department, National Research Centre, Egypt</footnote>

<footnote><sup>d</sup> Material Engineering Lab, Central Laboratories Network, National Research Centre, Egypt</footnote>

<footnote><sup>e</sup> ChimEco UMR5021 CNRS-UM, Montpellier, France</footnote>

<footnote><sup>f</sup> Cellulose and Paper Department, National Research Centre, Egypt</footnote>

<footnote>* Corresponding author at: ICGM, University of Montpellier-CNRS-ENSCM, Montpellier, France.</footnote>

<footnote>** Corresponding author at: Cellulose and Paper Department, National Research Center, Cairo.</footnote>

<footnote>E-mail addresses: peter.hesemann@umontpellier.fr (P. Hesemann), ns.salem@nrc.sci.eg (N.S. El-Sayed).</footnote>
lightweight, and portable technologies.

Whereas earlier research directed mainly their attention toward fabricating symmetric and asymmetric supercapacitor configurations to optimize performance, the current research is now focusing on the revolution of new, smart electrode materials that can operate at high voltages and/or innovating novel electrolytes. Recent advancements have shifted toward enhancing the intrinsic electrochemical stability of electrode materials and broadening their potential window. These innovations aim to significantly boost both the energy and power densities of supercapacitors, paving the way for next-generation energy storage solutions.

The surging demand for advanced energy storage solutions has led to significant breakthroughs in the design of innovative materials for advanced energy storage units. One such promising route involves the development of electrically conductive electrodes based on biopolymers [6,7]. The choice of electrode material plays a crucial role in determining the device's energy density, power density and overall efficiency [8–10]. Sugarcane bagasse which is a by-product of sugar production, is a highly abundant, rich in cellulose and serves as a low-cost renewable resource for producing cellulose-based materials [11–13]. With its fibrous structure, bagasse pulp can be modified into carbon-rich hydrogels and other materials, which aiming to enhance its mechanical stability and electrochemical properties, particularly for supercapacitor applications [7,9]. The use of bagasse pulp in sustainable energy storage contributes to waste valorization and advances eco-friendly production of supercapacitor electrodes.

These electrode materials leverage the renewable and biodegradable nature of biopolymers, making them eco-friendly options for energy storage applications. Additionally, biopolymer-based materials can provide a good porosity, essential for efficient charge storage and enhanced capacitance in supercapacitor devices [3,4,14–16]. Carboxymethyl Cellulose (CMC) is a water-soluble derivative of cellulose [15,17] that offers environmental and cost benefits. CMC is used in electrode fabrication for electrochemical devices. As a binder, CMC improves cycling performance and capacity retention in silicon-based electrodes compared to conventional binders [18]. Its amphiphilic nature, with hydroxyl and carboxylate groups, allows for modification into conducting materials, enhancing properties such as hydrogen bonding, swelling, and ion dispersion [19]. These properties make CMC a promising material for various electrochemical applications, including batteries and energy storage devices [20].

The electrochemical properties of CMC-based devices can be further enhanced by incorporating functional components. The selection of electrode components plays a critical role in determining the energy density, power density, and overall efficiency of these devices. To enhance performance, researchers have explored a diverse array of nanofillers, including graphene, reduced graphene oxide, activated carbon, single- and multi-walled carbon nanotubes, transition metal oxides and sulphides, conducting polymers, and hybrid nanocomposites with various polymeric matrices for fabricating supercapacitor electrodes [21–27]. Among the studied conducting nanofillers, particular attention has been paid in the use of nickel oxide (NiO) [20,28,29]. NiO is a p-type semiconductor with a band gap of 3.5 eV, which provides high pseudocapacitance through reversible $Ni^{2+}/Ni^{3+}$ redox transitions. Due to its remarkable electrical properties, thermal stability, and low electron affinity [30–32], NiO has been studied for various applications, including optoelectronics, gas sensors, resistive-switching memories, memory devices, and electrodes in lithium-ion batteries [33,34]. To highlight the effect of loading NiO within polymer matrices on the electrochemical properties of the formed devices, numerous studies have shown that metal oxide loading can enhance thermal stability, alter optical band gaps, and affect electrical conductivity. For example, Costa et al. [35] recently demonstrated that PMMA/NiO nanocomposites exhibited capacitive behavior and increased specific capacitance up to 407 µF.g⁻¹ with 5 % NiO content. Similarly, Basappa et al. found that the incorporation of NiO nanoparticles within PVC/PMMA blends films

improves the electrochemical performance of the resulting materials [36]. Aftab et al. [37] investigated the effects of NiO@rGO and NiO@CNT hybrid nanoparticles in polymer solar cells and X-ray detectors, reporting enhanced charge carrier capacities and exciton dissociation properties. These studies highlight that the incorporation of NiO in polymer matrices improves the electrochemical properties and performance of various electronic devices and allows increasing the power conversion efficiency (PCE) for the conversion of sunlight into electricity up to 6.42 %.

The combination with reduced graphene oxide (rGO) advantageously modifies the properties of NiO. The deposition of NiO NPs on graphene sheets allows a better dispersion and avoids agglomeration of the nanoparticles. Furthermore, reduced graphene oxide (rGO) significantly contributes to electric double-layer capacitance (EDLC), enhances conductivity, and improves ion and electron transport. The combination of these materials yields a hybrid system that simultaneously offers high charge storage and structural stability. Owing to their superior electrochemical performance, rGO@NiO composites have been implemented in various fields, including batteries, sensors, and electrochromic films [38–42]. To cite some examples, Xu et al. reported that NiO-rGO composites display a specific capacitance up to 171.3 F g⁻¹ at a current density of 0.5 A g⁻¹ [38]. Bu et al. showed that nickel oxide (NiO) and reduced graphene oxide (rGO) composite materials display a maximum specific capacitance of 461 F g⁻¹ at the discharge current density of 0.21 A g⁻¹ in 6 M KOH electrolyte. The authors concluded that the strong enhancement of the total specific capacitance of the composite is due to synergistic effects between the two components [39]. NiO@rGO nanocomposites also found applications in hydrogen gas sensing [40] and as magnetocaloric materials, due to an inherent magnetocaloric effect [41]. These diverse applications of rGO@NiO composites originate from their unique structural properties and well-defined electrochemical characteristics, making them promising materials for future technological advancements.

In the context of the elaboration of novel biosourced and sustainable materials for energy storage, we report novel electrically conductive materials based on carboxymethyl cellulose/poly(acrylamide-co-AMPS) systems. These ionic supports were further loaded with NiO nanoparticles, rGO, or rGO/NiO composites. The resulting materials were characterized by FTIR, XRD, SEM, and TGA. Electrochemical performance evaluated via EIS, CV, and GCD, revealed high specific capacitance, demonstrating their potential for sustainable and efficient energy storage in advanced supercapacitors. The combination of CMC loaded with both NiO NPs and rGO creates a synergistic effect [43] with NiO NPs offering high capacitance due to their pseudocapacitive behavior, and rGO providing excellent electronic conductivity together with high mechanical stability. The synergy between all components of the composite materials holds the potential to address key challenges in supercapacitor technology, including maximizing specific capacitance, ensuring mechanical stability, and facilitating charge transport. It presents a multifaceted approach that allows achieving a balance between energy density and power density.

Here, our results show that CMC is a suitable matrix polymer for the elaboration of electroactive materials via the incorporation of rGO@NiO composites within a CMC matrix. The mechanical robustness of the materials was improved via reticulation with acrylamide (AM) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) copolymers. Furthermore, the incorporation of NiO NPs and rGO within the electrode material enhances both chemical stability and electrochemical performance, preventing degradation during prolonged cycling in the supercapacitor.

## 2. Materials and methods

### 2.1. Materials

2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS), N,N'-
methylenebisacrylamide (MBA), and potassium persulfate (KPS) were purchased from Sigma Aldrich. Acrylamide (AM) was purchased from Fluka. Graphite, potassium permanganate, nickel chloride hexahydrate, hydrazine hydrate, potassium hydroxide, and hydrogen peroxide were analytical reagents acquired from Alfa Aeser. Local bleached bagasse pulp was purchased from Qena Company for the Paper Industry, Qena City, Egypt.

## 2.2. Synthesis of CMC

Carboxymethyl cellulose (CMC) was prepared following the previously described procedure [44]. 10.00 g of bleached sugarcane bagasse pulp was placed in a 750 mL glass beaker before adding 10.00 g of NaOH dissolved in solution and 150 mL of distilled water. The beaker was incubated in a water bath at 35 °C for 1 h with mechanical stirring at 150 rpm. Subsequently, a mixture of 10.00 g NaOH dissolved in 50 mL of DW, 11.70 g of chloroacetic acid, and 350 mL of 95 vol% ethanol was added to the beaker. The resulting suspension was heated at 75 °C for 1.5 h with agitation at 200 rpm to complete the carboxymethylation. After cooling to room temperature, the mixture was neutralized with hydrochloric acid until pH 7, filtered, dissolved in water, and reprecipitated with 95 v% ethanol three times to remove all salts and impurities. The resulting solid was freeze-dried for 24 h to obtain the CMC as a white powder. Its degree of substitution (DS) and structure have been determined via FT-IR, NMR, and TGA (Figs. S1-S3).

## 2.3. Synthesis of NiO nanoparticles, reduced graphene oxide (rGO) and rGO@NiO composite

NiO NPs were prepared following a hydrothermal synthesis strategy [45,46]. Typically, 4.75 g of NiCl₂·6H₂O (20 mmol) were dissolved in 75 mL DI water under stirring for 30 min at 90 °C. Then, 7.20 g (120 mmol) of urea were added to the solution, and stirring was continued 1.5 h before transfer into a 100 mL Teflon-lined stainless-steel autoclave, sealing and annealing at 180 °C for 16 h. After cooling, the formed light green solid was filtered off, washed with deionized water till neutral pH and ethanol, and finally dried at 95 °C for 2 h. After calcination under air at 400 °C for 3 h, we obtained 1.27 g (85 %) a dark green powder.

Reduced graphene oxide (rGO) was prepared in a two-step sequence via oxidation of graphite followed by the reduction of the intermediate formed graphene oxide [47,48]. In a three-neck round-bottom flask maintained at 50 °C, a solution comprising potassium permanganate (18.00 g), graphite (3.00 g), and a mixture of sulfuric acid and phosphoric acid (360:40 mL) was introduced. The resulting mixture underwent agitation in an oil bath until achieving homogeneity. Subsequently, the mixture was cooled in an ice bath to room temperature. Deionized water (400 mL) and hydrogen peroxide (3 mL, 30 %) were gradually added to the flask. The resulting suspension underwent centrifugation at 5000 rpm for 10 min, and the supernatant was discarded. The precipitate was washed with deionized water, ethanol, and 30 % hydrochloric acid until neutral. The obtained precipitate was dried at 85 °C in a vacuum oven and sieved through a 325-mesh screen to get the graphene oxide (GO). For the reduction process, 100 mL of distilled water containing 1.00 g of GO was ultrasonically dispersed, and the suspension was sonicated for an hour. Subsequently, a reduction mixture (1 mL hydrazine hydrate +3 mL ammonia solution (30 %) + 10 mL water) was added with stirring. The resulting suspension was rinsed with distilled water and allowed to dry overnight to obtain rGO. Confirmation of the reduction of GO was achieved through XRD and FTIR (Figs. S4, S5).

For the synthesis of rGO@NiO NPs, the same method for NiO NPs was repeated by incorporating 100 mg of rGO into the starting solution of NiCl₂·6H₂O. The suspension was subjected to sonication for 30 min before the addition of urea, followed by annealing and calcination as the same procedure of the synthesis of NiO NPs described above. [45,46,49] We obtained 1.35 g (80 %) as dark grey powder. The rGO@NiO NPs

composite was characterized by XRD, SEM, and TEM.

## 2.4. Preparation of CMC-poly(AM-co-AMPS) materials

Materials were prepared following previously described procedures [3,4]. Briefly, 100 mg of potassium persulfate (KPS) was introduced into a carboxymethyl cellulose (CMC) aqueous solution (1.00 g in 25 mL of distilled water) and stirred magnetically for 15 min at 60 °C. Subsequently, a mixture of 2.00 g of acrylamide (AM) and 1.00 g of 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) neutralized using few drops of 40 % solution of KOH was added to start grafting polymerization. The pH was adjusted to 5.5 and the temperature to 70 °C, under stirring for 15 min. Then, 200 mg of N,N'-methylenebisacrylamide (MBA) in 5 mL DI water was introduced, the pH was maintained at 5.5, and stirred for an additional 3 h. The resulting hydrogel was subjected to washing by soaking in boiled distilled water then boiled acetone 3 times to eliminate any residual unreacted monomers, crosslinker, and homopolymers. Finally, the resulting material was cut into circular pieces with 1 cm thickness and 4 cm diameter, and freeze-dried overnight to get a brittle solid white material X₀.

Several other materials were synthesized with different concentrations of rGO, NiO NPs, and rGO@NiO NPs composite. The compositions were labeled as XG₁, XG₂, XG₂.₅, XNi₂.₅, and XGNi₂.₅, corresponding to 1, 2, and 2.5 % rGO, 2.5 % NiO Nps, and 2.5 % rGO@NiO Nps, respectively. The synthesis process followed the previous protocol, stopping just before the addition of the cross-linker. At this stage, specific quantities of the particles were introduced into the reaction mixture. For XG₁, 40 mg of rGO were added, while 80 mg were used for XG₂, and 100 mg for XG₂.₅. Similarly, 100 mg of NiO NPs were incorporated for XNi₂.₅, and 100 mg of the rGO@NiO Nps composite for XGNi₂.₅. Before being introduced to the reaction, the particles were dispersed using a probe sonicator for 20 min to ensure uniform distribution. Following the particle dispersion, N,N'-methylenebisacrylamide (MBA) was introduced into the mixture containing the monomers in an aqueous solution. This entire mixture was continuously stirred for 3 h at 70 °C to ensure thorough integration of the components. After the reaction, the resulting materials underwent washing and lyophilization.

## 2.5. Characterization methods

### 2.5.1. X-ray diffraction (XRD)

The X-ray diffraction (XRD) patterns of the materials were examined using a Malvern Analytical X'Pert MRD diffractometer equipped with a Cu K-alpha source suitable for reflectometry and X-ray diffraction measurements on thin layers. The XRD patterns were recorded over a diffraction angle range of 2θ from 10 to 80° using 1.54 Å as wavelength. The crystal size was estimated using the Debye-Scherrer formula.

$$D = \frac{K\lambda}{\beta\cos\theta}$$

where k = 0.89, λ is the wavelength of the Cu-Kα radiations, β is the full width at half maximum, and θ is the angle obtained from 2θ values corresponding to the maximum intensity peak in the XRD pattern.

### 2.5.2. Fourier transform infrared spectroscopy (ATR-FTIR)

The FTIR spectra were obtained using Nicolet Impact-400 FT-IR spectrophotometer in Attenuated Total Reflectance (ATR) mode.

### 2.5.3. Scanning electron microscopy with energy dispersive electron spectroscopy (EDX - SEM)

The spatial elemental distribution of the materials was investigated through non-destructive Energy Dispersive X-ray (EDX) analysis, utilizing an EDX unit attached to a scanning electron microscope (JSM 6360LV, JEOL/Noran). Surface morphology images of various samples were captured using an accelerating voltage of 10–15 kV.
### 2.5.4. Thermogravimetric analysis (TGA)

TGA was conducted on an STA 6000 Perkin Elmer analyzer. The analysis involved heating the samples from room temperature to 800 °C at a rate of 10 °C/min under air.

### 2.6. Electrochemical characterization

The Voltalab 40 electrochemical workstation was employed to evaluate the electrochemical characteristics of all the samples. The electrochemical properties of the synthesized material were evaluated using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) in a three-electrode system as illustrated. [50] The EIS measurements were conducted from 0.01 Hz to 100 kHz with an amplitude of 5 mV, while the GCD and CV experiments were performed in a solution of 1 M H₂SO₄ from 0 to 0.8 V. The electrodes were coated onto a conductive substrate (nickel foam) to form the working electrode. The working electrode was as-prepared by mixing the active material. The as-prepared samples were prepared through the mixture of 80 wt%, with 10 wt% carbon black, which acted as a conducting agent, and 10 wt% of polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone (NMP), which held the samples together. The counter electrode and reference electrode were Pt wire and Ag/AgCl, respectively. The weight of the active material in the electrodes was approximately (3.2 mg ± 0.2 mg). The specific capacitances (CGCD, F g⁻¹) of individual electrodes have been calculated from the GCD curve using the following eq. 1 [51]:

$$C_{\text{GCD}} = \frac{I\Delta t}{m\Delta V} \qquad (1)$$

The discharge current represents (I), the discharge time represents (Δt), the electrode's surface mass represents (m), and the applied potential window represents (ΔV).

Following these steps, the SSC devices were assembled: The SSC devices were created by pressing together two 1 × 1 cm² nickel foam-coated active material with a polyvinyl alcohol (PVA)-H₂SO₄ gel electrolyte. The electrodes were then dried out overnight in a fume hood. The energy density and power density of the constructed supercapacitors were calculated using Eqs. (2) and (3):

$$E = \frac{1}{2} C_{\text{GCD}} \Delta V^2 \qquad (2)$$

$$P = \frac{3600E}{\Delta t_s} \qquad (3)$$

where E and P correspond to the energy density (Wh.kg⁻¹) and power density (W.kg⁻¹), respectively.

## 3. Results and discussion

The rationale behind fabricating matrix materials based on carboxymethyl cellulose (CMC) grafted with acrylamide (AM) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) lies in their function as conductive components upon polymerization, enhancing the overall electronic conductivity of the final material. This enhancement is crucial for improving the charge transfer kinetics in supercapacitors. Additionally, the high hydrophilicity of CMC facilitates good ion accessibility and mobility within the material matrix, parameters that are essential for efficient ion diffusion, and results in improved capacitance and energy storage performance. The inclusion of CMC along with AM and AMPS together with crosslinking with N,N'-methylenebisacrylamide (MBA) ensures mechanical robustness. Finally, the incorporation of NiO NPs and rGO is supposed to improve the electrochemical properties, NiO NPs offering high capacitance due to their pseudocapacitive behavior, and rGO providing excellent electronic conductivity and mechanical stability. It has been shown that the incorporation of NiO NPs and rGO within the electrode material can enhance its chemical stability,

preventing degradation during prolonged cycling in the supercapacitor [38,52].

### 3.1. Synthesis of the conductive fillers

Firstly, the conductive fillers rGO, NiO NPs, and rGO@NiO NPs have been synthesized following previously reported protocols [38,39,42].

The synthesis of rGO was performed in a two-step sequence involving the oxidation of graphite, leading to the formation of graphene oxide (GO) in a first step. Then, this intermediate material was reduced with hydrazine hydrate. The XRD pattern of the prepared rGO (Fig. S4) shows the characteristic peak at 2θ = 21° of reflection plane (002). The successful reduction of GO is further evidenced by the disappearance of the sharp (001) diffraction peak at 2θ = 11°, which is associated with the ordered oxygen-functionalized layers in GO [53]. This confirms the restoration of a partially graphitic structure in rGO, indicative of the removal of oxygen-containing functional groups. These diffractograms are in good agreement with the reported results in the literature [54].

The chemical modifications of the oxidation-reduction sequence were also monitored via infrared spectroscopy. The FTIR spectra of the prepared graphene oxide (GO) (Fig. S5) showed a characteristic peak at 1733 cm⁻¹, corresponding to the stretching vibration of carbonyl groups. A broad peak around 3426 cm⁻¹ corresponds to the OH vibrations that can be attributed to carboxylic acid, hydroxyl groups, or physisorbed water. Other absorption bands in the spectrum at 2923 and 2853 cm⁻¹ correspond to the stretching vibration of the aliphatic C–H bonds group. The absorption band at 1628 cm⁻¹ can be attributed to the C=C stretching vibrations. Finally, the band at 1040 cm⁻¹ points to the presence of C–O–C motifs in the material. The presence of these oxygen-containing groups in the GO material confirms the successful oxidation of graphite [55].

In contrast, the spectrum of the rGO shows a decrease or disappearance of the intensities of all peaks related to oxygen-containing groups compared to those observed in GO. This indicates the effective removal of oxygen functionalities during the reduction process. [55] This reduction in oxygen-related peaks together with the increase of the intensity of the absorption band related to aromatic C–H bonds (ν < 3000 cm⁻¹) highlights the successful restoration of the sp² carbon network in rGO, this contributes to its electrical conductivity and structural stability [56].

The thermogravimetric analysis of rGO (Fig. S6) shows a weight loss of ~6 wt% due to the elimination of physisorbed water below 150 °C. The decomposition of rGO starts around 300 °C, and the complete decomposition of rGO is reached at 590 °C with a residual mass of 18.9 %, mainly due to the presence of inorganic impurities. Indeed, the EDX analysis of rGO (Fig. S7) reveals the presence of mineral components such as Si, Mn, Fe, and some more, resulting in the formation of the corresponding oxides during calcination.

SEM micrographs of rGO (Fig. S8) show the characteristic lamellar-type microstructure of graphene-derived materials.

The synthesis of NiO NPs was performed via a hydrothermal synthesis starting from NiCl₂·6H₂O [38,42,57]. The X-ray diffractogram of the formed NiO NPs (Figs. 3, S9) reveals a crystalline structure with diffraction peaks at 2θ = 37.3°, 43.3° and 63.0°, characteristic for the (111), (200) and (220) crystal planes of bulk NiO NPs, respectively [58]. These diffraction peaks correspond to the face-centred cubic (FCC) crystalline phase of NiO NPs, and are consistent with its standard diffractogram (JCPDS, No. 04–0835). However, the presence of large and featureless peaks in the range of 2θ = 20–30° indicates the presence of a significant amount of an amorphous nickel oxide phase. The particle size of NiO NPs was estimated to be approximately 20 nm using the Debye-Scherrer formula. The average crystallite size, calculated from the (111), (200), and (220) reflections using the Debye–Scherrer equation (Table S1), was 19.15 nm, with crystallinity degree (CrD) of 27,884,64 and the corresponding microstrain from Williamson–Hall analysis was approximately 0.005, confirming the highly ordered nature of NiO
nanoparticles [59]. The crystallite size estimation was confirmed via SEM imaging (Fig. S10), indicating that the material consists of agglomerated nanoparticles of a diameter around 20 nm. EDX confirmed that the materials consist nearly exclusively of nickel and oxygen (Fig. S11), and thermogravimetric measurements indicated very low mass loss (< 1 %) by heating to 550 °C, mainly due to the elimination of physisorbed water below 200 °C (Fig. S12).

The synthesis of the composite rGO@NiO NPs followed a similar method to the synthesis of NiO NPs mentioned above. As described in the literature, this composite was obtained via a hydrothermal formation of NiO NPs in the presence of rGO [38–42]. The formed rGO/NiO NPs phase was annealed to give the rGO@NiO NPs composite material. The XRD pattern of the formed rGO@NiO NPs material (Fig. 3) reveals slightly broadened peaks of NiO NPs at 2θ = 37.2° and 43.3°, with FWHM values of 0.8° and 1.0°, respectively. In comparison, the pure NiO NPs phase displays sharper peaks with FWHM values of 0.4° and 0.6°. In contrast to NiO, the rGO@NiO nanocomposite exhibited an average crystallite size of 8.93 nm, with a higher microstrain of 0.010, indicating a smaller particle size and higher lattice distortion likely due to the interaction between NiO and the rGO matrix. The CrD for rGO@NiO is 2853,467, reflecting a partial loss in long-range order due to the incorporation of disordered carbon domains [60]. This structural interplay between crystalline and amorphous phases is beneficial for enhancing electrochemical performance through synergistic conductivity and active surface interactions [38]. SEM and TEM (Figs. S15 and S16) indicate the formation of nanometric NiO NPs with well-dispersed rGO sheets. This structural interaction between rGO and NiO NPs suggests the formation of a well-integrated composite, where the rGO scaffold favours the dispersion of NiO NPs [61].

Finally, TGA analysis of rGO@NiO NPs shows a mass loss of approx. 2 % in the range of 150–550 °C (Fig. S17), most probably due to the degradation of the carbonaceous rGO part in the composite.

## 3.2. Synthesis of the CMC-based solid electrolytes

The matrix materials were synthesized following previously established protocols [3,4] through the radical grafting polymerization of AM and AMPS in the presence of CMC. The process was completed via crosslinking with MBA. To achieve the electrochemical conductors, variable amounts of rGO, NiO NPs or rGO@NiO NPs composites were added during the polymerization procedure, yielding the materials X₀, XG₁, XG₂, XG₂.₅, XNi₂.₅, and XGNI₂.₅ after freeze-drying (Table 1).

The TGA curves for the materials X₀, XG₂.₅, XNi₂.₅, and XGNI₂.₅ are shown in Fig. 1. The thermograms of all materials show a similar shape. They reveal three distinct thermal degradation regions. A slight weight loss (~7 %) around 100 °C is due to the elimination of physically adsorbed water. The significant weight loss between 200 and 600 °C corresponds to the thermal decomposition of the polymer matrix and all organic components within the materials. Complete degradation is reached around 500 °C. The materials show a noticeable residual mass due to the presence of sodium cations in CMC. The residual weight of 10.6 % in the case of material X₀ can therefore be explained by the formation of Na₂O. The other materials display higher residual mass mainly due to the addition of NiO NPs in the materials XNi₂.₅ and XGNI₂.₅.

The FT-IR spectra of CMC and the composite materials with variable

<table><caption>**Table 1**

Composition of the synthesized materials.</caption><thead><tr><th>Material</th><th>Loaded particle</th><th>Added amount (wt%)</th></tr></thead><tbody><tr><td>X<sub>0</sub></td><td>–</td><td>–</td></tr><tr><td>X<sub>G1</sub></td><td>rGO</td><td>1</td></tr><tr><td>X<sub>G2</sub></td><td>rGO</td><td>2</td></tr><tr><td>X<sub>G2.5</sub></td><td>rGO</td><td>2.5</td></tr><tr><td>X<sub>Ni2.5</sub></td><td>NiO NPs</td><td>2.5</td></tr><tr><td>X<sub>GNI2.5</sub></td><td>rGO@NiO NPs</td><td>2.5</td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Thermogravimetric analysis curves under air stream of X₀, XG₂.₅, XNi₂.₅ and XGNI₂.₅ materials.</figcaption></figure>

rGO and NiO NPs contents are given in Fig. 2. In the spectrum of CMC, a broad band at ~3200 cm⁻¹, and the bands observed between 2915 and 2935 cm⁻¹, can be attributed to NH stretching vibrations, hydrogen-bonded interactions, and aliphatic CH groups respectively [62]. The absorption band at 1547 cm⁻¹ associated with the asymmetric stretching vibrations of carboxylate (COO⁻) groups [63]. The successful introduction of poly(AM) and poly(AMPS) is further supported by absorption bands at 1617 and 1609 cm⁻¹, corresponding to the stretching vibrations of overlapping carbonyl groups in the polymer chains [64]; The spectra of the materials of the X series do not display any difference, most probably due to the small amount of the NiO NPs, rGO or rGO@-NiO NPs composite added.

The X-ray diffractograms of the materials are given in Fig. 3. The patterns exhibited a single broad peak at 2θ ~22°, indicative of their amorphous nature. For XNi₂.₅ and XGNI₂.₅, distinct diffraction peaks of low intensity can be seen at 2θ values of ~37.3°, 43.4°, and 63.0°, thus confirming the presence of crystalline NiO NPs in the materials.

The SEM micrographs of X₀, XG₂.₅, and XGNI₂.₅ are shown in Fig. 4. The surface morphology examination of X₀ unveiled a sponge-like shape structure present in all materials. The incorporation of rGO and/or NiO NPs does not significantly modify the architecture of the materials. Energy Dispersive X-ray (EDX) (Fig. S18) reveals the presence of carbon, oxygen, nitrogen, and sulphur within the materials, all originating from the polymer matrix. The successful loading of rGO and NiO NPs onto the

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** FT-IR spectra of CMC and the materials X₀, XG₂.₅, XNi₂.₅ and XGNI₂.₅.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** X-ray diffractograms of rGO, NiO NPs, rGO@NiO NPs, X₀, X<sub>G2.5</sub>, X<sub>Ni2.5</sub>, and X<sub>GNI2.5</sub> materials.</figcaption></figure>

material matrix, which is monitored via the appearance of small Ni signals at the EDX spectrum of the sample X<sub>GNI2.5</sub>. Overall, this excerpt highlights the characterization and functionalization of the materials through surface morphology examination and elemental distribution analysis.

### 3.3. Electrochemical performance of the materials

This section delves into the detailed results obtained from several electrochemical analyses, including cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). Each technique offers a unique perspective, enabling us to delve into the intricate electrochemical characteristics of the material. The CV profiles offer insights into the redox reactions and charge storage mechanisms, GCD analyses provide information on the specific capacitance and energy density, while EIS unveils the conductivity and charge transfer kinetics of the material composite.

### 3.4. Cyclic voltammetry

A three-electrode configuration with a 1 M H₂SO₄ electrolyte was constructed to investigate the electrochemical characteristics of electrodes. The CV profiles, as shown in Fig. 5, display distinctive redox peaks, indicating the occurrence of reversible Faradaic processes during the potential sweep. These peaks signify pseudocapacitive behavior predominantly attributed to NiO nanoparticles (NPs), while the broad and rectangular features reflect the contribution of the electric double-layer capacitance (EDLC) from the reduced graphene oxide (rGO) network. The shape and position of the redox peaks provide insights into the capacitive and pseudo-capacitive contributions within the material [65]. The CV profiles of the X₀, X<sub>G2.5</sub>, X<sub>Ni2.5</sub>, and X<sub>GNI2.5</sub> electrodes at 5 mV s⁻¹ are displayed in Fig. 5a. The CV profiles of X<sub>GNI2.5</sub> electrodes exhibit an increase in capacitance as well as more distinct redox peaks than other electrodes, demonstrating that the materials' electrochemical features are enhanced through the addition of rGO and NiO NPs. The incorporation of rGO into the material electrode may have increased its surface area and electrochemical activity [58], which in turn might clarify why X<sub>GNI2.5</sub> electrodes outperform X₀ and other electrodes. The appearance of well-defined redox peaks at higher scan rates signifies the pseudo-capacitive behavior associated with the NiO NPs [66]. The redox reactions occurring at these peaks contribute additional charge storage capacity, enhancing the overall energy density of the supercapacitor electrode [67]. The incorporation of CMC enhances the overall stability and mechanical integrity of the material structure.

The CV profiles of the X<sub>GNI2.5</sub> electrode at different scan speeds ranging from 5 to 100 mV s⁻¹ are presented in Fig. 5b. It is clear that the electrodes exhibit excellent scanning capability because the redox current density increases with the scan rate [68]. Within low rates, the redox peaks of X<sub>GNI2.5</sub> could potentially be observed. The redox peaks at ~0.45 V (anodic) and ~0.30 V (cathodic) correspond to the reversible redox transition of Ni²⁺ ↔ Ni³⁺ (NiO ↔ NiOOH). However, as the scan rate moves up, the amplitudes of these peaks go down because there doesn't appear to a sufficient period for the ions to diffuse. The broad hump in the CV curves at lower scan rates indicates the presence of a significant capacitive response, characteristic of the electrochemical double-layer capacitance (EDLC) arising from the conductive rGO component. The broadening of the hump with increasing scan rates suggests effective charge storage and rapid ion adsorption-desorption kinetics, essential for high-performance supercapacitors. The CV profiles exhibit a robust and stable response, indicating that CMC contributes to maintaining the structural integrity during repeated charge-discharge cycles [69,70]. The CV profile for X<sub>GNI2.5</sub> exhibits distinct features compared to the other samples, which we attribute to potential morphological differences in the distribution and size of NiO nanoparticles, as well as the interaction between NiO and reduced graphene oxide (rGO). X<sub>GNI2.5</sub> might have a unique morphology that affects its surface area and porosity, leading to changes in the electrochemical behavior. This could result in either a less efficient ion diffusion or an altered double-layer capacitance, which would manifest as a deviation in the CV curve. Another contributing factor may be the specific concentration of NiO in X<sub>GNI2.5</sub>. A higher concentration of NiO NPs could lead to agglomeration or clustering of nanoparticles, which might reduce the effective surface area available for electrochemical reactions. This could explain the observed difference in the redox peaks and overall CV response, as the electrochemical activity of NiO depends on its dispersion within the composite material. Additionally, X<sub>GNI2.5</sub> could exhibit a higher degree of NiO crystallinity, which might lead to a more sluggish Faradaic process due to the reduced availability of active sites for charge transfer. This would also be reflected in a less prominent or shifted redox peak in the CV profile. The unique behavior observed in X<sub>GNI2.5</sub> CV profile could also be influenced by the interaction between NiO and rGO. If the NiO particles in this sample are not well integrated with rGO, or if the rGO sheets are less effective at conducting electrons to the NiO, this could reduce the overall electrochemical performance, leading to differences in the CV curve. The observed capacitive and pseudo-capacitive behaviours, coupled with the extended potential window and electrochemical stability, position the X<sub>GNI2.5</sub> material as a promising candidate for high-performance supercapacitor electrodes. The combination of CMC, rGO, and NiO NPs imparts a balanced electrochemical performance, catering to the dual requirements of energy density and power density in supercapacitor applications [43]. These insights are pivotal for tailoring the material's composition and structure for optimized supercapacitor applications, moving closer to the development of high-performance and sustainable energy storage devices.

### 3.5. Galvanostatic charge-discharge (GCD)

The galvanostatic charge-discharge results provide a detailed understanding of the electrochemical performance of the as-prepared electrodes [71]. GCD profiles, as represented in Fig. 6, display symmetric charge and discharge curves, indicative of the capacitive nature of X<sub>GNI2.5</sub> [72]. The symmetric and stable CD curves, coupled with high capacitance retention and favourable energy/power density characteristics, emphasize the material's potential for high-performance supercapacitor electrodes. The graphs in Fig. 6a, illustrate the GCD characteristics of the X₀, X<sub>G2.5</sub>, X<sub>Ni2.5</sub>, and X<sub>GNI2.5</sub> electrodes at 0.5 A g⁻¹. As presented in Fig. 6a, the GCD curves exhibit quasi-triangular shapes with minor deviations due to Faradaic redox reactions. The observed plateau region in the charge-discharge profile of the X<sub>GNI2.5</sub> electrode (Fig. 6a) appears around 0.30–0.45 V, corresponding to the redox couple
<figure><img src="image_6.png" /><figcaption>Fig. 4. SEM images of a,b) $X_{0}$ c,d) $X_{G2.5}$ e,f) $X_{GNi2.5}$.</figcaption></figure>

of $Ni^{2+}/Ni^{3+}$. This plateau matches the redox peaks identified in the CV curves, confirming the contribution of the NiO pseudocapacitive mechanism. Meanwhile, the nearly linear regions outside the plateau originate from the EDLC behavior of rGO. Consequently, reducing the internal electrical resistance of $X_{GNi2.5}$ is a possible consequence of adding rGO to it. These insights inform the optimal strategies for tailoring the material's composition and morphology, paving the way for the development of advanced and sustainable energy storage solutions. The CD results affirm the practical viability of $X_{GNi2.5}$ material as a high-performance supercapacitor electrode material. The combination of structural stability, rate capability, and charge storage capacity positions the material for diverse applications requiring efficient energy storage and rapid charge-discharge cycles.

Fig. 6b shows the GCD curves of the $X_{GNi2.5}$ electrode at varying current densities, demonstrating stable and symmetric profiles. The specific capacitance values decrease gradually with increasing current density, as expected, due to limited ion diffusion and reduced time for redox reactions at high rates. However, the retention of curve symmetry and profile shape indicates superior rate performance and reversibility, further aided by the conductive rGO matrix and the stable CMC scaffold.

The observed electrochemical performance of the as-prepared electrodes is attributed to a synergistic energy storage mechanism involving both electric double-layer capacitance (EDLC) and pseudocapacitance. The CV curves exhibit quasi-rectangular shapes with noticeable redox peaks, while the GCD profiles are non-linear, both of which confirm the presence of Faradaic reactions alongside capacitive behavior. Specif-
<figure><img src="image_7.png" /><figcaption>Fig. 5. Comparison of CV of as-prepared electrode material at 5 mV s⁻¹ in H₂SO₄, b) CV of Xₐ₄Ni₂.₅ at different scan rates in H₂SO₄ electrolyte.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6. Comparison of CD of as-prepared electrode material at 0.5 A g⁻¹ in H₂SO₄, b) CD of Xₐ₄Ni₂.₅ at different current densities in H₂SO₄ electrolyte.</figcaption></figure>

ically, the NiO component contributes significantly to pseudocapacitance through reversible surface redox transitions between Ni²⁺ and Ni³⁺ states, as represented by the reaction:

$$NiO + OH^{-} \leftrightarrow NiOOH + e^{-}$$

This reaction enhances charge storage via fast and reversible electron transfer processes. Meanwhile, the reduced graphene oxide (rGO)

provides a conductive matrix and large surface area that enables rapid ion adsorption/desorption at the electrode-electrolyte interface, contributing to EDLC behavior. Carboxymethyl cellulose (CMC) enhances the structural integrity of the electrode, ensuring uniform dispersion of NiO and rGO while facilitating electrolyte penetration and ion mobility. The combined action of these components leads to improved specific capacitance, good rate capability, and superior

<figure><img src="image_9.png" /><figcaption>Fig. 7. Relation between current density and specific capacitance, b) capacity retention of as-prepared material electrodes.</figcaption></figure>
cycling stability. This hybrid mechanism aligns well with previous reports on composite electrode materials with dual charge storage pathways [73].

Fig. 7 shows the specific capacitance, which was determined from the GCD profiles of all the electrodes, at current densities ranging from 0.5 A g⁻¹ to 5 A g⁻¹. The specific capacitance of the X<sub>GNI2.5</sub> electrode is 518.1 F g⁻¹, which is greater than the capacitance of CMC (X<sub>0</sub>, 430.7 F g⁻¹), rGO (X<sub>G2.5</sub>, 483.5 F g⁻¹), NiO NPs (X<sub>Ni2.5</sub>, 497.6 F g⁻¹). In comparison to other material electrodes, X<sub>GNI2.5</sub> demonstrates improved rate capability as its capacitance maintains 55.2 % of the initial value when the current density is increased from 0.5 to 5 A g⁻¹.

Rate capability tests under varied current densities showcased the material's ability to maintain high specific capacitance even at elevated charging rates. CD curves obtained at varying current densities offer insights into the rate capability of the material. This feature is crucial for applications demanding rapid energy storage and release. The maintenance of a similar shape and symmetry at different current densities indicates the material's capability to handle dynamic charging and discharging conditions. The material's stability under dynamic charge-discharge cycles reinforces its potential for long-term use. At lower current densities, the charge-discharge curves exhibit symmetric and well-defined profiles, indicating efficient charge storage and release. Even at high current densities, the material demonstrates a remarkable ability to maintain a stable and symmetric charge-discharge behavior. The specific capacitance values calculated at each current density provide quantitative insights into the rate capability of the material. The ability of the material to sustain high specific capacitance at elevated current densities is indicative of its excellent rate performance. The rate capability is attributed to the synergistic effects of CMC, rGO, and NiO NPs. CMC provides structural stability, while rGO facilitates rapid charge transport. The pseudo-capacitive behavior of NiO NPs contributes to the charge storage capacity, ensuring a balanced performance. Its ability to maintain high specific capacitance at different current densities positions it as a versatile and efficient energy storage solution for applications requiring dynamic and rapid charge-discharge cycles. The analysis of capacitance retention over extended cycles showcases the ability of the material to maintain a high level of charge storage capacity without significant degradation. The GCD studies were conducted at current densities of 5 A g⁻¹ in order to further evaluate the electrochemical characteristics of the CMC, rGO, NiO NPs and X<sub>GNI2.5</sub> electrodes as shown in Fig. 7. By comparison to CMC electrode, X<sub>GNI2.5</sub> electrodes have better cycling stability, retaining 79.4 % and 89.3 % of their initial capacitance after 5000 cycles respectively. High capacitance retention is indicative of the material's durability and suitability for practical supercapacitor applications. The improved cycling performance may be due to the fact that the CMC, rGO, and NiO NPs work together so well; this combination prevents the material electrode from collapsing.

### 3.6. Electrochemical impedance spectroscopy (EIS)

EIS is a powerful technique for analysing the impedance behavior of supercapacitor electrodes. It provides valuable information about the charge transfer kinetics, electrode-electrolyte interface, and overall electrochemical performance [74]. EIS analyses depicted low charge transfer resistances, signifying efficient electron transport within the electrode. The Nyquist plot displayed a semicircle at high frequencies, indicating the combined capacitive and pseudo-capacitive charge storage mechanisms. Fig. 8 reveals the impedance response of the material electrode. The semi-circular arc in the high-frequency region represents the charge transfer resistance (Rct), while the linear portion at lower frequencies provides insights into the Warburg impedance associated with ion diffusion. The inclination of the Warburg region in the low-frequency region offers information about the ion diffusion within the material. A steeper slope indicates efficient ion diffusion, suggesting rapid charge storage and release capabilities. The diameter of the semi-

<figure><img src="image_10.png" /><figcaption>Fig. 8. Nyquist plot of as-prepared material electrodes X₀, X<sub>G2.5</sub>, X<sub>Ni2.5</sub> and X<sub>GNI2.5</sub>.</figcaption></figure>

circular arc in the high-frequency region corresponds to the charge transfer resistance (Rct). A smaller semicircle indicates lower charge transfer resistance, signifying improved electron transport at the electrode-electrolyte interface. The intersection of the semicircle with the real axis represents the equivalent series resistance (ESR) and provides insights into the overall capacitance behavior. A lower ESR implies reduced energy losses and improved capacitive performance. The electrolyte solution exhibits significant ionic conductivity due to the exceptionally small ESR, and the electrode X<sub>GNI2.5</sub>, which has been constructed with a significant overall conductivity. It is clear that the CMC loaded rGO@NiO NPs electrode receives the lowest Rct value when compared with the other X₀ electrode (15.8 Ω), X<sub>G2.5</sub> (8.5 Ω), X<sub>Ni2.5</sub> (6.1 Ω), X<sub>GNI2.5</sub> (2.1 Ω). The pseudo-capacitive behavior of NiO NPs is reflected in the charge transfer processes. Therefore, the X<sub>GNI2.5</sub> electrode shows the most effective electrochemical performance, according to the EIS analysis, which agrees with the CV and GCD curves.

The incorporation of NiO NPs into the CMC/rGO composite material plays a significant role in improving the electrochemical performance of the resulting supercapacitor electrodes. The presence of NiO contributes to several key aspects of the electrochemical behavior. NiO NPs exhibit pseudocapacitive behavior, which involves reversible faradaic redox reactions at the electrode-electrolyte interface. This significantly enhances the capacitance of the material, as the number of active sites for charge storage increases. The specific capacitance of the composite material is significantly enhanced by the incorporation of NiO NPs. NiO promotes ion intercalation and boosts the overall capacitance. Our results, as shown in the CV and GCD profiles, demonstrate a substantial increase in specific capacitance for the CMC/rGO/NiO composite compared to the pure CMC material. These findings align with similar reports where NiO-enhanced composites exhibited improved capacitance values [75,76].

Although NiO itself has relatively low conductivity, the addition of reduced graphene oxide (rGO) significantly improves the electrical conductivity of the composite material. rGO facilitates faster electron transport, which is reflected in the low charge transfer resistance and efficient ion diffusion pathways observed in the EIS results. The outstanding electrochemical performance, coupled with the cost-effectiveness and biocompatibility of CMC, positions the synthesized material as a promising candidate for practical supercapacitor applications. As a consequence, the synthesized X<sub>GNI2.5</sub> exhibits exceptional electrochemical performance, showcasing its potential as a high-performance supercapacitor electrode material. The synergistic combination of CMC, rGO, and NiO NPs contributes to the material's structural
integrity, electrical conductivity, and charge storage capabilities. These findings pave the way for the development of advanced, cost-effective, and sustainable supercapacitor technologies.

A symmetric supercapacitor (SSC) was designed by sandwiching two sheets of $X_{GNI2.5}$ electrode that were filled with polyvinyl alcohol (PVA)-$H_2SO_4$, with the possible application of the electrode in energy storage devices to be considered (Fig. 9). The SSC device's CV characteristics at various scan rates are presented in Fig. 9a, it is important to point out

that the CV profiles demonstrate a nearly rectangular shape at low scan rates, nevertheless, reveal clear signs of distortion as the scan rate increases. This indicates that the supercapacitor makes utilization of two an electrical double layers and pseudo-capacitance. The GCD curves of the SSC devices at various current densities (1–10 A g⁻¹) are presented in Fig. 9b. Consistent with the findings of CV studies, it is evident that all GCD curves exhibit a somewhat distorted triangular shape, demonstrating the presence of pseudo-capacitance. The highest specific

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** (a) CV of SSC bases on the $X_{GNI2.5}$ electrode, b) CDV of SSC bases on the $X_{GNI2.5}$ electrode, c) relation between capacitance of the SSC $X_{GNI2.5}$ calculated from different current densities, d) Nyquist curves of the SSC $X_{GNI2.5}$, e) relation between logarithm plot of cathodic and anodic peak currents and scan rates, (f) capacitance retention of the SSC $X_{GNI2.5}$ for 5000 cycles.</figcaption></figure>
capacitance recorded by the SSC device, as illustrated in Fig. 9c, is 103.9 F g⁻¹ at 1 A g⁻¹. Capacitance remains at 77.9 % of the initial value even when the current density increases from 1 to 20 A g⁻¹, indicating good rate performance. The SSC device's Nyquist plot is displayed in Fig. 9d. The results of the equivalent circuit fitting show that the supercapacitor experiences rapid charge transfer in the high-frequency zone and ion diffusion in the low-frequency region, as indicated by a small semicircle at around 0.74 Ω and a straight line nearly perpendicular to the x-axis, respectively. The relationship between current (i) and scan rate (v) was employed to investigate the charge storage mechanism of the SSC device:

$$i = a v^b$$

With the two constants a and b. The diffusion-controlled process is represented by b = 0.5, while the non-diffusion-controlled capacitive process is represented by b = 1. Most of the charge storage seems to be due to contributions from the double layer and surface redox reactions, as seen in Fig. 9e, where the anodic b-value is 0.81 and the cathodic b-value is 0.72. When it comes to energy storage devices, the cycle life is a further significant trait for practical applications. Fig. 9f shows that the self-assembled SSCs have good cycling stability, maintaining 91.1 % of their original capacitance after 5000 cycles at 10 A g⁻¹. Furthermore, the specific capacitance values are employed to calculate the energy as well as power densities of the SSC device (Fig. 9). It is worth noting that the SSC device's maximum energy density approaches 98.97 W h Kg⁻¹ at a power density of 399.89 W g⁻¹.

The data presented above reveal that the X<sub>GNI2.5</sub> electrode represents an exceptionally robust structure, exhibits greater stability, and has outstanding electrochemical activity. It is conceivable that the subsequent factors contribute to the outstanding performance of X<sub>GNI2.5</sub> electrode. To start, adopting CMC as rGO synthesis allowed for high mass loading and reduced aggregation. Secondly, rGO considerably facilitates electron transport throughout the charge-discharge cycle. Finally, the CMC, rGO, and NiO NPs all interact strongly and create a web of interconnected structures. Finally, ions and charges are better stored and transported due to the increased specific surface area.

## 4. Conclusion

This study successfully demonstrated the valorization of sugarcane bagasse pulp into high-performance supercapacitor materials. Crystallographic analysis confirmed NiO crystallinity (~84.8 %) and rGO@NiO nanostructure (~28.1 %), with corresponding microstrain values indicating efficient interfacial interactions. These structural properties directly correlate with improved charge transfer, as observed in CV, GCD, and EIS measurements. The synthesized X<sub>GNI2.5</sub> material-based electrodes, containing a combination of carboxymethyl cellulose (CMC), reduced graphene oxide (rGO), and nickel oxide (NiO NPs), exhibited exceptional electrochemical performance, making them promising candidates for energy storage applications. The electrochemical analysis of a symmetric supercapacitor (SSC) device based on X<sub>GNI2.5</sub> electrodes revealed a high specific capacitance of 103.9 F g⁻¹ at 1 A g⁻¹, with a retention of 77.9 % even when the current density increased to 20 A g⁻¹, indicating a strong rate performance. Moreover, the SSC demonstrated outstanding cycling stability, maintaining 91.1 % of its initial capacitance after 5000 cycles at 10 A g⁻¹. The device also achieved an impressive maximum energy density of 98.97 Wh kg⁻¹ and a power density of 399.89 W kg⁻¹, further confirming its high-performance capabilities. These results highlight the synergistic effects between CMC, rGO, and NiO NPs, where rGO enhances electron transport and NiO NPs contribute to pseudocapacitance, resulting in improved charge storage and rapid ion diffusion. The integration of these components into the CMC matrix leads to a stable, high-capacitance material suitable for scalable, sustainable supercapacitor technologies. Future research can focus on further optimization of

material composition and large-scale production processes to promote the development of eco-friendly, high-performance supercapacitors for industrial energy storage applications.

Crystallographic analysis confirmed NiO crystallinity (~84.8 %) and rGO@NiO nanostructure (~28.1 %), with corresponding microstrain values indicating efficient interfacial interactions. These structural properties directly correlate with improved charge transfer, as observed in CV, GCD, and EIS measurements.

## CRediT authorship contribution statement

Mohamed A. Abdelfadeel: Writing – original draft, Investigation, Data curation. Sherief A. Al Kiey: Writing – original draft, Investigation, Data curation. Peter Hesemann: Writing – review & editing, Supervision, Project administration. Ahmad Mehdi: Writing – review & editing, Supervision. Samir Kamel: Writing – review & editing, Project administration, Conceptualization. Ahmed K. El-Ziaty: Writing – review & editing, Supervision. Naglaa Salem El-Sayed: Writing – review & editing, Supervision, Project administration, Methodology, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgement

The authors gratefully acknowledge the financial support provided by the Institut Français d'Égypte (IFE) for the doctoral research. This support was pivotal in facilitating the progress and completion of the PhD project.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijbiomac.2025.144703.

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