Effect of the Electrolyte on the Production of Graphene Oxide by Electrochemical Exfoliation from Discharged Batteries

Abstract

Graphene derivatives are known for their exceptional properties and are essential for commercial applications, ranging from electronics to energy storage and generation. This study investigates the effect of different acidic electrolytes (sulfuric acid (H₂SO₄), hydrochloric acid (HCl), and nitric acid (HNO₃)) on the synthesis of graphene oxide (GO) via electrochemical exfoliation. Characterization techniques confirm that all exfoliated samples contain a mixture of GO and reduced graphene oxide (rGO). X-ray diffraction reveals that HNO₃ promotes the highest GO formation, as indicated by a sharp peak at 2θ = 11.6°, suggesting increased interlayer spacing. X-ray photoelectron spectroscopy shows a higher oxygen content (greater carbon-to-oxygen (C/O) ratio), confirming a greater presence of oxygenated groups in HNO₃-treated samples. Raman spectroscopy differentiates GO and rGO through characteristic bands. Ultraviolet-visible absorption spectroscopy further supports these findings, as the HNO₃-treated sample exhibits a strong absorption band at 262 nm, indicating partial restoration of electronic conjugation, while the 300 nm band is absent in H₂SO₄-treated samples, confirming smaller amount oxygenated groups. Scanning electron microscopy reveals morphological changes, with stacked GO layers and incomplete exfoliation. No single-layer graphene was observed. These results highlight how electrolyte choice significantly affects GO structure and composition, guiding its optimization for applications in energy storage, electronics, and materials science.

Keywords:
electrolytic solution; discharged battery cell; eco-friendly; electrochemical exfoliation; graphene oxide

Introduction

Different types of batteries are essential in daily life but often contain hazardous substances, including heavy metals, which pose significant environmental risks if not disposed of properly. Although regulations encourage recycling, a large number of batteries are still incinerated or sent to landfills, leading to the loss of valuable materials, such as graphite rods. These graphite rods, however, present an opportunity for sustainable recycling, as they can be repurposed to produce graphene and its derivatives, including graphene oxide (GO) and reduced graphene oxide (rGO).¹ This approach not only reduces environmental impact but also adds value by transforming waste into materials that can be used in the production chain.² The process of obtaining graphene from graphite involves overcoming the van der Waals forces binding the carbon sheets together in the three-dimensional structure of graphite.³ Graphene and its derivatives possess remarkable properties, including high conductivity, thinness, strength, flexibility, and malleability.⁴ In the scientific and industrial context, researchers and industries have shown great interest in these materials, particularly in materials science,⁴ energy storage,⁵ supercapacitors,⁶ and sensors.⁷ The industry predicts a demand of over 4000 tons per year by 2026.⁸ Pure graphene is a two-dimensional molecule comprising a perfectly flat sheet of sp²-hybridized carbon atoms.⁹ This sheet is highly compact, with a single carbon layer arranged in a hexagonal honeycomb structure through σ and π bonds.⁹

GO, the oxidized form of graphene, is obtained by oxidizing graphite, resulting in a structure containing various oxygen-containing functional groups, such as hydroxyl, carboxyl, carbonyl, and alkoxy. While GO sheets predominantly consist of sp²-hybridized carbon atoms, the presence of oxygenated groups can cause sp³ hybridization, leading to displacement below or above the graphene plane.¹⁰,¹¹ Structural imperfections such as defects, wrinkles, disorder, impurities, and fragmentation can be found in GO sheets due to the oxidation process.¹² These imperfections can influence the properties of GO. To mitigate or eliminate these imperfections, GO can be reduced to rGO. The reduction process involves removing the oxygenated functional groups, resulting in a material with graphene-like properties that can be modified.¹² Recent research¹³-¹⁵ highlights that synthesized materials, such as GO and rGO, continue to be explored for various applications. One key area of study is their use as nanofillers to enhance the physical and mechanical properties of medium-density fiberboards.¹³ Additionally, these materials are being investigated for their impact on the optical properties of green light-emitting conjugated copolymers,¹⁴ as well as in studies of linear and nonlinear optical properties.¹⁵ For industrial applications, graphene and its derivatives must be produced cost-effectively compared to existing materials. Achieving this goal poses a significant challenge, necessitating synthesis processes that are economically viable, scalable, and yield high-quality products.¹⁶,¹⁷

Two main synthesis methods are currently under discussion: (i) bottom-up processes which construct graphene from individual molecules, with techniques such as chemical vapor deposition and epitaxial growth on silicon carbide substrates being prominent;¹⁸,¹⁹ (ii) top-down processes, which deconstruct complex structures to obtain graphene, with methods such as mechanical exfoliation and chemical exfoliation using the Hummers method being widely used.²⁰ Electrochemical exfoliation, among top-down processes, is a straightforward method for preparing graphene. It involves few process steps, minimal chemical reagents, short synthesis times, and low costs, making it suitable for large-scale production with high quality and yield.²¹,²² However, exfoliation of graphite in liquid electrolytes at ambient conditions commonly introduces O into the carbon lattice of graphene and breaks the exfoliated graphene into small pieces.²³ Liu et al.²⁴ discuss synthesis methods for graphene materials with high C/O ratios from four aspects: graphite electrodes, equipment engineering, electrolytes, and additional reduction methods. This scientific review provides an understanding of the wide range of parameters involved in electrochemical exfoliation, as well as the challenges associated with the different synthesis routes of GO and rGO. In this case, the parameter related to the electrolyte used in electrochemical exfoliation, a range of researchers synthesized their materials in acidic electrolytes (sulfuric acid H₂SO₄), with variations in their concentrations.²⁵-²⁹

These reported studies help to understand the steps of the process via acidic electrolytes, thus advancing research regarding the acidic electrochemical environment and proposing variations in the type of acidic electrolyte to be used. In this context, this study aims to obtain GO and rGO through electrochemical exfoliation, followed by ultrasonic treatment, to evaluate the effect of different exfoliating acid electrolytes, all at a concentration of 0.5 mol L^–1. The effectiveness of the process is evaluated by physicochemical characterization techniques, such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, UV-Vis spectroscopy and scanning electron microscopy (SEM).

Experimental

Solutions and reagents

All acid solutions were prepared using deionized water with a resistivity exceeding 18.2 MΩ cm. Sulfuric acid (H₂SO₄), hydrochloric acid (HCl) and nitric acid (HNO₃) sourced from the manufacturer Synth were utilized to prepare the electrolyte solutions for the exfoliation process. Isopropyl alcohol, purchased from Dinâmica LTA, was used to dilute the samples and prepare them for UV-Vis measurements. The graphite rods, with an initial diameter of 4.0 mm, a height of 47.0 mm, and a weight of 1.0 g, and served as working electrodes in the electrochemical exfoliation processes.

Electrochemical exfoliation procedure

The electrochemical exfoliation process involved an electrolyte, an electric current conductor, and electrodes serving as the anode and cathode, representing the oxidation and reduction reactions, respectively. The graphite exfoliation process typically ranged from a few minutes to several hours to produce a few grams of graphene on a laboratory scale, with the quality being moderately dependent on operational parameters such as potential, graphite precursor, and electrolyte used.

The graphite rod was then used as the anode and cathode in an electrochemical cell. 50 mL of the acid solutions used previously were used as the electrolytic medium in the electrochemical exfoliation. A potential of 7.0 V DC (direct current) was then applied with power supply model M10-AD370-6 from MCP Lab Electronics through the electrodes for 120 min. After this process, the solutions were transferred to an Erlenmeyer flask and sent to the ultrasonication process, for a period of 60 min at 90% power (Ultrasonique QR550) with an ultrasonic frequency of 20 kHz and power of 550 Watts (4 mm diameter titanium micro tip). The material obtained from the electrochemical exfoliation process was separated in a Daiki model 80-2B centrifuge at approximately 4000 rpm and washed thoroughly with ultrapure water to neutral pH using Falcon tubes (25 mL) and pH scale indicator strips. The material obtained was then dried in Petri dishes at 60 °C for 72 h in an Ethik Technology model 400-5ND oven.

Characterization techniques

Scanning electron microscopy

The morphology of the samples was investigated using a scanning electron microscope model Zeiss EVO MA10 scanning electron microscope.

X-ray diffraction

The X-ray diffraction patterns were collected on PANalytical Empyrean multi-purpose X-ray diffractometer using Cu Kα radiation (λ = 0.154 nm) in the 2θ range of 5° to 80° with angular step of 0.02° continuous (scanning rate).

X-ray photoelectron spectroscopy

To correlate the chemical structure with the physical properties of the exfoliated materials, the samples produced in this study were characterized by X-ray photoelectron spectroscopy (XPS) at the National Nanotechnology Laboratory (LNNano-CNPEM). XPS measurements were performed using a Thermo Scientific K-Alpha photoelectron spectrometer equipped with an Al Kα X-ray source. The C1s spectra were fitted using a pseudo-Voigt profile function with a Shirley background model.

Raman spectroscopy

Raman analysis was performed using a HORIBA LabRAM HR Evolution Raman microscope. Spectra were collected over a wavenumber range of 75 to 4000 cm^–1. A 532 nm excitation laser was used with a power of 50 mW, a 3.2% neutral density filter, and a 100× objective lens. The exposure time per acquisition window was 20 s, with 4 accumulations used to obtain the averaged spectrum.

Ultraviolet-visible absorption spectroscopy

The main electronic transitions in electrochemically exfoliated samples were investigated using UV-Vis. Measurements were performed with a Thermo Scientific Evolution 201 spectrometer, scanning wavelengths from 200 to 800 nm, with an integration time of 0.2 s and a spectral resolution of 1 nm. All samples were pre-diluted in isopropyl alcohol with a dilution factor of (2 mg mL^–1) to ensure that absorbance values remained within the linear detection range of the spectrophotometer.

Results and Discussion

Table 1 presents the data obtained from the electrochemical exfoliation, including information on the sample, yield, exfoliated material, and process losses. Yield refers to the mass of exfoliated material as a percentage of the initial graphite mass. It is calculated as the mass of exfoliated material divided by the mass of the initial graphite. The exfoliated material percentage represents the fraction of graphite that underwent exfoliation during the 2-h process.

Table 1
Exfoliated material, yield and losses

Electrolyte (0.5 mol L-1)	Exfoliated material / %	Yield / %	Loss / %
H2SO4	35.0	95.0	5.0
HCl	32.0	94.3	5.7
HNO3	33.0	93.5	6.5

The study found that the exfoliation process achieved a yield greater than 94% for all tested electrolytes. When using an electrolyte concentration of 0.5 mol L^–1, more than 30% of the graphite was successfully exfoliated. Throughout the process, losses of less than 7% were observed, primarily due to handling and washing steps required to neutralize the pH. Additionally, some material loss may have occurred due to the presence of nanoscale particles that could not be fully recovered. This limitation is attributed to the centrifugation process, where particles may have been lost with the supernatant due to equipment constraints (maximum speed of 4000 rpm).

Scanning electron microscopy (SEM)

The morphology of graphite and exfoliated materials was examined through SEM images acquired at a magnification of 20 k×, as shown in Figure 1. Figure 1a presents the micrograph of the graphite rod before exfoliation, where a rough, agglomerated, but well-compacted and heterogeneous surface can be observed. Conversely, the SEM images of the electrochemical exfoliated samples displayed in Figures 1b-1d present well-spaced layered morphology confirming the exfoliation of graphite rod. In the specific case of sample exfoliated using H₂SO₄ electrolyte defective thick layers show some roughness. The more defective morphology observed in SEM images during electrochemical exfoliation using sulfuric acid (H₂SO₄) compared to nitric acid (HNO₃) or hydrochloric acid (HCl) can be attributed to the differences in the intercalation and oxidation mechanisms of these electrolytes.³⁰-³² H₂SO₄ promotes the intercalation of sulfate ions (SO₄^2–) between the graphite layers. These ions are larger and more aggressive than nitrate (NO₃^–) or chloride (Cl^–) ions, leading to greater expansion and disruption of the graphite structure. In this study, no single-layer graphene structures were found; however, the structures obtained from the exfoliation processes appear to be stacked in layers with varying degrees of thickness. This result may be caused by incomplete oxidation of the graphite, leading to only partial exfoliation of the layers and the detachment of bulk graphite pieces.

Figure 1
SEM imagens (20k× magnification) of graphite: (a) before electrochemical exfoliation and (b-d) after exfoliation in 0.5 mol L^–1 acid solutions (b) sulfuric acid, (c) hydrochloric acid, and (d) nitric acid.

X-ray diffraction analysis

Figure 2 presents the X-ray diffraction (XRD) patterns of graphite and electrochemically exfoliated samples. The XRD pattern of graphite displays a sharp peak at 2θ = 26.7°, corresponding to the (002) diffraction plane. This peak confirms the presence of a well-ordered layered structure with an interplanar spacing of 0.33 nm along the (002) orientation.³³,³⁴ In contrast, the XRD patterns of the materials exfoliated using different acid electrolytes differ from that of pristine graphite. As shown in Figure 2, the (002) peak at 26.7° remains present in all diffractograms; however, its intensity decreases in the electrochemically exfoliated samples. This reduction in peak intensity indicates a decrease in the orderly stacking of graphene layers, consistent with the exfoliation of graphite.³⁵,³⁶

Figure 2
XRD patterns of graphite and electrochemically exfoliated samples.

Additionally, a broad peak centered at 2θ = 11.6° was observed in samples exfoliated using HCl and HNO₃ electrolytes. This peak corresponds to the (001) diffraction plane of graphene oxide (GO), with an interplanar distance of 7.62 Å.³³,³⁷,³⁸ The intensity of this peak is more pronounced in the sample exfoliated with HNO₃, suggesting that oxygen-containing functional groups were introduced at the graphite edges during electrochemical exfoliation, leading to GO formation.³⁹-⁴²

In contrast, for the sample exfoliated using H₂SO₄, the peak at 2θ = 11.6° becomes nearly imperceptible. This result can be attributed to the oxidizing strength of the acids and the electrochemical environment they create. HNO₃ is a strong oxidizing agent, which promotes extensive oxidation of graphite during exfoliation. On the other hand, H₂SO₄, while a strong acid, has weaker oxidizing properties compared to nitric acid. During exfoliation by H₂SO₄, it causes less oxidation of graphite, resulting in the reduction of oxygen functional groups, characteristic of rGO.⁴³

The low intensity peaks at 2θ = 44.5° (101) and 55.0° (004) are remaining signs of the crystalline structure of graphite after exfoliation. Therefore, we can conclude from the XRD analysis that electrochemically exfoliated samples are formed by a mixture of phases; GO, rGO and remaining graphitic structures. The percentage of GO is more significant for acidic HNO₃ solution, while for the samples exfoliated using H₂SO₄ and HCl electrolytes the formation of GO phase is reduced.

X-ray photoelectron spectroscopy (XPS)

Figure 3 presents the C1s core-level XPS spectra of graphite and electrochemically exfoliated samples. The C1s spectrum of graphite exhibits two main contributions at 284.5 and 286.5 eV. The more intense peak at 284.5 eV corresponds to C=C/C–C bonds resulting from sp²/sp³ hybridizations, while the peak at 286.5 eV is attributed to C–OH species. In the spectra of electrochemically exfoliated samples, additional oxidized carbon species appear, including carbonyl (C=O) at 287.0 eV and carboxyl (HO–C=O) at 289.0 eV.⁴⁴,⁴⁵

Figure 3
C 1s XPS spectra obtained for the exfoliating electrolytes: (a) graphite without exfoliation, (b) H₂SO₄, (c) HCl, (d) HNO₃.

Table 2 summarizes the percentage composition of each chemical species present in the samples. A significant increase in the atomic percentage of C=O and HO–C=O species is observed for the sample exfoliated using HNO₃ as the electrolyte. This result aligns with the XRD analysis, which indicates that the formation of the GO phase is favored during electrochemical exfoliation with HNO₃. Notably, studies⁴⁶-⁴⁸ using synthesis conditions similar to those in this work typically report lower concentrations of carbonyl (C=O) groups in GO. Therefore, as outlined in the Experimental section, the combination of electrospinning and high-power ultrasonication, along with the optimized electrolyte conditions, enabled the synthesis of a GO material with a distinct stoichiometric composition compared to conventional GO.

Table 2
Characterization of the components of the high-resolution XPS spectrum in the C 1s region for the materials that underwent electrospinning with variations in acid electrolytes. The relative atomic percentages were obtained by fitting the spectrum using Lorentzian-Gaussian curves and a Shirley background

Electrolyte (0.5 mol L-1)	C sp2 / %	C=O / %	HO-C=O / %	C-OH / %	π → π* (shake-up)
Graphite (no exfoliation)	96.3	-	-	3.7	-
H2SO4	71.5	11.2	10.9	6.4	-
HCl	65.7	20.2	6.2	7.9	-
HNO3	52.5	26.4	8.0	8.6	4.33

• -: not detected.

Table 3 shows the atomic percentages of carbon and oxygen obtained from the survey XPS spectra (see Figure 4) and the C/O ratios calculated for the graphene derivatives.

Table 3
Atomic percentages of carbon and oxygen and the C/O% ratio obtained from XPS survey spectra

	Electrolyte H2SO4 (0.5 mol L-1)	Electrolyte HCl (0.5 mol L-1)	Electrolyte HNO3 (0.5 mol L-1)
Carbon / %	83.3	78.6	75.2
Oxygen / %	16.0	15.4	23.9
C/O (%) ratio	5.20	5.10	3.15

Figure 4
XPS survey spectra of (a) graphite without exfoliation and exfoliated samples using different electrolytes: H₂SO₄ (b), HCl (c), and HNO₃ (d).

According to the proportion (C/O) verified, we can infer that with the use of the HNO₃ electrolyte, a higher percentage of oxygenated groups is produced in the electrochemical exfoliation, characteristic of GO, and the electrolytes (H₂SO₄ and HCl) produce reduced amounts of oxygenated groups.

Raman spectroscopy

Figure 5 shows the Raman spectra of the exfoliated samples. The most intense band present in the Raman spectrum of graphite without exfoliation is located at 1580 cm^-1 and corresponds to the G band which arises from the vibrations of sp²-hybridized bonds in the hexagonal lattice of graphitic carbon. The D band is located at 1360 cm^-1 is a forbidden mode indicative of defects and disorder in the structure owing to the presence of sp³-hybridized carbon.⁴⁹ The overtone of this peak, the 2D band, appears at approximately 2720 cm^-1, and its position is related to the number of graphene layers.⁴⁹

Figure 5
Raman spectra of the exfoliated materials.

The Raman spectra of the electrochemical exfoliated have these same bands, but they differ in some aspects. In addition to the D, G and 2D bands, other signals appear that are induced by the presence of defects and disorders: the D + G and D + D’ bands at 2940 and 3210 cm^-1, respectively.⁵⁰ In contrast to graphite, in the Raman spectrum of the electrochemical exfoliated samples the D and G bands are intense, broad and displaced from their positions in relation to graphite, being centered at approximately 1340 and 1590 cm^-1, respectively. The 2D band appears around 2690 cm^-1. Although its intensity is low, it is still possible to estimate the number of layers and the consequent degree of exfoliation of the samples. The position of the 2D band. As observed by Cançado et al.⁵¹ in their work on quantifying defects in graphene via Raman spectroscopy at different excitation energies. The authors report also that an increase in disorder in graphene causes the D band to shift to lower wavenumbers (red shift). The shift of the G band to higher wavenumbers (blue shift) is associated with an increase in the population of sp³ carbons. In fact, the graphite oxidation and exfoliation process cause a rupture in the graphene network, which leads to the formation of small isolated domains of sp² carbons that vibrate at higher frequencies than the graphite G-band.⁵²,⁵³ Mowry et al.⁵⁴ studied in situ Raman spectroscopy and thermal analysis in the formation of graphene, and in their results, they attributed the blue shift of the G band to distortions in the graphene crystal lattice caused by the oxygen groups present in the GO structure. The shifts and broadening of the D and G bands show that graphene oxide is a highly disordered material compared to the starting material. In order to discover the degree of disorder and defects in the structure of the prepared materials, the intensity ratio of the D and G bands (I_D/I_G) is calculated, where values close to zero indicate a crystalline material and an increase in this value indicates a loss of crystallinity and, consequently, a more disorganized structure.⁵⁵ The work by Tu et al.⁵⁶ uses the I_2D/I_G < relationship, which provides data on the quality of the material based on the number of graphene layers, with values above I_2D/I_G > 2 indicating a monolayer graphene structure, 1 > _2D/I_G for bi-layer graphene and 0.5 ≤ I_2D/I_G < 1 for graphene with few layers and I_2D/I_G for graphene with several layers. Based on the aforementioned relationships, the data obtained from the Raman spectra is shown in Table 4.

Table 4
Data extracted from the Raman spectra of the materials produced

Electrolyte (0.5 mol L-1)	Band D		Band G		Band 2D		I2D/IG	ID/IG	Number of layersa
	Position / cm-1	ID	Position / cm-1	IG	Position / cm-1	I2D
H2SO4	1346.9	9548.9	1590.9	9484.7	2693.2	3226.8	0.34	1.00	multi
HCl	1350.1	5144.2	1586.2	5324.5	2693.2	1832.5	0.34	0.96	multi
HNO3	1348.5	7239.6	1581.6	7365.2	2689.2	2269.1	0.31	0.98	multi

• ^a
  Classification obtained through InstaNANO.⁵⁷

Comparing the results of the I_2D/I_G ratio in Table 4, it was observed that the materials produced using acid electrolytes all have structures with multiple graphene layers (ca. 0.31 to 0.34). In a study by Yu et al.⁵⁸ comparing the type of graphite immersion solution, a difference was found between the final material, where immersion in an acidic solution result in a final structure with few layers, while immersion in water results in several layers. This result can be explained by the hydrophobic nature of the graphite electrode, which makes it difficult for anions in solution to access vacancies and interstices.⁵⁹,⁶⁰ This restriction is removed by the presence of acid in these places, which consequently makes the material more hygroscopic and contributes to exfoliation.⁵⁹,⁶⁰ It was therefore found that the materials synthesized by electrochemical exfoliation using acid electrolytes have a greater degree of disorder and multiple layers.

UV-Vis absorption spectroscopy

Figure 6 shows the UV-Vis of the electrochemical exfoliated samples. As can be seen, the characteristic bands are observed in these spectra. The intense bands at 212 nm result from π → π* transitions, whose higher absorptivity explains the intensity difference compared to n → π* transitions.⁶¹ In materials that have been exfoliated in acid electrolytes (HNO₃ and HCl), a broad, low-intensity band is also observed at 300 nm, attributed to the n → π* transitions of the carbonyl and carboxyl groups (C=O and HO-C=O, respectively).⁶² This transition involves the promotion of an electron (Ione-pair) from a non-bonding orbital of the oxygen atom to an anti-bonding π* orbital through the absorption of a photon. For the material exfoliated with hydrochloric acid electrolyte, a broad, low-intensity band can be seen at 250 nm. This absorption corresponds to the π → π* transitions of the C=C bonds of the aromatic rings.⁶² With the use of the HNO₃ electrolyte, at approximately 262 nm it shows a narrow, high-intensity band. This behavior indicates that the electronic conjugations within the GO sheets are partially restored during the GO synthesis process, which increases the amount of sp² carbon in the network.⁶³ The band at 300 nm does not appear with the use of the electrolyte H₂SO₄, which demonstrates the decrease in the amounts of oxygenated groups in this material. Unlike graphene, which is a zero-gap semiconductor, the process of oxidation and exfoliation of graphite results in the opening of an optical gap between the valence and conduction bands of GO.⁶⁴ Depending on the degree of functionalization, GO can have insulating, semiconducting or semi-metallic properties.⁶⁵,⁶⁶

Figure 6
UV-Vis spectra of samples exfoliated using H₂SO₄, HCl, and HNO₃ as electrolytes.

Effect of electrolytes for GO/rGO preparation

The proposed process of electrochemical exfoliation of graphite from AA batteries in graphene (acid medium) is a complex phenomenon. With this in mind, researchers investigate processes involving ion intercalation, chemical oxidation, gas evolution, bubble formation, mechanical expansion and exfoliation reactions.⁶⁴,⁶⁵ In the research carried out, the electrolytes in use were: sulfuric acid, nitric acid and hydrochloric acid. The penetration power between graphite layers may be associated with the effect of anions derived from acidic electrolytes (sulfate, SO₄^2–, nitrate, NO₃^–, and chloride, Cl^–). These mentioned parameters are related when an electrical potential is applied to the electro exfoliation medium. When an electrical potential is applied to the graphite electrode, anodic oxidation of water generates hydroxyl ions (OH^-) or oxygen radicals (O^•).⁶⁵,⁶⁶ The hydroxylation and oxidation of ions and radicals at the edges of the graphite open up sites that allow the intercalation of anions. During this process, water molecules also co-intercalate between the graphite layers with the anion under analysis. Studies⁶⁴,⁶⁵ suggest that reduction of the SO₄^2- anion, water oxidation and carbon corrosion can generate gaseous species such as SO₂, O₂, CO and CO₂. The generated gaseous species can further expand the distance between the graphite layers, resulting in the separation and production of graphene sheets. In the article,⁶⁵ the role of anions on electrochemical exfoliation of graphite into graphene in aqueous acids the authors showed greater efficiency with sulfate salts, which can be attributed to the lower standard reduction potential of SO₄^2- (+0.20 V) compared to those of other anions (0.96 V for NO₃^-, 1.36 V for Cl^-) vs hydrogen electrode. They hypothesized that the favorable formation of SO₂ gas from SO₄^2- compared to other NO or Cl₂ gases are responsible for effective exfoliation in sulfate-containing substances solution.⁶⁶ In the case of HNO₃, O₂ gas was mainly produced and relatively small amounts of CO₂ and CO were detected during the exfoliation process. A large amount of O₂ gas may be due to multiple reactions. In addition to the water oxidation reaction, NO₃^- anions can be electrochemically oxidized by the following reaction:⁶⁷

(1) ${\text{NO}}_{\text{3}}\to {\text{NO}}_{\text{2}}+\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\text{O}}_{\text{2}}+{\text{e}}^{-}$

Previous studies⁶⁸ have shown that the hydrolysis of generated NO₂ can result in the production of NO. In the case of HCl, Cl₂ was the main gaseous product during the electrochemical process. Unlike other anions present in acidic electrolytes, chloride ions were oxidized, generating Cl₂ gas. Thus, the low exfoliation efficiency of graphite in HCl may be attributed to the formation of Cl₂ before the intercalation reaction occurs. This initial discussion in the presentation of the results becomes important to identify the difference in the graphite exfoliation mechanism for each type of anion, thus corroborating the discussion of the physical-chemical analyses.

Conclusions

This study demonstrates the significant influence of electrolyte choice on the electrochemical exfoliation of graphite for graphene oxide (GO) synthesis. Among the tested acidic electrolytes, nitric acid (HNO₃) proved to be the most effective in promoting GO formation, as confirmed by XRD and XPS analyses, which indicated increased interlayer spacing and a higher oxygen content. Raman and UV-Vis absorption spectroscopy further supported these findings, highlighting structural and electronic variations among the samples. SEM revealed morphological transformations, with evidence of incomplete exfoliation and stacked GO layers. These insights contribute to the understanding of electrochemical exfoliation mechanisms and provide valuable guidance for tailoring GO properties for specific applications in energy storage, electronics, and advanced materials.

Supplementary Information

Supplementary data are available free of charge at http://jbcs.sbq.org.br as PDF file.

Acknowledgments

Eliseu S. Junior and Leonardo A. Veltrone are grateful for the CNPq scholarships. The authors are grateful to CNPq (405065/2021-3) and Fundação Araucária (No. PBA2022011000128) for financial support. We thank the Brazilian Nanotechnology National Laboratory (LNNano, CNPEM). The authors would like also to thank to the Federal University of Latin American Integration (UNILA) for financial supporting and facilities, PRPPG, PPGIES and SACT.

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Edited by

• Editor handled this article:
  Cristiane Luísa Jost (Associate)

Publication Dates

• Publication in this collection
  06 June 2025
• Date of issue
  2025

History

• Received
  07 Mar 2025
• Accepted
  13 May 2025