Progress and perspectives in the electrochemical
synthesis of carbon nanomaterials

Javier Quílez-Bermejo, Emilia Morallón, Diego Cazorla-Amorós, A. Celzard, Vanessa Fierro

► To cite this version:

Javier Quílez-Bermejo, Emilia Morallón, Diego Cazorla-Amorós, A. Celzard, Vanessa Fierro. Progress and perspectives in the electrochemical synthesis of carbon nanomaterials. Carbon, In press, 237, pp.120151. 10.1016/j.carbon.2025.120151. hal-05011390

HAL Id: hal-05011390

https://hal.univ-lorraine.fr/hal-05011390v1

Submitted on 29 Mar 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

Check for
updates

# Progress and perspectives in the electrochemical synthesis of carbon nanomaterials

Javier Quílez-Bermejo<sup>a, *</sup> <sup>†</sup>, Emilia Morallón<sup>b, †</sup>, Diego Cazorla-Amorós<sup>c, †</sup>, Alain Celzard<sup>a, d</sup>,
Vanessa Fierro<sup>a, **</sup>

## ABSTRACT

The synthesis of carbon materials traditionally relies on energy-intensive processes that often involve high-temperature treatments of carbon-rich precursors derived from fossil fuels. These methods pose significant environmental challenges, making it imperative to develop more sustainable and cost-effective alternatives. Recent advances in electrochemical synthesis offer a promising pathway for the production of carbon nanomaterials using milder conditions and renewable feedstock, such as biomass and small organic molecules. Inspired by the current surge in interest in the C–C coupling reactions in the electrochemical CO₂ reduction reaction, researchers have begun to explore the potential of electrochemical methods for the synthesis of carbon nanomaterials. This comprehensive review consolidates key findings in electrochemical carbon science, focusing on methodologies such as electrochemical functionalization, graphite exfoliation, molten salt-assisted carbonization, and the electrochemical conversion of small organic molecules. By highlighting the synergistic potential between electrochemistry and carbon science, this review aims to provide new insights and a foundational framework for advancing sustainable carbon nanomaterial synthesis, contributing to the development of future renewable energy technologies.

## 1. Introduction

The synthesis of carbon materials requires highly energy-intensive processes [1,2]. These materials are indeed mainly produced industrially by high-temperature heat treatments of carbon-rich precursors, which does not overcome the significant environmental limitations linked to the use of fossil fuels in our highly demanding economy [2–6]. It is therefore imperative to reduce the energy costs associated with the preparation of these carbon materials and to replace petrochemical feedstocks with other precursors of natural origin, such as biomass, in order to remain competitive in an environmentally sustainable future [7–10].

Various works carried out in the field of carbon science during the 1970s and 1980s led to key results in the synthesis of carbon nanomaterials and their applications, particularly in the electrochemical synthesis of carbon materials [11–14]. These works claimed that the conversion of halide-based carbon-rich precursors into carbon materials of nanometric diameter was possible through electrochemical processes operating under reducing conditions. Unfortunately, these pioneering

works did not have a significant impact, mainly due to the scarcity, toxicity and exorbitant cost of the precursor materials, combined with less stringent environmental requirements at the time. The current boom in the CO₂ reduction reaction (CO₂RR) [15,16] has revitalized this subject, as the electrochemical reduction of carbon dioxide molecules has now revealed the generation of small hydrocarbons via the C–C reaction of two or more adsorbed CO₂ molecules (C–C coupling reactions) [17–20]. These findings are relevant since CO₂ is one of the most stable molecules in nature, with a bond dissociation energy of 805 kJ mol⁻¹ [21–23]. Therefore, the cleavage of its C=O bonds requires aggressive catalytic conditions. It should be noted that many other organic molecules in nature have weaker C–O bonds than CO₂, making them more amenable to C–C coupling reactions. This, in turn, facilitates the formation of carbon domains with reduced energy requirement and, maybe, using more environmentally friendly feedstocks.

Besides the CO₂RR, the electrochemical exfoliation of graphite by the application of an external voltage is an important step in the synthesis of graphenic materials. As a result, there is an emerging focus in new electrochemical routes for the preparation of carbon materials. The

<footnote><sup>a</sup> Université de Lorraine, CNRS, IJL, F-88000, Épinal, France</footnote>

<footnote><sup>b</sup> Departamento de Química Física and Instituto de Materiales, Universidad de Alicante, Ap. 99, 03080, Spain</footnote>

<footnote><sup>c</sup> Departamento de Química Inorgánica and Instituto de Materiales, Universidad de Alicante, Ap. 99, 03080, Spain</footnote>

<footnote><sup>d</sup> Institut Universitaire de France (IUF), F-75231, Paris, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail addresses: javier.quilez-bermejo@univ-lorraine.fr (J. Quílez-Bermejo), vanessa.fierro@univ-lorraine.fr (V. Fierro).</footnote>
development of advanced carbon nanomaterials through electrochemical methods has indeed attracted significant interest due to their potential in renewable energies and green technologies. This interest is fueled by the advantages of cost-effectiveness and environmentally friendly synthesis using non-polluting reagents. Previous reviews have explored specific aspects of this field, such as the electrochemical exfoliation of graphite or its application to the CO₂ reduction reaction [24–26]. However, the literature lacks a comprehensive integration of these methodologies, as well as novel approaches, such as molten salt-assisted synthesis and the electrochemical conversion of small organic molecules.

The present comprehensive review aims to address the significant challenges associated with the tailor-made production of carbon nanomaterials from an interdisciplinary perspective, focusing on a variety of electrochemical methodologies. These include electrochemical functionalization of carbon materials, electrochemical exfoliation of graphite, molten salt-assisted electrochemical carbonization and the electrochemical synthesis of carbon materials from small organic molecules and alcohols (Fig. 1). Unlike previous studies, this review not only highlights the mechanistic understanding of these methodologies, but also identifies critical areas for progress, such as scalability, environmental impact and techno-economic assessments for the development of next-generation electrochemical systems.

We believe this in-depth review will provide new insights into the synergistic potential between electrochemistry and carbon science, helping to shape the future landscape of renewable and sustainable energy technologies. With these aspirations in mind, this review aims to lay the fundamental groundwork for this pioneering field of carbon science.

## 2. Electrosynthesis as an environment-friendly alternative

Electrochemical synthesis plays a key role in many scientific disciplines. Its applications cover a wide range of fields, including organic chemistry, pharmaceuticals and materials science. Notably, it offers a sustainable and environment-friendly alternative to traditional chemical synthesis methods, as it minimizes the use of hazardous reagents and reduces waste production. This situation has evolved considerably thanks to the significant advances that have facilitated the emergence of novel electrochemical reactions in recent years. For instance, recent studies have highlighted the potential of electrochemical methods for valorizing microplastics, demonstrating their ability to control various reaction parameters and upcycle plastic waste into value-added products in a more environmentally-friendly manner [27]. Furthermore, the ease and accessibility of electrochemical facilities have made electro-synthesis an enduring tool that has spanned years, decades and even

centuries in many branches of science. Moreover, electrochemical methodologies can be easily adapted to large-scale production, enabling efficient and cost-effective synthesis.

In the 19th century, electrolysis began to find industrial applications for the large-scale synthesis of chemical reagents. Notable examples from this period include the well-known chloroalkali or the Hall-Héroult processes, used to produce chlorine gas and aluminum, respectively, marking the beginning of electrochemical synthesis two centuries ago. Subsequently, numerous examples have demonstrated the usefulness of electrochemistry as a tool for manufacturing large quantities of value-added chemical reagents. This is especially remarkable in the field of organic and pharmaceutical chemistry, where electrochemical synthesis of organic compounds has been used in numerous organic reactions, including metoxymethylation, chemoselective cathodic reduction, decarboxylative etherification or catalytic hydrogenation, among others [28]. Nevertheless, the community of organic chemistry scientists recognizes that electrochemical synthesis is a "powerful, yet underdeveloped synthetic approach" due to the complexity of the electrochemical settings and fundamentals [29].

All these works over the years have prompted materials, organic chemistry and electrochemistry scientists to focus their efforts on the electrochemical synthesis of new materials, such as metal-organic frameworks (MOFs) or, more recently, carbon materials. The electrochemical synthesis of MOFs has emerged as a green and powerful strategy and has been widely addressed in the literature [30,31]. While previous review works have addressed these electrochemical synthesis methods individually [24,32,33], this review uniquely compiles the most relevant advances across all these approaches. In doing so, it reveals overarching trends, challenges and opportunities in the field, providing a holistic understanding and comparison of electrochemical methods for the electrochemical synthesis of carbon materials.

## 3. Electrochemical synthesis of carbon materials

One of the most fascinating characteristics of carbon materials is the wide range of properties they can exhibit, depending on the synthesis method, precursors or functionalization procedures. The physicochemical properties of carbon materials, including pore texture, are critical for their applications in various fields. Therefore, the ability to control and tailor these properties is essential for their successful use [34]. Traditionally, the most common methods for producing carbon materials involve the carbonization or hydrothermal treatment of carbon-rich precursors, such as polymers, coals or resins. However, these methods do not fully address the limitations about the climate change and global warming currently facing our society. Other synthesis methods include laser ablation, carbohydrate dehydration and microwave-assisted treatments, among others. For readers interested in more detailed information about these traditional synthetic routes, we recommend consulting the following literature [2,35–40].

Electrochemistry has long been linked to carbon science, and is a powerful tool for the functionalization of carbon materials. By applying a controlled potential in the presence of heteroatom-containing precursors, functional groups can be easily introduced onto the surface of carbon materials. Additionally, carbon materials have also played a major role in energy conversion and storage technologies, serving as electrodes in supercapacitors, electrocatalysts in fuel cells or metal-air batteries, and as biosensors [41–48]. The electrochemical synthesis of carbon materials typically involves the use of precursors that undergo electrochemical reactions to form carbon-based products. However, this approach has rarely been associated with the production of carbon materials. It is worth mentioning, though, that over the years, occasional contributions have been made to the preparation of carbon materials by electrochemical methods, albeit with limited impact within the carbon community prior to the 2000s.

In recent years, a significant evolution has taken place with the development of electrochemical exfoliation of graphite into graphene-

<figure><img src="image_4.png" /><figcaption>Fig. 1. Illustration of the most important content of the present review article.</figcaption></figure>
based materials, carbon functionalization and molten salt-assisted electrochemical synthesis of carbon materials. These electrochemical methods have represented unique opportunities, elevating the role of electrochemistry in carbon science, particularly in the preparation of carbon materials [49,50]. These new synthesis methods have generated considerable scientific interest in exploring novel electrochemical routes for products based on carbon materials. Consequently, the concept of “electrochemical carbonization” has reemerged, referring to the preparation of carbon materials from carbon-rich organic precursors by a bottom-up approach using a controlled potential in an electrochemical cell. This section examines in detail the role of electrochemistry in carbon science, with particular emphasis on innovative electrochemical strategies and methods for the production and functionalization of carbon materials.

### 3.1. Electrochemical functionalization of carbon materials

This section focuses on the surface modification of pre-existing carbon materials rather than their direct synthesis, in order to highlight the versatility and impact of electrochemical techniques in carbon science, not only to create new materials, but also to broaden the functionality and applicability of existing ones. Although electrochemical functionalization is not, strictly speaking, a synthetic route for producing carbon materials, it is a powerful tool for tuning the properties of carbon materials. Before discussing electrochemical procedures for functionalizing carbon materials, it is worth highlighting the differences between covalent and non-covalent functionalization strategies.

* Covalent functionalization involves the formation of chemical bonds between pristine carbon material and organic or inorganic functionalities [51]. This method typically disrupts the π-system of the carbon material, which can compromise the structure of the original material while introducing new properties (Fig. 2a). However, for

many applications, it is crucial to retain the pristine structure of carbon materials, making covalent functionalization less desirable.

* Non-covalent functionalization preserves the carbon structure, relying mainly on the interaction of the π clouds of the carbon layers and other molecules (Fig. 2a) [51]. Additionally, non-covalent functionalization can involve various other interactions, such as hydrogen bonds, non-polar gas-π, H-π, anion-π, cation-π and carbon-ligand interactions.

Electrochemical functionalization of carbon materials can be achieved via either covalent or non-covalent interactions depending on factors such as the carbon material, functionalization agents, applied potential or current, and the electrolyte used [51,52]. Electrochemical methods offer a green and cost-effective strategy for creating functional groups on carbon materials, using renewable energy and non-toxic reagents [53]. This approach represents a significant advance in materials science for the functionalization of carbon materials, as traditional methodologies often rely on harsh chemicals and treatments, typically involving heat treatments in the presence of heteroatom-containing chemicals or atmospheres.

Non-covalent electrochemical functionalization is a widely used technique for growing polymer thin films on the surfaces of carbon materials such as carbon nanotubes (CNTs) or graphene [54,55]. This process involves π-π interactions, where the π cloud of the polymer chain physically interacts with the carbon walls without damaging the pristine carbon structure. As a result, this method improves wettability and interactions with various solvents. Non-covalent electrochemical functionalization has proved effective with a variety of polymers, including aniline-based polymers, among others [56,57]. This effectiveness is demonstrated in the electrochemical polymerization of aminophosphonic and aniline groups on the surface of single-walled CNTs (SWCNTs), for instance. As observed in the shift of the redox peak position in cyclic voltammograms of Fig. 2b [56], the precursor nature

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** (a) Illustration of the covalent modification of a carbon nanotube (CNT) by the introduction of oxygen moieties after the application of acidic oxidizing agents (left side) and the non-covalent modification of a carbon nanotube by π-π interaction with a polymer (right side). (b) Cyclic voltammograms of single-walled CNTs (SWCNTs) electrochemically modified at 1.2 V vs RHE with 1 mM aniline, 1 mM 2-aminophenilphosphonic acid and 4-aminophenilphosphonic acid in a monomer-free 0.5 M H₂SO₄ solution at 50 mV s⁻¹. Reproduced with permission from (54). (c) Cyclic voltammograms of carbon materials obtained by heat treatment of polyaniline electrodeposited onto graphite/SiO₂ composites after 10 s, 30 s and 5 min of chronoamperometric electrochemical synthesis in a 0.1 M H₂SO₄ solution at 50 mV s⁻¹. Reproduced with permission from (56).</figcaption></figure>
significantly influences the electrochemical properties of post-functionalized CNTs. The quasi-rectangular shape from the original SWCNTs is still observed, however each polymer shows different redox contributions depending on its chemical configuration.

In addition, the polymerization of small N-containing molecules can be used to control the thickness of carbon materials. For instance, polyaniline has been electrodeposited on titania/graphite and silica/graphite mesoporous composites via chronoamperometric experiments in an aniline-containing acidic solution [58]. By controlling the electropolymerization time, the amount of polyaniline deposited on the composite surface could be precisely regulated. The material was then heat-treated at 900 °C to obtain thin carbon films. Electro-polymerization time is directly related to carbon thickness and electrochemical behavior, as observed in Fig. 2c. Covalent electrochemical functionalization is much more commonly used than the non-covalent approach, because the chemical changes it induces in carbon materials have a greater impact. Although some structural properties may be compromised, the introduction of heteroatom-containing moieties into the carbon matrix significantly enhances electrical conductivity, creates reactive sites and improves gas and energy storage capabilities, for example, which are of great interest for many applications [59].

## 3.2. Electrochemical exfoliation of graphite: top-down approach

In the electrochemical synthesis of carbon materials, top-down approaches involve the breakdown of macroscopic bulk material into nanosized carbon particles [60,61]. Compared with bottom-up synthesis methods, the main advantage of top-down approaches lies in the use of abundant precursors that are easily sourced from nature, in the cost-effectiveness of synthesis protocols, and in the numerous opportunities to fine-tune the properties of carbon nanomaterials by adjusting the characteristics of the pristine bulk carbon material. Over the years, significant advances have been made in the electrochemical exfoliation

of carbon materials, enabling the production of carbon nanomaterials with unique and desirable properties.

The production of graphenic materials by electrochemical exfoliation of graphite has proven to be more energy-efficient and environmentally friendly than traditional methods. Notably, the production of monolayer graphene has an energy impact around five orders of magnitude greater than the production of bulk graphene by electrochemical exfoliation [62]. However, if the power comes from renewable energy, the CO₂ emissions associated with graphene production can be reduced to 4.39·10⁻⁴ kg CO₂ per gram of graphene through electrochemical exfoliation. Assuming electrochemical exfoliation in sodium acetate and potassium hydroxide at applied voltages of either 9 or 15 V, a techno-economic analysis (TEA) and life-cycle assessment (LCA) revealed that electricity input is the dominant factor determining life cycle environmental impact [62]. The highest energy efficiency was achieved at 15 V with a KOH electrolyte, with a value of 1.85 MJ·g⁻¹graphene, compared with 0.38 MJ·g⁻¹graphene for chemical reduction, 0.28 MJ·g⁻¹graphene for thermal reduction, and only 10 kJ·g⁻¹graphene in the case of chemical oxidation processes [62].

Recent advances in electrochemical exfoliation methodologies have shown impressive efficiency, with yields close to 95%, underscoring the effectiveness of this approach [63]. Additionally, significant progress has been made in scaling up this synthesis method, with recent studies reporting batch productions of up to 10 kg under mild conditions [64].

As mentioned above, the main application of electrochemical exfoliation in carbon science is the production of graphene from bulk graphite [60,61,65]. Graphite readily allows for the intercalation of various chemical species between its graphitic layers when a voltage is applied. The exfoliation process occurs via two primary mechanisms: anodic exfoliation, which takes advantage of oxidation processes, and cathodic exfoliation, driven by reduction conditions (Fig. 3a). When graphite is used as the cathode electrode, positively charged ions (e.g., Li⁺) intercalate into the graphite sheets, causing the interlayer spacing

<figure><img src="image_6.png" /><figcaption>Fig. 3. (a) Schematic illustration of cathodic and anodic exfoliation mechanisms. The scheme also includes the addition of functionalizing agents after or during exfoliation. Reproduced with permission from 48. (b) Photographs of the electrochemical solution during electrochemical exfoliation of graphite at a fixed potential of 10 V in a Na₂MoO₄ solution at different times. Reproduced with permission from 61. (c) CND size (represented by circle diameter) as a function of time and voltage. Each color represents distinct electrochemical conditions, apart from time and voltage. CND size is proportional to the diameter of the circle and a white circle is added as a reference. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
to increase. If the voltage is sufficiently high, this process ultimately results in the exfoliation of the graphite into single and few-layer graphite. On the other hand, when graphite is used as the anode, negatively charged ions (e.g., $SO_4^{2-}$) intercalate between graphite layers, expanding the space between the layers and producing graphene. During this process, treatment with functionalizing agents can be applied to fine-tune the properties of the resulting graphene.

This expansion of the graphite electrode and the resulting darkening of the electrolyte is easily observed (Fig. 3b) [66]. Electrochemical exfoliation of graphite offers many advantages over chemical and traditional methods, such as lower cost for mass production, absence of harsh and hazardous chemicals, and production of high-quality graphene, among other benefits. Additionally, this method allows the resulting graphene to be functionalized by incorporating functionalizing agents directly during the process (as shown in Fig. 3a) [50]. This approach facilitates precise control over the functional groups introduced into graphene, eliminating the need for an additional post-functionalization step after synthesis. Furthermore, functionalities can be modulated according to the requirements of the final application by varying potential, current, electrolyte and functionalizing agents [50, 67–70].

Although electrochemical exfoliation is most often associated with graphite/graphene-based materials, it has also been effectively applied to other materials [71], such as $MoS_2$ [72] and silver nanorods [73]. Carbon nanodots (CNDs) have also been widely produced using electrochemical methods in the scientific literature [24]. This is achieved by optimizing electrochemical conditions and using pristine bulk carbon electrodes [74–76], although CNDs can also be obtained from carbon fibers [77]. When electrochemical exfoliation conditions are strong enough, not only do intercalation and exfoliation occur in graphite-like materials, but cleavage of the carbon layers can also take place. This cleavage is caused by oxidation of the carbon layers due to the electrolyte and the high voltage applied. The resultant corrosion of the carbon layers produces CNDs, with unique properties useful for fluorescence, biosensing and other applications.

Nevertheless, it seems difficult to draw definitive conclusions on the influence of electrochemical parameters on CND properties. As shown in Fig. 3c, there is no close correlation between voltage and time during the production of CNDs using top-down methodologies. This suggests that other factors, such as carbon precursor, electrolyte nature and concentration, play an important role in determining the properties of the resulting CNDs. However, it can be observed that there is indeed a correlation when the same electrochemical conditions are applied (blue circles), where higher time leads to smaller CND size.

With regard to graphenic materials, it is worth mentioning the significant differences in the application of electrochemical exfoliation under anodic versus cathodic approaches, as the characteristics of the resulting carbons and the underlying mechanisms are highly dependent on the electrochemical conditions (Fig. 3c) [78].

### 3.2.1. Anodic exfoliation

Electrochemical exfoliation under anodic conditions, based on the intercalation of anions between graphitic layers, has been the subject of much recent research. Various compounds have been proposed as intercalating agents, and the choice of the latter has a significant impact on the properties of the resulting carbon material. For example, organic and inorganic salts, inorganic acids and bases, and ionic liquids have all demonstrated their effectiveness as intercalating agents in anodic exfoliation. Various sulfate salts have proved particularly successful as exfoliating agents under anodic conditions, as demonstrated by Parvez et al. [79]. The authors described the mechanistic behavior as follows: when an applied potential is used in the presence of sulfate salts, the graphite layers undergo depolarization and expansion due to the oxidation of the graphite edges, which facilitates the entry of the intercalating agents between the graphitic layers. Finally, the oxidation of water (and, to a lesser extent, the reduction of sulfate ions) in the

interlayer space produces gases that promote complete exfoliation of the graphene sheets.

A major drawback of anodic conditions is the oxidation of carbon materials during exfoliation, which can lead to undesirable changes in the properties of carbon nanomaterials. However, recent advances in electrolyte combinations and applied potentials have enabled better control of the oxygen content of the resulting carbon materials. Munuera et al. [80] have demonstrated that low oxygen content and fewer defects could be obtained by selecting appropriate types of pristine bulk graphite. Additionally, ordered graphene nanosheets can be obtained using suitable carbon materials, such as graphite foil, and carefully selected electrolytes. Nevertheless, a significant amount of oxygen species can also be beneficial for anchoring metal nanoparticles, as there is a strong interaction between oxygen atoms and metal particles [81–83]. It is therefore crucial to select precisely the conditions for electrochemical exfoliation in anodic mode to obtain the most desirable conditions for specific applications.

### 3.2.2. Cathodic exfoliation

The use of the cathodic mode for electrochemical exfoliation offers a notable advantage over the anodic mode, as the intercalated cations are non-oxidizing, which helps preserve the $sp^2$ hybridization of the carbon layers [65]. However, cathodic conditions also present certain drawbacks. A major concern is the hydrogen evolution reaction (HER), which occurs during electrochemical exfoliation of graphite electrodes under cathodic conditions. The production of hydrogen bubbles plays a crucial role in the exfoliation process, as these bubbles generate mechanical stresses at the carbon/electrolyte interface [78,84]. This effect is particularly pronounced, as some carbon materials are highly efficient electrocatalysts for the HER, with this behavior becoming more pronounced at potentials that are more negative. Therefore, when evaluating electrochemical cathodic exfoliation, it is essential to consider hydrogen production carefully, as part of the current may be related to the HER rather than solely to exfoliation of the carbon material.

The selection of the electrolyte used in anodic and cathodic electrochemical exfoliation has a significant impact on the properties of the resulting carbon materials [32]. Aqueous solutions containing $H_2SO_4$ are the most widely used electrolytes for the electrochemical exfoliation of graphite. $SO_4^{2-}$ anions easily intercalate into graphene layers and generate large amounts of gaseous products that promote exfoliation [85,86]. Nevertheless, the aqueous electrolytes $H_3PO_4$, $H_2C_2O_4$ or $HClO_4$ display similar electrochemical performance. However, the use of salts and alkaline electrolytes can strongly modify the properties of the resultant graphene through the reaction of ions with radicals generated during graphite expansion and exfoliation [87]. Moreover, the use of electrolytes containing heteroatoms can facilitate not only the exfoliation process but also the functionalization of carbon materials, thereby improving their processability. Heteroatoms such as phosphorus [88], nitrogen [75,89,90] and sulfur [91], among others, have been shown to integrate into the carbon framework during the electrochemical exfoliation of graphite in the presence of heteroatom-containing molecules. Additionally, organic compounds have also been introduced into aqueous electrolytes to improve the quality of graphene [92] and to enhance exfoliation efficiency [93].

### 3.3. Molten salt-assisted electrochemical synthesis of carbon materials

Molten salts, which are ionic liquids with melting points above 200 °C, have long been recognized as a promising route to large-scale materials production, especially for pure metals or alloys. The well-known Hall-Héroult method for aluminum production, for instance, relies on the electrolysis of alumina in molten cryolite. In recent years, multiple ionic liquids have been used in the preparation of various metals due to their unique properties. These ionic liquids serve: i) as accelerating agents for the conversion of carbon precursors into carbon materials; ii) as physical barriers, preventing undesirable interactions
between carbon precursors and the atmosphere; and iii) as excellent conductors of mass and heat, which are crucial for facilitating various electrochemical or chemical reactions. At temperatures above their melting point, ionic crystals transform into their respective ionic liquid state, creating an essential reaction medium. This transformation significantly improves the carbonization of carbon precursors, reducing energy requirements compared with traditional methods for achieving a similar degree of carbonization.

The molten salt-assisted electrochemical route has emerged as a promising strategy for the synthesis of carbon materials [94–99]. This method enables the use of a wide variety of carbon precursors, including organics, graphite-like materials, metal carbides and carbon-containing gases. Furthermore, molten salts facilitate the creation of pores, which are crucial in carbon science for applications where textural properties are essential, such as supercapacitors or adsorbents. Molten salts have emerged as a highly promising strategy for the graphitization of carbonaceous materials, whether from CO₂ or amorphous carbons. Unlike conventional graphitization methods, which typically require temperatures of around 2700 °C and consequently high energy demands, molten salts significantly reduce the graphitization temperature to approximately 700 °C [100]. This substantial reduction in temperature not only lowers the energy costs of the process, but also enables more cost-effective equipment to be used, as the extreme conditions associated with traditional technologies are no longer necessary. Additionally,

although other methodologies, such as microwave treatments or the use of metal catalysts in graphitization, can also achieve lower graphitization temperatures, only the molten salt approach has demonstrated, or is expected to demonstrate, the ability to reach the mass production scales of traditional technologies [100]. This unique combination of cost-effectiveness, scalability and reduced energy demand makes molten salts a transformative technology for the large-scale production of carbon-based materials.

This approach has also attracted considerable attention in molten salt-assisted CO₂ reduction reaction (CO₂RR), since the absence of water and protons in molten salts improves selectivity towards solid carbon products or CO gas, while avoiding hydrogen and hydrocarbon formation [95]. The molten salt-assisted CO₂RR can proceed along two distinct pathways: i) a 2-electron pathway from CO₂ to CO; or ii) a 4-electron pathway, which reduces CO₂ to solid carbon. Li₂CO₃ and CaCO₃ are among the most commonly used molten salts for the electrochemical conversion and utilization of CO₂ due to their stability in the molten state over a wide temperature range, from 400 to 900 °C. While Li₂CO₃ demonstrates a strong preference for CO production, CaCO₃ exhibits high selectivity for carbon deposition [101]. Notably, CNTs, graphene or carbon nanospheres have been successfully synthesized by the electrochemical conversion of CO₂ in molten salts. Moreover, thin graphite sheets have been prepared by the direct conversion of CO₂ in the presence of anhydrous CaCl₂ molten salt at 850 °C and 2.6 V

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** (a) Graphene sheets obtained from the capture and electrochemical conversion of CO₂ at 750 °C on a Cu cathode. Reproduced with permission from (96). (b) SEM images of carbon nanotubes obtained from molten salt-assisted CO₂RR at 730 °C in molten Li₂O. Reproduced with permission from (100). (c) Schematic representation of the electrochemical setup for graphitization of amorphous carbon materials in molten CaCl₂. (d) XRD patterns of graphitized amorphous carbons at different voltages for 2 h at 820 °C. Reproduced with permission from (101).</figcaption></figure>
[102]. Graphene-like materials have also been obtained from the capture and electrochemical conversion of CO₂ in molten CaCl₂-NaCl-CaO salts (Fig. 4a) [103]. Nevertheless, this process requires not only molten salts, but also the application of high temperatures (400–800 °C) and pressures (10 atm of CO₂).

To avoid the complications associated with high pressures, such as more tedious, expensive and complex electrochemical setups, the use of molten carbonate salts has shown promise for molten salt-assisted CO₂RR for the synthesis of carbon materials [98]. The electrolysis of carbonates can produce carbon nanostructures by similar mechanisms. Specifically, CO₃²⁻ is reduced through a 4-electron pathway to form solid carbon and three O²⁻. The first successful electrosynthesis of carbon materials from carbonates dates back to 1966, using molten Li₂CO₃-Na₂CO₃-K₂CO₃ salts at 600 °C under a continuous flow of CO₂ and Ar [104]. Since then, numerous studies have explored the electrochemical synthesis of carbon materials from molten carbonates [98]. Temperature plays a significant role in the electrodeposition yield from molten carbonate salts. An interesting study demonstrated that carbon materials obtained at around 500 °C has a larger surface area (585 m² g⁻¹) than those obtained at 700 °C at the same voltage (280 m² g⁻¹) [105]. Concerning electrolyte composition [106], it was demonstrated that carbon electrodeposition in a Li₂CO₃ electrolyte occurs a lower voltage compared to Li₂CO₃-Na₂CO₃-K₂CO₃ eutectic melt at the same temperature (750 °C). The authors attributed the importance of Li₂CO₃ to the thermodynamic stability of the Li₂O intermediate, which is produced before metallic Li. However, in the absence of Li⁺ cations, other carbonates, such as Na₂CO₃, allow metal deposition prior to carbon electrodeposition.

With regard to nanostructured carbons, CNTs have been successfully synthesized by electrolysis of molten binary and ternary Li-Ba-Ca carbonate salts at cell voltages between 4 and 6 V and temperatures ranging from 550 to 650 °C (Fig. 4b) [107]. Interestingly, the type of cation present in the carbonate has a significant influence on CNT diameter. The authors demonstrated that CNTs obtained in the Li-Ca-Ba system exhibited a smaller diameter of around 50 nm, while those obtained from Li-Ba carbonates presented a diameter of around 200 nm. Fig. 4b shows that CNTs often contain small Ni nanoparticles originating from the Ni anode. These nanoparticles leach into the electrolyte at high potentials and temperatures, leading to contamination of the solid carbon material. CNTs are the most commonly produced nanostructured carbon materials using this methodology. However, by adjusting certain parameters of the electrochemical synthesis process, it is possible to obtain other carbon materials, such as carbon nanopowders [105].

Molten salts are also frequently used to graphitize amorphous carbons, which traditionally require very high temperatures (2500–3000 °C) in an inert atmosphere for graphitization [94]. Molten salts, on the other hand, provide a low-temperature environment that enables amorphous carbon materials to be graphitized at temperatures below 1000 °C. This process is carried out by cathodic polarization of an assembled amorphous carbon pellet at temperatures ranging from 600 to 1000 °C (Fig. 4c) [108]. The application of a voltage between the two electrodes and the high temperatures facilitates the rearrangement of carbon atoms from a disordered to an ordered structure. Using molten salt-assisted electrochemical graphitization, a highly disordered carbon material can be transformed into a well-ordered, graphitic carbon material at 820 °C, as clearly shown by the XRD patterns of the materials (Fig. 4d) [108]. The degree of graphitization also depends on the temperature of the molten salt process, with higher temperatures resulting in a greater degree of graphitization. Additionally, applying a lower voltage increases the polarization of the carbon electrode, leading to stronger conversion to a graphitic structure. This method has also been successfully applied to biomass-derived carbon precursors, although higher temperatures of the order of 1000 °C are required to achieve similar degrees of graphitization [97,109].

Molten salt-assisted electrochemical processes have also shown promise for the electrochemical splitting of methane to produce

hydrogen. This process, which is sometimes integrated with in situ CO₂ conversion [110], offers a clean, energy-efficient and cost-effective alternative for hydrogen production. It significantly outperforms traditional methane steam reforming technology in terms of carbon emissions and energy consumption. Life cycle assessment (LCA) studies reveal that CO₂ emissions from electrochemical methane splitting are 1.525 tCO₂/tH₂, which is comparable to water splitting (1.246 tCO₂/tH₂) and substantially lower than methane steam reforming (6.038 tCO₂/tH₂) [110].

The techno-economic viability of molten salt-assisted processes has been widely demonstrated in power-to-heat solutions for the industrial sector, offering potential OPEX savings of up to €3 million per year compared with conventional technologies [111]. Moreover, the latter study highlighted that electricity markets will play a decisive role in shaping the future cost of this technology, with price reductions anticipated as renewable energy sources become more widespread and integrated. However, the molten salt-assisted electrochemical method is still in its early stages of development, and no comprehensive techno-economic analysis is currently available.

### 3.4. Electrochemical carbonization: bottom-up approach

The bottom-up approach to the electrochemical synthesis of carbon nanomaterials, closely linked to organic chemistry, offers the possibility of producing nanoscale carbon materials more economically and with greater control over their physicochemical properties, structure and composition. This approach involves building carbon materials from organic molecules or alcohols. Despite its promising advantages, this method is less commonly used than electrochemical exfoliation of bulk carbon materials, as it is still in the early stages of development in this emerging field. So much so that all reviews on the electrochemical synthesis of carbon nanomaterials classify the electrochemical approach solely as a top-down method, focusing on the electrochemical exfoliation of bulk materials [32,50,112-115], while overlooking the fact that electrochemistry can also be used for the electrochemical carbonization of organic molecules. This underlines the novelty and the lack of understanding in this emerging branch of materials science.

We would like to highlight here that the CO₂ reduction reaction is another bottom-up electrochemical process, in which CO₂ is converted into carbon-based chemicals of significant industrial value. However, this reaction has already been extensively studied and numerous reviews are already available on the subject. Readers interested in this field will find detailed information elsewhere [116,117]. Additionally, this review does not include results related to CO₂ reduction, as large-scale production of carbon-based materials beyond C1 and C2 products is uncommon, except in molten salt methodologies.

Although organic chemistry has a long and rich history, the use of applied voltage or current to convert small organic molecules or alcohols into carbon nanomaterials has only gain in appeal in recent years. Nevertheless, it is worth mentioning that the concept of electrochemical carbonization of organic molecules dates back to the 1980s, when the electrochemical reduction of halogenated compounds was first reported [11–14]. In these pioneering studies, the reduction of halogens in halogen-containing carbon precursors by electrochemical techniques was used to prepare polymer-like carbon nanostructures through C–C coupling reactions. The first discovery of electrochemical carbonization was made in 1981, where polytetrafluoroethylene (PTFE) was electrochemically reduced in the presence of lithium amalgam, resulting in the formation of a hydrocarbon film and lithium fluoride via a polymeric anion radical [13]. Over the following years, the electrochemical carbonization of various other precursors, such as Nafion® [11], hexafluorobenzene, tetradecafluorohexane [12] and hexachlorobutadiene, 3-diene [118], was demonstrated, with or without the presence of metal amalgams. However, the use of these halogenated compounds is impractical for scaling up the production of carbon materials due to their scarcity, toxicity and high cost. As a result, these methods had minimal
impact within the scientific community because they are not feasible for large-scale applications. For broader applicability, halogenated molecules need to be replaced by organic molecules found in nature and that are more abundant.

After several decades without revisiting the electro-reduction of carbon precursors, a very interesting article was published in 2010 that

brought electrocarbonization back into the spotlight. In this studies [119,120], the authors indirectly discovered the electrochemical synthesis of CNDs while investigating the production of graphene by electrochemical exfoliation of graphite under anodic conditions in a NaOH/EtOH electrolyte. They observed that the loss of graphite during the electrochemical exfoliation was not equal to, but less than, the

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** (a) Setup for the electrochemical synthesis of carbon dots from ethanol. Two platinum foils were used as anode and cathode. (b) TEM images of carbon nanodots (CNDs) obtained from ethanol at an applied potential of 3.0 V and (c) corresponding particle size distribution. (d) UV absorbance spectra of CNDs obtained at 3.0, 4.5, 6.0 and 7.5 V under 365 nm excitation. Reproduced with permission from (111). (e) Scheme illustrating the effect of voltage and time on CND size reported in the literature; CND size is proportional to the diameter of the circle. CND size is proportional to the diameter of the circle and a white circle is added as a reference. (f) Chronoamperometric curve and resulting quantum yields plotted as a function of reaction time at an applied potential of 3 V in a 3 M $NH_3$ solution containing 0.5 M aspartic acid, and (g) photographs of the solution after application of the fixed potential at different working times. Reproduced with the permission from (115).</figcaption></figure>
amount of the graphene obtained. This discrepancy could only be explained by the electrochemical synthesis of CNDs from ethanol. This was corroborated in their subsequent work in 2011 [121], where they synthesized fluorescent CNDs by one-step, NaOH-assisted electrochemical process. They used a voltage of 25–40 V and platinum rods as electrodes for both the anode and cathode. These electrochemical conditions facilitated the production of a significant amount of fluorescent CNDs with stable and intense visible emission and, for the first time, carbon nanomaterials were thus produced by electrocarbonization of halogen-free carbon precursors, opening a new paradigm in carbon science and electrosynthesis.

In addition to the successful production of carbon materials by electrocarbonization, this process has demonstrated its ability to synthesize high-value nanomaterials such as CNDs. CNDs have emerged as highly promising nanomaterials due to their unique combination of properties and potential applications. Their nanometric scale (diameter less than 10 nm) leads to unique changes in physicochemical properties, such as fluorescence properties, high solubility, high resistance to photobleaching and low biological toxicity, making them valuable materials for multiple applications.

Since then, several works have been published on electrochemical carbonization. In 2014, an in-depth study was conducted on the electrochemical conditions of carbonization in a NaOH/EtOH solution at various applied voltages (electrochemical setup in Fig. 5a, [122]). In this study, the authors attributed CND formation to the electrochemical oxidation of alcohol. However, this finding contradicts previous research, which has extensively discussed the fact that carbon nanoparticle formation is generally associated with reduction reactions of organic molecules. The use of a single-compartment electrochemical cell can lead to a poor understanding of electrochemical carbonization mechanisms, as the products from the anode and cathode cannot be distinguished in the resulting electrolyte. Nevertheless, these studies provide valuable information. In particular, the applied voltage plays a key role in determining the size of the CNDs during electrochemical carbonization of EtOH (see below).

Indeed, the higher the applied voltage, the larger the size of the CNDs, as confirmed by HRTEM images (Fig. 5b and c). The excellent control that electrochemistry offers over the size of the CNDs enables their fluorescence properties to be tuned for different applications according to their nanoscale diameter (as shown in Fig. 5d). The authors also observed that fluorescence quantum yields and maximum emission wavelengths varied with the size of the CNDs produced by electrochemical carbonization of EtOH. EtOH-derived CNDs have also shown promising activities for $Fe^{3+}$ assays by detecting $Fe^{3+}$-induced fluorescence quenching [123]. Similarly, other carbon sources such as propanol, butanol, ethylene glycol and glycerin have been studied, and they produced CNDs with comparable fluorescence properties [122].

Glycine has also shown promise as a carbon source for the production of CNDs via electrochemical carbonization [124]. In the latter study, glycine was introduced into an aqueous $NH_4OH$ solution, and a voltage of 10 V was applied between the two platinum electrodes. The authors succeeded in producing fluorescent CNDs of 2.4 nm diameter and attributed CND formation to electro-oxidation, electro-polymerization, electrochemical carbonization and passivation mechanisms. Moreover, they demonstrated the significant impact of electrochemical conditions on electrochemical carbonization and the properties of the CNDs obtained. For example, the use of other aqueous electrolytes, such as NaOH or $Na_3PO_4$, results in faster electrochemical carbonization, producing larger CNDs that, in turn, exhibit reduced photoluminescence. Furthermore, the concentration of $NH_4OH$ was studied to understand better the mechanisms underlying electrochemical carbonization. It is worth mentioning here that neither glycine nor $NH_4OH$ alone can lead to CND formation: both are required for successful electrochemical carbonization. Interestingly, increasing the $NH_4OH$ concentration enhances the photoluminescence effect of the resulting CNDs up to a maximum of 3 M. The authors attributed this effect to the increased

stability of glycine in the presence of $OH^-$ anions. Above this concentration, further increases in $NH_4OH$ molarity led to higher pH values, which hinder the reaction between carboxylates and ammonium groups, thereby reducing the efficiency of the carbonization process. Nevertheless, no particular attention has been paid to the role of the nitrogen heteroatom in the resulting CNDs. Moreover, an in-depth study on the electrochemical carbonization of 1-propanol to produce CNDs was carried out in 2016 [125]. The CNDs produced in this study exhibited interesting luminescence properties and were also used as highly active bioelectrochemical sensors of dopamine and epynaphryne.

Only a few studies have investigated the bottom-up electrochemical carbonization of organic precursors to produce CNDs, and each study used different electrodes, electrolyte nature and concentration, carbon precursors, voltage and time. It is therefore difficult to draw definitive conclusions on which parameters have the greatest impact. Nonetheless, an overall assessment (Fig. 5e) shows that voltage and time are obvious factors; with higher voltage or longer time leading to larger CNDs.

After studying the most common alcohols, researchers turned their attention to the electrochemical carbonization of nitrogen-containing organic molecules. This approach aims to improve the properties of CNDs by doping or functionalizing them with nitrogen species. Nitrogen can indeed modify the band gap and electron density of carbon nanomaterials by localizing charge density and creating an electron-withdrawing effect on adjacent carbon atoms. As an effective doping agent, nitrogen has proven to be a promising heteroatom for carbon materials in electrocatalytic applications. In this context, numerous studies have focused on the electrochemical carbonization of various nitrogen precursors, with different routes proposed to prepare N-doped carbon nanomaterials via an electrochemical bottom-up approach.

For example, with the aim of modulating carbon properties at the atomic-scale, Niu et al. proposed the synthesis of N-doped CNDs using amino acids, since nitrogen- and carbon-containing materials are highly biocompatible [126]. L-cysteine, L-aspartic acid, L-serine, L-methionine, L-histidine and L-arginine were introduced into a single-compartment cell in aqueous $NH_3$ solution, and a potential from 1 to 10 V was applied for 2 h. Fig. 5f shows the dependence of the quantum yield on reaction time at the applied potential of 3 V. Moreover, the current was recorded and plotted as a function of time during chronoamperometric analysis. Interestingly, the highest quantum yield of the suspension obtained during electrochemical carbonization coincides with the maximum current value during chronoamperometric synthesis. This is clearly observed in Fig. 5g, in which significant color changes were observed in the electrochemical suspension, as shown in the images during chronoamperometry. As a result, the authors claimed that electrochemical carbonization could be used to achieve the highest quantum yield. Similar trends were observed for all the amino acids studied, with slight variations in the optimum time for the highest quantum yield. The oxidation states of the nitrogen species were studied by XPS, and the results obtained showed the contribution of two main peaks at 399.8 and 400.5 eV, which are assigned to C-N and N-H-type species, respectively.

Urea was introduced as a source of nitrogen and carbon in an aqueous sodium nitrate solution [127]. Again, two platinum foils were used as anode and cathode in a single-compartment electrochemical cell during the 1-h electrochemical carbonization at 5 V. Under these conditions, N-doped CNDs with diameters from 1.0 to 3.5 nm were obtained, with promising performance for sensitive $Hg^{1+}$ detection. CNDs were also obtained by electrochemical carbonization of a mixture of acetonitrile and 1-butyl-3-methyl-imidazolium hexafluorophosphate at an applied voltage of 15 V, using two platinum foils as cathode and anode [128]. The authors proposed that, due to the positive charge of the cation in the ionic liquid, the final product is formed at the cathode by a reductive mechanism. Nitrogen species from the cation in the carbon materials were detected as N-C=C and quaternary N species, and phosphorus from the anion was also observed in XPS P 2p spectra. The resulting N- and P-co-doped CNDs have shown excellent photo-stability and low cytotoxicity, and they have been efficiently applied to
Fe³⁺ detection. Moreover, co-doping, or multi-doping, of carbon materials has been widely used for multiple applications, such as electrocatalysts [44], electrode for supercapacitors [129] or electrochemical sensors [130,131], but electrochemical carbonization is still in its infancy and different combinations of heteroatoms and oxidations states have yet to be assessed.

Another target for study is the combination of CNDs with metal-
containing reagents. Like heteroatoms, metals can also modify most of
the properties of carbon materials, enabling unprecedented results in
multiple applications, such as ORR/OER/HER electrocatalysts for
instance Refs. [132,133]. CNDs were obtained by electrochemical
carbonization of EtOH in sodium hydroxide at different concentrations
and an applied voltage of 13 V for 1 h. Subsequently, a homogeneous
suspension of the resulting carbon nanomaterials in an aqueous
Ag⁺/Cu²⁺ solution was maintained in the dark at room temperature to
obtain core-shell silver-copper oxide supported on CNDs [134]. The
resulting materials showed high activity towards the oxygen reduction
reaction (ORR), with an onset potential of approximately 0.88 V vs RHE.
In addition, the introduction of metal in the CNDs also led to high
selectivity towards the 4-electron transfer pathway, which is desirable
for efficient ORR in fuel cells and metal-air batteries.

Nevertheless, Hongrui et al. presented compelling evidence that the electrochemical carbonization of alcohols can take the synthesis of carbon nanomaterials even further [135]. Indeed, not only were CNDs synthesized via the electrochemical reduction of organic compounds, but the same kind of method was also used for the preparation of nanodiamond-like carbon materials via the electrochemical reduction of acetic acid. This reaction takes place under ambient conditions, using Ag/Cu electrodes as the cathode electrode and Pt foil as the anode electrode in the presence of water and ionic liquid. The combination of ionic liquids, along with operation at room temperature and pressure, positions this an exciting topic to pursue in the coming years.

As discussed in this section, the electrochemical conversion of organic compounds into carbon nanomaterials has so far been limited to the production of carbon nanodots (CNDs) using Pt electrodes, which significantly limits the potential of this technology. Additionally, CNDs are relatively new materials, and assessing the economic viability of this strategy compared with other approaches, such as hydrothermal routes, remains challenging. This technology and the materials associated are still in their infancy and hold great promise. However, it is currently difficult to determine its full potential from a techno-economic point of view. Further research is needed to evaluate its feasibility and competitiveness in a broader technological and economic context.

4. Conclusions & perspectives

In the age of renewable and green technologies, electrochemistry and carbon-based materials play an indisputable role at the core of multiple energy conversion and storage devices. Carbon materials doped or not, are one of the most promising alternatives to commercially available electrodes containing precious metals, as they are inexpensive and easy to implement. Together, carbon science and electrochemistry might potentially lead to significant changes in our modern society by limiting carbon emissions and reducing anthropogenic CO₂ in the atmosphere.

In the past, electrochemistry in carbon science has been used to fine-
tune some of the properties of carbon materials through the function-
alization of carbons in electrochemical cells. However, electrochemistry
as a tool for the synthesis of carbon nanomaterials went unnoticed until
the rise of the electrochemical exfoliation of bulk graphite, which
brought about a paradigm shift in the scientific community for materials
scientists. Electrochemical exfoliation of bulk carbon materials emerged
as a novel and eco-friendly method for obtaining carbon nanomaterials,
offering a simple and cost-effective alternative to traditional ap-
proaches. The use of anodic or cathodic conditions, along with a wide
range of electrolytes and carbon precursors, offer unlimited possibilities
for obtaining carbon materials with diverse surface chemistry and

properties.

Recently, the bottom-up electrochemical approach has re-emerged as a precise tool for producing tailor-made carbon-based nano-materials. While electrochemical reduction of halogenated organic compounds was briefly explored in the 1980s, it turned out that most mechanisms were based on dehydration, condensation and carbonization reactions, which occur when a fixed reduction potential is applied to the cathode of an electrochemical cell. In recent years, electrochemical carbonization of organic molecules has arisen as a promising strategy for obtaining highly tunable CNDs. By carefully controlling the electrochemical conditions during this process, researchers have achieved excellent control over the diameter, chemical and photoluminescence properties of the resulting CNDs. Moreover, some studies have explored the modification of CND properties by preparing N-doped carbon nanomaterials using nitrogen- and carbon-containing precursors. However, many critical aspects of this new field remain unresolved, hindering progress in electrochemical carbonization. We have put together several key questions that need to be addressed to move this important topic forward.

- To date, carbon nanomaterials have only been synthesized in single-compartment electrochemical cells, giving rise to considerable debate about the reaction mechanisms underlying the electrochemical carbonization of organic molecules. Some researchers suggested that electrochemical oxidation of organic molecules at the anode was responsible for the carbonization process. However, early literature and recent findings in electrochemistry and carbon science suggest that electrochemical reduction at the cathode is actually the driving force behind electrochemical carbonization. To investigate this question further, it is essential to carry out experiments using a two-compartment electrochemical cell. This setup will allow us to determine whether CNDs are obtained at the cathode, the anode or, alternatively, whether both electrodes are involved in the electrochemical carbonization process.

- All electrochemical carbonization experiments to date have been conducted using two platinum foils as anode and cathode. However, platinum is not the most effective electrocatalyst for facilitating carbon-carbon coupling reactions. In the CO₂RR, copper is known to offer significant advantages for the production of larger C–C hydrocarbons. Therefore, if the aim is to produce not only CNDs but also larger forms of carbon materials, it is crucial to explore and study alternative electrocatalysts, particularly transition metal-based electrocatalysts, and more particularly, Cu-based electrocatalysts.

- Electrochemical carbonization has mainly been performed using single organic molecules, such as methanol or ethanol. However, introducing several organic molecules into the process could result in carbon materials with novel and improved properties, especially if one of the molecules contains a heteroatom that could modify the electronic properties of the resulting material.

Electrochemical carbonization of organic molecules is therefore still
in its infancy, but it offers significant potential for fine-tuning the
properties and production of carbons in materials science. In this over-
view, we have presented the fundamentals of this emerging field, along
with a brief review of its current progress and future prospects.

CRediT authorship contribution statement

**Javier Quílez-Bermejo:** Writing – review & editing, Writing – original draft, Conceptualization.
**Emilia Morallón:** Writing – review & editing.
**Diego Cazorla-Amorós:** Writing – review & editing.
**Alain Celzard:** Writing – review & editing, Conceptualization.
**Vanessa Fierro:** Writing – review & editing, 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.

Acknowledgements

This study was partially supported by the French PIA project "Loraine Université d'Excellence", reference ANR-15-IDEX-04-LUE and the TALiSMAN project funded by ERDF (2019–000214). The authors would like to thank PID2021-1230790B-100 project funded by MCIN/AEI/ 10.13039/501100011033 and "ERDF/EU".

Data availability

No data was used for the research described in the article.

References

[1] K. Du, J. Li, Towards a green world: how do green technology innovations affect total-factor carbon productivity, Energy Policy 131 (2019) 240–250.

[2] J. Quílez-Bermejo, S. Pérez-Rodríguez, A. Celzard, V. Fierro, Progress in the use of biosourced phenolic molecules for electrode manufacturing, Front. Mater. 9 (2022) 810575.

[3] G. Lan, J. Yang, R.P. Ye, Y. Boyjoo, J. Liang, X. Liu, Sustainable carbon materials toward emerging applications, Small Methods 5 (2021) 2001250.

[4] Q. Ma, Y. Yu, M. Sindoro, A.G. Fane, R. Wang, H. Zhang, Carbonbased functional materials derived from waste for water remediation and energy storage, Adv. Mater. 29 (2017) 1605361.

[5] M.A. Yahya, Z. Al-Qodah, C.W.Z. Ngah, Agricultural bio-waste materials as potential sustainable precursors used for activated carbon production: a review, Renew. Sustain. Energy Rev. 46 (2015) 218–235.

[6] M.M. Titirici, R.J. White, N. Brun, V.L. Budarin, D.S. Su, F. del Monte, J.H. Clark, M.J. MacLachlan, Sustainable carbon materials, Chem. Soc. Rev. 44 (2015) 250–290.

[7] Y. Jing, Y. Guo, Q. Xia, X. Liu, Y. Wang, Catalytic production of value-added chemicals and liquid fuels from lignocellulosic biomass, Chem 5 (2019) 2520–2546.

[8] W. Dessie, X. Luo, M. Wang, L. Feng, L. Yunhui, Z. Wang, Z. Yong, Z. Qin, Current advances on waste biomass transformation into value-added products, Appl. Microbiol. Biotechnol. 104 (2020) 4757–4770.

[9] M.L. Divya, S. Natarajan, Y.S. Lee, V. Aravindan, Biomass-derived carbon: a value-added journey towards constructing high-energy supercapacitors in an asymmetric fashion, ChemSusChem 12 (2019) 4353–4382.

[10] S.K. Tiwari, M. Bystrzejewski, A. De Adhikari, A. Huczko, N. Wang, Methods for the conversion of biomass waste into value-added carbon nanomaterials: recent progress and applications, Prog. Energy Combust. Sci. 92 (2022) 101023.

[11] L. Kavan, F.P. Dousek, J. Weber, Electrochemical reduction of nafion 117, J. Electroanal. Chem. Interfacial Electrochem. 208 (1986) 109–116.

[12] L. Kavan, F.P. Dousek, Reaction of hexafluorobenzene and tetra-decafluorohexane with lithium amalgam, J. Fluor. Chem. 41 (1988) 383–391.

[13] J. Jansta, F.P. Dousek, Electrochemical reduction of polymeric carbon, Electrochim. Acta 26 (1981) 233–237.

[14] V.H. Brecht, F. Mayer, H. Binder, ESCA-Untersuchungen an geätzten polytetrafluorothylen-folien, Angew. Makromol. Chem. 33 (1973) 89–100.

[15] J. Li, H. Heidarpour, G. Gao, M. McKee, H. Bemana, Y. Zhang, C.T. Dinh, A. Seifitokaldani, N. Kornienko, Heterogeneous electrosynthesis of C-N, C-S and C-P products using CO2 as a building block, Nat. Synth. 3 (2024) 809–824.

[16] J. Gao, A. Bahmanpour, O. Krocher, S.M. Zakeerudding, D. Ren, M. Gratzel, Electrochemical synthesis of propylene from carbon dioxide on copper nanocrystals, Nat. Chem. 15 (2023) 705–713.

[17] B. Martínez-Sanchez, J. Quílez-Bermejo, D. Cazorla-Amorós, E. Morallón, Electrocatalysis with metal-free carbon-based catalysts, in: Carbon-Based Met. Free Catal., 2022, pp. 213–244.

[18] M. Li, H. Wang, W. Luo, P.C. Sherrell, J. Chen, J. Yang, Heterogeneous single-atom catalysts for electrochemical CO2 reduction reaction, Adv. Mater. 32 (2020) 2001848.

[19] Y. Zou, S. Wang, An investigation of active sites for electrochemical CO2 reduction reactions: from in situ characterization to rational design, Adv. Sci. 8 (2021) 2003579.

[20] J. Fu, K. Liu, H. Li, J. Hu, M. Liu, Bimetallic atomic site catalysts for CO2 reduction reactions: a review, Environ. Chem. Lett. 20 (2022) 243–262.

[21] W. Nie, C.C.L. McCrory, Strategies for breaking molecular scaling relationships for the electrochemical CO2 reduction reaction, Dalton Trans. (2003) (2022). In press.

[22] W. Lai, Z. Ma, J. Zhang, Y. Yuan, Y. Qiao, H. Huang, Dynamic evolution of active sites in electrocatalytic CO2 reduction reaction: fundamental understanding and recent progress, Adv. Funct. Mater. 32 (2022) 2111193.

[23] W. Lai, Y. Qiao, J. Zhang, Z. Lin, H. Huang, Design strategies for markedly enhancing energy efficiency in the electrocatalytic CO2 reduction reaction, Energy Environ. Sci. (2022). In press.

[24] D. Rocco, V.G. Moldoveanu, M. Feroci, M. Bortolami, F. Vetica, Electrochemical synthesis of carbon quantum dots, Chemelectrochem 10 (2023) e02202104.

[25] D.V. Vasilyev, P.J. Dyson, The role of organic promoters in the electroreduction of carbon dioxide, ACS Catal. 11 (2021) 1392–1405.

[26] C. Zhu, N.W.J. Ang, T.H. Meyer, Y. Qiu, L. Ackermann, Organic electrochemistry: molecular syntheses with potential, ACS Cent. Sci. 7 (2021) 415–431.

[27] J. Li, J. Chen, L. Zhang, J. Matos, L. Wang, J. Yang, Electrocatalytic upcycling of plastic waste: progress, challenges, and future, Electron 2 (2024) e63.

[28] M.C. Leech, A.D. Garcia, A. Petti, A.P. Dobbs, K. Lam, Organic electro synthesis: from academia to industry, React. Chem. Eng. 5 (2020) 977–990.

[29] A. Shatskiy, H. Lundberg, M.D. Karkas, Organic electrosynthesis: applications in complex molecule synthesis, Chemelectrochem 6 (2019) 4067–4092.

[30] H. Ren, T. Wei, Electrochemical synthesis methods of metal-organic frameworks and their environmental analysis applications: a review, Chemelectrochem 9 (2022) e02200196.

[31] M.V. Varsha, G. Nageswaran, Review - direct electrochemical synthesis of metal organic frameworks, J. Electrochem. Soc. 167 (2020) 155527.

[32] F. Liu, C. Wang, X. Sui, M.A. Riaz, M. Xu, L. Wei, Y. Chen, Synthesis of graphene materials by electrochemical exfoliation: recent progress and future potential, Carbon Energy 1 (2019) 173–199.

[33] L. Kavan, Electrochemical carbon, Chem. Rev. 97 (1997) 3061–3082.

[34] J. Urzúa, P.S. Poon, J. Matos, Biomass-derived graphene and nanostructured carbons: a review for electrochemical applications, J. Non-Cryst. Solids 626 (2024) 122779.

[35] M. Zhu, G. Diao, Review on the progress in synthesis and application of magnetic carbon nanocomposites, Nanoscale 3 (2011) 2748–2767.

[36] G.K. Yogesh, S. Shukla, D. Sastikumar, P. Koinkar, Progress in pulsed laser ablation in liquid (PLAL) technique for the synthesis of carbon nanomaterials: a review, Appl. Phys. Mater. Sci. Process 127 (2021) 810.

[37] P. Joshi, P. Riley, S. Gupta, R.J. Narayan, J. Narayan, Advances in laser-assisted conversion of polymeric and graphitic carbon into nanodiamond films, Nanotechnology 32 (2021) 432001.

[38] J. Lee, J. Kim, T. Hyeoun, Recent progress in the synthesis of porous carbon materials, Adv. Mater. 18 (2006) 2073–2094.

[39] J. Sun, Z. Wu, C. Ma, M. Xu, S. Luo, W. Li, S. Liu, Biomass-derived tubular carbon materials: progress in synthesis and applications, J. Mater. Chem. A 9 (2021) 13822–13850.

[40] B. Szczesniak, J. Phuriragpitikhon, J. Choma, M. Jaroniec, Recent advances in the development and applications of biomass-derived carbons with uniform porosity, J. Mater. Chem. A 8 (2020) 18464–18491.

[41] M. Wu, J. Liao, L. Yu, R. Lv, P. Li, W. Sun, R. Tan, X. Duan, L. Zhang, F. Li, J. Kim, K.H. Shin, H.S. Park, W. Zhang, Z. Guo, H. Wang, Y. Tang, G. Gorgolis, C. Galiotis, J. Ma, Roadmap on carbon materials for energy storage and conversion, Chem. Asian J. 15 (2020) 995–1013, 2020.

[42] Y. Huang, Z. Tang, S. Zhou, H. Wang, Y. Tang, D. Sun, H. Wang, Renewable waste biomass-derived carbon materials for energy storage, J. Phys. D Appl. Phys. 55 (2022) 313002.

[43] J. Xiao, J. Hand, C. Zhang, G. Ling, F. Kang, Q.H. Yang, Dimensionality, function and performance of carbon materials in energy storage devices, Adv. Energy Mater. 12 (2022) 2100775.

[44] J. Quílez-Bermejo, E. Morallón, D. Cazorla-Amorós, Metal-free heteroatom-doped carbon-based catalysts for ORR. A critical assessment about the role of heteroatoms, Carbon N. Y. 165 (2020) 434–454, https://doi.org/10.1016/j. carbon.2020.04.068.

[45] A.F. Quintero-Jaime, J. Quílez-Bermejo, D. Cazorla-Amorós, E. Morallón, Metal free electrochemical glucose biosensor based on N-doped porous carbon material, Electrochim. Acta 367 (2021) 137434.

[46] J. Quílez-Bermejo, S. Pérez-Rodríguez, R.L.S. Ganevesi, D. Torres, E. Morallón, D. Cazorla-Amorós, A. Celzard, V. Fierro, Easy enrichment of graphitic nitrogen to prepare highly catalytic carbons for oxygen reduction reaction, Carbon N. Y. 196 (2022) 708–717.

[47] J. Quílez-Bermejo, K. Strutyński, M. Melle-Franco, E. Morallón, D. Cazorla-Amorós, On the origin of the effect of pH in ORR for non-doped and edge-type quaternary N-doped metal-free carbon-based catalysts, ACS Appl. Mater. Interfaces 12 (2020) 54815–54823.

[48] J. Quílez-Bermejo, E. Morallón, D. Cazorla-Amorós, On the deactivation of N-doped carbon materials active sites during oxygen reduction reaction, Carbon N. Y. 189 (2022) 548–560.

[49] M. Toyoda, S. Hou, Z.H. Huang, M. Inagaki, Exfoliated graphite: room temperature exfoliation and their applications, Carbon Lett 33 (2023) 335–362.

[50] P. Yu, S.E. Lowe, G.P. Simon, Y.L. Zhong, Electrochemical exfoliation of graphite and production of functional graphene, Curr. Opin. Colloid Interface Sci. 20 (2015) 329–338.

[51] V. Georgakilas, M. Otyepka, A.B. Bourlinos, V. Chandra, N. Kim, K.C. Kemp, P. Hobza, R. Zboril, K.S. Kim, Functionalization of graphene: covalent and non-covalent approaches, derivatives and applications, Chem. Rev. 112 (2012) 6156–6214.

[52] H. Chen, C. Li, L. Qu, Solution electrochemical approach to functionalized graphene: history, progress and challenges, Carbon N. Y. 140 (2018) 41–56.

[53] D. Bélanger, J. Pinson, Electrografting: a powerful method for surface modification, Chem. Soc. Rev. 40 (2011) 3995–4048.
[54] C. Gonzalez-Gaitán, R. Ruiz-Rosas, E. Morallón, D. Cazorla-Amorós, Electrochemical methods to functionalize carbon materials, in: Chem. Funct. Carbon Nanomater., 2015, pp. 231–261.

[55] A. Quintero-Jaime, D. Cazorla-Amorós, E. Morallón, Electrochemical functionalization of carbon nanomaterials and their application in immobilization of enzymes, Nanomater. Biocatal. (2021) 67–103.

[56] B. Martinez-Sanchez, J. Quílez-Bermejo, E. San-Fabián, D. Cazorla-Amorós, E. Morallón, On the mechanism of electrochemical functionalization of carbon nanotubes with different structures with aminophenylphosphonic acid isomers: an experimental and computational approach, J. Mater. Chem. A 10 (2022) 7271–7290.

[57] A. Quintero-Jaime, D. Cazorla-Amorós, E. Morallón, Effect of surface oxygen groups in the electrochemical modification of multi-walled carbon nanotubes by 4-amino phenyl phosphonic acid, Carbon N. Y. 165 (2020) 328–339.

[58] J. Quílez-Bermejo, E. Morallón, D. Cazorla-Amorós, Polyaniline-derived N-doped ordered mesoporous carbon thin films: efficient catalysts towards oxygen reduction reaction, Polymers 12 (2020) 2382.

[59] B. Cardenas-Benitez, I. Djordjevic, S. Hosseini, M.J. Madou, S.O. Martinez-Chapa, Review—covalent functionalization of carbon nanomaterials for biosensor applications: an update, J. Electrochem. Soc. 165 (2018) 103–117.

[60] J.I. Paredes, J.M. Munuera, Recent advances and energy-related applications of high quality/chemically doped graphene obtained by electrochemical exfoliation methods, J. Mater. Chem. A 5 (2017) 7228–7242.

[61] Y. Fang, X. Li, J. Li, C. Yao, H.Y. Hoh, X. Hai, J. Lu, C. Su, Janus electrochemical exfoliation of twodimensional materials, J. Mater. Chem. A 7 (2019) 25691.

[62] M. Cossutta, J. McKechnie, S.J. Pickering, A comparative LCA of different graphene production routes, Green Chem. 19 (2017) 5874–5884.

[63] J.W. Lee, M. Kim, W. Na, S.M. Hong, C.M. Koo, Fabrication of high quality graphene nanosheets via a spontaneous electrochemical reaction process, Carbon N. Y. 91 (2015) 527–534.

[64] Y. Nishina, Mass production of graphene oxide beyond the laboratory: bridging the gap between academic research and industry, ACS Nano 18 (2024) 33264–33275.

[65] C.D. Jaimes-Paez, E. Morallón, D. Cazorla-Amorós, Few layers graphene-based electrocatalysts for ORR synthesized by electrochemical exfoliation methods, Energy 278 (2023) 127888.

[66] J. Wu, H. Wang, J. Qiu, J. Shao, K. Zhang, L. Yan, Electrochemical exfoliation for few-layer graphene in molybdate aqueous solution and its application for fast electrothermal film, Prog. Nat. Sci. Mater. Int. 30 (2020) 312–320.

[67] D.B. Shinde, V.K. Pillai, Electrochemical preparation of luminescent graphene quantum dots from multiwalled carbon nanotubes, Chem. Eur. J. 18 (2012) 12522–12528.

[68] C.T.J. Low, F.C. Walsh, M.H. Chakrabarti, M.A. Hashim, M.A. Hussain, Electrochemical approaches to the production of graphene flakes and their potential applications, Carbon N. Y. 54 (2013) 1–21.

[69] J. Shen, Y. Zhu, X. Yang, C. Li, Graphene quantum dots: emergent nanolights for bioimaging, sensors, catalysis and photovoltaic devices, Chem. Commun. 48 (2012) 3686–3699.

[70] J. Lu, J.X. Yang, J. Wang, A. Lim, S. Wang, K.P. Loh, One-pot synthesis of fluorescent carbon nanoribbons, nanoparticles, and graphene by the exfoliation of graphite in ionic liquids, ACS Nano 3 (2009) 2367–2375.

[71] D. Kang, T.W. Kim, S.R. Kubota, A.C. Cardiel, H.G. Cha, K.S. Choi, Electrochemical synthesis of photoelectrodes and catalysts for use in solar water splitting, Chem. Rev. 115 (2015) 12839–12887.

[72] S. Garcia-Dalí, J.I. Paredes, J.M. Munuera, S. Villar-Rodil, A. Adawy, A. Martinez-Alonso, J.M.D. Tascón, Aqueous cathodic exfoliation strategy toward solution-processed and phase-preserved MoS2 nanosheets for energy storage and catalytic applications, ACS Appl. Mater. Interfaces 11 (2019) 36991–37003.

[73] J.J. Zhu, X.H. Liao, X.N. Zhao, H.Y. Chen, Preparation of silver nanorods by electrochemical methods, Mater. Lett. 49 (2001) 91–95.

[74] L. Tian, F. Chen, H. Ding, X. Li, X. Li, The influence of inorganic electrolyte on the properties of carbon quantum dots in electrochemical exfoliation, J. Electroanal. Chem. 878 (2020) 114673.

[75] K. Chu, J.R. Adsetts, S. He, Z. Zhang, L. Yang, J.M. Wong, D.A. Love, Z. Ding, Electrogenerated chemiluminescence and electroluminescence of N-doped graphene quantum dots fabricated from an electrochemical exfoliation process in nitrogen-containing electrolytes, Chem. Eur. J. 26 (2020) 15892–15900.

[76] J. Joseph, A.A. Anappara, White-light-emitting carbon dots prepared by the electrochemical exfoliation of graphite, ChemPhysChem 18 (2017) 292–298.

[77] Y. Yan, H. Li, Q. Wang, H. Mao, W. Kun, Controllable ionic liquid-assisted electrochemical exfoliation of carbon fibers for the green and large-scale preparation of functionalized graphene quantum dots endowed with multicolor emission and size tunability, Journla Mater. Chem. C 5 (2017) 6092–6100.

[78] A.M. Abdelkader, A.J. Cooper, R.A.W. Dryfe, I.A. Kinloch, How to get between the sheets: a review of recent works on the electrochemical exfoliation of graphene materials from bulk graphite, Nanoscale 7 (2015) 6944.

[79] K. Parvez, Z.S. Wu, R. Li, X. Liu, R. Graf, X. Feng, K. Müllen, Exfoliation of graphite into graphene in aqueous solutions of inorganic salts, J. Am. Chem. Soc. 136 (2014) 6083–6091.

[80] J.M. Munuera, J.I. Paredes, S. Villar-Rodil, M. Ayán-Varela, A. Pagán, S.D. Aznar-Cervantes, J.L. Cenis, A. Martínez-Alonso, J.M.D. Tascón, High quality, low oxygen content and biocompatible graphene nanosheets obtained by anodic exfoliation of different graphite types, Carbon N. Y. 94 (2015) 729–739.

[81] R. Kou, Y. Shao, D. Mei, Z. Nie, D. Wang, C. Wang, V.V. Viswanathan, S. Park, I. A. Aksay, Y. Lin, Y. Wang, J. Liu, Stabilization of electrocatalytic metal nanoparticles at metal-metal oxide-graphene triple junction points, J. Am. Chem. Soc. 133 (2011) 2541–2547.

[82] S. Guo, X. Pan, L. Yu, X. Bao, Dispersion of metal nanoparticles on carbon nanotubes with few surface oxygen functional groups, Mater. Lett. 65 (2011) 1522–1524.

[83] J. Quintero-Ruiz, R. Ruiz-Rosas, J. Quílez-Bermejo, D. Salinas-Torres, D. Cazorla-Amorós, E. Morallón, Preparation of Pt/CNT thin-film electrodes by electrochemical potential pulse deposition for methanol oxidation, C - j, Carbon Res 7 (2021) 32.

[84] M.H. Dalal, C.Y. Lee, G.G. Wallace, Cathodic exfoliation of graphite into graphene nanoplatelets in aqueous solution of alkali metal salts, J. Mater. Sci. 56 (2020) 3612–3622.

[85] K. Parvez, Z.S. Wu, R. Li, R. Graf, X. Feng, K. Müllen, Exfoliation of graphite into graphene in aqueous solutions of inorganic salts, J. Am. Chem. Soc. 136 (2014) 6083–6091.

[86] J. Liu, C.K. Poh, D. Zhan, L. Lai, S.H. Lim, L. Wang, X. Liu, N.G. Sahoo, C. Li, Z. Shen, J. Lin, Improved synthesis of graphene flakes from the multiple electrochemical exfoliation of graphite rod, Nano Energy 2 (2013) 377–386.

[87] L. Wu, W. Li, P. Li, S. Liao, S. Qiu, M. Chen, Y. Guo, Q. Li, C. Zhu, L. Liu, Powder, paper and foam of few-layergraphene prepared in high yield by electrochemical intercalation exfoliation of expanded graphite, Small 10 (2014) 1421–1429.

[88] L. Zu, X. Gao, H. Lian, X. Cai, C. Li, Y. Zhong, Y. Hao, Y. Zhang, Z. Gong, Y. Liu, X. Wang, X. Cui, High electrochemical performance phosphorus-oxide modified graphene electrode for redox supercapacitors prepared by one-step electrochemical exfoliation, Nanomaterials 8 (2018) 417.

[89] O. Ustavytska, Y. Kuryts, V. Koshechko, V. Pokhodenko, One-step electrochemical preparation of multilayer graphene functionalized with nitrogen, Nanoscale Res. Lett. 12 (2017) 175.

[90] Y. Yang, W. Shi, R. Zhang, C. Luan, Q. Zeng, C. Wang, S. Li, Z. Huang, H. Xiao, X. Ji, Electrochemical exfoliation of graphite into nitrogen-doped graphene in Glycine solution and its energy storage properties, Electrochim. Acta 204 (2016) 100–107.

[91] N. Parveen, M.O. Ansari, S.A. Ansari, M.H. Cho, Simultaneous sulfur doping and exfoliation of graphene from graphite using an electrochemical method for supercapacitor electrode materials, J. Mater. Chem. A 4 (2016) 233–240.

[92] S. Yang, S. Brüller, Z.S. Wu, Z. Liu, K. Parvez, R. Dong, F. Richard, P. Samori, X. Feng, K. Müllen, Organic radical-assistedelectrochemical exfoliation for the scalable production ofhigh-quality graphene, J. Am. Chem. Soc. 137 (2015) 13927–13932.

[93] K.S. Rao, J. Sentilnathan, H.W. Cho, J.J. Wu, M. Yoshimura, Soft processing of graphene nanosheets by glycine-bisulfate ionic-complex-assisted electrochemical exfoliation of graphite for reduction catalysis, Adv. Funct. Mater. 25 (2015) 298–305.

[94] Y. Chen, M. Wang, J. Zhang, J. Tu, J. Ge, S. Jiao, Green and sustainable molten salt electrochemistry for the conversion of secondary carbon pollutants to advanced carbon materials, J. Mater. Chem. A 9 (2021) 14119.

[95] A. Yu, G. Ma, J. Ren, P. Peng, F.F. Li, Sustainable carbon materials and fuels: recent advances of CO2 conversion in molten salts, ChemSusChem 13 (2020) 6229–6245.

[96] Z. Pang, G. Li, X. Xiong, L. Ji, Q. Xu, X. Zou, X. Lu, Molten salt synthesis of porous carbon and its application in supercapacitors: a review, J. Energy Chem. 61 (2020) 622–640.

[97] I.L. Egun, H. He, D. Hu, G.Z. Chen, Molten salt carbonization and activation of biomass to functional biocarbon, Adv. Sustain. Syst. 6 (2022) 2200294.

[98] M.A. Hughes, J.A. Allen, S.W. Donne, Carbonate reduction and the properties and applications of carbon formed through electrochemical deposition in molten carbonates: a review, Electrochim. Acta 176 (2015) 1511–1521.

[99] K. Kang, B. Liu, G. Yue, H. Ren, K. Zheng, L. Wang, Z. Wang, Preparation of carbon quantum dots from ionic liquid modified biomass for the detection of Fe3 + and Pd2+ in environmental water, Ecotoxicol. Environ. Saf. 255 (2023) 114795.

[100] F. Zhu, J. Ge, Y. Gao, S. Li, Y. Chen, J. Tu, M. Wang, S. Jiao, Molten salt electro-preparation of graphitic carbons, Explorations 3 (2023) 20210186.

[101] Z. Fan, Z.F. Gao, X. Zhou, Z. Peng, S. Deng, Q. Lu, Y. Wang, W. Yang, X. Chen, Electrochemical CO2 fixation in molten salts: a pathway towards the fabrication of exceptional carbon-based materials for pollutant removal, J. Mater. Sci. Technol. 213 (2025) 174–189.

[102] L. Hu, Y. Song, J. Ge, J. Zhu, S. Jiao, Capture and electrochemical conversion of CO2 to ultrathin graphite sheets in CaCl2-based melts, J. Mater. Chem. A 3 (2015) 21211–21218.

[103] L. Hu, Y. Song, S. Jiao, Y. Liu, J. Ge, H. Jiao, J. Zhu, J. Wang, H. Zhu, D.J. Frey, Direct conversion of greenhouse gas CO2 into graphene via molten salts electrolysis, ChemSusChem 9 (2016) 588–594.

[104] M.D. Ingram, B. Baron, G.J. Janz, The electrolytic deposition of carbon from fused carbonates, Electrochim. Acta 11 (1966) 1629–1639.

[105] H.V. Ijije, C. Sun, G.Z. Chen, Indirect electrochemical reduction of carbon dioxide to carbon nanopowders in molten alkali carbonates: process variables and product properties, Carbon N. Y. 73 (2014) 163–174.

[106] R.C. Lawrence, Carbon from Carbon Dioxide via Molten Carbonate Electrolysis: Fundamental Investigations, 2013.

[107] Z. Li, D. Yuan, H. Wu, W. Li, D. Gu, A novel route to synthesize carbon spheres and carbon nanotubes from carbon dioxide in a molten carbonate electrolyzer, Inorg. Chem. Front. 5 (2018) 208–216.

[108] X. Jin, R. He, S. Dai, Electrochemical graphitization: an efficient conversion of amorphous carbons to nanostructured graphites, Chem. Eur. J. 23 (2017) 11455–11459.
[109] W.L. Song, S. Li, G. Zhang, J. Tu, H.S. Chen, S. Jiao, Cellulose-derived flake graphite as positive electrodes for Al-ion batteries, Sustain. Energy Fuels 3 (2019) 3561.

[110] Z. Fan, W. Xiao, Electrochemical splitting of methane in molten salts to produce hydrogen, Angew. Chem. Int. Ed. 600 (2021) 7664–7668.

[111] S. Trevisan, B. Buchbjerg, R. Guedez, Power-to-heat for the industrial sector: techno-economic assessment of a molten salt-based solution, Energy Convers. Manag. 272 (2022) 116362.

[112] C. Backes, A.M. Abdelkader, C. Alonso, A. Andrieux-Ledier, R. Arenal, J. Azpeitia, N. Balakrishnan, E. Al, Production and processing of graphene and related materials, 2D Mater. 7 (2020) 022001.

[113] P. Kumar, S. Singh, B.K. Gupta, Future prospects of luminescent nanomaterial based security inks: from synthesis to anti-counterfeiting applications, Nanoscale 8 (2016) 14297–14340.

[114] P. Zuo, X. Lu, Z. Sun, Y. Guo, H. He, A review on syntheses, properties, characterization and bioanalytical applications of fluorescent carbon dots, Microchim. Acta 183 (2016) 519–542.

[115] A. Khayal, V. Dawane, M.A. Amin, V. Tirth, V.K. Yadav, A. Algahtani, S.H. Khan, S. Islam, K.K. Yadav, B.H. Jeon, Advances in the methods for the synthesis of carbon dots and their emerging applications, Polymers 13 (2021) 3190.

[116] M. Gattrell, N. Gupta, A. Co, A review of the aqueous electrochemical reduction of CO2 to hydrocarbons at copper, J. Electroanal. Chem. 594 (2005) 1–19.

[117] M. Jun, J. Kindu, D.H. Kim, M. Kim, D. Kim, K. Lee, S.I. Choi, Strategies to modulate the copper oxidation state toward selective C2+ production in the electrochemical CO2 reduction reaction, Adv. Mater. 36 (2024) 2313028.

[118] H. Nishihara, H. Harada, M. Tateishi, K. Ohashi, K. Aramaki, New approach to graphite synthesis: electrochemical carbonization of hexachlorobuta-1,3-diene, J. Chem. Soc. Faraday. Trans. 87 (1991) 1187–1192.

[119] H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, S. Lian, C.H.A. Tsang, X. Yang, S. T. Lee, Water-Soluble fluorescent carbon quantum dots and photocatalyst design, Angew. Chem. Int. Ed. 49 (2010) 4430–4434.

[120] H. Li, X. He, Y. Liu, H. Huang, S. Lian, S.T. Lee, Z. Kang, One-step ultrasonic synthesis of water-soluble carbon nanoparticles with excellent photoluminescent properties, Carbon N. Y. 49 (2011) 605–609.

[121] H. Li, H. Ming, Y. Liu, H. Yu, X. He, H. Huang, K. Pan, Z. Kang, S.T. Lee, Fluorescent carbon nanoparticles: electrochemical synthesis and their pH sensitive photoluminescence properties, New J. Chem. 35 (2011) 2666–2670.

[122] J. Deng, Q. Lu, N. Mi, H. Li, M. Liu, M. Xu, L. Tan, W. Xie, Y. Zhang, S. Yao, Electrochemical synthesis of carbon nanodots directly from alcohols, Chem. Eur J. 20 (2014) 4993–4999.

[123] P. Miao, Y. Tang, K. Han, B. Wang, Facile synthesis of carbon nanodots from ethanol and their application in ferric(III) ion assay, J. Mater. Chem. A 3 (2015) 15068–15073.

[124] C.I. Wang, W.C. Wu, A.P. Periasamy, H.T. Chang, Electrochemical synthesis of photoluminescent carbon nanodots from glycine for highly sensitive detection of hemoglobin, Green Chem. 16 (2014) 2509–2514.

[125] T.C. Canevari, M. Nakamura, F.H. Cincotto, F.M. de Melo, H.E. Toma, High performance electrochemical sensors for dopamine and epinephrine using nanocrystalline carbon quantum dots obtained under controlled chronoamperometric conditions, Electrochim. Acta 209 (2016) 464–470.

[126] F. Niu, Y. Xu, J. Liu, Z. Song, M. Liu, J. Liu, Controllable electrochemical/ electroanalytical approach to generate nitrogen-doped carbon quantum dots from varied amino acids: pinpointing the utmost quantum yield and the versatile photoluminescent and electrochemiluminescent applications, Electrochim. Acta 236 (2017) 239–251.

[127] Y. Hou, Q. Lu, J. Deng, H. Li, Y. Zhang, One-pot electrochemical synthesis of functionalized fluorescent carbon dots and their selective sensing for mercury ion, Anal. Chim. Acta 866 (2015) 69–74.

[128] F. Niu, Y. Xu, M. Liu, J. Sun, P. Guo, J. Liu, Bottom-up electrochemical preparation of solid-state carbon nanodots directly from nitriles/ionic liquids using carbon-free electrodes and the applications in specific ferric ion detection and cell imaging, Nanoscale 8 (2016) 5470.

[129] J. Castro-Gutierrez, A. Celzard, V. Fierro, Energy storage in supercapacitors: focus on tannin-derived carbon electrodes, Front. Mater. 7 (2020) 217.

[130] N. Sohal, B. Maity, S. Basu, Recent advances in heteroatom-doped graphene quantum dots for sensing applications, RSC Adv. 11 (2021) 25586–25615.

[131] S. Kaushal, M. Kaur, N. Kaur, V. Kumari, P.P. Singh, Heteroatom-doped graphene as sensing materials: a mini review, RSC Adv. 10 (2020) 28608–28629.

[132] J. Quflez-Bermejo, A. Daouli, S.G. Dalí, Y. Cui, J.C. Andrea Zitolo, M. Emo, M. T. Izquierdo, W. Mustain, M. Badawi, A. Celzard, V. Fierro, Electron transfer from encapsulated Fe3C to the outermost N-doped carbon layer for superior ORR, Adv. Funct. Mater. (2024) 2403810.

[133] J. Quflez-Bermejo, S. García-Dalí, A. Daouli, A. Zitolo, R. Canevesi, M. Emo, M. T. Izquierdo, M. Badawi, A. Celzard, V. Fierro, Advanced design of metal nanoclusters and single atoms embedded in C1N1-derived carbon materials for ORR, HER, and OER, Adv. Funct. Mater. (2023) 2300405.

[134] H. Im, S. Noh, J.H. Shim, Spontaneous formation of core-shell silver-copper oxide by carbon dot-mediated reduction for enhanced oxygen electrocatalysis, Electrochim. Acta 329 (2020) 135172.

[135] N. Hongrutai, R. Nganglumpoon, S. Watmanee, P. Pinthong, P.Á. Szilágyi, M. M. Titirici, J. Panpranot, In-situ electrodeposited Ag/Cu for electrochemical reduction of acetic acid to nanodiamond under ambient conditions, Mater. Today Chem. 30 (2023) 101509.