Plasma Modification of Technical Carbon with Nitrogen and Sulfur-Containing Functional Groups for Application in Catalytic Systems

Abstract

This study presents an effective plasma treatment method for doping technical carbon by nitrogen- and sulfur-containing functional groups. Nitrogen incorporation shifted the oxygen reduction reaction onset potential by 0.25 V and increased the limiting current by 1 mA cm⁻², while sulfuration showed a more pronounced effect, with a 0.31 V shift in onset potential and an increase in the limiting current to 6.23 mA cm⁻². These enhancements are attributed to the formation of additional active sites and improved surface properties. The proposed plasma-based approach is simple, scalable, and environmentally friendly, minimizing the use of hazardous reagents and eliminating the need for multistep processes. This method demonstrates the potential for industrial applications using commercially available raw materials such as technical carbon and to be extended to other carbon-based materials.

1. Introduction

The oxygen reduction reaction (ORR) is a crucial process in energy conversion and storage systems, such as hydrogen-air fuel cells and metal-air batteries [1,2,3]. However, traditional electrocatalysts for ORR, based on noble metals like platinum (Pt), have several drawbacks, including high cost, low resistance to contamination, and limited durability [4,5]. Consequently, many research groups are focused on developing and utilizing new materials with low noble metal content [6,7,8,9,10,11,12] or non-metallic materials, such as carbon-based ones, which exhibit high activity in ORR [5,13].

The doping of carbon materials by heteroatoms typically involves the modification of polymer precursors with sulfur- and nitrogen-containing [1,2,14,15] groups, followed by pyrolysis to obtain carbon materials [1,5,15,16,17,18,19], This approach is often complex, time-consuming, and costly. An alternative option is the modification of commercially available precursors using various methods, such as chemical vapor deposition. Both approaches enable the variation of sulfur content on the material’s surface as well as structural modification. However, these methods are often limited by their multistep naturethe need for strict control of conditions, and the use of potentially non-ecofriendly substances [1,2,20,21].

In this work, nitrogen- and sulfur-containing functional groups were introduced to the surface of technical carbon using the one-step plasma–electrochemical method. The primary reason for selecting technical carbon is its commercial availability and low cost. For instance, technical carbon is widely used across various sectors of the industry. The availability of commercially produced raw materials for efficient non-metallic ORR catalysts enables efficient scaling of the technology. On the other hand, plasma technology is a practical and readily available way to synthesize and modify electrocatalysts due to its unique property [22,23].

2. Materials and Methods

2.1. Materials Preparation

The modification of technical carbon-carbon black (CB), purchased by Xiamen Tob new energy technology CO (BET Surface Area: 62 m²/g, Carbon Content: >99.5%), was carried out using a microplasma discharge between an electrode and the liquid surface. The counter electrode was immersed in the liquid, where the formation of cations, anions, radicals, and electrons involved in redox processes took place [24,25].

To ensure the reproducibility and reliability of the results, each experiment was repeated at least three times under the same conditions. The carbon was nitrated in a dispersion of ammonium nitrate solution, prepared from 1 g of carbon, 2.5 g of ammonium nitrate, and 10 g of water. Graphite rods were used as both cathode and anode. The anode was positioned above the solution, while the cathode was immersed in the liquid. The process was initiated using a DC power source at a voltage of 500–600 V under vacuum conditions, with a current of 50 mA (galvanostatic mode). The treatment lasted for 5 h, during which the evaporation of the liquid was compensated by the addition of water to maintain a constant volume. The electrolyte solution temperature was a critical parameter, necessitating rigorous monitoring and control. A thermocouple, affixed to the exterior of the electrolysis cell, facilitated continuous temperature measurement. The solution temperature was maintained below 50 °C, the boiling point of the water at the operating pressure range of −0.8 to −0.9 atm. To actively regulate the temperature, a 1 L water jacket was incorporated around the cell, through which approximately 0 °C water was circulated and periodically exchanged during the plasma–liquid electrolysis process. The resulting product (CBNO₂) was washed, filtered, and dried under vacuum at 80 °C for 12 h. Each sulphonation experiment was also repeated at least three times to ensure consistency and reproducibility. During the process, ammonium nitrate acted as a nitrogen source in various oxidation states. Radicals formed in the plasma discharge, including peroxynitrites and other intermediates, were effectively incorporated into the technical carbon surface, modifying its properties [26].

Sulphonation was performed under similar conditions. A solution was prepared from 1 g of technical carbon and 200 mL of 1 M sulfuric acid. The process was carried out in galvanostatic mode with a current of 15 mA and a voltage of 500–600 V. The treatment lasted for 2 h, after which the product (CBSO₃) was washed with distilled water, filtered, and dried under vacuum at 80 °C for 24 h. Electrochemical cell cooling was not required under these experimental conditions, as the combination of a larger electrolyte volume and a reduced working current maintained the solution temperature below 50 °C, the electrolyte’s boiling point. Each sulphonation experiment was also repeated at least three times to ensure consistency and reproducibility. During the plasma discharge process, radicals such as OH⁻ and hydrated electrons with high oxidative and reductive potentials were formed [25,27]. These active species oxidized sulfite anions, leading to the formation of SO₃ radicals, which interacted with the carbon surface, forming sulfonic groups and additional functional groups.

2.2. Characterization of the Materials

Surface morphology and particle size analysis were performed using micrographs obtained from an AURIGA CrossBeam scanning electron microscope (Carl Zeiss Group, Oberkochen Germany). X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Thermo Fisher Scientific Escalab 250Xi (Waltham, MA, USA) spectrometer with AlKα radiation (photon energy 1486.6 eV). Spectra were recorded in constant pass energy mode: 50 eV for core-level spectra and 100 eV for survey spectra, using a spot size of 650 μm and a total energy resolution of 0.2 eV. A dual-mode charge compensator (a combination of low-energy electrons and argon) was used to counteract surface charge caused by photoelectron emission. The experiments were conducted at room temperature under ultrahigh vacuum conditions (~10⁻⁹ mbar).

2.3. Electrochemical Experiments

Electrochemical measurements were performed using a BioLogic VMP3 (Grenoble, France) potentiostat and an Autolab RDE device (Herizau, Switzerland). The experiments were conducted in a three-electrode cell with a saturated AgCl/KCl electrode as the reference electrode, a platinum wire as the counter electrode, and a rotating disk electrode (RDE) as the working electrode. The electrolyte (0.1 M KOH/H₂O) was purged with argon or oxygen for 10 min prior to the experiments and kept under gas flow during the measurements. In the RDE experiments, continuous oxygen purging of the electrolyte was used to maintain saturation. In the presented manuscript, potentials were converted to the reversible hydrogen electrode (RHE) scale by adding 0.978 V. The RDE method was employed to evaluate the activity of the materials in the oxygen reduction reaction (ORR). The disk rotation speed was set at 5000 rpm.

3. Results and Discussions

3.1. Plasma Treatment

To obtain modified carbon materials, a liquid-phase discharge, or microplasma discharge, between an electrode and the liquid surface, was employed under reduced pressure conditions [25,27]. The counter electrode was immersed in the liquid (Figure 1). During nitration, the plasma discharge (Figure 1), active species such as hydroxide radicals, hydrated electrons, hydrogen radicals, and more complex compounds like peroxynitrite HONOO were formed. Hydroxide radicals exhibit high oxidative potential, allowing them to interact with the surface of carbon, forming carboxyl and hydroxyl groups. Hydrated electrons and hydrogen radicals, on the other hand, have reductive properties and facilitate the incorporation of nitrogen or sulfur atoms through reduction reactions with the nanotube surface. For instance, in a medium containing nitric acid, the formed peroxynitrite HONOO contributes to the formation of nitrogen-containing groups, such as pyrrolic and graphitic functionalities, on the carbon surface [26]. Various species formed at the plasma–liquid interface are participating in redox processes not only at the interface but also within the liquid [25].

In the case of sulphonation, according to the literature, the main product during anodic plasma combustion is OH⁻ radicals, which are converted into hydrogen peroxide [28]. Consequently, the oxidation of sulfite anions by these radicals, leading to the formation of SO₃ radicals, may be more likely and efficient. Simultaneously with sulphonation, carboxylation of the surface also occurs, which may further enhance the activity of the resulting catalyst.

Sulphonation in sulfuric acid proceeds according to reactions (1–4). During plasma combustion, both within the bulk and at the water interface, species with high oxidative potential, such as OH⁻ radicals (E⁰ = +2.85 V), and species with extremely high reductive potential, such as hydrated electrons $e_{\mathrm{aq}}^{-}$ (E⁰ = −2.87 V) and hydrogen radicals H• (E⁰ = −2.30 V) [29], are generated. When these species interact with sulfate anions, oxidizing agents can form that modify the carbon material.

The authors of studies [30,31,32] suggest that hydrogen radicals participate in the formation of the sulfonating agent SO₃ by reducing H₂SO₄ according to Equation (1). Subsequently, SO₃ actively interacts with the surface of the carbon material according to Equation (2), followed by conversion into -SO₃H as described in Equation (3).

H• + H₂SO₄ ⟶ SO₃ + H₃O⁺ + e⁻

(1)

$$\mathrm{C}+{\mathrm{S}\mathrm{O}}_3\to \mathrm{C}–{\mathrm{S}\mathrm{O}}_3\bullet$$

(2)

$$\mathrm{C}–{\mathrm{S}\mathrm{O}}_3\bullet +{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}–{\mathrm{S}\mathrm{O}}_3\mathrm{H}+\mathrm{O}\mathrm{H}\bullet$$

(3)

3.2. Characterization

Scanning electron microscopy (SEM) was used to study the surface of the carbon, providing information on morphological changes on the surface before and after plasma treatment. Micrographs of the initial material CB clearly show that the particle surface has a smooth and uniform structure (Figure 2a). After plasma treatment, a significant increase in surface roughness is observed, indicating structural changes caused by the incorporation of functional groups into the CSB surface structure (Figure 2b,c). Nitrogen-containing plasma treatment likely led to the formation of functional groups, such as amine and pyrrolic groups, on the particle surface. Similarly, treatment in a sulfur-containing medium resulted in the formation of sulfur-containing groups, including sulfonic (-SO₃H) and thiol (-SH) groups. The morphological changes in the surface after plasma treatment can be attributed to the activation of the carbon matrix under the influence of highly reactive species, such as OH• and SO3• radicals, as well as the physicochemical effects of the plasma itself. The formation of new functional groups increases surface roughness and enhances the electrochemically active surface area of the material. Thus, the SEM analysis confirms that plasma treatment of carbon-based materials in nitrogen- or sulfur-containing environments is an effective method for modifying the surface structure, thereby improving the functional properties of the material.

To analyze the surface composition, the original and modified carbon materials were examined using X-ray photoelectron spectroscopy (XPS) (Figure 3). This method allowed for the determination of the elemental surface composition (

Table 1. Elemental composition of the surface of the original and modified carbon materials.

Sample	C1s, Atomic %	O1s, Atomic %	N1s, Atomic %	S2p, Atomic %
CB	77.34	22.66
CBNO2	91.10	8.48	0.23
CBSO3	81.83	12.98		0.92

) and the identification of changes associated with the incorporation of nitrogen- and sulfur-containing functional groups following plasma treatment. The surface of the original material contained only carbon and oxygen (Table 1).

After nitration via plasma treatment, nitrogen-containing groups appeared on the surface. The nitrogen in the CBNO₂ material is present in pyrrolic and graphitic configurations, corresponding to peaks at 399.4 eV and 401.2 [18]. A peak at 402.5 eV corresponding to N-O species was observed in the spectra [19] (Figure 3a). The species on the surface of the materials proved the oxidation of the surface groups during the plasma treatment. The presence of nitrogen on the surface indicates the successful incorporation of nitrogen into the material’s structure, demonstrating the effectiveness of the plasma treatment method in modifying the surface chemistry. Such changes in the XPS spectrum of the modified sample CBNO₂ confirm the successful modification of the surface with nitrogen-containing groups via plasma treatment and their integration into the carbon structure, enhancing the material’s catalytic and functional properties.

After the modification of the initial material CB, the surface composition includes carbon, oxygen, and sulfur. In the S_2p spectrum of the modified material CBSO₃, two peaks are present at 169.4 eV and 168.2 eV, which correspond to C-S and C-SO₃H bonds, respectively [33] (Figure 3b). The presence of these peaks indicates the effective incorporation of the functional group -SO₃H into the surface structure of the carbon material. Thus, the assumption regarding the possibility of effective modification of carbon material through anodic plasma processing in a sulfite solution has been confirmed.

3.3. Electrochemical Measurements

The catalytic activity of the obtained catalysts was studied in an alkaline electrolyte. (Figure 4).

The modification of carbon-based materials with nitrogen-containing groups results in a positive shift in the reaction onset potential by 0.25 V at a current density of 0.5 mA cm⁻² compared to the original material, indicating an enhancement in the catalytic activity of the material. Additionally, the limiting current increases by 1 mA cm⁻², which is attributed to an increased number of active sites on the material’s surface, facilitating more efficient reaction processes. These active sites are likely nitrogen-containing groups on the catalyst surface.

The effect of plasma surface treatment is even more pronounced in the case of sulfur-containing groups. For the CBSO₃ sample, the onset potential of the reaction at 0.5 mA cm⁻² shifts positively by 0.31 V relative to the original material. Moreover, the limiting current increases sixfold compared to the original material, reaching 6.23 mA cm⁻². This significant improvement in catalytic activity is likely due to both the increased electrochemically active surface area and the formation of additional catalytically active sites resulting from the incorporation of sulfur-containing functional groups.

The obtained data are consistent with the results of material characterization and confirm the effectiveness of the developed plasma treatment method for enhancing the activity of carbon-based materials in the oxygen reduction reaction.

4. Conclusions

This work demonstrates an effective approach to modifying carbon using plasma treatment to introduce nitrogen- and sulfur-containing functional groups. Plasma treatment successfully incorporated nitrogen-containing and sulfur-containing groups into the structure of carbon materials, as confirmed by XPS and SEM data. Scanning electron microscopy revealed increased surface roughness after treatment, which was attributed to the introduction of functional groups and an increase in the electrochemically active surface area, positively impacting the catalytic activity of the materials.

Nitrogen-containing groups were identified in pyrrolic and graphitic configurations, while sulfur-containing groups were represented by C-S and C-SO₃H functionalities. Nitrogen-containing groups shifted the oxygen reduction reaction onset potential by 0.25 V and increased the limiting current by 1 mA cm⁻². Sulphonation of carbon materials demonstrated an even more pronounced effect: the ORR onset potential shifted by 0.31 V, and the limiting current increased by approximately 5 mA cm⁻² compared to the original material.

The use of plasma modification methods eliminates complex multistep processes and minimizes the use of non-ecofriendly reagents. The developed plasma treatment method significantly enhances the catalytic activity of carbon materials, making them competitive for use in energy conversion systems such as hydrogen-air fuel cells and metal-air batteries. The simplicity of scaling and the availability of raw materials, such as technical carbon, make the proposed method promising for industrial applications.