Electrochemistry of Carbon Materials: Progress in Raman Spectroscopy, Optical Absorption Spectroscopy, and Applications

This paper is dedicated to the discussion of applications of carbon material in electrochemistry. The paper starts with a general discussion on electrochemical doping. Then, investigations by spectroelectrochemistry are discussed. The Raman spectroscopy experiments in different electrolyte solutions are considered. This includes aqueous solutions and acetonitrile and ionic fluids. The investigation of carbon nanotubes on different substrates is considered. The optical absorption experiments in different electrolyte solutions and substrate materials are discussed. The chemical functionalization of carbon nanotubes is considered. Finally, the application of carbon materials and chemically functionalized carbon nanotubes in batteries, supercapacitors, sensors, and nanoelectronic devices is presented.


Introduction
Carbon nanotubes include single-walled carbon nanotubes (SWCNTs) with unique properties [1,2] and multi-walled carbon nanotubes (MWCNTs). SWCNTs are sorted and separated to obtain uniform electronic properties [3], and the channels of SWCNTs are filled [4,5] with substances [6,7] for the investigation of kinetics and electronic properties [8][9][10][11][12][13][14][15] for applications ( Figure 1). The SWCNTs are filled with inorganic compounds , molecules , and elementary substances . Inorganic substances are introduced inside carbon nanotubes by the gas phase and liquid phase methods. Molecules are filled inside carbon nanotubes by the gas phase and solution methods. Elementary substances are incorporated inside carbon nanotubes by the gas, solution, and melt methods. The electronic properties of chemically modified carbon material are investigated by Raman spectroscopy, near-edge X-ray absorption fine structure spectroscopy (NEXAFS), optical absorption spectroscopy (OAS), and photoemission spectroscopy.
A high current carrying capacity, long cycling stability, excellent electrical conductance, and good capability in rapid charge and discharge make SWCNTs electrodes possible, leading to a high performance [109].
The electrochemical doping allows for the alteration of the doping level [110][111][112]. The shift in the Fermi level is proportional to the applied voltage [112,113]. The experimental parameters are varied to achieve perfect conditions [112].
In 1999, the first example of electrochemical doping appeared [114]. The methods that investigated the modified electronic properties are voltamperometry [115][116][117][118][119] and spectroelectrochemistry [110,113,. Voltamperometry allows for evaluating the reversibility of the p-and n-doping of SWCNTs by applied voltage using the cyclic voltamerograms [112]. Combining the spectroscopic investigations, such as Raman spectroscopy and A high current carrying capacity, long cycling stability, excellent electrical conductance, and good capability in rapid charge and discharge make SWCNTs electrodes possible, leading to a high performance [109].
The electrochemical doping allows for the alteration of the doping level [110][111][112]. The shift in the Fermi level is proportional to the applied voltage [112,113]. The experimental parameters are varied to achieve perfect conditions [112].
In 1999, the first example of electrochemical doping appeared [114]. The methods that investigated the modified electronic properties are voltamperometry [115][116][117][118][119] and spectroelectrochemistry [110,113,. Voltamperometry allows for evaluating the reversibility of the p-and n-doping of SWCNTs by applied voltage using the cyclic voltamerograms [112]. Combining the spectroscopic investigations, such as Raman spectroscopy and optical absorption spectroscopy, with electrochemical charging allows for the evaluation of the modifications of the electronic properties of SWCNTs [143].
The aim of this review is to summarize the reports on the investigations of the electrochemical properties of carbon material. In Section 2, the results of the voltamperometry are discussed. In Section 3, the results of the spectroelectrochemistry with Raman spectroscopy are considered, i.e., measuring the spectra with electrochemical charging. In Section 4, the results of the spectroelectrochemistry with optical absorption spectroscopy are presented. In Section 5, investigations of chemically functionalized carbon nanotubes are highlighted. In Section 6, applications of carbon material in electrochemical devices are discussed.

Voltamperometry
For voltamperometry, it is important that the MWCNTs do not destroy under electrochemical charging. MWCNTs are stable up to a ~±2 eV applied potential. This allows for one to use them for voltamperometry measurements. Measurements are conducted at different scan rates. The data present the dependence of the voltage on the capacity. The insertion of the electrolyte happens. For MWCNTs, a capacity of up to ~1000 mAh/g was observed. The aim of this review is to summarize the reports on the investigations of the electrochemical properties of carbon material. In Section 2, the results of the voltamperometry are discussed. In Section 3, the results of the spectroelectrochemistry with Raman spectroscopy are considered, i.e., measuring the spectra with electrochemical charging. In Section 4, the results of the spectroelectrochemistry with optical absorption spectroscopy are presented. In Section 5, investigations of chemically functionalized carbon nanotubes are highlighted. In Section 6, applications of carbon material in electrochemical devices are discussed.

Voltamperometry
For voltamperometry, it is important that the MWCNTs do not destroy under electrochemical charging. MWCNTs are stable up to a~±2 eV applied potential. This allows for one to use them for voltamperometry measurements. Measurements are conducted at different scan rates. The data present the dependence of the voltage on the capacity. The insertion of the electrolyte happens. For MWCNTs, a capacity of up to~1000 mAh/g was observed.
In Ref. [116], the electrochemical experiments were performed with lithium insertion into nanotubes. High values of irreversible capacity C irr (from 460 to 1080 mAh/g) were obtained. The authors plotted the dependence of C irr on the mesopore volume, which showed the linear behavior [116]. Figure 2 presents the voltamperometric data of lithium insertion and extraction in MWCNTs to test electrodes from carbon tubular form. They show the typical behavior of tubular carbon [116]. The irreversible capacity values of all forms of tubular carbon C irr are extremely high. It is believed that one factor that makes the value of C irr is the solid electrolyte interphase (SEI) formation. In Ref. [144], the voltamperometry data of MWCNT at different scan rates are presented ( Figure 4). Until 1.5 V (open-circuit voltage (OCV)) and about 1 V, there are the reductions of chemicals, and from about 0.5 V until 0 V, there is the lithium insertion.  In Ref. [144], the voltamperometry data of MWCNT at different scan rates are presented ( Figure 4). Until 1.5 V (open-circuit voltage (OCV)) and about 1 V, there are the reductions of chemicals, and from about 0.5 V until 0 V, there is the lithium insertion. Above 1.5 V, the region of charging of the double layer was observed, which testified to the good electrochemical properties [114]. In Ref. [144], the voltamperometry data of MWCNT at different scan rates are presented ( Figure 4). Until 1.5 V (open-circuit voltage (OCV)) and about 1 V, there are the reductions of chemicals, and from about 0.5 V until 0 V, there is the lithium insertion. Above 1.5 V, the region of charging of the double layer was observed, which testified to the good electrochemical properties [114].

Spectroelectrochemistry with Raman Spectroscopy
The electrochemical experiments are combined with Raman spectroscopy and optical absorption spectroscopy. Yet, other methods are possible.
To present the method, in the spectroelectrochemical technique, SWCNTs serve as a working electrode. The spectrum is measured under an applied potential. The spectra obtained at different applied voltages are plotted for comparison and tracing changes.
In Ref. [118], the authors studied the Raman spectra of SWCNTs upon electrochemical charging at applied potentials from −0.2 to −0.8 eV and from +0.2 to +1.4 eV. Figure 5 shows the RBM and tangential displacement modes (TDM) of the resonance Raman spectra of a buckypaper in a 1 M NaCl aqueous solution acquired at a 514 nm laser [118].
In Ref. [118], the authors studied the Raman spectra of SWCNTs upon electrochemical charging at applied potentials from −0.2 to −0.8 eV and from +0.2 to +1.4 eV. Figure 5 shows the RBM and tangential displacement modes (TDM) of the resonance Raman spectra of a buckypaper in a 1 M NaCl aqueous solution acquired at a 514 nm laser [118]. The authors of Ref. [123] investigated the RBM-band of Raman spectra. In the RBM band, there is the peak at 180 cm −1 . In the G-band, there is the peak of the tangential modes (TM) at 1590 cm −1 . Figure 6 shows experimental Raman spectra (solid lines) of the RBM at zero bias ( Figure 6a   In Ref. [127], the potential dependent Raman spectra (excited at 2.18 eV) of HiPco SWCNT were obtained. There are intermediate frequency modes (IFM) in the spectrum. Figure 7 shows the dependence of IFM modes on electrochemical charging from −1.8 V to 0 V and from 0 V to 1.2 V. The dispersive IFM modes are shifted stronger than the nondispersive IFM modes upon p-doping, whereas n-doping has the opposite effect. Thus, one laser wavelength is enough to recognize dispersive and non-dispersive features [127]. In Ref. [127], the potential dependent Raman spectra (excited at 2.18 eV) of HiPco SWCNT were obtained. There are intermediate frequency modes (IFM) in the spectrum. Figure 7 shows the dependence of IFM modes on electrochemical charging from −1.8 V to 0 V and from 0 V to 1.2 V. The dispersive IFM modes are shifted stronger than the Nanomaterials 2023, 13, 640 6 of 34 non-dispersive IFM modes upon p-doping, whereas n-doping has the opposite effect. Thus, one laser wavelength is enough to recognize dispersive and non-dispersive features [127].
at zero bias (open circles) and at −0.08 V (solid line). Reprinted from Ghosh S, Sood A K, Rao C N R J. Appl. Phys. 92 1165 (2002), with the permission of AIP Publishing [123].
In Ref. [127], the potential dependent Raman spectra (excited at 2.18 eV) of HiPco SWCNT were obtained. There are intermediate frequency modes (IFM) in the spectrum. Figure 7 shows the dependence of IFM modes on electrochemical charging from −1.8 V to 0 V and from 0 V to 1.2 V. The dispersive IFM modes are shifted stronger than the nondispersive IFM modes upon p-doping, whereas n-doping has the opposite effect. Thus, one laser wavelength is enough to recognize dispersive and non-dispersive features [127]. Thus, the spectroelectrochemistry technique is a modern, useful, and powerful method based on the Raman spectroscopy method. The substances for electrodes should be stable in electrolyte solutions, and they should not destroy under applied voltages. Raman maps are plotted using the obtained data; they are the dependence of the Raman peak position on an applied voltage. In Raman maps of carbon nanotubes, the modifications of the electronic properties are observed.
In Ref. [118], electrochemical experiments were conducted using the ITO substrate and aqueous 0.1 M KCl saturated with nitrogen as the electrolyte solution. Figure 8 shows the OAS spectra of SWCNTs measured with applied potentials from 0 V to 0.8 V [118]. The spectra contain absorption bands at 1800 nm (0.68 eV), 1000 nm (1.3 eV), and 700 nm (1.9 eV). Upon electrochemical charging, there is the suppression of the absorption bands. This was explained by a shift in the Fermi level.
In Ref. [133], the in situ measurement of SWCNT films under electrochemical charging was performed. Figure 9 shows the OAS spectra at constant electrode potentials [133]. liquids [125] were used.
In Ref. [118], electrochemical experiments were conducted using the ITO substrate and aqueous 0.1 M KCl saturated with nitrogen as the electrolyte solution. Figure 8 shows the OAS spectra of SWCNTs measured with applied potentials from 0 V to 0.8 V [118]. The spectra contain absorption bands at 1800 nm (0.68 eV), 1000 nm (1.3 eV), and 700 nm (1.9 eV). Upon electrochemical charging, there is the suppression of the absorption bands. This was explained by a shift in the Fermi level. In Ref. [133], the in situ measurement of SWCNT films under electrochemical charging was performed. Figure 9 shows the OAS spectra at constant electrode potentials [133].
In Ref. [182], the covalent functionalization of double-walled carbon nanotubes (DWC-NTs) with aryldiazonium salt was performed, and it was shown that the functionalization is reversible upon thermal treatment. The DWCNT transistors were constructed based on the functionalized DWCNTs, and the assignment of the metallicity of the inner and outer walls of DWCNTs was conducted ( Figure 10). Nanomaterials 2023, 12, x FOR PEER REVIEW 9 of 37
In Ref. [182], the covalent functionalization of double-walled carbon nanotubes (DWCNTs) with aryldiazonium salt was performed, and it was shown that the functionalization is reversible upon thermal treatment. The DWCNT transistors were constructed based on the functionalized DWCNTs, and the assignment of the metallicity of the inner and outer walls of DWCNTs was conducted ( Figure 10). In Ref. [183], the outer walls of SWCNTs were selectively oxidized by oleum and nitric acid. Figure 11 shows the transmission electron microscopy (TEM) data of oxidized nanotubes. In Figure 11A,B low-and high-magnification images of DWCNTs treated with 5 mL of a solution for 24 h are shown. In Figure 11C,D DWCNTs treated with 10 mL of a solution for 2 h are presented [183]. In Ref. [183], the outer walls of SWCNTs were selectively oxidized by oleum and nitric acid. Figure 11 shows the transmission electron microscopy (TEM) data of oxidized nanotubes. In Figure 11A,B low-and high-magnification images of DWCNTs treated with 5 mL of a solution for 24 h are shown. In Figure 11C,D DWCNTs treated with 10 mL of a solution for 2 h are presented [183].
In Ref. [183], the samples were further investigated at a fixed reaction time of 2 h with nitric acid (Figure 12A-C) and an increasing reaction time using 5 mL of HNO 3 ( Figure 12D-F). In Figure 12B, it is seen that the relative solubility of DWCNTs is increased by increasing the amount of acid. In Figure 12C, it is visible that the nanotubes become more defective, because the ratio of the peaks in the Raman spectra of I D /I G is increased. By increasing the reaction time ( Figure 12D-F), the solubility and defectiveness of nanotubes increase, too. This is caused by the appearance of more carboxylic groups on the surface of DWCNTs with oxidation. This leads to more solubility and more defects [183]. In Ref. [183], the samples were further investigated at a fixed reaction time of 2 h with nitric acid (Figure 12A-C) and an increasing reaction time using 5 mL of HNO3 ( Figure  12D-F). In Figure 12B, it is seen that the relative solubility of DWCNTs is increased by increasing the amount of acid. In Figure 12C, it is visible that the nanotubes become more defective, because the ratio of the peaks in the Raman spectra of ID/IG is increased. By increasing the reaction time ( Figure 12D-F), the solubility and defectiveness of nanotubes increase, too. This is caused by the appearance of more carboxylic groups on the surface of DWCNTs with oxidation. This leads to more solubility and more defects [183].   In Ref. [183], the samples were further investigated at a fixed reaction time of 2 h with nitric acid (Figure 12A-C) and an increasing reaction time using 5 mL of HNO3 ( Figure  12D-F). In Figure 12B, it is seen that the relative solubility of DWCNTs is increased by increasing the amount of acid. In Figure 12C, it is visible that the nanotubes become more defective, because the ratio of the peaks in the Raman spectra of ID/IG is increased. By increasing the reaction time ( Figure 12D-F), the solubility and defectiveness of nanotubes increase, too. This is caused by the appearance of more carboxylic groups on the surface of DWCNTs with oxidation. This leads to more solubility and more defects [183].  In Ref. [184], DWCNTs were fluorinated using (1) gaseous F 2 at 200 • C, (2) a mixture of BrF 3 and Br 2 at room temperature, and (3) radio frequency CF 4 plasma. Figure 13a shows the relative ratio of fluorine plotted versus the synthesis temperature. Figure 13b shows the relative ratio of oxygen plotted versus the experiment temperature. Figure 13c demonstrates the high-resolution TEM image of DWCNTs fluorinated by F 2 at 200 • C [184].
In Ref. [184], DWCNTs were fluorinated using (1) gaseous F2 at 200 °C, (2) a mixture of BrF3 and Br2 at room temperature, and (3) radio frequency CF4 plasma. Figure 13a shows the relative ratio of fluorine plotted versus the synthesis temperature. Figure 13b shows the relative ratio of oxygen plotted versus the experiment temperature. Figure 13c demonstrates the high-resolution TEM image of DWCNTs fluorinated by F2 at 200 °C [184].
(c)  In Ref. [185], the covalent modification of DWCNTs was performed to control the sidewall chemistry. Figure 14a shows the schematics of the covalent functionalization. Figure 14b demonstrates the optical absorption spectrum of DWCNTs before and after the functionalization with diazonium salts. It is visible that the covalent functionalization leads to a charge transfer in DWCNTs [185].
In Ref. [185], the covalent modification of DWCNTs was performed to control the sidewall chemistry. Figure 14a shows the schematics of the covalent functionalization. Figure 14b demonstrates the optical absorption spectrum of DWCNTs before and after the functionalization with diazonium salts. It is visible that the covalent functionalization leads to a charge transfer in DWCNTs [185].

Gas Sorption on Carbon Nanotubes
Gas sorption on carbon nanotubes is possible. It can be reversible depending on the experimental conditions [187,188]. This allows for using carbon nanotubes as gas sensors, biosensors, and sensors for liquids. The reversibility of adsorption is the main characteristic for the implementation in sensors.

Gas Sorption on Carbon Nanotubes
Gas sorption on carbon nanotubes is possible. It can be reversible depending on the experimental conditions [187,188]. This allows for using carbon nanotubes as gas sensors, biosensors, and sensors for liquids. The reversibility of adsorption is the main characteristic for the implementation in sensors.
In Ref. [210], nitrogen-doped carbon nanotubes were boron-doped. Figure 15  In Figure 16, the surface chemical compositions of B,N-CNTs calculations with XPS are shown. Figure 16a shows the C 1s XPS spectra of samples. Figure 16b shows the B1s XPS spectra of samples. Figure 16c shows the N 1s XPS spectra of the samples [210]. Nanomaterials 2023, 12, x FOR PEER REVIEW 13 of 37   In Figure 16, the surface chemical compositions of B,N-CNTs calculations with XPS are shown. Figure 16a shows the C 1s XPS spectra of samples. Figure 16b shows the B1s XPS spectra of samples. Figure 16c shows the N 1s XPS spectra of the samples [210].

Intercalation of Nanotube Bundles
The intercalation of carbon nanotube bundles means incorporating simple substances and chemical compounds to the space between nanotubes in the bundles. The intercalation with p-and n-dopants was demonstrated. The electronic properties of SWCNTs were investigated by Raman spectroscopy, optical absorption spectroscopy, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy techniques .

Filling of Carbon Nanotubes
OAS spectroscopy is used to identify the charge transfer in filled SWCNTs . Figure 17 shows the OAS spectra of pristine and cobalt bromide-filled SWCNTs [146]. The modification of spectra such as the change in the intensity of peaks, the shift of peaks, the alteration of peak profiles, and the disappearance and appearance of peaks testifies to the charge transfer in filled SWCNTs.

Intercalation of Nanotube Bundles
The intercalation of carbon nanotube bundles means incorporating simple substances and chemical compounds to the space between nanotubes in the bundles. The intercalation with p-and n-dopants was demonstrated. The electronic properties of SWCNTs were investigated by Raman spectroscopy, optical absorption spectroscopy, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy techniques .

Filling of Carbon Nanotubes
OAS spectroscopy is used to identify the charge transfer in filled SWCNTs . Figure 17 shows the OAS spectra of pristine and cobalt bromide-filled SWCNTs [146]. The modification of spectra such as the change in the intensity of peaks, the shift of peaks, the alteration of peak profiles, and the disappearance and appearance of peaks testifies to the charge transfer in filled SWCNTs.
Raman spectroscopy is used to identify the charge transfer and electronic and vibronic properties of filled SWCNTs [181][182][183][184][185]. Figure 18 shows the Raman spectra of pristine and cobalt iodide-filled SWCNTs obtained at different laser wavelengths [238]. The modification of spectra such as the change in the intensity of peaks, the shift of peaks, and the alteration of peak profiles testifies to the charge transfer in filled SWCNTs.
XPS is used to identify the direction and value of the Fermi level shift of the filled SWCNTs . Figure 19 shows the XPS spectra of pristine and gallium selenide-filled SWCNTs [162]. The C 1s XPS spectra showed the shift of the peak and the change in its width. UPS was also used as a direct method of the investigation of the Fermi level shift in filled SWCNTs . Raman spectroscopy is used to identify the charge transfer and electronic and vibronic properties of filled SWCNTs [181][182][183][184][185]. Figure 18 shows the Raman spectra of pristine and cobalt iodide-filled SWCNTs obtained at different laser wavelengths [238]. The modification of spectra such as the change in the intensity of peaks, the shift of peaks, and the alteration of peak profiles testifies to the charge transfer in filled SWCNTs. XPS is used to identify the direction and value of the Fermi level shift of the filled SWCNTs . Figure 19 shows the XPS spectra of pristine and gallium selenidefilled SWCNTs [162]. The C 1s XPS spectra showed the shift of the peak and the change in its width. UPS was also used as a direct method of the investigation of the Fermi level shift in filled SWCNTs . Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) was applied to analyze the local interactions between encapsulated substances and SWCNTs . Figure 20 shows the NEXAFS spectra of the pristine and sulfur-containing sample without (SWCNT/S-1) and with sonication (SWCNT/S-2) before and after light illumination (SWCNT/S-1i, SWCNT/S-2i) [239].
The schematics of the modification of the electronic properties of filled SWCNTs are shown in Figure 21  . Near-edge X-ray absorption fine structure spectroscopy analyze the local interactions between encapsulated substan
In Ref. [251], SWCNT field effect transistors (FET) can be used to reveal changes in the chemical potential. The gate-voltage dependence of the nanotube conductance was measured. This is because of the interaction of the molecules with the electrode and their redox chemistry. Figure 22 shows the threshold voltage shifts of SWCNT FET (red squares, left axis) and the open-circuit potential between the working and reference electrode (blue triangles, right axis) [251].
Nanomaterials 2023, 12, x FOR PEER REVIEW In Ref. [251], SWCNT field effect transistors (FET) can be used to reveal cha the chemical potential. The gate-voltage dependence of the nanotube conductan measured. This is because of the interaction of the molecules with the electrode an redox chemistry. Figure 22 shows the threshold voltage shifts of SWCNT FE squares, left axis) and the open-circuit potential between the working and referen trode (blue triangles, right axis) [251]. The authors of Ref. [252] made high perform FET-transistor form semicon SWCNTs with a diameter of 1.9 nm. Figure 23a shows an optical image of the tra They used six source electrodes. Figure 23b shows an atomic force microscopy (AF age of a tube in the FET. Figure 23c demonstrates the schematic of the electroly measurement [252]. The authors obtained very high device mobilities and transco ances. This makes semiconducting SWCNTs very useful for electronic applicatio sensing (Figure 23d) [252]. The authors of Ref. [252] made high perform FET-transistor form semiconducting SWCNTs with a diameter of 1.9 nm. Figure 23a shows an optical image of the transistor. They used six source electrodes. Figure 23b shows an atomic force microscopy (AFM) image of a tube in the FET. Figure 23c demonstrates the schematic of the electrolyte gate measurement [252]. The authors obtained very high device mobilities and transconductances. This makes semiconducting SWCNTs very useful for electronic applications and sensing (Figure 23d) [252].
In Ref. [268], the nitrogen-doped carbon (NC) and modified-with-ammonia nitrogendoped carbon (mNC) were tested as working electrodes of supercapacitors in threeelectrode cells using 1 M H 2 SO 4 and 6 M KOH electrolytes. Figure 24 shows the electrochemical performance of the samples, which show excellent characteristics [268].
In Ref. [262], the kinetics of sodium storage were studied in brominated activated in alkali porous nitrogen-doped carbon. Figure 25 shows the current-potential curves of Br-aPNC (Figure 25a) measured at scan rates of 0.1-1.0 mV s −1 and the log(i)-log(v) plots ( Figure 25b) obtained for oxidation and reduction peaks [262]. This behavior improves the storage and high-rate capability of carbon materials in sodium-ion batteries.
In Ref. [263], annealed nitrogen-doped carbon showed an excellent performance in both lithium-ion batteries and sodium-ion batteries. Figure 26a,c shows the current vs. potential measurements at various scanning rates. The diffusion and pseudocapacitive contributions to the electrochemical storage for different scan rates are shown in Figure 26b,d. It is visible that by increasing the scan rate, the pseudocapacitive contribution increases [263]. In Ref. [268], the nitrogen-doped carbon (NC) and modified-with-ammonia nitrogendoped carbon (mNC) were tested as working electrodes of supercapacitors in three-electrode cells using 1 M H2SO4 and 6 M KOH electrolytes. Figure 24 shows the electrochemical performance of the samples, which show excellent characteristics [268]. In Ref. [275], the electrochemical properties of nitrogen-doped carbon material were investigated. Figure 27a shows the specific capacitance plotted vs. scan rate for pristine nitrogen-doped carbon (N-C) and that after hydrothermal treatment in water (N-Cw) and ammonia solution (N-Ca). Figure 27b,c show the current vs. voltage measurements at scan rates of 5 and 10 mV s −1 [275]. In Ref. [262], the kinetics of sodium storage were studied in brominated activated in alkali porous nitrogen-doped carbon. Figure 25 shows the current-potential curves of Br-aPNC (Figure 25a) measured at scan rates of 0.1-1.0 mV s −1 and the log(i)-log(v) plots ( Figure 25b) obtained for oxidation and reduction peaks [262]. This behavior improves the storage and high-rate capability of carbon materials in sodium-ion batteries.  In Ref. [262], the kinetics of sodium storage were studied in brominated activated in alkali porous nitrogen-doped carbon. Figure 25 shows the current-potential curves of Br-aPNC ( Figure 25a) measured at scan rates of 0.1-1.0 mV s −1 and the log(i)-log(v) plots ( Figure 25b) obtained for oxidation and reduction peaks [262]. This behavior improves the storage and high-rate capability of carbon materials in sodium-ion batteries. In Ref. [263], annealed nitrogen-doped carbon showed an excellent performance in both lithium-ion batteries and sodium-ion batteries. Figure 26a,c shows the current vs. potential measurements at various scanning rates. The diffusion and pseudocapacitive contributions to the electrochemical storage for different scan rates are shown in Figure  26b,d. It is visible that by increasing the scan rate, the pseudocapacitive contribution increases [263]. In Ref. [275], the electrochemical properties of nitrogen-doped carbon material were investigated. Figure 27a shows the specific capacitance plotted vs. scan rate for pristine nitrogen-doped carbon (N-C) and that after hydrothermal treatment in water (N-Cw) and ammonia solution (N-Ca). Figure 27b,c show the current vs. voltage measurements at scan rates of 5 and 10 mV s −1 [275]. In Ref. [276], the electrochemical properties of pristine, sonicated, and fluorinated SWCNTs were investigated. Figure 28 shows the current density vs. potential measurements for pristine (SW), split (SW_DC), and fluorinated F-SW, and the F-SW_DC electrodes at a scan rate of 100 mV s −1 show a nearly rectangular shape. Figure 28 also shows the capacitance retention vs. cycle number measurements [276]. In Ref. [276], the electrochemical properties of pristine, sonicated, and fluorinated SWCNTs were investigated. Figure 28 shows the current density vs. potential measurements for pristine (SW), split (SW_DC), and fluorinated F-SW, and the F-SW_DC electrodes at a scan rate of 100 mV s −1 show a nearly rectangular shape. Figure 28 also shows the capacitance retention vs. cycle number measurements [276].  [275].
In Ref. [276], the electrochemical properties of pristine, sonicated, and fluorinated SWCNTs were investigated. Figure 28 shows the current density vs. potential measurements for pristine (SW), split (SW_DC), and fluorinated F-SW, and the F-SW_DC electrodes at a scan rate of 100 mV s −1 show a nearly rectangular shape. Figure 28 also shows the capacitance retention vs. cycle number measurements [276]. In Ref. [277], the electrochemical properties of SWCNTs filled with red phosphorous were investigated. Figure 29a,d show the current vs. potential dependence of P-filled SWCNTs (P@SWCNT/P) and those treated by filtering, sonication, and drying samples (P@SWCNT). Figure 29b,e show the log(current) vs. log(scan rate). Figure 29c,f demonstrate the diffusion and capacitive contributions for different scan rates [277].
the terms and conditions of the Creative Commons Attribution (CC BY) license [276].
In Ref. [277], the electrochemical properties of SWCNTs filled with red phosphorous were investigated. Figure 29a,d show the current vs. potential dependence of P-filled SWCNTs (P@SWCNT/P) and those treated by filtering, sonication, and drying samples (P@SWCNT). Figure 29b,e show the log(current) vs. log(scan rate). Figure 29c,f demonstrate the diffusion and capacitive contributions for different scan rates [277].

Conclusions
The filled nanotubes show great promise in many areas [13,253] and are set to advance functional materials of the future. For instance, carbon nanotubes have been used with metal oxides as cheap and stable nanocatalysts in the alcohol oxidation processes for fuel cells [282]. There are many filling materials [6,7, that can create devices with an appropriate efficiency, intensity, and color. They are not patented yet because their construction requires five to ten years. I think that the filling of single-walled carbon nanotubes is a very promising achievement; there are no other ways to create devices than to start from laboratory synthesis in a flask, and the applications in car lights are the most recent. The state corporations in Russia, Europe, the USA, and different countries are interested in developments [283][284][285][286][287][288][289][290][291][292][293][294][295][296][297][298][299][300][301][302].

Conclusions
The filled nanotubes show great promise in many areas [13,253] and are set to advance functional materials of the future. For instance, carbon nanotubes have been used with metal oxides as cheap and stable nanocatalysts in the alcohol oxidation processes for fuel cells [282]. There are many filling materials [6,7, that can create devices with an appropriate efficiency, intensity, and color. They are not patented yet because their construction requires five to ten years. I think that the filling of single-walled carbon nanotubes is a very promising achievement; there are no other ways to create devices than to start from laboratory synthesis in a flask, and the applications in car lights are the most recent. The state corporations in Russia, Europe, the USA, and different countries are interested in developments [283][284][285][286][287][288][289][290][291][292][293][294][295][296][297][298][299][300][301][302].  Acknowledgments: M.V.K. acknowledges the coauthors of the reviewed paper.

Conflicts of Interest:
The author may have a conflict of interest with Andrei Eliseev (Lomonosov Moscow State University). The funders had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript; or in the decision to publish the results.