Preparation and Electrochemical Properties of Functionalized Multi-Walled Carbon Nanotubes @ Carbon Quantum Dots @ Polyaniline Ternary Composite Electrode Materials

: Based on various carbon nano materials, the ternary composite functionalized carbon nanotubes (FMWCNTs) @ carbon quantum dots (CQDs) @ polyaniline (PANI) was prepared by in-situ polymerization and hydrothermal method. The carbon-based material was made into an electrode sheet. The morphology and microscopic nanostructures were characterized by FTIR, ﬁeld emission scanning electron microscopy and ﬁeld emission transmission electron microscopy. Cyclic voltammetry and the galvanostatic charge discharge method was adapted to study the electrochemical properties of these active materials. Our results showed that the speciﬁc capacitance of FMWCNTs @ CQDs @ PANI was as high as 534 F / g, while it was 362 F / g, 319 F / g and 279 F / g for PANI @ FMWCNTs, PANI @ CQDs and polyaniline. This means that the speciﬁc capacitance of FMWCNTs @ CQDs @ PANI is increased by 47.5%, 67.4% and 91.4% comparing with the capacitance of PANI @ FMWCNTs, PANI @ CQDs and polyaniline, respectively. Moreover, the speciﬁc capacitance retention rate of the ternary active electrode after 1000 times of constant current charge and discharge cycle reached 86%, while it was 60% for PANI @ FMWCNTs, 72% for PANI @ CQDs and 65% for polyaniline.


Introduction
The research on carbon-doped materials has received more and more attention from academia. Especially, the interaction between conductive polymers, such as polyaniline (PANI) and carbon nanomaterials, such as multi-walled carbon nanotubes (MWCNTs) and carbon quantum dots (CQDs), has been researched hotspots since its discovery [1]. Due to the unique physical and chemical properties and their being easy to synthesize and obtain, MWCNTs are broadly applied for the preparation of various composite materials [2,3]. The electrical conductivity, thermal stability and mechanical strength of the composite, which included nanotubes and conjugated polymers, has been improved since plenty of work has been devoted to the development of see-through materials [4]. Especially after the combination of MWCNTs and conductive polymer PANI and other carbon nanomaterials, it can be used in supercapacitors [5] and effective adsorbents [6].

Materials and Preparation Method
We prepared a new composite electrode material by using CQDs, PANI and functionalized CNTs in this experiment. Under the power of 560 W, 200 mL CQDs were prepared by microwave method with carbon source glycerin (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and trisodium phosphate (Fuchen Chemical Reagent Co., Ltd., Tianjin, China) and the reaction time was set at 25 min. The MWCNTs (Main Range of Diameter: 10-30 nm, Length: >2 µm, Purity: >97%, Ash: <3%, Special Surface Area: 160-200 m 2 /g) were oxidized and refluxed for 7 h in concentrated nitric acid vapor at 110 • C to get FMWCNTs. The composite material PANI @ CQDs and PANI @ FMWCNTs was prepared by in-situ polymerization under acidic and ice bath conditions for 24 h, while the FMWCNTs @ CQDs was prepared with hydrothermal method at 150 • C for 3 h. Based on the composite material FMWCNTs @ CQDs, the ternary composite material FMWCNTs @ CQDs @ PANI was obtained by the following steps. First, the composite powder was synthesized with in-situ polymerization under acidic and ice bath conditions for 24 h. Second, it was rinsed with absolute ethanol and deionized water to neutral. Third, vacuum filtration, drying and grinding was completed in turn. To obtain the electrode sheet, FMWCNTs @ CQDs @ PANI, acetylene black and polyvinylidene fluoride (PVDF) were grinded with the mass ratio at 8:1:1. Then, appropriate N-methylpyrrolidone (NMP) was added dropwise according to the mass ratio and grinded for additional 15 min. After that, the grinded materials were evenly applied to a stainless-steel mesh (304 stainless steel wire mesh), pressed and dried.

Characterization
The type characteristics and material structure of the functional groups were studied and analyzed by conventional tests (7800-350 cm −1 ) in FTIR (Nicolet iSr50, USA). The electrochemical performance of these materials was analyzed using an electrochemical workstation (CHI660e, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) to measure the CP curve, Cyclic voltammetry curve and constant current cycle curve.

FTIR Analysis
The FTIR spectra of the ternary composites FMWCNTs @ CQDs @ PANI, the composite materials PANI @ FMWCNTs and PANI @ CQDs, and the monolithic material PANI are shown in Figure 1. The absorption peaks of 3851 cm −1 , 3728 cm −1 and 3626 cm −1 in the composite material PANI @ FMWCNTs spectrum correspond to free OH, while the absorption peak 1655 cm −1 corresponds to C=O stretching vibration. The 1644 cm −1 , 1402 cm −1 and 1047 cm −1 of the composite material PANI @ CQDs spectrum correspond to C=O stretching vibration, O-H blending and C-O stretching, respectively [1,6,16,[28][29][30]. It can be seen from the figure that the absorption peak is broad and strong for the composite sample, but a very weak spectrum for pure PANI, which may be related to the interaction between PANI and CNS, involves "charge transfer" [31]. When FMWCNTs and CQDs are attached to the surface of PANI, the intensity of the band will be increased in different degrees while introducing the characteristic functional groups. For the ternary composite FMWCNTs @ CQDs @ PANI, the functional group type and peak intensity have been strengthened, and the absorption peak is also clearer. In the FTIR spectrum, the composite material's spectrum contains the characteristic peaks of PANI, indicating that the generated PANI is wrapped on FMWCNTs and CQDs after polymerization with the aniline monomer. The inhibited growth and suppressed atomic vibration resulted in some absorption peaks in the composite material spectrum being similar, and absorption peaks of hydroxyl functional groups also appeared in the PANI @ FMWCNTs spectrum. Considering all the factors, the composite material was proven to be successfully prepared.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 10 The type characteristics and material structure of the functional groups were studied and analyzed by conventional tests (7800-350 cm −1 ) in FTIR (Nicolet iSr50, USA). The electrochemical performance of these materials was analyzed using an electrochemical workstation (CHI660e, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) to measure the CP curve, Cyclic voltammetry curve and constant current cycle curve.

FTIR Analysis
The FTIR spectra of the ternary composites FMWCNTs @ CQDs @ PANI, the composite materials PANI @ FMWCNTs and PANI @ CQDs, and the monolithic material PANI are shown in Figure 1. The absorption peaks of 3851 cm −1 , 3728 cm −1 and 3626 cm −1 in the composite material PANI @ FMWCNTs spectrum correspond to free OH, while the absorption peak 1655 cm −1 corresponds to C=O stretching vibration. The 1644 cm −1 , 1402 cm −1 and 1047 cm −1 of the composite material PANI @ CQDs spectrum correspond to C=O stretching vibration, O-H blending and C-O stretching, respectively [1,6,16,[28][29][30]. It can be seen from the figure that the absorption peak is broad and strong for the composite sample, but a very weak spectrum for pure PANI, which may be related to the interaction between PANI and CNS, involves "charge transfer " [31]. When FMWCNTs and CQDs are attached to the surface of PANI, the intensity of the band will be increased in different degrees while introducing the characteristic functional groups. For the ternary composite FMWCNTs @ CQDs @ PANI, the functional group type and peak intensity have been strengthened, and the absorption peak is also clearer. In the FTIR spectrum, the composite material's spectrum contains the characteristic peaks of PANI, indicating that the generated PANI is wrapped on FMWCNTs and CQDs after polymerization with the aniline monomer. The inhibited growth and suppressed atomic vibration resulted in some absorption peaks in the composite material spectrum being similar, and absorption peaks of hydroxyl functional groups also appeared in the PANI @ FMWCNTs spectrum. Considering all the factors, the composite material was proven to be successfully prepared.  Figure 2 shows the constant current charge and discharge curves of the electrode sheets prepared by the monolithic material PANI, the composite materials PANI @ CQDs and PANI @ FMWCNTs, Figure 1. FTIR spectra of FMWCNTs @ CQDs @ PANI, PANI @ FMWCNTs, PANI @ CQDs and PANI. Figure 2 shows the constant current charge and discharge curves of the electrode sheets prepared by the monolithic material PANI, the composite materials PANI @ CQDs and PANI @ FMWCNTs, and the ternary composite material FMWCNTs @ CQDs @ PANI in a 1 M H 2 SO 4 electrolyte. At a current density of 0.5 A/g, the specific capacitances of the four materials were measured at 279 F/g, 319 F/g, 362 F/g, and 534 F/g, respectively. Compared to PANI, PANI @ CQDs, and PANI @ FMWCNTs, the specific capacitance of FMWCNTs @ CQDS @ PANI was increased by 91.4%, 67.4% and 47.5%, respectively. What is more, the capacitance retention rate of FMWCNTs @ CQDS @ PANI reached 86% after 1000 cycles, which was higher than 72% of PANI @ CQDs, 60% of PANI @ FMWCNTs and 65% of pure PANI. In addition, it can be seen from the constant current charge-discharge curves of the electrode materials PANI, PANI @ CQDs and PANI @ FMWCNTs that their charging time is much longer than the discharge time, while the constant current charge and discharge curve of the ternary electrode material shows a near perfect state-almost symmetry [32]. This phenomenon reflects the synergistic effect between monomers [1,29]. More importantly, compared with PANI, PANI @ CQDS and PANI @ FMWCNTs, FMWCNTs @ CQDS @ PANI electrode has obvious linear deviation on GCD curve, indicating that FMWCNTs @ CQDs @ PANI has significant pseudo capacitance characteristic [16]. The reason for this is likely to be manifested in two aspects: (I) as the nature of the material itself is not good enough, the dispersion is poor in the process of preparing the electrode material, and the specific surface area is limited, thereby making its electrochemical performance is affected; (II) because of its large electrical resistance, the electrochemical reversibility is poor, which affects the cycle life and capacity attenuation of the electrode.

Electrochemical Performance
Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 10 and the ternary composite material FMWCNTs @ CQDs @ PANI in a 1 M H2SO4 electrolyte. At a current density of 0.5 A/g, the specific capacitances of the four materials were measured at 279 F/g, 319 F/g, 362 F/g, and 534 F/g, respectively. Compared to PANI, PANI @ CQDs, and PANI @ FMWCNTs, the specific capacitance of FMWCNTs @ CQDS @ PANI was increased by 91.4%, 67.4% and 47.5%, respectively. What is more, the capacitance retention rate of FMWCNTs @ CQDS @ PANI reached 86% after 1000 cycles, which was higher than 72% of PANI @ CQDs, 60% of PANI @ FMWCNTs and 65% of pure PANI. In addition, it can be seen from the constant current chargedischarge curves of the electrode materials PANI, PANI @ CQDs and PANI @ FMWCNTs that their charging time is much longer than the discharge time, while the constant current charge and discharge curve of the ternary electrode material shows a near perfect state-almost symmetry [32]. This phenomenon reflects the synergistic effect between monomers [1,29]. More importantly, compared with PANI, PANI @ CQDS and PANI @ FMWCNTs, FMWCNTs @ CQDS @ PANI electrode has obvious linear deviation on GCD curve, indicating that FMWCNTs @ CQDs @ PANI has significant pseudo capacitance characteristic [16]. The reason for this is likely to be manifested in two aspects: (I) as the nature of the material itself is not good enough, the dispersion is poor in the process of preparing the electrode material, and the specific surface area is limited, thereby making its electrochemical performance is affected; (II) because of its large electrical resistance, the electrochemical reversibility is poor, which affects the cycle life and capacity attenuation of the electrode. The curves obtained by charging and discharging the four electrode materials PANI, PANI @ CQDs, PANI @ FMWCNTs and FMWCNTs @ CQDs @ PANI in a current density of 5 A/g and 1 M H2SO4 electrolyte for 20 cycles are shown in Figure 3a-d. It can be seen that the time taken for the 20 cycles of the PANI electrode is only 300 s, while the time taken for the 20 cycles of the PANI @ CQDs electrode and PANI @ FMWCNTs electrode is more than 1000 s and 1200 s separately. There is no significant difference between the two composite materials. However, the time taken for the 20-cycle of the ternary active electrode has reached more than 2750 s, which is more than twice the time spent on the binary active electrode. In addition, the specific capacitance decreases with the increase in the number of cycles [33]. There are two possible reasons: (I) the electrolyte decomposes on the surface The curves obtained by charging and discharging the four electrode materials PANI, PANI @ CQDs, PANI @ FMWCNTs and FMWCNTs @ CQDs @ PANI in a current density of 5 A/g and 1 M H 2 SO 4 electrolyte for 20 cycles are shown in Figure 3a-d. It can be seen that the time taken for the 20 cycles of the PANI electrode is only 300 s, while the time taken for the 20 cycles of the PANI @ CQDs electrode and PANI @ FMWCNTs electrode is more than 1000 s and 1200 s separately. There is no significant difference between the two composite materials. However, the time taken for the 20-cycle of the ternary active electrode has reached more than 2750 s, which is more than twice the time spent on the binary active electrode. In addition, the specific capacitance decreases with the increase in the number of cycles [33]. There are two possible reasons: (I) the electrolyte decomposes on the surface of the electrode material during the electrolysis process [34]; (II) the dissolution and stripping of the active material and the reduction in the active center during the electrolysis process [35]. specific capacitance. Therefore, it is concluded that the electrical activity of ternary composite is better than that of PANI, PANI @ CQDs and PANI @ FMWCNTs.

( )
For its specific capacitance retention ratio (Figure 4), it can be seen that the composite material PANI @ FMWCNTs has poor performance of the electrode material after multiple charge and discharge comparing PANI @ CQDs, even lower than the retention rate of pure PANI. According to the cyclic voltammetry curve in Figure 5, there is a significant redox peak. The possible reason is a sharp decay of the cycle life of the electrode material, as that the electrolyte has a large irreversible capacity during multiple charge and discharge processes, and the embedding of solvent molecules directly leads to the serious collapse of the electrode material.  According to Formula (1), there is a direct relationship between the discharge time and the specific capacitance. Therefore, it is concluded that the electrical activity of ternary composite is better than that of PANI, PANI @ CQDs and PANI @ FMWCNTs.
For its specific capacitance retention ratio (Figure 4), it can be seen that the composite material PANI @ FMWCNTs has poor performance of the electrode material after multiple charge and discharge comparing PANI @ CQDs, even lower than the retention rate of pure PANI. According to the cyclic voltammetry curve in Figure 5, there is a significant redox peak. The possible reason is a sharp decay of the cycle life of the electrode material, as that the electrolyte has a large irreversible capacity during multiple charge and discharge processes, and the embedding of solvent molecules directly leads to the serious collapse of the electrode material. To further compare the electrochemical capacitive behaviors of PANI, PANI @ CQDs, PANI @ FMWCNTs and FMWCNTs @ CQDs @ PANI electrodes, CV tests were carried out at 10 mv s −1 , as shown in Figure 5. It can be seen from the figure that there is a significant redox peak in the FMWCNTs @ CQDs @ PANI curve in the interval of 0.4-0.5 V, indicating that the redox reaction is reversible. It can also be observed that the cyclic voltammetry curve is approximately embodied as an isosceles triangle, which reflects the combination of the electric double layer capacitance and the tantalum capacitance of the composite material, good reversibility [32,36]. It is also obvious that FMWCNTs @ CQDs @ PANI electrode has much higher redox peak current, suggesting increased specific capacitance. There is also a significant redox peak in the PANI @ CQDs and PANI @ FMWCNTs cyclic voltammetry curve. The two types of electrodes show the same pair of redox peaks, which are attributed to redox transition of PANI between CQDs and FMWCNTs [37]. In comparison, the redox peak in the pure PANI is very weak [38], indicating that the redox reaction is less reversible. Combined with the conclusions obtained in Figure 2, the electrical activity and reversibility of the PANI @ FMWCNTs electrode in the early stage are better than those in the later stage. In addition, from the active area of the curve, the electrical activity of the ternary active material is better than the composite materials PANI @ CQDs and PANI @ FMWCNTs, as well as pure polyaniline, with strong energy storage capacity [32]. Furthermore, it shows that the compounding of three active materials has been successful. Therefore, the conclusion about the specific capacitance obtained in the constant current charge and discharge curve of Figure 2 was verified: ternary active electrode > binary active electrode > pure polyaniline. Figure 6 shows the cyclic voltammetric curves of PANI, PANI @ CQDS, PANI @ FMWCNTs and FMWCNTs @ CQDs @ PANI at a series of scanning rate ranging from 5 to 50 mv s −1 . It can be seen that: (I) the specific capacitance decreases with the increase in scanning rate (Table 1), which may be due to the superior charge mobilization per unit time [39]; (II) the response current increases with the increase in scanning rate, which shows that the composite has good electrochemical reversibility and high-power characteristics [32].  To further compare the electrochemical capacitive behaviors of PANI, PANI @ CQDs, PANI @ FMWCNTs and FMWCNTs @ CQDs @ PANI electrodes, CV tests were carried out at 10 mv·s −1 , as shown in Figure 5. It can be seen from the figure that there is a significant redox peak in the FMWCNTs @ CQDs @ PANI curve in the interval of 0.4-0.5 V, indicating that the redox reaction is reversible. It can also be observed that the cyclic voltammetry curve is approximately embodied as an isosceles triangle, which reflects the combination of the electric double layer capacitance and the tantalum capacitance of the composite material, good reversibility [32,36]. It is also obvious that FMWCNTs @ CQDs @ PANI electrode has much higher redox peak current, suggesting increased specific capacitance. There is also a significant redox peak in the PANI @ CQDs and PANI @ FMWCNTs cyclic voltammetry curve. The two types of electrodes show the same pair of redox peaks, which are attributed to redox transition of PANI between CQDs and FMWCNTs [37]. In comparison, the redox peak in the pure PANI is very weak [38], indicating that the redox reaction is less reversible. Combined with the conclusions obtained in Figure 2, the electrical activity and reversibility of the PANI @ FMWCNTs electrode in the early stage are better than those in the later stage. In addition, from the active area of the curve, the electrical activity of the ternary active material is better than the composite materials PANI @ CQDs and PANI @ FMWCNTs, as well as pure polyaniline, with strong energy storage capacity [32]. Furthermore, it shows that the compounding of three active materials has been successful. Therefore, the conclusion about the specific capacitance obtained in the constant current charge and discharge curve of Figure 2 was verified: ternary active electrode > binary active electrode > pure polyaniline. Figure 6 shows the cyclic voltammetric curves of PANI, PANI @ CQDS, PANI @ FMWCNTs and FMWCNTs @ CQDs @ PANI at a series of scanning rate ranging from 5 to 50 mv·s −1 . It can be seen that: (I) the specific capacitance decreases with the increase in scanning rate (Table 1), which may be due to the superior charge mobilization per unit time [39]; (II) the response current increases with the increase in scanning rate, which shows that the composite has good electrochemical reversibility and high-power characteristics [32].

Conclusions
In this experiment, FTIR, electron microscopy and electrochemical studies have shown that the preparation of ternary composite electrode materials has been successful. After the polymerization of carbon nanomaterials and conductive polymer PANI, its electrochemical performance was greatly improved, especially in the constant current cycle charge and discharge curve. Its specific capacitance is much higher than pure PANI and composite electrode materials, and the cycle stability is also greatly improved. On the other hand, the doped composite material effectively improves the performance degradation caused by agglomeration, slows down the energy decay, enhances its dispersibility in solution, increases the specific surface area, improves its stability, and further enhances its electrical activity and reversibility. In addition, a composite material composed of an electroactive material with a high specific surface area and a carbon material significantly increases the energy density of the electrode material and is also a major cause of an increase in specific capacitance. Finally, during the electrolysis process, the problem of the collapse of the electrode sample after PANI composite requires further study.

Conflicts of Interest:
The authors declare no conflict of interest.