Ultrasonication-mediated nitrogen-doped multiwalled carbon nanotubes involving carboxy methylcellulose composite for solid-state supercapacitor applications

In this study, a novel nanohybrid composite containing nitrogen-doped multiwalled carbon nanotubes/carboxymethylcellulose (N-MWCNT/CMC) was synthesized for supercapacitor applications. The synthesized composite materials were subjected to an ultrasonication-mediated solvothermal hydrothermal reaction. The synthesized nanohybrid composite electrode material was characterized using analytical methods to confirm its structure and morphology. The electrochemical properties of the composite electrode were investigated using cyclic voltammetry (CV), galvanic charge–discharge, and electrochemical impedance spectroscopy (EIS) using a 3 M KOH electrolyte. The fabricated composite material exhibited unique electrochemical properties by delivering a maximum specific capacitance of approximately 274 F g−1 at a current density of 2 A g−1. The composite electrode displayed high cycling stability of 96% after 4000 cycles at 2 A g−1, indicating that it is favorable for supercapacitor applications.


Scientific Reports
| (2021) 11:9918 | https://doi.org/10.1038/s41598-021-89430-x www.nature.com/scientificreports/ electrochemical properties via synergistic effects. Therefore, carbon-based composite electrodes are beneficial for supercapacitors, sensors, electrocatalysts, and gas sensor applications [9][10][11] . Carbon nanotubes have been predominantly reported worldwide in the last decades owing to their outstanding mechanical properties, thermal stabilities, and electrical conductivities 12 . CNTs with improved properties have been used as potential materials for supercapacitors in the presence of electrolytes 13 . The electrochemical properties of CNTs have been of great interest in recent years because of their fast recharge, long-term cycling performances, and high power densities 14,15 . Kumar et al. 16 created an Fe 3 O 4 /RGO nanosheet hybrid electrode for supercapacitor applications. This hybrid composite exhibited a capacitance of approximately 455 F g −1 at 8 mV s −1 and excellent cyclic stability. In addition, cobalt oxide electrodes on nanocrystalline CNT/polypyrrole (PPy) composites have been used in supercapacitor applications 17 .
Kalam et al. 18 reported a CNT/PVA gel-based electrode material and showed that at a specific capacitance applied voltage and value of 150 mV s −1 and 219 F g −1 , respectively, it had improved electrochemical properties. Karthika et al. 19 described the construction of a CNT/PVA gel electrolyte on polyester paper, which showed a specific capacitance of approximately 276 F g −1 at 5 mV s −1 with excellent retention. Moreover, CMC has favorable properties such as water solubility, biocompatibility, non-toxicity, and chemical stability, which makes it an excellent electrode material for supercapacitor applications [19][20][21] . Shi et al. 22 reported a conductive polyaniline (PANI) hydrogel in the presence of bacterial cellulose materials for antimicrobial applications. The microcrystalline cellulose-PPy hydrogel composite via a dendritic network was structured to fabricate the electrode for supercapacitor and sensor applications. Li et al. 23 reported a CMC/PANI composite hydrogel, which acquired high conductivity and excellent mechanical properties 23 . In this study, the porous materials of nitrogen-doped multiwalled carbon nanotubes/carboxymethylcellulose (N-MWCNT/CMC) composites were fabricated using a hydrothermal process for symmetric and asymmetric supercapacitor applications. The composite was confirmed using structural, morphological, and electrochemical applications in the presence of an electrolyte.

Experimental section
Materials. Analytical grade Na-CMC with a number-average molecular weight of 90,000, MWCNTs, urea, nitric acid, potassium permanganate, ammonium hydroxide, ethanol, potassium hydroxide, and double-distilled (DD) water were purchased from Sigma-Aldrich Pvt. Ltd. (Korea). The composite materials were synthesized for supercapacitor applications.

Methods. Synthesis of N-MWCNT composite.
The MWCNTs were synthesized based on previous studies 24 .
The nitrogen doping process of the MWCNTs is shown in detail. The estimated amounts of 2 and 2.4 g of MWC-NTs and urea, respectively, were dispersed in 200 mL of DD water, and the mixture was continuously sonicated for 4 h to complete the dispersion. The resultant solution was heated at 80 °C for 6 h and then purified using ethanol/water (1:1). The reaction mixture was then purified at 90 °C in a vacuum oven for 24 h to analyze its structural, morphological, and electrochemical properties.

Synthesis of the N-MWCNT/CMC composite.
A mixture of N-MWCNT (0.5 g) and CMC (2 g) in 200 mL of DD water was vigorously stirred at 95 °C for 4 h. Subsequently, 20 mL of ammonia was added to the reaction solution, and the solution was stirred at the same temperature for another 12 h. The reaction mixture was purified using ethanol at 200 °C for 12 h using a vacuum oven. The resultant product was transferred to an alumina crucible and heated at 400 °C for another 8 h to complete the formation of the hybrid composite, which was ready to be characterized. The synthesis process of the N-MWCNT/CMC composite is described in Fig. 1.

Materials characterization.
Fourier-transform infrared spectroscopy (FTIR, Model number, PerkinElmer) was used to analyze the functionality of the N-MWCNT/CMC composite. Raman spectral examination of the composite was conducted using an RM200 confocal Raman microscope (alpha300R). The nanostructures of the composite were ascertained using the powder X-ray diffraction (XRD) patterns (Rotaflex RU-200B, RIGAKU) and a source of Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) analysis of the elemental composition was conducted using an ESCALAB 250Xi instrument in the presence of Al Kα radiation. The morphologies of the composites were characterized using field emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Japan) and high-resolution transmission electron microscopy (HR-TEM, JEM-2010F, Japan).
Electrochemical measurements. The CV results of the composite were characterized using a three-electrode configuration containing Pt, Ag/AgCl, and working electrodes (N-MWCNT/CMC). Electrochemical analysis was performed using CV, GCD, and EIS, and was conducted on an Auto lab PGSTAT302N in Metrohm, Netherlands. The (75:20:10) electrode material was fabricated using polytetrafluorethylene and forced into a current collector of nickel wire (1 cm × 1 cm). The electrochemical properties of the N-MWCNT/CMC composite electrode were characterized using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) analysis via a 3 M KOH electrolyte. The electrochemical properties of the N-MWCNT/CMC composite electrode were characterized using CV, GCD, and EIS analyses. The CV was conducted in a potential range between 0.0 and 0.6 V versus Ag/AgCl at different scan rates in the presence of an electrolyte. The constant current charge-discharge test was conducted at different current densities of 2 to 6 A g −1 in the potential range of 0.1 to 0. The morphological analysis of the N-MWCNT and N-MWCNT/CMC composites was conducted using FE-SEM, and FE-SEM EDS results are displayed in Fig. 6. Based on the SEM results, small globular particles, such as cauliflower, accumulated and were allocated on the N-MWCNT surface on a nanometer scale. The SEM-EDX results suggest that the composition of elements such as carbon (93.93%), oxygen (4.86%) and nitrogen (1.21%) is successfully doped via hydrothermal reaction. These well-organized composites are assembled with a higher surface area and smaller diffusion, which can improve the electrochemical properties of the composites [28][29][30] .
The morphologies of the N-MWCNT and N-MWCNT/CMC composites, which were examined using HR-TEM are shown in Fig. 7. Based on the TEM images, the composite contained bundles of nanotubes and exhibited a crystalline behavior of the CMC in the range of approximately 20-100 nm. Moreover, nitrogen was predominant on the surface of the nanotubes. The N-MWCNT/CMC composite is a well-defined nanostructure with carbon nanotubes at the nanometer scale and sheet-like morphology, as shown in Fig. 7d. Therefore, the N-MWCNT/CMC composite with a conductive core improved the remarkable properties shown in the electrochemical analysis. The selected area electron diffraction (SAED) pattern of the composite is shown in Fig. 7, which shows the nanocrystalline behavior of the N-MWCNT and composite samples. The results indicated that the precise rings proved the nano polycrystalline morphologies of the N-MWCNT and CMC composite.   (Fig. 9a) at various scan rates indicated an excellent electrochemical supercapacitor behavior. Figure 8b shows the GCD curves of the N-MWCNT/CMC composite electrodes in the presence of 3 M KOH solution at current densities of 2, 3, 4, 5, and 6 A g −1 . The results indicated that an approximately similar potentialtime behaviors occurred, which indicated that the GCD process of the composite sample was a reversible reaction in terms of their electrochemical properties 31 . The specific capacitance of the composite materials was calculated from the following equation 32 : C (F g −1 ) represents the specific capacitance, I (A) is the galvanostatic current used for GCD, t (s) is the time denoted as the discharge cycle, V (V) is the potential difference of the discharge, and m (g) is the mass of the electrode material. Therefore, the composite materials were subjected to superior www.nature.com/scientificreports/ capacitance (~228 F g −1 ) than that of the pristine N-NMWCNT materials. The significant improvement in the specific capacitance of the composite changed the size of the nanoparticles, redox activity, and electronic conductivity of the CNT and CMC. There are numerous works on conducting polymers involving CMC composites and carbon materials that were used in supercapacitor applications 32 . It was reported that the capacitance of PANI/ CMC can reach up to 451.25 F g −1 by employing 20wt% CMC composites in conjunction with carbon materials. Also, they reported that at a current density of 1 A g −1 exceeding 1000 cycles, the capacitance reaches up to 300 F g −1 . Similarly, other work specified that the capacitance of PPy/CMC nanospheres can reach up to 184 F g −1 by employing at current density of 0.25 Ag −1 . Additionally, the 80% retentive cyclic stability has attained for a hybrid electrode with CMC template after 200 cycles which is efficient for the supercapacitor applications 33 . Babu IM et al. 34 described on well-ordered mesoporous Co 3 O 4 /CMC nanoflake assembly likewise provides limited diffusion route for ions and hasten operative charge carriage that reflects in elevated capacitance of 298 °C g −1 at 1 A g −1 , exceptional stability of 90% maintenance after 5000 cycles and little charge passage resistance of 0.5 Ω in three electrode schemes. Moreover, they reported the design of asymmetric supercapacitor operating at voltages 0-1.2 V which exhibits appropriate electrochemical performance at 2 A g −1 with an energy density of 18 W h kg −1 . These results support the prospective competencies of incorporated Co 3 O 4 & CMC which could be employed as an efficient electrode in the forthcoming supercapacitor applications.
The composite electrode showed a maximum capacitance of approximately 228 F g −1 at a current density of 2 A g −1 , indicating an exceptional rate capability in the presence of a 3 M KOH electrolyte. Figure 8c shows the plot of the specific capacitance (Vs) at various current densities. The capacitance of the composite decreased from approximately 228, 205, 158, 150, and 143 F g −1 , and the current density varied from 2 to 6 A g −1 . Figure 9d The phase angle for the impedance plot of the composite electrodes was observed to be higher than 45° at low frequencies, suggesting electrochemical capacitive properties. Therefore, the results showed that the composite electrode was favorable for supercapacitor properties 35-51 . Discussion. The N-MWCNT/CMC composite was studied using a two-electrode system containing a PVA/ H 2 SO 4 gel electrolyte for supercapacitor applications, as shown in Fig. 9. The CV curves of the composite electrode were obtained at various scan rates of 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 mV s −1 . As seen in Fig. 9,  Figure 9b shows that the GCD test was conducted for the device at current densities of 2, 3, 4, 5, and 6 A g −1 with corresponding specific capacitances of approximately 256, 160, 151, 132, and 128 F g −1 , respectively. Before the GCD cycles, the EIS test was conducted in a frequency range of 0.01-105 Hz, which demonstrated an internal resistance of 9.68 Ω and increased after 4000 cycles to Rct-65 Ω, as shown in the Nyquist plot in Fig. 9d. The results showed that the equivalent series circuit was used for the electrochemical properties. Furthermore, the GCD cyclic stability test was conducted at a 2 A g −1 scan rate of up to 4000 cycles, which confirmed that it retained 96% of the initial capacity. Based on the mechanical strain, the specific capacitance value decreased by a 4% loss in capacitance, and the remaining 96% retained its initial capacitance with a coulombic efficiency of approximately 98% of the overall cyclic stability.

Conclusion
In conclusion, the present work proved that a sonication-assisted hydrothermal process can synthesize an N-MWCNT/CMC composite. The synthesized composite materials exhibited excellent morphologies and porosities, which improved their specific capacitance values. The electrochemical properties of the composite materials showed the highest specific capacitance values of approximately 274 F g −1 at a current density of 2A g −1 in the presence of 3 M KOH and PVA/H 2 SO 4 gel electrolytes. This improvement in specific capacitance values was attributed to the excellent penetration and enhanced utilization of the active surface, which is favorable for supercapacitor applications. The excellent structural, cyclic, and electrochemical properties at a higher current rate with approximately 96% cyclic stability were useful in fabricating a wide range of supercapacitor applications. This further proved that composite materials can be used in real supercapacitor applications using two-electrode performance via a PVA/H 2 SO 4 gel electrolyte. Therefore, the composite materials can be synthesized at a low cost and the electrodes can be efficiently fabricated for promising supercapacitor applications in the future.