Sonication-supported synthesis of cobalt oxide assembled on an N-MWCNT composite for electrochemical supercapacitors via three-electrode configuration

The Co3O4@N-MWCNT composite was synthesized by a sonication-supported thermal reduction process for supercapacitor applications. The structural and morphological properties of the materials were characterized via Raman, XRD, XPS, SEM–EDX, and FE-TEM analysis. The composite electrode was constructed into a three-electrode configuration and examined by using CV, GCD and EIS analysis. The demonstrated electrochemical value of ~ 225 F/g at 0.5 A/g by the electrode made it appropriate for potential use in supercapacitor applications.

www.nature.com/scientificreports/ reported that morphological patterns such as mesopores, nanocubes, nanospheres and nanorods exert different impacts on the fabrication of electrode materials 17 . Apart from morphological influence, crystal size, aspect ratio, orientation, and crystalline density also play a vital role in the enhancement of electrochemical phenomena 17 . Aligned Co 3 O 4 nanowires on nickel foam electrode materials synthesized by a hydrothermal process achieved a capacitance of ~ 750 F/g at 0.5 A/g. The flower-like nanostructure of cobalt oxides/carbon electrode materials registered a capacitance of ~ 330 F/g at 0.5 A/g in the solvothermal process 18 . The porous morphology of the Co 3 O 4 film synthesized via electrode deposition showed a maximum capacitance of ~ 443 F/g at 0.5 The nanocrystalline morphology of Co 3 O 4 material-based electrodes exhibited improved electrochemical properties with improved cyclic stability [19][20][21][22] .
The carbon nanotubes (CNTs) are widely used as support for active metal nanoparticles for catalytic properties due to thier outstanding resistance to challenging the reaction and surface properties for chemical functalization and nitrogen doping process. These surface functional groups can be used to modify the catalytic performance of the metal nanoparticles for various potential applications. In particular, the N-MWCNTs are found to be a auspicious support for catalysis and improved electrochemical properties [23][24][25] . A higher dispersion of the supported metals can be accomplished on N-MWCNTs than on nitrogen-free CNTs, which was attributed to a higher amount of surface nucleation sites and to the formation of some individual sections around the N-rich sites, allowing efficient anchoring of metal nanoparticles for various potential applications.Frackowiak et al. reported that the specific capacitance of MWCNTs was increased from (80 to 135) F g [−1 26 , while treating MWC-NTs with acid electrolyte and other KOH electrolytes is almost 90 F g −1 . The pristine cobalt oxide is not constant value of the specific capacitance, the outcome values changes accordingly the morphology and electrolyte. The specific capacitances almost is (~ 150-225) F g −1 in the nanocrystalline morphology with KOH electrolyte [27][28][29] .
The following publications showed different outcome values of cobalt oxides with different approach of the electrochemical reaction. The following reports were increased or decreased the specific capacitances, cyclic stability depends on the synthetic route, morphologies of the materials and types of electrolyte. Based on the literature, we synthesised Co 3 O 4 @N-MWCNT composite via sonication assisted thermal reduction process and the outcome specific capacitance 225 F/g at 0.5 A/g with excellent retention in the 5000 cycles. Therefore, the synthesized composite materials useful for supercapacitor application with excellent cyclic retention. These surface functional groups can be used to tailor the catalytic performance of supported metal nanoparticles for various potential applications. Nitrogen-doped carbon nanotubes (N-MWCNT) are found to be a promising support for hydrogenation catalysts and amended the electrochemical properties. A higher dispersion of the supported metals can be accomplished on N-MWCNTs than on nitrogen-free CNTs, which was attributed to a higher amount of surface nucleation sites and to the formation of some individual sections around the N-rich sites, allowing efficient anchoring of metal nanoparticles for various potential applications [30][31][32][33] .
Based on the amended benefits accompanied by Co 3 O 4 -based materials, we designed and synthesized cobalt oxide@nitrogen-doped multiwalled carbon nanotube composite by sonication-mediated thermal reduction processes and explored their supercapacitor applications. This composite displayed a high specific capacitance of ~ 225 F/g at 0.5 A/g and excellent cyclic stability in the presence of a 3 M KOH electrolyte.

Co 3 O 4 @N-MWCNT synthesis.
In a typical synthesis of a Co 3 O 4 @N-MWCNT composite, 0.8 g of nitrogen-doped MWCNTs was diffused in 200 ml of double DD water via sonication for 2 h. To this end, 0.3 mol of cobalt acetate tetrahydrate and Co(CH 3 COO) 2 ·4H 2 O were added, followed by 20 mL of 30% ammonia, and the whole solution was roused at 90 °C for 12 h. At this point, the whole reaction combination was transferred to an autoclave reactor, and the thermal reduction process was carried out at 200 °C for 8 h. The preceipitated Co 3 O 4 @N-MWCNT composite material was filtered and washed with a 1:1 solution of DD water/ ethanol repetitively and purified at 95 °C for 12 h. This dried composite was stored in an airtight bottle and subjected to structural, morphological, and electrochemical studies. Schematic illustration of synthetic protol for Co 3 O 4 @N-MWCNT composite is shown in Fig. 1.
Fabrication of electrodes for supercapacitor study. The composite was fabricated via a three-electrode configuration, and electrochemical studies were performed by CV, GCD, and EIS analysis. The composition of active material (80 wt%), 10 wt% conducting carbon black as the conductive agent and 10 wt% polyvinylidene fluoride (PVDF) binder were mixed with N-methyl pyrrolidinone (NMP) via sonication to complete the uniform slurry. After that, this composite slurry was coated uniformly on a strip of nickel wire (1 × 1 cm −2 ) current collector and dried in an oven at 90 °C for 10 h. Then, the slurry was coated on the nickel wire for the working electrode in the CV analysis. counter electrode is (platinum wire) and reference electrode (Ag/AgCl) for electrochemical studies.

Results and discussion
The Raman spectra of the Co 3 O 4 @N-MWCNT composite results are shown in Fig. 2a. The Raman shifts at ∼1340, ∼1572, and 2675 cm −1 can be attributed to the three distinct types of peaks of the N-MWCNT composite. The D band is attributed to the lattice defect that highlights the phonon mode of vibration from the N-MWCNT surface. The G band represents the C-C (vibrational modes), and E 2g symmetry denotes the doubly degenerated phonon modes 23 Fig. 5a,b.
The electrochemical properties of various nanostructured cobalt oxides and N-MWCNT material electrodes were studied for supercapacitor applications 26,27 . Furthermore, the electrical properties of the composite were investigated via three-eleectrode configuration by using CV, GCD, and EIS analysis. In this configuration, the working electrode (Co 3 O 4 @N-MWCNT, reference electrode (Ag/AgCl) and Pt act as counter electrodes in the presence of 3 M KOH electrolyte at room temperature. The CV results of the synthesized composite electrodes at different applied scan rates are shown in Fig. 8a-e. Electrochemical CV curves (Fig. 8a) are obtained for 5, 10, 20, 50, 100, 150, and 200 mV/s with a potential window from 0.8 to 1.4 at different applied scan rates. In this result, well-defined rectangular peaks were observed due to the enhancement of the electrochemical properties of the composite materials. While increasing the scan rates, the peak intensities increase towards higher potentials. This is corroborated by the rapid electrochemical charge-discharge obtained between the active composite material and electrolyte surface, as shown in reactions 1 and 2. Generally, the rate competence was mainly reliant on three routes: (i) electrolyte and ion diffusion, (ii) electrode surface, and ion adsorption, and (iii) charge transfer and electrode. Based on the three steps, with increasing scan rate, the reaction rate relatively lowers and decreases the specific capacitance. Therefore, the characteristic CV curves did not change significantly, which   27 .
Cobalt oxides are involved in charge transfer or electron transfer reactions in the presence of a 3 M KOH electrolyte. The resulting anodic and cathodic curves were defined as the electrochemical EDLC behaviour of the materials in the CV analysis. The reaction mechanisms are connected with previously reported cobalt oxide materials via different electrolytes. The CV peaks significantly vary on the mode of morphological, surface, and structural properties of composite materials. Therefore, cobalt oxides are potential materials involved in electrochemical reactions for pseudocapacitance applications 28,29 . Figure 8b demonstrates the GCD data of the composite results and indicates symmetrical triangular curves at current densities of 0.5, 1, 1.5, 2,5 and 10 A g −1 , which signifies EDLC behaviour in the fixed potential range of (0.8 to 1.4) V. Increasing the applied current density above 10 A/g reduces the EDLC behaviour and increases the resistance properties. Hence, the GCD curves after 10 A/g diverging linearity indicate a decrease in the EDLC behaviour of the composite. Therefore,   Table 1.
The cyclic stability of the composite electrode (Fig. 8d) shows that 0.5 A/g for 5000 continuous GCD cycles and 2.2% loss occurred, representing excellent cyclic stability in the strong electrolyte 3 M KOH. Figure 8e demonstrates the EIS results of the composite materials. This result authorizes the electrochemical properties of The specific capacitance of the synthesized composites was comparable to considerably increases in the stability of cobalt oxide-based composite electrodes reported previously (Table 1) [40][41][42][43][44] . Although it is challenging to arrange a clear-cut comparison of the electrode materials, the data in Table 1 were recorded using a range of parameters, such as the fabrication of electrodes, synthesis methods, capacitances observed at different current densities/scan rates, and cyclic stabilities [45][46][47][48][49][50] . In the present work, the composite demonstrated excellent electrochemical performance compared to the electrodes reported previously due to the good capacitance at a higher current density of 0.5 A g −1 and the high cyclic stability, losing only 2.2% of its initial capacitance after 5000 cycles. The resulted morphological and electrochemical behaviour of the composite materials are compared to previously syntheized electrodes published in the literature [45][46][47][48][49][50] .

Conclusion
The synthesized Co 3 O 4 @N-MWCNT composite materials were constructed in a three-electrode configuration for supercapacitor applications. The capacitance value and cycling stability were enhanced up to 5000 cycles with a retension of 97.8% (2.2% loss at 0.5 A g −1 ). The enhancement of the composite electrode was due to the electrolyte distribution and morphological, surface and controlled synthesis, which was developed for supercapacitor applications. The composite electrode exhibited a marked specific capacitance (~ 225 F/g at 0.5 A/g), exceptional cyclic stability, improved morphological properties, and excellent cyclic retention.