Facile synthesis of NiCo2 O4/rGO nanowire array electrodes for supercapacitor cathodes

In this study, NiCo2O4/rGO nanowire arrays (NWAs) were effectively synthesized, and these arrays were subsequently employed as components in supercapacitors. The phase structure of the materials was analyzed using X-ray diffraction spectroscopy, and the morphology of the materials was observed by scanning and transmission electron microscopy. By comparing with NiCo2O4 nanowire arrays without graphene oxide, it was found that the surface of NiCo2O4 arrays grown directly on nickel foams had a large number of accumulations, whereas the surface agglomerations of NiCo2O4/rGO nanowire arrays complexed with reduced graphene oxide were significantly improved. The NiCo2O4/rGO electrode with improved agglomeration phenomenon shows better performance. The specific capacitance of NiCo2O4/rGO NWAs is 2311 F·g-1 at 1 A·g-1 and remains at 1211 F·g-1 even at 50 A·g-1. In addition, the capacitance retention of this material could reach 83.5 % after 3,000 cycles at 10 A·g-1, demonstrating excellent multiplicative performance and cycling stability.


Introduction
Supercapacitors are gaining prominence in the realm of energy storage owing to their exceptional energy storage capabilities.It has high specific capacitance and high power density and is environmentally friendly.Supercapacitors have a little longer life compared to traditional batteries.The current commercial supercapacitor can work normally at -40℃ ~ 80℃, while the working temperature range of a lithium battery is -20℃ ~ 60℃, so it is more flexible compared with a lithium battery.In addition, it also features high power density, specific capacitance, and the ability to accept huge charge-discharge currents while maintaining a high-efficiency level.These advantages give supercapacitors a broad spectrum of applications in hybrid energy systems, electric vehicles, mobile communications, information technology, and national defense technology [1][2][3][4].Hence, conducting fundamental theoretical investigations and applied supercapacitor studies is extremely important.When voltage is applied to the electrodes of double-layer supercapacitors, the positive electrode attracts negative charges while the negative electrode attracts positive charges.As a result, the surface of the electrodes exhibits two layers of charges produced by the ions in the electrolyte.Therefore, conducting polymers is often used as electrode materials for this type of supercapacitor because their porous structure provides a large specific surface area that enhances the ability to adsorb and store ions in the electrolyte.The operational mechanism of pseudo-capacitor supercapacitors relies on the expeditious redox reaction at the electrode interface.At present, NiCo 2 O 4 electrodes still present challenges for supercapacitor applications, such as poor conductivity that hinders the transfer of electrons [5][6], whereas graphene oxide possesses remarkable properties such as large surface area and high conductivity [7].The findings show that the simple composite of graphene material with NiCo 2 O 4 does not significantly improve the electrode activity of NiCo 2 O 4 material.Traditional electrodes were prepared by making a slurry of active material and using the adhesive to stick the active material to the collector.However, the presence of the adhesive could limit electron transfer.So, by following the traditional way of preparing the electrode, the performance of the electrode is always limited and does not have a high upper limit.In this paper, we present the growth of ordered arrays of NiCo 2 O 4 /rGO nanowires on Ni foam utilizing a simple hydrothermal technique.The Ni foam, loaded with the arrays, can be utilized directly as electrodes for supercapacitors.A comparative analysis was conducted between the electrodes composed of NiCo 2 O 4 nanowire arrays and those composed of NiCo 2 O 4 /rGO nanowire arrays generated straight on Ni foam substrates.The latter demonstrated the following clear advantages: (1) There is almost no particle aggregation on the Ni foam surface, and only a significant portion of the particles are scattered on the electrode surface so that the array material on the electrode surface can be almost completely exposed to the electrolyte.( 2) Due to the nanowires remaining relatively independent of each other, with little or no site of entanglement, the electrolyte can fully contact each nanowire and diffuse into the interior of the electrode with little resistance.

Material and methods
The schematic diagram in Figure 1 illustrates the synthesis process employed to fabricate NiCo 2 O 4 and NiCo 2 O 4 /rGO nanowire arrays.The solvents and chemicals utilized in this experiment were of reagent grade and did not require additional purification.We clean the Ni foam substrate before synthesizing.After the cleaning was completed, it was placed on the inner wall of the PTFE liner.Subsequently, Ni(NO 3 ) 2 ꞏ6H 2 O, Co(NO 3 ) 2 ꞏ6H 2 O, and urea were mixed inside deionized water in the ratio of 1:2:6, where Ni(NO 3 ) 2 ꞏ6H 2 O was 1 mmol.Separately, a clean beaker was prepared, and 32 ml of ethanol and 80 mg of graphene oxide (GO) were added to the beaker and ultrasonicated for half an hour.After ultrasonication, the graphene oxide solution was transferred to the mixed solution.Finally, the mixed solution was relocated to a PTFE liner containing Ni foam and subjected to 120°C for 6 h.Then, the NiCo 2 O 4 /rGO precursor was extracted.The precursor underwent multiple rounds of washing with ethanol and water.Once drying was completed, the material underwent calcination in a mediumtemperature oven at 350°C for 2 h to obtain Ni foam-supported NiCo 2 O 4 /rGO nanowire arrays.The preparation steps for NiCo 2 O 4 NWAs are the same, except for excluding GO.

Results and discussion
The Ni foam with material grown was first sonicated, and then the powder down from the sonication was XRD tested.Figures 2(a It can be found that the accumulation of the surface of Ni foam with the addition of GO has been significantly improved.Before the inclusion of GO, the outermost layer of the electrode was scattered with a large number of particles, which were closely connected, and the surface of the nickel foam was hardly in direct contact with the electrolyte.After adding the Ni foam, surface particles are significantly reduced, and the outermost layer of the Ni foam has almost no particles agglomerated together, allowing it to come into full contact with the electrolyte.This is because graphene oxide has a significant surface area, which results in a high number of active sites on its surface.These active sites can physically or chemically bind to other materials, preventing them from being attracted to the surface.Additionally, the surface functional groups of graphene oxide carry an electric charge, typically negative.These charges can generate electrostatic repulsion between particles, preventing electrostatic attraction between particles, which also helps to reduce accumulation between particles.Figures 2(c) and 2(f) show the images of NiCo 2 O 4 nanowire arrays and NiCo 2 O 4 /rGO nanowire arrays, and it can be seen that the two materials keep relatively independent and grow uniformly on the Ni foam, with sufficient gaps between the materials.Therefore, when this electrode is in operation, the electrolyte can easily penetrate the interior of the electrode and make contact with the electrode material.4(h)) under the same conditions.By comparison, it can be seen that under the same conditions, the NiCo 2 O 4 /rGO electrode with graphene oxide added exhibits higher specific capacitance, higher capacitance retention, and higher stability.This is because the addition of oxidized graphene enhances the dispersion of materials on the electrode surface, reducing the accumulation between materials.Consequently, the outermost layer of the Ni foam has almost no particles agglomerated together, which allows it to have full contact with the electrolyte.

Conclusions
we propose a straightforward and unique technique for producing ordered NiCo 2 O 4 /rGO nanowire arrays.After the material was fabricated into electrodes, it performed excellently under different operating conditions.The findings indicate that the addition of graphene oxide makes the material on the electrode surface more dispersed and reduces the occurrence of aggregation between the materials.The nanowires on the electrode surface are readily exposed to the electrolyte, and the diffusion of the electrolyte onto the material's surface encounters minimal resistance.This significantly enhances the performance of the electrode.The NiCo 2 O 4 /rGO electrodes, prepared in advance, demonstrated 2311 Fꞏg -1 at 1 Aꞏg -1 .Furthermore, even after 3,000 cycles at 10 Aꞏg -1 , the electrodes exhibited a capacity retention of 83.5%.These results indicate that the nanowire arrays composing the NiCo 2 O 4 /rGO material have the potential to be applied in energy storage devices with high efficiency.
) and 2(d) show the test result graphs.These results show that the diffraction peaks of NiCo 2 O 4 and NiCo 2 O 4 /rGO corresponded to the diffraction peaks of the corresponding PDF standard cards, indicating that the materials were pure phases without any impurities.The morphologies of NiCo 2 O 4 and NiCo 2 O 4 /rGO after SEM examined calcination, and the images in Figurers 2(b) and 2(e) show the agglomerations of NiCo 2 O 4 and NiCo 2 O 4 /rGO on the Ni foam skeleton at the same magnification.

Figure 2 .
Figure 2. (a and d) XRD patterns of NiCo 2 O 4 and NiCo 2 O 4 /rGO scraped from Ni foam; SEM images of NiCo 2 O 4 (b and e) and NiCo 2 O 4 /rGO (c and f) electrode.To further demonstrate the nanowire arrays grown on Ni foam are NiCo 2 O 4 and NiCo 2 O 4 /rGO, the materials were subjected to TEM tests by analyzing the HR-TEM images (Figures 3(a) and 3(d)), lattice spacing (Figures 3(b) and 3(e)), and SAED (Figures 3(c) and 3(f)).Figures 3(a) and 3(d) clearly show that both NiCo 2 O 4 and NiCo 2 O 4 /rGO are long linear, where NiCo 2 O 4 is about 500 nm long while NiCo 2 O 4 /rGO is about 100 nm long, which is also attributed to the incorporation of graphene oxide.The lattice spacing measured in Figure 3(b) is 2.86 nm, correlating to the crystal plane (220) of NiCo 2 O 4 , and the diffraction rings in the diffractogram (Figure 3(c)) correspond to the (220), (311), (400), and (440) crystallographic planes, respectively.Similarly, the lattice spacing measured in Figure 3(e) is 1.17 nm, correlating to the crystal plane (444) of NiCo 2 O 4 , and the diffraction rings in the diffractogram

Figure 3 .
Figure 3. (a and b) TEM images of NiCo 2 O 4 nanowires; (c) SAED pattern of an individual NiCo 2 O 4 nanowire; (d and e) TEM images of NiCo 2 O 4 /rGO nanowires; (f) SAED pattern of an individual NiCo 2 O 4 /rGO nanowire.To further explore which NiCo 2 O 4 and NiCo 2 O 4 /rGO nanoarray materials have the best capacitance properties as supercapacitor electrode materials, electrochemical tests were performed using 3 M KOH as the electrolyte, and Ni foams grown with the two materials were directly used as the integrated electrodes.Figures 4(a) and 4(d) show the cyclic voltammograms of NiCo 2 O 4 and NiCo 2 O 4 /rGO electrodes at various scan rates, respectively.Each voltammogram exhibits typical pseudocapacitance characteristics.The specific capacitances of NiCo 2 O 4 and NiCo 2 O 4 /rGO electrodes at 1 Aꞏg -1 can be obtained by calculating the area of the GCD (Figures 4(b) and 4(f)) curves for NiCo 2 O 4 and NiCo 2 O 4 /rGO electrodes as 1351 Fꞏg -1 and 2311 Fꞏg -1 , respectively.Figures 4(c) and 4(g) show the plots of the multiplicity curves of NiCo 2 O 4 and NiCo 2 O 4 /rGO electrodes.The curve variation shows that the capacitance of the NiCo 2 O 4 /rGO electrodes changes less with the change in current density.Another factor to consider when evaluating the performance of electrode materials in supercapacitors that undergo frequent charging and discharging is their ability to sustain optimal performance over extended periods of operation.The capacity retention rate of the NiCo 2 O 4 /rGO electrode is 83.5% after 3000 cycles (Figure4(d)) at 10 Aꞏg -1 .In comparison, the capacitance retention rate of the NiCo 2 O 4 electrode is only 83% after 1000 cycles (Figure4(h)) under the same conditions.By comparison, it can be seen that under the same conditions, the NiCo 2 O 4 /rGO electrode with graphene oxide added exhibits higher specific capacitance, higher capacitance retention, and higher stability.This is because the addition of oxidized graphene enhances the dispersion of materials on the electrode surface, reducing the accumulation between materials.Consequently, the outermost layer of the Ni foam has almost no particles agglomerated together, which allows it to have full contact with the electrolyte.

Figure 4
Figure 4 CV and GCD curves of NiCo 2 O 4 (a and b) and NiCo 2 O 4 /rGO (e and f) electrode.Specific Capacitance and of Cyclic stability curves NiCo 2 O 4 (c and d) and NiCo 2 O 4 /rGO (g and h) electrode.