Effect of rGO Coating on Interconnected Co3O4 Nanosheets and Improved Supercapacitive Behavior of Co3O4/rGO/NF Architecture

In this study, the effect of reduced graphene oxide (rGO) on interconnected Co3O4 nanosheets and the improved supercapacitive behaviors is reported. By optimizing the experimental parameters, we achieved a specific capacitance of ~1016.4 F g−1 for the Co3O4/rGO/NF (nickel foam) system at a current density of 1 A g−1. However, the Co3O4/NF structure without rGO only delivers a specific capacitance of ~520.0 F g−1 at the same current density. The stability test demonstrates that Co3O4/rGO/NF retains ~95.5% of the initial capacitance value even after 3000 charge–discharge cycles at a high current density of 7 A g−1. Further investigation reveals that capacitance improvement for the Co3O4/rGO/NF structure is mainly because of a higher specific surface area (~87.8 m2 g−1) and a more optimal mesoporous size (4–15 nm) compared to the corresponding values of 67.1 m2 g−1 and 6–25 nm, respectively, for the Co3O4/NF structure. rGO and the thinner Co3O4 nanosheets benefit from the strain relaxation during the charge and discharge processes, improving the cycling stability of Co3O4/rGO/NF.


Introduction
In recent years, extensive efforts have been dedicated to research related to supercapacitors owing to their higher power densities, longer cycling performance than Li-ion batteries, and larger energy densities than conventional dielectric capacitors [1][2][3]. Supercapacitors have a huge potential in applications requiring high-density power and long cycling lifetime such as electric vehicles and portable electronics [4].
Co 3 O 4 , an important supercapacitor material, has the advantages of high theoretical capacitance (*3560 F g -1 ) [5], low cost, environmental friendliness, and high chemical stability in alkaline electrolytes. It has, thus, attracted much attention recently. Geng et al. [6] prepared porous Co 3 O 4 nanoplates with a specific capacitance of *231 F g -1 at a current density of 1 A g -1 using a facile reflux method. Naveen et al. [7] synthesized Co 3 O 4 /graphene nanosheets by a chemical method and a high specific capacitance of *650 F g -1 at a scan rate of 5 mV s -1 . However, there still remain some challenges in the practical applications of Co 3 O 4 as a high capacity electrode such as poor conductivity and cycling stability, and relatively lower experimental specific capacitances than the theoretical value.
As one kind of nanostructured carbon material, reduced graphene oxide (rGO) has been extensively investigated because of its superior mechanical and electronic properties, high specific surface area, and reasonable chemical stability [8]. These properties make graphene a preferred material for use in supercapacitors and Li-ion batteries as electrode materials and/or active material supporters [9][10][11][12][13][14][15][16]. Therefore, supercapacitors combining nanostructured Co 3 O 4 and rGO can be expected to deliver high power and energy densities and long cycling lifetime [15,17].
In this study, interconnected Co 3 O 4 nanosheets anchored onto rGO-coated nickel foam (NF) are facilely synthesized using a green, simple, and low-cost approach. The effect of rGO on the microstructure and improved supercapacitive behaviors of Co 3 O 4 nanosheets are investigated here. Because of the large specific area of *87.8 m 2 g -1 and a more optimal mesopore size distribution of *4-15 nm, the Co 3 O 4 /rGO/NF architecture delivers higher specific capacitances of *1016.4 and 767.1 F g -1 at current densities of 1 and 5 A g -1 , respectively. In comparison, the Co 3 O 4 /NF structure has a relatively smaller specific area of *67.1 m 2 g -1 and a less optimal mesopore size distribution of *6-25 nm, resulting in lower specific capacitances of *520.0 and 485.8 F g -1 at current densities of 1 and 5 A g -1 , respectively. Moreover, Co 3 O 4 /rGO/NF has excellent stability, with *95.5% capacity retention at a high current density of 7 A g -1 even after 3000 cycles. This can be attributed to the thinned Co 3 O 4 nanosheets and the presence of rGO, improving the electrical and mechanical properties of the Co 3 O 4 /rGO/NF system. For Co 3 O 4 /NF, the capacity retention after 3000 cycles at the corresponding current density is about 84.4%.

Experimental
2.1 Preparation of the Co 3 O 4 /rGO/NF Architecture Figure 1 shows a schematic illustration of the preparation procedure of the Co 3 O 4 /rGO/NF architecture. First, the hydrothermal reduction process is used to prepare rGOcoated NF. After sequential ultrasonic cleaning in acetone, ethanol, and deionized (DI) water each for 10 min, the NFs are dried in air. GO aqueous solution (1.0 mg mL -1 ) is prepared by dispersing GO, which is prepared by the modified Hummers method [18], into DI water under ultrasonication for 30 min. Then, the GO aqueous solution (10 ml, 1.0 mg mL -1 ), ascorbic acid (L-AA, 0.02 g), and cleaned NFs of size 2 9 3 cm 2 are placed into a beaker, which is then heated up to 95°C and kept for 5.0 h for GO reduction and rGO coating onto the NFs. After the beaker is cooled down to room temperature, the products are taken out and dried at 50°C for 3 h [19].
Then, Co(OH) 2 nanosheets are electrochemically deposited at 70°C in a three-electrode cell using rGOcoated NF as the working electrode, a platinum mesh (surface area: 2 9 2 cm 2 ) as the counter electrode, and Ag/ AgCl (sat. KCl) as the reference electrode. The aqueous electrolyte consists of 0.02 M Co(NO 3 ) 2 Á6H 2 O and 0.2 M NH 4 Cl. The electrodeposition potential is set at -3.0 V. After 500 s of electrodeposition, the resultant green foam is carefully washed using ethanol and DI water several times and finally dried in air. Then, the product is calcined at 250°C for 2 h in a quartz tube to change Co(OH) 2 into interconnected Co 3 O 4 nanosheets [20]. A reference structure of interconnected Co 3 O 4 nanosheets anchored onto NF is prepared following a similar process but without the rGO coating procedure, to determine the role of rGO.
Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) tests are conducted using an electrochemical workstation having a three-electrode configuration. Co 3 O 4 /rGO/NF or Co 3 O 4 /NF is employed as the working electrode using a 1 M KOH electrolyte. The applied potential window of CV measurements ranges from 0.0 to 0.6 V. GCD is performed at a constant current over a fixed potential range of 0.0-0.5 V. The capacitance retention test is performed from 0.0 to 0.5 V at a constant current density of 7 A g -1 for 3000 cycles. EIS measurement is performed under an AC voltage with a 5 mV amplitude over a frequency range of 0.01-100 kHz. The specific capacitance C s (F g -1 ) is calculated from the GCD data using C s = (I4t)/(m4V), where I is the charge-discharge current, Dt is the discharge time, m is the mass of Co 3 O 4 , and DV is the potential change. Figure 2a shows the XRD pattern of the Co(OH) 2 /rGO/ NF structure. The peaks presented in the spectrum match well with those in the standard crystallographic spectrum of layered a-Co(OH) 2 (JCPDS 46-0605) [21], except for the peaks at 44.5°and 51.8°attributed to the NF substrate (JCPDS 87-0712). Figure 2b exhibits the XRD spectra of the products of Co(OH) 2 /rGO/NF and Co(OH) 2 /NF after calcination at 250°C for 2 h, and the XRD pattern of the rGO/NF structure serves as a reference. It is evident that except for the diffraction peaks from NF, all the other peaks are consistent with the (220), (311), (511), and (440) planes in the standard Co 3 O 4 pattern (JCPDS 42-1467) [22]. It is worth noting that there are no obvious XRD signals of rGO, possibly because of the low mass loading or destruction of regular stacks of rGO [23]. Figure 2c shows the micro-Raman spectra of rGO/NF, Co 3 O 4 /NF, and Co 3 O 4 /rGO/NF composites. Two bands located around 1348.6 and 1609.5 cm -1 are assigned to the D and G band of rGO, respectively, indicating that GO has been successfully reduced [24]. For Co 3 O 4 -containing samples, five peaks of the crystalline Co 3 O 4 phase corresponding to the A 1g (691.5 cm -1 ), F 2g (617.7 cm -1 ), F 2g (522.8 cm -1 ), E g (482.7 cm -1 ), and F 2g (193.6 cm -1 ) modes are evident [25], and no obvious characteristic peaks related to Co(OH) 2 can be observed. Accordingly, both XRD and Raman spectra indicate that Co(OH) 2 is completely changed to Co 3 O 4 after the aforementioned thermal treatment.

Results and Discussion
SEM images of rGO/NF, Co 3 O 4 /NF, and Co 3 O 4 /rGO/ NF are shown in Fig. 3. It is obvious that the rGO coating onto NF is relatively uniform during the hydrothermal reducing process (see Fig. 3a). Figure 3b, c exhibits the top-view SEM images of Co 3 O 4 /NF and Co 3 O 4 /rGO/NF, respectively. Interconnected Co 3 O 4 nanosheets can be synthesized on both NF and rGO-coated NF. This indicates that the growth of Co 3 O 4 nanosheets is dependent more on the growth condition [26] rather than on the substrates. However, compared to the Co 3 O 4 nanosheets on NF, the nanosheets on rGO-coated NF have a higher density and lower dimension because of the roughened NF surface due to rGO. Moreover, the corresponding high-magnification SEM images shown in the inset in Fig. 3b, c clearly show that the Co 3 O 4 nanosheets on rGO-coated NF are thinner than the ones on NF. Accordingly, compared to Co 3 O 4 /NF, a higher specific surface area can be expected for Co 3 O 4 / rGO/NF. BET measurement is conducted to examine this hypothesis. Figure 3d shows [27,28]. The polycrystalline structure of the obtained Co 3 O 4 nanosheets is further confirmed by the selected area electron diffraction (SAED) pattern (see Fig. 4c). The homocentric diffraction rings (from the inside to the outside) can be assigned to the (220), (311), (400), (511), and (440) planes of Co 3 O 4 [29]. The elemental distribution in the Co 3 O 4 nanosheets is also characterized by scanning transmission electron microscopy (STEM) (see Fig. 4d) and X-ray elemental mapping images (see Fig. 4e-g). The elemental mapping further indicates that Co 3 O 4 nanosheets were synthesized successfully. Figure 5a presents the CV data for NF, rGO/NF, Co 3 O 4 / NF, and Co 3 O 4 /rGO/NF under a scanning rate of 3 mV s -1 and a potential range of 0.0-0.6 V. It is remarkable that the CV area of Co 3 O 4 /rGO/NF is evidently larger than that of Co 3 O 4 /NF owing to the high specific area of Co 3 O 4 /rGO/ NF as mentioned above. The redox peaks for the Co 3 O 4containing samples originate from the conversion of different cobalt oxidation states. Here, it is worth noting that two redox peaks also emerge for NF and rGO/NF, which are attributed to the oxidized surface of NF during heat treatment of Co(OH) 2 . However, the contribution of the surface nickel oxide and rGO is not taken into consideration in the following study because of their negligible capacities compared to the Co 3 O 4 -containing samples. This is verified by the calculated areal specific capacitances of 84.0, 120.7, 627.3, and 1107.5 mF cm -2 for NF, rGO/NF, Co 3 O 4 /NF, and Co 3 O 4 /rGO/NF, respectively, as exhibited in Fig. 5b. The GCD results of Co 3 O 4 / NF and Co 3 O 4 /rGO/NF are exhibited in Fig. 5c, d with current densities of 1, 3, 5, 7, 9, and 10 A g -1 and a potential window of 0.0-0.5 V. For each charge/discharge current density, Co 3   and Co 3 O 4 /NF. This is because of the insufficient supply of active material [31] and severe polarization [32] at higher current densities. Hence, how to improve the supercapacitive behaviors from these two aspects will be the main consideration in a future study.
To evaluate the cycling performance, the electrochemical stability of Co 3 O 4 /rGO/NF and Co 3 O 4 /rGO is tested using the GCD technique. The cycling performance of Co 3 O 4 / rGO/NF at a current density of 7 A g -1 is recorded over the potential range of 0.0-0.5 V (see Fig. 5e). For the Co 3 O 4 / rGO/NF structure, *95.5% of the initial specific capacitance can be retained after 3000 cycles even at such a high current density compared to other related reports [7,33]. However, *84.4% of the initial specific capacitance is retained after the same cycling period for Co 3 O 4 /rGO. To understand this excellent cycling stability, SEM characterization is performed on the Co 3 O 4 /rGO/NF sample after 3000 cycles and is shown in the inset of Fig. 5e. It is obvious that even after 3000 cycles, the microstructure is maintained very well, showing no evident change compared to the microstructures before cycling (see Fig. 3c). It is noted that during the first cycle, there is a significant increase in the specific capacitance. Previous reports mentioned this phenomenon as an activation process [34]. However, such a strong 'activation' has been rarely reported and needs to be further investigated to reveal the underlying mechanism.
EIS is used to further understand the electrical properties of the related material/structural system. Figure 5f shows the Nyquist plots of Co 3 O 4 /rGO/NF and Co 3 O 4 /NF. The linear portion of the Nyquist plots in the low-frequency region corresponds to the Warburg impedance, which is related to electrolyte diffusion into the pores in the electrodes. If the impedance plot increases sharply and tends to become a vertical line, it indicates a pure capacitive behavior. In the high-frequency region, the Z'-intercept represents the equivalent series resistance (ESR) including the ionic resistance of the electrolyte, intrinsic resistance of the substrate, and contact resistance at the interface of the active material and current collector [35]. The ESR of Co 3 O 4 /rGO/ NF and Co 3 O 4 /NF is *1.18 and 1.22 X, respectively (see the inset in Fig. 5f), indicating a lower solution resistance and Faradaic resistance for the Co 3 O 4 /rGO/NF architecture. In this study, the improved performance of Co 3 O 4 /rGO/NF is mainly attributed to the introduction of rGO, which not only provides effective electrolyte accessible channels, thus shortening the ion diffusion distance, but also improves the specific surface area of Co 3 O 4 and optimizes the mesopore size distribution for facilitating an enhancement in the capacitance performance.

Conclusions
This work reports the effect of rGO on the interconnected Co 3   microstructures of the interconnected Co 3 O 4 nanosheets, including an increased specific surface area and a more optimal mesopore size distribution, which result in the specific capacitance of the Co 3 O 4 /rGO/NF architecture being higher that that of the Co 3 O 4 /NF structure. Further, the Co 3 O 4 /rGO/NF structure possesses excellent cycling stability owing to the improved mechanical and electrical properties associated with the thinned Co 3 O 4 nanosheets and incorporation of rGO.