Enhanced Activity of Hierarchical Nanostructural Birnessite-MnO2-Based Materials Deposited onto Nickel Foam for Efficient Supercapacitor Electrodes

Hierarchical porous birnessite-MnO2-based nanostructure composite materials were prepared on a nickel foam substrate by a successive ionic layer adsorption and reaction method (SILAR). Following composition with reduced graphene oxide (rGO) and multiwall carbon nanotubes (MWCNTs), the as-obtained MnO2, MnO2/rGO and MnO2/rGO-MWCNT materials exhibited pore size distributions of 2–8 nm, 5–15 nm and 2–75 nm, respectively. For the MnO2/rGO-MWCNT material in particular, the addition of MWCNT and rGO enhanced the superb distribution of micropores, mesopores and macropores and greatly improved the electrochemical performance. The as-obtained MnO2/rGO-MWCNT/NF electrode showed a specific capacitance that reached as high as 416 F·g−1 at 1 A·g−1 in 1 M Na2SO4 aqueous electrolyte and also an excellent rate capability and high cycling stability, with a capacitance retention of 85.6% after 10,000 cycles. Electrochemical impedance spectroscopy (EIS) analyses showed a low resistance charge transfer resistance for the as-prepared MnO2/rGO-MWCNT/NF nanostructures. Therefore, MnO2/rGO-MWCNT/NF composites were successfully synthesized and displayed enhanced electrochemical performance as potential electrode materials for supercapacitors.


Introduction
Recently, supercapacitors (SCs) have attracted widespread attention and research due to their advantages, such as high specific capacitance, high power density, good cycle life, and low maintenance cost [1][2][3]; however, the storage energy density for such devices is not as good as that found for lithium-ion batteries, which, thus, limits applications. In addition, the pore size, specific surface area, surface functional groups, and conductivity for an electrode active material can also affect the specific capacitance of a supercapacitor [4,5]. Therefore, improving nanostructure electrode materials with high specific capacitance to increase their specific capacitance and energy density is a focus of research for energy storage devices [6][7][8].
Manganese dioxide (MnO 2 ) has the advantages of low cost, high environmental safety and high theoretical capacitance (1380 F·g −1 ) in aqueous electrolytes and is considered to be one of the most composite electrode materials on nickel foam (NF), such as MnO 2 /NF, MnO 2 /rGO/NF and MnO 2 /rGO-MWCNT/NF composite cathode materials to improve the agglomeration for such electrode materials. The specific surface area, pore size and distribution for the electrode materials were studied, and mixtures of rGO and MWCNT with different weight ratios were studied to investigate the electrochemical properties of the composite electrodes.

Preparation of MnO 2 -Based/NF Electrode
A graphene sheet weighing 0.25 g was weighed and added into 50 mL DI water. After ultrasonic mixing for 1 h, ammonia water was used to adjust the pH at approximately 11. Then, the as-prepared GO suspension was put in an autoclave and kept at 180 • C for 12 h. After cooling, the suspension solution was turned into rGO. Finally, it was dried in a freeze dryer for 24 h. The properties of the as-obtained materials were characterized (Supplementary Materials, Figure S1).
The nickel foam (1 cm × 1 cm) was washed with acetone, ultrasonic vibration, and deionized water, and then dried. Then, 0.5 mg mL −1 of safranin was mixed in DI water with an initial mixture of rGO and MWCNT at a concentration of 0.6 mg·mL −1 in DI water. The weight ratio for rGO and MWCNT was 1:1. The mixture was then subjected to an ultrasonic vibrator for 30 min to form a uniform suspension rGO-MWCNT ink.
The cleaned NF was immersed in reduced graphene oxide (rGO), MWCNT or dispersed rGO-MWCNT ink, and then rinsed with deionized water for 40 s to obtain rGO, MWCNT or rGO-MWCNT adhered to the NF substrate to prepare rGO/NF, MWCNT/NF or rGO-MWCNT/NF, respectively. SILAR technology was used to coat the film onto the substrate. NF, rGO/NF, MWCNT/NF or rGO-MWCNT/NF was respectively placed into Mn 2+ solution (0.01 M MnSO 4 solution) and MnO 4 solution (0.01 M KMnO 4 solution), and then rinsed and air-dried. Mn 2+ was oxidized and deposited layer by layer, which then underwent a redox reaction with MnO 4 − . Through the SILAR process, MnO 2 , MnO 2 /rGO, MnO 2 /rGO-MWCNT was attached to the Ni substrate, respectively. This procedure was repeated five times. Finally, the as-deposited electrode was annealed at 200 • C for 1 h. Figure 1 shows a schematic for the preparation of MnO 2 /rGO-MWCNT/NF material via the successive ion layer adsorption and reaction method.
rGO-MWCNT/NF was respectively placed into Mn 2+ solution (0.01 M MnSO4 solution) and MnO4solution (0.01 M KMnO4 solution), and then rinsed and air-dried. Mn 2+ was oxidized and deposited layer by layer, which then underwent a redox reaction with MnO4 − . Through the SILAR process, MnO2, MnO2/rGO, MnO2/rGO-MWCNT was attached to the Ni substrate, respectively. This procedure was repeated five times. Finally, the as-deposited electrode was annealed at 200 °C for 1 h. Figure 1 shows a schematic for the preparation of MnO2/rGO-MWCNT/NF material via the successive ion layer adsorption and reaction method.

Electrochemical Characterization of the As-Obtained MnO 2 -Based/NF Electrodes
All the electrochemical properties were investigated using a conventional three-electrode electrochemical cell equipped with the as-prepared MnO 2 -based/NF electrode (1.0 cm × 1.0 cm) as the working electrode, a platinum plate (1.0 cm × 1.0 cm) as the counter electrode, and an Ag/AgCl as the reference electrode, and a 1.0 M Na 2 SO 4 aqueous solution as the electrolyte.
The specific capacitance can be evaluated by the CV test using the following Formula (1): where Q is the area of the CV curve, V is the scan rate (V·s −1 ), m is the mass of the electrode active material (g), and ∆U is the voltage range (V). The specific capacitance was obtained according to the discharge curve of the GCD test by using the Formula (2): where i (A) is the discharge current, ∆t (s) is the discharge time, ∆V(V) is the discharging potential difference, and m (g) is the mass of the loaded active material (MnO 2 ). Electrochemical impedance spectra (EIS) were measured at open-circuit voltage, with a bias of 10 mV for frequencies ranging from 100 kHz to 0.01 Hz, to analyze the electron transport properties.

Characterization of the MnO 2 -Based Electrode Materials
The deposition of thin films was done by the following the steps to grow nucleation sites, and then ions reacted to produce the as-deposited material on the substrate, in the SILAR process. In this Nanomaterials 2020, 10, 1933 5 of 16 case, Mn 2+ from manganese sulfate was adsorbed on the nickel foam and reacted with MnO 4 − from potassium permanganate to produce MnO 2 .
In the preliminary experiment, MnO 2 prepared under different manganese ion concentrations was characterized by XRD, BET, CV and GCD analyses. The results showed that δ-MnO 2 crystals were obtained at a lower concentration of MnSO 4 (0.01 M), and γ-MnO 2 crystals were prepared at concentrations of MnSO 4 higher than 0.05 M. Because δ-MnO 2 crystal possessed a larger BET specific surface area and better pore properties, it exhibited better electrochemical characteristics (Supplementary Materials, Figure S2), therefore, preparation of δ-MnO 2 was conducted on 0.01 M MnSO 4 in this study. Figure 2 shows the XRD analysis for the MnO 2 -based electrode materials prepared by the SILAR process. The results show that a weak diffraction peak can be mainly attributed to the prepared sample, which indicates low crystallinity. Nevertheless, it can still be clearly found that the diffraction peaks at 2θ values of 37.2 • and 66.8 • were due to the (111) and (020) crystal planes of the birnessite-type MnO 2 (JCPDS 18-0802, Joint Committee on Powder Diffraction Standards (JCPDs), Newtown Square, PA, USA) [27] and that the peak width of the peak intensity was not strong. It is revealed that the MnO 2 material prepared by the SILAR method was not highly crystalline and that the crystallinity was not perfect. A clear diffraction peak was observed at a 2θ value of 22 • for the as-deposited MnO 2 /rGO material, showing the presence of rGO [28]; the small peak on the right of 22 • , indicates that a small amount of unreacted graphite carbon existed in the GO. According to XRD analysis for the as-deposited MnO 2 /rGO-MWCNT/NF electrode material, the diffraction peak observed at a 2θ value of approximately 22 • was characteristic of rGO; the peak observed at a 2θ value of approximately 25 • was the characteristic peak for the MWCNTs. By using Bragg's Law, nλ = 2d sinθ, the interplanar spacing for rGO and MWCNT was calculated to be 0.40 nm and 0.36 nm, respectively. Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 16 the MnO2 material prepared by the SILAR method was not highly crystalline and that the crystallinity was not perfect. A clear diffraction peak was observed at a 2θ value of 22° for the as-deposited MnO2/rGO material, showing the presence of rGO [28]; the small peak on the right of 22°, indicates that a small amount of unreacted graphite carbon existed in the GO. According to XRD analysis for the as-deposited MnO2/rGO-MWCNT/NF electrode material, the diffraction peak observed at a 2θ value of approximately 22° was characteristic of rGO; the peak observed at a 2θ value of approximately 25° was the characteristic peak for the MWCNTs. By using Bragg's Law, nλ = 2d sinθ, the interplanar spacing for rGO and MWCNT was calculated to be 0.40 nm and 0.36 nm, respectively. Birnessite MnO2 has a two-dimensional (2D) layered structure, consisting of MnO6 octahedrons shared with edges. The spacing of this layered structure is approximately 7 Å and it is located in the middle layer area, which is considered to be favorable for the transport of metal ions or water [29]. The specific capacitance of the relatively open layered structure of δ-MnO2 is much higher than that of β-MnO2 and γ-MnO2 [30]. A detailed comparison of the XRD diffraction peaks for the three different electrode materials composed of MnO2 (such as the two diffraction peaks at 2θ values of 37.2° and 66.8°) shows that the diffraction peak due to the MnO2/rGO-MWCNT/NF electrode material Birnessite MnO 2 has a two-dimensional (2D) layered structure, consisting of MnO 6 octahedrons shared with edges. The spacing of this layered structure is approximately 7 Å and it is located in the middle layer area, which is considered to be favorable for the transport of metal ions or water [29]. The specific capacitance of the relatively open layered structure of δ-MnO 2 is much higher than that of β-MnO 2 and γ-MnO 2 [30]. A detailed comparison of the XRD diffraction peaks for the three different electrode materials composed of MnO 2 (such as the two diffraction peaks at 2θ values of 37.2 • and 66.8 • ) shows that the diffraction peak due to the MnO 2 /rGO-MWCNT/NF electrode material broadened and shifted to a low diffraction angle; it can be presumed that the addition of MWCNT widened the crystal planes in the prepared electrode structure (such as the (111) and (020) crystal planes).
TEM was used to study the morphologies of the MnO 2 -based materials. Figure 3 shows the TEM analysis for the as-obtained MnO 2 , MnO 2 /rGO and MnO 2 /rGO-MWCNT materials. As shown in Figure 3a,b, it can be clearly seen that the prepared MnO 2 layered structure was loosely arranged. In addition, the selected region electron diffraction image shows an indistinct electron diffraction ring (upper right corner of Figure 3a), indicating that the prepared MnO 2 had poor crystallinity.  TEM analysis of the MnO2/rGO/NF nanocomposite electrode is shown in Figure 3c,d. Wrinklelike features for graphene can be observed. Because the MnO2/rGO/NF was prepared by the SILAR process, the graphene could restack, resulting in excessive surface agglomeration, so it was difficult to observe the regular structure of MnO2.
Figure 3e,f shows the TEM analysis for the prepared MnO2/rGO-MWCNT/NF nanocomposite electrode. It indicated that the surface of the film showed 3D flowerlike structures staggered with each other (Supplementary Materials, Figure S3). The flowerlike hierarchical nanostructure maintains good integrity because the CNTs prevent rGO from restacking; therefore, the prepared MnO2/rGO-MWCNT/NF materials have more uniform dispersion and homogeneity [31]. Following examination by high-resolution TEM, the lattice patterns for rGO and MWCNT were observed, which showed the interplanar spacing for rGO and MWCNT to be approximately 0.40 nm and 0.35 nm, respectively. TEM analysis of the MnO 2 /rGO/NF nanocomposite electrode is shown in Figure 3c,d. Wrinklelike features for graphene can be observed. Because the MnO 2 /rGO/NF was prepared by the SILAR process, the graphene could restack, resulting in excessive surface agglomeration, so it was difficult to observe the regular structure of MnO 2 .
Figure 3e,f shows the TEM analysis for the prepared MnO 2 /rGO-MWCNT/NF nanocomposite electrode. It indicated that the surface of the film showed 3D flowerlike structures staggered with each other (Supplementary Materials, Figure S3). The flowerlike hierarchical nanostructure maintains good integrity because the CNTs prevent rGO from restacking; therefore, the prepared MnO 2 /rGO-MWCNT/NF materials have more uniform dispersion and homogeneity [31]. Following examination by high-resolution TEM, the lattice patterns for rGO and MWCNT were observed, which showed the interplanar spacing for rGO and MWCNT to be approximately 0.40 nm and 0.35 nm, respectively. This result is very consistent with the XRD results discussed in the previous section.
Furthermore, it was also found that the MWCNTs attach to the inside of MnO 2 , which indicates structural stability for the MnO 2 /rGO-MWCNT/NF nanocomposite electrode. Such an electrode structure can shorten the ion/electron transport length between the electrode and the electrolyte and increase the contact surface area. The 3D hierarchical flowerlike MnO 2 /rGO-MWCNT/NF nanocomposite electrode can be expected to show an excellent electrochemical performance. The energy dispersive spectrum (EDS) (Figure 3g) indicates that the MnO 2 /rGO-MWCNT material is composed of Mn, Ni, O and C, elements, except for Cu from the supporting Cu grid. It exhibits that the atomic composition of Mn and C is 15.4 at. % and 38.9 at. %, respectively; means that the loading amount of MnO 2 in the MnO 2 /rGO-MWCNT material is approximately 74 wt. %. Furthermore, the Mn element in the electrode dispersed well in the hybrid material (Figure 3h,i), indicating that MnO 2 has good dispersibility in the rGO-MWCNT. BET specific surface area and pore size distribution analysis for MnO 2 , MnO 2 /rGO and MnO 2 /rGO-MWCNT materials scraped from the as-deposited electrodes is shown in Figure 4. Figure 4a shows the N 2 isotherm adsorption-desorption analysis for the MnO 2 -based materials. The results show a typical type IV isotherm. The sample has a clear triangular hysteresis loop and a steep absorption between 0.4-0.9 P/P o , showing highly interconnected holes, and the material has a narrow opening and a wide structure [32].  Figure 4. Figure 4a shows the N2 isotherm adsorption-desorption analysis for the MnO2-based materials. The results show a typical type IV isotherm. The sample has a clear triangular hysteresis loop and a steep absorption between 0.4-0.9 P/Po, showing highly interconnected holes, and the material has a narrow opening and a wide structure [32].  Table 1 shows the powder properties of MnO2, MnO2/rGO and MnO2/rGO-MWCNT materials. For the table, the specific surface area and pore size distribution were obtained by using the BET equation; the pore volume and the average pore diameter were obtained from the branch-curves for the adsorption isotherm according to the Barrett-Joyner-Halenda (BJH) equation. The results show that the BET specific surface areas for the MnO2, MnO2/MWCNT, MnO2/rGO, and MnO2/rGO-MWCNT materials were 155.7 m 2 ·g −1 , 102.6 m 2 ·g −1 , 132.8 m 2 ·g −1 , and 167.7 m 2 ·g −1 , respectively and that the BJH pore sizes were 4.6 nm, 5.6 nm, 6.7 nm, and 13.6 nm, respectively. MnO2/rGO-MWCNT showed the largest BET specific surface area, which is attributed to the loosely arranged structure; the MnO2/rGO specific surface area is small because graphene restacks.

Materials
Surface Area (m 2 ·g −1 ) Pore Volume (cm 3 ·g −1 ) Pore Size (nm) MnO2 155.7 0.4 4.6  Table 1 shows the powder properties of MnO 2 , MnO 2 /rGO and MnO 2 /rGO-MWCNT materials. For the table, the specific surface area and pore size distribution were obtained by using the BET equation; the pore volume and the average pore diameter were obtained from the branch-curves for the adsorption isotherm according to the Barrett-Joyner-Halenda (BJH) equation. The results show that the BET specific surface areas for the MnO 2 , MnO 2 /MWCNT, MnO 2 /rGO, and MnO 2 /rGO-MWCNT materials were 155.7 m 2 ·g −1 , 102.6 m 2 ·g −1 , 132.8 m 2 ·g −1 , and 167.7 m 2 ·g −1 , respectively and that the BJH pore sizes were 4.6 nm, 5.6 nm, 6.7 nm, and 13.6 nm, respectively. MnO 2 /rGO-MWCNT showed the largest BET specific surface area, which is attributed to the loosely arranged structure; the MnO 2 /rGO specific surface area is small because graphene restacks. In contrast, the MnO 2 /rGO exhibits the smallest specific surface area. It can be explained by combined with the results of Raman shifts (Supplementary Materials, Figure S1). It indicates that the relative intensity ratio of the D-band to G-band, defined as R = I D /I G , of the commercial GO sheet was found to be 0.86. This indicates that the GO showed the characteristic of high regularity and low randomness. Then, the as-obtained rGO by the hydrothermal method showed a highly stacked sp 2 structure because of the elimination of the oxygen-containing functional groups, causing an increase in the reduction of graphite and a shift in the G band to 1585.5 cm −1 .
Furthermore, the R-value of MnO 2 /rGO material increased after the SILAR process. That was presumably because some impurities and defects were introduced during the preparation. Moreover, the composited MnO 2 particles also damaged the graphene order. Additionally, in the preparation of MnO 2 /rGO material, rGO was sequentially immersed in the MnSO 4 and KMnO 4 solutions by the SILAR process. Due to the high ionic strength of the solutions, the graphene was restacked and more agglomerated, resulting in specific surface area decrease. Figure 4b shows the pore distribution analysis for MnO 2 , MnO 2 /rGO and MnO 2 /rGO-MWCNT materials. It can be seen from the figure that the pore distributions for the three different electrode materials were 2-8 nm, 5-15 nm and 2-75 nm, respectively. Due to the wide distribution of the MnO 2 /rGO-MWCNT material composition, it indicates the existence of a hierarchical porous structure. Especially for the MnO 2 /rGO-MWCNT material, the addition of MWCNT and rGO enhances the superb distribution of micropores, mesopores and macropores, and greatly improves the electrochemical performance.
The electrolyte ions' size and the porous structure of the electrode are the key factors for the specific capacitance. The small size of the pores confines the accessible pores for the ions from the electrolyte solution. Su et al. [33] indicated that it is very important to meet the balance between the pore size and the ion size of the electrolyte for supercapacitors. However, an electrode material with hierarchical pore structure and high ion-accessible surface area that is helpful to ion transportation, and leading to high capacitance. It has been previously indicated that an ideal electrode material should have a hierarchical porous structure consisting of large pores (greater than 50 nm) used as an ion buffer reservoir, mesopores (2-50 nm) used for ionic migration and micropores (less than 2 nm) used for charge storage [34].
Xu et al. [35] also studied the supercapacitive properties of MnO 2 electrode in Li 2 SO 4 , Na 2 SO 4 and K 2 SO 4 electrolyte, respectively. They indicated that the charging-discharging rate can be affected by the size of the cation, the size of the hydrated cation, the mobility of the cation, and the adsorption-desorption rate. The lithium ion (Li + ) and hydrated lithium ion (Li h + ) radius was 0.69 Å and 6 Å, respectively; sodium ion (Na + ) and hydrated sodium ion (Na h + ) radius was 1.02 Å and 4 Å, respectively, and potassium ion (K + ) and hydrated potassium ion (K h + ) radius was 1.38 Å and 3 Å, respectively. Therefore, Na + ion may possess a moderate diffusion rate in MnO 2 matrix, a moderate adsorption-desorption rate, and a moderate mobility in aqueous solutions, resulting in the large capacitance and fast charging/discharging rate. Thus, in the as-obtained MnO 2 /rGO-MWCNT material, MWCNT helps prevent rGO from restacking and increases the specific surface area. The material has a highly hierarchical porous structure, which provides effective transportation for electrons and ions. MnO 2 /rGO-MWCNT/NF as an electrode in Na 2 SO 4 electrolyte exhibits the higher specific capacitance as shown on Section 3.2 electrochemical properties.

Electrochemical Properties
These MnO 2 -based/NF materials were directly manufactured into working electrodes without using a binder to reveal their capacitance performance as shown in Figure 5. Figure 5a shows the analysis for the CV characteristics obtained for MnO 2 /NF, MnO 2 /MWCNT, MnO 2 /rGO/NF and MnO 2 /rGO-MWCNT/NF in a 1 M Na 2 SO 4 electrolyte at a scan rate of 5 mV s −1 , respectively. The figure indicates the different electrodes with quasi-rectangular and quasi-symmetric CV curves. Among them, the area enclosed by the CV curve for the MnO2/rGO-MWCNT/NF electrode is much larger than the curve areas for the MnO2/NF, MnO2/MWCNT/NF and MnO2/rGO/NF nanocomposite materials, showing that the MnO2/rGO-MWCNT/NF electrode had an extremely high specific capacitance.
In the CV curve, MnO2/NF, MnO2/MWCNT/NF and MnO2/rGO/NF shows obvious gradient patterns in the voltage ranges of 0.0 V to 0.1 V and 0.7 to 0.8 V, individually, while the curve for the MnO2/rGO-MWCNT/NF can be observed to show an almost vertical CV characteristic curve with a near rectangular and symmetrical shape, indicating that the conductivity of MnO2/rGO-MWCNT/NF was greatly improve by the homogeneous mixing of rGO and MWCNT with MnO2, demonstrating excellent electrochemical performance. In addition, the CV curve areas for MnO2/rGO-MWCNT/NF are much larger than the CV curve areas for the MnO2/rGO/NF, MnO2/MWCNT/NF and MnO2/NF composites, respectively. It is speculated that both MnO2 and MWCNT can be used as spacers to prevent aggregation of rGO, which is conducive towards obtaining a higher electric double layer capacitance for rGO. Furthermore, rGO and MWCNT act as electronic conduction channels to increase the conductivity of MnO2. This is proved by the EIS analysis below, which shows a low contact electrical resistance for the MnO2/rGO-MWCNT/NF electrode.
Na2SO4 was used as electrolyte in this study, and the MnO2-based/NF electrodes showed a quasirectangular shape in CV curves measured at different scan rates, indicating the capacitance characteristics for the MnO2 deposited onto the NF electrode [36]. The CV curve for the MnO2-based electrode in Na2SO4 electrolyte is unlike that expected from an electric double-layer capacitor (EDLC); In the CV curve, MnO 2 /NF, MnO 2 /MWCNT/NF and MnO 2 /rGO/NF shows obvious gradient patterns in the voltage ranges of 0.0 V to 0.1 V and 0.7 to 0.8 V, individually, while the curve for the MnO 2 /rGO-MWCNT/NF can be observed to show an almost vertical CV characteristic curve with a near rectangular and symmetrical shape, indicating that the conductivity of MnO 2 /rGO-MWCNT/NF was greatly improve by the homogeneous mixing of rGO and MWCNT with MnO 2 , demonstrating excellent electrochemical performance. In addition, the CV curve areas for MnO 2 /rGO-MWCNT/NF are much larger than the CV curve areas for the MnO 2 /rGO/NF, MnO 2 /MWCNT/NF and MnO 2 /NF composites, respectively. It is speculated that both MnO 2 and MWCNT can be used as spacers to prevent aggregation of rGO, which is conducive towards obtaining a higher electric double layer capacitance for rGO. Furthermore, rGO and MWCNT act as electronic conduction channels to increase the conductivity of MnO 2 . This is proved by the EIS analysis below, which shows a low contact electrical resistance for the MnO 2 /rGO-MWCNT/NF electrode. Na 2 SO 4 was used as electrolyte in this study, and the MnO 2 -based/NF electrodes showed a quasi-rectangular shape in CV curves measured at different scan rates, indicating the capacitance characteristics for the MnO 2 deposited onto the NF electrode [36]. The CV curve for the MnO 2 -based electrode in Na 2 SO 4 electrolyte is unlike that expected from an electric double-layer capacitor (EDLC); the CV characteristic curve for an EDLC shows a nearly ideal rectangle [37]. The quasirectangular shape observed for the CV curve is a characteristic of the reversible surface redox reaction of MnO 2 , the oxidation of Mn(III) to Mn(IV) and the reduction from Mn(IV) to Mn(III) [38]. Figure 5b shows GCD characterization for the as-obtained MnO 2 /NF, MnO 2 /MWCNT/NF, MnO 2 /rGO/NF and MnO 2 /rGO-MWCNT/NF electrodes at 1 A·g −1 . The longer discharge time of the MnO 2 /rGO-MWCNT/NF electrode indicates that its capacitance was higher than that of the MnO 2 /NF, MnO 2 /MWCNT/NF and MnO 2 /rGO/NF electrodes, which is consistent with the results obtained from CV tests. In particular, compared with the as-prepared electrodes, the MnO 2 /rGO-MWCNT/NF electrode exhibits highly linear and almost symmetrical charge and discharge curves, revealing that the IR potential drop for MnO 2 /rGO-MWCNT/NF is less noticeable. The as-obtained MnO 2 /rGO-MWCNT/NF electrode has a maximum specific capacitance of 416 F·g −1 at a low current density of 1 A·g −1 . Figure 5c further compares the relationship between the specific capacitance and current density determined from GCD examination. It can be found that, as the current density increases, the capacitance retention for the MnO 2 /rGO-MWCNT/NF electrode was better than that for the MnO 2 /NF, MnO 2 /MWCNT/NF and MnO 2 /rGO/NF electrodes. At a high current density of 10 F·g −1 , the specific capacitance of the MnO 2 /rGO-MWCNT/NF electrode remained at 250 F·g −1 , while the MnO 2 /NF, MnO 2 /MWCNT/NF and MnO 2 /rGO/NF electrodes showed a capacitance of 176 F·g −1 , 215 F·g −1 and 232 F·g −1 , respectively.
There was no oxidation peak/reduction peak in the CV curves and the GCD discharge curve of 0.0-0.2 V indicates a similar characterization of slope variation of the time for the as-deposited MnO 2 -based electrode as Na 2 SO 4 used for electrolyte (Figure 5a,b). This is due to the charge transfer reaction between MnO 2 and Na 2 SO 4 electrolyte, which is related to the pseudo-capacitance behavior [39].
Contrast to the MnO 2 electrode in KOH electrolyte, the oxidation peak/reduction peak appears in the CV curve, and the steep slope at the end of the GCD discharge curve [40]. It reported that redox reaction peaks were visible (in the CV curves), indicating that the process of energy storage was mainly associated with a pseudocapacitance mechanism and not the reaction between the Mn 4+ and OH − in the electrolyte [41]. The redox mechanism of MnO 2 in KOH electrolyte are reversible insertion/extraction of K + in MnO 2 as Formula (3) [42]: (3) Figure 6 shows the cycling charge-discharge test for the MnO 2 /NF, MnO 2 /rGO/NF and MnO 2 /rGO-MWCNT/NF electrodes in a 1 M Na 2 SO 4 electrolyte at a constant current of 4 A·g −1 . The results clearly show that the specific capacitance of the MnO 2 /rGO-MWCNT/NF electrode decreased at 10,000 cycles of charging and discharging; however, this decrease was smaller than that found for the MnO 2 /rGO/NF and MnO 2 /NF electrodes, respectively. The MnO 2 /NF, MnO 2 /MWCNT/NF, MnO 2 /rGO/NF and MnO 2 /rGO-MWCNT/NF electrodes exhibit a capacitance retention of 62.4% (from 194 to 121 F·g −1 ), 78.8% (from 201 to 158 F·g −1 ), 80.2% (from 223 to 179 F·g −1 ), and 85.6% (from 302 to 259 F·g −1 ), respectively. results clearly show that the specific capacitance of the MnO2/rGO-MWCNT/NF electrode decreased at 10,000 cycles of charging and discharging; however, this decrease was smaller than that found for the MnO2/rGO/NF and MnO2/NF electrodes, respectively. The MnO2/NF, MnO2/MWCNT/NF, MnO2/rGO/NF and MnO2/rGO-MWCNT/NF electrodes exhibit a capacitance retention of 62.4% (from 194 to 121 F·g −1 ), 78.8% (from 201 to 158 F·g −1 ), 80.2% (from 223 to 179 F·g −1 ), and 85.6% (from 302 to 259 F·g −1 ), respectively. In this study, SILAR technology was used to prepare a MnO 2 /rGO-MWCNT/NF electrode onto rGO-MWCNT composite coated foamed nickel substrates. The material was found to show excellent cycle stability, which verifies that the charge storage reaction of the supercapacitor is reversible and that the electroactive material is stably adsorbed onto the substrate (current collector). MnO 2 /rGO-MWCNT/NF can maintain high cyclic stability, which can be mainly attributed to the synergy effect between rGO, MWCNT and MnO 2 .
Kong et al. [43] indicated that in the use of graphene nanosheets (GNS) to produce electrode materials, the aggregation and restacking of GNS will hinder the migration of electrolyte ions onto the interface, resulting in a substantial decrease in electrochemical performance. When multiwall carbon nanotubes (MWCNTs) are introduced, the graphene layer can be dispersed and the diffusion coefficient for the ions in the material can be effectively improved. In addition, the synergy effect between GNS and MWCNT is conducive to increasing the contact area between the electrode material and the electrolyte, providing a rich electroactive site for pseudocapacitance; such a hierarchical porous structure can effectively shorten the Na + diffusion path. Sun et al. [44] studied a MnO 2 /rGO/Ni composite foam electrode exhibiting good supercapacitor performance. It was pointed out that this excellent performance was closely related to the inherent hierarchical nanostructured porous MnO 2 /rGO composite material grown onto the foamed Ni framework.
In this study, the SILAR process was used to apply layer-by-layer coating technology to prepare MnO 2 /rGO-MWCNT/NF electrodes. In addition to the aforementioned characteristics [43,44], SILAR is more capable of producing a high surface area material with a higher electrolyte diffusion rate; in addition, rGO-MnO 2 electrodes prepared using SILAR technology can be used to manufacture lightweight and ultrasmall supercapacitor devices. It can induce the material to be more uniformly dispersed, increasing the capacity to build MnO 2 /rGO-MWCNT/NF electrodes that demonstrate excellent cycle durability and excellent electrochemical performance.
To study the electrochemical mechanism for the MnO 2 -based composite electrode materials showing good supercapacitor properties, MnO 2 /NF, MnO 2 /MWCNT/NF, MnO 2 /rGO/NF and MnO 2 /rGO-CNT/NF electrodes were prepared and subjected to EIS analysis, as shown in Figure 7. EIS measurements were taken in the frequency range from 100 kHz to 0.01 Hz. The results were displayed using Nyquist plots, which are divided into three different regions: dispersed, increasing the capacity to build MnO2/rGO-MWCNT/NF electrodes that demonstrate excellent cycle durability and excellent electrochemical performance.
To study the electrochemical mechanism for the MnO2-based composite electrode materials showing good supercapacitor properties, MnO2/NF, MnO2/MWCNT/NF, MnO2/rGO/NF and MnO2/rGO-CNT/NF electrodes were prepared and subjected to EIS analysis, as shown in Figure 7. EIS measurements were taken in the frequency range from 100 kHz to 0.01 Hz. The results were displayed using Nyquist plots, which are divided into three different regions:  In the high frequency region, the intercept at the real axis (Z 0 ) represents the equivalent series resistance (ESR), including the ionic resistance of the electrolyte, the inherent resistance of the substrate, and the contact resistance of the active material/current collector interface [45]. The span of the semicircular arc in the mid-high frequency region represents the charge transfer resistance (R ct ) at the electrode/electrolyte interface, also known as the Faraday resistance [46,47]. In the low-frequency region, the impedance represents the diffusion resistance for the electrolyte ions in the holes of the electrode. If the impedance graph increases sharply and tends to become a vertical line, a characteristic of pure capacitance behavior is indicated [48].
As shown in Figure 7, at high frequencies, the intercepts (R E ) for the curve and real axis for the MnO 2 /rGO-MWCNT/NF composite electrode, MnO 2 /rGO/NF, MnO 2 /MWCNT/NF and MnO 2 /Ni electrodes were determined to be 1.5 Ω, 1.7 Ω, 1.9 Ω and 2.1 Ω, representing a good contact between the electrode and the electrolyte, respectively. Especially, the MnO 2 /rGO-MWCNT/NF composite electrode shows the smallest equivalent resistance, demonstrating that the electrode has better conductivity. In particular, the as-obtained MnO 2 /rGO-MWCNT/NF composite electrode shows a vanishing semicircular arc-shaped impedance in the high-medium frequency region, indicating that the charge transfer resistance (R ct ) for the electrode is extremely low and that the ion diffusion path is very short. This result has hardly been observed previously in the high-to-medium frequency region, which is similar to the study of Liu et al. [49].
In the low-frequency region, the MnO 2 /rGO-MWCNT/NF electrode shows a straight line with a steep slope, indicating that the capacitance performance is very close to that of an ideal supercapacitor [50]. Additionally, in this region, the slope of the impedance curve for the MnO 2 /rGO/NF electrode is not as steep as that for the other electrodes, which may be due to the relatively worse dispersion of the rGO in the MnO 2 /rGO/NF electrode.
In this study, the as-obtained MnO 2 /rGO-MWCNT/NF electrode exhibited an extremely low impedance, which is attributed to the high homogeneity and nanostructure of the hierarchical porous composites grown on the nickel foam. The addition of MWCNTs leads to high aggregation but high specific surface area rGO is easily dispersed, unclogging the electronic conductive channels, resulting in an extremely small (even difficult to observe) arc span for the MnO 2 /rGO-MWCNT/NF electrode. In addition, the porous hierarchical structural MnO 2 /rGO-MWCNT/NF electrode exhibits an equivalent series resistance (R E ) that is lower than that of the other electrodes. This result further shows that the composite MnO 2 /rGO-MWCNT/NF electrode has faster kinetics compared to the MnO 2 /rGO/NF, MnO 2 /MWCNT/NF and MnO 2 /NF composite electrode, which is beneficial towards improving the capacitance performance of the composite material, especially at high charge/discharge rates for the supercapacitor [51,52].
In addition, comparing the electrochemical properties of MnO 2 /MWCNT/NF and MnO 2 /rGO/NF electrodes, it is found that MnO 2 /rGO/NF exhibits relatively good specific capacitance; however, the capacitance retention of MnO 2 /MWCNT/NF electrode seems to be relatively stable. It is postulated that in the MnO 2 /rGO/NF electrode, the MnO 2 nanoparticles make the rGO exhibit relatively good exfoliations, which makes the MnO 2 deposition relatively dispersed, and the material has a higher specific surface area. The electrode possesses a better pore structure makes Na 2 SO 4 electrolyte easily adsorbed on the electrode, which facilitates migration and diffusion; resulting in a larger CV curve area and a higher specific capacitance value. In contrast, in MnO 2 /MWCNT/NF, it is possible that MnO 2 is relatively easy to firmly adhere to the wall of the MWCNT, and the electrode microstructure is relatively strong, therefore, the capacitance retention during the charge-discharge cycling is relatively stable. However, the detailed differences between MnO 2 /MWCNT/NF and MnO 2 /rGO/NF need to be studied more accurately. Table 2 shows a comparison of electrochemical performance of MnO 2 -based electrode in the literatures [6,[53][54][55][56]. Galvanostatic charge-discharge measurement results revealed that the composite with hybrid MnO 2 /rGO-CNT exhibited the specific capacitance of 416 F·g −1 at 1 A·g −1 in a 1 M Na 2 SO 4 electrolyte and an excellent capacitance retention of 85.6% at 10,000 charge-discharge cycles. The capacitance retention is quite higher than that of the previously studied electrode material. Combining the above results, the hybrid nanostructural rGO-MWCNT and MnO 2 material on nickel foam enables fast electron and ion transportation and further improves electrochemical performance. The addition of MWCNTs leads to high aggregation but also to the easy dispersion of rGO. The 3D network structure of the MnO 2 /rGO-MWCNT/NF electrode was found to exhibit an excellent pore distribution, which can facilitate passage of electrons, charge storage and electron transportation. Therefore, MnO 2 /rGO-MWCNT/NF composites were successfully synthesized, which display enhanced electrochemical performance as potential electrode materials for supercapacitors.

Conclusions
SILAR technology was used to construct 3D δ-MnO 2 -based/foamed nickel electrodes. A hierarchical porous MnO 2 nanocomposite electrode material was supported on rGO-coated and rGO-MWCNT-coated nickel foam by convenient and simple "immersion and drying", respectively. Because MWCNTs can effectively enable rGO to form a stable dispersion, they are beneficial for the uniform deposition of MnO 2 onto the substrate.
The synergetic combination of rGO-MWCNT and pseudocapacitance MnO 2 material onto nickel foam enables fast electron and ion transportation and further improves electrochemical performance. The as-prepared MnO 2 /rGO-MWCNT/NF electrode was found to exhibit extremely low impedance due to the high uniformity and nanostructured material properties of the porous composite material grown onto the nickel foam. The addition of MWCNTs leads to high aggregation but also to the easy dispersion of high specific surface area rGO. The 3D network structure of the MnO 2 /rGO-MWCNT/NF electrode was found to exhibit an excellent pore distribution, which can facilitate passage of electrons, charge storage and electron transportation.
The as-deposited MnO 2 /rGO-MWCNT/NF hierarchical porous nanostructural electrode exhibited a high specific capacitance of 416 F·g −1 at 1 A·g −1 in 1 M Na 2 SO 4 . After 10,000 charge and discharge cycles, the capacitance retention reached 85.6%. Therefore, this high-performance and convenient fabrication method provides excellent prospects for energy storage applications.

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