Development of Novel and Ultra-High-Performance Supercapacitor Based on a Four Layered Unique Structure

This paper presents an electrode with a core/shell geometry and a unique four-layered porous wrinkled surface for pseudocapacitive supercapacitor applications. To design the electrode, Ni foam was used as a substrate, where the harmonious features of four constituents, ZnO (Z), NiS (N), PEDOT:PSS (P), and MnO2 (M) improved the supercapacitor electrochemical performance by mitigating the drawbacks of each other component. Cyclic voltammetry and galvanostatic charge discharge measurements confirmed that the ZNPM hybrid electrode exhibited excellent capacitive properties in 2 M KOH compared to the ZNP, ZN, and solely Z electrodes. The ZNPM electrode showed superior electrochemical capacitive performance and improved electrical conductivity with a high specific capacitance of 2072.52 F g−1 at 5 mA, and a high energy density of 31 Wh kg−1 at a power density of 107 W kg−1. Overall, ZNPM is a promising combination electrode material that can be used in supercapacitors and other electrochemical energy conversion/storage devices.

SCs need to have high energy storage capacity. Therefore, to construct SC electrodes, conducting polymers, carbon allotropes, and transition metal oxides/hydroxides are generally chosen because of their high theoretical specific capacitance [17][18][19][20][21][22][23]. On the other hand, the materials have their advantage and disadvantages. Current applications require a large energy density and high specific capacitance. To fulfil this requirement, the most promising action is to mix pseudocapacitive materials with highly conductive materials [24][25][26][27][28].
Various metal oxides and metal sulfides have pseudocapacitive properties [29][30][31][32][33]. Among the many metal oxide/hydroxide electrodes, RuO 2 is the most well-known pseudocapacitive material with a specific capacitance as high as 1300 F g −1 [34][35][36]. Albeit, it is expensive, cannot be used in 90 • C for 1 h. After maturation of the seed layer, the samples were annealed at 65 • C in air for approximately 5 h. To grow the ZnO NRs, 0.015 M zinc nitrate hexahydrate (Zn(NO 3 ) 2 ·6H 2 O) and hexamethylenetetramine (HMT) (C 6 H 12 N 4 ) solutions were mixed at 95 • C for 15 h, and a seeded substrate was immersed into this aqueous solution. The samples were washed with deionized (DI) water and annealed at 65 • C for 5 h. The NF with the deposited ZnO NRs is the Z electrode.
To design the second electrode, NiS was deposited on the Z electrode, which is termed as the ZN electrode. To synthesize the NiS nanoparticles, the host precursor materials, such as 0.1 M Nickel(II) Chloride Hexahydrate (NiCl 2 ·6H 2 O) and 0.4 M thiourea, were dissolved in 40 mL of ethylene glycol. Subsequently, 0.2 M 3-MPA was added to the above mixture with stirring. The mixture was stirred for 60 min to complete the dissolution of the added chemicals. The mixture was then transferred to a Teflon lined autoclave and subjected to a hydrothermal treatment at 150 • C for 10 h. The autoclave was cooled naturally to room temperature and the as-obtained NiS material was filtered and washed two to three times with DI water and ethanol. The NiS was dried in a vacuum oven at 55 • C for 4 h. The resulting materials (NiS) were used to prepare the supercapacitor electrodes. To produce the ZN electrode, a second layer of NiS paste was applied to the Z electrode, which was then placed in a vacuum and dried for 4 h at 55 • C.
A third layer was coated on the ZN electrode to construct the ZNP electrode. ZN electrodes were immersed in a PEDOT:PSS solution for 10 min, which coated the ZN electrodes with polymer. Subsequently, the polymer coated electrode was kept in a vacuum for 12 h at 60 • C. MnO 2 is one of the most promising materials for supercapacitor practical applications. To utilize its pseudocapacitive properties, it was applied to the ZNP electrode. The ZNP was coated with MnO 2 nanoparticles. A 10 mM aqueous KMnO 4 solution was prepared and the ZNP electrodes were dipped in it. The electrodes were characterized using a variety of techniques. The specific capacitance and energy and power densities were calculated from the electrical measurements. ZNPM showed the highest specific capacitance of all of the electrode presented in this paper.

Characterization
The morphology was examined by scanning electron microscopy (SEM, Hitachi: SU-70, Tokyo, Japan) at an accelerating voltage of 5.0 kV and the elemental composition was analyzed by energy dispersive X-ray spectroscopy (EDX, KBSI, Busan, Korea). Transmission electron microscopy (TEM, Jeol Corporation, JEM 2011, Tokyo, Japan) images were recorded at the Busan KBSI centre at an accelerating 200 kV. The oxidation states of the elements in the four electrodes was examined by X-ray photoelectron spectroscopy (XPS, KBSI, Busan, Korea).
Cyclic voltammetry (CV) was conducted using a SP-150 Biologic science instrument three electrode cell; Ag/AgCl, Pt wire, and the four electrodes (Z, ZN, ZNP and ZNPM) were used as the reference, counter and working electrode, respectively, in a 2 M KOH electrolyte. The energy, power, and capacitance were calculated by galvanostatic charge-discharge (GCD) at different current densities. A Biologic potentiostat/galvanostat/EIS was used for electrochemical impedance spectroscopy (EIS, Paris, France).

Schematic Illustration
Firstly, ZnO NRs were prepared by facile CBD method. In the first step, ZnO seed layer was deposited on the Ni foam (Scheme 1a). Secondly, ZN electrode was fabricated by deposit NiS particles on the Z electrode (Scheme 1b). Thirdly, To coat the ZN electrode with polymer, the prepared electrode were dip-coated into PEDOT:PSS solution for 10 min. The polymer coated electrode was dried in a vacuum and resultantly a rough wrinkled surface (ZNP electrode, Scheme 1c). Lastly, a fourth layer of MnO 2 nanoparticle was applied on ZNP electrode by drop casted using 10 mM KMnO 4 , which is termed as ZNPM (Scheme 1d).   Figure 1(a1) shows the morphology of the wellaligned ZnO NRs (Z) and the resulting cone-like morphology formed densely on the substrates. Higher magnifications revealed the hexagonal end surface of the NRs and their length and diameter were approximately 1 to 3 μm, respectively.

Surface Morphology
On the other hand, SEM showed that the surface of the NRs was rough after NiS (ZN) deposition. The rough texture of the nanoscale NiS coatings on these NRs were observed (Figure 1b, Figure 1(b1)). NiS was well crystallized with uniform nanospheres, 700 to 800 nm in diameter. NF is inactive in the KOH electrolyte and has a three-dimensional structure that contributes to the high specific surface area and escalates the adhesion of the NiS film. The strong adhesion plays a key role in thin film deposition, which affects the capacitance. The good adhesion of the material depends on the good wettability of the bonded surfaces by the liquid adhesive [55]. When PEDOT:PSS was deposited, the structure and surface adhesion was enhanced slightly. Figure 1c,d presents SEM images of ZNP and ZNPM films, respectively. Figure 1c shows a compact and rough morphology for the PEDOT:PSS film on ZN that exhibits a wrinkled surface morphology and a typical porous network structure Figure 1(c1). Figure 1d presents the ZNPM film, which shows that the material spreads out over the NF substrate. ZNPM and their corresponding high magnification images (a1, b1, c1 and d1). TEM ( Figure 2a) revealed ZnO NRs with a mean diameter of approximately 1-3 μm. The distribution of ZnO NRs is shown in the magnified image ( Figure 2(a1)). When NiS was coated on ZnO, the TEM image of the prepared ZnO NRs revealed a smooth surface. The outer layers were composed of a solid Z core and shell. Figure 2c presents a representative TEM image of the obtained ZNP. The strong difference in contrast was observed in the nanoparticles from ZN to ZNP, where NRs were converted to distributed particles. These products were shorter than the ZnO NRs, indicating that they had broken into smaller sections during the chemical conversion process ( Figure  2d).  On the other hand, SEM showed that the surface of the NRs was rough after NiS (ZN) deposition. The rough texture of the nanoscale NiS coatings on these NRs were observed (Figure 1b, Figure 1(b1)). NiS was well crystallized with uniform nanospheres, 700 to 800 nm in diameter. NF is inactive in the KOH electrolyte and has a three-dimensional structure that contributes to the high specific surface area and escalates the adhesion of the NiS film. The strong adhesion plays a key role in thin film deposition, which affects the capacitance. The good adhesion of the material depends on the good wettability of the bonded surfaces by the liquid adhesive [55]. When PEDOT:PSS was deposited, the structure and surface adhesion was enhanced slightly. Figure 1c,d presents SEM images of ZNP and ZNPM films, respectively. Figure 1c shows a compact and rough morphology for the PEDOT:PSS film on ZN that exhibits a wrinkled surface morphology and a typical porous network structure Figure 1(c1). Figure 1d presents the ZNPM film, which shows that the material spreads out over the NF substrate. ZNPM and their corresponding high magnification images (a1, b1, c1 and d1). TEM ( Figure 2a) revealed ZnO NRs with a mean diameter of approximately 1-3 µm. The distribution of ZnO NRs is shown in the magnified image ( Figure 2(a1)). When NiS was coated on ZnO, the TEM image of the prepared ZnO NRs revealed a smooth surface. The outer layers were composed of a solid Z core and shell. Figure 2c presents a representative TEM image of the obtained ZNP. The strong difference in contrast was observed in the nanoparticles from ZN to ZNP, where NRs were converted to distributed particles. These products were shorter than the ZnO NRs, indicating that they had broken into smaller sections during the chemical conversion process (Figure 2d).  XPS was carried out for ZN and the results are shown in Figure 3a. The survey spectrum of ZN revealed S2p, C1s, O1s, Ni2p3, Zn2p3 and N1s at 162.89, 284.6, 531.14, 855.5, and 1072.12 eV, respectively. The O1s, C1s, and N1s peaks were attributed to exposure of the fabricated electrode to air. The binding energy of NiS at 853.2 and 870.5 eV corresponds to the presence of Ni2p3, and for the S2p spectrum of NiS. The binding energy at 161.4 eV suggests that S exists as S 2− , and the peaks for S2p1/2 at 162.5 eV can be assigned to sulphur ions with low coordination at the NiS surface [56]. The peaks at 164.2 eV and 165.3 eV correspond to the sulfur atom of PEDOT and the higher binding energy peak at 168.5 eV corresponds to the sulfur atom present in PSS (Figure 3b) [57]. Figure 3c shows the survey spectrum of the ZNPM electrode, where the binding energy peaks at 642.48 eV for Mn 2p3 is shown along with Ni2p3 (855.16) and S2p (167.3). The survey spectrum was similar to those of ZN and ZNP, where the addition of Mn2p3 can be seen. The high resolution XPS scan of Mn 2p, showed two peaks at 642.48 and 653.32 eV, which were assigned to Mn 2p3/2 and Mn 2p1/2 respectively, and were separated by 10.84 eV.   XPS was carried out for ZN and the results are shown in Figure 3a. The survey spectrum of ZN revealed S2p, C1s, O1s, Ni2p3, Zn2p3 and N1s at 162.89, 284.6, 531.14, 855.5, and 1072.12 eV, respectively. The O1s, C1s, and N1s peaks were attributed to exposure of the fabricated electrode to air. The binding energy of NiS at 853.2 and 870.5 eV corresponds to the presence of Ni2p3, and for the S2p spectrum of NiS. The binding energy at 161.4 eV suggests that S exists as S 2− , and the peaks for S2p1/2 at 162.5 eV can be assigned to sulphur ions with low coordination at the NiS surface [56]. The peaks at 164.2 eV and 165.3 eV correspond to the sulfur atom of PEDOT and the higher binding energy peak at 168.5 eV corresponds to the sulfur atom present in PSS (Figure 3b) [57]. Figure 3c shows the survey spectrum of the ZNPM electrode, where the binding energy peaks at 642.48 eV for Mn 2p3 is shown along with Ni2p3 (855.16) and S2p (167.3). The survey spectrum was similar to those of ZN and ZNP, where the addition of Mn2p3 can be seen. The high resolution XPS scan of Mn 2p, showed two peaks at 642.48 and 653.32 eV, which were assigned to Mn 2p3/2 and Mn 2p1/2 respectively, and were separated by 10.84 eV. XPS was carried out for ZN and the results are shown in Figure 3a. The survey spectrum of ZN revealed S2p, C1s, O1s, Ni2p3, Zn2p3 and N1s at 162.89, 284.6, 531.14, 855.5, and 1072.12 eV, respectively. The O1s, C1s, and N1s peaks were attributed to exposure of the fabricated electrode to air. The binding energy of NiS at 853.2 and 870.5 eV corresponds to the presence of Ni2p3 , and for the S2p spectrum of NiS. The binding energy at 161.4 eV suggests that S exists as S 2− , and the peaks for S2p 1/2 at 162.5 eV can be assigned to sulphur ions with low coordination at the NiS surface [56]. The peaks at 164.2 eV and 165.3 eV correspond to the sulfur atom of PEDOT and the higher binding energy peak at 168.5 eV corresponds to the sulfur atom present in PSS (Figure 3b) [57]. Figure 3c shows the survey spectrum of the ZNPM electrode, where the binding energy peaks at 642.48 eV for Mn 2p3 is shown along with Ni2p3 (855.16) and S2p (167.3). The survey spectrum was similar to those of ZN and ZNP, where the addition of Mn2p3 can be seen. The high resolution XPS scan of Mn 2p, showed two peaks at 642.48 and 653.32 eV, which were assigned to Mn 2p 3/2 and Mn 2p 1/2 respectively, and were separated by 10.84 eV.

Electrochemical Studies
The electrochemical capacitive performance of the bare ZnO and hybrid structures, ZN, ZNP, and ZNPM, were evaluated by CV and GCD techniques. Figure 5a-d shows the CV curves of ZN, ZNP, and ZNPM, respectively. The CV curves were obtained in a 2 M KOH aqueous solution. All the electrodes delivered a pair of redox peaks, indicating that all electrodes exhibit Faradic pseudo capacitance behaviour. Furthermore, due to polarization of the electroactive materials, the anodic and Electronics 2018, 7, 121 7 of 14 cathodic peaks were slightly shifted towards higher and lower potentials under the increased scan rates. The ZNPM curves showed a rectangular and symmetric shape indicating a rapid and reversible redox reaction similar to that of an ideal supercapacitor. ZNPM showed the best performance compare to other three electrodes. This is because the ZNPM electrode-coated material provides a large active surface area, which allows smoother electron flow and shows the largest redox peak current density, which has oxidation and reduction currents are 121.98 mA and −86.33 mA, respectively. The enclosed area in the CV curve was highest with the ZNPM electrode, indicating that ZNPM has superior capacitance to the other three electrodes.

Electrochemical Studies
The electrochemical capacitive performance of the bare ZnO and hybrid structures, ZN, ZNP, and ZNPM, were evaluated by CV and GCD techniques. Figure 5a-d shows the CV curves of ZN, ZNP, and ZNPM, respectively. The CV curves were obtained in a 2 M KOH aqueous solution. All the electrodes delivered a pair of redox peaks, indicating that all electrodes exhibit Faradic pseudo capacitance behaviour. Furthermore, due to polarization of the electroactive materials, the anodic and cathodic peaks were slightly shifted towards higher and lower potentials under the increased scan rates. The ZNPM curves showed a rectangular and symmetric shape indicating a rapid and reversible redox reaction similar to that of an ideal supercapacitor. ZNPM showed the best performance compare to other three electrodes. This is because the ZNPM electrode-coated material provides a large active surface area, which allows smoother electron flow and shows the largest redox peak current density, which has oxidation and reduction currents are 121.98 mA and −86.33 mA, respectively. The enclosed area in the CV curve was highest with the ZNPM electrode, indicating that ZNPM has superior capacitance to the other three electrodes. Two main factors for the practical use of supercapacitors are the specific capacitance and energy density. GCD experiments were carried out on the Z, ZN, ZNP, and ZNPM electrodes to examine the specific capacitance and energy density in the potential range of −1 to 0.4 V at different current densities. The results are shown in Figure 6a-d, respectively. We have performed GCD analysis for bare nickel foam and observed very low specific capacitance which is negligible ( Figure S1). The nonlinear variation of the potential versus charge/discharge time clearly shows that the discharge time of the ZNPM electrode was significantly higher than that of the other three electrodes, including the visible enhanced symmetrical profiles at the ZNPM electrodes, indicating an increase in the Two main factors for the practical use of supercapacitors are the specific capacitance and energy density. GCD experiments were carried out on the Z, ZN, ZNP, and ZNPM electrodes to examine the specific capacitance and energy density in the potential range of −1 to 0.4 V at different current densities. The results are shown in Figure 6a-d, respectively. We have performed GCD analysis for bare nickel foam and observed very low specific capacitance which is negligible ( Figure S1). The non-linear variation of the potential versus charge/discharge time clearly shows that the discharge time of the ZNPM electrode was significantly higher than that of the other three electrodes, including the visible enhanced symmetrical profiles at the ZNPM electrodes, indicating an increase in the conductivity in the core supporting nanowires. The specific capacitance can be calculated from the following Equation (1): where Cs is the specific capacitance (F/g); i is the discharge current (A); ∆t is the discharge time (s); ∆V is the potential window (V); and m is the mass of the electrode active material. The Cs values were calculated from the discharge curves in Figure 6a-d and the result is plotted in Table 1. Table 1 lists the specific capacitance of the electrodes at current densities of 5-100 mA. The increase in current density was attributed to the decrease in specific capacitance. This is because of the diffusion effect of OH − ions within the electrode at high current densities.
conductivity in the core supporting nanowires. The specific capacitance can be calculated from the following Equation (1): where Cs is the specific capacitance (F/g); i is the discharge current (A); Δt is the discharge time (s); ΔV is the potential window (V); and m is the mass of the electrode active material. The Cs values were calculated from the discharge curves in Figure 6a-d and the result is plotted in Table 1. Table 1 lists the specific capacitance of the electrodes at current densities of 5-100 mA. The increase in current density was attributed to the decrease in specific capacitance. This is because of the diffusion effect of OH − ions within the electrode at high current densities. A comparison of the GCD curves of Z, ZN, ZNP and ZNPM electrodes at 5 mA were shown in Figure S2. At a low current density of 5 mA, the specific capacitance of Z, ZN, ZNP and ZNPM were approximately 24.77, 919.44, 1716.08, and 2072.52 F g −1 , respectively. A specific capacitance of fourlayer electrodes exhibited higher showing improved performance for the four layer electrode compare to the three, two and bare ZnO nanowires. The ZN electrode showed higher specific capacitance due to the NiS porous structure. Remarkably, specific capacitance of ZNPM (2072.52 F g −1 at 0.71 A g −1 ) is superior to previously reported WO3@CuO (284 F g −1 at 1 A g −1 ) [58], N-MWCNT/Co3O4 (406 F g −1 at 2 A g −1 ) [59], ZnO/activated carbon (155 F g −1 at 0.5 A g −1 ) [60], ZnO/C (820 F g −1 at 1 A g −1 ) [61], G-ZnO (122.4 F g −1 at 2.13 A g −1 ) [62]. On the other hand, the ZNP electrode surface morphology was updated by the wrinkled and porous network structure. This assists the electrolyte ion permeability, leading to a rapid redox reaction between the electrode and electrolyte. ZNP and ZNPM showed similar surface morphologies, so that almost identical diffusion was expected. The presence of MnO2 results in a high energy density and large charge transfer-reaction. The poor conductivity of MnO2 was overcome by combining it with highly conductive PEDOT:PSS. Therefore, the increased electrochemical performance could be attributed to the development of conductivity in the core-supporting nanowire material. The relatively high specific capacitance of A comparison of the GCD curves of Z, ZN, ZNP and ZNPM electrodes at 5 mA were shown in Figure S2. At a low current density of 5 mA, the specific capacitance of Z, ZN, ZNP and ZNPM were approximately 24.77, 919.44, 1716.08, and 2072.52 F g −1 , respectively. A specific capacitance of four-layer electrodes exhibited higher showing improved performance for the four layer electrode compare to the three, two and bare ZnO nanowires. The ZN electrode showed higher specific capacitance due to the NiS porous structure. Remarkably, specific capacitance of ZNPM (2072.52 F g −1 at 0.71 A g −1 ) is superior to previously reported WO 3 @CuO (284 F g −1 at 1 A g −1 ) [58], N-MWCNT/Co 3 O 4 (406 F g −1 at 2 A g −1 ) [59], ZnO/activated carbon (155 F g −1 at 0.5 A g −1 ) [60], ZnO/C (820 F g −1 at 1 A g −1 ) [61], G-ZnO (122.4 F g −1 at 2.13 A g −1 ) [62]. On the other hand, the ZNP electrode surface morphology was updated by the wrinkled and porous network structure. This assists the electrolyte ion permeability, leading to a rapid redox reaction between the electrode and electrolyte. ZNP and ZNPM showed similar surface morphologies, so that almost identical diffusion was expected. The presence of MnO2 results in a high energy density and large charge transfer-reaction. The poor conductivity of MnO 2 was overcome by combining it with highly conductive PEDOT:PSS. Therefore, the increased electrochemical performance could be attributed to the development of conductivity in the core-supporting nanowire material. The relatively high specific capacitance of the ZNPM hierarchical architecture can be attributed to the four materials combining the characteristic unique structures, providing rapid ion and electron transfer.
EIS plots (Figure 7) of Z and ZNPM electrodes provided more information on their electrochemical performance. The Nyquist plot includes a partial semicircle in the high-frequency region indicates the charge transfer resistance (R ct ) and a straight line at the low-frequency region represents the repaid ion/electron transportation. The equivalent series resistance (R S ) can be calculated from the intercepts of the high-frequency semicircle on the real axis. The R S (0.39 Ω) and R ct (0.62 Ω) of Z were higher compared with ZNPM based electrode (R S = 0.30 Ω and R ct = 0.50 Ω). The low values of R S and R ct convey that all four layers of the material on the Ni substrate stick very well together and have excellent electrochemical conductivity, rapid electron transfer kinetics, and diffusion of ions at the interface of electrode/electrolyte. In addition, the slope of the plot at low frequency for ZNPM electrode was steeper than Z electrode, revealing faster kinetics of the ions diffusion process. The ZNPM electrode-based supercapacitor performance was improved considerably compared to the Z electrode. One of the reasons is the equivalent series resistance for ZNPM, which was much lower than Z. EIS plots (Figure 7) of Z and ZNPM electrodes provided more information on their electrochemical performance. The Nyquist plot includes a partial semicircle in the high-frequency region indicates the charge transfer resistance (Rct) and a straight line at the low-frequency region represents the repaid ion/electron transportation. The equivalent series resistance (RS) can be calculated from the intercepts of the high-frequency semicircle on the real axis. The RS (0.39 Ω) and Rct (0.62 Ω) of Z were higher compared with ZNPM based electrode (RS = 0.30 Ω and Rct = 0.50 Ω). The low values of RS and Rct convey that all four layers of the material on the Ni substrate stick very well together and have excellent electrochemical conductivity, rapid electron transfer kinetics, and diffusion of ions at the interface of electrode/electrolyte. In addition, the slope of the plot at low frequency for ZNPM electrode was steeper than Z electrode, revealing faster kinetics of the ions diffusion process. The ZNPM electrode-based supercapacitor performance was improved considerably compared to the Z electrode. One of the reasons is the equivalent series resistance for ZNPM, which was much lower than Z.   The energy-power performance delivered by the ZNPM was much higher than that of the Z electrode, as shown in Figure 8. This was attributed to the densities through the charge and discharge curves at different current densities evaluated using the following Equations (2) and (3): P (kW/kg), E (Wh/kg) and t(s) are the power density, energy density, and discharge time, respectively. Based on above data, even at a high-power density of~2142 W/kg, the ZNPM electrode showed an energy density of~6.9 Wh/kg while the Z electrode was only~0.041 Wh/kg. On the other hand, at low power density,~150 W/kg, the energy density for ZNPM and Z was~13 Wh/kg and~0.83 Wh/kg, respectively. The energy density decreased to~12.17 Wh/kg. The energy-power performance of ZNPM was higher than that of Z due to the higher specific capacitance registered at ZNPM. The cycling stability of the prepared ZNP and ZNPM electrodes was additional inspected by GCD tests at a current density of 5 mA for 3000 cycles. As shown in Figure S3b, the specific capacitance of the ZNPM based electrode remains to be 1954.38 F g −1 after 3000 cycles with a retention of 94.3% of the initial specific capacitance (2072.52 F g −1 ), which is higher compared to ZNP electrode (86.1% retention). Indicating that the charge/discharge process for the ZNPM electrode is substantially reversible, the materials architecture and the pore accessibility for electrolyte ions are highly stable upon long-term cycling tests. Moreover, we have included field emission scanning electron microscopy images for the ZNPM after 3000 cycles in KOH solution ( Figure S4). Still we can see many nanoparticles spread on nickel foam substrate which shows that the ZNPM electrode is effective for supercapacitor application.
curves at different current densities evaluated using the following Equations (2) and (3): 1 2 ∆ 1000 3600 (2) P (kW/kg), E (Wh/kg) and t(s) are the power density, energy density, and discharge time, respectively. Based on above data, even at a high-power density of ~2142 W/kg, the ZNPM electrode showed an energy density of ~6.9 Wh/kg while the Z electrode was only ~0.041 Wh/kg. On the other hand, at low power density, ~150 W/kg, the energy density for ZNPM and Z was ~13 Wh/kg and ~0.83 Wh/kg, respectively. The energy density decreased to ~12.17 Wh/kg. The energy-power performance of ZNPM was higher than that of Z due to the higher specific capacitance registered at ZNPM. The cycling stability of the prepared ZNP and ZNPM electrodes was additional inspected by GCD tests at a current density of 5 mA for 3000 cycles. As shown in Figure S3b, the specific capacitance of the ZNPM based electrode remains to be 1954.38 F g −1 after 3000 cycles with a retention of 94.3% of the initial specific capacitance (2072.52 F g −1 ), which is higher compared to ZNP electrode (86.1% retention). Indicating that the charge/discharge process for the ZNPM electrode is substantially reversible, the materials architecture and the pore accessibility for electrolyte ions are highly stable upon long-term cycling tests. Moreover, we have included field emission scanning electron microscopy images for the ZNPM after 3000 cycles in KOH solution ( Figure S4). Still we can see many nanoparticles spread on nickel foam substrate which shows that the ZNPM electrode is effective for supercapacitor application.

Conclusions
ZnO NRs were used as a base for fabricating four pivotal components; ZNPM, for hybrid core/shell structure electrodes in supercapacitor applications. Each layer of a new component on the electrode increased the specific capacitance compared to the single component layer. The measured specific capacitance of Z, ZN, ZNP, and ZNPM was 24.77 F g −1 , 919.44 F g −1 , 1716.08 F g −1 , and 2072.52 F g −1 , respectively.
The interconnected arrangements of four layered unique structure of ZNPM electrode resulted in less resistance to ion transportation, large surface area, super electronic conductivity, as well as improved cycling stability, rate capability, and high energy and power densities. Overall, the ZNPM electrodes exhibited better electrochemical performance than the other electrodes. This technique of

Conclusions
ZnO NRs were used as a base for fabricating four pivotal components; ZNPM, for hybrid core/shell structure electrodes in supercapacitor applications. Each layer of a new component on the electrode increased the specific capacitance compared to the single component layer. The measured specific capacitance of Z, ZN, ZNP, and ZNPM was 24.77 F g −1 , 919.44 F g −1 , 1716.08 F g −1 , and 2072.52 F g −1 , respectively.
The interconnected arrangements of four layered unique structure of ZNPM electrode resulted in less resistance to ion transportation, large surface area, super electronic conductivity, as well as improved cycling stability, rate capability, and high energy and power densities. Overall, the ZNPM electrodes exhibited better electrochemical performance than the other electrodes. This technique of producing electrode materials will allow the fabrication of more promising electrode materials for future supercapacitor applications.
Supplementary Materials: The following are available online at http://www.mdpi.com/2079-9292/7/7/121/s1. Figure S1. GCD curve of bare nickel foam in KOH solution. Figure S2. A comparison of the GCD curves of Z, ZN, ZNP and ZNPM electrodes at 5 mA. Figure S3. Cyclic stability of the ZNP and ZNPM electrodes in 3 M KOH solution for 3000 cycles. Figure S4. Field emission scanning electron microscopy images of after 3000 cycles in KOH electrolyte.