Electrodeposition manganese oxide on Ni foam loaded graphene for high-performance supercapacitor

The Ni foam used in our experiments (110 PPI, 480 g m⁻², 0.3 mm thick) was purchased from Kunshan Vesta Electronics Co. Ltd Oily slurry of graphene (5 wt%) was acquired from Suzhou Graphene Nanotechnology Co. Itd. Manganous acetate (Mn(CH₃COO)₂), Sodium sulfate(Na₂SO₄) and absolute ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd, China. Deionized water (DI) was used in all the experiments. All other reagents were of analytical grade without further purification.

1 ml Oily slurry of graphene was dispersed into 5 ml of absolute ethanol, and ultrasonically treated by a gentle bath for 1 h to form solution A. 0.05 mol l⁻¹ Mn(CH₃COO)₂ and 0.05 mol l⁻¹ Na₂SO₄ were dissolved in 30 ml deionized water at ambient temperature to form homogeneous solution B for electrodeposition.

Firstly, the Ni foam (1 * 1 cm) was carefully cleaned in the DI, hydrochloric acid, DI, absolute ethanol and DI in ultrasonic bath for 5 min, respectively and dried at 60 °C for 30 min In order to remove the nickel oxide layer and oily impurities that may exist on the outer surface. After that, the prepared Ni foam was repeatedly immersed in the solution A for 5 times and dried at 60 °C for 30 min to obtain G/Ni foam. Then used the high precision electronic balance to measure the weight of the sample and recorded it. Next, the prepared G/Ni foam as positive electrode, the platinum wire as negative electrode were deposited at room temperature for 10 min at a current density of 0.01 A cm⁻² to acquire the MnO₂/G/Ni foam electrode. Finally, the obtain electrode was washed in deionized water for 5 min and dried at 60 °C for 1 h and weight the sample again, in order to calculate the mass of deposited active substances.

In this study, the time of electrodeposition MnO₂ were varied (5, 10, 15 and 20 min) for the other samples at the same reaction conditions to obtain the different MnO₂/G/Ni foam electrodes. At last, the samples were respectively named as MnO₂/G/Ni-x (x = 5, 10, 15 or 20).

The surface characteristic of theas-preparedMnO₂/G/Ni foam was carried out using scanning electron microscopy(SEM, JSM-6360LA). The crystallographic information of the as-prepared MnO₂/G/Ni was examined by x-ray diffraction(XRD, Rigaku, RINT2000, Japan) operated at 220 mA and 45 kV with Cu Kα radiation in the range of 10–70 with step size 0.02. X–ray photoelectron spectroscopy(XPS, Axis Ultra DLD, Kratos, UK) was used to analyze the chemical composition and the energy state of samples with Al-K alpha radiation (hv = 14, 866 eV). The weight of all samples is measured 3 times by an electronic analytical balance (Sartorius CPA225D) with a accuracy of 100 mg.

To estimate the influence of MnO₂ on the electrochemical performance of the electrode materials, we used a three-electrode cell system to measure the cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge (GCD)byCHI604E electrochemical workstation and by Zahner Ennium electrochemical workstation, respectively. Platinum plate was used as the counter electrode and a saturated calomel electrode was used as the reference electrode. All electrochemistry measurements were measured in 1 M Na₂SO₄ aqueous solution as the electrolyte.

The microstructure and surface area of the MnO₂/G/Ni foam electrode have an important influence on the electrochemical performance of supercapacitor [17]. Figure 1 shows the surface morphologies of pure Ni, G/Ni foam and theMnO₂/G/Ni foam electrodes observed by SEM under different resolutions. The pure Ni foam with a lot of crisscross sponge like three-dimensional macroporous structure have more pores and larger specific surface area and the relevant expand the size of the images were displayed in figure 1(a). As seen in figure 1(b), the randomly aggregated graphene was loaded on the Ni foam presented a thin and wrinkled as sheet shape. Figures 1(c) and (d) show a low-magnification and a high-magnification SEM image of MnO₂ nanoparticles deposited on G/Ni foam, respectively. It can be clearly seen that the nanoparticles were uniformly and firmly adhere on the G/Ni foam to form a layer on the surface and evenly covered the graphene sheets. Figure 2 reveals a representative EDS spectrum of theMnO₂/G/Ni foam electrode. Through the Mn and O elements in proportion to prove the MnO₂ been deposited on the G/Ni foam without other metal contamination. The C and Ni peaks are originated from the grapheme coated on Ni foam.

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In order to further confirm the crystal phase of the as-prepared material, XRD measurement was carried out. As shown in figure 3, the XRD patterns of the MnO₂/G/Ni foam composite. Apart from three strong peaks at around ∼44.5°, ∼51.9° and ∼76.4° from the Ni foam, which overlaps with the basal reflection of graphene, all the other diffraction peaks at 2θ values of 12.26, 25.5, 37.52, 32.26 and 67.19 were respectively indexed to the (110), (101), (100)and (004) crystal planes of MnO₂. This study indicates that MnO₂ had deposited on the G/Ni foam material surface and no other impurities are observed. Reveal the effect of MnO₂ on the G/Ni foam. The intensity of carbon diffraction peak decreased with the introduction of MnO₂.

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Moreover, the XPS full spectra used to analyze the surface composition of MnO₂/G/Ni foam composite also evidence the existence of C, O, and Mn in figure 4(a). In recent research has shown that the graphene oxide has C1s core level spectra which contain C=C bond, C–C bond, C–O bond, O–C=O bond and C=O bond which is similar to the reduced graphene oxide. Figure 4(b) shows the C1s core level spectra of MnO₂ loaded on graphene with four peaks. It is corresponding to C–C/C=C (284.42 eV), C–O(285.86 eV), C=O(286.40 eV) and O–C=O(288.36 eV), respectively. The result is in consistent with previous research [16]. The three components of the O1s are show in figure 4(c), binding energies at 529.38 and 530.88 eV assigned to O-Mn bond and O–C bond. As shown in figure 4(d) the Mn2p3/2 and Mn2p1/2 peak located at 641.78 eV and 653.78 eV, respectively. The separation of peak energy is 11.9 eV, which agree well with previously reported data for MnO₂ revealing that the oxidation state of Mn is +4. Further demonstrate for the existence of MnO₂ on the G/Ni foam [18].

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The CV curves of pure Ni foam, pure MnO₂, G/Ni foam andMnO₂/G/Ni foam in 1 M Na₂SO₄solution at a scan rate of 50 mV s⁻¹ are shown in figure 5(a). It can be seen clearly that the CV of pure Ni foam shows as a straight line in the picture and the CV of G/Ni foam has little change compare to the CV of pure Ni foam, indicating that they had little capacitance compare with the MnO₂/Ni and MnO₂/G/Ni electrode. The CV of the MnO₂/Ni electrode shows obvious redox peaks due to pseudocapacitance characteristics of MnO₂. The CV of MnO₂/G/Ni electrode exhibited resemble rectangle shape can be seen clearly in picture, indicating the most ideal capacitive performance and fast and reversible redox peaks. It should be attributed to the synergistic effect of MnO₂ particles and graphene makes the graphene sheets loaded on Ni foam provide surface area for MnO₂ and enhance the low conductivity of MnO₂ particles to increase specific capacitance. Figure 5(b) compares CV information for electrodes of different electrodeposition time in the range of 0–0.8 V at a scan rate of 20 mV s⁻¹, the results show that the CV area increases gradually with the increase time of deposition MnO₂. When the deposition time is more than 10 min, there is little obvious difference in CV between the electrodeofMnO₂/G/Ni-15 and MnO₂/G/Ni-20. The result shows that with excessive mass loading, the structure is tightly packed and the electrochemical active surface area is limited, leading to the specific capacitance reduce. It proves that the electrode of MnO₂/G/Ni-10 possesses the highest specific capacitance which reached1997.5 F/g according equation (1),

$$\begin{eqnarray}&&{\rm{C}}=\displaystyle \frac{1}{mv\left(Va-Vc\right)}{\int }_{Vc}^{Va}I\left(V\right)dV\end{eqnarray} \tag{ 1 }$$

Where m is the mass of the active material (g) and v is a scan rate (V/s), (Va − Vc) is sweep potential window (V) and I is the applied current (A). Figure 5(c) is CVs of the MnO₂/G/Ni-10 electrode in the range from 0 to 0.8 V at the scan rate of 5, 10, 20, 50 and 100 mV s⁻¹ in 1 M Na₂SO₄. It is obvious to see that the shape of the CV curve expand with the increase of scan rates, the area of curve increases and still retain the symmetry of the CV curve. It indicated excellent supercapacitor behavior of MnO₂/G/Ni-10 electrode. The specific capacitance of as-prepared electrodes was calculated from the CVs with different scan rates, as shown in figure 5(d). With the scan rate increase, the specific capacitance of the as-prepared electrodes decrease, revealing that scan rates greatly influence the diffusion of ions into or out of MnO₂ nanoparticles. When the scan rate is low, the ions can easily reach both the inner and outer surface of the MnO₂ nanoparticles. Under such condition and due to the completion of the so-called insertion reaction, a near-ideal capacitive behavior is observed for the electrode material, which is reflected by the rectangular CVs. With increase of scan rate, the diffusion of ions can more likely happen on the outer surface of the MnO₂ nanoparticles, which make the deeper area unoccupied and leads to incomplete insertion reaction. This reduces the contribution of the particles in the total capacitance and makes the electrode material show a huge capacity loss, which is reflected by the deviation of CVs from the ideal rectangle shape [19].

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The EIS was used to research the fundamental behavior of the prepared electrode materials in the frequency range from 100 kHz to 0.01 Hz and the Nyquist diagram was obtained. The Nyquist diagram is divided into the low-frequency region and the high-frequency region to analyze the response of components performance in frequency domain. The straight line in the low-frequency region expresses the process of ions diffusion in the electrolyte [20]. The semicircle in the high frequency region expresses the process of charge transfer at the interface of electrode-electrolyte [21]. The diameter of semicircle represents the charge transfer resistance (R_ct) at the electrode interface [22]. Another feature in the high frequency is the intercept on the real impedance (Z⁻¹) axis yields the electrolyte resistance (R_e) [23]. Figure 6 shows the fitted impedance spectra of as-prepared electrodes, respectively. The curve of the MnO₂/G/Ni-10 electrode is more vertical than other electrodes in the low-frequency. This indicating that the capacitive behavior with small diffusion resistance and near-ideal electric double-layer [24]. In the inset of figure 6, each electrode has a small semicircle at high frequency and the diameter of semicircle of MnO₂/G/Ni-10 electrode is the lower than others, implying the MnO₂/G/Ni-10 electrode with more specific capacitance than others. The EIS analysis clearly showed that the electrochemical behavior of MnO₂/G/Ni-10 electrode was better than others.

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The enhanced electrochemical performance of the MnO₂/G/Ni-10 electrode is further conformed by GCD and the GCD curves of each electrode over the voltage window of 0−0.8 V at current densities of 2 A g⁻¹ are shown in figure 7(a). The specific capacitance of these curves were calculate according the equation (2):

$$\begin{eqnarray}&&{\rm{C}}=\displaystyle \frac{I{\rm{\Delta }}t}{m{\rm{\Delta }}V}\end{eqnarray} \tag{ 2 }$$

Where C(F/g) is the specific capacitance of as-prepared electrode, I(A) represents the charge-discharge current, Δt(s) is the discharge time, m(g) is mass of the electroactive material, and ΔV(V) stands for the potential window. At current densities of 2 A g⁻¹, the calculated specific capacitance values of MnO₂/G/Ni-x (x = 5, 10, 15 and 20 min) electrodes are 945, 1465.25, 1315 and 1265 F g⁻¹ respectively. It can be concluded that the MnO₂/G/Ni-10 electrode shows higher specific capacitance than other electrodes, which is consistent with the results of CV and EIS measurements. In figure 7(b), the charge–discharge curve of MnO₂/G/Ni-10 electrode at different current densities maintains the triangle shape at high current density, confirms the high rate capacity of the electrode. The specific capacitance of the MnO₂/G/Ni-10 electrode decreases from 1465.25 F g⁻¹ at 2 A g⁻¹ to 998.25 F g⁻¹ at 10 A g⁻¹, with the retention of 68.13%, which is higher than those of MnO₂/G/Ni-5(60.85%), MnO₂/G/Ni-15 (64.87%) and MnO₂/G/Ni-20(61.26%) electrodes as shown in figure 7(c). The cycling stability is important to supercapacitor [25]. Figure 7(d) shows the capacitance retention of the MnO₂/G/Ni-10 electrode for 5000 cycles at a current density of 10 A g⁻¹. The electrode has excellent electrochemical stability with only 13.91% deterioration after 5000 cycles.

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To further confirm the performance of as-prepared electrode, an asymmetric supercapacitor(ASC) was built using the G/Ni foam and MnO₂/G/Ni foam as the negative electrode and positive electrode, respectively. The ASC used 1 M Na₂SO₄ as the electrolyte and used resin film to separate two electrodes. Similarly, the device was tested by CV, GCD and EISmeasurements. Figure 8(a) shows the CV curves at a scan rate of 20 mV s⁻¹ shows the stable electrochemical potential windows of the electrodes. Figure 8(b) shows the made ASC still has the excellent capacitive behavior with rectangular CV curves at the cell voltages from 0.8 to 2.0 V. The CVs of the ASC at different scan rates is plotted in figure 9(a). It can be seen clearly, the shape of CVs keep unchanged although the scanning rate reach to 100 mV s⁻¹, indicated low contact resistance in the equipment. Figure 9(b) depicts the GCD curve at various currents ranging from 2 to 10 A g⁻¹ within the potential window of 0–1.8 V. When the current density reach to 2 A g⁻¹, the shape of the charge curve towards nonlinear characteristics, further indicating that the charge storage mainly originated from Faradic redox reactions and verifying that the pseudocapacitance behavior. Figure 9(c) shows that the capacitance retention rate of the capacitor was 79.5% under the condition of3000 cycles at current density of 8 A g⁻¹. The corresponding Nyquist plot which was obtained by EIS test (figure 9(d)) is to evaluate the electrochemical stability of the ASC. The plots possesses excellent vertical in the low-frequency and very small semicircle at high frequency. Figure 10 shows the Ragone plots of the device. The power density was obtained from different energy density is calculated by GCD curves according to equations (2), (3) and (4):

$$\begin{eqnarray}&&{\rm{E}}=\displaystyle \frac{1}{2}C{({\rm{\Delta }}V)}^{2}\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&{\rm{P}}=\displaystyle \frac{E}{t}\end{eqnarray} \tag{ 4 }$$

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${\rm{\Delta }}V$ (V) stands for the charging potential window, t is the charge time (s). It showed that the device could achieve a high energy density 113.95 Wh kg⁻¹ at a power density of 1800 W kg⁻¹, which much higher than that of the researchers using MnO₂ or graphene as electrode materials, as shown in Table 1. At last, when the supercapacitor was fabricated, after it was charged for 30 s at 20 A g⁻¹, the voltage can reach to 2.7 V (figure 10) and light up four red LEDs in series for 10 min (figure 10). The results certify the immense potential of the MnO₂/G/Ni electrode in electrochemical supercapacitors.