An advanced asymmetric supercapacitor based on a novel ternary graphene/nickel/nickel oxide and porous carbon electrode with superior electrochemical performance

Abstract

A novel ternary system composed of graphene, nickel and nickel oxide (RGO/Ni/NiO) has been constructed as a positive electrode for the first time via a facile route. The RGO/Ni/NiO composite exhibits excellent electrochemical performance, and could achieve a high specific capacitance of 1410 F g⁻¹ at 1 A g⁻¹ and 1020 F g⁻¹ at a high current density of 15 A g⁻¹. Meanwhile, porous carbon (PC) as a negative electrode has been synthesized by using activated carbon (AC) with further KOH activation. With the above two kinds of electrode materials, an advanced asymmetric supercapacitor of RGO/Ni/NiO//PC is assembled and studied. The optimized asymmetric supercapacitor displays a superb performance with a maximum specific capacitance of 183.8 F g⁻¹ and high energy density of 65.3 W h kg⁻¹. Even at a high power density of 8000 W kg⁻¹, it can maintain 42.2 W h kg⁻¹, indicating remarkable rate performance. Additionally, the RGO/Ni/NiO//PC asymmetric supercapacitor retains a high specific capacitance of 120 F g⁻¹ without capacitance loss after 3000 cycles at 8 A g⁻¹, demonstrating its long cycle life. This work provides new insight into the design of RGO/Ni/NiO as a positive electrode to fabricate high-performance asymmetric supercapacitors. The obtained RGO/Ni/NiO//PC shows superior electrochemical performance, suggesting a promising application in energy-storage devices.

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Introduction

Nowadays, energy storage has drawn global attention due to the ever-rising energy and environmental problems concerning the depletion of fossil fuels and global warming.^1–5 Of all the energy-storage devices, supercapacitors are considered to be one of the most highly promising candidates because of their high power density, low cost, and long cycle life.^6,7 Nevertheless, their low energy density is still a big challenge for supercapacitors to be implemented in practical applications.⁸ Therefore, it is necessary to explore advanced supercapacitors with high energy and power density simultaneously.^9,10 According to the formula E = 0.5CV², advanced supercapacitors should have a much wider operating voltage window. Meanwhile, each individual electrode needs to achieve maximization of the specific capacitance.

As well known, the asymmetric supercapacitors could enlarge operating voltage window compared to the symmetric supercapacitors.^11,12 Thus assembling an asymmetric supercapacitor is a good method to get a higher voltage, further improve the energy density. Beyond the high voltage, the asymmetric supercapacitor possesses other advantage. It is composed of two types of electrodes with different energy-storage mechanisms.¹³ The first one is capacitor-type electrode, which involves a non-faradic process.¹⁴ The other is battery-type electrode, which bears a faradic redox reaction for energy storage.¹⁵ In this asymmetric supercapacitor, capacitor-type electrode ensures large power density, superior rate performance and long circle life due to its fast charge/discharge. The battery-type electrode offers high energy density. Therefore, the asymmetric supercapacitor could make best use of the advantage of both electrodes, further extremely improving the electrochemical performance for practical application. For the capacitor-type electrode, carbon materials are commonly used as negative electrode material owing to its high rate performance at fast charge/discharge.^16–19 As for the battery-type electrode, metal oxides, such as NiO,^20–22 Ni(OH)₂,^23,24 NiCo₂O₄,^25,26 Co₃O₄,²⁷ and MnO₂,²⁸ have been investigated widely because of their high charge storage from redox reaction. Recently, many studies have been focused on the combination of carbon anode and metal oxide cathode to fabricate asymmetric supercapacitors. Xie et al.²⁹ assembled an asymmetric supercapacitor with activated carbon and the Co₃O₄–RGO nanocomposite in 6 M aqueous KOH solution as electrolyte. The asymmetric supercapacitor with a voltage window of 0–1.5 V can exhibit energy density of 35.7 W h kg⁻¹ at low power density of 225 W kg⁻¹. Fan et al.³⁰ developed an asymmetric supercapacitor using activated carbon nanofibers as anode and graphene/MnO₂ composite as cathode in an aqueous Na₂SO₄ electrolyte. It can be cycled reversibly in the voltage range of 0–1.8 V, and exhibits energy density of 8.2 W h kg⁻¹ at a high power density of 16.5 kW kg⁻¹. Though devising asymmetric supercapacitors can improve the performance significantly, it is hard to get high energy and power density simultaneously in the previous reports.^29–34 Thus, it is a major topic of interest to exploit novel electrode materials with excellent specific capacitance to construct asymmetric supercapacitors.

In order to fulfill the demand of high specific capacitance, carbon materials as negative electrode should own large specific surface area, appropriate pore size distribution and short pore length. These properties could make carbon materials form a large amount of electrical double layers to facilitate the transport of electrolyte ions, further increase the energy storage capacitance. Meanwhile, the metal oxides as positive electrode should possess high theoretical specific capacitance and good conductivity. They would offer facile electron transport paths for faradic redox reaction and more effective availability of active materials, thus boost the electrochemical performance.

In this work, we successfully fabricated a high-performance asymmetric supercapacitor based on graphene/Ni/NiO and porous carbon (PC) electrodes with high energy and power density. In this asymmetric supercapacitor, PC with high specific surface area as anode exhibits better electrochemical performance than commercial activated carbon (AC). At the same time, a novel ternary system composed of RGO, Ni, and NiO was prepared as cathode for the first time. Ni/NiO nanocomposites of ∼30 nm were in situ grown and anchored on graphene sheets directly. Among these, RGO is a great substrate for Ni/NiO nanocomposites, which improves the dispersity of Ni/NiO and makes the ternary system a network. Ni metal as an outstanding conductive material increases electron transport rate during faradic redox reaction. NiO provides high electrochemical capacitance on account of its good redox, and charge storage property. The ternary components influence the capacitance values synergistically. As a consequence, the RGO/Ni/NiO composite shows great electrochemical characteristic as positive electrode with a high specific capacitance of 1410 F g⁻¹ at 1 A g⁻¹ and 1020 F g⁻¹ at 15 A g⁻¹. In order to achieve high energy and power densities, an advanced high-voltage asymmetric supercapacitor of RGO/Ni/NiO//PC is successfully fabricated and studied. The optimized asymmetric supercapacitor displays high energy density of 65.3 W h kg⁻¹ at power density of 400 W kg⁻¹. Even at a high power density of 8000 W kg⁻¹, it can remain 42.2 W h kg⁻¹. This work constitutes the first using of RGO/Ni/NiO to assemble asymmetric supercapacitor. And the obtained RGO/Ni/NiO//PC with high power and energy density, suggests a promising application in energy-storage devices.

Experimental

Synthesis of RGO/Ni/NiO composite

In a typical synthesis, GO made by a modified Hummers method was dispersed in glycol.³⁵ 100 mL GO/glycol dispersion with a concentration of 0.25 mg mL⁻¹ was mixed with 600 mg Ni(NO₃)₂·6H₂O. The suspension was heated to 190 °C, to which 25 mL of 32 mg mL⁻¹ NaBH₄–glycol solution was added. The mixture was kept stirring at 190 °C for 1 h. After reaction, the sample was washed with distilled water and ethanol several times and then dried in vacuum at 100 °C overnight. The as-prepared RGO/nickel alkoxide precursor was further calcined at 300 °C for 1 h in Ar to obtain RGO/Ni/NiO. To get the optimized electrochemical performance, GO dispersion with two other concentration of 0.1 mg mL⁻¹ and 0.4 mg mL⁻¹ was used to prepare the ternary composites by the same procedure. The obtained samples were denoted as RGO/Ni/NiO-10 and RGO/Ni/NiO-40, respectively. As comparison, the Ni/NiO composite was synthesized by the same procedure without adding graphene.

Synthesis of porous carbon

Activated carbon (AC, chemically pure, Shanghai Dahe Chemical Reagent Ltd.) was chosen as an initial sample and then processed by KOH activation.³⁶ Typically, the mixture of activated carbon and KOH (mass ratio of 1 : 5) was heated to 800 °C with a heating rate of 1.25 °C min⁻¹ in Ar and kept 1 h at this temperature. Subsequently, the product was washed with hydrochloric acid and distilled water until the pH = 7, and then dried in vacuum at 100 °C overnight. The obtained sample was denoted as PC (porous carbon).

Characterization

The structures of the products were examined by X-ray power diffraction (XRD) on Rigaku D/Max-2550 V diffractometer using Cu Kα radiation. The morphologies and textures were tested on scanning electron microscope (SEM JEOL S-4800) equipped with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM JEOL 2100) equipped with the selected area electron diffraction (SAED). Raman spectroscopy was conducted on a DXR Raman Microscope with a 532 nm excitation length. The specific surface area was measured by Brunauer–Emmett–Teller (BET) method at 77 K in N₂ atmosphere using Micromeritics ASAP 2010 surface area analyzer.

Electrochemical measurements

For fabrication of the positive electrodes, the electroactive material, acetylene black and polytetrafluoroethylene (PTFE) binder (weight ratio of 70 : 20 : 10) were mixed to slurry and pressed onto a nickel foam (1 cm × 1 cm) current collector. Each working electrode contained about 3 mg of electrode material. For the anode material PC, a slightly different weight ratio was used (80 : 10 : 10). For the tests of each individual electrode, all measurements were performed in a three-electrode system. A platinum wire and a Hg/HgO electrode were used as counter electrode and reference electrode, respectively. To assemble an asymmetric supercapacitor, the RGO/Ni/NiO cathode and PC anode were dipped in 6 M KOH aqueous solution (Scheme 1). The optimized mass ratio of cathode/anode was evaluated to be 3/5 according to the electrochemical tests. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out on a Parstat 2273 electrochemical station (Princeton applied research CO., Ltd, USA). EIS measurements were conducted in the frequency range from 0.01 to 100 kHz at a constant dc bias potential of 0.2 V with an ac perturbation of 5.0 mV. Galvanostatic charge/discharge tests were performed on a LAND CT2001A cell measurement system. The specific capacitance (C) of both electrodes was calculated from the corresponding galvanostatic discharge curves according to the following equation:
where I stands for the current, m is the mass of active materials in the electrode, V represents the voltage difference, Δt is the discharge time. The energy density (E) of asymmetric supercapacitor was calculated by the formation:

E = 0.5CV²

Scheme 1 Schematic show of the fabricated asymmetric supercapacitor based on RGO/Ni/NiO as cathode and PC as anode.

The power density (P) of asymmetric supercapacitor was calculated using the formula:

Results and discussion

Characterization of RGO/Ni/NiO composite as cathode material

The RGO/Ni/NiO composite was synthesized by a facile two-step route. Firstly, the graphene/nickel alkoxide precursor is prepared by using NaBH₄–glycol as a novel alkaline precipitant, and it subsequently followed by one-step pyrolysis of the precursor to obtain RGO/Ni/NiO composite directly (Scheme 2). In order to analyze the reaction process, a series of characterization were carried out. As illustrated in Fig. 1, the nickel alkoxide which phase has been proved by our previous work is composed of 3D spherical spongy shape with the size of ∼200 nm.³⁷ The spongy structure is bestrewed with many rippled-shaped interconnected thin nanoflakelets. For the RGO/nickel alkoxide, the nickel alkoxide nanoflakelets are inserted upright on the surface of graphene homogenously, forming a unique sandwich structure of nickel alkoxide–graphene–nickel alkoxide. When we calcined the precursors at 300 °C in Ar, the novel products are obtained. XRD patterns show that both samples present six peaks (Fig. 2). The peaks of NiO marked by “■” are indexed to (111), (200) and (220) planes in the standard spectrum (PDF#47-1049). The peaks of Ni marked by “●” are in good agreement with (111), (200) and (220) planes in the standard spectrum (PDF#65-0380).³⁸ No other peaks are observed, proving that the composites have no impurity phases. It is worth to note that the nickel alkoxide and RGO/nickel alkoxide directly turn into Ni/NiO and RGO/Ni/NiO composites after pyrolysis, respectively. To prove the reduction of GO, the XRD pattern (Fig. S1†) and Raman spectroscopy (Fig. S2†) were detected. The XRD pattern of dried GO represents the characteristic peak around 12°, which is indicative of the expanded interlayer distance owing to the oxygen-containing functional groups. On the contrary, the RGO shows only a broad peak at 24°, implying that GO is readily reduced.³⁹ Meanwhile, the Raman spectroscopy reveals the increasing ratio of D band (∼1347 cm⁻¹) to G band (∼1589 cm⁻¹) from 1.18 to 1.58, confirming the reduction of GO, in consistent with the previous reports.^40,41 The above results demonstrate that a novel ternary system of RGO/Ni/NiO composite is achieved by an easy one-step pyrolysis method.

Scheme 2 Schematic illustration for the formation of RGO/Ni/NiO composite.

Fig. 1 SEM images of (a) nickel alkoxide (b) RGO/nickel alkoxide, TEM images of (c) nickel alkoxide (d) RGO/nickel alkoxide.

Fig. 2 XRD patterns of Ni/NiO and RGO/Ni/NiO.

SEM and TEM images were observed to study the morphology and structure of RGO/Ni/NiO composite. As shown in Fig. 3a, RGO/Ni/NiO retains the sandwich structure, in which RGO is a flaky substrate and Ni/NiO nanocomposites are anchored on the both surface of RGO uniformly. From Fig. 3b, it can be seen that nanoparticles of ∼30 nm and thin nanoflakelets connected together, which forms a unique Ni/NiO nanoparticle/nanoflakelet structure on the surface of RGO. To further investigate the distribution of Ni and NiO, line scanning images of RGO/Ni/NiO composite were given in Fig. S3.† The area corresponding to the nanoparticles possesses more nickel elements. And the nanoflakelet region contains more oxygen elements. This result indicates that the nanoparticles are mainly composed of Ni, and the nanoflakelets are mostly NiO. Ni and NiO as mixture are dispersed on the RGO homogeneously. The similar conclusion can be obtained by TEM (Fig. 3c and d). The SAED pattern of RGO/Ni/NiO composite discloses the coexistence of Ni and NiO, in agreement with the XRD data. The HRTEM image shows two kinds of lattice distance, corresponding to the (111) plane of Ni and (200) plane of NiO respectively, further confirming the mixture structure of Ni and NiO. As comparison, the SEM images of pure Ni/NiO composite were also observed. Nanocomposites of 40–50 nm are agglomerated into irregular porous ball of ∼200 nm, which size is in accordance with that of nickel alkoxide (Fig. S4†). Compared to pure Ni/NiO composite, nanocomposites on the surface of RGO/Ni/NiO display better dispersity and smaller size. It is reasonable that the adding of RGO decreases the size and improves the uniformity of nanocomposites due to the dispersing nucleation assisted by the oxygen groups of RGO. Moreover, the novel morphology and structure of RGO/Ni/NiO composite would increase effective utilization of active material and boost the specific capacitance, which is more favorable to electrochemical performance than that of pure Ni/NiO composite.

Fig. 3 SEM (a) and HRSEM images (b) of RGO/Ni/NiO. (c) TEM and (d) HRTEM images of RGO/Ni/NiO. Inset is SAED image of RGO/Ni/NiO.

BET analysis was characterized to further study the structure of RGO/Ni/NiO and Ni/NiO composite (Fig. 4). Nitrogen adsorption/desorption isotherms reveals that the specific surface areas are 161 m² g⁻¹ and 65 m² g⁻¹ for RGO/Ni/NiO and Ni/NiO, respectively. The corresponding pore size distributions plots of both samples given in the insets state that the pore sizes are 3–4 nm and RGO/Ni/NiO composite possesses larger pore volume calculated by the BJH method. The higher surface area and pore volume of RGO/Ni/NiO could be ascribed to prevention of the aggregation for nanocomposites by the addition of RGO, in good agreement with the conclusion of SEM and TEM. As well known, the large specific surface area, pore volume and suitable pore size distribution in the range of 2–5 nm is favorable for electrochemical reaction.⁴² The large specific surface area and pore volume can provide the high availability of electrode materials to electrolyte. Meanwhile, the proper pore size facilitates better diffusion and accession of electrolyte ions through pore channels for efficient redox reactions during the charge storage process. From this point, RGO/Ni/NiO composite, with higher specific surface area, pore volume and appropriate pore size distribution, is expected to be a novel and promising positive electrode material.

Fig. 4 Nitrogen adsorption–desorption isotherms (with the BJH pore size distributions plots in the insets) measured at 77 K for positive electrode material Ni/NiO (a) and RGO/Ni/NiO (b).

To confirm the outstanding advantage of the novel ternary as positive electrode material, the electrochemical performance was performed by cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy. The CV curves (Fig. 5a) are recorded at 5 mV s⁻¹ in the potential range of 0–0.65 V. The anodic and cathodic peaks corresponded to the reversible reaction of Ni²⁺/Ni³⁺, which provides active center for generating pseudocapacitance. Furthermore, the CV curve of RGO/Ni/NiO is wider than that of Ni/NiO, demonstrating its better capacitance. The galvanostatic charge/discharge curves of RGO/Ni/NiO and Ni/NiO were measured to further discuss in detail at various current densities in a voltage window of 0.05–0.55 V. The nonlinear galvanostatic discharge curves (Fig. 5b and c) suggest pseudocapacitance behavior of the electrodes. In addition, the specific capacitances of both samples are calculated based on total weight of all the components from the galvanostatic discharge curves. As determined in Fig. 5d, the specific capacitance of RGO/Ni/NiO composite is 1410 F g⁻¹ at 1 A g⁻¹. Even Ni/NiO composite can attain the specific capacitance of 1080 F g⁻¹ at 1 A g⁻¹, higher than the previously reported NiO (Table 1). It is obvious that the introduction of nickel metal promotes the specific capacitance extremely due to the improvement of conductivity. Besides, the RGO/Ni/NiO composite shows better capacitance and rate performance compared with Ni/NiO. The RGO/Ni/NiO composite could achieve 1020 F g⁻¹ at a high current density of 15 A g⁻¹, 72% of that at 1 A g⁻¹. While the capacitance of Ni/NiO composite is 660 F g⁻¹ at 15 A g⁻¹, only 61% of that at 1 A g⁻¹. This result demonstrates that the unique ternary RGO/Ni/NiO is more appropriate to use as the positive electrode material than Ni/NiO. Several features make the unique ternary building for high capacitance and ultrafast energy storage and release. The Ni/NiO nanoparticle/nanoflakelet structure possesses great electrochemical activity and conductivity. The intimate bonding between Ni and NiO affords facile electron transport path. More importantly, the introduction of RGO prevents the aggregation of Ni/NiO nanoparticle/nanoflakelet, further improves the effective utilization of Ni/NiO nanocomposites. It is key to the high capacitance and rate performance. To investigate the effect of the adding amount of RGO on the electrochemical performance, lower and higher concentration GO dispersion was used to synthesize the composite, named RGO/Ni/NiO-10 and RGO/Ni/NiO-40 (Fig. S5†). With the increase of RGO, the specific capacitance is 1280, 1410 and 1254 F g⁻¹ at 1 A g⁻¹, 810, 1020 and 930 F g⁻¹ at 15 A g⁻¹ for RGO/Ni/NiO-10, RGO/Ni/NiO and RGO/Ni/NiO-40, respectively. Besides, the corresponding capacitance retention is 63.3%, 72% and 74.2%. It can be seen that with the increase of adding amount of RGO, the capacitance retention improves gradually. This is because the increase of RGO improves the dispersity of Ni/NiO, thus enhances the electrochemical performance at high current density. In conclusion, the RGO/Ni/NiO possesses the best overall performance, testifying the importance of appropriate adding amount of RGO. In addition, the circle life of electrode materials is always a key factor. Hence, the stability of the capacitance performance of RGO/Ni/NiO and Ni/NiO composite is evaluated by testing the cycle performance at 4 A g⁻¹. In the first several hundred cycles, the specific capacitance of the electrodes increases. This could be attributed to the penetration of electrolyte ions and gradual activation of the active materials.²⁶ After 1000-cycle test, the RGO/Ni/NiO could deliver a high specific capacitance of 1468 F g⁻¹. These results reveal that the high specific capacitance and excellent cycle stability are achieved in RGO/Ni/NiO, further demonstrating the advantage of RGO/Ni/NiO as positive electrode. To better estimate the advantage of RGO/Ni/NiO, the electrodes were subjected to AC impedance measurements to quantitatively evaluate their intrinsic resistance.

Fig. 5 Electrochemical characterizations: (a) CV curves at 5 mV s⁻¹. (b) Galvanostatic charge/discharge curves of RGO/Ni/NiO. (c) Average specific capacitance of RGO/Ni/NiO and Ni/NiO at different current densities. (d) Cycle performance at 4 A g⁻¹.

Table 1 A comparison of various published NiO supercapacitors

Samples	Technique	Capacitance (F g^−1)	Current load	Ref.
NiO	Microwave	585	5 A g^−1	20
NiO	Microwave	598	5 mV s^−1	45
NiO	Hydrothermal	555	1 A g^−1	22
NiO	Hydrothermal	390	5 A g^−1	21
NiO	Hydrothermal	348	5 mV s^−1	46
NiO	Reflux	710	1 A g^−1	47
NiO	Reflux	718	2 mV s^−1	48

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Fig. 6 exhibits the complex-plane impedance plots of the electrodes with potential amplitude of 0.2 V. All the plots are composed of two distinct parts, an arc at high frequency and a linear tail at low frequency. The arc of high frequency could be attributed to the faradic charge-transfer resistance during the surface redox reaction.^43,44 It could be seen that the radius of the arc of RGO/Ni/NiO and Ni/NiO are almost identical for the two electrodes. This is ascribed to the excellent electrical conductivity of RGO/Ni/NiO and Ni/NiO due to the introduction of the nickel. Besides, the linear tails at the low frequency derive from the transportation of OH– within the channels of the electrode during redox reactions. Compared with Ni/NiO, RGO/Ni/NiO electrode shows distinctive near-straight vertical line, demonstrating lower diffusion resistance. It is reasonable because the introduction of RGO improves the dispersity of Ni/NiO, and then increases the available specific areas. The novel ternary structure is helpful to the diffusion of the OH–. In general, RGO/Ni/NiO possesses superior conductivity and more effective specific areas, which make it a more suitable candidate as asymmetric supercapacitor.

Fig. 6 Complex plane impedance plots of RGO/Ni/NiO and Ni/NiO.

Characterization of porous carbon as anode material

Before assembling an asymmetric supercapacitor, the negative electrode material PC was also prepared and characterized. The PC was prepared by activating of commercial AC using KOH. As shown in Fig. 7, after KOH activation, the PC achieves a high specific surface area of 2500 m² g⁻¹ and pore volume of 1.28 cm³ g⁻¹. Nevertheless, AC only obtains relatively low specific surface area of 2163 m² g⁻¹ and pore volume of 1.05 cm³ g⁻¹. In addition, a narrow pore size distribution of 1–3 nm is obtained in both carbon materials. The higher specific surface area, pore volume and appropriate pore size distribution are beneficial to improve availability of electrode material and transport rate of electrolyte ion. Based on the above results, it is highly expected that PC will exhibit an excellent electrochemical performance compared to AC. The electrochemical tests of negative electrodes were performed in a three-electrode cell. Fig. S6† shows the galvanostatic charge/discharge curves of PC in a voltage window of −1 to 0 V. It can be clearly observed that all the curves are highly linear and symmetrical at various current densities. This is a typical characteristic of an ideal capacitor. The specific capacitance of both electrodes is compared in Fig. 7c. The PC can achieve a superior specific capacitance of 322 F g⁻¹ at 1 A g⁻¹, much better than AC (268 F g⁻¹). Even at a high current density of 10 A g⁻¹, the PC can sustain a high value of 290 F g⁻¹. Furthermore, the PC electrode exhibits an excellent long life cycle with no capacitance loss after 1000 cycles. As a conclusion, the PC as negative electrode could deliver a better electrochemical performance because of its high specific surface area, large pore volume and proper pore size.

Fig. 7 (a) Nitrogen adsorption–desorption isotherms for positive electrode material PC and AC as contrast. (b) Pore size distribution of PC and AC as contrast. (c) Average specific capacitance of PC and AC at different current densities. (d) Cycle performance of PC at 4 A g⁻¹.

Asymmetric supercapacitor based on RGO/Ni/NiO composite and PC

As consideration above, with the high specific capacitance of the redox character over RGO/Ni/NiO composite and the fast charge/discharge property of the electrical double-layer storage for PC material, an asymmetric supercapacitor of RGO/Ni/NiO//PC was constructed using these materials as the positive and negative electrodes (Scheme 1), respectively. In order to optimize high energy and power density of the supercapacitor, the working voltage window and the mass ratio of the electrodes were evaluated to make the best use of both electrode materials. To confirm the optimized working voltage, CV curves at different operation voltage were tested. As shown in Fig. 8a, the redox peaks are corresponded to the faradic reaction of RGO/Ni/NiO, which offers high specific capacitance. It can be seen that the area of CV curves enlarges sharply as the operation voltage increases, demonstrating the improvement of specific capacitance. Based on the calculation, the specific capacitance of the RGO/Ni/NiO//PC increases from 51.3 F g⁻¹ to 142.4 F g⁻¹ at 5 mV s⁻¹ along with the operation voltage from 1.2 to 1.6 V. Thus 1.6 V is chosen as the operation voltage of the asymmetric supercapacitor. Additionally, the best mass ratio of the two electrodes must be adjusted, because the specific capacitance of each individual electrode is different at various current densities. For supercapacitors, the charge balance between the cathode and anode will follow the equation Q₊ = Q₋, where Q represents the total charge. Based on the formula:

Q = C × V × m

Fig. 8 Capacitive performances of the asymmetric supercapacitor RGO/Ni/NiO//PC in 6.0 M KOH electrolyte: (a) CV curves of different operation voltages at the scan rate of 5 mV s⁻¹ (the inset corresponds to the specific capacitance). (b) Galvanostatic charge/discharge curves at different current densities. (c) Average specific capacitance at different current densities. (d) Cycle performance at 8 A g⁻¹.

The mass ratio of the two electrodes can be expressed as follows:

From the above formula, the mass ratio of RGO/Ni/NiO : PC should be in the range of 3/4 to 3/6. Consequently, the performance of asymmetric supercapacitors was measured in the different mass ratios of RGO/Ni/NiO to PC, as depicted in Fig. 8c. The asymmetric supercapacitor at the mass ratio of RGO/Ni/NiO : PC = 3/5 delivers a maximum specific capacitance of 183.8 F g⁻¹ at the current density of 1 A g⁻¹. And the specific capacitance is higher than the other mass ratios at various current densities. Thus the mass ratio of 3/5 is selected as the optimized ratio to construct the asymmetric supercapacitor. Under the best working voltage and mass ratio, the measurement of RGO/Ni/NiO//PC asymmetric supercapacitor was performed in 6 M KOH aqueous solution. Fig. 8b shows the galvanostatic charge/discharge curves at different current densities. It is worth noting that the slopes of curves are not linear, proving that the asymmetric supercapacitor displays the charge/discharge performance of the combination of capacitor-type and battery-type electrode. Additionally, the RGO/Ni/NiO//PC remains specific capacitance of 120 F g⁻¹ after 3000 cycles at 8 A g⁻¹, exhibiting its long cycle life.

Fig. 9 shows the Ragone plots of the as-fabricated the RGO/Ni/NiO//PC and PC//PC measured in the voltage window of 0–1.6 V. The energy density and power density are calculated based on the total mass of both electrodes. It can be noted that both energy density and power density of RGO/Ni/NiO//PC are much higher than those of PC//PC (calculated from Fig. S7†). The RGO/Ni/NiO//PC with a cell voltage of 1.6 V can achieve an energy density of 65.3 W h kg⁻¹ at a power density of 400 W kg⁻¹, and still maintain 42.2 W h kg⁻¹ at a high power density of 8000 W kg⁻¹. While the PC//PC exhibits only 9.2 W h kg⁻¹ at 500 W kg⁻¹. This means that RGO/Ni/NiO//PC possesses huge advantage as energy-storage device. Moreover, compared with the performance of other asymmetric supercapacitors reported previously, such as the Co₃O₄/RGO//activated carbon system,²⁹ the graphene/MnO₂//CAN system,³⁰ the Ni(OH)₂/graphene//porous graphene system,³¹ the nickel oxide/carbon system,³² it is worthy to note that at the same power density, the RGO/Ni/NiO//PC possesses the highest energy density. Considering its superior performance, relatively low cost and high safety, this asymmetric supercapacitor is highly competitive with Li-ion batteries.

Fig. 9 Ragone plot related to energy and power densities of the RGO/Ni/NiO//PC asymmetric supercapacitor () and PC//PC symmetric supercapacitor () operated at 1.6 V in comparison to asymmetric supercapacitors reported previously, namely Ni(OH)₂/graphene//porous graphene (), nickel oxide/carbon (), Co₃O₄/RGO//activated carbon () and graphene/MnO₂//CAN ().

The superior electrochemical performance of the fabricated RGO/Ni/NiO//PC asymmetric supercapacitor can be attributed to the follow reasons. (i) In the RGO/Ni/NiO positive electrode, RGO acts as the support for the Ni/NiO nanocomposites grown on the sheets, improving their dispersity. Ni maintains the high electrical conductivity of the overall electrode. The ternary components influence the capacitance values synergistically, leading to the remarkable capacitance of the positive electrode. (ii) PC with high specific surface area, large pore volume and proper pore size as the negative electrode facilitates the transport of electrolyte ions and provides a larger area for charge-transfer reactions, ensuring high power density and excellent rate performance. (iii) The positive and negative electrodes are assembled into asymmetric supercapacitor with a wide operation voltage window. It combines the advantage of both electrodes and improves the power and energy density simultaneously. Thus, pairing up RGO/Ni/NiO and PC hybrid materials for asymmetrical supercapacitor represents a new approach to high-performance energy storage.

Conclusions

In summary, an advanced asymmetric supercapacitor based on a novel ternary RGO/Ni/NiO and PC electrodes with superior electrochemical performance was successfully fabricated. In this asymmetric supercapacitor, RGO/Ni/NiO as a novel ternary system, was prepared to use as positive electrode for the first time. The RGO/Ni/NiO composite exhibits a high specific capacitance of 1468 F g⁻¹ after 1000 cycles and excellent rate performance, making it one of the most promising cathode materials for applications. The asymmetric supercapacitor assembled by the unique ternary system and PC could make the best advantage of the two electrodes, achieving high power and energy density simultaneously. Additionally, the RGO/Ni/NiO//PC exhibits long cycle life, remaining specific capacitance of 120 F g⁻¹ with no capacitance loss after 3000 cycles at 8 A g⁻¹. From this work, we open up a viewpoint that the unique ternary building of reduced graphene oxide, metal and metal oxide possesses significant advantage as positive electrode materials. Moreover, we constitute the first using of RGO/Ni/NiO as positive electrode, and the fabricated asymmetric supercapacitor shows promising applications in energy-storage devices.

Acknowledgements

This work is supported by the National Basic Research Program of China (2012CB932303), the National Natural Science Foundation of China (Grant No. 51172261) and the supportion from the committee of Shanghai Science and Technology (13XD1403900).

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Footnote

† Electronic supplementary information (ESI) available: XRD patterns of RGO and GO, Raman spectra patterns of GO, RGO and RGO/Ni/NiO, line scanning images of RGO/Ni/NiO, SEM images of Ni/NiO, galvanostatic charge/discharge curves and average specific capacitance of RGO/Ni/NiO-10 and RGO/Ni/NiO-40 at different current densities, galvanostatic charge/discharge curves of PC and specific capacitance of PC//PC symmetry supercapacitor. See DOI: 10.1039/c5ra18976d

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