Freestanding and flexible graphene wrapped MnO₂/MoO₃ nanoparticle based asymmetric supercapacitors for high energy density and output voltage

Abstract

Asymmetric supercapacitors (ASC) based on freestanding membranes with high energy density and high output voltage by a simple pre-reduced and vacuum filtering method are reported. Reduced graphene oxide (rGO) coated MnO₂ nanospheres and rGO coated MoO₃ nanoparticle composites are selected as the positive and the negative materials of the devices, respectively. The ASC has a high operation voltage window of 2.0 V in a hydrogen electrolyte, a high energy density of 34.6 mW h cm⁻³ at a power density of 100 mW cm⁻³, and a high volumetric capacitance of 62.7 F cm⁻³ at a current density of 0.1 A cm⁻³. Especially, the ASC exhibits an excellent cycling performance of 94.2% capacitance retention after over 3000 cycles. This strategy of designing the hybridized structure for freestanding and flexible ASC provides a promising route for next-generation supercapacitors with high energy density and high output voltage.

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Introduction

Bendable freestanding films have emerged to garner major interest recently because they have various potential applications in energy storage, catalysis, the environment and sensing.^1–3 Particularly, flexible energy storage devices such as supercapacitors (SC) and batteries, which extensively rely on such bendable film electrodes, are receiving more and more attention with the increasing demand for wearable and portable consumer electronics.^4,5 However, these SC suffer from a lower energy density, which is an obstacle to their potential applications.^6,7 According to the equation E = 0.5 CU², the increase of energy (E) can be achieved by maximizing the specific capacitance (C) and the operation voltage (U).⁸ Organic and solid electrolyte based ASC can effectively increase the operation voltage to realize energy density boost. However, their low capacitance and unfriendly electrolyte undermine their overall effectiveness for ASC.

MnO₂ and MoO₃ are both the most thoroughly investigated for supercapacitor applications because of their remarkable theoretical specific capacitance, natural abundance, low cost and environmental friendliness.^9–12 Fortunately, due to the big work function difference (4.4 eV of MnO₂ and 6.9 eV of MoO₃, respectively), it is easy to acquire widest potential window while choose MnO₂ as a positive electrode material and MoO₃ as a negative electrode material.^13–16 However, the poor electrical conductivity seriously hinders to achieve the theoretical specific capacitance in the experiment. Graphene, as a two dimensional carbon material, has drawn considerable attention as an electrode material for supercapacitor because of its unique properties such as high electrical conductivity, flexibility, mechanical strength, superior chemical stability and broad electrochemical window.¹⁷ Unfortunately, freestanding films composed of graphene nanostructure are readily to be restacked into “graphite” due to the tremendous van der Waals attraction during the fabrication process, which leads to poor penetration of electrolyte into the graphene nanosheets and a small active surface area.¹⁸ Some reporters want to escape this problem by making a gel.^19,20 However, it is obvious that it is unacceptable for flexible and freestanding film with high volumetric energy density.²¹ The method of making sandwich structure from graphene and pseudocapacitance materials is valid to alleviate the problem, regrettably, those works with high energy density are rarely reported.^22–24

Herein, we fabricated a flexible ASC based on rGO coated MnO₂ nanoparticles (MnO₂@rGO) for the positive electrode and rGO wrapped nanostructure MoO₃ nanoparticles (MoO₃@rGO) for the negative electrode by simply pre-reduced and vacuum filtering method. The unique nanostructure of graphene is to maintain the flexibility and electronic transmission path of the electrode material.²⁵ The clearance of different graphene sheets were used to be ion diffusion channel. In addition, MnO₂ and MoO₃ nanoparticles are wrapped by rGO not only to prevent rGO to reunite but also to improve the contact between nanoparticles and rGO. Thus we will benefit for the reaction of Faradic pseudocapacitance, electronic transaction and cyclic stability. Since freestanding graphene-based films have shown more applications compared to individual graphene sheets,^26,27 and such unique nanostructure to composite freestanding films into ASC has not reported, the freestanding rGO-wrapped nanostructure film (MnO₂@rGO/MoO₃@rGO) ASCs were fabricated. The positive electrode MnO₂@rGO shows a high volume capacitance 343 F cm⁻³, and the negative electrode shows volume capacitance 200 F cm⁻³. Importantly, the ASC acquire a high volumetric capacitance of 62.7 F cm⁻³ at a current density of 0.1 A cm⁻³, excellent cycling performance of 94.2% capacitance retention after over 3000 cycles, and a high energy density of 34.6 mW h cm⁻³ at a power density of 100 mW cm⁻³. Compared to others researchers on freestanding graphene matrix composite nanostructure ASC, our ASC is flexible and significantly improves both energy density and power density.

Experimental

Materials

GO was prepared by oxidation of natural graphite powder by using a modified Hummers' method. It is easy to prepare the MnO₂ nanoparticles by modified hydrothermal reduction method. By a simply pre-reduced and vacuum filtration procedure, the flexible and freestanding positive electrode material is acquired. The same procedure to fabrication freestanding graphene@MoO₃ films.

The morphologies, structure, and chemical composition of the samples were characterized by high-resolution field emission SEM (FEI Nova Nano-SEM 450), TEM (FEI Titan G2 60-300), and XRD (PANalytical B.V. X'Pert PRO).

Electrochemical measurements

All the electrochemical measurements were carried out in a two-electrode system at room temperature using Autolab PGSTAT302N (Metrohm AG). The electrochemical impedance spectroscopy was carried out at 10 mHz to 100 kHz with a potential amplitude of 10 mV. The electrochemical tests of the individual electrode were performed in a three electrode cell, in which carbon electrode and Ag/AgCl electrode was used as the counter and reference electrode, respectively. The electrochemical measurements of the ASC were carried out in a two electrode cell at room temperature in 1 M LiCl electrolyte. All of the above electrochemical measurements were carried out by a biologicMP3 electrochemical workstation.

Result and discussion

Characterizations of positive electrode materials

The preparation process for ASC based on rGO is illustrated in Fig. 1. Freestanding membranes are prepared by a simple vacuum filtration and pre-reduced technique. When the films were dried completely, they are cut to a suitable area and then and been annealed, finally assembled the ASC. Particularly, the pre-reduced process before vacuum filtering is necessary, otherwise the freestanding flexible films will become rough and fragile (more details of the experiment are given in ESI†). The morphologies and detailed structures of MoO₃@rGO and MnO₂@rGO composites are characterized by both SEM and TEM observations. Fig. 2a shows the optical image of freestanding MnO₂@rGO compound film with wonderful flexibility. TEM image of pure MnO₂ particles is shown in Fig. 2b. The diameters of the particles are about 100–160 nm. SEM image Fig. 2c shows the freestanding film surface of the MnO₂ nanoparticles which were wrapped by rGO. The average diameters of fusiform MnO₂@rGO particles are around 110–190 nm.

Fig. 1 Fabrication process of ASC devices based on graphene@MnO₂ as positive and graphene@MoO₃ as negative electrodes in 1 M LiCl electrolyte.

Fig. 2 (a) Optical image of MnO₂@rGO film with expected flexibility, (b) TEM image of pure MnO₂ morphology, (c) SEM image of the stacking MnO₂ particles wrapped by rGO, (d and e) low and high magnification TEM images of MnO₂@rGO composite, (f) XRD patterns of pure MnO₂, pure rGO and MnO₂@rGO.

Adjacent graphene films as a framework support freestanding structure. The low and high magnification TEM images of MnO₂@rGO composites demonstrate the typical crystalline texture of MnO₂ nanoparticles which are tightly enwrapped by graphene sheets (Fig. 2d and e). The X-ray diffraction (XRD) pattern samples are presented in Fig. 2f. The samples are poorly crystallized and a wide peak at 37.5° can be clearly observed. This can be ascribed to the (211) diffraction peak of α-MnO₂ (ICDD-JCPDS card no. 44-0141).

Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) curses are generally used to characterize the capacitive behavior of an electrode material. Fig. 3a shows the type CV curves of a freestanding MnO₂@rGO film (the resistance is 30.14 Ω cm⁻², the resistivity is 18.84 × 10⁻³ Ω cm⁻¹), approaching the ideal capacitive behavior. This is ascribed to the highly conductive graphene wrapped on the surface of MnO₂ nanoparticles. According to Y. Gogotsi and P. Simon's perspective,²⁸ the volumetric capacitance (or areal capacitance) of our devices is calculated. The freestanding film exhibits high specific volumetric capacitances of 268 F cm⁻³ and 110 F cm⁻³ at scan rate 5 mV s⁻¹ and 100 mV s⁻¹ in 1 M LiCl aqueous solution (based on the total volume of the free-standing MnO₂@rGO film), respectively. The specific energy density (E, W h cm⁻³) and power density (P, W cm⁻³) for a supercapacitor cell can be calculated using the following equations: E = 0.5 CU² and P = Et⁻¹, where C is the specific capacitance of SC, U is voltage change, t is discharge time. Fig. 3b shows the Ragone plot for the energy density and the power density of the freestanding MnO₂@rGO film, which is calculated from Fig. 3a. The GCD curves of the freestanding MnO₂@rGO film show nearly linear and symmetrical curves (Fig. 3c), thus demonstrate the excellent capacitor performance of the sample.

Fig. 3 Electrochemical performance of freestanding MnO₂@rGO electrode in a three-electrode configuration: (a) CV cures of composite at different scan rates, (b) Ragone plots of our electrode at different scan rates, (c) GCD curves at various current density, (d) specific volume capacitance at various current density.

Fig. 3d exhibits the specific capacitances from GCD curves of the freestanding MnO₂@rGO film. The freestanding film exhibits high specific volumetric capacitances of 343 F cm⁻³ and 250 F cm⁻³ at a current density of 1 A g⁻¹ and 10 A g⁻¹ in 1 M LiCl aqueous solution.

Characterizations of negative electrode materials

In spite of abundant researches on positive electrodes, studies on the negative electrode materials with transition metal oxides have been limited due to the unsatisfactory capacitive performance or high cost of several types of electrode materials.^29,41–43 MoO₃ is low cost and layer-structured material, which facilitates the injection of different electrolyte ions into the free spaces, thus creating better electrochemical properties.^30,31

The corresponding detailed structures of pure MoO₃ nanoparticles and MoO₃@rGO composites surface were also individually characterized by both SEM and TEM. Fig. 4a shows the optical image of freestanding MoO₃@rGO film with wonderfully flexibility. SEM images Fig. 4b and c demonstrate nanostructure of pure MoO₃ nanoparticles and MoO₃@rGO nanoparticles. It is obvious that MoO₃ nanoparticles were coated by soft graphene. What is more, the low and high magnification TEM images of MoO₃@rGO composites demonstrate that the typical MoO₃ nanoparticles were tightly enwrapped by graphene sheets (Fig. 4d and e). Fig. 4f shows the typical XRD patterns of the pure MoO₃ with very sharp diffraction peaks (JCPDS no.05-0508), indicating a highly crystalline orthorhombic structure (α-MoO₃).³²

Fig. 4 (a) Optical image of flexible freestanding MoO₃@rGO film; (b) SEM image of pure MoO₃ nanoparticles, (c) SEM image of freestanding MoO₃@rGO composite, (d and e) the detailed structure of the composite using low and high magnification TEM, (f) XRD patterns of pure MoO₃.

The electrochemical tests of the electrode materials were performed in a three-electrode cell using aqueous 1 M LiCl electrolyte. The freestanding film of MoO₃@rGO composite material exhibits excellent capacitive performance with potential window of −0.9 to 0 V in Fig. 5. The CV measurements show excellent negative volumetric capacitance characteristics with various scan rates in Fig. 5a. Fig. 5b exhibits the Ragone plot for freestanding MoO₃@rGO electrode (the resistance is 19.86 Ω cm⁻², the resistivity is 19.41 × 10⁻³ Ω cm⁻¹). To further quantify the capacitance of the as-obtained hybrid film electrode, a series of GCD measurements are carried out at different current densities, and the typical discharge curves are presented in Fig. 5c and d. The freestanding MoO₃@rGO film electrode was able to yield a high volume capacitance of 200 F cm⁻³ at 1 A cm⁻³ based on the total electrode volume. When the current density increased to 10 A cm⁻³, its capacitance remained at 95 F cm⁻³.

Fig. 5 Electrochemical performance of freestanding MoO₃@rGO electrode in a three-electrode configuration, (a) CV cures of composite at different scan rates, (b) Ragone plots of our electrode at different scan rates, (c) GCD curves at various current density, (d) specific volume capacitance at various current density.

Hybridized aqueous asymmetric supercapacitor

To further investigate the capacitive performance of the flexible freestanding electrodes in a full cell set-up, an ASC is assembled in our study by using a freestanding MnO₂@rGO film as the positive electrode and a freestanding MoO₃@rGO film as the negative electrode in 1 M LiCl aqueous electrolyte. Because more negative potential and positive potential can be achieved, both hydrogen and oxygen evolution reactions are supposed to be kinetically limited on these transition metal oxide and graphene composites.^39,40 As a consequence, the operation voltage window can be extended from −0.1 to 1.2 V for MnO₂ nanoparticles and from −1.3 to 0.2 V for MoO₃ nanoparticles in 1 M LiCl aqueous electrolyte. In this way, we can still avoid the decomposition of aqueous electrolyte under an operation voltage 2.0 V, reaching a safe performance of both electrodes during long cycling.

In order to verify the high power density and energy density of the ASC, CV measurements and discharge curves are showed in Fig. 6. By expressing the total cell voltage as the sum of the potential range of MnO₂@rGO and MoO₃@rGO electrodes, the hybridized nanostructured ASC can be operated up to 2 V. Fig. 6a shows the CV curves of an optimized ASC full cell measured at different scan rates between 0 and 2.0 V in 1 M LiCl electrolyte. These CV curves exhibit distorted rectangular shapes indicating both pseudocapacitance and EDLC contributions. In order to further evaluate the performance of unit cell, we measure discharge curves at various current density (Fig. 6b). Since the ASC two electrodes connect in series, the voltage of ASC to be imposed has no change, the ASC volume capacitor is slightly (base on the total electrode volume) less than 1/4 volume capacitor of each electrode. Fig. 6c show volumetric capacitor of ASC at different current density. The specific capacitance calculated from the GCD curves at a current density of 0.1 A cm⁻³ is 62.7 F cm⁻³. Profiting from the unique and suitable structure, the ASC exhibits high power density and energy density (Fig. 6d).^33–35 The ASC with a cell voltage of 2.0 V exhibits an energy density of 34.6 mW h cm⁻³ (corresponding energy density 25.2 W h kg⁻¹) at a power density of 100 mW cm⁻³ (corresponding energy density 72.8 W kg⁻¹). Electrochemical impedance spectroscopy study (EIS) analysis is a principle method to examine the fundamental behavior of electrode materials for supercapacitor. The Nyquist plots reveal the impedance of ASC in the frequency range 100 000–0.01 Hz at an open circuit potential with an AC perturbation of 10 mV (Fig. 6e). It exhibits a negligible 45° Warburg region, indicating fast ion transport at the interface of active materials-electrolyte. In the low frequency portion of the spectrum, the impedance spectra tend towards almost a vertical line where the imaginary part of impedance increases rapidly, showing the perfect capacitive behaviour of the ion diffusion in the electrode structure.³⁶ Long cycle life is an important requirement for supercapacitor. Fig. 6f shows the cycle life test during 3000 cycles for ASC which was carried out by repeating the CV test between 0 and 2.0 V at a scan rate of 50 mV s⁻¹. The capacitance retention is 94.2% when the ASC cell stands 3000 cycles, which is much better than other pseudocapacitors.^37–39 It is worth noting that the specific capacitance sharply increases at the initial 400 cycles, which is probably related to an improvement in the surface wetting of the electrode by electrolyte and activation of the active materials.⁴⁰ This can be rationalized by hydrophobic nature of graphene.

Fig. 6 (a) CV curves of the ASC based on MnO₂@rGO and MoO₃@rGO composite electrodes, (b) discharge curves of the ASC at different current densities, (c) volumetric capacitances of the ASC at different current density, (c) CV curves of the SASC at different scan rates, (d) comparison of the volumetric and gravimetric capacitances of ASC with other carbon electrodes in aqueous electrolytes, (e) Nyquist plots of the ASC, (f) cycle performance of the optimized ASC within a voltage window of 2.0 V at a scan rate of 50 mV s⁻¹.

Conclusions

We successfully develop an ASC using freestanding MnO₂@rGO and MoO₃@rGO film as positive and negative electrode, respectively. We demonstrate that coupling the MnO₂@rGO and MoO₃@rGO composites can produce ASCs with high energy and high output voltage in aqueous electrolyte. This hybridized nanostructure ASC presented here aims at freestanding, flexible and high energy density for applications which also maintains low cost and using environmental friendly materials practical applications. The simple fabricating process and the cell configuration of the ASC provide a promising research direction for low cost, environmental friendly, freestanding, flexible, high energy density, and high output voltage supercapacitors.

Acknowledgements

This work was supported by the National Basic Research Program (no. 2011CB933300) of China, the National Natural Science Foundation of China(no. 11204093 and 11374110), and the Fundamental Research Funds for the Central Universities (HUST: no. 2014TS124).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: See DOI: 10.1039/c5ra06152k

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