A three-dimensional MnO₂/graphene hybrid as a binder-free supercapacitor electrode

Abstract

Highly aligned manganese dioxide (MnO₂) nanowall arrays electrodeposited onto Ti sheets are used as substrates to grow graphene (GR) through chemical vapor deposition (CVD), thus forming a three-dimensional (3D) MnO₂/GR hybrid composite. Furthermore, a 3D MnO₂/GR hybrid with different structures and properties has been prepared at different temperatures. The as-prepared hybrid materials could be directly used as supercapacitor electrodes without any binder and conductive additive, and fully maintain the high conductivity and high specific area of GR, and large pseudocapacitance of MnO₂ nanowall arrays. In aqueous electrolytes, the hybrids show a high specific capacitance of ∼326.33 F g⁻¹ with good cycling stability at the scan rate of 200 mV s⁻¹ and high energy density of 23.68 W h kg⁻¹ while maintaining high power density of 7270 W kg⁻¹. The preparation method provides a novel method to fabricate 3D graphene-based composite materials, and the as obtained hybrid electrode demonstrates its potential applications in supercapacitors.

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1. Introduction

In response to the gradual depletion of fossil fuels and the increasing severity of environmental pollution problems, developing sustainable and renewable energy storage devices has become increasingly urgent in order to meet future demands. Supercapacitors are regarded as efficient and environmentally friendly energy storage and supply technology for its high power density and low pollution. Such “high power-density” devices, however, have low energy density compared to batteries. To improve the performance and promote the commercialization of supercapacitors, it is important to explore new electrode materials with low cost, high performance and environmental benignity. Carbon-based materials, as supercapacitor electrodes are widely concerned^1–7 due to their low cost and good electrical conductivity. Thereinto graphene (GR) is the ideal electrode materials because of the outstanding electrical conductivity, large specific surface area and mechanical properties. So far many methods have been developed to prepare graphene^8–12 including chemical vapor deposition (CVD),⁸ micromechanical cleavage,^9,10 and chemical reduction of graphite oxide (GO).^11,12 Among these methods, chemical reduction of GO is the predominant method in the mass production of graphene for industrial applications. However, GR exhibits an undesired specific surface area (200–500 m² g⁻¹) far below its theoretical value (2675 m² g⁻¹) due to the intensive stacking among these two dimensional sheets. To solve this problem, 3D GR has become a hot research topic^13,14 because this structure could sufficiently maintain the features of individual GR and generate a large specific surface area. Yu et al.¹³ reported a 3D mechanically strong, electrically conductive, and thermally stable self-assembled GR hydrogel (SGH) with a high specific capacitance, which was prepared from GO by a one-step hydrothermal method, however, the as-prepared composite suffered from poor conductivity with the presence of a large amount of water. Huang et al.¹⁴ fabricated assembled embossed chemically modified GR film by using polystyrene colloidal particles as a sacrificial template and explored its use in supercapacitor electrodes. Nevertheless, this three-dimensional GR structure is unstable and easy to collapse.

Currently, metal oxides have been extensively applied as the spacer to separate GR nanosheets in view of their high pseudocapacitance.^15–17 Among metal oxides, manganese dioxide (MnO₂) is regarded as a promising candidate for supercapacitors due to its low cost, high natural abundance, high theoretical capacity (∼1370 F g⁻¹), and non-toxicity.^18,19 More significantly, unlike other electrode materials, which should be used in strong acidic or alkaline electrolytes, MnO₂ can be used in neutral aqueous electrolytes, and hence can meet the requirements for “green electrolyte” in supercapacitors. Furthermore, compared to some traditional backing materials such as Ni, Co, Pt, Ru etc.,²⁰ MnO₂ used as a substrate to grow GR is beneficial to reduce CVD reaction temperature, however, the low surface area and poor electronic conductivity (10⁻⁵ to 10⁻⁶ S cm⁻¹) remain the major problems. One promising approach is to fabricate 3D MnO₂/GR composite structures, where the hybrid can obtain the synergistic effect of individual constituents. Various approaches to synthesize 3D MnO₂/GR electrodes have been reported,^15–17 including physical mixing, microwave-assisted method, chemical co-precipitation and electrochemical deposition. For instance, MnO₂/GR electrodes synthesized by soft chemical route exhibited a specific capacitance (SC) of 210 F g⁻¹ (ref. 15) and those produced by microwave-assisted method displayed SC of 310 F g⁻¹.¹⁶ Chen et al.¹⁷ reported a maximum energy density of 30.4 W h kg⁻¹ and power density of 5 kW kg⁻¹ using GR/MnO₂/GR hybrid cells.

In this work, an effective two-step approach is developed to fabricate high-quality but low cost 3D MnO₂/GR hybrids through one-step CVD and an electrodeposition method. The so-obtained 3D MnO₂/GR hybrid presents a high specific capacitance of 326.33 F g⁻¹ at the scan rates of 200 mV s⁻¹ with capacitance retention of ∼92% and a high energy density of 23.68 W h kg⁻¹ after 1000 cyclic voltammogram (CV) cycles.

2. Experimental

2.1 Preparation of substrate of aligned MnO₂ nanowall arrays

The precursor solutions were prepared by dissolving manganese acetate, Mn(CH₃COO)₂·4H₂O (99+%, Alfa Aesar) and anhydrous sodium sulfate, Na₂SO₄ (99%, J. T. Baker) into deionised (DI) water in a concentration of 0.15 M for both precursors. The solution was stirred at room temperature for 1 h to ensure the complete dissolution of both solutes prior to the electrodeposition experiments.

Schematic illustration of electrodeposition of the MnO₂ nanowall arrays was presented in Fig. S1.† Highly aligned MnO₂ nanowall arrays were obtained by cathodic electrodeposition onto a Ti substrate with an exposed surface area of 1 cm². Ti sheets were rinsed with ethanol and acetone before electrochemical deposition. The anode and cathode of the adjustable constant voltage source is connected to two same Ti sheets, separately (the horizontal distance between the two sheets is 2 cm). The Ti sheets were soak in a mixed aqueous solution of manganese acetate and sodium sulfate. The deposition was induced by a change of the voltage (voltage was maintained between 5.0–8.0 V). The deposition time was controlled within 15 min.

2.2 Fabrication of MnO₂/GR composites

The preparation process was shown in Scheme 1. The aligned MnO₂ nanowall arrays on the surface of Ti substrate were placed into the tube furnace and heated to 600, 700 °C, individually at the heating rate of 15 °C min⁻¹ under Ar (200 sccm) and H₂ (100 sccm). 0.25 mL of benzene used as carbon source was injected into the furnace at the rate of 0.01 mL min⁻¹ for 30 min. After 5 min of reaction-gas mixture flow, the samples were rapidly cooled to room temperature at a rate of 100 °C min⁻¹ under Ar (200 sccm) and H₂ (100 sccm). A series of 3D MnO₂/GR composites were prepared and denoted as MnO₂/GR600, MnO₂/GR700, MnO₂/GR800, respectively.

Scheme 1 Synthetic scheme of the 3D MnO₂/GR composite.

2.3 Material characterization

The morphologies and microstructures of the samples were examined by scanning electron microscopy (SEM; XL30), transmission electron microscopy (TEM; Hitachi H-600), and High Resolution TEM (HRTEM). The crystal structures of the as-prepared products were characterized by an X-ray diffractometer (XRD; D/Max 2500 V PC⁻¹, Cu-Kα radiation) with a scan speed of 2 min⁻¹. Raman spectra were collected through a Renishaw 2000 model confocal microscopy Raman spectrometer with a CCD detector and a holographic notch filter (Renishaw Ltd, Gloucestershire, U.K.) at ambient conditions, using the radiation of 514.5 nm from an air-cooled argon ion laser to excite the SERS.

2.4 Electrochemical measurements

The electrochemical experiments were carried out using a two-electrode system at ambient temperature, with 1.5 M Li₂SO₄ as the electrolyte solution. The as-synthesized materials were directly used as the working electrode and the counter electrode, respectively. Electrochemical measurements were all conducted on a CHI 660D electrochemical workstation (Shanghai, Chenhua). The CV curves of the electrodes were measured between 0 and 1.2 V at different scan rates from 50 to 400 mV s⁻¹. The galvano-static cycling for each electrode was performed in the potential range from 0 to 1 V at different current densities of 1, 1.5, 2, 2.5 and 3 A g⁻¹, and electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 100 kHz to 1 Hz.

3. Results and discussion

3.1 Structure and morphology

Fig. 1A presents the photographs of Ti, MnO₂, MnO₂700 (MnO₂ heated in 700 °C) and MnO₂/GR composites at different temperatures. It can be seen that the white original Ti plate changes to yellow after electrodeposition of MnO₂. After GR grown on the surface of MnO₂ through CVD, the color of the materials changes gradually into black, which indicates that the surface of MnO₂ is coated with a layer of carbon. In order to investigate whether the morphology of MnO₂ is destroyed at high temperature, a comparison of the pristine MnO₂ and heated MnO₂ is necessary. Fig. 1B and C show two SEM images of MnO₂ before and after the heating process (700 °C). Clearly, electrodeposition synthesis of the flower-like MnO₂ nanostructures can be observed before and after the heating process, which demonstrates that the morphology of MnO₂ is not destroyed after the heating process. More significantly, the flower-like MnO₂ nanostructures can provide the high accessibility of electrolytic ions for shorten diffusion paths, which is beneficial to obtain good electrochemical capacitance performance. Unfortunately, when the MnO₂ sample is heated to 800 °C (Fig. S2†), the flower-like MnO₂ nanostructures has been destroyed. Fig. 1D and E show the SEM images of GR grown on the surfaces of MnO₂ nanowall substrates at different temperatures (600 and 700 °C), respectively. The surfaces of MnO₂ nanowall substrates are both covered with a layer of GR, which will be further confirmed in the Raman test.

Fig. 1 (A) Photographs of Ti, MnO₂, MnO₂700 (MnO₂ heated in 700 °C), and MnO₂/GR composites at different temperature. (B and C) SEM images of MnO₂ before and after the heating. (D and E) SEM images of 3D MnO₂/GR in 600 and 700 °C, respectively.

The XRD patterns of the GR grown on the surface of the MnO₂ nanowall substrates at 600 and 700 °C are shown in Fig. 2A. Two characteristic peaks of the MnO₂ at 2θ around 37° and 66° are displayed, which can be indexed to birnessite-type MnO₂ (JCPDS 42-1317).²¹ Furthermore, from the XRD analysis, the d-spacing of MnO₂ is about 7.0 Å, which is consistent with the high magnification TEM image (Fig. 3B) and the previous report.²² The diffraction peaks at 2θ around 43° that is in keeping with the literature report²³ indicates that GR has been obtained successfully on the surface of the MnO₂ nanowall arrays after CVD.

Fig. 2 (A) XRD patterns of the MnO₂ nanowall electrodeposited and the GR grown on the surface of the MnO₂ nanowall substrates at 600 and 700 °C, respectively. (B) Raman spectra of pure GR (700 °C) after the MnO₂ nanowall was removed and the GR grown on the surface of the MnO₂ nanowall substrates at 600 and 700 °C, respectively.

Fig. 3 (A) Low and (B) high magnification TEM images of MnO₂/GR700 composite, showing the graphene covered on the surface of the MnO₂ nanowall. (C) The TEM images of pure graphene after the MnO₂ nanowall was removed. (D) The scanning electron microscopic elemental map of 3D MnO₂/GR composite.

The structure of 3D MnO₂/GR composites are further confirmed by Raman spectra (Fig. 2B). Except from the D band (about 1350 cm⁻¹) and G band (1590 cm⁻¹) associated with GR, the major Raman bands located at 500–700 cm⁻¹ for MnO₂/GR sample matches well with the major vibrational features of MnO₂ compounds reported previously.²² Furthermore, compared to the value of I_D/I_G at 600 °C, the lower value at 700 °C indicates fewer defects. Consequently, the following tests are based on MnO₂/GR700.

The microstructure of the MnO₂/GR700 composite is further investigated by TEM (see Fig. 3). Obviously, the GR sheets are closely attached on the surface of the MnO₂ nanowall arrays (Fig. 3A). The flower-like morphology is the characteristic of the birnessite-type MnO₂.^24,25 As shown in Fig. 3B, the interplanar spacing of MnO₂ nanowall arrays and the GR sheets are measured to be 0.7 and 0.34 nm, respectively, which are in line with the literature report for birnessite-type MnO₂.²⁴ To investigate the morphology of GR, the TEM image of GR is examined by etching away MnO₂ with 3 M HCl. The ultrathin morphology of the derived GR can be seen clearly in Fig. 3C. The related EDS spectrum (Fig. 3D) reveals the Mn, O signal from MnO₂ as expected. The C signal is derived from the GR and the Ti signal is derived from the Ti sheets.

3.2 Electrochemical properties

To evaluate the electrical behaviour of the as-fabricated 3D MnO₂/GR700 hybrids, the electrochemical capacitive performance for each type of electrode is measured by CV, galvanostatic charge–discharge (GCD) and EIS tests. The CV profiles of electrodeposition of MnO₂700 nanowall arrays (Fig. S3†) at different scan rates from 50 to 400 mV s⁻¹ show nearly rectangular shapes, indicating good pseudocapacitive behavior attributed to a continuous and reversible faradaic redox transition of MnO₂ over the potential range. Furthermore, Fig. 4A shows the CV curves of the MnO₂/GR700 hybrid at various scan rates ranging from 50 to 400 mV s⁻¹ within an electrochemical window from 0 to 1.2 V. As the scan rate increases, the current response also increases without any obvious change in the shape of the CV curves, which presents a good rate performance. The rectangular and symmetric shape of the CV curve is also observed at a high scan rate of 400 mV s⁻¹ due to the low contact resistance of the electrode. Fig. 4B displays a comparison of CV curves of MnO₂, MnO₂/GR600, MnO₂/GR700 and MnO₂/GR800 hybrids at a scan rate of 200 mV s⁻¹. The specific capacitance can be calculated according to the following equation:^26,27
where C_m is the specific capacitance (F g⁻¹), I is the response current (A), m (∼0.1 mg) is the mass of the electroactive materials (g), and C_m is the potential window during the CV measurements process (V). The specific capacitances of MnO₂, MnO₂/GR600, MnO₂/GR700 and MnO₂/GR800 electrodes are 183.23 F g⁻¹, 208.76 F g⁻¹, 326.33 F g⁻¹ and 136.28 F g⁻¹, respectively. Hence, we can confirm that the GR supplies a large double layer capacitance while MnO₂ provides a large pseudocapacitive contribution to the capacitance of MnO₂/GR. In addition, more than 60% decrease in the capacitance of MnO₂/GR800 supports the SEM analysis of MnO₂/GR800 hybrid (Fig. S2†). Therefore, the optimal temperature for the growth of GR on MnO₂ substrates should be 700 °C. Furthermore, the picture in Fig. 4C is the CV curve of MnO₂ at the scan rate of 100 mV s⁻¹. It presents a bigger voltage drop compared with the MnO₂/GR hybrid, which implies that the internal resistance has decreased largely after the growth of GR on the surface of the MnO₂ nanowall arrays. This is because GR in the hybrid acts as electronic conductive channels to increase the electrical conductivity, which matches well with the GCD curves of the MnO₂/GR (Fig. 4D). Fig. 4D shows GCD curves for MnO₂, MnO₂/GR700 electrodes at the current density of 3.0 A g⁻¹. It can be seen that the curve are highly linear and symmetrical without obvious voltage drop compared with the MnO₂, indicating a rapid I–V response, an excellent electrochemical reversibility and little overall resistance of this material (more data about the GCD curves of MnO₂/GR700 electrode at different current densities of 1, 1.5, 2, 2.5 and 3 A g⁻¹ are given in Fig. S4 in the ESI†). It can be seen from Fig. S4† that all the GCD curves of MnO₂/GR700 at different current densities are highly linear and almost typical isosceles triangular in shape, which further demonstrates that the electrode has ideal capacitive characteristics and excellent electrochemical reversibility. EIS measurements are carried out in a frequency range of 100 kHz to 1 Hz to further evaluate the electrochemical and structural characteristics of the electrode material (Fig. 4E). Compared with the MnO₂, the EIS Nyquist plot of the MnO₂/GR700 has a shorter arc located at high frequencies followed by an inclined line with a slope about 45°. The span of the arc is indicative of the charge-transfer resistances of the electrode materials.²⁸ It can make clear from Fig. 4E that, the span of the arc in Nyquist plot decreases when GR is added, which is consistent with the results of the CV and GCD studies. Obviously, the low resistances of MnO₂/GR700 electrode should stem from their intrinsic high conductivity, rich porous 3D network and the flower-like morphology of MnO₂. Fig. 4F displays the cyclic stability of MnO₂/GR700 at the scan rate of 200 mV s⁻¹ for 1000 cycles. It is clear that the MnO₂/GR700 electrode presents long-term cycle stability with capacitance retention of ∼92% after 1000 CV cycles (approximately 300 F g⁻¹). These results fully reflect the merit of the 3D architecture cherished by the MnO₂/GR electrode.

Fig. 4 (A) CV curves of the MnO₂/GR700 composite at different scan rates. (B) Comparison CV curves of the MnO₂/GR600, MnO₂/GR700, MnO₂/GR800 and MnO₂ at the scan rate of 200 mV s⁻¹. (C) CV curves of MnO₂ with absence of GR at the scan rate of 100 mV s⁻¹. (D) Galvanostatic charge–discharge of the MnO₂/GR700 and MnO₂. (E) Ragone and Nyquist plots of the supercapacitors based on the MnO₂/GR700 and MnO₂. (F) Cycle life curves of the supercapacitors based on MnO₂/GR700 composites at the scan rate of 200 mV s⁻¹.

Energy and power densities are two crucial parameters for evaluating the electrochemical performance of supercapacitors. The energy density (E in W h kg⁻¹) and power density (P in W kg⁻¹) of the electrode can be further evaluated from the CV curves at different scan rates using the following eqn (1) and (2), respectively:²⁹

P = E/t (2)

where C_m is the specific capacitance (F g⁻¹), ΔV is the potential window during the CV measurements process (V) and t is the discharge time of the CV curves (s). Fig. 5 exhibits the energy and power density for the MnO₂/GR700 hybrid electrode, where the reported data of some typical energy storage device such as batteries³⁰ and MnO₂–carbon based supercapacitors.^18,31–36 By comparison, our work displays a high energy density of 23.68 W h kg⁻¹ while maintaining high power density of 7270 W kg⁻¹ at the scan rate of 200 mV s⁻¹, which can be comparable with the previously reported high-performance energy-storage devices (the related data are displayed in the Table S1 in the ESI†). In addition, more data for large mass loading (1 and 3 mg) are also given in Fig. S5(A and B) in the ESI.† Obviously, the results are consistent with the analysis results of Fig. 4A, and the energy and power densities can reach to 24.86, 25.13 W h kg⁻¹ and 6997, 6812 W kg⁻¹ at scan rate of 200 mV s⁻¹, respectively, which also displays high-performance electrochemical energy-storage of 3D MnO₂/GR700 electrode. Thus, it is to be expected: the as-fabricated 3D MnO₂/GR hybrid has enormous potential applications in supercapacitors.

Fig. 5 Ragone plot of the energy density vs. power density for batteries and carbon-based supercapacitors.

4. Conclusions

In summary, a novel strategy has been put forward to fabricate 3D MnO₂/GR hybrid through chemical vapor deposition on the substrate of aligned MnO₂ nanowall arrays that were electrochemically deposited onto Ti sheets. An optimal condition of preparation has been also obtained by comparing the electrical behaviour of MnO₂/GR hybrid electrodes fabricated at different temperatures. The as-synthesized MnO₂/GR700 electrode shows a specific capacitance of 326.33 F g⁻¹ at the scan rate of 200 mV s⁻¹ and can be cycled reversibly in the voltage region of 0 to 1.2 V. Moreover, it can maintain an energy density of 23.68 W h kg⁻¹ even at the power density of 7270 W kg⁻¹, which indicates the high-efficiency energy storage of MnO₂/GR700 hybrid. We expect this work will open up new opportunities for the application of low-cost, high-efficiency 3D GR-based electrodes materials in new energy electric vehicle.

Acknowledgements

This work was supported by the Specialized Research Fund for the Doctoral Program of Higher Education (20116102110014), National Natural Science Foundation of China (Grant No.: 51172184), the Key Science and Technology Program of Shaanxi Province, China (2013K09-03), and the Innovation Fund of China Aerospace Science and Technology (CASC200906).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra14411f

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