Binder-free three-dimensional porous Mn₃O₄ nanorods/reduced graphene oxide paper-like electrodes for electrochemical energy storage

Abstract

This research demonstrates novel flexible and binder-free Mn₃O₄ nanorods (NRs)/reduced graphene oxide (rGO) hybrid papers with unique three-dimensional nanoporous networks were fabricated by filtration and a hydrothermal reduction process, where rGO acts not only as a flexible substrate but also as an electron conductor. The three-dimensional nanoporous networks were generated by the homogeneous intercalation of Mn₃O₄ NRs into the lamellar rGO layers, which exhibited excellent mechanical stability and provided electrically conducting channels to promote electrolyte penetration when used as electrodes for Li-ion batteries (LIBs) and supercapacitors. The prepared Mn₃O₄ NRs/rGO hybrid lamellar papers demonstrated excellent cyclic retention with the specific capacity of 669.6 mA h g⁻¹ after 100 cycles in LIBs, which is 9 times higher than 65.8 mA h g⁻¹ of γ-MnOOH/Mn₃O₄ mixed phase nanorods. Additionally, the three-dimensional porous hybrid Mn₃O₄ NRs/rGO papers also exhibit superior specific capacitance of 204.2 F g⁻¹, two times higher than that of γ-MnOOH/Mn₃O₄ mixed phase nanorods, and only decreases by 10% after 2000 cycles in the supercapacitor. These Mn₃O₄ NRs/rGO papers hold promising potential for flexible electrochemical energy storage devices.

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Introduction

Flexible Li-ion batteries (LIBs) and supercapacitors have attracted great attention in portable electronics, rolled-up displays, implantable devices, and other applications.^1–3 Carbon-based materials, such as carbon nanotubes (CNTs), carbon spheres, carbon fibers, and graphene have been extensively developed to construct high performance flexible LIBs and supercapacitors.^4–7 To further enhance their electrochemical performance, different transition metal oxides such as Co₃O₄, NiO, CuO and Fe₂O₃ were used to modify the surface chemistry of the carbon materials.^8–11 Because of the relatively low cost, non-toxicity and high specific capacitance, manganese oxides are particularly attractive as potential candidates for electrochemical energy storage.^12–14 Compared with other types of manganese oxides, Mn₃O₄ shows a stable crystal structure and high theoretical specific capacity (937 mA h g⁻¹), which has potential applications as anode materials for LIBs.¹⁵ However, there are two suspending problems that may greatly prohibit the application of Mn₃O₄ anode materials in high-performance LIBs. Firstly, the huge volume change during Li insertion/extraction can lead to the agglomerations and mechanical instabilities of Mn₃O₄. Secondly, low electrical conductivity of Mn₃O₄ cause rather low rate capability.^16–18 Reduced graphene oxide (rGO) is a promising substrate to fabricate flexible electrode due to its good conductivity and mechanics. The introduction of rGO to CuO, SnO₂, Fe₃O₄, Mn₃O₄, results in a remarkable improvement in electrochemical performance.^19–22 Binder-free Mn₃O₄ nanoparticles/rGO paper electrode prepared by Duan et al. exhibited excellent mechanical stability and enhanced ion transportation.²² Dai and Yang groups reported Mn₃O₄ nanoparticles/rGO composite with good rate capability and cycling stability for LIBs applications, and they attribute to the strong interactions between the graphene substrates and the Mn₃O₄ nanoparticles.^23,24 but there is rare report about the flexible Mn₃O₄ one-dimensional nanostructures/rGO hybrid papers used as anode electrode in LIBs. Our group recently reported a flexible CuO NSs/r-GO paper with much higher specific capacity and excellent cycling performance than the pristine CuO in supercapacitor and LIBs, respectively.^19,25 This can be ascribed to the nanoporous structure composed of CuO NSs spaced constructive r-GO layers, which provides an efficient electrically conducting channels in favor of electrolyte ion penetration and utilizes the surface area of rGO sheets as much as possible. In this paper, based on the above research, unique nanoporous Mn₃O₄ nanorods (NRs)/rGO paper electrode was prepared by hydrothermal treating the β-MnOOH nanofibers (NFs)/GO composite paper at 180 °C. The β-MnOOH NFs/GO composite paper were prepared through vacuum filtering from the mixture of β-MnOOH NFs and GO sheets. The GO sheets provided a confine space for the in-situ conversion of β-MnOOH (NFs) to Mn₃O₄ NRs. The sandwiched network in the Mn₃O₄ NRs/rGO paper can effectively prevent the aggregation of graphene nanosheets and provide electrically conducting channels for the fast electron transfer between the Mn₃O₄ NRs and charge collectors. The flexible Mn₃O₄ NRs/rGO paper showed a capacity of 204.2 F g⁻¹ and demonstrated excellent cyclic retention with the specific capacity of 669.6 mA h g⁻¹ after 100 cycles when served as binder-free electrodes for supercapacitors and LIBs, respectively, which are above 2 times and 9 times higher than those of the electrode converted hydrothermally from β-MnOOH NFs under the same conditions.

Experimental

Materials

Mn(NO₃)₂ and aminoethanol (AE) were purchased from Sigma-Aldrich and used without purification. Graphite powder (99.95%) was purchased from Aladdin. The polycarbonate (PC) membranes (Whatman) with a pore size of 400 nm, and effective diameters of 47 mm (porosity: 10%) were used for the preparation of the flexible films by filtration.

Synthesis of β-MnOOH NFs and GO

The β-MnOOH NFs were synthesized by the method reported previously.²⁶ In brief, an aqueous solution of 0.4 mM Mn(NO₃)₂ was mixed with an equal volume of 0.8 mM AE aqueous solution under vigorous stirring for one day to form β-MnOOH NFs. GO was prepared from graphite powder by a modified Hummers' method as described elsewhere.^25,27 Then the as-synthesized GO was dispersed ultrasonically in distilled water (with a concentration of 0.2 mg mL⁻¹). The pH value of the dispersion was adjusted near neuter using NaOH and HCl solutions.

Synthesis of Mn₃O₄ NRs/rGO

The route of preparation of β-MnOOH NFs/GO composite is demonstrated in Scheme 1. A certain amount of β-MnOOH floccule dispersion was mixed with GO dispersion under vigorous stirring. Then, flexible β-MnOOH NFs/GO lamellar paper was fabricated from the mixture by a simple filtration method using a SIBATA filtration system and peeled off from the PC membrane. Finally, the obtained free-standing β-MnOOH NFs/GO paper were hydrothermally reduced at 180 °C for 10 h. After that, the paper was washed alternatively with ethanol and distilled water several times before dried at 60 °C for 4 h. In order to obtain the optimal electrochemical performance, three samples with mass ratios of β-MnOOH NFs to GO of 2 : 1, 1 : 1 and 1 : 2 were fabricated, and named as Mn₃O₄ NRs/rGO-2-1, Mn₃O₄ NRs/rGO-1-1 and Mn₃O₄ NRs/rGO-1-2 after hydrothermal treatment, respectively. Pure β-MnOOH NFs were also hydrothermally treated at 180 °C for 10 h for reference.

Scheme 1 Schematic of the synthesis procedure of Mn₃O₄ NRs/rGO paper.

Electrode preparation and electrochemical characterization

For LIBs, the hybrid Mn₃O₄ NRs/r-GO paper with weight of 2.1 mg was directly used as the working electrode without binder in a coin-type cell with a lithium foil counter and reference. The electrolyte employed in the cell was 1 M LiPF₆ in a 50 : 50 (w/w) mixture of ethylene carbonate and diethyl carbonate. The galvanostatic (GV) charge–discharge performances of the cell were tested in the voltage range of 0.01–3.0 V under a constant current density of 100 mA g⁻¹ by EQ-BST8-WA battery-test system.

For supercapacitor, 1.9 mg Mn₃O₄ NRs/rGO paper was binder-freely used as the work electrode and sandwiched between two pieces of foam Ni sheets. The capacitance measurement was conducted by a CHI 660D electrochemical workstation in a 1 M Na₂SO₄ aqueous solution with a platinum counter electrode and an Ag/AgCl reference electrode. Cyclic voltammetry (CV), constant-current GV charge–discharge curves, and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical performances of Mn₃O₄ NRs/rGO papers.

Characterization

The phases of the samples were characterized by X-ray diffraction (XRD) using an X'Pert PRO (PANalytical, Netherlands) instrument with Cu Kα radiation. The morphologies and structures were characterized using scanning electronic microscopy (SEM) (Hitachi S-4800) and transmission electron microscopy (TEM, Philips CM200). Further evidence for the composition of the product was inferred from X-ray photoelectron spectroscopy (XPS), using an ESCALAB_250Xi X-ray photoelectron spectrometer with Al Kα X-ray as the excitation source. The specific surface area was calculated by Brunauer–Emmett–Teller (BET) method using a Quantachrome Autosorb-1 apparatus.

Results and discussion

Morphology and structures

The XRD patterns in Fig. 1a present the evolution of β-MnOOH NFs, β-MnOOH NFs/GO before and after the hydrothermal treatment. For β-MnOOH NFs, diffraction peaks at 2θ values of 19.3°, and 38.4° are observed, which match well with the crystal planes of the (002) and (312) of β-MnOOH as reported before.²⁶ After hydrothermal treatment at 180 °C for 10 h, tetragonal β-MnOOH is converted into a mixture phase of monoclinic γ-MnOOH (JCPDS 41-1379) and tetragonal Mn₃O₄ (JCPDS 24-0734) with the nearly 4 : 1 of ratio of γ-MnOOH to Mn₃O₄. From the XRD patterns of β-MnOOH NFs/GO composite paper, a new diffraction peak at 10.2° is indexed to GO, indicating the GO nanosheets have been successfully composited with β-MnOOH NFs. After hydrothermal treatment, the appearance of the (002) diffraction at 24.4° for Mn₃O₄ NRs/rGO paper gives evidence that the GO was successfully reduced to rGO during the hydrothermal process. In addition, the peaks marked by triangles in Mn₃O₄ NRs/rGO correspond to Mn₃O₄, which indicate the β-MnOOH NFs are successfully transformed to Mn₃O₄ NRs. No other impurity manganese oxide or hydroxide phases are observed. Zhang and Li et al.^28,29 used reductive organic solvent such as ethylene-diamine, ethylene glycol and propylene glycol to transform γ-MnOOH nanowires to Mn₃O₄ nanoparticles and big nanorods under hydrothermal conditions. In our experiments, pure water was used. Without rGO, pure Mn₃O₄ phase could not be obtained. But pure Mn₃O₄ are formed when GO presents. Hence, the GO plays an important role in the formation of pur-phase Mn₃O₄ NRs. The D and G bands in Raman spectra of β-MnOOH NFs/GO and Mn₃O₄ NRs/rGO are shown in Fig. 1b. The higher I_D/I_G ratio of Mn₃O₄ NRs/rGO suggests that abundant defects were introduced into the rGO during the reduction of exfoliated GO.^19,25 XPS spectra (Fig. 1c) of the Mn₃O₄ NRs/rGO paper shows the presence of C, O, and Mn elements. The high-resolution spectrum of Mn 2p (Fig. 1d) exhibits two peaks at 653.2 and 641.4 eV, corresponding to Mn 2p_1/2 and Mn 2p_3/2 levels of Mn₃O₄, respectively. The splitting width of 11.8 eV between the Mn 2p_1/2 and Mn 2p_3/2 is in accordance with the previously reported for Mn₃O₄.^24,30 The Mn 3s (Fig. 1e) with splitting width 5.5 eV, further confirms the presence of Mn₃O₄.³¹ The C 1s spectrum in Fig. 1f can be fitted into three peaks at 284.4, 285.9, and 288.2 eV, which are arising from C–C, C–O, and C=O bonds, respectively.^19,30 The intensities of C–O and C=O peak are weaker than that of C–C indicates that most oxygen-containing functional groups are removed after hydrothermal treatment, which is in good agreement with the XRD and Raman results.^25,30,31

Fig. 1 (a) XRD patterns of β-MnOOH NFs and β-MnOOH NFs/GO before and after the hydrothermal treatment, respectively; (b) Raman spectra of β-MnOOH NFs/GO and Mn₃O₄ NRs/rGO paper; (c) XPS spectra of the survey scan; (d) Mn 2p, (e) Mn 3s and (f) C 1s region of the Mn₃O₄ NRs/rGO paper.

Fig. 2a and b show the SEM images of β-MnOOH NFs before and after hydrothermal treatment, respectively. The as-synthesized β-MnOOH NFs are uniform with an average diameter of 15 nm and length up to 1.5 μm. After hydrothermal treatment, a large amount of rod-like nanostructures can be observed. They have an average diameter of about 80 nm and length of around 1 μm. After mixing β-MnOOH NFs with GO sheets and assembled to hybrid paper by filtration, Fig. 2c clearly demonstrates that the β-MnOOH NFs are uniformly distributed on the GO surface. After hydrothermal treated, Mn₃O₄ NRs are combined well with rGO as observed in Fig. 2d. On the most out surface, big nanorods are formed with diameter of about 80 nm similar to that formed in the pure β-MnOOH NFs case (Fig. 2b). However, the cross-section SEM images of Mn₃O₄ NRs/rGO in Fig. 2e and f indicate that the diameters of the nanorods formed in the inner spaces between the rGO sheets are uniform in diameter and smaller than that on the top surface. This indicates that the lamellar stacked GO sheets provided a nanoconfinement reaction space for the in-situ conversion of β-MnOOH NFs to the fine Mn₃O₄ NRs. This is further confirmed by the TEM results. The thickness of the film is ∼10 μm. From the fractured edges of the sandwich films, lamellar structures are obviously seen. The rGO sheets are intercalated by the Mn₃O₄ NRs to form sandwich structures. This unique porous structure could favor the electrolyte ions transport between the electrochemical active Mn₃O₄ NRs and rGO sheets. The inset in Fig. 2f shows that the paper had excellent flexibility. In addition, The N₂ adsorption/desorption isotherms of Mn₃O₄ NRs/rGO and γ-MnOOH/Mn₃O₄ NRs are shown in Fig. 3. Mn₃O₄ NRs/rGO presents the surface area of 34.8 m² g⁻¹, higher than 14.9 m² g⁻¹ of rGO²⁵ and 24.6 m² g⁻¹ of γ-MnOOH/Mn₃O₄ NRs. This could be attributed to the porous structure, in which Mn₃O₄ NRs expand the space between the rGO sheets.

Fig. 2 SEM images of β-MnOOH NFs before (a) and after (b) hydrothermal treatment, (c) β-MnOOH NFs/GO, and (d) Mn₃O₄ NRs/rGO, (e and f) cross-section SEM images of Mn₃O₄ NRs/rGO, which show good flexibility (the inset in (f)).

Fig. 3 Nitrogen adsorption and desorption isotherms of Mn₃O₄ NRs/rGO-1-1(a) and γ-MnOOH/Mn₃O₄ NRs (b).

Fig. 4 shows the TEM images of the original β-MnOOH NFs/GO paper and Mn₃O₄ NRs/rGO papers, respectively. In both case, the one dimensional structures are tightly and uniformly attached on the GO and rGO sheet surfaces. The Mn₃O₄ NRs is 30 nm in diameter, which is bigger than 15 nm of MnOOH NFs. This is in agreement with the SEM results. The insets in Fig. 4a and b are the corresponding selected area electron diffraction (SAED) results and assigned to β-MnOOH and Mn₃O₄, respectively, in consistent with the above XRD results.

Fig. 4 TEM images of β-MnOOH NFs/GO (a) and Mn₃O₄ NRs/rGO. (b) The insets are their corresponding SAED patterns.

LIBs performances

Coin cells were fabricated to measure the LIBs performance of Mn₃O₄ NRs/rGO hybrid films. Fig. 5a shows the CV curves of Mn₃O₄ NRs/rGO-1-1 electrode in the range of 0.01–3.0 V at a scanning rate of 0.1 mV s⁻¹. In the first cycle, the cathodic peak at 1.17 V can be attributed to the formation of a solid electrolyte interphase (SEI) layer mainly owing to the decomposition of electrolyte on the Mn₃O₄ surface. The reduction peak at 0.05 V was attributed to the decomposition of Mn₃O₄ into metallic Mn and Li₂O.^32,33 During the subsequent cycles, the shift to 0.37 V of reduction peak suggests the homogeneous porous structure of the Mn₃O₄ NRs/rGO is highly reversible. The peak intensity and integral areas of the third and forth cycles are similar to those of the second one, suggesting the good reversibility of lithium insertion and extraction reactions. The lithium storage mechanism between Li and Mn₃O₄ can be described by the following electrochemical conversion reactions:^32,34

Mn₃O₄ + 8Li⁺ + 8e⁻ ↔ 4Li₂O + 3Mn (1)

Fig. 5 (a and b) CV curves and charge–discharge curves of Mn₃O₄ NRs/rGO-1-1. (c) Cycling performance of Mn₃O₄ NRs/rGO-2-1, Mn₃O₄ NRs/rGO-1-1, Mn₃O₄ NRs/rGO-1-2, and γ-MnOOH/Mn₃O₄ NRs. (d) Cycling performances of Mn₃O₄ NRs/rGO hybrid papers at different rates.

Fig. 5b shows the charge–discharge profiles of the 1^st, 5^th and 10^th for Mn₃O₄ NRs/rGO-1-1 at a current density of 100 mA g⁻¹. The first discharge and charge capacities are 1299 and 731.2 mA h g⁻¹, respectively. The discharge capacity was much higher than the theoretical capacity of Mn₃O₄ (937 mA h g⁻¹) owing to the formation of the SEI layer and possible interfacial Li⁺ storage. The similar charge–discharge curves implied the electrochemical reactions proceed into the stable cycling stages.

Fig. 5c shows the cyclic performance of β-MnOOH NFs/GO, γ-MnOOH/Mn₃O₄ NRs and Mn₃O₄ NRs/rGO hybrid electrodes. It can be seen that the discharge capacity is maintained 184.0, 65.8, 572.5, 669.6 and 357.2 mA h g⁻¹ after 100 cycles for β-MnOOH NFs/GO-1-1, γ-MnOOH/Mn₃O₄ NRs, Mn₃O₄ NRs/rGO-2-1, Mn₃O₄ NRs/rGO-1-1 and Mn₃O₄ NRs/rGO-1-2, respectively. It is clearly that the performance of Mn₃O₄ NRs/rGO hybrid electrode in LIB is about ten times as that of γ-MnOOH/Mn₃O₄ NRs and 4 times as that of β-MnOOH NFs/GO electrodes, respectively. β-MnOOH NFs/GO-1-1 showed lower capacitance due to the poor electrical conductivity of GO and β-MnOOH. Evidently, the reduced GO greatly improves the cyclic stability of Mn₃O₄ NRs/rGO hybrid electrodes. The optimal performance, observed in Mn₃O₄ NRs/rGO-1-1, is an initial discharge capacity of 1299 mA h g⁻¹ and a first columbic efficiency of 56.3%. And the efficiency increases in subsequent cycles and finally stabilizes at nearly 100% after 3^rd cycle, confirming a good reversibility. The good cycling performance and highly reversible capacity may be attributed to the following three reasons. First, in the unique lamellar structure, rGO can serve as the continuous conductive substrate and effectively decrease the inner resistance of the electrode. Second, Mn₃O₄ NRs significantly expand the inter-space between the rGO sheets to form 3-dimension porous networks, this can provide fast transport pathways for the electrolyte ions to enhance interaction between Mn₃O₄ NRs and the rGO sheets. Third, the porous structure with more space can accommodate the volume change and suppress the aggregation of Mn₃O₄ NRs upon Li⁺ insertion/extraction. The increased capacity after the 40^th cycle is usually a result of the reversible growth of a polymeric gel-like film.^32,35,36 Besides, the rate performances (Fig. 5d) of these three Mn₃O₄ NRs/rGO electrodes were measured at different current densities, Mn₃O₄ NRs/rGO-1-1 exhibits the best capacity retention over 685 mA h g⁻¹ in the first 10 cycles at 100 mA g⁻¹, and this value decreases with increasing discharge rate. After returning the rate of 0.1 A g⁻¹, the specific capacitance of Mn₃O₄ NRs/rGO-2-1, Mn₃O₄ NRs/rGO-1-1 and Mn₃O₄ NRs/rGO-1-2 are recovered to 446.0, 566.1 and 283.6 mA h g⁻¹, respectively. The rate performances reflected that increasing electrical conductivity by introducing rGO could improve the rate stability of Mn₃O₄. The electrochemical results indicate that the graphene based conductive networks are beneficial to relax the volume expansion and improve electrochemical properties.

The Mn₃O₄ NRs/rGO electrodes were also fabricated in high performance supercapacitor and evaluated by CV and charge-discharge in three-electrode systems. Fig. 6a shows the CV curves of γ-MnOOH/Mn₃O₄ NRs and Mn₃O₄ NRs/rGO samples at the scan rate of 2 mV s⁻¹. The specific capacitance is in proportional to the area under the CV curves. It is clearly shown that the capacitance of Mn₃O₄ NRs/rGO hybrid paper is much larger than that of γ-MnOOH/Mn₃O₄ NRs. The capacitance calculated from the CV curves at different scan rates are shown in Fig. 6b. The hybrid electrodes after the introduction of rGO exhibit higher specific capacitance. Of all, Mn₃O₄ NRs/rGO-1-1 also shows the highest capacitance of 204.2 F g⁻¹ at the scan rate of 2 mV s⁻¹. The capacitances of the rGO,²⁵ γ-MnOOH/Mn₃O₄ NRs, Mn₃O₄ NRs/rGO-2-1 and Mn₃O₄ NRs/rGO-1-2 samples are 66.0, 109.8, 119.2, and 138.6 F g⁻¹, respectively. The less specific capacitance of Mn₃O₄ NRs/rGO-2-1 is possibly ascribed to the lower electric conductivity. However, due to the lower capacity of rGO than Mn₃O₄, the high rGO content in Mn₃O₄ NRs/rGO-1-2 also shows an adverse effect on the electrochemical performance. The excellent performance of the Mn₃O₄ NRs/rGO-1-1 hybrid paper is attributed to the synergistic integration of the two functional nanomaterials. Besides, the inter-space between the rGO sheets can be expanded by Mn₃O₄ NRs, and facilitate electrolyte penetration and interaction with rGO as much as possible.^7,37

Fig. 6 (a) CV curves of Mn₃O₄ NRs/rGO-2-1, Mn₃O₄ NRs/rGO-1-1, Mn₃O₄ NRs/rGO-1-2, and γ-MnOOH/Mn₃O₄ NRs at the scan rate of 2 mV s⁻¹ (b) capacitances of the above four products at different scan rates derived from CV curves. (c) GV constant-current charge–discharge performance of Mn₃O₄ NRs/rGO-1-1. (d) The corresponding capacitances plot from (c); (e) cycling performances of Mn₃O₄ NRs/rGO-1-1 at the current density of 5 A g⁻¹. (f) Nyquist plot of the EIS of the Mn₃O₄ NRs/rGO-1-1 and γ-MnOOH/Mn₃O₄ NRs. The inset shows the equivalent circuit diagram of different elements from the EIS analysis.

Fig. 6c shows GV charging–discharging curves of Mn₃O₄ NRs/rGO-1-1 at different current density. Almost linear and nearly symmetric charge and discharge curves reveal excellent electrochemical reversibility and charge–discharge properties. The corresponding specific capacitance was 176.2, 168.5, 155.3 and 138.2 F g⁻¹ at 1, 2, 5 and 10 A g⁻¹ current density (in Fig. 6d), respectively. The cycling stability of Mn₃O₄ NRs/rGO-1-1 electrode is investigated at the current density of 5 A g⁻¹ as shown in Fig. 6e. After 2000 cycles, the specific capacitance only decreases 10% to 139.8 F g⁻¹.

To verify that the rGO are responsible for the good performance, electrochemical impedance spectroscopy (EIS) measurements were carried out on the γ-MnOOH/Mn₃O₄ NRs and Mn₃O₄ NRs/rGO-1-1 over a frequency range of 100 kHz-0.01 Hz. The Nyquist plots obtained from the γ-MnOOH/Mn₃O₄ NRs and Mn₃O₄ NRs/rGO-1-1 are shown in Fig. 6f. The equivalent circuit (inset in Fig. 6f) was constructed to analyze the impedance spectra. The diameter of the semicircles in the curve represents the charge transfer resistance (R_ct), which is mainly generated at the interface between the electrode and the electrolyte.^6,38 Obviously, the smaller semicircle in Mn₃O₄ NRs/rGO-1-1 electrode means charge transfer resistance decreases in the presence of conductive rGO, indicating that the rGO facilitate the conductivity and the reaction dynamics of the electrodes.

Conclusions

In this study, a series of flexible Mn₃O₄ NRs/rGO hybrid electrodes have been prepared by filtration assembly process and subsequently hydrothermal reduction process. Mn₃O₄ NRs was uniformly sandwiched between the rGO layers and formed unique porous structures. As flexible and binder-free electrodes, Mn₃O₄ NRs/rGO achieved high specific capacitance of 204.2 F g⁻¹ in supercapacitor and demonstrated excellent cyclic retention of 669.6 mA h g⁻¹ after 100 cycles in LIBs. These performances are significantly improved from 66.0 F g⁻¹ and 65.8 mA h g⁻¹ of γ-MnOOH/Mn₃O₄ NRs, respectively, and comparable to those of Mn₃O₄ nanoparticles/rGO composite electrodes but remarkably higher than pure Mn₃O₄ anodes.^22–24 The excellent electrochemical performance is ascribed to the 3D porous structures, which provides a large number of conductive channels to favor the penetration of electrolyte, and the increased electrical conductivity by introduction of rGO. We believe the binder-free Mn₃O₄ NRs/rGO hybrid electrodes hold the promise potential for supercapacitors and lithium ion batteries.

Acknowledgements

This work was supported by the National Natural Science Foundations of China (NSFC 21003105, 21271154, 51272232), Doctoral Fund of Ministry of Education of China (20110101110028), and the project-sponsored by SRF for ROCS, SEM.

Notes and references

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