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Self-Assembly Synthesis and Electrochemical Performance of Partially Reduced Graphene Oxide Supported Hierarchical MnO2 Nanonocomposite for Supercapacitor.

Byline: Muhammad Ishaq, Maher Jabeen and Weiming Song

Summary: Partially reduced graphene oxide supported hierarchical birnessite-type manganese oxide (MnO2) nanonocomposite (MnO2-pGO) with large surface area (116 m2 g-1) were successfully synthesized by a facile ultrahigh dilute vesicle solution approach, via surfactant Hexadecyltrimethylammonium bromide (CTAB) and sodium dodecylbenzenesulfonate (SDBS), as a structure-directing agent. The resultant structure exhibit hierarchical porous MnO2 nanocluster, which were self-assembled from elongated nanorods and grafted successfully on the surface of the pGO nanosheets. Furthermore, the obtained MnO2-pGO nanocomposite was found to exhibit favorable electrochemical activity showing ultra-high specific capacitance of (282 F/g at 0.5 A/g), the enhanced rate capability of (67.7%) at 10 A/g), the most stable capacitance retention (91.4 %of its initial capacitance was retains after 5000 cycles, with ~100% Coulombic efficiency).

The result suggests that the approach is not only effective to deposit MnO2 over the pGO sheets, but also offered great promise to prepare other pGO-metal oxide composite electrode materials for supercapacitor.

Keywords: MnO2, pGO, Vesicle solution, Supercapacitors.

Introduction

Recently, energy storage and conversion technology has become a fundamental research and development center, because of fossil fuels depletion and incredible serious environmental issues caused by their combustion [1]. The growing need of electronic devices has led researchers to focus on novel and smart electrochemical material designs having unique properties of high redox capability, low-cost, and eco-friendliness [2]. To date, several effective configurations have been designed to construct electrochemical materials for supercapacitors [3]. Among which majorities are integrated by two-dimensional layered materials, including metal dichalcogenides (MoS2, MoSe2, and WS2), [4, 5] electrically conducting polymers, [6] transition metal (Ni, Mn, Co, Fe, Ru) oxides/hydroxide and carbon-based graphene have attracted substantial attention because of pseudocapacitive behaviors as well as their high electrical properties across the electrode/electrolyte interface.[2, 7-13]

Within this class, manganese dioxide (MnO2), containing transition metal is considered as one kind of the most promising energy materials for pseudocapacitors [14, 15]. Nevertheless the electrochemical performances (ECP) of MnO2 are often hampered by their relatively low electric conductivity, deprived mechanical stability and bad cycling stability [16].

To remedy the above limitations, Co-assembly of MnO2 and carbon-based nanomaterial's, such as activated carbon, [17] carbon nanotubes, [18] carbon nanofibers, [19] and graphene, [20] have been used in constructing 2D/3D nanocomposite, which exhibits enhanced ECP. Among the various carbon-based materials, graphene and its chemical derivatives, aroused as potential substrates for assembling nanocomposite for energy-based applications, due to its superior electrochemical reactivity, high conductivity and exceptional electrode stability [21].Within this class, graphene oxide (GO) has revealed excessive coordinating potential with other materials to form GO based electrode material, due to the presence of oxygenic functional groups on the basal aromatic lattice [22, 23]. Nevertheless, GO undergoes poor electrical conductivity due to biased sp3 hybridized geometry [24].

In contrast excellent conductivity is accomplished in case of reduced graphene oxide (RGO) with planar sp2 hybridization, but this could happen by reducing the surface oxygen functional groups. Very recently, a new class of graphene derivative known as partially reduced graphene oxide (pGO) has materialized, wherein unlike reduced graphene oxide (RGO), pi-pi (I-I) conjugations are restored only partially. Thus, pGO hold the benefits of both GO and rGO at the same time.[25]

Yan et al [26]. Reported that the ECP of partially reduced graphene oxide nanosheet (prGON) are superior to that of the reduced graphene, owing to the remaining oxygen functional groups in prGON, facilitated the reactive sites for pseudocapacitance and enhance its capacitance and life time performance.

On the basis of the above considerations, in this work, partially reduced graphene oxide supported hierarchical birnessite-type manganese oxide (MnO2) nanocomposite (MnO2-pGO) were successfully synthesized by a facile ultrahigh dilute vesicle solution approach. Vesicles are usually spherical,(10 nm to 50 um diameter), enclosed and hollow lamellar aggregates with an arched bilayer membrane, consist of amphiphilic molecules, with an exclusive self-assembling properties and are widely explored as a template to synthesize novel nanostructured materials.[27,28] The structural and supercapacitive properties of the MnO2-pGO nanpcomposite are briefly explored and discussed. In comparison, pure manganese oxide (MnO2) was also fabricated by the same procedure.

More importantly, the electrochemical studies of the MnO2-pGO composite electrode exhibited significantly higher specific capacitance value (282 F/g), the better rate capability (67.7%) at 10 A/g), the most stable capacitance retention (98 %) after 5000 cycles, with ~100% Coulombic efficiency) and low RCT value (2.032 a| before and 2.733 a| after 5000 charge discharge cycles from impedance measurements), making it the most favorable to be used an electrode energy materials for supercapacitor applications.

Experimental

Chemicals

Crude flake graphite powder (45 um, 99%), sulfuric acid (H2SO4, 98.08%), sodium nitrate (NaNO3), potassium permanganate (KMnO4), hydrogen peroxide (H2O2 30%), hexadecyl trimethyl ammonium bromide (CTAB), sodium dodecyl benzene sulfonate (SDBS), ascorbic acid, manganese(II) sulfate monohydrate (MnSO4.H2O, 99%), and hydrochloric acid (HCL 36 %), were obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. Water ultra-purified (UP) filtered resistivity (18.2 Ma| *cm) were utilized throughout the experimentation.

Synthesis of pGO

Graphite oxide (GrO) was prepared from crude flake graphite by modified Hummers method.[29,30] pGO was prepared from the as-prepared GrO in the following typical process. First, 0.075g of GO was added into 75 ml of ultrapure water (1mg/ml) followed by 0.5 h of sonication. Then, 0.15g of ascorbic acid was added into the exfoliated graphene oxide suspension followed by another 10 min of stirring. The GO suspension was than refluxed in sonics ultrasonic processor at 90AdegC (750 W, 20 kHz) with 40% amplitude for 0.5 h. Finally, the prepared pGO was washed using ultra-purified water through filtration.

Synthesis of MnO2-pGO nanocomposite

The MnO2-pGO nanocomposite was synthesized through ultrahigh dilute vesicle solutions route as shown in Error! Reference source not found. Typically, first CTAB and SDBS (1: 2 molar ratios; 0.028 mol/L surfactant) aqueous solutions in twice-distilled water were prepared by weighting the surfactants and warming the dispersion to 40AdegC for 24 h, respectively. Than a 6.5 mmol portion of MnSO4.H2O and 3.8mmol of KMnO4, were dissolved in 15 and 10 ml of UP water. Second, the as-prepared pGO was redispersed in 75 ml of the vesicle solution and was exfoliated via a Sonics ultrasonic processor (750 W, 20 kHz) with 40% amplitude for 1 h. Next the solution of MnSO4.H2O and KMnO4 were added drop wise into the GO-vesicle solution under stirring and the reaction was carried out at 45AdegC for 12h. Finally the resultant dark-brown slurry of MnO2-pGO were decanted and washed with UP water and ethanol to remove the organic compound prior to drying at 60 AdegC for 24 h in oven.

For comparison, pure MnO2 was fabricated in the absence of GO with the other same experimental condition, respectively.

Morphology and structural characterization

The phase purity of MnO2-pGO and pure MnO2 nanocomposite materials were analyzed using MAC Science MXP18 diffractometer. The Fourier transform infrared spectroscopy (FT-IR) measurement of the samples was recorded on Nicolet 6700 spectrophotometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on Kratos Axis Ultra DLD instrument photoelectron spectroscope system. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) images were acquired using a JEOL JSM-6360 electron microscope, Transmission electron microscopy (TEM) was performed on a FEI-TECHNI-G2 microscope. Raman spectra were run on an Ultra Resonance Raman spectroscopy (HORIBA Jobin Yvon S.A.S Lab RAM HR evo 800) with 32 nm line of an Ar ion laser as an excitation source. The Brunauer-Emmett-Teller (BET) specific surface area was measured using an automatic volumetric sorption analyzer (Quantachrome, Autosorb-1, USA) apparatus.

characterization

Electrochemical characterizations were conducted in a three-electrode system on electrochemical analyzer (CHI660e, Shaanxi, China) at room temperature. A saturated calomel electrode (SCE), 1cm x 1cm platinum gauze and 1 M Na2SO4 aqueous solution were used as reference, counter electrodes and electrolyte, respectively. The working electrodes were fabricated by grinding mixing the obtained products, acetylene black as conductive material and poly(tertrafluoroethylene) (PTFE) as binder material with a mass ratio of 7.5: 1: 1. The mixture was dispersed in ethanol by ultrasonication for 15 minutes to form homogeneous slurry. The resultant slurry was coated onto the precleaned nickel foam current collector (1cm x 1cm). The electrode was than dried at 60 AdegC for 12 h in vacuum to evaporate solvent.

Finally, the active materials coated nickel foam was pressed at 10 MPa for 10 min to (CV) and galvanostatic charge/discharge experiment were studied in the optimized potential window of -0.1 to 0.9 V at various scan rate and current densities. Electrochemical impedance spectroscopy (EIS) was tested over the frequency range of 100 kHz to 0.01 Hz with an ac voltage of 5.0 mV. The specific capacitance C (F/g) was calculated from the discharge curves according to the following equations.[31]

Cs = I x I t / (IV x m) (1)

where I is the constant discharge current, It is the time of a discharge cycle, and IV is the potential drop during discharge[32], m is the mass of the active materials (g).

Results and Discussion

Physicochemical characterizations of materials

Fig 2 demonstrates XRD patterns of the samples. The most intensive sharp peak (001) centered at around 2I, = 10.7Adeg (JCPDS card 75-1621) was observed, which is the characteristic reflection peak for GO, indicating an increase in interlayer d-spacing from 0.34 to 0.82 nm. This increase in the interlayer is caused by chemical oxidation and exfoliation of pristine graphite sheets. [33] The peaks seemed in the pattern of pure MnO2 and MnO2-pGO nanocomposite agreed well with the regular compound K-birnessite MnO2 (JCPDS card 80-1098) with the diffraction peaks appeared at 2I,=11.7Adeg (001), 21.8Adeg (002), 36.7Adeg (100), 41.8Adeg (102) 55.5 (103), and 66. 1Adeg (110) respectively, indicating that MnO2 was generated. [34] In addition the interlayer spacing of (001), (002), (100), (102), (103), and (110) reflection planes were nearly 0.75, 0.40, 0.24, 0.2, 0.16 and 0.14 nm attributed to the presence of k+ and H2O molecules in the interlamination.[35]

In addition, the intensity of (002), (102), and (103) plane reflection peaks in the MnO2-pGO composite weakened after hybridizing with pGO. This could be ascribed to the MnO2 crystals deposited on pGO sheets which inhibit themselves from growing to be complete crystals, result much lower crystallinity.[36]

The FT-IR spectra of the samples are shown in Fig 3. The spectrum of GO illustrating the -OH stretching vibration at 3428 cm-1, the C=O stretching mode at 1701 cm-1, the aromatic C=C stretching at 1627 cm-1, the C-O (carboxyl) stretching mode at 1365 cm-1, the C-O (epoxyl) stretching mode at 1230 cm-1 and the C-O (alkoxyl) stretching vibration at 1085 cm-1.[33] After hybridization, the characteristic peaks of -OH (3428), C=C (1627), C-OH (1469), C-O (epoxyl, 1365), C-O (alkoxyl, 1085) and Mn-O (515 cm-1) were detected in the spectrum of the composite, suggesting the successful synthesis of the MnO2-pGO. In addition, the peak intensity of these functional groups are reduced, or even diminish (at around 3428 and 1230 cm-1 for C=O and C-O (epoxyl), demonstrating partial reduction of GO into pGO.[37] The existence Mn-O bond suggests that MnO2 were produced and grafted on the surface of pGO, during the reaction process.[38]

Moreover, the simultaneous manifestation of bands in the region 2920 and 2851 cm-1 nearby in the MnO2-pGO belong to methyl groups (C-H), which might be due to residual surfactant even after washing with water and ethanol. In the XPS survey-scan spectrum of MnO2-pGO nanocomposite (Fig. 4a), the typical C 1s, O 1s, and Mn 2p peaks were observed, suggesting the presence of GO and MnO2 in composite. Notably, K is also detected on the survey spectrum since K+ is a balance charge of negatively-charged MnO2 layers. In Fig. 4b and Fig. S1, the C 1s spectra of the MnO2-pGO and GO, is deconvoluted into several symmetrical peaks. Four peaks centered at 284.0 (C=C), 285.9 (C-OH/C- O -C), 287.5 (C=O), and 288.4 eV (HO- C =O) were observed in both of Fig. 4. b and Fig. S1 [39].

In case of MnO2-pGO, the peak intensity of aromatic C=C (284.4 eV) is dramatically increased while that of (C-OH/C- O -C), (C=O), and (HO- C=O) are partially decreased or even reduced (C=O), as the reaction time prolonged, which is consistent well with the results of FT-IR analysis. In the XPS spectra of O 1s (Fig 4c) the peak located at 531.0 eV is assigned to the oxygen bonded with manganese (O-Mn) [40] and the peak appeared at 529.0 eV is attributed to oxygen bonded with carbon (O-C).[41] In Fig 4(d) appearance of two peaks centered at 653.7 and 642.0 eV can be ascribed to Mn (2p1/2) and Mn (2p3/2), signifying the existence of Mn2+ oxidation state in the composite respectively. They have a spin-energy separation of 11.7 eV, which correlates with earlier reported data of Mn (2p1/2) and Mn (2p3/2) in MnO2 [42]. In addition, according to the survey scan of XPS analysis, the mass of C, Mn, O and K was about 36.52%, 18.6%, 42.98%, and 1.9%, respectively.

The corresponding energy-dispersive X-ray spectrometry (EDX) mapping and spectrum analysis of MnO2-pGO composite Fig. 1(B) unambiguously confirms the uniform distribution and the manifestation of Mn, O and C elements, and the homogeneous distribution of MnO2 over the GO surface in the MnO2-pGO composite[43]. The atomic percentages (see Table.S1) of C, O and Mn are 43.170, 34.293 and 22.536, respectively.

The typical Raman spectra of the GO, pure MnO2 and MnO2-pGO nanocomposite are shown in Fig. 5. In Fig. 5(a) the two diagnostic peaks of GO, the D and G band are observed around 1345 cm-1 and 1597 cm-1, assign to the breathing mode of aromatic six member sp2 carbon rings and the bond stretching mode of sp2 carbon atoms[44].The Sharp peak at 644 cm-1 is ascribed to the Mn-O lattice vibration perpendicular to the direction of MnO6 octahedral double chain of MnO2 [45], which is in well agreement with pure MnO2 shown in Fig. 5(b). The analysis further endorses the yield of MnO2 and graphene oxide composite have been accomplished.

The SEM images of pure of pure MnO2 and MnO2-pGO nanocomposite are shown in Fig. 6. The low and high resolutions images of pure MnO2, (Fig. 6 a, b), depicts hierarchical porous MnO2 nanocluster, which were self-assembled desultorily from elongated nanorods with an average size of nearly ~57-190 nm. The SEM image of MnO2-pGO composite in Fig. 6(c and d) showed that the elongated nanostructures of MnO2 (Fig. S2 the other images) are grafted and homogeneously covered all the surface of pGO sheets. The reflection well agreement with the homogeneous distribution of the Mn, O and C elements, concluded from the corresponding EDS mapping analysis Fig. 1 (B). Furthermore, the pGO sheets exhibit porous folded crumples structure (Fig. 6c), which are favorable for providing fast electrolyte diffusion channels with a low activation barrier than that of a smooth graphene sheets [46].

Moreover, the TEM image in Fig. 7 (a low and the inset high magnification) shows pure MnO2 exhibits rode-like morphology with an average diameter of ~3-12 nm and an average length of ~ 19.7 to 49.9 nm. The TEM image of MnO2-pGO nanocomposite in Fig. 7 (b and c) revealed the layered structure of pGO sheets appeared in a transparent swollen state, uniformly coated with MnO2 nanorods, corresponding well to the results of SEM analysis. The image also suggest robust collaboration between pGO sheets and MnO2 layers through electrostatic attraction, physisorption or charge-transfer interactions.[47] As can be seen in Fig. 7 (d), HR-TEM image displays the well-defined lattice fringes with interplaner distance of 0.24 nm and 0.14nm. This result is agreement well with the interplanar spacing between the (100) and (110) crystal planes of K-birnessite MnO2 [48] and similarly consistent with XRD results.

For further characterization of the porous structure of the prepared pure MnO2 and MnO2-pGO composite the N2 adsorption-desorption isotherm and the pore size distribution curves were probed by measuring nitrogen adsorption-desorption isotherms. As shown in Fig. 8, both the samples (pure MnO2 and MnO2-pGO composites) were found to exhibit similar, type IV adsorption/desorption isotherm, with an H1-type hysteresis loop, suggesting mesoporous nature of the samples.[49] The textural properties and parameters, such as BET specific surface area (SABET), the total pore volumes (VT) and pore size (PSBJH) of the composite are calculated and are precised in Table-1. The BET specific surface area of MnO2-pGO is (121.0 m2 g-1), which is greater than pure MnO2 (101.6 m2 g-1).

Moreover, the pore size distribution calculated from PSBJH shows that the pores created by both the pure MnO2 and MnO2-pGO nanocomposite (as shown inset image of Fig. 9), are mesoporous structure with an average pore size of ~5.1 and ~6.0 nm. The corresponding pore volume is 1.301 and 1.301 m3g-1 for pure MnO2 and the MnO2-pGO nanocomposite, respectively. It is reasonable to believe that the introduction of pGO in the composite enhanced the aggregation of MnO2 particles on its surface(pGO), thus leading to a high surface area which are beneficial to the kinetics of electrochemical process.[50].

Table-1: Summary of nitrogen sorption porosimetry studies parameters of as-synthesized pure MnO2 and MnO2-pGO nanocomposites.

###S.ABET###Pore volume###Mean pore size

Sample

###(m2 g-1)###(cm3 g-1)###(nm)

Pure MnO2###101.60###1.301###5.120

MnO2-pGO###121.60###1.625###6.066

Electrochemical studies

Fig 9 (a) shows typical cyclic voltammograms (CV) of pure MnO2 and the MnO2-pGO nanocomposite at the sweep rate of 5 mV/s. Rectangular shape and mirror image of CV loop is a fingerprint for the capacitance behavior of any material.[51] It can be seen that, both the curves exhibits an obvious rectangular nature of the CV plot, which confirms the redox reactions are proceeding during the charge/discharge processes. Based on the perception anticipated by chan.et.al., [52] alkali metal cations (Na+) could rapidly intercalated / de-intercalated into the bulk of MnO2, as soon as the electrode was dipped into the electrolyte (1 M Na2SO4), so the reversible redox reactions could be represented as;

MnO2 + H+ + e- a MnOOH (2)

MnO2 + Na+ + e- a MnOONa (3)

(MnO2) surface + Na+ a MnOONa+ (4)

Moreover, the integral areas of the CV curve for MnO2-pGO nanocomposite is greater than that of pure MnO2 electrode (MnO2-pGO > pure MnO2), suggesting higher specific capacitance (SC) and fast charge circulation within the electrode surface. Fig. 9 (b) shows the comparative CV curves of pure MnO2 and MnO2-pGO nanocomposite at a very high scan rate of 100 mV/s. It is observed that, the CV curve of pure MnO2 electrode shows quit deviation from the rectangular shape at a high scan rate of 100 mV/s. However, MnO2-pGO nanocomposite electrode display CV curve of nearly rectangular shape, with no severe distortion of the symmetry, showing a comparatively good capacitive behavior under the high scan rate of 100 mV/s, which were mainly ascribed to the synergistic effect between conductive pGO and MnO2 as reported earlier.[43]

The galvanostatic charge-discharge (GCD) curves of pure MnO2 and the MnO2-pGO nanocomposite at 0.5Ag-1 are demonstrated in Fig. 9(c). The GCD curves of both the electrode shows linear and triangular symmetry, indicating an ideal capacitive behavior during the redox process.[32] The discharge time for MnO2-pGO nanocomposite electrode comparatively longest, suggesting good energy storage performance. At a current density of 0.5 A/g, the SC was estimated to be 223 F/g for pure MnO2 and 282 F/g for MnO2-pGO (as shown inset of Fig. 9c). Fig. 9 (d) illustrate the SC of pure MnO2 and MnO2-pGO at various current densities of 0.5, 1, 2, 4, and 8 A/g. MnO2-pGO revealed obviously higher specific capacitances than that of the pure MnO2 at any current density. The SC of MnO2-pGO was calculated to be 282, 266, 259, 236, and 188 F/g with retention rates of 66.7 %. Similarly, the SC of pure MnO2 was 223, 176, 147, 103, and 71 F/g, with retention rates of 32.0 %.

Consequently, the rate capability of MnO2-pGO nanocomposite electrode was remarkably enhanced compared with pure MnO2 electrode. It can be attributed to the excellent electron conductivity and electrode stability of pGO, in the nanocomposite.

On the basis of the above information, MnO2-pGO electrode was selected for further detail electrochemical analysis. Fig. 10 (a) shows the rate-dependent CV curves for MnO2-pGO nanocomposite electrode with the scan rates range of 5 50 mV/s. It can be seen that, at a low scan rate of 5,10 and 20 mV/s the CV curves show standard rectangular shape, signifying ideal pseudocapacitive performance and low charge contact resistance.[41] Furthermore, with increasing scan rate (50-100 mV/s), the total current increases in the CV curves, clearly indicating the shape does not change significantly, suggesting a good rate capability and diffusion-controlled phenomenon. [53] Fig. 10 (b) illustrates the GCD curve of MnO2-PGO nanocomposite electrode at different current densities range of 0.5 8 A/g. It can be observed that, all the charge curves are highly linear to their corresponding discharge counterpart, signifying the Faradaic reactions along with double layer contribution.[47]

Furthermore, the long discharging time of the curve at 0.5 A/g, demonstrated higher SC, hence verifying the CV results. In addition, the SC decreasing as the increasing current densities, which could be attributed to the high resistance and lower Faradaic redox reactions under higher current densities.[21] The enhanced electrochemical performance of MnO2-pGO predominantly attributed to its unique nanoarchitectures (hierarchical elongated nanorod) of MnO2 obtained in vesicle solution route are around 3-12 nm in diameter, which provides an efficient surface area for effective pseudocapacitive reaction between MnO2 and electrolyte.

The excellent support of pGO carbon matrix allowed the strong deposition of MnO2 nanoparticles, which enhanced the mechanical strength of nanocomposite materials and MnO2 could act as a spacer to avoid restacking of pGO sheets, thus forming a 3D network structure with large electrochemically active surface area leads to better conductivity and effective diffusion of electrolyte in the nanocomposite.

The resistive characteristics of the samples electrodes (MnO2 and MnO2-pGO), was analyzed by electrochemical impedance analysis (EIS), are illustrated in Fig. 11. The Nyquist plots of both the sample electrodes exhibit an imperfect small arc in the high-frequency region and a nonstraight line in the low-frequency regime. In general, for a typical electrode-electrolyte system, the semicircle diameter in the high-frequency regime represents charge-transfer resistance (RCT) at the electrode/electrolyte interface [54], while the slope of the line intercept to the real axis (between 45Adeg to 90Adeg) related to capacitive nature and ions diffusion resistance in the samples.[55] As can be seen the enlarge image in the inset part of Fig. 11, the diameter increased in order of MnO2-pGO < MnO2 and the RCT were calculated to be 2.032 a| for MnO2-pGO and 2.147 a| for MnO2.

The slope of MnO2-pGO slightly greater than pure MnO2, indicating that MnO2-pGO embrace the lowest RCT, the good capacitive performance, and the lowest ions diffusion resistance, [56] compared to pure MnO2. The results further suggest that MnO2-pGO possessed the enhanced electrochemical performance suitable to be used electrode energy material of supercapacitor applications.

Meanwhile a long term cycling stability and Coulombic efficiency test was investigated upto 1200 cycles for pure MnO2 and 5000 cycles for MnO2-pGO nanocomposite at current density of 4 A/g and the results are demonstrated in Fig. S3 and Fig. 12. After 1200 and 5000 cycles, pure MnO2 and MnO2-pGO nanocomposite remained 64.0% and 91.4% of their initial specific capacitance with ~100% Coulombic efficiency, respectively. MnO2-pGO nanocomposite reveals clearly the excellent long term retention capability, demonstrating high electron and electrolyte transportation in the nanocomposite.[57] Furthermore, Nyquist was implemented to explore the motive for the enhanced specific capacitance and stable capacitance retention of MnO2-pGO nanocomposite before and after a 5000 cycles test. The Nyquist plots of MnO2-pGO before and after cycles test are shown in Fig. S4.

As can be seen in the high frequency region, (the inset part of Fig. S4), there is not substantial changes arise in the diameter of the curves before and after 5000 cycles test, although there is slight decrease can be observed in the slope line intercept to the real axis. The RCT were calculated to be 2.032 a| before and 2.733 a| after 5000 cycles for MnO2-pGO.

Conclusion

In conclusion, nanocomposite of MnO2-pGO was successfully synthesized using vesicle solution approach, via surfactant hexadecyltrimethyl ammonium bromide (CTAB) and sodium dodecyl benzene sulfonate (SDBS), as a structure-directing agent. Our analysis revealed that manganese oxide (MnO2) nanocluster, which were self-assembled from elongated nanorods were generated on pGO sheet during the prolonged reaction time. More importantly, the fabricated MnO2-pGO nanocomposite endow excellent ECPs, like high SC (282 F g-1), the good rate capability (67.7%) at 10 A/g), the stable capacitance retention (91.4 %after 5000 cycles), and the ~100% Coulombic efficiency, making it to become a promising electrode energy material to be used in supercapacitor application.

Acknowledgment

We thankfully acknowledge the support of this work by the National Natural Science Foundation of China (No.21501104) the Natural Science foundation of Heilongjiang Province (B2015014)

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