ous symmetric pseudocapacitor based on highly graphitized onion-like carbon / birnessite-type manganese oxide nanohybrids †

We present a study on the pseudocapacitive properties of birnessite-type MnO2 grafted on highly graphitized onion-like carbon (OLC/MnO2). In a three-electrode setup, we evaluated two different substrates, namely a platinum disc and nickel foam. The OLC/MnO2 nanohybrid exhibited a large specific capacitance (Csp) of 295 and 323 F g 1 (at 1 A g ) for the Pt disc and Ni foam, respectively. In addition, the Ni foam substrate exhibited much higher rate capability (power density) than the Pt disc. A symmetrical two-electrode device, fabricated with the Ni foam, showed a large Csp of 254 F g , a specific energy density of 5.6 W h kg , and a high power density of 74.8 kW kg . These values have been the highest for onion-based electrodes so far. The device showed excellent capacity retention when subjected to voltage-holding (floating) experiments for 50 h. In addition, the device showed a very short time constant (s 1⁄4 40 ms). This high rate handling ability of the OLC/MnO2 nanohybrid, compared to literature reports, promises new opportunities for the development of aqueous-based pseudocapacitors.


Introduction
Supercapacitors are advanced systems for electrochemical energy storage. 1,24][5][6] Over the last years, supercapacitors have attracted tremendous attention due to their excellent properties such as high power density, long cycle ability, high efficiency, and relying on abundantly available carbon materials. 7,8Considering energy and power performance, supercapacitors play a key role as intermediates between batteries and electrolytic capacitors 9 and nd widespread applications for fast charge-discharge and uninterrupted power supply applications as well as in combination with batteries in hybrid systems. 10There have been extensive studies of varieties of carbon materials for supercapacitor electrodes because of their large specic surface area (SSA), high conductivity, facile availability, and chemical stability. 5,11Some of the best performing carbon materials include activated carbon, 12 carbon nanotubes (CNTs), 13 graphene, 14,15 carbon nanobers (CNFs), 16,17 and carbon aerogels. 184][25] The major attractions stem from the ability to prepare them on a large scale by thermal annealing of nanodiamonds and the superior power handling ability. 26OLCs are described as multi-shell fullerenes 27 that, unlike fullerenes, exhibit a high electrical conductivity commonly in the range of 2-4 S cm À1 . 25However, the limited surface area of OLCs (200-600 m 2 g À1 ) has also resulted in limited double-layer capacitance (usually between 25 and 50 F g À1 , equivalent up to 2 W h kg À1 at 1 V). 25,26LCs derived from thermal treatment of nanodiamonds (NDs) 28 are highly graphitic spherical particles (5-10 nm) that consist of several concentric carbon shells. 29Alternative synthesis methods may also yield larger OLCs with diameters of more than 10 nm (ref.19 and 30) and include condensation of carbon vapor 31 or electron beam irradiation. 32However, thermal annealing of NDs 33 at temperatures between 1000 and 2000 C is currently the preferred technique to synthesize OLCs since large amounts of material can be obtained per run. 34Also, a narrow size distribution of the ND precursor translates into a narrow size distribution of the resulting onion-like carbons. 21Birnessite-type MnO 2 (in this paper referred to as just "MnO 2 ") exhibits a two-dimensional layered structure (see ESI, Fig. S1 †) displaying edge-sharing MnO 6 octahedra in the sheets and metal cations (for example K + ) and water molecules in the interlayer region.Hence, an appropriate chemical representation would be K x Mn 2 O 4 $yH 2 O (with x # 0.5 and y # 1.5). 357][38][39][40][41][42] However, because of its low electrical conductivity (10 À6 to 10 À5 S cm À1 ) and poor power handling capability, the electrochemical performance of MnO 2 electrodes is rather low, which signicantly limits its potential applications as high-power supercapacitor electrodes. 435][46] However, the resulting performance strongly depends on the quality and properties of such carbon/metal oxide hybrid materials.
This work, for the rst time, reports the electrochemistry of MnO 2 integrated with highly graphitized OLCs derived from NDs (herein abbreviated simply as OLC/MnO 2 nanohybrid) as a high-power pseudocapacitor in a neutral aqueous medium (1 M Na 2 SO 4 ).Previous studies related to MnO 2 with "carbon onions" were carried out using low-graphitized materials obtained from either claried butter ("Ghee") 30 or phenolicformaldehyde resins with much larger particle diameters (tens of nm). 42Whilst these initial reports are encouraging, we show in this study that by using highly graphitized OLCs, OLC/MnO 2 nanohybrids exhibit a very high power density ($75 kW kg À1 ).In addition, our devices show excellent capacitance retention upon long-hour voltage-holding and very low equivalent distribution resistance (EDR z 3 U cm 2 ) with a response time of just a few milliseconds.

Experimental section
Precursor and synthesis of OLC and OLC/MnO 2 OLC was synthesized from nanodiamond (ND) powder with a purity of 98-99% (NaBond Technologies) and thoroughly characterized as recently described. 47Briey, ND powder was placed in a closed-lid cylindrical graphite crucible (30 mm in diameter and 20 mm in height) and thermally annealed in a water-cooled high temperature vacuum furnace with tungsten heaters (Model: 1100-3580-W1, Thermal Technology Inc.).The heating and cooling rates were both 15 C min À1 and the chamber pressure ranged between 10 and 100 mPa.The nal OLC was annealed at 1750 C for 3 h.The OLC/MnO 2 nanohybrid material was prepared using the conventional hydrothermal reduction technique.Typically, 40 mg of OLC was dispersed by sonication in 30 mL of 0.02 M KMnO 4 (Merck), and the mixture (pH ¼ 7.05) was reuxed at 130 C in an oil bath for 24 h with continuous magnetic stirring.The resultant dispersion was then centrifuged and washed several times with deionized water, and nally dried at 60 C overnight in a vacuum oven.All chemicals were of analytical grade and used as received.Deionized water was used throughout the synthesis process.

Structural characterization
Surface morphology characterization of the samples was obtained using a JSM-7500F (JEOL, Japan) scanning electron microscope (SEM) operated at 3.0 kV.Energy dispersive X-ray spectra (EDX) were recorded with an EDX system (Oxford Instruments) at 5 different positions.The chemical composition was calculated using the AZtec energy analysis soware (Oxford Instruments).Transmission electron microscopy (TEM) samples were prepared by dispersing powders in ethanol and placing the dispersion over a copper grid with a lacey carbon lm.All measurements were carried out with a 2100F microscope (JEOL) operating at 200 kV.X-ray diffraction (XRD) patterns of the samples were collected using an X'Pert-Pro MPD diffractometer (PANalytical) with theta/theta geometry (step width: 0.0263 s À1 ), operating a copper tube at 40 kV and 40 mA.The instrumental resolution function was characterized with the NIST SRM 660a (LaB 6 ) standard.The patterns were recorded in the range of 5-148 2q.Qualitative phase analysis of the samples was conducted using Bruker EVA soware using the PDF database.
Raman spectra were recorded with a Renishaw inVia Raman microscope using a Nd-YAG laser with an excitation wavelength of 532 nm and a grating with 1800 lines mm À1 yielding a spectral resolution of ca.1.2 cm À1 .The spot size on the sample was in the focal plane ca. 2 mm using an output power of 0.5 mW.Spectra were recorded for 30 s and accumulated 50 times to eliminate cosmic rays and to obtain a high signal-to-noise and signal-to-background ratio.Peak tting was achieved by employing Lorentzian peaks assuming four components for the carbon spectrum between 1000 and 1800 cm À1 .Fourier infrared spectroscopy (FTIR) analyses were carried out using a Perkin Elmer FT-IR spectrophotometer.OLC and OLC/MnO 2 nanohybrids were analyzed as KBr pellets (10 scans).
X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Kratos Axis Ultra-DLD system (Shimadzu) with monochromated Al Ka radiation (1486.6 eV).Binding energies were calibrated using the containment carbon (C 1s at 284.6 eV).The spectra analysis was performed with the XPS Peak 4.1 program and a Shirley function was used to subtract the background.The metal oxide content in the nanohybrid was determined by thermogravimetric analysis (TGA) using an STA Jupiter 449 C (Netzsch) in an Ar/O 2 atmosphere at a temperature scan rate of 10 K min À1 .
Nitrogen gas sorption measurements were made with a Quantachrome Autosorb iQ system.The samples were outgassed at 150 C for 10 h under vacuum conditions.Gas sorption was performed in liquid nitrogen (À196 C) with a relative pressure range of 10 À7 to 0.95 in 68 steps.The specic surface area (SSA) was calculated with the ASQwin-soware using the Brunauer-Emmett-Teller (BET) equation 48 in the relative pressure range of 0.01-0.2.We also calculated the SSA and pore size distribution (PSD) via quenched-solid density functional theory (QSDFT) 49 with a hybrid model for slit and cylindrical pores and pore size between 0.56 and 37.5 nm.

Electrochemical characterization
All electrochemical measurements were carried out using a Bio-Logic VMP 300 potentiostat/galvanostat using either a threeelectrode (half-cell) or a two-electrode (full cell) conguration.For the three-electrode conguration a Pt disc and nickel foam were used as substrates for the working electrodes.For the three-electrode conguration using a Pt disc, a custom-built three-electrode cell (cf.ref. 47) was used.The working electrode was prepared by drop-casting 7.1 mg mL À1 OLC colloidal dispersion (10 mass% polyvinylidenuoride, PVDF in ethanol) or 9.0 mg mL À1 OLC/MnO 2 nanohybrid colloidal dispersion (in anhydrous N-methyl-2-pyrrolidone, NMP) onto a Pt disc (diameter: 12 mm, thickness: 100 mm, purity 99.99%, Carl Schaefer) and dried at 80 C overnight in a vacuum oven at 20 mbar to remove the solvent.Polytetrauoroethylene (PTFE) bound (5% in total electrode mass) activated carbon (YP50F, Kuraray Chemical) served as a counter electrode and was largely oversized in charge capacity as compared to the working electrode.A platinum wire (diameter 1 mm, purity 99.99%, Carl Schaefer) served as a pseudo-reference electrode.For the nickel foam based three-electrode conguration, the nickel foam (Celmet: thickness ¼ 1.6 mm, surface area ¼ 7500 m 2 m À3 , cell size ¼ 0.5 mm, 48-52 cells per inch) was cleaned prior to use, in a 1 M HCl solution, washed with a copious amount of deionized water to a neutral pH, and dried under vacuum.It was pasted with a mixture of OLC/MnO 2 nanohybrid, carbon black (CB, Degussa), and polyvinylidene uoride (PVDF) (mass% of 80 : 15 : 5 respectively, homogeneously mixed with a few drops of anhydrous N-methyl-2-pyrrolidone using a paste pestle and mortar).The CB and PVDF served as a conductive additive and a binder, respectively.The electrode was then dried at 80 C overnight in a vacuum oven, and pressed to a thickness of 250 mm.The electrode was cut into a piece of 1 cm Â 1 cm, while the mass loading was typically 1 mg cm À2 for the Pt disc and nickel foam.An oversized glassy carbon plate (1.6 Â 1.6 cm 2 ) was used as the counter electrode and Ag/AgCl (3 M KCl) as the reference electrode.The two-electrode conguration used the nickel foam as substrate.Both the positive and negative electrodes used nickel foam coated with the OLC/MnO 2 nanohybrid, obtained as described for the three-electrode above.The resulting slurry was coated onto the nickel foam substrate ($3 cm 2 ) with a spatula using an average mass loading of 1 mg cm À2 .Symmetric cells were also prepared using nickel foam loaded with only OLC.In all experiments, 1 M Na 2 SO 4 was used as the electrolyte and a porous glass ber (Whatman Grade GF/D Glass Microber Filters, Sigma-Aldrich) served as the separator.For the three-electrode conguration, cyclic voltammetry was performed at various scan rates (2-100 mV s À1 ).Voltage-holding (oating) experiments were performed for 10 h at 0.8 V, then galvanostatically charged-discharged between 0.0 and 0.8 V at 1 A g À1 , repeating the process ve times (i.e., a total of 50 h).Electrochemical impedance spectroscopy (EIS) data were obtained between 100 kHz and 10 mHz with a perturbation amplitude (rms value) of the AC signal of 2 mV.Every EIS experiment was performed aer allowing the cell to equilibrate for 5 min at the chosen xed potential.
The specic capacitance (C sp ) of the half-cells, obtained from CV and galvanostatic discharge curves, was evaluated using the established equations ( 1) and (2), respectively.
where i (A) is the current, DV (V)/Dt (s) the slope of the discharge curve, and m (g) the mass of the active electrode, and V (V) is the voltage obtained during charge.Note that the iR drop ranged from 3.2 to 1.1 U at current densities of 0.1-10 A g À1 .The specic capacitance (C sp ), maximum specic power density (P max ) and specic energy density (E sp ) for the full cells (symmetric devices) were evaluated from the slope of the charge-discharge curves using eqn (3)-( 6). 1 where i (A) is the applied current, DV (V)/Dt (s) the slope of the discharge curve and m (g) the total mass of both electrodes, C (F) the calculated capacitance, V (V) is the maximum voltage obtained during charge, and R s is the equivalent series resistance (ESR).

SEM, TEM, and gas sorption analysis
The surface morphologies of OLC and the OLC/MnO 2 nanohybrid studied using SEM are shown in Fig. 1a and c and using TEM in Fig. 1b and d.The primary particle size of carbon onions is in the range of a few nanometers as seen from the TEM images in agreement with our previous ndings. 47This primary particle size is maintained for the OLC/MnO 2 nanohybrid.Compared with OLC, OLC/MnO 2 hybrid nanoparticles exhibited an obviously different morphology.The selected area electron diffraction (SAED) pattern in the inset of Fig. 1d shows lattice fringes for crystalline MnO 2 and circular lattice shells for OLC. 30ather than a monolayer or multilayer coating of each carbon onion, an effective OLC/MnO 2 nanohybrid was obtained with nanodomains of highly mixed graphitic carbon and metal oxide.The amount of metal oxide was determined by TGA to represent 47 mass% of the OLC/MnO 2 nanohybrid material (see ESI, Fig. S2 †).Also, the TGA data show the excellent thermal stability of OLC with an onset of oxidation at around 630 C as a result of the highly graphitic character of carbon onions synthesized at 1750 C.
Fig. 2 shows nitrogen gas sorption data for OLC and the OLC/MnO 2 nanohybrid.As we see, OLC/MnO 2 exhibits a QSDFT SSA of 122 m 2 g À1 with a distribution of micropores (<2 nm) and mesopores (between 2 and 50 nm).This represents a severe loss in the specic surface area compared to OLC with a QSDFT SSA of 391 m 2 g À1 and is mostly related to the higher molecular mass and higher density of MnO 2 in addition to pore blocking. 50et, Fig. 2b shows that the overall pore size distribution is preserved aer the addition of MnO 2 at a lower total pore volume.
XRD, Raman, FTIR, EDX, and XPS studies Fig. 3 illustrates the Raman spectra, X-ray diffraction patterns, and FTIR spectra of OLC and the corresponding OLC/MnO 2 nanohybrid material.The presence of MnO 2 is conrmed by a strong Raman signal at around 565 cm À1 , 51 (Fig. 3a).The presence of the OLC in the hybrid from the XRD analysis is conrmed by Raman peaks associated with the carbon D-mode (1350 cm À1 ) and G-mode (1590 cm À1 ) of OLC/MnO 2 .Peak analysis (Fig. 3a) shows that the hydrothermal synthesis only insignicantly changes the OLC structure: both the D-and G-mode remain almost unchanged.In particular, the I D /I G ratios before and aer MnO 2 deposition are almost identical to the values of 1.20 and 1.25, respectively.The FWHM values for both the D-mode and the G-mode were measured to be 73.1 cm À1 and 69.8 cm À1 before the deposition and to be 78.5 cm À1 and 65.4 cm À1 aer the MnO 2 deposition.The only minor change related to the carbon signal is identied at around 1100 to 1200 cm À1 which may indicate the formation of a small amount of functionalized carbon. 52FTIR was used to study further the electrode materials as shown by Fig. 3c.The well pronounced peak at 550 cm À1 is due to the Mn-O-Mn asymmetric stretching vibration.The broad peak at 3450 cm À1 is assigned to hydroxyl groups which suggests that there are water molecules in the interlayers (see also the structure given in the ESI, Fig. S1 †). 53rom the XRD patterns of the OLC/MnO 2 nanohybrid (Fig. 3b), the peak at around 26 2q is associated with the (002) plane of graphitic carbon and it can be observed also in OLC/MnO 2 diffractograms indicating the presence of carbon in the nanohybrid.The other peaks can be indexed to birnessitetype MnO 2 (PDF 42-1317).All diffraction peaks of the metal oxide are broadened which indicates the nanocrystalline nature of the MnO 2 with an average coherence length (domain size) in the range of 5-10 nm.The calculated carbon d-spacing for the (002) plane is 0.352 nm and remains at that value with or without the presence of MnO 2 .This represents a small increase in lattice spacing compared to an ideal graphite crystal (i.e., 0.344 nm) as is well-known for the carbon onion structure. 52hemical analysis conrms the presence of birnessite, meaning, not of pure MnO 2 but of a material following the average formula K x Mn 2 O 4 $yH 2 O. Semi-quantitative analysis of OLC EDX spectra (Fig. 4a and Table 1) shows less than 0.2 mass% of impurities alongside ca. 9 mass% of surface oxygen.The metal oxide shows an average molar Mn : K ratio of 4.6 : 1 which is somewhat larger than the maximum stoichiometric value of 4 : 1.The small difference might indicate the presence of minor amounts of residual KMnO 4 .Yet, we note that the previously reported non-carbon content of around 47 mass% is in agreement with our EDX data (54.3 mass%).Only minor impurities of Si and Na can be detected which stem from impurities in the KMnO 4 .XPS analysis of OLC/MnO 2 (Fig. 4b) shows the binding energy peaks of Mn and C. The Mn 2p region consisted of a spin-orbit doublet with Mn 2p 1/2 and Mn 2p 3/2 having binding energies of 654.2 eV and 642.3 eV, respectively. 54][57] From the XPS survey scan, we also see the presence of signicant amounts of K in addition to Mn, C, and O.

Comparative performance of half-cells with Pt disc or Ni foam
Fig. 5 and 6 compare the electrochemical performance of the three-electrode congurations using either a platinum disc (Fig. 5) or nickel foam (Fig. 6) as current collectors.The CV curves of OLC (Fig. 5a) are characteristic of double-layer capacitive materials, while the CV curve of the OLC/MnO 2 nanohybrid shows redox-peaks indicative of faradaic reactions (Fig. 6a).The same conclusions can be drawn from the galvanostatic charge-discharge proles (Fig. 5b and c vs. Fig.6b and  c). 58,59We also see a high power handling ability of the materials with a comparatively small drop in the specic capacitance of OLC/MnO 2 (335-180 F g À1 ) as a function of the current density (0.3-32 A g À1 ), Fig. 6d.
The key ndings from Fig. 5 and 6 may be summarized as follows: (i) the specic capacitance of OLC/MnO 2 is more than a magnitude higher than that of OLC for both current collectors (i.e., Ni foam and a Pt disc); (ii) both types of current collectors gave an essentially similar specic capacitance at different current densities (e.g., 250 F g À1 at 5 A g À1 ); and (iii) the stable voltage window for the Pt disc is narrower (0-0.5 V) than that of  the nickel foam (0-1.0V).The nickel foam alone only insignificantly contributes to the charge storage mechanism (see ESI, Fig. S3 †).We also note that carbon onions alone, that is without the presence of MnO 2 , only exhibit a low specic capacitance of 12 F g À1 .
As summarized in Table 2, the maximum C sp values for our 3-electrode tests (335-408 F g À1 between 0.1 and 0.3 A g À1 ) are much higher than those recorded in the literature.The impressive value (603 F g À1 at 10 A g À1 ) for electrodeposited MnO 2 -nanopillars reported by Yu et al. 73 for their exible nanostructured electrode obtained by combined sputtercoating and electrodeposition (PAN/Au-Pd/MnO 2 , i.e., comprised of a cocktail of polyacrylonitrile polymer and very expensive precious metals of palladium and gold) may, amongst other factors, be related to the thin lm nature of their system and the mass of active materials used in their calculations.Our values are somewhat comparable to those of the recent work by Ruoff et al. 39 involving the elaborate preparation of mesoporous nanotubes assembled from interwoven ultrathin birnessite-type MnO 2 nanosheets.Note that our result is much higher than that of the "OLC"/MnO 2 ($190 F g À1 at 0.2 A g À1 ) reported by Wang et al., 42 and the disparity can be related to the high graphitization of our OLC.

Symmetric pseudocapacitor with a nickel foam substrate
Further investigation of the OLC and OLC/MnO 2 as a full cell symmetric supercapacitor was carried out using nickel foam as the current collector considering its lower cost and better performance at half-cell experiments compared to platinum.Fig. 7 shows CVs (Fig. 7a and c) and galvanostatic chargedischarge curves (Fig. 7b and d) of the OLC and OLC/MnO 2 nanohybrid.In agreement with the three-electrode experiment, two-electrode data of the OLC/MnO 2 nanohybrid show much higher gravimetric capacitance compared to OLC electrodes.The OLC/MnO 2 is capable of cycling at very high current densities (up to 10 A g À1 , Fig. 7d), yielding a high specic capacitance suitable for high power energy storage applications.Table 3 summarizes the values of the capacitance parameters obtained in comparison with the literature, and it is evident that the OLC/MnO 2 nanohybrid exhibits higher performance (in terms of power density or rate capability) than many state-of-the-art MnO 2 -based pseudocapacitors.Note that there has been no known report on symmetric supercapacitors based on birnessite-type MnO 2 in the literature so far; yet, the latter is important to transition to actual devices.
Voltage-holding (or oating) experiments represent a reliable analysis method for establishing the long-term stability of supercapacitor electrodes. 60,61In this work, the OLC/MnO 2 Table 2 Comparison of specific capacitance of various MnO 2 -based three-electrode systems nanohybrid exhibited excellent stability during voltage-holding over 50 h at 1 A g À1 (see ESI, Fig. S4 †).This performance has been illustrated by the gradual decrease in the specic capacitance as the current is kept constant at high potential, retaining ca.200 F g À1 (i.e., approximately 90% of its initial capacitance of 220 F g À1 ).The excellent stability of the OLC/MnO 2 nanohybrid showed that this device can be charged and discharged without signicant deterioration.These values correspond to a maximum specic energy of 5.6 W h kg À1 and an excellent power density of 74.8 kW kg À1 .The improved performance of this hybrid symmetric pseudocapacitor is attributed to the combination of the high electrical conductivity of OLC and the highly reversible redox reactions (pseudocapacitance) arising from the nanostructured MnO 2 material.
EIS data were acquired prior to and post-oating experiments for the OLC/MnO 2 nanohybrid material (Fig. 8 and ESI, Fig. S5 †) and OLC alone (see ESI, Fig. S6 †).The equivalent distributed resistance (EDR), comprising both the equivalent series resistance (ESR) and the ionic resistance within the porous structure (i.e., RC semicircle), was obtained by extrapolating the vertical portion of the plot to the real axis.The OLC/MnO 2 device shows a lower EDR (3.1 U cm 2 ) compared to the OLC alone (7.8 U cm 2 ).However, the RC semicircle for the OLC/MnO 2 is slightly bigger ($1.8 U cm 2 ) than that of the OLC alone ($1.2 U cm 2 ), meaning that the ionic resistance within the porous structure of the pure EDLC (OLC alone) is increased for the OLC/MnO 2 pseudocapacitor.From the Bode plots, the phase angle for the pure OLC is À85 (which is close to the  À90 for an ideal EDLC) compared to the OLC/MnO 2 which is À80 , further indicating the pseudocapacitive behavior of the OLC/MnO 2 device.The knee frequency (f o , f ¼ À45 ) describes the maximum frequency at which the capacitive behavior is dominant, and is a measure of the power capability of a supercapacitor; the higher the f o the more rapidly the supercapacitor can be charged and discharged or the higher the power density that can be achieved from the supercapacitor.The values of f o were ca. 25 Hz for the OLC/MnO 2 (time constant $ 40 ms) and 5 Hz (time constant $ 0.2 s) for the OLC, which further corroborates the higher power performance of the OLC/MnO 2 over its OLC counterpart.It is important to note that the f o values remain approximately the same for both devices before and aer 50 h voltage holding.This result shows that most of the stored energy in OLC/MnO 2 is accessible up to 25 Hz, that is, the energy output available on the millisecond time scale.It should be stated here that most commercially available supercapacitors, including those designed for higher power applications, operate at frequencies less than 1 Hz. 62

Conclusions
This work investigated the electrochemical performance of highly graphitized onion-like carbon integrated with nanostructured birnessite-type MnO 2 materials (OLC/MnO 2 ) when used as a symmetrical pseudocapacitor device.From the halfcell experiment, the OLC/MnO 2 nanohybrid exhibited better performance when using Ni foam as the current collector (in terms of specic capacitance and rate capability) compared to a Pt disc substrate.Based on its excellent performance, Ni foam was used to fabricate the OLC/MnO 2 symmetric pseudocapacitor.The device gave excellent electrochemical performance with a specic capacity of 408 F g À1 , specic energy density of 5.6 W h kg À1 , power density of 74.8 kW kg À1 , capacity retention upon long-hour voltage-holding and cycling, very low equivalent distributed resistance (EDR z 3 U cm 2 ), and very short RC time constant (40 ms).Using such a nanohybrid material, it is possible to overcome the main limitation of MnO 2 , namely its poor electrical conductivity (10 À6 to 10 À5 S cm À1 ) and to exploit its main advantages, namely low-cost, high abundance, and environmental friendliness, for high power energy storage devices.Indeed, the electrochemical properties of OLC/MnO 2 nanohybrids as high-rate energy storage devices have great potential for the development of high power aqueous-based supercapacitors that can be deployed for high-power technological applications.

Fig. 1
Fig. 1 SEM images of (a) OLC and (c) the OLC/MnO 2 nanohybrid; TEM images of (b) OLC and (d) the OLC/MnO 2 nanohybrid.The inset is the corresponding SAED pattern of (d).

Fig. 8
Fig. 8 Nyquist plot for the OLC/MnO 2 symmetric pseudocapacitor before and after 50 h voltage holding experiments.The inset is the expanded portion of the high frequency region.Electrolyte: aqueous 1 M Na 2 SO 4 .

Table 1
Chemical composition of OLC and the OLC/MnO 2 nanohybrid measured by EDX in mass% and atom%

Table 3
Comparison of electrochemical performance of some MnO 2 -based aqueous symmetric electrochemical capacitors a max (V) C sp (F g À1 ) E sp (W h kg À1 ) P max (kW kg À1 ) # EDR (U cm 2 )a Key: GN ¼ graphene nanosheet; GF ¼ graphene foam; CNT ¼ carbon nanotube; DNTA ¼ double-walled nanotube array; CNOs ¼ carbon nanoonions; PDDA: polydiallyldimethylammonium chloride.#TheEDR (equivalent distributed resistance) values were obtained prior to stability studies, and were converted to the U cm 2 based on the information we extracted from the cited reports.