MnOx nanocomposites with superior electrochemical performance for supercapacitors

Journal of Alloys and Compounds 729 (2017) 9e18

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Environment-benign synthesis of rGO/MnOx nanocomposites with superior electrochemical performance for supercapacitors Zhenhong Luan a, Yan Tian a, Ligang Gai a, *, Haihui Jiang a, Xiumei Guo a, Yang Yang b, ** a Institute of Advanced Energy Materials and Chemistry, School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology, Jinan 250353, People's Republic of China b NanoScience Technology Center, Department of Materials Science and Engineering, University of Central Florida, Orlando, FL 32826, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 July 2017 Received in revised form 9 September 2017 Accepted 11 September 2017 Available online 15 September 2017

Chemical oxidation synthesis of graphene oxide (GO) through modified Hummers methods has been widely employed for producing graphene- or reduced GO (rGO)-based advanced functional materials such as rGO/MnOx nanocomposites. However, the manganese species in GO colloids are usually washed out during GO synthesis through modified Hummers methods, causing manganese waste and environmental risk. In this paper, we report preparation of rGO/MnOx nanocomposites, in which MnOx is composed of Mn2O3, Mn3O4, and MnO2 components, through anneal treatment of the precursor counterparts obtained by simple pH tuning of GO colloids. The rGO/MnOx nanocomposites exhibit superior electrochemical performance for supercapacitors. rGO/MnOx-5, derived from GO colloids with pH 5, exhibits a high gravimetric discharge capacitance (Cdis) of 191 F g
1 at 20 A g
1 and a high capacitance retention (82.9%) relative to Cdis at 1 A g
1. Furthermore, typical symmetric supercapacitor cells made from rGO/MnOx-5 show a high areal capacitance (172 mF cm
2) and excellent capacitance retention (96.6%) at 2 A g
1 (10 mA cm
2) for 20,000 cycles, holding great potential for practical applications. The superior electrochemical performance of rGO/MnOx nanocomposites is attributed to multiple charge storage mechanisms in association with the coexistence of mixed-valent manganese oxides. © 2017 Elsevier B.V. All rights reserved.

Keywords: GO colloids rGO/MnOx nanocomposites Electrochemical performance Supercapacitors

1. Introduction Since the discovery of graphene by creating flakes with one atom thickness from a lump of bulk graphite with sticky tape [1], graphene and graphene-based nanocomposites have attracted considerable interest due to their potential applications in photocatalysis [2], sensors [3], solar cells [4], lithium-ion batteries [5], fuel cells [6], and supercapacitors [7]. So far, many methods have been developed for synthesis of graphene such as graphite exfoliation [1,8], chemical vapor deposition [9], arc-discharge [10], templating synthesis [11], solvothermal synthesis [12], and reduction of graphene oxide (GO) [13e15]. As well documented in literature, GO obtained through modified Hummers methods [16] is widely employed as the raw material for producing reduced graphene oxide (rGO)-based advanced functional materials [15,17e21]. For modified Hummers methods [22], GO colloids are formed

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Gai), [email protected] (Y. Yang). http://dx.doi.org/10.1016/j.jallcom.2017.09.115 0925-8388/© 2017 Elsevier B.V. All rights reserved.

through three steps: (1) intercalation of H2SO4/HSO
4 into graphite interlayers to form graphite intercalation compound (GIC); (2) conversion of GIC into pristine graphene oxide (PGO) through diffusion-controlled oxidation by KMnO4/H2SO4; and (3) conversion of PGO into GO layers by reaction of PGO with H2O2. To make full conversion of PGO into GO, 2e4 wt equiv of KMnO4 are needed with respect to 1 wt equiv of graphite [22,23]. GO powders are obtained by freeze-drying of GO colloids after being washed and centrifuged several times. Using 2 wt equiv of KMnO4 and 1 wt equiv of chemically expanded flake graphite in concentrated H2SO4, ultralarge GO nanosheets can be obtained via a simple filtration process, where the residual acid containing metal ions can be easily removed [23]. In view of the high weight ratio of KMnO4/graphite (2) for GO synthesis [22,23], a large amount of manganese species exist in the acid eluent, causing manganese waste and environmental risk. rGO-based manganese oxide (rGO/MnOx) nanocomposites have been subjected to extensive research owing to their wide applications in sensors [24], electrocatalysis [25], lithium-ion batteries [17,21], and supercapacitors [18e20,26e33]. In most of the cases, the rGO component is derived from GO prepared by modified

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Hummers methods and the MnOx component results from additional manganese sources. Unfortunately, the in situ manganese source derived from KMnO4 oxidant for GO synthesis is usually ignored by researchers, resulting in complicated and high-cost synthesis of rGO/MnOx nanocomposites. To take advantage of the in situ manganese source during the chemical oxidation synthesis of GO, Qiu's group developed an air-oxidation method for synthesis of GO/Mn3O4 nanocomposites [32,33]. After plasma treatment [32] or hydrazine reduction [33], rGO/Mn3O4 nanocomposites can be obtained from the GO/Mn3O4 counterparts. However, the electrochemical performance of rGO/Mn3O4 nanocomposites is not satisfactory for supercapacitors [29,32,33]. For example, the specific capacitance of rGO/Mn3O4 is 179 F g
1 at 0.1 A g
1 and the capacitance retention is 91.7% relative to the initial value (156 F g
1) after 800 cycles when tested in a three-electrode system at 0.2 A g
1, using saturated Na2SO4 as the electrolyte [32]. Here, we report a simple synthesis of rGO/MnOx nanocomposites, in which MnOx is composed of Mn2O3, Mn3O4, and MnO2 components. The synthetic strategy is illustrated in Fig. 1. When NaOH aqueous solution is added into GO colloids, the newly formed manganese hydroxides are instantly dissolved due to strong acid intensity of GO colloids. With continued addition of NaOH solution, some solid manganese species occur. The solid manganese species tend to be anchored on GO nanosheets due to strong capturing ability of oxygen-containing functional groups upon GO towards metal oxides [34], resulting in formation of precursor of the target product (Fig. 1a). The precursor was obtained after centrifugal and freeze-drying treatment (Fig. 1b), and then subjected to anneal treatment in Ar to offer the target product (Fig. 1c). When tested as electrode materials for supercapacitors, rGO/MnOx nanocomposites exhibit superior electrochemical performance. 2. Experimental 2.1. Preparation of rGO/MnOx nanocomposites rGO/MnOx nanocomposites were prepared by annealing the precursors derived from GO colloids with pH tuning in the range of 3e7 through addition of NaOH aqueous solution. GO colloids were synthesized through a modified Hummers method [35]. In brief, 1 g of expanded graphite (10e30 mm, Nanjing, China) was grounded with 20 g of NaCl for 20 min. The mixture was washed thoroughly with distilled water to remove NaCl. Graphite powders were collected by filtration, and dried in a vacuum oven at 40  C for 10 h. The dried powders were mixed with 20 mL of concentrated H2SO4 and stirred at 0  C for 12 h, followed by addition of 0.1 g of NaNO3 and 3 g of KMnO4. The mixture was stirred at 0  C for 2 h, and the stirring was continued at 40  C for 1 h. After that, the mixture was transferred into a 500 mL of beaker, followed by careful addition of 150 mL of warm water (60  C) with constant stirring. Bright yellow GO colloids were formed in the beaker after addition of 20 mL of 30 wt% H2O2.

The pH of GO colloids was tuned at room temperature by adding NaOH aqueous solution (7.5 mol L
1) with constant stirring. The designated pH values of 3, 5, and 7 were determined through a pH meter (PHS-3C, Shanghai, China). The precipitates were collected by centrifugal separation, washed with distilled water several times, and then freeze dried, yielding composite precursor powders. rGO/MnOx nanocomposites were produced by annealing the precursor powders in Ar at 450  C for 2 h, with a ramp rate of 2  C min
1. The as-obtained rGO/MnOx samples are named rGO/MnOxP, where P denotes the pH values of GO colloids. For comparison, rGO was prepared by annealing freeze-dried GO powders derived from GO colloids after being thoroughly washed with distilled water, without addition of NaOH. 2.2. Characterization X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer with Cu Ka radiation (l ¼ 1.5406 Å), operating at 40 kV and 40 mA. Raman spectra were collected at room temperature on a Renishaw inVia plus laser Raman spectrometer with laser excitation at 632 nm. Scanning electron microscopy (SEM) and mapping images were taken on a Hitachi S-4800 field-emission scanning electron microscope. Transmission electron microscopy (TEM) images were taken on a JEOL JEM-2100 high-resolution transmission electron microscope, operating at an accelerating voltage of 200 kV. Fourier transform infrared (FT-IR) spectra were collected on a Shimadzu IRPrestige-21 infrared spectrometer using pressed KBr discs. The FT-IR spectra were recorded with resolution of 4 cm
1 over the range of 4000e400 cm
1. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Fisher Scientific Escalab 250 X-ray photoelectron spectrometer, using monochromatic Al Ka radiation (1486.6 eV). Binding energies for high-resolution spectra were calibrated by setting C1s at 284.8 eV. Nitrogen adsorption/ desorption isotherms were collected at 77.3 K, using a Micromeritics TriStar II 3020 sorption analyzer. The BrunauereEmmetteTeller (BET) method was utilized to calculate the specific surface areas (SBET). Pore size distributions (PSD) were determined from adsorption data through the BarretteJoynereHalenda (BJH) model. Before recording the nitrogen sorption isotherms, the samples were degassed at 200  C for 12 h. 2.3. Assembly of pouch supercapacitor cells and electrochemical tests The electrochemical properties of rGO/MnOx samples were first evaluated in a three-electrode system to select the samples with better electrochemical performance for actual supercapacitor cells. A platinum counter electrode, Hg/HgO electrode reference electrode, and 6 mol L
1 KOH aqueous electrolyte were used. The working electrode was prepared by coating sample slurries onto a stainless steel cloth. The sample slurries were prepared by grinding the active material, acetylene black, and polyvinylidene fluoride

Fig. 1. Illustration of rGO/MnOx formation: (a) manganese species anchored on GO nanosheets during pH tuning with concomitant formation of the precursor; (b) precursor powders obtained after separation and freeze-drying treatment; and (c) rGO/MnOx formation after anneal treatment.

Z. Luan et al. / Journal of Alloys and Compounds 729 (2017) 9e18

with weight ratio of 8:1:1 in N-methyl-2-pyrrolidone. The asprepared electrodes were dried in a vacuum oven at 80  C overnight, and then double-rolled to ensure close contact between the active material and the current collector. The mass of active material upon the current collector was 2e5 mg, and the coating area was 1 cm2. Before electrochemical measurements, the working electrodes were immersed into the electrolyte at room temperature for 4 h. A pouch symmetric supercapacitor cell was fabricated by stacking two working electrodes with a Whatman GF/D glass fiber separator saturated with electrolyte, and heat-sealed in a Mylar bag (DuPont, America). The mass of active materials was close for the two electrodes in a supercapacitor cell. Electrochemical tests were performed on a CHI 660E electrochemical workstation (Shanghai CH Instruments Co., China). The electrochemical properties were evaluated in terms of cyclic voltammograms (CVs), galvanostatic charge/discharge (GCD), electrochemical impedance spectroscopy (EIS), rate capability, and/or cycling stability. In a three electrode, the gravimetric discharge capacitance (Cdis, F g
1) was calculated according to equation (1) expressed as [15]:

Cdis ¼

I Dt mDV

I Dt mDV

Fig. 2. XRD spectra of the samples: (a) rGO; (b) rGO/MnOx-3; (c) rGO/MnOx-5; (d) rGO/ MnOx-7; the diffraction peaks marked with A, (, and C belong to MnO2, Mn3O4, and Mn2O3, respectively.

(1)

where I is discharge current (A), Dt the discharge time (s), m the mass of active material on the working electrodes (g), and DV the potential window (V). In a two electrode, the gravimetric discharge capacitance of the cell (Ccell,dis, F g
1) was calculated according to equation (2) expressed as [27]:

Ccell;dis ¼

11

(2)

where m is the total mass of active material on the two electrodes (g). The specific energy (Ecell, W h kg
1) and specific power (Pcell, W
1 kg ) are determined through equations expressed as [26]:

Ecell ¼

Ccell; dis DV2 2  3:6

(3)

Pcell ¼

Ecell Dt

(4)

MnOx-based electrode materials for supercapacitors. To further confirm the structural information, Raman spectra were collected as shown in Fig. 3. The strong and broad peaks centered around 1332 and 1595 cm
1 are associated with the A1g vibration mode of disordered carbon (D band) and the E2g vibration mode of graphitic carbon (G band), respectively [17,19]. The two bands can be deconvolved into four peaks through Lorentzian functions (Table S1) [37]. Taking rGO/MnOx-5 as an example, four peaks centered at 1100 (D4), 1334 (D1), 1550 (D3), and 1595 cm
1 (G) can be resolved (Fig. 3c, green lines). The peaks at 1334 and 1595 cm
1 correspond to the sp2 domains while the other peaks correlate with the sp3-type carbon [37]. The appearance of a small peak at 647 cm
1 and a weak peak around 363 cm
1 in Fig. 3d is due to MneO vibrations in crystalline MnOx [17,27,38]. This result indicates a relatively higher MnOx concentration in rGO/MnOx-7 [38]. The weight percentage of MnOx component in rGO/MnOx-3, rGO/MnOx-5, and rGO/ MnOx-7 accounts for ca. 23.0%, 29.5%, and 33.8% in sequence, estimated by the TG analysis (Fig. S1) [39]. The increasing MnOx

where Dt is the discharge time (h).

3. Results and discussion 3.1. Structure, composition, and morphology Fig. 2 shows XRD spectra of the samples. rGO spectrum exhibits a broad peak centered at 24.8 , corresponding to the (002) plane with a d-spacing of ca. 0.36 nm for rGO (Fig. 2a) [13,18]. Also, there are broad peaks centered at ca. 25 in rGO/MnOx spectra (Fig. 2bed), indicating the existence of rGO component in the composites. Due to the altervalent states of elemental Mn, three manganese oxides can be discerned. The diffraction peaks marked in Fig. 2d can be indexed to cubic MnO2 (JCPDS 42-1169), cubic Mn3O4 (JCPDS 04-0732), and hexagonal Mn2O3 (JCPDS 33-0900), respectively. As well documented in literature [36], the coexistence of mixed-valent manganese oxides enables multiple charge storage mechanisms with much smaller internal resistance and fast charge transfer kinetics, a situation that facilitates to improve the electrochemical performance of

Fig. 3. Raman spectra of the samples: (a) rGO; (b) rGO/MnOx-3; (c) rGO/MnOx-5; (d) rGO/MnOx-7.

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concentration with increasing pH is consistent with the Raman result. The intensity ratio of ID/IG is considered a measure of graphitization degree, defects, and size of sp2 domains in rGO [17,19,29]. The lower ID/IG value, the higher degree of graphitization [17,19]. In the present case, the ID1/IG values for rGO/MnOx nanocomposites are close to 1.5, lower than that of 1.6 for rGO (Table 1). This result indicates that the precursor of MnOx upon GO can enhance the graphitization degree of rGO during anneal treatment, a situation that is beneficial for improving the electronic conductivity of the nanocomposites [19]. Fig. 4 shows SEM images of the samples. Compared with individual rGO (Fig. S2), the nanocomposites exhibit rGO nanosheets enchasing spherical MnOx nanopariticles with size in the range of 10e40 nm. With increasing solution pH to 7, aggregated MnOx nanopariticles with size close to 90 nm occur on the surface of rGO sheets (Fig. 4f). In addition, elemental mapping images of rGO/ MnOx-5 confirm the existence of C, O, Mn (Fig. 4g), and K, Na, S (Fig. S3). The latter three elements arise from the reagents of KMnO4, NaCl/NaOH, and H2SO4 during sample synthesis. MnOx nanopariticles residing on rGO sheets can also be discerned from the TEM images (Fig. 5a,b,e). High-resolution TEM images reveal coexistence of nanocrystals with lattice fringe of 0.263 (Fig. 5d), 0.252 (Fig. 5c), and 0.243 nm (Fig. 5f), corresponding separately to the d311, d110, and d311 of crystalline Mn3O4, Mn2O3, and MnO2. The indistinct lattice fringes for Mn3O4 and Mn2O3 are due to the low degree of crystallinity of MnOx components. These results are in consistence with XRD results. 3.2. Surface chemistry Because the energy storage of supercapacitors mainly involves electrochemical double-layer capacitance (EDLC) and pseudocapacitance associated with redox reaction on the surface of active materials, surface chemistry of rGO/MnOx samples was examined by the FT-IR and XPS techniques. Fig. 6a shows FT-IR spectra of the samples. In rGO spectrum, the peaks centered at 3430, 1641, and 1407 cm
1 arise separately from v(OeH) stretching, d(OeH) bending, and g(OeH) deforming vibrations of surface-adsorbed water [14]. The other peaks correspond separately to v(C]O) stretching (1732 cm
1), v(C]C) stretching (1523 cm
1), v(CeOH)/ v(CeOC) stretching (1050, 868 cm
1), and g(CeH) out-of-plane bending (670 cm
1) vibrations [14,40], due to incomplete removal of oxygen-containing functional groups upon GO. In rGO/MnOx spectra, however, the peaks related to C]O/CeO groups (1732, 868 cm
1) are greatly reduced. This is a result of further reduction of oxygen-containing functional groups, a situation that facilitates to improve the graphitization degree of rGO in nanocomposites. This inference is in concert with the Raman result. A noteworthy point is that the peak positions of v(C]C) and v(CeO) in rGO/MnOx spectra blue shift to 1550e1590 and 1112 cm
1, respectively, compared with those in rGO spectrum. This result indicates strong combination between rGO and MnOx, rather than blending of the two components. In addition, the peak at 617 cm
1 in rGO/MnOx spectra is attributed to v(MneO) stretching vibrations. rGO/MnOx XPS survey spectra confirm the existence of elemental C, O, Mn, and traces of impurity elements introduced

during sample synthesis (Fig. S4a). The high-resolution C 1s spectra can be deconvolved into four components by Gaussian functions (Table S2). Taking rGO/MnOx-5 as an example, four peaks centered at 284.8, 285.9, 287.2, and 289.9 eV can be resolved (Fig. 6b, green lines), corresponding to sp2 C, sp3 C, CeO, and OeC]O, respectively [28,30]. Also, four components can be resolved in rGO/MnOx O 1s spectra (Table S2). The fitting lines for rGO/MnOx-5 O 1s spectrum (Fig. 6c, cyan lines) exhibit four peaks centered at 530.4, 531.8, 533.6, and 535.8 eV correspond separately to MneO, C]O, CeOH/ C, and surface adsorbed water [17,40]. The concentration of MneO component in rGO/MnOx O 1s increases with increasing pH (Table S2). This result is also reflected by the rGO/MnOx Mn 2p spectra (Fig. S4b), where the Mn 2p component is more easily discerned from rGO/MnOx-7 compared with rGO/MnOx-5 and rGO/ MnOx-3. The fitting lines for rGO/MnOx-5 (Fig. 6d, magenta lines) exhibit two broad peaks centered at 642.2 and 653.5 eV, corresponding separately to Mn 2p3/2 and Mn 2p1/2 [17e19,26,41]. The peak for Mn 2p3/2 can be further deconvolved into two components at ca. 641.8 and 643.1 eV, corresponding separately to Mn(IV) and Mn(III) species (Fig. 6d, green lines) [19]. The concentration ratio of Mn(IV)/Mn(III) slightly increases as the MnOx concentration in the composites increases (Table S2). Also, the splitting width of Mn 2p doublets varies in the range of 11.6e11.9 eV (Table S2), indicating different compositions of surface MnOx species on the composites [17,18]. Further, Mn 3s spectra exhibit splitting doublets due to parallel spin coupling between 3d and 3s electrons during photoelectron ejection [18,28,42]. The valence of Mn can be estimated by an equation of DE ¼ 7.88e0.85n, where DE is the splitting width (eV) and n the average valence [18]. In the present case, the DE values are 5.2, 4.9, and 4.7 eV, presenting the average valence of 3.2, 3.5, and 3.7 for Mn in rGO/MnOx-3, rGO/MnOx-5, and rGO/MnOx-7, respectively. 3.3. BET analysis Fig. 7a shows nitrogen sorption isotherms of the samples. The existence of hysteresis loops in the relative-pressure range of 0.4e0.98 indicates coexistence of mesopores and macropores [43]. The distribution of the pores is reflected by the PSD plots (Fig. 7b), where mesopores are dominant. The textural properties of the samples are provided in Table 1. It is found that the SBET values of rGO/MnOx are close to each other yet higher than that of rGO, due to attachment of MnOx nanoparticles on rGO. The low SBET of rGO in the present case is attributed to the anneal treatment, which may cause severe stacking between rGO layers. The mesopores mainly arising from intervals between MnOx nanoparticles serve as reservoirs of electrolyte. This facilitates to enhance charge transfer rate and, hence, to improve rate capability of the electrode materials [43]. On the basis of the above analysis, rGO/MnOx nanocomposites with rGO nanosheets enchasing MnOx nanoparticles are easily prepared by annealing the precursor counterparts, which are obtained by simple pH tuning of GO colloids. MnOx nanoparticles mixed in MnO2, Mn3O4, and Mn2O3 exhibit strong combination with rGO. With increasing pH, the concentration of MnOx and the average valence of Mn are increased. Compared with individual

Table 1 Physicochemical properties of the samples. sample

SBET (m2 g
1)

total pore volume (cm3 g
1)

average pore diameter (nm)

ID1/IG

rGO/MnOx-3 rGO/MnOx-5 rGO/MnOx-7 rGO

11.3 14.5 12.9 3.2

0.06 0.1 0.08 0.04

21.3 26.5 24.5 9.5

1.50 1.49 1.45 1.60

Z. Luan et al. / Journal of Alloys and Compounds 729 (2017) 9e18

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Fig. 4. SEM images of the samples: (a,d) rGO/MnOx-3; (b,e) rGO/MnOx-5; (c,f) rGO/MnOx-7; (g) mapping images of rGO/MnOx-5.

rGO, the graphitization degree of rGO in nanocomposites and the SBET of rGO/MnOx are enhanced, a situation that is beneficial for rGO/MnOx serving as electrode materials for supercapacitors. 3.4. Electrochemical performance Fig. 8a shows the CVs of rGO/MnOx samples tested in a three-

electrode system at a scan rate of 10 mV s
1. For comparison, we also performed electrochemical tests on rGO/MnOx samples prepared by annealing the precursor powders at 400 and 500  C, where the precursor powders were obtained by pH tuning at 5. Hereafter, the two samples are named rGO/MnOx-5-400 and rGO/MnOx-5500, respectively. The CVs deviate from rectangular shape and there are redox peaks in CVs. These results indicate coexistence of EDLC

Fig. 5. TEM images of rGO/MnOx-5: (a,e) low magnification; (b) image corresponding to the squared area in a; (c) high-resolution image corresponding to the circled area in b; (d) high-resolution image corresponding to the squared area in b; (f) high-resolution image corresponding to the squared area in e.

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Fig. 6. (a) FT-IR spectra; (b) C 1s spectra; (c) O 1s spectra; (d) Mn 2p and Mn 3s spectra of rGO/MnOx-5.

Fig. 7. (a) Nitrogen isotherms; (b) PSD plots; inset is magnification PSD plots in the range of 0e10 nm.

Z. Luan et al. / Journal of Alloys and Compounds 729 (2017) 9e18

15

Fig. 8. (a) CVs collected at 10 mV s
1; (b) CVs of rGO/MnOx-5 at different scan rates; (c) plots of redox peak current versus scan rates; (d) GCD curves recorded at 1 A g
1; (e) plots of Cdis versus current densities; (f) Nyquist plots; inset is the magnified Nyquist plots.

and pseudocapacitance [44], due to coexistence of rGO and MnOx with multivalent states of elemental Mn [30,36]. The deviation in redox peak position in rGO/MnOx CVs indicates discrepancy in concentration of MnO2, Mn3O4, and Mn2O3 species upon rGO, in concert with the XPS analysis. Among the CVs, the area of CVs ranks in sequence of rGO/MnOx-7 > rGO/MnOx-5 > rGO/MnOx-3 > rGO/ MnOx-5-400 > rGO/MnOx-5-500. This result indicates: (i) increasing the concentration of MnOx species in nanocomposites improves the

specific capacitance of rGO/MnOx; (ii) annealing the precursor powders of rGO/MnOx-5 at 450  C results in rGO/MnOx-5 nanocomposite with superior electrochemical performance to rGO/ MnOx-5-400 and rGO/MnOx-5-500. With respect to rGO/MnOx-5 CV, two redox peak pairs occur at
0.23/e0.41 and
0.68/e0.70 V. The small potential separation between anodic and cathodic peaks for transition metal oxides is indicative of rapid energy storage in active materials [45]. The shape of rGO/MnOx-5 CVs remains

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Z. Luan et al. / Journal of Alloys and Compounds 729 (2017) 9e18

unchanged (Fig. 8b) while the redox peak current linearly increases as the scan rate increases from 1 to 20 mV s
1 (Fig. 8c). This result indicates the redox reactions follow a surface-controlled rather than a diffusion-controlled process [46]. The small inflections in the GCD curves reflect pseudocapacitive

contribution from the MnOx species (Fig. 8d). The existence of inflections in GCD curves is a common feature for electrode materials with contribution of pseudocapacitance especially tested in a three-electrode system [40]. The relatively flat plateaux in rGO/ MnOx-7 GCD curves are indicative of intercalation

Fig. 9. Electrochemical performance of a symmetric supercapacitor cell: (a) CVs; (b) GCD curves; (c) cycling performance, inset is the GCD curves of the initial five and the last five cycles at 2 A g
1 for 20,000 cycles; (d) Nyquist plots and fitted equivalent circuit; (e) Ragone plot; (f) LED lightened with two cells in series.

Z. Luan et al. / Journal of Alloys and Compounds 729 (2017) 9e18

pseudocapacitance [47]. This is attributed to more MnOx species on rGO/MnOx-7. The values of Cdis at 1 A g
1 are calculated to be 309, 264, 140, 127, and 85 F g
1, corresponding separately to rGO/MnOx7, rGO/MnOx-5, rGO/MnOx-3, rGO/MnOx-5-400, and rGO/MnOx-5500, in concert with the CV result. At the current densities lower than 8 A g
1, the Cdis values for rGO/MnOx-7 are larger than those for the other samples (Fig. 8e). However, with increasing the current density to 20 A g
1, the Cdis values for rGO/MnOx-7, rGO/MnOx5, rGO/MnOx-3, rGO/MnOx-5-400, rGO/MnOx-5-500, and rGO are 120, 191, 92, 64, 45, and 66 F g
1, presenting capacitance retention (relative to Cdis at 1 A g
1) of 38.8%, 82.9%, 75.3%, 58.1%, 60.6%, and 67.5% in sequence. This result indicates rGO/MnOx-5 possesses the best rate capability among the samples. This is attributed to the small and uniform MnOx particle size, relatively larger SBET and total pore volume, and medium MnOx concentration and average valence of Mn, all of which may provide a synergistic effect on the enhanced electrochemical properties of rGO/MnOx-5. Also, rGO/ MnOx-7, rGO/MnOx-5, and rGO/MnOx-3 exhibit superior electrochemical performance to rGO. This is attributed to the multiple charge storage mechanisms with much smaller internal resistance and fast charge transfer kinetics offered by the coexistence of mixed-valent manganese oxides, as mentioned before [36]. Also, the enhanced graphitization degree of rGO, improved SBET, and strong combination between rGO and MnOx contribute positively to the electrochemical performance of rGO/MnOx. The electrode kinetics of the samples is revealed by the Nyquist plots (Fig. 8f), where the intercepts on the realistic axis at the high frequency end represent the equivalent series resistance (ESR) consisting of bulk electrolyte resistance (Re), internal resistance of the active materials, and interfacial contact resistance between the active materials and current collector [19,40]. The depressed semicircles in the middle frequency region correspond to the charge transfer resistance (Rct), and the inclined lines in the low frequency region represent the Warburg impedance (Zw) associated with ion diffusion in the bulk of the electrode [19,40]. It is found that rGO/ MnOx-5 spectrum exhibits relatively lower values of ESR and Rct, and the inclined line of rGO/MnOx-5 approaches nearer to the imaginary axis compared with the other plots, suggesting that rGO/ MnOx-5 is more suitable for application in supercapacitors. On the basis of the above analysis, we can infer that: (1) annealing the rGO/MnOx precursor powders at the temperature of 450  C facilitates to achieve rGO/MnOx with enhanced electrochemical performance; (2) rGO/MnOx-7, rGO/MnOx-5, and rGO/ MnOx-3 exhibit superior electrochemical performance to rGO at the current densities ranging from 0.5 to 20 A g
1; (3) among the samples, rGO/MnOx-5 exhibits better rate capability and larger Cdis when tested at elevated current densities (>8 A g
1); (4) rapid energy storage occurs in rGO/MnOx-5 and the redox reactions contributing pseudocapacitance follow a surface-controlled mechanism; and (5) the Cdis and rate capability of rGO/MnOx-5 are superior to those of carbon/MnOx analogues in recent reports (Table S3) [18,28,29,32,33,38,48e50]. To put rGO/MnOx nanocomposites into practical applications, rGO/MnOx-5 was selected as the active material for assembly of symmetric pouch cells. The thickness of the coating on the current collector is ca. 50e90 mm (Fig. S5), much larger than the recommended value of 15 mm for commercial supercapacitors [51]. Fig. 9a shows the CVs of a typical supercapacitor cell at different scan rates in a potential range of 0e1 V. The CVs exhibit a quasi-rectangular shape at scan rates ranging from 1 to 20 mV s
1 and there are broad peaks in CVs proportional to scan rate. These results indicate coexistence of EDLC and pseudocapacitance [44]. With increasing scan rate, the CVs deviate from rectangular shape (Fig. S6), due to internal resistance of the electrodes [48]. The nearly symmetrical GCD curves (Fig. 9b) indicate excellent electrochemical reversibility

17

and capacitive performance of the cell [18,48]. The Ccell,dis values are calculated to be 50.1, 49.3, 46.2, 41.2, 35.6, and 28.8 F g
1 when tested at 0.1, 0.2, 0.4, 1, 2, and 4 A g
1, respectively. If areal capacitance (Ccell,a ¼ IDt/ADV) is taken into account, where A is the geometric area of the two electrode (cm2), the Ccell,a values are calculated to be 250.5, 246.5, 231, 206, 178, and 144 mF cm
2 when tested at 0.5, 1, 2, 5, 10, and 20 mA cm
2, respectively. The cycling performance of the cell was tested at 1 and 2 A g
1, respectively. After 10,000 cycles at 1 A g
1 (5 mA cm
2), the cell retains a Ccell,dis value of 38 F g
1 (190 mF cm
2), presenting capacitance retention of 92.2% (Fig. 9c). When tested at 2 A g
1 (10 mA cm
2) for 20,000 cycles, the cell retains a Ccell,dis value of 34.4 F g
1 (172 mF cm
2), rendering capacitance retention of 96.6% (Fig. 9c). The high capacitance retention can be reflected by the tiny difference in GCD curves between the initial five and the last five cycles (Fig. 9c, inset). Furthermore, the electrode kinetics reflected by the Nyquist plots remains almost unchanged during charge/ discharge cycles (Fig. 9d). The Nyquist plots can be well fitted to an equivalent circuit consisting of Re, Rct, Zw, Cdl, and Cps (Fig. 9d, inset) [18,19]. The Cdl and Cps denote EDLC and pseudocapacitance, respectively. After 20,000 cycles, the ESR slightly increases by 0.12 U while the Rct and Zw remain nearly unchanged. This result supports the excellent cycling performance of the cell. The specific energy and specific power of the cell are calculated as shown by the Ragone plot (Fig. 9e). The cell can deliver Ecell values of 6.96, 6.85, 6.42, 5.72, 4.94, and 4 W h kg
1 with corresponding Pcell values of 50, 100, 200, 500, 1000, and 2000 W kg
1. Even after 20,000 cycles at 2 A g
1, the cell can deliver an Ecell value of 4.8 W h kg
1 with corresponding Pcell of 1 kW kg
1. The energy storage/conversion performance of the cell is superior or comparable to that of symmetric supercapacitor cells made from graphene/MnOx analogues in recent report (Fig. 9e, inset) [30,31,48]. The functionality of the supercapacitor cells is evidenced that a blue LED (2.8e3.2 V, 20 mA) can be lightened by two cells in series with duration more than 2 min (Fig. 9f and Video S1), after charging 1 min. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.jallcom.2017.09.115. 4. Conclusion In summary, manganese species, usually washed out during GO synthesis through modified Hummers methods, can be effectively recycled by simple pH tuning of GO colloids to offer rGO/MnOx nanocomposites with superior electrochemical performance for supercapacitors. The concentration of MnOx component and the average Mn valence can be controlled by tuning the pH of GO colloids. The coexistence of Mn2O3, Mn3O4, and MnO2 in MnOx is beneficial for rGO/MnOx nanocomposites with enhanced electrochemical performance. Symmetric supercapacitor cells made from rGO/MnOx-5 exhibit excellent cycling performance. After 20,000 cycles at 2 A g
1, the typical cell provides capacitance retention of 96.6% and delivers an Ecell value of 4.8 W h kg
1 with corresponding Pcell of 1 kW kg
1. The electrochemical performance of rGO/MnOx nanocomposites presented here is superior or comparable to that of carbon/MnOx analogues in recent reports. Acknowledgment This research was financially supported by National Natural Science Foundation of China under Grant No. 51272143. Appendix A. Supplementary data Supplementary data related to this article can be found at http://

18

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