Journal of Alloys and Compounds 812 (2020) 152124

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Journal of Alloys and Compounds

journal homepage: http://www.elsevier.com/locate/jalcom

Facile fabrication of binder-free reduced graphene oxide/MnO2/Ni

foam hybrid electrode for high-performance supercapacitors
Zhiyong Zhao, Ting Shen, Zhihua Liu, Qishi Zhong, Yujun Qin*
Department of Chemistry, Renmin University of China, Beijing, 100872, China

a r t i c l e i n f o a b s t r a c t

Article history: A novel three-dimensional reduced graphene oxide aerogel and MnO2 (rGO/MnO2) hybrid is prepared via
Received 21 May 2019 a mild method and freeze-drying treatment, followed by an electrodeposition process. The character-
Received in revised form ization results reveal that the deposited MnO2 is homogeneously anchored on the graphene sheets,
30 August 2019
which is served as binder-free electrode material to fabricate high-performance supercapacitor. The
Accepted 1 September 2019
specific capacitance of rGO/MnO2 on Ni foam reaches 288 F g
1 at 0.5 A g
1. The assembled symmetrical
Available online 2 September 2019
rGO/MnO2/Ni supercapacitor exhibits a maximum energy density of 26.82 Wh kg
1 and a maximum
power density of 8.61 kW kg
1, whose capacitance retention maintains 94.7% over 1000 cycles. Moreover,
Keywords:
MnO2
it can successfully activate different LEDs after being charged. These appreciable performances are pri-
Graphene aerogel marily put down to the cooperative effect of porous structure of rGO/MnO2 and pseudocapacitive
Electrochemical deposition property of MnO2, which provides adequate electroactive sites and facilitates the electron/ion transfer
Supercapacitor during the electrochemical processes.
Binder-free © 2019 Elsevier B.V. All rights reserved.

1. Introduction graphene-based materials, especially aerogels, have been success-

fully synthesized for sundry promising applications, including
Graphene is a unique two-dimensional (2D) atomic crystal catalysis [19], electrochemical sensing [20] as well as energy stor-
made up of single-layer carbon atoms in the form of sp2 hybridi- age or conversion [21,22].
zation, which has aroused widespread concern and intensive Among numerous applications of 3D graphene materials,
research on account of its outstanding characteristics such as car- supercapacitors are deemed to be one of the most prospective as-
rier mobility (~200000 cm
2 V
1 s
1), Young modulus (~1.1 TPa) pects and have obtained plenty of attention by virtue of their long
and high specific surface area (2630 m2 g
1) [1e4]. These distinc- cycle life, splendid power density as well as brief charge and
tive merits allow graphene to be studied in varieties of fields [5e7], discharge time [23,24]. Nevertheless, using graphene alone as the
particularly in new energy area including photocatalysis [8], supercapacitor electrode material can only achieve low energy
supercapacitors [9], lithium ion batteries [10,11], and solar energy density, which is confined by the charge storage of electrical
materials [12e14] to name but a few. However, the application of double-layer [25]. The pseudocapacitive materials, another super-
graphene is usually restrained by the aggregation between mono- capacitor electrode materials, possess high energy density and high
layer graphene because of the robust p-p stacking interaction. In specific capacitance obtained by highly reversible Faraday redox
consequence, a legion of efforts have been dedicated to con- reaction [24], however also suffering from low power delivery
structing diverse structures to expand the practical applications of owing to the poor conductivity [26]. Considering the minuses and
graphene in recent years [15]. For instance, the 3D hierarchical pluses of both, the modification/combination of graphene with
architectures (aerogels, foams or sponges), the frameworks where pseudocapacitive materials has come into the front stage, including
graphene sheets are connected, have been greatly developed on conducting polymers/graphene [27,28] and transition metal ox-
account of the advantages of multichannel transport network, big ides/graphene [28,29], which have exhibited excellent capacitive
surface area, and good conductivity [16e18]. So far, plentiful 3D performances because of their cooperative effects.
In the midst of many pseudocapacitive materials as reported,
MnO2 is the widest used one because of its superb pseudocapaci-
tance, availability in abundance, low cost and non-toxicity [30]. As
* Corresponding author.
an efficient method for preparing reduced graphene oxide (rGO)
E-mail address: yjqin@ruc.edu.cn (Y. Qin).

https://doi.org/10.1016/j.jallcom.2019.152124
0925-8388/© 2019 Elsevier B.V. All rights reserved.
2 Z. Zhao et al. / Journal of Alloys and Compounds 812 (2020) 152124

aerogel, hydrothermal process is also usually used to fabricate rGO/ 2.3. Preparation of rGO/MnO2/Ni hybrid electrode
MnO2 aerogel for supercapacitors. For instance, Dong et al. pre-
pared N-doped rGO/MnO2 composite by a one-step hydrothermal Ni foam was cut into slices (20  10  1.0 mm), and the resulting
process, with the capacitance up to 220 F g
1 [31]. Chai et al. re- rGO aerogel was ground thoroughly and then coated uniformly on
ported a hydrothermal and low-temperature thermal deposition one side of the Ni foam slice without using any adhesives and
method to fabricate rGO/MnO2 material possessing the capacitance conductive agents. The mass loading of rGO aerogel is ~1 mg and
of 192.2 F g
1 and good capacitance retention [32]. Alternatively, the coating area is around 1 cm2. Subsequently, the rGO-covered Ni
electrodeposition is a relatively simple and efficacious method for foam (rGO/Ni) was compressed into thin calendered electrode by
preparing graphene/MnO2 composite, and the growth rate and physical pressing (10 MPa). MnO2 was coated on rGO/Ni (with the
morphology of MnO2 can be well controlled [33]. For instance, Park bare Ni side covered with insulated rubber tape) by galvanostatic
et al. have assembled rGO/MnO2 composites through an electro- electrodeposition using a three-electrode system, where the rGO/
chemical deposition, which revealed the good rate capability of Ni slice served as the working electrode (WE), a Ag/AgCl electrode
70.29% in the supercapacitor [34]. as the reference electrode (RE), and a platinum plate electrode as
After all, there remain some challenges ahead in preparing such the counter electrode (CE). Before electrodeposition, the rGO/Ni
MnO2-modified graphene electrode materials. On one side, the slice was immersed into the plating solution which was comprised
hydrothermal procedure, with a high-temperature and high- of 10 mM Na2SO4 and 0.1 M Mn(CH3COO)2 for 2 h under vacuum so
pressure process, is usually involved in the preparation of the that the rGO aerogel can fully absorb the plating solution. The
graphene/MnO2 aerogels, which limits the application of the ma- electrodeposition was carried on with an anodic current of 1 mA at
terial to some degree due to the rigorous reaction conditions. different times (10, 20 and 30 min), after which the slice was rinsed
Another question that comes to mind is that in most cases, the and dried in vacuum oven (60  C, 2 h) to gain the rGO/MnO2/Ni
adhesives and conductive agents are necessary for fabricating the hybrid electrode finally. For comparison experiments, MnO2 was
supercapacitor electrode to ensure the good adhesion between electrodeposited on the pure Ni foam in the same method, marked
matrices and active materials, as well as the total conductivity of as MnO2/Ni.
electrode material, and the further perplexed process inevitably
reduces the content of the active materials and decreases the cor- 2.4. Electrochemical measurements
responding energy density [35]. As a result, it is still imperative to
explore more convenient and effective way to prepare graphene/ All of the electrochemical tests were conducted in three-
MnO2 hybrid as high-performance supercapacitor electrode electrode (half-cell) or two-electrode (full-cell) system with an
material. electrochemical workstation (CHI660E, Shanghai, China). In three-
In the current paper, we report the facile preparation of novel electrode system, our tests were carried out in 1 M Na2SO4, where
rGO/MnO2 hybrid materials including the preparation of rGO aer- the binder-free rGO/MnO2/Ni, rGO/Ni and MnO2/Ni samples acted
ogel through a mild method and the electrochemical deposition of as the WE. Moreover, the Ni foam also served as current collector
MnO2. In the glass bottle, the mixed solution of graphene oxide during electrochemical tests. In two-electrode tests, the symmet-
(GO) and reductant underwent a mild “hydrothermal” process rical supercapacitor (SSC) was fabricated with two identical rGO/
(40  C) in place of normal high temperature and pressure. The rGO MnO2/Ni electrodes that were segregated by polypropylene film
hydrogel obtained was lyophilized to get rGO aerogel, which was (NKK, Japan), which was soaked in 1 M Na2SO4 before assembling.
ground and directly coated on the Ni foam, with the electrodepo- The electrochemical characteristics were evaluated in the
sition of MnO2 followed. The highly conductive rGO matrix and aforesaid electrochemical workstation by means of cyclic voltam-
sturdy interaction between rGO sheet and MnO2 enable the as- metry (CV), galvanostatic charge-discharge (GCD) and electro-
prepared rGO/MnO2/Ni hybrid to be utilized as binder-free elec- chemical impedance spectroscopy (EIS). The specific capacitance
trode to assemble symmetrical supercapacitor, which shows can be computed by the GCD curves as following equations:
satisfying electrochemical performance.
I Dt 2I Dt
half cell : Csp ¼ ; full cell : C 0sp ¼
mDV mDV
2. Experimental
in which I (A) represents the discharge current, Dt (s) represents the
2.1. Materials discharge time, m represents the mass load of the electrode ma-
terials, and DV (V) represents the discharging potential window
Graphite flake (99.8%) and manganese (II) acetate tetrahydrate after deducting the IR drop. The energy density (E) and power
were purchased from Alfa Aesar. L-ascorbic acid (L-AA) was pro- density (P) of the SSC were computed using the following formulas:
vided by Sinopharm Chemical Reagent Co., Ltd. Additional chem-
icals were bought from Beijing Chemical Works. All chemical 1 1000 E 3600
E ¼  C 0sp  ðDVÞ2  ;p ¼ 
reagents in these experiments were directly used without any 2 3600 t 1000
treatment.

2.2. Synthesis of rGO aerogel 2.5. Materials characterization

GO was prepared by a modified Hummers’ method [36]. In order A Hitachi SU8010 scanning electron microscope (SEM) appen-
to synthesis the rGO aerogel, GO suspension (4 mg mL
1, 5 mL) was ded with an energy dispersive spectroscopy (EDS) was utilized to
poured into a flasket and the pH was regulated to 9 with 0.1 M study the microstructure of the samples. Transmission electron
NaOH, where L-AA (60 mg) was then added under continuous microscope (TEM) images were captured by an FEI Tecnai G2 F20 U-
agitation. Subsequently, the suspension was transferred into a glass TWIN at 200 kV. Raman spectra were measured on a Horiba Xplora
bottle, which was kept for 16 h at 40  C. The hydrogel obtained was plus Raman spectrometer using excitation laser of 532 nm. A Shi-
rinsed with deionized water. Later, a vacuum freeze-drying (e madzu XRD-7000 was used to record the X-ray diffraction (XRD)
60  C) process was adapt to get the final rGO aerogel. results with Cu Ka radiation. The chemical structure was analyzed
 Z. Zhao et al. / Journal of Alloys and Compounds 812 (2020) 152124 3

by X-ray photoelectron spectroscopy (XPS), which was investigated MnO2@10, rGO/MnO2@20 as well as rGO/MnO2@30. As we can see,
by a Thermofisher ESCALAB 250Xi with 300 W Al Ka radiation. The the amount and size of the spherical MnO2 gradually increase with
N2 adsorption and desorption testing was conducted in a liquid the growth of deposition time until they completely overlap on the
nitrogen bath at 77 K applying a MicrotracBEL Belsorp-Mini II. The rGO sheet after 30 min. According to the morphological observation
specific surface areas (SSA) of the samples were calculated by and the electrochemical performances to be mentioned below, we
Brunauer-Emmett-Teller method. The pore volume and pore size selected 20 min as the optimum condition. As shown in Fig. 2b and
distribution were acquired through Barrett-Joyner-Halenda the inset, after 20 min of electrochemical deposition, the rGO/
method. MnO2@20 sample maintains the structural framework of the rGO
aerogel with MnO2 uniformly attached on the smooth rGO sheets.
3. Results and discussion Additionally, Fig. 2c and d expressly present SEM images of rGO/
MnO2@20 at high magnifications to scrutinize the morphology of
Schematic fabrication of the rGO/MnO2/Ni hybrid can be seen in MnO2. We can clearly see that the flower-shaped spheres with the
Fig. 1, which is described in experimental section in detail. Hydro- diameter of ~400 nm consisting of multiple petaloid MnO2 nano-
thermal process is generally required during the preparation of rGO sheets. The EDS mapping results of Mn, O and C (Fig. 2c inset)
aerogel, along with the reduction of GO by diverse reductant like further affirm the homogenous distribution of the MnO2 on rGO
LiAlH4, NaBH4, hydrazine, etc. In our case, the mild reductive L-AA sheets. These MnO2 nanosheets interlace together and closely cling
was used as reducing agent at 40  C, where high pressure was to the surface of the rGO sheet, thus it is expected that the rGO/
abandoned and no toxic gas was released during the formation of MnO2@20 is equipped with more porous structure and larger sur-
the hydrogel precursor. Besides, dehydroascorbic acid, the oxidized face areas, which was certified by the results of nitrogen adsorp-
product of the reaction, served as a protective reagent to prevent tion/desorption isotherm displayed in Fig. S2. Compared with the
the stacking of rGO sheet as well [37]. Finally, the rGO aerogel was rGO aerogel (134 m2 g
1), the SSA of rGO/MnO2 was 204 m2 g
1.
obtained via a freeze-drying method, and the corresponding digital Besides, the calculated total pore volume of rGO/MnO2@20 was
photos recording the transformation of GO solution into a cylin- 0.998 cm3 g
1, bigger than that of rGO aerogel (0.698 cm3 g
1).
drical monolith of rGO aerogel is demonstrated in Fig. S1. Our as- Upon our results above, we can safely learn the MnO2 deposited on
prepared rGO aerogel has an apparent porous structure with the rGO sheets did increase the SSA and therefore provided ample
SSA of 134 m2 g
1, allowing it to be used as an excellent matrix for active sites as charge transfer pathways during the reactions.
the electrodeposition of MnO2. As obviously plotted in Fig. S2, the Moreover, the mesoporous configuration of the rGO/MnO2@20
isotherm curves are typical IV-type isotherm, which displays a hybrid can provide more electrolyte storage to facilitate electron/
distinct hysteresis loop in middle and high pressure regions be- ion transfer in the oxidation-reduction reaction process, which can
tween adsorption curve and desorption curve, manifesting the make the utmost of the pseudocapacitive property of MnO2 [38].
mesoporous feature of the as-prepared rGO aerogel. Such meso- Simultaneously, the increasing SSA and pore-size distribution are
porous structure could enable the WE to get access to electrolyte beneficial to the enhancement of the rate capability and cycle
easily and facilitate the diffusion of ions during electrochemical ability of the hybrid [31].
reactions. TEM was also conducted to better investigate the structure of
The microstructure of rGO aerogel and rGO/MnO2 was studied rGO/MnO2@20. As shown in Fig. 2e, the MnO2 nanoflakes are, quite
by SEM in detail. Fig. 2a displays the surface morphology of our evidently, evenly deposited on the rGO surface. Notably, in the
pristine rGO aerogel, which presents typical porous and connected process of preparing TEM samples, the MnO2 nanoflakes remain
graphene structure. The cross-section image shown in the inset sturdily anchored on the rGO surface and not stripped off even after
presents a hierarchical structure with hole size of several microns, intense ultrasonication treatment, which indicates a robust inter-
which could insure the thorough infiltration of the plating solution action between rGO sheets and MnO2. Such uniformly dispersed
in the following MnO2 deposition. Subsequently, the influence of MnO2 nanoflakes attached tightly on the graphene could give rise
electrochemical deposition time on the size and morphology of to a larger surface area, which is in conformity with the previous
MnO2 was explored. Fig. S3 shows the various surface morphology analysis consequence (Fig. S2). As discussed above, the 3D structure
of rGO/MnO2 corresponding to different deposition time (10, 20 established by MnO2 and the rGO aerogel would mitigate the
and 30 min). The samples are respectively marked as rGO/ stacking of graphene during the electrochemical measurement to
some extent, providing rapid Naþ transport pathways and therefore
improving pseudocapacitive performances. Besides, as presented in
Fig. 2f, the lattice fringes have been clearly observed by high-
resolution TEM, illustrating the interlayer spacing of 0.14 and
0.24 nm, which are indexed to (020) and (110) crystal planes of the
birnessite-type MnO2 and coincide well with the following XRD
data (Fig. 3a).
XRD patterns for the GO, rGO and rGO/MnO2@20 samples are
illustrated in Fig. 3a. For rGO aerogel, the characteristic peak of
pristine GO at 10 has vanished and the observed broad peaks at
23 and 43 are attributed to the (002) and (100) crystal planes of
the graphitic carbon (JCPDS 75e1621), respectively, indicating that
GO is fully reduced to rGO. After 20 min of electrodeposition, except
for the characteristic peak of rGO, the broad peaks around 37 and
66 could be assigned to the monoclinic lamellar structure of the
birnessite-type MnO2 (JCPDS 42e1317) [39]. It is well known that
broad peaks are relevant to low degree of crystallization. Based on
the XRD results, we can come to the consequence that the elec-
trochemically deposited MnO2 is a poorly crystallized compound
Fig. 1. Schematic illustration for the preparation of rGO/MnO2/Ni hybrid electrode. with amorphous portions. What's more, the peak at 43 almost
4 Z. Zhao et al. / Journal of Alloys and Compounds 812 (2020) 152124

Fig. 2. SEM images of (a) rGO aerogel and (b) rGO/MnO2@20 at front view and lateral view (inset). (ced) SEM images of the rGO/MnO2@20 surface under high magnifications (inset
c: relevant EDS mapping results of Mn, O and C). (eef) TEM and high-resolution TEM images of rGO/MnO2@20.

Fig. 3. (a) XRD patterns and (b) Raman spectra of GO, rGO and rGO/MnO2@20.

cannot be seen after 20 min of electrodeposition, which manifests and 1596 cm
1, respectively, attributed to D and G bands, where D
that entire surface of rGO sheets is well coated by the MnO2. band can be assigned to the disordered structural defect and G band
Raman measurements were employed to attest the reduction represents the E2g mode of sp2 carbon. When it comes to rGO and
degree of GO and the existence of MnO2 after electrochemical rGO/MnO2, the spectra of both display analogous peaks of D and G
deposition. Fig. 3b displays Raman spectra of GO, rGO and rGO/ bands. We find that ID/IG value rises after GO was reduced by L-AA,
MnO2@20.The spectrum of GO exhibits two distinct peaks at 1358 which may put down to the formation of smaller pieces of
 Z. Zhao et al. / Journal of Alloys and Compounds 812 (2020) 152124 5

graphene sheets after the reduction [40]. As for rGO/MnO2@20, two area of CV profile is known to represent its corresponding specific
newly emerging peaks concentrated at 586 and 631 cm
1 are capacitance. We can clearly see that the MnO2 functionalization
attributed to MneO stretching vibration and symmetric stretching gives rise to the increasing of current density and enlarging of in-
vibration in the basal plane of MnO6 sheet [41], verifying that the tegrated area, which is owing to the excellent pseudocapacitive
MnO2 was successfully anchored onto rGO sheets. Moreover, the ID/ properties of MnO2. Obviously, rGO/MnO2@20/Ni electrode ex-
IG intensity of rGO/MnO2@20 is almost unchanged compared with hibits better performance than rGO/MnO2@10/Ni electrode owing
that of rGO, which manifests that the introduction of MnO2 doesn't to the increasing MnO2 deposition. While the rGO/MnO2@30/Ni
substantially undermine the structure of graphene matrix. The electrode shows inferior performance compared with rGO/
collective evidences identify the excellent structure of the rGO/ MnO2@20/Ni electrode, which could be ascribed to the excessive
MnO2@20 on Ni foam, which is obligated to the improved elec- accumulation of MnO2 upon longer deposition time, resulting in
trochemical performance. the reducing in the conductivity of materials and deteriorating of
XPS were carried out to further characterize the surface infor- the corresponding capacitive features. The capacitive properties of
mation of rGO/MnO2@20 hybrid. As Fig. 4a shows, the XPS survey the electrodes can also be appraised by GCD tests. As demonstrated
spectrum displays typical peaks of C 1s, O 1s, Mn 3s, as well as Mn in Fig. S4b, the relevant results acquired at 1 A g
1 give the Csp
2p, suggesting the existence of MnO2 on rGO sheets. Fig. 4b displays values of 56, 85, 237 and 169 F g
1 for the rGO/Ni, rGO/MnO2@10/
the deconvolution spectrum of Mn 2p, where the peaks centred at Ni, rGO/MnO2@20/Ni and rGO/MnO2@30/Ni electrodes, respec-
653.9 and 642.3 eV are fitted into Mn 2p1/2 and Mn 2p3/2, severally. tively. Obviously, the variation of Csp value for the samples keeps in
The energy difference of 11.7 eV illustrates the state of Mn4þ, which line with the CV results owing to the same reasons. Thus, we
conforms to other literature [42]. Fig. 4c presents the C 1s decon- selected 20 min as the optimized deposition time and the following
volution spectrum, where the dominating peak is owing to C]C characterization on the supercapacitors was based on rGO/
bonds, located at 284.8 eV. Other peaks at 285.6, 286.5 and 288.5 eV MnO2@20/Ni electrode.
can be ascribed to CeOH, CeOeC and OeC]O, respectively [43,44]. Subsequently, the electrochemical properties of rGO/MnO2@20/
The presence of MnO2 can be further validated by the O 1s spec- Ni were systematically studied, and for comparison, electrochem-
trum, which is broken into three dominated peaks in Fig. 4d. The ically deposited MnO2 for 20 min (MnO2@20) on Ni foam was also
peaks centred at 529.9, 531.3 and 532.3 eV could be owing to prepared, as well as rGO/Ni sample. Fig. 5a exhibits the different CV
MneOeMn, MneOeH and HeOeH banding configurations, curves of rGO/Ni electrode, MnO2@20/Ni electrode as well as rGO/
respectively [45]. MnO2@20/Ni electrode, which were conducted within the potential
To evaluate the capacitive features of as-prepared binder-free interval of 0e1 V at 50 mV s
1. Distinctly, as for rGO/Ni electrode,
rGO/MnO2/Ni hybrid electrode, CV tests in a three-electrode system the configuration of CV curve takes on a typical rectangular shape
were conducted. First, the impact of deposition time on the with no conspicuous redox peaks, demonstrating the trait of
capacitive performances was explored so as to obtain the optimal double-layer capacitance. Moreover, the curve of MnO2@20/Ni
MnO2 electrodeposition time. Fig. S4 compares the CV curves of electrode presents a quasi-rectangle bearing a couple of small
rGO/Ni electrode and rGO/MnO2/Ni electrode under different redox current peaks (0.5 and 0.7 V), because that Naþ ions undergo
deposition time (10, 20 and 30 min) at 50 mV s
1. The integrated the oxidation-reduction reaction of intercalation and

Fig. 4. (a) XPS survey spectrum of rGO/MnO2@20. XPS spectra of (b) Mn 2p, (c) C 1s and (d) O 1s in high-resolution.
6 Z. Zhao et al. / Journal of Alloys and Compounds 812 (2020) 152124

evidently larger output current than the latter. The increase of

current density and curve area for rGO/MnO2@20/Ni electrode is on
account of the improved SSA of rGO/MnO2@20 hybrid. Moreover,
the remained rectangular shape of the CV curve indicates contin-
uous and highly reversible oxidation-reduction reactions occurring
during the CV process [46], which is attributed to the good pseu-
docapacitive property of the uniformly distributed MnO2.
Additionally, we compared the GCD properties of aforesaid
three kinds of samples, Fig. 5b presents GCD curves of these elec-
trodes conducted within the potential interval from 0 to 1 V at
1 A g
1. Apparently, GCD curve of rGO/MnO2@20/Ni electrode is
almost symmetric in comparison to those of the first two elec-
trodes, which demonstrates that the rGO/MnO2@20/Ni has higher
columbic efficiency owing to highly reversible oxidation-reduction
reactions. Furthermore, the rGO/MnO2@20/Ni electrode possesses
observably longer charge-discharge time and markedly higher Csp
value. To be specific, the Csp value of rGO/MnO2@20/Ni (237 F g
1)
at 1 A g
1 was much larger than those of MnO2@20/Ni (105 F g
1)
and rGO/Ni (56 F g
1), and the consequence fits right with the
outcome of CV measurements.
To have an insight into the electrochemical characteristics of
rGO/MnO2@20 hybrid, the EIS measurements were also conducted.
Fig. 5c shows Nyquist plots of the above-mentioned three kinds of
electrodes with enlarged plots in the inset, all of which consist of a
semi-circular curve at high frequency zone and a tilt line at middle-
to-low frequency zone. Meanwhile, the equivalent circuit is drawn
in Fig. S6, where Rs represents the equivalent series resistance, Rct
represents the charge transfer resistance, Zw represents the War-
burg impedance, Cd represents the double-layer capacitance, and CL
is the pseudocapacitive capacitance. As we expected, on the one
hand, the Rs value of the rGO/MnO2@20/Ni electrode (2.26 U) can
be seen at the intersection of the real axis, smaller than that of
MnO2@20/Ni (3.54 U) but a bit bigger than that of rGO/Ni (1.02 U),
indicating a relatively low internal resistance. On the other, a low
Rct value of the rGO/MnO2@20/Ni electrode can be proved by the
small semicircle in the Nyquist plot. The good parameters of rGO/
MnO2@20/Ni electrode root in the conducting rGO scaffold and the
excellent combination with MnO2. What's more, the Nyquist plot of
rGO/MnO2@20 is nearly straight in low frequency range, illus-
trating the fast ion diffusion and excellent capacitor behavior.
Rate capability is another vital criterion to estimate the prop-
erties of supercapacitor materials. Fig. 6a presents rate-dependent
CV curves of rGO/MnO2@20/Ni electrode by altering the scan rate
(5e100 mV s
1). Evidently, the CV curves have negligible shape
change and still present relatively rectangular profiles, indicating
the distinguished rate capability of our electrode. Meanwhile, cor-
responding GCD curves at various current densities (0.5e10 A g
1)
remain almost symmetric, as shown in Fig. 6b. What's more, the
electrode achieves a Csp of 288 F g
1 at 0.5 A g
1, which can remain
at 200 F g
1, even though at a high current density of 10 A g
1. The
decline of Csp upon the rising current density could be attributed to
Fig. 5. (a) CV curves at a scan rate of 50 mV s
1, (b) GCD curves at a current density of the difficulty of Naþ diffusion and insufficient oxidation-reduction
1 A g
1 for the rGO, MnO2@20/Ni and rGO/MnO2@20/Ni electrodes. (c) Nyquist plots of reactions at a high current density [47]. For comparison, Fig. S5
corresponding electrodes (inset: relevant datas in high-middle frequency zone). exhibits the GCD curves of MnO2@20/Ni, rGO/Ni, rGO/MnO2@10/
Ni and rGO/MnO2@30/Ni electrodes at varying current densities,
and their relevant Csp values are listed Table S1, which further de-
deintercalation in MnO2 surface or bulk phase (MnO2 þ Naþ þ e

fines the superior capacitive performance of rGO/MnO2. Thus, it can
! MnOONa), accordingly realizing the pseudocapacitive charge
be speculated from all these results that the unique 3D mesoporous
storage of the supercapacitor. Nevertheless, the reversibility of the
structure of rGO/MnO2@20 is much in favour of Naþ diffusion into
oxidation-reduction reaction for MnO2@20/Ni is not so good, due to
the active material and charge transfer for oxidation-reduction
the absence of rGO as the superduper matrix. As a result, the overall
reaction, consequently resulting in the still invertible capacitive
electrochemical properties of MnO2@20/Ni electrode are not very
performance at high rates.
satisfactory. Fortunately, after the MnO2 is deposited uniformly on
Considering the remarkable electrochemical and capacitive
rGO sheets, the CV shape of rGO/MnO2@20/Ni electrode is quite
properties of the rGO/MnO2@20 hybrid, the SSC with the structure
analogous to the rectangular shape of rGO/Ni electrode with
of Ni/rGO/MnO2@20//MnO2@20/rGO/Ni and the polypropylene
 Z. Zhao et al. / Journal of Alloys and Compounds 812 (2020) 152124 7

Fig. 6. (a) CV curves and (b) GCD curves for the rGO/MnO2@20/Ni electrode with different rates.

film as separator was assembled to further study the practical po- connected two and three SSC cells in series to enlarge the potential
tentials. Firstly, GCD measurements of the SSC in a two-electrode window for actual tests, which were charged to 0e2 V and 0e3 V,
condition at distinct current densities were carried out and the severally, and corresponding charge-discharge graphs at the steady
graph is presented in Fig. 7a. We find that a high C'sp value of current (1 mA) are presented in Fig. 7d. Our two and three cells in
189 F g
1 is attained at 0.5 A g
1, still delivering 161 F g
1 at series can successfully light up yellow and white LEDs, respectively,
10 A g
1, realizing a good capacitance retention of 85.2%. Fig. 7b after being charged, and the videos can be seen in Videos S1e2.
(inset is the sketch diagram of SSC) presents the Ragone plot of our Finally, the performance of our SSC is compared with the reported
as-assembled SSC, revealing a maximum energy density of ones from other MnO2-based materials, as listed in Table S2, which
26.82 Wh kg
1 and maximum power density of 8.61 kW kg
1. demonstrates the superior properties of MnO2@20/rGO/Ni as
Moreover, the cycling stability measurement was carried out at supercapacitor electrode. However, in comparison with other Ni-
10 A g
1 over 1000 cycles. The SSC could achieve an appreciable foam-based supercapacitors (Table S3), our electrode performs
capacitance retention of 94.7% in the end, as shown in Fig. 7c. This is not so well, especially in the energy density, indicating the focus in
also evidently seen from the inset of Fig. 7c, which illustrates part of our future work.
the GCD curves of SSC in the cycling process. Furthermore, we

Fig. 7. (a) GCD curves of the SSC at the current density from 0.5 to 10 A g
1. (b) Ragone plots for the SSC (inset: schematic of the SSC structure). (c) Cycling stability of the SSC (inset:
the GCD curve of the SSC at 10A g
1). (d) GCD curves of the different SCCs in series and the white LED activated by the SSC (the inset).
8 Z. Zhao et al. / Journal of Alloys and Compounds 812 (2020) 152124

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