MnO2 and biomass-derived 3D porous carbon composites electrodes for high performance supercapacitor applications

Article abstract of DOI:10.1016/j.jallcom.2018.01.108

MnO₂/biomass-derived porous carbon (BPC) composites have been prepared by a hydrothermal method, in which the BPC 3D porous carbon structure was based on a banana peel. The banana peel, after freeze drying, can maintain its hierarchical natural porous structure, which provides enough growth space for MnO₂ and reduces the agglomeration of MnO₂ particles. The MnO₂/BPC composites were characterized by XRD, FT-IR, XPS, TGA, SEM, TEM, BET. The electrochemical performance of the composites was tested in three-electrode supercapacitors using 1 M Na₂SO₄ aqueous solution as an electrolyte. Due to the large amounts of hierarchical pores and large pore volume, the as-prepared composites exhibited good electrochemical performance. Electrochemical measurements indicated that the MnO₂/BPC composites applied in supercapacitors had a specific capacitance of 139.6 F g^?1 at 300 mA g^?1, and exhibited a good cycling stability with a capacitance retention ratio of 92.3% after 1000 cycles (at 1 A g^?1). The MnO₂/BPC composites with 3D porous structure are promising materials in the application of supercapacitors.

Full text of DOI:10.1016/j.jallcom.2018.01.108

Accepted Manuscript
MnO and biomass-derived 3D porous carbon composites electrodes for high
2
performance supercapacitor applications
Guijun Yang, Soo-Jin Park
PII:
S0925-8388(18)30109-9
10.1016/j.jallcom.2018.01.108
JALCOM 44572
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 11 November 2017
Revised Date: 6 January 2018
Accepted Date: 8 January 2018
Please cite this article as: G. Yang, S.-J. Park, MnO and biomass-derived 3D porous carbon
2
composites electrodes for high performance supercapacitor applications, Journal of Alloys and
Compounds (2018), doi: 10.1016/j.jallcom.2018.01.108.
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ACCEPTED MANUSCRIPT
Graphical abstract
Activitation
KOH
Freeze dryer
12 h
Banana peel
Activated slices
Dried slices
MnO2
Hydrothermal
method
Stirring
30 min
120 ℃, 12 h
KMnO4 + Urea
solution
Product
Porous carbon

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Manuscript submitted to "journal of alloys and compounds" as a review paper.
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MnO2 and biomass-derived 3D porous carbon composites electrodes
for high performance supercapacitor applications
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Guijun Yang and Soo-Jin Park*
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Department of Chemistry, Inha University, 100 Inharo, Incheon, Korea
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*Corresponding author. Tel.: +82-32-876-7234, Fax: +82-32-867-5604.
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E-mail addresses:sjpark@inha.ac.kr (S. -J. Park)
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Abstract
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MnO2/biomass-derived porous carbon (BPC) composites have been prepared by a
hydrothermal method, in which the BPC 3D porous carbon structure was based on a
banana peel. The banana peel, after freeze drying, can maintain its hierarchical natural
porous structure, which provides enough growth space for MnO2 and reduces the
agglomeration of MnO2 particles. The MnO2/BPC composites were characterized by
XRD, FT-IR, XPS, TGA, SEM, TEM, BET. The electrochemical performance of the
composites was tested in three-electrode supercapacitors using 1 M Na2SO4 aqueous
solution as an electrolyte. Due to the large amounts of hierarchical pores and large pore
volume, the as-prepared composites exhibited good electrochemical performance.
Electrochemical measurements indicated that the MnO2/BPC composites applied in
supercapacitors had a specific capacitance of 139.6 F g-1 at 300 mA g-1, and exhibited a
good cycling stability with a capacitance retention ratio of 92.3% after 1000 cycles (at
1A g-1). The MnO2/BPC composites with 3D porous structure are promising materials in
the application of supercapacitors.
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Keywords: porous carbon structure, hierarchical pores, MnO2, supercapacitor.
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1. Introduction
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With the development of national economies, the worldwide fossil-fuel
consumption is growing rapidly. However, with the depletion of traditional energy
sources and the contribution of the use of fossil-fuels to environmental pollution, the
development of alternative energy sources has become an urgent matter for the whole
world. Compared to secondary batteries, supercapacitors (SCs) possess distinct
advantages, such as a high power density, larger capacitance, and long cycling life; they
are also environmentally friendly [1-3]. Although supercapacitors have a high power
density, their characteristic low energy density is still a problem that needs to be solved.
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In recent years, many efforts have been made to develop various methods to further
enhance the energy density of supercapacitors. Carbon materials such as porous carbon
[4, 5], graphene [6, 7], activated carbon [8, 9], carbon nanotubes (CNTs) [10, 11],
carbon aerogels [12, 13] and carbon nanofibers [14, 15] have been the most commonly
studied electrode materials for use in supercapacitors. However, their stability is
relatively poor, and the lower specific capacitance of these materials tends to limit the
capacity of the capacitor. In addition, conductive polymers are commonly used
electrode materials in supercapacitors. Currently, the conductive polymer materials used
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as electrodes in supercapacitors mainly include PANI [16-18] and PPy [19-21]. The
oxidation and reduction reaction can take place in the conductive polymer and lead to a
larger Faraday pseudo-capacitance. In recent years, transition-metal oxides such as
RuO2 [22-24], Fe2O3 [25-27], V2O5 [28-30], Co3O4 [31-33], NiO [34-36] and MnO2
[37-39] have also attracted extensive attention as electrode materials. The reversible
redox reaction in these materials results in high electronic conductivity. Moreover, the
electrode redox reaction not only occurs on the surface of the electrode, but also in the
interior of the electrode, which can lead to an improvement in the specific capacitance
of the electrode. As a result, metal oxide materials have a higher energy density than
carbon materials and better electrochemical stability than conductive polymer materials.
Among these various electrode materials, manganese oxide (MnO2) has received
attention due to its high theoretical capacitance (1370 F g-1), abundant reserves, low
toxicity and low cost [40, 41]. However, the experimental capacitance is much lower
than the theoretical capacitance due to its poor electronic conductivity (10-5-10-6 S cm-1)
and insufficient contact surface area. This can significantly reduce the specific
capacitance of MnO2 and hinder its large-scale commercial application. Many studies
have been performed to prepare MnO2/carbon composites materials to improve
electrochemical performance. Recently, Zhang et al. [42] used a novel method to
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prepare three-dimensional (3D) hierarchical, porous reduced graphene oxide
(rGO)/MnO2/carbon foam (CF) nanocomposites. The nanocomposites had a high
specific capacitance and excellent cycle life. Zheng et al. [43] reported the synthesis of
MnO2/carbonized cotton textile composites by a hydrothermal method. These materials
exhibited high specific capacitance, good electronic conductivity, good cycle stability
and excellent flexibility. In addition, Chen et al. [44] reported the synthesis of
MnO2/graphdiyne oxide (GDYO) composites by an in-situ redox reaction with different
carbon materials as templates, which showed a specific capacitance of 170 F g-1 at 10 A
g-1 and a 98 % capacitance retention after 3000 charge/discharge cycles. Therefore, it
follows that these carbon composites materials can significantly improve the
electrochemical performance of MnO2.
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In this work, we prepared petal-like, nanosheet MnO2/carbon composites by using
an activated banana peel-derived porous carbon as a template. The banana peel contains
abundant cellulose and pectin, naturally forms biomass carbon with a 3D porous
network structure after carbonization at high temperature. The biomorphic 3D porous
structure with large surface area can not only load the MnO2 active materials, but also
enhance electron and mass transport, thus leading to improved electrochemical
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performance.
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2. Experimental
2.1. Preparation of MnO2
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All chemical reagents were analytical grade and used without further purification.
MnO2 powder was synthesized by a hydrothermal method with KMnO4 (99.9%, Aldrich)
and MnSO4·H2O (99.9%, Aldrich) as raw materials. The mole ratio of KMnO4:
MnSO4·H2O was 3:2. First, a certain amount of KMnO4 and MnSO4·H2O were
dissolved in 60 mL of deionized water and stirred for 20 min, the purple solution was
transferred to a 100 mL Teflon-lined autoclave and maintained at 140 °C for 6 h. Finally,
the black product was dried at 80 °C for 12 h after being washed with deionized water
and ethanol many times.
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2.2. Preparation of 3D porous carbon
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Activated banana peel-derived porous carbon (BPC) was synthesized by a common
carbonation method. The banana were bought from supermarket and the country of
origin is Philippines. The collected banana peel was washed by diluted alcohol and
deionized water to remove the grease and impurities on the surface. Typically, the raw
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banana peel was firstly cut into small pieces and immersed in 2 M KOH for 24 h. Next,
the banana peel pieces were dried by a freeze-drying technique for 12 h. The banana
peel pieces with vacuum freeze drying technique can maintain its natural porous
structure. The dried precursor was calcinated at 900 °C for 2 h under argon atmosphere.
The obtained product was washed with a dilute HCl solution to remove residual KOH.
Finally, the porous carbon was dried at 80 °C for 12 h after washed with deionized
water and ethanol.
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2.3. Synthesis of the MnO2/BPC composites
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The MnO2/BPC composites were synthesized by a hydrothermal method. A given
amount of KMnO4 and urea were dissolved into 60 mL of deionized water. The
previously described, as-prepared 3D porous carbon was immersed in the KMnO4
solution. The mixture was transferred into a 100 mL Teflon-lined autoclave and
maintained at 120 °C for 12 h. The product was carefully washed with deionized water
and dried at 60 °C for 12 h.
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2.4. Structural characterization
To evaluate the characteristics of the obtained samples, X-ray powder diffraction
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(XRD, Bruker D2 PHASER) was employed to analyze the morphological composition
of the samples. Infrared spectra were obtained using Fourier transform infrared
spectroscopy (FT-IR, Jasco PS-4000). Scanning electron microscopy (SEM, Model
SU8010, Hitachi Co., Ltd.) and field-emission transmission electron microscopy (FE-
TEM, JEOL JEM-2100F) were used to characterize the structure and morphology. X-
ray photoelectron spectroscopy (XPS, ESCALAB MK-II, VG Scientific Co.) was used
to verify the chemical composition and valence states of the elements in the samples.
The thermal stability was studied by thermogravimetric analysis (TGA, TG 209 F3,
NETZSCH, Germany) at a heating rate of 10 °C min-1 under air atmosphere. The
specific surface areas (SSA) of the composites were investigated by the Brunauer-
Emmett-Teller (BET) method at 77 K. The pore size distributions were calculated by the
density functional theory (DFT) model. The total pore volume was determined at a
relative pressure of P/P0=0.99.
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2.5. Electrochemical measurements
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Cyclic voltammetry (CV) testing and galvanostatic charge/discharge (GCD)
measurements were performed on a Ivium electrochemical workstation at room
temperature. Here, a three-electrode system included prepared samples as the working
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electrode, a platinum (Pt) wire as a counter electrode and a saturated calomel electrode
(SCE) as the reference electrode. The electrolyte was 1 M Na2SO4 aqueous solution. In
order to prepare working electrodes, the as-prepared samples were mixed with ketjen
black and a PTFE binder in a weight ratio of 80:10:10 in N-Methyl-2-Pyrrolidone
(NMP) solution. Afterwards the homogeneous slurry was coated onto a nickel foam and
dried at 60 °C for 12 h to obtain the working electrodes.
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3. Results and discussion
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The crystallographic structure and phase purity of the BPC, MnO2 and the
MnO2/BPC composites samples are shown in Fig. 1a. There are two broad diffraction
peaks at 26° and 60° 2θ for BPC, which shows its amorphous carbon nature. The four
broad peaks of the MnO2/BPC composites at around 26°, 37.6°, 42.8° and 56.9°
2θ correspond to the (001), (100), (101) and (431) crystal planes of MnO2 (JCPDS no.
44-0141). The diffraction peak of MnO2 at around 24°, 2θ is broadened due to the
overlap of the amorphous BPC peak. No impurity peak was detected for the sample. In
addition, the XRD diffraction peaks show weak intensity which indicates poor
crystallization of MnO2. Compared with high crystalline material, the poor
crystallization leads to more transportation channels, and results in a better
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electrochemical performance [45].
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FT-IR spectroscopy was used to measure the functionalization of the pure MnO2
and MnO2/BPC composites. As illustrated in Fig. 1b, the absorption peaks of pure
MnO2 at 3414 cm-1 and 1401cm-1 correspond to the bending vibration band and
stretching vibration band of -OH, respectively. The absorption peaks at 1647 cm–1 and
651 cm–1 were assigned to the Mn-O stretching vibration band. As shown in the FT-IR
of the MnO2/BPC composites, we can find strong peaks belonging to Mn-O stretching
vibration band which indicates the existence of MnO2 in the MnO2/BPC composites.
XPS was employed to measure the chemical properties of the prepared MnO2/BPC
composites. The XPS survey spectra and core-level spectra of C 1s, Mn 2p and O 1s
peaks are shown in Fig. 2. As shown in Fig. 2a, the XPS full survey scan spectrum
presents signals of Mn, O and C elements, which indicates the presence of MnO2 and
carbon. Fig. 2b shows the high-resolution C 1s spectrum with four individual peaks,
which can be fitted to the binding energy (BE) of sp2 C-C (284.7 eV), C-O (285.6 eV),
C-O-C (286.4 eV) and O-C=O (288.5 eV) [46], respectively. As seen from Fig. 2c, there
are two peaks at 654.8 eV and 643.2 eV, corresponding to Mn 2p1/2 and Mn 2p3/2 with a
spin energy separation of 11.6 eV, suggesting that the main oxidation state of Mn is +4,
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associated with the MnO2 structure [47]. There are three individual peaks in the high-
resolution O 1s spectrum (Fig. 2d), which represent O-Mn bonds (529.9 eV) for
tetravalent oxide MnO2 and O-C (531.5 eV)/C=O (531.3 eV) bonds on the surface of
carbon [48].
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TGA measurement of MnO2/BPC composites was carried out from room
temperature to 900 °C under air atomosphere to determine the content of MnO2 in
MnO2/BPC composites. As shown in Fig. S1, the TGA curve of MnO2/BPC composites
can be divided into three stages, the first weight loss of 3.5 % at a temperature range of
room temperature to 150 °C due to the dehydration of the sample. The second weight
loss of 4.7 % at 150 °C-400 °C in TGA curve corresponds to the decomposition of
hemicellulose and cellulose in biochar. The third weight loss of 3.5 % at 400 °C and
completed at around 600 °C corresponds to combustion of carbon. From the TGA curve,
we can estimate the mass loading of MnO2 in MnO2/BPC composites are 29.7 %.
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The morphology and 3D structure of the prepared BPC, MnO2 and MnO2/BPC
composites samples were characterized by SEM. The morphology of BPC is shown in
Fig. 3a. A loose 3D network structure can be seen and the channels are orderly, which is
due to freeze drying preserving the integrity of the structure. As shown in Fig. 3b, the
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channels are approximately 30 ꢀm in diameter which can provide sufficient growth
space for the MnO2. This 3D network structure of carbon can serve as a kind of
substrate for electrode materials. The channels can load a large amount of active
materials and the carbon structure can also improve electrical conductivity. Moreover, it
can greatly increase the contact area with the electrolyte, which allows for the
electrolyte permeate and better transport of electrolyte ions. Fig. 3c is the SEM image of
the MnO2 sample which was prepared by the hydrothermal method. The MnO2 particles
with irregular shape are severely agglomerated, which will lead to poor electrochemical
performance. The morphology of the as-prepared MnO2/BPC composites is shown in
Fig. 3d-f. Compared with Fig. 3b, the MnO2/BPC composites have a similar structure as
compared to the pure BPC. This indicates that the morphology of BPC was not
significantly changed after the MnO2 was loaded. As can be seen from Fig. 3e, the 3D
network of the BPC is uniformly covered with a large amount of small petals. Further
shown in Fig. 3f, the petal-like nanosheets are composed of MnO2 distributed evenly on
the surface.
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Fig. 4 shows the TEM image of the BPC and the MnO2/BPC composites. The BPC
sample (Fig. 4a) exhibits a thin, sheet-like morphology. The edge of the BPC sample
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was magnified and shown in Fig. 4b and shows that there are many pores smaller than
10 nm in diameter on its surface. Fig. 4c shows TEM image of MnO2/BPC composites,
MnO2 particles gather together to form clusters with occasional folds and crinkles. From
Fig. 4d, we find that MnO2 covers the surface of the BPC uniformly. MnO2 directly
grown on the conductive carbon substrate can significantly reduce the interfacial
resistance between MnO2 and the BPC conductive carbon substrate, thus leading to
better electrical conductivity of the electrode.
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The KOH activation method is the most effective activation method due to its high
carbon yield, but the activation mechanism between KOH and banana peel is
complicated. Generally, the reaction may be as follows [49]:
4ꢀꢁꢂ + ꢃ → ꢀꢄꢃꢁꢅ + ꢀꢄꢁ + 2ꢂꢄ
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Here, considering the decomposition of potassium hydroxide and the reduction of
carbon, the following reactions are given.
2ꢀꢁꢂ → ꢀꢄꢁ + ꢂꢄꢁ
ꢂꢄꢁ + ꢃ → ꢃꢁ + ꢂꢄ
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ꢂꢄꢁ + ꢃꢁ → ꢃꢁꢄ + ꢂꢄ
ꢀꢄꢁ + ꢃꢁꢄ → ꢀꢄꢃꢁꢅ
ꢀꢄꢁ + ꢂꢄ → 2ꢀ + ꢂꢄꢁ
ꢀꢄꢁ + ꢃ → 2ꢀ + ꢃꢁ
ꢀꢄꢃꢁꢅ + 2ꢃ → 2ꢀ + 3ꢃꢁ
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A large amount of gas is produced in the reaction process, which is helpful to the
formation of pores. Originally, KOH activation actively occurs at 600 °C or more
higher. But the banana peels contain high amount of cellulose, The cellulose is
gradually graphitized at above 800 °C, therefore we choose 900 °C to ensure complete
graphitization of cellulose.
Fig. 5 shows representative nitrogen adsorption/desorption isotherms and the
corresponding Barrett-Joyner-Halenda (BJH) pore size distribution of the BPC,
MnO2/BPC composites and MnO2 samples. The quantity of nitrogen adsorbed showed
that the three samples possess large amount of pores. Based on the International Union
of Pure and Applied Chemistry (IUPAC) classification system, the nitrogen adsorption
isotherm of BPC sample showed a typical Type-I, indicating its micropore structure. In
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contrast, the adsorption isotherms of MnO2/BPC composites and MnO2 showed typical
type-ꢀ isotherms, which indicates that the two samples contain both micropore and
mesopore structures. Especially for the MnO2/BPC composites, Nitrogen uptake
significantly at low partial pressures (P/P0 < 0.1 and P/P0 > 0.4), indicating the
mesoporous and microporous structure of the composites. The corresponding pore size
distribution further reveals that a hierarchical, multi-pore structure has been successfully
prepared, which corresponds to the results of TEM. MnO2/BPC composites and MnO2
samples exhibit typical hierarchical pore size distribution with a highest peak centering
at ca. 1.3 nm, as well as a lower peak centering at ca. 14 nm. It can be clearly observed
that the majority pores of the two samples are micropores and mesopores. In
comparison, BPC sample exhibits a obviously peak at ca. 200 nm, the pore sizes of BPC
are located in the region of micropores and macropores. As can be seen from the
nitrogen desorption isotherms, the adsorption capacity of MnO2/BPC composites is
lower than the BPC, this is due to MnO2 covering the surface of the BPC substrate,
blocking the majority of the pores. There are more micropores in the MnO2/BPC
composites leading to higher surface area and larger pore volume, resulting in better
electrochemical performance.
Electrochemical performance of the BPC, MnO2 and the MnO2/BPC composites
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electrodes were performed in 1 M Na2SO4 electrolyte by using cyclic voltammetry
(CV) and galvanostatic charge/discharge (GCD) measurements in a three-electrode
configuration. Fig. 6a shows CV curves of MnO2 and the MnO2/BPC composites in
the potential range from 0 to 1 V at a scan rate of 20 mV s-1.It can be seen that the two
CV curves show nearly rectangular shape, suggesting reversible electrochemical
performance. As shown in Fig. 6b, the distortion of the CV curves has no obvious
variation even at higher scanning rate, demonstrating that the system of electrodes has
a lower resistance and a more desirable electrochemical behavior. Fig. S2 shows CV
curves of the BPC samples at different scan rates from 5 to 30 mV s-1 with the voltage
window of 0-1 V. As shown in Fig. S2, the CV curves of the BPC are nearly
rectangular in shape. But compared to the CV curves of the MnO2/BPC composites
(Fig. 6b), the current density of the MnO2/BPC composites improves considerably
due to the pseudo-capacitive properties of MnO2. Fig. 6c exhibits the GCD curves of
MnO2 and the MnO2/BPC composites at 1 A g-1 with a potential window from 0 to 0.9
V. The MnO2/BPC composites maintain longer discharge time than MnO2, indicating
the capacitance is much higher than that of the MnO2 electrode. In order to further
measure the electrochemical performance, the charge/discharge performance of the
MnO2/BPC composites, at various current densities from 300 mA g-1 to 10 A g-1,were
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carried out and are shown in Fig. 6d. The charge/discharge curves are nearly
triangular in shape, suggesting a typical, electrical double-layer, capacitive behavior
with high columbic efficiency. The specific capacitance of the electrode can be
calculated according to the following formula.
Cꢆ = I∆t/m∆V
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Where Cs is the specific capacitance (F g-1), I is the charge/discharge current (A),
∆t is the discharge time, m is the mass of the active material (g) and ∆V is the
potential window (V). The specific capacitance of the MnO2/BPC composites
decreases from 139.6 to 70 F g-1 with increasing the current density from 300 mA g-1
to 10 A g-1, respectively. Fig. S3 shows the charge/discharge curves of the BPC and
MnO2 at different current densities. Compared to the MnO2/BPC composites, the BPC
and MnO2 exhibit shorter discharge times. Fig. S4 shows the electrochemical
performance of YP-50F activated carbon, which is a commercial reference sample. By
comparation, the BPC exhibits a higher specific capacitance than YP-50F activated
carbon in 1M Na2SO4 electrolyte. The specific capacitance of MnO2 and MnO2/BPC
composites electrodes at different current densities were calculated and are shown in
Fig. 6e. In fact, the charge-storage mechanism of the MnO2-based electrode in neutral
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solution is as follows.
MnOꢄ + Hꢇ + eꢈ ↔ MnOOH
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The process of the MnO2 charge-storage includes the intercalation/de-
intercalation reaction of H+ and the surface adsorption/desorption of electrolyte
cations (H+) during the redox reaction process. As can be seen from the SEM images,
the carbon-structure substrate provides enough growth space for the MnO2 particles,
which can avoid the aggregation of the MnO2 particles and provide more sites for
rapid, faradic redox reactions, thus improving the electrochemical performance. Such
results are consistent with the CV curves. Furthermore, the long-term cycling stability
of MnO2 and the MnO2/BPC composites electrodes are measured at 1 A g-1 for 1000
cycles. It can be clearly seen from Fig. 6f that the capacitance retention of MnO2 and
the MnO2/BPC composites electrodes are 89.5% and 92.3% after 1000 cycles. The
MnO2/BPC composites electrode shows a better cycling performance than the MnO2
electrodes, suggesting a good electrochemical stability of the MnO2/BPC composites
electrode. The MnO2/BPC composites electrode has a higher specific capacitance and
better charge/discharge rate capability, which is mainly due to the 3D porous carbon
network structure, in which the MnO2 and BPC act synergistically for improved
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electrochemical performance.
4. Conclusions
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In summary, banana peel has been used to prepare a hierarchically, naturally
porous carbon substrate for energy storage applications. Petal-like nanosheets of MnO2
were grown directly on the surface of the BPC by a simple hydrothermal method. The
as-prepared MnO2/BPC composites electrode exhibits a specific capacitance of 139.6 F
g-1 at 300 mA g-1, even exhibiting a specific capacitance of 70 F g-1 at the current
density of 10 A g-1, with specific capacitance retention of 92.3% after 1000 cycles. The
3D carbon substrate can improve the electrochemical performance of MnO2 due to its
hierarchical porous structure, which provides growth space for the MnO2, serves as a
conductive channel for electrons, and benefits the diffusion of electrolyte ions on the
surface of the electrode. The banana-peel derived supercapacitors are good candidates
for the development of composites materials with high electrochemical performance.
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Acknowledgements
This research was supported by The Leading Human Resource Training Program
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of Regional Neo industry through the National Research Foundation of Korea (NRF)
funded by the Ministry of Science, ICT and future Planning (grant number) (NRF-
2016H1D5A1909732).
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Figure Captions
459
460
Scheme 1. Schematic illustration for the preparation of the BPC and the petal-like
nanosheets hierarchical MnO2/BPC composites electrode.
461
462
Fig. 1. (a) XRD patterns of the BPC, MnO2 and the MnO2/BPC composites and (b) FT-
IR spectra of MnO2 and the MnO2/BPC composites.
463
464
Fig. 2. (a) XPS survey and high-resolution XPS spectra for (b) C 1s, (c) Mn 2p and O 1s
of the as-prepared MnO2/BPC composites sample.
465
466
Fig. 3. SEM images of (a, b) the BPC at different magnifications, (c) MnO2 particles
and (d, e, f) the MnO2/BPC composites at different magnifications.
467
468
Fig. 4. TEM images of (a, b) the BPC at different magnifications and (c, d) the
MnO2/BPC composites at different magnifications.
469
470
Fig. 5. (a) Nitrogen adsorption/desorption isotherms and (b) pore size distributions of
BPC sample, MnO2 sample and MnO2/BPC composites.
471
472
473
474
475
476
477
478
Fig. 6. (a) CV curves of MnO2 and the MnO2/BPC composites electrode at a scan rate
of 5 mV s-1 (b) CV curves of the MnO2/BPC composites electrode at different scan rates
(5, 10, 20 and 30 mVs-1) (c) Charge/discharge curves of MnO2 and the MnO2/BPC
composites electrode at 1A g-1 (d) Charge/discharge curves of the MnO2/BPC
composites electrode at different current densities (0.3, 0.5, 1, 2, 5 and 10 A g-1) (e)
Specific capacitance of MnO2 and the MnO2/BPC composites electrode at different
current densities calculated from charge/discharge curves (f) Cycling performance of
MnO2 and the MnO2/BPC composites electrode at a current density of 1 A g-1.
479
480
Fig. S1. TGA curve of MnO2/BPC composites measured from room tempereature to
900 oC under air atmosphere.
481
482
Fig. S2. CV curves of the BPC samples at different scan rates (5, 10, 20 and 30 mV s-1)
with the voltage window of 0-1 V.
483
484
Fig. S3. Charge/discharge curves of (a) the BPC and (b) MnO2 at different current
densities.
485
486
Fig. S4. Charge/discharge curves and CV curves of YP-50F with the voltage window of
0-1 V in 1 M Na2SO4 electrolyte.
25

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Activitation
KOH
Freeze dryer
12 h
Banana peel
Activated slices
Dried slices
MnO2
Hydrothermal
method
Stirring
30 min
120 ℃, 12 h
KMnO4 + Urea
solution
Product
Porous carbon
Scheme 1. Schematic illustration for the preparation of BPC and petal-like hierarchical MnO2/BPC composites .

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(a)
(b)
MnO2
MnO2/C composites
1401
1056
1647
MnO2/C composites
MnO2
651
3414
1401
1647
651
BPC
1056
1140
3414
4000 3500 3000 2500 2000 1500 1000 500
10
20
30
40
50
60
70
80
-1
2θ (degree)
Wave numbers (cm )
Fig. 1. (a) XRD patterns of BPC, MnO2 and MnO2/BPC composites and (b) FT-IR spectra of MnO2 and
MnO2/BPC composites.

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Raw
Baseline
Sum
C 1s
Mn 2p
O 1s
(a)
(b)
C-C
C-O
C-O-C
O-C=O
C 1s
1200 1000
800
600
400
200
0
282
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Binding energy (eV)
Binding energy (eV)
O 1s
Raw
Baseline
Sum
O-Mn
C-O
C=O
(c)
(d)
Mn 2p3/2
643.2 eV
Mn 2p1/2
654.8 eV
11.6 eV
650
660
655
645
640
635
526
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530
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538
Binding energy (eV)
Binding energy (eV)
Fig. 2. (a) XPS survey and high-resolution XPS spectra for (b) C 1s, (c) Mn 2p and (d) O 1s of the
as-prepared MnO2/BPC composites sample.