Coherent Mn O -carbon nanocomposites with enhanced energy ... · Coherent Mn 3 O 4-carbon...

Nano Res

1

Coherent Mn3O4-carbon nanocomposites with

enhanced energy storage capacitance

Chaofeng Liu1, Huanqiao Song1, Changkun zhang1, Yaguang Liu1, Cuiping Zhang1, Xihui Nan1, and

Guozhong Cao1,2 ( )

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0837-4

http://www.thenanoresearch.com on June 10, 2015

© Tsinghua University Press 2015

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Nano Research

DOI 10.1007/s12274-015-0837-4

TABLE OF CONTENTS (TOC)

Coherent Mn3O4 nanocomposites with enhanced

energy storage capacitance

Chaofeng Liu1, Huanqiao Song1, Changkun zhang1,

Yaguang Liu1, Cuiping Zhang1, Xihui Nan1, and

Guozhong Cao1,2 *

1 Beijing Institute of Nanoenergy and Nanosystems,

Chinese Academy of Sciences, Beijing, 100083, China 2 Department of Materials Science and Engineering,

University of Washington, Seattle, Washington, 98195,

USA

Sonochemical method was adopted to synthesize coherent

Mn3O4-activated carbon nanocomposites for fabricating high

energy density Li ion capacitor. Extreme conditions derived

from sonication irrad iated ethanol to catalyze reducing reaction

in synthesis process and redox react ion in Mn3O4 contributed to

the enhanced capacitance of nanocomposites.

Coherent Mn3O4-carbon nanocomposites with

enhanced energy storage capacitance

Chaofeng Liu1, Huanqiao Song1, Changkun zhang1, Yaguang Liu1, Cuiping Zhang1, Xihui Nan1, and

Guozhong Cao1,2 ( )

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

Sonochemical,

Mn3O4,

Li ion capacitor,

Activated carbon,

Nanocomposite

ABSTRACT

Nanostructured Mn3O4 was introduced to activated carbon by a novel

sonochemical reaction and the resulting nanocomposites were investigated as

supercapacitor electrodes. Not only does the sonication catalyze the redox

reaction, but also promote the diffusion of the precursors , resulting in the

formation of coherent nanocomposites with Mn3O4 nanoparticles grown and

uniformly distributed inside the mesopores of activated carbon. In addition, the

extreme local condition in sonochemical synthesis resulted in excessive amount

of divalent manganese ions and oxygen vacancies. This novel microstructure

endows the sample with a superior performance, with a specific capacitance of

150 F g-1 against 93 F g-1 of activated carbon at a charge/discharge rate of 100 mA

g-1, and the Li ion capacitor delivered an energy density of 68 Wh Kg-1 against

41 Wh Kg-1 of activated carbon capacitor at a power density of 210 W kg-1.

1 Introduction

Supercapacitors as electrical energy storage devices

bridge the gap between conventional dielectric

capacitors and batteries for the high energy and

power densities they possess [1, 2]. Generally,

supercapacitors can be classified into two categories

of electrical double layer capacitor (EDLC) and

pseudo-capacitor on the basis of charge storage

mechanism of electrode materials [3, 4]. EDLC is

based on the accumulation of charges at interfaces

between electrodes and electrolytes, and thus, the

capacitance is strongly dependent on the surface area

and porous structure of electrode materials.

Pseudo-capacitor relies on a pseudo-redox reaction

which derives from electron transfer between the

electrode and the electrolyte. Transition metal oxides

(TMOs), carbonaceous materials decorated with

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2 Nano Res.

functional groups, and conductive polymers are all

candidates for redox reactions [2, 5, 6]. However,

both metal oxides and conductive polymers often

exhibit relatively poor cyclic stability and less power

density, which limits their practical applications [7].

Activated carbons (ACs) have abundant sources,

possess large surface area, and good chemical

stability, leading to their wide utilization as electrode

materials in supercapacitors. Surface area of ACs

determines the formation of the electric double layer

dictating the speed of pseudo-capacitive reaction.

Pore size and pore morphology are also the key

parameters affecting their electrochemical

performance [8]. Surface functionalization of ACs

plays an important role on surface wettability and

introduction of pseudo-capacitance [9, 10]. Methods

for surface modification on ACs can be categorized

into three types: chemical bonding of heteroatoms

[10-14], deposition of metal coating [15] and

incorporation of transition metal oxides [2, 16].

TMOs as electrode materials can offer high

pseudo-capacitance through Faradaic reactions.

Tailoring the microstructure and tuning the

crystallinity of materials with given chemical

composition and crystal structure are effective

approaches to improve their electrochemical

properties and enhance the performance of devices

made of such materials [2, 7]. Combining these

merits of TMOs with the large surface area and

excellent electrical conductivity of carbonaceous

materials has attracted extensive attention to design

and fabricate higher capacitive electrode materials

[17]. For example, introduction of MnO2 nanoneedles

to AC obtained excellent electrochemical capacitance,

which is attributed to the synergistic effect of both

the AC porous structure and redox reaction of MnO2

nanoneedles [18]. The similar results have been

reported in RuO2/AC [19] and Fe3O4/AC [20].

Various synthesis and processing methods have been

developed or studied for the fabrication of desired

nanostructures and microstructures to capitalize the

synergistic merits of both ACs and TMOs.

Sonochemical synthesis is known as an efficient,

environmental benign, and cost effective approach

for the fabrication of nanomaterials or the

modification of the surface texture of materials.

During the sonochemical synthesis process,

ultrasound can induce unique hot spots of a possible

temperature above 5,000 K, pressures exceeding

1,000 bars, and heating/cooling rate in excess of 10 10

K s-1 [21]. Amorphous manganese oxide coated

carbon has been synthesized through sonochemical

method from a mixture of NaMnO4 and carbon

delivered an initial capacity of 273 mAh g-1 at a

charge/discharge rate of 100 mA g-1 in the voltage

window of 1.5-4.0 V for Li ion battery [22]. Lee et al.

adopted sonochemical method to synthesize 3D

graphene/ nanoparticles (NPs) foam and displayed a

high specific capacitance of 421 F g-1 at 100 mA g-1

and 97% capacitance retention at a charge/discharge

rate of 20 A g-1 [23]. Besides, the localized spots with

extremely high temperature and pressure generated

by ultrasonication easily create abrupt

supersaturation inducing high density of nucleation

and subsequent uniformly sized nanoparticles [24].

This work reports sonochemical synthesis of coherent

Mn3O4 - AC nanocomposites and their excellent

electrochemical properties when used as

supercapacitor electrodes in organic electrolyte.

Mn3O4 is characterized with high theoretical

capacitance, low cost, abundance, and is

environmental friendly [25]. Organic electrolyte, 1M

LiPF6 in ethylene carbonate/dimethyl carbonate was

used for its high operating voltage so as to obtain

increased energy density [26, 27]. The phases and

porous structure of resulting nanocomposites were

characterized by means of X-ray diffractometry

(XRD), scanning electron microscopy (SEM), and

nitrogen sorption analysis. The electrochemical

performances were investigated with cyclic

voltammetry, galvanostatic cycling, and

electrochemical impedance spectroscopy (EIS). The

reaction mechanism of sonochemical synthesis and

the relationships between the processing,

microstructure, and electrochemical properties of the

resulting coherent Mn3O4 -AC nanocomposite

(labeled as AC-Mn) have been discussed.

2 Experimental section

2.1 Synthesis

Coherent trimanganese tetraoxide (Mn3O4) - carbon

nanocomposite was synthesized via a sonication

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3 Nano Res.

assisted mechanical stirring solution method using

potassium permanganate (KMnO4, Beijing chemical

works) and activated carbon derived from coconut

shell (AC, TF-B520, Shanghai SinoTech Investment

Mangement CO., LTD) as raw materials. First, 1.50g

AC was dispersed into 150 ml absolute ethyl alcohol

by mechanical stirrer and sonicated by ultrasonic

cleaning bath (720 W, 40 KHz, Kunshan, China) with

5 min. 0.12 g KMnO4 was dissolved into 12 ml

de-ionized water. After the pre-treatment, AC

dispersion liquid was stirred continuously and the

solution of KMnO4 was added into it with the

sonicated treatment for 30 min. The product was

filtered and washed by absolute ethyl alcohol and

de-ionized water several times in order to remove the

by-products. At last, the product was dried by

vacuum oven with 150 °C for 5 h. A comparison

sample was also synthesized under otherwise the

same conditions without the sonication.

2.2 Structural characterization

X-ray diffraction (XRD) analyses of the samples

were carried out using a Marcogroup

diffractometer (MXP21 VAHF) with a Cu-Kα

radiation source (λ=1.54056 Å). The microstructures

of samples were investigated by cold field emission

scanning electron microscope (SEM, HITACHI

SU8200), high resolution transmission electron

microscope (HR-TEM, Tecnai G2) and the total

surface area was determined using nitrogen

sorption analyses via Micromeritics surface area

and porosity analyzer (ASAP 2020 HD88, USA).

The degas condition was set to 250 °C with 4 h

under vacuum of 500 μmHg (～0.67 mbar), and all

adsorption–desorption measurements were

conducted at liquid nitrogen temperature. For the

mesopore surface area, pore volume, and pore

diameter, the Barrett-Joyner-Halenda (BJH) method

was adopted, whereas the specific surface area and

the pore volume of micropores were calculated

using t-method. XPS analysis was carried out on a

K-Alpha 1063 instrument using monochromatic Al

Ka X-ray source operated at 72 W. The peaks of

Mn2p were fitted with Gaussian–Lorentzian

functions for separating the information from two

oxidation states (Mn2+ and Mn3+). The relative

atomic ratio of Mn2+ and Mn3+ was calculated from

the corresponding area ratios of these fits. The

content of oxide was measured on TGA/DTA

instrument (Mettler-Toledo STAR system,

TGA/SDTA) with a flow of oxygen at 50 sccm/min

in 40-800 °C and the heating rate is 10 °C/min.

2.3 Electrochemical characterization

Electrochemical tests were carried out using 2032

coin cells assembled in an argon-filled glovebox in

which both the content of oxygen and water are

below 0.5ppm. To prepare working electrodes, a

mixture of active material, carbon black and

poly-(vinyl difluoride) (PVDF) at a weight ratio of

80:10:10 was pasted on an Al foil. The mass loading

of the active material on each electrode disk was

2.0-3.0 mg cm-2. The electrolyte was composed of 1 M

LiPF6 dissolved in a mixture of ethylene

carbonate/dimethyl carbonate (EC/DMC, 1:1 vol %).

A cellulose film (TF4840) from NKK Corporation was

used as the separator. To investigate the

electrochemical properties of electrode materials, the

half cells were adopted and lithium foil was used as

the counter electrode. Meanwhile, Li ion capacitors

were fabricated in which commercial Li4Ti5O12 was

employed as anodes for studying the performance of

cathode materials.

Galvanostatic charge-discharge tests of assembled

cells were preformed on a Land CT2001A system

(Wuhan, China) under various current densities.

The measured voltage range for half cells and Li ion

capacitors are 2-4V (vs. Li/Li+) and 0-2.8V (vs.

Li4Ti5O12), respectively. Cyclic voltammograms

(CVs) were conducted on a Solartron SI 1287 at the

scanning rate of 0.1mV/s. All electrochemical

measurements were carried out at room

temperature. The current densities for the half or


4 Nano Res.

full cells were all determined based on the mass of

active materials on cathodes. The specific energy

and specific power values of full cells were

calculated as follows:

I/mΔVP

tPE

2/)( minmax VVV

where I is the discharge current, t is the discharge

time, m is the mass of the active materials in both

electrodes, Vmax is the potential at the beginning of

discharge after the IR drop, and Vmin is the potential

at the end of discharge.

3 Results and discussion

Figure 1a-c compares the XRD patterns of activated

carbon, the sample synthesized from KMnO4 and

activated carbon through sonochemical reaction and

the reference sample from KMnO4 and activated

carbon without sonication in synthesizing process. A

broad peak sited between 40 and 48° might be

attributed to AC as reported in literature. [28] While

the XRD pattern (Fig.1b) of manganese oxide -

carbon sample through sonochemical reaction can be

indexed to pure Mn3O4 with a tetragonal structure

(space group I41/amd, JCPDS card No.24-0734),

albeit with low intensity of characteristic peak at ~36°

due to the low content of Mn3O4 of ~3.84 wt%, in

addition to the broad hump between 40° and 48° for

carbon, which indicates that the product is a mixture

of Mn3O4 and carbon. It is worthy of notice that the

formed Mn3O4 with a thermodynamically more

favorable/stable than MnO2, is commonly reported in

the literatures through the following [29-31] .

332224 2KHCOCOK4MnOOH3C4KMnO (1)

By contrasting experimental conditions, MnO2 was

formed from KMnO4 and carbon, when the reaction

temperature was controlled at 73 [29] or 90 °C [30].

Our own experiments also confirmed the formation

of MnO2 (space group C2/m, JCPDS card No.42-1317)

from KMnO4 and carbon without sonication,

displayed in the XRD pattern of Fig.1c, which is in a

good agreement with the literature [29, 30]. In order

to ensure that the weak and broad peaks exhibited in

Fig.1a-c not be mistaken as amorphous, a series of

AC-Mn samples were prepared and the distinct

characteristic peaks were detected when the content

of Mn3O4 increases (Fig.S1a in the ESM), and AC-Mn

sample with heavy amount of oxide was consisted of

MnO2 without sonication treatment (Fig.S1b in the

ESM). In addition, the result obtained from X-ray

photoelectron spectroscopy (XPS) analyses shown in

Fig.1d further confirmed the sample from sonication

consisted of Mn3O4 . The XPS spectrum revealed an

increased energy separation of 11.8eV between the

Mn2p1/2 and Mn2p3/2 peaks. The actual metal

oxidation state in the sample was identified through

fitting the XPS peaks with Gaussian functions. The

Mn2p3/2 peak can be fitted by three Gaussian

functions and the simulative peaks located at 640.8,

642.2 and 644.8eV, respectively. The peak at 642.2eV

derives from Mn3+ ions and the other two peaks

corresponds to Mn2+ ions as widely reported in

literatures [32, 33]. Besides, the ratio of Mn2+ and

Mn3+ ions was estimated through utilizing the

intensities from the simulative peaks area and found

to be 0.35 which suggested the content of Mn2+ is

slightly higher than its stoichiometric level,

indicating that a little excessive reduction of the raw

material KMnO4 . Based on defects chemistry and

charge conservation law, excessive Mn2+ will occupy

the sites belonged to Mn3+ in a stoichiometric Mn3O4

and this would create oxygen vacancies in the

crystall lattice. XPS spectra of AC-Mn synthesized

without sonication are shown in Fig. S2 (in the ESM),

and the energy separation is 11.7eV between Mn 2p1/2

and Mn 2p3/2, which agrees well with the data

reported in literature [34, 35]. In addition, the energy

separation related to Mn3s has been well established

to be 4.94 eV as documented in literature [36], which

confirmed the sample of AC-Mn consisted of MnO2

from the method without sonication. Comparison of

the results of AC-Mn with and without sonication


5 Nano Res.

together with the literature data strongly suggested

and demonstrated that sonochemical synthesis

resulted in a very different phase and nano- and

microstructures of manganese oxides (Mn3O4 instead

of MnO2). Although the exact mechanism of the

formation of Mn3O4 is a subject of further study, the

local extreme reaction conditions or parameters

generated by ultrasonication are likely the causes, as

briefly discussed below. In general, ultrasound does

not directly interact with reactive substance on a

molecular level because the wavelength ranges (10 to

10−4 cm) are far above the atomic or molecular

dimensions. The origin of sonochemical reaction

derives from the result of concentrated ultrasonic

energy: acoustic cavitation [24, 37]. The effect of

cavitation causes bubbles and the collapse of bubbles

could generate enormously high local temperatures

and pressures. Suslick et al. have used

sonoluminescence spectra to evaluate the hot spot

irradiated from cavitation bubbles in various media

[38, 39]. The effective hot spot temperatures are in

excess of 4,000 and 5,000 K in aqueous and

nonaqueous media, respectively. At the same time,

the collapse of cavitation bubbles generates a shock

wave which can reach a pressure of 60,000 bars in

water [40]. Such extreme conditions and its abrupt

nature (an extremely short duration) can easily alter

the local thermodynamics and kinetics inducing and

promoting less common chemical reactions and the

formation of chemicals or compounds that would

otherwise not form. In addition, it is known that

ultrasonic irradiation of water or ethanol generates

highly reactive radicals, such as H• and OH•, which

can initiate and promote the reduction reaction as

well [41]. Therefore, the formation of Mn3O4 may be

explained through the following reactions. OHHOH2 (2)

HOCHCHOHCHCH 2323 (3)

MnOOHHMnO2 (4)

2243 OO6HO4Mn12MnOOH (5)

Highly reactive H• radicals with reducing nature

produced from ultrasonic irradiation in water at the

beginning. These H• radicals could react with MnO2

derived from KMnO4 and activated carbon to form

MnOOH, and this reaction is consistent with the

capacitive charge storage mechanism of MnO2 [42].

At last, the local extreme conditions provide the

thermodynamics for transforming MnOOH to Mn3O4

which is more thermodynamically stable [43].

Therefore, it is not difficult to understand the slightly

high content of Mn2+ in the sonication product of

AC-Mn as characterised through XPS because highly

active radicals might provide an excessively reducing

condition for transforming Mn4+ to Mn2+ . Tuning the

solution components and sonication time further

verified the reducing effect of ethanol and extreme

condition irradiated from cavitation to catalyze

chemical reactions, thus, the optimized conditions

were explored for studying coherent AC-Mn

composites (Fig.S3 in the ESM). On the basis of

reaction (1), KMnO4 consumes 0.47wt% carbon in the

initial synthesizing process. MnO2 transforms to

Mn3O4 leading to a mass loss of 12.6wt% in the oxides.

As a result, Mn3O4 has a weight ratio of 3.84% in the

AC-Mn sample. To further confirm the actual content

of Mn3O4, differential scanning calorimetry/

thermogravimetric analyses (DSC/TG) were

conducted and the results are shown in Fig. 1e. The

remaining weight ratio is 5.02% which is higher than

the calculated results of 3.84% based on equation (1).

Two possible reasons may attribute to this difference,

one relates to the oxidation of Mn3O4 transformed to

Mn2O3 (Fig.S4 in the ESM) during the measurement

due to the directly contacted of a small amount of

sample with sufficient flowing oxygen, and the other

derives from the possible nonuniformity of AC

particles which affects the loading of Mn3O4 in a

small mount of the sample used in the test.


6 Nano Res.

Comparing with bare AC, a coating grown and

covered on the carbon surface in the sample of

AC-Mn (Figs.S5a-b in the ESM). Considering the

fact that there is only 3.84 wt% of Mn3O4 and Mn3O4

has a much high density so its volume fraction

would be even smaller than 3.84%, it is too hard to

observe the morphology and distribution of Mn3O4

in AC because some Mn3O4 may deposit on the

internal surface of mesopores of AC. However, the

leaf-like matter also can be found on the external

surface of AC (Fig.S5b in the ESM). In order to

confirm its composition, EDS was adopted to verify

it is the targeting product Mn3O4 rather than

terminations or other byproducts (Figs.S5c-e in the

ESM). The morphologies of oxides are also

determined by the synthesis conditions. The key

factor mentioned above is sonication treatment

adopted in the synthesizing process and the

growing process of oxide could be explained as

follows. At the beginning stage, ultrasound

irradiation activates the surface sites of AC and

ethanol molecular adsorb on them to form reactive

sites for KMnO4. The radicals reduce the

intermediate product from KMnO4 and AC to

produce Mn3O4 and more secondary radicals can

form because of higher ethanol concentration

leading to higher reduction rate [44]. At the same

time, ethanol acts as structure directing agent to

control the growth of Mn3O4 . Therefore, the faster

reduction rate and limited growth direction force

the nucleation of Mn3O4 to grow preferentially. The

directing effects of ethanol were also reported in

literature in sonochemical synthesis of metal

nanoparticles [45, 46]. In order to demonstrate the

morphological difference of sonochemical samples,

samples synthesized without sonication were

studied by means of SEM with results shown in

Fig.S5f (in the ESM), in which the oxide distributes

randomly and the particles are irregular and

non-uniform. In addition, KMnO4 contacts the

surface of activated carbon and reacts easily with

the protruding positions, such as edges, corners and

pointedness, because such convex surface possesses

higher surface energy and higher solubility than

both flat and concave surfaces [47], and thus is

more readily to react and/or be removed as

schematically illustrated in Figure 1f. Similarly,

microsized pores would be more favorable for the

deposition of Mn3O4 than mesopores, as the

deposition on the concave surface with smaller

radius (microsized pores) offers greater reduction of

Gibbs free energy than deposition inside mesopores.

Thus, micropores are more likely to be obstructed

by the deposition of Mn3O4. When sonication was

applied, a shock wave generated from a collapsing

bubble can reach a velocity of 4,000 m s -1 in water

[40], empowering the reactive ions with adequate

motion energy for transferring to the internal

surface of pores. Jin et al. have synthesized a

composite consisted of MnO2 and carbon nanotubes

(CNTs), in which KMnO4 reacts with CNTs to form

MnO2 not only on the surface defect sites, but also

on the internal surface of CNTs because a nanoscale

microelectrochemical cell appeared leading to the

separation between formation sites of MnO2 and

consuming sites of carbon [47]. Therefore, the shock

wave supplies propulsion for mass transportation

and the electrochemical cell ensures the charges

transfer on the formation sites of reactive materials

causing Mn3O4 to nucleate in the internal

mesopores. The nitrogen sorption isotherms of

samples AC and AC-Mn shown in Figure 2d

revealed characteristics of Type I adsorption curves

standing for the surface area ratio of micropores is

in the majority and an appreciable pore volume

decrease. Table 1 compares and shows the BET

surface area decreases from AC of 1783.9 m2 g-1 and

AC-Mn of 1496.5 m2 g-1, or a 16.1% reduction and

the total pore volume decreases from 0.99 cm3 g-1 to

0.83 cm3 g-1, approximately 16.2% reduction;

however, the pore size remains almost unchanged.

On the assumption that all Mn3O4 mixed with AC

mechanically, the mixture of AC with 3.84 wt%

Mn3O4 would result in a specific surface area of

Figure 1. XRD patterns of (a) AC and (b) AC-Mn with sonication and (c) AC-Mn without sonication in the synthesizing process.

The red arrows mark the difference of main peaks positions from two samples. (d) XPS curves of AC-Mn with sonication and the

peaks were simulated with Gaussian functions. (e) DSC/TG curves of AC-Mn sample from sonication. (f) Schematic illustration of

formation process of Mn3O4 on AC. KMnO4 reacts with pointedness of pores preferentially and consumes the carbon to form Mn3O4

deposition. (g) HR-TEM image of AC-Mn from sonication treatment. (h) Nitrogen sorption curves of two samples and the total

surface area of AC-Mn decreases, compared with that of AC.

1715.4 m2 g-1 (less than 4% reduction) and the pore

volume of 0.95 cm3 g-1 (less than 4% reduction) with

unchanged pore size. The significantly larger

decrease in both specific surface area and pore

volume supports the hypothesis that the Mn3O4

were deposited inside the pores as reported to

incorporate SnO2 into CMK-3 pore channel [48].

Such changes are a result of the complex

combination of the reduction reaction of KMnO4 by

carbon with the partial removal of carbon and the

deposition of Mn3O4 as schematized in Fig.1f.

HR-TEM image supports this explanation in details


8 Nano Res.

as shown in Fig.1g, it can be clearly observed some

mesopores in AC as marked by the blue arrow, and

the apparent lattice fringes with inter-planar

spacing of 2.37 and 2.88 Å , these fringes agree well

with the planar distance between (004) and (200)

planes of Mn3O4 calculated from standard XRD data,

respectively. This interface of Mn3O4 and AC

implies that the two phases formed coherent

nanocomposite.

Table 1. Specific surface area, micropores volume and diameter of mesopores of samples

Sample ID SBET (m2 g-1) SMeso (m

2 g-1) SMicro (m2 g-1) Vtotal (cm3 g-1) VMeso (cm3 g-1) VMicro (cm3 g-1) DMeso (nm)

AC 1784 1121 663 0.97 0.61 0.38 2.17

AC-Mn 1497 962 535 0.83 0.52 0.31 2.17

Mixture 1715 0.95 2.17

Generally, AC is a typical EDLCs material with

huge surface area, while Mn3O4 displays

pesudocapacitance from redox reaction similar to

other manganese oxides. Although Mn3O4 possess a

spinel structure rather than the layer structure as

MnO2, charge storage reactions are very similar in

two crystals. An intercalation/chemisorption of Li+

goes through bulk or on the surface of MnO2 with

the valence variation of Mn ions between Mn(IV)

and Mn(III) as follows [42]:

MnOOLieLiMnO2 (6)

The pesudocapacitance reaction of Mn3O4 in org

anic electrolyte can be described in the followin

g equation, similar to what reported in literatur

e for MnOx(OLi)y [49]:

δδδ LiMnOeLiMnO 1.331.33 (7)

The charge/discharge curve of AC-Mn displays a

characteristic of typical EDLC electrode (Fig.2a),

with an excellent specific capacitance of 150.8 F g-1,

much higher than that of AC, 95.2 F g-1 at 100 mA g-1

(Fig.2b). Cyclic voltammgram (CV) curves of

samples AC-Mn and AC were presented in Fig.2c.

AC exhibits a typical rectangular characteristic of

EDLC; while AC-Mn shows a broad oxidation

peaks at 3.8 V, indicating that redox reaction

occurred in the process of energy storage. Similar

effect has been observed in the Cu decorated AC

sample due to the mixed state of Cu0-Cu2+ which

caused electrochemical reactions [15]. As the

current densities increase, the specific capacitance

of AC-Mn decreases faster than AC, at a current

density of 1000 mA g-1, AC-Mn and AC have almost

identical specific capacitance. The specific

capacitance of AC-Mn returns to the initial level

when the charge current density reduces to 100 mA

g-1, demonstrating an excellent cyclic stability. The

relative rapid decrease in capacity with an

increased charge current density may be

attributable to strong polarization and lower

electrical conductivity of Mn3O4 limiting the

transport process. The similar tendency were also

observed in other AC-Mn composites (Fig.S6 in the

ESM). Nyquist plots of EIS data of both AC-Mn and

AC samples are shown in Fig.2d. Surface charge

resistance is displayed as a semicircle at higher

frequencies and the semicircle diameter of AC-Mn

is larger than that of AC. The slope of the curve at

low frequencies stands for Warburg impedance,

which signifies the electrolyte diffusion in the

porous electrode. Both samples have an almost

ideal vertical line, suggesting good capacitive

performance as well as similar porous structure,

which corroborates the results from nitrogen

sorption analyses. A slight difference can be


9 Nano Res.

distinguished from the phase angles of two curves

at low frequencies from the Bode plot as seen in the

inset, where the phase angle of AC-Mn is less than

that of AC at 1 mHz, indicating it has a higher

diffusion resistance because the angle is close to

-90° standing for the ideal capacitor [50]. This high

diffusion resistance can be attributed to the redox

reactions [51] of Mn3O4, contributing to

pseudocapacitance. All the results revealed Mn3O4

can endow AC with an enhanced capacitance, most

significantly at low current densities. Better

capacitance and rate capability are expected when

the amount of Mn3O4 is tailored.

Figure 2. (a) Galvanostatic charge/discharge curves of AC and AC-Mn. (b) Specific capacitance of samples measured in the half

cells where metal lithium was used as a counter electrode and the measuring voltage window is 2-4 V. The AC-Mn possesses

superior specific capacitances compared with those of AC. (c) CV curves of AC and AC-Mn samples measured at the scanning rate

of 0.1 mV s-1. (d) Nyquist and Bode plots of samples. The measuring frequencies range is 105-10-3Hz and the voltage amplitude is

10mV.

Li-ion capacitors were fabricated, in which

commercial Li4Ti5O12 (LTO)was adopted as anodes

for its own excellent rate capability and stable cycling

performance [52]. The capacitance match of cathode

and anode materials is required in an asymmetric

capacitor [50]. In the present study, the mass ratio of

cathode and anode were calculated and settled at 3:1

and 5:1 in AC-Mn//LTO and AC//LTO capacitors,

respectively. The mass ratio was based on the charge

balance of both electrodes as follows [50].

EC

EC

m

m (8)

where C is specific capacitance, ΔE the potential

window during the charge/discharge process and m

is the mass of the electrode. The specific capacitance

of Li4Ti5O12 was also measured for calculating the

mass ratio on both electrodes in asymmetric


10 Nano Res.

capacitors (Fig.S7 in the ESM). The charge/discharge

curves of Li ion capacitors measured at a current

density of 100mA g-1 are shown in Fig.3a; the

capacitor with AC-Mn cathode has a higher specific

capacitance of 106 F g-1 than 68 F g-1 with AC cathode.

The results are in markedly good agreement with the

results measured in half cells. It is noticed that the

characteristic of discharge curves also exhibited

typical EDLC rather than pesudocapacitor, but that

does not mean the mechanism of charge storage is

only surface adsorption. Defects might play an

important role in ions or charges storage, especially

in the present work, oxygen vacancies created in the

Mn3O4 crystal lattice can provide more active sites to

accelerate surface reactions and facilitate phase

transformation that occurred during Li ion

insertion/extraction as widely reported [53-55].

Introduction defects in compounds can tune their

electrochemical characters effectively, such as

capacitor behavior of materials becomes more

dominant with decreasing crystallite size [40] or

amorphous state [47] ,especially at nanoscale.

Figure 3. (a) The charge/discharge of Li ion capacitors where AC or AC-Mn used as cathodes and Li4Ti5O12 as anodes at 100mA/g.

(b) Ragone plot for Li ion capacitors at various charge/discharge rates in the working potential range of 0-2.8V. The data were

calculated based on the total mass of the active materials on both electrodes. The performances of capacitors based on Mn3O4 were

collected for the parallel comparison [59, 60], demonstrating that AC-Mn//LTO with a competitive performance. (c) The cycling

performance of Li ion capacitor at 500 mA g-1.

The Ragone plot in Figure 3b was obtained from

charge/discharge measurements of Li ion capacitors

at a variety of power densities based on the total

mass of both electrodes. The plot summarizes and

compares the electrochemical performances of two

capacitors with AC and AC-Mn as cathodes,

respectively. The energy densities at a low power

density, 210 W Kg-1, for AC-Mn//LTO Li ion

capacitor and AC//LTO system are about 61 and 48

Wh Kg-1, respectively. But at a higher power density

of 1,500 W Kg-1, the AC//LTO hybrid capacitor

exhibited an energy density of 3.9 Wh Kg-1 against


11 Nano Res.

3.1 Wh Kg-1 for AC-Mn//LTO hybrid system. It

could be explained that Mn3O4 on the surface of AC

could not react quickly enough to deliver charges

for storing or releasing of energy, one of the

disadvantages of batteries. At any rate, these

specific energy density values for Li ion capacitors

reveal synergistic effects relative to the literature

values for EDLCs (about 8.5 Wh Kg-1 at 270 W

Kg-1 )[56, 57] or hybrid capacitors (3Wh Kg-1 at

1,500W Kg-1) constructed by Li4Ti5O12 and AC [58].

The cycling performance of Li ion capacitor was

also tested with 1000 cycles at 500 mA g-1 and no

appreciable degradation appeared as shown in

Fig.3c, demonstrating that the coherent Mn3O4 as

electrode material has an excellent cycling stability.

The results above reveal that coherent

Mn3O4-activated carbon nanocomposite possessed

an improved specific capacitance and exhibited

considerable energy densities. Combining the

merits of batteries materials with EDLCs carbon

materials provides a reliable method to enhance the

performance of cathode materials in Li ion

capacitors.

4 Conlusions

A facile, environmentally benign and low cost

synthesis method was investigated to fabricate a

superior capacitive material: coherent

Mn3O4-activated carbon nanocomposite, which has

an excellent specific capacitance of 106 F g-1 and an

energy density of 68 Wh Kg-1 in the Li ion full

capacitor without device optimization. The superior

properties derived from effective merits

combination of AC and Mn3O4. This combination

can be attributed to the sonochemical method

which provides extreme local conditions to anchor

nanostructured Mn3O4 in the wall of mesopores and

external surface of AC; the extreme local condit ions

in sonochemical synthesis resulted in an excessive

amount of divalent manganese ions and oxygen

vacancies promoting the electrochemical reactions.

The higher voltage window of an organic system

and higher capacitance of AC-Mn are contributed to

produce novel devices with higher energy and

power densities simultaneously.

Acknowledgements

This work was supported by the "thousands talents"

program for pioneer researcher and his innovation

team, China. This work was also supported by the

National Science Foundation of China (51374029),

Program for New Century Excellent Talents in

University (NCET-13-0668), Fundamental Research

Funds for the Central Universities

(FRF-TP-14-008C1) and China Postdoctoral Science

Foundation (2014M550675).

Electronic Supplementary Material: Supplementary

material ( XRD patterns related to phase evolution of

manganese oxides with various amount of KMnO4,

solution and sonication time. XPS of AC- Mn sample

synthesized without sonication. Elements mapping

of AC-Mn sample, electrochemical performances of

Li4Ti5O12. ) is available in the online version of this

article at http://dx.doi.org/10.1007/s12274-***-****-*

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