Manganese Oxide-based Materials

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This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 16971721 1697

Cite this:Chem. Soc. Rev., 2011, 40, 16971721

Manganese oxide-based materials as electrochemical

supercapacitor electrodes

Weifeng Wei,ab Xinwei Cui,a Weixing Chena and Douglas G. Ivey*a

Received 28th September 2010

DOI: 10.1039/c0cs00127a

Electrochemical supercapacitors (ECs), characteristic of high power and reasonably high

energy densities, have become a versatile solution to various emerging energy applications.

This critical review describes some materials science aspects on manganese oxide-based materials

for these applications, primarily including the strategic design and fabrication of these electrode

materials. Nanostructurization, chemical modification and incorporation with high surface

area, conductive nanoarchitectures are the three major strategies in the development ofhigh-performance manganese oxide-based electrodes for EC applications. Numerous works

reviewed herein have shown enhanced electrochemical performance in the manganese oxide-based

electrode materials. However, many fundamental questions remain unanswered, particularly with

respect to characterization and understanding of electron transfer and atomic transport of the

electrochemical interface processes within the manganese oxide-based electrodes. In order to fully

exploit the potential of manganese oxide-based electrode materials, an unambiguous appreciation

of these basic questions and optimization of synthesis parameters and material properties are

critical for the further development of EC devices (233 references).

1. Introduction

Sustainable and renewable energy resources are being intensively

pursued owing to the diminishing supply of fossil fuels and

climate change. Consequently, rapid growth in renewable

energy production from sun and wind, as well as the develop-

ment of electric vehicles (EVs) or hybrid electric vehicles

(HEVs) with low CO2emissions, is occurring. Since renewable

sources from sun and wind generally have on-peak and off-peakload variations and EVs/HEVs have a driving range of

150200 miles before charging is required, electrochemical

energy storage systems such as rechargeable batteries and electro-

chemical capacitors (ECs) are receiving increasing consideration.1

A Ragone plot (Fig. 1) illustrates power density against

energy density for the most important electrochemical energy

storage systems.2,3 ECs, with a combination of high power and

a Department of Chemical and Materials Engineering,University of Alberta Edmonton, Alberta, Canada T6G 2G6.E-mail: [email protected]; Fax: +1 780-492-2881;Tel: +1 780-492-2957

b Department of Materials Science and Engineering,Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Weifeng Wei

Weifeng Wei is currently a

postdoctoral researcher in

the Department of Materials

Science and Engineeringat the Massachusetts Institute

of Technology (MIT). He

received his PhD in Materials

Engineering from the University

of Alberta (2009). His research

interests include materials

development for electro-

chemical energy storage devices

(rechargeable batteries and

supercapacitors).

Xinwei Cui

Xinwei Cui received a Bachelor

of Science degree in Materials

Engineering from the Univer-

sity Science and TechnologyBeijing in 2005 and a PhD in

Materials Engineering from

the University of Alberta in

2010. He is currently a research

associate in the Department of

Chemical and Materials

Engineering at the University

of Alberta. His research

focuses on nanostructured

materials, particularly carbon

nanomaterials for applica-

tions in electrochemical energy

storage and conversion.

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1698 Chem. Soc. Rev., 2011, 40, 16971721 This journal is c The Royal Society of Chemistry 2011

reasonably high energy density, are a versatile solution to a

variety of emerging energy applications. The energy stored in

the ECs is either capacitive or pseudocapacitive in nature. The

capacitive (non-Faradaic) process is based on charge separa-

tion at the electrode/electrolyte interface, while the pseudo-

capacitive (Faradaic) process relies on redox reactions that

occur in the electrode materials. The most widely used active

electrode materials are carbon, conducting polymers and

transition metal oxides.414 Among these electrode materials,manganese oxides, characterized by high specific capacitance

and their low-cost, abundance and environmentally friendly

nature, have attracted significant interest as active electrode

materials for ECs.

The pioneering work on the pseudocapacitive behavior of

manganese oxide in an aqueous solution was published in 1999

by Lee and Goodenough.13,14 This was followed by several

studies to establish the charge storage mechanism in manganese

oxide electrodes. Pseudocapacitive (Faradic) reactions

occurring on the surface and in the bulk of the electrode are

the major charge storage mechanisms for manganese oxides.

The surface Faradaic reaction involves the surface adsorption

of electrolyte cations (C+ = H+, Li+, Na+ and K+) on the

manganese oxide:15,16

(MnO2)surface+ C+ + e2 (MnOOC)surface (1)

The bulk Faradaic reaction relies on the intercalation or

deintercalation of electrolyte cations in the bulk of the

manganese oxide:15,16

MnO2 + C+ + e2 MnOOC (2)

It is noted that, in both charge storage mechanisms, a redox

reaction between the III and IV oxidation states of Mn ions

occurs. In general, hydrated manganese oxides exhibit specific

capacitances within the 100200 F g1 range in alkali saltsolutions, which are much lower than those for RuO2 ECs.

Thus far, further advancements in current MnO2-based super-

capacitors are constrained by MnO2electrode material limita-

tions with limited specific capacitance (low energy density),

lack of structural stability and long-term cyclability, and low

rate-capacity.

The improvement pursued in active materials mainly concerns

high reversible capacitance, structural flexibility and stability,

fast cation diffusion under high chargedischarge rates,

and environmental friendliness. As a transition metal

element, manganese can exist as a variety of stable oxides

(MnO, Mn3O4, Mn2O3, MnO2)17,18 and crystallize in various

types of crystal structures, as shown in Fig. 2 and Table 1.1929

Associated with a wide diversity of crystal forms, defect

chemistry, morphology, porosity and textures, manganese

oxides exhibit a variety of distinct electrochemical properties.

These structural parameters play a crucial role in determining

Fig. 1 Ragone plot (specific power vs. specific energy) for variouselectrochemical energy storage devices. (Reproduced from ref. 2;

reprinted with permission. Copyright 2008, Macmillan Publishers

Limited.)

Weixing Chen

Weixing Chen is an associate

professor in the Department

of Chemical and Materials

Engineering at the University

of Alberta. He is interested in

fabricating carbon nanotube

arrays and their applications,in addition to corrosion and/

or environmentally induced

cracking of materials used in

energy and petrochemical

processing industries.

Douglas G. Ivey

Douglas Ivey is a professor in

the Department of Chemical

and Materials Engineering at

the University of Alberta and

director of the Alberta Centre

for Surface Engineering and

Science (ACSES). He receivedhis PhD in Engineering Mate-

rials from the University of

Windsor (Canada) in 1985.

His research focuses on

applying high resolution micro-

structural characterization

techniques to understanding

the relationships between

materials structure, properties

and processing. Recent work has focused on developing electro-

chemical techniques to deposit thin films and thicker coatings for

a wide range of potential applications, from microelectronics to

MEMS to fuel cells to supercapacitors.

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and optimizing the electrochemical properties when manganese

oxides are applied as electrode materials. Investigation of the

influence on these characteristics of manganese oxides on

capacitor performance is the basis of a rational design of improved

electrode materials. Extensive efforts have been dedicated to adjust

synthesis conditions to obtain manganese oxides with desirable

morphologies, defect chemistry (cation distributions and oxida-

tion states) and crystal structures to improve the subsequent

capacitance and power characteristics.3037

The capacitance of thick MnO2 electrodes is ultimately

limited by the poor electrical conductivity of MnO2. On the

other hand, EC device performance using a planar ultrathin

configuration is restricted because of low mass loading. In

this case, incorporation of other metal elements into MnO2compounds has also been extensively studied in recent years

to enhance their electrical conductivity and charge-storage

capability. The chemical modification of MnO2 electrodes

can be generally divided into two categories: one is mixed

oxide electrodes containing other transition metal elements,

such as Ni, Cu, Fe, V, Co, Mo and Ru.11,3843 The other type

of a modified MnO2 electrode was realized through doping

with small amounts of other metallic elements such as

Al, Sn and Pb.4446 The corresponding electrochemical

properties indicate that the manipulation of defect chemistry

by chemical modification has significant influence on the

electronic conductivity and, in turn, on the specific capacitance

and rate capacity.

Another method to compensate for the poor electrical

conductivity of thick MnO2 electrodes is to deposit a thin

MnO2layer on the surface of a porous, high surface area, and

electronically conducting structure, which can provide good

electrochemical performance with high mass-loading of the

Fig. 2 Schematic representation of the crystal structure of manganese oxides. (a) Rock salt; (b) spinel (Mn3O4); (c) bixbyite (Mn2O3);

(d) pyrolusite b-MnO2 (rutile-type) (note the single chains of edge-sharing octahedra); (e) ramsdellite (diaspore-type) ([MnO6] octahedra form

infinite double layers); (f) phyllomanganate (birnessitebuserite family of layered MnO2). In this idealized representation there are alternate layers

of full and empty octahedral sites. (2df, adapted from ref. 36, reprinted with permission. Copyright 2006, The Electrochemical Society.)

Table 1 Crystal structure of manganese oxides1929

Type Crystal structure Description

MnO19 Rock salt, Fm3m Face centered cubic (FCC) lattice with a 6 : 6 octahedral coordination.Mn3O4

20 Tetragonal spinel,I41/amd Metal cations occupy 1/8 of the tetrahedral sites and 1/2 of theoctahedral sites and there are 32 oxygen anions in the FCC unit cell.

Mn2O321 (bixbyite) Body-centered cubic, Ia3 Body-centered cubic (BCC) unit cell with 16 formula units per unit cell

a-MnO222 (psilomelane) Monoclinic, A2/m Cross-linking of double or triple chains of the [MnO6] octahedra,

resulting in two-dimensional tunnels within the lattice.b-MnO2

23 (pyrolusite) Rutile structure, P42/mnm Rutile structure with an infinite chain of [MnO6] octahedra sharing

opposite edges; each chain is corner-linked with four similar chains.b-MnO2

24 (ramsdellite) Pbnm Closely related to rutile except that the single chains of edge-sharingoctahedra are replaced by double chains.

g-MnO225,26 (nsutite) An irregular intergrowth of layers of pyrolusite and ramsdellite.

Z-MnO2 Different fromg-MnO2only in crystallite size and the concentrationof microdomains of pyrolusite within the ramsdellite matrix.

d-MnO227,28 (phyllomanganate) Birnessite,R3m Layered structure, containing infinite two-dimensional sheets

of edge-shared [MnO6] octahedra.e-MnO2

29 Defective NiAs,P63/mmc Hexagonal close packing of anions, with Mn4+ statistically distributed

over half the available octahedral interstices.

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MnO2 phase. The porous architectures could be carbon

nanofoams, templated mesoporous carbon, nanographite,

and nanotube assemblies. In such a hybrid electrode configu-

ration, the carbon substrates act as highly conductive current

collectors, the interconnected porosity serves as a continuous

pathway for electrolyte diffusion, and the nanoscopic active

MnO2 phase shortens solid-state transport distances for ions

into the oxide materials.47 Recent efforts to integrate carbon

and MnO2 have primarily focused on depositing nano-

scale MnO2 onto carbon nanotubes by using a variety of

approaches, including physical mixing of the components,48

thermal decomposition,49 chemical deposition using precursors

such as permanganate,50,51 and electrochemical deposition.52,53

In addition to their poor electrical conductivity, mechanical

issues such as low structural stability and flexibility and

electrochemical dissolution of active materials exist for

MnO2 electrodes, resulting in degraded long-term electro-

chemical cyclability. To address electrochemical dissolution,

a self-limited growth process based on the electropolymeriza-

tion of o-phenylenediamine has been developed.54,55

The

resulting polymer conformally coats the oxide nanoscale net-

work, serving as an effective barrier to the electrolyte thereby

protecting the underlying MnO2nanoarchitecture from chemical

dissolution. The underlying metal oxide remains electro-

chemically accessible but significant reduction in conductivity is

observed.55 Many efforts have also been attempted to incorporate

conductive polymers (polyaniline, polypyrrole and polythiophene)

and their derivatives to get mixed MnO2polymer composite

electrodes with desirable morphologies.5659 The excellent

electronic conductivity, high stability, and mechanical flexibility

of applied conductive polymers enable improved electro-

chemical and mechanical properties for MnO2polymer com-

posite electrodes for ECs.59

Manganese oxide-based electrodes, either fabricated as a

single nanostructured component with desirable physicochemical

features or assembled with conductive polymers and porous

carbon architectures, open new possibilities for the develop-

ment of advanced ECs. The strategies can be generalized as the

following options:

1. Chemical and structural modification of manganese oxide

materials to introduce more electrochemically active sites for

the redox reaction between the Mn(III) and Mn(IV).

2. Shortening of the transport path length for both

electrons and cations by using porous, high surface area,

and electronically conducting carbon architectures.

3. Addressing of the low structural stability and flexibility

and electrochemical dissolution of active materials through

application of conductive polymers in manganese oxide

materials.

There are some reviews that cover different topics in

the development of ECs;3,6066

however, to the best of our

knowledge, none are dedicated to the development of

manganese oxide-based electrochemical supercapacitors. This

paper categorizes and reviews the most important related

works and achievements for manganese oxide-based ECs

published in the last ten years. Current challenges and future

strategies will be discussed. Herein, we focus on the materials

science of manganese oxides and manganese oxide-based

composites used as electrodes in ECs. Some other important

aspects such as device engineering or electrolyte development

are not included in this work.

2. Manganese oxide electrodes

2.1 Powder electrodes

2.1.1 Amorphous MnO2 powder electrodes. In the first

study on the capacitive behavior of manganese dioxide publishedin 1999 by Lee and Goodenough, amorphous hydrated

manganese dioxide powders were prepared by reacting

KMnO4 with Mn(CH3COO)2 in water through the following

reaction:13,14

Mn(VII) + 3/2Mn(II) - 5/2Mn(IV) (3)

It was found that the amorphous hydrated MnO2 powder

electrodes exhibited ideally capacitive behavior in KCl, NaCl,

and LiCl aqueous solutions. This was followed by several

studies that used similar KMnO4 reducing procedures with

different reducing agents, including MnSO4,15,30,67 potassium

borohydride,68 sodium dithionite,68 sodium hypophosphite

and hydrochloric acid,68 aniline,69 and ethylene glycol32 to

make hydrated MnO2 powders. In addition to water-based

procedures, organic solvent-assisted reduction of KMnO4was investigated. Typical cases are reduction of KMnO4 in

an AOT/iso-octane solution,70 a H2O/CCl4 interface, and a

ferrocene/chloroform solution.31,71 In the first case, surfactant

sodium bis(2-ethylhexyl)sulfosuccinate (AOT) was used as

both the dispersant and reducing agent. KMnO4 aqueous

solution was dispersed in iso-octane by AOT to form nano

droplets of the water phase, and then KMnO4was reduced by

AOT.70 The other two processes involved interfacial reactions

occurring at the aqueous/organic interface. The formation

of MnO2

at the interface of aqueous KMnO4

/ferrocene in

chloroform can be described as follows. With the presence of

sodium dodecylsulfate (SDS), the micelles containing KMnO4were formed and arranged at the aqueous/organic interface.

Ferrocene molecules contacted the permanganate at the core

of the micelles and redox took place at the interface to form

MnO2 particles in the micelles.31

The as-prepared hydrated MnO2 powders are generally

amorphous or poorly crystalline in nature. A representative

X-ray diffraction (XRD) pattern for as-prepared hydrated

MnO2 powders is depicted in Fig. 3a.32,69 The hydrated

MnO2 powders maintain their amorphous characteristics up

to 300 1C (Fig. 3b and c), whereas a-MnO2 with a tunnel

structure matching JCPDS 44-0141 appears for samples

heat treated at temperatures higher than 400 1C f o r 3 h

(Fig. 3df).69 Similar structural evolutions during heat treat-

ment were also observed in other studies. For instance,

Belanger et al. demonstrated decomposition ofa-MnO2 into

a-Mn2O3 (bixbyite-C, Ia3 space group) at 400 1C, with the

well-crystallized a-Mn2O3 becoming the main phase at

600 1C.30 Devaraj and Munichandraiah also showed the struc-

tural evolution from a-MnO2 (as-prepared at B400 1C) to

a-Mn2O3 (500800 1C) and then to Mn3O4 (900 1C).72

Along with the structural evolution occurring during heat

treatment, the morphology and chemistry of the hydrated

MnO2powders change significantly. Fig. 4 shows SEM images

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of the morphology of MnO2 annealed at different temperatures.69

Amorphous MnO2 exhibits highly clustered granules of

varying size between five and tens of nanometres, as shown

in Fig. 4a. As the annealing temperature is increased to 300 1C,

gradual growth of nanorods begins (Fig. 4c and d). Upon

further heating to 500 and 600 1C, well-defined nanorods of length

500750 nm and diameter 50100 nm are seen (Fig. 4e and f).32,69

Note that the morphological change from particles to nanorods is

also evident in the hydrated MnO2powders prepared through

a microemulsion method.72 Through X-ray photoelectron

spectroscopy (XPS) analysis, it has been well established that

the as-prepared MnO2 powders contain a significant amount

of hydrated content (hydrated trivalent MnOOH and residual

structural water).15,32 The hydrated content in the MnO2powders was reduced considerably by increasing the annealing

temperature.32 At the same time, the specific surface area of

the MnO2 powders declines dramatically with increasing

annealing temperature and holding time, as shown in

Table 2.32,72

The electrochemical properties of the hydrated MnO2powder electrodes are evaluated using cyclic voltammetry

(CV), galvanostatic chargedischarge, and electrochemical

impedance spectroscopy (EIS). The specific capacitance C

(F g1) of the MnO2 powder electrodes is determined by

obtaining the voltammetric charge (Q) from the cyclic voltammo-

grams or the galvanostatic chargedischarge curves. Then Qis

divided by the mass of the active materials in the electrodes (m)

and the width of the potential window DE.

C= Q/(mDE) (4)

The specific capacitance of the hydrated MnO2powder electrodes is rather sensitive to their microstructure

(surface area) and hydrated content. An increase in the surface

area will enhance the specific capacitance of the amorphous

MnO2 powder electrodes.15 As illustrated in Table 2, it was

found that the surface area and specific capacitance decreased

slightly when heated to 300 1C.32,72 For MnO2 powders

prepared through a microemulsion method, a dramatic

decrease in the surface area and specific capacitance occurred

for samples annealed at 300 and 400 1C, together with a

structural change.32,72 For MnO2 powders prepared through

reduction with ethylene glycol, however, a rapid reduction insurface area and specific capacitance occurred at 500 1C, which

was associated with an a-MnO2 to Mn2O3 phase transforma-

tion. Assuming that specific capacitance is closely related to

surface area, secondary pores, and water content, which

supports the proposed surface effect for the operating mecha-

nisms of an electrode material for supercapacitors, these

properties decrease with increasing temperature.

2.1.2 Crystalline MnO2 powder electrodes. As described

above, there are many types of crystal structures occurring in

manganese oxides, whose structural frameworks consist of

MnO6 octahedra sharing vertices and edges. The stacking of

the MnO6

octahedra enables the building of 1D, 2D, or 3D

tunnel structures, which can be seen in Fig. 2. The different

crystal structures can be described by the size of the tunnel

determined by the number of octahedra subunits (n m), as

indicated in Fig. 2. The tunnels can be filled with either water

molecules or cations such as Li+, Na+, K+, and Mg2+, so the

crystalline manganese oxides are expected to demonstrate

interesting electrochemical properties in a mild aqueous

electrolyte.

The first systematic study comparing the capacitive properties

of MnO2powders with various crystal structures was published

by Brousseet al. in 2006.36 The MnO2powders were prepared

through co-precipitation and solgel techniques under

Fig. 3 XRD patterns of as-prepared and annealed MnO2 samples.

(a) Dried in air and annealed at 50 1C, (b) 200 1C, (c) 300 1C,

(d) 400 1C, (e) 500 1C, and (f) 600 1C for 3 h in air. (Reproduced

from ref. 69, reprinted with permission. Copyright 2008, The Electro-

chemical Society.)

Fig. 4 SEM images of MnO2 (a) as prepared and dried at 50 1C in

air (inset shows energy-dispersive X-ray spectrum) and annealed at

(b) 200 1C, (c) 300 1C, (d) 400 1C, (e) 500 1C, and (f) 600 1C for 3 h inair. Arrows in (c) indicate initiation of nanorods. (Reproduced from

ref. 69, reprinted with permission. Copyright 2008, The Electro-

chemical Society.)

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different synthesis conditions to achieve a variety of crystal

structures such as a-, b-, d-, g-, and l-type crystal structures.

The relationship between the crystal structure, BET surface

area and specific capacitance can be seen in Table 3.36 It was

revealed that the capacitance of the different materials depends

strongly on the crystalline structure, i.e., the size of the tunnels

that limits the intercalation of cations.36 Birnessite d-MnO2with a 2D tunnel structure doped with potassium has relatively

high specific capacitance values (110 F g1) even with moderate

BET surface area (17 m2 g1). 1D tunnel structures, such as

b- or g-MnO2, provide only a pseudoFaradaic surface

capacitance that is closely related to the BET surface area of

the crystalline materials. 3D tunnel structures containing

l-MnO2 show intermediate electrochemical performance

between birnessite and 1D tunnel structures.36

More recently, another systematic study dealing with the

effects of crystallographic forms of MnO2 on the electro-

chemical performance was conducted by Ghodbane et al.37

A series of MnO2 allotropic phases were prepared. The 1D

structures included pyrolusite, ramsdellite, cryptomelane,

Ni-doped todorokite (Nitodorokite), and OMS-5. The 2D

and 3D structures were birnessite and spinel, respectively.37

Fig. 5 illustrates the relationship between the crystal structure,

BET surface area, and specific capacitance.37 It was discovered

that the 3D-type spinel showed the highest capacitance, followed

by the 2D layer birnessite sample. For the 1D tunnel group, a

larger cavity corresponded to a larger capacity.37 The com-

parison of the BET surface area and electrochemical results

also showed that the specific surface area has a limited impact

on the capacitance of MnO2 electrodes. Charge storage in

prepared MnO2 materials is mainly Faradaic.37 The specific

capacitance correlates with the ionic conductivity of the MnO2powder, which is clearly related to the crystallographic

microstructure.37

Hydrothermal or solvothermal synthesis is an interesting

technique to prepare materials with different nanoarchitectures

including nanoparticles, nanorods, nanowires, nano-urchins,

and nanotubes by properly choosing the reaction temperature

or time, or the active fill level or solvent used for the reaction.

For instance, Subramanian et al. reported a hydrothermal route

based on aqueous solutions of MnSO4H2O and KMnO4.73,74

Through varying the hydrothermal time, evolution from a

distinct plate-like morphology to nanorods was observed in

as-prepared MnO2 nanocrystals, as shown in Fig. 6a and b.73

Xuet al.reported another simple hydrothermal process, based

on KMnO4, sulfuric acid and Cu scraps, for preparing

a-MnO2 hollow spheres and hollow urchins.75 The hollow

Table 2 Structure type, surface area (m2 g1), and specific capacitance (F g1) for MnO2at different temperatures32,72

MnO2 (microemulsion) MnO2(reducing with ethylene glycol)

StructureSurface area,SBET

a/m2 g1 Capacitance/F g1 StructureSurface area,SBET

a/m2 g1 Capacitance/F g1

As-prepared d-MnO2 230 250 a-MnO2 145 300200 1C d-MnO2 212 225 a-MnO2 146 260300 1C d-MnO2 194 207 a-MnO2 119 254

400 1C a-MnO2 36 85 a-MnO2 16 245500 1C a-MnO2 34 73 Mn2O3 8 70600 1C a-MnO2 22 61 Mn2O3 1 40

a The surface area of MnO2powders was measured using BrunauerEmmettTeller (BET) technique.

Table 3 Relationship between the crystal structure, BET surface area, and specific capacitance36

Compound Structure SBET/m2 g1 C/F g1 Scan rate/mV s1 Electrolyte

co-MnO2 a-MnO2 200 150 5 0.1 M K2SO4Ambigel H2SO4 a-MnO2 208 150 5 0.1 M K2SO4Ambigel H2O a-MnO2 8 125 5 0.1 M K2SO4l-MnO2 l-MnO2 35 70 5 0.1 M K2SO4g-MnO2 g-MnO2 41 30 5 0.1 M K2SO4b-MnO2 b-MnO2 1 5 5 0.1 M K2SO4

Birnessite H2O Birnessite d-MnO2 17 110 5 0.1 M K2SO4Birnessite H2SO4 Birnessite d-MnO2 89 105 5 0.1 M K2SO4Birnessite Birnessite d-MnO2 3 80 5 0.1 M K2SO4

Fig. 5 Comparison of the specific capacitance, ionic conductivity and

BET surface area of various MnO2 structures. (Reproduced from

ref. 37, reprinted with permission, copyright 2009, American Chemical

Society.)

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sphere or urchin structured a-MnO2materials possess a highly

loose, mesoporous cluster structure consisting of thin plates or

nanowires (Fig. 6c) and exhibit enhanced rate capacity and

cyclability.75 Similar morphologies were also observed in

a-MnO2 prepared from KMnO4 and nitric acid-containing

reactants, and in a-MnO2 and e-MnO2 nanostructures

prepared at a low temperature (110 1C).76,77 A modified

hydrothermal process based on Mn(CH3COO)24H2O and

K2S2O8 was also introduced to synthesize Mn3O4 and

MnOOH single crystals utilized as EC electrode materials.78

The morphology and crystal structure of the Mn3O4 and

MnOOH single crystals are shown in Fig. 6d and e.78

Similarly, single crystalline Mn3O4 nano-octahedrons were

synthesized by an ethylenediaminetetraacetic acid disodium

salt (EDTA-2Na) assisted hydrothermal route.79 More recently,

lamellar birnessite d-MnO2

with different interlayer spacings

was prepared by hydrothermal synthesis, as shown in Fig. 6f.

The microstructure was found to be controlled by changing

the pH value of the initial reaction system.77

In addition to the hydrothermal technique, low temperature

reduction processes were also developed to prepare crystalline

MnO2 powders. Rod-shaped MnO2, consisting of botha- and

g-MnO2, was synthesized under high viscosity conditions

at a temperature of 65 1C.80 Similar rod-shaped MnO2 with

ad-type structure was also prepared through a room tempera-

ture precipitation route based on a MnSO4+ K2S2O8solution,

as shown in Fig. 7a.81 Monodisperse manganese oxide flower-

like nanostructures have been prepared facilely at 40 1C and

ambient atmosphere using KMnO4 and formamide

(HCONH2).82 Low-cost layered manganese oxides with the

rancieite structural type (Fig. 7b) were prepared by reduc-

tion of KMnO4 or NaMnO4 in acidic aqueous medium at

temperatures of 20 1C, 60 1C or 100 1C for 10 h, followed by

successive proton- and alkali-ion exchange reactions.83 Sono-

or microwave sources were also introduced into the synthesis

process of crystalline MnO2 powders.84,85 A typical nanobelt-

like morphology can be observed in the MnO2 powders

prepared by a microwave-assisted process.86 Template-assisted

solgel is another effective way to prepare crystalline MnO2powders with distinct morphologies. Through this technique,

Wang et al. synthesized MnO2 nanowires or nanorods with a

hollandite type a-MnO2 structure, as shown in Fig. 7c.87

Moreover, a solution combustion technique based on

Mn(NO3)2 and C2H5NO2 was developed to obtain plate-like

and sphericale-MnO2 particles. A representative SEM image

of the morphology of plate-likee-MnO2is shown in Fig. 7d.86

Table 4 summarizes the synthesis conditions, physicochemical

features, and specific capacitances of crystalline MnO2 prepared

with different techniques. Highly scattered results were reported

on the specific surface area and capacitance of nanocrystalline

MnO2 powder electrodes. An unambiguous relationship

among the synthesis conditions, physicochemical features,

and specific capacitance has still not been obtained.

2.1.3 Summary of amorphous and crystalline MnO2powder

electrodes. Different MnO2 powders, prepared by chemical

co-precipitation, hydrothermal/sonothermal, solgel and solution

combustion techniques, have been investigated as possible

active electrode materials for electrochemical capacitors in

mild aqueous electrolytes. For the amorphous and crystalline

MnO2 compounds, a large variation in specific capacitance

values was reported. In the literature one can find that

many factors influence the charge storage process, including

porosity, morphology, defect chemistry (cation distributions

Fig. 6 Crystalline MnO2 with plate-like (a), nanorod (b), hollow

sphere, urchin (c), cubic (d), nanowire (e), and lamellar (f) morpho-

logies prepared from hydrothermal processes. (a and b, reproduced

from ref. 73, reprinted with permission, copyright 2005, American

Chemical Society; c reproduced from ref. 75, reprinted with

permission, copyright 2007, American Chemical Society; d and e,

reproduced from ref. 78, reprinted with permission, copyright 2008,

American Chemical Society; f, reproduced from ref. 77, copyright

2010, Elsevier Ltd.)

Fig. 7 Crystalline MnO2 with various morphologies prepared from

low temperature reduction processes. (a) Reproduced from ref. 81,

reprinted with permission, copyright 2009, American Chemical

Society; (b) reproduced from ref. 83, copyright 2010, Elsevier Ltd.;

(c) reproduced from ref. 87, copyright 2010, Elsevier Ltd.; (d) reproduced

from ref. 86, copyright 2010, Elsevier Ltd.

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and oxidation states), crystal structure, and residual water

content. It should be pointed out that there is still no standard

uniform structure and morphology available for MnO2materials. A comparison between electrodes consisting of

various kinds of MnO2 powders is rather difficult since

MnO2 powders produced under different conditions exhibit

different structures and properties. It is believed that both

surface and bulk (crystal structure and microstructure) pheno-

mena are actively involved in the charge storage process. A

rational design to maximize the electrochemically active sites

for redox reactions through increasing BET surface area and

obtaining opened structures is important to further increase

the energy storage density of MnO2 powders.

A major disadvantage of packing MnO2powders is the low

electronic conductivity to sustain high rate chargedischarge

processes. Hence, in a typical MnO2 powder electrode, an

electrically conductive enhancer, most commonly a high

surface area graphitic carbon, must be incorporated into the

powder electrode to improve its performance and a polymeric

binder for mechanical stability. The total amount of carbon

and organic binder (inactive components) ranges from 15 to

35% in weight and up to 70% of the total electrode volume,

which will undoubtedly sacrifice the gravimetric and volumetric

energy densities of the MnO2 powder electrode. The

volumetric capacity is of interest for manufacturers as those

materials are studied with the aim of compact applications.

2.2 Thin-film MnO2 electrodes

Because of the interest for more fundamental studies and

potential applications as micro-scale energy storage systems

such as integrated devices, thin film or MnO2coating electrodes

have been intensively explored recently. In this case, a

manganese oxide thin layer with desirable physical features

is directly assembled on a current collector through a variety

of techniques, including solgel dip-coating,16,33 anodic/cathodic

electrodeposition,9092 electrophoresis,9396 electrochemical

formation of manganese, and sputtering-electrochemical

oxidation.34

2.2.1 Solgel coating. Pang et al. conducted the first

research on solgel processing of thin film MnO2 electrodes

for EC application in 2000.16,33 Stable colloidal MnO2 was

prepared by reducing Mn(VII) (potassium permanganate)

with Mn(II) (manganous perchlorate) in an alkaline aqueous

medium,16,33 reducing tetrapropylammonium permanganate

with 2-butanol or97 adding solid fumaric acid to 0.2 M

NaMnO4,98 mixing manganese acetate with a citric acid

containing n-propyl alcohol at room temperature,99 reacting

KMnO4with H2SO4solutions,100,101 or reducing KMnO4with

polyacrylamide (PAM) and polyvinyl alcohol (PVA).90

Solgel-derived nanoparticulate MnO2 thin films were then

formed by either dip-coating or drop-coating colloidal

MnO2 directly onto conductive substrates, followed by

calcination at various temperatures.

The calcination temperature was found to have significant

influence on the surface morphology, specific surface area, and

specific capacitance of solgel derived MnO2

thin films. The

highest surface areas and specific capacitances were normally

achieved by calcinating the MnO2 thin films at temperatures

ranging from 200 to 300 1C.90 The possible reason is that

calcination at proper temperatures can generate high porosity

and a well-defined pore size distribution through evaporation

of the adsorbed water, solvent, and organic molecules, but

without further densification occurring at higher temperatures.

The specific capacitance of the solgel derived MnO2thin films

is also sensitive to thin film thickness. For instance, the

ultrathin MnO2 deposits (tens to hundreds of nanometres

thick) deliver specific capacitances as high as 700 F g1.90

The specific capacitance for thicker MnO2 films, i.e., higher

Table 4 Synthesis conditions, physicochemical features, and subsequent specific capacitance of crystalline MnO2

Technique Synthesis conditions Morphology Structure SBET/m2 g1 C/F g1

Hydrothermal MnSO4H2O+ KMnO4,140 1C

73,74Plate-like, nanorods a-MnO2 100150 72 to 168

(200 mA g1)Hydrothermal KMnO4+ sulfuric acid and

Cu scraps, 110 1C75

Hollow spheres,hollow urchins

a-MnO2 52108 147 (5 mV s1)

Hydrothermal KMnO4+ nitric acid, 110 1C76 Urchin-like a-MnO2 80119 86152 (5 mV s

1)Hydrothermal MnSO4 + K2S2O8 + sulfuric acid,

110 1C88

Urchin-like, clew-like a-MnO2, e-MnO2 46120 (5 mV s1)

Hydrothermal Mn(CH3COO)2 + K2S2O8, 120 1C78 Cubes and nanowires Mn3O4, MnOOH B170 (500 mV s

1)Hydrothermal a-NaMnO2 + nitric acid, 120 1C

77 Lamellar d-MnO2 241 (2 mA cm2)

High viscosityprocess

KMnO4+ MnCl2 + PG + PAM,65 1C

80Rod-shaped a-MnO2, g-MnO2 389 (10 mV s

1)

Room temperatureprecipitation

MnSO4 + K2S2O881 Rod-shaped d-MnO2 201

Low temperaturereduction

KMnO4+ formamide, 40 1C82 Nanoflower Cubic MnO2 (Fd3m) 225.9 121.5 (1000 mA g

1)

Low temperaturereduction

KMnO4or NaMnO4 in acidsat 20100 1C

83Layered Rancieite structure 11206 17112 (2 mV s1)

So noc hemistry KB rO3 + MnSO4 + 24 kHzultrasound84,85

Spherical particles g-MnO2 118344

Microwave-assistedemulsion

KMnO4+ oleic acid + microwave88 Belt-like d-MnO2 277 (0.2 mA cm

2)

Solgel process Manganese acetate + citric acid,80 1C

89Nanorods g-MnO2 317 (100 mA g

1)

Solution combustion Mn(NO3)2+ C2H5NO286 Plate-like e-MnO2 2343 71123

(1000 mA g1)

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mass loading, is generally limited to 100200 F g1, which

confirms that only a thin layer of MnO2, an electrochemically

active layer, is involved in the redox process.90

2.2.2 Anodic electrodeposition. Anodic electrodeposition

involves oriented diffusion of charged reactive species through

an electrolyte when an electric field is applied, and oxidation of

the charged species on the deposition surface that also serves

as an electrode. For the anodic electrodeposition of thin film

MnO2, the electro-oxidation of Mn(II) species occurs on the

anode surfaces, shown as follows:

Mn2+ + 2H2O - MnO2 + 4H+ + 2e (5)

Pang et al. potentiostatically prepared the first electro-

deposited thin film MnO2 electrode for ECs in 2000.90 This

was followed by more than 30 papers dealing with anodic

electrodeposition of manganese dioxide thin film electrodes used

in ECs.35,91,92,102143 Most of these studies focused on varying

the deposition parameters to obtain MnO2 thin films with a wide

range of water contents, oxidation states, and Brunauer

EmmettTeller (BET) surface areas and, in turn, to achieveenhanced electrochemical performance, e.g., high capacitance,

long-term cycling behavior, and fast charge/discharge rates.

Electrode materials with three-dimensional (3D), meso-

porous and ordered/periodic architectures are desirable for

the penetration of electrolyte and reactants into the entire

electrode matrix. Among the efforts, therefore, morphology-

controlled growth has attracted much interest to obtain

more accessible electroactive sites and shorter cation

diffusion lengths in MnO2 electrodes. Morphology-controlled

growth is generally achieved through controlling the

deposition parameters, filling template membranes, or using

etched, nanoporous substrates. In the first case, the most

common surface morphology in MnO2 electrodes throughgalvanostatic or potentiostatic modes is a porous, three-

dimensional (3D) fibrous network, as shown in

Fig. 8a.33,102,108,110,115,116,118,119,125,128,130132 Another type of

surface morphology through a template-free process is free-

standing micro- and nano-scale fibers, rods and interconnected

nanosheets prepared either by using a dilute electrolyte

(Fig. 8bd)134,135 or applying cyclic voltammetry (Fig. 8e

and f).35,108,118 In the second case, anodic Al oxide (AAO)

templates or lyotropic liquid crystalline (LLC) phases are

applied to direct the MnO2 deposition.120,129,133,136 MnO2

electrodes with oriented nanofibrous, nanotubular and

mesoporous ravine-like morphologies are attained, as shown

in Fig. 9.

120,129,133,136

For electrodeposition on etched, nano-porous substrates, selective dissolution of copper (Cu) from a

NiCu alloy layer is conducted to obtain a nanoporous Ni

substrate, followed by anodic electrodeposition of MnO2.138

Schematic illustrations for preparing a high-porosity Mn oxide

electrode are depicted in Fig. 10a.138 Fig. 10bd show SEM

micrographs of the as-deposited NiCu alloy film, the nano-

porous Ni film, and the high-porosity Mn oxide electrode.138

This idea was also applied to obtain high-porosity Mn oxide

deposits based on micro-etched duplex stainless steel (DSS).139

2.2.3 Cathodic electrodeposition. Cathodic deposition of

manganese oxide thin films can be realized through two

electrochemical processes, depending on the reactions taking

place at the cathode surfaces. One method is electrogenera-

tion of the base, including reactions that consume H+

ions

Fig. 8 Typical surface morphologies for MnO2 electrodes prepared

through template-free anodic electrodeposition processes. (a) Reproduced

from ref. 106, reprinted with permission. Copyright 2003, The

Electrochemical Society; (b) reproduced from ref. 134, reprinted with

permission. Copyright 2010, Elsevier Ltd.; (c) and (d) reproduced from

ref. 135, reprinted with permission. Copyright 2010, Elsevier Ltd.;

(e) reproduced from ref. 118, reprinted with permission. Copyright

2007, Elsevier Ltd.; (f) reproduced from ref. 35, reprinted with

permission. Copyright 2005, American Institute of Physics.

Fig. 9 MnO2 nanostructures prepared by template-assisted

anodic electrodeposition. (a) Reproduced from ref. 133, reprinted

with permission. Copyright 2009, The Royal Society of Chemistry;

(b) reproduced from ref. 120, reprinted with permission. Copyright

2006, Elsevier Ltd.; (c) reproduced from ref. 136, reprinted with

permission. Copyright 2010, Elsevier Ltd.; (d) reproduced from

ref. 129, reprinted with permission. Copyright 2008, Elsevier Ltd.

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or electrolysis of water. These reduction reactions cause an

increase in the pH of the electrolyte adjacent to the cathode,

either by the consumption of H+ ions or the generation of

OH ions, which compete with the metal ion reduction reac-

tion and become the overriding reactions. As a result, metal

deposition does not take place; instead the metal ion deposits

in the form of a hydroxide on the cathode as:

Mn2+ + 2(OH) = Mn(OH)2k (6)

Subsequent thermal annealing will convert the manganesehydroxide into stable manganese oxides by a dehydration

process. Zhitomirsky et al. fabricated very smooth and amorphous

MnOx films for electrochemical supercapacitors from poly-

ethylenimine (PEI) or chitosanMnCl2solutions, as shown in

Fig. 11a.91,92 A similar cathodic process based on manganese

acetate-containing solutions was developed to prepare Mn3O4films with a porous/nanoflake hierarchical architecture

(Fig. 11b).140

The other cathodic process involves electro-reduction of

Mn(VII) species occurring on the cathode surfaces as

MnO4 + 2H2O + 3e- MnO2 + 4OH

(7)

This reaction was employed for the fabrication of nano-

structured manganese dioxide films by galvanostatic, pulse,

and reverse pulse electrodeposition.141145 The deposition rate,

composition, and microstructure of the deposits are dependent

on the concentration of active MnO4 species. Electrodeposition

from a relatively dilute solution containing 0.02 M NaMnO4resulted in the formation of fibrous films with a birnessite-

type crystal structure, while smoother and amorphous

films were obtained using 0.1 M NaMnO4 solutions.141145

Typical surface morphologies of electrodeposited MnO2films prepared under different conditions are compared in

Fig. 11c and d.143

2.2.4 Electrochemical oxidation of Mn films. Broughton

et al.developed a new procedure making use of physical vapor

deposition (PVD) with a glancing vapor incidence angle

(GLAD) in order to produce a chevron-type porous Mn

metallic structure, which was then electrochemically oxidized

to produce a manganese oxide electrode for ECs.34,146150

The

primary concept of thin-film structural control using GLAD

deposition relies on self-shadowed columnar growth that is

manipulated by angular and rotational control of the substrate

with respect to the evaporation source during the deposition.34

Metallic manganese films were deposited using the GLAD

deposition apparatus, as shown schematically in Fig. 12a.34

Fig. 12b and c show the typical as-deposited chevron-type Mn

metallic film (Fig. 12b)147 and electrochemically oxidized

petal-shaped MnO2 (Fig. 12c).149 The porous chevron-type

structure has been converted into thin sheet-like, hydrated

MnO2. In addition to the electrochemical oxidation process,

the chevron-type Mn metallic films were also thermally

oxidized in air to obtain manganese oxide layers. In this case,

the zig-zag architecture was replaced by a porous oxide

structure with little zigzag texture remaining, as shown in

Fig. 12d.147 The specific capacitance obtained from the

electrochemically oxidized manganese films was generally

higher than that from thermally oxidized samples, which

further confirms that the electrochemically active sites in

manganese oxide are closely related to a hydrated and porous

structure.148

In addition to the GLAD deposition process, an electro-

chemical deposition process was also developed to deposit metallic

manganese films from a BMPNTf2 ionic liquid.151153 The

electrodeposited Mn films (Fig. 13a) were anodized in Na2SO4aqueous solution by various electrochemical methods such

as potentiostatic, galvanostatic, and cyclic voltammetry (CV),

and transformed to Mn oxides with different physical charac-

teristics, as shown in Fig. 13bd.151 The Mn oxide anodized

Fig. 10 (a) Schematic for preparing a high-porosity Mn oxide

electrode based on micro-etched conductive substrates, (b)(d) SEM

micrographs of the as-deposited NiCu alloy film, nanoporous Ni

film, and high-porosity Mn oxide electrode. (Reproduced from

ref. 138, reprinted with permission. Copyright 2008, Elsevier Ltd.)

Fig. 11 Cathodic deposition of manganese oxide thin films based on

different cathodic reactions. (a) and (b) Electrogeneration of base;

(c) and (d) electro-reduction of Mn(VII) species. (a, reproduced from

ref. 91, reprinted with permission. Copyright 2006, Elsevier Ltd.; b,

adapted from ref. 140, reprinted with permission. Copyright 2009, The

Electrochemical Society; c and d, reproduced from ref. 143, reprintedwith permission. Copyright 2008, Institute of Materials, Minerals and

Mining.)

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under the CV condition had the largest surface area, highest

hydrous state, and lowest Mn valence state and showed thehighest specific capacitance.

2.2.5 Electrophoretic deposition. Another electro-assisted

coating technique is electrophoretic deposition (EPD). EPD is

achieved through the motion of charged particles in suspen-

sions towards an electrode and deposition under an external

electric field. Bath compositions for EPD include various

additives, which provide stabilization and charging of inorganic

particles in the suspensions. The electrophoretic deposition of

MnO2 films involves a two-step process: firstly, a stable

suspension of charged MnO2 particles is prepared by the

reduction of KMnO4aqueous solutions with various reducing

agents. Electrophoretic deposits are subsequently obtained

under potentiostatic or galvanostatic conditions.9396 The

deposition variables can be voltage or current density, solution

concentration, pH value, and solution temperature, but the

physicochemical features of EPD MnO2 films are primarily

dictated by the formation process of charged MnO2 suspen-

sions. Highly porous thin films containing nanofibers and

equiaxed particles are prepared through manipulation of the

formation parameters of charged MnO2 suspensions, as

shown in Fig. 14.95,96 Comparable electrochemical properties

(specific capacitance and cycle life) are obtained from the EPD

MnO2 films.

2.2.6 Summary on the development of thin film MnO2electrodes

The fundamentals and technique approaches in synthesizing

thin film or MnO2

coating electrodes have been summarized in

this chapter. Four groups of coating synthesis methods,

including solgel, electrochemical deposition, electrochemical

oxidation of metallic Mn films, and electrophoretic deposition,

have been reviewed and discussed in detail. Solgel techniques

involve either dip-coating or drop-coating colloidal MnO2directly onto conductive substrates, followed by calcination

at various temperatures. Calcination at elevated temperatures,

however, limits the types of substrate materials and the scope

of phase structures in which MnO2 nanostructures can be

deposited. By contrast, electrochemical deposition and electro-

phoretic deposition are two room temperature techniques,

opening up the prospect of deposition on various inexpensive

substrates including plastic substrates. Electrophoretic deposi-

tion is achieved through the motion of charged particles in

suspensions and deposition on the electrode surfaces. In this

case, the MnO2 particles are prepared before electrophoretic

deposition, so this technique itself cannot change the physico-

chemical characteristics of the MnO2 phase. The electro-

chemical deposition technique has some distinct advantages

for growing nanostructured electrode materials. It is generally

applicable to obtain metallic-, oxide-, and polymer-based

nanostructured materials. The composition, defect chemistry,

and even crystal structure can be manipulated through adjusting

solution concentration, solution pH value, and applied over-

potential or current density.

Fig. 12 (a) Schematic illustration of evaporation source and substrate

geometry during GLAD deposition of Mn films; (b) as-deposited

chevron-type Mn metallic films; (c) chevron-type Mn metallic films

after full electrochemical oxidation; (d) chevron-type Mn metallic films

after full thermal oxidation. (a, adapted from ref. 34, reprinted with

permission. Copyright 2002, The Electrochemical Society; b and d,

reproduced from ref. 147, reprinted with permission. Copyright 2003,

Kluwer Academic Publishers; c, adapted from ref. 149, reprinted with

permission. Copyright 2006, The Electrochemical Society.)

Fig. 13 SEM micrographs of (a) as-deposited Mn, (b) Mn oxide

potentiostatic electrode, (c) Mn oxidegalvanostatic electrode, and

(d) Mn oxidecyclic voltammetry electrode. (Reproduced from

ref. 151, reprinted with permission. Copyright 2008, Elsevier Ltd.)

Fig. 14 Highly porous thin films containing nanofibers (a) and

equiaxed particles (b) prepared through manipulating the forma-

tion parameters of charged MnO2 suspensions. (a, reproduced from

ref. 95, reprinted with permission. Copyright 2008, Elsevier Ltd.;

b, reproduced from ref. 96, reprinted with permission. Copyright

2009, Elsevier Ltd.)

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3. MnO2-based composite electrodes with other

elements and conducting polymers

Poor electrical conductivity (B105 O cm) has been reported

for micrometre-thick birnessite-type MnO2.55 The specific

capacitance and power characteristics of MnO2 electrodes

are ultimately limited by the high charge-transfer resistance.

It is well established that the transition metal oxides are

semiconducting in nature. Incorporation of other metal elements

into MnO2 compounds is a potential route to enhance the

electrical conductivity and charge-storage capability of

manganese oxides by introducing more defects and charge

carriers. The other consideration for alleviating the poor

electronic conductivity of MnO2 electrodes is to tailor the

electrode architecture,i.e., applying an ultrathin layer of MnO2on the surface of a porous, high surface area and electronically

conducting structure to shorten the electron transport distance.

This can produce good electrochemical performance without

sacrificing the mass-loading of the MnO2 phase. The porous

architectures can be carbon nanofoams, templated mesoporous

carbon, and nanotube assemblies.

In addition to their poor electrical conductivity, another

important issue is the electrochemical cyclability of MnO2electrodes. Active material dissolution during electrochemical

cycling has been well recognized in some investigations,16,33,130,131

which accounts for the major capacitance loss of the MnO2electrodes. A direct route to protect active MnO2 material

from electrochemical dissolution is to apply a coating on the

MnO2 nanoarchitecture as an effective barrier to Mn cation

permeation while allowing the electrolyte to be accessible.

Mechanical issues, such as low structural stability and flexibility,

also exist in MnO2electrodes resulting in degraded long-term

electrochemical cycle life. Hsieh et al. reported that capaci-

tance fading can be attributed to gradual mechanical failure of

the electrode materials caused by cyclic volumetric variations

of the oxide particles upon cycling.154 To enhance the mecha-

nical stability and flexibility, many efforts have also been

attempted to incorporate conductive polymers (polyaniline,

polypyrrole, and polythiophene) and their derivatives to get

mixed MnO2polymer composite electrodes with desirable

morphologies.5659 The excellent electronic conductivity, high

stability and mechanical flexibility of applied conductive polymers

enable improved electrochemical and mechanical properties of

MnO2polymer composite electrodes for ECs.5659

3.1 MnO2MeOx composite electrodes

3.1.1 MnNi mixed oxide compounds.Nickel oxides made

by the solgel method or electrochemical oxidation have been

evaluated as active materials for ECs, but they normally

exhibit low specific capacitances of 5064 F g1.155,156 MnNi

mixed oxide electrodes for EC application were first synthesized

through reduction of KMnO4 with Ni(II) acetatemanganese

acetate reducing solutions.44 Another chemical route, based

on thermal decomposition of the precursor obtained by

chemical co-precipitation of Mn and Ni transition metal salts,

was also developed to prepare MnNi and MnNiCo oxide

composites.157,158 By introducing 20% NiO into MnO2, the

specific capacitance increased from 166 F g1 for MnO2 to

210 F g1 for the Mn/Ni mixed oxide. A higher rate capacity

was also observed for the Mn/Ni mixed oxides, and the

enhanced properties were attributed to the increased surface

area of the mixed oxide due to the formation of micropores.44

Electrochemical co-deposition of Mn/Ni mixed oxides from

Mn(II) and Ni(II) containing solutions was also applied toprepare thin film electrodes. The rate of anodic deposition for

manganese oxide is much larger than that for Ni oxide under

potentiostatic conditions, as shown in the linear sweep

voltammograms in Fig. 15a.38 The Ni content of binary

MnNi oxide electrodes is raised by applying a relatively

positive potential and a high Ni2+/Mn2+ ratio (e.g., 10 : 1)

during co-deposition. Addition of Ni oxide into the MnO2electrodes changed the shape of the grains from spherical into

flat, as shown in Fig. 15b and c, but a similar fibrous feature

was maintained at higher magnification.38 The a-(Mn,Ni)xOywith B18 wt% Ni showed higher capacity, better electro-

chemical reversibility, and high-power characteristics in a

mixed electrolyte consisting of Na2SO4 and NaOH withpH of 10.1.

38

Potentiodynamic methods are another effective electro-

chemical strategy to realize co-deposition of nanostructured

and microporous nickelmanganese oxides onto inexpensive

stainless steel substrates.39 A scan rate of 200 mV s1 between

the potential limits of 0.0 and 1.4 V was employed for the

deposition. At high scan rates, the deposition rate is low, and

accordingly, more than 300 cycles were required to deposit the

required amount of manganesenickel oxides with an atomic

ratio of 65 : 35. Enhanced electrochemical properties were

observed in this type of mixed oxide electrode. For instance,

specific capacitance (SC) values of 621 F g1 and 377 F g1

were obtained for CV scan rates of 10 and 200 mV s1

,respectively.39 These values of electrochemical performance

are much higher than those obtained with just MnO2. Since

the mass loading of the manganesenickel oxides was not

reported, it is difficult to compare with other systems.

3.1.2 MnCo mixed oxide compounds. Cobalt oxides are

another candidate material for electrochemical supercapacitor

applications.8,159161 However, it is noted that cobalt oxides

have a narrower operation potential window of approximately

0.5 V and require a basic working electrolyte for supercapacitor

applications, compared with manganese oxide-based electrode

materials. Incorporation of Co oxides in Mn oxides has been

Fig. 15 (a) Linear sweep voltammograms measured at 2 mV s1

in (1) 0.01 M MnCl2 and (2) 0.1 M NiCl2; SEM micrographs of

(b) a-MnOxnH2O and ( c) a-(Mn,Ni)xOynH2O. (Adapted from


chemical Society.)

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attempted to further improve their pseudocapacitive performance.

Prasad and Miura deposited nanostructured and microporous

cobaltmanganese oxide (CMO) onto inexpensive stainless

steel substrates by potentiodynamic methods, and found that

addition of Co oxide has a beneficial impact on improving

specific capacitances of MnO2.39 Similar improvement in

electrochemical properties of CoMn oxides was observed in

another study.162 However, neither the corresponding

physicochemical features were characterized nor was the

electrochemical mechanism addressed.

Some other studies using potentiostatic deposition of mixed

MnCo oxides exhibited different tendencies. For instance,

Chuang and Hu investigated the effects of solution pH values

on the surface morphology and chemistry of as-deposited

mixed MnCo oxides.163 Adjusting the pH of the plating

solution varied the surface morphology and Co/Mn content

ratio in the deposited oxides, but the specific capacitance

of these oxides essentially remained unchanged.163 By

contrast, Changet al. reported that the addition of Co oxides

modified the oxide surface from a fibrous shape to a rather

smooth morphology (Fig. 16a and b), which may account

for the significant reduction in the specific capacitance of the

as-deposited mixed MnCo oxide with a high Co content

(>15 wt%) (Fig. 16c).164,165 However, the addition of Co

oxides effectively inhibited the irreversible anodic dissolution

of the deposited oxide during electrochemical cycling in

aqueous KCl electrolyte (Fig. 16d).164,165 This suggests a great

improvement in electrochemical stability for mixed MnCo

oxide electrodes.

In addition to electrochemical routes, a radio-frequency

sputtering procedure was used to prepare manganesecobalt

oxide thin films.166 Through the control of the flow rate of

oxygen, sputtering pressure, sputtering power, and annealing

temperature, mixed oxide layers with petal-shaped morpho-

logy were obtained, as shown in Fig. 17.166 A maximum

specific capacitance of 256 F g1 was retained at the 2000th

cycle of potential cycling, demonstrating long-term operational

stability and good specific capacitance at a high sweep rate of100 mV s1.166

3.1.3 MnFe mixed oxide compounds. Iron possesses

various valence states making it a promising electrode material

for supercapacitors. MnFe2O4, a MnFe mixed oxide material,

was recently found to exhibit pseudocapacitance in aqueous

NaCl solutions. MnFe2O4 was synthesized via a precipitation

technique in basic aqueous solutions with a MnSO4 to FeCl3ratio of 2 : 1, followed by a subsequent calcination process at

different temperatures for 2 h in a N2 atmosphere.167 It was

found that large capacitances are associated with the Mn2+

ions in the tetrahedral sites in the spinel structure. Pseudo-

capacitance was observed only for crystalline, rather than

amorphous MnFe2O4 treated at temperatures o 200 1C, as

shown in the cyclic voltammograms (Fig. 18) taken from MnFe

mixed oxide electrodes annealed at different temperatures.167

This tendency is essentially different from what was observed

for MnO2 electrodes, but a clear understanding has not been

reached yet.

Another technique to incorporate iron oxides into manga-

nese oxide electrodes is electrochemical co-deposition.168170

MnFe oxides were prepared on graphite substrates by anodic

deposition from 0.25 M Mn acetate aqueous solutions with

various amounts of FeCl3 up to 0.15 M.168170 Experimental

results indicated that the incorporated iron was present as

di- and tri-valent oxides.In situX-ray absorption spectroscopy

(XAS) results (Fig. 19) confirm that the pseudocapacitive

behavior of the MnFe oxides is associated with the reversible

variations in the oxidation states of both the Mn and Fe

cations during electrochemical cycling.170 It has been found

that through introduction of Fe oxide, the degree of change in

the Mn oxidation state increases from 0.70 to 0.81 within the

potential range of 01 V. The specific capacitance of Mn90Fe10oxide (255 F g1) is higher than that of the pure Mn oxide

(205 F g1). This experimental evidence suggests that Fe

addition enhances the electrochemical performance through

improving the electrical conductivity of manganese oxides.170

Fig. 16 (a) and (b) SEM micrographs of the oxides deposited in

Mn(CH3COO)2 plating solutions with 0 and 0.2 M Co(CH3COO)

additions; (c) cyclic voltammograms of the various oxide electrodes

measured in 2 M KCl electrolyte at a potential scan rate of 5 mV s1;

(d) variations in the specific capacitance vs. the CV cycle number for

various oxide electrodes. Curves ae present the oxides deposited with

0, 0.05, 0.1, 0.15, and 0.2 M Co(CH3COO)2 additions, respectively.

(Adapted from ref. 164, reprinted with permission. Copyright 2008,

Elsevier Ltd.)

Fig. 17 SEM micrograph of the MnCo oxide electrode prepared by

radio-frequency sputtering. (Adapted from ref. 166, reprinted with

permission. Copyright 2010, The Electrochemical Society.)

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A two-step spray pyrolysis-electrophoretic deposition

(SP-EPD) process was employed to prepare nanocrystalline

MnFe mixed oxide electrodes.171 The iron-containing

manganese oxide powders were synthesized from manganese

acetate and iron nitrate solutions at 400 1C. The as-processed

powders were subsequently deposited onto graphite substrates

via electrophoretic deposition. Structural analysis revealed

that the as-deposited coatings exhibited a nanocrystalline

Mn3O4 phase. The specific capacitance of the as-deposited

Mn-oxide coating was increased from 202 F g1 to 232 F g1

when 2 at% Fe was added.171 Moreover, enhanced electro-

chemical cyclability was observed for the iron-added coatings

(B78% of the initial maximum capacitance) compared with a

pure MnO2 electrode (B60% of its maximum value).171

3.1.4 MnX (X = Pb, V, Ru, Mo and Al) mixed oxides.In

addition to Ni, Co and Fe, other metallic elements have been

introduced with the intention to further improve the electro-

chemical performance of Mn oxide electrodes. Lead oxides

were added into Mn oxide electrodes through reduction of

KMnO4 with Pb(II) acetatemanganese acetate reducing

solutions.44 By introducing 20% Pb into MnO2, the specific

capacitance increased from 166 F g1 estimated for MnO2 to

185 F g1 for Mn/Pb mixed oxides, which was attributed to

the increased surface area of the mixed oxide due to the

formation of micropores.44

Vanadium oxide (V2O5) was introduced into MnO2 thin

film electrodes by electro-oxidation of Mn2+ precursors in

aqueous solution with VO3.40,172 XRD analysis confirmed

that the presence of vanadate ions inhibits the growth of the

Mn oxide lattice, leading to an amorphous-like phase in the

as-deposited mixed MnV oxide.40,172 The infrared spectrum

exhibited bands attributable to V2O5, suggesting that protona-

tion and dehydration of VO3 occur to form the polymeric

structure. The electron spin resonance results suggest that

Mn3+ ions do not exist in the oxide network, but pair with

unreacted VO3.40,172 When annealed at elevated tempera-

tures, the formation of Mn2+ occurs only in the mixed MnV

oxide films. This implies that electron transfer from thermally

generated V4+ cations to neighboring Mn cations occurs.40,172

Enhanced voltammetric response of the heat treated Mn/V

oxide film was observed in a borate solution, compared with

that of pure manganese oxide, which can be attributed to the

higher electrical conductivity of the mixed MnV oxide

films.40,172

Rutheniummanganese oxide (RunMn1nOx) has been

prepared by oxidative co-precipitation through mixing of Mn(VII)

(potassium permanganate), Mn(II) (manganese acetate), and

Fig. 18 CV curves of coprecipitated electrodes consisting of MnFe2O4/carbon black powders treated at different temperatures: (a) 50 1C and

200 1C, (b) 300 1C, 350 1C, 400 1C and 500 1C. Scan rate was 20 mV s1. (Adapted from ref. 167, reprinted with permission. Copyright 2005,

The Electrochemical Society.)

Fig. 19 (a) Twenty-one serialin situMnK edge XAS spectra for Mn90Fe10oxide measured at various applied potentials. (b) The dependence of

the Mn oxidation state with respect to the applied potential, obtained from (a). (Adapted from ref. 170, reprinted with permission. Copyright 2008,

Elsevier Ltd.)

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Ru(III) (ruthenium chloride) in a neutral aqueous solution at

room temperature.42 The proposed formation reactions are as

follows:42

2MnO4 + 3Mn2+ + 2H2O - 5MnO2 + 4H

+ (8)

Ru3+ + 3H2O - Ru(OH)3 + 3H+ (9)

4Ru(OH)3

+ O2- 4RuO

2+ 6H

2O (10)

At an appropriate calcination temperature (e.g., 170 1C), the

mixed RuMn oxide powders are in a hydrous amorphous

state with improved electrochemical properties (higher specific

capacitance and lower charge transfer resistance, as shown in

the CV curves in Fig. 20a and impedance spectra in Fig. 20b),

when compared with pure Mn oxide.42 The reduced charge

transfer resistance and enhanced voltammetric response observed

in the mixed RuMn oxide powders can be attributed to a

higher electronic conductivity induced by the Ru-doping.

In addition to chemical co-precipitation, a co-electrospinning

technique was employed to synthesize MnOxRuO2composite

fiber mats through two isolated spinnerets.173 During the

electrospinning process, two types of precursor solutions,

based on Mn acetylacetonate + PVAc and RuCl3 + PVAc,

were transferred simultaneously into separate syringes

mounted on the electrospinning apparatus, as schematically

shown in Fig. 21a.173 The as-electrospun and calcinated

MnOxRuO2 fiber mats at different temperatures are shown

in Fig. 21bd. The electrochemical performance using the

co-electrospun MnOxRuO2 fiber mats calcined at 300 1C

exhibited a high specific capacitance of 208.7 F g1 at a scan

rate of 10 mV s1.173 The co-electrospinning technique shows

high versatility to preparing composite electrodes with both

high surface activity and high conductivity for EC applications.

MnMo mixed oxide thin films were deposited anodically

on a platinum substrate by cycling the electrode potential

between 0 and +1.0 Vvs.Ag/AgCl in aqueous Mn(II) solutions

containing molybdate anions (MoO42).43 In this process,

only Mn(II) cations were electro-oxidized to form MnO2.

The MoO42 ions were incorporated into MnO2 by protona-

tion and dehydration,43 which is similar to the formation

of MnO2V2O5 mixed oxides mentioned above.40,172 Cyclic

voltammetry of the Mn/Mo oxide electrode in an aqueous

0.5 M Na2SO4 solution exhibited pseudocapacitive behavior

with a higher capacitance and better rate capability than that

for pure Mn oxide, most likely due to an increase in electrical

conductivity of the mixed oxide film.43

Aluminium (Al) is another element that has been added into

manganese oxide electrodes.45,174 Al-doped MnO2compounds

were synthesized either by an electrochemicalhydrothermal

route45,174 or through a high energy ball-milling technique.45,174

The electrochemicalhydrothermal technique can be regarded

as anodic electrodeposition in an autoclave at solution tempera-

tures ranging from 80 1C and 140 1C.45,174 The Al-substituted,

g-MnO2materials exhibited a higher specific capacitance than

non-substituted MnO2, consistent with the beneficial effect of

substitution on the surface area.45,174 For high energy ball-

milling, a mixture of pure aluminium and manganese dioxide

powder with a desired atomic ratio was put into a stainless

steel vessel with steel balls at a ball-to-powder weight ratio of

20 : 1. The high energy ball-milling was conducted at 250 rpm

for 50 h. The Al(0.05)/Mn(0.95)O2 electrode showed the largest

Fig. 20 (a) Cyclic voltammograms and (b) Nyquist plots for RunMn1nOx (n = 0.1) and MnO2electrodes at different applied potentials in the

frequency range of 102104 Hz. Both powders were calcined at 170 1C. (Adapted from ref. 42, reprinted with permission. Copyright 2009, Elsevier

Ltd.)

Fig. 21 (a) Schematic diagram of the co-electrospinning process used

to synthesize MnOxRuO2 fiber mats. (b) SEM image of as-spun

MnOxRuO2/PVAc composite fiber mats prepared by electrospinning.

(c) SEM image of MnOxRuO2fiber mats calcined at 300 1C. (d) SEM

image of MnOxRuO2 fiber mats calcined at 400 1C. (Adapted from


chemical Society.)

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specific capacitance among the MnO2samples with various Al

contents.45,174 Excellent electronic conductivity for Al doped

MnO2 was considered to be responsible for the enhanced

electrochemical performance of doped MnO2.45,174

3.1.5 Summary of incorporation of other metal elements in

MnO2. A core challenge in EC design is the identification

of suitable electro-active materials. The poor electrical con-

ductivity of MnO2 limits the specific capacitance and power

characteristics of MnO2 electrodes through high charge-

transfer resistance. Metal elements, including Ni, Fe, Co,

V, Mo, Al, Pb, and Ru, can be incorporated into MnO2compounds to enhance the electrical conductivity and

charge-storage capability by introducing more defects and

charge carriers. Enhancement of specific capacitance was

observed for Ni-, Fe-, V-, Mo-, Al-, Pb-, and Ru-containing

MnO2 electrodes, and was mainly attributed to the improved

electrical conductivity of the composite electrodes. The

amount of metal additives has significant effects on the

electrochemical properties. For instance, for MnFe mixed

oxides, the specific capacitance increases with addition of Fe

oxides and reaches a maximum value at a Mn : Fe ratio of

9 : 1. A further increase in the amount of Fe oxide may cause a

reduction in the specific capacitance.170 However, contradictory

results have been reported for the effect of Co oxide additions

on the specific capacitance of MnO2electrodes. Some research

work exhibited beneficial effects by introducing Co oxides,

whereas other studies showed an opposite tendency.39,162 It is

interesting to note that the addition of Co oxides may signifi-

cantly suppress the irreversible dissolution of the deposited

oxide during electrochemical cycling in aqueous KCl electro-

lyte, resulting in a great improvement in the electrochemical

stability of mixed MnCo oxide electrodes. Thus far, however,

an unambiguous understanding on the effect of introducing

metal elements to form mixed oxide electrodes on the sub-

sequent electrochemical properties is lacking, which is critical

to optimizing manganese oxide-based electrodes for EC

application.

Note that similar chemical modification routes have been

extensively applied to improve the electrochemical properties

of MnO2-based batteries such as Li/MnO2. Layered LiMnO2and spinel LiMn2O4are the most attractive positive electrode

materials for the Li/MnO2 batteries. Layered LiMnO2 has

high theoretical capacity but poor capacity retention induced

by structural instability (phase transition from the layered

structure to spinel).175,176 Partial substitution of Mn with Co

or Ni has been employed to improve the electrochemical

reversibility of layered LiMnO2 electrodes.175,177179 For the

stable spinel LiMn2O4, on long term cycling, slight capacity

fading was also observed in the Li/MnO2batteries, which was

attributed to slow dissolution of the LiMn2O4 electrode in

the electrolyte and the onset of the JahnTeller distortion

for a deeply discharged LiMn2O4 electrode.180182 There are

various strategies to alleviate or eliminate the dissolution

and JahnTeller distortion issues, including the substitution

of JahnTeller ion Mn3+ by some other trivalent cations

(such as Al3+, Fe3+, Ni3+, Co3+, or Cr3+)183187 and surface

modification of the LiMn2O4 electrode with some oxide

coatings (such as ZrO2 or AlPO4).188,189 Generally, chemical

modification in Li/MnO2 batteries aims to address primarily

the structural instability of electrode materials during deep

insertion/extraction of Li, which is distinct from MnO2ECs since fast surface or near-surface redox reactions are

predominant in ECs. However, the chemical modification

strategies for Li/MnO2 batteries may shed light on further

improvement of electrochemical cyclability in MnO2 ECs.

3.2 MnO2polymer composite electrodes

3.2.1 MnO2poly(o-phenylenediamine) (PPD). During

electrochemical cycling, manganese oxide electrodes undergo

a reductive-dissolution process when exposed to even mildly

acidic and near-neutral electrolytes, generating soluble Mn(II)

species.190192

MnO2 + H+ + e- MnOOH (11)

MnOOH + 3H+ + e- Mn + 2H2O (12)

Therefore, MnO2 is limited to neutral or basic aqueous

electrolytes, and the specific capacitance and rate capacity of

MnO2

electrodes are diminished by employing less-desirable

insertion cations such as Li+, Na+, and K+, compared with

the fast H+ insertion process. To overcome the limitation

associated with MnO2, a self-limiting electropolymerization

strategy has been developed to apply ultrathin, conformal

poly(o-phenylenediamine) (PPD) coatings onto nanostructured

MnO2ambigel electrodes.54,55 Self-limited PPD films, electro-

deposited from an aqueous borate buffer solution with

pH = 9 on planar indium-tin oxide (ITO) electrodes, have a

thickness of less than 10 nm. These hybrid PPDMnO2nanoarchitectures are stable in aqueous acid electrolytes upon

electrochemical cycling. The ultrathin PPD coating serves as

an effective barrier to the electrolyte, protecting the underlying

MnO2 nanoarchitecture from electrochemical dissolution.

54,55

However, based on conductive-probe atomic force micro-

scopy (CP-AFM) results, the hybrid PPDMnO2 electrode

structures were found to exhibit degraded electronic con-

ductivity, compared with native MnO2ambigel films supported

on indium tin oxide (ITO) substrates.54,55 Fig. 22 shows the

topography and current images for uncoated and PPDcoated

MnO2 ambigel films supported on ITO.55 Following the self-

limiting electrodeposition of the PPD coating onto the MnO2ambigel, a 20-fold reduction in conductivity was observed in

the hybrid structures. These results demonstrate the feasibility

of constructing hybrid configurations with metal oxides and

polymers for ECs; however, a polymer coating based on

poly(o-phenylenediamine) is not an optimal choice due to itspoor electrical conductivity.

3.2.2 MnO2polyaniline (PANI). PANI is one of the most

promising electronically conducting polymers with potential

applications as ECs because of its easy synthesis, high stability,

and good conductivity. The addition of PANI can be either

chemical or electrochemical. The first MnO2PANI composite

electrode was prepared through a two-step electrochemical

route: nanostructured MnO2was potentiodynamically deposited

on a polyaniline (PANI) matrix synthesized through an

electrochemical method.56 Fig. 23a and b show the morpho-

logy of the as-deposited PANI matrix and the MnO2PANI

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composite.56 Optimization of thickness of the PANI film and

the amount of MnO2deposition leads to a specific capacitance

of 715 F g1 and an energy density of about 200 Wh kg1 at a

chargedischarge current density of 5 mA cm2, as shown in

Fig. 23c.56 By contrast, in another study, a PANI coating was

electrochemically polymerized on MnO2 nanoparticles.193 To

enhance the interaction of MnO2 and PANI, the MnO2nanoparticles were modified by triethoxysilylmethyl N-substituted

aniline (ND42) before the PANI electro-polymerization.

Significant enhancement of specific capacitance in the

PANINDMnO2 film was observed, compared with the

PANIMnO2 film prepared in a similar manner, indicating

that the presence of the coupling reagent can improve the

electrochemical performance of PANIMnO2 composite

films.193

In addition to the two-step electrochemical method, electro-

chemical co-deposition was applied to prepare MnO2PANI

composite electrodes.194 Electro-codeposition was conducted

on carbon cloth from solutions of aniline and MnSO4through

potential cycling between 0.2 and 1.45 V (vs. SCE). The

codeposition of PANI with MnO2 produced larger effective

areas for hybrid films with fibrous structures. The films

obtained displayed pseudocapacitive behavior from 0 to

0.65 V vs. SCE in an acidic aqueous solution (1.0 M NaNO3at pH = 1). A specific capacitance of 532 F g1 at 2.4 mA cm2

discharging current and a coulombic efficiency of 97.5% over

1200 cycles with 76% capacitance retention were achieved.194

Chemical polymerization is another common route to

incorporate PANI into MnO2 electrodes. A MnO2PANI

composite was prepared through oxidizing an aniline thin film

with a KMnO4solution on a porous carbon electrode.195 At a

rate of 50 mV s1, the carbon/MnO2PANI composite electrode

and the MnO2PANI film produced specific capacitances as

high as 250 and 500 F g1, respectively. The specific capaci-

tance maintained 61% of the initial specific capacitance after

Fig. 22 Simultaneously acquired topography and current images of

uncoated and PPD-coated MnO2 ambigel films supported on ITO.

(a) and (b) topography and current images of an uncoated sample;

(c) and (d) topography and current images of a PPD-coated sample.

Below each image is the corresponding plot along the white lines

across the AFM images. (Reproduced from ref. 55, reprinted with

permission, copyright 2006, American Chemical Society.)

Fig. 23 (a) and (b) Morphology of the as-deposited PANI matrix and MnO2PANI composite; (c) specific capacitance and specific energy of

MnO2PANI ele ctrodes at various d ischarge c urrent de nsities wi th various specific masses of MnO2. The specific mass of PANI was kept constant

at 4 mg cm2. (Adapted from ref. 56, reprinted with permission. Copyright 2004, The Electrochemical Society.)

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5000 cycles mainly due to slow dissolution of MnO2 in the

electrolyte.195 Another similar oxidative polymerization process,

with aqueous ammonium

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