MnO2 nanomaterials for flexible supercapacitors ...ce.sysu.edu.cn/UploadFiles/File/201609/63610866309406613470532.pdf2 via non-three-dimensional (3D) ... a KLGHEI of Environment and

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MnO2 nanomaterials for flexible supercapacitors:performance enhancement via intrinsic andextrinsic modification

Teng Zhai,ab Xihong Lu,a Fuxin Wang,a Hui Xia*b and Yexiang Tong*a

Increasing power and energy demands for next-generation portable and flexible electronics have raised critical

requirements (flexibility, stretch-ability, environmental friendliness, lightweight, etc.) for the energy storage

devices. Flexible supercapacitors (SCs), as one of the most promising next-generation energy storage devices,

have stimulated intensive interest owing to their outstanding features including small size, low weight, ease of

handling, excellent reliability, and high power density. Manganese oxide (MnO2), has attracted much interest in

the development of flexible SCs with high electrochemical performance. Yet, the poor electronic and ionic

transport in MnO2 electrodes still limits its promotion in practical applications. This review aims to describe the

recent progress in the application of MnO2 materials in the development of flexible SCs and summarizes

the intrinsic modification of MnO2 via crystallinity, crystal structure, and oxygen vacancy introduction and the

extrinsic modification of MnO2 via non-three-dimensional (3D) and 3D flexible conductive scaffolds for high

performance flexible SCs. Moreover, we also discuss briefly on the current challenges, future directions, and

opportunities for the development of high-performance MnO2 based flexible SCs.

1. Introduction

There has been an increasing demand for fossil fuel originatedenergy in the past thirty years. However, it may not be possible

to provide sufficient energy as the world population continuesto grow and the limited amount of fossil fuels has begun todiminish.1 Sustainable energy sources including solar energy,wind, and tidal energy have emerged as promising candidates toreplace the conventional fossil fuel.2 Since the supply is inter-mittent and strongly dependent on the natural environment, thedevelopment of these sustainable energy sources has been greatlylimited by their storage. Moreover, the highly efficient, low costenergy storage devices for these kinds of intermittent renewableenergies are still one of the greatest challenges. Supercapacitors(SCs), also known as electrochemical capacitors (ECs), have

a KLGHEI of Environment and Energy Chemistry, MOE of the Key Laboratory of

Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical

Engineering, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of

China. E-mail: [email protected] Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and

Technology, Nanjing, 210094, China. E-mail: [email protected]

Teng Zhai

Teng Zhai received his PhDdegree in Physical Chemistryfrom Sun Yat-sen University in2015. He is now an AssistantProfessor in Herbert GleiterInstitute of Nanoscience,Nanjing University of Scienceand Technology. His researchinterests include the synthesis ofmetal oxide nanomaterials andits application in energy storage.

Xihong Lu

Xihong Lu received his BS degree(2008) and his PhD degree (2013)in Physical Chemistry from SunYat-Sen University, P. R. China.Then he joined the Sun Yat-SenUniversity as an associate pro-fessor in the school of chemistryand chemical engineering at SunYat-Sen University. His researchinterests focus on the developmentof functionally nanostructuredmaterials for applications insupercapacitors, Li ion batteries,and photocatalysts.

Received 11th August 2015,Accepted 25th September 2015

DOI: 10.1039/c5nh00048c

rsc.li/nanoscale-horizons

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110 | Nanoscale Horiz., 2016, 1, 109--124 This journal is©The Royal Society of Chemistry 2016

been recognized for more than fifty years and considered as oneof the energy storage systems with the most potential.3–5 Withthe rapid growth of electronics, there is increasing demand forflexible or space-saving electronic devices such as wearableelectronics, mobile phones, and flexible displays.6,7 To catchup with the rapid growth of the demand for portable flexibleelectronics, it is essential to develop high performance andreliable flexible energy storage devices. In this regard, flexiblesupercapacitors with high energy density, high power density(high-rate current output/input), and high cycling stability mustfacilitate practical use of portable flexible electronics. However, itremains a major concern to achieve improved electrochemicalperformance and ultrahigh flexibility simultaneously.

According to the charge storage mechanism of electrodematerials in SCs, they can be generally divided into two categories:

electrical double-layer capacitors (EDLCs) and pseudocapacitors.8

Fig. 1 compares the specific capacitances of various types ofelectrode materials based on these two mechanisms for SCs.The EDLC mechanism based carbon materials store the electricalcharge via a physical charge separation process, which is stronglydependent on the pore size distribution and high surface area ofthe carbon materials. In this regard, highly conductive andchemically stable carbon materials with large surface area, suchas graphene,9–11 carbon nanotubes (CNTs),12,13 and activatedcarbon,14 have been widely used as electrode materials for EDLCsand achieve specific capacitances of around 50–350 F g�1. Due totheir conductive electrochemical properties and physical storageprocess, carbon material based EDLCs can achieve high power(410 kW kg�1) and long term cycle stability (4105 cycles).However, they suffer from low capacitance derived low energy

Fig. 1 Reported specific capacitances for electrode materials. Reprinted from ref. 15 with the permission from interface, Copyright 2008 – TheElectrochemical Society.

Hui Xia

Hui Xia received his PhD degree inAdvanced Materials for Micro- andNano- Systems from Singapore-MIT Alliance, National Universityof Singapore in 2008. He is now aProfessor in School of MaterialsScience and Engineering, NanjingUniversity of Science and Tech-nology. His research interestsinclude electrode materials forlithium-ion batteries and super-capacitors, fabrication of all-solid-state thin film microbatteries, andgas sensors.

Yexiang Tong

Yexiang Tong is currently aprofessor in School of Chemistryand Chemical Engineering at SunYat-Sen University, P. R. China.Prof. Yexiang Tong received hisBS in General Chemistry in 1985,MS in Physical Chemistry in1988, and PhD in OrganicChemistry in 1999 from SunYat-Sen University. He joinedSun Yat-Sen University as anAssistant Professor of Chemistryin 1988. His current researchfocuses on the electrochemical

synthesis of alloys, intermetallic compounds and metal oxidenanomaterials, and investigation of their applications for energyconversion and storage.

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density (1–10 W h kg�1). In contrast, conducting polymer ormetal oxide based pseudocapacitors can deliver much higherspecific capacitance due to the fact that charges are stored viafast and reversible faradic reactions at the surface and bulk ofthe electrode materials. This will certainly lead to higher energydensity (20–50 W h kg�1) in comparison with EDLCs. It shouldbe noted that, due to the slow diffusion of ions within thebulk of the electrode and a relatively slower charge storagemechanism compared with simple adsorption/desorption,the pseudocapacitors usually possess a relatively lower powerdensity (0.5–2 kW kg�1). In addition, they often begin to degradeunder less than one or two thousand cycles due to changes intheir physical structure and dissolution problems. Therefore,significant efforts have been devoted in the past years to theconcerns of pseudocapacitors.

Among various pseudocapacitive materials, manganeseoxide (MnO2) is characterized by a low-cost (compared withhighly expensive RuO2) material with a high theoretical capa-citance (B1370 F g�1) in a wider working potential window(B0.8 to 1 V vs. Ag/AgCl in aqueous electrolytes). Over the pastdecades, MnO2 has become one of the most widely investigatedpseudocapacitive electrode materials. Latest developments inMnO2 based supercapacitors have been recently reviewed.4,16

However, the practical capacitance is around 200–600 F g�1 at amass loading of less than 1 mg cm�2,17 so the full potential ofMnO2 for SCs still needs to be further exploited. Intensiveexplorations have shown that one major issue crucial forMnO2 based SCs to achieve fascinating capacitive performanceis fast electron and ion transportation in materials or at interface.To solve this problem, considerable efforts have been devotedto the crystallinity,18,19 crystal structure,20–24 morphology, nanoor micro structural design,25–31 and vacancy introduction32–36

of MnO2 to pursue high-performance MnO2 based SCs. Fortu-nately, progress has been made within recent years. Herein, wereview the recent progress in the application of MnO2 materialsin the development of flexible supercapacitors and discussbriefly the current challenges, future directions and opportu-nities for the development of high-performance MnO2 basedflexible SCs.

2. Evaluation of electrodes and SCs

Cyclic voltammetry (CV) and galvanostatic charge/discharge(GCD) measurements are two main techniques to evaluate theelectrochemical properties of SCs such as specific or areal/volumetric capacitance, rate capability, cycling stability, etc.Generally, the specific or areal/volumetric capacitance can becalculated via eqn (1) and (2) as follows:

CV curves:

Cs ¼Q

mDV¼ S

2vmDV(1)

GCD curves:

Cs ¼Q

mDV¼ I

DtmDV

(2)

where Cs (F g�1) is the specific capacitance, Q (C) is the averagecharge during the charging and discharging process, m (g) isthe mass loading of the active materials, DV (V) is the potentialwindow, S (A V) is the integrated area of the CV curve, v (V s�1)is the scan rate, I (A) is the constant discharging current, and Dt(s) is the discharging time. For areal (Ca) and volumetric (Cv)capacitance, the m in the calculations should be replaced by thearea (cm2) and volume (cm3), respectively.

The average gravimetric power density (Ps) and energy density(Es) were calculated by using the following eqn (3) and (4):

Es ¼1

2CsV

2 (3)

Ps ¼Es

Dt(4)

The corresponding volumetric average power density andvolumetric energy density can be obtained by replacing Cs with Cv.

Large values of specific capacitance are always favoured forpursuing the corresponding high energy densities. It should benoted that the mass loading and charge/discharge rate arecrucial for the evaluation of specific capacitance. High specificcapacitance at higher mass loading and charge/discharge rate,which means high areal capacitance and output power, is moremeaningful for SCs at a practical device level. With respect to highenergy density, the unquestioning pursuit of a large voltagewindow is also meaningless. High coulombic efficiency and highlystable and reversible reactions in the selected voltage windoware key parameters for evaluations.

3. High-performance MnO2 basedflexible electrodes

Since the first report about the supercapacitor behavior of MnO2 byLee and Goodenough in 1999,37 it has attracted intensive attentionand is considered as one of the most promising active materials forSCs due to its high theoretical capacitance (B1300 F g�1), low cost,environmentally friendly features.38,39 The high capacitance ofMnO2 mainly originates from its pseudocapacitance, which canbe attributed to the faradic reactions occurring at the surface andsubsurface in the bulk of the MnO2 phase. Previous reports40,41

have indicated that the adsorption/desorption (at surface) orintercalation/deintercalation (at subsurface) of protons andcations are involved in the charge storage process, as well asthe transition between Mn(III) and Mn(IV):

MnIII(x+y)MnIV

1�(x+y)OOCxHy 2 MnIVO2 + xC+ + yH+ + (x + y)e�

(5)

where C+ = Li+, Na+, K+. Since both the electrons and ions (protons,and/or cations) are involved in the charge storage process, it iscrucial to achieve high ionic and electronic conductivity inMnO2 electrodes.

Fig. 2a presents the schematic of cyclic voltammetry (CV) fora single MnO2 electrode in a mild aqueous electrolyte (0.1 MK2SO4).42 The dotted lines highlight the successive multiplesurface redox reactions occurring during the charge/discharge

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process in the potential window 0–1 V vs. Ag/AgCl. The oxida-tion from Mn(III) to Mn(IV) accounts for the upper (red) part,while the reverse reduction relates to the lower (blue) part.Clearly, the fast, reversible, and successive multiple reactionstake place throughout the potential range 0–1 V, leading to awell-defined rectangular shaped CV curve (analogous to EDLCbehavior). It has been reported that the reversible redox transi-tion of Mn(II)/Mn(III) may also be involved in the charge storageprocess.4,43 Very recently, Yu-Ting Weng et al. investigated indetail the transitions between Mn(III)/Mn(II), Mn(IV)/Mn(III) withinthe electrode potential window�1–1 V vs. Ag/AgCl.43 Fig. 2b showsthe schematic expression of various charge-storage mechanismsfor MnO2 as a function of potential. At a low scan rate of 2 mV s�1,the battery behavior of transition between Mn(II)/Mn(III) accountsfor B1/2 of the charge storage. However, the attribution ofMn(II)/Mn(III) decreases sharply when the scan rate increases to100 mV s�1, indicating that achieving high ionic and electronicconductivities is the major issue for MnO2 to achieve fascinat-ing capacitive performance. Unfortunately, the MnO2 alwayssuffers from poor electronic (10�6 to 10�3 S cm�1) and ionicconductivities, resulting in a poor specific capacitance of around300–400 F g�1 (B1 mg cm�2), which is far from its theoreticalcapacitance of B1300 F g�1.44,45

As reported in the literature in recent years, both the intrinsicmodification (crystal structure and crystallinity, microstructuralconstruction, and valence introduction) and extrinsic modifica-tion (non-3D and 3D carbon and metal oxide based composites)were used to synthesize MnO2 with improved ionic and electronicproperties for high performance flexible SCs.

3.1 Intrinsically modified MnO2 electrodes for flexible SCs

3.1.1 Crystallinity and crystal structure. Similar to othertransition metal oxides, crystallinity is one of the importantfactors to intrinsically tune the capacitive performance ofMnO2.18,46,47 MnO2 with lower crystallinity possesses a porousmicrostructure (higher surface area) and fast ion transportation.

However, it will also result in a lower electrical conductivity.Thus, one optimized point with appropriate ion transportationand electrical conductivity can be found via tuning the crystallinityof MnO2 electrodes. With respect to the crystallinity, the anneal-ing temperature is a common effective approach to achieveoptimal electrochemical properties. Wei et al.18 annealed the asprepared MnO2 at 200 1C and found that the calcination lead tothe reduced porosity and improved crystallinity. For crystallizedMnO2, a series of crystalline structures, including a-, b-, d-MnO2

(birnessite), etc., have been investigated in detail.20–24,48 Amongthem, a- and b-MnO2 possess a tunnel structure: a mixture of2 � 2 octahedral and 1 � 1 octahedral units for a-MnO2

(Fig. 3a); 1 � 1 octahedral units for b-MnO2 (Fig. 3a). Fig. 3bshows the typical XRD patterns of a-MnO2 and b-MnO2. a-MnO2

shows a pure tetragonal phase [space group: I4/m (87)] withlattice constants a = 9.7847 Å and c = 2.8630 Å (JCPDS 44-0141),while the b-MnO2 presents a pure tetragonal phase [space group:P42/mnm (136)] with lattice constants a = 4.3999 Å and c = 2.8740 Å(JCPDS 24-0735). It has been reported that the crystalline a-MnO2

achieved a bulk specific capacitance of B200 F g�1,41 whilethe crystalline b-MnO2 phase exhibited a bulk capacitance ofB10 F g�1.23 The tunnel sizes of these two crystal structures areproposed as the explanation for this phenomenon.49,50 Until veryrecently, Matthias J. Young and coworkers41 have presented forthe first time the detailed charge storage mechanism of a-MnO2

and explained the capacity differences between a- and b-MnO2

using a combined theoretical electrochemical and band struc-ture analysis. Fig. 3c shows the absolute band edge energies ofa-MnO2 and b-MnO2 and the electrochemical scanned potentialwindow (SPW) for MnO2 in aqueous electrolyte at a pH of 7.4.The charge-switching states were induced by interstitial cationsin a-MnO2 through stabilization of Mn–O antibonding orbitalsfrom the a-MnO2 conduction band. Moreover, the cationsstabilize high energy dangling O 2p bonds resulting from Mnvacancies. d-MnO2 (birnessite), as a 2D material with an openlayered structure (inset of Fig. 3d), has received intensive

Fig. 2 (a) Schematic of cyclic voltammetry for a MnO2 electrode in a mild aqueous electrolyte (0.1 M K2SO4). The dotted lines highlight the successivemultiple surface redox reactions occurring during the charge/discharge process: oxidation from Mn(III) to Mn(IV) accounts for the upper (red) part, whilethe reverse reduction relates to the lower (blue) part. Adapted with permission from ref. 42, Copyright Nature Publishing Group 2008. (b) A schematicexpression of various charge-storage mechanisms as a function of potential; B: battery behavior; PC: pseudocapacitance; (IV), (III), and (II): Mn ionshaving valences of 4, 3, and 2, respectively. Adapted with permission from ref. 43, Copyright Wiley-VCH 2015.


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attention in recent years.51,52 Especially for planar supercapacitors,which enable the entire device to be much thinner and flexible,quasi-2D graphene like materials are more favorable.53–55 Lele Pengand coworkers22 reported for the first time a high-performancein-plane SC based on hybrid nanostructures of quasi-2D ultrathind-MnO2/graphene nanosheets. Fig. 3d shows the XRD patternof the monoclinic potassium birnessite (JCPDS 80-1098). Thepeaks of (002) and (006) facets observed in the pattern indicate awell-defined c-orientation, which can facilitate the fabricationof c-oriented thin films. Fig. 3e shows the schematic descriptionof the 2D planar ion transport favored within the 2D d-MnO2/graphene hybrid structures. The d-MnO2 nanosheets integratedon graphene can introduce more electrochemically active sur-faces for absorption/desorption of electrolyte ions. Besides, itcan also bring additional interfaces at the hybridized interlayerareas to facilitate charge transport during the charging/dischargingprocess. The specific capacitance of the as prepared planar super-capacitors based on the 2D d-MnO2/graphene hybrid at differentscan rates (from 50 to 400 mV s�1), is shown in Fig. 3f. Thed-MnO2/graphene based planar supercapacitor exhibits enhancedspecific capacitance (254 F g�1 at 0.5 A g�1) in comparison with theone based on graphene (B140 F g�1 at 0.5 A g�1). Furthermore, theformer achieved a better rate capability (B78%, 0.5 to 10 A g�1),which is substantially higher than that of graphene-basedelectrodes (B44%, 0.5 to 10 A g�1), confirming the enhancedcharge transportation.

3.1.2 Nanostructural and microstructural construction. Dueto the high surface area, the nanostructured MnO2 electrodescan achieve substantially improved electrochemical performancecompared with the bulk MnO2 electrodes. Thus, the constructionof nanostructural MnO2 electrodes is an effective approach toachieve fast ion transport for MnO2. Up to now, MnO2 withvarious morphologies, such as nanorods,56–58 nanoflower arrays,59

nanosheets,19 nanowires,60 etc., have been intensively reported.Accordingly, the specific surface area of nanostructured MnO2

electrodes ranges from 20 to 150 m2 g�1. Lai and his coworkers28

synthesized MnO2 with nanosheet (NS) and nanorod (NRs)morphologies on highly conductive NiCo2O4-doped carbonnanofibers (NCCNFs). Fig. 4a and b present the illustrationof the electron and ion transport pathways in the NiCo2O4-NCCNF@MnO2 NS and NCCNF@MnO2 NRs hybrid membranes.The construction of the nanostructured MnO2 on the surface ofNCCNFs effectively prevents the MnO2 nanoparticles fromaggregation, which will not only increase their specific surfacearea but also provide more active sites for the ionic adsorption.Furthermore, Xu et al.25 reported a flexible asymmetric super-capacitor (ASC) based on the MnO2 nanoflower@carbon nanofiber (CNF) cathode and CNF anode (Fig. 4c and d).25 Theflower-like MnO2 material with a porous structure enables fast iontransportation via significantly shortening the diffusion paths,and consequently ensures a high electrochemical performance.A good rate capability, with about 54% retention of the specific

Fig. 3 (a) Schematic crystal structure of a-MnO2 and b-MnO2. (b) XRD patterns of a-MnO2 and b-MnO2. Reprinted from ref. 104 with permission fromCopyrightr 2002, American Chemical Society. (c) Absolute band edge energies of a-MnO2 (left) and b-MnO2 (center) and the electrochemical scannedpotential window (SPW) for MnO2 in aqueous electrolyte at pH = 7.4 (right). Reprinted from ref. 41 with permission from Copyrightr 2015, AmericanChemical Society. (d) The XRD pattern of the bulk birnessite. The inset is a schematic crystal structure of d-MnO2 (birnessite). (e) A schematic descriptionof the 2D planar ion transport favored within the 2D d-MnO2 /graphene hybrid structures. (f) Specific capacitance of planar supercapacitors based on the2D d-MnO2/graphene hybrid at different scan rates (from 0.5–10 A g�1). The insets show the different bending states (folded and rolled) of planarsupercapacitors. Reprinted from ref. 22 with permission from Copyrightr 2013, American Chemical Society.


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capacitance as the current density increases from 3 to 30 mA cm�2,was achieved by the flower-like MnO2 on CNFs. It should be notedthat this value is achieved at a high mass loading of 3.1 mg cm�2,which is more persuasive. Significantly, the as assembled ASCdevice also shows outstanding mechanical flexiblity. Fig. 4eillustrates that the bending states exert a negligible effect on itselectochemical properties. Finally, the MnO2 nanoflower/CNF based ASC device delivers a maximum energy density of11.3 W h kg�1 at a power density of 352.6 W kg�1, which iscomparable and higher than other reported values.

Besides the nano-structured construction of MnO2 materials,the micro-structured construction of MnO2 electrodes is anothereffective method for MnO2 to achieve fast ion transport duringthe charge storage process. The fiber or wire based SCs havebeen considered to possess distinct advantages over the planarcounterparts in the development of reconfigurable, lightweight,and portable electronics.30,61–63 Significantly, a wire-shaped devicecan be co-woven using the well-developed textile technology.64

Xu et al.31 fabricated an all-solid-state fiber-shaped ASC viawrapping a conducting carbon paper on a MnO2 nanoflowercoated nanoporous gold wire. The MnO2 nanoflower coatednanoporous gold wire electrode exhibits an outstanding capa-citance retention of B88.2% as the current density increasesfrom 0.5 to 8 mA cm�2, indicating its high electronic and ionicconductivities. Fig. 5a shows the CV curves of the as fabricatedfiber-shaped ASC device under different bending states collectedat a scan rate of 100 mV s�1. Negligible electrochemical perfor-mance changes can be observed for the fiber-shaped device

under different bending states (01, 901, and 1801). Moreover,the devices are woven into textile structures to show the greatpotential for future flexible electronic devices (Fig. 5b).

Aiming at applications in wearable electronics, the stretchability has become an important factor for flexible fiber shapedSCs. However, the stretchable fiber shaped SCs are still rarelyreported or still suffer from low capacitance, low working potential(usually 0.8 V).65–67 Until very recently, Xu and his co-workers30

assembled a stretchable fiber-shaped ASC constituted of anMnO2/CNT hybrid fiber positive electrode, a CNT fiber negative

Fig. 4 Illustration of the electron and ion transport pathways in the (a) NiCo2O4-doped carbon nanofiber (NCCNF)@MnO2 nanosheet hybrid membraneand (b) NCCNF@MnO2 nanorod hybrid membrane. Reprinted from ref. 28 with permission from Copyrightr 2015, American Chemical Society.(c) The SEM image of MnO2 nanoflowers on carbon nanofibers. (d) A schematic illustration of the as-assembled MnO2 nanoflower based flexible SCs. Thedigital images present its flexibility. (e) CV curves (collected at 200 mV s�1) of the as-assembled flexible SCs under flat and bending states. (f) Ragoneplots of the MnO2 nanoflower based device. The values reported for other ASCs are added for comparison. Adapted with permission from ref. 25,Copyright Wiley-VCH 2015.

Fig. 5 (a) CV curves of the fiber-shaped ASC device under different bendingstates collected at a scan rate of 100 mV s�1. The insets are the photo imagesof the device under different bending states (01, 901, 1801). (b) The photo-graph of the fiber-shaped device integrated with conventional cotton yarns.Reprinted from ref. 31 with the permission from the Tsinghua UniversityPress and Springer-Verlag Berlin Heidelberg 2014.


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electrode and a KOH–PVA electrolyte (Fig. 6a). By taking advan-tage of the stable working potential of the MnO2/CNT fiberpositive electrode (0–0.5 V vs. Ag/AgCl) and CNT fiber negativeelectrode (�0.4 to �1 V vs. Ag/AgCl), the voltage window of theas assembled ASC device was extended to 1.5 V. A high specificcapacitance of B157.53 mF cm�1 at 50 mV s�1 was delivered inan extended voltage window of 0–1.5 V. Remarkably, a cyclictensile strain of up to 100% presents negligible effects on theelectrochemical performance of the stretchable ASC device.As shown in Fig. 6b, the redox peaks are still well maintainedat different tensile strains (0%, 40%, 70%, and 100%) and evenafter 20 mechanical stretching–releasing cycles (MSRCs), indi-cating the negligible effects of stretch states to the capacitiveperformance. The minor capacitance change for the stretchedASC device at various scan rates again confirms its high stretchability (Fig. 6c). Moreover, the specific capacitance still retainsmore than 99% after 10 000 galvanostatic charge/dischargecycles, indicating the long cycling stability.

3.1.3 Oxygen vacancy introduction. The introduction of oxygenvacancies into MnO2 is believed to be helpful for improving theconductivity and charge storage, and thus enhancing its electro-chemical performances.68–70 The presence of oxygen vacancies inMnO2 would lead to the charge compensation by changes ofoxidation states of Mn ions (Mn2+, Mn3+, and Mn4+), which willfurther result in variation of charge carrier density and make thedeficient MnO2 more conductive.105,106 Moreover, Song et al.69

performed in situ X-ray absorption near edge spectroscopy (XANES)and density functional theory (DFT) calculations to gain insightsinto charge storage mechanisms of mixed-valent MnOx. Due tothe Mn3+ 3d and the O 2p orbitals, excess negative charges can

be spilled onto the neighboring O atoms, which means thatmore charges can be stored in deficient MnO2. In recent years,the researchers have developed two major strategies to intro-duce oxygen vacancies into MnO2. The first one is atomicdoping with lower valence state impurity.71–74 Chen et al.75

proposed a strategy to facilitate the formation of oxygen vacan-cies in ramsdellite-MnO2 (R-MnO2) via the introduction oflower valence-state doping, which is effective to enhance theconductivity and activity of R-MnO2. Fig. 7a shows the chargedensity difference of (010) and (110) surfaces of Zn-dopedramsdellite-MnO2 with oxygen vacancies, presenting that theoxygen vacancies are positively charged, and the electrontransfer to near Mn or Zn atoms. Consequently, the chargedsurface Mn atoms will serve as active sites with better chemicalactivities. Moreover, the dangling bond density by coordinatelyunsaturated Mn will increase as the coordination number ofMn atoms near the vacancies decreases, which will enhance thesurface activity.76 In other words, as shown in Fig. 7b, enhancedelectron diffusion to the surface can be achieved by the bulk Zndopants. The surface oxygen vacancies will draw the electronsto the reaction sites, where the oxygen vacancies and reducedMn ions will improve the activity of the electrode reactionsin SCs. Kang et al.77 also reported the remarkably improvedconductivity of MnO2 via atomic doping of Au atoms (Fig. 7c)and the Au-doped MnO2 film exhibited substantially improvedcapacitive performance.

Annealing in de-oxygen or reducing in ambient atmosphereis the second effective approach for the introduction of oxygenvacancies into MnO2.36,70 Zhai et al. demonstrated an effectivestrategy, annealed MnO2 in hydrogen atmosphere (Fig. 8a), to

Fig. 6 (a) Photo images of the as assembled stretchable ASC device. (b) CV curves at different stretching states. (c) Specific capacitance variations withscan rates. (d) Cycling performance of the asymmetric supercapacitor at a current density of 2.8 mA cm�2 after 20 MSRCs. Reprinted from ref. 30 withpermission from the Copyrightr 2015, American Chemical Society.


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intrinsically improve the conductivity and capacitive perfor-mance of MnO2 by inducing oxygen vacancies.36 Fig. 8b showsthe areal capacitance and surface Mn oxidation state of hydro-genated MnO2 (H-MnO2) as a function of the hydrogenatedtemperature. It could be observed that the H-MnO2 samplesexhibit a higher areal capacitance in comparison with theuntreated MnO2 sample. Moreover, H-MnO2 hydrogenated at

250 1C with moderate oxygen vacancies exhibits the highestareal capacitance of 0.22 F cm�2 (449 F g�1) at 0.75 mA cm�2.Significantly, a solid-state ASC based on the H-MnO2 andreduced graphene oxide was assembled and exhibited a higherenergy density and power density (0.25 mW h cm�3 at 1.01 W cm�3)over other reported SC devices (Fig. 8c). Finally, to test the feasibilityof the H-MnO2//RGO ASC device as an energy storage device in

Fig. 7 (a) The charge density difference of (010) and (110) surfaces of Zn-doped ramsdellite-MnO2 with the oxygen vacancies of site I. The blue andyellow areas highlight the electron loss and gain, respectively. (b) A schematic illustration of the effects on the MnO2 materials from Zn2+ doping.Reprinted from ref. 75 with the permission from Royal Society of Chemistry 2015. (c) A sketch of the fabrication process of the Au-doped MnO2

electrodes. Reprinted from ref. 77 with the permission from 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 8 (a) Schematic diagram illustrating the growth process for preparing hydrogenated MnO2 (H-MnO2) NRs on carbon cloth substrate. (b) Arealcapacitance and surface Mn oxidation state of H-MnO2 as a function of hydrogenated temperature. (c) Ragone plots of the H-MnO2//RGO ASC devicemeasured in the gel electrolyte. The values reported for other SC devices are added for comparison. (d) Photo images of the cloth model and the zoom-in image of wearable ASCs sewing on the cloth model. Reproduced from the ref. 36 with the permission from 2014 Elsevier Ltd.


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Tab

le1

Typ

ical

resu

lts

of

cap

acit

ive

pe

rfo

rman

ceo

fe

lect

rod

es

and

SCs

bas

ed

on

diff

ere

nt

pla

tfo

rmsu

pp

ort

s

Act

ive

mat

eria

lsC

ond

uct

ive

scaff

old

Cap

acit

ance

Mas

slo

adin

gFl

exib

ilit

y(c

apac

itan

cere

ten

tion

%)

En

ergy

den

sity

rete

nti

onof

SCs

En

ergy

den

sity

(E)

Pow

erd

ensi

ty(P

)C

ycle

stab

ilit

yR

ef.

Mn

O2

Gra

phen

e/C

NT

pape

r48

6.6

Fg�

1

at2

mV

s�1

—N

egli

gibl

eva

riat

ion

ofel

ectr

ical

con

du

ctiv

ity

(100

0be

nd

ing

cycl

es)

B40

%(p

ower

den

sity

form

150

to22

50W

kg�

1)

24.8

Wh

kg�

1B

150

Wkg�

1—

80

Mn

O2

3Dh

ollo

wst

ruct

ure

dgr

aph

ene

B28

0F

g�1

at2

mV

s�1

0.42

mg

cm�

292

%(2

00be

nd

ing

cycl

es)

B55

%(P

form

62to

2500

Wkg�

1)

6.8

Wh

kg�

162

Wkg�

181

.2%

afte

r50

00cy

cles

78

Mn

O2

3Dgr

aph

ene/

CN

T34

3.1

Fg�

1

at2

mV

s�1

0.21

mg

cm�

2—

B56

.8%

(Pfo

rm17

0.5

to22

727

Wkg�

1)

33.7

1W

hkg�

117

0.5

Wkg�

195

.3%

afte

r10

00cy

cles

90

Mn

O2

Gra

phen

efo

am42

2.5

Fg�

1

at1

Ag�

1—

B97

%af

ter

ben

din

g18

01fo

r10

0ti

mes

.B

57.2

%(P

form

453.

6to

9188

.1W

kg�

1)

31.8

Wh

kg�

145

3.6

Wkg�

184

.4%

afte

r10

000

cycl

es82

Mn

O2

H-T

iO2

nan

o-w

ires

@ca

rbon

clot

h

449.

6F

g�1

at10

mV

s�1

0.23

mg

cm�

2N

egli

gibl

eva

riat

ion

attw

iste

dan

dbe

nd

ing

stat

esB

56.7

%(P

form

3500

0to

4500

0W

kg�

1)

0.30

mW

hcm�

3

(59

Wh

kg�

1)

0.23

Wcm�

3

(45

kWkg�

1)

91.2

%af

ter

5000

cycl

es85

Mn

O2

nan

orod

Gra

phen

efi

lm20

9F

g�1

at1

mV

s—

B97

.2%

atth

ebe

nt

stat

esB

24%

(Pfo

rm10

1.5

to24

500

Wkg�

1)

50.8

Wh

kg�

110

1.5

Wkg�

181

%af

ter

1000

cycl

es87

Mn

O2

Few

wal

led

CN

Tpa

per

203

Fg�

1

at2

mV

s�1

——

89%

(Pfo

rm13

0to

7800

Wkg�

1)

23.9

Wh

kg�

17.

8kW

kg�

195

%af

ter

2000

cycl

es88

Ppy/

Mn

O2

Car

bon

fibe

r—

—99

.76%

wh

enit

was

roll

edu

pB

26%

(Pfo

rm0.

05to

2W

cm�

3)

6.16

mW

hcm�

30.

05W

cm�

386

.7%

afte

r10

00cy

cles

84

Mn

O2

CN

Fpa

per

525

mF

cm�

2

(110

Fg�

1)

at3

mA

cm2

3.1

mg

cm�

2Y

es80

.5%

(Pfo

rm35

2.6

to33

70W

kg�

1)

43.4

mWh

cm�

2

(11.

3W

hkg�

1)

1.35

mW

cm�

2

(352

.6W

kg�

1)

85%

afte

r40

00cy

cles

25

Cu

Co 2

O4@

Mn

O2

nan

owir

eC

arbo

nfa

bric

s32

7F

g�1

at1.

25A

g�1

0.5

mg

cm�

2N

osi

gnif

ican

td

evia

tion

sat

0,30

,60

,90

1

70%

(Pfo

rm0.

4745

to31

20m

Wcm�

2)

94.3

Wh

cm�

20.

4757

mW

cm�

2Lo

ng-

term

cycl

ing

life

over

3000

cycl

esin

diff

eren

tbe

nt

stat

es

89

Mn

O2

Poly

eth

ylen

ete

reph

thal

ate

4.72

mF

cm�

2

at5mA

cm�

2

—SC

No

obvi

ous

chan

gebe

nt

at0,

901

——

—85

.5%

capa

cita

nce

rete

nti

onaf

ter

500

cycl

es91

Ppy/

Mn

O2

CN

Tte

xtil

e46

1.0

Fg�

1

at0.

2A

g�1

—SC

96.2

%of

init

ial

valu

eaf

ter

750

000

ben

din

gcy

cles

—31

.1W

hkg�

122

.1kW

kg�

193

.8%

capa

cita

nce

rete

nti

onaf

ter

1000

0cy

cles

92

Mn

O2

Stai

nle

ssst

eel

mes

h66

7F

g�1

at5

mV

s�1

——

—93


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wearable electronics, the as-fabricated flexible ASC was knittedinto laboratory clothing as a demonstration (Fig. 8d).

3.2 Extrinsically modified MnO2 electrodes for flexible SCs

In addition to intrinsic modification of MnO2, the extrinsiccombination of MnO2 with highly conductive materials, such ascarbon based materials,78–82 metal oxides or conductive poly-mer based materials,83–85 has also been investigated extensivelyfor flexible SCs. As the flexible SCs raised critical requirementsfor these highly conductive supports, the supports for MnO2 arerequired to have high mechanical integrity upon bending orfolding and lightweight property and excellent electrochemicalproperties.86 Herein, we review in detail the MnO2 materialsgrown on non-3D and 3D conductive and flexible supports forflexible SCs.

3.2.1 Non-3D scaffold supported MnO2 for flexible super-capacitors. To meet the demands of flexible supercapacitors,several kinds of non-3D supports including CNTs or graphenepaper,78,80,87,88 carbon fabrics,84,85,89 polyethylene terephtha-late (PET), transition metal oxide or conductive polymer nano-wires,38,85 are possible choices for flexible supercapacitorelectrodes. Table 1 summarized the typical results of capacitiveperformance of electrodes and SCs based on different platformsupports. Though good capacitive performance as well as out-standing flexibility has been achieved, the low surface area ofthese flat supports limited the further improvements of powerand energy density for flexible SCs.

Qiu and his co-workers91 recently developed for the firsttime a novel Au@MnO2 core–shell nanomesh structure on aflexible polymeric substrate. Fig. 9a shows the illustration ofthe fabrication of flexible SCs based on the flexible Au@MnO2

nanomesh electrode. The average diameter of the applied

polystyrene (PS) particle was around 700 nm and the as preparedAu nanomesh film with a thickness of 50 nm achieved a sheetresistance of 13–18 O sq�1, indicating a higher surface area(compared with platform supports) and good conductivities.Moreover, a flexible SC (Fig. 9c) based on the Au@MnO2

nanomesh electrode was assembled to explore the advantagesof this novel design. A stable coulombic efficiency of greater than95% over 1000 cycles was obtained at flat and bent states,indicating the stable Au nanomesh supports and strong couplingbetween the Au nanomesh and MnO2 nanosheets. However, thesurface area of Au nanomesh flexible supports still cannotbe compared with the one-dimensional (1D) nanostructuredsupports. Furthermore, the use of Au may hinder the widerange of applications of the Au nanomesh. Lu et al.85 developeda high-performance and flexible solid-state ASC device basedon 1D core–shell nanowire (NW) electrodes (Fig. 10a). Thehydrogen-treated TiO2 (denoted as H-TiO2) NWs were adoptedas the core (conducting scaffold) to support electrochemicallyactive MnO2. Fig. 10b presents the CV curves collected forH-TiO2, MnO2, TiO2@MnO2 and H-TiO2@MnO2 electrodes ata scan rate of 100 mV s�1. Obviously, the H-TiO2@MnO2

electrode exhibits a substantially higher current density thanthe values obtained for the MnO2 and TiO2@MnO2 electrodes,which can be attributed to the increased surface area throughthe H-TiO2 nanostructured supports and the enhanced chargetransport for TiO2 after hydrogenation. As the scan rate increasedfrom 10 to 200 mV s�1, the H-TiO2@MnO2 electrode retained54.6% of its capacitance, which is also substantially higher thanthat of the MnO2 (29.4%) and TiO2@MnO2 (43.4%) electrodes,again confirming the enhanced charge transport during thecharge/discharge process. Moreover, the electrode materialsdirectly grown on carbon cloth endow the ASC device with

Fig. 9 (a) Schematic illustration for the fabrication of flexible SCs based on the Au@MnO2 nanomesh. (b) A schematic diagram of ion diffusion pathwaysand charge transport channels of the Au@MnO2 nanomesh electrode. (c) Photo images of the Au@MnO2 nanomesh electrode based SCs sealed in aPDMS film, flat and bent. (d) Cyclic stability and coulombic efficiency of the device at flat and bending states. Reprinted from ref. 91 with the permissionfrom 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.


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outstanding flexibility (Fig. 10d). The as assembled flexible ASCdevice delivered a maximum energy density of 0.30 mW h cm�3

(59 W h kg�1) (Fig. 10e), which is higher than most of thereported SSCs.11,79,94,95

Similar to other transition metal oxides, MnO2 also suffersfrom the delamination problems caused by the large volumechanges during charge/discharge cycles, and hence a decreasein electrochemical stability.96,97 To prevent the delamination ofnanostructured MnO2 electrodes, researchers have tried to coata thin conductive layer on the top of MnO2.92,98 Yun et al.92 usedthe polypyrrole (Ppy) conductive polymer to coat on the top ofMnO2 nanoparticles to prevent delamination (Fig. 11a). With theprotected conductive polymer layer, the Ppy–MnO2/CNT electrodeachieved improved cycling stability, with 93.8% normalized capa-citance after 10 000 cycles (Fig. 11b), which is higher than that ofthe MnO2/CNT electrode. Significantly, the Ppy–MnO2/CNT basedSCs exhibited excellent flexibility (Fig. 11c). As shown in Fig. 11d,the Ppy–MnO2/CNT based SCs still well retained the almostoriginal capacitive performance even after 750 000 bending cycles,with capacity retention of 98.9, 98.4, and 96.2% of initial valueafter 250 000, 500 000, and 750 000 bending cycles, respectively.

3.2.2 3D scaffold supported MnO2 for flexible supercapa-citors. The design of 3D architectures for electrodes has beenconsidered as a highly efficient approach because of is ability toreduce ‘‘dead surface’’ of active materials and porous channelfacilitated ionic diffusion process. In recent years, 3D constructedMnO2 electrodes have attracted particular interest compared tolower dimensional structures.99 The 3D constructed MnO2 electro-des usually consist of MnO2 active materials and conductive 3Dscaffolds assembled from low-dimensional units (including one ormore types of 0D, 1D, and 2D nanomaterials), which can retain theintrinsic properties of the low-dimensional structure as well as fasterand more efficient electronic and ionic transport. Graphene based3D architectures are one of the most used flexible 3D scaffolds forMnO2 based SCs owing to the graphene resulted high conductivityand surface area. In addition, its lightweight feature resulted highactive mass ratio is quite meaningful for SCs at practical application.For example, Zhai et al.100 developed an electrochemical capa-citor using a highly conductive 3D graphene hollow (3DGH)structure (Fig. 12a) as a current collector to pursue a high activematerial ratio for the MnO2 based cathode and V3S4 anode. Theas prepared 3DGH exhibited an outstanding flexibility (Fig. 12b).

Fig. 10 (a) Schematic diagram illustrating the growth processes for H-TiO2@MnO2 and H-TiO2@C core–shell NWs on a carbon cloth substrate. (b) CVcurves collected for H-TiO2, MnO2, TiO2@MnO2 and H-TiO2@MnO2 electrodes at a scan rate of 100 mV s�1. (c) Specific capacitance of these electrodesas a function of the scan rate. (d) CV curves collected at a scan rate of 100 mV s�1 for the ASC device under flat, bent, and twisted conditions. The insetsare the device pictures under test conditions. (e) Ragone plots of the ASC devices measured in aqueous and gel electrolytes. The values reported forother SC devices are added for comparison. Reprinted from ref. 85 with the permission from 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.


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Finally, V3S4@3DGH and MnO2@3DGH were assembled intoan ASC (Fig. 12c) with a high active material ratio of 24% anddelivered a remarkable energy density of 7.4 W h kg�1 (basedon the weight of the entire device) at an average power densityof 3000 W kg�1. He and his co-workers78 also fabricated a

flexible symmetrical SC (of weight less than 10 mg and thicknessB0.8 mm; Fig. 12d) consisting of a sandwich structure of twopieces of 3D graphene/MnO2 composite network. The 3D graphenenetwork facilitates the synthesis of electrodes with a high activematerial ratio (B92.9% of the mass of the entire electrode).

Fig. 11 (a) Schematic illustration of a Ppy–MnO2-coated textile SC. (b) Cycling stability of the Ppy–MnO2-coated textile electrode. (c) A schematic of thebending test performed on the Ppy–MnO2-coated textile SC. (d) CV curves of the supercapacitor under 13% bending strain collected at 0, 250 000,500 000, and 750 000 bending cycles. Reprinted from ref. 92 with permission from Copyrightr 2015, American Chemical Society.

Fig. 12 (a) Photo images of 3D graphene/Ni foam (3DG/NFs) and 3D hollow graphene (3DGH) samples. The insets are the mass of each sample (samplearea, 6 cm2). (b) An illustration of the flexibility of the 3DGH sample. (c) A schematic illustration of the V3S4/3DGH//MnO2/3DGH device. Reprinted fromref. 100 with permission from Copyrightr 2015, American Chemical Society. (d) Digital photographs show the flexible SC at bent states. (e) Cyclingstability of the as assembled flexible SC collected at a current density of 1.5 mA cm�2 for 5000 cycles. The inset shows its cycling performance forbending cycles with a bending angle of 901. Reprinted from ref. 78 with permission from Copyrightr 2012, American Chemical Society.


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Moreover, the 3D graphene/MnO2 composite network-basedsymmetrical SC delivered an acceptable cycling stability perfor-mance even at bending angle of 901 (Fig. 12e). It should benoted that the mass loading of active materials for the 3Dgraphene network to assemble into the flexible SC is usuallylimited at B1 mg cm�2.78,100 The electrode will be more rigid ata high active material loading level, which will severely limit itsfurther progress. Xia et al.101 reported a new type of 3D porousand thin graphite foam (GF) as a light and conductive substratefor the growth of metal oxide core/shell nanowire arrays to formintegrated electrodes. The porous GF possesses a porosity ofB99.8% and a super high surface area of B980 m2 g�1, whichresult in its lightweight (B4 mg cm�3) and excellent scaffoldfor active materials (Fig. 13a–d). In addition to its excellentconductivity, the GF based electrode presents high specificcapacitance (based on the mass of the whole weight of theelectrode), good rate capability, and further enhanced energy/power density (Fig. 13e).

4. Summary and prospect

Manganese oxide (MnO2) has stimulated intensive interest dueto its application in the development of flexible supercapacitorsowing to its outstanding features such as low-cost, high theo-retical capacitance, and wide working potential window. How-ever, one of the crucial challenges for the use of MnO2 as anelectrode for flexible supercapacitors is its poor electronic andionic conductivities, which have hindered its wider applicationin SCs. To overcome this challenge, intensive efforts have beendevoted. In this review, recent advances in intrinsically andextrinsically modified MnO2 have been summarized, including

the approach to tune the crystallinity, synthesis of MnO2 withcertain targeted crystal structure, oxygen vacancy introduction,construction of flexible non-3D and 3D architecture compo-sites. The integration of MnO2 into low-dimensional supportswith high surface areas, and highly conductive and flexible 3Dscaffolds could achieve high specific capacitance close to thetheoretical value and good rate capability. While, the massloading of MnO2 in most of the reports is limited below1 mg cm�2, which is meaningless to flexible SCs at a practicallevel since it will result in a super low volumetric capacitance.Moreover, the interfacial problems between the MnO2 and theconductive support are still rarely investigated. Due to theintroduction of oxygen vacancies into MnO2, the electronicconductivity is enhanced significantly. However, the concen-tration of oxygen vacancies cannot be introduced controllably.In particular, there is still lack of a facile, efficient method tointroduce oxygen vacancies into bulk MnO2 instead of just thesurface. Given these unresolved problems or challenges, thefollowing aspects might be possible ways to develop reallyapplicable MnO2 electrodes with ultrahigh energy density andpower density for flexible SCs:

1. The mass loading of commercial level (B10 mg cm�2) forMnO2 active materials has already been achieved via variousapproaches.17,102 However, the specific capacitance is still toofar from the theoretical value (B1370 F g�1), i.e., MnO2/Graphene gel/NFs (234 F g�1, 13.61 mg cm�2, 10 mV s�1),17

graphene–MnO2 composites (147 F g�1, 9.6 mg cm�2, 2 mV s�1),78

MnO2-CNT-textile (337 F g�1, 8.3 mg cm�2, 0.05 mV s�1),102 MnO2-PEDOT:PSS composites (196 F g�1, 8.5 mg cm�2, 0.5 mA cm�2),103

even comparable with commercial activated carbon. The inte-gration with high-surface area carbon or other conductivematerials is a common approach for MnO2 to achieve high

Fig. 13 (a–d) Schematics of the fabrication process of thin 3D porous graphite foams (GF) and GF based Co3O4/PEDOT-MnO2 core/shell nanowirearrays. (e) Specific capacitance of four positive electrodes as a function of current density (it should be noted that the capacitance of a positive electrodeis calculated based on the mass of the whole electrode). (f) Ragone plot of four positive electrodes. (g) Photos of the as assembled flexible asymmetricsupercapacitor device. Reprinted from ref. 101 with permission from Copyrightr 2014, American Chemical Society.


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capacitive performance at a high mass loading level. However,since the limitation of MnO2 with high mass loading is theelectronic transport in its phase, the intrinsic modificationshould be the efficient way. In this regard, further investigationmight better be focused on the oxygen vacancy introduction topursue fast electronic transport and high capacitance at a highmass loading level.

2. Introduction of oxygen vacancies, as one of the mostefficient methods to intrinsically improve the electronic con-ductivities, has drawn intensive attention in recent years.Various approaches including annealing in de-oxygen atmo-sphere, atomic or ion doping are adopted to introduce oxygenvacancies into the MnO2 active materials. Enhanced electro-chemical properties of MnO2 have been achieved by the oxygenvacancy introduction. However, these kinds of methods canonly introduce the deficiency into the surface area of MnO2.With respect to MnO2 at a high mass loading, the contributionof the oxygen vacancies is limited to the electrochemicalperformance of the MnO2 electrodes. Thus, further investiga-tions of new methods to introduce the oxygen vacancies intothe bulk or inactive phase of MnO2 might be valuable for MnO2

based flexible SCs.3. The mechanical properties of 3D conductive scaffolds are

still a major concern for their application in flexible SCs. Theflexible, wearable electronics have raised critical requirementssuch as stretch ability, flexibility and twist ability for the flexibleSCs and also the electrode. Nevertheless, the 3D scaffolds forMnO2 active materials in the current study involve the con-struction of 3D structures. Their flexibility is usually realized viaimpregnation of elastic polymers like PMMA,78 which willremarkably hinder the electron transport and prevent the porousstructure from the electrolyte ions. Therefore, new modificationsof existing 3D scaffolds such as 3D graphene foam or graphenehydrogel and/or construction of a new type of 3D scaffold need tobe developed.

Author contributions

The manuscript was written through the contributions of allthe authors. All the authors have given approval to the finalversion of the manuscript.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

We acknowledge the financial support of this work received by theNatural Science Foundation of China (21403306 and 21273290),Guangdong Province Natural Science Foundation for DistinguishedYoung Project (2014A030306048), Foundation for Youth InnovativeTalents in Higher Education of Guangdong (2014KQNCX003) andThe Research Foundation of IARC-SYSU (201408).

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