Flexible solid-state electrochemical supercapacitors

journal homepage: /locate/nanoenergyAvailable online at REVIEWFlexible solid-state electrochemical supercapacitorsPeihua Yang,Wenjie Mai nDepartment of Physics and Siyuan Laboratory,Jinan University,Guangzhou510632,ChinaReceived21April2014;received in revised form27May2014;accepted28May2014Available online12June2014KEYWORDSEnergy storage;Flexible;Solid-state;SupercapacitorAbstractWith the proliferation of microelectronic devices,the need for portable power supplies isevidently increasing.Possible candidates for micro energy storage devices are Li-ion batteriesand supercapacitors.Among them,theflexible solid-state supercapacitors combined withexceptionally long cycle life,high power density,environmental friendliness,safety,flexibilityand stability,afford a very promising option for energy storage applications.This paper reviewsflexible solid-state electrochemical supercapacitors and the performance metrics.A betterpractice by calculating released energy to evaluate material and device performance isproposed.In addition,an overview is given of the electrolyte and various electrode materialsthat are suitable forflexible solid-state supercapacitors.The review summarizes the recentresearch focusing on novel configurations offlexible solid-state supercapacitors for energystorage and applications,such as freestanding,asymmetric,interdigitated,andfiber-basedstly,some discussions on future research are presented.&2014Elsevier Ltd.All rights reserved.IntroductionDevices for the future electronics will be twistable anddeformable,thereby enabling applications that would beimpossible to achieve by using the hard,rigid electronics oftoday.Energy storage devices possessingflexibility,light weight,and even safety may meet the large proliferation of consumerelectronics.Sustainable energy research is attracting increasingattention as our planet facing enormous challenges related toenvironment and energy.Electrochemical capacitors,or namedas supercapacitors(SCs),combined with exceptionally longcycle life,very high power density and enhanced energydensity,afford a smart maneuver[1,2].Figure1a shows theRagone plot for the most important energy storage systems.SCsare used to provide high power in a short time,whereasbatteries(commonly Li-ion battery,LIB)are preferred for long-time operation of a pared with liquid electrolytebased SCs,solid-state SCs have many advantages,such as/10.1016/j.nanoen.2014.05.0222211-2855/&2014Elsevier Ltd.All rightsreserved.n Corresponding author.Tel.:+862085224386x315.E-mail address:wenjiemai@(W.Mai).Nano Energy(2014)8,274–290portability ,environmental friendliness,flexibility and stability ,which can broaden the application area of SCs [3,4].Therefore the need for solid-state SCs arises,which expedites the development of stand-alone microelectronic devices and enhance the applicability in the in vivo systems.T o enhance the power and energy density of these solid-state SCs new advanced concepts have been proposed,which are based on the exploration of new materials and architectures.In this review ,an overview will be given of various aspects of flexible solid-state SCs.Firstly ,a brief context will be outlined by illustrating the mechanism and performance metrics of SC electrodes and devices.Secondly ,the electrolyte of solid-state SCs will be discussed.In the third part of this review ,various commonly used flexible SC electrode materials will be described and compared.In the subsequent section,several cases of advanced concepts will be presented together .Although the present state-of-the-art flexible solid-state SCs does not yet allow a full comparison of the various approaches as only very few working devices have been demonstrated,it is interesting to highlight the advances that were already made and the remaining challenges that are still ahead.This may serve as a source of inspiration for further research and development in this interesting new scienti fic field.MechanismsIn general,SCs can be classi fied into electric double layer capacitors (EDLCs)and pseudocapacitors according to differ-ent charge storage mechanisms.EDLCIn EDLC,electrostatic energy storage is achieved by separa-tion of charges in a Helmholtz double layer at the interface between the surface of a conductive electrode and an electrolyte.The distance of the static separation of charges in a double-layer is on the order of a few Angstroms which is extremely small [5].The key to reaching high capacitance by charging the double layer is using high speci fic surface area (SSA)and electronically conducting electrodes [2].During thedischarge process,the voltage of an ideal EDLC decreases linearly ,but an ideal LIB remains constant (Figure 1b).As a result,the energy stored in ideal EDLC is proportional to the voltage squared,whereas the energy stored in ideal LIB is just proportional to the voltage.In addition,the voltage provides a convenient measure of the state of charge in EDLC,but not in LIB [6].In the condition of EDLC,carbon materials and their derivatives have been investigated and used in EDLC electrodes.The EDLCs generally store charges through surface absorption (up to 0.17–0.20electrons per atom at accessible surface),so they reveal high power density ,long cycle life but low energy density [1].PseudocapacitorIn contrast,in pseudocapacitors,electrochemical energy storage is associated with electron transfer ,which is Faradic in origin,involving the passage of charges across the double layer ,achieved by surface redox reactions with speci fically adsorbed ions from the electrolyte,intercalation of atoms in the layer lattice or electrosorption,underpotential deposition of hydrogen or metal adatoms in surface lattice sites which result in a reversible faradaic charge-transfer (ca.2.5electrons per atom at accessible surface)[1,7].Pseudocapacitors usually possess much higher energy density than EDLCs.The materials primarily for pseudoca-pacitor mainly are metal oxides/nitrides/sul fides and con-ducting polymer .For instance,the pseudocapacitive behavior of RuO 2is generally ascribed to a series of fast,reversible electron-transfer reactions that are coupled with adsorption of protons at or near the electrode surface (Figure 1c)[5].Performance metricsSingle electrodeFor a single electrode,its speci fic capacitance,C s (F/g)can be calculated from the charge –dischargecurves:Figure 1(a)Speci fic power density against speci fic energy density,also called a Ragone plot,for various electrical energy storage devices.Reproduced with permission from [2].Copyright 2008,Nature Publishing Group.(b)Difference of charge and discharge relationships for a capacitor and a battery.Reproduced with permission from [1].Copyright 1999,Springer Science +Business Media New York.(c)Schematic of charge storage via the process of electrochemical double-layer capacitance and pseudocapacitance.Reproduced with permission from [5].Copyright 2011,Cambridge University Press.275Electrochemical supercapacitorsC s ¼I Δt ð1Þwhere I is the discharge current,Δt is the discharge time,ΔU is the potential window,and m is the active material mass of the electrode.For the electrodes under the same testing condition (the same mass,potential window,and discharge current),a longer discharge time usually repre-sents a larger speci fic capacitance of the material.C s can also be calculated from the cyclic voltammogram (CV)curves:C s ¼Q m ΔUð2Þwhere Q is the average of the absolute values of charges during the charging and discharging processes,and ΔU is the potential window [8].Because the enclosed area S in the CV loop can be obtained by using:S ¼I I d U ¼I Iv d t ¼v I I d t ¼2vQ ð3Þwhere v is the potential scan rate,the enclosed area in the CV curve is proportional to the average charge Q at a certain scan rate according to the equation.So Eq.(2)can be replaced by:C s ¼S 2vm ΔUð4ÞIt can be seen that for the electrodes under the same testing condition (the same mass,potential window,and scan rate),a bigger CV loop usually means a larger speci fic capacitance of the material.For CV curves with a rectangular shape (common for EDLC,Figure 2a),becauseS ¼2I ΔU ð5Þwhere I is the constant current under the potential scan rate.Eq.(4)can be simpli fied to:C s ¼I mvð6ÞIn this condition,Eqs.(1)and (6)are similar ,but the speci fic capacitances (C s )calculated by charge-discharge curve and CV curve can be different due to their different current densities.In addition,discussions above,also commonly used in many publications [2,8–10],actually assume that the capa-citance is constant within the entire voltage window.However ,in reality,the situation can be very complicated and the capacitance can change signi ficantly at different voltage range (Figure 2b and d).Here,we propose a new metrics for evaluating performance of SC device from the energy point of view and discuss it in detail in the part named “Better practice for evaluating SC performance ”.DeviceA typical SC unit cell is a charge-storage device similar to batteries in design and manufacturing.As shown in Figure 3,an SC device is comprised of two electrodes (active materials on current collectors),an electrolyte,and a separator that electrically isolates two electrodes [11].Figure 3shows that the SC device can be treated as two capacitors in series,each of which represents one elec-trode.If the capacitances of the two electrodes,i.e.positive and negative,are constant and can be expressed as C p and C n ,respectively,the overall capacitance (C )oftheFigure 2Electrochemical characteristics distinguish an ideal and a practical SC.CV curves of an ideal SC (a)where the current is constant (I )and a practical SC (b)with variable current.(c)Galvanostatic charge and discharge behavior for an ideal SC is linear and symmetric,but (d)it can be distorted for a practical SC.P .Yang,W.Mai276SC device can be expressed as:1C ¼1C p þ1C nð7ÞIdeally,for symmetrical electrochemical capacitors,the two capacitor electrodes in Figure 3are identical (C p =C n ).However ,because of the capacity of the pseudocapacitor electrode is generally many times greater than that of the electric double layer capacitor electrode,the pseudocapa-citor electrode provides options to construct asymmetric electrochemical capacitors with enhanced energy densities [5].In this case,capacitances of two electrodes are different (C p a C n ).The operating voltage of the asym-metric design can be larger because the two electrodes have different working potentials.For this purpose,it must be considered that the charges at positive (q +)and negative (q À)electrodes should be balanced [12–14].q ¼C s m ΔU ð8Þq þ¼q Àð9Þm þm À¼C s ÀΔU ÀC s þΔU þð10Þwhere m +,C s +,ΔU +and m À,C s À,ΔU Àare the mass,speci fic capacitance,and the previous working potential window of the positive electrode and negative electrode,respectively.In some speci fic cases,the parameter m can be replaced by area (C s should be substituted by areal capaci-tance at the same time)for practical application.The most important component in an SC is the electrode material.Although the research to improve the electrode performance of nanomaterials has dramatically increased [15],it is still controversial for accurately determining a material's performance for use as an SC electrode.Various techniques and experimental procedures are currently being employed leading to wide variations in reported results.Recently,Stoller et al.[16]and Gogotsi et al.[17]discussed practice methods that will enable the more accurate determination and reporting of an SC device performance.Symmetric SCWhen a symmetric SC is charged,a voltage will build up across the two electrodes.The capacitance (C ,F)of the SC device is usually calculated from their charge –discharge curves:C ¼I d U =d tð11Þwhere I is the discharge current,t is the discharge time and U is the device potential.The value of d U /d t can be calcu-lated from the slope of the discharge curve,only when the curve is linear [16],and the Eq.(11)can be simpli fied to:C ¼I Δt ΔUð12Þwhere Δt means the discharging time from fully chargedvoltage U to zero.For non-linear curve (Figure 2d),we will discuss them in detail in the part named “Better practice for evaluating SC performance ”.For an ideal symmetric SC,the speci fic capacitance,C s (F/g)for the active material can be derived from the capacitance of the device:C s ¼4C Mð13Þwhere M is the total mass of active materials in an SCdevice.The multiplier of 4adjusts the capacitance of the device and the combined mass of two electrodes to the capacitance and mass of the active material in a single electrode [16].The energy density (E ),maximum power density (P m )and average power density (P a )of an ideal SC device can be expressed as:E ¼CU 22M ð14ÞP m ¼U 2s ð15ÞR s ¼U IR ΔI ¼U IR2I ð16ÞP a ¼E ð17Þwhere C is capacitance calculated before,U is the cell voltage,U IR is the IR drop,ΔI is the current change at charging/discharging switch,Δt is the discharge time,and R s is the equivalent series resistance and can be obtained from the fitting of IR drop vs current [18].If replaced M ,the weight of the active material with the total weight of the real device,you can obtain the overall energy densityandFigure 3Principles of a single-cell double-layer capacitor and illustration of the potential drop at the electrode/electrolyte interface.Reproduced with permission from [11].Copyright 2000,Elsevier .277Electrochemical supercapacitorspower density of the real SC,which can be much lower due to considering the weight of current collector ,electrolyte,separator ,etc .If the volume or area is more important for the targeted application,the electrode material's volume or area may be substituted for mass [17].It can be seen that both energy and power densities are proportional to the square of voltage;therefore,increasing the voltage may be another effective method to improve the SC's energy and power densities.To increase the SC's device voltage within the electrolyte's stability window,selecting electrode materials and optimizing device architectures can achieve high cell voltages.Asymmetric SCUsually ,the asymmetric SC involves the pseudocapacitor ,so in most cases,the CV curve has no longer rectangular shape and the discharge part in charge –discharge curves is no longer linear either .This means that C is not a constant during operation.The proper parameter to represent the actual performance of the supercapacitor becomes controversial.The capacitance calculation for pseudocapcacitors with non-linear charge –discharge curves will be much more complicated than that for symmetric carbon-based capaci-tors with linear charge –discharge curves.If people still use Eq.(12),it will cause large confusion.For example,using Eq.(12),the linear ,concave and convex discharging curves (Figure 4)with the same starting and ending points of three different devices will give the same values of C .This convex kind of discharging curves can be seen in literatures [19,20].In fact,for non-linear curves,d U /d t varies with voltage,which means the C is no longer a constant during charging or discharging.Some researchers suggested,especially for pseudocapacitive devices,it is important to calculate capacitance using the typical operating voltage range for the application that the device will be used.A better characterization of an asymmetric SC device is to calculate the capacitance based on the usable voltage operating range of the device.For practical applications of hybrid devices,most SCs will be operated in the range of U max to 1/2U max instead of be completely discharged,so some researchers suggested calculating the capacitance based on the voltage range between the U max and approximately 1/2U max in discharge curve [16,21].In this regard,a practicalmethod using two data points from the discharge curve is recommended:d U d t ¼U max À12U max T max 12T maxð18Þwhich reduces the impact from the varying capacitance,and it is practically realizable for an actual application.Indeed,C obtained using this practice can distinguish the different performances of the linear ,concave and convex discharging curves with the same starting and ending points of three different devices in some degree.Better practice for evaluating SC performanceIn this part,we are trying to propose a better practice for evaluating SC performance directly from energy point of view.The charges stored in three different devices are the same in Figure 4,but people can see they are obviously different in performance.It is not reasonable to continue to use Eq.(12)because the convex curve implies holding much more energy than the concave curve.Previously,people were not using clear and proper metrics to indicate such difference.Based on the discharge curves in Figure 4,the total energy that can be released from the device during discharging can be expressed as:E t ¼Z U d Q ¼Z UI d t ¼I Z U d t ¼IS ð19Þwhere I is the discharge current and S is the enclosed area of discharge curve and coordinate axis.The charges stored in three different devices are the same in Figure 4,but they actually contain different energy (E t 3o E t 2o E t 1).Energy instead of the varying capacitance is the indicator re flecting the true performance of SC.Therefore,we propose here that total stored energy instead of the capacitance should be calculated using Eq.(19)to evaluate the performance of the SC device or even SC electrodes in the future.People may choose upper and lower integral limits to calculate the usable stored energy according to the preferred working potential windows of the device.This practice should be better than previous ones and applicable to both electrode materials and devices.However ,because people would prefer to directly use speci fic capacitance and capacitance to evaluate electro-chemical performance of materials and devices,respec-tively,these two values can be deduced backward.For example,to calculate an effective device capacitance (C eff )within some de fined working potential window (e.g.U max to 0),firstly the E t should be obtained by using Eq.(19),and then C eff can be obtained from:E t ¼1C eff U2ð20ÞIt should be noted that the calculated C eff will be different if the device is operated at other working potential windows (for example,from U max to 1/2U max ).We believe many researchers have considered the above issue in the charge –discharge curves during their researches,but to the best of our knowledge,this is the first time for this method to be explicitly proposed.Because thethirdFigure 4Different discharging curves with the same startingand ending points.The charge released from the devices is same,but the total energy released is different (E t 3o E t 2o E t 1).P .Yang,W.Mai278kind of curves (Figure 4)are much more frequently observed,people would obtain a smaller value of capaci-tance using our method from energy perspective than using the previous method from charge perspective.Similarly,the total energy can be obtained in CV dis-charged part by using equation:E t ¼Z P d t ¼Z IU d t ¼1v ZIU d U ð21Þwhere P is instantaneous power ,I is the discharge current,U is the voltage,and v is the potential scan rate [22].Actually,it is relatively complicated to use Eq.(21),so Eq.(19)is recommended for calculating the released total energy in SC.ElectrolytesElectrolytes for solid-state SC can be classi fied into differ-ent types.The first type is aqueous polymeric gels [23].Polyvinyl alcohol (PVA)/H 3PO 4proton conducting polymer blend was made as a pioneer and the ionic conductivity could reach at 2.2Â10À5S/cm [24].Since then many types of gel electrolytes have been explored to satisfy the speci fic condition,for instance,PVA/H 2SO 4[25],PVA/KOH [26],polyethyleneoxide (PEO)/LiClO 4[27],poly(methylmethacry-late)(PMMA)/LiClO 4[28],polybenzimidazole/H 3PO 4[29],PVA/Na 2SO 4-silica [30],PVA/NaNO 3[31],PVA/LiCl [32],and so on.Generally,the neutral electrolytes like PVA/LiCl have a wider range of applications in pseudocapacitors,espe-cially for metal oxides based SCs (ZnO [33],V 2O 5[34],etc .).Another example is Na fion,a typical per fluoro-alkylsulfonate-based ionomer .Na fion can not only be used as membrane,but also as effective ionomer [35–37].An all-solid-state flexible SC prepared through the assembly of Na fion-functionalized reduced graphene oxide (rGO)thin films and solvent-cast Na fion electrolyte membranes showed a 2-fold higher speci fic capacitance and rate cap-ability compared to those of all-solid-state graphene SCs [38].The second type of electrolytes is nonaqueous polymeric gels [23].They consist of either plasticized polymer com-plexes with electrolytic salts or polar polymer matrices swollen with organic electrolyte solutions,and have been many combinations of polymer –salt –liquid systems so far proposed as the gel electrolytes of SCs.As an example,a polymeric gel electrolyte composed of PEO-modi fied PMMA swollen with propylene carbonate (PC)that dissolves tetra-ethylammonium tetra fluoroborate (TEABF 4)was success-fully applied in an all-solid-state EDLC system [39].Because the organic solvents such as acetonitrile or PC can operate under much higher potential windows,the cell voltage can reach 2.7–3.7V [40].According to Eq.(14),the energy density of SC device will be greatly increased.The third type is mainly inorganic solid materials.A Li 2.94PO 2.37N 0.75(Lipon)electrolyte film can be applied between ruthenium oxide electrodes [41].An all-solid-state thin film SC was fabricated with tungsten co-sputtered ruthenium oxide electrodes and Lipon electrolyte [42].The effective reaction of the solid-state SC can be presented by RuO 2+x Li +(from the Lipon)+x e À2Li x RuO 2.It was shown that the room-temperature charge –dischargebehavior of the thin film SC is similar to that of a bulk-type SC.If the SC was fabricated on flexible substrate,an excellent device with flexibility and durability SC device will be realized.Other inorganic electrolytes such as phosphotungstic acid/Al 2(SO 4)3Á18H 2O [43],and Li 2S –P 2S 5[44]have wide application in solid-state SC systems as well.In addition,the solid-state electrolytes for LIB can work as useful reference,which promote the research of solid-state electrolytes in SC systems [45].It is discovered that ionic conductivity of the electrolyte is improved when added mediators.For example,a quasi-solid-state SC was assembled by using PVA/KOH –KI as gel electrolyte and activated carbons electrodes [46].The introduction of KI increases the ionic conductivity of electrolyte,and improves the pseudocapacitance of the electrode,increasing by 74.28%compared to the PVA/KOH system at the same current density.Electrode materialsCarbon materialsHigh surface area carbon materials mainly include activated carbon [47,48],porous carbon [49],carbide-derived carbon [50],onion-like carbon [22],carbon aerogels [51],carbon nanotubes (CNTs)[52,53],carbon shell [54],graphene [55],and graphene quantum dots [56].CNT ,especially the single-walled carbon nanotube (SWCNT),has intrinsically excellent properties as active materials such as high SSA,high conductivity,high flexibility,regular pore structures,and electrochemical stability.Woong et al.fabricated all-solid-state flexible SCs using CNTs,regular of fice papers,and ionic-liquid-based silica gel electrolyte [57].Kaempgen et al.designed a kind of flexible electrodes made by coating CNTs on of fice papers by a drop-dry method [58].The maximum power and energy density of the SCs were 164kW/kg and 41Wh/kg,respectively.SWCNT network film printed on the flexible plastic substrate (polyethylene terephthalate,PET)was demonstrated as an easy-fabrication SC that showed very high energy and power densities.The SWCNTs formed an entangled random net-work on the PET (Figure 5a).The SWCNT networks and the gel electrolyte were sandwiched together constructing an SC device (Figure 5b).CV and charge –discharge curves showed good electrochemical stability and capacitance in the SWCNT networks (Figure 5c,d).Graphene-based materials are attractive because of their excellent mechanical and electrical properties as well as exceptionally high surface area [59,60].Recently,the intrinsic capacitance of single-layer graphene was reported to be $21μF/cm 2[61];this value now sets the upper limit for EDLC capacitance for all carbon-based materials.A direct laser reduces the graphite oxide (GO)to laser-scribed graphene (LSG)films,which are mechanically robust,show high electrical conductivity (1738S/m)and high speci fic surface area (1520m 2/g)[62].In order to evaluate the performance of the all-solid-state LSG-SC for flexible energy storage,a device placed under different bending conditions is shown in Figure 6.These SCs can be bent arbitrarily without degrading performance.Modi fica-tion or composite of graphene and other capacitive279Electrochemical supercapacitorsmaterials may be a good approach into the rational design and fabrication of all-solid-state flexible SCs based on graphene materials.Graphene aerogels (GAs)represent a new class of ultra-light and porous carbon materials that are associated with high surface-area-to-volume ratios.A simpli fied prototype device of high-performance all-solid-state SC based on three-dimensional nitrogen and boron co-doped monolithic gra-phene aerogels was achieved with using PVA/H 2SO 4gel electrolyte [25].The fabricated device exhibited not only minimized thickness,but also showed high speci fic capaci-tance (E 62F/g),good rate capability ,enhanced energy density (E 8.65Wh/kg)and power density (E 1.6kW/kg).Three dimensional graphene exhibits exceptional electrical conductivity and mechanical robustness,making it an excel-lent material for flexible energy storage devices.Xu et al.reported with a 120μm thick graphene hydrogel thin film flexible SC exhibited excellent capacitivecharacteristics,Figure 6(a)A schematic diagram of the all-solid-state LSG-SC.Inset is a digital photograph showing the flexibility of the device.(b)A comparison between performances of LSG-SC using gelled versus aqueous electrolytes.(c)Bending the device has almost no effect on its performance,as seen in these CVs collected at a scan rate of 1000mV/s.Reprinted with permission from [62].Copyright 2012,American Association for the Advancement ofScience.Figure 5(a)Scanning electron microscopy (SEM)image of as-deposited SWCNT networks.(b)Thin film SC using sprayed SWCNT films on PET as electrodes and a PVA/H 3PO 4based polymer electrolyte as electrolyte.(c)CV curves and (d)galvanostatic charge –discharge curve measured with a current density of 1mA/cm 2(30mA/mg)of an SWCNT SC device.Reprinted with permission from [58].Copyright 2009,American Chemical Society.P .Yang,W.Mai280including a high specific capacitance of186F/g,an unpre-cedented areal specific capacitance of372mF/cm2,low leakage current(10.6μA),excellent cycling stability,and extraordinary mechanicalflexibility[63].Porous graphene(PG)on carbon cloth via an electro-phoretic deposition process was used as electrodes for flexible all-solid-state SCs[64].The macroscopic porous morphology of carbon cloth is favorable for the ion diffusion and electron transport.The excellent mechanical stability andflexibility of PG on carbon cloth ensure the device with goodflexibility.The resultantflexible SCs showed high specific capacitance,good cycling stability,and enhanced energy density and power density(1.64Wh/kg and0.67kW/ kg).Another type of carbon,exfoliated graphite(EG),is proposed as an electrode material for solid-state electro-chemical capacitors[65].The EG-based capacitors assembled with a solid electrolyte show high areal capaci-tance in the range of0.74–0.98mF/cm2.Metal oxides/nitrides/sulfidesIn general,metal oxides such as ruthenium oxide(RuO x), manganese oxide(MnO x),nickel oxide(NiO x),cobalt oxide (CoO x),iron oxide(FeO x)and titanium oxide(TiO2),can provide higher energy density in SC applications than conventional carbon materials[18].They not only store energy like electrostatic carbon materials but also exhibit electrochemical faradaic reactions between electrode materials and ions within appropriate potential windows. In thefield of metal oxide inflexible solid-state SC,RuO x and MnO x are mainly discussed here.Among the transition metal oxides,RuO x has been themost extensively studied candidate due to its wide potentialwindow,highly reversible redox reactions,metallic typeconductivity,remarkably high specific capacitance,goodthermal stability,long cycle life,and high rate capability[66].As a typical pseudocapacitive material,solution-basedbinder-free synthetic approach of RuO2films enabled an all-solid-state SC.The all-solid-state nature combined withsuperior electrochemical performance makes RuO2-baseddevice an excellent candidate for power sources in portableelectronics[67].In another example,an all-solid-state SCusing Nafion solid polymer electrolyte membrane and RuO2combined Nafion ionomer electrode[68].The SC exhibits arelatively stable capacitance during10,000cycles of opera-tion(about30%loss).This implies that RuO2based all-solid-state SC with a Nafion membrane can be operated withacceptable stability.However,the high costs of RuO x haveprompted the search for less-expensive metal oxides thatcan also display pseudocapacitance.In comparison,MnO x is often considered as the promisingtransition metal oxide for pseudocapacitors due to its hightheoretical specific capacitance(1100–1300F/g),low cost,environmental benignity and abundant resource[8,69,70].To overcome the drawback of poor ionic conductivity,MnO xwas usually composited with CNTs,graphene,and conduc-tive scaffold[71–74].Yuan et al.reported a highlyflexiblesolid-state SC based on a carbon nanoparticles(CNPs)/MnO2nanorods hybrid structure using PVA/H3PO4electrolyte(Figure7)[75].The device exhibited good electrochemicalperformance with an energy density of 4.8Wh/kg at apower density of14kW/kg.Inspired by this work,Yanget al.developed a new kind of worm-like amorphous MnO2 Figure7(a)Optical photographs of the fabricated solid-state SC device.(b)CV curves for a CNPs/MnO2nanorod SC at different curvatures.(c)Galvanostatic charging–discharging curves for CNPs/MnO2nanorod solid-state SC at different current densities.(d)Cycling stability of a CNPs/MnO2nanorod SC device over10,000cycles.Reprinted with permission from[75].Copyright2012, American Chemical Society.281 Electrochemical supercapacitors。