Current Technology of Supercapacitors: A Review

PRIYANKA SHARMA 1,2 and VINOD KUMAR1,3

1.—Department of Physics, Chandigarh University, Gharuan, Mohali, Punjab 140413, India.2.—e-mail: priyankaphysics2@gmail.com. 3.—e-mail: vinod.kumar@cumail.in

A supercapacitor is a solid-state device that can store electrical energy in theform of charges. It represents an advancement in the field of energy storage,as it overcomes many of the shortcomings of batteries. This paper presents anoverview of the various types of supercapacitors, electrode materials, andelectrolytes, and the future of supercapacitors. Due to their high storagecapacity, supercapacitors are commonly used in portable electronic devicessuch as MP3 players and mobile phones, and in hybrid vehicles and otherapplications. In electrical and hybrid vehicles, supercapacitors are increas-ingly used as provisional energy storage for regenerative braking. Variousmaterials are used in electrodes to boost the performance of the supercapac-itor. This review presents details regarding the materials and electrolyte, andthe improvements in the field of supercapacitors.

Key words: Power density, energy storage system, hybrid vehicles,renewable energy sources, charging and discharging

INTRODUCTION

Energy storage systems play a significant role inour world.1,2 Specific energy, specific power, life-time, dependability, and protection are the mostimportant parameters when selecting an energystorage device.3 Most of our energy needs are met byfossil fuels, which are very harmful to the environ-ment. Solar and wind energy, on the other hand, arevery clean and abundant renewable energy sources.However, the irregular properties of wind andsunlight are the main reason for power variationsin wind rotors or stators and solar setups, so it iscritical to develop ways to acquire the power foruse,4 which creates the need for good storagedevices. Many countries store large amounts ofwater for hydroelectric energy. However, it has beenfound that the construction of large dams can affectthe earth’s rotation and the displacement of poles.For examples, a huge dam with three power gener-ators was recently constructed in China. Because ofthe enormous mass of water stored in the dam, itwas discovered that it caused a change in the earth’s

rotation and a shifting of the poles by 1 cm. Theconventional energy storage medium is lead-acidbatteries. These suffer from low energy density,high self-discharge, limited charge–discharge cycle,and short life. Also, they contain poisonous lead andharmful acids. Lithium-ion batteries are used inportable electronic devices such as mobile phones,tablets, and laptops. Batteries can store largeamounts of energy, but they also suffer from severalshortcomings including self-discharge, short life-time, weight, and temperature sensitivity.

In the case of batteries, chemical combustiontakes place, and the energy from this reaction canbe stored. When load resistance is connected acrossthe terminals of a battery, electrical energy isreleased. This allows the electrode material torespond electrochemically with the ions necessaryfor the reactions to occur through the electrolyte inwhich the electrodes are dipped. The functionalenergy is stored in the battery, and it can beindicated by VQ, where V is the voltage of the celland Q is the charge on the electrode shifted to theload in a chemical reaction. A supercapacitor—alsoknown as an electrochemical capacitor—is an elec-trical energy storage device that is assembled muchlike a battery.5 Many researchers have discussedthe structure of the various capacitors in their(Received September 11, 2019; accepted January 31, 2020;

published online March 12, 2020)

Journal of ELECTRONIC MATERIALS, Vol. 49, No. 6, 2020

https://doi.org/10.1007/s11664-020-07992-4� 2020 The Minerals, Metals & Materials Society

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research papers.6 Most of the problems of tradi-tional batteries can be eliminated by using super-capacitors. Supercapacitors can be charged very fast(in seconds). They also have high efficiency, longlife, light weight, and good power density. Most ofthe common problems are resolved by using thisdevice.7 The positive and negative electrodes areeach coated with different materials such as carbonor graphene. This layer allows only the movement ofions and avoids electric contact. Supercapacitorsstore electrical energy in the form of chargescommonly due to the formation of the double-layercapacitor structure at the boundary between theelectrodes and the electrolyte. This energy storageprocess includes no chemical reaction or chemicalcombustion, apart from fast and reversible faradaicreactions or charge transfer reactions on the surfaceof the electrode, and also gives the value of capac-itance. The properties of electrostatic charge trans-port result in a high degree of recyclability.8 A highcapacitance value is dependent on the area, i.e., thelarger the surface area, the greater the capacitance,and is also determined by the electrode materialsthat are used in the supercapacitor and theirproperties (e.g., conductivity and porosity).Advanced electrode materials have been an area ofmuch study, and the latest progress was periodi-cally reviewed in.9,10 With the exception of biofuels,renewable energy is largely provided as electricity.There are various practical phases for electrochem-ical storage, including batteries, fossil fuel, andelectrochemical supercapacitors, that are very use-ful for the development of storage systems.11,12 Theperformance of supercapacitors depends on theelectrolyte, electrode, and separator used in thedevice. Different parameters play different roles inthe working of batteries, capacitors, and superca-pacitors. First, with regard to weight, batteries aremuch heavier than capacitors and supercapacitors,and so they can be difficult to employ. The secondparameter is the charge method. This charging and

discharging can be done when the voltage is appliedto the terminals of the battery. The power density ofsupercapacitors is much larger than both batteriesand conventional capacitors, and both conventionalcapacitors and supercapacitors discharge rapidly(Table I). The energy density of batteries is greaterthan that of capacitors, but the lifetime of batteriesis shorter. Another advantage of supercapacitors isthat they are environmentally friendly, as there isno need for chemical combustion, unlike batteries.Different types of supercapacitors, namely electro-chemical double-layer capacitors (EDLCs), pseudo-capacitors, and hybrid capacitors, have differentcharacteristics. EDLCs have low energy density,but their power density is higher than that ofpseudocapacitors. Supercapacitor life cycle isanother important parameter. Generally, capacitorstability testing involves subjecting the device to anumber of charge and discharge cycles and thencomparing the capacitance before and after the fieldcycling stress.13

A conventional capacitor stores energy in the formof electric charge. Capacitors can be divided intothree types. The first and simplest capacitor is anelectrostatic capacitor with a dry separator.7 It hasvery low capacitance and is used in frequencytuning and filtering.7 The size of electrostaticcapacitors varies from picofarads to microfarads.The second type is the electrolytic capacitor, whichhas higher capacitance than an electrostatic capac-itor.7 A wet separator is used between the twoelectrodes in these capacitors, and they are used forfiltering and buffering of the signals. The last type isthe supercapacitor, with capacitance measured infarads, thousands of times that of an electrolyticcapacitor (Fig. 1).7 Supercapacitors are one of thecomponents in power electronics for renewableenergy systems. Because of their excellent proper-ties, including high electrical capacitance, superca-pacitors are very important devices for electricalenergy storage. Supercapacitor technology is an

Table I. Comparison between batteries, capacitors and supercapacitors

Parameters Batteries Capacitors Supercapacitors

Weight Large weight (10 gto> 10 kg)

Lower weight (1–100 g) Lower weight (1–2 g)

Charge method Current and voltage Voltage across terminals i.e. from abattery

Voltage across terminal i.e. from abattery

Power delivered Constant voltage over longtime period

Rapid discharge, linear orexponential voltage decay

Rapid discharge, linear orexponential voltage decay

Charge/dis-charge time

Large Less Very less

Lifetime 150–1500 cycles > 100 k cycles > 100 k cyclesChemical reac-

tionsChemicals are required No chemicals required No chemicals required

Temperaturesensitive

More temperature sensitive Excellent temperature performance Excellent temperatureperformance

Current Technology of Supercapacitors 3521

area where there is still a need for better under-standing of the processes involved and impact withregard to the present information.14

The performance of a supercapacitor depends onvarious factors including electrode material, elec-trolyte, and separator. The material should havehigh capacitance values and a large surface area.Electrodes used for commercial supercapacitors aregenerally based on activated carbon material, whichis an important material in this technology becauseof its low density, and carbon electrodes can befabricated with a very large exposed surface area.The fabrication and commercialization of superca-pacitors has been undertaken by several companies,but further development of the manufacturingtechnology is still needed.14 Supercapacitors areideal for any application having a short load cycleand high dependability requirements, such asenergy recapture sources including load cranes,forklifts, and electric vehicles. Other applicationsexploit a supercapacitor’s ability to nearly instan-taneously absorb and release power, such as factorypower backup.15,16

SUPERCAPACITORS

Supercapacitors are used to store energy and areuseful to some extent compared with batteries. Theflexibility of supercapacitors, the materials used forthe electrode and electrolyte, and the charge storageprocess all govern supercapacitor performance. Inorder to address the problems related to the highconsumption of unsustainable fossil fuels, the devel-opment of new economical and environmentallyfriendly energy sources is critical.17,18 However,energy storage resources are not sufficient for largestorage. As a result, there has been a growinginterest in high-power and high-energy-densitystorage systems.19

Energy storage systems (ESSs) are very impor-tant in dealing with the unstable environment ofrenewable energy sources and increasing the powertransferred into the system from sources such aswind and solar power.19 The energy density ishigher in the case of EDLCs than with conventionalcapacitors due to their large surface area cross-

section and very small charge separation distances.Because of this, they can attain high capacitancevalues. In the current period, supercapacitors havedeveloped as viable energy storage devices thatovercome most of the problems found in traditionalbatteries. Supercapacitors are used in many appli-cations such as electric buses and renewable energysystems.5 Supercapacitors are rated in farads; anominal value available is 500 F. The charging timeof a supercapacitor is very low.6 New research isbeing conducted on electrolyte and electrode mate-rial, which is yielding improvements.

Types of supercapacitors

1. Electrochemical double-layer capacitor2. Pseudocapacitor3. Hybrid capacitor

Electrochemical Double-Layer Capacitors(EDLCs)

These capacitors store energy electrostatically. AnEDLC has no dielectric. There are two electrodesand an electrolyte. In a conventional capacitor

C ¼ e0era =d: ð1Þ

Experimentally, capacitance can be calculatedfrom the difference for the relation

C ¼ T=R ð2Þ

where C is the capacitance, e0 and er are theabsolute permittivity for air and permittivity ofthe dielectric medium used between the two plates,a is the area of the electrodes, d is the distancebetween two electrodes, R is resistance, and T is thetime taken to discharge the capacitor (Fig. 2).

According to Eq. 1, capacitance is proportional tothe surface area of the electrodes and inverselyproportional to the gap between the electrodes.Therefore, if a material has a large surface area, itgenerates large capacitance. The materials used inthe fabrication of the electrode play an importantrole in achieving high capacitance. Activated carbon

Fig. 1. Structure of capacitors: (a) dry capacitor, (b) electrolytic capacitor, (c) supercapacitor

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is has become an important electrode material givenits large surface area, which enables the EDLC toachieve high capacitance.20 Absorption and desorp-tion of ions on the surface layers contribute tocharge and discharge.20 The high surface-to-volumeratio arises from the porosity. The pore size shouldbe roughly twice the ion size to allow coverage of thepore walls. There is no relationship between thespecific surface area and the specific capacitance.Therefore, an increase in surface area withincreased pore size does not mean that the capac-itance will also increase.21 When voltage is appliedto the electrodes, ions migrate to the surface of thedouble layer, and the capacitor is charged.20 Con-versely, ions are repelled when discharging a capac-itor. This is how the EDLC charges and discharges.In the electrolyte solution, ions are diffused throughthe membrane or layer into the pores of theelectrode having an opposite charge. However, theelectrode prevents the recombination of positive andnegative charges, as shown in Fig. 2. Thus, chargeis generated on both electrodes. The double layersare joined, resulting in increased surface area andreduced distance between electrodes, permittingEDLCs to achieve much greater strength densitythan conventional capacitors.15,16 The performanceof a supercapacitor depends on the type of elec-trolyte. Aqueous and organic electrolytes are typi-cally used. The choice depends on the electrodes andcharge collectors.

Pseudocapacitors

The performance of a supercapacitor depends ontwo types of energy storage: electrostatic attractionand faradaic reactions. In this capacitor, a transi-tion metal oxide or conducting polymer is used aselectrode material.6 These capacitors can storeenergy faradaically through the transfer ofcharges. These faradaic processes also permit

pseudocapacitors to achieve much higher capaci-tance and electric current density than EDLCs.Electrodes with pseudocapacitance are able toincrease on charging and shrink on discharging,which is the main reason for poor mechanicalstability and short cycle life in capacitors.19 Theneed to develop high-energy supercapacitors whilemaintaining high power and long cycle life hasgiven rise to numerous reports on the combinationof rechargeable batteries and supercapacitors forcreating electrode materials with new reactionmechanisms for ultrafast charge/discharge and highperformance. Through the physical control of elec-trode materials, scientists have discovered theexistence of a pseudocapacitance contribution insome electrode materials for metal-ion batteries,which is referred to as intercalation pseudocapaci-tance.21 There are two types of materials used in apseudocapacitor to store energy: conducting poly-mers and metal oxides.

Conducting Polymers

Conducting polymers are one of the best materialsfor use as electrodes, given their high capacitanceand conductivity and low cost compared with car-bon-based electrode materials.22 A particularlyattractive polymer configuration is the n-p-type,which has one negatively charged (n-doped) and onepositively charged (p-doped) electrode. This con-ducting polymer offers greater capacity for energystorage and higher strength density. However, n-doped conducting polymer materials have preventedthose pseudocapacitors from reaching their capabil-ity.23 Also, it is clear that the mechanical stress onconducting polymers at some point of redox reac-tions affects the stability of these pseudocapacitorsthrough many charge–discharge cycles. This lowcycling stability has impeded the development ofconducting polymer pseudocapacitors (Fig. 3).

Fig. 2. Diagram of electric double-layer capacitor (type ofsupercapacitor)

Fig. 3. Schematic diagram of pseudocapacitor

Current Technology of Supercapacitors 3523

Metal Oxides

Metal oxides have also been investigated as apromising electrode material for pseudocapacitors.Among these, ruthenium oxide has been found toexhibit excellent properties. Capacitance is attainedvia the intercalation (insertion and removal) ofprotons into its amorphous structure. In its hydrousform, ruthenium oxide can achieve higher capaci-tance than that of carbon-based and conductingpolymer materials, and the equivalent series resis-tance (ESR) is lower than that of other electrodematerials. As a result, higher energy and powerdensity is possible with ruthenium oxide pseudoca-pacitors compared with other EDLCs and conduct-ing polymer pseudocapacitors.23 However, metaloxide is very expensive. Therefore, alternativessuch as nickel or manganese oxide can be used forsupercapacitor electrodes to reduce costs. Goodperformance has been found with manganese oxide.

Hybrid Capacitors

Hybrids are distinguished by their constant elec-trical features at high frequency.24 Hybrid capaci-tors combine the properties of EDLCs andpseudocapacitors in one device, which can use bothfaradaic and non-faradaic processes. They also havehigher energy and power density than EDLCs.23

These capacitors are made of irregular electrodes.Lithium-ion capacitors are a good example of ahybrid capacitor. A� and Li+ are the two ions in theelectrolyte for charging and discharging (reaction),as shown in Fig. 4. Most studies thus far in Russiahave used nickel and lead oxides as materials forelectrodes with positive charge. Activated carbonmaterial is generally used for the fabrication ofnegative electrodes. Negative carbon electrodematerial is thicker than that of the positive bat-tery-like electrode, because the capacity per unitvolume of the carbon to store the charge is signif-icantly lower than that of the positive electrode

material. Hybrid capacitors fabricated in Russia arelarge, with relatively thick electrodes, and canachieve capacitance of 3000–15000 F/cell.5 In mostapplications, charge and discharge times are 10–20 min, with peak power density of approximately300 W/kg for high-efficiency discharge. Thus theirperformance resembles that of a battery rather thana supercapacitor. For devices using nickel oxide,energy density for charge/discharge varies accord-ing to the voltage range, from approximately1.5 W h/kg at 0.8–1.3 to 8–10 W h/kg for 0.8–1.6 V, with the significant increase in energy den-sity as a result of the pseudocapacitance in thecarbon electrode at higher voltages. Devices usinglead oxide for the positive electrode achieve approx-imately 10–20 W h/kg in a voltage range of 0.7–1.8 V.5 Some work has been reported in the case ofthin-film hybrid capacitors. The performance ofthese hybrid capacitors is dependent on the designand thickness of the electrodes. It is very difficult toaccurately determine the power density because ofthe contribution of resistance. Based on the lowresistance of thin-film electrodes using aqueouselectrolytes, the power density values lie withinsome kilowatts per kilogram, making them envi-ronmentally friendly. Various materials can be usedas the electrode in supercapacitors, including acti-vated carbon, activated charcoal, activated carbonfibers, carbon nanotubes, carbide-derived carbon,graphene, polymers, and oxides.6 Hybrid capacitorsare classified into three categories differentiated bytheir electrode alignment, i.e., composite, asymmet-ric, and battery-type, as discussed below.23 Appli-cations for hybrid capacitors include computerservers, security cameras, and backup devices.24

Composite

Composite electrodes are fabricated using poly-mers such as carbon nanotubes and polypyrroleconducting polymer or metal oxides. Carbon-basedmaterials facilitate a capacitive double layer ofcharge and also provides a high surface area thatincreases the interaction between the depositedpseudocapacitive material and electrolyte.23 Fara-daic reactions result in increased capacitance. Thuspseudocapacitive materials are able to enhance thecapacitance of the electrode through faradaicreactions.25

Asymmetric

The asymmetric electrode capacitor combinesfaradaic and non-faradaic phenomena through theconnection of an EDLC electrode with a pseudoca-pacitor electrode— in other words, combining thebattery-type and capacitor-type electrodes. Themain advantage of asymmetric electrode capacitorsis high energy and power density due to differentpotential changes in the two electrodes.23 Althoughconducting polymer electrodes have large capaci-tance values, they also have much lower voltage,

Fig. 4. Diagram of hybrid supercapacitor

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stability, and resistance. By coupling the two elec-trodes, asymmetric hybrid capacitors reduce theextent of this trade-off to achieve higher energy andpower density than similar EDLCs.25

Battery-Type

Battery-type electrodes feature a unique combi-nation of a supercapacitor electrode with a batteryelectrode. This configuration demonstrates the needfor both higher-energy capacitors and higher-powerbatteries, combining the properties of batteries andrecharging times of supercapacitors,23 and achiev-ing the properties of both supercapacitor and bat-tery in a single cell.26 Only a small number ofstudies have been carried out on battery-typehybrids. The existing data suggest that thesehybrids may be able to bridge the gap betweensupercapacitors and batteries.25

ENERGY STORAGE SYSTEMS

Energy is essential for survival, and thus devicesare needed that can store a large amount of backupenergy. Batteries and capacitors are capable ofbackup energy storage, but they have numerousdrawbacks. They pollute the environment due tointernal chemical combustion. The life of a naturalsource of energy is not long enough to providesufficient energy for survival. In the case of non-polluting energy sources, they can provide veryclean energy but are not constantly available. Theexploration of renewable energy sources such assolar, wind, and biofuels has expanded rapidly. Aheightened awareness of global climate change hasalso motivated various types of governmental ini-tiatives to promote the development of renewableenergy choices.13 Supercapacitors are able to handlehigh power rates comparable to batteries. Thepower obtained from supercapacitors is much lessthan that of electrolytic capacitors. These devicesare fascinating, as they are an intermediate devicebetween electrolytic capacitors and batteries whichare able to store energy in the form of charges.19

Electric double-layer capacitors (EDLCs), pseudo-capacitors, and hybrid capacitors can be illustratedbased on the storage process or mobile configura-tion. EDLCs are based on the high specific surfacearea of the electrode material, with active materialssuch as carbon or charcoal able to achieve highercapacitance values relative to conventional capaci-tors. Pseudocapacitors use conducting polymer ormetal oxide-based electrodes, and sometimes func-tionalized porous carbon, combining both electro-static and pseudocapacitive charge storageprocesses.19 Although supercapacitors providehigher power for a given volume,27 they are unableto store the same amount of charge as batteries,which are typically 3–30 times higher.27 For thisreason, supercapacitors are better suited for appli-cations in which a rapid high power charge isimportant but high energy storage capacity is not

required.19 A renewable electricity technology plantwith its equal energy storage device can act as aconsistent strength era plant. Capacitors storeenergy through the separation of charges. Energyis stored in a thin layer of dielectric material that issandwiched between metal plates that serve asterminals for the devices.5 In a battery, energy isstored chemically through the electrodes, andenergy is produced by connecting the load acrossthe terminals. Supercapacitors function the same asbatteries. Supercapacitors can work on the sameprinciple as capacitors and can be used to storeenergy. The only difference is that the charges arenot accumulated on the surface but on the conduc-tive interface of the electrolyte and electrolyte.28

Energy storage systems can be classified on thebasis of their applications in terms of ‘‘power’’ and‘‘energy density’’ as follows:

� Pumped hydroelectric energy storage� Superconducting magnets� Flywheels� Supercapacitors and batteries

ELECTRODE MATERIALS

The electrode material plays an important role insupercapacitor performance. The material shouldprovide high capacitance in order to achieve the bestperformance results for the supercapacitor. Thecapacitance of a supercapacitor is reliant on theeffective surface area of its electrode materials.However, not all of the effective area is fullyavailable for the electrolyte/electrode interaction,and thus the capacitance for electrode materials isnot directly proportional to their effective surfacearea.29 For this reason, the electrochemically avail-able area can be referred to as the electrochemicalactive surface area.30 The electrochemical activesurface area is determined by the pore size of theconducting materials, which can be tuned simply bythe introduction of nanostructures. Studies havereported achieving maximum capacitance at 0.7 nm.As the pore size is increased, the distance betweenthe pores increases and capacitance decreases.13

Finally, the capacitance, cross-section area, andpore size are dependent on each other. In compar-ison with conventional materials, graphene is ableto provide increased capacitance, although its highcost is a substantial drawback. Another advantageis that thin and flexible supercapacitors may bedesigned with this method.31 The purity of thematerial is critical for supercapacitors, because itstrongly affects both their leakage current and cyclelife. Impurities in either the electrode material orthe electrolyte lead to undesirable leakage currentand resulting self-discharge of the supercapacitor.While it is easy to develop devices for which the self-discharge does not significantly affect performance,great care must be taken to reduce the leakage

Current Technology of Supercapacitors 3525

current such that the device can be held unused formany days. There is only one change in voltagewhen it is charged near its rated voltage.5 Next, thevarious electrode materials will be discussed, asfollows:

� Carbon-based materials� Metal oxides� Polymers

Carbon-Based Materials

Carbon materials including activated carbon, car-bon nanotubes (CNTs), and graphene are commonlyused as cathodes for supercapacitors because oftheir large surface area and extremely high con-ductivity and stability.32–42 Our research grouprecently described various graphene-basednanocomposite materials for application in sensors,biosensors, solar cells, and biofuel cells.28,43–45 Theability to achieve ultrahigh capacity using carboncloth (CC) as high-energy anodes offers new designand manufacturing potential.46 Carbon cloth is verycheap and has high conductivity and superb flexi-bility, and therefore can be used as an electrodematerial for flexible supercapacitors.34,42,46–48 Incarbon materials, instead of charge storage in thebulk of the capacitive material, charges are typicallystored in an electrochemical double-layer that isformed at the interface between the electrode andthe electrolyte, and so the capacitance is largelydependent on the surface area available to theelectrolyte ions. The electrochemical performance isinfluenced mainly by the specific surface area, poresize distribution, pore shape and structure, electri-cal conductivity, and surface functionality, amongwhich the most important factors affecting theperformance of the carbon material are the specificsurface area and pore size.49

Activated Carbon

Activated carbon is the most suitable carbon-based material for the fabrication of supercapacitorelectrodes, due to its large surface area, goodconductivity, low cost, and easy availability. Acti-vated carbons are derived from various carbon-containing materials (i.e. waste, coal, nutshells, andwood) through physical and chemical process, whichare originated at different temperatures. Physicalactivation involves the thermal treatment of carbonprecursors at high temperature with the help ofreducing and oxidizing agents. A chemical process isconducted under lower-temperature conditionsusing chemical agents such as hydroxide or metalchloride, depending on the activation method andthe precursors used in fabrication.13 There aremany issues related to the specific surface areawith respect to capacitance for activated carbon. For

example, with graphene, the reduction of grapheneoxide results in low surface area and poor pore sizedistribution, leading to lower capacitance. It hasbeen reported that even with a large availablesurface area, the specific capacitance is lower thanthe theoretical capacitance of an EDLC.13 EDLCperformance is dependent on factors other thansurface area, including pore size distribution, struc-ture, and shape, as well as surface functionality andconductivity. Additionally, the higher specific areaof activated carbon will lead to an increase in theactive surface area, which can result in increasedelectrolyte decomposition, especially in organicmedia. Therefore, improved porosity and activesurface functionality is necessary to optimize per-formance.50 Many functionalities can affect elec-trode properties. For example, surfacefunctionalities can affect the wettability of theelectrode surface, which can also provide extracapacitance.51,52

Carbon Nanotubes

Generally, carbon nanotubes can be classified intotwo types: a single graphite sheet curled intocylindrical form constitutes a single-walled carbonnanotube, whereas multi-walled carbon nanotubescontain many single-walled carbon nanotubes withdifferent diameters but with the same center. Dueto their interesting electrochemical features includ-ing high specific capacitance, stability under highcurrent loads, and low internal resistance, carbonnanotubes make excellent polarizable electrodes.13

Carbon nanotubes and carbon nanofibers are pro-duced via the breakdown of specific hydrocar-bons.9,53 Using different parameters, it is possibleto obtain different nanostructures that can modifytheir crystalline order.53 Recent research advancesprovide the opportunity to further explore the use ofcarbon nanotubes as electrode material in anEDLC.10,54–56 Their performance is stronglyaffected by two factors, micropores and internalresistance, both of which can reduce the value ofspecific capacitance from the theoretically estimatedvalue. ‘‘Consequently, researchers are now trying toprogress flexible carbon fiber hybrid electrodes totake benefit of their synergistic effects.13’’ Imagesfrom scanning electron microscopy show an entan-gled mat of carbon nanotubes.57

Graphene

Graphene has been the subject of extensiveresearch. Specifically, the properties of single-layergraphite make it an important material in elec-trodes for energy applications, and electrochemicalenergy storage in particular.58 Graphene has thehighest surface area among carbon materials, andpossesses many other properties that are useful inthe fabrication of supercapacitors, including highelectrical and thermal conductivity, light weight,and strong chemical stability. The combination of

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chemical, physical, and mechanical properties makegraphene-based material more attractive for energystorage.59 Graphene oxide is the originator for theformation of graphene-based material. It is a newform of carbon material. The functionality of agraphene surface has a significant impact on thespecific capacitance.60 Due to van der Waals forces,the surface area and energy density of graphene arereduced when restacking occurs.

In order to realize the benefit of high surface areato achieve high capacitance, it is important thatboth sides of the graphene layer are capable offorming an electrochemical double-layer.58 Thevalue of capacitance for both graphene and acti-vated carbon is controlled by the space chargecapacitance in the majority of the material. It isvery difficult to form full electrochemical doublelayers on both sides of graphene. However, gra-phene may be helpful for electrode materials used inelectrochemical devices for storing energy. The highsurface area and incomparable strength of thematerial together with its optical properties andgood conductance may result in new and improvedelectrodes.58 Three-dimensional graphene-basedframeworks (3DGFs) represent an emerging classof ultra-light and porous carbon materials with aunique structure and intriguing properties thatinclude an interconnected macroporous structure,large surface area, low mass density, and excellentelectrical conductivity and stability, used in foams,aerogels, and sponges.46 Recent studies have inves-tigated graphene oxide membranes as materials forcritical gas separation challenges. Studies haveshown that hydrogen has much higher permeabilitythan other atmospheric gases such as oxygen andnitrogen, which paves the way for use as anatomically thin, selective barrier layer for sensitivehydrogen storage materials. Related studies haveshown that reduction of graphene oxide to formreduced graphene oxide results in a furtherdecrease in the presence of water, while maintain-ing desirable gas permeability characteristics.61

Metal Oxides

Metal oxides have high specific capacitance,which makes them suitable for the fabrication ofelectrodes.19 RuO2, IrO2, MnO2 are some examplesof metal oxides.19 Most of the related research hasbeen conducted with ruthenium oxide. The capaci-tance in the case of ruthenium oxide is attainedthrough the insertion and removal of protons in itsamorphous structure. Because ruthenium oxide isvery expensive, alternatives such as manganese andferrous oxides are being explored for supercapacitorelectrodes because of their similarities to the prop-erties of ruthenium. However, manganese oxide andother metal oxides are poor conductors, and theoverall performance of metal oxides in supercapac-itors is very low. For this reason, compositesincluding metal oxides have been investigated for

improved performance.14 Cobalt oxide, nickel oxide,and other metal oxides and various composites havebeen explored as potential electrode material.14

Effective electrode–electrolyte interactions requirethe fast transmission of ionic species in the bulkelectrode and at the electrode/electrolyte interface,which gives these materials high specific capaci-tance. In addition to metal oxides, composites ofmetal oxides and other materials such as man-ganese oxide-graphene are also commonly consid-ered for supercapacitor applications.13

Polymers

Polymers are generally poor conductors of elec-tricity, and thus they are being replaced in manyapplications by doped polymers, which have higherconductivity and other improved properties. Poly-mers have unique properties that make them usefulin the fabrication of supercapacitors. When anexternal electric field is applied, the redox state ofthe electrode is changed with the treatment ofpolymer film. ‘‘Polymer can form inert layers onmetal surfaces with semiconductor band struc-ture.62 In order to make an electrode, conductingpolymer should be grown on the surface of thecurrent collector. Through this electrochemical for-mation mechanism, the electrode can be p-doped orn-doped. On charging or discharging, the electrodedopant ions move in or out of the polymer electrode,forming an electric double layer. This material’scharging mechanism is requested to be pseudoca-pacitive rather than double layer charging, attain-ing very high capacitances.62’’ Rectangular polymerelectrodes are present in most capacitors, but in thecase of a typical cyclic voltammogram (graph afterthe electrochemical experiment) of a polymer, anelectrode is not like a capacitor.19

Polyaniline (PANI) is a conducting polymer thatis a good candidate for energy storage devices. Inthis case, supercapacitor electrodes are fabricatedby mixing PANI nanostructures with binders suchas polyvinylidene fluoride and conductive additives.However, the PANI nanostructures can be alteredand inert materials introduced as a result of thebinder deposition on the electrode surface, thusnecessitating a single, direct separation process tofabricate self-supporting PANI nanostructures.13

Another example of a conducting polymer ispolypyrrole. Its unique properties, including highconductivity and thermal stability, fast charge/discharge, low cost, and high energy density, makeit an excellent candidate for use as electrode instorage devices. Polypyrrole is one of the best-conducting polymers because its monomer can beeasily oxidized, and is also water-soluble and easilyavailable. The major advantages with polypyrroleused as an electrode material are its high energydensity, unstable electrochemical doping, and sim-ple electrochemical process capability.13

Current Technology of Supercapacitors 3527

The different types of polymers are shown inTable II.

The table shows that different polymers havedifferent melting points, strength, and density. Wedescribed only three polymers that are commonlyused for electrode material, i.e., linear, branched,and cross-linked polymers. The density and tensilestrength of linear and crossed-linked polymers arevery high as compared with branched polymers.63,64

ELECTROLYTES

In every device, there is a source of conduction.For example, a dielectric is present in a conven-tional capacitor. Similarly, supercapacitors containan electrolyte that is used for the conduction ormovement of ions. The important parameters forselecting an electrolyte are the size and type of ions,electrode materials, concentration, and the ion andsolvent interaction. Electrolytes can affect capaci-tance, cycle life, and energy or power density.13 Asupercapacitor’s performance is dependent on thetype of electrolyte.60 The electrolyte concentrationshould be sufficient to eliminate depletion problemsat some point, mainly in the case of organicelectrolytes.65 If the electrolyte reservoir is verysmall relative to a large electrode surface area, theoverall performance of the supercapacitor may beinadequate. Important properties in electrolytes arethe coefficient of temperature and conductivity,which primarily affect the equivalent series resis-tance (ESR) of the supercapacitor. Thermodynamicpotential stability is also very important, and itsvalue for non-aqueous electrolytes is higher than foraqueous electrolytes.19 ‘‘Aqueous electrolytes canoffer high value of capacitance and conductivity, buttheir working voltage is narrow due to decomposi-tion voltage. Inorganic liquids and organic elec-trolytes can work at high voltages, but they sufferfrom poor ionic conductivity.13’’ The electrolyte ionicconductivity depends on the internal resistance ofsupercapacitors. It is important that the size of theions present in the electrolyte should be equal to orless than the pore size of the electrode material. Forexample, if the material used is carbon and theelectrolyte is NaOH, then the pore size of the carbonshould be greater than the size of the ion of theelectrolyte present, which will result in high capac-itance and power density. In some cases, however,the freezing point and viscosity of electrolytes also

affect the thermal stability of the supercapaci-tor.66,67 No single electrolyte can meet all require-ments. For example, both high capacitance andionic conductivity can be attained by aqueouselectrolytes, but the lower breakdown voltage leadsto lower operating voltage of aqueous electrolytes.While organic and ionic liquid electrolytes provide awide operating voltage, they typically suffer fromlarge internal resistance. Consequently, a largedrawback of aqueous electrolytes is their smallvoltage window, which can be as low as about 1.2 V,much lower than that of organic electrolytes.49

Aqueous and Organic Electrolytes

There are two main types of electrolytes used insupercapacitors: aqueous and organic electrolytes.Aqueous electrolytes have more restricted mobilevoltage than organic electrolytes. Organic elec-trolytes have a larger resistance than aqueouselectrolytes, and thus they have much lower powercapacity. The aqueous electrolyte provides largercapacitance than the organic electrolyte due to itshigh conductivity and small radius between ions.19

Ionic liquid and organic electrolytes are usuallyused for composite cleaning actions under a con-trolled environment to stay clear of moisture. All ofthese properties of aqueous electrolytes stronglydescribe the model. The high conductivity of theaqueous electrolyte is favorable for reducing equiv-alent series resistance (ESR) which gives highpower density supercapacitors. The most commonlyused aqueous electrolytes are KOH, sulfuric acid,and phosphoric acid.68 Acetonitrile (ACN) andpropylene carbonate are two solvents commonlyused in organic electrolytes. Acetonitrile can liquefylarge portions of salts, but it is toxic, while propy-lene carbonate-based electrolytes have huge volt-age, temperature, and comparatively properconductivity.65 ‘‘Acetonitrile can dissolve largeramounts of salt than other solvents, but suffersfrom environmental and toxic problems. Propylenecarbonate based electrolytes are friendly to theenvironment and can offer a wide electrochemicalwindow, a wide range of operating temperature, aswell as good conductivity. Salts with less symmetricstructures have lower crystal-lattice energy andincreased solubility. However, one issue whichshould be kept in mind is that the water contentin organic electrolytes must be kept below 3–5 ppm.

Table II. Properties of different polymers

Polymer DensityTensilestrength Melting point Examples

Linear High High High High-density polymersBranched Low Low High Low-density polymersCross-

linkedHigh High Do not melt at high

temperatureVulcanized rubber, urea-formaldehyde

resins

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Otherwise, an electrochemical supercapacitor’s volt-age will be significantly reduced.49’’ Organic elec-trolytes produce a large voltage window ascompared with aqueous electrolytes. The aqueouselectrolytes is classified into three categories: alka-line, acid, and neutral solutions.68

Acid Electrolyte

A number of acids are used as electrolytes, withsulfuric acid among the most common in superca-pacitors. The conductivity (or movement) of the ionsis dependent on the concentration. The ionic con-ductivity of an electrolyte can be quickly reducedwhen the concentration is sharply reduced orincreased. Due to the small potential window ofaqueous electrolytes, the energy density of EDLCsis significantly reduced. The energy density ofsupercapacitors in aqueous electrolytes can beincreased with the use of a hybrid supercapacitor.The combination of two different electrodes indifferent working potentials can increase the func-tioning potential window in an aqueouselectrolyte.68

Alkaline Electrolyte

Because acid electrolytes are not suitable for allmetal materials, alkaline electrolytes have becomean increasingly important alternative. The mostcommon alkaline aqueous electrolyte is potassiumhydroxide (KOH), which provides better ionic con-ductivity. The energy density of EDLC-based super-capacitors in aqueous KOH is similar to thatreported for sulfuric acid electrolytes. This has ledto many efforts by researchers to increase the EDLCmaterial energy density in the alkaline electrolyteby increasing the capacitance and the operatingvoltage.68

Neutral Electrolyte

The most important properties of neutral elec-trolytes are their larger working potential and lesscorrosive features. Several types of neutral elec-trolytes have been used in supercapacitor studies,including LiCl, Na2SO4, NaCl, KCl, and K2SO4.Sodium sulfate is the most common neutral elec-trolyte used in electrochemical reactions, whileMnO2 is the most common pseudocapacitive mate-rial, and has been widely studied in neutral elec-trolytes. Neutral electrolytes have also been utilizedfor asymmetric supercapacitor devices, providing awider potential window for achieving high energydensity. In addition to solving the problems ofelectrochemical supercapacitor corrosion, neutralaqueous electrolytes offer a low-cost and environ-mentally friendly alternative to enhance the perfor-mance of electrochemical supercapacitors, withlarger operating voltage and energy density,although challenges remain with regard to achiev-ing improved cycle stability.68

Ionic Liquids

‘‘Low-temperature ionic liquids (ILs) are pureorganic salts containing no solvents with meltingpoints below 100�C.19 If the liquid state is main-tained at ambient temperature, they are termedroom-temperature ionic liquids (RTILs). RTILs areof interest to supercapacitors because they are non-volatile, poorly combustible, and heat-resistant,with these properties being very unusual andunfeasible with conventional solvents.19 In RTILs,at least one ion usually has a delocalized charge andone component is organic, which prevents theformation of a stable crystal lattice. Properties suchas melting point, viscosity, and conductivity arecontrolled by both the substituents on the organicion. Many ionic liquids can be and have beenestablished with the large variation of physicochem-ical properties. For this reason, ionic liquids havebeen termed designer solvents.19’’

SIGNIFICANCE OF SUPERCAPACITORS

Supercapacitors are very useful devices for stor-ing energy. In transportation, electric car manufac-turers are seeking to obtain a range comparable tothat of internal combustion-driven cars, about 100miles, which will require the right battery/superca-pacitor. Because the energy density of supercapac-itors is bridging the gap between the batteries andcapacitors, the automotive industry will likely soonbegin replacing chemical batteries with supercapac-itors. Supercapacitors have been widely used as theelectrical equivalents of flywheels in commercialsystems such as energy reservoirs. Common areas ofapplication include wind turbines and electrical andhybrid vehicles. In electrical and hybrid vehicles,supercapacitors are increasingly used as temporaryenergy stores for regenerative braking. A keyfeature is their ability to store energy in a verysmall interval of time, with good charge–dischargerates as compared with batteries (time constant,i.e., RC). Supercapacitors currently represent analternative for a system that injects power for 1 h,but they would not be a solution for a system thatinjects power for an entire day. Their lack of abilityto store large amounts of energy and the highnumber of modules needed adds significantly totheir cost.28

BASIC APPLICATIONS

Supercapacitors are most commonly used inbackup power applications. An early-design circuitusing micro-power amplifiers and a Farad-rangesupercapacitor was able to run for more than 2 h ona single charge, and was also able to be charged veryquickly, in just a few seconds, in comparison withhours needed for conventional rechargeable batter-ies. The application was also very environmentallyfriendly, as the large number of charge–dischargecycles meant that there were no disposable parts

Current Technology of Supercapacitors 3529

throughout the operating life of the device. Due totheir high storage capacity, supercapacitors arefinding increased use in portable electronic devicessuch as MP3 players and mobile phones. In trans-portation, electric car manufacturers are working toachieve a range comparable to internal combustion-driven cars, about 500 miles, which demands theright battery/supercapacitor. As the energy densityof supercapacitors is closing the gap with batteries,it can be predicted that automotive production willbegin using supercapacitors as a replacement forchemical batteries in the near future. Supercapac-itors have been widely used as the electrical equiv-alents of flywheels in energy storage systems, andare commonly applied in wind turbines and electri-cal and hybrid vehicles. In electrical and hybridvehicles, supercapacitors are increasingly used astemporary energy stores for regenerative brakingand other applications including power condition-ers, smart meters, welders, energy harvesting,inverters, audio systems, power generators, LEDsystems, diesel engines, cooking equipment, andelectric solenoids.

Other Applications:Most supercapacitors are used in industrial appli-

cations. Uninterruptible power supply (UPS) sys-tems are used for critical loads by transferring thepower supply to the backup energy storage when apower disturbance takes place. Rechargeable bat-teries are always the primary choice owing to theircomparatively high energy density. Therefore, thereare various disadvantages associated with batteries,such as low power density and limited cycle life. Bymaintaining supercapacitors at a suitable charge,they can meet impulse power demands to suppressthe battery current transients, and therefore extendthe service life of the overall energy storage.69

Electronic power devices are an integral part ofsystems for power-level bridging and AC–DC orDC–AC conversion. Large backup storage systemsrequires batteries, although DC links usually useDC capacitors for short-term charge storage. Super-capacitors can provide an alternative in powerelectronic converter applications.69 Hybrid energystorage systems combine more than one energystorage device with complementary characteristics,especially in terms of energy and power, to achieveimproved performance and reduced size in compar-ison with single systems. Supercapacitors are anideal complement to high-energy but slow-responseenergy storage devices, such as fuel cells andrechargeable batteries, owing to their fast responsetime and extremely long life span.69

Supercapacitors are also a good choice as energybuffer for adjustable variable drives (AVDs) andother industrial applications. AVDs have grown inpopularity as the required inverter cost hasdropped, and they are efficient and flexible.69

FUTURE PROSPECTS

Supercapacitors as energy storage devices areimportant for a diverse range of applications includ-ing hybrid vehicles, military warheads, communi-cation devices, uninterruptible power supplies,mobile phones, laser technology, and solar cellenergy storage. Supercapacitors offer many advan-tages, such as high power density and quick chargeand discharge processes. There are two main diffi-culties for supercapacitors, high cost and low energydensity, which must be overcome without sacrificingtheir long cycle life and exceptional rate perfor-mance.69 As the energy density of supercapacitors isbridging the gap with batteries, it could be expectedthat in the near future the automotive industry willimplement supercapacitors as a replacement forchemical batteries. Supercapacitors have beenwidely used as the electrical equivalents of fly-wheels in energy storage systems. The most imme-diate future application for supercapacitors is inenergy storage and rapid charging. They will beused to power medical implants such as pacemakersand knee implants, providing power to instrumentsfor extended cycles. Supercapacitors will be used forrobots and unmanned aerial vehicles due to theirquick charging capacity. Other applications includesupercapacitor-based engine starters and chargingstations, and in renewable energy systems. Operat-ing costs can be reduced by applying higher oper-ating voltages. With high cell electromotive force,specific-voltage systems will be possible with fewercells in series, leading to reduced load of externalvoltage-residue circuits. Many of the propertiesdepend on the electrode material used in thesupercapacitor, and much research is devoted todeveloping low-cost and high-performance energystorage devices. One cost-effective method is the useof waste materials such as carbon produced fromcoal combustion for electrodes fabrication, althoughcommercialization of these electrodes will requirefurther research on storage techniques anddevices.13

CONCLUSION

A capacitor is able to store potential energy in theform of an electric charge, with capacitance valuesvarying from picofarad to microfarad. Supercapac-itors are graded in farads, which is a thousandtimes that of an electrolytic capacitor.7 The energydensity of a supercapacitor is more than a capacitorbut less than a battery. Different electrode materi-als such as activated carbon, carbon nanotubes,graphene, and polymers are used to study thebehavior and enhance the performance of superca-pacitors. Graphene has recently been used aselectrode material inside the supercapacitorbecause of its high electrical and thermal conduc-

Sharma and Kumar3530

tivity, light weight, large surface area, and strongchemical stability. The performance of the superca-pacitor is dependent on the electrolyte.19 Electrolyteconcentration should be sufficient to overcome thereduction (depletion) problem through the charge ofthe supercapacitor, particularly for organic elec-trolytes.65 Aqueous and organic electrolytes are thetwo main types of electrolytes used in supercapac-itors. Different organic or ionic liquids are also usedin the electrode material as a binder.

Future research can be carried out on newmaterial that yields better results than grapheneand also provides energy density comparable to thatof batteries. Additionally, hydroxides can be used inelectrode material to enhance supercapacitor con-ductivity and performance. The two issues forsupercapacitors, high cost and low energy density,must be overcome, while maintaining their longcycle life. An important advantage of supercapaci-tors is that they are symmetric with respect tocharge and discharge throughout their completeoperating voltage range, and have recycling stabil-ity as high as 90%. When comparing the powercharacteristics of supercapacitors and batteries, thecomparisons should be made for the same charge–discharge efficiency. The peak power capacity ofbatteries is often quoted for discharge at thematched impedance point at which only half of theenergy from the battery is in the form of electricalenergy to the load, and the other half is dissipatedwithin the battery as heat. A key strategy forovercoming the problem of low energy density is theuse of new, improved materials for electrochemicalsupercapacitor electrodes.

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