micromachines
Review Recent Studies on Supercapacitors with Next-Generation Structures
Juho Sung and Changhwan Shin * Department of Electrical and Computer Engineering, Sungkyunkwan University, Suwon 16419, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-31-290-7694
Received: 27 November 2020; Accepted: 16 December 2020; Published: 18 December 2020
Abstract: Supercapacitors have shown great potential as a possible solution to the increasing global demand for next-generation energy storage systems. Charge repositioning is based on physical or chemical mechanisms. There are three types of supercapacitors—the electrochemical double layer, the pseudocapacitor, and a hybrid of both. Each type is further subdivided according to the material used. Herein, a detailed overview of the working mechanism as well as a new method for capacitance enhancement are presented.
Keywords: supercapacitor; electrical double layer capacitor; EDLC; negative capacitance; energy storage system; pseudocapacitor; micropores; nanopores; ultracapacitor; Helmholtz model
1. Introduction The Fourth Industrial Revolution, which facilitates the emergence of a hyper-connected society, is the basis for data processing between people and things as well as things and things. The essence anchored in a host of electronic devices highlights the energy consumption problem to address the ever-increasing energy demand. However, the challenge confronted (based on expenditure) is proving hard to overcome. Self-powered devices are mostly based on lithium-ion batteries, which were proposed three decades ago. They demonstrate superior properties as compared to energy storage systems (ESSs) based on other materials with the highest energy storage capacity; however, the world no longer refers to them as “perfect batteries”. Moreover, the progress of battery technology would be considered as a stumbling block to the development of electrically powered industry in some parts of the world. Insufficient cycles to deliver energy owing to membrane shrinkage by shutdown, power density, and thermal stability are now more noteworthy aspects in particular fields. Most importantly, the energy storage capacity, which is considered a strength, is no longer increasing. The major frameworks to solve these issues are classified as follows: (1) ultra-low-power devices based on energy efficiency with various techniques to overcome the theoretical limit of energy consumption efficiency with constant energy capability [1–4] and (2) focusing on developing the next generation of ESSs to complement lithium-ion batteries that have reached their innovation limit in a specific area [5–7]. There are various requirements to implement these next-generation ESSs, but to date some systems to satisfy certain conditions remain nonexistent [8]. Of these, capacitors using the extreme surface area and distance from the electrode to the charged ions layer on the electrolyte have particularly attracted attention owing to their superior specific power. Capacitors with superior characteristics (called supercapacitors), which are unavailable in conventional batteries, exhibit excellent functionality in many areas, including power density, charge/discharge cycles, operation over a wide temperature range, and reliability, which have been noted as limitations of batteries [8–13]. However, based on energy density, it is evident that supercapacitors must be studied intrinsically.
Micromachines 2020, 11, 1125; doi:10.3390/mi11121125 www.mdpi.com/journal/micromachines Micromachines 2020Micromachines, 11, 1125 2020, 11, x 22 of of 25 25
MicromachinesMicromachines[8,9,10,11,12,13]. 2020, 11 2020, x , 11, x However, based on energy density, it is evident that supercapacitors must2 of 25 be 2 of studied 25 Among aintrinsically. few solutions for overcoming the limitations of low energy density, the main area of [8,9,10,11,12,13].[8,9,10,11,12,13].Among However, However, a few based solutions based on energy on for energy overcoming density, density, it is theevident it islimitations evident that supercapacitors that of lowsupercapacitors energy must density, mustbe studied the be mainstudied area of interestMicromachines involves 2020, 11, x the development of new materials or structures for the electrodes of2 of supercapacitors. 25 intrinsically.intrinsically.interest involves the development of new materials or structures for the electrodes of supercapacitors. The electrode materials in laboratories and industries are typically reliant on carbon, metal oxides, [8,9,10,11,12,13].AmongTheAmong However, a electrodefew a solutions few based materialssolutions on for energy overcoming forin laboratories overcomingdensity, the it is limitationsand evidentthe industrilimitations that of es supercapacitorslow are of energy lowtypically energy density, reliant must density, the be on studiedmain carbon,the mainarea metal ofarea oxides,of mixed metal oxides, and conductive polymers. Since the onset of supercapacitor manufacturing, carbon intrinsically.interestinterest involvesmixed involves themetal development the oxides, development and of newconductive of materials new materials polymers. or structures or structuresSince for the the foronset electrodes the ofelectrodes supercapacitor of supercapacitors. of supercapacitors. manufacturing, materialsAmongThe electrodeThe have a fewelectrodecarbon been solutionsmaterials usedmaterials materials becausefor in havelaboratoriesovercoming in beenlaboratories of their used andthe broad because limitations industriand surface industri ofes their areof areaeslow brtypically areoad energy [7 typically, surface14 ,15reliant density,]. area Metalreliant on [7,14,15]. thecarbon, oxideson main carbon, Metalmetal area are mainlymetal oxidesofoxides, oxides, are used mainly for pseudocapacitors,interestmixed involvesmixed metalused the metal oxides, development andfor oxides,pseudocapacitors, due and to conductiveand theirof newconductive high materials andpolymers. capacity due polymers. or to structures Sincetheir and Since highlowthe for onset resistancecapathe the city onsetelectrodesof supercapacitorand of they lowsupercapacitor ofcan resistancesupercapacitors. be manufacturing, attractive they manufacturing, can as be electrode attractive materialsThe electrodecarboncarbon which materials materialsas materialselectrode have have in a laboratories higherhavebeenmaterials usedbeen energy usedbecause whichand because industri and ofhave their power ofesa brtheir higherareoad than typicallybr surface oaden conventionalergy surface areareliant and [7,14,15]. area poweron [7,14,15]. carbon, carbon-based Metalthan metal Metal oxidesconventional oxides,oxides are supercapacitors. mainly are carbon-based mainly Formixed conductiveused metal usedfor oxides, pseudocapacitors,supercapacitors. for polymers, pseudocapacitors, and conductive a reduction-oxidationFor and conductive dueandpolymers. to due their topolymers, Since theirhigh processcapahighthe a onset cityreduction-oxidationcapa and is cityof used supercapacitorlow and toresistance low store resistance process and theymanufacturing, release iscanthey used be can charges.attractiveto bestore attractive and Various release as electrodecharges. materials Various which conductive have a higherpolymers en ergyhave andbeen power widely than studied conventional because they carbon-based are easy to produce conductivecarbon materialsas polymerselectrode have been materials have used been because which widely ofhave their studied a br higheroad becausesurface energy area they and [7,14,15]. arepower easy Metal than to produceoxidesconventional are and mainly arecarbon-based inexpensive supercapacitors.supercapacitors.and are For inexpensive conductive For conductive compared polymers, polymers, to a other reduction-oxidation a materialsreduction-oxidation fabricated process forprocess issupercapacitor used is usedto store to storeandelectrodes release and release [11]. comparedused for pseudocapacitors, to other materials and due fabricated to their high for supercapacitor capacity and low electrodes resistance they [11]. can be attractive as electrodecharges.charges. materials Various VariousRecently, conductivewhich conductive ahave new polymers atheory higher polymers havewas energy proposedhavebeen and beenwidely power withwidely studied athan layerstudied becauseconventional combination because they are they carbon-basedof easy aare ferroelectric toeasy produce to produce exhibiting Recently, a new theory was proposed with a layer combination of a ferroelectric exhibiting negative supercapacitors.and areand inexpensive arenegative For inexpensive conductive capacitance compared compared polymers, toand other todielectric, a other reduction-oxidationmaterials materials which fabricated couldfabricated processfor enhance supercapacitor for is supercapacitor usedthe capacitance to store electrodes and electrodes th releaseat [11]. had [11]. been hitherto capacitancecharges. VariousRecently, andconsideredRecently, conductive dielectric, a new a astheorynew polymersan which theoryinherent was could haveproposedwas limitation. beenproposed enhance widelywith In withadditiona the layerstudied capacitancea layercombination to because previously combination they that of exhibiting area had ferroelectricof easy a been ferroelectric to the produce hitherto ability exhibiting exhibitingto considered handle low- asand an arenegative inherent inexpensivenegative capacitancepower limitation. capacitancecompared technologies and to dielectric, Inand other addition dielectric,[1,2,3,4], materials which to ferroelectricwhich facould previouslybricated could enhance for mateenhance supercapacitor exhibiting therials capacitance the with capacitance the electrodesnegative abilitythat hadth capacitanceat[11]. to beenhad handle beenhitherto low-powereffectshitherto have technologiesRecently,consideredconsidered a [demonstrated 1 newas–4 an], theory ferroelectricasinherent an inherent wasthe limitation. abilityproposed materialslimitation. of InESSs with addition In within a additioncombination layer negativeto previouslycombination to previously with capacitance exhibiting dielectric of exhibitinga ferroelectric elayers.theffects ability the The have ability exhibiting to theory handle demonstrated to handleis basedlow- low- on ionic the polarization, which leads to ferroelectric effects, as well as dipolar polarization. It has evinced an abilitynegativepower of capacitance ESSspower technologies in technologies combination and dielectric,[1,2,3,4], [1,2,3,4], with whichferroelectric dielectric ferroelectric could enhance layers.mate rialsmate Thethe rialswith capacitance theory withnegative isnegative based th atcapacitance had oncapacitance ionicbeen hithertoeffects polarization, effects have have which demonstrateddemonstratedimproved the ability the capacitance, ability of ESSs of ESSswhichin combination in resulted combination inwith a theoretical withdielectric dielectric layers.increase layers. The in storagetheory The theory is energy based is based ofon up ionic onto twoionic times leadsconsidered to ferroelectric as an inherent eff limitation.ects, as well In addition as dipolar to previously polarization. exhibiting It has the evinced ability to an handle improved low- capacitance, polarization,polarization,and which also whichexperimentally leads leadsto ferroelectric to ferroelectricachieved effects, an increaseeffects, as well as in as wellstorage dipolar as dipolarenergy polarization. ofpolarization. up to 1.5It has times It evinced has [16]. evinced This an presents an whichpower resultedtechnologies in a theoretical[1,2,3,4], ferroelectric increase inmate storagerials energywith negative of up tocapacitance two times effects and also have experimentally demonstratedimprovedimproved the acapacitance, new ability capacitance, possibility of ESSs which to inwhich increaseresultedcombination resulted capacitancein a theoretical within a theoreticaldielectric which increase can layers. increase overcome in storageThe in theorystorage the energy limitations is energy based of up onof to upsupercapacitors. ionictwo to times two times The achieved an increase in storage energy of up to 1.5 times [16]. This presents a new possibility to increase polarization,and alsoand which experimentally alsoresults experimentally leads were to experimentally ferroelectric achieved achieved an effects,increase confirmed, an increase as in well storage as in ascomprehensively storage dipolar energy energy polarization.of up of todesc up 1.5 ribedto times It 1.5 has intimes [16]. Sectionevinced This[16]. 4. presentsThisan presents capacitanceimproveda new capacitance,a possibilitynew which possibilityThis can towhich articleincrease overcome to resultedincrease not capacitance only the incapacitance describes a limitations theoretical which the which can increaseanalysis of overcome can supercapacitors. overcome ofin supercapacitorsstorage the limitations the energy limitations The of basedof results up supercapacitors. toof on supercapacitors.two werevarious times experimentally electrochemical The The confirmed,and alsoresults experimentallyresults were asdouble-layer comprehensively wereexperimentally experimentally achieved models confirmed,an describedbut increase confirmed, also introducesas in comprehensivelyinstorage as Section comprehensively newenergy4. ways of desc up to improvetoribed desc 1.5 ribed timesin Sectioncapacitance in [16]. Section 4.This beyond 4.presents the fatal limit by a newThis possibilityThis article articleutilizingThis to not increasearticle not only negative only not capacitance describes describesonly capacitance describes thewhichthe analysis materials.the can analysis overcome of of supercapacitors supercapacitorsof supercapacitors the limitations based basedof based on supercapacitors. various on various various electrochemical electrochemical The electrochemical The characteristics of capacitors, supercapacitors, and batteries, which are typical ESSs, are double-layerresultsdouble-layer weredouble-layer experimentally models models models but but confirmed, also also but introduces introduces also asintroduces comprehensively new new waysnew ways ways to toimprove desc improveto ribedimprove capacitance in capacitanceSection capacitance beyond4. beyond beyond the fatal the the limitfatal fatal limitby limit by by utilizingutilizing negativedescribed negative capacitance in Table capacitance 1. materials.The indicatorsmaterials. for specific energy density, specific power density, charge storage utilizingThis article negative not only capacitance describes materials. the analysis of supercapacitors based on various electrochemical double-layerThe models characteristicsmechanism,The characteristics but also cycling introducesof capacitors, of performance capacitors, new supercapacitors, ways stsupercapacitors, toorage improve region, and capacitance charge batteries,and batteries, temperature, beyond which which theare fataldischarge typical are limit typical ESSs, bytemperature, ESSs, are are and The characteristics of capacitors, supercapacitors, and batteries, which are typical ESSs, are utilizingdescribed negativedescribed galvanostaticin Tablecapacitance in Table 1. The 1.discharge materials.indicators The indicators curves for specific for are specific presented.energy en density,ergy In density, the specific case specific powerof galvanostatic power density, density, charge discharge charge storage storagecurves, this describedThemechanism, characteristicsmechanism, inallows Table cycling1 us .cycling of to Theperformance figurecapacitors, indicatorsperformance out that supercapacitors,storage the forst capacitororage region, specific region, and chargeand energy su chargebatteries,percapacitor temperature, density, temperature, which could discharge specific are use dischargetypical the powertemperature, total ESSs, temperature, stored density,are energy and chargeand rapidly storagedescribedgalvanostatic mechanism, ingalvanostatic Tablethrough 1. discharge The cycling the dischargeindicators slope curves performance of forcurves curves. are specific presented. are In encontrast,presented. storageergy In density, th region,the eIn battery case the specific chargeofcase shows galvanostatic powerof temperature, galvanostatica relatively density, discharge chargeuniform discharge discharge storagecurves, state. curves, temperature, this this andmechanism, galvanostaticallowsallows uscycling to usfigure toperformance discharge figure out that out the that curvesst capacitororage the capacitor areregion, and presented. su chargeandpercapacitor su percapacitortemperature, In the could case could usedischarge ofthe use galvanostatic total the temperature, storedtotal stored energy discharge energyand rapidly rapidly curves, Table 1. Description of the properties of energy storage systems. thisgalvanostatic allowsthroughthrough us thedischarge to slope the figure slope of curves curves. out of curves. thatare In presented.contrast, the In capacitorcontrast, th eIn battery ththee and batterycase shows supercapacitor of showsgalvanostatic a relatively a relatively uniform could discharge uniform use state. curves, the state. total this stored energy rapidlyallows us through to figureEnergy out the thatStorage slope the System of capacitor curves. and In su contrast,percapacitor Capacitors the batterycould use shows the total Supercapacitors a relativelystored energy uniform rapidly state. Batteries through the slopeSpecific of curves. energyTable In density contrast,Table 1. Description (Wh 1. Description thkge−1 )battery of the ofproperties shows the0.01 propertiesto 0.1a relativelyof energy of energy storage uniform storage systems. 0.1state. tosystems. 50 10 to 200 Specific power density (W kg−1) 103 to ~107 1 to ~106 10 to 100 EnergyEnergy Storage Storage System System Capacitors Capacitors Supercapacitors Supercapacitors Batteries Batteries Table 1. Description of the properties of energy storageCharge Separation systems. Faradaic Specific energy density (Wh kg−1) −1 0.01 to 0.1 0.1 to 50 10 to 200 SpecificCharge energyTable storage density 1. Description mechanism (Wh kg ) of the properties0.01Charge to 0.1 of Separation energy storage Charge systems. 0.1 adsorption to 50 (desorption) 10 toIntercalation 200 Specific power density (W kg−1) −1 3 37 7 6 6 Specific power density (W kg ) 10 to ~1010 to ~10 1 toIntercalation ~101 to ~10 (deintercalation) 10 to 100 10(deintercalation) to 100 EnergyEnergy Storage Storage System System Capacitors Capacitors Supercapacitors Supercapacitors Batteries Batteries Storage region Surface ChargeCharge Separation SeparationSurface FaradaicFaradaic Surface to bulk SpecificCharge energy storage density mechanism (Wh kg−1) 1 0.01 to 0.1 0.1 to 50 10 to 200 Specific energyChargeCycling density storage performance mechanism (Wh kg− ) Charge0.01Charge Separation to 0.1 SeparationInfinite Charge Charge adsorption adsorption 0.1 to (desorption) 50>500,000 (desorption) IntercalationIntercalation 10 to 200500~2000 Specific power density (W kg−1) 1 103 to ~1037 7 Intercalation1 to ~106 (deintercalation)6 10 to(deintercalation) 100 Specific powerCharge density temperature (W kg− ) (°C) 10 to ~10 −20 to 100 Intercalation1 to ~10 (deintercalation)−40 to 65 (deintercalation)10 to 100 0 to 45 Storage region Surface Charge SeparationSurface FaradaicSurface to bulk StorageDischarge region temperature (°C) Surface−20 to 100 Charge SeparationSurface−40 to 65 SurfaceFaradaic to− 20bulk to 60 ChargeCycling storage performancemechanism Charge SeparationInfinite Charge adsorption>500,000 (desorption) Intercalation500~2000 Charge storageCyclingCharging mechanism performance time (sec) Charge SeparationInfinite10− 6 to 10−3Charge adsorption>500,000 (desorption) 1 to 10 Intercalation500~2000103 to 105 ChargeCharge temperature temperature (°C) (°C) −20 to 100−20 to 100Intercalation Intercalation (deintercalation)−40 to (deintercalation) 65−40 to 65 (deintercalation) 0(deintercalation) to 45 0 to 45 StorageStorageDischarge region regionDischarge temperature temperature (°C) (°C) Surface−20Surface to 100−20 to 100 Surface−40 Surfaceto 65−40 to 65 Surface to −bulk20 Surface to 60−20 to to 60 bulk CyclingCyclingCharging performance performanceCharging time (sec) time (sec) Infinite10Infinite −6 to 10−63 to 10−3 >500,000 1 >to500,000 10 1 to 10 500~200010 3 to 500~2000101053 to 105 Charge temperature (°C) −20 to 100 −40 to 65 0 to 45 Charge temperature (◦C) 20 to 100 40 to 65 0 to 45 Discharge temperature (°C) −20 to −100 −40 to 65 − −20 to 60 Discharge temperature (◦C) 20 to 100 40 to 65 20 to 60 Charging time (sec) 10−6 to 10− −63 3 1 to 10 − 10 3 to 105 − 3 5 Charging timeGalvanostatic (sec) discharge curves10 − to 10− 1 to 10 10 to 10
GalvanostaticGalvanostaticGalvanostatic discharge discharge discharge curves curves curves
Galvanostatic discharge curves Note: Galvanostatic discharge curves of supercapacitors were classified into three types: (1) EDLC, (2) Pseudocapacitors (redox), and (3) Pseudocapacitors (intercalation).
2. Classification of Supercapacitors
Supercapacitors, which are also referred to as double-layer capacitors, gold capacitors, power caches, power capacitors, ultracapacitors, or super condensers, are classified into three types, depending on the charge storage mechanism: (1) the electrical double-layer capacitor (EDLC), which is based on a Micromachines 2020, 11, 1125 3 of 25 charge storage mechanism that physically stores charges on the electrode surface with an electrical double layer without causing irreversible chemical reactions [8–11,17]; (2) a pseudocapacitor, which is operated by a chemical reaction that relies on a mechanism that stores charge only in a limited area based on surface, unlike a battery, wherein the reaction occurs on the spacious electrode region from the surface to the bulk region; and (3) hybrid capacitors that adopt both mechanisms, composed of EDLC electrodes and electrodes based on chemical mechanisms, such as pseudocapacitors or batteries, which can exhibit the characteristics of both systems and provide intermediate performances. Here again, they are subdivided according to the material of the electrode or electrolyte (see Figure1)[18]. Notably, EDLCs can withstand millions of cycles owing to the mechanism that stores charge on the electrode surface with an electrical double layer based on a constantly reversible capability. This property allows the device to not only exceed the life of the system but also the quality of the device compared to batteries. The reversible capability does not stir deviation up in the electrode, thereby eliminating the swelling that occurs in typical redox reactions found in spacious amounts of battery active materials during charge and discharge cycles. Supercapacitor electrodes undergo electrochemical dynamics through polarization resistance, and no rate limiting occurs within the bulk range as in redox battery electrodes. Based on the EDLC structure, many research teams have spent their efforts on improving the charge storage quality regarding the high specific surface area of porous materials, which are mainly used for electrodes. Until pseudocapacitors based on intercalation were embraced, pseudocapacitors have also been synonymously used with “surface redox reactions”. Redox-active materials, such as metal oxides, mixed metal oxides, sulfides, selenides, nitrides, conductive polymers, carbon materials, and composite with carbon materials, have been used on the active electrode. The energy based on the chemical reaction of a solid substance is mainly stored in chemical bonds—that is, ionic bonds, covalent bonds, and hydrogen bonds. When exploited in electrochemical reactions, solid materials with high bond energies are converted into low bond energy materials to release electricity. Electrical and chemical potentials in the electrolyte can be reversibly converted by an interaction between charged/discharged electrode materials and chemical reactions. Pseudocapacitors do not depend on metal chemistry in the bulk area; thus, there is less risk of metal plating, which poses significant performance degradation, failure mechanism, and safety issues that can lead to short-circuiting and uncontrollable energy chemical reactions [6,18–20]. The material conventionally used in the active region of the electrode is described in Table2. Active carbon, carbon nanotube(CNT)/graphene, metal oxide pseudocapacitance, metal oxide faradaic reactions, and conducting polymers are described according to their benefits, issues, and electrode descriptions [21,22].
Table 2. Properties of various electrode materials [21–23].
Electrode Materials Advantages Issues and Challenges Electrode Description high surface area (over 1000 m2 g 1) low specific capacitance, Activated carbons − AC cost, chemical stability, and availability conductivity 2 1 high surface area (over 2600 m g− ), volumetric capacitance, thermal conductivity, flexibility, agglomeration, GO CNT/Graphene chemical, thermal, mechanical stability nano–micro MWCNT corrosion resistance transformation, ion accessibility, Metal oxides- high specific capacitance, intrinsic stability, MnO , RuO pseudocapacitance porosity, adhesion, conductivity 2 2 relatively high cost Co O NiO, CuO, Fe O high specific capacitance, long cycles life, stability, 3 4, 2 3, Metal TiO , etc. high conductivity, good electrochemical barren reveres, 2 oxides/hydroxides-faradaic Metal hydroxides reversibility, and high rate capability relatively high cost (Ni(OH)2, Co(OH)2), relatively high storage capacity, low cost, conductivity, Conducting polymers low environmental impact, porosity PANI, PPy, PTh stability high voltage window, Micromachines 2020, 11, x 4 of 24
Co3O4, NiO, CuO, Metal high specific capacitance, long cycles life, stability, Fe2O3, TiO2, etc. oxides/hydroxides- high conductivity, good electrochemical barren reveres, Metal hydroxides faradaic reversibility, and high rate capability relatively high cost (Ni(OH)2, Co(OH)2), relatively high storage capacity, low cost, conductivity, MicromachinesConducting2020 polymers, 11, 1125 low environmental impact, porosity PANI, PPy, PTh4 of 25 stability high voltage window,
Pseudocapacitors based on redox reactions as well asas otherother chemicalschemicals exhibitexhibit higherhigher specificspecific capacitancecapacitance propertiesproperties thanthan EDLCsEDLCs basedbased on physicalphysical charge storage operations. However, there are advantages andand disadvantagesdisadvantages ofof energyenergy storagestorage viavia physicalphysical andand bulkbulk chemicalchemical reactions.reactions. The conceptconcept ofof exploitingexploiting bothboth physicalphysical andand chemicalchemical mechanismsmechanisms is presentedpresented asas hybridhybrid supercapacitors.supercapacitors. This This is is a a combination combination of of materials commonly usedused inin pseudocapacitor-basedpseudocapacitor-based electrodes–electrolytes with EDLC-based electrodes electrodes and and lithium lithium electrode–electrolytes electrode–electrolytes with with EDLC EDLC - -basedbased electrodes. The The second second approach approach is is considered considered as a a lithium-ion lithium-ion capacitor, which mainly entails thethe useuse ofof metal metal oxides oxides or or mixed mixed metal metal oxides oxides and and carbon carbon electrodes electrodes to construct to construct the cell.the cell. Owing Owing to this to architecturethis architecture of arrangement, of arrangement, the storage the storage cell functions cell functions in a wider in a workingwider working potential potential window, window, thereby exhibitingthereby exhibiting a largercapacitance a larger capacitance value. This value. approach Thisprovides approach a 2–3-timesprovides highera 2–3-times energy higher density energy than thatdensity of traditional than that of EDLCs traditional [21]. EDLCs [21].
Figure 1. ClassificationClassification of supercapacitors [[24].24].
3. Electrical Double-Layer Capacitors (EDLCs)
3.1. Theory of the Electrochemical Double Layer As thethe namename itselfitself indicates,indicates, supercapacitorssupercapacitors are devicesdevices thatthat maximizemaximize thethe capacitancecapacitance ofof conventionalconventional capacitors. These These devices based on physicalphysical charge storage withwith electricalelectrical operationoperation exhibit aa chargecharge outputoutput like like capacitors; capacitors; thus, thus, the the capacitance capacitance “C “”C depends” depends on on the the dielectric dielectric constant constantεr d ofεr of the the electrolyte, electrolyte, the the eff effectiveective thickness thickness “ “”d” of of the the charge charge separation separation layer layer fromfrom electrodes,electrodes, andand thethe accessible surfacesurface “ “AA”” through through the the distance distance between between the the electric electr layersic layers based based on Helmholtzon Helmholtz theory theory and theand porous the porous structure structure [21]. [21]. InIn conventionalconventional capacitors,capacitors, surfacesurface areaarea AA isis aa two-dimensionaltwo-dimensional single-planarsingle-planar electrode,electrode, whilewhile thethe supercapacitor supercapacitor is is a athree-dimensional three-dimensional porous porous electrode—i.e., electrode—i.e., the the surface surface area area is maximized. is maximized. The Thedielectric dielectric layer layer of a of supercapacitor a supercapacitor comprises comprises a single a single or or several several molecular molecular layers, layers, so so “ “dd”” is is only a smallsmall molecularmolecular distance,distance, ranging ranging from from a fewa few Å Å to to several several Å. Å. This This is a is huge a huge diff erence,difference, considering considering that anthat electrostatic an electrostatic capacitor capacitor has an has insulator an insulator layer layer that reachesthat reaches up to up several to several microns. microns. The charge separation layer is formed with ions when a charged object is placed in an electrolyte. The mutual interfacial structure between the electrodes creates a certain structure. The balancing counter charge on the charged surface of this object is formed correspondingly in the electrode and concentrated near the surface. Various theories have been proposed to explain the structure wherein charges are formed, and a complete theory explaining all the mechanical contents remains Micromachines 2020, 11, x 5 of 24
The charge separation layer is formed with ions when a charged object is placed in an electrolyte. The mutual interfacial structure between the electrodes creates a certain structure. The balancing Micromachinescounter charge2020, on11, 1125the charged surface of this object is formed correspondingly in the electrode5 ofand 25 concentrated near the surface. Various theories have been proposed to explain the structure wherein charges are formed, and a complete theory explaining all the mechanical contents remains undetermined. There are theories for understanding the charge separation layer, as represented in undetermined. There are theories for understanding the charge separation layer, as represented in Figure2. Figure 2.
FigureFigure 2.2. ModelsModels ofof thethe electrochemicalelectrochemical doubledouble layer.layer. ((aa)) HelmholtzHelmholtz model;model; ((bb)) Gouy–ChapmanGouy–Chapman oror didiffuseffuse model; model; ( c(c)) Stern Stern model; model; ((dd)) GrahameGrahame model;model; ((ee)) Bockris,Bockris, Devanathan, and and Muller Muller model model [25–27] [25–27]..
TheThe HelmholtzHelmholtz model model is theis the first first theory theory to provide to provide an approximation an approximation for the arrangementfor the arrangement between thebetween electrode the electrode and layer and of ions layer called of ions the called Helmholtz the Helmholtz layer or plane layer with or plane a finite with distance. a finite However, distance. thereHowever, are some there problems are some with problems interactions with thatinteractions occur further that occur away fromfurther the away electrode from which the electrode include thewhich electrolyte include concentrationthe electrolyte andconcentration all the given and environments, all the given environments, besides the double besides layer the arisingdouble fromlayer thearising interface from (Figurethe interface2a) [ 25 (Figure,28]. Chapman 2a) [25,28]. did Chap not takeman thisdid intonot take account this andinto proposedaccount and an electricalproposed double-layeran electrical theorydouble-layer and considered theory and the considered applied potential the applied and electrolyte potential concentration. and electrolyte Gouy concentration. suggested thatGouy a layersuggested of positive that a ionic layer charge of positive that counteractsionic charge the that charge counteracts affecting the the charge charged affecting electrode the appearscharged inelectrode the electrolyte appears surrounding in the electrolyte the charged surrounding solid, but the that charged ions didsolid, not but appear that ions only did on thenot surfaceappear withonly aon di fftheusion surface layer with based a ondiffusion the Boltzmann layer based distribution on the thatBoltzmann demonstrated distribution the concentration that demonstrated of charged the ionsconcentration with the counter of charged ions ions near with the surface. the counter This ions is called near thethe disurface.ffuse layer. This However,is called the this di modelffuse layer. also hasHowever, problems. this Becausemodel also ions has are problems. expressed Because as point ions charges, are expressed the specific as size point of thecharges, ion is the not specific considered. size Theof the theory ion is does not notconsidered. recognize The that theory the finite does space not re ofcognize ions cannot that the exist finite near space the electrode.of ions cannot Secondly, exist itnear fails the when electrode. applied Secondly, to highly chargedit fails when double applied layers to (Figure highly2b) charged [ 25,29– double34]. Stern layers modified (Figure the 2b) di ff[25,29–usion double34]. Stern layer modified to integrate the diffusion the theories double of Helmholtz layer to andintegrate Gouy–Chapman the theories and of Helmholtz proposed the and compact Gouy– layerChapman presented and byproposed the improved the compact understanding layer presented of the architectures by the improved of the ions understanding with a specific of sizethe andarchitectures limiting space of the between ions with the a specific electrode size and and electrolyte, limiting space but also between extended the electrode the diffusion andlayer electrolyte, to the bulkbut also layer. extended This distance the diffusion is generally layer considered to the bulk as la theyer. radius This ofdistance the ions. is generally Consequently, considered the potential as the chargesradius of and the concentration ions. Consequently, of the di theffusion potential portion char ofges the and layer concentration are sufficiently of the low diffusion to justify portion treating of ionsthe layer with pointare sufficiently charges (Figure low 2toc) justify [ 25,35 –treating37]. Grahame ions with developed point charges the theory (Figure of electrical 2c) [25,35–37]. double layersGrahame based developed on four regions—the the theory inner of electrical Helmholtz do planeuble inlayers front ofbased the electrode on four with regions—the small molecular inner distances;Helmholtz the plane outer in Helmholtz front of the plane, electrode which with consists small of molecular Helmholtz distances; layers; the the di outerffusion Helmholtz region; and plane, the bulkwhich region consists suggested of Helmholtz by Stern. layers; The the theory diffusion of specific region; adsorptionand the bulk was region applied. suggested This by bond Stern. occurs The ontheory the surfaceof specific ofthe adsorption metal electrode. was applied. The negativeThis bond ions occurs are adsorbedon the surface on the of surfacethe metal of electrode. the electrode The regardlessnegative ions of its are charge, adsorbed not byon the surface electrical of force, the electrode but via chemicalregardless interaction. of its charge, The not ions by referred the electrical to as ghost ions act as pseudocapacitive particles. Cations rarely stick to the electrode. The adsorption of ghost ions would be reduced while keeping the solution and electrodes clean. The capacitor centered on the reaction by specifically adsorbed ions is called a pseudocapacitor, which is found at the interface Micromachines 2020, 11, 1125 6 of 25 between the electrolyte and electrode (Figure2d) [ 25–27]. Bockris, Devanathan, and Muller’s model shows a phenomenon for the preponderance of solvent molecules around the interface. Solvated molecules, anions, cations, and electrodes were suggested, and the dipole moment was considered for both the charge separation and diffusion layers. The model is acceptable for the values applied to understand high-current-power EDLCs. However, the concept cannot perfectly explain carbon-based models with porous structures (Figure2e) [25,38].
3.2. Variation of Capacitance According to Pore Size and Structure Although the theory of electric double layers on plane surfaces has been clarified, problems arise when applying electric double layers to pores in actual spaces. The characteristics of ion electrical adsorption in porous structures make the process of charge storage difficult to deal with. Understanding the principle of charge storage through the formation of layers by cations and anions remains insufficient to explain experimental results when applied in nanopores. Generally, the inverse effect of pore size on capacitance is also known. Ions have different movement patterns with pores compared to the surface or bulk region. The specific surface area, which exhibits an evincive connection between the pore sizes, is important in order to understand its impact on specific capacitance. Additionally, it has been an attractive subject of study to numerous research groups in the past two decades. The capacitance generally increases proportionally to the surface area, but in the micropore region a difference in increasing value occurs. As the movement of ions has architectures with a finite size, the potential charges are greatly affected by the size of the pores. Each ion that plays a role in the storage of charge has a specific size, thereby allowing access to structures above a certain scale. Even if the surface area is increased, in regions where the pore size is too small for the solvated ion to approach it does not fully contribute to the layer, which should interact with electrodes. The specific and volumetric capacitance values indicate significant increases and are inversely commensurate to the nonlinear proportion depending on the chlorination temperature, which is largely involved in pore size control [39,40]. It is important to adjust the temperature, density, and intensity values for each material. The targets that constitute the electric double layer with the ions in the electrode layer are activated carbon and graphene, which have two-dimensional structures, and carbon nanotubes (CNTs), which have three-dimensional structures. Activated carbon changes the pore size depending on the temperature. Because the size of the pores determines the number of ions that can pass through the bilayer, if the conductivity is greatest at one point in the nano-meter range, then it decreases rapidly at higher and lower temperatures based on that state [39–44]. The reasons for this are as follows. First, when they are smaller than the largest point, solvated ions do not have the minimum length to form a double layer. That is, the optimal pore size may vary depending on the scale of the electrolyte ions (Figure3)[ 42–44]. According to their characteristics, there is a change in the interaction mechanism between the electrodes and ions. To understand this phenomenon, micropores are analyzed from various perspectives. The first aspect concerns layer formation as a cylinder (Figure3). There are many inquiries regarding the interaction between the electrolyte and the electrode fabricated with carbon material to adequately describe the electrostatic capacity depending on the pore shape and size. The analysis of capacitance for a cylindrical model is as follows, and it is analyzed by Gauss’s law (Figure3c): Z b Q Vba = ( )dr (1) − a 2πεrε0rL Z Q b dr Vba = (2) −2πεrε0L a r Q b Vba = ln . (3) −2πεrε0L a Micromachines 2020, 11, x 7 of 24