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Pb

1 s10 s100 s 0.1 s1000 s

0.01

0.10

1.00

10.00

100.00

1 10 100 1'000 10'000 100'000 1'000'000 10'000'000

Power density [W/kg]

10000 s

BOOSTCAP

Al El Capacitors

NiCd

Li+

NiMH

Film-Foil-Capacitor

E'max

P'max,0

(E'α, P'α,0)

Properties and applications of supercapacitorsFrom the state-of-the-art to future trends

Adrian Schneuwly, Roland Gallay

montena components SA, 1728 Rossens, SwitzerlandTel.: +41 26 411 22 ; Fax: +41 26 411 25 25 ; Email: adrian.schneuwly@montena.com

I. Abstract

Electrochemical double-layer capacitors, alsoknown as supercapacitors or ultracapacitors,are electrical storage devices, which have arelatively high energy storage densitysimultaneously with a high power density.Recent developments in basic technology,materials and manufacturability have madesupercapacitors an imperative tool for short-term energy storage in power electronics. Withmuch higher energy density than today'scapacitors and none of the problemsassociated with conventional batterytechnology, supercapacitors give an access tonew power electronic and industrial storageapplications.

The paper presents basic supercapacitortechnology, component specific properties aswell as state-of-the-art product applications.The problematic nature of supercapacitorseries connection for higher voltageapplications is touched on. The review alsodeals with an energy storage system, which isbased on the hybridization of rechargeablebatteries and supercapacitors, with a suitabledesigned electronic interfacing arrangement inorder to obtain a very high energy densitydevice with a high power performance and along lifetime. Finally, an overview over futuretrends regarding the supercapacitor technologyas well as application scenarios, mainly in thetraction domain, is given.

II. Introduction

Supercapacitors are energy storage deviceswith very high capacity and a low internalresistance. In a supercapacitor, the electricalenergy is stored in an electrolytic double-layer.Therefore such energy storage devices are

generally called electrochemical double-layercapacitors (EDLC). Helmholtz has discoveredthe storage process, based on the separationof charged species in an electrolytic doublelayer, in 1879. ECDLs or supercapacitors (i.e.supercaps) are also known as ultracapacitors,Boostcaps , Bestcaps etc. Supercapacitorsare attractive for their high energy and powerdensities, their long lifetime as well as theirgreat cycle number. In addition to the highspecific power the energy storage insupercapacitors is reversible in contrast toconventional batteries [1].

The electronic applications need passivecomponents to store the electrical energy involume and weight as small as possible. Thechoice of the storage device type depends inparticularly on the speed of the storageprocess, in other words on the power requiredby the application.

Figure 1: TheRagone plot shows the energy density vs. thepower density

Actually, while the slower storage processesmay be performed with batteries, the fasterones have to be done with capacitors. Ingeneral the electrical energy storage devicesare of 3 types: faradaic batteries, electrostatic

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capacitors and magnetic inductors. Thesituation may be well summarized by thefollowing so-called Ragone plot (Figure 1).Detailed informations on the Ragone plot aregiven in chapter 4.The energy density in abattery may rise to 150 Wh/kg. This is about 10times higher than the highest expected value ofa supercapacitor. The power density in abattery has difficulty to reach 200 W/kg and istherefore about 20 times smaller than theexpected supercapacitor performance. Thebatteries suffer from several weaknesses,which exhibit a rapid decrease of theirperformances. The origins may be the fastcharge-discharge cycles or the coldenvironmental temperature. The batteries havealso a limited lifetime and require expensivemaintenance.Through the different capacitor technologytypes, the EDLC presents the highest energydensity. Dielectric and electrolytic capacitors,as well as ceramic capacitors show very highpower densities but very low energy densities.Compared to batteries, capacitors reveal muchlonger lifetimes and cyclabilities. In terms ofpower and energy density the supercapacitorfills up the gap between the batteries and theclassical capacitors, allowing new applications.The properties of the different energy storagedevices are presented in Table 1.

Capacitors EDLC BatteriesEnergy density [Wh/kg] 0.1 3 100Power density [W/kg] 107 3'000 100Time of charge [s] 10-3-10-6 0.3-30 >1’000Time of discharge [s] 10-3-10-6 0.3-30 1’000-

10’000Cyclability [1] 1010 106 1'000Typical lifetime [years] 30 30 5Efficiency [%] >95 85-98 70-85

Table 1 : Storage component propertycomparisons

III. Technological aspects ofsupercapacitors

3.1 Cell construction

A supercapacitor cell basically consists of twoelectrodes, a separator, and an electrolyte(Figure 2). The electrodes are made up of ametallic collector, which is the high conducting

part, and of an active material, which is the highsurface area part. The two electrodes areseparated by a membrane, the separator,which allows the mobility of the charged ionsbut forbids the electronic conductance. Thiscomposite is subsequently rolled or folded intoa cylindrical or rectangular shape and stackedin a container. Then the system is impregnatedwith an electrolyte. The electrolyte may be ofsolid state, organic or aqueous type, dependingon the application power requirement. Theworking voltage of supercapacitor isdetermined by the decomposition voltage of theelectrolyte and depends mainly on theenvironmental temperature, the current intensityand the required lifetime. The capacitance ofan EDLC can be very large, e.g. severalthousands of Farads, thanks to the very smalldistance which separates the opposite chargesat the interfaces between the electrolyte andthe electrodes and thanks to the very hugesurface of the electrodes.

Figure 2: Supercapacitor sketch

In the following, the different materials used insupercapacitor are discussed more in detail.

3.1.1 Electrode

Since the capacitance is proportional to thesurface area, electrochemical inert materialswith the highest specific surface area areutilized for supercapacitor electrodes in order toform a double layer with a maximum number ofelectrolyte ions. As high surface activematerials, metal oxides, carbon and graphiteare the most interesting. The main difficultiesare to find cheap materials, which are

Electrolyte Separator

Electrodes

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chemically and electrically compatible with theelectrolyte. Capacitors for high energyapplications require electrodes made of highsurface area activated carbon with appropriatesurface and pore geometry. Carbonaceousmaterials commonly are activated carbonfibers, carbon black, active carbon, carbonfibers, carbon gel, skeleton carbon,mesocarbon as well as microbeads. The bestcarbon electrodes have surfaces as high as3'000 m2 per gram of material. The electrodecapacitance increases linearly with the carbonsurface area and may reach a capacitance of250 F/g. They are usually prepared from highsurface area carbon powders or fibers. Thepowders are applied, e.g. as a paste on themetallic current collector. Such anarrangement, however, leads to a considerablecontact resistance between grains and betweenthe grains and the support. In order toovercome these problems, pressure has to beapplied or the carbon powder has to be mixedwith metal fibers or powders in order toincrease conductivity.

Recently several capacitors using high surfacearea electrodes composed of RuO2 basedcomposites have been introduced [14]. Suchdevices have e.g. the size of a credit card andhave a capacitance of 200-300 mF at 4-8 V.The RC time constant is about 5 ms.

3.1.2 Electrolyte

As mentioned above the electrolyte may be ofthe solid state, organic or aqueous type.Organic electrolytes are produced by dissolvingquaternary salts in organic solvents. Theirdissociation voltage may be greater than 2.5 V.Aqueous electrolytes are typically KOH orH2SO4, presenting a dissociation voltage of only1.23 V. The energy density is thus about 4times bigger for an organic electrolyte. As aconsequence of the quadratic dependence ofthe energy density of the capacitor on thecapacitor’s voltage use of an organic electrolytewould be desirable. However, if power densityis important, the increase in the internalresistance (ESR) due to the lower electrolyteconductivity has to be considered as well. Theelectrolyte solution should therefore providehigh conductivity and adequate electrochemicalstability to allow the capacitor being operated atthe highest possible voltages.

Earlier work [2] indicated TEATFB inacetonitrile as the best performing organicelectrolyte system for EDLC applications.Depending on the molarity conductivities up to60 mS/cm are possible. Covalent Associatesintroduced novel electrolytes known as ionicliquids. The advantage of these electrolytes aremanifold, they are non-corrosive, the typicalconductance is about 8 mS/cm, and theelectrolytes can be used up to hightemperatures of about 150 °C. Blending withacetonitrile results in a conductance of 60mS/cm.

3.1.3 Separator

Many of the commercial available separatorsare designed for battery use mainly. Hence anaccurate evaluation of the separator is essentialto achieve the exceptional performance ofEDLCs. If organic electrolytes are used,polymer (typically PP) or paper separators areapplied. With aqueous electrolytes glass fiberseparators as well as ceramic separators arepossible. The separator allows the transfer ofthe charged ions but forbids the electroniccontact between the electrodes.

The basic principles to obtain a competitiveEDLC is to collect all the followingperformances: high ionic electrolyteconductance, high ionic separator conductance,high electronic separator resistance, highelectrode electronic conductance, largeelectrode surface, low separator andelectrodes thickness.

In Table 2 an overview over materialspecifications for best performance EDLCsusing an organic electrolyte are given (basedon model calculations).

Active layer thickness 100 µmVolumetric Capacitance 100 F/cm3

Electrolyte 0.05 S/cm,Cell Voltage 2.5 VCurrent Collector Thickness 25 µmSeparator Porosity 50 %Separator Thickness 25 µmDistributed Resistance in Pores 10 x electrolyte

Table 2: EDLC best performance parameters3.1.4 Material research

Most of the recently published works [3-6] dealwith developing and testing of new electrodematerials on laboratory scale devices.Carbonaceous materials in their various forms

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have been studied as electrodes for theconstruction of EDLC energy storage devices.Activated carbon composites and fibers [7] withsurfaces up to 3'000 m2/g, measured by gasadsorption, as well as carbon aerogels [8] withsurface areas up to 850 m2/g have beeninvestigated. Unfortunately a significant part ofthe surface area resides micropores (<2nm)which are not accessible to the electrolyte ions.The remaining surface area electrochemicallyaccessible (meso and macropores) reveals acapacitance well below the estimated values.Therefore the pore size distribution togetherwith the surface area are important for thedetermination of the double layer capacitance.Since Iijima’s original work [9], nanostructuredmaterials such as carbon nanotubes have beenrecognized as a material with promisingapplications in chemistry and physics. Severalmethods were developed to synthesizenanotubes as arc-discharge process [10, 11],chemical vapor deposition (CVD) i.e. pyrolisisof hydrocarbons [12], laser ablation and avariety of combinations of the above mentionedmethods [13]. Several requirements still remainto be fulfilled before the development of ananotube-based technology, in particular theproduction of macroscopic quantities ofnanotubes can be achieved. RecentlyEmmenegger et al. [11] have realized EDLCelectrodes with well aligned carbon nanotubes

films synthesized with a pyrolytic method on athin aluminum substrate (Figure 3).

Figure 3: Carbon nanotube electrode [11]3.2 Devices

Two construction types of supercapacitordevices are generally used: the monopolar celldesign and the bipolar cell design.

The monopolar device shows very highcapacitance due to a high surface area. Thedevices are assembled by winding or stackingin parallel identical electrodes, separator andcollector foils. The voltage of the device isidentical to that of the single cell. It is alsopossible to produce asymmetric devices withone supercapacitor electrode and one batteryelectrode such as NiOx or MnO2 [14]. The ideais to double the capacitance of the devicebecause only the supercapacitor-electrodedetermines the capacitance now. In additionthe cell voltage is higher due to the redoxpotential of the battery electrode. As anadvantage the voltage of such supercapacitorsduring discharge decreases only slightly.Energy densities of up to 20 Wh/kg and powerdensities of up to 5 kW/kg are possible.Another type of asymmetric device is the hybridcapacitor by Evans [15]. He uses RuO2 for ahigh capacitance electrode and a Ta electrolyticcapacitor electrode. This device was made formilitary applications and resisted severalthousand g acceleration.

A high productivity is obtained by winding thedifferent monopolar cell components with aprecise control of all the parameters.

The bipolar devices allow a high voltage, lowresistance and low capacitance [16]. Suchsupercapacitors are built up by stacking severalsingle cells in series into one case. The sealingof such devices is very sophisticated, as theelectrolyte must be prevented from contactingone cell with another.

3.3 The winding technology

Many production and deposition methods existto produce supercapacitor electrodes. Theextrusion of charged polymers, produced bymixing activated carbon in a polymer matrice, isa possible way to produce good performanceand low cost carbon electrodes forsupercapacitors. The advantages of thisprocess are a continued fabrication processresulting in a high productivity and therefore inlow costs. The extruded carbon foil is veryhomogeneous, can be produced in variablesizes and is self-supporting.

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Different processes are applied to assemblethe electrodes, the separator and the currentcollector layers.

Advantages of the winding technology are avery reliable process, a high productivity andtherefore low costs. The winding technologyallows variable sizes and designs of the devicesas the composite is subsequently rolled orfolded into a cylindrical or credit card shape.Due to a good control of the foil tensions duringthe winding, it is possible to achieve a lowinternal resistance ESR.

Figure 5: Montena metar winding machine

IV. Properties of supercapacitors

4.1 Capacitor equivalent circuit

The capacitor may be modelized by theelectrical circuit presented in Figure 6. Theequivalent series resistance ESR limits thecurrent and is responsible for the electricallosses. To get high power, it is absolutelynecessary to have a low series resistance. Theparallel resistance Rp is responsible for thecapacitor self discharge time. Its value must beas high as possible to limit the leakage current.The time constant τ of the self-discharge is τ =Rp C.

4.2 Ragone plot

The Ragone plot is a chart, which gives therelation between the energy density and thepower density of a storage system. It is used tocompare the performances of the differentstorage types like batteries, supercapacitorsand conventional capacitors.

Before to give the equation of the Ragonecurve let us define

C the capacitance [F]uc the voltage on the theoretical capacitor [V]RS the capacitor internal resistance ESR [Ω]RL the load resistance [Ω]P the power [W]P0 the initial power during a discharge at constant RL

Pmax,0 the initial maximum power [W]M the capacitor massτ0 the technology time constant: τ0 = RS Cτ the application time constant: τ = (RL + RS) Ci the current: i(t) = C duc/dtu the voltage on the physical cap. u(t) = uc(t) -RSi(t)Emax the max. energy stored in the cap. Emax = C Uo

2 / 2

4.2.1 Ragone plot coordinates

The parameter of the Ragone plot is actuallythe load resistance RL. It does mean that theplot shows the energy and the power, whichcan be dissipated for the different values of theload resistance. To simplify the relations weintroduce the parameter α defined by therelation τ = α τ0, RL + RS= α RS. If RL isconstant, the initial peak power at time t=0 maybe written as

Pα,ο = Uo2 (α - 1) / α2 / Rs

The maximum peak power Pmax,0 is available attime t=0 when RS = RL , and it is equal to

Pmax,o = Emax / 2 το = Uo2 / 4 Rs

Only a part of the stored energy is available forthe load. The amount is equal to the load powerintegration from t = 0 to t = ∞.

Eα = Emax (α -1) / α

ESR

Rp

L

C

UUc

Figure 6: Supercapacitor equivalent circuit

Figure 4: Supercapacitor flat winding

Electrodes

Separator

Current Collector

Connections

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The Ragone plot coordinates for a given RL areconsequently

E’α = Eα /M = Emax (α -1) / α / M

P’α,ο = Pα,ο /M= 2 Emax (α - 1) / α2 / τo /M= 4 Pmax,o (α - 1) / α2 /M

The numerical values of these coordinates areusually given per unit of component weight.

4.2.2 Available power

When the load resistance is smaller than theinternal resistance, the current will be higher,but the voltage drop on the load is stronglyreduced. The consequence is that a smallerpower may be delivered to the load.

On the right side of the Ragone plot (Figure 1),the curve is interrupted on a point of maximumpower density P’max,0 corresponding to a loadresistance equal to the capacitor internalresistance. On the left side, in the "low" powerrange the load resistance is much bigger thanthe capacitor internal resistance.

4.2.3 Available energy

The first observation, which may be done, isthat at the maximum power rate, only half ofthe stored energy is available. The second halfis dissipated in the capacitor internal resistance.In this high power condition, the current is veryimportant and the losses in the capacitor areproportional to the internal resistance time thesquare of the current.

The second observation is that in the "low"power condition almost all the energy E’max isavailable for the load. The losses are very smallbecause for the same capacitor internalresistance, the current is very small.All the above remarks are valid for the primaryand secondary batteries.The losses inside the capacitor are

Epα = Emax / αPpα,ο = 2 Emax / α2 / τo

The efficiency η, which is the ratio between theenergy used in the load to the total energycontent, is equal to η = 1? - 1 / α = RL /(RL + RS)

4.3 Performance of supercapacitors

In Table 3 the performances of selectedsupercapacitors available today is given.

Companies Voltage[V]

Capacity[F]

Energy[Wh/kg]

Power[W/kg]

Asahi Glass (J) 2.5 3000 7 0.4Econd (R) 500 0.3 56Elna Comp. (J) 5.5 1Epcos (D) 2.3 2700 2.5 2.7Evans (US) 100 200 1.1Matsushita (J) 2.3 470 1.1 0.35Maxwell (US) 2.3 2700 2.5 2.7Montena (CH) 2.5 800 2.4 2.8Nec (Tokin) (J) 5.5 1.3 0.5Polystor (USA) 2.5 7 4

Table 3 : Performance of supercapacitors [17]

The actual detailed performance of themontena Boostcap ultracapacitors are given inTable 4.

BCAP0005

BCAP0007

Rated voltage [V] 2.5 2.5Max. voltage [V] 2.8 2.8Capacitance [F] 800 1'400DC serial resistance [mW] 2.4 1.6Time constant [s] < 2 < 2DC parallel res. (24h,25°C)

[W] > 2'000 > 2'000

Self discharge current [mA] < 2 < 2Spec. energy (@ 2.5V) [Wh/kg] 2.4 4.3Spec. peak power(@ 2.5V)

[W/kg] 2'200 3'450

Weight [g] 290 280Volume [l] 0.255 0.255Operating temperature [°C] - 35 to 65 - 35 to 65Storage temperature [°C] -35 to 65 -35 to 65

Table 4 : Performance of montena Boostcapultracapacitor

4.3.1 Performance in future

The EDLC goal is to reach the highest energyand power densities to get the smallestcomponent volume and weight for a givenapplication. Concretely the goals of montenahave been fixed to 5 Wh/kg for the energydensity and to 5 kW/kg for the power density.

To meet these values, it is necessary toincrease capacitance density, to reduce theESR and to increase the cell operating voltage.The efforts are concentrated on the study of theelectrode surface and accessibility, the grain tograin and grain to support contacts, the

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separator materials, and the electrolyticdecomposition voltage.

V. Applications

Today small size supercapacitors as forexample gold caps from Tokin are widely usedas maintenance-free power sources for ICmemories and microcomputers [18]. Amongnewly proposed applications for large sizesupercapacitors are load leveling in electric andhybrid vehicles as well as in the tractiondomain, the starting of engines, applications inthe telecommunication and power quality andreliability requirements for uninterruptablepower supply (UPS) installations. In generalsupercapacitors may be adapted to thefollowing two application domains.

The first one corresponds to the high powerapplications, where the batteries have norepresentative access. The EDLCs, thanks totheir high power capability, will allow newopportunities for power electronics. Allapplications where short time power peaks arerequired can be provided by these capacitors.Typical examples where a big current isrequired during a short time are the fast energymanagement in hybrid vehicles or the startingof heavy diesel engines

The second one corresponds to the low powerapplications, where the batteries could be moresuitable but are at the origin of maintenanceproblems or of insufficient lifetimeperformance. The supercapacitors, even if theyare much bigger, bring enough advantages tosubstitute the batteries. In this field, the UPS aswell as security installations are the mostrepresentative examples.

5.1 Typical applications

The ECDL capacitors may be used whereverhigh power delivery or electrical storage isrequired. The following examples give anoverview over typical supercapacitorapplications.

5.1.1 Starter

Today the energy that is required to crank asmall or big engine is stored in either Pb or Ni-Cd batteries. Because of their high internalresistance, which limits the initial peak current,

they have to be oversized. The fast batterydischarging and the cold environmentaltemperature affect heavily their properties. Thesupercapacitors have a better power behaviorand a better environmental acceptance.

5.1.2 Hybrid vehicle

The mean power of a small car is about 30 kWand its peak power should be about 60 kW.The supercapacitors may supply the power tothe electrical vehicle required to meet the cityroad traffic conditions. Nevertheless in most ofthe cases there is a need for an additionalbattery to insure a certain amount of autonomyand to reach the range requirement.

Since 1991 in the department of electricalengineering of the HTA Lucerne (University ofapplied Science of Central Switzerland)concepts for hybrid vehicles have beendeveloped and realized [19]. In April 1997, asone of the first vehicles in the world, the « Blue-Angel » prototype-vehicle was able torecuperate energy by means ofsupercapacitors in a so-called SAM (SuperAccumulator Module) (Figure 7 and 8).Figure 7: Blue-Angel of the HTA Lucernepulling an 85t ADtranz locomotive [19]

ECN Netherlands is active in a fuelcell/supercapacitor project for an electricscooter. The fuel cell has a power of 1.2 kWand is mounted in a trailer pulled by thescooter. The supercapacitor has a 15 kWsenergy. The supercapacitors were mounted onthe scooter replacing the former battery. Thefeasibility of the fuel cell plus supercapacitorconcept is demonstrated in this project. Hondaannounced at the vehicle conference 2000 ofGeneva that the first fuel cell vehicles will beintroduced on the market in 2003.

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5.1.3 UPS

The Uninterruptable Power Supplies (UPS)may find some economical interests by usingthe ECDL capacitors, thanks to the suppressionof an inverter and to the suppression of themaintenance. The energy supply during alimited time, at a voltage much higher than thatof batteries, is easier to perform with thesecapacitors.

5.1.4 Toy applications

Another domain are toy applications, where thetotal running time is typically not longer than 10hours [20]. A supercapacitor designed for tenyears or several 100'000 cycles is notoptimized for such application, lowerperformance is thoroughly sufficient. For shortterms the largest markets are for devices with <12 V and only around 2004 the market fordevices with > 48 V will have grown to thesame size and will give opportunities for thesupercapacitor market.

5.1.5 GSM applications

During the short 0.5 ms pulse of 1 A, thebattery voltage drops considerably. If it is belowa certain limit, the phone is not longer operable.With a supercapacitor the voltage drop isreduced significantly and it takes much longeruntil the critical low voltage is reached duringthe pulse. In essence the operation time of thephone is extended.

5.1.6 Other applications

There are a variety of other very interestingapplications, which also emphasizes theeconomically interesting aspects of thesupercapacitors for high-power densityapplications. Additional applications may befound in:

ú elevators, cranes or pallet trucks in theelectric transportation domain

ú handtools or flashlightsú radars and torpedoes in the military domainú defibrillators and cardiac pacemakers in the

medicinal domainú pulsed laser and welding in the industryú memory supplies in phones or computers.

5.2 SAM

Most of the applications, where supercapacitorsmay be involved, need an additional battery toinsure a certain amount of autonomy. Thanksto the great know-how of the HTA-Lucerne inapplications with supercapacitors and theirintegrated intelligent electronic controlmanagement systems, montena componentsand the HTA-Lucerne became partners indeveloping the already mentioned SAM (SuperAccumulator Module), consisting of thehybridization of supercapacitors and batterieswith an intelligent control for universalapplications (Figure 9) [19].The parallel connection of the ECDL with abattery must be done in a clever way becausebatteries and capacitors have a fundamentaldifferent dynamic behavior. The batteries arefaradaic (redox) storage devices. During theircharging or discharging the voltage remains inprinciple constant. The capacitors areelectrostatic storage devices. The voltagechanges proportionally to the charge Q.

5.3 Voltage repartition

The main difficulty with the supercapacitors istheir extreme low operating voltage. In the case

of organic electrolyte for example this voltagelays between 2 and 3 Volts (with some hope to4 Volts in future). Contrarily most powerfulapplications need much higher voltages up to700 V. The main reasons of these operating

Figure 8: Hybrid vehicle sketch

Figure 9: Supercapacitor and battery in a SuperAccumulator Module (SAM) [18]

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voltages are, on one side the current reductionwhich requires smaller and lighter conductors,and on the other side, the voltage drop in thesemiconductors which is proportionally muchless important in comparison to a 2.5 Vapplication. To reach the required applicationvoltage the super-capacitors are connected inseries to form a "system".

2 electrodynamics laws govern the voltagerepartition between the single cells. Incontinuous operation the voltage is distributedthrough the cells in function of their parallelresistance. Those, which have a higherresistance, are submitted to a higher voltage. Acell with a small resistance has a behaviorwhich looks like a short. In transient operationthe voltage is distributed through the cells infunction of their capacitance. Those, whichhave a higher capacitance, need more time tochange their voltage.If all the cells had the same parallel resistanceand the same capacitance, the voltage wouldbe equal on each of them. It is actually not thecase because several factors induce ascattering of the supercapacitor properties:

ú the manufacturing process (5-10%)ú the temperature gradient in the systemú the cell aging

To prevent the decompostion of the electrolyte,which causes a supercapacitor failure, thevoltage between the electrodes must bemaintained under the electrolyte decompositionpotential. This later depends on the electrolytetype, but also on the materials which are usedfor the electrodes, the conductors and the can.The voltage repartition is not only important forsecurity reason but also to assure an equalaging of the cells. The problem is well knownfor electrolytic capacitor where the voltagerepartition is generally controlled with resistors.It does mean finally that each supercapacitorcell must be controlled or protected.

The different methods, which are used toequilibrate the voltage through the cells, areeither based on external parallel resistances, onZener diodes or on an active power electronic[21]. In the case of the external resistance, theyhave to be about typically 10 time less resistivethen the internal parallel resistances in order todominate them. The losses are therefore muchmore important. The typical system efficiencymay be estimated to 16%. In the case of Zenerdiodes, the principle is that the diode start to be

conducting when the voltage exceed the redactvoltage. This method may be considered as acell protection. When the system is working atvoltage lower then the nominal one, the cellvoltage distribution is not corrected. The agingwill be different from one cell to the other. Theefficiency is improved to 90%. With the activeelectronic (Figure 10), the system efficiencymay rise to 97%. The losses are due to thetransistor resistance.

Figure 10 : Electronic schema and setup of theactive voltage sharing with Boostcaps [21]

The costs of the system are strongly related tothe electronic components choice, especially tothe current they have to manage. This laterdepends on the level of the cell propertiesunbalancing. Performant manufacturing pro-cesses and a good temperature repartition inthe system is very important to reduce costs.

To determine the electronic components size itis necessary to analyze the type of voltagestresses in the application. Generally steadystate and transient stresses are superposed.The parallel resistance distribution is importantfor capacitors, which are permanently charged.The time constant to reach a steady statebehavior is given by T = Rp C which is typicallyequal to about 500 hours.

VI. Conclusions

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Supercapacitors may be used wherever highpower delivery or electrical energy storage isrequired. Therefore numerous applications arepossible. The use of supercapacitors allows acomplementation of normal batteries. Incombination with batteries the supercapacitorsimprove the maximum instantaneous outputpower as well as the battery lifetime. In order toincrease the voltage across a supercapacitordevice, a series connection is needed. Anactive voltage repartition device has beendefined which ensures no over-voltage over anysupercapacitor and an optimal efficiency.

VII. Acknowledgements

The authors gratefully acknowledgecollaborations with A. Züttel, Ch. Emmeneggerand L. Schlapbach of the University of Fribourg,L. Diederich and P. Milani from the University ofMilano, R. Kötz, S. Müller and M. Bärtschi fromthe PSI in Villigen, V. Härri, P. Erni and S.Egger from the HTA in Lucerne and A. Ruferfrom the LEI EPF in Lausanne. The authors arevery much indebted to the CREE-RDP, PSEL,BFE and CTI, which are supporting the EDLCresearch.

VIII. References

[1] A. Schneuwly, R. Gallay, Proc. PowerConversion PCIM 1999

[2] D. Ue, K. Ida, S. Mori, J. Electrochem.Soc. 141, 1994, 2989

[3] A. Yoshida, S. Nonaka, I. Aoki, A. Nishino,J. Power Sources 60, 1996, 213

[4] Y. Kibi, T. Saito, M. Kurata, J. Tabuchi, A.Ochi, J. Power Sources, 60, 1996, 219

[5] H. Shi, Electroch. Acta, 41, 1996, 1633[6] L. Diederich, P. Milani, A. Schneuwly, R.

Gallay, Applied Physics Letters 75, Nr 17,1999, 2662

[7] I. Tanahashi, A. Yoshida, A. Nishino, J.Electrochem. Soc. 137, 1990, 3052

[8] S.T. Mayer, R.W. Pekala, J.L. Kasch-mitter, J. Electrochem. Soc. 140, 1993,446

[9] S. Iijima, Nature 354, 1991, 56[10] T.W. Ebbesen, Nature 358, 1992, 220

[11] C. Emmenegger, A. Züttel, L. Schlapbach,A. Schneuwly, R. Gallay, Applied SurfaceScience, to be printed

[12] M.Endo, K. Takeuchi, S. Igarashi, K.Kobori, M. Shiraishi, H.W. Kroto, J. Phys.Chem. Solids 54, 1993, 1841

[13] P.L. McEwen, Nature 393, 1998, 16[14] A. Beliakov, ELIT company, 9th Seminar

ECDL Deerfield Beach FL, 1999[15] D. Evans, 9th Seminar ECDL Deerfield

Beach FL, 1999[16] R. Kötz, Proc. 7th Seminar ECDL

Deerfield Beach FL, 1994[17] Proc. 7th - 9th Seminar ECDL Deerfield

Beach FL, 1997-1999[18] O. Haas and E.J. Cairns, Annu. Rep.

Prog. Chem., Sect. C, 95, 1999, 163[19] V. Härri, SEV/VSE 1, 1999, 25[20] J. Nickerson, Fullpower Technologies, 9th

Seminar ECDL Deerfield Beach FL, 1999[21] P. Barrade, S. Pittet, A. Rufer, Proc.

Power Conversion PCIM 1999