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AT THE EDGE130 UltracapacitorsChallenge the BatteryJohn M. MillerA new generation of devicesthat store accumulatedelectrical charges in the vastinterstices of the nanoworldwill soon achieve energy-stor-age densities rivaling thoseoffered by the long-dominantsource of bottled electricalenergy.

NATURE WALK138 Giants of the DeepAlessandro De MaddalenaFamous for their colossal sizeand power, whales aremammals that inhabit a widerange of aquaticenvironments, navigating andcommunicating insophisticated ways.

IMPACTS144 Agent of MassProtection andBeautificationAndrew ChristopherImportant throughout historyand especially in theRenaissance, paint coveredthe Titanic, and still todaypreserves not only the EiffelTower, but also cars, bridges,ships, and white picketfences.

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UltracapacitorsChallenge theBatteryJohn M. Miller

Traditionally quick and powerful but energypoor, capacitors have transmuted into quick,powerful, and energy-rich storage deviceswhose first applications are likely to be inhybrid electric vehicles and backup powersupplies.

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Known for storing a short-livedjolt of electricity essential to thesuccessful operation of electricalcircuits in devices and appliancesranging from PCs to microwaveovens, cell phones, and televi-sions, the capacitor is in themidst of a major, ongoingupgrade of its energy storagecapabilities. After nearly two cen-turies in which batteries havebeen the obvious choice for stor-ing usable amounts of energy,high-end capacitors, known asultracapacitors, are poised tochallenge them in a growingrange of applications.

“In fuel cell vehicles, ultraca-pacitors have demonstrated a

higher recovery of energy frombraking than batteries, are con-siderably lighter, have a longereconomic life, and are more envi-ronmentally friendly in theirmanufacture and disposal,” saysPierre Rivard, president andCEO of Hydrogenics of Missis-sauga, Ontario, a clean powergeneration company with a focuson fuel cells.

Looking beyond applicationsin cars, he continues, “Whenpaired with fuel cells in stop-and-go mobility applications, suchas forklifts, ultracapacitors pro-vide burst power for lifting andacceleration and enable regener-ative braking; in backup power

applications [ranging from hos-pitals to office buildings, facto-ries, and homes], they provideinstantly available short-term

bridge power. In manyapplications they bufferpower demand peaks,allowing our scalablefuel cell systems to beoptimized for size andlow cost.”

Honda Motor Company isusing ultracapacitors in its FCXhybrid fuel cell vehicle, a few testmodels of which are already onthe road in California. Accordingto a spokesman for Honda, “Uti-lizing ultracapacitors, we havegained an edge in energy effi-ciency and throttle responsive-ness over competitors that arepursuing the hybrid battery/fuelcell model.”

In February 2004, Maxwell

Technologies of San Diegoannounced that it has contractedto provide ultracapacitors for 27hybrid gasoline-electric busesbeing built for Long Beach Tran-sit of Long Beach, California.Beyond these already superlativeultracapacitors, yet another gen-eration with 10 times moreenergy-storage capacity wasrecently announced.

Battery’s older brother

The capacitor has come along way since it was invented in1745 as a liquid-filled glass jarwith a layer of foil wrappedaround the outside. Throughtheir generations of technological

improvement, capacitors haveprogressed from a laboratorycuriosity to an important labora-tory instrument and, throughoutthe twentieth century, a key com-ponent of electrical circuits. Thebasic principle underlying thecapacitor’s operation is that ofcharge storage: Positivelycharged particles collect on onesurface and negatively chargedparticles on a second nearby, butelectrically separate, surface. The

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■ Above: Investigating possibleautomotive applications forultracapacitors, MIT researcherRiccardo Signorelli here is setting upa test of the charge and dischargebehavior of a 3,000-farad capacitor,whose stored energy is about one-eighth that of a D cell battery. Right:A high-resolution scanning electronmicrograph of the ultracapacitor’selectrode material reveals a tinyfraction of its highly involutedsurface, on which charged speciesform a double layer.

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two surfaces are called electrodes.Capacitors store electricalcharges in static form for lateruse.

Three main factors deter-mine how much electrical energya capacitor can store: the elec-trode surface area; the electrodeseparation distance; and theproperties of the insulating layerseparating the electrodes. Thehistory of capacitors has beenwritten by numerous scientists,who have discovered the princi-ples of capacitor operation andimproved their storage capacityby increasing the electrode sur-face area, decreasing the elec-trode separation distance, andimproving the insulating layer.

The physics of electricityevolved in tandem with improve-ments to the original jar accu-mulator, which was soon calledthe Leyden jar after the citywhere it was invented. An earlyand important technical improve-ment involved replacing the fluidelectrode with a layer of foil lin-ing the jar. Other important

developments included replacingthe jar’s enclosing glass wall witha glass plate, which in turn wasreplaced by thinner and more pli-able insulating materials. On aparallel path, electrode materialsbecame thinner.

These developments openedthe way for the spiral-woundcapacitor invented in 1926 byRobert Sprague. To make it,Sprague simply rolled together apair of thin conducting foils (theelectrodes) separated by a paperinsulating sheet, or dielectric.

During the early 1980s, ITWPaktron of Lynchburg, Virginia,and Siemens (now Epcos) ofMunich developed stacked filmcapacitors for use in consumerelectronics, automobiles, andappliances. Called polymer mul-tilayer capacitors, such units aresimply stacks of several thousandpairs of conducting plates, eachseparated by an insulator. Bothspiral-wound and polymer multi-layer capacitors are examples ofelectrostatic capacitors, which arebased on the original concept of

two physically distinct electrodesseparated by a distinct insulatinglayer.

Electrostatic capacitors arewidely used today in virtuallyevery electronic item, from con-sumer appliances and toys toelectronic boards in computers forPCs and spacecraft. In most ofthese applications, capacitors aretiny ceramic bricks attacheddirectly to the electronic circuitboards. The ability to store smallamounts of electricity and releasethem quickly makes capacitorsessential components, along withtransistors and resistors, of mostelectrical circuits.

Capacitors’ Achilles’ heel

The Leyden jar’s property ofreleasing all of its stored electricalenergy in a sudden spurt nodoubt inspired scientists to seek atechnology that could release asustained current. This technol-ogy, the battery, was invented in1800 by Alessandro Volta, an Ital-ian physicist.

As Volta and numerous otherscientists improved its perfor-mance, the battery quickly sup-planted the Leyden jar’s descen-dants. It has reigned as the

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■ Capacitors of one of the threedifferent types represented hereperform essential roles in mostelectronic circuits. Note theirrelatively small capacitance rating,such as .22 microfarads (220billionths of a farad) and 1,000microfarads (one-thousandth of afarad).

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preferred technology for storingelectricity for nearly 200 years.The battery doesn’t store sepa-rated charged particles; instead,it stores charge through chemicalchanges at the electrodes. Thanksto the chemical changes, the bat-tery can save large amounts ofelectrical energy and releasethem as a sustained current. Onthe downside, just as the batteryis slow to take on its full charge,it is also slow to release thecharge.

Ongoing improvements

The electrolytic capacitor wasdeveloped in the 1930s by theCornell-Dubilier Electric Corpo-ration in New Jersey. Thinkingout of the box, the company’s sci-entists and engineers introduceda new way of designing capacitorsfeaturing three major enhance-ments:

● Expanded surface area:The surface of one aluminumelectrode was etched with acid,leaving it roughened and pock-marked and offering more surfacearea on which to accumulatecharge.

● Shrunken insulator thick-ness: After the electrode surfacewas etched, it was oxidized tocover it with an insulating layerof aluminum oxide that separatestwo layers of charges.

● A liquid (actually paste-like) electrolyte electrode: Theroughened and oxidized surfaceof the aluminum electrode wasimmersed in an electrolyte, asolution whose dissolved mole-cules are readily ionized. Theelectrolyte in effect becomes anextension of the second electrode,the enclosing wall of the capaci-tor.

Although an electrolyticcapacitor looks different than anelectrostatic capacitor, it nonethe-less exhibits all the characteris-tics of an electrostatic device: ithas one conductive electrode sep-arated from a second conductiveelectrode by a thin dielectric.Here, the operative word is thin.In an electrostatic capacitor, forcomparison, the insulator may bea thin plate of glass or ceramic, asheet of wax paper, or a piece ofmica. As these materials aremade thinner, however, they soonreach a limit—about one-tenth ofa millimeter (10-3 meters)—basedon their inherent brittleness andlimited ability to withstand avoltage.

In comparison, electrolyticcapacitor designs reduce the insu-lating layer’s thickness dramati-cally by growing a thin film of alu-mina (Al2O3) over all themicroterrain of the etched alu-minum foil. This resulting insu-lating layer is only a few microm-

eters (10-6 meters) thick, so thatcharged species on opposite sidesof it are separated by no morethan a micron (10-6 meters). Forelectrolytic capacitors, the etchedand oxidized metal foil is both anelectrode and the insulating layer.

The electrolytic capacitor’ssecond electrode is its containerwall and the organic electrolytethat is in contact with the wall.The electrolyte permeates a sep-arator material (between the con-tainer wall and the foil electrode)and wets the coiled-up and etchedmetal foil. Such conductive elec-trolytes consist of a paste madewhen boric acid dissolves in andreacts with glycol, the thick liq-uid commonly used in antifreeze.The high ratio of surface area (onthe etched foil surface) to smallcharge-separation distance(across the aluminum oxide layer)accounts for the electrolyticcapacitors’ ability to store muchmore electricity than comparablysized electrostatic capacitors.

Going ultra

Ultracapacitors embody anotherround of innovations beyond theelectrolytic capacitors. Thecharge-separation distance inultracapacitors (more techni-cally known as electrochemicaldouble-layer capacitors) hasbeen reduced to literally the

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Electrostatic capacitors are widelyused in virtually every electronicitem, from consumer appliances toelectronic boards in computers.

dimensions of the ions them-selves within the electrolyte.Here, charges are not separatedby millimeters or micrometers(microns) but by a few nanome-ters. In our three examples, rang-ing from electrostatic capacitorsto electrolytic capacitors to ultra-capacitors, the charge-separationdistance has in each instancedropped by three orders of mag-nitude, from millimeters (10-3

meters) to microns (10-6 meters)to nanometers (10-9 meters).

Coupling the ultrasmall sep-aration distance with a relativelyvast surface area, in ultracapaci-tors the ratio of available surfacearea to charge-separation dis-tance has grown to an amazing10 raised to the twelfth power. Itis this ratio, in fact, that makescapacitors “ultra.” The ability tohold opposite electrical charges instatic equilibrium across molecu-lar spacing is the key feature.[For more details about how

ultracapacitors achieve molecu-lar-scale charge separation, see“Making a Capacitor Ultra” onpage 133.]

The developmental pathleading to today’s ultracapacitorsoriginated in the work of Stan-dard Oil of Ohio Research Center(SOHIO) in the early 1960s.SOHIO researchers discoveredthat two pieces of activated car-bon immersed in an aqueous elec-trolyte solution and connectedacross the terminals of a batteryacted as a capacitor. Later,SOHIO’s scientists explored theuse of organic electrolytes, but atthe time (early 1970s) there wasreally no market for such devicesand little understanding of whatwas happening in them.Nonetheless, this new type ofcapacitor worked very well.

SOHIO licensed its double-layer capacitor technology, as itcame to be known, to NEC in1971. During the 1980s Mat-

sushita Electric Companypatented a method of manufac-turing ultracapacitors havingimproved electrodes. As design-ers became more familiar withthe technology, applications pro-liferated, especially for the coincell types of ultracapacitors suchas those manufactured in Japanby Nippon Electric Company,Elna/Asahi Glass, and Mat-sushita. (Coin cell capacitors aresimilar in appearance to thesmall batteries common towatches, cameras, and portableelectronics.)

In early coin, or “button,”cells, one electrode was the shal-low aluminum can forming thebase of the cell. The second elec-trode was the combined unit ofthe disk-shaped aluminum lidplus an attached porous carbonpellet formed by pressingtogether activated carbon powderand dilute sulfuric acid. In thesecells, the carbon pellet and alu-minum can are electrically iso-lated from each other by an ion-permeable separator. Contactbetween the can and lid isblocked by a rubber gasket.

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■ Connected to a voltage source, adouble-layer capacitor (anultracapacitor) forms a double layerof charged species at the surface ofeach electrode. At the negativeelectrode, positive ions from theelectrolyte pair up with electronstrapped below the electrode surface.The opposite pairing occurs at thepositive electrode, where thepositive charges below the surfaceare holes vacated by the electronsthat have accumulated at thenegative electrode.

The Double-Layer Capacitor

Battery

Electrode Electrode

Electrolyte

Throughout the 1980s and ’90s,manufacturing of ultracapacitorswas primarily an art.

Up, down, and everywhere

Giving numbers to trends incapacitor performance and costsrequires some capacitor lan-guage: capacitance and farad.Capacitance refers to the capaci-tor’s unique ability to store elec-trostatic energy (which is differ-ent than the electrochemicalenergy stored by the battery). Afarad is the unit measure ofcapacitance. Today’s ultracapaci-tors achieve capacitances rangingup to 2700 farads, while thewhole family of capacitors offerscapacitances ranging down tomicrofarads (10-6 farads), nano-farads (10-9 farads), and evenpicofarads (10-12 farads).

Recently, automated assem-bly techniques have replaced thelabor-intensive aspects of ultra-capacitor manufacturing andcosts have decreased substan-tially. For instance, in mid-1980a 2.3-volt ultracapacitor rated at470 farads and manufactured byPanasonic (Matsushita Electric)cost roughly $2 per farad. Today,that same ultracapacitor wouldcost one-twentieth as much at 10cents per farad, and costs con-tinue to decrease rapidly as ongo-ing automation replaces handassembly. According to informedsources, when ultracapacitorcosts decrease by another factorof 20 to below 0.5 cents per farad,these components will be afford-able in mass-market automotive

applications.Scientists are busy on the

frontiers of ultracapacitorresearch, pushing up the capaci-tance rating and pushing downthe costs. In October 2003,JEOL Ltd. in Tokyo announcedan improved ultracapacitor it

refers to as a nanogate or nano-carbon capacitor. This new com-ponent has an energy densityof 50–75 watt-hours per kilo-gram, more than 10 times that ofexisting ultracapacitors. Thedevice features two carbon elec-trodes formed of a new, patented

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Making a Capacitor UltraUltracapacitors resemble batteries in having two electrodesimmersed in an electrically responsive liquid, the electrolyte.Applying a potential (voltage) across the ultracapacitor’s electrodespolarizes the electrolyte, with roughly half of the electrolyte mole-cules transferring an electron to the other half. The resulting pos-itive and negative ions migrate via the impressed electric field totheir respective electrodes. There, although they form a chargedlayer on the surface and the electrode is oppositely charged, no elec-trons are exchanged across the electrode surface due to the elec-trode’s electrochemical properties. Contact between the two elec-trodes is blocked by a porous separator.

Although the electrodes appear to be a lightweight, solid layerof carbon, examination at the nanometer scale reveals a vastlabyrinth of interconnected, nearly uniformly sized caverns whosewalls all become charged when a voltage is applied across the twoelectrodes.

The physicists’ model of conduction-band electrons in metalshelps explain what happens inside the carbon when the voltage isapplied. All of the involuted surface area of each electrode becomesan energy-level boundary. Just beneath the surface of the negativeelectrode, for example, is a conduction band occupied by a horde ofroving electrons that lack the energy to escape from the surface. Ina similar band at the positive electrode, “holes,” or electron vacan-cies, rove beneath the surface but are unable to capture electronsfrom outside.

When positively charged electrolyte ions form a layer on thesurface of the negative electrode, electrons beneath the surface pairup with them. These two layers of separated charges, then, are acapacitor storing static charge. Similarly, at the positive electrode,holes pair up with negative ions, forming a second electronic dou-ble layer that itself is a capacitor. Electrochemists and engineersdescribe capacitors based on this design concept as electrochemicaldouble-layer capacitors.

For each of the two electrochemical double layers, the negativeand positive charges are separated by only half the diameter of theelectrolyte ions. This molecular-scale charge-separation distance,coupled with the great surface area of the activated carbon elec-trodes, yields the ultracapacitor’s extreme storage capabilities.

—J.M.M.

material whose uniqueness liesin its high porosity and accessi-bility for storing ions. The com-pany’s goal is to start shippingsamples of nanogate capacitorsby the end of 2004.

Even further out on theexperimental edge, researchersare exploring the possibility ofusing carbon nanotubes for ultra-capacitor electrodes. The impor-tance of carbon nanotubes lies intheir uniform nanoscopic pores(about 0.8 nanometers in diame-ter), which could in theory storemuch more charge than thenanogate capacitors if the nan-otubes could be properly assem-

bled into macroscale units.The leading manufacturers

of ultracapacitors today areMaxwell Technologies in theUnited States, NESS CapacitorCompany in South Korea, Oka-mura Laboratory in Japan, andEPCOS in Europe. These compa-nies manufacture carbon-carbon,or symmetric, ultracapacitors.That is, both electrodes haveidentical construction. There aresome differences in the organicsalts and solvents used, and thisis where ultracapacitor manufac-turing becomes proprietary.

According to Andrew Burkeof the Institute for Transportation

Studies, University of Californiaat Davis, the carbon-carbon ultra-capacitors all have relatively sim-ilar ratings. The ultracapacitorsproduced by the companies listedabove are rated at 2.5–2.7 volts,with specific capacities clusteredat 5 farads/gram (packaged prod-uct). By contrast, the newnanogate device exhibits a capac-itance of 30 farads/gram.

Applications of ultracapacitors

Ultracapacitors are now findingtheir way into automotive andutility applications as energystorage components. Utilitieshave interest in ultracapacitorsas replacements for batterybanks that are being used tobuffer short-term outages on thepower grid. There are also appli-cations of ultracapacitors inuninterruptible power sourceslocated on the premises of criti-cal-load utility customers such as

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■ Left:The ultracapacitor bankcarried on the Honda FCX fuel cellcar comprises 160 ultracapacitors,as shown in the cutaway modelmounted on a demonstration base ofthe car. With each ultracapacitorcarrying a 2.7-volt charge, the entirebank delivers electricity at more than400 volts. The red fuel tank directlybelow the ultracapacitor containscompressed hydrogen, with whichthe car’s fuel cell generateselectricity for storage in theultracapacitor or for powering theelectric motor that drives the wheels.Left below: In the FCX, theultracapacitor bank (yellow arrow) ispositioned behind the rear seat, asshown in the FCX cut-body model.

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hospitals, banking centers, air-port control towers, and cellphone towers. The ultracapacitorbank would supply a continuousflow of power to such customersduring the critical secondsbetween a utility outage andbringing a standby diesel-engine-driven generator on line.

Perhaps the most pervasiveapplication of ultracapacitors aspower components is beginningto show up in fuel cell–poweredautomobiles, a few of which arebeing manufactured by HondaMotor Company, as mentionedearlier, and also by Toyota, Gen-eral Motors, and others for leaseto cities in the United States andelsewhere. The performance pro-files of ultracapacitors and fuelcells are highly complementary,especially for powering vehiclesdriving in stop-and-go traffic.Fuel cells provide the sustainedenergy as it is needed, but theyfall short in delivering the burstenergy needed for starting andaccelerating. Ultracapacitorsexcel at providing exactly thoseshort bursts of energy and also atreceiving and storing energybursts produced by regenerativebraking.

Ready for the lithium-ion bigleague?

Lithium-ion batteries of the typeused in cell phones, laptops, andgasoline-electric hybrid cars andfuel cell vehicles are very ener-getic compared to ultracapacitors.Typically, today’s commerciallyavailable ultracapacitors deliver

only one-tenth the energy of acomparable-weight battery, butbecause they deliver energy muchfaster than a battery does, theyare gaining ground in a marketalready strongly courted by bat-tery manufacturers.

The announcement of thenanogate capacitor heralds theimminent arrival of the Leydenjar’s descendants into thelithium-ion big league. Whennanogate capacitors enter themarketplace they will offer therapid charge and discharge prop-erties of ultracapacitors alongwith the energy storage capacityof batteries. This is amazing fora device that is simply an accu-mulator of electric charge.

When capacitors can store asmuch energy as batteries whileavoiding much of the environ-mental threat posed by the met-als (such as lead, nickel, cad-mium, and mercury) required torun the battery’s electrochemicalprocess, a new era of energy fortransportation will begin. Costs,of course, will need to come down,and the devices will need to beproven functional and highly reli-able in daily use.

As each of these obstacles ismet and overcome, the new ultra-capacitors coupled with fuel cellswill be a major factor in the shifttoward automotive systems thatare environmentally friendly andfuel efficient. Beyond automo-biles, as well, the new technologyseems likely to infiltrate thenooks and crannies occupied bytoday’s batteries, ranging fromflashlights to cell phones and lap-

top computers. In the twenty-firstcentury, the capacitor may finallyget the respect that until now hasbeen claimed by its youngerbrother.■

John M. Miller, owner of J-N-J MillerDesign Services, in Cedar, Michigan,holds 44 patents on various aspects ofautomotive power and propulsion sys-tems. He chairs the KiloFarad Interna-tional Education and Outreach work-ing group devoted to promotingultracapacitor technology. He retiredfrom Ford Motor Company’s ScientificResearch Laboratories in 2002. Theauthor extends his thanks to the follow-ing for helpful discussions and com-ments: Richard Smith Sr., MaxwellTechnologies; Robert Waterhouse, AmtekResearch; Joel Schindall, MIT; TomSaunders, ITW Paktron; and ClaudeLetournou, KiloFarad International.

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On the Internet

EDUCATION AND OUTREACH

KILOFARAD INTERNATIONAL

www.kilofarad.org

MANUFACTURERS

EUROPE: EPCOSwww.epcos.com/web/home/html/home_e.html

JAPAN: OKAMURA LABORATORY;JEOLwww.okamura-lab.comwww.jeol.co.jp

RUSSIA: ESMA-CAPACITOR

www.esma-cap.com

SOUTH KOREA: NESS CAPACITOR

COMPANY

www.nesscap.com/index.html

U.S.: MAXWELL TECHNOLOGIES

www.maxwell.com/ultracapacitors/index.html