introduction to fuel cell technology

7/28/2019 introduction to fuel cell technology

1/12

Introduction to fuel

byBrianCookWhereas the 19th century was the century of the steam engine and the 20th century was the century oftheinternal combustion engine, it is likely that the 21st century will be the century Ofthe uel cell.Full cells are now on the vergeofbeing introduced commercially, revolutionising the way we presentlyproduce power. Fuel cells can use hydrogen as afuel, ofering the prospectofsupplying theworld with clean, sustainable electrical power.

fuel cell by definition is an electrical cell,whch unhke storage cels can becontinuously fed with a fuel so that theA lectrical power output is sustained

indefinitely. It converts hydrogen, or hydrogen-containing fbels, &rectly into electrical energy plusheat through the electrochemical reaction of hydrogenand oxygen into water. The process is that ofelectrolysis in reverse:overal reaction: 2H2(gas)+0 2 gas)+2H20 +energy(1)Because hydrogen and oxygen gases are electro-chemically converted into water, fuel cels have manyadvantages over heat engines. These include: hghefficiency, virtually silent operation and, ifhydrogen is the fuel, there are no pollutantemssions. If the hydrogen is produced fromrenewable energy sources, then the electricalpower produced can be truly sustainable.

Thet wo principal reactions in the burning ofany hydrocarbon he1are the formation of waterand carbon &oxide.Asthe hydrogen content inafuel increases, the formation ofwater becomesmore significant, resulting in proportionallylower emssions of carbon dioxide (Fig. 1).Asfuel use has developed through time, thepercentage of hydrogen content in the fues hasincreased. It seemsanatural progression that thefuel of the futurewill be 100%hydrogen.

in 1839. The principle was dxovered by accidentduring an electrolysis experiment. When SirW&amdisconnected the battery from the electrolyser andconnected the two electrodes together, he observed acurrent flowing in the opposite drection, consumingthe gases of hydrogen and oxygen (Fig. 2). He calledthis device a gas battery. His gas battery consistedofplatinum electrodes placed in test tubes of hydrogenand oxygen, immersed in a bath of ddute sulphuricacid. It generated voltages of about 1V.In 1842 Groveconnecteda number of gas batteries together in seriesto forma gas chain. He used the electricity producedfrom the gas chain to power an electrolyser, splittingwater into hydrogen and oxygen (Fig. 3). However,due to problems of corrosion of the electrodes andinstabdity of the materials, Groves fuel cell was not

Historyof fuel cellsThe kas battery

SirWaam Grove(1811-96),aBritish ayerand amateur scientist, developed the first fuel cell Fig. 1 Trends in th e use of fu els. As fuel has developed throug htime, the p ercentage of h ydro gen content in the fu el has increasedENGINEERING SCIENCE AND EDUCATION OURNA L DECEMBER 2002

205

Authorized licensed use limited to: LIVERPOOL JOHN MOORES UNIVERSITY. Downloaded on September 18, 2009 at 11:47 from IEEE Xplore. Restrictions apply.


2/12

Fig. 2 Principle of an electrolyser (lef t) and a fue l cell (see Reference 2)

practical.Asaresult, there was little research and furtherdevelopment oL@el cels for many years to follow.The 'Bacon uel cell'

Significant work on fuel cels began again in the1930s,by Francis Bacon, a chemical engineer at theUniversity of Cambridge, UK. In the 1950sBaconsuccessfully produced the first practical fuel cell, whichwas an &&ne version (Fig. 4). I t used an alkalineelectrolyte (moltenKOH) instead of dhte sulphuric

acid. The electrodes were constructed of poroussintered nickel powder so that the gases could di&ethrough the electrodes to be in contact with theaqueous electrolyte on the other side of the electrode.Thsgreatly increased the contact area contact betweenthe electrodes, the gases and the electrolyte, thusincreasing the power density of the fuel cell. Inaddition, nickel was much less expensive thanplatinum. The chemical reactions in the alkaline fuelcell are:

- 12

Fig. 3 Grove's 'gas battery' ( 1839)produced a vol tage of abo ut 1V (left); Grove's 'gas chain' po w eri ng an electrolyser ( 1842)(see Reference3)ENGINEERING SCIENCEAND EDUCATION JOURNAL DECEMBER 2002

206



3/12

anode reaction: 2H2+4OH-+4&O+ 4e- (2)cathode reaction: 0 2 +4e- +2H20+40H- (3)overall reaction: 2Hz +0 2 -+2H20 (4)Fuel cells-forNASAFor space applications, fuel cells have the advantageover conventional batteries in that they produce severaltiniesas much energy per equivalent unit ofweight. Inthe 1960s, International Fuel Cells in Windsor,Connecticut, USA, developed a fuel cell power plantfor the Apollo spacecraft. The plant, located in theservice module of the spacecraft, provided bothelectricity as well as drinking waterfor the astronauts on their journeyto the moon. It could supply 1.5kWof continuous electrical power. Fuelcell performance during the Apollomssions was exemplary. Over10000 hours of operation wereaccumulated in 18 nlissions,without a single in-flight incident(Internet source: IFC).

In the 1970s, International FuelCells developed a more powerfiialkaline fuel cell for NASA's SpaceShuttle Orbiter (Fig. 5). TheOrbiter uses three fuel cell powerplants to supply a l the electricalneeds during bght. There are nobackup batteries on the SpaceShuttle, and as such, the fuel cellpower plants nust be highly reliable.The power plants are fuelled byhydrogen and oxygen fromcryogenic tanks and provide bothelectricalpower and drinking water.Each fuel cell is capableofsupplying12kW continuously, and up to

Fig. 4 Bacon's lab or ato rya t the Department o fChemical Engineering,Universi ty of Cambridge(1955). A f uel cell can beseen being assembled onth e leftof the pic ture(photo: cou rtesy ofDepart men t of ChemicalEngineering, Universi ty o fCambridge)

16kW for short periods. The Orbiter ukts represent asignificant technology advance over Apollo, producingabout ten times the power froma sindar sized package.In the Shuttle progranme, the fuel cels havedemonstrated outstanding reliabhty (over 99%availabhty). Todate, they have flown on 106nlissionsand clocked up over 82000 hours of operation(Internet source: NASA).Alkaline uel cells or terrestrial applicationsCompared with other typesof fuel cells, the alkalinevariety offered the advantage of a high power to weight

Fig. 5 NASA Space Shuttle Orbiter fue l cell, on e of thr ee fuel cells aboar d theSpace Shut tle. These fuel cells prov ide all the electricity as well as drin king w aterwh en th e Space Shuttle is in flig ht . It produces 12kW(e) and occupies 154 litres(photo: cour tesy of NASA)ENGINEERING SCIENCE AND EDUCATION OURNAL DECEMBER 2002

207



4/12

Fig. 6 Two p rototype automobi les pow ered bv Ballard fuel cells, the NECAR 5 an d JEEP Commander, from DaimlerChrysler(photo: cour tesy of DaimlerChrysler)ratio. This was primarily due to intrinsically fasterlanetics for oxygen reduction to the hydroxyl anion inan alkahne environment. Therefore alkaline fuel celswere ideal for space applications. However, forterrestrial use, the primary dmdvantage of these celsisthat of carbon dioxide poisoning of the electrolyte.Carbon &oxide is not only present in the air but asopresent in reformate gas, the hydrogen rich gasproduced from the reformation of hydrocarbon fuels.In the poisoning of an &&ne fuel cell, the carbondioxide reacts with the hydroxide ion in the electrolyteto form a carbonate, thereby reducing the hydroxideionconcentration in the electrolyte. This reduces theoverall efficiency of the fuel cell. The chemicalequation for carbon &oxide reacting with apotassiumhydroxide electrolyteis:

Because of the complexity of isolating carbon dioxidefiom the &&ne electrolyte in fuel cels for terrestrialapplications, most &el cell developers have focusedtheir attention on developing new types usingelectrolytes that are non-alkaline. These fuel celsinclude: solid oxide fuel cells (SOFC), phosphoric acidfuel cels (PAFC), molten carbonate fuel cels (MCFC)and polymer electrolyte membrane or protonexchange membrane (PEM) fuel cells.PEM uel cellIn the early 1960s, General Electric (GE) aso madea significant breakthrough in fuel cell technology.Through the work of Thomas Grubb and LeonardNiedrach, the company invented and developed thefirst polymer electrolyte membrane (PEM) fuel cell. Itwas initially developed under aprogramme with the

USNavys Bureau of Ships andUS Army Signal Corpsto supply portable power for personnel in the field.Attracted by the possibility of using GEs PEM fuel

cell on the Apollo mssions, NASA tested its potentialto provide auxiliary power onboard its Geminispacecrafi. The Gemini space programme consisted of12flights in preparation for the Apollo mssions to themoon. For lunar fights, a longer power source wasrequired than could be provided by batteries, whichhad been usedonprevious space flights. Unfortunately,the GE fuel cell, model PB2, encountered technicaldifficulties prior to launch, including the leakage ofoxygen through the membrane. As aresult the Geminimissions 1to 4 flewonbatteries instead.

The GE fuel cel was redesigned andanew model, theP3, successhlly operated on the Gemini fights6 to 12.The Gemini fuel cel power plant consisted oftwo &el celbattery sections, each capable of producinga maximumpower of about1000W (Internet source: NASA).A further limitation of the GE PEM fuel cell at thattime was the large quantity of platinum requiredas acatalyst on the electrodes. The cost of PEM fuel celswas prohbitively high, restricting its use to spaceapplications.In 1979, Geoffrey Ballard,a Canadian geophysicist,chemist Keith Prater and engineer Paul Howardestablished the company Ballard Power Systems. In theearly 1980s, Ballard took the abandoned GE fuel cell,whose patents had expired and searched for ways toimprove its power and build it out of cheapermaterial ^

Worlung on a contract fiom the CanadianDepartment of National Defence, Ballard developedfuel cels with a significant increase in power densitywhile reducing the amount of platinum required. Fromthese developments it was recognised that fuel cels

ENGINEERING SCIENCE AND EDUCATION JOURNA L DECEMBER 2002208

Authorized licensed use limited to: LIVERPOOL JOHN MOORES UNIVERSITY. Downloaded on September 18, 2009 at 11:47 f rom IEEE Xplore. Restrictions apply.


5/12

could be made smaller, more powerful and che?plyenough to eventually replace conventional powertechnologies.

Balard Power Systems has since grown to becomerecognised as a world leader in PEM fuel celtechnology, developing fuel cels with power outputsranging from 1kW for portable and residentialapplications, through to250kW for distributrd power.Ballad has formed alliances with a wide range ofcompanirs, ncluding DainderChrysler,Ford, EBARAin Japan andALSTOM in France.

In the late 1980s and early 1990s Los AlamosNational Laboratory and Texas A&M Uuiversity asomade significant developments to the PEM fuel cell.They aso found ways to signiticantly reduce theamount ofplatinum required and developeda niethodto h i t catayst poisoning due to the presenceof traceimpurities in the hydrogen fuel (Internet source: LosAlamos National Laboratory).Fuel cell app licationsPanspur tat ion

The Cahfornia Low Emssion Vehicle Program,administered by the California Air Resources Board(CARB),has been a large incentive for automobilemanufacturers to actively pursue fuel cel development.This programme requires that, beginning in2003,2%of passenger cars delivered for sae in California frommedium or large-sized manufacturers must be zeroemssion vehicles, called ZEVs. Either automobilespowered by batteries or those powered by fuel celsinert these requirements, as thc only output of ahydrogen fuel cell is pure water.

TheNECAR 5 (Fig. 6) is the latest prototype fuelcell automobile by DaimlerChrysler. This automobileis fuelled with liquid methanol which is converted intohydrogen and carbon dioxide through use of anonboard fuel processor. The vehicle has virtually nopollutant emssions of sulphur dioxide, oxides ofnitrogen, carbon nionoxide or particulates, the primarypollutants of the internal combustion engine. Theeficiency of a fuel cell engine is abouta factor of twohigher than that of an internal combustion engine andthe output of carbon dioxide, when fuelled with ahydrocarbon fuel, is considerably lower.

The NECAR 5 drives and fees like a 'normal; car.It has a top speed of over 150hdh (90mph), with apower output of75kW (100bp). t is alsobelieved thatthis vehicle wlll require less maintenance. It combinesthe low emssion levels, the quietness and the smooth-ness associated with electric vehicles, while deliveringaperformance sinularto that ofan automobile with aninternal combustion enginr.

In April I Y99 the California Fuel Cell Partnershipwas developed. Foundmg members includedDainderChrysler. he CaliforniaAn Resources Board,the Cahfornia Energy Commission, Balard PowerSystems, Ford, Shell and Texaco. The primary goas ofthe partnership are to:

Fig. 7produ ced by Bal lard Power Systems, prov ides 250kW h eatand electr ici ty which i s enough po wer for a smal l bui ld ing,a school or a comm uni ty of up to 50 homes (photo:courtesy o f Ballard Power Systems)Demonstrate vehiclr technology by operating andtesting the vehicles under real-world conditions inCalifornia.Demonstrate the viability of alternative fuelinfrastructure technology, including hydrogen andmethanol stations.Explorr the path to conunercialisation, fromidentifylng potential problcms to developingsolutions.Increase public awareness and enhance opinionabout fuel cell electric vehicles, preparing the marketfor commercialisation.

PEM fuel-cel l distr ibuted p ow er plant. This unit,

Fig. 8 Fuel-cell cogeneration pow er plant fo r resid entialappl ications, providing 7kW heat and electrici ty, enoug hpow er for a modern energy ef f ic ient home (photo:courtesy of Plug Power)ENGINEEKING SCIENCE AND EDUCATION JOURNA L DECEMBER 2002

209



6/12

Fig. 9 Prototype po rtable uel cell providing 50Welectrical power , produced by Helioc entris. The fuel cell iscontained in the upper compartment; h e hydrogen isstored in a m etal hydride canister in the lowercompartmentSince then new participants include General Motors,Honda, Hyundai, Nissan, Toyota, Volkswagen, BritishPetroleum, Exxon Mobil, US Department of EnergyandUSDepartment ofTransportation. Todate five ofthe world's leading auto manufacturers haveannounced that they plan to introduce fuel cellautomobilesbeginning in the 2003to 2005timeframe.

There are aso plans{or buses and truck, al poweredwith fuel cell engines. In 2000, Ballard completed atwo-year program of in-service field testing with sixfuel cell buses, thrrc in Vancouver, British Colunibia,and three in Chicago. Objectives of the 6eld testincluded gathering data on performance andmaintenance for me in Ballard's future heavy-dutyengine designs, assessing public acceptance, anddetermining the needs of transit authorities and usersof the buses.

The results of the tests were exeniplary-the sixbuses travelled alnlost 75000 mles and carried over200000passengen. Thirty new transit buses poweredby Balard's heavy-duty fuel cell engines will beintroducedto ten European cities beginning in 2003for additional demonstration. The resulting data will beused to further develop a commercial fuel cell bus.Dbtriburedpowerpxerdtion

Electrical energy demands throughout the world arecontinuing to increase. In Canada the demand isgrowing at an annual rate of approximately 2.6%.InAmerica the rate is about 2.4%?, and in developingcountries it is appmximatcly 696" How can theseenergy demands be niet responsibly and safely?Distributed power plants using fuel cels can providepart ofthe solution.

Distributed or 'decentrahsed' power plants,coutrastrd with centmised power plants, are plantslocated close to the consumer, with the capabilityof providing both heat and electrical power(a combination known as 'cogeueration'). Heat, thebyproduct ofelectrical power generation, is transferredfromthe fuel cell to a heat exchanger. The exchangertransfers the heat to a water supply, providing hot watcrto local customers. The overall efficiency of acogeneration system can he iu excess of SOY,,comparatively high compared to a system producingelectricity alone. An increase in efficiency naturallycorresponds to a decreasein fuel consuniption.

Distributed power plants have many additionaladvantages. For example, they can provide power to aremote location without the need of transportingelectricity through transmssion lines from a centralplant. Thereisalso an efficiency benefit in that the costof transporting fuel is more than ofiet by theelinunation of the electrical lossesof transmssion. Theability to quickly build up a power infrastructure indeveloping nations is ohen cited. Using fuel cell powerplants obviates the need for an electrical grid.

Distributed power plants can provide either primaryor back-up power. In primary applications theycan provide base-load power, operating virtuallycontinuously from the consumption of natural gas,reducing the demand from the electrical grid. This notonly decreases the cost ofdisplaced power, but can alsoresult in a reduction of demand charges imposed by theutility Should the power plant provide an excess ofelectricity, the excess can be fed back into the electricalgrid, resulting in additional savings.

In case of a power outagc on the grid, a distributedpower plant can continuetopmvidt! power to cssentidservices; eliminating the need for both anuninterruptible power supply(UPS),presently handledby lead-acid battery banks, and a standby generator, forexteiided period? of power outage. An additionalqualityof a fuel cell power plant forUPSapplicationsis that the average 'down time' is anticipated to below, 3.2 to 32 seconds per year against typically ninehours for a conventional battery-hank UPS (Internrtsource: HDR Engineering).For industries where UPSsystems are critical, suchas banking, minimising dowiitime is of utmost importance.

Other applications for fuel cell dmributed powerplants arealsopossible, e.g. stand-alone back-up powergenerators. The PEM fuel cell plant can be started inseconds, supplying power for as long as required fromstored hydrogen, producing electrical power clearllyand virtually silently.Shown in Fig. 7 is a prototype l i d cell distributedpower plant, by Ballard Power Systems. This unitpmvides 2SOkW of electricity and an cyuivalentamount ofheat. This senough power for a communityof about SO homes, or a small hospital or a remoteschool. This particular unit incorporates a fueprocessor so that natural gas cau be used asa fiiel. Thefuel processor convrro the natural gas, through the

ENGINEERING SCIENCE AND EDUCATION JOURNAL 1)ECEMBEK 2002210



7/12

process of reformation, into ahydrogen-rich gas composedprimarily of hydrogen and carbondioxide. The hydrogen is used bythe fuel cell and the carbon dioxideis released into the atmosphere.Eventuallyasan infrastructure forhydrogen develops, these unitscould be powered with hydrogendirectly without the need fora fuelprocessor. Ballard Power ispresently field-testing five of theseunits in the United States,Germany, Japan and Switzerland,with four more units planned for2002. Testing is expected tocontinue until 2004.Residential power

Fuel cell power plants are asobeing developed by severamanufacturers to provideelectricity and heat to single-fadyhomes. Fuelled by either naturalgas or propane, these plants will beable to supply base-load power oral the electricity required by amodern-day home.

Balard Power Systems, incollaboration with its associate

compressed hydrogen lithium-ionbattery lead-acid batterystorage density by volume

300 rc>g 100c

500 compressed hydrogen lithium-ionbattery lead-acidbattery

storage density by weight

Fig. 10 Comparison o f the energy densi ty o f compressed hydro gen (3000 psi)against l i thium-ion and lead-acid batteriescompany Ebara Ballard, partner Ebara Corporation,and co-developer Tokyo Gas, has developed a 1kWfuel cel generator designed to supply both base-loadelectrical poweras wel as heat to a dwelling. This unitcan aso be fuelled by natural gas. It does not provideenough power to supply the total electrical demands ofa residence, but it does shift a portion of the demandfrom the electrical grid to natural gas. The electricalefficiency of this fuel cell system is ratedat42% and theheat efficiency sratedat43%. Therefore the combinedcogeneration efficiency of the system can beas hgh as85%. This particular generator is targeted at theJapanese residential market. Balards goal is tocommence saes of these units in2004.

Plug Power, based in Latham New York hasdeveloped a new fuel cell power plant that supplies5kW of electricity plus heat, using natural gasas a fuel(Fig. 8).Dependmg on the size and location of thehouse this could be enough power to supply theelectrical demand of a modern energy efficient home.Initially these fuel cell power plants will be operated inparallel with the grid. Eventually they will be able tooperate either grid parallel or grid independent,possibly supplying the entire power for a modernhome. Plug Power is presently installing and testingthese units in selected sites throughout North America,Europe and Japan. In 2001,75units were delivered toLong Island Power Authority (LIPA) to supplement thegrid and provide learning experience for LIPAemployees. This experience will enable LIPA to

purchase and install a larger number of units inavarietyof locations in the future.Portable power

Several manufacturers are aso developing fuel cellpower supplies for portable applications, providmg afew watts up to severa kilowatts of electricity (Fig.9).Fuelled by stored natural gas, propane, methanol orhydrogen gas, portable fuel cels may one day replaceboth gasoline and desel-engine generators or portableapplicationsas well as conventional batteries for usessuch as remote lighting, laptop computers and mobilephones.

Compared with engine-driven mobile electricalgenerators, fuel cels have the significant advantage ofbeing quiet and having low emssions.As they have fewmoving parts (only external pumps and fans) theyoperate virtually silently. If stored hydrogen is the fuel,again the only emssion is pure water.

A significant advantage of thehe1cell over its batterycounterpart is that of its energy density (Fig. 10).Portable power packs using fuel cels can be lighter andsmaller in volume for an equivalent amount of energy,particularly the direct methanol &el cell. Note that thecomparison here is the he1 tank. The uel cell makes sense when the energy storage requiredby an application represents many hours o operation at ullpower. The durability o batteries i n this sort o applicationis at best afew hours. The size, weight, and cost o energy

ENGINEERING SCIENCE AND EDUCATION JOURNAL DECEMBER 2002211



8/12

I 1

IFig. 11 Prototype direct meth ano l uel cel l used as al i th ium battely charger prov ides up t o 20W electricalpo wer (photo: courtesy of Jet PropulsionLaboratorieslNASA)storage f.r a fuel cel power plant easily out-competesbatteries. You do have thefixed cost (andsize and wekht)of theplant, whch is afunction o power. 7his is why it isimportant to note that theadvanrageoffue cels isfor lowpower, hkh eneryy applicationr. (Ric Pow of PowConsulting,2001)Rechargeable batteries will discharge over time; thecolder the ambient temperature the quicker they

Fig. 12 Schematic of a single PEM fuel cell. Wh en anelectrical loa d s attached across the anode and thecathode of the fuel cel l a redox reaction occurs.Th ework in g vol tage produced by one cell in th is process isbetween 0.5 an d O W, epend ing on the l oad. To createpractical wo rking voltages, individ ual uel cel ls are stackedtog ether in series to form a fuel cel l stack

discharge. Also the charge capacity of a rechargeablebattery decreases with the number of times of chargeand discharge. Conversely, provided that the hydrogensupply is seaed correctly, a fuel cel willnot dischargeover time, maintaining its fu l charge capacity almostindefinitely.Direct methanol fuelcelswere invented and initiallydeveloped at the et Propulsion Laboratory in Pasadena,California. They were designed to supply electricity forfield troops in the Armed Forces and for applicationswith NASA (Fig. 11).The direct methanol, fuel celhas the advantage over the hydrogen fuel cell in thatthey can use a liquid fuel, i.e. methanol without theneed for external reforming. Liquid Fue iseasy to storeand hasahigh energy density compared to compressedhydrogen. At present, the direct methanol fuel celsuffers From relatively low efficiency and hi gh cost,owing to required platinum loadmg compared to thatofthe hydrogen fuel cell. However, as this improves, itis expected that the direct methanol fuel cell will playa leading role in providing power for portable andpossibly transportation applications.

Ballad Power Systems, Motorola, the Los AIaniosNational Laboratory and Manhattan Scientific are alactively pursuing the development of the directmethanol fuel cell. Motorola claim thataportable cellphone wU he able to remain fully charged on standbyfor a month rather than &ys. The company has alsoannounced that it plans to have its versioncommercially avalable in three to five years.The science of the PEM fuel cellThe chemstryo a singlecel

In a PEM fuel cell, hvo haf-cel reactions take placesimultaneously, an oxidation reaction (lossofelectrons)at the anode andareduction reaction (gan ofelectrons)at the cathode. These two reactions make up the totaloxidation-reduction (redox) reaction of the fuel cell,the formation of water from hydrogen and oxygengases.As in an electrolyser, the anode and cathode areseparated by an electrolyte, which alows ions to betransferred &om one side to the other (Fig. 12). Theelectmlyte in a PEM fuel cell is a solid acid supportedwithin the membrane. The solid acid electrolyte issaturated with water so that thr transportof ions canproceed. The chemical reactions for a PEM fuel cellare:anode reaction: &+2H+ +2e- (6)overall reaction: H2+XOo2+H20 (8)cathode reaction:XO2+2e- +2Hi+H?Og) (7)At the anode, the hydrogen molecules first come intocontact with a platinum catayst on the electrodesurface. The hydrogen molecules break apart, bondingto the platinumsudaceforming weak H-Pt bonds.Asthe hydrogen molecule is now broken the oxidationreaction can proceed. Each hydrogen atom reeases its

ENGINEERING SCIENCE AND EDUCATION OURNA L DECEMBER 2002212



9/12

electron, which traves around the external circuit tothe cathode (it is this flow of electrons that is referredto as electrical current). The remaining hydrogenproton bonds with a water molecule on the membranesurface, forming a hydronium ion (H@+). Thehydronium ion traves through the membrane materialto the cathode, eaving the platinum catayst site free forthe next hydrogen molecule.

At the cathode, oxygen molecules come into contactwith a platinum catayst on the electrode surface. Theoxygen molecules break apart bondmg to the platinumsurface forming weak 0-Pt bonds, enabling thereduction reaction to proceed. Each oxygen atom thenleaves the platinum catayst site, combining with twoelectrons (which have travelled through the externalcircuit) and two protons (whch have travelled throughthe membrane) to form one molecule of water. Theredox reaction has now been completed. The platinumcatayst on the cathode electrode is agan f?ee for thenext oxygen molecule to arrive.

Ths exothermic reaction, the formation of waterfromhydrogen and oxygen gases, has an enthalpy of-286kJ of energy per mole of water formed. The &eeenergy avadable to perform work decreases as afunction of temperature. At 25"C, one atmosphere, thefree energy avadable to perform work is about-237kJ/mole. Thsenergy is observedaselectricity andheat.Polymer electrolyte (orproton exchange) membrane ('EM)The membrane material used in a PEM cell is apolymer. PEMs are generally producedinlarge sheets.The electrode catayst layer sapplied to both sides, andis cut to the appropriate size. A single PEM sheet istypically 50-175pm thck, or around the thckness of2-7 sheets of paper.A common PEM material used today is Nafion.Developed n the 1970s by Dupont, Nafion consists ofpolytetrafluoroethylene (PTFE) chains, commonlyknown as Teflon, forming the backbone of themembrane. Attached to the Teflon chains, are sidechains endmg with sulphonic acid (HS03) groups(Fig. 13).A close-up view of the membrane materialshows long, spaghetti-like chain molecules withclusters of sulphonate side chains (Fig. 14). Aninteresting feature of this material is that, whereas thelong chain molecules are hydrophobic (repel water),the sulphonate side chains are hghly hydrophylic(attract water).

For the membrane to conduct ions efficiently thesulphonate side chains must absorb large quantities ofwater. Withn these hydrated regions, the hydrogenions of the sulphonic acid groups can then move heely,enabling the membrane to transfer hydrogen ions, inthe form of hydronium ions from one side of themembrane to the other.Cell voltage and ej i cii encyIf the hel , cell was perfect at transferring chemicalenergy into electrical energy, the ideal cell voltage

polytetrafluoroethylene(PTFE) chainsF F F F F F F F F F F F F F F

- C - C - C - C - C - C - C - C - C - C - c - c - c - c - c -F F F F F F F O F F F F F F F

I I I I I I I I I I I I I I II I I I I I I I I I I I I I I

Fig. 13 Chemical structure of a PEM fuel cel l membrane-lon g chains of PTFE (Teflon) w it h side chain endin g wi thsulphonic acid (HSOs) (source: Refer ence 2)(thermodynamic reversible cell potential) of thehydrogen he1 cell would beat25"C, one atmosphere,1.23V. As the he1 cell heats up to operatingtemperature, around 80C the ideal cell voltage dropsto about 1.18V. However, there are many limitingfactors that reduce the &el cell voltage further. Thevoltage out of the cell is a good measure of electrical

Fig. 14 Close-up o f a PE M fuel cell m embrane. The Figureshows long spaghetti - l ike chain m olecules of Teflonsurrounding c lusters o f hydrated regions around th esulpho nate side chains. The Teflon chains form th ebackbone of th e membrane. The hy drated regions aroundth e su lphon ate side chains become th e electrolyte (source:Reference 2)ENGINEERING SCIENCE AND EDUCATION OURNA L DECEMBER 2002

213



10/12

Fig. 15 Graph comparingcarbon dio xide emissionsof cars, using diff eren ttypes o f f uel sources(reprinted by p ermissionfrom Pembina Insti tu te). aCar with internalcombu stion engine; b Fuelcell car wi th h ydro genproduced from Alber taelectr ic grid (coalgeneration); c Fuel cell carwi t h onbo ard gasol inereformer;d Fuel cell carw i th onboard methano lreformer; e Fuel cell carw i th hydrogen producedfrom natural gas(dis tr ibuted rom urbanretai l o utlets); f Fuel cellcar wi th h ydrogenproduced from natural gas

' (made at large refineries)

300

25020015010050

0

-

- ~ -- - -- - -

1

efficiency; the lower the voltage, the lower theelectrical efficiency and the more chemical energy isreleased in the formation of water and transferred intoheat.

The primary losses that contribute to a reduction incell voltage are:

Actiuation losses: Activation losses are a result of theenergy required to initiate the reaction. This is aresult of the catalyst. The better the catayst the lessactivation energy is required. Platinum forms anexcellent catalyst; however, there is much researchunderway for better materials. A limiting factor topower density avalable froma fuel cell is the speedat which the reactions can take place. The cathodereaction (the reduction of oxygen) is about 100times slower than that of the reaction at the anode,thus it is the cathode reaction that limits powerdensity.Fuel crossouer and internal currents: Fuel crossover andinternal currents are a result of fuel that crossesdirectly through the electrolyte, from the anode tothe cathode without releasing electrons through theexternal circuit, thereby decreasing the efficiency ofthe fuel cell.Ohmic losses: Ohmic losses are a result of thecombined resistances of the various coniponents ofthe fuel cell. Ths includes the resistance of theelectrode materials, the resistance of the electrolytemembrane and the resistance of the variousinterconnections.Concentration losses (aso referred to as 'masstransport'): These losses result from the reduction ofthe concentration of hydrogen and oxygen gases atthe electrode. For example, following the reactionnew gases must be made immedately avalableatthecatayst sites. With the build up of water at thecathode, particularly at high currents, catayst sitescan become clogged, restricting oxygen access. I t is

therefore important to remove this excess water,hence the term mass transport.

Direct methanol fuel cellA direct methanol fuel cell aso usesaPEM membrane.However, other cataysts in addtion to platinum arerequired on the anode sideof the membrane to breakthe methanol bond in the reaction forming carbondioxide, hydrogenionsand free electrons. Aswith thehydrogen fuel cell, the free electrons flow from theanode of the cell through an external circuit to thecathode and the hydrogen protons are transferredthrough the electrolyte nienibrane. At the cathode theGee electrons and the hydrogen protons react withoxygen to form water. The chemical reactions of thedirect methanol fuel cell are:anode reaction: CH30H+H20+COZ+6H' +6e- (9)cathode reaction: 31202+6Hf+6e-+ 3H20 (10)overal reaction: CH30H+3/202-+C02+2H20 (11)Where will the hydrogen come from?One of the most important questions to be asked is:where will the hydrogen come from?A very interestingstudy published by the Pembina Institute, based inCalgary, Alberta, compared total carbon dioxideemssions of fuel cell vehicles using hydrogen producedfroma variety ofmethods (Fig.15).The results clearlyshow that the choiceas to which method will be usedto produce the hydrogen will be a criticalenvironmental decision.For example, if hydrogen is produced from theelectrolysis of water and the electrolysers are poweredfrom the electrical grid, whereby the electricity isproduced from a coal burning power station, then therewill be no reduction in carbon dioxide emssionscompared with the leves of the present day internal

ENGINEERING SCIENCE AND EDUCATION OURNAL DECEMi3ER 2002214



11/12

combustion engine. In fact,there will be an increase inmetals and pollutants into theenvironment. If on the otherhand the electrolyser ispowered from a renewableenergy source, through useof a solar panel, a windturbine or a hydroelectricturbine, there will be noemssions of carbon dioxide.As the fuel cell industrygrows there will likely bedifferent approaches toniahng hydrogen in variousareas of the world, de-pending on their localenergy production methods.Reformation of hydrocarbonfuclsFor the short term,because of the abundanceofnatural gas, the avalability ofmethanol and propane, andthe lack of a hydrogeninfrastructure, it is expectedthat hydrocarbon fues willbe the dominant fues for

solar cel

wind

micro hydro

Fig. 16 Electr ical pow er from renewab le energy sources. In the past, the l imiting factorsof renewable energy have been the s torage and transpor t of th at energy. Wi th the use ofan electrolyser, a met ho d of storing and transp orting hy drog en gas, and a fuel cel l ,electr ical pow er from renewab le energy sources can be del ivered wher e and w henrequired, cleanly, effic ientl y and sustainably

stationary fuel cel applications. For as long as thesefues are cheaply avalable, reformation of ahydrocarbon fuel is the most cost efficient method ofproducing hydrogen. In the reformation of ahydrocarbon fuel, however, there is an emssion ofcarbon &oxide. Although carbon dioxide is notconsidered a pollutant, controversy exists that man-made emssions niay contribute to global warming.Renewabl e energy systems

Hydrogen can be produced sustainably with noemssion of carbon &oxide from renewable energysystems.An example of sucha system is the use of asolar panel, awind turbine oramicro-hydro generatorto convert the ra&ant energy of sunhght into electricalpower, which drives an electrolyser. The electrolyserbreaks apart water producing hydrogen and oxygengases. The hydrogen is stored for use by the fuel cell andthe oxygen s released into the atmosphere. Thus whenthe sun shines, the wind blows or the waterflows, theelectrolyser can produce hydrogen.A power system incorporating hydrogen fromrenewable sources anda fuel cell is a closed system,asnoneof the products or reactants, water, hydrogen andoxygen, is lost to the outside environment. The water

consumed by the electrolyserisconverted to gases. Thegases are converted back to water. The electrical energyproduced by the solar panel is transferred to cheinicalenergy n the form ofgases. The gases can be stored andtransported, to be reconverted back to electricity(Fig.16).

These systems are truly sustainable, for as long asthere is sunlight there can be electrical power, avalablewhere and when required.Biological methods

Research and development is talung place on theproduction of hydrogen from biological methods(biohydrogen). For example, Dr. A. Melis at theUniversity of California, Berkeey, has discovered ametabolic switch in common green agae(chlamydomonas reinhardtill that causes the agae tooxidise water and to produce pure hydrogen gas whensulphur nutrients are depleted froin the growthmedium. This and other biohydrogen mechanisms arepresently in the R&D stage but may oneday providethe world with an additional sourceof hydrogen.Benefits and ob staclesBen$tsFuel cells are e$icient: They convert hydrogen and

oxygen directly into electricity, water and heat, withno combustion in the process. The resultingefficiency sbetween50and80%,about double thatof an internal combustion engine.Fuel (ells are clean: If hydrogen is the fue, there are nopollutant emssions &om a fue cel itseE only theproduction of pure water. In contrast to an internalcombustion engine,abel cel produces no emssionsofsulphur dioxide, which can lead to acidrain,nor nitrogenoxides whch produce smog nor dust particulates.

ENGINEERING SCIENCE AND EDUCATION OURNAL DECEMBER 2002215



12/12

Fuel cells are quiet: A bel cell itself has no movingparts, althoughafuel cell system may have pumps andfans. As a result, electrical power is producedrelatively silently. Many hotels and resorts in quietlocations, for example, could replace dese1 enginegenerators with &el cels for both main power supplyor for backup power in the event of power outages.Fuel cells are modular: That is, fuel cels of varyingsizes can be stacked together to meet a requiredpower demand. As mentioned earlier, fuel cellsystems can provide power overa large range, &oma few watts to megawatts.Fuel cels are environmentally safe: They producenohazardous waste products, and their only byproductis water (or water and carbon dioxide in the case ofmethanol cels) and heat.Fuel cels may giveusthe opportunity to provide theworld with sustainable electrical power.

ObstaclesAt present there are many uncertainties to the success offuel cells and the development of ahydrogen economy:

Fuel cels must obtain mass-market acceptance tosucceed: Thls acceptance depends largely on price,reliabhty, longevity of fuel cells and the accessibhtyand cost of fuel. Compared to the price of presentday alternatives e.g. desel-engine generators andbatteries, fuel cells are comparatively expensive. Inorder to be competitive, fuel cels need to be mass-produced and less expensive materials developed.An infvastructure for the mass-market availability ofhydrogen, or methanol uel initial ly, must also develop:At present there is no infi-astructurein place foreither of these fuels. As it is we must rely on theactivities of the oil and gas companies to introducethem. Unless motorists are able to obtain fuelconveniently and affordably, a mass market formotive applicationswill not develop.A t present a largeportion of the investment in uel celsand hydrogen technology has come from automanujicturers:However, if fuel cels prove unsuitablefor automobiles, new sources of investment for &elcells and the hydrogen industry wdl be needed.Changes in government policy could also derailfuel cell and hydrogen technology development:At present stringent environmental laws andregulations, such as the Cahfornia Low EmissionVehcle Program, have been a great encouragementto these fields. Deregulation laws in the uthtyindustry have been a large impetus for thedevelopment of distributed stationary powergenerators. Should these laws change it could create

ConclusionAs our demand for electrical power grows, it becomesincreasinglyurgent to find new ways ofmeeting t bothresponsibly and safey. In the past, the limiting factorsof renewable energy have been the storage andtransport of that energy. With the use of fuel cels andhydrogen technology, electrical power from renewableenergy sources can be delivered where and whenrequired, cleanly, efficiently and sustainably.AcknowledgmentsThe author would &e to thank Rachel Browne forsupplying the graphs and drawings and editing the text,and RicPow,h m ow Consulting, Vancouver, Canada,for reviewing the paper and providmg techcal advice.References1 CONNIHAN, M. A,: Dictionary of energy (Routledge2 LARMINIE, J ., and DICKS, A,: Fuel cel systems3 BERRY, M., and MACDONALD A.: Energy through4 KOPPEL,T.:Powering hef uture(JohnWiley and Sons, 1999)5 INTERNATIONAL ENERGY AGENCY Energypoliciesof IEA countries (OECD Publications, 1997)6 KHATIB,H.: Electrica powerindeveoping countries,PowerEngineeringJournal, October 1998,12, (lo),pp. 239-2477 COLELL, H.: Solar hydrogen technology (Heliocentris,1998)8 MELIS,A,,and HAPPE, T.: Hydrogen production: green agaeasasourceofenergy,PlantPhysiology, 2001,127:pp. 740-7489 THOMAS, S.,and ZALBOWITZ, M.: Fuel cels, greenpower &osAlamos Nanonal Laboratory, 1999, booklet)

and Kegan Paul, 1981)explained (John Wiley& Sons,2000)hydrogen Heliocentris

internet sourcesBalard Power System: http://www.balard.com/25Ok-stationary.aspHDR Engineering and Architecture: hap://www.hdrinc.com/information/search.asp?PageID=476IFC:http://www.in~rnationalfuelcells.com/spacedefe~e/eritage.shtmJet Propulsion Laboratory:http://www2.jpl.nasa.gov/til~unages/captiom/p486OO.txtLosAlamos NationaLaboratory:http://www.lanl.gov/worldview/science/features/hece l.htmNASA: Gemini:http: / / sci enCe.ksc. nasa. gov/hi story/ gemgemjn-Space Shuttle Orbiter: http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/ sts-eps. tmPembina Institute:http://www.pembina.org/pubs/pa ibecel.pdfPlug Power:http://www.plugpower.com/technology/Smithsonian Institution: http: / /amricanh story.si .edu/csr/ fuel cel l s/pem/pemmain.hm0 EE: 2002

v/Fmini-v.htd

supplies to the world platinum market are &om cells and hydrogen technol ib equipment for schools andSouth Africa, Russia and Canada. Shortages of universities. Mr Cook has a background in science komUniversityof British Columbia and electrical technology, andhas a specia interest in environmental sustanability. He can becontactedat: [email protected] anticipated; however, changes ingovernment policies could afect the supply.

ENGINEERING SCIENCE AND EDUCATION OURNA L DECEMBER 2002216
http://www.ballard.com/25Okhttp://hap//www.hdrinc.comhttp://www.lanl.gov/worldviewhttp://www.lanl.gov/worldviewhttp://science.ksc.nasa.gov/history/gemihttp://science.ksc.nasa.gov/shuttlehttp://www.pembina.org/pubs/pahttp://www.plugpower.com/technologyhttp://americanhistory.si.edu/csr/fuelcellsmailto:[email protected]:[email protected]://americanhistory.si.edu/csr/fuelcellshttp://www.plugpower.com/technologyhttp://www.pembina.org/pubs/pahttp://science.ksc.nasa.gov/shuttlehttp://science.ksc.nasa.gov/history/gemihttp://www.lanl.gov/worldviewhttp://hap//www.hdrinc.comhttp://www.ballard.com/25Ok

introduction to fuel cell technology

Documents

Transcript of introduction to fuel cell technology