Fuel Cells Green Power

8/2/2019 Fuel Cells Green Power

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The authors acknowledge the following review ers:

Los Alamos National LaboratoryShimshon Got tesfeldCharles F. KellerSteffen M ller-HolstAntonio Redondo

Office of Advanced Automotive Technologies,U.S. Department of Energy

JoAnn Milliken

LA-UR-99-3231

Los Alamos National Laboratory, an affirmativeaction/ equal opportunity employer, is operatedby the University of California for the USDepartment of Energy under contract W-7405-ENG-36. All company names, logos, and productsmentioned herein are trademarks of theirrespectiv e companies. Ref erence t o any specificcompany or product is not be construed as anendorsement of said company or product by theRegents of the University of California, theUnited States Government, the US Department ofEnergy, nor any of their employees. The LosAlamos National Laboratory strongly supportsacademic freedom and a researchers right topublish; as an institution, however, theLaboratory does not endorse the viewpoint of apublication or guarantee its technical correctness.


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1

Green Power

Whats Inside

2. A Brief History

4. The Very Basics

and More

5. SomeComparisons

7. The Polymer ElectrolyteMembrane Fuel Cell

15 . Other Types

of Fuel Cells

19 . What About Fuel?

21 . Applicationsfor Fuel Cells

24 . Hydrogen as A Fuel

27 . Getting To CleanerTransportation

28 . Oil Reserves,Transportation,and Fuel Cells

29 . Climat e Change,Greenhouse Gases,

and Fuel Cells:W hat is the Link?

32 . A Once-In-A-LifetimeOpportunity

Fuel Cells Green Power

w as w ritten by Sharon Thomasand Marcia Zalbowitz atLos Alamos National Laboratoryin Los Alamos, New Mexico

Designed by Jim Cruz

This publication has been funded by the

Off ice of Advanced Automotive TechnologiesOffice of Transportation TechnologiesEnergy Eff iciency and Renew able Energy

U.S. Department of Energy

The 3M Foundation

For more information:w w w .education.lanl.gov/ resources/ fuelcells


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Lamps, burning coaloil and coal gas, litthe living rooms ofmost homes of theearly 1900s. Butw hen electric lightbulbs replaced thosesmoky, smellysources of illumina-tion, homes becamebrighter, cleaner, andsafer. At first onlythe w ealthy couldafford electric lights.But as the demandwent up and the costw ent dow n, moreand more of thepopulation were ableto a fford electriclighting even though

there was plentyof coal to continuelighting buildings inthe usual w ay. Thebetter technologywon.

D.S. Scott and W. Hafele.

The Coming HydrogenAge: Preventing World

Climate Disruption.International Journal of

Hydrogen Energy. Vol.

15, No. 10, 1990.

A Brief History

A lthough fuel cells have been around since 1 83 9, it took 1 20 yearsuntil NASA demonstrated some of their potential applications inproviding power during space flight. As a result of these successes, inthe 1960s, industry began to recognize the commercial potential offuel cells, but encountered t echnical barriers a nd high investment costs

fuel cells wer e not economically competit ive w ith ex isting energytechnologies. Since 1984, the Of fice of Transportat ion Technologies atthe U.S. Department of Energy has been supporting research anddevelopment of fuel cell technology, and as a result, hundreds ofcompanies around the w orld are now w orking tow ards making fuel

cell technology pay of f. Just as in the commercializat ion of the electriclight bulb nearly one hundred years ago, todays companies arebeing driven by technical, economic, and social forces such as highperformance characteristics, reliability, durability, low cost, andenvironmental benefits.

In 1839, William Grove, a British jurist and amateur physicist, first discovered theprinciple of the fuel cell. Grove ut ilized four large cells, each containing hydrogenand oxygen, to produce electric power which was then used to split the water inthe smaller upper cell into hydrogen and oxygen.

I cannot but regard the experiment

as an important one...

William Grove writing to Michael Faraday,October 22, 1842


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3

The mission of our global fuel cell project

center is nothing less than to make us the leader in

commercially viable fuel cell powered vehicles.

Harry J. Pearce, Vice Chairman,

Board of Directors, General Motors.May 1998

In March 1998, Chicago became the firstcity in the world to put pollution-free,hydrogen fuel cell powered buses intheir public transit system.(Courtesy: Ballard Power Systems)

The FutureCar Challenge, sponsored bythe U.S. Department of Energy, presentsa unique assignment to students from

North Americas top engineering schools:convert a conventional midsize sedaninto a super efficient vehicle withoutsacrificing performance, utility, andsafety. In 19 99 , the Virginia Techteam entered the competition with afuel cell vehicle.

The first fuel cell powered bicycleto compete in the American Tour-de-Sol.(Courtesy: H-Power)

The automobile,it is fair to say, changed the industrial andsocial fabric of the United States and most

countries around the globe. Henry Ford epitomized Yankee ingenuityand the Model T helped create the open road, new horizons, abundant andinexpensive gasoline...and tailpipe exhaust. More people are driving morecars today than ever before more than 200 million vehicles are on the roadin the U.S. alone. But the car has contributed to our air and water pollution

and forced us to rely on imported oil from the Middle East, helping to createa significant trade imbalance. Today many people think fuel cell technologywill play a pivotal role in a new technological renaissance just as theinternal combustion engine vehicle revolutionized life at the beginning ofthe 20th century. Such innovation would have a global environmental andeconomic impact.

In todays world,

solving environmental problems

is an investment, not an expense.

William Clay Ford, Jr.Chairman and CEO, Ford Motor Company,September 1998

Fuel cells are not just laboratory curiosities. While there is much work thatneeds to be done to optimize the fuel cell system (remember, the gasolineinternal combustion engine is nearly 120 years old and still being improved),hydrogen fuel cell vehicles are on the road now. Commuters living inChicago and Vancouver ride on fuel cell buses. You can take a ride around

London in a fuel cell taxi and even compete in the American Tour de Solon a fuel cell bicycle. Every major automobile manufacturer in the worldis developing fuel cell vehicles. To understand why fuel cells have receivedsuch attention, we need to compare them to existing energy conversiontechnologies.


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Where the Actionin Fuel Cells is Today

Allied SignalVolvo

BallardDaimlerChryslerDetroit EdisonDuPontShellFordGeneral MotorsHondaMazdaGeorgetow n UniversityCase Western Reserve UniversityLos Alamos National LaboratoryMotorolaPenn State UniversityPrinceton UniversityRolls-RoyceArgonne National LaboratorySanyoDAISSiemensBritish GasPlug Pow erUniversity of M ichiganTexas A&M UniversityARCO

EpyxInternational Fuel CellsH-PowerEnergy PartnersHydrogen BurnerW.L. GoreA.D. LittleInstitute of Gas TechnologyVairexElectrochemGinerJet Propulsion LaboratoryToyota

University of CaliforniaExxonWestinghouseRenault3MNissanBMWPSA Peugeot CitronTexacoUniversity of FloridaTokyo Electric Pow er

(This is just a partial list)

Carnot Cycle vs. Fuel Cells

T he theoretical thermodynamic derivation of Carnot Cycle showsthat even under ideal conditions, a heat engine cannot convert allthe heat energy supplied to it into mechanical energy; some of theheat energy is rejected. In an internal combustion engine, the engineaccepts heat from a source at a high temperature (T1), converts part ofthe energy into mechanical work and rejects the remainder to a heat

sink at a low temperature (T2). The greater the t emperature differencebetween source and sink, the greater the efficiency,

Maximum Efficiency = (T1 T2) / T1

where the temperatures T1 and T2 are given in degrees Kelvin.Because fuel cells convert chemical energy directly to electrical energy ,this process does not involve conversion of heat to mechanical energy .Therefore, fuel cell efficiencies can exceed the Carnot limit even whenoperating at relatively low temperatures, for example, 80C.

The Very Basics

A fuel cell is an electrochemical energyconversion device. It is tw o to threetimes more efficient than an internalcombustion engine in converting fuelto power.

A f uel cell produces electricity, w ater,and heat using fuel and oxygen in theair.

Water is the only emission whenhydrogen is the fuel.

As hydrogen flow s into the fuel cell onthe anode side, a platinum catalyst facili-tat es the separation of the hydrogen gas

into electrons and protons (hydrogen ions). The hydrogen ions passthrough the membrane (the center of the fuel cell) and, aga in with thehelp of a platinum catalyst, combine with oxygen and electrons on thecat hode side, producing w ater. The electrons, w hich cannot passthrough the membrane, flow from the anode to t he cat hode through anexternal circuit containing a motor or other electric load, which con-sumes the power generated by the cell.

The voltage from one single cell is about 0.7 volts just about enoughfor a light bulb much less a car. W hen the cells are stacked in series,the operating voltage increases to 0.7 volts multiplied by the numberof cells stacked.

Hydrogen Catalyst

Cathode (+)

Water

Oxygen

Anode (-)

ElectronsProtons

Membrane/Electrolyte


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5

W

hat internal combustion

engines, batteries, and fuelcells have in common is theirpurpose: all are devices that convertenergy from one form to another.As a starting point, lets consider theinternal combustion engine usedto power virtually all of the carsdriven on U.S. highways today.These engines run on noisy, hightemperature explosions resultingfrom the release of chemical energy

by burning fuel with oxygen fromthe air. Internal combustion en-gines, as well as conventional utilitypower plants, change chemicalenergy of fuel to thermal energy togenerate mechanical and, in the caseof a power plant, electrical energy.Fuel cells and batteries are electro-chemical devices, and by their verynature have a more efficient conver-sion process: chemical energy is

converted directly to electricalenergy. Internal combustion enginesare less efficient because they includethe conversion of thermal to me-chanical energy, which is limited bythe Carnot Cycle.

If cars were powered by electricitygenerated from direct hydrogen fuelcells, there would be no combustioninvolved. In an automotive fuel cell,

hydrogen and oxygen undergo arelatively cool, electrochemicalreaction that directly produceselectrical energy. This electricitywould be used by motors, includingone or more connected to axles usedto power the wheels of the vehicle.The direct hydrogen fuel cell vehiclewill have no emissions even duringidling this is especially importantduring city rush hours. There are

How do Fuel Cells Compare toInternal Combustion Engines and Batteries?

some similarities to an internal

combustion engine, however. Thereis still a need for a fuel tank andoxygen is still supplied from the air.

Many people incorrectly assume thatall electric vehicles (EVs) are pow-ered by batteries. Actually, an EV isa vehicle with an electric drive trainpowered by either an on-boardbattery or fuel cell. Batteries andfuel cells are similar in that they

both convert chemical energy intoelectricity very efficiently and theyboth require minimal maintenancebecause neither has any movingparts. However, unlike a fuel cell,the reactants in a battery are storedinternally and, when used up, thebattery must be either recharged orreplaced. In a battery-powered EV,rechargeable batteries are used.With a fuel cell powered EV, the fuel

is stored externally in the vehicles

fuel tank and air is obtained from

the atmosphere. As long as thevehicles tank contains fuel, the fuelcell will produce energy in the formof electricity and heat. The choiceof electrochemical device, battery orfuel cell, depends upon use. Forlarger scale applications, fuel cellshave several advantages over batter-ies including smaller size, lighterweight, quick refueling, and longerrange.

The polymer electrolyte membrane(PEM) fuel cell is one in a family offive distinct types of fuel cells. ThePEM fuel cell, under considerationby vehicle manufacturers around theworld as an alternative to theinternal combustion engine, will beused to illustrate the science andtechnology of fuel cells.

The P2000, from Ford Motor Company, is a zero-emission vehicle that utilizes a directhydrogen polymer electrolyte fuel cell. (Courtesy of Ford Motor Co.)

Hydrogen Fuel Cell Car

Chemical Energy

(gaseous flow) Electrical Energy

FuelCells

HydrogenTank

Mechanical Energy

TractionInverterModule

Turbo-

compressor

Hydrogen is

supplied to the Fuel Cells

Air is supplied to

the fuel cells by

turbocompressor

The traction inverter module

converts the electricity f or

use by the electric motor/

transaxles

The electric motor/

transaxle converts the

electric energy into the

mechanical energy

which turns the wheels

Oxygen from the air and

hydrogen combine in the

fuel cells to generate electricity

that i s sent to the t raction

inverter module

ElectricMotor/

Transaxle


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References:

Bill McKibben, A Special Moment inHistory. The Atlantic Monthly,May, 1998 .

James J. MacKenzie, Oil as a Finite

Resource: W hen is GlobalProduction Likely to Peak?World Resources Institute, March1996.

Colin Campbell and Jean H. Laherrere The End of Cheap Oil . ScientificAmerican, March 1998.

Busting Through wit h Ballard.Financial Times EnergyWorld. Number Five, W inter1998.

Jacques Leslie. The Dawn of theHydrogen Age. W ired. October,1997 .

McGraw-Hill Dictionary of Scientificand Technical Terms. 1 994 .

M otor Vehicle Facts and Figures.American AutomobileM anufacturers Association. 19 98 .

Resources:

World Resources Institutehttp:/ / ww w.wri.org

International Fuel Cellshttp:/ / ww w.hamilton-standard.com/ ifc-onsi

United Nations Framework Conventionon Climate Changehttp:/ / w w w.unfccc.de/

Linkages http:/ / ww w.iisd.ca/linkages/

Environmental Protection Agency ,Alternative Fuelshttp:/ / ww w.epa.gov/ omsww w

U.S. Fuel Cell Councilhttp:/ / w w w.usfcc.com

Fuel Cells 2000http:/ / fuelcells.org

Future Opportunities

Polymer electrolyte membrane fuel cells arelimited by the temperature range over w hichw at er is a liquid. The membrane must containwater so that the hydrogen ions can carry thecharge wit hin the membrane. Operat ing poly-

mer electrolyte membrane fuel cells attemperatures exceeding 100C is possibleunder pressurized conditions, r equired to keepthe w ater in a liquid state, but shortens thelife of the cell. Currently, polymer electroly temembranes cost about $1 00 / square foot.Costs are ex pected to decrease significantlyas the consumer demand for polymer electro-lyt e membrane fuel cells increases.

Remaining Challenges:

producing membranes not limit ed by thetemperature range of liquid w ater,possibly based on another mechanism forprot onic conduction

reducing membrane cost by developingdifferent membrane chemistries

Structure of Polymer Electrolyte Membranes

T he polymer electrolyte membrane is a solid, organic polymer,usually poly[ perfluorosulfonic] acid. A typical membrane material,such as NafionTM , consists of three regions:

(1 ) the Teflon-like, fluorocarbon backbone, hundreds of repeating CF2 CF CF2 units in length,

(2 ) the side chains,O CF2 CF O CF2 CF2 , w hich connect t hemolecular backbone to the third region,

(3 ) the ion clusters consisting of sulfonic acid ions, SO3-

H+.

The negative ions, SO3-, are permanently attached to the side chain

and cannot move. How ever, w hen the membrane becomes hydrat edby absorbing water, t he hydrogen ions become mobile. Ion movementoccurs by prot ons, bonded to w ater molecules, hopping fr om SO3

-site

to SO3- site w ithin the membrane. Because of this mechanism, the

solid hydrated electrolyte is an excellent conductor of hydrogen ions.

CF2 CF CF2l

Ol

CF2l

CF CF3l

Ol

CF2l

CF2l

SO3-

H+

Chemical structureof membrane

material;NafionTM by DuPont

Definitions:

Electrochemical reaction: A reactioninvolving the transfer ofelectrons from one chemicalsubstance to another.

Fuel cell: An electrochemical devicethat continuously converts thechemical energy of externallysupplied fuel and oxidant directlyto electrical energy.

Oxidant: A chemical, such as oxygen,that consumes electrons in anelectrochemical reaction.

Electrolyte: A substance composed ofpositive and negative ions,

Ion: An atom that has acquired anelectrical charge by the loss or gainof electrons.

1 micron = 10 -6 m, 10 - 4 cm, 10 -3 or0.001 mm = 1 m

Polymer: A substance made of giantmolecules formed by the union ofsimple molecules (monomers).

Thermal: Pertaining to heat
http://www.wri.org/http://www.wri.org/http://www.hamilton-standard.com/ifc-onsihttp://www.hamilton-standard.com/ifc-onsihttp://www.hamilton-standard.com/ifc-onsihttp://www.unfccc.de/http://www.unfccc.de/http://www.unfccc.de/http://www.iisd.ca/linkages/http://www.iisd.ca/linkages/http://www.iisd.ca/linkages/http://%20www.epa.gov/omswwwhttp://%20www.epa.gov/omswwwhttp://%20www.epa.gov/omswwwhttp://www.usfcc.com/http://www.usfcc.com/http://fuelcells.org/http://fuelcells.org/http://fuelcells.org/http://www.usfcc.com/http://%20www.epa.gov/omswwwhttp://www.iisd.ca/linkages/http://www.unfccc.de/http://www.hamilton-standard.com/ifc-onsihttp://www.wri.org/


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7

As little as 10 years ago, vehicles powered byfuel cells seemed more science fiction than fact.

Today, development of fuel cell technology for transpor-

tation is made possible due to the polymer electrolytemembrane fuel cell. This type of fuel cell is also knownas the proton exchange membrane fuel cell, the solidpolymer electrolyte (SPETM) fuel cell and simply, poly-mer electrolyte fuel cell. It is often referred to simply asthe PEM fuel cell. The center of the fuel cell is thepolymer electrolyte membrane. For all five families offuel cells, it is the electrolyte that defines the type of fuelcell, so the discussion of the polymer electrolyte mem-brane fuel cell should logically begin with its electrolyte,the membrane.

The Polymer Electrolyte Membrane

An ordinary electrolyte is a substance that dissociatesinto positively charged and negatively charged ions inthe presence of water, thereby making the water solu-tion electrically conducting. The electrolyte in apolymer electrolyte membrane fuel cell is a type ofplastic, a polymer, and is usually referred to as a mem-brane. The appearance of the electrolyte varies

depending upon the manufacturer, but the most preva-lent membrane, Nafion TM produced by DuPont,resembles the plastic wrap used for sealing foods. Typi-cally, the membrane material is more substantial thancommon plastic wrap, varying in thickness from 50 to175 microns. To put this in perspective, consider that apiece of normal writing paper has a thickness of about 25microns. Thus polymer electrolyte membranes havethicknesses comparable to that of 2 to 7 pieces of paper.In an operating fuel cell, the membrane is well humidi-fied so that the electrolyte looks like a moist piece of

thick plastic wrap.

Polymer electrolyte membranes are somewhat unusualelectrolytes in that, in the presence of water, which themembrane readily absorbs, the negative ions are rigidlyheld within their structure. Only the positive ionscontained within the membrane are mobile and are free

to carry positive charge through the membrane. Inpolymer electrolyte membrane fuel cells these positiveions are hydrogen ions, or protons, hence the term

proton exchange membrane. Movement of the hydro-gen ions through the membrane, in one direction only,from anode to cathode, is essential to fuel cell operation.Without this movement of ionic charge within the fuelcell, the circuit defined by cell, wires, and load remainsopen, and no current would flow.

Because their structure is based on a TeflonTM backbone,polymer electrolyte membranes are relatively strong,stable substances. Although thin, a polymer electrolytemembrane is an effective gas separator. It can keep the

hydrogen fuel separate from the oxidant air, a featureessential to the efficient operation of a fuel cell. Al-though ionic conductors, polymer electrolytemembranes do not conduct electrons. The organicnature of the polymer electrolyte membrane structuremakes them electronic insulators, another featureessential to fuel cell operation. As electrons cannotmove through the membrane, the electrons produced atone side of the cell must travel, through an externalwire, to the other side of the cell to complete the circuit.It is in their route through the circuitry external to the

fuel cell that the electrons provide electrical power torun a car or a power plant.

The Polymer Electrolyte M embrane Fuel Cell


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The Electrodes

All electrochemical reactions consist of two separate reactions: an oxidation half-reaction occurring at the anode anda reduction half-reaction occurring at the cathode. The anode and the cathode are separated from each other by theelectrolyte, the membrane.

In the oxidation half-reaction, gaseous hydrogen produces hydrogen ions, which travel through the ionically con-

ducting membrane to the cathode, and electrons which travel through an external circuit to the cathode. In thereduction half-reaction, oxygen, supplied from air flowing past the cathode, combines with these hydrogen ions andelectrons to form water and excess heat. These two half-reactions would normally occur very slowly at the lowoperating temperature, typically 80C, of the polymer electrolyte membrane fuel cell. Thus, catalysts are used onboth the anode and cathode to increase the rates of each half-reaction. The catalyst that works the best on eachelectrode is platinum, a very expensive material.

The final products of the overall cell reaction are electric power, water, and excess heat. Cooling is required, in fact,to maintain the temperature of a fuel cell stack at about 80C. At this temperature, the product water produced atthe cathode is both liquid and vapor. This product water is carried out of the fuel cell by the air flow.

Electrochemistry of Fuel Cells

Oxidation half reaction 2H2 4H+ + 4e-

Reduction half reaction O2 + 4H+

+ 4e-

2H2O

Cell reaction 2H2

+ O2

2H2O

T he physical and electrochemical processes that occur at eachelectrode are quite complex. At the anode, hydrogen gas (H 2 )must diffuse through tortuous pathways until a platinum (Pt) particleis encountered. The Pt cata lyzes the dissociation of t he H2 moleculeinto tw o hydrogen atoms (H) bonded to t w o neighboring Pt atoms.Only then can each H atom release an electron to form a hydrogen ion(H

+). Current flow s in the circuit as these H

+ions are conducted

through the membrane to the cathode while the electrons pass fromthe anode to the outer circuit and then to the cathode.

The reaction of one oxygen (O 2) molecule at the cathode is a 4

electron reduction process (see above equat ion) w hich occurs in amulti-step sequence. Expensive Pt based cat alyst s seem to be theonly catalysts capable of generating high rates of O 2 reduction at therelatively low temperatures (~ 80C) at which polymer electrolytemembrane fuel cells operat e. There is still uncerta inty regarding themechanism of t his complex process. The performance of t he polymerelectroly te membrane fuel cells is limited primarily by the slow rat e ofthe O 2 reduction half reaction which is more than 100 times slowerthan the H2 oxidation half reaction.

Definitions:

Catalyst: A substance t hatparticipates in a reaction,increasing its rate, but is notconsumed in the reaction.

Current: The flow of electric chargethrough a circuit.

Electrode: An electronic conductorthrough w hich electrons areexchanged with the chemicalreactants in an electrochemical cell.

Electron: An elementary particlehaving a negative charge.

1 nanometer: = 10-9

m = 10-7

cm= 1 0

-6mm = 10

-3m = 1 nm

Oxidation half reaction: A process inw hich a chemical species changes t o

another species with a morepositive charge due to the releaseof one or more electrons. It canoccur only w hen combined wit h areduction half reaction.

Reduction half reaction: A process inw hich a chemical species changesto another species with a lesspositive charge due to theaddition of one or more electrons.It can occur only w hen combinedw ith an ox idation half reaction.


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9

Polymer electrolyt e membrane w ith porous electrodes thatare composed of platinum particles uniformly supported

on carbon particles.

Why a Fuel Cell Goes Platinum

T he half reactions occurring at each electrode can only occur at ahigh rat e at t he surface of the Pt cat alyst . Platinum is uniquebecause it is sufficiently reactive in bonding H and O intermediates asrequired to facilitate the electrode processes, and also capable ofeff ectiv ely releasing the intermediate t o form the final product. Forex ample, the anode process requires Pt sites t o bond H atoms w hen

the H2 molecule reacts, and these Pt sites next release the H atoms,as H + + e-

H2 + 2Pt 2 Pt-H2 Pt H 2 Pt + 2 H

++ 2e

-

This requires optimized bonding to H atoms not too w eak and nottoo strong and this is the unique feature of a good cataly st. Real-izing that the best catalyst for the polymer electrolyte membrane fuelcell is expensive, lowering Pt catalyst levels is an on-going effort.One of t he best w ays t o accomplish this is to construct t he cat alystlayer w ith the highest possible surfa ce area . Each electrode consistsof porous carbon (C) to w hich very small Pt particles are bonded. The

electrode is somewhat porous so that the gases can diffuse througheach electrode t o reach the cat alyst . Both Pt and C conduct e lectronsw ell, so electrons are able to move freely through the electrode. Thesmall size of the Pt particles, about 2 nanometers in diameter, resultsin an enormously large total surface area of Pt that is accessible togas molecules. The total surfa ce presented by t his huge number ofsmall particles is very large even when the total mass of Pt used issmall. This large Pt surface area allow s the electrode reactions toproceed at many Pt surface sites simultaneously. This high dispersionof the catalyst is one key to generating significant electron flow, i.e.current, in a fuel cell.

Water and Fuel Cell Performance

W ater management is key to effective operation of apolymer electroly te membrane fuel cell. Although w at eris a product of the fuel cell reaction, and is carried out of the cellduring its operation, it is interesting that both the fuel and airentering the fuel cell must still be humidified. This additional w at erkeeps the polymer electroly te membrane hydrated. The humidity

of the gases must be caref ully controlled. Too litt le wat er preventsthe membrane fr om conducting t he H+ ions well a nd the cellcurrent drops.

If the air flow past the cathode is too slow, t he air cant carry allthe water produced at the cathode out of the fuel cell, and thecathode floods. Cell performance is hurt because not enoughoxygen is able to penetrate the excess liquid water to reach thecathode catalyst sites.


Impurities often present in the H2 fuelfeed stream bind to the Pt catalyst

surface in the anode, preventing H 2oxidation by blocking Pt catalyst sites.Alternative catalysts which can oxidizeH2 while remaining unaffected byimpurities are needed to improve cellperformance.

The rate of the oxygen reductionprocess at the air electrode is quite loweven at the best Pt catalysts developedto date, resulting in significantperformance loss. Alternat ive cat alyst sthat promote a high rate of oxygen

reduction are needed to further enhancefuel cell perf ormance.

Future alternative catalysts must be lessexpensive t han Pt to low er the cost ofthe cell.

9

Polymer

ElectrolyteMembrane

Pathway(s) allowingconduction of electrons

Pathway(s) allowingconduction of hydrogen ions

Pathway(s) allowingaccess of gas tocatalyst surface

Carbon

Platinum


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TheMembrane/ Electrode

Assembly

T

he combination of anode/membrane/cathode is

referred to as the membrane/electrode assembly.The evolution of membrane/electrode assemblies inpolymer electrolyte membrane fuel cells has passedthrough several generations. The original membrane/electrode assemblies were constructed in the 1960s forthe Gemini space program and used 4 milligrams ofplatinum per square centimeter of membrane area(4 mg/cm2). Current technology varies with the manu-facturer, but total platinum loading has decreased fromthe original 4 mg/cm2 to about 0.5 mg/cm2. Laboratoryresearch now uses platinum loadings of 0.15mg/cm2 .

This corresponds to an improvement in fuel cell perfor-mance since the Gemini program, as measured byamperes of current produced, from about 0.5 amperesper milligram of platinum to 15 amperes per milligramof platinum.

The thickness of the membrane in a membrane/elec-trode assembly can vary with the type of membrane.The thickness of the catalyst layers depends upon howmuch platinum is used in each electrode. For catalystlayers containing about 0.15 mg Pt/cm2, the thickness of

the catalyst layer is close to 10 microns, less than half thethickness of a sheet of paper. It is amazing that this

membrane/electrode assembly, with atotal thickness of about 200 microns or 0.2millimeters, can generate more than halfan ampere of current for every squarecentimeter of membrane/electrode assem-bly at a voltage between the cathode andanode of 0.7 volts, but only when encasedin well engineered components backinglayers, flow fields, and current collectors.

M embrane/ electrode assembly


Optimization of membrane/ electrodeassembly ( M EA) construction is on- going.

Fundamental research into the catalyst layer/membrane interface is needed to furtherunderstand the processes involved in currentgeneration. New M EA designs w hichw ill increase f uel cell performanceare needed. As alw ays, the science andtechnology of M EAs are interconnected;whether improved understanding will leadto better MEA design or a different designwill lead to improved understanding remainsto be seen.

M aking a M embrane/ Electrode Assembly

M embrane/ electrode assembly construction varies greatly,but the following procedure is one of several used at LosAlamos National Laboratory where fuel cell research is activelypursued. The cat alyst mat erial is first prepared in liquid inkform by thoroughly mixing together appropriate amounts ofcatalyst (a powder of Pt dispersed on carbon) and a solution of

the membrane material dissolved in alcohols. Once the ink isprepared, it is applied to the surface of the solid membrane ina number of differ ent w ays. The simplest method involvespainting the catalyst ink directly onto a dry, solid piece ofmembrane. The w et catalyst layer and the membrane areheated until the cata lyst lay er is dry. The membrane is thenturned over and the procedure is repeated on the other side.Cat alyst lay ers are now on both sides of the membrane. Thedry membrane/ electrode assembly is next rehydrated byimmersing in lightly boiling dilute acid solution to also ensurethat the membrane is in the H

+form needed for fuel cell opera-

tion. The final step is a thorough rinsing in distilled water. Themembrane/ electrode assembly is now ready for insertion into

the fuel cell hardw are.

0.2mm

AnodeCathode

Polymer electrolyt e membrane


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11

TheBacking Layers

The hardware of the fuel cell, backing layers,flow fields and current collectors, is designed to

maximize the current that can be obtained from a

membrane/electrode assembly. The so-called backinglayers, one next to the anode, the other next to thecathode, are usually made of a porous carbon paper orcarbon cloth, typically 100 to 300 microns thick (4 to 12sheets of paper). The backing layers have to be made ofa material, such as carbon, that can conduct the electronsexiting the anode and entering the cathode.

The porous nature of the backing material ensureseffective diffusion of each reactant gas to the catalyst onthe membrane/electrode assembly. In this context,

diffusion refers to the flow of gas molecules from aregion of high concentration, the outer side of thebacking layer where the gas is flowing by in the flowfields, to a region of low concentration, the inner side ofthe backing layer next to the catalyst layer where the gasis consumed by the reaction. The porous structure ofthe backing layers allows the gas to spread out as itdiffuses so that when it penetrates the backing, the gaswill be in contact with the entire surface area of thecatalyzed membrane.

Enlarged cross-section of a membrane/ electrode assembly showing structural detai ls.

The backing layers also assist in watermanagement during the operation ofthe fuel cell; too little or too muchwater can cause the cell to cease opera-tion. The correct backing materialallows the right amount of water vaporto reach the membrane/electrodeassembly to keep the membranehumidified. The backing material alsoallows the liquid water produced at thecathode to leave the cell so it doesnt

flood. The backing layers are oftenwet-proofed with Teflon to ensurethat at least some, and hopefully most,of the pores in the carbon cloth (orcarbon paper) dont become cloggedwith water, which would prevent rapidgas diffusion necessary for a good rateof reaction to occur at the electrodes.

Electrode

Pathways for

gas access

to electrode

Backing layerBacking layer Membrane/ electrode assembly

Cathodebacking

Anodebacking

PolymerElectrolyteMembrane

M embrane/ electrode assembly wit h backing layers.


14/36

P ressed against the outer surface of each backinglayer is a piece of hardware, called a plate, whichoften serves the dual role of flow field and currentcollector. In a single fuel cell, these two plates are thelast of the components making up the cell. The platesare made of a light-weight, strong, gas-impermeable,electron-conducting material; graphite or metals arecommonly used although composite plates are nowbeing developed.

The first task served by each plate is to provide a gasflow field. The side of the plate next to the backinglayer contains channels machined into the plate. Thechannels are used to carry the reactant gas from thepoint at which it enters the fuel cell to the point atwhich the gas exits. The pattern of the flow field in theplate as well as the width and depth of the channels havea large impact on the effectiveness of the distribution ofthe reactant gases evenly across the active area of themembrane/electrode assembly. Flow field design alsoaffects water supply to the membrane and water removal

from the cathode.

The second purpose served by each plate is that ofcurrent collector. Electrons produced by the oxidationof hydrogen must be conducted through the anode,through the backing layer and through the plate beforethey can exit the cell, travel through an external circuitand re-enter the cell at the cathode plate.

With the addition of the flow fields and current collec-tors, the polymer electrolyte membrane fuel cell is now

complete. Only a load-containing external circuit, suchas an electric motor, is required for electric current toflow, the power having been generated by passinghydrogen and air on either side of what looks like apiece of food wrap painted black.

The FlowFields / Current

Collectors

Photographs of stainless steelflow fields/ current collectors

Micrograph of a milled carbon-fiber

composite flow -f ield. Height, w idthand spacing of channels = 0.8mm(0.032 inch)

A single polymer electrolyte membrane fuel cell.

Hydrogenflow field

Air (oxygen)flow field

Hydrogenoutlet

Water and ai r

e-e-

Anodebacking MEA

Cathodebacking

Cathodecurrentcollector

Anodecurrent

collector


15/36

13

Efficiency, Power and Energy of Polymer Electrolyte Membrane Fuel Cell

Energy conversion of a fuel cell can be summarized in the following equation:

Chemical energy of fuel = Electric energy + Heat energy

A single, ideal H2 / air fuel cell should provide 1.1 6 v olts at zero current ( opencircuit conditions), 80C and 1 a tm gas pressure. A good measure of energy

conversion efficiency for a fuel cell is the ratio of the actual cell voltage to the theo-retical max imum voltage f or the H 2 / air reaction. Thus a fuel cell operating at 0 .7 V isgenerating about 6 0% of the max imum useful energy av ailable from the fuel in theform of electric power. If t he same fuel cell is operated at 0 .9 V, about 77 .5 % of themax imum useful energy is being delivered as electricity. The remainingenergy (40 % or 22 .5 % ) w ill appear as heat. The characteristic performance curv efor a fuel cell represents the DC voltage delivered at the cell terminals as a function ofthe current density, total current divided by area of membrane, being drawn from thefuel cell by the load in the external circuit

The power (P), expressed in units of w att s, delivered by a cell is the product of the current (I) draw n and the terminal voltage (V) at that cur-rent (P = IV ). Pow er is also the rate at w hich energy ( E) is made available (P = E/ t) or conversely, energy, ex pressed in units of w att -hours, isthe pow er ava ilable over a time period (t) ( E = Pt ). As the mass and volume of a fuel cell system are so important , additional terms are alsoused. Specific pow er is the ratio of t he power produced by a cell to the mass of the cell; power density is the ratio of t he power produced by acell to the volume of the cell. High specific pow er and power density ar e important for t ransportat ion applications, to minimize the weight andvolume of the fuel cell as well as to minimize cost.

Derivation of Ideal Fuel Cell Voltage

Prediction of the maximum available voltage from a fuel cell process involves evaluation of energy differences between the initial state of reac-tants in the process (H2 +1/ 2 O2) and the final state (H 2O). Such evaluation relies on thermodynamic functions of stat e in a chemical process,

primarily the Gibbs free energy. The maximum cell voltage ( E) for the hydrogen/ air fuel cell reaction ( H 2 + 1/ 2 O2 H2O) at a specific tem-

perature and pressure is calculated [ E = - G/ nF] , where G is the Gibbs free energy change for the reaction, n is the number of moles ofelectrons involved in the reaction per mole of H 2 , and F is Faradays constant, 96 , 4 87 coulombs (joules/ volt), the charge tra nsferred per moleof electrons.

At a constant pressure of 1 atmosphere, the Gibbs free energy change in the fuel cell process (per mole of H 2) is calculated from the reaction

temperature (T), and from changes in the reaction enthalpy ( H) and entropy ( S)

G = H - T S= - 285 ,800 J (298 K)( -163.2 J/ K)= - 237,200 J

For the hydrogen/ air fuel cell at 1 atmosphere pressure and 25 C (29 8 K), the cell voltage is 1.2 3 V.

E = - G/ nF= - (- 237 ,200 J/ 2 x 96 ,487 J/ V)= 1.23 V

As temperature rises from room temperature to that of an operating fuel cell (8 0C or 353 K), the va lues of H and S change only slightly,

but T changes by 55 . Thus the absolute value of G decreases. For a good estimat ion, assuming no change in the values of H and S

G = - 285,800 J/ mol (353 K) ( -163.2 J/ mol K)= - 228,200 J/ mol

Thus, the maximum cell voltage decreases as well (for the standard case of 1 atm), from 1.23 V at 25C to 1.18 V at 80C

E = - ( -2 2 8 ,2 0 0 J/ 2 x 9 6 ,4 8 7 J/ V )= 1.18 V

An additional correction for air, instead of pure oxygen, and using humidified air and hydrogen, instead of dry gases, further reduces themaximum voltage obtainable from the hydrogen/ air fuel cell to 1. 16 V at 8 0 C and 1 at mosphere pressure.

13

0

Current Density (mA/ cm2)

CellVoltage(mV)

1200

800

600

200

0200 600 800 1000 1400 1600

400

400 1200

1000

l

ll

Graph of voltage vs. current density of a hydrogen/ airpolymer electrolyte membrane fuel cell.


16/36

Rate of Heat Generation in an Operating Fuel Cell

Assume a 100 cm2 fuel cell is operat ing, under t ypical conditions

of 1 atmosphere pressure and 80C, at 0.7 V and generating0.6 A/ cm

2of current, for a t otal current of 6 0 A. The ex cess

heat generated by this cell can be estimated as follows:

Power due to heat = Total pow er generated Electrical power

Pheat = Ptotal Pelectrical= (V ideal x Icell) (V cell x Icell)= (V ideal Vcell) x Icell= (1.16 V 0.7 V) x 60 A= 0 .46 V x 60 coulombs/ sec. x 6 0 seconds/ min.= 1650 J/ min

This cell is generating about 1.7 kJ of excess heat every minuteit operates, w hile generating about 2. 5 kJ of electric energyper minute.

S ince fuel cells operate at less than 100% efficiency,the voltage output of one cell is less than 1.16 volt.As most applications require much higher voltages thanthis, (for example, effective commercial electric motorstypically operate at 200 300 volts), the required voltageis obtained by connecting individual fuel cells in series toform a fuel cell stack. If fuel cells were simply lined-upnext to each other, the anode and cathode currentcollectors would be side by side. To decrease the overallvolume and weight of the stack, instead of two currentcollectors, only one plate is used with a flow field cut

into each side of the plate. This type of plate, called abipolar plate, separates one cell from the next, withthis single plate serving to carry hydrogen gas on oneside and air on the other. It is important that the bipolarplate is made of gas-impermeable material. Otherwisethe two gases would intermix, leading to direct oxidationof fuel. Without separation of the gases, electrons passdirectly from the hydrogen to the oxygen and theseelectrons are essentially wasted as they cannot berouted through an external circuit to do useful electricalwork. The bipolar plate must also be electronically

conductive because the electrons produced at the anodeon one side of the bipolar plate are conducted throughthe plate where they enter the cathode on the other sideof the bipolar plate. Two end-plates, one at each end ofthe complete stack of cells, are connected via the externalcircuit.

In the near term, different manufacturers will provide avariety of sizes of fuel cell stacks for diverse applications.The area of a single fuel cell can vary from a few squarecentimeters to a thousand square centimeters. A stack

can consist of a few cells to a hundred or more cellsconnected in series using bipolar plates. For applicationsthat require large amounts of power, many stacks can beused in series or parallel combinations.

A 3 cell fuel cell stack with two bipolar plates and two end plates.

The PolymerElectrolyte M embrane

Fuel Cell Stack

Polymer electrolyte membrane fuel cell stack.(Courtesy: Energy Partners)

End-plateEnd-plate Bipolar plates

Hydrogenflow fi elds

Airflow fi elds

e- e-


17/36

15

T here are several other types of polymer electrolytemembrane fuel cells for transportation applications,although none have reached the same stage of develop-ment and simplicity as the hydrogen/air.

Reformate/ Air Fuel Cell

In addition to the direct hydrogen fuel cell, research iscurrently underway to develop a fuel cell system thatcan operate on various types of hydrocarbon fuels including gasoline, and alternative fuels such as metha-

nol, natural gas, and ethanol. Initially, this fuel-flexiblefuel strategy will enable reformate/air fuel cell systemsto use the exisiting fuels infrastructure. A hydrogen/airpolymer electrolyte membrane fuel cell would be fueledfrom an onboard reformer that can convert these fuelsinto hydrogen-rich gas mixtures. Processing hydrocar-bon fuels to generate hydrogen is a technical challengeand a relatively demanding operation.

Hydrocarbon fuels require processing temperatures of700C - 1000C. Sulfur, found in all carbon-based fuels,and carbon monoxide generated in the fuel processor,must be removed to avoid poisoning the fuel cellcatalyst. Although the reformate/air fuel cell lacks thezero emission characteristic of the direct hydrogen fuelcell, it has the potential of lowering emissions signifi-cantly vs. the gasoline internal combustion engine. Thenear-term introduction of reformate/air fuel cells isexpected to increase market acceptance of fuel celltechnology and help pave the way for the widespreaduse of direct hydrogen systems in the future.

Other Types of PolymerElectrolyte M embrane

Fuel Cell Systems

Computer model of 50kW fuel cell stack with reformer.(Courtesy: International Fuel Cells)

Processing Hydrocarbon Fuels into Hydrogen

As long as hydrogen is difficult to store on a vehicle, on-boardfuel processors will be needed to convert a hydrocarbon fuel,such as methanol or gasoline, to a H2 rich gas for use in the fuelcell stack. Currently, steam ref orming of methanol to H2 is theconventional technology, although partial oxidation of gasoline toH2 is attractive because of the gasoline infrastructure already inplace in most countr ies. Both types of fuel processors are complexsystems.

The steam reforming of methanol involves the reaction of steamand pre-vaporized methanol at 20 0 C (gasoline requires tempera-tures over 800C) to produce a mixture of H 2 , carbon dioxide(CO2), carbon monoxide (CO), and excess steam. This mixt urepasses through another reactor, called a shift reactor, which usescatalysts and w ater t o convert nearly all of t he CO to CO2 as wellas additional H 2 . There can be a third stage in w hich air is in-jected into the mix ture in a t hird type of rea ctor, t he preferentialoxidat ion reactor. Ox ygen in the air reacts wit h the remaining COover a Pt-containing catalyst to convert CO to CO 2 . The final gasmixture contains about 7 0% H2 , 24% CO 2 , 6% nitrogen (N2)

and traces of CO.

W ith the part ial oxidation reformer system, liquid fuel is firstvaporized into a gas. The gas is then ignited in a part ial oxidat ionreactor which limits the amount of air so that primarily H2 , COand CO2 are produced from t he combustion. This mixt ure ispassed through a shift reactor to convert the CO to CO 2 and thenthrough a preferential oxidation reactor to convert any remainingCO to CO2 . Conventional partial oxidat ion tak es place at~1 00 0 C and cataly tic partial oxidation takes place at ~7 00 C.The final reformate composition is about 42 % N 2 , 3 8 % H2 ,18% CO2 , less than 2% CH4 and traces of CO.


18/36

Diagram of reformate/ air fuel cell engine ut ilizing liquid methanol as fuel.

Hydrogen (H2) CO2and H2

Cooling pump

Fresh Air

Coolingair out

Coolingair in

Water producedby fuel cells

Fuel Cells

Compressor/expander

Cooler

Reformerand

catalyticburner

Gaspurification

Vaporizer

Methanoltank

Watertank

The Fuel Cell Engine

Fuel cell stacks need to be integrated into a complete fuel cellengine. A fuel cell engine must be of appropriate w eight andvolume to fit into the space typically av ailable for car engines. Impor-tantly, the operation of the entire engine must maintain the near zeroemissions and high efficiency of f uel cells. Finally , all t hese require-ments must be met with components that are both inexpensive anddesigned for low cost, high volume manufacturing.

References:

Fuel Cell Handbook on CD(4

thEdition), U.S. Department of

Energy, Office of Fossil Energy,Federal Energy TechnologyCenter. November 1998.

C.E. (Sandy) Thomas, et.al.Integrated Analysis of Hy drogenPassenger VehicleTransportation Pathw ays.Nat ional Renew able EnergyLaboratory, M arch, 19 98 .

Shimshon Gott esfeld. PolymerElectrolyt e Fuel Cells. Advancesin Electrochemical Science and

Engineering, V ol.5, W iley-V CH,1997.Fritz R. Kalhammer, et. al. Status

and Prospects of Fuel Cells asAutomotiv e Engines. Preparedfor the State of California AirResources Board, July, 1998.

Regenerative Fuel Cell SystemTestbed Program for Governmentand Commercial Applications.http:/ / w w w .lerc.nasa.gov/w w w / RT1 9 95 / 5 0 0 0/5420p.htm

Solar-Powered Plane Flies to NewRecord Height One StepCloser to a Commercial Sat elliteSubstitute. http:/ /www.aerovironment.com

Resources:

A.J. Appleby and F.R. Foulkes. FuelCell Handbook. Van NorstandReinhold, New York: 19 89 .

S.R. Narayanan, G. Halpert, et.al.

The Status of Direct MethanolFuel Cells at the Jet PropulsionLaborat ory. Proceedings of the37

thAnnual Pow er Source

Symposium, Cherry Hill, N.J.,June 17, 1 99 6.

Department of Defense Fuel CellDemonstration Programhttp:/ / dodfuelcells.com/

Energy Eff iciency/ Renew able EnergyNetwork http:/ /www.eren.doe.gov

Hydrogen & Fuel Cell Letterhttp:/ / mhv.net/ ~hfcletter

Hydrogen and the Materials of aSustainable Energy Future

http:/ / education.lanl.gov/RESOURCES/ h2

Karl Kordesh and Gunter Sinander.Fuel Cells and Their Applications.VCH Publishers, New York, 1996.

DOE Office of TransportationTechnologies htt p:/ /www.ott.doe.gov

United Sta tes Council For Automotiv eResearch http:/ / w w w.uscar.org/

Arthur D. Littlehttp:/ / w w w.arthurdlittle.com
http://www.lerc.nasa.gov/www/RT1995/5000/5420p.htmhttp://www.lerc.nasa.gov/www/RT1995/5000/5420p.htmhttp://www.lerc.nasa.gov/www/RT1995/5000/5420p.htmhttp://www.lerc.nasa.gov/www/RT1995/5000/5420p.htmhttp://www.lerc.nasa.gov/www/RT1995/5000/5420p.htmhttp://www.lerc.nasa.gov/www/RT1995/5000/5420p.htmhttp://www.aerovironment.com/http://www.aerovironment.com/http://www.aerovironment.com/http://www.aerovironment.com/http://www.aerovironment.com/http://dodfuelcells.com/http://dodfuelcells.com/http://dodfuelcells.com/http://www.eren.doe.gov/http://www.eren.doe.gov/http://www.eren.doe.gov/http://mhv.net/~hfcletterhttp://mhv.net/~hfcletterhttp://education.lanl.gov/resources/h2/http://education.lanl.gov/resources/h2/http://education.lanl.gov/resources/h2/http://education.lanl.gov/resources/h2/http://www.ott.doe.gov/http://www.ott.doe.gov/http://www.ott.doe.gov/http://www.uscar.org/http://www.uscar.org/http://www.arthurdlittle.com/http://www.arthurdlittle.com/http://www.arthurdlittle.com/http://www.uscar.org/http://www.ott.doe.gov/http://education.lanl.gov/resources/h2/http://mhv.net/~hfcletterhttp://www.eren.doe.gov/http://dodfuelcells.com/http://www.aerovironment.com/http://www.lerc.nasa.gov/www/RT1995/5000/5420p.htm


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17

Electrochemistry of a Direct Methanol Fuel Cell

The electrochemical reactions occurringin a direct methanol fuel cell are:Anode CH3OH + H2O CO2 + 6 H

++ 6e

-

Cathode 3/ 2 O2 + 6H+

+ 6e-

3 H2O

Cell reaction CH3OH + 3 / 2 O2 CO2 + 2 H2O

Mixing Tank

MeOH + H2O

Air exhaust

Air

Airoutlet

CO2 vented

CondenserFan

Methanol Tank

D

M

FC

stack

Emissions from Fuel Cell Engines

The potential of fuel cells to provide zero or near zeroemissions has been a significant force in the developmentof the technology over the past 30 years. Direct hydro-gen/air systems (utilizing on-board hydrogen storage)are the only fuel cells having zero emissions from the

tailpipe. On-board processing of gasoline, methanol andother carbon-based fuels into hydrogen rich gas can bedone with minute amounts of tailpipe emissions andwater and CO

2as the major by-products. Over the next

few years, near-zero emissions and performance willcontinue to improve.

Schematic of direct methanol fuel cell system.(Courtesy: Los Alamos National Laboratory)

The Opel Zafira, by General Motors, reforms methanol to operatea polymer electrolyt e membrane fuel cell. This vehicle has nearlyzero emissions of sulfur dioxide and nitrogen oxides and the carbondioxide output is expected to be 50 % less than a comparableinternal combustion engine vehicle. (Courtesy: General Motors)

Direct Methanol Fuel Cell

As its name implies, methanol fuel is directly used in this fuel cell. In thedirect methanol fuel cell, as in the hydrogen/air fuel cell, oxygen from the

surrounding air is the oxidant, however, there is no oxidation of hydrogen.Liquid methanol is the fuel being oxidized directly at the anode.

Direct methanol fuel cell technology, unique as a lowtemperature fuel cell system not utilizing hydrogen, isstill relatively new compared to hydrogen/air polymerelectrolyte fuel cell technology, with several challengesremaining. Recent advances in direct methanol fuel cellresearch and development have been substantial, withthe direct methanol fuel cell achieving a significantfraction of the performance of direct hydrogen/air fuel

cells. However, there are critical obstacles to be over-come. To achieve high current, the necessary amount ofexpensive platinum catalyst is still much greater than theamount used in hydrogen/air polymer electrolyte fuelcells. Methanol fuel crosses through the membrane fromthe anode to the cathode; this undesired methanolcrossover decreases the performance of the air cathodeand wastes fuel.

The advantages of supplying methanol directly to thefuel cell are significant with consumer acceptance of a

liquid fuel being high on the list. While a new ormodified infrastructure would be required to supplylarge quantities of methanol, there are some methanolpumps already available. Importantly, a direct methanolfuel cell system does not require a bulky and heavyhydrogen storage system or a reforming subsystem.This advantage, in terms of simplicity and cost, meansthe direct methanol fuel cell system presents an attrac-tive alternative to hydrogen or reformate-fed systems. Inaddition, a direct methanol fuel cell is considered a zeroemission vehicle.
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Renew able regenerat ive fuel cell utiliz ing the energy source of the sunto produce power (Courtesy: Aerovironment)

In the near term, fuelavailability could bean important reason

for operating fuel cellvehicles on gasoline.Today, oil r efiners in

the U.S. are spending

over $10 billionto comply with

reformulated gasolineregulations to help

lower tailpipeemissions. Fuel cell

vehicles operat ing onalternative fuels

will require new andexpensive fueling

infrastructures.However,

alternative fuels will

provide superiorenvironmental

performance. In thelong term, incrementalinvestments in a new

domestic fuelinfrastructure will be

necessary for the21

stcentury.

RFG :Reformulated

Gasoline

M 1 0 0 :Methanol

E 10 0 :Ethanol

H2 :Hydrogen

CNG :Compressed

Natural Gas

Regenerative Fuel Cell

A regenerative fuel cell, currently being developed for utility applications, uses hydrogen andoxygen or air to produce electricity, water, and waste heat as a conventional fuel cell does.However, the regenerative fuel cell also performs the reverse of the fuel cell reaction, usingelectricity and water to form hydrogen and oxygen. In the reverse mode of the regenerativefuel cell, known as electrolysis, electricity is applied to the electrodes of the cell to force the

dissociation of water into its components.

The closed system of a regenerative fuel cell could have a significant advantage because itcould enable the operation of a fuel cell power system without requiring a new hydrogeninfrastructure. There are two concerns to be addressed in the development of regenerativefuel cells. The first is the extra cost that would be incurred in making the fuel cell reversible.

Unmanned solar plane powered by a renewable regenerativefuel cell (Courtesy: Aerovironment)

The second drawback to theoperation of the regenerativefuel cell is the use of gridelectricity to produce the

hydrogen. In the United States,most electricity comes fromburning fossil fuels. The fossilfuel electricity hydrogenenergy route generates signifi-cantly more greenhouse gasesthan simply burning gasoline inan internal combustion engine.Although the concept of aregenerative fuel cell is attrac-tive, until renewable electricity,

e.g. electricity from solar orwind sources, is readilyavailable, this technologywill not reduce greenhousegas emissions.
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21/36

19

Characteristics of Potential Fuel Cell Fuels

Production

RFG

M 100

E 100

H2

CNG

StorageCost est./

gal. eq SafetyDistribution

Infrastructure

Environmental

Attributes

Large existingproduction operation

Uses importedfeedstock

No energy security ortrade balancebenefits

Conventionalstorage tanks

Low flashpoint

Narrow flammabilitylimits

Potentiallycarcinogenic wheninhaled

Existinginfrastructureand distributionsystem

Reduction ingreenhouse gases

Much lower reactivehydrocarbon and

sulfur oxideemissions thangasoline

Abundantdomestic/importednatural gas feedstock

Can be manufacturedrenewably fromdomestic biomass -not currently beingdone

Requires specialstorage because fuelcan be corrosive torubber, plastic andsome metals

Toxic and can beabsorbed through theskin

No visible flame

Adequate trainingrequired to operatesafely

Infrastructureneeds to beexpanded

High greenhousegas emissionswhen manufacturedfrom coal

Zero emissionswhen maderenewably

Made from domesticrenewable resources:corn, wood, rice, straw,waste, switchgrass.Many technologiesstill experimental

Production fromfeedstocks are energyintensive

Requires specialstorage becausefuel can becorrosive to rubber,plastic and somemetals

Wide flammabilitylimit

Adequate trainingrequired to operatesafely

Less toxic thanmethanol andgasoline

Nearly noinfrastructurecurrentlyavailable

Food/fuelcompetition athigh productionslevels

Zero carbon dioxideemissions as a fuel

Significant emissionsin production

Domesticmanufacturing:

Steam reforming ofcoal, natural gas ormethane

Renewable solar

Compressed gascylinders

Cryogenic fuel tanks

Metal hydrides

Carbon nanofibers

Currently storagesystems are heavyand bulky

Low flammability limit

Disperses quickly whenreleased

Nearly invisible flame

Odorless and colorless

Non-toxic

Adequate trainingrequired to operate safely

Needs newinfrastructure

High emissionswhen manufactured

from electrolysis

Lower emissionsfrom natural gas

Zero emissionswhen manufacturedrenewably

$.05-.15morethangasoline

$1.10-$1.15

$.79-$1.91

Abundantdomestic/importedfeedstock

Can be made fromcoal

CNG needs to becompressed duringrefueling and requiresspecial nozzles toavoid evaporativeemissions

Stored in compressedgas cylinders

Low flashpoint

Non-carcinogenic

Dissipates into the air in

open areas

High thermal efficiency

Adequate trainingrequired to operate safely

Limitedinfrastructure

Non-renewable

Possible increasein nitrogen oxide

emissions

$.85

$.90

U.S. Congress, Office of Technology Assessment, Replacing Gasoline-Alternative Fuels for Light-Duty Vehicles OTA-E-364, September, 199 0.

Union of Concerned Scientists, Summary of Alternative Fuels, 1991.U.S. Department of Energy, Taking an Alternative Route, 1994.

National Alternative Fuel and Clean Cities Hotline: http:/ / www.afdc.doe.govJason Mark . Environmental and Inf rastructure Trade-Offs of Fuel Choices for Fuel Cell Vehicles. Future Transportation Technology Conference,

San Diego, CA. August 6-8, 1997. SAE Technical Paper 972693.

19
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A laptop computer using a fuel cell power source can operate for up to 20 hourson a single charge of fuel. (Courtesy: Ballard Power Systems)

References:

Natural Gas Fuel Cells, FederalTechnology Alert, Federal EnergyManagement Program. U.S.Department of Energy,November 19 95 .

J.L. Preston, J.C. Trocciola, and R.J.Spiegel. Utiliza tion and Control ofLandfill Methane by Fuel Cellspresented at U.S. EPA GreenhouseGas Emissions & MitigationResearch Symposium, June 27-29,19 95 , W ashington, D.C.

G.D. Rambach and J.D. Snyder. AnExamination of the CriteriaNecessary for Successful WorldwideDeployment of Isolated, RenewableHydrogen Stationary PowerSystems. XII W orld HydrogenEnergy Conference, Buenos Aires,June, 1998.

Keith Kozloff, Power to Choose.Frontiers of Sustainability. Wor ldResources Institute, Island Press,1997.

Resources:

Federal Energy Technology Centerhttp:/ / ww w .fetc.doe.gov/

World Fuel Cell Councilhttp:/ / members.aol.com/ fuelcells/

M ichael Corbett . Opportunities inAdvanced Fuel Cell Technologies:

Volume One, Stationary PowerGeneration 1998 2009, Kline &Co., Inc. Fairfield, N.J. August,1998 .

Definitions:

Quad: A unit of heat energy, equal to10

15British thermal units.

The w orld's first prototype polymer electrolyt e membrane fuel cell (on theright) used to provide a ll residential power needs for a home in Latham,New York. This 7kW unit is attached to a power conditioner/ storage unitthat stores excess electricity. (Courtesy: Plug Power)
http://www.fetc.doe.gov/http://www.fetc.doe.gov/http://members.aol.com/fuelcells/http://members.aol.com/fuelcells/http://members.aol.com/fuelcells/http://www.fetc.doe.gov/


23/36

21

4 Times Square in New York City is one of thefirst office buildings in the U.S. t o be powered

by a 200 kW phosphoric acid fuel cell system(Courtesy: International Fuel Cells)

Fuel cells are becoming analternative choice for ruralenergy needs

In places w here there are noexisting pow er grids

Where pow er supply is oftenunreliable

In remote locations that arenot accessible to power lines

Fuel cells were developed for and have long been used in the space programto provide electricity and drinking water for the astronauts. Terrestrialapplications can be classified into categories of transportation, stationary orportable power uses.

Polymer electrolyte membrane fuel cells are well suited to transportationapplications because they provide a continuous electrical energy supply fromfuel at high levels of efficiency and power density. They also offer theadvantage of minimal maintenance because there are no moving parts in thepower generating stacks of the fuel cell system.

The utility sector is expected to be an early arena where fuel cells will bewidely commercialized. Today, only about one-third of the energy con-sumed reaches the actual user because of the low energy conversionefficiencies of power plants. In fact, fossil and nuclear plants in the U.S vent21 quads of heat into the atmosphere more heat than all the homes andcommercial buildings in the country use in one year! Using fuel cells forutility applications can improve energy efficiency by as much as 60% whilereducing environmental emissions. Phosphoric acid fuel cells have beengenerally used in the initial commercialization of stationary fuel cell systems.These environmentally friendly systems are simple, reliable, and quiet. Theyrequire minimal servicing and attention. Natural gas is the primary fuel,

however, other fuels can be used including gas from local landfills, pro-pane, or fuels with high methane content. All such fuels are reformed tohydrogen-rich gas mixtures before feeding to the fuel cell stack. Over 200(phosphoric acid fuel cells) units, 200 kilowatts each, are currently in opera-tion around the world. Fuel cell manufacturers are now developing smallscale polymer electrolyte fuel cell technology for individual home utility andheating applications at the power level of 2-5 kilowatts because the potentialfor lower materials and manufacturing costs could make these systemscommercially viable. Like the larger fuel cell utility plants, smaller systemswill also be connected directly to natural gas pipe lines not the utility grid.In addition to these small scale uses, polymer electrolyte fuel cell technology

is also being developed for large scale building applications.

Distributed power is a new approach utility companies are beginning toimplement locating small, energy-saving power generators closer to wherethe need is. Because fuel cells are modular in design and highly efficient,these small units can be placed on-site. Installation is less of a financial riskfor utility planners and modules can be added as demand increases. Utilitysystems are currently being designed to use regenerative fuel cell technologyand renewable sources of electricity.

PotentialApplicationsfor Fuel Cells


24/36

Comparison of Five Fuel Cell Technologies

T he electrolyte defines the key properties, particularly operating tempera-ture, of the fuel cell. For this reason, fuel cell technologies are named bytheir electrolyte. Four other distinct types of fuel cells have been developed

in addition to the polymer electrolyte membrane fuel cell:

alkaline fuel cells phosphoric acid fuel cells molten carbonate fuel cells solid oxide fuel cells

These fuel cells operate at different temperatures and each is best suited toparticular applications. The main features of the five types of fuel cells aresummarized in chart form.

Other Fuel Cell Technologies

Fuel Cell

Polymer Electrolyt e/Membrane (PEM)

Alkaline (AFC)

Phosphoric Acid (PAFC)

Molten Carbonate (MCFC)

Solid Oxide (SOFC)

Electrolyte

Solid organicpolymerpoly-perfluorosulfonicacid

Aqueous solution ofpotassium hydrox idesoaked in a matrix

Liquid phosphoricacid soaked in amatrix

Liquid solution oflithium, sodium and/or potassium carbon-ates, soaked in amatrix

Solid zirconium oxideto which a smallamount of ytrria isadded

OperatingTemperature (C)

60 - 100

90 - 100

175 - 200

600 - 1000

600 - 1000

ElectrochemicalReactions

Anode: H22H+ + 2e-

Cathode: 1/ 2 O2 + 2H+ + 2e-H

2O

Cell: H2

+ 1/ 2 O2 H2OAnode: H

2+ 2(OH) -2 H

2O + 2e

Cathode: 1/ 2 O2 + H2O + 2e- 2(OHCell: H

2+ 1/ 2 O2 H2O

Anode: H22H+ + 2e-

Cathode: 1/ 2 O2 + 2H+ + 2 e- H2OCell: H

2 + 1/ 2 O2 H2O

Anode: H2 + CO32 -

H2O + CO2 + Cathode: 1/ 2 O2 + CO2 + 2 e- CO32 -Cell: H

2+ 1/ 2 O2 + CO2 H2O

(CO2

is consumed at cathode and produced at an

Anode: H2

+ O2 - H2O + 2e-

Cathode: 1/ 2 O2 + 2e - O2 -Cell: H

2+ 1/ 2 O2 H2O


25/36

23

Applications

electr ic utilityportable powertransportation

militaryspace

electr ic utilitytransportation

electr ic utility

electr ic utility

Advantages

Solid electrolyte reducescorrosion & management problems

Low temperature Quick start-up

Cathode reaction faster in alkalineelectrolyt e so high performance

Up to 85 % efficiencyin co-generation of electricityand heat

Impure H2 as fuel

High temperature advantages*

High temperature advantages* Solid electrolyte advantages

(see PEM)

Disadvantages

Low temperature requiresexpensive catalysts

High sensitivity to fuel impurities

Expensive removal of CO2 from fueland air streams required

Pt catalyst Low current and pow er Large size/ w eight

High temperature enhancescorrosion and breakdow n of cellcomponents

High temperature enhancesbreakdow n of cell components

*High t emperature advantages include higher ef ficiency, and t he flex ibility t o use more ty pes of f uels and inexpensive catalysts as thereactions involving breaking of carbon to carbon bonds in larger hydrocarbon fuels occur much faster as the temperature is increased.

23


26/36

C=Carbon H=Hydrogen

Trends in energy use: Hydrogen-to-Carbonrat io increases as w e become less dependenton carbon-based fuels.(Cour tesy : Wired 10/ 97)

H ydrogen is the most attractive fuel for fuel cells having excellentelectrochemical reactivity, providing adequate levels of power densityin a hydrogen

/air system for automobile propulsion, as well as having zero

emissions characteristics.

Historically, the trend in energy use indicates a slow transition from fuelswith high carbon content, beginning with wood, to fuels with more hydro-gen. Fossil fuels release varying quantities of carbon dioxide into theatmosphere coal having the highest carbon content, then petroleum, andfinally natural gas the lowest carbon dioxide emitter per thermal unit.Hydrogen obviously releases no carbon dioxide emissions when burned.

Hydrogen (H2) is the most abundant element in the universe, although

practically all of it is found in combination with other elements, for ex-

ample, water (H2O), or fossil fuels such as natural gas (CH4 ). Therefore,hydrogen must be manufactured from either fossil fuels or water before itcan be used as a fuel. Today, approximately 95% of all hydrogen is producedby steam reforming of natural gas, the most energy-efficient, large-scalemethod of production. Carbon dioxide (CO

2) is a by-product of this

reaction.

CH4 + 2H2 O 4H2 + CO2

Hydrogen can also be produced by gasification of carbon containing materi-als such as coal although this method also produces large amounts of

carbon dioxide as a by-product. Electrolysis of water generates hydrogenand oxygen.

H2O H2 + 1 / 2 O2

The electricity required to electrolyze the water could be generated fromeither fossil fuel combustion or from renewable sources such as hydro-power, solar energy or wind energy. In the longer term, hydrogengeneration could be based on photobiological or photochemical methods.

While there is an existing manufacturing, distribution, and storage infrastruc-

ture of hydrogen, it is limited. An expanded system would be required ifhydrogen fuel were to be used for automotive and utility applications.

In 1809, an amateur inventorsubmitted a patent for this hydrogen car.

Hydrogen As a FuelWood Coal Oil NaturalGas Hydrogen

C

H


27/36

25

While a single hydrogen production/distribution/storage system may not be appropriate for the diverseapplications of fuel cells, it is certainly possible that acombination of technologies could be employed to meetfuture needs. All of the system components are cur-rently available but cost effective delivery anddispensing of hydrogen fuel is essential. If hydrogen

were to become available and affordable, this wouldreduce the complexity and cost of fuel cell vehicles enhancing the success of the technology.

Hydrogen Economy is an energy system based uponhydrogen for energy storage, distribution, and utiliza-tion. The term, coined at General Motors in 1970,caught the imagination of the popular press. During theoil crisis in the early 70s, the price of crude oil sharplyincreased, concern over stability of petroleum reservesand the potential lack of a secure energy source grew,

and government and industry together developed plansand implementation strategies for the introduction ofhydrogen into a world energy system. However, thelessening of tensions in the Middle East led to a loweringof crude oil prices and the resumption of business asusual. Petroleum has continued to be the fuel of choicefor the transportation sector worldwide.

Hydrogen fuel has the reputation of being unsafe.However, all fuels are inherently dangerous howmuch thought do you give to the potential dangers of

gasoline when you drive your car? Proper engineering,education, and common sense reduce the risk in anypotentially explosive situation. A hydrogen vehicle andsupporting infrastructure can be engineered to be as safeas existing gasoline systems. Dealing with the perceptionand reality of safety will be critical to the successful wideintroduction of hydrogen into our energy economy.

References:

Peter Hoffmann. The Forever Fuel:The Story of Hydrogen. Boulder:Westview Press, 1981.

James Cannon. HarnessingHydrogen: The Key toSustainable Transportat ion.

New York: I nform, 19 95 .Strategic Planning for the HydrogenEconomy: The HydrogenCommercializat ion Plan.National Hydrogen Association.November, 1996.

U.S. Department of Energy.Hydrogen Program Plan,FY 199 3 - FY 1997.

Richard G. Van Treuren. Odorless,Colorless, Blameless.Air & Space, M ay, 19 97 .

National Hydrogen Association

http:/ / ww w.ttcorp.com/ nha/advocate/ ad22z epp.htm

Resources:

HyWeb http:/ / ww w.HyWeb.deHydrogen InfoNet

http:/ / ww w.eren.doe.gov/hydrogen/ infonet.html

The New Sunshine Programhttp:/ / ww w.aist.go.jp/ nss/text/ wenet.htm

C.E. Thomas. Hydrogen VehicleSafety Report. NationalTechnical Information Service.U.S. Department of Commerce.Springfield, Virginia. July, 1997.

Shell has established a Hydrogen Economy team dedicated

to investigate opportunit ies in hydrogen manufacturing and

new fuel cell technologies...

Chris Fay, Chief Executive, Shell UK

25
http://www.ttcorp.com/nha/advocate/ad22zepp.htmhttp://www.ttcorp.com/nha/advocate/ad22zepp.htmhttp://www.ttcorp.com/nha/advocate/ad22zepp.htmhttp://www.hyweb.de/http://www.eren.doe.gov/hydrogen/infonet.htmlhttp://www.eren.doe.gov/hydrogen/infonet.htmlhttp://www.eren.doe.gov/hydrogen/infonet.htmlhttp://www.aist.go.jp/nss/text/wenet.htmhttp://www.aist.go.jp/nss/text/wenet.htmhttp://www.aist.go.jp/nss/text/wenet.htmhttp://www.eren.doe.gov/hydrogen/infonet.htmlhttp://www.aist.go.jp/nss/text/wenet.htmhttp://www.eren.doe.gov/http://www.hyweb.de/http://www.ttcorp.com/nha/advocate/ad22zepp.htm


28/36

Front page of The New York Times, M ay 7 , 1 93 7, the day after the explosionof the Hindenberg. (Enhanced color photograph courtesy: National Hydrogen Association)

Dont paint your ship with rocket fuelAt 7 :30 , on the evening of M ay 6 , 1 93 7, the Hindenburg dirigible w as destroyed by f ire and explosions as it w as about t o land inLakehurst, New Jersey. 62 passengers survived and 35 lost their lives. The Hindenburg was nearing its landing site during anelectric storm. According to observers on the ground, the dirigible began to drift past its landing position, and after a brief delay ,the ship star ted t o valv e hydrogen into w hat w as highly charged outside air. This combination of factors could prompt severecorona activ ity on a

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