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SAE TECHNICALPAPER SERIES 2000-01-1528

Advanced Power Sources for aNew Generation of Vehicles

Steven G. ChalkU.S. Department of Energy,

Office of Transportation Technologies

H. Jackson HaleConsultant

Fred W. WagnerEnergetics, Incorporated

Reprinted From: 2000 Future Car Congress Proceedings CD-ROM

2000 Future Car CongressArlington, Virginia

April 2-6, 2000



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2000-01-1528

Advanced Power Sources for a New Generation of Vehicles

Steven G. ChalkU.S. Department of Energy, Office of Transportation Technologies

H. Jackson HaleConsultant

Fred W. WagnerEnergetics, Incorporated

ABSTRACT

The U.S. Department of Energy (DOE) and the U.S.

automotive industry are collaborating on research anddevelopment of advanced compression ignition directinjection (CIDI) engine technology and polymerelectrolyte membrane (PEM) fuel cells for automotiveapplications. Under the auspices of the Partnership for aNew Generation of Vehicles (PNGV), the partners aredeveloping technologies to power an automobile that canachieve up to 80 miles per gallon (mpg), while meetingcustomer needs and all safety and emissionsrequirements. Research on enabling technologies forCIDI engines is focusing on advanced emissions controlto meet the proposed stringent Environmental ProtectionAgency emissions standards for oxides of nitrogen (NO x)and particulate matter (PM) in 2004, while retaining thehigh efficiency and other traditional advantages of CIDIengines. DOE is teaming with U.S. diesel enginecompanies, catalyst manufacturers, and nationallaboratories to develop integrated emissions controlsystems. This paper will present emissions andefficiency data for several advanced diesel fuelalternatives when burned in a Daimler Benz Model OM611 CIDI engine. Progress achieved in lean NO x catalystand particulate trap emissions control systems will alsobe discussed. DOE and the automotive industry are alsoinvesting heavily in research and development of PEMfuel cells to meet the requirements of the PNGV. Twocritical challenges facing fuel cell systems are addressingfuel processing issues including the removal of sulfurfrom fuel and lowering the overall cost of fuel cellsystems. A cost estimate will be presented for a 50-kWPEM fuel cell system for automobiles based ontechnology available in year 2000. The high riskchallenges facing the development of CIDI engine andPEM fuel cell technologies for the PNGV necessitate anactive government role. This paper provides a statusreport on the PNGV program and an overview of thetechnical accomplishments and challenges facing theAdvanced Combustion and Emission Control R&D and

Fuel Cells for Transportation Programs of the Departmentof Energy.

INTRODUCTIONThe Partnership for a New Generation of Vehicles(PNGV) is presented with the difficult challenge ofdeveloping, by the year 2004, a production prototype ofan up-to-80-mpg family sedan that will meet customers’needs for quality, performance, and utility as well assafety and emissions requirements. Key to achieving thegoal of 80-mpg is the development of clean, advancedpower sources with high energy conversion efficiencies.The PNGV has identified two primary candidates for theenergy conversion system on the up-to-80-mpgproduction prototype vehicle: the compression ignitiondirect injection (CIDI) diesel engine, and the polymerelectrolyte membrane (PEM) fuel cell. To achieve thetarget of up to 80-mpg, the energy conversion systemsare likely to be part of a hybrid electric propulsion system.

Figure 1. Shows the timetable for the PNGV Program.

CIDI ENGINES FOR PNGV – The CIDI engine is anattractive candidate for the PNGV because it has a veryhigh energy conversion efficiency, and a technical andmanufacturing maturity that is in line with the PNGVprogram schedule (see Figure 1). The engine also hasexcellent evaporative and cold start emissions and low



operating hydrocarbon (HC) and carbon monoxide (CO)emissions. However, it does face serious challenges inmeeting the Environmental Protection Agency’s (EPA’s)proposed stringent emissions standards for oxides ofnitrogen (NO x) and particulate matter (PM) in 2004, andin reducing the weight and cost of the power plant. Oneof the critical challenges facing CIDI engines is the trade-off between emissions of NO x and particulates. Intoday’s diesel engines, strategies to reduce NO x (i.e.,exhaust gas recirculation and retarded injection timing)result in increased particulate emissions. Conversely,methods to reduce PM (i.e., higher operatingtemperatures and more complete in-cylinder mixing) tendto increase NO x emissions. This situation is particularlydifficult given that significant reductions are required inboth particulate and NO x emissions.

The applicable EPA Tier 2 target for particulates is 0.01grams/mile (g/mi). Emissions of particulates from today’sCIDI engines exceed this target by more than a factor ofsix. Current engine-out NO x emissions for CIDI enginesare 0.5 g/mi or greater whereas the target is the Tier 2,120,000-mile standard of 0.07 g/mi. This standard willrequire emissions control with NO x conversionefficiencies of approximately 90 percent. Technologiescurrently being demonstrated exhibit NO x conversionefficiencies of about 40 percent and require near sulfur-free fuel. To achieve these ambitious emissions targetswill require emissions control for both NO x and PM, aswell as significant changes in fuels. Fuel composition canhave significant impacts on engine-out emissions and theperformance of emissions control systems.

FUEL CELLS FOR PNGV – The PNGV is developingfuel cell technology because of its potential for high

efficiency, near-zero emissions, and fuel flexibility.Despite many advances in recent years, it is likely thatthe timeframe for developing fuel cell technology utilizingconventional type fuels will extend beyond the current2004 deadline of the PNGV program. Fuel cells are likelyto be the long-term replacement for internal combustionengines in automobiles and other transportation systems,but many important technical issues, including cost,remain to be solved before fuel cells can become a viablealternative.

Gaseous hydrogen (H 2) is the ideal fuel for fuel cells, butbecause of the difficulty of storing H 2 on board thevehicle, the lack of infrastructure for distributing H 2, andits high cost, its use is not viable in the near term formass-produced light-duty vehicles. To serve as atransition, it is necessary to use liquid hydrocarbon fuels,which must be reformed to produce hydrogen-rich gas forthe fuel cell stack. Reformation of hydrocarbon fuelsintroduces a host of complex issues.

DOE’S ADVANCED COMBUSTION ANDEMISSION CONTROL R&D PROGRAM

In support of the PNGV, DOE’s Office of AdvancedAutomotive Technologies (OAAT) is sponsoring a

research and development (R&D) program to developadvanced CIDI-enabling technologies for automobilesand light truck applications. The program works closelywith the PNGV’s four-stroke direct injected (4SDI)engines technical team to identify R&D priorities,establish technical targets, and review progress. Thefocus of the program is on advanced emission-controltechnologies that will enable CIDI engines to meet futureemissions standards while maintaining the high efficiencyand other advantages of the diesel engine. The DOEprogram targets for the CIDI engine system forautomobiles and light trucks are shown in Table 1.Emissions targets for the CIDI engine system areespecially challenging. The remainder of this section is adiscussion of new CIDI program activities and theprogress made in meeting the challenges facing fuelsand emissions control for CIDI engines.

Table 1. DOE Targets for the CIDI Engine System

NEW CIDI ACTIVITIES – In 1998, it became clear that toachieve stringent standards for NO x and PM emissionsfor CIDI engines, emissions reduction must be pursuedwithin the context of an integrated system. That is,

improvements in in-cylinder combustion and fuelsreformulation must be synergistically combined withemission-control systems. The National ResearchCouncil Committee Panel, which is chartered withannually reviewing the progress and direction of thePNGV, stated that considerably more effort andresources should be devoted to exhaust-gasaftertreatment of NO x and particulates. The Panel alsostated that the PNGV should consider greatly expandingits efforts to involve catalyst manufacturers.

In October 1998, OAAT released a solicitation and in late1999 awarded 2 ½ year contracts to teams lead by



Cummins Engine Company and Detroit DieselCorporation for the development of components foremission-control subsystems. The Cummins-Engelhardteam is evaluating parallel prototype hardwarecombinations for reducing NO x and PM emissionsincluding a lean NO x catalyst, plasma-assisted catalyst,and an adsorber catalyst using intermittent richconditions. Each system will be coupled with amicrowave regenerated catalyzed soot filter to control PMemissions. The Detroit Diesel-Johnson Matthey team ispursuing two parallel approaches to achieve NO xreduction including a NO x catalyst and a lean NO x trap.Their system will be combined with a continuouslyregenerating trap for PM reduction. Efforts will be closelycoordinated with R&D at the national laboratories.Deliverables include the development of completeemission-control subsystems for CIDI engines that arecandidates for PNGV. These subsystems will operate ona low sulfur diesel fuel and will meet interim emissionstargets of ≤ 0.2 g/mi NO x and ≤ 0.02 g/mi particulates.Deliverables will be provided to Oak Ridge NationalLaboratory (ORNL) for independent verification and may

be provided to automotive manufacturers for subsequenttesting and evaluation. In addition, the emissions controlsubsystems will be scaled for light-duty truckapplications.

FUELS FOR CIDI ENGINES – During the past severalyears, a substantial research effort has been focused onthe evaluation of diesel fuels which appear to have thepotential to reduce engine emissions. DOE has fundedSouthwest Research Institute (SWRI) to conduct dieselfuels tests using one of the worlds most advancedproduction automotive diesel engines. Daimler Benzprovided their Model OM 611 engine which iscommercially produced and used in their stock EuropeanClass C automobile. This direct injection, 2.2 liter dieselengine includes advanced engineering features such ashigh-pressure, common-rail injection, turbocharger with

intercooling, variable EGR, variable intake swirl, andelectronic controls. This engine is similar in many waysto the designs evolving for the PNGV automobiles. Theinitial testing at SWRI was conducted without changingthe engine tuning from the factory settings, and using afull spectrum of speed load conditions. The fuels testedincluded the following:

2D EPA Certification FuelCARB Pseudo California Certification FuelLS Low-Sulfur Low Aromatics DieselFT100 Fischer-Tropsch Synthetic DieselFT20 Twenty percent blend of Fischer-Tropsch in

LS fuelB20 Twenty percent blend of Biodiesel (produced

from soybean feedstock) with the LS fuelDMM15 Fifteen-percent blend of Methylal in LS fuel

Results from these early tests have been reported bySWRI in their Interim Report TFLRF No. 338, publishedin November 1998. Further analysis by SWRI hasrevealed that there is a strong correlation between thereduction of particulate matter (PM) and the hydrogen/

carbon (H/C) ratio and the oxygen content of the fuel (seeFigure 2). If tests continue to support this trend, it wouldappear that selection of diesel fuels for the future shouldinclude fuels having the highest practical H/C ratio and

Figure 2. PM reduction is correlated with the hydrogen/ carbon ratio and the oxygen content of thefuels.

Figure 3. For the same operating conditions, thermal efficiency varies little for all of the test fuels.



the maximum practical oxygen content. This trend alsoappears to indicate that there are some limitations to thereduction of particulate emissions through improvementsin fuel composition alone. As shown in Figure 3, theseinitial tests also show that there are only minor variationsin the thermal efficiency for all of the tested fuels whenthey are burned at the same operating power level(speed and torque) – with the exception of the non-tunedidle conditions.

Testing at SWRI is continuing to determine whetherfurther emissions reductions can be achieved byoptimizing the engine for each fuel by electronicallytuning exhaust gas recirculation, rail pressure, andinjection timing. The procedure includes a set ofstatistically designed tests which cover the practicallimits of the engine operating conditions and the availabletest variables. For each fuel, the test results areanalyzed to generate engine emissions responsesurfaces as shown in Figure 4. Comparable responsesurfaces are generated for emissions of PM, NO x, HC,CO, and specific fuel consumption. Further statisticalanalysis of this data enables the selection of engineoperating parameters that should be used to achieve theoverall optimum engine performance and criteriaemissions. Although various optimization scenarios canbe analyzed with this approach, the DOE has initiallyelected to achieve the minimum NO x and PM, whilesacrificing not more than 10 percent of the engine fueleconomy. This work will lead to a set of emissions andperformance maps which will be used in a vehicleperformance model to ascertain the criteria emissionsand vehicle performance for a PNGV type vehicleoperating under the driving cycles as prescribed by EPA.The automotive companies are evaluating their newly

evolving CIDI engines using fuels similar to those beingtested at SWRI. They have elected to use a different testand optimization approach; when their test resultsbecome available, they will be compared with the SWRItest results.

Figure 4. Representative NO x emissions map showseffect of electronically controlled engineparameters.

The DOE is also sponsoring SWRI to evaluate thetailpipe emissions of a stock European Mercedes C220automobile which uses the same engine model as the

one used in the above testing. For the initial testing,three fuels (LS, FT100, and DMM15) were selected fromthe original group.

All of these fuels had less than 10 ppm of sulfur to avoiddamage to the stock emissions control system providedon the vehicle. Each of the fuels were evaluated usingthe current EPA FTP driving cycle, the European ECEdriving cycle and the EPA proposed US06 driving cycle.These tests are continuing, so the results are preliminaryand subject to later revision. However, the early resultsare illustrated in Figure 5, which shows the PM and NO xlevels that were achieved for the three driving cycles. Forcomparison purposes, the proposed EPA Tier 2Standards are shown in the small box in the lower cornerof the chart. This preliminary data shows that theproposed EPA US06 driving cycle results in the highestemissions. For the current EPA FTP driving cycle, theNOx emissions for all three fuels are about half of theresults achieved with the US06 driving cycle, and the PMis reduced significantly when using DMM15. Evenfurther reductions of PM and NO x result when using theEuropean ECE driving cycle. Finally, Figure 5 clearlyshows that the attainment of the proposed EPA Tier 2emissions standards will require a major breakthrough inthe emissions control systems of the future.

Figure 5. Vehicle emissions are strongly influenced byselection of the standard driving cycle.

Based on the early test results from SWRI, DOE is alsofunding additional integrated fuels research at severalNational Laboratories and SWRI, in collaboration with theautomotive and fuels industries, to gain furtherunderstanding of the emissions reductions that can bederived from oxygenated fuels. The industrialparticipants are working collectively to recommend themost promising and practical oxygenated diesel fuels forCIDI engines. Sandia National Laboratories (SNL) isinvestigating the in-cylinder processes to determine theimportance of oxygen concentrations in reducing sootformations. SNL will use these results in thedevelopment of a technological and scientific informationbase that is needed to optimize in-cylinder combustionprocesses while using oxygenated fuels. LawrenceLivermore National Laboratory (LLNL) is developing animproved understanding of the chemical kinetics thatprovide links between the combustion stages in dieselengines; this will lead to an improved understanding of



the effects of oxygenates in diesel fuels. LLNL researchto date indicates that the products of rich pre-mixedignition are strong soot precursors. Laboratory researchresults also indicate that the total amount of oxygen in thefuel determines the suppression of soot precursors.However, there seems to be little correlation between richpre-mix conditions and the formation of NO x. In addition,DOE is sponsoring research at the SWRI to determinewhether any of these new fuels will generate combustionproducts that have high levels of toxicity. All of thisresearch information is critically important for thesuccessful transition to CIDI engines in automobiles andlight-duty trucks.

EMISSIONS CONTROL SYSTEMS FOR CIDI ENGINES – Conventional NO x reduction catalyst technology used ongasoline engines cannot be applied to CIDI engines dueto excess oxygen in the exhaust. As such, OAAT issupporting research focusing on lean NO x catalysts forCIDI engines applicable to the PNGV. New catalystformulations are being targeted to achieve 1) high NO xconversion over steady-state and transient operation of

the FTP cycle, 2) low light-off temperatures that arecritical for lower temperature CIDI exhaust applications,and 3) wider operating temperature windows from 100 ° Cto 350 ° C.

Figure 6. SNL is comparing NO x reduction results forvarious Pt-based/HTO:Si catalysts.

Sandia National Laboratories has developed and scaled-up platinum-based hydrous metal-oxide (HMO)-supported catalysts for NO x reduction. As shown inFigure 6, SNL has identified new silica-doped hydroustitanium oxide-supported platinum (Pt/HTO:Si) catalystswhich lower light-off temperature and widen thetemperature window of appreciable NO x reduction.Material A represents the new dopant added to the

baseline Pt/HTO:Si material.Screening experiments at Los Alamos NationalLaboratory (LANL) on microporous support materialshave resulted in the discovery of a new family of zeolite-based catalysts that exhibit high NO x reduction over abroad temperature range. LANL has undertakenoptimization of preparation procedures for the catalystswhich involves the choice of zeolite support, supportpretreatment, active metal catalyst, and the method ofexchange. Figure 7 illustrates the importance of supportpretreatment procedures (Treatments A-D) on two

different zeolite supports used to fabricate catalysts withthe same active metal and exchange method. Support 1prepared by pretreatment method A demonstratesbetween 100 and 85 percent NO x conversion between125 ° and 600 ° C. Support 2 prepared by pretreatmentmethod C has demonstrated greater than 95 percentNOx conversion between 250 ° C and 500 ° C.

Figure 7. LANL's choice of zeolite support and supportpretreatment procedure significantly impactsNOx conversion efficiency.

Figure 8. Zeolite based catalyst activity is largelyunaffected below 300 ° C despiteprogressively severe treatments of thecatalyst power.

Figure 8 presents the results of LANL’s progressivetreatment process on a single zeolite-based catalyst.This catalyst was initially tested fresh, then pretreated for8 hours at 650 ° C with 5 percent steam (tested), steampretreated for 4 hours at 750 ° C (tested), and steampretreated for 2 hours at 850 ° C prior to final testing.Results indicate that the 8-hour steam pretreatment haslittle effect on catalytic properties; subsequent steam

pretreatments progressively diminish activity above300 ° C. Throughout the pretreatments, low temperatureactivity (below 300 ° C) remains largely unchanged withinexperimental error ( ± 5 percent conversion at highconversion levels). Figure 9 presents data obtained fromcatalysts prepared by placing the pretreated support on acordierite monolith by slurrying the support in eitherwater or alumina (Disperal). The active metal componentwas then ion-exchanged onto the coated monolith, dried,and calcined prior to testing. Both slurrying methodslead to NO x conversion results similar to the unsupportedcatalyst. 1



Figure 9. The effectiveness of a zeolite based catalystprepared by slurrying a pretreated supporton a cordierite monolith is similar to theunsupported catalyst.

While it is generally agreed that achieving the emissiontarget for NO x is the greatest barrier facing CIDI engines,targets for particulates are also very demanding. Thissituation is made worse by the fact that many CIDIengines may have to employ in-cylinder NO x reducingstrategies such as exhaust gas recirculation that tend toexacerbate particulate emissions. To achieve the targetof 0.01 g/mile for particulates will require particulatereduction close to 90 percent. While particulate traps area relatively well-proven technology, they must stilldemonstrate better efficiency, reduced pressure dropsacross the filter, and improved regeneration methods,and must be made more durable and cost-effective.

Industrial Ceramic Solutions, LLC (ICS) has developedsilicon carbide fibers highly efficient in convertingmicrowaves to heat energy for regeneration of particulatetraps (see Figure 10). ICS, in conjunction with Oak RidgeNational Laboratory and other industrial suppliers, hasdemonstrated an exhaust filter system that can achieveregeneration temperatures of greater than 600 ° C atengine idle conditions. Experimental results indicate thefilter removes between 80 and 90 percent of dieselparticulates. Efficiency can be improved to greater than90 percent by reducing pore size through the mediapapermaking process. Three microwave cleanings of thefilter cartridge (at engine idle speed) subsequent to eachof three one-hour particulate loadings has resulted in a97 percent regeneration efficiency. Additionally, test cellresults show the pressure drop across the ceramic papercartridge is significantly lower (approximately 0.4 kPa)than that experienced by existing extruded cordieritewall-flow filters (approximately 2 kPa). 2

Figure 10. In less than one minute, ICS’s silicon carbidefibers can reach 800 ° C in a 2.45 GHzmicrowave field (under ambient airconditions).

Table 2. Ambitious Program Targets for the PNGV’sIntegrated Fuel Cell Power Systems

The emissions challenges facing the use of CIDI enginesin light-duty vehicle applications are daunting, requiringan approximately 90 percent reduction in bothparticulates and NO x as compared to the current state-of-the-art. It is clear that fuel modification can only gopart of the way and that highly effective emissions controlsystems are required. As discussed earlier, progress isbeing made in these areas, however significantchallenges remain before the CIDI engine can become aviable future option for mass produced automobiles andlight trucks.

DOE’S FUEL CELLS FOR TRANSPORTATIONPROGRAM

Under the auspices of the PNGV, the U.S. Department ofEnergy is a major participant in an ambitious, cost-shared, government-industry R&D program to developautomotive fuel cell power system technologies. Thesetechnologies are expected to be highly efficient with lowor zero emissions, cost-competitive, and to operate onconventional and alternative fuels. The program targetsfor integrated fuel cell power systems are shown in Table2. The following sections present an overview of theprogress made and challenges for fuel cells in severalkey areas, including improving their efficiency, fuels, andlowering overall system costs.

EFFICIENCIES OF FUEL CELL SYSTEMS – Table 3presents an analysis by Argonne National Laboratory

(ANL) of the challenges to meeting efficiency targets forfuel cell systems. The program’s target efficiencies of 48percent at 25 percent power (12.5 kW) and 38 percent atfull power (50 kW) will require fuel cell stack voltages ofapproximately 0.9 volts and 0.77 volts, respectively. Fuelcell stacks typically operate at cell voltages of about 0.6-0.7 volts, at higher voltages power density and costbecome significant challenges. Additionally, this requiresthe fuel processor to achieve an overall efficiency of 80percent, which is a major challenge when usingpetroleum-based fuels.



Table 3. ANL’s System Analysis Identifying Componentand Operating Challenges to Achieve Program Targets

Epyx is developing fuel-flexible partial oxidation (POX)fuel processing technologies for automotive PEM fuel cellsystems. Figure 11 presents the thermal conversionefficiency and percent hydrogen (dry) in the reformate ofEpyx’s fuel processor when operating on CaliforniaPhase II gasoline. The efficiency shown is at the exit ofthe preferential oxidation (PROX) carbon monoxide (CO)clean up system which functions to maintain CO levels inthe reformate at less than 10 ppm. The Epyx reformerdemonstrates efficiencies in excess of 70 percent over aturndown of four to one; similar efficiencies have beendemonstrated at higher turndown. These efficiencies areclose to the maximum theoretical efficiency indicating ahighly optimized reformer design. 3

Figure 11. Epyx's multifuel processor demonstrates highthermal conversion efficiency and hydrogenconcentrations when operating on gasoline.

Current state-of-the-art fuel processing subsystems

combined with fuel cell stack subsystems lead to anefficiency around 35 percent. Presently, these systemsconsist of experimental units made up of componentswhich have yet to be integrated into a complete system;no automotive size system operating on petroleum-basedfuels currently exists. DOE’s current focus is to developthis system and to improve overall functionality whileachieving a total system efficiency of 30 to 35 percent.Once the system has been fully integrated and isoperating, work will commence to increase overallsystem efficiency to 40 percent.

Figure 12. PNNL is using engineered microstructures inits fuel processing technology.

Pacific Northwest National Laboratory (PNNL) is usingengineered microstructures to develop ultra-compactreactors, separators, and heat exchangers to facilitate thereforming of liquid hydrocarbon fuels (see Figure 12).PNNL has developed a 50-kW e-capacity microchannelgasoline vaporizer for partial oxidation (POX) and/orautothermal reformers, which has met performancegoals at a volume of only 0.3 liters. This technology hasbeen transferred for testing to the industry fuel processordevelopers Epyx and Hydrogen Burner Technology.PNNL is also developing and testing the first proof-of-principle, single-cell, microchannel steam reformer.Steam reforming operates at lower temperatures(≈ 650 ° C) than POX or autothermal reforming and atgreater than atmospheric pressure without a compressor,while producing higher concentrations of hydrogen in theproduct stream. Fuel cells can operate more efficientlywith a pure hydrogen feed. Figure 13 illustrates PNNL’ssuccess in using an integral microchannel reactor/heatexchanger leading to fast kinetics for steam reforming.The conversion of iso-octane (a surrogate for gasoline)and selectivity to hydrogen are both about 90 percentwith a reactor residence time of just 2.3 milliseconds.The drygas content of hydrogen is between 65 and 70percent compared to a drygas content of 35 to 40percent traditionally obtained with POX and autothermalreactors.

Figure 13. Data obtained from PNNL’s microchannelsteam reforming system using iso-octanedemonstrates conversion and selectivity ofabout 90 percent.

In the future, the DOE Fuel Cells for TransportationProgram will target higher-risk development of high-



catalysts. Typically, fuel cell stack operating temperaturesare limited to 80 ° C. Key advantages would be obtainedfrom the development of an inexpensive, high-temperature membrane operating at 100 ° C-150 ° C thatsustains current densities comparable to today’smembranes and does not require significanthumidification. This membrane would enhance COtolerance and facilitate heat rejection permitting adramatic reduction in the size of the condenser andradiator. As mentioned earlier, higher operating voltagesare required to meet efficiency targets for fuel cellsystems. The development of improved oxygenreduction electrocatalysts with greater kinetics would bebeneficial because the most significant contributor to cellvoltage loss is polarization on the cathode. Additionally,advanced oxygen catalysts could reduce air compressorrequirements in fuel cell systems, thereby improvingefficiency through reduced parasitics. 4

FUELS FOR FUEL CELLS – The DOE Fuel Cells forTransportation program is pursuing a fuel-flexible strategyenabling the conversion of petroleum-based fuels,

ethanol, methanol, and natural gas into hydrogen-richstreams for fuel cells (see Figure 14). This fuels strategyaccepts the reality of today’s fuel infrastructure whilecreating pathways for alternative fuels. Today and forperhaps the next 50 years, petroleum-based fuels are aprimary focus because of their easy availability and lowcost. Although from a vehicle perspective it is lessenvironmentally compatible than a zero-emissions,direct-hydrogen fuel cell vehicle, a petroleum-basedgasoline-like PEM fuel cell vehicle is expected to havemuch greater efficiency and far lower emissions than areallowable under the strictest California standardsproposed. Ethanol and methanol are part of the DOEfuels strategy because they are renewable and can bemade from domestic energy sources, providing the mostoil displacement. Natural gas is included in the fuelsstrategy to maintain synergism with DOE stationary andportable power applications and for potential heavyvehicle applications. These alternative fuels provide atransition to a hydrogen-based refueling system.

Figure 14. DOE’s fuel strategy for fuel cells is fuel-flexible and emphasizes domestic energysources.

Desirable characteristics of fuels for fuel cells includehigh hydrogen to carbon ratios which lead to highconversion efficiencies (see Table 4) and high hydrogencontent in the fuel stream. High oxygenate concentra-

tions and low aromatics tend to reduce coke formationand fuels containing no sulfur eliminate the requirementfor on-board removal systems. Another importantconsideration is that the type and size of hydrocarbonmolecules in the fuel can influence the ease ofreforming. 5

Fuel cells do not require pure hydrogen to operateefficiently and it is acceptable for reformate to containsubstantial carbon dioxide, water, and nitrogenconcentrations. However, reformate cannot containimpurities or contaminants that poison the anode. As aresult, fuels for fuel cells must contain very low levels ofcontaminants and must be sufficient to preserve catalystactivity for more than 100,000 miles.

Table 4. Theoretical Fuel Conversion Efficiencies as aFunction of Hydrogen-to-Carbon Ratios.

An important new aspect of the DOE Fuel Cells forTransportation program is determining the effects of fuelconstituents, additives, and impurities on theperformance and durability of fuel processors and fuelcell stacks. ANL and LANL are examining petroleum-based and alternative fuels (methanol, ethanol, dimethylether, and Fischer-Tropsch liquids) to identify appropriatefuel compositions for use with fuel cells. ANL isconducting autothermal reforming of major constituentsin a microreactor to examine by-product formation,catalyst deactivation, and the dependence of hydrogenyield and conversion efficiency on reactor temperatureand residence time. LANL is exploring fuel processingreactions with reacting (catalyzed) surfaces and withhomogenous (non-catalyzed) mixtures. LANL has alsosuccessfully completed its studies on mitigating the effect

of certain impurities and constituents on fuel cell stacks,including ammonia, methane, hydrogen sulfide, chloride,aromatics, and olefins.

Sulfur poisoning is one of the biggest issues facing theuse of petroleum-based fuels in fuel cell systems.Gasoline sold at the present time in the United Stateshas an average sulfur content of 347 ppm with spikes ashigh as 1000 ppm. In December 1999, theEnvironmental Protection Agency announced that thesulfur content of gasoline must be reduced to 30 ppmwith a maximum of 80 ppm by year 2004. In the reducingenvironment of the fuel reformer, the organic sulfur in



gasoline is converted primarily to hydrogen sulfide (H 2S).For autothermal reforming, the concentration of H 2Sresulting in reformate is approximately one-tenth thesulfur content of the fuel. However, even 3 ppm of sulfurpoisons many catalysts, including many envisioned forthe fuel processing chain. For example, a maximumconcentration of 50 ppb is recommended for copper-zincoxide catalysts for the water-gas shift reaction.

Figure 15. The graph illustrates the weight of ZnOrequired to lower H 2S concentration inreformate to less than 1 ppm based solely onZnO loading capacity.

Tests at ANL are demonstrating that catalysts forautothermal reforming do have some tolerance to sulfur.Initial tests with premium gasoline and diesel fuel showno degradation during short-term testing (of 8 to 20hours); 20-hour testing with iso-octane containing 300ppm and 1000 ppm sulfur support these earlier results.However, H 2S is known to irreversibly poison the platinumanode in fuel cell stacks at concentrations of 1 ppm.

Therefore, it may be necessary to remove H 2S fromreformate even when using low sulfur gasoline.

ANL is investigating adsorption with chemical reaction forreducing H 2S concentrations to below 1 ppm inreformate. Here, H 2S is reacted with a solid, generally ametal oxide, which “traps” the sulfur by converting it into astable metal sulfide. Zinc oxide (ZnO) is a candidateadsorbent due to favorable equilibrium thermodynamicsfor zinc sulfide (ZnS) formation with a H 2S equilibriumpartial pressure of less than 1 ppm under typical fuelprocessing conditions. ZnO is used in many industrialprocesses, including ammonia synthesis, to reduce H 2S

concentrations to as low as 50 ppb.Based solely on the loading capacity of ZnO, Figure15illustrates the weight of ZnO required for a sulfuradsorption bed for an automotive PEM fuel cell processorusing gasoline with different sulfur concentrations.However, to appropriately size an adsorption bed, masstransfer and chemical reaction rates that depend uponspace velocity and power rating must also be considered(see Table 5). For a bed based on peak power (50 kW e),approximately 10 times more ZnO is required than basedsolely on loading capacity. When sized for averagepower (10kW e), ZnO requirements are approximately the

same as those when based on loading capacity. At thepresent time, long-term performance has not beenestablished for a sulfur adsorption bed based on averagepower with periodic operation at peak power. Inresponse, ANL is researching ways to improve the rate ofH2S uptake by ZnO and is determining the optimal powerrating on which to base the size of the absorbent bedwhile still meeting size and weight constraints establishedfor fuel processors for automotive PEM fuel cell systems.

Table 5. The Weight of ZnO Required to Lower H 2SConcentration in Reformate to less than 1 ppm forCommercial ZnO Adsorbents Based on Power andSpace Velocity

COSTS OF FUEL CELL SYSTEMS – Another primechallenge for fuel cells is to reduce the cost ofcomponents and subsequently of integrated fuel cellpower systems from today’s $300/kW to the programtarget of $50/kW for high volume production. Membraneelectrode assemblies (MEAs) are the core of the fuel cellstack and require very high quality and production yieldsto achieve automotive cost and performance targets.The 3M Company is developing novel MEAs based onnanostructured thin film catalysts and support systems.Figure 16 shows a scanning electron micrograph of the

company’s high-surface-area, catalyst-coatednanostructure supports before transfer to a PEM surfaceto form a 3-layer MEA. Most importantly, the 3MCompany is developing MEAs in parallel with highvolume process development to meet the PNGV costtarget of $10/kW. 3M is also developing proprietarycatalysts based on the nanostructured films for optimizedCO tolerance. Figure 17 illustrates the performancecomparison of a platinum ruthenium (PtRu)-basedsystem operating on pure hydrogen and 50 ppm carbonmonoxide with a 0.5 percent air bleed.

Figure 16. A scanning electron micrograph shows 3M’snano-structured catalyst-coated film.



Figure 17. This chart shows the performancecomparison of 3M CO tolerant anode catalystbased on microstructured films using pilotproduction processes.

One of the most expensive components in the fuel cellstack system is the solid graphite, bipolar separatorplates. The Institute of Gas Technology (IGT) isdeveloping low-cost, compression-molded, carboncomposite bipolar separator plates and a conceptual

design for their mass production. IGT has successfullyselected, blended, and optimized inexpensive rawmaterials to meet or exceed all electrical, chemical, andphysical property targets. As shown in Figure 18, theperformance of the molded composite bipolar separatorplates (300 cm 2) is comparable to state-of-the-artmachined graphite plates. At a production level of500,000 units per year, the program goal of $10/kW isexpected to be met. The raw material cost is estimatedat $1.46 per pound accounting for $4/kW. IGT hastransferred this bipolar plate technology to Stimsonite,which has fabricated molds for pilot production and joinedwith ENDESCO in a joint venture called PEM Plates,

LLC.

Figure 18. The performance of an IGT molded graphitebipolar separator plate is similar to that of amachined graphite plate.

In 1999, Arthur D. Little developed a comprehensive costestimate for a 50 kW PEM fuel cell system for passengervehicles based on technology available in year 2000 andassuming high production volumes (500,000 units peryear). This cost estimate for the baseline system is thefirst part of a multi-year effort to assess the impact andprogress of technology advances on overall system

costs. The first step in the process was to define aplausible configuration for the fuel cell system andrealistic operating parameters on which to base the costestimate. An overall system efficiency of 35-40 percentand water self-sufficiency were required, which had astrong influence on the design and specification of thesystem (see Figure 19). Table 6 presents the systemparameters chosen which led to a projected overallsystem efficiency of 37.1 percent at peak power basedon lower heating value. The actual drive cycle efficiencycould be higher depending on the efficiency of thecompressor/expander at partial load. Table 7 shows theallocation of components to the fuel processing and fuelcell sub-systems, as well as balance-of-plant.

Figure 19. PNGV requirements for high systemefficiency and water self sufficiency impactedthe configuration of the fuel cell system.

Table 6. PNGV Requirements and DOE SpecificationsEstablished Many System Parameters and Design Goalsfor the Cost Model

The factory cost estimate of the fuel cell system andcomponents includes fixed and variable manufacturingcosts, but excludes corporate expenses (e.g. R&D, sales,marketing, G&A) and profits. A bottom-up manufacturingmodel identifying critical manufacturing operations wasused to estimate the cost of major fuel processor and fuelcell components. The cost of the balance of the systemcomponents (e.g. heat exchangers, control valves,sensors) is based on discussions with suppliers. The costof components such as valves and sensors not readilyavailable nor currently suitable for high-volumeproduction are based on best estimates by Arthur D.Little.

SystemRequirements

Fuel CellModule

Fuel Cell Stack

Fuel flexible(gasoline)

• System efficiency35-40%

• Water self-sufficiency

• 3 atmosphereoperation

• Turbocompressor / expander

• 50kW e

netJ 300

volts @fullpower

• 80°C• Reforma

te fuel

• 0.8 volts percell

J 310 mA/cm 2

currentdensity

J One coolingplate per cell

J Total power56 kW e

• 85% H 2

utilization

• Specified by DOE J Established duringtechnology assessment

SystemRequirements

Fuel CellModule

Fuel Cell Stack

• Fuel flexible(gasoline)

• System efficiency35-40%

• Water self-sufficiency

• 3 atmosphereoperation

• Turbocompressor / expander

• 50kW e

netJ 300

volts @fullpower

• 80°C• Reforma

te fuel

• 0.8 volts percell

J 310 mA/cm 2

currentdensity

J One coolingplate per cell

J Total power56 kW e

• 85% H 2

utilization

• Specified by DOE J Established duringtechnology assessment



Table 7. The Allocation of Components to the Various Subsystems for Estimating Fuel Cell Costs

The cost modeling indicates that the overall system cost

for baseline year 2000 is $14,700 or $294/kW. The costof the fuel cell subsystem represents 60 percent of theoverall system cost and the fuel processor subsystemabout 30 percent, while the balance-of-plant andassembly account for the rest (see Table 8). The fuel cellstack represents approximately 80 percent of the fuel cellsub-system cost, with the integrated tailgas burner($465), compressed air supply ($860), and stack coolingsystem ($480) accounting for the remainder. The tailgasburner incorporates emissions control, steam generation,and fuel vaporization. The membrane electrodeassemblies account for 75 percent of the total fuel cellstack cost (see Table 9). The precious metal content in

the membrane electrode assemblies (Anode – Ru/Pt 0.2/ 0.4 mg/cm 2; Cathode – Pt 0.4 mg/cm 2; total of 180 gramsof Pt per stack) accounts for 75 percent of the cost of themembrane electrode assemblies. The cost projection forthe perfluorosulfonic acid membrane 6 is $50/m 2, whilethe cost for injection molded bipolar interconnect plates 7

was estimated at $20/kW. Requirements for highefficiency operation leading to modest power density(250 mW/cm 2) is a major factor in the high stack cost. Ifoptimized for high power output, the stack cost would bereduced by up to half. High power lowers the unit cellvoltage but increases the current density resulting in anet decrease in fuel cell materials. The active volume isapproximately inversely proportional to the power density.

Table 10 presents the cost break down for the fuelprocessing subsystem. Thermal, water, and steammanagement; controls; and packaging contribute 70percent to the overall cost of this subsystem.Consequently, improved system design and engineeringwill be required to reduce the cost of peripheral systemsin fuel processors, as well as improved catalysttechnology to reduce bed size and extend life.

Table 8. The Domination of Fuel Cell and Fuel

Processing Subsystems on the Total Cost of Fuel CellSystems

Table 9. Membrane Electrode Assemblies are 75Percent of Fuel Cell Stack Cost

Fuel Processor Subsystem Fuel Cell Subsystem Balance-of-Plant

Reformate Generator@ Autothermal Reformer@ High Temperature

Shift@ Sulfur Removal@ Low Temperature Shift@ Steam Generator@ Air Preheater@ Steam Superheater@ Reformate Humidifer

Fuel Supply@ Fuel Pump@ Fuel Vaporizer

@ Fuel Cell Stack (UnitCells)

@ Stack Hardware@ Fuel Cell Heat

Exchanger@ Compressor/

Expander@ Anode Tailgas Burner@ Sensors & Control

Valves

@ Startup Battery@ System

Controller@ System

Packaging@ Electrical@ Safety

Reformate Conditioner@ NH3 Removal@ PROX@ Anode Gas Coller@ Economizers (2)@ Anode Inlet Knockout

Drum

Water Supply@ Water

Separators (2)@ Heat Exchanger@ Steam Drum@ Process Water

Reservoir

@ Sensors & Control Valves for each section

Fuel Processor Subsystem Fuel Cell Subsystem Balance-of-Plant

Reformate Generator@ Autothermal Reformer@ High Temperature

Shift@ Sulfur Removal@ Low Temperature Shift@ Steam Generator@ Air Preheater@ Steam Superheater@ Reformate Humidifer

Fuel Supply@ Fuel Pump@ Fuel Vaporizer

@ Fuel Cell Stack (UnitCells)

@ Stack Hardware@ Fuel Cell Heat

Exchanger@ Compressor/

Expander@ Anode Tailgas Burner@ Sensors & Control

Valves

@ Startup Battery@ System

Controller@ System

Packaging@ Electrical@ Safety

Reformate Conditioner@ NH3 Removal@ PROX@ Anode Gas Coller@ Economizers (2)@ Anode Inlet Knockout

Drum

Water Supply@ Water

Separators (2)@ Heat Exchanger@ Steam Drum@ Process Water

Reservoir

@ Sensors & Control Valves for each section



Table 10. The Cost of Thermal, Water, and SteamManagement, Controls, and Packaging for the FuelProcessing Subsystem

Cost modeling indicates that materials and purchasedcomponents make up approximately 80 percent of thetotal factory cost of the fuel cell system (based on highproduction volumes). Table 11 breaks down thesecomponents and materials by categories. Catalysts forstacks and fuel processing represent the largest materialcost and are dominated by platinum (total Pt=210 g).

Purchased components such as heat exchangers,sensors, valves, controllers, the compressor/expander,pumps, and motors contribute 20 percent to the totalsystem cost. Simplifications in design and alternativetechnologies could potentially lower these componentcosts. The next phase of the cost modeling will focus onopportunities for cost reduction achievable throughtechnology improvements such as reduced preciousmetal loadings, increased fuel cell power density and fuelprocessing catalyst activities, and higher temperaturemembranes.

Table 11. The Cost of Purchased Materials and

Components for the Fuel Cell System

CONCLUSION

The Department of Energy is working cooperatively toaddress key challenges facing the development oftechnologies for CIDI engines and PEM fuel cells for theup-to-80 mpg production prototype vehicle of the PNGV.The most significant challenge facing CIDI engines is toreduce emissions of both particulates and NO x by up to90 percent. Research data indicates that aggressive fuel

modifications have the potential to reduce particulateemissions by up to 50 percent. As such, it has becomeclear that fuel modifications must be synergisticallycombined with emissions control systems (such asparticulate traps) to achieve targeted particulate levels.With regards to lean NO x catalysis, recent research dataindicates significant progress is being achieved. Thisdata shows the potential for 80 to 100 percent NO xconversion over a broad temperature range applicable toCIDI engines for light duty vehicles.

Improving fuel processing and reducing overall systemcosts are two primary challenges facing the use of PEMfuel cell technology in mass produced light duty vehicles.Researchers are closing in on achieving the targetedefficiency of 80 percent for fuel processors whenoperating on petroleum-based fuel. However, the effectsof fuel constituents, additives, and impurities on theperformance and durability of fuel processors and fuelcell stacks remains an issue, with sulfur poisoning beinga primary challenge. A multi-year effort is underway toassess the impact and progress of technology advanceson overall fuel cell system costs. Based on the results ofthe cost analyses, the DOE program will targettechnology improvements in those areas with thegreatest potential for long-term cost reduction.

ACKNOWLEDGMENTS

The authors of this paper would like to recognize thesignificant contribution made by Eric Carlson of Arthur D.Little Corporation in providing the information andnarrative on the cost estimate for the 50-kW PEM fuel cellsystem for passenger vehicles. The authors would alsolike to recognize the contribution of John P. Kopasz andTheodore Krause of Argonne National Laboratory fortheir information on Challenges of Working with GasolineFuels – The Sulfur Issue.

REFERENCES

1. Ott, K., Los Alamos National Laboratory, personalcommunications, December 1999.

2. Nixdorf, R., Industrial Ceramic Solutions, LLC,personal communications, January 2000.

3. Prabhu, S., Epyx Corporation, personalcommunications, December 1999.

4. Miller, J., Argonne National Laboratory, personalcommunications, September 1999.



5. Sutton, R., P. Davis, N. Vanderborgh, and S. Ahmed,Fuel Issues for Fuel Cell Systems, WindsorWorkshop, Toronto, Ontario, Canada, June 8, 1999.

6. Bartelt, J. Satisfying the Commercial Requirementsfor Alternative Energy Applications, Workshop onAdvanced PEM Fuel Cell Membranes andMembrane Electrode Assemblies for Non-Conventional Fuels, Clemson University, April 28,1998.

7. Busick, D.N., and M.S. Wilson, Composite BipolarPlates, Annual Meeting DOE Fuel Cells forTransportation – National Laboratory R&D Program,Argonne, IL , June 23-25, 1999.

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