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40 The Electrochemical Society Interface • Winter 2008

0.1-1 W 2-5 W 25-150 W 500-1000 W

MP3 player

Mobile Phone

Future Soldier Materials Handling

15-30 W

LaptopComputers

High-Energy Portable Fuel Cell Power Sources

by S. R. Narayan and Thomas I. Valdez

F IG. 1. Portable power applications require high-energy power sources.

T

he energy density requirementfor portable power sources isever on the increase. Technology

companies are working to find ways toenhance the run time of mobile devicessuch as portable computers, MP3 players,and mobile phones. For example, thevideo-streaming offered in present-day mobile phones is a power-hungryfeature that reduces the run-time forphones powered with even the mostadvanced of lithium-ion batteries. TheU.S. military has also had a persistentdemand for compact, long-life powersources for meeting the field needs ofthe soldier. The future-day soldier willbe outfitted with high-tech electronicsthat significantly increase awareness of

the combat environment. Radios, night-vision devices, portable computers,and personal cooling systems are allexamples of power-hungry devices to beused by the future-day soldier. There arealso other areas that need high-energyportable power sources: (a) extendingthe run-time of materials handlingequipment, (b) meeting on-boardpower needs in recreational vehicles,and (c) powering remote electronicsequipment. Achieving longer run-timeswill eliminate the need for the frequentreplacement of discharged batteries,reduce the downtime resulting from

recharging or replacing batteries,and eliminate the logistics neededfor recharging batteries. Some of theapplications of portable power sourcesand the corresponding power levels areshown in Fig. 1.

For mobile devices, the energydensity target is to increase the devicerun-time by five to ten times, without

increasing the size or mass of the powersource. A recent call for proposals fromthe U.S. Department of Energy (DoE)identifies these targets. To meet theenergy and power density targets forfuture mobile devices, it would bedesirable to achieve a specific energyof 500 Wh/kg or greater, an energydensity of 1000 Wh/liter or greater,and a specific power not less than 50W/kg. Typical power levels in mobiledevices vary from 1-25 W with run-times varying from 10 to100 h. Further,any next-generation power source formobile devices must include batteryfeatures, namely, quiet operation,ruggedness, ease-of-use, operability inany orientation, operability over a wide

temperature range (-40oC to +60oC), andoperability in confined environments.There may also be other requirementssuch as underwater operation that arespecific to military applications. Forlarge portable systems such as thoseused in material handling equipment,the high-energy power source suppliesthe base load and charges a batteryduring the off-period or serves as apower back-up. Typical power levels forthis category of applications vary from50 to 1000 W and the power densitycan be as low as 10 W/kg. The target isusually to increase the run-time of large

portable systems by at least five timesthe current values.The above requirements present

significant technical challenges.However, in the last decade or so,there have been focused efforts in thecommercial and government sectors todevelop fuel cell based power sourcesoperating on high-energy fuels such

as hydrogen, methanol, and propaneto meet the requirements of next-generation portable power sources.

Types of Fuel Cells

Portable fuel cell power sourcescan be categorized by the fuel andtype of fuel cell used for operation.Methanol can be directly utilized in apolymer-electrolyte membrane (PEM)type of fuel cell; this type of fuelcell is known as a direct methanolfuel cell (DMFC). The typical operatingtemperatures for DMFCs are in therange of 40-60oC. Methanol can also bereformed to hydrogen at about 300oC,and the resulting “reformate” fuel is

then supplied to a PEM fuel cell thatoperates at 180oC. Sodium borohydridecan be decomposed to hydrogen atambient temperatures; the liberatedhydrogen is then utilized in a PEM fuelcell. Butane and JP-8 are processed tohydrogen at 600oC and the productsare fed to a solid oxide fuel cell (SOFC)that operate with internal temperaturesas high as 1000oC. Thus, each fuelcombined with an appropriate fuel celltype results in a different portable fuelcell power source. These differences aresummarized in Table I.

When the topic of high-energy

density portable power sources wasreviewed in 2002-2003,1,2 the directmethanol fuel cell was primarily thetechnology of interest. The research atthat time was focused on demonstratingvarious types of cells and compactstacks, and proving the feasibility ofsystem concepts. Much of this initialwork from the conception of the



The Electrochemical Society Interface • Winter 2008 41

technology to the demonstration ofstack and systems was carried out in theU.S. under research programs fundedby the Department of Defense at the Jet Propulsion Laboratory (JPL), LosAlamos National Laboratory (LANL),U.S. industry, and universities3-5 and byresearch groups in the UK and India.6 Since then there has been substantialprogress in the commercialization ofportable DMFC systems particularly

suitable for use by the military. Also,there has been more development ofsystems based on the other fuel celltypes listed in Table I.

Direct Methanol FuelCell Systems

DMFC systems are based on theability to achieve the complete electro-oxidation of methanol to carbon dioxidein a liquid-fed PEM cell. In a DMFC,methanol is diluted into an aqueoussolution and fed to the anode of thefuel cell. Air or oxygen is supplied to

the cathode of the fuel cell. DMFCsystems are configured to allow for thecontinuous supply of fuel solution at theappropriate concentration to the stack,removal of by-product water and carbondioxide, recovery of water needed forthe anode reaction, and rejection of theheat generated. A schematic of a DMFCsystem is shown in Fig. 2. This figurealso shows the internal construction ofa 300 W system fabricated at JPL.

In a DMFC, nano-particulateplatinum-ruthenium anode catalystsare the choice for the electro-oxidationof methanol. Platinum black is the

F IG. 2. Schematic of a DMFC system and the internal distribution of components in a 300 W DMFCsystem fabricated at JPL.

Table I. Characteristics of various fuel cell types suitable for portable power sources.

Fuel Cell Type Fuel

FreeEnergyContentof FuelWh/kg

FuelProcessing

requirements

InternalOperating

temperatureof fuel cell Electrolyte Catalysts

Direct Oxidation Methanol 6,000 None 45 - 60 oC

ProtonConductingmembrane

(Nafion)

Pt-Ru onanode, Pt oncathode

Indirect MethanolFuel Cell

Methanol 6,000Steam reformingat 250-300oC

180- 200oC

Hightemperaturepolymerelectrolyte

Pt on anodeand cathode

Solid oxide FuelCell

Propane, Butane, JP-8

10,000Partial oxidationat 650oC

800- 1000oCYttria-StabilizedZirconia

Nickel onanode andperovskiteoxide oncathode

Hydrogen/airHydrogen storedin metal andchemical hydrides

3,000-9,000

Catalyticdecompositionat near ambient

temperature

45- 60oC

Protonconductingmembrane

(Nafion)

Pt on anodeand cathode



42 The Electrochemical Society Interface • Winter 2008

F IG. 3. Jenny and EFOY DMFC Systems from Smart Fuel Cell. (Graphics reproduced with permissionfrom SFC Inc.)

arayan, et al .ontinued from previous page)

preferred catalyst for the cathode.Unsupported precious metal catalystat the loading level of 2 to 4 mg/cm2 is needed to achieve practical powerdensities in DMFCs. There is a growinginterest in developing alternate catalystswith reduced platinum loading, butthe progress in this area has been

rather slow. The phenomenon ofmethanol crossover is another issuebeing addressed in DMFC research.7 Nafion®-based membranes are still themost widely used polymer electrolyte inDMFCs. Multi-layer Nafion membraneshave been shown to provide 50%reduction in methanol cross-over overstandard Nafion 117 membranes.8 Othermembrane materials such as composites

with cross-linked polystyrene sulfonicacid have been shown to be quiteefficient at suppressing methanolcrossover from the anode to the cathode.9 Suppressing methanol cross-overwithout compromising on the protonconductivity is a technical challenge.Research on alternate membranes

continues to be actively pursuedworldwide since the benefit of reducedmethanol cross-over and reducedmembrane cost are very important tothe widespread deployment of DMFCsystems. Many commercial entitiesare now offering membrane-electrodeassemblies (MEAs) for DMFCs.

Smart Fuel Cell Inc. of Germany hasbeen leading the development of small

packaged DMFC systems in the rangeof 15 to 150 W. Several thousand unitsof this company’s fuel cell systems arealready aboard recreational vehicles,boats, and vacation homes. DMFCsystems present a clear advantage overconventional technologies such asgenerators by being quiet and non-

polluting. The SFC units shown inFig. 3 are in two ranges of power levels,namely, 25 W and 65 W typified bythe Jenny Tactical and the EFOY 1600product lines, respectively.

Metrics on the power source andfuel storage are summarized in TableII. The specific energy of a Jenny withone fuel cartridge is 290 Wh/kg. Thespecific energy of the EFOY 1600,paired with the 5 and 28 L fuel tanks,are 388 and 805 Wh/kg respectively.These specific energy values clearlyexceed that of lithium ion batteries. Itis important to note that higher specificenergy values can be achieved whenthe fuel mass fraction of the systempackage increases. Consequently,specific energy and specific powervalues quoted for portable fuel cellsystems must be qualified by the powerlevel and the run-time. This type ofdescription is principally different fromthat of rechargeable batteries whereinthe specific energy and specific powervalues do not change dramatically withthe energy content of the battery.

Toshiba Corporation, MTI, Sony, andSamsung have claimed developmentof small DMFC systems for portableelectronics applications and some of

these companies have exhibited theirprototypes at various trade-shows.

Power Source Fuel System Specifications

System

FuelCellType Fuel

Power(W)

Volume(L)

Mass(kg)

Volume(L)

Mass (kg)

EnergyOutput(Whr)

RunTime,h

SpecificPower(W/kg)

PowerDensity(W/L)

EnergyDensity(Whr/kg)

EnergyDensity(Whr/L)

SFCENNY

DMFC Methanol 25 2.8 1.3 0.6 0.36 480 19 15.1 7.4 289.2 141.2

EFOY1600-M5

DMFC Methanol 65 24.00 7.3 7.8 4.3 4,500 69 5.6 2.0 387.9 141.5

EFOY1600-M10

DMFC Methanol 65 24.00 7.3 14.00 8.4 9,100 140 4.1 1.7 579.6 239.5

EFOY1600-M28

DMFC Methanol 65 24.00 7.3 41.5 24.00 25,200 388 2.1 1.0 805.1 384.7

Oorja

Pac

DMFC Methanol 1,000 51.00 125 22.00 17.6 20,000 20 7.0 13.7 140.3 274.0

Ultra-CellXX25

IndirectPEM

Methanol-Water

25 1.48 1.24 0.24* 0.131 166 7 18.2 16.9 121.1 112.2

able II. Summary of performance characteristics of methanol fuel cell systems.

*Fuel cartridge integrated within the system



However, these systems have not yetbeen commercialized. The technologyadvances for the next-generation ofportable DMFC systems aim at reducingthe size and increasing the power den-sity. Some of the approaches toimprovement include simplifying oreliminating the components requiredfor liquid circulation, gas handling,and thermal management. This isachieved by either using a diffusionfeed of the methanol reactant or byhaving multifunctional componentsin the balance-of-plant. Most of

these modifications to make a DMFCsystem compact, limit the poweroutput. Reducing stack mass is anotherapproach being pursued for advancingportable DMFC systems. One approachto reduce stack mass is to use monopolarconfigurations that eliminate the use ofheavy bipolar plates. The power densityof the stack can be increased by at leastthree times by this approach. Details ofsuch designs are not easily available inthe public domain. However, an exampleof a 5 W DMFC system developedat JPL that incorporates a lightweightmonopolar stack is shown in Fig. 4. One

of the more straightforward approachesto increasing power density of DMFCpower source is to construct a hybrid ofthe fuel cell with high-power lithiumion battery.10 In this case the batteryis sized to handle the peak powerdemands, and the fuel cell is sized tohandle the base load and charge thebatteries during off-peak periods.

Another commercially availableDMFC system is the OorjaPacTM manufactured by Oorja Protonics Inc.The OorjaPac is a 1 kW DMFC powersource designed to be an onboardbattery charger for material handlingvehicles. The unit is mounted inside the

battery tray of the vehicle and the fuelcell continuously charges the vehiclesbattery. The power metrics for the fuelcell system alone are 10 W/kg and 15W/L. In the OorjaPac, six gallons ofmethanol provide 20,000 Wh of energy,enough for a whole day of operation.The fuel-to-electric efficiency of thissystem is about 20%. Several unitshave passed extensive field testing todemonstrate over 2000 h of durability.

Indirect Methanol Fuel Cell

A compact portable power sourcethat uses methanol and an indirect fuelcell configuration has been developedby Ultracell Corporation. The UltraCellXX25TM shown in Fig. 5 supplies 20 Wcontinuously with a peak capability of25 W.

The XX25 is aimed at providing powerto field computers and communicationsequipment for extended periods. Theunit has a total volume of 1.5 L andmass of 1.24 kg. It incorporates a 240mL methanol-water fuel cartridge onits side that can be hot-swapped torefuel. A fueled UltraCell XX25 has

specific energy of 121 Wh/kg with theuse of a single fuel cartridge. Additionalcartridges each offer about 180 Wh ofenergy and weigh about 350 g. Thus,with 9 fuel cartridges, specific energyvalues as high as 400 Wh/kg can beachieved for a 72 h mission. Thistype of implementation emphasizesagain the role of the fuel fraction indetermining the specific energy of fuelcell based systems. Since this fuel cellincorporates a steam reformer and ahigh temperature polymer membranefuel cell, the system takes about 12min to reach steady state operation

and to deliver full power. However, the

F IG. 4. Lightweight monopolar stack implemented in a portable 5 W DMFC system.

F IG. 5. 25 W Ultracell XX25 Reformed Methanol Portable Power Source. (Graphics reproduced with permission from Ultracell Inc.)

packaging of this unit is impressive inthat it is rugged and contains high-temperature components assembledin a compact configuration. The U.S.military is in the process of field testingthese units for soldier applications.

Larger systems based on the indirectmethanol fuel cell concept are beingoffered by Idatech and ProtonexInc. and are in the initial stages ofdeployment as commercial products.

The iGen™ is IdaTech’s 250 W portablefuel cell. This system uses a methanol-water fuel processor in conjunctionwith a metallic separation membrane toproduce pure hydrogen that is then fedto a PEM fuel cell. The Valta™ M250 isa system designed by Protonex Inc. toprovide a nominal 250 W also uses areformed methanol feed in conjunctionwith a PEM fuel cell. These 250 Wsystems aim at providing continuouspower for battery charging, replacingconventional generators for backuppower, and providing auxiliary powerfor boats and recreational vehicles.

Because of the use of a reformer, thesesystems tend to be larger, heavier,and have longer start up times than acorresponding DMFC system.

Direct Hydrogen Fuel Cell

Hydrogen-fueled systems were amongthe first high-power density portable fuelcell concepts to be developed. However,hydrogen systems have always had tocontend with the logistics of supplyingthe fuel and the inability to storeadequate fuel to achieve reasonable runtimes. Jadoo Systems has commercialized

the N-GenTM

a 100 W system that uses





12. S. R. Narayanan, T. Valdez, N. Rohatgi, J. Christiansen, W. Chun, G. Voecks,and G. Halpert, in Proton Conducting Membrane Fuel Cells II , S. Gottesfeldand .T Fuller, eds., PV 98-27, p.316, The Electrochemical SocietyProceeding Series, Pennington, NJ(1998).

About the Authors

S. R. NARAYAN is the Technical GroupSupervisor and Fuel Cell Team Leaderfor the Electrochemical TechnologiesGroup at the NASA Jet PropulsionLaboratory. During the last 18 yearshe has focused on advancing fuel celltechnology for aerospace, military, andtransportation applications. Dr. Narayanis currently the Vice-Chair of the ECSEnergy Technology Division and may bereached at [email protected].

THOMAS I. VALDEZ is a Senior Memberof the engineering staff in theElectrochemical Technologies Group at

the Jet Propulsion Laboratory, Pasadena,California. Mr. Valdez has been ad-vancing the performance of catalystsand membrane electrode assemblies forDMFC, PEM, and electrolysis systems,and has been developing fuel-batteryhybrid power sources. He may be reachedat [email protected].

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