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Volume 2 Issue 1 Feb/Mar 2005Bipolar plates: materials matterAPUs for the trucking industryHigh-temperature membranesThe creation of a supply chain

An IOP Emerging Technology Review

THE

FUEL CELL REVIEWCOMPETIT IVE INTELLIGENCE ON HYDROGEN AND FUEL CELL TECHNOLOGIES

From innovation to manufacturing

ISSN 1743-3029

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THE

FUEL CELL REVIEWVolume 2 Issue 1 February/March 2005

Advanced bipolar plates p23

Steel interconnects in SOFCs p11

High-temperature membranes p8

5 News & Analysis

Fuel-cell APUs help haulage industrythink clean ● German firms flex theircollective muscle ● Energy efficiencyboosts industrial chlorine production● Clean cars, trucks ready to roll ● Howodorants can make hydrogen safer● Membranes a hit at high temperatures

11 R&D Focus

Is steel the real deal in SOFC systems?● French centre puts automotive fuel cells to the test ● Titania nanotubes yield ‘solar hydrogen’ from water

13 Patents

Gillette ● Johnson Matthey ● FranklinFuel Cells ● Medtronic Physio-ControlCorporation ● Ford Motors/DetroitEdison ● The University of WesternOntario ● General Motors ● Materials and Electrochemical Research

29 Technology Tracking

SOFCs: transitioning from innovation to manufacturing ● A new spin on theregenerative fuel cell ● Onboard fuelreforming ● GM teams with Sandia onhydrogen storage ● The importance of test and measurement

33 Products & Services

EscoVale ● Nextech ● Precision FlowTechnologies ● Zahn Electronics

15 THEORYBipolar plates: the lungs of the PEM fuel cellDAN BRETT AND NIGEL BRANDON

Advances in materials and design are delivering low-cost, high-performance bipolar plates forpolymer-electrolyte-membrane fuel cells. Indeed,improvements in bipolar-plate technology couldyield the next big leap in fuel-cell performance.

25 MARKETSFuel cells and the automotive supply chainATAKAN OZBEK

The creation of hydrogen-fuelling infrastructures,fundamental innovation, technology transfer andwholehearted government backing are going to beneeded to fast-track the fuel-cell industry’s movestoward full-scale commercialization.

DEPARTMENTS FEATURES

T H E F U E L C E L L R E V I E W | F E B R U A R Y / M A R C H 2 0 0 5 | F C R . I O P . O R G 3

Cover: From innovation to manufacturing p29. Ceres Power of theUK reckons its advanced electrolyte materials and proprietary‘power chip’ designs will make intermediate-temperature solid-oxide fuel cells a commercial proposition. The cover picture showssome of the company’s early-development metal bipolar plates.

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NEWS &ANALYSIS

Also in this section

6 Industrial power recovery

6 German firms think ahead

7 Hydrogen fuel and safety

8 Engineered membranes

Can the trucking industry clean up its act? Transportation

Auxiliary power units based on solid-oxide fuel cells might help it to do so.


“There are two rules in the trucking industry:the first is don’t change anything, and the sec-ond is don’t forget rule one.” That wry sound-bite is from someone who should know,namely Stephen Lasher, associate principaland manager of the Hydrogen and Fuel CellsUnit at TIAX LLC, a collaborative R&D com-pany based in Cambridge, Massachusetts, US.“The point is that the trucking industry is tra-ditionally conservative, but for good reason,”he told The Fuel Cell Review. “Downtime canmake or break the industry, so reliability andmaintainability of new technologies is criticalto profitability.” Yet despite this conservativeapproach to new technologies, Lasher stillbelieves that long-haul trucks could be amongthe early adopters of fuel-cell technology.

Lasher and TIAX’s Suresh Sriramulu andKristine Isherwood have joined forces withresearchers at the Institute for TransportationStudies (ITS) at the University of California,Davis to evaluate the use of fuel cells in auxil-iary power units (APUs) in long-haul trucks.TIAX believes that solid-oxide fuel cells(SOFCs) could reduce the fuel consumption,emissions and noise associated with generat-ing electricity when the truck is stationary.

Today, most trucks idle the main dieselengine to provide “hotel” and auxiliary electri-cal power, a process which is highly inefficientas well as noisy. Now, thanks to the high cost offuel and the growing number of anti-idling reg-ulations, the trucking industry is slowly accept-ing the benefits of installing APUs – be theypowered by a much smaller (and hence lowerfuel-consumption) diesel internal combustionengine (ICE) or, in the longer term, a fuel cell.Typical hotel and auxiliary loads within a truckinclude the cabin heating and cooling system,coffee maker, refrigerator, electric blanket andengine cooling fan. Power requirements rangefrom 100 W to 2100 W, meaning that a fuel-cellAPU would need to be in the 5 kW range.

Looking at the numbersAs part of the study, TIAX and ITS used severallinked models to determine the performance,cost and efficiency of conceptual SOFC APUs.

The ITS researchers used a modified ADVISORdrive-cycle model to estimate fuel savings andemission reductions associated with APU adop-tion. They first looked at typical auxiliary-powerrequirements of working trucks as a function ofload duration. The average power requirementover a duty cycle was found to be 1.8 kW, with apeak power requirement of 4.6 kW. The major-ity of the load is between 2.5 and 3 kW. Themodified ADVISOR model revealed that a con-ceptual SOFC offers lower fuel consumptionand emissions than an ICE-based APU.

The benefits were even greater when com-pared to the idling of the main diesel engine.The SOFC APU would achieve an 85% reduc-tion in fuel consumption and carbon dioxideemission, and a greater than 99% reduction inthe emission of nitrogen oxides and carbonmonoxide. Other benefits include the potentialfor approximately 18 dB reduction in cabinnoise and 25 dB reduction in external noise. Theinstalled volume of such an SOFC was esti-mated to be 100 l, which should be acceptablefor most applications.

TIAX used this data to calculate the timerequired to recover the cost of the APU, and con-cluded that a payback period of 1–2 years couldbe feasible if the long-term projected installedcost of $580/kW is achieved. The calculationassumes that the cost of diesel is $1.40/gallon,

and that the APU would be used in lieu of engineidling for 6 h per day, 300 days per year.

While this sounds good in theory, there aremany practical problems to overcome. Not leastof these is the processing of diesel fuel intohydrogen to feed the fuel cell. “There are reportsof limited success in reforming diesel but thetechnology is not ready for prime-time yet,”Suresh Sriramulu told The Fuel Cell Review. Thereare two key challenges surrounding diesel – itshigh sulphur content and the presence of heavyhydrocarbons, which are difficult to convertinto hydrogen without the formation of soot.

A key problem with desulphurization tech-niques is that they are not easily reversible on-board a vehicle. This means that the sorbentsystem must be replaced on a regular basis,increasing maintenance costs. Replacementtimes are a function of the size of the sorbentbed, however, and could in principle bedesigned to coincide with the regular mainte-nance of the truck.

TIAX also identified challenges regardingthe fuel-cell stack itself, including power den-sity, reliability and thermal management.However, Sriramulu stresses that these chal-lenges are currently being addressed by the USDepartment of Energy’s Solid State EnergyConversion Alliance (SECA), which combinesthe expertise of government, researchers and

Keep on trucking: the high cost of fuel and the growing number of anti-idling regulations meanthat the trucking industry is slowly accepting the benefits of APUs.

Nedstack, a fuel-cell technology companybased in Arnhem, the Netherlands, is workingto develop a niche application for its hydrogenfuel cells in the production of chlorine. Theproject is a joint venture with the chemicalcompany Akzo Nobel, and is subsidized by theDutch government’s “Energy saving throughinnovation” scheme.

Chlorine is commonly produced throughthe electrolysis of brine, a process that yieldscaustic-soda lye and hydrogen as by-products.Typically, one-third of the power expended inthe electrolysis process is wasted in the pro-duction of hydrogen. Now, however, Nedstackand Akzo have come up with an innovativeway of reducing this energy loss: by installingpolymer-electrolyte-membrane (PEM) fuel-cell stacks that consume all of the hydrogen togenerate electric power. This power is then feddirectly back into the electrolyser, improvingthe efficiency of the process by around 20%.

Although the work is still in its early stages,

the partners’ long-term goal is to install a50 MW PEM stack at a full-scale chlorine elec-trolysis plant. Currently, only a small pilot sys-tem is running, based on a 20 kW stack.However, this plant has successfully demon-strated the principle and later this year a200 kW module will go live. This second-phasedemonstrator will implement identical tech-nology to the planned 50 MW project, con-struction of which is scheduled to start in 2007.

Crunch time will come in mid-2006, whenan investment decision regarding the 50 MWinstallation will be made, based largely on theresults of the second-phase demonstrator. Thekey metrics are cost, long-term efficiency anddurability. “The target for cost is 7250 per kilo-watt,” explained Erik Middelman, CEO ofNedstack. He believes that reductions in mate-rial costs and an increase in production vol-umes at Nedstack make that target achievableby 2007. System-reliability requirements forfull-scale application are in excess of 40 000 h.So far, stacks have failed to exceed 25 000 h ofoperation, but Middelman is again confidentthat improvements in raw materials and con-struction will see longevity extended.

If it does go ahead, the 50 MW system will use2000 fuel-cell stacks. With five Akzo chlorineplants in the Netherlands and hundreds of elec-trolysis plants worldwide, Nedstack may havehit on a lucrative niche for its fuel-cell products. Jonathan Wills

industry in the development of SOFCs (see TheFuel Cell Review October/November 2004 p34).SECA aims to have a working SOFC stack readyfor APU applications by 2007, and a completesystem a few years later. SECA member Delphi,for example, is developing a 5 kW APU forlong-haul trucks, and Sriramulu says that “theyare making good progress”.

One big reason why TIAX believes that fuelcells will find early applications in truck APUsis that the cost:performance requirements arenot as stringent as in other potential applica-tions. “Both stationary power and vehicle-power-train applications have very stringentsystem-cost requirements,” explained Lasher.The power-train systems target is about$30/kW (factory cost), whereas the TIAX studysuggests that the system price could be morethan 10 times higher for APUs. What’s more,power-train applications require very fast start-up (less than 30 s), which is currently a challengefor fuel cells. However, the APU start-upprocess could begin before the main engine isshut down. In addition, the fuel cell could bekept warm during the operation of the mainengine, making start-up easier.

Despite the conservative nature of the truck-ing industry, Lasher believes that the haulageindustry is “coming around to the fact thatthere will be further regulations regardingengine idling – and they will have to act”.Hamish Johnston

Germany is, by some margin, Europe’s engineroom when it comes to fuel-cell research,development and technology transfer. Yet interms of industry representation and govern-ment lobbying, a duplication of effort andlack of joined-up thinking seems to be ham-pering progress. The root of the problem isthat Germany has 21 fuel-cell associations,incorporating more than 300 companies, andeach has its own take on what’s best for theindustry’s near- and long-term development.

“Everyone is speaking very loud [and] doinglots of good work, but everyone has a slightlydifferent approach,” said Werner Tillmetz,head of ZSW, the Centre of Solar Energy andHydrogen Research based in Ulm. “[But] thegovernment would like to speak to one bodyin the fuel-cell industry, [so] we have to workout how we are going to realize this one voice.”

In December last year, Tillmetz and his col-leagues set about addressing the situation

when they invited representatives from theGerman fuel-cell community to come togetherto thrash out a common strategy. The outcomeof that effort is a paper, supported by all 21associations, outlining a broad-based allianceof German fuel-cell companies. Known as BZB(Brennstoffzellenbündis Deutschland), themain function of the alliance is to lobby politi-cians in Berlin for legislative and financial sup-port. That support could take the form of stategrants, tax advantages and guaranteed rates forelectricity produced by fuel cells.

“There is no formal structure, there is just anagreement between all parties,” Tillmetz toldThe Fuel Cell Review. “Think about the energy weare using today, be it crude oil, nuclear or windpower; there was always a huge impact fromgovernment. Many people ask [fuel cells] to becompetitive with existing incumbent tech-nologies, but this is impossible [at present]because these incumbent technologies have

T H E F U E L C E L L R E V I E W | F E B R U A R Y / M A R C H 2 0 0 5 | F C R . I O P . O R G

NEWS & ANALYSIS

German fuel-cell companies coordinate their approach and plan for the future.

Germany unites on fuel cells

A more efficient takeon chlorine production

6

Government and industry

Stationary power

Energy wasted in the industrial productionof chlorine can be captured by fuel cells andfed back into the parent process.

ZAP, the automotive-technology pioneerbased in Sacramento, California, US, recentlysigned an exclusive distribution agreementwith Anuvu, also of Sacramento, for the latter’spatented Power-X hydrogen fuel-cell-enginesystems. The engines will be used to provide afuel-cell power option to the electric ZAPWorld Car and DaimlerChrysler’s petrol Smartcar, which is distributed by ZAP in the US. Aninitial $10 m order for 1100 fuel-cell engineswas confirmed in January. In 2003, Anuvustarted selling vehicles with a fuel cell/batteryhybrid based on the Nissan Frontier pick-up(pictured). The list price: $99 995. ●

Cleaner cars


NEWS & ANALYSIS

7

been developed through strong governmentalsupport and legislation.”

BZB points to countries such as Japan, theUS, Canada and China, where coordinatedefforts at a government level are pushing fuelcells through to commercial introduction.Japan, in particular, has a well defined roadmapfor the mass-introduction of fuel cells. By 2010,it plans to install stationary plants with a totalpower of 2.1 GW, as well as put 50 000 fuel-cellvehicles on the road. If appropriate politicalsupport is forthcoming, BZB claims that fuel-cell plants with a total capacity of 15 MW a yearcould be built in Germany from 2006, rising to1.3 GW by 2015.

For further information about BZB, contactJohannes Schiel at [email protected] Wills

Scientists at the Japan Automobile ResearchInstitute (JARI) in Tsukuba have identifiedthree strong-smelling hydrocarbon com-pounds that could be used as additives inhydrogen fuel, enabling its detection by smell.Their work, which is attracting interest fromseveral leading car manufacturers, could oneday help to ease public concerns over the safetyof hydrogen fuel and fuel-cell vehicles.

The JARI team is researching hydrogen addi-tives by testing their effects on fuel-cell per-formance, with chemicals selected on the basisof properties such as pungency, boiling pointand toxicity. To date, the researchers have eval-uated 26 chemical additives: 10 sulphurouscompounds commonly added to natural gasand liquefied petroleum gas (LPG); and 16 non-sulphurous compounds.

Based on the current legislation for naturalgas in Japan, the estimated legal concentra-tion (ELC) of odorant was calculated for addi-tives in hydrogen fuel. The performance of aJARI standard fuel cell was then carefullymonitored while hydrogen containing vari-ous additives at ELC was used as fuel. All of thesulphurous chemicals were quickly elimi-nated, as the sulphur was found to poison theelectrode catalysts and caused rapid deterio-ration in performance. Eight compoundswere found to have very little or no effect onfuel-cell performance at ELC, but three ofthese odorants had to be ruled out because oftheir tendency to condense at high pressures(for example, in storage cylinders).

Higher-concentration tests on the five

remaining compounds showed that all wouldbe detrimental to fuel-cell performance ifallowed to accumulate in the system. However,three compounds stood out above the others:2,3-butanedione; ethyl isobutyrate; and 5-eth-ylidene-2-norbornene. For all of these addi-tives, fuel-cell performance returned tonormal once their concentration in the feedfuel was reduced. According to JARI, the bestof the three is ethyl isobutyrate, which has astrong, sweet smell. However, the scientistsacknowledge that in practical applications itmay be more useful to select an unpleasant-smelling additive (such as 2,3 butanedione or

5-ethylidene-2-norbornene). Further work at JARI will evaluate the diffu-

sion properties of the additives, as well as issuessuch as cost and availability. In the long term,JARI’s goal is to “improve the safety of handlinghydrogen and to pave the way for the wide-spread use of fuel-cell vehicles”. Jonathan Wills

Mixing hydrogen fuel with pungent additives could improve safety. But will it impair performance?

Why a bad odour can be goodHydrogen safety

Whichever way you look at it, hydrogen is atricky gas to deal with. It’s highly volatile andflammable; it can explode when mixed with airin certain concentrations; and it also leaksthrough small orifices at a faster rate than anyother gas – 2.8 times faster than methane and3.3 times faster than air.

Like natural gas and LPG, pure hydrogen isodourless. However, legislation requires theaddition of odorous chemicals to natural gasand LPG to allow their detection by smell atconcentrations below combustible levels.Although no equivalent legislation yet existsfor hydrogen fuels, it seems likely that odorousadditives will become a requirement – if onlyto provide an additional layer of safety.

“As far as we know,” said JARI researcherShogo Watanabe, “there aren’t any plans toimplement legislation regarding the odour ofhydrogen fuel as yet, [largely] because there isno suitable odorant for fuel-cell applications.However, the requirement is clear.”

The sense of smell

Performance monitoring: JARI’s standardproton-exchange-membrane fuel cell was usedin the evaluation of 26 hydrogen additives.

The Ontario provincial government hasannounced two technology initiatives that willbenefit Canada’s burgeoning fuel-cell industry:a new Ontario centre of excellence (OCE) forenergy and a dedicated fuel-cell innovationprogramme. In both cases, the focus will be oncommercialization, moving products throughto the manufacturing stage and linking smalland medium-sized enterprises with the researchcommunity and venture capital.

The OCE will see C$8 m ($6.5 m) ingovernment investment over four years andwill build on the success of the province’s fourestablished OCEs (in photonics, information

technology, earth and space technology andmaterials/ manufacturing), which attracted acombined C$24 m in private investment lastyear alone. The fuel-cell innovationprogramme will benefit from a further C$3 min annual funding, specifically for fuel-cell R&Dprogrammes through 2007/8.

The fuel-cell programme will also foster linkswith the US states of New York, Ohio andMichigan where state governments areproviding aggressive support for hydrogen andfuel-cell activities. ● Further information can be found at www.fuel-cells.2ontario.com and www.oce-ontario.org.

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NEWS & ANALYSIS

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On 22 January, the former German ChancellorHelmut Kohl presented the 25th InnovationAward for German Industry to a Frankurt/Mainstart-up called PEMEAS Fuel Cell Technologies.The significance of the award is twofold. First,it highlights the importance that Europe’sbiggest economy attaches to fuel cells and sus-tainable technologies in general. Second, it rec-ognizes the progress that PEMEAS is makingtowards its stated vision “to become a marketleader in the manufacture and sale of mem-brane–electrode assemblies (MEAs) for high-temperature polymer-electrolyte-membrane(PEM) fuel cells”.

Although PEMEAS is a start-up, the businesscan trace its origins back more than 10 years tothe fuel-cell activities of the former HoechstGroup. That legacy has enabled the company,which now numbers 40 staff, to assemble acomprehensive portfolio of intellectual prop-erty (110 patents and patent applications). Italso claims to be the only commercial supplierof an MEA that can operate at temperatures ofup to 200 °C. The product in question, theCeltec-P MEA, is optimized for use in reform-ate-based fuel cells – systems that incorporatean integrated reforming unit so that they canrun on fuels other than pure hydrogen. In thisregard, methanol or natural gas are easy tostore, easy to transport and have a high energydensity in the liquid state.

Hot stuffThere’s a downside to traditional reformate-based PEM fuel cells: the reformation processgenerates large amounts of CO (of the order of30 000 ppm), which few MEAs can handle.Because the CO poisons the platinum catalyst,a complicated gas-cleaning process is required,adding to the component count and cost. TheCeltec-P looks like an elegant design fix. TheMEA can tolerate CO concentrations of up to5% with only a 5% reduction in power per-formance compared to that of a clean gas refer-ence. Conventional PEM fuel cells, whichrequire operating temperatures of less than100 °C, might be able to handle CO levels of0.005% with a reduction in performance of15% compared to clean gases (or CO levels ofup to 0.01% CO if the designers are prepared totake a performance penalty of 30%).

The numbers speak for themselves. Celtec-Pincreases the CO tolerance of the MEA assem-bly by almost three orders of magnitude. That

performance enhancement is made possiblebecause the MEA can operate at temperaturesof up to 200 °C. The higher-temperature regimespeeds up desorption of CO from the surface ofthe platinum catalyst, which in turn minimizesany disruption to the oxidation–reductionreactions going on in the cell.

“Celtec contains the same basic catalystmaterials as conventional fuel cells, so the[enhanced] CO tolerance of MEAs is exclu-sively due to this temperature effect,” explainedCarsten Henschel, director of market develop-ment for PEMEAS. It’s also worth noting thatthe water-management system can be elimi-nated or simplified considerably as humidifi-cation of the reaction gas is not required.

In conventional PEM MEAs, the upper limiton operating temperature is fixed at close to100 °C – because liquid water supports theionic conductivity of the membrane. TheCeltec-P MEA gets round this problem thanksto a membrane that is based on the heat-resist-ant polymer polybenzimidazole, while phos-phoric acid provides the ionic conductivity.

According to Henschel, the Celtec-P MEA canachieve a power density of more than200 mW/cm2 using reformate fuel with a COconcentration of 2000 ppm. The MEA has alsobeen tested for more than 11 000 h with adegradation rate of less than 6 µV/h. To date,the main concern has centred around possibleleakage of phosphoric acid, though Henscheltold The Fuel Cell Review that the acid evapora-tion rate has been measured in the laboratoriesof both PEMEAS and one of its customers.These studies predict a lifetime of more than40 000 h for the MEAs.

Routes to marketPEMEAS was formed in April last year whenparent company Celanese, the internationalchemicals group, decided to spin off its fuel-cellactivities. With backing from Celanese and aconsortium of blue-chip investors, PEMEAShas so far secured financing of around t20 m.The money has been used to good effect, withCeltec-P technology already being evaluated bycustomers and incorporated into prototypes.

In November, for example, Plug Power of theUS reported promising results for a stationary-power generation system based on the Celtec-PMEA. At the same time, PEMEAS is pushing itshigh-temperature MEA for applications inmicro fuel cells for portable electronic devices.The company has a long-standing partnershipwith US mobile-phone maker Motorola, whichis evaluating prototype reformed-methanolfuel cells based on Celtec-P.

In the long term, PEMEAS plans to com-mercialize other advanced membranes andMEA product lines. Its latest membrane offer-ing, the Celtec-V, is based on the same polymeras Celtec-P but uses an alternative electrolyteto enable operation between 40 and 160 °C.This broad temperature range could proveattractive to the automotive industry. For now,though, Celtec-V is targeted at direct-methanol fuel-cell (DMFC) applications.Henschel claims that several DMFC develop-ment companies and research teams are test-ing the MEA and that initial results lookencouraging. “Celtec-V technology can beoperated at a broad range of methanol con-centrations,” he added. “It shows a very lowmethanol cross-over and it has alreadyachieved the performance of standard DMFCMEAs, even with a non-optimized electrode.” Siân Harris

A heat-resistant polymer membrane enhances the performance of reformate-based fuel cells.

It’s time to turn up the heatComponents

Window of opportunity: PEMEAS says that itsCeltec-P MEA is currently being evaluated by anumber of partner companies, including PlugPower and Motorola.

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Espoo, Finland: Scientists at theTechnical Research Centre ofFinland (VTT) have shown thatinterconnectors for solid-oxidefuel-cell (SOFC) stacks can deliversatisfactory oxidation resistanceand electrical performance whenfabricated from commonly avail-able standard-steel alloys. Althoughthe work is in its early stages, VTTresearch scientist Juha Veivo andteam believe that steel could help todrive down the cost of prototypeSOFC stacks and systems.

As part of its initial programme,VTT evaluated a hydrogen-fuelledSOFC constructed from a range ofdedicated interconnector steels, aswell as employing interconnectorsfabricated from standard steels. Inthe trials, a Finnish grade of ferriticsteel performed surprisingly wellcompared to new high-chromiumsteels from Germany and Japan.

One of the main problems facingSOFCs is production costs. Andalthough costs can be expected tofall with increasing volumes, poordurability and short lifespan needto be addressed in critical parts of

the cells. On top of that, high oper-ating temperatures (800–1100 °C)necessitate the use of expensiveceramic interconnectors betweencells in SOFC stacks – which, apartfrom being costly, are also brittleand difficult to engineer.

Recently, however, new SOFCtypes have emerged that can oper-ate at reduced temperatures(650–750 °C). These intermediate-temperature designs open up thepossibility of employing cheap steelmaterials in cell-stack intercon-

nectors. In addition to lower costand greater ease of fabrication,“metallic materials allow muchmore freedom in design”, explainedVeivo. “Typically, ceramic inter-connectors were sintered directlyto the cell, but steel interconnectorscan be assembled separately andmade more easily to satisfy otherfunctions such as structural sup-port and gas flow-channelling.”

So what’s the next step for theVTT team? “It would generally bean advantage to reduce [SOFCoperation] temperature furtherfrom the point of view of strengthand oxidation resistance of theinterconnector materials,” saidVeivo. “Development of new steelsfor this purpose is of some interestto manage contact resistance,harmful element diffusion into theceramic parts and thermoelasticcompatibility with the cell [elec-trode and electrolyte] ceramics”.

VTT is a partner in the EuropeanHydrogen and Fuel Cell TechnologyPlatform and is involved in severalprojects relating to SOFCs. Fordetails, visit www.HFPeurope.org.

R&D FOCUS


Highlights of cutting-edge research, development and innovation.

Steel: the real deal for SOFCs

Belfort, France: In the two yearssince it was established, France’sNational Fuel Cell Test Platform forAutomotive Applications hasnotched up an impressive list ofheavyweight clients, includingdomestic car makers Peugeot andRenault and the likes of Delphi andAir Liquide from further down thesupply chain. Yet the facility, locatedat the University of TechnologyBelfort Montbéliard (UTBM), iskeen to stress its inclusive creden-tials, with a remit that extends fromlarge-scale car manufacturers tosmall independent companies anduniversity research groups.

The test platform boasts the abil-ity to “completely replicate the envi-ronment of a fuel cell in a car”claims Daniel Hissel, an associateprofessor of UTBM and seniorresearcher at the platform. The facil-

ity includes three fuel-cell testinglaboratories: one lab dedicated tosystems operating below 10 kW,and the other two for peak powersof up to 200 kW. Users can also takeadvantage of facilities for evaluat-ing pressurized hydrogen storageand the changing electrical loads offuel-cell vehicles. In addition, there

is a vibration-testing facility that cangenerate frequencies of vibration inthree dimensions between 5Hz and2 kHz. The offering is completed bya climatic chamber that can cyclethe environmental temperaturebetween –45 and 130 °C.

In short, says Hissel, the Frenchtest platform offers conditionsbeyond the extremes that might beencountered in even the most exact-ing field demonstrations – and at afraction of the cost needed to putprototype fuel-cell vehicles on theroad. The platform is particularlyuseful for small companies andresearch groups that lack theresources to construct such a testfacility themselves.

The centre is jointly managed bythe Laboratory of Electrical andEngineering Systems (L2ES) fromUTBM and the University of

Franche-Comté, and the NationalTechnological Research Centre(CNRT-INEVA). L2ES manages thescientific and technological aspectsof the platform while CNRT-INEVAis responsible for the promotion ofthe platform and the developmentof partnerships between industriesand universities.

So how do prospective cus-tomers gain access to the facility? “Ifa company wants to test here, thereare two possibilities,” explainedHissel. “They can either rent part ofthe platform for the duration of thetests, or make a research contract,in which case the company simplyprovides the fuel cell and tests willbe carried out by staff at the centre.”

For more information, visit theL2ES website at http://l2es.utbm.fror contact Daniel Hissel via e-mailat [email protected].

Solid prospects: metallic SOFCinterconnectors could help toreduce fabrication costs as well as improve performance.

Automotive fuel cells are put through their paces

Shake it up: the vibration table cansupport 3D performance analysis.

Nanotubes yield‘solar hydrogen’Phi ladelphia, PA: Researchersaround the world have beenattempting to harness solar powerto cleave water molecules. The aimis to generate hydrogen “for free”with no impact on the environ-ment – a big step towards conceiv-ing a viable hydrogen economy.

Now, a team from PennsylvaniaState University in the US has con-structed a material from titania(TiO2) nanotubes that they claim ismore than 90% efficient at har-nessing the UV fraction of solarradiation, and 6.8% efficient atextracting hydrogen from water.The conversion efficiency is thehighest recorded for a titania-based photoelectrochemical cell,according to a paper published inNano Letters (2005 5 191).

The key to making titania nano-tubes that efficiently harness solarenergy is controlling the thicknessof the nanotube walls. Nanotubes224 nm long with 34 nm-thickwalls were found to have a quan-tum absorption efficiency threetimes that of 120 nm-long nan-otubes with 9 nm-thick walls.


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Hydride storage – two tanks better than oneThe problems associated with the storage and supply of hydrogenfor automotive applications must be addressed if a functioninghydrogen economy is to move from vision to reality. In a new takeon the problem, engineers at Johnson Matthey, UK, have come upwith a hybrid hydrogen-storage system comprising two separatestorage tanks, both containing solid-state hydrogen-storagematerials (WO 2005/012164). The first tank contains an AB5 typehydride material which can be activated to release hydrogen at arelatively low temperature. The second tank contains an MgH2 typematerial, which has a much higher storage capacity but also a higheractivation temperature (>300 °C). At start-up, a portion of thehydrogen released from the first tank is used to activate hydrogenrelease from the second tank, which is then made available to a fuelcell or combustion engine.

Catalytic interconnector boosts SOFCsEngineers from Franklin FuelCells, US, have designed aninterconnector with catalyticreforming properties for solid-oxide fuel cells (SOFCs). Thecomponent is grooved to allowgas/liquid flow and coated with acatalyst to assist in thereformation of hydrocarbonmolecules (WO 2005/011019).The invention would give SOFCs

the ability to function directly on higher hydrocarbon fuels.Furthermore, the invention would give SOFCs improvedtemperature management, since the endothermic reformationprocess will cool sections of the fuel cell, giving a more uniformoverall temperature distribution.

Defibrillators given the shock treatmentMedtronic Physio-Control Corporation, US, has designed a fuel-cellpower system for portable external defibrillators (WO 2004/108213).In the design, the fuel cell powers a processor and user interface as wellas charging an energy-storage circuit (such as a capacitor) that can bedischarged through the patient as a defibrillation shock. Currently,portable external defibrillators rely on batteries, which often have alimited shelf-life and can require frequent conditioning cycles.Replacing batteries with fuel cells would result in a more reliabledefibrillator requiring less routine maintenance, says Medtronic.

Waste gases can generate useful energyA US collaboration involving the Ford Motor Company and theDetroit Edison Company has developed a technique for convertingvolatile organic compounds into electrical energy(WO 2005/007567). The invention relates to a method and device thatconcentrates a dilute hydrocarbon gas using a concentrator into agaseous or liquid concentrated fuel. The concentrated fuel is thenconverted into a reformate using a reformer and made into energy bymeans of an energy-conversion device. Various agricultural,manufacturing and contamination-remediation processes produce astream of low-concentration hydrocarbon gas as an unwantedby-product. Currently, these waste products are flared or burned,which means that all of the energy they contain is lost.

Fullerenes enhance electrolyte propertiesScientists at the Materials and Electrochemical Research Corporationin the US have found that the addition of small amounts of fullerenematerials to polymer electrolytes can improve their low-relative-humidity proton-conductivity properties (WO2004/112099). Theinvention could help to overcome some of the problems associatedwith existing polymer electrolytes, in particular with respect to theirlow operating-temperature range.

Iron chemistry powers up biofuel cellsThe bacterium Acidithiobacillus ferroxidans is the key to a new biofuel celldesigned by researchers at the University of Western Ontario, Canada(WO 2005/001981). The fuel cell is based on the cathodic reduction offerric to ferrous ions with a fuel such as hydrogen. Regeneration of ferricions is achieved by bacteria in a bioreactor. Generation of electricalenergy is coupled with the consumption of carbon dioxide from theatmosphere and its transfer into microbial cells.

Imides have potential for hydrogen storageGeneral Motors is one of several leading car makers looking todevelop practical solid-state hydrogen-storage materials for on-boardvehicle applications. Among other materials, the US manufacturer isinvestigating metal-imide compounds which readily absorbhydrogen to form metal-amide compounds (WO2005/005310).Amides with suitably chosen metal cations enable the liberation ofhydrogen upon gentle heating. One of the systems underconsideration is lithium imide (Li2NH), which absorbs hydrogen toyield lithium amide (LiNH2) and lithium hydride (LiH). In preliminaryexperiments, hydrogen equivalent to 6.5% by weight has beenliberated from a mixture of lithium imide and hydride.

PATENTS


Fuel cells: the best a man can get?Gillette, the US manufacturer of shaving products and disposablegoods, has developed a hybrid power supply that incorporates a fuelcell, a rechargeable battery and balancing electronics to ensure aconstant current discharge (WO 2005/011045). The set-up employs acommon interface that connects on one side with a fuel cartridge(containing methanol, for example), an external battery or a mainspower adapter. Internally, a switching-type DC/DC boost-typeconverter channels electrical power from the fuel cell or externalbattery to a rechargeable cell. Operation of the converter iscontrolled, in part, by a feedback loop incorporating a fuel-cell-current sensor/comparator. If commercialized, it seems likely thatthe power supply will find applications in electric razors and otherportable electronic equipment.

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The pick of the latest international patent applications.

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FEATURE: COMPONENTS

MEMBRANE AND CATALYST development typically take centre-stage in any discussion of fuel-cell technology, so much so thatrelatively little attention has been paid in the published litera-ture to another vital fuel-cell component, the bipolar plate.Now, however, advances in engineering materials and sophis-ticated flow-field designs are set to change all that, suggestingthat pioneering work on bipolar plates may well bring aboutthe next significant improvement in fuel-cell performance bylowering the size, weight and cost of stacks.

The bipolar plate (also referred to as the flow-field plate,duplex electrode, current collector, convection plate or inter-connect) is one of the fundamental building blocks of fuel-celloperation. It must carry fuel and air to the respective electrodesand disperse the reactants in an even and controlled mannerwhile removing excess product water – it is for this reason thatit’s often referred to as the “lung” of the fuel cell. It must alsoconduct electronic current from each electrode; guaranteecomplete separation of air and fuel; provide mechanical sup-port and strength to the membrane–electrode assembly(MEA); handle transmission of seal stresses; incorporate inter-nal manifolds; and help moderate the stack temperature.

Figure 1 shows how bipolar plates and MEAs are assembledto form the fuel-cell stack and describes the main features of atypical plate. The flow channels are located in the central regionof the plate, in the area that contacts the active catalyst of theanode and cathode. The open channels are typically of the orderof 1 mm in diameter and depth, although this can vary signifi-cantly depending on the size, design and number of channels inthe plate. The areas of the flow-field that make direct contactwith the gas-diffusion layer (GDL) are called “lands”, and it is atthese points that current flows to and from the MEA electrodes.There are various flow-field designs aimed at improving factorssuch as reactant transport to the catalyst, removal of productwater, internal hydration, plate strength and back pressure,among other things. In the corners of the plate are manifoldsthat run through the length of the stack and carry reactant andproduct to and from each bipolar plate, as well as taking waterto cooling plates (positioned intermittently throughout thestack). In addition, features such as positioning holes and inte-grated gasket grooves are often present.

Bipolar plates: the lungsof the PEM fuel cell

DAN BRETT AND NIGEL BRANDON

Advances in materials and design are delivering low-cost, high-performance bipolar plates for polymer-electrolytefuel cells. Indeed, improvements in bipolar-plate technology could yield the next big leap in fuel-cell performance.

Top: position of the MEA and bipolar plate within a fuel-cell stack.Bottom: the main features of a typical bipolar-plate flow-field.The open channels are typically of the order of 1 mm in diameterand depth, although this can vary significantly depending on thesize, design and number of channels in the plate. There are variousflow-field designs, all aimed at improving factors such as reactanttransport to the catalyst, removal of product water, internalhydration, plate strength and back pressure.

1. Going with the flow

membrane

Stack

bipolarplate cathode anode

gas-diffusionlayer (GDL)

membrane–electrodeassembly (MEA)

e–

H+

H+

H+

H+

... ...

coolantmanifold

opposite electrodeexhaust manifold

exhaust manifoldland

channel

reactantmanifold

opposite electrodereactant manifold

positioninghole

integrated gasketgrooves

Typical bipolar plate flow-field

e–

e–

e–

e–

e–

e–

e–

Bipolar plates are a significant factor in determining thegravimetric and volumetric power density of a fuel cell, typi-cally accounting for more than 80% of the weight of a stack andalmost all of the volume. Consequently, if component design-ers can reduce the weight of the plates they can remove someof the performance burden from other components (by reduc-ing the amount of platinum required in the MEA, for instance).At the same time, the price of the raw material and the oftencomplex processing that goes into manufacturing bipolarplates makes them one of the most expensive parts of the fuelcell.1 Projections range from 12% to 68% of the total stack cost,though it is generally accepted that bipolar plates shouldamount to no more than 33% of cell cost for automotive appli-cations (typically less than $10/kW or approximately $2 perplate). For component developers, the challenge is therefore toreduce the weight, size and cost of the bipolar plate while main-taining the desired properties for high-performance operation.

Which material?The multiple roles of the bipolar plate and the challenging envi-ronment in which it operates means that the material fromwhich it is made must possess a particular set of properties.2 Theideal material should combine the following characteristics:● High electrical conductivity Especially in the through-planedirection; a target of over 100 S/cm has been set by the USDepartment of Energy (DOE).● Low contact resistance with the GDL Depending on the platematerial and thickness, the contact resistance with the GDLcan dwarf the resistance of the plate itself. ● Good thermal conductivity Efficient removal of heat from theelectrodes is vital for maintaining an even temperaturedistribution and avoiding hotspots. ● Thermal stability The trend towards higher-temperatureoperation (as high as 200 ºC) places constraints on certaincarbon–polymer composites. ● Gas impermeability To avoid potentially dangerous andperformance-degrading leaks.

● High mechanical strength So as to be physically robust andsupport the MEA.● Corrosion resistance Bipolar plates operate in a warm, dampenvironment while simultaneously exposed to air and fuelover a range of electrical potentials (ideal conditions forcorrosion to occur).● Resistance to ion-leaching If metal ions are released from theplate they can displace protons in the membrane and lowerthe ionic conductivity.● Thin and lightweight proportions While still accommodatingthe flow channels and maintaining mechanical stability.● Low cost and ease of manufacturing. ● Environmentally benign Recyclability is a particular concern.

These requirements are a challenge for any class of material,and none fits the profile exactly. The relative merits of each aresummarized in table 1, with examples of their physical param-eters shown in table 2. A summary of the main bipolar-platematerials developers is provided in figure 2.

GraphiteBipolar plates have traditionally been made from graphitic car-bon impregnated with a resin or subject to pyrolytic impregna-tion (a thermal treatment that seals the pores to a depth of ~7 µminto the surface of the graphite) to render them gas-imperme-able.3 Such materials, available from the likes of POCO Graphite(Texas, US) and SGL Carbon (Germany), offer high electronicand thermal conductivities, low contact resistance, corrosionresistance and ease of machining. If pyrolytic impregnation isused, they can operate at temperatures as high as 450 °C; how-ever, resin-impregnated graphite is limited to 150 °C. Althoughstill used in some state-of-the-art stacks and for prototypingflow-field designs, standard graphite plates are being super-seded by various metallic and carbon composites – materialsthat are more robust, thinner and less expensive to manufacture.

On the other hand, flexible graphite is now being used exten-sively. For example, GRAFCELL from GraphTech (Ohio, US) is aflexible graphite bipolar plate found in the majority of fuel-cell


COMPONENTS

16

2. Bipolar plates: the main players

Carbon–carbonPorvair Fuel Cells

ORNL

Composites

Carbon–polymer

DuPont Fuel CellsICM Plastics

Quantum CompositesSchunk

SGL CarbonZBT

BMC Inc.Entegris

Fraunhofer ICTGTI

H2 ECOnomyHuntsman(Vantico)

ICM PlasticsNedstackPlenco

QuantumComposites

SchunkSGL Carbon

Liquid-crystalpolymer

(injection moulded)Ticona (Celanese)DuPont Fuel Cells

FormingC A Lawton (compression moulding)

Century Engineering (RingExtruder for injection moulding)Precision Micro (stamping, photochemical machining)Metro Mold & Design (machining and mould design)

Tech-Etch (photo etching)FCCI (machining and moulding)Morgan Fuel Cells (ElectroEtch)

Metallic(stainless steel, aluminium, nickel alloy, titanium alloy)

DANAGas Technology Institute

GenCell Corp.Sumitometals

NuveraUlbrich

Graphite(resin or pyrolytic impregnation)

POCO GraphiteSGL Carbon

Flexible graphiteGrafTech (GRAFCELL)

Metallic coatings/treatment

INEOS Chlor Ltd.(PEMcoat)

TIMCAL(TIMREX LB)ORNL/NREL

(Thermal nitridation)

Thermoset (compression mould)Thermoplastic (injection mould)

Bipolar platefabrication


COMPONENTS

17

vehicles. Flexible graphite is made by “expanding” naturalgraphite with the help of an intercalating agent and heat treat-ment – a process that increases the spacing between the planesof the graphite structure by as much as 80 times. The expandedform is then compressed to the desired density and pressed toform the bipolar plate. Flexible graphite meets basic cost targetsand has the advantage of very low contact resistance and density(typically 1 g/cm3). It also has excellent sealing characteristics.The downside is that it is mechanically weaker than other bipo-lar-plate materials and has a relatively high gas permeability.

MetallicMetallic bipolar plates, used by stack manufacturers such as DeNora (Italy) and Dana Corporation (Ohio, US), have the genericadvantages of high electrical conductivity and gas imperme-ability.4 They are also cheaper than graphite in the quantitiesrequired. Probably the most important benefit, though, is thatthe resultant stack can be smaller and lighter than one madefrom graphite. This is because it is possible to use very thinsheets of metal (as thin as 100 µm) to separate the reactantswhile still maintaining sufficient strength. Intelligent Energy(UK) has demonstrated outstanding power densities of over2.5 kW/l using metallic bipolar plates with a thickness of0.5 mm. Figure 3 compares the mass of a 33 kW stack made

from graphite and metallic bipolar plates and shows howpressed metal reduces the overall plate thickness.5

The main disadvantage of metallic plates is that they tend tocorrode in the hostile fuel-cell environment. This is detrimen-tal to performance for several reasons: (i) surface oxide forma-tion significantly increases the contact resistance between theplate and the GDL (although the surface oxide can, in itself, actto prevent further corrosion); (ii) the corrosion processchanges the morphology of the surface (potentially reducingthe contact area with the GDL); and (iii) corrosion leads to therelease of cations that degrade membrane performance.6

The bulk resistance of metallic bipolar plates is very low andnot the limiting factor in determining the material’s suitability,especially since metallic plates can be made so thin. Instead,cost, corrosion resistance, ease of manufacture and contactresistance are the key performance metrics. Although variousmetals have been evaluated, and many qualify in terms ofmaterial properties, only a relatively small number meet costtargets. The four with the most potential are: ● Aluminium Has the advantages of low density, low cost and thefact that it is easily formed. However, its corrosion resistance isa result of a passivating oxide film that reduces surfaceconductivity to such an extent as to disqualify aluminium as aplate material unless treated with a suitable coating. ● Titanium Has a low density and, like aluminium, exhibitsexcellent corrosion resistance due to an insulating oxide film.It also has the advantage of being able to diffusion-bond withitself, allowing complicated flow-field designs to beconstructed by overlaying several layers. The drawback is thattitanium is more expensive than aluminium and stainlesssteel (though the prohibitive pricing may be addressed by atechnique known as electrodeoxidation, currently beingdeveloped by QinetiQ in the UK). ● Nickel Does not form a protective oxide layer and in theenvironment of a fuel cell would corrode severely. Nickelmust be alloyed with chromium or subjected to surfacetreatment to make it a feasible choice. ● Stainless-steel alloys Are a common low-cost class of metalcombining high strength and corrosion resistance. Theenvironment within a fuel cell is very challenging, however,and corrosion is still a problem for low-chromium stainlesssteels. Despite this, unmodified stainless steels have yieldedpromising results from certain alloys. For example, the GasTechnology Institute (Illinois, US) has reported more than20 000 h of operation from its patented austenitic alloy. Thismetal has no coating, a corrosion rate of less than the US DoEtarget (<16 µA/cm2), and only 10% greater performance losscompared with graphite. Meanwhile, the suitability ofdifferent austenitic stainless-steel alloys has been investigatedat the National Renewable Energy Laboratory (NREL,Colorado, US).7 Decreased contact resistance and corrosioncurrent are both observed for steels with increasing Crcontent; SS349 is cited as a particularly promising alloy.

Generally speaking, the use of uncoated stainless steel can beconsidered a feasible option, especially if direct contact withthe polymer membrane can be avoided and the water in the sys-

The big benefit of metallic bipolar plates is that they make for asmaller and lighter fuel-cell stack than graphite plates. Top:weight comparison of a 33 kW stack using coated aluminiumand graphite bipolar plates. Bottom: thickness comparison of agraphite or moulded plate and a stamped metal plate.

3. Graphite versus metallic

19.4 kg 35.2 kg

coated aluminium graphite

electrodeend-plate

membranecurrent collector

gasketbipolar plate

Graphite/moulded plate

1.5 mm

stamped metal plate

0.1 mm0.5 mm

(a)

(b)

tem is pure and remains close to pH neutral. The problem is thathigh-chromium steel is expensive, which means that adoptinga metal-coating strategy promises to be more cost-effective. Inthis regard, work is under way to develop low-cost protectivecoatings that avoid ion leaching, prevent corrosion, improvecontact resistance and extend the lifetime of metal plates.

Coatings for metallic bipolar platesTo overcome the disadvantages of metallic bipolar plates, vari-ous coating and surface treatments have been proposed. Theideal properties should be: ● High electrical conductivity and low contact resistance withthe GDL.● Corrosion resistance.● Strong adherence to the metal substrate and resistance tocracking, blistering and pinhole formation.● Resistance to ion leaching from the metal into the membrane.● Similar thermal expansion coefficient (TEC) to theunderlying metal to avoid delamination.● Low cost and simple to apply to the metal plate.

Precious metals have been trialled as coatings for steel and alu-minium. Despite its expense, gold with a thickness of less than2 µm coated on aluminium is claimed by some researchers to bean economically feasible alternative to graphite, although stillrelatively expensive.5 The main problem with metallic coatingson metal plates is delamination of the layer caused by a mismatchin TEC. One way to buffer this disparity is to build up a number oflayers of different metals to ensure a gradation of TEC.

In itself, coating an active metal with a more noble metal is adubious strategy for corrosion protection. If both metals areexposed to an electrolyte (acidic water in the fuel cell) at pinholesand defects in the protecting layer, the metal further down theelectrochemical series will be oxidized and dissolve. This processcan be avoided by having a faultless layer (difficult to achieve inpractice) or by reducing the driving force of the reaction by usingmetals close to each other in the electrochemical series. The latterhas been investigated by workers at Physical Sciences Inc.(Massachusetts, US), who used a number of coatings ranging inelectrochemical series from the active substrate to that of car-bon.5 However, as with the TEC buffering technique, this adds tothe material and processing cost of the coating.

Elsewhere, a team from INRS-Energie et Matériaux andMcGill University in Canada has reported a method of deposit-ing a graphitic protective coating directly onto stainless steel.8

The coating is composed of three layers, the middle being aspray-coated graphite, with the top and bottom layers compris-ing a high-carbon-content polymer which upon pyrolysis at750 ºC forms a dense 70–100 µm-thick coating with electrical-and corrosion-resistance properties similar to that of graphite.In Europe, TIMCAL Graphite & Carbon (Switzerland) is usingits TIMREX range of aqueous-based graphite dispersions to pro-vide an easily applied coating for metallic plates. The companyclaims these coatings adhere strongly to the substrate with highelectrical conductivity and corrosion resistance.

Another variation on the theme is the growth of nitrides oncertain metals to impart high electrical conductivity and cor-


COMPONENTS

18

Graphite Metallic Coated metallic

Advantages ● High electrical conductivity ● High electrical conductivity ● As for metallic● Corrosion-resistant ● High thermal conductivity● Low contact resistance with GDL ● High strength● Good prototyping material ● High-temperature operation ● High-temperature operation for pyrolytic ● Gas-impermeable

impregnation (450 °C), ca. 150° for resin ● Thin platesimpregnation ● Amenable to a range of processing and forming

techniques● Easily recyclable

Disadvantages ● Flow-field machining required (expensive) ● Prone to corrosion ● Extra processing and ● Material is expensive ● Form insulating oxides (increased contact resistance) expense● Permeable to hydrogen (requires impregnation) ● Ion leaching (cations degrade membrane performance)● Brittle ● Corrosion-resistant metals and alloys are expensive● Must be made thick ● Corrosion-resistant coating may be necessary

Processing ● Computerized numerical control (CNC) milling ● CNC milling ● Depends on nature of options ● Stamping/embossing coating and order in

● Foaming which coating is applied● Die forging● EtchingNote: the range of forming methods applicable dependson the metal and size of plate

Table 1. Relative merits of bipolar-plate materials


COMPONENTS

19

rosion resistance. A collaboration between NREL and OakRidge National Laboratory (ORNL, Tennessee, US), for exam-ple, has developed a method of thermal nitridation that pro-duces corrosion-resistant coatings designed for rapid andinexpensive manufacturing.9 The process involves heating pre-viously stamped or machined bipolar plates to 1100 ºC in anatmosphere of nitrogen for 1–2 h. The nitride forms directly ontitanium, chromium and alloys thereof. In tests on Ni-50Cralloy, a high-integrity layer forms with negligible surface oxideand no significant ion leaching from the bulk; contact resist-ance is also improved compared to the untreated metal.

Such a designer alloy is far too expensive for fuel-cell applica-tions, however, and lower-Cr alloys (such as SS348) do not ben-efit from the same degree of corrosion protection. Nevertheless,a collaboration involving GenCell Corporation (Connecticut,US), NREL and ORNL is aiming to apply the nitridation processto commercial alloys. Cost estimates for the process are in therange of $0.1–1 per plate depending on size, shape and quantity.

Finally, INEOS Chlor Ltd (UK) has developed PEMcoat, a suiteof products that provide corrosion-resistant coatings and sur-face treatments for nickel, titanium and stainless steel. Theprocess for stainless steel is particularly promising, since it canbe applied to low-cost variants such as SS316 and 316L. Anddelamination is not an issue because the treatment is a modifi-cation to the surface rather than a coating procedure. Reportsclaim that reduced contact resistance, by virtue of the surfacetreatment, accounts for almost a doubling of the area-specificpower output compared to unmodified SS316. The treatment

for Ni/Ti has demonstrated operation in a working stack inexcess of 10 000 h, and for stainless steel in excess of 7000 hwith no decrease in performance. Clearly, such a product couldgo a long way towards securing a place for metallic bipolarplates in next-generation fuel-cell designs.

Carbon–carbon compositesCarbon–carbon composites were originally developed for theApollo space programme. Today, though, they are used in appli-cations such as aircraft brakes and turbine rotors, where strengthand compatibility with high-temperature operation are essen-tial. Those same characteristics are now being exploited in bipo-lar plates. More than this, it’s the combination of strength, lowdensity, chemical stability, high electrical and thermal conduc-tivity, and the ability to operate at temperatures in excess of400 ºC that make carbon–carbon technology a promising devel-opment route.10 In 2001, for example, Porvair Fuel Cells Inc.(North Carolina, US) licensed carbon–carbon composite tech-nology developed at ORNL and has subsequently invested over$6 m, with the aid of the US DOE, to scale up its productionprocess to handle more than 300 plates per hour.

Carbon–carbon composites are made of a carbon matrixreinforced with carbon fibres. The Porvair preparation involvesan initial slurry-moulding process, in which the slurry is com-posed of carbon fibres (approximately 400 × 10 µm) suspendedin a water/phenolic-resin mixture. The phenolic binder imparts“green strength” to the component after curing, yielding a pre-form that can be pressed to form the flow-field features. This is

Carbon–carbon composite Carbon-polymer compositeThermoset Thermoplastic

Advantages ● High electrical conductivity ● Higher temperature operation than ● Injection moulding lends itself to ● High thermal conductivity thermoplastic manufacturing automation● Lightweight ● Fast cycle-time ● Fast cycle time● High-temperature operation ● Flow-field introduced during moulding ● Flow-field introduced during moulding● High strength ● Low contact resistance ● Low contact resistance● Highly corrosion- and chemical-resistant● Flow-field introduced during stamping

of preform

Disadvantages ● Long and expensive chemical vapour ● Relatively low electrical conductivity ● Low electrical conductivity when using impregnation (CVI) process (bulk processing standard thermoplasticsand automation set to lower price) ● Limited to low-temperature operation

● Injection moulding difficult at highgraphite loadings

● Generally less chemically stable than thermoset resins

Processing ● Slurry moulding (potential for continuous ● Compression moulding ● Injection mouldingoptions “paper making” processing) ● Post-moulding CNC milling of blank ● Compression moulding

● Pressing (of preform) ● Post-moulding CNC milling of blank● Stamping (of preform)● CNC milling of blank

Table 1. Relative merits of bipolar-plate materials (cont.)

followed by chemical-vapour infiltration (CVI), a process thatdeposits carbon at the near-surface of the material, making theplate impermeable to reactants and greatly enhancing surfaceconductivity. The CVI process involves treating the componentwith a gaseous mixture containing methane at a temperatureof 1500 ºC, which causes carbon formation to occur around theoriginal carbon fibres. The elevated temperature of the CVIprocess also causes pyrolysis of the phenolic binder, resultingin a purely carbon–carbon composite material (figure 4).

Carbon–carbon plates can operate at much higher tempera-tures than carbon–polymer composites owing to the all-carbonnature of their design. They have low density (typically around1.2–1.3 g/cm3) because of the retained porosity at the core of thecomponent; they are also significantly stronger and more elec-trically conductive than carbon–polymer composites. A poten-tial problem with the process is loss of dimensional tolerancecaused by shrinkage in the mould – though Porvair’s improve-ments to the process have achieved an impressive capability ofapproximately ±0.013 mm on the thickness uniformity and±0.13 mm on length and width (depending on size and designof the plate). Accuracy of bipolar-plate thickness is particularlyimportant, since tolerances accumulate as the plates are stackedand can lead to leakage of seals.

The main disadvantage of carbon–carbon composites is theirlonger processing time compared with carbon–polymermoulded plates. It’s this feature, together with the high-temper-ature CVI process, that makes the product potentially expen-sive. Porvair, for its part, claim that carbon–carbon processingcosts are not prohibitive if plates are manufactured in sufficientvolumes. Cost estimates of $10/kW are predicted for manufac-ture in quantities exceeding 1 million units per annum.

Carbon–polymer compositesCarbon–polymer composites are made by incorporating car-bonaceous material into a polymer binder, thereby producinga material that can be formed by injection or compressionmoulding. These plates are low-cost, lightweight and amenableto rapid processing since the flow-field geometry can bemoulded directly into the composite. However, the perform-ance of carbon–polymer composites has, until recently, tendedto be inferior to that of other materials – largely a result of theirlow electronic conductivity. Improving that conductivity is aquestion of getting as much graphite into the composite as pos-sible without affecting the mouldability. In this respect, electri-cal conductivity has increased from as little as 2.4 S/cm forcarbon-black and polyvinylidene-fluoride (PVDF) compositesto over 300 S/cm for composites with graphite loadings as highas 93% (reported by the Gas Technology Institute, for example).

It’s worth noting that carbon–polymer plates can be formedin thinner sheets than carbon–carbon composite or graphiteplates; they also have lower contact resistances than metallicplates, which offsets the lower bulk conductivity. The choice andproportion of carbonaceous material, polymer binder, solventand other mechanical and conductivity-enhancing componentsis vital in achieving the desired properties of the plate and deter-mining the cost and processing conditions. The carbonaceous

material can be in the form of natural graphite, synthetic graphiteor non-graphitic carbon, such as coke. The graphite loading (typ-ically 60–80%) and particle size (typically 50–200 µm) have a pro-nounced effect on the electrical and thermal properties and easeof processing. The polymer binder is generally either a thermo-plastic (a plastic that repeatedly softens when heated and hard-ens when cooled) or thermoset resin (a plastic that undergoespermanent chemical change during heating or curing such thatthe polymer chains cross-link and the material becomes rigid).The composite may also have components added to reinforce thestructure and improve mechanical strength – for example, glass,graphite fibres or KEVLAR. Components may also be added tomodify the hydrophobic/hydrophilic properties of the compos-ite to aid removal of water from within the flow channels.

Which plastic? Thermoplastic versus thermoset The choice of polymer binder is governed by the chemical com-patibility with the fuel-cell environment, creep resistance,operating temperature of the fuel cell (thermal stability), vis-cosity when loaded with conductive filler, moulding processand cost. (Depending on the composition, the resin can


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The fabrication of carbon–carbon composite bipolar plates bychemical vapour infiltration (CVI), as pioneered by Porvair FuelCells, US. The CVI process deposits carbon at the near-surface ofthe material, making the plate impermeable to reactant andsignificantly enhancing the surface conductivity.

4. Carbon–carbon technology

milledcarbon fibre

phenolicresin

stirred slurry

vacuumpump

pressing/stamping

preform

vacuummould (150 °C)

methane/argon

bipolar plate

chemical vapourinfiltration (CVI)1350–1500 °C


COMPONENTS

21

account for the majority of the cost of the composite.) Most carbon–polymer composites use thermoset resins (e.g.

phenolics, epoxies, polyester and vinyl ester). Epoxy is a popu-lar example of a thermoset resin used in carbon–polymer bipo-lar plates. Its low viscosity means that it can accommodate ahigher proportion of conductive filler; it also facilitates themoulding process, allowing more intricate features and higherdimensional tolerance to be achieved. Furthermore, the settingchemistry of epoxy has the advantage that no volatiles arereleased during the cure and the resulting polymer is highlycross-linked and chemically resistant.

Thermoplastics (e.g. PVDF, polypropylene, polyethylene)have been applied to a lesser extent because they are generallynot so chemically stable as thermosets and must operate atlower temperatures to avoid melting. What’s more, higher-vis-cosity thermoplastics are limited with respect to the amountof graphite that can be added, so as to maintain the desiredmoulding properties at processing temperature (typically200–250 °C). This means that thermoplastic-based compositessuffer from lower electronic conductivity than other technolo-gies. The main advantage of using thermoplastics, however, isthat they can be injection-moulded and are therefore more con-ducive to automated manufacturing.

Recently, a special type of thermoplastic known as a liquid-crystal polymer (LCP) has been used to produce a composite witha carbon content of 85%. This composite, developed by TiconaEngineering Polymers (Germany) and moulded by SGL Carbon,is based on the former’s Vectra LCP, which can accommodate ahigh carbon content (owing to the low viscosity of the polymer).The high carbon content improves the electrical and thermal

conductivity, while ensuring that the polymer remains amenableto injection moulding. Furthermore, the structure of the LCPmakes the material stronger, stiffer, of higher dimensional sta-bility and more chemically resistant than amorphous thermo-plastics; it also enables operation at temperatures of up to 200 ºC.

Following this innovation, Ticona has demonstrated a stackmade exclusively of engineering thermoplastics (see photo-graph p23). Using carbon–LCP bipolar plates and end-plates ofFortron (polyphenylene sulphide), Ticona claims a 50% lower-ing of stack cost relative to metallic and graphite products – andthat’s despite the higher cost of LCPs over other plastics.Elsewhere, DuPont of the US is also developing a compositebased on its Zenite LCP.

Adding detail: getting flow channels into the plateIn order to get the reactant to the electrodes, channels need tobe formed in the bipolar plate. Integrated cooling channels andfeatures such as manifolds and gasket grooves are also oftenrequired. The manufacturing method for the plate, and partic-ularly the forming of the flow-field geometry, is vital in deter-mining the cost, throughput, level of detail, dimensionaltolerance and range of materials that can be processed.

Manual or computerized numerical control (CNC) routing isthe most common way in which prototypes are produced (andis the method most commonly applied for graphite plates,although it can be employed for most bipolar-plate materials).The machining tolerance is high when using the CNC method,and individual changes to flow-channel designs can be accom-modated with ease. The big drawback is that the time requiredto machine each plate is too long for large-scale manufactur-

Source Bulk Moulding Compounds SGL Carbon SGL Carbon Porvair POCO Graphite SS316LInc. (BMC 940) SIGRACET (BBP 4) SIGRACET (PPG 86) Fuel Cells (AXF 5Q) (unimpregnated)

Type Thermoset Thermoset Thermoplastic Carbon–carbon Graphite Metalliccomposite

Density (g/cm3) 1.82 1.97 1.84 1.2–1.3 1.78 8(20% porous)

Electronic 100* 200* 55.6* >500* 680.3 13513conductivity (S/cm) 50† 41.6† 18.2†

Coefficient of thermal 30 5.8 49 2 7.9 16expansion ×10–6 (K–1)

Thermal conductivity 18.5† 20.8 14 >35* 95 16.3(W/m K) (at 85 °C)

Flexural strength 407.8 407.8 358.6 420–500 878.8(kg/cm2)

Permeability coefficient 2.5 3.5 <0.2 Gas (10–5 (cm3/cm2 s) tight

Max operating 200 ≤180 ≤80 >400 >400 (pyrolytic 1400temperature (°C) glass transition impregnation) 150 m.p.

(resin impregnation)* In-plane property † Through-plane property

Table 2. Physical properties of bipolar-plate materials

ing; the long tool paths mean that a typical 140 × 140 mm platecould take up to 15 min to rout. The lengthy cycle time, capitalcost of CNC machines and tool wear make machining of thissort an expensive forming method.

An alternative way forward could be provided by anapproach called ElectroEtch. This etching technique, developedby Morgan Fuel Cells (UK), allows complicated flow-fields to beformed at a fraction of the cost of CNC milling and up to 30times faster. The process involves blasting an air-abrasiveagainst a masked plate to form complicated flow-field features(down to 200 µm). An example is the fractal-like Biomimeticflow-field that Morgan has developed.

To sum up: metallic plates lend themselves to a wide range offorming techniques, including casting, embossing and etch-ing.4 Embossing is fast and inexpensive; however, channeldepth is limited as excessive embossing pressure can constrictthe metal and result in plate failure. The key advantage of car-bon-composite technologies is that the flow-field design isintroduced during the moulding process. For carbon–polymercomposites, the moulding process is performed by compres-sion or injection moulding (figure 5).

Compression versus injection moulding Compression moulding is primarily used with thermoset-resin-based composites, although it can also be used forthermoplastics. In this process, the carbon-thermoset bulk-moulding compound is added to a heated mould (at greater than200 ºC) where it “flows” to fill the cavity while subjected to acompressive pressure from a hydraulic piston. The limit on pro-cessing time for the thermoset is the curing process (polymercross-linking) and the cooling time for thermoplastics.

Thermosets can be removed from the mould while hot, whereasthermoplastics need to be cooled below their melting pointbefore removal. C A Lawson Co. (Wisconsin, US) has developeda special compression-moulding press that can handle the poorflowability of highly filled composites. The press can be auto-mated and builds to a full pressure of 700 tonnes in 0.65 s. Highlyaccurate plate dimensions are achieved by using a rigid framewith very low platen deflection.

Injection moulding, used for thermoplastics and LCPs,involves the addition of powder or pellet feed via a hopper intoan Archimedean-type screw situated inside a heated chamber;the plastic melts in the chamber and is fed into the mould athigh pressure by the screw and hydraulic ram. The melt fills theheated mould and is removed after cooling. Sophisticated injec-tion-moulding machines make manufacturing automationand mass-production easier, as well as ensuring short process-ing times (as little as 30 s per plate) and high dimensional toler-ance. However, in order to achieve reasonable electricalconductivities, the thermoplastic must be highly loaded withgraphite, which causes the melt to flow poorly. This compli-cates the moulding process and exposes the screw and mouldto excessive wear.

That said, the injection-moulding industry is well establishedand responding positively to the bipolar-plate challenge. Forexample, Century Engineering (Michigan, US) has developed a12-screw “RingExtruder” that it claims will revolutionize theway in which bipolar plates are made. Meanwhile, QuantumComposites (Ohio, US), with the help of machinery fromFerromatik Milacron (Germany), has been the first to demon-strate injection moulding of a thermoset-based carbon-poly-mer composite (PEMTEX) based on vinyl ester.


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Processes for makingcompression-moulded (a) andinjection-moulded (b) bipolarplates. In compressionmoulding, the carbon-thermoset bulk-mouldingcompound is added to a heatedmould, where it “flows” to fillthe cavity while subjected to acompressive pressure from ahydraulic piston. Injectionmoulding, which is used forthermoplastics and LCPs,involves the addition ofpowder or pellet feed via ahopper into an Archimedean-type screw situated inside aheated chamber.

5. Breaking the mould

meltarchimediean

screw

hopper

granularcomposite

screw motorand gears

hydraulicram

heating zone(heaters individually programmable)

parting line

bipolar platemould

ejectorpins

mould cavity

nozzle

(a)

(b)

alignmentpin

bipolar plate mouldthermoset

composite charge

hydraulic pistonmoulded

bipolar plateheated platens

ejector pins


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Which technology will prevail?The lack of a reliable benchmark for performance and the rapidrate of development make it difficult to say which type of bipo-lar-plate material is best. Since fuel cells will penetrate such adiverse range of markets, the question is not so much whichtechnology will prevail, but which technology best suits theintended application. For example, light weight and high per-formance are more important for automotive and mobileapplications, whereas durability and low cost are more impor-tant for stationary applications.

Right now, flexible graphite plates have the greatest share ofthe automotive market, largely because they are used inBallard’s Mark 900 Series stacks. Metal plates are achievingexcellent volumetric and gravimetric power densities owing tothe thinness of pressed plates. Novel stainless-steel alloys andcoated metal plates are overcoming the problem of corrosionand extending lifetimes to technologically meaningful levels.Carbon–carbon composites have many advantages, butwhether automation and large-scale production will realize thecost target has yet to be seen. And carbon–polymer bipolarplates have come a long way in terms of increasing bulk con-ductivity and dimensional tolerance.

Just as significant is the fact that the plastics-mouldingindustry has identified the huge potential market for bipolarplates and is adapting machinery to accommodate the highgraphite loadings required for adequate conductivity withhigh dimensional tolerance. The competition between ther-moset and thermoplastics and compression and injectionmoulding is wide open. At present, plastics companies aremainly concentrating on compression moulding of ther-moset composites. However, if injection-moulding machinescan be made to handle high graphite loading without signifi-cant component wear, as seems to be the case, then the lure oflarge-scale automation pulls in favour of injection mouldingin the future. ●

Further reading 1. V Mahta and J Smith Cooper 2003 Review and analysis of PEM fuel cell

design and manufacturing J. Power Sources 114 32. 2. J Cooper 2004 Design analysis of PEMFC bipolar plates considering

stack manufacturing and environmental impact J. Power Sources 129152.

3. K Roßberg and V Trapp 2003 Graphite-based bipolar plates Handbookof Fuel Cells – Fundamentals, Technology and Applications vol. 3 eds.W Vielstich, H A Gasteiger and A Lamm (John Wiley & Sons Ltd.) 308.

4. J Wind, A LaCroix, S Braeuninger, P Hedrich, C Heller and M Schudy2003 Metal bipolar plates and coatings Handbook of Fuel Cells –Fundamentals, Technology and Applications vol. 3 eds. W Vielstich,H A Gasteiger and A Lamm (John Wiley & Sons Ltd.) 294.

5. A S Woodman, E B Anderson, K D Jayne and M C Kimble 1999Development of corrosion-resistant coatings for fuel cell bipolar platesProceedings of the American Electroplaters and Surface Finishers Society 6 21.

6. D A Shores and G A Deluga 2003 Basic materials corrosion issuesHandbook of Fuel Cells – Fundamentals, Technology and Applications vol. 3 eds.W Vielstich, H A Gasteiger and A Lamm (John Wiley & Sons Ltd.) 273.

7. H Wang, M A Sweikart and J A Turner 2003 Stainless steel as bipolarplate material for polymer electrolyte membrane fuel cells J. PowerSources 115 243.

8. N Cunningham, D Guay, J P Dodelet, Y Meng, A R Hill and A S Hay2002 J. Electrochem. Soc. 149 A905.

9. H Wang, M P Brady, G Teeter, J A Turner 2004 Thermally nitridedstainless steels for polymer electrolyte membrane fuel cell bipolarplates J. Power Sources 138 86.

10. T M Bessmann, J W Klett, J J Henry, Jr. and E Lara-Curzio 2000Carbon/carbon composite bipolar plate for proton exchangemembrane fuel cells J. Electrochem. Soc. 147 4083.

Dan Brett is a research associate specializing in fuel-cell research in theDepartment of Chemical Engineering at Imperial College London. NigelBrandon holds the Shell Chair in Sustainable Development in Energy atImperial College London. He is a visiting professor at the University ofConnecticut’s Global Fuel Cell Center and chairman of the Energy Futuresprogramme at Imperial College London.

Left: bipolar plate made from Ticona’s Vectra liquid-crystal-polymer composite. Right: the ‘all plastic’ stack with Fortron endplates.

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The 207th ECS Meeting will be held at the Québec CityConvention Centre, located in downtown Québec City(1000 Blvd. René-Lévesque Est, Québec G1R 2B5 Canada).

This major international conference offers a unique blend of elec-trochemical and solid-state science and technology in a variety offormats including oral presentations, poster presentations, exhibits,panel discussions, and short courses.

In order to offer a wide range of hotels to suit individual travelbudgets, special rates have been reserved at several hotels for partic-ipants attending this meeting. We are financially responsible for allof the meeting space used at the Québec City Convention Centre.The amount charged for the meeting space is contingent upon thetotal number of guestrooms reserved using our special conventionrates. All hotel rates are quoted in Canadian Dollars and are as fol-lows:

Québec Hilton - $169 CDN Single or DoubleDelta Québec - $169 CDN Single or Double

Le Chateau Frontenac - $209 CDN Single of Double

Note at the time of publication the above rates are equivalent to$131/$162 in U.S. Dollars; €106 and €132 in Euros!

The Québec City and Area Tourism and Convention Bureauhas been appointed as the housing agency for this meeting to assistyou with your hotel reservations. Contact the Central HousingBureau directly by phone at 418.641.6419, fax: 418.641.6578, ore-mail: [email protected] for more informationor to book your accommodations.

Short CoursesAs of press time, the following short courses are planned to be heldin conjunction with the meeting: Electrochemical Nanotechnology,by S. Lipka (University of Kentucky); Impedance Spectroscopy:Theory and Applications, by M. Orazem (University of Florida);Molecular Electronics, by W. Weber (Infineon Tech.) and M.Mayor (University of Basel); Solid Oxide Fuel Cells, by A. Virkar(University of Utah) and S. Adler (University of Washington).

May 15-20, 2005Québec CityQuébec CityQuebec CityQuébec CityQuébec City

ECS • The Electrochemical Society65 South Main Street

Pennington, New Jersey 08534-2839 USA Tel 609.737.1902 • Fax 609.737.2743

E-mail: [email protected] • Web: www.electrochem.org

Symposium TopicsOver 49 topics in the most exciting areas of solid-state and

electrochemical science and technology!

◗ Batteries, Fuel Cells, and Energy Conversion◗ Corrosion, Passivation, and Anodic Films◗ Electrochemical/Chemical Deposition and Etching◗ Electrochemical Synthesis and Engineering◗ Physical and Analytical Electrochemistry◗ Dielectric Science and Materials◗ Semiconductor Devices, Materials, and Processing◗ Fullerenes, Carbon Nanotubes, and Carbon Nanostructures◗ Nanotechnology, Nanomaterials, and Nanoscience◗ Bioelectrochemistry and Biomedical Applications◗ Sensors and Display Materials◗ Student Poster Session

THE

FUEL CELL REVIEW

REPRINTSBenefit from reprints of editorial featuringyour company. Reprints of items from The Fuel Cell Review are valuable for usein presentations, as reference material,and as sales and marketing tools.

We offer a high-quality service, customizing reprintsto meet your needs and at very competitive rates.

For more information and a quote, contact:

Simon AllardiceTel: +44 (0)117 930 1284E-mail: [email protected] fcr.iop.org


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FEATURE: MARKETS

DOES IT MAKE sense to talk about a fuel-cell supply chain, anymore than it does to talk about a fuel-cell industry? Whilethere are nearly 100 operational fuel-cell vehicles being putthrough their paces around the world, these vehicles remainlargely in the realm of research, proof-of-concept and proto-type projects. Conversely, even if fuel cells remain very much a“proto-industry”, many serious companies are collectivelysinking billions of R&D dollars into the technology, with theclear expectation of seeing a viable, profitable, high-volumeindustry develop in the fullness of time.

There are three major markets, or potential markets, for fuelcells: micro (think of laptop computers), stationary (distributedelectricity generation) and automotive. It is essentially the samefuel cell being used in each case, but each application has a differ-ent set of operational parameters, which alter the performancecharacteristics required from fuel-cell systems. This article dealsonly with the automotive market and seeks to evaluate wherethings stand today and what the prospects are for the future.

There are many promising, smallish companies in the sector,producing and researching materials, components and sys-tems. Trouble is, they’re not making any profit – yet. To survive

in this fuel-cell economy, companies have to be able to losemoney for a long period of time, which means that only largecompanies with a high level of commitment are able to sustainthe financial load. Because the production volumes that werepredicted three years ago haven’t materialized so far, compa-nies in the sector have had to prepare themselves financially forthe long haul – chiefly by cutting costs. Ballard Power Systems,FuelCell Energy and Plug Power are three high-profile compa-nies that have been forced to adjust their operations in this way.

Sound technologyThe fact that it’s going to take longer than anticipated to developan automotive fuel-cell industry does not mean in any way thatthere is a question-mark hanging over the technology. If therewere, those billions of R&D dollars would not be spent eachyear. What’s more, the amount of funding going into fuel-cellprogrammes is increasing year on year, and there is a high levelof interest from venture-capital firms and other financial com-panies, as well as from large technology companies interestedin buying up smaller players.

But even Ballard, the largest fuel-cell manufacturer, now has

Hydrogen fuel cells and the automotive supply chain

ATAKAN OZBEK

Fundamental innovation, technology transfer and wholehearted government backing are going to be needed to fast-track the fuel-cell industry’s moves towards full-scale commercialization.

The road is long: large public fleets are the perfect place for governments to provide financial support for the fuel-cell industry.

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a market capitalization of less than a billion dollars (around$750 m in mid-January, which is about 5% of the level itreached at the peak of the technology boom in 2000). On amore encouraging note, DaimlerChrysler and Ford MotorCompany have both recently increased their stakes in Ballard(to about 25% and 17% respectively) by completing an invest-ment of approximately $45 m. In a way, this strategic moveillustrates both DaimlerChrysler’s and Ford’s ongoing com-mitment to the Canadian manufacturer, as well as increasingthe two car makers’ clout in Ballard’s day-to-day operationsand longer-term roadmap.

For a company with very deep pockets, such as GeneralMotors, an investment like this is not a problem, which is why itstill spends in excess of $200 m on fuel-cell development everyyear, as well as maintaining more than 100 engineers workingon different aspects of fuel-cell systems. Yet the fact that thereare hundreds more companies with a foothold in the embry-onic fuel-cell supply chain – and that’s before there are anycommercial products – is equally encouraging. Most of thecompanies in the sector are doing their planning on two levels:one plan for the next year or two, and one for the mid- to long-term. It is clear that most of them are not really expecting to seeradical commercial developments in the next five years.

What’s needed?What parameters characterize the automotive fuel cell? To becommercially successful, a fuel-cell system has to give what aninternal combustion engine gives to today’s vehicles – andmore. On the technology end, the fuel-cell stack must have anoperational life of at least 5000 h (it is currently around 1500 h).The device should have a start-up time of less than 5 s; majorauto companies are confident start-up time will not be a show-stopper, a position shared by the team here at ABI Research.Just as important, the automotive fuel cell should operate withequal efficiency at temperatures from below freezing to desertheat. There are also peak-load requirements specific to vehiclesthat must be met during operation. In summary, a fuel-cell sys-tem needs to have the following characteristics to be usable intransportation applications: high-temperature, high-pressureoperation; high variable-duty cycles; and low humidity.

It is the vehicle manufacturers, however, who will have thefinal say over the technical specifications of the fuel-cell powerplants, because it is their products that will be in the hands ofconsumers. As such, it is they who will ultimately decide whichcomponents and which materials will make up a fully maturefuel-cell supply chain.

So what are the most common materials and componentsthat will go into the fuel-cell supply chain as it develops? A fuel-cell stack can be thought of as a sandwich in which bipolarplates represent the “bread”, while membrane–electrodeassemblies, gas-diffusion layers and seals provide the “filling”.The stack will include materials such as thermoplastics, elas-tomers, lithium and nickel, carbon black and graphite.Fortunately, some of the largest suppliers of these materials –companies such as 3M, Dow Corning and DuPont – are alreadyimportant suppliers to the automotive sector, and they are car-

rying out intensive research in an effort to make these materialslighter, stronger and cheaper.

Things are not so straightforward when it comes to the cata-lyst, although platinum, the most commonly used fuel-cell cat-alyst, is already found in the automotive supply chain inside thecatalytic converter. The big problem with platinum is its price:it costs about $900 per ounce (28 g). At current efficiency lev-els, it takes about 2 g of platinum to produce 1 kW of power.With a typical motor vehicle demanding anything from 50 to100 kW, that translates to a price of somewhere between $1600and $3200 per vehicle – just for the platinum. Clearly, the cost ofthe platinum alone renders current fuel-cell designs unfeasibleand uneconomical, and the goal of researchers at many com-panies is to reduce the amount of platinum required by a factorof 10, down to 0.2 g/kW.

Others are seeking an answer to the platinum problem bysubstituting cheaper metals (such as palladium) or nanocom-posite oxide materials. But the lower catalytic efficiency of suchalternatives has so far prevented any other catalysts from find-ing widespread adoption. It is encouraging that so muchresearch into catalysts is under way, yet this also has a down-side: it prevents us from developing a reliable picture of whatfuel-cell systems may look like several years from now. Thatuncertainty increases the discomfort felt by all stakeholders inthe industry, and results in a lot of uninformed negative pub-licity that questions the basic viability of the technology. Suchspeculation is hurting the fledgling industry at present, andslowing its progress.

Over time, economies of scale will obviously help to drivedown the costs of fuel-cell materials and components.Meanwhile, Nissan’s chief executive officer Carlos Ghosnrecently estimated the cost of the company’s prototype fuel-cell vehicle to be about $1.5 m. And it’s safe to say that fuel-cellvehicles will remain very expensive until at least 5000–10 000


MARKETS

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Chicken and egg: to ensure long-term viability, a hydrogenproduction and distribution system needs to be built, even whilethe search for an economically sustainable fuel-cell vehicle goes on.

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MARKETS

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units are being manufactured annually. In fact, “real” manufac-turing volumes for a mass-market motor vehicle start at100 000, and by the time a million fuel-cell vehicles are beingbuilt every year, all the economies of scale that might beexpected from such volumes should become visible.

When it comes to the prototypes and test beds currentlybeing constructed, a supply chain related to other industrieshas already sprung up. Its elements vary with the capacity ofthe manufacturer. For example, if General Motors wants tobuild three fuel-cell-powered vehicles for a trial, it can do thewhole job in-house, including the fuel cells. A company likeDaimlerChrysler, on the other hand, would probably order afuel-cell system from, say, Ballard. Ballard in turn would makesome system elements itself, but for components such asgraphite plates, for instance, it would turn to existing supplierswith established businesses and revenues in other verticalindustries (GrafTech International in this case). Such compa-nies are using their existing supply-chain relationships andmodifying them to fit the fuel-cell production model.

And then there’s hydrogenIt really is impossible to talk about an automotive fuel-cell sup-ply chain without also considering the hydrogen supply chain.No matter how advanced and how economical your vehicle, it’snot going anywhere without widespread availability of hydro-gen in an easily usable form. Fortunately, hydrogen is a com-mon substance with long-established supply chains in manyindustries. But before it gets to a pump that a driver can use, anumber of questions will have to be answered. Who is going toproduce the hydrogen? Where will it come from? Will we usenatural gas? If so, what kind of production technology will beemployed? Will the current natural-gas infrastructure be suf-ficient to meet demand, or must it be upgraded?

And what about the last link in that supply chain: the con-

sumer at the pump? Demonstration projects are examiningissues such as whether we will have dedicated hydrogen sta-tions (as at Munich airport and in Iceland), or just anotherpump at a conventional gasoline filling station (as in aWashington DC trial, where General Motors is supplying thevehicles and Shell the hydrogen). The latter option is easier andmore achievable in the short term, but it would mean consid-erable alteration to existing infrastructure.

And what about transmission of hydrogen fuel? Who isgoing to deliver it to the location where it will be used? Willhydrogen be stored on site, or will we build pipelines for deliv-ery? If it’s the latter, all pipeline companies should be watchingdevelopments closely, because these are questions involvingbillions of dollars of potential revenues. It’s a classic chicken-and-egg situation: for long-term viability, a hydrogen distribu-tion system needs to be built, even while the search for aneconomically sustainable fuel-cell vehicle goes on.

The race is for the fleetTo ensure popular acceptance, the answers to these questionsshould demand minimal change in the consumer’s driving andrefuelling habits. But the individual buyer – the biggest prize inmonetary terms – is also the hardest to win and hold on to.Individuals are likely to be the last adopters of fuel-cell vehicles,not the first. So fuel-cell vehicles will find their first market infleet applications. And the biggest fleets are run by big munici-pal governments – New York City, for instance, has about 4500buses in the metropolitan area.

Large public fleets are the perfect place for governments tobegin extending the financial support that this proto-industryneeds to kick-start its engine. They are also a very visible plat-form for governments that have the political will to addressenvironmental issues and – except in the US and Australia – towork toward meeting Kyoto Protocol commitments. Trial proj-ects reflecting such government support already exist. In theEuropean Union, the Clean Urban Transport for Europe(CUTE) project has been running 27 Mercedes-Benz Citarofuel-cell buses (with Ballard power plants) in nine Europeancities. In related projects, several of the buses are being put intoservice in the Ecological City Transport System (ECTOS) inReykjavik, Iceland, and a further three in the SustainableTransport Energy Project (STEP) in the Australian city of Perth(see The Fuel Cell Review August/September 2004 p7).

What’s important in these small field trials is determiningwhat kind of fuel-cell system must be used in vehicles for theauto makers to feel comfortable that they can use and buildthem. As of early November 2004, the CUTE, ECTOS and STEPfuel-cell buses had covered a collective distance of 42 000 km,consuming about 44 000 kg of hydrogen in the process. That’sa lot of hydrogen that needs to be paid for and a lot of front-loaded investment. In the end, of course, fuel-cell and hydro-gen supply chains must be profitable and self-sustaining.

All of this begs another leading question. Right now, howdo fuel-cell vehicles stack up against conventional gasolinevehicles? Fuel cells are nearly twice as efficient as internalcombustion engines at converting fuel to usable energy, and

Fuel for thought: one of the biggest R&D challenges is therealization of a practical on-board hydrogen-storage system.Current storage systems are too heavy, too large and too costly.

BM

W

scientists and engineers believe they can achieve considerablybetter performance. But hydrogen, in spite of being the mostabundant element and the product of an already huge indus-try, currently costs three to five times as much as petrol toproduce. Only when manufacturing volumes increase fur-ther will that cost come down.

An industry in fluxOn environmental grounds, however, fuel cells would seemat first glance to be a “no-brainer”. The main by-product offuel-cell reactions is water. But many different analyticalmodels are being used to assess the net effect of fuel cells onthe atmosphere. The National Academy of Sciences in the UShas reached mixed conclusions, using a “well-to-wheel”model adapted from the oil economy (i.e. a model that breaksdown the hydrogen supply chain into segments from pro-duction to fuel tank to where the rubber meets the road). Inits study, fuel-cell systems proved to be very clean from tankto wheel. But the academy notes that the methods of produc-ing hydrogen that we have today – based largely on process-ing of hydrocarbon feedstocks – actually produce moreatmospheric pollution than is saved by replacing the internalcombustion engine.

That means that unless governments take the lead, lendingfinancial support and dragging the oil companies and auto-motive manufacturers along, the process of developing this

industry will take much longer than necessary. Put anotherway: the phased inclusion of large numbers of fuel-cell vehi-cles into public fleets will be a major driver for the sector in thenext 10 to 15 years.

And let’s not forget that a huge amount of R&D is being done.Who’s to say that someone isn’t going to emerge from a labo-ratory somewhere soon with a catalyst material that does a bet-ter job than platinum at a tenth of its cost? That would reallychange the whole picture. Other promising avenues of investi-gation include, for example, carbon nanostructures for hydro-gen storage. If storing hydrogen at 10 000 psi suddenly becamefeasible, we would see cars that could easily travel 300 or400 miles on a tank of the gas.

That said, it remains unlikely that any “magic” invention willchange everything at a stroke. More likely, fuel cells willbecome part of the technology mainstream through multiple,incremental breakthroughs in materials, components andinfrastructure. Only when all the pieces fall into place – perhaps10 years from now – will we see the emergence of a true, indus-trial-sized fuel-cell supply chain. ●

Atakan Ozbek is principal analyst, energy research, at ABI Research, an analystfirm specializing in technology markets. ABI is based in Oyster Bay, New York,US, and has offices in London and Hong Kong. Further information about thegroup’s latest study on the automotive fuel-cell supply chain may be found atwww.abiresearch.com/reports/AFSC.html.


MARKETS

28

http://www.abiresearch.com/reports/AFSC.html

http://www.hydrogenexpo.com

http://www.abiresearch.com/reports/AFSC.html

TECHNOLOGYTRACKING

Also in this section

30 Regenerative fuel cells

31 On-board fuel reforming

31 Hydrogen-storage R&D

32 Test and measurement

From innovation to manufacturing

Some big bets are being placed on the future ofsolid-oxide fuel cells (SOFCs). The US Depart-ment of Energy’s Solid State Energy ConversionAlliance (SECA) is a case in point; it is spendingaround $50 m a year to fast-track the commer-cialization of these multikilowatt powersources (The Fuel Cell Review October/November2004 p38). If true believers like SECA have gotit right, SOFCs are destined for great things instationary industrial and residential power gen-eration, auxiliary power for trucks and cars, andall manner of military applications.

For now, though, SOFCs remain a “show me”technology, with plenty of technical obstaclesyet to be overcome. The SOFC design is basedon a ceramic electrolyte with doped oxide cath-odes; the anodes typically comprise a ceramic-nickel mixture. Oxygen in the air reacts withelectrons at the cathode to create oxide ions.These ions migrate to the anode through thesolid electrolyte, which must be as thin as pos-sible to reduce resistance. At the anode, theoxide ions react with hydrogen to producewater – a process that also generates energy.

Since conventional electrolyte materialssuch as yttria-stabilized zirconia only performwell at high temperatures, SOFCs have in themain been designed to operate at between 800and 1000 °C. This is good news in somerespects – for example, SOFCs can work with awide range of fuels and there is no need forexpensive precious-metal catalysts. Trouble is,high-temperature operation also comes with adownside. There are considerable thermal-management challenges in this regime, notleast the need for costly heat-resistant materi-als in balance-of-plant components and thedifficulties associated with the sealing ofceramic components to obtain gas tightness.

Leap of faithThose challenges were very much to the foreback in 2001 when a team of senior fuel-celland materials scientists at Imperial College,London, put their collective know-howbehind a university start-up called CeresPower. Throughout the 1990s, the researchershad been working on advanced electrolyte

materials that would enable SOFCs to operatesuccessfully at temperatures of 500–600 °C.At these temperatures, low-cost stainless steelcan be used for the balance-of-plant compo-nents, while the benefits of high-temperatureoperation (i.e. multiple fuels and no precious-metal catalyst) are retained.

The impetus behind the creation of Cereswas a materials-science breakthrough identi-fying a method for depositing an electrolytecalled cerium gadolinium oxide (CGO) onto astainless-steel support. Building on the origi-nal patented design from Imperial, Ceres hasdeveloped significant expertise in manufac-turing techniques and ink formulations, aswell as the engineering of complete systemsfor its target market applications.

Four years on and Ceres Power is a product-development company that specializes in whatit calls the “power chips” at the heart of theSOFC. These chips are essentially stainless-steel wafers coated with CGO electrolyte andthe fuel-cell electrodes. The latter are based onstandard SOFC materials – the anode is aceramic–nickel mixture (cermet) and the cath-ode is based on lanthanum-strontium-cobaltferrite. Durability testing on cells and stacksindicates that the technology can operate formany thousands of hours with minimumdegradation (and using widely available fuelslike natural gas and liquid-petroleum gas).

The fuel cells are also robust when it comesto thermal cycling between extremes of tem-perature, coping with hundreds of rapid cyclesfrom room temperature to full power at 600 °C.“This is very unusual for SOFCs,” claims PeterBance, Ceres’ chief executive officer, adding thatthe key to the durability of the fuel cells is thatthe underlying materials are so well matchedacross the intermediate temperature range.

Right now, Ceres is concentrating its effortson technology transfer and commercializa-tion. Development of advanced materials willcontinue, but the company’s technical priori-ties in the short term mirror its commercialgoals. In other words, Ceres wants to validateits manufacturing process and to further thedesign, demonstration and testing of proto-

Product development

Advanced electrolyte materials could make solid-oxide fuel cells a more compelling proposition.


A clear roadmap: after listing on London’sAlternative Investment Market in Novemberlast year, Ceres Power is focusing on prototypedevelopment, testing and the validation of itsmanufacturing process.


TECHNOLOGY TRACKING

30

type fuel-cell systems alongside partner com-panies. Significantly, the financial picturelooks solid for the foreseeable. Two venture-financing rounds in 2001 and 2003 yielded atotal of £10 m ($18.6 m) and helped to generatemomentum for early-stage development. In2004, staff numbers doubled (to around 30),and recruitment is ongoing. In particular,Bance and his team are looking for engineerswith a track record in manufacturing scale-up(not surprising, given that work is now underway on the commissioning of a pilot-scalemanufacturing plant at Ceres’ headquarters).

Last November, things moved up a gear whenCeres listed on London’s Alternative Investment

Market (a secondary stock exchange specificallyfor new growth businesses). The stock offeringwas oversubscribed and generated around£16 m of development capital. As well as fund-ing the manufacturing side of the business, themoney will be used to build long-term develop-ment partnerships with systems integrators,component suppliers and other manufacturers.In recent months, for example, Ceres has beenworking with the industrial-gases giant BOC toevaluate the use of liquid-petroleum gas in itsSOFCs. Further collaborations are in the works.

Clearly, Ceres has negotiated the tricky tran-sition from the rarefied confines of academia tocommercial independence. Beyond giving

Imperial College a minority stake in exchangefor its core intellectual property, the start-uphas not kept any special links with its parent.However, the company does collaborate withthe university on specific mid- and long-termR&D projects, with Ceres owning any newintellectual property in its core areas of activity.“The umbilical cord was cut when we spun outand got our first round of venture-capital fund-ing,” says Bance.

Time will tell whether Ceres is capable ofpulling off the most difficult transition of all –from product-development hot-house to man-ufacturing powerhouse.Siân Harris and Joe McEntee

It uses metal-hydride materials in place ofcostly noble-metal catalysts in the fuel anode.It requires hydrogen fuel but is also capable ofoperating for several minutes without any fuelat all. It can run “backwards” and store energywithin the fuel-cell stack at 80% efficiency andwithout the need for electrolysis of water. Whatis it? It’s a radical rethink of fuel-cell first prin-ciples from the Ovonic Fuel Cell Company ofRochester Hills, Michigan, US.

The key to Ovonic’s “regenerative” fuel cell isthe design of the electrodes, which are able tostore energy in a similar way to nickel-metal-hydride batteries. Hydrogen gas diffusesthroughout the anode and is absorbed by theelectrode material; in a similar way, oxygen isstored in the metal-oxide-based cathode.Electrical current evolves through the exchangeof protons from the anode to the cathode via anelectrolyte. However, given an excess of hydrideand oxide in the anode and cathode respec-tively, the cell is able to operate without exter-nal fuel or oxygen for as long as 11 min at 50%peak current (5 min at 100% peak current).

Ovonic claims that the regenerative design iscompatible with the full range of fuel-cellchemistries (alkaline, polymer-electrolyte,phosphoric acid, solid oxide and molten car-bonate). That said, the initial developmenteffort is focusing around an alkaline electrolyte(potassium hydroxide) that facilitates theintroduction of metal-hydride materials intothe hydrogen-fuel anode and ensures morefavourable kinetics at the metal-oxide air cath-ode (as well as allowing the introduction ofnon-noble metal catalysts at the cathode).

In terms of technical specifications, how-

ever, Ovonic still has a significant amount ofwork to do if its regenerative cell is to becomecompetitive with the latest polymer-elec-trolyte-membrane fuel cells (PEMFCs). “Thetechnology is still fairly immature, but we areconfident that we can be competitive withPEMFCs on power-to-weight,” explainedDennis Corrigan, president of Ovonic. The cellis currently capable of generating around70 W/kg compared to approximately 1 kW/kgfor state-of-the-art PEMFCs. Corrigan contin-ued: “In terms of current density, the leadingPEMFCs are achieving about 1 A/cm2, whilewe are currently at 250 mA/cm2.”

While there’s room for improvement onpower performance, the Ovonic cell has plentyof plus points, not least its instant-start capa-bility (of the order of microseconds) and itslow-temperature performance (with 50% peakpower at 0 °C and operation to below –20 °C).

Durability and lifetime studies also look prom-ising, with individual electrode half-cell testsexceeding 5000 h of continuous operation atrated currents.

Prototype multicell products will be ready toship later this year. In the near term, Ovonic isbetting that its regenerative fuel cell will findapplications in uninterruptible power supplies,back-up power for telecommunications net-works and military systems. Further down theline, Corrigan hopes car manufacturers willbuy into the regenerative capabilities of thedesign, not least its ability to capture and utilizeregenerative braking energy at high efficiency.

The Ovonic Fuel Cell Company currentlyholds 11 US patents on the technology, with afurther 20 applications pending. The companyis a subsidiary of the Ovonic Energy ConversionDevices (ECD) Group.Jonathan Wills

Question: when is a fuelcell not a fuel cell?Answer: when it thinks it’s a battery.

Innovation

The Ovonic fuel cell uses metal-hydride battery technology to support regenerative operation(i.e. the ability to accept regenerative braking energy in an automotive application). Here, thefuel-cell stack operating at 100 mA/cm2 is interrupted by a 100 mA/cm2 pulse. During the pulse,the cell voltage increases to about 1.25 V, indicating the storage of energy without electrolysis ofwater. Following the pulse, the cell exhibits an elevated voltage as a result of the stored energy.

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TECHNOLOGY TRACKING

31

With national networks of hydrogen fuellingstations likely to be rolled out over a period ofdecades rather than years, it’s clear that car mak-ers and infrastructure companies will need tomake some transitional moves if fuel-cell vehi-cles are to build any kind of critical mass in theinterim. One of those evolutionary steps couldinvolve the reforming of gasoline to hydrogenon board fuel-cell vehicles. The favouredapproach uses a fuel processor based on steamreforming, in which hydrocarbon fuel reactswith steam at high temperatures over a catalystto produce hydrogen and carbon monoxide.

Steam reforming is the most cost-effectivemethod for generating hydrogen, and also themost efficient, with conversion rates ofbetween 70 and 80%. The technique is widelyused in large-scale industrial applications – inpetrochemical refineries and chemical plants,for example. Trouble is, it does not follow thatsimply downsizing an industrial-grade steamreformer will result in an appropriate technol-ogy for on-board hydrogen generation.

Now, however, a team of engineers fromExxonMobil Corporate Research in Annan-dale, New Jersey, US, has come up with analternative take on hydrogen production –one that they claim could be compatible withboth on-board vehicle applications and large-scale distributed fuel-cell power systems. Theinnovation, which they call a pressure swingreformer (PSR), relies on a steam-reformingprocess to generate high-pressure, undilutedsyngas (a hydrogen/CO mix) from low-pres-sure air at very high efficiency. It’s all aboutsimplification, not least the fact that the PSRperforms the heat transfer inside the reactor(within the catalyst bed), thereby eliminatingmany of the troublesome heat-transfer stepsfound in conventional steam reformers.

“For on-vehicle applications, previous efforts[on fuel reforming] have all suffered from theinability to meet very challenging performancetargets: size, start-up time, start-up energy and system simplicity,” explained ExxonMobilspokesperson Paul Berlowitz. “Typically, fuelprocessing for fuel-cell vehicle applicationsemploys a reforming reactor followed by severalCO chemical-conversion stages. Each reactorrequires an intermediate heat exchanger, furthercomplicating the system.”

Exxon claims that the PSR overcomes thesedrawbacks by reducing the overall componentcount in the system. The reformer and the inlet

and exit heat exchangers are all incorporatedinto the PSR reactor bed. By enabling separa-tion, the high-pressure, undiluted syngas out-put eliminates all of the CO clean-up reactorsand heat exchangers, and results in a purehydrogen product suitable for the fuel-cellstack. As a result, claims Berlowitz, the PSR canbe integrated directly into the fuel-cell stack,ensuring “system simplicity and reducingoverall size, weight and cost”.

The heart of the PSR is a single chamberenclosing a solid heat-exchange bed. Most ofthe chemistry occurs around the interface with

the reforming catalyst, which is localizedwithin a section of the bed. Combustion andreformation reactions occur in a cycle, sequen-tially heating and cooling the bed. At the sametime, the reactor oscillates between low pres-sure during combustion and higher pressureduring reformation.

The reaction scheme generates a heat “bub-ble” (1000–1200 °C) within the bed. This bub-ble expands and contracts, promoting highreformation rates whilst allowing the inputand output gases to remain relatively cool (lessthan 400 °C). The resulting high-pressure syn-gas then passes through a separation system(such as pressure-swing adsorption or mem-branes similar to those used in industrialreforming) to yield a high-purity hydrogenproduct that can be used directly in a polymer-electrolyte-membrane fuel cell.

Although development is still at an earlystage, the ExxonMobil team has demonstratedencouraging fuel-conversion yields (morethan 95%) at reasonable temperatures (lessthan 1200 °C) for a methane feed. Similarresults have been obtained for liquid-petro-leum conversion. Start-up time for the PSR isestimated at around 20 s, and less than 30 s forthe entire fuel processor.

Berlowitz concluded: “We have used steady-state and dynamic system modelling to evalu-ate the PSR system against the on-board vehicletargets developed by the US Department ofEnergy for 2010 and 2015. The results show anintegrated PSR-based system has the potentialto meet stringent targets for size, start-up timeand overall system efficiency.” Jonathan Wills

ExxonMobil reckons on-board fuel reforming is edging closer to becoming a practical proposition.

Reforming the reformersHydrogen production

General Motors (GM) and Sandia NationalLaboratories have embarked on a $10 m collab-oration to develop advanced hydrogen-storagemethods based on metal-hydride materials.Over the next four years, researchers in Detroit,Michigan, and Livermore, California, will team

up in a bid to realize a prototype solid-statehydrogen “tank” based on sodium alanate.

For now, the goal is to develop a hydrogentank capable of storing more on-board fuelthan traditional compressed-gas or liquid-hydrogen approaches. The end-game, however,is to find a way of storing enough on-boardhydrogen to enable a fuel-cell vehicle to matchor exceed the driving range of a petrol or diesel-fuelled internal-combustion-engine vehicle.

“Hydrides have shown a lot of early promiseto one day increase the range of fuel-cellvehicles,” explained Jim Spearot, director of theGM advanced hydrogen-storage programme.“We know a lot of research still needs to bedone, both on the types of hydrides we use and

The deep secrets of solid-state storage

Advanced materials

Can US researchers come up with a viablesystem for in-vehicle hydrogen storage?They’re certainly going to try.

Heat-bubble evolution within the catalyst bed(a). During regeneration, the heat bubbleexpands and moves to the left (b); duringreforming, the heat bubble contracts and movesright (c). The high temperature cycles aroundthe centre, leaving the inlet and outlet cool.

reform

catalytic(reforming)

non-catalytic(recuperation)

regen

(a)

(b)

(c)


TECHNOLOGY TRACKING

32

A deeper understanding of fundamental elec-trochemical processes will be essential ifresearchers are to capitalize on advances inmaterials science and component-level inno-vations in the next generation of fuel cells,batteries and supercapacitors. In this regard,electrochemical impedance spectroscopy(EIS) stands out from all other electrochemi-cal analysis techniques, chiefly because – inmarked contrast with traditional DC analyti-cal techniques – information regarding ohmiclosses, reaction kinetics and mass-transferprocesses can be characterized in a singleexperiment.

Those DC techniques involve perturbing thesystem under study by applying either a largepotential difference or a current, and then mon-itoring the resultant current–voltage curves. It’sa straightforward process and any deviationsfrom the expected results are easy to spot. Thiscan be as far as it goes, however. Being DC, anyfundamental change within the cell will only bemeasured as a change in voltage or current, soit is often difficult to make the transition fromidentifying the existence of a problem to work-ing out its cause. Furthermore, there is a

concern that any analytical technique necessi-tating a large DC perturbation will result intransient measurements which are not truly

representative of the cell characteristics underthe required load conditions.

EIS provides a powerful way forward by

The root causes of underperformanceTest and measurement

While AC impedance studies provide a powerful tool for analysing fuel-cell behaviour, development engineers need to knowexactly what they’re looking at if they are to get the most from their experimental data.

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Data from an AC impedance study of a 1 W direct-methanol fuel cell operated at 10 mA. In bothcurves, Z’ and Z’’ are the ‘in’ and ‘out’ of the phase components of the cell impedance (derivedfrom mathematical manipulation of the current and voltage). Their relationship shows whetherthe cell is performing as it should. Figure (a) is the initial plot for the cell, with two impedance arcs.The high-frequency arc (left-hand side) corresponds to the electrode kinetic response; the widthof the arc is a measure of the rate of the electrochemical reaction. This is followed by a low-frequency arc (<1 Hz) which can be attributed to water management. Figure (b) shows the samecell after it has been operated at 10 mA for 1 h. Only a single arc is now present, which suggeststhat the cell has become flooded with water and that this is limiting performance.

the tanks we store them in, [but] we think ourwork with Sandia will get us another stepcloser to our goal.”

Initially, the researchers will analyse severaltank designs using thermal and mechanicalmodelling. Control systems, heat managementand tank size/shape will all be consideredbefore the most promising tank configurationsare subjected to rigorous safety testing aheadof prototype fabrication.

Hydride storage works on the principle thatsome metal alloys and other metallic materi-als are able to absorb large amounts of hydro-gen by forming metal-hydride compounds.When heated, the hydrogen is released andthe material reverts back to its original, dehy-drogenated state. In this way, solid-state mate-rials could be used as refillable hydrogen-fuelstores for both stationary and mobile fuel-cellapplications.

Sodium alanate (NaAlH4) is one of the mostpromising metal hydrides for hydrogen-stor-age applications. Currently, the material sys-tem can achieve around 3 wt% reversiblehydrogen storage at reasonable rates when dis-charged at more than 100 °C and when doped

with small amounts of titanium-based com-pounds and other agents. In separate studies,scientists are evaluating alternative storage mate-

rials. For example, magnesium-modifiedLi-amide potentially offers 10.4 wt% reversiblestorage at operating temperatures of less than200 °C; and LiBH4 desorbs three of the four Hatoms per molecule (13.5 wt%) upon melting at280 °C (although it requires heating to 700 °Cand 200 bar pressures to recharge).

Nevertheless, GM–Sandia and others have along way to go before a commercially viablemetal-hydride storage system becomes practi-cal. For starters, the temperature at whichhydrogen is released is currently too high andtherefore consumes too much energy. Equally,the time taken to “refuel” or reabsorb hydrogenis too long – around 30 min for the best-per-forming materials.

The research conducted through theGM–Sandia partnership is privately fundedand independent from that related to Sandia’sparticipation in the Metal-Hydride Centre ofExcellence. The Centre of Excellence, which isbeing funded through a US Department ofEnergy “Grand Challenge”, aims to develop anew class of materials capable of storinghydrogen safely and economically. Jonathan Wills

Thermal management: Sandia engineer Terry Johnson sets up a test apparatus that,when verified, will generate external heat toimprove the overall energy density of thehydrogen-storage medium.

BU

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FUEL CELL REVIEWFeature: Intermediate-temperature SOFCs: the technology challenge Subhash Singhal et al., Pacific Northwest National Laboratory, US

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applying a small sinusoidal perturbation (cur-rent or potential) at a wide range of frequen-cies on top of the fuel cell’s normal behaviour.The principle is based on AC impedance the-ory, which looks at the response of a circuit toalternating current or voltage as a function offrequency. The analogue of AC circuit theoryis used to characterize the electrochemicalsystem in terms of an equivalent-circuitmodel. What’s more, because small excitation

amplitudes are typically used (5–10 mV rms),EIS does not disturb the fuel cell under testand so provides a more realistic picture of theoperational performance.

Unsurprisingly, a number of test and meas-urement companies are already battling formarket share, pushing EIS analysers of varyingsophistication. John Harper, an applicationsspecialist at UK-based Solartron Analytical,one of the leading vendors of EIS gear, is clear

about the benefits of AC impedance studies.“We apply a range of frequencies over singleexperiments,” he said, adding that a typicalSolartron analyser can apply frequencies from1 MHz down to 10 µHz.

This is important, says Harper, because thefactors governing electrochemical reactionsoccur on very different timescales. For exam-ple, the kinetics of the oxidation and reductionreactions can be fast (typically measured athigh- to mid-frequencies of between about10 kHz and 1 Hz), while changes in water con-tent generally happen much more slowly (usu-ally observed at frequencies of less than 1 Hz).In addition, evaluation of ohmic losses mayrequire analysis in the >10 kHz region.Changing the frequency of the applied currentenables designers to decipher the timescale onwhich a problem is occurring and get to theroot of that problem more quickly.

Too much information?There are drawbacks, however. For starters,EIS results are more complicated to interpret,because AC techniques yield a lot more infor-mation than their DC counterparts. Changesin the output signal at different frequenciesrelate to different phenomena, plus there aresubstantial variations in the underlying behav-iour from one fuel-cell type to another.

High-temperature systems, such as solid-oxide fuel cells, do not suffer from water-man-agement issues, but they can experienceproblems with component stability (at operat-ing temperatures as high as 1000 °C). On theother hand, water management is a bigheadache in lower-temperature proton-exchange-membrane (PEM) fuel cells, whilecells that undergo internal fuel reformation canbe hampered by carbon-monoxide poisoningof the precious-metal catalyst. And to compli-cate matters further, all of these fuel-cell typescan be studied by EIS using the same analyser.

“Because it is such a powerful technique, itdoes require a deeper understanding [com-pared to DC studies],” Harper conceded. “Butthe benefits far outweigh the time taken inlearning.” For this reason, Solartron believes itis vital that test and measurement vendors edu-cate the market on the subtleties of EIS andhow to get the best results from the technique.

For the time being, Solartron’s EIS productsare aimed at the R&D community, althoughHarper believes that there will be a major rolefor AC impedance testing in emergingfuel-cell applications too. “In the PEM fuelcells that will be used in cars, for example, itwill be crucial to monitor gas-diffusionmembranes and water content during opera-tion,” he explained. Siân Harris

http://www.stewart-warner.com

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