current technology of fuel cells systems

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CURRENT TECHNOLOGY OF FUEL CELL SYSTEMS

Ali T-RaissiArundhati Banerjee

Kenneth G. SheinkopfFlorida Solar Energy Center1679 Clearlake RoadCocoa, Florida 32922407-638-1000FAX 407-638-1010

ABSTRACTA great deal of research is taking place on fue l cells, which usehydrogen and oxygen as the fuel. One of the reasons for thisinterest is that fuel cells offer the best criteria for meeting therequirements of zem emission vehicles, and thus are expected tobe the prime users of hydrogen in the nea r future.There are presently six different types of fuel cell technologiesavailable- phospho:ic acid fuel cells, proton exchange memb ranefuel cells, alkaline fuel cells, molten carbonate fuel cells, solidoxide fuel cells, and direct methanol-air fuel cells.This paper looks at each of these six types and gives a briefoverview of the technologies and their present state ofdevelopment. The suitability of the various types toward use inthe transportation sector is also studied.Because the selection of a fuel storage method is highlydependent on basic q ui re me nt s of operational characteristics, thestatus of the fuel cell techn ologies is discusse d with respec t to theirbasic operating principle, acceptable contamination level,economics, and suitability toward transportation.To be used in the transportation sector, fuel cells must me et thedemands of rapid startup, fast pickup, high power density, greaterfuel efficiency, easy and safe handling, high lifetime and low cost.Unfortunately, none of the six types can satisfy all of thesedemands at this time, but each has its ow n advantage s and benefits.This paper categorizes each type as to their strengths andweaknesses in meet:.ng hese needs.

to batteries, fuel cells are made of tw o electrodes with a conductiveelectrolyte sandwiched in between but unlike a battery, a fuel celldoes not require recharging. It produces electrical energy as longas fuel is supplied to it. The fue l for the fuel cell can be hydrogenor any other hydrogen-containing compound which onreprocessing can produce hydrogen. At the hydrogen electrode(anode), hydrogen ions(protons) and electrons are formed.Protons migrate through the electrolyte to the oxygen electrode(cathode) while electrons move through an extemal circuit to aload and then retum to the cathode. At the oxygen electrode,oxygen, hydrogen and electrons combine to form wa ter. Thus, byforcing the electrons tfi take an extemal path, low temperaturedirect energy conversion is achieved*.Thermodynamically, a fuel cell converts the Gibbs free energychange (*G) of a electrochemical reaction to electrical energyaccording to the following equation:

aG = nFBErwhere E, = reversible potential of the cell, n = no. of electrons aridF =Faradays constant. Considering the m ost comm only referredreaction of the fuel cell -- the reaction between hydrogen aridoxygen to produce water -- (H 2 + 1/2 O,=H,O), *Go is 56.32kcdm ole, n is 2 and therefore E, turns out to be 1.23volt (under

INTRODUCTIONThis paper looks at the six major types of fuel cells and givestheir basic operating principles, a cceptable contamination levels,technological status, and suitability in the transport sector.Fuel cells are e1ec:trochemical devices which directly convertchemical energy to electrical energy without combustion. Similar

David L., Handbook cfBatteries and Fuel Cells, McGraw Hil l 1984.Swain, D.H. and Appleby, A.J., Fuel Cells and Other Long RangeTechnology Options fo r Electric Vehicles: Knowledge Gaps andDevelopmen t Priorities, in The Urban Electric Vehicle, proceedings ofan International Conference, Stockholm, Sweden, Ma y 25-27, 1992, pp.457-468, OECD Document.

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standard conditions (i,e.,T=25C ,P, = Poz= 1 atm, liquid Hz03 ).It is this electrical energy which can b e effectively used to pow erthe motor of an electric vehicle.The operation of a fuel cell involves startup, fuel and air deliverycontrol as a function of the load. Th e products are heat and waterwhich have to be removed from the system. In this regard, the fuelcell can be referred to as an electrochemical engine forelectridhybrid vehicles.In practice, the efficiencies of fuel cells range from 40% t3 65%based on the lower heating value of hydrogen. Under op erationalconditions, the voltage output falls due to polarization effects. Tomake a useful voltage for practical purposes, multiple cells areconnected in electrical series and are referred to as a fuel cell stack.The fuel cell stack and its necessary auxiliaries are collectivelycalled fuel cell systems. The major accessories include thermalmanagement system, fuel supply and storage subsystems.There are presently six different types of fu el cell technologies:

1)2) Proton exchan ge memb rane fuel cell3) Alkaline fuel cells (AFC)4)5)6)

Phosphoric acid fuel cell (PAFC)(PEMFC)Solid Oxide Fuel cell (SOFC)Molten carbonate fuel cell (MOFC)Direct melhanol-air fuel cell (DMAFC)

The following classification is based on the type of electrolyteused in the fuel cells. Because the selection of a fuel storagemethod is highly dependent on basic requirem ents of operationalcharacteristics, the status of fuel cell technologies is discussedwith respect to their basic operating principle, acceptablecontamination level, technological status and economics andsuitability towards transportation sectors.

PHOSPHORIC ACID FUEL CELLS (PAFCs)Basic Operating Principle

The basic compon ents of a phosphoric acid fuel cell (PA FC) arethe electrodes consisting of finely dispersed Pt catalyst on carbonpaper, S i c matrix holding the phosphoric acid and a bipolargraphite plate with flow channels for fuel and oxidant. Theoperating temperature ranges between 160-220C and it can useeither hydrogen or hydrogen produced from hydrocarbons(natura1gas) or alcohols as the anodic reactant. In the case of hydrogenproduced from a reformer with air as the anodic reactant, atemperature of 200C and a pressure of 8 atm are required forbetter performance. PAFC s are advantageous from a thermalmanagem ent point of view. Th e rejection of waste heat andproduct water is very efficient in this system and the waste heat at200C can be used efficiently for the endothermic steam-reforming reaction4.

Acceptable Contamination Levels

3Appleby,A.J. an d Foulkes, F.R.,Fuel Cell Handbook, Va n NostrandReinhold, N.Y., 1989.4U.S. Department of Energy, Phosphoric Acid Fuel Cells,DE93000003,Nov. 92.

The system is extremely sensitive to CO and H,S which arecomm only present in the reformed fuels. A major challenge forusing reformed fuel, therefore, lies in the removal of C O to a levelof 180C. Removal of sulfur is still essential.TechnoloPical Statu sThe PAFC system is the most advanced fuel cell system forterrestrial applications. Its major use is in on-site integratedenergy systems to provide electrical power in apartments,shopping centers, etc. These fuel cells are com mercially availablein the range from 24V, 250 watt portable units to 200kW on-sitegenerators. PAFC systems of 0.5 - 1.OMW are being d evelopedfor use in stationary power plants of 1-11 MW capacity.Using natural gas as the fuel, a 200kW system is $2875/kW(1992 $). The power density of PA FC is 200 mW /cm2and theprojected capital costs are $1000-$150 0/kW5. (The projectedpower density for 36 kW brassboard PAFC fuel cell stack has beenreported to be 0.12 kW /kg and 0.16 kW/L.)TransportationSectorPAFCs are currently being considered for use in heavy dutyvehicles. Their major problem for use in vehicular application istheir slow start-up (since the cell has to be h eated to over 200 C),high costs and excessive weight. Since PAFCs work best at aconstant output, their application will be b etter in hybrid system swhere a battery or other device m eets the high-pow er demands ofacceleration. PAFC s stand their best chance of success in heavyduty vehicles or locomotives. The 200kW unit can be used forlong-range bus or truck applications while the larger megawattcapacity units are planned to be used as the power plant of a long-range locomotive unit.

PROTON EXCHANGE MEMBRANE FUEL CELLS(PEMFC)/SP(E)FCBasic Operating PrinciplePEM fuel cells, also called solid polymer electrolyte fuel cells --SP(E)FC -- use a proton (hydrogen ion) conducting membranewhich stays sandwiched between two platinum-catalyzed porouselectrodes728. Initially, these mem branes were based onpolystyrene, but at present a Teflon-based product Nafion is

5Snnivasan,. S. et.al, Overview of Fuel Cell Technology, in Fuel CellSystem, edited by Leo J.M.J. Blomen and Michel N. Mugenva, PlenumPress, N.Y. 1993.%hi, C.V., Glemm, D.R. and Abens, S.G.,Developmentof a fuel cellpower source for bus, proceedingsof th e 2Sh ntersociety EnergyConversion Engineering Conference, v o l 3 , edited by PIA. Nelson,W.W. Schertz, and R.H. Till, American Institute of Chemical Engineers,N.Y. 1990, pp. 308-313.U.S. Department of Energy, Proton-Exchange MembranceFuel CellProgram, DE93000009, Nov. 1992.8Gottesfeld,S . Polymer Electrolyte Fuel Cells, submitted to 11International Seminar on Primary an d Secondary Battery Technologyand Applications, Feb. 28-March 3, 1994, Deerfield Beach, Florida(private communication).

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used. This offers high stability, high oxygen solubility and highmechanical strength. The cell operating temperature is quite low(80-90C) and opepating pressures can be in the range of 1-8atmosphere. The fuel cell requires humidified hydrogen andoxygen for its operation. The pressures, in general, are maintainedequal on either side of the membrane . Operation at high pressureis necessary to attain high power densities, particularly w hen airis chosen as the anodic reac tant.Acceptable Contamination LevelThe major contaminant of the PEMFC system is carbonmono xide. Even a trace amount of CO drastically reduces theperformance levely~'o t is best to use a fuel w hich is virtually freeof CO for PEMFC. On the other hand, it is tolerant toward CO,contamination.Technological Stat@PEMFCs have high power density (350 mW/cmz) and are nowcomme rcially available in power ranges (IOOW-5OOW) with thecorresponding capital cost ranging from approximately $5,676-$13,000. The lOkW to 200 kW power plants have been developedand demonstrated successfully but are waiting for large scaleproductionY.los". he projected ca pital cost is approximately $200-$300/kW, given a 10-20 fold reduction in the membrane andcatalyst cost and also considering large scale productions.Therefore, the major challenge ahead is to find a low-cost catalyst,low-cost membrane and an efficient water management optionwithin the cell assembly.Transportation SeemDue to their high power de nsity, rapid startup and variable poweroutput, PEMFCs are suitable for use in the transportation sector.They are considered the best choice as far as present day availablefuel cell technologiw are concerned. Their high power densityand small size makes them primary candidates for light-dutyvehicles, though they are also used for heavy-duty vehicles.PEMFCs are also being developed for space and underwatertransportation applications.ALKALINE FUEL CELLS (AFCS)

and heat is difficult at these low temperatures. In space shuttles,closed loop hydrogen circulation as well as dielectric liquidcirculation is used for heat managem ent. Some of the terrestrialfuel cells are process air-cooled.Acceptable Contamination LevelAlkaline fuel cell can operate only with pure H, and pure 0,.Even a small level (


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advantage as far as fuel se lect ion is c~ nc er ne d ~ , ~ .he presentday estimated capital cost is $1500/kW but is expected to bereduced with improvements in technology.Transportation SectorAlthough the high power density and potential for intemal fuelprocessing of SOFC is very attractive from the transportation pointof view, the high operating temperature and long warm-up timerules out their use in vehicular applications.

DIRECT METHANOL-AIR FUEL CELLS (DMAFCS)DM AFC s are the least developed among all the fuel celltechnologies. Thou gh methanol itself has simpler storagerequirements that hydrogen lacks and is simpler to make andtransport, its electrochemical activity is on e-third that of hy drogen.Also, the conversion takes place at lower temperature (200C)compared to other hydrocarbons and therefore the contaminantproblem is higher than other fuel cells. The technology is

underdeveloped for present consideration.MOLTEN CARBONATE FUEL CELLS (MCFCS) SUMMARYBasic Operatinq PrincipleIn the molten carbonate fuel cell, a molten alkali carbonatemixture, retained in a porous lithium aluminate matrix, is used asthe electrolyte. Th e conducting species is carbonate ionsj. Theoperating temperature is in the range of 600 - 800C,high enoughto produce suitable qualities of w aste heat. This waste heat can beused for fuel processing and cogeneration, a bottoming cycle, orintemal reforming of methan e . MCFCs normally have 75% fuel(hydrogen) utilization.The advanced form of MCFC referred to as internal reformingmolten carbonate fuel cell (IRMCFC) may consume lowerhydrocarbons (CH,) directlyi6. It is intrinsically efficient since theheat produced at the anode is used for reformation ofhydrocarbons. Normally their efficiencies are 50% or higher.Acceptable Contamination LevelMCFCs do not suffer from CO poisoning and, in fact, can useCO in the anode gas as the fuel. They a re extremely sensitive (1ppm) to the presence of sulfur in the reformed fuel or oxidant gasstream. The presence of HCl, HF, HBr, etc. causes corrosion,while trace metals can spoil the electrodes. The presence ofparticulates of coay fine as h in the reformed fuel can clog the gaspassages.Technoloqical Statusapplication is in power generation and cogeneration.projected capital cost is ap proximately $lOOO/kW.

MCF C technologies are still under development. Their bestTh e

Transportation Sectoras yet.MCF Cs have not been considered for transportation application

l3Nguyen, Q. Minh, Ceram ic Fuel Cells, J. Am. Ceram. Soc., vol. 76,no . 3, 1993 pp. 563-588.Dollar, W.J. and Parker, W.J., Fuel Cell Technology - Into The OS,"4Ene rw World, vo l . 199 , June 1992 . pp . 11-14.

Terda, S . and Horiuchi, N., E valuation and Target Values forMaterials Used for Advanced M CFC Stacks, Conversion Technologies- Electrochemical Conversion in P roceedings of Th s 27Ih IntersocietyEnergy Conversion Engineering Conferen ce, JECEC 92, Sa n Diego,CA, vol. 3 1992, pp. 3.275 -3.279.I6Masauki, Miyazaki et. Al., Development of an Indirect IntemalReforming Molten Carbonate Fuel Cell Stack, ibid, pp. 3.287-3.292

A summary of the above discussion, operational characteristicsand technolog ical status of the six types of fuel cells is given inTable 1.For their acceptance into the transportation sector, fuel cells m ustmeet the demand s of rapid startup (preferably co ld), fast pickup,high power d ensity (light weight, compact), greater fuel efficiency,easy and safe handling, high lifetime and low cost (comparable togasoline). Unfortunately, none of the abov e fuel cell technologiescan satisfy all of these dema nds today. Howev er, among thepresent day available technologies are:. The PEMFC meets the demands of rapid startup,acceleration, high power density. But until its cost islowered by 10-20 times, it will not be economicallyacceptable. PEM FC w ill be best for light-duty vehicleapplications.. The PA FC is also suitable for vehicular application, butits application is limited to b uses and trucks because ofits size and weight. Also, battery support is needed foracceleration since it can only produc e a constant output.. Finally, the AFC stands a better chance fortransportation use as far as weight and startup areconcerned. Its use is limited because of it s inability totolerate CO, contamination in the fuel and oxidant.

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TABLE 1. OPERATIONAL CHARACTERISTICS AND TECHNOLOGICAL STATUS OF VARIOUS FUEL CELLS 'J'

Type of Operating, Choice of Oxidant Impurity Power Density P rojected Rated Fuel Lifetime Capital CostsFuel Temp("C:m Fuel (mW /cmz) Pow er Level Efficieucy ProjectedCell (present), (kW) (h) Projected

( $ k W )rojected

PAFC 160-200 hydrogen oxygen, CO,natural gas air HISmcthanol

PEMFC 80-90 hydrogen, oxygen, CO,methanol air H2S

SOFC 800-1000 hydrogen, oxygen, H Snatural gas, aircoal

AFC 60-90

MCFC 660

pure pure co2hydrogen oxygen

200,250 100-5000 40 >4Q,ooo 1000

350, >600 1-1000 45 >40,000 >200

240, 300 100-100,000 >50 r40,000 1500

100-200,>300 10-100 40 >10,000 >200

hydrogen oxygen, S,H,S, 100,>200 1000-100,000 50-75 >40,000 1000natural gas, air HC1, HI,coal

DMAFC not reported methanol _ __ not 40 , >I00 1-100reported 30 >10,000 >zoo

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current technology of fuel cells systems

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