14Fuel CellsXiao-Zi Yuan and Haijiang Wang

14.1Introduction

A fuel cell is an electrochemical device that produces electricity from an externallysupplied fuel and oxidant. Unlike the internal combustion engine (ICE), whichconverts the chemical energy of the fuel into mechanical energy, a fuel cell convertsthe fuel�s chemical energy directly into electric energy. The hydrogen–air fuel cell isthe most popular, using hydrogen as the fuel and oxygen from air as the oxidant.Besides hydrogen, other fuels, includingmethanol, ethanol, and natural gas, can alsobe used directly in fuel cells. The most common method of fuel cell classification isbased on the electrolyte used in the fuel cell. According to this system, fuel cells areusually classified into the five most common types: polymer electrolyte membranefuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), moltencarbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC).

Energy conversion within a fuel cell is realized through electrochemical reactions.The fuel is oxidized at the anode and gives up electrons, which travel through anoutside circuit to reach the cathode, where oxygen is reduced by the electrons to formwater. When electrons pass through an electric load that is connected to the outsidecircuit, electric power is generated. In a hydrogen–air fuel cell, water and heat are theonly byproducts, and therefore fuel cells are very environment-friendly powergeneration devices. Indeed, the fuel cell is very likely to replace the ICE in thefuture, due to the �greenhouse gas� effect and the ever-increasing pollution arisingfrom the combustion of fossil fuels. In addition, the energy conversion efficiencythrough a fuel cell is much higher than that through an ICE. A fuel cell has manyadvantages, such as silent operation, high power density, quick recharge (refueling),minimal maintenance, and broad application. Such features have been the drivingforce behind the extensive worldwide research activities into fuel cell technologyduring the past two decades.

However, the fuel cell is, in fact, not a new technology. As early as 1839, WilliamRobert Grove, the father of the fuel cell, discovered that reversing the electrolysis of

Handbook of Combustion Vol.1: Fundamentals and SafetyEdited by Maximilian Lackner, Franz Winter, and Avinash K. AgarwalCopyright � 2010 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-32449-1

j333

water could produce electricity when the correct catalysts were used. A few years later,he developed a bank of 50 fuel cells, which he called the �gaseous voltaic battery� [1].Unfortunately, this great discovery did not lead to any further development for almosta century because the extremely low power density limited the cells� practicalapplications. This impasse was not broken until 1937, when Francis T. Bacon startedto develop fuel cells that could have practical applications. In 1959, he successfullydeveloped and demonstrated a 5 kW fuel cell that powered a welding machine, acircular saw, and a forklift [2]. At approximately the same time, Willard ThomasGrubb and Leonard Niedrach at General Electric (GE) began the development ofpolymer electrolyte membrane (PEM) fuel cells. The successful application of fuelcell technology in the 1960s spacemissionwas nodoubt an immense boost to fuel celldevelopment. Thenewera of fuel cell technology development began in the late 1980sand early 1990s, when Ballard Power Systems made rapid breakthroughs with theirPEM fuel cell technology. Since then, fuel cell technology development, demonstra-tion, and commercialization have been advancing rapidly, and a whole new fuel cellindustry has emerged [1].

14.2Theory

14.2.1Principles

To elucidate the principles of fuel cells, the PEM fuel cell can be taken as an example.Figure 14.1 illustrates the key components and structure of a PEM fuel cell and itsoperational principle.

Figure 14.1 Diagram of the polymer electrolyte membrane fuel cell principle. CL: catalyst layer;GDL: gas diffusion layer. Modified from Ref. [3], with permission from Elsevier.

334j 14 Fuel Cells

As shown in Figure 14.1, the key part of a single PEM fuel cell, known as themembrane electrode assembly (MEA), consists of a PEM with an anode catalystlayer (CL) and a cathode CL on either side. Adjacent to each CL is a gas diffusionlayer (GDL). The PEM functions as a proton conductor and a separator of gases. Onthe anode side of the cell, hydrogen fuel is delivered to the anode through theflow channels of the anode plate. Similarly, on the cathode side of the cell,oxygen from the air is delivered to the cathode through the flow channels of thecathode plate.

At the anode, the hydrogen oxidation reaction (HOR) occurs:

H2 ! 2Hþ þ 2e� ð14:1ÞAt the cathode, the oxygen reduction reaction (ORR) takes place:

1=2O2 þ 2Hþ þ 2e� !H2O ð14:2ÞThe overall reaction of the fuel cell is:

H2 þ 12O2 !H2O= ð14:3Þ

Under standard conditions, the anode potential is E0a ¼ 0:00 V versus standard

hydrogen electrode (SHE), while the cathode potential is E0c ¼ 1:229 V versus SHE.

Therefore, the theoretical cell voltage under standard conditions can be calculated asE ¼ E0

c�E0a ¼ 1:229 V. Under other conditions, the theoretical cell voltage can be

expressed as:

E ¼ 1:229�0:85� 10�3ðT�298:15Þþ 4:3085� 10�5T ½ln ðpH2Þþ 1=2 ln ðpO2Þ�ð14:4Þ

where T is the temperature in Kelvins, and pH2 and pO2 are the partial pressure (inatm) for hydrogen and oxygen, respectively. The actual value of the cell voltage isalways lower than the theoretical value due to the combined effects of fuel crossover(hydrogen permeates through the electrolyte to the cathode) and parasitic oxidationreactions occurring at the cathode.

Single cells produce less than 1Vof electricity, which is far from enough to power avehicle. To generate a useful voltage, multiple cells must be assembled into a fuel cellstack. This can be achieved in a parallel and/or a seriesmode to supply feed gas to thestacks. For example, in the case of a parallel gas supply for a PEM fuel cell stack, allcells are fed inparallel froma commonhydrogen/air inlet. In the serial configuration,the gas from the outlet of the first cell is fed to the inlet of the second cell, and so on,until the last cell, which helps to prevent nonuniform gas distribution. In order toavoid a large pressure drop, this arrangement can be used only for stacks with a smallnumber of fuel cells. In addition to the stack, practical fuel cells such as those in fuelcell vehicles (FCVs) require several other subsystems and components to function asa system. Generally speaking, most fuel cell systems contain subsystems forprocesses such as hydrogen reforming or hydrogen purification, air supply (whichincludes air compressors or blowers as well as air filters), water management, andthermal management [1].

14.2 Theory j335

14.2.2Thermodynamics

14.2.2.1 Heat of ReactionThe overall reaction of a fuel cell described in Equation 14.3 is an exothermic process:

H2 þ 1=2 O2 !H2OðlÞþ 286 kJmol�1 ð14:5ÞThe heat or enthalpy change of this reaction is calculated by the difference in the

enthalpy change of formation between the products and reactants. Equation 14.5 isvalid only at 25 �C, and the value of 286 kJmol�1 is known as hydrogen�s higherheating value (HHV). The heat of this reaction will become 242 kJmol�1, which isknown as hydrogen�s lower heating value (LHV), if hydrogen is combusted withoxygen and produces water vapor (101.3 kPa) at 25 �C [4].

14.2.2.2 Energy EfficiencyEngine efficiency or, more broadly, the efficiency of an energy conversion device, canbe defined in a number of ways, but it is usually done by comparing the useful energyoutput with the energy input. As the ICE is a heat engine, its efficiency is limited bythe Carnot cycle, with the overall efficiency being determined by the differencebetween the lower and upper operating temperatures of the engine. For example, ifthe upper temperature of the heat engine isT1 and the lower operating temperature isT2 (which is assumed to be not lower than room temperature), then the Carnotefficiency, which defines the engine�s maximum efficiency, can be calculated by:

Carnot efficiency ¼ T1�T2

T1� 100% ð14:6Þ

where both the temperatures are in Kelvins. Thus, the greater the temperaturedifference between the upper and the lower operating temperatures, the greater thethermodynamic efficiency. Therefore, a high thermal stability of the engine materialcan allow a higher upper operating temperature, and thus a higher efficiency. For asteam turbine operating at 300 �C (573K) with water exhausted at 50 �C (323K), theCarnot efficiency is 44%. Usually, the actual efficiency of an ICE is much lower thanthe Carnot efficiency due to its nonideal thermodynamic process. As a result, theaverage efficiency of ICEs is about 18–20%.

For a fuel cell, the useful energy output is the electrical energy produced, and theenergy input is the heat of the hydrogen combustion reaction. The heat of thisreaction at 25 �C is 286 kJmol�1 (HHV) or 242 kJmol�1 (LHV). Assuming that all oftheGibbs free energy can be converted into electrical energy, the theoretical efficiencyof a fuel cell using the HHV is:

g ¼ DG0

DH0¼ 237:1 kJmol�1

286 kJmol�1 ¼ 83%ð25 �C; 1 atmÞ ð14:7Þ

where DG0 is the change in the Gibbs free energy of the reaction (the differencebetween the Gibbs free energy of the products and of the reactants) under standard

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conditions, and DH0 is the standard enthalpy change of this reaction (25 �C, 1 atm),which is the heat of the reaction under standard conditions. If the numerator anddenominator in Equation 14.7 are both divided by nF, then the fuel cell efficiency canbe expressed as a ratio of two potentials:

g ¼ DG0

DH0¼

�DG0

nF�DH0

nF

¼ 1:231:48

¼ 83% ð25 �C; 1 atmÞ ð14:8Þ

where 1.23V is the theoretical cell voltage and 1.48V is the thermal-equivalent voltagecorresponding to hydrogen�sHHV (this value becomes 1.25V if based on hydrogen�sLHV). Note that the voltage of the fuel cell is related to the change in the Gibbs freeenergy of the reaction:

DG0 ¼ �nFE ð14:9Þwhere F is the Faraday constant (96 485Cmol�1), E is the theoretical cell voltage (alsoknown as reversible cell voltage), and n is the number of electrons transferred in thereaction.

Figure 14.2 compares the theoretical efficiency of a fuel cell with the Carnotefficiency at the standard pressure, but at different temperatures. It can be clearlyseen from the figure that as the operating temperature increases, the Carnotefficiency increases but the efficiency of the fuel cell decreases. At about 700 �C,the two types of energy conversion device have the same theoretical efficiency. Above700 �C, the combustion engine is more efficient, but below 700 �C the fuel cell

20

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40

50

60

70

80

90

100

10009008007006005004003002001000

Temperature (°C)

Th

eore

tica

l eff

icie

ncy

(%

)

Fuel cell vapor

product

Carnot efficiency, 50°C

exhaust

Fuel cell liquid product

Figure 14.2 Theoretical efficiency of aH2 fuel cell at standard pressure basedonHHV. Reproducedfrom Ref. [5], with permission from John Wiley & Sons Inc.

14.2 Theory j337

efficiency is higher than the Carnot efficiency, and the lower the temperature, thegreater the efficiency difference.

The actual efficiency of a fuel cell is defined as the actual voltage divided by thethermal-equivalent voltage [5]:

Cell efficiency ¼ mfVcell

1:48� 100% ðbased on HHVÞ ð14:10Þ

and

Cell efficiency ¼ mfVcell

1:25� 100% ðbased on LHVÞ ð14:11Þ

where mf is the fuel utilization coefficient. The addition of this coefficient inEquations 14.10 and 14.11 is necessary because a small portion of the fuel fed tothe cell simply passes through the cell unconsumed. The fuel utilization coefficient isdefined as:

mf ¼Fuel reactedFuel input

ð14:12Þ

The fuel utilization depends on the design of the fuel cell. For open-end andperiodic purge designs, fuel utilization can reach about 90%, whereas for dead-endand fuel recycling designs it can reach almost 100%.

As will later become apparent, the actual cell voltage changes with the load, as doesthe actual cell efficiency. For PEM fuel cells intended for vehicles, the nominaloperational current density is approximately 1Acm�2, at which the cell voltage isabout 0.65V. Thus, the fuel cell efficiency under the nominal operational condition isabout 44%. Since vehicles operate at different loads, on average the actual efficiencycan be higher than 50%, based on current PEM fuel cell technology. The target forcommercial fuel cell products for transportation applications has been set by theUnited States Department of Energy (DOE) at 60%.

14.2.3Reaction Kinetics

14.2.3.1 The Butler–Volmer EquationThe reactions in a fuel cell are typical electrochemical reactions. The reaction kineticscan be described by the Butler–Volmer equation, which relates the current to theelectrode potential:

i ¼ i0 exp�anFgRT

� ��exp

ð1�aÞnFgRT

� �� �ð14:13Þ

where

i ¼ the current density (A cm�2)i0 ¼ the exchange current density (A cm�2)R ¼ the universal gas constant (8.314 Jmol�1 K)T ¼ the temperature (K)F ¼ the Faraday constant (96 485Cmol�1)

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n ¼ the electron transfer numbera ¼ the transfer coefficientg ¼ the overpotential (V)

The exchange current density is a measure of how quickly the redox coupleexchanges electrons with the electrode at equilibrium, which is dependent on manyfactors, including: the nature of the redox couple; the properties of the electrode; themedium in which the reaction takes place; and the reaction conditions. The higherthe exchange current density, the faster the electrode kinetics. On a platinumelectrode in a PEM fuel cell, the exchange current density for the HOR is about0.1 A cm�2, and for the ORR about 6mAcm�2 [6]. The HOR is clearly much fasterthan the ORR, which is why in a PEM fuel cell the cell polarization is assumed to beentirely due to the cathode reaction. The transfer coefficient is a measure of theposition of the activated complex along the reaction coordinates for an elementaryelectron transfer reaction, which has the value of 0.5 for most such reactions. Forcomplicated electron transfer reactions, such as the HOR and ORR in fuel cells thatinvolve many elementary reaction steps, the transfer coefficient has lost its originalphysical meaning. It may involve many processes, such as the adsorption equilib-rium constant of the reactants and the desorption equilibrium constant of theproducts. It has been found that on a platinum electrode the transfer coefficientis a temperature-dependent parameter forO2 reduction, but forH2 oxidation it seemsto be independent of temperature; a value of 0.5 has been widely reported [1].

At low overpotential, the Butler–Volmer equation can be simplified as:

�g=i ¼ RTnFi0

¼ Rct ð14:14Þ

where Rct is the charge transfer resistance, an important kinetic parameter thatdescribes the speed of the electrode reaction. The current–potential relationshipdescribed in Equation 14.14 indicates that when the overpotential is low, it is linearlydependent on current density.

At very high overpotential, the Butler–Volmer equation can be simplified in adifferent form:

g ¼ aþ b log i ð14:15Þwhere

a ¼ 2:303RTanF

log i0 ð14:16Þ

b ¼ � 2:303 RTanF

ð14:17Þ

Equation 14.15 is the well-known Tafel equation, an empirical expression relatingthe overpotential to the current for an electrode reaction. The Tafel current–potentialrelationship indicates that, when the overpotential is high it is linearly dependent onthe logarithmof the current density. The slope is called theTafel slope, and the steeper

14.2 Theory j339

it is, the slower the reaction kinetics. For ORR in a PEM fuel cell, the Tafel slope isabout 70mVper decade at 80 �C, a typical operational temperature for PEM fuel cells.

14.2.3.2 Polarization CurveThemost common approach to characterize the performance of a fuel cell is to recordthe polarization curve [voltage–current (V–I) curve] by measuring the cell voltage atdifferent current densities, or vice versa. Figure 14.3 shows the typical polarizationcurves of a PEMFC designed by the National Research Council of Canada�s Institutefor Fuel Cell Innovation (NRC-IFCI), measured at different operating temperatures.TheMEAused for this fuel cell wasmade of SGLGDL and Gore 5510 catalyst-coatedmembrane (CCM). The cell was operated under the conditions of zero back-pressure,100% humidification for both anode and cathode, a constant hydrogen flow rate of 5SLPM (standard liter per minute), and a constant air flow rate of 25 SLPM.

As can be seen from Figure 14.3, the cell voltage drops when current densityincreases such that, at a specific current density, the higher the cell voltage the betteris the fuel cell�s performance. As the power output is the product of the voltage andthe current, the power density of the fuel cell can easily be plotted from thepolarization curves. Unlike the polarization curves, that monotonically drop withincreasing current density, the power density curves always go through a maximum(usually the nominal design point), at which the fuel cell has the highest poweroutput.

As operating temperature increases, cell voltage rises, even though there is a slightdecrease in the theoretical cell voltage, as mentioned above. This result is dueto increased electrode kinetics, a higher proton conductivity of the membrane,

Figure 14.3 Polymer electrolyte membrane fuel cell polarization curves.

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increased mass transport, and better water management. Therefore, a high-temper-ature PEM fuel cell is quite appealing, provided that the component materials aredurable.

14.2.3.3 Voltage LossesThe voltage loss (voltage drop from the open circuit voltage; OCV), shown inFigure 14.3, is due to the three distinct electrode processes that occur in the fuelcell when current density increases:

. Activation polarization: This refers to an overpotential arising from electrodekinetics, and is mainly dependent on the CL structure.

. Ohmic polarization: This refers to the voltage drop due to ionic and electricresistances; the major contributors to ohmic polarization are membrane resis-tance and contact resistances between the layers.

. Concentration polarization: This is due to mass transport limitations; the GDLstructure, flow field design, and water management are the most importantfactors affecting concentration polarization.

The three polarizations dominate in different current density regions, so that thepolarization curve is usually divided into three regions. As shown in Figure 14.3, atlow current densities (e.g.,<200mAcm�2), the cell voltage drops exponentially withthe current density, due mainly to the sluggish kinetics of the ORR. In the middleregion (i.e., current density between 200 and 1000mAcm�2), the voltage dropsalmost linearly; the voltage loss in this region ismainly caused by ohmic resistance. Athigh current densities (e.g.,>1000mAcm�2) a sharp voltage drop is observable, dueto themass transport limitations of the reactant gas through the pore structure of theGDLs andCLs [1]. In summary, the output voltage of an operating single cell,Ecell, canbe expressed as:

Ecell ¼ EOCV�gact�gohmic�gcon ð14:18Þwhere Ecell is the cell voltage under a certain operating condition, EOCV represents thefuel cell�sOCV,gact is the kinetic loss (activation polarization),gohmic is the ohmic loss(ohmic polarization), and gcon is themass transport loss (concentration polarization).For a typical PEM fuel cell designed for transportation applications, the kinetic lossrepresents the biggest voltage loss under nominal operating conditions. Therefore,improving catalyst activity can effectively improve fuel cell performance.

14.3Types of Fuel Cell

To date, several different types of fuel cell have been developed, ranging from low-temperature to high-temperature, and from solid polymer electrolyte to ceramicelectrolyte. The major types of fuel cell are PEMFC, AFC, PAFC, MCFC, and SOFC,based on the electrolyte used, and the optimum operating temperatures of these fuelcells are shown in Figure 14.4. In general, the PEMFC, AFC, and PAFC are operated

14.3 Types of Fuel Cell j341

at temperatures less than 300 �C (hence, they are called low-temperature fuel cells),whereas MCFC and SOFC are operated at high temperatures (hence high-temper-ature fuel cells). The advantages and disadvantages of these fuel cells are compared inTable 14.1.

14.3.1PEMFCs

The key components of a PEMFC, as seen in Figure 14.1, include the membrane,CLs, GDLs, and flow field plates. The materials for these key components in state-of-the-art PEM fuel cells are summarized below.

Figure 14.4 Fuel cell types and their optimum operating temperatures [7].

Table 14.1 Advantages and disadvantages of different fuel cell types.

Fuel cell type Advantages Disadvantages

PEMFC . Rapid start-up . Noble metal catalyst required. High power density . Gas humidification required. Simple construction . Low contaminant tolerance. Easy operation. Wide applications

AFC . Fast ORR kinetics, so non-noblemetal catalyst can be used

. CO2 contamination

. Strong corrosive electrolytePAFC . High CO tolerance . Strong corrosive electrolyte

. Suitable for electricity and heatcogeneration

. High material stability required

. Limited to stationary applicationsMCFC . Fuels other than hydrogen can be

used directly

. Severe materials requirements

. Noble metal catalysts not required

. Increased corrosion

SOFC . Fuels other than hydrogen can beused directly

. Slow start-up

. High power density . High material stability required

. Noble metal catalysts not required . Complicated construction

. Suitable for electricity and heatcogeneration

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The Nafion� membrane, a sulfonated perfluorinated polymer membrane pro-duced by DuPont, is the most commonly used PEM for this type of fuel cell [8]. Thewater content of the membrane drastically affects its conductivity, causing a drasticincrease in proton conductivity. At 70 �C, when the membrane is fully humidified,the conductivity approaches 0.1 S cm�1. This water-content-dependent conductivityof the Nafionmembrane limits the PEMFC�s operating temperature to below 100 �C,whilst a need for external humidification complicates the fuel cell system. In order toreduce the membrane�s ionic resistance, thin membranes in the range of 18–25mmare commonly used. Attempts have been made to raise the PEM fuel cell operatingtemperature up to 300 �C by replacing the Nafion membrane with an acid-dopedpolybenzimidazole (PBI) membrane. However, the limited durability of the PBI fuelcell makes its future uncertain. Strategies used by the DOE to develop high-temperature membranes have included the development of novel hydrocarbonmembranes and the modification of perfluorinated membranes.

The PEMFC requires a noble metal catalyst such as Pt for the anode and cathodereactions, and carbon-supported platinum appears to be the only catalyst suitable forhigh-power density PEM fuel cells. Decades of effort have led to a significantreduction in the loading of the platinum catalyst, from �10mg cm�2 to �0.4mgcm�2, while at the same time the cell performance has greatly improved. Yet, even atthis low platinum loading the catalyst constitutes 55% of the fuel cell system cost,such that its replacement is the most important area of research in PEMFCdevelopment. Although numerous Pt alloy catalysts have been studied, few haveexhibited catalytic activity comparable to platinum. The use of various nonpreciousmetal catalysts has also been reported, but their catalytic activities have been too low toenable any practical applications of PME fuel cells in the near future.

TheGDLused for PEM fuel cells is usually a carbon-based porousmaterial, such ascarbon fiber paper or carbon cloth, about 0.1–0.5mm thick. Typically, this has a dual-layer structure, comprised of a macroporous carbon substrate and a thin micropo-rous layer. The GDL plays several roles: collecting current; physically supporting thecatalyst layer; and providing the transport media for gases, water, and heat.

Another key component of PEM fuel cells is the bipolar plate. This is commonlymade from carbon composite or stainless steel, though each material has its ownadvantages and it is difficult to predict which will prevail commercially. Thefunctions of the bipolar plate include separating the reactant gases, providing flowfields for the reactant gases and coolant, and transporting water and heat. The flowfield design of bipolar plates can significantly affect PEM fuel cell performance.Common flow field designs include serpentine channel, straight channel, andinterdigitated flow fields.

14.3.1.1 H2/Air PEMFCsA H2/air PEMFC system consists of the PEMFC stack and the balance of plant,including the hydrogen system, air system, anode and cathode humidifier, andcooling system. During operation, hydrogen is introduced into the anode by thehydrogen system, while air is introduced into the cathode by the air system. Thehydrogen gas for PEMFCs must be quite pure, containing <10 ppm CO. A state-of-

14.3 Types of Fuel Cell j343

the-art PEM fuel cell can produce power of about 0.65Wcm�2 active area; hence,small PEM fuel cells can be fabricated that produce several watts of power, and largercells that produce megawatts. The PEM fuel cell has very broad applications,including vehicles, stationary power sources, and portable electronics.

14.3.1.2 Direct Liquid Fuel Cells (DLFCs)Liquid fuels such as methanol, ethanol, and formic acid have much higher energydensities than hydrogen. As these fuels can be fed directly to the anode of a PEM fuelcell to produce power, they are termeddirect liquid fuel cells (DLFCs), and include thedirect methanol fuel cell (DMFC), direct ethanol fuel cell (DEFC), and direct formicacid fuel cell (DFAFC). As the direct oxidation of liquid fuel at the anode is kineticallya very difficult electrode process, the DLFCs typically operate at very low currentdensities. Therefore, unlike the H2/air fuel cell, DLFCs are usually used to generatepower in the range of several watts to several hundreds of watts. Interest in DLFCsstems from the demand for high-power batteries in increasingly sophisticatedportable electronics. For example, a DMFC can generate five times the power densityof the most advanced lithium ion battery, and it can be �recharged� in a second (i.e.,refueled by replacement with a new fuel cartridge).

The materials used in DLFCs are very similar to those in H2/air PEMFCs, exceptthat a much higher platinum catalyst loading is needed at the anode to facilitate theliquid fuel oxidation reaction. In terms of design, the DLFC stack design is alsosimilar to that of H2/air PEMFCs. A DLFC does not require external humidificationbecause an aqueous solution is used for the anode reaction; nor does it require acooling system, as so little heat is generated. A DLFC often uses air-breathing ratherthan a forced air flow.

The electrochemical reactions in a DMFC are as follows:

Anode reaction : CH3OHþH2O!CO2 þ 4Hþ þ 6e� ð14:19Þ

Cathode reaction : O2 þ 4Hþ þ 4e� ! 2H2O ð14:20Þ

Overall reaction : 3=2O2 þCH3OH! 2H2OþCO2 ð14:21ÞThe theoretical cell voltage of a DMFC under standard conditions is 1.20 V.

Currently, the DMFC is the most promising DLFC and has attracted the mostresearch attention; however, its widespread use has raised concerns due to the toxicityof methanol.

Ethanol, on the other hand, is a very safe, renewable biofuel that is ideal forDLFCs,although the anode oxidation of ethanol is more challenging than that of methanol.The electrode reactions of a DEFC are as follows [9]:

Anode reaction : CH3CH2OHþ 3H2O! 2CO2 þ 12Hþ þ 12e� ð14:22ÞCathode reaction : 3O2 þ 12Hþ þ 12e� ! 6H2O ð14:23Þ

Overall reaction : CH3CH2OHþ 3O2 ! 2CO2 þ 3H2O ð14:24Þ

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The theoretical cell voltage of a DEFC under standard conditions is 1.145 V. So far,the DEFC has proved to be less popular than the DMFC because an effective anodecatalyst has not yet been found; however, if a breakthroughwere to occur then the useof DEFCs would surely surpass that of DMFCs.

In the case of a DFAFC, the electrode reactions are as follows:

Anode reaction : HCOOH!CO2 þ 2Hþ þ 2e� ð14:25Þ

Cathode reaction : 3O2 þ 12Hþ þ 12e� ! 6H2O ð14:26Þ

Overall reaction : HCOOHþ 1=2 O2 !CO2 þH2O ð14:27ÞThe theoretical cell voltage of a DFAFC under standard conditions is 1.4 V [10]. As

can be seen from the above reactions, water is not involved in the anode reaction of aDFAFC, and therefore the fuel concentration can be as high as 90%. Thus, the powerdensity of a DFAFC is much greater than that of a DMFC or DEFC. In addition, theoxidation reaction for formic acid is much faster. Whilst these are attractivefeatures [11, 12], formic acid is toxic (like methanol) and is also highly corrosive,which consequently greatly limits DFAFC applications.

14.3.2Alkaline Fuel Cell (AFC)

The AFC uses concentrated KOH solution absorbed into a porous matrix as theelectrolyte, and therefore charge transport within the electrolyte is effected by themovement of OH� ions from the cathode to the anode. WhenH2 and air are used asfuel and oxidant, respectively, the efficiency of the AFC may be as high as 60%. TheAFC can operate over a wide temperature range, from sub-zero degrees Celsius toabout 250 �C. The operational principle of an AFC, as well as its cell structure and keycomponents, are shown in Figure 14.5 [13].

The electrochemical reactions of an AFC are as follows:

Cathode reaction : O2 þ 2H2Oþ 4e� ! 4OH�E0 ¼ 0:401 V ð14:28Þ

Anode reaction : H2 þ 2OH� ! 2H2Oþ 2e� E0 ¼ �0:828 V ð14:29Þ

Overall reaction : O2 þ 2H2 ! 2H2O Ecell ¼ 1:229 V ð14:30ÞThe theoretical cell voltage is the same for both AFCs and PEMFCs, but in an

alkaline environment theORR ismuch faster. Therefore, awide range of catalysts canbe used in AFCs, since not only platinum groupmetals but also low-cost metals suchas Ni and Ag can catalyze the ORR. The fatal drawback of the AFC is its sensitivity toCO2; exposure to CO2 will gradually convert the potassium hydroxide into potassiumcarbonate, thereby eliminating the electrolyte�s ionic conductivity. In order to avoiddamage from CO2, both the fuel and air must be vigorously scrubbed. Cell designsusing a circulating electrolyte have also been applied to prevent destruction of the

14.3 Types of Fuel Cell j345

electrolyte by CO2. In addition to the CO2 problem, the highly corrosive electrolytemakes the chemical stability of the cell components a major issue.

The AFCwas selected by the US space program during the 1960s, and remained avery competitive technology throughout the 1970s and 1980s. Subsequently, how-ever, due to the rapid advancement of PEMFCs, AFCs have fallen out of favor.Nonetheless, this type of cell does have its advantages and still has the potential tosucceed in certain applications, as it can provide high power density and has a longlifetime [13].

14.3.3Phosphoric Acid Fuel Cell (PAFC)

Theelectrolyteused inaPAFC isphosphoric acid immobilizedwithinaporousmatrix(e.g.,aTeflon-bondedsiliconcarbide).APAFCoperatesbetween150and200 �C,whilethe electrode reactions are the same as those that occur in a H2/air PEMFC. Thetechniques for fabricating PAFC electrodes borrow heavily from AFC technology.During the early research stages, noblemetal blacks (unsupportednoblemetals)wereused as the electrode catalyst, but currently carbon-supported Pt or Pt alloy electro-catalysts are commonly used for both the electrodes, so as to reduce the catalystloadings. As in theH2/air PEMFCs, porous carbonfiber paper and graphite plates arecommonlyusedas thegasdiffusion layerand thebipolarplates, respectively.ThePEMfuel cell with the PBI membrane is actually a reproduction of the PAFC design [14].

Figure 14.5 Alkaline fuel cell structure. Reproduced from Ref. [13], with permission fromInternational Journal of Hydrogen Energy.

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Unlike PEMFCs, PAFCs are much less susceptible to CO impurities in thehydrogen stream. In addition, the operating temperature of 150–200 �C reducesthe complexity of the power plants and makes the fuel cell suitable for cogeneration.However, the electrolyte�s corrosive nature and the high operating temperatureintroduce more material challenges [7].

The PAFC is a relativelymature technology, andwas the first fuel cell technology tobe commercialized. The PC-25 PAFCmanufactured by UTC Fuel Cells, a division ofUnited Technology Corporation, was the first available commercial fuel cell unit, andserved as a model for fuel cell applications. The PC-25 has been installed in a widevariety of environments, including hospitals, hotels, large office buildings,manufacturing sites, and wastewater treatment plants [15].

14.3.4Molten Carbonate Fuel Cell (MCFC)

The MCFC operates at approximately 650 �C in order to achieve sufficient conduc-tivity in the carbonate electrolyte. However, low-cost metal cell components must beused in such heat. The high operating temperature offers several advantages,including a high efficiency, the direct use of various fuels, and no need for noblemetal catalysts in the electrochemical oxidation and reduction reactions. However, amajor challenge for MCFCs is electrolyte management to retain long-term perfor-mance.Other issues include hardware corrosion, cathode dissolution, and lowpowerdensity.

The state-of-the-art cell structure of a MCFC is depicted in Figure 14.6, along withthe anode and cathode reactions. The overall cell reaction of a MCFC is:

H2 þ 12 O2 þCO2 !H2OþCO2= ð14:31Þ

During the mid-1960s, precious metals were frequently used as electrode materi-als, but soon were replaced by Ni-based alloys at the anode, and oxides at the cathode.Themajor challenges with Ni-based anodes and NiO cathodes are structural stability(sintering and mechanical deformation of the porous Ni-based anode under com-pressive load leads to performance decay) and NiO dissolution. Since themid-1970s,the materials for the electrodes and electrolyte (molten carbonate/LiAlO2) haveremained the same. One important achievement in MCFC technology during the1980s was an improvement of the electrolyte structure such that, over the past thirtyyears, the performance of a single cell has increased from about 10mWcm�2 to>150mWcm�2. Typical MCFCs will generally operate in the range of 100–200mAcm�2 at a cell voltage of 750–900mV.

MCFCs have been developed for natural gas and coal-based power plants usedin industrial, electrical utility, and military applications. To date, MCFC stackswith cell areas up to 1m2 and with a power output of over 250 kW have beendeveloped and produced, for example, the model FCE-300 (manufactured by FuelCell Energy)[15].

14.3 Types of Fuel Cell j347

14.3.5Solid Oxide Fuel Cells (SOFCs)

SOFCs operate at 600–1000 �C. The electrolyte consists of solid oxides throughwhichoxide ion transports the current flow. SOFCs are composed entirely of solid-statematerials, the most commonly used solid electrolyte being yttria-stabilized zirconia(YSZ), an ionically conducting oxidemembrane. The typical anode isNi-ZrO2 cermet(e.g., YSZ/Ni), and the typical cathode is a perovskite mixed conductor (e.g.,LaxSr1–xMnO3, LaxSr1–xFeO3, LaxSr1–xCrO3). SOFCs have two types of cell designs,namely tubular and planar. Three examples of tubular cell design in SOFCs, a uniquefeature of these cells, are shown in Figure 14.7a–c [15].

Due to their high efficiency, low sensitivity to fuel impurities, and fuel flexibil-ity [16], SOFCs have been used in large-scale stationary power plants, smaller home-scale power plants, and portable power generators [17], with a capacity range of 2 kWto 100 MW. Although an SOFC system is not the first choice for transport applica-tions, small-scale systems have been developed for use as auxiliary power units incars [18]. Unfortunately, the high operating temperature of SOFCs requires highly

Figure 14.6 State-of-the-art cell structure of a molten carbonate fuel cell [15].

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stable materials, whilst high cost and low durability/reliability represent the twomajor barriers for their commercialization. Thus, the key technical challenges forSOFCs today are to develop suitable low-cost materials and low-cost fabricationprocesses for ceramic materials. While PEMFC research is heading towards high

Figure 14.7 Three examples of tubular solid oxide fuel cell design [15]. (a) Current flow around thetube; (b) Current flow along the tube; (c) Segmented in series.

14.3 Types of Fuel Cell j349

operating temperatures (>100 �C), SOFC research is attempting to reduce theoperating temperature (to 500–850 �C). Low-temperature SOFCs have many advan-tages, such as awider choice of low-cost componentmaterials, improved stability, andincreased design flexibility [19].

14.4Fuel Cell Applications

Since the start of the twentieth century, ICEs have been used to power vehicles andgenerators to generate electricity. Attempts to apply fuel cells in power generationbegan over twenty years ago. Because fuel cells can generate power over a wide range,from a fraction of a watt to hundreds of kilowatts, they can be used in almost anyapplication. For example, fuel cells have been applied to local distribution powerstations (>1MW), to large transportation vehicles such as submarines and buses(100 kW–1MW), to transportation vehicles such as cars and motorcycles, to back-uppower (1 kW–100 kW), to simple riding devices such as bicycles, scooters, andwheelchairs (1 kW–10 kW), to uninterrupted power supply (UPS) (100W–1 kW),and to portable power devices, ranging from military equipment to cell phones(<100W) [1].

Aside from the specialized examples of fuel cells in military and space programs,fuel cell usages can be categorized into four main groups: stationary; transportation;back-up; and portable power. PEMFCs can be utilized in all of these fields, especiallytransportation, because the cells can be fabricated for various power ranges. Indeed,numerous prototype automobiles, buses, utility vehicles, scooters, and bicycles havealready been developed using PEMFCs as power sources. The use of DMFCs inportable electronics, such as laptop computers, video cameras, and mobile phones,has also been demonstrated [18]. Fuel cells in stationary power generation offertremendous flexibility in power supply, from individual homes or complex buildingsto entire communities. PAFCs have been favored for stationary applications with acombined heat and power cogeneration. Fuel cells are more attractive as back-uppower generators than ICE generators (due to noise, fuel, reliability, and mainte-nance considerations) or batteries (due to weight, lifetime, and maintenance con-siderations), while small fuel cells used as portable power generators offer severaladvantages over conventional batteries [1]. Some examples of these fuel cell applica-tions are shown in Figure 14.8.

In addition to the principal application of generating electricity for variouspurposes, fuel cells have one other special feature – namely, that they can beused to create useful materials at the fuel cell anode or cathode, while simulta-neously producing power, rather than consuming it. The principle of a fuel cellreactor that concurrently produces value-added chemicals and energy is shown inFigure 14.9. In this case, a chemical and electricity cogeneration system mainlyconsists of a conventional fuel cell or fuel cell reactor, an external load, and asubsystem to recover the product chemicals. From the economic and/or environ-

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mental point of view, this fuel cell application could become commercially attractivefor industries.

By using fuel cells, a variety of chemicals can be produced including inorganicmaterials and organic compounds (e.g., hydrocarbons, benzene, alcohols, ketones,and their derivatives). Reactions in fuel cells involve hydrogenations [22–24],dehydrogenations, halogenations, and oxidations, and these reactions are normallyquite selective. Consequently, hydrogen peroxide and valuable organic chemicals canbe obtained from PEMFCs, while AFCs can also be used to produce hydrogenperoxide. The selective oxidation of hydrocarbons and aromatic compounds, and theproduction of industrial compounds such as cresols, have been reported for PAFCs,whilst acetaldehyde with high product selectivity from ethanol oxidation can beachieved by using MCFCs. High yields of valuable industrial inorganic compoundssuch as nitric oxide can be produced with SOFCs [21, 25].

Figure 14.8 Examples of fuel cell applications.(a) Fuel cell buses: BallardMark 902� FC stack;(b) Stationary fuel cell: EBARA Ballard 1 kWJapan Cogeneration System; (c) Backup power:

the air-cooled Ballard Mark 1020 ACS fuel cellstack; (d) Lift trucks: BallardMark 9 SSL fuel cellstack [20]. Image courtesy of Ballard PowerSystems, Inc.

14.4 Fuel Cell Applications j351

Figure 14.9 Principle of a fuel cell reactor for chemical and energy cogeneration. Reproduced fromRef. [21], with permission from Elsevier.

14.5Outlook

Fuel cells have the ability to directly convert the chemical energy of fuels such ashydrogen, methanol, ethanol, formic acid, and methane into electricity. Thesetechnologies have several advantages over direct combustion devices, including highefficiency, low/zero emissions, and high power density, and such features will surelylead to fuel cells becominghighly competitivewith ICEs in the future. In addition, therapid depletion of fossil fuels, coupled with increasing concerns over global warmingand environmental pollution, are making powerful calls for an accelerated commer-cialization of fuel cell technology. Indeed, today almost all major automotivecompanies are developing FCVs. Demonstrations of fuel cell technology relating totransportation and other areas have been quite successful, with two examples ofhydrogenandfuel celldemonstrationprojectsbeing theHydrogenHighway inBritishColumbia, Canada, and the California Hydrogen Highway Network [1]. These twodemonstration and market development programs are aimed at promoting theapplication of fuel cell technology by providing a vehicle-fueling infrastructure.Today, several barriers to the commercialization of fuel cell technology remain, the

main one being a lack of any hydrogen infrastructure, including hydrogen produc-tion, transport, and storage, and fueling stations. In order to develop a hydrogeninfrastructure similar to the current fossil fuel infrastructure will require not only along time but also enormous capital investment. In addition, the cheapest hydrogencurrently derives from natural gas reforming, and using fossil fuels means that thewell-to-wheel efficiency is less competitive. Likewise, the production of hydrogenfrom fossil fuels does not help to reduce CO2 emissions, and therefore a paralleldevelopment of low-cost hydrogen production from alternative technologies isrequired. The electrolysis of water by hydro, wind, nuclear, and photovoltaic power

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each represent very promising possibilities for low-cost hydrogen sources; hydrogenfrom renewable biomass would also be low-cost.Another challenge for fuel cell commercialization is their cost which, at present, is

still much greater than that of an ICE [26]. Low-temperature fuel cells require noblemetals as catalysts, and Pt-based catalysts –which are very expensive – constitute themajor cost of a PEMFC, although other key components such as the membrane,bipolar plates, and GDL are also costly. Last, but not least, fuel cell durability is one ofthe most important technical challenges to show fuel cells as viable commercialproducts. The required lifetimes for fuel cells vary, depending upon their application.For example, a total lifetime of at least 40 000 h and 8000 h of uninterrupted serviceare required for stationary applications, and a lifetime of at least 20 000 h and 6000 his required for buses and automobiles, respectively [27, 28]. At present, PEMFCtechnology can achieve around 2000 h for cars and around 10 000 h for stationarygenerators. It is also important to consider the durability of the materials to be usedwhen developing low-cost components.In order to help bridge the gap between gasoline vehicles and FCVs in the future,

somemanufacturers are developing vehicles that burn hydrogen instead of gasolinein ICEs, to reduce automotive energy consumption and CO2 emissions [29]. As thefuel contains no carbon, there are no CO, CO2, or hydrocarbons in the exhaust, andthe toxic emissions are expected to be very low, with NOx levels <50 ppm [30]. Theenergy efficiency of a hydrogen ICE is said to be 20–25%,which is better than that of agasoline ICE because it can run at a lean air-to-fuel ratio and a higher compressionratio. A hydrogen ICE can also be controlled without a throttle, and is, therefore,claimed to be a lower-cost alternative to a fuel cell. Another benefit is that hydrogenICE automobiles can be started in weather that is too cold even for gasoline engines,and can use hydrogen that contains impurities, without damaging the engine. Today,BMW and the Ford Motor Company are the two leading developers of hydrogen-fuelled ICE vehicles and, in fact, are simultaneously developing hydrogen ICE andfuel cell engine vehicle technologies [29].Other alternatives aimed at bridging the gap between gasoline vehicles and FCVs

are hybrid vehicles, which include ICE–battery, ICE–fuel cell, and fuel cell–batteryhybrids.Unfortunately, hydrogen-fuelled ICE vehicles and hybrid vehicles are merely

transitional technologies, and to make FCVs more competitive with conventionalICEvehicles, R&Dactivitiesmust be focused not only on the fuel cell technology itselfbut also on the entire energy chain, including the hydrogen infrastructure. However,to accomplish this difficult task, FCV market penetration will be needed on aninternational scale, which in turn will require significant investments from bothindustry and government [31].

14.6Summary

Unlike an ICE, which converts the chemical energy of fuels tomechanical power, fuelcells convert the chemical energy of fuels directly into electric power, with the

14.6 Summary j353

potential for wide applications in transportation, stationary power, back-up power,portable electronics, and military and space programs. Fuel cells have much higherenergy conversion efficiency than ICEs; they also have the advantages of low/zeroemissions, high power density, silent operation, and quick refueling. The fivemajor fuel cell types are the PEMFC, AFC, PAFC, MCFC, and SOFC, classifiedaccording to the electrolyte used. Except for the PAFC, which is already consideredto be a commercial fuel cell product, the other types are all still in the developmentstage. The major technical challenges for fuel cells are their cost and durability,although the lack of a fueling infrastructure currently represents a huge obstacleblocking their commercialization. Nevertheless, there is no doubt that FCVs repre-sent the ultimate solution to energy needs in the post-fossil fuel era. The problemhere is that the urgency to protect the world against global warming and environ-mental pollution has left no spare time to accelerate the commercialization of fuel celltechnology.

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