Fuel Cell Systems Explained - from and for SET students

-Fuel Cell Systems Explained-(Summary of chapter 1-4, 7, 8)1. Introduction1.1 Hydrogen fuel cells – basic principlesDefinition: A fuel cell is an electrochemical device that converts a supplied fuel to electrical energyand heat continuously, so long as reactants are supplied to its electrodes. Neither the electrodes northe elctrolyte are consumed by the operation of the cell (page 17).Example: Acid electroclyte fuel cellAnode: hydrogen gas ionises, releasing electrons and creating H + ions.2H 2 → 4 H + + 4e -Cathode:oxygen reacts with the electrons from the electrode and H + ions fromthe electrolyte to form water.O 2 + 4 H + + 4e - → 2H 2 OAn acid electrolyte allows only H + ions to pass through. The e - have to pass through an externalelectric circuit. Certain polymers also allows only positiv ions to pass and can be used aselectrolyte, so called proton exchange membranes PEM.Note:For all primary batteries and therefore for fuel cells, are the electrons flowing from the anode to thecathode. Thus the anode is electrical negative and the cathode is electrical positive.Cations: positive ionsAnions: negative ions1.2 What limits the current?First the activation energy has to be exceeded for an exothermic reaction taken place. If teprobability of a molecule having enough energy is low, the reaction rate is slow.The reaction rate can be accelerated by using a catalyst raising the temoerature increasing the electrode surface area (for fuel cells)

The chemical reactions have to take place at the surface of the electrodes, due to removing orsupplying electrons. These reactions of the fuel with the electrolyte at the electrode is called a threephase contact. The reaction rate is propotional to the effective surface area of the electrodes.Requirements for the electrodes: high effective surface area (highly porous material) incoporate with a catalyst endure high temperatures in a corrosive environment1.3 Connecting cells in series – bipolar plateA single fuel cell generates only a small voltage drop (0.7V). In order to generate a useful voltageseveral cells are connected into series. Such a connection is called a stack. The simplest way tocreate a series connection would be to connect the edge of each anode to the cathode of the nextcell. The problem of such a system is, that the electrons would have to travel through the wholeelectrode, thus the ohmic resistance would be relative high.A better alternative are bipolare plates. These plates are made of a conducting material (Graphite,stainless steel, some ceramic materials) with connections areas over the surface as well as gassupply channels for the electrodes. The bipolare plates are complex parts and difficult tomanufacture, thus the most expensive parts of a fuel cell.In order to optimize the design of bipolare plates several aspects counteracts each other: good electric contact requires a large contact area and a thin plate, to reduce the ohmicresistance. This lead to small narrow gas channels thus bad gas distribution over the electrode and lowgas flow rates or high operation pressueres.1.4 Gas supply and coolingA major problem of manufacture fuell cells is leakage of the reactant gases. In order to seperate thegases strictly from each other are the electrrodes sealed at the edges with a gasket.External manifolds:gas is supplied through an external lid, that covers the whole side of the stack.+ simple structure- cooling of the system. Needs a higher air supply rate for cooling. Makes the cell more inefficient- higher leakage probability. The gasket is not pressed evenly onto the electrodes.Internal manifolds:gas is supplied into the stack through additional supply channels in the bipolare plates.- That requires larger and more complex bipolare plates with extra channels for the reactants supply.These channels are connected to the the gas channels along the electrodes.+ solid stack with the gases fed in at the same ends of the electrical connections.+ additional cooling channels along the bipolare plates.+ lower leakage probability

1.5 Fuel cell typesBesides the practical issuses fuel cells have to fundamental technical problems: slow reaction rate, low current and power hyrogen is not readily available as fuelThere are six viable systems of fuel cells:TypeAlkalineAFCProton exchangemembranPEMFCDirect methanolDMFCPhosporic acidPAFCMoltencarbonateMCFCMobileIonOperatingtemp. [°C]Anode and cathodereactionsApplication and notesOH - 50-200 Space vehiclesH + 30-100 2H 2 →4H + +4e - (a)O 2 +4H + +4e - →2H 2 O (c)Vehicles and mobileapplications, low power CHPH + 20-90 Portable electronic systems,low power but long runningtimeH + ca. 220 For small CHP plants(hundreds of kW), firstcommercial oneCO 32-ca. 650For medium to large CHP (upto MW)Solid OxideSOFCO 2- 500-1000 For all sizes of CHP (from kWto multi MW)Advantages and disadvantages are discussed later.1.6 Other cells – Some fuel cells, some not1. Biological fuel cellsuse an organic fuel, like methanol or ethanol, and enzymes to produce energy. Not yetcommercial2. Metal air cells (batteries)at the negative electrode the metal reacts with an alkaline electrolyte to form metal oxide orhydroxide. The released eelctrons pass though an external electric circuit to the air cathode,where water and oxygen reacts to hydroxyl ions (see alkaline fuel cell). Metal oxide isdissolved into the electrolyte, which have to be renewed after some time. Consumption ofanode and electrolyte.Very good energy densitiy: commercial for low power applications with long running time.Research for higher power for electric vehicles.3. Reodx flow cells or regenerative fuel cellsFor charging the reactants are removed from the electrodes and stored in a tank. Fordischarging the electrolytes are supplied back to the electrodes. Is used for very largecapacity rechargeable batteries. The electrocolyte changes during operation and the systemcannot work idenfinitely.

1.7 Other parts of fuel cellsThe core of a fuel cell power system consists of electrodes, electrolyte and bipolar plate. Theadditional parts, also called balance of plant BOP, make a up a large contribution to the wholesystem. They depent on the type of the fuel cell and the used fuel. Circulation of the reactants: pumps, blowers, compressors, intercoolers Electricity output: power electronics, as DC/DC converters, DC/AC inverters Electric motors to drive the devices Fuel storage Fuel processing, desulphurisation Control valves and pressure regulation Controll unit, especially for start-up and shutdown at high temperature devices Cooling system Heat exchanger and pre-heater for high temperature cells humidification of reactant in PEM1.8 Figures used to compare systemsFor the fuel cell: Current density to compare electrodes and electrolytes Specific operating voltage (0.6 – 0.7V) Power densityNote: Electrodes do not scale up properly. If the area is doubled, the current will often not double.The reasons are not yet understood, but realted to even delivery of reactants and removal ofproducts from all over the face of the electrode.For the whole generation system: Power density Specific power cost per kWh Lifetime (difficult to specify)Performance and MTBF (mean time between failures) depends on the age of the electrode andelectrolyte. 'Percentage deterioration per hour' Gradual decline in voltage in mV/1000h (somtimes) Efficiency1.9 Advantages and applicationsDisadvantages: mainly the costsAdvantages: EfficiencyGenerally more efficient than combustine engines. Independant of the size of the system.Useful for small local CHP plants SimplicityThe basics of a fuel cell is very simple, with few moving parts. Can lead to highly reliableand long-lasting systems. Low emissionsUsing hydrogen leads to pure water as by-product. A fuel cell can operate with zeroemissions. CO 2 emission for H 2 production is not considered. SilenceFuel cells operate very silent, even with extensive fuel processingUsing H 2 as fuel can count for disadvantage or advantage. In a future perspective with a limitationof fossil fuels, H 2 will be a major fuel.

Fuel cells can be used successfully in CHP plants or in mobile power systems, like vehicles orelectronic equipment.2. Efficiency and open circuit voltgage2.1 Energy and the EMF of the hydrogen fuel cellThe chemical energy in and output of a fuel cell is not easily derived. Parameters as enthalpy,Hemholtz function, Gibbs free energy or exergy are used. Gibbs free energyenergy available to do external work, neglecting any work done by changes in pressureand/or volume. In case of a fuel cell are the moving electrons in the electric circuit meantwith external work. Exergyis all external work that can be extracted, including that due to pressure and volume changes EnthalpyGibbs free energy plus the energy connected to the entropy.The chemical energy has similarities to mechanical potential energy.1. Reference energy level can be defined as almost anywhere. Working with chemical reactionsthe reference level with zero energy is defined as pure elements, in the normal state, atstandard conditions (25°C or 298.15K, 0.1MPa).Using this convention, the terms 'Gibbs free energy of formation', G f , and 'enthalpy offormation' is used. In case of a hydrogen fuel cell operating at standard temperature and pressurethe Gibbs free energy of formation at the input is zero (useful simplification).2. Energy difference between in and output is the important value. In a fuel cell, the change ofthe Gibbs free energy of formation, ΔG f , is the energy released. Given by the equationΔG f = G f of products - G f of reactantsIt is convenient to use the molar specific Gibbs free energy of formation, written g̅ f.

Note:The mole is a measure of the amount of a substance in respect to its molar mass.H 2 molar mass: 2amu1 mole = 2gH 2 O molar mass: 18amu1 mole = 18gOne mole of a substance always has the same number of molecules, given by the Avogardro'snumber N = 6.022 x 10 23 .The Faraday's constant gives the charge of one mole of electrons.F = N x e = 6.022 x 10 23 x 1.602 x 10 -19 C = 96485 CUsing the basic reaction of the hydrogen/oxygen fuel cell, it can be seen that one mole of H 2 andhalf a mole O 2 deliver one mole of H 2 O. However, note that the Gibbs free energy of formation isnot constant. It depends on the temperature and state (liquid or gas). When the values are negative itmeans that energy is released.Electromotive force EMF = reversible open circuit voltage OCFIn a reversible process in the fuel cell, all Gibbs free energy would be converted into electricalenergy. The basic reaction provides two electrons to the electric circuit for each molecule of H 2used. So for one mole of H 2 , 2N of electrons pass through the circuit.Flowing charge: -2Ne = -2F [C]The electric work done equals the product of charge times voltage E. In a reversible process, theelectric work also equals the Gibbs free energy released.Given is the electromotive force EMF or reversible open circuit voltage of the hydrogen fuelcell.In practice the voltage would be lower, due to the fact that some irreversibilities also apply evenwhen no current is drawn from the cell.In generalwith z is the number of electrons per molecule of fuel.2.3 Efficiency and efficiency limitsThe Gibbs free energy is the part of chemical energy which is converted into electrical energy.However, it is insufficient to calculate the efficiency of a fuel cell only with this part of thechemical energy of the reactants by taken the irreversibilities into account, due to the fact that theGibbs free energy depends on the temperature, the pressure and other factors.In order to compare a fuel cell with other fuel burning technologies, is the maximum possbileefficiency derived by the ratio of the electrical energy produced per mole of fuel and the change ofenthaply of formation, that means the energy amount released as heat by burning the fuel.The maximum possible efficiency level, also named thermodynamic efficiency, is given byη max = Δg̅ f / Δh̅ f x 100%Note that Δh̅ f depends on the state of the substance, liquid or gas. For gas it is called the higherheating value HHV, for liquid it it called the lower heating value LHV. The difference betweeenboth is the molar enthalpy of vaporisation. Any statement of efficiency should say whether it relates

to the HHV or LHV. If the information is not given, probably the the LHV is used.Table 2.2 shows the theoretical efficiency limit of a hydrogen fuel cell in respect of the operatingtemperature. However in practice there are some differences. Voltage losses are less at higher temperatures, so the fuel cell voltage is higher at hightemperatures. Waste heat of higher temperature cells can be used. Fuel cells do not always have a higher efficiency limit than heat engines.The mentioned decline in maximum possible efficiency with temperature for the hydrogen fuel cellis not exactly occur for other cell types.2.4 Efficiency and the fuel cell voltageThe operation voltage of a fuel cell can be related to its efficiency.For 100% efficient hydrogen fuel cell, the enthalpy of formation is converted into electric energy.Considering the fact that not all fuel is used during operation, the fuel utilisation coefficient isintroduced.This is equivalent to the ratio of fuel cell current and the current that would be obtained if all thefuel were reacted. The fuel cell efficiency in respect to the HHV is therefore given byA good estimation for µ f is 0.95.

2.5 The effect of pressure and gas concentration2.5.1 The Nernst equationAs mentioned in 2.1 is the Gibbs free energy change dependant on the temperature and in a morecomplex relation on the reactant pressure and concentration.In a chemical reaction every reactant and product have their own 'activity'. In case of an ideal gas,the 'activity' is definded asa = P / P 0withP: partial pressure of the gasP 0 : standard pressure, 0.1MPaThe activity of a gas is proportional to its partial pressure. The produced water in a fuel cell caneither be a liquid or steam. In case of liquid water, it is a reasonable approximation to assumea H2O =1.The activities of the reactants and products of a chemical reactionjJ + kK → mMmodify the Gibbs free energy change, given by the equationwith Δg̅ f 0 : molar Gibbs free energy change at standard pressure (from gas tables)R = 8.314 j/Kmol molar gas constantT: temperatureIt can be seen, that when the activities of the reactants increase, Δg̅ f becomes more negative, somore energy is released. On the other side, if the activity of the product increase, Δg̅ f becomes lessnegative, so less energy is released.Substitute this equation in the equation of the EMF gives the Nernst equation.with E 0 : EMF at standard pressureThe Nernst equation gives the EMF dependant on the product and reactant activities. The derivedEMF, also called Nernst voltage, is the reversible cell voltage that would exist at a giventemperature and pressure.Using the rules of logarithmic functions an the definition of activity, the equation can be simplified.In nearly all cases will be the pressures partial pressures, that is, the gases will be part of a mixture.It often appears that the pressures at cathode and anode are the same, to simplify the design.Note:In a mixture of gases, the total pressure is the sum of all the partial pressures of the components.From the gas law equation it can be shown, that the volume fraction, molar fraction and pressurefraction of a gas mixture are all equal.Example: CH 4 + H 2 O → 3H 2 + CO 2

The products are three parts of H 2 and one part of CO 2 by moles and volume. If thereaction takes place at 0.1MPa the partial pressures can be derivedP H2 = ¾ x 0.1MPa = 0.075MPaP CO2 = ¼ x 0.1MPa = 0.025MPa2.5.3 Fuel and oxidant utilisationAs air passes through a fuel cell, the oxygen is used, and so the partial pressure will reduce.Similarly, the fuel partial pressure will often decline, as the proportion of fuel reduces abd reactionproducts increase. That makes the logarithmic term in the Nernst equation smaller, and so the EMFwill fall. This circumstance will vary over the cell and will be worst at the fuel outlet as the fuel isused. Due to the conducting material of the bipolar plate the voltage is constant over the cell, thatmeans that the current will vary. The current density will be lower nearer the exit where the fuelconcentration is lower. The RT term indicates that the voltage drop due to fuel utilisation will begreater for high temperature fuel cells.A high system efficiency normally requires a high fuel utilisation. That counteracts the abovedescribed behaviour. The cell voltage, hence the cell efficiency will fall with higher fuel utilisation.Therefore, fuel and oxygen utilisation is an important aspect in the system design and thus needscareful optimisation.2.5.4 System pressureWhen the system pressure is known, the Nernst equation can be rewritten, usingP H2 = αP with α constant depending on the molar masses and concentration of H 2P O2 = βP with α constant of O 2P H2O = δP with α constant of H 2 OasIt can be seen, thet the EMF of the fuel cell is depending on the system pressure P. A hihgherpressure leads to a higher voltage. For high temperature fuel cells, for example SOFC at 1000°C,the predictions of the Nernst equation correlate with practise. For lower temperatures, as in a PAFCat 200°C, the voltage change is even bigger. This occurs due to increasing pressure also reduces thelosses at the electrodes, especially at the cathode. A similar effect occurs from switching from air tooxygen (β). The voltage losses at the cathode decline for pure oxygen.

2.6 Summary reversible OCV or EMF max efficiencyη max = Δg̅ f / Δh̅ f x 100% efficiency of working H 2 fuel cell relative to HHV Nernst equationThe pressure and the concentration of the reactants affects the Gibbs free energy, and thusthe voltage, given by the Nernst equation.3. Operational fuel cell voltage3.1 IntroductionThe operating voltage of a fuel cell is always below the theoretical open circuit voltage derived inchapter 2. Figure 3.1 shows the performance of a typical hydrogen fuel cell operating at 70°C andnormal air pressure.Note: even the open circuit voltage is less than the theoretical value there is a rapid initial fall afterwards the voltage falls less rapidly and mor linear there may be a higher current density at which the voltage falls rapidly againFor a higher temperature fuel cell the graph changes. The reversibel 'no loss' voltage is smaller, butthe difference between the 'no loss' voltage and the operational voltage is less, due to the initialvoltage drop is significant smaller.

Figure 3.2 shows the performance of a SOFC operating at 800°C.Note: the OCV is equal or just a litle less than the theoretical vallue the intial fall in voltage is very small, and the graph more linear there may be a higher current density at which the voltage falls rapidly againThe focus of chapter 3 is on the questions: What causes the voltage to fall below the reversiblevalue and how can this situation be improved?3.2 TerminologyCommonly five terms are used to describe this voltage difference: Overvoltage or overpotential Polarisation Irreversibility Losses Voltage dropEvery term is used in the book!3.3 Fuel cell irreversibilities – causes of voltage dropThe characteristic shape of the voltage/current density graphs results from four majorirreversibilities.1. Activation lossescaused by the slowness of the reactios at the electrode. Voltage is used to drive the reactions. Nonlinear.2. Fuel crossover and internal currentscaused by unused fuel diffusion and electron flow through the electrolyte. Small losses.3. Ohmic or resistive lossescaused by the ohmic resistance of the electrodes and interconnections.4. Mass transport or concentration lossescaused by a change in the reactants concentration at the surface of the electrodes. Modelled by theNernst equation.

3.4 Activation losses3.4.1 The Tafel equationThe Tafel equation and plots are experimental results. Tafel observed, that the overvoltage at thesurface of an electrode follows similar pattern for different chemical reactions: The plot of thevoltage against the logarithmen of the current density is approximatly a straight line.Tafel plotsTafel equationThe constant A is higher for slow chemical reactions. The exchange current density i 0 is higher ifthe reaction is faster. The current density i 0 can be considered as the current density at which theovervoltage begins to move from zero. The equation is only true for i 0 > i.3.4.2 The constants in the Tafel equationFor a hydrogen fuel cell, the constant A is given bywith α: charge transfer coefficientThe charge transfer coefficient varies from 0 to 1 and amount of electrical energy that is used tochange the electrochemical reaction rate. The value of α depends on the electrode material.The impact of the temperature in the equation is far outweighted by the effect of i 0 with changingtemperature and thus is not considered. Indeed we shall see, that the key to making the activationovervoltage as low as possible is i 0 , which can vary by several orders of magnitude.Even at zero current density the electrochemical reaction at the electrodes is taken place, but thereverse reaction is also taken place at the same rate. There is an equilibrium.Thus, there is an continual forwards and backwards flow of electrons from and to the electrolyte.This current density is the exchange current density i 0 . If i 0 is high, that means, that the surface ofthe electrode is more active, and thus it is easier to direct the current densit in a particular direction.The exchange current density is crucial in controlling the performance of the fuel cell electrode andshould be as high as possible.

In order to get the current instead of the voltage the Tafel equation is cahnged in the Butler-Vollmer equationThe importance of i 0 can be seen in the following graph for values of 0.01, 1.0 and 100mA/cm 2 .Measurements of i 0 shows a great variation in respect of the e;ectrode material. For hydrogen fuelcells are the values at the oxygen electrode (cathode) lower by a factor of 10 5 . Therefore is theovervoltage at the anode negligible compared to the cathode.For other fuel cells, for example a DMFC, the anode overvoltage is not negligible. In these cases theequation for the total overvoltage combine the voltage drops at both electrodes. I can be expressedaswith3.4.3 Reducing the activation voltageIn order to reduce the activation voltage it is most important to increase i 0 . This can be done by Raising the cell temperature Using more effective catalysts Increasing the roughness of the electrode (increased surface) Increasing reactant concentration (pure O 2 instead of air)(more effective occupation of the surface by reactants) Increasing the pressure(more effective occupation of the surface by reactants)Increasing the value of i 0 . Has the effect of raising cell voltage by a constant amount at mostcurrents, and so lead to raising OCV. The last two points explain the difference between theareticaland practical OCV.In low and medium temperature fuel cells, activation overvoltage is the most importantirreversibility and cause a voltage drop at the electrodes. Mainly at the cathode for hydrogen fuel.At higher temperatures and pressures the activation overvoltage becomes less important.

3.5 Fuel crossover and internal currentsEven if the electrolyte is designed toconduct only ions, it always occurs that a small amount ofelectrons pass through it. In practical fuel cells the fact that a certain amount of fuel diffuse unusedthrough the electrolyte is more important. The unused fuel will react directly with oxxygen at thecathode and no current is produced. These effects – fuel crossover and internal current – areessentially equivalent. One diffused hydrogen molecule effects in the same way as two electrons ofinternal current. The amount of these losses will be very small in respect to the irreversebilities.However,, in low temoerature fuel cells it causes a very noticeable drop of OCV, about 0.2V (20%)for a PEM. Noticeable in the steep initial falls of the voltage plots.A small change in fuel crossover and/or internal current, caused for example, by a change inhumidity of the electrolyte, can cause a large change in the OCV.Measurements of the internal current are problematic, but can be done by the measurement of thefuel consumption. The consumption rate of hydrogen [moles/s] is related to the current by thefollowing equation.The equation of the cell voltage can be refined, including the internal current density i n .To conclude, the effect of internal current is much less for high temperature cells, because i 0 ismuch higher. For low temperature cells internal current and fuel diffusion is usually not of greatimportance in terms of operating efficiency, but it has a marked effect on the OCV.3.6 Ohmic lossesThe losses occured by the electrical resistance of the electrodes and the interconnection and by theresistance against the flow of ions in the eelctrolyte. The ohmic voltage loss is important in all typesof fuel cells, especially in SOFC, and is mainly caused by the electrolyte. However the electricalresistance of the electrodes and bipolar plates is not negligible.The voltage drop is given by Ohm's law, using the current density and the area-specific resistance.ΔV ohm = i x rwith i in [mA/cm 2 ]r in [kΩcm 2 ]There are three ways of reducing the internal resistance: highly conductiv electrode material design and material of the bipolar plate or interconnection electrolyte as thin as possible(but have to prevent shorting of the electrodes, wide enough to provide a flow of electrolyte,physical robust, due to electrodes are built on it)3.7 Mass transport and concentration lossesDue to the electrochemical reactions at the eelctrodes, hydrogen and oxygen are consumed. Thatleads to a change in concentration, which leads on the other hand to a change in partial pressure ofthe reactants, which results in a voltage drop over the cell. The pressure changes depend on thecurrent drawn from the cell and the physical characteristics of the gas supply system, relating to thefluid resistance, the flow rate and the design of the gas supply along the electrodes.There is no overall satisfactory analytical solution to this problem. One theoretical approach ismentioned, which shows the effect of reducing the partial pressure on the OCV.

Assumption:Introduction a limiting current density i 1 at which the hydrogen consumption rate is equal to themax. supply rate. At i 1 the pressure would have reached zero.P 1 is the pressure when i is zero. If we assume a linearly fall from P 1 to P(i 1 ), tha the pressure at anycurrent density is given bySubstitution in above equation delivers the voltage drop due to mass transport.The negative sign is due to the fact that it is a voltage drop.The term RT/2F will be different for each reactant and is in general substitute by the constant B.This theoretical approach has many limitations, like: supply of air instead of pure oxygen lower temperature cells mixed fuel production and removal of reaction products build-up nitrogen in air systemsAn empirical approach is more favourable, because it provides an equation which delivers muchbetter results and will be used in the rest of the chapter.With m is about 3x10 -5 V and n is about 8x10 -3 cm/mA.The mass transport or concentration losses are important when Hydrogen is supplied by a reformer. The reaction rate on a demand change is too slow. The supply at the air cathode is not well circulated. Build-up nitrogen can block the oxygen supply at high currents. Water is not removed qickly enough in PEMFC.3.8 Combining the irreversebilitiesThe operating voltage of a fuel cell at a current density i is given bywith E: reversible OCVi n : internal and fuel crossover equivalent current densityA: slope of the Tafel linei 0 : exchange current densitym & n: constants in the mass-transfer overvoltager: area-specific resistanceThis equation can be simplified, due to i n is usually very small and has little impact on operatinglosses of fuel cells at working currents, so it will be cancelled. The term of the activationovervoltage can be rewritten as

The second part is constant and is included in a real, practical OCV.Finally we get a simple equation, that fits excellent results in respect to real fuel cells.Example:3.9 The charge double layerWhenever two differnet materials are in contact, there is a build-up of charge on the surfaces or acharge transfer over the junction. In electrochemical systems, the charge double layer is formed bydiffusion effects, by the reactions between electrons in the electrodes and ions in the electrolyte andby an applied voltage. The probability of electrochemical reaction taking place depends on thedensity of the different charges at the double layer; more charges lead to higher current. The chargeseperation at the double layer generates an electric voltage, in case of a fuel cell the activationovervoltage. Adding catalysts to the electrodes increases the probability of a reaction; a highercurrent canflow without such a build-up voltage. The charge double layer can be seen as an electriccapacitor, a storage of electric energy. Due to the capacitive behaviour of the double layer, will afuel cell react on a cureent change in two steps.1. Immediate change in operating voltage due to the internal resistance.2. Smoothly move to the final equilibrium value due to the capacitance.3.10 Distinguishing the different irreversebilitiesHow can the different kinds of voltage losses be distinguished in respect to conditions they mainlyoccur? Eperiments using specialised electrochemical test equipment such as half cells Electrical impedance spectroscopyAC current with variable frequency is applied to the cell, the voltage is measured and theimpedance is calculated. Higher frequencies means less impedance of the capacitors.Plots of the impedance over the frequencies shows the values of the equivalent electric circuitelements, thus the mass transport and activation losses can be seperated. Current interrupt techniqueis an alternative, which delivers accurate quantitative results but also quick qualitative indications.It can be performed using low cost electronic equipment.A current is applied and suddenly cut of. The voltage will follow in the above mentioned steps:1. Immediate change in operating voltage due to the internal resistance.2. Smoothly move to the final equilibrium value due to the capacitance.The ohmic losses and the activation overvoltage can be seperated from each other. The followinggraph shows a sketch of the voltage change.

To summarize the irreversibilities due to their importance in different conditions. Concentration and mass transport losses are important only at higher currents. The should bevery small in well designed systems. In low temperature hydrogen fuel cells activation overvoltage and ohmic losses areimportant Fuel cells using mixed fuel, as methanole, the activation overvoltage at anode and cathodedominates. In higher temperature fuel cells the ohmic losses are the major factor.4. Proton Exchange Membrane Fuel Cell PEMFCA PEMFC, also called solid polymer fuel cell, consists of an ion conducting polymer electrolytebonded at each side with a catalysed porous electrode. These membrane electrode assemblies MEAsare very thin and connected in series with bipolare plates. The mobile ion is an H + ion or a proton.The basic reactions are at the anode2H 2 → 4 H + + 4e -and at the cathodeO 2 + 4 H + + 4e - → 2H 2 OThe polymer electrolyte works at low temperature, so that the fuel cell can start very quickly.Further advantages are, that the design can be very compact, no corrosive fluid hazards are used andthe cell works in every direction. This means that the PEMFC is very suitable for a wide range ofapplications, like vehicles and portable electronics, but can also be used in CHP power plants.Similarities for applications: electrolyte electrode structure and catalystDifferences for applications: water management cooling the fuel cell connecting the cells in series operating pressure used reactants

4.2 How the polymer electrolyte worksThe basicly used material for the electrolyte is fluoroethylene. The established industrial polymerelectrolyte is Nafion from the company Dupont. The construction of the electroclyte materialconsists of the following steps. The starting material is the polymer polyethylene.1. The hydrogen in PE is substituted through fluorine, to get polytetrafluoroethylene PTFE,also known as Teflon. This process is called perfluorination.PTFE is durable and resistance against chemical attacks. It is also highly hydrophobic, waterrepellent.2. The PTFE polymer is sulphonated, that means a side chain, ending with sulphonic acidHSO 3 is added to the molecul. The resulting structure is caleed anionomer, due to the ionicalbond of the H + and SO 3 - ions. The ionic bondage results in a clustering of the side chains inthe overall structure.Another important property is, that HSO 3 is highly hydrophyllic,attracts water.In conclusion, we are creating water attracting areas in a water repellent material. Within thesehydrated regions, H + ions are relatively weakly bonded and are able to move (dilute acid). Even ifthese areas are seperated, it is still possible for the H + ions to move along the the supporting longmolecules.From the fuel cell use, the main features of Nafion are: chemically highly resistant mechanically strong, thin films are possible acidic absorb large quantities of water if well hydrated, H + ions are able to move quite freely through the material4.3 Electrodes and electrode structureThe electrodes in a PEMFC are essentially the same, a high porous conducting material withplatinum as catalyst. To built the single cells, first the platinum catalyst is formed of small platinumparticals, which are added onto bigger carbon-based particals. In the seperate electrode method, thispowder is fixed on corbon cloth or carbon paper, which builts the basic structure of the electrode.Also PTFE is added due to its hydrophobicproperty, so that the water as the reaction productis expelled from the surface. Additional thecarbon cloth/paper acts as a gas diffusion layerand diffuses the gas onto the catalysts. To formthe fuel cell, two of these electrodes are hotpressed on each side of the electrolyte membran.The result is a complete MEA.The alternative method involves building theelectrode directly onto the electrolyte. Theplatinum on carbon catalyst is fixed directly to theelectrolyte by rolling or spraying. The electrode isdirectly manufactured on the membrane.Then thegas diffusion layer is added ,which provides theelectrical connection between the carbonsupportedcatalyst and the bipolar plate, carriesthe water away and also forms a protective layerfor the thin catalyst layer. The final result is shown in figure 4.7, beginning from the left side:electrolyte molecules, platinum/carbon katalyst, carbon cloth or paper.In order to increase the performance the electrolyte material is spread out over the catalyst, topromote the three-phase contact between, the reactant gas, electrolyte abd electrode catalyst.

4.4 Water management in the PEMFCWtaer management is a crucial aspect in designing a PEMFC. The PEM requires a sufficient watercontent, the proton conductivity is directly proportional to it. On the other side, should theelectrodes and the gas diffusion layer not be flooded and blocked. In an ideal case, the productwater would diffuse evenly through the electrolyte and keep it at a correct level of hydration.But there are several complications: Electro-osmotic dragH + moving from anode to cathode, drag H 2 O with them, especially at high current densities. Theanode side of the PEM can dry out. Drying effect of air at high temperaturesAt approximatly 60°C the air will always dry out the elctrodes faster than H 2 O is produced.Solution: Humidify the reactants before entering the fuel cell. Correct water balance throughout the whole cellDry air may ente the cell, but it can become saturated in the cell and can not dry off any moreexcess water. Important for designing larger stacks.Fortunately, all water movements are predictable and controllable. Water production and water dragare directly proportional to the current. The water evaporation can be predicted. The back diffusionof water from cathode to anode depends on the thickness of th PEM and the relative humidity ofeach side. The preceded external of humidification of the reactants is also controllable.4.4.2 Air flow and water evaporationIn a PEMFC it is common to remove the product water by the air flow through the cell (expect thespecial case: using pure O 2 ). That means, that air is always fed at an higher rate into the cell, thanneeded to provide the O 2 for the reactions (> stoichiometric rate). In practice, the stoichiometry λwill be at least 2.Problems arise because the drying effect of air is highly non-linear in relation to the temperature.The amount of water vapour in air varies greatly, depending on temperature, location, weatherconditions and other factors. One way of describing it is the humidity ratio, the ratio of watervapour in respect to the other gases in air, also called absolute humidity or specific humidity.ω = m w / m awith m w : mass of waterm a : mass of dry airThe relative humidity of air gives a good impression of the drying effect of air and is defined asΦ = P w / P satwith P w : partial pressure of waterP sat : saturated vapour pressure (the air is fully humiditied, it can not hold any more water)The problem in case of a fuel cell is, that P sat varies with temperature in a highly non-linear way. Itincreases more rapidly with incresing temperature.

Another way of describing the water content is the dew point, the temperature to which the air haveto be cooled in oeder to reach saturation.Example: P w = 12.35kPa → dew point T = 50°C (see table 4.1)Sometimes it is necessary to humidify the gases going into the cell, so the mass of H 2 O added hasto be calculated. To do so, note that the mass of a species in a mixture is proportional to the productof molecular mass and partial pressure of the species.And so we get withP = P a + P wNote that the mass of water is inversely proportional to the total air pressure P, so higher airpressure requires less added water to achieve the same humidity.4.4.3 Humidity of PEMFC airThe air flow in a PEMFC must be dry enough to evaporate the product water but also wet enoughnot to dry the PEM out, thus it have to be carefully controlled. The humidity should be between 80and 100%. In the following section a theoretical formula is derived to set up proper conditions forthe humidity required.The partial pressure of a gas is proprtional to the number of molecules (molar fraction)due to the assumption, that all water is removed by the cathode air supply, the following equationscan be used.

With Pe: electrical Power of the stackVc: voltage of a single cellṅ O2 : supply or use rate of oxygenλ: air stoichiometryThe exit flow rate of the non-oxygen components of air is the same as in the inlet. The non-oxygencomponents amount to 79% of the air. This will be graeter than the oxygen molar flow rate by thefactor 0.79/0.21 = 3.76.Substituting the molar flow rates in the first equation deliversThe water vapour pressue at the outlet only depends on the air stoichiometry and the air pressure atthe exit. This eqution gives the worst case conditions, not water vapour in the inlet.If the realtive humidity of the excess air is too dry, it could be changed by lowering temperature → increase losses lowering air flow rate and hence λ → reduce cathode performance increasing pressure → more enrgy for the compressorNone of these options is really attractive.Another option is to extract the water of the excess air and supply it to the reactants at the inlet. Thisrequires additional equipment, extra costs, but is often justified by an increased performance. Thewater pressure can be derived, taking the humidity of the inlet air into account, is given bywith4.4.4 Running PEM fuel cells without extra humidificationIt is possible to run a PEM fuel cell at the right conditions without external humidification, bychoosing suitable operating temperatures and air flowrates.

Note that humidities are lower at greater airflow. At higher temperatures the relative humidity fallssharply. Note that if the relative humidity of the excess air is below 100% the fuel cell has given allits water, but the air is not yet saturated. That means the fuel cell dries out. On the other side arelative humidity greater than 100% is basically impossible, because the air stream would containcondensed water and the electrons would become flooded.At temperatures above 60°C, the relative humidity of the exit air is below 100% at all reasonablevalues of λ. Above this temperature extra humidification of the reactants is essential in PEMFCoperating. This causes the difficulties in choosing the optimum operating temperature; highertemperatur means better performance but also increasing humidification problems.The key is to set the stoichiometry so that the relative humidity of exit air is about 100% and designa balanced water distribution within the cell.

4.4.5 External humidification - principlesSmall fuel cells can be operated without external humidification, they will perform, even with thecorrect air flowrate and temperature, muc =h worse than a identical humidified cell. Operatingtemperatures over 60°C are desirable to reduce the cathode activation losses. Considering theadditional equipment and costs for extra humidification, it is still more economic to operate the fuelcell at maximum possible power density.However, water as a additional input is not desirable. The water has to be extracted from the exit airflow. That requires a water condensation or seperation system in the exit path. The intrnal recyclingof water also ensures the purity of the water.Note: air & hydrogen are often humidified. Process involvs evaporating water in the incoming gas. This will cool the gas. Desired inpressurized systems. Benefits of increasing humidity improve the performance especially at operating at higherpressures.4.4.6 External humidification – methodsIn this operation process no industrial standard has been yet emerged. In the following section eightdifferent methods will be outlined.1. In laboratory test systems the reactants are humidified by bubbling them through water(sparging)2. Direct injection of water as a spray in th reactants flow.Easy to control, but expensive and inefficient due to additional pumps and valves.Cools the gas.3. Metal foam to make a fine water spray.No pumps and other devices needed.4. Direct watering of the PEMFC (used in the past)Wicks are constructed as part of the gas diffusion layer and draw water directly into the cell. Selfregulating,because no water is drawn if the wicks are saturated.Problem: Sealing the cell and no cooling effect5. Direct injection of liquid water and distribution by the reactants.Requires an efficient flow field design of the bipolar plate. Good results are reported, though thereactants have to be driven at pressure.Problem: no cooling effectAll the mentioned methods requires liquid water. There are other methods which don't need a watercondensation and seperation system.6. Rotating piece of water absorbing materialAbsorbs water in the exit, rotates and evaporates it in the inlet.Problem: system is bulky, needs to be powered and controlled.7. MembraneExiting warm, damp air passes the membrane, water is condensed, liquid water passes through themembrane and is evaporated by the dry gas at the inlet. A PEM membrane can be used.Advantage: evenly spread humidity, simple, no moving partsDisdvantage: no control8. Self-humidification, by using a modified electrolyte which also produces water. A catalyst isadded in the PEM, some amount of the reactants diffuse through the electrolyte and combineto water. The improved performance justifies the fuel loss.

4.5 PEMFC cooling and air supply4.5.1 Cooling using the cathode air supplyPEM reaches normally efficincies of around 50%, so half of the chemical energy is changed in heat.The heat produced by a PEM, if the product water is evaporated within the cell isThe way the heat is removed depends greatly on the size of the fuel cell, beolow 100W it is possibleto cool the cell and evaporate the water by air, without using any fan, only due to a open cell design.The fact that damp air is less dense than dry air aids the circulation process. For more compacttypes a small fan blows the reactants and cooling air through the cell, though a large proportion ofheat is lost through natural convection and radiation.For systems with a power higher than 100W, seperate cooling is needed.4.5.2 Seperate reactant and cooling airIs best shown with a specific example. fuel cell power P e [W] single cell voltage V c = 0.6V operating temperature T = 50°C cooling air enters at 20°C and leaves at 50°C (best possible case!) 40% of generated heat is removed by the air, rest is radiated. Entry air humidity 70%40% of the heat rate (equation above) is removed by air of the specific heat capacity c p , with theflowing rate ṁ and a temperature change ΔT.With c p = 1004 J/kgKΔT = 30KV c = 0.6VThe reactant air flow rate is given (see appendix A2.2).If the reactant air and cooling air are the same, the equation would be the same.This gives an exit air humidity at 50°C of 26%, that means that the relative humidity decreases, andso the PEM will quickly dry out. In order to reduce λ to a desired value of 3 to 6, the air flow ratehas to be reduced and a seperate cooling system is required. Seperate cooling is achieved by addingcooling channels in the bipolar plates of adding additional cooling plates in the cell. Air coolingworks for cell powers up to 2 kW.4.5.3 Water cooling of PEMFCFor larger fuell cells water cooling is more useful, due to the higher cooling effect of water. Theoperation process are basically the same as for external air cooling. Cooling channels or additionalcooling plates distribute the water in a seperate cooling circuit in the cell. Water cooling is, forexample, in cases of heat recovering in small CHP plants more preferable.

4.6 PEMFC connection – the bipolare plateBipolar plates are difficult to manufacture, and made up almost all volume and mass of a fuel cell,thus they are also a high proportion of the cost of a PEMFC stack.4.6.2 Flow field patterns on the bipolar platesIn the bipolar plate the reactants are fed over the electrode in a fairly dimple pattern or parrallelgrooves. The parallel system has the problem that impurities, like notrogen, can block one groove,and therefore leading an area of the electrode unsupplied. This leads to a more serpentine pattern.The problem with this pattern is the longer path length abd the large numbers of turns in the gasflow. That means more work and thus more energy is needed to push the gas through.The grooves are very narrow, less than 1mm 2 . In order for water droplets not to form, the system isarranged so that the pressure drop along a channel is greater than the surface tension holding awater droplet in place. That has to work even when the gas suuply is stopped. The latestdevelopment by Ballard is a rectangular plate, several times longer than wide, with long straightchannels, which have a high pressure drop and no inefficient bends and turns.4.6.3 Making bipolar plates for PEMFCsThe bipolare plate has many tasks in the cell. Collecting and conducting the current from the anodeto the cathode of the next cell, evenly distributing the fuel to the anode and oxygen/air to thecathode and sometimes carrying a cooling fluid. Furthermore keeping all the gases and fluids apartfrom each other and contain the reactant gases within the cell.Requirements: electric conductivity > 10 S/cm heat conductivity > 20 W/mK with additional cooling and > 100 W/mK withoutgas permeability < 10 -7 mbarL/scm 2 corrosion resistant stiff, flexural strength > 25 Mpa low costs slim for min stack volume light for min stack weight short production timeThe materials and production method used, varies considerably. In terms of easier manufacture theplates are often made in two halves. Then, the cooling channels can be cutted and don't have to bedrilled into the plate. The most commonly used materials and methods are: machining of graphit paperellectrically conductive, reasonably easy to machine, low densityDisadvantages: long machining time, brittle so careful handling needed, porous so a few mmthickness is needed inection molding of graphite filled polymercheap, short production timeDisadvantages: poor conductivity, needs further developments compression moulding of a graphite and thermoplastic polymer granuleshigher proportion of graphit, so better conductivity, but not so fast as injection moulding. Carbon-carbon compositethe complex shape of the surface is moulded and then applied to a solid carbon back plate, in orderto get a higher stiffness, lower gas premeability and sufficiently flat plate. Stainless steelgood electrical and heat conductor, not porous so thin, can be machined easily.Disadvantages: heavy, expensive Perforated or foamed metal

flow field is made of foamed metal, with a sponge-like structure. Then a thin solid metal plate isadded between two foamed plates. A corrocsive resistant coating is added.Cooling plates can be made in the same way. One porous metal slice between two solid plates.Advantage: already available material, cutting and moulding the plastic deals only process steps4.6.4 Other topologiesFuel cell stacks construction using bipolar plates gives very good electrical connection between thecells, but are expensive and difficult to manufacture. In cases of lower current densitiesacompromise with higher electrical resistance but therefore cheaper and simpler is helpful. Such acompromise is shown in figure 4.20. A system of three cells in series, consisting a cheap plasticbody and requires only on chamber for the fuel and on for the air. It is not a compact system, but thepotential leaks are much reduced and due to the free circulation, humidification is not a problem.Another problem is the possible imbalance of this system. The single cells are some way apart. Ifone cell becomes rather warmer, this would lead to more rapid water evaporation, and so to higherresistance and higher temperatures. The cell got in a vicious circle and would end up dried.4.7 Operating pressureRunning a fuel cell at higher pressure will increase the power of the cell, mainly by reducing thecathode activation losses. But it also involves consumption of power for the additional equipment ascompressors and turbines. A typical value of power consumption is about 20%.4.7.2 Simple cost/benefit analysis of higher operating pressuresWe have seen, that ΔV is proportional to the logarithm of the pressure rise.We also saw, that the activation overvoltage is related to the exchange current by a logarithmicfunctio. So we can say, that if the pressure is increased from P 1 to P 2 , then there is a gain in voltageand power.with C: voltage gain constant, depeding on the change of i 0 with the pressure, R, T and F.I: currentn: number of cellsOn the other side we have the power consumption of the compressor and the electric propulsion.

with T 1 : entry temperatureη c : compressor efficiencyη m : motor and drive efficiencyṁ: air flow rateThe air flow rate is connected to the electric power, the cell voltage and the air stoichiometry, bySubstituting the electric power by nIV c and the air parameters c p and γ, we get the power losses.The highlighted part is the voltage loss.We can get now the voltage change for a rise of the pressure byThe following plot shows two graphs, a realistic and optimistic one, for the different parameters.It can be seen, that for the realistic values, the nett voltage will always be less for higher pressures.The made derivation is a quite simple one and a lot of other factors have to be included to get arealistic view of the effect of ibcreased pressures.4.7.3 Other factors affecting the choice of pressureGains: fuel reforming systemreduction in size, the waste heat of the burner can be used to drive the compressor humidificationeasier for hot inlet air, that needs cooling. Compression goes always with a temperature rise, about150°C. Less water is needed to humidify the air and cool it to suitable temperature.It is very difficult to arrange proper PEMFC humidification ta temperatures above 80°C unless thesystem is pressurized in larger fuel cells the flow paths of the reactants is long and narrow, so a certain pressure isalready needed to drive the gases through the cell.Disadvantages: size, weight, cost of the additional equipment. Rising noise, due to the operation of compressors and electric motors.Using pure oxygen nstead of air, the choice of operating pressure will be made under completedifferent aspects and can be much higher.

4.8 Reactant compositionIn larger systems the hydrogen fuel will frequently come from some kind of fuel reforming system.These systems nearly always involve a reaction, like the water shift reaction, producing carbonmonoxide. CO has a great negative effect on the performance of the cell, by poisoning theelectrodes. CO covers the platinum, preventing the hydrogen fuel from reaching it.This happenseven in very low concentrations, as 10ppm CO.In the end CO has to be removed of the fuel stream, which requires additionel processing. Small quantities, around 2%, of hydrogen or oxygen are added to the fuel stream. This reactswith the CO. However, any oxygen not reacting with CO will react with hydrogen, and thuswasting fuel. Gas clean-up unit is used.Pure oxygen is mainly used in air independant systems, as submarines or spacecraft. Advantages: no loss OCV rises, due to higher partial pressure of oxygen (Nernst equation) actication overvoltage decreases limiting current increases, thus reducing the mass transport and concentration losses,because of the absence of nitrogen.7. Medium and high temperature fuel cellsEven if the no loss OCV decreases with increasing temperature outweigh the advantages thedisadvantages at high temperatures. Electrochemical reactions take place more quickly. Noble metal catalysts are often notneeded. Enough heat in the exit gases to extract hydrogen from readily available fuels. Heat of the exit gases can be used for heating buildings, processes and other facilities, inCHP plants. Heat of the exit gases can be used to drive turbines and generate electricity.Three types of medium and high temperature cell are considered. PAFC, MCFC and SOFC.PAFC:operates at around 200°C and needs a noble metal catlyst, and so, like the PEM is poisoned by anyCO in the fuel gas, thus its fuel processing system is necessarily complex.MCFC:operates at around 650°C. The main problem is the degradation of cell components over longperiods. But it shows great promises for the use in CHP plants.SOFC:operates between 650°C and 1000°C. Since it is a solid state device, it has many advantages due toits mechanical simplicity and can be used in a wide range of applications.The advantages mentioned above consider mostly the use of the waste heat. PAFC, MCFC andSOFC must always be thought of as a integral part of a complete fuel processing and heatgenerating system. The common features of all three types are: used fuel needs processing fuel utilisationfuel is a mixture, in which the hydrogen is used. This means that the concentration will decrease,what reduces the local cell voltage. Exit gases carry a large amount of heat, which can be further converted into electricity and

thus leads to higher efficiency. Heat can also be used to preheat fuel and oxidant.Ensure high electrical and thermal efficiency and reduce the exergy losses are the key aspects indesigning such a system.7.2 Common features7.2.1 An introduction to fuel reformingThe production of hydrogen from hydrocorabons usually involves steam reforming.The steam reforming reaction with natural gas is an endothermic reaction, that means heat needs tobe suuplied for a suficient reaction rate. In most cases fuel reforming takes place at about 500°Cand can be used from the exhaust gases of the fuel cell itself. The PAFC requires additional burningof fuel to achieve that temperature, what reduces the effcicncy to 40-45%. For the MCFC and theSOFC efficiencies over 50% can be achieved.In a PAFC as in a PEM must the reformed gas further processed with the gas shift reaction, due toreducing the CO content in the gas. An aspect considering all fuel cells using hydrocarbonat,asnatural gas, is the desulphurisation of the fuel .Even small amounts of sulfur deactivate theelectrodes. This proces also needs heating up the fuel over 350°C. The higher operationtemperatures of theMCFC and the SOFC allows the basic steam reaction taken place in the fuel cellstack itselfs. Furthermore the steam neede is already present in the stack. The exhaust steam at theanode is used for the fuel reforming. That simplifies the design of MCFC and SOFC.7.2.2 Fuel utilisationFuel utilisation has always to be taken into account when the hydrogen is fed in a mixture to thestack. As the gas mixture passes through the cell, the hydrogen is utilised, the other componentssimply pass through unconverted, and so the hydrogen concentration falls. At the end of the cell thepartial pressure of hydrogen will be very low and thus lower the cell voltage.From the Nernst equation we can get the voltage change in respect of a change in hydrogenpressure.If the partial pressure of hydrogen falls from P1 to P2, the voltage change will always be negative.The RT term means that this drop will be greater for higher temperatures. The same occur for theoxygen utilisation from the air. Figure 7.1 shows the effect of it in four cases.1. Standard hydrogen fuel cell operating at 1bar with pure hydrogen and oxygen.2. Using air at the cathode and a mixtue of 4 parts hydrogen and 1 part carbon dioxide at theanode.3. Nernst exit voltage for 80% fuel utilisation and 50% air utilisation.4. Nernst exit voltage for 90% fuel utilisation and 50% air utilisation.

Sometimes the voltage drop can be reduced, by using opposite flow direction (counteer-flow), sohigh oxygen concentration at the point of the cell with low hydrogen concentration. A factor ignoredyet, is that in MCFC and SOFC the product steam ends up at the anode, hydrogen is replaced bysteam. This will also cause a fall of the OCV.The important conclusion is, that in the case of reformed fuel containing carbon dioxide or wheninternal reforming is applied, all the hydrogen can nevver be consumed in the fuel cell itself. Somehydrogen must pass right through the cell and can be used later to provide energy to process the fuelor to be burnt to increase the heat energy. To find an optimum, extensive modelling is needed.7.2.3 Bottoming cyclesBottoming cycles mean the use of waste heat from a fuel cell axhaust gases to drive some kind ofheat engine, for example boiler and steam turbine or a gas turbine. These combined cycle systemsoffer an elegant way of producing very efficient systems of generating electricity.However, the Gibbs free energy for hydrogen decreases with temperature. So the highest theoreticalpossible efficiency is at ambient temperature.On the other side are the losses high at abient temperature and so the practical efficiency would bewell below that limit. To reduce the voltage losses we have to raise the temperature, but this reducesthe Gibbs free energy.If we assume now a revrsible process operating at high temperature, the waste heat can also beconverted into electricity. Using thermodynamic basics and the Carnot limit we get the samemaximum efficiency as mentioned above. Figure 7.2 shows the efficiency limit of fuel cell, a heatengine and a combined cycle. Note that the temperature limit of the heat engine is the combustiontemperature of the fuel.A fuel cell operating at around 800-1000°C can approach the theoretical maximum efficiency, as

minimum difference. This so-called pinch temperature defines the target for the optimumprocess design, since in a real system heat cannot transferred from above or below thistemperature.Other considerations taken into account are the materials of the BOP components and theirmechanical layout.7.4 The molten carbonate fuel cell MCFCThe electrolyte is a molten mixture of alkali metal carbonates, usually lithium and potassium orlithium and sodium, which is retained in a ceramic matrix of LiAlO 2 . At the operation temperatureof 600°-700°C the alkali carbonates form a highly conductive molten salt, with CO 3 2- as mobile ion.Unlike other fuel cells, CO 2 and O 2 needs to be supplied at the cathode. Figure 7.9 shows theprincipal function of a MCFC with the electrode reactions.The Nernst reversible potential, taken the CO 2 transfer into account, is given byIn practice the CO 2 produced at the anode is externally recycled to the cathode. The anod exhaustgas is fed to a burner, which converts any unused fuel into water and CO 2 . The exit flow of theburner is then supplied with fresh air to the cathode inlet. The process also pre-heat the reactant air.Another method of reuse the CO 2 is a membrane seperator. The advantage of this method is unusedfuel gas can be recycled to the anode or used for other purposes. If an external supply of CO 2 isalready existing this can also be used.MCFC doesn't need nobel metal catalysts, due to the high operating temperature. Nickel (anode)and nickel oxide (cathode) is used.Furthermore it is also able to convert CO directly and reform hydrocarbon fuels directly. The highoperating temperature of MCFCs provides the opportunity for achieving higher overall systemefficiencies and greater flexibility in use of available fuels compared to low temperature cells.Unfortunately, the higher temperatures and the aggresive medium of molten carbonate electrolytealso place severe demands on the corosion stability and life of the cell components.7.4.2 Implications of using a molten carbonate electrolyteMCFC uses a liquid electrolyte that is immobilised in a porous ceramic matrix. The electrolyteinterfacial boundaries are established by a balance in capillar pressures in the MCFC. By properlyco-ordinating the pore diameters in the electrodes and in the electrolyte matrix, the electrolytedistribution is established. The smaller pores in the matrix remain completely filled, while thebigger pores in the electrodes are partially filled. The electrolyte management is a critical point toachieve hich performance and endurance.

7.4.3 Cell components in a MCFCElectrolyteMCFC electrolytes contain typically 60wt% of carbonate constrained in a matrix of 40wt%LiOALO 2 . The ohmic resistance of the electrolyte and especially the ceramic matrix has animportant effect on the operating voltage. They account for around 70% of the ohmic losses, whichdepends mainly on the thickness t of the electrolyte according toΔV = 0.533 x tThe ceramic matrices can nowadays be made quite thin (0.25 – 0.5mm), by using tapecastingmethods, what counteracts the long-term stability, which is obtained with thicker materials.Note that, in contrast to the other types, the final preperation is carried out once the stackcomponents are assembled. The whole package is heated up slowly. At its melting temperature thecarbonate is absorbed by the matrix. That leads to a significant shrinkage of the stack. Damages dueto the shrinkage have to be provided. In addition, a reducing gas has to be supplied to the anode,while heating up, to ensure that the nickel anode remains in the reduced state.Heating and cooling of a MCFC needs time to avoid cracking the electrolyte matrix. It is alsonecessary to protect the anode at lower temperatures of oxidation with inert gas. Therefore they arebest used in continous processes.AnodesAnodes are made of porous sintered NiCr/NiAl alloy, with a thickness of 0.4 to 0.8mm and aporosity of 55 – 75%. Cr is added in order to reduce the sintering of the Ni during cell operation. Itleads on the other side to bigger pores, so less surface area and finally to a performance drop, due toredistribution of carbonate from the electrolyte. Cr also reacts with the Li in the electrolyte ovr thetime. To overcome this, Al is added, to improve the creep resistance in the anode and reduceelectrolyte losses. The anodes have achieved a commercial acceptable stability but are veryexpensive.A high surface area of the anode is not required, because the anode reaction is fast in respect to thecathode reaction. Because of this the anode is partially flooded to provide a reservoir for carbonate.Nowadays, using the tape cast structure it is possible to control the pore distribution; Small poresclose to the electrolyte, larger pores near the fuel gas channels. However, long term electrolyte lossis still a significant problem with MCFC.CathodesThe major problem with the cathode is that the nickel oxide has a small, but significant, solubility inmolten carbonate, thus metallic nickel can precipitate out in the elctroclyte, that can cause internalshort circuits with a subsequent loss of power. The precipitated nickel can act as a sink for nickelions, which promates further dissolution. This problem can be reduced by using basic carbonates,operating at atmospheric pressure and low CO 2 partia; pressure.Non-porous componentsThe bipolare plates for a MCFC are usually made of stainless stell, which is coated at the anode sidewith nickel, due to the reducing environment at the anode, and with a thin layer of aluminium toavoid corrosion. The sealing is achieved by connecting the bipolare plate with the liquid electrolyteoutside the electrochemically active area.7.4.5 Internal reformingThe principal variations of internal reforming in a MCFC are direct internal reforming DIR andindirect internal reforming IIR. DIR offers a higher cell performance. The product of the reformingreaction, hydrogen, is directly consumed by the electrochemical reaction, therefore the reformingreaction is shifted in the forward direction. This leads to a higher conversion of hydrocarbon thanwould normally expected. A thigh fuel utilisation in a DIR MCFC nearly 100% of methane is

converted into hydrogen at operating temperature. IR eliminates the costs of an external reformerand thus systems efficiency is improved, but the cell configuration can get more complex.IR is only possible with an additional metal catalyst, due to the low anode surface area. Keyrequirements for the MCFC reforming catalyst area-specific sustained activity to achieve the desired cell performance and lifetime resistance to poisons in the fuel resistance to alkali/caronate poisoning7.4.6 Perfomance of MCFCThe performance curve of a MCFC is defined by cell pressure, temperature, gas composition andutilisation. State of the art MCFCs operate in the range of 100 – 200mA/cm 2 and 750 – 900mV percell.Influence of pressureThe reversible voltage change in respect to a pressure change is given by the Nernst equation. Inpractice the voltage gain is greater because of the reduced cathode polarisation. Increasing theoperating pressure ofMCFCs result in enhanced cell voltages because of the increase in the partialpressure of the reactants, increase in gas solubilities and increase in mass transport. Anotheradvantage of a pressurized system is that the exiting gases can be used to drive a gas turbine. On theother side compressing requires a lot of power and undesirable side reactions can occur. Higherpressure also inhibits the steam reformation if internal reforming is used. In practice the benefits ofincreasing pressure only outweigh the disadvantages up to 5bar.To provide gas crossover between the electrdodes and leakage the pressure differences between theelectrodes as well as to the outside should be as small as possible. Therefore, the stack is enclosedin a pressure vessel.Influence of temperatureThe reversible voltage of a MCFC decreases with higher temperature due to the decrease of Gibbsfree energy and the change in equilibrium gas composition at the anode. Under real conditions theeffect of cathode polarisation dominates; Increasing temperature, decreasing cathode polarisationand this leads to higher operating voltages of the MCFC. However above 650°C the effect isnegligible small and undesired side effects, as electrolyte evaporation and corrosion, rise. Generally650°C is considered as an optimum operating temperature.7.5 The solid oxide fuel cell SOFC