fuel cell systems explained_summary (1)

7/28/2019 Fuel Cell Systems Explained_Summary (1)

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-Fuel Cell Systems Explained-(Summary of chapter 1-4, 7, 8)

1. Introduction

1.1 Hydrog en fuel cel ls

basic pr inc ip les

Definition: A fuel cell is an electrochemical device that converts a supplied fuel to electrical energy

and heat continuously, so long as reactants are supplied to its electrodes. Neither the electrodes nor

the elctrolyte are consumed by the operation of the cell (page 17).

Example: Acid electroclyte fuel cell

Anode: hydrogen gas ionises, releasing electrons and creating H+ ions.

2H2 4 H+ + 4e-

Cathode: oxygen reacts with the electrons from the electrode and H+ ions from

the electrolyte to form water.

O2 + 4 H+ + 4e- 2H2O

An 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 as

electrolyte, so called proton exchange membranes PEM.

Note:For all primary batteries and therefore for fuel cells, are the electrons flowing from the anode to the

cathode. Thus the anode is electrical negative and the cathode is electrical positive.

Cations: positive ions

Anions: negative ions

1.2 What l imits th e current?

First the activation energy has to be exceeded for an exothermic reaction taken place. If te

probability 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)


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The chemical reactions have to take place at the surface of the electrodes, due to removing or

supplying electrons. These reactions of the fuel with the electrolyte at the electrode is called a three

phase 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 environment

1.3 Connect in g cel ls in series bipo lar plate

A single fuel cell generates only a small voltage drop (0.7V). In order to generate a useful voltage

several cells are connected into series. Such a connection is called a stack. The simplest way to

create a series connection would be to connect the edge of each anode to the cathode of the next

cell. The problem of such a system is, that the electrons would have to travel through the whole

electrode, 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 gas

supply 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 supp ly and coo l ing

A major problem of manufacture fuell cells is leakage of the reactant gases. In order to seperate the

gases 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


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1.5 Fuel cell ty pes

Besides the practical issuses fuel cells have to fundamental technical problems:

slow reaction rate, low current and power hyrogen is not readily available as fuel

There are six viable systems of fuel cells:

Type Mobile

Ion

Operating

temp. [C]Anode and cathode

reactions

Application and notes

Alkaline

AFC

OH-

50-200 Space vehicles

Proton exchange

membran

PEMFC

H+

30-100 2H24H++4e- (a)

O2+4H++4e-2H2O (c)

Vehicles and mobile

applications, low power CHP

Direct methanol

DMFC

H+

20-90 Portable electronic systems,

low power but long running

time

Phosporic acid

PAFC

H + ca. 220 For small CHP plants

(hundreds of kW), first

commercial one

Molten

carbonate

MCFC

CO32-

ca. 650 For medium to large CHP (up

to MW)

Solid Oxide

SOFC

O - 500-1000 For all sizes of CHP (from kW

to multi MW)

Advantages and disadvantages are discussed later.

1.6 Other cells Some fuel cel ls, som e not

1. Biological fuel cellsuse an organic fuel, like methanol or ethanol, and enzymes to produce energy. Not yet

commercial

2. Metal air cells (batteries)at the negative electrode the metal reacts with an alkaline electrolyte to form metal oxide or

hydroxide. 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 is

dissolved into the electrolyte, which have to be renewed after some time. Consumption of

anode 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. For

discharging the electrolytes are supplied back to the electrodes. Is used for very large

capacity rechargeable batteries. The electrocolyte changes during operation and the system

cannot work idenfinitely.


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1.7 Other parts o f fuel cells

The core of a fuel cell power system consists of electrodes, electrolyte and bipolar plate. The

additional parts, also called balance of plant BOP, make a up a large contribution to the whole

system. 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 PEM

1.8 Figur es used to com pare systemsFor the fuel cell:

Current density to compare electrodes and electrolytes Specific operating voltage (0.60.7V) Power density

Note: 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 of

products 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 and

electrolyte. 'Percentage deterioration per hour'

Gradual decline in voltage in mV/1000h (somtimes) Efficiency

1.9 Advantages and app licat ions

Disadvantages:

mainly the costsAdvantages: Efficiency

Generally 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 reliable

and long-lasting systems.

Low emissionsUsing hydrogen leads to pure water as by-product. A fuel cell can operate with zero

emissions. CO2 emission for H2 production is not considered.

SilenceFuel cells operate very silent, even with extensive fuel processing

Using H2 as fuel can count for disadvantage or advantage. In a future perspective with a limitation

of fossil fuels, H2 will be a major fuel.


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Fuel cells can be used successfully in CHP plants or in mobile power systems, like vehicles or

electronic equipment.

2. Efficiency and open circuit voltgage

2.1 Energy and the EMF of the hyd rogen fu el cel l

The 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 pressure

and/or volume. In case of a fuel cell are the moving electrons in the electric circuit meant

with external work. Exergy

is 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, at

standard conditions (25C or 298.15K, 0.1MPa).

Using this convention, the terms 'Gibbs free energy of formation', Gf, and 'enthalpy offormation' is used. In case of a hydrogen fuel cell operating at standard temperature and pressure

the 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, Gf, is the energy released. Given by the equation

Gf= Gfof products - Gfof reactants

It is convenient to use the molar specific Gibbs free energy of formation, written g f.


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Note:The mole is a measure of the amount of a substance in respect to its molar mass.

H2 molar mass: 2amu

1 mole = 2g

H2O molar mass: 18amu

1 mole = 18g

One 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 1023

x 1.602 x 10-19

C = 96485 C

Using the basic reaction of the hydrogen/oxygen fuel cell, it can be seen that one mole of H2 and

half a mole O2 deliver one mole of H2O. However, note that the Gibbs free energy of formation is

not constant. It depends on the temperature and state (liquid or gas). When the values are negative it

means that energy is released.

Electromotive force EMF = reversible open circuit voltage OCF

In 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 2

used. So for one mole of H2, 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, the

electric 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 even

when no current is drawn from the cell.

In general

with z is the number of electrons per molecule of fuel.

2.3 Effic iency and eff ic iency l im its

The 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 the

chemical energy of the reactants by taken the irreversibilities into account, due to the fact that the

Gibbs 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 possbile

efficiency derived by the ratio of the electrical energy produced per mole of fuel and the change of

enthaply 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 by

max = g f/ h fx 100%

Note that h 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 betweeen

both is the molar enthalpy of vaporisation. Any statement of efficiency should say whether it relates


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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 operating

temperature. 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 cell

is not exactly occur for other cell types.

2.4 Effic iency and the fuel cel l vo ltage

The 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 is

introduced.

This is equivalent to the ratio of fuel cell current and the current that would be obtained if all the

fuel were reacted. The fuel cell efficiency in respect to the HHV is therefore given by

A good estimation for f is 0.95.


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2.5 The effect of pressu re and gas c onc entrat ion

2.5.1 The Nernst equation

As mentioned in 2.1 is the Gibbs free energy change dependant on the temperature and in a more

complex 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 as

a = P / P0

with P: partial pressure of the gas

P0: standard pressure, 0.1MPa

The activity of a gas is proportional to its partial pressure. The produced water in a fuel cell can

either be a liquid or steam. In case of liquid water, it is a reasonable approximation to assume

aH2O=1.

The activities of the reactants and products of a chemical reaction

jJ + kK mM

modify the Gibbs free energy change, given by the equation

with g f0: molar Gibbs free energy change at standard pressure (from gas tables)

R = 8.314 j/Kmol molar gas constant

T: temperature

It can be seen, that when the activities of the reactants increase, g f becomes more negative, somore energy is released. On the other side, if the activity of the product increase, g f becomes lessnegative, so less energy is released.

Substitute this equation in the equation of the EMF gives the Nernst equation.

with E0: EMF at standard pressure

The Nernst equation gives the EMF dependant on the product and reactant activities. The derived

EMF, also called Nernst voltage, is the reversible cell voltage that would exist at a given

temperature 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 pressure

fraction of a gas mixture are all equal.

Example: CH4 + H2O 3H2 + CO2


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The products are three parts of H2 and one part of CO2 by moles and volume. If the

reaction takes place at 0.1MPa the partial pressures can be derived

PH2 = x 0.1MPa = 0.075MPa

PCO2 = x 0.1MPa = 0.025MPa

2.5.3 Fuel and oxidant utilisation

As 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 reaction

products increase. That makes the logarithmic term in the Nernst equation smaller, and so the EMF

will fall. This circumstance will vary over the cell and will be worst at the fuel outlet as the fuel is

used. Due to the conducting material of the bipolar plate the voltage is constant over the cell, that

means that the current will vary. The current density will be lower nearer the exit where the fuel

concentration is lower. The RT term indicates that the voltage drop due to fuel utilisation will be

greater for high temperature fuel cells.

A high system efficiency normally requires a high fuel utilisation. That counteracts the above

described 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 pressure

When the system pressure is known, the Nernst equation can be rewritten, using

PH2 = P with constant depending on the molar masses and concentration of H2PO2= P with constant of O2PH2O= P with constant of H2O

as

It can be seen, thet the EMF of the fuel cell is depending on the system pressure P. A hihgher

pressure leads to a higher voltage. For high temperature fuel cells, for example SOFC at 1000C,

the predictions of the Nernst equation correlate with practise. For lower temperatures, as in a PAFC

at 200C, the voltage change is even bigger. This occurs due to increasing pressure also reduces the

losses at the electrodes, especially at the cathode. A similar effect occurs from switching from air to

oxygen (). The voltage losses at the cathode decline for pure oxygen.


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2.6 Summary

reversible OCV or EMF

max efficiency max = g f/ h fx 100% efficiency of working H2 fuel cell relative to HHV

Nernst equationThe pressure and the concentration of the reactants affects the Gibbs free energy, and thus

the voltage, given by the Nernst equation.

3. Operational fuel cell voltage

3.1 Introduc t ion

The operating voltage of a fuel cell is always below the theoretical open circuit voltage derived in

chapter 2. Figure 3.1 shows the performance of a typical hydrogen fuel cell operating at 70C and

normal 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 again

For 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 initial

voltage drop is significant smaller.


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Figure 3.2 shows the performance of a SOFC operating at 800C.

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 again

The focus of chapter 3 is on the questions: What causes the voltage to fall below the reversible

value and how can this situation be improved?

3.2 Termino log y

Commonly five terms are used to describe this voltage difference:

Overvoltage or overpotential Polarisation Irreversibility Losses Voltage drop

Every term is used in the book!

3.3 Fuel cell irreversib il i t ies causes of vol tage drop

The characteristic shape of the voltage/current density graphs results from four major

irreversibilities.

1. Activation lossescaused by the slowness of the reactios at the electrode. Voltage is used to drive the reactions. Non-

linear.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 the

Nernst equation.


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3.4 Act ivat ion loss es

3.4.1 The Tafel equation

The Tafel equation and plots are experimental results. Tafel observed, that the overvoltage at the

surface 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 plots

Tafel equation

The constant A is higher for slow chemical reactions. The exchange current density i0 is higher if

the reaction is faster. The current density i0 can be considered as the current density at which the

overvoltage begins to move from zero. The equation is only true for i0 > i.

3.4.2 The constants in the Tafel equation

For a hydrogen fuel cell, the constant A is given by

with :charge transfer coefficient

The charge transfer coefficient varies from 0 to 1 and amount of electrical energy that is used tochange the electrochemical reaction rate. The value of depends on the electrode material.The impact of the temperature in the equation is far outweighted by the effect of i0 with changing

temperature and thus is not considered. Indeed we shall see, that the key to making the activation

overvoltage as low as possible is i0, which can vary by several orders of magnitude.

Even at zero current density the electrochemical reaction at the electrodes is taken place, but the

reverse 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 i0. If i0 is high, that means, that the surface of

the 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 and

should be as high as possible.


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In order to get the current instead of the voltage the Tafel equation is cahnged in the Butler-

Vollmer equation

The importance of i0 can be seen in the following graph for values of 0.01, 1.0 and 100mA/cm2.

Measurements of i0 shows a great variation in respect of the e;ectrode material. For hydrogen fuel

cells are the values at the oxygen electrode (cathode) lower by a factor of 105. Therefore is the

overvoltage 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 the

equation for the total overvoltage combine the voltage drops at both electrodes. I can be expressed

as

with

3.4.3 Reducing the activation voltage

In order to reduce the activation voltage it is most important to increase i0. 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 O2 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 i0. Has the effect of raising cell voltage by a constant amount at most

currents, and so lead to raising OCV. The last two points explain the difference between thearetical

and practical OCV.

In low and medium temperature fuel cells, activation overvoltage is the most important

irreversibility 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.


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3.5 Fuel cros sov er and internal currents

Even if the electrolyte is designed toconduct only ions, it always occurs that a small amount of

electrons pass through it. In practical fuel cells the fact that a certain amount of fuel diffuse unused

through the electrolyte is more important. The unused fuel will react directly with oxxygen at the

cathode 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 in

humidity 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 the

fuel consumption. The consumption rate of hydrogen [moles/s] is related to the current by the

following equation.

The equation of the cell voltage can be refined, including the internal current density in.

To conclude, the effect of internal current is much less for high temperature cells, because i0 is

much higher. For low temperature cells internal current and fuel diffusion is usually not of great

importance in terms of operating efficiency, but it has a marked effect on the OCV.

3.6 Ohmic loss es

The losses occured by the electrical resistance of the electrodes and the interconnection and by the

resistance against the flow of ions in the eelctrolyte. The ohmic voltage loss is important in all types

of 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.

Vohm = i x rwith i in [mA/cm2]

r in [kcm2]

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 concentrat ion lo sses

Due to the electrochemical reactions at the eelctrodes, hydrogen and oxygen are consumed. That

leads to a change in concentration, which leads on the other hand to a change in partial pressure of

the reactants, which results in a voltage drop over the cell. The pressure changes depend on the

current drawn from the cell and the physical characteristics of the gas supply system, relating to the

fluid 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 is

mentioned, which shows the effect of reducing the partial pressure on the OCV.


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Assumption:

Introduction a limiting current density i1 at which the hydrogen consumption rate is equal to the

max. supply rate. At i1 the pressure would have reached zero.

P1 is the pressure when i is zero. If we assume a linearly fall from P1 to P(i1), tha the pressure at anycurrent density is given by

Substitution 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 systems

An empirical approach is more favourable, because it provides an equation which delivers much

better 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 Combinin g th e irreversebi l i t ies

The operating voltage of a fuel cell at a current density i is given by

with E: reversible OCV

in: internal and fuel crossover equivalent current density

A: slope of the Tafel line

i0: exchange current density

m & n: constants in the mass-transfer overvoltage

r: area-specific resistance

This equation can be simplified, due to in is usually very small and has little impact on operating

losses of fuel cells at working currents, so it will be cancelled. The term of the activation

overvoltage can be rewritten as


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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 doub le layer

Whenever two differnet materials are in contact, there is a build-up of charge on the surfaces or a

charge transfer over the junction. In electrochemical systems, the charge double layer is formed by

diffusion effects, by the reactions between electrons in the electrodes and ions in the electrolyte and

by an applied voltage. The probability of electrochemical reaction taking place depends on the

density of the different charges at the double layer; more charges lead to higher current. The charge

seperation at the double layer generates an electric voltage, in case of a fuel cell the activation

overvoltage. Adding catalysts to the electrodes increases the probability of a reaction; a higher

current 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 a

fuel 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 Dist inguish ing the dif ferent irreversebi l i t ies

How can the different kinds of voltage losses be distinguished in respect to conditions they mainly

occur?

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 the

impedance is calculated. Higher frequencies means less impedance of the capacitors.

Plots of the impedance over the frequencies shows the values of the equivalent electric circuit

elements, 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.


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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 PEMFC

A PEMFC, also called solid polymer fuel cell, consists of an ion conducting polymer electrolyte

bonded at each side with a catalysed porous electrode. These membrane electrode assemblies MEAs

are 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 anode

2H2 4 H+ + 4e-and at the cathode

O2 + 4 H+ + 4e- 2H2O

The 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 and

the cell works in every direction. This means that the PEMFC is very suitable for a wide range of

applications, like vehicles and portable electronics, but can also be used in CHP power plants.

Similarities for applications:

electrolyte electrode structure and catalyst

Differences for applications:

water management cooling the fuel cell connecting the cells in series operating pressure used reactants


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4.2 How th e polymer electrolyte work s

The basicly used material for the electrolyte is fluoroethylene. The established industrial polymer

electrolyte is Nafion from the company Dupont. The construction of the electroclyte material

consists 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, water-

repellent.

2. The PTFE polymer is sulphonated, that means a side chain, ending with sulphonic acidHSO3 is added to the molecul. The resulting structure is caleed anionomer, due to the ionical

bond of the H+ and SO3- ions. The ionic bondage results in a clustering of the side chains in

the overall structure.Another important property is, that HSO3 is highly hydrophyllic,

attracts water.

In conclusion, we are creating water attracting areas in a water repellent material. Within these

hydrated regions, H+ ions are relatively weakly bonded and are able to move (dilute acid). Even if

these 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 material

4.3 Electrodes and electrode s tructur e

The electrodes in a PEMFC are essentially the same, a high porous conducting material with

platinum 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, this

powder is fixed on corbon cloth or carbon paper, which builts the basic structure of the electrode.

Also PTFE is added due to its hydrophobic

property, so that the water as the reaction product

is expelled from the surface. Additional the

carbon cloth/paper acts as a gas diffusion layer

and diffuses the gas onto the catalysts. To form

the fuel cell, two of these electrodes are hot

pressed on each side of the electrolyte membran.

The result is a complete MEA.The alternative method involves building the

electrode directly onto the electrolyte. The

platinum on carbon catalyst is fixed directly to the

electrolyte by rolling or spraying. The electrode is

directly manufactured on the membrane.Then the

gas diffusion layer is added ,which provides the

electrical connection between the carbon-

supported catalyst and the bipolar plate, carries

the water away and also forms a protective layer

for 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, to

promote the three-phase contact between, the reactant gas, electrolyte abd electrode catalyst.


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4.4 Water management in t he PEMFC

Wtaer management is a crucial aspect in designing a PEMFC. The PEM requires a sufficient water

content, the proton conductivity is directly proportional to it. On the other side, should the

electrodes and the gas diffusion layer not be flooded and blocked. In an ideal case, the product

water 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 H2O with them, especially at high current densities. The

anode side of the PEM can dry out.

Drying effect of air at high temperaturesAt approximatly 60C the air will always dry out the elctrodes faster than H2O 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 more

excess water. Important for designing larger stacks.

Fortunately, all water movements are predictable and controllable. Water production and water drag

are 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 of

each side. The preceded external of humidification of the reactants is also controllable.

4.4.2 Air flow and water evaporation

In a PEMFC it is common to remove the product water by the air flow through the cell (expect the

special case: using pure O2). That means, that air is always fed at an higher rate into the cell, than

needed to provide the O2 for the reactions (> stoichiometric rate). In practice, the stoichiometry will 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, weather

conditions and other factors. One way of describing it is the humidity ratio, the ratio of water

vapour in respect to the other gases in air, also called absolute humidity or specific humidity.

= mw / mawith mw : mass of water

ma: mass of dry air

The relative humidity of air gives a good impression of the drying effect of air and is defined as

= Pw / Psatwith Pw : partial pressure of water

Psat : 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 Psat varies with temperature in a highly non-linear way. Itincreases more rapidly with incresing temperature.


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Another way of describing the water content is the dew point, the temperature to which the air have

to be cooled in oeder to reach saturation.

Example: Pw = 12.35kPa dew point T = 50C (see table 4.1)

Sometimes it is necessary to humidify the gases going into the cell, so the mass of H2O added hasto be calculated. To do so, note that the mass of a species in a mixture is proportional to the product

of molecular mass and partial pressure of the species.

And so we get with P = Pa + Pw

Note 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 air

The air flow in a PEMFC must be dry enough to evaporate the product water but also wet enough

not to dry the PEM out, thus it have to be carefully controlled. The humidity should be between 80

and 100%. In the following section a theoretical formula is derived to set up proper conditions for

the 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 equations

can be used.


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With Pe: electrical Power of the stack

Vc: voltage of a single cell

O2: supply or use rate of oxygen: air stoichiometry

The exit flow rate of the non-oxygen components of air is the same as in the inlet. The non-oxygen

components amount to 79% of the air. This will be graeter than the oxygen molar flow rate by the

factor 0.79/0.21 = 3.76.

Substituting the molar flow rates in the first equation delivers

The water vapour pressue at the outlet only depends on the air stoichiometry and the air pressure at

the 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 reduce cathode performance increasing pressure more enrgy for the compressor

None 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. This

requires additional equipment, extra costs, but is often justified by an increased performance. The

water pressure can be derived, taking the humidity of the inlet air into account, is given by

with

4.4.4 Running PEM fuel cells without extra humidification

It is possible to run a PEM fuel cell at the right conditions without external humidification, by

choosing suitable operating temperatures and air flowrates.


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Note that humidities are lower at greater airflow. At higher temperatures the relative humidity falls

sharply. Note that if the relative humidity of the excess air is below 100% the fuel cell has given all

its water, but the air is not yet saturated. That means the fuel cell dries out. On the other side a

relative humidity greater than 100% is basically impossible, because the air stream would contain

condensed water and the electrons would become flooded.

At temperatures above 60C, the relative humidity of the exit air is below 100% at all reasonable

values of . Above this temperature extra humidification of the reactants is essential in PEMFCoperating. This causes the difficulties in choosing the optimum operating temperature; higher

temperatur 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 design

a balanced water distribution within the cell.


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4.4.5 External humidification - principles

Small fuel cells can be operated without external humidification, they will perform, even with the

correct air flowrate and temperature, muc =h worse than a identical humidified cell. Operating

temperatures over 60C are desirable to reduce the cathode activation losses. Considering the

additional equipment and costs for extra humidification, it is still more economic to operate the fuel

cell at maximum possible power density.However, water as a additional input is not desirable. The water has to be extracted from the exit air

flow. That requires a water condensation or seperation system in the exit path. The intrnal recycling

of 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 in

pressurized systems.

Benefits of increasing humidity improve the performance especially at operating at higherpressures.

4.4.6 External humidification methods

In this operation process no industrial standard has been yet emerged. In the following section eight

different 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. Self-

regulating, because no water is drawn if the wicks are saturated.

Problem: Sealing the cell and no cooling effect

5. 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 the

reactants have to be driven at pressure.

Problem: no cooling effect

All the mentioned methods requires liquid water. There are other methods which don't need a water

condensation 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 the

membrane and is evaporated by the dry gas at the inlet. A PEM membrane can be used.

Advantage: evenly spread humidity, simple, no moving parts

Disdvantage: no control

8. 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 combine

to water. The improved performance justifies the fuel loss.


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4.5 PEMFC cooling and air sup ply

4.5.1 Cooling using the cathode air supply

PEM 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 is

The way the heat is removed depends greatly on the size of the fuel cell, beolow 100W it is possible

to 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 compact

types a small fan blows the reactants and cooling air through the cell, though a large proportion of

heat 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 air

Is best shown with a specific example. fuel cell power Pe [W] single cell voltage Vc = 0.6V operating temperature T = 50C cooling air enters at 20C and leaves at 50C (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 cp, with the

flowing rate and a temperature change T.

With cp = 1004 J/kgK

T = 30KVc = 0.6V

The 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 50C of 26%, that means that the relative humidity decreases, and

so the PEM will quickly dry out. In order to reduce 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 adding

cooling channels in the bipolar plates of adding additional cooling plates in the cell. Air cooling

works for cell powers up to 2 kW.

4.5.3 Water cooling of PEMFC

For larger fuell cells water cooling is more useful, due to the higher cooling effect of water. The

operation process are basically the same as for external air cooling. Cooling channels or additional

cooling plates distribute the water in a seperate cooling circuit in the cell. Water cooling is, for

example, in cases of heat recovering in small CHP plants more preferable.


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4.6 PEMFC connectio n the bipo lare plate

Bipolar 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 plates

In the bipolar plate the reactants are fed over the electrode in a fairly dimple pattern or parrallel

grooves. 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 gas

flow. That means more work and thus more energy is needed to push the gas through.

The grooves are very narrow, less than 1mm2. In order for water droplets not to form, the system is

arranged so that the pressure drop along a channel is greater than the surface tension holding a

water droplet in place. That has to work even when the gas suuply is stopped. The latest

development by Ballard is a rectangular plate, several times longer than wide, with long straight

channels, which have a high pressure drop and no inefficient bends and turns.

4.6.3 Making bipolar plates for PEMFCs

The bipolare plate has many tasks in the cell. Collecting and conducting the current from the anode

to the cathode of the next cell, evenly distributing the fuel to the anode and oxygen/air to the

cathode and sometimes carrying a cooling fluid. Furthermore keeping all the gases and fluids apart

from 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 without gas permeability < 10-7 mbarL/scm2

corrosion resistant stiff, flexural strength > 25 Mpa low costs slim for min stack volume light for min stack weight short production time

The materials and production method used, varies considerably. In terms of easier manufacture the

plates are often made in two halves. Then, the cooling channels can be cutted and don't have to be

drilled into the plate. The most commonly used materials and methods are:

machining of graphit paperellectrically conductive, reasonably easy to machine, low density

Disadvantages: long machining time, brittle so careful handling needed, porous so a few mmthickness is needed

inection molding of graphite filled polymercheap, short production time

Disadvantages: 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 order

to 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


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flow field is made of foamed metal, with a sponge-like structure. Then a thin solid metal plate is

added 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 steps

4.6.4 Other topologies

Fuel 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 densitiesa

compromise with higher electrical resistance but therefore cheaper and simpler is helpful. Such a

compromise is shown in figure 4.20. A system of three cells in series, consisting a cheap plastic

body and requires only on chamber for the fuel and on for the air. It is not a compact system, but the

potential 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. If

one cell becomes rather warmer, this would lead to more rapid water evaporation, and so to higher

resistance and higher temperatures. The cell got in a vicious circle and would end up dried.

4.7 Operat ing p ressure

Running 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 as

compressors and turbines. A typical value of power consumption is about 20%.

4.7.2 Simple cost/benefit analysis of higher operating pressures

We have seen, that V is proportional to the logarithm of the pressure rise.

We also saw, that the activation overvoltage is related to the exchange current by a logarithmic

functio. So we can say, that if the pressure is increased from P 1 to P2, then there is a gain in voltage

and power.

with C: voltage gain constant, depeding on the change of i0 with the pressure, R, T and F.

I: current

n: number of cells

On the other side we have the power consumption of the compressor and the electric propulsion.


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with T1: entry temperature

c: compressor efficiencym: motor and drive efficiency

: air flow rateThe air flow rate is connected to the electric power, the cell voltage and the air stoichiometry, by

Substituting the electric power by nIVc and the air parameters cpand , 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 by

The 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 a

realistic view of the effect of ibcreased pressures.

4.7.3 Other factors affecting the choice of pressure

Gains:

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, about

150C. 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 80C unless the

system 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 complete

different aspects and can be much higher.


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4.8 Reactant compos it ion

In 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 carbon

monoxide. CO has a great negative effect on the performance of the cell, by poisoning the

electrodes. CO covers the platinum, preventing the hydrogen fuel from reaching it.This happens

even 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 thus

wasting 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 cells

Even if the no loss OCV decreases with increasing temperature outweigh the advantages the

disadvantages 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, in

CHP 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 200C and needs a noble metal catlyst, and so, like the PEM is poisoned by any

CO in the fuel gas, thus its fuel processing system is necessarily complex.

MCFC:

operates at around 650C. The main problem is the degradation of cell components over long

periods. But it shows great promises for the use in CHP plants.

SOFC:

operates between 650C and 1000C. Since it is a solid state device, it has many advantages due to

its 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 and

SOFC must always be thought of as a integral part of a complete fuel processing and heat

generating system. The common features of all three types are:

used fuel needs processing fuel utilisation

fuel 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


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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 in

designing such a system.

7.2 Comm on features

7.2.1 An introduction to fuel reforming

The production of hydrogen from hydrocorabons usually involves steam reforming.

The steam reforming reaction with natural gas is an endothermic reaction, that means heat needs to

be suuplied for a suficient reaction rate. In most cases fuel reforming takes place at about 500C

and can be used from the exhaust gases of the fuel cell itself. The PAFC requires additional burning

of fuel to achieve that temperature, what reduces the effcicncy to 40-45%. For the MCFC and the

SOFC 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 to

reducing 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 the

electrodes. This proces also needs heating up the fuel over 350C. The higher operation

temperatures of theMCFC and the SOFC allows the basic steam reaction taken place in the fuel cell

stack itselfs. Furthermore the steam neede is already present in the stack. The exhaust steam at the

anode is used for the fuel reforming. That simplifies the design of MCFC and SOFC.

7.2.2 Fuel utilisation

Fuel utilisation has always to be taken into account when the hydrogen is fed in a mixture to the

stack. As the gas mixture passes through the cell, the hydrogen is utilised, the other components

simply 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 hydrogen

pressure.

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 the

oxygen 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.


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Sometimes the voltage drop can be reduced, by using opposite flow direction (counteer-flow), so

high oxygen concentration at the point of the cell with low hydrogen concentration. A factor ignored

yet, is that in MCFC and SOFC the product steam ends up at the anode, hydrogen is replaced by

steam. This will also cause a fall of the OCV.The important conclusion is, that in the case of reformed fuel containing carbon dioxide or when

internal reforming is applied, all the hydrogen can nevver be consumed in the fuel cell itself. Some

hydrogen must pass right through the cell and can be used later to provide energy to process the fuel

or to be burnt to increase the heat energy. To find an optimum, extensive modelling is needed.

7.2.3 Bottoming cycles

Bottoming cycles mean the use of waste heat from a fuel cell axhaust gases to drive some kind of

heat engine, for example boiler and steam turbine or a gas turbine. These combined cycle systems

offer an elegant way of producing very efficient systems of generating electricity.

However, the Gibbs free energy for hydrogen decreases with temperature. So the highest theoretical

possible efficiency is at ambient temperature.

On the other side are the losses high at abient temperature and so the practical efficiency would be

well below that limit. To reduce the voltage losses we have to raise the temperature, but this reduces

the Gibbs free energy.

If we assume now a revrsible process operating at high temperature, the waste heat can also be

converted into electricity. Using thermodynamic basics and the Carnot limit we get the same

maximum efficiency as mentioned above. Figure 7.2 shows the efficiency limit of fuel cell, a heat

engine and a combined cycle. Note that the temperature limit of the heat engine is the combustion

temperature of the fuel.

A fuel cell operating at around 800-1000C can approach the theoretical maximum efficiency, as


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well as heat engines. Figure 7.3 sketches a possible combined cycle system based on an SOFC.

The work done by the two compressors are the major losses in the system. Assuming realistic

values such a system can reach efficiencies of around 60%. Working at higher pressures offers the

possibilties to reach even higher pressuers.

7.2.4 The use of heat exchangers -exregy and pinch technology

In fuel cell systems are process streams that need heating and others need cooling. A challenge for

the designer is to use the available heat in the most efficient way. The hot gas streams, which need

to be cooled deliver their heat over heat exchangers to the streams which need it.

Exergy may be defined as a kind of energy which can be converted into work, like the potential

energy. Energy is conserved in all processes, exergy only in reversible processes. Real processes are

irreversible, so that exergy is always partly consumed to give enthaply. The exergy is measured by

the change out of the thermodynamic equilibrium (reference state).

Note: The higher the temperature, the higher the exergy.

For a SOFC and PEMFC having the same power output and efficiency, the heat, so the enthaply

content, of the exhaust streams will be the same. However the heat produced by the SOFC would be

at higher temperature and therefore has a higher exergy. It can be converted into usefl energy.

Pinch analysis or pinch technology is a methodology that can be applied to fuel cell systems for

deciding the optimum arrangement of heat exchangers and other units so as to minimise the exergy

loss.

1. Defining the required chemical processes and set up a system configuration, showing all hotand cold process streams. Calculating the enthalpy content for each process and building

heating and cooling curves.2. Sum them together to derive a total heating and a total cooling curve.3. Slid the curves together at the y-axis and note the point the curves pinch together, so the


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minimum difference. This so-called pinch temperature defines the target for the optimum

process design, since in a real system heat cannot transferred from above or below this

temperature.

Other considerations taken into account are the materials of the BOP components and their

mechanical layout.

7.4 The molten carbonate fuel cell MCFCThe electrolyte is a molten mixture of alkali metal carbonates, usually lithium and potassium or

lithium and sodium, which is retained in a ceramic matrix of LiAlO2. At the operation temperature

of 600-700C the alkali carbonates form a highly conductive molten salt, with CO32- as mobile ion.

Unlike other fuel cells, CO2 and O2 needs to be supplied at the cathode. Figure 7.9 shows the

principal function of a MCFC with the electrode reactions.

The Nernst reversible potential, taken the CO2 transfer into account, is given by

In practice the CO2 produced at the anode is externally recycled to the cathode. The anod exhaust

gas is fed to a burner, which converts any unused fuel into water and CO2. The exit flow of the

burner is then supplied with fresh air to the cathode inlet. The process also pre-heat the reactant air.

Another method of reuse the CO2 is a membrane seperator. The advantage of this method is unused

fuel gas can be recycled to the anode or used for other purposes. If an external supply of CO2 is

already 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 system

efficiencies and greater flexibility in use of available fuels compared to low temperature cells.

Unfortunately, the higher temperatures and the aggresive medium of molten carbonate electrolyte

also place severe demands on the corosion stability and life of the cell components.

7.4.2 Implications of using a molten carbonate electrolyte

MCFC uses a liquid electrolyte that is immobilised in a porous ceramic matrix. The electrolyte

interfacial boundaries are established by a balance in capillar pressures in the MCFC. By properly

co-ordinating the pore diameters in the electrodes and in the electrolyte matrix, the electrolyte

distribution is established. The smaller pores in the matrix remain completely filled, while the

bigger pores in the electrodes are partially filled. The electrolyte management is a critical point toachieve hich performance and endurance.


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7.4.3 Cell components in a MCFC

ElectrolyteMCFC electrolytes contain typically 60wt% of carbonate constrained in a matrix of 40wt%

LiOALO2. The ohmic resistance of the electrolyte and especially the ceramic matrix has an

important effect on the operating voltage. They account for around 70% of the ohmic losses, which

depends mainly on the thickness t of the electrolyte according to

V = 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 stack

components are assembled. The whole package is heated up slowly. At its melting temperature the

carbonate is absorbed by the matrix. That leads to a significant shrinkage of the stack. Damages due

to 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 also

necessary to protect the anode at lower temperatures of oxidation with inert gas. Therefore they are

best used in continous processes.

AnodesAnodes are made of porous sintered NiCr/NiAl alloy, with a thickness of 0.4 to 0.8mm and a

porosity of 5575%. 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 to

redistribution of carbonate from the electrolyte. Cr also reacts with the Li in the electrolyte ovr the

time. To overcome this, Al is added, to improve the creep resistance in the anode and reduce

electrolyte losses. The anodes have achieved a commercial acceptable stability but are very

expensive.

A high surface area of the anode is not required, because the anode reaction is fast in respect to the

cathode 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 pores

close to the electrolyte, larger pores near the fuel gas channels. However, long term electrolyte loss

is still a significant problem with MCFC.

Cathodes

The major problem with the cathode is that the nickel oxide has a small, but significant, solubility in

molten carbonate, thus metallic nickel can precipitate out in the elctroclyte, that can cause internal

short circuits with a subsequent loss of power. The precipitated nickel can act as a sink for nickel

ions, which promates further dissolution. This problem can be reduced by using basic carbonates,

operating at atmospheric pressure and low CO2 partia; pressure.

Non-porous components

The bipolare plates for a MCFC are usually made of stainless stell, which is coated at the anode side

with nickel, due to the reducing environment at the anode, and with a thin layer of aluminium to

avoid corrosion. The sealing is achieved by connecting the bipolare plate with the liquid electrolyte

outside the electrochemically active area.

7.4.5 Internal reforming

The principal variations of internal reforming in a MCFC are direct internal reforming DIR and

indirect internal reforming IIR. DIR offers a higher cell performance. The product of the reforming

reaction, 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 than

would normally expected. A thigh fuel utilisation in a DIR MCFC nearly 100% of methane is


34/34

converted into hydrogen at operating temperature. IR eliminates the costs of an external reformer

and 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. Key

requirements 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 poisoning

7.4.6 Perfomance of MCFC

The performance curve of a MCFC is defined by cell pressure, temperature, gas composition and

utilisation. State of the art MCFCs operate in the range of 100 200mA/cm2 and 750900mV percell.

Influence of pressureThe reversible voltage change in respect to a pressure change is given by the Nernst equation. In

practice the voltage gain is greater because of the reduced cathode polarisation. Increasing the

operating 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. Another

advantage of a pressurized system is that the exiting gases can be used to drive a gas turbine. On the

other side compressing requires a lot of power and undesirable side reactions can occur. Higher

pressure also inhibits the steam reformation if internal reforming is used. In practice the benefits of

increasing pressure only outweigh the disadvantages up to 5bar.

To provide gas crossover between the electrdodes and leakage the pressure differences between the

electrodes as well as to the outside should be as small as possible. Therefore, the stack is enclosed

in a pressure vessel.

Influence of temperature

The 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 the

effect of cathode polarisation dominates; Increasing temperature, decreasing cathode polarisation

and this leads to higher operating voltages of the MCFC. However above 650C the effect is

negligible small and undesired side effects, as electrolyte evaporation and corrosion, rise. Generally

650C is considered as an optimum operating temperature.

7.5 The solid oxide fuel cell SOFC

fuel cell systems explained_summary (1)

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