Fuel Cell Lectures

8/11/2019 Fuel Cell Lectures

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Fuel Cell Thermodynamics


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Fuel Cells Basics

Fuel cells convert chemical energy directly into electrical energy.

Difference with batteries: fuel cells require a fuel to flow in order to produceelectricity.

Heat is produced from chemical reaction and not from combustion.

Types of fuel cells:Proton exchange membrane (PEMFC)Direct Methanol fuel cell (DMFC)Alkaline fuel cell (AFC)Phosphoric acid fuel cell (PAFC) (*)

Molten-carbonate fuel cell (MCFC) (*)Solid-oxide fuel cell (SOFC) (*)

(*) Suitable for microgrids.


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Fuel cells operation

Example: PEMFCThe hydrogen atoms electron and proton are separated at the anode.Only the protons can go through the membrane (thus, the nameproton exchange membrane fuel cell).

Hydrogen Oxygen

Water

2 2 2H H e

Heat

2 21/ 2 2 2 1O H e H O

Membrane(Nafion)

Catalyst (Pt)Anode (-)

Catalyst (Pt)Cathode (+)

dc current

2 2 22 2 ( 1.23 )

rO H H O E V


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Fuel cell thermodynamics

The first law of thermodynamics:

The energy of a system is conserved

In conservational fields, potential functions change depend only on initial andfinal values. Hence,

For a closed system (control masssystem), such as a piston

(The total energy change equals the sum of the change in internal energy, thechange in kinetic energy, and the change in potential energy)

Q W dE

Q W E

Change of heatprovided to the

system

Change ofwork provided

by the system

Change ofsystems total

energy

E U K P


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For an open system with mass flow across its boundaries (control volume),such as a steam turbine

pV represents the work to keep the fluid flowing (p is pressure and V is

volume). Hence, if a magnitude called enthalpyHis defined as

Then,

If we use the 1stlaw of thermodynamics for a stationary control volume (i.e.

the kinetic and potential energies are constant in time, then

Thus, the enthalpy is the difference between the heat and the work involved

in a system such as the one defined immediately above.

( )E U K P pV

H U pV

H E K P

H Q W


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If the change in enthalpy is negative, heat is liberated and the reaction occursspontaneously (contrary to endothermic reactions that requires to apply heat in

order for the reaction to occur).

In the anode:In the cathode:

Hence, in a PEMFC, 285 kJ/mol are converted into heat (Q) and electricity(W).

Entropy: it is a property that indicates the disorder of a system or how

much reversible is a process. This last definition relates entropy to

energy quality.

In a reversible isothermal process involving a heat transfer Qrevat a

temperature T0, the entropy is defined as


2 2 2 , 0H H e H kJ

2 21/ 2 2 2 1 , 285.8O H e H O H kJ

0

revQS

T


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In all processes involving energy conversion or interactions Sis non-

negative.

S

is zero only in reversible processes..

For any process then

The = in the above relationship will give us the minimum amount of heat

Qminrequired in a process.

From the enthalpy definition a fuel cell can be considered as a system like thefollowing one


QS

T

H

QQ W


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The maximum possible efficiency for a fuel cell is, then

An alternative derivation involves using Gibbs Free Energy

The definition of entropy is relates with the 2ndLaw of Thermodynamics. One

of its interpretations is that it is impossible to convert all the energy related withirreversible processes, such as heat or chemical energy, into work.

Hence, it is possible to define a magnitude with units of energy called GibbsFree Energy that represents the reversible part of the energy involved in the

process.

Hence, for fuel cells, the electrical work represents the Gibbs Free Energy andthe maximum possible energy conversion efficiency is

max

G

H

minmax

1 QWH H


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From tables:

In the anode:In the cathode:

And from slide #6 Hequals 285 kJ/mol. Thus,

The Gibbs Free Energy can also be used to calculate the output voltage of anideal fuel cell. Since the Gibbs Free Energy equals the electrical work, and the

electrical work equals the product of the charge and voltage, then

whereFis the Faraday constant (charge on one mole of electrons) the factor

of two represents the fact that two electrons per mole are involved in thechemical reaction.

2 2 2 , 0H H e G kJ

2 21/ 2 2 2 1 , 237.2 /O H e H O G kJ mol


max 237.2 0.83285.8

GH

2o

W G FE


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Thus,

and sinceF = 96,485 C/mole and G = -237.2 kJ/mole, then

E0is also denoted byEr, the reversible voltage.

This is the voltage that can be obtained in a single ideal PEMFC when thethermodynamic reaction limitations are taken into account. I.e., this is theoutput voltage of a single ideal PEMFC when it behaves as an ideal voltagesource. However, additional energy loosing mechanisms further reduce thisvoltage.

2o GE

F

( 237200)

1.229 1.23(2)(96,485)oE V


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The Tafel equation yields the cells output voltageEcconsidering additionalloosing mechanisms:

The first term is the reversible cell voltage (1.23V in PEMFCs)

The last term represents the ohmic losses, where iis the cells current density,and ris the area specific ohmic resistance.

The second term represent the losses associated with the chemical kineticperformance of the anode reaction (activation losses). This term is obtained

from the Butler-Volmer equation and its derivation is out of the scope of thiscourse.In the second term, i0is the exchange current density for oxygen reaction andbis the Tafel slope:

log( )

RTb

n e

PEMFC output: Tafel equation

0log( / )c rE E b i i ir


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In the last equationRis the universal gas constant (8.314 Jmol-1K-1), Fis the

Faraday constant, Tis the temperature in Kelvins, nis the number of electronsper mole (2 for PEMFC), and is the transfer coefficient (usually around 0.5).Hence, b is usually between 40 mV and 80 mV.

The Tafel equation assumes that the reversible voltage at the cathode is 0 V,which is only true when using pure hydrogen and no additional limitations, suchas poisoning, occur.

The Tafel equation do not include additional loosing mechanisms that aremore evident when the current density increases. These additional mechanismsare:

Fuel crossover: fuel passing through the electrolyte without reactingMass transport: hydrogen and oxygen molecules have troubles reachingthe electrodes.

Tafel equation also assumes that the reaction occurs at a continuous rate.

PEMFC output: Tafel equation


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PEMFC electrical characteristics

Maximum poweroperating point

Er= 1.23 V

Activation lossregion Ohmic loss region

(linear voltage to currentrelationship)

Mass transport loss region

Er =1.23V

b=60mV,

i0=10-6.7Acm-2

r=0.2cm2

Actual PEMFCs efficiency vary between 35% and 60%


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PEMFC electrical characteristics

This past curve represent the steady stateoutput of a fuel cell.

The steady stateoutput depends on the fuel flow:

Amrhein and Krein Dynamic Simulation for Analysis of Hybrid Electric Vehicle

System and Subsystem Interactions, Including Power Electronics


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Hydrogen production

Hydrogen needs to be produced, and sometimes it also needs to betransported and/or stored. Hydrogen is not a renewable source of energy.

Hence, FC are alternative sources of energy.

Methods for hydrogen production:

Methane Steam Reforming (MSR)It uses natural gas

Two-step process:1)

endothermic reaction (needs heat)2)

exothermic reaction (provides heat)

75 % to 80 % efficient.Partial oxidation (POX)

It also uses natural gas or other hydrocarbonand/or

POX is compact and has faster dynamic response than MSR, but MSRprovides higher hydrogen concentration.

4 2 23CH H O CO H

2 2 2CO H O CO H

4 2 21/ 2 2CH O CO H 4 2 2 22CH O CO H


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Hydrogen production

More methods for hydrogen production:

Electrolysis of waterWater molecules can be separated using electricity. But we use electricityto produce hydrogen to produce electricity again.Pure water is in many places an scarce resource.The electricity for the electrolysis needs to be produced and the waterneeds to be purified (soft de-ionized water is needed).

Reaction:

Electricity can be obtained at a large scale from nuclear reactors but thehydrogen needs to be stored and transported, and nuclear fuel is not a

renewable source of energy.At a VERY small scale wind or solar power can be used, but this energy isavailable only when there is wind or sunlight.

Gasification of Biomass, Coal or WastesThese methods are still a long way into the future.

2 2 22 2H O O H


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Hydrogen Storage

Hydrogen atoms are the lightest and smallest of all elements. For this reason,it is very difficult to keep hydrogen from escaping confined environments such

as tanks or pipes.

Since an effort (i.e. work) needs to be done to keep hydrogen stored, storinghydrogen implies loosing efficiency.

Some storage methods:Pressure Cylinders: Some efficiency is lost in the compressingprocessLiquid Hydrogen: it requires lowering the hydrogen temperature to20.39 K. This process already reduces 1/3 of the efficiency.

Metal Hydrides: These are compounds of hydrogen and Magnesium,titanium and other metals. Efficiency is low to medium and lot of heat isgenerated when the hydrogen is released, but these compounds arevery easy to store in the form of soils.Carbon nano-fibers: New technology.


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PEMFC Technology and issues

Expected life of PEMFC is very short (5,000 hours).

The most commonly used catalyst (Pt) is very expensive.

The most commonly used membrane (Nafiona sulfonated tetrafluorethylenecopolymer is also very expensive).

PEMFCs are very expensive.

CO poisoning diminishes the efficiency. Carbon monoxide (CO) tends to bindto Pt. Thus, if CO is mixed with hydrogen, then the CO will take out catalystspace for the hydrogen.

Hydrogen generation and storage is a significant problem.

Additional issues to be discussed when comparing other technologies:dynamic response and heat production.


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The main advantage is that they use a liquid fuel.Reactions:

AnodeCathode

Voltages: 0.046 V at anode, 1.23 V at cathode, 1.18 V overall.

Methanol has high energy density so DMFC are good for small portableapplications.

Issues:Cost

Excessive fuel crossover (methanol crossing the membrane)Low efficiency caused by methanol crossoverCO poisoningLow temperature productionConsiderable slow dynamic response

Direct Methanol Fuel Cells (DMFC)

3 2 26 6CH OH H O CO H e

2 21/ 2 2 2O H e H O


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One of their main advantages is their long life in the order of 40,000 hours.

The phosphoric acid serves as the electrolyte.

The reactions are the same than in a PEMFC. Hence, the reversible voltage is1.23 V

The most commercially successful FC: 200 kW units manufactured by UTC

They produce a reasonable amount of heat

They support CO poisoning better than PEMFC

They have a relatively slow dynamic response

Relative high cost is an important issue

Phosphoric Acid Fuel Cells (PAFCs)


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The main advantage is that their cost is relatively low (when considering the

fuel cell stack only without accessories.Reactions:

AnodeCathode

Developed for the Apollo program.

Very sensitive to CO2poisoning. So these FCs can use impure hydrogen butthey require purifying air to utilize the oxygen.

Issues:

Cost (with purifier)Short life (8000 hours)Relatively low heat production

Alkaline Fuel Cells (AFCs)

2 22 2 2H OH H O e

2 21/ 2 2 2 2O H O e OH


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One of the main advantages is the variety of fuels and catalyst than can beused.Reactions:

AnodeCathode

They operate at high temperature. On the plus side, this high temperature

implies a high quality heat production. On the minus side, the high temperaturecreates reliability issues.

They are not sensitive to CO poisoning.

They have a relatively low cost.

Issues:Extremely slow startupVery slow dynamic response

2

2 3 2 2 2H CO H O CO e

2

2 2 31/ 2 2O CO e CO

Molten Carbonate Fuel Cells (MCFCs)


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Solid Oxide Fuel Cells (SOFCs)

One of the main advantages is the variety of fuels and catalyst than can beused.

Reactions:AnodeCathode

They operate at high temperature with the same plus and minus than inMCFCs.

They are not sensitive to CO poisoning.

They have a relatively low cost.

They have a relatively high efficiency.

They have a fast startup

The electrolyte has a relatively high resistance.

2

2 2 2H O H O e

2

21/ 2 2O e O


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Comparison of the most common technologies

PEMFC DMFC AFC PAFC MCFC SOFC

Fuel H2 CH3OHH2 H2

H2, CO, CH4,hydrocarbons

H2, CO, CH4,hydrocarbons

ElectrolyteSolid polymer

(usually Nafion)Solid polymer

(usually Nafion)

Potasiumhydroxide

(KOH)

Phosporicacid (H3PO4

solution)

Lithium andpotassiumcarbonate

Solid oxide(yttria,

zirconia)

Charge carried inelectrolyte

H+ H+ OH- H+ O2-

Operationaltemperature (oC) 50100 50 - 90 60 - 120 175200 650 1000

Efficiency (%) 3560 < 50 3555 3545 4555 5060

Unit Size (KW) 0.1500 2.5

Installed Cost ($/kW) 4000 > 5000 < 1000* 30003500 8002000 1300 - 2000

Fuel cell technologies

2-

3CO

* Without purifier

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Fuel Cell Lectures