FUEL CELLS: INTRODUCTION - UMONS · PEMFC Proton exchange membrane fuel cell Type Electrolyte Same...

19/05/200819/05/2008

FUEL CELLS:

INTRODUCTION

M. OLIVIER

[email protected]

22

Two electrochemical half reactions :

OHeHO

eHH

22

2

222

1

22

↔++

+↔

−+

−+

These reactions are spatially separated:Electrons: flow through an external circuit (electrical current)Electrolyte : allows ions to flow but not electrons

At a minimum: Fuel cell = two electrodes separated by an electrolyte

A SIMPLE FUEL CELLA SIMPLE FUEL CELL

33

Energy = the ability to do work - [J] or [cal]

Power= rate at which energy is expended or produced Power is a rate = amount of energy used or produced per second - [W = J/s]

Volumetric power density = amount of power that can be supplied by a device per unit volume [W/cm3] or [kW/m3]

Gravimetric power density = amount of power that can be supplied by a device per unit mass [W/g] or [kW/kg]

Volumetric energy density = amount of energy that can be supplied by a device per unit volume [Wh/cm3] or [kWh/m3]

Gravimetric energy density = amount of energy that can be supplied by a device per unit mass [Wh/g] or [kWh/kg]

DEFINITIONSDEFINITIONS

44

- Producing electricity as long as they are supplied with fuel

- Far more efficient than combustion engines

- No moving parts and so silent

- Undesirable products such as NOx, SOx and particulate emissions are virtually zero

- Unlike batteries, fuel cells allow easy independent scaling between power (determined by the fuel cell size) and capacity (determined by the fuel reservoir size).

- Higher energy densities compared to batteries and can be quickly recharged by refueling (no recharge)

FUEL CELL ADVANTAGESFUEL CELL ADVANTAGES

55

- Cost

- Power density

- Fuel availability and storage: hydrogen or alternatives such as gasoline, methanol (difficult to use directly)

- Operational temperature compatibility concerns:susceptibility to environmental poisons and durability under start-stop cycling

FUEL CELL DISADVANTAGESFUEL CELL DISADVANTAGES

66

Solid oxide fuel cellSOFC

Molten carbonate fuel cellMCFC

Alkaline fuel cellAFC

Phosphoric acid fuel cellPAFC

Direct methanol fuel cellDirect ethanol fuel cell

DMFCDEFC

Proton exchange membrane fuel cellPEMFC

ElectrolyteType

Same electrochemical principles but operate at different temperature regimens, incorporate different materials and often differ in their fuel tolerance and performance characteristics

FUEL CELL TYPESFUEL CELL TYPES

77

ANODE = OXYDATION CATHODE = REDUCTION

OXIDATION = process where electrons are liberated by the reaction

REDUCTION = process where electrons are consumed by the reaction

OHeHO

eHH

22

2

222

1

22

↔++

+↔

−+

−+

For hydrogen-oxygen fuel cell:

- The anode is the electrode where the hydrogen oxidationreaction (HOR) takes place

- The cathode is the electrode where the oxygen reductionreaction (ORR) takes place

ELECTROCHEMICAL CONCEPTSELECTROCHEMICAL CONCEPTS

88

Be carefull… Anodes and cathodes can be eitherpositive or negative

Galvanic cell = Anode is negative and cathode ispositive

Electrolytic cell = Anode is positive and cathode isnegative

ELECTROCHEMICAL CONCEPTSELECTROCHEMICAL CONCEPTS

99

Major steps involved in producing electricity:

1) Reactant delivery (transport)

2) Electrochemical reaction

3) Ionic conduction through the electrolyte and electron transport through the external circuit

4) Product removal

BASIC FUEL CELL OPERATIONBASIC FUEL CELL OPERATION

1010

Current –voltage (i-V) curve: voltage output of the fuel cell for a given current output

Irreversible losses:

- Activation losses (due to electrochemical reaction)

- Ohmic losses (due to ionic and electronic conduction)

- Concentration losses (due to mass transport)

concohmicactthermoEV ηηη −−−=

FUEL CELL PERFORMANCEFUEL CELL PERFORMANCE

1111

Power density curve: power density delivered by a fuel cell as a function of the current density

[ ]2−

=

cmW

ViP

FUEL CELL PERFORMANCEFUEL CELL PERFORMANCE

1212

FUEL CELL AND THE ENVIRONMENTFUEL CELL AND THE ENVIRONMENT

1313

Key to understand the conversion of chemical energy into electrical energy

Thermodynamics yields the theoretical boundaries of what is possible with a fuel cell = « ideal case »

Understanding real fuel cell performance requires a knowledge of kinetics in addition to thermodynamics

FUEL CELL THERMODYNAMICSFUEL CELL THERMODYNAMICS

1414

• Internal Energy (U): The energy needed to create a system in the absence of changes in temperature and volume

• Enthalpy (H): The energy needed to create a system plus the work needed to make room for it (from zero volume)

• Helmholtz Free Energy (F): The energy needed to create a system minus the energy that you can get from the system’s environment due to spontaneous heat transfer (at constant temperature)

• Gibbs Free Energy (G): The energy needed to create a system and make room for it minus the energy that you can get from the environment due to heat transfer


1515


1616

Implies equilibrium

A reversible fuel cell voltage = voltage produced by a fuel cell at the thermodynamic equilibrium

To distinguish between reversible and nonreversible fuel cell voltages

E = reversible (thermodynamically predicted) fuel cell voltage

V = operational (nonreversible) fuel cell voltage


� REVERSIBILITY

1717

NnMmBbAa +→+

The maximum heat energy that we can extract from a fuel is given by the fuel’s enthalpy of reaction

For a general reaction:

[ ] [ ])()()()(00000

BhbAhaNhnMhmh ffffrxn ∆+∆−∆+∆=∆

Enthalpy of reaction (in STP) = computed from the difference between the molar formation enthalpies of the products and the reactants


� WORK POTENTIAL OF A FUEL : ENTHALPY OF REACTION

1818


� WORK POTENTIAL OF A FUEL : ENTHALPY OF REACTION

1919

G : the net energy you had to transfer to create the system = the maximum energy that you could ever get back out of the system

Gibbs free energy = the exploitable energy potential (Work potential)

sThg ∆−∆=∆


� WORK POTENTIAL OF A FUEL : GIBBS FREE ENERGY

At T = constant

rxnelec gW ∆−=

2020

∆G > 0 : Nonspontaneous (energitically unfavourable)

∆G = 0 : Equilibrium

∆G < 0 : Spontaneous (energetically favourable)


� GIBBS FREE ENERGY AND REACTION SPONTANEITY

Spontaneity is no guarantee that a reaction will occur, nor doesit indicate fast a reaction will occur

STUDY OF THE KINETIC BARRIERS

Ex: Diamond from graphite

2121

Welec = E Q and Q = n F

OHOH 2222

1→+

∆g = -237,2 kJ mol-1 (liquid water - STP)

∆g = -228,6 kJ mol-1 (gaseous water - STP).


� GIBBS FREE ENERGY AND VOLTAGE

EFng −=∆

VnF

gE rxn 23,1

964002

0002370

0 =×

−=

∆−=

The highest voltage attainable from H2-O2 fuel cell at STP

Most feasible fuel cell reactions have reversible voltage in therange of 0,8 – 1,5 V

2222


� STANDARD ELECTRODE POTENTIALS

STANDARD ELECTRODE POTENTIAL TABLES

∑= 00

reactionshalfCell EE

( ) 229,12442

1

000,022

0

22

0

2

+=→+++

−=+→

−+

−+

EOHeHO

EeHH

229,12

1 0

222 +=→+ cellEOHOH

Electrode potential tables list all reactions as reduction reactions.

Any thermodynamically spontaneous electrochemical reaction will have a positive cell potential.

To obtain the reverse reaction: an external voltage must be applied.

2323


� UNDER NON-STANDARD-STATE CONDITIONS

Reversible voltage variation with temperature

( )

( )0

0 TTnF

sEE

nF

s

dT

dE

EFng

sdT

gdS

dT

dG

T

p

pp

−∆

+=

∆=

−=∆

∆−=

∆−=

For the familiar H2-O2

cell, ∆srxn= - 44,43 J/mol

For every 100°C increase in cell temperature, there is an approximate 23 mV decrease in cell voltage

Should we operate a fuel cell a the lowest temperature ?

The answer is NO.

Kinetic losses tend to decrease with increasing temperature.

2424




2525

0,92-177,4Gaseous1000

1,04-199,6Gaseous600

1,14-220,4Gaseous200

1,17-225,3Gaseous100

1,23-237,2Liquid25

E (V)∆G (kJ.mol-1)WaterTemperature(°C)




2626



Reversible voltage variation with pressure

( )

nF

v

dp

dE

EFng

vdp

gdV

dp

dG

T

TT

∆−=

−=∆

∆=

∆=

If the volume change of the reaction is negative, then the cell voltage will increase with increasing pressure.

Pressure has a minimal effect on reversible voltage.

Pressurizing a H2-O2 fuel cell to 3 atm H2 and 5 atm O2

increases the reversible voltage by only 15 mV.

2727



2828


� UNDER NON-STANDARD-STATE WITH CONCENTRATION

Nernst Equation

Concept of chemical potential

µi0= the reference chemical potential of species i at standard

conditions

ai = the activity of the species i

When we change the concentration of chemical species in a fuel cell, we are changing the free energy of the system.

This change in turn changes the reversible voltage.

ijnPTi

in

G

≠

∂

∂=

,,

αµ

iii aRT ln0 += µµ

2929



Nernst Equation

Concept of chemical potential

This Nernst equation is the centerpiece of fuel cell thermodynamics.

( )

∏∏

∑ ∑

−=−=

−=∆

+∆=∆

+↔+

+==

i

i

tsreac

products

b

BA

n

N

m

M

b

BA

n

N

m

M

i i

iiiii

a

a

nF

RTE

aa

aa

nF

RTEE

EFng

aa

aaRTgg

nNmMbBA

dnaRTdndG

ν

ν

µµ

tan

0

1

0

1

0

0

lnln

ln

1

ln

3030



Nernst Equation

For the familiar hydrogen-oxygen fuel cell reaction

Pressurizing the fuel cell in order to increase the reactant gaspartial pressures will increase the reversible voltage.

OHOH 2222

1→+

21

0

21

0

22

22

2

1ln

2

ln2

OH

OH

OH

ppF

RTEE

aa

a

F

RTEE

−=

−=

Below 100°C

3131



Nernst Equation

Perhaps, we are worried that almost fuel cells operate on air instead of pure oxygen.

How much does this affect the reversible voltage of a room temperature H2-O2 fuel cell?

Operation in air drops the reversible voltage by only 10 mV.

( )( )( )( ) ( )( )

VE 219,121,01

1ln

964002

15,298314,8229,1

21=−=

3232


� IDEAL REVERSIBLE FUEL CELL EFFICIENCY

EFFICIENCY ε = The amount of useful energy that can be extracted from the process relative to the total energy evolved by that process:

h

work

energytotal

energyuseful

∆==ε

83,0286

3,237, =

−

−=

∆

∆=

h

gfcthermoε

3333


� REAL (PRACTICAL) FUEL CELL EFFICIENCY

The real efficiency of a fuel cell is always less than the reversible thermodynamic efficiency.

1) Voltage losses

2) Fuel utilization losses

fuelvoltagethermoreal εεεε =

• The voltage efficiency of the fuel cell εvoltage= the ratio of the real operating voltage of the fuel cell (V) to the thermodynamics reversible voltage (E)

E

Vvoltage =ε

3434


� REAL (PRACTICAL) FUEL CELL EFFICIENCY

• The fuel utilization efficiency εfuel = accounts that not all of the fuel provided to a fuel cell will participate in the electrochemical reaction. The ratio of the fuel used by the cell to generate electric current versus the total fuel providedto the cell.

λυε

1==

fuel

fuel

nFi

νfuel = rate at which fuel is supplied to the fuel cell (mol/s)

λ = stoichiometric factor

∆

∆=

∆

∆=

λνε

1

E

V

h

gnFi

E

V

h

g

fuel

real

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FUEL CELLS: INTRODUCTION - UMONS · PEMFC Proton exchange membrane fuel cell Type Electrolyte Same...

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