university of cassino and southern lazio

UNIVERSITY OF CASSINO AND SOUTHERN LAZIOUNIVERSITY OF CASSINO AND SOUTHERN LAZIO

Fuel cellsFuel cells

Date of birth: 1839Paternity: Sir William GroveFirst applications: 50s and 60s

Converts the energy of a fuel by oxidation as a heat engine.

It transfers ions (positive or negative) like a battery or any other electrochemical device.

Oxidant

Oxidant

Fuel

FuelProducts

e-

H+

Anode (-), Anode (-), oxidation oxidation (transfer of electrons)(transfer of electrons)

Cathode (+), Cathode (+), reduction reduction (acquisition of (acquisition of electrons)electrons)

Generalities and operating principleGeneralities and operating principle

The electrochemical reaction that takes place in a fuel cell consists of two semi-reactions, one of oxidation, in which a fuel oxidises by transferring electrons and increasing its oxidation state, and one of reduction, in which an oxidiser is reduced by acquiring electrons transferred by the fuel and reducing its oxidation state.

The electrolyte is the characterising element of the fuel cell. It can be solid or liquid and determines the charge of the ions which, through it, migrate from one electrode to another.

The catalyst has the purpose of promoting the creation of ions and, therefore, assumes a fundamental importance in low temperature fuel cells.

The electrodes, which must be porous in such a way as to constitute a three-phase interface in which the reactants, the electrolyte and the catalyst come into contact. In a fuel cell, the anode is the negative electrode at which oxidation occurs, while the cathode is the positive electrode at which reduction occurs.In the case of cells with liquid electrolyte, the catalyst covers the surface of the pores of the electrodes with a thin film that allows the diffusion of the reagents. The risk to be avoided is the so-called flooding, which consists of an excessive thickness of the film and which blocks the operation of the cell.

Elements of a fuel cellElements of a fuel cell

StackStack

Electrolyte

Bipolar plate

Cathode

Anode

Structure of fuel cells and stacksStructure of fuel cells and stacks

Electrolyte Temperature

AFC Alkaline Fuel Cell Potassium hydroxide 60-250°C 45-65%

PEMFC Proton Exchange Membrane Fuel CellProton exchange membrane 45-55%DMFC Direct Methanol Fuel Cell

DEFC Direct Ethanol Fuel Cell

PAFC Phosphoric Acid Fuel Cell Phosphoric acid 60-220°C 40-50%

MCFC Molten Carbonate Fuel CellMolten carbonates 600-650°C 45-55%

DFC Direct Fuel Cell

SOFC Solid Oxide Fuel Cell Zirconium dioxide 850-1000°C 45-55%

Theoretical efficiency

60-80°C (160 °C)

There are also coal, ammonia and microbial fuel cells.

Classification of fuel cellsClassification of fuel cells

Anodic semireactionAnodic semireaction Cathodic semireactionCathodic semireaction

AFCs were developed by the English scientist Bacon in the late '50s. Their electrolyte is an alkaline solution dispersed on a porous matrix that acts as a support. Usually it is a solution of potassium hydroxide (KOH) whose concentration depends on the operating temperature: if the latter is high (T≈250 °C) then the concentration is also high (>85%), whereas it is lower (35-45%) for T≈120 °C.

Alkaline fuel cellsAlkaline fuel cells

OH-

OH-

OH-

OH-

H2

H2

H2

-anodo catodo

+

O2

H2OO2

H2

H2

e- e-

OH-

OH-

OH-

OH-

H2

H2

H2

-anodo catodo

+

O2

H2OO2

H2

H2

e- e-

anode cathode

H 2+2OH -⇒2H 2O+2e- 1

2O2+H 2O+2e -

⇒2OH -

The basic material for electrodes are nickel and silver. Catalysts can be made from noble metals, but also from much cheaper materials, such as nickel or metal oxides.

The purity of the reactants is very important and the use of air is allowed only after elimination of the carbon dioxide which could react with the hydroxide to form potassium carbonate:

The heat generated during the fuel cell operation is often removed by electrolyte recirculation and its external cooling.

The power density can reach 8 kW/m3. The start up is fast even in cold weather, just a few minutes. Life of 10,000-15,000 hours have already been demonstrated.

Because of intolerance to impurities they are used almost exclusively for aerospace and military applications.

2 KOH+CO2⇒K2CO3+H 2O

Alkaline fuel cellsAlkaline fuel cells

Cathodic semireactionCathodic semireactionAnodic semireactionAnodic semireaction

They are characterized by an electrolyte consisting of a perfluorinated sulfonic membrane, whose proton conductivity is almost proportional to the water content. Therefore the management of the flow of water is a critical aspect because it is necessary to guarantee the humidification, avoiding, at the same time, the flooding of the electrodes.

In the last decade alternative membranes for high temperature PEMFC (around 160 °C) has been developed in order to eliminate the issues connected with water management as well as to increase the heat recovery options.

H+

H+

H+

H+

H2

H2

H2

-anodo catodo

+

O2

H2O

O2

H2

H2

e- e-

H+

H+

H+

H+

H2

H2

H2

-anodo catodo

+

O2

H2O

O2

H2

H2

e- e-

anode cathode

H 2⇒2H++2e - 1

2O2+2H+

+2e -⇒H 2O

Proton exchange membrane fuel cellsProton exchange membrane fuel cells

The electrodes are deposited on the membrane and consist of a porous and conductive diffusive layer (graphite + PTFE) and a catalytic layer (C/PTFE-catalyst).

The catalyst used is platinum, or one of its alloys (Pt-Ru, Pt-Sn), which is permanently poisoned by sulfur and, in a temporary way, by carbon monoxide possibly present in the fuel.

Generally the cooling of the cell is carried out by means of refrigeration plates placed between the cells and supplied with water (optionally added with anti-freezing liquid) which must be demineralized in such a way to avoid conducting electric current. There are also air and humidified air refrigeration systems.

Since the only liquid present is the water generated at the cathode, there are no corrosion problems.


Electrolytic membrane

Anode Cathode

Anodiccollector

Cathodiccollector

Active anodic layer

Active cathodic layer

Polymer

Carbon, PTFE

Carbon, PTFE, Pt, polymer

Electrical conductor

Advantages:

➢ high electrical efficiency (>45%);➢ high stack power density (>1 kW/l, >1 kW/kg);➢ speed of cold start (about one minute);➢ constructive simplicity;➢ absence of corrosion problems.

Disadvantages:

➢ high Pt load (0.2-0.4 mg/cm2);➢ sensitivity of the catalyst to CO, even in

traces;➢ need for a cooling system to remove

excess heat;➢ difficulty in thermal integration between

fuel treatment system and stack;➢ difficulty in managing the water

produced.


The electrolyte is dispersed over a porous silica matrix (SiO2) and is pure phosphoric acid

(H3PO4) which achieves a good ionic conductivity around 150 °C.

Being acid cells, they also transfer protons.


H+

H+

H+

H+

H2

H2

H2

-anodo catodo

+

O2

H2O

O2

H2

H2

e- e-

H+

H+

H+

H+

H2

H2

H2

-anodo catodo

+

O2

H2O

O2

H2

H2

e- e-

anode cathode

H 2⇒2H++2e -

12O2+2H+

+2e -⇒H 2O

Phosphoric acid fuel cellsPhosphoric acid fuel cells

The anode consists of Pt (0.10 mg/cm2) bound to PTFE on a carbon support. At a low temperature, it presents the risk of carbon monoxide poisoning (CO<1%). The cathode is analogous, but with a higher concentration of Pt (0.50 mg/cm2). The bipolar plates are in graphite.

Thanks to the higher temperature, they can directly use the gas produced by the reforming of hydrocarbons. The heat generated can be used either to produce the steam used in the reforming process or for external thermal utilities (sanitary hot water and domestic heating).

The starting procedure is longer than that of lower temperature cells: 1 to 4 hours.

PAFCs have enjoyed a period of good spread, exceeding a total installed capacity of 100 MW worldwide, with a plant of 11 MW, some of 5 MW and a large number of 200 kW electric cogeneration units (PC25). However, despite the fact that they have been for about twenty years the only fuel cells mature for the market, manufacturers stopped investing in this technology, considering it less promising than others.

Phosphoric acid fuel cellsPhosphoric acid fuel cells

The electrolyte is a solution of liquid alkaline carbonates (Li2CO3–K2CO3 62-38% on a porous

ceramic matrix of γ-LiAlO2) with high ionic conductivity.

The anode is in Ni-Cr or Ni-Al, whereas the cathode is in lithium-plated NiO and the bipolar plate in metal alloys (Incoloy825, steel 310S or 316) with protective materials.


H2O

H2

H2

H2

-anodo catodo

+

O2

O2

H2

H2

e- e-

CO3--

CO3--

CO3--

CO3--

CO2

CO2

CO2

CO

H2O

H2

H2

H2

-anodo catodo

+

O2

O2

H2

H2

e- e-

CO3--

CO3--

CO3--

CO3--

CO2

CO2

CO2

CO

anode cathode

12O2+CO2+2e-

⇒CO3- -H 2+CO3

- -⇒H 2O+CO2+2e -

Molten carbonate fuel cellsMolten carbonate fuel cells

Critical issue: Boudouard reaction 2CO⇒CO2+C(s)

Advantages:

➢ not expensive catalysts thanks to faster reaction kinetics;➢ high efficiency (45-55%);➢ high flexibility with regard to fuels;➢ possibility of cogeneration at temperatures of industrial interest or to realize combined

plants (with turbomachinery).

Disadvantages:

➢ dissolution of the lithium-oxide cathode;➢ sintering of the nickel anode;➢ corrosion of metal components;➢ cost of materials for the high T;➢ poor duration;➢ long starting time (5-10 hours);➢ need to recycle CO2 from the anode to the cathode.

Molten carbonate fuel cellsMolten carbonate fuel cells

For molten carbonated fuel cells, methane is a diluent. Therefore, although it is possible to use fuel containing methane, it flows through the anode without reacting and without producing any useful effect. This is not of great importance when the fuel used is a coal gas, since the methane content is low, whereas it requires a reforming process when the starting fuel is natural gas.

Thanks to the high temperature, the use of internal reforming is a valid alternative. In such a case the anodic compartment is equipped with a catalyst suitable for the reforming reaction: usually it is Ni supported on MgO or on LiAlO2. The thermal energy required by the reforming

reaction is provided by the anodic semi-reaction which also provides the necessary steam.

The result is a better thermal balance of the cell and an almost total conversion of the methane since the progressive consumption of hydrogen continuously shifts the reforming reaction towards the products. It follows that the system is and more efficient and also cheaper since the external reformer is missing.

Molten carbonate fuel cells: internal reformingMolten carbonate fuel cells: internal reforming

electrolyte

anode

cathode

The electrolyte is zirconium oxide stabilized with yttrium oxide, which, at high temperature, is a good conductor of oxygen ions.The anode is a cermet based on nickel-zirconium oxide capable of catalyzing the reforming of methane and inhibiting the sintering of the metal, whereas the cathode is in lanthanum manganite doped with strontium.


H2O

H2

H2

H2

-anodo catodo

+

O2

O2

H2

H2

e- e-

O--

CO2

CO2

O--

O--

O--

O--

CO

H2O

H2

H2

H2

-anodo catodo

+

O2

O2

H2

H2

e- e-

O--

CO2

CO2

O--

O--

O--

O--

CO

anode cathode

12O2+2e-

⇒O- -H 2+O- -⇒H 2O+2e -

Solid oxide fuel cellsSolid oxide fuel cells

Advantages:

➢ high tolerance to impurities;➢ lack of problems of evaporation and loss of liquid;➢ Not expensive catalysts thanks to faster reaction kinetics;➢ high efficiency (45-55%);➢ high flexibility with regard to fuels;➢ possibility of cogeneration at temperatures of industrial interest or to realize

combinedplants (with turbomachinery).

Disadvantages:

➢ presence of thermomechanical stresses that imply a greater difficulty in the realization of the stack;

➢ long starting time (5-10 hours);➢ costs for high temperature resistant materials.

Solid oxide fuel cellsSolid oxide fuel cells

Tubular solid oxide fuel cellsTubular solid oxide fuel cells

Planar and monolithic solid oxide fuel cellsPlanar and monolithic solid oxide fuel cells

interconnections

interconnections

cathodeelectrolyteanode

exhaust

air

fuel

Source ENEA

AFC PEMFC PAFC MCFC SOFCfuel fuel fuel fuel fuel

CO <10 ppm < 1% fuel fueldiluent diluent diluent fuel

product diluent

product diluent diluent product product

diluent diluent diluent diluent diluent

? ? ? <1% <0.1%

? <50 ppm <1 ppm <1 ppmHCl ? ? ? <1 ppm <1 ppm

H2

CH4 diluent or fuel

CO2 ~0%

H2O

N2

NH3

H2S & COS ~0%

Compatibility with fuelsCompatibility with fuels

Fuel cell AFC PEMFC PAFC MCFC SOFC

Electrolyte KOH Nafion

Ion

Temperature [°C] 50-200 50-80 200 650 600-10000.7-8.1 2.6-3.8 0.8-1.9 0.1-1.5 1.5-2.6

Reforming External External or internalStack efficiency [%] 45-60 45-55 40-50 45-55 45-55System efficiency [%] 41 55-65 55-65Startup min min 1-4 hours 5-10 hours 5-10 hours

Phosphoric acid

Molten carbonate

Zirconium dioxide

OH- H+ CO3

- -O- -

Power density [kW/m2]

SummarySummary

Efficiency is less sensitive to unit size and load reduction.

Chemicalenergy

Electricenergy

Mechanicalenergy

Thermalenergy

combustion thermodynamiccycle

electrochemicalconversion

electromechanicalconversion

Efficiency is higher.

Energy performanceEnergy performance

0% 20% 40% 60% 80% 100%0%

10%

20%

30%

40%

50%

MCI PEMFC

PotenzaE

ffici

enza

Power

Eff

icie

nc

y

IMTV supercritici

E

ffic

ien

cy

(%

)

Power (MW)

Em

issi

on

s

Tier II Euro III (2000) Euro IV (2005) Fuel cells 2004 gasoline diesel gasoline diesel hydrogen methanol

High efficiency and non-polluting products➢ Zero or very low chemical impactAbsence of moving parts➢ QuietenessHigh efficiency and exhaust discharged into the atmosphere➢ Reduced thermal impact

H 212O2 H 2O

CO12O2CO2

Environment performanceEnvironment performance

Coal Oil Nat. gas Fuel cells

Dust (mg/kWh)Hydrocarbons (mg/kWh)

Modularity

Cost saving for cosntruction

Reduced environmental impact

Possibility of cogenerationFlexibility towards fuel

Rapid load tracking capabilityAutomated management

Ease of site selection

Inexpensive management

Plant performancePlant performance

1 2 3 4 5 6 7 8 9 100

102030405060708090

100Richiesta Convenzionale Pile a combustibile

tempo

po

ten

za r

ich

iest

a o

inst

alla

ta (

%)

time

Po

wer

re

qu

ired

/in

sta

lled

(%

)

Required Conventional Fuel cells

ElectrochemicalElectrochemicalsystemsystem

(constant (constant p, T)p, T)

H2OH2

O2

A first approach calculation of the energy balance is possible through the use of classical thermodynamics. We initially consider the fuel cell as a system reversible, isothermal, isobar and site of electrochemical reactions, and we apply the I Principle of Thermodynamics.

dU=δQ−δ L=T⋅d S−p⋅dV−dLel

dH=dU+ p⋅dV=T⋅dS−dLel

δQ=T⋅d S δ L=p⋅dV +dLel

dG=dH−T⋅dS=−dLel

Thermodynamic analysis of a reversible systemThermodynamic analysis of a reversible system

Considering a generic chemical reaction:

in which Ai are the reacting species and Bj the species produced and αi and βj the respective

stoichiometric coefficients, we have:

K being the chemical equilibrium constant. And:

wherein n are the moles of electrons, F is the Faraday constant (that is the charge brought by a mole of electrons) and E is the voltage. Therefore:

And finally:

E=−ΔGn⋅F

E o=−

ΔGo

n⋅F=

R⋅Tn⋅F

ln K

E=E o+R⋅Tn⋅F

lnΠ aAi

α i

Π aBi

βi

Le=−ΔG=n⋅F⋅E

Σαi Ai⇒Σβ j B j

ΔG=ΔGo+R⋅T⋅ln

Π aB j

β j

Π aAi

α i ΔGo

=−R⋅T⋅ln K

reversible electrical potential under standard conditions

The Nernst equationThe Nernst equation

By replacing the partial gaseous pressures to the activities, it follows that:

and passing to the molar fractions:

If the water is generated in the liquid state:

Finally, for molten carbonate cells:

Standard values for the water formation reaction are:

E=E o+R⋅Tn⋅F

⋅lnpH 2

⋅√ pO2

pH 2O

E=E o+R⋅Tn⋅F

⋅ln √P+R⋅Tn⋅F

⋅lnxH 2

⋅√ xO 2

xH 2O

E=E o+

3⋅R⋅T2⋅n⋅F

⋅ln (P)+R⋅Tn⋅F

⋅ln (xH 2⋅√ xO2

)

E=E o+R⋅Tn⋅F

⋅ln (√P )+R⋅Tn⋅F

⋅lnxH 2

⋅xCO2 (C )

⋅√ xO2

xH 2O⋅xCO 2( A)

Δ H 298o

=−285.19 kJ /mol ΔG298o

=−237.19 kJ /mol Eo=1.229V

The Nernst equation: particular casesThe Nernst equation: particular cases

Coming to a real system, one must take into account the irreversibilities which obviously have a negative effect. The voltage produced by a fuel cell is therefore lower than that given by the Nernst equation.

The voltage reduction is called polarization and is atributable to three different components:➢ohmic polarization, due to the internal resistance of the fuel cell, both towards the electrons

that run through the electrodes, and towards the ions that pass through the electrolyte;➢activated polarization due to the energy required to break the molecular bonds of the reactants

and those between the reactant and catalyst atoms;➢concentrated polarization, due to the limitation of the mass transport velocity that manifests

itself in the slow diffusion of the gases in the electrodes and ions in the electrolyte: the consumption of the reactants and ions near the electrode generates, inside the cell itself, a concentration battery which originates a back-electromotive force.

V=E−ΔV o−ΔV a−ΔV c

IrreversibilitiesIrreversibilities

The internal resistance consists of two components:➢ the resistance of the electrolyte to the passage of ions ➢ the resistance of the electrodes to the passage of electrons.

The Ohm's law is valid for both components and therefore the ohmic polarization varies linearly

with the current density:

The dependence of the polarization activated by the current density is expressed by an empirical

relation, valid for polarizations above 50 mV and called the Tafel equation:

wherein α and jo are two empirically determined parameters and called, respectively, charge

transfer coefficient and exchange current density. The activated polarization is affected by the

type of reaction, the type of catalyst and the temperature.

ΔV o= I⋅Ro

ΔV a=R⋅T

n⋅F⋅α⋅ln

jj0

Ohm's law and Tafel’s equationOhm's law and Tafel’s equation

➢ D is the diffusion coefficient of the reacting ions;➢ ci is the concentration of the reacting ions at the

electrolyte-electrode interface;➢ cb is the concentration of the reacting ions

outside the diffusion layer;➢ d is the thickness of the diffusion layer.

If ci=0 then:

with jL representing the limit current density. The Nernst equation applied to the electrolyte,

respectively for open and closed circuit, is:

and

A concentration battery is then created whose tension is:

elettrolita e l

t

r o d

e

t

o

e l

t

r o d

e

t

o

b c

c i

d d c i

jn⋅F

=D⋅(cb−ci)

d

ΔV c=

R⋅T⋅lncb

ci

n⋅F=

−R⋅T⋅ln(1−jjL )

n⋅F

jL=n⋅F⋅D⋅cb

d→

ci

cb

=1−jjL

E=E o+R⋅T⋅ln cb

n⋅FE=E o

+R⋅T⋅ln ci

n⋅F

Fick's lawFick's law

electrolyte

ele

ctr

od

e

ele

ctr

od

e

Globally, the voltage of a fuel cell varies with the current density according to the equation:

which contains a constant term, a linear term, a logarithmic and an exponential term.

The latter is negligible if you do not get too close to the limit current density: since the operating

conditions suggested by the manufacturers are always far from this limit, the term related to the

concentrated polarization is often omitted.

The logarithmic term, on the other hand, is more significant at low current density values and

then progressively lose importance with respect to the linear term. Moreover, due to its nature, it

is strongly related to temperature and becomes almost negligible at high temperatures.

Therefore the following relationships are often taken as valid:

respectively for low and high temperature fuel cells.

V=E− I⋅Ro−R⋅Tn⋅F

⋅[ 1α⋅ln

jj 0

+ ln (1−jj L )]

V=a−b⋅ j−c⋅ln j e V =a−b⋅ j

The global polarisationThe global polarisation

The characteristic curveThe characteristic curve

Current density (mA/cm2)

Ce

ll v

olt

ag

e (

V)

Reversible voltage

Total loss Activated polarisation

Ohmic polarisation

Concentrated polarisation

ExamplesExamples

Current [A]

Po

we

r [W

]

Curva di polarizzazione Stack Nexa

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50

Corrente [A]

H2 100%

Stack Nexa

Current [A]

V

olt

ag

e [

V]

Curva di polarizzazione Mark1030 V3

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30 35 40 45

Corrente [A]

prova

Dati Ballard Power System

Syngas: H2 72%, balance CO2, N2, CH4

Stack Mark1030 V3

Current [A]

Vo

lta

ge

[V

]

0

200

400

600

800

1000

1200

1400

1600

0 10 20 30 40 50

Corrente [A] Current [A]

Po

we

r [W

]

Due to the existence of irreversibilities we define a voltage efficiency that can be increased by acting on temperature and pressure (as well as on the composition):

The value of the open circuit voltage is higher for the low temperature cells, but also the polarizations are higher. In fact, in addition to the presence of activated polarization, the ohmic polarization is also higher due to the unfavourable effect of low temperature on the ionic resistance of the electrolyte.

200 400 600 800 1000 1200 14000,6

0,7

0,8

0,9

1

1,1

1,2

1,3Reversibile AFCPEMFC PAFCMCFC SOFC

Temperatura [K]

Ten

sion

e [V

]

ηV=VE

The influence of temperatureThe influence of temperature

Temperature [K]

Vo

lta

ge

[V

]

The influence of pressure on voltage and power density is favorable, both considering the reversible voltage and taking into account the polarizations. However, the voltage increase with the pressure is rapidly damped, whereas the construction problems increase as well as the power consumption of auxiliaries.

The influence of pressureThe influence of pressure

Current density (mA/cm2)

Current density (mA/cm2) Power density (mW/cm2)

Po

we

r d

en

sit

y (

mW

/cm

2)

Power density (mW/cm2) Power density (mW/cm2)

Ce

ll v

olt

ag

e (

V)

Ce

ll e

ffic

ien

cy

A current efficiency is also defined:

wherein v is the molar flow of fuel actually consumed in the reaction, φf is the utilization of the

fuel and ηF is the efficiency of Faraday which takes into account the different reactions that can

lead to the fuel oxidation: if only one type of reaction is possible then ηF is unitary, but if

different reactions can take place with different value of the number of electrons involved then ηF

is lower than unity. The contribution of ηF is, however, generally negligible, so that η

I≈φ

f.

Not all the fuel can be used because a high consumption negatively affects both E and the value of the threshold j

L at which the concentrated polarization is manifested. For the same reason, the

oxidant can not be used in full, but this has no negative effect on the efficiency of the fuel cell.

The only exception is PEMFC with a dead-end anode: in fact, if a PEMFC is fed with pure hydrogen, inside the anodic compartment remains only pure hydrogen and, therefore, it can be used completely. In reality there are impurities that gradually accumulate, in addition to water that can cross the membrane, and therefore periodic purges are needed.

ηI=I

n⋅F⋅v⋅ϕf=ηF⋅ϕ f

Fuel and oxidant utilisationsFuel and oxidant utilisations

university of cassino and southern lazio

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