Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Fuel cells

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

StrutturaConsists of three layers, one above the other: The first layer is the anode, the second an electrolyte and the third layer is the cathode. Anode and cathode serve as catalyst. The layer in the middle consists of a carrier structure whichabsorbs the electrolyte.

In different types of fuel cells differentsubstances are used as electrolyte. Some electrolytes are liquid and some are solid with a membrane structure.

Because one cell generates only low voltageseveral cells get stacked according to the requested voltage. This arrangement is called"stack". Many cells combined are called a fuel cell

stack. The bipolar plates (dark blue) seperatethe cells and avoid electric connections.

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Struttura

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Principio di funzionamentoThe fuel cell reverses the process of electrolysis which isknown from school. In the process of electrolysis byapplying electric power water isdecomposed into the gaseouscomponents oxygen and hydrogen.

The fuel cell takes exactly these two substances and converts them to water again. In theory the same amount of energy which has been used for the electrolysis is set free by this conversion. In practice insignificant losses are causedby different physical-chemicalprocesses.

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Tipi di celleFuel cells use a chemical reaction, rather than combustion (burning a fuel), to produce electricity in a process that is the reverse of electrolysis. In electrolysis, and electric current applied to water produces hydrogen and oxygen. By reversing this process, hydrogen and oxygen are combined in the fuel cell to produce electricity and water.

Fuel cells are really a family of technologies; there are several major types of fuel cells, differentiated by the type of electrolyte they use.

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Confronto

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Applicazioni

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

PEMPolymer Electrolyte Fuel Cell (PEFC) or Proton Exchange Membrane Fuel Cell (PEMFC)The electrolyte in this type of fuel cell is an ion exchange membrane made of some type of polymer thatis a good conductor of protons. This type of fuel cell runs at low temperatures (usuallyaround 80-degrees Celsius), with electrical efficienciesof about 45%, and is the primary candidate forautomotive, small stationary, and portable power applications. PEMFCs require very pure hydrogen as the fuel.

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

PEM FC stackThe proton exchange membrane fuel cell -PEMFC is easy to handle. It is very light, it isvery efficient and as reaction gas it requiresonly atmospheric oxygen instead of pure oxygen. The hydrogen has to be generated in a reformer. PEM fuel cells are very sensitive to carbonmonoxide (CO). This gas might block the anodecatalyst and subsequently lead to a reducedperformance. The electrolyte consists of a solid protonexchange membrane (PEM) made fromsulphonated polymer. The power output of a PEM fuel cell can becontroled very dynamically. Therefore it isperfectly suitable for mobile applications and decentralised power plants. Among the development of fuel cells the PEMFC is most paramount at the moment. One reason is the cell`s enormous potential to bemass produced.

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Principio di funzionamento

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Principio di funzionamentoStep 1Inside the two seperate gas supplycicuits the gaseous oxygene and hydrogen flow into the gas area and the catalyzer.

Step 2While getting in contact with the catalyzer the hydrogen molecules(H2) are splitted into two H+protons. At the same time eachhydrogen atom sends out one electron.

Step 3The protons move through the electrolyte (membrane) to the cathode area.The electrolyte is an isolatorwhich is not permeable forelectrons

Step 4Dhe electrons move from the anode to the cathode and cause anelectric current. This electriccurrent supplies an electriccapacitor with electric power.

Step 5Respectively four electronsrecombine with one hydrogenmolecule at the cathode.

Stept 7The oxygene ions give theirelectrons to the two protons and oxidize to water.

Step 6The now generated oxygene ionshave a negative load. They move tothe positiv loaded protons.

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Caratteristica elettrica (1cm2)

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

PAFCPhosphoric Acid Fuel Cell (PAFC) - The electrolyte in this type of fuel cell is phosphoric acid, concentrated to100%. PAFCs have an operating temperature of about100-220 degrees Celsius, and achieve an electricalefficiency of about 37-42%. Buses and stationaryapplications currently use PAFCs.

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

MCFCMolten Carbonate Fuel Cell (MCFC) The electrolyte in this type of fuel cell is usually a combination of alkali carbonates, retained in a ceramicmatrix. The MCFC operates at 600-700 degrees Celsius. The high temperature enables the end user to utilize boththe electricity and the thermal energy generated by the fuel cell, resulting in electrical efficiencies of more than 70 percent. MCFCs are well-suited to large-scalestationary applications, and are currently beingdemonstrated for powering buildings. High-temperaturefuel cells can more easily use a wide range of fuelswithout using a "fuel reformer."

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

SOFCSolid Oxide Fuel Cell (SOFC) The electrolyte in the SOFC is a solid, nonporous metal oxide. At temperatures over 650 degrees Celsius, the SOFC can utilize a hydrocarbon fuel directly, withoutreforming, similar to the MCFC. Also similar to the MCFC, the SOFC generates both electricity and usablethermal energy. High-temperature SOFCs are being demonstrated forstationary power applications, while low-temperatureSOFCs are also being looked at for automotiveapplications.

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

AFCAlkaline Fuel Cell (AFC) - This was one of the first modern fuel cells to be developed, and was used toprovide on-board electric power for the Apollo space vehicle.

The electrolyte in this fuel cell is Alkaline (KOH). AFCs require pure hydrogen and pure oxygen as the reactants. The operating temperature for this type of fuel cell isaround 200 degrees Celsius.

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

AltreOther Types of Fuel Cells - There are other types of fuel cellsthat are relatively newer to the family of fuel cells. The Direct Methanol Fuel Cell (DMFC) is very similar to the PEMFC, but it isable to directly utilize liquid methanol at the anode. There is also a Regenerative Fuel Cell, which contains a membrane that can bothelectrolyze water into hydrogen and oxygen and, with the flick of a switch, recombine the two elements, producing electricity and water. In a Metal Air Fuel Cell, zinc pellets and an alkalineelectrolyte are circulated through the fuel cell stack and are combined with oxygen from air to create electricity, heat and zincoxide (in a solution of potassium zincate). The zincate can beregenerated in a separate process into fresh zinc pellets.

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Fuel Cell Power Plant vs. the “Internal Combustion Engine”

Source: Scientific American

Hydrogen-powered fuel cell engine:TWICETWICE the efficiency with ZEROZERO emissions

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

HydroGen3 Fuel Cell Car

Fuel cell power module

Hydrogen storage

Traction system

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

AutomotiveA 65 kW electric motor acceleratesthe A-Class to a 140 km/h top speedThe whole fuel cell system is accommodated in the sandwich floor construction of the A-Class with long wheelbase. The car’s fuel tanks contain hydrogen compressed under 350-bar pressure and give the A-Class Fuel Cell an operating range of some 150 kilometres. Hydrogen consumption is equivalent to 4.2 litres of diesel per 100 kilometres.The electric motor generates 65 kW, accelerating the compact Mercedes from 0 to 100 km/h in 16 seconds and on to a maximum speed of 140 km/h –enough power to equip the Fuel Cell for everyday conditions out on the road.

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

La collocazione dei principali elementi

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Rendimenti (1/2)The conventional way to compare the efficiency of vehicles is bythe distance they can travel per unit of fuel (km/l or mpg). However, this method presents difficulties if the cars being compared run on different kinds of fuel such as hydrogen or natural gas. In such cases, we need to use a yardstick that not only measures how efficiently the car itself uses energy (tank to wheel), but also how efficiently the energy is obtained and transported to the car's tank (well to tank). This overall measure of efficiency is called "well to wheel." Well-to-tank: Efficiency with which the fuel is obtained, processed, stored and transported to the vehicle's tank.Tank-to-wheel: Efficiency with which the fuel in the vehicle's tank is consumed and converted into vehicle motion at the wheels.

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Rendimenti (2/2)

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Fuel Cell BenefitsCleanFuel cells produce low or zero emissions. EfficientFuel cells are highly efficient at converting fuel to electrical energy. Plus, the heat produced as a by-product of that conversion process can be usedto generate even more energy or for area heating.QuietFuel cells are quiet, making them suitable for residential areas. SmallA high power density means fuel cells are relatively compact. Low MaintenanceWhile fuel cell systems usually have some moving parts, actual fuel cellshave no moving parts, making them more reliable, and less costly tomaintain than power sources that do. SustainableFuel cells can be powered by hydrogen, the most abundant element in the universe. Hydrogen can be produced from a variety of sources, includingfossil fuels, natural gas, methanol, and various renewable energy sources.

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Quale futuro per l’idrogeno?

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Road map - 1

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Road map -2

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Road map -3

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Caratteristica elettrica (PEM)Tensione in condizioni ideali (reversibili)

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Perdite di attivazionePer bassi valori di corrente si ha un brusco calo di tensione espresso dalla equazione di Tafel. Più è piccolo i0 più bruscamente (cioè per piccoli valori di i) cresce ΔVact.

⎟⎟⎠

⎞⎜⎜⎝

⎛⋅=Δ

0act i

ilnAV

A è grande se la reazione elettrochimica è lenta.i0 rappresenta la densità di corrente in corrispondenza della quale ΔVact diviene significativa.Tale caduta di tensione si manifesta specialmente al catodo. Si può dimostrare ed osservare che si riduce aumentando la temperatura e la pressione ed usando ossigeno puro invece dell’aria.

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Corrente internaAnche a circuito aperto la cella è interessata da una corrente!

Si può infatti verificare che elettroni (invece che ioni) fluiscano nell’elettrolita, e che idrogeno fluisca attraverso l’elettrolita fino al catodo, reagendo lì con l’ossigeno a causa del catalizzatore e non determinando corrente attraverso il circuito esterno. Se in è la densità di corrente interna, la caduta di tensione a bassissime correnti si esprime modificando le cadute di attivazione.

⎟⎟⎠

⎞⎜⎜⎝

⎛ +⋅=Δ

0

n

iiilnAV

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Caduta ohmicaSono cadute di tensione legate alla resistenza elettrica di elettrodi ed elettrolita.Se r è la resistenza specifica per 1cm2 di cella ed i, come in precedenza, è la densità di corrente:

irVohm ⋅=Δ

Questa caduta di tensione può essere ridotta operando su materiali e forme degli elettrodi e riducendo lo spessore dell’elettrolita (ma questo è possibile fino ad un certo punto, altrimenti si rischia che i due elettrodi vengano a contatto).

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Perdite per concentrazioneAd elevata densità di corrente si verifica che il consumo dei reagenti (idrogeno ed ossigeno) è più rapido della loro sostituzione agli elettrodi. La diminuzione di concentrazione determina una riduzione delle pressioni parziali e quindi una caduta di tensione. Sperimentalmente si ha (m ed n sono costanti dipendenti dal tipo di cella e dalle condizioni operative):

)inexp(mVtrans ⋅⋅=Δ

In celle PEM l’acqua può incrementare queste perdite perché funge da barriera per l’ossigeno.

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

In totale…

Tale equazione viene usualmente semplificata trascurando in (ma a quel punto è valida per densità di corrente non troppo basse)

)inexp(mi

iilnAirEV

VVVEV

0

n

transactohm

⋅⋅−⎟⎟⎠

⎞⎜⎜⎝

⎛ +⋅−⋅−=

Δ−Δ−Δ−=

( ) )inexp(milnAirEV oc ⋅⋅−⋅−⋅−=

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

Un esempio: PEM Ballard Mark V(35 celle-5kW))

Eoc [V] = 1.031r [kΩcm2] = 2.45e-4A [V] = 0.03m [V] = 2.11e-5n [cm2mA-1] = 8e-3

0 200 400 600 800 10000.5

0.6

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0.8

0.9

1

1.1

1.2

1.3

[mA/cm2]

[V]

0 200 400 600 800 10000

100

200

300

400

500

600

[mA/cm2]

[W]

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

L’elettronica di potenza

Prof. G.Spagnuolo – D.I.I.I.E. – Università di Salerno

P&O