Week 5 Flashcards by Heba Ali

What is a fuel cell

converts chemical energy directly into electricity

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What are fuel cells composed of

-2 electrodes (one positive (cathode) and one negative (anode). Reactions to produce electricity occur at electrodes

-Electrolyte-carries charge particles from one electrode to another

-Catalyst-speeds up reactions at electrodes

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What fuels are required in basic fuel cells

Hydrogen and oxygen

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What happens at anode in basic fuel cell

Hydrogen atoms enter at anode, get stripped of electrons, become ionised, carries positive charge.
negative electrons carry current around circuit

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What happens at cathode in basic fuel cell

Oxygen enters fuel cell at cathode, combines with electrons returning from electrical circuit and hydrogen ions from anode

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What is a fuel stack

Multiple fuel cells stacked to increase voltage

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What type of current do fuel cells provide?

DC- used to power motors, lights or electrical appliances.

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Advantages of fuel cells

No pollution
oxygen abundant in air

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disadvantages of fuel cells

Hydrogen not readily available
difficult to store hydrogen

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what are alkaline fuel cells (AFC)

operate on compressed hydrogen and oxygen to produce electricity

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what electrolyte is used in alkaline fuel cells (AFC)

KOH- potassium hydroxide

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What are the electrodes made of in alkaline fuel cells

porous carbon plates

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what catalyst is used in alkaline fuel cells

platinum

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what happens at the anode in an AFC

hydrogen gas combines with hydroxide ions to produce water vapour. Result- electrons flow out of anode and produce current.

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What happens at the cathode in an AFC

oxygen and water and returning electrons form hydroxide ions which are recycled back to anode.

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What is efficiency and operating conditions of AFC

Efficiency= 70%
Operating temp: 150-200 C

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Disadvantages of AFC

-require pure hydrogen and platinum electrode catalysts (expensive)
-Can leak
-easily poisoned by CO2

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What are molten carbonate fuel cells (MCFC)

Uses high temp compounds of salt as electrolyte eg-carbonates

what is the efficiency and operating temp for MCFC

Efficiency=60-80%
Operating temp= 650 C

How are carbonates used in MCFC

solid at room temp- exhibit ion conductivity at molten state- hence used as electrolyte

Advantages of MCFC

-Limited damage from carbon monoxide poisoning
-Waste heat recycled to make more electricity
Nickel electrode catalysts inexpensive
-High efficiency
-Fast reaction

what is the electrode material in MCFC

Nickel

what is catalyst in MCFC

nickel

What are disadvantages of MCFC

-High operating temp- not safe to use domestically
-Carbonate ions from electrolyte used up in reactions- CO2 needs to be injected to keep it running
-Corrosion
-Liquid electrolyte=handling problems
-Matrix cracking due to high temps

What electrolyte is used in phosphoric acid fuel cells (PAFC)

Phosphoric acid

What is the efficiency and operating temp in PAFC

efficiency = 40-80% temp= 150-200 C

Adv of PAFC

can tolerate Carbon monoxide concentration of 1.5% broadening choice of fuel used

Disadv of PAFC

-Sulfur must be removed -Platinum electrode and catalyst needed= expensive -Internal parts must withstand corrosive acid

What is the electrode in PAFC

Platinum

What is the catalyst in PAFC

Platinum

What happens at the anode in proton exchange membrane fuel cell (PEMFC)

Frees electrons from hydrogen and conducts them in circuit. Has channels that disperse hydrogen gas over catalyst surface.

What happens at cathode of PEMFC

Has channels to distribute oxygen to surface to surface of catalyst. Conducts electrons back from circuit to catalyst to recombine with hydrogen ions and oxygen to form water.

What is the electrolyte in PEMFC

proton exchange membrane. uses polymer electrolyte (thin sheet), only conducts positively charged ions (blocks electrons).

What is the catalyst in PEMFC

Platinum powder thinly coated on carbon paper (rough and porous to increase surface area).

What is the efficiency and operating temperature for PEMFC

Efficiency= 35-45% Temp= 80 C

Adv of PEMFC

High efficiency (when running on hydrogen and no air pressurisation) Flexible electrolyte that does not leak or crack Low temp (suitable for homes and cars)

Disadv of PEMFC

Fuels must be purified Platinum expensive Easily poisoned by carbon monoxide Start up time required Hydrogen storage is difficult

what is a solid oxide fuel cell (SOFC)

Uses hard ceramic compound of metal oxides as electrolyte.

What happens at cathode in SOFC

Oxygen molecules from air are split into oxygen ions with addition of four electrons. Ions are conducted through electrolyte, combine with hydrogen at anode, releases 4 electrons. Electrons travel in circuit producing electricity and heat.

Operating temp of SOFC

700-1000 C

Explain the benefit of planar SOFC over tubular

Planar is thin ceramic sheets which have lower electrical resistance compared to tubular arrangement- hence higher efficiency.

ADV of SOFC

-Solid electrolyte- no pumps required to circulate electrolyte -Hydrogen and carbon monoxide can be used in cell (doesn't get poisoned)- readily use natural gas, diesel, gasoline and alcohol. -Gas can be reformed inside cell (PEMFC needs external reformer) -Air compression not required (simpler, quiet, higher efficiency)

What is the catalyst for SOFC

Cobalt and nickel