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Sustainability > Week 5 > Flashcards

Week 5 Flashcards

1
Q

What is a fuel cell

A

converts chemical energy directly into electricity

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2
Q

What are fuel cells composed of

A

-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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3
Q

What fuels are required in basic fuel cells

A

Hydrogen and oxygen

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4
Q

What happens at anode in basic fuel cell

A

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

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5
Q

What happens at cathode in basic fuel cell

A

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

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6
Q

What is a fuel stack

A

Multiple fuel cells stacked to increase voltage

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7
Q

What type of current do fuel cells provide?

A

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

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8
Q

Advantages of fuel cells

A

No pollution
oxygen abundant in air

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9
Q

disadvantages of fuel cells

A

Hydrogen not readily available
difficult to store hydrogen

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10
Q

what are alkaline fuel cells (AFC)

A

operate on compressed hydrogen and oxygen to produce electricity

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11
Q

what electrolyte is used in alkaline fuel cells (AFC)

A

KOH- potassium hydroxide

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12
Q

What are the electrodes made of in alkaline fuel cells

A

porous carbon plates

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13
Q

what catalyst is used in alkaline fuel cells

A

platinum

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14
Q

what happens at the anode in an AFC

A

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

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15
Q

What happens at the cathode in an AFC

A

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

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16
Q

What is efficiency and operating conditions of AFC

A

Efficiency= 70%
Operating temp: 150-200 C

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17
Q

Disadvantages of AFC

A

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

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18
Q

What are molten carbonate fuel cells (MCFC)

A

Uses high temp compounds of salt as electrolyte eg-carbonates

19
Q

what is the efficiency and operating temp for MCFC

A

Efficiency=60-80%
Operating temp= 650 C

20
Q

How are carbonates used in MCFC

A

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

21
Q

Advantages of MCFC

A

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

22
Q

what is the electrode material in MCFC

A

Nickel

23
Q

what is catalyst in MCFC

A

nickel

24
Q

What are disadvantages of MCFC

A

-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

25
Q

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

A

Phosphoric acid

26
Q

What is the efficiency and operating temp in PAFC

A

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

27
Q

Adv of PAFC

A

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

28
Q

Disadv of PAFC

A

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

29
Q

What is the electrode in PAFC

A

Platinum

30
Q

What is the catalyst in PAFC

A

Platinum

31
Q

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

A

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

32
Q

What happens at cathode of PEMFC

A

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.

33
Q

What is the electrolyte in PEMFC

A

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

34
Q

What is the catalyst in PEMFC

A

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

35
Q

What is the efficiency and operating temperature for PEMFC

A

Efficiency= 35-45%
Temp= 80 C

36
Q

Adv of PEMFC

A

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)

37
Q

Disadv of PEMFC

A

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

38
Q

what is a solid oxide fuel cell (SOFC)

A

Uses hard ceramic compound of metal oxides as electrolyte.

39
Q

What happens at cathode in SOFC

A

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.

40
Q

Operating temp of SOFC

A

700-1000 C

41
Q

Explain the benefit of planar SOFC over tubular

A

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

42
Q

ADV of SOFC

A

-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)

43
Q

What is the catalyst for SOFC

A

Cobalt and nickel

44
Q
A

Sustainability (8 decks)

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