Week 5 Flashcards
What is a fuel cell
converts chemical energy directly into electricity
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
What fuels are required in basic fuel cells
Hydrogen and oxygen
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
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
What is a fuel stack
Multiple fuel cells stacked to increase voltage
What type of current do fuel cells provide?
DC- used to power motors, lights or electrical appliances.
Advantages of fuel cells
No pollution
oxygen abundant in air
disadvantages of fuel cells
Hydrogen not readily available
difficult to store hydrogen
what are alkaline fuel cells (AFC)
operate on compressed hydrogen and oxygen to produce electricity
what electrolyte is used in alkaline fuel cells (AFC)
KOH- potassium hydroxide
What are the electrodes made of in alkaline fuel cells
porous carbon plates
what catalyst is used in alkaline fuel cells
platinum
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.
What happens at the cathode in an AFC
oxygen and water and returning electrons form hydroxide ions which are recycled back to anode.
What is efficiency and operating conditions of AFC
Efficiency= 70%
Operating temp: 150-200 C
Disadvantages of AFC
-require pure hydrogen and platinum electrode catalysts (expensive)
-Can leak
-easily poisoned by CO2
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
