Energy transfers - fuel cells

Question 1

Fuel cells are used in spacecraft for several reasons:

Answer 1

• The water produced is not wasted - the astronauts drink it.
• They are lightweight; normal batteries are heavier, so the spacecraft can use the saving to carry a bigger payload.
• They are compact.
• There are no moving parts.

Question 2

The car industry is developing fuel cells for a variety of reasons.

Answer 2

• There are no carbon dioxide emissions when fuel cells are used in cars.
• Fossil fuels are non-renewable, so car manufacturers are researching alternatives that are renewable and cause less pollution.
• The main product of a hydrocarbon-powered fuel cell is water, which is not a pollutant.
• There is a large source of hydrogen available by decomposing water.

Question 3

What are the current mobile sources of electricity?

Answer 3

The battery and internal combustion engine.

Question 4

What are fuel cells especially useful for?

Answer 4

Mobile energy sources.

Question 5

Advantages of hydrogen-oxygen fuel cells.

Answer 5

• There is direct energy transfer. Energy is converted directly from chemical energy in the fuel into electrical energy. The energy does not have to be converted into heat first.
• There are fewer production stages. Every new stage in any process increases the energy losses. In the fuel cells themselves the energy conversion is all done in a single stage.
• They are more efficient as there are fewer stages and almost all the energy is converted.
  *

Question 6

However, there are also problems with hydrogen-oxygen fuell cells.

Answer 6

• Poisonous catalysts are used that have to be disposed of at the end of the life-time of the cell.
• Fossil fuels are burnt to produce the hydrogen and oxygen needed.

Question 7

Getting energy from fuel

Answer 7

• Energy is only released when the fuel reacts reacts with oxygen in the air. This reaction is endothermic.
• If the fuel cell uses hydrogen, the reaction is: 2H₂ + O₂ ⇒ 2H₂O
  
  • When hydrogen reacts with oxygen by burning, chemical energy is give out as heat.
  • A fuell cell converts chemical energy directly into electrical energy.
• The overall reaction in a hydrogen-oxygen fuell cell is: 2H₂ + O₂ ⇒ 2H₂O with electrons being exchanged from the cathode to the anode.

Question 8

What does the fuel contain?

Answer 8

• The fuel (hydrogen) and oxygen have chemical energy as a store inside them.
• When the fuel reacts some of the chemical energy is given out. This is shown in an energy-level diagram.

Question 9

What happens at the electrodes?

Answer 9

• At the negative electrode the hydrogen releases electrons. This is an oxidation reaction:
  
  • 2H₂ ⇒ 4H⁺ + 4e^-.
• At the positive electrode electrons are taken in. This is a reduction reaction:
  
  • 4H⁺ + O₂ + 4e^-. ⇒ 2H₂O

Question 10

What is a redox reaction?

Answer 10

A reaction where electrons are gained and lost is called of redox reaction.