F325: Module 2: Fuel Cells

Question 1

I) How does a proton exchange membrane cell work?

Answer 1

At the negative electrode the hydrogen dissociates into H+ ions and electrons.

The ions diffuse through an ion-permeable polymer membrane (the electrolyte) to the other electrode where they react with O2 to produce water.

The overal reaction is: H2(g) + 1/2O2(g) => H2O(l)

Question 2

I) How does an Alkaline fuel cell work?

Answer 2

An alkaline electrolyte is used (potassium hydroxide)
4e- + 4H2O ==> 2H2 + 4OH-
4e- + 2H2O + O2 ==> 4OH-
Overall: 2H2 + O2 ==> 2H2O

Question 3

I) How does a Direct methanol fuel cell work?

Answer 3

…

Question 4

I) Describe the function of the three main components of the fuel cell.

Answer 4

The Negative Electrode - H2 is oxidised to produce electrons and protons.

The Electrolyte - Consists of materials such as aqueous alkali or a proton-permiable membrane.

The Positive Electrode - O2 reacts with H+ ions and is reduced.

Question 5

I) What fuels can be used to power fuel cells?

Answer 5

Hydrogen gas

Hydrogen-rich fuel such as methanol

Question 6

What is hydrogen economy and what factors need considering if we are to adopt hydrogen as fuel?

Answer 6

Hydrogen economy is the proposed use of hydrogen as a major fuel to produce energy.

The factors are:
- There are problems with the safety, storage and handling of hydrogen.

• Hydrogen is an energy carrier and not a source of energy. Energy has to be used to extract it from water (by electrolysis) or other sources.
• It has not yet gained general acceptance as a fuel, and FCVs remain more expensive than their fossil-fuel counterparts.
• There is currently no infrastructure designed to supply hydrogen as fuel.

Question 7

(4 marks)

Discuss two disadvantages and advantages of using fuel cells for energy rather than fossil fuels.

Answer 7

• H2 is difficult to store
• H2 is difficult to manufacture initially/ limited life cycle of H2 adsorber
• Only water formed
• Greater efficiency

Question 8

I) How does an Acidic fuel cell work?

Answer 8

The fuel cell uses an acidic electrolyte.
2e- + 2H+ ==> H2
4e- + 4H+ + O2 ==> 2H2O
Overall: 2H2 + O2 ==> 2H2O

Question 9

I) What are some of the advantages of Fuel Cells over conventional petrol or diesel powered vehicles?

Answer 9

LESS POLLUTION AND LESS CO2 (Pure hydrogen only emits H2O whilst hydrogen rich fuels only produce small amounts of air pollutants and CO2.)

GREATER EFFICIENCY

Question 10

I) What are some of the disadvantages of Fuel Cells over conventional petrol or diesel powered vehicles?

Answer 10

STORING AND TRANSPORTING HYDROGEN

LIMITED LIFE CYCLE OF ADSORBER

LIMITED LIFETIME

HIGH PRODUCTION COST

USE OF TOXIC CHEMICALS IN THEIR PRODUCTION

Question 11

I) How can Hydrogen be stored in fuel cells?

Answer 11

As a liquid under pressure.

Adsorbed on the surface of a solid material.

Adsorbed within a solid material.