Fuel cells revision

Why do the 2 half reactions occur in seperate compartments?

It prevents direct contact between the oxidant and reductant

What are the 2 main functions of a salt bridge?

• Maintains the neutrality of the 2 half cells ; prevents the + charge accumulating at anode and - charge at the cathode

• Completes the circuit by creating an internal circuit

Primary cells

Cannot be recharged

Secondary cells

Can be recharged

Fuel cells

Function continuously

Why must electrodes be both conducting and porous?

Allows hydrogen and oxygen gas to come into contact with the ions in the electrolyte and allow for the half-reactions to occur at the surface

Function of catalysts

Provides an alternative reaction pathway by lowering the activation energy of a reaction

Advantages of fuel cells

• Convert chemical energy directly to electrical energy

• Can use a variety of fuels

• When hydrogen is used as a fuel, no GHG are released

Disadvantages of fuel cellls

• Require a constant fuel supply

• Expensive

• The hydrogen used to source the fuel cells is mainly sourced from fossil fuels

Equation for steam reforming in the presence of a nickel catalyst

CH4(g) + H2O(g) → CO(g) + 3H2(g)

Steam reforming in the presence of a copper or iron catalyst

CO(g) + H2O(g) → CO2(g) + H2(g)

2 types of fuel cells

Alkaline fuel cell and acidic fuel cell

Oxidation equation for alkaline fuel cell

H2(g) + 2OH-(aq) → 2H2O(l) + 2e-

Reduction equation for alkaline fuel cell

O2(g) + 2H2O(l) + 4e- → 4OH-(aq)

Oxidation equation for acidic fuel cell

2H2(g) → 4H+(aq) + 4e-

Reduction equation for acidic fuel cell

O2(g) + 4H+(aq) + 4e- → 2H2O(l)

Overall equation for both alkaline and acidic fuel cell

2H2(g) + O2(g) → 2H2O(l)

Why are fuel cells more efficient than coal fired power stations or combustion engines?

Fuel cells convert chemical energy directly to electrical energy whereas coal fired power stations and combustion engines have multiple energy transformations which means energy is lost

Advantages of fuel cells

Converts chemical energy directly to electrical energy

Hydrogen fuel cells produce water and heat as by products which means no greenhouse gases are released

Electricity can be generated on site

Disadvantages of fuel cells

Expensive

Require a constant fuel supply

Issues with storage and safety of hydrogen fuel

What is the main source of greenhouse gases and pollutants?

Burning fossil fuels

How do we reduce greenhouse gas and pollutant emissions?

Increase energy efficiency

Reduce fossil fuel use and pollutant emissions

Limitations to using biofuels

Crop based fuels use land that is used for food production

Solid biofuels are low quality

Biogas contains less methane which contains lower energy per gram than natural gas

What are 2 ways to produce sustainable hydrogen?

Using renewable electricity to convert water to hydrogen

Collecting biogas from landfill sites and convert the methane gas to hydrogen through steam reforming

What are current storage methods for hydrogen?

Liquid hydrogen and compressed hydrogen

What is a microbial fuel cell (MFC)

A device that converts organic material to electrical energy via microorganisms

How do Microbial Fuel Cells work?

Microorganisms form a film on the surface of the anode

They oxidise the organic material to produce CO2, protons and electrons

The electrons are transferred to the anode

What is the purpose of the membrane in MFCs?

Separates the anode and cathode and allows ions to balance charge

Advantages of MFCs

Operate at room temp

Use low grade waste 

Useful for sustainable wastewater treatment and low power electricity generation