1 of 30© Boardworks Ltd 2010. 2 of 30© Boardworks Ltd 2010.

1 of 30 © Boardworks Ltd 2010


Electrochemical cells

Redox reactions involve the transfer of electrons between two species. The flow of electrons is an electrical current.

An electrochemical cell is a store of chemical energy in a closed system.

Redox reactions can therefore be used to generate electrical current. This is the basis of batteries and fuel cells.

All reactants and products are contained within the casing of the cell.


Rechargeable and non-rechargeable cells

Primary cells can only be used once.

Secondary cells can be recharged and reused.

Smaller cells contain fewer reactants and produce less electrical energy. However, the reaction voltage doesn’t change with cell size.

There are two types of cell: non-rechargeable and rechargeable. They are also known as primary and secondary cells.


How does a cell work?


Primary cells

A primary cell can only be used once because it transfers stored chemical energy into electrical energy by a non-reversible chemical reaction.

Primary cells are usually cheaper to buy.

They are more reliable as they do not discharge much when they are not in use. This makes them more useful for applications such as smoke detectors. Alkaline

dry cell

Carbon–zinc dry cell


Secondary cells


Reactions in a zinc–carbon cell


Primary or secondary cells?


Batteries

A battery is a closed system which contains the high energy reactants and the low energy products in a sealed unit.

By connecting more than one cell together, the electromotive force is increased. The battery voltage is the sum of the voltages of each of the cells.

When a conductor is connected across the two terminals, a circuit is formed and electrons flow through it.

A battery is more than one cell connected together in a series arrangement.


Lead-acid battery


Calculate the e.m.f


Fuel cells

A fuel cell converts chemical energy into electrical energy in a similar fashion to a battery. However, a fuel cell is an open system – reactants flow in and products flow out, rather than being stored in the cell.

Fuel cells are used in spacecraft and in hydrogen powered cars.


Hydrogen fuel cells

Hydrogen fuel cells have been used in spacecraft for many years. New developments are incorporating them into cars to replace the internal combustion engine.

In a hydrogen fuel cell, hydrogen and oxygen gases react to form water.

At the anode:

At the cathode:

Overall equation:

2H2(g) 4H+(aq) + 4e–

4H+(aq) + O2(g) + 4e– 2H2O(l)

2H2(g) + O2(g) 2H2O(l)


How does a hydrogen fuel cell work?


Label the fuel cell


Production of hydrogen

Hydrogen is a very reactive element and must be produced by a chemical reaction before it can be used in a fuel cell.

Although a hydrogen fuel cell is not directly polluting, processes used to produce hydrogen can be. Electricity for electrolysis is often produced by fossil fuel power stations. Carbon dioxide is produced from the reaction of hydrocarbons with steam.

There are currently two main methods of manufacturing hydrogen:

reaction of hydrocarbons with steam

electrolysis of acidified water.


Transporting and storing hydrogen


Hydrogen-rich fuels

With the difficulty in transporting and storing hydrogen, some fuel cell vehicles use hydrogen-rich fuels which are converted to hydrogen by an onboard ‘reformer’.

The reformer uses temperatures of 250–300°C to favour reactions generating hydrogen gas.

Hydrogen-rich fuels include methanol, natural gas and petrol.


Direct methanol fuel cells


Reactions in a direct methanol fuel cell


Ethanol fuel cell

Currently, platinum-based catalystsare used to oxidize ethanol, butdon’t achieve complete oxidation. Cheaper, more efficient catalysts capable of fully oxidizing ethanol are needed.

Ethanol is a less toxic and more energy-dense alcohol than methanol. Research into developing an ethanol fuel cell is ongoing.

Ethanol can be made from renewable sources, such as the fermentation of sugar. These energy resources are considered carbon neutral – the amount of carbon dioxide they release is equal to the amount absorbed during growth.


Fuel cell vs. conventional vehicles


Hydrogen economy


Modern breath alcohol testers

Fuel cell technology has been applied to breath alcohol testers. Ethanol in a person’s breath is oxidized into carbon dioxide and water, producing a measurable electrical current.

ethanol from breath

platinum anode

acid–electrolyte membrane

platinum cathode

water

electrical current

oxygen


Glossary


What’s the keyword?


Multiple-choice quiz

1 of 30© Boardworks Ltd 2010. 2 of 30© Boardworks Ltd 2010.

Documents

Transcript of 1 of 30© Boardworks Ltd 2010. 2 of 30© Boardworks Ltd 2010.