Commercial Applications of Electrochemical Cells (Oxford AQA International A Level Chemistry)
Revision Note
Written by: Richard Boole
Reviewed by: Stewart Hird
Commercial Cells
Electrochemical cells can be used as a commercial source of electrical energy
Cells can be:
Non-rechargeable (irreversible) cells
Rechargeable cells
Fuel cells
The type of cells used in commercial applications depend on the:
Required voltage
Required current
Size
Cost
Although it is commonly used incorrectly, the term battery should be used to refer to a collection of cells
A car battery is correct, because it is a collection of six cells joined together
Non-rechargeable cells
The Daniell cell
The Daniell cell was one of the earliest electrochemical cells and consisted of a simple metal-metal ion system
It was invented by British chemist John Daniell in 1836
The Daniell cell consists of:
A zinc rod immersed in a solution of zinc sulfate
A copper cylinder filled with copper sulfate solution
A porous pot that separates the copper sulfate from the zinc sulfate
The zinc acts as the negative electrode and the copper is the positive electrode
The half-cell reactions are
Zn (s) → Zn2+ (aq) + 2e- Eꝋ = -0.76 V
Cu2+ (aq) + 2e- → Cu (s) Eꝋ = +0.34 V
The cell generates an electromotive force of:
Ecellꝋ = Ereductionꝋ - Eoxidationꝋ
Ecellꝋ = (+0.34) - (-0.76) = +1.10 V
The overall reaction is:
Zn (s) + CuSO4 (aq) → Cu (s) + ZnSO4 (aq)
However, the cell is impractical to use as a portable device because of the hazardous liquids in the cell
Zinc-carbon cells
Zinc-carbon cells are the most common type of non-rechargeable cells
Zinc-carbon cells consist of:
A zinc casing
A paste of ammonium chloride which acts as an electrolyte as well as the positive electrode
A carbon rod which acts as an electron carrier in the cell
The zinc acts as the negative electrode and the ammonium chloride is the positive electrode
The half-cell reactions are
Zn (s) → Zn2+ (aq) + 2e- Eꝋ = -0.76 V
2NH4+ (aq) + 2e- → 2NH3 (g) + H2 (g) Eꝋ = +0.74 V
The cell generates an electromotive force of:
Ecellꝋ = Ereductionꝋ - Eoxidationꝋ
Ecellꝋ = (+0.74) - (-0.76) = +1.50 V
The overall reaction is:
2NH4+ (aq) + Zn (s) → 2NH3 (g) + H2 (g) + Zn2+ (aq)
One disadvantage is that as the cell discharges, the zinc casing eventually wears away and the corrosive contents of the electrolyte paste can leak out
Rechargeable Cells
Rechargeable cells employ chemical reactions which can be reversed by applying a voltage greater than the cell voltage, causing electrons to push in the opposite direction
There are many types of rechargeable cells, but common ones include:
Lead-acid batteries
NiCad cells
Lithium cells (covered in more detail in the next section)
Lead-acid batteries
Lead-acid batteries consist of six cells joined together in series
The cells use lead metal as the negative electrode and and lead(IV) oxide as the positive electrode
The electrolyte is sulfuric acid
The half-cell reactions are:
Pb (s) + SO42- (aq) ⇌ PbSO4 (s) + 2e- Eꝋ = -0.36 V
PbO2 (s) + 4H+ (aq) + SO42- (aq) + 2e- ⇌ PbSO4 (s) + 2H2O (l) Eꝋ = +1.70 V
Each cell generates an electromotive force of:
Ecellꝋ = Ereductionꝋ - Eoxidationꝋ
Ecellꝋ = (+1.70) - (-0.36) = +2.06 V
The overall reaction is:
PbO2 (s) + 4H+ (aq) + 2SO42- (aq) + Pb (s) → 2PbSO4 (s) + 2H2O (l)
In a commercial car battery, the six cells in series give a combined voltage of about 12 V
When the car is in motion, the generator provides a push of electrons that reverses the reaction and regenerates lead and lead(IV) oxide
Lead-acid batteries are designed to produce a high current for a short period of time, hence their use in powering a starter motor in car engines
The disadvantage of lead-acid batteries is that:
They are very heavy
They contain toxic lead and lead(IV) oxide
The sulfuric acid electrolyte is very corrosive
This presents challenges of disposal when lead-acid batteries come to the end of their useful life
NiCad cells
NiCad stands for nickel-cadmium and these cells are available in many standard sizes and voltages so they can replace almost any application of traditional zinc-carbon cells
Although they are more expensive cells, the fact they can be recharged hundreds of times means they are commercially viable
The positive electrode consists of cadmium and the negative electrode is made of a nickel(II) hydroxide-oxide system
The half-cell reactions are:
Cd (s) + 2OH- (aq) → Cd(OH)2 (s) + 2e- Eꝋ = -0.82 V
NiO(OH) (s) + H2O (l) + e- → Ni(OH)2 (s) + OH- (aq) Eꝋ = +0.38 V
The cell generates an electromotive force of:
Ecellꝋ = Ereductionꝋ - Eoxidationꝋ
Ecellꝋ = (+0.38) - (-0.82) = +1.20 V
The overall reaction is:
2NiO(OH) (s) + 2H2O (l) + Cd (s) → 2Ni(OH)2 (s) + Cd(OH)2 (s)
Cadmium is a toxic metal so the disposal of old NiCad cells is also an environmental issue
Lithium Cells
Lithium ion cells power the laptop or mobile device you are probably reading this on
The Noble Prize for Chemistry in 2019 was awarded to John B. Goodenough, M. Stanley Whittingham and Akira Yoshino for their work on lithium ion cells that have revolutionised portable electronics
Lithium is used because it has a very low density and relatively high electrode potential
Lithium cells consists of:
A positive lithium cobalt oxide electrode
A negative carbon electrode
A porous polymer membrane electrolyte
The polymer electrolyte cannot leak since it is not a liquid or paste, which presents advantages over other types of cells
The cell consists of a sandwich of different layers of lithium cobalt oxide and carbon
When the cell is charged and discharged the lithium ions flow between the negative and the positive through the solid electrolyte
The half-cell reactions on discharge are:
Li (s) → Li+ (s) + e– Eꝋ = -3.05 V
Li+ (s) + CoO2 (s) + e– → Li + (CoO2) – (s) Eꝋ = +1 V
The cell generates an emf of between 3.5 V and 4.0 V and the overall reaction is
Li (s) + CoO2 (s) → Li + (CoO2) – (s) Eꝋcell ~ +3.5
NiCad cells have a problem called the memory effect in which they gradually begin to lose their charge after repeated charge cycles when the cell is not fully discharged
The cells appear to 'remember' their lower state of charge
Lithium-ion cells do not have this problem so can be topped up without any loss of charge
Some of the problems with lithium ion cells:
A global shortage of lithium is likely to make lithium ion cells unsustainable as the current demand for lithium exceeds the supply
If cells are not recycled but thrown away in landfills, then a huge amount of lithium becomes lost to future generations
Reports of lithium ion cell fires have raised concern about the safety of these batteries in electronic devices; it is a reminder to us that lithium is a very reactive element in Group 1 of the periodic table, which is why it has a high electrode potential
Fuel Cells
A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode
These cells are becoming more common in the automotive industry to replace petrol or diesel engines
As the fuel enters the cell it becomes oxidised which sets up a potential difference or voltage within the cell
Different electrolytes and fuels can be used to set up different types of fuel cells
An important cell is the hydrogen-oxygen fuel cell which combines both elements to release energy and water
The fuel cell consists of:
A reaction chamber with separate inlets for hydrogen and oxygen gas
An outlet for the product - water
An electrolyte of aqueous sodium hydroxide
A semi-permeable membrane that separates the hydrogen and oxygen gases
The half equations are
2H2 (g) + 4OH– (aq) → 4H2O (l) + 4e– Eꝋ = -0.83 V
O2 (g) + 2H2O + 4e– → 4OH– (aq) Eꝋ = +0.40 V
The cell generates an electromotive force of:
Ecellꝋ = Ereductionꝋ - Eoxidationꝋ
Ecellꝋ = (+0.40) - (-0.83) = +1.23 V
The overall reaction is found by combining the two half equations and cancelling the common terms:
2H2 (g) + O2 (g) → 2H2O (l)
Benefits
Water is the only reaction product, so fuel cells present obvious environmental advantages over other types of cells
The reaction is the same as hydrogen combusting in oxygen, but since the reaction takes place at room temperature without combustion, all the bond energy is converted into electrical energy instead of heat and light
There are no harmful oxides of nitrogen produced, which are usually formed in high temperature combustion reactions where air is present
Fuel cells have been used on space craft, where the product can be used as drinking water for astronauts
Risks and problems
Hydrogen is a highly flammable gas and the production and storage of hydrogen carries safety hazards
Very thick walled cylinders and pipes are needed to store hydrogen which has economic impacts
The production of hydrogen is a by-product of the crude oil industry, which means it relies on a non-renewable, finite resource
Until a cheap way is found to make hydrogen, its widespread use in fuel cells will be limited
Hydrogen has high energy density, that is, the amount of energy contained in 1g of the fuel is high compared to other fuels, but because it is a gas, its energy density per unit volume is low which means larger containers are needed compared to liquid fuels
Examiner Tips and Tricks
One difference between fuel cells and other cells is that the cell operates continuously as long as there is a supply of hydrogen and oxygen; the energy is not stored in the cell.
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