Cells and Batteries

Syllabus Statements:

• C.5.1 Describe how a hydrogen–oxygen fuel cell works. (Include the relevant half-equations in both acidic and alkaline electrolytes.)

• C.5.2 Describe the workings of rechargeable batteries. ( Include the relevant half-equations. Examples should include the lead–acid storage battery, the nickel–cadmium (NiCad) battery and the lithium-ion battery.

• C.5.3 Discuss the similarities and differences between fuel cells and rechargeable batteries.

Fuel Cells

• A fuel cell is a device that converts chemical energy directly into electrical energy.

• The most widely used fuel cell is a hydrogen/oxygen fuel cell.

• This is used in the space programme!!

• The electrodes are made of porous carbon impregnated with a catalyst

• (either Pd or Pt for the negative electrode; Pt for the positive electrode)

• Notice that I’ve been very careful not to talk about anode and cathode.

• I’ll explain why later!

• The chemical reactants are supplied from an external source (gas tanks!) directly into the electrodes.

• The electrodes are surrounded by an electrolyte of hydroxide solution (KOH or NaOH)

• Let’s see a diagram!

• At the –ve electrode:

• Hydrogen reacts to give water and surplus electrons

• 2H2 (g)+ 4OH-(aq) 4H2O(l) + 4e-

• Everything has been doubled to make it easier to balance the overall equation!

• The hydrogen has been oxidised• Why?

• The electrons generated flow round the circuit to the other electrode.

• O2(g) + 2H2O(l) + 4e- 4OH-(aq)

• The oxygen is reduced• Why?

• What is the overall reaction?

• 2H2(g) + O2(g) 2H2O(l)

• Notice that the hydroxide concentration isn’t changed.

• This type of fuel cell generates about 1.23V

• The hydroxide electrolyte can be replaced with acidic conditions

• In this case the half equations are:• At the –ve electrode:

• 2H2(g) 4H+(aq) + 4e-

• At the +ve electrode:

• O2(g) + 4H+(aq) 2H2O(l)

• Make sure you can give either set of half equations

• A small aside!• Why was I so sneaky about not

mentioning anode and cathode?• Well when you were told that the

anode was the +ve electrode and the cathode was the –ve electrode,• We lied to you!

• The anode is the electrode where oxidation occurs.

• In electrolysis this is the +ve electrode• But in cells, this is the –ve electrode!!

• The cathode is the electrode where reduction occurs

• In electrolysis this is the –ve electrode• But in cells this is the +ve electrode• You will study this more in the oxidation and

reduction module

• See what I did there?• Eh – oh = A O = anode oxidation

• Advantages:• Fuels cells are very efficient compared to

conventional cells (70 – 80% efficient)• They produce no greenhouse gases• No “thermal pollution”• The water produced can be drunk (useful in

the space program)• They are light weight

Advantages and Disadvantages of Fuel Cells

• Disadvantages:• Gases ( H2 ; O2 )are hard to store and handle• The H2 is often produced from the electrolysis

of water, which requires fossil fuels to be burned.

• They often experience technical problems (leaks, corrosion, failure of the catalyst)

• They are ****** expensive!

Summary

Rechargeable Batteries

• Primary cells produce electricity through chemical reactions BUT CANNOT BE RECHARGED.

• These are discussed in detail in the oxidation and reduction topic.

• Secondary cells CAN be recharged• You need to be aware of the details of– The Lead Acid Battery– Nickel Cadmium Batteries (NiCad)– Lithium Ion Batteries

The Lead Acid Battery

• Used in cars• Anode in made of lead plates• Cathode is made of lead(IV) oxide• The electrolyte is sulphuric acid• 5.2 mol / dm3 since you asked!

• The half equations are:

• Pb(s) + SO42-

(aq) PbSO4(s) + 2e-

• Is this the negative electrode or positive electrode?

• Negative – because it generates electrons• Is it the anode or the cathode?• It is the anode because the lead is oxidised (it

loses electrons)• And . . .

• At the positive electrode:

• PbO2(s) + 4H+(aq) + SO4

2-(aq) + 2e- PbSO4(s) +

2H2O(l)

• Lead is reduced – why?• Goes from Pb(IV) to Pb(II)• So this is the • Cathode

• What’s the overall reaction:• Pb(s) + PbO2(s) + 2H2SO4(aq) 2PbSO4(s) +

2H2O(l)

• Uses up sulphuric acid• The electrolyte gets less concentrated• The condition of the battery can be checked

by measuring the strength of the acid• We don’t bother titrating it (phew!) -

• The whole point of a rechargeable battery is that it can be recharged (DUH!)

• This is done by passing electricity through it

• PbSO4 + 2e- Pb + SO42-

• Pb(II) Pb(0) Lead is reduced

• PbSO4 + 2H2O PbO2 + 4H+ + SO42- + 2e-

• So what’s the overall reaction?

• 2PbSO4(s) + 2H2O(l) Pb(s) + PbO2(s) + 2H2SO4(aq)

• Notice that this uses up water and regenerates sulfuric acid

• Lead and lead oxide are also regenerated

• In practice the battery is usually charged by the alternator (a small generator) whilst it is still in the car.

• Each cell produces 2V; car batteries are 12V, so 6 cells are connected in series to make a car battery

• Advantages:• Easily recharged• Can deliver a large amount of energy for a

short time

• Disadvantages:• Heavy• Acid can spill

NiCad batteries

• Known as “dry cells”• Produce about 1.4V• The electrolyte is KOH• They are called “dry cells” because the

electrolyte is either soaked onto a paper separator or made into a paste

• At the –ve electrode• Cd(s) + 2OH-

(aq) Cd(OH)2(s) + 2e-

• Cd is oxidised (loses electrons)• Hence this is the anode• At the +ve electrode• NiO(OH)(s) + H2O + e- Ni(OH)2 + OH-

• Ni is reduced• Ni(III) Ni(II)• Therefore this is the cathode

So what’s the overall reaction?

• Cd + 2NiO(OH) + 2H2O Cd(OH)2 + 2Ni(OH)2

• (All solids - except water!)

• The reducing and oxidising agents are regenerated by recharging

• Cd(OH)2 + 2Ni(OH)2 Cd + 2NiO(OH) + 2H2O

• Remember - only the half equations show the electrons. We cancel them out in the overall reaction!

• Advantages:• Light• Easy to transport• Long life

• Disadvantages:• Expensive (compared to lead - acid batteries)• Lower voltage than lead – acid batteries• Cd is toxic and must be disposed of carefully• Memory effect

• Memory effect:• If a NiCad cell is recharged without being

completely discharged, then an unreactive surface can form on the electrodes, which can stop the recharging

Lithium Ion Batteries

• These are now used in laptops, mobile phones etc.

• They are complicated; high tech; prone to bursting into flames!

• Li is a reactive metal and should be able to generate lots of electrical energy

• BUT Li quickly gets covered in an oxide layer, which stops it making contact with an electrolyte, so the cell won’t work.

• To overcome this problem, lithium ion cells don’t contain lithium metal

• They contain mobile lithium ions• The –ve electrode is made of graphite• Lithium ions can enter the carbon lattice to

form LiC6

• The +ve electrode is a metal compound e.g MnO2 or CoO2 or NiO2

• At the –ve electrode• LiC6 Li+ + 6C + e-• (oxidation, hence anode)

• The electrolyte is usually an organic solvent which can carry the lithium ions to the +ve electrode

• Li+ + e- + MnO2 LiMnO2

• This is reduction

• What’s the overall reaction?

• LiC6 + MnO2 6C + LiMnO2

• The cell is powered by the overall movement of lithium ions

• The process can be reversed by passing a current in the opposite direction

• The big advantage is that there is no memory effect!

• They generate about 3.6V• This technology is as new to me as it is to you!• If you want to know more you’ll have to research it

yourself!!!

Discuss the similarities and differences between fuel cells and

rechargeable batteries

• Similarities:• Both convert chemical energy into electrical

energy• Both make use of spontaneous redox

reactions

• Differences:• Batteries are energy storage devices; fuel cells

are energy conversion devices• Fuel cells require a constant supply of

reactants. Batteries are a closed system• Batteries can be recharged – but cannot

generate electricity whilst being recharged. Fuels cells can operate continually and so have a longer operating life.

• Fuel cells have inert electrodes.• Fuel cells are more expensive!!!!!

Did we address all the syllabus statements?

• C.5.1 Describe how a hydrogen–oxygen fuel cell works. (Include the relevant half-equations in both acidic and alkaline electrolytes.)

• C.5.2 Describe the workings of rechargeable batteries. ( Include the relevant half-equations. Examples should include the lead–acid storage battery, the nickel–cadmium (NiCad) battery and the lithium-ion battery.

• C.5.3 Discuss the similarities and differences between fuel cells and rechargeable batteries.