Chapter 27 - Cells and Batteries

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Chapter 27 – Cells and Batteries
Primary Cells
• Batteries such as dry cells, alkaline cells and
button cells have one common feature; they
are non-rechargeable.
• Cells that cannot be recharged are called
primary cells.
• In primary cells, the products slowly migrate
away from the electrodes and are consumed
by side reactions occurring in the cells.
Zinc-Carbon Dry Cell
• This was the first small scale source of electrical
energy.
• An electrolyte composed of a moist paste of zinc
chloride and ammonium chloride and it plays the
same role as the salt bridge.
• At the anode (-), oxidation of the zinc
case produces the electrons.
• At the cathode (+), reduction of
manganese dioxide.
Zinc-Carbon Dry Cell cont…
• A new cell produces about 1.5 volts, but this diminishes
during use.
• To maintain a net forward reaction, the soluble reaction
products must migrate away from the electrodes.
• During use, the build up of products at the electrodes slows
and can stop the forward reaction.
• This is known as polarisation.
• If a cell is allowed to rest, some of the products migrate
away from the electrodes.
• However, once the cell reaches equilibrium, the cell will be
flat.
Alkaline Cells
• Have largely replaced Zinc-Carbon Cells.
• The chemical reaction within an alkaline cell is similar to the Zinc-Carbon
cell but the construction is totally different.
• The alkaline cell is optimised for performance and longevity.
• At the anode, zinc powder around the central metal rod is oxidised and
once the ion is formed it reacts immediately with the OH- ions in the
electrolyte to form zinc hydroxide.
• At the cathode, manganese dioxide is reduced.
• The improvements in this cell give it about five times the life of
the Zinc-Carbon cell.
Button Cells
• Are used in very small devices.
• There are two main types: silver-zinc cells and
lithium cells.
• Lithium button cells produce about 3 volts
during discharge and silver-zinc give an almost
constant 1.6 volts.
Rechargeable Cells and Batteries
• Rechargeable cells are known as secondary cells or
accumulators.
• To recharge a cell, the products of the reaction must be
converted back into the original reactants.
• This is done by connecting the cell to a charger, a source of
electrical energy, which has a potential difference greater
than the potential difference of the cell.
• Electrical energy supplied is converted into chemical energy
in the cell.
• In order for it to be possible to regenerate the reactants,
the products formed in the cell during discharge must
remain in contact with the electrodes in a convertible form.
Car Batteries
• Lead-acid batteries are the most widely used
secondary cells.
• This is a car battery.
• Although they appear to be a single unit, they
are actually six separate cell connected
together in a series.
• Each cell contains three positive electrodes
sandwiched between four negative
electrodes.
Car Batteries cont…
• The electrodes are separated by a porous
separator.
• The positive electrodes consist of a lead grid
packed with lead oxide.
• Negative electrodes consist of a lead grid packed
with powdered lead.
• A solution of sulfuric acid acts as the electrolyte.
• Each cell has a potential difference of just over 2
volts, a car battery has six of these so the total
potential difference is about 12volts.
Car Batteries cont…
• At the anode:
– Pb(s) + SO42-(aq)  PbSO4(s) + 2e-
• At the cathode:
– PbO2(s) + SO42-(aq) + 4H+(aq) + 2e-  PbSO4(s)+
2H2O(l)
• The overall equation:
– Pb(s) + PbO2(s) + 2SO42-(aq) + 4H+(aq) 
2PbSO4(s) + 2H2O(l)
The product of both electrode reactions forms a
solid on the surface of the electrodes, enabling
the battery to be recharged.
Car Batteries cont…
• To recharge the battery, the electrode
reactions are reversed.
• The alternator is used to force electrons into
the battery’s negative terminal and draw
them out at the positive terminal.
Nickel-based Cells and Batteries
• The nickel based cells consist of a coiled
anode, porous separator and cathode
immersed in a concentrated KOH electrolyte.
• At the anode:
– For nickel-cadmium cells: Cd(s) + 2OH-(aq) 
Cd(OH)2(s) + 2e– For nickel-metal hydride cells: MH(s) + OH-(aq) 
M(s) + H2O(l) + e-
• At the cathode:
– NiOOH(s) + H2O(l) + e-  Ni(OH)2(s) + OH-(aq)
Nickel-based Cells and Batteries
• The electrode reactions are fully reversible.
• This enables the reactants to be regenerated
when the cell is recharged.
• Most brands of nickel-cadmium cells are
capable of 1000 discharge-recharge cycles.
• Nickel-metal hydride cells will do around 500
cycles.
Fuel Cells
• Cells can be constructed in which the reactants are
supplied continuously allowing constant production
of electrical energy.
• These devices are called fuel cells.
• Fuel cells transform chemical energy directly into
electrical energy.
• At the anode:
– H2(g) + 2OH-(aq)  2H2O(l) + 2e-
• At the cathode:
– O2(g) + 2H2O(l) + 4e-  4OH-(aq)
• The overall reaction:
– 2H2(g) + O2(g)  2H2O(l)
Fuel Cells cont…
• Each cell produces about one volt.
• Higher voltages are obtained by
connecting a number of fuel cells in
series.
• The only by-products are water and
heat.
• The has been development of an acid
fuel cell – this cell uses concentrated
phosphoric acid as the electrolyte
and air as the source of oxygen. It
uses hydrogen but does not require it
to be pure.
Fuel Cells cont…
• The use of fuel cells in transportation
improves fuel efficiency and reduces
greenhouse gas and other emissions.
• Stationary fuel cell systems are used to
generate electricity for domestic, commercial
and industrial purposes.
• A methanol powered fuel cell has been
developed for use with portable electronic
equipment such as mobile phones.
Advantages of Fuel Cells
• Convert chemical energy directly to electrical
energy, better efficiency.
• Water is a by-product, no greenhouse gases
released.
• Will generate electricity for as long as fuel is
supplied, in conventional batteries electricity
production stops once the reaction reaches
equilibrium.
• Use a variety of different fuels.
• Electricity can be generated on site.
Disadvantages of Fuel Cells
• Require a constant fuel supply.
• They are expensive.
• Some types of fuel cell use expensive electrolytes
and catalysts.
• Fuel cells generate a direct current (DC), electrical
appliances used in homes and industry rely on
alternating current (AC).
• The effectiveness of some fuels cells are affected by
impurities in the hydrogen fuel.
• Use of fuel cells in transport is limited by lack of
facilities for hydrogen storage and distribution.
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